Small Molecule Acceptors Research Articles

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.

Ternary solar cells via ternary polymer donors and third component PC71BM to optimize morphology with 13.15% efficiency

The third component played an increasingly significant role in polymer solar cells (PSCs), especially for the morphological impact. Here, we use a ternary polymer donor (TBFCl50-BDD) and organic small molecule acceptor (BTP-4F) as the main material and introduce a third component (PC71BM) to enhance the morphology of the photoactive layer and result in improved device performance. The TBFCl50-BDD:BTP-4F based PSCs only exhibited an efficiency of 10.58%, while The power conversion efficiency of ternary device (TBFCl50-BDD:BTP-4F:PC71BM) reached 13.15%. Importantly, the introduction of PC71BM can not only improve the morphology of the active layer and promote more efficient exciton transport, but also influence the molecular arrangement for better crystallization performance. This work offers an effective method for optimization of the morphology to enhance the properties of PSCs.

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.

Design of All-Fused-Ring Electron Acceptors with High Thermal, Chemical, and Photochemical Stability for Organic Photovoltaics

High-performance donor–acceptor electron acceptors containing 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (INCN)-type terminals are labile toward photooxidation and basic conditions, and ...

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.

Brominated Small-Molecule Acceptors with a Simple Non-fused Framework for Efficient Organic Solar Cells

Recently, non-fullerene organic solar cells (OSCs) have experienced a rapid increase in efficiency, benefiting from the development of small-molecule acceptors (SMAs). The widely studied SMAs for h...

Small-molecule acceptors with long alkyl chains for high-performance as-cast nonfullerene organic solar cells

Abstract Three fused-ring small-molecule electron acceptors, IDTC16-IC, IDTC16-Th, and IDTC16-4F, were designed and synthesized by introducing indacenodithiophene (IDT) as the electron-donating core and 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (IC), fluorinated IC, and a thiophene-based unit as the electron-withdrawing end group. Here, instead of the commonly used n-hexyl or n-hexylphenyl side chains, n-hexadecyl peripheral substituents were employed at the IDT core to study the influence of alkyl groups on photovoltaic performance of the nonfullerene acceptors. The introduction of flexible n-hexadecyl group endowed the three acceptors with excellent solubility in common organic solvents. All the three acceptors presented strong absorption ranging from 450 nm to 720 nm in solution with high molar extinction coefficients. As a result, the as-cast organic solar cells (OSCs) based on IDTC16-IC and the wide bandgap polymer donor PM6 exhibited a power conversion efficiency (PCE) of 5.12%. The OSCs based on PM6:IDTC16-Th and PM6:IDTC16-4F showed much better photovoltaic performance with PCEs of 8.76% and 8.55%, respectively. The PCE values were improved to 5.89%, 9.09%, and 9.42% for the PM6:IDTC16-IC, PM6:IDTC16-Th, and PM6:IDTC16-4F OSCs, respectively, with the addition of the solvent additive 1,8-diiodooctane. These findings demonstrate that the combination of alkyl chains at the fused rings and fluorination or aromatic structure change of the terminal groups leads to greatly enhanced photovoltaic performance of nonfullerene acceptors through improving the photophysical, molecular orbital, and film morphological properties.

High-Molecular-Weight Electroactive Polymer Additives for Simultaneous Enhancement of Photovoltaic Efficiency and Mechanical Robustness in High-Performance Polymer Solar Cells.

