Reducing Voltage Losses in Cascade Organic Solar Cells while Maintaining High External Quantum Efficiencies

Abstract

High photon energy losses limit the open‐circuit voltage (VOC) and power conversion efficiency of organic solar cells (OSCs). In this work, an optimization route is presented which increases the VOC by reducing the interfacial area between donor (D) and acceptor (A). This optimization route concerns a cascade device architecture in which the introduction of discontinuous interlayers between alpha‐sexithiophene (α‐6T) (D) and chloroboron subnaphthalocyanine (SubNc) (A) increases the VOC of an α‐6T/SubNc/SubPc fullerene‐free cascade OSC from 0.98 V to 1.16 V. This increase of 0.18 V is attributed solely to the suppression of nonradiative recombination at the D–A interface. By accurately measuring the optical gap (Eopt) and the energy of the charge‐transfer state (ECT) of the studied OSC, a detailed analysis of the overall voltage losses is performed. Eopt – qVOC losses of 0.58 eV, which are among the lowest observed for OSCs, are obtained. Most importantly, for the VOC‐optimized devices, the low‐energy (700 nm) external quantum efficiency (EQE) peak remains high at 79%, despite a minimal driving force for charge separation of less than 10 meV. This work shows that low‐voltage losses can be combined with a high EQE in organic photovoltaic devices.

Boron Subphthalocyanines and Silicon Phthalocyanines for Use as Active Materials in Organic Photovoltaics

“…inserted rubrene as an interlayer between the alpha‐sexithiophene (α‐6T)/Cl‐Cl n BsubNc layers in an α‐6T/Cl‐Cl n BsubNc/Cl‐BsubPc device developed by Cnops et al . (discussed in next section) . The insertion did successfully achieve their goal of improving the V OC , and it also slightly increases the BsubNc contribution to photocurrent.…”

“…In 2014, Cnops et al . demonstrated the validity and high performance of a three‐layer energy cascade device architecture (Figure ), which became the basis of several follow‐up studies . Cl‐Cl n BsubNc and Cl‐BsubPc were applied as dual electron acceptors paired with α‐6T, whereby exciton dissociation follows a two‐step process.…”

“…In a unique approach to applying BsubPc as an interlayer in a charge cascade OPV, Nikolis et al . inserted varying thicknesses of PhO‐BsubPc at the α‐6T/Cl‐Cl n BsubNc interface of the α‐6T/Cl‐Cl n BsubNc/Cl‐BsubPc OPV developed by Cnops et al . Slight increases in performance were observed, but only for the 2 nm layer of PhO‐BsubPc.…”

“…A subsequent study aimed to reduce losses in V OC by introducing an interlayer at the α‐6T/Cl‐Cl n BsubNc interface . Interlayer materials that were investigated consisted of DIP, phenoxy BsubPc (PhO‐BsubPc), and the previously mentioned rubrene.…”

Effects of the Number of Bromine Substitution on Photovoltaic Efficiency and Energy Loss of Benzo[1,2‐b:4,5‐b′]diselenophene‐based Narrow‐Bandgap Multibrominated Nonfullerene Acceptors

“…[24][25][26][27][28][29][30][31] Moreover, compared to the fullerene-based devices, NF-SMAs based PSCs generally exhibit lower energy loss (E loss ) owing to the weak driving force for charge separation, leading to more balanced J sc and V oc and high PCE. [32][33][34][35][36] Recently, the PCE of NF-SMAs based binary PSCs has reached more than 14% with proper pair of polymer donors/NF-SMAs and device optimizaion. [37,38] Among various types of NF-SMAs, planar and linear acceptor-donor-acceptor (A-D-A) type NF-SMAs have aroused great interest and became the most successful design strategy for high performance NF-SMAs.…”