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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.

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.

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.

In this article, we look, from the carrier perspective, at the effect of the double grading in a CIGS absorber, in order to understand its benefit compared to a single grading. We focus on the double grading effects on the conduction band edge, generation rate and carrier collection. We first show that, when the minimum bandgap position (notch position) is outside of the absorber depletion region, a local maximum of the conduction band edge appears at the back of the depletion region, creating a hump, which hinders the collection of electrons. Interpreting the effect of the grading as an effective electric field, we show that the hump is caused by the competition between the absorber internal electric field and the grading created effective electric field. From this, we deduce that the notch should be positioned in the depletion region. As expected, the generation rate increases significantly at the notch position. However, studying carrier collection and the recombination rate, we show that, even if in the ideal case we should observe a total collection of carriers generated in the depletion region, the recombination rate in the SCR increases significantly when the notch is in it. We propose that this is caused by the increased carrier generation close to recombination centres. The ideal notch position is, therefore, a compromise between being close enough to the front to have a maximal carrier collection and avoiding the creation of a hump, while minimizing the carrier recombination.

The carrier lifetime in CdTe is strongly limited by the nonradiative recombination via defects. Here, deep level defects in CdTe thin-film solar cells are revealed by transient photo-capacitance (TPC) measurement. A broad defect band centered at 1.07 eV above the valance band is identified at 90 K. The defect signal is reduced with the insertion of the CdSe layer between the CdS/CdTe heterojunction. The TPC signals are rapidly quenched with increased temperature, which suggests that this deep level defect is highly possible to act as an effective recombination center. Based on the thermal quenching model, the activation energy (E a) of the defect is estimated to be ∼0.2 eV. With the configuration coordinate model, the temperature-dependent TPC signal and the corresponding electronic transition process can be well interpreted. All the observations strongly indicate that the introduction of Se atoms into CdTe is promising to suppress the formation of deep defects.

The deep red phosphor Sr4(PO4)2O:Eu2+, which has an excitation peak around 450 nm for blue LED applications, is reported. This behavior is unusual for most phosphate phosphors. The crystal structure of Sr4(PO4)2O:Eu2+ is found to be monoclinic P21 and isotypic with Ca4(PO4)2O:Eu2+, which also shows deep red emission. Sr4(PO4)2O:Eu2+ has a larger lattice volume than Ca4(PO4)2O:Eu2+, but their emission and excitation spectra at room temperature are very similar. The key factors are discussed for achieving a large redshift of the 5d levels of Eu2+ ion in order to emit red light. In particular, the importance of the anion polarizability and the distortions of the metal coordination polyhedra are discussed, including the effective coordination number. Importantly, Sr4(PO4)2O:Eu2+ lacks the yellow emission at 77 K, which is found in Ca4(PO4)2O:Eu2+. The differences in thermal quenching behavior for Eu2+ dopants in Sr4(PO4)2O:Eu2+ and Ca4(PO4)2O:Eu2+ are attributed to the degree of auto/photo-ionization due to differences in the band gaps of these compounds. The importance of the large band gap of the host lattice in avoiding non-radiative processes of energy relaxation was confirmed.

In order to characterize solar cell modules, patterning a sub-module into small-sized devices by photolithography are required. For CIGSSe sub-modules fabricated by sulfurization-after-selenization (SAS), the small-size device performance was considerably deteriorated (from 19.3 % to 11.3 % for power conversion efficiency (PCE)) as compared to the sub-module after traditional photolithography process. We found that, undesirable Cu 2-x Se phase was introduced into the film by the traditional photolithography process. By utilizing an optimized photolithograph recipe, the PCE as high as 19.5 % (nearly the same as the sub-module) for the patterned small-size unit cells were achieved. This allows us to perform specific characterizations to the small-size device in order to evaluate the properties of the sub-modules.

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.

