Three lactone-based rigid semiconducting polymers were designed to overcome major limitations in the development of n-type organic thermoelectrics, namely electrical conductivity and air stability. Experimental and theoretical investigations demonstrated that increasing the lactone group density by increasing the benzene content from 0 % benzene (P-0), to 50 % (P-50), and 75 % (P-75) resulted in progressively larger electron affinities (up to 4.37 eV), suggesting a more favorable doping process, when employing (N-DMBI) as the dopant. Larger polaron delocalization was also evident, due to the more planarized conformation, which is proposed to lead to a lower hopping energy barrier. As a consequence, the electrical conductivity increased by three orders of magnitude, to achieve values of up to 12 S cm and Power factors of 13.2 mu Wm(-1) K-2 were thereby enabled. These findings present new insights into material design guidelines for the future development of air stable n-type organic thermoelectrics.

Electron spin dephasing and relaxation due to hyperfine interaction with nuclear spins is studied in an InAs/GaAs quantum dot ensemble as a function of temperature up to 85 K, in an applied longitudinal magnetic field. The extent of hyperfineinduced dephasing is found to decrease, whereas dynamic nuclear polarization increases with increasing temperature. We attribute both effects to an accelerating electron spin relaxation through phonon-assisted electron-nuclear spin flip-flops driven by hyperfine interactions, which could become the dominating contribution to electron spin depolarization at high temperatures.

We demonstrate a general approach via spin engineering that is capable of not only deactivating defect-mediated efficient non-radiative carrier recombination channels in a semiconductor that are harmful to photonic and photovoltaic device performance, but also adding new room-temperature (RT) spin functionalities that are desirable for future spintronics and spin-photonics but so far unachievable otherwise. This approach exploits the Pauli Exclusion Principle that prohibits occupation of a non-degenerate defect level by two spin-parallel electrons, thereby providing spin blockade of carrier recombination via the defect level. The success of the approach is demonstrated in the dilute nitride of Ga(In)NAs, which holds promises for low-cost, highly efficient lasers for fiber-optic communications as well as for multi-band and multi-junction solar cell applications. First we identify that Ga_i self-interstitials and their complexes are the most common grown-in defects found in Ga(In)NAs grown by both molecular beam epitaxy (MBE) and metalorganic chemical vapour deposition (MOCVD). They provide a dominant non-radiative shunt path for non-equilibrium carriers, leading to low efficiencies of light-emitting and photon-charge carrier conversion. Spin blockade is shown to lead to a giant enhancement by up to 800% in light emission intensity at RT.Furthermore we show that via spin engineering these seemingly harmful defects can be turned into advantages by adding unconventional defect-enabled spin functionalities that are highly effective at RT, including some of the fundamental building blocks essential for future spintronics. We demonstrate efficient defect-engineered spin filtering in Ga(In)NAs, which is capable of generating a record-high degree (> 40%) of electron spin polarization at RT [Nature Materials 8, 198 (2009), Phys. Rev. B 89, 195412 (2014)]. We also provide the first experimental demonstration of an efficient RT spin amplifier based on defect engineered Ga(In)NAs with a spin gain up to 2700% [Adv. Materials 25, 738 (2013)]. Such a spin amplifier is shown to be capable of amplifying a fast-modulating input spin signal while truthfully maintaining its time variation of the spin-encoded information [7]. By taking advantage of the spin amplification effect, we show that Ga(In)NAs can be employed as efficient RT spin detectors, with spin detection efficiency well exceeding 100% [8,9]. By combining the spin-filtering effect and hyperfine coupling, we further achieve the first realization of RT nuclear spin hyperpolarization in semiconductors via conduction electrons [Nature Communications. 4, 1751 (2013)], relevant to nuclear spin qubits. We believe that such defect-enabled spin functionalities could potentially provide an attractive, alternative solution to the current and important issues on RT spin injection, spin amplification and spin detection in semiconductors for future spintronics.

Fine-structure splitting (FSS) of excitons in semiconductor nanostructures is a key parameter that has significant implications in photon entanglement and polarization conversion between electron spins and photons, relevant to quantum information technology and spintronics. Here, we investigate exciton FSS in self-organized lateral InAs/GaAs quantum-dot molecular structures (QMSs) including laterally aligned double quantum dots (DQDs), quantum-dot clusters (QCs), and quantum rings (QRs), by employing polarization-resolved microphotoluminescence (μPL) spectroscopy. We find a clear trend in FSS between the studied QMSs depending on their geometric arrangements, from a large FSS in the DQDs to a smaller FSS in the QCs and QRs. This trend is accompanied by a corresponding difference in the optical polarization directions of the excitons between these QMSs, namely, the bright-exciton lines are linearly polarized preferably along or perpendicular to the [11̅0] crystallographic axis in the DQDs that also defines the alignment direction of the two constituting QDs, whereas in the QCs and QRs, the polarization directions are randomly oriented. We attribute the observed trend in the FSS to a significant reduction of the asymmetry in the lateral confinement potential of the excitons in the QRs and QCs as compared with the DQDs, as a result of a compensation between the effects of lateral shape anisotropy and piezoelectric field. Our work demonstrates that FSS strongly depends on the geometric arrangements of the QMSs, which effectively tune the degree of the compensation effects and are capable of reducing FSS even in a strained QD system to a limit similar to strain-free QDs. This approach provides a pathway in obtaining high-symmetry quantum emitters desirable for realizing photon entanglement and spintronic devices based on such nanostructures, utilizing an uninterrupted epitaxial growth procedure without special requirements for lattice-matched materials combinations, specific substrate orientations, and nanolithography.

