CdS/CdTe thin‐film solar cells with Cu‐free transition metal oxide/Au back contacts

Superstrate CdS/CdTe thin‐film solar cells with Cu‐free transition metal oxide (TMO)/Au and Au‐only back contacts have been fabricated. The TMOs include MoO_3‐x, V₂O_5‐x, and WO_3‐x. The incorporation of the TMO buffer layers at the back contacts resulted in significant improvement on open‐circuit voltage (V_OC) as compared with the cells with Cu‐free Au‐only back contacts. Among the cells using TMO buffer layers, the ones with MoO_3‐x buffer layers exhibited the best performance, yielding an efficiency of 14.1% under AM1.5 illumination with V_OC of 815 mV and a fill factor of 67.9%. Though the performance is slightly behind the best reference cell with a Cu/Au back contact fabricated in our lab with V_OC of 844 mV, fill factor of 76.3%, and efficiency of 15.7%, the use of Cu‐free back contacts may lead to improved long‐term cell stability. Copyright © 2013 John Wiley & Sons, Ltd.     

Application of copper thiocyanate for high open‐circuit voltages of CdTe solar cells

Copper thiocyanate (CuSCN) has proven to be a low‐cost, efficient hole‐transporting material for the emerging organic–inorganic perovskite solar cells. Herein, we report that CuSCN can also be applied to CdTe thin‐film solar cells to achieve high open‐circuit voltages (V_OCs). By optimizing the thickness of the thermally evaporated CuSCN films, CdTe cells fabricated by close space sublimation in the superstrate configuration have achieved V_OCs as high as 872 mV, which is about 20–25 mV higher than the highest V_OC for the reference cells using the standard Cu/Au back contacts. CuSCN is a wide bandgap p‐type conductor with a conduction band higher than that of CdTe, leading to a conduction band offset that reflects electrons in CdTe, partially explaining the improved V_OCs. However, due to the low conductivity of CuSCN, CdTe cells using CuSCN/Au back contacts exhibited slightly lower fill factors than the cells using Cu/Au back contacts. With optimized CdS:O window layers, the power conversion efficiency of the best CdTe cell, using CuSCN/Au back contact, is 14.7%: slightly lower than that of the best cell (15.2%) using Cu/Au back contact. Copyright © 2015 John Wiley & Sons, Ltd.     

Ultrathin CdTe Solar Cells with MoO3−x /Au Back Contacts

We report on the fabrication and characterization of CdTe thin-film solar cells with Cu-free MoO_3?x /Au back contacts. CdTe solar cells with sputtered CdTe absorbers of thicknesses from 0.5 to 1.75 μm were fabricated on Pilkington SnO₂:F/SnO₂-coated soda–lime glasses coated with a 60- to 80-nm sputtered CdS layer. The MoO_3?x /Au back contact layers were deposited by thermal evaporation. The incorporation of MoO_3?x layer was found to improve the open circuit voltage (V _OC) but reduce the fill factor of the ultrathin CdTe cells. The V _OC was found to increase as the CdTe thickness increased.     

Stable CdTe thin film solar cells with a MoOx back‐contact buffer layer

MoO_x thin films were employed as a buffer layer in the back contact of CdTe solar cells. A monograined CdS layer was employed as the window layer to reduce light absorption. The insertion of a MoO_x buffer layer in the back contact greatly reduced the Schottky barrier leading to increased fill factor and open‐circuit voltage. A CdTe solar cell, with an efficiency as high as 14.2%, was fabricated. The use of a MoO_x buffer layer made it possible to fabricate high‐efficient CdTe solar cell with much less Cu in the back contact, thus greatly enhancing the cell stability. Copyright © 2015 John Wiley & Sons, Ltd.     

