Re-Examining Open-Circuit Voltage in Dilute-Donor Organic Photovoltaics

Abstract

Dilute-donor (DD) organic photovoltaic (OPV) devices comprise a low concentration of donor molecules in an acceptor matrix. The open-circuit voltage (V oc) of these devices is commonly higher than their bulk heterojunction (BHJ) counterparts and has been attributed to Schottky barrier heights between the anode and the acceptor matrix or reduced bimolecular recombination due to smaller donor/acceptor interfacial areas. Here, we examine the V oc of a variety of small-molecule and polymer donors, all at 5 wt %, in both fullerene and non-fullerene acceptors by performing photovoltaic (PV) and electroluminescence (EL) measurements on the same devices. We find a substantial V oc variation for different donors in the same acceptor matrix, indicating that DD V oc cannot be adequately explained by the Schottky barrier or interfacial area. V oc values of fullerene DD devices vary linearly with the band gap value as determined from the intercept of PV external quantum efficiency and EL spectra. In contrast, the non-fullerene acceptor (NFA) DD devices show V oc variation with donors despite having the same band gap. These results show that V oc of DD OPVs is predominantly determined by non-radiative voltage loss, similar to BHJ OPVs. NFA-based DD devices show promises for low voltage loss and high charge generation.

Dilute Donor Organic Solar Cells Based on Non-fullerene Acceptors

“…However, perhaps what is most remarkable is that the low donor content devices still exhibit good efficiency. Indeed, compared with fullerene OPV devices with comparably low donor content, the performance of these non-fullerene OPV devices represents a significant improvement to the ≈3% PCE typically reported when taking into account differences in molecular mass. , We also compared the performance of blend devices with neat Y6 and ITIC-4F active layers with otherwise identical architecture and processing parameters. The Y6 homojunctions exhibited a markedly lower J SC than devices with even the lowest concentration of PM6 along with a FF approximately half that of the 1:10 PM6 : Y6 blend ratio devices.…”

“…The general approach to achieve the most efficient BHJ-based devices is to have similar concentrations of the donor and acceptor materials in a binary blend. However, there have been several reports of an alternative device architecture that can still give reasonable efficiencies in which the donor component is at a much lower concentration in the film relative to the acceptor. − The original and indeed almost all reports related to these so-called low donor content or “homeopathic” devices utilize a fullerene as the electron acceptor and therefore the primary component of the blend. Given the low donor content in the blend, there has been considerable debate as to how free charges are generated and extracted ,,− since hole mobility in fullerenes can be strongly affected by trapping and in some cases below measurable limits. , Now that NFA OPV devices are more efficient than fullerene-based BHJ solar cells, we were interested to see whether low donor content NFA devices with higher efficiencies than those containing fullerenes could be achieved.…”

Contributions of Charge Generation Versus Transport to Photocurrent in Dilute-Donor Organic Solar Cells with Non-fullerene Acceptors

“…Instead, we aim to understand the mechanism(s) of photocurrent generation at extremely low donor concentrations. In addition, the different V oc values are determined by the acceptor alone according to radiative and non-radiative recombination …”