Isopropanol-treated PEDOT:PSS as electron transport layer in polymer solar cells

Isopropanol (IPA)-treated poly(3,4-ethylenedioxithiophene):poly(styrene sulfonate) (PEDOT:PSS) was applied as a new electron transport layer (ETL) in P3HT:PCBM bulk heterojunction polymer solar cell (BHJ-PSC) devices for the first time, revealing the electron transport property of IPA-treated PEDOT:PSS in sharp contrast to the well known hole transport property of the untreated PEDOT:PSS. Under the optimized condition for incorporating PEDOT:PSS ETL, the power conversion efficiency (PCE) of the ITO/untreated PEDOT:PSS (HTL)/P3HT:PCBM/IPA-treated PEDOT:PSS (ETL)/Al device (3.09%) is quite comparable to that of the reference ITO/untreated PEDOT:PSS (HTL)/P3HT:PCBM/Al device without any ETL (3.06%), and an annealing treatment of PEDOT:PSS ETL at 120 °C for 10 min led to a PCE of 3.25%, which even slightly surpasses that of the reference device, revealing the electron transport property of IPA-treated PEDOT:PSS. The electron transport property of IPA-treated PEDOT:PSS is interpreted by the lowering of the work function of PEDOT:PSS upon IPA treatment and incorporation as ETL as probed by scanning Kelvin probe microscopy (SKPM).     

Stable organic photovoltaic with PEDOT:PSS and MoOX mixture anode interfacial layer without encapsulation

Herein, we report about an efficient and stable organic photovoltaic that uses a poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS) and molybdenum oxide (MoO_X) mixture for the anode interfacial layer, and that can reach 4.43% power conversion efficiency (PCE) under AM1.5 conditions. Utilizing PEDOT:PSS:MoO_X (1:1), the shelf lifetime of poly(3-hexylthiophene) (P3HT), and indene-C₆₀ bisadduct (ICBA)-based solar cells without encapsulation, can be realized with only a 25% deterioration after 672 h of storage in air. Furthermore, we compare the photovoltaic performance of the P3HT:ICBA-based organic photovoltaic with PEDOT:PSS, and PEDOT:PSS:MoO_X, in which PEDOT:PSS:MoO_X has outperformed the others. In addition, the water vapor transmission rate of PEDOT:PSS:MoO_X is 0.17 gm/(m² day), which is much less than that of PEDOT:PSS.     

High efficiency bulk heterojunction organic solar cell by using high conductivity modified PEDOT: PSS as a buffer layer

In this paper,bulk heterojunction solar cells with poly-(3-hexylthiophene)(P3HT):[6,6]-phenyl-C61-butyric-acid-methylester(PCBM) as an active layer and modified poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)(PEDOT:PSS) as a buffer layer are fabricated.The buffer layer is modified by adding 1% to 5% dimethyl sulfoxide(DMSO) into PEDOT:PSS solution before spin-coating.The conductivity of modified PEDOT:PSS and the performance of solar cells with modified PEDOT:PSS are measured.The highest conductivity of modified PEDOT:PSS with 4% DMSO can achieve 89.693 S/cm.The performance of organic solar cell with PEDOT:PSS modified by 4% DMSO is the best.The 4% DMSOmodified-PEDOT:PSS cell has a power conversion efficiency of 3.34%,V oc of 5.7 V,J sc of 14.56 mA/cm 2 and filling factor(FF) of 40.34%.     

Ionic-Liquid Induced Morphology Tuning of PEDOT:PSS for High-Performance Organic Electrochemical Transistors

The ability to operate in aqueous environments makes poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate), PEDOT:PSS, based organic electrochemical transistors (OECTs) excellent candidates for a variety of biological applications. Current research in PEDOT:PSS based OECTs is primarily focused on improving the conductivity of PEDOT:PSS film to achieve high transconductance (g_m). The improved conductivity and electronic transport are attributed to the formation of enlarged PEDOT-rich domains and shorter PEDOT stacking, but such a change in morphology sacrifices the ionic transport and, therefore, the doping/de-doping process. Additionally, little is known about the effect of such morphology changes on the gate bias that makes the maximum g_m ($V_G^{Peak}$), threshold voltage (V_T), and transient behavior of PEDOT:PSS based OECTs. Here, the molecular packing and nanostructure of PEDOT:PSS films are tuned using ionic liquids as additives, namely, 1-Ethyl-3-methylimidazolium (EMIM) as cation and anions of chloride (Cl), trifluoromethanesulfonate (OTF), bis(trifluoromethylsulfonyl)imide (TFSI), and tricyanomethanide (TCM). It is demonstrated that an optimal morphology is realized using EMIM OTF ionic liquids that generate smaller fibril-like PEDOT-rich domains with relatively loose structures. Such optimal morphology improves ion accessibility, lowering the gate bias required to completely de-dope the channel, and thus enabling to achieve high transconductance, fast transient response, and at lower gate bias window simultaneously.     

