Contact angle measurements: an empirical diagnostic method for evaluation of thin film solar cell absorbers (CuInS2)

An empirical diagnostic method for the evaluation of solar cell grade CuInS₂ absorbers has been developed. The method involves the measurement of the contact angle between water and the CuInS₂ absorber before fabrication of a solar cell. The contact angle is expected to depend upon local inhomogeneity, chemical composition and surface morphology of the CuInS₂ absorber. The variation of these factors on the surface is supported with scanning electron micrographs, chemical analyses, laser scanning photocurrent mapping of various CuInS₂ absorbers and measurements of the solar cell performance. The contact angle has been found to be different at different places on the CuInS₂ surface. Empirically, it was found that for high conversion efficiency solar cells (>8–10.5%), the contact angle on CuInS₂ absorbers ranges between 53° and 63°. For low conversion efficiency solar cells (<6%), it is between 48° and 50°. Therefore, it is seen that contact angle measurements on CuInS₂ absorbers can be used to assess the quality of CuInS₂ absorbers prior to solar cell fabrication.     

Study of the Mo/CuInS2/ZnS system by TEM

This work presents results from a study carried out on the Mo/CuInS₂/ZnS stacked layers, using high-resolution transmission electron microscopy (HRTEM). This system will be used later for the fabrication of solar cells with Mo/CuInS₂/ZnS/TCO structure, where the layers conforming it will perform as an electrical contact, absorber layer and buffer layer, respectively. The layers of the Mo/CuInS₂/ZnS system were deposited sequentially on soda lime glass substrates. The Mo film was deposited by DC magnetron sputtering, the CuInS₂ (CIS) layer was grown by co-evaporation of precursors in a two-stage process and the ZnS was deposited by co-evaporation and by CBD (chemical bath deposition) using a solution containing zinc acetate, sodium citrate, ammonia and thiourea.The performed study provided significant information regarding crystalline structure, grain boundaries and defects visualization of each one of the layers as well as of the Mo/CuInS₂ and CuInS₂/ZnS interfaces.     

Preparation and characterization of CuInS2 thin films solar cells with large grain

The CuInS₂ films with a maximum thickness of about 9 μm and a maximum atomic Cu/In ratio (as-deposited precursor) of 3.0 were prepared, and, to prevent peeling from substrate, were heat treated during Cu/In evaporation and/or intercalated with very thin Pt or Pd (between Mo and CuInS₂ layers). Thus, we could prepare the films with very large grain. It is also worth noting that the large grain films were easily optimized by chemical etching of the films using a thick film and Cu-rich composition. Therefore, the absorber for high-efficiency solar cells can be prepared by varying over a wide range of composition and thickness of precursor. The characterization of CuInS₂ absorbers with various film thickness and compositions were investigated and related with the performance of the photovoltaic device.     

CuInS2 thin-films solar cells fabricated by sulfurization of oxide precursors

CuInS₂ thin-films were prepared by sulfurization of Cu---In---O precursors in H₂S gas. X-ray diffraction patterns showed that In₂O₃ phases did not remain in the CuInS₂ films sulfurized in a H₂S and H₂ atmosphere, whereas In₂O₃ phase remained in the films sulfurized in a H₂S and Ar atmosphere. The performance of CuInS₂ solar cells were studied as a function of the H₂ gas pressure during sulfurization. The open-circuit voltage, short-circuit current and fill factor increased with increasing the H₂ gas pressure. The conversion efficiency of the CuInS₂ solar cells is strongly affected by the reduction of the Cu---In---O precursors.     

Preparation and properties of CuInS2 thin film prepared from electroplated precursor

Thin CuInS₂ films were prepared by sulfurization of Cu/In bi-layers. First, the precursor layer was electroplated onto the treated surface of Mo-coated glass. Observation of the cross-section prepared by focused ion beam (FIB) etching revealed that the void-free film was initially formed on the top surface of the precursor layer and continued to grow until the advancing front of the film reached the Mo layer. The nucleation of voids near the bottom of the CuInS₂ film followed. To determine whether the condition of the Cu/In alloy influences the CuInS₂ quality we investigated the Cu/In alloy state using FIB. We found that the annealed precursor of low Cu/In ratio (1.2) has several voids in the mid position in the layer compared with Cu-rich precursor (1.6). The cross-sectional view of the Cu-rich absorber layer is uniform compared with the low copper absorber layer. Thin film solar cells were fabricated using the CuInS₂ film (Cu/In ratio: 1.2) as an optical absorber layer. It was found that the optimization of a sulfurization period is important in order to improve the cell efficiency. We have not yet obtained good results with high Cu-rich absorber because of a blister problem. This blister was found before sulfurization. So, we are going to solve this blister problem before sulfurization.     

