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The study aims to investigate the issues embedded in the secondary-level teacher recruitment system from the structural perspective, which affect the motivation of the teaching profession, appointment and retention, and teacher development. The study applied the qualitative method and identified the issues that cause skill teacher shortage through causal relation analysis. The study used interviews with policymakers and administrators to identify the causes of the shortage of high-quality teachers in secondary institutions and discerned the solutions to ensure quality secondary education in Bangladesh. Adverse appointment and retention systems; and weak and stagnant teacher development facilities, were identified as the demotivating factors for taking the teaching profession. The solutions were found to enhance the attraction for teaching: firstly, by increasing salary, upholding teachers’ position in society and educational administration, and improving the working environment. The appointment and retention systems should be friendly by selecting and facilitating quality teachers. Lastly, a comprehensive and interconnected teacher development system should be initiated to ensure high-quality teachers at the secondary level.

As the multimedia classroom gained enormous importance from the government of Bangladesh to modernize the educational system, this study initiated the objective to investigate the state of those classrooms at secondary schools. The study used a mixed-method approach with qualitative and quantitative data to explore the execution and possible improvements for those multimedia classes. Five secondary schools were selected purposively from each of the two districts named Sylhet and Cumilla. From these ten schools, hundred students and hundred assistant teachers were selected randomly to conduct ten focus group discussions (FGD) with each of the two groups. Moreover, ten interviews were conducted with the head teachers. Besides those, classrooms were observed while the multimedia was in use, along with a case study in a school. Although head teachers and assistant teachers mentioned many classes using multimedia, the students' words and observed class information gave alternative facts. Almost similar types of outcomes were found regarding digital content development, monitoring, and mentoring activities. While teachers were optimistic about the quality of digital content and classroom teaching, researchers found those to be less standard. In terms of monitoring the multimedia classroom, the majority of head teachers were more concerned about the number of multimedia used in the classroom due to administrative obligations than the quality.

This work proposes a simple simulation method for optimization of n-i-p perovskite solar cell (PSCs) via SCAPS-1D and aims to achieve high-performance devices. Nowadays, the carrier recombination induced by heavy defects in bulk and interfaces is one of the main obstacles which restricts the PSCs efficiency and meanwhile is harmful to device stability. Here we modify the MAPbI3 device through a series of structural and basic optimization, including thickness of each layer, carrier diffusion length, interface recombination, doping concentration and overall series resistance. Through the modified simulation, high-performance MAPbI3 device with suppressed recombination and optimized structure is realized, resulting in an encouraging power conversion efficiency (PCE) of 20.09%, enhanced Voc of 1.087 V, Jsc of 22.56 mA/cm2 and FF of 78.5%. These findings unveil the critical effect of defect suppression on PSCs and offer a simple method to achieve high-performance device.

Technical issues which remain in CuInSe2-based solar cells are reviewed. A study of single-crystalline Cu(In,Ga)Se2film devices, carrier recombination analysis, and effects of alkali-metal doping and silver-alloying are particularly focused on.

Incorporation of Ag into a crystalline lattice of kesterite Cu2ZnSnS4 thin film was found to be effective for improvements of its photoelectrochemical properties for water reduction.

Iron pyrite [cubic FeS2 (cFeS2)] is considered as an earth-abundant and low-cost thin-film photovoltaic material. However, the conversion efficiency of cFeS2-based solar cells remains below 3%. To elucidate this limitation, we evaluate the optical and electrical characteristics of cFeS2 single crystals that are grown using the flux method, thus providing us an understanding of the electron transport behavior of cFeS2 single crystals. The oxide layer on the surface of cFeS2, which can possibly have an influence on the electrical characteristics of cFeS2, is removed prior to characterization via optical spectroscopy and electrical transport measurement. The optical property of cFeS2 was found to have both indirect and direct transitions. We also observed the presence of a band tail below the conduction band. The obtained electrical transport behavior indicates that cFeS2 bulk exhibits a high defect density and a disordered phase, thus leading to the hopping conduction mechanism. Our results will pave the way for the development of photovoltaic applications with iron pyrite.

