Effect of Thermal Annealing of Atomic-Layer-Deposited AlOx/ Chemical Tunnel Oxide Stack Layer at the PEDOT : PSS/n-type Si Interface to Improve its Junction Quality

埼玉大学博士(学術)xxiv, 106pHigh-efficiency c-Si solar cells are attracting interest in research, in which a metal oxide or organic polymer thin film having electron/hole transporting ability is bonded to crystalline Si （c-Si） as an anode and a cathode. In our previous study, a transparent and high hole-transporting conductive polymer poly（3,4-ethylenedioxy thiophene）:poly（styrene sulfonate） （PEDOT:PSS） was used to plane n-type crystal Si （We have achieved an efficiency of 13.3% for pristine PEDOT:PSS device）, and have achieved an efficiency of 15.5% by using an antireflection film together. Furthermore, we have prototyped 10 series modules for 2 cm2 and 4-inch size elements and have demonstrated the potential as an independent power supply for surveillance cameras. However, in order to further improve the device performance, it is necessary to promote the passivation of the c-Si/PEDOT:PSS anode-cathode interface and further enhance the internal electric field at the anode-cathode interface. In this doctoral dissertation, aluminum oxide （AlOx）, which is expected to have high Si passivation ability and high fixed charge density, among high-k materials with high dielectric constant, and PEDOT:PSS/n-Si interface were selected. The effect of increasing the electric field strength at the interface was investigated by inserting the film into the PEDOT:PSS/n-Si interface. Firstly, by arranging the amorphous （a-） AlOx films prepared by atomic layer deposition （ALD） in islands, the efficiency of hole collection at the anode, and the improvement of AlOx to increase the electric field strength at the anode interface. Secondly, Si passivation by short-time heat treatment （RTA） at 425℃ in a reduced pressure environment and an atmospheric pressure heat treatment （FGA） in an N₂/H₂ environment of a coated structure of a 1-2 nm thick AlOx layer and a chemically oxidized SiOx layer. And the effects on PEDOT:PSS/n-Si interface local chemical bond state, electronic structure, and device performance were investigated. This doctoral dissertation consists of 6 chapters. The outline of each chapter is described below: Chapter 1, "Introduction", describes the background of research and development of crystalline Si solar cells and the purpose of this paper. In Chapter 2, the effect of a TiOx layer as a hole blocking layer at the Si/Al interface on the back cathode of a c-Si solar cell is discussed from steady-state photocurrent and current transient response characteristics. Specifically, focusing on TiO₂, which has a small work function, a titanium oxide （TiO₂） synthesized by hydrolysis of TiCl₂ was used to insert a TiOx layer with a thickness of 1-2 nm at the n-Si/Al interface by spin coating. It has been demonstrated that the device structure is effective in improving the conversion efficiency and the efficiency of collecting photo-generated carriers in the 600-1200 nm region as compared with the device performance without insertion. Furthermore, in order to quantitatively evaluate the hole blocking ability, the hole current pushed back into the c-Si by the hole blocking layer by instantaneously applying the reverse bias from the steady current when the forward bias was applied in the dark. We established a Transient Reverse Recovery measurement to determine the recombination velocity S from the waveform. As a result of a comparative study of this technique with existing techniques for evaluating the recombination rate, such as the QSSPC and μ-PCD methods, it has been clarified that it can be sufficiently used as a technique for evaluating the performance of the hole blocking layer. Chapter 3, “Experimental procedure”, describes the fabrication method and the evaluation method of the ultra-thin aluminum oxide （AlOx） film, which is expected as a high dielectric constant material （high-k）, by the ALD method for the purpose of enhancing the electric field strength at the PEDOT:PSS/n-Si anode interface. It also describes the photolithography process for forming a 15 × 15 μm² island array, chemical oxide layer formation, RTA, and FGA heat-treatment process. About the in-plane distribution of minority carrier lifetime （τeff） by μ-photoconductive decay method （μ-PCD method）, X-ray photoelectron spectroscopy （XPS） method, and infrared absorption spectroscopy （FTIR） evaluation method for the above ultrathin films. In Chapter 4, 4.1 describes the fundamental physical properties of amorphous （a-）AlOx insulator thin films produced by an alternate supply of TMA[Al（CH₃）₃] and water by the ALD method. Film formation was performed by using TMA, water supply time, time sequence, substrate temperature as variables, the film thickness, surface morphology, and local chemical bonding state were evaluated. In addition, the physical properties of n-Si interface bonding were evaluated by short-time heat treatment （RTA） at 425°C for 15 minutes in a reduced pressure environment after film formation. As a result, it was clarified that the surface roughness and the Al（OH） bond remaining in the film were reduced most at the substrate temperature of 200°C, and a dense amorphous structure was formed. In 4.2, in order to achieve both passivation of the c-Si surface and hole trapping ability, a 15 × 15 μm2 size of 20 nm thick