Doctor of Philosophy

dissertationIn the last decade, the organic/inorganic hybrid perovskites have emerged as one of the most promising classes of contenders for low-cost solar cells, owing to extraordinary power conversion efficiencies of more than 22%, and convenient solution fabrication processes. Moreover, the hybrid perovskite systems have found applications in many other fields, such as light emitting diodes, lasers, field effect transistors, and spintronics. In this dissertation, by using a variety of transient and steady-state optical measurements, we have investigated the photoexcitations in different organic/inorganic hybrid perovskite systems, including three- (3D) and two- (2D) dimensional thin films and single crystals. In 3D hybrid perovskites, we have identified the excitons/photocarriers duality nature in the lead iodine perovskite at room temperature (RT), which is consistent with its small exciton binding energy (Eb) less than kBT for T=300K. For lead bromide and lead cloride perovskites with larger Eb , excitons have been verified as the the primary photoexcitations. These results show that the branching ratio between the photogenerated excitons and free carriers is determined by the exciton binding energy in the hybrid perovskites. Due to the anistropic teragonal structure phase, we have also observed the polarization memory (POM) dynamics of both excitons and free carriers in the lead iodine perovskite, from which we have estimated the long exciton diffusion length at 150nm. In addition, the effects of nano-morphologies on photoluminescence and magnetic properites in perovskites have also been studied. In 2D hybrid perovskites, we have found a strong photoinduced absorption (PA) from the primary exciton state to the upper branch of the Rashba-splitting band, from which we have obtained a giant Rashba-splitting in this compound, with the Rashba-splitting energy, ER= 40/pm 5 meV and Rashba-splitting parameter αR=1.6/pm 0.1 eV·Å that is among the highest values reported so far. We have extended our measurements to lead bromide perovskite single crystals, which show much lower trap state densities and longer exciton lifetimes in the bulk than in the film samples. Our observation of a fast photobleaching (PB) and a corresponding rise of PA in the near-surface regime reveals the process of the exciton recombination on the surface and diffusion to the interior of the bulk

Temperature-dependent modulated reflectance of InAs/InGaAs/GaAs quantum dots-in-a-well infrared photodetectors

We present a photoreflectance (PR) study of multi-layer InAs quantum dot (QD) photodetector structures, incorporating InGaAs overgrown layers and positioned asymmetrically within GaAs/AlAs quantum wells (QWs). The influence of the back-surface reflections on the QD PR spectra is explained and a temperature-dependent photomodulation mechanism is discussed. The optical interband transitions originating from the QD/QW ground- and excited-states are revealed and their temperature behaviour in the range of 3–300 K is established. In particular, we estimated the activation energy (∼320 meV) of exciton thermal escape from QD to QW bound-states at high temperatures. Furthermore, from the obtained Varshni parameters, a strain-driven partial decomposition of the InGaAs cap layer is determined

Doctor of Philosophy

dissertationThe present work reports studies of the ultrafast photoexcitations in various pristine n-conjugated polymers as well as compounds of polythiophene/fullerene blends, which act as the active layer of donor/acceptor in organic photovoltaic applications. The main technique used is the ultrafast (∼150 fs) transient photomodulation (PM) spectroscopy in the range of 0.25 to 2.5 eV using two different laser systems. In addition, two-photon-absorption and electroabsorption have also been complementary used. In organic photovoltaic studies, two different donor polymers namely, Regio-Regular-poly(3-hexylthiophene) (RR-P3HT) that forms lamellae, and Regio-Random-poly(3-hexylthiophene) (RRa-P3HT) that forms lamellae with lesser extent have been compared. The transient PM measurement of the most efficient RR-P3HT/fullerene blend shows that the decay of exciton does not result in the generation of polarons in the donor and acceptor materials, as assumed by the present model of charge dissociation in photovoltaic devices. On the contrary, the decay of exciton fits very well to the build-up of charge-transfer (CT) state in the fullerene phase, which indicates the migration of the photoexcited exciton in the polymer phase to the fullerene nano-domains. The transient PM measurement of RRa-P3HT/fullerene blend, which does not form phase-separated nano-domains, shows the formation of a CT state at the interface following by ultrafast geminate recombination. The transient PM measurement of poly(phenylene-vinylene) (PPV) derivatives show that in 2-methoxy-5-(2'-ethylhexyloxy) PPV (MEH-PPV) film there are two kinds of primary photoexcitations, namely, intrachain exciton and excimer, but only intra-chain exciton in other PPV derivative polymers. Furthermore the high-pressure study of MEHPPV film shows two kinds of polymer chain orders: isolated-chains and closely packed-chains. The high pressure mainly affects photoexcited excimers in the closely packed-chains. In contrast there is no pressure effect on the photogenerated intrachain excitons in the isolated-chains. The other π-conjugated polymers investigated in this dissertation are platinum(Pt)-containing conjugated polymers. The different parity excited states of these Pt-polymers are studied by electroabsorption and two-photon-absorption spectroscopies. These results show that, even after the incorporation of Pt atoms into the backbone of conjugated polymer, the behavior of the excited states is similar to regular π-conjugated polymers

