Progress In Organic Solar Cells Research Articles

Advancements in non‐fullerene acceptors for organic solar cells: Brief review of research trends

AbstractIn the last few decades, fullerene‐based acceptors have been extensively utilized in organic solar cells (OSCs). However, OSCs based on fullerenes suffer from low photocurrent and photovoltage due to their poor light absorption capacity and limited tunability of energy levels, which impedes efficiency improvements. Recently, non‐fullerene acceptors (NFAs) have enabled rapid progress in OSCs, owing to their favorable properties such as strong light absorption, facile energy level tuning, and improved charge transport characteristics. These features have led to rapid efficiency enhancements. This review discusses the latest research trends related to NFAs, covering several factors that can be applied in the development of NFAs. Finally, we suggest prospects and challenges for high‐performance NFA‐based OSCs on the path to commercialization.

Manipulating Crystal Stacking by Sidechain Engineering for High‐Performance N‐Type Organic Semiconductors

AbstractY‐series non‐fullerene acceptors (NFAs) have achieved great progress in organic solar cells (OSCs). Most research attention is currently paid to their molecular engineering to improve the efficiency of OSCs. However, as n‐type organic semiconductors, the relationship between their molecular packing structures and charge transport properties is mostly ignored. Herein, it is clarified how the molecular packing of Y‐series NFAs fundamentally determines their charge transport properties by manipulating their crystal stacking via sidechain engineering. Therefore, branched alkyl/alkoxy substitutions are taken on a reference NFA (Y6‐1O), affording three derivatives, namely 1OBO‐1, 1OBO‐2, and 1OBO‐3. Results show that while the replacement of branched alkyl/alkoxy sidechains has little impact on optical properties and energy levels, it can change crystal stacking motifs significantly. The single crystal of Y6‐1O with all linear sidechains forms a 2D‐brickwork structure and shows lower mobility. In contrast, 1OBO‐2 with all branched sidechains exhibits a favorable 3D interpenetrating porous network, displaying an electron mobility of 1.42 cm2 V−1 s−1 in single‐crystal organic field‐effect transistors (SC‐OFETs). This value is among the highest for NFA‐based n‐type OFETs. This study not only reveals the fundamental structure–property relationships of Y‐series NFAs, but also suggests the potential of Y‐series NFAs for high‐performance n‐type organic semiconductors.

Mastering morphology of non-fullerene acceptors towards long-term stable organic solar cells

Despite the rapid progress of organic solar cells based on non-fullerene acceptors, simultaneously achieving high power conversion efficiency and long-term stability for commercialization requires sustainable research effort. Here, we demonstrate stable devices by integrating a wide bandgap electron-donating polymer (namely PTzBI-dF) and two acceptors (namely L8BO and Y6) that feature similar structures yet different thermal and morphological properties. The organic solar cell based on PTzBI-dF:L8BO:Y6 could achieve a promising efficiency of 18.26% in the conventional device structure. In the inverted structure, excellent long-term thermal stability over 1400 h under 85 °C continuous heating is obtained. The improved performance can be ascribed to suppressed charge recombination along with appropriate charge transport. We find that the morphological features in terms of crystalline coherence length of fresh and aged films can be gradually regulated by the weight ratio of L8BO:Y6. Additionally, the occurrence of melting point decrease and reduced enthalpy in PTzBI-dF:L8BO:Y6 films could prohibit the amorphous phase to cluster, and consequently overcome the energetic traps accumulation aroused by thermal stress, which is a critical issue in high efficiency non-fullerene acceptors-based devices. This work provides insight into understanding non-fullerene acceptors-based organic solar cells for improved efficiency and stability.

Direct Arylation Synthesis of Small Molecular Acceptors for Organic Solar Cells.

In recent years, small molecular acceptors (SMAs) have extensively promoted the progress of organic solar cells (OSCs). The facile tuning of chemical structures affords SMAs excellent tunability of their absorption and energy levels, and it gives SMA-based OSCs slight energy loss, enabling OSCs to achieve high power conversion efficiencies (e.g., >18%). However, SMAs always suffer complicated chemical structures requiring multiple-step synthesis and cumbersome purification, which is unfavorable to the large-scale production of SMAs and OSC devices for industrialization. Direct arylation coupling reaction via aromatic C-H bonds activation allows for the synthesis of SMAs under mild conditions, and it simultaneously reduces synthetic steps, synthetic difficulty, and toxic by-products. This review provides an overview of the progress of SMA synthesis through direct arylation and summarizes the typical reaction conditions to highlight the field's challenges. Significantly, the impacts of direct arylation conditions on reaction activity and reaction yield of the different reactants' structures are discussed and highlighted. This review gives a comprehensive view of preparing SMAs by direct arylation reactions to cause attention to the facile and low-cost synthesis of photovoltaic materials for OSCs.

