Electrode For Organic Solar Cells Research Articles

Flexible organic solar cells using an oxide/metal/oxide trilayer as transparent electrode

Organic solar cells (OSCs) have attracted much attention as a clean and renewable energy convention system, owning to the low-cost and easy-processing nature of organic semiconductors. While indium tin oxide (ITO) is commonly used in OSCs as the transparent conductive electrode, the rising cost of indium, the high temperature process and the poor flexibility of ITO, make it incompatible with large-scale roll-to-roll manufacture of OSCs. In this paper, the MoO3/thin metal/MoO3 trilayer structure was used to replace the ITO electrode in OSCs. The optical and electrical properties of the trilayer were shown to depend on the material and thickness of the intermediate metal layer. The maximum power conversion efficiency of up to 2.5% under simulated 1 sun AM 1.5 solar illumination was achieved for OSCs based on poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), compared to a maximum efficiency of 3.1% for the ITO-based devices. Moreover, due to the flexible nature of the trilayer structure, the OSCs with the trilayer electrode exhibited good mechanical flexibility. The efficiency of the flexible device was only reduced by ∼6% from its original performance after 500 bending cycles with a bending radius of 1.3cm. Therefore, the performance of the ITO-free devices on rigid/flexible substrates suggests that this oxide/metal/oxide trilayer electrode is a promising ITO replacement in OSCs.

Metallic nanomesh electrodes with controllable optical properties for organic solar cells

We have fabricated a metallic nanomesh using nanosphere lithography and metal evaporation. The metallic nanomesh has a precisely controlled nanostructure showing excellent uniformity with hexagonally arrayed periodic circular holes. A P3HT:PCBM organic solar cell, with the gold nanomesh electrode, demonstrates a high fill factor of 61% and a considerable power conversion efficiency of 3.12%. Electromagnetic simulation indicates that the optical properties of the metallic nanomesh can be optimized for organic photovoltaic devices by tuning the film thickness, hole diameter, and periodicity. These results show the promising potential of using a metallic nanomesh as the transparent electrode in organic solar cells.

Cu-based multilayer transparent electrodes: A low-cost alternative to ITO electrodes in organic solar cells

Copper-based multilayer transparent electrodes (MTEs) consisting of a dielectric-metal-dielectric (DMD) structure are explored as a replacement for indium tin oxide (ITO) electrodes in an effort to realize truly low-cost organic solar cells (OSCs). The outer dielectric is shown to play an important role not only in tuning the optical properties of MTEs but also in improving the electrical characteristics of Cu layers, thus providing both high transmission and low sheet resistance. We demonstrate that devices with the optimized Cu-based MTEs can have power conversion efficiency that is as high as 87% of that of ITO-based conventional cells, despite the absorption by Cu in the visible spectrum.

Large-scale, ultra-dense and vertically standing zinc phthalocyanine π–π stacks as a hole-transporting layer on an ITO electrode

Vertically standing π–π stacks play a key role in advancing the charge transporting properties in the fields of some organic materials and devices such as organic solar cells, organic light-emitting diodes and photodetector. However, realization of large-scale, ultra-dense and vertically standing π–π stacks of organic semiconductors is still a big challenge. By using an amide armed ZnPc–COOH molecule, we show herein a facile solution deposition method to prepare large-scale (>2 × 3 mm2), ultra-dense (completely covering the ITO surface) and vertically standing π–π stacks through supramolecular self-assembly. These vertically standing π–π stacks show a high conductivity and hole mobility, of the order of 10−3 S cm−1 and 10−3 cm2 V−1 s−1, respectively, and act as the hole-transporting layer on the ITO electrode in organic solar cells.

