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-C ₇₁-butyric acid methyl ester (PC₇₁BM); [6,6]-phenyl-C ₆₁-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 PC ₇₁BM-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 IC ₆₀BA 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. The effect of modulating the driving force for charge separation via photoinduced hole transfer from fullerene excitons in polymer:fullerene solar cells is measured. Poor photoinduced hole transfer is identified as a major limiting factor for photocurrent generation in indene-C₆₀-bis-adduct devices. These results provide a guide to materials design and device engineering for highly efficient organic solar cells.