Improvement in stability of poly(3-hexylthiophene-2,5-diyl)/[6,6]-phenyl-C61-butyric acid methyl ester bulk heterojunction solar cell by using UV light irradiation

A study of organic solar cells based on photoactive blends of the conjugated regioregular-poly(3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) with different UV-light treatments is presented. As expected, air exposure of an unencapsulated P3HT:PCBM solar cell is observed to result in rapid degradation of device efficiency. In order to ease this degradation, we found that exposing poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) to UV light may reduce the degradation and preserve good performance. Samples with PEDOT:PSS exposed to UV light show better long-run stability than the pristine cells. The active layer exposed to UV light shows the poorest performance and degrades rapidly. From the initial value, the efficiency decreased by 56% and 35% for pristine cells and cells with PEDOT:PSS exposed to UV light, respectively. It has been found that device half-life was 650 and 400 h for the samples with and without UV treatment, respectively. The trend in device performance was explained by observed changes in work function of the PEDOT:PSS layer and decreased absorption intensity of P3HT:PCBM.     

Light-induced degradation of the P3HT-based solar cells active layer

We report on the effects of long-term artificial accelerated ageing on the active layer of organic solar cells in the absence of oxygen. The samples were composed of a bulk heterojunction formed by poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM) deposited on poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). First, a set of experiments was performed to study the modifications resulting from prolonged exposure to UV-vis light. A gradual decrease in absorbance was recorded, and TEM results clearly indicated that the initial morphology was unstable upon long irradiation times. Second, we revealed that the annealing temperature of PEDOT:PSS strongly influenced the degradation of the active layer. Indeed, an increase in the PEDOT:PSS annealing temperature resulted in an important improvement in stability. Third, a comparison was made between different active layers obtained by changing the P3HT type, polymer:fullerene weight ratio and solvent nature. As expected, the polymer:fullerene weight ratio was shown to significantly impact the degradation kinetics. The ageing effects on the photovoltaic properties were then explored, and extrapolation of the data to outdoor exposure is also discussed.     

Influence of doped PEDOT:PSS on the performance of polymer solar cells

The influence of anode buffer layers of doped poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) on the performance of solar cells made from blends of poly(3-hexylthiophene) and [6,6]-phenyl-C₆₁-buytyric acid methyl ester has been investigated. Different concentration of ethylene glycol were added into the PEDOT:PSS solution to increase its conductivity. The surface roughness of the doped PEDOT:PSS film was changed, which was examined by atomic force microscopy. The best doped device with a power conversion efficiency of 4.39% as compared to 3.41% for the pristine device has been achieved. The enhanced PEDOT:PSS conductivity improved the short circuit current and fill factor of the doped device. The almost constant open circuit voltage indicated the well-established ohmic contact between the anode and active layer irrespective of the doping of the PEDOT:PSS. The changed surface roughness of the doped PEDOT:PSS film did not correlate with the morphology of the consequent active layer and the resultant device performance.     

A polymer photodiode using vapour-phase polymerized PEDOT as an anode

We report the photovoltaic properties of devices made using a highly conducting polymer electrode, from vapour-phase polymerized poly (3,4-ethylenedioxy) thiophene (VPP PEDOT) on glass substrate as an anode and a polyfluorene copolymer poly[2,7-(9,9-dioctyl-fluorene)-alt-5,5-(4′,7′-di-2thienyl-2′,1′3′-benzothiadiazole)] (APFO-3) mixed with [6,6]-phenyl-C₆₁-butyric acid methylester (PCBM) in the ratio of 1:4 as the active layer. The device performance was compared with that of devices made with PEDOT-PSS on glass substrates. The surfaces of VPP PEDOT were imaged using atomic force microscopy (AFM).     

Effect of strong base addition to hole-collecting buffer layer in polymer solar cells

We report a brief study on the effect of strong base addition to the hole-collecting buffer layer [poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)] on the performance of polymer solar cells made using blend films of poly(3-hexylthiophene) and soluble fullerene. A concentrated aqueous solution of sodium hydroxide (NaOH) was added to the PEDOT:PSS solution to decrease its acidity. The optical absorption spectra of modified buffer layers were measured to investigate the influence of NaOH addition on the spectral shape, while the surface of modified buffer layers was examined using atomic force microscopy. Results showed that the acidity of PEDOT:PSS solutions was remarkably reduced by adding the NaOH solution. However, the performance of solar cells was slightly degraded, which has been attributed to the decreased charge transportability as evidenced from the dark current-voltage characteristics.     