The effect of solvent treatment on the buried PEDOT:PSS layer

Solvent treatment has been widely used to improve the device performance of both Organic Light Emitting Diodes (OLEDs) and Polymer Solar Cells (PSCs). One of the proposed mechanisms is the modification of the buried PEDOT:PSS layer underneath the organic active layer by the permeating solvent. By measuring the lateral electric conductivity of the PEDOT:PSS layer, the 3 orders of magnitude's enhancement on the conductivity after solvent treatment confirms that the solvent permeates through the top organic active layer and modifies the PEDOT:PSS layer. Using a “peel-off” method, the buried PEDOT:PSS layer is fully exposed and studied by UV–vis spectra, XPS spectra, and c-AFM images. The data suggest that the permeating solvent dissolves PSS, changes PEDOT:PSS′ core-shell structure into a linear/coiled structure, and moves PSS from the bulk to the surface. As a result, PEDOT becomes more continuous in the bulk. The continuous conducting PEDOT-rich domains create percolating pathways for the current which significantly improve electric conductivity.     

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.     

Directional dependence of electron blocking in PEDOT:PSS

The directional dependence of electron blocking by poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is investigated in organic photovoltaic devices. In a conventional OPV architecture we find that a doped interlayer forms between poly(3-hexylthiophene) (P3HT) and the PSS-rich top layer of spin-coated PEDOT:PSS films. In an inverted OPV architecture, we find no mixing between PEDOT:PSS and P3HT, which is due to the lower concentration of PSS in bulk PEDOT:PSS than is found in the PSS-rich top layer. Through electrical measurements of conventional and inverted photovoltaic devices we show that the interlayer is necessary for PEDOT:PSS to be electron blocking. This result implies that PEDOT:PSS is not intrinsically electron blocking and that its directional anisotropy must be considered when comparing the advantages and disadvantages of conventional and inverted architecture photovoltaic devices.     

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).     

ITO-free organic solar cells using highly conductive phenol-treated PEDOT:PSS anodes

Phenol as one of the most polar solvent was used to enhance the conductivity of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) films. The conductivity of PEDOT:PSS films improved to 1193 S/cm after treatment with phenol vapor and 1054 S/cm after treatment with phenol drop. The treated films also showed high transmittance in the visible region which is one of the crucial factors for optoelectronic devices such as organic solar cells and light emitting diodes. The mechanism of conductivity enhancement of treated thin PEDOT:PSS films was investigated by atomic force microscopy (AFM) and UV/Vis spectrophotometer. The AFM images showed that the ratio of PEDOT to PSS at top most of the surface was increased for treated film. Rearrangement of PEDOT segment throughout the film and hence conformational changes are the reasons for enhancement of conductivity. The modified PEDOT:PSS films were used as electrode for ITO-free organic solar cells (OSCs). These ITO-free OSCs showed almost equal operation to those for ITO electrodes.     

Highly Conductive PEDOT:PSS Electrode with Optimized Solvent and Thermal Post‐Treatment for ITO‐Free Organic Solar Cells

Highly conductive poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) films as stand‐alone electrodes for organic solar cells have been optimized using a solvent post‐treatment method. The treated PEDOT:PSS films show enhanced conductivities up to 1418 S cm^?1, accompanied by structural and chemical changes. The effect of the solvent treatment on PEDOT:PSS has been investigated in detail and is shown to cause a reduction of insulating PSS in the conductive polymer layer. Using these optimized electrodes, ITO‐free, small molecule organic solar cells with a zinc phthalocyanine (ZnPc):fullerene C₆₀ bulk heterojunction have been produced on glass and PET substrates. The system was further improved by pre‐heating the PEDOT:PSS electrodes, which enhanced the power conversion efficiency to the values obtained for solar cells on ITO electrodes. The results show that optimized PEDOT:PSS with solvent and thermal post‐treatment can be a very promising electrode material for highly efficient flexible ITO‐free organic solar cells.     

An ammonia modified PEDOT: PSS for interfacial engineering in inverted planar perovskite solar cells

Poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS) is one of the most widely used hole transport layers (HTL) in inverted perovskite solar cells (PSCs) due to its simple solution-processed ability, high transparency, and conductivity. However, PEDOT:PSS-based devices suffer a lower open-circuit voltage (V_oc) than devices with the conventional structure. To address this issue, we fabricated ammonia-modified PEDOT:PSS films by simply doping PEDOT:PSS solution with different ratio of ammonia. The acidity of PEDOT:PSS can be neutralized by the doped ammonia, which inhibits the ion-exchange reaction between PSS-H and CH₃NH₃I, thus retarding the reduction of the work function for PEDOT:PSS to some extent. As a result, a superior power conversion efficiency (PCE) of 15.5% was obtained for the device based on the ammonia-doped PEDOT:PSS HTL than that of the pristine PEDOT:PSS-based device. We ascribe the PCE enhancement to the increased V_oc and fill factor (FF), which is attributed not only to the better energy-level alignment between the ammonia-modified PEDOT:PSS film and perovskite layer but also to the increased grain size and crystallinity of perovskite film.     

