Zooming in on perovskites sheds light on solar cell efficiency boost
First invented in 2009, perovskite solar cells are improving at an astonishing rate, having already surpassed the 20 percent efficiency mark in the lab. A new finding at the Lawrence Berkeley National Laboratory (LBNL) could now reveal their full potential, approaching their theoretical efficiency limit of 31 percent – far better than commercially available panels.
Perovskites are a family of materials that share a specific crystalline structure, from which they derive properties like ferroelectricity and superconductivity. Perovskite-based solar cells are taking the industry by storm because they are cheap and simple to make, and their performance is, after a short few years of development, already matching that of long established silicon-based cells.
This quick rise to prominence is even more impressive when considering that scientists still don't fully understand the cause of the crystals' high efficiency.
Researchers Sibel Leblebici and her team set out to shed more light on this by analyzing the performance of single perovskite crystals, and their findings point to new ways of raising the cells' efficiency by as much as 50 percent.
The surface of the active (photon-absorbing) layer of a perovskite cell features grains about 200 nanometers in length. Each grain sports multi-angled facets that collectively behave like billions of tiny solar cells, all connected in parallel.
Leblebici and team carefully mapped the performance of the facets, using a new conductive atomic force microscopy technique that left the material undamaged (where the microscope tip approaches the surface at each pixel, takes a measurement, withdraws and moves to the next pixel).
Surprisingly, the scientists found great variability between the performance of each facet, even adjacent ones. About five percent of the measured area was non-functioning, while in 35 percent of the area the efficiency was very high, approaching the theoretical maximum.
The scientists believe that the performance of single facets could depend chiefly on two factors – the orientation of the facets and the presence of tiny defects close to other charge-carrying layers of the cell. Controlling the direction in which the crystals are grown and removing defects through chemical passivation could therefore bring dramatic results.
"If the material can be synthesized so that only very efficient facets develop, then we could see a big jump in the efficiency of perovskite solar cells, possibly approaching [the theoretical limit of] 31 percent," said Leblebici.
If some of the other problems that have plagued perovskite solar cells so far – chief among these their real-world durability – are addressed, this advance could lead to dramatically cheaper and more efficient solar cells and LEDs.
The scientists are now exploring the correlation between facet heterogeneity and power conversion efficiency in high-performance films.
The study appears in a recent edition of the journal Nature Energy.