Dependence of carrier lifetime on Cu-contacting temperature and ZnTe:Cu thickness in CdS/CdTe thin film solar cells

Cu diffusion from a ZnTe:Cu contact interface can increase the net acceptor concentration in the CdTe layer of a CdS/CdTe photovoltaic solar cell. This reduces the space-charge width (W_d) of the junction and enhances current collection and open-circuit voltage. Here we study the effect of Cu concentration in the CdTe layer on carrier lifetime (τ) using time-resolved photoluminescence measurements of ZnTe:Cu/Ti-contacted CdTe devices. Measurements show that if the ZnTe:Cu layer thickness remains constant and contact temperature is varied, τ increases significantly above its as-deposited value when the contacting temperature is in a range that has been shown to yield high-performance devices (~ 280° to ~ 320 °C). However, when the contacting temperature is maintained near an optimum value and the ZnTe:Cu thickness is varied, τ decreases with ZnTe:Cu thickness.     

Cu-related recombination in CdS/CdTe solar cells

Cu used in the back contact of CdS/CdTe solar cells is known to improve contact behavior and open-circuit voltage. A study of devices made with varying Cu amounts confirmed these observations. However, Cu was also found to be deleterious to current collection. Time-resolved photoluminescence measurements of CdTe devices show that carrier lifetime decreased with increased Cu concentration. Drive-level-capacitance-profiling and low-temperature photoluminescence suggest this decrease in lifetime was associated with increased recombination center density introduced by Cu in the CdTe layer. The resulting impact of increased Cu on device performance was a voltage-dependent collection of photogenerated carriers that reduced fill-factor.     

Transmission electron microscopy study of dislocations and interfaces in CdTe solar cells

We report on our transmission electron microscopy study of dislocations and interfaces in CdTe solar cells. The atomic structure of dislocations formed inside CdTe grains have been determined by atomic-resolution transmission electron microscopy. We discuss the electronic properties of the dislocations and explore the effects of oxygen on the interdiffusion at CdS/CdTe interface. We find that the presence of oxygen in either CdS or CdTe suppresses the interdiffusion at the CdS/CdTe interface. We have further investigated interdiffusion at the CdS/Zn₂SnO₄ interface. We find that Zn diffuses into CdS from Zn₂SnO₄ and Cd diffuses into Zn₂SnO₄ from CdS. The possible effects of the interdiffusion are discussed. Finally, we have examined the distribution of intentionally introduced Cu at the CdTe/CdS junction, and we find that Cu is distributed uniformly in the CdS layer.     

Phase control of CuxTe film and its effects on CdS/CdTe solar cell

Phase control is critical for achieving high-performance CdTe cells when Cu_xTe is used as a back-contact for CdTe cells. Cu_xTe phases are mainly controlled by the Cu/Te ratio, and they can also be affected by post-heat-treatment temperature. Although Cu₂Te has the highest conductivity, it is unstable and provides more Cu diffusion into the CdS and CdTe films. Cu diffusion into the CdS causes “cross-over”, and Cu diffusion into the CdTe film creates Cu-related defects that lower photogenerated carrier lifetime and result in voltage-dependent collection. A “recontact” experiment clearly indicated that the mechanism giving rise to “roll-over” is the formation of Cu-related oxides, rather than the loss of Cu on the back-contact.     

An effective method of Cu incorporation in CdTe solar cells for improved stability

Thin film CdTe solar cells of the superstrate configuration have been fabricated in order to study the effect of Cu on device stability. The study focused on two distinct sets of solar cells: in one set of devices Cu was introduced during the formation of the back contact, by sputtering a small thickness of Cu onto the CdTe surface prior to the application of a graphite electrode; for the second set of devices Cu was introduced in CdS by briefly immersing the CdS films in a CuCl solution prior to the deposition of CdTe with the back contact electrode being sputtered Mo. The solar cells were light soaked under approximately AM1.5 conditions for nearly 700 h during 4 h ON/4 h OFF cycles. Device degradation correlated well with the amount of Cu for the devices with Cu in the back contact. Cells with larger amounts of Cu exhibited larger degradation, suggesting that the amount of Cu utilized during the back contact formation must be minimized. On the other hand, a number of devices fabricated without any Cu in the back contact, but with Cu in the CdS, exhibited nearly no degradation during the light soaking process suggesting that in addition to the amount of Cu used for the fabrication of CdTe cells, the method of incorporating this element is also critical in achieving long term device stability.     

