Alex Niemegeers

Effects of the Au/CdTe back contact on IV and CV characteristics of Au/CdTe/CdS/TCO solar cells

A simple analytical theory is presented to explain the measured roll over and cross over behaviour of the IV characteristics of thin film CdTe solar cells. It involves a classical description of the CdS/CdTe junction and the CdTe/back contact structure and is extended with a new description of minority carrier current in the CdTe contact region. This extension is crucial in describing the light dependence of the forward IV curves, and hence cross over. The same model also explains the measured CV curves. It is shown that analysis of the capacitance measurement can yield additional information about the doping density of CdTe in the vicinity of the contact. A relationship between the fill factor of the solar cell and the barrier height of the back contact is derived; this relation is useful as a new, practical criterion for the quality of the back contact. The results of this simple analytical model are confirmed by full numerical calculations of the dc and ac characteristics.

Model for electronic transport in Cu(In,Ga)Se2 solar cells

Temperature-dependent measurements of the current–voltage characteristics and of the junction admittance of ZnO/CdS/Cu(In,Ga)Se2 heterojunction solar cells are presented, together with numerical modelling of these experimental results. We explain the cross-over between dark and illuminated current–voltage characteristics currently observed for this type of device by the impact of the defect chalcopyrite layer at the surface of the Cu(In,Ga)Se2 absorber. Our model assumes an illumination-dependent voltage drop across a defect layer with a thickness of 15 nm to explain the cross-over. The voltage drop results from the electrical dipole made up of donor-like states at the interface between the defect layer and CdS and deep acceptor states in the defect layer itself. The illumination dependence of this voltage drop is explained by photogenerated holes trapped by the deep acceptor states in the defect layer. Numerical simulations have been carried out using the program SCAPS-1D in order to verify our model assumptions. From our model, indirect conclusions are derived concerning the maximum conduction band offsets between CdS and the defect layer and between CdS and ZnO. Copyright

On the CdS/CuInSe2 conduction band discontinuity

Recent calculations of the electron affinity difference between CdS and CuInSe2 indicate that the conduction band (CB) minimum of CuInSe2 is below the CB minimum of CdS. As a consequence, a spike occurs in the CB at the CdS/CuInSe2 interface. Such a spike is commonly considered as in conflict with good photovoltaic performance of heterojunction solar cells. It is outlined here that the simple assumption of thermionic emission across the junction can explain an unimpeded electron transport in the case of an n+p structure (n‐type window, p‐type absorber), even when a spike in the CB occurs.

Calculation of CIS and CdTe module efficiencies.

Abstract An accurate and fast method to calculate the efficiency of Cu(In,Ga)Se2 (CIGS) and CdTe thin-film solar modules is presented here. This comprises a new method to calculate the fill factor as a function of discrete and distributed series resistance, and of shunt conductance: a three-dimensional, third-order polynomial approximation is presented, and the expansion of the coefficients as a power series of 1/Voc is given. Analytical expressions are presented which fit experimental data of the optical absorption in ZnO as a function of its thickness or sheet resistance. Together with a calculation outline of the series and shunt effects of the module integration, this constitutes a practical module design tool. This is illustrated with results of dependence of module efficiency on cell length, window and absorber sheet resistance, interconnect contact resistance, “softness” of the cell I–V curve, and absorber material (CIGS or CdTe). Optimal or critical values for these parameters are given.

Defects in Cu(In, Ga) Se2 semiconductors and their role in the device performance of thin-film solar cells

This contribution is a summary of an international, interdisciplinary workshop dedicated to defects in chalcopyrite semiconductors and their relation to the device characteristics of thin-film solar cells, held on 3–5 June 1996 in Oberstdorf, Germany. Results of different characterization methods were brought together to identify common observations. The comparison of results from electrical defect spectroscopy and luminescence investigations confirmed the presence of energetic distributions of defects throughout the bandgap of chalcopyrite thin films. Electrical defect spectroscopy detects a defect about 280 meV above the valence band edge of Cu(In, Ga)Se2 regardless of the preparation conditions of the sample. In a solar cell the density of this defect depends on the operation conditions. This observation might be related to the migration of copper in an electric field, which occurs even at room temperature. Other defects appear to be related to processing or impurities. Photoluminescence decay measurements yield time constants of several nanoseconds under low injection conditions. Modelling of the current–voltage characteristics of Cu(In, Ga)Se2-based thin-film cells suggests that compensating acceptor states in the CdS or at the heterointerface are responsible for the frequently observed cross-overs between the dark and illuminated curves.© 1997 John Wiley & Sons, Ltd.

Numerical modelling of AC-characteristics of CdTe and CIS solar cells

A complete electrical characterisation of thin-film solar cells necessitates the analysis of capacitance vs. voltage measurements at different frequencies and illumination intensities. The authors developed a fully numerical device simulation tool for polycrystalline CdTe and CuInSe/sub 2/ solar cells, which carries out frequency-domain calculations. Numerical simulations of I-V and C-V characteristics of CdTe cells are compared with measurements. It is shown that capacitance-voltage measurements not only confirm the thesis that a back contact barrier limits the current at high forward bias-they also yield additional information on the CdTe doping in the vicinity of the contact. The numerical model has also been applied to CuInSe/sub 2/. The authors indicate that especially the doping profiles which are deduced from C-V data may be misinterpreted when interface states are present at the heterojunction.

Influence of illumination conditions on the design of thin-film modules

The most important, and most accessible design parameter of a series integrated thin-film photovoltaic module is the cell length lc. Its optimal value lcopt is a trade off between area loss, and series and shunt losses. If the design is performed at standard reporting conditions (i.e. constant one sun illumination), too low a value of lcopt results. When the design is done for a realistic histogram of the incoming sun power Pin, a larger value of lcopt follows, and an efficiency gain of a few tenths of a percent (absolute) is obtained. A climate model, enabling to calculate the histogram of Pin for given cloud conditions is presented here. A model to calculate the module parameters was presented before (Burgelman and Niemegeers, Sol. Energy Mater. Sol. Cells, June 1998). Both models are then applied together. As a result, the optimal value lcopt and the efficiency gain are calculated as a function of the location (geographical latitude) of the panel, both in clear sky and in clouded conditions.

Growth studies of CuInSe/sub 2/ using Cu-Se fluxes

The latest developments in CIGS/CdS/ZnO solar cells show the importance of understanding the growth mechanisms of the absorber layer. For this purpose, the authors studied the influence of Cu, CuSe, Cu/sub 2/Se and Se as additives to a CuInSe/sub 2/ base material. They found that, in a selenium rich atmosphere, the copper rich phase forms CuSe, which acts as a flux in a liquid phase sintering (above its melting temperature of 523/spl deg/C). Testing their findings to the selenization of glass/Mo/In(Se)/Cu structures led to marked grain growth with grains up to 20 /spl mu/m.