Double Asymmetric Core Optimizes Crystal Packing to Enable Selenophene-based Acceptor with Over 18 % Efficiency in Binary Organic Solar Cells.

Side-chain tailoring is a promising method to optimize the performance of organic solar cells (OSCs). However, asymmetric alkyl chain-based small molecular acceptors (SMAs) are still difficult to afford. Herein, we adopted a novel asymmetric n-nonyl/undecyl substitution strategy and synthesized two A-D1 A'D2 -A double asymmetric isomeric SMAs with asymmetric selenophene-based central core for OSCs. Crystallographic analysis indicates that AYT9Se11-Cl forms a more compact and order intermolecular packing compared to AYT11Se9-Cl, which contributed to higher electron mobility in neat AYT9Se11-Cl film. Moreover, the PM6 : AYT9Se11-Cl blend film shows a better morphology with appropriate phase separation and distinct face-on orientation than PM6 : AYT11Se9-Cl. The OSCs with PM6 : AYT9Se11-Cl obtain a superior PCE of 18.12 % compared to PM6 : AYT11Se9-Cl (17.52 %), which is the best efficiency for the selenium-incorporated SMAs in binary BHJ OSCs. Our findings elucidate that the promising double asymmetric strategy with isomeric alkyl chains precisely modulates the crystal packing and enhances the photovoltaic efficiency of selenophene-incorporated SMAs.

Optimized Crystal Framework by Asymmetric Core Isomerization in Selenium-Substituted Acceptor for Efficient Binary Organic Solar Cells.

Both the regional isomerization and selenium-substitution of the small molecular acceptors (SMAs) play significant roles in developing efficient organic solar cells (OSCs), while their synergistic effects remain elusive. Herein, we developed three isomeric SMAs (S-CSeF, A-ISeF, and A-OSeF) via subtly manipulating the mono-selenium substituted position (central, inner, or outer) and type of heteroaromatic ring on the central core by synergistic strategies for efficient OSCs, respectively. Crystallography of asymmetric A-OSeF presents a closer intermolecular π-π stacking and more ordered 3-dimensional network packing and efficient charge-hopping pathways. With the successive out-shift of the mono-selenium substituted position, the neat films give a slightly wider band gap and gradually higher crystallinity and electron mobility. The PM1 : A-OSeF afford favourable fibrous phase separation morphology with more ordered molecular packing and efficient charge transportation compared to the other two counterparts. Consequently, the A-OSeF-based devices achieve a champion efficiency of 18.5 %, which represents the record value for the reported selenium-containing SMAs in binary OSCs. Our developed precise molecular engineering of the position and type of selenium-based heteroaromatic ring of SMAs provides a promising synergistic approach to optimizing crystal stacking and boosting top-ranked selenium-containing SMAs-based OSCs.

Intramolecular Chloro-Sulfur Interaction and Asymmetric Side-Chain Isomerization to Balance Crystallinity and Miscibility in All-Small-Molecule Solar Cells.

Intramolecular Cl-S non-covalent interaction is introduced to modify molecular backbone of a benzodithiophene terthiophene rhodamine (BTR) benchmark structure, helping planarize and rigidify the molecular framework for improving charge transport. Theoretical simulations and temperature-variable NMR experiments clearly validate the existence of Cl-S non-covalent interaction in two designed chlorinated donors and explain its important role in enhancing planarity and rigidity of the molecules for enhancing their crystallinity. The asymmetric isomerization of side-chains further optimizes the molecular orientation and surface energy to strike a balance between its crystallinity and miscibility. This carefully manipulated molecular design helps result in increased carrier mobility and suppressed charge recombination to obtain simultaneously enhanced short-circuit current (Jsc ) and fill factor (FF) and a very high efficiency of 15.73 % in binary all-small-molecule organic solar cells.

Regioisomer-Free Difluoro-Monochloro Terminal-based Hexa-Halogenated Acceptor with Optimized Crystal Packing for Efficient Binary Organic Solar Cells.

