Lanchao Ma

A bipolar small molecule based on indacenodithiophene and diketopyrrolopyrrole for solution processed organic solar cells

A new linear A–D–A type low band gap small molecule (IDT-2DPP) based on 4,4,9,9-tetrakis(4-hexylphenyl)-4,9-dihydro-s-indaceno[1,2-b:5,6-b′]dithiophene (IDT) and 2,5-bis(2-ethylhexyl)-3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (DPP) was designed and synthesized by Pd-catalyzed Stille coupling reaction. IDT-2DPP exhibits good solubility and good thermal stability with a decomposition temperature of 395 °C. IDT-2DPP shows strong absorption from 500 to 700 nm with a high molar extinction coefficient of 1.3 × 105 M−1 cm−1 at the absorption peak (640 nm) in chloroform solution. The HOMO and LUMO levels of IDT-2DPP were estimated to be −5.11 and −3.32 eV, respectively. Solution processed bulk-heterojunction solar cells using IDT-2DPP as a donor material blending with PC71BM as an acceptor yielded a power conversion efficiency of 2.82%, and solar cells using IDT-2DPP as an acceptor material blending with P3HT as a donor yielded a power conversion efficiency of 0.83% with a high Voc of 1.17 V.

Nanometer‐scale recording on an organic‐complex thin film with a scanning tunneling microscope

Nanometer‐scale recording on an organic‐complex thin film with a scanning tunneling microscope (STM) under ambient conditions is demonstrated. The recording marks are made by applying external voltage pulses between the tip and the highly ordered pyrolytic graphite substrate. A 30×30 nm2 STM image with recorded marks is given. The average recorded mark is 1.3 nm in diameter, which corresponds to a data storage density of about 1013 bits/cm2. The current–voltage characteristics measured by the STM show an insulator behavior for the unrecorded regions, and a conductor behavior for the recorded regions, which indicates that the data are recorded by local change of the electrical property of the films.

Data storage with 0.7 nm recording marks on a crystalline organic thin film by a scanning tunneling microscope

Ultrahigh density data storage on a novel organic thin film by scanning tunneling microscope (STM) under ambient conditions is demonstrated. The material, N-(3-nitrobenzylidene)p-phenylenediamine (NBPDA), is used for preparing thin film by vacuum evaporation method. Crystalline NBPDA films with electrical bistability are obtained by this method. Recording experiment on the films is made by applying voltage pulses between the STM tip and substrate. The recorded marks are 0.7 nm in size, corresponding to a storage density of 1014 bit/cm2. Current–voltage characteristic measurement shows that the resistance of the unrecorded region of the NBPDA films is much higher than that of the recorded region. The mechanism of recording is discussed.

Perylene and naphthalene diimide polymers for all-polymer solar cells: a comparative study of chemical copolymerization and physical blend

Five copolymers, having 4,4,9,9-tetrakis(4-hexylphenyl)-indaceno[1,2-b:5,6-b′]-dithiophene as a donor unit, and perylene diimide (PDI) and/or naphthalene diimide (NDI) as acceptor moieties, were synthesized by Stille coupling copolymerization, and used as electron acceptors in solution-processed polymer solar cells (PSCs). All five copolymers exhibited broad absorption in the region of 300–800 nm. The LUMO energy level of the resulting copolymers was from −3.90 to −3.77 eV and the HOMO energy level had little variation from −5.65 to −5.57 eV. Among binary blend PSCs using P3HT as a donor and these polymers as acceptors, PPDI25-co-NDI75-based devices (P3HT : PPDI25-co-NDI75 = 3 : 1, w/w) yielded the best power conversion efficiency (PCE) of up to 1.54%. Among ternary blend PSCs using P3HT as a donor and PDI polymer PPDI100 and NDI polymer PNDI100 as coacceptors, the P3HT : PPDI100 : PNDI100 (3 : 0.25 : 0.75, w/w) ternary blend afforded the best PCE of 0.83%. All ternary blends based on P3HT : PPDI100 : PNDI100 showed decreased VOC, JSC, FF and PCE compared to the corresponding binary blends based on P3HT : PPDI-co-NDI.

Ultrahigh density data storage from local polymerization by a scanning tunneling microscope

Ultrahigh density data storage from local polymerization on an organic thin film is demonstrated by using a scanning tunneling microscope (STM) operating in air. An organic monomer material, which may become electrical conductive by polymerization, is selected as data storage material. Films prepared by the monomer material are used for data recording. By applying a high electric field with the STM tip to realize local polymerization, highly stable recorded patterns with molecule-sized recorded marks have been performed. One recorded mark corresponds to a polymeric molecule in the film. The marks are 0.8 nm in size. The nearest distance between two recorded marks is 1.2 nm. Having been read 2000 times the recorded patterns show no discernible change.