Photoreflectance and surface photovoltage spectroscopy of beryllium-doped GaAs/AlAs multiple quantum wells

We present an optical study of beryllium delta-doped GaAs/AlAs multiple quantum well (QW) structures designed for sensing terahertz (THz) radiation. Photoreflectance (PR), surface photovoltage (SPV), and wavelength-modulated differential surface photovoltage (DSPV) spectra were measured in the structures with QW widths ranging from 3 to 20 nm and doping densities from 2×10(10) to 5×10(12) cm(–2) at room temperature. The PR spectra displayed Franz-Keldysh oscillations which enabled an estimation of the electric-field strength of ~20 kV/cm at the sample surface. By analyzing the SPV spectra we have determined that a buried interface rather than the sample surface mainly governs the SPV effect. The DSPV spectra revealed sharp features associated with excitonic interband transitions which energies were found to be in a good agreement with those calculated including the nonparabolicity of the energy bands. The dependence of the exciton linewidth broadening on the well width and the quantum index has shown that an average half monolayer well width fluctuations is mostly predominant broadening mechanism for QWs thinner than 10 nm. The line broadening in lightly doped QWs, thicker than 10 nm, was found to arise from thermal broadening with the contribution from Stark broadening due to random electric fields of the ionized impurities in the structures. We finally consider the possible influence of strong internal electric fields, QW imperfections, and doping level on the operation of THz sensors fabricated using the studied structures. © 2005 American Institute of Physic

Nonlinear Optical Spectroscopy of Solid/Solid Interfaces

Three-wave mixing (3WM) spectroscopy is an exciting and relatively unexplored probe of buried solid interfaces. It possesses long penetration depths characteristic of most optical methods and intrinsic interface specificity characteristic of second-order optical processes. In this thesis we present frequency domain measurements of the ZnSe/GaAs(OOl) heterojunction by second-harmonic (SH) and sum-frequency (SF) generation. Our experiments reveal an unusual three-wave mixing resonance that arises as a result of virtual transitions between an interfacial quantum well state and the ZnSe valence band. The interfacial quantum well was brought about by interdiffusion of Zn (Ga) into GaAs (ZnSe) during sample growth. The observation introduces a new class of nonlinear optical phenomena at interfaces that can provide useful information about band profiles, diffusion and defects along the boundary of two semiconductors. We have found that this interfacial SH resonance is sensitive to a variety of structural phenomena. In essence any process that modifies the band profile near the junction will affect the strength of the resonance. We have observed the variation of interface SH spectra with respect to lattice strain relaxation and to surface reconstruction of the buried GaAs. In addition, using a newly developed photomodulationSHG (PSHG) technique, we have exploited this sensitivity to determine the nature and relative density of interface charge traps as a function of substrate surface reconstruction. The PSHG method was also used to study free charge trapping mechanisms at ZnSe/GaAs(OOl) heterointerfaces. Our measurements determined that the interfacial trap-centers are mainly hole-traps with lifetime of 35 sec. In the course of carrying out these experiments we also observed interference in reflected second harmonic generation from two adjoined nonlinear slabs. A theory for the phenomena was presented and was used to understand our experimental results with ZnSe/GaAs(OOl) heterostructures. This interference phenomena was introduced as a new methodology to measure the second-order susceptibility of thin overlayer materials

