Detrimental Effects of ?Universal? Singlet Photocrosslinkers in Organic Photovoltaics

The use of singlet photocrosslinkers based on diazirine or azide has been investigated for organic photovoltaics (OPVs) to stabilize the morphology of mixed bulk heterojunction (BHJ) films. The photocrosslinking reactions in BHJs have major 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 pi-conjugated backbone of the non-fullerene acceptor to form trace amounts of byproducts that cause severe 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.