Heavily Doped poly(3,4‐ethylenedioxythiophene) Thin Films with High Carrier Mobility Deposited Using Oxidative CVD: Conductivity Stability and Carrier Transport

The transparent conductingpoly(3,4‐ethylenedioxythiophene) (PEDOT) is of interest for various optoelectronic device applications. Here, the conductivity stability of PEDOT processed using oxidative chemical‐vapor‐deposition (oCVD) with FeCl₃ as an oxidant is primarily dominated by the change in carrier density when aged in air. To establish the mechanism for the conductivity decrease, the changes in carrier density and carrier mobility of PEDOT films are separately monitored using an AC Hall Effect measurement system. The measured electrical properties reveal that a decrease in carrier density dominates the conductivity decrease during annealing. X‐ray diffraction analysis made on the HBr‐ and MeOH‐rinsed PEDOT samples identifies the Fe‐related dedoping phase of Fe(OH)₂ and provides the dedoping mechanism. The carrier transport study demonstrates heavily doped oCVD PEDOT with the carrier density higher than ~10²⁰ cm^–3, and in this regime, an increase in carrier density yields lower carrier mobility which shows that the carrier transport is governed by the ionized impurity scattering mechanism due to increased dopant counter‐anions. These findings of the mechanisms for PEDOT conductivity decrease and carrier transport behavior may be important to organic optoelectronic device applications that show a strong effect of air‐exposure and low‐temperature annealing on the device stability and performance.     

Impedance spectroscopy on copper phthalocyanine diodes with surface-induced molecular orientation

Molecular orientation and packing motif governs charge-transport property of organic semiconductor films, especially for planar small molecules. We analyze the surface-induced orientation of copper phthalocyannine (CuPc) molecules deposited on graphene or poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) covered indium-tin-oxide (PEDOT:PSS/ITO). The CuPc films deposited on graphene are templated with preferential face-on stacking, whereas the molecules on PEDOT:PSS/ITO crystallize with edge-on ordering. Static current–voltage measurement and small-signal impedance spectroscopy are combined to elucidate the structural impact on the electrical response when those films are part of a rectifying diode. The graphene-templated diode shows enhanced out-of-plane hole conduction as compared to the diode with a PEDOT:PSS/ITO contact. Equivalent circuits describing charge injection and transport properties are proposed.     

Metal salt modified PEDOT:PSS as anode buffer layer and its effect on power conversion efficiency of organic solar cells

The effects of metal chlorides such as LiCl, NaCl, CdCl₂ and CuCl₂ on optical transmittance, electrical conductivity as well as morphology of PEDOT:PSS films have been investigated. Transmittance spectra of spun PEDOT:PSS layers were improved by more than 6% to a maximum of 94% in LiCl doped PEDOT:PSS film. The surface of the PEDOT:PSS films has exhibited higher roughness associated with an increase in the electrical conductivity after doping with metal salts. The improvement in the physical properties of PEDOT:PSS as the hole transport layer proved to be key factors towards enhancing the P3HT:PCBM bulk heterojunction (BHJ) solar cells. These improvements include significantly improved power conversion efficiency with values as high as 6.82% associated with high fill factor (61%) and larger short circuit current density (∼18 mA cm⁻²).     

Thermoelectric Performance of Poly(3,4-Ethylenedioxy-thiophene)/Poly(Styrenesulfonate) Pellets and Films

The thermoelectric performance of poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS) pellets and free-standing PEDOT/PSS films, prepared from PEDOT/PSS solution containing the additives dimethyl sulfoxide or ethylene glycol, have been systematically investigated. It has been found that the electrical conductivity of free-standing PEDOT/PSS films is invariably much higher than that of PEDOT/PSS pellets, while there is no distinct change in the Seebeck coefficient. The highest electrical conductivity of a free-standing PEDOT/PSS film can be up to 300 S cm⁻¹, five to six times higher than that of PEDOT/PSS pellets (55 S cm⁻¹). The thermal conductivity was measured over a wide temperature range, indicating that PEDOT/PSS has extremely low thermal conductivity. The figure of merit (ZT) of free-standing PEDOT/PSS films with good environmental stability can be up to 10⁻², an order of magnitude higher than that of pressed PEDOT/PSS pellets (10⁻³).     

