Solar Energy Materials and Solar Cells

Published by Elsevier
Online ISSN: 0927-0248
Publications
Conference Paper
A three-stage process starting with the deposition of (In,Ga)<sub>2</sub>Se<sub>3</sub> precursor films has been successful in the fabrication of graded band-gap Cu(In,Ga)Se<sub>2</sub> thin films. In this work we examine (1) the reaction of Cu+Se with (In,Ga)<sub>2 </sub>Se<sub>3</sub>, which leads to a spontaneous grading in the Ga content as a function of depth through the film, and (2) modification of the Ga content in the surface region of the film through a final deposition of In+Ga+Se. We show how band-gap grading can be enhanced by the formation of nonuniform precursors, how counterdiffusion limits the degree of grading possible in the surface region, and how the Cu<sub>x </sub>Se secondary phase acts to homogenize the film composition
 
Conference Paper
Several factors that may affect the net discoloration rate of the ethylene-vinyl acetate (EVA) copolymer encapsulants used in crystalline-Si photovoltaic (c-Si PV) modules upon accelerated exposure have been investigated by employing UV-visible spectrophotometry, spectrocolorimetry, and fluorescence analysis. A number of laminated films, including the two typical EVA formulations, A9918 and 15295, were studied. The results indicate that the rate of EVA discoloration is affected by the: (1) curing agent and curing conditions; (2) presence and concentration of curing-generated, UV-excitable chromophores; (3) UV light intensity; (4) loss rate of the UV absorber, Cyasorb UV 5311; (5) lamination; (6) film thickness; and (7) photobleaching rate due to the diffusion of air into the laminated films. In general, the loss rate of the UV absorber and the rate of discoloration from light yellow to brown follow a sigmoidal pattern. A reasonable correlation for net changes in transmittance at 420 nm, yellowness index, and fluorescence peak area (or intensity) ratio is obtained as the extent of EVA discoloration progressed
 
Conference Paper
The actual efficiency of photovoltaic generators is often lower than predicted by standard test conditions (STC) or standard operating conditions (SOC). This is caused mainly by an underestimation of reflection losses and solar cell temperature in the module. The authors describe how, in order to obtain more accurate results in predicting the performance of PV-modules, the parameters influencing incoming (optical parameters) and outgoing power flow (electrical and thermal parameters) were investigated by simulation and some verifying experiments at the University of New South Wales and the Australian desert
 
Conference Paper
A high-efficiency AlGaAs/Si tandem solar cell is fabricated by metal-organic chemical vapor deposition (MOCVD). It consists of a Al<sub>0.15</sub>Ga<sub>0.85</sub>As top cell and a Si bottom cell. The crystalline quality of the Al<sub>0.15</sub>Ga<sub>0.85</sub>As heteroepitaxial layer grown on Si is improved using a high-temperature growth process (800°C) and thermal cycle annealings (300~900°C). The quantum efficiency of the Si bottom cell in the long wavelength region is improved by back surface field. The conversion efficiencies of the tandem solar cell under AMO and 1 sun measurement conditions with 4-terminal and 2-terminal configuration are 20.0% and 19.0%, respectively. The conversion efficiencies of the tandem solar cell with graded-band-gap emitter Al<sub>x</sub>Ga<sub>1-x</sub>As layer achieved 20.6% and 19.9% under same condition with 4-terminal and 2-terminal configuration, respectively
 
Conference Paper
Optimised thicknesses of the individual layers in any thin film a-Si:H solar cell can be obtained using optical analysis methods. By minimising the reflectance using an optimal antireflection coating layer and selecting a suitable rear contact material, a significant increase in the photon collection efficiency at long wavelengths can be achieved. The effect of the variation in thicknesses of different layers on the performance characteristics of a cell is discussed. The calculated results and analyses show that the present method can be used directly to design any thin film solar cell with an optimised structure for a desired possible high efficiency
 
Conference Paper
Silicon solar cell efficiencies of 17.1%, 16.4%, 14.8%, and 14.9% have been achieved on FZ, Cz, multicrystalline (mc-Si), and dendritic web (DW) silicon, respectively, using simplified, cost-effective rapid thermal processing (RTP). These represent the highest reported efficiencies for solar cells processed with simultaneous front and back diffusion with no conventional high-temperature furnace steps. Appropriate diffusion temperature coupled with the added in-situ anneal resulted in suitable minority-carrier lifetime and diffusion profiles for high-efficiency cells. The cooling rate associated with the in-situ anneal can improve the lifetime and lower the reverse saturation current density (J<sub>o</sub>), however, this effect is material and base resistivity specific. PECVD antireflection (AR) coatings provided low reflectance and efficient front surface and bulk defect passivation. Conventional cells fabricated on FZ silicon by furnace diffusions and oxidations gave an efficiency of 18.8% due to greater short wavelength response and lower J<sub>o</sub>
 
Conference Paper
Improvements in the stabilized efficiency of single-junction and multi-junction a-Si solar cells are studied. It has been found that the stabilized efficiency can be improved by suppressing the oxygen and nitrogen content in the i-layer. The defect density and conversion efficiency are strongly correlated with the content of hydrogen having an SiH<sub>2</sub> configuration rather than with the total hydrogen content in a-Si i-layers. A practical estimation method, which considers the variation of the I-V characteristics of each component cell, is also developed in order to optimize multi-junction cells. The world's highest stabilized efficiency of 8.8% for a single-junction a-Si solar cell and 10.6% for an a-Si/a-SiGe double-junction solar cell are achieved for 1 cm<sup>2</sup> cells
 
