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Naphthodithiophene‐Based Nonfullerene Acceptor for High‐Performance Organic Photovoltaics: Effect of Extended Conjugation

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Abstract

Naphtho[1,2-b:5,6-b′]dithiophene is extended to a fused octacyclic building block, which is end capped by strong electron-withdrawing 2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile to yield a fused-ring electron acceptor (IOIC2) for organic solar cells (OSCs). Relative to naphthalene-based IHIC2, naphthodithiophene-based IOIC2 with a larger π-conjugation and a stronger electron-donating core shows a higher lowest unoccupied molecular orbital energy level (IOIC2: −3.78 eV vs IHIC2: −3.86 eV), broader absorption with a smaller optical bandgap (IOIC2: 1.55 eV vs IHIC2: 1.66 eV), and a higher electron mobility (IOIC2: 1.0 × 10−3 cm2 V−1 s−1 vs IHIC2: 5.0 × 10−4 cm2 V−1 s−1). Thus, IOIC2-based OSCs show higher values in open-circuit voltage, short-circuit current density, fill factor, and thereby much higher power conversion efficiency (PCE) values than those of the IHIC2-based counterpart. In particular, as-cast OSCs based on FTAZ: IOIC2 yield PCEs of up to 11.2%, higher than that of the control devices based on FTAZ: IHIC2 (7.45%). Furthermore, by using 0.2% 1,8-diiodooctane as the processing additive, a PCE of 12.3% is achieved from the FTAZ:IOIC2-based devices, higher than that of the FTAZ:IHIC2-based devices (7.31%). These results indicate that incorporating extended conjugation into the electron-donating fused-ring units in nonfullerene acceptors is a promising strategy for designing high-performance electron acceptors.

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... The extended conjugation has been found to decrease the LUMO energy which results in the low band-gap and higher shift in λ max values 36 . The red shift in λ max value along with strong light absorption has resulted to lower the excitation energy, and ultimately reinforce the electron-acceptor property of the molecules 37 . ...
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... Fused ring electron acceptors are being extensively studied because of advantages such as the ability to tune energy levels, stronger absorption in the UV−visible region, and greater thermal stability. 20,21 Small-molecule donors are preferable to polymer donors possessing a well-defined structure, easier purification, and perfect batch-to-batch replicability, and these are important parameters in the commercialization of all-small-molecule organic SCs (ASM-OSCs). For achieving a higher PCE of ASM-OSCs, several criteria should be met such as the complementary absorption of donors and acceptors, a welloptimized structure of the blended film, and matched molecular energy levels of donors with that of acceptors. ...
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... Because the BDT unit is easy to be chemically processed, it is easy to adjust the molecular energy level and optical band gap, and improve the electron mobility. The BDT-based 2D conjugate structure was also constructed to improve the solubility of materials [11][12][13][14], So building NF-SMA based on BDT is expected to achieve high PCE. In addition, using near-infrared (NIR) absorbing materials to widen the photocurrent response area to achieve high-performance OSC is an ICEMEE 2020 IOP Conf. ...
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The use of photovoltaic technologies has been regarded as a promising approach to converting solar energy to electricity and mitigating the energy crisis, among which organic photovoltaics (OPVs) have attracted broad interest owing to their solution processability, flexibility, light weight and potential for large-area processing. The development of OPV materials, especially electron acceptors, has been one of the focuses in recent years. Compared with fullerene derivates, n-type nonfullerene molecules have some unique merits, such as synthesis simplicity, high tunability of the absorption and energy levels, and small energy loss. In the last five years, organic solar cells based on n-type nonfullerene molecules have achieved a significant breakthrough in the power conversion efficiency from approximately 4% to over 17%, superior to fullerene-based solar cells; meanwhile, n-type nonfullerene molecules have created brand new opportunities for application of OPVs in some special situations. This Perspective analyzes the key design strategies of high-performance n-type molecular photovoltaic materials and highlights instructive examples of their various applications, including in ternary and tandem solar cells towards high efficiency, semitransparent solar cells towards power generating building facades and windows, and indoor photovoltaics for driving low power consumption devices. Moreover, to accelerate the pace towards commercialization of OPVs, the existing challenges and future directions are also discussed from the perspectives of efficiency, stability and large-area fabrication.
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We designed and synthesized a series of fused-ring electron acceptors (FREAs) based on naphthalene-fused octacyclic cores end-capped by 3-(1,1-dicyanomethylene)-5,6-difluoro-1- indanone (NOICs) using a bottom-up approach. The NOIC series shares the same end groups and side chains, as well as similar fused-ring cores. The butterfly effects, arising from different methoxy positions in the starting materials, impact the design of the final FREAs, as well as their molecular packing, optical and electronic properties, charge transport, film morphology, and performance of organic solar cells. The binary-blend devices based on this NOIC series show power conversion efficiencies varying from 7.15% to 14.1%, due to the different intrinsic properties of the NOIC series, morphologies of blend films, and voltage losses of devices.
Article
Alkylthienyl decorated two-dimensional (2D) conjugated electron-donating (D) unit such as 4,8-bis(2-alkylthienyl)benzo[1,2-b:4,5-b’]dithiophene has been widely used for designing highly efficient photovoltaic materials. However, the thienyl side chains are less involved in the backbone Frontier molecular orbitals (FMOs). In this paper, we replace the side-chains from tri(n-propyl)silylthienyl to tri(n-propyl)silylethynyl and report two alkynyl-decorated 2D conjugated narrow-bandgap (~1.4 eV) nonfullerene acceptors, named ISI-4F and ISI-4Cl. In a striking contrast to the thienyl-linked counterpart, the ethylnyl is significantly involved in the backbone FMOs of the new acceptor molecules, enhancing the absorptivity and reducing the energy levels. With PM6 as the donor polymer, 12.5% and 12.1% efficiencies are obtained. The VOC are as high as 0.87 – 0.88 V. The tetrachlorinated acceptor supplies a larger fill factor (67.9% vs. 62.2%) while a smaller short-circuit current-density (22.8 vs. 20.5 mA/cm2) than the tetrafluorinated counterpart does. When introduced as acceptor guests of PM6:Y6, 16.1% and 15.9% efficiencies are achieved. These results demonstrate the trialkylsilylalkynyl can be potential side-chain candidates for designing highly efficient acceptor materials.
Article
Porphyrin and its derivatives play important roles in natural energy conversion application. In this work, we designed and synthesized three novel acceptor-donor-acceptor (A-D-A) type small molecules (namely Por-Cu-IC, Por-Cu-ICF and Por-Cu-ICFF) with Cu(II)-porphyrin as central electron-donating core and fluorinated 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile as the electron-withdrawing end groups. These new molecules show broad absorption spectra from 300 to 900 nm with strong intramolecular charge transfer absorption spectra around at 800 nm, and optical bandgap of about 1.4 eV. Organic solar cells (OSCs) based on these new molecules as non-fullerene electron acceptors achieved power conversion efficiencies from 0.5% to 2.44%. The OSCs were characterized by several techniques, including density functional theory (DFT), space-charge limited current (SCLC), photoluminescence spectra (PL) and atomic force microscopy (AFM). These results demonstrate a systematic study of Cu(II)-porphyrin molecules, which could be used to design molecules for use in organic electronics.
