Anode In Organic Solar Cells Research Articles

A donor:hole-transport layer alloy for high-efficiency and stable binary organic solar cells with promoted hole collection and suppressed recombination

A donor-based nanoparticle (NPs)-incorporated hole-transport layer was synthesized for fine-tuning the energy level of anodes in organic solar cells. The conventional and inverted devices achieved record efficiencies of 19.9% and 17.2%, respectively.

Facile fabrication of conductive graphitic carbon films as transparent electrodes in organic solar cells by ion beam irradiation of polystyrene films and carbonization

Conductive graphitic carbon films were fabricated from insulating polystyrene (PS) by ion beam irradiation and subsequent thermal treatment, and used as a transparent anode for organic solar cells (OSCs). Analyses of optical and chemical properties revealed that the thermally resistant oxidized carbon clusters in the PS precursors (spin-coated on quartz) were efficiently generated by ion beam irradiation at a fluence of 2 x 1016 ions/cm2, and transformed to graphitic carbon thin films through high-temperature treatment. The graphitic carbon films exhibited thickness-dependent conductivity (the highest conductivity of 374 S/cm), while their work function (5.20 eV) and surface roughness (0.560–0.596 nm) were unaffected by the thickness of the films. Furthermore, the introduction of the 10.2 nm-thick graphitic carbon films into poly(3-hexylthiophene) (P3HT): [6,6]-phenyl-C61 butyric acid methyl ester (PCBM)-based OSCs resulted in a power conversion efficiency (PCE) of 1.40%, demonstrating the feasibility of using the graphitic carbon films as transparent electrodes.

Comparative degradation analysis of V2O5, MoO3 and their stacks as hole transport layers in high-efficiency inverted polymer solar cells

The use of V2O5, MoO3 and their stacks as the buffer anode in organic solar cells greatly improves the lifetime of the photovoltaic devices, with a remaining PCE of over 50% of the initial PCE after 19 600 h (about 2 years).

Flexible ITO‐Free Organic Photovoltaics on Ultra‐Thin Flexible Glass Substrates with High Efficiency and Improved Stability

The ability for organic solar cells to be conformable and bendable enables them to be used in a broad range of applications. Indium tin oxide (ITO) or PEDOT:PSS on plastic substrates such as poly(ethylene terephthalate) or poly(ethylene napthalate) (PET or PEN) have been used for transparent conductive electrodes (TCEs) for flexible devices. However, ITO is brittle and when used on flexible substrates is prone to cracking, and the acidity of PEDOT:PSS can lead to corrosion of the ester‐based plastic substrates and cause device degradation. In this work, TCEs based on modified high‐conductivity PEDOT:PSS on 100 μm‐thick flexible glass substrates are used as the anode for organic solar cells. The optimized PEDOT:PSS TCE anode on flexible glass has a sheet resistance of ≈30 Ω/sq and a transmission of ≈77% at 550 nm, with a broad transmission window between 300 and 800 nm. The best PEDOT:PSS on flexible glass‐based organic solar cell has a power conversion efficiency (PCE) of 8.0%, which is higher than devices comprising ITO or PEDOT:PSS on PEN, which have PCEs of 6.4% and 5.8%, respectively. It is also found that the ultra‐thin glass devices can be scaled, with 1.6 cm2 flexible cells having an efficiency of 5.2%.

Improvement of MoO3/Ag/MoO3 multilayer transparent electrodes for organic solar cells by using UV–ozone treated MoO3 layer

