Electron Donor Materials Research Articles

Cover Picture: Novel low‐band gap non‐fullerene acceptors based on IDIC core as potential photovoltaic materials (BKCS 9/2024) Radhiha Ravindran, Inchan Kim, Yun‐Hi Kim, Soon‐Ki Kwon

Organic photovoltaics (OPVs) attracted a lot of research attention owing to its advantages such as convenient processing technique, unsubstantial weight, low cost, flexibility and use of environmentally benign materials. In general, these are bulk heterojunction (BHJ) OPVs obtained through a solution process and the large‐area OPV production is achieved via roll‐to‐roll process. In OPVs, the BHJ active layer consists of electron donor (D) and electron acceptor (A) materials blended together by dissolving them in a common organic solvent. The new low band gap non‐fullerene based acceptors could be used as potential candidates for the fabrication of OPVs in combination with suitable donor polymers. More details are available in the article by Radhiha Ravindran, Inchan Kim, Yun‐Hi Kim, Soon‐Ki Kwon image

Molecular Design and High‐Throughput Virtual Screening of Electron Donor and Non‐fullerene Acceptors for Organic Solar Cells

The complicated trilateral relationships among molecular structures, properties, and photovoltaic performances of electron donor and acceptor materials hinder the rapid improvement of power conversion efficiency (PCE) of organic solar cells (OSCs). Herein, the database of 310 donor and non‐fullerene acceptor pairs is constructed and 39 molecular structure descriptors are selected. Four kinds of machine learning (ML) algorithms random forest (RF), extra trees regression, gradient boosting regression trees, and adaptive boosting are applied to predict photovoltaic parameters. The coefficient of determination, Pearson correlation coefficient, mean absolute error, and root mean square error are adopted to evaluate ML performance. The results show that the RF model exhibits the best prediction accuracy. The Gini important analysis suggests the fused ring and aromatic heterocycles are critical fragments in determining PCE. The molecular unit sets are constructed by cutting each donor and acceptor molecules in database. The 31 752 D‐π‐A‐π type donor molecules and 5 455 164 A‐π‐D‐π‐A type acceptor molecules are designed by recombination of molecular units, and 173 212 367 328 donor–acceptor pairs are generated by combining the newly designed donor and acceptor molecules. Based on the predicted PCE using the trained RF model, 42 donor–acceptor pairs exhibit the predicted PCE > 16%, in which the highest PCE is 16.24%.

Dilute Donor Organic Solar Cells Based on Non-fullerene Acceptors.

The advent of small molecule non-fullerene acceptor (NFA) materials for organic photovoltaic (OPV) devices has led to a series of breakthroughs in performance and device lifetime. The most efficient OPV devices have a combination of electron donor and acceptor materials that constitute the light absorbing layer in a bulk heterojunction (BHJ) structure. For many BHJ-based devices reported to date, the weight ratio of donor to acceptor is near equal. However, the morphology of such films can be difficult to reproduce and manufacture at scale. There would be an advantage in developing a light harvesting layer for efficient OPV devices that contains only a small amount of either the donor or acceptor. In this work we explore low donor content OPV devices composed of the polymeric donor PM6 blended with high performance NFA materials, Y6 or ITIC-4F. We found that even when the donor:acceptor weight ratio was only 1:10, the OPV devices still have good photoconversion efficiencies of around 6% and 5% for Y6 and ITIC-4F, respectively. It was found that neither charge mobility nor recombination rates had a strong effect on the efficiency of the devices. Rather, the overall efficiency was strongly related to the film absorption coefficient and maintaining adequate interfacial surface area between donor and acceptor molecules/phases for efficient exciton dissociation.