Factors Affecting the Stability of CdTe/CdS Solar Cells Deduced from Stress Tests at Elevated Temperature

CdTe/CdS solar cells were subjected to heat stress at 200 °C in the dark under different environments (in N₂ and in air), and under illumination (in N₂). We postulate that two independent mechanisms can explain degradation phenomena in these cells: i) Excessive Cu doping of CdS: Accumulation of Cu in the CdS with stress, in the presence of Cl, will increase the photoconductivity of CdS. With limited amounts of Cu in CdS, this does NOT affect the photovoltaic behavior, but explains the crossover of light/dark current–voltage (J–V) curves. Overdoping of CdS with Cu can be detrimental to cell performance by creating deep acceptor states, acting as recombination centers, and compensating donor states. Under illumination, the barrier to Cu cations at the cell junction is reduced, and, therefore, Cu accumulation in the CdS is enhanced. Recovery of light‐stress induced degradation in CdTe/CdS cells in the dark is explained by dissociation of the acceptor defects. ii) Back contact barrier: Oxidation of the CdTe back surface in O₂/H₂O‐containing environment to form an insulating oxide results in a back‐contact barrier. This barrier is expressed by a rollover in the J–V curve. Humidity is an important factor in air‐induced degradation, as it accelerates the oxide formation. Heat treatment in the dark in inert atmosphere can stabilize the cells against certain causes of degradation, by completing the back contact anneal.     

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.     

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.     

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.     

Beam injection methods for characterizing thin‐film solar cells

II–VI and I–III–VI solar cells are promising for future thin‐film photovoltaics. In this paper, the roles of electron‐beam‐induced current (EBIC) and cathodoluminescence in evaluating the influence of interfaces on those solar cells are reviewed. CdTe and Cu(In,Ga)Se₂ (CIGS) are the absorbers of the cells investigated. For CdTe/CdS solar cells, a detailed study has been conducted of the effects of grain boundaries and the Te/CdTe or ZnTe:Cu/CdTe interfaces for back‐contacting. For CIGS solar cells, we have investigated different buffer layer schemes, showing that these interfaces are critical in the definition of the mechanisms for carrier collection. Copyright © 2002 John Wiley & Sons, Ltd.     

Flexible CdTe solar cells on polymer films

Lightweight and flexible CdTe/CdS solar cells on polyimide films have been developed in a ‘superstrate configuration’ where the light is absorbed in CdTe after passing through the polyimide substrate. The average optical transmission of the approximately 10‐μm‐thin spin‐coated polyimide substrate layer is more than ∼75% for wavelengths above 550 nm. RF magnetron sputtering was used to grow transparent conducting ZnO:Al layers on polyimide films. CdTe/CdS layers were grown by evaporation of compounds, and a CdCl₂ annealing treatment was applied for the recrystallization and junction activation. Solar cells of 8·6% efficiency with V_oc = 763 mV, I_sc = 20·3 mA/cm² and FF = 55·7% were obtained. Copyright © 2001 John Wiley & Sons, Ltd.     

Novel Cu Nanowires/Graphene as the Back Contact for CdTe Solar Cells

Cu‐nanowire‐doped graphene (Cu NWs/graphene) is successfully incorporated as the back contact in thin‐film CdTe solar cells. 1D, single‐crystal Cu nanowires (NWs) are prepared by a hydrothermal method at 160 °C and 3D, highly crystalline graphene is obtained by ambient‐pressure CVD at 1000 °C. The Cu NWs/graphene back contact is obtained from fully mixing the Cu nanowires and graphene with poly(vinylidene fluoride) (PVDF) and N‐methyl pyrrolidinone (NMP), and then annealing at 185 °C for solidification. The back contact possesses a high electrical conductivity of 16.7 S cm^?1 and a carrier mobility of 16.2 cm² V^?1 s^?1. The efficiency of solar cells with Cu NWs/graphene achieved is up to 12.1%, higher than that of cells with traditional back contacts using Cu‐particle‐doped graphite (10.5%) or Cu thin films (9.1%). This indicates that the Cu NWs/graphene back contact improves the hole collection ability of CdTe cells due to the percolating network, with the super‐high aspect ratio of the Cu nanowires offering enormous electrical transport routes to connect the individual graphene sheets. The cells with Cu NWs/graphene also exhibit an excellent thermal stability, because they can supply an active Cu diffusion source to form an stable intermediate layer of CuTe between the CdTe layer and the back contact.     

