Exponential Optical Absorption Edge of PM6, Y6, and PM6:Y6 Bulk Heterojunction in Organic Photovoltaic Materials Revealed by Photoacoustic and Absorbance Spectroscopies

Organic photovoltaics (OPVs) have attracted much attention, and they have seen rapid recent progress, achieving high photovoltaic conversion efficiencies (PCEs) of 20% using the combination of the donor polymer PM6 and the nonfullerene acceptor Y6 in a bulk heterojunction structure (PM6:Y6 BHJ). However, in spite of the excellent performance of PM6:Y6 BHJ OPVs, the reported open-circuit voltage (Voc) is still low, and the Voc loss is observed to depend on charge-carrier nonradiative recombination within the absorber of the OPVs. One potential source for the nonradiative recombination is sub-band gap optical absorption (Urbach tail). The purpose of this study is to focus specific attention on the Urbach tail regions in a PM6 single junction (SJ), Y6 SJ, and PM6:Y6 BHJ on different substrate surfaces (i.e., glass and ITO) by applying photoacoustic (PA) along with absorbance (Abs) spectroscopies to consider the affected electronic structures and nonradiative recombination. Our analysis suggests that there are discrepancies between the PA and Abs spectra in the Urbach tail regions in PM6 SJ, Y6 SJ, and PM6:Y6 BHJ on both glass and ITO substrates, indicating a difference in the efficiency for heat generation through nonradiative recombination, particularly in the case of PM6:Y6 BHJ. Our results suggest that the discrepancy relates to the PA theory.