Structure and optical properties of polycrystalline Cd1−xZnxTe thin films prepared by simultaneous evaporation

Cd_1–xZn_xTe films were deposited by simultaneous evaporation of CdTe and ZnTe. These Cd_1–xZn_xTe films were of cubic phase, and strongly (1 1 1) oriented as deposited. Predominant direct optical transitions were observed and the band gap varied with zinc content in a non-linear way. A structure development of CdS/CdTe/ZnTe : Cu solar cells with a Cd_1–xZn_xTe buffer layer was proposed.     

具有复合背接触层的 CdTe多晶薄膜太阳电池

为了提高CdTe太阳电池的背接触性能,用共蒸发法制备了ZnTe：Cu和Cd1-xZnxTe多晶薄膜。研究结果表明：Cd1-xZnxTe多晶薄膜的能隙与锌含量呈二次方关系,ZnTe：Cu多晶薄膜能隙随着掺Cu浓度的增加而减小。分别用ZnTe／ZnTe：Cu和Cd1-xZnxTe／ZnTe:Cu复合膜作为背接触层,既能修饰异质结界面,改善电池的能带结构,又能防止Cu原子向电池内部扩散。因此获得了面积0．502cm^2,转换效率为13．38％的CdTe多晶薄膜太阳电池。     

Structural,optical, and electrical properties of ZnTe:Cu thin films by PLD

In this work, ZnTe and ZnTe:Cu films were obtained by pulsed laser deposition using the co-deposition method. ZnTe and Cu₂Te were used as targets and the shots ratio were varied to obtain 0.61, 1.47, 1.72, and 3.46% Cu concentration. Doping of ZnTe films with Cu was performed with the purpose of increasing the p-type carrier concentration and establishing the effect of concentration of Cu on structural, optical, and electrical properties of ZnTe thin films to consider their potential application in electronic devices. According to X-ray diffraction, X-ray photoelectron spectroscopy, UV–visible spectroscopy, and Hall effect results, ZnTe and ZnTe:Cu films correspond to polycrystalline zinc–blende phase with preferential orientation in (111) plane. Optical characterization results indicate that as-deposited films (band gap?=?2.16 eV) exhibit a band gap decrease as function of the increase of Cu concentration (2.09–1.64 eV), while, annealed films exhibit a decrease from 1.75 to 1.46 eV, as the Cu concentration increases. Lastly, Hall effect results show that ZnTe films correspond to a p-type semiconductor with a carrier concentration of 3?×?10¹³ cm^?3 and a resistivity of 1.64?×?10⁵ Ω?cm. ZnTe:Cu films remain like a p-type material and present an increasing carrier concentration (from 3.8?×?10¹⁵ to 1.26?×?10¹⁹ cm^?3) as function of Cu concentration and a decreasing resistivity (from 7.01?×?103 to 2.6?×?10^?1 Ω cm). ZnTe and ZnTe:Cu thin films, with the aforementioned characteristics, can find potential application in electronic devices, such as, solar cells and photodetectors.     

