Regioisomeric Non-Fullerene Acceptors Containing Fluorobenzo[c][1,2,5]thiadiazole Unit for Polymer Solar Cells

Influence of the –CN substitution position on the performance of dicyanodistyrylbenzene-based polymer solar cells

We developed four novel copolymers based on DCB units with differently positioned –CN groups and investigated their effects on the film morphology and performance of non-fullerene polymer solar cells.

TD-DFT benchmark for UV-visible spectra of fused-ring electron acceptors using global and range-separated hybrids

The accuracy of Time-Dependent Density Functional Theory in predicting the vertical absorption wavelength of 50 widely-used fused-ring electron acceptors (FREAs) has been investigated by considering the solvent effects.

Recent Progress in Molecular Design of Fused Ring Electron Acceptors for Organic Solar Cells

The quest for sustainable energy sources has led to accelerated growth in research of organic solar cells (OSCs). A solution-processed bulk-heterojunction (BHJ) OSC generally contains a donor and expensive fullerene acceptors (FAs). The last 20 years have been devoted by the OSC community to developing donor materials, specifically low bandgap polymers, to complement FAs in BHJs. The current improvement from ≈2.5% in 2013 to 17.3% in 2018 in OSC performance is primarily credited to novel nonfullerene acceptors (NFA), especially fused ring electron acceptors (FREAs). FREAs offer unique advantages over FAs, like broad absorption of solar radiation, and they can be extensively chemically manipulated to tune optoelectronic and morphological properties. Herein, the current status in FREA-based OSCs is summarized, such as design strategies for both wide and narrow bandgap FREAs for BHJ, all-small-molecule OSCs, semi-transparent OSC, ternary, and tandem solar cells. The photovoltaics parameters for FREAs are summarized and discussed. The focus is on the various FREA structures and their role in optical and morphological tuning. Besides, the advantages and drawbacks of both FAs and NFAs are discussed. Finally, an outlook in the field of FREA-OSCs for future material design and challenges ahead is provided.

Recent Progress in Fused-Ring Based Nonfullerene Acceptors for Polymer Solar Cells

The progress of bulk-heterojunction (BHJ) polymer solar cells (PSCs) is closely related to the innovation of photoactive materials (donor and acceptor materials), interface engineering, and device optimization. Especially, the development of the photoactive materials dominates the research filed in the past decades. Photoactive materials are basically classified as p-type organic semiconductor donor (D) and an n-type organic semiconductor acceptor (A). In the past two decades, fullerene derivatives are the dominant acceptors for high efficiency PSCs. Nevertheless, the limited absorption and challenging structural tunability of fullerenes hinder further improve the efficiency of PSCs. Encouragingly, the recent progresses of fused-ring based A-D-A type nonfullerene acceptors exhibit great potential in enhancing the photovoltaic performance of devices, driving the power conversion efficiency to over 13%. Such kind of nonfullerene acceptors is usually based on indacenodithiophene (IDT) or its extending backbone core and end-caped with strong electron-withdrawing group. Owing to the strong push-pulling effects, the acceptors possess strong absorption in the visible-NIR region and low-lying HOMO (highest occupied molecular orbital) level, which can realize both high open-circuit voltage and short-circuit current density of the devices. Moreover, the photo-electronic and aggregative properties of the acceptors can be flexibly manipulated via structural design. Many strategies have been successfully employed to tune the energy levels, absorption features, and aggregation properties of the fused-ring based acceptors. In this review, we will summarize the recent progress in developing highly efficient fused-ring based nonfullerene acceptors. We will mainly focus our discussion on the correlating factors of molecular structures to their absorption, molecular energy levels, and photovoltaic performance. It is envisioned that an analysis of the relationship between molecular structures and photovoltaic properties would contribute to a better understanding of this kind of acceptors for high-efficiency PSCs.

Small-Molecule Electron Acceptors for Efficient Non-fullerene Organic Solar Cells

The development of organic electron acceptor materials is one of the key factors for realizing high performance organic solar cells. Compared to traditional fullerene acceptor materials, non-fullerene electron acceptors have attracted much attention due to their better optoelectronic tunabilities and lower cost as well as higher stability. Non-fullerene organic solar cells have recently experienced a rapid increase with power conversion efficiency of single-junction devices over 14% and a bit higher than 15% for tandem solar cells. In this review, two types of promising small-molecule electron acceptors are discussed: perylene diimide based acceptors and acceptor(A)-donor(D)-acceptor(A) fused-ring electron acceptors, focusing on the effects of structural modification on absorption, energy levels, aggregation and performances. We strongly believe that further development of non-fullerene electron acceptors will hold bright future for organic solar cells.