The development of small-molecule acceptors (SMAs) has significantly enhanced the power conversion efficiency (PCE) of polymer solar cells (PSCs); however, the inferior mechanical properties of SMA-based PSCs often limit their long-term stability and application in wearable power generators. Herein, we demonstrate a simple and effective strategy for enhancing the mechanical robustness and PCE of PSCs by incorporating a high-molecular-weight (MW) polymer acceptor (PA, P(NDI2OD-T2)). The addition of 10–20 wt % PA leads to a more than 4-fold increase in the mechanical ductility of the SMA-based PSCs in terms of the crack onset strain (COS). At the same time, the incorporation of PA into the active layer improves the charge transport and recombination properties, increasing the PCE of the PSC from 14.6 to 15.4%. The added PAs act as tie molecules, providing mechanical and electrical bridges between adjacent domains of SMAs. Thus, for the first time, we produce highly efficient and mechanically robust PSCs with a 15% PCE and 10% COS at the same time, thereby demonstrating their great potential as stretchable or wearable power generators. To understand the origin of the dual enhancements realized by PA, we investigate the influence of the PA content on electrical, structural, and morphological properties of the PSCs.

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...

Mapping the Side-Chain Length of Small-Molecule Acceptors towards the Optimal Hierarchical Morphology in Ternary Organic Solar Cells

Using multiple light-absorbing materials to realize a broader and better absorption spectrum in multi-component organic photovoltaics has achieved significant success to obtain high power conversion efficiency. Meanwhile, the good materials combinations with matched electronic structure and proper blend morphology for charge generation and transport are of primary importance for implementation of the multi-component strategy. Hierarchical morphology has been clearly demonstrated to improve all performance parameters in ternary organic photovoltaics but shows strong dependence on the molecular structures. Here we develop four small-molecule electron acceptors with different alkyl chain lengths to find the optimal solution of alkyl chain towards the defined hierarchical morphology and carry out a clear and comprehensive investigation of the alkyl chain length effects on the structure–morphology–device performance relationships in ternary blends. There is a positive correlation between the power conversion efficiencies of the four ternary systems and their short-circuit current density parameters, manifesting the significance of distinguishing optimal alkyl side chain length of small-molecule electron acceptors for defined hierarchical morphology to afford efficient carrier generation. The non-optimal side chains would retard the BTR crystallization and make the PC71BM domain sizes incontrollable, leading to a morphology without a defined hierarchy. Such a detailed mapping of the alkyl side chain length of small-molecule electron acceptors provides new insight into the materials combinations for the next-step high-performance multi-component organic photovoltaics.

Importance of Terminal Group Pairing of Polymer Donor and Small‐Molecule Acceptor in Optimizing Blend Morphology and Voltage Loss of High‐Performance Solar Cells

AbstractAs a variety of non‐fullerene small molecule acceptors (SMAs) have been developed to improve power conversion efficiency (PCE) of organic solar cells (OSCs), the pairing of the SMAs with optimal polymer donors (PDs) is an important issue. Herein, a systematic investigation is conducted with the development of the SMA series, named C6OB‐H, C6OB‐Me, and C6OB‐F, which contain distinctive terminal substituents –H, –CH3, and –F, respectively. These SMAs are paired with two PDs, PBDT‐H and PBDT‐F. Interestingly, the PD/SMA pairs with similar terminal groups yield enhanced molecular compatibility and energetic interactions, which suppress voltage loss while improving blend morphology to enhance simultaneously the open–circuit voltage, short–circuit current, and fill factor of the OSCs. In particular, the OSC based on the PBDT‐F:C6OB‐F blend sharing fluorine terminal groups achieves the highest PCE of 15.2%, which outperforms those of PBDT‐H:C6OB‐F (10.1%) and PBDB‐F:C6OB‐H OSCs (11.2%). Furthermore, the PBDT‐F:C6OB‐F OSC maintains high PCEs with active layer thicknesses between 85 and 310 nm. In contrast, the PCE of PBDT‐H:C6OB‐F‐based OSC already drops by 80% from 10.1% to 2.1% when the active layer thickness increases from 100 to 200 nm. This study establishes an important PD/SMA pairing rule in terms of terminal functional groups for achieving high‐performance OSC.