ALD a-AlOx with different lattice spacing was formed on the c-Si surface by photolithography. We investigated the PEDOT:PSS/a-AlOx/n-Si junction characteristics and device performance of spin-coated 80 nm thick PEDOT:PSS layer on the island array. The Si passivation ability was improved, and the diffusion potential was increased to 1.4V with the increase of the a-AlOx/PEDOT:PSS area ratio, but the fill factor in the solar cell element was significantly decreased by the increase of the parallel resistance, which deteriorated the conversion efficiency. Therefore, in Section 4.3, we investigated the effect of inserting a 2-3 nm thick AlOx ultrathin layer formed by the ALD method as a tunnel layer and Si passivation. In the RTA of 20-nm-thick a-AlOx, τeff was reduced from 150 μs to 15-30 μs. On the other hand, τeff can be improved to 600-700 μs by FGA treatment of a-AlOx/ch-SiOx/n-Si coated structure with ch-SiOx of 1-2 nm thickness inserted at the a-AlOx/n-Si interface. It is revealed that, from the evaluation of the sheet resistance, as a result of FGA processing of the coated structure, it decreases from PEDOT:PSS/c-Si junction 162Ω/□ to PEDOT:PSS/a-AlOx/ch-SiOx/n-Si coated structure 105Ω/□. In addition that, from the evaluation of the capacitance-voltage （C-V） characteristics, the fixed charge density was changed from 3.2 × 10¹² cm-² to 5.7 × 10¹² cm-² by applying FGA processing from a-AlOx/n-Si to a-AlOx/ch-SiOx/n-Si coated structure, and the interface state density decreased from 4.5 × 10¹¹ cm-²eV-¹ to 2 × 10¹¹ cm-²eV-¹. Moreover, the conversion efficiency of the device on the planarized n-Si with PEDOT:PSS/a-AlOx/ch-SiOx/n-Si coated structure as anode is 13.08% without insertion and 14.91% （open voltage: 0.645V, Fill factor: 0.77, short-circuit current density increased to 30 mA/cm²）. In Chapter 5, the electronic structure of the interface in the a-AlOx/ch-SiOx/n-Si layered structure by ch-SiOx insertion and RTA and FGA treatment was evaluated by XPS, UV spectroscopy, and Kelvin probe method. FGA treatment revealed that ch-SiOx contained a large proportion of Si* complexes near 103 eV, which did not belong to Si+, Si²+, Si³+, and Si⁴+, compared to RTA. In RTA, the oxidation of the a-AlOx layer on the surface became dominant, whereas in FGA, the oxidation of the ch-SiOx layer was promoted as the reduction of the AlOx layer progresses, and as a result, the passivation ability of the c-Si surface was improved. Besides, the band level diagram of the PEDOT:PSS/a-AlOx/ch-SiOx/n-Si interface was determined, and the a-AlOx/ch-SiOx/n-Si coated structure was inserted, and the subsequent FGA treatment was performed to form the anode interface. It was clarified for the first time that the electric field strength was enhanced. In Chapter 6, we summarized the doctoral dissertation, gave conclusions, and mentioned future prospects.Abstract ......................................................................................................... III Acknowledgments ................................................................................................. VIII List of Publications and Presentations ............................................................................. X List of Tables ................................................................................................... XII List of Figures ................................................................................................. XIII List of Abbreviations .......................................................................................... XVIII Table of Contents ................................................................................................. XX Chapter 1 .......................................................................................................... 1 Introduction ....................................................................................................... 1 1.1 Research background ............................................................................................ 1 1.2 Crystalline-Si/Organic Heterojunction Solar Cells ....................................................... 3 1.3. Background of the Solution-Processed Hybrid PVs .................................................... 3 1.4 Motivation of this study ....................................................................................... 5 1.5 Outline of This Dissertation .................................................................................. 10 Bibliography ...................................................................................................... 13 Chapter 2 ......................................................................................................... 23 Effect of TiO₂ as a Hole Blocking Layer in the PEDOT:PSS/n-Si Heterojunction Solar Cells .......................... 23 2.1. Introduction ................................................................................................. 23 2.2 Experimental Details .......................................................................................... 24 2.2.1 Solution-processed TiO₂ and the device fabrication .............................................. 24 2.3 Characterizations ............................................................................................. 26 2.3.1 XPS study ................................................................................................... 