Synthesis, Properties, and Photovoltaic Characteristics of Fluoranthenedione-containing Nonfullerene Acceptors for Organic Solar Cells

The progress of organic solar cells (OSCs) largely depends on the development of nonfullerene acceptors (NFAs) based on electron-accepting π-conjugated compounds. Therefore, the creation of its building unit is important to tune the properties and increase the OSC performance. In this contribution, a new electron-accepting building unit, fluoranthenedione (FDO), was designed by extending the π-conjugation of representative indene-1,3-dione (IDO) framework. The electron-accepting π-conjugated compound (FL-FDO) composed of thiophene-linked fluorene (FL) as a central unit and FDO as terminal units was designed and synthesized. Compared to the corresponding IDO-containing NFAs, the introduction of FDO led to the red-shifted absorption and increased frontier orbital energy levels. The OSCs based on FL-FDO and poly(3-hexylthiophene) (P3HT) as a donor showed improved power conversion efficiency, compared to the P3HT/FL-IDO-based devices. These results indicate that the FDO unit has the potential to act as a promising terminal unit of NFAs.

Recent advances of non‐fullerene organic solar cells: From materials and morphology to devices and applications

AbstractThe innovation of non‐fullerene acceptors (NFAs) enables the rapid progress of organic solar cells (OSCs) in power conversion efficiencies to over 19%, endowing OSCs with great potential toward real‐world application. In this critical review, we outline the recent advances of NFA‐based OSCs ‐ from ITIC‐ to Y6‐family, to exemplify the structure–performance correlations, and cover from material chemistry to nanomorphology controlling. In addition, we point out the possible degradation mechanisms behind the NFA‐based devices and strategies for mitigating the stability issues. With OSC efficiencies approaching 20% benchmark, increasing attention has been built‐up toward the technology's applications. Therefore, we describe the opportunities and challenges in the promising applications, mainly on semitransparent and flexible OSCs for commercial photovoltaics. Finally, we provide a summary and perspective to point out the primary challenges the OSC technology is facing toward commercialization.image

Over 1000 nm photoresponse with cyclopentadithiophene-based non-fullerene acceptors for efficient organic solar cells

Non-fullerene acceptors have a pivotal role in the progress of organic solar cells (OSCs). Though, optical characteristics most of NFAs is quite limited within the range of 600–900 nm, limiting the short-circuit density (Jsc) of the OSCs devices. To address this issue, we have designed nine different C-shaped cyclopentadithiophene (CPDT)-based acceptor–donor-acceptor (A-D-A) structured NFAs for OSCs. To this end, various end-cap functional units have been used to further modify the reference molecule (C0) to improve the optoelectronics and photophysical characteristics of these materials. For this, various advanced quantum mechanical density functional theory (DFT) and time dependent (TD-DFT) approached used to explore the hidden potential of these materials (R, CA1-CA9). The characterizations associated with frontier molecular orbitals (FMOs), transition density Matrix (TDM) analysis, open-circuit voltages (Voc), electron-hole reorganizational energies, fill factor and optical properties of all the investigated molecules are calculated at DFT and TD-DFT-B3LYP/6-31G. In addition to all these calculations the PTB7-Th:C5 complex study (D:A) has also been investigated to check out the charge-transfer from donor polymer (PTB7-Th) to NFA C5 at the D:A interface. The results of these investigations reveal that all the designed A-D-A molecules (C1-C9) with electron-rich core and with electron-deficient end-capped acceptors are perfect for efficient tuning of optical and electrical parameters. With such approach, we can design desired photovoltaic materials by tuning their FMO energy levels, binding energy, and reorganizational energies, Voc, fill factor and absorption maxima characteristics. Therefore, these deigned materials (C1-C9) are highly recommended for synthesis to develop an efficient OSCs. And, this approach could be use effectively to design desire properties materials for the future development of an efficient photovoltaic materials.