Synergistic strategies to unveil the dual influence of additive and annealing treatment on the morphology of isatin-derived low bandgap small molecule donor in solution-processed organic solar cells

A novel organic small molecule, 5-(4-(diphenylamino)phenyl)-1-octylindoline-2,3-dione (TPI) containing an alkyl chain substituted electron accepting isatin (indoline-2,3-dione) and strong electron withdrawing N, N-diphenylaniline units have been synthesized through Suzuki coupling reaction. The molecular structure of TPI is confirmed by NMR, FT-IR, and Mass spectroscopy techniques. The TPI molecule exhibits a low band gap of approximately ∼ 2.03 eV which is also supported by the DFT results. The influence of additive and heat treatment on the electronic, morphological, and photovoltaic performance of organic solar cells was investigated meticulously. The TPI donor exhibits complementary light absorption with fullerene acceptor in combination with DIO and TA which efficiently absorb the photon in the 300–730 nm range. The TPI was later used as an electron donor material for the fabrication of organic solar cells. Remarkably, the morphology-dependent optimized TPI:PC71BM (1:3.0) exposed excellent power conversion efficiency (PCE) of about ∼ 5.56 % with enhanced VOC of ∼ 0.811 V, JSC of 11.81 mA cm−2 and FF of 55.3 % fabricated in addition with DIO and thermal annealing treatment. Most importantly, the unencapsulated devices maintained > 96 % of the initial PCE value after 240 h under ambient temperature. Our findings indicate that isatin-derived small molecules with simple architecture propose great potential for promising BHJ-OSCs material.

Triselenasumanene-based organic molecules as donor and hole-transporting materials for next-generation solar cells

The optoelectronic and charge transport properties of triselenasumanene-based organic molecules were studied using electronic structure calculations to explore their potential as a hole-transporting material (HTM) in perovskite solar cells (PSCs) and as electron donors in organic solar cells (OSCs). Based on the recently synthesized triselenasumanene (SESUM) molecule, nine molecules (SESUMXn, where X = anthracene (A), tetracene (T) and pentacene (P), n = number of attached A, T and P, i.e. 1, 2 and 3) were designed by extending the π-conjugation. The newly designed molecules possessed suitable energy-level alignment with the energy bands of commonly used perovskite material, CH3NH3PbI3, in PSCs, and are suitable as electron-donor material with the commonly used acceptor material, PC61BM, in OSCs. The calculated ionic state properties and electrophilicity index also showed that the studied molecules were potential donor materials for OSCs. Further, the calculated open circuit voltage and fill factor of SESUM and anthracene- and tetracene-substituted SESUM molecules were comparable with those of recently reported donor molecules. The calculated absorption spectra, effective hole transfer integral and stacking angle distribution showed the suitability of SESUMX2 and SESUMX3 molecules as HTM in PSCs. The results of periodic density functional theory calculations showed that the tri-substituted SESUM molecules, SESUMX3 (where, X = A, T and P), were effectively adsorbed on the CH3NH3PbI3 surface and abstracted the hole carrier from the perovskite surface.

Understanding photochemical degradation mechanisms in photoactive layer materials for organic solar cells.

Over the past decades, the field of organic solar cells (OSCs) has witnessed a significant evolution in materials chemistry, which has resulted in a remarkable enhancement of device performance, achieving efficiencies of over 19%. The photoactive layer materials in OSCs play a crucial role in light absorption, charge generation, transport and stability. To facilitate the scale-up of OSCs, it is imperative to address the photostability of these electron acceptor and donor materials, as their photochemical degradation process remains a challenge during the photo-to-electric conversion. In this review, we present an overview of the development of electron acceptor and donor materials, emphasizing the crucial aspects of their chemical stability behavior that are linked to the photostability of OSCs. Throughout each section, we highlight the photochemical degradation pathways for electron acceptor and donor materials, and their link to device degradation. We also discuss the existing interdisciplinary challenges and obstacles that impede the development of photostable materials. Finally, we offer insights into strategies aimed at enhancing photochemical stability and discuss future directions for developing photostable photo-active layers, facilitating the commercialization of OSCs.

Green solvent-processed organic solar cells based on a small-molecule Ir(iii) complex as electron donor materials

Through the synergistic involvement of an environmentally friendly solvent and high temperature spin-coating process, the power conversion efficiency for small-molecule Ir(iii) complex based organic solar cells could be significantly improved.