CuInS2/In2S3 thin film solar cell using spray pyrolysis technique having 9.5% efficiency

Copper indium sulfide (CuInS₂)/In₂S₃ solar cells were fabricated using spray pyrolysis method and high short circuit current density and moderate open circuit voltage were obtained by adjusting the condition of deposition and thickness of both the layers. Consequently, a relatively high efficiency of 9.5% (active area) was obtained without any anti-reflection coating. The cell structure was ITO/CuInS₂/In₂S₃/Ag. We avoided the usual cyanide etching and CdS buffer layer, both toxic, for the fabrication of the cell.     

The influence of sodium on the properties of CuInS2 thin films and solar cells

The controlled incorporation of sodium into the absorber layer of CuInS₂ solar cells improved cell performance remarkably. Without toxic KCN treatment, conversion efficiencies of over 6% were achieved by sulfurization of sodium-containing precursors. We also investigated the characteristics of the sodium-incorporated CuInS₂ films by intentional addition and diffusion from a soda-lime glass. The ternary compound semiconductor of NaInS₂ was found to form mainly on the surface of each of the CuInS₂ films.     

Role of substrate temperature in controlling properties of sprayed CuInS2 absorbers

Optimization of substrate temperature of spray pyrolysed CuInS₂ absorber is discussed along with its effect on the photoactivity of junction fabricated. For CuInS₂ thin films, properties like crystallinity, thickness and composition showed progressive behavior with substrate temperature. X-ray photoelectron spectroscopic depth profile of all the samples showed that the concentration of copper on the surface of the films is significantly lesser than that in the bulk thus avoiding need for toxic cyanide etching. Interestingly, samples prepared at 623 K had higher conductivity compared to those prepared above and below this temperature. Also, the low energy transition, in addition to the direct band gap which was observed in other samples were absent in films prepared at 623 K. From thermally stimulated conductivity studies it was seen that shallow levels present in this sample contribute to its improved conductivity. Also, CuInS₂/In₂S₃ bilayer prepared at this substrate temperature showed higher photoactivity than those prepared at other temperatures.     

Electrodeposited CuInS2-based thin-film solar cells

CuInS₂ thin films were prepared by sulfurization of electrodeposited Cu–In precursors. Morphological improvement enabled us to fabricate the solar cells using electrodeposited Cu–In precursors. The photovoltaic property of a conversion efficiency of 1.3% was obtained.     

Electrical characterization of Al, Ag and In contacts on CuInS2 thin films deposited by spray pyrolysis

The specific contact resistivity (ρ_C) for aluminum (Al), silver (Ag) and indium (In) metallic contacts on CuInS₂ thin films was determined from I-V measurements, with the purpose of having the most appropriate ohmic contact for TCO/CdS/CuInS₂ solar cells; ρ_C was measured using the transmission line method (TLM) for the metallic contacts evaporated on CuInS₂ thin films deposited by spray pyrolysis with ratios x=[Cu]/[In]=1.0, 1.1, 1.3 and 1.5 in the spray solution. The results show that In contacts have the lowest ρ_C values for CuInS₂ samples grown with x=1.5. The minimum ρ_C was 0.26 Ω cm² for the In contacts. This value, although not very low, will allow the fabrication of CuInS₂ solar cells with a small series resistance.     

