Oligothiophene-based small molecules with 3,3′-difluoro-2,2′-bithiophene central unit for solution-processed organic solar cells

Two new oligothiophene-based small molecules, namely DRCN6T-F and DRCN8T-F, with 3,3′-difluoro-2,2′-bithiophene as the central building block and 2-(1,1-dicyanomethylene)-rhodanine as end groups, were designed and synthesized. Compared to their non-fluorinated counterparts DRCN6T and DRCN8T, DRCN6T-F and DRCN8T-F exhibit enhanced intermolecular interactions and lower HOMO energy levels. However, PCEs of 2.26% and 5.07% were obtained for DRCN6T-F and DRCN8T-F based optimized devices, respectively, lower than those of non-fluorinated molecules DRCN6T and DRCN8T. The relatively poor performance for the DRCN6T-F and DRCN8T-F were mainly caused by their low short-circuit current densities, due to their unfavorable morphologies and low charge carrier mobilities.     

A new dithienosilole-based oligothiophene with methyldicyanovinyl groups for high performance solution-processed organic solar cells

A new linear dithienosilole-based oligothiophene end-capped with methyl and electron-withdrawing dicyanovinyl groups, DTS(Oct)₂-(2T-DCV-Me)₂, was prepared in good yield. This oligomer exhibited broad absorption spectra in bulk down to the near-IR region with the optical edge at 900 nm, resulting in an initially high power conversion efficiency of 5.44% in solution-processed organic solar cells using PC₇₁BM as an acceptor.     

A small molecule with selenophene as the central block for high performance solution-processed organic solar cells

A solution-processable A–D–A structure small molecule donor material called DRCN7T-Se with selenophene as the central block was synthesized. Conventional bulk-heterojunction solar cell devices based on DRCN7T-Se and PC₇₁BM were optimized by thermal annealing and an excellent power conversion efficiency of 8.30% was achieved under AM 1.5G irradiation (100 mW cm⁻²).     

Two different donor subunits substituted unsymmetrical squaraines for solution-processed small molecule organic solar cells

Two unsymmetrical squaraines (USQs) with different donor (D) subunits as photovoltaic materials, namely USQ-11 and USQ-12, were designed and synthesized to investigate the effect of different D subunits on the optoelectronic properties of USQs for the first time. The two USQs compounds were characterized for optical, electrochemical, quantum chemical and optoelectronic properties. By changing the two different D subunits attached to the squaric acid core from 2,3,3-trimethylindolenine to 2-methylbenzothiazole, the HOMO energy levels could be tuned with a stepping of 0.07 eV, and quite different solid state aggregations (H- or J-aggregation) were observed in the thin film by UV-Vis absorption spectra, which were attributed to their distinct steric effects and dipole moments. Solution-processed bulk-heterojunction small molecule organic solar cells fabricated with the USQ-11/PC₇₁BM (1:5, wt%) exhibited extremely higher PCE (4.27%) than that of the USQ-12/PC₇₁BM (2.78%). The much enhanced PCE should be attributed to the simultaneously improved V_oc, J_sc and FF.     

An effective bilayer cathode buffer for highly efficient small molecule organic solar cells

Novel organic/ultrathin low work function metal bilayer cathode buffers for small molecule organic solar cells are proposed. Ultrathin low work function metal layers possess a high built-in electric field for effective carrier extraction and a high cathode reflectivity for maximum absorption in the photoactive layers. This leads to a significant increase of short circuit current density and fill factor of cells. By integrating this bilayer cathode buffer with DTDCTB:C₆₀ small molecular heterojunction, the device exhibits a high power conversion efficiency of up to 5.28%, which is an improvement of 22% compared to a device with a traditional single organic layer buffer.     

Acenaphtho[1,2-b]quinoxaline diimides derivative as a potential small molecule non-fullerene acceptor for organic solar cells

We designed and synthesized a small molecule acenaphtho[1,2-b]quinoxaline diimide derivative AQI-T2 as an electron-accepting material for non-fullerene organic solar cells. This molecule exhibits a relatively broad absorption band from 300 to 650 nm, with a moderately low-lying lowest unoccupied molecular orbital energy level of −3.64 eV. Non-fullerene organic solar cells with conventional structure using PTB7-Th as the electron donor and AQI-T2 as the electron acceptor exhibited moderate photovoltaic performances. The best performance was attained from the pristine device, which showed a power conversion efficiency of 0.77% with a relatively high open-circuit voltage of 0.86 V, a short circuit current of 2.04 mA cm⁻² and a fill factor of 43.98%. These results indicated that this n-type molecule can be a promising electron-accepting material for non-fullerene organic solar cells.     

