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RESEARCH ARTICLE |  OCTOBER 08 2024
Solar spectral response of photochromic YHO films
Per-Anders Hansen   ; Elbruz Murat Baba 
AIP Advances 14, 105316 (2024)
https://doi.org/10.1063/5.0224875
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AIP Advances ARTICLE pubs.aip.org/aip/adv
Solar spectral response of photochromic
YHO films
Cite as: AIP Advances 14, 105316 (2024); doi: 10.1063/5.0224875
Submitted: 29 June 2024 • Accepted: 23 September 2024 •
Published Online: 8 October 2024
Per-Anders Hansena) and Elbruz Murat Baba
AFFILIATIONS
Solar Energy Materials and Technology Department, Institute for Energy Technology, Kjeller, Norway
a)Author to whom correspondence should be addressed: Per-Anders.Hansen@ife.no
ABSTRACT
Photochromic yttrium oxyhydride, YHO, has excellent properties as a responsive solar control material, in particular due to its color-neutral
darkening. In this work, we have investigated YHO’s spectral response to the solar AM 1.5 spectrum to evaluate how the total photon activa-
tion flux changes as a function of thickness and wavelength. The calculations are based on ellipsometry measurements in the 240–2000 nm
range. Our results show that the UV light absorption saturates at around 400 nm thick film, while blue light (>400 nm) continues to provide
photoactivation well beyond 1 μm thickness. Understanding the spectral response and absorption per unit time of activating photons as a
function of material thickness is important for evaluating the material or coating for applications.
© 2024 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution-NonCommercial 4.0
International (CC BY-NC) license (https://creativecommons.org/licenses/by-nc/4.0/). https://doi.org/10.1063/5.0224875
Photochromic YHO films has been researched for over a
decade.1 An up-to-date literature review of, the history of, and
perspectives on photochromic YHO can be found in Refs. 2 and
3. Its unique colour-neutral reversible photodarkening makes it a
very attractive material for self-darkening windows and solar con-
trol applications. YHO can be photodarkened mainly by UV and
blue light, but even yellow light has been shown to cause photo-
darkening.4 The optical properties of YHO depend critically on the
synthesis parameters, in particular with respect to the bandgap and
absorption spectrum.5 In addition, general LnHO can be deposited
using lanthanides other than Y,6 adding even more variability to
optical properties. In combination, this means that YHO’s response
to full-spectrum sunlight can be significantly different between sam-
ples. In addition, the photochromic response is also dependent on
both wavelength and intensity. This makes it challenging to com-
pare literature results when different excitation wavelengths have
been used.4,7 Equally important is that YHO films’ response to sun-
light under realistic conditions may be very different from lab results
based on an often-monochromatic light source. Moldarev et al.7
investigated environmental dependency of YHO samples with thick-
nesses between 630 and 1773 nm using excitation sources between
450 and 600 nm. It was reported that the environmental dependency
of bleaching is related to the relation between excitation light and the
material’s bandgap.
In this work, for the first time, we have investigated how the
optical properties of YHO combine with the AM1.5 solar spectrum.
We have determined n(λ) and k(λ) in the 240–2000 nm range by
spectroscopic ellipsometry, and we have calculated the wavelength
dependent absorption using a film based on k(λ). This is further
used to calculate the total amount of absorbed activating photons
in the AM1.5 spectrum and the contribution per wavelength, in
addition to the total absorption as a function of film thickness.
Note that the physical properties of YHO and LnHO films depend
strongly on synthesis conditions5 and on the thickness and crystallite
sizes.8 Thus, films grown to achieve significantly different bandgaps
(high or lower) will require updated k(λ) in the calculations provided
here. In this work, we simplify the thickness dependent calculations
by assuming that k(λ) is constant with thickness.
The YHO thin films were synthesized following two main steps:
First, YH2−x metallic films were deposited in a Kurt J. Lesker PVD
75 sputter deposition unit using a hydrogen (5N) and argon (5N)
gas mixture with a 1:10 flux ratio in pulsed DC mode under 5 mTorr.
Depositions were made using 3-in. yttrium targets with 99.9% purity
under 200 W power. Oxidation of the deposited samples was per-
formed by exposing the samples to ambient air, while no oxygen was
introduced during deposition. Spectroscopic ellipsometry measure-
ments was conducted on a J. A. Woollam VASE, in the wavelength
range of 240–2000 nm and with angles of 60–80○ in steps of 5○.
