Article

Exceeding conversion efficiency of 26% by heterojunction interdigitated back contact solar cell with thin film Si technology

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Abstract

We have developed heterojunction interdigitated back contact solar cell with conversion efficiency of 26.6% (designated area: 180 cm²) independently confirmed by Fraunhofer Institute for Solar Energysystem Callab. Compared to our previous record efficiency (26.3%), the 0.3% absolute improvement can be regarded as ~10% reduction of remaining losses to the theoretical limit (~ 29%). We discuss the analyzed cell properties together with our recent progress to predict how far we can go in reducing the remaining losses in silicon photovoltaic.

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... Many research laboratories with expertise in thin-film silicon photovoltaics joined the effort in the past 15 years, following the decline of this technology for large-scale energy production. Their success suggests that strong synergies exist between the two fields 57,79,[115][116][117][118] . A key feature of such silicon heterojunction (SHJ) devices (Fig. 3g,h) is their high V oc (typically 730-750 mV) (TaBle 1). ...
... Efficiencies above 21% (two-side contacted) and 22% (all-rear contacted) were demonstrated in 'dopant-free' architectures (not using doped silicon to form the contact) 153,154 . Parasitic light absorption in a-Si:H is totally eliminated in interdigitated back contact devices, for which even light absorbed in a front intrinsic a-Si:H layer contributes to photocurrent 116,153,155 . This structure has enabled the highest efficiency silicon solar cells since 2015 (reFs 116,156 ). ...
... Parasitic light absorption in a-Si:H is totally eliminated in interdigitated back contact devices, for which even light absorbed in a front intrinsic a-Si:H layer contributes to photocurrent 116,153,155 . This structure has enabled the highest efficiency silicon solar cells since 2015 (reFs 116,156 ). Process complexity precludes industrialization, but significant simplifications of the manufacturing process were demonstrated 81,82 . ...
Article
Crystalline silicon (c-Si) photovoltaics has long been considered energy intensive and costly. Over the past decades, spectacular improvements along the manufacturing chain have made c-Si a low-cost source of electricity that can no longer be ignored. Over 125 GW of c-Si modules have been installed in 2020, 95% of the overall photovoltaic (PV) market, and over 700 GW has been cumulatively installed. There are some strong indications that c-Si photovoltaics could become the most important world electricity source by 2040–2050. In this Review, we survey the key changes related to materials and industrial processing of silicon PV components. At the wafer level, a strong reduction in polysilicon cost and the general implementation of diamond wire sawing has reduced the cost of monocrystalline wafers. In parallel, the concentration of impurities and electronic defects in the various types of wafers has been reduced, allowing for high efficiency in industrial devices. Improved cleanliness in production lines, increased tool automation and improved production technology and cell architectures all helped to increase the efficiency of mainstream modules. Efficiency gains at the cell level were accompanied by an increase in wafer size and by the introduction of advanced assembly techniques. These improvements have allowed a reduction of cell-to-module efficiency losses and will accelerate the yearly efficiency gain of mainstream modules. To conclude, we discuss what it will take for other PV technologies to compete with silicon on the mass market. Crystalline silicon solar cells are today’s main photovoltaic technology, enabling the production of electricity with minimal carbon emissions and at an unprecedented low cost. This Review discusses the recent evolution of this technology, the present status of research and industrial development, and the near-future perspectives.
... In previous sections, an interdigitated back-contact cell with passivating and carrier-selective POLO contacts was developed, which on one hand maximizes the photo-generating current through the back-contact configuration and on the other hand maximizes the open-circuit voltage by using passivating and carrier-selective contacts. Solar cells of this type have already achieved an efficiency of 26.7% with a-Si/c-Si heterojunction contacts on n-type wafers [396] and an efficiency of 26.1% with POLO contacts on p-type wafers [156]. As a result, the potential of single-junction silicon solar cells with a theoretical limiting efficiency of 29.5% [28,29] is nearly exploited 1 . ...
... The R F should be kept low for narrow band gap top cells. In summary, bottom cells for highly efficient reverse-connected tandem cells should be designed similar to high efficiency IBC single junction solar cells with low R Z values [156,157,396,417]. Large-scale industrial-type IBC cells with a total series resistance of 0.4 Ωcm 2 and an efficiency of 25.2% as reported by Smith et al. [189] are an excellent choice for industrialization of reverse-connected tandem cells. ...
Thesis
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This thesis investigates back-contacted (IBC) bottom solar cells with passivating and carrier-selective POLO contacts with three terminals (3T-POLO-IBC cell). Such cells form the foundation of monolithic three-terminal tandem solar cells. This novel tandem solar cell enables the use of sub-cells with mismatched photocurrents. Thus, this tandem solar cell technology platform offers the flexibility with respect to subcell material selection, the ease of fabrication, and a robustness to spectral variations of incident light over the course of the day and year. Three building blocks of the 3T POLO IBC bottom solar cell, which are based on each other, are examined: First, the passivating and carrier-selective POLO contact. Second, the integration of POLO contacts on the rear side of a solar cell. Third, the principle of operation of a bottom cell with three terminals. In the first part, the process of charge carrier extraction at selective contacts to the photoabsorber is theoretically explored. The selectivity of a contact is defined on the basis of (reaction) kinetic considerations at the contact in terms of the rate ratio of desired processes to undesired processes. The extraction efficiency of charge carriers at the contact is derived as the ratio of the external voltage versus the internal voltage from a thermodynamic point of view. To emphasize the unifying nature of the definitions in this thesis, the existing literature definitions are calculated from the definitions in this thesis. The extraction efficiency is related to the selectivity coefficient of the contact and the limiting efficiency of a silicon solar cell with given contact selectivity is calculated accordingly. After the detailed theoretical investigation on selectivity, the properties of n+ and p+ POLO contacts are examined. Low saturation current densities between 2 fA/cm² and 18 fA/cm² and contact resistivities between 0.4mOhmcm² and 10mOhmcm² are found at the same time. It is shown that the efficient carrier transport of majority carriers is ensured by pinholes in the interfacial oxide. The resulting logarithmic selectivity coefficient of POLO contacts is determined to be above 15, which is one of the highest values measured. This makes POLO contacts predestined for solar cells with the highest efficiencies. POLO contacts are integrated on the rear side of a back-contact cell with POLO contacts for both polarities. Thereby, the p+ and n+ doped poly-Si on the backside of the solar cell form a parasitic graded p+n+ junction within the defect-rich poly-Si with a carrier lifetime of a few picoseconds. The arising recombination limits the achievable efficiency of the POLO-IBC cell to about 18%. For this reason, the parasitic junction is removed during the cell fabrication process by wet-chemically introducing a trench between the n+- and p+-doped poly-Si regions. The POLO-IBC cell with isolated n+- and p+ poly-Si regions achieves a certified efficiency of 24.25%. For the last part, a third POLO contact is added to the POLO-IBC cell and the 3T-IBC bottom cell is studied in detail using current-voltage measurements. First, the different realization options for a 3T tandem solar are sorted and the corresponding nomenclature is presented. Two different 3T IBC bottom cell architectures are identified. The first one – the unijunction bottom solar cell – contains a single minority carrier contact and two majority carrier contacts. The second one – the bipolar junction bottom solar cell – on the other hand, has two minority carrier contacts and a single majority carrier contact. Both 3T bottom cell architectures are fabricated based on a modified POLO-IBC fabrication process. The principles of operation and loss mechanisms are elucidated using J-V measurements on illuminated devices and by means of analytical modeling. The experiments show that the third contact of a 3T unijunction and bipolar junction bottom cell allows the collection or injection of additional minority or majority carriers from or into the bottom cell. Ideally, the power output of such a 3T bottom cell is nearly independent of the current density applied by the top cell. Therefore, no current matching of both subcells is required. However, the transport of majority carriers or minority carriers through the unijunction or bipolar junction bottom cell causes a loss, which, however, can be made negligible by a specific design of the bottom cell. The design rules are explained in detail. After the detailed investigations, a 3T unijunction bottom cell with a textured n+-POLO front contact with an efficiency of 20.3% and a simplified screen-printed PERC-like 3T bipolar junction bottom cell with 14.4% are developed. The latter is an attractive approach to utilize the dominant PERC technology in a low-cost tandem solar cell with maximum energy yield. Finally, the first 3T GaInP//POLO-IBC tandem cell demonstrator is fabricated with an efficiency of 27.3% and a net efficiency gain of 0.9% is demonstrated compared to the 2T operation of the 3T tandem cell.
... A RECORD conversion efficiency of 26.6% has recently been reported for amorphous silicon/crystalline silicon (a-Si:H/c-Si) hetero-junction solar cells (SHJ) [1]. Such an exceptional performance is attributed to the high carrier injection selectivity demonstrated by the a-Si:H/c-Si hetero contact [2], [3] especially when an ultrathin intrinsic a-Si:H(i) Manuscript spacer layer is inserted between the p-type doped a-Si:H(p) layer and the c-Si substrate [4]. ...
... It is worth mentioning that the simulated results in Fig. 6 are obtained assuming drift-diffusion hole transport across the thermionic emission barrier at the hetero-interface At open circuit terminals, the total current through subcircuit S is zero such that J Sp , J ph,S , and J Dn are looping and a voltage drop V S,OC is established on the terminals of DS. The loop equation (1) in that case becomes ...
Article
A bipolar equivalent circuit model is developed for silicon hetero-junction silicon solar cells (SHJ) with a Schottky contact between the p-type amorphous silicon and the transparent conducting oxide (TCO). The Schottky barrier is treated as a bipolar collector junction through which holes are transported by thermionic emission, while electrons reaching the barrier are swiftly collected. The electron current is shown to suffer an Early effect especially in cells with low a-Si:H(p) doping level. The results of the equivalent circuit analysis in the dark and under illumination perfectly match both the J-V characteristics resulting from AFORS-HET device simulations and the measured J-V characteristics of real cells, with low and with high doping levels in a-Si:H(p). The model provides a clear interpretation for the role of the Schottky diode in the degradation of the cell open circuit voltage and in the S-shape distortion of the J-V characteristics under illumination. Such degradations disappear if hole tunneling through the Schottky contact prevails.
... Later, Panasonic reported another new record conversion efficiency of 25.6% by reducing recombination rate and optical gain, using the Interdigitated Back Contact (IBC) design [29]. However, in 2017, Kaneka Corporation broke this record by attaining a conversion efficiency of 26.63%, which is the world record for cell efficiency value until now [30]. The evolution of SHJ solar cell conversion efficiencies over the years can be found in Figure 1.8. ...
... 30. Mean values of C -/Si -SI ratios calculated using ToF-SIMS depth profile data directly relate with the measured optical band gap and resistivity values. ...
... High-quality purified and thin absorber (i.e., kerf loss reduction) [171][172][173];  Introduction of high-quality surface passivation with better light transmission and electronically tunneling layers of polycrystalline silicon on oxide (POLO) technology and its derivative technologies such as tunnel oxide passivated contact (TOPCon) harmonized with PERC technology forming PERx/TOPCON/(PERC+) solar cells, which may benefit from existing PERC production facilities [174][175][176][177]. The most recent SHJ solar cell research on passivated contacts investigates a shift of local selective contact from very thin a-Si: H to a new concept of self-doped and high/low work function (hole/electron) adapted materials. ...