2,9-Dimethyl-4,7-diphenyl-1,10-phenanthroline (C26H20N2), known as bathocuproine (BCP), is a commonly used cathode buffer layer in conventional structure organic solar cells (OSCs). We demonstrated that BCP layer can also be used as a buffer layer in inverted structure OSCs. Unfortunately, the device exhibited an anomalous kink in the current density–voltage (J–V) characteristics, namely, an S-shaped J–V curve, leading to a low fill factor and low power conversion efficiency (PCE). To improve device performance, Ag-doped BCP layer (Ag:BCP) was used to replace the BCP layer. The results showed that the Ag:BCP layer can eliminate the S-kink in the J–V curve, resulting in a large improvement of fill factor and PCE. The origin of the S-shaped J–V curve was demonstrated to originate from the charge accumulation at the fullerene (C60)/BCP interface. On the contrary, the C60/Ag:BCP interface has favorable electronic properties with beneficial gap states for the transport of free carriers. Together with the good conductivity of Ag:BCP layer and the smooth morphology properties, the device performance was greatly improved by Ag:BCP buffer layer.

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.

Upconversion (UC) emissions in the hexagonal phase of NaYF 4 :10%Er 3+ /NaYF 4 core/shell nanocrystals have been observed by off-resonant excitation at 1620 nm, whose energy is lower than that corresponding to the lowest f–f transition of Er 3+ ( 4 I 15/2 → 4 I 13/2 ). The UC emission spectrum consists of five main peaks at around 1540 ( 4 I 13/2 → 4 I 15/2 ), 980 ( 4 I 11/2 → 4 I 15/2 ), 800 ( 4 I 9/2 → 4 I 15/2 ), 660 ( 4 F 9/2 → 4 I 15/2 ) and 540 nm ( 4 S 3/2 → 4 I 15/2 ). The infrared absorption spectrum shows a weak peak at around 1600 nm which is separated from the 4 I 15/2 → 4 I 13/2 absorption peak by 315 cm −1 , and phonon bands of NaYF 4 has been observed at about 300 cm −1 in the Raman spectrum. From these results, the UC emission is interpreted to involve a phonon-assisted excitation process. The UC emission intensity under off-resonant excitation at 1620 nm exhibits a much stronger temperature dependence than that under resonant excitation at 1550 nm, which is consistent with the phonon-assisted interpretation.

A novel synthesis method for hexagonal (β)-phase NaYF 4 :Er nano-crystals (NCs) which showed up-conversion (UC) from infrared to visible spectral region was developed. The NaYF 4 :Er NCs were synthesized in oleic acid (OA) and 1-octadecene (ODE) with Y 2 (CO 3 ) 3 · x H 2 O, Er 2 (CO 3 ) 3 · x H 2 O, Na 2 CO 3 and NH 4 F as precursors. This proposed method was simple and less toxic compared with generally used method so far. The XRD results showed that the molar ratio of OA/ODE and the temperature were key factors for phase control of NaYF 4 :Er NCs. The UC emission spectra were obtained with the emission wavelength at about 980 nm ( 4 I 11/2 → 4 I 15/2 ), 800 nm ( 4 I 9/2 → 4 I 15/2 ), 660 nm ( 4 F 9/2 → 4 I 15/2 ) and 540 nm ( 4 S 3/2 → 4 I 15/2 ) from Er 3+ ions, by excitation wavelength of 1550 nm. The slope values, n , in the pump-power dependence, showed that the emission at 980 and 800 nm were generated by 2-step UC and at 660 nm and 540 nm were 3-step UC. The optical process for the UC excitation was discussed.

Hydrogen isotope tracer experiment is taken on the a-C:H + D films prepared by the reactive RF magnetron sputtering including CH 4 and D 2 . The nine different samples were prepared with changing the partial pressure of D 2 in the process gas. The concentrations of H and D relative to C in the films were determined by ERDA and RBS. It was found that the total amount of H + D concentration is constant at ~ 22.7%. This means that hydrogen makes a chemical bond with one of four hands of C, and little amount of H atoms exist in the interstitial site. The D concentration in the films linearly increases with increasing the relative partial pressure P D 2 /( P D 2 + P CH 4 ). This implies that the activated CH 4 and D 2 molecules stick to the growing films and release the H and D atoms. XPS results show that all the prepared samples have the same sp 3 -rich structure.