Conference Paper
Solar cells based on Cu(In,Ga)Se<sub>2</sub> were prepared replacing the `standard buffer layer' CdS with a In<sub>x</sub>(OH,S) <sub>y</sub> thin film. The film is deposited in a chemical bath (CBD) process using an aqueous solution containing InCl<sub>3</sub> and thioacetamide. X-ray photoemission spectroscopy measurements were performed in order to characterize the growth kinetics and the chemical composition. The influence of different concentrations of InCl<sub>3 </sub> and thioacetamide in the solution on the electrical properties of the solar cells was studied by measuring the j-V characteristics and the spectral quantum efficiencies. Capacitance-voltage (C-V) measurements indicate that the high V<sub>oc</sub> values of devices with the novel buffer layer are correlated with narrower space charge widths and higher effective carrier concentrations in the absorber materials. The achieved conversion efficiency of approximately 15% using the cadmium free In<sub>x</sub>(OH,S)<sub>y</sub> buffer demonstrates the potential of this process as an alternative to the standard chemical bath deposition of CdS
 
Conference Paper
The alignment of energy bands in a Mo/CIS/CdS/ZnO solar cell structure is presented. Special attention is paid to the surface chemistry of the molybdenum coated substrate. In this study we have performed in situ analyses on polycrystalline thin films starting from the molybdenum coated soda lime glass. The different films have been deposited sequentially under ultra high vacuum conditions and analyzed with photoelectron spectroscopy techniques. We have found that Mo surface is significantly oxidized. Air annealing at 200°C leads to a diffusion of sodium through the Mo layer. Mo-O and Mo-Se compounds are formed and present during the evolution of the Mo-CIS interface. Band lineups have been determined for the CIS/CdS/ZnO interfaces of the solar cell structure. We have found no indication for band discontinuities deteriorating the solar cell device performance
 
Conference Paper
This paper explores the possibility of measuring the open-circuit voltage as a function of wavelength as a tool for device characterization. Computer simulations show that the spectral response of the open-circuit voltage exhibits a similar dependence to the spectral response of the short-circuit current. Experimental studies on silicon solar cells confirmed the strong spectral dependence of the open-circuit voltage. The spectral measurements have been performed using a quasi-steady state open-circuit voltage method, which also allows to determine the spectral response of the maximum power voltage. The advantages of this new technique over conventional spectral response measurements include its applicability directly after junction formation and its simple apparatus.
 
Conference Paper
This paper addresses the problem of preparing a good p-layer contact with zinc oxide as TCO (transparent conducting oxide). Our approach was to deposit pin cells with different p-layer recipes on ZnO coated SnO<sub>2</sub>:F and on uncoated SnO<sub>2</sub>:F in one run. The pin cells prepared on the ZnO surface exhibit a lower fill factor (FF). Our experiments demonstrate that the hydrogen interaction with the ZnO surface plays the most decisive role for the ZnO/p contact. We explain the observed effects using a band diagram of the ZnO/p interface and show that the accumulation layer at the ZnO surface-caused by atomic hydrogen in the plasma-is responsible for the low FF in pin cells. Based on this model the contact problem is solved by introducing a μc-n-Si intralayer between ZnO and p layer resulting in an identical high FF on both ZnO and SnO<sub>2</sub> substrates
 
Conference Paper
Preliminary research into the development of single-junction Ga <sub>x</sub>In<sub>1-x</sub>As thermophotovoltaic (TPV) power converters is reviewed. The devices structures are grown epitaxially on single-crystal InP substrates. Converter band gaps of 0.50 -0.74 eV have been considered in accordance with modeling calculations. A 1-sun, AMO efficiency of 12.8% is reported for a lattice-matched, 0.74-eV converter. Converters with lower band gaps are fabricated using lattice-mismatched, compositionally graded structures. Functional TPV converters with good performance characteristics have been demonstrated for band gaps as low as 0.5 eV
 
Conference Paper
Thin films based on Cu(In,Ga)Se<sub>2</sub> prepared on alkali free substrates are compared to films prepared on soda lime glass. On the latter, the presence of sodium species as detected with X-ray photoelectron spectroscopy is correlated with an enhanced formation of Se-O, In-O and Ga-O bonds at the surface after several days air exposure. The electrical conductivity is also one order of magnitude higher on soda lime glass and solar cells prepared on molybdenum-coated substrates exhibit increased open circuit voltages. Capacitance-voltage characteristics on junctions prepared on alkali free substrates show an increased space charge width. The observations can be explained in terms of an increased net acceptor density in polycrystalline Cu(In,Ga)Se<sub>2</sub> when prepared on soda lime glass substrates. An alkali-metal-promoted oxidation of the surface is discussed
 
