Absorption and emission processes in organometal trihalide perovskites

Among the various solution processed semiconductors, organometal halide perovskites represent a noteworthy class of materials thanks to their unique combination of optoelectronic properties: efficient charge transport, favorable emission properties, strong light absorption and optical gap tunability stand for the key features of these novel semiconductors and make them appealing for the realization of a new generation of low-cost solar cells and optical emitters. Recently, scientists have devoted large efforts to study perovskites as absorbers in solar cells, reaching energy conversion efficiencies even higher than 20%. However, in spite of such an intense research, fundamental aspects of the photophysics remain still elusive, generating lively debates among researchers from all over the world. One of the more interesting debates concerns the nature of the photoexcited species. Being organometal perovskites hybrid materials, theoretically it is not clear if the excited states are dominated by bound or unbound electron-hole states. Initial publications on optical properties assumed that perovskites exhibit excitonic properties, feature typical of organic materials. On the contrary, recent optical spectroscopy reports converge to state that the majority of band-edge optical excitations at room temperature are free carriers, as it happens in inorganic semiconductors. Whether photoexcitations result in bound or unbound electron-hole pairs is not a problem of minor importance, because the design of optoelectronic devices strongly depends on it. As an example, if light absorption gives rise to bound electron-hole states, a heterojunction is necessary to split charges and produce a current flow in a solar cell, while such architecture is not necessary if electron-hole states are unbound, since only an electric field is needed to separate charges. Another debate concerns the physical reasons that support the prevalence of free carriers over bound electron-hole pairs. A small value of the exciton binding energy could justify such finding, as thermal excitation at room temperature could ionize excitons. Consequently, a precise and reliable determination of the exciton binding energy results to be of primary importance. However, exciton binding energies have been reported from less than 5 meV to over 50 meV and it has even been suggested that ionic screening could decrease the exciton binding energy with temperature. At the present, a consensus concerning the exciton binding energy is lacking. In addition to the potential use in photovoltaics (PV), organometal perovskites could be employed also as active media in light emitting devices. Different works reported amplified spontaneous emission (ASE) at injected carrier densities comparable to ones typical of organic semiconductors, making perovskites promising for the realization of a new generation of lasers. However, such reports demonstrated ASE only under impulsive excitation, a regime far from the continuous operation of real lasers, where different parasitic and warming processes are involved. Hence, further investigations are needed to state if perovskites can be actually used in future laser devices. Herein, we will discuss in details experimental results obtained by optical spectroscopy measurements, providing both a deep understanding of the photophysical properties of organometal perovskites and the possible solution to the unsolved problems we mentioned before. Chapter 1 deals with a general description of perovskites and their potential applications, while both the techniques and experimental setups with which we carried out optical spectroscopy experiments are described in Chapter 2. After, we will start discussing the nature of the photoexcitations of organometal trihalide perovskites in Chapter 3, where we will show that an electron-hole plasma exists in a wide excitation range, ranging from light intensities much smaller than those typical of solar illumination to those typical to obtain light amplification. The issue concerning the determination of an accurate exciton binding energy is addressed in Chapter 4, where we will apply the Elliot’s theory of Wannier excitons to study the absorption spectra at the band-edge. Finally, Chapter 5 concerns the study of optical amplification, where optical thermometry and rate equations will be used to estimate the magnitude of the warming processes that are established under cw regime