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.     

Effects of morphologies of (Cd+Te) powders on the properties of sintered CdS/CdTe solar cells

Polycrystalline CdS/CdTe solar cells have been prepared by coating and sintering a CdS slurry and a (Cd+Te) slurry. CdS layers were first formed on borosilicate glass substrates at 600°C in nitrogen and then CdTe layers were formed on the sintered CdS layers at 625°C in nitrogen. The (Cd+Te) slurry contained (Cd+Te) powders mixed in a ball mill for 12–220 h instead of more expensive CdTe powders. The shape of cadmium particles changed from spherical to plate-like and the diameter of the plate-shaped particles became smaller as the ball-milling time increased. In addition, a compound CdTe started to form during a long milling time. The sintered CdTe layers were more compact as the diameter of plate-shaped cadmium particles decreased. However, cracks developed in the sintered CdTe layer when the diameter was small ( 2 m). The efficiency of sintered CdS/CdTe solar cells increased with decreasing particle diameter and then decreased with further decrease in particle diameter. The highest efficiency of 12.1% was achieved using a mixture of (Cd+Te) powders which had plate-shaped cadmium particles with a diameter of 5 m. The results suggest that high-efficiency sintered CdS/CdTe solar cells can be fabricated by using CdTe slurry from the mixture of (Cd+Te) powders with an inexpensive ball-milling process.     

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.     

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.     

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.     

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.     

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 %).     

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.     

Electrical properties of the CdS/InP solar cell for photovoltaic applications

n-CdS/p-InP solar cells have been prepared by deposition of n-CdS thin films using thermal evaporation technique onto p-type InP <100>. The I–V characteristics of the CdS/InP heterojunctions in dark condition were studied in the 298–350 K temperatures range for charge transport mechanism investigation. It has been established that in the entire temperatures range, the charge transport mechanism is determined by recombination processes in the depletion region. The CdS/InP heterojunction solar cells obtained using this technique and characterized under illumination condition have showed a conversion efficiency of 11% at I_sc = 10 mA/cm², V_oc = 0.7 V.     

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.     

The formation of different phases of CuxTe and their effects on CdTe/CdS solar cells

Material studies and device applications of Cu_xTe in an NREL-developed CdTe solar cell structured as glass/Cd₂SnO₄/ZnSnO_x/CdS/CdTe are presented. The Cu_xTe primary back contact was formed by evaporating a Cu layer with various thicknesses at room temperature on HNO₃/H₃PO₄ (NP) solution etched CdTe layer. A post-annealing was then followed. The structural evolution and electrical properties of Cu_xTe were investigated. Cu/Te ratio and post-annealing temperature are two processing parameters in this study. The Cu_xTe phases are mainly controlled by the Cu/Te ratio. After a post-annealing at a low temperature, such as 100 °C, no Cu_xTe phase transformation from its as-deposited phase was observed. A post-annealing treatment at a higher temperature, such as 250 °C, can reveal the stoichiometric Cu_xTe phases based on the Cu/Te ratio used in the devices. But a post-annealing at a further higher temperature, such as 400 °C, resulted in a complicated Cu_xTe phase appearance. CuTe, Cu_1.4Te, and Cu₂Te are three major phases detected by X-ray diffraction (XRD) for different Cu thickness application annealed at 250 °C. Application of Cu thicker than 60 nm degrades open-circuit voltage (V_oc) and shunting resistance (R_sh), but increases series resistance (R_s). The correlation between device performance and the Cu_xTe back contact illustrates that the process used for forming the Cu₂Te back contact failed to produce good fill factor (FF) and also introduced higher barrier height. The best device was observed for a back contact with a mixed Cu_1.4Te and CuTe phases.     

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.     

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.     

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.     

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.     

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.     

C-V calculations in CdS/CdTe thin films solar cells

Polycrystalline thin film CdS/CdTe heterojunction solar cells are important candidates for large scale photovoltaic applications. In this work we use a C-V (capacitance vs. voltage) theoretical method for the determination of the interface charge density σ and band discontinuity ΔEv of the CdS/CdTe heterojunction. The methodology is based on three cardinal equations: i) line up of the bands relative to the common Fermi level (at equilibrium) or the quasi-Fermi level (when voltage is applied), ii) charge neutrality and iii) the total capacitance of the heterostructure. We used CdS/CdTe solar cells, grown in our laboratory by the chemical bath deposition (for CdS film) and the close space vapor transport (for CdTe film) techniques. The interface parameters σ, and ΔEv are determined from C-V fitting between the calculated and the measured curve. The methodology presented in this study is general and can be applied to semiconductor-semiconductor and semimetal-semiconductor heterojunctions.     

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.