Passivation films with SU-8 polymers for organic solar cell protection from ultraviolet ray

This paper investigates passivation films for organic solar cells (OSCs) in terms of the ultraviolet (UV) rays that the SU-8 film employs. An SU-8 film was deposited on the back side of an OSC sample, and the performance of the OSC was measured in terms of the UV intensity variation. The UV intensity was 10 and 30 mW/cm² at 365 nm. When 30 mW/cm²of UV was irradiated for 5 min, the V_oc and FF of the OSC without SU-8 film significantly decreased to 70% and 74%, respectively, from the pristine condition; leading to an efficiency decrease of approximately 48%. However, the V_oc and FF of OSC with passivation film slightly decreased to 93% and 98%, respectively.     

Characteristics of flexible indium tin oxide electrode grown by continuous roll-to-roll sputtering process for flexible organic solar cells

The preparation and characteristics of flexible indium tin oxide (ITO) electrodes grown on polyethylene terephthalate (PET) substrates using a specially designed roll-to-roll sputtering system for use in flexible organic solar cells are described. It was found that both electrical and optical properties of the flexible ITO electrode were critically dependent on the Ar/O₂ flow ratio in the continuous roll-to-roll sputter process. In spite of the low substrate temperature (<50 °C), we can obtain the flexible ITO electrode with a sheet resistance of 47.4 Ω/square and an average optical transmittance of 83.46% in the green region of 500-550 nm wavelength. Both X-ray diffraction and field emission scanning electron microscopy analysis results showed that all flexible ITO electrodes grown on the PET substrate were amorphous with a very smooth and featureless surface, regardless of the Ar/O₂ flow ratio due to the low substrate temperature, which is maintained by a cooling drum. In addition, the flexible ITO electrode grown on the Ar ion-beam-treated PET substrates showed more stable mechanical properties than the flexible ITO electrode grown on the wet-cleaned PET substrates, due to an increased adhesion between the flexible ITO and the PET substrates. Furthermore, the flexible organic solar cell fabricated on the roll-to-roll sputter-grown flexible ITO electrode at an optimized condition exhibited a power conversion efficiency of 1.88%. This indicates that the roll-to-roll sputtering technique is a promising continuous sputtering process in preparing flexible transparent electrodes for flexible solar cells or displays.     

Engineering of hybrid interfaces in organic photovoltaic devices

In this study, we engineer and investigate the interface structure and chemistry at the indium tin oxide (ITO) anode (front-side electrode) as well as at the Mg−Ag cathode (back-side electrode) in metal phthalocyanine (MePc)/C₆₀ organic solar cells (OSCs).For the front-side electrode, Zn-phthalocyaninetetraphosphonic acid (Zn-PTPA) and Sn-phthalocyanine axially substituted with tartaric acid (Sn-PTA) have been used for the surface termination of ITO coated glass substrates. Both terminations yielded OSCs with higher fill factors and open circuit voltages, thus increasing the power conversion efficiency by 33% and 67%, respectively. A possible influence of a chemisorbed Zn-PTPA on the film growth of the adjacent ZnPc absorber in the vicinity of the hybrid interface is discussed using X-ray reflectivity and near edge X-ray absorption fine structure data. Distinct effects of the Zn-PTPA and Sn-PTA terminations on the electronic properties of the ITO surface were found by X-ray photoelectron spectroscopy (XPS) measurements at the valence band edge. We demonstrate the possibility to engineer the hybrid interface without additional buffer.For the back-side electrode we report the formation of buffer-free charge carrier selective Mg−Ag cathodes, which are applied for bulk heterojunction organic absorbers consisting of copper phthalocyanine (CuPc) donor and fullerene C₆₀ acceptor materials. The chemical and structural properties of the CuPc:C₆₀/Mg−Ag interface are investigated by element depth profiling using secondary ion mass spectrometry (SIMS), grazing incidence X-ray diffraction analysis (GI-XRD) and XPS.We demonstrate that an optimum charge carrier selectivity is achieved with Mg:Ag/Ag cathode structures, where the Mg:Ag alloy layer has a composition close to that of Ag₃Mg. In addition, Mg diffusion into CuPc:C₆₀ layer is observed. As a result, an interaction between Mg and Cu²⁺ with a concurrent change in oxidation state of both metals takes place. However, no formation of MgPc is observed.The findings of this work are discussed against the background of the performance and electrical properties of the corresponding MePc/C₆₀-based organic solar cells.     

