Eco-Compatible Solvent-Processed Organic Photovoltaic Cells with Over 16% Efficiency

Substitution Effect on Thiobarbituric Acid End Groups for High Open-Circuit Voltage Non-Fullerene Organic Solar Cells

Recent advances in non-fullerene acceptors (NFAs) have resulted in significant improvement in the power conversion efficiencies (PCEs) of organic solar cells (OSCs). In our efforts to boost open-circuit voltage (V_OC) for OSCs, the molecular design employing thiobarbituric acid (TBTA) end groups and an indacenodithieno[3,2-b]thiophene (IDTT) core gives rise to NFAs with significantly raised lowest unoccupied molecular orbital (LUMO) energy level, which, when paired with PCE10, can achieve V_OC's over 1.0 V and decent PCEs that outperform the equivalent devices based on the benchmark ITIC acceptor. While the use of a TBTA end group is effective in tuning energy levels, very little is known about how the alkyl substitution on the TBTA group impacts the solar cell performance. To this end, TBTA end groups are alkylated with linear, branched, and aromatic sidechains to understand the influence on thin-film morphology and related device performances. Our study has confirmed the dependence of solar cell performance on the end-group substituents. More importantly, we reveal the presence of an ideal window of crystallinity associated with the medium-length hydrocarbon chains such as ethyl and benzyl. Deviation to the shorter methyl group makes the acceptor too crystalline to mix with the polymer donor and form proper domains, whereas longer and branched alkyl chains are too sterically bulky and hinder charge transport due to nonideal packing. Such findings underline the comprehensive nature of thin-film morphology and the subtle end-group effects for the design of non-fullerene acceptors.

Efficient Ternary Organic Solar Cells with a New Electron Acceptor Based on 3,4-(2,2-Dihexylpropylenedioxy)thiophene

In this work, a ternary blend strategy based on PBDB-T and two small molecular acceptors (IDTT-OB and IDT-PDOT-C6) is demonstrated to simultaneously improve the photocurrent and reduce the voltage loss in organic solar cells (OSCs). The improved photocurrent is partially due to a broad absorption spectrum of the active layer. In addition, we find that the ternary system possesses a higher degree of crystallinity, smaller domain size, higher domain purity, and higher and more balanced charge-carrier mobilities in comparison with the two corresponding binary systems. The reduced voltage loss in the ternary device is mainly due to a lower energy loss (E_loss) of charge carriers. We achieve a E_loss of only 0.50 eV, which is one of the lowest values reported for the ternary nonfullerene OSCs. Our results have demonstrated that all photovoltaic parameters of ternary OSCs can be simultaneously improved by elaborately selecting the three active layer components.

Crystallography, Morphology, Electronic Structure, and Transport in Non-Fullerene/Non-Indacenodithienothiophene Polymer:Y6 Solar Cells

Emerging nonfullerene acceptors (NFAs) with crystalline domains enable high-performance bulk heterojunction (BHJ) solar cells. Thermal annealing is known to enhance the BHJ photoactive layer morphology and performance. However, the microscopic mechanism of annealing-induced performance enhancement is poorly understood in emerging NFAs, especially regarding competing factors. Here, optimized thermal annealing of model system PBDB-TF:Y6 (Y6 = 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) decreases the open circuit voltage (V_OC) but increases the short circuit current (J_SC) and fill factor (FF) such that the resulting power conversion efficiency (PCE) increases from 14 to 15% in the ambient environment. Here we systematically investigate these thermal annealing effects through in-depth characterizations of carrier mobility, film morphology, charge photogeneration, and recombination using SCLC, GIXRD, AFM, XPS, NEXAFS, R-SoXS, TEM, STEM, fs/ns TA spectroscopy, 2DES, and impedance spectroscopy. Surprisingly, thermal annealing does not alter the film crystallinity, R-SoXS characteristic size scale, relative average phase purity, or TEM-imaged phase separation but rather facilitates Y6 migration to the BHJ film top surface, changes the PBDB-TF/Y6 vertical phase separation and intermixing, and reduces the bottom surface roughness. While these morphology changes increase bimolecular recombination (BR) and lower the free charge (FC) yield, they also increase the average electron and hole mobility by at least 2-fold. Importantly, the increased μ_h dominates and underlies the increased FF and PCE. Single-crystal X-ray diffraction reveals that Y6 molecules cofacially pack via their end groups/cores, with the shortest π-π distance as close as 3.34 Å, clarifying out-of-plane π-face-on molecular orientation in the nanocrystalline BHJ domains. DFT analysis of Y6 crystals reveals hole/electron reorganization energies of as low as 160/150 meV, large intermolecular electronic coupling integrals of 12.1-37.9 meV rationalizing the 3D electron transport, and relatively high μ_e of 10^-4 cm² V^-1 s^-1. Taken together, this work clarifies the richness of thermal annealing effects in high-efficiency NFA solar cells and tasks for future materials design.