27 2.3.2 Minority Carrier Lifetime ................................................................................... 28 2.3.3 Transient reverse recovery (Trr) measurement ................................................................ 28 2.4 Results and discussion ........................................................................................ 30 2.4.1 Photovoltaic performance of solar cells ............................................................... 30 2.4.2 Junction property at the Si/TiO₂ cathode interface monitored by the Trr characterization .................... 33 2.3 Summary and conclusions................................................................................... 34 Bibliography ...................................................................................................... 35 Chapter 3 ......................................................................................................... 39 Experimental Procedure and Characterization Method ..................................................... 39 3.1 Experimental Procedure......................................................................................... 39 3.1.1 Fabrication process of PEDOT:PSS/n-Si heterojunction solar cell ................ 39 3.1.2 Deposition of AlOx on c-Si by Atomic Layer Deposition (ALD) .................... 40 3.1.2.1 Principle of AlOx deposition by ALD .......................................... 40 3.1.2.2. Sample preparation ....................................................................................... 41 3.1.2.3 AlOx film deposition by ALD ............................................................................. 42 3.1.3 Preparation of AlOx island by UV photolithography process .................................. 43 3.1.3.1 Sample preparation ........................................................................................ 43 3.1.3.2 Preparation of AlOx island by UV photolithography process .... 44 3.1.3.3 Device fabrication PEDOT:PSS/n-Si heterojunction solar cells with AlOx island.................. 47 3.1.4 Fabrication process PEDOT:PSS/n-Si heterojunction solar cell with ultrathin AlOx/ch-SiOx (1~3 nm) ........... 48 3.1.4.1 Fabrication process of ch-SiOx at the AlOx/n-Si interface ................ 48 3.1.4.2 Fabrication process PEDOT:PSS/n-Si heterojunction solar cells with AlOx/ch-SiOx stack layer ............... 48 3.2 Characterization method ....................................................................................... 50 3.2.1 Micro-photoconductive decay (μ-PCD) ................................................................. 50 3.2.2 Forming gas annealing (FGA) and Rapid thermal annealing (RTA)method ...................................... 51 3.2.3 Atomic Force Microscopy (AFM) ........................................................................... 51 3.2.4 Fourier-transform infrared spectroscopy (FTIR) ..................................................... 52 3.2.5 Capacitance-Voltage (C-V) Profiling ...................................................................... 54 3.2.6 X-ray electron spectroscopy (XPS) method ........................................................... 55 3.2.7 Photoemission Yield Spectroscopy in Air (PYSA) .................................................... 56 3.2.8 Four-probe method for sheet resistance measurement .......................................... 58 Bibliography ...................................................................................................... 60 Chapter 4 ......................................................................................................... 62 ALD-AlOx related result and discussion ........................................................................ 62 4.1 Fundamental properties of AlOx film deposited by ALD ............................................. 62 4.1.1 ALD- AlOx film characterization .......................................................................... 62 4.1.1.1 ALD- AlOx film thickness .................................................................................. 62 4.1.1.2 AFM study ................................................................................................. 65 4.2 Effect of ALD-AlOx island at the PEDOT:PSS/n-Si interface property by the UV photolithography process ......... 69 4.2.1 Effect of AlOx island at the PEDOT:PSS/n-Si interface for different area ratio of AlOx and PEDOT:PSS ........ 69 4.2.2 Effect of AlOx island at the PEDOT:PSS/n-Si interface for different donor density substrate ................. 73 4.3 Effect of thermally annealed atomic-layer-deposited AlOx/chemical tunnel oxide stack layer at the PEDOT:PSS/n-type Si interface to improve its junction quality ....................................................................... 77 4.3.1 Effect of FGA and RTA at the stack layer of ALD-AlOx/ch-SiOx /c-Si .......................................... 77 4.3.1.1 Effective lifetime of ALD-AlOx/ch-SiOx stack layer on c-Si................................................. 77 4.3.1.3 Study of XPS for the ALD-AlOx/SiOx stack layer on c-Si with and without FGA. .............................. 81 4.3.1.3 Effect of RTA and FGA on the ALD-AlOx with and without ch-SiOx (1~3nm) by FTIR spectra .................... 85 4.3.1 PV performance of FGA treated ALD-AlOx/ch-SiOx stack layer with the PEDOT:PSS/n-Si heterojunction solar cell .................... 