AIP Advances 14, 105316 (2024); doi: 10.1063/5.0224875 14, 105316-1
© Author(s) 2024
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AIP Advances ARTICLE pubs.aip.org/aip/adv
FIG. 1. Sample structure used in VASE modeling, along with the thicknesses of
the layers.
The VASE data were modeled as a layered sample, as illustrated
in Fig. 1, with oscillator models for the YHO and top oxide lay-
ers. The glass was measured without coatings and modeled using
a Cauchy model. For the top oxide, an YHO film on glass was oxi-
dized at 150 ○C for 30 min in air to obtain a transparent Y2O3-like
film. This film is assumed to approximate the YHO top oxide layer in
terms of optical properties. This Y2O3-like film was modeled using a
Sellmeier model. The YHO top oxide was modeled using an effec-
tive media approximation (EMA) of the Y2O3-like film model and
air to account for differences in density between the deliberately
oxidized film and the naturally created YHO top oxide layer. The
YHO film was modeled using a Tauc–Lorentz oscillator to deter-
mine the YHO bandgap. Ellipsometry is very sensitive to even weak
absorption, and handling of the sample in room light for a short
duration is sufficient to cause a small darkening that is not visible to
the eye but still significant for VASE. A very broad Gaussian oscilla-
tor centered at 1.7 eV was used for the darkened state. The Gaussian
oscillator approximates the absorption of the darkened YHO well.1
A thorough description of VASE modelling of photochromic YHO
can be found in our recently accepted article.9 For the absorp-
tion calculations in this work, only the Tauc–Lorentz oscillator
is considered based on the assumption that photons absorbed
FIG. 2. VASE Psi data and model. Note that the gap in the data around 1400 nm
is due to a blocking band in the UV fiber used.
FIG. 3. VASE Delta data and model. Note that the gap in the data around 1400 nm
is due to a blocking band in the UV fiber used.
by the darkening state do not contribute to further darkening
of YHO.
The materials’ oscillator models and sample structure (Fig. 1)
fit the VASE Psi and Delta data very well, as seen in Figs. 2 and 3.
The modeled n(λ) of the YHO top oxide is shown in Fig. 4,
along with the deliberately oxidized sample and the glass substrate.
The deliberately oxidized sample indeed shows a refractive index
similar to what is expected of an Y2O3 film.
The optical properties of the YHO layer, n(λ) and k(λ), are
shown in Fig. 5. The Tauc–Lorentz absorption is seen as the large
k-peak below 440 nm, while the broad darkening state covers the
whole spectrum at longer wavelengths.
The layer thicknesses, n(λ) and k(λ), for YHO and the EMA
ratio between oxidized YHO and air are optimized to obtain a good
FIG. 4. n(λ) of the oxidized YHO, YHO top oxide layer, and glass substrate.
AIP Advances 14, 105316 (2024); doi: 10.1063/5.0224875 14, 105316-2
© Author(s) 2024
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AIP Advances ARTICLE pubs.aip.org/aip/adv
FIG. 5. Modeled n(λ) and k(λ) of YHO, consisting of a Tauc–Lorentz oscillator for
the bandgap and a very broad Gaussian for the darkened state.
fit of the VASE Psi and Delta data. The obtained YHO k(λ) forms
the basis of the following absorption calculations.
From k(λ), one can calculate the absorption coefficient and the
total absorption through a film as a function of wavelength and
depth into the film. The relation between k(λ) and the absorption
coefficient α(λ) is given by Eq. (1). From α(λ), the Beer–Lambert law
can be used to calculate the total amount of absorbed photons as a
function of depth into the material,
α(λ) = 2π
λ
k(λ). (1)
The Beer–Lambert law can also be used to perform the reverse cal-
culation, to calculate how thick a film would be needed for a certain
total absorption. A useful parameter for understanding the spectral
response of a thin film to the spectrum of the incident light is the
FIG. 6. t90(λ) of the YHO film based on k(λ) and the photon flux of the AM1.5
spectrum.
FIG. 7. Photon absorption as a function of film thickness and photon wavelength.
film thickness required for 90% absorption, t90(λ).10 This is shown
in Fig. 6. In this figure, we can see that if we want to darken this
YHO film with monochromatic 350 nm light, a film thickness of
about 600 nm is required to absorb 90% of this light. Note that in
real applications such as solar control films, it may not be neces-
sary to absorb all the UV photons to obtain a sufficient darkening
rate. The t90(λ) parameter is in any case useful for understand-
ing the spectral response of the material toward different light
sources.
