Hybrid inverted organic photovoltaic cells based on nanoporous TiO2 films and organic small molecules

Solar cells based on nanoporous TiO₂ films with an inverted structure of indium tin oxide (ITO)/TiO₂/copper phthalocyanine (CuPc):fullerene (C₆₀)/CuPc/poly(3,4-oxyethyleneoxythiophene):poly(styrene sulfonate) (PEDOT:PSS)/Au were fabricated. The best overall photovoltaic performance undergoing a series of device optimization was achieved with the device of ITO/dense TiO₂ (30 nm)/nanoporous TiO₂ (130 nm)/C₆₀:CuPc (1:6 weight) (20 nm)/CuPc (20 nm)/PEDOT:PSS (50 nm)/Au (30 nm). The device using the nanoporous TiO₂ films has better photovoltaic properties compared to those using dense TiO₂ films. Higher photovoltaic performances were obtained by introducing a coevaporated layer of C₆₀:CuPc between TiO₂ and CuPc. The stability of inverted structure was better than that of the normal device, which gives a promising way for fabrication of solar cells with improved stability.     

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.     

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.     

Effect of solution processed graphene oxide/nickel oxide bi-layer on cell performance of bulk-heterojunction organic photovoltaic

We report the solution processed graphene oxide (GO), NiO_x and GO/NiO_x bi-layer used as an anode interfacial layer in organic bulk-heterojunction solar cells. The bulk-heterojunction solar cells using GO, NiO_x and GO/NiO_x bi-layer exhibited the conversion efficiency of 2.33%, 3.10% and 3.48%, respectively. The cell efficiency is correlated with the matching of energy levels between ITO, hole transport layer and P3HT and thus a well-matched stack layer of ITO/GO/NiO_x/P3HT:PCBM/LiF/Al shows the best cell efficiency of 3.48% with the J_SC of 8.71 mA/cm², V_OC of 0.602 V and FF of 66.44%.     

Efficiency enhancement of polymer solar cells by incorporating a self-assembled layer of silver nanodisks

We report the efficiency enhancement of polymer solar cells by incorporating a silver nanodisks' self-assembled layer, which was grown on the indium tin oxide (ITO) surface by the electrostatic interaction between the silver particles and modified ITO. Polymer solar cells with a structure of ITO (with silver nanodisks)/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) (Clevious P VP AI 4083)/poly(3-hexylthiophene):[6,6]-phenyl-C61 butyric acid methyl ester (P3HT:PC₆₁BM)/LiF/Al exhibited an open circuit voltage (V_OC) of 0.61±0.01 V, short-circuit current density (J_SC) of 9.24±0.09 mA/cm², a fill factor (FF) of 0.60±0.01, and power conversion efficiency (PCE) of 3.46±0.07% under one sun of simulated air mass 1.5 global (AM1.5G) irradiation (100 mW/cm²). The PCE was increased from 2.72±0.08% of the devices without silver nanodisks to 3.46±0.07%, mainly from the improved photocurrent density as a result of the excited localized surface plasmon resonance (LSPR) induced by the silver nanodisks.     

Performance of a new polymer electrolyte incorporated with diphenylamine in nanocrystalline dye-sensitized solar cell

A new solvent-free composite polymer electrolyte consisting of poly(ethylene oxide) (PEO) incorporated into diphenyl amine (DPA) along with KI and I₂ has been developed. The current-voltage characteristics of this nanocrystalline dye-sensitized solar cell measured under simulated sunlight with 1.5 AM at 60 mW/cm² have indicated that this cell generates a photocurrent of 10.2 mA/cm², together with a photovoltage of 810 mV and fill factor of 0.47 yielding an overall energy conversion efficiency of 6.5%. This result suggests that the electron donicity of DPA influences the interaction of nanocrystalline TiO₂ electrode and I⁻/I₃⁻ electrolyte, leading to a high performance of the fabricated solar cell.     