86 Bibliography ...................................................................................................... 91 Chapter 5 ......................................................................................................... 93 Band alignment at the PEDOT:PSS/a-AlOx/ch-SiOx/c-Si interface ..................................................... 93 5.1 Determination of band offset and band alignment of ALD- AlOx/SiOx stack layer on n-Si substrate ............... 93 5.3.4 Effect of FGA treated ALD-AlOx/ch-SiOx stack layer at the PEDOT:PSS/n-Si interface .......................... 98 Bibliography ..................................................................................................... 102 Chapter 6 ........................................................................................................ 104 Summary and future work ........................................................................................ 104 5.1 Summary and conclusion .................................................................................... 104 5.2 Future Work .................................................................................................. 106指導教員 : 白井肇textapplication/pdfdoctoral thesi

Overcoming the performance limitations of industrial silicon solar cell by laser doping technology

Global photovoltaic (PV) market harvested rapid growth annually during the past two decades. As the primary part of this rapid growth, most of Silicon (Si) wafer-based solar cells employ screen printing(SP) technology for metallization on both polarities of P-type CZ Si wafers and the solar cell efficiency is around 18%. This efficiency is significantly lower than that of high efficiency solar cells developed in research laboratory. The fabrication of high efficiency solar cells in laboratory usually involves multiple expensive processes and high quality silicon material which are not suitable for production. Compared to the complicated and expensive laboratory process for high efficiency solar cell, SP technology is a simple and robust process. However, such technology also has some disadvantages such as poor blue response, high shading issue, low aspect ratio of metal fingers and insufficient rear surface passivation.Selective emitter (SE) technology is one possible way to further increase solar cell efficiency by harvesting more blue light. SE technology is of high interest to the PV industry in recent past. One of the cost effective method of achieving SE is by using laser doping. The laser doped selective emitter (LDSE) solar cell developed at the University of New South Wales (UNSW) combines laser doping and self-aligned light induced plating (LIP) technologies which make it one of the most feasible solutions for industrial selective emitter solar cell structure. The main advantage of LDSE technology is the simultaneously creation of dielectric layer patterning and localized heavy doping without an extra high temperature process or any other masking processes which are required by other technologies. In this thesis, LDSE technology was employed to improve the conversion efficiency of solar cells using a commercial available continuous wave (CW) green laser. The basic laser theory was reviewed and laser induced defects by CW laser were studied. The impact of different laser parameters and dielectric layer combinations on the morphology of laser scanned region was investigated. The process optimization of standard LDSE solar cells was presented. The focus of optimization was given to two key processes: laser doping and light induce plating in the LDSE solar cell process. Wide ranges of parameters were investigated in detail for each process, such that systematic improvements were demonstrated. The influences of different parameters on final solar cell devices were studied through the use of techniques such as light I-V curve, spectral response, scanning electron microscope, focus ion beam etc. As a result of optimizations, a final solar cell device with efficiency >19% was achieved on p-type 1 Ω cm Czochralski (CZ) silicon wafer.The other very important aspect of improving solar cell efficiency is the rear surface design. The aluminum (Al) back surface field (BSF) used in SP solar cells only has moderate passivation effect. Such Al BSF was identified as a limiting factor for standard LDSE solar cells. To overcome this problem, a stack dielectric layer of silicon dioxide (SiO2) and silicon oxynitride (SiOxNy) was developed. Such stack layer demonstrated excellent surface and bulk passivation ability on commercial grade p-type 1 Ω cm CZ Si wafer with carrier lifetime of 670µs and implied open circuit voltage (iVoc) of 735 mV. The significance of rear surface passivation has been realized by researchers worldwide as a key approach for high efficiency solar cell designs and several dielectric layers were successfully developed to passivate Si surface and get high carrier lifetime. However, challenge lies in making local contact opening on the dielectric layers or forming local BSF (LBSF) without massive jeopardize passivation ability using industry suitable approach rather than expensive photolithograph which is normally employed in laboratory. As a local heating process, laser doping could achieve dopants diffusion and dielectric layer patterning in step without causing massive degradation