Influence of Alkyl Substitution Position on Wide-Bandgap Polymers in High-Efficiency Nonfullerene Polymer Solar Cells

Two wide-bandgap (WBG) conjugated polymers (PBPD-p and PBPD-m) based on phenyl-substituted benzodithiophene (BDT) with the different substitution position of the alkyl side chain and benzodithiophene-4,8-dione (BDD) units are designed and synthesized to investigate the influence of alkyl substitution position on the photovoltaic performance of polymers in polymer solar cells (PSCs). The thermogravimetric analysis, absorption spectroscopy, molecular energy level, X-ray diffraction, charge transport and photovoltaic performance of the polymers are systematically studied. Compared with PBPD-p, PBPD-m exhibits a slight blue-shift but a deeper highest occupied molecular orbital (HOMO) energy level, a tighter alkyl chain packing and a higher hole mobility. The PBPD-m-based PSCs blended with acceptor IT-4F shows a higher power conversion efficiency (PCE) of 11.95% with a high open-circuit voltage (V_oc ) of 0.88 V, a short-circuit current density (J_sc ) of 19.76 mA cm^-2 and a fill factor (FF) of 68.7% when compared with the PCE of 6.97% with a V_oc of 0.81 V, a J_sc of 15.97 mA cm^-2 and an FF of 53.9% for PBPD-p. These results suggest that it is a feasible and effective strategy to optimize photovoltaic properties of WBG polymers by changing the substitution position of alkyl side chain in PSCs.

Delocalization of exciton and electron wavefunction in non-fullerene acceptor molecules enables efficient organic solar cells

A major challenge for organic solar cell (OSC) research is how to minimize the tradeoff between voltage loss and charge generation. In early 2019, we reported a non-fullerene acceptor (named Y6) that can simultaneously achieve high external quantum efficiency and low voltage loss for OSC. Here, we use a combination of experimental and theoretical modeling to reveal the structure-property-performance relationships of this state-of-the-art OSC system. We find that the distinctive π-π molecular packing of Y6 not only exists in molecular single crystals but also in thin films. Importantly, such molecular packing leads to (i) the formation of delocalized and emissive excitons that enable small non-radiative voltage loss, and (ii) delocalization of electron wavefunctions at donor/acceptor interfaces that significantly reduces the Coulomb attraction between interfacial electron-hole pairs. These properties are critical in enabling highly efficient charge generation in OSC systems with negligible donor-acceptor energy offset.

Chlorination: An Effective Strategy for High-Performance Organic Solar Cells

This work summarizes recent developments in polymer solar cells (PSCs) prepared by a chlorination strategy. The intrinsic property of chlorine atoms, the progress of chlorinated polymers and small molecules, and the synergistic effect of chlorination with other methods to elevate solar conversions are discussed. Halogenation of donor-acceptor (D-A) materials is an effective method to improve the performance of PSCs, which mainly affects the push-pull of electrons between donor and acceptor units due to their strong electron-withdrawing capabilities. Although chlorine is less electronegative than fluorine, it can form very strong noncovalent interactions, such as Cl···S and Cl···π interactions, because its empty 3d orbits can help to accept the electron pairs or π electrons. This synergistic effect of electronegativity together with the empty 3d orbits of chlorine atoms leads to increased intramolecular and intermolecular interactions and a much stronger capability to down-shift the molecular energy levels. This work is intended to support a better understanding of the chlorination strategy to modify the material properties, and thus improve the performance of solar devices. Eventually, it will provide the research community with a clearer pathway to choose proper substitution methods according to different situations for high and stable solar energy conversion.

