Investigation of low resistance transparent MoO3/Ag/MoO3 multilayer and application as anode in organic solar cells

Depending on the resistivity and transmittance, transparent conductive oxides (TCO) are widely used in thin film optoelectronic devices. Thus doped In₂O₃ (ITO), ZnO, SnO₂ are commercially developed. However, the deposition process of these films need sputtering and/or heating cycle, which has negative effect on the performances of the organic devices due to the sputtering and heat damages. Therefore a thermally evaporable, low resistance, transparent electrode, deposited onto substrates room temperature, has to be developed to overcome these difficulties. For these reasons combination of dielectric materials and metal multilayer has been proposed to achieve high transparent conductive oxides. In this work the different structures probed were: MoO₃ (45 nm)/Ag (x nm)/MoO₃ (37.5 nm), with x = 5-15 nm. The measure of the electrical conductivity of the structures shows that there is a threshold value of the silver thickness: below 10 nm the films are semiconductor, from 10 nm and above the films are conductor. However, the transmittance of the structures decreases with the silver thickness, therefore the optimum Ag thickness is 10 nm. A structure MoO₃ (45 nm)/Ag (10 nm)/MoO₃ (37.5 nm) resulted with a resistivity of 8 × 10^− 5 Ω cm and a transmittance, at around 600 nm, of 80%. Such multilayer structure can be used as anode in organic solar cells according to the device anode/CuPc/C₆₀/Alq₃/Al. We have already shown that when the anode of the cells is an ITO film the introduction of a thin (3 nm) MoO₃ layer at the interface anode (ITO)/organic electron donor (CuPc) allows reducing the energy barrier due to the difference between the work function of ITO and the highest occupied molecular orbital of CuPc [1]. This property has been used in the present work to achieve a high hole transfer efficiency between the CuPc and the anode. For comparison MoO₃/Ag/MoO₃/CuPc/C₆₀/Alq₃/Al and ITO/MoO₃/CuPc/C₆₀/Alq3/Al solar cells have been deposited in the same run. These devices exhibit efficiency of the same order of magnitude.     

Effect of solvent vapor treatment on photovoltaic properties of conducting polymer/C60 interpenetrating heterojunction structured organic solar cell

The effect of solvent vapor treatment on the characteristics of a photovoltaic cell with a structure of indium–tin–oxide (ITO)/C₆₀/poly (3-hexylthiophene) (PAT6)/Au was studied. Interpenetrating film of C₆₀ and PAT6 fabricated by using chloroform (CF) as a solvent was exposed to vaporized CF atmosphere at different concentrations, temperatures and time periods before an evaporation of metal electrode. Solvent vapor treatment caused an increase in the open-circuit voltage, the fill factor and the external quantum efficiency; these were discussed by taking the surface morphology and crystallinity of the PAT6 layer which underwent a change after it was treated by this method.     

Light-emitting characteristics of organic light-emitting diodes with the MoOx-doped NPB and C60/LiF layer

The hole ohmic properties of the MoO_x-doped NPB layer have been investigated by analyzing the current density-voltage properties of hole-only devices and by assigning the energy levels of ultraviolet photoemission spectra. The result showed that the performance of organic light-emitting diodes (OLEDs) is markedly improved by optimizing both the thickness and the doping concentration of a hole-injecting layer (HIL) of N, N′-diphenyl-N, N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine (NPB) doped with molybdenum oxide (MoO_x) which was inserted between indium tin oxide (ITO) and NPB. For the doping concentration of above 25%, the device composed of a glass/ITO/MoO_x-doped NPB (100 nm)/Al structure showed the excellent hole ohmic property. The investigation of the valence band structure revealed that the p-type doping effects in the HTL layer and the hole concentration increased at the anode interfaces cause the hole-injecting barrier lowering. With both MoO_x-doped NPB as a hole ohmic contact and C₆₀/LiF as an electron ohmic contact, the device, which is composed of glass/ITO/MoO_x-doped NPB (25%, 5 nm)/NPB (63 nm)/Alq₃ (37 nm)/C₆₀ (5 nm)/LiF (1 nm)/Al (100 nm), showed the luminance of about 58,300 cd/m² at the low bias voltage of 7.2 V.     

