Solar Energy Materials & Solar Cells 230 (2021) 111234
Available online 20 June 20210927-0248/© 2021 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
Advances in upconversion enhanced solar cell performance 
Amr Ghazy a, Muhammad Safdar a,b, Mika Lastusaari c, Hele Savin d, Maarit Karppinen a,* 
a Department of Chemistry and Materials Science, Aalto University, FI-00076, Espoo, Finland 
b Picosun Oy, Tietotie 3, FI-02150, Espoo, Finland 
c Department of Chemistry, University of Turku, FI-20014, Turku, Finland 
d Department of Electronics and Nanoengineering, Aalto University, FI-00076, Espoo, Finland   
A R T I C L E  I N F O   
Keywords: 
Solar cells 
Upconversion 
Luminescence 
Lanthanides 
Atomic layer deposition 
Photonics 
A B S T R A C T   
Photovoltaics (PV) is the leading renewable energy harvesting technology. Thus, there is a remarkable strive to 
enhance the light harvesting capability of the state-of-the-art solar cells. The major issue common to all solar cell 
types is that they utilize only a limited portion of the solar spectrum, mostly in the visible range, as the active 
semiconductor materials suffer from intrinsic light absorption thresholds. As a result, photons below and above 
these threshold values do not contribute to the electricity generation. A plausible solution to enhance the per-
formance is to integrate the PV cell with an upconverting (UC) component capable of harvesting lower energy 
photons in the infrared (IR) range and emitting visible light. The concept was first introduced in 1990s, but major 
progress in the field has been made in particular in the recent few years. In this overview our intention is to 
provide the readers with a comprehensive account of the progress in the research on the UC-enhanced solar cells. 
Lanthanide ions embedded in different host lattices constitute the most important UC material family relevant to 
the PV technology; we first summarize the design principles and fabrication routes of these materials. Then 
discussed are the different approaches taken to integrate the UC layers in actual PV device configurations. 
Finally, we will highlight the most prominent results obtained, give some future perspectives and outline the 
remaining challenges in this scientifically intriguing and application-wise important field.   
1. Introduction 
The expected global transition to sustainable energy has massively 
increased the demand for renewable energy sources such as the solar 
energy. The production of solar cells for the photovoltaics (PV) industry 
has been growing constantly already for several decades, and the pros-
pects promise continuous growth in future too [1]. The most common 
solar cells are based on semiconducting materials and in order to convert 
solar energy into electricity as efficiently as possible, it is important to 
choose a semiconductor material with an appropriate band gap that 
matches the solar spectrum. However, a single material can be optimal 
only for a specific wavelength range and consequently, the solar cell 
materials are often divided into different categories based on the cor-
responding material properties. 
The dominant technology category today in the PV market is based 
on crystalline silicon (c-Si). This is due to the abundancy of the raw 
material as well as the highly developed mass-production infrastructure 
that have benefitted from the silicon microfabrication development. 
Silicon has also a rather ideal band gap (1.1 eV), which matches well 
with the maximum-intensity wavelength of the solar spectrum. Conse-
quently, c-Si cells have a theoretical power conversion efficiency limit of 
~30% [2]; the experimentally confirmed photoconversion efficiencies 
are already close to this limit, the current record being 26.7% [3]. In 
addition to c-Si, so-called thin-film solar cells have a long history. 
Among this category, amorphous silicon (a-Si) has been studied a lot 
since it was first reported by Carlson and Wronski in 1976 [4]. These a-Si 
cells have a wide band gap (~1.8 eV) making the technology ideal for 
specific applications including indoors use, although the total power 
conversion efficiency has remained at a modest level of ~12% [2]. 
More recently, new materials have emerged as potential alternatives 
to replace the silicon-based cells. First, dye sensitized solar cells (DSSC) 
were invented in 1991 by O’Regan and Gratzel aiming to provide much 
lower material costs combined with a cheap and simple manufacturing 
technology [5]. More recently, an organohalide perovskite sensitizer in 
a DSSC cell was reported in 2009 [6], and as a result so-called perovskite 
solar cells (PSC) emerged as a sub-branch of the DSSC technology. 
Initially the power conversion efficiency in these emerging technologies 
was rather low, around 3–4% [7], but the growth has been enormous in 
* Corresponding author. 
E-mail address: maarit.karppinen@aalto.fi (M. Karppinen).  
Contents lists available at ScienceDirect 
Solar Energy Materials and Solar Cells 
journal homepage: www.elsevier.com/locate/solmat 
https://doi.org/10.1016/j.solmat.2021.111234 
Received 27 April 2021; Received in revised form 5 June 2021; Accepted 7 June 2021   
Solar Energy Materials and Solar Cells 230 (2021) 111234
2
a relatively short time; the highest conversion efficiency of over 20% 
was reported already in 2015 [8]. While these results are highly prom-
ising, there are still issues related to upscaling the technology and the 
long-term stability as compared to the state-of-the-art silicon-based solar 
cells. 
Despite the progress made in enhancing the performance of the 
different solar cell technologies, the fundamental issues related to the 
semiconductor materials themselves (in particular their band gaps) still 
restrict the conversion efficiency prospects of the PV technology. Firstly, 
the photons with a higher energy than the band gap, i.e. the solar ra-
diation often in the ultraviolet (UV) range, result in so-called charge 
carrier thermalization, which causes major energy losses as heat [9]. 
Another major loss mechanism is the limited absorption of the 
state-of-the-art PV semiconductors in the photon wavelengths below the 
band gap, typically in infrared (IR) part of the solar spectrum [10]. The 
upper wavelength limit for the IR absorption varies with the solar cell 
type according to its semiconductor material (Fig. 1) [11,12]. The c-Si 
solar cells have the smallest band gap which allows them to absorb up to 
1100 nm of the solar radiation, but even these cells incur more than 19% 
loss in efficiency due to this incapability to fully utilize the IR part of the 
solar spectrum. For other types of solar cells, these losses are even 
higher, amounting up to 50% [10]. 
Traditionally, the aforementioned losses (both above and sub- 
bandgap) have been addressed by stacking multiple materials on top 
of each other with varying band gaps. Such multi-junction cells, often 
based on III-V semiconductors, have reached higher power conversion 
efficiencies even above 47% [13]. However, this technology category 
suffers from high fabrication and material costs and typically highly 
concentrated sunlight is needed to reach such high efficiencies. 
Recently, pricewise a more competitive technology emerged that com-
bines only two different materials, hence the name tandem cells. 
Currently the most promising option seems to be to combine c-Si and 
perovskite solar cells. Some commercial activities are already estab-
lished around this approach [14]. 
An alternative means for the utilization of non-absorbed sub-band 
gap photons is to take advantage of so-called photon upconversion (UC) 
materials. Instead of having charge-collection junction made of a low 
band gap material, they are based on separate luminescent materials 
that are placed in the vicinity of the charge-collection structures. 
Upconversion is an anti-Stokes luminescence process exhibited by 
certain materials that are capable of absorbing low-energy photons 
typically in the near-infrared (NIR) range and then converting the en-
ergy into higher energy photons, typically in the visible (Vis) range. This 
phenomenon has shown immense potential towards several applica-
tions, including bioimaging, photodynamic therapy, sensors and display 
technology [15–18]. Regarding PV applications, the UC materials could 
considerably enhance the energy harvesting capability of the PV cells as 
they extend the useable solar spectrum range to the NIR photons in 
single junction solar cells (Fig. 1). A clear advantage of this approach is 
that no major adjustments or changes are needed for the current PV 
technologies to integrate them with the UC layers [19]. Indeed, 
upconversion in photovoltaics has already been demonstrated in several 
PV technologies, e.g. first in GaAs solar cells by Gibart et al. [20], later in 
c-Si cells by Trupke et al. [9], and most recently also in DSSCs by Shan 
and Demopoulos [21]. It is interesting to note that before the UC phe-
nomenon itself was demonstrated experimentally, its use was suggested 
theoretically for the detection of infrared quanta in an IR quantum 
counter. That is essentially exactly what the PV-UC combination is set to 
do, to detect IR photons. 
