Shubham Duttagupta

A Systematic Loss Analysis Method for Rear-Passivated Silicon Solar Cells

By combining commonly available solar cell characterization methods with easy-to-prepare test structures and partially processed rear-passivated solar cells from the production line, we show that various cell loss mechanisms can be quantified in exquisite detail to generate process-related diagnostics. An example monocrystalline silicon localized back surface field solar cell type is examined using a systematic routine that breaks down the factors limiting open-circuit voltage, short-circuit current, and fill factor (FF) to identify the cell structures headroom for improvement.

Progress in Surface Passivation of Heavily Doped n-Type and p-Type Silicon by Plasma-Deposited AlO

We report an outstanding level of surface passivation for both n⁺ and p⁺ silicon by AlO_x/SiN_x dielectric stacks deposited in an inline plasma-enhanced chemical vapor deposition (PECVD) reactor for a wide range of sheet resistances. Extremely low emitter saturation current densities (J_0e) of 12 and 200 fA/cm² are obtained on 165 and 25 Ω/sq n⁺ emitters, respectively, and 8 and 45 fA/cm² on 170 and 30 Ω/sq p⁺ emitters, respectively. Using contactless corona-voltage measurements and device simulations, we demonstrate that the surface passivation mechanism on both n⁺ and p ⁺ silicon is primarily due to a relatively low interface defect density of <;10¹¹ eV^-1cm^-2 in combination with a moderate fixed negative charge density of (1-2) × 10¹² cm^-2. From advanced modeling, the fundamental surface recombination velocity parameter is shown to be in the order of 10⁴ cm/s for PECVD AlO_x/SiN_x passivated heavily doped n⁺ and p⁺ silicon surfaces.

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State-of-the-art surface passivation results are obtained on undiffused p-type commercial-grade Czochralski Si wafers with effective surface recombination velocity S_eff values of ~8 cm/s and implied open-circuit voltage iV_oc values of up to 715 mV with an industrially fired dielectric stack of silicon oxide and silicon nitride (SiO_x/SiN_x) deposited in an industrial inline plasma-enhanced chemical vapor deposition reactor. We are able to controllably vary the total positive charge density Q_total in the stack by more than one order of magnitude (10¹¹-10¹² cm^-2) with no impact on midgap interface state density D_{it,m idgap} (5 × 10¹¹ eV^-1· cm^-2) by altering the deposition temperature of the SiO_x layer in the stack. We show experimentally that, for inversion conditions, S_eff scales with the inverse square of the charge density 1/Q²_total, which is in good agreement with theory. Based on the measured injection-level-dependent minority carrier lifetimes and the total positive charge densities, it is shown that films with higher positive charge density have higher 1-sun Voc and fill factor (FF) potential. Large-area alloyed aluminum local back surface field solar cells confirmed this by showing higher conversion efficiency by 0.17% absolute due to improved cell V_oc and FF of the solar cells featuring a SiO_x/SiN_x stack with a higher Q_total.

/SiN

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.

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Extremely low emitter saturation current density (J_0e) values of 6 and 45 fA/cm², respectively, are reported for 220 and 30 Ω/sq planar p⁺ boron emitters passivated by an AlO_x/SiN_x stack deposited in an industrial plasma-enhanced chemical vapor deposition (PECVD) reactor. The thermal activation of the AlO_x films is performed in a standard industrial fast firing furnace, making the developed passivation stack industrially viable. For textured p⁺ emitters the J_0e values are found to be 1.5 - 2 times higher compared to planar emitters. This excellent surface passivation is attributed to a high negative charge density of -(3-6)×10¹² cm^-2 in combination with a low interface defect density of ℒ10¹¹ eV^-1cm^-2. Assuming a short-circuit current density of 40 mA/cm² and the ideal diode law, the J_0e result for the 80 Ω/sq emitter represents a 1-sun open-circuit voltage limit of 736 mV at 25°C.

