# CuInSe\({}_{2}\)-Based Near-Infrared Photodetector
## 1 Introduction  
Nowadays, near-infrared (NIR) photodetectors which can convert optical signals to electrical signals are getting considerable interest for night-vision imaging, optical fiber communications, biomedical imaging, vegetation monitoring, and security applications due to the strong propagation and low attenuation characteristics of NIR light [1; 2; 3; 4; 5; 6]. Conventionally, InGaAs and Ge-based photodetectors with cut-off wavelengths exceeding 1500 nm have been used for NIR photodetectors. Si-based photodetectors are challenging when used to detect the NIR wavelength range and show low external quantum efficiency due to their low cut-off wavelength of 1100 nm [7; 8]. However, Ge-based photodetectors suffer from a high dark current (\(\sim\)10 mA/cm\({}^{2}\)) that is orders of magnitude higher than the \(\sim\)0.5 mA/cm\({}^{2}\) of InGaAs photodetectors. This leads to low signal-to-noise ratio (S/N) and responsivity [9; 10; 11]. InGaAs-based photodetectors have issues related to expensive manufacturing processes [12; 13]. Recently, Cu(In, Ga)Se\({}_{2}\) (CIGS) thin films have become considered one of the most prominent candidates for NIR photoactive layers [14; 15; 16].  
Polycrystalline copper-indium-gallium-selenide (CIGS) thin films exhibit very promising properties such as competitive stability, a high absorption coefficient of \(\sim\)10\({}^{5}\) cm\({}^{-1}\) (two orders of magnitude higher than that of Si), and an adjustable direct energy bandgap (1.04-1.67 eV) [17; 18]. CIGS has gained a reputation as a highly efficient thin film solar cell. The CIGS solar cells have reached a power conversion efficiency (PCE) of 23.35% at laboratory scale [19]. The performance improvement in CIGS solar cells has proven the potential utilization of CIGS thin films for NIR photodetectors. The CIGS thin films can be deposited on top of various substrates such as glass, polyimide, stainless steel, and silicon (Si) so that a CIGS thin film can be utilized to realize NIR photodetectors on various material platforms. Because little research has been reported regarding CIGS as photodetectors, more effort is needed to meet the requirement of the cut-off wavelength needed for operating in the NIR region [20; 21; 22].  
In the work reported in this paper, copper-indium-selenide (CuInSe\({}_{2}\), CISe) thin films were investigated as NIR photoactive layers. To achieve high PCE with CIGS, Ga content is added to the CISe thin film to form Ga grading. As the Ga content increases, the bandgap of CIGS increases and shows tunable bandgap characteristics [23; 24]. The CIGS thin film optimized for photovoltaic applications contains 8.5% of Ga. When this amount of Ga is incorporated into a CIGS thin film, its cut-off wavelength is limited to 1059.9 nm. To extend the pho-detection capability to the longer wavelength, CISe thin film without Ga is interesting. Therefore, we aimed to fabricate CISe thin film with a bandgap near 0.95 eV. A structure of Al/Ni/aluminum-doped ZnO/i-ZnO/CdS/CISe/Mo was grown on a soda-line-glass (SLG) substrate to realize a CISe-based photodetector able to detect optical wavelengths longer than 1100 nm. The bandgap energies of the CISe thin films were tuned by varying the Cu/In ratio. The device performance of these CISe-based photodetectors was investigated via the morphological and optical properties of the CISe thin films.