Photovoltaic (PV), which converts sunlight into electricity, is a promising solution to the energy and environmental crisis we are facing right now. In this dissertation, we are focusing on next generation semiconductors as the photoactive materials, i.e. organic and orgainic/inorganic hybrid perovskite semiconductors, to achieve cost effective and energy efficient solar cell technology.

Organic semiconductor always shows narrow absorption compared with the conventional inorganic semiconductors, which is one of the limiting factors of the organic photovoltaic (OPV). In the second chapter, we solve this problem by employing the multi-absorber bulk heterojunction (BHJ) device architecture, in which different polymer absorbers are blended together to cover panchromatic absorption. A comparison study reveals the working mechanism of this novel device, and the material selection rule is summarized. Eventually, the 8.7% power conversion efficiency is realized in ternary BHJ solar cell. Besides the narrow absorption, the organic BHJ layer is usually very thin (~100 nm) due to its limited carrier mobility, hence the absorption is insufficient in the photoactive layer. In the third chapter, we utilize the plasmonic effect of the metal nanoparticle to enhance the absorption of the OPVs. The addition of spectrally tuned SiO2-Au nanorods leads to improved photocurrent and device efficiency. On the other hand, the narrow absorption of organic materials also creates new device possibility that traditional semiconductor can't deliver. One of the good examples is the visibly semitransparent solar cells. In the fourth chapter, we develop this idea by engineering transparent top electrode and incorporate photonic distributed bragg reflector to further improve the device efficiency without compromising the visible transparency.

Organic/inorganic hybrid perovskite semiconductor attracts incredible attentions in the past few years. It originates from the excitonic semiconductor family but delivers superior photovoltaic device performance than the typical excitonic solar cell e.g. dye sensitized solar cells and organic solar cells. However, the photophysics of this type of material is still mysterious in many aspects. In the fifth chapter, we try to understand its photophysical property by some basic characterization methods. We discover that the photo excitations in this materials varies with different crystal size. Besides, we observe the photoluminescence lifetime and intensity improve dramatically after exposing to the moisture. The improved photophysical property eventually results in greatly enhanced photovoltaic device performance. In addition to the solar cell, we successfully demonstrates the ultral high photodetectivity based on the perovskite materials, the results are included in sixth chapter.

The barrier to utilize solar generated electricity mainly comes from their higher cost relative to fossil fuels. However, innovations with new materials and processing techniques can potentially make cost effective photovoltaics. One such strategy is to develop solution processed photovoltaics which avoid the expensive vacuum processing required by traditional solar cells. The dissertation is mainly focused on two absorber material system for thin film solar cells: chalcopyrite CuIn(S,Se)2 (CISS) and kesterite Cu2ZnSn(S,Se)4 organized in chronological order.

Chalcopyrite CISS is a very promising material. It has been demonstrated to achieve the highest efficiency among thin film solar cells. Scaled-up industry production at present has reached the giga-watt per year level. The process however mainly relies on vacuum systems which account for a significant percentage of the manufacturing cost. In the first section of this dissertation, hydrazine based solution processed CISS has been explored. The focus of the research involves the procedures to fabricate devices from solution. The topics covered in Chapter 2 include: precursor solution synthesis with a focus on understanding the solution chemistry, CISS absorber formation from precursor, properties modification toward favorable device performance, and device structure innovation toward tandem device.

For photovoltaics to have a significant impact toward meeting energy demands, the annual production capability needs to be on TW-level. On such a level, raw materials supply of rare elements (indium for CIS or tellurium for CdTe) will be the bottleneck limiting the scalability. Replacing indium with zinc and tin, earth abundant kesterite CZTS exhibits great potential to reach the goal of TW-level with no limitations on raw material availability. Chapter 3 shows pioneering work towards solution processing of CZTS film at low temperature. The solution processed devices show performances which rival vacuum-based techniques and is partially attributed to the ease in controlling composition and CZTS phase through this technique. Based on this platform, comprehensive characterization on CZTS devices is carried out including solar cells and transistors. Especially defects properties are exploited in Chapter 4 targeting to identify the limiting factors for further improvement on CZTS solar cells efficiency. Finally, molecular structures and precursor solution stability have been explored, potentially to provide a universal approach to process multinary compounds.

