Small Molecule Donor Research Articles

15.51 % efficiency all-small-molecule organic solar cells achieved by symmetric thiazolyl substitution

Two new small molecule donors of SW1 and SW2 are designed and synthesized by employing thiophene and thiazole substituents as the side groups of benzo[1,2-b:4,5-b′]dithiophene central unit, respectively. Due to the more electron-withdrawing feature and enhanced intermolecular stacking of SW2 with respect to that of SW1, the all-small-molecule (ASM) organic solar cells (OSCs) based on SW2 afford a much better power conversion efficiency up to 15.51 %, along with both enlarged open-circuit voltage and fill factor. Further systematic investigation demonstrates that thiazole could act as an excellent building block to optimize the energy levels, molecular stacking as well as microscopic morphologies in ASM systems, leading to the highly efficient ASM-OSCs.

Furan-based liquid-crystalline small-molecule donor guest improving the photovoltaic performance of organic solar cells with amorphous packing

Introducing liquid-crystalline small-molecule donors (SMDs) into binary systems based on the strong intermolecular interactions of SMDs is a facile and effective strategy to tune the active layer morphology and improve the performance of organic solar cells (OSCs). Contrary to conventional understanding, this research proposes a new strategy for ternary OSCs implicating that “weakly crystalline materials can also optimize the morphology of the active layer and improve the OSCs performance”. Herein, we designed and synthesized two liquid-crystalline SMDs, Z1 and Z2, based on benzodifuran (BDF) units. The amorphous Z2-incorporated ternary devices present an unexpectedly improved power conversion efficiency (PCE) > 18% with good stability. By contrast, the highly ordered Z1-based ternary devices possess a significantly depressed efficiency. Multiple characterizations reveal that the Z2-based ternary blend films possess improved miscibility and efficient charge transport. This novel strategy for the selection of the third component is significant for the fabrication of high-efficiency ternary OSCs.

Understanding morphology effects on fill factor losses in dilute-donor organic solar cells

Experimental results of organic solar cells with low donor concentrations using small molecule donors have displayed significantly lower fill factors (FFs) compared to dilute-donor solar cells (DDSCs) with polymer donors. We perform experiments and kinetic Monte Carlo simulations, to understand the observed FF discrepancy and how FF can be improved. Our results reveal that small molecule DDSCs collect holes from the region of the active layer near the anode whereas polymer DDSCs collect holes from a deeper volume inside the active layer. This enlarged collection region is facilitated by the morphology of polymer chains extending from the anode into the active layer. The chains permit holes to hop along the donor sites to the anode with no barrier. Small molecule DDSCs, in contrast, require a large electric field to transfer holes from isolated donor sites back to the acceptor matrix to reach the anode. Collections in small molecule DDSCs are thus constrained to photogenerated holes on donors near the anode. We propose strategies to increase DDSC FF to levels comparable to bulk-heterojunction organic solar cells by decreasing the donor-acceptor highest occupied molecular orbital energy offset, or by engineering the active layer morphology so that a higher density of donors are in proximity/contact with the anode.

Thiol-Activated 1,2,4-Thiadiazolidin-3,5-diones Release Hydrogen Sulfide through a Carbonyl-Sulfide-Dependent Pathway.

Recent efforts have expanded the development of small molecule donors that release the important biological signaling molecule hydrogen sulfide (H2S). Previous work on 1,2,4-thiadiazolidin-3,5-diones (TDZNs) reported that these compounds release H2S directly, albeit inefficiently. However, TDZNs showed promising efficacy in H2S-mediated relaxation in ex vivo aortic ring relaxation models. Here, we show that TDZNs release carbonyl sulfide (COS) efficiently, which can be converted to H2S by the enzyme carbonic anhydrase (CA) rather than releasing H2S directly as previously reported.

