Synthesis of Cu(In,Ga)Se2 absorber using one-step electrodeposition of Cu-In-Ga precursor

One-step Cu-In-Ga electrodeposition on Mo substrate is carried out by potentiostatic method in acidic aqueous media. The applied potential, the pH and the nature of the electrolyte are determined to obtain adequate precursor composition. The electrodeposit is found highly dendritic, due to Cu diffusion-controlled deposition. Selenization at temperatures ranging from 450 to 600 °C leads to Cu(In,Ga)Se₂ (CIGS) absorber. The influence of selenization temperature and duration on Ga distribution as well as on CIGS crystallinity is discussed. Although the precursor is dendritic, relatively compact absorbers can be obtained. The best solar cell, achieved on 0.1 cm², shows 9.3% efficiency (V_oc 456 mV; j_sc 33 mA cm⁻²; FF 62%).     

Simple indoline based donor–acceptor dye for high efficiency dye-sensitized solar cells

A simple metal-free donor–acceptor type sensitizer U01, bearing strong electron donor indoline-triphenylamine was synthesized for panchromatic sensitization of TiO₂ nanocrystalline film. Photovoltaic properties of U01 showed remarkably enhanced light harvesting due to the presence of strong electron donor and robust structure. The new U01 sensitized solar cell exhibited a photovoltaic performance: a short-circuit photocurrent density (J_sc) of 10.70 mA cm⁻², an open-circuit photovoltage (V_oc) of 0.758 V and a fill factor (FF) of 0.74, corresponding to an overall conversion efficiency of 6.01% under standard global AM 1.5 solar light condition. Our results suggest that indoline-triphenylamine based robust D–A molecular architecture is a highly promising class of panchromatic sensitizers for improvement of the performance of dye-sensitized solar cells (DSCs).     

Cu2ZnSnS4 thin film solar cells from electroplated precursors: Novel low-cost perspective

Thin-film solar cells based on Cu₂ZnSnS₄ (CZTS) absorbers were fabricated successfully by solid-state reaction in H₂S atmosphere of electrodeposited Cu-Zn-Sn precursors. These ternary alloys were deposited in one step from a cyanide-free alkaline electrolyte containing Cu(II), Zn(II) and Sn(IV) metal salts on Mo-coated glass substrates. The solar cell was completed by a chemical bath-deposited CdS buffer layer and a sputtered i-ZnO/ZnO:Al bilayer. The best solar cell performance was obtained with Cu-poor samples. A total area (0.5 cm²) efficiency of 3.4% is achieved (V_oc = 563 mV, j_sc = 14.8 mA/cm², FF = 41%) with a maximum external quantum efficiency (EQE) of 80%. The estimated band-gap energy from the external quantum efficiency (EQE) measurements is about 1.54 eV. Electron backscatter-diffraction maps of cross-section samples revealed CZTS grain sizes of up to 10 µm. Elemental distribution maps of the CZTS absorber show Zn-rich precipitates, probably ZnS, and a Zn-poor region, presumably Cu₂SnS₃, close to the interface Mo/CZTS.     

Effects of LiF/Al back electrode on the amorphous/crystalline silicon heterojunction solar cells

To improve the quantum efficiency (QE) and hence the efficiency of the amorphous/crystalline silicon heterojunction solar cell, we have employed a LiF dielectric layer on the rear side. The high dipole moment of the LiF reduces the aluminum electrode's work–function and then lowers the energy barrier at back contact. This lower energy barrier height helps to enhance both the operating voltage and the QE at longer wavelength region, in turn improves the open-circuit voltage (V_oc), short-circuit current density (J_sc), and then overall cell efficiency. With optimized LiF layer thickness of 20 nm, 1 cm² heterojunction with intrinsic thin layer (HIT) solar cells were produced with industry-compatible process, yielding V_oc of 690 mV, J_sc of 33.62 mA/cm², and cell efficiencies of 17.13%. Therefore LiF/Al electrode on rear side is proposed as an alternate back electrode for high efficiency HIT solar cells.     

