Improved power conversion efficiency of bulk heterojunction poly(3-hexylthiophene):PCBM photovoltaic devices using small molecule additive

We report that the power conversion efficiency (PCE) of the bulk heterojunction organic photovoltaic device based on poly(3-hexylthiophene):[6,6]-phenyl-C₆₁-butyric acid methyl ester (P3HT:PCBM) blend was improved by incorporating a small molecule SM having absorption band in the longer wavelength region. SM is a small molecule containing thienothiadiazole central unit with terminal cyanovinylene 4-nitrophenyl at both sides, which were connected to the central unit via a thiophene ring. The combination of SM with P3HT and PCBM allows not only a broad band absorption up to longer wavelength, but also tuning the inter-energy level leading to a higher short circuit current (J_sc) and open circuit voltage (V_oc). The device based on the as cast P3HT:PCBM:SM exhibits a PCE of 3.69%, which is higher than the device based on P3HT:PCBM and SM:PCBM blends. The overall PCE of the device based on thermally annealed blend is further improved up to 4.1%. The improvement of the PCE has been attributed to a better charge transport in the device, due to the increased crystallinity of the blend through thermal annealing.     

Enhancing the short-circuit current and efficiency of organic solar cells using MoO3 and CuPc as buffer layers

Efficient bulk-heterojunction (BHJ) (regioregular poly (3-hexylthiophene) (P3HT): (6, 6)-phenyl C₆₁ butyric acid methyl ester (PCBM)) solar cells were fabricated with molybdenum trioxide (MoO₃) and copper phthalocyanine (CuPc) as buffer layers. The insertion of MoO₃ layer was found to be critical to the device performance, effectively extracting holes to prevent the exciton quenching and reducing the interfacial resistance because of alignment of energy levels. The introduction of CuPc buffer layer was observed to be ameliorative for device performance, further enlarging the visible absorption spectra range of the devices. The effect of the MoO₃ and CuPc layer thickness on device performance was studied. The optimized thickness was achieved when MoO₃ layer was 12 nm and CuPc layer was 6 nm, resulting in optimized power conversion efficiency (PCE) of 3.76% under AM1.5G 100 mW/cm² illumination.     

Improving the current density Jsc of organic solar cells P3HT:PCBM by structuring the photoactive layer with functionalized SWCNTs

Several works concerning the incorporation of carbon nanotubes (CNTs) in bulk polymer RR-P3HT (regio-regular poly(3-hexylthiophene-2,5-diyl)):PCBM (methanofullerene phenyl–C₆₁–butyric-acid–methyl-ester) heterojunction have been already reported by a number of research groups. The optical and electrical properties of organic cells have been extensively studied. We investigated the incorporation of functionalized single wall carbon nanotubes (SWCNTs) into the matrix of P3HT:PCBM photovoltaic (PV) cells. The photovoltaic characteristics of the cells depend on the concentration of SWCNT. The incorporation of low concentrations of SWCNT in the photoactive layer increases the current density J_sc before annealing and it can reach above 9 mA/cm². We attribute the improved PV performances to partial crystallization of the RR-P3HT. As revealed by XRD studies and confirmed by the absorbance spectra, which exhibit the typical shoulder at 600 nm and absorbance in the near infrared region. Interestingly, we observe also that doping the P3HT:PCBM active layer by the functionalized SWCNTs increases the open circuit voltage V_oc.     

Widening of harvesting layer and area of P3HT/PCBM bulk-heterojunction photovoltaic cells

We have fabricated P3HT/PCBM based bulk-heterojunction photovoltaic cells with P3HT layer as the hole transport layer and PCBM layer as the electron transport layer between electrode and blended P3HT/PCBM layer in order to widen the photon harvesting layer. Current density has increased by about 1 mA/cm² by the insertion of P3HT layer and the resulting conversion efficiency has been improved by about 20%. We have also fabricated a centimeter-scale active area with an efficiency of ∼1%.     

Hybrid inverted bulk heterojunction solar cells with nanoimprinted TiO2 nanopores

Photovoltaic devices with highly ordered nanoporous titanium dioxide (titania; TiO₂) were fabricated to improve the photovoltaic performances by increasing TiO₂ interface area. The nanoimprinting lithography technique with polymethyl methacrylate (PMMA) mold was used to form titania nanopores. The solar cell with poly(3-hexylthiophene) (P3HT):[6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM) active layer on nanoporous titania showed higher power conversion efficiency (PCE) of 1.49% than on flat titania of 1.18%. The improved efficiency using nanoporous titania is interpreted with the enhanced-charge separation and collection by increasing the interface area between TiO₂ and active layer.     

