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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.

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.

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...

Development of transparent-conducting oxide (TCO) back contact for Cu(In,Ga)Se2 (CIGS) absorber is crucial for bifacial CIGS photovoltaics. However, inherent GaOx formation at the TCO/CIGS interface has hampered the photocarrier extraction. Here, by controlling the Na doping scheme, we show that the hole transporting properties at the indium–tin oxide (ITO)/CIGS back contact can be substantially improved, regardless of the GaOx formation. Na incorporation from the glass substrate during the GaOx forming phase created defective states at the interface, which allowed efficient hole extraction from CIGS, while post Na treatment after GaOx formation did not play such a role. Furthermore, we discovered that an almost GaOx-free interface could be made by reducing the underlying ITO film thickness, which revealed that ITO/CIGS junction is inherently Schottky. In the GaOx-free condition, post-Na treatment could eliminate the Schottky barrier and create ohmic junction due to generation of conducting paths at the i...

To enhance the conversion performance of solar cells, a quantitative and depth-resolved elemental analysis of photovoltaic thin films is required. In this study, we determined the average concentration of the major elements (Cu, In, Ga, and Se) in fabricated Cu(In,Ga)Se2 (CIGS) thin films, using inductively coupled plasma atomic emission spectroscopy, X-ray fluorescence, and wavelengthdispersive electron probe microanalysis. Depth profiling results for CIGS thin films with different cell efficiencies were obtained using secondary ion mass spectrometry and Auger electron spectroscopy to compare the atomic concentrations. Atom probe tomography, a characterization technique with sub-nanometer resolution, was used to obtain three-dimensional elemental mapping and the compositional distribution at the grain boundaries (GBs). GBs are identified by Na increment accompanied by Cu depletion and In enrichment. Segregation of Na atoms along the GB had a beneficial effect on cell performance. Comparative analyses of different CIGS absorber layers using various analytical techniques provide us with understanding of the compositional distributions and structures of high efficiency CIGS thin films in solar cells.

Achieving favorable band profile in low-temperature-grown Cu(In,Ga)Se

The effect of pre-annealing on the phase formation behavior and efficiency of CZTS thin film solar cell prepared by sulfurization of sputtered Zn/(Cu,Sn) metal precursor with H 2 S gas was investigated. Precursor with stacking structure of Zn/(Cu,Sn) was deposited by sputtering of Cu, Zn, and Sn metal targets. The depth profile of metal elements and cell efficiency of the sulfurized CZTS films with H 2 S were observed to be critically dependent on the pre-annealing conditions. For the CZTS film prepared by sulfurization in N 2 -5 vol.% H 2 S at 550 °C after pre-annealing at 350 °C in Ar, segregation of SnS phase at the surface region was observed to be pronounced. When the pre-annealing was performed at 350 °C in N 2 -5 vol.% H 2 S, however, uniform depth profile of metal elements with a small amount of CuS phase was observed. The CuS phase was disappeared with increase in the pre-annealing temperature in N 2 -5 vol.% H 2 S. The phase formation behavior influenced by pre-annealing condition was observed to affect solar cell performance of the CZTS thin film synthesized at 550 °C in N 2 -5 vol.% H 2 S. In contrast to the CZTS thin film prepared with pre-annealing at 350 °C in Ar showing bad efficiency (∼0.93%), the CZTS solar cells fabricated with pre-annealing at 450 °C in H 2 S shows higher efficiency of 3.04%. By the optimization of Zn layer thickness, solar cell efficiency of 4.40% was obtained in the CZTS thin film prepared with pre-annealing at 450 °C in N 2 -5 vol.% H 2 S. This phenomenon was due to the change in the secondary phase formation behavior during sulfurization of the Zn/(Cu,Sn) metal precursor with various pre-annealing conditions.

