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.
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.
A complete electrical characterisation of thin film solar cells necessitates the analysis of capacitance vs. voltage measurements at different frequencies and illumination intensities. We developed a fully numerical device simulation tool for polycrystalline CdTe and CulnSe, solar cells, which carries out frequency domain calculations.Numerical simulations of /(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 CulnSe,. We indicate that especially the doping profiles which are deduced from C(V) data, may be misinterpreted when interface states are present at the heterojunction.
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