Se2 Thin Films Research Articles

Thin Ag Precursor Layer-Assisted Co-Evaporation Process for Low-Temperature Growth of Cu(In,Ga)Se2 Thin Film.

Achieving favorable band profile in low-temperature-grown Cu(In,Ga)Se2 thin films has been challenging due to the lack of thermal diffusion. Here, by employing a thin Ag precursor layer, we demonstrate a simple co-evaporation process that can effectively control the Ga depth profile in CIGS films at low temperature. By tuning the Ag precursor thickness (∼20 nm), typical V-shaped Ga gradient in the copper indium gallium diselenide (CIGS) film could be substantially mitigated along with increased grain sizes, which improved the overall solar cell performance. Structural and compositional analysis suggests that formation of liquid Ag-Se channels along the grain boundaries facilitates Ga diffusion and CIGS recrystallization at low temperatures. Formation of a fine columnar grain structure in the first evaporation stage was beneficial for subsequent Ga diffusion and grain coarsening. Compared to the modified co-evaporation process where the Ga evaporation profile has been directly tuned, the Ag precursor approach offers a convenient route for absorber engineering and is potentially more applicable for roll-to-roll fabrication system.

Effect of calcium incorporation on properties of Cu(InGa)Se2 thin film and solar cell

Cu(InGa)Se2 (CIGS) thin films with Ca doping from CaF2 film synthesized onto quartz glass substrates via a two-step process were studied. XRD peak splitting ascribed to graded compositional distribution existed in pristine CIGS film but disappeared in those with Ca doping. Depth profiles verified that the Ca element mainly existed in the near surface region of CIGS film with CaF2 post-deposition treatment (PDT) while diffused throughout the absorber with CaF2 pre-deposition. The grain sizes in surface of CIGS films without doping and with CaF2 PDT were much larger than those with CaF2 pre-deposition. The Fermi level of film with pre-deposited CaF2 showed no change while that of film with CaF2 PDT exhibited lower energy compared to the pristine film. Performance degradation due to inferior Mo/CIGS interface was observed in device with CaF2 pre-deposition. Inversely, PDT of CaF2 enhanced the device performance with an absolute efficiency of 2.2 %. SCAPS-1D was used to simulate the CIGS devices for further study. It showed a shallower defect position in solar cell with CaF2 PDT, giving explanation for its better performance.

Solar cells fabricated by spray pyrolysis deposited Cu2CdSnS4 thin films

Cu2ZnSnS4 (CZTS) occurs in nature and is the most likely alternative to Cu(In,Ga)Se2 (CIGS) thin film solar cells, which started to spread increasingly all over the world. CZTS is a very suitable absorber layer material for thin film solar cells due to containing cheap material. It has a 1.5 eV band gap that is appropriate for theoretical Shockley-Queisser limit values, and the large absorption coefficient (104 cm−1). Substitution of different metals and usage of low cost and easy controlled deposition systems may present advantages.Cu2CdSnS4 (CCTS) is a semiconductor with a band gap of 1.37 eV, and it has a large absorption coefficient over 104 cm−1 that makes it a possible photovoltaic material. Its structure is similar to CZTS. In this study, Cu2CdSnS4 thin films were deposited by ultrasonic spray method at various substrate temperatures as an alternative absorber layer. The structural features of the thin films were determined by X-ray diffraction (XRD), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS) surface morphology by Scanning Electron Microscopy (SEM) and optical band gaps by UV-VIS-NIR data. A CCTS-based solar cell with 1.14% power conversion efficiency was obtained using the most appropriate thin film according to optical and structural properties.

Optimazation of sputtered n-type GaN/InGaN for Cu(In,Ga)Se2 thin film solar cells

Cu(In1-xGax)Se2 thin film solar cell with 3.64% efficiency has been demonstrated by using n-Type GaN/In0.3Ga0.7N. The CIGSe thin film was prepared by co-sputtering with two targets of Cu-In and Ga2Se3+Sb2S3. CIGSe films on the Mo/glass substrates were made by co-sputtering technique at 100 °C with a metal target of Cu-In operated at 15 W and a cermet target of Ga2Se3+Sb2S3 at 55 W, followed by thermal annealing at 600 °C for 1 h. The properties of the CIGSe thin film was studied by measuring the electrical, structural and crystal structure. Solar cell devices were designed by depositing ∼50 nm GaN layer and ∼300 nm In0.15Ga0.85N or In0.3Ga0.7N followed by coating front contact with 300–400 nm indium-tin-oxide (ITO).

