Higher molecular weight leads to improved photoresponsivity, charge transport and interfacial ordering in a narrow bandgap semiconducting polymer

Increasing the molecular weight of the low-bandgap semiconducting copolymer, poly[(4,4-didoecyldithieno[3,2-b:2â,3â-d]silole)-2,6- diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl], Si-PDTBT, from 9 kDa to 38 kDa improves both photoresponsivity and charge transport properties dramatically. The photocurrent measured under steady state conditions is 20 times larger in the higher molecular weight polymer (HM_n Si-PDTBT). Different decays of polarization memory in transient photoinduced spectroscopy measurements are consistent with more mobile photoexcitations in HM_n Si-PDTBT relative to the lower molecular weight counterpart (LM_n Si-PDTBT). Analysis of the current-voltage characteristics of field effect transistors reveals an increase in the mobility by a factor of 700 for HM_n Si-PDTBT. Near edge X-ray absorption fine structure (NEXAFS) spectroscopy and grazing incidence small angle X-ray scattering (GISAXS) measurements demonstrate that LM _n Si-PDTBT forms a disordered morphology throughout the depth of the film, whereas HM_n Si-PDTBT exhibits pronounced π-π stacking in an edge-on configuration near the substrate interface. Increased interchain overlap between polymers in the edge-on configuration in HM_n Si-PDTBT results in the higher carrier mobility. The improved optical response, transport mobility, and interfacial ordering highlight the subtle role that the degree of polymerization plays on the optoelectronic properties of conjugated polymer based organic semiconductors. Increasing the molecular weight of the low-bandgap semiconducting copolymer, Si-PDTBT, leads to increased interfacial order of thin films. Whereas lower molecular weight (LM_n) Si-PDTBT shows no ordering, higher molecular weight (HM_n) Si-PDTBT exhibits π-π stacking of the conjugated backbone in an edge-on configuration near the interface with the substrate, which is appropriate to significantly increase the carrier mobility in the FET devices.