In recent years, several reviews have been published that discuss the 
chemistry and synthesis of UC materials [22], strategies used to tune the 
UC emission [23], and the progress in UC-enhanced DSSC and perov-
skite solar cells [24,25]. In this review, our intention is to provide a brief 
but comprehensive account of the state-of-the-art and the future pros-
pects of the UC materials in photovoltaics. Majority of the studies on the 
UC materials in the context of the PV technologies have involved 
lanthanide-based UC materials [26,27]. Therefore, we start by discus-
sing the UC properties of different lanthanide ions and the host materials 
used for them in Section 2. Then, before addressing the state-of-the-art 
of UC-integrated solar cells in Section 4, we devote Section 3 for 
different approaches taken to integrate the UC layers in PV devices and 
evaluating the performance enhancement in practice. Finally, Section 5 
is a short summary of the current state and an outlook for the future 
perspectives and challenges in this scientifically exciting and industri-
ally important field. 
2. Lanthanide-based upconverting materials 
2.1. Upconversion of lanthanides 
Lanthanides (Ln) are the group of 14 elements in the Periodic 
Table after lanthanum, starting from cerium (Z ˆ 58) up to lutetium (Z 
ˆ 71) with the 6s25d14fn electron configuration. For all these elements, 
the trivalent state is the most stable, where the 6s and 5d orbitals are 
empty and the 4f orbitals are partially filled. The 4f orbitals are strongly 
shielded from the coordination environment by the more spatially 
extended 5s and 5p orbitals. Because of the shielding, the Ln3‡ ions 
exhibit characteristic narrow-band emission (Fig. 2). Moreover, since 
the f–f transitions are parity forbidden (no change in dipole moment), 
the excited states have long lifetimes [28]. This is an essential feature for 
enabling the stacking of photons required for upconversion. 
There are several mechanisms for Ln3‡ based upconversion (Fig. 3). 
In the excited state absorption (ESA) mechanism, one Ln3‡ ion 
sequentially absorbs two photons so that the first excitation yields a 
metastable intermediate state which then absorbs a second photon to 
emit a single high energy photon [29]. The energy transfer upconversion 
(ETU) mechanism comprises two Ln3‡ ions, one of them being a sensi-
tizer and the other acting as an activator. The sensitizer ion absorbs a 
photon to reach an excited metastable state and thereafter transfers the 
energy non-radiatively to the activator. While the activator is still in the 
Fig. 1. Parts of the solar spectrum utilized (in blue) and lost (in red) of different 
solar cell types, and the absorption ranges (in green) of the upconverting Ln3‡
ions, sketched according to Ref. [10]. (For interpretation of the references to 
colour in this figure legend, the reader is referred to the Web version of 
this article.) 
A. Ghazy et al.                                                                                                                                                                                                                                  
Solar Energy Materials and Solar Cells 230 (2021) 111234
3
excited state, another photon absorbed by the sensitizer is transferred to 
the activator, which is then excited to a higher excited state [30]. A 
radiative relaxation of the activator results in the emission of an 
upconverted higher energy photon. Typically, Yb3‡ is used as a sensi-
tizer due to its high absorption cross-section, and the position of its f-f 
transition that lies around 980 nm, which facilitates energy transfer to 
the most common activator ions, e.g. Er3‡, Ho3‡ and Tm3‡ [31]. In the 
third UC mechanism, i.e. cooperative energy transfer (CET), two sensi-
tizers individually absorb low energy photons and simultaneously 
transfer their energy to the activator to cooperatively excite it for a 
subsequent emission of the upconverted photon [32,33]. 
2.2. Host materials and synthesis strategies 
The interest towards lanthanide luminescence started already in the 
1880s [34], and during the 1940s, 1950s and 1960s the theoretical 
background was formulated. Since those times, numerous different 
compounds doped with lanthanides have been synthesized just for the 
sake of basic research. After the establishment of the UC phenomenon in 
the 1960s also upconversion has been reported in a wealth of different 
Ln-based materials [26]. In fact, the following efficient lanthanide 
luminescence hosts (where the Ln host is typically Y, La, Gd, or Yb) were 
characterized for (Yb3‡,Er3‡) UC emission already by the early 1970s 
[35]: LnF3, LiLnF4, NaLnF4, BaLnF4, LnOX (X ˆ F, Cl, Br), Ln2O3, LnBO3, 
YAlO3, YGaO3, Ln3BO6, Y3Al5O10, Y3Ga5O10, Y2GeO5, Y2GeO7, YTiO5, 
Y2Ti2O7, YPO4, YAsO4, YVO4, YTaO4, LnNbO4, Y3TaO7, Y3NbO7, 
YTa3O9, YNb3O9, Y2TeO6, Y2WO6, and NaLn (WO4)2. In practice, all 
materials that can be doped with lanthanides can also be doped with e.g. 
Yb3‡, Er3‡, Ho3‡ and Tm3‡ to obtain the UC emission with at least some 
intensity. 
In a practical UC material, where the Ln3‡ ions are embedded in a 
suitable host lattice, the crystal structure and composition of this host 
lattice lay the grounds for the UC efficiency by affecting the site sym-
metry of the lanthanides and offering suitable energy transfer routes as 
well as inhibiting the radiation-less de-excitation. Also, the absorption 
cross-sections, concentrations and homogeneity in distribution of the 
lanthanide dopants affect the UC properties of the material [12]. To 
allow easy doping of the Ln3‡ ions, at least one of the cation species in 
the host lattice should be of similar size, and preferably also valence, to 
the Ln3‡ dopants. An important requirement for the host lattice is that it 
provides an asymmetrical crystal field to allow the f-f transitions of the 
Ln3‡ ions [36]. Also, an optimal host material should possess low 
phonon energies to minimize the non-radiative emissions [34], and be 
thermally and chemically stable, and transparent to the IR photons [35]. 
One possibility for such thermally and chemically stable host lattices are 
metal oxides, but the drawback is their high phonon energy. On the 
other hand, metal fluorides have been recognized as more efficient host 
material candidates owing to their lower phonon energy. In a very early 
review, the upconversion pioneer Auzel [35] demonstrated that the best 
fluorides may be even 100-fold more efficient in UC emission than the 
best oxides. Unfortunately, the fluorides are often hygroscopic, which 
may be a limitation [37]. For example, YF3:Yb
3‡,Er3‡ was initially 
proven to yield four times as high UC efficiency as NaYF4:Yb
3‡,Er3‡
[38], but the less hygroscopic tetrafluoride has established a position as 
the leading UC host. This flagship material was first highlighted in 1972 
in two separate studies: by Menyuk et al. [39] for the cubic α-NaYF4, and 
by Kano et al. [40] for the hexagonal β-NaYF4. Due to its stronger UC 
emission, the hexagonal form is currently considered more promising 
[41], and thus it is being studied vigorously for many different fields of 
UC applications. 
Fig. 2. Energy levels and emission lines seen for trivalent lanthanide ions [23].  
Fig. 3. Important UC mechanisms relevant for Ln3‡ ions.  