Utilizing Benzotriazole-Fused DAD-Type Heptacyclic Ring to Construct n-Type Polymer for All-Polymer Solar Cell Application

Polymerizing small-molecule acceptor is a valid method for combining the advantages of small molecule acceptors and polymer acceptors. Here, a weak electron-deficient unit, benzo[d][1,2,3]-triazole (BTA) was fused to get the heptacyclic ring of dithienothiophen[3,2-b]-pyrrolobenzo[d][1,2,3]-triazole (TPBTA), which was utilized as DAD unit to synthesize a A′-DAD-A′-D′ type polymer acceptor (named as A702). After being blended with classic p-type polymer PBDB-T, a high PCE of 11.84% with an open-circuit voltage (VOC) of 0.9 V was achieved. Our results indicate the heptacyclic ring containing BTA unit can be regarded as an attractive option to build high-efficient PAs.

Single-Component Organic Solar Cells with Competitive Performance

Organic semiconductors with chemically linked donor and acceptor units can realize charge carrier generation, dissociation and transport within one molecular architecture. These covalently bonded chemical structures enable single-component organic solar cells (SCOSCs) most recently to start showing specific advantages over binary or multi-component bulk heterojunction concepts due to simplified device fabrication and a dramatically improved microstructure stability. The organic semiconductors used in SCOSCs can be divided into polymeric materials, that is, double-cable polymers, di-block copolymers as well as donor–acceptor small molecules. The nature of donor and acceptor segments, the length and flexibility of the connecting linker and the resultant nanophase separation morphology are the levers which allow optimizing the photovoltaic performance of SCOSCs. While remaining at 1–2% for over a decade, efficiencies of SCOSCs have recently witnessed significant improvement to over 6% for several materials systems and to a record efficiency of 8.4%. In this mini-review, we summarize the recent progress in developing SCOSCs towards high efficiency and stability, and analyze the potential directions for pushing SCOSCs to the next efficiency milestone.

Semi‐Transparent Organic Photovoltaics: Sequential Deposition of Donor and Acceptor Provides High‐Performance Semitransparent Organic Photovoltaics Having a Pseudo p–i–n Active Layer Structure (Adv. Energy Mater. 13/2021)

In article number 2003576, Kung-Hwa Wei and co-workers demonstrate that semi-transparent organic photovoltaics with a sequential deposited (SD) active layer—individually deposited polymer donor layer and a small-molecule acceptor layer—forming pseudo p–i–n structures have larger power conversion efficiency and transmittance values than those for the devices with bulk heterojunction (BHJ) structures, and the enhancements for SD versus BHJ devices increase with the decreasing active layer thickness.

End-capped engineering of bipolar diketopyrrolopyrrole based small electron acceptor molecules for high performance organic solar cells

Non-fullerene small acceptor molecules demonstrate promising photovoltaic properties supported the progress of organic solar cells (OSCs). In organic photovoltaics, non-fullerene small acceptor compounds are valuable than traditional fullerene-based acceptor molecules for their good contribution in organic solar cells. This research generally focusses on Bipolar Diketopyrrolopyrrole (F(DPP)2B2) based small electron acceptor compounds used for organic solar cells (OSCs). For this purpose, four newly developed A-D-A (Acceptor-Donor –Acceptor) type electron accepting compounds named as V1, V2, V3, and V4 are planned for organic solar cells to study their optoelectronic characteristics. MPW1PW91/6-31G(d,p) basis set was used for calculations of various geometric parameters. Among all these, newly designed molecules V1 is proven an appropriate compound for OSCs applications due to the photovoltaic characteristics like low band gap between HOMO and LUMO (1. 88 eV) and V3 molecule shows broader absorption λmax in the gas phase (741.62 nm) and chloroform solvent (785.95 nm). Molecule V2 exhibits the lowest value of mobility of electrons (0.0806 Eh). All the designed compounds exhibit better open-circuit voltage, fewer energies of excitations, the highest value of absorption, greater dipole moment, equivalent binding energies, well-organized transport of electron and hole as compared to reference R. Thus, these molecules are suggested to researchers for the future development of highly efficient OSCs.

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.