Ultraviolet ozone (UVO) treatment of molybdenum trioxide (MoO3) appears to be a simple and efficient method for obtaining highly continuous and smooth silver (Ag) thin films in the thermally evaporated MoO3/Ag/MoO3 (MAM) multilayered structure as transparent electrodes for small-molecule organic solar cells (OSCs). It is observed that UVO treatment can oxidatively modify the non-stoichiometric MoO3 (or MoO3-x) surfaces, further increasing the Mo6+/Mo5+ composition ratio and work function of MoO3-x. Importantly, the use of UVO treatment for the MoO3 bottom layer effectively improves the wettability of Ag on MoO3 and enhances the lateral growth of Ag thin film, resulting in a reduction of the percolation threshold thickness of the continuous Ag layer. Due to the formation of an ultrathin Ag interlayer with a continuous and smooth surface morphology, the MAM multilayered electrode after UVO treatment of MoO3 for 3 min has excellent optical and electrical properties, including a high maximum transmittance of 89.1% and a low sheet resistance of 8.0 Ω/sq. When the optimal UVO-treated MoO3/Ag (7.5 nm)/MoO3 films are used as the anode in OSCs with the copper phthalocyanine (CuPc)/fullerene (C60) planar heterojunction structure, the OSCs have a power conversion efficiency of 0.55%, which is 2.0 and 1.2 times higher than that of devices with untreated MoO3/Ag (7.5 nm)/MoO3 (0.27%) and MoO3/Ag (10 nm)/MoO3 electrodes (0.46%), respectively, and competitive with that of indium-tin-oxide-based devices. Because of almost full surface coverage of the Ag interlayer, UVO treatment of MoO3 in MAM multilayered electrodes can improve charge carrier injection/extraction at the anode contact and hence improve the photovoltaic performance of OSCs.

Optical and electrical properties of short-pitch solar cells with finite-difference frequency-domain method

Organic solar cells (OSCs) have attracted intensive attention in recent years due to their distinct advantages of rich material resources, easy fabrication, and good flexibility. The standard structure of OSCs consists of an anode, an active layer and a cathode. Indium tin oxide (ITO) is often used as a transparent anode. However, the indium in ITO is not only very low in content, but also can penetrate into other layers of OSCs and affect the battery life. The ITO is not suitable for flexible OSCs because of its brittleness. Therefore, researchers have been trying to find alternatives to ITO, which should have transparent and flexible electrodes. The multilayer film consisting of MoO3/Ag/MoO3 is a very promising candidate as an alternative of ITO to work as the transparent anode in OSCs. However, in MoO3/Ag/MoO3 based thin OSCs structure, the absorption of light is quite poor. Here, we introduce a short-pitch metallic grating in which there are used the surface plasmon polaritons (SPPs) to enhance the light absorption of the active layer. The finite-difference frequency-domain method is used to solve the Maxwell's equations and semiconductor equations for revealing the optical and electrical properties of OSCs. As is well known, the contradiction between the long light absorption path and the short exciton diffusion length results in a relatively low power conversion efficiency (PCE) of the OSCs. Metallic gratings can be introduced into conventional OSCs for improving the light absorption due to the surface plasmon resonance. The light absorption can be enhanced compared with that in the conventional OSCs without metallic gratings. At the same time, the small periodic structure is introduced into the MoO3/Ag/MoO3 anode-based OSCs. The small spacing between gratings creates a strong interaction between two adjacent metal nanowalls. These nanostructures and metal nanostructures will further enhance the light absorption. In this work, it is proposed that short-pitch metallic gratings be introduced into the MoO3/Ag/MoO3 anode-based OSCs for improving the light absorption and PCE. It is found that the light absorption of plasmonic structure with short-pitch metallic gratings can be greatly enhanced compared with standard structure without metallic gratings. Meanwhile, with an optimal groove width of 4 nm, PCE is improved by 49% compared with the case with the planar structure. These results contribute to better developing the ITO free OSCs.

Absorption Enhancement in Thin Organic Solar Cells With MoO3/Ag/MoO3 Transparent Anode Based on Short-Pitched Metallic Grating

The multilayer film consisting of MoO3/Ag/MoO3 (shortened as MAM) is very promising as an alternative of indium-tin oxide (ITO) to work as the transparent anode in organic solar cells (OSCs). In MAM-based thin OSCs, the absorption of light is quite poor. Here, we propose to apply a short-pitched metallic grating into the MAM-based OSC for improving the absorption in its thin active layer. Numerical calculations reveal that the obtained enhancement of integrated absorption in the active layer is more than 500% with respect to the equivalent planar control. The field distributions at the two peaks of the normalized absorption spectrum indicate that propagating surface plasmon polaritons (SPPs), as well as the horizontal metal-insulator-metal (MIM) waveguide mode, play the main role of trapping light into the active material. It is expected that the proposal could contribute to the development of the efficient ITO free thin OSCs.

