The effect of temperature on the charge transport and transient absorption properties of K27 sensitized DSSC

The charge transport and transient absorption properties of K27 dye-sensitized solar cell have been investigated. The current–voltage (I–V) characteristics of the solar cell were analyzed by the thermionic emission theory. The ideality factor, barrier height and series resistance values of the solar cell were determined. The ideality factor higher than unity indicated the presence of non-ideal behavior in current–voltage characteristics at lower voltages. At the higher voltages, the charge transport mechanism for the solar cell is controlled by a space-charge limited current (SCLC) with an exponential distribution of traps. The built potential values are determined from capacitance–voltage plot and were found to be 0.14 and 0.58 V, respectively. The transient absorption data of K27 DSSC device suggest that the fast and slow phases are taking place. While the fast phase corresponds to regeneration of the dye cation by the iodide redox couple, the slow phase corresponds to the decay of long-lived I₂⁻/ TiO₂ electron absorption. The best conversion efficiency for K27 DSSC was found to be 0.317% under 100 mW/cm² (FF=0.584, V_oc=480 mV, I_sc=1.131 mA). The photocurrent results indicate that the photogeneration of charge carriers is a monophotonic process.     

Influence of aminothiazole additives in I/I3 redox electrolyte solution on Ru(II)-dye-sensitized nanocrystalline TiO2 solar cell performance

The influence of aminothiazole additives in acetonitrile solution of an I⁻/I₃⁻ redox electrolyte on the performance of a bis(tetrabutylammonium)cis-bis(thiocyanato)bis(2,2′- bipyridine-4-carboxylic acid, 4′-carboxylate)ruthenium(II) (N719) dye-sensitized TiO₂ solar cell was studied. The current–voltage characteristics were investigated under AM 1.5 (100 mW/cm²) for nine different aminothiazole compounds. The aminothiazole additives tested had varying influences on the solar cell performance. Most of the additives enhanced the open-circuit photovoltage (V_oc), but reduced the short circuit photocurrent density (J_sc) of the solar cell. Both the physical and chemical properties of the aminothiazoles were computationally calculated in order to determine the reasons that the additive influenced solar cell performance. The larger the calculated partial charge of the nitrogen atom in the thiazole, the higher the V_oc value. The V_oc value increased as the dipole moment of aminothiazoles in acetonitrile increased. Moreover, the V_oc of the solar cell also increased as the size of the aminothiazole molecules decreased. These results suggest that the electron donicity of the aminothiazole additives influenced the interaction with the TiO₂ photoelectrode, which altered the dye-sensitized solar cell performance.     

Influence of alkylaminopyridine additives in electrolytes on dye-sensitized solar cell performance

The influence of alkylaminopyridine additives on the performance of a bis(tetrabutylammonium)cis-bis(thiocyanato)bis(2,2′-bipyridine-4-carboxylic acid, 4′-carboxylate)ruthenium(II) dye-sensitized TiO₂ solar cell with an I⁻/I₃⁻ redox electrolyte in acetonitrile was studied. The current–voltage characteristics were measured for more than 20 different alkylaminopyridines under AM 1.5 (100 mW/cm²). The alkylaminopyridine additives tested had varying effects on the performance of the cell. All the additives decreased the short circuit photocurrent density (J_sc), but increased the open-circuit photovoltage (V_oc) of the solar cell. Molecular orbital calculations imply that the dipole moment of the alkylaminopyridine molecules influences the J_sc of the cell and that the size, solvent accessible surface area, and ionization energy all affect the V_oc of the cell. The highest V_oc of 0.88 V was observed in an electrolyte containing 4-pyrrolidinopyridine, which is comparable to the maximum V_oc of 0.9 V for a cell consisting of TiO₂ electrode and I⁻/I₃⁻ redox system.     

