Preparation and properties of CdS/CdTe thin film solar cell produced by periodic pulse electrodeposition technique

CdS/CdTe solar cells have been prepared by periodic pulse electrodepositionmethod. 10.8% efficient cell was made with open circuit voltage (V_oc)≈753mV, short-circuit current (J_sc)≈23.6 mA/cm² and fill factor (FF)≈0.61. Current-voltage-temperature measurments showed the variation of ideality factor (A) from 1.88 at 344 K to 4.49 at 202 K whereas voltage factor (α) was almost constant above 276 K. The junction transport is possibly dominated by a tunneling mechanism. Capacitance measurements gave the value of diffusion potential as 1.2 eV, ionized charged density as 5.9 × 10¹⁵ cm⁻³ and number of interface states (N_IS) as 2.8 × 10¹¹ cm⁻² eV⁻¹ at zero volt bias. Measurements of open circuit voltage (V_oc) with temperature gave the value of barrier height as 1.42 eV.     

Characterisation of CdCl2 treated electrodeposited CdS/CdTe thin film solar cell

An over 10% efficient electrodeposited CdS/CdTe solar cell has been prepared after CdCl₂ treatment. The open circuit voltage, V_oc, short-circuit current, J_sc and fill factor, FF were 758 mV, 21 mA cm⁻² and 0.65 respectively. The diode factor calculated from current-voltage-temperature measurements changed from 1.54 at 324 K to 2.64 at 146 K. The voltage factor, α ranged from 22.83 at 324 K to 29.46 at 146 K. Data from current-voltage-temperature measurements agrees with the model of Miller and Olsen and indicates that the current transport was a combination of tunneling and interface recombination. Capacitance-voltage-temperature measurements showed that capacitance decreased with increasing frequency and increased with temperature. Capacitance was insensitive to temperature indicating an intrinsic or low-doped depletion layer. The density of interface states was found to be 6.4 × 10¹⁰ cm⁻² eV⁻¹ at 293 K. The carrier concentration of CdTe calculated from Mott-Schottky plot was 1.5 × 10¹⁶ cm⁻³.     

Electronic and photovoltaic properties of Al/p-Si/copper phthalocyanine photodiode junction barrier

Al/p-Si/copper phthalocyanine photovoltaic device has been fabricated and characterised by current–voltage and capacitance–voltage measurements. Electrical properties of the device were determined by current–voltage characterizations under dark and illumination conditions. The density distribution of the interface states of the photodiode was found to vary from 8.88×10¹² eV⁻¹ cm⁻² in E_ss-0.54 eV to 4.51×10¹² eV⁻¹ cm⁻² in E_ss-0.61 eV. The device shows a photovoltaic behaviour with a maximum open circuit voltage Voc of 0.16 V and short-circuits current I_sc of 0.45 μA under 3500 lux light intensity.     

Effect of proton irradiation on electrical properties of CuInSe2 thin films

High-energy proton irradiation (380 keV and 1 MeV) on the electrical properties of CuInSe₂ (CIS) thin films has been investigated. The samples were epitaxially grown on GaAs (0 0 1) substrates by Radio Frequency sputtering. As the proton fluence exceeded 1×10¹³ cm⁻², the carrier concentration and mobility of the CIS thin films were decreased. The carrier removal rate with proton fluence was estimated to be about 1000 cm⁻¹. The electrical properties of CIS thin films before and after irradiation were studied between 80 and 300 K. From the temperature dependence of the carrier concentration in CIS thin films, we found N_D=9.5×10¹⁶ cm⁻³, N_A=3.7×10¹⁶ cm⁻³ and E_D=21 meV from the fitting to the experimental data on the basis of the charge balance equation. After irradiation, a defect level was created, and N_T=1×10¹⁷ cm⁻³ for a fluence of 3×10¹³ cm⁻², N_T=5.7×10¹⁷ cm⁻³ for a fluence of 1×10¹⁴ cm⁻² and E_T=95 meV were also obtained from the same fitting. The new defect, which acted as an electron trap, was due to proton irradiation, and the defect density was increased with proton fluence.     

Photoelectrochemical properties of rhodamine-C18 sensitized p-CuSCN photoelectrochemical cell (PEC)

Surface states in p-type Cu^I thiocyanate (CuSCN) were detected from I−V characteristics, diffuse reflectance spectra, and photocurrent action spectra. The p-CuSCN films are sensitized by rhodamine with octadecyl-alkyl chain, and the sensitized photocurrent is observed with the visible light illumination. In spite of the surface states in p-CuSCN, the maximum photocurrent quantum efficiency (gf_max) at λ = 560 nm, in 1 × 10⁻⁴ M KI + I₂ solution, pH = 6, reached 8.6%, where the surface dye concentration of photocathode Cu/p-CuSCN/Dye was 1.1 × 10¹⁴ molecules cm⁻². Photocathodes were biased at −0.25 V versus AgCl/Ag to give a zero dark current. From the variation of φ values with the reduction potential of electron acceptors, the cathodic sensitization mechanism presented is further confirmed.     