... A high-quality SHJ absorber features high-quality passivation of symmetric structure with an enhanced open-circuit voltage close to 750 mV [181,[188][189][190]. Hence, more focus on present silicon solar cell challenges will emerge from this technology. Despite SHJ-based technologies having less recombination loss in comparison with widely industrialized technologies such as passivated emitter and rear cell (PERC), as shown in Figure 5a, and aluminum-back surface field (Al-BSF) [174,176,177], SHJ technology encounters optical loss due to the intrinsic and doped hydrogenated amorphous silicon and TCO bilayers [191]. ...
Preprint
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The unprecedented development of perovskite-silicon (PSC-Si) tandem solar cells in the last five years has been hindered by several challenges towards industrialization, which require further research. The combination of the low cost of perovskite and legacy silicon solar cells serve as primary drivers for PSC-Si tandem solar cell improvement. For the perovskite top-cell, the utmost concern reported in the literature is perovskite instability. Hence, proposed physical loss mechanisms for intrinsic and extrinsic instability as triggering mechanisms for hysteresis, ion segregation, and trap states, along with the latest proposed mitigation strategies in terms of stability engineering, are discussed. The silicon bottom cell, being a mature technology, is currently facing bottleneck challenges to achieve power conversion efficiencies (PCE) greater than 26.7%, which requires more understanding in the context of light management and passivation technologies. Finally, for large-scale industrialization of the PSC-Si tandem solar cell, the promising silicon wafer thinning, and large-scale film deposition technologies could cause a shift and align with a more affordable and flexible roll-to-roll PSC-Si technology. Therefore, this review aims to provide deliberate guidance on critical fundamental issues and configuration factors in current PSC-Si tandem technologies towards large-scale industrialization. to meet the 2031 PSC-Si Tandem road maps market target.
... Photovoltaics were first investigated six decades ago in order to harness solar energy. During this investigation, numerous materials such as Si, CIGSSe, CdTe, CZTSSe, Perovskites, and other materials appeared as absorbent layers in solar cells, and Si solar cells now dominate the PV industry [ 1 ] . CIGSSe and CdTe-based solar cells have demonstrated higher efficiency in thin film solar cells [ 3 [ ,] 2 ] but indium (In) is rare in CIGSS, and the toxicity of cadmium (Cd) in CdTe has prevented their widespread usage. ...
... The reason for the decrease of Jsc at the density of defects of 1E10cm -2 with the increase in the thickness of the buffer layer is due to the large numbers of photons having wavelengths less than 500 nm being absorbed before they reach the absorption layer, This contributes to a decrease in the generation of electron-hole pairs in the absorption layer and this was evident in Figure a. 8 in the region. (1). As for the defect density of 1E14cm -2 , we showed a decrease in the quantum efficiency with an increase in thickness in regions (1) and (2) of Figure 6.b, this is due to the increase in surface recombination and the decrease in the diffusion length of the charge carriers, and thus the decrease of Jsc. ...
Article
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The simulation model used in this study is the heterojunction solar cell with SnS absorption layer using the AFORS-HET simulation program. Where the effect of interface defect density (Nit) and the location of these levels within the interface on the electrical and optical properties was studied. Through the study, we learned the effect of the locations of the energy levels for defects within the junction, and it was found that the largest effect of the defects is within the locations of the deep energy levels, D.deep traps and A.deep traps within the junction. After that, the effect of interface defect density and its relationship to thickness and impurities concentration of both the buffer and absorption layer were studied, as the increase in the thickness of the absorption layer indicated a clear decrease in the effect of the density of the interface cases, otherwise the increase in the thickness of the buffer layer did not reduce the effect of these defects. The effect of the interface defect density increases with increasing the concentration of impurities in the SnS absorption layer, the effect of the density of the interlayer defects decreases with the increase in the concentration of impurities in the buffer layer.
... The purpose of the present work is to understand the transport mechanisms underlying photovoltaic devices based on SHJ technology by simulating at atomistic resolution amorphous-crystalline heterointerfaces. In recent years, the SHJ solar cells reached the highest efficiency of 26.6% (Yoshikawa 2017), mainly due to the passivation contacts. In these devices, intrinsic hydrogenated amorphous silicon (a-Si:H) was used to passivate the Si surface and the p/n-type doped hydrogenated amorphous silicon was employed to select the transport carriers. ...
Conference Paper
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The microscopic mechanisms of transport and recombination mechanisms in silicon heterojunction solar cells are still poorly understood. The purpose of the present work is to understand the transport mechanisms underlying photovoltaic devices based on silicon heterojunction technology by simulating at atomistic resolution amorphous-crystalline heterointerfaces. We have used classic molecular dynamics simulations to build up realistic c-Si/a-Si:H/c-Si interface at different temperatures. The ab initio characterization has been executed on selected configurations to monitor the electronic properties of the c-Si/a-Si:H/c-Si interface. The electron transmission is calculated at different temperatures based on the non-equilibrium Green functions approach and its behavior is correlated to the evolution of the intragap states. The whole outlined process will allow designing more efficient silicon solar cells belonging to the silicon heterojunction technology.
... Полученная спектральная дисперсия квантового выхода с.э. HIT хорошо сопоставляется с литературными данными [6,7]. На рис. 3 показано различие между значениями квантового выхода с.э. ...
Article
Описана установка для измерения спектральной дисперсии квантового выхода в широком температурном диапазоне. Установка может отслеживать изменение мощности светового потока и вносить корректировку при расчете значения квантового выхода. Отличительной особенностью установки является отсутствие второго светового канала для мониторинга изменения световой мощности. Это позволяет отказаться от механически подвижных частей, что упрощает устройство и не требует синхронизирующих устройств.
... Crystalline silicon solar cells have reached an efficiency of 26.6%, and perovskite solar cells have achieved a PCE of 25.2% [1,2]. Fabrication of large-area perovskite solar cells using low-cost materials is an active area of photovoltaic research [3,4]. ...
Article
Full-text available
This paper presents the fabrication of large-area four-terminal (4 T) perovskite-Si solar cells. Large-area semi-transparent perovskite solar cells were fabricated by utilizing a thin copper layer as the low-cost transparent electrode. Carrier selective contact (CSC)-based Si solar cell was also fabricated with molybdenum oxide (MoOx) hole selective layer. Large-area semi-transparent perovskite solar cells (PSCs) with active areas 1 cm² and 2 cm² showed a power conversion efficiency (PCE) of 5.07% and 4.10%, respectively. The CSC Si solar cell displayed a PCE of 3.42%. The CSC-Si cell exhibited an efficiency of 2.24% under filtered light when placed under the semi-transparent perovskite top cell. The four-terminal effect was also demonstrated with a commercially available monocrystalline-Si solar cell. The efficiency of commercial c-Si solar cell was 14.11% under 100 mW cm⁻² illumination and 10.08% under filtered light. The combined efficiency values of the 4 T configurations with perovskite top cell and the bottom CSC Si cell, and the commercial c-Si solar cell, were 7.31% and 15.15%, respectively. These values were more than the individual cell efficiencies. Graphical Abstract
... [8][9][10] The last few years have witnessed the remarkable progress of passivating contacts based on doped-silicon layers, which have boosted the PCE of single-junction c-Si solar cells to over 26%. 11,12 However, doped-Si-based passivating contacts require complex and capital-intensive deposition processes involving toxic gasses and could induce undesired optical losses. 13 Hence, passivating contacts based on wide bandgap metal compounds (generally E g > 3 eV) become attractive alternatives because on one hand their hightransparency reduces parasitic absorption and on the other hand they can be easily deposited in cost-effective manners. ...
Article
Full-text available
High work function vanadium oxide (V2OX, X < 5) is expected to induce strong upward band bending at crystalline silicon (c-Si) surface thus selectively collect photogenerated hole-carriers. However, the performance of c-Si solar cells employing V2OX-based hole-selective contacts is still under expectation. Herein, we improve the hole-selectivity of V2OX in combination with NiOX. The innovative V2OX/NiOX stack shows reduced contact resistivity but deteriorated minority carrier lifetime due to undesired interfacial reaction between V2OX and NiOX. Inserting an ultrathin SiOX interlayer suppresses the reaction and preserves the high work function of V2OX. A remarkable power conversion efficiency of 22.03% (fill factor of 83.07%) was achieved on p-type c-Si solar cells featuring a full-area V2OX/SiOX/NiOX rear contact, which is so far the highest value reported for V2OX-based selective contacts. Our work highlights the significance of implementing p-type transition-metal-oxides to boost the selectivity of V2OX and the like.
... III-V multijunction solar cells are very efficient, repeatedly breaking conversion records in recent years (Geisz et al. 2020), but still expensive for terrestrial applications. Conversely, silicon solar cells whose technology dominates the terrestrial market (Battaglia et al. 2016) are approaching a standstill having essentially reached their practical efficiency limit (Yoshikawa et al. 2017). Therefore, the integration of III-V semiconductors on silicon substrates has been the target of numerous research lines from the 1980s (Suzuki et al. 1991, Hayashi et al. 1994a based on the premise of high performance III-V semiconductor multijunction solar cells combined with the low-cost advantages of large area silicon substrates (Hayashi et al. 1994b, Kurtz et al. 2008, Supplie et al. 2018, Essig et al. 2017. ...
Article
This paper studied how the chemical content of halogen atom impacts the structural, electrical, optical and photovoltaic characteristics of double perovskite Cs2Sn(Br1−xIx)6. When the iodide (I) content of x was increased, the lattice constant went up with a slight deviation from Vegard's linear law. Upon enhancing I concentration, the energy bandgap (Eg) fades. In fact, by rising the I content x from 0.00 to 1.00 in 0.25 increments, the Eg obtained values using the Perdew-Burke-Ernzerhof Generalized-Gradient Approximation (PBE-GGA) functional decrease from 1.413 to 0.210 eV, and those determined using the modified Becke Johnson generalized gradient approximation (mBJ-GGA) decrease from 2.497 to 0.934 eV. Calculated dielectric functions, refractive index, and absorption coefficient show a red shift at higher I content. Spectroscopic limited maximum efficiency (SLME) is 33.015%, which corresponds to the I content x = 0.557, closely approaching the Shockley-Queisser limit (33.16%).
... In an era of photovoltaic technology, silicon is an efficient light absorber with relatively high-power conversion efficiency and best stability [1]. But high processing cost to obtain pure silicon has been remained as a challenge since its usage for photovoltaic purpose. ...
Article
Full-text available
Outstanding improvement in power conversion efficiency (PCE) over 25% in a very short period and promising research developments to reach the theoretical PCE limit of single junction solar cells, 33%, enables organic–inorganic perovskite solar cells (OIPSCs) to gain much attention in the scientific and industrial community. The simplicity of production of OIPSCs from precursor solution either on rigid or flexible substrates makes them even more attractive for low-cost roll-to-roll production processes. Though OIPSCs show as such higher PCE with simple solution processing methods, there are still unresolved issues, while attempts are made to commercialize these solar cells. Among the major problems is the instability of the photoactive layer of OIPSCs at the interface of the charge transport layers and /or electrodes during prolonged exposure to moisture, heat and radiation. To achieve matched PCE and stability, several techniques such as molecular and interfacial engineering of components in OIPSCs have been applied. Moreover, in recent times, engineering on additives, solvents, surface passivation, and structural tuning have been developed to reduce defects and large grain boundaries from the surface and/or interface of organic–inorganic perovskite films. Under this review, we have shown recently developed additives and passivation strategies, which are strongly focused to enhance PCE and long-term stability simultaneously.