A. Ghazy et al.                                                                                                                                                                                                                                  
Solar Energy Materials and Solar Cells 230 (2021) 111234
4
A common way to apply the Ln3‡-based UC materials is to synthesize 
them in nanoparticle form, motivated by the fact that in NPs the non- 
radiative losses are usually lower and the UC quantum yield higher 
[42]. However, the lattice strain, surface defects and solvent inclusion 
with high phonon energies may cause quenching of the UC luminescence 
[43]. To address these issues, specific core-shell NP structures have been 
developed [24]. In these CS-NPs, the UC core is isolated by an inert shell. 
The quality of the UC luminescence process may vary based on the shell 
characteristics, including the chosen material and the thickness [44]. In 
addition to different types of nanoparticles, UC materials are typically 
also produced e.g. as microcrystalline powder, thin films, glass and glass 
ceramics. In Table 1, we have collected some examples of recently 
investigated UC materials (matrix, dopants) and also their synthesis 
methods, and the form in which the material is applied. It is to be noted 
that in the time period 2017–2020, the number of scientific articles and 
proceedings papers dealing with UC has maintained a steady pace of ca. 
2000 publications per year. Thus, the material list presented below is by 
no means exhaustive. 
Recently, UC thin films have become more appealing and important 
due to their compatibility towards solid-state optical devices and ap-
plications. Thin films have been fabricated from UC NPs using solution- 
based methods such as spin-coating [45] and dip-coating [46], but the 
drawback is that the thus prepared films are prone to contain traces of 
solvent impurities. Alternately, gas-phase deposition techniques, such as 
metal-organic chemical vapor deposition (MO-CVD) and atomic layer 
deposition (ALD) [47], could provide an attractive way to produce 
precisely thickness-controlled UC thin films on a variety of substrates 
[48]. Such UC thin films can offer some unique features over the NPs, 
such as conformality and improved adhesion to the substrate in practical 
devices [49]. 
3. Integration of upconversion layers in solar cell devices 
3.1. Integration strategies 
Several ways of fabricating and incorporating the UC layer within the 
solar cell structure have been experimented. Typically, the UC material 
is produced in a powder or nanoparticle form (as summarized in 
Table 1), and then applied as a thin coating using e.g. spin-coating, dip- 
coating techniques, or deposited directly from gaseous precursors. 
In Fig. 4, we show representative schemes for the placement of the 
UC layer within different PV cell types. For example, Markose et al. [52] 
used the coprecipitation method for synthesizing (Yb3‡,Er3‡)-doped 
Y2O3, YOF and YF3 UC phosphors which were then integrated in a back 
reflector layer of thin-film a-Si solar cells fabricated through 
plasma-enhanced chemical vapor deposition (PECVD). In another 
investigation, (Yb3‡,Ho3‡)-doped Y2O3 UC layers were prepared sol-
vothermally, and combined with N3 (C26H16N6O8RuS2) dye-sensitized 
TiO2-based DSSC cells fabricated by spin-coating; adding the UC layer 
was done by introducing the electrolyte solution by clamping both the 
FTO glass plates [87]. 
Li et al. [88] used pulsed laser deposition (PLD) technique to deposit 
NaYF4:Yb
3‡,Er3‡/NaYF4:Yb
3‡,Tm3‡ (55-nm UC layer) on their PSC 
device. Most recently, we demonstrated the use of the ALD technique in 
depositing (Er,Ho)2O3 (45–60 nm UC layer) on a silicon substrate, which 
was mounted to the backside of a bifacial c-Si solar cell [49]. Using 
bifacial c-Si cells [89,90] in photon upconversion (as compared to fully 
metallized backside cells) [91,92] has the benefit that there is no need to 
adjust the commercial cell fabrication process at all but the upconver-
sion layer can be just placed to the bottom of the finished cell. Additional 
back reflector can be used to guide the luminescent photons back to the 
cell. 
It is important to note that a precise control over the film thickness of 
the UC layer is critical to achieve optimal UC emission [49,93,94]. 
Hence, the advanced gas-phase deposition techniques are clearly bene-
ficial over the solution techniques as they allow for a nanoscale 
Table 1 
Examples of important host lattices for Ln-doped UC materials including the 
synthesis method and the form the material is applied: powder (P), nanoparticle 
(NP), core-shell nanoparticle (CS-NP), thin film (TF), glass/ceramics (G), or 
nanorod (NR).  
Host material Ln 
dopant 
Form Synthesis 
method 
Ref. 
Oxides 
Y2O3 Yb
3‡, 
Er3‡, 
Eu3‡
P Solvothermal [50] 
Er3‡ P Sol-gel [51]  
Yb3‡, 
Er3‡
P Co-precipitation [52] 
(Yb,Er)2O3 Yb
3‡, 
Er3‡
TF ALD [48] 
Er2O3 Ho
3‡ TF ALD [49] 
Lu2O3 Yb
3‡, 
Er3‡
P, NP Combustion [53] 
SiO2 Yb
3‡, 
Er3‡
TF Dip-coating [46] 
SiO2–TiO2 Ho3‡ TF Sol-gel & Spin- 
coating 
[54] 
Yb3‡, 
Er3‡, 
Eu3‡
TF Sol-gel [55] 
Au@TiO2 Yb
3‡, 
Er3‡
CS- 
NP 
Hydrothermal [56] 
anatase-TiO2 Yb
3‡, 
Er3‡
TF Sol-gel & Spin- 
coating 
[45] 
Mg2SiO4 Er
3‡ TF Co-precipitation [57] 
Gd4.67Si3O13 Tm
3‡ P Solid state 
synthesis 
[58] 
NaY9(SiO4)6O2 Yb
3‡, 
Tm3‡
P Solid state 
synthesis 
[59] 
Yb3‡, 
Ho3‡
P   
ZnO Yb3‡, 
Er3‡
NP Solution- 
combustion 
[60] 
CaCe2(MoO4)4 Yb
3‡, 
Er3‡
NP Sol-gel [61] 
Gd0.5Na0.5MoO4 Ho
3‡, 
Yb3‡
NP Co-precipitation [62] 
SrAl2O4 Eu
3‡, 
Yb3‡
NR Electrospinning [63] 
Ba0.85Ca0.15Ti0.9Zr0.1O3 Er
3‡ TF Chemical 
solution 
deposition 
[64] 
TeO2–GeO2–Na2O–Nb2O5–Er2O3 Er3‡ G Melt-quenching [65] 
NaBi(WO4)2 Yb
3‡, 
Er3‡
P Solid state 
synthesis 
[66] 
Na8Al6Si6O24(Cl,S)2 Yb
3‡, 
Er3‡
P Solid state 
synthesis 
[67] 
Oxyfluorides 
YOF Yb3‡, 
Er3‡
P Co-precipitation [52] 
Y2Te6O15–BaF2–YF3 Yb3‡, 
Er3‡
G Melt-quenching [68] 
Fluorides 
YF3 Yb
3‡, 
Er3‡
P Co-precipitation [52] 
NaYF4 Yb
3‡, 
Er3‡
NP Microemulsion [69] 
β-NaYF4 Yb3‡, 
Tm3‡
TF Sol-gel [70] 
Cu1.8S@NaYF4@NaYF4 Yb
3‡, 
Er3‡
CS- 
NP 
Solvothermal [71] 
NaYF4@NaPO3–Na2O–NaF Yb3‡, 
Er3‡
NP in 
G 
Melt-quenching [72] 
Li(Gd,Y)F4 Yb
3‡, 
Er3‡
NP Thermal 
decomposition 
[73] 
KLaF4 Yb
3‡, 
Er3‡/ 
Tm3‡/ 
Ho3‡
NP Solvothermal [74] 
BaYF5 NP Co-precipitation [75] 
(continued on next page) 
A. Ghazy et al.                                                                                                                                                                                                                                  
Solar Energy Materials and Solar Cells 230 (2021) 111234
5
thickness control. The ALD technology in particular is highly promising 
owing to its unique characteristics, such as relatively mild deposition 
conditions and the large-area uniformity and conformality of the 
resultant thin films [47]. Moreover, ALD cycles for metal species can be 
combined with MLD (molecular layer deposition) cycles for organic 
molecules to grow amorphous or crystalline metal-organic materials 
[95,96], where the ultrathin organic layers could work e.g. for 
sensitization. The ALD and ALD/MLD techniques have already been 
used for the fabrication of various Ln-based thin films with interesting 
photoluminescence and upconversion properties [86,97–105] though 
mostly not yet integrated with actual solar cell structures. 