Environmentally compatible 3‐dimensional star‐shaped donor materials for efficient organic solar cells

Small molecule (SM)-based star-shaped materials are getting tremendous attention to be employed as hole transport materials for organic/perovskite solar cells (OSCs) due to their tunable energy levels, intense absorption ability, and their three-dimensional (3D) charge transport properties. Herein, four-armed SM donors (BD1-BD10) with benzodithiophene (BDT) as the central-core unit have been efficiently designed and then characterized theoretically to investigate their optical and optoelectronic behavior. These four-armed designed star-shaped (BD1-BD10) materials exhibited deeper HOMO levels and higher extinction coefficients, which tend to offer better phase separation morphology during blend formation. These molecules (BD1-BD10) and reference (R) have been fully characterized theoretically with advanced quantum chemical approaches. The photophysical and optoelectronic characteristics have been investigated with density functional theory (DFT) and time-dependent (TD-DFT) calculations. The alignment of frontier molecular orbitals (FMOs), optical characteristics, open-circuit voltages, the density of states (DOS), transition density matrix (TDM), and reorganization energies of holes and electrons in these materials have been investigated. BD3 shows the highest absorption (λmax) of 571.33 nm, with an optical bandgap at 3.30 eV, respectively. Moreover, a complex study of BD3/PC61BM unveiled the process of remarkable charge shifting at the donor-acceptor interface. Therefore, our proposed strategy is a prerequisite for designing desirable photovoltaic molecules for efficient organic/perovskite SCs and light-emitting diodes (LEDs).

Self-Immolative Polymer-Based Chemiluminescent Nanoparticles for Long-Term In Vivo Imaging of Reactive Oxygen Species

Chemiluminescence (CL) provides a favorable technique for reactive oxygen species (ROS) imaging with high specificity and negligible background noise; however, the short duration limits its clinical applications in disease diagnosis. In this work, we serendipitously discover that the nanoparticles (NPs), consisting of a self-immolative polymer (SIP) and hydrophobic dyes, induce sustained CL, which is available for long-term in vivo ROS imaging. In response to ROS, SIP undergoes stepwise head-to-tail degradation and sustainably transfers energy to the dyes to emit CL. With this unique domino-like degradation mechanism, the SIP-based CL-NPs generate highly efficient ROS-selective CL signals for at least 6 h, superior to those provided by CL-NPs containing small-molecule CL donors. When intravenously administered, the SIP-based CL-NPs clearly and precisely illuminate the ROS-abundant regions in four typical inflammatory disease models for hours. These findings suggest that SIP-based CL-NPs are potential contrast agents for in vivo ROS imaging and disease diagnosis.

The use of machine learning, density functional theory, and molecular dynamics simulations for the designing and screening of efficient small molecule donors for organic solar cells

AbstractIndeed, a proper understanding of materials is necessary to get the full benefit from them. For this purpose, multiscale computational modeling is the ultimate need. For machine learning analysis, data is collected from the literature. Machine learning analysis is performed using molecular descriptors as independent parameters and power conversion efficiency (PCE) as dependent property. Various machine learning models are tried. The support vector machine (SVM) model has outperformed others. New donor materials that are small molecules are designed using both well‐known and new building blocks. Their PCE is predicted using a SVM model. The top 10 small molecule donors are further studied using density functional theory calculations. Their electronic behavior is studied. Reorganization energy, exciton binding energy and transfer integral are also calculated. Finally, the best three small molecule donors are selected for molecular dynamics simulations. Molecular packing and mixing of active layer materials is studied using radial distribution function. Our proposed framework has the ability to design potential donor materials in short time with marginal computational cost.

Recent Advances in Single‐Junction Organic Solar Cells

Single-junction organic solar cells (OSCs) have made significant progress in recent years. Innovations in material design and device optimization have improved the power conversion efficiencies to over 19%. In this review, based on recent advances, we discuss molecular design strategies to tune the absorption spectrum, energy level, and intermolecular aggregation as well as highlight the role of molecular electrostatic potential in decreasing energy loss. Then, we introduce the latest progress in four types of OSCs composed of different donor:acceptor combinations: polymer donor:small-molecule acceptor, all-polymer, all-small-molecule, and small-molecule donor:polymer acceptor OSCs. Finally, the challenges of OSCs in practical applications, including material cost, stability, and multi-function integration, are discussed.