Solvent-free one-pot two component synthesis. Tuning of optoelectronic properties of fluorescent phloroglucinol-aryl Schiff base compounds

A series of new aryl-Schiff base derivatives were synthetized from the one-pot approach: two component condensation reaction in absence of solvent, in quantitative yields with a high atom economy under green conditions. All of the derivatives were fully characterized by NMR, vibrational absorption analysis, UV/vis absorption and fluorescence spectroscopy, and high-resolution mass spectrometry. The aryl-Schiff bases predominantly present the enol-imine tautomer in acetone-d6, while both enol-imine and keto-imine tautomer readily concur in dimethyl sulphoxide-d6. The emission of the series could be tailored by substituting electron donor or electron acceptor groups in the aryl-Schiff bases from fluorescent to not fluorescent molecules, respectively. Fluorescent molecules emit blue, cyan and yellow color with quantum yields in the 2.4–6.2 % range and average lifetime of ∼0.2 ns. The large Stokes’ shift for the molecule bearing a methylalcoxy (3248 cm−1) and diethyl amine (5017 cm−1) suggests a possible intramolecular charge transfer process as also corroborated by cyclic voltammetry, and explaining their low fluorescence quantum yield. The aryl-Schiff base substituted with the methylalcoxy (2b) was used as an electron donor material in photovoltaic devices in configuration: ITO/PEDOT:PSS/2b:PC61BM/Fields metal. The device built under atmospheric conditions displayed an intrinsic power conversion efficiency of PCE ∼0.4 %, with a Voc of 420 mV and Jsc 4.28 mA/cm2.

All-small-molecule organic solar cells based on cyclometalated tris-Ir(III) complexes as donor materials

Cyclometalated tris-Ir(III) complexes with six-coordinated octahedral configuration are beneficial to reduce aggregation for improved film morphology in organic solar cells (OSCs). Herein, two homoleptic tris-Ir complexes of eBz3Ir and eTBz3Ir showing majority absorption from UV to light-blue range are designed as electron donor materials. Owing to the extended π-conjugation by an extra thiophene ring in each ligand, eTBz3Ir exhibits red-shifted and stronger absorption as well as better charge transport property than eBz3Ir. Therefore, when employing the fused-ring non-fullerene acceptor Y6 or L8-BO to fabricate all-small-molecule OSCs (ASMOSCs), eTBz3Ir blended devices demonstrate higher efficiency than eBz3Ir, with maximum power conversion efficiency (PCE) achieving at 9.48/7.55% for Y6 and 10.48/6.46% for L8-BO devices, respectively. To our knowledge, over 10% PCE is among the highest values for binary ASMOSCs based on organometallic Ir complexes as photovoltaic materials. Such uncompetitive solar-light absorption but relatively high PCE indicates homoleptic Ir complexes with octahedral configurations are expected as promising photoactive materials for high-efficiency ASMOSCs.

Dioxaborin Compounds with Terminal Carbazole Groups: Enhancing Electron Acceptor in Organic Photovoltaics

AbstractThree donor−acceptor−donor dioxaborin compounds containing carbazoles as the terminal electron donor groups were synthesized. Their electronic and photovoltaic properties were compared with those of an analog with terminal triphenylamine groups. The HOMO and LUMO levels of the N‐phenylcarbazole derivative were 0.3 and 0.1 eV lower, respectively, than those of the triphenylamine analog. During the evaluation of organic photovoltaic characteristics, the N‐phenylcarbazole derivative exhibited a power conversion efficiency (PCE) of 2.06 % when combined with a conducting polymer (PTB7‐Th). Contrarily, the triphenylamine analog exhibited a PCE of 2.85 % when combined with a fullerene acceptor (PC61BM). The results showed that the N‐phenylcarbazole and triphenylamine derivatives functioned as electron acceptor and donor materials, respectively. The conversion from the electron donor to the acceptor was achieved via a slight change in the structure of the terminal donor groups of the dioxaborin compounds. This study will prove valuable for the development of nonfullerene acceptors.

