Cadmium telluride films and solar cells

CdTe thin films for solar cell applications have been deposited by close-spaced vapor transport and by hot-wall vacuum evaporation. As-deposited films are p-type with hole densities that increase to values of 1 × 10¹⁶cm^-3with increasing substrate temperature. A variety of experimental results can be interpreted either in terms of doping by native defects such as cadmium vacancies or doping by diffusion from the graphite substrate, with evidence for self-compensation. Many CdS/CdTe/graphite solar cells have been prepared by vacuum evaporation of CdS onto thin-film CdTe, which have low values ofJ_{O} sim 10^{-9}A/cm²and high values ofJ_{SC} sim 17mA/cm². The open-circuit voltage is low at 0.48 V for CdS deposition at 300° C, but increases with decreasing CdS deposition temperature. The highest efficiency prepared to date is 6.4 percent. Tile efficiency is limited at present by the fill factor, associated with a total series resistivity in the light of the order of 10 Ω-cm². Supporting research on low-resistance contacts to p-type CdTe, grain boundary properties and passivation in p-type CdTe bicrystals and thin films, and high-resolution transmission electron microscopy of junction interfaces is briefly described.     

Structural mechanisms of optimization of the photoelectric properties of CdS/CdTe thin-film heterostructures

Comparative studies of the effect of chloride treatment of CdS/CdTe thin-film heterostructures on the output characteristics of ITO/CdS/CdTe/Cu/Au solar cells and the crystal structure of their base CdTe layer are carried out. Structural mechanisms determining variation in the efficiency of photoelectric processes in ITO/CdS/CdTe/Cu/Au thin-film solar cells produced by varying the thickness of the CdCl₂ layer during the chloride treatment are suggested. It is shown for the first time by X-ray diffractometry that the metastable hexagonal CdTe phase transforms into a stable cubic modification during the chloride treatment. This circumstance provides a substantial improvement in the photoelectric properties of CdS/CdTe thin-film heterostructures.     

Problems of efficiency of photoelectric conversion in thin-film CdS/CdTe solar cells

The available data are generalized and new results of investigation of losses of photoelectric energy conversion in CdS/CdTe thin-film solar cells are reported. The requirements concerning the electrical characteristics of the material, for minimizing the electric losses and providing effective radiation absorption in the active region of the diode structure, are discussed and refined. It is shown to what extent the incomplete collection of photogenerated charge carriers is determined by recombination both at the CdS/CdTe interface (based on the continuity equation taking into account the surface recombination) and in the space-charge region (based on the Hecht equation). The comparison of the calculated and experimental results shows that, in general, both types of recombination losses are important but can be virtually eliminated by the choice of parameters of both the barrier structure and the material used. The limiting values of the short-circuit current density and efficiency of the CdS/CdTe solar cell are discussed.     

Dependence of the efficiency of a CdS/CdTe solar cell on the absorbing layer’s thickness

On the basis of the continuity equation, the spatial distribution of photogenerated excess electrons in the neutral region of the CdTe layer in a CdS/CdTe heterostructure is analyzed taking into account recombination at the rear surface of the layer. It is demonstrated that, owing to diffusion, excess electrons penetrate deep into the CdTe layer at distances far exceeding the effective penetration length for solar radiation. Calculations of the short-circuit current indicate that, for electron lifetimes of 10^?10–10^?9 s, typical of thin-film CdS/CdTe solar cells, recombination losses are insignificant if the CdTe layer’s thickness amounts to 3–4 μm but increase dramatically if the thickness is below 1–1.5 μm. In order to eliminate recombination losses in more efficient solar cells where the electron lifetime is ≥10^?8 s the absorbing CdTe layer needs to be much thicker.     

Ion implantation as a production technique

This paper discusses the development of ion implantation techniques for the production of high efficiency n-on-p silicon solar cells. Although the process is still being optimized, ion-implanted cells are already competitive with diffusion produced cells, with air mass zero (AMO) efficiencies of 11 percent having been achieved. A high-current production machine capable of producing 10 000 cells/week has been constructed and is being applied to further cell development. The process has been applied to dendritic material with AMO efficiencies of > 9.3 percent having been achieved. Using an ion beam high-vacuum sputtering process, cells have been fabricated with 1 mil fused SiO2integral cover slips and AMO efficiencies of > 10 percent.     

Environmental assessment of thin silicon solar cells from pilot production

Following the earlier demonstration of the performance capabilities of 4-mil silicon solar cells and the feasibility of using these cells on large flexible arrays of space vehicles, more than a thousand 4-mil cells have been fabricated in pilot production by four routes. The various types of cells that have been evaluated had solderless evaporated titanium-silver contacts in both a conventional and wraparound configuration, solderless evaporated titanium-silver contacts "overplated" with a layer of copper-gold, and solderless plated mickel-copper-gold contacts in a conventional and wraparound configuration. Both 1-by-2-cm and 2-by-2-cm, n on p cells have been manufactured from 1 and 10 Ω . cm boron-doped silicon. In every case, satisfactory production yields have been achieved. The above cells have been subjected to environmental conditions aimed at studying the effects of high-ambient humidity on the cell contacts during "shelf life" prior to launch and the degradation in performance from electron and proton irradiation encountered during long-term spiral transfer orbits to synchronous altitude. Specifically the problem of low-energy "synchronous altitude" proton irradiation of exposed bar and back contacts and the protection afforded by various forms of coatings has been investigated.     

