Singlet exciton fission, a multi-exciton generation process, in organic semiconductor solar cells

Abstract

Organic semiconductor photovoltaics hold the promise of cheap production and low manufacturing setup costs. The highest efficiency seen in research labs, ~10% today, is still too low for production. In this work we explore implementations of a multiple exciton generation process, singlet exciton fission, to work around the Shockley-Queisser limit, according to which, all single junctions cells have a theoretical efficiency limit of 33.7%. This is the first implementation of a singlet fission photovoltaic. We measured a singlet fission efficiency of 72% at room temperature. We showed that singlet fission can be implemented in bulk heterojunction photovoltaics, which is an important result since some of the highest efficiency organic photovoltaics in the last 5 years have been bulk heterojunction structures. Secondly, we showed that the magnetic field effect can be used as a probe to investigate triplet dissociation in singlet fission devices. Thirdly, we implemented singlet fission photovoltaics, using the singlet fission material pentacene as donor and low bandgap infrared-absorptive lead chalcogenide quantum dots as acceptors. Singlet fission can enhance the efficiency of organic photovoltaics only if the fission material is paired with an absorptive low-energy-gap material. We find that pentacene triplet excitons dissociate at the pentacene/quantum dot heterojunctions with an internal quantum efficiency of 35%. Lastly, we investigate a series of materials to find a better acceptor in singlet fission photovoltaics using the methods and some results from the previous two investigations. We investigate device structures that pair pentacene and 6,13 diphenyl-pentacene as singlet fission donors with C60 , perylene diimides, PbS quantum dots and PbSe quantum dots as acceptors.