Baochen Liao

Excellent c-Si surface passivation by low-temperature atomic layer deposited titanium oxide

In this work, we demonstrate that thermal atomic layer deposited (ALD) titanium oxide (TiOx) films are able to provide a—up to now unprecedented—level of surface passivation on undiffused low-resistivity crystalline silicon (c-Si). The surface passivation provided by the ALD TiOx films is activated by a post-deposition anneal and subsequent light soaking treatment. Ultralow effective surface recombination velocities down to 2.8 cm/s and 8.3 cm/s, respectively, are achieved on n-type and p-type float-zone c-Si wafers. Detailed analysis confirms that the TiOx films are nearly stoichiometric, have no significant level of contaminants, and are of amorphous nature. The passivation is found to be stable after storage in the dark for eight months. These results demonstrate that TiOx films are also capable of providing excellent passivation of undiffused c-Si surfaces on a comparable level to thermal silicon oxide, silicon nitride, and aluminum oxide. In addition, it is well known that TiOx has an optimal refractive index of 2.4 in the visible range for glass encapsulated solar cells, as well as a low extinction coefficient. Thus, the results presented in this work could facilitate the re-emergence of TiOx in the field of high-efficiency silicon wafer solar cells.In this work, we demonstrate that thermal atomic layer deposited (ALD) titanium oxide (TiOx) films are able to provide a—up to now unprecedented—level of surface passivation on undiffused low-resistivity crystalline silicon (c-Si). The surface passivation provided by the ALD TiOx films is activated by a post-deposition anneal and subsequent light soaking treatment. Ultralow effective surface recombination velocities down to 2.8 cm/s and 8.3 cm/s, respectively, are achieved on n-type and p-type float-zone c-Si wafers. Detailed analysis confirms that the TiOx films are nearly stoichiometric, have no significant level of contaminants, and are of amorphous nature. The passivation is found to be stable after storage in the dark for eight months. These results demonstrate that TiOx films are also capable of providing excellent passivation of undiffused c-Si surfaces on a comparable level to thermal silicon oxide, silicon nitride, and aluminum oxide. In addition, it is well known that TiOx has an optimal refract...

The effect of light soaking on crystalline silicon surface passivation by atomic layer deposited Al2O3

The effect of light soaking of crystalline silicon wafer lifetime samples surface passivated by thermal atomic layer deposited (ALD) Al2O3 is investigated in this paper. Contrary to other passivation materials used in solar cell applications (i.e., SiO2, SiNx), using thermal ALD Al2O3, an increase in effective carrier lifetime after light soaking under standard testing conditions is observed for both p-type (∼45%) and n-type (∼60%) FZ c-Si lifetime samples. After light soaking and storing the samples in a dark and dry environment, the effective lifetime decreases again and practically returns to the value before light soaking. The rate of lifetime decrease after light soaking is significantly slower than the rate of lifetime increase by light soaking. To investigate the underlying mechanism, corona charge experiments are carried out on p-type c-Si samples before and after light soaking. The results indicate that the negative fixed charge density Qf present in the Al2O3 films increases due to the light soa...

Deposition temperature independent excellent passivation of highly boron doped silicon emitters by thermal atomic layer deposited Al2O3

In this work, we demonstrate that by using H2O based thermal atomic layer deposited (ALD) Al2O3 films, excellent passivation (emitter saturation current density of ∼28 fA/cm2) on industrial highly boron p+-doped silicon emitters (sheet resistance of ∼62 Ω/sq) can be achieved. The surface passivation of the Al2O3 film is activated by a fast industrial high-temperature firing step identical to the one used for screen printed contact formation. Deposition temperatures in the range of 100-300 °C and peak firing temperatures of ∼800 °C (set temperature) are investigated, using commercial-grade 5″ Cz silicon wafers (∼5 Ω cm n-type). It is found that the level of surface passivation after activation is excellent for the whole investigated deposition temperature range. These results are explained by advanced computer simulations indicating that the obtained emitter saturation current densities are quite close to their intrinsic limit value where the emitter saturation current is solely ruled by Auger recombinatio...

