Acceptor-donor-acceptor-based small molecules with varied crystallinity: processing additive-induced nanofibril in blend film for photovoltaic applications

Synthesis and Characterization of a Soluble A-D-A Molecule Containing a 2D Conjugated Selenophene-Based Side Group for Organic Solar Cells

A new acceptor-donor-acceptor (A-D-A) small molecule based on benzodithiophene (BDT) and diketopyrrolopyrrole (DPP) is synthesized via a Stille cross-coupling reaction. A highly conjugated selenophene-based side group is incorporated into each BDT unit to generate a 2D soluble small molecule (SeBDT-DPP). SeBDT-DPP thin films produce two distinct absorption peaks. The shorter wavelength absorption (400 nm) is attributed to the BDT units containing conjugated selenophene-based side groups, and the longer wavelength band is due to the intramolecular charge transfer between the BDT donor and the DPP acceptor. SeBDT-DPP thin films can harvest a broad solar spectrum covering the range 350-750 nm and have a low bandgap energy of 1.63 eV. Solution-processed field-effect transistors fabricated with this small molecule exhibit p-type organic thin film transistor characteristics, and the field-effect mobility of a SeBDT-DPP device is measured to be 2.3 × 10^-3 cm² V^-1 s^-1 . A small molecule solar cell device is prepared by using SeBDT-DPP as the active layer is found to exhibit a power conversion efficiency of 5.04% under AM 1.5 G (100 mW cm^-2 ) conditions.

Nanoimprinting-induced molecular orientation in poly(3-hexylthiophene) nanogratings and its extraordinary retention after thermal annealing

Face-on orientation of poly(3-hexylthiophene) nanogratings induced by nanoimprinting and its remarkable retention after thermal annealing.

Facile synthesis of low band-gap DPP–EDOT containing small molecules for solar cell applications

Four donor–donor–acceptor–donor–donor type small molecules were synthesized (by direct alkylation) for photovoltaic applications.

Design of Diketopyrrolopyrrole (DPP)-Based Small Molecules for Organic-Solar-Cell Applications

After the first report in 2008, diketopyrrolopyrrole (DPP)-based small-molecule photovoltaic materials have been intensively explored. The power conversion efficiencies (PCEs) for the DPP-based small-molecule donors have been improved up to 8%. Furthermore, through judicious structure modification, DPP-based small molecules can also be converted into electron-acceptor materials, and, recently, some exciting progress has been achieved. The development of DPP-based photovoltaic small molecules is summarized here, and the photovoltaic performance is discussed in relation to structural modifications, such as the variations of donor-acceptor building blocks, alkyl substitutions, and the type of conjugated bridges, as well as end-capped groups. It is expected that the discussion will provide a guideline in the exploration of novel and promising DPP-containing photovoltaic small molecules.

The Fabrication of Ordered Bulk Heterojunction Solar Cell by Nanoimprinting Lithography Method Using Patterned Silk Fibroin Mold at Room Temperature

The performance of organic solar cell is greatly determined by the nanoscale heterojunction morphology, and finding a practical method to achieve advantageous nanostructure remains a challenge. We demonstrate here that ordered bulk heterojunction (OBHJ) solar cell can be fabricated assisted by a simple, cost-effective nanoimprinting lithography method using patterned silk fibroin film mold at room temperature. The P3HT nanogratings were achieved by nanoimprinting lithography (NIL) process, and phenyl-C61-butyric acid methyl ester (PCBM) was spin-coated on the top of P3HT nanogratings. The conducting capacity of P3HT nanograting film has little difference compared with the unimprinted film in the vertical direction, due to the same edge-on chain alignment. However, it can be found that the fabrication of OBHJ nanostructure using room temperature NIL technique with patterned silk fibroin mold is able to promote optical absorption, interfacial area, and bicontinuous pathway. Therefore, the ordered heterojunction morphology plays an important part in improving device performance due to efficient exciton diffusion, dissociation, and reducing charge recombination rate.