Cu-Ag bi-layer films in dielectric/metal/dielectric transparent electrodes as ITO free electrode in organic photovoltaic devices

Among ITO alternative, dielectric/metal/dielectric multilayer structures are one of the most often studied possible substituent. However, if their optical and electrical properties are systematically investigated it is not the same with regard to their mechanical properties. In the present manuscript we have studied the properties of ZnS/Cu/Ag/ZnS, ZnS/Cu/Ag/MO3 (with M = Mo or W) structures. With a maximum transmission of 90% and a sheet resistance of 5 Ω/sq the optimum structure exhibits a figure of merit of 82 10−3 Ω−1 when λ = 600 nm. Beyond these standard measures we proceeded to the study of the mechanical properties of the multilayer structures. The inner and outer bending tests show that the ZnS/Cu/Ag/ZnS (or MO3) structures are more flexible than ITO, while their responses to scotch tests show that they exhibit a large adhesion to the substrate, glass or plastic. The scratching adhesion test puts in evidence that the adhesion to the substrate of the Ag layer is smaller than that of ZnS/Cu/Ag/ZnS, which is smaller than that of ITO. On the other hand, this test shows that the ZnS/Cu/Ag/ZnS (no cracks for L = 25 N) is less brittle than ITO (cracks L = 15N). Finally, when used as anode in organic solar cells, the structure ZnS/Cu/Ag/WO3 allows achieving the best efficiency, similar to that obtained with ITO.

Transparent Conductive ITO/Ag/ITO Electrode Deposited at Room Temperature for Organic Solar Cells

We investigated the optical and electrical properties of room-temperature-deposited indium-tin-oxide (ITO)/Ag (19 nm)/ITO multilayer films as a function of ITO layer thickness. The optical and electrical properties of the ITO/Ag/ITO films were compared with those of high-temperature-deposited ITO-only films for use as an anode in organic solar cells (OSCs). The ITO/Ag/ITO multilayer films had sheet resistances in the range 5.40–5.78 Ω/sq, while the ITO-only film showed 14.18 Ω/sq. The carrier concentration of the ITO/Ag/ITO films gradually decreased from 2.01 × 1022 to 7.20 × 1021 cm−3 as the ITO thickness increased from 17 nm to 83 nm. At 530 nm, the transmittance of the ITO/Ag/ITO (50 nm/19 nm/50 nm) films was ~90%, while that of the ITO-only film gave 96.5%. The multilayer film had a smooth surface with a root mean square (RMS) roughness of 0.49 nm. Poly (3-hexylthiophene) (P3HT):[6,6]-phenyl-C61 butyric acid methylester (PCBM) bulk heterojunction (BHJ)-based OSCs fabricated with the ITO/Ag/ITO (50 nm/19 nm/50 nm) film showed a power conversion efficiency (PCE) (2.84%) comparable to that of OSCs with a conventional ITO-only anode (3.48%).

Enhanced conductivity of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) film by acid treatment for indium tin oxide-free organic solar cells

An acid treatment is used in the enhancement of the conductivity of the conducting polymer poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) thin film, which is often used as the anode in organic solar cells. There are three types of acid treatment for PEDOT:PSS thin film: hydrochloric, sulfuric, and phosphoric acid treatments. In this study, we examine and compare these three ways with each other for differences in conductivity. Hydrochloric acid results in the highest conductivity enhancement, from 0.3 to 1109 S/cm. We also discuss the optical transmittance, conductivity, surface roughness, surface morphology, and stability, as well as the factors that can influence device efficiency. The devices are fabricated using an acid-treated PEDOT:PSS thin film as the anode. The highest power conversion efficiency was 1.32%, which is a large improvement over that of the unmodified organic solar cell (0.21%). It is comparable to that obtained when using indium tin oxide (ITO) as an electrode, ca. 1.46%.