n-Type Molecular Photovoltaic Materials: Design Strategies and Device Applications

The use of photovoltaic technologies has been regarded as a promising approach for converting solar energy to electricity and mitigating the energy crisis, and among these, organic photovoltaics (OPVs) have attracted broad interest because of 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 non-fullerene molecules have some unique merits, such as synthetic simplicity, high tunability of the absorption and energy levels, and small energy loss. In the last 5 years, organic solar cells based on n-type non-fullerene molecules have achieved a significant breakthrough in the power conversion efficiency from approximately 4% to over 17%, which is superior to those of fullerene-based solar cells; meanwhile, n-type non-fullerene molecules have created brand new opportunities for the 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, high-efficiency semitransparent solar cells for power-generating building facades and windows, and indoor photovoltaics for driving low-power-consumption devices. Moreover, to accelerate the pace toward commercialization of OPVs, the existing challenges and future directions are also reviewed from the perspectives of efficiency, stability, and large-area fabrication.

Progress in Stability of Organic Solar Cells

The organic solar cell (OSC) is a promising emerging low-cost thin film photovoltaics technology. The power conversion efficiency (PCE) of OSCs has overpassed 16% for single junction and 17% for organic-organic tandem solar cells with the development of low bandgap organic materials synthesis and device processing technology. The main barrier of commercial use of OSCs is the poor stability of devices. Herein, the factors limiting the stability of OSCs are summarized. The limiting stability factors are oxygen, water, irradiation, heating, metastable morphology, diffusion of electrodes and buffer layers materials, and mechanical stress. The recent progress in strategies to increase the stability of OSCs is surveyed, such as material design, device engineering of active layers, employing inverted geometry, optimizing buffer layers, using stable electrodes and encapsulation materials. The International Summit on Organic Photovoltaic Stability guidelines are also discussed. The potential research strategies to achieve the required device stability and efficiency are highlighted, rendering possible pathways to facilitate the viable commercialization of OSCs.

Reducing the Singlet-Triplet Energy Gap by End-Group π-π Stacking Toward High-Efficiency Organic Photovoltaics

To improve the power conversion efficiencies for organic solar cells, it is necessary to enhance light absorption and reduce energy loss simultaneously. Both the lowest singlet (S1) and triplet (T1) excited states need to energertically approach the charge-transfer state to reduce the energy loss in exciton dissociation and by triplet recombination. Meanwhile, the S1 energy needs to be decreased to broaden light absorption. Therefore, it is imperative to reduce the singlet-triplet energy gap (ΔE_ST ), particularly for the narrow-bandgap materials that determine the device T1 energy. Although maximizing intramolecular push-pull effect can drastically decrease ΔE_ST , it inevitably results in weak oscillator strength and light absorption. Herein, large oscillator strength (≈3) and a moderate ΔE_ST (0.4-0.5 eV) are found for state-of-the-art A-D-A small-molecule acceptors (ITIC, IT-4F, and Y6) owing to modest push-pull effect. Importantly, end-group π-π stacking commonly in the films can substantially decrease the S1 energy by nearly 0.1 eV, but the T1 energy is hardly changed. The obtained reduction of ΔE_ST is crucial to effectively suppress triplet recombination and acquire small exciton dissociation driving force. Thus, end-group π-π stacking is an effective way to achieve both small energy loss and efficient light absorption for high-efficiency organic photovoltaics.