Recent progress in organic solar cells (Part II device engineering)

Recent progress in organic solar cells (Part II device engineering)

Novel Star‐Shaped Benzotriindole‐Based Nonfullerene Donor Materials: Toward the Development of Promising Photovoltaic Compounds for High‐Performance Organic Solar Cells

The development of novel photovoltaic materials for solar cell applications is a fascinating area of current research. Star‐shaped materials with promising photovoltaic features have attracted scientists for boosting the progress of organic solar cells (OSCs). Herein, seven novel star‐shaped molecules (DA1‐DA7) are developed quantum chemically from the experimentally synthesized BTI(2 T‐DCV‐Hex)3 molecule. The open‐circuit voltage (Voc), transition density matrix heat maps, density of state (DOS), overlap DOS, frontier molecular orbital, UV−visible, binding energy (Eb), hole (λh) and electron (λe) reorganizational energy, and highest occupied molecular orbital (HOMO)donor−lowest unoccupied molecular orbital (LUMO)PC61BM charge transfer analysis are performed to explore the optoelectronic properties. Newly developed molecules exhibit promising optoelectronic features with reduced energy gap (2.43−1.97 eV), transition energy (1.89−1.45 eV), λe (0.00149328−0.00101405 Eh), λh (0.0066471−0.0028843 Eh), broadened λmax (655−856 nm), and high Voc (2.07−1.65 V), as compared with reference BTI(2 T‐DCV‐Hex)3 values 2.82 eV, 2.28 eV, 0.00176501 Eh, 0.0060877 Eh, and 544 nm, 1.65 V, respectively. The developed molecules have proficient hole and electron transfer mobilities and can serve as best candidates when blended with PC61BM film. These eye‐catching results recommend the novel star‐shaped compounds for future development of high‐performance OSCs.

Recent progress in organic solar cells based on non-fullerene acceptors: materials to devices

This review presents the recent progress in organic solar cells based on non-fullerene acceptors, with a wide coverage from material synthesis and processing to interface engineering, device structure, large-area fabrication, and device stability.

Recent progress in organic solar cells (Part I material science)

Recent progress in organic solar cells (Part I material science)

Asymmetric Non-Fullerene Small-Molecule Acceptors toward High-Performance Organic Solar Cells.

Applying an asymmetric strategy to construct non-fullerene small-molecule acceptors (NFSMAs) in organic solar cells (OSCs) plays a vital role in the development of organic photovoltaic materials. In the past several years, taking advantage of the larger dipole moment and stronger intermolecular interactions, asymmetric NFSMAs have witnessed tremendous progress in OSCs with a power conversion efficiency of over 18%. From a structural point of view, besides the possible changes in the conformation effect on molecular packing, asymmetric acceptors can also achieve a balance between the solubility and the crystallinity. Herein, we systematically investigate the structure–property–performance relationships of asymmetric NFSMAs that have recently emerged and try to clarify the feasibility and practicality of an asymmetric strategy for the design of higher-performance NFSMAs. Finally, we put forward our views and a concise outlook on the asymmetric strategy.

Structural optimization of acceptor molecules guided by a semi-empirical model for organic solar cells with efficiency over 15%

Despite much progress in organic solar cells (OSCs), higher efficiency is still the most desirable goal and can indeed be obtained through rational design of active layer materials and device optimization according to the theoretical prediction. Herein, under the guidance of a semi-empirical model, two new non-fullerene small molecule acceptors (NF-SMAs) with an acceptor-donor-acceptor (A-D-A) architecture, namely, 6T-OFIC and 5T-OFIC, have been designed and synthesized. 6T-OFIC exhibits wider absorption spectrum and a red-shifted absorption onset (λonset) of 946 nm due to its extended conjugation central unit. In contrast, 5T-OFIC with five-thiophene-fused backbone has an absorption with the λonset of 927 nm, which is closer to the predicted absorption range for the best single junction cells based on the semi-empirical model. Consequently, the device based on 5T-OFIC yields a higher power conversion efficiency (PCE) of 13.43% compared with 12.35% of the 6T-OFIC-based device. Furthermore, an impressive PCE of 15.45% was achieved for the 5T-OFIC-based device when using F-2Cl as the third component. 5T-OFIC offers one of a few acceptor cases with efficiencies over 15% other than Y6 derivatives.

End-capped engineering of bipolar diketopyrrolopyrrole based small electron acceptor molecules for high performance organic solar cells

Non-fullerene small acceptor molecules demonstrate promising photovoltaic properties supported the progress of organic solar cells (OSCs). In organic photovoltaics, non-fullerene small acceptor compounds are valuable than traditional fullerene-based acceptor molecules for their good contribution in organic solar cells. This research generally focusses on Bipolar Diketopyrrolopyrrole (F(DPP)2B2) based small electron acceptor compounds used for organic solar cells (OSCs). For this purpose, four newly developed A-D-A (Acceptor-Donor –Acceptor) type electron accepting compounds named as V1, V2, V3, and V4 are planned for organic solar cells to study their optoelectronic characteristics. MPW1PW91/6-31G(d,p) basis set was used for calculations of various geometric parameters. Among all these, newly designed molecules V1 is proven an appropriate compound for OSCs applications due to the photovoltaic characteristics like low band gap between HOMO and LUMO (1. 88 eV) and V3 molecule shows broader absorption λmax in the gas phase (741.62 nm) and chloroform solvent (785.95 nm). Molecule V2 exhibits the lowest value of mobility of electrons (0.0806 Eh). All the designed compounds exhibit better open-circuit voltage, fewer energies of excitations, the highest value of absorption, greater dipole moment, equivalent binding energies, well-organized transport of electron and hole as compared to reference R. Thus, these molecules are suggested to researchers for the future development of highly efficient OSCs.