Interface characterisation of ZnSe/CuGaSe2 heterojunction

The ZnSe/CuGaSe₂ heterojunctions were fabricated by flash evaporation technique of CuGaSe₂ onto the (110) surface of ZnSe crystals. CuGaSe₂ layers had thickness 2–4 μm and showed a hole concentration up to (1.5–18.0)×10¹⁸ cm⁻³ and mobility μ4–24 cm² V⁻¹ s⁻¹ at 300 K. The charge carrier concentration in ZnSe crystals at 300 K was n=5.6×10¹⁶ cm⁻³ and their mobility μ=300 cm² V⁻¹ s⁻¹. The investigated ZnSe/CuGaSe₂ heterojunctions have at the interface an intermediate layer with a thickness of 450–750 Å and a linear graded band gap as well as an i-ZnSe compensated layer with a thickness of 1–2 μm and resistivity ρ10⁸–10⁹ Ω cm. The i-ZnSe layer is highly compensated due to the presence of Cu acceptor impurities. In this layer the Fermi level position E_c−F₀690 meV and a trap level position E_t−F₀17 meV were determined. The total trap concentration in the i-ZnSe layer is N_t5×10¹⁴ cm⁻³. The mean free path of excited charge carriers in the graded band gap region was calculated as λ55 Å. On the basis of experimental data analysis of electrophysical properties of both ZnSe/CuGaSe₂ heterojunctions and constituent materials the energetic band diagram of the investigated heterostructures is proposed. The current transport mechanism through ZnSe/CuGaSe₂ heterojunctions is consequently elucidated.     

Reduction of defects of polycrystalline silicon thin films by heat treatment with high-pressure H2O vapor

An improvement of electrical properties of pulsed laser crystalllized silicon films was achieved by simple heat treatment with high-pressure H₂O vapor. The electrical conductivity of 7.4×10¹⁷ cm⁻³ phosphorus-doped 50-nm-thick pulsed laser crystallized silicon films was markedly increased from 1.6×10⁻⁵ S/cm (as crystallized) to 2 S/cm by heat treatment at 270°C for 3 h with 1.25×10⁶ Pa H₂O vapor because of reduction of density of defect states localized at grain boundaries. Spin density was reduced from 1.7×10¹⁸ cm⁻³ (as crystallized) to 1.2×10¹⁷ cm⁻³ by heat treatment at 310°C for 3 h with 1.25×10⁶ Pa H₂O vapor.     

Radiation resistant AlGaAs/GaAs concentrator solar cells with internal Bragg reflector

The problem of increasing efficiency, reliability and radiation resistance of solar cells based on AlGaAs/GaAs heterostructures can be solved by using an internal Bragg reflector. The Bragg reflector as a back surface reflector and as a back surface potential barrier which allows to conserve the high photosensitivity in the long- and middle-wavelength parts of the spectrum after electron and proton irradiation. The effect of base doping and base thickness on the radiation resistance of AlGaAs/GaAs solar cells with the internal Bragg reflector has been investigated. Concentrator solar cells efficiency and related parameters before and after 3 MeV electron irradiation at the fluence up to 3×10¹⁵ cm⁻² are represented. A base doping level of 1×10¹⁵ cm⁻³ and base thickness in the range 1.1–1.6 μm give an EOL AM0 efficiency of 15.8% (BOL–22%) at 30 Suns concentration after exposure to 1×10¹⁵ cm⁻² electron fluence. This EOL efficiency is among the highest reported for GaAs single-junction concentrator cells under AM0 conditions. Making the base doping level lower and the base thinner allows retaining a j_EOL/j_BOL ratio of 0.96 upon exposure up to 3×10¹⁵e/cm² 3 MeV electron fluence. These results are additionally supported by the modeling calculations of the relative damage coefficient.     

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.     