Fine-structure splitting (FSS) of excitons in semiconductor nanostructures is a key parameter that has significant implications in photon entanglement and polarization conversion between electron spins and photons, relevant to quantum information technology and spintronics. Here, we investigate exciton FSS in self-organized InAs/GaAs quantum molecular structures (QMSs) including laterally-aligned double quantum dots (DQDs), quantum-dot clusters (QCs) and quantum rings (QRs), by employing polarization-resolved micro-photoluminescence spectroscopy. We find a clear trend in FSS between the studied QMSs depending on their geometric arrangements, from a large FSS in the DQDs to a smaller FSS in the QCs and QRs with an overall higher geometric symmetry. This trend is accompanied by a corresponding difference in the optical polarization directions of the excitons between these QMSs, namely, the bright-exciton lines are linearly polarized preferably along or perpendicular to the [11̅0] crystallographic axis in the DQDs that also defines the alignment of the two constituting QDs, whereas in the QCs and QRs the polarization directions are randomly oriented. We attribute the observed trends in the FSS to a significant reduction of the anisotropic strain field in the high symmetry QCRs and QCs as compared with the low-symmetry  DQDs. Our work demonstrates that FSS can be effectively controlled by geometric engineering of the QMSs, capable of reducing FSS even in a strained QD system to a limit similar to strain-free QDs. This approach provides a new pathway in obtaining high-symmetry quantum emitters desirable for realizing photon entanglement and spintronic devices based on such nanostructures, without special requirements for lattice-matched materials combinations, specific substrate orientations and nanolithography.

Engineering a low singlet-triplet energy gap (Delta E-ST) is necessary for efficient reverse intersystem crossing (rISC) in delayed fluorescence (DF) organic semiconductors but results in a small radiative rate that limits performance in LEDs. Here, we study a model DF material, BF2, that exhibits a strong optical absorption (absorption coefficient = 3.8 x 10(5) cm(-1)) and a relatively large Delta E-ST of 0.2 eV. In isolated BF2 molecules, intramolecular rISC is slow (delayed lifetime = 260 mu s), but in aggregated films, BF2 generates intermolecular charge transfer (inter-CT) states on picosecond timescales. In contrast to the microsecond intramolecular rISC that is promoted by spin-orbit interactions in most isolated DF molecules, photoluminescence-detected magnetic resonance shows that these inter-CT states undergo rISC mediated by hyperfine interactions on a similar to 24 ns timescale and have an average electron-hole separation of >= 1.5 nm. Transfer back to the emissive singlet exciton then enables efficient DF and LED operation. Thus, access to these inter-CT states, which is possible even at low BF2 doping concentrations of 4 wt%, resolves the conflicting requirements of fast radiative emission and low Delta E-ST in organic DF emitters.

In this work, we developed four porphyrin-based small molecular electron acceptors for non-fullerene organic solar cells, in which different side groups attached to the porphyrin core were selected in order to achieve optimized performance. The molecules contain porphyrin as the core, perylene bisimides as end groups, and the ethynyl unit as the linker. Four side groups, from 2,6-di(dodecyloxy)phenyl to (2-ethylhexyl)thiophen-2-yl, pentadecan-7-yl, and 3,5-di(dodecyloxy)phenyl unit, were applied into the electron acceptors. The new molecules exhibit broad absorption spectra from 300 to 900 nm and high molar extinction coefficients. The molecules as electron acceptors were applied into organic solar cells, showing increased power conversion efficiencies from 1.84 to 5.34%. We employed several techniques, including photoluminescence spectra, electroluminescence spectra, atomic force microscopy, and grazing-incidence wide-angle X-ray to probe the blends to find the effects of the side groups on the photovoltaic properties. We found that the electron acceptors with 2,6-di(dodecyloxy)phenyl units show high-lying frontier energy levels, good crystalline properties, and low nonradiative recombination loss, resulting in possible large phase separation and low energy loss, which is responsible for the low performance. Our results provide a detailed study about the side groups of non-fullerene materials, demonstrating that porphyrin can be used to design electron acceptors toward near-infrared absorption.