Nonfullerene/Fullerene Acceptor Blend with a Tunable Energy State for High-Performance Ternary Organic Solar Cells

Ternary blending is an effective strategy for broadening the absorption range of the active layer in bulk heterojunction polymer solar cells and for constructing an efficient cascade energy landscape at the donor/acceptor interface to achieve high efficiencies. In this study, we report efficient ternary blend solar cells containing an acceptor alloy consisting of the indacenodithiophene-based nonfullerene material, IDT2BR, and the fullerene material, phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM). The IDT2BR materials mix fully with PC₇₁BM materials, and the energy state of this phase can be tuned by varying the blending ratio. We performed photoluminescence and external quantum efficiency studies and found that the ternary charge cascade structure efficiently transfers the photogenerated charges from the polymer to IDT2BR and finally to PC₇₁BM materials. Ternary blend devices containing the IDT2BR:PC₇₁BM acceptor blend and various types of donor polymers were found to exhibit power conversion efficiencies (PCEs) improved by more than 10% over the PCEs of the binary blend devices.

Nonfullerene Acceptor with "Donor-Acceptor Combined π-Bridge" for Organic Photovoltaics with Large Open-Circuit Voltage

In this work, a kind of "donor-acceptor (D-A) combined π-bridge" based on the regioselective reactivity of monofluoro-substituted benzothiadiazole (FBT) to link a thiophene ring has been designed to construct a new A-π-D-π-A-type small molecular acceptor (IDT-FBTR) with indacenodithiophene (IDT) as a central core (D) and 3-octyl-2-(1,1-dicyanomethylene)rhodanine as an electron-withdrawing terminal group (A). Because of the strong intramolecular push-pull electron effect, the IDT-FBTR shows a strong and broad intramolecular charge-transfer absorption band in the range of 500-750 nm. Especially, as an electron-deficient FBT unit (A') and an electron-rich thiophene ring (D') in "D-A combined π-bridge" exert an "offset effect" to regulate the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) energy levels of the molecule, a relatively high LUMO energy level can be maintained for IDT-FBTR that is helpful to enhance the open-circuit voltage ( V_oc) for highly efficient organic solar cells (OSCs). Therefore, the optimized OSC device based on IDT-FBTR as the acceptor and PTB7-Th as the donor shows a much high V_oc of 1.02 V with a relatively low E_loss of 0.56 eV and a best power conversion efficiency of 9.14%.

Intra-molecular Charge Transfer and Electron Delocalization in Non-fullerene Organic Solar Cells

Two types of electron acceptors were synthesized by coupling two kinds of electron-rich cores with four equivalent perylene diimides (PDIs) at the α-position. With fully aromatic cores, TPB and TPSe have π-orbitals spread continuously over the whole aromatic conjugated backbone, unlike TPC and TPSi, which contain isolated PDI units due to the use of a tetrahedron carbon or silicon linker. Density functional theory calculations of the projected density of states showed that the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) for TPB are localized in separate regions of space. Further, the LUMO of TPB shows a greater contribution from the orbitals belonging to the connective core of the molecules than that of TPC. Overall, the properties of the HOMO and LUMO point at increased intra-molecular delocalization of negative charge carriers for TPB and TPSe than for TPC and TPSi and hence at a more facile intra-molecular charge transfer for the former. The film absorption and emission spectra showed evidences for the inter-molecular electron delocalization in TPB and TPSe, which is consistent with the network structure revealed by X-ray diffraction studies on single crystals of TPB. These features benefit the formation of charge transfer states and/or facilitate charge transport. Thus, higher electron mobility and higher charge dissociation probabilities under J_sc condition were observed in blend films of TPB:PTB7-Th and TPSe:PTB7-Th than those in TPC:PTB7-Th and TPSi:PTB7-Th blend films. As a result, the J_sc and fill factor values of 15.02 mA/cm², 0.58 and 14.36 mA/cm², 0.55 for TPB- and TPSe-based solar cell are observed, whereas those for TPC and TPSi are 11.55 mA/cm², 0.47 and 10.35 mA/cm², 0.42, respectively.

Post-Treatment-Free Main Chain Donor and Side Chain Acceptor (D-s-A) Copolymer for Efficient Nonfullerene Solar Cells with a Small Voltage Loss

Main chain donor and side chain acceptor (D-s-A) copolymers are an important branch of the D-A copolymer family. However, the development of D-s-A copolymers significantly falls behind the alternative D-A copolymers, especially for organic solar cells, because a breakthrough in device performance is not yet obtained with a reported power conversion efficiency (PCE) of 2%-4%. Herein, a newly developed D-s-A copolymer PDRCNBDT, bearing 2-(1, 1-dicyanomethylene) rhodanine pendant group as the donor material, delivers a high PCE of 5.3% for nonfullerene solar cells. To the best of our knowledge, this is the best value reported for D-s-A copolymers to date. The improved PCE is observed to be associated with a very small energy loss (E_loss ) of 0.57 eV, accompanied by a high open-circuit voltage (V_oc ) of 1.015 eV. It is important to note that this efficient D-s-A copolymer is employed in organic solar cells (OSCs), free of additive and annealing treatments.

Synthesis and photovoltaic properties of a non-fullerene acceptor with F-phenylalkoxy as a side chain

A new acceptor–donor–acceptor (A–D–A) non-fullerene acceptor (ITIC-FOR) was synthesized and applied in non-fullerene solar cells with a PCE of up to 7.33%.