Effect of modified ITO substrate on electrochromic properties of polyaniline films

In this work, we report the morphological and electrochromic properties of electrochemically synthesized polyaniline (PANI) thin films on bare and modified indium–tin oxide (ITO) glass substrates. In the last case, the surface of ITO glass was covered by a self-assembled monolayer of N-phenyl-γ-aminopropyl-trimethoxysilane (PAPTS). Atomic force microscopy images and perfilometry show that smoother and thinner PANI films were grown on PAPTS-modified ITO substrates. PANI-based electrochromic devices (ECDs) were assembled by using a viscous polymeric electrolyte (PE) of LiClO₄ and polymethyl methacrylate (PMMA) co-dissolved in a mixture of propylene and ethylene carbonate. The architectural design of the devices was glass/ITO/PANI/PE/ITO/glass. A dual ECD was also prepared by collocating a poly(3-methylthiophene) (P3MT) thin film as a complementary electrochromic element. The effect of the PAPTS-modified ITO substrate is reflected in a higher optical transmittance at bleach state and a little less color change at 550 nm of PANI-based ECDs.     

Low resistance and highly transparent ITO–Ag–ITO multilayer electrode using surface plasmon resonance of Ag layer for bulk-heterojunction organic solar cells

We comprehensively investigated the electrical, optical, structural, mechanical, interfacial, and surface properties of ITO–Ag–ITO (IAI) multilayer electrodes grown on glass substrates by linear facing target sputtering (LFTS) for bulk-heterojunction organic solar cells (OSCs). Although the single ITO electrode with a thickness of 150 nm showed a fairly high sheet resistance of 34 Ω/square, the IAI multilayer electrode exhibited a very low sheet resistance of 4.4 Ω/square due to the low resistivity of the inserted Ag layer. Without using a substrate heating or post-annealing process, we were able to obtain an IAI multilayer electrode with a low sheet resistance, comparable to that of a crystalline ITO electrode, using the room-temperature LFTS process. In addition, the surface plasmon resonance (SPR) and antireflection of the optimized Ag layer significantly increased the optical transmittance of the IAI multilayer. It was found that the optimization of the thickness of the Ag layer is very important to obtain transparent IAI multilayer electrodes, because the SPR effect is critically affected by the Ag morphology. Moreover, the OSC fabricated on the optimized IAI electrode with an Ag thickness of 16 nm showed a higher power conversion efficiency (3.25%) compared to that prepared on the amorphous ITO electrode (2.35%), due to its low sheet resistance and high optical transmittance at 400–600 nm, which corresponds to the absorption wavelength of the organic active layer. This indicates that IAI multilayer electrodes grown by LFTS are promising transparent conducting electrodes for OSCs or flexible OSCs due to their very low resistivity and high optical transmittance.     

Large-scale fabrication of CdS nanorod arrays on transparent conductive substrates from aqueous solutions

This paper reported the fabrication of CdS nanorod arrays on transparent conductive tin-doped In₂O₃ (ITO) glass substrate by combining electrodeposition and hydrothermal processes. One thin layer of CdS particles on substrates was prepared by electrodeposition as seeds, upon which well-aligned nanorod arrays were hydrothermally grown in large scale. The morphology, structure and phase composition of CdS nanorod arrays were examined by X-ray diffraction, field-emission scanning electron microscopy, energy-dispersive X-ray spectroscopy, transmission electron microscopy (TEM), and high-resolution TEM. The optical absorption spectra and the temperature-dependent photoluminescence of CdS nanorod arrays were also studied. An organic/inorganic hybrid solar cell prototype device was fabricated based on the as-prepared CdS nanoarrays grown on the transparent ITO electrode (ITO/CdS arrays/poly[2-methoxy-5-(20-ethylhexyloxy)-1,4-phenylene-vinylene]/Au). The power conversion efficiency was 0.06%, showing an obvious photovoltaic effect.     