... III-V multijunction solar cells are very efficient, repeatedly breaking conversion records in recent years (Geisz et al. 2020), but still expensive for terrestrial applications. Conversely, silicon solar cells whose technology dominates the terrestrial market (Battaglia et al. 2016) are approaching a standstill having essentially reached their practical efficiency limit (Yoshikawa et al. 2017). Therefore, the integration of III-V semiconductors on silicon substrates has been the target of numerous research lines from the 1980s (Suzuki et al. 1991, Hayashi et al. 1994a based on the premise of high performance III-V semiconductor multijunction solar cells combined with the low-cost advantages of large area silicon substrates (Hayashi et al. 1994b, Kurtz et al. 2008, Supplie et al. 2018, Essig et al. 2017. ...
Article
III-V compound semiconductors and SiGe alloys can be combined to develop multijunction solar cells on Silicon substrates with optimum bandgap combinations. Current implementations of such devices have reached efficiencies over 20%, using thick –and thus costly– buffer layers which induce the appearance of cracks in large area samples. As a strategy to mitigate these two issues (thick buffers and cracking), a GaAsP/SiGe tandem solar cell has been developed employing group IV reverse graded buffer layers grown on Ge/Si virtual substrates with a subsurface Silicon porous layer. Reverse buffer layers facilitate a reduction in the threading dislocation density with limited thicknesses but can also induce cracks. To minimise this, a porous silicon layer has been incorporated close to the Ge/Si interface so that the ductility of this layer suppresses crack propagation. In terms of solar cell performance, this porous layer reduces the problem of cracks, not totally supressing them though. Accordingly, the low shunt resistance observed in previous designs has been increased thus improving solar cell efficiency, which is still notably behind designs using thicker forward graded buffer layers. The first results of this new architecture are presented here.
... The passivation quality of a-Si/c-Si heterojunction is excellent which allowed to achieve the actual world record efficiency of 26.7% for silicon solar cells. 11 However, the significant parasitic absorption of light in the a-Si layers made it necessary to use an interdigitated back contact (IBC) cell structure. Bothsides contacted heterojunction cells were commercialized by Sanyo and then later by Panasonic. ...
Article
Full-text available
Passivating contacts based on poly-Si/SiOx structures also known as TOPCon (tunnel oxide passivated contacts) have a great potential to improve the efficiency of crystalline silicon solar cells, resulting in more than 26% and 24% for laboratory and industrial cells, respectively. This publication gives an overview of the historical development of such contact structures which have started already in the 1980s and describes the current state-of-the-art in laboratory and industry. In order to demonstrate the great variety of scientific and technological research, four different research topics are addressed in more detail: (i) the superior passivation quality of TOPCon structures made it necessary to re-parametrize intrinsic recombination in silicon, (ii) the control of diffusion of dopants through the intermediate SiOx layer is essential to optimize passivation and transport properties, (iii) single-sided deposition of the poly-Si layer would reduce process complexity for industrial TOPCon cells, and (iv) silicon-based tunnel junctions for perovskite–silicon tandem cells can be fabricated using the TOPCon technology.
... The interdigitated back-contact (IBC) cells employing this structure possess a high open-circuit voltage (V oc ) and have achieved a world record PCE of 26.6%. 1 Another approach is based on the tunnel oxide (SiO 2 ) and doped poly-Si (tunnel oxide passivated contacts [TOPCon]), which has achieved a remarkably high efficiency of 25.1%. 2 However, the undesired parasitic absorption in a-Si:H layers results in inevitable current losses. Furthermore, these techniques employed capital-intensive systems (such as plasma enhanced chemical vapor deposition [PECVD]) and flammable gases, such as silane and boron and phosphorus gases, all of which make it indispensable to search for alternative CSCs. ...
Article
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Designing effective carrier-selective contact is a prerequisite for high-efficiency crystalline silicon (c-Si) solar cells. Compared to doped silicon thin films, wide-band-gap transition metal oxides (TMOs) feature low parasitic absorption, but their carrier selectivity and passivation being poor leads to a mediocre cell efficiency. Herein, we introduce a NiOx/MoOx bilayer as an efficient hole-selective contact in c-Si solar cells. A power conversion efficiency (PCE) of 21.31% is achieved using NiOx/MoOx bilayer, outperforming cells with a single layer of NiOx or MoOx. Upon depositing NiOx on MoOx, interfacial reactions modify the stoichiometry and defect chemistry in both oxides, leading to a band alignment beneficial for hole selectivity. By inserting a SiOx tunneling layer on c-Si surface to further suppress recombination, we achieve a PCE of 21.60% (fill factor 83.34%). Our work highlights a promising approach to improve the performance of dopant-free c-Si solar cells by employing cost-effective TMOs as hole-selective contact.
... Tracking the point of maximum power generation in a SPV system is critical and several techniques are available [16][17][18][19][20][21]. An appropriate Maximum Power Point Tracking (MPPT) technique is usually used to extract the maximum possible power from the PV panel. ...
Article
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Solar Photovoltaic (SPV) and wind energy are two major sources of renewable energy that are intermittent in nature. A hybrid system consisting of SPV and Wind Energy Conversion System (WECS) can meet the energy needs as either of the source continues to generate energy, in the absence of the other that is reliable and cost effective. The paper presents detailed mathematical modelling, simulation and performance analysis of a single cell SPV and Permanent Magnet Synchronous Generator (PMSG) based WECS. These two sources are integrated to form a hybrid renewable energy system. The hybrid model is simulated and the overall performance is analyzed using MATLAB/Simulink for varying temperature/irradiation conditions and varying wind speeds for SPV and WECS respectively. Performance characteristics of the hybrid PV and WECS are presented. The paper also presents as a case study, the annual energy generation for a chosen location from individual SPV and WECS systems based on the real time data collected from Solacast website. This study evaluates the feasibility of implementation of a hybrid system at the site.
... The selected architecture is based on interdigitated back-contacted (IBC) c-Si solar cells. Such technology currently holds the highest conversion efficiency (η) not only at cell level at laboratory scale 20,21 but also at module level among all commercially available technologies. 22 Moreover, as both metallic contacts are at the back side of the cells, modules based on such architecture result in a pleasant and homogeneous esthetic. ...
Article
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Building Integrated Photovoltaic systems can produce a significant portion of the energy demand of urban areas. Despite their potential, they remain a niche technology that architects and project engineers still find esthetically limited. The dark blue or black color of standard photovoltaic panels is considered inappropriate for restoration projects of historic buildings and represents a major constraint on the development of new projects. This work will provide insight into how the use of optic filters can offer new pathways for architectural acceptance of photovoltaic panels. Optic filters selectively reflect or transmit light by interference and can be designed and fabricated using cost‐effective and industrially compatible processes. By using in‐house developed ray tracing software coupled with TCAD Sentaurus, more than 400 colors were obtained, and their impact on the opto‐electrical performance of interdigitated back‐contacted solar cells was studied. Results show a maximum efficiency loss of 1.6% absolute at the perpendicular incidence of light on the range of obtained colors when compared with a standard dark blue solar cell. Simulations for different angles of incidence showed that the current reduction on the standard device could be modeled using a cosine relationship. The colored cells, however, deviated significantly from this relationship. We propose that the angular behavior of any cell (colored or standard) could be simulated by modifying the effective irradiance with scaling factors equal to the ratios of the photogenerated current at any angle with respect to the value at normal incidence. We demonstrate that this approach accurately models the effect of the color filter and allows for an easy transition from a bare cell to an encapsulated one. Due to the spectral effect of the filter, we developed both a spectrally resolved optical model and a two‐dimensional finite volume transient thermal model. In case of the optical model, we demonstrate an accuracy in the prediction of the reflectance produced by the color with values of mean bias error (MBE) between 2.0% and 3.9%. As for the thermal model, it was validated by first analyzing a standard model under conditions of nominal operating cell temperature and then comparing its results with published scientific literature. Later, we compare its prediction against 2 weeks of measurements. In both cases the thermal model proves an adequate accuracy, yielding differences below 1.5°C with respect to other scientific works and an MBE value of 0.89°C as well as a root‐mean‐square error value of 2.10°C for the entire measurement period. With the validated models, we studied the effect of the encapsulation on the color perception. We present two options of color filters. The first one produces relatively low reflectance losses and presents relative annual direct current (DC) energy losses of up to 6.4% for Delft, in the Netherlands, and up to 5.9% for Alice Springs in Australia. However, this first option has very poor color brightness. The second studied filter produces highly saturated bright colors. Improving brightness can increase the annual DC relative losses up to 13.7% and 13.5% for Delft and Alice Springs, respectively. Overall, we demonstrate that colored filters based on multilayer optical stacks are a versatile option for coloring cells that allow a good compromise between esthetics and performance. We present a modeling framework for performance assessment of colored solar cells. Sentaurus TCAD simulations showed that application of optic filters to produce colored interdigitated back‐contacted (IBC) solar cells produces relatively low impact on efficiency. In‐house developed ray tracing software was used to study the behavior of colored cells. The proposed methods demonstrate that cell to system modeling is possible using scaling factors and the two‐diode model. Simulations show that colors can produce relative annual yield losses ranging from 2% to 11%.
... heterojunction-with-intrinsic-thin-layer (HIT) solar cell [2]. The former can enhance the incident light by all electrodes on rear surface and the later can reduce recombination loss by well-passivated contact. ...
... The former is usually formed on n-type c-Si wafers with intrinsic and doped amorphous Si passivating and carrierselective stacks and currently holds the c-Si efficiency record at 26.7%. 25 On the other hand, TOPCon/POLO solar cells feature a thin tunnel layer of silicon oxide (SiO x ) embedded between the doped poly-Si and the c-Si absorber and currently have reached efficiencies exceeding 26% on p-type c-Si wafers. 26,27 Though these technologies are at an advanced level in the development of c-Si solar cells, there still exist several challenges associated with the fabrication complications as well as various fundamental limitations arising mainly from the doped Si layers. ...
Article
The present study investigates the application of hole-selective transition metal oxide (TMO) layers (MoOx, V2Ox, and WOx) with silver (Ag) as full-area rear contact to 22.5 μm-thick low-quality Cz p-type c-Si solar cells. Thin films of metal oxides are deposited directly on p-type c-Si by thermal evaporation at room temperature. The large work function of these TMOs creates strong accumulation at the interface with p-type c-Si, which allows only holes to transport and simultaneously suppress the interfacial recombination current density (J0) and contact resistivity (ρc). The current generation and losses of 22.5 μm-thick solar cells with different hole-selective TMO/Ag at the rear are simulated. The presence of TMO/Ag at the rear is found to significantly reduce parasitic light absorption at longer wavelengths which becomes more pronounced for ultrathin wafers, providing significant advantages over conventional Al contact. The best device performance was attained by the MoOx/p-type c-Si solar cells, demonstrating a considerably high efficiency (η) of 14% with Voc of 555 mV, FF of 76.0%, and Jsc of 33.2 mA/cm². Furthermore, the present work is the first to employ MoOx, V2Ox, and WOx as rear contact in ultrathin p-type c-Si solar cells.
... Some other advantages of IBC architecture include easy module fabrication and lower series resistance due to higher metal fraction on the rear side [3]. Kaneka Corporation has reported a conversion efficiency of 26.7% [4] for its heterojunction IBC, ISFH has reported a conversation efficiency of 26.1% on its POLO solar cells [5], SunPower has reported an efficiency of 25% [6] on the SunPower X-Series technology, and SPIC has reported efficiency in excess of 23.5% on its low-cost bifacial IBC ZEBRA technology [7]. ...