An evaporated CdSe layer was incorporated with a very thin CdS layer (40 nm) to optimize the performance of CdTe solar cells. Se alloys with narrower bandgaps were formed in the CdTe deposition and annealing process. With the incorporation of the CdSe layer at the optimal thickness, the highest short-circuit current density was obtained because of the simultaneous high carrier collection in the short-wavelength region and extended carrier collection in the long-wavelength region. Although narrow bandgap Se alloys were introduced, the open-circuit voltage was also significantly increased. As suggested by dark current–voltage measurements, the current leakage paths induced by the thin CdS layer can be significantly suppressed by the incorporation of the CdSe layer. The redshift of PL emission confirms the formation of the Se alloys. As revealed by transient photo-capacitance measurement, the Se alloys are less defective compared with that of CdTe, which is also in accordance with longer carrier lifetimes extracted from photoluminescence decays. The formation of Se alloys prior to the CdTe layer is favorable for the collection of photogenerated carriers. We attribute the performance enhancement to the formation of Se alloys, which are helpful to suppress the leakage current and enhance carrier collection. The incorporation of CdSe with thin CdS layers is validated to improve the diode quality and promising for further device performance improvement.

Monoclinic BiVO4 is a promising material for realizing low-cost visible-light water splitting. Here, we report the incorporation mechanism of Zr into solution-processed BiVO4. Characterization of t...

Photocarrier recombination dynamics of BiVO4 powders synthesized at different temperatures were studied by temperature-dependent steady-state and time-resolved photoluminescence (PL). Structural an...

A selective fabrication method for monoclinic-scheelite (m-s) BiVO4 and tetragonal-zircon (t-z) BiVO4 thin films using radio fRequency (RF) sputtering from a single target was developed. The kinetic energy of the sputtered atoms was controlled by varying the sputtering power to obtain BiVO4 films with m-s and t-z crystalline phases. Although the band gap of the t-z BiVO4 phase (3.0 eV) was larger than that of m-s BiVO4 (2.5 eV), the deposited t-z BiVO4 films showed a comparable photocurrent density (1.5 mA cm−2) at 1.23 V versus the reversible hydrogen electrode (400 W Xe lamp). This was mainly because of the reduced sputtering damage in the t-z BiVO4 crystal, which originated from the low sputtering power as well as the deep valence-band position in t-z BiVO4 that enabled the efficient utilization of the photocarriers. This work provides insights into crystalline phase control using the particle kinetic energy in sputtering.

Chalcopyrite CuInSe

Silicon (Si) films were deposited on low-cost graphite substrates by the electrochemical reduction of silicon dioxide nanoparticles (nano-SiO2) in calcium chloride (CaCl2), melted at 855 °C. Cyclic voltammetry (CV) was used to analyze the electrochemical reduction mechanism of SiO2 to form Si deposits on the graphite substrate. X-ray diffraction (XRD) along with Raman and photoluminescence (PL) results show that the crystallinity of the electrodeposited Si-films was improved with an increase of the applied reduction potential during the electrochemical process. Scanning electron microscopy (SEM) reveals that the size, shape, and morphology of the Si-layers can be controlled from Si nanowires to the microcrystalline Si particles by controlling the reduction potentials. In addition, the morphology of the obtained Si-layers seems to be correlated with both the substrate materials and particle size of the feed materials. Thus, the difference in the electron transfer rate at substrate/nano-SiO2 interface due to different applied reduction potentials along with the dissolution rate of SiO2 particles during the electrochemical reduction process were found to be crucial in determining the microstructural properties of the Si-films.

