Non-fullerene Small Molecule Acceptors Research Articles

Thermoplastic Elastomer Tunes Phase Structure and Promotes Stretchability of High-Efficiency Organic Solar Cells.

Top-performance organic solar cells (OSCs) consisting of conjugated polymer donors and nonfullerene small molecule acceptors (NF-SMAs) deliver rapid increases in efficiencies. Nevertheless, many of the polymer donors exhibit high stiffness and small molecule acceptors are very brittle, which limit their applications in wearable devices. Here, a simple and effective strategy is reported to improve the stretchability and reduce the stiffness of high-efficiency polymer:NF-SMA blends and simultaneously maintain the high efficiency by incorporating a low-cost commercial thermoplastic elastomer, polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene (SEBS). The microstructure, mechanical properties, and photovoltaic performance of PM6:N3 with varied SEBS contents and the molecular weight dependence of SEBS on microstructure and mechanical properties are thoroughly characterized. This strategy for mechanical performance improvement exhibits excellent applicability in some other OSC blend systems, e.g., PBQx-TF:eC9-2Cl and PBDB-T:ITIC. More crucially, the elastic modulus of such complex ternary blends can be nicely predicted by a mechanical model. Therefore, incorporating thermoplastic elastomers is a widely applicable and cost-effective strategy to improve mechanical properties of nonfullerene OSCs and beyond.

Selenium-Containing Organic Photovoltaic Materials.

ConspectusOrganic photovoltaics (OPVs) with a photoactive layer containing a blend of organic donor and acceptor species are considered to be a promising technology for clean energy owing to their unique flexible form factor and good solution processability that can potentially address the scalability challenges. The delicate designs of both donors and acceptors have significantly enhanced the power conversion efficiency of OPVs to more than 18%. Nonfullerene small-molecule acceptors (NFAs) have played a critical role in enhancing the short-circuit current density (JSC) by efficiently harvesting near-infrared (NIR) sunlight. To take full advantage of the abundant NIR photons, the optical band gap of NFAs should be further reduced to improve the performance of OPVs. Incorporating highly polarizable selenium atoms onto the backbone of organic conjugated materials has been proven to be an effective way to decrease their optical band gap. For example, a selenium-substituted NFA recently developed by our group has attained a JSC of approximate 27.5 mA cm-2 in OPV devices, surpassing those of most emerging photovoltaic systems. Inspired by this advance, we concentrate on the topic of selenium-containing materials in this Account to incite readers' interest in further exploring this series of materials.In this Account, we first compare the differences among chalcogen heterocycles and discuss the influence of fundamental electronic behavior on the collective photoelectrical properties of the resulting materials. The superior features of selenium-substituted materials are summarized as follows: (1) The large covalent radius of selenium can diminish the π-orbital overlap, rendering enhanced quinoidal resonance character and a narrowed optical band gap of resulting materials. (2) The selenium atom is more polarizable than sulfur owing to its larger and looser outermost electron cloud, enabling enhanced intermolecular Se-Se interaction and increased charge carrier mobility of relevant materials in the solid state. We then focus on summarizing the design rules for various categories of selenium-containing materials including polymer donors, small-molecule acceptors, and polymer acceptors, especially those composed of ladder-type polycyclic units. The motivation for incorporating selenium atoms into these materials and the structure-property relationships were thoroughly elucidated. Specifically, we discuss the changes in the optical band gap, charge carrier mobility, and molecular packing induced by selenium substitution and correlate the effects of these changes with the exciton behaviors, energy loss, and nanoscale film morphology of corresponding OPV devices. Furthermore, we point out the intrinsic stability of selenium-containing materials under maximum-power-point tracking and long-term photo- or thermostress and indicate their potential use in semitransparent and tandem solar cells. At the end, the prospect of future research focuses and the possible applications of selenium-containing materials in the OPV field are discussed.

