A solution state diode using semiconductor polymer nanorods with nanogap electrodes

A solution state polymer diode, which uses regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT):dichlorobenzene solution as the semiconductor between highly doped p-type silicon and aluminum electrodes has been built. Electrodes separated by a 40 nm gap enable intra-chain charge carrier transfer through the lengths of single polymer chains. This prevents chain to chain hopping and chain entanglements, increasing carrier mobility. The degradation with time and hysteresis effects of the diodes are measured. An optimal P3HT solution concentration of 6 mg ml(-1) is found. A current density of at least 300 mA cm(-2) is achieved, indicating at least a six-fold improvement in carrier mobility compared to previously fabricated solid state P3HT diodes.     

Enhanced performance of Au/P3HT/ (bilayer dielectrics)/Si thin-film-transistor having gold nanoparticles chemically bonded to P3HT

The charge transport enhancement of using poly(3-hexylthiophene) covalently bonded with gold nanoparticles as an active layer in a bottom-gate/top contact, Au/(P3HT)/(bilayer dielectric)/Si OTFT device has been studied. P3HT was synthesized via the modified Grignard metathesis method and functionalized to have a thiol terminal (P3HT(SH)). Gold nanoparticles (AuNPs) with sizes ranging from 2 to 10 nm were then formed via a reduction of HAuCl4 in the presence of P3HT(SH). Compared to the pristine P3HT, the AuNPs-containing P3HT composite materials have a higher energy level of HOMO and a smaller electrochemical bandgap E(g)chem. The smaller bandgap enhances the charge carrier mobility and the higher HOMO energy level indicates a reduced barrier and an increased injection rate for charge carrier at the source contact. Furthermore, the threshold voltage V(T) of AuNPs-containing P3HT samples remain nearly unchanged and their saturation current I(D) and the field-effect mobility are higher. An OTFT device fabricated with a composite sample containing 1.30% AuNPs has a carrier mobility and saturation current nearly two time higher than pristine P3HT.     

Effect of titanium oxide-polystyrene nanocomposite dielectrics on morphology and thin film transistor performance for organic and polymeric semiconductors

Previous studies have shown that organic thin film transistors with pentacene deposited on gate dielectrics composed of a blend of high K titanium oxide-polystyrene core-shell nanocomposite (TiO₂-PS) with polystyrene (PS) perform with an order of magnitude increase in saturation mobility for TiO₂-PS (K = 8) as compared to PS devices (K = 2.5). The current study finds that this performance enhancement can be translated to alternative small single crystal organics such as α-sexithiophene (α-6T) (enhancement factor for field effect mobility ranging from 30-100× higher on TiO₂-PS/PS blended dielectrics as compared to homogenous PS dielectrics). Interestingly however, in the case of semicrystalline polymers such as (poly-3-hexylthiophene) P3HT, this dramatic enhancement is not observed, possibly due to the difference in processing conditions used to fabricate these devices (film transfer as opposed to thermal evaporation). The morphology for α-sexithiophene (α-6T) grown by thermal evaporation on TiO₂-PS/PS blended dielectrics parallels that observed in pentacene devices. Smaller grain size is observed for films grown on dielectrics with higher TiO₂-PS content. In the case of poly(3-hexylthiophene) (P3HT) devices, constructed via film transfer, morphological differences exist for the P3HT on different substrates, as discerned by atomic force microscopy studies. However, these devices only exhibit a modest (2×) increase in mobility with increasing TiO₂-PS content in the films. After annealing of the transferred P3HT thin film transistor (TFT) devices, no appreciable enhancement in mobility is observed across the different blended dielectrics. Overall the results support the hypothesis that nucleation rate is responsible for changes in film morphology and device performance in thermally evaporated small molecule crystalline organic semiconductor TFTs. The increased nucleation rate produces organic polycrystalline films with small grain size which are better connected and exhibit lower barriers for charge transport and as such higher field effect mobilities are measured in these devices.     