Charge-transfer and other excitonic state in conjugated polymer : fullerene blends-implication in photovoltaics

"May 2014."Dissertation Supervisor: Dr. Suchi Guha.Includes vita.Over the last few decades there has been tremendous progress in organic photovoltaics (OPVs), with efficiencies reaching over 10%. Still, many factors including the origin and the dynamics of charge carrier involved are debatable. New and sensitive techniques are constantly being devised to identify the origin of free charges. At the same time, a lot of research has also been devoted to synthesize low bandgap material such that its absorption spectra overlap with that of the solar spectrum. The most important hindrance in organic semiconductors is the formation of bound electron-hole (exciton) charge pair upon photoexcitation. Additional energy is required to dissociate the bound pair to generate free charges for photovoltaic application. The most popular and efficient way to dissociate excitons is to fabricate a bulk heterojunction solar cell, which comprises of a blend of at least two polymers: one donor and the other acceptor. It is very well established that the presence of fullerene (acceptor) helps in transfer of the negative charges from the donor polymer to fullerene, making the exciton slightly less bound. The nanometer scale islands further help in migration of charges. A crucial aspect of our studies has been evaluating the role of various excitonic states such as charge-transfer and triplet excitonic states in device efficiencies. The focus of this work was on diketopyrrolopyrrole (DPP)- based donor-acceptor (D-A) type conjugated copolymers which have low bandgap energies and have been known to show high efficiency in organic photovoltaics. These copolymers have D-A unit present in the same chain, which lowers the optical bandgap of the material. Variation of either the donor or the acceptor fraction offers an option to tune the optical bandgap by using the same D-A chromophores. The D-A configuration also results in the separation of positive and negative charges within the same polymeric chain, which is the intramolecular charge-transfer excitonic state. We analyze the intramolecular charge-transfeIncludes bibliographical references (pages 125-136)

Doctor of Philosophy

dissertationIn this work, we used the pump-probe photomodulation (PM) spectroscopy technique to measure the transient PM spectrum and decay kinetics in various n - conjugated polymers (PCPs) films and blends. Using two ultrafast laser systems, we covered a broad spectral range from 0.25 - 2.5 eV in the time domain from 200 fs to 1 ns with 120 fs time resolution. We also used continuous wave (CW) photomodulation spectroscopy, photoluminescence (PL), electro-absorption, and doping-induced absorption to study the photoexcitations and other optical properties of PCPs and guest/host blends. In particular, we studied two different types of Poly(thienylenevinylene) polymer derivatives. One polymer type is the ordered region-regular (RR) and regio-random (RRa) - PTV in which the dark exciton, 2Ag, is the lowest excited state. In these polymers, the photoexcited exciton shows very fast decay kinetics due to the internal conversion to the dark exciton, which results in weak PL emission; thus, these two polymers are nonluminescent. The other PTV derivative is the imide - PTV which is more luminescent due to the proximity of 1Bu and 2Ag states, that results in longer decay kinetics and a difference between the calculated value of the QEPL (9%) and the measured one (1%). We also demonstrate transient strain spectroscopy in RR - PTV thin films, where the ultrafast energy release associated with the exciton decay gives rise to substantial static and dynamic strains in the film that dramatically influences the film's transient PM response. We also study the photophysics of poly(dioctyloxy) phenylenevinylene polymer with different isotopes, where we substituted hydrogen (H - polymer) by deuterium (D -polymer), and 12 C by 13C isotopes. From the transient decay kinetics measurements, we found that the exciton recombination in DOO - PPV consists of two processes. These are: intrinsic monomolecular, and exciton-exciton annihilation (bimolecular). In the D - polymer, different probe frequencies of the main exciton photoinduced absorption band (PA1) show a variety of decay kinetics that result from various photoexcitations that contribute to the spectrum. Comparing the transient PM spectrum at 1 ns time delay to the CW PM shows the formation of triplet excitons, which is possible due to singlet fission of mAg (at 2.9 eV) into two triplets (2 X 1.4 eV). In the last part of this thesis, we summarize our studies of organic light emitting diodes (OLED) devices based on a host/guest blend of polyfluorene polymer that is mixed with various percentages of Ir(btp)2acac molecules. In this mixture, the PFO (host) shows blue fluorescence, whereas the Ir-complex (guest) has red phosphorescence emission; thus, OLED based on this mixture can serve as a "white OLED." Since the PFO emission spectrum perfectly matches the absorption band of the Ir-complex, it induces an efficient energy transfer from the PFO host to the Ir-complex guest molecules, which we tried to time resolve by the transient PM method

