Defect tolerance in “hot carriers” for solar cell materials
Controlling defects in semiconductors is critical to achieving efficient performance in applications such as solar cells. Conventionally, semiconductors have to be made defect-free, but lead-halide perovskite materials have been shown to be capable of tolerating defects. This means that efficient performance can be achieved using simple, cost-effective fabrication methods.
Currently, however, we only understand defect tolerance for electrons and holes at the edges of the bandgap (so-called “cold carriers”). We don’t understand much about whether “hot carriers” (electrons and holes with energy above the band-edge) can also exhibit defect tolerance. Being able to harness these hot carriers could allow solar cells to surpass their theoretical limits in efficiency by converting some of their energy to electrical energy, instead of losing it all as heat.
Now, in a study published this week in Nature Communications, a joint team from the Universities of Oxford, Cambridge and Imperial College London have revealed that the defect tolerance of cold carriers can be extended to hot carriers, and developed a physical model to account for their kinetics.
Lead-halide perovskite materials have attracted significant attention over the past decade as an emerging material for solar cells. The power conversion efficiency (PCE) of lead iodide-based perovskite photovoltaics has now reached a certified value of 26.7% under 1-sun illumination, (the maximum radiative PCE limit is around 30%).
An important reason for this is defect tolerance, which reduces irreversible energy losses due to defects, despite them being present in high concentrations. To date, the discussion of defect tolerance has primarily focussed on cold carriers, i.e., charge-carrier recombination from the band-edges. The defect tolerance of hot carriers in halide perovskites is not currently known. This is because the community has largely been focussed on understanding the influence of optical and acoustic phonons on hot carrier cooling processes, with the assumption that defects have minimal impact. There are also contradictory results in the literature, because there has been no systematic investigation into how the defects influence hot carrier cooling.
A new study published this week in Nature Communications addresses this important gap in the field by investigating how intentionally introduced traps in halide perovskites affect hot carrier cooling. The leading authors of this paper, Dr. Junzhi Ye, Dr. Navendu Mondal and Prof. Robert L.Z. Hoye have discovered that:
- The defect tolerance of hot carriers is strongly correlated to that of cold carriers. That is, hot carriers in lead-halide perovskites are not universally defect tolerant but depend on the energetic position of the defect state in the bandgap. Shallow traps are benign for both cold and hot carriers, whereas deeper traps enhance non-radiative recombination, causing the faster hot carrier cooling, along with reducing the effects of the hot phonon bottleneck and Auger reheating;
- Rather than an indirect trapping process via the formation of cold carriers, their work shows that hot carriers are directly captured by traps. They show this through state-of-the-art intraband specific three-pulse transient absorption spectroscopy, in which a near-infrared “push” pulse is used to re-excite the cold-carriers generated by the pump-laser, allowing them to exclusively monitor the intraband relaxation of hot carriers without any interference from any other effects. Unlike traditional approaches, this technique naturally allows them to study all of the materials under the same experimental conditions (irrespective of their bandgaps, defect densities).
- By developing a new kinetic model for hot carrier cooling, they quantified the relationship between trap density, trap saturation, and the hot carrier cooling rate. This model allows them to verify that hot carrier trapping is a direct process and not via a cold-state intermediate, and implies that hot carrier lifetimes can be governed by the complex interplay of carrier-carrier, carrier-phonon and carrier-defect interactions This model is generally applicable and can be used to study hot carrier cooling in systems going beyond the halide perovskites.
You can read more about this study in Nature Communications. https://doi.org/10.1038/s41467-024-52377-4