All-small-molecule organic solar cells with over 14% efficiency by optimizing hierarchical morphologies

Electron-deficient TVT unit-based D-A polymer donor for high-efficiency thick-film OSCs

As the power conversion efficiencies of organic solar cells (OSCs) have been improved continuously in recent years, more attention will be paid to the industrial production and practical application of OSCs. However, there are still many problems to be solved in the process of large-scale production. Among them, reducing the costs of the materials and enhancing the film-thickness tolerance of the active layer are the two key points. Therefore, it is urgent to develop organic semiconductor materials which are easy to synthesize and suitable for the construction of high-efficiency, thick-film OSCs. In this work, we have focused on the (E)-2-[2-(thiophen-2-yl)vinyl]thiophene (TVT) unit because of its unique coplanar structure. And we noticed that TVT was mostly used as an electron-donating unit in the previous reports. However, we have modified TVT into electron-withdrawing unit by the introduction of fluorine atoms/ester groups. And two new donor-acceptor (D-A) copolymers have been obtained by combining the electron-withdrawing TVT unit with benzo[2,1-b:4,5-b']dithiophene (BDT) unit. Among them, the polymer based on the ester modified TVT unit presents excellent photovoltaic performance by virtue of its good solubility and preferable molecular stacking mode, and the corresponding devices also show extraordinarily high-thickness tolerance. The emergence of this new electron-withdrawing TVT unit will undoubtedly further promote the development of low-cost, high-efficiency, thick-film OSCs.

Modulating Crystal Packing, Film Morphology, and Photovoltaic Performance of Selenophene-Containing Acceptors through a Combination of Skeleton Isomeric and Regioisomeric Strategies

Here, we report a series of acceptor-donor-acceptor (A-D-A) architecture isomeric acceptors (SeCT-IC, CSeT-IC, and CTSe-IC), which have an identical electron-deficient terminal A-group and three different central D-cores with the selenophene at the innermost, relatively outer, and outermost positions of the central core, respectively. From CSeT-IC to the atom regioisomer of CTSe-IC and to the conjugated skeleton isomer of SeCT-IC, the optical band gap of neat films continuously reduced and highest occupied molecular orbitals (HOMO) gradually upshifted with changing the selenophene from relatively outer position to the outermost position and to the innermost position of the central core. More importantly, the single-crystal structure and the GIWAXS measurements revealed that CTSe-IC presents the closest π-π stacking distance, the largest CCL, and the best molecular order and crystallinity, which led to the highest electron mobility in neat films. Furthermore, the J71:CTSe-IC blend film presents a more ordered film morphology with more proper phase separation domain size, more dominant face-on orientation, and relatively higher and more balanced electron-hole mobilities in comparison with that of J71:SeCT-IC and J71:CSeT-IC. Consequently, the J71:CTSe-IC-based organic solar cell gave a superior power conversion efficiency (PCE) of 11.59%, which was obviously higher than those for J71:SeCT-IC (10.89%) and J71:CSeT-IC (8.52%). Our results demonstrate that the acceptor with selenophene in the outermost position led to significantly enhance the PCE. More importantly, rational modulation of the central fused core in combination with the conjugated skeleton isomeric method and the atom regioisomeric method provides an effective way to understand the structure-crystallinity-photovoltaic property relationship of selenophene-based regioisomers.

Indacenodithiophene-based small-molecule donor with strong crystallinity for efficient organic solar cells

Two indacenodithiophene (IDT)-based small-molecule analogues (IDBT and IDBT-Cl) are designed as donor materials for organic solar cells. Relative to the amorphous IDBT-Cl, the IDBT with strong crystallinity shows overall better photovoltaic performance when blended with a Y6 acceptor. The results demonstrate the great potential of IDT units in designing efficient small-molecule donors.

18.5% Efficiency Organic Solar Cells with a Hybrid Planar/Bulk Heterojunction

The donor:acceptor heterojunction has proved as the most successful approach to split strongly bound excitons in organic solar cells (OSCs). Establishing an ideal architecture with selective carrier transport and suppressed recombination is of great importance to improve the photovoltaic efficiency while remains a challenge. Herein, via tailoring a hybrid planar/bulk structure, highly efficient OSCs with reduced energy losses (E_loss s) are fabricated. A p-type benzodithiophene-thiophene alternating polymer and an n-type naphthalene imide are inserted on both sides of a mixed donor:acceptor active layer to construct the hybrid heterojunction, respectively. The tailored structure with the donor near the anode and the acceptor near the cathode is beneficial for obtaining enhanced charge transport, extraction, and suppressed charge recombination. As a result, the photovoltaic characterizations suggest a reduced nonradiative E_loss by 25 meV, and the best OSC records a high efficiency of 18.5% (certified as 18.2%). This study highlights that precisely regulating the structure of donor:acceptor heterojunction has the potential to further improve the efficiencies of OSCs.

