Triplet Acceptors with a D‐A Structure and Twisted Conformation for Efficient Organic Solar Cells

Linqing Qin,Kai Wang,Hui Huang,Xingzheng Liu,Xinhui Lu,Yuanping Yi,Lei Yang,Jianwei Yu,Kangwei Wang,Feng Gao,Hao Chen,Jianlong Xia,Xiaoxi Wu,Yuhao Li,Miaofei Huang,Fenggui Zhao,Xin Zhang

doi:10.1002/ange.202006081

Abstract

AbstractTriplet acceptors have been developed to construct high‐performance organic solar cells (OSCs) as the long lifetime and diffusion range of triplet excitons may dissociate into free charges instead of net recombination when the energy levels of the lowest triplet state (T1) are close to those of charge‐transfer states (3CT). The current triplet acceptors were designed by introducing heavy atoms to enhance the intersystem crossing, limiting their applications. Herein, two twisted acceptors without heavy atoms, analogues of Y6, constructed with large π‐conjugated core and D‐A structure, were confirmed to be triplet materials, leading to high‐performance OSCs. The mechanism of triplet excitons were investigated to show that the twisted and D‐A structures result in large spin–orbit coupling (SOC) and small energy gap between the singlet and triplet states, and thus efficient intersystem crossing. Moreover, the energy level of T1 is close to 3CT, facilitating the split of triplet exciton to free charges.

Highlights

organic solar cells (OSCs) have rapidly developed over the past decades owing to their advantages of low cost, flexibility, and light weight.[1]
The results revealed that the D-A structure with nonplanar conformation reduced the DEST and facilitated the intersystem crossing (ISC), yielding triplet excitons efficiently
High-performance OSCs based on these acceptors were fabricated to afford efficiencies of over 15 %, demonstrating that the triplet excitons were generated and split in the blend films to contribute to the power conversion efficiency (PCE), supported by magneto-photocurrent and transient spectroscopy

Summary

Introduction

OSCs have rapidly developed over the past decades owing to their advantages of low cost, flexibility, and light weight.[1]. One of important strategies to minimize DEST is combining nonplanar donor (D) and acceptor (A) units in conjugated systems, which has been employed to construct various organic room-temperature phosphorescence[8] and thermally activated delayed fluorescence materials.[9] these type of triplet materials have never been used for OSCs since their twist structures usually led to weak light absorption intensities and low charge transport mobilities, which is detrimental for efficiency of OSCs. two twisted-conformation molecular semiconductors with A-D-A’-D-A structure (H1 and H2), analogues of Y6,[2c,10] were constructed based on a large p-conjugated fused core (Figure 1 b,c), which were shown to be triplet acceptors with strong light absorption, supported by steady and transient photoluminescence, and absorption spectroscopy, electron paramagnetic resonance (EPR), magneto-photocurrent (MPC), and time-dependent density function theory (TD-DFT). High-performance OSCs based on these acceptors were fabricated to afford efficiencies of over 15 %, demonstrating that the triplet excitons were generated and split in the blend films to contribute to the PCE, supported by magneto-photocurrent and transient spectroscopy

Results and Discussion

Conclusion

Conflict of interest