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.

Contact Design for Silicon Heterojunction Solar Cells

Luca Massimiliano Antognini

Today more than ever the world needs clean energy sources and thus a fast deployment and scaling up of the photovoltaic industry. In this context improving solar cell efficiency plays a major role. In order to achieve the maximum single junction efficiency for this material, technologies featuring carrier selective passivating contacts are foreseen to be the next successors of actual industrial cells. Among them, the silicon heterojunction (SHJ) solar cell is a promising technology which demonstrated efficiencies up to 26.7%. Its main specificity consists in the use of intrinsic and doped amorphous silicon (a-Si:H) to provide surface passivation and carrier selectivity to the c-Si, which allows to reach very high operating voltages in comparison to other c-Si based technology. The major drawbacks of SHJ solar cell is the large parasitic absorption occurring in those layers. To address this challenge, a strong focus on alternative non-silicon based thin films with wider band gap has built up in the literature. A first part of this thesis focuses on reviewing the fundamental process behind carrier selectivity and discuss the ability of different models from the literature to draw informations directly from the peculiar shapes of IV curves commonly observed in the solar cells featuring newly developed materials. We first discuss the simple four ideal diodes model of Roe et al to explain the occurrence of S-shapes depending on the contact quality. We discuss from a theoretical point of view how to improve the model by modifying the ideal diodes by other circuit elements as well as adding bulk recombinations. We test the theory experimentally and identified modifications that should be incorporated in the model.In a second part of the thesis, we explore the development of doped nano-crystalline silicon (nc-Si) layers to replace classical amorphous silicon layers. We investigate the influence of TMB and BF3 as dopant sources on the transparency and contact properties of nc-Si:H(p) layers as well as their integration in solar cells. We expose the roles of both gas to modify the crystallinity of the layer and find different optimum for both of them to lead high efficiencies, illustrated by a certified 23.9%-efficient solar cell. We report also on further optimization steps of this front junction architecture resulting in certified efficiency of 24.44%.We explore next the n-type nc-Si layers for application as window layer. In particular, we perform thickness and doping series to unravel the layer properties along its growth direction. In order to improve the low doping of the nucleation zone of nc-Si:H(n) at the i/n interface, a thin a-Si:H(n) buffer layer is introduced and shown to improve the passivation, selectivity and contact resistivity. Finally, we also report on the beneficial effect of an additional SiOx capping layer which improves both the reflection properties as well as all contact properties, broadening the optimal parameter window. The absorption loss in the front silicon layers remains however the highest share of losses, and therefore we explore theoretically the opportunity of using thinner front side layers or using thin TCO / silicon nitride bilayers to reduce the absorption losses. We finish the discussion by presenting an optimization roadmap, showing a possibility for double-side contacted SHJ cells to reach efficiencies above 26% without requiring any patterning or localization step.

EPFL2022

Transparent Passivating Contacts for Front Side Application in Crystalline Silicon Solar Cells

Josua Andreas Stückelberger

In this thesis, we present the development and characterization of novel approaches to carrier-selective passivating contacts for crystalline silicon (c-Si) solar cells. In our first approach, we benefit from a wide-bandgap material, namely mixed-phase silicon oxide (SiOx). High hydrogen dilution during the plasma-enhanced chemical vapor deposition of a SiOx layer leads to the growth of vertically oriented silicon filaments embedded in a silicon oxide matrix. The resulting mixed-phase material combines transparency and vertical conduction through the layer. We propose a layered contact structure starting with an ultra-thin chemically grown SiOx (~1.2 nm) at the c-Si wafer surface, followed by a phosphorus-doped bilayer of the mixed-phase SiOx, finishing with a nanocrystalline silicon layer that ensures contact with the metallization. With such a stack, an electron-selective passivating contact with a saturation current density J0 of 5.3 fA/cm2 is achieved on n-type wafers with hydrogenation, whereas on p-type wafers the contact reached 5.8 fA/cm2 even without hydrogenation. The contact is analyzed in detail with respect to its structure and its change upon thermal treatment. Two different growth regions are detected: one with the vertically oriented silicon inclusions and a second one in which the silicon phases have more lateral growth. Crystallization and atomic redistribution are analyzed during in-situ annealing in an electron microscope, leading to a better understanding of the change in distribution of phosphorus and oxygen as well as the nucleation sites. The influence of the annealing temperatures and dwell times as well as the initial doping concentration on the resulting in-diffused region and the passivation quality is reported. We use simulations to relate the influence of these parameters on passivation to the different recombination mechanisms. From the analyzed phosphorus doping profiles, an interesting observation is reported. The chemical oxide varies in thickness and density depending on the wafer polarity as well as the base resistivity. Proof-of-concept cells including this novel structure demonstrate the suitability for electron-selective passivating contacts at the device level with conversion efficiencies of 19.0% on planar wafers and 20.1% on textured wafers. Promisingly high short-circuit current densities (Jsc) of 35 mA/cm2 (40 mA/cm2) on planar (textured) with low parasitic absorption indicate its potential as a front contact in next-generation solar cells. Additionally, the beneficial feature of the smooth refractive index change within the layers that leads to a low reflectance over a broad wavelength range, similar to a built-in anti-reflection coating, is presented. A second approach to reach highly transparent electron-selective passivating contacts is demonstrated by the use of a plasma process based on the precursor gases phosphine (PH3), silicon tetrafluoride (SiF4), hydrogen (H2), and argon (Ar). The beneficial effect of fluorine in terms of passivation without the need for additional hydrogenation processes is shown, reaching implied open-circuit voltages of 730 mV on planar surfaces and 707 mV on textured surfaces. These layers are implemented in first proof-of-concept working devices with efficiencies of 19.5% on single-side-textured wafers and Jsc values of up to 40 mA/cm2.

