Microfabrication for solar-to-hydrogen devices

In this thesis microfabrication techniques for the development of solar-to-hydrogen devices were studied.

‘We searched for novel device architectures,’ says Pieter Westerik. ‘Especially the right combination and arrangement of semiconductor materials is crucial. Catalysts and gas separators should not interfere too much with light absorption. Also good ion conduction without reducing the illuminated semiconductor area was investigated.’

New techniques were developed for patterning the sidewalls of the silicon structures. ‘Combinations of special lithography etching techniques were used,’ Pieter says. ‘For example, we succeeded in accurately producing wafer-scale arrays of the same 3D monocrystaline structures in parallel. By producing millions of the same structures, a record high efficiency of 10,8 % for silicon wire-based photocathodes was   reached.’

Pieter: ‘By covering only the tips of the pillars, we optimized the catalyst-covered area without obstructing the illumination surface reaction area. This successful result was published in Nature Energy journal. Our Mesoscale Chemical Systems Group, led by Professor Han Gardeniers, collaborated on this with the Molecular Nanofabrication Group, led by Professor Jurriaan Huskens.’

In another study an integrated device architecture was studied. Pieter: ‘Basically this was a solar panel immersed in water. By using only 1 % of the surface area for micropores, we managed to achieve good conduction through the water between the front and back side of the device.’ 

In this PhD thesis, it was also investigated whether a photo-absorber tandem configuration with silicon and hematite, could be made fully integrated and used for providing all the necessary current and voltage for splitting water. A Ti99Nb1 connection layer was developed, and tandem operation was shown for the combination of a monocrystaline silicon buried junction solar cell with a reactively sputtered hematite photoanode.

‘Certainly, promising possibilities for improving the performance are present,’ Pieter concludes. ‘But it remains uncertain whether such tandem devices can ever compete with silicon-based triple cells as the driving force for water electrolysis.’

Strategic decisions

During his PhD, Pieter learned to work as an independent researcher. ‘It was a great feeling they considered me capable of making my own strategic decisions within the project,’ Pieter says. ‘This is a special experience when performing PhD research. It is comparable, I reckon, with starting your own company. It is great being able to work on a complex and challenging project for four years, at the beginning of your career.’

Practical considerations

Several device architectures were considered. All devices used photoelectrodes made out of earth-abundant elements with some form of microstructuring, to circumvent certain trade-offs.

‘In choosing materials and fabrication methods, I always let practical considerations prevail,’ Pieter says. ‘These kind of devices hold promise for solar hydrogen with a reasonable price. However, the general feeling is that triple cells are favorable, as the electrolyzer part requires less integration because these cells do not depend on a semiconductor/liquid junction. Also separate optimization is possible in further product development of such PV-electrolysis devices.’

Shopping for knowledge

After his PhD Defense Pieter started working at Petersburg Consultants in Doorwerth.

‘A career in academics I consider a harsh road to travel,’ he says. ‘To strive for a Professor position one day, you need to be very ambitious. At Petersburg Consultants I will work on high-voltage cable applications, for various partners from within industry. Working customer-oriented is quite new for me. However, I feel confident that my working experience, research and communication skills will be enough to solve complex issues and projects. To be of economic value in a direct way will certainly add to my job happiness. During the PhD project I enjoyed to search collaboration partners and proceed in research step by step. Shopping for knowledge outside your area of expertise always is fun to do.’