Fano resonance is a fundamental physical process that strongly affects the electronic transport, optical, and vibronic properties of matter. Here, we provide the first experimental demonstration of its profound effect on spin properties in semiconductor nanostructures. We show that electron spin generation in InAs/GaAs quantum-dot structures is completely quenched upon spin injection from adjacent InGaAs wetting layers at the Fano resonance due to coupling of light-hole excitons and the heavy-hole continuum of the interband optical transitions, mediated by an anisotropic exchange interaction. Using a master equation approach, we show that such quenching of spin generation is robust and independent of Fano parameters. This work therefore identifies spin-dependent Fano resonance as a universal spin loss channel in quantum-dot systems with an inherent symmetry-breaking effect.

The ultimate goal for singlet fission is that each photo-excited singlet exciton, S1, will result in two triplet excitons with unity yield. However, the singlet fission is now recognized to be complicated, involving bright/dark excited states of different spin multiplicity. Identifying the role of such states is vital to optimize singlet fission yield but difficult due to their elusive spectral signature. Here, we develop an experimental protocol based on a refined magneto-optical probe to access the fast time evolution of various excited states. In diphenylhexatriene crystal, the S1 is found to undergo two competing processes?to form one of the two dark triplet pair intermediates having different exchange energies or to form a bright state, Sx, exhibiting excimer-like delayed photoluminescence. Our result provides a clear picture of a competition event in singlet fission, which is beneficial for the development and tailoring of singlet fission materials with high yield.

We review our recent results from optical spin orientation studies of In(Ga)As/GaAs quantum dots (QD) and QD molecular structures (QMSs), which shed light on some critical issues in spin injection and spin detection in these semiconductor nanostructures.

Semiconductor quantum dot (QD) structures are considered as promising building block for spintronic applications with the advantage of prolonged spin relaxation time owing to 0D character of confined carriers or excitons. However, feasible application is haunted by severe spin injection loss from its adjacent barrier layers and its mechanism is still not fully understood. Here, we show that exciton spin injection in self-assembled InAs/GaAs QD molecular structures (QMSs) is dominated by localized excitons confined within the QD-like regions of the wetting layer (WL) and GaAs barrier layer surrounding QD structures. The origin of spin injection loss is attribute to finite anisotropic exchange interaction (AEI) of the localized excitons subjected to asymmetric confinement potential in the injection layers. As a result, the AEI of the injected excitons and, thus, the spin injection efficiency is determined to be correlated with the overall geometric symmetry of QMSs, which hold strong influence on the confinement potential of the localized excitons in the surrounding barrier layers. Our results shed light on the microscopic origin of the spin injection loss in QD structures. More importantly, they offer a useful guideline to significantly improve spin injection efficiency by optimizing the lateral arrangement of QMSs and overcome a major challenge in the QD based spintronic device applications.

We investigate ensemble electron spin dephasing in self-assembled InGaAs/GaAs quantum dots (QDs) of different lateral sizes by employing optical Hanle measurements. Using low excitation power, we are able to obtain a spin dephasing time T-2* (in the order of ns) of the resident electron after recombination of negative trions in the QDs. We show that T-2* is determined by the hyperfine field arising from the frozen fluctuation of nuclear spins, which scales with the size of QDs following the Merkulov-Efros-Rosen model. This scaling no longer holds in large QDs, most likely due to a breakdown in the lateral electron confinement. (C) 2015 AIP Publishing LLC.

Dynamic nuclear polarization (DNP) is well recognized as being important in spintronics and quantum-information processing. DNP gives rise to high nuclear spin polarization that not only can prolong electron-spin lifetime by generating an Overhauser field (OHF), but also has fertilized the idea to explore nuclear spin qubits. In strained quantum-dot structures (QDSs), a nuclear spin is coupled to a strain field via its quadrupole moment. It has been shown that such nuclear quadrupole interaction (NQI) can be used to achieve appreciable DNP and hence electron-spin polarization. Here, we uncover magneto-optical anomalies from a series of laterally arranged (In,Ga)As QDSs that arise from the NQI and DNP in these nanostructures. We find that the principal axis of NQI in symmetry-lowered QDSs significantly deviates from the growth direction, resulting in tilting of OHF with an angle exceeding 37 degrees. The resulting transverse component of OHF is probed with respect to the crystallographic orientations and its influence on the DNP and ensemble spin dephasing is analyzed. We show that a high-symmetry electronic confinement potential for excitons does not guarantee a high-symmetry NQI for nuclei within the same nano-object, thereby calling for correlated optimization in the symmetry of the electronic confinement potential and that of the nuclear spin bath. Our results underline the role of oblique NQI in electron-spin decoherence and depolarization, which has so far largely been overlooked. This work thus sheds light on design rules for engineering the electronic and spin landscape of QDSs for better performance of DNP desirable for applications in spintronics and quantum computation.