Picosecond laser scribing of bilayer molybdenum thin films on flexible polyimide substrate

Molybdenum (Mo) is a commonly used material as the electrical back contact for Cu(In,Ga)Se2 (CIGS) thin film solar cells taking advantages in electrical, structural and chemical properties. Bilayer structures have been actively examined to achieve Mo films of high conductivity with good substrate adhesion, which will also enable monolithically integrated solar cell modules on flexible platforms by combining laser scribing technology. However, reports on the bilayers on flexible substrates and their scribing by laser are very limited.In this study, we report the characteristics of bilayer Mo films deposited on a flexible polyimide substrate and the impact on the performance of picosecond laser scribing. Bilayer Mo films were deposited over a range of sputtering pressures and discharge powers as well as top-bottom layer thickness ratios, and laser scribing behavior was compared in the practical scribing speed range of the order of m/s. It is shown that microstructural and interfacial characteristics set by top and bottom Mo layers deposition parameters are critical factors influencing laser scribing quality.

Effect of Nanosecond Laser Beam Shaping on Cu(In,Ga)Se2 Thin Film Solar Cell Scribing

The Cu(In,Ga)Se2 (CIGS) thin film solar cell is a promising material architecture considering its high photovoltaic (PV) efficiency at low material cost. Recently, the authors demonstrated all lase...

Innovative industrial Cu(In,Ga)Se2 thin film solar cell with high characterization using nanoparticles structure

This paper presents new design of ZnO/CdS/CIGS/Mo nanocomposites thin film solar cell based on individual and multiple nanocomposites absorber layer. A theoretical analysis is presented for enhancing the efficiency and characterization of individual and multiple nanocomposites CdS/Cu (In, Ga) Se2 thin film solar cell. Moreover, this paper is achieving high efficiency with a thinner CIGS layer for reducing the direct materials usage and there by the materials costs by adding various metallic nanoparticles in CIGS absorber layer .

Innovative industrial Cu(In,Ga)Se2 thin film solar cell with high characterization using nanoparticles structure

This paper presents new design of ZnO/CdS/CIGS/Mo nanocomposites thin film solar cell based on individual and multiple nanocomposites absorber layer. A theoretical analysis is presented for enhancing the efficiency and characterization of individual and multiple nanocomposites CdS/Cu (In, Ga) Se2 thin film solar cell. Moreover, this paper is achieving high efficiency with a thinner CIGS layer for reducing the direct materials usage and there by the materials costs by adding various metallic nanoparticles in CIGS absorber layer .

Multifunctional metal oxide electrodes: Colour for thin film solar cells

Building integrated photovoltaics (BIPV) is an essential part to reduce the CO2 footprint of metropolitan areas. Currently, full integration of photovoltaic elements in a building is a very costly and complex undertaking, as it usually requires expensive custom modules. In order to increase the market share of BIPV in the residential mass market, a low-cost, flexible technical process to change the appearance of solar elements is required. Transparent conductive electrodes consisting of an oxide-metal-oxide (OMO) stack of thin layers have been optimized for application in thin film solar cells. Here the OMO stack is multifunctional: It provides the transparent front contact electrode and at the same time allows tuning of the module colour. This has several advantages compared to other colouring techniques, i.e. coloured glass or additional interlayers. The OMO colouring does not require an additional process step, and with sputtering an already existing deposition method is used. By varying the thickness of the oxide layers it is possible to change the reflected spectrum of the stack and with it the module colour. In this publication, we present how the optical model of the OMO stack, that is necessary for precise tuning of the colour, is first developed for OMO/glass samples and then report the changes necessary to adapt the OMOs for use on Cu(In,Ga)Se2 thin film solar cells.

Effects of Cu(In,Ga)3Se5 defect phase layer in Cu(In,Ga)Se2 thin film solar cells

Cu(In,Ga)3Se5 (135-CIGS) layers with various thicknesses were deposited on the surface of Cu(In,Ga)Se2 (112-CIGS) photon absorber in the fabrication of CIGS thin film solar cells. This significantly affects the shift of the optical band gap energy from 1.15 eV (112-CIGS) to 1.19 eV, with only 10 nm thick of 135-CIGS capping layer, leading to the increase in open-circuit voltage (Voc) of the solar cells. The optical transmission spectra show no sign of single 135-CIGS layer. The maximum Voc of 670 mV was obtained from 5 to 10 nm thick 135-CIGS capping layer on 112-CIGS compared to 646 mV of only 112-CIGS. The power conversion efficiencies of the devices covered with 135-CIGS with thickness of less than 80 nm are slightly lower than that of the uncovered 112-CIGS solar cells due to lower generated photocurrents. The solar cell parameters become dramatically deteriorate with thicker 135-CIGS capping layer. The XRD also shows the shift of the diffraction peak toward larger 2θ without peak broadening or splitting when the thickness of 135-CIGS is increased. The external quantum efficiency (EQE) measurements indicate the shift of absorption threshold towards shorter wavelength when the thickness of 135-CIGS is increased, which is consistent with the optical transmission measurements.

