Yang, Shihang = 銅銦鎵硒太陽能電池中白光退火系統的設計 / 楊世航. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2009. / Includes bibliographical references (p. 87-91). / Abstract also in Chinese. / Yang, Shihang = Tong yin jia xi tai yang neng dian chi zhong bai guang tui huo xi tong de she ji / Yang Shihang. / Chapter 1 --- Introduction to Photovoltaics --- p.1 / Chapter 1.1 --- "Developments, markets and forecasts" --- p.1 / Chapter 1.2 --- The physics of solar cells --- p.2 / Chapter 1.2.1 --- Light Absorption --- p.2 / Chapter 1.2.2 --- Charge Carrier Separation --- p.6 / Chapter 1.2.3 --- Solar Cell I-V Characteristics --- p.7 / Chapter 1.3 --- Classifications of Solar Cells --- p.10 / Chapter 1.3.1 --- Crystalline silicon solar cell --- p.10 / Chapter 1.3.2 --- Thin film solar cells --- p.12 / Chapter 1.3.3 --- Organic and polymer solar cells --- p.12 / Chapter 1.4 --- "Cu(In,Ga)Se2 Solar Cells" --- p.13 / Chapter 1.4.1 --- State of the art --- p.13 / Chapter 1.4.2 --- Material properties --- p.14 / Chapter 1.4.3 --- Basic processing steps --- p.15 / Chapter 2 --- Equipment design --- p.24 / Chapter 2.1 --- System design concepts --- p.24 / Chapter 2.2 --- Sample transfer chamber --- p.26 / Chapter 2.3 --- Co-evaporation chamber --- p.28 / Chapter 2.3.1 --- Load-lock chamber --- p.28 / Chapter 2.3.2 --- Co-evaporation chamber --- p.31 / Chapter 2.4 --- Sputtering chambers --- p.34 / Chapter 2.4.1 --- Mo sputtering chamber --- p.34 / Chapter 2.4.2 --- Three targets sputtering chamber --- p.36 / Chapter 2.5 --- Other chambers --- p.38 / Chapter 3 --- Design of Rapid Thermal Processing System --- p.42 / Chapter 3.1 --- Introduction to RTP --- p.42 / Chapter 3.1.1 --- History and current status of RTP --- p.42 / Chapter 3.1.2 --- Advantages of RTP system compared to conventional furnaces --- p.45 / Chapter 3.2 --- Computational simulation for RTP system design --- p.47 / Chapter 3.2.1 --- Introduction to Ansys Fluent --- p.47 / Chapter 3.2.2 --- Model setup steps --- p.54 / Chapter 3.2.3 --- Physical principles --- p.57 / Chapter 3.2.4 --- Models setup and comparisons --- p.62 / Chapter 3.3 --- Rapid thermal processing system --- p.76 / Chapter 3.3.1 --- Se deposition chamber --- p.76 / Chapter 3.3.2 --- Quartz chamber --- p.78 / Chapter 3.3.3 --- Lamp frame --- p.79 / Chapter 4 --- Conclusions --- p.83 / Chapter 4.1 --- RTP heater design --- p.83 / Chapter 4.2 --- Future prospect --- p.83 / Bibliography --- p.87

Bulk heterojunction (BHJ) organic photovoltaic (OPV) is one of the most promising techniques to generate electricity with advantages of flexibility, solution processing and capability for large area device fabrication. Although the power conversion efficiency (PCE) of BHJ solar cells has already achieved over 13%, there are still problems remain to be solved. This thesis presents the binary and ternary organic BHJ devices with alternative donor:acceptor (D:A) ratios, and the charge transport properties and electronic interactions in their BHJ films. In a high performance BHJ solar cell, the commonly optimized D:A weight ratio is about 1:x, where x is commonly in excess of 1.5, when PC71BM is used as the acceptor. We demonstrated how to achieve high PCEs of BHJ solar cells by enriching the D:A weight ratios. The PCEs of the re-optimized cells were improved for the PTB7:PC71BM, PCDTBT:PC71BM, PDTSTPD:PC71BM devices. Current-voltage (JV) and admittance spectroscopy (AS) measurements indicate enhanced hole mobilities for the polymer-rich BHJs based on PTB7, PCDTBT, and PDTSTPD. At the same time, although the relative weight ratio of PC71BM is reduced, the electron mobilities are maintained due to the dispersion of fullerene domains by increased DIO concentrations. The active layer thickness of most optimized BHJ solar cells is about 100nm. The thin active layer is unfavorable for optical absorption and film coating. We employed a ternary strategy to address this problem, and the thick-film BHJ devices can retain 90% PCEs of their optimized thin-film devices. Three model systems were studied, involving PTB7:PC71BM, PTB7-Th:PC71BM and P3HT:PCBM BHJs. Into these BHJs, a ternary component, p-DTS(fbtth2)2 (DTS) is introduced. With DTS, the corresponding thick film devices have significantly improved PCEs. The ternary component DTS improves hole mobility and reduces sub-bandgap trap states. Both observations are well correlated with improved FFs of the ternary BHJ cells. Photothermal deflection spectroscopy (PDS) and 1H nuclear magnetic resonance (1H NMR) results indicate that DTS behaves as conducting bridges in between two neighboring polymer segments. Most lab-based BHJ solar cells are optimized by their power conversion efficiencies (PCEs). We challenge this conventional view by showing that BHJ cells using fullerene acceptors should be optimized by their fill-factors (FFs). With the optimized-FF approach, BHJ cells tend to have higher fullerene content when compared to the BHJ cells that are optimized by PCEs. The FF-optimized BHJ cells have slightly reduced PCEs (due to smaller Jscs) compared to the PCE-optimized cells. Yet, FF-optimized cells enjoy a much better thermal stability. We demonstrate that these FF-optimized BHJs possess better-balanced electron-to-hole mobility ratios due to weakly field-dependent electron mobilities. The improved mobility ratio suppresses carrier recombination. Our results suggest that BHJ cells optimized by their PCEs should be meta-stable, and other D:A ratios should be considered for practical BHJ cell development.

