Development of small-molecule materials for high-performance organic solar cells

Theoretical Investigation of the Effects of Aldehyde Substitution with Pyran Groups in D-π-A Dye on Performance of DSSCs

This work investigated the substitution of the aldehyde with a pyran functional group in D-π-aldehyde dye to improve cell performance. This strategy was suggested by recent work that synthesized D-π-aldehyde dye, which achieved a maximum absorption wavelength that was only slightly off the threshold for an ideal sensitizer. Therefore, DFT and TD-DFT were used to investigate the effect of different pyran substituents to replace the aldehyde group. The pyran groups reduced the dye energy gap better than other known anchoring groups. The proposed dyes showed facile intermolecular charge transfer through the localization of HOMO and LUMO orbitals on the donor and acceptor parts, which promoted orbital overlap with the TiO2 surface. The studied dyes have HOMO and LOMO energy levels that could regenerate electrons from redox potential electrodes and inject electrons into the TiO2 conduction band. The lone pairs of oxygen atoms in pyran components act as nucleophile centers, facilitating adsorption on the TiO2 surface through their electrophile atoms. Pyrans increased the efficacy of dye sensitizers by extending their absorbance range and causing the maximum peak to redshift deeper into the visible region. The effects of the pyran groups on photovoltaic properties such as light harvesting efficiency (LHE), free energy change of electron injection, and dye regeneration were investigated and discussed. The adsorption behaviors of the proposed dyes on the TiO2 (1 1 0) surface were investigated by means of Monte Carlo simulations. The calculated adsorption energies indicates that pyran fragments, compared to the aldehyde in the main dye, had a greater ability to induce the adsorption onto the TiO2 substrate.

Using a molecular additive to control chiral supramolecular assembly and the subsequent chirality transfer process

Hierarchical assembly of chiral molecules is achieved through the introduction of molecular additives, which enables the chiral assembly of nanosheets into helical nanorods with inverted chirality. Moreover, the hierarchical assembly of chiral molecules in the presence of a molecular additive can lead to the subsequent chirality transfer from a molecular system to nanoparticle assemblies.

Advances in Flexible Organic Photodetectors: Materials and Applications

Future electronics will need to be mechanically flexible and stretchable in order to enable the development of lightweight and conformal applications. In contrast, photodetectors, an integral component of electronic devices, remain rigid, which prevents their integration into everyday life applications. In recent years, significant efforts have been made to overcome the limitations of conventional rigid photodetectors, particularly their low mechanical deformability. One of the most promising routes toward facilitating the fabrication of flexible photodetectors is to replace conventional optoelectronic materials with nanomaterials or organic materials that are intrinsically flexible. Compared with other functional materials, organic polymers and molecules have attracted more attention for photodetection applications due to their excellent photodetection performance, cost-effective solution-fabrication capability, flexible design, and adaptable manufacturing processes. This article comprehensively discusses recent advances in flexible organic photodetectors in terms of optoelectronic, mechanical properties, and hybridization with other material classes. Furthermore, flexible organic photodetector applications in health-monitoring sensors, X-ray detection, and imager devices have been surveyed.

Efficient Organic Solar Cells Based on Non-Fullerene Acceptors with Two Planar Thiophene-Fused Perylene Diimide Units

We designed and synthesized two non-fullerene acceptors (CDT-TFP and C8X-TFP), which comprise a central 4H-cyclopenta[2,1-b:3,4-b']dithiophene (CDT) as the bridge and two thiophene-fused perylene diimide (TFP) units. The bulky side chains, such as the 4-hexylphenyl side chains, on the CDT bridge can effectively prevent the acceptor molecules from forming large aggregates, and the π-π stacking of the terminal planar TFP units can form effective electron transport pathways when blending with the donor polymers. These non-fullerene acceptors are used to fabricate organic solar cells (OSCs) by blending with the regioregular middle bandgap polymer reg-PThE. The as-cast devices based on reg-PThE:CDT-TFP show the best power conversion efficiency (PCE) of 8.36% with a V_oc of 1.10 V, J_sc of 12.43 mA cm^-2, and an FF of 61.4%, whereas the analogue perylene diimide (PDI) dimers (CDT-PDI) that comprise two PDI units bridged with a CDT unit show only a 2.59% PCE with a V_oc of 0.92 V, J_sc of 6.82 mA cm^-2, and an FF of 41.5%. Our results have demonstrated that the non-fullerene acceptors comprising planar PDI units can achieve excellent photovoltaic performance and provide meaningful guidelines for the design of PDI-based non-fullerene electron acceptors for efficient OSCs.

