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Absorption and Emission Processes in Organometal Trihalide Perovskites

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Absorption and Emission Processes in Organometal Trihalide Perovskites UNIVERSITÀ DEGLI STUDI DI CAGLIARI FACOLTÀ DI SCIENZE MATEMATICHE, FISICHE E NATURALI SCUOLA DI DOTTORATO IN FISICA XXVIII CICLO Absorption and Emission Processes in Organometal Trihalide Perovskites Supervisor: PhD Candidate: Prof. Michele Saba Michele Cadelano ANNO ACCADEMICO 2014/2015 Ad Anna e alla mia famiglia CONTENTS 1 Introduction 1 2 Optical spectroscopy 5 2.1 Introduction . .5 2.2 Light propagation in an optical medium . .5 2.2.1 Light absorption . .7 2.2.2 Rayleigh scattering . .8 2.3 Electronic bands in crystals . .9 2.4 Interband transitions in semiconductors . 11 2.4.1 Absorption in direct gap semiconductors . 12 2.5 Excitons . 14 2.6 Recombination in direct gap semiconductors . 17 2.6.1 Photoluminescence . 19 2.6.2 Amplified spontaneous emission . 20 3 Perovskites 23 3.1 Introduction . 23 3.2 Crystal structure . 23 3.3 Methylammonium lead trihalide perovskites . 24 3.3.1 Applications . 26 3.3.2 Sample fabrication . 28 4 Experimental techniques 31 4.1 Introduction . 31 4.2 Linear Absorption Spectroscopy . 31 4.2.1 UV-Vis absorption setup . 33 4.3 Photoluminescence Spectroscopy . 33 i 4.3.1 Ultrafast PL Spectroscopy setup . 35 4.3.2 PL Spectroscopy setup under cw pumping . 38 4.3.3 PL Spectroscopy setup under quasi-cw pumping . 38 5 Excitons vs free carriers 41 5.1 Introduction . 41 5.2 Radiative recombination processes . 42 5.3 Saha equation . 44 5.4 Quantum yield . 47 5.5 Analysis of the recombination processes . 49 5.6 Steady-state photoluminescence . 51 6 Exciton binding energy 55 6.1 Introduction . 55 6.2 Elliot’s theory of Wannier excitons . 56 6.2.1 Comparison with literature . 59 6.3 f-sum rule . 62 7 Optical amplification 67 7.1 Introduction . 67 7.2 ASE under fs excitation . 68 7.3 ASE under ns excitation . 69 7.4 Warming processes . 71 7.4.1 Rate equation model . 74 7.5 Comparison with nitride semiconductors . 79 8 Conclusion and outlook 81 A Radiation-matter Interaction 85 A.1 Einstein coefficients . 85 A.1.1 Absorption . 85 A.1.2 Stimulated emission . 86 A.1.3 Spontaneous emission . 86 A.2 Light dispersion . 86 A.2.1 Diffraction grating . 86 A.2.2 Condition for maxima . 87 ii A.2.3 Angular dispersion and resolving power . 88 A.2.4 Blazed grating . 88 A.2.5 Monochromator . 89 B Laser devices 91 B.1 Laser emission . 91 B.2 Pulsed lasers . 92 B.3 Nd:YAG and Nd:YLF lasers . 93 C Light detection 95 C.1 Quantum efficiency . 95 C.2 Responsivity . 95 C.3 p-n junction . 96 C.3.1 Photodetectors . 97 C.4 Microchannel plate . 97 C.5 Detectors . 98 C.5.1 Charge-Coupled Device (CCD) . 98 C.5.2 Streak camera . 99 C.5.3 Gated intensified CCD camera . 101 D Samples 103 D.1 Sample fabrication . 103 D.1.1 Spin coating . 103 D.1.2 Samples for absorption measurements . 103 D.1.3 Samples for optical amplification measurements . 105 D.2 Sample characterization . 106 D.2.1 X-ray diffraction . 106 D.2.2 Scanning probe microscopy . 107 Acknowledgements 111 References 115 iii iv 1|I NTRODUCTION 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, favor- able 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 [1-19]. Recently, scientists have devoted large efforts in studying perovskites as absorbers in solar cells, reaching energy con- version efficiencies even higher than 20%. However, in spite of such an intense research, fundamental aspects of the photophysics remain still elu- sive, generating lively debates among researchers from all over the world. [20-29]. One of the more interesting debates concerns the nature of the pho- toexcited species. Being organometal perovskites hybrid materials, the- oretically it is not clear if the excited states are dominated by bound or unbound electron-hole states. First publications showed perovskites ex- hibiting excitonic properties, feature typical of organic materials. On the contrary, recent optical spectroscopy reports converge to state that the ma- jority of band-edge optical excitations at room temperature are free carriers, like happens in inorganic semiconductors [6,22,30]. If photoexcitations re- sult 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 1 2 Introduction