Multiple Exciton Generation in Nanoparticle Solar Cells
Solar cells based on single semiconductors as the active layer are typically restricted in efficiency by the Shockley-Queisser limit, since the energy of solar photons in excess of the semiconductor bandgap is dissipated as heat. In semiconductor nanoparticles, however, it has been shown that high-energy photons can produce multiple pairs of charge carriers, a process known as multiple exciton generation (MEG). Whilst this process has been demonstrated spectroscopically, there are very few reports of quantum efficiencies in excess of 100% in devices. Challenges involve (i) separating the extra charges before rapid Auger recombination occurs; (ii) ensuring that the device itself has good photovoltaic performance when operating in the normal (single-exciton) regime; and (iii), for practical implementation, ensuring that the threshold for MEG is as close as possible to twice the bandgap. We have recently obtained results, with device quantum efficiencies substantially in excess of 100%. The strategies we have employed include controlling the particle shape (rods of PbSe) and use of new materials such as PbTe to improve MEG yields. This work is at an early stage, and there is much to be done to translate encouraging quantum efficiencies at high photon energies into a useful boost to power conversion efficiency in practical devices. This project will investigate and solve these challenges. Areas that will be addressed include: (i) Characterising the efficiency of charge separation from multiple excitons by a combination of spectroscopic and device measurements, moving beyond the current focus on the initial yield of multiple excitons; (ii) Further developing our understanding of structure-property relationships for charge generation following MEG, investigating the roles of particle shape and surface coverage; and (iii) improving the open-circuit voltage of cells based on MEG by controlling nanoparticle surface defects and electrode interactions without interfering with the MEG process.