Development of high-voltage photovoltaic micro-devices for an energy supply to micromachines

A submodule of photovadtaic micro-devices which generates more than 200 volts with an area of 1 cm2 has been developed to directly drive piezoelectric or electrostatic actuators which will be used as microactuators. The submodule consists of 95 micro cells ( unit cell size : about 0.5 m m x 2.0 mm ) interconnected in series, and produces a submodule open circuit voltage (Vac) of 207 volts, short circuit current (Isc) of 36.6p A, maximum output power (Pmax) of 4.65mW and fill factor (F.F.) of 0.615 under Air Mass ( A M ) 1 . 5 , 100mW/cmz illumination. Each micro cell has aln a-Si triple stack construction and produces V,, of 2.3 volts, and short circuit current density (Jsc) of 6.5 mA/cm2. The series connection for the micro cells is precisely processed by a focused laser beam, thereby significantly reducing the invalid area of the submodule. To evaluate the use of this submoduleas a microactuator's power source, i t was confirmed tlhat a piezoelectric polymer could be directly driven by the electrical output of the submodule. 1. I NTR 0 DUCTI <IN Recently several energy supply methods for micromachines, such as the microwave, photon and microbattery energy supply, have been investigated. Among them, the photon energy supply is the most suitable as an energy supply for micromachines because of the following advantages; (1) Wireless energy supply (2) Simultaneous supply for plural micromachines (3) Applicable both in water and in air (4) Semipermanent operation under photon energy (5 ) Suitable for producing a high voltage On the other hand, many different types of actuation principles have been proposed for the micromachine's actuator. However, most of this work has focused on electrostatic[l] or piezoelectric drives[2]. Although some work has been done to decrease the electric voltage requirement, electrostatic or piezoelectric actuators usually require a drive voltage ranging from tens to hundreds of volts. In order to meet this energy requirement for actuators, the energy source must be developed to generate a correspondingly high-voltage in a small area. Concerning the photon energy supply, J.B. Lee et al have already reported a high-voltage solar cell array which generates 150 volts in 1 cm2 [ 31. However, the efficiency of the solar cell array is not very high (less than 0.2 %). We have newly applied a laser processing technique to the fabrication of photovoltaic microdevices. As a result, we have developed high-voltage photovoltaic micro-devices which generate more than 200 volts with an area of 1 cm2 and offer very good output characteristics. This paper describes the structure, the fabrication process and the characteristics of the high-voltage photovoltaic micro-devices and the development of the laser micro-processing technique. An actuation demonstration using this devices to drive a piezoelectric actuator is also described. 2.HIGH-VOLTAGE PHOTOVOLTAIC MICRO-DEVICES 2.1 Structure The energy source for micromachines should satisfy two requirements at the same time. (1) Generate high enough voltage to drive an electrostatic or piezoelectric actuator (2) Take up a small area In order to meet these requirement, we decided the following design criteria.