Depletion layer characteristics of Al/microcrystalline chlorophyll a/Ag sandwich cells

The capacitance measurements of Al/microcrystalline chlorophyll a/Ag sandwich cells have been carried out at various frequencies in the dark and under illumination. The voltage-dependent capacitance, C, and the linear Schottky plots obtained at only low frequencies, 0.1 Hz, and under illumination suggest that the depletion layer mainly consists of trapped charges that are not able to follow the variations in applied voltage at high frequencies 100 Hz. From the linear Schottky plots, the depletion parameters, e.g., built-in potential, width at zero bias, and space charge density have been determined in the dark and under illumination (11 μW cm⁻²), which are, respectively, 620 mV, 300 Å and 2 × 10²³ m⁻³, and 700 mV, 205 Å and 4.78 × 10²³ m⁻³. The ageing of the cells in ambient air has been found to have dramatic effect on the capacitance characteristics of the cells. The diffusion of oxygen and presence of water vapour are most probably responsible for an inhomogeneous space charge developing within the depletion layer and the decrease of space charge density with time, respectively.     

Electron irradiation and thermal annealing effect on GaAs solar cells

This paper describes the effect of electron irradiation and thermal annealing on LPE AlGaAs/GaAs heterojunction solar cells with various p/n junction depths. The electron irradiation experiments were performed with energy of 3 MeV, fluences ranging from 1×10¹⁴ to 5×10¹⁵ e/cm². The results obtained demonstrate that the irradiation-induced degradation of performances of the cells is mainly in the short circuit current and could be mostly recovered by annealing at 260°C for 30 min. Four electron traps, E_c−0.24 eV, E_c−0.41 eV, E_c−0.51 eV, E_c−0.59 eV, were found by DLTS analysis, only two shallow levels of which could be removed by the annealing.     

Surface passivation at a SiO2/n-layer interface

Effects of surface passivation at a SiO₂/phosphorus-doped layer (n⁺-layer) front interface were investigated. Two kinds of cells with different surface concentration were fabricated. Surface potential at the interface was changed by applying bias voltage (V_F). Both open-circuit voltage and short-circuit current of the cell with n⁺-layer concentration of 3 × 10¹⁸ cm⁻³ depended on V_F. Internal quantum efficiency of this cell in short- and medium-wavelength range was changed by applying V_F. It was shown that cell performance was improved by the accumulation of electrons at the interface. To consider the work function difference between a material on the SiO₂ film and the n⁺-layer is important, and cell performance can be further improved by applying V_F to passivate the SiO₂/n⁺-layer interface.     

Effect of 8 MeV electron irradiation on electrical properties of CuInSe2 thin films

Radiation damages due to 8 MeV electron irradiation in electrical properties of CuInSe₂ thin films have been investigated. The n-type CuInSe₂ films in which the carrier concentration was about 3×10¹⁶ cm⁻³, were epitaxially grown on a GaAs(0 0 1) substrate by RF diode sputtering. No significant change in the electrical properties was observed under the electron fluence <3×10¹⁶ e cm⁻². As the electron fluence exceeded 10¹⁷ e cm⁻², both the carrier concentration and Hall mobility slightly decreased. The carrier removal rate was estimated to be about 0.8 cm⁻¹, which is slightly lower than that of III–V compound materials.     

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.     

Thermogalvanic conversion of heat to electricity

A thermogalvanic (nonisothermal) cell was constructed for carrying out power conversion efficiency measurements. The design departed from that of traditional thermogalvanic cells which have largely been used only for studies of open-circuit voltage. The cell was used to obtain temperature coefficients, ∂E/∂T, of the open circuit voltage and power conversion efficiencies, Φ, for an interelectrode temperature difference, ΔT, of 20 K, using various redox couples. The values obtained were the following: Cu²⁺/Cu (1.0 mol dm⁻³), ∂E/∂T = 785 μV K⁻¹; Zn²⁺/Zn (1.0 mol dm⁻³), ∂E/∂T = 790 μV K⁻¹; Fe phen(CN)₄⁻/Fe phen(CN)₄²⁻ (10⁻³ mol dm⁻³), ∂E/∂T = 1046 μV K⁻¹, Φ = 4.17 × 10⁻⁵%; Fe(CN)₆³⁻/Fe(CN)₆⁴⁻ (0.07 mol dm⁻³), ∂E/∂T = 1600 μV K⁻¹, Φ = 1.4 × 10⁻²%. More detailed studies of the latter system when [Fe(CN)₆³⁻] = [Fe(CN)₆⁴⁻] = 0.26 mol dm⁻³ and [KCl] = 0.80 mol dm⁻³, using platinum electrodes, with ΔT = 20 K, gave a current density of 1.45 mA cm⁻² and a power conversion efficiency, Φ, of 2.8 × 10⁻²%. This approaches 0.5% of the maximum theoretical efficiency of a Carnot engine operating across the same temperature difference.     

