Chang Eun Song

New TIPS-substituted benzo[1,2-b:4,5-b ']dithiophene-based copolymers for application in polymer solar cells

Novel triisopropylsilylethynyl (TIPS)-substituted benzodithiophene-based copolymers, poly[4,8-bis(triisopropylsilylethynyl)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl-alt-4,6-(2-ethylhexyl-thieno[3,4-b]thiophene-2-carboxylate)] (P1), poly[4,8-bis(triisopropylsilylethynyl)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl-alt-[4,6-{(1-thieno[3,4-b]thiophen-2-yl)-2-ethylhexan-1-one}] (P2), and poly[4,8-bis(triisopropylsilylethynyl)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl-alt-4,6-(2-ethylhexyl(3-fluorothieno[3,4-b]thiophene)-2-carboxylate)] (P3), were designed and synthesized for use in polymer solar cells (PSCs). We describe the effects of the different acceptor segment side groups on the optical, electrochemical, field-effect hole mobility, and photovoltaic characteristics of the resulting TIPS-based copolymers. The side groups in the copolymers were found to significantly influence the carrier mobilities and photovoltaic properties of the copolymers. The field-effect mobilities of the holes varied from 9 × 10−5 cm2 V−1 s−1 in P2 to 3 × 10−3 cm2 V−1 s−1 in P1. Under optimized conditions, the TIPS-based polymers showed power conversion efficiencies (PCEs) for the PSCs in the range of 3.16–5.76%. Among the TIPS-based copolymers studied here, P1 showed the best photovoltaic performance, with an open-circuit voltage (Voc) of 0.82 V, a short-circuit current density (Jsc) of 12.75 mA cm−2, a fill factor (FF) of 0.55, and a power-conversion efficiency of 5.76% using a P1:PC71BM blend film as the active layer under AM 1.5G irradiation (100 mW cm−2).

DPP-based small molecule, non-fullerene acceptors for “channel II” charge generation in OPVs and their improved performance in ternary cells

We synthesized three diketopyrrolopyrrole-thiophene-based small molecules (p-, m-, and o-DPP-PhCN) substituted with electron-withdrawing cyanide groups on both end phenyl rings in different positions. The physical properties of the oligomers varied based on the position of the CN groups. Compared to m- and o-DPP-PhCN, the p-DPP-PhCN film had a more red-shifted, strong UV absorption (λmax = 670 nm). p-DPP-PhCN also exhibited a relatively well-aligned arrangement in the X-ray diffraction pattern, owing to a high degree of molecular packing in p-DPP-PhCN. Such an exceptionally strong aggregation of p-DPP-PhCN is expected to give rise to strong molecular orbital interactions and a subsequent decrease in the energy band gap (Eg). p-DPP-PhCN has a lower optical Eg (1.75 eV) than m- and o-DPP-PhCN (∼1.80 eV). Organic photovoltaic cells with the structure ITO/PEDOT:PSS/poly(3-hexylthiophene) (P3HT):DPP-PhCN/LiF/Al were fabricated. Two D/A-type binary cells using p- or o-DPP-PhCN showed similar power conversion efficiencies (PCEs) of 0.5% although the device parameters were different. A high open circuit voltage of 1.09 V in P3HT:o-DPP-PhCN comes from a high-lying lowest unoccupied molecular orbital energy level of o-DPP-PhCN. In contrast, the relatively high short circuit current density of P3HT:p-DPP-PhCN can be explained by the red-shifted UV absorption and superior molecular packing in p-DPP-PhCN. Furthermore, the maximum photocurrent response (13%) of P3HT:p-DPP-PhCN was observed at the λmax of p-DPP-PhCN. In other words, the light absorption of p-DPP-PhCN contributes to the photocurrent along with the absorption of P3HT (e.g., “channel II” charge generation). Finally, a PCE of 1.00%, more than twice that of binary cells, was achieved in the D/A/A-type ternary cells composed of P3HT, p-, and o-DPP-PhCN. The contribution of the electron acceptor to the photocurrent of the devices was enhanced by adding a second acceptor. Improved film morphology and better charge separation were observed in the ternary cells.

A simple structured and efficient triazine-based molecule as an interfacial layer for high performance organic electronics

Achieving the state-of-the-art performance of solution processable and flexible organic electronics requires efficient, stable, and cost-effective interfacial layers (ILs). Here, we report an alcohol soluble phosphine oxide functionalized 1,3,5-triazine derivative (PO-TAZ) as an IL, which remarkably tailors the work function of conductors including metals, transparent metal oxides and organic materials, making it an ideal candidate for an interfacial material in organic electronics. Consequently, PO-TAZ thin films enable the fabrication of organic and organic–inorganic (perovskite) solar cells with power conversion efficiencies of 10.04% and 16.41%, respectively, and n-channel organic field-effect transistors with an electron mobility of 8 cm2 V−1 s−1. Owing to the low-cost processing associated with PO-TAZ and the tremendous improvement in device performances as compared to the devices without PO-TAZ along with ambient stability, PO-TAZ is a good choice for efficient organic electronics in large area printing processes.

Highly efficient uniform ZnO nanostructures for an electron transport layer of inverted organic solar cells

A highly uniform and predesigned ZnO nanostructure fabricated by single step direct nanoimprinting was used as the efficient electron transport layer (ETL) in inverted bulk heterojunction organic solar cells. Improved photovoltaic cell efficiency with long-term stability can be observed due to the large interface between the active layer and nanostructured ZnO ETL.