Inverted and transparent polymer solar cells prepared with vacuum-free processing

Inverted transparent polymer solar cells were fabricated by sequentially depositing several organic layers from fluids, on ITO/glass substrates. ITO was used as a cathode to collect electrons. The photovoltage of these diodes can be increased by up to 400 mV by inserting a buffer layer of polyethylene oxide between ITO and the active layers, which results in 4-fold enhancement of power conversion efficiency under the illumination of 100 mW/cm² simulated AM1.5 solar light. The enhancement of V_oc is consistent with the work function change between ITO and ITO/PEO measured by photoelectron spectroscopy. Solar cell production without vacuum processing may lower production costs.     

Water and oxygen induced degradation of small molecule organic solar cells

Small molecule organic solar cells were studied with respect to water and oxygen induced degradation by mapping the spatial distribution of reaction products in order to elucidate the degradation patterns and failure mechanisms. The active layers consist of a 30 nm bulk heterojunction formed by the donor material zinc-phthalocyanine (ZnPc) and the acceptor material Buckminsterfullerene (C₆₀) followed by 30 nm C₆₀ for additional absorption. The active layers are sandwiched between 6 nm 4,7-diphenyl-1,10-phenanthroline (Bphen) and 30 nm N,N′-diphenyl-N,N′-bis(3-methylphenyl)-[1,1′-biphenyl]-4,4′-diamine p-doped with C₆₀F₃₆ (MeO-TPD:C₆₀F₃₆), which acted as hole transporting layer. Indium-tin-oxide (ITO) and aluminum served as hole and electron collecting electrode, respectively. Time-of-flight secondary ion mass spectrometry (TOF-SIMS) and X-ray photoelectron spectroscopy (XPS) in conjunction with isotopic labeling using H₂¹⁸O and ¹⁸O₂ provided information on where and to what extent the atmosphere had reacted with the device. A comparison was made between the use of a humid (oxygen free) atmosphere, a dry oxygen atmosphere, and a dry (oxygen free) nitrogen atmosphere during testing of devices that were kept in the dark and devices that were subjected to illumination under simulated sunlight. It was found that water significantly causes the device to degrade. The two most significant degradation mechanisms are diffusion of water through the aluminum electrode resulting in massive formation of aluminum oxide at the BPhen/Al interface, and diffusion of water into the ZnPc:C₆₀ layer where ZnPc becomes oxidized. Finally, diffusion from the electrodes was found to have no or a negligible effect on the device lifetime.     

Organic photovoltaic devices: influence of the cell configuration on its performences

On the basis of our thin film technology we have proceeded to the study of different cell configurations: two-layers D/A organic solar cells deposited by vacuum evaporation and bulk D/A heterojunction material based on a discontinuous D/A network thin film obtained by spin coating. We have also tested different transparent conductive oxides (TCO: ITO, ZnO). These TCO films have been used as under or upper electrode. The organic materials were zinc-phthalocyanine (Zn-Pc) or poly vinyl(N-carbazole) (PVK) as electron donor and 1,4-diaminoanthraquinone (1,4-DAAQ) as electron acceptor. A PEDOT/PSS thin film was often intercalated between the organic and the TCO in order to improve the current characteristics.Results can be summarized as follows:

• The cells efficiency does not depend strongly on the nature of the TCO used in the present work.

