Dye sensitized solar cells on paper substrates

This article reports for the first time in the literature, a dye sensitized solar cells with 1.21% efficiency (V_oc=0.56 V, J_sc=6.70 mA/cm² and F.F.=0.33) on paper substrates. The current dye sensitized solar cell technology is based on fluorine doped SnO₂ (FTO) coated glass substrates. The problem with the glass substrate is its rigidity and heavy weight. Making DSSCs on paper opens the door for both photovoltaic and paper industries. The potential of using mature paper making and coating technologies will greatly reduce the current PV cost. Paper substrate based DSSCs not only offer the advantages of flexibility, portability and lightweight but also provide the opportunities for easy implantation to textile. In this study, a low temperature process is developed to coat uniform nickel on paper substrate as the metal contact to replace the traditional expensive FTO. The Ni paper showed excellent conductivity of 8-10 Ω/□. It is found that the control of metal oxide electrode morphology is critical to solar cell performance. The TiO₂ film has the tendency to crack on Ni coated paper, which resulted in the shunt of the device and no solar cell efficiency was obtained. ZnO film on the other hand had good morphology tolerance on Ni coated paper and yielded solar cell efficiency of 1.21% (V_oc=0.56 V, J_sc=6.70 mA/cm² and F.F.=0.33) under AM 1.5 (activation area is 0.16 cm²). The control sample of ZnO solar cell on FTO glasses has the efficiency of 2.66% (V_oc=0.64 V, J_sc=9.97 mA/cm² and F.F.=0.42).     

Synthesis conditions, light intensity and temperature effect on the performance of ZnO nanorods-based dye sensitized solar cells

We present in this work a careful study of the different parameters affecting vertically-aligned ZnO-nanorods (NRs) based dye sensitized solar cells (DSCs). We analyze the effect of synthesis conditions, light intensity, UV light and working temperature, and correlated them to the final photovoltaic properties of the DSC. Although similar studies can be found in the literature for DSCs based on TiO₂, this work is, to our knowledge, the first detailed study carried out for DSC based on vertically-aligned ZnO nanorods. The ZnO NRs were grown between 1.6 and 5.2 μm long. Electrodes made with 1.6 ± 0.2 μm thickness were used to analyze parameters such as synthesis conditions, light intensity (800-1500 W m⁻²), UV light irradiation and temperature (25-75 °C). We have also carried out initial analysis of the solar cell lifetime under continuous light irradiation at 45 °C, and analyzed the ZnO electrode before and after testing. The best photovoltaic response was characterized by a power conversion efficiency of 1.02%, with J_sc of 3.72 mA cm⁻², V_oc of 0.603 V and 45% FF (at 72 °C), for a ZnO NR electrode of 5.2 μm thickness. Comparison of our power conversion efficiency values with published data is also presented, as well as a brief discussion on the possible reasons behind the low power conversion efficiency observed for these type of solar cells.     

Preparation of Cu(In,Ga)Se2 thin films from Cu–Se/In–Ga–Se precursors for high-efficiency solar cells

Improved preparation process of a device quality Cu(In,Ga)Se₂ (CIGS) thin film was proposed for production of CIGS solar cells. In–Ga–Se layer were deposited on Mo-coated soda-lime glass, and then the layer was exposed to Cu and Se fluxes to form Cu–Se/In–Ga–Se precursor film at substrate temperature of over 200°C. The precursor film was annealed in Se flux at substrate temperature of over 500°C to obtain high-quality CIGS film. The solar cell with a MgF₂/ITO/ZnO/CdS/CIGS/Mo/glass structure showed an efficiency of 17.5% (V_oc=0.634 V, J_sc=36.4 mA/cm², FF=0.756).     

