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For the Bright Future-Bulk Heterojunction Polymer Solar Cells with Power Conversion Efficiency of 7.4%
Advanced Materials
Abstract
The photovoltaic performance of polymer bulk heterojunction solar cells is studied systematically. Using a new benzodithiophene polymer (PTB7) and PC 71BM (see figure) a power conversion efficiency of 7.4% has been achieved in PTB7/PC71BMblend film, indicating a great potential and bright future for polymer solar cells (FF = fill factor, PCE = power-conversion efficiency). (Figure Presented).
... Organic solar cells (OSCs) as an emerging thin-film photovoltaic technology have gained tremendous attention during the past three decades, due to their attractive properties of light weight, semitransparency, lowtemperature fabrication, and so on [1][2][3][4][5][6][7][8][9][10]. As of now, the power conversion efficiencies (PCEs) of singleheterojunction OSCs have risen to the level of 19% [11,12]. ...
... V. Given E g preset to the reported value of 1.43 eV [11], J SC = 26.7 mA cm −2 , and V OC,rad = 1.098 V, σ is calculated to be 0.062 eV via equations (6)(7)(8). With preset E g of 1.43 eV, deduced σ of 0.062 eV, and preset μ eff of 2.8 × 10 −3 cm 2 V −1 s −1 , SIM 1 gives a PCE of 19.0%. ...
... mA cm −2 , and V OC,rad = 1.066 V, σ is calculated to be 0.048 eV via equations (6)(7)(8). With preset E g = 1.37 eV, deduced σ = 0.048 eV, preset μ eff = 8.5 × 10 −4 cm 2 V −1 s −1 , SIM 6 gives a PCE of 18.56%. ...
The bandgap and energetic disorder of photoactive layer are of great importance to analyzing the energy losses of organic solar cells (OSCs). However, the accurate determinations of these two parameters have yet to be realized so far. Here, an improved analytic model based on Shockley equation is provided to simulate the photovoltaic performance of nonfullerene OSCs with efficiencies of ~19%, whereby the bandgap and energetic disorder as fitting parameters are deduced. The modeling indicates that the radiative voltage loss is major, relative to the nonradiative one. The accurate quantification of the bandgap and energetic disorder relies on the accurate experimental measurement of charge carrier mobilities of photoactive layer. The simulations show that the state-of-art nonfullerene photoactive layers have bandgaps of ~1.35 to 1.37 eV and energetic disorders of ~0.11 eV. In order to improve the efficiencies of OSCs to over 20%, it is proposed to decrease the energetic disorders and/or increase the charge-carrier mobilities of photoactive layers.
... The benzo[1,2-b:4,5-b′]dithiophene (BDT) unit proved to be an important electron-rich component for high-performing organic solar cells. The PTB series polymers uses a BDT and thieno[3,2-b]thiophene (TT) unit, developed by Yu and coworkers through the Stille polycondensation method with a Pd(PPh3)4 catalyst mixed with toluene/DMF solvent at 120 °C to enhance the organic solar cell performance [25,26]. The high molecular weight can be achieved for the PTB series polymers due to their excellent solubility, which permits them to stay in the solution until the large polymer chain growth. ...
The charge carrier formation and transport in the pristine polymers as well as in the polymer–fullerene blend is still a hot topic of discussion for the scientific community. In the present work, the carrier generation in some prominent organic molecules has been studied through ultrafast transient absorption spectroscopy. The identification of the exciton and polaron lifetimes of these polymers has led to device performance-related understanding. In the Energy Gap Law, the slope of the linear fit gradient (γ) of lifetimes vs. bandgap are subjected to the geometrical rearrangements experienced by the polymers during the non-radiative decay from the excited state to the ground state. The value of gradient (γ) for excitons and polarons is found to be −1.1 eV−1 and 1.14 eV−1, respectively. It suggests that the exciton decay to the ground state is likely to involve a high distortion in polymer equilibrium geometry. This observation supports the basis of Stokes shift found in the conjugated polymers due to the high disorder. It provides the possible reasons for the substantial variation in the exciton lifetime. As the bandgap becomes larger, exciton decay rate tends to reduce due to the weak attraction between the holes in the HUMO and electron in the LUMO. The precise inverse action is observed for the polymer–fullerene blend, as the decay of polaron tends to increase as the bandgap of polymer increases.
... In CdTe, nickel creates a deep acceptor level. In this section, we report the simulation of back contacts WF absolute values ranging from 5 eV (carbon) to 5.8 eV (Ag electrode modified with pure FDT) [24]: C (5 eV) and Ni, Au Simulations reveal that, metals with very high work functions which form Schottky contacts with small barriers and can thus act effectively as Ohmic-like contacts at normal working temperature leading to more stable devices. The higher the work function, the more efficient the cell ( figure 11). ...
This work is a theoretical contribution to improving the performance of CdTe-based thin-film solar cells (TFSC) by optimising the collection of photons in the absorber structure. The basic data are retrieved from experimental reference work and the reference structure is as follows: CdS/CdTe/ZnTe with an efficiency of 20.16%, where ZnTe is used as a BSF to limit backward recombination. The first approach is to incorporate a ZnTe thin layer at the CdS/CdTe heterojunction, to subdivide the CdTe active layer into two (02) sub-layers and to identify the optimum structure as a function of their position in the stack. Investigating the work function of back contact materials enables to better enhance the device′s performance and stability. To take into consideration the discontinuities in the material properties, grain boundaries and performance loss factors, the impacts of charge carrier capture cross sections, bulk and interfacial defects are investigated. SCAPS software is employed for all the numerical modelling, which enables to calculate the current-voltage (J-V), power-voltage (P-V), external quantum efficiency (EQE) and other PV parameters and to draw energy band diagram to better appraise charge carrier transportation. The doping level in the CdTe active layer, the thicknesses and external temperature are also investigated to optimize our device properties. In terms of the obtained fill factor (FF) and efficiency (PCE), the performances were improved with the following structure CdS/CdTe/ZnTe/CdTe/ZnTe, FF = 81.6% and PCE = 23.45%, with 500 nm thickness of CdTe. These results are opening a promising new perspective in high efficiency CdTe TFSC.
... Following this concept, many low-gap polymers built using the donor-acceptor approach have been developed and have made it possible to obtain cells that can reach up to 7% conversion efficiency [54]. ...
... M has been used widely in the literature to understand differences between measured spectra and reference spectra, for example when calibrating solar simulators [132], testing new PV materials [133], and for PV performance analysis under varying spectral irradiance conditions [134,135,136]. ...
