Microscopic Signature of the Interfacial Charge Transfer States and their Relevant Spin-Dependent Processes in Organic Photovoltaics
Yuttapoom Puttisong a b, Weimin M Chen b, Feng Gao b, Yuxin Xia b, Irina A Buyanova b, Olle Inganäs b
a University of Cambridge - UK, The Old Schools, Trinity Ln, Cambridge CB2 1TN, UK, Cambridge, United Kingdom
b Linköping University, Sweden, SE-581 83, Linköping, Sweden
International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV16)
Swansea, United Kingdom, 2016 June 29th - July 1st
Organizers: James Durrant, Henry Snaith and David Worsley
Oral, Yuttapoom Puttisong, presentation 069
Publication date: 28th March 2016

Organic photovoltaics (OPV) is promising for low-cost, printable, flexible and light-weight solar energy harvesting. Efficient solar energy conversion in OPV devices relies on physical nature of photo-generated charges at the polymer donor and fullerene accepter hetero-interface. An important milestone for improving device efficiency is our understanding on the microscopic nature of interfacial charge transfer (CT) states contributing to both charge generations and energy losses. In this contribution, we will focus on our recent studies on direct probing of the interfacial CT states and their spin-dependent processes via CT state precursors. Here we shall shows, by combining selective optical excitation and detection with state-of-the-arts of magnetic resonance, we provide microscopic insight into the lowest CT exciton state (CT1) in model polymer-fullerene solar cells. We are able to estimate the electron-hole (e-h) separation of the CT1 exciton to be about 1 nm, within the physical dimension of a one-polymer-one-fullerene unit. The size of the CT1 exciton is found to be identical in the TQ1:PCBM blends regardless of the fullerene load and aggregation that are known to affect the degree of delocalization of high-energy CT excited states. We therefore conclude that the degree of the localization of the lowest CT exciton state is not responsible for the observed different efficiency of the TQ1:PCBM solar cells with different fullerene loads. In addition, we also provide experimental evidence that CT1 can mediate charge loss by facilitating intersystem crossing between the singlet and triplet of CT1, trapping and bimolecular recombination of separated charges via CT1, and electron back transfer from CT1 to the polymer triplet. We further propose the physical principle and possible pathways to turn CT1 from a loss channel of photo-generated charges into a charge generation channel and provide comments for the energy loss issues at open-circuit condition.



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