The Importance of Quantifying the Composition of the Amorphous Intermixed Phase in Organic Solar Cells

Abstract

The relation of phase morphology and solid‐state microstructure with organic photovoltaic (OPV) device performance has intensely been investigated over the last twenty years. While it has been established that a combination of donor:acceptor intermixing and presence of relatively phase‐pure donor and acceptor domains is needed to get an optimum compromise between charge generation and charge transport/charge extraction, a quantitative picture of how much intermixing is needed is still lacking. This is mainly due to the difficulty in quantitatively analyzing the intermixed phase, which generally is amorphous. Here, fast scanning calorimetry, which allows measurement of device‐relevant thin films (<200 nm thickness), is exploited to deduce the precise composition of the intermixed phase in bulk‐heterojunction structures. The power of fast scanning calorimetry is illustrated by considering two polymer:fullerene model systems. Somewhat surprisingly, it is found that a relatively small fraction (<15 wt%) of an acceptor in the intermixed amorphous phase leads to efficient charge generation. In contrast, charge transport can only be sustained in blends with a significant amount of the acceptor in the intermixed phase (in this case: ≈58 wt%). This example shows that fast scanning calorimetry is an important tool for establishing a complete compositional characterization of organic bulk heterojunctions. Hence, it will be critical in advancing quantitative morphology–function models that allow for the rational design of these devices, and in delivering insights in, for example, solar cell degradation mechanisms via phase separation, especially for more complex high‐performing systems such as nonfullerene acceptor:polymer bulk heterojunctions.

A High‐Performance Nonfused Wide‐Bandgap Acceptor for Versatile Photovoltaic Applications

“…The well‐developed morphology is beneficial for the charge transport and thus suppressing the bimolecular charge recombination. [ 49,50 ] The miscibility between the donor and nonfused NFAs are further investigated via contact angle measurement. The contact angle images are shown in Figure S14, Supporting Information, and the detailed calculation processes are demonstrated in Supporting Information.…”

A History and Perspective of Non‐Fullerene Electron Acceptors for Organic Solar Cells

“…[99] Similarly, if domains are so large that excitons could not diffuse to the discrete D/A interfaces, mixed domains with distributed D/A contacts greatly help with exciton quenching and charge generation. [100,101] It is thus likely that there are a number of slightly different morphologies that could provide excellent and comparable performance, i.e., there is no single ideal BHJ morphology.…”

Comparing the microstructure and photovoltaic performance of 3 perylene imide acceptors with similar energy levels but different packing tendencies

“…More specifically, our data shows that PIPB has a stronger tendency to pack into ordered structures when blended with PBDBT, which likely results in a more phase separated morphology that encompasses isolated, nonpercolated microislands that penalize charge transport and, hence, the device efficiency. 57 The results summarized above have focused on films and OSC devices prepared from solutions of the components in CF : CB 3 : 1 VR mixtures. In order to investigate the origin of the relatively low efficiencies, we studied the effect of the solvent on the performance of the systems under consideration.…”