Heat light soaking (HLS) has been known to impact the photovoltaic parameters of Cu(In,Ga)Se2 (CIGS) solar cells for a long time. Recently, the focus shifted to the effect of the procedure on alkali fluoride-treated CIGS. Here, we investigate the impact of long-term HLS on the open-circuit (V OC) loss in high-efficiency CIGS with potassium fluoride (KF) and sodium fluoride (NaF) post-deposition treatment (PDT). HLS is shown to increase the net doping density, however, the subsequent improvement of the V OC is lower than expected. Using an analysis based on the SQ theory, we show that HLS reduces the nonradiative recombination rate in the bulk but increases the one at the interface. We present a model to explain the increase of interface recombination. We further demonstrate that a combination of HLS and KF/NaF-PDT is necessary to enhance the positive impacts of HLS and mitigate the detrimental ones leading to high-efficiency CIGS devices (22%).

This study presents a novel chemical process for the extraction of high purity silica from diatomite. The process involves leaching of raw diatomite with acid prior to alkali extraction, solubilization of treated diatomite with alkali solution to form sodium silicate solution, and precipitation of silica gel by adding sulfuric acid. Besides, the process includes washing of silica gel with hot solution of 6 M hydrochloric acid for three hours. The morphology and the chemical composition of the raw diatomite, the derivative products, and the final purified silica were determined by scanning electron microscopy, X-ray fluorescence, X-ray diffraction, and inductively coupled-plasma optical emission spectroscopy. The results indicate that raw diatomite contains over 80 wt% of amorphous micro-porous silica (SiO2) with particle size, 10–35 μm. We were able to enrich the content of silica in diatomite to 90.2 wt% by step-wise acid leaching. Moreover, in this study, boron (B) was removed from silica slurry using mannitol as a complexing compound. The elimination ratio of boron based on this method was about 70%. Incorporation of mannitol in the final stage of the acid washing of silica gel led to a silica product with lower B content. The final purified silica is 99.99 wt% (four nines) pure.

We have reported electrodeposition of silicon films on metal substrates obtained through electrochemical reduction of silicon dioxide nanoparticles (SiO2-NP) at high temperature of 855 °C in the calcium chloride (CaCl2) melt. Electrodeposition was conducted using a three-electrode based electrochemical cell. Reduction of the SiO2 was found possible with applying a negative potential of 0.9 V or more negative on the metal substrate with respect to the graphite reference electrode. Mechanism of the reduction and electrodeposition process was discussed using cyclic voltammetry (CV), and chronoamperograms (CA) techniques in relation to the applied reduction potentials during experiments. Raman spectroscopy, and X-ray diffraction method confirmed formation of the Si-film on the silver (Ag) substrate using the electrodeposition technique. Effect of the various reduction potentials on the properties of the formed Si layer was studied using Raman spectroscopy, photoluminescence (PL), and Scanning electron microscopy (SEM). Crystallinity of the electrodeposited Si-films were found to be correlated with the reduction potential. Nanocrystalline Si (nc-Si) film was obtained through electrodeposition with lower reduction potentials, while higher reduction potential was found to be effective to get Si-films with uniform crystalline quality, and better morphology. Effect of the substrate materials on the electrochemical reduction of SiO2 was also investigated.

For copper indium gallium selenide [Cu(In1−x,Gax)Se2, CIGS]-based solar cells, defect states or impurity phase always form due to both the multinary compositions of CIGS film and the difficulty of controlling the growth process, especially for high Ga concentration. To further improve device performance, it is important to understand such formation of impurity phase or defect states during fabrication. In the work presented herein, the formation mechanism of impurity phase Cu2−δSe and its depth profile in CIGS film with high Ga content, in particular CuGaSe2 (i.e., CGS), were investigated by applying different growth conditions (i.e., normal three-stage process and two-cycle three-stage process). The results suggest that impurity phase Cu2−δSe is distributed nonuniformly in the film because of lack of Ga diffusion. The formed Cu2−δSe can be removed by etching the as-deposited CGS film with bromine-methanol solution, resulting in improved device performance.