High-efficiency photovoltaic cells with wide optical band gap polymers based on fluorinated phenylene-alkoxybenzothiadiazole

A series of semi-crystalline, wide band gap (WBG) photovoltaic polymers were synthesized with varying number and topology of fluorine substituents. To decrease intramolecular charge transfer and to modulate the resulting band gap of D–A type copolymers, electron-releasing alkoxy substituents were attached to electron-deficient benzothiadiazole (A) and electron-withdrawing fluorine atoms (0–4F) were substituted onto a 1,4-bis(thiophen-2-yl)benzene unit (D). Intra- and/or interchain noncovalent Coulombic interactions were also incorporated into the polymer backbone to promote planarity and crystalline intermolecular packing. The resulting optical band gap and the valence level were tuned to 1.93–2.15 eV and −5.37 to −5.67 eV, respectively, and strong interchain organization was observed by differential scanning calorimetry, high-resolution transmission electron microscopy and grazing incidence X-ray scattering measurements. The number of fluorine atoms and their position significantly influenced the photophysical, morphological and optoelectronic properties of bulk heterojunctions (BHJs) with these polymers. BHJ photovoltaic devices showed a high power conversion efficiency (PCE) of up to 9.8% with an open-circuit voltage of 0.94–1.03 V. To our knowledge, this PCE is one of the highest values for fullerene-based single BHJ devices with WBG polymers having a band gap of over 1.90 eV. A tandem solar cell was also demonstrated successfully to show a PCE of 10.3% by combining a diketopyrrolopyrrole-based low band gap polymer.

Impact of the Crystalline Packing Structures on Charge Transport and Recombination via Alkyl Chain Tunability of DPP-Based Small Molecules in Bulk Heterojunction Solar Cells.

A series of small compound materials based on benzodithiophene (BDT) and diketopyrrolopyrrole (DPP) with three different alkyl side chains were synthesized and used for organic photovoltaics. These small compounds had different alkyl branches (i.e., 2-ethylhexyl (EH), 2-butyloctyl (BO), and 2-hexyldecyl (HD)) attached to DPP units. Thin films made of these compounds were characterized and their solar cell parameters were measured in order to systematically analyze influences of the different side chains of compounds on the film microstructure, molecular packing, and hence, charge-transport and recombination properties. The relatively shorter side chains in the small molecules enabled more ordered packing structures with higher crystallinities, which resulted in higher carrier mobilities and less recombination factors; the small molecule with the EH branches exhibited the best semiconducting properties with a power conversion efficiency of up to 5.54% in solar cell devices. Our study suggested that tuning the alkyl chain length of semiconducting molecules is a powerful strategy for achieving high performance of organic photovoltaics.

A non-fullerene acceptor with a diagnostic morphological handle for streamlined screening of donor materials in organic solar cells

Utilizing the N-annulated PDI acceptor PDI–DPP–PDI, a simple air-processed and air-tested organic photovoltaic device fabrication procedure has been established to streamline the screening of donor materials. Post-deposition solvent vapour annealing of the active layer blend results in a preferential re-organization dictated by PDI–DPP–PDI and is responsible for significant increases in device performance. Employing this method, a series of low-cost and scalable donor polymers were screened to help select promising candidates as alternatives to the expensive high-performance donor polymer PTB7-Th. Universal improvements in performance upon solvent vapour annealing with a range of donor polymers highlighted the advantages of PDI–DPP–PDI and this post-deposition treatment. This facile screening protocol identified PDTT-BOBT as the most suitable PTB7-Th alternative in the surveyed series, with the best device efficiencies reaching 4.5% PCE compared to control devices with PTB7-Th at 4.6% PCE.

Low band gap diketopyrrolopyrrole-based small molecule bulk heterojunction solar cells: influence of terminal side chain on morphology and photovoltaic performance

Two new low band gap small molecules BDT(DPP-TTHex)2 and BDT(DPP-TT)2 with different terminal side chain are designed and synthesized for bulk heterojunction solar cells. BDT(DPP-TTHex)2 is flanked by hexyl at head and tail of backbone while BDT(DPP-TT)2 is free attachment. Both small molecules exhibit similar optical, electrochemical properties and charge carrier mobilities. Nevertheless, BDT(DPP-TT)2 performs optimal film morphology leading to higher short circuit current (JSC) comparing with BDT(DPP-TTHex)2, thus 5.12% power conversion efficiencies (PCE) of BDT(DPP-TT)2 is achieved, higher than 2.36% PCE of BDT(DPP-TTHex)2 under AM1.5G illumination (100 mW cm−2).

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.

High-Performance CH3NH3PbI3-Inverted Planar Perovskite Solar Cells with Fill Factor Over 83% via Excess Organic/Inorganic Halide

The reduction of charge carrier recombination and intrinsic defect density in organic-inorganic halide perovskite absorber materials is a prerequisite to achieving high-performance perovskite solar cells with good efficiency and stability. Here, we fabricated inverted planar perovskite solar cells by incorporation of a small amount of excess organic/inorganic halide (methylammonium iodide (CH3NH3I; MAI), formamidinium iodide (CH(NH2)2I; FAI), and cesium iodide (CsI)) in CH3NH3PbI3 perovskite film. Larger crystalline grains and enhanced crystallinity in CH3NH3PbI3 perovskite films with excess organic/inorganic halide reduce the charge carrier recombination and defect density, leading to enhanced device efficiency (MAI+: 14.49 ± 0.30%, FAI+: 16.22 ± 0.38% and CsI+: 17.52 ± 0.56%) compared to the efficiency of a control MAPbI3 device (MAI: 12.63 ± 0.64%) and device stability. Especially, the incorporation of a small amount of excess CsI in MAPbI3 perovskite film leads to a highly reproducible fill factor of over 83%, increased open-circuit voltage (from 0.946 to 1.042 V), and short-circuit current density (from 18.43 to 20.89 mA/cm2).