Semiconductor quantum-dot (QD) structures are promising for spintronic applications owing to strong quenching of spin relaxation processes promoted by carrier and excitons motions. Unfortunately, spin injection efficiency in such nanostructures remains very low and the exact physical mechanism for the spin loss is still not fully understood. Here, we show that exciton spin injection in self-assembled InAs/GaAs QDs and quantum-dot molecular structures (QMSs) is dominated by localized excitons confined within the QD-like regions of the wetting layer (WL) and GaAs barrier layer immediately surrounding QDs and QMSs that in fact lack the commonly believed 2D and 3D character with an extended wavefunction. We identify the microscopic origin of the observed severe spin loss during spin injection as being due to a sizable anisotropic exchange interaction (AEI) of the localized excitons in the WL and GaAs barrier layer, which has so far been overlooked. We find that the AEI of the injected excitons and thus the efficiency of the spin injection processes are correlated with the overall geometric symmetry of the QMSs, as the latter largely defines the anisotropy of the confinement potential of the localized excitons in the surrounding WL and GaAs barrier. These results pave the way for a better understanding of spin injection processes and the microscopic origin of spin loss in QD structures, which in turn provides a useful guideline to significantly improve spin injection efficiency by optimizing the lateral arrangement of the QMSs thereby overcoming a major bottleneck in spintronic device applications utilizing semiconductor QDs.

Water-based conductive inks are vital for the sustainable manufacturing and widespread adoption of organic electronic devices. Traditional methods to produce waterborne conductive polymers involve modifying their backbone with hydrophilic side chains or using surfactants to form and stabilize aqueous nanoparticle dispersions. However, these chemical approaches are not always feasible and can lead to poor material/device performance. Here, we demonstrate that ground-state electron transfer (GSET) between donor and acceptor polymers allows the processing of water-insoluble polymers from water. This approach enables macromolecular charge-transfer salts with 10,000x higher electrical conductivities than pristine polymers, low work function, and excellent thermal/solvent stability. These waterborne conductive films have technological implications for realizing high-performance organic solar cells, with efficiency and stability superior to conventional metal oxide electron transport layers, and organic electrochemical neurons with biorealistic firing frequency. Our findings demonstrate that GSET offers a promising avenue to develop water-based conductive inks for various applications in organic electronics. Chemical approaches to improve aqueous dispersions of conjugated polymers are limited by the feasibility of modifying the backbone or lead to poor performance. Here, Liu et al. show that ground-state electron transfer in donor:acceptor blends aids aqueous dispersion, for high conductivity and solubility.

The interface between electron-donating (D) and electron-accepting (A) materials in organic photovoltaic (OPV) devices is commonly probed by charge-transfer (CT) electroluminescence (EL) measurements to estimate the CT energy, which critically relates to device open-circuit voltage. It is generally assumed that during CT-EL injected charges recombine at close-to-equilibrium energies in their respective density of states (DOS). Here, we explicitly quantify that CT-EL instead originates from higher-energy DOS site distributions significantly above DOS equilibrium energies. To demonstrate this, we have developed a quantitative and experimentally calibrated model for CT-EL at organic D/A heterointerfaces, which simultaneously accounts for the charge transport physics in an energetically disordered DOS and the Franck-Condon broadening. The 0-0 CT-EL transition lineshape is numerically calculated using measured energetic disorder values as input to 3-dimensional kinetic Monte Carlo simulations. We account for vibrational CT-EL overtones by selectively measuring the dominant vibrational phonon-mode energy governing CT luminescence at the D/A interface using fluorescence line-narrowing spectroscopy. Our model numerically reproduces the measured CT-EL spectra and their bias dependence and reveals the higher-lying manifold of DOS sites responsible for CT-EL. Lowest-energy CT states are situated similar to 180 to 570 meV below the 0-0 CT-EL transition, enabling photogenerated carrier thermalization to these low-lying DOS sites when the OPV device is operated as a solar cell rather than as a light-emitting diode. Nonequilibrium site distribution rationalizes the experimentally observed weak current-density dependence of CT-EL and poses fundamental questions on reciprocity relations relating light emission to photovoltaic action and regarding minimal attainable photovoltaic energy conversion losses in OPV devices.

We provide direct evidence for a spin-active V4+ defect center, likely in the form of a VO2+ complex, predominantly introduced in single crystals of vanadium-doped Cs2NaInCl6 halide double perovskites grown by the solution-processed hydrothermal method. The defect has C-4v point group symmetry, exhibiting an electron paramagnetic resonance (EPR) spectrum arising from an effective electron spin of S = 1/2 and a nuclear spin of I = 7/2 (corresponding to V-51 with nearly 100% natural abundance). The determined electron g-factor and hyperfine parameter values are g(perpendicular to)= 1.973, g(parallel to) = 1.945, A(perpendicular to) = 180 MHz, and A(parallel to) = 504 MHz, with the principal axis z along a < 001 > crystallographic axis. The controlled growth of V-doped Cs2NaInCl6 in an oxygen-free environment is shown to suppress the V4+ EPR signal. The defect model is suggested to have a VOCl5 octahedral coordination, where one of the nearest-neighbor Cl- of V is replaced by O2-, with octahedral compression along the V-O axis. This VO complex formation competes with the isolated V3+ substitution of In3+, which in turn provides a means for the charge-state tuning of V ions. This finding calls for a better understanding and control of defect formation in solution-grown halide double perovskites, which is critical for optimizing and tailoring material design for solution-processable optoelectronics and spintronics.

