Resolving Atomic‐Scale Interactions in Nonfullerene Acceptor Organic Solar Cells with Solid‐State NMR Spectroscopy, Crystallographic Modelling, and Molecular Dynamics Simulations

Abstract

Fused‐ring core nonfullerene acceptors (NFAs), designated “Y‐series,” have enabled high‐performance organic solar cells (OSCs) achieving over 18% power conversion efficiency (PCE). Since the introduction of these NFAs, much effort has been expended to understand the reasons for their exceptional performance. While several studies have identified key optoelectronic properties that govern high PCEs, little is known about the molecular level origins of large variations in performance, spanning from 5% to 18% PCE, for example, in the case of PM6:Y6 OSCs. Here, a combined solid‐state NMR, crystallography, and molecular modeling approach to elucidate the atomic‐scale interactions in Y6 crystals, thin films, and PM6:Y6 bulk heterojunction (BHJ) blends is introduced. It is shown that the Y6 morphologies in BHJ blends are not governed by the morphology in neat films or single crystals. Notably, PM6:Y6 blends processed from different solvents self‐assemble into different structures and morphologies, whereby the relative orientations of the sidechains and end groups of the Y6 molecules to their fused‐ring cores play a crucial role in determining the resulting morphology and overall performance of the solar cells. The molecular‐level understanding of BHJs enabled by this approach will guide the engineering of next‐generation NFAs for stable and efficient OSCs.

Side Chain Length and Interaction Mediated Charge Transport Networks of Non-Fullerene Acceptors for Efficient Organic Solar Cells

“…Molecular dynamics simulations (MDs) were conducted to estimate how the varied alkyl chains would affect the planarity and intermolecular stacking of among these NFAs. − As shown in Figure S2, BTP-4Cl-C9-12, BTP-4Cl-C9-16, and BTP-4Cl-C9-20 exhibit dihedral angles of 4.9° (11.9°), 5.6° (12.0°), and 13.1° (14.4°), suggesting that elongating the alkyl chain length of NFAs could reduce their molecular planarity. The stacking and aggregation of these NFAs were further elucidated using the Polymorph module in Materials Studio .…”

Low Voltage‐Loss Organic Solar Cells Light the Way for Efficient Semitransparent Photovoltaics

“…The SSNMR results indeed establish the molecular origins for the different optoelectronic properties, which are due to the different degrees of D-A intermix and local domain separation for the PM2:ITIC-Th and PM2: PC 61 BM blends, consistently with the recent SSNMR studies of organic semiconductors and BHJ blends. [45,[65][66][67] Next, the relative degrees of energetic order of the CT states in the blends were analyzed. Previous work has suggested that many systems which obtain high photovoltaic performance despite low energetic offsets achieve this via highly ordered domains, which facilitate both charge generation and transport.…”

“…The SSNMR results indeed establish the molecular origins for the different optoelectronic properties, which are due to the different degrees of D‐A intermix and local domain separation for the PM2:ITIC‐Th and PM2:PC 61 BM blends, consistently with the recent SSNMR studies of organic semiconductors and BHJ blends. [ 45,65–67 ]…”

A Theoretical Perspective on the Thermodynamic Stability of Polymer Blends for Solar Cells: From Experiments to Predictive Modeling

“…Surface and bulk characterization techniques, as well as theoretical simulations, have been employed to understand how phase separation and other morphological features affect the PCE in these blends. [16][17][18][19][20][21][22][23] Despite the great progress of organic solar cells, the photovoltaic market is still dominated by crystalline silicon (c-Si) technologies due to the material availability, its excellent electronic charge transport properties, and PCEs >24% for large module areas >1 m 2 . Moreover, the lifetime of c-Si solar cells typically exceeds 25 years [24,25] : thermal and photochemical stability of inorganic semiconductors is in fact generally higher than that of organic counterparts.…”

“…Surface and bulk characterization techniques, as well as theoretical simulations, have been employed to understand how phase separation and other morphological features affect the PCE in these blends. [ 16–23 ]…”