SnS Quantum Dots as Hole Transporter of Perovskite Solar Cells

Perovskite solar cells have achieved comparable power conversion efficiencies as commercial silicon cells, making the issue of long-term operation stability as the most critical scientific factor toward industrial realization. In this work, we introduce SnS quantum dots (QDs) as a new inorganic hole transporting material (HTM) to perovskite solar cells. The SnS QDs decorated with the oleylamine (OAm), oleic acid (OA), and trioctylphosphine (TOP) ligands are prepared through the traditional nonaqueous solvothermal method. Therefore, the as-synthesized SnS QDs can be orthogonally processed onto the top of a triple cation perovskite film, exhibiting a good surface coverage and an excellent hole extraction ability. With careful device engineering towards film thickness, annealing procedure, and ligand exchange on the SnS layer, we have obtained a power conversion efficiency (PCE) of 13.7%. Compared with the 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (spiro-OMeTAD)-based control device, the SnS-based perovskite solar cell presents a better air stability, showing unaltered device performance after 1000 h storage under ambient conditions.

Electrical Losses Mitigation in Silicon Heterojunction Solar Cells

Laurie-Lou Senaud

To overcome the worldwide challenges of climate change, photovoltaics is foreseen to play a significant role in the world electricity production. Nowadays, single junction crystalline silicon (c-Si) based solar cells hold the largest share of the global photovoltaic market. To achieve their maximum energy conversion efficiency, carrier-selective passivating contacts have been demonstrated to be the most promising technologies. Among them, silicon heterojunction (SHJ) solar cells are expected to further reduce the cost of photovoltaics. Indeed, they have achieved record efficiency thanks to high c-Si bulk quality and excellent full area surface passivation. However, despite remarkable performance, the material layers used to build SHJ cells are at the origin of significant trade-offs between electrical and optical properties. In particular, the properties coupling arising between the layers can cause significant electrical transport losses, strongly capping the SHJ device performance. The aim of this thesis is to accurately describe, quantify, and mitigate the electrical losses occurring in SHJ solar cells by means of advanced methodologies and characterization methods.Key outcomes of this work are fourfold. First, we introduce a generalized and unambiguous description of the concept of contact using the terminology of shell. Then, we propose a characterization methodology using two new approaches named top-down and bottom-up, designed to independently track the properties of the material layers used to build the shells, as well as the performance of the solar cells incorporating the shells. This was shown to allow one to accurately investigate and mitigate the electrical and optical losses affecting solar cells. Secondly, we present transfer length method (TLM) measurements under variable illumination as a novel advanced characterization method to study the electrical transport losses in SHJ devices. We demonstrated (i) that illumination, i.e., the injected carrier density inside the c-Si bulk, has a strong impact on the contact resistivity (rc), (ii) the importance of measuring rc under maximum power point conditions for a relevant characterization of transport losses, and (iii) how the dependence of the rc on the injected carrier density enables the comparison of the illumination response of different SHJ shells. In addition, we conducted preliminary investigations of the applicability of TLM under illumination to measure p-type shell parts on n-type c-Si wafers. Thirdly, we engineered various material layers and studied their properties. Multilayering the thin hydrogenated silicon and the TCO layers demonstrated a high potential for electrical losses mitigation by (i) improving the properties coupling in the shell, (ii) addressing the constraints of the device architecture, and (iii) mitigating the trade-offs impacting solar cell performance. Finally, the last outcome is the integration of the most promising shells in actual baseline solar cells. This resulted in fill-factors up to 83.2%, for 6-inch SHJ cells, and in efficiencies up to 25.45% for SHJ IBC cells. Finally, combining optimal electrical and optical shell properties, an impressive efficiency of 24.24% was achieved using an aluminium-doped zinc oxide as rear TCO. The advanced methodologies and characterization methods presented in this work are foreseen to drive the understanding and optimization of other solar cell technologies such as, e.g. TOPCon or tandem solar cells.

EPFL2021

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

Monolithic Perovskite-Silicon Tandem Solar Cells: From the Lab to Fab?

Christophe Ballif, Antonin Faes, Fan Fu, Yi Hou, Quentin Thomas Jeangros, Chien-Jen Yang

This review focuses on monolithic 2-terminal perovskite-silicon tandem solar cells and discusses key scientific and technological challenges to address in view of an industrial implementation of this technology. The authors start by examining the different crystalline silicon (c-Si) technologies suitable for pairing with perovskites, followed by reviewing recent developments in the field of monolithic 2-terminal perovskite-silicon tandems. Factors limiting the power conversion efficiency of these tandem devices are then evaluated, before discussing pathways to achieve an efficiency of >32%, a value that small-scale devices will likely need to achieve to make tandems competitive. Aspects related to the upscaling of these device active areas to industry-relevant ones are reviewed, followed by a short discussion on module integration aspects. The review then focuses on stability issues, likely the most challenging task that will eventually determine the economic viability of this technology. The final part of this review discusses alternative monolithic perovskite-silicon tandem designs. Finally, key areas of research that should be addressed to bring this technology from the lab to the fab are highlighted.