Christiane Becker studied physics at University of Karlsruhe and University of Heidelberg, from where she obtained her diploma (MSc) in physics in 2002 with distinction. After graduation as a PhD from University of Karlsruhe in 2006 and a PostDoc stay at Karlsruhe School of Optics and Photonics, she joined Helmholtz-Zentrum Berlin in 2007. Since 2012 she is head of the Young Investigator Group “Nanostructured Silicon for Photovoltaics and Photonics”. She received an appointment as professor in experimental physics at University of Applied Sciences HTW Berlin in 2014. In 2017 she was visiting scientist at NanoLund in Lund University, Sweden. Her research activities lie primarily with the development of silicon nanostructures for photovoltaic and photonic applications.
Title and Abstract of the Speech:
Light management in thin-film silicon and perovskite-silicon tandem solar cells
Christiane Becker1, David Eisenhauer1, Klaus Jäger1, Phillip Manley1, Sven Burger2, Daniel Amkreutz1, Steve Albrecht1, Bernd Rech1
1Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Albert-Einstein-Str. 16, 12489 Berlin, Germany
2Zuse Institute Berlin, Takustr. 7, 14195 Berlin, Germany
Light management is a key element for boosting the efficiency of photovoltaic devices via minimizing optical losses. Tailored nano- and microstructures, which are implemented into the solar cell, have proven to exhibit excellent broadband antireflective as well as light trapping properties. The main technological challenge is the design of nanostructured interfaces enabling efficient light trapping without negatively affecting the electronic absorber material quality or interfaces in the solar cell. Here, we present light management strategies based on nanoimprint-lithography for two different emerging solar cell technologies:
(1) Liquid phase crystallization (LPC) of 5 – 40 μm thick silicon films being grown and crystallized directly on a glass substrate is a promising technology avoiding current challenges of wafer based solar cells, namely high material losses and handling issues particularly arising at very low wafer thicknesses. Scanning a line-shaped laser beam across silicon films on glass leads to the formation of large-grained polycrystalline material comparable with multi-crystalline silicon wafers, which enables solar cells with open-circuit voltages exceeding 650 mV. However, the highest demonstrated efficiency of state-of-the-art LPC silicon solar cells is still only about 14% due to incomplete light absorption, mainly caused by reflection losses at the planar front interfaces of the device. Therefore, we developed a smooth anti-reflective three-dimensional (SMART) texture based on titanium and silicon oxides, leading to a morphologically flat but optically rough layer system. The SMART texture was found to excellently overcome the trade-off between optical gain and texture-induced electronic losses with open-circuit voltage values up to 650 mV and an increased short-circuit current density compared to optimized planar devices, which were simultaneously processed. This paves the way towards efficiencies around 18%.
(2) Perovskite–silicon tandem solar cells are currently one of the most investigated concepts for overcoming the theoretical limit of the power conversion efficiency of silicon solar cells. For a state-of-the art monolithic device design with planar interfaces, strong reflection and parasitic absorption limit device performance even after optimization of the layer thicknesses. As the realization of perovskite top cells on silicon bottom cells with pyramidal texture is challenging, we investigated shallow two-dimensional sinusoidal nanotextures. In a first attempt, we experimentally realized such sinusoidal structured perovskite solar cells for application in monolithic tandems on nanoimprinted substrates. Numerical simulations reveal that sinusoidal nanotexturing of the perovskite top cell allows current matching to be reached, an increase of current density by more than 1.6 mA/cm2 in each sub cell and a power conversion efficiency of 31.8% at the standard perovskite bandgap.