Quantitatively Analyzing the Influence of Side Chains on Photovoltaic Properties of Polymer−Fullerene Solar Cells

Abstract

Conventional wisdom dictates that the band gap and energy levels of a conjugated polymer are primarily determined by the molecular structure of the conjugated backbone, while the solubilizing alkyl chains should have a negligible impact on these properties. Hence the side chains should have minimal impact on the short circuit current (Jsc) and open circuit voltage (Voc) of corresponding polymer based bulk heterojunction (BHJ) solar cells. Contrary to the “conventional wisdom”, we demonstrate that the side chain of a low band gap polymer (PNDT-DTBT) significantly impacts the observed Voc and Jsc of the corresponding BHJ solar cell with variations as much as 100%, depending upon the length and shape of these alkyl chains. The observed difference in Voc and Jsc is quantitatively correlated with a pre-exponential dark current term, Jso, which accounts for the intermolecular interactions in the polymer/PCBM blends.