Photoinduced Hole Transfer Becomes Suppressed with Diminished Driving Force in Polymer-Fullerene Solar Cells While Electron Transfer Remains Active

Device performance and photoinduced charge transfer are studied in donor/acceptor blends of the oxidation-resistant conjugated polymer poly[(4,8-bis(2-hexyldecyl)oxy)benzo[1,2-b:4,5-b]dithiophene)-2,6-diyl-alt-(2,5-bis(3-dodecylthiophen-2-yl)benzo[1,2-d;4,5-d]bisthiazole)] (PBTHDDT) with the following fullerene acceptors: [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM); [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM); and the indene-C60 bis-adduct IC60BA). Power conversion efficiency improves from 1.52% in IC60BA-based solar cells to 3.75% in PC71BM-based devices. Photoinduced absorption (PIA) of the PBTHDDT:fullerene blends suggests that exciting the donor polymer leads to long-lived positive polarons on the polymer and negative polarons on the fullerene in all three polymer fullerene blends. Selective excitation of the fullerene in PC71BM or PC61BM blends also generates long-lived polarons. In contrast, no discernible PIA features are observed when selectively exciting the fullerene in a PBTHDDT/IC60BA blend. A relatively small driving force of ca. 70 meV appears to sustain charge separation via photoinduced hole transfer from photoexcited PC61BM to the polymer. The decreased driving force for photoinduced hole transfer in the IC60BA blend effectively turns off hole transfer from IC60BA excitons to the host polymer, even while electron transfer from the polymer to the IC60BA remains active. Suppressed hole transfer from fullerene excitons is a potentially important consideration for materials design and device engineering of organic solar cells.