Degradation in next-generation passivating contact solar cells

The solar photovoltaics industry is quickly transitioning from the p-type passivated emitter with rear contact (PERC) cells to n-type-based cell architectures that incorporate various forms of passivating contacts. By reducing surface recombination, these passivating contacts allow for much higher open-circuit voltages and efficiencies. However, recent studies have shown that various forms of stability and reliability may limit their field performance. This thesis aims to advance the current understanding of these instabilities by investigating the roles of passivation layers and processing conditions on these structures. In this thesis, by employing advanced hydrogenation techniques, heterojunction solar cells are investigated to understand their long-term stability. Using a combination of high-intensity illumination and temperature, the transient nature of defects present in commercial heterojunction cells is studied. An observation of a light-induced degradation not previously reported is evaluated, with an analysis of the influences of illumination and temperature on the defect kinetics carried out. The techniques previously established in the literature for identifying defects such as light and elevated temperature-induced degradation (LeTID) and boron-oxygen light-induced degradation (BO-LID) are adapted to study passivating oxide contacts on n- and p-type silicon wafers. The roles of processing conditions such as firing temperature, and passivation layers on the front, rear, and interface layers are studied. An assessment of the long-term stability of p- and n-type silicon wafers with polysilicon layers is made, with some techniques to improve the kinetics of degradation and recovery presented

The role of hydrogen in firing and light-soaking effects on doped polysilicon passivating contacts for silicon solar cells

High-temperature firing treatment used in commercial screen-printed solar cells can directly deteriorate the surface passivation, and indirectly impact the long-term stability of the cell performance by triggering degradation in both the surface and the silicon bulk. This thesis studies the firing and long-term performance of polycrystalline silicon on silicon oxide (poly-Si/SiOx) passivating contacts, and light and elevated temperature induced degradation (LeTID) in various types of silicon wafers. This thesis first examines the firing stability of n-type phosphorus-doped and p-type boron-doped poly-Si/SiOx structures. N-type poly-Si exhibits a substantial increase in the recombination current density parameter J0 after firing at 800oC, with the extent of degradation sensitive to numerous factors, such as the firing temperature, the phosphorus diffusion conditions, and the dielectric coating layers. In comparison, p-type poly-Si shows higher firing stability than n-type poly-Si. With the assistance of various characterization tools, such as Fourier transform infrared (FTIR) and secondary ion mass spectrometry (SIMS), it is found that hydrogen diffusion during firing determines the final passivation quality. An optimum amount of hydrogen surrounding the interfacial SiOx is beneficial to achieve high-level surface passivation after firing, while an excess or insufficient amount of hydrogen is detrimental. The improved firing stability in p-type poly-Si could be owing to the lower effective hydrogen diffusivity, preventing the accumulation of excess hydrogen, which contributes to the degradation observed in n-type poly-Si. This thesis then studies the long-term stability of the poly-Si/SiOx passivating contacts and silicon wafers. Firing can activate a degradation and a subsequent recovery in the surface passivation of n-type poly-Si upon light soaking at temperatures between 75oC and 200oC, with the extent of the degradation depending on the samples, such as the SiNx coating layers and the poly-Si films, and the test conditions, such as the light-soaking temperature and the light intensity. Hydrogen could help with the regeneration, evident from the observation that samples with silicon nitride (SiNx) films removed after firing suffer a significantly larger degradation without showing recovery. Moreover, firing can lead to degradation in the silicon bulk upon light soaking at elevated temperature, which is known as LeTID. This thesis compares LeTID on p-type boron-doped and n-type phosphorus-doped mono-like silicon and float zone silicon. While all the studied materials exhibit degradation, n-type materials generally suffer a lower degradation extent than their p-type counterparts. Moreover, LeTID is affected by the SiNx film properties, with the degradation extent increasing with the Si-N bond density measured by FTIR and the refractive index of the SiNx films given by ellipsometry, which indicates that hydrogen is also responsible for LeTID. The degree of degradation is found to be reduced by phosphorus diffusion gettering and decreasing SiNx deposition temperature, revealing a potential solution to mitigate LeTID

The impact of silicon nano-texture morphology on solar cell integration, performance, and degradation