Article
The thermal oxidation of CuInSe<sub>2</sub> single crystals and bulk polycrystalline plates of n- and p-type conductivity with polished facets have been carried out at elevated temperatures 400 to 610°C in dry oxygen/air atmosphere. All the crystals demonstrated an enhancement in hole conductivity near the surface. Additionally, transparent layers of In<sub>2</sub>O<sub>3</sub> chemical composition have been grown on the surface. Electron-probe analysis, Rutherford backscattering, X-ray diffraction, optical and photoelectric experimental data have provided a basis to develop a physico-chemical model thermal oxidation of CuInSe<sub>2</sub> crystals. The model includes the In<sub>2</sub>O<sub>3</sub> compound appearance, V<sub>Se </sub>, V<sub>Cu</sub> vacancy absorption, Se<sub>i</sub>, Cu<sub>i</sub> extra atom and Cu<sub>x</sub>Se generation. The chemical reactions are accompanied by arising dislocations and mechanical stresses at the In<sub>2</sub>O<sub>3</sub>/CuInSe<sub>2</sub> interface
 
Conference Paper
This paper reports the effect of solvent and dopant impurities on the performance of LPE silicon solar cells. For LPE layers grown from Sn and In based solutions and having similar surface morphology and resistivity, the performance of solar cells made on LPE layers grown from In was always higher than that of cells made on LPE layers grown using Sn as solvent. Consistently higher performance was also obtained from solar cells fabricated upon Ga-doped LPE layers than from cells made on Al-doped LPE silicon. The best cell was fabricated upon a Ga-doped LPE layer grown from In solution and had a total area efficiency of 16.4% confirmed by Sandia measurements. The observed phenomena are explained on the basis of Hall mobilities and minority carrier lifetimes of LPE layers grown from different solutions, and also the oxidation difference of these layers during cell processing
 
Conference Paper
Presents results of the role of the oxygen partial pressure (p<sub>02</sub>) used on the properties exhibited by doped μc silicon oxycarbide films produced by a two consecutive decomposition and deposition chamber (TCDDC) system, where a spatial separation between the plasma and the growth regions is achieved. The films produced are highly conductive and transparent with suitable properties for optoelectronic applications. The film's structure, composition, morphology and optoelectronic properties were analysed by means of grazing X-ray diffraction, electron secondary chemical analysis (ESCA), secondary ion mass spectroscopy (SIMS) as a function of temperature (T) and optical absorption (ranging from 300 to 80 K) measurements
 
Conference Paper
For the first time the Si/C interface in poly-Si photovoltaic thin films on graphite substrate is described. The isostatically pressed graphite is covered with a 3-5 μm thick amorphous silicon layer which is recrystallized by the ZMR method (zone melting recrystallization by means of a line electron beam). During ZMR the molten silicon penetrates into the graphite pores, while at the interface β-SiC particles with a size of 50-1000 nm were formed (TEM, SEM analysis). Furthermore there exists a “reaction zone” where Si, SiC and C are found by electron diffraction. As a consequence the poly-Si layer shows an excellent adhesion to the substrate. Since there is no continuous electrically insulating SiC layer formed, highly boron doped seed layers show an ohmic contact to the graphite. The investigated poly-Si thin films on graphite substrate offer a great potential for photovoltaic application after epitaxy up to 50 μm by LPE or CVD
 
Conference Paper
Sub-bandgap spectral response measurements on silicon solar cells are used to characterise the infrared response of present devices, and to investigate the impurity photovoltaic (IPV) effect for improving their infrared response. The former has, aside from establishing a baseline case, led to an improved determination of the subgap absorption coefficient of crystalline silicon. Absorption coefficient values as low as 10<sup>-7</sup> cm<sup>-1</sup> have been determined, revealing structure due to 3- and 4-phonon assisted absorption. The influences of free carrier absorption, bandgap narrowing, and the Franz-Keldysh effect on cell infrared response are considered. Investigation of the IPV effect of indium in high efficiency bulk and thin film cells reveals that indium improves their infrared response. The cross-section for electron photoemission from the indium level, a crucial parameter for modelling indium's IPV effect, is determined
 
Conference Paper
A new method for the fabrication of a columnar, multicrystalline silicon layer on a graphite substrate is presented. This method basically involves three process steps: (1) deposition of a thin (3-5 μm) silicon layer; (2) zone melting recrystallization of this layer with a line electron beam as the heat source to form a multicrystalline seed layer; and (3) thickening of the seed layer by high temperature, epitaxial chemical vapour deposition (CVD) to a thickness of 20-40 μm. The recrystallization leads to (110)[112]-textured silicon seed layers if sufficiently high scan velocities are applied. The degree of deviation from the ideal (110)[112]-texture increases with decreasing scan velocity. The doping level of the seed layer is found to be only weakly affected by the zone melting recrystallization. The epitaxial layer grown on top of the seed layer exhibits a columnar grain structure
 
Conference Paper
Thin film crystalline silicon solar cells can only achieve high efficiencies if light trapping can be used to give a long optical path length, while simultaneously achieving near unity collection probabilities for all generated carriers. This necessitates a supporting substrate of a foreign material, with refractive index compatible with light trapping schemes for the silicon. The resulting inability to nucleate growth of crystalline silicon films of good crystallographic quality on such foreign substrates, prevents the achievement of high efficiency devices using conventional single junction solar cell structures. The parallel multijunction solar cell provides a new approach for achieving high efficiencies from very poor quality material, with near unity collection probabilities for all generated carriers achieved through appropriate junction spacing. Heavy doping is used to minimise the dark saturation current contribution from the layers, therefore allowing respectable voltages. The design strategy, corresponding advantages, theoretical predictions and experimental results are presented
 