Fabrication of Cu2ZnSnS4 films by sulfurization of Cu/ZnSn/Cu precursor layers in sulfur atmosphere for solar cells

Cu₂ZnSnS₄ (CZTS) absorbers were grown by sulfurization of Cu/ZnSn/Cu precursors in sulfur atmosphere. The reaction mechanism of CZTS formation from the precursor was analyzed using XRD and Raman spectroscopy. The films with a single phase CZTS were formed at 560 and 580 °C by sulfurization for 30 min. The film grown at 560 °C showed bi-layer morphology with grooved large grains on the top and dense small grains near the bottom of the film. On the other hand, the film grown at 580 °C showed large grains with grooves that are extended from surface top to bottom of the film. The solar cell fabricated with the CZTS film grown at 560 °C showed the best conversion efficiency of 4.59% for 0.44 cm² with V_oc=0.545 V, J_sc=15.44 mA/cm², and FF=54.6. We found that further improvement of the microstructure of CZTS films can increase the efficiency of CZTS solar cells.     

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 study of vapor CdCl2 treatment by CSS in CdS/CdTe solar cells

We report the effect of CdCl₂ vapor treatment on the photovoltaic parameters of CdS/CdTe solar cells. Vapor treatment allows combining CdCl₂ exposure time and annealing in one step. In this alternative treatment, the CdS/CdTe substrates were treated with CdCl₂ vapor in a close spaced sublimation (CSS) configuration. The substrate temperature and CdCl₂ powder source temperature were 400 °C. The treatment was done by varying the treatment time (t) from 15 to 90 min. Such solar cells are examined by measuring their current density versus voltage (J-V) characteristics. The open-circuit voltage (V_oc), short circuit current density (J_sc) and fill factor (FF) of our best cell, fabricated and normalized to the area of 1 cm², were V_oc = 663 mV, J_sc = 18.5 mA/cm² and FF = 40%, respectively, corresponding to a total area conversion efficiency of η = 5%. In cells of minor area (0.1 cm²) efficiencies of 8% have been obtained.     

Fabrication of organic photovoltaic cells with double-layer ZnO structure

The fabrication process of a photovoltaic cell with a structure of indium-tin-oxide (ITO)/double ZnO/C₆₀/poly(3-hexylthiophene) (PAT6)/Ag has been investigated. The C₆₀/PAT6 heterojunction of this cell was fabricated by spin-coating a chloroform solution of PAT6 onto the C₆₀ thin film formed on double-layer ZnO-coated ITO. The fabrication of this double-layer ZnO was a new method, which was a composite of a sputtered ZnO layer and oriented zinc oxide nanograins layer fabricated at low temperature (343 K). Insertion of the double-layer ZnO in the photovoltaic cells produced enhanced performance with the power conversion efficiency of 1.31% under AM1.5 illumination.     

Fabrication of CuInSe2 films and solar cells by the sequential evaporation of In2Se3 and Cu2Se binary compounds

Cu₂Se/In_xSe(x≈1) double layers were prepared by sequentially evaporating In₂Se₃ and Cu₂Se binary compounds at room temperature on glass or Mo-coated glass substrates and CuInSe₂ films were formed by annealing them in a Se atmosphere at 550°C in the same vacuum chamber. The In_xSe thickness was fixed at 1 μm and the Cu₂Se thickness was varied from 0.2 to 0.5 μm. The CuInSe₂ films were single phase and the compositions were Cu-rich when the Cu₂Se thickness was above 0.35 μm. And then, a thin CuIn₃Se₅ layer was formed on the top of the CuInSe₂ film by co-evaporating In₂Se₃ and Se at 550°C. When the thickness of CuIn₃Se₅ layer was about 150 nm, the CuInSe₂ cell showed the active area efficiency of 5.4% with V_oc=286 mV, J_sc=36 mA/cm² and FF=0.52. As the CuIn₃Se₅ thickness increased further, the efficiency decreased.     