Study of CuInS2/ZnS/ZnO solar cells, with chemically deposited ZnS buffer layers from acidic solutions

Thin film solar cells based on CuInS₂/ZnS/ZnO have been prepared with ZnS buffer film of different thickness. ZnS films are grown by chemical bath deposition (CBD) from acidic solutions of ZnSO₄ and thioacetamide (TA). The change of the growth rate with time is studied by means of the quartz crystal microbalance. Films with different thickness show variable physical, chemical and morphological properties. The structure is studied with X-ray diffraction, showing different crystallinity with deposition time. The absorption coefficient depends also on the CBD deposition time, and shows absorption edges between 2.70 and 3.65 eV. The compositional analysis carried out with XPS (surface) and EDAX (bulk). Bulk composition reflects highly stoichiometric films, with Zn/S ratios close to unit. Preliminary results with CuInS₂-based solar cell show efficiencies around 5%, lower than usually found with standard CdS buffer films (around 9%).     

Influence of In2S3 film properties on the behavior of CuInS2/In2S3/ZnO type solar cells

Solar cells of CuInS₂/In₂S₃/ZnO type are studied as a function of the In₂S₃ buffer deposition conditions. In₂S₃ is deposited from an aqueous solution containing thioacetamide (TA), as sulfur precursor and In³⁺. In parallel, variable amounts of In₂O₃ are deposited that have an important influence on the buffer layer behavior. Starting from deposition conditions determined in a preliminary study, a set of parameters is chosen to be most determining for the buffer layer behavior, namely the solution temperature, the concentration of thioacetamide [TA], and the buffer thickness. The solar cell results are discussed in relation with these parameters. Higher efficiency is attained with buffer deposited at high temperature (70 °C) and [TA] (0.3 M). These conditions are characterized by short induction time, high deposition rate and low In₂O₃ content in the buffer. On the other hand, the film deposited at lower temperature has higher In₂O₃ content, and gives solar cell efficiency sharply decreasing with buffer thickness. This buffer type may attain higher conversion efficiencies if deposited on full covering very thin film.     

Characterization of CuInS2 thin films prepared by electrodeposition and sulfurization with photoluminescence spectroscopy

Potentiostatic electrodeposition and sulfurization techniques were used to prepare polycrystalline CuInS₂ thin films. X-ray diffraction and photoresponse measurements in a photoelectrochemical cell (PEC) revealed that photoactive polycrystalline CuInS₂ films can be deposited on Ti substrate. Photoluminescence (PL) spectroscopy was used to investigate the prepared thin films and optically characterize them. PL spectra revealed the defect structure of the samples with an acceptor energy level at 109 meV above the valance band and a donor energy level at 71 meV below the conduction band. The CuInS₂ thin films prepared in this investigation are observed to be In-rich material with n-type electrical conductivity.     

Effect of Ga incorporation in sequentially prepared CuInS2 thin film absorbers

Thin film CuInS₂:Ga solar cell absorber films were prepared by sequential evaporation of Cu–In–Ga precursors and sulfurization in sulfur vapor. The depth distribution of Ga was found to be highly inhomogeneous caused by CuGaS₂ phase segregation at the back contact. Depending on overall Ga content and sulfurization temperature a quaternary CuGa_xIn_1−xS₂ compound formed exhibiting a shift in absorber lattice constant and band gap. Micro Raman measurements showed that crystal quality was also affected by Ga. Open-circuit voltages well above 800 mV were achieved while sustaining high fill factors of 71%.     

A novel one-step electrodeposition to prepare single-phase CuInS2 thin films for solar cells

Copper indium disulfide (CuInS₂) thin films have been successfully prepared on Ni substrates using a novel one-step potentiostatic electrodeposition combined with a potassium hydrogen phthalate (C₈H₅KO₄) complexing agent, accompanied by annealing at 350 °C. Electrodeposition in the solution of Cu and In salts and sodium thiosulfate (Na₂S₂O₃) containing an adequate concentration of C₈H₅KO₄ (e.g., [C₈H₅KO₄]=23 mM) provides thin films comprised of a CuInS₂ single phase as the bulk composition, without forming Cu_xS secondary phases. In addition to the effect on bulk-phase compositions, the adjustment of [C₈H₅KO₄] causes variation in morphology and atomic composition of the film surface. The surface states of the films change from the Cu-rich rough surface at low [C₈H₅KO₄] (15 mM) to the In-rich smooth surface at high [C₈H₅KO₄] (23 mM). The higher [C₈H₅KO₄] induces the grains constructing the film to interconnect and form a densely packed CuInS₂ film without voids and pinholes. The single-phase and void-free CuInS₂ film shows a band gap of 1.54 eV, satisfying the requirement of the absorber layers in solar cells. The electrical properties tests denote its n-type conductivity with a resistivity of 9.6×10⁻⁵ Ω cm, a carrier concentration of 2.9×10²¹ cm⁻³ and a carrier mobility of 22.2 cm² V⁻¹ s⁻¹.     

