Processing Methods for Obtaining a Face-On Crystalline Domain Orientation in Conjugated Polymer-Based Photovoltaics

Abstract

The polymer chain orientation and degree of crystallinity within a polymer:fullerene bulk heterojunction (BHJ) photovoltaic can greatly impact device performance. In general, a face-on chain orientation is preferred for charge conduction through sandwich-structure photovoltaic devices, but for many conjugated polymers, an edge-on conformation is energetically favored. In this work, we examine the effects of different processing techniques on photovoltaics based on the poly[4,8-bis(2-ethylhexyloxy)-benzol[1,2-b:4,5-b′]dithiophene-2,6-diyl-alt-4-(2-ethylhexyloxy-1-one)thieno [3,4-b]thiophene-2,6-diyl] (PBDTTT-C):[6,6]-phenyl-C71-butyric-acid-methylester (PC71BM) materials combination. We examine the extent of polymer crystallinity and crystalline domain orientation using both traditional blend-casting (BC), where the polymer and fullerene are cast from a single, codissolved solution, as well as sequential processing (SqP), where the polymer film is deposited first, and then the fullerene is infiltrated into the polymer film in a second solution processing step. We show using two-dimensional grazing-incidence wide-angle X-ray scattering (GIWAXS) that BC leads to a disordered, isotropic polymer network in the resulting BHJ film with a correspondingly poor device efficiency. By contrast, SqP preserves the preferred face-on chain orientation seen in pure polymer films, yielding higher short-circuit currents that are consistent with the increased hole mobility of face-on oriented polymer chains. We also study the effects of the widely used processing additive 1,8-diiodooctane (DIO) on polymer chain orientation and crystallinity in photovoltaic devices made by both processing techniques. We show that DIO results in increased polymer crystallinity, and in devices made by BC, DIO also causes a partial recovery of the face-on PBDTTT-C domain orientation, improving device performance. The face-on chain orientation in SqP devices produces efficiencies similar to those of optimized BC devices made with DIO but without the need for solvent additives or other postprocessing steps.