Supramolecular Reorientation During Deposition Onto Metal Surfaces of Quasi-Two-Dimensional Langmuir Monolayers Composed of Bifunctional Amphiphilic, Twisted Perylenes

Supramolecular dye structures, which are often ruled by π-π interactions between planar chromophores, crucially determine the optoelectronic properties of layers and interfaces. Here, we present the interfacial assembly of perylene monoanhydride and monoimide that do not feature a planar chromophore but contain chlorine substituents in the bay positions to yield twisted chromophores and hence modified π-stacking. The assembly of the twisted perylene monoanhydride and monoimide is driven by their amphiphilicity that ensures proper Langmuir layer formation. The shielding of the hydrophilic segment upon attaching an alkyl chain to the imide moiety yielded a more rigid Langmuir layer, even though the degrees of freedom were increased due to this modification. For the characterization of the Langmuir layer's supramolecular structure, the layers were deposited onto glass, silver, and gold substrates via Langmuir-Blodgett (LB) and Langmuir-Schaefer (LS) techniques and were investigated with atomic force microscopy and surface-enhanced resonance Raman spectroscopy (SERRS). From the similarity between all SERR spectra of the LS and LB layers, we concluded that the perylenes have changed their orientation upon LB deposition to bind to the silver surface of the SERRS substrate via sulfur atoms. In the Langmuir layer, the perylenes, which are π-stacked with half of the twisted chromophores, must already be inclined and cannot achieve full parallel alignment because of the twisting-induced steric hindrance. However, upon rotation, the energetically most favorable antiparallel aligned structures can be formed and bind to the SERRS substrate. Thus, we present, to the best of our knowledge, the first fabrication of quasi-two-dimensional films from twisted amphiphilic perylene monoimides and their reassembly during LB deposition. The relation between the molecular structure, supramolecular interfacial assembly, and its adoption during adsorption revealed here is crucial for the fabrication of defined functionalizations of metal surfaces, which is key to the development of organic (opto)electronic devices.

13.4 % Efficiency from All-Small-Molecule Organic Solar Cells Based on a Crystalline Donor with Chlorine and Trialkylsilyl Substitutions

How to simultaneously achieve both high open-circuit voltage (Voc ) and high short-circuit current density (Jsc ) is a big challenge for realising high power conversion efficiency (PCE) in all-small-molecule organic solar cells (all-SM OSCs). Herein, a novel small molecule (SM)-donor, namely FYSM-SiCl, with trialkylsilyl and chlorine substitutions was designed and synthesized. Compared to the original SM-donor FYSM-H, FYSM-Si with trialkylsilyl substitution showed a decreased crystallinity and lower highest occupied molecular orbital (HOMO) level, while FYSM-SiCl had an improved crystallinity, more ordered packing arrangement, significantly lower HOMO level, and predominant "face-on" orientation. Matched with a SM-acceptor Y6, the FYSM-SiCl-based all-SM OSCs exhibited both high Voc of 0.85 V and high Jsc of 23.7 mA cm-2 , which is rare for all-SM OSCs and could be attributed to the low HOMO level of FYSM-SiCl donor and the delicate balance between high crystallinity and suitable blend morphology. As a result, FYSM-SiCl achieved a high PCE of 13.4 % in all-SM OSCs, which was much higher than those of the FYSM-H- (10.9 %) and FYSM-Si-based devices (12.2 %). This work demonstrated a promising method for the design of efficient SM-donors by a side-chain engineering strategy via the introduction of trialkylsilyl and chlorine substitutions.

Introducing Siloxane-Terminated Side Chains in Small Molecular Donors for All-Small-Molecule Organic Solar Cells: Modulated Molecular Orientation and Enhanced Efficiency

