The controlling influence of the Cu2S optical absorption coefficient on the short-circuit currents of Cu2S/CdS solar cells

Cu2S is a p-type defect semiconductor and is the main optical absorber-current generator in the Cu2S/CdS solar cell. This cell undergoes large reversible changes in its short-circuit current depending upon the ambient to which it is exposed. While a large number of mechanisms have been proposed for this effect, we find that it can be accounted for solely on the basis of changes in the absorption coefficient of the Cu2S, as controlled by the position of the Fermi level in the degenerate material. We have calculated the relation between the absorption coefficient and sheet resistance for degenerate Cu2S, and compared the results to existing experimental data on material prepared in the same way as solar cells. We find good agreement between the experimental data and our calculations if Cu2S is acting as a direct-gap semiconductor. Based upon the magnitude of the experimental changes in absorption coefficient with sheet resistance we have calculated the expected change in short-circuit current that would be produced in a typical Cu2S/CdS solar cell. The changes in short-circuit current calculated are on the order of 20-40 percent for ρ/d changing by an order of magaitude. These results are in agreement with the results on actual cells.     

The design and fabrication of CdS/Cu2S cells of 8.5-percent conversion efficiency

Thin-film polycrystalline CdS/Cu2S cells with energy conversion efficiencies exceeding 8.5 percent have been developed. The improvement over previously achieved efficiencies is the result of cell design and fabrication changes made in response to detailed quantitative modeling and analysis of the cell operation.     

The design and fabrication of thin-film CdS/Cu2S cells of 9.15-percent conversion efficiency

Thin-film polycrystalline CdS/Cu2S cells with energy conversion efficiencies in sunlight of up to 9.15 percent and areas of ∼1 cm²have been developed. The improvement over previously achieved efficiencies is due to the development of techniques to separately measure and minimize fill factor losses. Specific design and fabrication changes based on a detailed quantitative analysis of the cell operation, were introduced to correct series resistance, shunt conductance and field effect losses. Further increases in efficiency can be expected from the development of a planar junction thin-film CdS/Cu2S cell.     

Nondestructive measurement of solar cell sheet resistance using a laser scanner

Experimental data have shown that a laser scanner can be used as a probe to make nondestructive measurements of solar cell sheet resistance width an accuracy of several percent. The photovoltaic response from cells with controlled sheet resistance was measured using the scanner and compared with the theoretical predictions made by other workers. Several limitations in this technique are identified and a measurement methodology is suggested.     

Reliability of low-cost CdS/Cu2S solar cells

Accelerated life tests show that CdS/Cu2S solar-cell lifetimes increase with lower temperature, cyclic light and a nitrogen-gas ambient. A normal Arrhenius type of temperature dependence was found. Extrapolated lifetime for cells kept below 50°C in nitrogen gas with earth deployment exceeded twenty years. The probable decay mechanisms are thermal diffusion, formation of recombination centres and voltage-induced decomposition.     

Photoluminescence properties of polycrystalline ZnO/CdS/CuInGaSe2 solar cells at a low temperature

Emission properties of highly efficient polycrystalline solar cells based on ZnO/CdS/CuInGaSe₂ thin films were studied at T=20 K. The edge photoluminescence band was observed for the reference device at a photon energy of 1.191 eV. This band vanishes after treating the unincapsulated device in a humid atmosphere (relative humidity of 85%) at an elevated temperature (85°C). Long-wavelength bands at 1.13 and 1.07 eV, which are associated with optical transitions via defect levels in the absorber film, preserve the intensity and spectral position. A decrease in the conversion efficiency of the solar cell after treatment is caused by the degradation of upper wide-gap films and a CdS-CuInGaSe₂ heterointerface.     

The photoresponse of CdS/CuInSe2thin-film heterojunction solar cells

The effect of light bias on the spectral current response and spectral capacitance characteristics of CdS/CuInSe2thin-film heterojunction solar cells has been investigated. Monochromatic light bias has been used to identify specific wavelength regions responsible for the spectral behavior seen under white light bias. Variations with light or voltage bias are consistent with the effect of the field on interface recombination in both high and low CdS resistivity devices. Devices with high CdS resistivity show spectrally dependent enhancement and quenching effects very similar to those reported for CdS/Cu2S devices in which the space charge region was primarily in the CdS. It is concluded that in high CdS resistivity devices the junction behavior is controlled by the photoconductive CdS as has been established in CdS/Cu2S cells. Low CdS resistivity CdS/CuInSe2devices show none of these effects.     

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.     

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.     

Effect of a rear contact on the electrical properties of the CdS/CdTe-based thin-film solar cells

The comparative study of dark current-voltage and capacitance-voltage characteristics of the CdS/CdTe/Cu/Au and CdS/CdTe/ITO thin-film solar cells is carried out. The physical properties of the p ⁺-CdTe/n ⁺-ITO rear contact are experimentally determined for the first time.     

