[Some symbols cannot be rendered in the following metadata – please see the PDF file for an accurate version of the Abstract] ¶ Recombination within the bulk and at the surfaces of crystalline silicon has been investigated in this thesis. Special attention has been paid to the surface passivation achievable with plasma enhanced chemical vapour deposited (PECVD) silicon nitride (SiN) films due to their potential for widespread use in silicon solar cells. The passivation obtained with thermally grown silicon oxide (SiO2) layers has also been extensively investigated for comparison. ¶ Injection-level dependent lifetime measurements have been used throughout this thesis to quantify the different recombination rates in silicon. New techniques for interpreting the effective lifetime in terms of device characteristics have been introduced, based on the physical concept of a net photogeneration rate. The converse relationships for determining the effective lifetime from measurements of the open-circuit voltage (Voc) under arbitrary illumination have also been introduced, thus establishing the equivalency of the photoconductance and voltage techniques, both quasi-static and transient, by allowing similar possibilities for all of them. ¶ The rate of intrinsic recombination in silicon is of fundamental importance. It has been investigated as a function of injection level for both n-type and p-type silicon, for dopant densities up to ~5x1016cm-3. Record high effective lifetimes, up to 32ms for high resistivity silicon, have been measured. Importantly, the wafers where commercially sourced and had undergone significant high temperature processing. A new, general parameterisation has been proposed for the rate of band-to-band Auger recombination in crystalline silicon, which accurately fits the experimental lifetime data for arbitrary injection level and arbitrary dopant density. The limiting efficiency of crystalline silicon solar cells has been re-evaluated using this new parameterisation, with the effects of photon recycling included. ¶ Surface recombination processes in silicon solar cells are becoming progressively more important as industry drives towards thinner substrates and higher cell efficiencies. The surface recombination properties of well-passivating SiN films on p-type and n-type silicon have been comprehensively studied, with Seff values as low as 1cm/s being unambiguously determined. The well-passivating SiN films optimised in this thesis are unique in that they are stoichiometric in composition, rather than being silicon rich, a property which is attributed to the use of dilute silane as a process gas. A simple physical model, based on recombination at the Si/SiN interface being determined by a high fixed charge density within the SiN film (even under illumination), has been proposed to explain the injection-level dependent Seff for a variety of differently doped wafers. The passivation obtained with the optimised SiN films has been compared to that obtained with high temperature thermal oxides (FGA and alnealed) and the limits imposed by surface recombination on the efficiency of SiN passivated solar cells investigated. It is shown that the optimised SiN films show little absorption of UV photons from the solar spectrum and can be easily patterned by photolithography and wet chemical etching. ¶ The recombination properties of n+ and p+ emitters passivated with optimised SiN films and thermal SiO2 have been extensively studied over a large range of emitter sheet resistances. Both planar and random pyramid textured surfaces were studied for n+ emitters, where the optimised SiN films were again found to be stoichiometric in composition. The optimised SiN films provided good passivation of the heavily doped n+-Si/SiN interface, with the surface recombination velocity increasing from 1400cm/s to 25000cm/s as the surface concentration of electrically active phosphorus atoms increased from 7.5x1018cm-3 to 1.8x1020cm-3. The optimised SiN films also provided reasonable passivation of industrial n+ emitters formed in a belt-line furnace. It was found that the surface recombination properties of SiN passivated p+ emitters was poor and was worst for sheet resistances of ~150./ . The hypothesis that recombination at the Si/SiN interface is determined by a high fixed charge density within the SiN films was extended to explain this dependence on sheet resistance. The efficiency potential of SiN passivated n+p cells has been investigated, with a sheet resistance of 80-100./ and a base resistivity of 1-2.cm found to be optimal. Open-circuit voltages of 670-680mV and efficiencies up to ~20% and ~23% appear possible for SiN passivated planar and textured cells respectively. The recombination properties measured for emitters passivated with SiO2, both n+ and p+, were consistent with other studies and found to be superior to those obtained with SiN passivation. ¶ Stoichiometric SiN films were used to passivate the front and rear surfaces of various solar cell structures. Simplified PERC cells fabricated on 0.3.cm p-type silicon, with either a planar or random pyramid textured front surface, produced high Voc’s of 665-670mV and conversion efficiencies up to 19.7%, which are amongst the highest obtained for SiN passivated solar cells. Bifacial solar cells fabricated on planar, high resistivity n-type substrates (20.cm) demonstrated Voc’s up to 675mV, the highest ever reported for an all-SiN passivated cell, and excellent bifaciality factors. Planar PERC cells fabricated on gettered 0.2.cm multicrystalline silicon have also demonstrated very high Voc’s of 655-659mV and conversion efficiencies up to 17.3% using a single layer anti-reflection coating. Short-wavelength internal quantum efficiency measurements confirmed the excellent passivation achieved with the optimised stoichiometric SiN films on n+ emitters, while long-wavelength measurements show that there is a loss of short-circuit current at the rear surface of SiN passivated p-type cells. The latter loss is attributed to parasitic shunting, which arises from an inversion layer at the rear surface due to the high fixed charge (positive) density in the SiN layers. It has been demonstrated that that a simple way to reduce the impact of the parasitic shunt is to etch away some of the silicon from the rear contact dots. An alternative is to have locally diffused p+ regions under the rear contacts, and a novel method to form a rear structure consisting of a local Al-BSF with SiN passivation elsewhere, without using photolithography, has been demonstrated.