Tuning PEDOT:PSS conductivity with iron oxidants

Controlling the electrical properties of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is crucial for its use in a wide range of energy and sensing applications. We have polymerized PEDOT:PSS using a new iron oxidant, hemin, and compared the resulting polymer to PEDOT:PSS polymerized with the iron oxidant, FeCl₃. We characterize these polymers with five different techniques: visible and near IR spectroscopy, Fourier transform infrared spectroscopy, electron spin resonance spectroscopy, four point probe conductivity measurements, and X-ray photoelectron spectroscopy. Although the elemental composition of both polymers is nearly identical, hemin-oxidized PEDOT:PSS is six orders of magnitude more conductive than FeCl₃-oxidized PEDOT:PSS. This difference is associated with a change in oxidation state of the polymer. In hemin-oxidized PEDOT:PSS, bipolarons are the dominant charge carrier species. In FeCl₃-oxidized PEDOT:PSS, polarons dominate. These results demonstrate that the properties of PEDOT:PSS can be controlled in a single step aqueous reaction by the choice of iron oxidant used for polymerization.     

Piezoresistive properties of PEDOT:PSS

Recent market studies mention the necessity to include sensors in the design of organic electronic devices in order to broaden the range of applications. It is therefore essential to identify potential organic mechanical sensor materials and to develop processes and methods to structure them and characterize their piezoresistive properties. Furthermore, it is also essential for organic electronic devices to know the change of resistance upon bending of flexible substrates. A material widely used in organic electronics is the complex of the intrinsically conductive polymer poly(3,4-ethylenedioxythiophene) and polystyrene sulfonate acid (PEDOT/PSS). In this paper first the fabrication of a polyimide (PI) membrane with integrated PEDOT/PSS strain gauges is presented. Upon a pressure difference the membrane is deflected and the resulting changes in resistance of the sensor elements are recorded. By applying a membrane mechanics model the resistance changes can be linked to the strain in the membrane and then the plane strain gauge factor k_PS for PEDOT/PSS of 0.48±0.07 at 36.6±3% rH can be determined.     

Enhanced performance in inverted polymer solar cells via solution process: Morphology controlling of PEDOT:PSS as anode buffer layer by adding surfactants

Inverted polymer solar cells were fabricated by adding the amphiphilic surfactant ‘Surfynol 104 series’ to Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as a anode buffer layer by solution process. With the introduction of Surfynol 104 series-added PEDOT:PSS, it was able to form a homogeneous film by adjusting the wettability of a hydrophobic poly(3-hexylthiophene) (P3HT):[6,6]-phenyl C₆₁-butyric acid methyl ester (PCBM) film. With decrease in series resistance (R_S) and increase in shunt resistance (R_SH), as a result, the short circuit current density (J_SC), open circuit voltage (V_OC) and fill factor (FF) of the optimized device were 10.2 mA/cm², 0.63 V and 61.3%, respectively, calculated the power conversion efficiency (PCE) was 4.0%. In addition, the air stability of the fabricated device was improved.     

Simultaneous enhancement of the efficiency and stability of organic solar cells using PEDOT:PSS grafted with a PEGME buffer layer

We report a simple processing method to simultaneously improve the efficiency and stability of organic solar cells (OSCs). Poly(4-styrene sulfonate)-doped poly(3,4-ethylenedioxy-thiophene (PEDOT:PSS), widely used as hole transport layer (HTL) in OSCs, tends to accelerate the degradation of devices because of its hygroscopic and acidic properties. In this regard, we have modified PEDOT:PSS to reduce its hygroscopic and acidic properties through a condensation reaction between PEDOT:PSS and poly(ethylene glycol) methyl ether (PEGME) in order to improve the efficiency and stability of OSCs. As a result, the power conversion efficiency (PCE) increased by 21%, from 2.57% up to 3.11%. A better energy level alignment by the reduced work function of the modified PEDOT:PSS with a highest occupied molecular orbital (HOMO) level of poly(3-hexylthiophene-2,5-diyl) (P3HT) is considered the origin of the improved the efficiency. The half-life of OSCs with PEDOT:PSS modified with PEGME buffer layer also increased up to 3.5 times compared to that of devices with pristine PEDOT:PSS buffer layer.     

