Electrical Losses Mitigation in Silicon Heterojunction Solar Cells

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.

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

Impact of the Oxygen Content on the Optoelectronic Properties of the Indium-Tin-Oxide Based Transparent Electrodes for Silicon Heterojunction Solar Cells

Brahim Aissa, Christophe Ballif, Mathieu Gérard Boccard, Jean Cattin, Jan Haschke, Akshath Raghu Shetty

Amorphous/crystalline silicon heterojunction (SHJ) solar cells technology is attracting tremendous attention in recent years due to its potential to achieve high power conversion efficiencies at low fabrication temperatures and using few process steps. However, the commercial mass production of this technology is still somehow restricted so far, which is mainly due to the high sensitivity of the SHJ solar cell parameters to the growth conditions. A significant distinctness between the SHJ configuration and the standard silicon wafer solar cell is the current collection scheme. Indeed, as the SHJ silicon wafer solar cell is limited by the low lateral conductivity of the thin-film-silicon layers used to form the contact, a transparent conductive oxide (TCO) is systematically employed to improve the carrier transport properties, whilst also acting as an antireflective coating (ARC) for the front side. From the variety of TCOs, indium tin oxide (ITO) is the most frequently used. In this work, we investigate the properties of ITO thin films grown by DC magnetron sputtering using different oxygen to total flow ratios [r(O-2) = O-2/(Ar+O-2)] ranging from 0.01 (1%) to 0.08 (8 %). Hall effect measurements together with optical spectrometry were carried out and were found to be drastically affected by the oxygen content. Furthermore, time of flight-secondary ion mass spectrometry (TOF-SIMS) was used to determine the depth profiling of indium, oxygen, tin, silicon, phosphorous, and hydrogen throughout the ITO and silicon layers forming the solar cell. Finally, silicon heterojunction devices were fabricated and the associated photovoltaic performance were evaluated as a function of the r(O-2) into the ITO electrodes. Lower oxygen flow ratio was found to yield the best performance which is attributed to lower parasitic resistive losses.

AMER INST PHYSICS2019

Solid state nanocrystalline titanium oxide photovoltaic cells

The dye solar cell, is a novel photoelectrochemical solar cell presenting unique advantages, as the use of low cost materials and the potential simplicity of manufacturing. However, a liquid electrolyte is actually required for the transport of the photogenerated positive charges. A major breakthrough is the replacing of the liquid electrolyte by a solid-state hole conducting material – the aim of this work. First, the metal/TiO2 junction was investigated, with either silver or gold as rectifying contact. Sensitization was demonstrated, as action spectra were measured using different dyes. The evaporated metal could not enter the pores of the nanocrystalline TiO2 to contact all the dye molecules, hence only low photovoltaic performances were obtained. Furthermore, these metal/TiO2 junctions showed a high sensitivity to humidity and to forward bias polarization. The use of an electrically conducting polymer, poly(bithiophene), was studied in a metal/polymer junction and in TiO2/polymer devices with flat and nanocrystalline TiO2 electrodes. The photoelectric characterizations indicated that electrochemically reduced poly(bithiophene) was able to inject electrons into TiO2, i.e. sensitizing TiO2, and it acted as a hole conducting layer. Short-circuit current densities of 2-3 mA/cm2 were measured at an irradiation of 1.2 W/cm2. The open-circuit voltages were in the 250-350 mV range, depending on the light intensity and temperature. The use of a transparent hole conducting material, CuI, which is a wide bandgap p-type semiconductor, was demonstrated in a nanocrystalline junction made of highly porous sensitized TiO2 and solution-cast CuI. A short-circuit current of 1 mA/cm2 was achieved under one sun simulated light; the open-circuit voltage was 600 mV and the light-to-power conversion efficiency was 0.3 %. The peak quantum efficiencies of 8 % in the UV, and 3 % in the visible, showed that the nanocrystalline junction was working inefficiently, probably due to the poor electrical contact between the TiO2 nanocrystallites and the CuI crystals. Furthermore, the sensitizer was not tuned to inject holes into the valence band of CuI. As conclusion, a controlled-capillary pore based sensitized device is suggested. In this concept, all the internal surface of the TiO2 electrode coated with sensitizer is contacted with the transparent hole conducting layer, hence improving the photovoltaic properties of the device.

EPFL1996