The obstructed diffusion of the I3 ion in mesoscopic TiO2 membranes

The diffusional permeability of I⁻₃ ion in acetonitrile in free standing TiO₂ membrane with a porosity of 55% was examined. The apparent diffusion coefficient, D_app at 25°C of the ion was found to be 3.4×10⁻⁶ cm^{2 −1}, an order of magnitude smaller than the free diffusion at the same temperature. The temperature dependency of D_app was measured in the range 0–30°C and analysed in terms of the Walden product. The diffusional activation energy was found to be 13.5 kJ/mol. The parameters of interest for the efficiency of mesoscopic wet solar cells are discussed. A back of an envelope calculation shows that although the obstructed diffusion coefficient of the I⁻₃ ion was an order of magnitude smaller than the free diffusion the diffusional flux is still sufficient to meet a current density of 50 mA cm⁻². At incident photon flux of 1 kW m⁻² and at a photopotential of 0.6 V this would correspond to a solar energy efficiency of approximately 30%.     

Photoelectrochemical parameters of In-doped thin CdSe layers

Photoanodes of polycrystalline CdSe layers, doped with different concentration of In are investigated. SEM microphotographs show that during the recrystallization process dense and homogenous layers with 50 μm sized crystallites are obtained. In order to obtain maximum performance the effect of In concentration on the photocurrent is analyzed and the optimal value of donor concentration (5×10¹⁶cm⁻³) is established. Applying Gartner-Butler's model for a semiconductor-electrolyte junction, the minority carrier diffusion length L (5.6 × 10⁻⁶ cm) and the donor concentration in the depletion layer N_d are determined for samples with optimal donor concentration. Direct preparation of well crystallized and suitably doped layers removes the necessity of sample posttreatment usually performed to increase their efficiency.     

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.     

Optimization of process parameters in a large-area hot-wire CVD reactor for the deposition of amorphous silicon (a-Si:H) for solar cell application with highly uniform material quality

Scale-up of a-Si:H-based thin film applications such as solar cells, entirely or partly prepared by hot-wire chemical vapor deposition (HWCVD), requires research on the deposition process in a large-area HWCVD system. The influence of gas supply and filament geometry on thickness uniformity has already been reported, but their influence on material quality is systematically studied for the first time. The optimization of deposition parameters for obtaining best material quality in our large-area HWCVD system resulted in an optimum filament temperature, T_fil≈1600°C, pressure, p=8 mTorr and silane flow, F(SiH₄)=100 sccm, keeping the substrate temperature at T_S=200°C. A special gas supply (gas shower with tiny holes of uniform size) and a filament grid, consisting of six filaments with an interfilament distance, d_fil=4 cm were used. The optimum filament-to-substrate distance was found to be d_fil–S=8.4 cm. While studying the influence of different d_fil and gas supply configurations on the material quality, the above-mentioned setup and parameters yield best results for both uniformity and material quality. With the setup mentioned, we could achieve device quality a-Si:H films with a thickness uniformity of ±2.5% on a circular area of 20 cm in diameter. The material, grown at a deposition rate of r_d≈4 Å/s, was characterized on nine positions of the 30 cm×30 cm substrate area, and revealed reasonable uniformity of the opto-electronic properties, e.g photosensitivity, σ_Ph/σ_D=(2.46±0.7)×10⁵, microstructure factor, R=0.17±0.05, defect densities, N_d(PDS)=(2.06±0.6)×10¹⁷ cm⁻³ and N_d(CPM)=(2.05±0.5)×10¹⁶ cm⁻³ (film properties are given as mean values and standard deviations). Finally, we fabricated pin solar cells, with the i-layer deposited on small-area p-substrates distributed over an area of 20 cm×20 cm in this large-area deposition system, and achieved high uniformity of the cell parameters with initial efficiencies of η=(6.1±0.2)% on the 20 cm×20 cm area.     

High efficiency AlxGa1−xAs/GaAs solar cells: Fabrication, irradiation and annealing effect

High efficiency Al_xGa_1−xAs/GaAs heteroface solar cells have been fabricated by an improved multi-wafer squeezing graphite boat liquid phase epitaxy (LPE) technique, which enables simultaneous growth of twenty 2.3 × 2.3cm² epilayers in one run. A total area conversion efficiency of 17.33% is exhibited (1sun, AMO, 2.0 × 2.0cm²). The shallow junction cell shows more resistance to 1 MeV electron radiation than the deep one. After isochronal or isothermal annealing the density and the number of deep level traps induced by irradiation are reduced effectively for the solar cells with deep junction and bombardment under high electron fluences.