Conference Paper
We used a 10-kW high-flux solar furnace (HFSF) to diffuse the front-surface n<sup>+</sup>-p junction and the back-surface p-p<sup>+ </sup> junction of single-crystal silicon solar cells in one processing step. We found that all of the HFSF-processed cells have better conversion efficiencies than control cells of identical structures fabricated by conventional furnace diffusion methods. HFSF processing offers several advantages that may contribute to improved solar cell efficiency: (1) it provides a cold-wall process, which reduces contamination; (2) temperature versus time profiles can be precisely controlled; (3) wavelength, intensity, and spatial distribution of the incident solar flux can be controlled and changed rapidly, (4) a number of high-temperature processing steps can be performed simultaneously; and (5) combined quantum and thermal effects may benefit overall cell performance. The HFSF has also been successfully used to texture the surface of silicon wafers and to crystallize a-Si:H thin films on glass
 
Conference Paper
The transitory and erratic nature of shunt currents, whether caused by light-soaking or electrical biasing, in amorphous Si (a-Si) single- and triple-junction solar cells has been a real puzzle and made the study of these cells difficult. The authors present a careful study of the time/voltage dependence of these current transients in several different a-Si solar cell structures and find they reveal more about the basic shunt mechanism. In single-junction cells, they see stepwise current changes that increase in size and number with reverse bias and can be removed with forward bias. This stepwise, on and off switching suggests a discrete shunt path conduction mechanism. The kinetics of these metastable shunt paths show that both the “on-state” and “off-state” possess memory. Cells without (Al)ZnO show no metastable switching. The authors associate the stepwise features with the textured substrate and the switching metastability with contact to (Al)ZnO. Switching in triple-junction cells occurs with hundreds of oscillations at each step
 
Article
4,7-Bis(2,3-dihydrothieno [3,4-b] [1,4] dioxin-5-yl)-2-dodecyl-2H-benzo [1,2,3] triazole (BEBT) was polymerized both electrochemically (ePBEBT) and chemically (cPBEBT). Since chemical polymerization enabled a soluble polymer in common organic solvents, a single layer electrochromic device of ePBEBT was constructed. The polymer cPBEBT was also used in bulk heterojunction (BHJ) solar cells as the active layer in combination with a soluble fullerene derivative, 1-(3-methoxycarbonyl)-propyl-1-1-phenyl-(6,6)C61 PCBM.
 
Article
A solution-processable and star-shaped molecule 4-((E)-2-(benzo[1,2,5]thiadiazol-4-yl)vinyl)-N,N-bis(4-((E)-2-(benzo[1,2,5]thiadiazol-7-yl)vinyl)phenyl)benzenamine (TPA-BT) has been designed and synthesized by palladium-catalyzed Heck reaction for the application in organic solar cells (OSCs). The molecule possesses a D–A structure with a triphenylamine core (donor unit) linked with three benzo[1,2,5]thiadiazole (acceptor unit) arms through double bonds. TPA-BT film shows a strong absorption peak in the visible wavelength range from 400 to 560 nm, which could be ascribed to the charge transfer band of the D–A structure of the molecule. The bulk-heterojunction OSCs with the device structure of ITO/PEDOT:PSS/TPA-BT:PCBM/Ca/Al (or Ba/Al) were fabricated by spin-coating the blend solution of TPA-BT and PCBM (1:3, w/w), in which TPA-BT was used as donor and PCBM as acceptor materials. The devices show a high open circuit voltage of ca. 0.9 V and a power conversion efficiency of 0.61%, under the illumination of AM 1.5, 100 mW/cm2. The results indicate that TPA-BT is a promising solution-processable organic photovoltaic material.
 
Article
Bulk heterojunction solar cells utilizing soluble phthalocyanine derivative, 1,4,8,11,15,18,22,25-octahexylphthalocyanine (C6PcH2) have been investigated. The active layer was fabricated by spin-coating the mixed solution of C6PcH2 and 1-(3-methoxy-carbonyl)-propyl-1-1-phenyl-(6,6)C61 (PCBM). The photovoltaic properties of the solar cell with bulk heterojunction of C6PcH2 and PCBM demonstrated the strong dependence of active layer thickness, and the optimized active layer thickness was clarified to be 120 nm. By inserting MoO3 hole transport buffer layer between the positive electrode and active layer, the FF and energy conversion efficiency were improved to be 0.50 and 3.2%, respectively. The tandem organic thin-film solar cell has also been studied by utilizing active layer materials of C6PcH2 and poly(3-hexylthiophene) and the interlayer of LiF/Al/MoO3 structure, and a high Voc of 1.27 V has been achieved.Highlights► Organic solar cells utilizing phthalocyanine derivative (C6PcH2) have been investigated. ► The photovoltaic properties demonstrated the strong dependence of active layer thickness. ► By inserting MoO3 buffer layer the FF and energy conversion efficiency were improved. ► The tandem solar cell has been investigated by utilizing C6PcH2 and poly(3-hexylthiophene). ► The optimized thickness was 120 nm, and a high Voc of 1.27 V has been achieved in the tandem cell.
 