Solution processed metal oxide semiconductors have attracted much attention as a promising class of materials to be used as the channel material in thin film transistors due to its transparency, high mobility, scalability, and low cost of manufacturing. Nevertheless, there are still major challenges in terms of processing, device performance and stability that need to be overcome before this process can be implemented for large scale production. In this thesis, several chemical additives and their effects on film formation, processing temperature and electrical parameters such as field effect mobility, on-off ratio and threshold voltage shifts under PBS stress tests were investigated. In particular, the addition of ethylene glycol, acetylacetone, and acetic acid were investigated in a metal oxide precursor solution. It was demonstrated that ethylene glycol significantly improved the wettability of the concentrated IGZO solution and resulted in a minimal contact angle of 3.8 degrees. This allowed for coating a concentrated metal

oxide precursor solution (0.5M) five times the baseline concentration while still maintaining high film formation quality. The addition of acetylacetone allowed for low annealing temperatures (less than 300�C) through the combustion synthesis route and serves as a protection group to prevent premature formation of metal oxide network in solution. Finally, the addition of acetic acid improved the solubility of the metal precursor in the solution and allowed for higher concentrations of metal precursor to be dissolved in solution, which becomes important if higher viscosity precursors for thick films are needed. The addition of these additives produced devices with near zero turn on voltage, excellent on-off ratio (>107), and superior stability (less than eight volts of threshold voltage shift at 10,000 seconds of PBS). The findings in this thesis present improved synthesis routes for solution processed semiconductors and open new possibilities for the fabrication of flexible electronic devices and next generation large scale consumer electronics.