Influence of alkylpyridine additives in electrolyte solution on the performance of dye-sensitized solar cell

The influence of alkylpyridines additive to an I⁻/I₃⁻ redox electrolyte in acetonitrile on the performance of a bis(tetrabutylammonium)cis-bis(thiocyanato)bis(2,2′-bipyridine-4-carboxylic acid, 4′-carboxylate)ruthenium(II) dye-sensitized TiO₂ solar cell was studied. I–V measurements were performed using more than 30 different alkylpyridines. The alkylpyridine additives showed a significant influence on the performance of the cell. All the additives decreased the short-circuit photocurrent (J_sc), but most of the alkylpyridines increased the open-circuit photovoltage (V_oc) and fill factor (ff) of the solar cell. The results of the molecular orbital calculations suggest that the dipole moment of the alkylpyridine molecules correlate with the J_sc of the cell. These results also suggest that both the size and ionization energy of pyridines correlate with the V_oc of the cell. Under AM 1.5 (100 mW/cm²), the highest solar energy conversion efficiency (η) of 7.6% was achieved by using 2-propylpyridine as an additive, which was more effective than the previously reported additive, 4-t-butylpyridine.     

Performance of p-type silicon-oxide windows in amorphous silicon solar cell

a-SiO_x films have been prepared using silane and pure oxygen as reactive gases in plasma CVD system. Diborane was introduced as a doping gas to obtain p-type conduction silicon oxide. Infrared absorption spectra show the incorporation of Si–O stretch mode around 1000 cm⁻¹. The optical bandgap increases with the oxygen to silane gas ratio, while the electrical conductivity decreases. Hydrogenated amorphous silicon solar cells have been fabricated using p-type a-SiO_x with around 1.85 eV optical bandgap and conductivity greater than 10⁻⁷ S/cm. The measured current–voltage characteristics of the solar cells under 100 mW/cm² artificial light are V_oc=0.84 V, J_sc=14.7 mA/cm², FF=0.635 with a conversion efficiency of 7.84%.     

Semiconductor-sensitized solar cells based on nanocrystalline In2S3/In2O3 thin film electrodes

A possibility of semiconductor-sensitized thin film solar cells have been proposed. Nanocrystalline In₂S₃-modified In₂O₃ electrodes were prepared with sulfidation of In₂O₃ thin film electrodes under H₂S atmosphere. The band gap (E_g) of In₂S₃ estimated from the onset of the absorption spectrum was approximately 2.0 eV. The photovoltaic properties of a photoelectrochemical solar cell based on In₂S₃/In₂O₃ thin film electrodes and I⁻/I₃⁻ redox electrolytes were investigated. This photoelectrochemical cell could convert visible light of 400–700 nm to electron. A highly efficient incident photon-to-electron conversion efficiency (IPCE) of 33% was obtained at 410 nm. The solar energy conversion efficiency, η, under AM 1.5 (100 mW cm⁻²) was 0.31% with a short-circuit photocurrent density (J_sc) of 3.10 mA cm⁻², a open-circuit photovoltage (V_oc) of 0.26 V, and a fill factor ( ff ) of 0.38.     

Highly efficient photon-to-electron conversion with mercurochrome-sensitized nanoporous oxide semiconductor solar cells

Dye-sensitized solar cells based on nanoporous oxide semiconductor thin films such as TiO₂, Nb₂O₅, ZnO, SnO₂, and In₂O₃ with mercurochrome as the sensitizer were investigated. Photovoltaic performance of the solar cell depended remarkably on the semiconductor materials. Mercurochrome can convert visible light in the range of 400–600 nm to electrons. A high incident photon-to-current efficiency (IPCE), 69%, was obtained at 510 nm for a mercurochrome-sensitized ZnO solar cell with an I⁻/I₃⁻ redox electrolyte. The solar energy conversion efficiency under AM1.5 (99 mW cm⁻²) reached 2.5% with a short-circuit photocurrent density (J_sc) of 7.44 mA cm⁻², a open-circuit photovoltage (V_oc) of 0.52 V, and a fill factor (ff) of 0.64. The J_sc for the cell increased with increasing thickness of semiconductor thin films due to increasing amount of dye, while the V_oc decreased due to increasing of loss of injected electrons due to recombination and the rate constant for reverse reaction. Dependence of photovoltaic performance of mercurochrome-sensitized solar cells on semiconductor particles, light intensity, and irradiation time were also investigated. High performance of mercurochrome-sensitized ZnO solar cells indicate that the combination of dye and semiconductor is very important for highly efficient dye-sensitized solar cells and mercurochrome is one of the best sensitizers for nanoporous ZnO photoelectrode. In addition, a possibility of organic dye-sensitized oxide semiconductor solar cells has been proposed as well as one using metal complexes.     

