Electrode For Organic Solar Cells Research Articles

Ultrathin Organic Solar Cells with Graphene Doped by Ferroelectric Polarization

Graphene has been employed as transparent electrodes in organic solar cells (OSCs) because of its good physical and optical properties. However, the electrical conductivity of graphene films synthesized by chemical vapor deposition (CVD) is still inferior to that of conventional indium tin oxide (ITO) electrodes of comparable transparency, resulting in a lower performance of OSCs. Here, we report an effective method to improve the performance and long-term stability of graphene-based OSCs using electrostatically doped graphene films via a ferroelectric polymer. The sheet resistance of electrostatically doped few layer graphene films was reduced to ∼70 Ω/sq at 87% optical transmittance. Such graphene-based OSCs exhibit an efficiency of 2.07% with a superior stability when compared to chemically doped graphene-based OSCs. Furthermore, OSCs constructed on ultrathin ferroelectric film as a substrate of only a few micrometers show extremely good mechanical flexibility and durability and can be rolled up into a cylinder with 7 mm diameter.

Highly transparent Nb-doped indium oxide electrodes for organic solar cells

The authors investigated the characteristics of Nb-doped In2O3 (INbO) films prepared by co-sputtering of Nb2O5 and In2O3 for use in transparent anodes for organic solar cells (OSCs). To optimize the Nb dopant composition in the In2O3 matrix, the effect of the Nb doping power on the resistivity and transparency of the INbO films were examined. The electronic structure and microstructure of the INbO films were also investigated using synchrotron x-ray absorption spectroscopy and x-ray diffraction examinations in detail. At the optimized Nb co-sputtering power of 30 W, the INbO film exhibited a sheet resistance of 15 Ω/sq, and an optical transmittance of 86.04% at 550 nm, which are highly acceptable for the use as transparent electrodes in the fabrication of OSCs. More importantly, the comparable power conversion efficiency (3.34%) of the OSC with an INbO anode with that (3.31%) of an OSC with a commercial ITO anode indicates that INbO films are promising as a transparent electrode for high performance OSCs.

Highly doped fullerene C60 thin films as transparent stand alone top electrode for organic solar cells

We report on transparent stand alone C60 top electrodes for small molecule based organic photovoltaic cells. All device layers (including bottom and top electrode) are thermally evaporated in vacuum, guaranteeing fast, reliable, high-purity and low-cost processing. Employing an organic compound as electrode avoids refractive index mismatch between contact and photo-active layer which results in an increased number of photons reaching the absorber layers. The conductivity of C60 is improved from 10−8S/cm to remarkable 4.97S/cm by optimized molecular n-type doping. Furthermore, it is shown that high doping concentrations change the absorption behavior of C60, leading to transmittance values over 70% for wavelengths larger than 600nm with a weak dependence on layer thickness. Organic solar cells employing such an optimized n-doped C60 top electrode (thickness 200nm, doping concentration 16wt%) exhibit power conversion efficiencies of 1.53%. Electrical device simulations based on a discretized equivalent circuit model reveal an efficient lateral charge transport in n-doped C60 electrodes over a few 100µm. In combination with a supporting grid electrode (usually employed for large area photovoltaic applications), n-doped C60 might replace ITO and fill the lack of transparent conductive top electrodes required for roll-to-roll processed semi-transparent organic devices or devices on opaque substrates.

Transparent electrode for organic solar cells using multilayer structures with nanoporous silver film

A transparent electrode consists of nanoporous silver thin film and molybdenum oxide was developed for transparent anode with advantages of low cost, large-area processing and room-temperature fabrication. High density nanoporous silver film was fabricated by controlling thickness and deposition rate during thermal evaporation. With silver thickness of 10nm and 5Å/s deposition rate, the MoOx/nanoporous Ag/MoOx multilayer structure achieved ~9Ω/sq sheet resistance and ~70% transparency. Compared to the indium—tin-oxide (ITO) anode, the nanoporous anode/SubPc/C60/BCP/Al photovoltaic device has an enhanced surface-plasmon absorption and a higher fill factor. Although its transparency is lower than ITO, the nanoporous anode achieved 3.45% power conversion efficiency, comparable to ITO-based devices.