High-Efficiency Ternary Organic Solar Cells Enabled by Synergizing Dicyanomethylene-Functionalized Coumarin Donors and Fullerene-Free Acceptors

The small-molecule-based donor/acceptor blend is an important component in bulk-heterojunction organic solar cells (OSCs). Although a variety of small-molecule donors have been developed in recent years, only a few of them have reached efficiencies over 15% using fullerene-free acceptors in OSCs. Therefore, it is important to develop high-performing molecular donor materials with ideal molecular orbital energy levels, complementary absorption to the acceptors, and good charge-transporting character. Herein, the design and development of two wide band gap functional coumarin-based donor materials C1-CN and C2-CN are presented, which are synthesized efficiently using readily accessible starting materials, an important criterion for the realization of cost-effective large area OSCs. The associated effect of excellent optoelectronic and thermal properties and frontier orbital energy levels enabled the application of the synthesized donors in solution-processed fullerene-free OSCs. Ternary OSCs fabricated with C1-CN:DBTBT-IC:Y6 displayed efficiencies up to 15.28%, one of the highest values reported for all-small-molecule OSCs with a low energy loss of 0.43 eV.

Blueshifting the Absorption of a Small‐Molecule Donor and Using it as the Third Component to Achieve High‐Efficiency Ternary Organic Solar Cells

Adding a small‐molecule donor (SMD) to state‐of‐the‐art nonfullerene organic solar cells (OSCs) is demonstrated as a useful strategy to construct ternary organic solar cells, as SMDs typically have high crystallinity and can tune charge transport properties of OSCs. However, the absorption of most SMDs overlaps with typical donor polymers (e.g., PM6), which is against the general guidelines of adopting materials with complementary absorption in ternary OSCs. Herein, the absorption of state‐of‐art SMDs (BTR‐Cl) by linking the beta position of the outer thiophene to the alpha position of the inner thiophene unit is intentionally blueshifted. The resulting molecule β‐S1 shows a maximum absorption peak at 505 nm in the film state, which exhibits wider bandgap and shows complementary absorption with the host system (PM6:Y6). The corresponding ternary OSCs with 20%wt β‐S1 show significantly enhanced efficiency from 16.2% to 17.1% due to the increased short‐circuit current (J SC) and improved fill factor (FF). Herein, an effective strategy to design SMDs with both wider bandgaps and higher crystallinity for high‐performance ternary OSCs is presented.

A novel chlorinated small molecule donor for efficient binary and ternary all-small-molecule organic solar cells

A new acceptor-donor-acceptor (A-D-A) type small molecule donor named BTEHR-CT has been developed, taking the chlorothienyl-substituted benzodithiophene (BDT) as the central unit and 3-(2-ethylhexyl)rhodanine as the terminal group. BTEHR-CT shows a low-lying highest occupied molecular orbital (HOMO) level of −5.56 eV and the suitable molecular aggregation benefiting from the introduction of chlorine atoms, which contributes to achieving excellent photovoltaic performances. Based on an efficient non-fullerene acceptor F–2Cl, the optimized binary device exhibits a high power conversion efficiency (PCE) of 11.13% with an open-circuit voltage ( V oc ) of 0.946 V, short-circuit current densities ( J sc ) of 17.05 mA cm −2 and FF of 0.690. With the incorporation of PC 71 BM, the ternary blend film based on BTEHR-CT:F–2Cl:PC 71 BM (weight ratio of 1:0.8:0.2) shows a fibrous morphology with suitable domain size and favorable phase separation after solvent vapor annealing (SVA) treatment, leading to a superior J sc of 18.53 mA cm −2 and fill factor (FF) of 0.692, thus a higher PCE of 12.05%. To our best knowledge, the PCEs of 11.13% and 12.05% are among the highest values reported to date for non-Y series acceptors based binary and ternary all-small-molecule organic solar cells (ASM-OSCs), respectively. The results indicate that the combination of careful molecular design and suitable morphology control is an effective strategy to achieve high performances for ASM-OSCs. • A new chlorinated small molecule donor named BTEHR-CT was designed and synthesized. • BTEHR-CT exhibits excellent optical and electrochemical properties. • The binary ASM-OSCs based on BTEHR-CT:F–2Cl gave a high PCE of 11.13%. • BTEHR-CT based ternary ASM-OSCs showed a higher PCE of 12.05% with the incorporation of PC 71 BM. • The PCEs are among the highest values reported to date for non-Y series acceptors based ASM-OSCs.

