# CuInSe\({}_{2}\)-Based Near-Infrared Photodetector
## 3 Results and Discussion
### CuInSe\({}_{2}\)-Based Photodetectors  
CISe layers with Cu/In ratios of 0.87, 0.93, 0.94, and 1.02 were utilized to fabricate the photodetectors. The Cu-rich CISe layers, with Cu/In ratios of 1.27 and 1.11, exhibited a Cu-Se compound-related high-defect density on their surfaces, which implies that they are unsuitable for device applications.  
The external quantum efficiency (EQE) curves of the CISe thin-film photodetectors with various Cu/In ratios are compared in Figure 5a. The cut-off wavelengths were extracted by extrapolating the EQE curves as shown in the inset of Figure 5a and were utilized to calculate bandgap energies. The bandgap energies of the CISe thin films were tuned by varying the Cu/In ratio from 0.87 to 1.02. As the Cu/In ratio decreased, the bandgap energy decreased, which indicates that an increased cut-off wavelength was realized. The longest cut-off wavelength (1309 nm) was obtained from the CISe thin film having the Cu/In ratio of 0.87. Figure 5b shows the bias current-voltage (I-V) characteristics of the CISe thin-film photodetector in the dark and under 1064 nm laser illumination whose power ranged from 0.83 to 2.33 mW. The CISe-based photodetector with the Cu/In ratio of 0.93 exhibited the dark current density of 28.82 mA/cm\({}^{2}\) and the differential resistance of 13.24 k\(\Omega\) at \(-\)0.4 V. The low differential resistance is due to the large shunt leakage current. The dark current and responsivity values of the CISe-based photodetector were compared with those of the previously reported CIGS-based photodetectors in Table 1. Further material growth optimization is required to reduce the dark current and to enhance the differential resistance of the CISe-based photodetectors. In the forward biased condition, the current was almost constant, exhibiting the photovoltaic mode of the device. In the reverse biased condition, the current increased gradually with increasing laser power densities up to 2.33 mW, exhibiting a linear response. This current linear response dependency on the incident optical power is favorable for the photodetectors. The responsivity of the CISe thin-film photodetector was 0.52 and 0.60 A/W at 0 V and \(-\)0.4 V, respectively.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Device Structure** & **Dark Current** & **Responsivity (A/W)** & **References** \\ \hline Solution-processed CIGS & 0.3 mA/cm\({}^{2}\) @ \(-\)0.4 V & 0.34 @ 980 nm, 1 mW/cm\({}^{2}\) & [14] \\ ZnO/CdS/Al\({}_{2}\)O\({}_{3}\)/CIGS & 22.2 nA/cm\({}^{2}\) @ 0 V & 0.54 @ 880 nm & [15] \\ Ga\({}_{2}\)O\({}_{3}\)/CIGS & 1 nA/cm\({}^{2}\) @ 0 V & – & [25] \\ CISe & 28.82 mA/cm\({}^{2}\) @ \(-\)0.4 V & 0.6 @ 1064 nm & This work \\ \hline \hline \end{tabular}
\end{table}
Table 1: Comparison of dark current values of CIGS and CISe-based photodetectors reported in previous literature.  
Figure 5: (a) External quantum efficiency (EQE) curves of the CISe photodetectors measured at the bias voltage of \(-\)0.4 V. The inset figure shows the method to estimate the cut-off wavelength (\(\lambda_{\text{c}}\)) from the EQE curve. (b) Current–voltage characteristics of the photodetectors with the Cu/In ratio of 0.93 measured under 1064 nm wavelength.