Implications of Crystallization Temperatures of Organic Small Molecules in Optimizing Nonfullerene Solar Cell Performance

Abstract

Crystallization behaviors of both small-molecule donors and acceptors are critical yet complicated factors that determine the microstructure of all small-molecule organic solar cells. To achieve desired photovoltaic performance, it is of vital importance to elucidate the role of key crystallization parameters of each component in optimizing the morphology of blend films. To this end, four binary blends based on crystalline donors with various crystallization temperatures (namely DRTT-R, DRTT-T, DRTT-2T, and DRTT-TT) and a weakly crystalline acceptor N3 were selected. We determined the crystallization onset temperature (T c,onset) and aggregation transition temperature of the small molecules in neat/blend films by temperature-dependent X-ray diffraction and UV–vis measurements. Based on the detailed analysis of molecular crystallization, film morphology evolution, and device performance, a strong correlation between the T c,onset of photoactive materials and optimal thermal annealing (TA) conditions for device performance was established across these systems. Guided by this finding, a two-step TA approach was used to realize DRTT-T:N3 blend films with highly ordered molecular packing and appropriate phase separation morphology, thereby yielding a high power conversion efficiency of 13.21%. Our study demonstrates that using simple crystallization parameters, that is, T c,onset, can rationalize the annealing protocols, which is instructive for promoting the performance of all small-molecule electronic blends.

“Twisted” small molecule donors with enhanced intermolecular interactions in the condensed phase towards efficient and thick-film all-small-molecule organic solar cells

“…The selection of post-treatments was assisted by our previous study on the correlation between the crystallization temperature of photoactive materials and optimal TA conditions for the ASM blends based on this type of molecule donors. 19 According to our previous study, the optimal device performance can be obtained via two-step TA for the blend system of DRTT-T: N3 in which the crystallization onset temperature ( T c,onset ) of DRTT-T is obviously lower than that of N3 . By contrast, for the blend systems with T c,onset of the donor component close to that of the acceptor, the optimal performance can be obtained via one-step TA.…”

“…Moreover, the selection of the optimal thermal annealing (TA) conditions for the ASM-OSCs can be assisted by evaluating the crystallization parameters of these small molecule donors and nonfullerene acceptor in neat and blend lms, which can reduce trial-and-error efforts on device optimization. 19 Compared to the small molecule donor with alkyl side chains on the central TT unit, the molecule DRTT-T with alkylthiophenyl side chains on TT delivered superior performance with a PCE of ∼13% 19 (Fig. 1b), which was related to the stronger molecular interactions in the lm state coming from the alkylthiophenyl side chains.…”

“…Nevertheless, for the ASM blend systems, small molecule donors and small molecule acceptors with relatively weak molecular interactions and limited conjugate length oen present high congurational entropy of mixing, and tend to form nely intermixed donor/ acceptor domains and homogeneous lm morphology without distinct phase separation. [18][19][20][21] Improving the crystallinity of the small molecule donors by enhancing molecular rigidity and planarity can allow the ASM blends to form phase-separated morphology. [22][23][24][25][26] However, because of the easier molecular motion of the small molecules, the small molecules with high crystallinity are likely to form large crystals and to undergo excessive aggregation, leading to over-sized phase separation.…”

Correlating the Photovoltaic Performance and Stability of the All-Small-Molecule Organic Solar Cells to Their Intermixed Phases Determined by Concentration-Dependent Ultraviolet–Visible Absorption Spectroscopy

“…Small molecules typically tend to aggregate more, 30 making it easier to tune the molecular stacking by post-thermal annealing [thermal annealing (TA)] and/or solvent vapor annealing (SVA). 31,32 Therefore, investigating the morphological changes during device fabrication and the thermal aging process will better guide the preparation of high-performance and stable ASM-OSCs. The microstructure of the active layer usually consists of three components: the donor phase, the acceptor phase, and the intermixed phase.…”

“…The first is the aggregation/crystallization of small molecules in the intermixed phase region, and the second is the change of molecular stacking. , Moreover, the molecular stacking behavior of small molecules differs from that of polymers. Small molecules typically tend to aggregate more, making it easier to tune the molecular stacking by post-thermal annealing [thermal annealing (TA)] and/or solvent vapor annealing (SVA). , Therefore, investigating the morphological changes during device fabrication and the thermal aging process will better guide the preparation of high-performance and stable ASM-OSCs.…”