Thin Film of Lithium and ZnO nanowires microgenerator : all Integrated for “perpetual Battery” on fLExible substrates – FLexIBLE

The project fits into the economic development strategy for alternative and sustainable sources of energy. The energy that we want to harvest in this project are ‘truly free’ mechanical sources like fluid flows (like wind or river), parasitic vibration (in engines or rotating machines, for example) or human movements (voluntary motion like walking or hand movements). The main applications are low consumption nomad electronics and remote wireless sensors.

The heart of the project is to develop a prototype that integrates, on the same flexible chip, a microgenerator that converts this ambient mechanical energy into electrical energy that can recharge a lithium battery, through a specific electrical converter. Since the efficiency of such a mechanical harvesting device strongly depends on the matching between: a piezoelectric microgenerator (called PG), the power management block (called PMB), the electrical load block (a Li battery called LiB), we propose in this project to have an integrative approach: studying and designing the whole energy conversion chain.

The geometry of the final prototype is already fixed: 5 x 2,5 cm2 on flexible substrate (ISO 7816 smart card international standard), corresponding to the dimensions of the flexible LiB currently being developed by ST. The objective is to reach 150 µW at the output of the PMB, which is the minimum power necessary to charge the LiB. Given the efficiency (around 55%) of existing PMBs at low current, the PG is asked to provide a minimum 280 µW to the PMB. This performance is consistent with small indoor photovoltaic cell (27cm2) that provides 200 µW 6 hours a day in office conditions (cf. PowerFilm). These harvesters already power existing applications : e.g. 3 activated sensors and a RF signal every 10 minutes. Hence, a PG working 8 hours a day and providing an average 280 µW can power the same loads as already addressed by the PowerFilm cell. Now if vibrating energy is only available 40 minutes a day, there is still niche applications where the 280 uW harvester can fit, like autonomous switch for lighting (cf. ECS300 from EnOcean : a simple message is sent 30 meters away every time a switch is pressed). So the 280 µW dimensioning covers realistic cases of load, with an energy availability ranging from 40 minutes to 8 hours.

GREMAN has a well-known expertise in all the aspects of piezoelectricity, and more specifically on piezoelectric nanowires for more than 5 years. It has started by modelling aspects and it is reaching now the level of ZnO-based PGs. The whole device is designed and fabricated in GREMAN: growth of the ZnO nanowires, device fabrication and characterization. GREMAN’s technology is now mature enough on silicon substrates to be optimised for specific applications, specific loads and specific flexible substrates.
STMicroelectronics Tours aims to be a world-class center of expertise for designing and manufacturing electronic components for providing storage solutions and intelligent energy management. ST Tours is now considering different energy harvesting solutions including photovoltaic, vibratory and thermoelectric means. In this context, ST Tours sells LiCoO2/LiPON/Li microbatteries. They are already relatively flexible, suitable for the ISO 7816 standard. Moreover, ST has already developed a PMB able to charge the Li microbattery. This PMB is a non flexible CMOS on silicon wafer prototype device, based on robust BCD6s technology (size 3 x 3 x 1 mm3). Finally, ST posses in Tours a “packaging lab” which has a long term expertise in reporting such devices on flexible modules, making the appropriate electrical interconnections.

All the equipment, the knowledge and the technological bricks to reach our goal are available on the same site shared by ST and GREMAN in Tours. The consortium GREMAN - ST Tours has a solid experience of cooperation through several projects and 22 joint CIFRE PhD projects, which have led to more than 50 publications and 10 patents.