Highly durable inverted-type organic solar cell using amorphous titanium oxide as electron collection electrode inserted between ITO and organic layer

An indium tin oxide/titanium oxide/[6,6]-phenyl C₆₁ butyric acid methyl ester:regioregular poly(3-hexylthiophene)/poly(3,4-ethylenedioxylenethiophene):poly(4-styrene sulfonic acid)/Au type organic solar cell (ITO/TiO_x/PCBM:P3HT/PEDOT:PSS/Au) with 1 cm² active area, which is called “inverted-type solar cell”, was developed using an ITO/amorphous titanium oxide (TiO_x) electrode prepared by a sol-gel technique instead of a low functional electrode such as Al. The power conversion efficiency (η) of 2.47% was obtained by irradiating AM 1.5G-100 mW cm⁻² simulated sunlight. We found that a photoconduction of TiO_x by irradiating UV light containing slightly in the simulated sunlight was required to drive this solar cell. The device durability in an ambient atmosphere was maintained for more than 20 h under continuous light irradiation. Further, when the air-stable device was covered by a glass plate with a water getter sheet which was coated by an epoxy-UV resin as sealing material, the durability was still higher and over 96% of relative efficiency was observed even after continuous light irradiation for 120 h.     

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.     

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.     

Synthesis and photovoltaic properties of a star-shaped molecule with triphenylamine as core and benzo[1,2,5]thiadiazol vinylene as arms

A solution-processable and star-shaped molecule 4-((E)-2-(benzo[1,2,5]thiadiazol-4-yl)vinyl)-N,N-bis(4-((E)-2-(benzo[1,2,5]thiadiazol-7-yl)vinyl)phenyl)benzenamine (TPA-BT) has been designed and synthesized by palladium-catalyzed Heck reaction for the application in organic solar cells (OSCs). The molecule possesses a D-A structure with a triphenylamine core (donor unit) linked with three benzo[1,2,5]thiadiazole (acceptor unit) arms through double bonds. TPA-BT film shows a strong absorption peak in the visible wavelength range from 400 to 560 nm, which could be ascribed to the charge transfer band of the D-A structure of the molecule. The bulk-heterojunction OSCs with the device structure of ITO/PEDOT:PSS/TPA-BT:PCBM/Ca/Al (or Ba/Al) were fabricated by spin-coating the blend solution of TPA-BT and PCBM (1:3, w/w), in which TPA-BT was used as donor and PCBM as acceptor materials. The devices show a high open circuit voltage of ca. 0.9 V and a power conversion efficiency of 0.61%, under the illumination of AM 1.5, 100 mW/cm². The results indicate that TPA-BT is a promising solution-processable organic photovoltaic material.     

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%.     

Large-area organic photovoltaic module—Fabrication and performance

Here we describe the fabrication of the largest (233 cm² total area) organic photovoltaic (OPV) module (polymer:fullerene) to be certified by the National Renewable Energy Laboratory (NREL). OPV solar cells were fabricated at Plextronics by spin coating a blend of poly 3-hexylthiophene-2,5 diyl (P3HT) and [6,6] phenyl C₆₁ butyric acid methyl ester (PCBM) on top of our hole transport layer (HTL), Plexcore^® OC. In laboratory-scale devices (0.09 cm²), this system routinely exhibits power conversion efficiencies exceeding 3.7%. This P3HT:PCBM active layer and HTL ink system was used to scale up to the larger area module (15.2 cm×15.2 cm module size, i.e. 233 cm² total area; 108 cm² active area), which was certified by NREL as having 1.1% total area efficiency (3.4% active area efficiency).     

Gravure printed flexible organic photovoltaic modules

In this letter, organic solar cell modules based on poly-3-hexylthiophene (P3HT) and [6.6]-phenyl-C61-butyric acid methyl ester (PCBM) blend films with a module active area of 15.45 cm² prepared by roll-to-roll (R2R) compatible gravure printing method are demonstrated. The gravure printed organic photovoltaic modules consist of eight serially connected solar cells in same substrate. Indium-tin-oxide (ITO) is patterned by screen printable etching paste. Hole injection layer and active layer are prepared by gravure printing method. All processing steps excluding cathode evaporation are performed in air. Electrical measurements are done to modules consisting of 5-8 serially connected solar cells. The photovoltaic modules comprising 5, 7 and 8 serially connected cells exhibit an active area power conversion efficiency of 1.92%, 1.79% and 1.68%, respectively (Oriel Sol3A Class AAA, AM1.5G, 100 mW cm⁻²).     