3.2. Figures of merit 
The evaluation of enhancement in solar cell performance due to 
upconversion can be reported with different indicators. From the final 
application point of view, the most informative parameter is naturally 
the power conversion efficiency (PCE), which describes the ratio be-
tween the energy produced by the solar cell and the input solar energy. 
Obviously, successful implementation of UC to the cells should be seen 
as higher PCE. However, PCE is affected by many different factors and it 
may hence be difficult to estimate the role of UC in the given cell ar-
chitecture. Therefore, it is often more useful to focus on the cell pa-
rameters that measure directly the impact of the UC process as these 
describe better the potential of the given UC material. 
One of such parameters is external quantum efficiency (EQE), which 
describes the ratio between the number of charge carriers (electrons) 
collected by a solar cell and the number of photons incident on the cell. 
EQE can be measured for each photon wavelength separately, which 
gives the advantage that one can focus on the EQE of the emitted photon 
wavelength originating from the luminescent material. For instance, in 
case of NaYF4:Er
3‡, the EQE is interesting only wavelength range of 
1480–1560 nm [106–109]. If EQE is increased with the given wave-
length, it means more charge carriers are generated and as a result the 
cell produces higher current. In fact, from the measured EQE one can 
calculate the short circuit current (Isc) for the solar cell (or current 
density, Jsc, is often preferred as it is independent on the cell area). 
Alternatively, one can also directly measure Jsc by measuring the 
photocurrent of the cell under zero bias voltage. Since Jsc is the 
parameter that is affected directly by the incoming photons, it is the 
most widely used parameter to describe the effectiveness of the UC 
process on the cell efficiency. 
The EQE (and Jsc) values are directly proportional to PCE, which 
means that only if all the other parameters remain unaffected, increase 
Table 1 (continued ) 
Host material Ln 
dopant 
Form Synthesis 
method 
Ref. 
Yb3‡, 
Er3‡, 
Tm3‡
Ba2LaF7 Nd
3‡, 
Yb3‡
G Melt-quenching [76] 
NaGdF4@NaGdF4@ZIF-8 Yb
3‡, 
Er3‡
CS- 
NP/ 
MOF 
Solvothermal [77] 
PbF2 Yb
3‡, 
Er3‡, 
Ho3‡
P Solid state 
synthesis 
[78] 
CaF2 Yb
3‡, 
Er3‡, 
Tm3‡
TF MO-CVD [79] 
CaF2 Yb
3‡, 
Er3‡, 
Tm3‡
TF MO-CVD 
Sol-gel 
[80] 
InF3–ZnF2–SrF2–BaF2 Er3‡ G Melt-quenching [81] 
Others 
YPO 
LaPO4 
Yb3‡, 
Er3‡
NP  [82] 
GdPO4     
LuPO4     
Al2O3–P2O5–SiO2-Yb2O3 Yb3‡ G Sol-gel [83] 
MoS2 Yb
3‡, 
Er3‡
P Hydrothermal [84] 
Y-pyrazine Yb3‡, 
Er3‡
TF ALD/MLD [85] 
(Yb,Er)-IR-806 Yb3‡, 
Er3‡
TF ALD/MLD [86]  
Fig. 4. Integration of UC layers in different solar cell types: a-Si (amorphous silicon), bifacial c-Si (crystalline silicon), DSSC (dye synthesized solar cell), and PSC 
(perovskite solar cell) [49,52,87,88]. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 
A. Ghazy et al.                                                                                                                                                                                                                                  
Solar Energy Materials and Solar Cells 230 (2021) 111234
6
in EQE or Jsc will be seen in PCE. Therefore, the EQE and Jsc values are 
much more informative parameters than PCE in convincing the com-
munity about the quality of the studied material in an UC process. 
4. State-of-the-art of upconversion-integrated solar cells 
The implementation of UC in solar cells is an area of increasing in-
terest since last few decades. The first positive results were reported by 
Gibart et al., in 1996, when they combined (Yb3‡,Er3‡) co-doped 
vitroceramic with an ultra-thin GaAs solar cell [20]. A sequential ab-
sorption and energy transfer of the IR photons from Yb3‡ to Er3‡
resulted in emission of a green photon that caused a photo-response in 
the 0.039 cm2 substrate-free GaAs cell. With an input excitation of 1W at 
1.39 eV by a Ti-sapphire IR laser, an EQE of 2.5% was measured. Later, 
integration of NaYF4:Er
3‡ UC layers with bifacial c-Si solar cells was 
demonstrated [110]. The polycrystalline UC samples were mixed into an 
optically transparent acrylic adhesive with similar refractive index and 
coupled to the rear of a bifacial silicon solar cell. An EQE of about 2.5% 
was obtained under 5.1 mW excitation at 1523 nm. Trupke et al. [111] 
theoretically predicted up to ⁓40% conversion efficiency using 
non-concentrated sunlight (AM1.5) for a silicon solar cell combined with 
UC. However, a practical demonstration with NaYF4:Er
3‡ UC phosphors 
gave an EQE close to 1% at 1550 nm, using a prototypical device setup. 
In recent years, the efforts in the field have rapidly grown to apply UC 
materials in different forms and combine UC with conventional as well 
as emerging solar cell technologies. 
Amorphous silicon solar cells: As mentioned in the introduction, a- 
Si cells are relatively low-cost and easy to fabricate with an additional 
advantage of high chemical stability. The relatively wide band gap of a- 
Si (⁓1.8 eV) implies that these devices can absorb only up to 700 nm of 
solar light, hence they suffer from greater transmission losses compared 
to some other solar cell technologies, such as crystalline Si (c-Si). The 
research related to UC enhancement of a-Si cells can be traced back to 
2010 [112–114], when de Wild et al. and Zhang et al. independently 
reported Er3‡-based NaYF4 nanocrystals coupled to the backside of a-Si 
solar cells. De Wild et al. [112,113] used 200–300 μm thick layers of 
β-NaYF4:Yb3‡,Er3‡ powder dispersed in polymethylmethacrylate 
(PMMA) and applied to the backside of a-Si cells using spin-coating; they 
recorded a 3x increase in the short-circuit current. On the other hand, 
Zhang et al. [114] implemented NaYF4 nanocrystals with 18% Yb
3‡ and 
2% Er3‡ doping to achieve an increase of ⁓6% in short-circuit current 
density for their a-Si cells, as shown in Fig. 5. Later, Chen et al. [115] 
applied β-NaYF4:Er3‡ powder dispersed in cyclohexane to produce a 
200 μm thick UC layer on a glass substrate, followed by physical 
placement of it below an a-Si cell. Under single-illumination with 60/80 
mW/cm2 980/1560 nm diode lasers, short-circuit currents of 0.3/0.01 
μA were measured, respectively. Under co-excitation by 60 mW 980 nm 
and 100 mW 1560 nm lasers, a current improvement to 0.54 μA was 
obtained, owing to the UC-induced enhancement. 