Heavy alkali-metal treatment of Cu(In,Ga)Se2 (CIGSe) absorbers has been emerging as a key process for achieving over 23% high conversion efficiencies in CIGSe solar cells. Here, we investigate the effect of rubidium fluoride post-deposition treatment (RbF-PDT) on the electronic and carrier recombination properties of narrow bandgap (narrowgap) gap and wide bandgap (widegap) CIGSe solar cells using thermal admittance spectroscopy (TAS), transient photocapacitance spectroscopy (TPC), as well as time-resolved photoluminescence (TRPL). We find that the activation energy of the main capacitance step in TAS spectra of narrowgap and widegap CIGSe solar cells reduces after RbF-PDT. On the other hand, capacitance–voltage (C–V) and temperature-dependent current–voltage (IVT) measurements demonstrate that the built-in potential, as well as the activation energy Ea, increases upon RbF-PDT both for narrowgap and widegap samples, pointing to reduced interface recombination. TPC revealed an appreciable reduction of the optical response of bulk defects in the narrowgap and widegap CIGSe, suggesting improvement of bulk properties after RbF treatment. TRPL confirmed that RbF-PDT significantly reduces carrier recombination in the bulk of narrowgap and widegap CIGSe absorbers and at the surface, leading to extended carrier lifetimes. Analysis of open-circuit voltage ( V O C ) losses due to nonradiative recombination in the bulk of the CIGSe showed a strong correlation between enhanced carrier lifetime and improved V O C for narrow gap CIGSe cells. In contrast, although we observed a substantial decrease of V O C losses in widegap CIGSe bulk, the analysis indicated that the key to photovoltaic performance enhancement is improved interface quality.

BiVO4 films were fabricated by radio frequency (RF) sputtering from a single target. The deposited BiVO4 films were found to be rich in Bi, and the reason for the Bi-richness was investigated. It was inferred from the Monte Carlo simulation that, during sputtering, the transfer process of target atoms through argon gas played a major role in this phenomenon. The transfer process resulted in an imbalanced ratio of Bi and V, arising from the difference in atom mass and interaction radius. The high RF power was found to be effective in adjusting the Bi/V ratio, influencing the sputtering yield. This type of preferential sputtering was maintained by the diffusion of target atoms from the bulk to the surface. BiVO4 films with monoclinic scheelite crystal structures were obtained at high RF power values and found to exhibit photocatalytic performances beneficial for photoanodic applications.

Effects of zirconium (Zr) doping into BiVO4 powder on its structural properties and photocatalytic activity for O2 evolution were examined. The formation of BiVO4 powder crystallized in a monoclinic scheelite structure (ms-BiVO4) was achieved when the sample was doped with a relatively small amount of Zr. The photocatalytic activity of Zr-doped ms-BiVO4 powder was much higher than that of non-doped ms-BiVO4. However, further doping caused a reduction of photocatalytic activity for O2 evolution due to the occurrence of structural alterations into tetragonal scheelite and tetragonal zircon structures. Similar effects of Zr doping were also observed for the photoelectrochemical (PEC) system based on BiVO4 thin films doped with various amounts of Zr. Thus, Zr doping was confirmed to be effective for improvements of photocatalytic and PEC functions of BiVO4 for water oxidation.

Defect study of the CuGaSe2 thin-films deposited with three-stage evaporation process under different Se-flux (PSe) conditions were performed using admittance spectroscopy (AS) technique. A dominant defect, A2 has been identified around 230~350 meV above the valance band (Ev) of the CuGaSe2. In general, density of defects was found to decrease with an increase in the PSe condition during deposition of the CuGaSe2 layer. Effect of the Se-flux on the performance of the fabricated solar-cells were discussed in relation to the defect-properties of the corresponding CuGaSe2 absorber-layer identified by admittance spectroscopy.

Silicon (Si) layer was formed on the silver (Ag) substrates using electrodeposition technique through electrochemical-reduction of the purified (99%) diatomaceous earth (SiO2) at high temperature of 855° C using calcium chloride (CaCl2) molten-salt. Electrodeposition was conducted using a three-electrode electrochemical -cell. Reduction of the SiO2was possible with applying negative potential of more than 0.7 volt on the Ag-electrode with respect to graphite reference-electrode. Raman spectroscopy and X-ray diffraction method confirm formation of the Si-layer on the Ag-substrate from SiO2using electrodeposition technique. Scanning electron microscopy indicates that deposited Si has morphology of rough surfaces.