Fabrication of organic field effect transistor by directly grown poly(3 hexylthiophene) crystalline nanowires on carbon nanotube aligned array electrode

We fabricated organic field effect transistors (OFETs) by directly growing poly (3-hexylthiophne) (P3HT) crystalline nanowires on solution processed aligned array single walled carbon nanotubes (SWNT) interdigitated electrodes by exploiting strong π-π interaction for both efficient charge injection and transport. We also compared the device properties of OFETs using SWNT electrodes with control OFETs of P3HT nanowires deposited on gold electrodes. Electron transport measurements on 28 devices showed that, compared to the OFETs with gold electrodes, the OFETs with SWNT electrodes have better mobility and better current on-off ratio with a maximum of 0.13 cm(2)/(V s) and 3.1 × 10(5), respectively. The improved device characteristics with SWNT electrodes were also demonstrated by the improved charge injection and the absence of short channel effect, which was dominant in gold electrode OFETs. The enhancement of the device performance can be attributed to the improved interfacial contact between SWNT electrodes and the crystalline P3HT nanowires as well as the improved morphology of P3HT due to one-dimensional crystalline nanowire structure.     

Hole transport in organic field-effect transistors with active poly(3-hexylthiophene) layer containing CdSe quantum dots

Hybrid field-effect transistors (FETs) based on poly(3-hexylthiophene) (P3HT) containing CdSe quantum dots (QDs) were fabricated. The effect of the concentration of QDs on charge transport in the hybrid material was studied. The influence of the QDs capping ligand on charge transport parameters was investigated by replacing the conventional trioctylphosphine oxide (TOPO) surfactant with pyridine to provide closer contact between the organic and inorganic components. Electrical parameters of FETs with an active layer made of P3HT:CdSe QDs blend were determined, showing field-effect hole mobilities up to 1.1×10^?4 cm²/Vs. Incorporation of TOPO covered CdSe QDs decreased the charge carrier mobility while the pyridine covered CdSe QDs did not alter this transport parameter significantly.     

Enhanced Electrical Properties in Carbon Nanotube/Poly （3-hexylthiophene） Nanocomposites Formed Through Non-Covalent Functionalization

Poly(3-hexylthiophene) (P3HT) has received much attention as a good candidate to replace inorganic semiconductors for flexible electronics due to its solution-processability. However, the low charge mobility of P3HT is an obstacle to its commercialization. To overcome this problem, we propose a new non-covalent functionalization method for carbon nanotubes (CNTs) for use in CNT/P3HT nanocomposites. By using modified pyrene molecules with hydrophobic long alkyl chains, the non-covalently functionalized CNTs can become well dispersed in hydrophobic solutions and organic semiconductor matrices. Fabrication of organic thin-film transistors (OTFTs) from the non-covalently functionalized CNT/organic semiconductor nanocomposites shows that our non-covalent functionalization method significantly reduces damage to CNTs during functionalization when compared with covalent functionalization by treatment with acids. The OTFTs show 15 times enhancement of field effect mobility (1.5 × 10⁻² cm²/(V·s)) compared to the mobility of OTFTs made from pure P3HT. This enhancement is achieved by addition of only 0.25 wt% of CNTs to P3HT.      

Electrical contact properties between the accumulation layer and metal electrodes in ultrathin poly(3-hexylthiophene)(P3HT) field effect transistors

Probing contact properties between an ultrathin conjugated polymer film and metal electrodes in field effect transistors (FETs) is crucial not only to understanding charge transport properties in the accumulation layer but also in building organic sensors with high sensitivity. We investigated the contact properties between gold electrodes and poly(3-hexylthiophene) (P3HT) as a function of film thickness using gated four-point sheet resistance measurements. In an FET with a 2 nm thick P3HT film, a large voltage drop of 1.9 V (V(D) = -3 V) corresponding to a contact resistance of 2.3 × 10(8) Ω was observed. An effective FET mobility of 1.4 × 10(-3) cm(2)/(V s) was calculated when the voltage drop at the contacts was factored out, which is approximately a factor of 3 greater than the two-contact FET mobility of 5.5 × 10(-4) cm(2)/(V s). A sharp decrease in the ratio of the contact resistance to the channel resistance was observed with increasing film thickness up to a thickness of approximately 6 nm, separating a contact limited regime from a charge transport limited regime. The origin of the large contact resistance observed in the device prepared with an ultrathin P3HT film is discussed in light of results from X-ray diffraction (XRD) and atomic force microscopy (AFM) studies.     

