ITO-free flexible organic solar cells with printed current collecting grids

The presence of a transparent conductive electrode such as indium tin oxide (ITO) limits the reliability and cost price of organic photovoltaic devices as it is brittle and expensive. Moreover, the relative high sheet resistance of an ITO electrode on flexible substrates limits the maximum width of a single cell. We have developed an alternative ITO-free transparent anode, based on solution processed high conductive PEDOT:PSS in combination with a printed current collecting grid. The screen printed silver grid demonstrates a typical sheet resistance of 1 Ω/□ with 6.4-8% surface coverage. The efficiency of a flexible device with an active area of 4 cm² with such a grid is much higher than a similar device based on ITO. Furthermore, as this composite anode is solution-processed, it is a step forward towards low-cost large area processing.     

Mechanical integrity of flexible InZnO/Ag/InZnO multilayer electrodes grown by continuous roll-to-roll sputtering

The mechanical integrity of a flexible ZnO-doped In₂O₃ (IZO)/Ag/IZO multilayer electrode deposited onto a flexible PET substrate using a continuous roll-to-roll (R2R) sputtering system was investigated by means of outer/inner bending, twisting and stretching tests. The R2R-sputtered flexible IZO (40 nm)/Ag (12 nm)/IZO (40 nm) electrode with a sheet resistance of 5.37 Ω/sq and an optical transmittance of 87.3% exhibited superior flexibility due to existence of a ductile Ag layer with high strain failure. It was noteworthy that the failure bending radius (5.5 mm) of the inner bending test was lower than that (6.5 mm) of the outer bending test due to complete separation of the cracks on the top IZO layer during the outer bending test. In addition, the twisting test showed that the resistance of the flexible IZO/Ag/IZO electrode began to increase at an angle of 26°. Furthermore the stretching test demonstrated that the strain failure of the IZO/Ag/IZO multilayer was 2.4%, which is higher than those of conventional flexible ITO electrodes. The mechanical integrity of the R2R-sputtered IZO/Ag/IZO multilayer indicated that hybridization of an oxide and Ag metal is a promising flexible electrode scheme for next generation flexible organic solar cells.     

Patternable solution process for fabrication of flexible polymer solar cells using PDMS

PH 500, a highly conducting poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS), is a typical conducting polymer anode material used in organic electric devices. However, it has the disadvantages of low conductivity and poor surface roughness and requires a patterning method for the electrode through including the laser and plasma. In this paper, therefore, the conducting polymer ink for a transparent anode was formulated by adding dimethyl sulfoxide (DMSO) and BYK-333 as the surfactant to enhance the conductivity and surface roughness. The conducting polymer anode was patterned through the application of a new patterning method that used polydimethylsiloxane (PDMS) on a flexible substrate. In addition, a photoactive layer was formed by applying the new patterning method to the conventional brush painting method in which patterning had previously been impossible. The resulting material was compared with the device fabricated by the spin coating method. The fabricated flexible polymer solar cells (PSCs) exhibited short-circuit current density (J_sc), open-circuit voltage (V_oc), fill factor (FF) and power conversion efficiency (PCE) values of 4.2 mA/cm², 0.878 V, 26.5% and 0.98%, respectively, which represented an efficiency improvement of 38% over those fabricated by the spin coating method. Meanwhile, the J_sc value was increased when the series resistance (R_s) decreased to 150 Ω cm².     

Organic solar cells with 2-Thenylmercaptan/AU self-assembly film as buffer layer

The interface between an electrode and the organic active layer is an important factor in organic solar cells (OSCs) that influences the power conversion efficiency (PCE). In this report, a buffer layer of 2-thenylmercaptan/Au self-assembly film is introduced into OSCs as a substitute for the poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT: PSS) layer. The electrode/active layer interface is meliorated by Au-S coordinate bond of self assembly after applying this buffer layer. The series resistance reduces from 20 Ω cm² in a device based on PEDOT:PSS to 10.2 Ω cm². Correspondingly, the fill factor (FF) increases from 0.50 to 0.64. Moreover, due to the dipole of this self-assembled layer, the open circuit voltage (V_oc) also increases slightly from 0.54 V to 0.56 V and the PCE reaches 2.5%.     