Benzo[1,2-b:4,5-b′]dithiophene and benzotriazole based small molecule for solution-processed organic solar cells

A novel deep HOMO A₁-π-A₂-D-A₂-π-A₁ type molecule (D(CATBTzT)BDT), which terminal electron-withdrawing octyl cyanoacetate group is connected to a benzo[1,2-b:4,5-b′]dithiophene (BDT) core through another electron-accepting benzotriazole block, has been synthesized, characterized, and employed as electron donor material for small molecule organic solar cells (SM-OSCs). By simple solution spin-coating fabrication process, D(CATBTzT)BDT/PC₆₁BM based OSCs exhibit a power conversion efficiency (PCE) of 3.61% with a high open-circuit voltage of 0.93 V. The D(CATBTzT)BDT based solar cells device also can show high FF of 72% with PCEs of 2.31% which is one of the best FF results for solution-processed SM-OSCs.     

Small molecules based on bithiazole for solution-processed organic solar cells

A series of donor-acceptor-donor small molecules (1-3) with bithiazole as acceptor unit, triphenylamine as donor unit and thiophene with different number (0, 1, 2) as bridge were synthesized by palladium(0)-catalyzed Suzuki or Stille coupling reactions. The thermal, optical, electrochemical, charge transport, and photovoltaic properties of these small molecules were examined. All compounds exhibit excellent thermal stability with decomposition temperatures (5% weight loss) over 390 °C in nitrogen atmosphere. As increasing the number of thiophene and π-conjugation length of molecule, the absorption maximum in film red shifts from 406 to 498 nm, the extinction coefficient increases from 1.35 × 10⁴ to 7.66 × 10⁴ M⁻¹ cm⁻¹, and the optical band gap decreases from 2.6 to 2.0 eV. The electron-donating thiophene and bithiophene in compounds 2 and 3 up-shift HOMO energy level from −5.42 (1) to −5.24 eV (2) or −5.22 eV (3), and down-shift LUMO energy level from −2.48 (1) to −2.84 eV (2) or −2.81 eV (3). The hole mobility of compound 3 is up to 3.6 × 10⁻⁴ cm² V⁻¹ s⁻¹, which is one order of magnitude higher than that of compound 2, but compound 1 shows no field-effect transistor performance. Solution-processed bulk heterojunction organic solar cells based on 1-3:PC₇₁BM (1:4, w/w) blend films exhibit increasing power conversion efficiency (up to 2.61%) as increasing thiophene unit number.     

Improved efficiency of solution processed small molecules organic solar cells using thermal annealing

A solution processable A-D-A-D-A structure small molecule DCAEH5TBT using a BT unit as the core has been designed and synthesized for application in BHJ solar cells. The device employing DCAEH5TBT/PC₆₁BM as active layer shows PCE of 2.43% without any post treatment. After thermal annealing (150 °C, 10 min), the PCE of this molecule based device increased to 3.07%, with J_sc of 7.10 mA/cm², V_oc of 0.78 V and FF of 55.4%, which indicates that high performance of solution processed small molecule based solar cells can be achieved using thermal annealing by carefully design molecule structure.     

Merocyanines for vacuum-deposited small-molecule organic solar cells

We report here synthesis and photovoltaic properties of three merocyanines dyes (DPPT, DTPT, 1-NPPT) which are functionalized with electron withdrawing thiazolidenemalononitrile and electron rich diarylamine functionalities. It is found that structural feature of the diarylamino groups has a profound effect on the physical properties such as the absorption spectrum, oxidation potential, and HOMO/LUMO energy levels. The compound DTPT containing a better electron-donating ditolyl group, exhibits red-shifted absorption with relatively higher molar extinction coefficient, indicating its better light-harvesting ability. Hole mobility of these compounds is found to be strongly dependent on the various intermolecular interactions. Interestingly, single crystal structures reveal that the crystal packing motifs are rather closely related to the observed hole mobility in a trend of DPPT > DTPT > 1-NPPT. Vacuum-processed small-molecule organic solar cells were fabricated using the title merocyanines as p-type materials (donor) in combination with fullerene (C₆₀ or C₇₀) as n-type material (acceptor) with various device configurations. Among them, the DPPT-based devices outperform the devices based on DTPT and 1-NPPT. The power conversion efficiency (PCE) of DPPT-based device was improved from 1.55% of a BHJ device to 2.63% of a PMHJ device and 3.52% of a PMHJ device without the thin donor layer.     