• The performance of the p–n bilayer ZnPc/1,4-DAAQ depends strongly on the surface roughness of the structure. The fill factor (FF) of the current–voltage characteristics varies from 0.2, when the structure is glass/TCO/(PEDOT/PSS)/ZnPc/1,4-DAAQ/Al, to 0.6, when the structure is glass/Al/DAAQ/ZnPc/ PEDOT/PSS/”mechanical contact”/TCO. It is shown that this behaviour is related to the columnar growth properties of the 1,4-DAAQ films.

• Best efficiency has been achieved with bulk D/A heterojunction material based on PVK/1,4-DAAQ blend (efficiency=0.5%). The electrochemical measure of its HOMO (5.7 eV) and LUMO (3.8 eV), by comparison with PVK (HOMO=5.7 eV, LUMO=2.2 eV), shows that these values should be optimized. Effectively, the electron affinity of the donor should be significantly smaller than that of the acceptor, while the ionization potential of the acceptor should be significantly greater than that of the donor, which is not the case presently.

Doubled layered ITO/SnO2 conducting glass for substrate of dye-sensitized solar cells

The effects of indium tin oxide (ITO) and ITO/SnO₂ conducting substrates on photovoltaic properties of dye-sensitized solar cells (DSCs) using nanocrystalline TiO₂ were studied. The decrease in fill factor of the DSCs was correlated to the increase in resistance of conducting substrate. The heat stability of ITO conducting glass was improved by depositing SnO₂ on ITO layer. The efficiency of the cells using double layered ITO/SnO₂ substrate remarkably increased comparing with that of the cells using ITO substrates. It is worth mentioning that increasing in sintering time, which enhanced the electronic contact between substrate and TiO₂, also modified the cell performance of MP-TiO₂ cells. Our experimental finding suggests that 3000 Å ITO substrate, which was covered by 1000 Å SnO₂ layer, exhibited the best properties for the DSCs.     

Organic donor, acceptor and buffer layers of small molecules prepared by OVPD technique for photovoltaics

Organic vapour-phase deposition (OVPD^®) is used for the growth of the organic solar cell component materials such as the donor copper phthalocyanine (CuPc), the acceptor fullerene C₆₀, and electron-conducting buffer layers of bathocuproine (BCP) on Si1 0 0 wafers and indium tin oxide (ITO) substrates on areas as large as 15×15 cm². By means of X-ray diffraction (XRD) analysis we show that under continuous operating conditions the source materials possess long-term stability. The CuPc, C₆₀ and BCP thin film morphology and structure are characterised using scanning electron microscopy and XRD analysis. We demonstrate CuPc thin films with a highly folded surface morphology suitable for the preparation of solar cells with an interpenetrating donor–acceptor interface. The XRD diffraction patterns of the CuPc and C₆₀ layers deposited under conditions appropriate for the preparation of organic solar cells show spectra typical for these materials. Mixed CuPc:C₆₀ layers with controlled constituent ratios and layer thickness are deposited for the preparation of organic solar cells. First ITO/CuPc:C₆₀/Al organic photovoltaic devices are prepared with an efficiency of 1% (conditions AM1.5).     

Influence of electrode on electrical conduction of ZincPhthalocyanine (ZnPc) thin films

The electrical conduction properties of ZincPhthalocyanine (ZnPc) thin films have been studied using copper, silver and aluminium electrodes. The sandwich structures were prepared by the thermal evaporation method. The I–V characteristics were investigated to identify the dominant charge transport mechanism in the films. Among all possible mechanisms, it was observed that the data fits well to the SCLC type of conduction in the Al/ZnPc/Al and Schottky type of conduction prevails in the Ag/ZnPc/Ag and Cu/ZnPc/Cu devices. The trap levels and its dependence of structure have been studied and results are discussed. The charge transport phenomenon in the ZnPc films seems to depend highly on the electrode material and temperature. The carrier mobility increases with increasing temperature whereas the density of trapped holes decreases with increasing temperature. The barrier height also decreases with increase in temperature. The influence of the temperature on the electrical parameters such as saturation current density (J_s), barrier height (Φ_b), density of states in the valence band edge N_d (m⁻³), the position of the Fermi level E_F (eV), ionized acceptor atom density N_e (m⁻³), activation energy ΔΦ (eV), mobilities of hole (μ₀) and the concentration of free holes in the valence band (n₀) have been discussed in detail.     