Metal halide perovskite solar cells have proven themselves as one of the most promising candidates to replace the currently well-commercialized silicon-based solar cells. Because of its unique energy band structure, it has merits such as high defect tolerance, favorable charge carrier mobility, and high absorption coefficient. However, the major issue that hinders the successful real-life application of the metal halide perovskite solar cell is its unsatisfactory stability. In Chapter One, I introduce the four basic elements that cause the instability of perovskite solar cells: light, heat, bias, and moisture. The perovskite material degradation mechanism behind each environment will be detailly illustrated. Ion migration suppression and device encapsulation can be regarded as the main solutions to enhance the operational lifetime of the perovskite solar cells. From interior to exterior, in the following chapters, I introduce the strategies we developed that are proven to be powerful to improve the stability of the metal halide perovskite solar cells.In Chapter Two, I introduce the first strategy of interior multivalent interstitial doping. It is a strategy originated inside the perovskite lattice. Cations with suitable sizes to occupy an interstitial site of perovskite crystals have been widely used to inhibit ion migration and promote the performance and stability of perovskite optoelectronics. However, the interstitial doping accompanies inevitable lattice strain to impair the long-range ordering and stability of the crystals to cause a sacrificial trade-off. In this chapter, I unravel the evident influence of the valence states of the interstitial cations on their efficacy to suppress the ion migration. Incorporation of a trivalent neodymium cation (Nd3+) effectively mitigates the ion migration in the perovskite lattice with a significantly reduced dosage (0.08%) compared to a widely used monovalent cation dopant (Na+, 0.45%). Less but better, the prototypical perovskite solar cells incorporated with Nd3+ exhibits significantly enhanced photovoltaic performance and operational stability. In Chapter Three, I discuss the defect passivation of the perovskite crystal, which constitutes one of the most commonly used strategies to fabricate highly efficient perovskite solar cells (PSCs). The durability of the passivation effects under harsh operational conditions has not been extensively studied regardless of the weak and vulnerable secondary bonding between the molecular passivation agents and perovskite crystals. Here, we incorporated strategically designed passivating agents to investigate the effect of their interaction energies with the perovskite crystals and correlated these with the performance and longevity of the passivation effects. We unraveled that the passivation agents with a stronger interaction energy are advantageous not only for effective defect passivation, but also to suppress defect migration. The prototypical PSCs treated with the optimal passivation agent exhibited superior performance and operational stability, retaining 81.9% and 85.3% of their initial performance under continuous illumination or nitrogen at 85 ℃ after 1008 hours, respectively while the reference device completely degraded during the time. This work provides important insights into designing operationally durable defect passivation agents for perovskite optoelectronic devices. In Chapter Four, we focus on the perovskite grain and the grain boundary density. Intrinsically, detrimental defects accumulating at the surface and grain boundaries limit both the performance and stability of perovskite solar cells. Small molecules and bulkier polymers with functional groups are utilized to passivate these ionic defects but usually suffer from volatility and precipitation issues, respectively. Starting from the addition of small monomers in PbI2 precursor, in this chapter, I introduce a polymerization-assisted grain growth (PAGG) strategy in the sequentially deposited method. With a polymerization process triggered during the PbI2 film annealing, the bulkier polymers formed will be adhered to the grain boundaries, remaining the previously established interactions with PbI2. After perovskite formation, the polymers anchored on the boundaries can effectively passivate under-coordinated lead ions and reduce defect density. As a result, we obtain a champion power conversion efficiency (PCE) of 23.0%, together with a prolonged lifetime where 85.7% and 91.8% of the initial PCE remains after 504-hour continuous illumination and 2208-hour shelf storage, respectively. In Chapter Five, I will go to the exterior of the perovskite solar cell and introduce a novel strategy of device encapsulation. Unstable nature against moisture is one of the major issues of metallic halide perovskite solar cell application. Thin-film encapsulation is known as a powerful approach to notably enhance the operational stability of perovskite solar cells in a humid environment. However, encapsulation layers with ideal gas barrier performance always require harsh fabrication conditions with high temperature and harmful precursors. For this reason, here we provide a mild encapsulation strategy to maintain the original performance of solar cell devices by utilization of ethylene glycol-induced immediate layer to minimize the damage of plasma-enhanced atomic layer deposition to perovskite solar cells. The organic-inorganic alternating encapsulation structure has exhibited a water vapor transmittance rate of 1.3 �10-5 g�m-2�day-1, which is the lowest value among the reported thin-film encapsulation layers of perovskite solar cells. Our perovskite solar cells have survived at 80% relative humidity and 30 �C for over 2000 hours while preserving 96% of their initial performance.

