Yunchuang Wang

A Series of Simple Oligomer-like Small Molecules Based on Oligothiophenes for Solution-Processed Solar Cells with High Efficiency

A series of acceptor-donor-acceptor simple oligomer-like small molecules based on oligothiophenes, namely, DRCN4T-DRCN9T, were designed and synthesized. Their optical, electrical, and thermal properties and photovoltaic performances were systematically investigated. Except for DRCN4T, excellent performances were obtained for DRCN5T-DRCN9T. The devices based on DRCN5T, DRCN7T, and DRCN9T with axisymmetric chemical structures exhibit much higher short-circuit current densities than those based on DRCN6T and DRCN8T with centrosymmetric chemical structures, which is attributed to their well-developed fibrillar network with a feature size less than 20 nm. The devices based on DRCN5T/PC71BM showed a notable certified power conversion efficiency (PCE) of 10.10% under AM 1.5G irradiation (100 mW cm(-2)) using a simple solution spin-coating fabrication process. This is the highest PCE for single-junction small-molecule-based organic photovoltaics (OPVs) reported to date. DRCN5T is a rather simpler molecule compared with all of the other high-performance molecules in OPVs to date, and this might highlight its advantage in the future possible commercialization of OPVs. These results demonstrate that a fine and balanced modification/design of chemical structure can make significant performance differences and that the performance of solution-processed small-molecule-based solar cells can be comparable to or even surpass that of their polymer counterparts.

Subtle Balance Between Length Scale of Phase Separation and Domain Purification in Small‐Molecule Bulk‐Heterojunction Blends under Solvent Vapor Treatment

A series of solvents with different solubilities for DR3TBDTT and PC71 BM, and different boiling points, is used for solvent vapor annealing (SVA) treatment to systematically investigate the solvent-morphology-performance relationship. The presence of solvent molecules inside bulk-heterojunction (BHJ) thin films promotes the mobility of both donor and acceptor molecules, leading to crystallization and aggregation, which are important in modulating morphology.

In situ sulfur deposition route to obtain sulfur–carbon composite cathodes for lithium–sulfur batteries

An in situ sulfur deposition route has been developed for synthesizing sulfur–carbon composites as cathode materials for lithium–sulfur batteries. This facile synthesis method involves the precipitation of elemental sulfur into the nanopores of conductive carbon black (CCB). The microstructure and morphology of the composites are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results indicate that most of the sulfur in the amorphous phase is chemically well-dispersed in the nanopores of the CCB. The sulfur content in the composites is confirmed using thermogravimetry analysis (TGA). The S–CCB composites with different sulfur content (52 wt%, 56 wt% and 62 wt%) deliver remarkably high initial capacities of up to 1534.6, 1357.4 and 1185.9 mA h g−1 at the current density of 160 mA g−1, respectively. Correspondingly, they maintain stable capacities of 1012.2, 957.9 and 798.6 mA h g−1 with the capacity retention of over 75.1% after 100 cycles, exhibiting excellent cycle stability. The electrochemical reaction mechanism for the lithium–sulfur batteries during the discharge process is investigated by electrochemical impedance spectroscopy (EIS). The significantly improved electrochemical performance of the S–CCB composite is attributed to the carbon-wrapped sulfur structure, which suppresses the loss of active material during charging–discharging and the restrained migration of the polysulfide ions to the anode. This facile in situ sulfur deposition method represents a low-cost approach to obtain high performance sulfur–carbon composite cathodes for rechargeable lithium–sulfur batteries.

Fine-Tuning the Energy Levels of a Nonfullerene Small-Molecule Acceptor to Achieve a High Short-Circuit Current and a Power Conversion Efficiency over 12% in Organic Solar Cells.

Organic solar cell optimization requires careful balancing of current-voltage output of the materials system. Here, such optimization using ultrafast spectroscopy as a tool to optimize the material bandgap without altering ultrafast photophysics is reported. A new acceptor-donor-acceptor (A-D-A)-type small-molecule acceptor NCBDT is designed by modification of the D and A units of NFBDT. Compared to NFBDT, NCBDT exhibits upshifted highest occupied molecular orbital (HOMO) energy level mainly due to the additional octyl on the D unit and downshifted lowest unoccupied molecular orbital (LUMO) energy level due to the fluorination of A units. NCBDT has a low optical bandgap of 1.45 eV which extends the absorption range toward near-IR region, down to ≈860 nm. However, the 60 meV lowered LUMO level of NCBDT hardly changes the Voc level, and the elevation of the NCBDT HOMO does not have a substantial influence on the photophysics of the materials. Thus, for both NCBDT- and NFBDT-based systems, an unusually slow (≈400 ps) but ultimately efficient charge generation mediated by interfacial charge-pair states is observed, followed by effective charge extraction. As a result, the PBDB-T:NCBDT devices demonstrate an impressive power conversion efficiency over 12%-among the best for solution-processed organic solar cells.

Investigation of the effect of large aromatic fusion in the small molecule backbone on the solar cell device fill factor

The structure and performance relationship in photovoltaic cells is still not fully understood, particularly in the case of controlling/optimizing the fill factor (FF). Here a pair of molecules DR2TDTCz and DR3TCz with similar backbone structures and varying conjugated central units were designed and synthesized, and their photovoltaic performance was studied and compared. The molecule DR2TDTCz, containing dithieno[3,2-b;6,7-b]carbazole (DTCz) as the central building block, with a carbazole ring in the center and two fused thiophene rings at the two sides of carbazole, exhibits improved solar light absorption and slightly narrow band gap, compared with the analogue system DR3TCz which has carbazole and two un-fused thiophene rings in the central building block. More importantly, it is found that introducing DTCz with thiophene fused 2,7-carbazole to replace 2,7-carbazole achieves a better molecular packing and favorable orientation, thus benefiting charge transport. As a result, the DR2TDTCz based device exhibits a power conversion efficiency (PCE) up to 7.03% with an impressively high FF of 75%, while the DR3TCz based device shows a PCE of 4.08% with a much lower FF of 54%. The results indicate that the FF can be tuned directly by the molecular structures and enlarged conjugation central core units could be beneficial to achieve high FF for the devices based on the acceptor–donor–acceptor (A–D–A) type small molecules.