EPFL2018

Full-Area Passivating Contacts with High and Low Thermal Budgets: Solutions for High Efficiency c-Si Solar Cells

Gizem Nogay

Today more than 90% of the global PV market is covered by c-Si solar cells which are limited by recombination losses at the metal-semiconductor interface. This recombination path can be avoided by separating the metal from the c-Si wafer by introducing a buffer layer that passivates electronically the wafer surface while still allowing the flow of one type of carrier. Such structures are called carrier selective passivating contacts. In the scope of this thesis, carrier selective passivating contacts with low and high thermal budgets are investigated with the purpose of achieving a good understanding of the junction formation, the working principle, limiting factors, and integration requirements. Silicon heterojunction (SHJ) solar cells are studied as representatives for solar cells processed with low thermal budget. Solar cells with interfacial oxide contacts are investigated as the cell concept for high thermal budgets. To be industrially relevant, contact formation of the latter is studied with p-type wafers. The carrier selective passivating contacts based on microcrystalline silicon (mc-Si:H) are studied for SHJ solar cells with low thermal budget. It is demonstrated that mc-Si:H layers improve the optical and electrical performance of the SHJ solar cells compared to cells with purely amorphous contacts. However, as the layer crystallinity increases with thickness, a trade¿off between short circuit current density and fill factor is encountered. By combining a MoOx front hole-contact and a mc-Si:H(n) rear electron-contact, this trade¿off is avoided. Applying an electrodeposited copper front metallization, a short circuit current density of 38.9 mA/cm2 and a fill factor of to 80% are obtained leading to an efficiency of 22.5%. The passivating hole contacts with high thermal budget are developed using a chemically grown oxide (SiOx) and boron doped silicon-carbide which is deposited under Si-rich conditions by plasma enhanced chemical vapour deposition. It is observed that introducing an undoped Si inter¿layer between SiOx and SiCx(p) is necessary to prevent chemical reaction between carbon atoms in the SiCx(p) and the adjacent SiOx, and consequently to obtain wellpassivated SiOx/c-Si interface. With this contact structure, an implied open circuit voltage value of 718 mV and a specific contact resistivity value of 17m cm2 are attained. The contact structure is tested at device level by realizing proof-of-concept hybrid solar cells that feature a high thermal budget rear hole-contact and a low thermal budget SHJ front electron-contact. With this concept, the fill factor of 81.8% is attained. In an effort to improve the hole-contact property further, in-situ incorporation of fluorine into the boron-doped hole-contact is explored. It is observed that in the as-deposited state, fluorine is evenly distributed within the contact layer of SiCx:F(p). Upon annealing, fluorine diffuses toward the c-Si wafer and accumulates at the interface where it reduces the defect states. With this contact structure, implied open circuit voltage values up to 735mV are achieved. The potential of the fluorinated hole-contact is investigated at device level by realizing both sides-contacted co-processed solar. For this purpose in-situ phosphorous doped SiCx(n) layers were developed as a front contact. Planar, both sides-contacted p-type solar cells attain an impressive fill factor value of 84% and an open circuit voltage of 727 mV.

EPFL2018

Transparent Passivating Contacts for Front Side Application in Crystalline Silicon Solar Cells

Josua Andreas Stückelberger

EPFL2018

Full-Area Passivating Contacts with High and Low Thermal Budgets: Solutions for High Efficiency c-Si Solar Cells

Gizem Nogay

EPFL2018

Contact Design for Silicon Heterojunction Solar Cells

Luca Massimiliano Antognini

EPFL2022