Hydrogen generated by conversion of sunlight into chemical energy is considered as a promising route to store solar energy on a large scale and offers a perspective to replace fossil carbon and hydrocarbons in the future. Presently, artificial leaf-type structures are under investigation to realize efficient and cheap energy conversion devices. A common feature of such structures is a membrane comprising light absorb­ing and catalytically active components immersed in an aqueous electrolyte and evolving hydrogen and oxygen at separate locations when illuminated by sunlight. From an economic and environmental point of view this device should consist of cheap, abundant and non-toxic elements.

To test such structures, we modified triple junction solar cells in superstrate configura­tion based on amorphous and microcrystalline silicon. The standard cell manufactured at HZB has an efficiency of 9.5%. However, without any modifications this cell is not stable in an electrolyte. By replacing the standard ZnO/Ag back contact by a Ti layer the efficiency of the PV cell drops to 5% due to a decreased specular reflectivity of titanium compared to silver, but the stability in a 0.5 M H₂SO₄ solution could be improved from a few minutes to more than 18 hours. Pt and RuO₂ nanoparticles were deposited as electrocatalysts on front and back contacts, respectively. A STH efficiency of 4.0% was achieved for this PV hybrid electrolyser [1]. We expect that further efficiency improvements will be possible by replacing the Ti layer by an electrochemically stable alternative metal of higher reflectivity.To lower the cost of the device, precious metal catalysts have to be replaced by alterna­tive electrocatalysts. In our system, we used MoS₂ as hydrogen evolving catalyst replacing the commonly used Pt. MoS₂ powders and thin films of different particle sizes and orientation, partially alloyed by nickel and cobalt, have been tested. Of special interest are carbon supported MoS₂ nanosheets using multi-walled carbon nanotube (MWCNT) of small diameters. By depositing a blend of MWCNT/MoS₂ and PEDOT:PSS on the backside of the solar cell, a STH efficiency of 3.7% was achieved. On the O₂-evolving side, RuO₂ was replaced by a calcium manganate electrode in order to demonstrate a stand-alone water splitting device, based entirely on earth-abundant elements. Since the MoS₂ / Ca_xMnO_y catalyst combination is only stable at pH 7, low electrolyte conductivity is presently restricting the efficiency of such a water splitting device to 1.5%.

[1]  D. Stellmach et al., in Materials and Processes for Energy, FORMATEX 2013, 880