Vacuum deposited ternary mixture organic solar cells

Ternary mixtures of photo-active organic materials are an intuitive approach to achieve enhanced photocurrent in organic solar cells (OSCs). In this work, we study ternary mixtures of vacuum deposited small molecules, complementing the recent surge of interest in solution processed ternary OSCs. The mixed layer composition is systematically varied to study all possible film configurations, and the resulting OSCs are successful in harvesting photocurrent from all three components to grant broad spectral photoresponse. However, the performance of the ternary OSC is generally less than the binary OSC, largely due to reduced fill factors. By examining ternary OSC transient photocurrents and multi-donor planar heterojunction devices, we demonstrate that the ternary OSC is strongly affected by the energy levels of its constituent materials, with small differences in the two donor materials’ highest occupied molecular orbitals degrading hole transport. The results stress the importance of fine molecular engineering for ternary OSCs, and further hint that the enhancements commonly observed in solution processed ternary OSCs may in part be associated with morphological variations that are not present in vacuum deposited OSCs. The research verifies that, by designing small molecules with specific energy levels, ternary OSCs provide an alternative pathway to low cost, high efficiency photovoltaics in lieu of more complicated device architectures.     

High open circuit voltage in efficient thiophene-based small molecule solution processed organic solar cells

We have synthesized and fully characterized an oligothiophene small organic molecule for its use as electron donor moiety in solution processed bulk-heterojunction organic solar cells. Our results show that device solvent annealing process of the conjugated oligothiophene molecule leads to a light-to-energy conversion efficiency of 3.75% under standard illumination conditions. The solar cell presents open-circuit voltage and fill factors as high as 1.01 V and 63.05% respectively, which are among the highest values obtained for small molecule solution processed organic solar cells.     

Linear and propeller-like fluoro-isoindigo based donor–acceptor small molecules for organic solar cells

Two donor–acceptor type fluoro-isoindigo based small molecule semiconductors are synthesized and their optical, electrochemical, thermal, and charge transport properties are investigated. The two molecular chromophores differ by their architecture, linear (M1) vs propeller-like (M2). Both molecules present a broad absorption in the visible range and a low optical HOMO–LUMO gap (∼1.6 eV). AFM images of solution-processed thin films show that the trigonal molecule M2 forms highly oriented fibrils after a few seconds of solvent vapor annealing. The materials are evaluated as electron donor components in bulk heterojunction organic solar cells using PC₆₁BM as the electron acceptor. The devices based on the propeller-like molecule M2 exhibit a high open-circuit voltage (around 1.0 V) and a power conversion efficiency of 2.23%.     

Enhancing the performance of solution-processed organic thin-film transistors by blending binary compatible small molecule semiconductors

Physical blending is a facile and effective way to improve the performance of solution processed organic thin-film transistors (OTFTs). Blending small molecule semiconductors with soluble polymers has been extensively studied in recent years. However, blending between binary small molecule semiconductors is rare due to the difficulty to obtain ideal thin films. Herein, we systematically investigate the blending effects on the morphologies of thin films and their field-effect performance by using two small molecule semiconductors, 2-phenyl[1]benzothieno[3,2-b][1]benzothiophene (Ph-BTBT) and 2-(4-dodecylphenyl) [1]benzothieno[3,2-b]benzothiophene, (C12-Ph-BTBT), which have the same aromatic skeleton. Molecular ordering and better crystallinity are observed in most of spin-coated blend thin films, thanks to the enhanced molecular interaction after blending. As a result, OTFTs based on blend thin films exhibit improved performance in most cases, with the highest average hole mobility about 1.5 cm² V⁻¹ s⁻¹ demonstrated. Further device performance improvements are demonstrated by blending polystyrene with Ph-BTBT and C12-Ph-BTBT blends. The results here indicate that blending between small molecule semiconductors with compatible fused ring structures may be a promising strategy to enhance the performance of organic transistors.     

Vacuum-deposited interconnection layers for tandem solar cells

Two different types of vacuum-deposited interconnection layers (ICLs) were investigated for tandem solar cells: (1) a pure metal oxide and (2) an organic matrix doped with conductive dopants. The optical and electrical properties of these ICLs were systematically studied and compared. Taking the characteristics of ICLs into consideration, optical design methodology for balancing the photocurrent of each sub-cell in the tandem cell is presented. According to the design, highly efficient small-molecule tandem solar cells with power conversion efficiencies up to 7.3%–7.4% were experimentally demonstrated in both devices utilizing pure metal oxide and organic matrix ICLs.     