Herein, we synthesized new hetero-halogenated end groups with well-determined fluorinated and chlorinated substitutions (o-FCl-IC and FClF-IC), and synthesized regioisomer-free small molecular acceptors (SMAs) Y-Cl, Y-FCl, and Y-FClF with distinct hetero-halogenated terminals, respectively. The single-crystal structures and theoretical calculations indicate that Y-FClF exhibits more compact three-dimensional network packing and more significant π-π electronic coupling compared to Y-FCl. From Y-Cl to Y-FCl to Y-FClF, the neat films exhibit a narrower optical band gap and gradually enhanced electron mobility and crystallinity. The PM6 : Y-FClF blend film exhibits the strongest crystallinity with preferential face-on molecular packing, desirable fibrous morphology with suitable phase segregation, and the highest and balanced charge mobilities among three blend films. Overall, the PM6 : Y-FClF organic solar cells (OSCs) deliver a remarkable efficiency of 17.65 %, outperforming the PM6 : Y-FCl and PM6 : Y-Cl, which is the best PCE for reported hetero-halogens-based SMAs in binary OSCs. Our results demonstrate that difluoro-monochloro hetero-terminal is a superior regio-regular unit for enhancing the intermolecular crystal packing and photovoltaic performance of hetero-halogenated SMAs.

A Synergistic Strategy of Manipulating the Number of Selenophene Units and Dissymmetric Central Core of Small Molecular Acceptors Enables Polymer Solar Cells with 17.5 % Efficiency.

A dissymmetric backbone and selenophene substitution on the central core was used for the synthesis of symmetric or dissymmetric A-DA'D-A type non-fullerene small molecular acceptors (NF-SMAs) with different numbers of selenophene. From S-YSS-Cl to A-WSSe-Cl and to S-WSeSe-Cl, a gradually red-shifted absorption and a gradually larger electron mobility and crystallinity in neat thin film was observed. A-WSSe-Cl and S-WSeSe-Cl exhibit stronger and tighter intermolecular π-π stacking interactions, extra S⋅⋅⋅N non-covalent intermolecular interactions from central benzothiadiazole, better ordered 3D interpenetrating charge-transfer networks in comparison with thiophene-based S-YSS-Cl. The dissymmetric A-WSSe-Cl-based device has a PCE of 17.51 %, which is the highest value for selenophene-based NF-SMAs in binary polymer solar cells. The combination of dissymmetric core and precise replacement of selenophene on the central core is effective to improve Jsc and FF without sacrificing Voc .

Two Well-Compatible Acceptors with Efficient Energy Transfer Enable Ternary Organic Photovoltaics Exhibiting a 13.36% Efficiency.

Organic photovoltaics (OPVs) are fabricated with PM6 as donor and T6Me, IT-2F, or their mixture as acceptor. A 13.36% power conversion efficiency (PCE) is achieved from the optimized ternary OPVs with 50 wt% IT-2F in acceptors, which is attributed to the enhanced photon harvesting of ternary active layers and improved exciton utilization efficiency through energy transfer from IT-2F to T6Me. The efficient energy transfer from IT-2F to T6Me can be confirmed from the photoluminescence spectra of neat and blend films, which may provide additional channels to enhance exciton utilization efficiency for achieving short-circuit current density (JSC ) improvement of ternary OPVs. It should be highlighted that the fill factor (FF) of ternary OPVs can be monotonously increased along with the incorporation of IT-2F, indicating the gradually optimized phase separation degree of ternary active layers. The third component IT-2F plays a key role in optimizing phase separation as a morphology regulator. Over 8% PCE improvement is achieved in the optimized ternary OPVs compared with the over 12% PCEs of the corresponding binary OPVs, respectively. This work indicates that the performance of ternary OPVs can be well optimized by carefully picking materials with good compatibility and complementary absorption spectra, as well as the appropriate energy levels.

One key issue in characterization of organic solar cells with solution processed interfacial layers.