Recent advances and prospects of asymmetric non-fullerene small molecule acceptors for polymer solar cells

Recently, polymer solar cells developed very fast due to the application of non-fullerence acceptors. Substituting asymmetric small molecules for symmetric small molecule acceptors in the photoactive layer is a strategy to improve the performance of polymer solar cells. The asymmetric design of the molecule is very beneficial for exciton dissociation and charge transport and will also fine-tune the molecular energy level to adjust the open-circuit voltage (V oc) further. The influence on the absorption range and absorption intensity will cause the short-circuit current density (J sc) to change, resulting in higher device performance. The effect on molecular aggregation and molecular stacking of asymmetric structures can directly change the microscopic morphology, phase separation size, and the active layer's crystallinity. Very recently, thanks to the ingenious design of active layer materials and the optimization of devices, asymmetric non-fullerene polymer solar cells (A-NF-PSCs) have achieved remarkable development. In this review, we have summarized the latest developments in asymmetric small molecule acceptors (A-NF-SMAs) with the acceptor–donor–acceptor (A–D–A) and/or acceptor–donor–acceptor–donor–acceptor (A–D–A–D–A) structures, and the advantages of asymmetric small molecules are explored from the aspects of charge transport, molecular energy level and active layer accumulation morphology.

A universal strategy via polymerizing non-fullerene small molecule acceptors enables efficient all-polymer solar cells with > 1 year excellent thermal stability

Record-holding polymer solar cells (PSCs) composed of polymer donors and Y-series non-fullerene small molecule acceptors have reached the efficiency threshold for commercialization, but this type of PSCs is often difficult to work continuously and stably at high temperatures close to 100 °C, limiting its outdoor application. Therefore, in this work, we tried to boost the thermal stability of PSCs based on Y-series acceptors Y5-F-Br by polymerizing Y5-F-Br obtaining PFA1, and establish a precise correlation between the molecular structure and device thermal stability. Consequently, the excellent film stability based on long-chain acceptor PFA1 effectively suppressed the thermally induced phase separation of PTzBI-oF:PFA1 blend film at high temperatures, resulting in over 98% thermal stability at 100 °C for over 1 year in the nitrogen-filled glovebox. Furthermore, the use of long-chain acceptor for improving thermal stability was also confirmed in other two commonly used PSCs, suggesting that polymerizing non-fullerene small molecule acceptors is an effective and universal strategy to boost the thermal stability of PSCs.

Efficient Ternary Polymer Solar Cells with Tunable Crystallinity and Phase Separation of Active Layers via Incorporating GHK-Cu.

The crystallinity of a nonfullerene small-molecule acceptor plays an important function in the bimolecular recombination and carrier transfer of polymer solar cells (PSCs). However, because of the competition between the donor (PBDB-T) and acceptor (ITIC) in processes of phase separation and crystallization, the PBDB-T preferentially forms a crystalline network, which limits the molecular diffusion of ITIC and leads to the weak crystallinity of ITIC, eventually restricting the photoelectric conversion efficiency (PCE) of PSCs. Therefore, in our work, a small-molecule biomaterial, Gly-His-Lys-Cu (SMBM GHK-Cu), is incorporated into binary PBDB-T:ITIC to construct a PBDB-T:ITIC:GHK-Cu ternary system. The addition of GHK-Cu increases ITIC crystallinity and promotes the formation in continuous single-phase domains of PBDB-T and ITIC, which creates an optimized bicontinuous network path to increase and balance charge transmission in PSCs. Meanwhile, GHK-Cu makes energy transfer from GHK-Cu to PBDB-T appreciably efficient, improving the photon capture and exciton-generation rate of PBDB-T. Moreover, it can form a complementary absorption spectrum with PBDB-T and ITIC, which enhances the PCE of ternary devices. Excitingly, the PCE of PSC-based PBDB-T:ITIC is enhanced from 10.28% to 12.07% via incorporating 0.1 wt % GHK-Cu into PBDB-T:ITIC, in which the enhanced open voltage (VOC) is 0.92 V, the short-circuit current (JSC) is 17.87 mA/cm2, and the fill factor (FF) is 73.4%. Meanwhile, the PCE of PSC-based PM6:Y6 is also enhanced from 15.21% for a binary PSC to 17.11% for ternary PSC-based PM6:Y6:0.1 wt % GHK-Cu. This work shows that the cheap and environmentally friendly GHK-Cu has great potential for application in tuning the crystallinity and phase separation of the active layer.

