Development of bulk heterojunction morphology by the difference of intermolecular interaction behaviors

The morphology of a bulk heterojunction can be controlled by adding a processing additive in order to improve its power conversion efficiency (PCE) in photovoltaic devices. The phase-separated morphologies of blends of PONTBT or P3HT with fullerene derivatives are systematically examined in the presence of processing additives that possess various alkane alkyl chain lengths or end-group electronegativities. We determined the morphologies of the bulk heterojunction layers by using atomic force microscopy (AFM) and grazing incidence wide angle X-ray scattering (GIWAXS). The photocurrent-voltage characteristics of the bulk heterojunction solar cells were found to be strongly dependent on the intermolecular interactions between the conjugated polymers, the fullerene derivatives, and the processing additives in the photoactive layer. The optimal PONTBT:fullerene derivative blend morphology was obtained with a processing additive, 1,3-diiodopropane (1,3-DIP), that possesses a short alkyl chain and an end group with weak electronegativity, and was found to exhibit a high fill factor (FF) and a high current density (J_SC). In contrast, in blends of P3HT with the fullerene derivative, PCEs with higher FF and J_SC values were achieved by incorporating the processing additive, 1,8-dibromooctane (1,8-DBrO), which has a long alkyl chain and a strong electronegative end group. Thus the selection of the processing additive with the aim of enhancing photovoltaic performance needs to take into account the intermolecular interaction of the conjugated polymer.