Simultaneous Increases in Electrical Conductivity and Seebeck Coefficient of PEDOT:PSS Films by Adding Ionic Liquids into a Polymer Solution

The electrical conductivity and Seebeck coefficient of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) films were simultaneously improved by adding an ionic liquid (IL) into a polymer solution of the polymers. The maximum electrical conductivity of such a PEDOT:PSS/IL film reached 174 S cm⁻¹, more than an order of magnitude higher than that of pure PEDOT:PSS film, and the maximum Seebeck coefficient was up to 30 μV K⁻¹, more than twice the value for pure PEDOT:PSS film. This behavior is different from conventional thermoelectric (TE) materials, whose TE properties are strongly correlated, such as increasing electrical conductivity with increasing carrier concentration, usually resulting in a logarithmic decrease in Seebeck coefficient. Atomic force microscopy images of the PEDOT:PSS/IL films indicated that the ILs induced formation of a particular three-dimensional structure of highly conducting PEDOT grains, resulting in improvement of the TE performance of PEDOT:PSS films.     

Organic Semiconductors for Thermoelectric Applications

The thermoelectric performance of thin films fabricated from two commercially available, highly conductive polymer formulations based on poly (3,4-ethylendioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) was investigated. In order to enhance the electrical conductivity, the high-boiling solvent dimethyl sulfoxide (DMSO) was added. By changing the content of DMSO the electrical conductivity was increased by a factor of two without changing the Seebeck coefficient or the thermal conductivity. We achieved ZT = 9.2 × 10⁻³ at room temperature upon the addition of 5 vol.% DMSO to the PEDOT:PSS formulation.     

Improvement of electrical conductivity of PEDOT:PSS films by 2-Methylimidazole post treatment

The electrical conductivity of poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) films was significantly improved without losing the optical transparency by treating the films with solution of 2-Methylimidazole in ethanol. The maximum electrical conductivity of such a thin film reached 930 S cm⁻¹, more than 1150 order of magnitude higher than that of pure PEDOT:PSS film. The mechanism of conductivity enhancement of treated thin PEDOT:PSS films was explored by atomic force microscopy (AFM) and UV/VIS spectrophotometer. The AFM scans show that the surface of the 2-Methylimidazole treated PEDOT:PSS layer is smoother than that of the pristine PEDOT:PSS thin film. Improvement in the morphology, electrical and optical properties of PEDOT:PSS films makes them highly suitable for numerous applications in optoelectronic devices.     

The application of oxidative chemical vapor deposited (oCVD) PEDOT to textured and non-planar photovoltaic device geometries for enhanced light trapping

In organic photovoltaics (OPVs), active layer thicknesses are limited by exciton diffusion length. Thus, non-planar surfaces and device architectures are desired for enhancing the light absorption in OPVs. The oxidative chemical vapor deposition (oCVD) process enables the formation of conformal films of conducting polymers on complex surface structures. oCVD poly(3,4 ethylenedioxythiophene) (PEDOT) is demonstrated to be compatible with a wide range of nano- to macro-scale textured and non-planar architectures that have been demonstrated to enhance light absorption in photovoltaics by various mechanisms of light trapping, such as lengthening optical pathways and taking advantage of reflective light bouncing. Here, conformal oCVD (PEDOT) layers are demonstrated over submicron features including submicron nanowedges and nanocones (with 70–100 nm groove depth and 100 nm pitch) and square gratings (50–350 nm groove depth and 139–833.3 nm pitch). In contrast, solution-applied PEDOT:PSS exhibits blanketing, thinning at the top of the features, and welling up of material in the bottom of the features, thus failing to conformally coat the complex surfaces. The application of oCVD PEDOT to macro-scale 3D OPV architectures was also explored. Enhanced active layer absorbance is shown using tetraphenyldibenzoperiflanthene (DBP) as the absorbing layer over the PEDOT.     

Enhanced Optical Property with Tunable Band Gap of Cross‐linked PEDOT Copolymers via Oxidative Chemical Vapor Deposition

Highoptical transmittance conjugated‐polymers with electrical conductivity are garnering much attention for the applications in organic optoelectronic devices including organic field‐effect‐transistors and solar cells. Polymers based on PEDOT are particularly promising candidates with high conductivity, uniform surface planarity and excellent ductility. In this work, homopolymer PEDOT deposited using oxidative chemical‐vapor‐deposition(oCVD) show the maximum conductivity of ≈3500 S/cm. However, their utility is limited due to the relatively low transmittance and abrupt decrease near the red edge in the visible regime. Here, the significantly improved optical properties achieved via tuning the bandgap of cross‐linked PEDOT copolymers using oCVD, offering a single‐step process for the synthesis and deposition of copolymer films, is reported. The cross‐linking monomers of biphenyl or anthracene are simultaneously evaporated with EDOT monomer and an oxidant(FeCl3) during the deposition. Poly(anthracene‐co‐EDOT)[p(ANTH‐co‐EDOT)] shows the superior transmittance (≈93%) to homopolymer PEDOT (≈80%) and poly(biphenyl‐co‐EDOT)[p(BPH‐co‐EDOT)] (≈88%). Additionally, copolymers show no transmission decay in the red edge regime unlike homopolymer PEDOT that presents an abrupt transmission falloff. An improvement in optical transmittance is in agreement with an increase in bandgap of materials (p(ANTH‐co‐EDOT), ≈2.3eV vs PEDOT, ≈1.8 eV). oCVD‐processed bandgap‐tunable PEDOT copolymers with enhanced transmittance may, therefore, have applications in organic optoelectronic devices that require high optical transparency.     