Density of interface states, excess capacitance and series resistance in the metal–insulator–semiconductor (MIS) solar cells

Dark and illuminatied current–voltage (I–V) characteristics of Al/SiO_x/p-Si metal–insulator–semiconductor (MIS) solar cells were measured at room temperature. In addition to capacitance–voltage (C–V) and conductance–voltage (G–V), characteristics are studied at a wide frequency range of 1 kHz–10 MHz. The dark I–V characteristics showed non-ideal behavior with an ideal factor of 3.2. The density of interface states distribution profiles as a function of (E_ss−E_v) deduced from the I–V measurements at room temperature for the MIS solar cells on the order of 10¹³ cm⁻² eV⁻¹. These interface states were responsible for the non-ideal behavior of I–V, C–V and G–V characteristics. Frequency dispersion in capacitance for MIS solar cells can be interpreted only in terms of interface states. The interface states can follow the a.c. signal and yield an excess capacitance, which depends on the relaxation time of interface states and the frequency of the a.c. signal. It was observed that the excess capacitance C_o caused by an interface state decreases with an increase of frequency. The capacitances characteristics of MIS solar cells are affected not only in interface states but also series resistance. Analysis of this data indicated that the high interface states and series resistance leads to lower values of open-circuit voltage, short-circuit current density, and fill factor. Experimental results show that the location of interface states and series resistance have a significant effect on I–V, C–V and G–V characteristics.     

Photovoltaic characteristics of CuInS2/CdS solar cell by electron beam evaporation

When a CuInS₂/CdS solar cell was fabricated by depositing CdS thin film with dopant In of 1.0 at% on ternary compound CuInS₂ thin film with the lowest resistivity of 5.59 × 10⁻² Ωcm, its best result was as follows: V_oc = 461 mV, I_sc = 26.9 mA, FF = 0.685, η = 5.66% under the illumination of 100 mW/cm². And its series resistance and lattice mismatch was 5.1 Ω and 3.2%, respectively.Besides, a 4 layer structure solar cell of -CuInS₂/high -CuInS₂/high -CdS/low - CdS has been fabricated. When thickness of high - CuInS₂ was 0.2 μm, its best result was as follows: V_oc = 580 mV, I_sc = 30.6 mA, FF = 0.697, η = 8.25%. An its series resistance and lattice mismatch were 4.3 Ω and 2.8%, respectively.     

The study on high efficient AlxGa1−xAs/GaAs solar cells

The investigation of Al_xGa_1−xAs/GaAs solar cells is carried out by means of both metalorganic chemical vapor deposition (MOCVD) and liquid-phase epitaxial (LPE) technique. The measurements of illuminated I–V characteristics, dark I–V characteristics and quantum efficiencies were performed for the GaAs solar cells made in author's laboratory. The measuring results revealed that the quality of materials in GaAs solar cell's structures is the key factor for getting high-efficient GaAs solar cells, but the effect of post-growth technology on the performances of GaAs solar cells is also very strong. The 21.95% (AM0, 2×27 cm², 25°C) high conversion efficiency in a typical GaAs solar cell has been achieved owing to improving the quality of materials as well as optimizing the post-growth technology of devices.     

Analysis of electrical parameters in heterojunctions based on poly 3-octylthiophene and cadmium sulfide thin films

Planar hybrid heterojunctions were built with poly 3-octylthiophene (P3OT) and chemical bath-deposited cadmium sulfide (CdS) thin films on a conductive glass substrate. The organic material, P3OT, acts as a light absorber and the inorganic one, CdS, as the electron acceptor. Two types of CdS films had been used: one is as-deposited and the other doped with HgCl₂. Heterojunctions were formed by casting a chemically synthesized P3OT solution onto CdS films. The P3OT film thickness was also varied for heterojunction studies. Current vs. potential (I–V) characterizations under dark and illumination conditions were performed for the P3OT/CdS heterojunctions under 88 mW/cm² irradiance level, which show photovoltaic effect with different open circuit voltage (V_OC) levels, being as high as 1 V for some devices. A parametric analysis of I–V curves details the effect of CdS resistivity and P3OT film thickness on series and shunt resistance of the heterojunctions.     

