Charge Carrier Dynamics in Non-Fullerene Acceptor-Based Organic Solar Cells: Investigating the Influence of Processing Additives Using Transient Absorption Spectroscopy

Abstract

In this study, we present a comprehensive investigation into the charge generation mechanism in bulk-heterojunction organic solar cells employing non-fullerene acceptors (NFAs) both with and without the presence of processing additives. While photovoltaic devices based on Y6 or BTP-eC9 have shown remarkable power conversion efficiencies, the underlying charge generation mechanism in polymer:NFA blends remains poorly understood. To shed light on this, we employ transient absorption (TA) spectroscopy to elucidate the charge transfer pathway within a blend of the donor polymer PM6 and NFAs. Interestingly, the charge carrier lifetimes of neat Y6 and BTP-eC9 are comparable, both reaching up to 20 ns. However, the PM6:BTP-eC9 blend exhibits substantially higher charge carrier generation and a longer carrier lifetime compared to PM6:Y6 blend films, leading to superior performance. By comparing TA data obtained from PM6:Y6 or PM6:BTP-eC9 blend films with and without processing additives, we observe significantly enhanced charge carrier generation and prolonged charge carrier lifetimes in the presence of these additives. These findings underscore the potential of manipulating excited species as a promising avenue for further enhancing the performance of organic solar cells. Moreover, this understanding contributes to the advancement of NFA-based systems and the optimization of charge transfer processes in polymer:NFA blends.

Keywords: charge carrier dynamics; non-fullerene acceptor; organic solar cell; processing additive; transient absorption spectroscopy.

Figure 1

The molecular structures of (a) Y6 and (b) BTP-eC9 as electron acceptors, and (c) PM6 as an electron donor; (d) J–V characteristics of PM6:Y6 and PM6:BTP-eC9 solar cells (inset: a device architecture).

Figure 2

UV-Vis absorption spectra of (a) PM6:Y6 and (b) PM6:BTP-eC9 blend films with and without additives including PM6 and Y6 neat films. Photoluminescence spectra of (c) PM6:Y6 and (d) PM6:BTP-eC9 blend films with and without additives excited at 600 nm, respectively.

Figure 3

(a) Schematic representation of the setup for nanosecond transient absorption spectroscopy and (b) general conceptual model for understanding charge carrier dynamics in organic solar cells. Energy level diagram illustrating the formation of multiple electronic CT states following CT from excited state S₁. All CT states either dissociate to charge-separated (CS) states or decay to the ground state S₀ (CT recombination). Separated charges may non-geminately recombine to the CT state. The presence of low-energy triplet states at the donor or acceptor enables triplet (T₁) back energy transfer from the CT state.

Figure 4

Transient absorption spectra of neat films of (a) PM6; (b) Y6; (c) BTP-eC9 probed at the visible region, 400–900 nm.

Figure 5

Transient absorption spectra of (a,b) PM6:Y6 and (c,d) PM6:BTP-eC9 films with and without additives probed at the visible region, 400–900 nm.

Figure 6

Transient absorption spectra of (a,b) PM6:Y6 and (c,d) PM6:BTP-eC9 films including neat films with and without additives, probed at 650 nm for (a,c) and at 850 nm for (b,d), respectively.

Figure 7

Transient absorption spectra of (a,b) PM6:Y6 and (c,d) PM6:BTP-eC9 films including neat films with and without additives probed at 550 nm for (a,c) and at 775 nm for (b,d), respectively.