in the passivation ability of dielectric layer. In this thesis, CW laser system was used to create double sided laser doped (DSLD) solar cell by performing phosphorous and boron laser doping to front and rear surface of industry p-type CZ Si wafer passivated by SiNx/SiO2 stack layers on the front surface and SiO2/SiOxNy stack layers on the rear surface. By optimizing the thermal stability of the dielectric layers, emitter sheet resistance and the laser doping parameters, over 700mV iVoc was achieved on CZ Si samples after laser process, prior to metallization. As proof-of-concept, DSLD solar cells were made on CZ samples. Open circuit voltage (Voc) in the range of 660mV was achieved with high short circuit current density. However, at this stage, the solar cells efficiency is limited by low fill factor (FF). The origin of low FF was investigated and possible causes and solutions were discussed. At the end of this thesis, DSLD solar cells with Voc of 680mV was achieved on p-type 1 Ω cm CZ silicon wafers by optimizing rear surface metallization process

High efficiency thin film silicon solar cells with novel light trapping : principle, design and processing

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2008.Includes bibliographical references.One major efficiency limiting factor in thin film solar cells is weak absorption of long wavelength photons due to the limited optical path length imposed by the thin film thickness. This is especially severe in Si because of its indirect bandgap. This thesis invents a novel light trapping scheme, the textured photonic crystal (TPC) backside reflector, which can enhance path length by at least several hundred times the film thickness for sufficient absorption. Physical principles and design optimization of TPC are discussed in detail. Thin film Si solar cells integrated with the new back reflector are successfully fabricated and significant efficiency enhancement is demonstrated.The new back reflector combines a one-dimensional photonic crystal as a distributed Bragg reflector (DBR) and reflection grating. The DBR achieves near unity reflectivity in a wide omnidirectional bandgap completely covering the wavelengths needing light trapping, and the grating can diffract light into large oblique angles and form total internal reflection against the front surface of the cell. The unique combination of DBR and grating tightly confines light inside the cell, effectively changing the path length from the thickness of the cell to its width.The back reflector parameters and the antireflection coating are systematically optimized for thin film Si solar cells through simulation and experiments. A 2 [mu]m thick cell can achieve 54% efficiency enhancement using the optimized design.For proof of concept, the TPC back reflector is integrated with thick crystalline Si solar cells (675 [mu]m thick), which demonstrate external quantum efficiency enhancement up to 135 times in the wavelength range of 1000-1200 nm.(cont.) To prove the theory on the intended application, top-contacted thin film Si solar cells integrated with the TPC back reflector are successfully fabricated using Si-on-insulator material through an active layer transfer technique. All cells exhibit strong absorption enhancement, similar to that predicted by simulation. The 5 [mu]m thick cells gained 19% short circuit current density improvement, despite machine problems during fabrication. The textured photonic crystal back reflector design can be applied directly to single and poly-crystalline Si solar cells, and its principle is broadly applicable to other materials systems.by Lirong Zeng.Ph.D

Application of amorphous silicon for photovoltaic silicon surface passivation

In recent years, the application of hydrogenated amorphous silicon (a-Si:H) to crystalline silicon (c-Si) solar cells for the purpose of surface passivation has begun to move rapidly forward following early innovations by Sanyo. The bulk of the research conducted throughout this thesis has been performed prior to this new drive in the development of a-Si:H/c-Si devices. Understanding the underlying principles and the essential physics concerning the interaction of these two materials has been often overlooked, making further improvements difficult, and limiting new technological developments therein. In this light, the strategies towards merging a-Si:H with c-Si to achieve high{u00AD} efficiency, low-cost photovoltaics are studied in this thesis, with a focus on the interface and lowering interface states. Plasma-Enhanced chemical vapour deposition (PECVD) of a-Si:H has commonly been an effective method for achieving uniform coverage of the c-Si surface. However, many deposition parameters have been reported as optimal, stemming from the limited range of experimental conditions examined. In this study, a more complete range of deposition conditions are tested, with the nature of the a-Si:H across a broad array of parameters being investigated. Ideally, a-Si:H layers which are most likely to result in high quality surface passivation should be deposited using temperatures of 225{u00B0}C, applied rf-power at 4W (SlmW/cm{u00B2} ), and partial pressure of 650mT. Notably, the ranges for deposition that can be ideally utilised, are with temperatures between 200{u00B0}C and 250{u00B0}C, rf-power up to 8W (100mW/cm{u00B2}), and partial pressures between 400mT and 750mT. Although