Unified model of fundamental limitations on the performance of silicon solar cells

Previous attempts to explain the substantial discrepancy between observed and predicted efficiencies in silicon solar cells are shown to have treated inadequately two important features of typical devices: 1) In the diffused region the electric-field distribution is much wider than generally believed and the field values away from the junction are generally higher; 2) Auger processes in heavily doped regions have a more pervasive impact than has been recognized. By incorporating a suitable modification of the junction model and a consistent treatment of Auger effects into the analysis, a unified model is developed for the principal limitations on the performance of Si solar cells. This model accounts for limits to Iscand Vocarising in either the front or base region. The present analysis reinterprets the "violet-cell" observations of the effect of the diffusion profile and presents an alternative to the bandgap-narrowing model of the heavy-doping effect on Voc. A new method is developed for evaluating the junction saturation current in heavily doped regions of such solar cells and transistor emitters.     

Recrystallization and sulfur diffusion in CdCl2-treated CdTe/CdS thin films

The role of CdCl₂ in prompting recrystallization, grain growth and interdiffusion between CdS and CdTe layers in physical vapor-deposited CdS/CdTe thin-film solar cells is presented. Several CdTe/CdS thin-film samples with different CdTe film thicknesses were treated in air at 415°C for different times with and without a surface coating of CdCl₂. The samples were characterized by scanning electron microscopy, transmission electron microscopy, energy dispersive x-ray spectroscopy, x-ray diffractometry and optical absorption. The results show that CdCl₂ treatment enhances the recrystallization and diffusion processes, leading to a compositional variation within the CdTe layer due to diffusion of sulfur from the CdS. The highest sulfur concentrations observed after 30 min treatments with CdCl₂ at 415°C are near the solubility limit for sulfur in CdTe. The compositional distributions indicated by x-ray diffraction measurements of samples with different CdTe thickness show that the S-rich CdTe_1−xS_x region lies near the CdTe/CdS interface. A multiple-step mixing process must be inferred to account for the diffraction profiles obtained. © 1997 John Wiley & Sons, Ltd.     

Open-circuit voltage,fill factor,and efficiency of a CdS/CdTe solar cell

The dependences of the open-circuit voltage, fill factor, and efficiency of the thin-film CdS/CdTe solar cell on the resistivity ρ and carrier lifetime τ in the absorbing CdTe layer were studied. In the common case in which the uncompensated acceptor concentration and the electron lifetime in the CdTe layer are within 10¹⁵–10¹⁶ cm⁻³ and 10⁻¹⁰–10⁻⁹ s, the calculation results correspond to the achieved efficiency of the best thin-film CdS/CdTe solar cells. It was shown that, by decreasing ρ and increasing τ in the absorbing CdTe layer, the open-circuit voltage, fill factor, and efficiency can be substantially increased, with their values approaching the theoretical limit for such devices.     

General properties of SnO2-GaAs and SnO2-Ge heterojunction photovoltaic cells

SnO2-Ge and SnO2-GaAs heterojunction formed by chemical vapor deposition have been investigated. The results show that (n-n) SnO2-GaAs, (n-p) SnO2-GaAs, and (n-n) SnO2-Ge heterojunctions give a photovoltaic effect, while (n-p) SnO2-Ge yields an ohmic contact. The measured short-circuit currents, arising mainly from semiconductor side, are comparable with those for "AMOS" and homojunction solar cells. The open-circuit voltage and fill factor and considerably less. The polarity observed in Vocand Iscis consistent with the band bending of a simple SnO2-semiconductor heterojunction energy-band diagram neglecting interface states. The ohmic behavior for (n-p) SnO2-Ge is attributed to the accumulation layer existing between SnO2conduction band and Ge valence band at the interface.     

Influence of the layer parameters on the performance of the CdTe solar cells

Influence of the layer parameters on the performances of the CdTe solar cells is analyzed by SCAPS-1D. The ZnO:Al film shows a high efficiency than SnO2:F. Moreover, the thinner window layer and lower defect density of CdS films are the factor in the enhancement of the short-circuit current density. As well, to increase the open-circuit voltage, the responsible factors are low defect density of the absorbing layer CdTe and high metal work function. For the low cost of cell production, ultrathin film CdTe cells are used with a back surface field (BSF) between CdTe and back contact, such as PbTe. Further, the simulation results show that the conversion efficiency of 19.28% can be obtained for the cell with 1-μm-thick CdTe, 0.1-μm-thick PbTe and 30-nm-thick CdS.     

