Effect of CsF interlayer on the performance of polymer bulk heterojunction solar cells

We fabricated polymer bulk heterojunction solar cells with blends of poly (2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV) and [6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM) by using CsF as an interlayer. Under illumination, the device with Al/CsF cathode exhibited a higher energy conversion efficiency compared to the Al/LiF cathode. The performance improvement with the Al/CsF cathode comes from the lower series resistance, which is almost constant (~6 Ω cm²) for all the CsF layer thicknesses included in the present study. The mechanism responsible for this phenomenon is attributed to the dissociation of CsF upon Al deposition to liberate Cs with a low work function, which reduces the interface resistance of the active layer/cathode and enhances the interior electric field for more efficient charge transport in the device.     

Photovoltaic properties of SnCl2Pc films and SnCl2Pc/pentacene heterostructures

The optical and photovoltaic properties of dichlorotin phthalocyanine (SnCl₂Pc) films and SnCl₂Pc/pentacene (Pn) heterostructures (HS) have been studied. Weak bands at 1.35, 1.52 and 2.05 eV have been found in absorption and modulated photoreflectance spectra of SnCl₂Pc films. These bands can be caused by the formation of charge transfer states. The low concentration of recombination centers of charge carriers has been formed on a free surface of SnCl₂Pc films. This concentration essentially decreases at air evacuation before vacuum deposition of a Pn layer. Therefore, interface with an insignificant recombination rate of charge carriers is formed for SnCl₂Pc/Pn HS.     

Increased photocurrent in bulk-heterojunction solar cells mediated by FeS2 nanocrystals

We found that the efficiency of bulk-heterojunction (BHJ) solar cells can be enhanced by incorporating a small amount of semiconductor FeS₂ nanocrystals (NCs) into the poly(3-hexylthiophene) (P3HT) and (6,6)-phenyl C61-butyric acid methyl ester (PCBM) based active layer. Through optical and nanoscale structure measurements, it is evident that low-cost and non-toxic FeS₂ NCs in such devices can efficiently improve charge carrier transport and exciton dissociation. This simple approach for increasing the photocurrent by NCs will be useful for accelerating the development of practical applications using organic solar cells.     

Surface ligand effects in MEH-PPV/TiO2 hybrid solar cells

TiO₂ nanorods (NRs) were synthesized by hydrolysis of titanium tetraisopropoxide (TTIP) using oleic acid (99%) as surfactant at low temperatures (80-100 °C) and are modified with different ligands: oleic acid (OLA), n-octyl-phosphonic acid (OPA) and thiophenol (TP) in order to investigate the effect of surface ligand on the excition dissociation and the charge transport in hybrid MEH-PPV/TiO₂ photovoltaic (PV) cells. The morphology and crystalline form of as-prepared TiO₂ NRs are examined by transmission electron microscopy (TEM), high-resolution TEM (HRTEM), X-ray diffraction (XRD) and Raman spectrometer (RS). The FTIR analysis confirms all the ligands coordinated with the Ti center of TiO₂ NRs. The optical properties of the modified TiO₂ NRs are characterized by UV-vis absorption spectra and photoluminescence (PL) spectra. Thiophenol modified TiO₂ NRs quench the PL of MEH-PPV more effectively than OLA-TiO₂ NRs and OPA-TiO₂ NRs. The power conversion efficiency of hybrid PV cells from thiophenol modified TiO₂ NRs and MEH-PPV is the highest among the investigated TiO₂ NRs.     

Efficient polymer bulk heterojunction solar cells with cesium acetate as the cathode interfacial layer

The enhanced performance of polymer solar cells based on regioregular poly(3-hexylthiophene) (P3HT) and methanofullerene [6,6]-phenyl C₆₁-butyric acid methyl ester (PCBM) blend was achieved by using cesium acetate (CH₃COOCs) as cathode buffer layer. Under 100 mW/cm² white light illumination, the device with 0.8 nm thick CH₃COOCs as cathode buffer layer exhibits power conversion efficiency (PCE) as high as (4.16 ± 0.02) %. Compared to the control devices without cathode buffer layer and with LiF as cathode buffer layer, the PCE is enhanced ∼100% and ∼31%, respectively. The introduction of the CH₃COOCs buffer layer effectively improves the photo-generated charge collection. The Kelvin Probe measurement shows that the work function of the CH₃COOCs is estimated to be −4.0 eV, which has an ideal energy band match with PCBM and a good property for electron collection. The static contact angle results indicated that the CH₃COOCs with the hydrophobic CH₃COO- group has an improved wettability between the buffer layer and the hydrophobic organic active layer surface, resulting in better interfacial contact and reduced contact resistance. The improved performance may be attributed to the dissociation of semi-conducting CH₃COOCs upon deposition to liberate Cs with a low work function, which reduces the interface resistance of the active layer and the cathode and enhances the interior electric field that may result in efficient charge transportation. Therefore, the CH₃COOCs interlayer could be a promising alternative to LiF to improve the efficiency of the electron collection of polymer bulk heterojunction solar cells.     