Enhancement of organic solar cell efficiency by patterning the PEDOT:PSS hole transport layer using nanoimprint lithography

In order to improve the conversion efficiency of organic photovoltaic (OPV) cells, nano-patterned poly (3,4-ethylenedioxythiophene) poly (styrenesulfonate) (PEDOT:PSS) was used as a hole transfer layer (HTL). Using nanoimprint lithography, a process that is easily applied to large-area substrates, a spherical array of PEDOT:PSS droplets was formed. The effect of the PEDOT:PSS nanostructure was characterized by optical and electrical measurements. Because the hemispherical array of PEDOT:PSS scatters light efficiently, absorption of the incident light increases when the nanostructured layer is employed. The conversion efficiency of the nano-patterned OPV cells is 25% larger than that of non-patterned OPV cells, due to the increase in short-circuit current (J_sc).     

Highly Conductive and Transparent PEDOT:PSS Films with a Fluorosurfactant for Stretchable and Flexible Transparent Electrodes

Highly conductive and transparent poly‐(3,4‐ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS) films, incorporating a fluorosurfactant as an additive, have been prepared for stretchable and transparent electrodes. The fluorosurfactant‐treated PEDOT:PSS films show a 35% improvement in sheet resistance (R_s) compared to untreated films. In addition, the fluorosurfactant renders PEDOT:PSS solutions amenable for deposition on hydrophobic surfaces, including pre‐deposited, annealed films of PEDOT:PSS (enabling the deposition of thick, highly conductive, multilayer films) and stretchable poly(dimethylsiloxane) (PDMS) substrates (enabling stretchable electronics). Four‐layer PEDOT:PSS films have an R_s of 46 Ω per square with 82% transmittance (at 550 nm). These films, deposited on a pre‐strained PDMS substrate and buckled, are shown to be reversibly stretchable, with no change to R_s, during the course of over 5000 cycles of 0 to 10% strain. Using the multilayer PEDOT:PSS films as anodes, indium tin oxide (ITO)‐free organic photovoltaics are prepared and shown to have power conversion efficiencies comparable to that of devices with ITO as the anode. These results show that these highly conductive PEDOT:PSS films can not only be used as transparent electrodes in novel devices (where ITO cannot be used), such as stretchable OPVs, but also have the potential to replace ITO in conventional devices.     

Photovoltaic Devices: Highly Conductive PEDOT:PSS Electrode with Optimized Solvent and Thermal Post‐Treatment for ITO‐Free Organic Solar Cells (Adv. Funct. Mater. 6/2011)

Highly conductive poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) films as stand‐alone electrodes for organic solar cells have been optimized using a solvent post‐treatment method. The treated PEDOT:PSS films show enhanced conductivities up to 1418 S cm^?1, accompanied by structural and chemical changes. The effect of the solvent treatment on PEDOT:PSS has been investigated in detail and is shown to cause a reduction of insulating PSS in the conductive polymer layer. Using these optimized electrodes, ITO‐free, small molecule organic solar cells with a zinc phthalocyanine (ZnPc):fullerene C₆₀ bulk heterojunction have been produced on glass and PET substrates. The system was further improved by pre‐heating the PEDOT:PSS electrodes, which enhanced the power conversion efficiency to the values obtained for solar cells on ITO electrodes. The results show that optimized PEDOT:PSS with solvent and thermal post‐treatment can be a very promising electrode material for highly efficient flexible ITO‐free organic solar cells.     

Hybrid photovoltaic structures based on amorphous silicon and P3HT:PCBM/PEDOT:PSS polymer semiconductors

Hybrid thin film photovoltaic structures, based on hydrogenated silicon (Si:H), organic poly(3-hexythiophene):methano-fullerenephenyl-C61-butyric-acid-methyl-ester (P3HT:PCBM) and poly(3,4ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) films, have been fabricated. Organic semiconductor thin films were deposited by spin-coating technique and were exposed to radio frequency plasma enhanced chemical vapor deposition (RF PECVD) of Si:H films at deposition temperature T_d = 160 °C. Different types of structures have been investigated: H1) ITO/(p)SiC:H /P3HT:PCBM/(n) Si:H, H2) ITO/PEDOT:PSS/(i)Si:H/(n) Si:H and H3) ITO/PEDOT:PSS/P3HT:PCBM/(i)Si:H/(n)Si:H. Short circuit current density spectral response and current-voltage characteristics were measured for diagnostic of the photovoltaic performance. The current density spectral dependence of hybrid structures which contains organic layers showed improved response (50–80%) in high photon energy range (hν ≈ 3.1–3.5 eV) in comparison with Si:H reference structure. An adjustment in the absorbing layer thickness and in the contact material for ITO/PEDOT:PSS/(i)Si:H/(n)Si:H structure, resulted in a remarkably high short circuit current density (as large as 17.74 mA/cm²), an open circuit voltage of 640 mV and an efficiency of 3.75%.