In this work, three small molecular donors (SMDs) S35, S35-1Si, and S35-2Si, with 3,5-difluorophenyl-substituted benzodithiophene as the central 2-dimensional unit to combine different numbers of siloxane-terminated side chain, were synthesized for all-small-molecule organic solar cells (ASM-OSCs). The three SMDs showed comparable film absorption peaks at 570 nm and optical band gaps of 1.8 eV. Relative to S35 and S35-1Si with symmetric alkyl side chains and asymmetric side chains on the central unit, respectively, the S35-2Si carrying two symmetric siloxane-terminated side chains displayed largely elevated melting and crystalline temperatures, lowered surface energy, and modulated molecular orientation. The three SMDs possessed edge-on dominated molecular orientations of their neat films; however, a big difference was found for their blend films with nonfullerene acceptor Y6. The S35:Y6 and S35-1Si:Y6 blends exhibited edge-on and face-on bimodal orientations but the S35-2Si:Y6 blend showed pure face-on orientation, indicating quite different donor:acceptor intermolecular interactions. Some large domains existed in the S35:Y6 and S35-1Si:Y6 blends, but could be suppressed by the S35-2Si:Y6 blend, leading to a more balanced charge transport. In ASM-OSCs, the two S35:Y6 and S35-1Si:Y6 active layers showed comparable power conversion efficiencies (PCE) of ∼12% but a much higher efficiency of 13.50% could be achieved with the S35-2Si:Y6 active layer. Our results suggest that the siloxane-terminated side chain is promising to regulate crystalline ability of a SMD, paving a way for high performance ASM-OSCs.

Accurate and Efficient Computation of Optical Absorption Spectra of Molecular Crystals: The Case of the Polymorphs of ROY

When calculating the optical absorption spectra of molecular crystals from first principles, the influence of the crystalline environment on the excitations is of significant importance. For such systems, however, methods to describe the excitations accurately can be computationally prohibitive due to the relatively large system sizes involved. In this work, we demonstrate a method that allows optical absorption spectra to be computed both efficiently and at high accuracy. Our approach is based on the spectral warping method successfully applied to molecules in solvent. It involves calculating the absorption spectrum of a supercell of the full molecular crystal using semi-local time-dependent density functional theory (TDDFT), before warping the spectrum using a transformation derived from smaller-scale semi-local and hybrid TDDFT calculations on isolated dimers. We demonstrate the power of this method on three polymorphs of the well-known color polymorphic compound ROY and find that it outperforms both small-scale hybrid TDDFT dimer calculations and large-scale semi-local TDDFT supercell calculations, when compared to the experiment.

High-Performance Ladder-Type Heteroheptacene-Based Nonfullerene Acceptors Enabled by Asymmetric Cores with Enhanced Noncovalent Intramolecular Interactions

Nonfullerene acceptors (MQ3, MQ5, MQ6) are synthesized using asymmetric and symmetric ladder-type heteroheptacene cores with selenophene heterocycles. Although MQ3 and MQ5 are constructed with the same number of selenophene heterocycles, the heteroheptacene core of MQ5 is end-capped with selenophene rings while that of MQ3 is flanked with thiophene rings. With the enhanced noncovalent interaction of O⋅⋅⋅Se compared to that of O⋅⋅⋅S, MQ5 shows a bathochromically shifted absorption band and greatly improved carrier transport, leading to a higher power conversion efficiency (PCE) of 15.64 % compared to MQ3, which shows a PCE of 13.51 %. Based on the asymmetric heteroheptacene core, MQ6 shows an improved carrier transport induced by the reduced π-π stacking distance, related with the increased dipole moment in comparison with the nonfullerene acceptors based on symmetric cores. MQ6 exhibits a PCE of 16.39 % with a V_OC of 0.88 V, a FF of 75.66 %, and a J_SC of 24.62 mA cm^-2 .

Computational Identification of Novel Families of Nonfullerene Acceptors by Modification of Known Compounds

We considered a database of tens of thousands of known organic semiconductors and identified those compounds with computed electronic properties (orbital energies, excited state energies, and oscillator strengths) that would make them suitable as nonfullerene electron acceptors in organic solar cells. The range of parameters for the desirable acceptors is determined from a set of experimentally characterized high-efficiency nonfullerene acceptors. This search leads to ∼30 lead compounds never considered before for organic photovoltaic applications. We then proceed to modify these compounds to bring their computed solubility in line with that of the best small-molecule nonfullerene acceptors. A further refinement of the search can be based on additional properties like the reorganization energy for chemical reduction. This simple strategy, which relies on a few easily computable parameters and can be expanded to a larger set of molecules, enables the identification of completely new chemical families to be explored experimentally.