Photovoltaic effect in arylenevinylene-co-pyrrolenevinylene (AVPV)

Arylenevinylene-co-pyrrolenevinylene (AVPV) is a promising candidate amongst the group of new photovoltaic materials. It is a low band gap organic material with a band gap of 1.84 eV and absorbs sunlight in 300-700 nm range. In this paper, we demonstrate the photovoltaic effect in an organic bulk heterojunction photovoltaic device based on the blend of AVPV as an electron donor and [6,6]-phenyl-C₆₁ butyric acid methyl ester (PCBM) as the acceptor. The short-circuit current density of the device was of the order of 0.55 μA cm⁻² with an open-circuit voltage of 0.7 V, measured under 1 sun illumination of AM 1.5 through a calibrated solar simulator. Fill factor was estimated to be 12%. Further, the tests conducted after 2 weeks showed that short-circuit current was 0.21 μA cm⁻² and open-circuit voltage was 0.5 V with a fill factor of 9.8%, suggesting the possibility of stable AVPV-based organic solar cell (OSC).     

Low bandgap carbazole copolymers containing an electron-withdrawing side chain for solar cell applications

A new series of low bandgap carbazole copolymers containing an electron-withdrawing moiety as a side chain, via Suzuki, Yamamoto, and Stille polymerization reactions has been synthesized. Their bandgaps and molecular energy levels can be tuned by copolymerizing with different conjugated electron-donating units. The resulting copolymers have low optical and electrochemical bandgaps. The optical bandgaps of the copolymers range from 1.79 to 1.24 eV. In order to investigate their photovoltaic properties, polymer solar cell devices based on low bandgap copolymers were fabricated with a structure of ITO/PEDOT:PSS/copolymers:PCBM/Al, under the illumination of AM 1.5 G, 100 mW/cm². The power conversion efficiencies (PCE) of the polymer solar cells based on these low bandgap copolymers were measured. The best performance was obtained by using PC-CARB as the electron donor and 6,6-phenyl C₇₁-butyric acid methyl ester (PC₇₁BM) as the electron acceptor. The PCE of the solar cell based on PC-CARB/P₇₁CBM (1:4) was 1.27% with an open-circuit voltage (V_oc) of 0.65 V, and a short-circuit current (J_sc) of 6.69 mA/cm².     

Performance of electron beam deposited tungsten doped indium oxide films as anodes in organic solar cells

Tungsten doped indium oxide (IWO) thin films have been investigated as an alternative to indium tin oxide (ITO) anodes in organic solar cells (OSCs). The surface morphology, electrical, and optical properties of the IWO films grown by electron beam deposition were studied as a function of oxygen flow rate. For 120 nm thick IWO films deposited on float glass substrates at 350 °C and oxygen flow rate of 35 sccm, an electrical resistivity of 4.78×10⁻⁴ Ω cm and average transmittance of over 78% between 400 and 2000 nm were obtained. OSCs based on poly(3-hexylthiophene) and [6,6]-phenyl C₆₁-butlyric acid methyl ester were prepared on glass/IWO electrodes and the device performance was investigated as a function of IWO films with different oxygen flow rates. OSCs fabricated on the optimum IWO anode (oxygen flow rate of 30-35 sccm) exhibited a power conversion efficiency of ∼3.5%, which is comparable with the same device made on commercial glass/ITO electrodes (3.75%).     

Synthesis and characterization of a novel fullerene derivative for use in organic solar cells

A novel fullerene derivative with an N-hexylphenothiazine moiety, PTZ-C_60, was synthesized and characterized. The new synthesized fullerene showed good solubility in common organic solvents such as toluene, chlorobenzene and 1, 2 dichlorobenzene. The synthetic product PTZ-C₆₀ was characterized by ¹H and ¹³C NMR, FT-IR and UV-vis spectroscopy. Photovoltaic devices were fabricated using the new fullerene derivative as the electron acceptor and P3HT as the electron donor. The configuration of the device was as follows: ITO/PEDOT:PSS/active layer/LiF/Al. The weight ratios of the electron donor to the acceptor in the active layer were 1:0.5, 1:0.7, and 1:1. The open-circuit voltage (V_oc) of the fabricated devices was found to be higher than that of devices based on C₆₀ because the LUMO energy level of the new fullerene derivative was higher than that of C₆₀. Further, the power conversion efficiency (PCE) of these devices was observed to be high when annealing was carried out at 150 °C for 5 min and the thickness of the active layer was 80 nm. The maximum V_oc, short-circuit current density, and PCE of the best device were 0.608 V, 4.393 mA/cm², and 1.29%, respectively.     

