Investigation of Metal and Metal Oxide Intermediate Layers in Organic Multi-Junction Solar Cells
Sebastian F. Hoefler a, Thomas Rath a, Gregor Trimmel a
a Graz University of Technology, Stremayrgasse 9, Graz, 8010, Austria
International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV16)
Swansea, United Kingdom, 2016 June 29th - July 1st
Organizers: James Durrant, Henry Snaith and David Worsley
Poster, Gregor Trimmel, 304
Publication date: 28th March 2016

Organic bulk-heterojunction solar cells comprising a blend of electron-donors (conjugated polymers) and electron-acceptors (fullerene derivatives) are promising alternative, renewable energy sources offering the possibility of a low-cost, solution-process fabrication method. To enhance solar cell performance and power conversion efficiencies, transmission and thermalization losses can be reduced by stacking sub-cells with complementary absorption behavior to obtain multi-junction solar cells. 

In this contribution, organic bulk-heterojunction-type solar cells were fabricated in normal device architecture with a thermally deposited molybdenum(VI) oxide (MoO3) hole-transport layer, a low-temperature solution-processed polymer:fullerene derivative blend as active layer and a cathode comprising a titanium interfacial layer and copper top electrode. Blends of various small and wide bandgap conjugated polymers with fullerene derivatives including benzothiadiazole-based small bandgap polymers with carbazole (PCDTBT), sila-fluorene (PSiF-DBT) and cyclopenta[2,1-b:3,4-b’]dithiophene (PCPDTBT) moieties, and the polyfluorene-based wide bandgap polymer F8T2, mixed with fullerene derivatives ([60]PCBM, [70]PCBM, ICBA) were evaluated with regard to their performance in organic solar cells. Bulk-heterojunction solar cells were optimized with regard to the polymer:fullerene derivative combination and donor:acceptor ratio.

The focus was on the investigation of PEDOT:PSS free multijungcion solar cells using thermally deposited intermediate layers based on main group and transition metals (aluminum, titanium, copper, silver, gold) and metal oxides (titania, zinc oxide, molybdenum(VI) oxide) as electron-transport material combined with MoO3 serving as hole-transport material in tandem solar cells. In addition, recombination layers were modified via additional metal (calcium, titanium) and alkali-metal compound (lithium fluoride, cesium carbonate) interfacial layers. Organic multi-junction bulk-heterojunction-type solar cells were fabrication in normal device architecture with a MoO3 hole-transport layer and a titanium/copper top electrode by stacking sub-cells in series via aforementioned intermediate layer combinations. Solar cells were characterized electronically with regard to their performance in J-V measurements and optically via UV/VIS spectroscopy.



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