Isomeric Small Molecule Donor with Terminal Branching Position Directly Attached to the Backbone Enables Efficient All‐Small‐Molecule Organic Solar Cells with Excellent Stability

Abstract

All‐small‐molecule organic solar cells (ASM‐OSCs) are challenging for their inadequate efficiency and device stability due to their more susceptive morphology. Herein, a family of isomeric small molecule donors (SMDs) is synthesized based on the benzodithiophene–terthiophene core with linear, 1st carbon, and 2nd carbon position branched butyl‐based rhodanine for ASM‐OSCs, respectively. The single crystal of thiophene‐substituted model T‐s‐Bu forms a more compact intermolecular packing with herringbone structure than slip‐layered packing‐based T‐n‐Bu and T‐i‐Bu. SM‐i‐Bu and SM‐s‐Bu show slightly blue‐shifted absorption and deepened HOMO levels in the neat film compared to SM‐n‐Bu. SM‐s‐Bu:BO‐4Cl blend films have distinct face‐on packing orientations and suitable fibrous phase separation along with more apparent microcrystals. Finally, SM‐s‐Bu:BO‐4Cl‐based device yields an improved power conversion efficiency of 16.06% compared to 15.12% and 8.22% for SM‐n‐Bu:BO‐4Cl and SM‐i‐Bu:BO‐4Cl, which is one of the top‐ranked results for BTR‐series SMDs in binary ASM‐OSCs. More importantly, the excellent storage stability with a T80 lifetime of over 1700 h and decent thermal stability is realized in SM‐s‐Bu:BO‐4Cl. This work highlights that the isomeric terminal alkyl with a branching point directly connected to the backbone for SMDs is a promising strategy for improving the crystal packing and film morphology and achieving highly efficient and stable ASM‐OSCs.

Stability of organic solar cells: toward commercial applications

“…Recently, Wang and co-authors have synthesized three isomeric SMDs with linear (SM- n -Bu), 1st carbon (SM- s -Bu), and 2nd carbon-position (SM- i -Bu) branched isomeric terminal alkyl chain-based rhodanine end groups. 51 The neat films of SMDs with branched terminal chains exhibited slightly blue-shifted absorption spectra, up-shifted LUMO levels, and down-shifted HOMO levels. The blend film based on SM- s -Bu:BO-4Cl showed the optimal blend morphology characterized by nanofiber-based phase separation with pronounced BO-4Cl microcrystals, resulting in the highest and balanced charge mobility and the weakest charge recombination.…”

High-Performance All-Small-Molecule Organic Solar Cells Fabricated via Halogen-Free Preparation Process

“…Bulk-heterojunction (BHJ) organic solar cells (OSCs) have been regarded as a promising energy conversion technology due to their unique characteristics of lightweight, flexibility, and solution processability. − In recent years, the advent of narrow band-gap small-molecule acceptors (SMAs), particularly Y6 and its derivatives, has significantly improved the power conversion efficiencies (PCEs) of OSCs based on polymer donors and small-molecule acceptors, surpassing 19%. − Unlike polymer photovoltaic materials, small-molecule photovoltaic materials offer advantages such as precisely defined chemical structure, minimal batch-to-batch variations, and easy purification. − As a result, there has been a remarkable focus on all-small-molecule organic solar cells (ASM-OSCs), which have shown substantial advancements. Specifically, the PCEs of binary ASM-OSCs have exceeded 17%, while ternary ASM-OSCs have surpassed 18%. ,− …”

Regulating the reorganization energy and crystal packing of small-molecule donors enables the high performance of binary all-small-molecule organic solar cells with a slow film growth rate

“…[20][21][22][23][24][25] With recent innovation of device-processing engineering and photovoltaic materials, particularly the development of Y-series acceptors, the power conversion efficiencies (PCEs) of the ASM-OSCs have surpassed 16%. [26][27][28][29][30] However, the PCEs of the ASM-OSCs still lag behind those of the polymer-based OSCs. 31,32 Therefore, engineering high-performance small molecule donors (SMDs) is crucial for enhancing the PCEs of the ASM-OSCs.…”

“…In A-D-A type SMDs with benzo[1,2-b:4,5-b 0 ]dithiophene (BDT) as the donor unit, the terminal group plays a crucial role in controlling the absorption, energy level and aggregation propensity, among other factors. 28,33,34 For instance, Ge's group reported a suite of SMDs using 1H-indene-1,3(2H)-dione, 2-ethylhexyl rhodanine and cyanoacetic acid esters as terminal groups, respectively. They found that varying the end-group can affect the packing morphology of the active layer, leading to different p-p interactions and distinct photovoltaic performance.…”