Monolithic perovskite silicon tandem solar cells with high-bandgap perovskite absorber exceeding 1.8 V open-circuit voltage
Alexander J. Bett a, Patricia S.C. Schulze a, Kristina M. Winkler a, Özde Kabakli a, Martin Bivour a, Ludmila Cojocaru b, Ines Ketterer a, Laura E. Mundt a, Leonard Tutsch a, Martin Hermle a, Stefan W. Glunz a b, Jan Christoph Goldschmidt a
a Fraunhofer Institute for Solar Energy Systems ISE, Heidenhofstrasse 2, 79110 Freiburg, Germany
b Freiburg Center for Interactive Materials and Bioinspired Technologies (FIT), Laboratory for Photovoltaic Energy Conversion, Department of Sustainable Systems Engineering (INATECH), University of Freiburg, Georges-Köhler-Allee 105, 79110 Freiburg, Germany
Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV19)
Roma, Italy, 2019 May 12th - 15th
Organizers: Prashant Kamat, Filippo De Angelis and Aldo Di Carlo
Oral, Kristina M. Winkler, presentation 102
Publication date: 11th February 2019

Perovskite silicon tandem solar cells have the potential to overcome the efficiency limit of 29.4% [1] of single junction silicon solar cells by reduction of thermalisation losses. In particular, we aim for monolithic tandem devices as it requires a lower number of transversely conductive layers and allows for more facile module integration compared to 4-terminal devices.

We use an n-type heterojunction silicon bottom solar cell with a pyramidal rear side texture. On both sides an intrinsic amorphous silicon layer is deposited by plasma enhanced chemical vapour deposition (PECVD) followed by a p doped and n doped amorphous silicon layer on the front and rear side, respectively. This silicon solar cell exhibits an implied open-circuit voltage (VOC) of over 700 mV. A perovskite top solar cell with regular n-i-p architecture is connected to the bottom solar cell via an indium doped tin oxide (ITO) recombination layer. To prevent degradation of the bottom solar cell, a low-temperature process is deployed for the top cell. As an electron contact we use evaporated compact TiO2 and a UV-treated mesoporous TiO2 scaffold [2]. Subsequently, a passivation layer of PCBM and PMMA is spin coated [3] followed by deposition of the stable mixed cation mixed halide perovskite FA0.75Cs0.25Pb(I0.8Br0.2)3 with an optical bandgap of 1.7 eV, which is in the optimal range for monolithic silicon-based tandem devices [4]. The front contact consists of Spiro-OMeTAD and directly sputtered ITO (sheet resistance 44 Ohm/sq). As we use a soft ITO deposition process no additional buffer layer is needed. Finally, a MgF2 antireflection coating was evaporated. The thicknesses of ITO and MgF2 have been optimized in order to achieve highest transmission. The perovskite top solar cells reach VOC values of ~1150 mV.

Our tandem devices achieve very high VOC values of >1.8 V and power conversion efficiencies (PCE) over 20% with negligible hysteresis on a defined cell area of 0.25 cm². The champion device exhibits over 21% PCE measured under 1 sun illumination at fixed maximum power point voltage for 30 min in ambient air.

The remarkably high VOC values exceed the VOC of recent record efficiency devices [5]. The slightly lower PCE is due to the Spiro-OMeTAD hole transport layer: parasitic absorption and the inappropriate refractive index causing reflection losses limit the short-circuit current of our devices. To overcome this limitation, we investigate alternative hole transport materials.

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