Light-intensity-dependent photoresponse time of organic photodetectors and its molecular origin

Organic photodetectors (OPDs) exhibit superior spectral responses but slower photoresponse times compared to inorganic counterparts. Herein, we study the light-intensity-dependent OPD photoresponse time with two small-molecule donors (planar MPTA or twisted NP-SA) co-evaporated with C60 acceptors. MPTA:C60 exhibits the fastest response time at high-light intensities (>0.5 mW/cm2), attributed to its planar structure favoring strong intermolecular interactions. However, this blend exhibits the slowest response at low-light intensities, which is correlated with biphasic photocurrent transients indicative of the presence of a low density of deep trap states. Optical, structural, and energetical analyses indicate that MPTA molecular packing is strongly disrupted by C60, resulting in a larger (370 meV) HOMO level shift. This results in greater energetic inhomogeneity including possible MPTA-C60 adduct formation, leading to deep trap states which limit the low-light photoresponse time. This work provides important insights into the small molecule design rules critical for low charge-trapping and high-speed OPD applications.

Thiophene–Perylenediimide Bridged Dimeric Porphyrin Donors Based on the Donor–Acceptor–Donor Structure for Organic Photovoltaics

Two D–A–D (D = donor, A = acceptor)-based small-molecule donors, TPDI-2P and F-TPDI-2P, are designed and synthesized for organic solar cells (OSCs), with two strong donor porphyrin units bridged by either an electron-deficient diethynyl-substituted thiophene–perylenediimide (TPDI) linker for the former or a diethynyl-fused TPDI linker for the latter; 3-ethylrhodanine units were then flanked symmetrically by phenylenethynylene π-linkers. Both compounds show strong absorption profiles from 400 to 800 nm, with a valley at 600 nm, attributed to the Soret and Q-bands of porphyrin units. Compared to TPDI-2P, F-TPDI-2P with a fused TPDI backbone exhibits high redshifted absorptions and low-lying energy levels due to increased coplanarity of the dimeric porphyrin units. As a result, the optimized photovoltaic device based on F-TPDI-2P/PC71BM has a decent power conversion efficiency (PCE) of 8.21%, whereas the device TPDI-2P/PC71BM has a slightly lower PCE (6.90%). The higher efficiency of F-TPDI-2P/PC71BM is mainly due to better photocurrent generation and smoother surface morphology with elevated charge carrier mobilities. The success of this molecular design strategy could be beneficial in the development of more efficient porphyrin-based donors for high-performance OSCs.

Crystallinity and Molecular Packing of Small Molecules in Bulk-Heterojunction Organic Solar Cells

Crystallinity has played a major role in organic solar cells (OSCs). In small molecule (SM) bulk-heterojunction (BHJ) OSCs, the crystallinity and crystalline packing of SM donors have been shown to have a dramatic impact on the formation of an optimum microstructure leading to high-power conversion efficiency (PCE). Herein we describe how crystallinity differs from polymers to SMs, and how the packing habits of SMs (particularly donors) in active layers of BHJ devices can be described as following two different main modes: a single crystal-like and a liquid crystal-like packing type. This notion is reviewed from a chronological perspective, emphasising milestone donor structures and studies focusing on the crystallinity in SM-BHJ OSCs. This review intends to demonstrate that a shift towards a liquid crystalline-like packing can be identified throughout the history of SM-BHJ, and that this shift can be associated with an increase in overall PCE.

