Effect of Bridging Atom Identity on the Morphological Behavior of Solution-Processed Small Molecule Bulk Heterojunction Photovoltaics

We examined the effects of changing the central bridging atom identity from carbon (d-CDT(PTTh2)2) to silicon (d-DTS(PTTh2)2) in the cyclopentadithiophene unit in a small molecule donor material. The substitution left the optical and electrical properties largely unchanged but significantly modified the melting/crystallization behavior and the formation of crystalline domains in thin film blends with PC71BM. Solar cells made with the d-CDT(PTTh2)2:PC71BM had efficiencies less than 1%, while thermally annealed solar cells made with d-DTS(PTTh2)2:PC71BM achieved efficiencies up to 3.4%. Morphological analyses of the active layer film morphology were done with polarized optical microscopy, grazing incidence wide-angle X-ray scattering, and transmission electron microscopy and showed that large (micrometer scale) crystals formed in the d-CDT(PTTh2)2 based films while smaller (25 to 50 nm) crystals formed in the d-DTS(PTTh2)2, largely explaining the difference in device performance. Thermally activated photocurrent was observed in devices suggest that the additional current at elevated temperatures results from thermally activated charge generation. Charge transfer excitons were also investigated using external quantum efficiency measurements. Sharper band tails for the small molecule donors suggest less disorder than in P3HT:PCBM and other polymer systems.