... [34] Particularly, single-component layer consisting either electron donor or acceptor is incorporated as active layer. We first explored most of the traditional materials such as P3HT, [35] PCBM, [36] PTB7-Th, [37] PBDB-T [38] and PM6. [39] However, these materials exhibit really poor device performance on such structure (Figure S1 and Table S1), which is consistent with previous report and quite within expectation considering their large exciton binding energy and poor ambipolar charge transport. ...
Comprehensive Summary
Since 1986, the donor‐acceptor (D:A) heterojunction has been regarded a necessity for high‐efficiency organic photovoltaics (OPVs), due to its unique advantage in compensating the intrinsic limitations of organic semiconductors, such as high exciton binding energy and poor ambipolar charge mobility. While this adversely causes tremendous non‐radiative charge recombination and instability issues, which currently become the most critical limits for commercialization of OPVs. Here, we present a concept‐to‐proof study on the potential of D:A heterojunction free OPV by taking advantage of recent progress of non‐fullerene acceptors. First, we demonstrate that the “free carriers” can be spontaneously generated upon illumination in an NFA, i.e. the 6TIC‐4F single layer. Second, the 6TIC‐4F layer also exhibits good ambipolar charge transporting property. These exceptional characteristics distinguish it from the traditional organic semiconductors, and relieve it from the reliance of D:A heterojunction to independently work as active layer. As a result, the subsequent OPV by simply sandwiching the 6TIC‐4F layer between the cathode and anode yields a considerably high power conversion efficiency ~ 1%. Moreover, we find the D:A heterojunction free device exhibits two order of magnitude higher electroluminescence quantum efficiency and significantly reduced V OC loss by 0.16 eV compared to those of the D:A BHJ structure, validating its promise for higher efficiency in the future. Therefore, our work demonstrates the possibility of using D:A heterojunction‐free device structure for high performance, that can potentially become the next game changer of OPV.
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... In the same year, the polymer donor PBDTTT-C (2009) was also reported by Huo et al. [47] Additionally, Liang et al. created PTB4 (2019) , PTB5 (2019) and PTB7 (2015) by introducing the functional groups 2-ethylhexyl acetate and "F" atom, octyl acetate and "F" atom, as well as octyl acetate into the TT unit, accordingly. [48,49] In comparison to the photoelectric performance of PBDTTT-E: PC 70 BM based OSC, PTB7: PC 71 BM based OSC obtained a higherV oc and J sc , which can be attributed to the optimized co-blending morphology formed by introducing solubilized alkyl chain 2-ethylhexyl. Afterward, Liang et al. announced three polymer donors PTB1 (2019) , PTB2 (2019) andPTB6 (2019) , [48] which were designed through introducing the side chain 1-methoxyoctane into the BDT unit as well as dodecyl acetate, 2-ethylhexyl acetate and 2-butyloctyl acetate into the TT unit, in that order. ...
In recent decades, the demand for clean and renewable energy has grown increasingly urgent due to the irreversible alteration of the global climate. As a result, efficient organic solar cells (OSCs) have gradually gained attention as a study hotspot. To break up this dilemma, this paper reviews the molecular design strategies of benzodithiophen (BDT)-based polymer and small molecule donor materials since their birth, focusing on the development of main-chain engineering, side-chain engineering and other unique molecular design paths. Up to now, the state-of-the-art power conversion efficiency ( PCE ) of binary OSCs prepared by BDT-based donor materials has approached 20%. Our work detailly discusses the potential relationship between the molecular changes of donor materials and photoelectric performance in corresponding OSC devices, thereby presenting a rational molecular design guidance for stable and efficient donor materials in future.
... The CQD ink can not only be applied for the fabrication of the heterojunction-structured CQDSC that the n-type CQD solid layer was stacked with the p-type CQD solid layer forming a p-n junction, but also be used for the construction of mixed CQDSCs, which was originated from the organic solar cells [189][190][191][192]. The n-type and p-type CQDs obtained from the LSLX method were mixed in the inks for the preparation of mixed CQD solid films [193]. ...
... It is worth noting that most additives, such as classical additive CN, tend to reduce the V OC of PM6:Y6 OSCs, as summarized in Figure S3 (Supporting Information), which is because these additives changed the phase separation of blend films. [11,34,35] However, the PM6:Y6 OSC processed with TT-Cl shows an improved V OC than the reference one. From the energy loss analysis, a direct deduction is the increased E CT reduces the voltage loss. ...
Although the advances in organic solar cells (OSCs) have been considerable, their efficiency is still limited by recombination losses. Photogenerated electrons and holes are generally bound as localized excitons in organic semiconductors. The transition from excitons into free charges requires diffusion and dissociation processes, in which parasitic recombination losses exist. Reducing these losses is necessary for highly efficient OSCs. The crystallization behavior of the active layers can influence the efficiency of exciton diffusion and dissociation. In this work, different additives are delicately designed to control the crystallization behavior. It is found that the crystallization quality of active layers can be improved by controlling the aggregation of nonfullerene acceptors. The π–π stacking of blend films becomes compact, meanwhile, the crystallization in the vertical direction is more uniform. These are beneficial to the diffusion and dissociation of excitons. As a consequence, recombination losses are reduced and power convention efficiencies (PCEs) are improved significantly. Meanwhile, the general applicability of the additive is demonstrated in various organic photovoltaic systems, in which a PCE of 19.3% is achieved in D18:BTP‐eC9‐4F OSCs. This work provides a facile strategy to reduce the recombination losses of excitons for efficient devices.
... BTA1 contains two rhodanine units as the end-capped groups and introduces a dicyanomethylene group in rhodanine producing BTA3. 21,22 PTB7 23,24 is the donor molecule. Table S1 summarizes the physical properties of the PTB7/BTA1 and BTA3 systems, where the fill factor is larger for PTB7/BTA3 than for PTB7/BTA1, and PTB7/BTA3 has the highest external quantum efficiency. ...
We studied photoinduced charge transfer (CT) states and their dissociation processes at the donor/acceptor (D/A) interface of PTB7/BTAx (x = 1 and 3) nonfullerene organic thin-film solar cells using density functional theory (DFT) and time-dependent DFT calculations. We focused on the CT distances and electron coupling in the CT state generated by photoexcitation and the Huang-Rhys (HR) factors that describe the nonadiabatic processes associated with vibronic interactions. The PTB7/BTA3 system with a large short-circuit current density (JSC) exhibited a large charge CT distance and electronic coupling. Contrastingly, the PTB7/BTA1 system with a low JSC has a large HR factor because of the low-wavenumber vibrational modes in the CT state of the D/A complex and is prone to nonadiabatic relaxation to the ground state. Systematic theoretical analysis of the excitonic states in the D/A complex has provided insight into the control of CT exciton dynamics, namely JSC and electron-hole recombination.
