Aging process of PEDOT:PSS dispersion and robust recovery of aged PEDOT:PSS as a hole transport layer for organic solar cells

Conducting p-type polymer of poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) has been widely used for organic optoelectronics, particularly as a hole transport layer for organic solar cells. While the aged PEDOT:PSS dispersion impacts device performance, the aging of PEDOT:PSS dispersion have not been well investigated. Moreover, the recovery process of aged (two-year-old) PEDOT:PSS dispersion has not been demonstrated yet. Herein, it is found that aqueous PEDOT:PSS dispersion undergoes extensive phase separation during the aging process, resulting in both nanoscale and macroscale hydrophobic PEDOT-rich agglomerates. When the aged PEDOT:PSS thin film is integrated into P3HT:PCBM organic solar cells, the PEDOT-rich agglomerates trap the photogenerated holes at the PEDOT:PSS/P3HT interface, resulting in poor extraction efficiency in organic solar cells. To recover a hole transport functionality from aged PEDOT:PSS, three different solvents such as isopropyl alcohol (C₃H₇OH), ethanol (C₂H₅OH) and methanol (CH₃OH) are investigated. Among them, it is found that isopropyl alcohol (IPA) yielded very uniform PEDOT:PSS thin film layer. This is because hydrophobic functional groups of IPA solvent facilitated the preferential solvation of phase separated hydrophobic PEDOT-rich agglomerates. However, when non-optimal concentration of IPA solvents was added into the aged PEDOT:PSS dispersion, the size of PEDOT-rich agglomerates was adversely enlarged. When organic solar cells were fabricated using more than a two-year-old PEDOT:PSS that was treated with IPA solvent, the resulting device performance of organic solar cells was fully recovered and became comparable or better than that of organic solar cells fabricated with fresh PEDOT:PSS.     

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.     

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).     

Preparation and thermoelectric properties of PEDOT:PSS coated Te nanorod/PEDOT:PSS composite films

PEDOT:PSS coated Te (PCTe) nanorod/PEDOT:PSS composite films were prepared by a drop-casting technique. H₂SO₄ treatment was employed to enhance thermoelectric (TE) properties of the composite films. The addition of PCTe nanorods increased both the electrical conductivity and the Seebeck coefficient of the composite films. An optimized power factor of 141.9 μW/mK² was obtained for the film containing 90 wt% PCTe nanorods treated with 12 M H₂SO₄ at room temperature, which was 2.75 times as high as that of the untreated composite film, corresponding to the electrical conductivity and Seebeck coefficient of 204.6 S/cm and 83.27 μV/K, respectively. XPS and GIWAXS analysis revealed the removal of insulating PSS units and the rearrangement of PEDOT chains after the H₂SO₄ treatment. Finally, a 9-leg TE generator prototype was fabricated using the optimized composite film. The maximum output power and area output power density produced from the prototype were 47.7 nW and 57.2 μW/cm², respectively, at the temperature difference of 40 K.     

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).     

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.     

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).     

In-situ modification of PEDOT:PSS work function using alkyl alcohols as secondary processing solvents and their impact on merocyanine based bulk heterojunction solar cells

The influence of a series of alkyl alcohols on the work function of PEDOT:PSS thin films is systematically investigated by Kelvin probe measurements. We show that the PEDOT:PSS work function can be increased stepwise from 5.2 eV for pristine PEDOT:PSS to 5.61 eV using either alcohols with different alkyl chain length or varying the amount of alcohol in mixtures with chlorobenzene. Moreover, we demonstrate the effect of work function modification on merocyanine based bulk heterojunction solar cells, resulting in improved values for the open-circuit voltage comparable to those obtained with high work function MoO₃. Thus, the processing method presented herein can potentially serve as a simple, alternative route to adjustable and high work function electrodes while maintaining processability from solution.     

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.     

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.     

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.     

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 performance inverted perovskite solar cells using PEDOT:PSS/KCl hybrid hole transporting layer

PEDOT:PSS is one of the most widely used hole transporting layer for inverted perovskite solar cells. Yet the performances of the corresponding perovskite solar cells are not satisfactory. Here, we demonstrate that KCl modified PEDOT:PSS film can promote the crystallization of perovskite film and enlarge the perovskite crystals. At the same time, KCl can diffuse into the perovskite film and effectively passivate the defects. As a result, inverted perovskite solar cells fabricated on 10 mg mL⁻¹ PEDOT:PSS/KCl films exhibit an average power conversion efficiency of 16.24 %, which is enhanced by 17.77 % compared with the reference perovskite solar cells. Open circuit voltage of 1.009 V and power conversion efficiency of 17.09 % have also been demonstrated using the optimized 10 mg mL⁻¹ PEDOT:PSS/KCl films.     

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.     

The enhancement of electrical and optical properties of PEDOT:PSS using one-step dynamic etching for flexible application

The conductivity enhancement of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) by dynamic etching process was investigated to introduce the outstanding and simplest method for soft electronics. Four different samples which were pristine PEDOT:PSS, PEDOT:PSS doped with 5 wt.% DMSO, PEDOT:PSS with dipping process, and PEDOT:PSS with dynamic etching process were prepared to compare the properties such as conductivity, morphology, relative atomic percentage, and topography. All samples were characterized by four point probe, current atomic force microscopy (C-AFM), X-ray photoelectron spectroscopy (XPS), and UV–visible spectroscopy. The conductivity of the sample with dynamic etching process showed the highest value as 1299 S/cm among four samples. We proved that the dynamic etching process is superior to remove PSS phase from PEDOT:PSS film, to flow strong current through entire surface of PEDOT:PSS, and to show the smoothest surface (RMS 2.28 nm). XPS analysis was conducted for accurate chemical and structural surface environments of four samples and the relative atomic percentage of PEDOT in the sample with dynamic etching was the highest as 29.5%. The device performance of the sample with the dynamic etching process was outstanding as 10.31 mA/cm² of J_sc, 0.75 eV of V_oc, 0.46 of FF, and 3.53% of PCE. All properties and the device performance for PEDOT:PSS film by dynamic etching process were the most excellent among the samples.     

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%.     

High-performance hole transport layer based on WS2 doped PEDOT:PSS for organic solar cells

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