Photocurrent enhancement of interlocked LB film dye layers in a p-CuSCN sensitised photoelectrochemical cell

Dye sensitised photoelectrochemical (PEC) cells based on Cu/p-CuSCN/LB films have been studied with mixed Langmuir Blodgett (LB) films as the dye layer. The effects of mixed layers were investigated in detail by observing the changes of optical absorption and photocurrent in a PEC cell configuration. Enhancements in both optical absorption and photocurrent were found when a mixture of octadecyl methylviolet–C₁₈ (M–C₁₈) and dioctadecyl rhodamine (C₁₈–R–C₁₈) were deposited using the LB technique on p-CuSCN wide band gap semiconductor. The maximum photocurrent quantum efficiency of the PEC cell reached ≈36% in KI (10⁻² M)+I₂ (10⁻⁴ M) electrolyte solution when mixed LB films were used as the dye layer. Photocurrent enhancement is believed to be the enhancement of light absorption of the dye layers due to the interlocking of M–C₁₈ between the double C₁₈ chains of rhodamine.     

Crystal violet dye-sensitized photocurrent by participation of surface states on p-CuSCN photocathode

Surface states on p-CuSCN are sensitized by aggregated crystal violet in addition to the cathodic sensitization process between semiconductor valence band and the excited monomeric dye molecules. Formation of surface states on bare p-CuSCN is confirmed by experimental results of photocurrent measurements, diffuse reflectance spectra and photoluminescence measurements. Photocurrent quantum efficiency obtained for the aggregated dyes sensitized the surface is 25% at λ=460 nm, and 14% for the monomeric dyes at λ=580 nm. From the variation of photocurrent quantum efficiency with surface concentration of crystal violet at various biased electrode potentials and diffuse reflectance spectra of dye coated p-CuSCN, it is found that the appearance of the sharp photocurrent quantum efficiency peak (λ=460 nm) is due to the sensitization process with the aggregated dye and that of the broad photocurrent quantum efficiency peak (λ=580 nm) is due to the sensitization process with the monomeric dye.     

Investigation of influence of redox species on the interfacial energetics of a dye-sensitized nanoporous TiO2 solar cell

The photoelectrochemical behaviours of dye-sensitized nanoporous TiO₂ solar cells are studied under influences of light intensity, redox couple concentration, temperature, different cations and water in the nonaqueous solution. The value of the ideality factor of dyed nanoporous TiO₂ film is determined to be 1.08. The diode behaviour of the dyed nanoporous TiO₂ film approaches an ideal rectification characteristic. The rate of the reaction of I₃⁻ with the electron at the surface of the dyed TiO₂ electrode is of first order, like the reduction of I₃⁻ at the Pt electrode. By analysis of the relationship of the photovoltage with temperature, the activation energies of the back-reaction for dyed nanoporous TiO₂ electrodes in different solutions are obtained. Cations of different kinds and water are found to modify the interfacial properties of the dyed TiO₂ electrode. Finally, a quantitative relationship between the short-circuit photocurrent and the light intensity, the I₃⁻ concentration is obtained and used to explain the diffusion-controlled photocurrent. The corrected diffusion coefficient of I₃⁻ is 5.4–6.2×10⁻⁶ cm²/s in a CH₃OCH₂CN solution.     

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.     

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.     

Photoelectrochemical response and differential capacitance of poly(3-methylthiophene)

Following the theory used to study the semiconductor/electrolyte interface the differential capacitance of poly(3-methylthiophene) films has been determined from measurements with a lock-in amplifier and by electrochemical impedance spectroscopy (EIS). According to our findings, the best results were obtained by EIS because the space charge capacitance can be separated from the other capacitances. Using Mott–Schottky plots (C⁻² vs. E) we obtained the flat band potential E_fb=80 mV and the carrier density N=6×10¹⁷ cm⁻³ for the PMeT film in contact with the electrolyte, where dissolved O₂ played the role of the electron acceptor. The determined width of the depletion layer is 0.04 μm. We also investigated the photoelectrochemical response of the PMeT film. The plot of the square of the photocurrent vs. potential yields E_fb=90 mV, in good agreement with the EIS measurement. The dependence of the photocurrent with the frequency of the incident light shows that PMeT has a long response time (order of ms), compared to an inorganic semiconductor. The band gap was also determined from the photocurrent spectra. The value obtained, for a direct transition is 1.9 eV and is coincident with the value obtained from the absorption spectra.     