Origin and Energy of Intra‐Gap States in Sensitive Near‐Infrared Organic Photodiodes

AbstractTrap states in organic semiconductors are notoriously detrimental to the performance of organic electronics. However, the origin and energetics of trap states remain largely elusive and under debate, especially for bulk‐heterojunction (BHJ) photodiodes consisting of electron donor and acceptor materials. Combining three sensitive techniques now enables locating the origin and energy of trap states in six state‐of‐the‐art polymer – non‐fullerene acceptor organic photodiodes (OPDs) with noise‐based specific detectivities exceeding 1013 Jones. Analyzing the temperature dependence of the reverse‐bias dark current density (Jd) identifies intra‐gap states in the polymers, lying 0.3−0.4 eV above the energy of the highest occupied molecular orbital, as being responsible for Jd. Sub‐bandgap external quantum efficiency spectra of donor‐only and acceptor‐only diodes confirm that intra‐gap states are much more abundant in the polymers. Likewise, responsivity measurements at ultra‐low light intensities (10−7 mW cm−2) show trap‐mediated charge recombination in BHJ and polymer‐only diodes, but not in acceptor‐only devices. The results imply that to further improve the specific detectivity of near‐infrared OPDs, the intra‐gap state energy, and density need to be reduced.

Benzodithiophene-based Acceptor-Donor-Acceptor-type compounds for highly efficient organic photovoltaic cells

In this study, the structural, optical and electronic properties of a conjugated A-D-A compound based on indandione and benzobitiophene, which is widely used in the production of organic solar cells, were theoretically calculated using different functionals of the Density Functional Theory (DFT) method. For this purpose, different parameters such as frontier molecular orbital (FMO), which is called the highest occupied molecular orbital HOMO and the lowest unoccupied molecular orbital LUMO, energy gap Egap (HOMO-LUMO), the density of states (DOS), the absorption maximum λmax, excitation energy Eex and transition density matrix (TDM) were calculated. In addition, reduced density gradient (RDG) and open circuit voltages VOC were simulated to evaluate hole-electron localization. According to the results obtained from this study, the investigated compound exhibits significant visual absorption and interesting electron transport properties due to its hard-flat structures. The open circuit voltage (VOC) of compounds with BTPF acceptors is 1.45 V. This study has shown that BTPF compound has great potential for integration into solar cells as an electron donor material and has provided evidence that the investigated material may be a potential photovoltaic material candidate.

End-capped modeling of dithienopicenocarbazole based derivatives for high-performance photovoltaic cells with optoelectronic response over 900 nm

Dithienopicenocarbazole (DTPC), as the inner core in the A-D-A configuration, has been recognized for its exceptionally small energy loss, narrow energy gap, and excellent power conversion efficiency. In this work, a series of six novel dithienopicenocarbazole-based molecules with wide absorption and remarkable opto-electrical properties, were designed as the new electron-donating materials for organic photovoltaic devices. (DI1-DI6) were derived by substituting end-capped moieties attached to the middle core (DTPC) with highly conjugated and electro withdrawing groups and insertion of π spacer between the donor part and the newly attached acceptor part. The findings demonstrated that the modified molecules had smaller excitation energies (Ex), a decreased energy gap (Eg), and have higher absorption (λmax) values in comparison to the DIR (reference structure). The greatest λmax (980 nm), smallest Ex (1.21 eV), and shortest Eg (1.63 eV) were all displayed by the DI4 molecule. In addition, due to its lowest RE value for the electron, DI4 might have best electron carrier ability among all. Furthermore, upon coupling each examined molecule with a PC61BM acceptor, the value of open circuit voltage (VOC) was computed. DI4 displayed the most suitable outcomes for the VOC (1.45 V), FF (0.91178) and normalized VOC (56.14). Therefore, all the altered structures, with special reference DI4, are strongly recommended to synthesize organic photovoltaic cells with exceptional optoelectronic and photovoltaic applications.

