Figure 6 (a) Photoelectron spectroscopy in air (PESA) curves for PTPD, PBiTPD, and Y6. PESA-inferred IPs are reported on the plots, and the traces are offset for clarity. (b) Energy level alignments (IP and EA) of electron acceptor Y6 and electron donors PTPD and PBiTPD. Reproduced with permission.[98] Copyright 2020, American Chemical Society.
As a derivative of TPD, Bi-thieno[3,4-c]pyrrole-4,6-dione (bi-TDP) has a larger planar backbone and much stronger electron-withdrawing ability, which make it a promising electron-deficient units for polymer semiconductors.[28,39,46,99,100] Its application in polymer donors was also investigated. The chemical structures of biTPD-based polymer donors are illustrated in Figure 7 and the photovoltaic properties are summarized in Table 3. Huang and coworkers synthesized two polymer donors PBiTPD and PTPD.[98] The biTPD-based polymer PBiTPD has a larger ionization potential (IP) value (ca. 5.20 eV) than PTPD (ca. 5.05 eV), which leads to a higher V OC values of the donor polymer PBiTPD in BHJ solar cells (Figure 6a). The first-level electron affinity (EA) values were 3.19 eV for PTPD, 3.45 eV for PBiTPD, and 4.30 eV for Y6 (Figure 6b), respectively. In addition, the GIWAXS results indicate that PBiTPD and PBiTPD: Y6 BHJ blend films exhibited more favorable face-on backbone orientation and stronger crystallinity. Therefore, the PBiTPD:Y6-based OSCs exhibited a PCE of 14.2 % that is much higher than 5.9 % for PTPD:Y6-based devices.