Voltage dependent photocurrent collection in CdTe/CdS solar cells

The voltage dependence of the photocurrent J_L(V) of CdTe/CdS solar cells has been characterized by separating the forward current from the photocurrent at several illumination intensities. J_L(V) reduces the fill factor (FF) of typical cells by 10–15 points, the open circuit voltage (V_OC) by 20–50 mV, and the efficiency by 2–4 points. Eliminating the effect of J_L(V) establishes superposition between light and dark J(V) curves for some cells. Two models for voltage dependent collection give reasonable fits to the data: (1) a single carrier Hecht model developed for drift collection in p‐i‐n solar cells in which fitting yields a parameter consistent with lifetimes of 10⁻⁹ s as measured by others; or (2) the standard depletion region and bulk diffusion length model fits almost as well. The simple Hecht‐like drift collection model for photocurrent gives very good agreement to J(V) curves measured under AM1·5 light on CdTe/CdS solar cells with FF from 53% to 70%, CdTe thickness from 1·8 to 7·0 µm, in initial and stressed states. Accelerated thermal and bias stressing increases J_L(V) losses as does insufficient Cu. This method provides a new metric for tracking device performance, characterizes transport in the high field depletion region, and quantifies a significant FF loss in CdTe solar cells. Copyright © 2007 John Wiley & Sons, Ltd.     

The impact of MOCVD growth ambient on carrier transport,defects, and performance of CdTe/CdS heterojunction solar cells

CdTe solar cells were fabricated by depositing CdTe films on CdS/SnO₂/glass substrates in various metalorganic chemical vapor deposition growth ambient with varying Te/Cd mole ratio in the range of 0.02 to 15. The short-circuit current density (J_sc) showed a minimum at a Te/Cd ratio of 0.1 and increased on both sides of this minimum. The open-circuit voltage (V_oc) was found to be the highest for the Te-rich growth ambient (Te/Cd∼6)and was appreciably lower (600 mV as opposed to 720 mV) for the stoichiometric and the Cd-rich growth conditions. This pattern resulted in highest cell efficiency (12%) on Te-rich CdTe films. Auger electron spectroscopy revealed a high degree of atomic interdiffusion at the CdS/CdTe interface when the CdTe films were grown in the Te-rich conditions. It was found that the current transport in the cells grown in the Cd-rich ambient was controlled by the tunneling/interface recombination mechanism, but the depletion region recombination became dominant in the Te-rich cells. These observations suggest that the enhanced interdiffusion reduces interface states due to stress reduction or to the gradual transition from CdS to CdTe. The hypothesis of reduced defect density in the CdTe cells grown in the Te-rich conditions is further supported by the high effective lifetime, measured by time-resolved photoluminescence, and the reduced sensitivity of quantum efficiency to forward/light bias.     

Copper migration in cdte heterojunction solar cells

CdTe solar cells were fabricated by depositing a Au/Cu contact with Cu thickness in the range of 50 to 150Å on polycrystalline CdTe/CdS/SnO₂/glass structures. The increase in Cu thickness improves ohmic contact and reduces series resistance (R_s), but the excess Cu tends to diffuse into CdTe and lower shunt resistance (R_sh) and cell performance. Light I-V and secondary ion mass spectros-copy (SIMS) measurements were performed to understand the correlations between the Cu contact thickness, the extent of Cu incorporation in the CdTe cells, and its impact on the cell performance. The CdTe/CdS/SnO₂/glass, CdTe/ CdS/GaAs, and CdTe/GaAs structures were prepared in an attempt to achieve CdTe films with different degrees of crystallinity and grain size. A large grain polycrystalline CdTe thin film solar cell was obtained for the first time by selective etching the GaAs substrate coupled with the film transfer onto a glass substrate. SIMS measurement showed that poor crystallinity and smaller grain size of the CdTe film promotes Cu diffusion and decreases the cell performance. Therefore, grain boundaries are the main conduits for Cu migration and larger CdTe grain size or alternate method of contact formation can mitigate the adverse effect of Cu and improve the cell performance.     

温度循环下CdS/CdTe太阳电池稳定性的研究

对CdS/CdTe太阳电池在温度循环下的稳定性进行了研究,测定了其I-V特性曲线,并与室温下的电池作了比较.结果表明:经温度循环后电池的转换效率、填充因子和短路电流密度都有不同程度的下降,而用ZnTe作背接触层的电池稳定性有所改善.     