Article
Current–voltage under illumination and quantum yield characteristics of an amorphous silicon/crystalline silicon hetero solar cell have been measured before and after exposure to high-energy (1.7 MeV) protons. A comparison of the measured wavelength-dependent quantum yield with calculated values enabled to determine the effective electron diffusion length of the crystalline silicon, that dropped from a value of 434 μm before to a value of 4 μm after irradiation with 5×1012 cm−2 protons. Good agreement has been obtained between measured and simulated data using DIFFIN,1 a finite-element simulation program for a-Si:H/c-Si heterojunction solar cells, enabling us to extract the depth profile of the recombination rate and the density of states distribution in the semiconductor layers before and after irradiation.
 
Article
In the present paper, we report on thin-film microcrystalline silicon solar cells grown at high deposition rates on back-reflectors with optimised light-scattering capabilities. A single-junction solar cell with a conversion efficiency of η=7.8% (2 μm thickness) was fabricated at a deposition rate of 7.4 Å/s. Another microcrystalline cell was successfully implemented in a “micromorph” tandem (i.e. a microcrystalline/amorphous tandem cell with n–i–p–n–i–p configuration); the resulting initial conversion efficiency was η=11.2%. A 4 μm thick single-junction cell at a deposition rate of 10 Å/s and with a conversion efficiency of η=6.9% was fabricated on a non-optimised substrate. Special attention is drawn to near-infrared spectral response and interface optimisation.
 
Article
We proposed a novel concept of organic two-layer photovoltaic devices with D–σ–A molecule (Donor subunit–σ bond–Acceptor subunit)/conducting polymer heterojunction. Au/PMeT (poly(3-methylthiophene))/NBPN (10-(p-nitrobenzyl)-2(10H)-phenazinone)/Al was fabricated as a prototype of D–σ–A/polymer photovoltaic device. The power conversion efficiency of this photodiode was 5.1 × 10 −2% under white illumination (8.46 mW/cm2). This value was larger than that of PMeT Schottky photodiode. This fact suggests that the concept of D–σ–A/polymer photovoltaic device is effective.
 
Article
In this paper an overview of the development of organic photovoltaics is given, with emphasis on polymer-based solar cells. The observation of photoconductivity in solid anthracene in the beginning of the 19th century marked the start of this field. The first real investigations of photovoltaic (PV) devices came in the 1950s, where a number of organic dyes, particularly chlorophyll and related compounds, were studied. In the 1980s the first polymers (including poly(sulphur nitride) and polyacetylene) were investigated in PV cells. However, simple PV devices based on dyes or polymers yield limited power conversion efficiencies (PCE), typically well below 0.1%. A major breakthrough came in 1986 when Tang discovered that bringing a donor and an acceptor together in one cell could dramatically increase the PCE to 1%. This concept of heterojunction has since been widely exploited in a number of donor–acceptor cells, including dye/dye, polymer/dye, polymer/polymer and polymer/fullerene blends. Due to the high electron affinity of fullerene, polymer/fullerene blends have been subject to particular investigation during the past decade. Earlier problems in obtaining efficient charge carrier separation have been overcome and PCE of more than 3% have been reported. Different strategies have been used to gain better control over the morphology and further improve efficiency. Among these, covalent attachment of fullerenes to the polymer backbone, creating so-called double-cable polymers, is the latest. The improved PCE of plastic solar cells combined with increased (shelf and operating) lifetime, superior material properties and available manufacturing techniques may push plastic PVs to the market place within a few years.
 
Article
The PV solar electricity market has shown an impressive 33 % growth per year since 1997 until today with market support programs as the main driving force. The rationales for this development and the future projections towards a 100 billion € industry in the 20ies by then only driven by serving cost competitively customer needs are described. The PV market, likely to have reached about 600 MW in the year 2003, is discussed according to its 4 major segments: consumer applications, remote industrial electrification, developing countries, grid-connected systems. While in the past, consumer products and remote industrial applications used to be the main cause for turnover in PV, in recent years the driving forces are more pronounced in the grid-connected systems and by installations in developing countries. Examples illustrating the clear advantage of systems using PV over conventional systems based e.g. on diesel generators in the rural and remote electrification sector are discussed. For the promotion of rural electrification combined with the creation of local business and employment, suitable measures are proposed in the context of the PV product value chain. The competitiveness of grid-connected systems is addressed, where electricity generating costs for PV are projected to start to compete with conventional utility peak power quite early between 2010 and 2020 if time-dependent electricity tariffs different for bulk and peak power are assumed. The most effective current pulling force for grid-connected systems is found to be the German Renewable Energy (EEG) feed-in law where the customers are focussing on yield, performance, and longlife availability.
 
Article
This paper presents a-Si:H and μc-Si:H p–i–n solar cells prepared at high deposition rates using RF (13.56 MHz) excitation frequency. A high deposition pressure was found as the key parameter to achieve high efficiencies at high growth rates for both cell types. Initial efficiencies of 7.1% and 11.1% were achieved for a μc-Si:H cell and an a-Si:H/μc-Si:H tandem cell, respectively, at a deposition rate of 6 Å/s for the μc-Si i-layers. A μc-Si:H cell prepared at 9 Å/s exhibited an efficiency of 6.2%.
 