Optical Multistability in High-Q Nonlinear Photonic Crystal Nanoresonators

Photonic crystals are prime candidates for photonic integrated circuit applications, for instance in the field of optical telecommunications and optical computing. In this thesis we experimentally investigate the optical nonlinear behavior of single and coupled nanoresonators on a waveguide in a photonic crystal membrane. Photonic crystal resonators typically have small mode volume and high quality factor, which optimizes the optical nonlinearity crucial for active photonic devices. Our photonic crystal is designed for near-infrared light (~1550nm). Thermo-optical nonlinearity is the dominating nonlinearity in these InGaP photonic crystal waveguides. We measure the thermo-optical timescale in the system and find that the decay time of a mode is related to its mode profile. Additionally, we present a nonlinear mode mapping technique to measure the mode profile. We use an out-of-plane blue pump beam to spatially scan the surface of the photonic crystal and probe the mode. The pump light is absorbed and serves as a local heat source. Nonlinear effects, caused by free carrier absorption of infrared input light, significantly improve the resolution with respect to linear mode map techniques. This provides a non-invasive far-field imaging technique. Lastly, we look at two coupled nanocavities that give rise to optical multistability in the nonlinear regime. We investigate the phase space of the system by exciting the system with a puls of the out-of-plane pump beam. It turns out the pump pulse triggers a state change that remains stable after the pump is turned off. In this way we, find different energy branches of the multistable system, where the position of the pump pulse determines the thermo-optical state the system stabilizes in. Using this effect we demonstrate a programmable multistable all-optical switch, where we walk through the multistable phase space of the system, addressing several thermo-optical stable states in the system by lowering or raising the internal energy of the hybridized cavities

Doctor of Philosophy

dissertationIn this work, we used the pump-probe photomodulation (PM) spectroscopy technique to measure the transient PM spectrum and decay kinetics in various π - conjugated polymers (PCPs) films and blends with appropriate molecular acceptors. Using two ultrafast laser systems, we covered a broad spectral range from 0.25 - 2.5 eV in the time domain from 100 fs to 1 ns with 150 fs time resolution. We also used continuous wave (CW) photomodulation spectroscopy, photoluminescence, electro-absorption, doping-induced absorption, and x-ray diffraction to study the excitations and other optical properties of PCPs and polymer donor-fullerene acceptor blends. In P3HT/PCBM blend with maximum domain separation we found that although the intrachain excitons in the polymer domains decay within ~ 10 ps, no charge polarons are generated at their expense. Instead, there is a build-up of charge-transfer (CT) excitons at the donor (D) - acceptor (A) interfaces that may dissociate into separated polarons in the D and A domains at a later time. Our results elucidate the charge photogeneration mechanism in polymer/fullerene blends, and unravel the important role of the binding energy in generating free charge polarons. We also studied the photophysics of a low band gap polymer, namely poly-thienophene-benzodithiophene seven (PTB7) film and its blend with acceptor [6,6] phenyl C71 butyric acid methyl ester [PC71BM]. In the CW PM spectrum of PTB7/PC71BM blend, clear signatures of polarons are observed. Whereas PA bands of triplet excitons and trapped polarons are observed in pristine PTB7 film. In the transient ultrafast PA spectrum of PTB7/PC71BM blend, surprisingly, we found singlet exciton, charge transfer exciton and a polaron band that are generated simultaneously; this is different from the transient PM spectrum of P3HT/PCBM blend. We also focused on the photophysics of DOO-PPV with different isotopes and their blends with PCBM. The transient ultrafast PA spectra of all isotope films are dominated by a singlet exciton. Similar results were observed in the isotope/PCBM blends. As in P3HT/PCBM blend, we do not observe the charge transfer exciton and polaron band immediately after photoexcitation, but they show up in the PM spectrum at a later time