Solution-processable semiconductors with antiferromagnetic (AFM) order are attractive for future spintronics and information storage technology. Halide perovskites containing magnetic ions have emerged as multifunctional materials, demonstrating a cross-link between structural, optical, electrical, and magnetic properties. However, stable optoelectronic halide perovskites that are antiferromagnetic remain sparse, and the critical design rules to optimize magnetic coupling still must be developed. Here, we combine the complementary magnetometry and electron-spin-resonance experiments, together with first-principles calculations to study the antiferromagnetic coupling in stable Cs-2(Ag:Na)FeCl6 bulk semiconductor alloys grown by the hydrothermal method. We show the importance of nonmagnetic monovalence ions at the B-I site (Na/Ag) in facilitating the superexchange interaction via orbital hybridization, offering the tunability of the Curie-Weiss parameters between -27 and -210 K, with a potential to promote magnetic frustration via alloying the nonmagnetic B-I site (Ag:Na ratio). Combining our experimental evidence with first-principles calculations, we draw a cohesive picture of the material design for B-site-ordered antiferromagnetic halide double perovskites.

Spintronics holds great potential for next-generation high-speed and low-power consumption information technology. Recently, lead halide perovskites (LHPs), which have gained great success in optoelectronics, also show interesting magnetic properties. However, the spin-related properties in LHPs originate from the spin-orbit coupling of Pb, limiting further development of these materials in spintronics. Here, we demonstrate a new generation of halide perovskites, by alloying magnetic elements into optoelectronic double perovskites, which provide rich chemical and structural diversities to host different magnetic elements. In our iron-alloyed double perovskite, Cs2Ag(Bi:Fe)Br-6, Fe3+ replaces Bi3+ and forms FeBr6 clusters that homogenously distribute throughout the double perovskite crystals. We observe a strong temperature-dependent magnetic response at temperatures below 30 K, which is tentatively attributed to a weak ferromagnetic or antiferromagnetic response from localized regions. We anticipate that this work will stimulate future efforts in exploring this simple yet efficient approach to develop new spintronic materials based on lead-free double perovskites.

Heterostructured two-dimensional colloidal nanoplatelets are a class of material that has attracted great interest for optoelectronic applications due to their high photoluminescence yield, atomically tunable thickness, and ultralow lasing thresholds. Of particular interest are laterally heterostructured core-crown nanoplatelets with a type-II band alignment, where the in-plane spatial separation of carriers leads to indirect (or charge transfer) excitons with long lifetimes and bright, highly Stokes shifted emission. Despite this, little is known about the nature of the lowest energy exciton states responsible for emission in these materials. Here, using polarization-controlled, steady-state, and time-resolved photoluminescence measurements, at temperatures down to 1.6 K and magnetic fields up to 30 T, we study the exciton fine structure and spin dynamics of archetypal type-II CdSe/CdTe core-crown nanoplatelets. Complemented by theoretical modeling and zero-field quantum beat measurements, we find the bright-exciton fine structure consists of two linearly polarized states with a fine structure splitting similar to 50 mu eV and an indirect exciton Lande g-factor of 0.7. In addition, we show the exciton spin lifetime to be in the microsecond range with an unusual B-3 magnetic field dependence. The discovery of linearly polarized exciton states and emission highlights the potential for use of such materials in display and imaging applications without polarization filters. Furthermore, the small exciton fine structure splitting and a long spin lifetime are fundamental advantages when envisaging CdSe/CdTe nanoplatelets as elementary bricks for the next generation of quantum devices, particularly given their ease of fabrication.

Semiconductor spintronics is one of the most interesting research fields that exploits both charge and spin properties for future photonics and electronic devices. Among many challenges of using spin in semiconductors, efficient generation of electron spin polarization at room temperature (RT) remains difficult. Recently, a new approach using defect-mediated spin filtering effect, employing -interstitial defects in Ga(In)NAs alloys, has been shown to turn the material into an efficient spin-polarized source capable of generating >40% conduction electron spin polarization at RT without an application of external fields. In order to fully explore the defectengineered spin functionalities, a better understanding and control of the spin filtering effects is required. This thesis work thus aims to advance our understanding, in terms of both physical and material insights, of the recently discovered spin filtering defects in Ga(In)NAs alloys. We have focused on the important issues of optimization and applications of the spin filtering effects.