Discharge amplified photo-emission from ultra-thin films applied to tuning work function of transparent electrodes in organic opto-electronic devices

A novel photoemission technique utilising localised discharge amplification of photo-yield is reported. It enables fast, accurate measurement of work function and ionisation potential for ultra-thin buffer layers vacuum deposited onto single and multilayer transparent conducting electrodes for organic solar cells and OLED's. Work function in most traditional transparent electrodes has to be raised to maximise charge transfer while high transmittance and high conductance must be retained. Results are presented for a range of metal oxide buffers, which achieve this goal. This compact photo-yield spectroscopy tool with its fast turn-around has been a valuable development aid since ionisation potential can vary significantly as deposition conditions change slightly, and as ultra-thin films grow. It has also been useful in tracking the impact of different post deposition cleaning treatments along with some storage and transport protocols, which can adversely reduce ionisation potential and hence subsequent device performance.

Temperature dependent growth morphologies of parahexaphenyl on SiO2 supported exfoliated graphene

The growth of small conjugated molecules on graphene is of increasing interest, since the latter bears the potential to serve as a transparent electrode for organic solar cells and light emitting diodes. Here, parahexaphenyl thin films have been grown by hot wall epitaxy on SiO2 supported exfoliated graphene. The arising morphologies—studied by atomic force microscopy—exhibit a strong dependence on deposition temperature. At temperatures from 280 K–333 K, islands consisting of almost upright standing molecules and needles composed from lying molecules coexist on the graphene flake. Between 363 and 423 K solely needles—consisting of lying molecules—are present on the graphene. The needles form well-ordered networks with relative orientation angles of ∼30°, ∼60°, and ∼90° reflecting the symmetry of the graphene substrate.

Organic Solar Cells: Improvement of Transparent Metal Top Electrodes for Organic Solar Cells by Introducing a High Surface Energy Seed Layer (Adv. Energy Mater. 4/2013)

Organic Solar Cells: Improvement of Transparent Metal Top Electrodes for Organic Solar Cells by Introducing a High Surface Energy Seed Layer (Adv. Energy Mater. 4/2013)

Influence of the addition of graphene-like materials on the thermophysical properties of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) thin film nanocomposites

Addition of few-layer graphene and graphite to poly(3,4-ethylenedioxy-thiophene):poly (styrenesulfonate) (PEDOT:PSS) is of great technological relevance for a host of applications, including electrodes for organic solar cells and the development of solution-processable conducting inks with improved thermal properties. In this paper we investigate the optical, electrical, morphological and thermal properties of nanocomposites formed by the incorporation of PEDOT:PSS into carbon-based thin films in which the thermal properties approach those of graphene. The properties of these nanocomposite thin films can be understood in the framework of a two-layer model, with a bottom layer formed by an effective medium of PEDOT:PSS and graphene-like flakes and a top layer of pure PEDOT:PSS. Strong dependence on the film thickness and morphology was found for all of the thermophysical properties we studied. We show a strong improvement in thermal conductivity due to the addition of graphene-like material to PEDOT:PSS.

Optimized ITO-free tri-layer electrode for organic solar cells

The optical properties of ZnO/Ag/ZnO (ZAZ) multilayer structures were numerically modeled and calculated by a FDTD method. Such tri-layers were also manufactured using an ion beam sputtering plant. A good agreement is obtained between modelizations and realizations. The impact of the oxide thicknesses on the optical properties of the ZAZ structures were experimentally and numerically investigated, and allow us to adjust the spectral position of the transmission maximum. The transmission of these structures is optimized up to around 74%, on the whole absorption spectral range of the photoactive P3HT:PCBM bulk heterojunction. The best electrode design is glass/ZnO (30nm)/Ag (14nm)/ZnO (30nm), which presents a sheet resistance of 7Ω/□. The optimized ZAZ structure was successfully integrated in an organic solar cell as anode. A photovoltaic efficiency of 2.58% is obtained and is compared to an organic solar cell integrating a traditional ITO anode with an efficiency of 2.99%. Numerical calculations of the intrinsic absorption inside each layer of the organic solar cells are performed. Alternative ITO-free electrodes for organic solar cells are demonstrated.