Effects of the Isomerized Thiophene-Fused Ending Groups on the Performances of Twisted Non-Fullerene Acceptor-Based Polymer Solar Cells

Recently, benefiting from the merits of small-molecule acceptors (NFAs), polymer solar cells (PSCs) have achieved tremendous advances. From the perspective of the structural characteristics of the π-conjugated acceptor-donor-acceptor (A-D-A) type of organic molecules, the backbone's planarity and the terminal groups and their substituents have strong influences on the performances of the constructed NFAs. Through enlarging the dihedral angle of the conjugated main chain of NFAs, a certain degree of enhancement of photovoltaic parameters has been achieved. To further probe the influences of ending groups on the performances of nonplanar NFAs, we synthesized two new NFAs i-cc23 and i-cc34 with isomerized thiophene-fused ending groups and a twisted π-conjugated main chain. Compared to i-cc23 containing the 2-(6-oxo-5,6-dihydro-4H-cyclopenta[b]thiophen-4-ylidene)malononitrile ending group, the acceptor i-cc34 containing 2-(6-oxo-5,6-dihydro-4H-cyclopenta[c]thiophen-4-ylidene)malononitrile has a relatively higher molar extinction coefficient, bathochromic-shifted absorption spectrum, and deepened energy levels. When mixed with PBDB-T in solar cells, the i-cc23-based device achieved an excellent open-circuit voltage (V_OC) of 1.10 V and a moderate power conversion efficiency of 7.34%. Although the V_OC of the i-cc34-related device was decreased to 0.96 V, the short-circuit current density and fill factor were improved, giving rise to an enhanced efficiency of 9.51%. Apart from the distinct photovoltaic performances, the two isomer-based devices exhibit a high radiative efficiency of 8 × 10^-4, leading to a very small nonradiative loss of 0.19 V. Our results emphasize the importance of the isomerized thiophene-fused ending groups on the performances of nonplanar NFA-based PSCs.

Nonfused Nonfullerene Acceptors with an A-D-A'-D-A Framework and a Benzothiadiazole Core for High-Performance Organic Solar Cells

Nonfullerene acceptors (NFAs) have contributed significantly to the progress of organic solar cells (OSCs). However, most NFAs feature a large fused-ring backbone, which usually requires a tedious multiple-step synthesis, and are not applicable to commercial applications. An alternative strategy is to develop nonfused NFAs, which possess synthetic simplicity and facile tunability in optoelectronic properties and solid-state microstructures. In this work, we report two nonfused NFAs, BTCIC and BTCIC-4Cl, based on an A-D-A'-D-A architecture, which possess the same electron-deficient benzothiadiazole central core but different electron-withdrawing terminal groups. The optical properties, energy levels, and molecular crystallinities were finely tuned by changing the terminal groups. Moreover, a decent power conversion efficiency of 9.3 and 10.5% has been achieved by BTCIC and BTCIC-4Cl, respectively, by blending them with an appropriate polymer donor. These results demonstrate the potential of A-D-A'-D-A type nonfused NFAs for high-performance OSCs. Further development of nonfused NFAs will be very fruitful by employing appropriate building blocks and via side-chain optimizations.

Flexible All-Solution-Processed Organic Solar Cells with High-Performance Nonfullerene Active Layers

All-solution-processed organic solar cells (from the bottom substrate to the top electrode) are highly desirable for low-cost and ubiquitous applications. However, it is still challenging to fabricate efficient all-solution-processed organic solar cells with a high-performance nonfullerene (NF) active layer. Issues of charge extraction and wetting are persistent at the interface between the nonfullerene active layer and the printable top electrode (PEDOT:PSS). In this work, efficient all-solution-processed NF organic solar cells (from the bottom substrate to the top electrode) are reported via the adoption of a layer of hydrogen molybdenum bronze (H_X MoO₃ ) between the active layer and the PEDOT:PSS. The dual functions of H_X MoO₃ include: 1) its deep Fermi level of -5.44 eV can effectively extract holes from the active layer; and 2) the wetting issues of the PEDOT:PSS on the hydrophobic surface of the NF active layer can be solved. Importantly, fine control of the H_X MoO₃ composition during the synthesis is critical in obtaining processing orthogonality between H_X MoO₃ and the PEDOT:PSS. Flexible all-solution-processed NF organic solar cells with power conversion efficiencies of 11.9% and 10.3% are obtained for solar cells with an area of 0.04 and 1 cm² , respectively.