... Este salto foi possível com a introdução de uma heterojunção dispersa entre materiais doadores e receptores (Figura 5). Atualmente essas células solares têm apresentado eficiência superior a 7 % (Liang, 2010). Além disso, acredita-se na possibilidade dessa tecnologia tornar-se competitiva principalmente devido à possibilidade de fabricação utilizando métodos de impressão tipo inkjet. ...
Em 2010 celebramos 14 anos de pesquisa na área de células solares de corante (dye-sensitized solar cells). Neste trabalho mostramos nossos esforços na otimização dos materiais componentes da célula solar e o aumento da eficiência dos dispositivos a partir do uso desses materiais. Apresentamos também a construção de um módulo de células solares, sendo o primeiro módulo de células de corante com eletrólito polimérico construído no Brasil. Em outra linha de pesquisa, estamos desenvolvendo células solares orgânicas (organic solar cells). O principal interesseneste tipo de tecnologia reside no fato de ser de baixo custo e permitir a fabricação de dispositivos totalmente flexíveis. Neste trabalho também mostramos a aplicação de novos materiais estratégicos nesse tipo de célula solar.
... Over the past two decades, OPV has experienced a considerable increase in interest as a potential energy source due to their semitransparent, flexible, and light-weight properties. In addition to their light harvesting properties, these devices can be fabricated using lowcost manufacturing processes and are suitable for the use of simple laboratory-environment deposition techniques, making research in this area readily accessible [1][2][3][4]. However, OPV devices traditionally exhibit lower efficiencies than their silicon counterparts and a significant challenge for the field is how to improve the PCE. ...
The performance of poly(3-hexylthiophene) (P3HT): phenyl-C61-butyric acid methyl ester (PCBM) organic photovoltaic (OPV) devices was found to be strongly influenced by environmental during preparation, thermal annealing conditions, and the material blend composition. We optimized laboratory fabricated devices for these variables. Humidity during the fabrication process can cause electrode oxidation and photo-oxidation in the active layer of the OPV. Thermal annealing of the device structure modifies the morphology of the active layer, resulting in changes in material domain sizes and percolation pathways which can enhance the performance of devices. Thermal annealing of the blended organic materials in the active layer also leads to the growth of crystalline for P3HT domains due to a more arrangement packing of chains in the polymer. Poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) acts as a hole transport layer in these P3HT:PCBM devices. Two commercially materials of PEDOT:PSS were utilizing in the optimization of the OPV in this research; high conductivity PEDOT:PSS-PH1000 and PEDOT:PSS-Al4083, which is specifically designed for OPV interfaces. It was demonstrated that OPVs were prepared with PEDOT:PSS-PH1000 have a less than the average performance of PEDOT:PSS-Al4083. The power conversion efficiency (PCE) decreased clearly with a reducing in masking area devices from 5 mm2 to 3.8 mm2 for OPVs based on PH1000 almost absolutely due to the reduced short circuit current (Jsc). This work provides a roadmap to understanding P3HT:PCBM OPV performance and outlines the preparation issues which need to be resolved for efficient device fabrication
... PCEs have seen a significant increase in activity as a result of the progress in both novel materials for the donor and acceptor. Recently, bulk heterojunction (BHJ) OPVs have achieved PCEs of more than 18% [391,392], with improvements of 150% over the past decade for mono-layer devices [393]. Although OPVs are a promising technology, they cannot be widely commercialized until PCEs are increased [394,395]. ...
Third-generation solar cells are designed to achieve high power-conversion efficiency while being low-cost to produce. These solar cells have the ability to surpass the Shockley–Queisser limit. This review focuses on different types of third-generation solar cells such as dye-sensitized solar cells, Perovskite-based cells, organic photovoltaics, quantum dot solar cells, and tandem solar cells, a stacked form of different materials utilizing a maximum solar spectrum to achieve high power conversion efficiency. Apart from these solar cells, other third-generation technologies are also discussed, including up-conversion, down-conversion, hot-carrier, and multiple exciton. This review provides an overview of the previous work in the field, alongside an introduction to the technologies, including their working principles and components. Advancements made in the different components and improvements in performance parameters such as the fill factor, open circuit voltage, conversion efficiency, and short-circuit current density are discussed. We also highlight the hurdles preventing these technologies from reaching commercialization.
... E Acceptor's LUMO illustrates the energy of excited state of PC 61 BM (chosen acceptor material), and 0.3 is an empirical factor [23][24][25]. Finally, the E Donor's HOMO represents the energy of ground state of all studied molecules [26,27]. K b donates Boltzmann constant (1.38*10 − 23 J/K), T represents temperature (300K), n is equal to 1, and q refers to the charge of 1.6*10 − 19 C. The quantity qVoc nK b T collectively represents the normalized V oc [28][29][30]. ...
Despite the substantial advancements in organic solar cells (OSCs), the best devices still have quite low efficiencies due to less focus on donor molecules. With the intention to present efficient donor materials, seven small donor molecules (T1-T7) were devised from DRTB-T molecule by using end-capped modeling. Newly designed molecules exhibited remarkable improved optoelectronic properties such as less band gap (from 2.00 to 2.23 eV) than DRTB-T having band gap of 2.57 eV. Similarly, a significant improvement in λmax values was noticed in designed molecules in gaseous medium (666 nm–738 nm) and solvent medium (691 nm–776 nm) than DRTB-T having λmax values at 568 nm and 588 nm in gas and solvent phase respectively. Among all molecules, T1 and T3 exhibited significant improvement in optoelectronic properties such as narrow band gap, lower excitation energy, higher λmax values and lower electron reorganization energy as compared to pre-existed DRTB-T molecule. The better functional ability of T1-T7 is also suggested by an improvement in open circuit voltage (Voc) of designed structures (1.62 eV–1.77 eV) as compared to R (1.49 eV) when PC61BM is used as an acceptor. So, all our newly derived donors can be employed in the active layer of organic solar cells to manufacture efficient OSCs.
