Ultrafast Dynamics Group
Frédéric Laquai's Group

Perovskite Solar Cells

Perovskite Solar Cells



Perovskite Solar Cells

Thin-film solar cells, such as solid-state dye-sensitized solar cells, bulk heterojunction organic photovoltaics or solar cells based on amorphous silicon are low-cost devices compared to crystalline silicon solar cells and they can be processed at low temperatures allowing shorter energy payback times. Recently, a novel solution-processed, low-cost and low-temperature thin film solar cell technology using organometallic halide 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 22% in less than a decade.

The perovskites show an absorption in the range from 350 to 800 nm due to their comparably small bandgap around 1.6 eV. Moreover, perovskite absorber layers have high absorption coefficient in the order of 105 cm-1. 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. These outstanding physical properties combined with the possibility of low temperature solution processing make this material class a promising for next-generation photovoltaic devices.

Figure 1. (a) SEM micrograph of perovskite absorber layer (inset: cross-sectional image of device) and (b) J-V graph of perovskite solar cell fabricated by our group.

Interestingly, until now only lead-based perovskite structures show these remarkable properties and the exact reason is not well understood yet. In ultrafast dynamics group, we are mainly working on planar CH3NH3PbI3 perovskite solar cells and we could get more than 16 % efficiency in this device architecture. Enhancing the PCE in the planar devices is crucial to make this technology competitive with current photovoltaic technologies since the planar device architecture is more practical for large scale production, flexible devices, and tandem applications. Thus, one aim of our research is to investigate the charge transport and recombination processes in the perovskite absorber layer and perovskite/charge transport layer interface using transient absorption spectroscopy (TAS), time-resolved photoluminescence (TRPL) spectroscopy, etc. Moreover, we focus on excited state dynamics in perovskite solar cells by transient optical spectroscopy, to gain a better understanding of the underlying photophysics and to guide the development of novel perovskites.

Figure 2. Transient absorption spectra of perovskite absorber layer on glass.