Large active layer thickness toleration of high-efficiency small molecule solar cells

High-efficiency organic solar cells with large active layer thickness toleration are in high demand to meet the challenges in feasible commercial production on a large scale. Generally, devices with thick active layers are preferred because they allow both the formation of a more uniform film and the effective utilization of incident light. In this work, solar cell devices with layer thicknesses ranging from 65 to 370 nm based on a small molecule donor DR3TSBDT and electron acceptor PC71BM were fabricated and the thickness dependence of the photovoltaic performance was systematically studied. High power conversion efficiencies (PCEs) were well-maintained in a wide layer thickness range, and for devices with layer thicknesses of 280 and 370 nm, PCEs that were off by only ∼8% and 20%, respectively, from the best PCE value of 9.95% at 120 nm were achieved. With systematic investigations, the well-maintained high performance is attributed to the fact that both the nearly ideal morphology (a bi-continuous interpenetrating crystalline nano-fibrillar structure) of the active layer and the hole mobility remained largely unchanged over the wide thickness range. Also as expected, with increasing thickness, larger transport resistance, charge recombination and transit times were observed, which made the fill factor lower. But these inferior factors were largely compensated by the increased current, and thus well-maintained high performance was achieved.

Investigation of the enhanced performance and lifetime of organic solar cells using solution-processed carbon dots as the electron transport layers

Easily prepared and stable solution-processed carbon dots (CDs) have been used and systematically investigated as the electron transport layers (ETLs) for both small-molecule and polymer-based solar cells. Significantly enhanced device performance and lifetime are observed. The enhanced performance is mainly driven by the improvements of the short circuit current (Jsc) and the fill factor (FF), caused by decreasing the work function of Al electrodes and series resistance, increasing shunt resistances, and balancing electrons and hole mobility. Therefore, the devices with CDs as the ETLs have higher charge transport and collection efficiency. In addition, lifetimes of the devices with CDs as the ETLs are also significantly improved, due to the much better air-stability of CD materials compared to LiF as the ETLs. And another reason is that it can efficiently prevent the formation of an unstable cathode contact for the diffusion of Al ions at the interface. These results indicate that CDs, relatively cheap and stable materials, have great potential to be promising ETL materials for industrial-scale manufacture of organic solar cells.

Fixing of highly soluble Br2/Br− in porous carbon as a cathode material for rechargeable lithium ion batteries

LiBr, as a representative of highly soluble electrochemically active materials, is fixed in nanopores of conductive carbon black (CCB). The Li/LiBr–CCB battery exhibits excellent high-rate capability to avoid slow solid-phase diffusion of Li ions in traditional solid cathode materials. The success will broaden the range of alternative materials for cathodes in LIBs and make them capable of providing both high power density and energy density.

Fine-tuning the side-chains of non-fullerene small molecule acceptors to match with appropriate polymer donors

Side-chain engineering of donor and acceptor materials is an important topic in the field of organic photovoltaics. The influence of side-chains in active layer molecules on the corresponding photovoltaic device performances is still elusive, especially for the devices based on non-fullerene small molecule acceptors. In this work, we designed and synthesized two non-fullerene acceptors (IDTT-BH and IDTT-OBH) using an indacenodithieno[3,2-b]thiophene (IDTT) moiety as the central building block and (2-3-oxo-2,3-dihydro-1H-cyclopenta[b]naphthalen-1-ylidene)malononitrile (NINCN) as the end-group, which have similar energy levels and optical absorption spectra, and differ in the side-chains. We systematically investigated the effect of the side-chains on the device performance based on these two non-fullerene acceptors pairing with different donor materials. For the devices with J71 and PDCBT as donor materials, IDTT-BH showed better PCEs of 11.05% and 10.35%, respectively. Notably, 10.35% efficiency is among the top values in PDCBT-based OSCs. While, the devices based on PBDB-T:IDTT-OBH showed a better PCE of 10.93% than that of IDTT-BH based devices. It was found that the side-chains of non-fullerene acceptors have an effect on tuning the morphology of blend films and thus affect the photovoltaic performance, and different donor materials should be paired with the acceptors with the most suitable side-chains to achieve the best photovoltaic performance.

A series of dithienobenzodithiophene based small molecules for highly efficient organic solar cells

Three acceptor-donor-acceptor (A-D-A) small molecules DCAODTBDT, DRDTBDT and DTBDTBDT using dithieno[2,3-d:2′,3′-d′]benzo[1,2-b:4,5-b′]dithiophene as the central building block, octyl cyanoacetate, 3-octylrhodanine and thiobarbituric acid as the end groups were designed and synthesized as donor materials in solution-processed photovoltaic cells (OPVs). The impacts of these different electron withdrawing end groups on the photophysical properties, energy levels, charge carrier mobility, morphologies of blend films, and their photovoltaic properties have been systematically investigated. OPVs device based on DRDTBDT gave the best power conversion efficiency (PCE) of 8.34%, which was significantly higher than that based on DCAODTBDT (4.83%) or DTBDTBDT (3.39%). These results indicate that rather dedicated and balanced consideration of absorption, energy levels, morphology, mobility, etc. for the design of small-molecule-based OPVs (SM-OPVs) and systematic investigations are highly needed to achieve high performance for SM-OPVs.