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.     

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.     

CdTe-HgCdTe叠层太阳电池CdS窗口层透射光谱性能研究

CdS窗口层光谱透射率的提高对CdTe-HgCdTe叠层太阳电池有效利用入射太阳光并增大电池的短路电流密度有重要的影响。通过研究化学水浴法、近空间升华法和磁控溅射法制备的CdS薄膜在CdCl2退火前后的光谱平均透过率和短路电流密度损失表明:在光谱区520~820 nm,化学水浴法制备的CdS薄膜在退火前后具有最高的光谱平均透过率,对应的CdTe顶电池有最小的短路电流密度损失;在光谱区820~1150和520~1150 nm,磁控溅射法制备的CdS薄膜在退火前后均具有最高的光谱平均透过率,对应的HgCdTe底电池和CdTe-HgCdTe叠层太阳电池有最小的短路电流密度损失。在光谱区520~820、820~1150和520~1150 nm,CdCl2退火可以显著增大CdS薄膜的光谱平均透过率,降低对应CdTe顶电池、HgCdTe底电池和CdTe-HgCdTe叠层电池的短路电流密度损失。     

Recent progress in thin-film cadmium telluride solar cells

Cadmium telluride (CdTe) with a room-temperature bandgap energy of 1.45 eV has been shown to be the most promising low-cost, thin-film photovoltaic material for terrestrial applications. Significant progress has been made during the past several years, and thin-film CdTe solar cells of > 1 cm² area with conversion efficiencies higher than 12% have been prepared by several techniques. Thin-film CdTe photovoltaic modules with 10% efficiency have also been produced. They are of the heterojuntion configuration using a transparent conducting semiconductor (TCS) as the window and p-CdTe as the absorber. In this paper, the potential window materials for thin-film CdTe solar cells are discussed. Thus far, cadmium sulphide (CdS) with a bandgap energy of 2.42 eV at room temperature has been found to be best suited for efficient CdTe solar cells. the deposition techniques for p-CdTe films capable of producing efficient solar cells, including close-spaced sublimation (CSS), electrodeposition, screen printing and spraying, are briefly reviewed, and the characteristics of the resulting solar cells are discussed. It is concluded that the efficiency of thin-film CdTe solar cells can be increased to 18-19% in the near-term, leading to 15-16.5% efficient modules.     

Efficient CdTe/CdS solar cells and modules by spray processing

Efforts have been directed toward production of efficient, large-area, low-cost photovoltaic modules based on the CdS/CdTe heterojunction, with the objective being to develop an improved materials technology and fabrication process for limited volume production of 1-ft² and 4-ft² CdS/CdTe photovoltaic modules. The present structure of the CdS/CdTe polycrystalline thin-film photovoltaic devices being produced is presented, along with its potential for efficiency improvement. Junction characterization studies are summarized. Module design is described, particularly with regard to encapsulation issue. Future developmental directions are discussed     

Transition resistances of ohmic contacts to p-type cdte and their time-dependent variation

Ohmic contacts to p-type CdTe are important for the development of solar cells based on this semiconductor, as for instance CdS/CdTe or ITO/ CdTe solar cells. Ohmic contacts to CdTe Bridgman crystals, doped with phosphorus, have been examined with respect to their resistivity dependence and their variation as a function of time. The ‘specific’ contact resistance r shows a linear dependence on the bulk resistivity; in addition, it is affected by the oxygen content of the CdTe. The lowest r obtained was 0.07Ω cm. With one exception, ali the contacts with nickel, gold and platinum deposited on different crystals show a more or less pronounced increase of r as a function of time.     

Modeling the defect distribution and degradation of ultrathin CdTe films

The defect distribution across an ultrathin film Cd Te layer of a Cd S/Cd Te solar cell is modelled by solving the balance equation in steady state. The degradation of the device parameters due to the induced defects during ion implantation is considered where the degradation rate is accelerated if the defect distribution is considerable.The defect concentration is maximum at the surface of the Cd Te layer where implantation is applied and it is minimum at the junction with the Cd S layer. It shows that ultrathin devices degrade faster if the defect concentration is high at the junction rather than the back region(Cd Te/Metal). Since the front and back contacts of the device are close in ultrathin films and the electric field is strong to drive the defects into the junction, the p-doping process might be precisely controlled during ion implantation. The modeling results presented here are in agreement with the few available experimental reports in literature about the degradation and defect configuration of the ultrathin Cd Te films.     

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.     

A one-dimensional theory of high base resistivity tandem-junction solar cells in low injection

A one-dimensional theoretical model of the Tandem-Junction Solar Cell (TJC) with high base resistivity and under low-level injection is derived. The model provides a theoretical basis for a previously published conceptual model and allows for the calculation of the spectral response and other performance parameters, namely, Isc, Voc, Pm,FF, and η, under variation of one or more of the geometrical and material parameters and 1-MeV electron fluence. Sample calculation results of computer simulation of this model are presented.