Precise Control of Nanostrcutres in Organic Photovoltaics beyond Mixed Bulk Heterojunction

Current organic photovoltaics (OPVs) achieve high conversion efficiencies through donor/acceptor interpenetrated bulk heterojunction (BHJ) structures. In general, BHJ structures are created by simply mixing two materials, electron donor and acceptor, but there are challenges in building nanostructures that are optimal for charge generation and their stability. For example, prolonged heat treatment causes donor/acceptor phase separation, which reduces conversion efficiency. More precise control of the nanostructures in OPVs beyond the simple mixed BHJ is needed to improve the efficiency and robustness of the devices.<br/>One strategy we have investigated is the use of photocrosslinkers. We have introduced singlet photocrosslinkers based on diazirine (2Dz) or azide (2Bx) into BHJ to stabilize the morphology of mixed BHJ films. The photocrosslinking reactions in BHJs have significant detrimental effects on the performance of OPVs. This is in striking contrast to previous reports on organic field-effect transistors, where these singlet crosslinkers can solidify the films without affecting the charge transport properties. Based on the OPV results with the layer-by-layer deposited films and the transferred films, we found that the crosslinking reactions had greater effects on the non-fullerene acceptor than on the donor polymer. The photoluminescence measurements suggested that the crosslinkers could react with the π-conjugated backbone of the non-fullerene acceptor to form trace amounts of by-products that cause strong exciton quenching. These results indicate that the use of the "universal" singlet photocrosslinkers in OPVs requires careful selection of the organic semiconducting materials and crosslinkers. In the talk, we will also discuss other strategies recently employed in our group to control nanostructures and molecular orientations in OPVs.