Zi En Ooi

High mobility organic thin film transistor and efficient photovoltaic devices using versatile donor–acceptor polymer semiconductor by molecular design

In this work, we report a novel donor–acceptor based solution processable low band gap polymer semiconductor, PDPP–TNT, synthesized via Suzuki coupling using condensed diketopyrrolopyrrole (DPP) as an acceptor moiety with a fused naphthalene donor building block in the polymer backbone. This polymer exhibits p-channel charge transport characteristics when used as the active semiconductor in organic thin-film transistor (OTFT) devices. The hole mobilities of 0.65 cm2 V−1s−1 and 0.98 cm2 V−1s−1 are achieved respectively in bottom gate and dual gate OTFT devices with on/off ratios in the range of 105 to 107. Additionally, due to its appropriate HOMO (5.29 eV) energy level and optimum optical band gap (1.50 eV), PDPP–TNT is a promising candidate for organic photovoltaic (OPV) applications. When this polymer semiconductor is used as a donor and PC71BM as an acceptor in OPV devices, high power conversion efficiencies (PCE) of 4.7% are obtained. Such high mobility values in OTFTs and high PCE in OPV make PDPP–TNT a very promising polymer semiconductor for a wide range of applications in organic electronics.

Solution processable bulk-heterojunction solar cells using a small molecule acceptor

We report a small-molecule electron-acceptor based on 2-vinyl-4,5-dicyanoimidazole [Vinazene™] for use in solution processed organic solar cells. The material has a favourably located LUMO level of −3.6 eV and absorbs strongly in the visible spectrum up to 520 nm—attractive properties compared to the widely used acceptor (6,6)-phenyl-C60-butyric acid methyl ester (PCBM). The Vinazene derivative was blended with a poly(2,7-carbazole) donor—chosen for its complementary absorption range and comparatively high-lying HOMO level of −5.6 eV—and incorporated into bulk heterojunction devices. The influence of the donor/acceptor composition and annealing temperature on device performance were then investigated. The best performing devices exhibited reasonable power conversion efficiencies of 0.75% and open-circuit voltages of more than 1.3 V, substantially higher than previously reported devices using small molecule acceptors.

Identifying Fundamental Limitations in Halide Perovskite Solar Cells.

The temperature dependence of the principal photovoltaic parameters of perovskite photovoltaics is studied. The recombination activation energy is in good agreement with the perovskites bandgap energy, thereby placing an upper bound on the open-circuit voltage. The photocurrent increases moderately with temperature and remains high at low temperature, reinforcing that the cells are not hindered by insufficient thermally activated mobility or carrier trapping by deep defects.

On the pseudo-symmetric current–voltage response of bulk heterojunction solar cells

We analyse the current–voltage (J–V) characteristics of bulk heterojunction solar cells containing poly(3,4-ethylenedioxythiophene):polystyrenesulfonate [PEDOT:PSS]-coated indium tin oxide [ITO] as the anode, a 1 : 1 blend by weight of poly(3-hexylthiophene-2,5-diyl) [P3HT] and phenyl-C61-butyric acid methyl ester [PCBM] as the active layer, and aluminium as the cathode. The J–V characteristics were recorded in the dark and under white-light illumination using a pulsed measurement scheme that ensured the two sets of measurements were obtained at equal temperatures. The dark current was subtracted from the photocurrent to obtain a corrected photocurrent Jph that excludes the photovoltage-induced injection current and is thus due solely to the flux of incident photons. The resultant Jph–V curves—which have a near-symmetric profile centred about the built-in potential VBI—provide critical information about the physical processes governing device operation. For the specific devices tested here, the open-circuit voltage VOC was 0.63 ± 0.002 V at an illumination level of ∼100 mW cm−2—some 50 ± 12 mV above the built-in potential of 0.58 ± 0.01 V. Hence, we find that VOC can exceed VBI by a considerable margin. A simple model is proposed to explain the shape of the Jph–V curves, and application of this model to the measured data indicates that one or both of the electrodes is preferentially selective towards its own carrier type—the anode to holes and/or the cathode to electrons. This ‘self-selectivity’—which results in a significant efficiency gain compared with devices that have non-selective electrodes—is consistent with vertical phase segregation in the active layer, yielding a P3HT-enriched region close to the anode and/or a PCBM-enriched region close to the cathode.