A facile chemical reduction approach for effectively tuning thermoelectric properties of PEDOT films

Poly(3,4-ethylenedioxythiophene)–tosylate–polyethylene glycol–polypropylene glycol–polyethylene glycol (PEDOT–Tos–PPP) films were prepared via a vapor phase polymerization (VPP) method. The films possess good electrical conductivity (1550 S cm⁻¹), low Seebeck coefficient (14.9 μV K⁻¹) and thermal conductivity (0.501 W m⁻¹ K⁻¹), and ZT ∼ 0.02 at room temperature (RT, 295 K). Then, the films were treated with NaBH₄/DMSO solutions of different NaBH₄ concentrations to adjust the redox level. After the NaBH₄/DMSO treatment (dedoping), the electrical conductivity of the films continuously decreased from 1550 to 5.7 S cm⁻¹, whereas the Seebeck coefficient steeply increased from 14.9 to 143.5 μV K⁻¹. A maximum power factor of 98.1 μW m⁻¹ K⁻² has been achieved at an optimum redox level. In addition, the thermal conductivity of the PEDOT–Tos–PPP films decrease from 0.501 to 0.451 W m⁻¹ K⁻¹ after treated with 0.04% NaBH₄/DMSO solution. A maximum ZT value of 0.064 has been achieved at RT. The electrical conductivity and thermal conductivity (Seebeck coefficient) of the untreated and 0.04% NaBH₄/DMSO treated PEDOT–Tos–PPP films decrease (increases) with increasing temperature from 295 to 385 K. And the power factor of the films monotonically increases with temperature. The ZT at 385 K of the 0.04% NaBH₄/DMSO treated film is 0.155.     

PEDOT electrodes for triboelectric generator devices

Triboelectric generators (TEGs) are devices that convert mechanical energy to electrical energy through triboelectric charging of different material surfaces at periodic contact. Typically, such devices consist of two dielectric contacting layers with electrodes attached on the non-contacting sides but alternatively, one material can simultaneously serve as both a contacting and an electrode material. In this work, we report the use of poly(3,4-ethylenedioxythiophene) (PEDOT) for TEG device were PEDOT film serves as both a contacting surface to PDMS and as an electrode. Two different PEDOT films were prepared on glass substrates by vapour-phase polymerization (VPP) and VPP combined with electropolymerization method and compared as TEG electrodes. Additionally, PEDOT/poly(1,6-hexanediol-co-citric acid) (PHC) composite films were prepared by using solution casting polymerization. These methods yielded PEDOT films with different morphology, surface roughness and conductivity. Best performance was demonstrated for the PEDOT film with the lowest surface roughness (1.88 nm RMS), prepared by VPP method, which generated peak current of 0.45 mA/m² and power density of 95 W/m², outperforming Sn doped In₂O₃ electrode approximately by threefold in the same experimental setup.     

Explaining the effects of processing on the electrical properties of PEDOT:PSS

By simultaneously measuring the Seebeck coefficient and the conductivity in differently processed PEDOT:PSS films, fundamental understanding is gained on how commonly used processing methods improve the conductivity of PEDOT:PSS. Use of a high boiling solvent (HBS) enhances the conductivity by 3 orders of magnitude, as is well-known. Simultaneously, the Seebeck coefficient S remains largely unaffected, which is shown to imply that the conductivity is improved by enhanced connectivity between PEDOT-rich filaments within the film, rather than by improved conductivity of the separate PEDOT filaments. Post-treatment of PEDOT:PSS films by washing with H₂SO₄ leads to a similarly enhanced conductivity and a significant reduction in the layer thickness. This reduction strikingly corresponds to the initial PSS ratio in the PEDOT:PSS films, which suggests removal and replacement of PSS in PEDOT:PSS by HSO₄⁻ or SO₄²⁻ after washing. Like for the HBS treatment, this improves the connectivity between PEDOT filaments. Depending on whether the H₂SO₄ treatment is or is not preceded by an HBS treatment also the intra-filament transport is affected. We show that by characterization of S and σ it is possible to obtain more fundamental understanding of the effects of processing on the (thermo)electrical characteristics of PEDOT:PSS.     

