THE INFLUENCE OF DEPOSITION AND PROCESSING CONDITIONS ON THE PROPERTIES OF EVAPORATED CdTe/ CdS LAYERS AND SOLAR CELLS

The influence of the deposition and CdCl₂ doping conditions on the properties of CdTe layers and on the performance of CdTe/CdS/ITO/glass solar cells is reported. Relatively high deposition temperatures (300-350°C) were found to enhance the reproducibility of the optical quality of the CdTe. Oxidation of CdTe layers during air annealing was observed and monitored by XRD. Conventional wet dip and CdCl₂ vapour doping of CdTe are compared. Methods for reducing the incidence of pinholes in the CdTe are described, the junction uniformity having been monitored by EBIC. The best solar cell made in this work had an efficiency of 9.87%. (V _oc = 0.696V, J _sc = 24.1 mA/cm², FF = 59%).     

Improved performance of CdTe thin film solar cells through controlling the initial stage of the CdTe layer deposition by close-spaced sublimation

Control of the initial stage of the CdTe deposition was investigated by modifying the substrate and the source temperature profiles in close-spaced sublimation (CSS) process for improving the performance of CdTe thin film solar cells. In the modified temperature profile, the substrate temperature was increased with higher rate than that of the source temperature. By using the modified temperature profile, the conversion efficiency was improved as compared with the conventional temperature profile, and 15.3% efficiency (V_oc: 0.811 V, J_sc: 26.3 mA/cm², FF: 0.718, 1 cm², AM 1.5) was achieved.     

Influence of electrolytes on the photovoltaic performance of organic dye-sensitized nanocrystalline TiO2 solar cells

We have studied the influence of electrolytes on the photovoltaic performance of mercurochrome-sensitized nanocrystalline TiO₂ solar cells using LiI, LiBr, and tetraalkylammonium iodides as the electrolyte. Short-circuit photocurrent density (J_sc) and open-circuit photovoltage (V_oc) depended strongly on the electrolyte. J_sc of 3.42 mA cm⁻² and V_oc of 0.52 V were obtained for the LiI electrolyte and J_sc of 2.10 mA cm⁻² and V_oc of 0.86 V were obtained for the Pr₄NI electrolyte. This difference in photovoltaic performance was due to the change in the conduction band level of the TiO₂ electrode. Large V_oc of 0.99 V was obtained for the LiBr electrolyte due to the large energy gap between the conduction band level of TiO₂ and the Br/Br₂ redox potential. Solar cell performance also depended strongly on organic solvent, suggesting that the physical properties of solvents such as Li ion conductivity and donor number affect photovoltaic performance.     

Improved Jsc in CIGS thin film solar cells using a transparent conducting ZnO:B window layer

A comparative study of the cell performance of CIGS thin-film solar cells fabricated using ZnO:Al and ZnO:B window layers has been carried out. ZnO:B films were deposited by RF magnetron sputtering using an undoped ZnO target in a B₂H₆–Ar gas mixture. The short-circuit current (J_sc) was found to improve upon the replacement of the ZnO:Al layer with ZnO:B layers. This improvement in J_sc is attributed to an increase in quantum efficiency due to the higher optical transmission of the ZnO:B layer in the near-infrared region. The best cell fabricated with a MgF₂/ZnO:B/i-ZnO/CdS/CIGS/Mo structure yielded an active area efficiency of 18.0% with V_oc=0.645 V, J_sc=36.8 mA/cm², FF=0.76, and an active area of 0.2 cm² under AM 1.5 illumination.     

Large-area high efficiency single crystalline silicon solar cells

This paper reports on a 100 cm² single crystalline silicon solar cell with a conversion efficiency of 19.44% (J_sc = 37.65 mA/cm², V_oc = 638 mV, FF = 0.809). The cell structure is as simple as only applying the textured surface, oxide passivation, and back surface field by the screen printing method. The comparison between cell performances of the CZ (Czochralski) and FZ (Floating zone) silicon substrates was investigated. The higher efficiency cells were obtained for the FZ substrate rather than the CZ substrate. The influence of the phosphorus concentration of the emitter on the cell efficiency has also been investigated. A good result was obtained when the surface concentration of phosphorus was 3 × 10²⁰ cm⁻³ and the junction depth was about 0.6 μm.     

