CuInS2 Thin Films Research Articles

Formation of CuInS2 in Spray Pyrolysis Process as Simulated by Thermal Analysis

The formation of CuInS2 thin films was studied by simulating spray pyrolysis conditions through thermogravimetric experiments performed both in air and helium. The solid precursor containing Cu, In, Cl, thiourea and H2O in a ratio 1:1:5:3:2 was obtained by evaporation of the aqueous spray pyrolysis solution. Thermal degradation of the precursor was found to be a complex process where CuInS2 is formed as the main crystalline product already at 250 °C in both atmospheres. In air, however, InOCl is also readily formed and at 650 °C In2O3 and CuO appear as final products. In thin films prepared at 320 °C the Cu:In:S ratio is correct but there are 1.5 wt.% Cl residues as shown by RBS and EDS analyses, respectively.

Smoothing of polycrystalline Cu(In,Ga)(Se,S)2 thin films by low-energy ion-beam etching

We present a study of the smoothing process of polycrystalline CuInS2 and CuGaSe2 thin films using ion-beam etching with low-energy nitrogen ions. The evolution of the surface roughness and morphology have been studied by atomic force microscopy with respect to the ion-beam angle of incidence and the ion dose. Raman scattering and photoluminescence spectroscopy were applied to investigate the surface damage induced by the ion-beam etching process. A drastic reduction of the surface roughness for both materials has been observed for ion-beam angles of incidence larger than 70° with increasing ion doses without significant surface damage. Using this technique the initial root-mean-square roughness of, e.g., CuGaSe2 of about 87 nm could be decreased to about 25 nm. Roughness analysis over different spatial wavelength regions by using the power spectral density reveals that smoothing occurs for all spatial wavelengths. Hence, ion-beam etching with nitrogen is a suitable method to polish polycrystalline CuInS2 and CuGaSe2 thin films for surface analysis techniques and depth profiling.

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 TS, and the ionic ratio R = Cu : In : S in the solution.

Transmission electron microscopy investigation and first-principles calculation of the phase stability in epitaxial CuInS2 and CuGaSe2 films

Ordering of metal atoms in CuInS2 and CuGaSe2 thin films grown by vapor-phase epitaxy on Si (001) and GaAs (001) substrates were studied using transmission electron microscopy and first-principles total energy calculations. Chalcopyrite and CuAu-like orderings of the metal atoms were observed coexisting in CuInS2 films, while only chalcopyrite ordering was found in CuGaSe2 films. First-principles total energy calculations find that the formation enthalpy difference between chalcopyrite and CuAu-like phases of CuInS2 is very small (2 meV/atom), while it is large for CuGaSe2 (9 meV/atom), indicating that the existence of CuAu-like phase in the nominally chalcopyrite semiconductors is controlled by bulk thermodynamics.

EFFECT OF THE SOLUTION PH ON THE GROWTH OF SPRAY-DEPOSITED CUINS2 THIN FILMS

The effect of solution pH on the growth of spray deposited CuInS2 thin films has been investigated. Solutions with pH 1.5, 2.4, 3.5 and 4.5 are used to deposit the films on glass substrates held at 550 K. The films have been characterized using optical absorption, X-ray diffraction (XRD) and transmission electron microscopy (TEM) analysis. At pH 1.5, no film formation is observed. Films deposited from 2.4 pH solution contain binary phase Cu2S. Near stoichiometric, single phase CuInS2 films with chalcopyrite structure are formed when solution pH=3.5. Films deposited from 4.5 pH solution are also found to be single phase, near stoichiometric CuInS2 and exhibited chalcopyrite structure. However, an additional optical absorption process is observed which is attributed to a sub-band gap absorption.

Negative-differential-resistance effects in amorphous CuInS 2 thin film

The microstructure and the optical properties of amorphous CuInS2 thin films deposited on glass substrates by a single source thermal evaporation technique, are characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission and reflection measurements. Amorphous CuInS2 thin films sandwiched between gold and copper electrodes, show current-controlled electronic switching. The switching is believed to be associated rather with electronic processes than the thermal ones.