The influence of post-annealing on the solar cell performance of Cu 2 ZnSnS 4 (CZTS) thin film prepared by sulfurization of Zn/(Cu,Sn) metal precursor with H2S gas was investigated. Two-step annealing process at 450°C and then at 550°C was adopted for the sulfurization of the stacked precursor in the mixed N2+H2S atmosphere. The post-annealing was performed at 300°C in air after the deposition of CdS buffer layer and Ni/Al grid. In comparison to the CZTS solar cell prepared without post-annealing showing lower efficiency of 4.30%, improved solar cell efficiency of 6.02% with increased Voc (590 mV) and FF (56.2%) was observed for the CZTS solar cell prepared with post-annealing. By the analysis of Voc as a function of temperature and frequency-dependent carrier concentration, the improved cell efficiency was believed to be due to the reduced absorber/buffer interface recombination and bulk defects in CZTS.

To enhance the conversion performance of solar cells, a quantitative and depth-resolved elemental analysis of photovoltaic thin films is required. In this study, we determined the average concentration of the major elements (Cu, In, Ga, and Se) in fabricated Cu(In,Ga)Se2 (CIGS) thin films, using inductively coupled plasma atomic emission spectroscopy, X-ray fluorescence, and wavelengthdispersive electron probe microanalysis. Depth profiling results for CIGS thin films with different cell efficiencies were obtained using secondary ion mass spectrometry and Auger electron spectroscopy to compare the atomic concentrations. Atom probe tomography, a characterization technique with sub-nanometer resolution, was used to obtain three-dimensional elemental mapping and the compositional distribution at the grain boundaries (GBs). GBs are identified by Na increment accompanied by Cu depletion and In enrichment. Segregation of Na atoms along the GB had a beneficial effect on cell performance. Comparative analyses of different CIGS absorber layers using various analytical techniques provide us with understanding of the compositional distributions and structures of high efficiency CIGS thin films in solar cells.

We investigated the effect of adding oxygen (O2) gas to argon (Ar) gas and varying the sputter target power during intrinsic ZnO sputtering on the photovoltaic performance of Cu(In,Ga)Se2 (CIGS) solar cells. Sputtering powers of 50, 100, 150 W were used in conjunction with an O2 fraction of 0 % or 2 % in the sputtering gas. Both increasing the power and adding oxygen improved the photovoltaic conversion efficiency, although the effect of the latter was more pronounced. The results showed that the open-circuit voltage, short-circuit current density, and fill factor were all improved, and the series resistance was reduced. Electroluminescence spectroscopy and electron-beam-induced current analysis showed that adding oxygen and increasing the power not only improved the CdS/CIGS junction properties, but also the CIGS bulk properties. The current-voltage characteristics at low temperature indicated that adding oxygen and increasing the power facilitated local secondary diode formation around the CIGS surface. It was also revealed that the sodium doping concentration was enhanced by oxygen addition and increased power, with this enhancement being larger for the former. The improvements in the CdS/CIGS junction and CIGS bulk properties were therefore tentatively attributed to an enhancement of the Na doping level.

The behavior of deep level defects by a voltage-induced stress for CuInGaSe 2 (CIGS) solar cells has been investigated. CIGS solar cells were used with standard structures which are Al-doped ZnO/i-ZnO/CdS/CIGSe 2 /Mo on soda lime glass, and that resulted in conversion efficiencies as high as 16%. The samples with the same structure were isothermally stressed at 100 °C under the reverse voltages. The voltage-induced stressing in CIGS samples causes a decrease in the carrier density and conversion efficiency. To investigate the behavior of deep level defects in the stressed CIGS cells, photo-induced current transient spectroscopy was utilized, and normally 3 deep level defects (including 2 hole traps and 1 electron trap) were found to be located at 0.18 eV and 0.29 eV above the valence band maximum (and 0.36 eV below the conduction band). In voltage-induced cells, especially, it was found that the decrease of the hole carrier density could be responsible for the increase of the 0.29 eV defect, which is known to be observed in less efficient CIGS solar cells. And the carrier density and the defects are reversible at least to a large extent by resting at room-temperature without the bias voltage. From optical capture kinetics in photo-induced current transient spectroscopy measurement, the types of defects could be distinguished into the isolated point defect and the extended defect. In this work, it is suggested that the increase of the 0.29 eV defect by voltage-induced stress could be due to electrical activation accompanied by a loss of positive ion species and the activated defect gives rise to reduction of the carrier density.