Article
We synthesized three fused-ring electron acceptors (m-ITBr, o-ITBr and IT-2Br) based on indacenodithieno[3,2-b]thiophene as the core unit and 2-(3-oxo-2,3-dihydroinden-1-ylidene)malononitrile with 1 or 2 bromine substituents as the end-capped unit. The effects of bromine atom number and substituted position on molecular film absorption, energy level, photovoltaic performance, charge transport and film morphology were systematically carried out. Compared with ITIC, m-ITBr, o-ITBr and IT-2Br all show red-shifted film absorption (550–750 nm) and downshifted highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels. The PM6:IT-2Br based devices exhibited power conversion efficiency (PCE) of up to 12.92%, which was much higher than the reference devices based on ITIC (8.98%). In addition, all three brominated devices achieved high efficiency over 11% (11.84% for m-ITBr devices and 11.14% for o-ITBr devices). Herein, it is also concluded that bromination on terminal groups plays a vital role in enhancing the photovoltaic performance of IDTT-based FREAs.
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The progress of all-polymer solar cells (all-PSCs) is often accompanied with morphology evolution of the active layer. However, the regulation of the morphology of the active layer often encounters obstacles,...
Article
Fullerene derivatives are classic electron acceptor materials for organic solar cells (OSCs) but possess some intrinsic drawbacks such as weak visible light absorption, limited optoelectronic property tunability, difficult purification and photochemical/morphological instability. Fullerene acceptors are a bottleneck restricting further development of this field. Our group pioneered the exploration of novel nonfullerene acceptors in China in 2006, and initiated the research of two representative acceptor systems, rylene diimide polymer and fused‐ring electron acceptor (FREA). FREA breaks the theoretical efficiency limit of fullerene‐based OSCs (~13%) and promotes the whole field to an unprecedented prosperity with efficiency of 20%, heralding a nonfullerene era for OSCs. In this review, we revisit 15‐year nonfullerene exploration journey, summarize the design principles, molecular engineering strategies, physical mechanisms and device applications of these two nonfullerene acceptor systems, and propose some possible research topics in the near future. This article is protected by copyright. All rights reserved.
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Considering the increased demand and potential of photovoltaic devices in clean, renewable electrical and hi-tech applications, non-fullerene acceptor (NFA) chromophores have gained significant attention. Herein, six novel NFA molecules IBRD1-IBRD6 have been designed by structural modification of the terminal moieties from experimentally synthesized A2-A1-D-A1-A2 architecture IBR for better integration in organic solar cells (OSCs). To exploit the electronic, photophysical and photovoltaic behavior, density functional theory/time dependent-density functional theory (DFT/ TD-DFT) computations were performed at M06/6-311G(d,p) functional. The geometry, electrical and optical properties of the designed acceptor molecules were compared with reported IBR architecture. Interestingly, a reduction in bandgap (2.528-2.126 eV), with a broader absorption spectrum, was studied in IBR derivatives (2.734 eV). Additionally, frontier molecular orbital findings revealed an excellent transfer of charge from donor to terminal acceptors and the central indenoindene-core was considered responsible for the charge transfer. Among all the chromophores, IBRD3 manifested the lowest energy gap (2.126 eV) with higher λ max at 734 and 745 nm in gaseous phase and solvent (chloroform), respectively due to the strong electron-withdrawing effect of five end-capped cyano groups present on the terminal acceptor. The transition density matrix map revealed an excellent charge transfer from donor to terminal acceptors. Further, to investigate the charge transfer and open-circuit voltage (V oc), PBDBT donor polymer was blended with acceptor chromophores, and a significant V oc (0.696-1.854 V) was observed. Intriguingly, all compounds exhibited lower reorganization and binding energy with a higher exciton dissociation in an excited state. This investigation indicates that these designed chromophores can serve as excellent electron acceptor molecules in organic solar cells (OSCs) that make them attractive candidates for the development of scalable and inexpensive optoelectronic devices. Solar energy is the most poised amongst the renewable sources to avert the climate crisis 1,2. Until now Silicon-based solar cells (SCs) have been used frequently due to their low toxicity, thermal stability, and comparatively impressive power conversion efficiency (PCE); but suffer from certain drawbacks, including the high cost of production, heavy weight (20-30 kg m −2), rigidity, as well as defined and unalterable HOMO-LUMO levels 3. To overcome these drawbacks, significant research effort has been put into more flexible, lightweight (0.5 kg m −2) organic solar cells (OSCs) 4. These show great promise but traditionally suffered from low efficiencies. OSCs are generally bulk heterojunction (BHJ) units where absorption layers blend donor and acceptor molecules. Over the OPEN 1
Article
Renewable energy sources are promising long-term solution to solve the energy supply crisis due to the excessive use of non-renewable fossil fuels. One of the options is solar energy, which can be harvested directly from sunlight using photovoltaic (PV) technology. In recent years, organic solar cells (OSCs) as the building blocks of organic PV technology have emerged in the PV field, enabling the realization of environmental-friendly and low-cost PV technology. However, issues related to efficiency performance still posed a major setback to commercialization of OSCs. In view of this, this study is conducted to present comprehensive understandings on how OSCs’ performance in terms of optical, electrical, morphological and mechanical properties can be improved through device engineering strategy (interface and electrode engineering strategy). In addition, the potential applications of OSCs achieved via device engineering strategy are also being explored. In summary, the studies conducted can be divided into three main parts. The first part focuses on improving OSCs’ performance through interface engineering strategy for the realization of high-performing OSCs. Interface engineering on sol-gel zinc oxide (ZnO) electron-transporting layer (ETL) was conducted by introducing additional oxadiazole-based electron-transporting material called 2-(4-tert-butylphenyl)-5-(4-biphenylyl)-1,3,4-oxadiazole (PBD) between ZnO ETL and photoactive layer. The significance of incorporating PBD on ZnO was demonstrated by investigating the change in optical, electrical and morphological properties of pristine ZnO ETL. The findings shown that additional PBD layer could improve pristine ZnO film’s conductivity, create better energy level alignment with the photoactive layer, smoothen ZnO film’s morphology and improve ZnO film’s hydrophobicity. All those factors crucially influenced the charge extraction, transport and recombination processes in OSCs, which were conducive for the enhancement in photovoltaic performance of ZnO/PBD-based device. In fact, through interface engineering strategy, inverted OSCs based on poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b’]dithiophene))-alt-(5,5-(1’,3’-di-2-thienyl-5’,7’-bis(2-ethylhexyl)benzo[1’,2’-c:4’,5’-c’]dithiophene-4,8-dione)] (PBDB-T donor) and 3,9-bis(2-methylene-((3-(1,1-dicyanomethylene)-6/7-methyl)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2’,3’-d’]-s-indaceno[1,2-b:5,6-b’]dithiophene (IT-M acceptor) could demonstrate ~7% increment in the photovoltaic performance from 10.8% (ZnO-based device) to 11.6% (ZnO/PBD-based device). The second part focuses on improving OSCs’ performance through electrode engineering strategy for the realization of high-performing flexible OSCs. Electrode engineering on poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) electrode was conducted by utilizing polyhydroxy compound dopant and gentle acid post-treatment method, specifically xylitol dopant and methanesulfonic acid (MSA) treatment. The significance of xylitol dopant and MSA treatment on PEDOT:PSS electrode was demonstrated by investigating the change in optical, electrical, morphological and mechanical properties of pristine PEDOT:PSS electrode. The findings shown that both doping and acid treatment on PEDOT:PSS electrode could improve the optical transparency of electrode, enhance electrode’s conductivity and modify electrode’s morphology. In addition, such treatment could also provide electrode a stronger adhesion ability with the substrate, which were effective for improving the mechanical stability of electrode against extreme mechanical deformation. All those factors promoted the realization of high-performing flexible OSCs based on PEDOT:PSS electrode. In fact, through electrode engineering strategy, conventional OSCs based on poly[(2,6-(4,8-bis(5-(2-ethylhexyl-3-fluoro)thiophen-2-yl)-benzo[1,2-b:4,5-b’]dithiophene))-alt-(5,5-(1’,3’-di-2-thienyl-5’,7’-bis(2-ethylhexyl)benzo[1’,2’-c:4’,5’-c’]dithiophene-4,8-dione)] (PBDB-T-2F/PM6 donor) and 2,2'-((2Z,2'Z)-((12,13-bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]thiadiazolo[3,4-e]thieno[2",3’':4’,5']thieno[2',3':4,5]pyrrolo[3,2-g]thieno[2',3':4,5]thieno[3,2-b]indole-2,10-diyl)bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (BTP-4F/Y6 acceptor) could demonstrate an excellent photovoltaic performance of 14.2% with remarkable mechanical robustness against bending and folding. The last part focuses on the application of device engineering, specifically electrode engineering as a continuation of study from the previous part. The desirable mechanical and optical properties of the engineered PEDOT:PSS could make PEDOT:PSS a great candidate for usage in foldable-flexible semi-transparent OSCs (FST-OSCs). FST-OSCs were fabricated similarly using engineered PEDOT:PSS electrode and PBDB-T-2F:Y6 photoactive layer system. As a result, high-performing FST-OSCs with over 10% efficiency and 21% average visible light transmittance, as well as excellent mechanical stability were obtained. The potential of such FST-OSCs for greenhouse application was investigated by incorporating them as part of roofs in the simulated greenhouse. Comparisons between plants grown under direct sunlight with FST-OSCs roof and those under direct sunlight yielded remarkably similar results in terms of branch sturdiness and hypertrophic leaves, proving the significance of electrode engineering strategy in realizing high-performing FST-OSCs for practical greenhouse applications.