Near-infrared small molecule acceptors based on 4H-cyclopenta[1,2-b:5,4-b']dithiophene units for organic solar cells

Two small-molecule non-fullerene acceptors (NFAs) CPDT-Me-2Eh and CPDT-4Cl-2Eh based on 4H-cyclopenta[1,2-b:5,4-b′] dithiophene as the electron-donating unit 2-(5/6-methyl-3-oxo-2,3-dihydro-1H-inden-1-ylidene) malononitrile and 2-(5,6-dichloro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile as the end group respectively were developed. Compared with CPDT-Me-2Eh, CPDT-4Cl-2Eh possesses red-shift absorption spectrum, lower HOMO and LUMO energy level, and higher electron mobility. Non-fullerene polymer solar cells based on PBDB-T: CPDT-4Cl-2Eh gave a power conversion efficiency (PCE) of 7.52%, with a fill factor (FF) of 64.46%, and a short-circuit current (Jsc) of 16.90 mA/cm2. As contrast, the cell based on PBDB-T: CPDT-Me-2Eh got a PCE value of 4.39%. Our results indicate that the small molecule acceptor containing the CPDT core can effectively extend the absorption spectrum to the near-infrared (NIR) region by chlorination of the capping group. Our work provides an easy routine to broaden the field of new NIR NFAs with high PCE based on non-fullerene OSCs.

Ternary organic solar cells with improved efficiency and stability enabled by compatible dual-acceptor strategy

To further elevate the power conversion efficiency (PCE) of organic solar cells (OSCs), ternary strategy is one of the most efficient methods via simply incorporating a suitable third component. Here, a nonfullerene small molecule acceptor MOITIC was incorporated into the state-of-art PM6:Y6 binary system to further enhance the photovoltaic performance. Detailed investigation revealed that MOITIC exhibited a good miscibility and compatibility with Y6, forming alloy-like acceptors in the ternary blends. The alloy-like phase promoted the phase separation and optimized the morphology of ternary blend, which afforded higher and more balanced carrier mobility and reduced charge recombination in devices. Moreover, the larger energy offset between PM6 and MOITIC:Y6 acceptor alloy is beneficial to enhance open-circuit voltage (Voc) of corresponding devices. As a consequence, the optimized ternary OSC (PM6:Y6:MOITIC = 1:1:0.1) showed a significantly increased PCE of 17.1% with simultaneously enhanced Voc of 0.882 V, short-circuit current density (Jsc) of 25.6 mA cm−2, and fill factor (FF) of 75.7%, which has about 9% enhancement compared to the control binary PM6:Y6 (15.7%). In addition, the optimized ternary device exhibited better stability. This work indicates that ternary strategy via combining two compatible small molecule acceptors is effective to simultaneously improve the efficiency and stability of OSCs.

Noncovalent interaction enables planar and efficient propeller-like perylene diimide acceptors for polymer solar cells

The development of non-fullerene small molecule acceptors enabled fast increasing top power conversion efficiency of polymer solar cells (PSCs). Perylene diimide (PDI) derivatives are among the most promising non-fullerene small molecule acceptors for their relatively straightforward accessible PDI chromophores readily deliver high electron mobility and intensive photon absorption in visible range. In this work, we demonstrated noncovalent interaction as a third strategy of connecting the PDI arms to the center core of propeller-like PDI derivatives other than flexible and all fused linkages through designing and synthesizing two novel PDI derivatives. Thiophene-annulated PDI arms were connected to a small electron withdrawing 1,3,5-triazine center core where N⋯S noncovalent conformational locks restricted the distortion of molecules. Fine tuning of the aggregation tendency of the PDI derivative by introducing N-annulation at the outer bay position of each PDI arms which achieved an outstanding power conversion efficiency reaching 10.52%, which is one of the top PCEs of PDI-based PSCs.