Design of All-Small-Molecule Organic Solar Cells Approaching 14% Efficiency via Isometric Terminal Alkyl Chain Engineering

Morphology is crucial to determining the photovoltaic performance of organic solar cells (OSCs). However, manipulating morphology involving only small-molecule donors and acceptors is extremely challenging. Herein, a simple terminal alkyl chain engineering process is introduced to fine-tune the morphology towards high-performance all-small-molecule (ASM) OSCs. We successfully chose a chlorinated two-dimension benzo[1,2-b:4,5-b′]dithiophene (BDT) central unit and two isomeric alkyl cyanoacetate as the end-capped moieties to conveniently synthesize two isomeric small-molecule donors, namely, BT-RO-Cl and BT-REH-Cl, each bearing linear n-octyl (O) as the terminal alkyl chain and another branched 2-ethylhexyl (EH) as the terminal alkyl chain. The terminal alkyl chain engineering process provided BT-RO-Cl with 13.35% efficiency and BT-REH-Cl with 13.90% efficiency ASM OSCs, both with Y6 as the electron acceptor. The successful performance resulted from uniform phase separation and the favorable combination of face-on and edge-on molecular stacking of blended small-molecule donors and acceptors, which formed a fluent 3D transport channel and thus delivered high and balanced carrier mobilities. These findings demonstrate that alkyl chain engineering can finely control the morphology of ASM OSCs, and provides an alternative for the optimal design of small-molecule materials towards high-performance ASM OSCs.

15.3% Efficiency All-Small-Molecule Organic Solar Cells Achieved by a Locally Asymmetric F, Cl Disubstitution Strategy

Single junction binary all-small-molecule (ASM) organic solar cells (OSCs) with power conversion efficiency (PCE) beyond 14% are achieved by using non-fullerene acceptor Y6 as the electron acceptor, but still lag behind that of polymer OSCs. Herein, an asymmetric Y6-like acceptor, BTP-FCl-FCl, is designed and synthesized to match the recently reported high performance small molecule donor BTR-Cl, and a record efficiency of 15.3% for single-junction binary ASM OSCs is achieved. BTP-FCl-FCl features a F,Cl disubstitution on the same end group affording locally asymmetric structures, and so has a lower total dipole moment, larger average electronic static potential, and lower distribution disorder than those of the globally asymmetric isomer BTP-2F-2Cl, resulting in improved charge generation and extraction. In addition, BTP-FCl-FCl based active layer presents more favorable domain size and finer phase separation contributing to the faster charge extraction, longer charge carrier lifetime, and much lower recombination rate. Therefore, compared with BTP-2F-2Cl, BTP-FCl-FCl based devices provide better performance with FF enhanced from 71.41% to 75.36% and J sc increased from 22.35 to 24.58 mA cm-2, leading to a higher PCE of 15.3%. The locally asymmetric F, Cl disubstitution on the same end group is a new strategy to achieve high performance ASM OSCs.

Efficient Electron Transport Layer Free Small-Molecule Organic Solar Cells with Superior Device Stability

Electron transport layers (ETLs) placed between the electrodes and a photoactive layer can enhance the performance of organic solar cells but also impose limitations. Most ETLs are ultrathin films, and their deposition can disturb the morphology of the photoactive layers, complicate device fabrication, raise cost, and also affect device stability. To fully overcome such drawbacks, efficient organic solar cells that operate without an ETL are preferred. In this study, a new small-molecule electron donor (H31) based on a thiophene-substituted benzodithiophene core unit with trialkylsilyl side chains is designed and synthesized. Blending H31 with the electron acceptor Y6 gives solar cells with power conversion efficiencies exceeding 13% with and without 2,9-bis[3-(dimethyloxidoamino)propyl]anthra[2,1,9-def:6,5,10-d'e'f ']diisoquinoline-1,3,8,10(2H,9H)-tetrone (PDINO) as the ETL. The ETL-free cells deliver a superior shelf life compared to devices with an ETL. Small-molecule donor-acceptor blends thus provide interesting perspectives for achieving efficient, reproducible, and stable device architectures without electrode interlayers.

Backbone Engineering with Asymmetric Core to Finely Tune Phase Separation for High-Performance All-Small-Molecule Organic Solar Cells

In order to obtain high-performance all-small-molecule organic solar cells (ASM-OSCs), it is crucial to exploit the available strategy for molecular design and to further understand key structure-property relationship that can rationally control the blend nanomorphology and influence the physical process. In this work, we design two small molecule donors FBD-S1 and TBD-S2 with identical electron-withdrawing units but various asymmetric central cores, which exhibit differing phase separation in Y6-based blend films. It is found that TBD-S2 with increased phase separation between donor and acceptor can lead to more favorable interpenetrating networks, effective exciton dissociation, and enhanced and more balanced charge transport. Importantly, a remarkable PCE of 13.1% is obtained for TBD-S2:Y6 based ASM-OSCs, which is an attractive photovoltaic performance for ASM-OSCs. This result demonstrates that the central core modification at the atomic level for small molecule donors can delicately control the phase separation and optimize photophysical processes, and refines device performance, which facilitate development in the ASM-OSC research field.

