Mechanically stacked and monolithically integrated perovskite/silicon tandems and the challenges for high efficiency

Colin Bailie^a, Michael McGehee^a, William Nguyen^a, Eric Hoke^a, Eric Johlin^b, Tonio Buonassisi^b, Austin Akey^b, Jonathan Mailoa^b

^aStanford University, Stanford, CA 94305, United States

^bMassachusetts Institute of Technology (MIT), Massachusetts Avenue, 77, Cambridge, United States

International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics 2015 (HOPV15)

Roma, Italy, 2015 May 11th - 13th

Organizer: Filippo De Angelis

Oral, Colin Bailie, presentation 098
Publication date: 5th February 2015

With the advent of high-bandgap perovskites, the opportunity now exists to make low-cost tandems with perovskites on top of silicon -- which has an ideal bandgap for the bottom cell in a double-junction tandem. We have prototyped a mechanically stacked tandem, achieving 17.9% certified efficiency. The top cell in this tandem consisted of a perovskite cell fabricated using a 2-step dip-conversion method and a silver nanowire mesh electrode to enable transparency. The top cell reached 77% transmission at 800nm, decreasing at longer wavelengths due to parasitic absorption in the doped spiro-OMeTAD layer and the fluorine-doped tin oxide layer. We developed a new method of depositing the silver nanowires onto the perovskite cell to achieve greater cell-to-cell performance consistency. We transfer a silver nanowire mesh originally sprayed onto a plastic film from the plastic to the top of the perovskite cell through mechanical force. A custom apparatus applies uniform air pressure across a diaphragm to press the nanowires from the plastic film into the spiro-OMeTAD layer. We have also prototyped a monolithically-integrated tandem on silicon, with the electrical connection between the two subcells enabled by band-to-band tunneling through carefully doped layers on the top of the silicon wafer. The perovskite cell was deposited using a 2-step dip-conversion method on top of the silicon wafer and a silver nanowire mesh was used as the transparent electrode. With a 1x1cm² aperture area, these monolithic tandems have achieved a V_OC over 1.6V and a steady-state efficiency of 13.7%. This tandem was current-limited by the perovskite top cell due to parasitic absorption in the doped spiro-OMeTAD layer and a lower IQE in the perovskite absorber when illuminated in this direction. From experience making perovskite/silicon tandems, we have identified key areas for improvement in these tandem architectures. We find that parasitic absorption of the spiro-OMeTAD, stability and increased V_OC of a perovskite with a 1.8eV bandgap, environmental stability of the perovskite top cell, quality and stability of the transparent electrode, and perovskite crystal quality are the primary challenges for the community in order to propel perovskite/silicon tandems into a high-efficiency (>25%) regime and reach commercial competitiveness.

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