Thermodynamic and kinetic insights for regulating molecular orientation in nonfullerene all‐small‐molecule solar cells

Abstract

The molecular orientation has a profound influence on the performance of organic solar cells. Both donor and acceptor adopt face-on orientation guarantees efficient exciton dissociation and charge transport, which is a key to achieving high device performance. However, the molecules usually adopt edge-on orientation in some blend systems, take small molecules based on an oligothiophene (DRCN5T): fused-ring electron acceptor based on indacenodithieno[3,2-b]-thiophene core and thienyl side-chains (ITIC-Th) blend, for instance, the orientation of DRCN5T is edge-on, which is detrimental to the photophysical process of the device. Herein, a solid additive strategy, that is, adding N2200 as a nucleus for DRCN5T, was proposed, which combined the nucleation process with molecular diffusivity, thus realizing the orientation transformation of DRCN5T from edge-on to face-on. Consequently, the device performance was significantly improved, and a clear relationship between molecular orientation and energy loss/ biomolecular recombination was established. More importantly, this study revealed not only the thermodynamic factors, including the crystallinity of solid additive, the lattice matching degree, and miscibility between DRCN5T and solid additive but also the kinetic parameter, such as the diffusivity of DRCN5T are very important to efficiently regulate the molecular orientation. Overall, this study presents the in-depth mechanism of orientation transformation via adding solid additives, which may provide a guideline for solid additive choices.

A review of nonfullerene solar cells: Insight into the correlation among molecular structure, morphology, and device performance

“…In P3HT:EH‐IDTBR blend, a new peak from acceptor appeared in out‐of‐plane direction as P3HT changed the packing order of EH‐IDTBR. Such structural features of IDTBR bring facile manipulation of morphology as indicated elsewhere 31,32 . As a result, P3HT:O‐IDTBR based device shows slightly better PCE (6.3%) compared to P3HT:EH‐IDTBR blend (6.0%) due to the higher J SC of 13.9 mA cm −2 91 …”

“…Such structural features of IDTBR bring facile manipulation of morphology as indicated elsewhere. 31,32 As a result, P3HT:O-IDTBR based device shows slightly better PCE (6.3%) compared to P3HT:EH-IDTBR blend (6.0%) due to the higher J SC of 13.9 mA cm −2 . 91 For the isomerization in Y6-based acceptors, Sun et al designed and synthesized branched alkyl chain L8-BO compared to linear-chain Y6.…”

“…[24][25][26] Recent studies have demonstrated that the ideal morphology should possess the following characteristics: (1) relatively small domain size, evaluated to be 10-20 nm, allows efficient diffusion of excitons to the donor/acceptor (D/A) interface within their limited lifetime; (2) higher domain purity prevents the recombination of exciton and carriers; (3) ordered molecular packing and face-on orientation are beneficial to charge transport; (4) favorable vertical phase separation structure provides transport pathways facilitating charge transport and extraction. [27][28][29][30][31][32][33][34] Considering the diversity of advanced NFAs, it is necessary to correlate the chemical structure of NFAs with morphology of active layer.…”

Recent Advances of Solid Additives Used in Organic Solar Cells: Toward Efficient and Stable Solar Cells

“…40 During TA, active layers are heated at temperatures above their glass transition temperatures for some time period, usually dozens of seconds to minutes. 41 With the help of TA, reorganization of both donor and acceptor occurs, which promotes the formation of an interpenetrating network. SVA is another effective method to optimize the morphology of the active layer.…”

“…At present, the universal methods mainly include thermal annealing (TA), − solvent vapor annealing (SVA), − solution state, − and adding solvent additives. − TA is a widely employed method to induce the crystallization and phase separation of active layers in OPVs . During TA, active layers are heated at temperatures above their glass transition temperatures for some time period, usually dozens of seconds to minutes . With the help of TA, reorganization of both donor and acceptor occurs, which promotes the formation of an interpenetrating network.…”