GaInNAsSb based solar cell structure has been grown using atomic hydrogen assisted molecular beam epitaxy (H-MBE). Transient photocapacitance (TPC) spectroscopy was applied to investigate the optically active defects in the Si-doped GaInNAsSb layer ( n -GaInNAsSb) of the structure. In addition to the interband transitions, a sub-band-gap optical transition was also determined using TPC spectroscopy. This sub-band-gap transition corresponds to an electron trap ( OE 1) at 0.78 eV below the conduction band ( E C ) edge of n -GaInNAsSb material. Thermally active defects were probed by the electrical measurement using admittance spectroscopy. Thus, a defect profile in the entire band gap of the n -GaInNAsSb film was obtained.

In order to precisely control the composition of quinary GaInNAsSb alloy, we investigated the incorporation behavior of constituent atoms during atomic hydrogen-assisted molecular beam epitaxial growth. The nitrogen (N) composition, in comparison of GaNAs and GaNAsSb, increased by the supply of antimony (Sb). However, addition of indium (In) decreases the N composition during Sb mediated growth of GaInNAsSb, which enables obtaining the same N composition when an adequate In composition is chosen. It was revealed that Sb incorporation was increased when (i) In composition decreased, (ii) Sb flux increased, (iii) growth temperature decreased, and (iv) growth rate increased. These results are thought to be related to the effect of competitive role among strain, coverage, desorption, and segregation. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Structural and electrical properties of polycrystalline CuGaSe2 thin films have been studied by changing the Ga/Cu ratio in the films. CuGaSe2 thin films with various Ga/Cu ratio were grown over Mo-coated soda-lime glass substrates. With the increase of Ga content in CuGaSe2, morphology of the films was found to deteriorate which is associated with the smaller grain size and the appearance of impurity phases presumably due to the phase transition from the chalcopyrite structure to the defect-related phase on the surface of the films. Properties of the Ga poor films were affected by the Cu rich secondary phases. Electrical properties of the films were strongly influenced by the structural properties and degraded with increasing the Ga/Cu ratio in the film. Device performances, fabricated with the corresponding CuGaSe2 films, were found to be correlated with the Ga/Cu ratio in the films and consistent with the observed structural and electrical properties.

Structural, electrical and optical properties of Al doped ZnO (Al:ZnO) thin film of various thicknesses, grown by radio-frequency magnetron sputtering system were studied in relation to the application as a window layer in Cu(In1−xGax)Se2 (CIGS) thin film solar cell. It was found that the electrical and structural properties of Al:ZnO film improved with increasing its thickness, however, the optical properties degraded. The short circuit current density, Jsc of the fabricated CIGS based solar cells was significantly influenced by the variation of the Al:ZnO window layer thickness. Best efficiency was obtained when CIGS solar cell was fabricated with electrically and optically optimized Al:ZnO window layer.

The influence of Se beam pressure on defect properties in Cu(In 1– x ,Ga x )Se 2 (CIGS)-based solar cells fabricated by the three-stage process was investigated by admittance spectroscopy. The Se beam pressure was varied from 1.3×10 −3 to 4.4×10 −3 Pa, at the position of the samples, by varying the Se cell temperature. The spectra for all samples show three distinct peaks denoted as α , β , and ζ with activation energies of 20, 150, and 300 meV, respectively. The trap density of ζ increased from 5×10 14 to 4×10 15 cm −3 with decrease in Se beam pressure; the trap densities of α and β did not change. These results suggest that the origin of ζ is related to the Se deficiency. The density of ζ seems to correlate with the cell efficiency. Therefore, it is important to control the Se beam pressure in order to obtain highly efficient solar cells.