High-efficiency fullerene free ternary organic solar cells based with two small molecules as donor

Herein, we have designed a ternary system comprising of two small molecules (B2 and B3), as donor and a narrow bandgap non-fullerene small molecule acceptor Y6. The chemical structures of B2 and B3 are close to each other but their absorption spectra are complementary with different energy levels. Using these small-molecules, a ternary organic solar cell was fabricated. The presence of B2 in the B3:Y6 blend increases the photon harvesting as well as also forms cascade energy level arrangement which benefits assisting the balancing between the dissociation of excitons into free charge carriers and their subsequent charge transfer between the two donors (B2 and B3) and the acceptor (Y6). Moreover, the photoluminescence spectroscopy analysis has revealed efficient energy transfer from B2 to B3, which resulted in the effective excitons utilization in ternary device. This way, we have been able to achieve over efficiency of about 12.88% with energy loss of 0.54 eV for the ternary device, greater than that for binary devices i.e., 8.73% with energy loss of 0.51 eV and 9.26% with energy loss of 0.61 eV for B2:Y6 and B3:Y6, respectively. This low energy loss is one of the lowest in the fullerene free ternary photovoltaic devices based on all small molecules (donor and acceptors).

Achieving additive-free organic solar cells by modulating the side-chain length of non-fullerene small molecule acceptors

After several years of development, the power conversion efficiencies (PCEs) of organic solar cells (OSCs) based on non-fullerene small molecule acceptors (NF-SMAs) can be more than 17 %. However, the addition of solvent additives, which could lead to complicated device processing and poor repeatability, is often the premise of obtaining high PCEs. Here, the impact of the side-chain length and solvent additives on device performance were investigated by synthesizing three NF-SMAs based on dithieno[3,2-b:2′,3′-d]pyrrole with 2-ethylhexyl (SNC2C4–F), 2-butyloctyl (SNC4C6–F) and 2-hexyldecyl (SNC6C8–F) attached to the N atom. The results showed that the acceptors with longer alkyl side chains were less likely to aggregate in the as-cast active layers. Consequently, the device based on SNC4C6–F, exhibiting the most suitable phase separation and morphology, demonstrated an ideal PCE of 10.71 %, which was higher than that of the other two systems. However, after being treated with 1-chloronaphthalene (1-CN), the morphologies of the active layers based on SNC2C4–F and SNC6C8–F were improved, and the corresponding devices also displayed increased PCEs (>10 %). On the other hand, the SNC4C6–F-based active layer treated with 1-CN showed a deteriorated morphology, leading to a reduced PCE. The findings of this work suggested that adjusting the length of alkyl side chains could achieve the optimal PCEs without additional additive treatment.

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.

Core effect on indacenodithieno[3,2-b]thiophene dimer based small molecule acceptors for non-fullerene polymer solar cells

Dimerization is a widely adopted approach to construct electron accepting materials for polymer solar cells (PSCs). Fullerene dimers were extensively explored with improved structural stability and perylene-diimide (PDI) dimers prevented the formation of undesired large crystals. However, dimers of indacenodithieno[3,2-b]thiophene (IDTT) chromophores have yet not been reported despite the great success in non-fullerene small molecule acceptors (NF-SMAs) such as ITIC. In this work, we designed and synthesized three IDTT dimers with conjugated cores or not and explored the core effect of introducing flexible dithiophene or rigid thienothiophene as linkage group. All the three IDTT dimers were found to be poorly crystallized and functional PSCs devices containing each of these dimers were successfully fabricated paring with donor polymer PBDB-T. The introduction of electron donating linkages raise energy levels of the dimers hence increased the open circuit voltages. Thienothiophene was found to be a more suitable linkage unit than dithiophene but the highest power conversion efficiency of 6.01% was recorded in devices based on dimer dIDTT without the linkage units. This work demonstrated the possibility of using IDTT chromophore to construct dimer NF-SMAs and revealed insights of core effect on their photovoltaic performance.

Fine-Tuning the Dipole Moment of Asymmetric Non-Fullerene Acceptors Enabling Efficient and Stable Organic Solar Cells.

Modifying molecular conjugation has been demonstrated as an effective strategy to enhance the photovoltaic performance of the non-fullerene small molecule acceptors (SMAs), which would regulate the molecular packing and nanoscale morphology in the active layer of organic solar cells (OSCs). Here, two novel SMAs PTIC-4Cl and PT2IC-4Cl are designed and synthesized by expanding the core unit of TB-4Cl in one or two directions. The effects of how to expand the conjugation length on the absorption property, energy levels, dipole moment, and solubility are studied via theoretical calculation and experiments. Compared to PT2IC-4Cl, PTIC-4Cl with a more asymmetric structure exhibits the larger dipole moment and enhanced intermolecular packing. The PTIC-4Cl-based OSCs exhibit a favorable morphology and balanced charge transport, thereby leading to the highest power conversion efficiencies. In addition, PTIC-4Cl-based devices show outstanding thermal and air stability. These results reveal that fine-tuning the dipole moment via rationally expanding the conjugation in asymmetric A-D1A'D2-A-type non-fullerene acceptors is critical to achieve high-performance OSCs.