A Grain Orientation-Independent Single-Step Saw Damage Gettering/Wet texturing Process for Efficient Silicon Solar Cells

Improving electrical and optical properties is important in manufacturing high-efficiency solar cells. Previous studies focused on individual gettering and texturing methods to improve solar cell material quality and reduce reflection loss, respectively. This study presents a novel method called saw damage gettering with texturing that effectively combines both methods for multicrystalline silicon (mc-Si) wafers manufactured using the diamond wire sawing (DWS) method. Although mc-Si is not the Si material currently used in photovoltaic products, the applicability of this method using the mc-Si wafers as it contains all grain orientations is demonstrated. It utilizes saw damage sites on the wafer surfaces for gettering metal impurities during annealing. Additionally, it can crystallize amorphous silicon on wafer surfaces generated during the sawing process to allow conventional acid-based wet texturing. This texturing method and annealing for 10 min allow for the removal of metal impurities and effectively forms a textured DWS Si wafer. The results show that the open-circuit voltage (ΔV_oc = +29 mV), short-circuit current density (ΔJ_sc = +2.5 mA cm⁻²), and efficiency (Δη = +2.1%) improved in the p-type passivated emitter and rear cells (p-PERC) manufactured using this novel method, as compared to those in the reference solar cells.     

Pyreno-chalcone dendrimers as an additive in the redox couple of dye-sensitized solar cells

Novel pyreno-chalcone dendrimers 1, 2, and 3 were synthesized and their ability to act as an additive in the redox couple (I⁻/I₃ ⁻) of dye-sensitized nanocrystalline TiO₂ solar cell has been tested. The redox couple doped with pyreno-chalcone dendrimer 3 gave a short circuit photocurrent density (J _sc) of 7.40 mA/cm², open circuit voltage (V _oc) of 820 mV, and a fill factor of 0.51, corresponding to an overall conversion efficiency (η) of 7.89% under 40 mW/cm² irradiation.     

Gel polymer electrolytes based on polyacrylonitrile and a novel quaternary ammonium salt for dye-sensitized solar cells

Gel polymer electrolytes were prepared by incorporating polyacrylonitrile (PAN) in a mixture of polysiloxane with quaternary ammonium side groups (PSQAS), ethylene carbonate (EC), propylene carbonate (PC) and iodine. The influence of PAN content on the ionic conductivity of gel polymer electrolytes and the charge-transfer kinetic performance in counterelectrode-electrolyte interface was investigated. The dye-sensitized solar cell with the gel polymer electrolyte containing 5 wt.% PAN showed the best photovoltaic performance; a maximum incident photon conversion efficiency of 63% at 520 nm was obtained, the short-circuit photocurrent density (J_sc), the open-circuit voltage (V_oc) and the fill factor (FF) were 7 mA cm⁻², 0.565 V and 0.65, respectively. The corresponding overall conversion efficiency (η) is 4.3%.     

Size dependent electron transfer from CdTe quantum dots linked to TiO2 thin films in quantum dot sensitized solar cells

In this present study, we demonstrate the size dependent charge transfer from CdTe quantum dots (QDs) into TiO₂ substrate and relate this charge transfer to the actual behavior of a CdTe sensitized solar cell. CdTe QDs was synthesized using mercaptopropionic acid as the capping agent. The conduction band offset for TiO₂ and CdTe QDs indicates thermodynamically favorable band edge positions for smaller QDs for the electron-transfer at the QD–TiO₂ interface. Time-resolved emission studies were carried out for CdTe QD on glass and CdTe QD on TiO₂ substrates. Results on the quenching of QD luminescence, which relates to the transfer kinetics of electrons from the QD to the TiO₂ film, showed that at the smaller QD sizes the transfer kinetics are much more rapid than at the larger sizes. I–V characteristics of quantum dot sensitized solar cells (QDSSC) with different sized QDs were also investigated indicating higher current densities at smaller QD sizes consistent with the charge transfer results. The maximum injection rate constant and photocurrent were obtained for 2.5 nm CdTe QDs. We have been able to construct a solar cell with reasonable characteristics (V_oc = 0.8 V, J_sc = 1 mA cm⁻², FF = 60%, η = 0.5%).     