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.     

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.     

Characterization of a-Si:H films via analysis of multiple photocurrent spectra

The spectral response of photoconductive materials applicable to solar cells and photosensors, is commonly characterized using different experimental approaches, such as photocurrent spectra normalized to incident light intensity, photoconductivity spectra measured at constant photon flux, the constant photocurrent method (CPM) [3–5], and the dependence of the photocurrent on the photon flux.In this work, a procedure is proposed, which allows the complementary results of the above mentioned approaches to be gained by analysing several photocurrent spectra, obtained by routine measurements at different illumination levels (Multiple Spectra). This procedure is used for investigation of two a-Si:H samples. The spectra, corresponding to a constant photon flux and a constant photocurrent, as well as the dependencies of the photocurrent on the photon flux at various wavelengths I−Φ spectral map) are derived from the Multiple Spectra and are subsequently used to obtain the following characteristics of the investigated samples: the subbandgap optical absorption spectra; the bulk density of “defect” states; the Urbach tail width; and the power index of the I−Φ)_λ dependencies for the bulk and for the interface regions.     

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.     

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.     

Preparation and characterisation of electrodeposited n-CdS/p-CdTe thin film solar cells

Cadmium sulphide and cadmium telluride films have been electrodeposited for n-CdS/p-CdTe solar cells. Cell efficiency varied considerably from 9.5% to 11.5% for each deposition set. The reverse saturation currents of 9.5% and 11.5% cells at 298 K were 25 and 6.7 nA cm⁻², respectively. The cells with higher efficiency has a lower number of interface states than the less efficient cells. The 11.5% cell had interface states (N_IS) of 3× 10¹⁰ cm⁻² eV⁻¹ at zero volt bias in dark and when it was illuminated with 35 mW cm⁻² light at zero volt bias N_IS increased by two orders to 1.2×10¹² cm⁻² eV⁻¹. At higher frequency the large voltage intercept of the Mott-Schottky plot indicates the existence of the near intrinsic layer of the polycrystalline heterojunction.     

A polymer gel electrolyte composed of a poly(ethylene oxide) copolymer and the influence of its composition on the dynamics and performance of dye-sensitized solar cells

A polymer gel electrolyte composed of a poly(ethylene oxide) derivative, poly(ethylene oxide-co-2-(2-methoxyethoxy) ethyl glycidyl ether), mixed with gamma-butyrolactone (GBL), LiI and I₂ is employed in dye sensitized solar cells (DSSC). The electrolyte is characterized by conductivity experiments, Raman spectroscopy and thermal analysis. The influence of the electrolyte composition on the kinetics of DSSC is also investigated by transient absorption spectroscopy (TAS). The electrolyte containing 70 wt.% of GBL and 20 wt.% of LiI presents the highest conductivity (1.9 × 10⁻³ S cm⁻¹). An efficiency of 4.4% is achieved using this composition. The increase in I_SC as a function of GBL can be attributed an increase in the mobility of the iodide (polyiodide) species. The increase in the yield of the intermediate species, I₂⁻, originating in the regeneration reaction, is confirmed by TAS. However, the charge recombination process is faster at this composition and a decrease in the V_oc is observed. Photovoltage decay experiments confirm an acceleration in charge recombination for the DSSC assembled with the electrolyte containing more GBL. Raman investigations show that in this electrolyte the I₅⁻/I₃⁻ ratio is higher. Theoretical calculations also indicate that the I₅⁻ species is a better electron acceptor.     

