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 (Jsc) due to increased absorption of normal incident photons but also from enhancement of open-circuit voltage (Voc) 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 Jsc and Voc 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