charges. Another debate concerns the physical reasons that support the preva- lence 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 pri- mary importance. However, exciton binding energies have been reported from less than 5 meV to over 50 meV [22,30-36] and it has even been sug- gested that ionic screening could decrease the exciton binding energy with temperature [33,34,37]. At the present, a consensus concerning the exciton binding energy is lacking. In addition to the potential use in photovoltaics, organometal perovskites could be employed also as active media in light emission devices. Different works reported amplified spontaneous emission (ASE) at injected carrier densities comparable to ones typical of organic semiconductors [17-19], 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 [17- 19,38-40], where different parasitic and warming processes are involved [19]. 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 op- tical 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 2 con- cerns the description of theoretical concepts useful to the reading, while Chapter 3 deals with a general description of perovskites and their poten- tial applications. Both the techniques and experimental setups with which we carried out optical spectroscopy experiments are described in Chap- ter 4. After, we will start discussing the nature of the photoexcitations of organometal trihalide perovskites in Chapter 5, where we will show that an electron-hole plasma exists in a wide excitation range, ranging from light intensities much smaller than 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 6, where we will 3 use a f-sum rule and the Elliot’s theory of Wannier excitons to study the absorption spectra at the band-edge. Finally, Chapter 7 concerns the study of optical amplification, where optical thermometry and rate equations will be used to estimate the magnitude of the warming processes establishing under cw regime. 4 Introduction 2|O PTICAL SPECTROSCOPY 2.1 Introduction The aim of this chapter is to introduce the basic concepts of optical spec- troscopy and discuss the processes involved in the interaction between light and materials. In details, we will describe the mechanisms concerning light absorption and emission, the electronic structure in crystals and the fea- tures that arise in absorption and emission spectra in direct-gap inorganic semiconductors. 2.2 Light propagation in an optical medium Considering a light beam of initial intensity I0 crossing an optical medium, some of the light will be reflected, part will propagate inside the material and part will be transmitted, as shown in Figure 2.1. Reflection is a process quantified by the reflectance R, which is defined as the ratio between the beam reflected intensity IR and the beam intensity I0 at the material surface. Transmission is quantified by the transmittance T, defined as the ratio between the beam transmitted intensity IT and I0. Fi- nally, the fraction of light that will be absorbed by the material is quantified by the absorbance A, defined by the ratio between IA and I0, where IA is the light intensity absorbed by the optical medium. Reflectance, transmittance and absorbance are connected by the following equation: R + T + A = 1 (2.1) In details, as the light beam propagates trough the material, the most common processes which can occur are refraction, absorption, luminescence 5 6 Optical spectroscopy Figure 2.1 | Light propagation in an optical medium. Refraction gives rise to a variation of the speed of light with a consequent bending of the progagating beam with respect to the initial direction, with no attenuation of intensity. Absorption of light result in a decrease of the beam intensity during propagation and can be accompanied by photoluminescence, which is light emission in all directions. Scattering gives rise to a redirection of light that results in a decrease of the beam intensity with propagation, as happens for absorption. and scattering. Refraction causes a reduction of the light velocity inside the material with respect to free space. The resulting beam will be bended at the interface between free space and the material, according to the Snell’s law, which defines the bending magnitude through the refraction index.
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