Well-aligned single-crystalline silicon nanowire hybrid solar cells on glass

Hybrid solar cells are fabricated on the glass substrate using well-aligned single-crystalline Si nanowires (SiNWs) and poly(3-hexylthiophene):[6,6]-phenyl-C₆₁-butyric acid methyl ester (P3HT:PCBM). Their key benefits are discussed. The well-aligned SiNWs are fabricated from Si wafer and transferred onto the glass substrate with the P3HT:PCBM. Such SiNWs provide uninterrupted conduction paths for electron transport, enhance the optical absorption to serve as an interesting candidate of the absorber, and increase the surface area for exciton dissociation. Our investigations show that SiNWs are promising for hybrid organic photovoltaic cells with improved performance by increasing the short-circuit current density from 7.17 to 11.61 mA/cm².     

Enhanced performance and stability in polymer photovoltaic cells using lithium benzoate as cathode interfacial layer

We report the enhanced performance and stability of polymer solar cells based on regioregular poly(3-hexylthiophene) (P3HT) and methanofullerene [6,6]-phenyl C₆₁-butyric acid methyl ester (PCBM) blend using lithium benzoate (C₆H₅COOLi) as cathode buffer layer between the active layer and the Al cathode. The effects of the C₆H₅COOLi thickness on the performance of polymer solar cell are also investigated. Under 100 mW/cm² white light illumination, the device with 1 nm thick C₆H₅COOLi as cathode buffer layer exhibits power conversion efficiency (PCE) as high as 3.41±0.07% and the device stability is greatly extended. Compared to the solar cell with LiF/Al cathode, the PCE is increased ca. 9.4%. Introduction of C₆H₅COOLi buffer layer effectively increases the shunt resistance and improves the photo-generated charge collection. The improved performance may attribute to the dissociation of semi-conducting C₆H₅COOLi upon deposition to liberate Li with a low work function, which reduces the interface resistance of the active layer and the cathode and enhances the interior electric field that may result in efficient charge transportion. In addition, the C₆H₅COOLi layer may serve as an effective oxygen and moisture diffusion barrier for the organic solar cells. Therefore, C₆H₅COOLi is a promising candidate as an interlayer to improve the efficiency of electron collection and to reduce the ambience influence on the stability of polymer solar cells.     

Integration of an M-phthalocyanine layer into solution-processed organic photovoltaic cells for improved spectral coverage

Photovoltaic devices made from blended poly(3,3?-didodecylquaterthiophene) (PQT-12) and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) incorporating an additional interlayer of M-phthalocyanine (M-Pc) have been characterized using current-voltage response, UV-visible absorption and external quantum efficiency. The introduction of H₂Pc, CuPc, ClInPc or TiOPc layers improves device performance compared to conventional bulk-heterojunction PQT-12:PCBM cells without M-Pc. Devices with M-Pc show increased absorption and free charge generation at longer wavelengths and have higher open circuit voltage. Polymorphic changes from solvent interaction are observed in TiOPc films during fabrication. Power conversion efficiencies of 0.79% are achieved for this modified bulk-heterojunction solar cell.     

Optimization of inverted tandem organic solar cells

Inverted tandem organic solar cells, consisting of two bulk heterojunction sub-cells with identical poly(3-hexylthiophene) (P3HT) and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C₆₁ (PCBM) active layer and a MoO₃/Ag/Al/Ca intermediate layer, have been presented and optimized. Indium tin oxide (ITO) modified by Ca acts as a cathode for electron collection and Ag is used as the anode for hole collection for the tandem device. A proper thickness of Ca (3 nm) forms a continuous layer, working as a cathode for the top sub-cell. MoO₃ as the anode buffer layer prevents exciton quenching and charge loss at the anode side, which could result in increase in interfacial resistance. The variance of sub-cell thickness adjusts the optical field distribution in the entire device, facilitating light absorption and good current matching in both sub-cells. The optimal inverted tandem device achieves a maximum power conversion efficiency of 2.89% with a short-circuit current density of 4.19 mA/cm², an open-circuit voltage of 1.17 V, and a fill factor of 59.0% under simulated 100 mW/cm² (AM 1.5G) solar irradiation.     