Dielectric Stacks

This paper presents a three-dimensional numerical analysis of homojunction/heterojunction hybrid silicon wafer solar cells, featuring front-side full-area diffused homojunction contacts and rear-side heterojunction point contacts. Their device performance is compared with conventional full-area heterojunction solar cells as well as conventional diffused solar cells featuring locally diffused rear point contacts, for both front-emitter and rear-emitter configurations. A consistent set of simulation input parameters is obtained by calibrating the simulation program with intensity dependent lifetime measurements of the passivated regions and the contact regions of the various types of solar cells. We show that the best efficiency is obtained when a-Si:H is used for rear-side heterojunction point-contact formation. An optimization of the rear contact area fraction is required to balance between the gains in current and voltage and the loss in fill factor with shrinking rear contact area fraction. However, the corresponding optimal range for the rear-contact area fraction is found to be quite large (e.g. 20-60 % for hybrid front-emitter cells). Hybrid rear-emitter cells show a faster drop in the fill factor with decreasing rear contact area fraction compared to front-emitter cells, stemming from a higher series resistance contribution of the rear-side a-Si:H(p+) emitter compared to the rear-side a-Si:H(n+) back surface field layer. Overall, we show that hybrid silicon solar cells in a front-emitter configuration can outperform conventional heterojunction silicon solar cells as well as diffused solar cells with rear-side locally diffused point contacts.

Dielectric Charge Tailoring in PECVD SiO

Multidimensional numerical simulation of boron diffusion is of great relevance for the improvement of industrial n-type crystalline silicon wafer solar cells. However, surface passivation of boron diffused area is typically studied in one dimension on planar lifetime samples. This approach neglects the effects of the solar cell pyramidal texture on the boron doping process and resulting doping profile. In this work, we present a theoretical study using a two-dimensional surface morphology for pyramidally textured samples. The boron diffusivity and segregation coefficient between oxide and silicon in simulation are determined by reproducing measured one-dimensional boron depth profiles prepared using different boron diffusion recipes on planar samples. The established parameters are subsequently used to simulate the boron diffusion process on textured samples. The simulated junction depth is found to agree quantitatively well with electron beam induced current measurements. Finally, chemical passivation on planar and textured samples is compared in device simulation. Particularly, a two-dimensional approach is adopted for textured samples to evaluate chemical passivation. The intrinsic emitter saturation current density, which is only related to Auger and radiative recombination, is also simulated for both planar and textured samples. The differences between planar and textured samples are discussed.

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We present state-of-the-art results on boron emitter passivation (J_0e <; 25 fA/cm² and S_n0 <; 400 cm/s) with industrially fired positively-charged low-temperature PECVD SiO_x/SiN_x dielectric stacks deposited in an industrial reactor. These films feature a very low fixed charge density (~ + 6×10¹⁰ cm^-2) and excellent interface quality (D_{it, midgap} of ~3×10¹⁰ eV^-1 cm^-2) after an industrial firing step. Based on contactless corona-voltage measurements and device simulation, we explain the mechanism of surface passivation to be dominated by chemical passivation rather than field-effect passivation. With excellent optical and passivation properties, these films are suitable for high-efficiency cost-effective industrial n-type silicon wafer solar cells.

/SiN

Excellent surface passivation of boron emitters is demonstrated for industrial plasma-enhanced chemical vapor deposited (PECVD) SiOx/AlOx stacks. Emitter saturation current densities of 39 and 34 fA/cm2, respectively, were achieved at 300 K on 80 Ω/sq boron emitters after activation by (i) a standard industrial firing process and (ii) a forming gas anneal followed by industrial firing. We find that the surface passivation by SiOx/AlOx stack can be effectively controlled by varying the SiOx layer thickness. This stack is directly applicable to certain high-efficiency solar cell structures, by optimising the SiOx thickness accordingly.

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Aluminum oxide has been highlighted as a promising surface passivation layer for p-type silicon surface. To-date, most of the studies have focused on aluminum oxide layers deposited with atomic layer deposition systems which have lower throughput than industrial plasma-based systems. In this study, the effects of deposition conditions on the electrical and optical properties of aluminum oxide deposited by an industrial plasma enhanced chemical vapor deposition system are presented. Low saturation current density of 1.9 fA/cm2 was achieved by as deposited layer on p-type Czochralski wafer. The most significant deposition process factor for high quality surface passivation was found to be the gas flow rate ratio between nitrous oxide and tri-methyl-aluminum.