Changes in the electronic structure of CuInS2 thin films by Na incorporation

For Na-free and Cu-deficient CuInS2 thin films with chalcopyrite structure, electron spectra of core levels and valence band were similar to each other in spite of a difference in Cu deficiency. Incorporation of Na with the CuInS2 films caused peak shifts by −0.5 eV for the core levels without changes in both linewidth and shape. The electron density of states, deeper by 1–5 eV than that from Cu d–S p hybridization centered at around 2.5 eV, increased by the Na incorporation, and increments in the electron density of states were larger for the films with larger the Cu deficiency. The Na incorporation yielded a surface layer expressed as (Na,Cu)InS2 on the Cu deficient CuInS2 films.

Characterization of CuInS2 thin films for solar cells prepared by spray pyrolysis

We have made a study of the chemical composition, the electrical, the optical and the structural properties of polycrystalline CuInS2 thin films prepared by spray pyrolysis to be used for thin film solar cells. These films were deposited starting from aqueous solutions with different chemical compositions ([Cu]/[In] and [S]/[Cu] ratios) and at different substrate temperatures. In all cases, the material is p-type with grains preferentially oriented in the (112) direction of the sphalerite structure. The electro-optical properties show a very strong dependence on the [Cu]/[In] ratio in the solution. Films with copper excess have smaller resistivity and better crystallinity than those which are stoichiometric or have indium excess. The results obtained in this work show the possibility of having CuInS2 thin films with a wide range of resistivity, a fact that could be important for making solar cells based on this material.

Growth and properties of CuInS2 thin films

Single phase copper indium disulphide (CuInS2) thin films of thickness between 60 nm and 650 nm with the chalcopyrite structure are obtained on NaCl and glass substrates by flash evaporation. The films were found to ben-type semiconducting. The influence of the substrate temperature on the crystallinity, conductivity, activation energy and optical band gap was studied. An improvement in the film properties could be achieved up to a substrate temperature of 523 K at a molybdenum source temperature of 1873 K.

Hydrogen Diffusion in Chalcopyrite Solar Cell Materials

ABSTRACTHydrogen diffusion in CuInSe 2 single crystals and CuInS2 thin films was studied by measuring the spreading of implantation profiles upon annealing. Deep implantation with an ion energy of 10 keV and sub-surface implantation with 300 eV were applied. The diffusion coefficients in both materials were found to be in the order of 10-14 to 10-13 cm2/s in the temperature range between 400 and 520 K.These fairly low diffusivities are typical for a trap and release transport process rather than intrinsic diffusion of interstitial hydrogen. In the polycrystalline CuInS2 films, hydrogen leaves the sample through the grain boundaries.

Thermal Decomposition of Copper(I) Thiocarbamide Chloride Hemihydrate

Two combinations of simultaneous thermoanalytical techniques (TG+DTA and TG+EGA) were used to study the thermal decomposition of the title compound in order to gain a better insight into the spray pyrolytic processes leading to Cu2-xS and CuInS2 thin films. After dehydration a complex sequence of reactions starts above 220°C leading through several intermediates to the formation of CuO in air at 1000°C. In an inert atmosphere Cu2S is formed which in helium above 800°C partly decomposes to Cu. XRD and FTIR were used to identify the intermediate solid phases which in air included CuCl, Cu2OSO4, Cu2OCl2 and CuSO4. EGA-FTIR confirmed the complex reaction mechanism with NH3, HCl, H2O, COS, CO2 and some HCN as main gaseous products under oxidative conditions.

Photoluminescence of CuInS2 thin films and solar cells modified by postdeposition treatments

The photoluminescence of CuInS2 thin films and solar cells is investigated as a function of postdeposition treatments for different temperatures and excitation intensities. Annealing in hydrogen atmosphere causes an increase of PL intensity at 1.445 eV by more than a factor of 100, while subsequent annealing in oxygen or air ambient passivates this transition, which is ascribed to a donor-acceptor pair recombination between a sulphur vacancy and a copper vacancy. A defect mechanism is suggested that assumes the passivation of sulphur vacancies by oxygen in grain surfaces which can be activated by hydrogen annealing.

Influence of the cooling rate on the electrical conductivity of coevaporated CuInS2 thin films

We investigated the influence of the cooling rate in a deposition process on the electrical conductivity σ of CuInS2 thin films for solar cells. In situ measurements were used in order to monitor the electrical conductivity of Cu-poor films during growth and cooldown. It is shown that the room-temperature electrical conductivity σRT depends significantly on the cooling rate of the film after deposition. σRT reaches 1×10−3 S cm−1 for a cooling rate of 2.0±0.2 K min−1 whereas for abrupt cooling and higher cooling rates σRT is reduced to 10−6 and 10−5 S cm−1, respectively.