To improve spin filtering efficiency, important material and defect parameters must be addressed. Therefore, in Papers I–III formation of the  defects in Ga(In)NAs alloys has been examined under different growth and post-growth treatment conditions, as well as in different structures. We found that the  defects were the dominant and important nonradiative recombination centers in Ga(In)NAs epilayers and GaNAs/GaAs multiple quantum wells, independent of growth conditions and post-growth annealing. However, by varying growth and post-growth conditions, up to four configurations of the  defects, exhibiting different hyperfine  interaction (HFI) strengths between defect electron and nuclear (e-n) spins, have been found. This difference was attributed to different interstitial sites and/or complexes of  . Further studiesfocused on the effect of post-growth hydrogen (H) irradiation on the spin filtering effect. Beside the roles of H passivation of N resulting in bandgap reopening of the alloys, H treatment was shown to lead to complete quenching of the spin filtering effect, accompanied by strong suppression in the concentrations of the  defects. We concluded that the observed effect was due to the passivation of the  defects by H, most probably due to the formation of H- complexes.

Optimizing spin filtering efficiency also requires detailed knowledge of spin interactions at the defect centers. This issue was addressed in Papers IV and V. From both experimental and theoretical studies, we were able to conclude that the HFI between e-n spins at the  defects led to e-n spin mixing, which degraded spin filtering efficiency at zero field.  Moreover, we have identified the microscopic origin of electron spin relaxation (T₁) at the defect centers, that is, hyperfine-induced e-n spin cross-relaxation. Our finding thus provided a guideline to improve spin filtering efficiency by selectively incorporating the  defects with weak HFI by optimizing growth and post-growth treatment conditions, or by searching for new spin filtering defect centers containing zero nuclear spin.

The implementation of the defect-engineered spin filtering effect has been addressed in Papers VI–VIII. First, we experimentally demonstrated for the first time at RT an efficient electron spin amplifier employing the  defects in Ga(In)NAs alloys, capable of amplifying a weak spin signal up to 27 times with a high cut-off frequency of 1 GHz. We further showed that the defectmediated spin amplification effect could turn the GaNAs alloy into an efficient RT optical spin detector. This enabled us to reliably conduct in-depth spin injection studies across a semiconductor heterointerface at RT. We found a strong reduction of electron spin polarization after optical spin injection from a GaAs layer into an adjacent GaNAs layer. This observation was attributed to severe spin loss across the heterointerface due to structural inversion asymmetry and probably also interfacial point defects.

Finally, we went beyond the generation of strongly polarized electron spins. In Paper IX we focused on an interesting aspect of using strongly polarized electron spins to induce strong nuclear spin polarization at RT, relevant to solid-state quantum computation using a defect nuclear spin of long spin memory as a quantum bit (qubit). By combining the spin filtering effect and the HFI, we obtained a sizeable nuclear spin polarization of ~15% at RT that could be sensed by conduction electrons. This demonstrated the feasibility of controlling defect nuclear spins via conduction electrons even at RT, the first case ever being demonstrated in a semiconductor.

The abilities to control and manipulate electron spin, especially in semiconductors, lead to many interesting proposals for spin-functional devices in future spintronics and quantum information technology. A key requirement for the success of these proposals is that the spin functionality should be operational at room temperature (RT), which remains as a great challenge. Very recently, spin-dependent recombination (SDR) via paramagnetic defects that dominate in carrier recombination, i.e Gai - interstitial defects in Ga(In)NAs alloys, has been shown to turn the material into a highly efficient defectengineered spin filter operating at RT and without requiring an external applied field. This finding has demonstrated the great potential of such a spin filter as an efficient spin source, which is capable of generating up to 90% electron spin polarization at RT.

Essential to realization of this attractive application in spintronics is a fundamental understanding of this alloy system and their related spin filtering defects. Therefore, factors controlling this spin filter must be studied, understood and optimized. In this licentiate thesis, we aim at optimization of the spin filtering effect in Ga(In)NAs alloys and the related quantum structures by studying influence of material fabrication techniques, post growth treatments and material structures. In paper I, we employed the optically detected magnetic resonance (ODMR) technique to study formation of Ga interstitial-related defects in Ga(In)NAs alloys. We showed that these spin-filtering defects are common grown-in defects in these alloys, independent of the employed fabrication techniques and post-growth annealing treatment. The defect formation was suggested to be thermodynamically favorable in the presence of nitrogen, possibly because of local strain compensation. In paper II, we further investigated the role of post-growth hydrogen treatment in the spin filtering effect in GaNAs epilayers and GaNAs/GaAs multiple quantum wells (QWs). From optical orientation studies, we found that the hydrogen treatment has led to nearly complete quenching of the spin filtering effect. Together with a detailed ODMR study and a rate equation analysis, the observed effect of hydrogen was attributed to hydrogen passivation of the spin filtering defects, likely by formation of complexes between the Gai-interstitial defects and hydrogen. This finding also ruled out the possibility of hydrogen as a part of the spin filtering defects in the as-grown materials, though hydrogen is known to be commonly present during the growth process.

