Close-space sublimation grown CdS window layers for CdS/CdTe thin-film solar cells

We report on the synthesis and characterization of CdS window layers grown by close-space sublimation (CSS) method for CdS/CdTe thin-film solar cells. Comparing with CdS window layers grown by other methods such as sputtering and chemical bath deposition, CSS-grown CdS layers can facilitate the consumption of CdS layers and suppress the diffusion of Te into CdS window layers. CSS-grown CdS layers exhibit much larger grains with faceted morphology. Due to large grains, CSS CdS layers must be grown thick enough to minimize the effects of pin-holes. The use of thicker CdS layer causes reduced blue response, resulting in current loss. Therefore, the thickness of CSS CdS window layer must be carefully optimized to achieve high efficiency. Our best small area dot cell using a CSS CdS window layer has exhibited a cell efficiency of about 14.2 % with an open circuit voltage (V_OC) of 806 mV, a short circuit current (J_SC) of 25.2 mA/cm², and a fill factor (FF) of 69.8 % under AM1.5 illumination and without an antireflection coating, slightly lower than our best reference cell using a sputtered CdS window layer (V_OC = 845 mV, J_SC = 24.5 mA/cm², FF = 76.8 %, and efficiency = 15.8 %).     

The effects of the growth parameters of the CdS window layer on the photovoltaic properties of MOCVD-grown CdS/CdTe solar cells

This paper forms part of an ongoing study aimed at producing high-efficiency polycrystalline photovoltaic cells by a single integrated process using metal organic chemical vapor deposition (MOCVD). Relationships between CdS growth variables, final microstructure and device performance parameters are established. CdTe grain sizes of 3–5 μm diameter can be achieved using CdS growth temperatures of 275 °C or below, even in the absence of nucleation delay. Short-circuit photocurrent depends on CdS growth temperature and dopant concentration.     

High efficiency solar cells with CdS window layers

This paper starts with the identification of the reasons for the continued research and development into new types of solar cell in the face of competition from relatively well-developed and established devices. CdS is a very commonly used material in thin film heterojunction solar cells and the reasons for this are discussed in terms of its readily available properties in thin film form.

Next, the required qualities of the absorber layer, in which photogeneration of excess minority carriers takes place, are examined and it is pointed out that both InP and CuInSe₂ are suitable materials. CdS has been deposited onto both single-crystal and polycrystalline substrates of InP by at least three methods and both the device performance and related fundamental information of cells prepared using these methods are reviewed. Likewise, devices based on CdS/CuInSe₂ prepared by various means are discussed and it is pointed out that most “all thin film” cells have efficiencies around 5% although more recently efficiencies greater than 9% have been achieved. This has been due to improved control over rates of deposition of the constituent elements of the absorber.

Finally, the present status of these classes of cell is discussed and it is pointed out that much more fundamental research is required if theoretically optimum efficiencies are ever to be attained.     

Dependence of efficiency of thin-film CdS/CdTe solar cell on parameters of absorber layer and barrier structure

Dependences of the open-circuit voltage, short-circuit current, fill factor, and efficiency of a CdS/CdTe solar cell on the resistivity and thickness of the p-CdTe absorber layer, the noncompensated acceptor concentration N_a-N_d, and carrier lifetime τ in CdTe, are investigated, and optimization of these parameters in order to improve the solar cell efficiency is performed. It has been shown that the observed low efficiency of CdS/CdTe solar cells is caused by the too short electron lifetime in the range of 10^− 10-10^− 9 s and too thin (3-5 µm) CdTe layer currently used for fabrication of CdTe/CdS solar cells. To achieve an efficiency of 28-30%, the resistivity and thickness of the CdTe absorber layer, the noncompensated acceptor concentration, and carrier lifetime should be ∼ 0.1 Ω·cm, ≥ 20-30 µm, ≥ 10¹⁶ cm^− 3, and ≥ 10^− 6 s, respectively.     

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.     

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.     