This review is focused on the current development in domain of organic photovoltaic cells (OPVs). Solar cells play a vital role for electricity production by converting sunlight to electric current. This paper presents an exhaustive literature review on advancements in field of OPVs. The solar cells, as a substitute for fossil fuels are, at the forefront in a wide range of research applications. The organic solar cells efficiency and operational lifespan made outstanding advancement by refining materials of the photoactive layer and presenting new inter-layers. The functioning of organic solar cells is centered on photoinduced electron transfer. Organic solar cell technology has immense potential owing to lower production cost and flexible characteristics. The latest advancement in the material engineering and sophisticated device structure have significantly improved the solar cells commercial feasibility. Further, we highlight the research and advancements of organic bioelectronics in powering numerous bio-medical electronic devices. The important challenges, engineering result, and forthcoming prospects driving the progress of OSCs are explored.
Due to the ability of organic materials to be processed in a high-throughput, solution phase, which would result in the generation of energy at a cheap cost, they have lately attracted a lot of attention for photovoltaic applications. The goal of hybrid solar cells which comprise organic as well as inorganic components is to take use of the inexpensive cell manufacture of organic photovoltaics while also reaping the benefits of the inorganic component, such as the ability to tailor the absorption spectrum. Even though hybrid solar cells have an ability to produce high PCEs, the efficiencies presently attained are very modest. The electronic structure of the inorganic component utilized as the electron acceptor in hybrid solar cells, in particular, has a critical role in the effectiveness of the device. An inorganic acceptor has an ideal electronic structural design. Four main categories of materials have been studied in this chapter: silicon, metal oxide nanoparticles, narrow bandgap nanoparticles, and cadmium compounds. This chapter examines potential PCE constraints for these devices, such as nanomorphology control and nanoparticle surface chemistry, and compares the electronic structure of these materials with the ideal design aspects of an inorganic material. There has been substantial advancement thanks to the numerous research investigations that have been conducted to identify the appropriate organic–inorganic hybrid material combinations and device architectures. The chapter provides a brief overview of the state of the art for bulk heterojunction organic–inorganic hybrid solar cells. Additionally, it identifies crucial research topics that must be addressed to enable commercialization of this technology.
In recent years, the progress of polymer solar cells (PSCs) has been greatly improved, with power conversion efficiencies (PCEs) exceeding 19%. In addition to major breakthroughs in the development of p-type and n-type materials, device engineering related to active layer morphology control plays a key role in improving efficiency. External treatments such as additives, thermal/solvent annealing, and interlayer modification are widely used to manipulate donor-acceptor bulk heterojunction (BHJ) layers. Volatile and non-volatile additives are considered to be a simple, efficient and effective method to control the morphology of active layers during spin-coating film-forming process. This review will summarize common and successful additives for fullerene and non-fullerene based PSCs.
Photovoltaic technology presents a sustainable solution to address the escalating global energy consumption and a reliable strategy for achieving net‐zero carbon emissions by 2050. Emerging photovoltaic technologies, especially the printable organic and perovskite solar cells, have attracted extensive attention due to their rapidly transcending power conversion efficiencies and facile processability, providing great potential to revolutionize the global photovoltaic market. To accelerate these technologies to translate from the laboratory scale to the industrial level, it is critical to develop well‐defined and scalable protocols to deposit high‐quality thin films of photoactive and charge‐transporting materials. Herein, the current state of printable organic and perovskite solar cells is summarized and the view regarding the challenges and prospects toward their commercialization is shared. Different printing techniques are first introduced to provide a correlation between material properties and printing mechanisms, and the optimization of ink formulation and film‐formation during large‐area deposition of different functional layers in devices are then discussed. Engineering perspectives are also discussed to analyze the criteria for module design. Finally, perspectives are provided regarding the future development of these solar cells toward practical commercialization. It is believed that this perspective will provide insight into the development of printable solar cells and other electronic devices.
Due to the high need for sustainable energy sources, there has been a tremendous increase in SC (solar cell) production and research in recent years. Despite the fact that inorganic SC has led the SC consumer market due to its exceptional efficiency, its expensive and difficult manufacture method makes it unaffordable. Hence alternative technology for SC has been explored by researchers to overcome the draw backs of inorganic SC fabrication. OSC (organic solar cell) alternatively known as polymer SC has the advantage of having lightweight, low production cost, and simple device structure. During the last few years, significant attention has been given in order to overcome the material and technological barriers in OSC devices to make them commercially viable. Buffer layers play a significant part in improving the power conversion efficiencies in OSCs, thus it is necessary to comprehend the underlying microscopic mechanisms that underlie the advancements in order to support the current qualitative knowledge. In this review article, we have studied extensively the impact of different BLs (buffer-layer) in enhancing the PCE (power conversion efficiency) and absorption capabilities of OSCs.
The ability of organic photovoltaics (OPVs) to be deposited on flexible substrates by roll-to-roll (R2R) processes is highly attractive for rapid mass production. Many research teams have demonstrated the great potential of flexible OPVs. However, the fabrication of R2R-coated OPVs is quite limited. There is still a performance gap between the R2R flexible OPVs and the rigid OPVs. In this study, we demonstrate the promising photovoltaic characteristics of flexible OPVs fabricated from blends of low bandgap polymer poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b′]dithiophene))-alt-(5,5-(1′,3′-di-2-thienyl-5′,7′-bis(2-ethylhexyl)benzo[1′,2′-c:4′,5′-c′]dithiophene-4,8-dione)] (PBDB-T) and non-fullerene 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene (ITIC). We successfully R2R slot-die coated the flexible OPVs with high power conversion efficiency (PCE) of over 8.9% under irradiation of simulated sunlight. Our results indicate that the processing parameters significantly affect the PCE of R2R flexible OPVs. By adjusting the amount of solvent additive and processing temperature, as well as optimizing thermal annealing conditions, the high PCE of R2R slot-die coated OPVs can be obtained. These results provide significant insights into the fundamentals of highly efficient OPVs for the R2R slot-die coating process.
The field of low bandgap organic conjugated polymers has found broad applications with efforts over the past decade. This article gives a brief overview of the recent progress of their synthesis and several applications for polymer solar cells, photodetectors, and bioimaging materials. Synthetic methods cover ground from Stille and Suzuki polycondensations to direct arylation polymerization. Donor–acceptor or ‘push–pull’ structures are generally necessary to lower bandgap of conjugated polymers, and this design strategy provides a rational approach to tune polymer energy levels as HOMO is mostly determined by donor units and conjugation, while LUMOs are determined by acceptor moieties. Special attention is given to the extraction of principles for efficient design of materials with desired properties. For efficient device performance, it is important to match the energy levels of functional materials with other device components and chosen applications, while maintaining solubility and good processability.
