Energy harvesting

Technical objectives:• Lead-free perovskite solar cell
• Energy efficiency: > 10%,
• Energy generation: > 6 μW/cm2 (indoor illumination conditions, 400 lux)
• Output voltage: 4 V

Metal-halide perovskite solar cells have shown a dramatic increase of  efficiency up to 25.2% within a few years, becoming the fastest developing solar technology in history. However, the photo-active perovskites are typically based on lead (Pb), that is water soluble and toxic, which infers environmental/health concerns. Lead-free alternatives have been introduced, but their success is still limited. For Sn2+-based perovskites the oxidation of Sn2+to Sn4+ is a critical source of instability, and barriers to protect the perovskite solar cells (PSCs) against environmental influences are required.

Sn-based perovskites have a lower band-gap compared to their Pb-based counterparts, and the Voc’s (open circuit voltage) of Sn-I based cells are typically below 0.6 V. On the contrary, Sn-free representatives typically have a relatively large band-gap. Surprisingly, the Voc’s of PSCs based on the latter materials are likewise comparatively low, which in part can be attributed to a poor interface engineering of contact materials, which do not properly match the electronic levels of the perovskites. Furthermore, the benign defect tolerance commonly quoted for Pb-based perovskites may not likewise apply to their Pb-free counterparts.

To tackle the above issues, we will follow two routes in parallel:

  1. Use Sn-based perovskite systems, which provide a high efficiency, such as FaSnI3 with dedicated additives, like guanidinium iodide, SnF2, and ethylenediammonium-iodide.
  2. Explore the opportunities of interface engineering in Pb- and Sn-free systems.

For all active systems we apply a dedicated recrystallization process by thermal imprint (see Figure 1). This way, we aim to achieve larger perovskite crystals, an overall improved crystal quality with less defects, and reduced non-radiative recombination. Larger crystals are also expected to provide improved stability. Depending on the Voc of the sub-cells (~ 1.4-0.5 V) a series connection of 3-8 cells is required to achieve 4 V operating voltage as needed by the TFC. To provide 4 V, the following concepts are considered.

  1. Connection of cells laterally, e.g. four 2 x 0.5 cm2 cells on 2 x 2 cm2 (for a single cell voltage of 1 V up to 4 V mini-module).
  2. Monolithic vertical integration of a multi-junction cell (added voltage of the sub-cells).
  3. A combination of (1)+(2).
Figure 1. a) Schematic of the thermal imprint process using a flat silicon stamp to recrystallize the as‐deposited cesium lead bromide layer. b,c) Atomic force microscopy of layers before (b) and after (c) thermal imprint. The layer thickness is 120 nm. d) Photograph of pristine and pressed layers on silicon substrate.