Influence of n-type chemical doping layer on the performance of organic photovoltaic solar cells

We report the performance improvement of organic solar cell by addition of an n-type chemical doping layer in organic bulk heterojunction device. The power conversion efficiency (PCE) of P3HT and PCBM-71 based polymer solar cells increases by adding a mixture of TCNQ (7,7,8,8-tetracyanoquinodimethane) and LCV (Leucocrystal violet) between active layer and cathode electrode. The PCE of the cell increases by 14% compared to the control cell with Al-only cathode electrode. The device with an organic n-doped layer shows the J_SC of 8.88 mA/cm², V_OC of 0.51 V, FF of 60.1%, and thus the PCE of 2.72% under AM1.5 illumination of 100 mW/cm².     

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.     

Enhancement of short current density in polymer solar cells with phthalocyanine tin (IV) dichloride as interfacial layer

The effect of n-type phthalocyanine tin (IV) dichloride (SnCl₂Pc) as cathode interfacial layer on the performance of poly[2-methoxy,5-(2′-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV):[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) bulk heterojunction solar cells (ITO/PEDOT:PSS/MEH-PPV:PCBM/SnCl₂Pc/LiF/Al) is investigated. Our results show that the integration of SnCl₂Pc into the solar cell not only enhances the exciton dissociation efficiency due to the formation of additional MEH-PPV/SnCl₂Pc exciton dissociation junction, but also improves the electron transport and collection due to the step-like electron injection barrier to cathode caused by SnCl₂Pc interlayer. The incorporation of 6 nm thick SnCl₂Pc interlayer leads to 15.7% improvement of the short circuit current density (J_SC), which in turn results in 15.2% improvement of power conversion efficiency (η) up to 2.49%. The results suggest that the employment of an n-type organic semiconductor like SnCl₂Pc as an interlayer is a promising strategy to improve the device performance of polymer solar cells.     

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.     

Effects of ITO precursor thickness on transparent conductive Al doped ZnO film for solar cell applications

Al doped ZnO (AZO) film was continuously deposited on ITO precursor on glass substrate by d.c. magnetron sputtering. The thickness of ITO was varied from 30 to 120 nm in order to investigate the effect of ITO thickness on crystallinity of AZO film. X-ray diffraction measurement shows that AZO film grown on ITO has an enhanced (0 0 2) preferred orientation as the ITO thickness was increased. The crystalline structure improvement of AZO film with an increase of ITO precursor thickness is due to the near-epitaxial growth of AZO on ITO precursor. As the ITO thickness was increased, mobility of AZO film by the Hall measurement was significantly increased from 5.4 cm²/V s (no ITO) to 23.6 cm²/V s (ITO 120 nm), and resistivity was about 81.7% improved from 1.99×10⁻³ to 3.63×10⁻⁴ Ω cm. The AZO films with ITO revealed excellent average transmission of visible (90.0%) and NIR (89.6%) regions, whereas those of AZO film without ITO were 82.1% and 88.1%, respectively. The haze values of AZO film with ITO of 90 and 120 nm are similar or higher than those of AZO film without ITO. The surface textured AZO film with ITO precursor is promising for optoelectronic applications such as the front TCO of thin film solar cells.     

Improvement in light harvesting and performance of P3HT:PCBM solar cell by using 9,10-diphenylanthracene

We have studied the effect of 9,10-diphenylanthracene (DPA) as a conjugated dye with different concentrations on light harvesting and performance of solar cell composed from poly (3-hexylthiophene) (P3HT):[6,6]-phenyl-C₆₁butyric acid methyl ester (PCBM) blend films. The dye concentration effect was investigated with optical absorption spectroscopy, photocurrent spectroscopy, and current density-voltage characteristic measurements on devices under AM1.5 white light illumination with intensity of 100 mW/cm². The incorporation of the conjugated DPA inside P3HT:PCBM blend improved the light harvesting, slightly, and conjugation length indicated from the optical absorption and external quantum efficiency spectra. By adding specific amounts of the DPA into P3HT:PCBM blend, the external quantum efficiency and solar cell performance parameters, i.e., short circuit current density, fill factor, and power conversion efficiency improved as a result of improvement in the light harvesting and charge carrier transfer taking place between P3HT and PCBM through the conjugated DPA molecules.     