Epitaxial Cu(In,Ga)Se2 Thin Films on Mo Back Contact for Solar Cells

Single crystal Mo thin films are successfully grown on single‐crystal sapphire {0001} substrates using a newly developed two‐step sputtering method. Cu(In1‐x,Gax)Se2 (CIGS) thin films are then epitaxially grown on the Mo thin films by three‐stage co‐evaporation. Following that, a CdS buffer layer is deposited on the epitaxial (epi)‐CIGS thin films by a chemical bath deposition (CBD) method, leading to the overall epitaxial relations such as CdS{111}//CIGS{112}//MoSe2{0001}//Mo{110}//sapphire {0001}. The Cd‐diffusion into the CIGS occurs to approximately 30 nm from the CIGS/CdS interface boundary together with depleted Cu at the CIGS surface region. This suggests that Cd diffuses through Cu vacancies in the intra‐grain rather than at the grain boundaries. Two distinct solar cells are fabricated using epi‐ and polycrystalline (poly)‐CIGS thin films deposited on sapphire substrates in the same deposition run. The open‐circuit‐voltage (VOC) of epi‐CIGS solar cell is found to be higher than poly‐CIGS solar cell. The net carrier concentration (NCV) derived from the capacitance‐voltage (C‐V) measurements of epi‐CIGS solar cell is 3.5 times higher than poly‐CIGS solar cell, suggesting one of the causes of higher VOC. This large difference in the NCV is discussed based on the results of low‐temperature photoluminescence measurements.

In situ monitoring of the permanent crystallization, phase transformations and the associated optical and electrical enhancements upon heating of Se thin films

In this work, the in situ structural transformations from amorphous to polycrystalline upon heating and the associated enhancements in the structural parameters of selenium thin films are studied by means of X-ray diffraction technique. The Se thin films which are grown onto ultrasonically cleaned glass substrate by the thermal evaporation technique under vacuum pressure of 10−5 mbar exhibits structural transformation from amorphous to polycrystalline near 353 K. The films completed the formation of the structure which includes both of the hexagonal and monoclinic phases at 363 K. It is observed that the hexagonal phase dominates over the monoclinic as temperature is raised. Consistently, the thermally assisted crystallization process is accompanied with increase in the crystallite size, decrease in the microstrain, decrease in defect density and decrease in the percentage of stacking faults. The scanning electron microscopy measurements also confirmed the crystallinity of selenium after heating. The time dependent reputations of the crystallization test has shown that the achieved phase transitions and enhancements in structural parameters are permanent in selenium. Optically, the crystallization process is observed to be associated with redshift in the absorption spectra and in the value of the energy band gap. Electrically, the in situ monitoring of the electrical conductivity during the heating cycle has shown that the electrical conductivity stabilizes and exhibit a decrease in the acceptor levels from 566 to 321 meV after the crystallization was achieved.

Formation, negative capacitance and negative conductance effects in Selenium stacked layers sandwiched with Ag nanosheets

In this work, we report the nature of formation in the presence and absence of Ag nanosheets being inserted between of two stacked layers of Se thin films which are grown onto Au substrates. The Se/Se and Se/Ag/Se films which are prepared by the thermal evaporation technique under vacuum pressure of 10−5 mbar are studied by means of x-ray diffraction, conductance and capacitance spectroscopy techniques in the frequency domain of 0.01–1.80 GHz. Metal inducted crystallization processes from amorphous to hexagonal phases are achieved by using the Au substrate. The presence of Ag nanosheets of thicknesses of 50 nm between two 500 nm thick stacked layers of Se strongly affects the structural parameters through increasing the lattice constants, the microstrain and the defect density and decreasing the crystallite size. While the two stacked layers of Se sandwiched between Au and In metals displayed negative conductance effect associated with resonance in the capacitance and maximum microwave cutoff frequency () of 0.68 GHz near 1.31 GHz, the insertion of Ag nanosheets forced the two stacked layers to exhibit higher positive conductance values and increased the values to 17.4 GHz. Ag nanosheets also caused negative capacitance (NC) effect in all the studied frequency domain. NC effect is associated with resonance-anti-resonance phenomena in the region of 1.33–1.37 GHz. The features of the selenium stacked layers make them attractive for use in microwave circuits as cavities, noise reducers, parasitic capacitance cancellers and bandpass filters.