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

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

Influence of the dipole moment on the photovoltaic performance of polymer solar cells employing non-fullerene small molecule acceptor

A new narrow bandgap non-fullerene acceptor with a DPP core and dicyanorhodanine terminals linked by selenophene spacers has been synthesized. Optical and electrochemical properties of MPU5 were examined and compared to that of its comparable MPU2, which contains thiophene linkers instead of selenophene. MPU5 has a lower optical bandgap (1.38 eV) than MPU2 (1.41 eV), this may be due to the higher electron richness and lower aromatici character of selenophene, comparative to thiophene, favoring a quinoidal structure and intramolecular charge transfer between push–pull moieties. MPU5 was employed as the electron acceptor in conjunction with a conjugated polymer (PCDTBT) as donor with a complementary absorption (350–650 nm) and high HOMO and low LUMO energy levels (–5.4 eV and –3.6 eV, respectively) for the fabrication of polymer solar cells using a bulk heterojunction active layer. The MPU5-based polymer solar cell enabled an overall power conversion efficiency (PCE) of 10.10% with an Eloss of 0.43 eV with an optimized bulk heterojunction active layer. These values are better than those for the MPU2 counterpart (PCE = 8.04%, Eloss = 0.56 eV). The higher PCE obtained for the MPU5-based polymer can be correlated with better balanced charge transport, appropriate active layer nanoscale morphology, efficient charge extraction and longer charge carrier lifetime, resulting in an improvement in the Jsc and FF values. The superior Voc value for the MPU5-based polymer may be related to the lower energy loss caused by the larger dipole moment and dielectric constant of MPU5, which result in a smaller exciton binding energy in the active layer.

Morphology Evolution Induced by Sequential Annealing Enabling Enhanced Efficiency in All-Small Molecule Solar Cells

Advances in small-molecule non-fullerene acceptors have led to increase in power conversion efficiencies (PCEs) of organic solar cells. Simultaneously, small molecules feature lower viscosity and b...

Tetraperylenediimide derivative as a fullerene-free acceptor for a high-performance polymer solar cell with the high-power conversion efficiency of 10.32% with open-circuit voltage over 1.0 V

Abstract A perylenediimide (PDI) tetramer non-fullerene small molecule acceptor consisting of four PDI units linked to tertraphenylethylene (TPhE) central moiety at their β-positions, denoted as TPhEPDI-4 was designed and synthesized. TPhEPDI-4 was combined with polymeric donor PTB7-Th to fabricate the polymer solar cells. TPhEPDI-4 exhibits an absorption profile ranging from 300 to 630 nm, which is complementary with the PTB7-Th, and the blend PTB7-Th: TPhEPDI-4 exhibits the absorption profile from 300 to 830 nm. The optimized solvent vapor annealed bulk heterojunction-based polymer solar cells showed overall power conversion efficiency of about 10.34% (with a high open-circuit voltage of 1.04 V and voltage loss of 0.53 eV), which is higher than that for the as-cast counterpart (6.12%). The improvement in the power conversion efficiency for solvent vapor annealed active layer-based polymer solar cell is originated from higher and balanced charge carrier mobilities suppressed by bimolecular recombination linked with efficient charge transport. The X-ray diffraction confirms the denser and ordered molecular packing after the solvent vapor annealing as well as crystal coherence length in the PTB7-Th: TPhEPDI-4 film.