The objective of the proposed research is to develop low-cost, screen-printed 20% efficient silicon solar cells. In the first part of this thesis, a ~19% efficient, screen-printed cell was fabricated using the commercially-dominant aluminum back surface field (Al-BSF) cell structure. Device modeling was then used to determine that increasing the efficiency to 20% required improvements in both back surface passivation and rear reflectance. In the second part of this thesis, a passivated, transparent boron BSF (B-BSF) structure was proposed as a high-throughput method for realizing these improvements. The first step in fabricating the proposed B-BSF cell involved the successful development of a water-based, spin-on solution of boric acid as a low-cost, non-toxic and non-pyrophoric alternative to common boron diffusion sources such as boron tribromide. A review of the literature shows that a common problem with boron diffusion is severe bulk lifetime degradation, with Fe contamination being commonly speculated as the cause. An experimental study was therefore devised in which the impact of boron diffusion and subsequent cell process steps on the bulk lifetime and bulk iron contamination was tracked. From this study, a model for boron diffusion-induced Fe contamination was developed along with methods for gettering Fe from the substrate. A key achievement of this thesis was the discovery of a novel, negatively charged, aluminum-doped spin-on glass (SOG) which can, in a short thermal step, simultaneously getter Fe and provide stable, high-quality passivation of planar, boron-diffused Si surfaces. Since past attempts at achieving low-cost, high-efficiency, boron-diffused cells have suffered from bulk lifetime degradation and difficulties with passivating a boron-diffused Si surface, the Al-doped SOG provides a solution to both challenges. Since a high rear reflectance is important for achieving high-efficiencies, an experimental study of various reflectors was undertaken and a silver colloid material was found which exhibits both high electrical conductivity and Lambertian reflectance >95%. The work on boric acid diffusion, iron gettering, surface passivation and rear reflectors was successfully integrated into a 20.2% efficient, screen-printed, B-BSF cell fabricated on 300 µm thick, p-type float-zone (FZ) Si wafers. Both device theory and modeling was used to show that, due to its well-passivated surfaces, this cell would suffer a large loss in efficiency due to light-induced degradation (LID) if it were fabricated on commercial p-type Czochralski (Cz) Si substrates. Since n-type Si substrates do not suffer from LID, the p-type process was slightly tweaked and applied to n-type FZ wafers, resulting in 20.3% efficient cells on 190 µm thick wafers. Computer modeling shows that both the p-type and n-type cells can maintain efficiencies of 20% for wafers as thin as 100 µm.