More recently, Markose et al. [52] investigated three different host 
lattices, Y2O3, YOF and YF3, for (Yb
3‡,Er3‡) co-doping, and found YF3 as 
the best matrix among the tested ones, for the performance enhance-
ment of a-Si cells. An improvement of 7.5% in short-circuit current 
density was recorded for YF3:Yb
3‡,Er3‡ UC layers on AM1.5G along 
with a single wavelength diode laser source of 980 nm. Liu et al. [116] 
reported an enhancement of 25%, as shown in Fig. 5 in the short-circuit 
current for a thin film a-Si solar cell by applying (Yb3‡,Er3‡) co-doped 
β-NaYF4 dispersed in a PMMA layer. 
Fig. 5. (a) I–V curve of the pioneering work done in 2009 using Yb3‡, Er3‡ co-doped β-NaYF4 in enhancing the performance of a-Si solar cells under AM 1.5G 
irradiation [114]. (b) I–V curve showing a 25% relative enhancement in Isc of a-Si solar cell by adding Yb3‡, Er3‡ co-doped β-NaYF4 UC layer [116]. (c) EQE curve of 
a-Si solar cell that shows a clear enhancement of the 900–1000 nm region absorption [116]. (d) Enhancement in the Jsc of a bifacial c-Si solar cell, achieved by 
introducing an ALD-grown UC (Er0.95Ho0.05)2O3 thin-film layer [49]. Reference cells are in black and UC-assisted cells are in red. (For interpretation of the references 
to colour in this figure legend, the reader is referred to the Web version of this article.) 
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Crystalline silicon solar cells: The narrow band gap of c-Si (⁓1.1 
eV) allows the c-Si cells to absorb photons up to 1100 nm. Hence, the 
employment of Yb3‡-based UC materials with the absorption maximum 
around 980 nm is not truly feasible; rather, other UC Ln ions should be 
challenged [117]. In this regard, Shalav et al. [86] in 2005 and Lahoz 
[94] in 2008 introduced Er3‡-doped NaYF4 microcrystals and 
Ho3‡-doped oxyfluoride glass ceramics for expanding the absorption of 
c-Si solar cells into the sub-bandgap region. These ions absorb at 
1150–1230 nm (Ho3‡) and 1450–1550 nm (Er3‡), and the emission of 
the UC photons takes place at <1000 nm. An attractive feature related to 
the use of Ho3‡ (instead of e.g. Er3‡) is that the solar intensity at ⁓1180 
nm is higher than at the longer wavelengths, indicating higher potential 
for the solar energy harvesting from this spectral range. An interesting 
idea is to use mixtures of two or more lanthanides in compatible con-
centrations to maximize the UC performance. For instance, (Ho3‡,Yb3‡) 
co-doped UC materials can provide stronger NIR luminescence due to 
the inter-ion energy transfer and sensitization [118]. 
Fischer et al. [108] reported the use of Er3‡-doped BaY2F8 UC layers 
to reach 8% EQE under 1520 nm monochromatic excitation; the relative 
solar cell efficiency enhancement was 0.55  0.14% at 94  17 suns. 
Monocrystalline BaY2F8:Er
3‡ shows less scattering than the typically 
used microcrystalline powders such as β-NaYF4:Er3‡. Incoming lower 
energy photons can enter deep into the monocrystalline material, 
resulting in higher extent of absorption from abroad spectral range. 
Considerably smaller enhancement in EQE, i.e. 0.4%, was achieved 
when Er3‡ doped tellurite glass was combined with a c-Si cell under 
1500 nm laser excitation [119]. More recently, we demonstrated a 3% 
improvement in short-circuit current density, as shown in Fig. 5, under a 
solar excitation equivalent to 16 suns, for a bifacial c-Si cell combined 
with ALD-grown (Er0.95Ho0.05)2O3 thin film [49]. 
Dye sensitized solar cells: The power conversion efficiency of 
DSSCs depends on the band gap of the dye used; for the most commonly 
used ruthenium dye the band gap is ~1.8 eV, which averts the cells from 
absorbing light beyond ~800 nm. Thus, DSSCs inherently suffer from a 
low PCE [24], with the record in PCE being 14.7% from the year 2015 
[120]. Hence, DSSCs are arguably the solar cell type that could benefit 
the most from the UC integration [10]. 
There are several approaches when incorporating the UC layers with 
DSSC, as seen in Fig. 6, one of them is adding the UC material to the TiO2 
layer. This approach was pioneered by Shan and Demopoulos in 2010 
[21], who fabricated a DSSC cell by incorporating (Yb3‡,Er3‡) co-doped 
LaF3 with the TiO2 layer of the cell. Recently, a similar approach was 
followed by Qin et al. [121] in their study where Ho3‡-doped NaYbF4 
was added to the TiO2 layer, increasing the PCE of the cell from 5.84% to 
7.52%, under one sun illumination. 
Another route to integrate UC materials to DSSC devices is to add the 
UC component as a rear reflector functional layer, thus serving the two 
purposes of light harvesting and reflection simultaneously. In 2011, Liu 
et al. [124] added (Yb3‡,Er3‡) co-doped Y3Al5O12 layer as a back 
reflector, and demonstrated that 980 nm laser irradiation resulted in an 
enhanced Jsc. More recently, a remarkable 33.5% relative enhancement 
under one sun illumination was reported for a DSSC device in which the 
UC (Yb3‡,Er3‡) co-doped CeO2, used as a back reflector and scattering 
layer at the back of a DSSC, was additionally doped with Fe3‡ [125]. 
An alternative approach is to employ the UC material as a counter 
electrode for the DSSC; this was reported by Li et al. [123] in 2014 who 
aimed to improve the performance of the cell using a lower cost counter 
electrode to replace Pt. Using (Yb3‡,Er3‡) co-doped fluorine tin oxide 
(FTO) as a counter electrode they could achieve a relative enhancement 
of 24% in Jsc and increase the PCE from 6.7% to 7.30%, compared to a 
standard Pt electrode, under one sun illumination. More recently, a 
remarkable 68.5% relative enhancement compared to a standard 
UC-free Pt electrode was reported for a SnS-carbon counter electrode 
prepared using (Yb3‡,Er3‡) co-doped CeO2, raising the PCE of the DSSC 
from 5.65% to 9.52%, under one sun irradiation [126]. Interestingly, in 
the same study adding a similar UC layer to the Pt-electrode based DSSC 
raised the PCE of the cell to 8.32%, which translates to a 47.3% relative 
enhancement. 
Perovskite solar cells: An attractive feature of PSCs is their large 
absorption coefficient [127]. However, the relatively wide band gap of 
~1.55 eV of the most common material components in PSCs limits the 
absorption of such cells to ~780 nm. This, on the other hand, makes the 
PSCs an ideal target for the integration of UC materials to broaden the 
absorption range [128]. Upconversion-enhanced PSC was first reported 
by Chen et al. [129] in 2016, who added (Yb3‡,Er3‡) co-doped LiYF4 as 
an external light converting layer to the cell, to enhance the PCE by 7.9% 
under a 7–8 sun illumination, as shown in Fig. 7. The same technique 
was used more recently in a study where a plasmon-enhanced NaYF4: 
Yb3‡,Er3‡/NaYF4:Yb
3‡,Tm3‡/Ag composite UC layer (Fig. 4) was found 
to enhance the PCE of the cell from 16.1% to 19.5% (21% increase), 
under one sun illumination [88]. Adding UC external layer to the rear 
side of the cell, showed better PCE enhancement than to the front side of 
Fig. 6. I–V curves and schematic diagrams of UC assisted DSSC using different approaches; (1) incorporating the UC phosphor in the TiO2 layer [87], (2) adding a UC 
rear reflector layer [122] and (3) replacing Pt counter electrode with a UC electrode [123]. 