Evaluation of poly(3-hexylthiophene)/polymeric insulator interface by charge modulation spectroscopy technique

We have investigated the sub-band gap states behaviors in P3HT MIS diodes and P3HT Schottky diode by the combination of C–V characteristics and the charge modulation spectroscopy (CMS) measurements. Single-layered heat-treated P3HT film sandwiched by ITO and Al electrodes behaves as a dielectric film, whereas it behaves as Schottky diode when oxygen molecules were doped. The thickness of depletion layer at Al/P3HT interface is controlled by external voltage, and there are two peaks at around 1.3 eV and 1.95 eV in CMS curves. On the other hand, P3HT MIS diodes deposited on polymeric gate insulators have three peaks in CMS curves at around 1.3, 1.6 and 1.95 eV respectively in accumulation. The first peak observed was attributed to the optical transition of polarons in isolated (1D) P3HT chain, and it was related to the field effect mobility and conjugation length in lamellae chains. The second peak observed was attributed to the optical transition of delocalized polarons in π-stacked (2D) P3HT chains, and the peak intensity became strong for highly crystallized P3HT film. The third peak was attributed to the deeply trapped charges in amorphous P3HT region, respectively.     

The synergistic effect of nanocrystal integration and process optimization on solar cell efficiency

This paper investigates the roles of semiconducting single-walled carbon nanotubes (SWNTs) and metallic SWNTs in the SWNT/poly(3-hexylthiophene) (P3HT)-based photovoltaic conversion system. SWNTs containing different fractions of semiconducting nanotubes were conjugated with P3HT by virtue of π-π interaction. The energy transfer and carrier transport mechanisms in the photovoltaic composites were experimentally investigated by optical absorption spectroscopy, photoluminescence spectroscopy and carrier mobility measurements. At low loading of SWNTs, a high percentage of semiconducting nanotubes result in diminished non-radiative decay of exciton and lower carrier mobility, causing higher open circuit voltage and lower photocurrent. At an optimized morphology, SWNT/P3HT/phenyl-C61-butyric acid methyl ester (PCBM) hybrid-based solar cells demonstrated much higher photocurrent than a reference solar cell (P3HT:PCBM) due to the improved carrier mobility. Further thermal annealing of the devices significantly increased the open circuit voltage to 610?mV, resulting in an 80% increase of power conversion efficiency in comparison to the reference solar cell. These results are expected to lay a foundation for the integration of various nanocrystals into solar cells for efficient photovoltaic conversion.     

Thermal annealing time effect on the performance of ambipolar organic light-emitting transistors based on conjugated polymer blends

Here we report the effect of thermal annealing time on the performance of ambipolar organic light-emitting transistors (OLETs) made using conjugated polymer blends. Regioregular poly(3-hexylthiophene) (P3HT) and poly(9,9-dioctylfluorenyl-2,3-diyl-co-1,4-benzo-2,1',3-thiadiazole) (F8BT) were chosen as a p-type and a n-type component, respectively. As a gate insulator, poly(4,4'-oxydiphenylene-pyromellitimide) (PMDA-ODA PI) was employed due to its high solvent resistance and thermal stability. Results showed that the present OLETs exhibited ambipolar characteristics even after thermal annealing. All devices showed almost identical field-effect mobility for both holes and electrons. The highest field-effect mobility was achieved for the OLET annealed at 130 degrees C for 60 min, which was assigned to the improved polymer-metal contact by thermal annealing leading to better charge injection.     

Investigation of self-assembled monolayer treatment on SiO2 gate insulator of poly(3-hexylthiophene) thin-film transistors

We have investigated self-assembled monolayer (SAM) treatment on SiO₂ gate insulator of poly(3-hexylthiophene) (P3HT) thin-film transistor (TFT), and demonstrated a correlation between mobility and surface free energy of the insulator. The device with lower surface free energy shows higher mobility. The docosyltrichlorosilane (DCTS)-treated device exhibits the best performance among the various SAM-treated devices examined. Field-effect mobility, on/off ratio and threshold voltage of the DCTS-treated P3HT TFT were 0.015 cm²/Vs, >10⁵ and −14 V, respectively.     