Flexible large area polymer solar cells based on poly(3-hexylthiophene)/fullerene

Photovoltaic devices based on regioregular poly(3-hexylthiophene) (P3HT) and ([6,6]-phenyl-C61-butyric acid methyl ester) (PCBM) were fabricated and characterized using 5×5 cm ITO polyester foils with an active cell area of 0.5×0.5 cm². The HOMO/LUMO of P3HT and PCBM were estimated from cyclic voltammetry data. The complete quenching of photoluminescence of P3HT after mixing with PCBM indicates an effective charge transfer from P3HT to PCBM. The absorption spectrum of a blend (1:3 wt%) of both components shows that there is no ground state doping. Following device parameters without any special postproduction treatment were determined: V_OC=600 mV, I_SC=6.61 mA/cm², FF=0.39 and η_AM1.5 (P_IN:100 mW/cm²)=1.54%.     

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.     

ITO-free inverted polymer solar cells using a GZO cathode modified by ZnO

A gallium-doped ZnO (GZO) layer was investigated and compared with a conventional indium-tin-oxide (ITO) layer for use as a cathode in an inverted polymer solar cell based on poly(3-hexylthiophene) (P3HT):[6,6]-phenyl-C₆₁ butyric acid methyl ester (PCBM) bulk heterojunctions (BHJ). By modifying the GZO cathode with a ZnO thin layer, a high power conversion efficiency (3.4%) comparable to that of an inverted solar cell employing the same P3HT:PCBM BHJ photoactive layer with a conventional ITO/ZnO cathode was achieved. This result indicates that GZO is a transparent electrode material that can potentially be used to replace high-cost ITO.     

Effect of P3HT:PCBM concentration in solvent on performances of organic solar cells

Focused on phase separation and morphologies of poly(3-hexylthiophene):[6,6]-phenyl C₆₁ butyric acid methyl ester (P3HT:PCBM) active layers, we studied the effect of preparation conditions of the active layer on photovoltaic performance by changing concentration of P3HT:PCBM in the solvent. The performances of the cells varied depending on concentration of P3HT:PCBM (1:1 ratio by weight) in solvent even with the same thickness. The P3HT:PCBM active layer is prepared in cell structure of ITO/PEDOT/P3HT:PCBM/Al by changing spin-coating speed with different concentrations (1, 2 and 3 wt%) in chlorobenzene. Here, it was found that both the P3HT:PCBM concentrations and spin-coating conditions affected the crystalline structure formation, interchain interaction, morphology and phase separation during drying process of solvent and subsequent annealing.     

Significantly improve photoelectrochemical performance of Ti:Fe2O3 with CdSe modification and surface oxidation

Significant interest has been arisen to explore photoanodes for full optical absorption spectrums and good stability in photoelectrochemistry. Herein CdSe is used to modify Ti:Fe₂O₃ photoanode forming Ti:Fe₂O₃/CdSe heterojunction. Combining with an air annealing treatment, Ti:Fe₂O₃/CdSe exhibits a 6.5 times higher photocurrent density that of the pristine Ti:Fe₂O₃ to achieve 3.25 mA cm^?2 at 1.2 V vs. RHE. The photoelectrochemical (PEC) stability of Ti:Fe₂O₃/CdSe annealed in air shows great improvement comparable to both unannealed and annealed ones in Ar. The enhancement mechanisms for both heterojunction and annealing are explored for fundamental insights, which reveal that the surface oxide layer can significantly increase the PEC stability of Ti:Fe₂O₃/CdSe photoanode. X-ray photoelectron spectra and transmission electron microscope results further confirms the surface oxidation on CdSe layer after annealing in air.     

The effect of porphyrin inclusion on the spectral response of ternary P3HT:porphyrin:PCBM bulk heterojunction solar cells

Cyanoporphyrins have been included into the active layer of bulk heterojunction poly (3-hexylthiophene) (P3HT): [6,6]-phenyl-C₆₁ butyric acid methyl ester (PCBM) solar cells. The amount of porphyrin, P3HT and PCBM were systematically varied and the characteristics of the devices from the corresponding active layers were recorded. The spectral responses of the devices showed that the addition of the porphyrin to the active layer broadened the absorption efficiency of the device and led to a porphyrin contribution to the photocurrent of the solar cell. The porphyrin molecules did not contribute to the photocurrent unless both P3HT and PCBM were present in the active layer. In most cases, the porphyrin was unable to contribute to the photocurrent after the devices had been annealed, suggesting changes to the morphology of the active layer.     

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.     