A. Ghazy et al.                                                                                                                                                                                                                                  
Solar Energy Materials and Solar Cells 230 (2021) 111234
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the PSC [130]. 
Another approach is to incorporate the UC material in the perovskite 
active layer. This was demonstrated in 2017 by Meng et al. [131] using 
β-NaYF4:Yb3‡,Er3‡ nanocrystals embedded within the perovskite layer 
in different amounts, raising the PCE of the cell from 12.1% to 18.60% as 
shown in Fig. 7 on average, to achieve a remarkable relative enhance-
ment of 54.4% compared to the control device, under one sunlight 
illumination. 
A third approach is to add the UC component to the hole transport 
layer of the PSC. This was explored by He et al. [132] who added 
β-NaYF4:Yb3‡,Er3‡ to replace the mesoporous TiO2 layer of the cell, thus 
raising the PCE of the cell from 12.88% to 17.78%, under one sun illu-
mination; an additional 0.35% increase in PCE was obtained by 980 nm 
laser irradiation. Later, Deng et al. [73] used the same technique adding 
UC Li(Gd,YF4:Yb
3‡,Er3‡ nano crystals with different concentrations, to a 
PSC, increasing the PCE of the cell from 14.7% to 18.3% (Fig. 7) and 
achieving almost 25% relative enhancement compared to the undoped 
cell under simulated one sun illumination. 
In Table 2, we have collected recent highlights of the solar cell per-
formance enhancements achieved with the UC integration. As a general 
comment, it should be noted that the Ln3‡ and transition metal (e.g. 
Fe3‡) dopants may, to certain extent, contribute to the observed 
enhancement due to their own photoluminescence effect. 
5. Conclusions and outlook 
In this brief overview, we have outlined the basic phenomena, ma-
terial design issues and integration strategies related to the utilization of 
upconversion materials for enhancing the performance of various types 
of solar cells through spectral manipulation. This research field has been 
strongly emerging in recent years, and for all the major types of solar 
cells promising results have already been reported. For example, a Jsc of 
20.5 mA/cm2 and 21.9 which reflects a ~79% and ~63% relative 
enhancement to respective control devices have been reported for DSSC 
[126,142], while for the PSC, Jsc has reached of 22.7 mA/cm
2, reflecting 
a ~40% relative enhancement than the control device [131]. Here the 
direct comparison of the given numbers may not be fully fair though, as 
there are various ways to measure and express the magnitudes of per-
formance enhancements; indeed, establishment of a more standard way 
of discussing the UC-driven enhancements could be one of the devel-
opment targets in future. 
The progress in the field is impressive considering that originally the 
UC phenomenon was realized only with lasers yielding very high exci-
tation photon densities, while the recent highlights for UC-enhanced 
solar cells have been demonstrated with actual sun light excitation. 
There is definitely still lot of space for improving the efficiency of the 
applied UC layers themselves. This research line should include not only 
the fine-tuning of the currently known UC materials but also innovative 
search for new UC host matrices. 
Lanthanide ions embedded in different host lattices constitute the 
most important UC material family relevant to the PV technology. For 
the required capability of harvesting lower energy photons in the NIR 
range and emitting visible light, the most advantageous lanthanide ions 
are the heaviest ones: Yb3‡, Tm3‡, Er3‡, Ho3‡, and Nd3‡. In practice, 
most of the studies have involved the combination of Yb3‡ and Er3‡ with 
the main absorption in the range 800–1000 nm; this NIR harvesting 
range fits quite well with the intrinsic absorption capability of the PV 
cells based on the wide-bandgap semiconductors (perovskite, dye- 
sensitized and organic solar cells). On the other hand, in the case of 
Si-based cells this wavelength range is already well absorbed by the 
narrow-bandgap Si itself (absorbs well up to 1100 nm). For c-Si, instead 
of Yb3‡, e.g. Ho3‡ together with Er3‡ could be more appropriate for 
efficient UC enhancement. Indeed, the major NIR absorption bands for 
Ho3‡ and Er3‡ are in the 1150–1230 and 1450–1580 nm ranges. In 
future works, more precise tuning of the UC Ln3‡ ion composition/ 
concentration should be targeted for each solar cell type separately. 
Another future challenge could be to more precisely investigate and 
discuss the true causes of enhancements observed with the UC materials. 
In reality, only a fraction of the absorbed photons is converted to the UC 
luminescence, and besides the UC emission there is always also regular 
photoluminescence present. Within the solar spectrum, many of the 
energy states of lanthanides can be excited which may result in regular 
photoluminescence in the visible wavelengths. This is a challenge 
regarding the deeper understanding and interpretation of the experi-
mental observations, but at the same time we should learn to take a full 
benefit of all the upconversion, downconversion and photoluminescence 
processes enabled by the different lanthanides (and even some d-block 
transition metals) to be able to harness all the possibilities for the best 
Fig. 7. I–V curves and schematic diagrams of UC assisted PSC using different approaches; (1) adding the UC as an additional active layer [129], (2) incorporating UC 
materials in the active perovskite layer [131], and (3) incorporating the UC material in the hole transport layer (HTL) of the cell [73]. 