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.

We have performed ultraviolet photoelectron spectroscopy measurements and density functional theory calculations to study the electronic structure at the interface between organic semiconductor (3-(4-biphenylyl)-4-phenyl-5-(4-tert-butyl phenyl)-1,2,4-triazole (TAZ)) and metals (Ca, Mg, Ag, and Au). The basic mechanism of interface states at organic–metal interfaces can be understood by controlling the injection of charge carriers at these interfaces. The position of highest occupied molecular orbital relative to the Fermi level and the magnitude of the interface dipole are measured for each organic–metal interface. For TAZ on Ca, Mg, and Ag, interface states are observed near the Fermi level. However, no interface state is observed for TAZ on Au. It is analyzed qualitatively that the interface state is formed due to interaction of TAZ lowest unoccupied molecular orbital composed of C2p and metal s levels. It is suggested that the interface state plays an important role in charge transport at the interface. The mechanism of formation of interface states and electrical properties are discussed.

The systematic variations in structural, optical and electrical properties of lead phthalocyanine (PbPc)-based solar cells with organic buffer layers were investigated. Transition of the PbPc crystal from a monoclinic phase to a triclinic one was observed when the buffer layers were changed. The structural properties of triclinic PbPc grown on the sexithiophene (6T) buffer layer was superior to those of PbPc grown on the other organic substrates. Since the crystal growth of organic layers is dominated by the anisotropic intermolecular interactions at organic hetero-interfaces, the highly oriented 6T buffer layer with the atomically flat morphology promotes the growth of well-ordered PbPc layers. The performance of the organic solar cells (OSCs) was in direct correlation with the structural and electrical properties of the PbPc single layers. The formation of the triclinic phase enhances the spectrum sensitivity at around 890 nm, suggesting the enhancement of the current density of OSCs. Thus, the control of both crystal quality and phase of PbPc layers using a proper organic buffer layer is effective in enhancing the device performance of OSCs.

The barrier formation at organic-metal interfaces was studied with density functional theory (DFT). We analyzed the induced density of states in the organic molecular gap and showed that it is high enough to control the barrier formation. We calculated the interface Fermi level for contact of BCP with various metals e.g. Ca, Mg, Al, Ag, and Au surfaces. We found our calculated result in consistent with experimental (UPS) result and concluded that the barrier formation is due to the charge transfer between the metal and the states induced in BCP molecular gap.

We used ultraviolet photoelectron spectroscopy (UPS) to study the electronic structure at the interface between organic semiconductor (CBP) and metals (Mg, Ag, and Au). Controlling the injection of charges at the interface requires a better understanding of the basic mechanism of the formation of interface states. In this context, photoelectron spectroscopy and density functional theory calculations were used to investigate the interaction of CBP with metal (111) surfaces. The position of HOMO relative to the Fermi level and the magnitude of the interface dipole were measured for each interface by UPS measurement. For CBP on Au, interface state (continuous state) was observed near the Fermi level by density functional theory calculations. However, no interface state was observed for CBP on Mg and Ag. It is suggested that the interface state plays an important role in charge transport at the interface. It was analyzed by density functional theory calculations that the interface state is formed due to interaction of CBP with metals and the position of the Fermi level varies strongly with the metal work function. The mechanism of formation of interface states and electrical properties were discussed.

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.

Interaction of bathocuproine (BCP) with low work function metals Ca and K was studied by density functional theory. New peaks known as interface states were observed near the Fermi level in BCP molecular gap due to interaction of metal (s) levels with BCP LUMO mainly composed of C (2p). It was analyzed that the charge injection becomes better at the interface between BCP and low work function electrodes. These findings indicate that the improvement in electrical characteristics is due to the improvement in charge injection with low work function metal electrodes. The effects of low work function on electrical properties were 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.