Improvement of Transparent Metal Top Electrodes for Organic Solar Cells by Introducing a High Surface Energy Seed Layer

AbstractWe present highly transparent and conductive silver thin films in a thermally evaporated dielectric/metal/dielectric (DMD) multilayer architecture as top electrode for efficient small molecule organic solar cells. DMD electrodes are frequently used for optoelectronic devices and exhibit excellent optical and electrical properties. Here, we show that ultrathin seed layers such as calcium, aluminum, and gold of only 1 nm thickness strongly influence the morphology of the subsequently deposited silver layer used as electrode. The wetting of silver on the substrate is significantly improved with increasing surface energy of the seed material resulting in enhanced optical and electrical properties. Typically thermally evaporated silver on a dielectric material forms rough and granular layers which are not closed and not conductive below thicknesses of 10 nm. With gold acting as seed layer, the silver electrode forms a continuous, smooth, conductive layer down to a silver thickness of 3 nm. At 7 nm silver thickness such an electrode exhibits a sheet resistance of 19 Ω/□ and a peak transmittance of 83% at 580 nm wavelength, both superior compared to silver electrodes without seed layer and even to indium tin oxide (ITO). Top‐illuminated solar cells using gold/silver double layer electrodes achieve power conversion efficiencies of 4.7%, which is equal to 4.6% observed in bottom‐illuminated reference devices employing conventional ITO. The top electrodes investigated here exhibit promising properties for semitransparent solar cells or devices fabricated on opaque substrates.

Efficient organic solar cells with solution-processed carbon nanosheets as transparent electrodes

We demonstrate that solution-processed carbon nanosheet (CNS) films can efficiently serve as transparent electrodes for organic solar cells (OSCs). The CNS was obtained by spin-coating of polyacrylonitrile (PAN) dissolved in dimethylformamide on quartz substrates, followed by stabilization and carbonization processes to convert polymer into CNS. The thickness of the newly developed CNS films was easily controlled by varying the PAN solution concentration. The polymer-converted CNS films were intensively examined for the feasibility of the use as transparent anodes in solar cells. This approach could be highly desirable for all-solution-processed or printed OSCs.

Application of solvent modified PEDOT:PSS to graphene electrodes in organic solar cells

Graphene has been proposed as a promising transparent conducting electrode material in organic photovoltaic (OPV) solar cells to substitute the widely used indium tin oxide (ITO). Various studies have reported OPV devices with graphene electrodes showing comparable performances to ITO-based OPV cells. However, the fabrication reliability or device yield has not been widely discussed. In our investigation it was found that graphene-based OPVs tend to have much lower device yield than the ITO-based ones if similar fabrication processes are used. One possible reason could be due to the challenge faced in obtaining a uniform coating of PEDOT:PSS hole injection layer (HIL) on graphene. In fact, several attempts have been made to overcome this challenge. In this work, we report a facile method of solvent modification of PEDOT:PSS to achieve a homogeneous coating on graphene. A significant enhancement in the device yield was observed as compared to devices using normal aqueous solution based PEDOT:PSS; nevertheless, it was still only ∼50%. Further improvement was made by adding an additional electron blocking layer (EBL), molybdenum trioxide, after the coating of PEDOT:PSS. The observation in this work calls for further investigation to understand the role played by the HIL or EBL in graphene-based OPV devices.

High-Conductivity Solution-Processed Carbon Nanotube Networks as Transparent Electrodes in Organic Solar Cells

ABSTRACTSolutions of individual, unbroken single-walled carbon nanotubes in organic solvent were fabricated in a reductive dissolution process. Transparent conductive films deposited from these organic inks gave a significantly higher conductivity to absorptivity ratio than those cast from an aqueous dispersion of carbon nanotubes. For example, films from the organic ink have achieved a sheet resistance of 250 Ω/□ with transmittance of 92% at 550 nm wavelength, compared to 76% transmittance for a 250 Ω/□ film from the aqueous dispersion. The promise of these films as transparent electrodes has been demonstrated by their incorporation into organic solar cells with power conversion efficiency of 2.3%, comparable to that of solar cells produced using indium tin oxide transparent electrodes.