Anatase TiO2 and ITO co-sputtered films for an indium-saving multicomponent electrode in organic solar cells

We report the characteristics of a Ti–In–Sn–O (Ti–ITO) multicomponent electrode prepared by co-sputtering of ITO and anatase TiO2 targets for reducing high-cost indium content in the anode layer of organic solar cells (OSCs). The dependence of electrical, optical, structural, and surface properties of the Ti–ITO multicomponent electrodes on the RF power was investigated in detail. The optimized Ti–ITO (80W TiO2 co-sputtered ITO) electrode exhibited a low sheet resistance of 18.06Ω/square, a high average optical transmittance of 80.61% and a root mean square roughness of 2.40nm. The anatase TiO2 phase in the ITO matrix prevents the bixbyite structure from attaining the preferred orientation, producing a smooth surface of the Ti–ITO electrode. Moreover, the OSC with the optimized Ti–ITO anode showed an open circuit voltage (0.60V), short circuit current (8.33mA/cm2), fill factor (63.99%) and power conversion efficiency (3.18%) comparable to those of OSCs with a reference ITO electrode. This indicates that the Ti–ITO multicomponent electrode is a promising indium-saving multicomponent electrode that to reduces the indium content in ITO electrodes for low-cost OSCs.

Aligned Carbon Nanotube Sheets for the Electrodes of Organic Solar Cells

Aligned carbon nanotube sheets are developed as a new family of electrodes to fabricate dye-sensitized solar cells. The energy conversion efficiency of the resulting cell is higher than the randomly dispersed carbon nanotube film and comparable with the platinum. Novel and flexible solar cells can be easily made from such carbon nanotube sheets with high potentials.

Environmental and economic assessment of ITO-free electrodes for organic solar cells

The use of Indium–Tin Oxide (ITO) as a transparent conductor in organic photovoltaic (OPV) devices has been shown to present a bottleneck for the technology due to the use of the rare metal Indium and also the energy intensive manufacturing processes required and subsequent high economic cost. This study discusses some of the alternative materials, which are being considered for use as transparent conductors in OPV modules. A life cycle and cost analysis of a number of the most promising ITO-free transparent conductors, namely: high conductivity PEDOT:PSS; a silver grid embedded in PEDOT:PSS; silver nanowires; single walled carbon nanotubes are completed in this study. The results show that there is great potential for reducing both the energy pay-back time and the cost of OPV modules by replacing ITO with an alternative transparent conductor.

Synthesis of mesoporous titania thin films with vertical pore channels and thick and crystalline walls

Mesoporous titania thin films with vertical pore channels and crystalline walls have been sought for a long time because such materials are considered to be useful in many applications, especially as electrodes in organic solar cells and dye-sensitized solar cells. In this paper, we report the syntheses and characterization of such materials. The syntheses are achieved by spin-coating of precursor solutions composed of TiCl 4 and a Pluronic triblock copolymer F-127 ((EO) 106(PO) 100(EO) 106 where EO is ethylene oxide and PO is propylene oxide) in 1-butanol in various ratios, followed by aging and calcination. The wall thickness can be varied in the range of 6–8 nm, larger than that obtained by using ethanol as the solvent, by controlling the composition of the precursor solution. The thick walls enable the use of high temperatures in the calcination, up to 500 °C in the present study, to induce the crystallization of the wall material into anatase. The pore structure of the 500 °C calcined films is characterized by vertical straight channels with smooth internal surfaces. The channels are arranged with the hexagonal symmetry when viewed from the top. The variable wall thicknesses are reflected in the band gaps of the films with the thickest one showing 3.29 eV.

Avoiding indium

Alternative electrode materials and device geometries that avoid the use indium tin oxide — an expensive and brittle material widely used for making transparent electrodes in organic solar cells — are now coming to fruition.

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)

AbstractHighly 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.

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

AbstractHighly 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.

