Evaluating Materials Design Parameters of Hole-Selective Contacts for Silicon Heterojunction Solar Cells

Silicon heterojunction (SHJ) solar cell efficiencies are limited by parasitic absorption from the hydrogenated amorphous silicon (a-Si:H) front contact, but this may be mitigated by selecting an alternative carrier selective contact material with a wider band gap. When choosing such a material as the hole-selective contact (“p-layer”), the alignment of the material's valence band edge energy ( EVB ) with that of crystalline silicon (c-Si) is an important criterion, but several other material parameters can also influence the band bending at the contact interface. In this article, we simulate an (n)c-Si/(i)a-Si:H/p-layer interface to explore the influence of six materials parameters in a variable p-layer on the SHJ performance. We find a strong influence on the fill factor (FF) from thickness, doping, and EVB , and on VOC from the interfacial defect density; notably, optimal EVB is ∼ 0.1 eV higher than the valence band edge energy of a-Si:H. Multiparameter sensitivity analyses demonstrate how performance is simultaneously influenced by EVB and doping; thus, both parameters should be optimized alongside one another. To assess the influence of these parameters experimentally, we grow p-type NiOx as a test-case p-layer, which shows that FFs decrease with the oxygen content likely from the increased misalignment of EVB . Although modest efficiencies are achieved experimentally (>7%), what is important is that our model simulates performance trends. With these results, we apply a materials discovery pipeline to suggest new materials (e.g., ZnTe and BeTe) to try as p-layers in the SHJ. This combination of simulations, experiments, and materials discovery informs a better understanding of contact selection in SHJ cells.

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

Light Generation and Harvesting in a van der Waals Heterostructure

Esther Alarcon Llado, Andras Kis, Volodymyr Koman, Oriol López Sánchez, Aleksandra Radenovic

Two-dimensional (2D) materials are a new type of materials under intense study because of their interesting physical properties and wide range of potential applications from nanoelectronics to sensing and photonics. Monolayers of semiconducting transition metal dichalcogenides MoS2 or WSe2 have been proposed as promising channel materials for field-effect transistors. Their high mechanical flexibility, stability, and quality coupled with potentially inexpensive production methods offer potential advantages compared to organic and crystalline bulk semiconductors. Due to quantum mechanical confinement, the band gap in monolayer MoS2 is direct in nature, leading to a strong interaction with light that can be exploited for building phototransistors and ultrasensitive photodetectors. Here, we report on the realization of light-emitting diodes based on vertical heterojunctions composed of n-type monolayer MoS2 and p-type silicon. Careful interface engineering allows us to realize diodes showing rectification and light emission from the entire surface of the heterojunction. Electroluminescence spectra show clear signs of direct excitons related to the optical transitions between the conduction and valence bands. Our p-n diodes can also operate as solar cells, with typical external quantum efficiency exceeding 4%. Our work opens up the way to more sophisticated optoelectronic devices such as lasers and heterostructure solar cells based on hybrids of 2D semiconductors and silicon.

Amer Chemical Soc2014

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.