The substituents on the intermediate electron-deficient groups in small molecular acceptors result appropriate morphologies for organic solar cells

The small molecule acceptors with the structure of acceptor-donor acceptor' donor-acceptor (A−DA'D−A) boost the power conversion efficiency of organic solar cells. Compared with multifused DA'D core with an electron-deficient unit at the center, the unfused core emerges low cost and high yield. Herein, we designed and synthesized three small molecule acceptors named as TR2F-IC4F, BTOC8-IC4F and, BTOC6C8-IC4F, with an unfused core containing one benzothiadiazole (BT) or triazole (TR) unit and two cyclopentadithiophene (CPDT) units. The nearly planar geometry is realized through noncovalent F⋯H, F⋯S and S⋯O interactions. These acceptors have broad near-infrared absorption. For devices with BTOC8-IC4F, TR2F-IC4F and BTOC6C8-IC4F, the highest PCEs are only 5.81%, 5.89% and 7.55%, respectively. For devices with BTOC6C8-IC4F, the PCEs are further improved to 10.01% for ternary cells, which add PC 71 BM into devices. In this work, we found with the increase of the substituents on the intermediate electron-donating groups, the planarity of the molecules become weaken, but can obtain more compatible with the donor molecules, which could result in the different film-forming properties and morphologies in blending films. This work provides a typical example of introducing unfused core into small molecule acceptors, which is important to design new solution-processable small molecule acceptors in the future. The small molecule acceptors named as TR2F-IC4F, BTOC8-IC4Fand BTOC6C8-IC4F with different substituents on the intermediate electron-deficient groups have different film-forming properties and morphologies in blending films, which correlate with the device performance. When PC 71 BM is used as the third component, the parameters of these devices can be further increased. The power conversion efficiency of the solar cell based on BTOC6C8-IC4F is improved from 7.55% to 10.01% by using PC 71 BM as the third component. • Changing the substituents on the intermediate electron-donating groups in the acceptor molecule can obtain different film-forming properties and morphologies in blending films. • The unfused core emerges low cost and high yield and the existence of noncovalent F⋯H and S⋯O interactions, obtain the planarity of molecules. • The PCEs can further improved for ternary cells, when add PC 71 BM into devices.

Efficient and Stable Nonfullerene Polymer Solar Cell Enabled by a Narrow-Bandgap, Nonplanar, Small-Molecule Acceptor with an Extended Main Chain

In the past several years, the small-molecule electron-acceptors with proper twisted main chain geometry have achieved promising performances in polymer solar cells (PSCs) not only because of the s...

Regio‐Regular Polymer Acceptors Enabled by Determined Fluorination on End Groups for All‐Polymer Solar Cells with 15.2 % Efficiency

AbstractPolymerization sites of small molecule acceptors (SMAs) play vital roles in determining device performance of all‐polymer solar cells (all‐PSCs). Different from our recent work about fluoro‐ and bromo‐ co‐modified end group of IC‐FBr (a mixture of IC‐FBr1 and IC‐FBr2), in this paper, we synthesized and purified two regiospecific fluoro‐ and bromo‐ substituted end groups (IC‐FBr‐o & IC‐FBr‐m), which were then employed to construct two regio‐regular polymer acceptors named PYF‐T‐o and PYF‐T‐m, respectively. In comparison with its isomeric counterparts named PYF‐T‐m with different conjugated coupling sites, PYF‐T‐o exhibits stronger and bathochromic absorption to achieve better photon harvesting. Meanwhile, PYF‐T‐o adopts more ordered inter‐chain packing and suitable phase separation after blending with the donor polymer PM6, which resulted in suppressed charge recombination and efficient charge transport. Strikingly, we observed a dramatic performance difference between the two isomeric polymer acceptors PYF‐T‐o and PYF‐T‐m. While devices based on PM6:PYF‐T‐o can yield power conversion efficiency (PCE) of 15.2 %, devices based on PM6:PYF‐T‐m only show poor efficiencies of 1.4 %. This work demonstrates the success of configuration‐unique fluorinated end groups in designing high‐performance regular polymer acceptors, which provides guidelines towards developing all‐PSCs with better efficiencies.