6,13-Dicyano pentacene-2,3:9,10-bis(dicarboximide) for solution-processed air-stable n-channel field effect transistors and complementary circuit

Aromatic diimides have attracted much attention due to their unique properties and potential applications for n-channel organic field effect transistors (OFETs). In this study, N,N′-dialkyl-6,13-dicyano pentacene-2,3:9,10-bis(dicarboximide) (1) was synthesized and used in n-channel OFETs. The photophysical, electrochemical and thermal properties of 1 were investigated. The low LUMO energy level (−4.15 eV) located within the air stability window ensures air-stable n-channel operation. Solution-processed top-contact thin film transistors based on such linear aromatic dicarboximide showed typical n-channel characteristics with a high electron mobility of up to 0.08 cm2 V−1 s−1 and a high Ion/Ioff ratio of 106 to 107. The devices exhibited good air stability when exposed to air for several months. Complementary inverters based on n-type 1 and p-type TIPS-pentacene were fabricated, which showed a maximum voltage gain (−dVOUT/dVIN) of 57.

Enhancing the performance of solution-processed bulk-heterojunction solar cells using hydrogen-bonding-induced self-organization of small molecules.

Small-molecule solar-cell performance is highly sensitive to the crystallinity and intermolecular connectivity of the molecules. In order to enhance the crystallinity for the solution-processed small molecule, it is possible to make use of carboxylic acid end-functional groups to drive hydrogen-bonding-induced π-π stacking of conjugated molecules. Herein, we report the synthesis and characterization of quarterthiophenes with carboxylic acid as end groups. The formation of hydrogen bonds between neighboring acid groups gives rise to a pseudo-polymeric structure in the molecules, which leads to substantial improvement in the organization and crystallinity of the active layers. This resulted in a four-fold increase in the hole mobility and a two-fold improvement in the performance of the solar cell device for the acid-functionalized molecule, compared to its ester analogue. More importantly, optimal device performance for the acid-functionalized molecule was achieved for the as-cast film, thereby reducing the reliance on thermal annealing and solvent additives.

Scanning photocurrent microscopy of lateral organic bulk heterojunctions

Scanning confocal photocurrent microscopy has been used to characterize carrier collection efficiency in lateral bulk heterojunction devices. By analyzing the photocurrent mappings within these devices, the lateral extents of the space charge regions has been measured and reported. Modulation via white light bias or increased voltage bias is also shown to increase the size of the space charge regions.

Analysis of photocurrents in lateral-geometry organic bulk heterojunction devices

Lateral-geometry organic bulk heterojunction (BHJ) devices resemble field-effect transistors without a gate and are useful for probing electrical characteristics of BHJ blends. The lateral structure allows physical access to the BHJ during device operation and lends itself well to device modelling. Photocurrent-voltage data from P3HT:PCBM-based lateral devices were found to possess features of space-charge limited (SCL) extraction. Numerical simulations indeed suggest a physical picture of SCL operation, effectively dividing the device into a photogenerative zone and a photoconducting zone. Based on these simulations, an analytical model was created to describe SCL photocurrents that account for channel lengths and bimolecular recombination.

Electrical characteristics of zinc oxide-organic semiconductor lateral heterostructure based hybrid field-effect bipolar transistors

Zinc oxide-organic semiconductor lateral heterostructure based field-effect bipolar transistors (FEBTs) having heterointerfaces approximately midway between the source and drain electrodes are fabricated and characterized. These hybrid FEBTs comprise zinc oxide (ZnO) and p-channel organic semiconductors [Pentacene and α-sexithiophene (6T)] supporting electron transport and hole transport on either side of the heterojunction, respectively. Current flow in the transistor channel is established as a result of carrier injection across the heterointerface followed by recombination. In steady state, such devices possess significant populations of holes and electrons in the transistor channel and operate in bipolar mode.

Field-dependent carrier generation in bulk heterojunction solar cells

We analyse the current–voltage (J–V) characteristics of bulk heterojunction devices based on two different materials systems: (i) poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene]:(6,6)-phenyl-C61-butyric acid methyl ester (MDMO-PPV:PCBM); and (ii) poly[N-(2′-decyltetradecyl)carbazole]-2,7-diyl: bis(2-(1-ethylhexyl-4,5-dicyanoimidazol-2-yl)vinyl) benzo[c]1,2,5-thiadiazole (PCz:EV-BT). The J–V characteristics were recorded in the dark and light using a pulsed technique that ensured the two sets of measurements were obtained at equal temperatures. In each case, the dark current was subtracted from the photocurrent to obtain a corrected photocurrent Jph that excludes the photovoltage-induced injection current and is thus due solely to the flux of incident photons. The Jph–V response of the MDMO-PPV:PCBM was consistent with a model in which all photogenerated excitons dissociate into free carriers irrespective of the internal field strength. The Jph–V response of the PCz:EV-BT device, by contrast, was consistent with a model in which an electric field is required to initiate appreciable charge separation. The different behaviour is attributable to the different energy offsets of the frontier orbitals responsible for charge separation: 1.3 eV for MDMO-PPV:PCBM compared to just 0.4 eV for PCz:EV-BT.