Doped graphene electrodes for organic solar cells

In this work graphene sheets grown by chemical vapor deposition (CVD) with controllednumbers of layers were used as transparent electrodes in organic photovoltaic (OPV)devices. It was found that for devices with pristine graphene electrodes, the powerconversion efficiency (PCE) is comparable to their counterparts with indium tin oxide(ITO) electrodes. Nevertheless, the chances for failure in OPVs with pristinegraphene electrodes are higher than for those with ITO electrodes, due to thesurface wetting challenge between the hole-transporting layer and the grapheneelectrodes. Various alternative routes were investigated and it was found thatAuCl3 doping on graphene can alter the graphene surface wetting properties such that auniform coating of the hole-transporting layer can be achieved and device successrate can be increased. Furthermore, the doping both improves the conductivityand shifts the work function of the graphene electrode, resulting in improvedoverall PCE performance of the OPV devices. This work brings us one step furthertoward the future use of graphene transparent electrodes as a replacement for ITO.

Rapid Patterning of Single‐Wall Carbon Nanotubes by Interlayer Lithography

High resolution patterning of single-wall carbon nanotubes is achieved using interlayer lithography. Minimum feature sizes of 6 and 0.7 μm are demonstrated on glass and silicon, using the negative photoresist SU-8 and the positive photoresist AZ7220, respectively. Films patterned using SU-8 have a good balance of transparency and sheet-resistance, and are shown to be effective electrodes for organic solar cells. Copyright © 2010 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim.

Spin‐ and Spray‐Deposited Single‐Walled Carbon‐Nanotube Electrodes for Organic Solar Cells

AbstractOrganic bulk‐heterojunction solar cells using thin‐film single‐walled carbon‐nanotube (SWCNT) anodes deposited on glass are reported. Two types of SWCNT films are investigated: spin‐coated films from dichloroethane (DCE), and spray‐coated films from deionized water using sodium dodecyl sulphate (SDS) or sodium dodecyl benzene sulphonate (SDBS) as the surfactant. All of the films are found to be mechanically robust, with no tendency to delaminate from the underlying substrate during handling. Acid treatment with HNO3 yields high conductivities >1000 S cm−1 for all of the films, with values of up to 7694 ± 800 S cm−1 being obtained when using SDS as the surfactant. Sheet resistances of around 100 Ω sq−1 are obtained at reasonable transmission, for example, 128 ± 2 Ω sq−1 at 90% for DCE, 57 ± 3 Ω sq−1 at 65% for H2O:SDS, and 68 ± 5 Ω sq−1 at 70% for H2O:SDBS. Solar cells are fabricated by successively coating the SWCNT films with poly(3,4‐ethylenedioxythiophene):poly(styrene sulphonate) (PEDOT:PSS), a blend of regioregular poly(3‐hexylthiophene) (P3HT) and 1‐(3‐methoxy‐carbonyl)‐propyl‐1‐phenyl‐(6,6)C61 (PCBM), and LiF/Al. The resultant devices have respective power conversions of 2.3, 2.2 and 1.2% for DCE, H2O:SDS and H2O:SDBS, with the first two being at a virtual parity with reference devices using ITO‐coated glass as the anode (2.3%).

Chemically-treated single-walled carbon nanotubes as digitated penetrating electrodes in organic solar cells

In this work, we fabricated and characterized organic photovoltaic (OPV) devices with hybrid composite anodes containing single-walled carbon nanotube (SWCNT) networks sandwiched between ITO and PEDOT : PSS. The SWCNTs used were initially grown on silicon wafers by a surfactant-free process based on ACCVD (alcohol catalytic chemical vapor deposition), and then transferred to glass substrates with pre-patterned indium-tin-oxide (ITO) electrodes for the fabrication of our OPV devices. We also integrated H2SO4/HNO3-treated and N2H4-treated SWCNT networks into OPV devices to investigate the effect of chemically-treated SWCNTs on OPV devices' performance. We found that open-circuit voltage (Voc) of our OPV devices was insensitive to SWCNTs' work function shifting caused by the employed chemical treatments, while their short-circuit current (Jsc) and power conversion efficiency (PCE) both varied in the order of: reference devices < devices with pristine SWCNT networks < devices with H2SO4/HNO3-treated SWCNT networks < devices with N2H4-treated SWCNT networks. In particular, we found that integration of N2H4-treated SWCNTs into the hybrid composite anode could enhance Jsc by 12% to 20% in P3HT : PCBM OPV devices (with PCE up to 4.02%). The improved performance in devices integrated with N2H4-treated SWCNTs can be attributed to (i) better crystallinity of the P3HT polymer, and (ii) increased hole-transport efficiency of the hybrid composite anode, both induced by the penetration/digitation of SWCNTs into the P3HT polymer layer.