Conference Paper
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With PERC technology approaching its efficiency limits, more companies and research institutes are interested in developing industrially feasible TOPCon and IBC cell architectures. Though the highest efficiency solar cells are realized on IBC cell structure, making them industrially viable is challenging. In this work, the process sequence for obtaining cost-effective IBC solar cells was studied. We investigate boron-doped emitter and phosphorus-doped FSF formation APCVD BSG and PSG layers, respectively. Lifetime samples with different dopant concentrations were exposed to different high-temperature co-annealing recipes. Also, the co-annealing was optimized to perform dual tasks: diffusion of dopants of both polarities and, at the same time, growing an in-situ SiO2 at the glass/Si interface for passivation purposes. To further optimize the APCVD IBC process sequence, a single wet-bench process after co-annealing was used to remove the dopant glasses using the in-situ grown SiO2 as the etch barrier. The passivation quality of lifetime test samples was lower than anticipated, and a systematic study pointed out that the source of low passivation quality is metal contamination.
... The highest efficiency (26.6%) among silicon single-junction solar cells (SCs) was achieved on the basis of a heterojunction between amorphous hydrogenated and monocrystalline silicon (a-Si:H/c-Si) [1], which simultaneously provides high selectivity of charge carriers and a low level of recombination at the interface. The disadvantage of a-Si:H is the parasitic absorption of solar radiation in the region of short wavelengths; therefore, to replace it, materials with wide bandgap are being sought that provide passivation and selection of charge carriers in the heterojunction; such structures are called "selective contacts" [2]. ...
... With great efforts from the photovoltaic (PV) research community, photoconversion efficiency (PCE) of silicon solar cells is now almost close to the practically achievable limit of 27.1% [1] and the theoretical limit of 29.56% [2]. The highest reported PCEs with tunnel oxide passivated contact (TOPCon) and heterojunction interdigitated back contact (HJ-IBC) structures were 25.8% [3] and 26.7% [4], respectively. In addition to increasing efficiency, silicon solar cells are reaching grid parity, while occupying about 95% of the PV market. ...
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In this work, nickel silicide was applied to tandem solar cells as an interlayer. By the process of thermal evaporation, a layer of NiOx, hole transport layer (HTL) was deposited on n+ poly-Si layer directly. Nickel silicide was simultaneously formed by nickel diffusion from NiOx to n+ poly-Si layer during the deposition and annealing process. The I–V characteristics of NiOx/n+ poly-Si contact with nickel silicide showed ohmic contact and low contact resistivity. This structure is expected to be more advantageous for electrical connection between perovskite top cell and TOPCon bottom cell compared to the NiOx/TCO/n+ poly-Si structure showing Schottky contact. Furthermore, nickel silicide and Ni-deficient NiOx thin film formed by diffusion of nickel can improve the fill factor of the two sub cells. These results imply the potential of a NiOx/nickel silicide/n+ poly-Si structure as a perovskite/silicon tandem solar cell interlayer.
... The external quantum efficiencies of the HIT SCs are shown with the correction introduced by the correction factor. The obtained spectral dispersion of the external quantum efficiency in the HIT SCs is in good agreement with the data in [6,7]. Figure 3 shows the difference between the SC quantum efficiencies with allowance for this factor. ...
... 36 Furthermore, c-Si devices suffer from significant Shockley−Read−Hall (SRH) recombination under low-light conditions. Although c-Si cells demonstrate large PCE (>25% 70,71 ) under AM 1.5, only 3−6% PCE is obtained under energy-efficient LED light. 14 Therefore, alternative absorbers with wider band gaps are required for efficient harvesting of indoor light spectra, which significantly differ from AM 1.5 spectra. ...
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Their unique quasi one-dimensional (Q1D) crystal structure and rapid power conversion efficiency (PCE) evolution evoke tremendous scientific and technological interest in antimony chalcogenide (Sb 2 X 3 , X = S, Se, or S x Se 1−x) photovoltaics (PVs). Solution processability, strong photon harvesting, readily tunable optoelectronic properties, exceptional physicochemical stability, nontoxicity, and earth-abundance are their key features, endorsing Sb 2 X 3 as next-generation PVs. Benign, self-healing grain boundaries and defect-tolerance add to their merits, empowering Sb 2 X 3 films to act like pseudo-single crystals. These semiconductors are born for flexible PVs, as their Q1D crystal structures aid ultrahigh flexibility and bending tolerance. Sb 2 X 3 solar cells are efficient in recycling indoor and ambient light; thus, they are promising as indoor PVs (IPVs). Sb 2 X 3 PVs exhibit potential to simultaneously solve the stability and toxicity issues faced by lead halide perovskite PVs and the cost issues faced by mainstream silicon PVs. Presently, a record certified PCE of 10.7% has been demonstrated by Sb 2 X 3 solar cells; thus, they are emerging as a promising low-cost alternative to the commercially available PV technologies. This review presents a unique perspective of the fundamentals, recent breakthroughs, challenges, and futuristic developments in this field, offering a fundamental guideline for the rational engineering, design, and fabrication of high-PCE Sb 2 X 3 solar cells. This review highlights Sb 2 X 3 based, large area, tandem, and flexible solar cells and explores the commercial viability of this technology from generic power production to niche markets.
... Shortly after, Kaneka further announced an improvement in the conversion efficiency of HBC solar cells as the best cell demonstrated a conversion efficiency of 26.6% (V oc = 740 mV, J sc = 42.5 mA/cm 2 and FF = 84.6%) with an active cell area of 179.7 cm 2 [12]. Recently, in October 2021 LONGi Solar has announced new record HJT solar cells with 26.3% efficiency based on M6 wafers [13]. ...
Article
Solar photovoltaics researchers have devoted enough time to improve the performance of various types of high efficiency crystalline silicon based solar cells including passivated emitter rear cells, hetrojunction with intrinsic thin-layer solar cells, interdigitated back contact solar cells, heterojunction with interdigitated back contact solar cells and tunnel oxide passivated contact solar cells. Out of these various high efficiency solar cells, tunnel oxide passivated contact (TOPCon) solar cells are gaining more interests due to possessing various advantages such as availability of raw material, easy process sequence, high efficiency potential etc. In this review article, we shall discuss the evolutionary development of this high efficiency TOPCon solar cell, the progress made by the researchers in various aspects to improve the cell efficiency, current status of commercialization and finally future scopes of works with possible challenges.
Article
The industry for producing silicon solar cells and modules has grown remarkably over the past decades, with more than a 100‐fold reduction in price over the past 45 years. The main solar cell fabrication technology has shifted over that time and is currently dominated by the passivated emitter and rear cell (PERC). Other technologies are expected to increase in market share, including tunnel‐oxide passivated contact (TOPCon) and heterojunction technology (HJT). In this paper, we examine the cost potential for using atomic layer deposition (ALD) to form transition metal oxide (TMO) layers ( MoOx, TiOx and aluminium‐doped zinc oxide [AZO]) to use as lower cost alternatives of the p‐doped, n‐doped and indium tin oxide (ITO) layers, respectively, the layers normally used in HJT solar cells. Using a bottom‐up cost and uncertainty model with equipment cost data and process experience in the lab, we find that the production cost of these variations will likely be lower per wafer than standard HJT, with the main cost drivers being the cost of the ALD precursors at high‐volume production. We then considered what efficiency is required for these sequences to be cost effective in $/W and discuss whether these targets are technically feasible. This work motivates further work in developing these ALD TMO processes to increase their efficiency towards their theoretical limits to take advantage of the processing cost advantage. Bottom‐up cost assessment of using atomic layer deposited (ALD) transition metal oxide (TMO) layers in silicon heterojunction (HJT) solar cells is presented. MoOx, TiOx and AZO are potentially lower manufacturing cost ($/cell) than the standard doped a‐Si and ITO layers used in HJT solar cells; key cost uncertainty is volume pricing of ALD precursors. For each ALD TMO layer, cost and efficiency requirements are identified for commercial competitiveness ($/W), and compared with theoretical limits—this provides targets for R&D efforts.
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Photovoltaic (PV) technology is ready to become one of the main energy sources of, and contributers to, carbon neutrality by the mid‐21st century. In 2020, a total of 135 GW of PV modules were produced. Crystalline silicon solar cells dominate the world's PV market due to high power conversion efficiency, high stability, and low cost. Silicon heterojunction (SHJ) solar cells are one of the promising technologies for next‐generation crystalline silicon solar cells. Compared to the commercialized homojunction silicon solar cells, SHJ solar cells have higher power conversion efficiency, lower temperature coefficient, and lower manufacturing temperatures. Recently, several new record efficiencies have been achieved. To meet the continued demand for high‐efficiency solar cells, expectations for large‐scale mass production of SHJ solar cells are rising. To approach the efficiency limit and industrialization of SHJ solar cells, serious attempts have been made, yielding higher short‐circuit current, open‐circuit voltage, and fill factor. In this article, these recent advancements are reviewed, which reveals the future roadmap for approaching the efficiency limit. From the authors’ point of view, silicon heterojunction (SHJ) solar cells are the candidate technique that can best approach the efficiency limit. The engineering of the amorphous/crystalline silicon interfaces still needs to be emphasized for improving both the open‐circuit voltage and fill factor. To address the optical losses, the exploration of the ultimate design of SHJ solar cells should be carried through.
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The electricity market from renewable energies is strongly driven by the pursuit of high energy conversion efficiency, which at present represents the most effective pathway to achieve substantial cost reductions. Silicon (Si) have been dominating the photovoltaic industry for decades, while the conversion efficiencies of Si single-junction solar cells are practically limited to around 27 %, and intrinsically constrained to 29.4 %. To tackle this long-term bottleneck, it is necessary to develop novel technologies and transfer them into industrial production. This paper commences with a review concentrating on two critical concepts enabling high-efficiency Si-based solar cells: passivating contacts and tandem technologies. Since the gradual evolution from full area Al back surface field cells to passivated emitter and rear contact cells, passivating contacts are considered as an essential concept to circumvent the recombination losses caused by the contacts. The theoretical background of the three prominent technologies for passivating contacts and their application prospects to solar cells are described in detail. The fundamental limit of single junction Si solar cells is attainable with the introduction of passivating contacts. To obtain conversion efficiencies greater than 30 %, upgrading Si with a high-bandgap tandem partner is a promising approach to improve the utilization of the solar spectrum, having the potential to produce efficiency surpassing the single junction Shockley–Queisser limit. Si is proven to be an ideal bottom cells material in tandem architectures due to its appropriate bandgap for the lower sub-cell and the advantage of compatibility with existing production lines, the technologies for crystalline Si as bottom-cell are already quite mature with a gigawatt scale. The two widely considered ideal options for the top-cell, i.e., III/V and perovskites, are summarized, respectively. Building on these two concepts, a clear technology route is provided to maximize energy conversion efficiency by integration of passivating contacts into Si based tandem solar cells. According to this discussion, guidelines for further developments of Si photovoltaics emerge clearly, proving that Si will continue to maintain its irreplaceable position in photovoltaics in the long term.