Chapter
Ionic liquids (ILs) are important media to effect various kinds of polymerizations. Ionic polymerizations were developed in IL solvents and that atom transfer radical polymerization catalysts are attached to ILs to make them more easily recoverable in living polymerizations. Conjugated polymers with metallic electrical transport were introduced around 1977. The rapidly expanding field of the usage of polymers for organic solar cells has been reviewed with respect to materials, processes and devices that significantly deviate from standard approaches. In the inverted architecture, the metal electrode collects holes, and the polymer‐rich layer that forms actually improves the performance of those solar cells. A triboelectric nanogenerator is a newly developed technique for harvesting mechanical energy from ambient environment with sparkly high output and extremely flexible structural designs. Although a polymer electrolyte fuel cell is a superior power source for electric vehicles, the high cost of this technology has become a barrier to its large‐scale commercialization.
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Two small-molecule non-fullerene acceptors (NFAs) CPDT-Me-2Eh and CPDT-4Cl-2Eh based on 4H-cyclopenta[1,2-b:5,4-b′] dithiophene as the electron-donating unit 2-(5/6-methyl-3-oxo-2,3-dihydro-1H-inden-1-ylidene) malononitrile and 2-(5,6-dichloro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile as the end group respectively were developed. Compared with CPDT-Me-2Eh, CPDT-4Cl-2Eh possesses red-shift absorption spectrum, lower HOMO and LUMO energy level, and higher electron mobility. Non-fullerene polymer solar cells based on PBDB-T: CPDT-4Cl-2Eh gave a power conversion efficiency (PCE) of 7.52%, with a fill factor (FF) of 64.46%, and a short-circuit current (Jsc) of 16.90 mA/cm². As contrast, the cell based on PBDB-T: CPDT-Me-2Eh got a PCE value of 4.39%. Our results indicate that the small molecule acceptor containing the CPDT core can effectively extend the absorption spectrum to the near-infrared (NIR) region by chlorination of the capping group. Our work provides an easy routine to broaden the field of new NIR NFAs with high PCE based on non-fullerene OSCs.
Article
The performance of organic solar cells based on nonfullerene acceptors is improving rapidly and nowadays the power conversion efficiencies of single-junction devices have achieved over 18%. Nonfullerene acceptors usually consist of electron-rich units and electron-deficient units, and the interaction between these two parts can widen the optical absorption, tune the energy levels and improve the film morphologies of acceptor materials. Among the various electron-withdrawing groups, cyano groups have been most used in the construction of electron-deficient units for nonfullerene acceptors. In this review, the electron-deficient units containing cyano groups for nonfullerene electron acceptors have been summarized, and the effects of these electron-deficient units on the photoelectric properties and photovoltaic performance of acceptor materials have been elaborated to extract some effective molecular design strategies for high-performance acceptor materials. This journal is
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To develop new central core structure, we designed and synthesized a naphthalene-fused octacyclic electron-rich block, named as NITT. With the NITT ladder-type core and two distinct terminal groups (IC-2F and CPTCN-Cl), two novel A-D-A-type non-fullerene acceptors (NFAs), namely NITT-BF and NITT-ThCl, were constructed. Two NFAs exhibit broad and intense absorption and high electron mobility due to the coplanar architecture. When blended with polymer donor PM6, NITT-BF-based device acquires more favorable morphology, higher exciton dissociation and charge collection efficiency, and higher and more balanced hole/electron mobilities, resulting in the significantly improved short-circuit current density (JSC) of 18.08 mA cm⁻² and fill factor (FF) of 72.38%. Additionally, both NITT-BF- and NITT-ThCl-based devices deliver very high open-circuit voltages (VOCs) (0.94 V for NITT-BF-based device and 0.97 V for NITT-ThCl-based device). Therefore, PM6:NITT-BF-based device yields an optimal PCE of up to 12.30%, which is much higher than that of PM6:NITT-ThCl-based device (9.58%). Notably, the high PCE of 12.30% is one of the best values for naphthalene-fused NFAs in organic solar cells (OSCs). These results indicate that the linear ladder-type NITT is a promising electron-donating core for constructing high-performance NFAs and terminal strategy is a good way to further boost the photovoltaic performance of OSCs.
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π‐Conjugated organic/polymer materials‐based solar cells have attracted tremendous research interest in the fields of chemistry, physics, materials science, and energy science. To date, the best‐performance polymer solar cells (PSCs) have achieved power conversion efficiencies exceeding 18%, mostly driven by the molecular design and device structure optimization of the photovoltaic materials. This review article provides a comprehensive overview of the key advances and current status in aggregated structure research of PSCs. Here, we start by providing a brief tutorial on the aggregated structure of photovoltaic polymers. The characteristic parameters at different length scales and the associated characterization techniques are overviewed. Subsequently, a variety of effective strategies to control the aggregated structure of photovoltaic polymers are discussed for polymer:fullerene solar cells and polymer:nonfullerene small molecule solar cells. Particularly, the control strategies for achieving record efficiencies in each type of PSCs are highlighted. More importantly, the in‐depth structure–performance relationships are demonstrated with selected examples. Finally, future challenges and research prospects on understanding and optimizing the aggregated structure of photovoltaic polymers and their blends are provided. A comprehensive overview of the key advances and current status in aggregated structure research of photovoltaic polymers was presented. This review provided a tutorial on the multi‐level aggregated structure and the characteristic parameters as well as the associated characterization techniques. To in‐depth understand the structure‐performance relationships, effective strategies to control the aggregated structure of photovoltaic polymers are summarized, with an emphasis on the systems of record‐high power conversion efficiencies.