Fabrication and opto-electric properties of ITO/ZnO bilayer films on polyethersulfone substrates by ion beam-assisted evaporation

Transparent conducting oxides bilayer films stacked by one 130-nm-thick indium tin oxide (ITO) top layer and one 75-nm-thick zinc oxide (ZnO) buffer layer were grown onto polyethersulfone (PES) substrates by ion beam-assisted evaporation. The effects of ion energy and ZnO buffer layers on the structural and opto-electric properties of ITO films were initially investigated. The as-deposited ZnO buffer layers show wurtzite (0 0 2) preferred orientation on the PES substrates with ion beam assistance. The results of X-ray diffraction reveal a marked increase in the crystallinity of the ITO films which use ZnO as a buffer layer material. A drop of ∼60% in electrical resistivity of the ITO film on the PES can be achieved by using ZnO buffer layer. The transmittance of the ITO/ZnO bilayer was not deteriorated due to the insertion of ZnO layer. The lowest electrical resistivity of 6.552×10⁻⁴ Ω-cm associated with the transmittance of ∼80% at the wavelength of 550 nm can be obtained for the ITO film on the ZnO-coated PES at ion energy of 60 eV. The ITO films on the ZnO-buffered PES with moderate control of ion energy have a promising future for the application of the contact layers for flexible solar cells.     

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.     

Nearly carbon-free printable CIGS thin films for solar cell applications

In order to fabricate low cost and printable CuIn_xGa_1−xSe_yS_2−y (CIGS) thin film solar cells, a precursor solution based method was developed. Particularly, in this method, nearly carbon-free CIGS film was obtained by applying a three-step heat treatment process: the first for the elimination of carbon residue by air annealing, the second for the formation of CIGS alloy by sulfurization, and the third for grain growth and densification in the CIGS film by selenization. The film also revealed very large grains with a low degree of porosity, similar to those produced by the vacuum based method. A solar cell device with this film showed current-voltage characteristics of J_sc=21.02 mA/cm², V_oc=451 mV, FF=47.3%, and η=4.48% at standard conditions.     

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%.     

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.     

Electrodeposited cuprous oxide on indium tin oxide for solar applications

Semiconducting cuprous oxide films were prepared by electrodeposition onto commercial conducting glass coated with indium tin oxide deposited by spraying technique. The cuprous oxide (Cu₂O) films were deposited using a galvanostatic method from an alkaline CuSO₄ bath containing lactic acid and sodium hydroxide at a temperature of 60°C. The film's thickness was about 4–6 μm. This paper includes discussion for Cu₂O films fabrication, scanning electron microscopy and X-ray diffractometry studies, optical properties and experimental results of solar cells. The values of the open circuit voltage V_oc of 340 mV and the short circuit current density I_sc of 245 μA/cm² for ITO/Cu₂O solar cell were obtained by depositing graphite paste on the rear of the Cu₂O layer. It should be stressed that these cells exhibited photovoltaic properties after heat treatment of the films for 3 h at 130°C. An electrodeposited layer of Cu₂O offers wider possibilites for application and production of low cost cells, both in metal–semiconductor and hetero-junction cell structures, hence the need to improve the photovoltaic properties of the cells.     

Fabrication of pentanary Cu(InGa)(SeS)2 absorbers by selenization and sulfurization

The objective of this study is to find the key factors to improve V_oc. In this study, pentanary Cu(InGa)(SeS)₂ absorbers were prepared by selenization and sulfurization or a sulfurization after selenization (SAS) method. It is found that the “sulfurization degree” defined as a function of temperature and holding time at the sulfurization step is a key factor to enhance the Ga diffusion and improve V_oc. It is also verified that increase in the temperature difference between selenization and sulfurization enhances the incorporation of S into the selenide absorber. Applying these findings related to Ga and S, V_oc of 642 mV/cell and efficiency of 14.3% are achieved on a 30 cm×30 cm-sized soda-lime glass substrate.     

Enhancement in the performance of dye-sensitized solar cells containing ZnO-covered TiO2 electrodes prepared by thermal chemical vapor deposition

A ZnO-covered TiO₂ (denoted as ZnO/TiO₂) film was prepared by incorporating a small quantity of particulate ZnO in a TiO₂ matrix by thermal chemical vapor deposition. When used in a dye-sensitized solar cell, an enhancement was observed in both short-circuit photocurrent (J_sc) and open-circuit voltage (V_oc) by 12% and 17%, respectively, relative to those of a cell containing a bare TiO₂ film. The observed J_sc enhancement is attributed to the increase in the surface area of the ZnO/TiO₂ film, and the V_oc enhancement to the formation of a potential barrier by ZnO at TiO₂/electrolyte interface. The films were characterized by FE-SEM, EDX, and XRD.     