All solution processed tandem polymer solar cells based on thermocleavable materials

Multilayer tandem polymer solar cells were prepared by solution processing using thermocleavable polymer materials that allow for conversion to an insoluble state through a short thermal treatment. The problems associated with solubility during application of subsequent layers in the stack were efficiently solved. Devices comprised a transparent front cathode based on solution processed zinc oxide nanoparticles, a large band gap active layer based on a bulk heterojunction between zinc oxide and poly(3-carboxydithiophene) (P3CT) followed by a layer of PEDOT:PSS processed from water. The second cell in the stack employed a zinc oxide front cathode processed on top of the PEDOT:PSS layer from an organic solvent, a low band gap active layer based on a bulk heterojunction between zinc oxide and the novel poly(carboxyterthiophene-co-diphenylthienopyrazine) (P3CTTP) followed by a layer of PEDOT:PSS again processed from water and finally a printed silver electrode. The devices were prepared without the use of fullerenes and vacuum steps and employ only thermal treatments and orthogonal solvents. The devices exhibited operational stability in air without any form of encapsulation.     

Light harvesting in organic solar cells

We present a methodology which allows designing photonic crystals slabs (PCs) able to couple incident light into “slow Bloch modes” (SBMs) and dealing with their incorporation in an organic solar cell (OSC). We theoretically study different structures based on the same couple of organic materials (poly-3-hexylthiophène (P3HT) as donor and [6,6]-phenyl-C61-butiryc acid methyl ester (PCBM) as acceptor): a 2D photonic crystal based on a perfectly ordered P3HT/PCBM blend (placed in the air), a 1D photonic crystal based on a nanostructured PEDOT:PSS (Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate)) layer embedded in a P3HT:PCBM host matrix (first placed in the air and then inserted in an organic solar cell) and finally a 1D photonic crystal based on a nanostructured P3HT:PCBM layer covered by a metallic electrode and inserted in an OSC ( with and without nanostructuration of the PEDOT:PSS layer). We show that the light coupling into SBMs in an OSC depends on vertical interferences and that optical spacers are needed. We then demonstrate that the P3HT:PCBM active layer nanostructuring covered by a thick metallic electrode exhibits the highest gain (4% in the 400–700 nm spectral range) thanks to a simultaneous optimisation of the optical properties of the photonic crystal (coupling of SBM) and of the stack of the organic solar cell (vertical interferences).     

Improvement of device performances by thin interlayer in conjugated polymers/methanofullerene plastic solar cell

Recent studies have reported that a thin interlayer between poly(3, 4-ethylene dioxythiophene)-poly(styrene sulfonic acid) (PEDOT: PSS) and an emissive polymer layer leads to a large increase in the performances of polymer light-emitting diodes (PLEDs) by preventing significant quenching of the radiative excitons at the PEDOT: PSS interface; therefore, acting as an efficient exciton-blocking layer. Using the similar idea, a thin interlayer was fabricated between PEDOT: PSS and the active layer of conjugated polymers/methanofullerene composites in a plastic solar cell. The interlayer consisted of a poly(fluorene)-based hole transporter spin-coated directly on top of the PEDOT: PSS layer. The devices with the interlayer exhibited a higher efficiency than in those without the interlayer.     

Gravure printing for three subsequent solar cell layers of inverted structures on flexible substrates

Inverted organic photovoltaic devices have been fabricated by gravure printing on a flexible substrate. In order to enable printing of multiple layers sequentially, a systematic study of wetting behaviour of each layer in the device is performed. Successful wetting of a hydrophobic P3HT:PCBM surface by a hydrophilic PEDOT:PSS ink is achieved with the addition of a surfactant/alcohol to the PEDOT:PSS ink and with oxygen plasma treatment. We are therefore able to print titanium oxide, poly(3-hexylthiophene) (P3HT) blended with [6,6]-phenyl C61 butyric acid methyl ester (PCBM) and poly-3,4-ethylenedioxythiophene:poly(styrene sulphonic acid) (PEDOT:PSS). As result we get for three printed layers a 0.6% power conversion efficiency.     

Effect of indium and tin contamination on the efficiency and electronic properties of organic bulk hetero-junction solar cells

Organic photovoltaic devices using an electrode of indium tin oxide (ITO) coated with a buffer layer of poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) exposed to controlled humidity during fabrication showed a 65-75% decrease in efficiency and displayed S-shaped J-V curves, changes, which are attributed to different levels of indium and tin migration into the PEDOT:PSS film. A distinct shift in the secondary electron cut-off in the UV Photoelectron spectra (UPS) of ITO/PEDOT:PSS samples exposed to controlled humidity indicate an increase of the dipole at the ITO/PEDOT:PSS interface, which could explain the appearance of S-shaped J-V curves. Additionally, the electron density at low binding energies is reduced for the humidity exposed PEDOT:PSS suggesting a second mechanism for decreased device performance.     

