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The III-nitride material system, i.e., (In, Ga, Al)N, which has a direct bandgap ranging from 0.7 eV to 6 eV, is well-suited for a wide range of optoelectronic devices. One of them would be the InGaN-based light-emitting diode (LED), which has become one of the leading general illumination technologies. The success of nitride LEDs has also sparked interest in other optoelectronic devices including photodetectors. This dissertation focuses on the development of both III-nitride photodetectors and LEDs.In recent years, increasing interest in visible light communication (VLC) has created demand for high-speed photodetectors in the visible light spectrum. InGaN-based photodetectors, which offer great wavelength-selective response due to its tunable bandgap, are promising candidates for optical receivers in VLC links. Instead of the InGaN double heterostructure devices, InGaN-based photodetectors with multiple quantum well (MQW) structure designs are favored due to their significant advantages in terms of growth. The speed limiting factors of such devices were studied to improve their speed performance. On top of the usual RC time constant, InGaN/GaN MQW photodetectors are also limited by carrier escape lifetimes due to carrier trapping in the QWs. By reducing the thickness of the quantum barriers, carrier escape lifetimes were reduced, resulting in a twofold improvement in the 3-dB bandwidth of the photodetectors. Despite the huge success of InGaN-based LEDs in solid-state lighting, one of the most enduring challenges that still limit the LEDs is the efficiency droop phenomenon, which refers to the decrease in the quantum efficiency with increasing injection current density. In conventional c-plane LEDs, polarization-induced electric fields further exacerbate the droop problem. The large internal electric fields in the quantum wells lead to a reduction in the overlap of the electron and hole wavefunctions. This lowers the recombination coefficients and causes an increase in carrier density at a given current density, leading to an early onset of efficiency droop due to the nonlinear Auger recombination. Additionally, the large internal electric fields also prevent the use of thick QW active region designs to reduce the carrier densities. One approach to reduce the internal electric field in c-plane QWs is through the use of doped barriers. However, the heavily doped Mg(Si)-doped p(n)-type GaN barriers also lead to a higher defect density. Growth optimization was performed with the aim of maximizing the field reduction and minimizing the detrimental impact of the doped barriers. With doped barriers, we demonstrated a 9-nm-thick single QW LED with a low efficiency droop. Biased photocurrent spectroscopy was also carried out to illustrate the effect of doped barriers on the internal electric fields. Device simulations were used in tandem with experimental results to guide the interpretation of the results. Lastly, differential carrier lifetime measurements were performed to determine the impact of doped barriers on the recombination coefficients of the LEDs. The improvement in the radiative coefficients in the LEDs with doped barriers, coupled with the blueshift of the emission wavelengths, indicates an enhancement in wavefunction overlap and a reduction of quantum confined Stark effect (QSCE) as a result of the reduced internal electric field.

Photovoltaics technology, or solar cell, has been developed for more than a half century. This idea that utilizes energy from an infinite resource is indeed fascinating and drives scientists and engineers to devote themselves to this field. It is one of the major renewable energy types which accounts for 2% of the global total electricity consumption in 2018 and this percentage is still growing in a steady trend. According to a publicized prediction, the solar power could supply 20% of the total electricity demand in 2030 because of drastically dropping price per watt. Although a brilliant future of solar cell is projected, there are still many unknown physical phenomena and engineering difficulties nonetheless. These challenges in photovoltaics are thus of our interest to work on. The state of art highest efficiency photovoltaics is multi-junction inorganic solar cell, which is used in astronautic application. Yet the complicated fabrication processes and high cost limit its usage. It is the reason that silicon solar cell with moderate efficiency and process rules the whole solar cell market. However, the world still needs a different type of solar cell to fulfill the requirements of low weight, flexible, transparent, non-toxic and solution process compatibility to accustom to the other environment conditions. Organic solar cell is one of the strong candidates for reaching this target. In this dissertation, I aim to achieve high efficiency organic solar cells in single-junction and multi-junction structure with different strategies. There are the modification of charge transport layer and interconnecting layer for a better Ohmic contact and fermi level pining as well as the sub-cell current matching by third photo-active component. In Chap. 3, the project for improving the transport properties of single junction devices has been explored. A high mobility Indium Oxide In2O3 as an universal electron transport layer was chosen and improved the original device efficiencies for 10%.　The physical fundamentals for efficiency improvement have been found that the crystalline In2O3 with highly aligned nanocrystallites can induce the crystallization of polymer into a preferential molecular packing for the charge transport and the charge extraction in the crystalline In2O3 devices is significantly faster than its counterpart devices.Based on the work principle of Fermi level pinning, organic tandem solar cells with a tunnel junction consisted of zirconium (IV) acetylacetonate (Zr‐Acac) has been developed and discussed in Chap. 4. The Zr‐Acac has a suitable low work function to pin the Fermi level of the interconnecting layer and hence, can effectively behave as the charge recombination function for the tandem device. Furthermore, Zr-Acac requires a least harmful preparation process method in the tandem devices. Comparing to the reference device, the Zr‐Acac interconnecting layer can effectively enhance the efficiency by 20%. The other strategies to improve the tandem organic solar cells with the ternary structure have been discussed in Chap. 5 and Chap. 6. Broadening the light absorption spectrum needs the optimization of current matching among different sub-cells with good interconnecting layer. Herein, it is firstly discovered that using ternary blend sub-cell in tandem device can finely optimize the current matching via tuning the materials composition in the ternary blend. The voltage-current trade-off in the tandem devices can be achieved. Due to the amorphous nature of the used small molecular, the phase separation in this ternary blend upon heating is sufficiently suppressed, resulting in a good thermal stability and viable for a better environment of interconnecting layer preparation.The device efficiency measured in the lab has reached 13% and the certified device efficiency is 11.5% by NREL, which achieves the world record as of October 2018. My conclusion and future perspective of these works on organic solar cells are summarized in Chap. 7.