Effects of tetramethylene sulfone solvent additives on conductivity of PEDOT:PSS film and performance of polymer photovoltaic cells

A solvent additive in PEDOT:PSS solution is one of many methods to improve the conductivity of the poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) films. We explore a new type of the solvent additive, namely tetramethylene sulfone (TMS), for the fabrication of the PEDOT:PSS conductive layer in the ITO/PEDOT:PSS/P3HT:PCBM/TiO_x/Al polymer photovoltaic cells, in comparison to a more common dimethyl sulfoxide (DMSO) solvent additive. At optimal conditions, the TMS additive at 10 wt.% has been found to enhance the conductivity of pristine PEDOT:PSS films from 0.04 S/cm up to approximately 189 S/cm, compared with the highest conductivity for the case of the DMSO additive at 15 wt.% of 117 S/cm. Possible mechanisms of this conductivity enhancement, relating to the polymer conformation and the film morphology, have been investigated by Raman spectroscopy, X-ray diffraction, atomic force microscopy, and transmission electron microscopy. The performance of the polymer photovoltaic cells fabricated with the solvent additives PEDOT:PSS films follows a similar trend to the conductivity of the films as a function of the additive concentration. The additives mainly lead to greater short circuit current density (J_sc) of the photovoltaic cells. The highest power conversion efficiency (PCE) of 2.24% of the device has been obtained with the 10 wt.% TMS additive of, compared to the PCE of 1.48% for the standard device without solvent additive.     

Transverse charge transport in inkjet printed poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS)

Transverse (z) alignment of PEDOT grains was demonstrated in inkjet printed PEDOT:PSS. This explained the superior transverse charge conduction mode in inkjet printed PEDOT:PSS films, best fitted by the Efros-Shklovskii 1D-VRH (variable range hopping) model in this study compared with spin coated PEDOT:PSS films, which have demonstrated layers of generally in-plane aligned PEDOT:PSS grains. The findings of this study, regarding the microstructure of inkjet printed PEDOT:PSS films and their transverse charge transport model, justify measurements of the transverse conductivity of inkjet printed films in this study being 600 times higher than that of spin coated films. In addition, it was found that the addition of 5 wt% DMSO in the printing PEDOT:PSS ink lowers the workfunction by 3% approximately.     

Indium zinc oxide ohmic contact to poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) induced by UV light

In this research, we investigated the I–V characteristics of ITO/PEDOT:PSS/InZnO devices for two sets of samples. The first set is composed of PEDOT:PSS as-prepared, while the second set is composed of PEDOT:PSS irradiated by UV light source. We found that UV irradiation improves the electrical conductivity of the fabricated devices and yields to ohmic contact. Based on the UPS measurements, it was found that UV irradiation leads to an increase in the work function and the enhancement of electrical conductivity of PEDOT:PSS films. XPS and AFM measurements indicate that conformational changes of the PEDOT:PSS films are responsible for this behavior. We also studied the effect of storage on the electrical properties of our devices. No significant changes of electrical characteristics have been found after storing the devices for a period of 30 days.     

Air-stable polythiophene-based thin film transistors processed using oxidative chemical vapor deposition: Carrier transport and channel/metallization contact interface

Oxidative chemical-vapor-deposition (oCVD) provides a facile route to polymerize and deposit insoluble monomers in thin film form. Here, we report on oCVD polythiophene (PT)-based organic thin film transistors (OTFTs) that present both high mobility and excellent stability over time in air. The measured field effect mobility (μ_FE) is ∼0.02 cm²/V sec with the low threshold voltage between −1 V and 0.3 V. Additionally the PT OTFTs show no evidence of performance degradation after 3 months exposure in air. The transmission line model (TLM) enables the determination of the specific contact resistance (ρ_C) of oCVD PT channel/metallization interface and reveals that ρ_C is improved with increasing gate bias. The oCVD PT channel conductivity (σ_ch) and carrier density (p) were evaluated from more than 100 devices using TLM measurements and the relation of σ_ch = qpμ_FE. Carrier transport analysis suggests that the charge screening effect governs hole carrier mobility in the carrier density regime below approximately 10¹⁸/cm³ where an increase in carrier density leads to higher mobility. We also demonstrate photo-conductivity of oCVD PT through an increase in device on-state current and the field effect mobility when the PT OTFT is illuminated. Strategies to further enhance the performance of the materials and devices are also suggested.     