Article
In the present communication, we report a rigorous analysis of optical, temperature, and bulk properties for the proposed configuration of p-Si/n-CdS heterojunction solar cell with ITO window layer. The proposed configuration here initially points out the investigation of bulk recombination by calibrating the concentrations of the defects of the p-Si absorber layer. Thus, solar cell efficiency must be improved favorably. Further, the effects of illumination and temperature dependence are also studied to monitor the signature of weather effect on the characteristic performance parameters. The obtained results show exciting behavior for two defects within the bandgap of the absorber layer. It is observed that the bulk recombination current (BRC) is very high for the higher concentrations of defects. Further, for the temperatures (in all weather conditions), the performance parameters as efficiency (η) and open-circuit voltage (Voc) show almost similar behavior with negative slop on increasing temperature, but short circuit current (Jsc) has the least variation. The fill factor first rises to room temperature and then falls for a higher temperature. Again, the illumination intensity of the incident radiation becomes crucial to affect the various performance parameters. Ultimately, we observed that the best performing efficiency of the proposed configuration at room temperature is 18.19 %.
Article
The efficiency of silicon heterojunction solar cells is limited by various factors including low surface passivation, parasitic absorption, and recombination losses. Herein, the surface passivation quality of crystalline silicon solar cells is improved by a hybrid passivation structure including a silicon heterojunction contact at the front side and a stack of tunneling oxide with n-type nano-crystalline silicon oxide (nc-SiOx(n)) passivating contact at the rear side. A passivation contact with thin silicon oxide (SiO2) and poly-silicon was previously proposed to enhance the rear surface passivation. In our study, the poly-silicon layer is swapped with the nc-SiOx(n) layer to improve the effective surface passivation, electrical properties, recombination losses, and carrier selectivity. The hybrid passivation structure shows significant passivation improvement with lifetime (τeff) of 2696 μs and implied open-circuit voltage (i-Voc) of 735 mV as compared with both-sides traditional silicon heterojunction (1650 μs, 719 mV) and tunneling passivation contact (2146 μs, 725 mV). The hybrid solar cell shows a potential performance as; open circuit voltage (Voc) = 724 mV, short circuit current (Jsc) = 38.95 mA/cm², fill factor (FF) of 75.9%, efficiency (η) = 21.4%. However, there is room to further improve the overall cell performance.
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Advanced doped‐silicon‐layer‐based passivating contacts have boosted the power conversion efficiency (PCE) of single‐junction crystalline silicon (c‐Si) solar cells to over 26%. However, the inevitable parasitic light absorption of the doped silicon layers impedes further PCE improvement. To this end, alternative passivating contacts based on wide‐bandgap metal compounds (so‐called dopant‐free passivating contacts (DFPCs)) have attracted great attention, thanks to their potential merits in terms of parasitic absorption loss, ease‐of‐deposition, and cost. Intensive research activity has surrounded this topic with significant progress made in recent years. Various electron‐selective and hole‐selective contacts based on metal compounds have been successfully developed, and a champion PCE of 23.5% has been achieved for a c‐Si solar cell with a MoOx‐based hole‐selective contact. In this work, the fundamentals and development status of DFPCs are reviewed and the challenges and potential solutions for enhancing the carrier selectivity of DFPCs are discussed. Based on comprehensive and in‐depth analysis and simulations, the improvement strategies and future prospects for DFPCs design and device implementation are pointed out. By tuning the carrier concentration of the metal compound and the work function of the capping transparent electrode, high PCEs over 26% can be achieved for c‐Si solar cells with DFPCs. Given the increasing interest in searching for high‐quality low‐cost passivating contacts for c‐Si solar cells, the fundamentals and development status of wide‐bandgap metal‐compound‐based passivating contacts are reviewed and the challenges and potential solutions in developing highly transparent passivating contacts with excellent carrier selectivity are discussed. Based on in‐depth data analysis and simulations, the improvement strategies for metal‐compound‐based passivating contacts design and device integration are pointed out.
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The year 2014 marks the point when silicon solar cells surpassed the 25% efficiency mark. Since then, all devices exceeding this mark, both small and large area, with contacts on both sides of the silicon wafer or just at the back, have utilized at least one passivating contact. Here, a passivating contact is defined as a group of layers that simultaneously provide selective conduction of charge carriers and effective passivation of the silicon surface. The widespread success of passivating contacts has prompted increased research into ways in which carrier‐selective junctions can be formed, yielding a diverse range of approaches. This paper seeks to classify passivating contact solar cells into three families, according to the material used for charge‐carrier selection: doped amorphous silicon, doped polycrystalline silicon, and metal compounds/organic materials. The paper tabulates their current efficiency values, discusses distinctive features, advantages, and limitations, and highlights promising opportunities going forth towards even higher conversion efficiencies. The widespread success of passivating contacts has prompted increased research into ways in which carrier‐selective junctions can be formed, yielding a diverse range of approaches for silicon solar cells. This paper seeks to classify passivating contact solar cells into three families, according to the material used for charge‐carrier selection: Doped amorphous silicon, doped polycrystalline silicon, and metal compounds/organic materials. The paper tabulates their current efficiency values, discusses distinctive features, advantages, and limitations, and highlights promising opportunities going forth towards even higher conversion efficiencies.
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In this work, we present a breakthrough in boronsilicate glass (BSG) passivated industrial tunnel oxide passivated contact (i-TOPCon) solar cells. We find that a high-temperature firing process significantly improves the front side BSG passivation quality; however, the use of such high-temperatures is undesirable for metallization as it could lead to more junction damage by the metal paste spikes. In this study, we present a simple and industrially viable method to resolve this dilemma. With a high-temperature industrial firing activation step to maximize the potential of BSG passivation, a low emitter saturation current (J0e) of 34 fA/cm² has been achieved, demonstrating excellent boron emitter passivation that is comparable to state-of-the-art SiO2 and Al2O3-based passivation methods on similar structures and boron emitters. Applying this solution to cell device, the open-circuit voltage (Voc) is improved by about 6 mV, corresponding to an absolute cell efficiency improvement of about 0.2%. Furthermore, after activating the BSG passivation, a lower temperature paste could be used at the rear side which further improves the Voc by around 3 mV. Combined together, an overall improvement of Voc close to 10 mV is achieved, propelling the cell Voc into the 690-mV era. The effectiveness of this solution was also verified in a mass production line, with average cell efficiencies of around 23.2% (0.5% more than the baseline) and a maximum cell efficiency and Voc of 23.4% and 693 mV, respectively. This work opens new routes for further improving conventional solar cell efficiencies, in particular for BSG-passivated TOPCon solar cells.
Thesis
The introduction of photovoltaic cells and light emitting diodes has defined this era of technology. Together they provide affordable renewable energy and high efficiency lighting. Despite all these benefits, current solid state materials are rapidly approaching theoretical device limits and will soon be unable to meet rising demands. In 2009, through a fusion of wet chemistry and nanotechnology, lead halide perovskite dots were identified. These novel materials exist as nanocrystalline semiconductors with bright, tunable absorption and emission wavelengths. As a result, perovskites are an intriguing candidate to work in tandem with solid state materials, promising to elevate device performance and efficiency. While an attractive prospect, such a combination is not without the significant challenges of material compatibility and stability, which must both be overcome to create a successful device. This thesis presents an overcoming of these obstacles by undertaking a detailed investigation of perovskite nanocrystals. Expertise in perovskite nanomaterials is developed to further their nanochemistry and optical properties with dramatic increases in photochemical lifetime obtained using polymeric encapsulation methods. The most promising candidate is then improved and brought to application in a device, resulting in a 100 % increase of Power Conversion Efficiency at UV wavelengths in a photovoltaic cell.
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Crystalline silicon (c-Si) solar cells using interdigitated back contact (IBC) configurations are one of the most promising candidates to reach the practical efficiency limits of c-Si solar cells. However, the complexity of the process flow hinders the mass production of the IBC cells with conventional doped regions. One of the simple fabrication methods is to introduce the dopant-free carrier-selective contacts, which utilizes the fabrication processes with low temperature, e.g., the thermal evaporation or the spin coating. In this paper, we investigated efficiency close to 20% silicon IBC solar cells with dopant-free asymmetric hetero-contacts. In this solar cell configuration, the high work function material MoOx was chosen as the hole transporting layer, while the low work function material LiF was chosen as the electron transporting layer, respectively. The simulation results indicate that the perspective efficiency exceeding 22% for this type of cells is achievable with the optimized pitch width and improved passivation quality of the contacts, which has a great potential for the industrialization of IBC solar cells with simple fabrication processes.
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We present a photon-counting simulation to evaluate the impact of top cell film quality on overall performance when designing stack tandem photovoltaic devices. We apply this model to the case where the bottom cell material is silicon in order to take advantage of the relatively low cost of manufacture already available. For the top cell, we approach material selection with an understanding that thin-film devices usually can’t reach the output voltage ideality that can be achieved with optimized single crystal devices. With limited thickness for the top cell, a range of photons with energies higher than the band gap may still not be absorbed in the top cell due to the normally smaller absorption coefficients just above the bandgap energy. We use the Tauc formalism for parameterizing near-edge optical absorption and investigate the effects of film thickness and bandgap on expected efficiency values of CdTe and MALI family materials, which are already known to be good candidates for top cell thin-film devices.
Chapter
Two-dimensional (2D) nanomaterials possess unique chemical, structural, optical, morphological, electronic, thermal, and mechanical properties and therefore attract extensive attention in energy-related applications such as supercapacitor, batteries, water splitting, thermal energy storage, and thermoelectric devices. Among the 2D materials, MXenes, graphene, black phosphorus, and transition metal dichalcogenides (TMDs) receive widespread research interest and are sought for energy-related applications. In this chapter, brief accounts are given to research literature for elucidating the preparation methods of MXenes, including chemical vapor deposition, hydrofluoric acid etching process, molten salt methods, and electrochemical etching methods. Further accounts are given to the works on enhancing the performance of electrochemical supercapacitor energy storage by fabricating conjugated microporous polymer/MXene composites, to the works on fabricating 2D TMDs MoS2 and their composites as supercapacitor electrode materials in energy storage applications, and to the works on enhancing thermoelectric power factor or reducing the thermal conductivity of graphene, black phosphorous, 2D TMDs as thermoelectric materials for energy conversion by taking advantage of their unique low-dimension electronic and thermal transport.
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Silicon (Si) solar cells are the dominant and well-developed solar technology holding more than 95% share of the photovoltaic market with efficiencies over 26%. Still, this value is far away from the Shockley-Queisser limit of 33.15% for single-junction devices. A prominent way to surpass this limit and boost the device performance is to combine different bandgap materials in a tandem configuration. The rapid emergence of perovskite solar cells (PSCs) as one of the most exciting research fields with over 25% efficiency has attracted the focus of the scientific community. The solution-processability, bandgap tunability, and outstanding optoelectronic properties of perovskites mark them a potential pair with silicon to develop tandem solar cells (TSCs). In nearly seven to eight years of development, silicon/perovskite TSCs have achieved record certified efficiencies of over 29%. This review emphasizes on two and four-terminal Si/perovskite TSCs. Initially, the advancement of efficiencies to date is discussed, including a comparison of numerous perovskite and silicon material choices. Then, the evolution of PSCs with Si (homojunction and heterojunction) bottom devices and their impact on the performance of TSCs is summarized. The suitable candidates for the perovskite and Si cells are proposed for Si/perovskite TSCs. Next, factors influencing the performance of tandems, such as fabrication issues on textured surfaces, parasitic absorption, reflection losses, nonideal perovskite absorber layer bandgap, device instability, and large-area fabrication, are highlighted. To reduce the electrical and optical losses for highly efficient tandems, an investigation of anti-reflecting coatings, current matching mechanisms, transparent electrodes, and recombination layers is discussed. Finally, based on these findings, future guidelines are proposed to boost the efficiencies beyond 30%. To the best of our knowledge, this is the first detailed study on two and four-terminal Si/perovskite TSCs. These findings would open new avenues for the research community with detailed information on silicon/perovskite tandem cells.