Insights into the Influence of Work Functions of Cathodes on Efficiencies of Perovskite Solar Cells

Though various efforts on modification of electrodes are still undertaken to improve the efficiency of perovskite solar cells, attributing to the large scope of these methods, it is of significance to unveil the working principle systematically. Herein, inverted perovskite solar cells based on indium tin oxide (ITO)/poly(3,4‐ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS)/CH₃NH₃PbI₃/phenyl‐C61‐butyric acid methyl ester (PC₆₁ BM)/buffer metal/Al are constructed. Through the choice of different buffer metals to tune work function of the cathode, the contact nature of the active layer with the cathode could be manipulated well. In comparison with the device using Au/Al as the electrode that shows an unfavorable band bending for conducting the excited electrons to the cathode, the one with Ca/Al presents a dramatically improved efficiency over 17.1%, ascribed to the favorable band bending at the interface of the cathode with the active layer. Details for tuning the band bending and the corresponding charge transfer mechanism are given in a systematic manner. Thus, a general guideline for constructing perovskite photovoltaic devices efficiently is provided.     

The effect of alcohol solvent treatment on the performance of inverted polymer solar cells

Significant performance change has been observed for inverted polymer solar cells (PSCs) with the structure of ITO/ZnO/P3HT:PC₆₁BM/MoO₃/Ag when the photoactive layer was rinsed by spin-coating alcohol solvent before the deposition of MoO₃ and Ag anode. As a result, isopropanol (IPA) treatment can dramatically enhance the device performance of inverted PSCs while methanol, ethanol, and 1-butanol treatment led to worse photovoltaic performance. The enhancing device performance should be attributed to the remove of PC₆₁BM near the top of the P3HT:PC₆₁BM active layer by spin-coating IPA due to better wetting with the photoactive layer, resulting in the power conversion efficiency (PCE) and short-circuit current (J_sc) increased from 3.96% and 8.97 mA/cm² to 4.49% and 9.92 mA/cm² for IPA treatment, respectively. This facile alcohol-treated route provides a promising method to enhance the device performance of inverted PSCs.     

Improvement of organic solar cell efficiency by solution-processed doping of pentacene in PEDOT:PSS layer

In the current research, organic solar cells (OSCs) with various concentrations of pentacene in Poly(ethylenedioxythiopene):Poly(styrenesulfonate) (PEDOT:PSS) interface layer were investigated for better hole extraction. The ITO/Pentacene?+?PEDOT:PSS/P3HT:PCBM/Al-fabricated solar cell fabricated via brush coating provides superior photovoltaic, electrical and optical characteristics when compared with the ITO/PEDOT:PSS/P3HT:PCBM/Al solar cell. The ITO/Pentacene?+?PEDOT:PSS/P3HT:PCBM/Al solar cells deliver a V_OC ~350?mV and 2.57% efficiency. It is observed that the optimized concentration of pentacene doping in PEDOT:PSS layer, along with an active layer of P3HT and PC₆₀BM, doubles the efficiency of the device, when compared with pristine PEDOT:PSS layer. The degradation studies of the fabricated bulk heterojunction OSCs reveal that the degrading abilities of ITO/Pentacene?+?PEDOT:PSS/P3HT:PCBM/Al solar cells are 60% more better than those of ITO/PEDOT:PSS/P3HT:PCBM/Al devices. Thus, this work will ultimately contribute toward fully solution processed painted device, which will provide low-cost manufacturing and improved stability of pentacene-based organic photovoltaics.     