A monolithic tandem solar cell consisting of crystalline Si (c-Si)/indium tin oxide (ITO)/CuGaSe2 (CGSe) was demonstrated by stacking a CGSe solar cell on a c-Si/ITO solar cell to obtain a photovoltaic conversion efficiency of about 10%. Electrical analyses based on cell-selective light absorption were applied to individually characterize the photovoltaic performances of the top and bottom subcells. Illumination at a frequency that could be absorbed only by a targeted top or bottom subcell permitted measurement of the open-circuit voltage of the target subcell and the shunt resistance of the non-target subcell. The cell parameters measured from each subcell were very similar to those of the corresponding single cell, confirming the validity of the suggested method. In addition, separating the light absorption intensities at the top and bottom subcells made us measure the bias-dependent photocurrent for each subcell. The series resistance of a c-Si/ITO/CGSe cell subjected to bottom-cell limiting conditions was slightly large, implying that the tunnel junction was a little resistive or slightly beyond ohmic. This analysis demonstrated that aside from producing a slightly resistive tunnel junction, our fabrication processes were successful in monolithically integrating a CGSe cell onto a c-Si/ITO cell without degrading the performances of both cells.

Kesterite Cu2ZnSnSe4 (CZTSe) thin films prepared by the selenization of mechanochemically synthe- sized Cu2ZnSnS4 (CZTS) nanocrystal films are systematically investigated as a function of the annealing time in terms of the phase purity, microstructure, composition, and device characteristics. It is shown that selenization for an extended time does not cause a noticeable amount of Sn loss or segregation of Zn-rich layers. Thus, the prolonged annealing leads to improvements (reduction) in the shunt conductance, reverse saturation current, and diode ideality factor. However, it also leads to a deterioration of the series resistance, of which influence turned out to overwhelm all of the aforementioned positive effects on the device performance. As a consequence, the CZTSe solar cell exhibits its highest efficiency (5.43%) at the shortest annealing time (10 min). Impedance spectroscopy is demonstrated to be of good use in detecting the change in the back contact of CZTSe solar cells during annealing. The impedance spectra of the CZTSe solar cells are analyzed in association with the microstructures of the back-contact electrodes, demonstrating that the increase in the series resistance is attributed to the formation of the resistive MoSe2 layer.

A buffer layer for PbZr x Ti 1− x O 3 (PZT) coating on nanocrystalline diamond (NCD) film was investigated to prevent the oxidation damage of NCD layer during ambient air annealing at high-temperature. As for the phase of buffer layer, metal nitride is more effective than pure metal for enhancing adhesion of PZT coating on NCD film. For the metal nitride-based buffer layer, the incorporation of Al and Si increases further the adhesive strength of PZT coating on NCD film. As a microstructural point of view, nanoscale multilayered structure was observed to contribute to the increase of adhesive strength between PZT and NCD. As a consequence, introducing thin (∼70 nm) composite buffer composed of Ti(Al)N/SiN x nanoscale multilayer with bilayer period of ∼5 nm as an intermediate layer between PZT coating and NCD film improved the high-temperature oxidation resistance of the NCD film and relevant adhesive strength of PZT coating even after the 900 °C air-annealing. The improved adhesion was attributed to the suppressed oxygen diffusion to the NCD film through the PZT layer at high temperature by the intervening nanoscale multilayer buffer.