Size effects (‘smaller is stronger’) are important aspects of the mechanical behavior of materials. Experimentally, they are usually probed by performing compression tests on micropillars of different sizes (cross-sectional areas). To overcome limitations associated with comparing different crystal structures and to better handle the influence of melting points in different metals, single crystal face centered cubic (fcc) micropillars of equiatomic binary, ternary, and quaternary medium-entropy alloy (MEA) subsystems of the quinary CrMnFeCoNi high-entropy alloy (HEA) were tested. All eight alloys investigated were single-phase solid solutions having the fcc crystal structure. Their melting temperatures varied only over a narrow range (1189-1462 °C). They exhibit a size-dependent critical resolved shear stress (CRSS ∝ d-n, where d is the pillar diameter, n is a power law exponent). The results show that, all else being equal, size effects scale inversely with friction stress. In contrast, there is no systematic dependence on configurational entropy, contrary to speculations in some earlier papers that solid solution strengthening would increase as the number of alloying elements increases. ‘Bulk’ CRSS values were estimated by extrapolating the measured CRSS values of pillars with diameters of approximately 1-8 μm to larger pillar sizes of 30 μm. Good agreement was found with available CRSS values of bulk single crystals. It is concluded that it is possible to obtain bulk CRSS values more reliably from micropillar tests than from the Taylor factor corrected yield strengths of bulk polycrystals.
Singlet fission (SF) is a spin–allowed process in which a higher–energy singlet exciton is converted into two lower‐energy triplet excitons via a triplet pair intermediate state. Implementing SF in photovoltaic devices holds the potential to exceed the Shockley‐Queisser limit of conventional single junction solar cells. Although great progress has been made in exploiting the underlying mechanism of SF over the past decades, the scope of materials capable of SF, particularly polymeric materials, remains poor. SF–capable polymer is one of the most potential candidates in the implementation of SF into devices due to their distinct superiorities in flexibility, solution processability, and the ability to achieve nanometer‐scale phase separation in thin films through self‐assembly. Notably, recent advancements have demonstrated high‐performance SF in isolated donor‐acceptor (D‐A) copolymer chains, leading to efficient dissociation of triplet pair states in thin films, yielding long‐lived free triplets. This review provides an overview of recent progress in the development of SF‐capable polymeric materials, with a significant focus on elucidating the mechanisms of SF in polymers and optimizing the design strategies for SF‐capable polymers. Additionally, the paper discusses the challenges encountered in this field and presents future perspectives. We expect that this comprehensive review will offer valuable insights into the design of novel SF‐capable polymeric materials, further advancing the potential for SF implementation in photovoltaic devices.
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Organic solar cells have emerged as promising alternatives to traditional inorganic solar cells due to their low cost, flexibility, and tunable properties. This mini review introduces a novel perspective on recent advancements in organic solar cells, providing an overview of the latest developments in materials, device architecture, and performance optimization. In contrast to existing literature, this review places a strong emphasis on the role of molecular engineering in achieving high power conversion efficiencies. It delves into the latest materials used in organic solar cells, including novel polymers and small molecules, showcasing their unique properties and potential for improved performance. Furthermore, the review explores cutting-edge device architectures, specifically tandem and multi-junction cells, which offer unprecedented opportunities for achieving higher efficiencies. The discussion on these advanced architectures highlights their potential and paves the way for future advancements in the field of organic solar cells. To maximize the performance of organic solar cells, this review also presents recent strategies for performance optimization, focusing on interface engineering, morphological control, and stability enhancement. By providing a comprehensive analysis of these strategies, the review enables readers to gain a deeper understanding of the underlying principles and techniques driving the improvement in device performance. By introducing this novel perspective on recent developments, this mini review offers researchers and practitioners a valuable resource for staying up-to-date with the latest advancements in organic solar cells. It not only presents the current state of the field but also identifies future directions and challenges, fostering further research and innovation in this rapidly evolving field.
As an economical solar energy conversion technology, organic photovoltaics (OPVs) are regarded as a promising solution to environmental problems and energy challenges. With the highest efficiency of OPVs exceeding 20%, the research focus will shift from efficiency-oriented aspects to commercialization-oriented aspects in the near future. Semi-transparent OPVs (STOPVs) are one of the most possible commercialized forms of OPVs, and have achieved power conversion efficiency over 14% with average visible light transmittance over 20% so far. In this tutorial review, we first systematically summarize the device structures, operating principles and evaluation parameters of STOPVs, and compare them with those of opaque OPVs. Then, strategies to construct high-performance STOPVs by cooperatively optimizing materials and devices are proposed. Methods to realize the scale-up of STOPVs in terms of minimization of electrode and interconnect resistance are summarized. The potential applications of STOPVs in multifunctional windows, agrivoltaics and floating photovoltaics are also discussed. Finally, this review highlights major challenges and research directions that need to be addressed prior to the future commercialization of STOPVs.
Alternated weak electron donating and electron withdrawing units are easy to achieve wide bandgap (WBG) π-conjugated polymer for non-fullerene polymer solar cells (PSCs), which can match well with low band...
Photovoltaic systems enable the sun’s energy to be converted directly into electricity using semiconductor solar cells. The ultimate goal of photovoltaic research and development is to reduce the cost of solar power to reach or even become lower than the cost of electricity generated from fossil and nuclear fuels. The power conversion efficiency and the cost per unit area of the phototvoltaic system are critical factors that determine the cost of photovoltaic electricity. Until recently, the power conversion efficiency of single-junction photovoltaic cells has been limited to approximately 33% - the socalled Shockley-Queisser limit.
This book presents the latest developments in photovoltaics which seek to either reach or surpass the Shockley-Queisser limit, and to lower the cell cost per unit area. Progress toward this ultimate goal is presented for the three generations of photovoltaic cells: the 1st generation based on crystalline silicon semiconductors; the 2nd generation based on thin film silicon, compound semiconductors, amorphous silicon, and various mesoscopic structures; and the 3rd generation based on the unique properties of nanoscale materials, new inorganic and organic photoconversion materials, highly efficient multi-junction cells with low cost solar concentration, and novel photovoltaic processes.
The extent to which photovoltaic materials and processes can meet the expectations of efficient and cost effective solar energy conversion to electricity is discussed. Written by an international team of expert contributors, and with researchers in academia, national research laboratories, and industry in mind, this book is a comprehensive guide to recent progress in photovoltaics and essential for any library or laboratory in the field.