Ionically Gated Small-Molecule OPV: Interfacial Doping of Charge Collector and Transport Layer

We demonstrate an improvement in the performance of organic photovoltaic (OPV) systems based on small molecules by ionic gating via controlled reversible n-doping of multi-wall carbon nanotubes (MWCNTs) coated on fullerene electron transport layers (ETLs): C₆₀ and C₇₀. Such electric double-layer charging (EDLC) doping, achieved by ionic liquid (IL) charging, allows tuning of the electronic concentration in MWCNTs and the fullerene planar acceptor layers, increasing it by orders of magnitude. This leads to the decrease of the series and increase of the shunt resistances of OPVs and allows use of thick (up to 200 nm) ETLs, increasing the durability of OPVs. Two stages of OPV enhancement are described upon the increase of gating bias V_g: at small (or even zero) V_g, the extended interface of ILs and porous transparent MWCNTs is charged by gating, and the fullerene charge collector is significantly improved, becoming an ohmic contact. This changes the S-shaped J-V curve via improving the electron collection by an n-doped MWCNT cathode with an ohmic interfacial contact. The J-V curves further improve at higher gating bias V_g due to the increase of the Fermi level and decrease of the MWCNT work function. At the next qualitative stage, the acceptor fullerene layer becomes n-doped by electron injection from MWCNTs while ions of ILs penetrate into the fullerene. At this step, the internal built-in field is created within OPV, which helps in exciton dissociation and charge separation/transport, increasing further the J_sc and the fill factor. The ionic gating concept demonstrated here for most simple classical planar small-molecule OPV cells can be potentially applied to more complex highly efficient hybrid devices, such as perovskite photovoltaic with an ETL or a hole transport layer, providing a new way to tune their properties via controllable and reversible interfacial doping of charge collectors and transport layers.

Fullerene/Non-fullerene Alloy for High-Performance All-Small-Molecule Organic Solar Cells

Organic solar cells (OSCs) that contain small molecules only were prepared with FG1 as the donor, a narrow band gap non-fullerene acceptor MPU4, and a wide band gap PC₇₁BM. The OSCs based on optimized FG1:MPU4 (1:1.2) and FG1:PC₇₁BM (1:1.5) active layers, respectively, gave power conversion efficiencies (PCEs) of 11.18% with a short circuit current (J_SC) of 19.54 mA/cm², open circuit voltage (V_OC) of 0.97 V, and fill factor (FF) of 0.59, and 6.62% with a J_SC of 12.50 mA/cm², V_OC of 0.84 V, and FF of 0.63%, respectively. A PCE of 13.26% was obtained from the optimized ternary FG1:PC₇₁BM:MPU4 (1:0.3:0.9) OSCs and this arises because of the boost in a J_SC of 21.91 mA/cm² and FF of 0.68. The V_OC of the ternary OSCs (0.89 V) lies between those for the OSCs based on FG1:MPU4 and FG1:PC₇₁BM, which indicates the formation of an alloy of the two acceptors. The increase in J_SC and FF in the ternary OSCs may result from the efficient energy transfer from PC₇₁BM to MPU4 as well as more charge-transfer donor/acceptor interfaces, enhanced charge carrier mobilities resulting in better adjusted charge transport, and lower bimolecular and trap-assisted recombination. The appropriate phase separation, increased crystallinity, and reduced π-π stacking distance in the ternary active layer are consistent with the enhancement in the FF for OSCs based on a ternary active layer. The results of this work suggest the merging of the fullerene acceptor into the non-fullerene acceptor to form a fullerene/non-fullerene acceptor alloy, and this may be a viable approach to obtain high-performance OSCs.

Bending and pinching of three-phase stripes: From secondary instabilities to morphological deformations in organic photovoltaics

Optimizing the properties of the mosaic nanoscale morphology of bulk heterojunction (BHJ) organic photovoltaics (OPV) is not only challenging technologically but also intriguing from the mechanistic point of view. Among the recent breakthroughs is the identification and utilization of a three-phase (donor-mixed-acceptor) BHJ, where the (intermediate) mixed phase can inhibit mesoscale morphological changes, such as phase separation. Using a mean-field approach, we reveal and distinguish between generic mechanisms that alter, through transverse instabilities, the evolution of stripes: the bending (zigzag mode) and the pinching (cross-roll mode) of the donor-acceptor domains. The results are summarized in a parameter plane spanned by the mixing energy and illumination, and show that donor-acceptor mixtures with higher mixing energy are more likely to develop pinching under charge-flux boundary conditions. The latter is notorious as it leads to the formation of disconnected domains and hence to loss of charge flux. We believe that these results provide a qualitative road map for BHJ optimization, using mixed-phase composition and, therefore, an essential step toward long-lasting OPV. More broadly, the results are also of relevance to study the coexistence of multiple-phase domains in material science, such as in ion-intercalated rechargeable batteries.