Physical properties of Bi doped CdTe thin films grown by CSVT and their influence on the CdS/CdTe solar cells PV-properties

The physical properties of Bi doped CdTe films, grown on glass substrates by the Closed Space Transport Vapour (CSVT) method, from different Bi doped CdTe powders are presented. The CdTe:Bi films were characterized using Photoluminescence, Hall effect, X-Ray diffraction, SEM and Photoconductivity measurements. Moreover, CdS/CdTe:Bi solar cells were made and their characteristics like short circuit current density (J_sc), open circuit voltage (V_OC), fill factor (FF) and efficiency (η) were determined. These devices were fabricated from Bi doped CdTe layers deposited on CdS with the same growth conditions than those used for the single CdTe:Bi layers. A correlation between the CdS/CdTe:Bi solar cell characteristics and the physical properties of the Bi doped CdTe thin films are presented and discussed.     

Effect of the purity of CdTe starting material on the impurity profile in CdTe/CdS solar cell structures

We have investigated CdTe/CdS/In₂O₃:F/glass solar cell structures using quantitative SIMS for profiling the impurity distribution from the CdTe free surface through to the glass substrate. Ion implanted CdTe standards were used. The effect of the purity of the CdTe starting material was determined by studying two structures grown from 7N and 5N source materials. Particular emphasis was placed on the potentially electrically active impurities that may originate from the CdTe starting material, and are likely to affect the CdTe/CdS solar cell performance. It was shown that Cu, Zn, Sn, Sb and Pb profiles had the same level and shape in the CdTe layer regardless of the purity of the starting material used, and were therefore not originating from the starting material. Cl, O, Na and Si showed higher levels for structures grown using 5N purity CdTe compared to those from 7N, and may, at least in part, be due to the CdTe starting material used. It was also postulated that at least some impurities (in addition to Cl) may partially come from the CdCl₂ treatment, and/or from the TCO (In) and glass (for Si and Na). Te and S interdiffusion at the CdTe/CdS interface was also shown to be enhanced when 5N CdTe source material is used as compared to 7N.     

Dependence of photovoltaic performance of solvothermally prepared CdS/CdTe solar cells on morphology and thickness of window and absorber layers

In the present work a new strategy for straightforward fabrication of CdS/CdTe solar cells, containing CdS nanowires and nanoparticles as a window layer and CdTe nanoparticles and microparticles as an absorber layer, are reported. CdS and CdTe nanostructures were synthesized by solvothermal method. X-ray diffraction analysis revealed that highly pure and crystallized CdS nanowires and nanoparticles with hexagonal structure and CdTe nanoparticles with cubic structure were obtained. Atomic force microscope and field emission scanning electron microscope images showed that CdS nanowires with length of several μm and average diameter of 35 nm, CdS nanoparticles with average particle size of 32 nm and CdTe nanoparticles with average particle size of 43 nm, were uniformly coated on the substrate by the homemade formulated pastes. Based on ultraviolet–visible absorption spectra, the band gap energies of CdS nanowires, CdS nanoparticles and CdTe nanoparticles were calculated 2.80, 2.65 and 1.64 eV, respectively. It was found that, the photovoltaic performance of the solar cells depends on thickness of CdTe and CdS films, reaching a maximum at a specific value of 6 μm and 225 nm, respectively. For such cell made of CdS nanowires and CdTe nanoparticles the V_OC, J_SC, fill factor and power conversion efficiency were calculated 0.62 V, 6.82 mA/cm², 59.7 and 2.53 %, respectively. Moreover, photovoltaic characteristics of the solar cells were dependent on CdTe and CdS morphologies. CdS/CdTe solar cell made of CdTe and CdS nanoparticles had the highest cell efficiency (i.e., 2.73 %) amongst all fabricated solar cells. The presented strategy would open up new concept for fabrication of low-cost CdS/CdTe solar cells due to employment of a simple chemical route rather than the vapor phase methods.     

硝磷酸腐蚀对CdTe薄膜性能及背接触层的影响

采用硝磷酸(NP)背表面刻蚀工艺并结合真空共蒸发法分别沉积了几种背接触层材料,研究了NP腐蚀对CdTe薄膜性能及背接触层的影响.结果表明:NP腐蚀后在CdTe薄膜上产生了富碲层;退火后,碲容易与背接触材料中的铜反应生成CuxTe.通过严格控制和优化腐蚀工艺,选择ZnTe/ZnTe:Cu作为背接触层材料,可制备出优异性能的CdTe太阳电池.     

