Quantification of Nanoscale Density Fluctuations in Hydrogenated Amorphous Silicon

Abstract

The nanostructure of hydrogenated amorphous silicon (a-Si:H) is studied by a combination of small-angle X-ray (SAXS) and neutron scattering (SANS) with a spatial resolution of 0.8 nm. The a-Si:H materials were deposited using a range of widely varied conditions and are representative for this class of materials. We identify two different phases which are embedded in the a-Si:H matrix and quantified both according to their scattering cross-sections. First, 1.2 nm sized voids (multivacancies with more than 10 missing atoms) which form a superlattice with 1.6 nm void-to-void distance are detected. The voids are found in concentrations as high as 6 10 19 cm -3 in a-Si:H material that is deposited at a high rate. Second, dense ordered domains (DOD) that are depleted of hydrogen with 1 nm average diameter are found. The DOD tend to form 10-15 nm sized aggregates and are largely found in all a-Si:H materials considered here. These quantitative findings make it possible to understand the complex correlation between structure and electronic properties of a-Si:H and directly link them to the light-induced formation of defects. Finally, a structural model is derived, which verifies theoretical predictions about the nanostructure of a-Si:H.

Imaging of Bandtail States in Silicon Heterojunction Solar Cells: Nanoscopic Current Effects on Photovoltaics

“…Dark current is governed by trap-assisted tunneling through localized states of the valence bandtail states, while PC occurs homogeneous across the SHJ area. The tail states are suggested to originate from dense ordered domains that have been reported in a-Si:H which can cluster into larger regions of up to 10 nm well in accordance with our experimental findings . The nanoscopic localization of the bandtail states of the valence band is found to be about 1 nm well in agreement with what has been estimated from theoretical simulations …”

“…Dark current is governed by trap assisted tunneling through localized states of the valence bandtail states while PC occurs homogeneous across the SHJ area. The tail states are suggested to originate from dense ordered domains that have been reported in a-Si:H 26 With a current collecting TCO layer, the observed nanoscopic effects will average out, leading to 𝑉 OC of 748 mV, in accordance with experiment and macroscopic device simulations.…”

“…wich can cluster into larger regions of up to 10 nm well in accordance with our experimental findings. The localization of the bandtail states of the valence band are around found to be about 1 nm well in agreement with what has been estimated from theoretical simulations26 .The nanoscopic I-V characteristics of SHJ devices is strongly influenced by fluctuating potentials caused by trapped and detrapping of localized charges at defects in the doped a-Si:H layer, producing local potential fluctuations and, thus, local 𝑉 OC above 0.8 V, in some few cases even higher than the bandgap. This observation does not violate the fundamental device paradigms of SHJ solar cells as neither the bandgap nor the local Fermi level is well defined on the nm level.…”

“…In this report, we will show that with nanometer conductive probes as provided by conduction atomic force microscopy (cAFM), we are able to resolve such nanoscopic effects in conventional crystalline silicon (c-Si) heterojunction (SHJ) solar cells with hydrogenated amorphous silicon (a-Si:H) as selective contact layers. − Different from previous cAFM on silicon, we probe the nanoscopic dark- and photocurrents (PCs) that flow vertically through the selective contact. , With this technique, we are able to detect local current–voltage ( I–V ) curves with V OC loc at room temperature (RT), which are as large as 1.2 V. We show that such large V OC loc are in accordance with thermodynamics and are a result of charge transfer and injection through a-Si:H bandtail- and defect-states at the charge-selective SHJ interface, which leads to stochastic charge trapping and hence to fluctuations of the nanoscopic device performance, which is observed as random telegraph noise (RTN) with up to 100% modulation. − This is orders of magnitude larger than what has been reported before for doped a-Si:H. − From the analysis of the cAFM images, we can reconstruct a density of states, which suggests that we image bandtail states in a-Si:H with a minimum localization of about 1 nm. We suggest that these states are induced by density fluctuations in the amorphous network that have recently been reported …”

“…We suggest that these states are induced by density fluctuations in the amorphous network that have recently been reported about 26 .…”