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, 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 1170 mV and fill factors of similar to 0.85 values were produced for initial growths and fabrications. This work has led to the identification of disadvantages of the tunnel junction approach, in the present manufacturing process. Recently, it has been shown that Fermi level pinning takes place at one of the four experimentally observed defect levels in CuInGaSe2/metal interfaces very similar to that of 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 situation is very similar to that of CdS/CdTe solar cells. It is becoming clear that Fermi level pinning due to defect levels dominates the performance in at least CdTe and CIGS thin film devices and future research should be directed to solving associated issues and hence improving the performance of PV solar cells. (C) 2009 Published by Elsevier B. V.