Toward efficient carbon nanotube/P3HT solar cells: active layer morphology, electrical, and optical properties

We demonstrate single-walled carbon nanotube (SWCNT)/P3HT polymer bulk heterojunction solar cells with an AM1.5 efficiency of 0.72%, significantly higher than previously reported (0.05%). A key step in achieving high efficiency is the utilization of semiconducting SWCNTs coated with an ordered P3HT layer to enhance the charge separation and transport in the device active layer. Electrical characteristics of devices with SWCNT concentrations up to 40 wt % were measured and are shown to be strongly dependent on the SWCNT loading. A maximum open circuit voltage was measured for SWCNT concentration of 3 wt % with a value of 1.04 V, higher than expected based on the interface band alignment. Modeling of the open-circuit voltage suggests that despite the large carrier mobility in SWCNTs device power conversion efficiency is governed by carrier recombination. Optical characterization shows that only SWCNT with diameter of 1.3-1.4 nm can contribute to the photocurrent with internal quantum efficiency up to 26%. Our results advance the fundamental understanding and improve the design of efficient polymer/SWCNTs solar cells.     

Improvement of photovoltaic properties by addition of a perylene compound in P3HT:PCBM BHJ system

The synthesized n-type perylene derivative, N,N'-bis-(4-bromophenyl)-1,6,7,12-tetrakis(4-n-butoxy-phenoxy)-3,4,9,10-perylene tetracarboxdiimide (PIBr), was applied as an additive to polymer solar cells (PSCs) with P3HT [poly(3-hexylthiophene)]:PCBM [[6,6]-phenyl C61-butyric acid methyl ester] blend films. Without post thermal annealing, a considerable improvement of about 98% in power conversion efficiency was achieved by the addition of 1 wt% PIBr into a P3HT:PCBM layer, when compared with that of reference cell without the additive. The results, in combination with relevant data from UV-Vis. absorption, photoluminescence, X-ray measurements and carrier mobility studies, revealed that the addition of the perylene compound within active layer contributed to more effective charge transfer and enhanced electron mobility.     

Highly stable carbon-based perovskite solar cell with a record efficiency of over 18% via hole transport engineering

Carbon-based perovskite solar cells show great potential owing to their low-cost production and superior stability in air, compared to their counterparts using metal contacts. The photovoltaic performance of carbon-based PSCs, however, has been progressing slowly in spite of an impressive efficiency when they were first reported. One of the major obstacles is that the hole transport materials developed for state-of-the-art Au-based PSCs are not suitable for carbon-based PSCs. Here, we develop a low-temperature, solution-processed Poly(3-hexylthiophene-2,5-diyl) (P3HT)/graphene composite hole transport layer (HTL), that is compatible with paintable carbon-electrodes to produce state-of-the-art perovskite devices. Space-charge-limited-current measurements reveal that the as-prepared P3HT/graphene composite exhibits outstanding charge mobility and thermal tolerance, with hole mobility increasing from 8.3 × 10⁻³ cm² V^-1 s^-1 (as-deposited) to 1.2 × 10^-2 cm² V^-1 s^-1 (after annealing at 100 °C) - two orders of magnitude larger than pure P3HT. The improved charge transport and extraction provided by the composite HTL provides a significant efficiency improvement compared to cells with a pure P3HT HTL. As a result, we report carbon-based solar cells with a record efficiency of 17.8% (certified by Newport); and the first perovskite cells to be certified under the stabilized testing protocol. The outstanding device stability is demonstrated by only 3% drop after storage in ambient conditions (humidity: ca. 50%) for 1680 h (non-encapsulated), and retention of ca. 89% of their original output under continuous 1-Sun illumination at room-temperature for 600 h (encapsulated) in a nitrogen environment.     

Ultrafast charge separation at a polymer-single-walled carbon nanotube molecular junction

We have investigated the charge photogeneration dynamics at the interface formed between single-walled carbon nanotubes (SWNTs) and poly(3-hexylthiophene) (P3HT) using a combination of femtosecond spectroscopic techniques. We demonstrate that photoexcitation of P3HT forming a single molecular layer around a SWNT leads to an ultrafast (～430 fs) charge transfer between the materials. The addition of excess P3HT leads to long-term charge separation in which free polarons remain separated at room temperature. Our results suggest that SWNT-P3HT blends incorporating only small fractions (1%) of SWNTs allow photon-to-charge conversion with efficiencies comparable to those for conventional (60:40) P3HT-fullerene blends, provided that small-diameter tubes are individually embedded in the P3HT matrix.     