Influence of ITO surface modification on the growth of CdS and on the performance of CdS/CdTe solar cells

We treated the surface of indium–tin oxide (ITO) substrates in two ways, (i) coating of thin insulating ITO layer or (ii) irradiation of the surface with accelerated ions, and investigated the change in sheet resistance (R_sh) and the water-contact angle (WCA). R_sh increased with the thickness of the insulating ITO layer or with the ion dose. WCA dropped as a result of the surface treatment to <15°. The microstructure, the surface morphology, the optical transmittance, and the stoichiometry of CdS improved with the surface treatment. CdS/CdTe solar cells showed a better performance as a result of ITO surface treatment.     

Flexible Ag electrode for use in organic photovoltaics

High efficiency organic photovoltaic cells discussed in literature are normally restricted to devices fabricated on glass substrates. This is a consequence of the extreme brittleness and inflexibility of the commonly used transparent conductive oxide electrode, indium tin oxide (ITO). This shortcoming of ITO along with other concerns such as increasing scarcity of indium, migration of indium to organic layer, etc. makes it imperative to move away from ITO. Here we demonstrate a highly flexible Ag electrode that possesses low sheet resistances even in ultra-thin layers. It retains its conductivity under severe bending stresses where ITO fails completely. A P3HT:PCBM blend organic solar cell fabricated on this highly flexible electrode gives an efficiency of 2.3%.     

ITO/ATO/TiO2 triple-layered transparent conducting substrates for dye-sensitized solar cells

A novel transparent conductive oxide film based on the triple-layered indium tin oxide (ITO)/antimony-doped tin oxide (ATO)/titanium oxide (TiO₂) has been developed for dye-sensitized solar cells by using radio frequency magnetron sputtering technique. Effects of the absence and presence of TiO₂ layer and the ITO layer thickness were investigated. Deposition of ATO layer was found to stabilize the thermal instability of ITO. Little change in sheet resistance and optical transmittance was observed by introduction of insulating thin TiO₂ layer on top of the ATO layer, whereas photovoltaic performance was significantly influenced. The conversion efficiency was improved from 4.57% without TiO₂ layer to 6.29% with TiO₂ layer. The enhanced photovoltaic performance with addition of TiO₂ layer was attributed mainly to the improved adhesion and partially to the reduced electron loss at the ITO/ATO conductive layer. Increase in the ITO layer thickness resulted in a slight decrease in photocurrent due to the reduced optical transmittance. When compared with the conventional fluorine-doped tin oxide (FTO), the ITO/ATO/TiO₂ conductive material exhibited similar photocurrent density but higher photovoltage and fill factor, resulting in better conversion efficiency.     

Electrical and photovoltaic properties of devices based on PbPc–TiO2 thin films

The electrical, optical and photovoltaic properties of organic–inorganic hybrid devices consisting of Al/TiO₂/PbPc/ITO and Al/PbPc/TiO₂/ITO structures have been investigated through analyzing the current–voltage characteristics, optical absorption and photocurrent action spectra of the devices. The combined presence of oxygen, light and an electric field in the photocurrent decay of Al/TiO₂/PbPc/ITO device have been studied. It is observed that under illumination, the oxygen radical anions and excitons are formed, which subsequently drift towards the interface with TiO₂, where an internal electric field is present. The excitons that reach to the interface are subsequently dissociated into free charge carriers due to the electric field present at the interface. The exciton diffusion length for PbPc calculated from the dependence of luminescence with the PbPc film is about 13 nm. We have also studied the effect of PbPc thickness and hole mobility on the device performance of organic photovoltaic device consisting of PbPc as an optically active layer, TiO₂ as the electron–transporting layer and ITO and Al used as electrodes. We have shown that the power conversion efficiency in the device is primarily limited by the short-exciton diffusion length combined with the low-hole mobility in PbPc layer. The model of charge transport in Al/TiO₂/PbPc/ITO device explained the experimental results where the total current density is a function of injected carriers at electrode–organic semiconductor surface, the leakage current through the organic layer and collected photogenerated current that results from the effective dissociation of excitons.     

