Yamin Zhang

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.

A simple small molecule as an acceptor for fullerene-free organic solar cells with efficiency near 8%

A simple small molecule acceptor named DICTF, with fluorene as the central block and 2-(2,3-dihydro-3-oxo-1H-inden-1-ylidene)propanedinitrile as the end-capping groups, has been designed for fullerene-free organic solar cells. The new molecule was synthesized from widely available and inexpensive commercial materials in only three steps with a high overall yield of ∼60%. Fullerene-free organic solar cells with DICTF as the acceptor material provide a high PCE of 7.93%.

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.

Organic and solution-processed tandem solar cells with 17.3% efficiency

Tailoring tandem organics Tandem solar cells can boost efficiency by using a wider range of the solar spectrum. The bandgap of organic semiconductors can be tuned over a wide range, but, for a two-terminal device that directly connects the cells, the currents produced must be nearly equal. Meng et al. used a semiempirical analysis to choose well-matched top- and bottom-cell active layers. They used solution processing to fabricate an inverted tandem device that has a power conversion efficiency as high as 17.4%. Science, this issue p. 1094 A semi-empirical analysis helped to optimize materials for a tandem organic solar cell with high power conversion efficiency. Although organic photovoltaic (OPV) cells have many advantages, their performance still lags far behind that of other photovoltaic platforms. A fundamental reason for their low performance is the low charge mobility of organic materials, leading to a limit on the active-layer thickness and efficient light absorption. In this work, guided by a semi-empirical model analysis and using the tandem cell strategy to overcome such issues, and taking advantage of the high diversity and easily tunable band structure of organic materials, a record and certified 17.29% power conversion efficiency for a two-terminal monolithic solution-processed tandem OPV is achieved.

A New Nonfullerene Acceptor with Near Infrared Absorption for High Performance Ternary‐Blend Organic Solar Cells with Efficiency over 13%

Abstract A new acceptor–donor–acceptor (A–D–A) type nonfullerene acceptor, 3TT‐FIC, which has three fused thieno[3,2‐b]thiophene as the central core and difluoro substituted indanone as the end groups, is designed and synthesized. 3TT‐FIC exhibits broad and strong absorption with extended onset absorption to 995 nm and a low optical bandgap of 1.25 eV. The binary device based on 3TT‐FIC and the polymer PTB7‐Th exhibits a power conversion efficiency (PCE) of 12.21% with a high short circuit current density (   J sc) of 25.89 mA cm−2. To fine‐tune the morphology and make full use of the visible region sunlight, phenyl‐C71‐butyricacid‐methyl ester (PC71BM) is used as the third component to fabricate ternary devices. In contrast to the binary devices, the ternary blend organic solar cells show significantly enhanced EQE ranging from 300 to 700 nm and thus an improved  J sc with a high value of 27.73 mA cm−2. A high PCE with a value of 13.54% is achieved for the ternary devices, which is one of the highest efficiencies in single junction organic solar cells reported to date. The results provide valuable insight for the ternary devices in which the external quantum efficiency (EQE) induced by the third component is evidently observed and directly contributed to the enhancement of the device efficiency.

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.