Over 15.7% Efficiency of Ternary Organic Solar Cells by Employing Two Compatible Acceptors with Similar LUMO Levels

Efficient organic solar cells (OSCs) are fabricated using polymer PM6 as donor, and IPTBO-4Cl and MF1 as acceptors. The power conversion efficiency (PCE) of IPTBO-4Cl based and MF1 based binary OSCs individually arrive to 14.94% and 12.07%, exhibiting markedly different short circuit current density (J_SC ) of 23.18 mA cm^-2 versus 17.01 mA cm^-2 , fill factor (FF) of 72.17% versus 78.18% and similar open circuit voltage (V_OC ) of 0.893 V versus 0.908 V. The two acceptors, IPTBO-4Cl and MF1, have similar lowest unoccupied molecular orbital levels, which is beneficial for efficient electron transport in the ternary active layer. The PCE of optimized ternary OSCs arrives to 15.74% by incorporating 30 wt% MF1 in acceptors, resulting from the simultaneously increased J_SC of 23.20 mA cm^-2 , V_OC of 0.897 V, and FF of 75.64% in comparison with IPTBO-4Cl based binary OSCs. The gradually increased FFs of ternary OSCs indicate the well-optimized phase separation and molecular arrangement with MF1 as morphology regulator. This work may provide a new viewpoint for selecting an appropriate third component to achieve efficient ternary OSCs from materials and photovoltaic parameters of two binary OSCs.

Recent progress in indoor organic photovoltaics

Among various potential applications of organic photovoltaics (OPVs), indoor power generation has great potential because of several advantages over outdoor light harvesting under 1 sun conditions. Commonly used indoor light sources have narrower emission spectra with lower intensity (by 3 orders of magnitude) as compared to the solar spectrum. Highly tunable optical absorption, large absorption coefficients, and small leakage currents under dim lighting conditions make OPVs promising candidates for indoor applications. For optimizing indoor photovoltaic materials and devices, several key issues (different from those under 1 sun conditions), such as developing new indoor photovoltaic materials and devices with suitable absorption spectra, large open-circuit voltages with low energy loss, minimized trap-mediated charge recombination and leakage currents, and device stability under indoor conditions, should be considered carefully. In this review, the recent progress in optimization of indoor photovoltaic materials and devices, and the key strategies to optimize the indoor photovoltaic characteristics will be summarized and discussed.

A "σ-Hole"-Containing Volatile Solid Additive Enabling 16.5% Efficiency Organic Solar Cells

Here we introduce a σ-hole-containing volatile solid additive, 1, 4-diiodotetrafluorobenzene (A3), in PM6:Y6-based OSCs. Aside from the appropriate volatility of A3 additive, the synergetic halogen interactions between A3 and photoactive matrix contribute to more condensed and ordered molecular arrangement in the favorable interpenetrating donor/acceptor domains. As a result, greatly accelerated charge transport process with suppressed charge recombination possibility is observed and ultimately a champion PCE value of 16.5% is achieved. Notably, the A3 treated OSCs can maintain a high efficiency of over 16.0% in a wide concentration range of A3 additive between 10 and 35 mg/mL. The A3-treated device shows excellent stability with an efficiency of 15.9% after 360-h storage. This work demonstrates that the σ-hole interaction can be applied to enhance the OSC performance and highlights the importance of non-covalent interactions in the optoelectronic materials.

Graphdiyne Derivative as Multifunctional Solid Additive in Binary Organic Solar Cells with 17.3% Efficiency and High Reproductivity

Morphology tuning of the blend film in organic solar cells (OSCs) is a key approach to improve device efficiencies. Among various strategies, solid additive is proposed as a simple and new way to enable morphology tuning. However, there exist few solid additives reported to meet such expectations. Herein, chlorine-functionalized graphdiyne (GCl) is successfully applied as a multifunctional solid additive to fine-tune the morphology and improve device efficiency as well as reproductivity for the first time. Compared with 15.6% efficiency for control devices, a record high efficiency of 17.3% with the certified one of 17.1% is obtained along with the simultaneous increase of short-circuit current (J_sc ) and fill factor (FF), displaying the state-of-the-art binary organic solar cells at present. The redshift of the film absorption, enhanced crystallinity, prominent phase separation, improved mobility, and decreased charge recombination synergistically account for the increase of J_sc and FF after introducing GCl into the blend film. Moreover, the addition of GCl dramatically reduces batch-to-batch variations benefiting mass production owing to the nonvolatile property of GCl. All these results confirm the efficacy of GCl to enhance device performance, demonstrating a promising application of GCl as a multifunctional solid additive in the field of OSCs.