Improvement of stability for organic solar cells by using molybdenum trioxide buffer layer

We demonstrated that the stability of organic solar cells (OSCs) under light irradiation is markedly enhanced by inserting a molybdenum trioxide (MoO₃) buffer layer between an anode layer of indium tin oxide (ITO) and a p-type layer of 5,10,15,20-tetraphenylporphyrin (H₂TPP) or N,N'-di(1-naphthyl)-N,N'-diphenylbenzidine (α-NPD). The use of the MoO₃ layer also enhanced open-circuit voltages and power conversion efficiencies of the OSCs due to an increase in built-in potential. From results of stability test of hole-only α-NPD devices, we concluded that the OSC degradation occurs near the ITO/p-type layer interface and that the use of the MoO₃ layer can prevent the degradation at this interface.     

High efficient inverted polymer solar cells with different annealing treatment

Inverted polymer solar cells (IPSCs) were fabricated with cesium carbonate (Cs₂CO₃) modified indium tin oxide (ITO) substrates as the electrode and molybdenum trioxide (MoO₃) modified Al as the anode. The Cs₂CO₃ dissolved in 2-ethoxyethanol was spin-coated on ITO substrates, showing snowflake-like morphology characterized by the scanning electron microscope (SEM). The absorption, X-ray diffraction as well as the morphology of the active layer were measured before and after annealing treatment. The IPSCs with annealing treatments on the active layers and MoO₃ layers exhibited the maximum power conversion efficiency (PCE) approaching to 2%, with open circuit voltage (V_oc) of 0.57 V, short circuit current density (J_sc) of 8.8 mA/cm² and fill factor (FF) of 38.7%. The performance of IPSCs was dramatically decreased by annealing treatment after the deposition of Al cathode, which may be due to the diffusion of Al atom crossing the MoO₃ layer forming new channels for charge carrier collection. However, the new channels are not beneficial to the charge carrier collection, which is demonstrated from that the J_sc of IPSCs was evidently decreased from 8.8 to 4.6 mA/cm² by annealing treatment after deposition Al layer. The annealing treatment after deposition of MoO₃ could improve the interfacial contact to aid in electron extraction.     

Enhancement of the power conversion efficiency of polymer solar cells by functionalized single-walled carbon nanotubes decorated with CdSe/ZnS core–shell colloidal quantum dots

In this paper, we demonstrated an enhanced performance of polymer solar cells by incorporating functionalized single-walled carbon nanotubes (SWCNTs) decorated with CdSe/ZnS core–shell colloidal quantum dots (CQDs) into copolymers of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) as active layer. Short-circuit current density and power conversion efficiency of the ITO/PEDOT:PSS/P3HT:PCBM:(CdSe/ZnS-SWCNTs)/Al solar cells can be enhanced by more than 31 and 23 %, respectively, as compared with the control device ITO/PEDOT:PSS/P3HT:PCBM/Al. This enhancement is due to the high electron-transporting ability of SWCNTs and the increased absorption of CdSe/ZnS CQD in visible region. It shows an applicable way to improve the efficiency of polymer solar cells by incorporating suitable quantity of CQDs-decorated SWCNTs with suitable kinds of CQDs and suitable acid treatment to the SWCNTs.     

Active-layer evolution and efficiency improvement of (CH<Subscript>3</Subscript>NH<Subscript>3</Subscript><Subscript>3</Subscript>Bi<Subscript>2</Subscript>I<Subscript>9</Subscript>-based solar cell on TiO<Subscript>2</Subscript>-deposited ITO substrate

We systematically investigated the development of film morphology and crystallinity of methyl-ammonium bismuth (III) iodide (MA₃Bi₂I₉) through onestep spin-coating on TiO₂-deposited indium tin oxide (ITO)/glass. The precursor solution concentration and substrate structure have been demonstrated to be critically important in the active-layer evolution of the MA₃Bi₂I₉-based solar cell. This work successfully improved the cell efficiency to 0.42% (average: 0.38%) with the mesoscopic architecture of ITO/compact-TiO₂/mesoscopic-TiO₂ (meso-TiO₂)/MA₃Bi₂I₉/2,2′,7,7′-tetrakis(N,N-di-4-methoxyphenylamino)-9,9′spiro-bifluorene (spiro-MeOTAD)/MoO₃/Ag under a precursor concentration of 0.45 M, which provided the probability of further improving the efficiency of the Bi³⁺-based lead-free organic–inorganic hybrid solar cells.