Preparation of flexible conductive composite electrode film of PEDOT:PSS/Aramid nanofibers via vacuum-assisted filtration and acid post-treatment for efficient solid-state supercapacitor

A highly conductive and flexible composite film of Poly (3,4-ethylenedioxythiophene):polystyrene sulfonate and aramid nanofiber (PEDOT:PSS/ANFs) is prepared by vacuum-assisted filtration and post-treated by acid. The composite displays excellent mechanical integrity under bending together with flexibility, properties being attributed to the strong attachment of PEDOT:PSS onto the surface of the ANFs via hydrogen bonding and the ANF structure, respectively. The conductivity of the prepared composite is progressively enhanced by the post-treatment using sulfuric acids (1 M H₂SO₄ and 1.5 M H₂SO₄), reaching 20–25 times higher than that of untreated film. This enhancement is traced to the removal of the insulating PSS group together with an analyzable change in crystallization of the PEDOT:PSS component. However, excessive use of acid treatment is seen to reduce the mechanical strength, and, thus, ultimate loss of conductivity after frequent bending (up to 1000 times), only having 59% conductivity retention with high concentration acid treatment (1.5 M H₂SO₄) compared to a high conductivity retention of 95% with 1.0 M H₂SO_4. Adopting the relatively weaker acid enables a balance to be reached between these crucial factors of electrical conductivity versus mechanical integrity. The prepared film of PEDOT: PSS/ANFs treated by acid as an electrode of supercapacitor shows good electrochemical performances, including good volumetric specific capacitance (83.5 F/cm³ with 1.0 M H₂SO₄ and 75 F/cm³ with 1.5 M H₂SO₄ at 0.5 A/cm^?3), cycle stability and capacitance retention of 83.3% and 87.5% after 2000 cycles, respectively. Furthermore, a solid flexible supercapacitor is finally assembled by the post-treatment of relatively low concentration acid with 1.0 M H₂SO_4. The configured supercapacitor displays excellent volumetric energy density of 23.44 mW h/cm² (power density of 399.95 mW/cm²) at a very wide operating potential window of 0–1.6 V and cycle stability. Therefore, it is quite feasible method to fabricate a highly conductive and flexible composite film using PEDOT:PSS and ANFs by vacuum filtration and acid post-treatment, which expects to be a promising flexible composite electrode material applied in the preparation of energy storage devices.     

Improved the efficiency of small molecule organic solar cell by double anode buffer layers

Small molecule organic solar cell with an optimized hybrid planar-mixed molecular heterojunction (PM-HJ) structure of indium tin oxide (ITO)/ poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT: PSS) doped with 4 wt% sorbitol/ pentacene (2 nm)/ copper phthalocyanine (CuPc) (10 nm)/ CuPc: C₆₀ mixed (20 nm)/ fullerene (C₆₀) (20 nm)/ bathocuproine (BCP) (10 nm)/Al was fabricated. PEDOT: PSS layer doped with 4 wt% sorbitol and pentacene layer were used as interlayers between the ITO anode and CuPc layer to help the hole transport. And then the short-circuit current (J_sc) of solar cell was enhanced by inserting both the PEDOT: PSS (4 wt% sorbitol) and the pentacene, resulting in a 400% enhancement in power conversion efficiency (PCE). The maximum PCE of 3.9% was obtained under 1sun standard AM1.5G solar illumination of 100 mW/cm².     

π-Conjugated polymer/GaN Schottky solar cells

We developed heterojunction-based Schottky solar cells consisting of π-conjugated polymers and n-type GaN. Poly (3,4-ethylendioxythiophene):poly (styrenesulfonate) (PEDOT:PSS) was used as the transparent Schottky contact material and their electrical properties were investigated in comparison with those of a polyaniline (PANI) Schottky contact. The PEDOT:PSS/n-GaN/sapphire (0 0 0 1) sample exhibited high-quality rectifying characteristics with a low reverse leakage current of less than 10⁻⁸ A/cm² at a reverse bias voltage of −3 V. While investigating the photovoltaic performance, it was observed that the open-circuit voltage of the PEDOT:PSS/n-GaN/sapphire (0 0 0 1) sample reached 0.8 V, which was much superior to the photovoltage reported for a conventional metal/GaN Schottky photodetector. We also confirmed that the PEDOT:PSS is as promising a material as PANI for π-conjugated polymer/GaN Schottky solar cells.     

Patternable polymer bulk heterojunction photovoltaic cells on plastic by rotogravure printing

We study the fabrication of poly(3-hexylthiophene)—P3HT and [6,6]-phenyl-C61 butyric acid methyl ester—PCBM based polymer bulk heterojunction photovoltaic cells using rotogravure printing. By studying the dependencies of device performance on material and process parameters including contact angles, ink concentrations, ink viscosities, solvent characteristics, and gravure printing parameters, optimized hole transport layers [poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)—PEDOT:PSS] and active layers (P3HT:PCBM) are printed, resulting in devices with power conversion efficiencies as high as 1.68% under AM 1.5 G and a spectrally matched intensity of 100 mW/cm².     

