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We previously developed perovskite/graphene solar cells with improved long-term stability. In this study, we investigated the effect of the number of layers on the characteristics of a solar cell by changing the number of graphene layers and further determined the optimum number of graphene layers. We showed that the interfacial recombination due to the damaged layer of the outermost perovskite layer and the band bending of the perovskite layer due to the change of the work function of graphene had a significant influence on the characteristics of the solar cell, especially the short-circuit current. A conversion efficiency of up to 9.4% was obtained as a result of optimizing the formation conditions of the perovskite layer with seven graphene layers.

The two-dimensional materials have the thickness of an atomic layer level and are expected as alternative materials for future electronics and optoelectronics due to their specific properties. Especially recently, transition metal monochalcogenides and dichalcogenides have attracted attention. Since these materials have a band gap unlike graphene and exhibit a semiconductor property even in a single layer, application to a new flexible optoelectronics is expected. In this study, the photovoltaic characteristics of a GaSe/MoSe2 heterojunction device using two-dimensional semiconductors, p-type GaSe and n-type MoSe2, were investigated. The heterojunction device was prepared by transferring GaSe and MoSe2 onto the substrate which the titanium electrodes were fabricated through a mechanical peeling method. The current–voltage characteristics of the GaSe/MoSe2 heterojunction device were measured in a dark condition and under light irradiation using a solar simulator. The irradiation light intensity was changed from 0.5 to 1.5 sun. It was found that when the illuminance was increased in this illuminance range, both the short-circuit current and the open-circuit voltage increased. The open-circuit voltage and the energy conversion efficiency were 0.41 V and 0.46% under 1.5 sun condition, respectively.

This study was conducted with the objective of improving the stability of perovskite solar cells by using the unique characteristics of graphene in order to facilitate the widespread application of such solar cells, for example, in multijunction devices. We consequently developed a new transfer method for graphene using vacuum lamination and, using graphene, successfully fabricated a perovskite solar cell that does not require a hole-transport layer. Initial stability tests indicated that the new device has better stability than a control perovskite solar cell using spiro-OMeTAD. Although the new solar cell design exhibited poorer cell performance than the control, we determined via modeling that its performance can be improved by modifying the interface state between perovskite and graphene or by modulating the work function of graphene.

The optoelectrical properties of bromide-based perovskite thin films were evaluated to develop an optical wireless power transmission system. CH3NH3PbBr3 (MAPbBr3) films were deposited by the anti-solvent method after substrate preheating. The low-temperature exciton emission and discontinuous peak shift from phase transitions in the emission spectra indicate the high optical quality of MAPbBr3 films on TiO2. The Urbach energy and diffusion length of the MAPbBr3 thin films on TiO2 were 22.7 meV and 1.14 μm, respectively. The films exhibited good solar cell performance under monochromatic light with an efficiency over 20%.

We developed a method of patterning graphene by plasma etching to improve the performance of graphene transparent conductive film. We succeeded in patterning the graphene multilayer film into the porous shape of the mask by optimizing the plasma etching gas type, pressure and power. As the plasma irradiation time was lengthened, the transmittance became high and saturated at a transmittance corresponding to the aperture ratio of the mask. A possibility of independently controlling the optical and electrical properties of the graphene film by etching under appropriate conditions was indicated.

We developed a perovskite solar cell using graphene as a hole transport layer. We succeeded in transferring graphene onto perovskite layer by using a vacuum bonding machine. The conversion efficiency of the perovskite solar cells using graphene achieved power conversion efficiency of 7.1%. Graphene cells showed higher stability than spiro-OMeTAD cells after light irradiation at high humidity. This is because graphene blocks the penetration of H_{2}O and O_{2} to the perovskite layer and plays also a role as a blocking layer.

Organic light emitting diodes (OLEDs) with nanostructured ultrathin layers inserted at the interface between electron- and hole-transport layers were investigated. The fundamental structure of the OLEDs fabricated by a vacuum evaporation method was indium-tin-oxide (ITO) anode/copper phthalocyanine (CuPc)/N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-diphenyl-4,4′-diamine (TPD)/8-hydroxyquinoline aluminum (Alq3)/LiF/Al cathode. Fullerene (C60) and rhodamine B (RhB) molecules were used as the nanosutructured ultrathin layers inserted at the interface between the Alq3 and TPD layers. The electroluminescent (EL) properties have been measured for the OLEDs with C60 and RhB ultrathin layers and the dependences on the thickness and the position of the inserted layers were examined. For the OLEDs with the C60 ultrathin layer, the improvements of the drive voltage and EL efficiency were observed. The OLED with the inserted C60 ultrathin film of a monolayer thickness showed the highest efficiency, which was twice as large as that without C60 layer. On the contrary, the improvements were not observed for the OLEDs with the RhB ultrathin layer.

Organic light-emitting diodes (OLEDs) with a nanostructured fullerene (C60) layer at and near the interface between the electron- and hole-transport layers were fabricated using a vacuum evaporation method. The OLEDs had a fundamental structure of indium-tin-oxide (ITO) anode/copper phtalocyanine (CuPc)/N,N'-dephenyl-N,N'-bis(3-methylphenyl)-1,1'-diphenyl-4,4'-diamine (TPD)/8-hydroxyquinoline aluminum (Alq3)/LiF/Al cathode. The electroluminescent (EL) properties have been investigated for the OLEDs with a nanostructured C60 layer at the TPD/Alq3 interface and at the different positions in the Alq3 layer near the TPD/Alq3 interface. The dependences of the EL properties on the thickness of the inserted C60 layer and on the position from the interface were examined. The OLEDs with a C60 monolayer inserted at the TPD/Alq3 interface showed the highest efficiency.