the ideal values are somewhat system specific, these broader ranges are common to many PECVD systems. Previously overlooked in many studies on a-Si:H and indeed most hydrogenated materials is the influence of hydrides on the surface passivation. A widespread belief is that layers hydrogen{u00AD} rich in their bulk are best for passivation, due to a plentiful source of hydrogen. Analysis of hydride density by IR-spectroscopy has revealed several interesting results which identify some misconceptions concerning surface passivation and the influence of hydrides. In particular, this thesis clarifies the function of the composition and distribution of hydrides throughout the layer and their influence on the quality of the surface passivation; the existence of bulk and interface regions within the a-Si:H layer; and the influence of deposition conditions on the composition and density of hydrides. Ideally, a hydride-rich interface region is shown to yield the most reliable results. The diffusion of hydrogen from within the a-Si:H bulk towards the interface with c-Si at an energy of l.SeV has been a widely accepted mechanism governing surface passivation. However, experimental evidence to support this preferential diffusion through a high-defect material such as a-Si:H has been somewhat absent. In this work, an Arrhenius relationship between temperature and surface passivation is revealed, providing evidence that disputes the a-Si:H bulk-diffusion hypothesis in favour of a surface-diffusion mechanism at the a-Si:H/c-Si interface. The thermally activated surface passivation is shown to have energy of 0.7 {u00B1} O.leV, below that required for bulk diffusion or spontaneous release of hydrogen. From this experimental study, new insight into a surface-related transport model governing the passivation of the c-Si surface by hydrogen already present at or near the interface is presented in this thesis. Determined in this physical model, is the relationship between the likelihood of hydrogen diffusion across a c-Si surface and temperature. Following from early experimentation using post-deposition thermal annealing to improve surface passivation by a-Si:H, a new plasma-enhanced chemical vapour deposition (PECVD) technique was developed as part of this work. Multi-Layer-PECVD involves the deposition of sub{u00AD} layers of a-Si:H with thermal cycling, to build up a total layer thickness. This technique of sub-layer deposition is shown to improve the control of hydride density, composition, surface coverage and reduce the inherent thin-film stresses for very thin a-Si:H layers. Comparison of layers deposited by ML-PECVD in-place of standard PECVD showed improved reliability and stability thanks to this new approach to deposition of a-Si:H. With a greater understanding for the properties of a-Si:H in passivating c-Si and improvements in deposition technique, stacked a-Si:H structures which combine n-type or p-type a{u00AD} Si:H with a thin intrinsic a-Si:H layer in a HiT-like design are investigated from the perspective of passivating c-Si. Results here show that high-quality surface passivation can be maintained, with recombination velocities and saturation current densities at the c-Si surface as low as 3cms{u207B}{u00B9} and averaged below 30fA/cm{u00B2} respectively, which are equivalent to those achieved with SiOx and SiN layers. In a world first application, the a-Si:H(i) and stacked a-Si:H layer structure have been applied in this thesis to the mc-Si surface; whereby, excellent surface passivation results are achieved using both n- and p-type mc-Si. Recombination velocities below lOOcms{u207B}{u00B9} using only a-Si:H(i) were reduced further to approximately 40cms{u207B}{u00B9} with stacked a-Si:H(i/n) or a-Si:H(i/n) layers, without a diffused emitter. In addition, low current saturation densities of 4.5 x 10{u207B}{u00B9}{u2074}Acm{u207B}{u00B2} and implied open{u00AD}circuit voltages of 670mV were achieved. In the case of 100{u03BC}m mc-Si, further reductions are shown to be possible, opening the doorway for simple, high-efficiency mc-Si based photovoltaic designs at low-cost. The work in this thesis has yielded an improved understanding relating to a-Si:H/c-Si devices. Fundamental misconceptions concerning the hydrogen passivation mechanism, hydride content and configuration have been identified and a more accurate understanding has been proposed. Although many of the principles in the Sanyo HIT design have recently been reproduced by other groups, the implications of this research remain applicable. Importantly, the research regarding the optimisation of a-Si:H, development of ML-PECVD and many of the preliminary findings of this research are focused on high-efficiency, low-cost next generation photovoltaic designs yet to be developed

Gallium Phosphide Integrated with Silicon Heterojunction Solar Cells