Solution processed interfacial layers are commonly employed in bulk heterojunction organic solar cells (OSCs) for better charge collection. PDIN interfacial layers were prepared by employing a static or dynamic spin coating method from PDIN methanol solution, and defined as the S-PDIN or D-PDIN layer. The OSCs with a S-PDIN layer exhibit 13.88% power conversion efficiency (PCE) with a virtual high short circuit density (JSC) of 26.45 mA cm-2 and relatively low fill factor (FF) of 58.94% during the current density versus voltage (J-V) measurement without a shadow mask. 12.56% PCE is achieved for OSCs with a D-PDIN layer, along with a JSC of 18.85 mA cm-2 and FF of 74.88%. Over 77% FFs are obtained for OSCs with a S-PDIN or D-PDIN layer during J-V measurement with a shadow mask, and both OSCs exhibit a very similar JSC and PCE. The virtual high JSCs and relatively low FF of OSCs with a S-PDIN layer may be due to the enhanced conductivity of PEDOT:PSS during preparation of the PDIN layer by the SSC method, which can be further confirmed from the OSCs with a methanol treated PEDOT:PSS layer. This work indicates that a well-balanced JSC and FF should be an important evaluating indicator for efficient OSCs, and an appropriate shadow mask is necessary to measure the J-V curves of OSCs with a solution processed interfacial layer.

Efficient Ternary Organic Solar Cells with Two Compatible Non-Fullerene Materials as One Alloyed Acceptor.

Efficient ternary organic solar cells (OSCs) are fabricated by employing a polymer PBT1-C as the donor and two non-fullerene materials, MeIC and MeIC2, as one alloyed acceptor. The optimized ternary OSCs with 30 wt% MeIC2 in acceptors achieve a power conversion efficiency (PCE) of 12.55%, which is much higher than that of 11.47% for MeIC-based binary OSCs and 11.41% for MeIC2-based binary OSCs. The >9.4% improvement in PCE is mainly attributed to the optimized photon harvesting and morphology of ternary active layers, resulting in the simultaneously improved short-circuit current and fill factor. Furthermore, good compatibility and similar lowest unoccupied molecular orbital energy levels of MeIC and MeIC2 are beneficial to form one alloyed acceptor for efficient electron transport in the ternary active layers. This work may provide new insight when selecting the third component for preparing efficient ternary OSCs.

Highly efficient polymer solar cells by step-by-step optimizing donor molecular packing and acceptor redistribution.

The dynamic drying process of the active layer should play a vitally important role in determining the performance of polymer solar cells (PSCs). Donor molecular packing and acceptor redistribution can be optimized by two successive post-treatments on the active layer. The blend films were freshly prepared by spin-coating method and then immediately transferred to a covered glass Petri dish to allow self-assembly of the donor molecules. The films were then treated with methanol or PFN-methanol solution to adjust the acceptor redistribution. In this study, power conversion efficiencies (PCEs) of PSCs with PffBT4T-2OD:PC71BM as the active layer were improved from 6.74% to 8.75% by employing 80 min for self-assembly and 20 s of methanol soaking. The PCE was improved even further to 9.72% by inserting a PFN interfacial layer. The performance improvement was mainly attributed to the optimized PffBT4T-2OD molecular packing during the self-assembly process, ideal vertical phase separation driven by methanol soaking and efficient charge collection by insertion of a PFN interfacial layer. The molecular packing and vertical phase separation were characterized by grazing incidence X-ray diffraction (GIXD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS), respectively. The experimental results solidly supported the effectiveness of the step-by-step optimization strategy.

Unique insight into phase separation in polymer solar cells from their electric characteristics.

A series of polymer solar cells (PSCs) were fabricated with indene-C60 bisadduct (ICBA) or [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) as an electron acceptor and with PBDT-TS1 as an electron donor. The donor/acceptor (D/A) phase separation was adjusted with different solution processing methods, consisting of cool (room temperature, 20 °C) solution, hot (70 °C) solution and the solutions with solvent additive 1,8-diiodideoctane (DIO). The champion PCE of PSCs with ICBA or PC61BM as an electron acceptor is 4.32% or 5.97% for the active layers prepared from hot solution with DIO additive or cool solution with DIO additive, respectively. The improved PCEs should be attributed to the optimized D/A phase separation in the active layers by adjusting the redistribution of PC61BM or the ICBA among the PBDT-TS1 networks. The degree of phase separation of the active layers with different acceptors was evaluated according to the current density-voltage (J-V) curves of hole-only and electron-only devices. The distribution of PC61BM or ICBA molecules in the normal direction can be simply judged from the symmetry degree of J-V curves of electron-only devices measured under the forward and reverse bias.

