Interfacial electronic structure of Cl₆SubPc non-fullerene acceptors in organic photovoltaics using soft X-ray spectroscopies

Abstract

In organic photovoltaics (OPVs), determining the energy-level alignment of a donor and an acceptor is particularly important since the interfacial energy gap between the highest occupied molecular orbital (HOMO) level of a donor and the lowest unoccupied molecular orbital (LUMO) level of an acceptor (E-E) gives the theoretical maximum value of the open-circuit voltage (V). To increase the E-E, non-fullerene acceptors, which have a lower electron affinity (EA) than C, are receiving increasing attention. In this study, we investigated the energy-level alignment at the interface of a boron chloride subphthalocyanine (SubPc) donor and a halogenated SubPc (ClSubPc) acceptor using soft X-ray spectroscopy techniques. The estimated E-E of ClSubPc/SubPc was 1.95 eV, which was significantly higher than that of 1.51 eV found at the interface of C/SubPc. This increased E-E was the origin of the enhanced V in OPVs. Additionally, we studied the molecular orientation of ClSubPc using angle-dependent X-ray absorption spectroscopy. The highly disordered ClSubPc molecules result in low carrier mobility, which contributes to the lower short-circuit current density of the ClSubPc acceptor OPVs than the C acceptor OPVs.

Hexachlorinated Boron(III) Subphthalocyanine as Acceptor for Organic Photovoltaics: A Brief Overview

“…[30], E G for the SubPc/Cl 6 SubPc pair was calculated to be equal to 1.83 eV, i.e., the value did not coincide with that from the Ref. [21]. Importantly, the optical bandgap E opt is smaller than the fundamental bandgap (the difference between the ionization potential and the electron affinity of a molecule) by the value of the electron-hole pair binding energy.…”

“…As seen from Table 1, the HOMO-LUMO values obtained by various methods/ research groups fluctuate within the range of ±0.2 eV. Moreover, different numbers can be reported in the articles by the same authors ( [20] vs. [28] or [21] vs. [22]). Such discrepancies are not surprising, considering the diversity of approaches/ methods used for estimation [36].…”

“…After the introduction of electron-withdrawing chlorine atoms on the periphery of a macrocycle, the Cl 6 SubPc molecule behaves as the electron acceptor with respect to the molecule on the other side of the heterointerface (e.g., unsubstituted SubPc [6,[18][19][20][21][22]), thereby participating in the separation of the photogenerated charges. In the bulk phase, addition of chlorine atoms to the conjugated macrocycle assists in getting the electron transporting organic semiconductors [7].…”

“…The energy level alignment at SubPc/Cl 6 SubPc heterojunction was recently discussed in detail [21] with the emphasis on the interface gap E G = E Donor HOMO -E Acceptor LUMO that determines the maximum achievable open-circuit voltage in a photovoltaic device with a D/A junction. The interface gap E G of 1.95 eV was derived from soft XPS and XAS, with the use of DFT ( Table 1) [21].…”

“…The energy level alignment at SubPc/Cl 6 SubPc heterojunction was recently discussed in detail [21] with the emphasis on the interface gap E G = E Donor HOMO -E Acceptor LUMO that determines the maximum achievable open-circuit voltage in a photovoltaic device with a D/A junction. The interface gap E G of 1.95 eV was derived from soft XPS and XAS, with the use of DFT ( Table 1) [21]. This value of E G is much greater than what is attainable through pairing the SubPc donor with the conventional N-type acceptor C 60 , which explains the popularity of Cl 6 SubPc among the so-called "non-fullerene" acceptors for organic photovoltaics [5,16,17,21,25,27,30,32,33].…”