Effects of junction formation conditions on the photovoltaic properties of sintered CdS/CdTe solar cells

Microstructures and properties of sintered CdS films on glass substrates and sintered CdTe films on polycrystal CdS substrates have been investigated. The CdS films, which contained 9 wt % CdCl₂ as a sintering aid and were sintered at 650° C for 1 h in nitrogen, are transparent and have an average grain size of 15m and an electrical resistivity of 0.5cm. The CdTe films, which were coated on the sintered CdS substrate and were sintered above 610° C for 1 h in nitrogen, have a dense structure with an average grain size larger than 5m. All polycrystal CdS/CdTe solar cells were fabricated by this successive coating and sintering method. The sintering temperature of CdTe films on the sintered CdS films was varied from 585 to 700° C. Compositional interfaces and p-n juctions are formed during sintering. The highest solar efficiency (7.18%) was found in a solar cell made by sintering the composite layer of glass-CdS-CdTe at 625° C for 1 h. A fabrication temperature below 610° C resulted in poor solar cell efficiencies due to the porous structure of the CdTe films and above 650° C also resulted in poor efficiencies due to the formation of a CdS_1-x Te_x layer at the interface and a large p-n junction depth.     

Ordered polycrystalline thin films for high performance CdTe/CdS solar cells

An ordered polycrystalline approach is proposed to overcome fundamental problems associated with random polycrystalline thin films, namely grain boundaries and inhomogeneity. The approach consists of two main steps: (1) the deposition of a patterned growth mask and (2) the selective-area deposition of the ordered polycrystals. The ordered polycrystalline approach was investigated using the CdTe/CdS material system. Experimental results demonstrate that SiO₂ and Si₃N₄ are effective growth masks and that temperature is a dominant parameter for selective-area deposition. PL and XRD characterization indicates that the ordered polycrystalline technique has the potential for improving the crystal quality and order of polycrystalline CdTe thin films. The approach appears to be fairly general and could be applied to other material systems.     

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.     

Effect of thickness: a case study of electrodeposited CdS in CdS/CdTe based photovoltaic devices

The effect of electrodeposition technique on CdS thickness incorporated in CdS/CdTe-based solar cell has been investigated using all-electrodeposited g/FTO/n-CdS/n-CdTe/p-CdTe multilayer device configuration. The optical, morphological and structural properties of the electroplated CdS were investigated for CdS thicknesses between 50 and 200 nm. The observed CdS bandgap ranges between 2.42 and 2.46 eV. The morphological analysis shows full coverage of underlying g/FTO substrate for all CdS thicknesses except for the 50 nm which shows the presence of gap in-between grains. The structural analysis shows a preferred orientation of H(101) for all the CdS thicknesses except the 50 nm thick CdS which shows either a weak crystallinity or an amorphous nature. The fabricated solar cell shows a maximum conversion efficiency of ~11 % using CdS thickness ranging between 100 and 150 nm. These results show that although low CdS thickness is desirable for photovoltaic application, the effect of nucleation mechanism of deposition technique should be taken into consideration.     

CdTe/CdS太阳电池光照和电子辐照研究

采用近空间升华法制备CdTe多晶薄膜,以ZnTe/ZnTeCu复合多晶薄膜作为背接触层,获得了转换效率为13.38%的CdTe/CdS太阳电池.用光强为100mW/cm2的卤钨灯对电池光照7天后,发现电池性能无明显变化.经能量为1.6MeV,辐照剂量为1013～1015电子/cm2的电子束辐照后,电池性能有不同程度的衰降,经真空150℃退火30min后,电池性能恢复到接近辐照前的水平.     

Development of electrodeposited ZnTe layers as window materials in ZnTe/CdTe/CdHgTe multi-layer solar cells

Zinc telluride (ZnTe) thin films have been deposited on glass/conducting glass substrates using a low-cost electrodeposition method. The resulting films have been characterized using various techniques in order to optimize growth parameters. X-ray diffraction (XRD) has been used to identify the phases present in the films. Photoelectrochemical (PEC) cell and optical absorption measurements have been performed to determine the electrical conductivity type, and the bandgap of the layers, respectively. It has been confirmed by XRD measurement that the deposited layers mainly consist of ZnTe phases. The PEC measurements indicate that the ZnTe layers are p-type in electrical conduction and optical absorption measurements show that their bandgap is in the range 2.10–2.20 eV. p-Type ZnTe window materials have been used in CdTe based solar cell structures, following new designs of graded bandgap multi-layer solar cells. The structures of FTO/ZnTe/CdTe/metal and FTO/ZnTe/CdTe/CdHgTe/metal have been investigated. The results are presented in this paper using observed experimental data.     