Highly sensitive thin film phototransistors based on a copolymer of benzodithiophene and diketopyrrolopyrrole

A new copolymer (P(DPP4T-co-BDT)) was synthesized by Stille coupling polymerization of 3,6-bis(5′-bromo-[2,2′-bithiophen]-5-yl)-2,5-bis(2-octyldodecyl)pyrrolo-[3,4-c]-pyrrole-1,4(2H,5H)-dione and 2,6-bis(trimethyltin)-4,8-dimethoxybenzo[1,2-b:3,4-b′]dithiophene. P(DPP4T-co-BDT) showed good solution processability, good thermal stability with decomposition temperature of >330 °C, and strong and broad absorption in the range of 500–900 nm. Field-effect transistors based on P(DPP4T-co-BDT) thin films exhibited a hole mobility of up to 0.047 cm2 V−1 s−1, an on/off current ratio of 106, and a threshold voltage of −5 V after thermal annealing at 200 °C. Thin film phototransistors based on P(DPP4T-co-BDT) exhibited a photoresponsivity of up to 4.0 × 103 A W−1 and a photocurrent/dark-current ratio of 6.8 × 105 under white light irradiation with a low light intensity (9.7 μW cm−2).

High-mobility, air stable bottom-contact n-channel thin film transistors based on N,N′-ditridecyl perylene diimide

Bottom-gate bottom-contact (BGBC) organic thin film transistors (OTFTs) based on N,N′-ditridecyl perylene diimide exhibit electron mobility as high as 3.54 cm2 V−1 s−1 in nitrogen, higher than that (1 cm2 V−1 s−1) of bottom-gate top-contact devices. The better performance of BGBC configuration in N2 is attributed to lower contact resistance, which is further reduced by thermal annealing. After thermally annealing the BGBC OTFTs at 180 °C, electron mobility as high as 3.5 cm2 V−1 s−1, current on/off ratio of 106 and threshold voltage of 9 V are achieved in air, and the mobility retains above 1 cm2 V−1 s−1 after storage for two months in air. Thermal treatment enhanced crystalline grains, reduced grain boundaries, and suppressed the adsorption of H2O and O2, leading to excellent performance in air.

Enhancing the organic thin-film transistor performance of diketopyrrolopyrrole–benzodithiophene copolymers via the modification of both conjugated backbone and side chain

We developed a synthetic strategy for enhancing organic thin-film transistor performances of polymer semiconductors through the modification of the side chain to optimize the stacking conformation and the conjugated backbone to decrease the π–π stacking distance of polymers. Our studies demonstrate that the role of bulky alkyl chains attached to the donor unit or the acceptor unit is especially crucial for molecular stacking and aggregation and thus the OTFT device performance of polymers. However, with a larger π–π stacking distance in the thin film, the polymer with a bulky alkyl chain attached at the acceptor (P2) shows almost two orders of magnitude higher mobility than that with a bulky alkyl chain attached at the donor (P1). The better performance for P2 is attributed to the bulky alkyl chain at the acceptor which allows more coplanarity of the P2 backbone in the solution state, which leads to self-assembly, and finally forms a highly ordered layer-by-layer lamellar packing for P2 during spin-coating. Further improved performances were obtained by introducing two thiophene units into the polymer backbone to give P3, due to closer π–π stacking and in-plane π-stacking alignment in the thin film and a higher HOMO energy level. Therefore, an optimized device performance was realized through subtle modification of the polymer structure, including both the main chain and the side chain, which provides an insight into structure–property relationships for high-mobility polymer semiconductors.

An isoindigo-bithiazole-based acceptor-acceptor copolymer for balanced ambipolar organic thin-film transistors

Balanced carrier transport is observed in acceptor-acceptor (A-A′) type polymer for ambipolar organic thin-film transistors (OTFTs). It is found that the incorporation of two electron-accepting moieties (BTz and IIG) into a polymer main chain to form A-A′ polymer PIIG-BTz could lower highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels and facilitate good molecular stacking of the polymer. Ambipolar transistor behaviour for PIIG-BTz, with the balanced hole and electron mobilities of 0.030 and 0.022 cm2 V−1 s−1 was observed in OTFT devices, respectively. The study in this work reveals that the utilization of acceptor-acceptor (A-A′) structure in polymer main chain can be a feasible strategy to develop ambipolar polymer semiconductors.