Article
Recent developments in industry on surface passivation, open the possibility of using less doped substrates in silicon solar cells. We investigate how the bulk resistivity affects the performance of silicon solar cells and the reliability of modules. Herein, n-type and p-type silicon heterojunction cells with bulk resistivities between 3-15000 Ωcm are studied. We measure the current-voltage characteristics of n-type cells across the resistivity range, and we find comparable responses to illumination intensities between 0.1-1 suns. The cells with bulk resistivities over 1000 Ωcm show breakdown voltages larger than -1000 V, almost two orders of magnitude higher than in typical commercial silicon solar cells. Although modules have bypass-diodes to prevent cells from going into breakdown, higher breakdown voltages can improve the reliability of modules in case of bypass-diode failure and reduce the module cost by easing the number of bypass-diodes required. Finally, we submit the cells to light soaking. The float-zone p-type cells with bulk resistivities over 10000 Ωcm are found to be less sensitive to light-induced degradation than cells with bulk resistivities below 10 Ωcm. The former shows a recovery in performance after a few hours of light soaking, while the latter recover only after dark annealing. This article is protected by copyright. All rights reserved.
Article
Carrier-selective crystalline silicon heterojunction (SHJ) solar cells have already reached superior lab-scale efficiencies. Besides judicious wafer thickness design, the optimal choice of passivation schemes and carrier-selective materials is essential for industry adoption. Appropriate reduction of process complexity and performance benefits through minimal recombination losses are key. Thus, along with well-designed back contacts, the development of low-temperature processable transparent passivating stacks that act as carrier-selective contacts (CSCs) is highlighted for their potential in circumventing the limited open-circuit photovoltage and contact-related losses in mainstream solar cells. In this review, effective passivation schemes deploying materials ranging from undoped metal oxides (MOs) to doped silicon are evaluated, with a focus on their significance for industrially viable passivating contact development. Passivation stack architectures with SiOx/heavily doped polycrystalline silicon (n+-/p+-poly-Si) realize the most attractive polysilicon-on-oxide (POLO) junctions and related schemes, e.g., combined with tunnel oxide passivated contact (TOPCon) and interdigitated back contact (IBC) solar cells. It is envisioned that the industrial trend is to eventually shift from the p-Si passivating emitter rear contact (PERC) and passivated emitter and rear polysilicon (PERPoly), towards TOPCon architectures, due to high manufacturing yields and compatibility with large-area metal screen printing and alternative bifacial designs.
Article
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A high‐performance semitransparent perovskite solar cell (PSC) with small photovoltage loss is highly desired to achieve efficient and stable perovskite/silicon tandem solar cells. Herein, a synergistic passivation strategy is developed to suppress the electronic defects at both the grain boundary and surface of a perovskite layer (Cs0.05FA0.82MA0.13Pb(I2.86Br0.14)). It is found that the incorporation of a small amount of sodium fluoride (NaF) into perovskite precursor solution modulates the crystallization process, which results in large crystal grains with enhanced conductivity at the grain boundaries. Meanwhile, a thin 2D perovskite layer is constructed on the surface of the 3D perovskite film by solution coating a layer of phenethylammonium iodide (PEAI), which passivates the surface defects and improves the stability of perovskite films. As a consequence, the optimized semitransparent p–i–n PSC delivers a high power conversion efficiency (PCE) of 17.55% with an open‐circuit voltage of 1.11 V. Combining the semitransparent PSCs with a silicon cell, the efficiency of the four‐terminal perovskite/silicon tandem solar cells reaches a PCE of 23.82%. Phenethylammonium iodide (PEAI) and sodium fluoride (NaF) are employed to passivate defects of triple‐cation mixed‐halide perovskite film synergistically, resulting in a significant power conversion efficiency (PCE) enhancement in four‐terminal perovskite/silicon tandem solar cells.
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The ever-increasing electricity demand from renewables has stimulated growth in the photovoltaic (PV) industry. Yet, while grid parity has already been achieved in several countries, a continued decline in module prices coupled with further efficiency improvements at an annual growth rate of $0.5% abs are needed to sustain its market growth. Mainstream PV technologies are still based on crystalline silicon (c-Si) wafers with heavily doped regions and directly metal-lized contacts. However, these cause band-gap narrowing, Auger recombination losses, and contact recombination losses. Passiv-ating contact (PC) technologies can overcome these limitations by decoupling surface passivation and contact formation requirements. Among PC technologies, amorphous silicon-based silicon heterojunction (SHJ) solar cells have established the world record power conversion efficiency for single-junction c-Si PV. Due to their excellent performance and simple design, they are also the preferred bottom cell technology for perovskite/silicon tandems. Nevertheless, SHJ technology accounts for only $2% of the current PV market share. In this review, we discuss the techno-economic challenges for large-volume SHJ manufacturing. In doing so, we highlight critical areas that need to be addressed for enabling tera-watt-scale SHJ deployment.
Article
Reducing indium consumption, which is related to the transparent conductive oxide (TCO) use, is a key challenge for scaling up silicon heterojunction (SHJ) solar cell technology to terawatt level. In this work, we developed bifacial SHJ solar cells with reduced TCO thickness. We present three types of In2O3‐based TCOs, tin‐, fluorine‐, and tungsten‐doped In2O3 (ITO, IFO, and IWO), whose thickness has been optimally minimized. These are promising TCOs, respectively, from post‐transition metal doping, anionic doping, and transition metal doping and exhibit different opto‐electrical properties. We performed optical simulations and electrical investigations with varied TCO thicknesses. The results indicate that (i) reducing TCO thickness could yield larger current in both monofacial and bifacial SHJ devices; (ii) our IWO and IFO are favorable for n‐contact and p‐contact, respectively; and (iii) our ITO could serve well for both n‐contact and p‐contact. Interestingly, for the p‐contact, with the ITO thickness reducing from 75 nm to 25 nm, the average contact resistivity values show a decreasing trend from 390 mΩ cm2 to 114 mΩ cm2. With applying 25‐nm‐thick front IWO in n‐contact, and 25‐nm‐thick rear ITO use in p‐contact, we obtained front side efficiencies above 22% in bifacial SHJ solar cells. This represents a 67% TCO reduction with respect to a reference bifacial solar cell with 75‐nm‐thick TCO on both sides. With double layer anti‐reflection coating, less transparent conductive oxide (TCO) use could produce more current in silicon heterojunction (SHJ) solar cells. Via appropriate contact engineering, TCO‐reduced option may reduce resistive losses in devices. Bifacial SHJ solar cells, which have 25‐nm‐thick front IWO in n‐contact and 25‐nm‐thick rear ITO in p‐contact, achieved front side efficiencies above 22%.
Article
In this work, the intrinsic hydrogenated amorphous silicon (a-Si:H) layer of silicon heterojunction (SHJ) solar cells was modified to improve carrier transport while maintaining excellent passivation of the c-Si absorber surface. The microstructure of different multilayer intrinsic a-Si:H films was measured and its influence on contact resistance and passivation quality was investigated thoroughly. We show that a pronounced trade-off between passivation and transport exists and that this trade-off is governed by the a-Si:H properties close to the c-Si surface. The fact that the same trend was observed for hole and electron contact suggests that the transport barrier formed by the interfacial a-Si:H layer is governed by a higher resistivity of the void-rich interfacial layer or a less pronounced induced junction and not by asymmetric hole or electron barriers (band offsets, tunnel efficiencies, …). Modified intrinsic layers have been tested on cell level, resulting in a series resistance (Rs) reduction by about 0.3 Ωcm² and an increase in fill factor (FF) by roughly 1.0 %abs. The power conversion efficiency (η) was improved by about 0.3 %abs with respect to our baseline. Further, the beneficial effect of a hydrogen plasma treatment (HPT) on passivation and transport of the hole contact was shown on device level.
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Ti3C2Tx MXene has recently attracted increasing attention in optoelectronics because of its good optical and electrical properties. Herein, we report a facile scheme of adjusting its electrical property by solution-processed SnCl4 treatment to realize the significantly improved performance of an n-type silicon (n-Si)/MXene Schottky junction solar cell prepared just by drop- casting the ethanol suspension of Ti3C2Tx MXene nanosheets onto the n-Si surface and subsequent natural drying. It is found that the optimal treatment by SnCl4 increases the electrical conductivity of the MXene layer and reduces the defects at the interface of Si and MXene. Thanks to its good compatibility with the micrometer- scale texture, the demonstration device of the pyramid-textured n- Si/SnCl4 -treated MXene nanosheets delivers a notable power conversion efficiency (PCE) of 9.3%, which is much higher than 4.7 and 4.8% for the pyramid-textured n-Si/MXene nanosheet device and the planar Si/MXene nanosheet device, respectively, both without SnCl4 treatment. Moreover, the SnCl4 -treated pyramid-textured n-Si/MXene nanosheet device exhibits significantly improved operation stability with ∼88% of its initial PCE, much higher than ∼67% for the control device without SnCl4 treatment, both after storage in air for 30 days.
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Back contact heterojunction (IBC-HIT) solar cells is one of the most promising technology for the upcoming generations of high efficiency crystalline-Silicon (c-Si) based photovoltaic modules [1] . However, the industrialization of the IBC-HIT technology is actually constrained by the complexity of the back side cell processing, which usually involves costly and time consuming photolithography steps. CEA-INES is currently developing a method based only on laser ablation for the structuration of IBC-HIT solar cells [2] . Laser ablation is indeed a fast and low cost technique that also allows the patterning of the back side amorphous (a-Si:H) layers on large area IBC-HIT solar cells. However laser irradiation might induce some damage at the c-Si/a-Si:H interface thus limiting the final performance of the devices. In this work, we compare the results obtained with our laser patterning process for different stack configurations and laser ablation conditions (532 nm and 355 nm). We will also discuss about the criteria used for the choice of the different materials and the laser ablation conditions needed in order to successfully pattern both the emitter and BSF (Back Surface Field) regions of the cell. Our best cell efficiency achieved up to now is 20.55% on an area of 18.11 cm 2 .
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Screen-printing provides an economically attractive means for making Ag electrical contacts to Si solar cells, but the use of Ag substantiates a significant manufacturing cost, and the glass frit used in the paste to enable contact formation contains Pb. To achieve optimal electrical performance and to develop pastes with alternative, abundant and non-toxic materials, a better understanding the contact formation process during firing is required. Here, we use in situ X-ray diffraction during firing to reveal the reaction sequence. The findings suggest that between 500 and 650 °C PbO in the frit etches the SiNx antireflective-coating on the solar cell, exposing the Si surface. Then, above 650 °C, Ag(+) dissolves into the molten glass frit - key for enabling deposition of metallic Ag on the emitter surface and precipitation of Ag nanocrystals within the glass. Ultimately, this work clarifies contact formation mechanisms and suggests approaches for development of inexpensive, nontoxic solar cell contacting pastes.