Next-generation and solution-processed thin-film solar cells have been attracted considerable attention because of their low cost, light weight, flexibility, and aesthetics. However, most of solution-processed thin-film solar cells are now focused on the use of photovoltaic absorbers containing the toxic element of Pb. In this study, eco-friendly silver-bismuth-iodide (Ag-Bi-I) thin-film photovoltaic devices with high open-circuit voltages (VOC) are developed by utilizing polythieno[3,4-b]-thiophene-co-benzodithiophene (PTB7) as the hole transport layer (HTL). The solution-processed AgBi2I7 semiconductor, which is an Ag-Bi-I ternary compound, exhibit features suitable for photovoltaic layers in thin-film solar cells, including a three-dimensional (3D) crystal structure, good surface morphology, and low optical bandgaps of 1.87 eV. Meanwhile, the solution-processed AgBi2I7 thin-film solar cell based on the PTB7 HTL exhibit a power conversion efficiency of 0.94% with an improved VOC value of 0.71 V owing to the deeper highest occupied molecular orbital (HOMO) energy level compared to that of poly(3-hexylthiophene-2,5-diyl) (P3HT). In other words, the VOC of the PTB7 HTL-based device is 20% higher than that of the P3HT HTL-based control device. Our results provide a new approach for increasing the VOC of eco-friendly Ag-Bi-I thin-film photovoltaics and indicate that further HTL engineering is necessary to simultaneously improve the VOC and performance of the devices.Graphical Abstract
Compared with perovskite solar cells and silicon solar cells, the excessive voltage loss (Vloss) becomes a stubborn stone that seriously hinders the further improvement of organic photovoltaic (OPV). Thus, many researchers focus on finding an effective material system to achieve high-performance OPVs with low Vloss. In recent 5 years, acceptor-donor-acceptor’-donor-acceptor (A-DA’D-A) type non-fullerene acceptors (NFAs) have attracted great attention because of their promising photovoltaic performance. Among them, A-DA’D-A type NFAs containing non-halogenated end group (NHEG) exhibit the large potential to achieve high open-circuit voltage (VOC) for the state-of-the-art OPVs, because of high-lying molecular energy levels and decreasing Vloss. In this review, we systematically summarize the recent development of A-DA’D-A type NHEG-NFAs and the impact of different NHEGs on the optoelectronic properties as well as the photovoltaic performance. In addition, we especially analyze the Vloss of NHEG-NFAs in the binary and ternary OPV devices. At last, we provide perspectives on the further molecular design and future challenges for this kind of materials as well as suggested solutions.
The notable wide bandgap conjugated polymer (WB‐CP) PM6 was optimized by introducing the third component of 4,8‐bis(tri‐iso‐propylsilicylethynyl)benzo[1,2‐b;4,5‐b’]dithiophene (BDT‐TIPS). With the increase of the BDT‐TIPS contents in the WB‐CPs, the maximal absorption peaks and on‐set‐band gap wavelengths of the WB‐CPs, are gradually blue‐shifted, alongside the successive deepening of the highest occupied molecular orbital (HOMO) from −5.54 eV to −5.68 eV. The more efficient OSCs with PCEs of 16.38 % were achieved in the OSCs from PM6‐TIPS5 paired with Y6 in contrast to that of 15.76 % for PM6 : Y6 based OSCs under AM1.5 sun stimulation (100 mA/cm²). Besides that, the investigations of the charge transporting, exciton dissociation and the recombination characteristics etc., were implemented and sugessted the improvement of the PCEs of PM6‐TIPS5 based OSCs was mainly ascribed to the subtly balancing of the HOMO energy levels and charge mobility of the PM6 by optimization of the feed ratio of the BDT‐TIPS.
Non-fullerene acceptors based organic solar cells represent the frontier of the field, owing to both the materials and morphology manipulation innovations. Non-radiative recombination loss suppression and performance boosting are in the center of organic solar cell research. Here, we developed a non-monotonic intermediate state manipulation strategy for state-of-the-art organic solar cells by employing 1,3,5-trichlorobenzene as crystallization regulator, which optimizes the film crystallization process, regulates the self-organization of bulk-heterojunction in a non-monotonic manner, i.e., first enhancing and then relaxing the molecular aggregation. As a result, the excessive aggregation of non-fullerene acceptors is avoided and we have achieved efficient organic solar cells with reduced non-radiative recombination loss. In PM6:BTP-eC9 organic solar cell, our strategy successfully offers a record binary organic solar cell efficiency of 19.31% (18.93% certified) with very low non-radiative recombination loss of 0.190 eV. And lower non-radiative recombination loss of 0.168 eV is further achieved in PM1:BTP-eC9 organic solar cell (19.10% efficiency), giving great promise to future organic solar cell research.
Grazing-incidence x-ray diffraction and atomic force microscopy were performed on bulk heterojunction regioregular poly(3-hexylthiophene) (RR-P3HT) [6,6]-phenyl-C71-butyric acid methyl esters spin-cast films with different film processing conditions to correlate the crystalline nanostructure of P3HT with the corresponding solar cell performance. The increase in long wavelength absorption for solvent annealed films is related to highly conjugated crystal structure of RR-P3HT phase-separated in the active layer. Upon thermal annealing, the solvent annealed 50-nm-thick device shows high solar cell performance with fill factor up to 73% and power conversion efficiency of 3.80%.
Converting solar energy into electricity provides a much-needed solution to the energy crisis the world is facing today. Polymer solar cells have shown potential to harness solar energy in a cost-effective way. Significant efforts are underway to improve their efficiency to the level of practical applications. Here, we report highly efficient polymer solar cells based on a bulk heterojunction of polymer poly(3-hexylthiophene) and methanofullerene. Controlling the active layer growth rate results in an increased hole mobility and balanced charge transport. Together with increased absorption in the active layer, this results in much-improved device performance, particularly in external quantum efficiency. The power-conversion efficiency of 4.4% achieved here is the highest published so far for polymer-based solar cells. The solution process involved ensures that the fabrication cost remains low and the processing is simple. The high efficiency achieved in this work brings these devices one step closer to commercialization.
