A solar cell  is an electronic device which directly converts sunlight into electricity. Light shining on the solar cell produces both a current and a voltage to generate electric power. This process requires firstly, a material in which the absorption of light raises an electron to a higher energy state, and secondly, the movement of this higher energy electron from the solar cell into an external circuit. The electron then dissipates its energy in the external circuit and returns to the solar cell. A variety of materials and processes can potentially satisfy the requirements for photovoltaic energy conversion, but in practice nearly all.

 Thermoelectric power generation is based on seebeck effect that was discovered in 1821. From the temperature difference of hot and cold side, we can harvest the electrical energy. To enhance the efficiency of thermoelectric generator, there are two key factors, which is temperature difference (ΔT) and dimensionless figure of merit (ZT). Temperature difference ΔT is system factor of thermoelectric generator that drives the charge carriers from hot side to cold side. Dimensionless figure of merit (ZT) is materials’s intrinsic properties that consists of three parameters (electrical conductivity, seebeck coefficient, thermal conductivity)

 Piezoelectric energy is drawing attention as the upcoming energy source. It is converting a little mechanical energy such as force, pressure and vibration that we may easily ignore into electric current and voltage. Piezoelectric harvesters can be employed as battery rechargers in various environments, such as industries, houses, the military and handheld or wearable devices. The possibility to avoid replacing exhausted batteries is highly attractive for wireless networks, in which the maintenance costs due to battery check and replacement are relevant. Another emerging field of application is biomedical systems, where the energy could be harvested from an off-the-shelf piezoelectric unit and used to implement drug delivery systems or tactile sensors. Recent research also includes energy conversion from the occlusal contact during chewing by means of a piezoelectric layer and from heart beats.