CuxS背接触层制备及在碲化镉薄膜太阳电池中应用研究

本文采用化学水浴法沉积CuxS薄膜,通过改变Cu元素比例研究其对碲化镉电池效率的影响。研究表明化学水浴法沉积的CuxS是非晶的,采用适当退火条件可以使其晶化,随着退火温度的提高,薄膜变得致密且结晶明显。CuxS薄膜厚度对电池性能有很大的影响,结果表明,随着CuxS薄膜厚度增加,电池性能先增加后减少。薄膜厚度为75nm时,CdS/CdTe电池性能最佳,达到了最高转化效率(η)为12.19%,填充因子(FF)为68.82%,开路电压(Voc)为820mV。     

Interdigitated back‐contact heterojunction solar cell concept for liquid phase crystallized thin‐film silicon on glass

We present an interdigitated back‐contact silicon heterojunction system designed for liquid‐phase crystallized thin‐film (~10 µm) silicon on glass. The preparation of the interdigitated emitter (a‐Si:H(p)) and absorber (a‐Si:H(n)) contact layers relies on the etch selectivity of doped amorphous silicon layers in alkaline solutions. The etch rates of a‐Si:H(n) and a‐Si:H(p) in 0.6% NaOH were determined and interdigitated back‐contact silicon heterojunction solar cells with two different metallizations, namely Al and ITO/Ag electrodes, were evaluated regarding electrical and optical properties. An additional random pyramid texture on the back side provides short‐circuit current density (j_SC) of up to 30.3 mA/cm² using the ITO/Ag metallization. The maximum efficiency of 10.5% is mainly limited by a low of fill factor of 57%. However, the high j_SC, as well as V_OC values of 633 mV and pseudo‐fill factors of 77%, underline the high potential of this approach. Copyright © 2015 John Wiley & Sons, Ltd.     

Preparation and characterization of pulsed laser deposited CdS/CdSe bi-layer films for CdTe solar cell application

CdS/CdSe bi-layer film was prepared by pulsed laser deposition with different substrate temperatures as an improved window layer for CdTe solar cells. The total thickness of each CdS/CdSe bi-layer film was about 70 nm, which could contribute to comparatively high transmittance of photons and, therefore, improving the photocurrent. Substrate temperature influenced the properties of the CdS/CdSe bi-layer films and the study showed that the bi-layer film prepared at 400 °C achieved the best optical transmittance and crystallinity. The crystal structure and optical transmittance of CdS/CdSe/CdTe stack before and after CdCl₂ annealing treatment were investigated by utilizing X-ray diffraction and UV/Vis spectrophotometer, respectively. It showed that further CdCl₂ annealing treatment improved the inter-diffusion of Se into CdTe, facilitating the formation of a CdTe_1−xSe_x alloy in the absorber layer. Comparing with CdTe, the alloy actually showed a smaller band gap which produced an obvious red shift of the absorption edge in long wavelength region. CdSe window layer was consumed by the inter-diffusion, while enhanced the short wavelength response in the range of 300–500 nm. The device based on CdS/CdSe window layer realized a J_SC enhancement due to the improved collection within both short and long wavelength regions accompany with a V_OC enhancement when compared to CdS/CdTe solar cell. The CdTe cell with CdS/CdSe bi-layer window deposited at 200 °C showed an efficiency of 13.47% with V_OC of 791 mV and J_SC of 27.40 mA/cm².     

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.     

Effects of Na and MoS2 on Cu2ZnSnS4 thin‐film solar cell

Cu₂ZnSnS₄ (CZTS)‐based materials have a useful band gap and a high absorption coefficient; however, their power conversion efficiency is low compared with that of CdTe and Cu(In,Ga)Se₂‐based solar cells. Two of the factors that strongly affect CZTS solar cell characteristics are the MoS₂ layer and the presence of defects. In this study, Mo back‐contact layers were annealed to control MoS₂ layer formation and the Na content in the Mo layer before the absorber precursor layer was deposited. The increase in oxygen content in the Mo layer suppressed MoS₂ layer formation. In addition, the increase in Na diffusion during the initial stage of the absorber precursor deposition decreased the defect density in the absorber layer and in the absorber–buffer interface. These results were verified through measurements of the external quantum efficiency, the temperature dependence of the open‐circuit voltage (V_OC), and admittance spectra. The current densities (J_SC) and V_OC, as well as the power conversion efficiencies, improved as the annealing temperature of the Mo layer increased, which suggests that CZTS solar cell characteristics can be improved by suppressing MoS₂ layer formation and increasing Na content in the Mo layer before deposition of the absorber precursor layer. Copyright © 2014 John Wiley & Sons, Ltd.     

