# Facial Recognition Method Based on Thin-Film Solar Cells
## 1 Introduction  
In recent years, facial recognition has become the most widely accepted biometric technology. Presently, it is the only technology among the various biometric techniques with a long-range performance. This recognition technology is realized by capturing facial images with photographic equipment for face model calibration, facial feature extraction, and feature matching. The most complex procedure, namely, feature matching, is performed by analyzing features in 2D images using a series of complicated algorithms, such as Eigenface [1], Fisherface [2], and Local Binary Pattern (LBP) [3; 4], to determine whether there is a match.  
However, facial recognition is a controversial biometric technology because facial feature matching is based on images captured by cameras across public spaces. Therefore, people may be monitored and photographed for face matching without their knowledge and consent. Such practices violate privacy and human rights [5]. We developed a facial recognition technology that does not rely on photographs or facial recognition algorithms to address this problem. Furthermore, we considered the global trend of green energy development and carbon-emissions reduction [6; 7]. Thus, we selected thin-film solar cells as sensing elements to replace traditional camera lenses, thereby introducing a new technique for facial recognition.  
In addition to the advantage of high photoelectric conversion efficiency in low light conditions [8; 9], thin-film solar cells can be utilized as detectors, as demonstrated in several studies. In 2017, [PERSON] et al. [10] developed a perovskite-based oxygen sensor to detect variations in oxygen concentration in a closed environment. The perovskite element in the sensor could increase the current with increasing oxygen concentration. [PERSON] found that perovskite elements could also be fabricated into humidity sensors [11]. The author successfully detected humidity changes through the capacitance variations of the perovskite element. Perovskite solar cells (PSCs) have been employed as refractometric sensors [12]. Similar light sensors also include the DSSC-based optical sensor developed by [PERSON] et al. [13] and the wide-band photodetector composed of flexible CIGS, which was developed by [PERSON] et al. [14].  
These studies were based on a single thin-film solar cell element, which can be easily customized into small- and medium-sized modules. The application of thin-film solar cells in small- and medium-sized self-powered products has not yet been realized but is anticipated.  
The facial recognition system developed in this study uses a 4 \(\times\) 3 thin-film solar cell array as the sensing element, with an overall sensing area of approximately B5 size. The thin-film solar cell array is self-powering because it absorbs ambient light to generate electricity for storage when the system is not operational. During the operation, the 4 \(\times\) 3 array receives reflected light after the user's face is illuminated by LEDs. The higher the intensity of the reflected light received, the greater the generated voltage or current. Twelve sets of voltage data were measured and then compared with pre-stored user data. This facial recognition system successfully distinguished six users, indicating the system's suitability for domestic biometric door locks. In such applications, thin-film solar cells can realize their new value, while reducing energy consumption.