Achieving high short-circuit current and fill-factor via increasing quinoidal character on nonfullerene small molecule acceptor

Data Cleansing and Sub-Unit-Based Molecular Description Enable Accurate Prediction of The Energy Levels of Non-Fullerene Acceptors Used in Organic Solar Cells

Non-fullerene acceptors (NFAs) have recently emerged as pivotal materials for enhancing the efficiency of organic solar cells (OSCs). To further advance OSC efficiency, precise control over the energy levels of NFAs is imperative, necessitating the development of a robust computational method for accurate energy level predictions. Unfortunately, conventional computational techniques often yield relatively large errors, typically ranging from 0.2 to 0.5 electronvolts (eV), when predicting energy levels. In this study, the authors present a novel method that not only expedites energy level predictions but also significantly improves accuracy , reducing the error margin to 0.06 eV. The method comprises two essential components. The first component involves data cleansing, which systematically eliminates problematic experimental data and thereby minimizes input data errors. The second component introduces a molecular description method based on the electronic properties of the sub-units comprising NFAs. The approach simplifies the intricacies of molecular computation and demonstrates markedly enhanced prediction performance compared to the conventional density functional theory (DFT) method. Our methodology will expedite research in the field of NFAs, serving as a catalyst for the development of similar computational approaches to address challenges in other areas of material science and molecular research.

Recent Research Progress in Indophenine-Based-Functional Materials: Design, Synthesis, and Optoelectronic Applications

This review highlights selected examples, published in the last three to four years, of recent advance in the design, synthesis, properties, and device performance of quinoidal π-conjugated materials. A particular emphasis is placed on emerging materials, such as indophenine dyes that have the potential to enable high-performance devices. We specifically discuss the recent advances and design guidelines of π-conjugated quinoidal molecules from a chemical standpoint. To the best of the authors' knowledge, this review is the first compilation of literature on indophenine-based semiconducting materials covering their scope, limitations, and applications. In the first section, we briefly introduce some of the organic electronic devices that are the basic building blocks for certain applications involving organic semiconductors (OSCs). We introduce the definition of key performance parameters of three organic devices: organic field effect transistors (OFET), organic photovoltaics (OPV), and organic thermoelectric generators (TE). In section two, we review recent progress towards the synthesis of quinoidal semiconducting materials. Our focus will be on indophenine family that has never been reviewed. We discuss the relationship between structural properties and energy levels in this family of molecules. The last section reports the effect of structural modifications on the performance of devices: OFET, OPV and TE. In this review, we provide a general insight into the association between the molecular structure and electronic properties in quinoidal materials, encompassing both small molecules and polymers. We also believe that this review offers benefits to the organic electronics and photovoltaic communities, by shedding light on current trends in the synthesis and progression of promising novel building blocks. This can provide guidance for synthesizing new generations of quinoidal or diradical materials with tunable optoelectronic properties and more outstanding charge carrier mobility.

Recent Progress in the Design of Fused-Ring Non-Fullerene Acceptors-Relations between Molecular Structure and Optical, Electronic, and Photovoltaic Properties

Organic solar cells are on the dawn of the next era. The change of focus toward non-fullerene acceptors has introduced an enormous amount of organic n-type materials and has drastically increased the power conversion efficiencies of organic photovoltaics, now exceeding 18%, a value that was believed to be unreachable some years ago. In this Review, we summarize the recent progress in the design of ladder-type fused-ring non-fullerene acceptors in the years 2018-2020. We thereby concentrate on single layer heterojunction solar cells and omit tandem architectures as well as ternary solar cells. By analyzing more than 700 structures, we highlight the basic design principles and their influence on the optical and electrical structure of the acceptor molecules and review their photovoltaic performance obtained so far. This Review should give an extensive overview of the plenitude of acceptor motifs but will also help to understand which structures and strategies are beneficial for designing materials for highly efficient non-fullerene organic solar cells.