In Paper III, we examined the effectiveness of the spin filtering effect in the GaNAs/GaAs QWs as a function of QW width. Even with rather narrow QW widths of 3-9 nm, the spin filtering effect was shown to be efficient. It was further revealed that the spin filtering effect is more efficient in the wider QWs. From studies of transient behavior of photoluminescence and ODMR, it was concluded that this was mainly due to an increase in the sheet concentration of the spin filtering defects.

We identify hyperfine-induced electron and nuclear spin cross-relaxation as the dominant physical mechanism for the longitudinal electron spin relaxation time (T-1) of the spin-filtering Ga-i(2+) defects in GaNAs alloys. This conclusion is based on our experimental findings that T-1 is insensitive to temperature over 4-300 K, and its exact value is directly correlated with the hyperfine coupling strength of the defects that varies between different configurations of the Ga-i(2+) defects present in the alloys. These results thus provide a guideline for further improvements of the spin-filtering efficiency by optimizing growth and processing conditions to preferably incorporate the Ga-i(2+) defects with a weak hyperfine interaction and by searching for new spin-filtering defects with zero nuclear spin.

Efficient and highly spin-dependent recombination processes are shown to not only turn GaNAs into an efficient spin filter but also to make it an excellent spin detector functional at room temperature (RT). By taking advantage of the defect-engineered spin-filtering effect, the spin detection efficiency is no longer limited by the fast spin relaxation of conduction electrons. This leads to a significant enhancement in the optical polarization of the spin detector, making it possible to reliably detect even very weak electron spin polarization at RT, as demonstrated by a study of spin loss during optical spin injection across a GaAs/GaNAs interface.

The first experimental demonstration of a spin amplifier at room temperature is presented. An efficient, defect-enabled spin amplifier based on a non-magnetic semiconductor, Ga(In)NAs, is proposed and demonstrated, with a large spin gain (up to 2700% at zero field) for conduction electrons and a high cut-off frequency up to 1 GHz.

Effects of hydrogen on the recently discovered defect-engineered spin filtering in GaNAs are investigated by optical spin orientation and optically detected magnetic resonance. Post-growth hydrogen treatments are shown to lead to nearly complete quenching of the room-temperature spin-filtering effect in both GaNAs epilayers and GaNAs/GaAs multiple quantum wells, accompanied by a reduction in concentrations of Ga(i) interstitial defects. Our finding provides strong evidence for efficient hydrogen passivation of these spin-filtering defects, likely via formation of complexes between Gai defects and hydrogen, as being responsible for the Observed strong suppression of the spin-filtering effect after the hydrogen treatments.

We show that circularly polarized emission light from InGaAs/GaAs quantum dot (QD) ensembles under optical spin injection from an adjacent GaAs layer can switch its helicity depending on emission wavelengths and optical excitation density. We attribute this anomalous behavior to simultaneous contributions from both positive and negative trions and a lower number of photo-excited holes than electrons being injected into the QDs due to trapping of holes at ionized acceptors and a lower hole mobility. Our results call for caution in reading out electron spin polarization by optical polarization of the QD ensembles and also provide a guideline in improving efficiency of spin light emitting devices that utilize QDs.

We show by electron spin resonance (ESR) and Raman spectroscopies that the crystal phase transition of the lead-free double-perovskite Cs2AgBiBr6 has a profound symmetry-breaking effect on the high spin states of, for example, a transition-metal ion Fe3+ and the vibrational modes. It lifts their degeneracy when the crystal undergoes the cubic-tetragonal phase transition, splitting the six-fold degenerate S = 5/2 state of Fe3+ to three Kramer doublets and the enharmonic breathing mode T-g of the MBr6 octahedra (M = Ag, Bi, Fe) into E-g + A(g). The magnitudes of both spin and Raman line splitting are shown to directly correlate with the strength of the tetragonal strain field. This work, in turn, demonstrates the power of the ESR and Raman spectroscopies in probing structural phase transitions and in providing in-depth information on the interplay between the structural, spin, and vibrational properties of lead-free double perovskites, a newly emerging and promising class of materials for low-cost and high-efficiency photovoltaics and optoelectronics.