Electronic structure of the interface between a bathocuproine (BCP) molecule and metal surface was studied by density functional theory. The bond energy of BCP with metal surface increased with decreasing work function of metals, that is, Au, Ag, Al, Mg, and Ca in this order. The charge transfer also increased with decreasing metal work function. It was analyzed that the Fermi level and the lowest unoccupied molecular orbital of BCP are composed of both BCP and metal orbital component, and electron transport properties across the BCP and metal is discussed based on these analyses.

The relationship between optical and structural properties of Er-doped GaN grown by molecular beam epitaxy on sapphire (0 0 0 1) substrates was studied. The Er-related luminescence with wavelength at around 558 nm originated from 4 S 3/2 to 4 I 15/2 transition was observed. The intensity of the Er-related luminescence increased abruptly with the Er concentration of about 2 at% and showed a maximum at about 4 at%. X-ray diffraction analyses and Rutherford backscattering measurements suggested that most of the Er ions are incorporated into substitutional lattice site with the Er concentration of up to 2 at%, and then begin to occupy interstitial sites. The incorporation site of Er was found to be dependent on the Er concentration. Based on these results, the optically active center of Er in GaN is discussed.

The authors report the effect of a low-temperature buffer layer of Cu 2 O (LTB-Cu 2 O) on the growth of Cu 2 O thin films. The samples were prepared by radio frequency-magnetron sputtering. It was found that with the introducing of LTB-Cu 2 O, the linewidth of the X-ray diffraction peak decreases and the Cu 2 O thin films have preferred (1 1 1) orientation. By introducing LTB-Cu 2 O, the grain size sufficiently increases and the surface flatness improves, as indicated from the atomic force microscopy measurement. Under the optimum growth conditions, hole mobility of 256 cm 2 V −1 s −1 , which is the highest value reported so far, with the hole concentration of 1×10 14 cm −3 was achieved.

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.

A degradation mechanism in InGaP/GaAs heterojunction bipolar transistor (HBT) was investigated. The current gain of the HBT grown at a V/III ratio of 0.7 for the base layer decreased when annealed at 670 °C. The samples grown at V/III ratios >1.0 showed constant photoluminescence intensity independent of annealing temperature, indicating that high thermal stability is obtained, while the V/III ratios 2 –H complexes in carbon-doped GaAs decreased when annealed at >550 and 650 °C, respectively. We suggest that the decomposition of the C 2 –H complex creates defect centers, which lead to the current gain reduction.

Deep levels in polycrystalline p-Cu 2 O/i-ZnO/n-ZnO/glass photovoltaic structures were studied by deep level transient spectroscopy (DLTS). Post-deposition crown-ether cyanide (CN) treatments of the samples were performed with the variation of time. DLTS spectra for all samples showed the presence of two traps localized at 0.2 eV and 0.5 eV from the top of the valence band. Effects of defect passivation and improvement of cell performance were demonstrated with the cyanide treated samples. Optimum time of cyanide treatment was found to be 3 min. Cell performance was increased from 0.4% to 0.7% for the samples undergoing the optimum post-deposition cyanide treatment.

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.

Luminescence spectra of Eu-doped GaN with varying Eu concentration and V/III ratio were investigated. Eu-related luminescence originating from the 5 D 0 - 7 F 2 transition of Eu 3+ was observed at about 622 nm. The luminescence basically consisted of three peaks, and the relative intensity of the three peaks was affected by the Eu concentration and the V/III, ratio. These results indicate that the Eu incorporation site is very sensitive to the Eu concentration and the V/III ratio.

