EXPERIMENTAL-ANALYSIS OF THE EFFICIENCY OF HETEROSTRUCTURE GAAS-ALGAAS SOLAR-CELLS

Some years ago Multiple Quantum Wells (MQW) solar cells were introduce d as an alternative to obtain high efficiencies. Based on the simple S hockley diode model, the short-circuit current could be increased with out loss in the open-circuit voltage. Applying the Detailed Balance Th eory, including radiative recombination in the i-region, leads to less optimistic predictions of the limiting efficiency of MQW cells. We pr esent an experimental study in order to compare the efficiency of MQW solar cells with heterostructure cells with graded Al compositions and single bandgap solar cells. Compared to the homojunction AlGaAs cell, an increase of the short-circuit current is observed by the incorpora tion of GaAs in the i-region. However, the open-circuit voltage is red uced by the implementation of GaAs, due to an increase of the non-radi ative recombination current. To estimate the maximum possible open-cir cuit voltage, the radiative recombination current is determined by mea suring the light emission as a function of the applied voltage. From t his experiment we conclude that the maximum possible open-circuit volt age of all the heterostructure cells is considerably lower than the ho mogeneous AlGaAs cell and close to the value of the GaAs cell, showing the relation between the open-circuit voltage and the smallest bandga p in the cell. The measured curves can be well predicted by calculatio ns based on the Detailed Balance Theory. We find no principal advantag e of MQW cells over cells with graded composition or single bandgap ce lls.