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With the significant progress of low bandgap non‐fullerene acceptors, the development of wide bandgap (WBG) donors possessing ideal complementary absorption is of crucial importance to further enhance the photovoltaic performance of organic solar cells. An ideal strategy to design WBG donors is to down‐shift the highest occupied molecular orbital (HOMO) and up‐shift the lowest unoccupied molecular orbital (LUMO). A properly low‐lying HOMO of the donor is favorable to obtaining a high open‐circuit voltage, and a properly high‐lying LUMO of the donor is conductive to efficient exciton dissociation. This work provides a new strategy to enlarge the bandgap of a polymer with simultaneously decreased HOMO and increased LUMO by increasing the polymer backbone curvature. The polymer PIDT‐fDTBT with a large molecular backbone curvature shows a decreased HOMO of −5.38 eV and a prominently increased LUMO of −3.35 eV relative to the linear polymer PIDT‐DTBT (EHOMO = −5.30 eV, ELUMO = −3.55 eV). The optical bandgap of PIDT‐fDTBT is obviously broadened from 1.75 to 2.03 eV. This work demonstrates that increasing the polymer backbone curvature can effectively broaden the bandgap by simultaneously decreasing HOMO and increasing LUMO, which may guide the design of WBG conjugated materials. In this work, an effective strategy is provided to design a large bandgap polymer by increasing the polymer backbone curvature. This strategy can reduce the effective conjugation length of the polymer, leading to a decreased highest occupied molecular orbital and a prominently increased lowest unoccupied molecular orbital, which has great potential in designing wide bandgap conjugated materials.
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A comprehensive study on the relationship between the structure and the photophysical and photovoltaic properties of acceptor-donor-acceptor (A-D-A) type nonfullerene acceptors (NFAs) is of pivotal importance to obtain design guidelines for high-performance organic photovoltaics (OPVs). In this study, we synthesized a D unit, NTT, in which the central benzene core, indacenodithieno[3,2-b]thiophene (IT), is replaced by naphthalene. Notably, NTTIC, an A-D-A type NFA with NTT as the D unit, showed a longer singlet exciton lifetime in the film than the IT-based benchmark NFA, ITIC. When paired with a conjugated polymer donor, PBDB-T, the NTTIC-based OPV device exhibited a higher power conversion efficiency (PCE = 9.95%) than the ITIC-based device (9.71%). Furthermore, due to the larger domain size of NTTIC in PBDB-T:NTTIC, the exciton diffusion (ED) at the donor/acceptor interface and the charge transfer (CT) were slower (14 ps in total) than those of PBDB-T:ITIC (7 ps in total). Nevertheless, the total efficiency of the ED and CT in PBDB-T:NTTIC was similar to that in PBDB-T:ITIC (95%) owing to the longer singlet exciton lifetime of the NTTIC film. These results demonstrate the high potential of naphthalene-cored D units of A-D-A type NFAs to achieve a long singlet exciton lifetime and a resultant high PCE in NFA-based OPVs.
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A new fused‐ring electron acceptor FNIC3 with dynamics controlled aggregation behavior was synthesized. FNIC3 shows strong absorption in 600–900 nm, HOMO/LUMO energy levels of −5.59/−4.04 eV, and electron mobility of 1.2 × 10⁻³ cm² V⁻¹ s⁻¹. The aggregation of FNIC3 shows strong dependency on film formation time. Prolongation of film formation time promotes the crystallization of FNIC3, leading to improved crystallinity and enlarged aggregate sizes. Aggregation of FNIC3 significantly influences the photovoltaic device parameters. Appropriate aggregation red‐shifts the absorption and improves the mobilities of the blend, which contributes to high photocurrent and fill factor thus high power conversion efficiency (PCE). Overaggregation leads to increased nonradiative energy loss and insufficient charge generation, resulting in decreased open‐circuit voltage and short‐circuit current density. The blends based on PM6:FNIC3 fabricated under proper film formation time exhibit a PCE of 12.3%, higher than those fabricated under short and long film formation time (10.0–10.5%).
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A thin film encapsulation layer was fabricated through two-sequential chemical vapor deposition processes for organic light emitting diodes (OLEDs). The fabrication process consists of laser assisted chemical vapor deposition (LACVD) for the first silicon nitride layer and laser assisted plasma enhanced chemical vapor deposition (LAPECVD) for the second silicon nitride layer. While SiNx thin films fabricated by LAPECVD exhibits remarkable encapsulation characteristics, OLEDs underneath the encapsulation layer risk being damaged during the plasma generation process. In order to prevent damage from the plasma, LACVD was completed prior to the LAPECVD as a buffer layer so that the laser during LACVD did not damage the devices because there was no direct irradiation to the surface. This two-step thin film encapsulation was performed sequentially in one chamber, which reduced the process steps and increased fabrication time. The encapsulation was demonstrated on green phosphorescent OLEDs with I–V-L measurements and a lifetime test. The two-step encapsulation process alleviated the damage on the devices by 19.5% in external quantum efficiency compared to the single layer fabricated by plasma enhanced chemical vapor deposition. The lifetime was increased 3.59 times compared to the device without encapsulation. The composition of the SiNx thin films was analyzed through Fourier-transform infrared spectroscopy (FTIR). While the atomic bond in the layer fabricated by LACVD was too weak to be used in encapsulation, the layer fabricated by the two-step encapsulation did not reveal a Si–O bonding peak but did show a Si–N peak with strong atomic bonding.
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As an effective molecular modification strategy, side chain engineering has been widely used in promoting the photovoltaic performance of non-fullerene acceptors. Herein, a novel non-fullerene small molecular acceptor i-IEOSi-4F comprising siloxane-terminated alkoxyl side chain was successfully designed and synthesized. The molecule shows an optical band gap of 1.53 eV, with large extinction coefficient of 2.36 × 10⁵ M⁻¹ cm⁻¹ in solution. Two fluorobenzotriazole based polymers J52 and PBZ-2Si with the same backbone units but different side chains were employed as the donor to construct the active layers that all can demonstrate suitable energy levels and complementary absorptions with i-IEOSi-4F. Relative to J52 only bearing alkyl side chain, PBZ-2Si with siloxane-terminated side chain could induce more balanced carrier transports and more favorable morphology, leading to a higher power conversion efficiency (PCE) of 12.66% with a good fill factor of 71.45%. The efficiency is 21% higher than that of 10.46% for the J52 based devices. Our results not only indicate that siloxane-terminated alkoxyl side chain is valuable for efficient non-fullerene acceptors, but also demonstrate that siloxane-terminated side chain on both polymer donor and small molecular acceptor is a useful combination to realize more efficient polymer solar cells.
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In pursuit of an alternative renewable and sustainable energy source to fossil fuels, polymer based photovoltaics has attracted considerable interests among researchers because of their lightweight, low cost and flexibility compared to the inorganic counterparts. The concept of bulk heterojunction (BHJ) device structure by blending both acceptor and donor materials as a one single active layer between the electrodes pioneered the polymer photovoltaics. However, the slow but steady enhancement of power conversion efficiency (PCE) with BHJ device having fullerene as the acceptor material was revolving around 5% until non-fullerene based materials were used as the acceptor material which led to ~16% PCE – mainly due to their wide range of visible and NIR absorbance capabilities. In addition, the recent trend of BHJ based polymer photovoltaics, with an organic perovskite and fullerene as donor and acceptor respectively, exhibit ~18% PCE under sunlight illumination which has raised the potential of polymer photovoltaics by thousand folds as the cheapest form of global energy source in coming years. However, this short review highlights the major advances in polymer based photovoltaics in terms of the choice of acceptors and donors, and discusses future directions and promising perspectives of polymer based photovoltaics.
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Two FREAs, IUIC-O and IUIC-T, based on an undecacyclic core were developed. IUIC-T having higher extinction coefficient, affords aligned energy levels with PBDB-T, finer nanoscale morphology and more orderly molecular...