Effect of electron irradiation on the properties of CdTe/CdS solar cells

Polycrystalline CdTe/CdS solar cells are used in space, as well as terrestrial, applications. The results of the studies on the effect of 8 MeV electron irradiation on p-CdTe/n-CdS thin film solar cells prepared by radio frequency (RF) sputtering are presented in this article. Solar cell parameters like short circuit current (I_sc), open circuit voltage (V_oc), fill factor (FF), conversion efficiency (η), saturation current (I_s) and ideality factor (n) have been considered. CdTe thin film solar cells exhibit good stability against electron irradiation up to 100 kGy.     

Structure, composition and optical properties of Cu2ZnSnS4 thin films deposited by Pulsed Laser Deposition method

Polycrystalline Cu₂ZnSnS₄ (CZTS) thin films have been directly deposited on heating Mo-coated glass substrates by Pulsed Laser Deposition (PLD) method. The results of energy dispersive X-ray spectroscopy (EDX) indicate that these CZTS thin films are Cu-rich and S-poor. The combination of X-ray diffraction (XRD) results and Raman spectroscopy reveals that these thin films exhibit strong preferential orientation of grains along [1 1 2] direction and small Cu_2−xS phase easily exists in CZTS thin films. The lattice parameters and grain sizes have been examined based on XRD patterns and Atom Force Microscopy (AFM). The band gap (E_g) of CZTS thin films, which are determined by reflection spectroscopy varies from 1.53 to 1.98 eV, depending on substrate temperature (T_sub). The optical absorption coefficient of CZTS thin film (T_sub=450 °C) measured by spectroscopic ellipsometry (SE) is above 10⁴ cm⁻¹.     

Preparation of Cu(In,Ga)(S,Se)2 thin films by sequential evaporation and annealing in sulfur atmosphere

Cu(In,Ga)(S,Se)₂ thin films with high Ga/III ratio (around 0.8) were prepared by sequential evaporation from CuGaSe₂, CuInSe₂, In₂Se₃ and Ga₂Se₃ compounds and then annealing in H₂S gas atmosphere. The annealing temperature was varied from 400 to 500 °C. These samples were characterized by means of XRF, EPMA, XRD and SEM. The S/(S+Se) mole ratio in the thin films increased with increase in the annealing temperature, keeping the Cu, In and Ga contents nearly constant. The open circuit voltage increased and the short circuit current density decreased with increase in the annealing temperature. The best solar cell using Cu(In,Ga)(S,Se)₂ thin film with Ga/(In+Ga)=0.79 and S/(S+Se)=0.11 annealed at 400 °C demonstrated V_oc=535 mV, I_sc=13.3 mA/cm², FF=0.61 and efficiency=4.34% without AR-coating.     

Air-stable triarylamine-based amorphous polymer as donor material for bulk-heterojunction organic solar cells

A new air-stable triarylamine-based amorphous polymer, TSP-T11, which consists of thiophene and triarylamine units, can be successfully utilized to fabricate bulk-heterojunction organic photovoltaics (OPVs) using PC₆₀BM or PC₇₀BM as acceptor materials. The highest level of performance of OPVs optimized at TSP-T11:PC₇₀BM (weight ratios of 1:4) with thicknesses of 68 nm exhibited an open circuit voltage (V_oc) of 0.75 V, a short circuit current (J_sc) of 8.03 mA cm⁻², and a power-conversion efficiency (PCE) of 2.22% under simulated air mass 1.5 solar irradiation at 100 mW cm⁻². Although TSP-T11 has a lower hole mobility (1.5×10⁻⁴ cm² V⁻¹ s⁻¹) than P3HT, the use of amorphous film of TSP-T11 as a donor material for OPVs offers advantages over the use of polycrystalline film of P3HT in terms of its air-stability and pinhole-free homogeneous morphology.