abstract: It has been a long-standing goal to epitaxially integrate III-V alloys with Si substrates which can enable low-cost microelectronic and optoelectronic systems. Among the III-V alloys, gallium phosphide (GaP) is a strong candidate, especially for solar cells applications. Gallium phosphide with small lattice mismatch (~0.4%) to Si enables coherent/pseudomorphic epitaxial growth with little crystalline defect creation. The band offset between Si and GaP suggests that GaP can function as an electron-selective contact, and it has been theoretically shown that GaP/Si integrated solar cells have the potential to overcome the limitations of common a-Si based heterojunction (SHJ) solar cells. Despite the promising potential of GaP/Si heterojunction solar cells, there are two main obstacles to realize high performance photovoltaic devices from this structure. First, the growth of the polar material (GaP) on the non-polar material (Si) is a challenge in how to suppress the formation of structural defects, such as anti-phase domains (APD). Further, it is widely observed that the minority-carrier lifetime of the Si substrates is significantly decreased during epitaxially growth of GaP on Si. In this dissertation, two different GaP growth methods were compared and analyzed, including migration-enhanced epitaxy (MEE) and traditional molecular beam epitaxy (MBE). High quality GaP can be realized on precisely oriented (001) Si substrates by MBE growth, and the investigation of structural defect creation in the GaP/Si epitaxial structures was conducted using high resolution X-ray diffraction (HRXRD) and high resolution transmission electron microscopy (HRTEM). The mechanisms responsible for lifetime degradation were further investigated, and it was found that external fast diffusors are the origin for the degradation. Two practical approaches including the use of both a SiNx diffusion barrier layer and P-diffused layers, to suppress the Si minority-carrier lifetime degradation during GaP epitaxial growth on Si by MBE were proposed. To achieve high performance of GaP/Si solar cells, different GaP/Si structures were designed, fabricated and compared, including GaP as a hetero-emitter, GaP as a heterojunction on the rear side, inserting passivation membrane layers at the GaP/Si interface, and GaP/wet-oxide functioning as a passivation contact. A designed of a-Si free carrier-selective contact MoOx/Si/GaP solar cells demonstrated 14.1% power conversion efficiency.Dissertation/ThesisDoctoral Dissertation Electrical Engineering 201

Optimization of processes for the rear side of monocrystalline silicon solar cells

En esta tesis, se ha estudiado la formación de un campo local superficial trasero (LBSF) de aluminio mediante el uso de diferentes pastas de dicho metal, las cuales contienen fritas en su composición para una mejor sinterización. Para la impresión de la pasta sobre los diferentes sustratos de silicio monocristalino de tipo p se han empleado dos técnicas diferentes. Por un lado, la serigrafía, técnica ampliamente usada en la industria fotovoltaica tanto para la formación de contactos frontales como posteriores. Por el otro lado, se ha utilizado también la técnica de dispensado, la cual permite una impresión de la pasta evitando el contacto con el sustrato, posibilitando la impresión sobre obleas muy finas de silicio evitando su rotura. Se han obtenido las resistencias específicas de contacto para las diferentes estructuras de aluminio creadas y los resultados se han comparado para diferentes pastas y sustratos. De esta manera, previa comprobación mediante imágenes microscópicas de cortes transversales de las muestras, se garantiza la formación del campo local superficial trasero. Con el objetivo de una mejora de la eficiencia de las células solares de silicio, se han depositado diferentes capas finas sobre sustratos de silicio mediante las técnicas de magnetrón sputtering y PECVD (Plasma Enhanced Chemical Vapor Deposition), con el fin de darle características pasivantes, como SiO2, SiNx y SiriON (silicon rich oxynitride); antirreflejantes, como SiNx; o de óxido conductor transparente para células de heterounión, como el caso de la capa de ZnO:Al. Para el estudio de dichas propiedades, se han empleado diversas técnicas, como QSSPC (Quasi-Steady State Photoconductance) para analizar la calidad de la pasivación y espectrofotometría para la medida de las propiedades ópticas. También se han empleado técnicas de caracterización de capa delgada para analizar su estructura y morfología. Sobre las capas finas depositadas sobre silicio, se ha realizado el depósito de las pastas de aluminio para, mediante el método FTC (fire through contact), formar un campo local superficial trasero. Este método se ha utilizado como alternativa a los que se suelen usar en la industria fotovoltaica, los cuales utilizan una serie de pasos que incrementan el coste del proceso total, como láseres o el depósito de resinas para la posterior eliminación por método químico antes de la impresión metálica. Se incluye un capítulo dedicado a la optimización del emisor de aluminio para células de contacto-trasero unión-trasera desarrollado en el Fraunhofer ISE (Alemania), con el objeto de aplicar las mismas técnicas de recubrimiento para el desarrollo de este nuevo tipo de celda solar y su posible transferencia a la producción industrial, atendiendo al proyecto de plan nacional de título Transferencia de las estructuras de alta eficiencia a la producción industrial

Enhanced Conducting Polymer PEDOT:PSS/ Silicon Hybrid Solar Cells: Optimization of Thin Film Properties and Heterojunction Interactions