Planar-diffused photovoltaic device based on the MEH-PPV/PCBM system prepared by solution process

A planar-diffused photovoltaic device based on Poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylenevinylene] (MEH-PPV) and [6,6]-phenyl C₆₁-butyric acid methyl ester (PCBM) has been prepared by a simple solution process, in which the PCBM organic solution was spin-coated onto the underlying MEH-PPV layer to fabricate the MEH-PPV/PCBM planar-diffused active layer. Investigation of the effects of active layer thicknesses and solvents on the performance of planar-diffused photovoltaic devices indicates that, with increasing the underlying MEH-PPV layer thickness, a gradual transition from absolutely penetrated to planar-diffused active layer structure occurs and the best power conversion efficiency is obtained for the device prepared by spin-coating a non-aromatic chloroform solution of PCBM onto the MEH-PPV layer, rather than device prepared by spin-coating the mixed solution of chloroform and chlorobenzene or aromatic chlorobenzene solution of PCBM onto MEH-PPV layer. Based on the photovoltaic performance, a structural model of planar-diffused active layer is proposed.     

Influence of ZnO interlayer on the performance of inverted organic photovoltaic device

Inverted organic photovoltaic devices with a structure of fluorine tin oxide (FTO)/ZnO/poly(3-hexylthiophene) (P3HT):[6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM)/Ag were fabricated, in which ZnO interlayer serves as an electron selective layer. The ZnO interlayer includes three different nanostructures: polycrystalline seed layer, polycrystalline seed layer/loose nanopillars and polycrystalline seed layer/dense nanopillars. The influences of the different ZnO interlayers on the device performance were investigated. It is concluded that the polycrystalline seed layer/loose nanopillars offer more interfacial area with the P3HT:PCBM blends and acts as a continuous conducting path to the cathode. Our results demonstrate that effective infiltration of the blends into the ZnO nanopillars is critical for optimizing the device performance.     

Ni–YSZ-supported tubular solid oxide fuel cells with GDC interlayer between YSZ electrolyte and LSCF cathode

Highly sinterable gadolinia doped ceria (GDC) powders are prepared by carbonate coprecipitation and applied to the GDC interlayer in Ni–YSZ (yttria stabilized zirconia)-supported tubular solid oxide fuel cell in order to prevent the reaction between YSZ electrolyte and LSCF (La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ) cathode materials. The formation of highly resistive phase at the YSZ/LSCF interface was effectively blocked by the low-temperature densification of GDC interlayer using carbonate-derived active GDC powders and the suppression of Sr diffusion toward YSZ electrolyte via GDC interlayer by tuning the heat-treatment temperature for cathode fabrication. The power density of the cell with the configuration of Ni–YSZ/YSZ/GDC/LSCF–GDC/LSCF was as high as 906 mW cm⁻², which was 2.0 times higher than that (455 mW cm⁻²) of the cell with the configuration of Ni–YSZ/YSZ/LSM(La_0.8Sr_0.2MnO_3−δ)–YSZ/LSM at 750 °C.     

Photoelectron spectroscopy and atomic force microscopy study of 1,2-dicyano-methanofullerene C60(CN)2 thin film for photovoltaic applications

1, 2-dicyano-methanofullerene (C₆₀(CN)₂) is a soluble fullerene derivative that has been reported to have stronger electron affinity than parent C₆₀. Ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) experiments were carried out on C₆₀(CN)₂ thin films spin coated on heavily doped n-type Si substrate. UPS spectra enabled the determination of the vacuum shift at the fullerene derivative/Si interface and the onset of the highest occupied molecular orbital (HOMO). From the UV-vis absorption spectra of C₆₀(CN)₂ thin films spin coated on quartz substrates, the optical band gap (E_g) and the onset of absorption were determined. These measurements allowed the determination of the lowest occupied molecular orbital (LUMO) position. The morphology of the deposited film was probed by AFM and reveals non-uniformity of the thin film. Open circuit voltage (V_oc) measurements on P3HT/C₆₀(CN)₂ based organic solar cell device are compared to the commonly used P3HT/PCBM device.     

Preparation of semiconducting copolymer and its application in nanocrystalline TiO2 solar cells

Abstract

The past decade has witnessed increasing attention in the nanocrystalline TiO₂ solar cells (TSSCs). In this work, we have studied a novel TiO₂/PCBM/PPy solar cell based on blends of the semiconducting copolymer polypyrrole (PPy) and [6,6]-phenyl C₆₁ butyric acid methyl (PCBM) coated titanium dioxide (TiO₂) nanocrystal film to substitute the I₃^?/I^? redox electrolyte and the dye using in DSSCs. The research by incident photon to current efficiency spectra shows that the TiO₂ films had a stronger absorption in 300–500 nm light range. The performance of the resulting photovoltaic devices was investigated, and the effects of the PCBM/PPy ratio by photocurrent–voltage characteristics were researched. By the optimised PCBM/PPy ratio was 3∶1, The TSSC exhibited a short circuit current of 1·28 mA cm^?2, an open circuit voltage of 0·788 V, a fill factor of 0·654 and a light to electric energy conversion efficiency of 0·622% under a simulated solar light irradiation of 100 mW cm^?2.     