Similarly, we can calculate the total absorption of an YHO thin
film as a function of both the wavelength of the incoming light and
the thickness of the film. This is shown in Fig. 7. Here, we see that a
film of 800 nm thickness will absorb all 300 nm photons, the majority
of 400 nm photons, and nearly no 450 nm photons.
This can be turned around to ask how the film thickness mat-
ters for different light sources, as shown in Fig. 8. If our light source
FIG. 8. Remaining light intensity as a function of depth into the film as a function
of photon wavelength.
AIP Advances 14, 105316 (2024); doi: 10.1063/5.0224875 14, 105316-3
© Author(s) 2024
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FIG. 9. Total amount of photons absorbed from the AM1.5 solar irradiance
spectrum as a function of film thickness, based on the combination of k(λ), AM1.5
solar spectrum, and Beer–Lambert law.
is a 400 nm laser, we see that there is still some light passing through
even for a 1000 nm film. For a 300 nm laser, on the other hand, it is
clear that film thicknesses above 300 nm will not provide any further
absorption. This is important when designing sample structures for
different applications. Depending on the light source in the applica-
tion area and the required absorption (i.e., some applications will not
require full absorption), the requirement for the YHO film thickness
can be very different.
Knowing k(λ), we can estimate the number of photons
absorbed for each wavelength for a given light source spectrum
and irradiance. By combining the ratio of absorbed photons per
FIG. 10. Photons absorbed from the AM1.5 spectrum as a function wavelength
and calculated film thickness.
wavelength from Fig. 8 and the AM1.5 solar spectrum from Fig. 6,
we can calculate the total number of photons absorbed as a func-
tion of film thickness and the contribution to this total from each
wavelength. This is shown in Figs. 9 and 10, respectively. Figure 9
shows that the total number of photons absorbed by an YHO
film increases as the film thickness increases, but the increase rate
becomes smaller as the thickness becomes large.
Figure 10 shows that for very thin films, the contribution to
the total photon absorption is quite evenly distributed over all wave-
lengths in the absorption range. This comes from combining YHO’s
absorption, which increases toward short wavelengths, with the
number of photons of the AM1.5 solar spectrum, which increases
toward longer wavelengths. As the film thickness increases up to
about 400 nm, it is seen that the absorption of UV photons saturates
and that additional film thicknesses will not give any more signifi-
cant contribution from this range. Incidentally, the AM1.5 spectrum
also has a step at 400 nm. For longer wavelengths (>400 nm), the
absorption has not saturated even for a 1 μm film, causing a contin-
uously increasing absorption that is based fully on >400 nm light.
An important point to note here is the increasing photon count
in AM1.5 in the visible range. This means that the bandgap of
YHO and the exact shape and length of the YHO absorption tail
matter significantly.
These results show that there is a strong difference in the contri-
bution and film thickness dependence for UV (<400 nm) and blue
(>400 nm) light. For thin films (t ≤ 200 nm), the contribution to
the total amount of absorbed photons is comparable for both the
UV and blue part of the AM1.5 spectrum. For thicker films, on the
other hand, the absorption of UV photons saturates at about 400 nm
thick films. The blue part continues to contribute with more pho-
ton absorption even beyond 1 μm films. This means that changes to
the YHO bandgap or the absorption tail going into the visible range
has significant effects on the total amount of absorbed photons, its
thickness dependence, and the contribution from the different parts
of the solar spectrum. It also means that the exact solar spectrum,
cloud conditions, etc., will have a significant impact on the photo-
darkening process. This is of importance when designing materials
for solar applications.
This work was partly performed within the project
Green Window, Project No. 332289, funded by the Research
Council of Norway, and partly supported by internal funds at the
Institute for Energy Technology.
AUTHOR DECLARATIONS
Conflict of Interest
The authors have no conflicts to disclose.
Author Contributions
Per-Anders Hansen: Conceptualization (lead); Data curation (lead);
Formal analysis (lead); Investigation (lead); Methodology (lead);
Writing – original draft (lead). Elbruz Murat Baba: Investigation
(supporting); Methodology (supporting); Writing – review & editing
(supporting).
AIP Advances 14, 105316 (2024); doi: 10.1063/5.0224875 14, 105316-4
© Author(s) 2024
 03 February 2025 11:30:43
AIP Advances ARTICLE pubs.aip.org/aip/adv
DATA AVAILABILITY
The data that support the findings of this study are available
from the corresponding author upon reasonable request.
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AIP Advances 14, 105316 (2024); doi: 10.1063/5.0224875 14, 105316-5
© Author(s) 2024
 03 February 2025 11:30:43


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