Article
Crystalline silicon solar cells 10–15 times thinner than traditional commercial c-Si cells with 14.9% efficiency are presented with modeling, fabrication, and testing details. These cells are 14 μm thick, 250 μm wide, and have achieved 14.9% solar conversion efficiency under AM 1.5 spectrum. First, modeling results illustrate the importance of high-quality passivation to achieve high efficiency in thin silicon, back contacted solar cells. Then, the methodology used to fabricate these ultra thin devices by means of established microsystems processing technologies is presented. Finally, the optimization procedure to achieve high efficiency as well as the results of the experiments carried out with alumina and nitride layers as passivation coatings are discussed.Graphical Abstract
 
Article
High-efficiency solar cells with thin CdS film have recently been developed. Semiconductive layers of CdS via the CVD method and of CdTe via the CSS method were deposited on an ITO/#7059 substrate. Cell performance depends primarily on the thickness of CdS film, and the conversion efficiency is highest for a CdS film thickness of around 60 nm. Since the CdS film thickness decreases by about 30% during deposition of the CdTe layer, a thickness of 95 nm is required to obtain a 60 nm-thick CdS film after deposition of a CdTe layer. By observing the CdS film during the CdTe deposition process, a decrease was detected before CdTe layer completely covers the surface of the CdS film. By optimizing the thickness of CdS film, an efficiency of 15.12% for the best cell under AM 1.5 verified at JQA was obtained. This fabrication process has good reproducibility; 92.5% of 1 cm2 solar cells fabricated under the same conditions have efficiencies above 14%.
 
Article
On the way to higher efficiencies, back contact solar cells seem to be a promising alternative to the conventional screen-printed solar cells. Especially, the metal wrap through (MWT) solar cell concept with only two additional process steps is appropriate for a fast transfer to industry. Hence, an industrially feasible process based on a new contact design was developed and tested at the pilot-line of the Photovoltaic Technology Evaluations Center (PV-TEC). A maximum cell efficiency of 16% is achieved. Compared with conventionally processed cells made of the same mc Si-block, an efficiency gain of 0.5% absolute is observed. Due to a cell interconnection on the back the serial resistance losses in the tabs decrease. Therefore, a fill factor of almost 77% and an efficiency of 15% for a MWT module prototype (16 MWT cells) is reached.
 
Article
On the way to higher efficiencies, back contact solar cells seem to be very promising. Especially, the metal wrap through (MWT) solar cell concept, with only three additional process steps when compared to conventionally processed cells, is appropriate for a fast transfer to industry. Hence, a pilot-line process based on a modified via-metallization step was set up. Therefore, a newly developed short suction step directly after the screen-printing process was established and characterized. Using this new via-metallization technique, cell efficiencies over 16% are reached on a multi-crystalline silicon (mc-Si) material. Compared with conventionally processed cells, an efficiency gain of 0.5% absolute is observed.
 
Article
We have fabricated 4 cm2 solar cells on String Ribbon Si wafers and edge-defined film-fed grown (EFG) Si wafers with using a combination of laboratory and industrial processes. The highest efficiency on String Ribbon Si wafer is 17.8% with an open circuit voltage (Voc) of 620 mV, a short circuit current density (Jsc) of 36.8 mA/cm2 and a fill factor (FF) of 0.78. The maximum efficiency on EFG Si is 18.2% with a Voc of 620 mV, a Jsc of 37.5 mA/cm2 and a FF of 0.78. These are the most efficient ribbon Si devices made to date, demonstrating the high quality of the processed Si ribbon and its potential for industrial cells. Co-firing of SiNx and Al by rapid thermal processing was used to boost the minority carrier lifetime of bulk Si from 3–5 μs to 70–100 μs. Photolithography-defined front contacts were used to achieve low shading losses and low contact resistance with a good blue response. The effects of firing temperature and time were studied to understand the trade-off between hydrogen retention and Al-doped back surface field (Al-BSF) formation. Excellent bulk defect hydrogenation and high-quality thick Al-BSF formation was achieved in a very short time (∼1 s) at firing temperatures of 740–750 °C. It was found that the bulk lifetime decreases at annealing temperatures above 750 °C or annealing time above 1 s due to dissociation of hydrogenated defects.
 
Article
Solar cells with efficiencies as high as 18.6% (1 cm2 area) and 17.5% (4 cm2 area) have been achieved by a process which involves impurity gettering and effective back-surface passivation on multicrystalline silicon (mc-Si) grown by the heat exchanger method (HEM). The former efficiency mark represents the highest reported solar cell efficiency on mc-Si to date. PCD analysis revealed that the bulk lifetime in certain HEM samples after phosphorus gettering can be as high as 135 μs. This increases the impact of the back-surface recombination velocity (Sb) on the solar cell performance. By incorporating a deeper aluminum back-surface field (Al-BSF), the Sb for solar cells in this study was lowered from 10,000 to 2000 cm/s (for 1 and 10 μm evaporated aluminum layers, respectively, alloyed in a conventional furnace) and finally to 200 cm/s (for screen printed Al-BSFs alloyed in an RTP furnace). It was observed that the screen-printed/RTP alloyed Al-BSF process raised the efficiency of both float zone and relatively defect-free mc-Si solar cells by lowering Sb. However, this process increased the electrical activity of extended defects so that mc-Si cells with a significant defect density showed degradation in performance.
 