Remarkable stability of the enhanced sharp photocurrent in methylviolet–C18 and rhodamine–C18 dye-sensitized photoelectrochemical cell with p-CuSCN

A remarkable stability in the sharp photo-current peak at 570 nm was found when p-CuSCN semiconductor photocathode was sensitized with double-dye LB films of rhodamine-C₁₈ (R-C₁₈) monolayers and methylviolet-C₁₈ (M-C₁₈) monolayers. At first, M-C₁₈ four monolayers were deposited on p-CuSCN and then, R-C₁₈ four monolayers were deposited on M-C₁₈ monolayers to make an molecular arrangement on the semiconductor (LB-(R-C₁₈(4)/M-C₁₈(4))/p-CuSCN). Such a photo-cathode exhibited a remarkable stability of steady-state photo-current in (10⁻² M) KI+(10⁻⁴ M) I₂ aqueous solution of pH=6.5, where the quantum efficiency reached was 45%, as estimated by correcting the dye absorption in the sharp photo-current peak at 570 nm. When four monolayers of M-C₁₈ and four monolayers of R-C₁₈ were separated with an arachidic acid LB films, sharp photo-current peak and its stability gradually decreased. This remarkable stability may be due to an efficient energy transfer process and an efficient charge separation caused by a strong interaction between the parallel electron-vibration planes of both M-C₁₈ and R-C₁₈ molecules.     

Photovoltaic activity in a ZnTe/PEO–chitosan blend electrolyte junction

A ZnTe/polymer junction has been fabricated and the photovoltaic properties studied. The polymer is a blend of 50 wt% chitosan and 50 wt% polyethylene oxide (PEO). The polymer blend was complexed with ammonium iodide (NH₄I) and some iodine crystals were added to the polymer–NH₄I solution to provide the I⁻/I³⁻ redox couple. The ionic conductivity of the polymer electrolyte is 4.32×10⁻⁶ S cm⁻¹ at room temperature. ZnTe was electrodeposited on ITO conducting glass. The polymer film was sandwiched between the ZnTe semiconductor and an ITO glass to form a ZnTe/polymer electrolyte/ITO photovoltaic cell. The open circuit voltage (V_oc) of the fabricated cells ranges between 300 and 400 mV and the short circuit current between 2 and 5 μA.     

Effect of electrolytes on the photovoltaic performance of a hybrid dye sensitized ZnO solar cell

The efficiency of dye sensitized solar cell depends on the number of factors such as impedance due to anions in the electrolytes, oxidation–reduction process of anions and size of cations of the electrolyte. This paper reports the effect of electrolytes on the photovoltaic performance of hybrid dye sensitized ZnO solar cells based on Eosin Y dye. The size of the cations has been varied by choosing different electrolytes such as LiBr+Br₂, LiI+I₂, tetrapropylammonium iodide +I₂ in mixed solvent of acetronitrile and ethylene carbonate. The impedance of anions has been determined by electrochemical impedance spectra. It is observed that Br⁻/Br₃⁻ offers high impedance as compared to I⁻/I₃⁻ couple. The oxidation–reduction reactions of electrolytes are measured by linear sweep voltammogram. It is found that Br⁻/Br₃⁻ is more suitable than an I⁻/I₃⁻ couple in dye sensitized solar cell (DSSC) in terms of higher open-circuit photovoltage production and higher overall energy conversion efficiency. This is attributed to more positive potential of the dye sensitizer than that of Br⁻/Br₃⁻. The gain in V_oc was due to the enlarged energy level difference between the redox potential of the electrolyte and the Fermi level (E_f) of ZnO and the suppressed charge recombination as well.     

Charge transport properties in the nanostructured ZnO thin film electrode – electrolyte system studied with time resolved photocurrents

The charge transport in nanoporous ZnO was studied by laser flash induced photocurrent transients. The results are discussed using a diffusion model and compared with previous results on TiO₂. The charge transport was highly dependent on the potential giving apparent diffusion coefficients for the electron ranging from 1×10⁻⁴ to 1×10⁻⁶ cm²/s with an applied bias of +100 and +300 mV vs. Ag/AgCl in ethanol, respectively. The electrolyte was 0.5 M LiClO₄ in ethanol. The potential dependence was much more pronounced for ZnO than for TiO₂. The charge transport was also dependent on the electrolyte giving a linear dependence between the conductivity of the electrolyte and the apparent electronic diffusion coefficient. The dependence of the light intensity was also studied. Intensity-dependent losses were observed.     

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.     

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.     

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.