Increasing Photostability of Inverted Nonfullerene Organic Solar Cells by Using Fullerene Derivative Additives

Abstract

Organic solar cells (OSCs) recently achieved efficiencies of over 18% and are well on their way to practical applications, but still considerable stability issues need to be overcome. One major problem emerges from the electron transport material zinc oxide (ZnO), which is mainly used in the inverted device architecture and decomposes many high-performance nonfullerene acceptors due to its photocatalytic activity. In this work, we add three different fullerene derivativesPC71BM, ICMA, and BisPCBMto an inverted binary PBDB-TF:IT-4F system in order to suppress the photocatalytic degradation of IT-4F on ZnO via the radical scavenging abilities of the fullerenes. We demonstrate that the addition of 5% fullerene not only increases the performance of the binary PBDB-TF:IT-4F system but also significantly improves the device lifetime under UV illumination in an inert atmosphere. While the binary devices lose 20% of their initial efficiency after only 3 h, this time is increased fivefold for the most promising ternary devices with ICMA. We attribute this improvement to a reduced photocatalytic decomposition of IT-4F in the ternary system, which results in a decreased recombination. We propose that the added fullerenes protect the IT-4F by acting as a sacrificial reagent, thereby suppressing the trap state formation. Furthermore, we show that the protective effect of the most promising fullerene ICMA is transferable to two other binary systems PBDB-TF:BTP-4F and PTB7-Th:IT-4F. Importantly, this effect can also increase the air stability of PBDB-TF:IT-4F. This work demonstrates that the addition of fullerene derivatives is a transferable and straightforward strategy to improve the stability of OSCs.

Simultaneously Achieving Highly Efficient and Stable Polymer:Non‐Fullerene Solar Cells Enabled By Molecular Structure Optimization and Surface Passivation

“…[ 24 ] By comparing the stability difference of the IT‐4F cells with different ZnO interlayer, we recently proved that photon generated hydroxyl radicals on the ZnO surface is the chemical reactive species that causes the breaking of the C═C bonds. [ 25 ] With the better understanding on the detailed degradation mechanism of the NFA cells, methods to improve device stability, including blending with fullerene molecules, [ 26 ] surface treatment with Lewis acids, [ 27 ] a thin PEI layer, [ 28 ] or a self‐assembled monolayer [ 29 ] were reported, supporting the proposed degradation mechanism. Despite these excellent research works in improving the stability of the cells, these cells showed lower initial efficiencies less than the optimized cell performance, and most of the cells showed a PCE lower than 15%.…”

Hole/Electron Transporting Materials for Nonfullerene Organic Solar Cells

“…In addition to the surface defects, ZnO exhibits intense photocatalytic activity, which induces chemical reactions at the ETL/active layer interface, especially when NFAs are used as the acceptors. [16] Such chemical reactions can dominate the degradation process for the NFA based OSCs. [17] Ma et al found that base treatment of ZnO caused poorer performance and accelerated the degradation of NF-OSCs, which was originated from enormous hydroxyl groups on the ZnO film.…”

“…[29] Yang et al reported a series of titanium diisopropoxide bis(acetylacetonate) (TIAA) based ETLs by changing the TA temperatures. [30] With increasing the temperature from 75 to 125 and 200 °C, the WF was lowered from À 3.7 eV of LT TIAA to À 3.9 eV of MT TIAA and À 4.1 eV of HT TIAA, owing to the 16 , denoted as Ti12) Ti atoms in the cores and alkyl groups on the surface as ETLs. [31] These TOCs have precise chemical structures with a single crystal, excellent solubility in methanol, well-aligned WF and could form smooth films on top of the active layer.…”

“…As a consequence, high efficiencies of 13.06 % and 17.29 % were obtained by using the blends of PM6: IT-4F and PM6:BTP-4Cl, respectively. In another work, they developed an electrolyte of CTOC-N-Br (Ti 32 O 16 -(EG) 32 (TMAc) 12 (DMG) 4 (MN) 4 (CH 3 O) 16 Br 4 ) by introducing carboxylic (R 2 COO) groups at the periphery of cyclic Ti-oxo cluster (CTOC) core and ligand exchange reaction. [32] CTOC-N-Br exhibited excellent solubility in methanol, well-aligned WF and good conductivity, resulting in a high PCE of 17.19 % in PM6: BTP-4Cl based OSCs.…”

“…In addition to the surface defects, ZnO exhibits intense photocatalytic activity, which induces chemical reactions at the ETL/active layer interface, especially when NFAs are used as the acceptors [16] . Such chemical reactions can dominate the degradation process for the NFA based OSCs [17] .…”