Computer simulation and modeling of graded bandgap CuInSe2/CdS based solar cells

This paper proposes the graded bandgap absorber material, Cu_1-xAg_xIn_1-y-zGa_yAl_z Se/sub 2(1-u$/ -/sub w/)S_2uTe_2w (CIS*) multinary system, to improve the low open-circuit voltage (V_OC) seen in CuInSe₂/CdS solar cells, without sacrificing the short-circuit current density (J_sc). It also proposes a p-i-n model for the CuInSe₂/CdS solar cell, where the intrinsic region is the graded bandgap CIS*. Reflecting surfaces are provided at the p-i and n-i interfaces to trap the light in the narrow intrinsic region for maximum generation of electron and hole pairs (EHP's). This optical confinement results in a 25-40% increase in the number of photons absorbed. An extensive numerical simulator was developed, which provides a 1-D self-consistent solution for Poisson's equation and the two continuity equations for electrons and holes. This simulator was used to generate J-V curves to delineate the effect of different grading profiles on cell performance. The effects of a uniform bandgap, normal grading, reverse grading, and a low bandgap notch have been considered. Having established the inherent advantages to these grading profiles an optimal doubly graded structure is proposed with grading between 1.5 eV and 1.3 eV regions which has V_OC=0.86 V, η=17.9%, FF=0.79 and J_sc=26.3 mA/cm² compared to 0.84 V, 14.9%, 0.76, and 23.3 mA/cm², respectively, for the highest efficiency 1.4-eV uniform bandgap cell. Replacing the thick CdS(2.42 ev) layer assumed in our simulations with a wide gap semiconductor such as ZnO(3.35 ev) increases all current densities by about 5 mA/cm², and increases the optimal calculated efficiency from 17.9% to roughly 21% for a doubly graded structure with a thickness of 1 μm and bandgaps ranging from 1.3 eV to 1.5 eV     

Optimization of vapor post‐deposition processing for evaporated CdS/CdTe solar cells

The effects of thermal annealing in conjunction with CdCl₂ vapor heat treatment on the properties of CdTe/CdS thin films and devices deposited by physical vapor deposition are reported. Results are compared for three treatment variations: high‐temperature anneal only, high‐temperature anneal followed by CdCl₂ vapor heat treatment and CdCl₂ vapor heat treatment only. X‐ray diffraction, transmission electron microscopy and scanning electron microscopy show improved crystallographic properties of the CdTe film and reduced CdS/CdTe interdiffusion when a high‐temperature anneal is used prior to CdCl₂ treatment. The CdTe/CdS solar cells fabricated using an anneal at 550°C in argon prior to the CdCl₂ vapor heat treatment exhibited improved electrical characteristics compared to cells fabricated with no anneal step, yielding an open‐circuit voltage exceeding 850 mV. Copyright © 1999 John Wiley & Sons, Ltd.     

Photosensitivity of thin-film ZnO/CdS/Cu(In, Ga)Se2 solar cells

The photoelectric properties of thin-film ZnO/CdS/Cu(In,Ga)Se₂ solar cells were studied by polarization photoactive absorption spectroscopy. It was shown that the thin-film solar cells have a high efficiency relative to the intensity of unpolarized radiation in the photon energy range from 1.2 to 2.5 eV. The induced photopleochroism coefficient P _I increases with the angle of incidence of the incident radiation as P _I ∼θ ² and at 70° it reaches 17–20% with photon energy 1.3 eV. Oscillations of the photopleochroism were also observed. These results are discussed taking into account the antireflection effect. The results obtained by us make it possible to use such solar cells as wide-band photosensors for linearly polarized radiation and for monitoring the production of high-efficiency, thin-film solar cells based on ternary semiconductors. Fiz. Tekh. Poluprovodn. 31, 806–810 (July 1997)     

Polarization photosensitivity of ZnO/CdS/Cu(In,Ga)Se2 solar cells

Results of the application of polarization spectroscopy of the photosensitivity of ZnO/CdS/Cu(In,Ga)Se₂ thin-film solar cells with different thicknesses of the CdS (50 and 100 nm) and ZnO (500 and 1000 nm) layers are considered. It is established that the induced photopleochroism coefficient is lowered while the quantum efficiency of photoconversion of the solar cells is raised by increasing the thickness of the front layer. The experimental conditions and spectral dependence of the induced photopleochroism are linked with the antireflection properties of the ZnO front layers. It is concluded that photosensitivity polarization spectroscopy can be used for rapid diagnostics of finished solar cells and to optimize their fabrication technology. Fiz. Tekh. Poluprovodn. 33, 484–487 (April 1999)     

1/f noise as a reliability estimation for solar cells

The 1/f noise from a forward biased dark solar cell is a non-destructive reliability estimation. The experimentally observed 1/f noise is compared with Kleinpenning's one-dimensional calculations for p-n diodes. At medium and low currents the 1/f noise of n⁺-p solar cells is about 50 times as large as predicted. Such deviations can be caused by non-uniformities in the large junction area. Local areas with lower built-in potentials at the junction lead to hot spots and reduced reliability. At large currents, reliability problems due to possible poor contacts can be studied from the proportionality between the noise and the square of the current.     