Branched chain polymeric metal complexes containing Co(II) or Ni(II) complexes with a donor–π–acceptor architecture: synthesis,characterization, and photovoltaic applications

Four donor–π–acceptor type polymeric metal complexes (PCo–F, PCo–B, PNi–F, and PNi–B) with Co(II) or Ni(II) complexes in the branched chain were synthesized by the Heck coupling and utilized as dyes for dye-sensitized solar cells (DSSCs). The structures, photophysical, electrochemicals, and thermal properties of the four dyes were investigated in detail, and the results showed that dye containing Ni(II) complex and alkoxy benzene unit benefited the generation of photocurrent and the open-circuit voltages. The polymeric metal complexes possess good thermal stability and exhibit good solubility in common organic solvents such as chloroform, THF, and toluene. The maximal power conversion efficiency of 1.21% (J _sc = 2.49 mA/cm², V _oc = 0.695 V, FF = 0.59) was obtained with a DSSCs based on PNi–B dye under simulated air mass 1.5 G solar irradiation.     

All-chemically deposited Bi2S3/PbS solar cells

We report on solar cells with a cross-sectional layout: TCO/window/Bi₂S₃/PbS, in which a commercial SnO₂ transparent conductive oxide (TCO-PPG Sungate 500); chemically deposited window layers of CdS, ZnS or their oxides; n-type Bi₂S₃ (100 nm) and p-type PbS (360-550 nm) absorber films constitute the cell structures. The crystalline structure, optical, and electrical properties of the constituent films are presented. The open circuit voltage (V_oc) and short-circuit current density (J_sc), for 1000 W/m² solar radiation, of these solar cells depend on the window layers, and vary in the range, 130-310 mV and 0.5-5 mA/cm², respectively. The typical fill factors (FF) of these cells are 0.25-0.42, and conversion efficiency, 0.1-0.4%.     

Photovoltaic fiber

The optoelectronically active optical fiber is demonstrated in this work. This fiber consists of dye sensitized solar cell (DSC) structure deposited on claddingless optical fiber. Both silica and plastic optical fibers are used as a substrate. Such a fiber converts light modes propagating in the modified cladding into electrical signal. DSC structure consisting of ZnO:Al transparent current collector layer, TiO₂ photoelectrode sensitized with ruthenium dye, gelatinized iodine electrolyte, and carbon-based counter electrode was deposited layer by layer on top of the optical fiber. Current density-voltage curves of photovoltaic (PV) fibers of different diameters are presented. Maximum obtained short circuit current, I_sc, was 26 nA/cm² and maximum open circuit voltage, V_oc, was 0.44 V. The fabrication issues and applications of the PV fiber are discussed in the article.     

Design and fabrication of InxGa1 − xN/GaN solar cells with a multiple-quantum-well structure on SiCN/Si(111) substrates

The fabrication and characterization of In_xGa_1 − xN/GaN-based solar cells that use In_xGa_1 − xN multiple quantum wells (MQWs) and a SiCN/Si(111) substrate are reported. Solar cell operation with a low dark current density (J_d), a high open-circuit voltage (V_oc), a high short-circuit current density (J_sc), and a high fill factor (FF) is demonstrated. It was found that the proposed device and fabrication technology are applicable to the realization of solar cells with a low J_d of 2.14 to 8.88 μA/cm², a high V_oc of 2.72 to 2.92 V, a high J_sc of 2.72 to 2.97 mA/cm², and a high FF of 61.51 to 74.89%. The device performance with various quantum-well configurations was investigated under an air mass 1.5 global solar spectrum. A high photovoltaic efficiency of 5.95% in the MQW sample over the p-i-n sample was observed.     