Non-fullerene acceptors based on multiple non-covalent interactions for low cost and air stable organic solar cells

Significant progresses have been made on organic solar cells (OSCs) in last few years, but the industrialization of OSCs is still hindered by high cost and poor device stability. Recently, the most successful OSCs are usually based on fused-ring small molecule non-fullerene acceptors (SM-NFAs) which often involve multiple complex synthetic steps and result in low overall yields and high cost. Herein, two easily accessible non-fused NFAs based on BT , CPDT and IC with different halogen atoms named BT-F and BT-Cl were designed and synthesized. Non-covalent interactions or conformation locks between BT and adjacent CPDT units provided planar conformations, which is beneficial for molecular packing and charge transport. When blended with low cost polymer donor PTQ-10 , optimal devices based on BT-F provided a PCE of 8.26%. A higher PCE of 8.65% was obtained from BT-Cl -based devices. Low voltage loss of 0.57 and 0.59 V were observed for BT-F - and BT-Cl -based devices. It is noteworthy that devices based on PTQ-10 : BT-F and PTQ-10 : BT-Cl exhibited excellent stabilities when stored in ambient atmosphere for 10 days without encapsulation. This work demonstrates an example of pursuing low cost OSCs with excellent air stability. • Two non-fused non-fullerene acceptors named BT-F and BT-Cl were reported. • When blend with PTQ-10, BT-F and BT-Cl show PCEs of 8.26% and 8.65%, respectively. • The devices exhibit excellent air stabilities without encapsulation.

Efficient Ternary Polymer solar cells based ternary active layer consisting of conjugated polymers and non-fullerene acceptors with power conversion efficiency approaching near to 15.5%

Ternary polymer solar cells employing two polymers (P2, PTB7-Th) and one non-fullerene small molecule acceptor (Y6) were constructed using a conventional structure. The PTB7-Th, P2 and Y6 exhibit complementary absorption spectra therefore, the ternary blend consists of these three can maximize the photon harvesting efficiency which is advantageous to boost of short circuit current density of polymer solar cell. After the optimization of the weight ratios between the two donors and keeping constant the weight concentration of acceptor and following solvent vapor annealing in THF for 40 s, P2:PTB7-Th: Y6 (0.3:0.7:1.5) based polymer solar cells attained the power conversion efficiency of 15.46%, superior to that for polymer solar cells based on binary active layers i.e., P2:Y6 (12.62%) and PTB7-Th:Y6 (12.84%), respectively. The open-circuit voltage for the P2:PTB7-Th:Y6 is about 0.88 V which is in between the values for P2:Y6 (0.94 V) and PTB7-Th:Y6 (0.85 V), consistent with the different highest occupied molecular orbital energy level for P2 (−5.38 eV) and PTB7-Th (−5.27 eV). The enhancement in the short circuit current may be associated with the broader absorption profile of the ternary active layer relative to binary counterparts while the enhanced value of fill factor may be originated from the balanced charge transport, suppressed recombination, and faster charge extraction.

Designing of small molecule non-fullerene acceptors with cyanobenzene core for photovoltaic application

A quantum chemical analysis of four star-shaped anisotropic acceptor molecules (A1-A4) having acceptor-acceptor′-acceptor (A-A′-A) architecture has been performed for solar cell applications. The designed acceptors consist of cyano benzene as central core, thiophene as π-bridge and 2-methylenemalononitrile (A1), 6-methylene-2-thioxo-1,3-thiazinan-4-one (A2), 2-(2-methylene-3-oxo-2,3-dihydro-1H-inden-1ylidene) malononitrile (A3) and 5-methylene-2-thioxothiazolidin-4-one (A4) as peripheral acceptor units. DFT and TD-DFT calculations were performed using selected MPW1PW91 to assess optical, electronic and charge transfer properties of acceptors (A1-A4) which were further characterized through frontier molecular orbitals (FMOs), DOS, reorganization energies, TDM and open-circuit voltage analysis. The results dictated that the substitution of different acceptor units on the three sides of cyano benzene core forming star-shaped molecules can fine tune the FMO energies, absorption properties (between 470 and 524 nm), band gap energies and reorganization energies in accordance with R. Among all acceptors, A3 exhibited exclusively high absorption value (524 nm), lowest band gap (2.46 eV) and lowest hole reorganization energy (0.0037 eV). Moreover, the acceptor molecules have comparable values of open-circuit voltage to R making them quite suitable for photovoltaic applications in OSCs. The electron-deficient core-based acceptors, in this study, open a new door for designing high-performance OSC acceptors.