Theoretical analysis of Cu2S—CdS solar cells with rough interfaces

It has been found experimentally that the junction interface of a Cu2S-CdS solar cell shows a rough structure. The Cu2S can form deep dips between the grains of the CdS layer. By using an idealized geometry for such a Cu2S layer, the minority-carrier concentration and hence the characteristics of such a solar cell will be determined. The numerical method will be based on an integral equation technique.     

Interfacial engineering of ZnO nanoarrays as electron transport layer for polymer solar cells

Highly uniform one-dimensional ZnO nanoarrays (ZnO NAs) fabricated by hydrothermal process were successfully explored as electron transport layer (ETL) for offering a direct and efficient path for electron transport in inverted polymer solar cells (PSCs). The inorganic CdS shell layer by in situ growth on the ZnO NAs surface was used to passivate and repair the surface defects of ZnO NAs. To further engineer the ZnO surface and improve the compatibility between ETL of inorganic ZnO/CdS core/shell and polymer blend contact junction, those organic molecules of 3,6,7,10,11-pentakis-(hexyloxy)-2-hydroxytriphenylene (TP-OH), 1-pyrenol (Py-OH) and 4′-(7-hydroxy-heptanoyl)-biphenyl-4-carbonitrile (BP-OH) were respectively spin-coated on the ZnO NAs/CdS surface to fabricate ETL based on ZnO/CdS/TP-OH NAs, ZnO/CdS/Py-OH NAs, ZnO/CdS/BP-OH NAs. The π–π interactions between the organic molecules and fullerene acceptors could lead to the well-organized distribution of active layer materials, which is in favor of the enhancement of electron selectivity and the reduction of recombination probability of electrons and holes. The incorporation of ZnO/CdS/Py-OH NAs as ETL into the inverted PSCs based on P3HT:PC₆₁BM resulted in a superior power conversion efficiency (PCE) of 4.2% with enhanced short circuit current (J_sc) and fill factor (FF), compared to 3.1% for bare ZnO NAs, due to the intermolecular close-stacking and relative stronger π–π interaction energy between Py-OH and fullerenes. In addition, the sensitized ZnO surface led to intimate interface between ETL and active layer, which would be in favor of increasing the stability of the device.     

The generation-recombination mechanism of charge transport in a thin-film CdS/CdTe heterojunction

An n-CdS/p-CdTe heterostructure is studied. The heterostructure is obtained using the sequential growth of CdS and CdTe layers by electrochemical deposition and closed-space sublimation, respectively. The measured current-voltage characteristics are interpreted in the context of the Sah-Noyce-Shokley generation-recombination model for the depletion layer of a diode structure. The theory quantitatively agrees with the experimental results.     

Model for the frequency-dependence of the capacitance in CdSCu2S solar cells

It is shown, theoretically and experimentally, that deep donor levels play an important part in the structure of the space charge region of CdSCu₂S solar cells. Both in the dark and under illumination, they are responsible for the frequency dependence of the capacitance, although their role in the compensation mechanism of diffused Cu-acceptors is different in both situations. These levels lie at about 0.5 eV under the conduction band, they show a spread in energy of about 0.2 eV, and have response times in the range of 10^?2 to 10^?6 sec.     

An investigation of photocurrent loss due to reflectance and absorption in CdTe/CdS heterojunction solar cells

An attempt is made to understand, quantify, and reduce the reflectance and photocurrent loss in CdTe solar cells. Model calculations are performed to determine the optimum thicknesses of CdS and SnO₂ films and anti-reflection (AR) coating on glass that can minimize the reflectance and enhance the performance of CdTe/CdS/SnO₂/glass solar cells. Photocurrent loss due to absorption in CdS films is also calculated as a function of CdS thickness. It was found that the current loss due to reflectance ando absorption is more sensitive to the CdS film when its thickness falls below 1500Å. Model calculations show that reducing the CdS thickness from 1500 to 600Å increases short-circuit current density ( J_sc) by 3 mA/cm² because of reduced reflectance as well as absorption. Further decrease in CdS thickness below 600Å increases reflectance but results in higher J_sc, because current gain due to reduced absorption in thin CdS offsets the current loss due to higher reflectance. Model calculations also indicate that J_sc is not sensitive to SnO₂ thickness above 4000Å. Finally, an optimum thickness for single layer MgF₂ AR coating on glass was calculated to be 1100Å, which should provide an additional increase of 0.7 mA/cm² in J_sc. Some of these results are also experimentally validated in this paper.