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Table 2 
Recent highlights of UC-enhanced solar cells. (NG: Not given)  
Host material Ln dopant Indicator Irradiation Ref. without UC Relative enhancem. Year/Ref 
a-Si 
Y2O3 
YOF 
YF3 
Yb3‡,Er3‡ JSC 98 suns 13.1 3.4% 4.8% 7.5% 2018 [52] 
β-NaYF4 Yb3‡,Er3‡ ISC-EQE (980 nm) AM1.5G 0.4–0 25% -‡0.05% 2018 [116] 
TeO2–PbF2 Yb3‡,Er3‡ EQE (980 nm) NG 4.25 ‡10.6% 2014 [133] 
β-NaYF4 Yb3‡,Er3‡,Gd3‡ EQE (980 nm) NG 0.02 ‡0.12% 2012 [134] 
β-NaYF4 Er3‡ JSC laser (980, 1560 nm) NG ‡0.3 μA, ‡0.1 μA, 2012 [115] 
β-NaYF4 Yb3‡,Er3‡ ISC AM1.5G 0.4 25% 2010 [114] 
β-NaYF4 Yb3‡,Er3‡ EQE (980 nm) laser 980 nm 0.01 ‡0.02% 2010 [113] 
β-NaYF4 Yb3‡,Er3‡ Jsc laser 980 nm 0.005 ‡0.005 2010 [112] 
c-Si 
(Er,Ho)2O3 Er
3‡,Ho3‡ ISC 16 suns 1021 3% 2021 [49] 
TeO2–WO3–ZrO2–Er2O3 Er3‡ EQE (1500 nm) laser 1500 nm 0.1 ‡0.3% 2017 [119] 
BaY2F8 Er
3‡ EQE (1520 nm) 94 suns ~0 ‡7.4% 2015 [108] 
PbF2 Er
3‡ EQE (1560 nm) NG 0 ‡0.38% 2014 [135] 
β-NaYF4 Yb3‡,Er3‡ EQE (1523 nm) 77 suns 0 ‡3.4% 2014 [136] 
β-NaYF4 Er3‡ EQE (1500 nm) ~1 sun 0 ‡1.69% 2014 [107] 
InF3–20ZnF2–20SrF2–20BaF2 Yb3‡,Er3‡ EQE (1480 nm) 864 suns 0.4 ‡0.1% 2013 [106] 
BaY2F8 Er
3‡ EQE (1557 nm) ~62 suns 0 ‡6.5% 2013 [109] 
β-NaYF4 Er3‡ PCE ~1 sun 7.68 8.6% 2012 [137] 
ZrF4–BaF2–LaF3–AlF3–NaF–InF3 Er3‡ EQE (1540 nm) NG NG ‡2.4% 2011 [138] 
β-NaYF4 Er3‡ EQE (1523 nm) NG 0 ‡5.1% 2010 [139] 
β-NaYF4 Er3‡ EQE (1523 nm) NG 0 ‡3.4% 2007 [140] 
β-NaYF4 Er3‡ EQE (1523 nm) NG 0 ‡2.5% 2005 [110] 
DSSC 
β-NaYF4 Ho3‡ PCE AM 1.5G 5.8 28.8% 2021 [121] 
BiYO3 Yb
3‡,Er3‡ PCE 1 Sun 5.7 8.2% 2021 [141] 
NaYbF4 Ho
3‡ PCE 1 Sun 6.4 46.1% 2021 [142] 
CaWO4@TiO2 Yb
3‡,Er3‡ PCE 1 Sun 6.9 21% 2020 [143] 
LiYF4 Yb
3‡,Er3‡ PCE AM 1.5G 5.0 51.1% 2020 [144] 
Y2O3 Ho
3‡,Yb3‡ PCE AM 1.5G 8.9% 10.3% 2020 [145] 
CeO2: Fe@TiO2 Yb
3‡,Er3‡ PCE AM 1.5G 5.5 33.5% 2020 [125] 
YbF3@TiO2 Ho
3‡ PCE AM 1.5G 6.4 21% 2020 [146] 
CeO2 Yb
3‡,Er3‡ PCE AM 1.5G 5.7 68.5% 2020 [126] 
La(OH)3 @TiO2 Yb
3‡,Er3‡ PCE AM 1.5G 6.4 38.0% 2019 [147] 
NaYF4 @TiO2 Yb
3‡,Tm3‡ PCE AM 1.5G 6.13 14.7 2019 [148] 
Y2O3 Ho
3‡,Yb3‡ PCE 1 Sun 16.8–5.8 30.0% 2019 [87] 
NaYF4 @TiO2 Yb
3‡,Er3‡ PCE AM 1.5 5.5 31.6% 2019 [149] 
CaCe2(MoO4)4 Yb
3‡,Er3‡ PCE AM 1.5 6.5 19.9% 2018 [61] 
ZnO Yb3‡,Er3‡ PCE AM 1.5G 8.3 8.3% 2018 [60] 
β-NaYF4 Yb3‡,Er3‡ PCE AM 1.5G 5.6 29.8% 2018 [150] 
β-NaYF4 Yb3‡,Er3‡/Eu3‡ PCE AM 1.5G 6.7 14.0% 2018 [151] 
β-NaYF4@YOF Yb3‡,Er3‡ PCE AM 1.5G 7.0 17.1 2018 [152] 
Y2O3@TiO2 Er
3‡ PCE 1 Sun 5.4 43.6% 2018 [153] 
CeO2 Yb
3‡,Er3‡ PCE AM 1.5G 5.5 27.0% 2017 [154] 
β-NaYF4@SiO2@TiO2 Yb3‡,Er3‡ PCE AM 1.5G 5.6 16.6% 2017 [94] 
Y2O3/Au@TiO2 Er
3‡ PCE AM 1.5G 6.8 27.6% 2016 [155] 
β-NaYF4@β-NaYF4 Yb3‡,Er3‡ PCE AM 1.5G 7.1 28.2% 2016 [156] 
NaYF4@SiO2@Au@TiO2 Yb
3‡,Er3‡ PCE AM 1.5G 6.1 28.1% 2016 [157] 
YbF3/TiO2 Ho
3‡ PCE AM 1.5G 6.5 24.6% 2016 [158] 
Gd2O3 Yb
3‡,Ho3‡ PCE 1 sun 6.7 10.4% 2016 [159] 
β-NaGdF4 Yb3‡,Er3‡ PCE AM 1.5G 3.6 19.44% 2016 [160] 
Gd2(MoO4)O3 Yb
3‡,Er3‡ PCE AM 1.5G 2.9 24.8% 2016 [161] 
β-NaYF4@SiO2@TiO2 Yb3‡,Er3‡ PCE AM 1.5G 8.67 5.0% 2016 [162] 
β-NaYF4 Yb3‡,Er3‡ PCE AM 1.5G 5.7 25.6% 2015 [163] 
Y2O3, Y3Al5O12 Er
3‡, Ce3‡ PCE AM 1.5G 5.2 35.4% 2015 [164] 
β-NaYF4@SiO2 Yb3‡,Er3‡ PCE AM 1.5G 3.7 6.8% 2015 [165] 
CeO2 Yb
3‡,Er3‡ PCE 1 sun 5.8 15% 2015 [166] 
β-NaGdF4 Yb3‡,Er3‡ PCE AM 1.5G 6.8 11.3% 2015 [167] 
β-NaYF4 Yb3‡,Er3‡,Tm3‡ PCE AM 1.5G 3.4 27.4% 2015 [168] 
Y2O3 Yb
3‡,Er3‡ PCE 1 sun 5.9 12.5% 2015 [169] 
ZnO/TiO2 Yb
3‡,Er3‡ PCE NG 3.0 35.8% 2015 [170] 
β-NaYF4@TiO2 Yb3‡,Er3‡ PCE 1 Sun 6.2 24.5% 2014 [171] 
β-NaYF4@ β-NaYF4 Yb3‡,Er3‡ PCE AM 1.5G 5.0 5.9% 2014 [172] 
β-NaYF4@SiO2 Yb3‡,Er3‡ PCE AM 1.5G 6.0 6.7% 2014 [173] 
FTO Yb3‡,Er3‡ PCE AM 1.5G 6.7 9.0% 2014 [123] 
F-doped TiO2 Yb
3‡,Er3‡ PCE AM 1.5G 6.3 40.1% 2014 [174] 
YF3 Yb
3‡,Er3‡ PCE AM 1.5G 5.3 26.6% 2014 [175] 
β-NaYF4@SiO2@Au Yb3‡,Er3‡ PCE AM 1.5G 7.2 14.8% 2014 [176] 
β-NaGdF4 Yb3‡,Er3‡ PCE AM 1.5G 5.8 21.4% 2014 [122] 
β-NaYF4@SiO2@TiO2 Yb3‡,Er3‡ PCE 1 Sun 7.8 18.1 2013 [177] 
TiO2 Er
3‡ PCE AM 1.5G 4.1 62.9% 2013 [178] 
TiO2-xFx Yb
3‡,Er3‡ PCE AM 1.5G 5.4 31.1% 2013 [179] 
(continued on next page) 
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possible utilization of these different energy conversion processes. 
From the practical performance point of view, the era of UC- 
enhanced PV is yet in its infancy, considering how far we are from the 
possibly achievable maximum values. Among the different PV technol-
ogies, the market leading c-Si technology could turn out to be the most 
rewarding for the UC enhancement efforts in the sense that the perfor-
mance of these cells is otherwise approaching a kind of saturation 
(current records around 27% versus the ~30% S-Q limit), meaning that 
the improvements expected through “standard” PV development efforts 
can be only fractional [211,212]. Optimistically, for the UC-enhanced 
c-Si PV technology the new target could be set much higher, even up 
to 40%, to truly shake the PV community. 
