## 3 Proposed Design Scheme  
In this section, different schemes are studied for the possible integration and use of PV for light energy harvesting in smartphones. PVs can be installed on the space available around the smartphone display. Smartphone accessories, such as a case (cover), are used to protect its exterior from scratches and damage. This cover/case exterior surface can also be used to install the PV cell. These techniques are more useful in charging the smartphone when no battery recharging system is available.  
This work proposes a miniaturized solar cells, which can be embedded inside the front display side glass cover of the smartphone, where there is some space available around the button, speaker, microphone, camera, and ALS, as shown in Figure 1 with red rectangles. This specialized solar cell can be manufactured to take advantage of the available contour area. The backside of the phone has a large empty area, but it faces the dark or rests on surfaces most of the time. Furthermore, mirrors and lenses are used in the photovoltaic system to concentrate the light on the PV cells, known as PV or solar concentrators. By installing the mirror on the inner surface of a book-like smartphone cover, a light concentrator can be produced. The user can change the angle of the mirrors to find the optimum PV energy harvesting orientation using light sensors as feedback, as shown in Figure 2. Additionally, when adjusting the light concentrator, the feedback of the ALS can be used together with PV cell output to find the maximum power point and inform the user of a maximum energy harvesting position.  
Harvesting energy for both indoor and outdoor environments requires the use of two types of solar cells optimized for each lighting condition [56]. A smartphone cover has a large enough area to occupy two types of PV cells, a miniaturized electronic circuit, and a small capacity battery, which will increase the thickness of the cover sheet to some extent. This will enable the traditional circuit presented in Figure 1 to be feasible and efficient enough to recharge the smartphone's battery to a certain level.  
The monocrystalline PV cell used for this design is model no. KXOB22-04 \(\times\) 3 L (IXYS), it can be used for both indoor and outdoor applications, and the rectangular-shaped cell has a size of 22 \(\times\) 7 \(\times\) 1.8 mm\({}^{3}\). The solar cell has an open-circuit voltage of 1.89 V and a short circuit current of 15 mA, and typical efficiency of 22% at standard testing values of \(\lambda\) = 1000 \(W\)/m\({}^{2}\) and 25 \({}^{\circ}\)C temperature, which is much higher than actual room lighting conditions. A total of four PV cells were used: two cells were connected in series to achieve a higher voltage, and then connected in parallel with two series-connected cells. A single power management circuit was used because small sized cells were used, and they were all expected to be in similar light conditions. Furthermore, using a chip with each solar cell increases the total power dissipation, lowering overall efficiency and requiring more space. The BQ25504 ultra-low-power boost converter for power management and working in a fractional open circuit voltage algorithm for MPPT was used [57].