Efficient triphenylamine photosensitizers with alkoxy- or fluorine-substituted phenylene spacer for dye-sensitized solar cells

Two new organic sensitizers (TP1–2) containing triarylamine donor and a cyanoacrylic acid acceptor bridged by alkoxy- or fluorine-substituted phenylene spacer have been synthesized and explored as a sensitizer in dye-sensitized solar cells (DSCs). The absorption spectra, electrochemical and photovoltaic properties of TP1–2 are extensively investigated. The DSC based on dye TP1 shows the best photovoltaic performance: a short-circuit photocurrent density (J _SC) of 13.5 mA cm⁻², an open-circuit photovoltage (V _OC) of 702 mV, and a fill factor (ff) of 0.68, corresponding to an overall conversion efficiency of 6.4% under standard global AM1.5 solar light conditions. The results demonstrate that structural modification of substituting groups on π-spacer is importance for realizing a high efficiency DSC.     

Thin film CdZnS/CuInSe2 solar cells by spray pyrolysis

Cd_1−xZn_xS/CuInSe₂ solar cells having efficiencies in the range of 2·3% were fabricated by spray pyrolysis. The best cell had the following parameters:V _oc = 305 mV,J _sc = 32 mA/cm², FF = 0·32 area = 0·4 cm² and efficiency = 3·149%.V _oc versus temperature measurements showed that the electron affinity difference was 0·22 eV. Forward dark current versus voltage curves were plotted and a possible current mechanism occurring in these cells has been proposed.     

Ag nanoparticle-decorated SiO2@TiO2 hierarchical microspheres improve the efficiency of dye-sensitized solar cells

SiO₂@TiO₂-Ag (STA) microspheres decorated with Ag nanoparticles (Ag NPs) were prepared and assembled into the photoanode. The photoanode composed of STA microspheres and TiO₂ nanoparticles (P25) was prepared by doctor blade method. UV–vis measurement indicates that the introduction of a few STA microspheres observably enhances the light scattering and capturing ability of the photoanode. The photoelectric conversion efficiency of the DSSCs with 2wt% STA photoanode increased to 7.4% from 4.3% comparing with pure P25 TiO₂ nanoparticles. The configuration DSSCs have the maximum short circuit current density (J_sc) of 16.0 mA cm^?2 and open-circuit voltage (V_oc) of 0.780 V, which are significantly higher than the pure TiO₂ DSSCs. The significant improvement of the DSSCs performance can be due to the synergistic effect of the superior light scattering of STA and the localized surface plasma resonance (LSPR) effect of Ag NPs modified on the microspheres surface.

     

Band gap profiles of intrinsic amorphous silicon germanium films and their application to amorphous silicon germanium heterojunction solar cells

Intrinsic amorphous silicon germanium (i-a-SiGe:H) films with V, U and VU shape band gap profiles for amorphous silicon germanium (a-SiGe:H) heterojunction solar cells were fabricated. The band gap profiles of i-a-SiGe:H were prepared by varying the GeH₄ and H₂ flow rates during the deposition process. The use of i-a-SiGe:H with band gap profile in an absorber layer for a-SiGe:H heterojunction solar cells was investigated. The solar cell using a VU shape band gap profile shows a higher efficiency compared to other shapes. The highest efficiency obtained for an a-SiGe:H heterojunction solar cell using the VU shape band gap profile technique was 9.4% (V_oc = 0.79 V, J_sc = 19.0 mA/cm² and FF = 0.63).     

The influence of base doping density on the performance of evaporated poly-Si thin-film solar cells by solid-phase epitaxy

The phosphorous base doping dependence of evaporated poly-Si thin-film solar cell by aluminium-induced crystallization solid-phase epitaxy (ALICE) has been investigated. It is found that the open-circuit voltage (V_oc) of the the poly-Si thin-film solar cell increases with the decrease of base doping density due to the defect-rich nature of poly-Si thin-film material and effectiveness of the back surface field. Meanwhile, the short-circuit current (J_sc) also increases with the decrease of the base doping density as a result of the reduced doping-induced defects. Therefore, the maximum V_oc and J_sc are simultaneously achieved when the lowest phosphorous base doping density (~ 5.5 × 10¹⁵ cm^− 3) is applied.     