Study of vortex glass-liquid transition in superconducting Fe(Te, Se) thin films on LaAlO3 substrates

Superconducting thin films of two different thicknesses have been fabricated on (100) LaAlO3 substrates using a polycrystalline target of composition Fe1.05Te0.50Se0.50 by the pulsed laser deposition technique. The onset of superconducting transition temperatures (TConset) at 0 T magnetic field of the grown thin films of thickness of ∼74 nm and ∼185 nm are ∼12.65 and 13.15 K, respectively. The upper critical field BC2(0) values have been calculated by the Ginzburg–Landau theory as well as by the Werthamer–Helfand–Hohenberg model, and the corresponding coherence lengths have been estimated. In the present work, the thermally activated energy (TAE) has been obtained using the conventional Arrhenius law as well as by a modified thermally activated flux flow (TAFF) theory. For both models, the TAEs of vortices show a crossover at a magnetic field of ∼2 T corresponding to the transition from the single vortex pinning regime to the collective vortex pinning regime. Based on the analysis of the field dependence of TAE, the planer/point defect dominating magnetic field regions have been identified. The analysis of the magnetotransport data with the modified TAFF model reveals the dominating three dimensional behavior of vortices for both thin films. Moreover, in the vortex phase diagram, a narrow vortex melting region and a vortex glass-liquid phase transition below BC2 have been observed for both of the grown thin films. Furthermore, the X-ray photoelectron spectroscopy results show that Fe is present in Fe2+ and Fe3+ chemical states in both thin films.

Enhancing photocurrent of Cu(In,Ga)Se2 solar cells with actively controlled Ga grading in the absorber layer

Highly efficient Cu(In,Ga)Se2 (CIGS) thin film solar cells are often fabricated with a double Ga grading along the thickness direction of the light absorber. In this work, through a progressive procedure of using time-dependent Ga and In deposition rates, it is manifested that how the detailed shape of the double Ga grading profile in terms of the depth of the V-shape, the width of the V-shape, and the spatial position of the minimum bandgap can be experimentally actively manipulated. Moreover, the effect of the aforementioned features of the Ga grading profile on the photo-generated current has been uncovered by the progressive improvement in the spectral response of the external quantum efficiency spectra. Within the effort of our optimization, a CIGS solar cell with a conversion efficiency of 20.3% is obtained, believed to be the best reported for CIGS solar cells without actively using alkali post deposition treatment technology. This work has provided a practical and operable route to actively control the Ga grading profile and turned the fabrication of high efficiency CIGS solar cell from an art to more a science.

Significantly Enhanced Detectivity of CIGS Broadband High-Speed Photodetectors by Grain Size Control and ALD-Al2O3 Interfacial-Layer Modification.

The Cu(In,Ga)Se2 (CIGS) thin film has been commercialized as solar cells with great success, but its application for photodetectors still faces some practical challenges, including low detectivity and long response time. In this paper, the structure of the Mo/CIGS/CdS/ZnO/ITO heterojunction has been fabricated, and satisfactory performances of high detectivity and fast response time have been achieved by suppressing the dark current and enhancing the carrier mobility. The controllable growth of CIGS grains is accomplished through optimizing the selenization process, demonstrating that bigger grain sizes resulted in higher carrier mobility and better response characteristics. Particularly, the high rise/decay speed of 3.40/6.46 μs is achieved. Furthermore, the interface of the CIGS/CdS heterojunction has been modified by the Al2O3 layer via the atomic-layer deposition (ALD) process. The dark current of the device is effectively suppressed by the ALD-Al2O3 layer, which remarkably drops from ∼10-7 to ∼10-9 A. As a consequence, the detectivity rises from 3.08 × 1011 to 1.84 × 1012 Jones. In addition, the ALD-Al2O3 layer shows a protective effect as well, which is positive for photoelectrical conversion. Besides, the wide linear dynamic range of 102.1 dB and large -3 dB bandwidth of 78 kHz are acquired. This work suggests that the CIGS-based heterojunction has great potential for high-performance thin-film photodetectors.

