Sequential Processing for Organic Photovoltaics: Design Rules for Morphology Control by Tailored Semi‐Orthogonal Solvent Blends

Abstract

Design rules are presented for significantly expanding sequential processing (SqP) into previously inaccessible polymer:fullerene systems by tailoring binary solvent blends for fullerene deposition. Starting with a base solvent that has high fullerene solubility, 2‐chlorophenol (2‐CP), ellipsometry‐based swelling experiments are used to investigate different co‐solvents for the fullerene‐casting solution. By tuning the Flory‐Huggins χ parameter of the 2‐CP/co‐solvent blend, it is possible to optimally swell the polymer of interest for fullerene interdiffusion without dissolution of the polymer underlayer. In this way solar cell power conversion efficiencies are obtained for the PTB7 (poly[(4,8‐bis[(2‐ethylhexyl)oxy]benzo[1,2‐b:4,5‐b′]dithiophene‐2,6‐diyl)(3‐fluoro‐2‐[(2‐ethylhexyl)carbonyl]thieno[3,4‐b]thiophenediyl)]) and PC61BM (phenyl‐C61‐butyric acid methyl ester) materials combination that match those of blend‐cast films. Both semicrystalline (e.g., P3HT (poly(3‐hexylthiophene‐2,5‐diyl)) and entirely amorphous (e.g., PSDTTT (poly[(4,8‐di(2‐butyloxy)benzo[1,2‐b:4,5‐b′]dithiophene‐2,6‐diyl)‐alt‐(2,5‐bis(4,4′‐bis(2‐octyl)dithieno[3,2‐b:2′3′‐d]silole‐2,6‐diyl)thiazolo[5,4‐d]thiazole)]) conjugated polymers can be processed into highly efficient photovoltaic devices using the solvent‐blend SqP design rules. Grazing‐incidence wide‐angle x‐ray diffraction experiments confirm that proper choice of the fullerene casting co‐solvent yields well‐ordered interdispersed bulk heterojunction (BHJ) morphologies without the need for subsequent thermal annealing or the use of trace solvent additives (e.g., diiodooctane). The results open SqP to polymer/fullerene systems that are currently incompatible with traditional methods of device fabrication, and make BHJ morphology control a more tractable problem.

Charge Generation and Recombination in an Organic Solar Cell with Low Energetic Offsets

“…To achieve this, a few routes may be possible: (i) It may be that film-formation techniques which do not rely on the self-assembly of the donor and acceptor materials from a homogeneous solution, such as sequential processing of the donor and acceptor, [81] can achieve improved BHJ morphologies. To achieve this, a few routes may be possible: (i) It may be that film-formation techniques which do not rely on the self-assembly of the donor and acceptor materials from a homogeneous solution, such as sequential processing of the donor and acceptor, [81] can achieve improved BHJ morphologies.…”

Drift-Diffusion Studies of Compositional Morphology in Bulk Heterojunctions: The Role of the Mixed Phase in Photovoltaic Performance

“…Since different donor/acceptor blends exhibit different phase separation kinetics and degrees of mixing in a uncontrollable way, it is highly likely that the performance impact of the mixed-phase is one of the root causes for the difficulty involved with optimizing new OPV donor/acceptor blends for device performance. This suggests that new fabrication techniques, such as sequential processing of donor/acceptor materials [62], are required to manipulate and control the degree of mixing in BHJ devices in a reproducible manner.…”

Panchromatic Sequentially Cast Ternary Polymer Solar Cells

“…While sequential casting has previously been used for the fabrication of binary PSCs, [29][30][31][32][33][34] it has never been utilized for casting stacks of different binary bulk heterojunction (BHJ) films, let alone for materials that form mechanical alloys. Our method gives rise to a favorable geometry and is simpler than lamination, but yields similar layered structures.…”