Manufacturing method for transparent electric windows using dye-sensitized TiO2 solar cells

The transparent electric windows based on dye-sensitized nanocrystalline TiO₂ solar cells have been prepared. The solar cell consists of dye-sensitized TiO₂ electrode with a TiO₂ layer of an about 8 μm thickness and of a 80×80 mm² active area, Pt counter electrode and redox electrolyte. The solar cell shows a transmittance of approximately 60% in the visible range and an open-circuit voltage (V_oc) of 0.64 V and a short-circuit photocurrent (J_sc) of 250 mA. A moderately transparent electric window composed of nine unit solar cells in series generates V_oc of 5.7 V and J_sc of 220 mA at one sun light intensity.     

High-efficiency CIGS solar cells with modified CIGS surface

CIGS films were treated in In–S aqueous solution for high-efficiency CIGS solar cells. The In–S aqueous solution contained InCl₃ and CH₃CSNH₂ (thioacetamide). The In–S treatment modified the CIGS surface favorably for high-efficiency CIGS solar cells as evidenced by the increase in V_oc, J_sc and FF. The In–S treatment formed thin CuInS₂ layer on the CIGS surface which contributes to the high efficiency and stable performance of the CIGS solar cell. The best cell showed an efficiency of 17.6% (V_oc=0.649 V, J_sc=36.1 mA/cm² and FF=75.1%) without any annealing and light soaking before I–V measurement.     

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.     

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.     

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.     

High efficiency silicon solar cells by plasma-CVD method

We have succeeded in obtaining excellent Back Surface Field (BSF) structures by depositing a highly doped silicon layer on the back side of the substrate even though the textured wafer is used. The substantial increase of the spectral response in longer wavelength has been observed and an increase of V_oc has also been found. The BSF formation technology, together with the low temperature grown thin a-Si emitter layer with appropriate bandgap, has enabled us to obtain a very high J_sc of 40.5 mA/cm², thus a high active area conversion efficiency of 18.9 % (V_oc = 0.592 V, FF = 0..789).     

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.     

p-doped μc-Si:H window layers prepared by hot-wire CVD for amorphous solar cell application

We report on boron-doped μc-Si:H films prepared by hot-wire chemical vapor deposition (HWCVD) using silane as a source gas and trimethylboron (TMB) as a dopant gas and their incorporation into all-HW amorphous silicon solar cells. The dark conductivity of these films was in the range of 1–10 (Ω cm)⁻¹. The open circuit voltage V_oc of the solar cells was found to decrease from 840 mV at low hydrogen dilution H-dil=91% to 770 mV at high H-dil =97% during p-layer deposition which can be attributed to the increased crystallinity at higher H-dil and to subsequent band edge discontinuity between μc-Si:H p- and amorphous i-layer. The short circuit current density J_sc and the fill factor FF show an optimum at an intermediate H-dil and decrease for the highest H-dil. To improve the conversion efficiency and the reproducibility of the solar cells, an amorphous-like seed layer was incorporated between TCO and the bulk p-layer. The results obtained until now for amorphous solar cells with and without the seed layer are presented. The I–V parameters for the best p–i–n solar cell obtained so far are J_sc=13.95 mA/cm², V_oc=834 mV, FF=65% and η=7.6%, where the p-layers were prepared with 2% TMB. High open circuit voltages up to 847 mV could be achieved at higher TMB concentrations.     

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.     