Inverted and transparent polymer solar cells prepared with vacuum-free processing

Inverted transparent polymer solar cells were fabricated by sequentially depositing several organic layers from fluids, on ITO/glass substrates. ITO was used as a cathode to collect electrons. The photovoltage of these diodes can be increased by up to 400 mV by inserting a buffer layer of polyethylene oxide between ITO and the active layers, which results in 4-fold enhancement of power conversion efficiency under the illumination of 100 mW/cm² simulated AM1.5 solar light. The enhancement of V_oc is consistent with the work function change between ITO and ITO/PEO measured by photoelectron spectroscopy. Solar cell production without vacuum processing may lower production costs.     

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.     

Pomegranate leaves and mulberry fruit as natural sensitizers for dye-sensitized solar cells

This study employs chlorophyll extract from pomegranate leaf and anthocyanin extract from mulberry fruit as the natural dyes for a dye-sensitized solar cell (DSSC). A self-developed nanofluid synthesis system is employed to prepare TiO₂ nanofluid with an average particle size of 25 nm. Electrophoresis deposition was performed to deposit TiO₂ nanoparticles on the indium tin oxide (ITO) conductive glass, forming a TiO₂ thin film with the thickness of 11 μm. Furthermore, this TiO₂ thin film was sintered at 450 °C to enhance the thin film compactness. Sputtering was used to prepare counter electrode by depositing Pt thin film on FTO glass at a thickness of 20 nm. The electrodes, electrolyte (), and dyes were assembled into a cell module and illuminated by a light source simulating AM 1.5 with a light strength of 100 mW/cm² to measure the photoelectric conversion efficiency of the prepared DSSCs. According to experimental results, the conversion efficiency of the DSSCs prepared by chlorophyll dyes from pomegranate leaf extract is 0.597%, with open-circuit voltage (V_OC) of 0.56 V, short-circuit current density (J_SC) of 2.05 mA/cm², and fill factor (FF) of 0.52. The conversion efficiency of the DSSCs prepared by anthocyanin dyes from mulberry extract is 0.548%, with V_OC of 0.555 V and J_SC of 1.89 mA/cm² and FF of 0.53. The conversion efficiency is 0.722% for chlorophyll and anthocyanin as the dye mixture, with V_OC of 0.53 V, J_SC of 2.8 mA/cm², and FF of 0.49.     

Polymer photovoltaic devices using highly conductive poly(3,4-ethylenedioxythiophene-methanol) electrode

In this work, modified poly(3,4-ethylenedioxythiophene) (PEDOT) was used as an anode in polymer photovoltaic devices (PVDs) based on poly(3-hexylthiophene) (P3HT):[6,6]-phenyl-C₆₀-butyric acid methyl ester (PCBM). We synthesized poly(3,4-ethylenedioxythiophene methanol) (PEDTM) with a transmittance of 87% (at 510 nm) and a conductivity of 700 S/cm. PEDTM was applied in photovoltaic devices as a hole transporting layer on indium-tin oxide (ITO) electrode as well as a direct anode layer. PVDs with PEDTM as hole transporting layers on ITO showed a very high short-circuit density of 14.87 mA/cm² and power conversion efficiency of 2.67% under an illumination of AM 1.5 G (100 mW/cm²). In addition, we also fabricated ITO-free PVDs using PEDTM as an anode, which exhibited a performance of 0.61% with a result of J_sc of 4.48 mA/cm², V_oc of 0.51 V, and FF of 27%.     

Hydrogen generation under sunlight by self ordered TiO2 nanotube arrays

Highly ordered TiO₂ nanotube arrays generate a considerable interest for hydrogen generation by an electrochemical photocell, since ordered architecture of nanotube arrays provides a unidirectional electric channel for electron's transport. Here, we report the hydrogen generation by highly ordered TiO₂ nanotube arrays under actual sunlight in KOH electrolyte. The two-electrode electrochemical cell included an adjustable anode compartment capable of tracing the trajectory of the sun and a set of alkaline batteries connected with a rheostat for application of external bias. The results showed that the photocurrent responses of nanotube arrays match well with the intensity of solar irradiance on a clear summer day. Addition of ethylene glycol into KOH electrolyte as a hole scavenger enhanced the rate of hydrogen generation. A maximum photocurrent density of 31 mA/cm² was observed at 13:30 h, by focusing the sunlight with an intensity of 113 mW/cm² on the surface of the TiO₂ nanotube arrays in 1 M KOH electrolyte with 10 vol% ethylene glycol under an applied bias of 0.5 V. The observed hydrogen generation rate was 4.4 mL/h cm² under the focalized solar irradiance with an intensity between 104 mW/cm² and 115 mW/cm² from 10:00 to 14:20 h.     

