The challenge to “Make solar energy affordable” is named first in the 14 grand challenges of the 21st Century presented by the National Academies. Success with this challenge could dramatically improve life for everyone. It is also the main target of the U.S. Department of Energy SUNSHOT Initiative to bring down the price of solar electricity to $0.06 per kilowatt-hour over its lifetime. Thin-film organic solar cells are well fit to reach this goal, since they are low-cost devices compared to crystalline silicon solar cells and they can be processed at low temperatures allowing shorter energy payback times. However, until very recently the power conversion efficiencies (PCE) of such solar cells were much lower than those of Si panels. However, recently, a novel solution-processed, low-cost and low-temperature thin film solar cell technology using perovskites as light absorbers has attracted huge attention, as they already outperform many other solar cell technologies now reaching power conversion efficiencies (PCE) of up to 20% in 2014. This PCE was achieved within 5 years of development of the organic-inorganic perovskite CH3NH3PbCl3-xIx which crystallizes in a ABX3 perovskite crystal structure. The perovskites show an absorption in the range from 400 to 850 nm due to their comparably small bandgap of 1.55 eV. This enables absorption of around 50% of the sunlight. In comparison to organic photovoltaics the hole and electron – forming the excited state, namely an exciton – are loosely bound (Wannier-type excitons) and therefore only a small amount of energy on the order of kbT is needed to separate the charge carriers. Consequently, a high open circuit voltage is possible. Recent studies revealed high charge carrier mobilities (in comparison to the traditional organic photovoltaic materials, such as P3HT and PCBM) and low recombination rates (well below Langevin recombination limit), both factors needed to achieve high efficiency solar cell. Moreover, when properly encapsulated, perovskite solar cells proved to be stable at least for several hundreds of hours. Overall, these outstanding physical properties combined with the possibility of solution processing make this material class extremely promising for next-generation photovoltaic devices.
We investigate mechanism of operation and stability of perovskite solar cells, namely:
- Electric charge transport, using various methods
- Optical properties, using ultra-fast spectroscopy
- Origin of defects in the perovskite film and their influence on overall performance
- Stability and lifetime of perovskite solar cells (origin of the hysteresis effect, effect of transport layers interfaces).