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The objective of this study is to optimize module technologies to obtain the lowest price per Watt peak ($/W p) ratio and the maximum power output of a flat-plate module for a given number of high-efficiency solar cells. Using B-doped p-type monocrystalline Cz silicon wafers, 500 pieces of full square[Formula: see text] solar cells with a passivated emitter and rear local contacts (PERC) were fabricated with an average efficiency of 20.6% by in-house measurement. The module includes half-cells for low interconnection losses, as well as a novel light-trapping scheme including light capture ribbon connected to the cells and a structured light reflective film between cells combined with an optimized large cell gap. The module using 60 pieces of the 20.6% efficient PERC solar cells has achieved a new world record, with a peak power output of 335.2 Wp in September 2014, demonstrating a large cell-to-module factor, which is defined as Pmmp of module divided by the sum of cell Pmmp. The CTM factor of the champion module is greater than 1.11.
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We analyze the optical losses that occur in interdigitated back-contacted amorphous/crystalline silicon heterojunction solar cells. We show that in our devices, the main loss mechanisms are similar to those of two-side contacted heterojunction solar cells. These include reflection and escape-light losses, as well as parasitic absorption in the front passivation layers and rear contact stacks. We then provide practical guidelines to mitigate such reflection and parasitic absorption losses at the front side of our solar cells, aiming at increasing the short-circuit current density in actual devices. Applying these rules, we processed a back-contacted silicon heterojunction solar cell featuring a short-circuit current density of 40.9 mA/cm2 and a conversion efficiency of 22.0%. Finally, we show that further progress will require addressing the optical losses occurring at the rear electrodes of the back-contacted devices.
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We report on the fabrication of back-contacted silicon heterojunction solar cells with conversion efficiencies above 21%. Our process technology relies solely on simple and size-scalable patterning methods, with no high-temperature steps. Using in situ shadow masks, doped hydrogenated amorphous silicon layers are patterned into two interdigitated combs. Transparent conductive oxide and metal layers, forming the back electrodes, are patterned by hot melt inkjet printing. With this process, we obtain high short-circuit current densities close to 40 mA/cm$^{2}$ and open-circuit voltages exceeding 720 mV, leading to a conversion efficiency of 21.5%. However, moderate fill factor values limit our current device efficiencies. Unhindered carrier transport through both heterocontact layer stacks, as well as higher passivation quality over the minority carrier-injection range relevant for solar cell operation, are identified as key factors for improved fill factor values and device performance.
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Recently, several parameters relevant for modeling crystalline silicon solar cells were improved or revised, e.g., the international standard solar spectrum or properties of silicon such as the intrinsic recombination rate and the intrinsic carrier concentration. In this study, we analyzed the influence of these improved state-of-the-art parameters on the limiting efficiency for crystalline silicon solar cells under 1-sun illumination at 25 °C, by following the narrow-base approximation to model ideal solar cells. We also considered bandgap narrowing, which was not addressed so far with respect to efficiency limitation. The new calculations that are presented in this study result in a maximum theoretical efficiency of 29.43% for a 110-μm-thick solar cell made of undoped silicon. A systematic calculation of the I--V parameters as a function of the doping concentration and the cell thickness together with an analysis of the loss current at maximum power point provides further insight into the intrinsic limitations of silicon solar cells.
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Accurate modeling of the intrinsic recombination in silicon is important for device simulation as well as for interpreting measured effective carrier lifetime data. In this contribution we study the injection-dependent effective carrier lifetime applying advanced surface passivation techniques based on Al2O3 or SiNx We show that in some cases the measured lifetime data significantly exceeds the previously accepted intrinsic lifetime limit proposed by Kerr and Cuevas [1]. To verify our measurements we independently perform lifetime measurements with different measurement techniques in two different laboratories. Based on effective lifetime measurements we develop an advanced parameterization of the intrinsic lifetime in crystalline silicon at 300 K as a function of the doping density and the injection level, which accounts for Coulomb-enhanced Auger recombination and Coulomb-enhanced radiative recombination.
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An accurate quantitative description of the Auger recombination rate in silicon as a function of the dopant density and the carrier injection level is important to understand the physics of this fundamental mechanism and to predict the physical limits to the performance of silicon based devices. Technological progress has permitted a near suppression of competing recombination mechanisms, both in the bulk of the silicon crystal and at the surfaces. This, coupled with advanced characterization techniques, has led to an improved determination of the Auger recombination rate, which is lower than previously thought. In this contribution we present a systematic study of the injection-dependent carrier recombination for a broad range of dopant concentrations of high-purity n-type and p-type silicon wafers passivated with state-of-the-art dielectric layers of aluminum oxide or silicon nitride. Based on these measurements, we develop a general parametrization for intrinsic recombination in crystalline silicon at 300 K consistent with the theory of Coulomb-enhanced Auger and radiative recombination. Based on this improved description we are able to analyze physical aspects of the Auger recombination mechanism such as the Coulomb enhancement.
Conference Paper
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This work demonstrates the feasibility and usefulness of a new method to analyse the quality of the rear contact of silicon solar cells separated from other ohmic loss channels as e.g. the resistive loss in the front contact grid. The measurement is based on SunsVoc data at high illumination densities between 1 and 1000 suns. Generally the rear contacts can be described as a Schottky diode with a shunt resistor in parallel. At 1 sun operation conditions the back contact is fully dominated by the shunt showing an ohmic behaviour. However, at high illumination densities the Schottky diode can not be shunted completely anymore resulting in an increasing voltage which is opposed to the pn junction voltage. Finally a reversal point in the SunsVoc characteristics can be observed, i.e. the voltage decreases with increasing illumination density. The evaluation of this characteristic behaviour is used to extract physical parameters like the barrier height of the contact. Additionally the contact quality is assessed for different contact types and base doping concentrations. The predicted contact quality is in good correlation with the measured fill factors of the cells.
Article
Our unique, high-efficiency c-Si solar cell, named the HIT cell, has shown considerable potential to improve junction properties and surface passivation since it was first developed. The improved properties in efficiency and temperature dependence compared to conventional p – n diffused c-Si solar cells are featured in HIT power 21TM solar cell modules and other applications which are now on the market. In the area of research, further improvement in the junction properties of the a-Si/c-Si heterojunction has been examined, and the highest efficiency to date of 20.1% has recently been achieved for a cell size of 101 cm². The high open circuit voltage exceeding 700 mV, due to the excellent surface passivation of the HIT structure, is responsible for this efficiency. In this paper, recent progress in HIT cells by Sanyo will be introduced. Copyright © 2000 John Wiley & Sons, Ltd.
Article
In the 1980s, advances in the passivation of both cell surfaces led to the first crystalline silicon solar cells with conversion efficiencies above 20%. With today's industry trend towards thinner wafers and higher cell efficiency, the passivation of the front and rear surfaces is now also becoming vitally important for commercial silicon cells. This paper presents a review of the surface passivation methods used since the 1970s, both on laboratory-type as well as industrial cells. Given the trend towards lower-cost (but also lower-quality) Si materials such as block-cast multicrystalline Si, ribbon Si or thin-film polycrystalline Si, the most promising surface passivation methods identified to date are the fabrication of a p–n junction and the subsequent passivation of the resulting silicon surface with plasma silicon nitride as this material, besides reducing surface recombination and reflection losses, additionally provides a very efficient passivation of bulk defects. Copyright © 2000 John Wiley & Sons, Ltd.
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Improving the photoconversion efficiency of silicon solar cells is crucial to further the deployment of renewable electricity. Essential device properties such as lifetime, series resistance and optical properties must be improved simultaneously to reduce recombination, resistive and optical losses. Here, we use industrially compatible processes to fabricate large-area silicon solar cells combining interdigitated back contacts and an amorphous silicon/crystalline silicon heterojunction. The photoconversion efficiency is over 26% with a 180.4 cm 2 designated area, which is an improvement of 2.7% relative to the previous record efficiency of 25.6%. The cell was analysed to characterize lifetime, quantum efficiency, and series resistance, which are essential elements for conversion efficiency. Finally, a loss analysis pinpoints a path to approach the theoretical conversion efficiency limit of Si solar cells, 29.1%.
Article
In this work, we present the results of the replacement of silver screen printing on heterojunction crystalline silicon (c-Si) solar cells with a copper metallization scheme that has the potential to reduce the manufacturing cost while improving their performance. We report for the first time silver-free heterojunction c-Si solar cells on 6-in. wafers. The conversion efficiency reached is a record 22.1% for c-Si technology for this wafer size (Voc = 729 mV, Jsc = 38.3 mA/cm², FF= 79.1%). The total power generated is more than 5 W for 1-sun illumination, which is a world record. Heat-damp reliability tests show comparable performance for mini-modules fabricated with copper metalized as for conventional silver screen printed heterojunction c-Si solar cells.
Article
Suns– $V_{{\rm{oc}}}$ measurements exclude parasitic series resistance effects and are, therefore, frequently used to study the intrinsic potential of a given photovoltaic technology. However, when applied to a-Si/c-Si heterojunction (SHJ) solar cells, the Suns–V $_{oc}$ curves often feature a peculiar turnaround at high illumination intensities. Generally, this turn-around is attributed to extrinsic Schottky contacts that should disappear with process improvement. In this paper, we demonstrate that this voltage turnaround may be an intrinsic feature of SHJ solar cells, arising from the heterojunction (HJ), as well as its associated carrier-transport barriers, inherent to SHJ devices. We use numerical simulations to explore the full current–voltage (J–V) characteristics under different illumination and ambient temperature conditions. Using these characteristics, we establish the voltage and illumination-intensity bias, as well as temperature conditions necessary to observe the voltage turnaround in these cells. We validate our turnaround hypothesis using an extensive set of experiments on a high-efficiency SHJ solar cell and a molybdenum oxide (MoO $_{x}$ ) based hole collector HJ solar cell. Our work consolidates Suns– $V_{{\rm{oc}}}$ as a powerful characterization tool for extracting the cell parameters that limit efficiency in HJ devices.
Article
In this work, we present the latest result of the Cu electroplated heterojunction by Kaneka Corporation. The top electrode on heterojunction crystalline silicon (c-Si) solar cells usually defined by silver screen printing is replaced with a copper metallization scheme that has the potential to reduce the manufacturing cost while improving the heterojunction performance. We show the progress on the heterojunction c-Si solar cells by Kaneka and report on heterojunction c-Si solar cells on 6 inch wafers with a conversion efficiency of 23.5% independently confirmed by ISE-CALLAB (Voc = 737 mV, Jsc = 39.97 mA/cm2, FF = 79.77%). Heat-damp reliability tests show comparable performance for mini-modules fabricated with copper metalized as for conventional silver screen printed heterojunction c-Si solar cells.
Article
This work explores the application of transparent nitrogen doped copper oxide (CuOx:N) films deposited by reactive sputtering to create hole-selective contacts for p-type crystalline silicon (c-Si) solar cells. It is found that CuOx:N sputtered directly onto crystalline silicon is able to form an Ohmic contact. X-ray photoelectron spectroscopy and Raman spectroscopy measurements are used to characterise the structural and physical properties of the CuOx:N films. Both the oxygen flow rate and the substrate temperature during deposition have a significant impact on the film composition, as well as on the resulting contact resistivity. After optimization, a low contact resistivity of ∼10 mΩ cm2 has been established. This result offers significant advantages over conventional contact structures in terms of carrier transport and device fabrication.
Article
Consolidated tables showing an extensive listing of the highest independently confirmed efficiencies for solar cells and modules are presented. Guidelines for inclusion of results into these tables are outlined, and new entries since January 2016 are reviewed. Copyright © 2016 John Wiley & Sons, Ltd.