All thermal-evaporated surface plasmon enhanced organic solar cells by Au nanoparticles

Au nanoparticles (NPs) are fabricated on indium-tin-oxide substrates by a thermal evaporation method and incorporated to an efficient small molecule organic solar cell (OSC). This renders an all thermal evaporated surface plasmon enhanced OSC. The optimized device shows a power conversion efficiency of 3.40%, which is 14% higher than that of the reference device without Au NPs. The improvement is mainly contributed to the increased short-circuit current which resulted from the enhanced light harvesting due to localized surface plasmon resonance of Au NPs and the increased conductivity of the device.     

Enhancing efficiency for additive–free blade–coated small–molecule solar cells by thermal annealing

Blade coating was successfully applied to realise high-efficiency small-molecule organic solar cells (OSCs) with a solution-processed active layer comprising a small organic molecule DR3TBDTT with a benzo[1,2–b:4,5–b′]dithiophene (BDT) unit as the central building block as the donor and [6,6]–phenyl–C71–butyric acid methyl ester (PC₇₁BM) as the acceptor. Using chloroform as the solvent, a DR3TBDTT/PC₇₁BM blend active layer without an additive was effectively formed through blade coating. The power conversion efficiency (PCE) of small organic molecule solar cells was enhanced by 3.7 times through thermal annealing at 100 °C. This method produces OSCs with a high PCE of up to 6.69%, with an open circuit voltage (V_oc) of 0.97 V, a short-circuit current density (J_sc) of 12.60 mA/cm², and a fill factor (FF) of 0.55.     

Room temperature solution-processed electron transport layer for organic solar cells

We present a new recipe for a solution-processed titanium oxide (TiO_x) based electron transport layer at room temperature. Due to its high chemical compatibility with all types of organic blends (semi-crystalline or amorphous) and it is good adhesion to both surfaces of glass/ITO substrate and the active layer (blend), the buffer layer is suitable for use in organic solar cell devices with conventional, inverted or multi-junction structures. The main goal of this recipe is producing with easiness an repeatable and stable precursor that will leads to titanium oxide buffer layer each time with the same quality. Since the processing of the titanium oxide layer itself does not require any initial or additional treatment before and after the coating, and can even be carried in air as well as under protective atmosphere, our room temperature solution-processed electron transport layer is highly versatile and very promising for cost effective mass production of organic solar cells.     

An asymmetric small molecule based on thieno[2,3-f]benzofuran for efficient organic solar cells

A new asymmetric small molecule, named R3T-TBFO, with 4,8-bis(2-ethylhexyloxy)-substituted thieno[2,3-f]benzofuran (TBF) as central donor block, has been synthesized and used as donor material in organic solar cells (OSCs). With thermal annealing (TA) and solvent vapor annealing (SVA) treatment, the blend of R3T-TBFO/PC₇₁BM shows a higher hole mobility of 1.37 × 10⁻⁴ cm² V⁻¹ s⁻¹ and a more balanced charge mobilities. Using a structure of ITO/PEDOT:PSS/R3T-TBFO:PC₇₁BM/ZrAcac/Al, the device with TA treatment delivered a moderate power conversion efficiency (PCE) of 5.63%, while device after TA + SVA treatment showed a preferable PCE of 6.32% with a high fill factor (FF) of 0.72.     

An oligothiophene dye with triphenylamine as side chains for efficient dye-sensitized solar cells

A novel oligothiophene-cyanoacrylic acid photosensitizer with two triphenylamine side chains (7T-2TPA) is designed and synthesized for dye-sensitized solar cells. 7T-2TPA exhibits broad (250-600 nm) and strong absorption (ε = 5.0 × 10⁴ L mol⁻¹ cm⁻¹ at 496 nm). The optical band gap (E_g) is estimated from the onset absorption edge to be 2.07 eV. The oxidation potential E_ox and reduction potential E_red vs NHE of the dye is 0.93 and −1.14 V, respectively. Dye-sensitized solar cell (DSSC) based on 7T-2TPA exhibits an open-circuit voltage (V_oc) of 724 mV, a short-circuit current density (J_sc) of 16.28 mA cm⁻², a fill factor (FF) of 0.684 and a power conversion efficiency of 8.06%. The efficiency of 8.06% is similar to that for widely used N719-based cell fabricated and measured under the same conditions.