Influence of growth temperature of transparent conducting oxide layer on Cu(In,Ga)Se2 thin-film solar cells

We have studied the influence of growth temperature (T_G) in the deposition of an indium tin oxide (ITO) transparent conducting oxide layer on Cu(In,Ga)Se₂ (CIGS) thin-film solar cells. The ITO films were deposited on i-ZnO/glass and i-ZnO/CdS/CIGS/Mo/glass substrates using radio-frequency magnetron sputtering at various T_G up to 350 °C. Both the resistivity of ITO and the interface quality of CdS/CIGS strongly depend on T_G. For a T_G ≤ 200 °C, a reduction in the series resistance enhanced the solar cell performance, while the p-n interface of the device was found to become deteriorated severely at T_G > 200 °C. CIGS solar cells with ITO deposited at T_G = 200 °C showed the best performance in terms of efficiency.     

Antimony assisted low-temperature processing of CuIn1 − xGaxSe2 − ySy solar cells

Application of the Sb-doping method to low-temperature (≤ 400 °C) processing of CuIn_1 − xGa_xSe_2 − yS_y (CIGS) solar cells is explored, using a hydrazine-based approach to deposit the absorber films. Power conversion efficiencies of 10.5% and 8.4% have been achieved for CIGS devices (0.45 cm² device area) processed at 400 °C and 360 °C, respectively, with an Sb-incorporation level at 1.2 mol % (relative to the moles of CIGS). Significant Sb-induced grain size enhancement was confirmed for these low processing temperatures using cross-sectional scanning electron microscopy, and an average 2-3% absolute efficiency improvement was achieved in Sb-doped samples compared to their Sb-free sister samples. With Sb inclusion, the CIGS film grain growth temperature is lowered to well below 450 °C, a range compatible with flexible polymer substrate materials such as polyimide. This method opens up access to opportunities in low-temperature processing of CIGS solar cells, an area that is being actively pursued using both traditional vacuum-based as well as other solution-based deposition techniques.     

Optimization of ALD-(Zn,Mg)O buffer layers and (Zn,Mg)O/Cu(In,Ga)Se2 interfaces for thin film solar cells

(Zn,Mg)O films, fabricated by atomic layer deposition, ALD, are investigated as buffer layers in Cu(In,Ga)Se₂-based thin film solar cells. Optimization of the buffer layer is performed in terms of thickness, deposition temperature and composition. High efficiency devices are obtained for deposition at 105-135 °C, whereas losses in open circuit voltage are observed at higher deposition temperatures. The optimal compositional region for (Zn,Mg)O buffer layers in this study is for Mg/(Zn + Mg) contents of about 0.1-0.2, giving band gap values of 3.5-3.7 eV. These devices appear insensitive to thickness variations between 80 and 600 nm. Efficiencies of up to 16.2% are obtained for completely Cd- and S-free devices with (Zn,Mg)O buffer layers deposited with 1000 cycles at 120 °C and having a band gap of 3.6 eV.     

Temperature dependence of the microstructure and resistivity of indium zinc oxide films deposited by direct current magnetron reactive sputtering

Indium zinc oxide (IZO) films were deposited as a function of the deposition temperature using a sintered indium zinc oxide target (In₂O₃:ZnO = 90:10 wt.%) by direct current (DC) magnetron reactive sputtering method. The influence of the substrate temperature on the microstructure, surface roughness and electrical properties was studied. With increasing the temperature up to 200 °C, the characteristic properties of amorphous IZO films were improved and the specific resistivity was about 3.4 × 10^− 4 Ω cm. Change of structural properties according to the deposition temperature was also observed with X-ray diffraction patterns, transmission electron microscopy, X-ray photoelectron spectroscopy, and atomic force microscopy. IZO films deposited above 300 °C showed polycrystalline phases evolved on the amorphous IZO layer. Very flat surface roughness could be obtained at lower than 200 °C of the substrate temperature, while surface roughness of the films was increased due to the formation of grains over 300 °C. Consequently, high quality IZO films could be prepared by DC magnetron sputtering with O₂/Ar of 0.03 and deposition temperature in range of 150-200 °C; a specific resistivity of 3.4 × 10^− 4 Ω cm, and the values of peak to valley roughness and root-mean-square roughness are less than 4 nm and 0.5 nm, respectively.     