Preparing polythiophene derivative with alternating alkyl and thioalkyl side chains via Kumada coupling for efficient organic solar cells

A polythiophene, namely PTST with alternating alkyl and thioalkyl side chains, is prepared by Kumada catalyst-transfer polycondensation. PTST can hierarchically pre-aggregate in solution, and then form a favorable morphology in organic solar cells.

Magnesium diethynylporphyrin derivatives with strong near-infrared absorption for solution-process bulk heterojunction organic solar cells

Magnesium diethynylporphyrin derivatives with strong near-infrared absorption were obtained. These derivatives possess electron rich units directly introduced to the porphyrin core. The electron rich units caused strong absorption on the near-infrared region due to an intramolecular charge transfer. Theoretical calculation also proved that the derivatives showed large oscillator strength at the Q band. As a donor material, such large absorption coefficient in the range of long wavelength region is a desirable characteristic for organic solar cells. Organic photovoltaic devices using these diethynylporphyrin derivatives gave a PCE of 2.91% in optimal conditions.

Optimization of the Bulk Heterojunction of All-Small-Molecule Organic Photovoltaics Using Design of Experiment and Machine Learning Approaches

All-small-molecule organic photovoltaic (OPV) cells based upon the small-molecule donor, DRCN5T, and nonfullerene acceptors, ITIC, IT-M, and IT-4F, were optimized using Design of Experiments (DOE) and machine learning (ML) approaches. This combination enables rational sampling of large parameter spaces in a sparse but mathematically deliberate fashion and promises economies of precious resources and time. This work focused upon the optimization of the core layer of the OPV device, the bulk heterojunction (BHJ). Many experimental processing parameters play critical roles in the overall efficiency of a given device and are often correlated and thus are difficult to parse individually. DOE was applied to the (i) solution concentration of the donor and acceptor ink used for spin-coating, (ii) the donor fraction, (iii) the temperature, and (iv) duration of the annealing of these films. The ML-based approach was then used to derive maps of the power conversion efficiencies (PCE) landscape for the first and second rounds of optimization to be used as guides to determine the optimal values of experimental processing parameters with respect to PCE. This work shows that with little knowledge of a potential combination of components for a given BHJ, a large parameter space can be effectively screened and investigated to rapidly determine its potential for high-efficiency OPVs.

Synergistic Optimization Enables Large-Area Flexible Organic Solar Cells to Maintain over 98% PCE of the Small-Area Rigid Devices

Slot-die coating is generally regarded as the most effective large-scale methodology for the fabrication of organic solar cells (OSCs). However, the corresponding device performance significantly lags behind spin-coated devices. Herein, the active layer morphology, flexible substrate properties, and the processing temperature are optimized synergistically to obtain high power conversion efficiency (PCE) for both the flexible single cells and the modules. As a result, the 1 cm² flexible devices produce an excellent PCE of 12.16% as compared to 12.37% for the spin-coated small-area (0.04 cm² ) rigid devices. Likewise, for modules with an area of 25 cm² , an extraordinary PCE of 10.09% is observed. Hence, efficiency losses associated with the upscaling are significantly reduced by the synergistic optimization. Moreover, after 1000 bending cycles at a bending radius of 10 mm, the flexible devices still produce over 99% of their initial PCE, whereas after being stored for over 6000 h in a glove box, the PCE reaches 103% of its initial value, indicating excellent device flexibility as well as superior shelf stability. These results, thus, are a promising confirmation the great potential for upscaling of large-area OSCs in the near future.

