Structure, dynamics and power conversion efficiency correlations in new low bandgap polymer:PCBM solar cells

Molecular packing and interfacial structures as well as photoinduced charge separation dynamics have been investigated in a group of recently developed bulk hetero-junction (BHJ) organic photovoltaic (OPV) material based on a series of poly-(thienothiophene-benzodithiophene) (PTBX) with power conversion efficiency (PCE) of from 2 to > 7% in solar cell devices. Correlations of structure, dynamics and PCE have been investigated through grazing incidence x-ray scattering (GIXS) measurements of PTBX:PCBM ([6,6]-phenyl-C61-butyric acid methyl ester) films revealed p-stacked polymer backbone planes oriented parallel to the substrate surface, in contrast to the p-stacked polymer backbone planes oriented perpendicular to the substrate surface in regioregular P3HT[poly(3-hexylthiophene)]:PCBM films. The photoinduced charge separation (CS) dynamics in the pristine and annealed films of PTBX:PCBM film are measured and rationalized with the intrinsic charge transfer characters of the copolymer. The combination of a small optical gap, push-pull intrinsic charge transfer character, fast CS rate, favorable orientation of the polymer backbone at the interface as well we high carrier mobilities in PTBX:PCBM films contribute to their relatively high PCE in solar cells. The systematic investigation revealed that donor and acceptor domain sizes do not affect the charge separation dynamics significantly but the charge carrier populations are strongly morphology dependent. Annealing PTBX:PCBM films reduced the device PCE by a factor of >2 due to reduced interfacial area between the electron donor and the acceptor. Consequently, quantum yields of exciton generation and charge separation in the annealed film are significantly reduced compared to those in the pristine film.