Advances in CdTe, CuInGaSe2 and GaAs/AlGaAs thin film PV solar cells

This paper summarises the latest developments in thin film solar cells based on CdTe, CuInGaSe2 and GaAs/AlGaAs absorber materials. After proposing a new model for CdS/CdTe solar cells in 2002, similar observations were noted for CuInGaSe2 based solar cells. It has been shown that the Fermi level pinning takes place at one of the four experimentally observed defect levels in pCuInGaSe2/metal interfaces very similar to that of n-CdTe/metal contacts. These levels are at 0.77, 0.84, 0.93 and 1.03 eV with ±0.02 eV error and are situated above the valence band maximum. As a result, discrete values of open circuit voltages are observed and the validity of the new model for CuInGaSe2 solar cells were published in 2009. It is becoming clear that Fermi level pinning due to defect levels dominates the performance in all CdTe, CuInGaSe2 and GaAs thin film devices and the future research should be directed towards resolving associated issues and hence improving the performance of PV solar cells. New designs based on graded bandgap multi-layer solar cells have been proposed for photovoltaic (PV) solar cell development. These new designs have been tested with well researched materials, GaAs/AlGaAs, and highest open circuit voltages of 1175 mV and fill factors of ~0.85 were produced for initial solar cell fabrications. The graded bandgap solar cells are capable of making use of surrounding heat radiation, via native defects using impurity PV effect. These devices produce Voc values in excess of 750 mV in complete darkness. This work has also led to the identification of disadvantages of the use of tunnel-junctions within tandem cells.