Controlling charge injection in organic field-effect transistors using self-assembled monolayers

We have studied charge injection across the metal/organic semiconductor interface in bottom-contact poly(3-hexylthiophene) (P3HT) field-effect transistors, with Au source and drain electrodes modified by self-assembled monolayers (SAMs) prior to active polymer deposition. By using the SAM to engineer the effective Au work function, we markedly affect the charge injection process. We systematically examine the contact resistivity and intrinsic channel mobility and show that chemically increasing the injecting electrode work function significantly improves hole injection relative to untreated Au electrodes.     

Efficient organic photovoltaic devices by using PEDOT:PSSs with excellent hole extraction ability

We have demonstrated the poly(3-hexyl-thiophene-1,5-diyl) (P3HT):[6,6]-phenyl C61-butyric acid methyl ester (PCBM) bulk heterojunction (BHJ) organic photovoltaic (OPV) devices on various poly(3,4-ethylene-dioxythiophene):poly(styrenesulfonate) (PEDOT:PSSs). The device with PEDOT:PSS of PH 500 adding 1% dimethyl sulfoxide (DMSO) showed the best performances in term of the fill factor and power conversion efficiency (PCE) than others. The hole extraction ability of PEDOT:PSS is very important to balance between holes and electrons mobility because the carrier mobility of PCBM (approximately 10(-4) cm2/Vs) is higher than that of P3HT (approximately 10(-6) cm2/Vs) in P3HT:PCBM BHJ structure. The optimized BHJ OPV with PEDOT:PSS of PH 500 adding 1% DMSO showed a short-circuit current density of 8.92 mA/cm2 and a PCE of 2.97%, which was nearly increased to 2.5 times than that of control device with PEDOT:PSS of P VP Al 4083.     

Morphological and optoelectronic characteristics of nanocomposites comprising graphene nanosheets and poly(3-hexylthiophene)

P3HT/graphene nanocomposite was prepared via in-situ reduction of exfoliated graphite oxide in the P3HT polymer matrix, where the exfoliated graphite oxide was formed beforehand via the oxidation of graphite via the Hummers method. The oxidation reaction not only imparts functional groups, such as C=O, C-OH, and C-O-C, to graphite but also causes exfoliation of the resulting graphite oxide. The functional groups render graphite oxide an additional, lower thermal degradation temperature (T(d)) and the exfoliation shifts the XRD pattern towards a much smaller angle. The oxidation of graphite into graphite oxide creates a pleated flaking morphology for graphite oxide as opposed to that of graphite. UV/Vis and photoluminescence (PL) spectra of P3HT/graphene nanocomposite indicate that the existence of graphene does not alter the UV/Vis and PL excitation characteristics of P3HT, and the P3HT/graphene composite has higher electron mobility, a smaller band gap and higher conductivity than the pristine P3HT.     

Photoinduced Self‐Assembled Nanostructures and Permanent Polaron Formation in Regioregular Poly(3‐hexylthiophene)

Solution processing of conjugated polymers into ordered self‐assembled precursors has attracted great interest in the past years owing to the ability to manipulate their structural and physical properties. Regioregular poly(3‐hexylthiophene) (P3HT) has become the benchmark polymer in this scenario, where ordered lamellar structures significantly improve carrier mobility of the thin films due to increased crystallinity, extended intrachain conjugation, and ordered interchain π‐stacking. Here, a new photoinduced approach is presented for the generation of highly ordered P3HT aggregate structures that is amenable to the use of visible light to control the aggregate formation. Strong intra‐ and interchain interactions in the solution precursors allow for permanent formation of localized and delocalized polarons that are stable for months. Spin‐coated thin films are found to preserve, in part, the morphological and physical properties of the aggregated P3HT solution precursors with high degree of crystallinity and short π‐stack interchain distances.