Quantitative phase analysis of Cu(In 1− x Ga x )Se 2 (CIGS) thin film grown over Mo coated soda lime glass substrates was studied by Rietveld refinement process using room temperature X-ray data at θ –2 θ mode. Films were found to contain both stoichiometric Cu(In 1− x Ga x )Se 2 and defect related Cu(In 1− x Ga x ) 3 Se 5 phases. Best fitting was obtained using crystal structure with space group I-42d for Cu(In 1− x Ga x )Se 2 and I-42m for Cu(In 1− x Ga x ) 3 Se 5 phase. The effects of Ga/III (=Ga/In+Ga= x ) ratio and Se flux during growth over the formation of Cu(In 1− x Ga x ) 3 Se 5 defect phase in CIGS was studied and the correlation between quantity of Cu(In 1− x Ga x ) 3 Se 5 phase and solar cell performance is discussed.

To clarify the mechanism of efficiency enhancement effect by potassium (K) treatment for Cu(In1− x Ga x )(Se1− y S y )2 (CIGSSe) solar cells, we investigated the physical properties of deep and shallow defect levels of CIGSSe solar cells without or with potassium fluoride treatment via using steady state photo-capacitance and admittance spectroscopy. The results show that the defect density of deep levels (such as 0.8 and 0.7 eV above valance band) are similar, suggesting that the K treatment does not drastically change the deep defect properties. For the shallow levels, the activation energy of interface-related shallow defect decreased from 183 to 120 meV upon K treatment, suggesting the modification of interface. The modified interface may be caused by the diffusion of potassium to grain boundaries and the formation of Cu-depleted layer in the near surface region of CIGSSe, resulting in the prolonged minority carrier lifetime. A new level with activation energy of 250 meV was found after K treatment, and it contributed to the increase of carrier density. These features well explained the enhanced device performance of CIGSSe solar cells by K treatment in terms of defects.

To understand the effect of bandgap grading on carrier recombination for Cu(In,Ga)Se2 (CIGS)-based solar cells in detail, samples with different bandgaps at the CIGS surface were fabricated by changing the Ga/(Ga + In) (GGI) ratio from 0.4 to 0 at the third stage of the conventional three-stage growth process. Optoelectronic characterizations, such as photoluminescence, temperature-dependent open-circuit voltage measurement and light-intensity-dependent current–voltage measurement, indicate that the photo-generated carriers move rapidly towards the location of the bandgap minimum, and the carrier recombination occurs mainly at this location. From simulation using a one-dimensional solar cell capacitance simulator (SCAPS-1D), a single-grade sample with the smallest bandgap on the surface of CIGS showed high recombination current at the surface, while the location of the maximum recombination current moved from the surface to the bulk for double-grade samples. This study suggests that controlling the bandgap grading is one way of suppressing recombination at the interface in CIGS-based solar cells.

Deep levels in Cu(In 1 − x ,Ga x )Se 2 (CIGS) are studied by transient photocapacitance (TPC) spectroscopy by varying the Ga concentration, x , from 0.38 to 0.7. The TPC spectra of CIGS thin-film solar cells at 140 K exhibited a defect level with an optical transition energy of about 0.8 eV. The spectrum shape in the sub-bandgap region is independent of the Ga concentration. Therefore, the optical transition energy to the defect level is almost constant with about 0.8 eV from the valence band. The TPC signals for defect level are quenched by increasing temperature. The activation energy of thermal quenching is estimated to be about 0.3 eV. The thermal and optical activation processes are explained using configuration coordinate diagram.

The effects of annealing under various atmospheres on the electrical properties of Cu(In,Ga)Se2 (CIGS) films and CdS/CIGS heterostructures were investigated. For CIGS films without CdS, the electrical properties of CIGS degraded under vacuum and O2 annealing, although such degradations were not observed under N2 annealing. For the CdS/CIGS heterostructures, the electrical properties of the junctions improved after annealing under all gas ambients. Therefore, CdS films prevent the chemical reactions at the CIGS surfaces and are necessary for effectively annealing the CIGS film. We observed a distinct correlation between the degradation of the electrical properties and increase in the defect density. Finally, we discussed the origin of the defect states.