High-Performance Ternary Polymer Solar Cells Enabled by a New Narrow Bandgap Nonfullerene Small Molecule Acceptor with a Higher LUMO Level

Obtaining a large open-circuit voltage (V_OC ) and high short-circuit current density (J_SC ) simultaneously is important in improving power conversion efficiency (PCE) of organic photovoltaics. The ternary strategy with using a higher lowest unoccupied molecular orbital (LUMO) level nonfullerene acceptor (NFA) guest can achieve increased V_OC , yet J_SC is decreased or maintained, so it's still a challenge to offer increased V_OC and J_SC values concurrently via the newly presented V_OC -increased ternary strategy. To overcome this issue, a new narrow bandgap NFA TT-S-4F is reported by introducing 3,6-dimethoxylthieno[3,2-b]thiophene (TT) as π-spacers to connect electron-rich core with terminal groups, so as to upshift the LUMO level and extend π-system. When adding 10% TT-S-4F into binary system based on PTB7-Th:IEICO-4F, the higher-LUMO-level of TT-S-4F, the increased charge mobilities, the reduced trap-assisted combination loss, and a finer nanofiber structure and increased phase separation size are obtained, which simultaneously promotes J_SC , V_OC , and fill factor (FF), thus obtaining an optimal PCE (12.5% vs 11.5%). This work illustrates that an extending conjugated backbone with large π-spacers and inclusion of alkylthiophenyl side-chains is a concept to synthesize NFA guests for use on the V_OC -increased ternary strategy that enables to realize simultaneously increased J_SC , V_OC , and FF.

Single graphene derivative layer as a hole transport in organic solar cells based on PBDB-T:ITIC

A layer of fluorinated reduced graphene oxide (FrGO), as an alternative hole transport (HTL) in organic solar cells (OSCs) based on a PBDB-T:ITIC active layer, is reported. OSC configuration is ITO/HTL/PBDB-T:ITIC/PFN/FM; FM is Field's metal, a eutectic alloy deposited at room atmosphere. PEDOT:PSS, FrGO/PEDOT:PSS, and FrGO are tested as HTLs; the average efficiencies of 8.8, 8.2, and 5.3%, respectively, are reached. Inhomogeneity of the FrGO layer is determined as the main factor that affects the photovoltaic behavior and stability. Device stability is very acceptable, sometimes with a superior behavior than data previously reported; FM also could potentially contribute to this enhanced stability.

13%-Efficiency Quaternary Polymer Solar Cell with Nonfullerene and Fullerene as Mixed Electron Acceptor Materials

In this article, we report 13%-efficiency quaternary polymer solar cell. By introducing bis-PC₇₁BM:PC₇₁BM into a known nonfullerene system-poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl))benzo[1,2- b:4,5- b']dithiophene)- co-(1,3-di(5-thiophene-2-yl)-5,7-bis(2-ethylhexyl)benzo[1,2- c:4,5- c']dithiophene-4,8-dione):3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone-methyl))-5,5,11,11-tetrakis(4- n-hexylphenyl)-dithieno[2,3 d:2',3' d']- s-indaceno[1,2 b:5,6 b']dithiophene (PBDB-T:IT-M), the quaternary solar cell significantly outperforms the nonfullerene binary and the ternary (PBDB-T:IT-M:fullerene) devices with a significant increase in the short-circuit current-density (18.2 vs 16.5 and 16.8-17.5 mA/cm²) and the fill factor (0.73 vs 0.67 and 0.707-0.726), and hence, large power conversion efficiency (13% for quaternary vs 11% for the binary and 12% for the ternary). Grazing incidence wide-angle X-ray scattering data indicate that both the polymer and IT-M phase crystallinity becomes greater upon introduction of PC₇₁BM as the forth additive into the host ternary PBDB-T:IT-M:bis-PC₇₁BM, which results in an increase in both the electron and hole mobilities, contributing to the J_sc enhancement. Our results indicate that the use of the forth fullerene component provides more choices and more mechanisms than the ternary systems for tuning the photon-to-electron conversion; therefore, sheds light on the realization of high-efficiency polymer solar cells by designing the multiacceptor components with aligned energy levels, complementary absorption spectra, and improved film morphologies.