CdS/CdTe叠层太阳电池的制备及其性能

CdS/CdTe太阳电池是薄膜太阳电池研究工作的一个重要方向.为了提高开路电压Voc、改善电池的光谱响应,进而提高电池的转换效率,在此提出CdS/CdTe叠层太阳电池结构.文中,叠层电池的顶电池由CdS/CdTe超薄层构成;底电池由CdS/CdTe薄膜层构成.经分析测试,实验制备的CdS/CdTe叠层太阳电池具有明显的叠层结构,开路电压最高达到了852mV,短路电流密度最大为13mA/cm2,填充因子最高为55.2%,这种叠层电池的效率达到了8.16%(0.071cm2).研究表明相对于传统的单层CdS/CdTe太阳电池,CdS/CdTe叠层电池的制备对研究如何提高CdS/CdTe太阳电池的光伏性能有一定的参考价值.     

CdS/CdTe叠层太阳电池的制备及其性能

CdS/CdTe太阳电池是薄膜太阳电池研究工作的一个重要方向.为了提高开路电压Voc、改善电池的光谱响应,进而提高电池的转换效率,在此提出CdS/CdTe叠层太阳电池结构.文中,叠层电池的顶电池由CdS/CdTe超薄层构成;底电池由CdS/CdTe薄膜层构成.经分析测试,实验制备的CdS/CdTe叠层太阳电池具有明显的叠层结构,开路电压最高达到了852mV,短路电流密度最大为13mA/cm2,填充因子最高为55.2%,这种叠层电池的效率达到了8.16%(0.071cm2).研究表明相对于传统的单层CdS/CdTe太阳电池,CdS/CdTe叠层电池的制备对研究如何提高CdS/CdTe太阳电池的光伏性能有一定的参考价值.     

A prediction of dislocation‐free CdTe/CdS photovoltaic multilayers via nano‐patterning and composition grading

Defects in multilayered films have long been a performance‐limiting problem for the semiconductor industry. For instance, CdTe/CdS solar cell efficiencies have had significant improvement in the past 15years or more without addressing the problem of high misfit dislocation densities. Overcoming this stagnation requires a fundamental understanding of interfacial defect formation. Herein, we demonstrate a new first principles‐based CdTe bond‐order approach that enables efficient molecular dynamics to approach the fidelity of density functional theory. Stringent quantum‐mechanical verification and experimental validation tests reveal that our new approach provides an accurate prediction of defects that earlier methods cannot. Using this new capability, we show that misfit dislocations in CdTe/CdS multilayers can be significantly reduced via nano‐patterning and composition grading and more importantly, dislocation‐free multilayers naturally arise when the pattern dimension is reduced below 90nm. Our predictive methods are generally applicable to other materials, highlighting a rational approach towards low‐defect semiconductor films. Copyright © 2015 John Wiley & Sons, Ltd.     

CdS/CdTe太阳电池的背接触

磷硝酸腐蚀是一种适宜于工业化生产的背表面刻蚀工艺.文中采用磷硝酸腐蚀CdTe薄膜,并用溴甲醇腐蚀作为对照实验,研究了两种腐蚀对材料性质的影响.随后用真空蒸发法分别沉积了四种背接触层,提出了适宜于工业化生产的背接触技术,并从实验和理论上对两种背接触结构的CdTe太阳电池进行了分析.     

Simulation approach for optimization of ZnO/c-WSe2 heterojunction solar cells

Taking into account defect density in WSe₂, interface recombination between ZnO and WSe₂, we presented a simulation study of ZnO/crystalline WSe₂ heterojunction (HJ) solar cell using wxAMPS simulation software. The optimal conversion efficiency 39.07% for n-ZnO/p-c-WSe₂ HJ solar cell can be realized without considering the impact of defects. High defect density (> 1.0 × 10¹¹ cm^-2) in c-WSe₂ and large trap cross-section (> 1.0 × 10^-10 cm²) have serious impact on solar cell efficiency. A thin p-WSe₂ layer is intentionally inserted between ZnO layer and c-WSe₂ to investigate the effect of the interface recombination. The interface properties are very crucial to the performance of ZnO/c-WSe₂HJ solar cell. The affinity of ZnO value range between 3.7-4.5 eV gives the best conversion efficiency.     