Structural control of the organic intrinsic layer (i-layer), which consists of a mixture of p- and n-type organic semiconductor materials, and the organic p–i–n triple-layer structure has been investigated. Significant modification of these structures, which used metal-free phthalocyanine (H2Pc) and 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) as the p- and n-type materials, respectively, was observed by introducing a modification layer deposited on a glass substrate. The orientation of the molecular planes of both H2Pc and PTCDA in the i-layer (H2Pc:PTCDA mixture) and p–i–n structure (H2Pc/i-layer/PTCDA) formed onto a PTCDA modification layer was parallel to the substrate surface. On the other hand, when H2Pc was used as the modification layer, the orientation of the molecular planes in the i-layer was found to be random. These results indicate that the internal structure of the organic mixed-layer is strongly affected by the organic layer-substrate interface. The enhancement in optical absorption observed for the orientation controlled p–i–n structure suggests the importance of the control of molecular orientation for improving the optical properties in organic device structures.

Defect levels in Cu(In 1 − x ,Ga x )Se 2 (CIGS) were studied by using admittance spectroscopy with varying Ga concentration of x from 0.38 to 1.0. Two distinct peaks ( α and ζ ) were detected from all of CIGS samples. Peak α could be attributed to the shallow acceptors. Peak ζ was closely correlated with the surface potential of CIGS; therefore, it may have been caused by traps near the surface of CIGS such as grain boundary defects. The intensity of peak ζ increases with increasing Ga concentration. Further, this intensity was correlated with the ideality factor of the Al/CIGS Schottky junction. We suggest that the degradation of the performance of CIGS solar cell with increasing Ga concentration may be concerned with defects corresponding to peak ζ ˙

Effects of V/III ratio on photoluminescence spectra of Eu-doped GaN grown by molecular beam epitaxy were investigated. The Eu-related luminescence originating from 5 D 0 – 7 F 2 transition of Eu 3+ was observed in the wavelength range of 620–624 nm under above-gap and below-gap excitations. Two luminescence peaks at 620.3 and 622.2 nm were observed in Eu-doped GaN with the Eu concentration of 1 at% under above-gap excitation, and a single peak at 620.3 nm was observed under below-gap excitation, suggesting multiple incorporation sites of Eu in GaN. The intensity ratio of the two peaks was found to vary with the V/III ratio. It was suggested that the incorporation site of Eu in Eu-doped GaN is sensitive to the V/III ratio.

We fabricated a thin film of Sm3+ doped ZnS by molecular-beam epitaxy (MBE) and investigated its photoluminescence properties. The excitation spectrum of the Sm3+ luminescence at 10 K is dominated by broad bands peaked at the free exciton energies, in contrast to many earlier reports on rare-earth-doped semiconductors. This result shows that the free exciton contributes significantly to the excitation of Sm3+ in our MBE sample, probably because the concentration of the trap centers which capture free excitons but do not transfer energy to Sm3+ is sufficiently low in our sample. The thermal quenching characteristic of the Sm3+ luminescence under interband excitation of the ZnS host is fitted well to a formula expressed by taking into account two deactivation channels. The origins of these deactivation channels are argued on the basis of previously proposed excitation mechanisms of the rare-earth ions in semiconductors.

To understand the degradation mechanism of organic solar cells (OSCs), the charge dynamics of conventional and inverted planar heterojunction OSCs based on boron subthalocyanine chloride (SubPc) and fullerene (C60) with identical buffers during the air exposure were investigated. The results of light intensity dependent open circuit voltage show that the bimolecular recombination is dominated in the fresh devices, regardless of the device structure. The appearance of transient peak in photocurrent after turn-on and the light intensity independent turn-off traces in transient photocurrent suggest that the rapid degradation of conventional device is due to the energy loss originated from the aggravated trap mediated recombination. In contrast, the half-lifetime of inverted device is ∼25 times longer than the conventional one. The improvement of stability is ascribed to the decrease of the trap generation possibility and the suppression of trap mediated recombination in the case of inverted structure, where the penetration of oxygen and water through buffer layer is avoided.

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.