Photovoltaic technology based on solution-processable organic solar cells (OSCs) provides a promising route towards a low-cost strategy to address the sharply increasing energy demands worldwide. However, up to date, the vast majority of solar cell reports have been based on spin-cast BHJ layers. Spin coating is not compatible with high speed and scalable coating processes, such as blade-coating and slot-die coating, which require the nanoscale morphology to be reproduced in scalable coating methods. And tolerance for thicker BHJ films would also facilitate high speed scalable coating. In the first part of this thesis, we investigate how pre-aggregating the conjugated polymer in solution impacts the charge transport in polymer films. We use P3HT in a wide range of molecular weights in different solvents of common use in organic electronics to investigate how they impact the aggregation behavior in the ink and in the solid state. By deliberately disentangling polymer chains via sonication of the solution in the presence of solvophobic driving forces, we show a remarkable ability to tune aggregation, which directly impacts charge transport, as measured in the context of field effect transistors. The second part of this thesis looks at the impact of the solution-coating method and the photovoltaic performance gap when applying modern BHJ inks developed for spin coating to scalable coating methods, namely blade coating. We ascribe this to significant differences in the drying kinetics between the processes. Emulating the drying kinetics of spin-coating was found to result in performance parity as well as morphological parity across several systems, resulting in demonstration of PTB7:PC71BM solar cells with efficiency of 9% and 6.5% PCEs on glass and flexible PET substrates, respectively. The last part of this thesis looks into going beyond performance parity by leveraging the differences of the scalable coating method to enable highly efficient thick solar cells which surpass the performance of spin-cast devices. High-speed wire-bar coating (up to 0.25 m/s) was used to produce OPV devices with power conversion efficiency (PCE) >10% and significantly outperforming devices prepared by spin-coating the BHJ layer for thicknesses >100 nm by maintaining a higher fill factor.

Both organic and perovskite solar cells (OSCs and PSCs, respectively) have shown remarkable progress in recent years reaching power conversion efficiencies (PCEs) of 17.6% and 25.2% for a single cell, respectively. These results were achieved by simultaneous advancements in organic and perovskite materials design and synthesis, as well as device and interfacial engineering. As these emerging photovoltaic technologies move closer to commercialization, further improvements in efficiencies and stability of the solar cells are needed. Interfaces in these thin-film solar cells have proven to be of tremendous importance for both device performance and degradation. This work is focused on studying recombination losses at the charge extracting layers in OSCs and PSCs and finding simple solution-processable ways of improving interfacial contacts. In the first part, we propose a simple way to improve the electron extracting properties of Phen-NaDPO, a small organic molecule widely used in OSCs, by mixing it with Sn(SCN)2. We show that this approach benefits morphology and charge transport, thus reducing recombination losses and improving overall performance of various bulk heterojunction OSCs and PSCs. In the second part, we describe the development of a multilayered system of electron transporting interlayers (ETLs) to improve the PCE and operational stability of PSCs. We sequentially deposit PC60BM, Al-doped ZnO (AZO), and small organic molecule triphenyl-phosphine oxide (TPPO), and study how the ETL properties and device performance change with each layer. We find that the trap-assisted recombination and energy level alignment in PSCs improve due to specific chemical interactions between PC60BM, AZO, and TPPO. The third part is divided into two and is focused on CuSCN, a wide bandgap inorganic molecular hole transporting material, and its application in OSCs. In the first half, we study the recombination and photogeneration processes in PC70BM-only OSCs. We demonstrate that CuSCN plays a crucial role in excitons dissociation and efficient charge transfer at the CuSCN/PC70BM interface. In the second half, we optimize CuSCN layers’ structural and electronic characteristics using a simple solvent engineering approach. We study how processing conditions affect the morphological, chemical, optical, and electronic properties of CuSCN and how they impact the OSCs’ performance.

No description available.