Doping effects for organic photovoltaic cells based on small-molecular-weight semiconductors

We have studied highly efficient organic photovoltaic (OPV) cells based on small-molecular-weight semiconductors: zinc phthalocyanine (ZnPc) and C60. To improve the efficiency furthermore, open-circuit voltage (V_oc) has to be increased. We reported that 5,6,11,12-tetraphenylnaphthacene (rubrene) produces the highest V_oc of 0.91 V by p–n heterojunction OPV cells with C60. In this paper, we report rubrene doping effects for OPV cells based on ZnPc to obtain higher V_oc.     

Characterization and optimization of indium tin oxide films for heterojunction solar cells

Well suited and reliable values of the optical and electrical properties of thin indium tin oxide (ITO) films are needed in order to choose the optimal deposition parameters and to perform reliable modeling for solar cells design. In this work, a new method will be presented to evaluate the ITO transparency directly on silicon substrates.The effects of each deposition parameter that influences the ITO transparency and conductivity force a trade-off in the frame of values useful for SHJ solar cells. The deposition of our optimized ITO film on a textured wafer yields a weighted average reflectance as low as 4.4±0.2%.The deposition of an MgF₂/ITO double-layer anti-reflection coating (DL-ARC) on textured cells increases the efficiency from 17.9%, measured immediately after contacts have been added to the ITO, to 18.4% after the MgF₂ deposition. An annealing step at 200 °C for 10 min proved to further increase the efficiency up to 18.9%, for a total gain of 1%.     

Performance of electron beam deposited tungsten doped indium oxide films as anodes in organic solar cells

Tungsten doped indium oxide (IWO) thin films have been investigated as an alternative to indium tin oxide (ITO) anodes in organic solar cells (OSCs). The surface morphology, electrical, and optical properties of the IWO films grown by electron beam deposition were studied as a function of oxygen flow rate. For 120 nm thick IWO films deposited on float glass substrates at 350 °C and oxygen flow rate of 35 sccm, an electrical resistivity of 4.78×10⁻⁴ Ω cm and average transmittance of over 78% between 400 and 2000 nm were obtained. OSCs based on poly(3-hexylthiophene) and [6,6]-phenyl C₆₁-butlyric acid methyl ester were prepared on glass/IWO electrodes and the device performance was investigated as a function of IWO films with different oxygen flow rates. OSCs fabricated on the optimum IWO anode (oxygen flow rate of 30-35 sccm) exhibited a power conversion efficiency of ∼3.5%, which is comparable with the same device made on commercial glass/ITO electrodes (3.75%).     

Rectification in CdS/TCO bilayers

The origin of process-induced rectification in CdS/ITO and CdS/SnO₂ bilayers has been investigated. Both pre-treatment of the transparent conducting oxide (TCO) substrates and post-growth treatment of the bilayers were explored for both oxidising and reducing conditions. In/CdS/TCO structures were used for I-V testing, and the CdS layers were verified as being pinhole-free using a test employing a rectifying Au/CdS contact. Whilst neither pre-oxidation nor reduction of any TCO substrate failed to induce rectification in CdS/TCO, oxidation of CdS always induced rectification, regardless of the substrate type. This was attributed to oxidation of CdS (confirmed by Auger electron spectroscopy), and the results are consistent with a band diagram postulated for the CdO/CdS/ITO structure. Recommendations are made for the fabrication of CdTe/CdS/TCO solar cells.