Energy Level Tuning of Non-Fullerene Acceptors in Organic Solar Cells

Abstract

The use of non-fullerene acceptors in organic photovoltaic (OPV) devices could lead to enhanced efficiencies due to increased open-circuit voltage (VOC) and improved absorption of solar light. Here we systematically investigate planar heterojunction devices comprising peripherally substituted subphthalocyanines as acceptors and correlate the device performance with the heterojunction energetics. As a result of a balance between VOC and the photocurrent, tuning of the interface energy gap is necessary to optimize the power conversion efficiency in these devices. In addition, we explore the role of the charge transport layers in the device architecture. It is found that non-fullerene acceptors require adjusted buffer layers with aligned electron transport levels to enable efficient charge extraction, while the insertion of an exciton-blocking layer at the anode interface further boosts photocurrent generation. These adjustments result in a planar-heterojunction OPV device with an efficiency of 6.9% and a VOC above 1 V.

Thiadiazole Fused Subporphyrazines as Acceptors in Organic Photovoltaic Cells

“…The rectification in the darkness is usually poor, most likely due to compensating (unwanted) contribution of the interactive 'acceptor/Al' junction. [1][2][3]18,19] Under illumination of 100 mW/cm 2 , a photovoltaic effect arises (Figure 2(a-d)), the corresponding parameters are summarized in Table 1. Significant photovoltage generated by the molecular heterojunction can be expected from the large difference between HOMO energy of donor (SubPcH 12 ) and LUMO energy of acceptor (see Figure 1).…”

“…These values are obtained from the B3LYP DFT calculations (pcseg-2 [20] basis sets) and should be treated with care when comparing to other sources. [1,6,21] They do represent only the relative positions of the energy levels in the series of subporphyrinoids used in this work. The highest U oc value in this series was measured on the cells containing SubPcF 12 as acceptor, but the best performing cell contains SubPzS 1 F 8 .…”

“…Fill factors in such cells typically suffer from the high parasitic resistances, basically serial resistance R s , and from the limited charge extraction at the cathode [1] (see, e.g. Figure 2(f)).…”

“…[13,14] The preparation of the archetypal photovoltaic cell was similar to that previously reported. [18,19] The cell scheme was 'glass/ ITO/MoO x /SubPcH 12 /acceptor/Al', where MoO x is a 10 nm substoichiometric molybdenum trioxide layer (buffer/workfunction modifier, [1,[5][6][7][8]19] see Figure S1), acceptor = SubPzS 3 F 0 , SubPzS 2 F 4 , SubPzS 1 F 8 , SubPcF 12 (or C 60 , see text), and Al is an aluminum top cathode (40 nm). The thickness of a p-type layer (SubPcH 12 ) was 20 nm, the thickness of an n-type (acceptor) layer was 30 nm.…”

“…Naturally, the ultimate stage of such a replacement leads, using the given abbreviation format, to SubPzS 0 F 12 , i.e., the well-known chloro[dodecafluorosub phthalocyaninato]boron(III), frequently termed as perfluorinated-SubPc. Following the existing tradition [1,2,5,6,15,16] this compound is denoted as SubPcF 12 in Chart 1.…”