Influence of pyrazole derivatives in I/I3 redox electrolyte solution on Ru(II)-dye-sensitized TiO2 solar cell performance

The influence of pyrazole additives in an I⁻/I₃⁻ redox electrolyte solution on the performance of a bis(tetrabutylammonium)cis-bis(thiocyanato)bis(2,2′-bipyridine-4-carboxylic acid, 4′-carboxylate)ruthenium(II) (N719) dye-sensitized TiO₂ solar cell was studied. The current–voltage characteristics of the cell were measured using 18 different pyrazole derivatives. All of the pyrazole additives enhanced the open-circuit photovoltage (V_oc) and the solar energy conversion efficiency (η), but reduced the short-circuit photocurrent density (J_sc). Most of the pyrazoles improved fill factor (ff). The physical and chemical properties of the pyrazoles were computationally calculated in order to elucidate the reasons for the additive effects on cell performance. The greater the partial charge of the nitrogen atom at position 2 in the pyrazole group, the larger the V_oc, but the smaller the J_sc values. As the dipole moment of the pyrazole derivatives increased, the V_oc value increased, but the J_sc value decreased. The V_oc of the cell also increased as the ionization energy of the pyrazoles decreased. These results suggest that the electron donicity of the pyrazole additives affected the interaction with the nanocrystalline TiO₂ photoelectrode, the I⁻/I₃⁻ electrolyte, and the acetonitrile solvent, which changed the Ru(II)-dye-sensitized solar cell performance.     

Characterization of CuInSe2/CdS thin-film solar cells prepared using CBD

CuInSe₂/CdS thin-film heterojunction solar cells were fabricated entirely by chemical bath deposition technique. The illuminated J−V characteristics of the devices prepared with different thicknesses of CdS and CuInSe₂ were studied. The typical solar cell parameters obtained for the best cell are: V_oc = 365 mV, J_sc = 12 mA/cm², FF = 61%, and η = 3.1% under an illumination of 85 mW/cm² on a cell of active area 0.1 cm². The J−V and C−V characteristics under dark condition and the spectral response were also studied for the best cell. The diode quality factor obtained is 1.7.     

CdTe/CdS Solar cells on flexible molybdenum substrates

Development of CdTe/CdS solar cells on flexible metallic substrates is highly interesting due to the light weight and flexible nature of the solar modules. We have deposited CdTe films onto flexible molybdenum substrates using close-spaced sublimation technique and the CdTe/CdS junction was developed by depositing a thin layer of CdS onto the CdTe substrate from a chemical bath. The devices were characterized by Current–voltage (I–V) and photocurrent spectroscopy techniques. Prior to the deposition of the transparent conducting layer, the devices were annealed in air at different temperatures and found that the devices annealed at 400°C have better photovoltaic parameters. The efficiency of a typical device under 60 mW cm⁻² illumination was estimated as 3.5%.     

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).     

Photosensitization of nanocrystalline TiO2 films by a polymer with two carboxylic groups, poly (3-thiophenemalonic acid)

Photovoltaic devices were assembled using a conducting polymer; poly (3-thiophenemalonic acid) sensitized TiO₂ electrodes and an electrolyte containing I₃⁻/I⁻ redox couple. This cell exhibited a short-circuit photocurrent (J_sc) of 6.65 mA cm⁻², an open circuit voltage (V_oc) of 355 mV and an efficiency of 1.5% under the illumination of 100 mW cm⁻² (AM 1.5). Addition of an ionic liquid, 1-methyl 3-n-hexylimidazolium iodide, into the electrolyte led to an improvement in the cell performances, achieving an overall efficiency of 1.8% under the same illumination. The average cell characteristics of the later devices are , with a fill factor of 0.65.     