ITO free MoO3/Au/MoO3 structures using Al2O3 as protective barrier between MoO3 and PEDOT:PSS in organic solar cells

A MoO₃/Au/MoO₃ structure with a protective barrier Al₂O₃ was developed to suppress the reactions between MoO₃ and the PEDOT:PSS film in organic solar cells (OSCs). Though the maximum optical transmittance of this structure was 66% at 550 nm wavelength, the power conversion efficiency of a MoO₃/Au/MoO₃/Al₂O₃/PEDOT:PSS based OSCs was 2.77%, comparable to the 2.89% of an ITO-based OSCs. The introduction of a very thin Al₂O₃ layer between the MoO₃ and the acidic PEDOT:PSS film effectively protected the MoO₃ from the acidic and water dispersed PEDOT:PSS film, increasing the J_sc, V_oc and FF of the structure above those of the MoO₃/Au/MoO₃/PEDOT:PSS structure. The Al₂O₃ (1 nm) introduced to the MoO₃/Au/MoO₃ structure improved J_sc because it suppressed the reactions between MoO₃ and PEDOT:PSS and lowered the work function of the PEDOT:PSS film. The MoO₃/Au/MoO₃/Al₂O₃ electrode was shown to be a promising replacement of ITO for use in flexible optoelectronic devices.     

Solvent treatment inducing ultralong cycle stability poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid) fibers as binding-free electrodes for supercapacitors

As one kind of conducting polymer composite, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS) has been widely used as an electrode for energy storage and conversion devices because of its optical transmittance, flexibility, and high electrical conductivity etc. Here, we prepared binding-free PEDOT:PSS fibers (PFs) electrodes with high capacitive performance for supercapacitors via a facile method followed by various solvent treatments. Dimethyl sulfoxide (DMSO)-treated electrodes displayed a better specific capacitance (C_s) of 202 F/g at 0.5 A/g with higher elongation at break, flexibility, and conductivity of 140.7 S/cm, compared to those of pristine PEDOT:PSS materials. More importantly, the DMSO-treated fibers possessed improved stability, which retained 105% of the initial C_s after 22 000 long cycles at 10 A/g. It is believed that the fabricated PFs will be promising organic electrodes for portable supercapacitors and other flexible electronic devices in the near future.     

Morphological and electrical effect of an ultrathin iridium oxide hole extraction layer on P3HT:PCBM bulk-heterojunction solar cells

An ultrathin iridium layer was treated with O₂-plasma to form an iridium oxide (IrO_x), employed as a hole extraction layer in order to replace poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS) in organic photovoltaic (OPV) cells with poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester (P3HT:PCBM). The IrO_x layer affects the self-organization of the P3HT:PCBM photo-active layer due to its hydrophobic nature, inducing a well-organized intraplane structure with lamellae oriented normal to the substrate. Synchrotron radiation photoelectron spectroscopy results showed that the work function increased by 0.57 eV as the Ir layer on ITO changed to IrO_x by the O₂-plasma treatment. The OPV cell with IrO_x (2.0 nm) exhibits increased power conversion efficiency as high as 3.5% under 100 mW cm⁻² illumination with an air mass (AM 1.5G) condition, higher than that of 3.3% with PEDOT:PSS.     

Gravure printing for three subsequent solar cell layers of inverted structures on flexible substrates

Inverted organic photovoltaic devices have been fabricated by gravure printing on a flexible substrate. In order to enable printing of multiple layers sequentially, a systematic study of wetting behaviour of each layer in the device is performed. Successful wetting of a hydrophobic P3HT:PCBM surface by a hydrophilic PEDOT:PSS ink is achieved with the addition of a surfactant/alcohol to the PEDOT:PSS ink and with oxygen plasma treatment. We are therefore able to print titanium oxide, poly(3-hexylthiophene) (P3HT) blended with [6,6]-phenyl C61 butyric acid methyl ester (PCBM) and poly-3,4-ethylenedioxythiophene:poly(styrene sulphonic acid) (PEDOT:PSS). As result we get for three printed layers a 0.6% power conversion efficiency.     