Quantum chemical simulations of benzothiadiazole (BT) based small molecule donor materials for efficient organic solar cells

Four new donor molecules based on benzothiadiazole (BT) core namely BTA1, BTA2, BTA3 and BTA4 have been designed with A-D-A configuration. Quantum chemical simulations represent that these molecules show the best optoelectronic properties just like model compound BTIC-2Cl-γCF3. BTA3 and BTA4 showed the red shifted absorption value (685 nm and 633 nm) respectively as well as lowest band gap 2.31 eV and 2.27 eV respectively. The findings of DOS and FMO for modified acceptormoieties have been found nearly identical, indicating that electron density is distributed over the whole molecule for all designed molecules. BTA3 has minimum electron mobility 0.0086897 eV that makes it a good candidate for efficient solar cell. Binding energy (Eb) of BTA1, BTA2 and BTA4 is less than that of BTIC-2Cl-γCF3 indicating greater dissociation of excitons. BTA4 has the highest dipole moment among all molecules. As a result, this would be the best molecule for fabricating solar cell sheets. These four newly designed A-D-A donor molecules are therefore proposed as appropriate donor materials for highly efficientorganic solar cell (OSC) devices.

Prophylactic Effect of Nitric Oxide Donors on Rat Models of EGFR Inhibitor‒Induced Cutaneous Toxicities

EGF receptor (EGFR) inhibitors have been established as first-line standard-of-care therapies for nonsmall cell lung cancer but are frequently accompanied by adverse dermatological effects, in particular, acneiform rash. There is no effective clinical intervention, partially because of its poorly understood etiology. In this study, we show that inhibition of EGFR initiated keratinocyte HaCaT cell cycle arrest and apoptosis, which fueled a robust secondary inflammatory response. Rats gavaged with EGFR inhibitor showed a phenotype similar to that of clinical patients, which was in line with the interrupted functions observed in HaCaT keratinocytes. We found that a nitric oxide donor, nitroglycerin, was a feasible treatment alternative for EGFR inhibitor‒induced rash. Restoration of epidermal extracellular signal‒regulated kinase and a reduction in signal transducer and activator of transcription 3 signaling through nitroglycerin treatment rescued the cellular functions that had been damaged invitro and further ameliorated the rash in rat models. In addition, the efficacy of nitroglycerin was superior to that of existing clinical interventions. These data highlighted the importance of epidermal EGFR signaling and led to the identification of a small-molecule nitric oxide donor as a mediator that can maintain EGFR pathway functions during anti-EGFR therapies, providing a therapeutic anchor point for adverse EGFRI-induced skin effects.

Regulation of Crystallinity and Vertical Phase Separation Enables High‐Efficiency Thick Organic Solar Cells

AbstractThick film organic solar cells have great potential for preparing large‐area devices in the future. However, unsuitable vertical donor/acceptor distribution, poor crystallinity of materials, and voltage losses remain unclear, limiting the performance of thick film devices. Here, a target ternary strategy by introducing a nematic liquid crystalline small molecule donor BTR‐Cl into the PM6:Y6 host system is used to reduce the performance degradation caused by the increase of active layer thickness. By incorporating 10% BTR‐Cl into the blend film, the voltage loss is minimized in both thin and thick ternary devices. Moreover, the power conversion efficiency of the ternary device is maintained at 15.28% compared to 12.94% of binary devices with an active layer thickness of 300 nm. It is benefited from the optimized crystallization behavior and more suitable vertical phase distribution driven by differences in miscibility between components. While ensuring sufficient phase separation, the charge transport is improved, it also prompts the balance of charge transport and reduces the non‐radiative recombination. This work demonstrates that the use of the third component to enhance the crystallization of the active layer and improve the vertical phase distribution is an effective method to build thick films for future large‐scale printing techniques.

