Different light sources, e.g., bulbs, LEDs, and the sun, have different spectral compositions and are indistinguishable to the human eye. Light spectral composition information is useful for increasing harvested energy [40]. It should be noted that the instantaneous power harvested by PV cells is proportional to the area (A) and spectral irradiance (E(\(\lambda\))) of an ambient light source as shown in Equation (1). \[P_{inst}=A\times V\int_{0}^{\infty}R(\lambda)E(\lambda)d\lambda \tag{1}\]  
where R(\(\lambda\)) is the spectral responsivity, V is the PV cells operating voltage, and \(\lambda\) is the ambient light wavelength. For recharging and data communication, currently available mobile phones use USB ports. This port allows a minimum current rating for charging: 2.5 mA in a suspended state, 100 mA when not configured and connected, and 500 mA to 1.5 A when configured [16]. When the current value drops from that threshold, the USB port is disconnected [41, 42]. This means that the current USB port charging system does not support low-power energy harvesting applications. The small sized photovoltaics cannot generate enough continuous power to directly match the smartphone or its wearable device's recharging requirements via a USB port. However, light energy harvesting can power some small functions of the smartphone or its associated wearable device, thereby increasing the standby and/or operating time [16]. In [PERSON] et al. [43], device-to-device communication was proposed to enhance smartphone battery life on LTE networks. Wearable and personal body sensor network (BSN) devices, such as headphones, watches, and medical and physiological monitoring sensors, use short-distance wireless communication so that they can be powered autonomously [44]. For wearable devices, the components should be small and thin in size, are able to be used at a convenient location, have an application-specific shape to match human contour, and may require flexibility or bending [45]. Wearable battery capacities range from 70 mAh to 300 mAh, and they have limited operational lifespans with finite energy. Energy harvesting can be used to prolong the operational lifetime of the device. In the case of wearable devices, PV cells can be installed at appropriate locations on the device, taking into consideration space and light intensity levels. Flexible solar energy harvesting is proposed for biometric monitoring capable of working indoors at an average of 320 Ix and generating 56 uW of electrical power [46]. A combination of a thermoelectric generator (TEG) and PV is recommended by [PERSON] et al. [47] for wearable devices for the head, and it was shown that PV generates eight times more power outdoors while TEG offers eight times more power in an office. PV can be used for standby power or to compensate for battery self-discharge [47]. PV cannot compete with TEG in wearable devices [24; 47]. Light energy harvesting can be used for extending the time between recharging micro-power electronics. Practically, PV energy harvesters can be used to supplement external battery charging [48]. This can be helpful in a scenario where low-power devices record and wirelessly transmit data after a long (\(\sim\)10 s) time interval [48]. OLED display requires 3 V and 20 mA pulse display-write currents [48]. For Bluetooth low energy (BLE) and ZigBee green power communication, data transmission requires 15 mA and reception requires 18 mA of current. The irradiance measurement for a person standing under an indoor light on different body conditions was performed by [PERSON] et al. [40]; on a person's hands, it was about 25 uW/cm\({}^{2}\), while on the head it was about 168 uW/cm\({}^{2}\). The power consumption of different sensor nodes for BSN is discussed in [PERSON] et al. [1].