In this work the operating properties of PEDOT:PSS - silicon hybrid solar cells were carefully studied. The motivation is to find a cost-effective alternative to some of the energy, environmental and sustainability issues the world is currently facing. Solar cells are already providing plentiful renewable energy but they remain constrained by inflexibility, weight, cost and efficiency. Overcoming these obstacles will allow these energy producing devices to become widespread and, along with them, new technologies and economies to emerge. Hybrid solar cells (HSCs) address these issues by junctioning conducting polymer with an inorganic semiconductor. The beneficial properties of these materials can be exploited to improve the cost effectiveness, flexibility, and ease of processing of the polymer, and the electron lifetime and diffusion length, and stability of the inorganic material. PEDOT:PSS and n-type silicon is one such HSC with easily reproducible high efficiencies around 12% that could greatly reduce the cost per watt ratio and other challenges associated with conventional silicon solar cells. In Chapter 2, the material properties of inorganic semiconductors, focusing on silicon, are introduced and the details of Schottky junctions, p-n junction and solar cells properties are discussed. These properties are compared and contrasted with conducting polymers, focusing on PEDOT:PSS, which function in a fundamentally different way from inorganics in that charge mobility is much more limited to intra or inter molecular transport. Finally the physics behind thin films and surfaces for the absorbance of light is examined. In Chapter 3, a novel method of increasing the conductivity of PEDOT:PSS was found by post-treating pre-deposited films with a 50 vol% ethylene glycol/methanol mixture. This post-treatment method more than doubled the conductivity to 1334 S/cm over the method of adding an ethylene glycol co-solvent to the PEDOT:PSS solution (637 S/cm). It also reduced the film thickness in half (51%) due to the removal of PSS. The treatment resulted in PEDOT to have a greater quinoidal character, and because of the decrease in PSS, more defined PEDOT containing nanodomains with the chains laying horizontal to the substrate. In Chapter 4, the optical properties of the films were studied using a single stack layer to model the reflectance of PEDOT:PSS on silicon and to determine the effects of film thickness on short circuit current density (JSC). Using the complex refractive index, the reflection of PEDOT:PSS films and silicon respectively, and the phase shifts found from fitting experimental transmittance and reflectance data, the external quantum efficiency (EQE) for the cells could be simulated. The JSC calculated from these results showed that JSC followed a sigmoidal curve and the highest value 28.3 mA/cm2 was obtained at a thickness of 85 nm. Interestingly, the post-treated cells had overall lower JSC with a maximum of 26.2 mA/cm2 at 63 nm due to the increased anisotropy, absorbance and series resistance. A comparative measure of resistivity was performed by removing the optical component from the quantum efficiency to reveal the electrical contributions and by fitting the data using a modified single diode equivalent circuit. This indicated that the resistivity was 75% higher for the post-treated films over the co-solvent films. Using the absorbance, reflectance, and EQE models for the optimal thickness of 85 nm, the optical generation and loss mechanisms could be calculated: 61.5% of the incident light was converted into current, 22.2% was lost to reflectance, 6.9% was absorbed by the film, 2.7 was absorbed by the rear electrode, and 6.7 was lost to recombination at the surface and in the bulk silicon. In Chapter 5, the PEDOT:PSS/silicon heterojunction was studied and the influence of passivation layers was examined using dark current density curves and the open-circuit voltage of the cell. It was discovered that the size of the native silicon oxide layer could be determined by the blue shift in the Raman Cα =Cβ band. It was noted that the native oxide continued to grow uninterrupted after PEDOT:PSS was deposited on hydrogen-terminated silicon resulting in a contaminated native oxide layer with decreased performance. It was concluded that the contamination at the surface, increased defects and Fermi level pinning could cause a decrease in band bending, leading to increased carrier recombination and poor performance. Allowing a controlled native oxide layer to grow to 2 nm in a clean environment increased the inversion layer and performance. The increase in bi-polaron modes with post-treatment together with the increased concentration of PEDOT and the effective density of acceptor states resulting from the removal of PSS, caused a stronger inversion and electron blocking at the interface. Finally in Chapter 5, P3HT was used as an interfacial layer between PEDOT:PSS and silicon. It was found that spin-coating a solution of P3HT dispersed in dichlorobenzene on silicon produced a homogeneous layer of small interconnected nanocrystallites. When applied to HSCs, no charge separation or transfer originating from the P3HT chromophore could be detected indicating the inversion layer existed entirely within the silicon substrate. However the larger open circuit voltage and change in dark saturation current undedicated the layer blocked electron majority carrier transfer from the silicon, increasing shunt resistance and open-circuit voltage. By combining the research of into short-circuit current density, and open-circuit voltage, it was concluded that the highest achievable efficiency of the setup used in this research was 11.8%