Reaction Rate Enhancement for Cu(In,Ga)Se2 Absorber Materials Using Ag-Alloying

The addition of Ag to Cu-Ga-In precursors for synthesizing (Ag,Cu)(In,Ga)Se2 (ACIGS) thin films has shown benefits including improved adhesion, greater process tolerance, and potential for improved device performance. In this study, reaction pathways to form Cu(In,Ga)Se2 (CIGS) and ACIGS were studied by time-progressive reactions at 450 °C in a 5% Ar/H2Se atmosphere followed by ex situ characterization. Results indicated that the addition of 25% Ag/(Ag+Cu) to the CIGS film reduces the reaction time by 50%. X-ray diffraction (XRD) analysis of CIGS films showed that the CuInSe2 phase initially formed after 3.5 min. The slow reaction of the stable γ-Cu9(In,Ga)4 phase, however, required more than 20 min to complete. Importantly, the addition of Ag to the CIGS film accelerated the reaction. Energy-dispersive X-ray spectroscopy shows that Ga/(Ga+In) grading occurs in the first 10 min of the reaction. XRD analysis showed that the chalcopyrite phase fully forms after 10 min and no significant changes were observed in samples selenized from 10–45 min. Reaction pathways of Ag-alloyed films were further characterized using in situ high temperature XRD analysis. The onset temperature of Se reaction was detected at 230 °C and a AgIn2 phase transformation to (Ag,Cu)In2 occurred during the early stage of the reaction.

Investigation of Mo:Na and Mo related back contacts for the application in Cu(In,Ga)Se2 thin film solar cells

In this study, sodium doped molybdenum (Mo:Na) and Mo thin films are deposited at different working pressures under the excitation of the radio frequency (RF) or direct current (DC) power supplies for application in Cu(In,Ga)Se2 (CIGS) thin film solar cells. The various material properties including dynamic deposition rate, electrical and optical properties, crystal structure and morphology are systematically investigated and compared for these RF and DC sputtered Mo:Na and Mo thin films. Mo and Mo:Na related back contacts, i.e. bottom Mo:Na layer prepared at a high working pressure and top Mo or Mo:Na thin film sputtered at a low working pressure are employed in CIGS thin film solar cells. CIGS thin film solar cells with Mo:Na (bottom, DC)/Mo:Na (top, DC) have a relatively high VOC, which is related to a relatively easy migration of Na passing through the DC Mo layers and entering into the CIGS thin film solar cells. A CIGS thin film solar cell of 14.0% has been achieved in this work.

Analysis of temperature-dependent transmittance spectra of Zn0.5In0.5Se (ZIS) thin films

Temperature-dependent transmission experiments of ZnInSe thin films deposited by thermal evaporation method were performed in the spectral range of 550–950 nm and in temperature range of 10–300 K. Transmission spectra shifted towards higher wavelengths (lower energies) with increasing temperature. Transmission data were analyzed using Tauc relation and derivative spectroscopy. Analysis with Tauc relation was resulted in three different energy levels for the room temperature band gap values of material as 1.594, 1.735 and 1.830 eV. The spectrum of first wavelength derivative of transmittance exhibited two maxima positions at 1.632 and 1.814 eV and one minima around 1.741 eV. The determined energies from both methods were in good agreement with each other. The presence of three band gap energy levels were associated to valence band splitting due to crystal-field and spin–orbit splitting. Temperature dependence of the band gap energies were also analyzed using Varshni relation and gap energy value at absolute zero and the rate of change of gap energy with temperature were determined.

In situ investigation of as grown Cu(In,Ga)Se2 thin films by means of photoemission spectroscopy

A UHV-transferred Cu(In,Ga)Se2 (CIGSe) thin film prepared by a three-stage process on an Mo-coated float glass was successfully investigated by detailed UPS/XPS and directly compared with air-transferred and cyanide etched samples from the same batch. Besides the fact that the valence band maximum of the UHV-transferred sample is located 1.19 eV below the Fermi level, pointing to a widened bandgap and type inversion, almost no oxygen or sodium was detected on the surface. After air transfer, Na- and O-containing species were found on the CIGSe surface that could be only partially removed upon cyanide etching. The residual and very low Na1s signal is slightly shifted, which is attributed to a modified band bending at the grain boundaries with Ga-, In-, and Se-related oxides still present on the CIGSe surface after the etching process. Thickness analysis reveals that both the sodium containing surface layer (after air-transfer) and the oxygen containing termination layer (after cyanide etching) are restricted to a few monolayers only. Finally, the topmost CIGSe layer appears to be Cu-free due to the fact that no oxidized Cu was found after air exposure. In addition, the Cu core level does not follow the surface termination induced shift of the valence band after the KCN treatment, pinning its maximum 0.77 eV below the Fermi level. These findings are ascribed to a Cu(In,Ga)7Se11 surface as deduced from XPS core level analysis.