Polymer or ''plastics'' solar cells have been an intensively studied area since the discovery of efficient electron transfer between polymers and fullerenes and the introduction of the bulk-heterojunction concept. The last few years have seen significant improvement in plastic solar cell performance through aggressive research on the regioregular poly(3-hexylthiophene) (RR-P3HT) : [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM) system. The morphology of the system is controlled through two major strategies which have proven effective in improving the device efficiency—thermal annealing and solvent annealing (slow growth). In this Feature Article, we review the recent progress on this material system. A detailed discussion on thermal annealing and solvent annealing approaches to improve device performance is presented, including a comparison between the two strategies. The effects of these two approaches on improving polymer crystallinity, light absorption in the polymer, carrier transport, blend film nano-morphology, etc. are summarized. We also include a brief discussion on accurate measurement and characterization techniques for polymer solar cells to correctly determine the efficiency by applying spectral mismatch factors. Future directions and challenges on polymer solar cell development are also discussed.
We report the fabrication and measurement of solar cells with 6% power conversion efficiency using the alternating co-polymer, poly[N-9′′- hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′, 1′,3′-benzothiadiazole) (PCDTBT) in bulk heterojunction composites with the fullerene derivative [6,6]-phenyl C 70 -butyric acid methyl ester (PC 70 BM). The PCDTBT/PC 70 BM solar cells exhibit the best performance of any bulk heterojunction system studied to date, with J SC 10.6mAcm 2, V OC 0.88V, FF0.66 and e 6.1% under air mass 1.5 global (AM 1.5G) irradiation of 100mWcm 2. The internal quantum efficiency is close to 100%, implying that essentially every absorbed photon results in a separated pair of charge carriers and that all photogenerated carriers are collected at the electrodes.
Fossil fuel alternatives, such as solar energy, are moving to the forefront in a variety of research fields. Polymer-based organic photovoltaic systems hold the promise for a cost-effective, lightweight solar energy conversion platform, which could benefit from simple solution processing of the active layer. The function of such excitonic solar cells is based on photoinduced electron transfer from a donor to an acceptor. Fullerenes have become the ubiquitous acceptors because of their high electron affinity and ability to transport charge effectively. The most effective solar cells have been made from bicontinuous polymer-fullerene composites, or so-called bulk heterojunctions. The best solar cells currently achieve an efficiency of about 5%, thus significant advances in the fundamental understanding of the complex interplay between the active layer morphology and electronic properties are required if this technology is to find viable application.
The self-organization of the polymer in solar cells based on regioregular poly(3-hexylthiophene) (RR-P3HT):[6,6]-phenyl C61-butyric acid methyl ester (PCBM) is studied systematically as a function of the spin-coating time ts (varied from 20-80s), which controls the solvent annealing time ta, the time taken by the solvent to dry after the spin-coating process. These blend films are characterized by photoluminescence spectroscopy, UV-vis absorption spectroscopy, atomic force microscopy, and grazing incidence X-ray diffraction (GIXRD) measurements. The results indicate that the p-conjugated structure of RR-P3HT in the films is optimally developed when ta is greater than 1 min (ts50 s). For ts < 50 s, both the short-circuit current (JSC) and the power conversion efficiency (PCE) of the corresponding polymer solar cells show a plateau region, whereas for 50
The carrier collection efficiency (ηc) and energy conversion efficiency (ηe) of polymer photovoltaic cells were improved by blending of the semiconducting polymer with C60 or its functionalized derivatives. Composite films of poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene) (MEH-PPV)
and fullerenes exhibit ηc of about 29 percent of electrons per photon and ηe of about 2.9 percent, efficiencies that are better by more than two orders of magnitude than those that have been achieved
with devices made with pure MEH-PPV. The efficient charge separation results from photoinduced electron transfer from the
MEH-PPV (as donor) to C60 (as acceptor); the high collection efficiency results from a bicontinuous network of internal donor-acceptor heterojunctions.
Polymer solar cells have the potential to become a major electrical power generating tool in the 21st century. R&D endeavors are focusing on continuous roll-to-roll printing of polymeric or organic compounds from solutionlike newspapersto produce flexible and lightweight devices at low cost. It is recognized, though, that besides the functional properties of the compounds the organization of structures on the nanometer levelforced and controlled mainly by the processing conditions applieddetermines the performance of state-of-the-art polymer solar cells. In such devices the photoactive layer is composed of at least two functional materials that form nanoscale interpenetrating phases with specific functionalities, a so-called bulk heterojunction. In this perspective article, our current knowledge on the main factors determining the morphology formation and evolution is introduced, and gaps of our understanding on nanoscale structure−property relations in the field of high-performance polymer solar cells are addressed. Finally, promising routes toward formation of tailored morphologies are presented.
A series of calculations were carried out to explore the percolation characteristics of the interpenetrating network in poly(3-hexylthiophene)/ fullerene bulk heterojunction materials, and to simulate the transport of charge carriers in an applied electric field. The network was found to be bi-continuous and forms two interpenetrating networks throughout the film, thereby enabling efficient charge carrier transport. The transport was found to be strongly dependent on the details of the tortuous pathways that are required to go between any two distant points on the network. Transmission electron microscopy (TEM) images represented a clear network structure that could be mathematically analyzed. Results provided insight into the network percolation characteristics and the charge carrier transport mobility under an applied electric field, and suggested an opportunity to improve the network morphology towards more efficient transport.
The mixed solvent approach has been demonstrated as a promising method to modify nanomorphology in polymer solar cells. This work aims to understand the unique role of the additive in the mixture solvent and how the optimized nanoscale phase separation develops laterally and vertically during the non-equilibrium spin-coating process. We found the donor/acceptor components in the active layer can phase separate into an optimum morphology with the additive. Supported by AFM, TEM and XPS results, we proposed a model and identified relevant parameters for the additive such as solubility and vapor pressures. Other additives are discovered to show the ability to improve polymer solar cell performance as well.
Solution-processed bulk-heterojunction solar cells have gained serious attention during the last few years and are becoming established as one of the future photovoltaic technologies for low-cost power production. This article reviews the highlights of the last few years, and summarizes today's state-of-the-art performance. An outlook is given on relevant future materials and technologies that have the potential to guide this young photovoltaic technology towards the magic 10% regime. A cost model supplements the technical discussions, with practical aspects any photovoltaic technology needs to fulfil, and answers to the question as to whether low module costs can compensate lower lifetimes and performances.