High quality Ge epitaxial layers on Si by ultrahigh vacuum chemical vapor deposition

Flat, relaxed Ge epitaxial layers with low threading dislocation density (TDD) of 1.94 × 10⁶ cm^− 2 were grown on Si(001) by ultrahigh vacuum chemical vapor deposition. High temperature Ge growth at 500 °C on 45 nm low temperature (LT) Ge buffer layer grown at 300 °C ensured the growth of a flat surface with RMS roughness of 1 nm; however, the growth at 650 °C resulted in rough intermixed SiGe layer irrespective of the use of low temperature Ge buffer layer due to the roughening of LT Ge buffer layer during the temperature ramp and subsequent severe surface diffusion at high temperatures. Two-dimensional Ge layer grown at LT was very crucial in achieving low TDD Ge epitaxial film suitable for device applications.     

The influence of Na on low temperature growth of CIGS thin film solar cells on polyimide substrates

The objective of this work is to study the influence of Na on the properties of Cu(In,Ga)Se₂ (CIGS) absorber layers and finished solar cell devices on polyimide substrates. For this study Na is added to 3-stage grown CIGS thin films by evaporation of a NaF precursor layer prior to the absorber deposition. The precursor layer modifies the CIGS growth kinetics. A stronger Ga-gradient and a decrease of grain size are observed when the Na content increases. An increase in V_oc for a higher Na concentration at a nominal growth temperature of T_sub,max = 500 °C during CIGS deposition is explained by a higher carrier density, as obtained by DLCP measurements. The higher carrier concentration for the higher Na content could be attributed to the reduction of a compensating donor. However, a low J_sc does not allow for an enhanced efficiency possibly due to a shorter depletion region, as observed by admittance spectroscopy, and effective diffusion length.     

Fabrication of transparent CuCrO2:Mg/ZnO p-n junctions prepared by pulsed laser deposition on glass substrate

Transparent p-n heterojunctions composed of zinc oxide, copper-chromium, and indium tin oxide films were fabricated by the pulsed laser deposition technique on a glass substrate. The effect of the deposition temperature of the p-CuCrO₂:Mg layer in the junction on photovoltaic properties was investigated. Post-annealing was performed to improve the crystallinity of the semiconductor layers deposited at a relatively lower temperature. The rectifying characteristics were observed in the current-voltage curves of the prepared junctions for both p- and n-layers as thin as 100 nm. A sample in which the copper-chromium oxide layer was deposited at 250 °C and annealed at 500 °C for 10 min exhibited the highest photovoltage—as large as 184 mV—under irradiation at λ ≈ 375 nm. The optical transmission of the p-n junction sample was 70% in the visible region.     

Thermodynamic prediction and experimental verification of optimal conditions for the growth of CuGa0,3In0,7Se2 thin films using close spaced vapor transport technique

The aim of this work is to predict optimal conditions for growing good quality crystalline thin films using close spaced vapor transport (CSVT) technique. A thermodynamic model was developed and tested for the Cu-Ga-In-Se-I system to describe the deposition of CuGa_0,3In_0,7Se₂ (CIGS). We considered a multiphase mixture containing 48 chemical compounds derived from the combination of the simple elements Cu, Ga, In, Se and I. The minimization of the Gibbs energy of the system was performed to calculate the composition of the mixture at the equilibrium state. In this way, the solid phase composition with possible impurities was predicted for various source temperature (T_S) and iodine pressure (P_I2). The conditions of stoichiometric and quasi-stoichiometric deposition are from 400 to 600 °C for T_S and from 30 Pa to 14 kPa for P_I2. These optimal conditions were tested experimentally at 475 °C, 500 °C, 525 °C and 550 °C. The elaborated CIGS thin films were of good quality as revealed by X-ray diffraction and scanning electron microscopy. The experimental results proved that the thermodynamic model is a helpful tool for the prediction of the optimal conditions in the CSVT process.     

Effect of the nanostructure on room temperature ferromagnetism and resistivity of undoped ZnO thin films grown by chemical vapor deposition

Undoped zinc oxide thin films having various types of morphology and nanostructure were deposited by metal organic chemical vapor deposition (MOCVD) on single-crystalline substrates. Water-assisted MOCVD process at low temperatures (300-500 °C) was applied along with conventional MOCVD in oxygen-containing atmosphere at 500 and 600 °C. The strong correlation between room-temperature ferromagnetism of the films, their electrical properties and morphology at nano-scale was demonstrated.     

Structural properties of Cu(In,Ga)Se2 thin films prepared from chemically processed precursor layers

We have developed a chemical process for incorporating copper into indium gallium selenide layers with the goal of creating a precursor structure for the formation of copper indium gallium diselenide (CIGS) photovoltaic absorbers. Stylus profilometry, EDX, Raman spectroscopy, XRD and SIMS measurements show that when indium gallium selenide layers are immersed in a hot copper chloride solution, copper is incorporated as copper selenide with no increase in the thickness of the layers. Further measurements show that annealing this precursor structure in the presence of selenium results in the formation of CIGS and that the supply of selenium during the annealing process has a strong effect on the morphology and preferred orientation of these layers. When the supply of Se during annealing begins only once the substrate temperature reaches ≈ 400 °C, the resulting CIGS layers are smoother and have more pronounced preferred orientation than when Se is supplied throughout the entire annealing process.     