Transparent electrode with ZnO nanoparticles in tandem organic solar cells

The transparent inter-electrodes with the p/n heterojunction consisting of the solution-processible ZnO nanoparticles as the n-type and the conventional hole injection layers (MoO₃ or PEDOT:PSS) as the p-type materials are studied for developing tandem organic solar cells employing different band gap active materials (i.e., P3HT:PCBM blend layer for larger band gap material in the bottom cell and ZnPc/C₆₀ bilayer for smaller band gap material in the top cell). For the ZnO/PEDOT:PSS inter-electrode, the V_OC corresponding to the sum of V_OC’s of the top and bottom unit cells is obtained, denoting that the two unit cells are successfully connected in series. For the ZnO/MoO₃ inter-electrode, the open-circuit voltage (V_OC) of the tandem cell is smaller than the sum of V_OC’s of the top and bottom unit cells, but it can be increased by inserting a very thin Al layer (∼3 nm) between ZnO and MoO₃ (ZnO/Al/MoO₃) as the recombination center for carriers.     

Highly durable inverted-type organic solar cell using amorphous titanium oxide as electron collection electrode inserted between ITO and organic layer

An indium tin oxide/titanium oxide/[6,6]-phenyl C₆₁ butyric acid methyl ester:regioregular poly(3-hexylthiophene)/poly(3,4-ethylenedioxylenethiophene):poly(4-styrene sulfonic acid)/Au type organic solar cell (ITO/TiO_x/PCBM:P3HT/PEDOT:PSS/Au) with 1 cm² active area, which is called “inverted-type solar cell”, was developed using an ITO/amorphous titanium oxide (TiO_x) electrode prepared by a sol-gel technique instead of a low functional electrode such as Al. The power conversion efficiency (η) of 2.47% was obtained by irradiating AM 1.5G-100 mW cm⁻² simulated sunlight. We found that a photoconduction of TiO_x by irradiating UV light containing slightly in the simulated sunlight was required to drive this solar cell. The device durability in an ambient atmosphere was maintained for more than 20 h under continuous light irradiation. Further, when the air-stable device was covered by a glass plate with a water getter sheet which was coated by an epoxy-UV resin as sealing material, the durability was still higher and over 96% of relative efficiency was observed even after continuous light irradiation for 120 h.     

Using modified poly(3,4-ethylene dioxythiophene): Poly(styrene sulfonate) film as a counter electrode in dye-sensitized solar cells

Dye-sensitized solar cells (DSSCs), assembling with nano-crystalline TiO₂ adsorbed cis-Ru(dcb)₂(NCS)₂ dye (known as N3) using polar solvent-treated poly(3,4-ethylene dioxythiophene): poly(styrene sulfonate) (PEDOT:PSS) coating on a conductive glass (fluorine-doped tin oxide, FTO) as a counter electrode, were studied. The conductivity of a bare PEDOT:PSS film was only 2±0.05 S/cm. However, the conductivities of PEDOT:PSS films treated with dimethyl sulfoxide (DMSO), N,N-dimethyl acetamide (DMAc), N,N-dimethyl formamide (DMF), and dichloromethane (DMC) reached 85±15, 45±10, 36±7, and 20±6 S/cm, respectively. In addition, carbon blacks (0.02, 0.1, 0.5, 1.0, 2.0 wt% with respect to PEDOT:PSS aqueous solution) were added into the DMSO-treated PEDOT:PSS solution (denoted as DMSO-PEDOT:PSS) to enhance the conductivity. Atomic force microscopy (AFM) images of PEDOT:PSS and various DMSO-PEDOT:PSS films coated on the FTO glasses were examined. The topographical images reveal that the increased surface roughness is responsible for the enhanced electrochemical property of the DMSO-PEDOT:PSS films. AC impedance technique was also employed to analyze the kinetics at the electrolyte/counter electrode interface. The DSSC using carbon black (0.1 wt%)-modified DMSO-PEDOT:PSS conductive coating as a counter electrode reached a cell efficiency of 5.81% under 100 mW/cm². This efficiency is higher than a DSSC using Pt as a counter electrode (5.66%).     