Improving the Photostability of Small-Molecule-Based Organic Photovoltaics by Providing a Charge Percolation Pathway of Crystalline Conjugated Polymer

Photostability of small-molecule (SM)-based organic photovoltaics (SM-OPVs) is greatly improved by utilizing a ternary photo-active layer incorporating a small amount of a conjugated polymer (CP). Semi-crystalline poly[(2,5-bis(2-hexyldecyloxy)phenylene)-alt-(5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole)] (PPDT2FBT) and amorphous poly[(2,5-bis(2-decyltetradecyloxy)phenylene)-alt-(5,6-dicyano-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole)] (PPDT2CNBT) with similar chemical structures were used for preparing SM:fullerene:CP ternary photo-active layers. The power conversion efficiency (PCE) of the ternary device with PPDT2FBT (Ternary-F) was higher than those of the ternary device with PPDT2CNBT (Ternary-CN) and a binary SM-OPV device (Binary) by 15% and 17%, respectively. The photostability of the SM-OPV was considerably improved by the addition of the crystalline CP, PPDT2FBT. Ternary-F retained 76% of its initial PCE after 1500 h of light soaking, whereas Ternary-CN and Binary retained only 38% and 17% of their initial PCEs, respectively. The electrical and morphological analyses of the SM-OPV devices revealed that the addition of the semi-crystalline CP led to the formation of percolation pathways for charge transport without disturbing the optimized bulk heterojunction morphology. The CP also suppressed trap-assisted recombination and enhanced the hole mobility in Ternary-F. The percolation pathways enabled the hole mobility of Ternary-F to remain constant during the light-soaking test. The photostability of Ternary-CN did not improve because the addition of the amorphous CP inhibited the formation of ordered SM domains.

Side-Chain Engineering of Benzodithiophene-Bridged Dimeric Porphyrin Donors for All-Small-Molecule Organic Solar Cells

Two new A-D-A small-molecule donors (C8T-BDTDP and C8ST-BDTDP) are prepared from benzodithiophene (BDT)-linked dimeric porphyrin (DP), which differ in side chains of BDT linkers with 4,8-bis[5-(2-ethylhexyl)thiophen-2-yl]benzo[1,2-b:4,5-b']dithiophene (C8T-BDT) for the former and 4,8-bis{5-[(2-ethylhexyl)thio]-2-thienyl}benzo[1,2-b:4,5-b']dithiophene (C8ST-BDT) for the latter. Both dimeric porphyrin donors show strongly UV-visible to near-infrared absorption. Compared to C8T-BDTDP, C8ST-BDTDP with an alkylthiothienyl-substituted BDT linker exhibits more intense absorption bands in the film and a lower highest occupied molecular orbital energy level. The blend film of the electron acceptor 6TIC with the respective dimeric porphyrin donor displays a broad photon response from 400 to 900 nm, unfortunately, with an absorption valley at ca. 600 nm. The device based on C8ST-BDTDP/6TIC demonstrates a promising power conversion efficiency (PCE) of 10.39% with a high short-circuit current density (J_SC) of 19.53 mA cm^-2, whereas the device based on C8T-BDTDP/6TIC shows a slightly lower PCE of 8.73% with a J_SC of 17.75 mA cm^-2. The better performance for C8ST-BDTDP/6TIC is mainly attributed to efficient charge dissociation and transportation because of the smooth surface morphology and highly ordered crystalline packing.

Planar D-π-A Configured Dimethoxy Vinylbenzene Based Small Organic Molecule for Solution-Processed Bulk Heterojunction Organic Solar Cells

A new and effective planar D-π-A configured small organic molecule (SOM) of 2-5-(3,5-dimethoxystyryl)thiophen-2-yl)methylene)-1H-indene-1,3(2H)-dione, abbreviated as DVB-T-ID, was synthesized using 1,3-indanedione acceptor and dimethoxy vinylbenzene donor units, connected through a thiophene π-spacer. The presence of a dimethoxy vinylbenzene unit and π-spacer in DVB-T-ID significantly improved the absorption behavior by displaying maximum absorbance at ~515 nm, and the reasonable band gap was estimated as ~2.06 eV. The electronic properties revealed that DVB-T-ID SOMs exhibited promising HOMO (−5.32 eV) and LUMO (−3.26 eV). The synthesized DVB-T-ID SOM was utilized as donor material for fabricating solution-processed bulk heterojunction organic solar cells (BHJ-OSCs) and showed a reasonable power conversion efficiency (PCE) of ~3.1% with DVB-T-ID:PC61BM (1:2, w/w) active layer. The outcome of this work clearly reflects that synthesized DVB-T-ID based on 1,3-indanedione units is a promising absorber (donor) material for BHJ-OSCs.