CuInS 2 thin film growth monitoring by in situ electric conductivity measurements

The growth of CuInS2 thin films by coevaporation has been monitored by in situ electrical conductivity measurements. Films with different cation ratio In/(In+Cu) were investigated. During the controlled cool-down period we obtain conductivity versus temperature data which are completed ex situ in the low-temperature region. The formation of the semimetallic CuS phase in Cu-rich films is found during the cool-down period at a substrate temperature of about 450 K. For In-rich films the dominance of charged grain boundary states is discussed.

Spray Chemical Vapor Deposition of CuInS2 Thin Films for Application in Solar Cell Devices

ABSTRACTChalcopyrite CuInS2 is a direct band gap semiconductor (1.5 eV) that has potential applications in photovoltaic thin film and photoelectrochemical devices. We have successfully employed spray chemical vapor deposition using the previously known, single-source, metalorganic precursor, (Ph3P)2CuIn(SEt)4, to deposit CuInS2 thin films. Stoichiometric, polycrystalline films were deposited onto fused silica over a range of temperatures (300–400 °C). Morphology was observed to vary with temperature: spheroidal features were obtained at lower temperatures and angular features at 400 °C. At even higher temperatures (500 °C), a Cu-deficient phase, CuIn5S8, was obtained as a single phase. The CuInS2 films were determined to have a direct band gap of ca. 1.4 eV.

Solar Cells Based on CuInS2 Thin Films through Sulfurization of Precursors Prepared by Reactive Sputtering with H 2S Gas

CuInS2 films were prepared using a two-stage process in which Cu–In–S precursors were sputtered in H2S/Ar with metal targets (Cu and In) and then annealed in H2S atmosphere. Control of S content in the precursor is essential for fabricating high-quality CuInS2 films and achieving high-efficiency solar cells. When S/(Cu+In) ratio in the precursor was about 0.3, CuInS2 films with good crystallinity and smooth surface were obtained, and consequently a CuInS2 solar cell with an efficiency of 6.3% has been achieved without KCN treatment.

太陽電池用ＣｕＩｎＳ２膜の作製

Photovoltaic-quarity CuInS2 thin films were prepared by means of a sulfurization of precursors under a S-vapor pressure. Conditions for preparing of single-phase films with a highly preferred (112) orientation are presented.

Hydrogen in thin films of CuInS2 solar cell material

The production of thin films of CuInS2 by the reactive annealing process in H2S atmosphere leads to the incorporation of hydrogen. Hydrogen concentrations of up to 5×1020 cm−3 were measured by means of a nuclear reaction analysis method. No negative influence on the electrical properties of the films containing hydrogen was observed. Instead, hydrogen, when incorporated in situ, seems to help to passivate electrically active lattice defects.

Formation of secondary phases in evaporated CuInS2 thin films: A surface analytical study

The surface composition of coevaporated Cu-In-S films for photovoltaic applications has been investigated by x-ray and ultraviolet photoelectron spectroscopy. A comparison of the front and back surface, i.e., the interface between the Cu–In–S film and the Mo back contact, is made for films with different stoichiometries. We find that In-rich films are covered at both surfaces by an In-rich second phase which has the cation ratio of CuIn3S5. The front surface of Cu-rich films shows a segregation of CuS that is not found at the back surface. The formation of binary and ternary second phases in nonstoichiometric CuInS2 films and their implications on the growth mechanism are discussed.

Preparation and properties of CuInS2 thin films

Abstract A thin film of chalcopyrite CuInS2 has been formed by sulfurization of a layered metallic precursor at 550°C in argon containing H2S. The Cu/In ratio of the film is ranged from 0.8 to 1.85. The resistivity of the film decreases by aging in air and then increases by annealing in vacuum or in air. The former effect can be attributed to gas absorption in the film which gives rise to an increase in the hole mobility, while the latter gives rise to gas desorption. The hole concentration of the films is of the order of 1020 cm−3 and is almost independent of aging. Electrical conduction in the thin film is discussed in terms of hole transport crossing over an intergranular potential barrier in the polycrystalline film.