5–20 MeV proton irradiation effects on GaAs/Ge solar cells for space use

This paper reports the high-energy proton irradiation effects on GaAs/Ge space solar cells. The solar cells were irradiated by protons with energy of 5–20 MeV at a fluence ranging from 1×10⁹ to 7×10¹³ cm⁻², and then their electric parameters were measured at AM0. It was shown that the I_sc, V_oc and P_max degrade as the fluence increases, respectively, but the degradation rates of I_sc, V_oc and P_max decrease as the proton energy increases, and the degradation is relative to proton irradiation-induced defect Ec−0.41 eV in irradiated GaAs/Ge cells.     

Improvement of a-Si solar cell properties by using SnO2:F TCO films coated with an ultra-thin TiO2 layer prepared by APCVD

TiO₂-overcoated SnO₂:F transparent conductive oxide films were prepared by atmospheric pressure chemical vapor deposition (APCVD) and an effect of TiO₂ layer thickness on a-Si solar cell properties was investigated. The optical properties and the structure of the TiO₂ films were evaluated by spectroscopic ellipsometry and X-ray difractometry. a-Si thin film solar cells were fabricated on the SnO₂:F films over-coated with TiO₂ films of various thicknesses (1.0, 1.5 and 2.0 nm) and I–V characteristics of these cells were measured under 1 sun (100 mW/cm² AM-1.5) illumination. It was found that the TiO₂ film deposited by APCVD has a refractive index of 2.4 at 550 nm and anatase crystal structure. The conversion efficiency of the a-Si solar cell fabricated on the 2.0 nm TiO₂-overcoated SnO₂:F film increased by 3%, which is mainly attributed to an increase in open circuit voltage (V_oc) of 30 mV.     

Solar cells using iodine-doped polythiophene–porphyrin polymer films

Wet-type organic solar cells containing 5,10,15,20-3-tetrathienylporphyrin (TThP) and polythiophene (PTh) films were fabricated. The TThP/PTh film was prepared on indium-tin-oxide (ITO) glass using an electrochemical polymerization method in an n-Bu₄NPF₆/CH₂Cl₂ solution. It was found that a small amount of iodine doping of the film improved the incident photon-to-electron conversion efficiency (IPCE) of a solar cell consisting of a TThP/PTh film and an aqueous electrolyte. A HOMO level measurement suggested that a modified HOMO level of the low iodine-doped TThP/PTh film allowed a fast electron transfer from PTh to a porphyrin moiety. To obtain further improvement, a sandwich-type solar cell using a 5% (w/w) aqueous solution of acetonitrile containing 0.05 M iodine and 0.5 M lithium iodide as an electrolyte was then fabricated. The solar cell absorbed light in the 300–800 nm wavelength range, converting this to a cathodic photocurrent with a maximum IPCE of 32% at 430 nm under irradiation of 5.0×10¹⁴ photon cm⁻² s⁻¹. This value is about 10 times higher than that of the solar cells using an aqueous electrolyte. The total energy conversion efficiency (η) of the solar cell under simulated sunlight reached 0.12% for 2.59 mW cm⁻² at AM1.5 and 0.05% for 100 mW cm⁻² at air mass 1.5.     

Bulk-heterojunction photovoltaic devices based on donor–acceptor organic small molecule blends

We present a systematic study on photovoltaic devices that combine an organic small molecule photoactive donor–acceptor bulk heterojunction system with controlled doping of the charge transport layers. The doped transport layers are formed using high vacuum co-evaporation deposition technique (i.e. co-sublimation of matrix and dopant). Solar cell devices have been fabricated based on zinc-phthalocyanine (ZnPc) as donor (D) and fullerene (C₆₀) as electron acceptor (A) with doped charge transport layers. The cells show a short circuit current, I_sc=1.5 mA/cm², an open circuit voltage, V_oc=450 mV, a fill factor, FF=0.5, and a power conversion efficiency, η_e=3.37% under sun (10 mW/cm²) white light illumination. In addition, these bulk-heterojunction photovoltaic devices were characterized under 1 sun (100 mW/cm²) white light illumination showing I_sc=6.3 mA/cm², V_oc=500 mV, and η_e=1.04%. We have observed that the performance of such ‘bulk-heterojunction’ photovoltaic devices is critically dependent on the transport properties of the interpenetrating network D/A system and doped charge transport layers.