Doping profiles in CdTe/CdS thin film solar cells

CdS/CdTe thin film solar cells showing comparable properties as commercial cells have been prepared by close space sublimation (CSS) in our own laboratory. We characterised the cells by capacitance-voltage profiling (C-V), thermal admittance spectroscopy (TAS), and thermally stimulated capacitance measurements (TSCAP). The doping profiles of the CdTe layer obtained by C-V measurements confirm the well known rise in dopant concentration with increasing depth if the usual evaluation procedure is employed. However, the TAS and TSCAP measurements reveal deep acceptors in the CdTe layer with a large concentration exceeding that of the shallow dopants. Under these conditions, C-V measurements are shown to yield an apparently rising dopant concentration even for homogeneous doping. A combined simulation of doping profiles measured at different temperatures using a fixed and uniform shallow and deep doping fits well to measured doping concentration. These results indicate how to get reliable information on the shallow dopant concentration.     

CdS annealing treatments in various atmospheres and effects on performances of CdTe/CdS solar cells

In this work, a systematic research on CdS annealing treatments under various atmospheres had been done to understand their effects on CdS/CdTe solar cells. CdS films were prepared by a standard CBD method and annealed under various atmospheres, including Ar, Ar+H₂, O₂, Ar+S and Ar+CdCl₂. Morphological, structural, optical and chemical properties were investigated using Atom force microscope (AFM), X-ray diffraction (XRD), UV–VIS spectroscopy and X-ray photoelectron spectroscopy (XPS). Annealing treatments enhanced modifications of morphology, structure and electrical properties of CdS films. AFM showed different surface morphologies and roughnesses of CdS films annealed under various atmospheres. XRD indicated the transition of CdS films from metastable cubic structure to stable hexagonal structure after annealing treatment, especially annealed in Ar+CdCl₂. From XPS analysis, Fermi levels of CdS films shifted closer to conduction band after annealing under O₂ and Ar+CdCl₂, while the levels shifted away from conduction band under Ar+H₂ and Ar+S. The relationships between those modifications by annealing treatments and effects on the performance of solar cells were discussed. Solar cell based on CdS annealed with Ar+CdCl₂ had the best performance due to the high n-doping of CdS layer introduced by annealing process.     

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.     

A microstructural study on the surface and interface of CdTe/CdS solar cells

The surface and interface properties of CdTe/CdS solar cells, including interfacial mixing, surface and interface geometrical morphology, CdTe grain size and preferential crystal orientation of CdTe layers were studied using Auger electron spectroscopy (AES) depth profiling, atomic force microscopy (AFM), Fourier transform infrared (FTIR) spectroscopy, optical reflectance (OR) and X-ray diffraction (XRD) techniques. The correlation between the surface and interface properties and CdTe/CdS solar cell performance was also investigated. AES depth profiling was used to analyse the interdiffusion between the CdTe and CdS layers. Atomic force microscopy (AFM) suggests that the interfacial geometrical morphology has a significant influence on the photovoltaic property of CdTe/CdS solar cells. Rough interfaces tend to increase the photovoltaic conversion efficiency of solar cells because of multiple reflections. X-ray diffraction shows that polycrystalline CdTe/CdS solar cells with higher efficiencies appear to be orientated with more (1 1 1) planes of CdTe parallel to the macrosurface, but CdTe single crystals with differently indexed surface planes show almost the same reflection behaviour. Further theoretical and experimental analyses are therefore needed to clarify this observation. This revised version was published online in July 2006 with corrections to the Cover Date.     

Unravelling complex nature of CdS/CdTe based thin film solar cells

Thin film solar cells based on CdS/CdTe hetero-structure has shown a drastic improvement changing from 16.5 to 22.1% efficiency during a short period of time from ~2013 to ~2016. This has happened in the industrial environment and the open research in this field has stagnated over a period of two decades prior to ~2013. Most of the issues of this hetero-structure were not clear to the photovoltaic (PV) community and research efforts should be directed to unravel its complex nature. Issues related to materials, post-growth treatment, chemical etching prior to metallisation and associated device physics are the main areas needing deeper understanding in order to further develop this device. After a comprehensive research programme in both academia and in industry on these materials, surfaces and interfaces and fully fabricated devices over a period of over three decades by the main author, the current knowledge as understood today, on all above mentioned complex issues are presented in this paper. Full understanding of this structure will enable PV developers to further improve the conversion efficiency beyond 22.1% for CdS/CdTe based solar cells.     

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.