Donor End‐Capped Alkyl Chain Length Dependent Non‐Radiative Energy Loss in All‐Small‐Molecule Organic Solar Cells

Abstract

A critical bottleneck for further efficiency breakthroughs in organic solar cells (OSCs) is to minimize the non‐radiative energy loss (eΔVnr) while maximizing the charge generation. With the development of highly emissive low‐bandgap non‐fullerene acceptors, the design of high‐performance donors becomes critical to enable the blend with the electroluminescence quantum efficiency to approach or surpass the pristine acceptor. Herein, by shortening the end‐capped alkyl chains of the small‐molecular donors from hexyl (MPhS‐C6) to ethyl (MPhS‐C2), the material obtained aggregation that was insensitive to thermal annealing (TA) along with condensed packing simultaneously. The former leads to small phase separation and suppressed upshifts of the highest occupied molecular orbital energy level during TA, and the latter facilitates its efficient charge‐transport at aggregation‐less packing. Hence, the ΔVnr decreases from 0.242 to 0.182 V, from MPhS‐C6 to MPhS‐C2 based OSCs. An excellent PCE of 17.11% is obtained by 1,8‐diiodoctane addition due to almost unchanged high Jsc (26.6 mA cm−2) and Voc (0.888 V) with improved fill factor, which is the record efficiency with the smallest energy loss (0.497 eV) and ΔVnr (0.192 V) in all‐small‐molecule OSCs. These results emphasize the potential material design direction of obtaining concurrent TA‐insensitive aggregation and condensed packing to maximize the device performances with a super low ΔVnr.

Efficient Small‐Molecule Organic Solar Cells by Modulating Fluorine Substitution Position of Donor Material

“…[ 12‐13 ] It is the reason that the state‐of‐art PCEs of SM‐OSCs are only approximate 17% up to now. [ 14‐17 ] Unlike conjugated polymer donors, the conjugated small molecules do not have pre‐aggregation ability in solution due to the lack of intermolecular entanglement and large π‐π interactions, meanwhile, the small‐molecule donors and NFAs are not prone to form an interpenetrating network in BHJ layer due to ineffective intermolecular π‐π stacking between donors and acceptors. [ 18‐21 ] Up to now, it is still a big challenging issue to achieve favorable BHJ morphology through the design of photoactive materials.…”

Enhanced Performance via π‐Bridge Alteration of Porphyrin‐Based Donors for All‐Small‐Molecule Organic Solar Cells

“…[ 1‐16 ] Among that, all‐small‐molecule organic solar cells (ASM OSCs) have caught great interest from related researchers owing to the advantages of small molecules, such as the clear molecular structure, convenience of purification, as well as the high reproducibility of the devices, and thus made a great progress in the power conversion efficiency (PCE). [ 17‐23 ] However, due to the similar acceptor‐donor‐ acceptor (A‐D‐A) structure of small molecule donors (SMDs) and nonfullerene acceptors (NFAs), excessive aggregation is prone to occurring in the process of film formation, which will cause undesirable blend film morphology. Generally, poor miscibility could lead to larger pure‐phase regions, reducing the efficiency of exciton dissociation and charge transportation, while strong miscibility may cause excessive mixing between SMDs and SMAs, which may bring bimolecular recombination.…”

Modulation of Alkyl Chain Length on the Thiazole Side Group Enables Over 17% Efficiency in All‐Small‐Molecule Organic Solar Cells

“…[ 15 ] Recently, with the emergence of the low bandgap small‐molecule acceptor Y6 and its derivatives, great progresses with PCEs over 17% has been witnessed in the SM‐OSCs using the A‐D‐A structured SMDs, but the performance of SM‐OSCs still lags behind PSCs. [ 16–19 ] As Proved by the development of OSCs in the past decades, material innovation is always the key to improving photovoltaic performance. [ 4,5,20 ] Thus, understanding the relationship between molecular structure and device performance to develop novel SMDs is highly desirable to enhance the PCEs of SM‐OSCs.…”