Plastic dye-sensitized photo-supercapacitor using electrophoretic deposition and compression methods

A plastic photo-rechargeable capacitor is studied using a three-electrode configuration, separating a flexible dye-sensitized solar cell (DSSC) and a supercapacitor by sharing a common Pt electrode. The thick and uniform TiO₂ film is formed by using commercially available TiO₂ nanocrystals, which are treated in an isopropyl alcohol without surfactant by the electrophoretic deposition (EPD) to deposit the mesoporous TiO₂ photoanode film with good adherence onto the plastic substrate. Afterward, a static mechanical compression technique as the post-treatment is employed to the electrophoretic deposited film in order to enhance the particles connection. In addition, a supercapacitor using PEDOT (poly(3,4-ethylenedioxythiophene)), which is potentiostatically electropolymerized to form a thick film, is fabricated to store the energy. The flexible DSSC part is fabricated with a TiO₂ film of 10.9 μm thickness and it can provide photoelectric conversion efficiency up to 4.37% under 1 sun illumination. The photocapacitor is made with such a flexible DSSC and a supercapacitor with ca. 0.5 mm thick PEDOT film, which provides a specific capacitance of 0.52 F cm⁻².     

The spectral sensitivity of PEDOT:PSS films

The sheet resistance of bare and encapsulated PEDOT:PSS films is investigated as a function of time when irradiated. A spectral sensitivity curve is extracted by monitoring the increase in sheet resistance over 500 h while the films are exposed to monochrome light in the spectral range of 290-417 nm at a power level of approximately 2 W/m². It is shown that light of wavelength λ<315 nm has to be strictly omitted to maintain the film's sheet resistance. The degradation can be significantly slowed down by taking proper encapsulation means. The impact of oxygen and humidity has to be blocked to avoid light induced oxidation reactions of PEDOT and subsequent conductivity losses. Alternatively, the conductivity can be maintained by the addition of stabilizing agents to the polymer dispersion.     

All solution processed tandem polymer solar cells based on thermocleavable materials

Multilayer tandem polymer solar cells were prepared by solution processing using thermocleavable polymer materials that allow for conversion to an insoluble state through a short thermal treatment. The problems associated with solubility during application of subsequent layers in the stack were efficiently solved. Devices comprised a transparent front cathode based on solution processed zinc oxide nanoparticles, a large band gap active layer based on a bulk heterojunction between zinc oxide and poly(3-carboxydithiophene) (P3CT) followed by a layer of PEDOT:PSS processed from water. The second cell in the stack employed a zinc oxide front cathode processed on top of the PEDOT:PSS layer from an organic solvent, a low band gap active layer based on a bulk heterojunction between zinc oxide and the novel poly(carboxyterthiophene-co-diphenylthienopyrazine) (P3CTTP) followed by a layer of PEDOT:PSS again processed from water and finally a printed silver electrode. The devices were prepared without the use of fullerenes and vacuum steps and employ only thermal treatments and orthogonal solvents. The devices exhibited operational stability in air without any form of encapsulation.     

Hybrid inverted organic photovoltaic cells based on nanoporous TiO2 films and organic small molecules

Solar cells based on nanoporous TiO₂ films with an inverted structure of indium tin oxide (ITO)/TiO₂/copper phthalocyanine (CuPc):fullerene (C₆₀)/CuPc/poly(3,4-oxyethyleneoxythiophene):poly(styrene sulfonate) (PEDOT:PSS)/Au were fabricated. The best overall photovoltaic performance undergoing a series of device optimization was achieved with the device of ITO/dense TiO₂ (30 nm)/nanoporous TiO₂ (130 nm)/C₆₀:CuPc (1:6 weight) (20 nm)/CuPc (20 nm)/PEDOT:PSS (50 nm)/Au (30 nm). The device using the nanoporous TiO₂ films has better photovoltaic properties compared to those using dense TiO₂ films. Higher photovoltaic performances were obtained by introducing a coevaporated layer of C₆₀:CuPc between TiO₂ and CuPc. The stability of inverted structure was better than that of the normal device, which gives a promising way for fabrication of solar cells with improved stability.     

The effect of introducing a buffer layer to polymer solar cells on cell efficiency

The effect of poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) as a buffer layer was investigated in polymer solar cells (PSCs). Four different types of PEDOT:PSS were used: PH, PH500 and their DMSO (dimethylsulfoxide)-doped counterparts. The efficiency of PSCs was independent of the electric conductivity of the buffer layer as a bulk property while it was significantly related to interfacial properties between the buffer layer and a bulk-heterojunction (BHJ) layer. The interfacial properties included charge transfer resistance (R_CT), hole mobility (μ_h) and contact angle (θ) of the solution of BHJ on the buffer layer. Lower R_CT, higher μ_h and smaller θ led to the higher fill factor (up to 72%), enabling highly efficient PSCs with efficiency (η)=4.25%.