Methods to accurately measure the current–voltage characteristics of organic solar cells under standard reporting conditions are presented. Four types of organic test cells and two types of silicon reference cells (unfiltered and with a KG5 color filter) are selected to calculate spectral-mismatch factors for different test-cell/reference-cell combinations. The test devices include both polymer/fullerene-based bulk-heterojunction solar cells and small-molecule-based heterojunction solar cells. The spectral responsivities of test cells are measured as per American Society for Testing and Materials Standard E1021, and their dependence on light-bias intensity is reported. The current–voltage curves are measured under 100 mW cm–2 standard AM 1.5 G (AM: air mass) spectrum (International Electrotechnical Commission 69094-1) generated from a source set with a reference cell and corrected for spectral error.
By applying the specific fabrication conditions summarized in the Experimental section and post-production annealing at 150 degrees C, polymer solar cells with power-conversion efficiency approaching 5 % are demonstrated. These devices exhibit remarkable thermal stability. We attribute the improved performance to changes in the bulk heterojunction material induced by thermal annealing. The improved nanoscale morphology, the increased crystallinity of the semiconducting polymer, and the improved contact to the electron-collecting electrode facilitate charge generation, charge transport to, and charge collection at the electrodes, thereby enhancing the device efficiency by lowering the series resistance of the polymer solar cells.
For almost two decades, the search of an intrinsically‐conductive organic metal has represented the major driving force for research on control of the band gap of extended π‐conjugated systems. However, the emergence of the application of π‐conjugated oligomers and polymers in field‐effect transistors, light‐emitting diodes, electrochromic devices and solar cells has introduced major changes in the chemistry of gap engineering. Besides controlled band gap, active materials for electronic and photonic applications must present appropriate absorption and/or emission properties, highest occupied and lowest unoccupied molecular orbital (HOMO and LUMO) energy levels and charge‐transport properties. The aim of this short review is to present an overview of the recent trends in this area in order to identify possible directions for future research.
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Conjugated polymers are attractive semiconductors for photovoltaic cells because they are strong absorbers and can be deposited on flexible substrates at low cost. Cells made with a single polymer and two electrodes tend to be inefficient because the photogenerated excitons are usually not split by the built-in electric field, which arises from differences in the electrode work functions. The efficiency can be increased by splitting the excitons at an interface between two semiconductors with offset energy levels. Power conversion efficiencies of almost 4% have been achieved by blending polymers with electron-accepting materials such as C-60 derivatives, cadmium selenide, and titanium dioxide. We predict that efficiencies higher than 10% can be achieved by optimizing the cell's architecture to promote efficient exciton splitting and charge transport and by reducing the band gap of the polymer so that a larger fraction of the solar spectrum can be absorbed.
Organic photovoltaics has come into the international research focus during the past three years. Up to now main efforts have focused on the improvement of the solar conversion efficiency, and in recent efforts 5% white light efficiencies on the device level have been realized. Despite this in comparison to inorganic technologies low efficiency, organic photovoltaics is evaluated as one of the future key technologies opening up completely new applications and markets for photovoltaics. The key property which makes organic photovoltaics so attractive is the potential of reel to reel processing on low cost substrates with standard coating and printing processes. In this contribution we discuss the economical and technical production aspects for organic photovoltaics.
The calibration and efficiency measurement procedures followed by the PV Devices and Measurements Branch at the Solar Energy Research Institute (SERI) are presented. The use of spectral irradiance measurements to calibrate reference cells used in these measurements has reduced the uncertainty in the calibration number to less than 1%. The use of the spectral mismatch index can reduce the errors in short-circuit current measurements to about 2% for a wide variety of test cell/reference cell combinations and reduce the need for a close match between the test cell and reference cell spectral responses or the source spectrum and standard spectrum. The effect of various spectra in use by the PV community on the short-circuit current is presented for a variety of cell spectral responses.
Inorganic semiconductor solar cells are well developed and are being deployed worldwide. However, the high cost of their manufacture limits their widespread acceptance as a source of renewable energy. The potential for low-cost manufacturing afforded by organic devices gives organic solar cells the potential to significantly impact the energy landscape, making them useful in a wide range of environments. This review discusses the advances made in developing organic cells. It covers the device structural approaches that have been used to prepare these devices as well as the design of materials for simultaneously achieving high solar absorptivity and optimal device structure for harvesting this electrical energy. Emphasis is on polymeric and composite polymeric/molecular materials.
This paper describes synthesis and photovoltaic studies of a series of new semiconducting polymers with alternating thieno[3,4-b]thiophene and benzodithiophene units. The physical properties of these polymers were finely tuned to optimize their photovoltaic effect. The substitution of alkoxy side chains to the less electron-donating alkyl chains or introduction of electron-withdrawing fluorine into the polymer backbone reduced the HOMO energy levels of polymers. The structural modifications optimized polymers' spectral coverage of absorption and their hole mobility, as well as miscibility with fulleride, and enhanced polymer solar cell performances. The open circuit voltage, V(oc), for polymer solar cells was increased by adjusting polymer energy levels. It was found that films with finely distributed polymer/fulleride interpenetrating network exhibited improved solar cell conversion efficiency. Efficiency over 6% has been achieved in simple solar cells based on fluorinated PTB4/PC(61)BM films prepared from mixed solvents. The results proved that polymer solar cells have a bright future.
A new low band gap semiconducting polymer, PTB1, was synthesized and found promising for solar energy harvesting. Simple polymer solar cells based on PTB1 and methanofullerene [6,6]-phenyl-C(71)-butyric acid methyl esters (PC(71)BM) exhibit a solar conversion efficiency of 5.6%. An external quantum efficiency of 67% and fill-factor of 65% are achieved, both of which are among the highest values reported for a solar cell system based on a low band gap polymer.
At present, solar energy conversion technologies face cost and scalability hurdles in the technologies required for a complete
energy system. To provide a truly widespread primary energy source, solar energy must be captured, converted, and stored in
a cost-effective fashion. New developments in nanotechnology, biotechnology, and the materials and physical sciences may enable
step-change approaches to cost-effective, globally scalable systems for solar energy use.
High charge-separation efficiency combined with the reduced fabrication costs associated with solution processing and the potential for implementation on flexible substrates make 'plastic' solar cells a compelling option for tomorrow's photovoltaics. Attempts to control the donor/acceptor morphology in bulk heterojunction materials as required for achieving high power-conversion efficiency have, however, met with limited success. By incorporating a few volume per cent of alkanedithiols in the solution used to spin-cast films comprising a low-bandgap polymer and a fullerene derivative, the power-conversion efficiency of photovoltaic cells (air-mass 1.5 global conditions) is increased from 2.8% to 5.5% through altering the bulk heterojunction morphology. This discovery can potentially enable morphological control in bulk heterojunction materials where thermal annealing is either undesirable or ineffective.
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