Optimization of interdigitated back contact silicon heterojunction solar cells: tailoring hetero‐interface band structures while maintaining surface passivation

Interdigitated back contact silicon heterojunction (IBC‐SHJ) solar cells have the potential for high open circuit voltage (V_OC) due to the surface passivation and heterojunction contacts, and high short circuit current density (J_SC) due to all back contact design. Intrinsic amorphous silicon (a‐Si:H) buffer layer at the rear surface improve the surface passivation hence V_OC and J_SC, but degrade fill factor (FF) from an “S” shape J–V curve. Two‐dimensional (2D) simulation using “Sentaurus device” demonstrates that the low FF is related to the valence band offset (energy barrier) at the hetero‐interface. Three approaches to the buffer layer are suggested to improve the FF: (1) reduced thickness, (2) increased conductivity, and/or (3) reduced band gap. Experimental IBC‐SHJ solar cells with reduced buffer thickness (<5 nm) and increased conductivity with low boron doping significantly improves FF, consistent with simulation. However, this has only marginal effect on efficiency since J_SC and V_OC also decrease due to poor surface passivation. A narrow band gap a‐Si:H buffer layer improves cell efficiency to 13.5% with unoptimized passivation quality. These results demonstrate that tailoring the hetero‐interface band structure is critical for achieving high FF. Simulations predicts that efficiences >23% are possible on planar devices with optimized pitch dimensions and achievable surface passivation, and 26% with light trapping. This work provides criterion to design IBC‐SHJ solar cell structures and optimize cell performance. Copyright © 2010 John Wiley & Sons, Ltd.     

Combinatorial study of NaF addition in CIGSe films for high efficiency solar cells

We report on a sodium fluoride (NaF) thickness variation study for the H₂Se batch furnace selenization of sputtered Cu(In,Ga) films in a wide range of Cu(In,Ga) film compositions to form Cu(In,Ga)Se₂ (CIGSe) films and solar cells. Literature review indicates lack of consensus on the mechanisms involved in Na altering CIGSe film properties. In this work, for sputtered and batch‐selenized CIGSe, NaF addition results in reduced gallium content and an increase in grain size for the top portion of the CIGSe film, as observed by scanning electron microscopy and secondary ion mass spectrometry. The addition of up to 20 nm of NaF resulted in an improvement in all relevant device parameters: open‐circuit voltage, short‐circuit current, and fill factor. The best results were found for 15 nm NaF addition, resulting in solar cells with 16.0% active‐area efficiency (without anti‐reflective coating) at open‐circuit voltage (V_OC) of 674 mV. Copyright © 2013 John Wiley & Sons, Ltd.     

CuIn1−xGaxSe2‐based thin‐film solar cells by the selenization of sequentially evaporated metallic layers

Polycrystalline CuIn_1−xGa_xSe₂ (CIGS) thin films were deposited by the non‐vacuum, near‐atmospheric‐pressure selenization of stacked metallic precursor layers. A study was carried out to investigate the influence of significant factors of the absorber on the solar cells performance. An efficiency enhancement was obtained for Cu/(In+Ga) atomic ratios between 0·93 and 0·95. The slope of the observed energy bandgap grading showed a strong influence on the V_OC and the short circuit current density J_SC. An increase of the Ga content in the active region of the absorber was achieved by the introduction of a thin Ga layer on the Mo back contact. This led to an improvement of efficiency and V_OC. Furthermore, an enhanced carrier collection was detected by quantum efficiency measurements when the absorber layer thickness was slightly decreased. Conversion efficiencies close to 10% have been obtained for these devices. Copyright © 2005 John Wiley & Sons, Ltd.     

Improved performance in ZnO/CdS/CuGaSe2 thin‐film solar cells

We report the growth and characterization of improved efficiency wide‐bandgap ZnO/CdS/CuGaSe₂ thin‐film solar cells. The CuGaSe₂ absorber thickness was intentionally decreased to better match depletion widths indicated by drive‐level capacitance profiling data. A total‐area efficiency of 9·5% was achieved with a fill factor of 70·8% and a V_oc of 910 mV. Published in 2003 by John Wiley & Sons, Ltd.