Quantum Dot Solar Cells with Nanoscale Barriers Around Dots: Experiment and Two-Diode Model Analysis

Incorporation of quantum dots into the p-n junction of the photovoltaic device provides numerous possibilities for nanoscale control of photoelectron processes via engineering the band structure and nanoscale potential profile. The band structure is determined by the size and shape of quantum dots and width of the wetting layer. The potential profile is formed by selective doping, which leads to the built-in dot charge. To study the effects of the band structure and nanoscale potential barriers on the photovoltaic performance we fabricated and investigated 3-μm base GaAs devices with various InAs quantum dot media and selective doping. All quantum dot devices show significant improvement in conversion efficiency in comparison with the reference cell. The data obtained have been analyzed in frame of the diode model. It was found that the two-diode model with the ideality factors of $n=1$ and $n=2$ well describes the scope of data. The essential $n=2$ component evidences that the Shockley-Read-Hall recombination plays a substantial role in recombination losses. Further optimization of solar cells should be aimed at the formation of potential profile with large barriers that separate quantum dot areas from high mobility conducting channels.