Understanding and Controlling the Evolution of Nanomorphology and Crystallinity of Organic Bulk‐Heterojunction Blends with Solvent Vapor Annealing

Abstract

Solvent vapor annealing (SVA) has been shown to significantly improve the device performance of organic bulk‐heterojunction solar cells, yet the mechanisms linking nanomorphology, crystallinity of the active layer, and performance are still largely missing. Here, the mechanisms are tackled by correlating the evolution of nanomorphology, crystallinity, and performance with advanced transmission electron microscopy methods systematically. Model system of DRCN5T:PC71BM blends are SVA treated with four solvents differing in their donor and acceptor solubilities. The choice of solvent drastically influences the rate at which the maximum device efficiency establishes, though similar values can be achieved for all solvents. The donor solubility is identified as a key parameter that controls the kinetics of diffusion and crystallization of the blend molecules, resulting in an inverse relationship between optimal annealing time and donor solubility. For the highest efficiency, optimum domain size and single‐crystalline nature of DRCN5T fibers are found to be crucial. Moreover, the π–π stacking orientation of the crystallites is directly revealed and related to the nanomorphology, providing insight into the charge carrier transport pathways. Finally, a qualitative model relating morphology, crystallinity, and device efficiency evolution during SVA is presented, which may be transferred to other light‐harvesting blends.

Using a fast hybrid pixel detector for dose-efficient diffraction imaging beam-sensitive organic molecular thin films

“…BHJ thin films consisting of DRCN5T small molecules blended in PC 71 BM, and processed by solvent vapor annealing in a CS 2 atmosphere for 720 s, were used as a reference sample in this study. An extended description of the sample preparation and comprehensive characterization has been reported in earlier studies [16,46].…”

“…To balance both spatial and angular resolution and to achieve a more homogeneous beam-specimen interaction, a semi-convergence angle α on the order of ∼1 mrad and a large defocus (z ≫ t) of a few micrometers are typically applied. The experiment optimization provided by 4D-SCED allows for the study of complex nano-crystalline structures of soft materials at few nm spatial resolution and enables the direct in situ observation of nanocrystals growth and coarsening in bulk heterojunction (BHJ) thin films [16,46].…”

All‐Green Solvent and Additive Combination Enables Efficient Nonfullerene Organic Solar Cells via Sequential Deposition Strategy

“…[36,[38][39][40] However, the random distribution of the donor and acceptor materials in the comingled spin coating process leads to a very sophisticated morphology manipulation process. [41] Despite the fact that researchers have developed methods such as thermal annealing, [42][43][44] additive engineering, [45][46][47] and solvent vapor annealing [48][49][50][51] to control the morphology of active layer, those complicated operation process is unfavorable for future large area preparation.…”

Role of 3,4-Ethylenedioxythiophene in the Enhancement of Optical and Charge Transport Properties of Low Band Gap Diketopyrrolopyrrole-Containing Metallooligomers Designed for Organic Solar Cells

“…2 The power conversion efficiency (PCE) significantly increases to 18%−19% when they are blended with nonfullerene acceptors (NFA) small molecules 3−5 like the blend PM6/Y6. 6 The use of new donor and acceptor materials with suitable hole/electron transporting layers, 7 the improvement of active layer morphology with thermal or solvent vapor annealing (SVA), 8,9 and the preparation of ternary systems 10 lead to significant improvements of the PCE nearly reaching 20%. 2,11,12 Further improvements are required to attain the current performances of commercial silicon-based solar cells (PCE = 26−27% 13,14 ).…”

“…The power conversion efficiency (PCE) significantly increases to 18%–19% when they are blended with nonfullerene acceptors (NFA) small molecules − like the blend PM6/Y6 . The use of new donor and acceptor materials with suitable hole/electron transporting layers, the improvement of active layer morphology with thermal or solvent vapor annealing (SVA), , and the preparation of ternary systems lead to significant improvements of the PCE nearly reaching 20%. ,, Further improvements are required to attain the current performances of commercial silicon-based solar cells (PCE = 26–27% , ). The main strategies are focused on structural modifications of the donors toward modulation of energy band levels and the decrease of the optical band gap ( E g opt ) to reduce energy losses and to broaden the spectral absorption range for an improvement of external quantum efficiency (EQE) and short circuit current ( J SC ) with an augmentation of open-circuit voltage ( V OC ).…”