The copper influence on the PL spectra of CdTe thin film as a component of the CdS/CdTe heterojunction

The influence of annealing in the presence of CdCl₂ and a thin copper layer deposited onto CdTe on the photoluminescence spectra of CdTe, as a component of CdS/CdTe heterojunction, has been studied for two excitation wavelengths: 0.337 μm and 0.6328 μm. The behavior of the PL was studied as a function of the measurement temperature and excitation intensity. At 0.6328 μm excitation, the interface PL consists of a known 1.43X band, and the chloride annealing enhances radiative transitions at 1.536 eV. The intensity of the 1.536 eV transitions increases when Cu is present. The PL of as-deposited CdTe films prepared in the presence of oxygen has the 1.45X band attenuated when excited with 0.337 μm excitation wavelength.     

Thin-film CdTe cells: Reducing the CdTe

Polycrystalline thin-film CdTe is currently the dominant thin-film technology in world-wide PV manufacturing. With finite Te resources world-wide, it is appropriate to consider the limits to reducing the thickness of the CdTe layer in these devices. In our laboratory we have emphasized the use of magnetron sputtering for both CdS and CdTe achieving AM1.5 efficiency over 13% on 3 mm soda-lime glass with commercial TCO and 14% on 1 mm aluminosilicate glass. This deposition technique is well suited to good control of very thin layers and yields relatively small grain size which also facilitates high performance with ultra-thin layers. This paper describes our magnetron sputtering studies for fabrication of very thin CdTe cells. Our thinnest cells had CdTe thicknesses of 1 μm, 0.5 μm and 0.3 μm and yielded efficiencies of 12%, 9.7% and 6.8% respectively. With thinner cells Voc, FF and Jsc are reduced. Current-voltage (J-V), temperature dependent J-V (J-V-T) and apparent quantum efficiency (AQE) measurements provide valuable information for understanding and optimizing cell performance. We find that the stability under light soak appears not to depend on CdTe thickness from 2.5 to 0.5 μm. The use of semitransparent back contacts allows the study of bifacial response which is particularly useful in understanding carrier collection in the very thin devices.     

硝磷酸腐蚀的CdTe太阳电池性能

CdTe薄膜的腐蚀是制作CdS/CdTe光伏电池的重要技术之一,本实验采用硝磷酸溶液（硝酸1%＋磷酸70%＋去离子水29%）腐蚀CdTe薄膜,通过XRD测试发现在CdTe膜上生成了碲层.随后,在腐蚀后的CdTe薄膜上分别沉积了几种结构的背接触层,并制备出相应结构的CdTe太阳能光伏电池.通过电池的光、暗I-V和C-V特性测试,以ZnTe/ZnTe：Cu/Ni为背接触的小面积太阳电池,其性能优于其它背接触的电池.实验结果表明器件性能与碲的生成和铜的扩散密切相关.     

Effects of CdCl2 in CdTe on the properties of sintered CdS/CdTe solar cells

Sintered CdS films on glass substrates with low electrical resistivity and high optical transmittance have been prepared by a coating and sintering method. All-polycrystalline CdS/CdTe solar cells with different microstructures and properties of the CdTe layer were fabricated by coating a number of CdTe slurries, which consisted of cadmium and tellurium powders, an appropriate amount of propylene glycol and various amounts of CdCl₂, on the sintered CdS films and by sintering the glass-CdS-(Cd + Te) composites at various temperatures. The presence of more than 5 wt% of CdCl₂ in the (Cd + Te) layer enhances the sintering of the CdTe film and the junction formation by a liquid-phase sintering mechanism. A low sintering temperature results in poor densification of the CdTe layer and the CdS-CdTe interface, whereas a high sintering temperature results in a deeply buried homojunction. The optimum temperature for the sintering of the CdTe layer and for junction formation decreases with increasing amount of CdCl₂. All-polycrystalline CdS/CdTe solar cells with an efficiency of 10.2% under solar irradiation have been fabricated by a coating and sintering method using cadmium and tellurium powders for the CdTe layer.     