Impact of ligand modification on the photovoltaic performance of cyclometalated Ir(III)-complexes based organic solar cells

In this work, systematical comparisons are performed between two Ir-complex based electron donor materials. Compared to the control TBzIr with alkylterthiophene coupled benzo[d]thiazole as cyclometalated ligand, the new-developed TBzRIr is designed by attaching additional strong electron-accepting dicyanorhodanine endcaps to the main ligands. The enlarged ligand conjugation length and amplified intraligand donor-acceptor interactions induces TBzRIr to show less efficient spin-orbital coupling with obviously weakened triplet emission and shorter triplet lifetimes than TBzIr. Notably, TBzRIr demonstrates beneficial physical parameters for organic solar cells with deeper energy levels, remarkably enhanced light-harvest capacity and visibly increased neat-film hole mobility than TBzIr. However, TBzRIr conversely yields inferior device efficiency than TBzIr in the corresponding organic solar cells, with sharply decreased power conversion efficiency from 5.36% for TBzIr:Y6 to 3.05% for TBzRIr:Y6. Besides on the influence of improved film morphologies for TBzIr:Y6, the benefits of stronger triplet feature of TBzIr is also expected to facilitate exciton dissociation, suppress charge recombination, and thus to enhance photovoltaic performance.

Latest Updates of Single-Junction Organic Solar Cells up to 20% Efficiency

Single-junction organic solar cells have reached a power conversion efficiency of 20% with narrow bandgap non-fullerene electron acceptor materials such as Y6, as well as with large band gap electron donor materials and their derivatives. The power conversion efficiency improvement of single-junction organic solar cells is a result of highly efficient light harvesting in the near-infrared light range and reduced energy losses with the most promising active layer layout currently available, Bulk-Heterojunction. Ternary blending is known to be the most advanced strategy to construct Bulk-Heterojunction structures in organic solar cells at present. In this review, we examine different devices based on Bulk-Heterojunction structures with efficient electron donors and acceptors. Then, we review the performance of binary and ternary organic solar cells with high power conversion efficiency, in conjunction with different anode and cathode interfaces used in recent studies of high-power conversion efficiency. Finally, we present perspectives on the future development of single-junction organic solar cells.

THEORETICAL STUDY OF NEW CANDIDATE ORGANIC MATERIALS FOR PHOTOVOLTAIC APPLICATIONS

Our work consists of a theoretical prediction, through DFT and TD-DFT methods, of the electronic and optical properties of six conjugated organic compounds used as electron donor materials in BHJ solar cells, of which PCBM is the acceptor material. This study is necessary to discuss the effect of substituents (donor units) on the different properties of these compounds, and to predict promising materials in organic solar cells using the AMPS-1D simulation software. The results obtained show that all the molecules have good geometric, electronic and optical properties, thus showing an increase in the power conversion efficiency of photovoltaic cells based on these materials, which reaches a value of 17% for the molecule P-Eth-TEdotT-A.

High-Performance Pseudo-Bilayer Organic Solar Cells Enabled by Sequential Deposition of D18/Y6 Chloroform Solution

Recently, sequential deposition (SD) fabricated pseudo-bilayer organic solar cells (OSCs) have achieved preferable performances compared to the long-term adopted bulk heterojunction (BHJ)-based devices, in which the solution-processing technique is the key to preparing the active layer of electron–donor materials and electron–acceptor materials. For devices with D18 as the electron–donor material and Y-serial small molecules as the electron–acceptor materials, however, one single solvent was seldom applied to prepare the corresponding high-performance SD OSCs. In this work, we adopted chloroform as the single solvent and successfully constructed high-performance SD OSCs based on D18 and Y6. Benefiting from the finely adjusting of the vertical phase distribution, the SD low molecular weight D18/Y6 pseudo-bilayer device realized high short-circuit current density (27.24 mA cm–2) and a champion efficiency of 17.94%, which surpassed the 17.36% value of the D18:Y16 device having a bulk heterojunction structure and the 7.81% result of the SD prepared Y6/D18-based device. This work demonstrates a single solvent strategy for fulfilling high-performance SD OSCs and reveals the reasons that account for the distinct performances of these OSCs fabricated by different approaches.

Designing Electron-Deficient Diketone Unit Based Non-Fused Ring Acceptors with Amplified Optoelectronic Features for Highly Efficient Organic Solar Cells: A DFT Study.