Quantitative analysis of the size effect of room temperature nanoimprinted P3HT nanopillar arrays on the photovoltaic performance

We develop a solvent-assisted room temperature nanoimprint lithography (SART-NIL) technique to fabricate an ideal active layer consisting of poly(3-hexylthiophene) nanopillar arrays surrounded by [6,6]-phenyl-C61-butyric acid methyl ester. Characterization by scanning electron microscopy, two-dimensional grazing incidence wide angle X-rays diffraction, and conducting atomic force microscopy reveals that the SART-NIL technique can precisely control the size of P3HT nanopillar arrays. With the decrease in diameters of P3HT nanopillar arrays, the P3HT nanopillar arrays exhibit a more preferable face-on molecular orientation, enhanced UV-vis absorption and higher conducting ability along the direction perpendicular to the substrate. The ordered bulk heterojunction film consisting of P3HT nanopillar arrays with a diameter of ∼45 nm (OBHJ-45) gives face-on orientation, a high interfacial area of 2.87, a high conducting ability of ∼130 pA and efficient exciton diffusion and dissociation. The polymer solar cell (PSC) based on an OBHJ-45 film exhibits a significantly improved device performance compared with those of PSCs based on the P3HT nanoapillar arrays with diameters ∼100 nm and ∼60 nm. We believe that the SART-NIL technique is a powerful tool for fabricating an ideal active layer for high performance PSCs.

A–D–A small molecules for solution-processed organic photovoltaic cells

The recent representative progress in the design and synthesis of A–D–A small molecules for organic solar cells is summarized.

Morphological effects on the small-molecule-based solution-processed organic solar cells

We report a proof-of-concept study on solution-processed organic solar cells (OSCs) based on [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) and structurally compact donor molecules which have dithiophene-phenazine-dithiophene (TH-P) and dithiophene-quinoxaline-dithiophene (TH-Q) configurations with decyloxy and methyl side groups, respectively. These molecules formed one-dimensional fibers through self-assembly via weak nonbonding interactions such as π-π and van der Waals interactions even during a fast solvent removal process such as spin-casting. Photophysical and thermal properties of the new donor molecules were characterized with UV-vis absorption and fluorescence spectroscopy, differential scanning calorimetry, and thermogravimetric analysis. The electrochemical data determined experimentally were correlated well with theoretical evaluations. The fibers from the two donor molecules showed distinct morphological differences, allowing for in-depth investigations into their influence on the OSC performance. A continuous three-dimensional network of endless one-dimensional nanofibers, with a width of 300-400 nm, were formed from TH-P regardless of the presence of PC61BM, affording spontaneous nanoscale phase separation that facilitates a large donor/acceptor interfacial area. Bulk (BHJ) and planar heterojunctions (PHJ) from TH-P/PC61BM showed a power conversion efficiency (PCE) of 0.38% and 0.30%, respectively, under optimum device conditions. Post thermal annealing led to the increased domain size and a major decrease in Jsc. Meanwhile, shorter, more rigid needles with a large thickness variation were formed from TH-Q. A continuous network of TH-Q was obtained by spin-coating only in the presence of PC61BM, and the PCE of TH-Q/PC61BM BHJ was found to be 0.36%. However, the PHJ showed poor device performance due to TH-Q's inability to form a continuous film by spin-coating. The present study suggests a basic molecular architecture to drive one-dimensional assembly and demonstrates the significance of fibrillation for small-molecule-based OSCs.

Supramolecular self-assembly and photovoltaic property of soluble fluorogallium phthalocyanine

Fluorogallium tetra-tert-butylphthalocyanine (ttbPcGaF) can self-organize into well-ordered cofacial structures to obtain one-dimensional (1D) supramolecular polymers in the solid state using a solution-coating technique. The ttbPcGaF-based BHJ OSC provided a profoundly improved power conversion efficiency (PCE) of 0.41%.