Fabrication of silver nanowire transparent electrodes using electrohydrodynamic spray deposition for flexible organic solar cells

Fabrication of a flexible organic solar cell is demonstrated with an anode that is free of tin-doped indium oxide (ITO) by electrohydrodynamic (EHD) spraying of silver nanowires (Ag NWs). This methodology is applicable to fabricate patterned Ag NW thin-film electrodes for organic solar cells (OSCs). By optimizing the spray parameters and post-processing conditions, transparent electrodes with sheet resistances of ∼11 Ω sq−1 (on glass) and ∼20 Ω sq−1 (on polyethylene terephthalate, PET) and DC to optical conductivities of ∼70 (on PET) can be obtained at an optical transmittance of ∼80%. Bulk heterojunction OSCs are demonstrated with patterned Ag NW films serving as bottom transparent anodes on both glass substrates and flexible PET substrates. The photoactive layers are based on low band-gap polymers, poly[(4,8-bis-(2-ethylhexyloxy)-benzo[1,2-b;4.5-b′]dithiophene)-2,6-diyl-alt-(4-(2-ehtylhexanoyl)-thieno[3,4-b]thiophene)]-2,6-diyl] and phenyl-C61-butyric acid methyl ester. Under AM 1.5 illumination, fabricated cells have high power conversion efficiencies of 5.27% (on glass) and 3.76% (on PET). This study indicates that a Ag NW electrode prepared by EHD spraying can serve as an alternative to the ITO electrode, which also enables its potential application in practical and flexible OSCs.

Effect of MoO3 doping power on the electrical, optical, and structural properties of MoO3-doped In2O3 anodes for organic solar cells

The authors investigated the effects of MoO3 doping power on the electrical, optical, and structural properties of MoO3-doped In2O3 (IMO) films for use as transparent electrodes in organic solar cells (OSCs). By optimizing the RF power of the MoO3 target (30 W), they obtained IMO films with sheet resistance of 24.57 Ω/sq and optical transmittance of 81.57%, comparable to conventional Sn-doped In2O3 (ITO) electrodes. In particular, the IMO film exhibited a high optical transmittance in the near-infrared (NIR) wavelength region due to the effects of Mo dopant with high Lewis acid strength and high mobility. In addition, strongly oriented (222) grains led to a smooth surface of the IMO electrode with root mean square roughness of 2.344 nm. The OSC fabricated on the optimized IMO electrode showed cell performances with a fill factor of 64.66%, a short circuit current of 8.058 mA/cm2, an open circuit voltage of 0.611 V, and a power conversion efficiency of 3.186%, which are nearly identical to OSCs with an ITO reference. This indicates that the IMO film is a promising alternative to the ITO electrode for the tandem OSC absorbing NIR wavelength region.

Enhanced ultraviolet response using graphene electrodes in organic solar cells

We demonstrate enhanced ultraviolet (UV) response in organic photovoltaic devices using few-layer graphene as the transparent and conducting electrode. This is compared to the same device stack fabricated on a conventional indium tin oxide (ITO) electrode. The enhanced UV response was confirmed by examining the photocurrent spectra over a wide spectral range and is consistent with the excellent UV transmittance of graphene over ITO. The results render the graphene electrode a promising alternative to ITO to enable applications for UV photodetectors or other optoelectronic devices with a wider optical window.

Highly near-infrared transparent GeO2-doped In2O3 electrodes for bulk heterojunction organic solar cells

We report on the characteristics of GeO2-doped In2O3 (IGO) films for use as transparent electrodes in organic solar cells (OSCs). The electrical, optical, and structural properties of IGO electrodes were investigated as a function of RF power and post-annealing conditions. At optimized conditions, the IGO electrode exhibited a low sheet resistance of 14.0Ω/square, a high optical transmittance of 86.9%, a root mean square roughness of 1.27nm and a work function of 5.2eV. In particular, the IGO film showed higher optical transmittance in the near-infrared region due to a lower free carrier concentration and higher mobility than conventional ITO electrodes. The higher Lewis acid strength of the Ge4+ (3.06) dopant, compared to that of a Sn4+ (1.62) dopant, led to higher mobility of the IGO films. In addition, we observed that the strongly oriented (222) grains in the IGO films enhanced carrier mobility and relaxation time. Furthermore, a bulk heterojunction OSC with the optimized IGO anode exhibited a good cell performance with a fill factor of 67.38%, a short circuit current of 8.438mA/cm2, an open circuit voltage of 0.606V, and a power conversion efficiency of 3.443%, which are comparable to OSCs with ITO anodes.