Study of buried junction and uniformity effects in CdTe/CdS solar cells using a combined OBIC and EQE apparatus

A study of junction position and uniformity in CdTe/CdS solar cells is reported in which the influence of excluding oxygen from the CdS layers was investigated. The samples were characterised with an optical beam induced current instrument capable of mapping the cell response in the range 400-900 µm at a resolution of 12.5 µm — either as a map or a quantum efficiency spectrum. For oxygen-free CdS, the junctions were always buried in the CdTe — at a depth presumed to be controlled by the chloride treatment. If CdS:O is used then shallow junctions result, indicating that such layers have a role in doping the devices. The wavelength dependence of the spatial uniformity of the cell's responses is also discussed.     

CdTe thin film solar cells with reduced CdS film thickness

A study was performed to reduce the CdS film thickness in CdTe thin film solar cells to minimize losses in quantum efficiency. Using close space sublimation deposition for CdS and CdTe a maximum efficiency of ~ 9.5% was obtained with the standard CdS film thickness of ~ 160 nm. Reduction of the film CdS thickness to less than 100 nm leads to poor cell performance with ~ 5% efficiency, mainly due to a lower open circuit voltage. An alternative approach has been tested to reduce the CdS film thickness (~ 80 nm) by depositing a CdS double layer. The first CdS layer was deposited at high substrate temperature in the range of 520-540 °C and the second CdS layer was deposited at low substrate temperature of ~ 250 °C. The cell prepared using a CdS double layer show better performance with cell efficiency over 10%. Quantum efficiency measurement confirmed that the improvement in the device performance is due to the reduction in CdS film thickness. The effect of double layer structure on cell performance is also observed with chemical bath deposited CdS using fluorine doped SnO₂ as substrate.     

The effect of impurities on the doping and VOC of CdTe/CdS thin film solar cells

The effect of introducing impurities in CdTe, namely antimony (Sb) and oxygen (O), on the net carrier concentration in CdS/CdTe solar cells and on their open-circuit voltage (V_OC) has been investigated. Oxygen was introduced in the CdTe films during the deposition of this layer by the close-spaced sublimation process. The total pressure was held constant at 1330 Pa (N₂ and O₂). The amount of oxygen was varied by varying its partial pressure. Antimony was introduced into CdTe using a post-deposition diffusion process. Following the deposition of CdTe a thin film (a few nm) of Sb was deposited onto the CdTe surface and subsequently heat-treated to cause in-diffusion of Sb. The temperature and time during the diffusion process were varied in the range of 300-525 °C and 20-160 min respectively. In both instances it was possible to vary (increase) the doping concentration in CdTe. The increase in doping was accompanied by an increase in V_OC. However, in all instances the doping in CdTe reached a maximum value, beyond which further increases were not possible leading to saturation in V_OC. The highest V_OC measured was similar to state-of-the-art values in the range of 800-830 mV, and the highest doping concentration measured was in the 10¹⁶ cm^− 3 range.     

Annealing effects on the chemical deposited CdS films and the electrical properties of CdS/CdTe solar cells

CdS layers grown by chemical bath deposition (CBD) are annealed in the oxygen and argon-hydrogen atmosphere respectively. It has been found that the open circuit voltage of the CdS/CdTe solar cell increases when the CBD CdS is annealed with oxygen before the deposition of CdTe by close spaced sublimation (CSS), while the performance of the solar cell decreases when the CBD CdS is annealed with argon-hydrogen. Electronic properties of the CdS films are investigated using X-ray photo-electron spectroscopy (XPS), which indicates that the Fermi level is shifting closer to the conduction band after annealing in the oxygen and consequently a higher open circuit voltage of the solar cell can be obtained.