Understanding How Processing Additives Tune the Nanoscale Morphology of High Efficiency Organic Photovoltaic Blends: From Casting Solution to Spun‐Cast Thin Film

Abstract

Adding a small amount of a processing additive to the casting solution of photoactive organic blends has been demonstrated to be an effective method for achieving improved power conversion effi ciency (PCE) in organic photovoltaics (OPVs). However, an understanding of the nano-structural evolution occurring in the transformation from casting solution to thin photoactive fi lms is still lacking. In this report, the effects of the processing additive diiodooctane (DIO) on the morphology of the established blend of PBDTTT-C-T polymer and the fullerene derivative PC 71 BM used for OPVs are investigated, starting in the casting solution and tracing the effects in spun-cast thin fi lms by using neutron/X-ray scattering, neutron refl ectometry, and other characterization techniques. The results reveal that DIO has no observable effect on the structures of PBDTTT-C-T and PC 71 BM in solution; however, in the spun-cast fi lms, it signifi cantly promotes their molecular ordering and phase segregation, resulting in improved PCE. Thermodynamic analysis based on Flory-Huggins theory provides a rationale for the effects of DIO on different characteristics of phase segregation due to changes in concentration resulting from evaporation of the solvent and additive during fi lm formation. Such information may help improve the rational design of ternary blends to more consistently achieve improved PCE for OPVs. and renewable next-generation energy sources. Remarkable progress has been made in improving the power conversion effi ciency (PCE) of OPVs above 11% by a combination of new materials synthesis, morphology optimization and advanced device engineering. [ 1,2 ] For high effi ciency solar cells, conjugated polymers are required to have a broad light absorption spectrum that effectively overlaps the solar spectrum for effective exciton generation, and high charge carrier mobility for efficient charge transport and collection. In order to meet these criteria, much effort has been expended to synthesize new conjugated polymers, in particular low band gap polymers. [ 3,4 ] Also, the PCE of an OPV device strongly depends on the nanoscale morphology of the electron donor (ED) and acceptor (EA) in the active layer. [ 5 ] The ideal morphology has been postulated to be the bulk heterojunction (BHJ), where the ED and EA form a nanoscale bicontinuous network. This nanoscale network ensures short path lengths for exciton diffusion, large interfacial area between ED and EA for effective exciton separation, and suffi cient network connectivity pathways for effective charge transport and collection. To achieve different BHJ morphologies, a variety of processing techniques have been explored such as

Breaking the 10% Efficiency Barrier in Organic Photovoltaics: Morphology and Device Optimization of Well‐Known PBDTTT Polymers

“…Researchers have shifted model materials for morphological understanding from PTB7 to other high-effi ciency PBDTTT polymers such as PBDTTT-C-T [ 36,55,56 ] and PTB7-Th. [ 57,58 ] For instance, Xiao et al [ 55 ] used a comprehensive set of characterization techniques including small-angle neutron/X-ray scattering and neutron refl ectometry to reveal the impact of DIO on the PBDTTT-C-T:PC 71 BM morphology from the cast solution to the solid fi lm; they found that DIO is critical for enhancing wileyonlinelibrary.com crystallinity and strongly improving both lateral and vertical phase segregation in the fi lm solidifi cation stage (see Figure 5 ).…”

Critical role of domain crystallinity, domain purity and domain interface sharpness for reduced bimolecular recombination in polymer solar cells

“…Nanomorphology plays a crucial role in BHJ solar cells as it dictates both charge generation and transport [5][6][7]. Nanomorphological features such as donor/acceptor crystallinity [8], vertical/ lateral phase separation [9,10], domain purity [11], orientation [12] and domain interfaces [13] can be tailored by optimizing the processing conditions to enhance device conversion efficiencies. Processing parameters such as solvent selection [8,14,15], annealing temperature [16], donor-acceptor mixing ratio [17,18], and coating techniques can be fine-tuned to attain high photovoltaic performance.…”

Nanophase Engineering of Organic Semiconductor-Based Solar Cells

“…Shao et al [19] also found that the DIO additives could significantly facilitate the molecular ordering in the spin-coated films. Two-dimensional (2D) grazing- [19].…”

“…The opposite structural evolution upon adding additive is determined by distinctive initial states without additives processing, which is related with the intrinsic factors of solubility, crystallization ability, and miscibility between BHJ components. Shao et al [19] observed that both lateral and vertical morphology of the PBDTTT-C-T:PC 71 BM blend films could be optimized by using a small amount of additive of 3 % DIO. Low-voltage energy filter transmission electron microscopy (EF-TEM) was applied to reveal the phase and morphology in PBDTTT-C-T: PC 71 BM blend films with DIO and without DIO.…”

“…Two-dimensional (2D) grazing- [19]. © (2014) WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim incidence wide-angle X-ray scattering (GIWAXS) was applied to reveal the molecular orientation, molecular ordering, as well as polymer crystallinity of the PBDTTT-C-T:PC 71 BM blend films.…”