Fiber laser annealing of brush-painted ITO nanoparticles for use as transparent anode for organic solar cells

We report on a fiber laser annealing process for brush-painted ITO nanoparticles, for use as transparent anodes in cost-efficient printable organic solar cells (OSCs). By simple brushing of ITO nanoparticle ink onto glass substrates followed by direct fiber laser annealing, we fabricated solution-processed ITO anodes with a sheet resistance of 56.79Ω/square and an optical transmittance of 85.77%. The electrical, optical, and structural properties of ITO nanoparticle electrodes were investigated as a function of laser scan speed under a nitrogen ambient. In addition, the detailed microstructure of the laser-annealed ITO electrode was examined to explain the conduction mechanism. OSCs fabricated on laser-annealed ITO electrodes exhibit an open circuit voltage of 0.59V, short circuit current of 9.02mA/cm2, fill factor of 53.30%, and power conversion efficiency of 2.81%. Successful operation of those OSCs with laser-annealed ITO electrode indicates that fiber laser annealing is a simple and cost-effective option to replace conventional energy-intensive furnace-based annealing processes.

Copper:molybdenum sub-oxide blend as transparent conductive electrode (TCE) indium free

Oxide/metal/oxide structures have been shown to be promising alternatives to ITO. In such structures, in order to decrease the high light reflection of the metal film it is embedded between two metal oxides dielectric. MoO 3- x is often used as oxide due to its capacity to be a performing anode buffer layer in organic solar cells, while silver is the metal the most often used [1]. Some attempts to use cheaper metal such as copper have been done. However it was shown that Cu diffuses strongly into MoO 3- x [2]. Here we used this property to grow simple new transparent conductive oxide (TCE), i.e., Cu: MoO 3- x blend. After the deposition of a thin Cu layer, a film of MoO 3- x is deposited by sublimation. An XPS study shows more than 50% of Cu is present at the surface of the structure. In order to limit the Cu diffusion an ultra-thin Al layer is deposited onto MoO 3- x . Then, in order to obtain a good hole collecting contact with the electron donor of the organic solar cells, a second MoO 3- x layer is deposited. After optimization of the thickness of the different layers, the optimum structure is as follow: Cu (12 nm) : MoO 3- x (20 nm)/Al (0.5 nm)/ MoO 3- x (10 nm).The sheet resistance of this structure is R sq = 5.2 Ω/sq. and its transmittance is T ma x = 65%. The factor of merit ϕ M = T 10 /R sq. = 2.41 × 10 -3 Ω -1 , which made this new TCE promising as anode in organic solar cells.

Embedment of nano-sized Ag layer into Ag-doped In2O3 films for use as highly transparent and conductive anode in organic solar cells

By inserting a nano-sized Ag layer between bottom Ag-doped In2O3 (AIO) and a top AIO layer, we were able to control the sheet resistance and optical transmittance of AIO films for application in organic solar cells (OSCs) as a transparent electrode. To optimize the AIO/Ag/AIO multilayer, we investigated the electrical, optical, structural and morphological properties of the AIO/Ag/AIO multilayer as a function of Ag interlayer thickness with a constant bottom and top AIO thickness of 35nm. The optimized AIO/Ag/AIO multilayer showed a much lower resistivity of 3.988×10−5Ωcm and a higher optical transmittance of 84.79% than the values (4.625×10−4Ωcm and 78.36%) of the single AIO film, due to the high conductivity of the metallic Ag layer and the antireflection effect of the symmetric AIO/Ag/AIO structure. In addition, we investigated the performances of OSCs with AIO/Ag/AIO electrodes as a function of Ag interlayer thickness to determine the optimal Ag thickness to produce a high power conversion efficiency (PCE) of the OSCs. Based on the PCE of the OSCs, we correlated the performance of the OSCs with the Ag interlayer thickness in the AIO/Ag/AIO multilayer and suggested a possible mechanism to explain the dependency of PCE on Ag thickness in AIO/Ag/AIO multilayer electrodes.

