Abstract
A study into the influence of direct polarity on the output parameters of ITO/CdS/CdTe/Cu/Au solar cells (SC) has been conducted. We have experimentally registered the effect of an electric field of direct polarity on the output parameters and light diode characteristics of ITO/CdS/CdTe/Cu/Au SCs, which underwent a degradation of efficiency. When a shaded SE is exposed for not less than 120 minutes to the electric field, induced by an external DC voltage of magnitude (0.5‒0.9) V, whose polarity corresponds to the forward bias of n-p heterojunction, there is an increase in efficiency coefficient. This becomes possible if, during degradation of the instrument structure, such defects did not have time to develop, which, over the specified time of exposure, lead to resettable alternating electric microbreakdowns. It has been established that an increase in efficiency coefficient comes at the expense of the increased density of a photocurrent, decreased sequential and increased shunt resistances of SC. Improvement of diode characteristics occurs due to several physical processes. When a SC is fed a forward bias voltage, an electric field forms inside the diode structure of SC, which amplifies the built-in electric field of the rear р-р + heterojunction and suppresses the built-in electric field of the frontal n + -p heterojunction. That occurs because the diodes are turned on towards each other. The magnitude of a forward bias voltage must not exceed the height of the potential barrier in a heterojunction. In this case, at the rear р-р + heterojunction and in its adjoining areas from both sides the processes will be intensified that are associated with the transport of copper atoms, the restructuring of complexes of point defects containing copper, and the phase transformations of Cu 1,4 Te into Cu 2-x Te. In addition, under the influence of the field induced by a forward bias voltage, the Cu Cd - particles from the depletion area of a CdS layer will start moving towards the absorber. That should reduce the resistance part of the CdS layer and lead to a decrease in the depletion area width from the absorber's side, thereby increasing the spectral sensitivity of SC in the shortwave and medium-wave fields of solar spectrum. Electrodiffusion of additional amount of Cu Cd - to the absorber must enhance the above-described and related effect of the increased spectral sensitivity and thus J ph of instruments. Based on the conducted research, we have constructed an algorithm for restoring the efficiency of ITO/CdS/CdTe/Cu/Au SCs and for rejecting the irrevocably degraded instrumental structures included in a running module
Highlights
It is known that effectiveness of the CdTe-based solar cells (SC) reduces during operation
We have experimentally registered the effect of an electrical field of direct polarity on the output parameters and the light diode characteristics of ITO/CdS/CdTe/Cu/ Au SC whose efficiency degraded
We have shown a possibility to improve the efficiency of film ITO/CdS/CdTe/Cu/Au SC after degradation by exposing them to a bias voltage of n-p heterojunction if the diode structure of such SC during degradation did not have time to form defects that over a specified time of exposure could lead to the alternating self-restoring electrical micro-breakdowns
Summary
It is known that effectiveness of the CdTe-based solar cells (SC) reduces during operation. The authors of paper [1] suggested two mechanisms in the degradation of such solar cells. The first is predetermined by the generation of defects in the area of semiconductor contact, caused by excessive charge carriers and defects of semiconductor layers. The second relates to an increase in the potential barrier at the rear ohmic contact. According to studies [2, 3], for the case of forming a tunnel rear contact, the degradation of SC is predetermined by the diffusion and interphase interactions between a nano-sized copper layer and a base layer [4]. Degradation of SC renders relevance to the search for structural-technological solutions aimed at improving the degradational stability of the specified instrumental structures
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