Article
All previous concepts for describing the effective local series resistance of really existing solar cells, as it can be measured e.g. by luminescence imaging, try to describe it by a single local number. In solar cells showing an inhomogeneous saturation current density, this results in different series resistance images for the dark and illuminated case. The reason is the distributed character of the series resistance and the different diode current profiles under these different conditions. In this work the well-known finite element concept is used for describing a solar cell, which contains separate resistors carrying horizontal and vertical currents. A strategy is proposed how to fit these resistors to results of electroluminescence and lock-in thermography images of a real solar cell, leading to separate images of the local horizontal grid resistance, which may also show broken gridlines, and the local vertical'lumped emitter contact resistance'. The latter lumps all resistive inhomogeneities of the cell, caused by a possibly inhomogeneous contact-, emitter-, grid-, bulk-, and back contact resistance. It will be shown that this description of the local series resistance reasonably describes both the dark and illuminated case, even in inhomogeneous multicrystalline silicon solar cells.
Article
With a global market share of about 90%, crystalline silicon is by far the most important photovoltaic technology today. This article reviews the dynamic field of crystalline silicon photovoltaics from a device-engineering perspective. First, it discusses key factors responsible for the success of the classic dopant-diffused silicon homojunction solar cell. Next it analyzes two archetypal high-efficiency device architectures – the interdigitated back-contact silicon cell and the silicon heterojunction cell – both of which have demonstrated power conversion efficiencies greater than 25%. Last, it gives an up-to-date summary of promising recent pathways for further efficiency improvements and cost reduction employing novel carrier-selective passivating contact schemes, as well as tandem multi-junction architectures, in particular those that combine silicon absorbers with organic-inorganic perovskite materials.
Conference Paper
Modelling and experimental results on IBC SHJ solar cells are presented in this paper. Different rear emitter designs are studied by 2D simulation and tested on experimental devices. IBC SHJ cells are fabricated with the SLASH process based on laser patterning steps, and the performance of such devices is shown to be limited by the rear emitter geometry (contacting scheme and total emitter fraction). On one hand IBC SHJ cells have to be carefully designed concerning the contacting scheme due to distributed series resistance effects. On the other hand SHJ technology allows a very high surface passivation level, and this reduces the influence of the emitter fraction on the cells performances.
Article
We have achieved a certified 25.1% conversion efficiency in a large area (151.9 cm2) heterojunction (HJ) crystalline Si (c-Si) solar cell with amorphous Si (a-Si)passivation layer. This efficiency is a world record in a both-side-contacted c-Si solar cell. Our high efficiency HJ c-Si solar cells are investigated from the standpoint of the effective minority carrier lifetime (τe), and the impact of τe on fill factor (FF) is discussed. The τe measurements of our high efficiency HJ c-Si solar cells reveal that τe at an injection level corresponding to an operation point of maximum power is dominated by the carrier recombination at the a-Si/c-Si interface. By optimization of the process conditions, the carrier recombination at the a-Si/c-Si interface is reduced, which leads to an improvement of the FF by an absolute value of 2.7%, and a conversion efficiency of 25.1% has been achieved. These results indicate that the reduction of carrier recombination centers at the a-Si/c-Si interface should be one of the most crucial issues for further improvement of FF even in the HJ c-Si solar cells with efficiency over 25%.
Article
An energy conversion efficiency of 25.1% was achieved in heterojunction back contact (HBC) structure Si solar cell utilizing back contact technology and an amorphous silicon thinfilm technology. A new patterning process was established, and it was applied to the fabrication process of HBC cells. In addition, the unique technology of the surface mount technology concept contributed to the superior performance of HBC cell. A short circuit current density (Jsc) and an open-circuit voltage (Voc) were 41.7 mA/cm2 and 736 mV, respectively. The high Jsc as well as the high Voc indicates the strength of HBC structure cell. Besides, a high fill factor of 0.82 was obtained, which shows that HBC structure cell does not have any fundamental critical losses caused from series resistance or shunt resistance. Such high values of I-V parameter means that the patterning process was properly performed.
Article
The contact resistance of amorphous Si (a-Si:H)/transparent-conducting oxide (TCO) is evaluated and analyzed in terms of the contribution to the series resistance (R-s) and fill factor (FF) in the Si heterojunction back-contact (HBC) solar cell. It is shown that p-a-Si:H (emitter) and n-a-Si:H (back surface field: BSF)/TCO contact resistance are of similar values (0.37-38 Omega cm(2)) which are much higher than those of doped crystalline Si/metal contacts used in conventional Si solar cells. Of some factors affecting R-s loss in the HBC solar cell, BSF/TCO contact is the most significant one when considering the contact area. By interleaving the n-type microcrystalline Si (n-mu c-Si) between n-a-Si:H and TCO, 6-inch HBC solar cell with 20.5% efficiency is obtained, which was attributed to the reduced R-s and improved FF. It is noteworthy that the variations of R-s and FF are well estimated by measuring BSF-contact resistance, and are close to the empirical data: reduction in R-s to 1.77 Omega cm(2) and the increase in FF by 6.0% compared to the cell without n-mu c-Si interface layer. The results indicate that there is much room for higher efficiency by reducing the emitter- and BSF-contact resistance. Nonetheless, the method developed here can be a powerful tool to analyze the resistance component in HBC cell.
Article
The crystalline silicon heterojunction structure adopted in photovoltaic modules commercialized as Panasonic's HIT has significantly reduced recombination loss, resulting in greater conversion efficiency. The structure of an interdigitated back contact was adopted with our crystalline silicon heterojunction solar cells to reduce optical loss from a front grid electrode, a transparent conducting oxide (TCO) layer, and a-Si:H layers as an approach for exceeding the conversion efficiency of 25%. As a result of the improved short-circuit current (Jsc), we achieved the world's highest efficiency of 25.6% for crystalline silicon-based solar cells under 1-sun illumination (designated area: 143.7 cm2).
Article
The interdigitated back contact (IBC) solar cells developed at the Australian National University have resulted in an independently confirmed (Fraunhofer Institut für Solare Energiesysteme (ISE) CalLab) designated-area efficiency of 24.4 ± 0.7%, featuring short-circuit current density of 41.95 mA/cm2, open-circuit voltage of 703 mV and 82.7% fill factor. The cell, 2 × 2 cm2 in area, was fabricated on a 230 µm thick 1.5 Ω cm n-type Czochralski wafer, utilising plasma-enhanced chemical vapour deposition (CVD) SiNx front-surface passivation without front-surface diffusion, rear-side thermal oxide/low-pressure CVD Si3N4 passivation stack and evaporated aluminium contacts with a finger-to-finger pitch of 500 µm. This paper describes the design and fabrication of lab-scale high-efficiency IBC cells. Characterisation of optical and electronic properties of the best produced cell is made, with subsequent incorporation into 3D device modelling used to accurately quantify all losses. Loss analysis demonstrates that bulk and emitter recombination, bulk resistive and optical losses are dominant and suggests a clear route to efficiency values in excess of 25%. Additionally, laser processing is explored as a means to simplify the manufacture of IBC cells, with a confirmed efficiency value of 23.5% recorded for cells fabricated using damage-free deep UV laser ablation for contact formation. Meanwhile all-laser-doped cells, where every doping and patterning step is performed by lasers, are demonstrated with a preliminary result of 19.1% conversion efficiency recorded. Copyright © 2014 John Wiley & Sons, Ltd.
Article
A new record conversion efficiency of 24.7% was attained at the research level by using a heterojunction with intrinsic thin-layer structure of practical size (101.8 cm2, total area) at a 98-μm thickness. This is a world height record for any crystalline silicon-based solar cell of practical size (100 cm2 and above). Since we announced our former record of 23.7%, we have continued to reduce recombination losses at the hetero interface between a-Si and c-Si along with cutting down resistive losses by improving the silver paste with lower resistivity and optimization of the thicknesses in a-Si layers. Using a new technology that enables the formation of a-Si layer of even higher quality on the c-Si substrate, while limiting damage to the surface of the substrate, the Voc has been improved from 0.745 to 0.750 V. We also succeeded in improving the fill factor from 0.809 to 0.832.
Article
Our unique, high-efficiency c-Si solar cell, named the HIT cell, has shown considerable potential to improve junction properties and surface passivation since it was first developed. The improved properties in efficiency and temperature dependence compared to conventional p – n diffused c-Si solar cells are featured in HIT power 21TM solar cell modules and other applications which are now on the market. In the area of research, further improvement in the junction properties of the a-Si/c-Si heterojunction has been examined, and the highest efficiency to date of 20.1% has recently been achieved for a cell size of 101 cm2. The high open circuit voltage exceeding 700 mV, due to the excellent surface passivation of the HIT structure, is responsible for this efficiency. In this paper, recent progress in HIT cells by Sanyo will be introduced. Copyright © 2000 John Wiley & Sons, Ltd.
Article
The construction of a 22.2% efficient single-crystal silicon solar cell fabricated at Stanford University is described. The cell dimensions were 3 x 5 mm and 100 microns thick with a base lifetime of 500 microseconds. The cell featured light trapping between a texturized top surface and a reflective bottom surface, small point contact diffusions, alternating between n-type and p-type in a polka-dot pattern on the bottom surface, and a surface passivation on all surfaces between contact regions.
Article
We present back-contacted amorphous/crystalline silicon heterojunction solar cells (IBC-SHJ) on n-type substrates with fill factors exceeding 78% and high current densities, the latter enabled by a SiNx /SiO2 passivated phosphorus-diffused front surface field. Voc calculations based on carrier lifetime data of reference samples indicate that for the IBC architecture and the given amorphous silicon layer qualities an emitter buffer layer is crucial to reach a high Voc, as known for both-side contacted silicon heterojunction solar cells. A back surface field buffer layer has a minor influence. We observe a boost in solar cell Voc of 40 mV and a simultaneous fill factor reduction introducing the buffer layer. The aperture-area efficiency increases from 19.8 ± 0.4% to 20.2 ± 0.4%. Both, efficiencies and fill factors constitute a significant improvement over previously reported values. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Article
Excitonic effects are known to enhance the rate of intrinsic recombination processes in crystalline silicon. New calculations for the limiting efficiency of silicon solar cells are presented here, based on a recent parameterization for the Coulomb-enhanced Auger recombination rate, which accounts for its dopant type and dopant density dependence at an arbitrary injection level. Radiative recombination has been included along with photon recycling effects modeled by three-dimensional ray tracing. A maximum cell efficiency of 29.05% has been calculated for a 90-μm-thick cell made from high resistivity silicon at 25°C. For 1 Ω cm p-type silicon, the maximum efficiency reduces from 28.6% for a 55-μm-thick cell in the absence of surface recombination, down to 27.0% for a thickness in the range 300–500 μm when surface recombination limits the open-circuit voltage to 720 mV. Copyright © 2002 John Wiley & Sons, Ltd.
Article
Consolidated tables showing an extensive listing of the highest independently confirmed efficiencies for solar cells and modules are presented. Guidelines for inclusion of results into these tables are outlined, and new entries since July 2011 are reviewed. Copyright © 2011 John Wiley & Sons, Ltd. Consolidated tables showing an extensive listing of the highest independently confirmed efficiencies for solar cells and modules are presented. Guidelines for inclusion of results into these tables are outlined, and new entries since July 2011 are reviewed.