Highly efficient copper-zinc-tin-selenide (Cu2ZnSnSe4 ; CZTSe) thin-film solar cells are prepared via the electrodepostion technique. A metallic alloy precursor (CZT) film with a Cu-poor, Zn-rich composition is directly deposited from a single aqueous bath under a constant current, and the precursor film is converted to CZTSe by annealing under a Se atmosphere at temperatures ranging from 400 °C to 600 °C. The crystallization of CZTSe starts at 400 °C and is completed at 500 °C, while crystal growth continues at higher temperatures. Owing to compromises between enhanced crystallinity and poor physical properties, CZTSe thin films annealed at 550 °C exhibit the best and most-stable device performances, reaching up to 8.0 % active efficiency; among the highest efficiencies for CZTSe thin-film solar cells prepared by electrodeposition. Further analysis of the electronic properties and a comparison with another state-of-the-art device prepared from a hydrazine-based solution, suggests that the conversion efficiency can be further improved by optimizing parameters such as film thickness, antireflection coating, MoSe2 formation, and p-n junction properties.

We demonstrate here that an improvement in the green density leads to a great enhancement in the photovoltaic performance of CuInSe2 (CISe) solar cells fabricated with Cu-In nanoparticle precursor films via colloidal solution deposition. Cold-isostatic pressing (CIP) increases the precursor film density by ca. 20%, which results in an appreciable improvement in the microstructural features of the sintered CISe film in terms of a lower porosity, a more uniform surface morphology, and a thinner MoSe2 layer. The low-band-gap (1.0 eV) CISe solar cells with the CIP-treated films exhibit greatly enhanced open-circuit voltage (V(OC), typically from 0.265 to 0.413 V) and fill factor (FF, typically from 0.34 to 0.55), compared to the control devices. As a consequence, an almost 3-fold increase in the average efficiency, from 3.0 to 8.2% (with the highest value of 9.02%), is realized. Diode analysis reveals that the enhanced V(OC) and FF are essentially attributed to the reduced reverse saturation current density and diode ideality factor. This is associated with suppressed recombination, likely due to the reduction in recombination sites at grain/air surfaces, intergranular interfaces, and defective CISe/CdS junctions. From the temperature dependences of V(OC), it is revealed that CIP-treated devices suffer less from interface recombination.

We fabricated metallic Cu–Zn–Sn (CZT) precursor thin films via electrochemical deposition from aqueous metal salt solution on Mo-coated soda-lime glass substrates, and the influence of the subsequent sulfurization condition on the morphology, composition and structure of the final Cu2ZnSnS4 (CZTS) thin films was investigated. A rapid thermal annealing equipment was used for a systematic control of the sulfurization process parameters. The as-deposited films are composed of binary metallic alloys, which can be converted to the highly crystalline CZTS phase after sulfurization at temperatures above 500 °C. The composition of the CZT film barely changes during the sulfurization, and a small amount of CuS-based secondary phases exists even at 550 °C. However, a quick post-annealing KCN treatment effectively and selectively removes the secondary phase, evidenced by the Raman spectroscopy and elemental.

The effects of varying the conditions for the chemical bath deposition (CBD) of cadmium sulfide (CdS) layers on CdS/Cu(In,Ga)Se2 (CIGS) hetero-junctions were investigated using photoluminescence (PL), electroluminescence (EL), deep level transient spectroscopy (DLTS), and red-light-illuminated current-voltage (I–V) measurements. We demonstrated that varying CBD-CdS conditions such as the temperature and time influenced the recombination pathways around the CdS/CIGS junction via the formation of different electronic defects, which eventually changed the photovoltaic conversion efficiency. As the CBD-CdS time and temperature were increased, the cell efficiency decreased. PL measurements revealed that this degradation of the cell efficiency was accompanied by increases in the defect-related recombination, which were attributed to the existence of donor defects around CdS/CIGS having an energy level of 0.65 eV below conduction band, as revealed by DLTS. Increasing distortions in the red-light-illuminated I–V characteristics suggested that the related defects might also have played a critical role in metastable changes around the CdS/CIGS junction. Because the CBD-CdS time and temperature were considered to influence the diffusion of impurities into the CIGS surface, the evolution of the efficiency, PL spectra, defect populations, and red-light-illuminated I–V characteristics observed in this work could be attributed to the diffusion of impurities during the CBD-CdS process.