Improvement of absorption in organic solar cells using tri-layer anode

The use of transparent conductive oxides (TCOs) in organic solar cells applications requires deposition at low temperatures and with a limited heat treatment. This process results in high resistivity materials. To achieve practical resistivity levels, we need to replace the conventional electrode with an TCO/Metal/TCO tri-layer anode. This material yielded the desired electrical properties without a significant effect on its optical properties. The use of these tri-layer anodes in organic solar cells led to the modification in optical field distribution in the structure of the solar cells, and improves the external power conversion efficiency by maximising the absorption into the photoactive layer.

Optimized multilayer indium-free electrodes for organic photovoltaics

Flexible multilayer electrodes that combine high transparency, high conductivity, and efficient charge extraction have been deposited, characterised and used as the anode in organic solar cells. The anode consists of an AZO/Ag/AZO stack plus a very thin oxide interlayer whose ionization potential is fine-tuned by manipulating its gap state density to optimise charge transfer with the bulk heterojunction active layer consisting of poly(n-3-hexylthiophene-2,5-diyl) and phenyl-C61-butyric acid methyl ester (P3HT:BC61BM). The deposition method for the stack was compatible with the low temperatures required for polymer substrates. Optimisation of the electrode stack was achieved by modelling the optical and electrical properties of the device and a power conversion efficiency of 2.9% under AM1.5 illumination compared to 3.0% with an ITO-only anode and 3.5% for an ITO:PEDOT electrode. Dark I–V reverse bias characteristics indicate very low densities of occupied buffer states close to the HOMO level of the hole conductor, despite observed ionization potential being high enough. Their elimination should raise efficiency to that with ITO:PEDOT.

Ag-Pd-Cu alloy inserted transparent indium tin oxide electrodes for organic solar cells

The authors report on the characteristics of Ag-Pd-Cu (APC) alloy-inserted indium tin oxide (ITO) films sputtered on a glass substrate at room temperature for application as transparent anodes in organic solar cells (OSCs). The effect of the APC interlayer thickness on the electrical, optical, structural, and morphological properties of the ITO/APC/ITO multilayer were investigated and compared to those of ITO/Ag/ITO multilayer electrodes. At the optimized APC thickness of 8 nm, the ITO/APC/ITO multilayer exhibited a resistivity of 8.55 × 10−5 Ω cm, an optical transmittance of 82.63%, and a figure-of-merit value of 13.54 × 10−3 Ω−1, comparable to those of the ITO/Ag/ITO multilayer. Unlike the ITO/Ag/ITO multilayer, agglomeration of the metal interlayer was effectively relieved with APC interlayer due to existence of Pd and Cu elements in the thin region of the APC interlayer. The OSCs fabricated on the ITO/APC/ITO multilayer showed higher power conversion efficiency than that of OSCs prepared on the ITO/Ag/ITO multilayer below 10 nm due to the flatness of the APC layer. The improved performance of the OSCs with ITO/APC/ITO multilayer electrodes indicates that the APC alloy interlayer prevents the agglomeration of the Ag-based metal interlayer and can decrease the thickness of the metal interlayer in the oxide-metal-oxide multilayer of high-performance OSCs.

