Low-Vapor-Pressure Solvent Additives Function as Polymer Swelling Agents in Bulk Heterojunction Organic Photovoltaics

Bulk heterojunction (BHJ) photovoltaics based on blends of conjugated polymers and fullerenes require an optimized nanoscale morphology. Casting BHJ films using solvent additives such as 1,8-diiodooctane (DIO), 1,8-octanedithiol (ODT), chloronapthalene (CN), or diphenyl ether (DPE) often helps achieve this proper morphology: adding just a few volume percent of additive to the casting solution can improve polymer/fullerene mixing or phase separation, so that solvent additives have become staples in producing high-efficiency BHJ solar cells. The mechanism by which these additives improve BHJ morphology, however, is poorly understood. Here, we investigate how these additives control polymer/fullerene mixing by taking advantage of sequential processing (SqP), in which the polymer is deposited first and then the fullerene is intercalated into the polymer underlayer in a second processing step using a quasi-orthogonal solvent. In this way, SqP isolates the role of the additives' interactions with the polymer and the fullerene. We find using ellipsometry-based swelling measurements that when adding small amounts of low-vapor-pressure solvent additives such as DIO and ODT to solutions of poly(3-hexylthiophene-2,5-diyl) (P3HT), poly[(4,4′-bis(3-(2-ethyl-hexyl)dithieno[3,2-b:″,3′-d]silole)-2,6-diyl-alt-(2,5-bis(3-(2-ethyl-hexyl)thiophen-2yl)thiazolo[5,4-d]thiazole)] (PSEHTT), or 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), the additives remain in the polymer film, leading to significant swelling. Two-dimensional grazing-incidence wide-angle X-ray scattering measurements show that the swelling is extensive, directly affecting the polymer crystallinity. When we then use SqP and cast phenyl-C₆₁-butyric acid methyl ester (PCBM) onto DIO-swollen polymer films, X-ray photoelectron spectroscopy and neutron reflectometry measurements demonstrate that vertical mixing of the PCBM in additive-swollen polymer films is significantly improved compared with films cast without the additive. Thus, low-vapor-pressure solvent additives function as cosolvent swelling agents or secondary plasticizers, allowing fullerene to mix better into the swollen polymer and enhancing the performance of devices produced by SqP, even when the additive is present only in the polymer layer. DIO and ODT have significantly different fullerene solubilities but swell polymers to a similar extent, demonstrating that swelling, not fullerene solubility, is the key to how such additives improve BHJ morphology. In contrast, higher-vapor-pressure additives such as CN and DPE, which have generally high polymer solubilities, function by a different mechanism, improving polymer crystallinity.