Low-Bandgap Non-fullerene Acceptors Enabling High-Performance Organic Solar Cells

Great progress in organic solar cells (OSCs) has been recently achieved owing to the advent of non-fullerene acceptors (NFAs). Indeed, low-bandgap NFAs ranging from 1.3 to 1.6 eV with broad absorpt...

Research progress of organic solar cells based on photonic crystals

With the rapid development of photovoltaic industry in recent years, organic solar cells have attracted much attention due to their advantages of low cost, light weight, capacity of batch production, simple production process and flexible performance. However, there are still some limitations hindering their commercialization process, including low photoelectric conversion efficiency and poor transmission color rendering. The introduction of photonic crystals provides a new way to solve these two problems. Starting from the optimization principle of photonic crystals, the effects of both one-dimensional photonic crystals and two-dimensional photonic crystals on organic solar cells, especially the short circuit current and photoelectric conversion efficiency, are systematically summarized in this paper. Then, we focus on the reasons for the performance improvement of organic solar cells based on one-dimensional photonic crystals and two-dimensional photonic crystals. The results of the experiments and characterization show that the performance improvement is mainly attributed to the photonic crystal acting as the reflector in the device. Photonic bandgap, a vivid property that the photonic crystals have, can block the light transmitting organic solar cells at a certain frequency. So, the light within the photonic bandgap is reflected back into the device, thus promoting the secondary absorption of light by the active layer which can result in the stronger light absorption capacity of the active layer, and then improving the performance of the device. In addition, the reason why one-dimensional photonic crystals can be used to regulate the color rendering of semitransparent organic solar cell is described in detail. This is of great significance to photovoltaic construction industry because semitransparent organic solar cells with excellent color rendering property can be widely used in it. However, due to the limitation of photonic crystal optimization mechanism, the reported applications so far have failed to improve the filling factor and open circuit voltage of the device, and due to the limitation of its own structure, three-dimensional photonic crystals have not been reported to be used in organic solar cells. Finally, by combining the existing research progress of organic optoelectronic devices, we look into the future research direction of organic solar cells based on photonic crystals.

Benzo[c][1,2,5]thiadiazole-fused pentacyclic small molecule acceptors for organic solar cells

Recently, A-DA′D-A-type small molecular acceptors (SMAs) with benzo [c][1,2,5]thiadiazole (BT)-fused central cores have shown tremendous progress in organic solar cells (OSCs). In this work, two new BT-fused pentacyclic core based SMAs, namely BTT-4F and BTTCN-Cl, were designed and synthesized. Compared to BTT-4F, BTTCN-Cl not only delivers a reduced bandgap but an elevated lowest unoccupied molecular orbital (LUMO) energy level, which leads to a higher open-circuit voltage (VOC) and thus a lower energy loss in PM6:BTTCN-Cl-based device. However, PM6:BTT-4F-based OSCs achieves a significantly higher power conversion efficiency (PCE) (10.03%) than that of BTTCN-Cl-based OSCs (6.56%) due to its higher short-circuit density (JSC) and fill factor (FF). More favorable morphology accompanying with higher and more balanced hole/electron mobilities was obtained in PM6:BTT-4F active layer, which well accounts for the high PCE in BTT-4F-based OSCs.

Designing Star-Shaped Subphthalocyanine-Based Acceptor Materials with Promising Photovoltaic Parameters for Non-fullerene Solar Cells.