Synergy of Liquid-Crystalline Small-Molecule and Polymeric Donors Delivers Uncommon Morphology Evolution and 16.6% Efficiency Organic Photovoltaics

Achieving an ideal morphology is an imperative avenue for enhancing key parameters toward high-performing organic solar cells (OSCs). Among a myriad of morphological-control methods, the strategy of incorporating a third component with structural similarity and crystallinity difference to construct ternary OSCs has emerged as an effective approach to regulate morphology. A nematic liquid-crystalline benzodithiophene terthiophene rhodamine (BTR) molecule, which possesses the same alkylthio-thienyl-substituted benzo moiety but obviously stronger crystallinity compared to classical medium-bandgap polymeric donor PM6, is employed as a third component to construct ternary OSCs based on a PM6:BTR:Y6 system. The doping of BTR (5 wt%) is found to be enough to improve the OSC morphology-significantly enhancing the crystallinity of the photoactive layer while slightly reducing the donor/acceptor phase separation scale simultaneously. Rarely is such a morphology evolution reported. It positively affects the electronic properties of the device-prolongs the carrier lifetime, shortens the photocurrent decay time, facilitates exciton dissociation, charge transport, and collection, and ultimately boosts the power conversion efficiency from 15.7% to 16.6%. This result demonstrates that the successful synergy of liquid-crystalline small-molecule and polymeric donors delicately adjusts the active-layer morphology and refines device performance, which brings vibrancy to the OSC research field.

An asymmetric end-capping strategy enables a new non-fullerene acceptor for organic solar cells with efficiency over 10

Two different terminal groups, rhodanine-flanked benzo[c][1,2,5]thiadiazole (BR) and 2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (IM2F), were connected to an indaceno[1,2-b:5,6-b']dithiophene (IDT) core to construct a new non-fullerene acceptor (IDTBF). Solar cells based on this acceptor exhibited promising photovoltaic performances with a power conversion efficiency (PCE) of up to 10.43%.

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.

Modulation of Donor Alkyl Terminal Chains with the Shifting Branching Point Leads to the Optimized Morphology and Efficient All-Small-Molecule Organic Solar Cells

Terminal group modification is one of the most influential factors for small-molecular donors compared with their polymer counterparts, resulting in an opportunity to optimize the morphology of all-small-molecule organic solar cells (ASM-OSCs). In this article, we report three novel small-molecular donors with branching points at the 1-, 2-, and 3-positions in alkyl terminal chains, called BSCl-C1, BSCl-C2, and BSCl-C3, respectively. Using IDIC-4Cl as the acceptor, the subtle branching position shift achieves a dramatic disparity in photovoltaic parameters, as indicated by the short circuit current (J_sc) changing from 4.9 to 20.1 to 14.2 mA cm^-2 and the fill factor varying from 33.9 to 71.3 to 67.0% for BSCl-C1, BSCl-C2, and BSCl-C3, respectively. The best device performance of 12.40% is obtained by the BSCl-C2:IDIC-4Cl system, which not only ranks among the top values reported to date but also exhibits low energy loss in systems that use IDIC as acceptors. The notable device performance based on BSCl-C2 is attributed to the optimized phase morphology caused by the strong molecular crystallinity and suitable intermolecular interaction with IDIC-4Cl. These results demonstrate that suitably tuning the branching position of terminal groups could promote the high performance of ASM-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.

Low-Bandgap n-Type Polymer Based on a Fused-DAD-Type Heptacyclic Ring for All-Polymer Solar Cell Application with a Power Conversion Efficiency of 10.7

An n-type polymer (A701) is designed and synthesized with an alternative A'-DAD-A'-D' backbone, where 1,1-dicyanomethylene-3-indanone (IC), dithienothiophen[3,2-b]-pyrrolobenzothiadiazole (TPBT), and benzodithiophene (BDT) are used as A', DAD, and D' units, respectively. A701 shows enhanced light absorption with a narrow bandgap of 1.42 eV and a high absorption coefficient of 6.85 × 10⁴ cm^-1 at 780 nm. It displays an uplifted LUMO (the lowest unoccupied molecular orbital) level of -3.80 eV. By introducing a high point solvent additive of 1,8-diiodooctane (DIO), all-polymer solar cells (all-PSCs) based on the PBDB-T:A701 blend exhibit efficient exciton dissociation, enhanced charge transport, and decreased bimolecular recombination. Thus, a high open-circuit voltage (V_OC) of 0.92 V, a short-circuit current (J_SC) of 18.27 mA cm^-2, and a fill factor (FF) of 0.64 are attained, affording an impressive power conversion efficiency (PCE) of 10.70%. The low voltage loss of 0.50 V and high efficiency of 10.7% are among the top values for all-PSCs. Our results indicate that the fused DAD-type heptacyclic ring can be utilized to construct not only nonfullerene small molecular acceptors but also promising polymer acceptors.