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The ring-expanded strategy in nonfullerene acceptors (NFAs) with the acceptor–donor–acceptor backbone has been reported to be an effective method to improve the fill factor (FF), open circuit voltage (VOC), and short circuit current (JSC) simultaneously in organic photovoltaics. However, design control is still missing in the ring-expanded strategy, and is urgently needed to further develop the origins and rules. To give insight into this strategy, a detailed theoretical study of the ring-expanded mechanism is performed on the systems comprising different 9,9′-bifluorenylidene-based cores and 1,1-dicyanomethylene-3-indanone group. Some main parameters involved in photoelectric conversion mechanism under the donor excitation (DE) and/or acceptor excitation (AE) are assessed by changing the position and size of ring-expanded modes. The results show that the external ring-expanded modes can not only maintain the original advantage as much as possible, variations in sizes and positions also offer them an opportunity to regulate the aforementioned parameters systematically, leading to better improvement regardless of AE or DE. Thus, the steady improvement in performance mentioned previously is the key to overcoming the negative correlation among FF, VOC, and JSC. This insight and discovery of the ring-expanded strategy provides new design approaches for the next generation of NFAs.
Article
NC-FIC nonfullerene acceptor, using a didodecylnaphthodithiophene (NDT) embedded octacyclic ladder-type donor NC, had been described previously. In this work, a new cross-shaped TC-FIC nonfullerene acceptor is designed by substitution of the NDT unit in the NC skeleton with a 2-dimensional didodecyltetrathienonaphthalene (TTN) unit. The 2-dimensional TC π-system with two vertically fused thiophenes strengthens electron-donating ability, promotes π-electron delocalization and enhances intramolecular donor-acceptor charge transfer and intermolecular π-π interactions. As a result, TC-FIC shows more red-shifted and widened absorption compared to the NC-FIC counterpart. The PBDB-T:TC-FIC: device exhibits a higher efficiency of 11.6% which greatly outperformed the corresponding PBDB-T:NC-FIC device with an efficiency of 7.52%. The dramatic 54% enhancement in efficiency is ascribed to the improved light-harvesting ability and enhanced charge mobilities which are benefited from the central 2-dimensional TTN moiety. By incorporating PC71BM to improve absorption and electron transport, the ternary-blend PBDB-T:TC-FIC:PC71BM device (1:1:1 in wt%) achieves a highest PCE of 13.49% with an enhanced Jsc of 23.80 mA/cm2, an FF of 64.4%, and a Voc of 0.88 V.
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A side-chain conjugation strategy in the design of nonfullerene electron acceptors is proposed, with the design and synthesis of a side-chain-conjugated acceptor (ITIC2) based on a 4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b']di(cyclopenta-dithiophene) electron-donating core and 1,1-dicyanomethylene-3-indanone electron-withdrawing end groups. ITIC2 with the conjugated side chains exhibits an absorption peak at 714 nm, which redshifts 12 nm relative to ITIC1. The absorption extinction coefficient of ITIC2 is 2.7 × 10(5) m(-1) cm(-1) , higher than that of ITIC1 (1.5 × 10(5) m(-1) cm(-1) ). ITIC2 exhibits slightly higher highest occupied molecular orbital (HOMO) (-5.43 eV) and lowest unoccupied molecular orbital (LUMO) (-3.80 eV) energy levels relative to ITIC1 (HOMO: -5.48 eV; LUMO: -3.84 eV), and higher electron mobility (1.3 × 10(-3) cm(2) V(-1) s(-1) ) than that of ITIC1 (9.6 × 10(-4) cm(2) V(-1) s(-1) ). The power conversion efficiency of ITIC2-based organic solar cells is 11.0%, much higher than that of ITIC1-based control devices (8.54%). Our results demonstrate that side-chain conjugation can tune energy levels, enhance absorption, and electron mobility, and finally enhance photovoltaic performance of nonfullerene acceptors.
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A fused hexacyclic electron acceptor, IHIC, based on strong electron-donating group dithienocyclopentathieno[3,2-b]thiophene flanked by strong electron-withdrawing group 1,1-dicyanomethylene-3-indanone, is designed, synthesized, and applied in semitransparent organic solar cells (ST-OSCs). IHIC exhibits strong near-infrared absorption with extinction coefficients of up to 1.6 × 10(5) m(-1) cm(-1) , a narrow optical bandgap of 1.38 eV, and a high electron mobility of 2.4 × 10(-3) cm(2) V(-1) s(-1) . The ST-OSCs based on blends of a narrow-bandgap polymer donor PTB7-Th and narrow-bandgap IHIC acceptor exhibit a champion power conversion efficiency of 9.77% with an average visible transmittance of 36% and excellent device stability; this efficiency is much higher than any single-junction and tandem ST-OSCs reported in the literature.
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A new polymer donor (PBDB-T-SF) and a new small molecule acceptor (IT-4F) for fullerene-free organic solar cells (OSCs) were designed and synthesized. The influences of fluorination on the absorption spectra, molecular energy levels and charge mobilities of the donor and acceptor were systematically studied. The PBDB-T-SF:IT-4F-based OSC device showed a record high efficiency of 13.1%, and an efficiency of over 12% can be obtained with a thickness of 100–200 nm, suggesting the promise of fullerene-free OSCs in practical applications.
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Fabricating organic solar cells (OSCs) with a tandem structure has been considered an effective method to overcome the limited light absorption spectra of organic photovoltaic materials. Currently, the most efficient tandem OSCs are fabricated by adopting fullerene-derivatives as acceptors. Here, we design a new NF-acceptor with an E(opt)g of 1.68 eV for the front sub-cells and optimize the phase separation morphology of a fullerene-free active layer with an E(opt)g of 1.36 eV to fabricate the rear sub-cell. The two sub-cells show a low Eloss and high external quantum efficiency (EQE), and their photo response spectra are complementary. In addition, an interconnection layer composed of ZnO and a pH-neutral self-doped conductive polymer PCP-Na and with a high light transmittance in the near-IR range was developed. Based on the highly optimized sub-cells and ICL, solution-processed fullerene-free tandem OSCs with an average PCE over 13% are demonstrated.
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To achieve efficient non-fullerene organic solar cells, it is important to reduce the voltage loss from the optical bandgap to the open-circuit voltage of the cell. Here we report a highly efficient non-fullerene organic solar cell with a high open-circuit voltage of 1.08 V and a small voltage loss of 0.55 V. The high performance was enabled by a novel wide-bandgap (2.05 eV) donor polymer paired with a narrow-bandgap (1.63 eV) small-molecular acceptor (SMA). Our morphology characterizations show that both the polymer and the SMA can maintain high crystallinity in the blend film, resulting in crystalline and small domains. As a result, our non-fullerene organic solar cells realize an efficiency of 11.6%, which is the best performance for a non-fullerene organic solar cell with such a small voltage loss.
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Inspired by the remarkable promotion of power conversion efficiency (PCE), commercial applications of organic photovoltaics (OPVs) can be foreseen in near future. One of the most promising applications is semitransparent (ST) solar cells that can be utilized in value-added applications such as energy-harvesting windows. However, the single-junction STOPVs utilizing fullerene acceptors show relatively low PCEs of 4%-6% due to the limited sunlight absorption because it is a dilemma that more photons need to be harvested in UV-vis-near-infrared (NIR) region to generate high photocurrent, which leads to the significant reduction of device transparency. This study describes the development of a new small-bandgap electron-acceptor material ATT-2, which shows a strong NIR absorption between 600 and 940 nm with an Eg(opt) of 1.32 eV. By combining with PTB7-Th, the as-cast OPVs yield PCEs of up to 9.58% with a fill factor of 0.63, an open-circuit voltage of 0.73 V, and a very high short-circuit current of 20.75 mA cm(-2) . Owing to the favorable complementary absorption of low-bangap PTB7-Th and small-bandgap ATT-2 in NIR region, the proof-of-concept STOPVs show the highest PCE of 7.7% so far reported for single-junction STOPVs with a high transparency of 37%.