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Study on the bulk junction type organic solar cells with double zinc oxide layer

The optical and photovoltaic properties of a photovoltaic cell with a structure of indium–tin oxide (ITO)/double ZnO/poly(3-hexylthiophene) (PAT6):PCBM/Ag have been investigated. The double layer ZnO was a composite of a sputtered ZnO layer and oriented zinc oxide nanopillars layer which was fabricated by a new method at low temperature (343 K). It is concluded that the double layer ZnO plays an important role in collecting photogenerated electrons and acts as a conducting path to the electrode. Insertion of the double layer ZnO in the photovoltaic cells produced enhanced performance with the power conversion efficiency of 1.42% under AM1.5 illumination.     

Efficient bulk heterojunction solar cells based on D–A copolymers as electron donors and PC70BM as electron acceptor

Two low band gap conjugated polymers P1 (alternating phenylenevinylene containing thiophene and pyrrole rings) and P2 (alternating phenylenevinylene with dithenyl (thienothiadiazole) segments) having optical band gap 1.65 eV and 1.74 eV, respectively, were used as electron donor along with the PC₇₀BM as electron acceptor for the fabrication of bulk heterojunction solar cells. The power conversion efficiency (PCE) of BHJ devices based on P1:PC₇₀BM and P2:PC₇₀BM cast from THF solvent is about 2.84% and 2.34%, respectively, which is higher than the BHJ based on PCBM as electron acceptor. We have investigated the effect of mixed (1-chloronaphthalene (CN)/THF) solvent, modification of PEDOT:PSS layer and inserting of TiO₂ layer, on the photovoltaic performance of polymer solar cell. We have achieved power conversion efficiency of 5.07% for the polymer solar cells having structure ITO/PEDOT:PSS (modified)/P1:PC₇₀BM (CN/THF cast)/TiO₂/Al. The effect of solvent used for spin coating, modification of PEDOT:PSS layer and inclusion of TiO₂ layer has been discussed in detail.     

Efficient and stable, structurally inverted poly(3-hexylthiopen): [6,6]-phenyl-C61-butyric acid methyl ester heterojunction solar cells with fibrous like poly(3-hexylthiopen)

We investigated an inverted organic photovoltaic device structure in which a densely packed ~ 100 nm thin TiO₂ layer on fluorine doped conducting glass serves as anode and poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate)/Au layer on top of the active layer serves as cathode. The active layer is comprised of a blend of poly(3-hexylthiopene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). The rectification behavior of such a device is improved significantly and injection losses are minimized compared to devices without any compact TiO₂ layer. Moreover, nanostructured P3HT active layer was achieved in-situ by spin coating concentrated pure P3HT and P3HT:PCBM blend and solar cell performances on thickness of the active layer were also investigated. For the inverted solar cells constructed with different concentrations of P3HT and PCBM keeping the P3HT:PCBM ratio 1:0.8 (wt.%), the highest short circuit current and efficiency was observed when the P3HT and PCBM concentration was equal to 1.5 (wt.%) and 1.2 (wt.%) respectively. This leads to highly stable and reproducible power conversion efficiency above 3.7% at 100 mW/cm² light intensity under AM 1.5 conditions.     