Heterojunctions of n-ZnO/p-Si solar cells were fabricated by RF sputtering ZnO:Al onto boron-doped (100) silicon (Si) substrates. Zinc Oxide (ZnO) films were also deposited onto soda lime glass for electrical measurements. Sheet resistance measurements were performed with a four-point-probe on the glass samples. Values for samples evacuated for 14 hours prior to deposition increased from 7.9 to 10.17 and 11.5 O/□ for 40 W, 120 and 160 W in RF power respectively. In contrast, those evacuated for 2 hours started with a higher value of 22.5 O/□, and decreased down to 7.6 and 5.8 O/□. Vacuum annealing was performed for both the glass and the Si samples. Current-voltage measurements were performed on the ZnO/Si junctions in the dark and under illumination. Parameters such as open-circuit voltage, Voc; short-circuit current, Isc; fill factor, FF; and efficiency, eta were determined. A maximum efficiency of 0.25% among all samples was produced, with an I sc of 2.16 mA, Voc of 0.31V and a FF of 0.37. This was a sample fabricated at an RF power of 80 W. Efficiency was found to decline with vacuum annealing. Furthermore, interfacial state density calculated based on capacitance-voltage measurements showed an increase in the value with vacuum annealing. The results found suggest that the interface states may be due to an interdiffusion of atoms, possibly those of Zn into the Si surface. The Electron Beam Induced Current (EBIC) method was used to determine diffusion length to be at a value ∼40--80 mum and therefore a minority carrier lifetime calculated of 3 musec. It was also used to determine the surface recombination velocity (SRV) of the fractured surface of the Si bulk from the fabricated solar cells. An SRV of ∼500 cm/sec was determined from the fractured Si surface, at a point located at 30 and 20 mum away from the junction interface.

The work deals with a comparison of existing and perspective types of passivation and anti-reflective coating for silicon solar cells. The theoretical part describes the appropriate methodology for the characterization of these layers and focuses on the passivation layers based on Al2O3. The practical part describes design and verification operations of the equipment for measuring of the amount of fixed charge in the passivation layers using corona discharge. It also describes the implementation of equipment and the results of indicative tests for positive and negative polarity of high voltage. The final part discusses the possibility of equipment improving.

The aim of the thesis is to suggest a comprehensive and inexpensive method of diagnosing the solar cell quality via a conventional digital camera. The following questions are answered, Can a conventionaldigital camera be used for diagnosing the quality of solar cells?; If so,is the experimental setup for the quality diagnosing constrained to a laboratory and single solar cells or can it be done in a private home for a full-sized solar panel?; What are the defects that can be observed in this experiment? A conventional digital camera has been modified to acquire the electroluminescence (EL) images and dark thermography(RevEL) images. The experiment has been done in two locations with different types of samples. Multi-crystalline p-type single solar cells are used during the laboratory experiment. In the experiment set up at the private home, a conventional solar panel with 36 quadraturemulti crystalline silicon solar cells, that are equivalent to 9 full solarcells, is used. The EL imaging has been performed under the forward bias whereas the dark thermography imaging has been performed under reverse bias. The contrast in EL images is due to the radiativeand non-radiative recombination of injected excess minority charge carriers. A large non-radiative recombination site produces a large dark area in the EL image. Similarly, the contrast in RevEL images is due to the generation of charge carriers that are associated with the non-radiative recombination sites in the depletion layer. A large defect area produces a large bright area in the RevEL image. Hence,the EL image and the RevEL image are some what inverted images of each other. It is also found that the IV characteristics and the semi-log curves are in a good agreement with the EL and the RevEL images. When the EL image is combined with the hand-on devices like a mobile camera and a macro lens, it reveals defected areas like finger-interruptions, microcracks, grain boundaries and planar defects. Whereas the RevEL images, when combined with the image processing software tool, reveal the morphology of the defected sites. This justifies the beauty and the simplicity of using an every day digitalcamera as a diagnostic tool for the quality control of the solar cell