Article
An efficiency of over 18% have been achieved in Cu(In,Ga)Se2 (CIGS) thin-film solar cells. Solar cell parameters were estimated for the cells with efficiencies of more and less than 18%. A diode quality factor n and forward current (saturated current) J0 of the cell with over 18% efficiency are lower than those with below 18% efficiency. This would be attributed to sufficient coverage of the CdS film with excellent uniformity as a buffer and/or window layer over the CIGS film because the process of CdS film formation was improved.
 
Article
This paper reports on the synthesis of SnO2 18 nm diameter colloidal suspension for the fabrication of nanoporous electrodes. The new suspension allows the fabrication of thick and homogeneous electrodes by simple one layer spreading; in contrast to the successive spin coating of the commonly used commercial suspension that results in a thin inhomogeneous electrode. When used in dye-sensitized solar cells, the new electrodes increase the light-to-energy conversion efficiency by a factor of 2.1 in comparison with standard commercial suspension based electrodes. The improvement is mostly the result of an increase of the photocurrent. This increase is attributed to the better electrolyte migration, and presumably also to an increase of the photoinjected electron diffusion rate in the electrode.
 
Article
The lifetimes of organic photovoltaic cells based on conjugated polymer materials were studied. The device geometry was glass:ITO:PEDOT:PSS:C12-PSV:C60:aluminium. To characterise and elucidate the parts of the degradation mechanisms induced by molecular oxygen, 18O2 isotopic labelling was employed in conjunction with time-of-flight secondary ion mass spectrometry. A comparison was made between devices being kept in the dark and devices that had been subjected to illumination under simulated sunlight (1000 W m−2, AM1.5) and this demonstrated that oxygen-containing species were generated throughout the active layer with the largest concentration towards the aluminium electrode. For devices that had been kept in the dark oxygen species were only observed at the immediate interface between the aluminium and the organic layer. The isotopic labelling allowed us to demonstrate that the oxygen comes from the atmosphere and diffuses through the aluminium electrode and into the device.
 
Article
W oxide films are of critical importance for electrochromic device technology, such as for smart windows capable of varying the throughput of visible light and solar energy. This paper reviews the progress that has taken place since 1993 with regard to film deposition, characterization by physical and chemical techniques, optical properties, as well as electrochromic device assembly and performance. The main goal is to provide an easy entrance to the relevant scientific literature.
 
Article
Mono- and poly-crystalline silicon solar cell modules currently represent between 80% and 90% of the PV world market. The reasons are the stability, robustness and reliability of this kind of solar cells as compared to those of emerging technologies. Then, in the mid-term, silicon solar cells will continue playing an important role for their massive terrestrial application. One important approach is the development of silicon solar cells processed at low temperatures (less than 300 °C) by depositing amorphous silicon layers with the purpose of passivating the silicon surface, and avoiding the degradation suffered by silicon when processed at temperatures above 800 °C. This kind of solar cells is known as HIT cells (hetero-junction with an intrinsic thin amorphous layer) and are already produced commercially (Sanyo Ltd.), reaching efficiencies above 20%. In this work, HIT solar cells are simulated by means of AMPS-1D, which is a program developed at Pennsylvania State University. We shall discuss the modifications required by AMPS-1D for simulating this kind of structures since this program explicitly does not take into account interfaces with high interfacial density of states as occurs at amorphous-crystalline silicon hetero-junctions.
 
Article
Despite the significant progress made in the field of electrochromic polymers, the multichromic facility of current knowledge is restricted. Therefore, as previously proven, electrochemical copolymerization of 1-benzyl-2,5-di(thiophen-2-yl)-1H-pyrrole (SNBS) and 3,4-ethylenedioxythiophene (EDOT) was used as a strategy to achieve desired multichromic properties, where the resultant copolymer displayed distinct color changes between claret red, yellow, green, and blue colors with short switching times and high optical contrast. As an application, absorption/transmission type electrochromic device with indium tin oxide (ITO)/copolymer/gel electrolyte PEDOT/ITO configuration was constructed, where copolymer and PEDOT functioned as the anodically and the cathodically coloring layers, respectively. Results implied the successive use of this copolymer in electrochromic device applications, since the device exhibited short switching times with a wide color variation upon applied potential.
 
Article
Polymers based on dithienyl-2,1,3-benzothiadiazole (TBT) have received strong attention as the donor materials of polymer photovoltaic cells (PVs), since they can have a low band gap. But soluble small organic molecules based on TBT have been rarely studied. This paper reports the synthesis of two small organic molecules based on TBT and their application as the donor materials of solution-processed bulk heterojunction organic photovoltaic cells (OPVs). These compounds were soluble in common organic solvents, such as chloroform, chlorobenzene and tetrahydrofuran. They have band gaps comparable to poly(3-hexylthiophene) (P3HT) and lower HOMO and LUMO (HOMO: highest occupied molecular orbital, LUMO: lowest unoccupied molecular orbital) levels than P3HT. These molecules and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) were used as the donors and acceptor to fabricate bulk heterojunction OPVs through solution processing. After optimization of the experimental conditions, power conversion efficiency (PCE) of 0.66% was achieved on the solution-processed OPVs under AM 1.5G, 100 mW cm−2 illumination.
 