Organic solar cells (OSCs) made of electron-acceptor and electron-donor materials have significantly developed in the last decade, demonstrating their enormous potential in cutting-edge optoelectronic applications. Consequently, we designed seven novel non-fused ring electron acceptors (NFREAs) (BTIC-U1 to BTIC-U7) using synthesized electron-deficient diketone units and reported end-capped acceptors, a viable route for augmented optoelectronic properties. The DFT and TDDFT approaches were used to measure the power conversion efficiency (PCE), open circuit voltage (Voc), reorganization energies (λh, λe), fill factor (FF), light harvesting efficiency (LHE) and to evaluate the potential usage of proposed compounds in solar cell applications. The findings confirmed that the photovoltaic, photophysical, and electronic properties of the designed molecules BTIC-U1 to BTIC-U7 are superior to those of reference BTIC-R. The TDM analysis demonstrates a smooth flow of charge from the core to the acceptor groups. Charge transfer analysis of the BTIC-U1:PTB7-Th blend revealed orbital superposition and successful charge transfer from HOMO (PTB7-Th) to LUMO (BTIC-U1). The BTIC-U5 and BTIC-U7 outperformed the reference BTIC-R and other developed molecules in terms of PCE (23.29% and 21.18%), FF (0.901 and 0.894), normalized Voc (48.674 and 44.597), and Voc (1.261 eV and 1.155 eV). The proposed compounds enclose high electron and hole transfer mobilities, making them the ideal candidate for use with PTB7-Th film. As a result, future SM-OSC design should prioritize using these constructed molecules, which exhibit excellent optoelectronic properties, as superior scaffolds.

Configuration-dependent photovoltaic properties of diketopyrrolopyrrole (DPP) based Ir complexes for organic solar cells

Organometallic complexes with diverse spatial configurations have been rarely investigated in organic solar cells (OSCs). In this work, two octahedral cyclometalated Ir(III) complexes of bis-heteroleptic DPPTBzIr and tris-homoleptic DPPTBz3Ir are designed from a highly planar diketopyrrolopyrrole (DPP)-containing ligand DPPTBz. Upon coordination, gradually increased optical absorption was observed with the maximum molar extinction coefficient (ε) of 3.80×104, 5.19×104 and 1.39×105 M-1 cm-1 for DPPTBz, DPPTBzIr and DPPTBz3Ir, respectively. Unexpectedly, owing to the large π-conjugation and strong intraligand donor-acceptor interactions from the cyclometalated DPPTBz ligand, no obvious triplet character is determined for the DPP-containing Ir complexes from both experimental and theoretical methods. When employed as electron donor materials for OSCs by blending with fused-ring electron acceptor N3, substantially improved power conversion efficiency (PCE) with average values from 0.74%, 2.2% to 3.41% are achieved for DPPTBz, DPPTBzIr and DPPTBz3Ir, respectively. The best photovoltaic performance for DPPTBz3Ir is originated from strengthened light harvest, suitable energy levels, manipulated blend morphologies and favorable carrier dynamics in the corresponding devices.

Low-Temperature Ammonia Synthesis on Iron Catalyst with an Electron Donor.

Haber-Bosch process produces ammonia to provide food for over 5 billion people; however, it is currently required to be produced without the use of fossil fuels to reduce global CO2 emissions by 3% or more. It is indispensable to devise heterogeneous catalysts for the synthesis of ammonia below 100-150 °C to minimize the energy consumption of the process. In this paper, we report metallic iron particles with an electron-donating material as a catalyst for ammonia synthesis. Metallic iron particles combined with a mixture of BaO and BaH2 species in an appropriate manner could catalyze ammonia synthesis even at 100 °C. The iron catalyst revealed that iron can exhibit a high turnover frequency (∼12 s-1), which is over an order of magnitude higher than those of other transition metals used in highly active catalysts for ammonia synthesis. This can be attributed to the intrinsic nature of iron to desorb adsorbed hydrogen atoms as hydrogen molecules at low temperatures.