The effect of C60 on the ZnO-nanorod surface in organic-inorganic hybrid photovoltaics

The electrical property of the organic-inorganic heterojunctions is one of the important factors affecting the performance of hybrid photovoltaic devices. We introduce a good electron conductor, C₆₀, to modify the surface of ZnO-nanorod arrays in a ZnO-nanorod/poly(3-hexylthiophene):(6,6)-phenyl C₆₁ butyric acid methyl ester (ZnO/P3HT:PCBM) hybrid photovoltaic structure. We found that there is ∼40% and ∼15% increment in the short circuit current (J_SC) and open circuit voltage (V_OC), respectively, for the device after C₆₀ modification. By probing the carrier dynamics and the surface property of ZnO-nanorods, the presence of the C₆₀ layer assists the exciton separation and passivates part of the defect states on the ZnO-nanorod surface. The charge transport property at the ZnO/polymer blend interface is, therefore, improved. As a result, higher charge concentration can transfer from the polymer blend to the ZnO-nanorods more effectively and subsequently travel to electrodes leading to the improved performance in the photovoltaic device.     

Photoelectrochemical study on charge transfer properties of nanostructured Fe2O3 modified by g-C3N4

Novel photocatalysts, which consist of two visible light responsive semiconductors including graphite-like carbon nitride (g-C₃N₄) and Fe₂O₃, were successfully synthesized via electrodeposition followed by chemical vapor deposition. The morphology of the g-C₃N₄/Fe₂O₃ can be tuned from regular nanosheets to porous cross-linked nanostructures. Remarkably, the optimum activity of the g-C₃N₄/Fe₂O₃ is almost 70 times higher than that of individual Fe₂O₃ for photoelectrochemical water splitting. The enhancement of photoelectrochemical activity could be assigned to the morphology change of the photocatalysts and the effective separation and transfer of photogenerated electrons and holes originated from the intimately contacted interfaces. The g-C₃N₄/Fe₂O₃ composites could be developed as high performance photocatalysts for water splitting and other optoelectric devices.     

Photovoltaic properties of polymer based organic solar cells adapted for non-transparent substrates

This paper explains a semi-transparent photovoltaic device structure using polymer based materials for light harvesting. Poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl C₆₁ butyric acid methyl ester (P3HT:PCBM) and poly(2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene) (MDMO-PPV:PCBM) as photoactive nano-layers were utilized and semi-transparent cells were compared with reference cells. Photoelectrical properties of developed devices were investigated. Also influencing factors of power conversion efficiency of devices were determined and possible application areas including solar harvesting textiles were discussed.     

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.     

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.     

Magnetic field effects on selective catalytic reduction of NO by NH3 over Fe2O3 catalyst in a magnetically fluidized bed

Selective catalytic reduction (SCR) of NO from simulated flue gas by ammonia with Fe₂O₃ particles as the catalyst was performed using a magnetically fluidized bed (MFB). X-ray diffraction (XRD) spectroscopy and Brunauer–Emmett–Teller (BET) method were used to analyze Fe₂O₃ catalyst. Important effects of magnetic fields were observed in the SCR of NO by ammonia over Fe₂O₃ catalyst. The apparent activation energies of SCR were reduced by external magnetic fields, and the SCR activity of Fe₂O₃ catalyst was improved with the magnetic fields at low temperatures. Thus the scope of temperature with high efficiency of NO removal was extended from 493–523 K to 453–523 K by magnetic fields. Magnetic fields of 0.01–0.015 T were suggested for NO removal on Fe₂O₃ catalyst with MFB. The results suggested that the magnetoadsorption of NO onto Fe₂O₃ surface together with NH₂ and NO free radicals effects induced by the external magnetic fields both acted to improve the rate of SCR of NO on Fe₂O₃ catalyst. On the other hand, magnetic field effects were also attributed to improved gas–solid contact in MFB.     

Comparison of magnetite/reduced graphene oxide nanocomposites and magnetite nanoparticles on enhancing hydrogen production in dark fermentation

Magnetite/reduced graphene oxide nanocomposites (Fe₃O₄-rGO NCs) and magnetite nanoparticles (Fe₃O₄ NPs) were added to enhance biohydrogen (bioH₂) production in dark fermentation. Concentration of supplements from 10 to 100 mg/L was appropriate to enhance bioH₂ production, and inhibition appeared once concentration exceeded 100 mg/L. The best bioH₂ yield was 198.30 mL/g glucose at 100 mg/L Fe₃O₄ NPs and 225.60 mL/g glucose at 100 mg/L Fe₃O₄-rGO NCs, which was 42.97% and 62.65% higher than that in the blank group, respectively. Both Fe₃O₄ NPs and Fe₃O₄-rGO NCs could intensify butyrate-type fermentation and change the hydrogen-producing microorganism cells morphology, but the enhancement effect of Fe₃O₄-rGO NCs was superior. Microbial community structure analysis showed that Clostridium-sensu-stricto-1 became more dominant ultimately by Fe₃O₄-rGO NCs.