Hybrid solar cells based on CuInS2 and organic buffer-sensitizer layers

Hybrid solar cells on the basis of CuInS₂ (CIS) photoabsorber on Cu-tape (CISCuT) in combination with organic buffer layers of Zn-phthalocyanine (ZnPc), ZnPc:fullerene (ZnPc:C₆₀) composite and conductive polymer buffer layers of poly(3,4-ethylenedioxythiophene) (PEDOT) doped with polystyrenesulfonate (PSS) were prepared using vacuum evaporation and spin-casting techniques. To prepare solar cells with an active area of 2 cm², the appropriate deposition parameters and thickness of ZnPc, ZnPc:C₆₀ and PEDOT-PSS layers were selected experimentally. For preparation of semitransparent contact-window layers, chromium and gold were evaporated on the surface of ZnPc, ZnPc:C₆₀ and PEDOT-PSS films. It was found that an intermediate chromium layer improves PV properties of the structures with organic buffer layers. The photosensitivity at small illumination intensities of complete structures with ZnPc and ZnPc:C₆₀ layers increased more than one order of magnitude in comparison with the structures where the PEDOT-PSS buffer layer was deposited. The presence of C₆₀ in the composite-buffer layer results in increased photoconductivity. The best structure with composite ZnPc:C₆₀ buffer layer showed an open-circuit voltage of 560 mV, a short-circuit current density of around 10 mA/cm² and a photoconversion efficiency of around 3.3% under the light illumination with an intensity of 100 mW/cm² from a tungsten-halogen lamp. The low transmission of the semitransparent chromium-gold window layer is the reason for relatively low current density.     

Development of air stable polymer solar cells using an inverted gold on top anode structure

We developed indium-tin-oxide/perylene diimide (or bathocuproine (BCP))/poly(2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene (MEH-PPV) and [6,6]-phenyl C₆₀ butyric acid methyl ester (PCBM) blend/copper phthalocyanine (CuPc)/Au interpenetrated network polymer solar cells in order to improve air stability. The stability properties of the cells were characterized by current-voltage measurements under the influence of light and air. We achieved long lifetime solar cells which work at least 2 weeks under ambient air conditions without encapsulation. Solar energy conversion efficiency of the cells decrease 30% of the first day value at the end of 2 weeks. Photocurrent absorption properties of the devices were also investigated.     

Aqueous Metal Oxide Inks for Modifying Electrode Work Function in Polymer Solar Cells

A method to prepare aqueous metal oxide inks for tuning the work function (WF) of electrodes is demonstrated. Thin films prepared from the metal oxide ink based on vanadium oxide (V₂O₅) nanoparticles are found to increase the WF of an indium‐tin‐oxide (ITO) electrode. ITO substrates modified with V₂O₅ films are applied as a hole selective layer (HSL) in polymer solar cells (PSCs) using a poly(3‐hexylthiophene) and [6,6]‐phenyl‐C61 butyric acid methyl ester blend as a photoactive layer. The PSCs prepared with V₂O₅‐modified ITO show better device performance, achieving a power conversion efficiency of 3.6%, demonstrating 15% enhancement compared to conventional ITO/poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT‐PSS) based devices. Furthermore, ITO/V₂O₅‐modified devices exhibit better ambient stability with 60% improvement in device lifetime than those using PEDOT:PSS as an HSL. This solution‐processable and highly stable WF‐modifying metal oxide film can be a potential alternative material for engineering interfaces in optoelectronic devices.     

Organic photovoltaic cells fabricated on a SnOx/Ag/SnOx multilayer transparent conducting electrode