The optical absorption efficiency (OAE) of thin crystalline Si (c-Si) solar cells was examined by using a direct absorption calculation algorithm based on the three-dimensional modeling and ray-tracing technique. The back contact was assumed to be made of ZnO based oxide layers with different optical constants and metallic electrode, and the front surface was assumed to be Lambertian. Simulation results showed that the insertion of non-absorptive oxide layer between c-Si and Al improved OAE, and that the relative amount of enhancement increased with decreasing refractive index, manifesting the reduction in absorption loss in Al electrode due to the increased total internal reflection. In the case of absorptive oxide layer, although increase in OAE was still attainable when compared with the back contact without oxide, the OAE was subdued significantly due to large absorption loss in oxide layer. The optimal oxide layer thickness was around 200 nm for non-absorptive oxide, and that of absorptive oxide decreased with increasing extinction coefficient. In the case of Ag metal contact, the enhancement of OAE due to the use of oxide layer was much less than the case of Al because of inherent high reflectivity and low absorption loss in Ag layer.

A low-cost, nonvacuum fabrication route for CuInSe2 and CuInS2 thin films is presented. To produce these films, binder-free colloidal precursors were prepared using Cu–In intermetallic nanoparticles that were synthesized via a chemical reduction method. The Cu–In alloy precursor films were transformed to CuInSe2 and CuInS2 by reactive annealing in chalcogen-containing atmospheres at atmospheric pressure. The as-synthesized nanoparticles and the annealed films were characterized by X-ray diffraction, transmission electron microscopy, scanning electron microscopy, energy dispersive X-ray spectrometry, electron probe X-ray microanalysis, Raman spectroscopy, and Auger electron spectroscopy depth profile measurements to elucidate the phase evolution pathway and the densification mechanism of the Cu–In–Se–S system. Solar cell devices made with CuInSe2 and CuInS2 absorbing layers exhibited power conversion efficiencies of 3.92% and 2.28%, respectively. A comparison of the devices suggested that the microstructur...

The electrical properties of Cu(In,Ga)Se2/Mo junctions were characterized with respect of MoSe2 orientation and Na doping level using an inverse transmission line method, in which the Cu(In,Ga)Se2 (CIGS)/Mo contact resistance could be measured separately from the CIGS film sheet resistance. The MoSe2 orientation was controlled by varying the Mo surface density, with the c-axis parallel and normal orientations favored on Mo surfaces of lower and higher density, respectively. The effect of Na doping was compared by using samples with and without a SiOx film on sodalime glass. The conversion of the MoSe2 orientation from c-axis normal to parallel produced a twofold reduction in CIGS/Mo contact resistance. Measurements of the contact resistances as a function of temperature showed that the difference in CIGS/Mo contact resistance between the samples with different MoSe2 orientations was due to different barrier heights at the back contact. Comparison between Na-doped and Na-reduced samples revealed that the contact resistance for the Na-reduced system was four times of that of the doped sample, which showed more pronounced Schottky-junction behavior at lower temperature, indicating that Na doping effectively reduced the barrier height at the back contact. Copyright © 2013 John Wiley & Sons, Ltd.

Polycrystalline ZnO thin films both undoped and doped with various types of impurities, which covered the wide carrier concentration range of 10 16 –10 21 cm − 3 , were prepared by magnetron sputtering, and their optical-band gaps were investigated. The experimentally measured optical band-gap shifts were analyzed by taking into account the carrier density dependent effective mass determined by the first-order nonparabolicity approximation. It was shown that the measured shifts in optical band-gaps in ZnO films doped with cationic dopants, which mainly perturb the conduction band, could be well represented by theoretical estimation in which the band-gap widening due to the band-filling effect and the band-gap renormalization due to the many-body effect derived for a weakly interacting electron-gas model were combined and the carrier density dependent effective mass was incorporated.