ZnO薄膜对于CdTe太阳电池的影响*

通过直流磁控溅射法在ITO薄膜上沉积的ZnO薄膜可以作为CdTe太阳电池的高阻层。通过XRD,可见-红外可见光谱仪和四探针法分析了制备薄膜的结构,光学和电学性质。通过紫外光电子能谱和X射线光电子能谱深度刻蚀法分析了ITO/ZnO和ZnO/CdS薄膜的界面性质。结果表明：ZnO 作为高阻层有良好的光学和电学性质。ZnO 薄膜降低了ITO和CdS之间的势垒。 制备出来电池有ZnO（没有ZnO）的能量转换效率和量子效率是12.77% (8.9%) 和 >90% (79%)。 进一步,通过AMPS-1D模拟分析了ZnO薄膜厚度对于CdTe太阳电池的影响。     

Deposition and doping of CdS/CdTe thin film solar cells

1% oxygen is incorporated into both CdS and CdTe layers through RF sputtering of CdS/CdTe thin film solar cells. The optical and electrical parameters of the oxygenated and O₂-free devices are compared after CdCl₂ treatment and annealing in ambient Ar and/or air. The effects of ambient annealing on the electrical and optical properties of the films are investigated using current-voltage characterization, field emission scanning electron microscopy, X-ray diffraction, and optical transmission spectroscopy. The 1% oxygen content can slightly increase the grain size while the crystallinity does not change. Annealing in ambient Ar can increase the transmission rate of the oxygenated devices.     

正置倒置异质结有机小分子太阳能电池

以MoO3为阳极修饰层,以Rubrene/C60为活性层,制备了正置和倒置异质结有机小分子太阳能电池。实验结果表明倒置器件的开路电压Voc、短路电流密度Jsc、填充因子FF和功率转换效率η比正置结构的器件分别提高了34%、20%、25%和102%。当插入BCP阴极缓冲层后,阻挡了热的Al原子对C60层的破坏,对倒置器件的性能没有明显的影响,但却显著改善了正置器件的性能,并分析了MoO3和BCP对倒置和正置器件的作用。     

多晶CdS/CdTe异质结界面的能带偏移

采用真空蒸发法制备了CdS和CdTe，并对其结构和光学性质进行了研究．原位制备了衬底沿(001)高度择优取向的CdS/CdTe异质结，研究了其结构、电子学性质．获得的CdS/CdTe半导体异质结的价带偏移ΔEV=0.98eV±0.05eV，导带偏移ΔEc=0.07±0.1eV．     

石墨烯/CdTe肖特基结柔性薄膜太阳能电池研究

利用Matlab仿真模拟了石墨烯/P-CdTe肖特基结太阳能电池的光电特性。结果表明,电池的短路电流密度Jsc为23.9×10–3A/cm2、开路电压Voc为0.64 V、填充因子FF为79.0,转换效率η高达12%。与传统的氧化铟锡(ITO)电极比较,石墨烯柔韧性好,同时具备高透光和高导电的特性,可替代ITO作为新型电极材料来制备柔性薄膜太阳能电池。     

有机体异质结太阳能电池的数值模拟

引入了一种可以直观展现出聚合物体异质结太阳能电池的伏安特性的数值模型,该模型是由双分子复合以及温度和电场效应下自由载流子产生的机制。这个在聚合物材料中的到很好的体现,该材料中空间电荷效应只起到微弱的作用,从而在模型中形成相对恒定的电场。此外,在短路条件下只有7%的自由载流子由于双分子复合而消失。该模型证明在PPV/PCBM太阳能电池中随着空穴迁移率的增加和减少受体0.5电子伏特将得到5.5%最高转换效率。     

CdTe solar cell in a novel configuration

Polycrystalline thin‐film CdTe/CdS solar cells have been developed in a configuration in which a transparent conducting layer of indium tin oxide (ITO) has been used for the first time as a back electrical contact on p‐CdTe. Solar cells of 7·9% efficiency were developed on SnO_x:F‐coated glass substrates with a low‐temperature (<450°C) high‐vacuum evaporation method. After the CdCl₂ annealing treatment of the CdTe/CdS stack, a bromine methanol solution was used for etching the CdTe surface prior to the ITO deposition. The unique features of this solar cell with both front and back contacts being transparent and conducting are that the cell can be illuminated from either or both sides simultaneously like a ‘bi‐facial’ cell, and it can be used in tandem solar cells. The solar cells with transparent conducting oxide back contact show long‐term stable performance under accelerated test conditions. Copyright © 2004 John Wiley & Sons, Ltd.     