Optimization of charge carrier transport balance for performance improvement of PDPP3T-based polymer solar cells prepared using a hot solution.

Polymer solar cells (PSCs), with poly(diketopyrrolopyrrole-terthiophene) (PDPP3T):[6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as the active layers, were fabricated using solutions of different temperatures. The best power conversion efficiency (PCE) of the PSCs prepared using a hot solution was about 6.22%, which is better than 5.54% for PSCs prepared using cool (room temperature) solutions and 5.85% for PSCs prepared using cool solutions with a 1,8-diiodooctane (DIO) solvent additive. The underlying reasons for the improved PCE of the PSCs prepared using a hot solution could be attributed to the more dispersive donor and acceptor distribution in the active layer, resulting in a better bi-continuous interpenetrating network for exciton dissociation and charge carrier transport. An enhanced and more balanced charge carrier transport in the active layer is obtained for the PSCs prepared using a hot solution, which can be determined from the J-V curves of the related hole-only and electron-only devices.

Enhanced performance of polymer solar cells by employing a ternary cascade energy structure.

We present a route to successfully tackle the two main limitations, low open circuit voltage (Voc) and limited short circuit-density (Jsc), of polymer solar cells (PSCs) based on poly(3-hexylthiophene) (P3HT) as an electron-donor. The indene-C60 bisadduct (ICBA) was selected as an electron acceptor to improve the open circuit voltage (Voc). The narrow band gap polymer poly[(4,8-bis-(2-ethylhexyloxy)-benzo[1,2-b:4,5-b']dithiophene)-2,6-diyl-alt-(4-(2-ethylhexanoyl)-thieno[3,4-b]thiophene)-2,6-diyl] (PBDTTT-C), as a complementary electron-donor material, was doped into the host system of P3HT:ICBA to form ternary cascade energy structured PSCs with increased Jsc. The power conversion efficiency (PCE) of P3HT:ICBA-based cells was improved from 3.32% to 4.38% by doping with 3 wt% PBDTTT-C with 1 min 150 °C annealing treatment. The 4.38% PCE of ternary PSCs is still larger than the 3.79% PCE of PSCs based on P3HT:ICBA with 10 minutes 150 °C annealing treatment.

Hot-Casting Strategy Empowers High-Boiling Solvent-Processed Organic Solar Cells with Over 18.5% Efficiency.

Most top-rank organic solar cells (OSCs) are manufactured by the halogenated solvent chloroform, which possesses a narrow processing window due to its low-boiling point. Herein, based on two high-boiling solvents, halogenated solvent chlorobenzene (CB) and non-halogenated green solvent ortho-xylene (OX), preparing active layers with the hot solution is put forward to enhance the performance of the OSCs. In situ test and morphological characterization clarify that the hot-casting strategy assists in the fast and synchronous molecular assembly of both donor and acceptor in the active layer, contributing to preferable donor/acceptor ratio, vertical phase separation, and molecular stacking, which is beneficial to charge generation and extraction. Based on the PM6:BO-4Cl, the hot-casting OSCs with a wide processing window achieve efficiencies of 18.03% in CB and 18.12% in OX, which are much higher than the devices processed with room temperature solution. Moreover, the hot-casting devices with PM6:BTP-eC9 deliver a remarkable fill factor of 80.31% and efficiency of 18.52% in OX, representing the record value among binary devices with green solvent. This work demonstrates a facile strategy to manipulate the molecular distribution and arrangement for boosting the efficiency of OSCs with high-boiling solvents.