Star-shaped three-dimensional (3D) twisted configured acceptors are a type of nonfullerene acceptors (NFAs) which are getting considerable attention of chemists and physicists on account of their promising photovoltaic properties and manifestly promoted the rapid progress of organic solar cells (OSCs). This report describes the peripheral substitution of the recently reported highly efficient 3D star-shaped acceptor compound, STIC, containing a 2-(3-oxo-2,3-dihydroinden-1-ylidene)malononitrile (IC) end-capped group and a subphthalocyanine (SubPc) core unit. The 3D star-shaped SubPc-based NFA compound STIC is peripherally substituted with well-known end-capped groups, and six new molecules (S1–S6) are quantum chemically designed and explored using density functional theory (DFT) and time-dependent DFT (TDDFT). Density of states (DOS) analysis, frontier molecular orbital (FMO) analysis, reorganization energies of electrons and holes, open-circuit voltage, transition density matrix (TDM) surface, photophysical characteristics, and charge-transfer analysis of selected molecules (S1–S6) are evaluated with the synthesized reference STIC. The designed molecules are found in the ambience of 2.52–2.27 eV with a reduction in energy gap of up to 0.19 eV compared to R values. The designed molecules S3–S6 showed a red shift in the absorption spectrum in the visible region and broader shift in the range of 605.21–669.38 nm (gas) and 624.34–698.77 (chloroform) than the R phase values of 596.73 nm (gas) and 616.92 nm (chloroform). The open-circuit voltages are found with the values larger than R values in S3–S6 (1.71–1.90 V) and comparable to R in the S1 and S2 molecules. Among all investigated molecules, S5 due to the combination of extended conjugation and electron-withdrawing capability of end-capped acceptor moiety A5 is proven as the best candidate owing to promising photovoltaic properties including the lowest band gap (2.27 eV), smallest λe = 0.00232 eV and λh = 0.00483 eV, highest λmax values of 669.38 nm (in gas) and 698.77 nm (in chloroform), and highest Voc = 1.90 V with respect to HOMOPTB7-Th–LUMOacceptor. Our results suggest that the selected molecules are fine acceptor materials and can be used as electron and/or hole transport materials with excellent photovoltaic properties for OSCs.

Designing indenothiophene-based acceptor materials with efficient photovoltaic parameters for fullerene-free organic solar cells.

Non-fullerene small molecular acceptors (NFSMAs) exhibit promising photovoltaic performance which promoted the rapid progress of organic solar cells (OSCs). In this study, an attempt is done to explore indenothiophene-based high-performance small molecular electron acceptors for organic solar cells. We have designed five acceptor molecules (M1-M5) with strong donor moiety indenothiophene linked to five different end-capped group acceptor moieties: diflouro-2-methylene-3-oxo-2,3-dihydroindene-1-ylidene)malononitrile (A1), 1-(dicyanomethylene)-2-methylene-3-oxo-2,3-dihydro-1H-indene-5,6-dicarbonitrile (A2), methyl-6-cyano-3-(dicyanomethylene)-2-methylene-1-oxo-2,3-dihydro-1H-indene-5-carboylate (A3), 2-(6-cyano-5-fluoro-2-methylene-3-oxo-2,3 dihydro-1H-indene-1-ylidene)malononitrile (A4), and (Z)-methyl 3-(benzo [c][1,2,5]thiadiazol-4-yl)-2-cyanoacrylate (A5) respectively. The structure-property relationship was studied and effects of structural modification on the optoelectronic properties of these acceptors (M1-M5) were determined systematically by comparing it with reference molecule R, which is recently reported as excellent non-fullerene-based small acceptor molecule. Among all designed molecules, M5 is proven as a suitable candidate for organic solar cell applications due to better photovoltaic properties including narrow HOMO-LUMO energy gap (2.11 eV), smallest electron mobility (λe= 0.0038eV), highest λmax values (702.82 nm in gas) and (663.09 nm in chloroform solvent) and highest open-circuit voltage (Voc = 1.49V) with respect to HOMOPTB7-Th-LUMOacceptor. Our results indicate that introducing more end-capped electron-accepting units is a simple and effective alternative strategy for the design of promising NFSMAs. This theoretical framework also proves that the conceptualized NFSMAs are superior and thus are recommended for the future construction of high-performance organic solar cell devices. Graphical abstract.

Progress of the key materials for organic solar cells

Organic solar cells have attracted academic and industrial interests due to the advantages like lightweight, flexibility and roll-to-roll fabrication. Nowadays, 18% power conversion efficiency has been achieved in the state-of-the-art organic solar cells. The recent rapid progress in organic solar cells relies on the continuously emerging new materials and device fabrication technologies, and the deep understanding on film morphology, molecular packing and device physics. Donor and acceptor materials are the key materials for organic solar cells since they determine the device performance. The past 25 years have witnessed an odyssey in developing high-performance donors and acceptors. In this review, we focus on those star materials and milestone work, and introduce the molecular structure evolution of key materials. These key materials include homopolymer donors, D-A copolymer donors, A-D-A small molecular donors, fullerene acceptors and nonfullerene acceptors. At last, we outlook the challenges and very important directions in key materials development.