Upscaling of integrated photoelectrochemical water-splitting devices to large areas

Photoelectrochemical water splitting promises both sustainable energy generation and energy storage in the form of hydrogen. However, the realization of this vision requires laboratory experiments to be engineered into a large-scale technology. Up to now only few concepts for scalable devices have been proposed or realized. Here we introduce and realize a concept which, by design, is scalable to large areas and is compatible with multiple thin-film photovoltaic technologies. The scalability is achieved by continuous repetition of a base unit created by laser processing. The concept allows for independent optimization of photovoltaic and electrochemical part. We demonstrate a fully integrated, wireless device with stable and bias-free operation for 40 h. Furthermore, the concept is scaled to a device area of 64 cm2 comprising 13 base units exhibiting a solar-to-hydrogen efficiency of 3.9%. The concept and its successful realization may be an important contribution towards the large-scale application of artificial photosynthesis.

device #1 before and after 3 hours of operation under illumination. A reduction of the fill-factor and a lowered open-circuit voltage can be seen from the curve after operation. Device degradation as well as increased temperatures could be responsible for such behavior since there was no control over the device temperature. Both measurements start from V = 0 V and a scan rate of 200 mV s -1 was employed.

Supplementary Figure 5 | Effect of electrolyte immersion. Plot of the PV element's I-V
characteristics of device# 1 dry and immersed in 1M KOH before operation. Both curves have differences in short-circuit current as well as in the open-circuit voltage. One explanation for these differences can be identified by a change of the device temperature by cooling due to the electrolyte which acts as a heat sink to a certain degree. Both measurements start from V = 0 V and a scan rate of 50 mV s -1 was employed.

Supplementary Figure 6 | Change of EC element properties. Comparison of the EC element´s I-V
properties from device #1 before and after 3 hours of operation under illumination in 1M KOH. After operation a lower overpotential is observed which can be explained be chemical changes of the nickel-foam surface. Oxidation and reduction potentials can be identified that were initially not apparent. A scan rate of 50 mV s -1 was employed. Please note that no preconditioning of the nickelfoam was done prior to the experiment.

Laser processing
Almost every thin-film photovoltaic technology makes use of laser processing for the series connection of solar cells. The series connection is required to lower the ohmic losses in the contacts. We often refer to it as the so called integrated series connection because the required laser processing steps are integrated in between the solar cell layer deposition steps.
A first laser process is required after the front contact deposition for cell stripe definition (called P1). After the silicon deposition a second laser processing step is required to selectively remove the absorber, exposing the underlying front contact (P2). Finally, in a last process step the back contact is locally removed by the laser for the final definition of the cell stripes (P3). Supplementary Figure 155 shows an illustration of a single solar module cell stripe with the interconnection structure in cross section.
The individual layers are removed selectively by laser irradiation through the glass substrate leading to an ablation of the material in scribe lines, parallel across the whole substrate. For the P1 process we used a Q-switched DPSS Nd:YVO 4 with a wavelength of 355 nm (third harmonic) and a pulse duration between 7-10 ns. The other two ablation processes P2 and P3 were realized with the use of a similar laser source but with a wavelength of 532 nm (secondary harmonic). This wavelength is favorable since silicon is highly absorbing in this spectral range while the front contact is highly transparent ensuring selective removal without severe damages. The area required for the series connection is no longer active for charge carrier generation and for thin-film silicon typically an area of 3-5% is lost (ratio ⁄ ). The additional laser processes are required for the fabrication of the presented device concept used similar patterning parameters as the P2 and P3 processes. For practical reasons the fillet to the front contact was 2 mm wide since manual patterning of the insulation epoxy was required (cf. Supplementary Figure 7).