Learning from Fullerenes and Predicting for Y6: Machine Learning and High‐Throughput Screening of Small Molecule Donors for Organic Solar Cells

In recent years, research on the development of organic solar cells has increased significantly. For the last few years, machine learning (ML) has been gaining the attention of the scientific community working on organic solar cells. Herein, ML is used to screen small molecule donors for organic solar cells. ML models are fed by molecular descriptors. Various ML models are employed. The predictive capability of a support vector machine is found to be higher (Pearson's coefficient = 0.75). The best small donors with fullerene acceptors are selected to pair with Y6. New small molecule donors are also designed taking into account quantum chemistry principles, using building units that are searched through similarity analysis. Their energy levels and power conversion efficiencies (PCEs) are predicted. Efficient small molecule donors with PCE > 13% are selected. This design and discovery pipeline provides an easy and fast way to select potential candidates for experimental work.

Symmetrically Fluorinated Benzo[1,2-b:4,5-b']dithiophene-Cored Donor for High-Performance All-Small-Molecule Organic Solar Cells with Improved Active Layer Morphology and Crystallinity.

Side-chain engineering is an efficient molecular design strategy for morphology optimization and performance improvement of organic solar cells (OSCs). Herein, a novel small-molecule donor C-2F, which owns a benzo[1,2-b:4,5-b']dithiophene (BDT) central unit with a symmetrically difluorinated benzene ring as a conjugated side chain, has been synthesized. The conjugated side chain possesses both the symmetry and halogenation effect in novel small molecular donor material. The photovoltaic devices were fabricated with N3 as an acceptor. C-2F:N3 based devices achieved an outstanding power conversion efficiency of 14.64% with a Jsc of 24.87 mA/cm2, a Voc of 0.85 V, and an FF of 69.33%. Then, we investigated the basic material properties, photovoltaic mechanism, and active layer morphology, and the results show that this molecular design strategy of the symmetrically difluorinated moiety as the conjugated side chain provides an effective method for fine-tuning the molecular stacking pattern and active layer phase separation morphology, to improve the all-small-molecule (ASM) OSCs' performances.

Single-bond-linked oligomeric donors for high performance organic solar cells

The effective design and synthesis of novel small-molecule donors (SMDs) is extremely essential for the in-depth study of the scientific problems of bulk heterojunction morphology and the improvement of photovoltaic performance in organic solar cells (OSCs). Importantly, developing a series of donors with different conjugated central donor (D) units is a remarkable strategy to obtain high-performance donors. Herein, two acceptor-donor-donor-acceptor (A-D-D-A) type oligomeric donors 2DTBDT and 2DTBDT-2T with two dithieno[2,3-d:2′,3′-d']benzo[1,2-b:4,5-b']dithiophene (DTBDT) as D units, without and with bithiophene as the π bridge respectively are designed and synthesized successfully. The central linked-DTBDT unit can provide a larger conjugated plane and promote π electron delocalization, which can effectively improve π-π interactions between donors and regulate the crystallinity. And we found that the π bridge provided 2DTBDT-2T with 12.31% efficiency that is already a high efficiency in OSCs, whereas 2DTBDT with merely 3.63% efficiency, both with 2,2′-((2Z,2′Z)-((12,13-bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]thiadiazolo[3,4-e]thieno[2,"3′’:4′,5′]thieno[2′,3′:4,5]pyrrolo[3,2-g]thieno[2′,3′:4,5]thieno[3,2-b]indole,10-diyl)bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene)) dimalononitrile (Y6) as the acceptor. We conjecture that the main reason for the different device performance may be ascribed to the different molecular stacking orientation of the oligomeric donors and morphology features of the donors:Y6 blend films. Compared to the predominant face-on orientation of the 2DTBDT neat film, the 2DTBDT-2T neat film performed a preferential edge-on orientation, which obtained a smoother surface, stronger crystallinity and more uniform phase separation in the 2DTBDT-2T:Y6 blend films with nanofiber structure, which delivered higher and more balanced carrier mobilities, the more efficient exciton dissociation and reduced biomolecule recombination, therefore obtaining better power conversion efficiencies (PCEs). We speculate that the transformation of molecular stacking orientation of oligomeric donors is possibly due to that π bridge extended and twisted the molecular structure of 2DTDBT-2T, resulting in an edge-on orientation relative to the substrate. These findings demonstrate that the single-bond-linked donor strategy is an alternative method to design the donors towards high-performance OSCs.