This thesis is a power generation study of a proposed CubeSat at Mississippi State University (MSU). CubeSats are miniaturized satellites of 10 x 10 x 10 cm in dimension. Their power source once in orbit is the sun during daylight and the batteries during eclipse. MSU CubeSat is equipped with solar panels. This effort will discuss two types of cells: Gallium Arsenide and Silicon; and which one will suit MSU CubeSat best. Once the cell type is chosen, another decision regarding the electrical power subsystem will be made. Solar array design can only be done once the choice of the electrical power subsystem and the solar cells is made. Then the power calculation for different mission durations will start along with the sizing of the solar arrays. In the last part the batteries are introduced and discussed in order to choose one type of batteries for MSU CubeSat.

As the power conversion efficiency (PCE) of single-junction solar cells approaching its theoretical limit, tandem solar cells have attracted great attention due to their ability to break this limitation. For example, the PCE of crystalline silicon-based solar cells (c-Si) reached 26.81% with an area of 274.4 cm2, approaching the theoretical limit of 29.4%. By combining the c-Si with perovskites, the theoretical PCE limitation of 29.4% can be further increased to 45%. The wide-bandgap (1.68 eV) inverted (p-i-n) perovskite solar cells (PSCs) are ideal candidates to integrate on top of narrow-bandgap solar cells to fabricate tandem solar cells, owing to the simple fabrication process and tunable bandgap. However, the PCE of wide-bandgap perovskite solar cells is limited by the severe open-circuit voltage loss due to non-radiative recombination arising from trap-assisted recombination and interfacial recombination. In this thesis, Poly[(9,9-bis[3-(trimethylammonium)propyl-2,7-fluorene)]-alt-2,7- (9,9-dioctylfluorene) diiodide (PFN-I), as modification layer between hole transport layer (HTL) and perovskite, was applied to efficiently passivate the interfacial defects, moderate the growth of perovskite crystal and modify the interfacial energy level alignment to enhance hole extraction. Through comprehensive characterization, it has been observed that the introduction of PFN-I into the system effectively reduces non-radiative recombination. Therefore, a PCE of 21.9% with an open-circuit voltage of 1.24 V and a fill factor of 80% was obtained for 1.68 eV-bandgap inverted PSCs.

For the fabrication of semiconductor devices, solar cells, and infrared detectors, thin film deposition methods are required. Of the deposition methods currently available including MBE, LPE, and MOCVD; MOCVD is preferred due to its relatively low cost per wafer, versatility, and high wafer throughput. Requirements which must be considered in the design of a deposition system are discussed. An MOCVD system is designed such that MOCVD can be carried out by plasma enhanced deposition (PED) or low pressure metal-organic chemical vapor deposition (LPMOCVD). As a result of the inherent characteristics of the two methods, a wide range of operational temperatures and pressures are possible. Software is developed for system control including a graphical display of the process schematic. The deposition of GaAS on Si is given as one possible application for this type system.

Optoelectronic characterisation techniques are assessed in their application to three solar cell systems. Charge injection barriers are found in PbS/ZnO colloidal quantum dot solar cells through the use of temperature dependent current-voltage and capacitance-voltage measurements. The injection barriers are shown to complicate the Mott-Schottky capacitance analysis which determines built-in bias and doping density. A model that incorporates depletion capacitance and a constant capacitance arising from the injection barriers is given to explain the Mott-Schottky plots. The junction mechanism at the PbS/ZnO interface is found to transition from excitonic to p-n behaviour based on the amount of UV photodoping the cell has received. External quantum efficiency analysis at different photodoping times reveals a growing charge collection region within the material, demonstrating the shift to p-n behaviour. This is further supported by the observance of depletion capacitance behaviour after, but not before, UV photodoping. Defects within GaAs cells containing InAs quantum dots are found to enhance the sub-bandgap performance of the cell using external quantum efficiency analysis. This is verified by illuminated current-voltage analysis using a 1000 nm high pass optical filter to block photons of larger energy than the bandgap. Using capacitance-voltage analysis, high temperature rapid thermal annealing is shown to induce defects in dilute nitride cells, which explains the drop in open circuit voltage compared to lower temperature annealed cells. The doping level of polymer solar cells exposed to air is found to increase with continued exposure using Mott-Schottky capacitance analysis. Current-voltage measurements show the formation of an Al2O3 barrier layer at the polymer/aluminium interface. The usefulness of capacitance-voltage measurements to probe the polymer/fullerene interface is investigated in thermally evaporated thiophene/C60 cells.

Crystalline silicon solar cells translate energy from the sun into electrical energy via the photoelectric effect. This technology has the potential to simultaneously reduce carbon emissions and our dependence on fossil fuels. The cost of photovoltaic energy, however, is still higher than the cost of electricity off of the grid which hampers this technologies adoption. Raising solar cell efficiency without significantly raising the cost is crucial to lowering the cost of photovoltaic produced energy. One technology which holds promise to increase solar cell efficiency is a selective emitter solar cell. In this work the benefit of selective emitter solar cells is quantified through numerical modeling. Further, the use of ultraviolet laser to create a laser doped selective emitter solar cell is explored. Through optimization of the laser doping process to minimize laser induced defects it is shown that this process can increase solar cell efficiency to over 19.1%. Additionally, 2D and 3D numerical modeling are performed to determine the limitations screen printed interdigitated back contact solar cells and the practical efficiency limit for crystalline Si solar cells.