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A new acceptor-donor-acceptor (A-D-A) type small molecule, namely NFBDT, with a heptacyclic benzodi(cyclopentadithiophene) (FBDT) unit based on BDT as the central building block, was designed and synthesized. Its optical, electrical, thermal properties and photovoltaic performances were systematically investigated. NFBDT exhibits a low optical bandgap of 1.56 eV resulting in wide and efficient absorption in the range from 600 to 800 nm, and suitable energy levels as an electron acceptor. With the widely used and successful wide bandgap polymer PBDB-T selected as the donor material, an optimized PCE of 10.42% was obtained for the PBDB-T:NFBDT-based device with an outstanding short-circuit current density of 17.85 mA cm-2 under AM 1.5G irradiation (100 mW cm-2), which is so far among the highest performance of NF-OSC devices. These results demonstrate that BDT unit could also be applied for designing NF-acceptors, and the fused-ring benzodi(cyclopentadithiophene) unit is a promising block for designing new and high performance NF-acceptors.
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A new fluorinated nonfullerene acceptor, ITIC-Th1, has been designed and synthesized by introducing fluorine (F) atoms onto the end-capping group 1,1-dicyanomethylene-3-indanone (IC). On the one hand, incorporation of F would improve intramolecular interaction, enhance the push–pull effect between the donor unit indacenodithieno[3,2-b]thiophene and the acceptor unit IC due to electron-withdrawing effect of F, and finally adjust energy levels and reduce bandgap, which is beneficial to light harvesting and enhancing short-circuit current density (JSC). On the other hand, incorporation of F would improve intermolecular interactions through CF···S, CF···H, and CF···π noncovalent interactions and enhance electron mobility, which is beneficial to enhancing JSC and fill factor. Indeed, the results show that fluorinated ITIC-Th1 exhibits redshifted absorption, smaller optical bandgap, and higher electron mobility than the nonfluorinated ITIC-Th. Furthermore, nonfullerene organic solar cells (OSCs) based on fluorinated ITIC-Th1 electron acceptor and a wide-bandgap polymer donor FTAZ based on benzodithiophene and benzotriazole exhibit power conversion efficiency (PCE) as high as 12.1%, significantly higher than that of nonfluorinated ITIC-Th (8.88%). The PCE of 12.1% is the highest in fullerene and nonfullerene-based single-junction binary-blend OSCs. Moreover, the OSCs based on FTAZ:ITIC-Th1 show much better efficiency and better stability than the control devices based on FTAZ:PC71BM (PCE = 5.22%).
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We design and synthesize four fused-ring electron acceptors based on 6,6,12,12-tetrakis(4-hexylphenyl)-indacenobis(dithieno[3,2-b;2',3'-d]thiophene) as the electron-rich unit and 1,1-dicyanomethylene-3-indanones with 0 to 2 fluorine substituents as the electron-deficient units. These four molecules exhibit broad (550-850 nm) and strong absorption with high extinction coefficients of (2.1-2.5) ×10(5) M(-1) cm(-1). Fluorine substitution down shifts LUMO energy level, red shift absorption spectrum, and enhance electron mobility. The polymer solar cells based on the fluorinated electron acceptors exhibit power conversion efficiencies as high as 11.5%, much higher than that of their nonfluorinated counterpart (7.7%). We investigate the effects of the fluorine atom number and position on electronic properties, charge transport, film morphology, and photovoltaic properties.
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Simutaneously high open circuit voltage and high short circuit current density is a big challenge for achieving high efficiency polymer solar cells due to the excitonic nature of organic semdonductors. Herein, we developed a trialkylsilyl substituted 2D-conjugated polymer with the highest occupied molecular orbital level down-shifted by Si–C bond interaction. The polymer solar cells obtained by pairing this polymer with a non-fullerene acceptor demonstrated a high power conversion efficiency of 11.41% with both high open circuit voltage of 0.94 V and high short circuit current density of 17.32 mA cm−2 benefitted from the complementary absorption of the donor and acceptor, and the high hole transfer efficiency from acceptor to donor although the highest occupied molecular orbital level difference between the donor and acceptor is only 0.11 eV. The results indicate that the alkylsilyl substitution is an effective way in designing high performance conjugated polymer photovoltaic materials.
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Nonfullerene acceptor FDICTF (2,9-bis(2methylene-(3-(1,1-dicyanomethylene)indanone))-7, 12- dihydro- 4, 4, 7, 7, 12, 12- hexaoctyl- 4H- cyclopenta[2″, 1″:5, 6;3″, 4″:5', 6'] diindeno[1, 2- b:1', 2'- b']dithiophene) modified by fusing the fluorene core in a precursor, yields 10.06% high power conversion efficiency, and demonstrates that the ladder and fused core backbone in A-D-A structure molecules is an effective design strategy for high-performance nonfullerene acceptors.
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A thieno[3,4-b]thiophene-based electron acceptor (ATT-1) is designed and synthesized. ATT-1 exhibits planar conjugated framework, broad absorption with large absorption coefficient, and slightly high LUMO energy level. Bulk-heterojunction (BHJ) solar cells based on PTB7-Th electron donor and ATT-1 electron acceptor delivered PCEs of up to 10.07%, which is so far highest for non-fullerene BHJ solar cells using PTB7-Th donor.
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Solution-processed organic photovoltaics (OPV) offer the attractive prospect of low-cost, light-weight and environmentally benign solar energy production. The highest efficiency OPV at present use low-bandgap donor polymers, many of which suffer from problems with stability and synthetic scalability. They also rely on fullerene-based acceptors, which themselves have issues with cost, stability and limited spectral absorption. Here we present a new non-fullerene acceptor that has been specifically designed to give improved performance alongside the wide bandgap donor poly(3-hexylthiophene), a polymer with significantly better prospects for commercial OPV due to its relative scalability and stability. Thanks to the well-matched optoelectronic and morphological properties of these materials, efficiencies of 6.4% are achieved which is the highest reported for fullerene-free P3HT devices. In addition, dramatically improved air stability is demonstrated relative to other high-efficiency OPV, showing the excellent potential of this new material combination for future technological applications.
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Organic solar cells have desirable properties, including low cost of materials, high-throughput roll-to-roll production, mechanical flexibility and light weight. However, all top-performance devices are at present processed using halogenated solvents, which are environmentally hazardous and would thus require expensive mitigation to contain the hazards. Attempts to process organic solar cells from non-halogenated solvents lead to inferior performance. Overcoming this hurdle, here we present a hydrocarbon-based processing system that is not only more environmentally friendly but also yields cells with power conversion efficiencies of up to 11.7%. Our processing system incorporates the synergistic effects of a hydrocarbon solvent, a novel additive, a suitable choice of polymer side chain, and strong temperature-dependent aggregation of the donor polymer. Our results not only demonstrate a method of producing active layers of organic solar cells in an environmentally friendly way, but also provide important scientific insights that will facilitate further improvement of the morphology and performance of organic solar cells.
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A nonfullerene electron acceptor (IEIC) based on indaceno[1,2-b:5,6-b′]dithiophene and 2-(3-oxo-2,3-dihydroinden-1-ylidene)malononitrile was designed and synthesized. IEIC exhibited good thermal stability, strong absorption in the 500–750 nm region with an extinction coefficient of 1.1 × 105 M−1 cm−1 at 672 nm, deep LUMO energy level (−3.82 eV) close to those of fullerenes, and a relatively high electron mobility of 2.1 × 10−4 cm2 V−1 s−1. Fullerene-free polymer solar cells (PSCs) based on the blends of the IEIC acceptor and a low-bandgap polymer donor PTB7-TH, using a perylene diimide derivative as a cathode interlayer, showed power conversion efficiencies (PCEs) of up to 6.31%, which is among the best PCEs reported for fullerene-free PSCs.
