Multiple Cases of Efficient Nonfullerene Ternary Organic Solar Cells Enabled by an Effective Morphology Control Method

Abstract

of multijunction architecture increases the absorption breadth of a solar cell device, the difficulty of device fabrication and therefore the cost for large-scale production are significantly raised. To maintain the simplicity of the single junction structure and in the meantime broaden the absorption range of the solar cell, ternary OSCs have been developed where two donors and one acceptor or one donor and two acceptors are blended together to form the active layer. Important advances have been realized for ternary OSCs with best PCEs reaching beyond 10%. [11,[15][16][17][18][19][20][21][22][23][24][25][26][27][28][29] However, one main constraint that slows down the development of ternary OSC is the difficulty in controlling the morphology of ternary blends. It is commonly accepted that the bulk-heterojunction (BHJ) morphology for a two-component system is already complex. [25,[30][31][32][33][34] A threephase model has been generally utilized to describe the complicated morphology where a relatively pure donor phase, a relatively pure acceptor phase, and a intermixed donor:acceptor phase coexist in the active medium. [35][36][37][38][39][40][41][42] Apparently, the morphology formed by ternary blends would be even more challenging to evaluate. Due to the absence of effective methods to control the morphology, most of the previous reports on ternary OSCs only focused on one single material combination, Ternary organic solar cells (OSCs) have attracted much research attention, as they can maintain the simplicity of the single-junction device architecture while broadening the absorption range of OSCs. However, one main challenge that limits the development of ternary OSCs is the difficulty in controlling the morphology of ternary OSCs. In this paper, an effective approach to control the morphology is presented that leads to multiple cases of efficient nonfullerene ternary OSCs with efficiencies of up to 11.2%. This approach is based on a donor polymer with strong temperature dependent aggregation properties processed from hot solutions without any solvent additives and a pair of small molecular acceptors (SMAs) that have similar surface tensions and thus low propensity to form discrete phases. Such a ternary blend exhibits a simplified bulk-heterojunction morphology that is similar to the morphology of previously reported binary blends. As a result, an almost linear relationship between V OC and film composition is observed for all nonfullerene ternary devices. Meanwhile, by carefully designing a control system with a large interfacial tension, a different phase separation and V OC dependence is demonstrated. This morphology control approach can be applicable to more material systems and accelerates the development of the ternary OSC field.

The Critical Impact of Material and Process Compatibility on the Active Layer Morphology and Performance of Organic Ternary Solar Cells

“…[64,65] Concurrently, tuning composition of the ternary blend was found to give rise to a change in the energy of the charge transfer states, which is in turn directly related to V oc . [18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33] This subtle optimization point is also evident in the series of TSC devices studied here. On the other hand, V oc is also highly limited by the intrinsic nonradiative recombination losses, and this issue has recently been rigorously investigated for binary blend systems.…”

“…[16] The second polymer is poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7diyl)-alt-(3,3′″-di-(2nonyltridecyl)2,2′;5′,2″;5″,2′″-quaterthiophen-5,5′″-diyl)] (PffBT4T-C 9 C 13 ) with a quaterthiophene donor unit and a difluorobenzothiadiazole acceptor unit and it gives high efficiency in binary BHJ solar cells. [18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33] This intriguing morphological changes are highly connected to the electrical properties and device FF. The LBG polymer has enabled the extension of absorption to 900 nm.…”

Versatile Ternary Approach for Novel Organic Solar Cells: A Review

“…The results indicate that PTB7‐Th can balance the mixing and phase separation due to thermal annealing and thus better regulate the morphology. Yan et al reported an effective way to control the nanomorphology of binary OSCs by the ternary strategy, leading to multiple cases of efficient non‐fullerene TOSCs . They presented three key factors.…”

Efficient Ternary Organic Solar Cells with Two Compatible Non‐Fullerene Materials as One Alloyed Acceptor

“…Figure a shows the chemical structures of used materials, two acceptors have similar chemical structures. The two acceptors with similar chemical structures may prefer to form one alloyed acceptor . As described in Figure b, the used materials exhibit complementary absorption spectra for sufficient photon harvesting of the ternary active layers.…”

“…The interfacial energy (γ MeIC‐MeIC2 ≈ 0.0038 mN m −1 ) between two acceptors is much smaller than those (γ MeIC‐PBT1‐C ≈ 0.2943 mN m −1 and γ MeIC2‐ PBT1‐C ≈ 0.2313 mN m −1 ) of between donor and acceptor. The low interfacial energy between MeIC and MeIC2 is beneficial to form an almost inseparable organic phase, the two acceptors may work well together as one alloyed acceptor in ternary OSCs . The energy levels of used materials and schematic diagram of alloy‐like state are exhibited in Figure b.…”