Transparent, flexible and low-resistive precision fabric electrode for organic solar cells

We report the use of conducting precision fabrics as transparent and flexible electrode for organic semiconductor-based thin film devices. Precision fabrics have well-defined mesh openings, excellent flexibility and are fabricated by high-throughput roll-to-roll manufacturing. Optimized fabrics reached light transmittance over 95% throughout the visible and near infrared spectra. A significant part of the transmitted light is scattered, which is particularly advantageous for solar cell applications. Surface resistivity is as low as ∼3 Ohms/square, which decreases Ohmic losses when scaling up to large area devices. We demonstrate that solar cells fabricated onto these electrodes show very similar characteristics to those prepared on ITO. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Effects of intrinsic ZnO buffer layer based on P3HT/PCBM organic solar cells with Al-doped ZnO electrode

Organic solar cell devices were fabricated using poly(3-hexylthiophene) (P3HT) and 6,6-phenyl C61-butyric acid methyl ester (PCBM), which play the role of an electron donor and acceptor, respectively. The transparent electrode of organic solar cells, indium tin oxide (ITO), was replaced by Al-doped ZnO (AZO). ZnO has been studied extensively in recent years on account of its high optical transmittance, electrical conduction and low material cost. This paper reports organic solar cells based on Al-doped ZnO as an alternative to ITO. Organic solar cells with intrinsic ZnO inserted between the P3HT/PCBM layer and AZO were also fabricated. The intrinsic ZnO layer prevented the shunt path in the device. The performance of the cells with a layer of intrinsic ZnO was superior to that without the intrinsic ZnO layer.

Ultrasonically sprayed and inkjet printed thin film electrodes for organic solar cells

Thin film pi-conjugated poly(3,4ethylenedioxythiophene): poly(styrenesulphonate) (PEDOT:PSS) as a hole transport layer on indium tin oxide is a key element in some of the most efficient organic photovoltaic and light emitting devices to date. Films are typically deposited by spincoating, which is not readily scalable. In this paper we investigate the critical parameters for both inkjet and ultrasonic spray deposition of PEDOT:PSS thin films on commercial indium tin oxide as a potentially scalable approach to contact formation. Inkjet parameters investigated include drop spacing and substrate temperature. Ultrasonic spray coating parameters investigated include substrate temperature and solution flow rate. We also show that the ink viscosity has a Newtonian character, making it well suited for inkjet printing. Films were characterized via optical profilometry, sheet resistance and atomic force microscopy. Optimized inkjet printed and ultrasonic sprayed PEDOT:PSS films were then compared to spincast layers in a prototypical bulk heterojunction photovoltaic device employing a poly(3-hexylthiophene) and [6,6]-PCBM (6,6-phenylC61-butric acid-methyl ester) blend as the absorber. Practically all three approaches produced devices of comparable efficiency. Efficiencies were 3.6%, 3.5% and 3.3% for spin, spray and inkjet depositions respectively.

Silver lining

Silver thin films with nanoscale holes could prove useful as optically transparent electrodes for organic solar cells

Transparent Carbon Films as Electrodes in Organic Solar Cells

Durchsichtig und aromatisch: Ein neuer chemischer „Bottom-up“-Ansatz liefert transparente Graphenfilme. Dabei werden synthetische Nanographenmoleküle thermisch zu riesigen polycyclischen aromatischen Kohlenwasserstoffen verknüpft, die miteinander zu großen Graphenschichten vernetzt sind. Solche Graphenfilme wurden als transparente Elektroden in organische Solarzellen eingebaut (siehe Bild).