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Recent progress in the development of polymer solar cells has improved power-conversion efficiencies from 3% to almost 9%. Based on semiconducting polymers, these solar cells are fabricated from solution-processing techniques and have unique prospects for achieving low-cost solar energy harvesting, owing to their material and manufacturing advantages. The potential applications of polymer solar cells are broad, ranging from flexible solar modules and semitransparent solar cells in windows, to building applications and even photon recycling in liquid-crystal displays. This Review covers the scientific origins and basic properties of polymer solar cell technology, material requirements and device operation mechanisms, while also providing a synopsis of major achievements in the field over the past few years. Potential future developments and the applications of this technology are also briefly discussed.
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We present the development and characterization of a dedicated resonant soft x-ray scattering facility. Capable of operation over a wide energy range, the beamline and endstation are primarily used for scattering from soft matter systems around the carbon K-edge (∼285 eV). We describe the specialized design of the instrument and characteristics of the beamline. Operational characteristics of immediate interest to users such as polarization control, degree of higher harmonic spectral contamination, and detector noise are delineated. Of special interest is the development of a higher harmonic rejection system that improves the spectral purity of the x-ray beam. Special software and a user-friendly interface have been implemented to allow real-time data processing and preliminary data analysis simultaneous with data acquisition.
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The photocurrent in conjugated polymer-fullerene blends is dominated by the dissociation efficiency of bound electron-hole pairs at the donor-acceptor interface. A model based on Onsager's theory of geminate charge recombination explains the observed field and temperature dependence of the photocurrent in PPV:PCBM blends. At room temperature only 60% of the generated bound electron-hole pairs are dissociated and contribute to the short-circuit current, which is a major loss mechanism in photovoltaic devices based on this material system.
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A wide bandgap small molecular acceptor, SFBRCN, containing a 3D spirobifluorene core flaked with a 2,1,3-benzothiadiazole (BT) and end-capped with highly electron-deficient (3-ethylhexyl-4-oxothiazolidine-2-yl)dimalononitrile (RCN) units, has been successfully synthesized as a small molecular acceptor (SMA) for nonfullerene polymer solar cells (PSCs). This SMA exhibits a relatively wide optical bandgap of 2.03 eV, which provides a complementary absorption to commonly used low bandgap donor polymers, such as PTB7-Th. The strong electron-deficient BT and RCN units afford SFBRCN with a low-lying LUMO (lowest unoccupied molecular orbital) level, while the 3D structured spirobifluorene core can effectively suppress the self-aggregation tendency of the SMA, thus yielding a polymer:SMA blend with reasonably small domain size. As the results of such molecular design, SFBRCN enables nonfullerene PSCs with a high efficiency of 10.26%, which is the highest performance reported to date for a large bandgap nonfullerene SMA.
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An effective way to improve the power conversion efficiency of organic solar cells is to use a tandem architecture consisting of two subcells, so that a broader part of the solar spectrum can be used and the thermalization loss of photon energy can be minimized. For a tandem cell to work well, it is important for the subcells to have complementary absorption characteristics and generate high and balanced (matched) currents. This requires a rather challenging effort to design and select suitable active materials for use in the subcells. Here, we report a high-performance solution-processed, tandem solar cell based on the small molecules DR3TSBDT and DPPEZnP-TBO, which offer efficient, complementary absorption when used as electron donor materials in the front and rear subcells, respectively. Optimized devices achieve a power conversion efficiency of 12.50% (verified 12.70%), which represents a new level of capability for solution-processed, organic solar cells.
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Technological deployment of organic photovoltaic modules requires improvements in device light-conversion efficiency and stability while keeping material costs low. Here we demonstrate highly efficient and stable solar cells using a ternary approach, wherein two non-fullerene acceptors are combined with both a scalable and affordable donor polymer, poly(3-hexylthiophene) (P3HT), and a high-efficiency, low-bandgap polymer in a single-layer bulk-heterojunction device. The addition of a strongly absorbing small molecule acceptor into a P3HT-based non-fullerene blend increases the device efficiency up to 7.7 ± 0.1% without any solvent additives. The improvement is assigned to changes in microstructure that reduce charge recombination and increase the photovoltage, and to improved light harvesting across the visible region. The stability of P3HT-based devices in ambient conditions is also significantly improved relative to polymer:fullerene devices. Combined with a low-bandgap donor polymer (PBDTTT-EFT, also known as PCE10), the two mixed acceptors also lead to solar cells with 11.0 ± 0.4% efficiency and a high open-circuit voltage of 1.03 ± 0.01 V.
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Five polymer donors with distinct chemical structures and different electronic properties are surveyed in a planar and narrow-bandgap fused-ring electron acceptor (IDIC)-based organic solar cells, which exhibit power conversion efficiencies of up to 11%.
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Low bandgap n-type organic semiconductor (n-OS) ITIC has attracted great attention for the application as acceptor with medium bandgap p-type conjugated polymer as donor in non-fullerene polymer solar cells (PSCs) because of its attractive photovoltaic performance. Here we report a modification on the molecular structure of ITIC by side chain isomerization with meta-alkyl-phenyl substitution, m-ITIC, to further improve its photovoltaic performance. Compared with its isomeric counterpart ITIC with para-alkyl-phenyl substitution, m-ITIC shows a higher film absorption coefficient, a larger crystalline coherence and higher electron mobility. These inherent advantages of m-ITIC resulted in a higher power conversion efficiency (PCE) of 11.77% for the non-fullerene PSCs with m-ITIC as acceptor and a medium bandgap polymer J61 as donor, which is significantly improved over that (10.57%) of the corresponding devices with ITIC as acceptor. To the best of our knowledge, the PCE of 11.77% is one of the highest values reported in literatures to date for non-fullerene PSCs. More importantly, m-ITIC-based device shows less thickness-dependent photovoltaic behavior than ITIC-based devices in the active-layer thickness range of 80~360 nm, which is beneficial for large area device fabrication. These results indicate that m-ITIC is a promising low bandgap n-OS for the application as acceptor in PSCs and the side chain isomerization could be an easy and convenient way to further improve the photovoltaic performance of the donor and acceptor materials for high efficiency PSCs.
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The structure evolution of oligomer fused-ring electron acceptors (FREAs) toward high efficiency of as-cast polymer solar cells (PSCs) is reported. First, a series of FREAs (IC-(1-3)IDT-IC) based on indacenodithiophene (IDT) oligomers as cores are designed and synthesized, effects of IDT number (1-3) on their basic optical and electronic properties are investigated, and more importantly, the relationship between device performance of as-cast PSCs and donor(D)/acceptor(A) matching (absorption, energy level, morphology, and charge transport) of IC-(1-3)IDT-IC acceptors and two representative polymer donors, PTB7-Th and PDBT-T1 is surveyed. Then, the most promising D/A system (PDBT-T1/IC-1IDT-IC) with the best D/A harmony among the six D/A combinations, which yields a power conversion efficiency (PCE) of 7.39%, is found. Finally, changing the side-chains in IC-1IDT-IC from alkylphenyl to alkyl enhances the PCE from 7.39% to 9.20%.
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A planar fused-ring electron acceptor (IC-C6IDT-IC) based on indacenodithiophene is designed and synthesized. IC-C6IDT-IC shows strong absorption in 500-800 nm with extinction coefficient of up to 2.4 × 105 M-1 cm-1 and high electron mobility of 1.1 × 10-3 cm2 V-1 s-1. The as-cast polymer solar cells based on IC-C6IDT-IC without additional treatments exhibit power conversion efficiencies of up to 8.71%.