Design Principles and Synergistic Effects of Chlorination on a Conjugated Backbone for Efficient Organic Photovoltaics: A Critical Review

The pursuit of low-cost, flexible, and lightweight renewable power resources has led to outstanding advancements in organic solar cells (OSCs). Among the successful design principles developed for synthesizing efficient conjugated electron donor (ED) or acceptor (EA) units for OSCs, chlorination has recently emerged as a reliable approach, despite being neglected over the years. In fact, several recent studies have indicated that chlorination is more potent for large-scale production than the highly studied fluorination in several aspects, such as easy and low-cost synthesis of materials, lowering energy levels, easy tuning of molecular orientation, and morphology, thus realizing impressive power conversion efficiencies in OSCs up to 17%. Herein, an up-to-date summary of the current progress in photovoltaic results realized by incorporating a chlorinated ED or EA into OSCs is presented to recognize the benefits and drawbacks of this interesting substituent in photoactive materials. Furthermore, other aspects of chlorinated materials for application in all-small-molecule, semitransparent, tandem, ternary, single-component, and indoor OSCs are also presented. Consequently, a concise outlook is provided for future design and development of chlorinated ED or EA units, which will facilitate utilization of this approach to achieve the goal of low-cost and large-area OSCs.

Nonfullerene Ternary Organic Solar Cell with Effective Charge Transfer between Two Acceptors

High power conversion efficiency can be realized by using a ternary bulk heterojunction with complementary absorption spectra in organic solar cells. However, as the development of nonfullerene acceptors with a broad absorption spectrum makes the absorption efficiency of the photovoltaic devices close to optimal, such a strategy needs modifying. In particular, charge transfer between the two acceptors is necessary to be considered. Herein, we purposely design a ternary system based on PTB7-Th:CO_i8DFIC:ITIC-4F. Though the presence of ITIC-4F in PTB7-Th:CO_i8DFIC could not broaden the absorption spectrum obviously, the formed cascade-energy-level alignment is beneficial for promoting and balancing exciton separation and charge transport between the donor and two acceptors and even between the acceptors. Insights into the charge transport route in the completed system are provided via using the techniques including photoluminescence spectroscopy and pump-probe photoconductivity spectroscopy. This work provides a new idea for designing highly efficient ternary organic solar cells.

Regulating the Packing of Non-Fullerene Acceptors via Multiple Noncovalent Interactions for Enhancing the Performance of Organic Solar Cells

Three noncovalently fused-ring electron acceptors (FOC6-IC, FOC6-FIC, and FOC2C6-2FIC) are synthesized. Single crystals of FOC6-IC and FOC2C6-2FIC are prepared, and structure analyses reveal that the molecular backbone can be planarized via the formation of the intramolecular noncovalent interactions. These acceptor molecules can be packed closely in the solid state via π-π stacking and static interactions between the central phenylene unit and the terminal group with a distance of 3.3-3.4 Å. Besides, multiple intermolecular noncovalent interactions can be observed in the single crystal structure of the fluorinated acceptor FOC2C6-2FIC, which help increase the crystallinity of acceptors and the charge mobility of the blends. Photovoltaic devices based on FOC2C6-2FIC give a power conversion efficiency of 12.36%, higher than 12.08% for FOC6-FIC and 10.80% for FOC6-IC.

