Electric Field Facilitating Hole Transfer in Non-Fullerene Organic Solar Cells with a Negative HOMO Offset

Abstract

The record high photoinduced current and power conversion efficiencies of organic solar cells (OSCs) should be attributed to the significant contribution of non-fullerene electron acceptors via hole transfer to electron donors and/or a pronounced decrease in energy losses for exciton dissociation by aligned highest occupied molecular orbitals (HOMOs) or lowest unoccupied molecular orbitals (LUMOs). However, the hole transfer mechanism in those highly efficient non-fullerene OSCs with small HOMO offsets has not been extensively studied and fully understood, yet. Herein, we comparatively study the hole transfer kinetics in two OSCs with a positive (0.05 eV) and a negative (−0.07 eV) HOMO offset (ΔHOMO) based on polymer donor PTQ10 paired with non-fullerene acceptors ZITI-C or ZITI-N. Short-circuit current densities (J sc ) of 20.42 and 12.81 mA cm −2 are achieved in the OSCs based on PTQ10:ZITI-C (ΔHOMO = 0.05 eV) and PTQ10:ZITI-N (ΔHOMO = −0.07 eV) with an optimized donor (D):acceptor (A) ratio of 1:1, respectively, despite the small and even negative ΔHOMO. Results from time-resolved transient absorption spectroscopy show slower hole transfer (14.3 ps) in PTQ10:ZITI-N than that (3.7 ps) in PTQ10:ZITI-C. To understand the decent J sc value in the OSCs of PTQ10:ZITI-N, the temperature and electric field dependences of hole transfer are investigated in low-donor-content OSCs (D:A ratio of 1:9) in which photocurrent is dominated by the contribution via hole transfer from ZITI-N to PTQ10. Devices based on PTQ10:ZITI-C and PTQ10:ZITI-N show similar free charge generation behavior as a function of temperature, whereas the external quantum efficiencies of the PTQ10:ZITI-N device exhibit a much stronger bias dependence than that of PTQ10:ZITI-C, which suggests that the electric field facilitates exciton dissociation in PTQ10:ZITI-N where the energetic driving force alone cannot efficiently dissociate excitons.

Origin of Hole Transport in Small Molecule Dilute Donor Solar Cells

“…Still, significant current densities are observed. Similar observations have been reported for dilute donor OSCs based on nonfullerene acceptors, [ 42 ] which are beyond the scope of this work. Increasing the HOMO offset increases the driving force for exciton dissociation, so the current increases as more excitons dissociate.…”

On the energy gap determination of organic optoelectronic materials: the case of porphyrin derivatives

“…To cite an example, the energy offsets play a key role in the charge separation in OPVs. [34][35][36][37] Since the advent of the new electron acceptors, commonly called non-fullerene acceptor (NFA), an intense debate is taking place regarding the need (or not) of an IP offset between the donor and acceptor materials for the charge transfer process when the acceptor is excited by the absorption of a photon. [37][38][39][40][41] Additionally, a simplistic comparison of energy levels measured for the isolated materials cannot fully describe the charge transfer process.…”

“…[34][35][36][37] Since the advent of the new electron acceptors, commonly called non-fullerene acceptor (NFA), an intense debate is taking place regarding the need (or not) of an IP offset between the donor and acceptor materials for the charge transfer process when the acceptor is excited by the absorption of a photon. [37][38][39][40][41] Additionally, a simplistic comparison of energy levels measured for the isolated materials cannot fully describe the charge transfer process. 42 Concerning the energy gap, different techniques are used to measure it both as a direct measurement, such as UV-Vis absorption, or as an indirect measurement, such as cyclic voltammetry (CV).…”

Studying Morphology Formation and Charge Separation in Organic Solar Cells

“…Consequently, the introduction of BHJ solar cells once led to drastic improvements in device performance, in comparison to the aforementioned Schottky junction and bilayer heterojunction devices. 60 Copyright 2020. American Chemical Society.…”

“…A sketch of (a) an optimized BHJ, and (b) a dilute BHJ where the donors are well-separated by the excessive acceptors. Adapted with permission 60. Copyright 2020.…”