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.     

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).     

Temperature dependence of Cu2ZnSn(SexS1−x)4 monograin solar cells

The temperature dependence of open-circuit voltage (V_oc), short-circuit current (I_sc), fill factor (FF), and relative efficiency of monograin Cu₂ZnSn(Se_xS_1−x)₄ solar cell was measured. The light intensity was varied from 2.2 to 100 mW/cm² and temperatures were in the range of T = 175-300 K. With a light intensity of 100 mW/cm²dV_oc/dT was determined to be −1.91 mV/K and the dominating recombination process at temperatures close to room temperature was found to be related to the recombination in the space-charge region. The solar cell relative efficiency decreases with temperature by 0.013%/K. Our results show that the diode ideality factor n does not show remarkable temperature dependence and slightly increases from n = 1.85 to n = 2.05 in the temperature range between 175 and 300 K.     

A dye-sensitized photo-supercapacitor based on PProDOT-Et2 thick films

A photo-rechargeable supercapacitor (photo-supercapacitor, or PSC) is studied using a N3-dye adsorbed TiO₂ photoelectrode and PProDOT-Et₂ poly(3,3-diethyl-3,4-dihydro-2H-thieno-[3,4-b][1,4]dioxepine) polymer films as supercapacitor materials for electron storage. The PSC device, comprising a dye-sensitized solar cell (DSSC) and a supercapacitor (SC), can store the photo-to-electric energy. The PProDOT-Et₂ films are potentiostatically electropolymerized to form thick films (ca. 0.5 mm) with a specific capacitance of ca. 6.5 F cm⁻². A symmetrical (p/p) supercapacitor, with PProDOT-Et₂ coated on both electrodes, is also characterized before fabricating the three-electrode PSC. The PSC is tested under light illumination of 100 mW cm⁻², and attaining a photocharged voltage of 0.75 V and a discharged energy density of 21.3 μWh cm⁻².     

SnPc:C60 bulk heterojunction organic photovoltaic cells with MoO3 interlayer

Infrared sensitive small molecule organic photovoltaic cells were fabricated using a tin (II) phthalocyanine (SnPc) and C₆₀ bulk heterojunction layer. Molybdenum oxide (MoO₃) was used as an interlayer between the ITO anode and light absorbing layers. We found that the short-circuit current of the photovoltaic cells with MoO₃ interlayer was substantially enhanced, resulting in a 45% enhancement in power conversion efficiency. The maximum power conversion efficiency is 1.31% under 1 sun standard AM1.5G solar illumination of 100 mW/cm².     

An investigation on the performance of a silver ionic solid electrolyte system for a new detergent-based nanocrystalline dye-sensitized solar cell

A new dye-sensitized solar cell containing a detergent-based solid-state composite electrolyte has been investigated. The electrolyte is composed of Triton-X (C₁₆H₂₆O₂)n, with a powder mixture of surface-treated carbon powder, a fast ion-conducting material (FCM) 40(Cu_1−xAg_xI)−30(Ag₂O)−30(WO₃), (x=0.2), I⁻/I₃⁻ redox couple and an iodine complexing agent. The introduction of this detergent-based solid-state composite electrolyte forms a mosaic-like surface for the smooth transfer of photo-ejected electrons between electrodes. The current-voltage characteristics of TiO₂ nanocrystalline dye-sensitized solar cells based on the above solid-state composite electrolyte measured under simulated sunlight with AM 1.5 at 40 mW/cm² have revealed a short circuit current of 0.7 mA, open circuit voltage of 588 mV and fill-factor of 0.4, thus yielding an over-all conversion efficiency of 0.4%. These results have suggested the origin of appreciably, high charge transfer and the suppressed reverse reaction of electrons as due to the presence of the detergent dispersed with suspensions of electron- and ion-conducting phases.     