Effects of Monofluorinated Positions at the End-Capping Groups on the Performances of Twisted Non-Fullerene Acceptor-Based Polymer Solar Cells

Recently, main-chain twisted small molecules are attractive as electron-acceptors in polymer solar cells (PSCs) for their upshifted molecular energy levels, enhanced extinction coefficients, and better charge extraction properties along with longer carrier lifetimes and lower recombination rates relative to their planar analogues, which are conducive to the power conversion efficiency (PCE) promotion of PSCs. To further probe the "structure-performance" correlation of main-chain twisted acceptors, in particular the monofluorine-substituted sites on the performances of the resultant acceptors, two new main-chain twisted small molecules were synthesized, in which a fluorine atom was introduced at different sites on the end-capping group 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (INCN). Although fine structural modification was adopted, quite different performances were obtained for the two acceptors. Compared to the 3-fluorinated analogue (i-IEICO-F3), a mixture of 4-fluorinated and 5-fluorinated isomers (i-IEICO-2F) exhibited a higher dipole moment, enlarged molar extinction coefficient with a bathochromic-shifted absorption region, suppressed charge recombinations with balanced charge mobilities, and slightly enhanced crystallinity. In combination with a fluorobenzotriazole-based medium-band gap polymer (J52), a high efficiency of 12.86% was resultantly achieved in an i-IEICO-2F-based device, which is superior to the result (7.65%) of the i-IEICO-F3 device, revealing the importance of monofluorinated positions on the performances of main-chain twisted non-fullerene acceptors.

Recent progress in wide bandgap conjugated polymer donors for high-performance nonfullerene organic photovoltaics

This feature article summarizes our recent achievements in the development of wide bandgap polymer donors as high-performance organic photovoltaics.

On the relations between backbone thiophene functionalization and charge carrier mobility of A–D–A type small molecules

Backbone thiophene functionalization: an efficient way to improve the charge carrier mobility of A–D–A type small molecules.

Effects of linear and branched side chains on the redox and optoelectronic properties of 3,4-dialkoxythiophene polymers

Identification of relevant structure–property relationships on solution-processable conjugated polymers have been shown to improve the performance of various redox properties.

Efficient organic solar cells based on a new “Y-series” non-fullerene acceptor with an asymmetric electron-deficient-core

Herein, a new “Y-series” non-fullerene acceptor, Y21, bearing an asymmetric electron-deficient-core (DA′D) and fluorinated dicyanomethylene derivatives as flanking groups, was designed and synthesized for organic solar cell applications.

Subtle Molecular Tailoring Induces Significant Morphology Optimization Enabling over 16% Efficiency Organic Solar Cells with Efficient Charge Generation

Manipulating charge generation in a broad spectral region has proved to be crucial for nonfullerene-electron-acceptor-based organic solar cells (OSCs). 16.64% high efficiency binary OSCs are achieved through the use of a novel electron acceptor AQx-2 with quinoxaline-containing fused core and PBDB-TF as donor. The significant increase in photovoltaic performance of AQx-2 based devices is obtained merely by a subtle tailoring in molecular structure of its analogue AQx-1. Combining the detailed morphology and transient absorption spectroscopy analyses, a good structure-morphology-property relationship is established. The stronger π-π interaction results in efficient electron hopping and balanced electron and hole mobilities attributed to good charge transport. Moreover, the reduced phase separation morphology of AQx-2-based bulk heterojunction blend boosts hole transfer and suppresses geminate recombination. Such success in molecule design and precise morphology optimization may lead to next-generation high-performance OSCs.

Electrospinning a Dye-Sensitized Solar Cell

Dye-sensitized solar cells (DSSCs) offer new possibilities to harvest solar energy by using non-toxic inexpensive materials. Since they can generally be produced on flexible substrates, several research groups investigated possibilities to integrate DSSCs in textile fabrics, either by coating full fabrics with the DSSC layer structure or by producing fiber-shaped DSSCs which were afterwards integrated into a textile fabric. Here we show a new approach, electrospinning all solid layers of the DSSC. We report on electrospinning the counter electrode with a graphite catalyst followed by a thin nonconductive barrier layer and preparing the front electrode by electrospinning semiconducting TiO2 from a polymer solution dyed with natural dyes. Both electrodes were coated with a conductive polymer before the system was finally filled with a fluid electrolyte. While the efficiency is lower than for glass-based cells, possible problems such as short-circuits—which often occur in fiber-based DSSCs—did not occur in this proof-of-concept. Since graphite particles did not fully cover the counter electrode in this first study, and the typical bathochromic shift indicating adsorption of dye molecules on the TiO2 layer was not observed, several ways are open to increase the efficiency in forthcoming studies.