Manipulating Alkyl Inner Side Chain of Acceptor for Efficient As-Cast Organic Solar Cells.

As-cast organic solar cells (OSCs) possess tremendous potential for low-cost commercial applications. Herein, five small-molecule acceptors (A1-A5) are designed and synthesized by selectively and elaborately extending the alkyl inner side chain flanking on the pyrrole motif to prepare efficient as-cast devices. As the extension of the alkyl chain, the absorption spectra of the films are gradually blue-shifted from A1 to A5 along with slightly uplifted lowest unoccupied molecular orbital energy levels, which is conducive for optimizing the trade-off between short-circuit current density and open-circuit voltage of the devices. Moreover, a longer alkyl chain improves compatibility between the acceptor and donor. The in situ technique clarifies that good compatibility will prolong molecular assembly time and assist in the preferential formation of the donor phase, where the acceptor precipitates in the framework formed by the donor. The corresponding film-formation dynamics facilitate the realization of favorable film morphology with a suitable fibrillar structure, molecular stacking, and vertical phase separation, resulting in an incremental fill factor from A1 to A5-based devices. Consequently, the A3-based as-cast OSCs achieve a top-ranked efficiency of 18.29%. This work proposes an ingenious strategy to manipulate intermolecular interactions and control the film-formation process for constructing high-performance as-cast devices.

Organic ultraviolet photodetector based on phosphorescent material.

A new type of organic visible-blind UV-PDs is demonstrated by utilizing phosphorescent material Ir(III)bis[(4,6-difluorophenyl)-pyridinato-N, C(2)'] picolinate (FIrpic) as the electron donor and [6,6]-phenyl-C-61-butyricacidmethylester (PCBM) as the electron accepter, respectively. The peak responsivity of the organic UV-PDs is as high as 140 mA/W, corresponding to an external quantum efficiency of about 48%, under 365 nm UV light with an intensity of 0.018 mW/cm(2). The distinct photoluminescence quenching of FIrpic is obtained by doping PCBM. The organic UV-PDs provide visible-blind performance with the strong photocurrent response in the UV-A region, the rise and fall times of less than one second, and linear response within the incident light-intensity range from 0.018 to 20 mW/cm(2).

Revealing the Sole Impact of Acceptor's Molecular Conformation to Energy Loss and Device Performance of Organic Solar Cells through Positional Isomers.

Two new fused-ring electron acceptor (FREA) isomers with nonlinear and linear molecular conformation, m-BAIDIC and p-BAIDIC, are designed and synthesized. Despite the similar light absorption range and energy levels, the two isomers exhibit distinct electron reorganization energies and molecular packing motifs, which are directly related to the molecular conformation. Compared with the nonlinear acceptor, the linear p-BAIDIC shows more ordered molecular packing and higher crystallinity. Furthermore, p-BAIDIC-based devices exhibit reduced nonradiative energy loss and improved charge transport mobilities. It is beneficial to enhance the open-circuit voltage (VOC ) and short-current current density (JSC ) of the devices. Therefore, the linear FREA, p-BAIDIC yields a relatively higher efficiency of 7.71% in the binary device with PM6, in comparison with the nonlinear m-BAIDIC. When p-BAIDIC is incorporated into the binary PM6/BO-4Cl system to form a ternary system, synergistic enhancements in VOC , JSC , fill factor (FF), and ultimately a high efficiency of 17.6% are achieved.

Two-Dimensional Conjugated Benzo[1,2-b:4,5-b']diselenophene-Based Copolymer Donor Enables Large Open-Circuit Voltage and High Efficiency in Selenophene-based Organic Solar Cells.

