Bhushan Sopori

Hydrogen in silicon: A discussion of diffusion and passivation mechanisms

Abstract A model for H diffusion and passivation is described that explains the experimental results from solar cell passivation, such as variations in the degree of passivation in substrates from different vendors, passivation due to forming gas anneals following Al alloying, and the effects of plasma enhanced chemical vapor deposition (PECVD) nitridation. Two major features of the model are inclusion of (i) a new H diffusion mechanism involving hydrogen-vacancy complex {V-H} formation, and (ii) surface damage that causes high solubity of H at the Si surface and dissociation of molecular H at low temperatures. The theoretical analysis, based on static potential energy surfaces at the ab-initio Hatree-Fock level, identifies some details of diffusion mechanisms.

Emissivity measurements and modeling of silicon-related materials: An overview

An overview of the emissivity measurements and modeling of silicon-related materials is presented. The experimental component of this investigation is based on results obtained utilizing spectral emissometry. An analysis of the comparison of the measured data with other similar approaches is made. In particular, the celebrated work of Sato is revisited to understand the implications of his study. Simulations of the temperature and wavelength dependent emissivity of silicon based on the semiempirical MULTIRAD model are presented. The influence of doping concentration, surface roughness, and coatings on the emissivity of silicon, as a function of temperature, is discussed.

Understanding light-induced degradation of c-Si solar cells

We discuss results of our investigations toward understanding bulk and surface components of light-induced degradation (LID) in low-Fe c-Si solar cells. The bulk effects, arising from boron-oxygen defects, are determined by comparing degradation of cell parameters and their thermal recovery, with that of the minority-carrier lifetime (τ) in sister wafers. We found that the recovery of t in wafers takes a much longer annealing time compared to that of the cell. We also show that cells having SiN:H coating experience a surface degradation (ascribed to surface recombination). The surface LID is seen as an increase in the q/2kT component of the dark saturation current (J02). The surface LID does not recover fully upon annealing and is attributed to degradation of the SiN:H-Si interface. This behavior is also exhibited by mc-Si cells that have very low oxygen content and do not show any bulk degradation.

Single heterojunction solar cells on exfoliated flexible ∼25 μm thick mono-crystalline silicon substrates

Mono-crystalline silicon single heterojunction solar cells on flexible, ultra-thin (∼25 μm) substrates have been developed based on a kerf-less exfoliation method. Optical and electrical measurements demonstrate maintained structural integrity of these flexible substrates. Among several single heterojunction ∼25 μm thick solar cells fabricated with un-optimized processes, the highest open circuit voltage of 603 mV, short circuit current of 34.4 mA/cm2, and conversion efficiency of 14.9% are achieved separately on three different cells. Preliminary reliability test results that include thermal shock and highly accelerated stress tests are also shown to demonstrate compatibility of this technology for use in photovoltaic modules.

A comparison of gettering in single- and multicrystalline silicon for solar cells

The differences in the impurity gettering between single and multicrystalline silicon are discussed. These differences arise from impurity-defect interactions that occur during thermal processing of multicrystalline material. A gettering model is proposed to explain the observed behaviour of gettering in multicrystalline cells.

Influence of distributed defects on the photoelectric characteristics of a large-area device

A network model is developed to analyze the effects of crystal defects on the performance of a large-area photovoltaic device. The model assembles local segments of the device as dark current generators whose values depend on the local defect densities. The values of the parameters for different defect densities are determined empirically.

Observation of enhanced hydrogen diffusion in solar cell silicon

We report the observation of higher diffusivity of hydrogen in some solar cell silicon compared to that in Czochralski and float zone wafers. SIMS profiles of hydrogen/deuterium, implanted at low energies and in a temperature range of 100–300 °C, are compared for a variety of different types of silicon substrates. In addition, a new technique that utilizes hydrogen decoration of dislocations was applied to directly verify long diffusion depths in some solar cell silicon. Higher diffusivity of hydrogen permits backside hydrogenation of solar cells to be carried out in less than 30 min with a significant improvement in the cell performance.

INCREASING SHORT MINORITY CARRIER DIFFUSION LENGTHS IN SOLAR-GRADE POLYCRYSTALLINE SILICON BY ULTRASOUND TREATMENT

We have found that ultrasound treatment (UST) has a profound effect on the recombination rate in as‐grown, B‐doped cast polycrystalline silicon wafers for photovoltaic applications. As determined by surface photovoltage measurements of the minority carrier diffusion length L, the UST increases the corresponding lifetime by almost an order of magnitude. The maximum enhancement takes place in the wafer regions with the shortest L values. For L≳20 μm, both positive and negative changes of L after UST are revealed at different wafer regions. The UST effect is temperature dependent and exhibits maximum influence at about 60 °C. Enhanced dissociation of Fe‐B pairs by UST is identified as a mechanism which leads to a negative change of large L values, and a complex post‐treatment relaxation. A positive change of L is attributed to the influence of ultrasound vibrations on crystallographic defects.

Optical modeling of a-Si solar cells

The authors describe applications of PV Optics to analyze the behavior of a metallic back-reflector on an a-Si solar cell. The calculated results from PV Optics agree well with the measured data on solar cells. Several unexpected results obtained from these calculations are qualitatively explained.

Studies on Backside Al-Contact Formation in Si Solar Cells: Fundamental Mechanisms

We have studied mechanisms of back-contact formation in screen-printed Si solar cells by a fire-through process. An optimum firing temperature profile leads to the formation of a P-Si/P + - Si/ Si-Al eutectic/agglomerated Al at the back contact of a Si solar cell. Variations in the interface properties were found to arise from Al-Si melt instabilities. Experiments were performed to study melt formation. We show that this process is strongly controlled by diffusion of Si into Al. During the ramp-up, a melt is initiated at the Si-Al interface, which subsequently expands into Al and Si. During the ramp-down, the melt freezes, which causes the doped region to grow epitaxially on Si, followed by solidification of the Si-Al eutectic. Any agglomerated (or sintered) Al particles are dispersed with Si. Implications on the performance of the cell are described.