Electronic structure and optical properties of CuGaS2 and CuInS2 solar cell materials

We report energy bands, density of states and optical properties of CuGaS₂ and CuInS₂ chalcopyrites. The electronic structure has been computed using linear combination of atomic orbitals (LCAO) scheme within density functional theory (DFT) and full-potential linearised augmented plane wave method. The energy bands, density of states, components of dielectric tensors and absorption coefficients are compared with the available data. It is seen that the present LCAO-DFT calculations reproduce the electronic properties of both the chalcopyrites in a reasonable way. The optical properties show more absorption of solar radiations for CuGaS₂ chalcopyrite, depicting its more usefulness in the solar cells.     

Sprayed CuInS2 films grown under Cu-rich conditions as absorbers for solar cells

CuInS₂ films were prepared by the spray pyrolysis method using either copper-rich solutions or the recrystallization of low-crystallinity film in the presence of an intentionally deposited Cu_xS layer. KCN-etched films were characterized by XRD, SEM and EDX. The Cu/In molar ratio of 1.5–4.0 in the solution resulted in well-crystallized CuInS₂ films with the mean crystallite size of 120 nm. SEM study showed nonuniform surface with irregularly placed large grain domains in the flat film. The two-step process resulted in a uniform film with the crystallite size of 50 nm. Films exhibited an In-rich composition. Solar cells based on a recrystallized absorber showed an improved quantum efficiency spectrum.     

Control of valence states by a codoping method in CuInS2

We deviate the valence and conduction band energies of stoichiometric CuInS₂ crystals based on ab initio electronic band structure calculations using the augmented spherical wave (ASW) method and discuss that at low doping levels, the Madelung energy is a good intrinsic parameter for stabilization of p- or n-type doped CuInS₂ crystals. We find that P and Sb atoms are eminently suitable dopants substituted for S atoms for p-type doped CuInS₂ crystals with lower resistivity from both the character of electronic states around E_F and the Madelung energy. A closer study of the nature of chemical bonds of CuInS₂ crystals using first-principles band structure calculation method reveals that In with polyvalence codoping for p-type CuInS₂ doped with P results in a decrease of the Madelung energy compared with CuInS₂: P, to be an effective method for stabilizing of its ionic charge distributions.     

CuInS2-Poly(3-(ethyl-4-butanoate)thiophene) nanocomposite solar cells: Preparation by an in situ formation route, performance and stability issues

In this contribution we present an in situ method for the preparation of CuInS₂-poly(3-(ethyl-4-butanoate)thiophene) (P3EBT) nanocomposite layers and their application in nanocomposite solar cells. A precursor solution containing copper and indium salts, thiourea and the conjugated polymer was prepared in pyridine, which was coated onto glass/ITO substrates followed by a heating step at 180 °C. The heating step induced the formation of the CuInS₂ nanoparticles homogeneously dispersed in the conjugated polymer matrix. The formation of the nanocomposite was investigated in situ by X-ray scattering techniques and TEM methods showing that nano-scaled CuInS₂ was formed. By addition of small amounts of zinc salt to the precursor solution, zinc containing CuInS₂ (ZCIS) was formed. ZCIS-P3EBT active layers exhibited higher V_OC than CuInS₂-P3EBT layers and showed efficiencies of about 0.4%. Additionally the stability of the solar cells was tested over a time scale of 172 h.     

Spray pyrolysis of CuInS2

Spray pyrolysis is a low-cost method of depositing thin films and is economically more attractive than other methods that have been used to produce stable CuInS₂ thin films. The electrical, optical and structural properties of the films, as prepared, are presented together with their evolution and with a variation of some fabrication parameters which are the fabrication temperature T_S, and the ionic ratio R = Cu : In : S in the solution.