A two-dimensional electron-rich fused-ring moiety (ClBDSe) based on benzo[1,2-b:4,5-b']diselenophene is synthesized. Three copolymers (PBDT-Se, PBDSe-T, and PBDSe-Se) are obtained by manipulating the connection types and number of selenophene units on the conjugated main chains with two 2D fused-ring units and two different π-bridges, respectively. In comparison with PBDT-Se and PBDSe-Se, PBDSe-T with benzo[1,2-b:4,5-b']diselenophene unit and thiophene π-bridge exhibits the deepest HOMO energy level and the strongest crystallinity in neat films. The PBDSe-T:Y6 blend film exhibits the best absorption complementarity, the most distinctive face-on orientation with proper phase separation, the highest carrier mobilities, and the lowest charge recombination among three blend films. Finally, the PBDSe-T:Y6-based device delivers an impressive power conversion efficiency (PCE) of 14.50 %, which is higher than those of PBDT-Se:Y6 and PBDSe-Se:Y6. Moreover, a decent open-circuit voltage (Voc ) of 0.89 V with a remarkably small energy loss of 0.44 eV is achieved for PBDSe-T:Y6. The efficiency of 14.50 % is the highest value for selenophene-containing copolymer-based binary organic solar cells (OSCs). This study provides evidence that introduction of 2D-benzo[1,2-b:4,5-b']diselenophene as a fused electron-rich unit with π-bridging into copolymeric donors is a valid strategy for providing high Voc and excellent PCE simultaneously in selenophene-based OSCs.

Semitransparent polymer solar cells with 12.37% efficiency and 18.6% average visible transmittance.

A series of opaque and semitransparent polymer solar cells (PSCs) were fabricated with PM6:Y6 as active layers, and 100 nm Al or 1 nm Au/(20, 15, 10 nm) Ag layer as electrode, respectively. The power conversion efficiency (PCE) of opaque PSCs arrives to 15.83% based on the optimized active layer with a thickness of 100 nm, resulting from the well-balanced photon harvesting and charge collection. Meanwhile, the 100 nm PM6:Y6 blend film exhibits a 50.5% average visible transmittance (AVT), which has great potential in preparing efficient semitransparent PSCs. The semitransparent electrodes were fabricated with 1 nm Au and different thick Ag layers, exhibiting a relatively high transmittance in visible light range and relatively low transmittance in near infrared range. The PCE and AVT of the semitransparent PSCs can be adjusted from 14.20% to 12.37% and from 8.9% to 18.6% along with Ag layer thickness decreasing from 20 to 10 nm, respectively, which are impressive values among the reported semitransparent PSCs.

Alloy-like ternary polymer solar cells with over 17.2% efficiency.

Ternary strategy has been considered as an efficient method to achieve high performance polymer solar cells (PSCs). A power conversion efficiency (PCE) of 17.22% is achieved in the optimized ternary PSCs with 10 wt% MF1 in acceptors. The over 8% PCE improvement by employing ternary strategy is attributed to the simultaneously increased JSC of 25.68 mA cm-2, VOC of 0.853 V and FF of 78.61% compared with Y6 based binary PSCs. The good compatibility of MF1 and Y6 can be confirmed from Raman mapping, contact angle, cyclic voltammetry and morphology, which is the prerequisite to form alloy-like state. Electron mobility in ternary active layers strongly depends on MF1 content in acceptors due to the different lowest unoccupied molecular orbital (LUMO) levels of Y6 and MF1, which can well explain the wave-like varied FF of ternary PSCs. The third-party certified PCE of 16.8% should be one of the highest values for single bulk heterojunction PSCs. This work provides sufficient references for selecting materials to achieve efficient ternary PSCs.

Asymmetrical Ladder-Type Donor-Induced Polar Small Molecule Acceptor to Promote Fill Factors Approaching 77% for High-Performance Nonfullerene Polymer Solar Cells.

In this work, an effectual strategy of constructing polar small molecule acceptors (SMAs) to promote fill factor (FF) of nonfullerene polymer solar cells (PSCs) is first reported. Three asymmetrical SMAs of IDT6CN, IDT6CN-Th, and IDT6CN-M, which own large dipole moments, are designed and synthesized. The PSCs based on three polar SMAs exhibit apparently higher FFs compared with their symmetrical analogues. The asymmetrical design strategy accompanied with side chain and end group engineering makes IDT6CN-Th- and IDT6CN-M-based nonfullerene PSCs achieve high power conversion efficiency with FFs approaching 77%.