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In the past two years, non-fullerene acceptors including polymers and small molecules have become the focus of many research efforts. Fullerene-free organic solar cells (OSCs) have shown efficiencies of up to 6.8% for solution-processed devices, and even up to 8.4% for vacuum-deposited devices, which have been significantly improved relative to those disclosed 2 years ago (generally <4%). Non-fullerene acceptor materials are a new focus in the OSC field. Tailoring extended fused-rings with electron-deficient groups is an effective strategy for design of acceptors. Here, very recent developments in several systems of fused ring-based electron acceptors, such as halogenated (sub or subna)phthalocyanine, imide-functionalized rylene, and linear fused-rings end capped with electron deficient blocks, are reviewed.
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A novel non-fullerene electron acceptor (ITIC) that overcomes some of the shortcomings, for example, weak absorption in the visible spectral region and limited energy level variability, of fullerene acceptors is designed and synthesized. Fullerene-free polymer solar cells (PSCs) based on the ITIC acceptor are demonstrated to exhibit power conversion efficiencies of up to 6.8%, a record for fullerene-free PSCs. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
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Developing novel materials and device architectures to further enhance the efficiency of polymer solar cells requires a fundamental understanding of the impact of chemical structures on photovoltaic properties. Given that device characteristics depend on many parameters, deriving structure-property relationships has been very challenging. Here we report that a single parameter, hole mobility, determines the fill factor of several hundred nanometer thick bulk heterojunction photovoltaic devices based on a series of copolymers with varying amount of fluorine substitution. We attribute the steady increase of hole mobility with fluorine content to changes in polymer molecular ordering. Importantly, all other parameters, including the efficiency of free charge generation and the coefficient of nongeminate recombination, are nearly identical. Our work emphasizes the need to achieve high mobility in combination with strongly suppressed charge recombination for the thick devices required by mass production technologies.
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Although fullerenes and their derivatives, such as PCBM, have been the dominant electron-acceptor materials in organic photovoltaic cells (OPVs), they suffer from some disadvantages, such as weak absorption in the visible spectral region, limited spectral breadth and difficulty in variably tuning the band gap. It is necessary to explore non-fullerene electron acceptors that will not only retain the favorable electron-accepting and transporting properties of fullerenes but also overcome their insufficiencies. After a decade of mediocrity, non-fullerene acceptors are undergoing rapid development and are emerging as a hot area of focus in the field of organic semiconductors. Solution-processed bulk heterojunction (BHJ) OPVs based on non-fullerene acceptors have shown encouraging power conversion efficiencies of over 4%. This article reviews recent developments in several classes of solution-processable non-fullerene acceptors for BHJ OPVs. The remaining problems and challenges along with the key research directions in the near future are discussed.
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The status of understanding of the operation of bulk heterojunction (BHJ) solar cells is reviewed. Because the carrier photoexcitation recombination lengths are typically 10 nm in these disordered materials, the length scale for self-assembly must be of order 10-20 nm. Experiments have verified the existence of the BHJ nanostructure, but the morphology remains complex and a limiting factor. Three steps are required for generation of electrical power: i) absorption of photons from the sun; ii) photoinduced charge separation and the generation of mobile carriers; iii) collection of electrons and holes at opposite electrodes. The ultrafast charge transfer process arises from fundamental quantum uncertainty; mobile carriers are directly generated (electrons in the acceptor domains and holes in the donor domains) by the ultrafast charge transfer (≈70%) with ≈30% generated by exciton diffusion to a charge separating heterojunction. Sweep-out of the mobile carriers by the internal field prior to recombination is essential for high performance. Bimolecular recombination dominates in materials where the donor and acceptor phases are pure. Impurities degrade performance by introducing Shockly-Read-Hall decay. The review concludes with a summary of the problems to be solved to achieve the predicted power conversion efficiencies of >20% for a single cell.
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A solar park based on polymer solar cells is described and analyzed with respect to performance, practicality, installation speed, and energy payback time. It is found that a high voltage installation where solar cells are all printed in series enables an installation rate in Watts installed per minute that far exceed any other PV technology in existence. The energy payback time for the practical installation of polymer solar cell foil on a wooden 250 square meter platform in its present form is 277 days when operated in Denmark and 180 days when operated in southern Spain. The installation and de-installation rate is above 100 m min−1, which, with the present performance and web width, implies installation of >200 W min−1. In comparison, this also exceeds the overall manufacturing speed of the polymer solar cell foil with a width of 305 mm which is currently 1 m min−1 for complete encapsulated and tested foil. It is also significant that simultaneous installation and de-installation which enables efficient schemes for decommissioning and recycling is possible. It is highlighted where research efforts should most rationally be invested in order to make grid electricity from OPV a reality (and it is within reach).
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Organic photovoltaic (OPV) technology has been developed and improved from a fancy concept with less than 1% power conversion effi ciency (PCE) to over 10% PCE, particularly through the efforts in the last decade. The signifi cant progress is the result of multidisciplinary research ranging from chemistry, material science, physics, and engineering. These efforts include the design and synthesis of novel compounds, understanding and controlling the fi lm morphology, elucidating the device mechanisms, developing new device architectures, and improving large-scale manufacture. All of these achievements catalyzed the rapid growth of the OPV technology. This review article takes a retrospective look at the research and development of OPV, and focuses on recent advances of solution-processed materials and devices during the last decade, particular the polymer version of the materials and devices. The work in this fi eld is exciting and OPV technology is a promising candidate for future thin fi lm solar cells.
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We show that rational functionalization of the naphthodithiophene core in copolymers based on naphthodithiophene and naphthobisthiadiazole improves the solubility without an alteration of the electronic structure. Surprisingly, the introduction of linear alkyl chains brings about a drastic change in polymer orientation into the face-on motif, which is beneficial for the charge transport in solar cells. As a result, the present polymers exhibit high power conversion efficiencies of up to ∼8.2% in conventional single-junction solar cells.
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The carrier collection efficiency (ηc) and energy conversion efficiency (ηe) of polymer photovoltaic cells were improved by blending of the semiconducting polymer with C60 or its functionalized derivatives. Composite films of poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene) (MEH-PPV) and fullerenes exhibit ηc of about 29 percent of electrons per photon and ηe of about 2.9 percent, efficiencies that are better by more than two orders of magnitude than those that have been achieved with devices made with pure MEH-PPV. The efficient charge separation results from photoinduced electron transfer from the MEH-PPV (as donor) to C60 (as acceptor); the high collection efficiency results from a bicontinuous network of internal donor-acceptor heterojunctions.
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We discuss the construction of a new SAXS/WAXS beamline at the Advanced Light Source at Lawrence Berkeley Laboratory. The beamline is equipped with a multilayer monochromator in order to obtain a high X-ray flux. The detrimental effects that the increased bandwidth transmitted by this monochromator could have on the data quality of the SAXS and WAXS patterns is shown to be negligible for the experimental program intended to be operated on this beamline.
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Inorganic semiconductor solar cells are well developed and are being deployed worldwide. However, the high cost of their manufacture limits their widespread acceptance as a source of renewable energy. The potential for low-cost manufacturing afforded by organic devices gives organic solar cells the potential to significantly impact the energy landscape, making them useful in a wide range of environments. This review discusses the advances made in developing organic cells. It covers the device structural approaches that have been used to prepare these devices as well as the design of materials for simultaneously achieving high solar absorptivity and optimal device structure for harvesting this electrical energy. Emphasis is on polymeric and composite polymeric/molecular materials.