Effects of GDC interlayer on performance of low-temperature SOFCs

To bridge the sintering temperature gap between the electrolyte and the cathode, low-temperature anode supported solid oxide fuel cells (SOFCs) with various thickness of electrolyte interlayer were fabricated and investigated. The porous thin interlayer was dip-coated and fired onto the dense Ce_0.9Gd_0.1O_1.95 (GDC) electrolyte surface. With humidified hydrogen as the fuel and air as the oxidant, the single cell with a 0.15 μm interlayer achieved the maximum power density (MPD) of 0.9 W cm⁻² at 600 °C. The higher open circuit voltages (OCVs) (>0.9 V at 600 °C) were obtained in this study. The impedance results showed that the porous interlayer not only improved the interfacial contact between electrolyte and cathode, but also increased electrochemically active surface area. The cathode/electrolyte polarization resistance exhibited minimum when a 0.15 μm interlayer was added. The apparent activation energies derived from the Arrhenius plots of interfacial polarization resistances were about the same when the added interlayer was thinner, which indicated that the reaction mechanism did not change. However, the corresponding values were higher as the thick interlayer was introduced, which could be ascribed to the retarded oxygen ion transfer in the added porous layer. The cell area specific resistance (ASR) obtained by linear fitting I–V plot in the region of 0.6–0.7 V was higher than the ohmic resistance tested at OCV condition, and it was potentially attributed to the increased oxygen partial pressure at the anode as well as the contribution from polarization resistance, i.e. polarization of mass transport.     

Effect of organic salt doping on the performance of single layer bulk heterojunction organic solar cell

The effect of organic salt, tetrabutylammonium hexafluorophosphate (TBAPF₆) doping on the performance of single layer bulk heterojunction organic solar cell with ITO/MEHPPV:PCBM/Al structure was investigated where indium tin oxide (ITO) was used as anode, poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEHPPV) as donor, (6,6)-phenyl-C61 butyric acid methyl ester (PCBM) as acceptor and aluminium (Al) as cathode. In contrast to the undoped device, the electric field-treated device doped with TBAPF₆ exhibited better solar cell performance under illumination with a halogen projector lamp at 100 mW/cm². The short circuit current density and the open circuit voltage of the doped device increased from 0.54 μA/cm² to 6.41 μA/cm² and from 0.24 V to 0.50 V, respectively as compared to those of the undoped device. The significant improvement was attributed to the increase of built-in electric field caused by accumulation of ionic species at the active layer/electrode interfaces.     

Improved performance of MEH-PPV/ZnO solar cells by addition of lithium salt

The paper reports the improved performance by addition of lithium bis(trifluoromethanesulfonyl)amide (LiTFSI) to poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV) in the hybrid solar cells consisting of MEH-PPV as an electron donor and vertically aligned ZnO nanorod array as an electron acceptor. Results show that, with increasing the weight ratio R of LiTFSI/MEH-PPV, the charge transfer efficiency at MEH-PPV/ZnO interface, the device short circuit current (J_sc) and open circuit voltage (V_oc) get increased for R ? 2/10, but decreased when R > 2/10, resulting in a peak power conversion efficiency of η = 0.48% for R = 2/10 at AM 1.5 illumination (100 mW/cm²). It is revealed that the increased J_sc is due to the improved charge transfer between the MEH-PPV and ZnO as a result of the interaction between LiTFSI and MEH-PPV, while the increased V_oc and the decreased charge recombination are attributed to the increased hole mobility of MEH-PPV; moreover, the decreased J_sc and V_oc at high R values are attributed to the morphology degradation in the active layer due to the high LiTFSI content.     

Bulk heterojunction organic solar cells utilizing 1,4,8,11,15,18,22,25-octahexylphthalocyanine

Bulk heterojunction solar cells utilizing soluble phthalocyanine derivative, 1,4,8,11,15,18,22,25-octahexylphthalocyanine (C6PcH₂) have been investigated. The active layer was fabricated by spin-coating the mixed solution of C6PcH₂ and 1-(3-methoxy-carbonyl)-propyl-1-1-phenyl-(6,6)C61 (PCBM). The photovoltaic properties of the solar cell with bulk heterojunction of C6PcH₂ and PCBM demonstrated the strong dependence of active layer thickness, and the optimized active layer thickness was clarified to be 120 nm. By inserting MoO₃ hole transport buffer layer between the positive electrode and active layer, the FF and energy conversion efficiency were improved to be 0.50 and 3.2%, respectively. The tandem organic thin-film solar cell has also been studied by utilizing active layer materials of C6PcH₂ and poly(3-hexylthiophene) and the interlayer of LiF/Al/MoO₃ structure, and a high V_oc of 1.27 V has been achieved.     

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.     

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.