Development of conjugated materials and optimization of their structure to improve the performance of eco-friendly processed organic solar cells

Abstract

i) C$_{70}$-based aqueous-soluble fullerene for the water composition-tolerant performance of eco-friendly polymer solar cells — Eco-friendly polymer solar cells (eco-PSCs) based on aqueous-soluble conjugated materials have recently received great research attention. In this work, we report the development of an aqueousprocessable C$_{70}$-based fullerene derivative (PC$_{71}$BO$_{15}$) for eco-PSCs processed by water/ethanol co-solvents. The desirable aggregation behavior and enhanced light absorption ability of PC$_{71}$BO$_{15}$ have enabled the fabrication of PPDT2FBT-A:PC$_{71}$BO$_{15}$-based eco-PSCs with power conversion efficiencies (PCEs) of up to 2.51%, which is the highest value reported to date for aqueous-processed PSCs. These PPDT2FBT-A:PC$_{71}$BO$_{15}$ eco-PSCs exhibit significantly higher PCEs than those of the reference PPDT2FBT-A:PC$_{61}$BO$_{15}$ devices over all device-processable water/ethanol compositions. At the optimal water/ethanol composition (v/v = 15 : 85), the PCE of the PPDT2FBT-A:PC$_{71}$BO$_{15}$ eco-PSCs is 73% higher than that of the PC$_{61}$BO$_{15}$-based counterparts, as a result of enhanced light absorption. Importantly, the PC$_{71}$BO$_{15}$-based eco-PSCs show much higher tolerance in their PCEs to the water/ethanol composition. For example, the PCEs of the PPDT2FBT-A : PC$_{71}$BO$_{15}$ eco-PSCs at a 30 : 70 water/ethanol ratio maintain 89% of the optimal performance at a 15 : 85 ratio, whereas the PC61BO15-based devices only maintain 45%. This large difference in terms of water-tolerant behavior is mainly related to the different aggregation behaviors between PC$_{71}$BO$_{15}$ and PC$_{61}$BO$_{15}$ in blend films, which are carefully investigated using electrical, optical and morphological characterizations. This high water composition-tolerance affords an excellent reproducibility of the PC$_{71}$BO$_{15}$-based eco-PSCs with eco/human-friendly aqueous processing under ambient conditions.ii) Green solvent-processed, high-performance organic solar cells achieved by outer side-chain selection of selenophene-incorporated Y-series acceptors — While the power conversion efficiencies (PCEs) of organic solar cells (OSCs) have been dramatically increased through the development of small molecular acceptors (SMAs), achieving eco-friendly solution processability of OSCs is a crucial prerequisite for their practical application. In this study, we develop three new, green solvent-processable SMAs (YSe–C3, YSe–C6, and YSe–C9) with different outer sidechains (n-propyl (C3), n-hexyl (C6), and n-nonyl (C9)), affording high-performance OSCs with nonhalogenated solvent (o-xylene)-processed active layers. Also, the impact of both outer and inner sidechain engineering of these SMAs on the performance of eco-friendly fabricated OSCs is systematically investigated. The outer side-chain structure has a much more significant impact than the inner sidechain. For example, the PM6:YSe–C6 blend affords high-performance OSCs with a power conversion efficiency (PCE) of over 16%, whereas the PCEs of the YSe–C3- and YSe–C9-based OSCs are only 11–14%. The lower PCEs of PM6:YSe–C3 and C9 are mainly attributed to reduced electron mobility and increased charge recombination, resulting from aggregate-containing non-optimal blend morphologies. Interestingly, the well-optimized morphology of the YSe–C6-based blend also affords OSC devices with active layer thickness-independent PCEs, up to a thickness of >400 nm, demonstrating the great potential for large-area device manufacturing via an eco-friendly printing process. Thus, optimizing the outer side-chain structure of Y-series SMAs is essential for producing green solvent-processed high performance OSCs.iii) Impact of Spacer Tuning in Oligoethyleneglycol-Based Y-Series Guest Acceptors on Green Solvent-Processed, High-Performance Ternary Organic Solar Cells — For the realization of green-solvent processable, and efficient organic solar cells (OSCs), the route to improve the processability of active layer materials has been limited to side-chain length extension. However, side chain extension can decrease crystallinity and electron mobility of conjugated materials, which would negatively influence the photovoltaic performance. Herein, we introduce three new, oligoethyleneglycol (OEG)-incorporated guest SMAs (Y-4C-4O, Y-6C-4O, Y-12C-4O) with different spacer-chains (n-butyl (4C), n-hexyl (6C), and n-dodecyl (12C)). When these acceptors are blended with Y6, the aggregation of Y6 in green-solvent (o-xylene) is reduced, resulting enough solubility of mixed Y-series acceptors. Alkyl spacer chain, which connects center Y-backbone and OEG chain, has significant impact on the performance of eco-friendly fabricated OSCs. The PM6:Y6:Y-4C-4O ternary blend affords the high-performance OSCs with a power conversion efficiency (PCE) of over 17%, whereas each PCE of the Y-6C-4O and Y-12C-4O-based ternary OSCs are only ~15% and ~11%. The lower PCEs of PM6:Y-6C-4O and Y-12C-4O devices are mainly attributed to reduced electron mobility and increased charge recombination, resulting from non-optimal molecular packing and blend morphologies. Enough procesability of Y6:Y-nC-4O (n = 4, 6, or 12) mixtures resulted in PCE tolerance over a wide range of guest acceptor ratio up to ~30%. Thus, optimizing the chemical structure of OEG-incorporated Y-series guest SMAs can be the novel route toward green solvent-processed high-performance OSCs.