Temperature dependent time resolved photoluminescence (TR-PL) measurements have been performed in similar structured kesterite Cu2ZnSn(S,Se)4 and chalcopyrite Cu(In,Ga)Se2 thin film absorbers. It is investigated that at 26 K the measured lifetime values for a set of CZTSSe samples are more than one order magnitude higher than at room temperature whereas in CIGS this difference is not significant. At room temperature the significantly lower photoluminescence emission and minority carrier lifetime for CZTSSe comparing to CIGS suggest that the dominant non-radiative recombination processes are associated with CZTSSe than CIGS and it can limit the efficiency of CZTSSe based solar cells.

CuGaSe2 thin films with different Cu/Ga ratio were studied using photoluminescence (PL) method. Measurement at 1.4 K shows asymmetrically broadened PL spectra for all the samples where PL-peaks shift to lower energy with a decrease of the Cu/Ga ratio. Excitation power and temperature dependent PL spectra were explained using donor to acceptor pair transition under the influence of potential fluctuation. Magnitude of the potential fluctuation was calculated from lower energy tail of the emission spectra. Effect of Cu-deficiency on the properties of the CuGaSe2 films and fabricated solar cells were discussed in relation to the potential fluctuation at the band-edge.

Defect properties of Cu 2 ZnSn(S x ,Se 1−x ) 4 (CZTSSe) were investigated by admittance spectroscopy (AS). Two defect states (labeled E A1 and E A2 ) were observed in CZTSSe (x=0.15) with different Cu/Sn ratio. When the Cu/Sn ratio increased from 1.75 to 1.95, the activation energy of E A1 and E A2 decreased and the defect densities increased. The capture cross sections of E A1 and E A2 defects are in the order from 10−16 cm2 to 10−18 cm2, indicating that these two defects possibly do not impact on device performance.

Room-temperature two-wavelength excited photoluminescence (PL) measurements have been performed in the kesterite Cu 2 ZnSnS 4 (CZTS) and Cu2ZnSn(S,Se)4 (CZTSSe) thin film absorbers. A defect level at 0.8 eV from the valence band and its properties are investigated. Two light sources of 635 nm and 1550 nm diode lasers, respectively, were used for above bandgap and 0.8 eV defect level excitation. The two-wavelength excited PL intensity was stronger than that only above-gap laser irradiation for the CZTS specimen. This phenomenon strongly suggests that the 0.8eV defect level acts as recombination center at room temperature. On the other hand, this defect may act as a trap in lower gap CZTSSe.

Cu(In0.52Ga0.48)Se2 solar cell performance with an MBE-grown ZnS buffer layer was studied. It was found that cell performance with the ZnS buffer layer was inferior compared to that with the CdS buffer layer; however, by insertion of a 50 nm CdS layer in addition to the ZnS buffer layer, the device performance was improved and was better than that with the CdS buffer layer. The higher device performance by using the ZnS/CdS double buffer layer may be due to the improved interface properties of the device.

The effect of Se/(Ga+In) ratio, expressed as Se/III, on the electrical, optical and structural properties of Cu(In,Ga)Se 2 , abbreviated as CIGS, thin film and on the solar cell performance was studied. CIGS films with various Se/III ratios were grown by the molecular beam epitaxy (MBE) process. With decreasing Se/III ratio, the resistivity of CIGS film was found to be increased and majority carrier hole concentration was decreased. This is probably due to the formation of Se-vacancy and/or vacancy complex. Again, the band-gap energy of CIGS film increased and the Cu content in the bulk of the film decreased with decreasing Se/III ratio despite the Ga/III ratio in the film remained almost unchanged. These results may suggest the formation of anti-site defects such as In Cu in CIGS. Solar cell performance degraded with decreasing Se/III ratio in CIGS absorber layer and the cause of the degradation was discussed.