Phonon-phonon and electron-phonon interactions underlie many fundamental transport properties like thermal conductivity and electrical mobility, and models of these properties provide information about the underlying microscopic interactions present in the materials. Many of these models use the Boltzmann transport equation where the choice of the expression for the collision integral is the most important and challenging aspect since it should capture all of the relevant interactions. In the past the expressions were semi-empirical, but in recent decades first principles models with no fitting parameters have become more commonplace, leading to discovery of new materials or providing deeper insights into the relevant mechanisms governing transport. This thesis presents first-principles calculations of thermal conductivity in polymer crystals, and charge transport at high electric fields in semiconductors in the Boltzmann transport framework. Polymers are thermally insulating in their typical amorphous form, but it is known that their thermal conductivity can be enhanced through drawing and aligning of their polymer chains. With perfect chain alignment, the structures can be described as polymer crystals, which tend to contain many atoms per unit cell. However, the conventional understanding of thermal transport in crystals predicts low thermal conductivity for complex, many atom unit cells. It is known from simple models that phonon focusing redirects the heat flow into the polymer chain direction, but the extent to which phonon focusing plays a role in setting the intrinsic upper limits of polymer thermal conductivity has not been assessed from a first principles standpoint. We calculate the ab initio lattice conductivity of polythiophene, a complex molecular crystal with 28 atoms per unit cell, using the temperature dependent effective potential (TDEP) method to obtain finite temperature phonon properties taking into account the large quantum nuclear motion of hydrogen atoms present in polymers. We find a high thermal conductivity due to phonon focusing and stiff branches that overcome the expected low phonon lifetimes. The phonon focusing aligns group velocities along the chain axis throughout the Brillouin zone, even for states with wave vector almost orthogonal to the chain axis. For charge transport, ab initio calculations focus almost exclusively on low field mobility, but technologically relevant phenomena like negative differential resistance manifest only at high fields far from equilibrium. Further, there are no ab initio calculations of non-equilibrium electronic noise, which differs qualitatively from transport observables at high fields. We report a methodological advance that obtains both the high-field transport properties and the non-equilibrium noise using an ab initio Boltzmann transport approach. Our method extends the collision integral to high fields by making physically motivated approximations to account for the non-linearities at high fields. Using our method, we calculate the high-field noise and transport properties in GaAs and find that the 1ph level of theory is inadequate. Thus, we implement an approximate form of higher order interactions where electrons are scattered consecutively by two phonons (2ph) and find that these 2ph processes qualitatively alter the energy relaxation of the electron system compared to 1ph scattering, resolving a long-standing discrepancy in the strength of intervalley scattering inferred from different experiments. We also calculate non-equilibrium electronic noise from first principles for the first time. However, we are not able to reproduce experimental trends, and we suggest that 2ph processes beyond our approximation may be necessary to obtain experimental agreement. Our calculation shows how noise provides a new observable against which the accuracy of first-principles methods can be measured.