Silicon nano-texture, also referred to as black silicon, has excellent light trapping properties and is a potential solution to overcome optical losses in silicon solar cell. However, integrating nano-textures into solar cells is challenging due to their altered behaviour when subjected to key fabrication steps resulting in lower energy conversion efficiencies. This thesis aims to identify the role of nano-texture morphology on solar cell fabrication process interaction, modify processes for improved cell performance, and study how nano-textures suppress cell degradation. A literature review highlights the processes that are affected by nano-textures: emitter formation, surface passivation, and metal contact formation. Furthermore, it is identified that nano-texture may suppress light-induced-degradation (LID), however, the root cause of this behaviour is not understood. While fast diffusing species have been identified as playing a critical role, the role of vacancies and interstitials is identified as requiring further study. The effect of reactive ion etching (RIE) texture morphology on optical and electrical performance is evaluated by fabricating passivated emitter and rear cell (PERC) solar cells. RIE texture morphology is varied with increasing etch time which increases texture height and surface area. It is shown that post-diffusion sheet resistance increases, surface recombination increases, and screen-printing metal contact performance decreases, with increasing etch time. To address these issues, the emitter formation is modified, and a laser doped selective emitter (LDSE) solar cell process is optimized for RIE textures. The RIE based LDSE solar cells performed poorly as compared to the planar control cells and the remaining physical and electrical issues are discussed. The role of nano-textures in LID suppression is investigated for RIE textures as well as for chemically formed nano-textures. The reduction in LID is shown with increasing texture surface area. It is concluded that the increase in surface area may cause point defects to disappear on the surface. To verify this, electron radiation is used on samples to create vacancies and test their role in LID. The radiation induced defect (RID) cause a large reduction in lifetime but are found to be significantly different from the LID causing defect. However, the RIDs are passivated with thermal processing only in the presence of hydrogen

Metal surface contamination in c-Si solar cell processing

Fe und Cu wurden als Schlüsselspezies für die Betrachtung von Oberflächenkontamination in der Prozessierung von c-Si-Solarzellen identifiziert. Studien mit gezielt aufgebrachten Metallkonzentrationen vor verschiedenen Passivierungs- und Diffusionsprozessschritten ergaben relativ hohe kritische Werte für Cu, außer bei thermischer Oxidation. Niedrige Werte wurden für beide Elemente vor Hochtemperaturschritten im n-Typ-Hocheffizienzprozess beobachtet, wobei sich die B-Diffusion als etwas weniger empfindlich darstellte. Temporäre Gettereffekte für Fe (in p-Typ-Si) und Cu (in n-Typ-Si) wurden beobachtet. Es zeigte sich, dass As-Cut-Wafer, unabhängig von der Sägeart (SiC-slurry oder Diamantdraht) sehr hohe Metallverunreinigungen (im Bereich 1*1011 – 5*1014 cm-2) in den Prozess einbringen. Das alkalische Ätzen verringert diese Menge kaum, was hohe Anforderungen an die anschließende Reinigung ergibt. Die Optimierung von HF/O3-Reinigungslösung für diese Flächen ergab beste Reinigungsergebnisse bei niedrigen HF-Konzentrationen, abhängig vom alkalischen Ätzschritt und anschließendem HCl/HF-Dip

Electrical properties of boron diffused emitters

The effective passivation of boron emitters is a critical precursor to the migration of the silicon solar cell industry to n-type substrates. With this requirement in mind, this thesis focuses upon the surface passivation quality of boron diffused emitters in crystalline silicon. The chief analytical approach applied involves the investigation of the dependence of surface recombination on the charge density. The investigation is undertaken on two structures, namely LPCVD Si{u2083}N{u2084}/SiO{u2082}/Si and PECVD SiNx/Si. Measurement of emitter saturation current (J{u2080}e) under strong accumulation provides a meaningful comparison of the interface quality of different samples, since the surface recombination is independent of surface doping and only weakly dependent on surface charge density. In all cases, the J{u2080}e measured following negative corona charge deposition in boron diffused samples is significantly reduced and saturated, even for samples with relatively high surface doping. Correlation of boron surface concentration and the Si/SiO{u2082} interface properties of oxides grown using low temperature wet-dry oxidation as an intermediate layer of an LPCVD Si{u2083}N{u2084}/SiO{u2082} stack is reported. The proportion of J{u2080}e due to the defects at the surface is estimated after the Auger recombination in the emitter bulk has been taken into account based on the emitter profiles. The Si/SiO{u2082} interface of boron diffused (111) samples with LPCVD Si{u2083}N{u2084}/SiO{u2082} is substantially inferior to the corresponding (100) surface, as indicated by higher effective surface recombination velocity (Sejf) under accumulation. For the range of boron surface concentrations investigated in this thesis (-"4x10{u00B9}{u2078} to 8x10{u00B9}{u2078} cm-{u00B3}), the interface properties of LPCVD Si{u2083}N{u2084}/SiO{u2082} on (100) and (111) surfaces are found to be insensitive to the changes in boron surface concentration. This conclusion is supported by electron paramagnetic resonance results, which illustrate that a comparable Pb defect densities exist at the various surfaces. A long term stability investigation on the thermal oxide and LPCVD Si{u2083}N{u2084}/SiO{u2082} passivated samples shows that the degradation observed after long term storage is due to moisture. The detrimental impacts of the moisture can mostly be avoided with the addition of an effective barrier such as a capping layer of LPCVD nitride. The application of negative charge on undiffused, PECVD SiNx passivated surfaces leads to a significant disagreement between the Sef! determined by two separate techniques. In particular, for samples not subjected to a high temperature anneal, significant discrepancies exist between Sef! extracted from effective lifetime (-ref!) and J{u2080}e data. The reason for this is not well understood, however, several possible mechanisms are discussed. On PECVD SiNx passivated samples featuring a boron diffused emitter, the higher J{u2080}e values under accumulation for boron diffused samples compared to that of undiffused samples implies that the boron diffusion introduces defects at the PECVD SiNx/Si interface of planar (100) and (111) surfaces. Further, the results of J{u2080}e under accumulation for samples within the range of surface concentrations and sheet resistances studied in this thesis suggest that the interface degradation is not a strong function of the boron surface concentration, the total boron dose, or the surface orientation

