Report
Teaser, summary, work performed and final results
Periodic Reporting for period 1 - APPEL (Approaching efficiency limits of perovskite solar cells by overcoming non-radiative recombination losses)
Teaser
Metal halide perovskites are currently viewed as new “wonder materials†due to the combination of their outstanding optical and electronic properties with the ease of processing as compared to similar direct bandgap semiconductor such as GaAs. Despite the meteoric rise of...
Summary
Metal halide perovskites are currently viewed as new “wonder materials†due to the combination of their outstanding optical and electronic properties with the ease of processing as compared to similar direct bandgap semiconductor such as GaAs. Despite the meteoric rise of the power conversion efficiency of perovskite solar cells (up to 20% in less than 5 years) there is still a substantial potential of improvement towards the theoretical efficiency limits. During the first research phase, most of the effort has been devoted to the development of deposition processes to produce the best crystalline thin films. This project has the ambition to initiate the second phase, in which fundamental understanding of the recombination losses in the perovskite and at heterojunctions with charge extraction layers will make possible to generate devices with efficiencies approaching the full potential of these semiconductors. A perfect solar cell should also be an excellent emitter, since 100% of the absorbed photons must recombine radiatively. Therefore, the original approach APPEL is to target primarily light emission (photoluminescence and electroluminescence) to understand the factors governing the recombination losses in solar cells. From the understanding of the fundamental recombination mechanisms at the heterojunctions, I will demonstrate highly efficient devices with >23% power conversion efficiency. Moreover, efficient light-emitting diodes (LED) will be produced alongside photovoltaic devices. This work will set the foundation of the future rational optimisation of metal halide perovskite devices in the same way that optimisation of light emission in GaAs led to the advent of devices tackling the theoretical limits.
Work performed
In the early part of the project we have concentrated on the study of fundamental optical properties and the mechanisms responsible for recombination in perovskite single crystals. We unravelled the role played by reabsorption and carrier diffusion in the apparent properties of the material and could estimate the that the defect density in single crystal was found to be only an order of magnitude smaller than for thin films (10e15 cm-3 vs 10e16 cm-3). We further used this understanding to reveal the nature of the states introduced by Bi3+ in perovskites. We also built the required tools to be able to understand the recombination mechanisms not only in thin film deposited on inert substrates but in partial and full devices stacks. For this purpose, we built a photoluminescence efficiency set-up and built a calibrated spectral electroluminescence set-up and a photothermal deflection spectroscopy set-up. This work helped to establish accurate photoluminescence efficiency measurements as the preferred technique to estimate the radiative efficiency in full operating devices.
Crucially, we applied several surface passivation techniques and studied their effect on recombination and full devices. We formulated a completely new approach that will be protect by a patent application. Other aspects of passivation have been explored as for example the role quaternary ammonium halide salts for bromide-containing perovskites and also the use of small alkali metal ions (e.g. Na+, K+) in high performing photovoltaic devices.
Finally, we demonstrated the benefits of improved understanding of recombination processes as well as the fine tuning of the surface of the perovskites via passivation, by fabricating photovoltaic devices with power conversion efficiency above 20%. This work has already generated a relatively large number of peer-reviewed publications (6) in high impact journals, which will be followed by more publications that have been submitted (6) or are currently in preparation (7).
Final results
This work demonstrated the key role played by the radiative recombination in optoelectronic devices based on lead halide perovskite. By studying nearly-ideal single crystals it showed the intrinsic limits of such materials and proved that thin-films based on these materials can be further improved by removing defects from the surfaces. We also showed various innovative ways to passivate the surfaces and illustrated their usefulness in photovoltaic and light-emitting devices. These results have foremost an impact on the scientific community and put the focus on the importance to optimise radiative efficiency to push the performance of the devices. This line is now currently used by many research groups in the field. Moreover, the project has had an impact on the industrial players working with such materials, either established PV companies but also for a spin-off company which creation is related to this work. Finally, via the presence on social network and general press articles, the general public as well as political leaders have learnt about the existence of a promising technology for the next generation of photovoltaic modules beyond silicon.