CuGaS 2 films have been grown on (1 0 0) GaAs and GaP wafers by the vacuum evaporation technique using constituent elementary substances as sources. RHEED patterns of the films that use 〈0 1 1〉 azimuths in a substrate show two different patterns in mutually orthogonal directions. In one direction, the observed pattern is a single pattern which illustrates that CuGaS 2 grows in the [0 0 1] orientation direction (the c -axis growth). In another direction, however, additional patterns appeared. This phenomenon indicates that one pair of the 180 rotated {1 1 2} twins exists in addition to c -axis growth in a film, and it also indicates that additional orientations except for c -axis growth are reduced in films grown on compound substrates as compared with ones on (1 0 0) Si.

As the basic study to realize a blue-color light-emitting diode using CuAlxGa1-xS2 crystals, heterojunction diodes were fabricated by the vacuum deposition of an n-type ZnyCd1-yS thin film on the {112} surface of a p-type CuGaS2 single crystal grown by the iodine transport method. It was found that the {0001} surface of ZnyCd1-yS is oriented in parallel to the substrate when the substrate is not heated, that the film is transparent to green and blue color, and that the resistivity is extremely low when y = 0.2 to 0.4. When single crystalline CuGaS2 is annealed in saturated sulfur vapor, the resistivity decreases. Next, the current-voltage characteristic of a diode was investigated. When sulfur-annealed CuGaS2 single crystal was used, the series resistance of the diode was low and the forward characteristic could be explained by using series and parallel resistances in the equivalent circuit. On the other hand, the reverse current showed almost an ohmic characteristic at low voltage but it increased nonlinearly with an increase of the reverse voltage. When y ≠ 0, a yellowish-orange or red light emission was observed when the diode was forward-biased. When y = 0, no light emission was observed although the electrical property of the diode was excellent.

Transparent p-type NiO films were prepared by reactive sputtering using the facing-target system under Ar-diluted O2 gas at T sub of 30 and 200 °C. The increasing intensity of dominant X-ray diffraction (XRD) peaks indicates improvements in the crystallinity of NiO films upon Cu doping. In spite of the crystallographic and optical changes after Cu-doping, the electrical properties of Cu-doped NiO films were slightly improved. Upon Ag-doping at 30 °C under low O2 concentration, on the other hand, the intensity of the dominant (111) XRD peaks was suppressed and p-type conductivity increased from ∼10−3 to ∼10−1 S cm−1. Finally, our Ag-doped NiO films were applied as the back contact of CdTe solar cells. CdTe solar cells with a glass/ITO/CdS/CdTe/NiO structure exhibited an efficiency of 6.4%, suggesting the high potential of using p-type NiO for the back-contact film in thin-film solar cells.

Reflectance anomaly and photoluminescence emission spectra of Ag(Al x Ga1-x )S2 epitaxial layers, which were grown in the range of xx. Because the oscillator energy and the damping constant coincided with the peak energy and the spectral full-width at half maximum of the emission respectively, the emission was identified as free exciton emission, indicating the good optical quality of the layers. Taking account of the lattice strain effect, the composition dependence of the A1-mode Raman scattering frequency of the layers coincided with that of the bulk crystals.

Lattice strain in AgGaS2 epitaxial layers of different thicknesses, which are grown on GaAs (100) by the multisource evaporation method, is described. Biaxial compressive mismatch strain in thin layers relaxes with increasing layer thickness, and biaxial tensile thermal strain becomes dominant in thick layers. The thin AgGaS2 epitaxial layers are completely oriented toward the c-axis, but the thick layers contain 180°-rotated { 112 } twin crystals. The generation of twin crystals is considered to cause partial relaxation of the thermal strain. The observed variation in the A1-mode frequency with thickness is fully explained by considering the layer thickness dependence of the lattice strain. Full width at half-maximum of the A1-mode Raman scattering line is affected by the spatial inhomogeneity of the strain magnitude in the depth direction. Raman scattering measurement of the A1 mode can be used as a sensitive technique to evaluate the lattice strain in AgGaS2.

The transmission and reflection spectra have been measured in CuGaS2 single crystals near the absorption edge in the temperature range 77–293 K. The absorption spectra for E⊥ Z calculated by assuming that the absorption band of the A-exciton is superposed on the absorption tail expressed as Urbach's rule fits the experimental spectra fairly well in that temperature range. Thus, the temperature dependence of the A-exciton energy (E 01) becomes clear. The reflectivity maximum coincides with E 01 only at low temperatures (< 100 K). Other parameters such as the damping constant, steepness factor and oscillator strength for the Γ5 exciton are also discussed.

The transmission and reflectance spectra for various x in CuAl x Ga1-x S2 single crystals have been measured with polarized light. The photon energies of excitons associated with the uppermost valence bands are determined by analyzing the exciton absorption spectra and the reflectance anomaly spectra at room temperature and at about 80 K. The exciton energies have an approximately quadratic dependence of x. The valence band splitting is predominantly due to the crystal field splitting. The splitting energy varies almost linearly with x.