Optimisation of Photon Recycling Effects in Strain-Balanced Quantum Well Solar Cells

Photon self-absorption or photon recycling (PR) can be used in strain balanced quantum well solar cells (SB-QWSC) to achieve higher device efficiencies. By growing SB-QWSCs on distributed Bragg reflectors (DBR) suppression of the device dark-current and changes in electro-luminescence (EL) spectra have been observed when compared to devices grown on GaAs substrates. The dark-current is suppressed in the region with ideality factor n=1, which corresponds to ideal Shockley and radiative recombination, while there is no difference in dark-currents in the region where the ideality factor n is ~2 and non-radiative Shockley-Read-Hall recombination is dominant. DBR SB-QWSCs in which the dark-current is dominated by radiative recombination benefit not only from enhancement of short-circuit current (J_sc) due to increased absorption of normal incident photons but also from enhancement of open-circuit voltage (V_oc) from PR. Experimental and modeling results of SB-QWSC devices will be presented in the paper. These results show that with optimal device design both the J_sc and V_oc can be increased significantly at the same time. Most importantly, to date PR has only been observed in dark-current measurements on fully metalised photodiode structures; but we will show current-voltage characteristics in devices under high concentration which reveal a suppression of the radiative dark-current in DBR devices compared to those without DBRs. The results presented here convincingly demonstrate, for the first time, the PR effect in SB-QWSCs under high concentration