Al and Ti/Al contacts on n-GaN

Al(60 nm) and Ti(40 nm)/Al(160 nm) metal layers have been deposited by thermal evaporation onto n-GaN epitaxial layers grown by metal organic chemical vapour deposition (MOCVD) on a c-plane sapphire substrate. The samples have been annealed at 300, 400, 700 or 900 °C for 10 min in vacuum. The microstructural and electrical properties of the contacts have been investigated by electron microscopy, X-ray diffraction and by current-voltage measurements. As-deposited Al and Ti/Al contacts were rectifying with Schottky barrier heights below 0.35 eV and 0.38 eV, respectively. After heat treatment at 300 °C and 400 °C both contacts exhibited linear current-voltage characteristics. After annealing at 700 °C Al contacts became rectifying with a barrier height of 0.42 eV, while Ti/Al contacts remained nearly linear at the same temperature. The electrical characteristics and XRD analysis indicated that the upper metal in Ti/Al contact diffused in the Ti layer already during deposition. Cross-sectional transmission electron microscopy revealed that in the case of Ti/Al contacts, the continuity of the Ti layers ceased when annealing above 700 °C. X-ray diffractions showed, that a Ti₂N interface phase formed in Ti/Al contacts at 700 and 900 °C, and an AlN interface phase developed in the same contact at 900 °C.     

High sensitivity and wide bandwidth image sensor using CuIn1 − xGaxSe2 thin films

We have fabricated a novel image sensor using Cu(In,Ga)Se₂ (CIGS). A combined process of dry etching using HBr and Ar gasses and wet etching using dilute HCl solution was developed as isolation process of CIGS photodiode deposited at 400 °C. Etchant residues of the dry etching, which consist of Cu complex, were almost completely cleaned using the wet etching process and favorable vertical side wall of CIGS films was obtained without mechanical damages. As a result, high performance image sensors with low leakage current of ~ 10^− 8 A/cm² and wide wavelength range up to ~ 1240 nm were achieved. The developed image sensor consisted of 352 × 288 pixels with 10 µm × 10 µm pixel sizes, was able to capture clear images of night scenes.     

Laser, buffer layer and CoCrPtOx-assisted low temperature fabrication process of a small-sized-CNT pattern by MPCVD

For improving compatibility with IC processes, this work presents a low temperature process (< 400 °C) to fabricate a small-sized-carbon nanotube (CNT) (< 6 graphene layers) pattern by buffer layer (AlN) and CoCrPtO_x catalyst precursor-assisted microwave plasma chemical vapor deposition (MPCVD). Without high temperature heating on the whole specimen, the low temperature process mainly results from selective local activation laser heating (≧ 600 °C) to form the catalyst nanostructures, which are beneficial to low temperature H-plasma treatment to form catalyst nanoparticles for CNT growth. The functions of the buffer layer and the catalyst precursor are to help the heat dissipation and the small-sized CNT formation.     

Raman spectroscopy of CuIn1 − xGaxSe2 for in-situ monitoring of the composition ratio

Metal organic vapor deposition (MOCVD) is a well known method for preparing high quality and large area CuIn_1 − xGa_xSe₂ (CIGS) absorber layers. Some in-situ non-contact monitoring systems are needed when CIGS absorber layers are manufactured in industry. In this study, CuInSe₂ (CIS) and CIGS thin films with different composition ratios, [Cu]/[In + Ga], were prepared by MOCVD using [Me₂In(μ-SeMe)]₂, hexafluoroacetylacetonate Cu(I) (3,3-dimethyl-1-butene), trimethyl gallium and dimethyle diselenide as the In-Se single source, Cu, Ga and Se precursors, respectively. The Raman shift spectra of the films with various composition ratios were analyzed to produce a basic algorithm that can determine the composition ratios of CIS and CIGS thin films indirectly.     

Flexible electrochromic devices based on crystalline WO3 nanostructures produced with hot-wire chemical vapor deposition

Crystalline WO₃ nanoparticles are employed in the development of flexible electrochromic (EC) devices. The nanoparticles are synthesized at high-density with a hot-wire chemical vapor deposition process where the hot filament provides the source of the tungsten metal. Polyethylene terephthalate coated with indium tin oxide is employed as a transparent flexible substrate. A simple electrophoresis technique is employed to deposit the WO₃ nanoparticles on the polymer, resulting in a uniform thin film. The EC performance is optimized for WO₃ particles that were baked at ~ 300 °C for 2 h prior to electrode fabrication. The transmittance is modulated between ~ 94% and ~ 28% without degradation for 100 cycles.     