Efficient and non-hysteresis CH3NH3PbI3/PCBM planar heterojunction solar cells

Highly efficient and non-hysteresis organic/perovskite planar heterojunction solar cells was fabricated by low-temperature, solution-processed method with a structure of ITO/PEDOT:PSS/CH₃NH₃PbI₃/PCBM/Al. The high-quality perovskite thin film was obtained using a solvent-induced-fast-crystallization deposition involving spin-coating the CH₃NH₃PbI₃ solution followed by top-dropping chlorobenzene with an accurate control to induce the crystallization, which results in highly crystalline, pinhole-free, and smooth perovskite thin film. Furthermore, it was found that the molar ratio of CH₃NH₃I to PbI₂ greatly influence the properties of CH₃NH₃PbI₃ film and the device performance. The equimolar or excess PbI₂ was facile to form a flat CH₃NH₃PbI₃ film and produced relatively uniform perovskite crystals. Perovskite solar cells (PSCs) with high-quality CH₃NH₃PbI₃ thin film showed good performance and excellent repeatability. The power conversion efficiency (PCE) up to 13.49% was achieved, which is one of the highest PCEs obtained for low-temperature, solution-processed planar perovskite solar cells based on the structure ITO/PEDOT:PSS/CH₃NH₃PbI₃/PC₆₁BM/Al. More importantly, PSCs fabricated using this method didn’t show obvious hysteresis under different scan direction and speed.     

Polymer selection toward efficient polymer/PbSe planar heterojunction hybrid solar cells

By using a series of polymers in the polymer/PbSe planar heterojunction hybrid solar cells (HSCs), we found that the open circuit voltage of HSCs showed a great improvement compared to that of PbSe Schottky junction solar cells, which might be attributed to the formation of interface dipole, resulting in decreased interfacial resistance, increased built-in electrical field, as well as reduced exciton recombination at interface. Meanwhile, polymers with higher PL quenching have more favorable hole transfer which lead to better device performance. In addition, the energy levels and surface energy of the polymers might largely affect their interaction with PbSe NCs, leading to different interfacial morphologies and influencing the charge transfer efficiency. Furthermore, the optimized HSCs showed a remarkable PCE of 5.31% which was the highest efficiency reported for polymer/PbSe based HSCs. We believe this HSC efficiency can be further improved by selecting polymers with rationally designed structures.     

CdTe/CdS thin film solar cells grown in substrate configuration

The ability to grow efficient CdTe/CdS solar cells in substrate configuration would not only allow for the use of non‐transparent and flexible substrates but also enable a better control of junction formation. Yet, the problems of barrier formation at the back contact as well as the formation of a p–n junction with reduced recombination losses have to be solved. In this work, CdTe/CdS solar cells in substrate configuration were developed, and the results on different combinations of back contact materials are presented. The Cu content in the electrical back contact was found to be a crucial parameter for the optimal CdCl₂‐treatment procedure. For Cu‐free cells, two activation treatments were applied, whereas Cu‐containing cells were only treated once after the CdTe deposition. A recrystallization behavior of the CdTe layer upon its activation similar to superstrate configuration was found; however, no CdTe–CdS intermixing could be observed when the layers were treated consecutively. Remarkably high V_OC and fill factor of 768 mV and 68.6%, respectively, were achieved using a combination of MoO₃, Te, and Cu as back contact buffer layer resulting in 11.3% conversion efficiency. With a Cu‐free MoO₃/Te buffer material, a V_OC of 733 mV, a fill factor of 62.3%, and an efficiency of 10.0% were obtained. Copyright © 2012 John Wiley & Sons, Ltd.     

Analysis of governing factors for photovoltaic loss mechanism of n‐CdS/p‐CdTe heterojunction via multi‐way data decomposition

An analytical framework for identifying key factors of the degradation of photovoltaic efficiency over time is presented. We demonstrate that, in many photovoltaic experimental settings, reliability data sets are easily cast in a multi‐ or N‐way format. We adopt a statistical technique, N‐way partial least squares, that generates a multi‐linear model using all of the data simultaneously. With this approach, we are able to model variables of interest such as cell efficiency while representing the data in a lower‐dimensional space in which salient features are more easily identified. We illustrate our approach with reliability data for CdS/CdTe heterojunction solar cell devices. Even with the inclusion of a noisy parameter such as the net acceptor density, and with a relatively small number of devices, we automatically identify key factors that are highly related to performance degradation. In particular, the conductance at the back contact is related to short stress‐time degradation (0–300 h), whereas the net acceptor density near the junction (at +0.08 V DC bias) is correlated with more gradual, long stress‐time degradation (300–1000 h). These notable degradation modes are explained with respect to our processing conditions and Cu‐diffusion in the cells. Copyright © 2013 John Wiley & Sons, Ltd.