Efficient hybrid carboxylated polythiophene/nanocrystalline TiO2 heterojunction solar cells

Efficient hybrid solar cells fabricated from TiO₂, novel carboxylated polythiophene poly (3-thiophenemalonic acid) P3TMA as sensitizer as well as hole conductor and poly (3-hexylthiophene) (P3HT) as hole transporter was described. UV-Vis absorption and morphology of the active layer were investigated. Device J/V characterizations with different P3HT layer thickness were measured and discussed. Efficiency improvements were observed in thinner P3HT layer thickness and with poly[3,4-(ethylenedioxy)-thiophene]:poly(styrene sulfonate) (PEDOT:PSS) as charge collection layer, and such device showed a short-circuit current density of 1.32 mA/cm², an open-circuit voltage of 0.44 V, a fill factor of 0.43, and a energy conversion efficiency of 0.25% at A.M. 1.5 solar illumination (100 mW/cm²).     

Morphological and electrical effect of an ultrathin iridium oxide hole extraction layer on P3HT:PCBM bulk-heterojunction solar cells

An ultrathin iridium layer was treated with O₂-plasma to form an iridium oxide (IrO_x), employed as a hole extraction layer in order to replace poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS) in organic photovoltaic (OPV) cells with poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester (P3HT:PCBM). The IrO_x layer affects the self-organization of the P3HT:PCBM photo-active layer due to its hydrophobic nature, inducing a well-organized intraplane structure with lamellae oriented normal to the substrate. Synchrotron radiation photoelectron spectroscopy results showed that the work function increased by 0.57 eV as the Ir layer on ITO changed to IrO_x by the O₂-plasma treatment. The OPV cell with IrO_x (2.0 nm) exhibits increased power conversion efficiency as high as 3.5% under 100 mW cm⁻² illumination with an air mass (AM 1.5G) condition, higher than that of 3.3% with PEDOT:PSS.     

Enhancing the durability of polymer solar cells using gold nano-dots

The durability of organic photovoltaic devices is improved by (a) replacing thermally labile poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) with gold nano-dots and (b) stabilizing the morphology of photoactive layers through thermally induced reaction. Gold nano-dots (Au-ND) (3–6 nm in diameter and 0.8 nm in height) were thermally deposited on ITO substrates prior to depositing a hole transporting layer (40 nm) of an azide-functionalized poly(3-hexylthiophene), P1, which was insolubilized by heating to 150 °C. A blend of P1 and [6,6]-phenyl C₆₁-butyric acid methyl ester (PCBM) was deposited and heated to 150 °C prior to the deposition of a Ca/Al cathode. The reaction of P1 with PCBM stabilized the bulk heterojunction film as evidenced by the suppression of crystallization of PCBM. Replacement of PEDOT:PSS with Au-ND, in combination with morphological stabilization, greatly improves the durability of PV devices under accelerated lifetime testing at 150 °C. Power conversion efficiencies (PCE) for the P1:PCBM devices stabilized at 1.25% after 28 h of accelerated testing at 150 °C, whereas conventional P3HT:PCBM devices on PEDOT/ITO dropped to 0.58% after only 7 h of accelerated testing. Prospects for similarly enhancing the durability of highly efficient PV devices are discussed.     

Air stable polymer photovoltaics based on a process free from vacuum steps and fullerenes

A polymer solar cell that can be stored under ambient conditions (25 °C and 35±5% relative humidity) in the dark for 6 months without noticeable degradation in performance is presented. The active layer is based on low-cost materials and is free from fullerenes. No vacuum steps are required for processing the device that employs an inverted device geometry, where the active layers in the device comprise a transparent cathode based on solution processed zinc oxide, an active layer based on a bulk heterojunction of zinc oxide nanoparticles and poly-(3-carboxydithiophene) (P3CT), a PEDOT:PSS layer and finally a printed silver based anode. No encapsulation was employed and the devices were robust and not sensitive to mechanical handling of the active layer and back electrode. The accelerated lifetime in air defined as 80% of the initial performance at continuous illumination (1000 W m⁻², AM1.5G, 72±2 °C, ambient atmosphere, 35±5% humidity) was typically 100 h and the devices were tested for 150 h. When keeping the same conditions and lowering the temperature, stable operation for hundreds of hours was possible. In terms of long-term stability, this performance is inferior to inorganic photovoltaics but the technology compares well and competes with small batteries in terms of capacity. The device efficiency more than doubled upon decreasing the incident light intensity from 1000 to 100 W m⁻².     

