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Zinc oxide (ZnO) is one of the best transparent conducting oxide (TCO) materials with a wide bandgap and good electrical and optical properties. Its low cost, nontoxicity and transparency in the optical region of the electromagnetic spectrum make it very promising candidate for solar cell applications. In this work, zinc acetate precursor was used to grow a ZnO thin film by using sol-gel spin-coating technique. The surface morphological study using scanning electron microscope (SEM) was carried out to confirm the growth pattern and crystal distribution. The optical properties, transmission (T), reflection (R), optical bandgap (Eg), refractive index (n), and extinction coefficient (k) were extracted and investigated to be used in the simulation of ZnO/Cu2O heterostructure solar cell, where ZnO thin film plays a double role: as the TCO window, as well as the emitter of the n-p junction. However, the solar cell showed weak external quantum efficiency (EQE) compared to those prepared by using zinc nitrate and diethyl zinc precursors. TCAD numerical simulation was used to clarify the origin of this weak EQE by taking into account two parameters. The first studied parameter is the root-mean-square interface roughness, RMS, in Haze modeling approach, H, which describes how much of incident light is scattered at the interface. The second studied parameter is the density of defects in the ZnO bulk with continuous distribution of states in its bandgap similar to an amorphous semiconductor made of tail bands and Gaussian distribution deep level bands. Consequently, and by adjusting and investigating the effect of the RMS and the constituents of the bandgap states, we were able to obtain a good agreement between simulated and measured EQE characteristics of the solar cell.

InGaN ternary alloys with their band gaps varying from 0.7 to 3.4 eV, are very promising for photodetector devices operating from UV to IR wavelength range. Using Silvaco–Atlas software, an In0.1Ga0.9N/GaN based p–i–n photodiode is designed and the J–V characteristics, the spectral responsivity, the frequency response and the cut‐off frequency as a function of InGaN thickness are studied. The photodiode exhibits a high reverse breakdown voltage of 38 V, a peak responsivity of 0.2 A W−1 at 0.343 μm wavelength and a cutoff frequency of 400 MHz under an applied reverse bias voltage of 2 V and for a 0.1 μm i‐InGaN layer, in good agreement with simulated and experimental results found in literature. It is found that an optimum i‐layer thickness of 1.5 μm for the maximum cutoff frequency of 4 GHz attributed to the predominance of the limitation in capacitance effect on the cutoff frequency at low i‐layer thickness and the transit time on the cuttoff frequency for high i‐layer thickness. For this i‐layer thickness, the highest peak responsivity about 0.244 A W−1 at 0.384 μm wavelength is achieved.

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The solar spectrum can be divided by tandem solar cells into several subcells that have different bandgaps which convert, more effectively, the light into electricity than the single cells. In this study, the simulation of the photovoltaic (PV) characteristics of a CZTS/CZTSe tandem solar cell, based on structures of copper zinc tin sulfide (CZTS) as a top cell and copper zinc tin selenide (CZTSe) as a bottom cell, was accomplished by using SCAPS-1D simulator under AM1.5 illumination. Initially, the simulation of single CZTS and CZTSe solar cells was performed to give efficiency of 14.37 % and 17.87 %, respectively, which are in good agreement with the literature results. Before feeding with filtered spectrum, the simulated PV parameters of the CZTS/CZTSe tandem solar cell are the conversion efficiency () of 20.68 % and the shortcircuit current density (Jsc) of 20.205 mA/cm 2 of the top and bottom cells with arbitrary normal thicknesses. Furthermore, and in order to reach the matching current, both top and bottom cells have been investigated at different thicknesses for tandem configuration after validation, where the performance of the top and bottom cells is at thicknesses ranged from 0.05-0.5 m and 0.1-1 m, respectively. The performance of the tandem solar cell is determined after filtered spectrum feeding and current matching. The Jsc of CZTS/CZTSe tandem solar cell is 20.33 mA/cm 2 for 0.255 m thick of the top, CZTS, cell and 0.8 m of the bottom, CZTSe, cell. The maximum  of 22.98 % is reached for tandem structure design with open circuit voltage (Voc) enhancing of 1.48 V.

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