Thin light-active layers of the CuInSe2 solar cell were prepared on Mo-coated sodalime glass substrates by one-step electrodeposition and post-annealing. The structure, morphology, and composition of CuInSe2 film could be controlled by deposition parameters, such as the composition of metallic precursors, the concentration of complexing agents, and the temperature of post-annealing with elemental selenium. A dense and uniform Cu-poor CuInSe2 film was successfully obtained in a range of parametric variation of electrodeposition with a constant voltage of -0.5 V vs. a Ag/AgCl reference electrode. The post-annealing of the film at high temperature above 500℃ induced crystallization of CuInSe2 with well-developed grains. The KCN-treatment of the annealed CuInSe2 films further induced Cu-poor CuInSe2 films without secondary phases, such as Cu2Se. The structure, morphology, and composition of CuInSe2 films were compared with respect to the conditions of electrodeposition and post-annealing using SEM, XRD, Raman, AES and EDS analysis. And the conditions for preparing device-quality CuInSe2 films by electrodeposition were proposed.

Laser scribing technology has been actively developed for thin-film solar cell fabrication taking a number of advantages over mechanical scribing. Its non-contact processing nature enables reliable and precise scribing processes. In particular, it is almost unavoidable to use laser scribing method for fabricating high-quality flexible thin-film solar cells. Despite the fundamental merits that laser scribing can offer, still a number of challenges should be addressed in order to replace the mechanical counterpart for wider range of thin-film solar cells. In this study, we explore optimal laser scribing conditions for copper–indium–gallium–selenide (CIGS) thin-film solar cells, especially based on flexible polyimide (PI) substrate in close comparison with that based on soda-lime glass substrate. Picosecond-pulsed laser of repetition rate up to 100 kHz and wavelength of 532 nm (~12 ps temporal pulse width) was mainly tested, and scribing speed in the range of 0.01–1 m/s was examined with a few different laser focal spot diameters (27, 34, and 62 μm). Main focus of this study is in understanding distinct laser scribing mechanisms for flexible substrate configurations, thereby finding out intrinsic optimal processing parameters. One of the most critical requirements is to prevent possible damage or deformation of underlying thin-film layer(s) or PI substrate. Effect of microstructures of thin films (in particular, Mo and CIGS) on the scribing behavior was also examined. In order to further improve the performance of the scribing process and reduce the laser power budget as well, mild gas injection scheme was tested.

The single-bath electrochemical deposition of CuInSe2 often leads to short-circuit behavior of the resulting solar cells due to the high shunt conductance. In this study, in an attempt to resolve this problem, the influence of the Se precursor concentration (CSe) on electrodeposited CuInSe2 films and solar cell devices is examined in the CSe range of 4.8 to 12.0 mM in selenite-based aqueous solutions containing Cu and In chlorides along with sulfamic acid (H3NSO3) and potassium hydrogen phthalate (C8H5KO4) additives. As CSe increases, the CuInSe2 layers become porous, and the grain growth of the CuInSe2 phase is restricted, while the parasitic shunting problem was markedly alleviated, as unambiguously demonstrated by measurements of the local current distribution. Due to these ambivalent influences, an optimal value of CSe that achieves the best quality of the films for high-efficiency solar cells is identified. Thus, the device prepared with 5.2 mM Se exhibits a power-conversion efficiency exceeding 10% with greatly improved device parameters, such as the shunt conductance and the reverse saturation current. The rationale of the present approach along with the physicochemical origin of its conspicuous impact on the resulting devices is discussed in conjunction with the electro-crystallization mechanism of the CuInSe2 compound.

The surface microstructures of molybdenum (Mo) back contacts were shown to play a crucial role in the preferred orientations of Cu(In,Ga)Se2 (CIGS) films. The lower surface density of Mo tends to drive the growth of CIGS films toward favoring a (220)/(204) orientation, attributed to the higher likelihood of a MoSe2 reaction. This work showed that the presence of a very thin layer on a Mo bilayer facilitated the tuning of the CIGS grain orientations from strongly favoring (112) to strongly favoring (220)/(204) without sacrificing the electrode conductivity. The efficiency of Na-doped CIGS cells was increased toward decreasing Mo surface density, that is, increasing (220)/(204) CIGS orientation. Although slight changes in Na doping found between different Mo surface properties could contribute in part, the comparison with Na-reduced CIGS cells showed that it was more likely due to the (220)/(204) orientation-related enhancement of CdS/CIGS junction characteristics, which were possibly attributed to a favorable CdS reaction and a reduction in the defect metastabilities. Copyright © 2012 John Wiley & Sons, Ltd.

