Unifying Charge Generation, Recombination, and Extraction in Low‐Offset Non‐Fullerene Acceptor Organic Solar Cells

Abstract

Even though significant breakthroughs with over 17% power conversion efficiencies (PCEs) in polymer:non-fullerene acceptor (NFA) bulk heterojunction organic solar cells (OSCs) have been achieved, not many studies have focused on acquiring a comprehensive understanding of the underlying mechanisms governing these systems. This is because it can be challenging to delineate device photophysics in polymer:NFA blends comprehensively, and even more complicated to trace the origins of the differences in device photophysics to the subtle differences in energetics and morphology. Here, a systematic study of a series of polymer:NFA blends was conducted to unify and correlate the cumulative effects of i) voltage losses ii) charge generation efficiencies, iii) nongeminate recombination and extraction dynamics, and iv) nuanced morphological differences with device performances. Most importantly, a deconvolution of the major loss processes in polymer:NFA blends and their connections to the complex BHJ morphology and energetics were established. An extension to advanced morphological techniques, such as solid-state NMR (for atomic level insights on the local ordering and donor:acceptor π-π interactions) and resonant soft x-ray scattering (for donor and acceptor interfacial area and domain spacings), provided detailed insights on how efficient charge generation, transport, and extraction processes can outweigh increased voltage losses to yield high PCEs.

Effect of Palladium‐Tetrakis(Triphenylphosphine) Catalyst Traces on Charge Recombination and Extraction in Non‐Fullerene‐based Organic Solar Cells

“…The energy levels of PTB7‐Th and IOTIC‐4F were taken from the literature. [ 54,55 ] The resulting energy band diagram of the PTB7‐Th, IOTIC‐4F, and Pd(PPh 3 ) 4 system is displayed in Figure 1b.…”

The Path to 20% Power Conversion Efficiencies in Nonfullerene Acceptor Organic Solar Cells

“…For efficient device operation, charge extraction must out-compete recombination processes, otherwise the J sc and FF of the OSC device will be negatively impacted; several studies have shown that if the time required for charges to be extracted (τ ex ) is faster than the time taken for charges to recombine (τ rec ), then excess losses due to nongeminate recombination can be mitigated. [10,[27][28][29][30][31] Therefore, understanding the PCE of an OSC entails accurately characterizing each of the charge generation, recombination, and extraction processes taking place following photoexcitation of the blend. From this discussion, it is clear that all of these major device photophysical processes are interlinked with the BHJ morphology.…”

“…[10,[63][64][65][66][67][68][69] The 1 H, 13 C, and 19 F chemical shifts and line-shapes are affected by local bonding environments, noncovalent interactions, and molecular conformations in π-conjugated materials. [29,[70][71][72][73] As a result, information about close molecular interactions and dipole-dipole coupled 1 H-1 H and 1 H-X pairs (where, X = 13 C, 15 N, and 19 F) at sub-nm to nm distances can be acquired. In addition, NMR techniques conducted in both in situ and ex situ conditions have been used to understand the phase transitions, crystallization, melting, and solid-to-solution transformations.…”

“…Due to its sensitivity to the local environments of nuclear spins, ssNMR has also been beneficial in characterizing ordered and disordered materials with molecular level resolution; as a result, this technique has been known to overcome the limitations faced by X-ray diffraction based methods. [29,77] However, characterizing materials containing multiple ordered and disordered domains can be challenging because NMR signals from different domains can overlap leading to broad spectra which can be difficult to analyze. For example, in polymers, statistical distributions of the local environments of chemically equivalent nuclei (e.g., 1 H atoms in polymer backbones that experience variations in local π-π packing) develop as broad chemical shift distributions.…”

The Role of Carbon Quantum Dots in Organic Photovoltaics: A Short Overview

“…By the way, a brief comparison with the recent advances in OSCs and DSSCs could be done in order to check the real advantages reprentented by the employment of CDs. In the case of OSCs, recently developed smallmolecules, known as non-fullerene acceptors (NFAs), have proven to be a better fullerene alternative than CDs in BHJ OSCs, reaching device PCEs of 17% for both a single junction and tandem cells [122,123] (ca. one order of magnitude higher than the ones reported for the cells with CDs).…”