Improvement of transparent silver thin film anodes for organic solar cells with a decreased percolation threshold of silver

Organic solar cells (OSCs) using thermally evaporated Ag thin films as the transparent anode are investigated with poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) films as the active layer. The optimal power conversion efficiency (PCE) of 2.57% is obtained for the device with the 11nm thick Ag layer, namely the percolation threshold of Ag. Then an interlayer of MoO3 between the Ag anode and glass substrate is introduced. Two different thicknesses of MoO3 (2nm or 10nm) are chosen here, corresponding to the unclosed or closed MoO3 layer, respectively. It is observed that the introduction of the MoO3 interlayer can effectively improve the wetting of Ag on the substrate and reduce the percolation threshold of Ag. When the MoO3 thickness is 10nm, since the surface energy of Ag (γ=1.25Jm−2) is higher than that of MoO3 (γ=0.06Jm−2), the Ag–Ag interactions are stronger than the Ag-substrate interactions, weakening the surface-modifying effect. In contrast, since the 2nm thick unclosed MoO3 layer may create preferred nucleation sites on the substrate to enhance the lateral growth of Ag film effectively, a very low sheet resistance of 9.32 Ω/□ and a relatively high transmittance are obtained for MoO3 (2 nm)/Ag (9 nm) anode. The PCE of corresponding OSCs can be improved to 2.71%, comparable to that of ITO-based reference OSCs (PCE of 2.85%). From our results, it is concluded that the performance of the Ag thin film electrode can be improved by decreasing the percolation threshold of Ag with a MoO3 interlayer (especially when the MoO3 layer is thin). It may be instructive for the further research of metal thin film electrodes.

Simple brush-painting of Ti-doped In2O3 transparent conducting electrodes from nano-particle solution for organic solar cells

In this work, we have demonstrated that simple brush-painted Ti-doped In2O3(TIO) films can be used as a cost effective transparent anodes for organic solar cells (OSCs). By the direct brushing of TIO nanoparticles ink and rapid thermal annealing (RTA), we can simply obtain TIO electrodes with a low sheet resistance of 28.25Ω/□ and a high optical transmittance of 85.48% under atmospheric ambient conditions. In particular, the brush painted TIO films showed a much higher mobility (33.4cm2/Vs) than that of previously reported solution-process transparent oxide films (1–5cm2/Vs) due to the effects of the Ti dopant with higher Lewis acid strength (3.06) and the reduced contact resistance of TIO nanoparticles. The OSCs fabricated on the brush-painted TIO films exhibited cell-performance with an open circuit voltage (Voc) of 0.61V, shot circuit current (Jsc) of 7.90mA/cm2, fill factor (FF) of 61%, and power conversion efficiency (PCE) of 2.94%. This indicates that brush-painted TIO film is a promising cost-effective transparent electrode for printing-based OSCs with its simple process and high performance.

Solution-processed copper-nickel nanowire anodes for organic solar cells.

This work describes a process to make anodes for organic solar cells from copper-nickel nanowires with solution-phase processing. Copper nanowire films were coated from solution onto glass and made conductive by dipping them in acetic acid. Acetic acid removes the passivating oxide from the surface of copper nanowires, thereby reducing the contact resistance between nanowires to nearly the same extent as hydrogen annealing. Films of copper nanowires were made as oxidation resistant as silver nanowires under dry and humid conditions by dipping them in an electroless nickel plating solution. Organic solar cells utilizing these completely solution-processed copper-nickel nanowire films exhibited efficiencies of 4.9%.

Control of Thickness of PEDOT Electrodeposits on Glass/ITO Electrodes from Organic Solutions and its Use as Anode in Organic Solar Cells

Poly-ethylendioxythiophene (PEDOT) was electropolymerized from the monomer EDOT in acetonitrile (ACN) containing Bu4N+ClO4-, BF4- or PF6- ions as supporting electrolyte. The electrode used was transparent electrodes (Glass/ITO) in order to generate the anode of an organic solar cell (OSC). Potentiodynamic and potentiostatic electropolymerization techniques were used to make the conducting polymer deposits (E-PEDOT), which were obtained as a thin film onto the ITO surface. It was possible to control the thickness of the electrodeposited films in the range of 15 to 200nm measured by AFM. With the thinner films (until 100nm), it was observed that its absorbance at 700nm was linearly dependent with their thickness and it was possible to obtain an equation that was used to measure the films thickness of future experiments. The E-PEDOT films were successfully used for constructing OSC's and the efficiency values found were equivalent or slightly superior to those found with the classical PEDOT:PSS anode.