Article
New low-band-gap copolymers, including thieno[3,2-b]thiophene and 2,1,3-benzothiadiazole, were synthesized as photovoltaic materials. Thiophene was introduced to provide extended π-conjugation length and charge transfer properties. A band gap (Egop=1.62 eV, Egec=1.51 eV) of this polymer was investigated through UV–vis spectroscopy and cyclic voltammetry. A bulk heterojunction structure of glass/indium tin oxide (ITO)/PEDOT:PSS/polymer-PCBM(1:3)/LiF/Al was fabricated for investigating photovoltaic properties. PC71BM was used as an acceptor material, due to its increased absorption in the visible region, in comparison with PC61BM. In this polymer, incident photon-to-current conversion efficiency (IPCE) was as high as 50%. Moreover, maximum power conversion efficiency (PCE) of up to 1.72% was achieved under AM 1.5 G conditions. It demonstrated relatively high VOC (0.67 V) and JSC (6.86 mA/cm2), while a low fill factor value (0.37) was obtained.
 
Article
A stable organic radical, 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO), was studied. We employed TEMPO as a cathodic radical provider in propylene carbonate (PC) and poly(3,4-propylenedioxythiophene) derivatives (PProDOT-Et2) as an anodic electrochromic (EC) thin film, which was obtained through electropolymerization. On assembling them together in a device, the electrochemical and optical performances of this hybrid electrochromic device (ECD) showed reversible cycling stability and high absorbance attenuation in the visible range. By selecting proper electrolytes (LiClO4/PC) and controlling the deposited charge of the PProDOT-Et2 thin film, it was possible to obtain a transmittance change (ΔT) of up to 59% at 590 nm with no noticeable degradation after operating between 0 and 0.9 V for 1000 cycles. Furthermore, an electrochemical quartz crystal microbalance (EQCM) was used to investigate ion migrations in the PProDOT-Et2 thin film, which influenced its long-term stability.
 
Article
Poly(2,5-thienylene vinylene) derivatives (PTVs) present a specific class of low band gap conjugated polymers of which the synthesis seems to be problematic. Recently, the dithiocarbamate route was introduced as a synthetic route towards PTV. However, the synthesis of alkyl-PTV derivatives failed on using lithium diisopropyl amide (LDA) as the base in the polymerisation reaction. We demonstrate that the use of lithium bis(trimethylsilyl)amide (LHMDS) instead of LDA allows for an efficient synthesis of alkyl-PTV derivatives. Indications are presented for the formation of an ordered structure in case of the 3-alkyl derivative, pointing to a regio-regular polymerisation process.
 
Article
A novel regioregular selenophene and thiophene-based poly(2,5-bis(3-dodecylthiophen-2-yl)-2′,2″-biselenophene) (PBTBS), was chosen as promising electron donor materials blended with [6,6]-phenyl-C61 butyric acid methyl ester(PCBM) to fabricate bulk heterojunction solar cells. The optical and photovoltaic properties of PBTBS were characterized and compared to those of poly(3,3″′-didedocyl-quaterthiophene)( PQT-12). The PBTBS showed a higher ionization potential (IP) and improved oxidative stability than PQT-12 through the cyclic voltammetry (CV) measurements, which was due to stronger electron-donating properties of selenophene than that of thiophene analogue. A comparison of photovoltaic properties of cells between two polymer donors PBTBS and PQT-12 were also carried out. The result showed that cells based on the PBTBS had the comparable photovoltaic parameters with those of the PQT-12. An open voltage (VOC) of 0.46 V, a short-circuit current (JSC) of 0.17 mA/cm2, a fill factor (FF) of 0.32 and a power conversion efficiency (η) of 0.34% were achieved at 100 mW/cm2 (AM 1.5). The external quantum efficiency of cells based on the PBTBS exhibited much better performance with around 3.4% at 540 nm compared with that of the PQT-12.
 
Article
The US Department of Energy (DOE), in collaboration with key stakeholders, initiated a strategy to accelerate the adoption of photovoltaics, biomass electric, and solar thermal electric technologies. The three major elements include technology development and validation, market conditioning, and joint venture projects. This paper details the activities associated only with photovoltaics, progress towards goals, and expected payoffs. Success is necessary to accelerate renewable technology acceptance and deployment towards meeting the world's demands for environmentally acceptable electricity generation.
 
Article
The suitability of a pigment for inclusion in “cool” colored coatings with high solar reflectance can be determined from its solar spectral backscattering and absorption coefficients. Pigment characterization is performed by dispersing the pigment into a transparent film, then measuring spectral transmittance and reflectance. Measurements of the reflectance of film samples on black and white substrates are also used. A model for extracting the spectral backscattering coefficient S and absorption coefficient K from spectrometer measurements is presented. Interface reflectances complicate the model. The film's diffuse reflectance and transmittance measurements are used to determine S and K as functions of a wavelength-independent model parameter σ that represents the ratio of forward to total scattering. σ is used to estimate the rate at which incident collimated light becomes diffuse, and is determined by fitting the measured film reflectance backed by black. A typical value is σ=0.8. Then, the measured film reflectance backed by white is compared with a computed value as a self-consistency check. Measurements on several common pigments are used to illustrate the method.
 
Top-cited authors
Claes Goran Granqvist
  • Uppsala University
Martin Green
  • UNSW Sydney
Mikkel Jørgensen
  • Technical University of Denmark
Kion Norrman
  • King Fahd University of Petroleum and Minerals