Large photocurrent generation of an ITO electrode modified with a red copper(II) complex

A film of copper complex [CuL](NO₃)₂ (L = 2,3,8,9-tetraphenyl-1,4,7,10-tetraazacyclododeca-1,3,7,9-tetraene) formed on indium-tin oxide (ITO) coated glass by the solvent evaporation of the acetonitrile solution of the complex onto the ITO substrate, was characterized by ultraviolet-visible absorption spectroscopy, cyclic voltammetry, scanning electron microscopy, and photoelectrochemistry. The photoelectrochemical cell by using [CuL](NO₃)₂ modified ITO, a saturated calomel electrode, and platinum wire as working, reference and counter electrodes respectively in 1 M Na₂SO₄, was found to exhibit a large prompt and reproducible cathodic photocurrent density of 71 μA/cm² under white light irradiation of 70 mW/cm² at an applied potential of −0.4 V, and an incident photon to current efficiency (IPCE) of 1.1% at λ = 660 nm. This Cu(II) complex photosensitizer has advantages of simple synthesis, low-cost, environmentally benign and good photoelectrochemical performance.     

Mechanical flexibility of transparent PEDOT:PSS electrodes prepared by gravure printing for flexible organic solar cells

The mechanical flexibility of transparent poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) films printed onto a flexible PET substrate using a gravure printing method was investigated using a lab-made bending test system. Gravure-printed PEDOT:PSS electrodes with a sheet resistance of 359 Ω/square and a transparency of 88.92% showed outstanding flexibility in several types of flexibility tests, including outer/inner bending, twisting and stretching. Notably, the PEDOT:PSS electrode had a constant resistance change (ΔR/R₀) within an outer and inner bending radius of 10 mm. In addition the stretched PEDOT:PSS electrode showed a fairly constant resistance change (ΔR/R₀) up to 4%, which is more stable than the resistance change of conventional amorphous ITO electrode. The twisting test revealed that the resistance of the PEDOT:PSS electrode began to increase at an angle of 36° due to delamination of the film from the PET substrate. Despite the high sheet resistance of the PEDOT:PSS electrode the flexible organic solar cells fabricated on the PEDOT:PSS electrode showed a power conversion efficiency of ∼2% (FF: 44.9%, V_o: 0.495 V and J_sc: 9.1 mA/cm²), indicating the possibility of using gravure printed PEDOT:PSS as a flexible and transparent electrode for printing-based flexible organic solar cells.     

Nano-structured Cu2O solar cells fabricated on sparse ZnO nanorods

Nano-structured Cu₂O/ZnO nanorod (NR) heterojunction solar cells fabricated on indium tin oxide (ITO)-coated glass are studied. Substrate film and NR density have a strong influence on the preferred growth of the Cu₂O film. The X-ray diffractometer (XRD) analysis results show that highly (2 0 0)-preferred Cu₂O film was formed when plating on plain ITO substrate. However, a highly (1 1 1)-preferred Cu₂O film was obtained when plating on sparse ZnO NRs. SEM, TEM and XRD studies on sparse NR samples indicate that the Cu₂O nano-crystallites mostly initiate its nucleation on the peripheral surfaces of the ZnO NRs, and are also highly (1 1 1)-oriented. Solar cells with ZnO NRs yielded much higher efficiency than those without. In addition, ZnO NRs plated on a ZnO-coated ITO glass significantly improve the shunt resistance and open-circuit voltage (V_oc) of the devices, with consistently much higher efficiency obtained than when ZnO NRs are directly plated on ITO film. However, longer NRs do not improve the efficiency due to low short-circuit current (J_sc) and slightly higher series resistance. The best conversion efficiency of 0.56% was obtained from a Cu₂O/ZnO NRs heterojunction solar cell fabricated on a 80 nm ZnO-coated ITO glass with V_oc=0.514 V, J_sc=2.64 mA/cm² and 41.5% fill factor.