Simulation of doping levels and deep levels in InGaN-based single-junction solar cell

Doping levels and deep levels in In_0.65Ga_0.35N single junction solar cells are studied theoretically, and simulation of cell properties is performed. Effective-mass approximation (EMA) is used to calculate the ionization energies and the radius of ground-state orbit for donors and acceptors in wurtzite In_0.65Ga_0.35N. The ionization energies of donors and acceptors are estimated to be about 15.5 and 92.9 meV, respectively. The validity of EMA to wurtzite InGaN alloy has also been discussed. AMPS-1D software is used to simulate the doping levels and deep levels in In_0.65Ga_0.35N single junction solar cells with assumption that the deep level is located at the middle of In_0.65Ga_0.35N band gap where the recombination is maximum. Band structure and concentration distributions of equilibrium carriers are obtained. The influence of deep level recombination on efficiency is estimated to be about 9.6% while recombination center concentration is 5 × 10¹⁵ cm⁻³, and capture cross section is 10⁻¹³ cm². The simulated results show that the increase of reverse saturation current and the decrease of open-circuit voltages (V _oc) and fill factor (FF) are mainly responsible for the decrease of the efficiency. Short-circuit current density (J _sc) is found to be not sensitive to deep level concentrations and capture cross sections. As the crystal quality of InGaN and p-type doping of In-rich InGaN may be the most important challenges for InGaN solar cells, this study is useful for the study of InGaN-based super-high efficiency solar cells.     

The effects of the thickness of TiO2 films on the performance of dye-sensitized solar cells

Nanocrystalline anatase TiO₂ thin films with different thicknesses (0.5-2.0 μm) have been deposited on ITO-coated glass substrates by a sol-gel method and rapid thermal annealing for application as the work electrode for dye-sensitized solar cells (DSSC). From the results, the increases in thickness of TiO₂ films can increase adsorption of the N3 dye through TiO₂ layers to improve the short-circuit photocurrent (J_sc) and open-circuit voltage (V_oc), respectively. However, the J_sc and V_oc of DSSC with a TiO₂ film thickness of 2.0 μm (8.5 mA/cm² and 0.61 V) are smaller than those of DSSC with a TiO₂ film thickness of 1.5 μm (9.2 mA/cm² and 0.62 V). It could be due to the fact that the increased thickness of TiO₂ thin films also resulted in a decrease in the transmittance of TiO₂ thin films thus reducing the incident light intensity on the N3 dye. An optimum power conversion efficiency (η) of 2.9% was obtained in a DSSC with the TiO₂ film thickness of 1.5 μm.     

Electrical and Optical Properties of (n)ZnO/(p)CdTe Heterojunction and Its Performance as a Photovoltaic Converter

Thin film heterojuctions of the type (n)ZnO/(p)CdTe with different doping concentration were prepared by vacuum evaporation, and their electrical and optical properties, both in dark and under illumination at room temperature as well as elevated temperatures, were studied. Different junction parameters such as ideality factors, barrier heights, Richardson constant, short-circuit current, etc. were determined from I–V characteristics. These parameters were found to change significantly on hydrogenation and annealing of the junctions and also on variation of temperature. The structures showed the change of the photovoltaic (PV) effect, giving a fill factor of 0.57 for hydrogen (H)-treated with an open-circuit voltage of 345 mV and a short-circuit current density of 75.72× 10⁻⁴ mA·cm⁻² and 0.42 for untreated with an open-circuit voltage of 244 mV and a short-circuit current density of 52.00× 10⁻⁴ mA·cm⁻² for doping concentrations of N _a=2.43× 10¹⁶/cm³(2.53% Sb doped CdTe) and N _d=3.88× 10¹⁵/cm³(2.74% Al doped ZnO). The junctions were found to possess a series resistance as high as 437 Ω for an untreated sample and 95 Ω for H-treated samples even under illumination. Proper doping, annealing, and hydrogenation are necessary to reduce the series resistance so as to achieve an ideal solar cell.Paper presented at the Seventh Asian Thermophysical Properties Conference, August 23–28, 2004, Hefei and Huangshan, Anhui, P. R. China.