The effect of hyperfine interaction (HFI) on the recently discovered room-temperature defect-enabled spin-filtering effect in GaNAs alloys is investigated both experimentally and theoretically based on a spin Hamiltonian analysis. We provide direct experimental evidence that the HFI between the electron and nuclear spin of the central Ga atom of the spin-filtering defect, namely, the Ga-i interstitials, causes strong mixing of the electron spin states of the defect, thereby degrading the efficiency of the spin-filtering effect. We also show that the HFI-induced spin mixing can be suppressed by an application of a longitudinal magnetic field such that the electronic Zeeman interaction overcomes the HFI, leading to well-defined electron spin states beneficial to the spin-filtering effect. The results provide a guideline for further optimization of the defect-engineered spin-filtering effect. DOI: 10.1103/PhysRevB.87.125202

Nuclear spin hyperpolarization is essential to future solid-state quantum computation using nuclear spin qubits and in highly sensitive magnetic resonance imaging. Though efficient dynamic nuclear polarization in semiconductors has been demonstrated at low temperatures for decades, its realization at room temperature is largely lacking. Here we demonstrate that a combined effect of efficient spin-dependent recombination and hyperfine coupling can facilitate strong dynamic nuclear polarization of a defect atom in a semiconductor at room temperature. We provide direct evidence that a sizeable nuclear field (~150 Gauss) and nuclear spin polarization (~15%) sensed by conduction electrons in GaNAs originates from dynamic nuclear polarization of a Ga interstitial defect. We further show that the dynamic nuclear polarization process is remarkably fast and is completed in <5 μs at room temperature. The proposed new concept could pave a way to overcome a major obstacle in achieving strong dynamic nuclear polarization at room temperature, desirable for practical device applications.

Effectiveness of the recently discovered defect-engineered spin-filtering effect is closely examined in GaNAs/GaAs multiquantum wells (QWs) as a function of QW width. In spite of narrow well widths of 3-9 nm, rather efficient spin filtering is achieved at room temperature. It leads to electron spin polarization larger than 18% and an increase in photoluminescence intensity by 65% in the 9 nm wide QWs. A weaker spin filtering effect is observed in the narrower QWs, mainly due to a reduced sheet concentration of spin-filtering defects (e.g., Ga-i interstitial defects).

Efficient generation, maintaining, manipulation and detection of electron spin polarization and coherence at room-temperature (RT) in semiconductors is a prerequisite for the success of future semiconductor spintronics. Potential spintronic devices are expected to be based on fundamental building blocks such as spin filters (or spin injectors or spin aligners), spin amplifiers and spin detectors. During the past decade spin filters and spin detectors have been a main focal point of intense research efforts in the field of semiconductor spintronics that have led to many innovative approaches and encouraging developments. In contrast, experimental developments in spin amplifiers have been extremely limited. At present, realization of efficient RT spin functionality remains to be a great challenge and a hotly pursued research topic.

In this work, we explore a new and unconventional approach of defect-enabled spin functionality in a non-magnetic semiconductor without requiring a magnetic layer or external magnetic fields. We demonstrated efficient defect-engineered spin filtering in Ga(In)NAs, which is capable of generating a remarkably high spin polarization degree (> 40%) of conduction electrons at RT. The highest spin polarization achieved to date by using this approach is up to 90 %. We also proposed a conceptually new spin amplifier by defect engineering and provided the first experimental demonstration of an efficient RT spin amplifier based on Ga(In)NAs with a spin gain up to 2700%! Such a spin amplifier is shown to be capable of amplifying a fast-modulating input spin signal while truthfully maintaining its time variation of the spin-encoded information, and is predicted to remain functional up to 1 GHz. By taking advantage of the spin amplification effect, we further showed that Ga(In)NAs can be employed as an efficient RT spin detector, with spin detection efficiency well exceeding 100%. Applications of such a spin-functional semiconductor material could potentially provide an attractive and viable solution to the current and important issues on RT spin injection, spin amplification and spin detection in semiconductors for future spintronics.

Recently discovered effect of spin-filtering and spin amplification in GaNAs enables us to reliably obtain detailed information on the degree of spin loss during optical spin injection across a semiconductor heterointerface at room temperature. Spin polarization of electrons injected from GaAs into GaNAs is found to be less than half of what is generated in GaNAs by optical orientation. We show that the observed reduced spin injection efficiency is not only due to spin relaxation in GaAs, but more importantly due to spin loss across the interface due to structural inversion asymmetry and probably also interfacial point defects.

In organic photovoltaics, efficient charge generation relies on our ability to convert excitons into free charges. Efficient charge separation from "energetic excitons" has been understood to be governed by delocalization effects promoted by molecular aggregation. A remaining puzzle is, however, the mechanism underlying charge generation via relaxed interfacial charge-transfer (CT) excitons that also exhibit an internal quantum efficiency close to unity. Here, we provide evidence for efficient charge generation via CT state absorption over a temperature range of 50-300 K, despite an intrinsically strong Coulomb binding energy of about 400 meV that cannot be modified by fullerene aggregation. We explain our observation by entropy-driven charge separation, with a key contribution from energy disorder. The energy disorder reduces the charge generation barrier by substantially gaining the entropy as electron hole distance increases, resulting in efficient CT exciton dissociation. Our results underline an emerging consideration of energy disorder in thermodynamic stability of charge pairs and highlight the energy disorder as a dominant factor for generating charges via the CT state. A discussion for a trade-off in harvesting charges from relaxed CT excitons is also provided.