Transparent conducting multilayer structured electrode of a few nm Ag layer embedded in tin oxide thin film SnO_x/Ag/SnO_x was fabricated on a glass by RF magnetron sputtering at room temperature. The multilayer of the SnO_x(40 nm)/Ag(11 nm)/SnO_x(40 nm) electrode shows the maximum optical transmittance of 87.3% at 550 nm and a quite low electrical resistivity of 6.5 × 10^− 5 Ω cm, and the corresponding figure of merit (T¹⁰/R_S) is equivalent to 3.6 × 10^− 2 Ω^− 1. A normal organic photovoltaic (OPV) structure of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate)/polythiophene:phenyl-C60-butyric acid methyl ester/Al was fabricated on glass/SnO_x/Ag/SnO_x to examine the compatibility of OPV as a transparent conducting electrode. Measured characteristic values of open circuit voltage of 0.62 V, saturation current of 8.11 mA/cm² and fill factor of 0.54 are analogous to 0.63 V, 8.37 mA/cm² and 0.58 of OPV on commercial glass/indium tin oxide (ITO) respectively. A resultant power conversion efficiency of 2.7% is also very comparable with the 3.09% of the same OPV structure on the commercial ITO glass as a reference, and which reveals that SnO_x/Ag/SnO_x can be appropriate to OPV solar cells as a sound transparent conducting electrode.     

Fabrication of selective-emitter silicon heterojunction solar cells using hot-wire chemical vapor deposition and laser doping

The Si heterojunction (HJ) solar cells were fabricated on the textured p-type mono-crystalline Si (c-Si) substrates using hot-wire chemical vapor deposition (HWCVD). In view of the potential for the bottom cell in a hybrid junction structure, the microcrystalline Si (μc-Si) film was used as the emitter with various PH₃ dilution ratios. Prior to the n-μc-Si emitter deposition, a 5 nm-thick intrinsic amorphous Si layer (i-a-Si) was grown to passivate the c-Si surface. In order to improve the indium-tin oxide (ITO)/emitter front contact without using the higher PH₃ doping concentration, a laser doping technique was employed to improve the ITO/n-μc-Si contact via the formation of the selective emitter structure. For a cell structure of Ag grid/ITO/n-μc-Si emitter/i-a-Si/textured p-c-Si/Al-electrode, the conversion efficiency (AM1.5) can be improved from 13.25% to 14.31% (cell area: 2 cm × 2 cm) via a suitable selective laser doping process.     

Suppression of Time‐Dependent Donor/Acceptor Interface Degradation by Redistributing Donor Charge Density

Poor operation stability is a major hurdle for the wide application of organic photovoltaic (OPV) devices. While most attention is given to environmental threats to device stability, we herein show evidence from X‐ray photoemission spectroscopy (XPS) of an intrinsic time‐dependent chemical reaction at a donor/acceptor interface. Albeit with impressive device performance from boron subphthalocyanine chloride (SubPc)/fullerene (C₆₀) interface, the forming boride bonds at its interface hinders the interfacial exciton dissociation and leads to device deterioration. Due to the high electron affinity of molybdenum oxide (MoO₃) film, the incorporation of MoO₃ layer under the SubPc film has strong electron‐drawing property and leads to charge‐transfer complex (CTC) formation at the MoO₃/SubPc interface. The resulting charge redistribution in SubPc molecules effectively suppresses the further interfacial reaction at SubPc/C₆₀ junction. Our results provide insight for new degradation mechanisms of OPV devices and corresponding stability control via charge redistribution in the donor film.     

Film morphology effects on the electrical and optical properties of bulk heterojunction organic solar cells based on MEH-PPV/C60 composite

The influence of film morphology on the electrical behaviour of an MEH-PPV/C₆₀ organic solar cells has been investigated. The dissociation of photogenerated charge pairs in composites of buckminsterfullerenes (C₆₀) in a conjugated polymer matrix (MEH-PPV) forming dispersed heterojunctions was studied at low C₆₀ acceptor concentrations to separate electron transfer from charge transport effects. The motivation of this study was to analyse the strong dependence of organic solar cell efficiencies on the morphology of the composite. Two effects controlling film morphology have been investigated; the first one being the influence of the fullerene concentration and the second one is the effect of the organic solvent used to deposit the photoactive layer. The sample morphology was studied using atomic force microscopy (AFM). Photoluminescence (PL) experiments and current–voltage (I–V) measurements were performed on the deposited photovoltaic film to investigate the influence of dispersion on the charge transfer process between MEH-PPV and C₆₀. An attempt to explain all the results will be presented.