Patternable polymer bulk heterojunction photovoltaic cells on plastic by rotogravure printing

We study the fabrication of poly(3-hexylthiophene)—P3HT and [6,6]-phenyl-C61 butyric acid methyl ester—PCBM based polymer bulk heterojunction photovoltaic cells using rotogravure printing. By studying the dependencies of device performance on material and process parameters including contact angles, ink concentrations, ink viscosities, solvent characteristics, and gravure printing parameters, optimized hole transport layers [poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)—PEDOT:PSS] and active layers (P3HT:PCBM) are printed, resulting in devices with power conversion efficiencies as high as 1.68% under AM 1.5 G and a spectrally matched intensity of 100 mW/cm².     

Lifetimes of organic photovoltaics: Combining chemical and physical characterisation techniques to study degradation mechanisms

Degradation mechanisms of a photovoltaic device with an Al/C₆₀/C₁₂-PSV/PEDOT:PSS/ITO/glass geometry was studied using a combination of in-plane physical and chemical analysis techniques: TOF-SIMS, AFM, SEM, interference microscopy and fluorescence microscopy. A comparison was made between a device being stored in darkness in air and a device that had been subjected to illumination under simulated sunlight (1000  W m^–2, AM1.5) in air. It was found that oxygen diffuses through pinholes in the aluminium electrode. If stored in air in the dark the oxidation is limited to the C₆₀ layer. Illumination accelerates the oxidation/degradation and thus expands the process to involve at least the underlying layer of C₁₂-PSV. Furthermore, it was found that particles are formed in the device during storage.     

Optimization of process parameters for high-efficiency polymer photovoltaic devices based on P3HT:PCBM system

Here, we report the fabrication of high-efficiency poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C₆₁-butyric acid methyl ester (PCBM) blend photovoltaic device. Process parameters like solvent, solvent drying conditions, electron donor to acceptor ratio and cathodes structures are optimized in making the devices. For the first time, we used cosolvent systems to make active layer of P3HT:PCBM composite and G-PEDOT:PSS, made by mixing 6 wt% glycerol to PEDOT:PSS, is used as a buffer layer. Highest efficiency of 4.64% was obtained for the device made with 1:0.7 ratio of P3HT to PCBM, o-dichlorobenzene:chloroform cosolvent, newly developed slow process and G-PEDOT:PSS. Film morphology is evaluated by atomic force microscopy (AFM). Time-of-flight (TOF) and incident photon-to-current conversion efficiency (IPCE) measurements are also performed for the best device.     

Improved the efficiency of small molecule organic solar cell by double anode buffer layers

Small molecule organic solar cell with an optimized hybrid planar-mixed molecular heterojunction (PM-HJ) structure of indium tin oxide (ITO)/ poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT: PSS) doped with 4 wt% sorbitol/ pentacene (2 nm)/ copper phthalocyanine (CuPc) (10 nm)/ CuPc: C₆₀ mixed (20 nm)/ fullerene (C₆₀) (20 nm)/ bathocuproine (BCP) (10 nm)/Al was fabricated. PEDOT: PSS layer doped with 4 wt% sorbitol and pentacene layer were used as interlayers between the ITO anode and CuPc layer to help the hole transport. And then the short-circuit current (J_sc) of solar cell was enhanced by inserting both the PEDOT: PSS (4 wt% sorbitol) and the pentacene, resulting in a 400% enhancement in power conversion efficiency (PCE). The maximum PCE of 3.9% was obtained under 1sun standard AM1.5G solar illumination of 100 mW/cm².     

Effects of solvent-treated PEDOT:PSS on organic photovoltaic devices

We investigated the effects of poly(3, 4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) films treated with methoxyethanol (ME) on the performance of polymer solar cells, and the effects were compared with the PEDOT:PSS films treated with dimethyl sulfoxide (DMSO). In particular, a correlation between the performance parameters of polymer solar cells and changes in the conductivity, surface morphology, and work function of treated PEDOT:PSS was investigated as a function of an added ME and DMSO ratio. The enhanced conductivity of the treated PEDOT:PSS improved the short circuit current and reduced the series resistance of solar cells. While the enhanced conductivity and surface roughness of the treated PEDOT:PSS also induced the large leakage current and sacrificed the device FF. The open circuit voltage was almost constant, although the slightly reduced voltage was observed.