It is known that the efficiency of a solar cell material can be correlated to its ability to emit light^[1]. The process of light absorption is essentially the inverse process of light absorption, therefore a material with a strong absorption coefficient should also have strong photoluminescence, with the limiting factor being the density of non radiative recombination pathways in the material. This makes the study of the photoluminescence a useful tool in gauging a material’s viability as a solar cell, as a good photoluminescence quantum yield (PLQY) then indicates a low density of traps or other loss pathways. Methylammonium Lead Halide Perovskites have recently emerged as an excellent new class of absorber for solar cells, and it can be shown that altering processing conditions to optimise the PLQY leads to a concomitant increase in overall device efficiency^[2].We perform an in depth study into the photoluminescence properties of lead halide perovskites with the aim of identifying new doping or processing techniques that could improve device performance. 

We first study the photoluminescence of CH₃NH₃PbI₃ in the ultrafast regime using optical gating techniques. We observe a clear rise time in the photoluminescence at different wavelengths, corresponding to the thermalisation of hot charges to the band edge – something not observed in organic semiconductors as emission comes from the exciton rather than free charge. We also use steady state photoluminescence spectra and PLQY to perform a study on the effects of different doping or preparation techniques on the perovskite. Perovskites are known to have an intensity-dependant PLQY, consistent with emission from a gas of free electrons and holes rather than bound electron-hole pairs, making a study of PLQY vs intensity a powerful method for observing loss mechanisms.

If sufficiently excited, CH₃NH₃PbI₃  can undergo amplified spontaneous emission under optical pumping. By solution deposition onto a UV nanoimprinted polymer grating, and optimising film quality to minimise scattering losses, we are able to create a distributed feedback perovskite laser with 4uJ/cm2 threshold under femtosecond pumping. The laser shows remarkable stablility. With a simple polymer encapsulation it can operate at repetition rates of 20kHz, and has a lasing half-life of 10⁸ pulses.

[1] Tvingstedt, K. et al. Radiative efficiency of lead iodide based perovskite solar cells. Sci. Rep. 4, 6071; DOI:10.1038/srep06071 (2014).

[2]Noel, N et al. Enhanced Photoluminescence and Solar Cell Performance via Lewis Base Passivation of Organic_Inorganic Lead Halide Perovskites, ACS Nano, 8, 10, 9815–9821, (2014)