Direct Observation of Increased Free Carrier Generation Owing to Reduced Exciton Binding Energies in Polymerized Small-Molecule Acceptors

Jinyuan Zhang; Jianxin Guan; Yaogang Zhang; Shucheng Qin; Qingye Zhu; Xiaolei Kong; Qing Ma; Xiaojun Li; Lei Meng; Yuanping Yi; Junrong Zheng; Yongfang Li

doi:10.1021/acs.jpclett.2c02337

Direct Observation of Increased Free Carrier Generation Owing to Reduced Exciton Binding Energies in Polymerized Small-Molecule Acceptors

J Phys Chem Lett. 2022 Sep 29;13(38):8816-8824. doi: 10.1021/acs.jpclett.2c02337. Epub 2022 Sep 15.

Authors

Jinyuan Zhang¹, Jianxin Guan², Yaogang Zhang^{1

3}, Shucheng Qin^{1

3}, Qingye Zhu², Xiaolei Kong^{1

3}, Qing Ma^{1

3}, Xiaojun Li¹, Lei Meng^{1

3}, Yuanping Yi^{1

3}, Junrong Zheng², Yongfang Li^{1

3

4}

Affiliations

¹ Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China.
² College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China.
³ School of Chemical Science, University of Chinese Academy of Sciences, Beijing 100049, China.
⁴ Laboratory of Advanced Optoelectronic Materials, Suzhou Key Laboratory of Novel Semiconductor Materials and Devices, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, Jiangsu, China.

PMID: 36107413
DOI: 10.1021/acs.jpclett.2c02337

Abstract

Energy loss caused by exciton binding energy (E_b) has become a key factor that restricts further advancement of organic solar cells (OSCs). Herein, we used transient mid-IR spectroscopy to study direct photogeneration of free charge carriers in small-molecule acceptors (SMAs) Y6 and IDIC as well as polymerized SMAs (PSMAs) PYFT and PZ1. We found that free carrier concentration is higher in PSMAs than in their corresponding SMAs, indicating reduced exciton E_b, which is then confirmed by ultraviolet photoelectron spectroscopy, low-energy inverse photoemission spectroscopy, and film absorption spectra measurements. The measured E_b values of PYFT and PZ1 are 0.24 and 0.37 eV, respectively, smaller than those of Y6 (0.32 eV) and IDIC (0.47 eV). This work not only provides a method to directly monitor the photogenerated free carriers in OSC materials but also demonstrates that polymerization is an effective strategy to reduce the E_b, which is crucial to decrease the energy losses in high-performance OSCs.