Oct 03, 2017

A team of researchers from The Hebrew University of Jerusalem in Israel, led by Prof. Lioz Etgar, investigated the optical and physical properties of bromide quasi 2D perovskites synthesized using different barrier molecules. The team reports on the high power conversion efficiency (PCE) and high open circuit voltage (Voc) of bromide-based quasi 2D perovskite solar cells.

High efficiencies of quasi 2D perovskite solar cells image

The various bromide quasi 2D perovskites were introduced into two PV cell configurations (with and without HTM). The use of the quasi 2D perovskite as an absorbing layer in PSCs reportedly yields improved efficiencies and open circuit voltage as compared to 3D PSCs. Different barriers in the quasi 2D structures have been shown to affect the photovoltaic performance; the cells' performance is reduced when increasing barrier length. However, the perovskite's hydrophilic character is suppressed with an increase in the chain length of the barrier molecule.

A high open circuit voltage of 1.37 V and close to 8% efficiency were observed for full bromide quasi 2D perovskite cells without the HTM with BA as the barrier, and an efficiency of 9.5% (one of the highest values reported) was observed for the same barrier with the HTM. All three barriers exhibited better PV performance and open circuit voltage than the 3D perovskite.

It was found that the energy gap and the exciton binding energy extracted using the Elliot formula do not change as a result of the barrier molecule. DFT and spin–orbit coupling calculations showed the decrease in electrical conductivity when the length of the barrier molecule is increased. Moreover, it was shown that when the charges are localized on the barrier molecule, better Voc is achieved, whereas when the charges are delocalized, better current density can be achieved. Finally, contact angle and stability measurements under extreme conditions were performed. The quasi 2D perovskite showed better stability than the 3D perovskite. The contact angle measurements clearly showed the hydrophobic nature of the quasi 2D perovskite compared with the 3D perovskite.

Bromide quasi 2D perovskites were synthesized using various long organic barriers (e.g., benzyl ammonium, BA; phenylethyl ammonium, PEA; and propyl phenylammonium, PPA). The influence of different barrier molecules on the quasi 2D perovskite's properties was studied using absorbance, X-ray diffraction, and scanning electron microscopy. No change was observed in the exciton binding energy as a result of changing the barrier molecule. Density functional theory (DFT) with spin–orbit coupling calculations showed that in the case of BA, holes are delocalized over the whole molecule, whereas for PEA and PPA, they are delocalized more at the phenyl ring.

This factor influences the electrical conductivity of holes, which is highest for BA in comparison with the other barriers. In the case of electrons, the energy onset for the nonzero conductivity is lowest for BA. Both calculations support the better PV performance observed for the quasi 2D perovskite based on BA as the barrier. Finally, using contact angle measurements, it was shown that the quasi 2D perovskite is more hydrophobic than the 3D perovskite. Stability measurements showed that cells based on the quasi 2D perovskite are more stable than cells based on the 3D perovskite.