Effects of quantum dot charging on photoelectron processes and solar cell characteristics

We present theoretical and experimental analysis of photocarrier kinetics in quantum dot (QD) solar cells. The measurements of the J–V characteristics reveal strong effects of QD charging by selective doping of the interdot space on the solar cell characteristics. We demonstrate that charging of QDs significantly increases electron coupling to sub-bandgap photons, provides effective harvesting of IR energy, and serve as an effective tool for manipulating the potential profile at the micro- and nanoscale. The potential well for electrons in InAs QDs is substantially deeper than that for holes and, due to major differences between the effective masses of electrons and holes, the electron level spacing is substantially larger than the level spacing for holes. Therefore, QDs act as deep traps for electrons but shallow traps for holes. Filling of QDs under illumination is determined by a condition of equality of electron and hole capture rates which is realized via strong exponential dependence of the capture rates on the potential barrier around a charged dot. Without adequate doping of the QD medium, QDs are filled by electrons from the n-doped junction area and deteriorate the solar cell performance. However, selective n-doping of the QD medium results in micro- and nanoscale potential profiles favorable for photovoltaic conversion. Potential barriers around charged QDs decrease the photoelectron capture processes and suppress recombination processes via QDs. The filling of QDs predominantly from dopants in the QD medium allows one to maintain the microscale potential profile analogous to that in the best conventional single-junction solar cells.