Assessment of effective-medium theories in the analysis of nucleation and microscopic surface roughness evolution for semiconductor thin films

Solar cells comprising methylammonium lead iodide perovskite (MAPI) are notorious for their sensitivity to moisture. We show that hydrated crystal phases are formed when MAPI is exposed to water vapour at room temperature and that these phase changes are fully reversed when the material is subsequently dried. The reversible formation of CH 3 NH 3 PbI 3 •H 2 O followed by (CH 3 NH 3 ) 4 PbI 6 •2H 2 O (upon long exposure times) was observed using time resolved XRD and ellipsometry of thin films prepared using 'solvent engineering', single crystals, and state of the art solar cells. In contrast to water vapour, the presence of liquid water results in the irreversible decomposition of MAPI to form PbI 2 . MAPI changes from dark brown to transparent on hydration; the precise optical constants of CH 3 NH 3 PbI 3 •H 2 O formed on single crystals were determined, with a bandgap at 3.1 eV. Using the single crystal optical constants and thin film ellipsometry measurements, the time dependent changes to MAPI films exposed to moisture were modelled. The results suggest that the mono-hydrate phase forms independently of the depth in the film suggesting rapid transport of water molecules along grain boundaries. Vapour phase hydration of an unencapsulated solar cell (initially J sc ≈ 19 mA cm -2 and V oc ≈ 1.05 V at 1 sun) resulted in more than a 90 % drop in short circuit photocurrent and around 200 mV loss in open circuit potential, but these losses were fully reversed after the cell was exposed to dry nitrogen for 6 hours. Hysteresis in the current-voltage characteristics was significantly increased after this dehydration, which may be related to changes in the defect density and morphology of MAPI following recrystallization from the hydrate. Based on our observations we suggest that irreversible decomposition of MAPI in the presence of water vapour only occurs significantly once a grain has been fully converted to the hydrate phase.

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“…In the particular case of top layer roughness, an EMA is used with identical volume proportion of both the top layer and voids. 22 …”

Section: The Bruggeman Approximationmentioning

confidence: 99%

Reversible Hydration of CH₃NH₃PbI₃ in Films, Single Crystals, and Solar Cells

et al. 2015

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“…In that case, however, expressions (12) and (13) do not hold since  effR may depend also on  1I . It must be mentioned that for strong absorption regions, different EMTs, lead to different compositional dependences of the real and imaginary parts of the dielectric function [19].…”

Section: Theorymentioning

confidence: 99%

Effective medium theories for composite optical materials in spectral ranges of weak absorption: the case of Nb₂O₅–SiO₂mixtures

Sancho‐Parramon¹,

Janicki²

2008

J. Phys. D: Appl. Phys.

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Abstract. The validity of effective medium theories (EMTs) for mixtures of dielectric materials in weak absorption regions is studied. Based on the Bergman spectral representation, it is possible to show that for any EMT the absorption properties of a mixture consist basically of scaling of the absorption properties of the material with highest absorption. The real part of the dielectric function remains unaffected by the absorption properties. Thin films consisting of Nb 2 O 5 -SiO 2 mixtures are characterized using optical measurements and the results are compared with the calculations of EMTs. The large discrepancies between the absorption properties observed experimentally and those calculated using EMTs are justified by the failure of these theories to predict a compositional dependence of relevant structural parameters, as the band gap energy or the width of localized states. This failure, however, affects the calculation of the refractive index in the weak absorption regions in a less significant degree.

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“…Nanocrystalline Si:H (nc-Si:H), denoted by  is represented by a model using a parametric critical point structure based on the joint density of states. 7 The optical properties of the components of the surface roughness layer are represented by a Bruggeman effective medium approximations 8 (EMA) consisting of variable fractions of bulk…”

Section: Spectroscopic Ellipsometrymentioning

confidence: 99%

Enhanced Growth Rate and Silane Utilization in Amorphous Silicon and Nanocrystalline-Silicon Solar Cell Deposition via Gas Phase Additives

Ridgeway¹,

Hegedus²,

Podraza³

2012

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Assessment of effective-medium theories in the analysis of nucleation and microscopic surface roughness evolution for semiconductor thin films

Cited by 262 publications

References 44 publications

Reversible Hydration of CH₃NH₃PbI₃ in Films, Single Crystals, and Solar Cells

Reversible Hydration of CH₃NH₃PbI₃ in Films, Single Crystals, and Solar Cells

Effective medium theories for composite optical materials in spectral ranges of weak absorption: the case of Nb₂O₅–SiO₂mixtures

Enhanced Growth Rate and Silane Utilization in Amorphous Silicon and Nanocrystalline-Silicon Solar Cell Deposition via Gas Phase Additives

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Assessment of effective-medium theories in the analysis of nucleation and microscopic surface roughness evolution for semiconductor thin films

Cited by 262 publications

References 44 publications

Reversible Hydration of CH3NH3PbI3 in Films, Single Crystals, and Solar Cells

Reversible Hydration of CH3NH3PbI3 in Films, Single Crystals, and Solar Cells

Effective medium theories for composite optical materials in spectral ranges of weak absorption: the case of Nb2O5–SiO2mixtures

Enhanced Growth Rate and Silane Utilization in Amorphous Silicon and Nanocrystalline-Silicon Solar Cell Deposition via Gas Phase Additives

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Product

Resources

About

Reversible Hydration of CH₃NH₃PbI₃ in Films, Single Crystals, and Solar Cells

Reversible Hydration of CH₃NH₃PbI₃ in Films, Single Crystals, and Solar Cells

Effective medium theories for composite optical materials in spectral ranges of weak absorption: the case of Nb₂O₅–SiO₂mixtures