Achieving Optimal Bulk Heterojunction in All-Polymer Solar Cells by Sequential Processing with Nonorthogonal Solvents

Developing efficient all-polymer solar cells (all-PSCs) has always been a long-standing challenge due to the unfavorable morphology caused by conventional blend casting (BC). Here, we first employ the methodology of sequential processing (SP) with nonorthogonal solvents to fabricate facilely all-PSCs. A highly crystalline polymer donor, PBDB-T, is used to construct a well-organized underlying film, while a new polymer, FPDI-BT1, is selected as the acceptor to be intercalated into the amorphous or semicrystalline regions of PBDB-T during the secondary deposition. By tuning the solvent composition for FPDI-BT1 processing, a bulk heterojunction-like configuration, rather than a traditional bilayer device, is obtained facilely without the need of further processing treatment. The extremely boosted power conversion efficiency of 7.15% from the SP device is achieved, which is more than twice as efficient as the BC analogue (3.57%). The results demonstrate that SP is a promising strategy to fabricate high-performance all-PSCs with tunable configurations of active layers.

Random Polymer Donor for High-Performance Polymer Solar Cells with Efficiency over 14

Constructing random copolymers has been regarded as an easy and effective approach to design polymer donors for state-of-the-art polymer solar cells (PSCs). In this work, we develop a naphtho[2,3-c]thiophene-4,9-dione-based copolymer PBN-Cl as a donor material for PSCs, and a moderate power conversion efficiency (PCE) of 11.21% is achieved with a relatively low fill factor (FF) of 0.615. We then incorporate a similar acceptor unit benzo[1,2-c:4,5-c']dithiophene-4,8-dione (BDD) into the polymeric backbone of PBN-Cl to tune its photovoltaic performance, and a significantly higher PCE of 14.05% is achieved from the random polymer PBN-Cl-B80 containing 80% BDD unit. The enhanced PCE of the PBN-Cl-B80-based device mainly relies on the higher FF value, resulting from the improved charge mobility properties, reduced bimolecular and trap-assisted recombination, and more appropriate phase separation. The results demonstrate a feasible strategy to tune the photovoltaic performance of polymer donors by constructing a random polymer with a compatible component.

Highly Efficient Indoor Organic Solar Cells by Voltage Loss Minimization through Fine-Tuning of Polymer Structures

Herein, we report a detailed study on the optoelectronic properties, photovoltaic performance, structural conformation, morphology variation, charge carrier mobility, and recombination dynamics in bulk heterojunction solar cells comprising a series of donor-acceptor conjugated polymers as electron donors based on benzodithiophene (BDT) and 5,8-bis(5-bromothiophen-2-yl)-6,7-difluoro-2,3-bis(3-(octyloxy)phenyl)quinoxaline as a function of the BDT's thienyl substitution (alkyl (WF3), alkylthio (WF3S), and fluoro (WF3F)). The synergistic positive effects of the fluorine substituents on the minimization of the bimolecular recombination losses, the reduction of the series resistances (R_S), the increment of the shunt resistances (R_Sh), the suppression of the trap-assisted recombination losses, the balanced charge transport, the finer nanoscale morphology, and the deeper highest occupied molecular orbital (E_HOMO) are manifested versus the alkyl and alkylthio substituents. According to these findings, the WF3F:[6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM)-based organic photovoltaic device is a rare example that features a high power conversion efficiency (PCE) of 17.34% under 500 lx indoor light-emitting diode light source with a high open-circuit voltage (V_OC) of 0.69 V, due to the suppression of the voltage losses, and a PCE of 9.44% at 1 sun (100 mW/cm²) conditions, simultaneously.