The III-Nitride materials system provides a fascinating platform for developing optoelectronic devices, such as solar cells and LEDs, which have the power to dramatically improve the efficiency of our power consumption and reduce our environmental footprint. Finding ways to make these devices more efficient is key to driving their widespread adoption. This dissertation focuses on the intersection of challenges in physics and metalorganic chemical vapor deposition (MOCVD) growth at the nanoscale when designing for device efficiency.In order to create the best possible InGaN solar cell, a multiple quantum well (MQW) active region design had to be employed to prevent strain relaxation related degradation. There were two competing challenges for MQW active region design and growth. First, it was observed current collection efficiency improved with thinner quantum barriers, which promoted efficient tunneling transport instead of inefficiency thermally activated escape. Second, GaN barriers could planarize surface defects in the MQW region under the right conditions and when grown thick enough. A two-step growth method for thinner quantum barriers was developed that simultaneously allowed for tunneling transport and planarized V-defects. Barriers as thin as 4 nm were employed in MQW active regions with up to 30 periods without structural or electrical degradation, leading to record performance. Application of dielectric optical coatings greatly reduced surface reflections and allowed a second pass of light through the device. This both demonstrated the feasibility of multijunction solar integration and boosted conversion efficiency to record levels for an InGaN solar cell.III-N LEDs have achieved state-of-the-art performance for decades, but still suffer from the phenomena of efficiency droop, where device efficiency drops dramatically at high power operation. Droop is exacerbated by the polarization-induced electric fields in InGaN quantum wells, which originate from a lack of inversion symmetry in GaN’s wurtzite crystal structure. These fields can be screened by using highly doped layers, but the extreme dopant densities predicted by simulation for complete screening may require using Ge as an alternative n-type dopant to Si. GaN:Ge layers with excellent electrical characteristics were grown by MOCVD with doping densities exceeding 1020 cm-3. However, their surface morphologies were very poor and they proved a poor screening dopant in LED structures. Using Si as the n-type screening dopant, LEDs with single QW active regions were grown, packaged, and tested. Biased photoluminescence showed strong evidence of complete polarization screening. The LEDs had low droop, but also low peak efficiencies. Possible explanations for trends in efficiency with varying QW width and field screening will be discussed.

Anisotropic two-dimensional materials possess intrinsic angle-dependent physical properties that originate from their low crystal symmetry. Yet, how these properties are affected by external impurities or structural defects in the material is still wholly unclear. Here, we address this question by investigating the electrical transport in the anisotropic layered model system germanium arsenide. First, we show that the ratio of conductivities along the armchair and zigzag crystallographic directions exhibits an intriguing dependence with respect to both temperature and carrier density. Then, by using a conceptually simple model, we demonstrate that this unexpected behavior is directly related to the presence of impurity-induced localized states in the band gap that introduce isotropic hopping conduction. The presence of this conduction mechanism in addition to the intrinsic band conduction significantly influences the anisotropic electrical properties of the material, especially at room temperature, i.e., at application-relevant conditions.

The origin of superconductivity in bulk SrTiO$_{3}$ is a mystery, since the non-monotonous variation of the critical transition with carrier concentration defies the expectations of the crudest version of the BCS theory. Here, employing the Nernst effect, an extremely sensitive probe of tiny bulk Fermi surfaces, we show that down to concentrations as low as $5.5 \times 10^{17}cm^{-3}$, the system has both a sharp Fermi surface and a superconducting ground state. The most dilute superconductor currently known has therefore a metallic normal state with a Fermi energy as little as 1.1 meV on top of a band gap as large as 3 eV. Occurrence of a superconducting instability in an extremely small, single-component and barely anisotropic Fermi surface implies strong constraints for the identification of the pairing mechanism., Final version

Nanowire diodes have been realized by employing an axial heterojunction between InAs and InSb semiconductor materials. The broken-gap band alignment (type III) leads to a strong rectification effect when the current-voltage (I-V) characteristic is inspected at room temperature. The additional insertion of a narrow InP barrier reduces the thermionic contribution, which results in a net decrease of leakage current in the reverse bias with a corresponding enhanced rectification in terms of asymmetry in the I-V characteristics. The investigated diodes compare favorably with the ones realized with p-n heterostructured nanowires, making InAs/InP/InSb devices appealing candidates to be used as building blocks for nanowire-based ultrafast electronics and for the realization of photodetectors in the THz spectral range.