Zn-Sn-O (ZTO) films are known to be able to form an amorphous phase, which provides a smooth surface morphology as well as etched side wall, when deposited by using the conventional sputtering technique and, therefore, to have a potential to be applied as transparent thin film transistors. In this study, ZTO thin films were prepared by using combined sputtering of ZnO and SnO2 targets, and the dependences of their electrical and optical properties on the composition and the deposition parameters were examined. The Sn content in the films was varied in the range of 35 ∼ 85 at.%. The deposition was carried out at room temperature, 150 and 300 °C, and the oxygen content in sputtering gas was varied from 0 to 1 vol.%. Sn-rich films had better electrical properties, but showed large oxygen deficiency when deposited at low oxygen partial pressures. ZTO films with Sn contents lower than 55 at.% had good optical transmission, but the electrical properties were poor due to very low carrier concentrations. A high Hall mobility of larger than 10 cm2/Vs could be obtained in the carrier density range 1017∼1020 cm−3, and the etching rate was measurable for films with Sn content up to 70 at.% when using a dilute HCl solution, indicating a good possibility of utilizing ZTO films for device applications.

In this study, a modified write-and-verify (WAV) scheme is proposed for improving the programming/erasing (P/E) endurance of multi-level cell (MLC) phase-change memory (PCM) using Ge-doped SbTe (GeST). A dual reference data read method is developed to detect the level margin decay during P/E cycling, and a trigger condition is designed to trigger self-repair for the degraded cells before any P/E error for the modified WAV scheme. Experimental results suggest that the modified WAV scheme effectively extends the P/E endurance of PCM using GeST during 4-level P/E by at least 10 times. The modified WAV scheme is expected to improve the endurance of MLC–PCM of system applications.

Hydrogen effects on deep level defects and a defect generation in proton implanted Cu(In,Ga)Se 2 (CIGS) based thin films for solar cell were investigated. CIGS films with a thickness of 3 μm were grown on a soda-lime glass substrate by a co-evaporation method, and then were implanted with protons. To study deep level defects in the proton implanted CIGS films, deep level transient spectroscopy measurements on the CIGS-based solar cells were carried out, these measurements found 6 traps (including 3 hole traps and 3 electron traps). In the proton implanted CIGS films, the deep level defects, which are attributed to the recombination centers of the CIGS solar cell, were significantly reduced in intensity, while a deep level defect was generated around 0.28 eV above the valence band maximum. Therefore, we suggest that most deep level defects in CIGS films can be controlled by hydrogen effects.

ZnO films co-doped with fluorine and hydrogen were prepared on Corning glass by radio frequency magnetron sputtering of ZnO targets with varying amounts of ZnF2 in H2/Ar gas mixtures of varying H2 content. The ZnO films' electrical, optical, and structural properties in combination with their compositional properties were investigated. A small addition of H2 to the sputtering gas caused a drastic increase of Hall mobility with a marginal increase in carrier concentration, indicating an effective passivation of grain boundaries due to hydrogenation. For further increase of H2 in sputter gas, the Hall mobility remained at a relatively constant level while the carrier concentration increased steadily. Most of the ZnO films co-doped with fluorine and hydrogen showed average transmittance higher than 83% in the 400-800 nm range, while the average absorption coefficients were lower than 600 cm(-1), implying very low absorption loss in these films. It was discovered that the fabrication of ZnO films with a Hall mobility higher than 40 cm2/Vs and a very low absorption loss in the visible range is possible by co-doping hydrogen and fluorine.