Synthesis and photovoltaic properties of a low bandgap donor-acceptor alternating copolymer with benzothiadiazole unit

A new donor-acceptor alternating copolymer as the donor material of the active layer in polymer solar cells has been synthesized. The alternating structure consisted of dithieno[3,2-b:2′,3′-d]thiophene (DTT) donor unit and 5,6-bis(tetradecyloxy)benzo-2,1,3-thiadiazole (BT) acceptor unit. Both units were confirmed by ¹H NMR and elemental analysis. Since the BT unit has long alkyoxyl side chains, the polymer was soluble in common organic solvents. Optoelectronic properties of the copolymer (PDTTBT) were investigated and observed by UV-vis, photoluminescence (PL) spectra, and cyclic voltammogram (CV). UV-vis spectrum exhibited a broad absorption band in the range of 300-750 nm and a low bandgap of 1.83 eV. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of PDTTBT could be determined from the data of CV and UV-vis spectrum. Based on the ITO/PEDOT:PSS/PDTTBT:PCBM/Al device structure, the power conversion efficiency (PCE) under the illumination of AM 1.5 (100 mW/cm²) was 0.113%. It was found that PCE of 0.301% could be acquired under the annealing condition at 150 °C for 30 min. In addition, solar cells fabricated with the 1,8-octanedithiol (OT) additive in the mixture solvent or adding TiO_x optical spacer show efficiencies significantly improved over 15%.     

Thin film solar cells of CdS/PbS chemically deposited by an ammonia-free process

A new type of solar cell with structure glass/ITO/CdS/PbS/conductive graphite was constructed and studied. Both window (CdS) and absorption (PbS) layers were deposited by means of the chemical bath deposition (CBD) technique. The maximum temperature employed during the solar cell processing was 70 °C and it did not include any post-treatment. In case of the CdS window layer, complexing agents alternative to ammonia were employed in the CBD process and their effects on the CdS films properties were studied. The solar cells are photosensitive in a large spectral range (all visible and near infrared regions); the cell with the area of 0.16 cm² without any special treatment has shown the values of open-circuit voltage V_oc of 290 mV and short circuit current J_sc of 14 mA/cm² with the efficiency η=1.63% (fill factor FF is 0.36) under illumination intensity of 900 W/m². It was found that the CBD-made PbS layer has a certain degree of porosity, which favorably affects its applicability in solar cell construction. The possible ways of device optimization, and in particular, the effect of the PbS grain size on its performance are discussed.     

Spray pyrolytically deposited nanoporous Ti doped hematite thin films for efficient photoelectrochemical splitting of water

Nanoporous hematite (α-Fe₂O₃) thin films doped with Ti⁴⁺ deposited by spray-pyrolysis were successfully used in photoelectrochemical splitting of water for solar hydrogen production. X-ray diffraction, field emission scanning electron microscopy, UV–visible absorption and photoelectrochemical studies have been performed on the undoped and Ti⁴⁺ doped hematite thin films. Morphology of α-Fe₂O₃ thin films was observed to be nanoporous, with increased porosity (pore size ∼12 to 20 nm) on increasing doping concentration. A significant decrease in the bandgap energy from 1.95 to 1.27 eV was found due to doping. α-Fe₂O₃ film doped with 0.02 M Ti⁴⁺ ions exhibited best solar to hydrogen conversion efficiency (photoconversion efficiency) of 1.38% at 0.5 V/SCE. Highest photocurrent densities of 0.34 mA/cm² at zero bias and 1.98 mA/cm² at 0.5 V/SCE were obtained by incorporating 0.02 M Ti⁴⁺ in α-Fe₂O₃, which are significantly larger than earlier reported values. Donor density (30.8 × 10²⁰ cm⁻³) and flatband potential (−1.01 V/SCE) obtained were also maximum for this sample. Hydrogen collected in 1 hr at Pt electrode with the best photoelectrode was 2.44 mL with 150 mW/cm² visible light source.