Isomery-Dependent Miscibility Enables High-Performance All-Small-Molecule Solar Cells

Nonfullerene polymer solar cells develop quickly. However, nonfullerene small-molecule solar cells (NF-SMSCs) still show relatively inferior performance, attributing to the lack of comprehensive understanding of the structure-performance relationship. To address this issue, two isomeric small-molecule acceptors, NBDTP-F_out and NBDTP-F_in , with varied oxygen position in the benzodi(thienopyran) (BDTP) core are designed and synthesized. When blended with molecular donor BDT3TR-SF, devices based on the two isomeric acceptors show disparate photovoltaic performance. Fabricated with an eco-friendly processing solvent (tetrahydrofuran), the BDT3TR-SF:NBDTP-F_out blend delivers a high power conversion efficiency of 11.2%, ranked to the top values reported to date, while the BDT3TR-SF:NBDTP-F_in blend almost shows no photovoltaic response (0.02%). With detailed investigations on inherent optoelectronic processes as well as morphological evolution, this performance disparity is correlated to the interfacial tension of the two combinations and concludes that proper interfacial tension is a key factor for effective phase separation, optimal blend morphology, and superior performance, which can be achieved by the "isomerization" design on molecular acceptors. This work reveals the importance of modulating the materials miscibility by interfacial-tension-oriented molecular design, which provides a general guideline toward efficient NF-SMSCs.

Inverted P3HT:PC61BM organic solar cells incorporating a π-extended squaraine dye with H- and (or) J-aggregation

In this report, we investigated the impact of adding a large π-extended squaraine dye to the classic bulk heterojunction P3HT:PC61BM system. The compound, SQIQ-A, is a bis-indole squaraine dye appended with acetylene-indoloquinoxaline end-capping units that exhibits strong optical absorption in thin film from 650 to 750 nm, beyond the absorption wavelength cut-off for P3HT:PC61BM films (ca. ∼650 nm). The dye SQIQ-A can form H- or J-aggregates (blue or red shifting the optical absorption profiles) via solvent vapour annealing with CHCl3 or THF, respectively, thus providing a simple method to control the morphology and optical properties of the molecule. Ternary blended films composed of P3HT:PC61BM:SQIQ-A (1:1:0.3 mass ratio) were investigated as-cast, after solvent vapour annealing, and after thermal annealing. Films were characterized using optical absorption spectroscopy, X-ray diffraction, atomic force microscopy, and polarized light microscopy. Solar cells were fabricated using an inverted architecture in air and analysed using current–voltage and external quantum efficiency measurements. It was found that solar cells thermally annealed at 130 °C for 10 min gave good power conversion efficiencies of 3%, similar to the control P3HT:PC61BM but with improved fill factors and noticeable photocurrent generation from 650 to 760 nm where the SQIQ-A molecule absorbs. Exposure to solvent vapour (CHCl3 or THF) results in aggregation of all components but specifically leads to the formation of micron-sized domains, lowering the overall photovoltaic performance.

On the theoretical prediction of fluorescence rates from first principles using the path integral approach

In this work, we present and implement the theory for calculating fluorescence rates and absorption and emission spectra from first principles, using the path integral approach. We discuss some approximations and modifications to the full set of equations that improve speed and numerical stability for the case when a large number of modes are considered. New methods to approximate the excited state potential energy surface are also discussed and it is shown that for most purposes, these can be used instead of a full geometry optimization to obtain the rates mentioned above. A few examples are presented and the overall performance of the method is discussed. It is shown that the rates and spectra computed in this way are well within the acceptable range of errors and can be used in future predictions, particularly for screening purposes, with the only limitation on size being that of the electronic structure calculation itself.