We investigated the effects of the microstructures of molybdenum (Mo) back contacts on sodium (Na) diffusion from sodalime glass into a Cu(In,Ga)Se2 (CIGS) absorber as a function of the sputter deposition pressure during preparation of the Mo contact layer. The surface characteristics of the Mo layers more significantly affected the diffusion of Na ions into the CIGS compared with the Mo bulk. The Na ion diffusion depended strongly on the amount of oxygen adsorbed onto the Mo layer surfaces. Secondary ion mass spectroscopy results showed that Na accumulated in a layer (Na–O compound) on the Mo surface (the CIGS/Mo interface), and this layer served as a primary source of Na ions diffusing into the CIGS. A trilayered Mo back contact structure was prepared in an effort to decouple the functions of electrical conductance and Na diffusion. The ability of this surface to control the Na concentration in a CIGS absorber is discussed. Copyright © 2012 John Wiley & Sons, Ltd.

The microstructures of molybdenum (Mo) thin films deposited at pressures from 3.3 to 10.3 mTorr were characterized, and the relationships between these microstructures and the properties of the films (residual stress and electrical resistivity) were investigated. In the low deposition pressure regime (region I, below 7 m Torr), the residual stress in the tensile direction increases with increasing pressure and the electrical resistivity increases gradually, but at high deposition pressures (region II, above 7 m Torr) the residual stress is reduced and the resistivity increases more steeply. These variations of the properties of the Mo films in the low pressure regime are due to the variation in grain size; the carrier mobility decreases due to increased grain boundary (GB) scattering and the tensile stress increases due to increased atomic attraction across the GBs. In contrast, the porosity of the Mo films increases significantly in the high pressure regime, as demonstrated with variable angle spectroscopic ellipsometry (VASE). Most of these pores are believed to be present along the grain boundaries of the Mo films, so their presence reduces the GB attraction and thus the tensile stress and enhances the carrier scattering. The high porosity of the Mo back contact was shown with secondary ion mass spectroscopy profiling to accelerate the Na diffusion from soda-lime glass into the Cu(In,Ga)Se 2 (CIGS) film.

ZnO films doped with Ga (GZO) of varying composition were prepared on Corning glass substrate by radio frequency magnetron sputtering at various deposition temperatures of room temperature, 150, 250 and 400 °C, and their temperature dependent photoelectric and structural properties were correlated with Ga composition. With increasing deposition temperature, the Ga content, at which the lowest electrical resistivity and the best crystallinity were observed, decreased. Films with optimal electrical resistivity of 2–3 × 10−4 Ω cm and with good crystallinity were obtained in the substrate temperature range from 150 to 250 °C, and the corresponding CGa/(CGa + CZn) atomic ratio was about 0.049. GZO films grown at room temperature had coarse columnar structure and low optical transmittance, while films deposited at 400 °C yielded the highest figure of merit (FOM) due to very low optical absorption despite rather moderate electrical resistivity slightly higher than 4 × 10−4 Ω cm. The optimum Ga content at which the maximum figure of merit was obtained decreased with increasing deposition temperature.

Fluorine and hydrogen co-doped ZnO:Al (AZO) films were prepared by radio frequency (rf) magnetron sputtering of ZnO targets containing 1 wt.% Al2O3 on Corning glass at substrate temperature of 150 °C with Ar/CF4/H2 gas mixtures, and the structural, electrical and optical properties of the as-deposited and the vacuum-annealed films were investigated. In as-deposited state, films with fairly low resistivity of 3.9–4 × 10−4 Ω cm and very low absorption coefficient below 900 cm−1 when averaged in 400–800 nm could be fabricated. After vacuum-heating at 300 °C, the minimum resistivity of 2.9 × 10−4 Ω cm combined with low absorption loss in visible region, which enabled the figure of merit to uplift as high as 4 Ω−1, could be obtained for vacuum-annealed film. It was shown that, unlike hydrogenated ZnO films which resulted in degradation upon heating in vacuum at moderately high temperature, films with fluorine addition could yield improved electrical properties mostly due to enhanced Hall mobility while preserving carrier concentration level. Furthermore, stability in oxidizing environment could be improved by fluorine addition, which was ascribed to the filling effect of dangling bonds at the grain boundaries. These results showed that co-doping of hydrogen and fluorine into AZO films with low Al concentration could be remarkably compatible with thin film solar cell applications.