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

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.

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

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

Defect passivation in zinc tin oxide: improving the transparency-conductivity trade-off and comparing with indium-based materials

Oscar Esteban Rucavado Leandro

Transparent conductive oxides (TCOs) are essential in technologies coupling light and electricity. Due to their good optoelectronic properties and the production scalability, Sn-doped indium oxide (In2O3:Sn) is the preferred TCO in industrial applications. Nonetheless indium is scarce in the Earth's crust and its availability might be compromised over the next decades. To address this issue, this doctoral project aims to (i) improve the optoelectronic properties of In-free TCOs seeking to match those of In2O3:Sn, and (ii) refine the optoelectronic properties of In2O3-based films to decrease the In consumption in applications. To accomplish this, we investigated the links between the defects, the microstructure and the optoelectronic properties of two families of TCOs, indium- and tin-based oxides. First, we studied the evolution in the optoelectronic properties and microstructure of amorphous zinc tin oxide (a-ZTO) when annealed up to 500C in oxidizing, neutral, and reducing atmospheres. We show that annealing in atmospheric pressure at temperatures > 300C decreases the detrimental subgap absorptance while increasing the electron mobility (mu). Thermal treatments in reducing atmospheres increase the free-carrier density (Ne) and the subgap absorptance. None of the thermal treatments resulted in important changes in the amorphous microstructure. Combining these results and density functional theory (DFT) calculations, oxygen deficiencies (VO) were identified as the source of detrimental subgap absorption. VO can act as donors but also as electron scattering centres. Based on these results, a-ZTO with ÎŒ up to 35 cm2/Vs, is demonstrated by high-temperature defect passivation. Profiting from the microstructure stability of a-ZTO, this material was used as a recombination junction in a tandem solar cell which require a high-temperature step in its processing. The passivation scheme might be problematic for temperature-sensitive technologies. Therefore, we demonstrated an alternative low-temperature passivation method, which relies on co-sputtering a-ZTO with SiO2 . Using two Sn-based oxides with different composition and microstructure: a-ZTO and SnO2 , and results of DFT calculations, we demonstrate that SiO2 contribution is twofold. (i) The oxygen from SiO2 passivate the VO in SnO2 and a-ZTO. (ii) The formation energy of the ionized VO is lowered by the silicon-atoms, enabling defects that do not contribute to the subgap absorptance. This passivation scheme improves the optical properties without affecting the electrical conductivity, overcoming the optoelectronic trade-off in Sn-based TCOs. Finally, we study an In-based high-ÎŒ TCO: Zr-doped In2O3. Films of In2O3:Zr with thickness from 15 nm-100 nm were sputtered in the amorphous state and annealed in different atmospheres. Annealing in air yields fully crystalline films with high transparency and a high-mu limited by phonon and ionized impurity scattering. 15 nm-thick films exhibit an average absorptance of < 0.5 % (between 390 nm-2000 nm) and an mu of 50 cm2/Vs increasing to 105 cm2/Vs for 100 nm-thick films. Alternatively, annealing in a neutral or reducing atmospheres result in higher mu for films thinner than 50 nm as a high Ne is maintained. The demonstration of thickness reduction while keeping high lateral mu makes In2O3:Zr is an alternative to reduce In in applications such as flexible displays, solar cells and light emitting diodes.

EPFL2019