Organic-inorganic metal halide perovskites (MHPs) for photovoltaic applications have emerged rapid progress since their first successful demonstration since a decade ago. The record power conversion efficiency (PCE) of lab-sized (typically <1 cm2) perovskite solar cells (PSCs) have risen from 14.1% to 25.5%, and small modules have reached 17.9% PCE in early 2021. With representative compositions including methylammonium lead triiodide (MAPbI3), formamidinium lead triiodide (FAPbI3), cesium lead triiodide (CsPbI3), or hybrids of these cations with mixed halide compositions (e.g. FAxCs1-xPbI3, FAxMA1-xPbI3-yBry), perovskite materials generally exhibit near infrared bandgap, ideal for single-junction solar cells. However, as the band edges formed by the hybridization of the electronic orbitals from the B-site lead and the X-site halide, the optical bandgap of perovskites could be precisely tuned by controlling the halide I/Br ratio, making configuring tandem photovoltaics with halide-perovskite possible which is also the core of this dissertation. Starting from Chapter 1, I will give a brief introduction on MHPs and tandem photovoltaics. The critical role of defect physics plays in halide perovskite solar cell’s performance and stability will also be discussed in this chapter, and I will introduce my early work at UCLA in delivering a certified 22.0% efficiency monolithic perovskite-CIGS tandem which is the origin of most of my ideas proceed to this thesis. After that, I will introduce strategies we developed that achieved highly efficient 4-terminal perovskite-Cu(In,Ga)(Se,S)2 and perovskite-silicon tandem solar cells in Chapter 2. Numerical simulation and analysis unraveled the efficiency losses in the perovskite sub-cell were still the bottlenecking factor in limiting the performance of perovskite-based tandem photovoltaics. An electronic defects passivation strategy at perovskite surface were developed utilizing a synergetic effect of post-treated surface fluoride and phenethylammonium iodide (PEAI) that mitigated surface traps. In Chapter 3, a more in-depth investigation of the surface reconstruction of perovskite during post-treatments was carried out at the microscale level based on a series of surface-sensitive or depth-resolved characterizations. A change of the perovskite surface could be induced by the commonly used post-treatment solvent, isopropyl alcohol (IPA), which directly affect the surface electronic affinity, surface termination, and surface defect feature. However, it was also found the IPA wash in fact assisted the ligand adsorption (e.g. PEA+) to the perovskite surface and thus enhanced their defect passivation effect. In Chapter 4, a set of in-situ or carefully designed ex-situ experiments were developed to distinguish the different formation dynamics and defect physics between the wide bandgap mixed-halide perovskite for tandem applications and the low bandgap tri-iodide perovskite for single junction solar cells, as the performance losses in the mixed-halide perovskite were significantly larger than their tri-iodide counterpart. The results show that the inclusion of bromide introduced a halide homogenization process during the perovskite growth stage from an initial bromide-rich phase towards the final target stoichiometry. We further elucidated a physical model that correlates the role of bromide with the formation dynamics, defect physics, and eventual optoelectronic properties of the film. After recognizing the critical role of surface properties and surface defects in halide perovskite, Chapter 5 will introduce a theoretical investigation of the formation and energy levels of perovskite surface defects. FAPbI3, which is the composition that delivered the record photovoltaic performance among the perovskite family, was chosen for this study. All surface point defects were considered under two most commonly seems terminations, namely PbI2 termination and FA-I termination. Insights of combining theoretical results and experimental outcomes were also discussed. In Chapter 6, I will introduce organic small molecules named Y1 and Y2 that also showed tremendous potential in tandem application. They are non-fullerene acceptors (NFAs) with near infrared absorption and could serve as an ideal absorber as rear cell. The optoelectronic properties of Y1 and Y2 were delicately tuned by the introduction of unconventional electron-deficient-core-based fused structure, and their single junction devices exhibited a low voltage loss of 0.57 V and high short-circuit current density of 22.0 mA cm−2, resulting in record-breaking power conversion efficiencies of over 13.4% (certified 12.6%).

Converting solar energy into electricity provides an effective solution to the energy crisis the world is facing today. Organic Photovoltaics have shown potential to harness solar energy in a cost-effective way. Extensive progress has been achieved in understanding OPV photophysical phenomena and in identifying key factors needed to improve organic solar cell device power conversion efficiency (PCE). The newly synthesized polymer BDTP-Bz-1 and BDTP-Bz-2 composed of alternating benzotriazole and phenyl substituted benzodithiophene (Figure 1), yield 8% PCEs when blended in a bulk-hetero-junction (BHJ) structure with electron acceptor materials such as ITCC. The donor polymer BDTP-Bz-2 showed better device performance including short circuit current density (Jsc), open circuit voltage (Voc), and External Quentum Efficiency (EQE), which can contribute to its preference for the “face-on” polymer backbone orientation in which the π-conjugated polymer backbone planes lie parallel to the substrate surface, resulting in maximal contacts between organic photoactive materials and the ITO anode coated with electron transport layer (ZnO). This orientation of the π-π stacked structures would favor charge transport across the interface. By AFM analysis, it was revealed that the BDTP-Bz-2 : ITCC active layer showed lower surface roughness (10.6 nm) than BDTP-Bz-1 : ITCC active layer (12.5 nm). Lower surface roughness also helped form better interface contact between the active layer and the electrode.