Ribbon Si solar cells with efficiencies over 18% by hydrogenation of defects

We have fabricated 4 cm² solar cells on String Ribbon Si wafers and edge-defined film-fed grown (EFG) Si wafers with using a combination of laboratory and industrial processes. The highest efficiency on String Ribbon Si wafer is 17.8% with an open circuit voltage (V_oc) of 620 mV, a short circuit current density (J_sc) of 36.8 mA/cm² and a fill factor (FF) of 0.78. The maximum efficiency on EFG Si is 18.2% with a V_oc of 620 mV, a J_sc of 37.5 mA/cm² and a FF of 0.78. These are the most efficient ribbon Si devices made to date, demonstrating the high quality of the processed Si ribbon and its potential for industrial cells. Co-firing of SiN_x and Al by rapid thermal processing was used to boost the minority carrier lifetime of bulk Si from 3–5 μs to 70–100 μs. Photolithography-defined front contacts were used to achieve low shading losses and low contact resistance with a good blue response. The effects of firing temperature and time were studied to understand the trade-off between hydrogen retention and Al-doped back surface field (Al-BSF) formation. Excellent bulk defect hydrogenation and high-quality thick Al-BSF formation was achieved in a very short time (1 s) at firing temperatures of 740–750 °C. It was found that the bulk lifetime decreases at annealing temperatures above 750 °C or annealing time above 1 s due to dissociation of hydrogenated defects.     

Photovoltaic characteristics of phosphorus-doped amorphous carbon films grown by r.f. plasma-enhanced CVD

The phosphorus-doped amorphous carbon (n-C:P) films were grown by r.f. power-assisted plasma-enhanced chemical vapor deposition at room temperature using solid phosphorus target. The influence of phosphorus doping on material properties of n-C:P based on the results of simultaneous characterization are reported. Moreover, the solar cell properties such as series resistance, short circuit current density (J_sc), open circuit current voltage (V_oc), fill factor (FF) and conversion efficiency (η) along with the spectral response are reported for the fabricated carbon based n-C:P/p-Si heterojunction solar cell were measured by standard measurement technique. The cells performances have been given in the dark I–V rectifying curve and I–V working curve under illumination when exposed to AM 1.5 illumination condition (100 mW/cm², 25 °C). The maximum of V_oc and J_sc for the cells are observed to be approximately 236 V and 7.34 mA/cm², respectively for the n-C:P/p-Si cell grown at lower r.f. power of 100 W. The highest η and FF were found to be approximately 0.84% and 49%, respectively. We have observed the rectifying nature of the heterojunction structures is due to the nature of n-C:P films.     

Photovoltaic effect in single-layer organic solar cell devices fabricated with two new imidazolin-5-one molecules

Organic solar cells were fabricated with two new imidazolin-5-one molecules as active layers. The use of imidazolin-5-ones, derivatives of a biomolecule chromophore, for photovoltaic applications is particularly attractive due to its biodegradable nature and tunable properties. Single-layer devices with two analogues of imidazolin-5-ones were prepared and characterized. Devices fabricated with one of the molecules as the active layer showed a maximum J_sc of 0.52 μA cm⁻² and V_oc of 0.68 V at an incident power of 20.32 mW cm⁻², while the other set of devices showed a maximum J_sc of 0.63 μA cm⁻² and V_oc of 0.57 V at the same incident power.     

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).     

Effect of copper diffusion on photovoltaic characteristics of CuGaSe2–GaAs cells

CuGaSe₂–GaAs heterojunctions were fabricated by fast evaporation of polycrystalline CuGaSe₂ from a single source on n-type GaAs substrates. The best CuGaSe₂–GaAs photocell (without an antireflective coating) exhibited an efficiency of 11.5%, J_sc=32 mA/cm², V_oc=610 mV and FF=0.60. The spectral distribution of photosensitivity of CuGaSe₂–GaAs junctions extends from 400 to 900 nm. The CuGaSe₂ films were characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM) techniques. XRD analysis indicated that the thin films were strongly oriented along the (1 1 2) plane. SEM studies of CuGaSe₂ films showed nearly stoichiometric composition with grain size about 1–2 μm. The energy dispersive X-ray spectroscopy (EDX) analysis of Cu concentration distribution in n-type GaAs showed that Cu diffused from the film into n-type GaAs during the growth process resulting in formation of the latent p–n homojunction in substrate. The diffusion coefficient of Cu in GaAs at growth temperature (520°C) estimated from EDX measurements was 6×10⁻⁸ cm²/s.     

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%.