Organic-inorganic perovskites are a class of solution-processed semiconductors holding promise for the realization of low-cost efficient solar cells and on-chip lasers. Despite the recent attention they have attracted, fundamental aspects of the photophysics underlying device operation still remain elusive. Here we use photoluminescence and transmission spectroscopy to show that photoexcitations give rise to a conducting plasma of unbound but Coulomb-correlated electron-hole pairs at all excitations of interest for light-energy conversion and stimulated optical amplification. The conductive nature of the photoexcited plasma has crucial consequences for perovskite-based devices: in solar cells, it ensures efficient charge separation and ambipolar transport while, concerning lasing, it provides a low threshold for light amplification and justifies a favourable outlook for the demonstration of an electrically driven laser. We find a significant trap density, whose cross-section for carrier capture is however low, yielding a minor impact on device performance.

Advances of optoelectronic devices based on methylammonium lead halide perovskites depend on understanding the role of excitons, whether it is marginal as in inorganic semiconductors, or crucial, like in organics. However, a consensus on the exciton binding energy and its temperature dependence is still lacking, even for widely studied methylammonium lead iodide and bromide materials (MAPbI3, MAPbBr3). Here we determine the exciton binding energy based on an f-sum rule for integrated UV-vis absorption spectra, circumventing the pitfalls of least-squares fitting procedures. In the temperature range 80-300 K, we find that the exciton binding energy in MAPbBr3 is EB = (60 ± 3) meV, independent of temperature; for MAPbI3, in the orthorhombic phase (below 140 K) EB = (34 ± 3) meV, while in the tetragonal phase the binding energy softens to 29 meV at 170 K and stays constant up to 300 K. Implications of binding energy values on solar cell and LED workings are discussed.

COMMUNICATIONRecent studies have shown amplifi ed spontaneous emission (ASE) under short pulse excitation conditions, with lasing threshold densities comparable to or even lower than those observed in state-of-the-art organic materials. [ 18,19,[30][31][32] Short pulse excitation is a very favorable regime for light amplifi cation, as carrier densities well above the threshold required for lasing can be easily injected. Yet light amplifi cation disappears just a few picoseconds after excitation, a transient regime that is far away from the working conditions of interest for applications. How long light amplifi cation can last in perovskite materials is the open question we address in this Communication.Here, we demonstrate ASE sustained over transients two orders of magnitude longer than the excited state lifetime. Through optical spectroscopy, we measure threshold densities for ASE as a function of the temperature of the environment and the duration of the exciting laser pulse. Particularly, we employ an optical thermometry technique to track the dynamics of the temperature of the electron-hole plasma and identify the runaway heating mechanism limiting the maximum achievable duration of light amplifi cation. We then discuss the conditions needed for true continuous wave operation of light amplifi cation.Light emission in methylammonium lead iodide (MAPbI 3 ) and methylammonium lead bromide (MAPbBr 3 ) thin fi lms has fi rstly been analyzed under 130 fs pulsed laser excitation (392 nm in wavelength), using a streak camera to detect timeresolved photoluminescence and a cooled charge-coupled device (CCD) camera for the time-integrated spectra. The resulting photoluminescence decays, shown in Figure 1 a, had characteristic decay times much longer than the pulse duration, few nanoseconds for MAPbBr 3 , tens of nanoseconds for MAPbI 3 , meaning that such an excitation regime can be considered as impulsive. ASE was demonstrated by a sharp peak (Figure 1 b,c) appearing in the low energy side of the emission spectrum for both fi lms once the excitation laser fl uence reached a threshold value; such a value turned out to be 26 µJ cm −2 per pulse for MAPbI 3 , a factor of two lower than for MAPbBr 3 (54 µJ cm −2 per pulse). The corresponding average excited population densities, as calculated by averaging the laser fl uence times the fi lm absorption coeffi cient over the fi lm thickness, were 4 × 10 18 cm −3 and 7 × 10 18 cm −3 for MAPbI 3 and MAPbBr 3 , respectively (see absorption coeffi cients in the Supporting Information paragraph). Films realized from different solution batches and with different age showed variations in ASE threshold fl uence, mainly due to the different optical losses occurring as a result of different morphology. All samples were therefore stored in vacuum and typically measured within the

Multilayered epitaxial nanofibers are exemplary model systems for the study of exciton dynamics and lasing in organic materials because of their well-defined morphology, high luminescence efficiencies, and color tunability. We use temperature-dependent continuous wave and picosecond photoluminescence (PL) spectroscopy to quantify exciton diffusion and resonance-energy transfer (RET) processes in multilayered nanofibers consisting of alternating layers of para-hexaphenyl (p6P) and α-sexithiophene (6T) serving as exciton donor and acceptor material, respectively. The high probability for RET processes is confirmed by quantum chemical calculations. The activation energy for exciton diffusion in p6P is determined to be as low as 19 meV, proving p6P epitaxial layers also as a very suitable donor material system. The small activation energy for exciton diffusion of the p6P donor material, the inferred high p6P-to-6T resonance-energy-transfer efficiency, and the observed weak PL temperature dependence of the 6T acceptor material together result in an exceptionally high optical emission performance of this all-organic material system, thus making it well suited, for example, for organic light-emitting devices.