The Electronic Properties of Defects in Silicon Solar Cells

This thesis is concerned with the measurements and interpretation of the electronic properties of defects in high-efficiency silicon solar cells. In Photovoltaic community, defects in crystalline silicon wafers or introduced during the solar cell fabrication can limit the performance and stability of the final devices. These defects are traditionally and widely studied using lifetime spectroscopy such as quasi-steady-state photoconductance decay measurements, in combination with Shockley-Read-Hall model to extract energy levels and capture cross section ratios of recombination active centres. In this thesis, the junction spectroscopy deep level transient spectroscopy is used for further understanding of the electronic properties of defects in silicon solar cells, with direct measurements of activation energy levels and majority carrier capture cross sections. As a result, insights of the defect formation/ dissociation mechanisms are provided and the physics features of the traps in silicon materials are understood for solar cell defect engineering to potentially achieve higher efficiency

Investigation of Bulk Defects in p-type Silicon Wafers and Solar Cells

While solar energy is already one of the cheapest methods for generating electricity, further improvements to solar cell efficiencies are needed to continue reducing the cost of photovoltaic (PV)-based energy along its current learning curve. Further improvements to the bulk lifetime of silicon wafers and cells are necessary, to keep up with rapid developments in the remaining regions of the solar cells i.e., surfaces and contacts. To achieve this, better identification of bulk defects in the dominant p-type silicon material is required. In this thesis, various characterization methods are developed and utilized to study two types of p-type bulk defects: recombination-active defects and minority carrier traps (traps). Photoconductance decay (PCD) measurements are used to investigate the boron-oxygen (BO)-related traps in boron (B)-doped Czochralski (Cz) silicon wafers. It is found that these traps are acting as the precursor of BO-related defects which cause light-induced degradation (LID) in B-doped Cz wafers. For the first time, the PCD measurement method is introduced as a potential method to investigate defects in their non-recombination-active form. The results from doping- and temperature-dependent PCD measurements suggest that the investigated BO-related traps have more than one energy level. The PCD method is also used to investigate traps in B-doped multicrystalline silicon wafers. It is shown that a set of slow traps are present in these wafers. These traps are temporarily removed after firing and they are re-generated by keeping the samples in the dark for several days or by dark annealing (DA) them for a short time. Further investigation of these traps shows that they are annihilated with laser annealing, and likely irreversible. However, no connection between removal of these traps and generation of light- and elevated temperature-induced degradation (LeTID)-related defects is observed. A deeper investigation of the observed traps suggests that grain boundaries are likely to be the cause. Newly emergent gallium (Ga)- as well as indium (In)-doped Cz silicon wafers are investigated. First, the presence of LID in these wafers is studied, suggesting that degradation happens at two stages for Ga-doped wafers and in a single stage for In-doped wafers. Moreover, defect annihilation with DA happens in two stages for both materials. The defect formation and annihilation activation energies for both Ga- and In-doped wafers are measured. The parameters of LID-related defects in Ga-doped wafers are extracted, suggesting that LeTID-related defects might be the source of degradation in this material. Traps in Ga-doped wafers are also studied. It is shown that these traps are removed with light soaking and re-generated with DA. However, they are not related to the observed LID in Ga-doped wafers. A new method for the extraction of defect parameters from completed and fully metalized solar cells is developed based on temperature-dependent Suns-Voc measurements, Suns Voc(T). This Suns Voc(T) method is validated by extracting the parameters of the well-known BO-related defects and comparing them to defect parameters extracted from lifetime structure and values published in the literature. The new Suns Voc(T) method is then used to extract the parameters of the defect causing LID in Ga-doped cells and the results are compared with the reported results in prior chapters and literature. A sensitivity analysis is performed for the Suns Voc(T) method, suggesting that the limited temperature range of the measurement system does not cause a systematic error in the extracted parameters of the two investigated defects