Deposition of fluorine doped indium oxide by atmospheric pressure chemical vapour deposition

We report the deposition of fluorine doped indium oxide by atmospheric pressure chemical vapour deposition (APCVD) using a previously unreported precursor combination; dimethylindium acetylacetonate, [Me₂In(acac)] and trifluoroacetic acid (TFA). This process is potentially scalable for high throughput, large area production.[Me₂In(acac)] is a volatile solid. It is more stable and easier to handle than traditional indium oxide precursors such as pyrophoric trialkylindium compounds.Cubic fluorine doped indium oxide (F.In₂O₃) was deposited at a substrate temperature of 550 °C with growth rates exceeding 8 nm/s. Resistivity was 8 × 10^− 4 Ω cm and transmission for a 200 nm film was > 80% with less than 1% haze.     

Synthesis and characterization of chalcopyrite quaternary semiconductor Cu (InxGa1 − x) S2 nanowires by electrospun route

Chalcopyrite Cu(In_0.8Ga_0.2)S₂ (CIGS) nanowires were synthesized by using a relatively simple and convenient electrospun process. From the reactions of CuCl, InCl_3, GaCl₃ and thiocarbamide as a precursor, semiconductor CIGS nanowires with diameter in the range of 60-80 nm were obtained for Polyvinylbutyral/CIGS precursor ratios of 40% at an applied voltage of 25 kV after annealing treatment at 600 °C for 3 h. A probable formation mechanism of chalcopyrite quaternary semiconductor nanowires is proposed based on a series of comparative experiments conducted under various reaction conditions.     

Highly efficient CIS solar cells and modules made by the co-evaporation process

Thin-film photovoltaic modules which use the chalcopyrite Cu(In,Ga)(Se,S)₂ (CIGS) as the light-absorbing layer have now entered the decisive industrial phase. Companies located mainly in Germany and Japan will produce more than 100 MWp CIGS modules in 2008, demonstrating that the CIGS technology has already achieved a certain maturity. Whereas key features of the technology are already well-optimized, there are several approaches to further improve the productivity of new lines. The ZSW operates a line for 30 × 30 cm² modules in which all process steps - from glass cleaning to module encapsulation - are being developed. A major goal of the development is the very fast and efficient transfer of promising new materials and processes from cells to the industrial module level. Therefore, ZSW is focusing on processes like the in-line co-evaporation method for CIS or chemical bath deposition for buffer layers to optimize the junction. We could demonstrate efficiencies close to 18% for small test cells and 14-15% for modules with modified processes. Different cell and material data from optoelectronic measurements and microscopic analysis will be presented in this contribution.     

Transparent conducting indium oxide thin films grown by low-temperature metal organic chemical vapor deposition

We have grown indium oxide thin films on silicon substrates at low temperature by metal organic chemical vapor deposition. Polycrystalline film growth could only be obtained at temperatures below 400 °C. Above 400 °C, metallic indium deposition dominated. We have investigated the effect of substrate temperature and reactor pressure on the film growth and structural properties in the range of 250-350 °C and 5 ^? 10³-4 ^? 10⁴ Pa. The film grown at 300 °C exhibited a resistivity of about 3.6 × 10^− 3 Ω cm and a maximal optical transmittance of more than 95% in the visible range. The film showed an optical band gap of about 3.6 eV.     

Hydrazine-based deposition route for device-quality CIGS films

A simple solution-based approach for depositing CIGS (Cu-In-Ga-Se/S) absorber layers is discussed, with an emphasis on film characterization, interfacial properties and integration into photovoltaic devices. The process involves incorporating all metal and chalcogenide components into a single hydrazine-based solution, spin coating a precursor film, and heat treating in an inert atmosphere, to form the desired CIGS film with up to micron-scaled film thickness and grain size. PV devices (glass/Mo/CIGS/CdS/i-ZnO/ITO) employing the spin-coated CIGS and using processing temperatures below 500 °C have yielded power conversion efficiencies of up to 10% (AM 1.5 illumination), without the need for a post-CIGS-deposition treatment in a gaseous Se source or a cyanide-based bath etch. Short-duration low-temperature (T < 200 °C) oxygen treatment of completed devices is shown to have a positive impact on the performance of initially underperforming cells, thereby enabling better performance in devices prepared at temperatures below 500 °C.