Voltage dependence of two-step photocurrent generation in quantum dot intermediate band solar cells

• Demonstration of two-step photocurrent in GaAs/AlGaAs quantum dot solar cell.
• Two-step photocurrent exhibits maximum at −0.3 V and decreases toward forward bias.
• Voltage-dependence discussed by recombination and escape rates using rate equations.
• Two-step signal strongly related to carrier population in QDs.
• Intermediate energy states achieved in strain-free QD with artificial wetting layer.

We studied in detail the voltage dependence of two-step photocurrent generation through a two-step process of absorbing sub-band gap photons of different photon energies in a GaAs/AlGaAs quantum dot Intermediate Band Solar Cell. Our experiments revealed that two-step photocurrent generation is largely dependent on voltage, and exhibits a maximum at −0.3 V. A notable feature is a monotonic decrease in two-step photocurrent in the forward bias region, where the operating point of the solar cell lies. Using a model of rate equations, we extracted the voltage dependence of the individual escape and recombination rates, and found that the decrease in two-step photocurrent in the forward bias region is related to a monotonic increase in recombination rate in the quantum dots with increasing bias.

A clear two-step photocurrent generation, the key operating principle of Intermediate Band Solar Cells (IBSCs), and its voltage-dependence are reported in a GaAs/AlGaAs quantum dot IBSC at room temperature. Intermediate energy states which limit thermal and tunneling escape of carriers, a necessary IBSC condition, are formed by droplet epitaxy quantum dots with an underlying artificial wetting layer. The two-step photocurrent signal exhibits a maximum at −0.3 V and decreases toward forward bias, which we analyzed using rate equations.Figure optionsDownload as PowerPoint slide