Oxide Sandwiched Metal Thin‐Film Electrodes for Long‐Term Stable Organic Solar Cells

AbstractOxide/silver/oxide multilayers as semitransparent top electrode for small molecule organic solar cells (OSCs) are presented. It is shown that two oxide layers sandwiching a central metal layer greatly improve the stability and lifetime of the organic solar cell. Thermally evaporated MoO3, WO3, or V2O5 layers are employed as an interlayer for subsequent silver deposition and significantly change the morphology of the ultrathin silver layer, improving charge extraction and electrodes series resistance. The transmittance of the electrode is increased by introducing oxide or oxide and organic multilayers as capping layer, which leads to higher photocurrent generation in the absorber layer. Application of 1 nm MoO3/11 nm Ag/10 nm MoO3/50 nm Alq3 multilayer electrodes in OSCs lead to an efficiency of 2.6% for a standard ZnPc:C60 cell, showing superior performance compared to devices with pure silver top contacts. The device lifetime is also strongly increased. MoO3 layers can saturate and stabilize the inner and outer metal surface, passivating it against most of the degradation mechanisms. With such an oxide/silver/oxide multilayer electrode, the time until the glass encapsulated OSC is degraded to 80% of its starting efficiency is enhanced from 86 h to approximately 4500 h compared to an OSC without an oxide interlayer.

Low-temperature synthesis of carbon nanotubes on indium tin oxide electrodes for organic solar cells

The electrical performance of indium tin oxide (ITO) coated glass was improved by including a controlled layer of carbon nanotubes directly on top of the ITO film. Multiwall carbon nanotubes (MWCNTs) were synthesized by chemical vapor deposition, using ultrathin Fe layers as catalyst. The process parameters (temperature, gas flow and duration) were carefully refined to obtain the appropriate size and density of MWCNTs with a minimum decrease of the light harvesting in the cell. When used as anodes for organic solar cells based on poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM), the MWCNT-enhanced electrodes are found to improve the charge-carrier extraction from the photoactive blend, thanks to the additional percolation paths provided by the CNTs. The work function of as-modified ITO surfaces was measured by the Kelvin probe method to be 4.95 eV, resulting in an improved matching to the highest occupied molecular orbital level of the P3HT. This is in turn expected to increase the hole transport and collection at the anode, contributing to the significant increase of current density and open-circuit voltage observed in test cells created with such MWCNT-enhanced electrodes.

Organic Solar Cells with Graphene Electrodes and Vapor Printed Poly(3,4-ethylenedioxythiophene) as the Hole Transporting Layers

For the successful integration of graphene as a transparent conducting electrode in organic solar cells, proper energy level alignment at the interface between the graphene and the adjacent organic layer is critical. The role of a hole transporting layer (HTL) thus becomes more significant due to the generally lower work function of graphene compared to ITO. A commonly used HTL material with ITO anodes is poly(3,4-ethylenedioxythiophene) (PEDOT) with poly(styrenesulfonate) (PSS) as the solid-state dopant. However, graphene's hydrophobic surface renders uniform coverage of PEDOT:PSS (aqueous solution) by spin-casting very challenging. Here, we introduce a novel, yet simple, vapor printing method for creating patterned HTL PEDOT layers directly onto the graphene surface. Vapor printing represents the implementation of shadow masking in combination with oxidative chemical vapor deposition (oCVD). The oCVD method was developed for the formation of blanket (i.e., unpatterened) layers of pure PEDOT (i.e., no PSS) with systematically variable work function. In the unmasked regions, vapor printing produces complete, uniform, smooth layers of pure PEDOT over graphene. Graphene electrodes were synthesized under low-pressure chemical vapor deposition (LPCVD) using a copper catalyst. The use of another electron donor material, tetraphenyldibenzoperiflanthene, instead of copper phthalocyanine in the organic solar cells also improves the power conversion efficiency. With the vapor printed HTL, the devices using graphene electrodes yield comparable performances to the ITO reference devices (η(p,LPCVD) = 3.01%, and η(p,ITO) = 3.20%).