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<p>Fracture of silicon solar cells in photovoltaic (PV) modules are widely reported and a wellknown issue in the PV industry, since it is exposed to adverse climatic conditions and varying temperature loads. A commercial silicon solar cell is mainly composed of four different layers. This thesis investigates delamination failure and thermal fatigue failure due to alternating temperature loads using finite element method (FEM) simulation.</p> <p><br></p> <p>The delamination of the encapsulant (EVA) layer and the cell interface was simulated using</p> <p>finite element (FE) simulations in the COMSOL Multiphysics software. The adhesion between the</p> <p>layers were modeled using the cohesive zone model (CZM). The CZM parameters such as normal</p> <p>strength and penalty stiffness were used for the bilinear traction-separation law for the cohesive</p> <p>model in a 90-degree configuration. The critical energy release rate (𝐺𝐺𝑐𝑐) was experimentally calculated as one of the CZM parameters. A uniaxial tensile test of the upper layer of the cell was conducted to determine the material properties of the solar cell layers, and that information was</p> <p>later used for FE simulations. To validate the simulation, we compared the peeling force graph</p> <p>from the experiment and FE simulation, and it was found both graphs showed a maximum peeling</p> <p>force of 120 N.</p> <p><br></p> <p>Finite element simulations were also conducted to predict the stress variations in the silicon</p> <p>solar cell layer due to alternating temperatures. An alternating temperature function was developed</p> <p>using triangular waveform equations in the COMSOL Multiphysics software. For this simulation,</p> <p>a 3D model of the cell with a 90-degree peel arm was used, like in the peeling simulation. A</p> <p>maximum stress of 7.31 x 10−3 𝑁/𝑚𝑚2 was observed on the encapsulant (EVA)/cell layer, but no</p> <p>delamination was observed for the given temperature range. In future work, we plan to explore the</p> <p>calculation of fatigue life using thermal simulation to predict the reliability of a solar cell.</p>

The objective of the research in this thesis is to develop manufacturable high-efficiency silicon solar cells at low-cost through advanced cell design and technological innovations using industrially feasible processes and equipment on commercial grade Czochralski (Cz) large-area (239 cm2) silicon wafers. This is accomplished by reducing both the electrical and optical losses in solar cells through fundamental understanding, applied research and demonstrating the success by fabricating large-area commercial ready cells with much higher efficiency than the traditional Si cells. By developing and integrating multiple efficiency enhancement features, namely low-cost high sheet resistance homogeneous emitter, optimized surface passivation, optimized rear reflector, back line contacts, and improved screen-printing with narrow grid lines, 20.8% efficient screen-printed PERC (passivated emitter and rear cell) solar cells were achieved on commercial grade 239 cm2 p-type Cz silicon wafers.

Photovoltaic manufacturing is material intensive with the cost of crystalline silicon wafer, used as the substrate, representing 40% to 60% of the solar cell cost. Consequently, there is a growing trend to reduce the silicon wafer thickness leading to new technical challenges related to manufacturing. Specifically, wafer breakage during handling and/or transfer is a significant issue. Therefore improved methods for breakage-free handling are needed to address this problem. An important pre-requisite for realizing such methods is the need for fundamental understanding of the effect of handling device variables on the deformation, stresses, and fracture of crystalline silicon wafers. This knowledge is lacking for wafer handling devices including the Bernoulli gripper, which is an air flow nozzle based device. A computational fluid dynamics model of the air flow generated by a Bernoulli gripper has been developed. This model predicts the air flow, pressure distribution and lifting force generated by the gripper. For thin silicon wafers, the fluid model is combined with a finite element model to analyze the effects of wafer flexibility on the equilibrium pressure distribution, lifting force and handling stresses. The effect of wafer flexibility on the air pressure distribution is found to be increasingly significant at higher air flow rates. The model yields considerable insight into the relative effects of air flow induced vacuum and the direct impingement of air on the wafer on the air pressure distribution, lifting force, and handling stress. The latter effect is found to be especially significant when the wafer deformation is large. In addition to silicon wafers, the model can also be used to determine the lifting force and handling stress produced in other flexible materials. Finally, a systematic approach for the analysis of the total stress state (handling plus residual stresses) produced in crystalline silicon wafers and its impact on wafer breakage during handling is presented. Results confirm the capability of the approach to predict wafer breakage during handling given the crack size, location and fracture toughness. This methodology is general and can be applied to other thin wafer handling devices besides the Bernoulli gripper.