Publication date: 1st April 2013
Traditional semiconductors used in photovoltaic devices produce one electron from each absorbed photon. On the other hand, new materials such as quantum dots, nanorods, carbon nanotubes and graphene can more efficiently convert high-energy photons into multiple electron-hole pairs through a process titled multiple exciton generation (MEG) provided that the energy of the photon is at least twice the bandgap of the absorber. This process has been shown to be more efficient in highly confined quantum dots than other forms of carrier multiplication (such as impact ionization) in bulk materials. Photovoltaic devices can benefit greatly from MEG by producing increased photocurrent from the multiple electrons and thus allowing a single junction solar cell to yield a theoretical maximum efficiency as high as 44% compared to 33% for bulk semiconductors. In this talk, we will present recent findings from incorporating PbSe quantum dots (QDs) into semiconducting arrays that make up the absorber layer in prototype solar cells. In these devices, MEG is confirmed by demonstrating the first solar cell with external quantum efficiency (EQE) exceeding 100% for solar relevant photon energies. The EQE in our device reaches a maximum value of 114% at 380 nm and we have employed an optical model to determine that the PbSe QD layer produces as many as 1.3 electrons per photon (on average) for these photons. These findings are compared to ultrafast time resolved measurements of carrier quantum yields where we find reasonable agreement. We will also discuss future directions for materials designs that increase the quantum yield through more efficient MEG.
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