Effects of thin film processing on pentacene/C60 bilayer solar cell performance

Solar cells based on pentacene/C₆₀ bilayer heterojunctions have been fabricated with a structure of ITO/poly(styrenesulfonate) (PEDOT:PSS)/pentacene (40 nm)/fullerene (C₆₀)(40 nm)/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) (10 nm)/Al. The effect of pentacene crystalline domain size on performance was investigated by controlling the pentacene deposition rate. The devices show improved light-to-electricity conversion efficiencies from 0.49% to 1.12% under an AM 1.5 solar simulator (100 mW/cm²), when the pentacene evaporation rate is in a range of 5 Å/s–0.5 Å/s. Atomic force microscopy (AFM) measurements show that the pentacene films deposited by a slow evaporation rate have larger crystalline domains and a fewer amorphous domains, compared to films obtained by faster evaporation rates. Upon thermal annealing at 200 °C for 1 min, there is merging of pentacene crystalline domains. These changes in film morphology impact the charge separation at the donor/acceptor interface and the hole and electron mobilities, and hence, directly affect the device performance.     

Effect of drying temperature on structural and electrical properties of PANI:PVDF composite thin films and their application as buffer layer for organic solar cells

Spin coated polyaniline:polyvinylidene-fluoride (PANI: PVDF) composites films were prepared by the optimized mass ratios of 5:95. The effect of drying temperature on the structure, morphology and electrical conductivity of PANI: PVDF films were studied, and their use as buffer layer for organic solar cells performance was explored. The PANI: PVDF film which dried at 90 °C exhibit higher β-phase content in PVDF (F_β=94%). Furthermore, solar cells conversion efficiency (η=3.06%) is better when using ITO/PVDF:PANI (dried at 90 °C) as photo-anode than that containing only ITO (η=0.96%) and ITO/PEDOT:PSS (η=1.78%).     

Harnessing Selective Exsolution of Sn Metal to Enhance Electrical Conductivity in Oxygen-Deficient Perovskite Stannates

The versatile application of newly discovered oxide semiconductors calls for developing a simple process to generate conducting carriers. High-temperature reduction treatment leads to electrical conduction in perovskite stannate semiconductors, but carrier concentration is poorly controlled and inconsistently reported in BaSnO_3−δ films after the reduction process so far. Here, a new strategy to enhance the electrical conductivity of BaSnO_3−δ films is demonstrated by exploiting selective exsolution of Sn metals in the perovskite framework. Due to strong dependence of conductivity on initial Sn/Ba cation ratio in the reduced BaSnO_3−δ films, interestingly, only Sn-excess BaSnO_3−δ films show a dramatic increase of carrier concentration ( ∆ n_3D  = 5–7 × 10¹⁹ cm⁻³) after high-temperature reduction; exceptionally high electrical conductivity (σ  ≈  6000 S cm⁻¹) is achieved in reduced Sn-excess (La, Ba)SnO_3−δ films, which exceed full activation of La dopants in untreated (La, Ba)SnO₃. By multiple characterizations combined with theoretical calculation, it is disclosed that a small fraction of segregated β-Sn nanoparticles is likely to contribute the additional source of n_3D in the BaSnO_3−δ matrix as a result of spontaneous charge transfer from the segregated β-Sn metallic phase to BaSnO_3−δ. These original results propose a simple strategy to further increase electrical conductivity in perovskite oxide semiconductors by non-stoichiometry-driven metal exsolution.     

Thermoelectric Properties of In<Subscript>0.3</Subscript>Ga<Subscript>0.7</Subscript>N Alloys

We report on the experimental investigation of the potential of InGaN alloys as thermoelectric (TE) materials. We have grown undoped and Si-doped In_0.3Ga_0.7N alloys by metalorganic chemical vapor deposition and measured the Seebeck coefficient and electrical conductivity of the grown films with the aim of maximizing the power factor (P). It was found that P decreases as electron concentration (n) increases. The maximum value for P was found to be 7.3 × 10⁻⁴ W/m K² at 750 K in an undoped sample with corresponding values of Seebeck coefficient and electrical conductivity of 280 μV/K and 93␣(Ω cm)⁻¹, respectively. Further enhancement in P is expected by improving the InGaN material quality and conductivity control by reducing background electron concentration.