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

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 (J(sc)) and open circuit voltage (V-oc) 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 V-oc and J(sc) 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 V-oc and J(sc) is quantitatively correlated with a pre-exponential dark current term, J(sc), which accounts for the intermolecular interactions in the polymer/PCBM blends.