Organophosphorus Derivatives as Cathode Interfacial-Layer Materials for Highly Efficient Fullerene-Free Polymer Solar Cells

Roles of cathode interfacial layer (CIL) for low work function metal cathode, which influences significantly the electron extraction and transport processes, are in current trends for improvement in the organic solar cell (OSC) performance. Two organophosphorus derivatives tetraphenylphosphonium bromide (QPhPBr) and ((2-(1,3-dioxan-2-yl)ethyl)triphenylphosphonium bromide) (TPhPEtBr) as CILs individually and with mixed binary layer with N719 were demonstrated. Tremendous improvement in photovoltaic performance with QPhPBr with an average power conversion efficiency, PCE, of 11.08% and TPhPEtBr with PCE of 10.20% as well as their binary layers with 11.61 and 10.74%, respectively, has been achieved using the PBDBT:ITIC blend active layer, in comparison to that of the bare Al cathode (7.37%). The maximum PCE of 12.0% is achieved with QPhPBr:N719 as the CIL, which is the highest value reported in the literature to date for PBDB-T:ITIC-based single junction binary fullerene-free OSCs, suggesting the potential of ionic organophosphorus derivatives and their binary blended mixtures with an ionic n-type organic semiconductor such as N719 used as CILs for realizing high-efficiency fullerene-free OSCs. Their efficient performance would be helpful for potential selection of CILs in OSCs.

Dithienonaphthalene-Based Non-fullerene Acceptors With Different Bandgaps for Organic Solar Cells

Compared to the traditional fullerene derivatives, non-fullerene acceptors show more tunable absorption bands as well as adjustable energy levels which are favorable for further PCE enhancement of organic solar cells. In order to enhance light-harvesting property of dithienonaphthalene (DTN)-based acceptors, we designed and synthesized two novel non-fullerene acceptors (DTNIF and DTNSF) based on a ladder-type DTN donor core flanked with two different acceptor units. In combination with a benchmark wide bandgap copolymer (PBDB-T), the best performance device based on DTNIF displayed a high PCE of 8.73% with a short-circuit current (J_sc) of 13.26 mA cm⁻² and a large fill factor (FF) of 72.77%. With a reduced bandgap of DTNSF, the corresponding best performance device showed an increased J_sc of 14.49 mA cm⁻² although only a moderate PCE of 7.15% was achieved. These findings offer a molecular design strategy to control the bandgap of DTN-based non-fullerene acceptors with improved light-harvesting.

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.

Tetraphenylphosphonium Bromide as a Cathode Buffer Layer Material for Highly Efficient Polymer Solar Cells

Here, we introduced the role of small organic molecule tetraphenylphosphonium bromide (QPhPBr) as an electron-transporting layer (ETL) material for fabricating high-efficiency bulk heterojunction polymer solar cells (PSCs). Their significantly higher power conversion efficiency (PCE) in well-known active layer devices (PTB7-Th:PC₇₁BM, PBDTTT-CT:PC₇₁BM, and P3HT:PC₇₁BM) was observed compared to that of the bare Al cathode. The use of N719 as an ETL was also demonstrated. Observed data reveal that QPhPBr-based devices exhibit high PCEs up to 9.18, 8.42, and 4.81% from PTB7-Th, PBDTTT-CT, and P3HT, respectively. For comparisons, the bare Al devices show PCEs of 5.37, 4.75, and 3.01%, respectively. Moreover, further enhancement of PSC efficiency (9.83, 8.69, and 5.35%) is achieved from mixed binary solution of N719:QPhPBr because of modulated adjustment of the work function of the Al electrode. Our results indicate the excellent function of tetraphenylphosphonium bromide and its binary blend as effective small-molecule organic materials to regulate the metal surface properties and the potential used as excellent cathode buffer layer materials for realizing high-efficiency PSCs.