Passivation of Boron-Doped Industrial Silicon Emitters by Thermal Atomic Layer Deposited Titanium Oxide

Passivation of p⁺ -doped silicon is demonstrated by using water (H₂O)-based thermal atomic layer-deposited titanium oxide (TiO_x) films. Emitter saturation current density (J_{0 e}) values below 30 fA/cm² are obtained on textured p⁺ -doped samples with a sheet resistance in the 80-120 Ω/sq range. This low emitter saturation current density would allow open-circuit voltages up to 720 mV when this TiO_x film is used in n-type silicon wafer solar cells with a front boron emitter. In addition, the optical properties of TiO_x make it an excellent option for use as antireflection coating on the silicon wafer solar cell after encapsulation. Thus, the results demonstrated in this paper could enable interesting new routes for future high-efficiency n-type silicon wafer solar cells.

An Improved Methodology for Extracting the Interface Defect Density of Passivated Silicon Solar Cells

The interface properties of the dielectric layer passivated silicon wafers can be characterized by capacitance-voltage, surface photovoltage, or contactless corona-voltage measurements. Conventionally, U-shaped interface defect density distributions Dit(E) are reported. However, in this study, the reported interface defect density toward the silicon conduction or valence band edges is proven to be an artefact, or at least strongly overestimated. This stems from the fact that the formula used for Dit extraction is valid only at very low temperatures, whereas measurements are typically performed at ~300 K. We propose an improved methodology for Dit(E) extraction, which can self-consistently reproduce the raw data of contactless corona-voltage measurements. Applying this advanced methodology, the interfaces of several dielectric layers passivated n-type Czochralski-grown silicon wafers are investigated. In addition, the extracted Dit(E) distributions of these samples are then used to analyze their effective carrier lifetime performance.

Understanding Surface Treatment and ALD AlOx Thickness Induced Surface Passivation Quality of c-Si Cz Wafers

In this paper, the passivation quality of crystalline silicon (c-Si) wafers, when passivated by atomic layer deposited aluminum oxide (ALD AlO<italic>_x</italic>), is investigated. Specifically, we investigated the effect of surface modification of the c-Si interface prior to the ALD AlO<italic>_x</italic> deposition (via −H and −OH termination of the c-Si wafer) over a large range of AlO<italic>_x</italic> thicknesses (0.4–80 nm). Fourier transform infrared (FTIR) studies confirmed the presence of −H and −OH termination on c-Si surface. In general, surface passivation on −OH terminated wafers was found to be better than their −H terminated counterparts, which is consistent with correspondingly measured density of interface defects, and density of fixed interface charge. In all cases, the formation of an additional interfacial SiO <italic>_x</italic> layer between the c-Si/AlO<italic>_x</italic> interface was revealed. The corresponding Si-oxide peaks, as measured by X-ray photoelectron spectroscopy (XPS), were found to be stronger for −OH terminated c-Si surfaces. The thickness of the interfacial SiO<italic>_x</italic> layer was measured by high-resolution transmission electron microscopy. Thus, despite the superior surface passivation of −OH terminated c-Si wafers, for the ALD AlO<italic>_x</italic> tunnel layers of 0.4–2 nm (i.e., on the order necessary to form passivated contacts for solar cells), there is a trade-off between the better passivation achieved by the thicker SiO<italic>_x</italic> interfacial layers (–OH termination), and the correspondingly higher tunneling resistance due to the overall larger effective tunnel layer thickness.

Surface passivation investigation on ultra-thin atomic layer deposited aluminum oxide layers for their potential application to form tunnel layer passivated contacts

The surface passivation performance of atomic layer deposited ultra-thin aluminium oxide layers with different thickness in the tunnel layer regime, i.e., ranging from one atomic cycle (~0.13 nm) to 11 atomic cycles (~1.5 nm) on n-type silicon wafers is studied. The effect of thickness and thermal activation on passivation performance is investigated with corona-voltage metrology to measure the interface defect density D it(E) and the total interface charge Q tot. Furthermore, the bonding configuration variation of the AlO x films under various post-deposition thermal activation conditions is analyzed by Fourier transform infrared spectroscopy. Additionally, poly(3,4-ethylenedioxythiophene) poly(styrene sulfonate) is used as capping layer on ultra-thin AlO x tunneling layers to further reduce the surface recombination current density to values as low as 42 fA/cm2. This work is a useful reference for using ultra-thin ALD AlO x layers as tunnel layers in order to form hole selective passivated contacts for silicon solar cells.