Density Functional Theory Investigations of D-A-D' Structural Molecules as Donor Materials in Organic Solar Cell

Squaraine core based small molecules in bulk heterojunction organic solar cells have received extensive attentions due to their distinguished photochemical properties in far red and infrared domain. In this paper, combining theoretical simulations and experimental syntheses and characterizations, three major factors (fill factor, short circuit and open-cirvuit voltage) have been carried out together to achieve improvement of power conversion efficiencies of solar cells. As model material systems with D-A-D' framework, two asymmetric squaraines (CNSQ and CCSQ-Tol) as donor materials in bulk heterojunction organic solar cell were synthesized and characterized. Intensive density functional theory computations were applied to identify some direct connections between three factors and corresponding molecular structural properties. It then helps us to predict one new molecule of CCSQ'-Ox that matches all the requirements to improve the power conversion efficiency.

1,3-Bis(thieno[3,4-b]thiophen-6-yl)-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione-Based Small-Molecule Donor for Efficient Solution-Processed Solar Cells

A small molecule TBTT-1 with 5-(2-ethylhexyl)-1,3-bis(2-(2-ethylhexyl)thieno[3,4-b]thiophen-6-yl)-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione (TBTT) as the central moiety was designed and synthesized for solution-processed bulk-heterojunction solar cells. TBTT-1 exhibits a broad absorption with a low optical band gap of approximately 1.53 eV in the thin film. An optimized power conversion efficiency (PCE) of 7.47% with a high short-circuit current of 14.95 mA cm^-2 was achieved with diphenyl ether (DPE) as additive, which is the highest PCE for TPD-based small-molecule solar cells. According to the detailed morphology investigations, we found that DPE processing helped to enhance π-π stacking and reduce the scales of phase separation, which led to improved exciton splitting and charge transport in BHJ thin film, and thus enhanced device performance.

Organic Optoelectronic Materials: Mechanisms and Applications

Organic (opto)electronic materials have received considerable attention due to their applications in thin-film-transistors, light-emitting diodes, solar cells, sensors, photorefractive devices, and many others. The technological promises include low cost of these materials and the possibility of their room-temperature deposition from solution on large-area and/or flexible substrates. The article reviews the current understanding of the physical mechanisms that determine the (opto)electronic properties of high-performance organic materials. The focus of the review is on photoinduced processes and on electronic properties important for optoelectronic applications relying on charge carrier photogeneration. Additionally, it highlights the capabilities of various experimental techniques for characterization of these materials, summarizes top-of-the-line device performance, and outlines recent trends in the further development of the field. The properties of materials based both on small molecules and on conjugated polymers are considered, and their applications in organic solar cells, photodetectors, and photorefractive devices are discussed.

Low-Bandgap Small-Molecule Donor Material Containing Thieno[3,4-b]thiophene Moiety for High-Performance Solar Cells

By replacing the central thiophene of STDR, a sepithiophene terminated with two 3-ethylrhodanine moieties, with 2-ethylhexyl 3-fluorothieno[3,4-b]thiophene-2-carboxylate, an A-D-Q-D-A-type small molecule has been developed for high-performance organic solar cells with improved photocurrent. STDR-TbT exhibits a significant bathochromic shift with a low optical bandgap of approximately 1.60 eV in the thin film. Accordingly, STDR-TbT shows broad external quantum efficiency spectral response up to 800 nm. A high short circuit current (Jsc) of 10.90 mA cm(-2) was achieved for STDR-TbT:PC71BM-based devices; this is significantly higher than that of STDR:PC71BM-based devices, Jsc: 5.61 mA cm(-2), with a power-conversion efficiency (PCE) of 5.05%. Compared with STDR-based devices, STDR-TbT-based devices show balanced charge carrier transport, better thin-film morphology, and favorable charge separation/collection.