Passivating contacts for homojunction solar cells using a-Si:H/c-Si hetero-interfaces

Crystalline silicon (c-Si) homojunction solar cells account for over 90% of the current photovoltaic market. However, further progress of this technology is limited by recombinative losses occurring at their metal-semiconductor contacts. The goal of this thesis is to develop passivating contacts to resolve this issue. The novel idea presented in this work is to insert an ultra-thin wide bandgap semiconductor-hydrogenated amorphous silicon (a-Si:H)-film underneath the metal to passivate the doped c-Si surface and suppress the recombination of minority charge carriers. Simultaneously, this layer should provide a contact to the metal allowing majority charge carrier transport. A transparent conductive oxide is additionally inserted between the a-Si:H layer and the metal to ensure efficient carrier collection. This concept is inspired by the silicon heterojunction solar cells, a technology characterized by extremely high open-circuit voltages. The development of these new passivating contacts requires two features: a homojunction, for charge separation, and a silicon heterojunction contact for improved passivation. In this thesis, we explicitly focus on large-area thin-film deposition technology for fabrication of our devices, guaranteeing the scalability of our findings. The main results of this thesis are then threefold. First, we show that, using low-temperature plasma enhanced chemical vapor deposition, a doped homo-epitaxial layer can be deposited to form the homojunction. Second, we develop passivating contacts and optimize them in silicon heterojunction solar cells. An in-depth analysis of the contact formation is provided, including a detailed investigation of the relevant interfaces in our proposed structure. Finally, combining these two technologies, we demonstrate a proof-of-concept for these passivating contacts. Highly doped phosphorus- and boron-doped c-Si surfaces are shown to be efficiently passivated by a-Si:H layers and a lower contact resistivity is obtained for our optimized passivating contacts on such doped surfaces compared to a heterojunction contact on lightly doped surfaces. We show that homojunction solar cells on diffused and ion-implanted wafers featuring such passivating contacts (called homo-hetero cells hereinafter) yield improved open-circuit voltages compared to conventional homojunction solar cells, due to reduction of recombination losses. Additionally, the temperature coefficient of such homo-hetero solar cells is lower. With these advantages, the homo-hetero solar cells outperform homojunction solar cells when operating at a cell temperature above 60 °C. This work contributes to the research and development of high-efficiency silicon solar cells by providing new insights on the properties of contact formation and a novel contact-type

Pulsed Laser Dielectric Ablation for Copper-plated Silicon Solar Cells

Copper-plated metallisation can act as an alternative to the dominant Ag screen-printing with a reduced material cost for Si solar cell manufacturing. However, concerns of laser-induced damage with carrier recombination and plated metal adhesion have contributed to a low market fraction of Cu-plated modules. This thesis aimed to further understand the contact formation process using light-induced plating (LIP) on selective openings formed by short pulse laser ablation of dielectrics on p-type Si solar cells including the passivated emitter and rear cell (PERC). A composite optical-thermal model was developed to explore the light-matter interaction between short laser pulses and textured silicon nitride (SiNx)/Si surfaces. It is shown that picosecond laser pulses of the longer wavelength of 532 nm result in melting depths of ~ 1 µm and the SiNx being primarily ablated by the indirect ‘spallation’ process, whereas for 266 nm pulses melting depths are predicted to be limited to < 150 nm and the direct ablation contributes to a greater extent in the removal of the SiNx antireflection coating. Longer nanosecond pulses result in less steep temperature gradients, and for 266 nm nanosecond pulses, an increased melting depth compared to picosecond pulses and hence are more appropriate for laser doping processes than dielectric ablation for contact formation. Surface chemistry and Ni nucleation were identified as key factors for the plated metal adhesion. Residual SiNx, resulting from incomplete laser dielectric ablation, was shown to hinder the formation of Ni silicides on both Al back surface field (BSF) and PERC cells and resulted in low busbar pull forces. Although busbar pull forces > 1 N mm-1 could be readily achieved on Al BSF cells with minimal electrical impact on the cells, PERC cells were more sensitive to laser damage and comparable busbar adhesion could not be achieved without significantly reduced VOC and pFF. It is shown that the use of pulsed LIP of Ni can be used to improve the busbar adhesion through enabling a greater density of Ni nucleates which can act as adhesive anchor points along busbars. However, the improvements in adhesion may not be sufficient to address the adhesion/electrical performance trade-off that appears to exist for higher efficiency PERC cells. Finally, this thesis shows that low pull forces can be measured even though bulk Si is fractured during 180° busbar pull tests. This suggests that an examination of the peeled interface is required before interpreting the busbar adhesion results, a finding that may also be relevant to the more dominant screen-printed cells produced industrially