From another perspective, it is difficult to see that the UC-enhanced 
solar cells could as such compete with the recent developments in the PV 
field, such as the tandem cells or new solar cell materials. Nevertheless, 
the UC components can be considered as a complementary technology 
which rather builds on top of the existing technologies providing extra 
boost to these technologies. A clear advantage is the relatively easy 
integration of the UC layers into the current PV cells, without drastically 
affecting the device configurations or process lines. Here the already 
existing strong c-Si PV infrastructure could be highly beneficial for the 
fast transition from the innovation to the commercial large-scale 
utilization. 
Finally, we like to emphasize the importance of the fabrication 
technique of the UC layers/coatings. The conventional solution-based 
spin-coating, spray-drying and electrodeposition methods offer limited 
control over the film thickness and conformality, and are less feasible 
from the industry points of view. Here advanced gas-phase thin film 
techniques could offer clear benefits, being for example free from sol-
vent impurities. The ALD technique in particular – known as the state-of- 
the-art gas-phase thin-film technology in conventional microelectronics 
– yields precisely thickness-controlled, conformal, and uniform coatings 
over large-area and complex substrate surfaces. The ALD technology is 
already well integrated with the silicon-based PV industry for the 
fabrication of metal oxide surface passivation layers [213,214], where it 
has proven its capability to fulfil the cost, throughput and performance 
criteria. Moreover, we already have the required well-behaving ALD 
processes available for a number of Ln-based thin films [215], including 
those with promising photoluminescence and upconversion properties. 
The ALD precursors employed are non-toxic and safe to handle, but 
naturally the lanthanides used as the UC activators in general are 
considered as critical raw materials, which should be taken into account 
considering their mass production for GW-scale PV factories. Despite the 
great prospects, efforts to integrate the ALD-grown UC films with actual 
PV cells have remained scarce. An exciting idea would be to develop a 
new ALD UC coating that could simultaneously work as the surface 
passivation layer. 
Table 2 (continued ) 
Host material Ln dopant Indicator Irradiation Ref. without UC Relative enhancem. Year/Ref 
β-NaYF4@TiO2 Yb3‡,Er3‡ PCE AM 1.5G 3.5 23.1% 2013 [180] 
β-NaYF4 Yb3‡,Er3‡ PCE AM 1.5G 2.7 4.4% 2012 [181] 
Y2O3 Yb
3‡,Er3‡ PCE 1 sun 5.8 19.9% 2012 [182] 
NaYF4 Yb
3‡,Er3‡ PCE AM 1.5G 1.6 11.9% 2011 [183] 
TiO2 Yb
3‡,Er3‡ PCE AM 1.5G 6.4 13.6% 2011 [184] 
Lu2O3 Yb
3‡,Tm3‡ PCE 1 sun 6.0 11.1% 2011 [185] 
β-NaYF4 Yb3‡,Er3‡ PCE AM 1.5G 5.5 26.6% 2011 [186] 
YF3 Yb
3‡,Er3‡ PCE 1 sun 5.8 35.5% 2011 [187] 
Y3Al5O12 Yb
3‡,Er3‡ Jsc laser 980 nm ~0 0.2% 2011 [124] 
LaF3–TiO2 Yb3‡,Er3‡ Isc AM 1.5G 6.2 2.4% 2010 [21] 
PSC 
NaCsWO3@ 
β-NaYF4@β-NaYF4 
Yb3‡,Er3‡ PCE AM 1.5G 16.1 18.0% 2021 [188] 
IR783: NaYF4@NaYF4 Yb
3‡,Er3‡,Nd3‡ PCE AM 1.5G 19.4 5.6% 2020 [189] 
TiO2 Yb
3‡,Er3‡ PCE AM 1.5G 8.3 16.5% 2020 [190] 
β-NaYF4@NaYF4 Yb3‡,Er3‡,Sc3‡ PCE AM 1.5G 17.4 15.8% 2019 [191] 
β-NaYF4@SiO2 Yb3‡,Tm3‡ PCE AM 1.5G 12.3 14.81% 2019 [192] 
Li(Gd,Y)F4 Yb
3‡,Er3‡ PCE AM 1.5G 14.7 24.9% 2019 [73] 
β-NaYF4 Yb3‡, Tm3‡ PCE AM 1.5G 15.8 15.2% 2019 [193] 
TiO2 Er
3‡ PCE AM 1.5G 9.1 16.5% 2018 [194] 
β-NaYF4 Ho3‡ PCE AM 1.5G 11.1 29.0% 2018 [195] 
KY7F22 Yb
3‡,Er3‡ PCE AM 1.5G 13.2 6.1% 2018 [130] 
TiO2 Yb
3‡,Er3‡ PCE AM 1.5G 14.0 17.9% 2018 [196] 
β-NaYF4@β-NaYF4 Yb3‡,Er3‡,Tm3‡ PCE AM 1.5G 16.1 19.3% 2018 [88] 
β-NaYF4@ SiO2 Yb3‡,Er3‡ PCE AM 1.5G 8.2 21.1 2018 [197] 
β-NaYF4 Yb3‡,Er3‡ PCE AM 1.5G 13.5 46.4% 2017 [131] 
TiO2 Yb
3‡,Er3‡ PCE AM 1.5G 10.3 20.8% 2017 [198] 
β-NaYF4/Li–Ag@SiO2 Yb3‡,Er3‡ PCE AM 1.5G 7.8 33.4% 2017 [199] 
β-NaYF4-IRM06 Yb3‡,Er3‡ PCE AM 1.5G 13.5 29.4% 2017 [200] 
mCu2 xSx@SiO2@Er2O3 Er
3‡ PCE AM 1.5G 16.2 9.9% 2017 [201] 
LiYF4 Yb
3‡,Er3‡ PCE 7-8 Suns 11 7.9% 2016 [129] 
β-NaYF4 Yb3‡,Er3‡ PCE AM 1.5G 12.9 9.8% 2016 [132] 
β-NaYF4 Yb3‡,Er3‡ PCE AM 1.5G 14.1 13.7% 2016 [202] 
β-NaYF4@β-NaYF4 Yb3‡, Tm3‡ PCE AM 1.5G 14.7 8.8% 2016 [203] 
Others 
(QDSSC)TiO2/CdS/β-NaYF4 Yb3‡,Er3‡ PCE 1 sun 2.14 106% 2020 [204] 
(QDSSC)TiO2 Yb
3‡,Er3‡ PCE AM 1.5G 2.9 20% 2020 [205] 
(GaAs) β-NaYF4 Yb3‡,Er3‡ PCE AM 1.5G 7.9 68.4% 2018 [206] 
(Polymer) β-NaYF4 Yb3‡,Er3‡ PCE AM 1.5G 3.1 6.5% 2014 [207] 
(Organic) β-NaYF4 Yb3‡,Er3‡ PCE laser (980 nm) 0.0048 29.1% 2012 [208] 
(Organic) Y2BaZnO5 Yb
3‡,Ho3‡ EQE (980 nm) laser (980 nm) 
~37 suns 
0.2 ‡4.95% 2012 [209] 
(GaAs) Y6W2O15 Yb
3‡,Er3‡ Isc laser (973 nm) 0 ‡3.58 μA 2012 [210] 
(GaAs) vitroceramic Yb3‡,Er3‡ PCE NG 0 ‡2.5% 1996 [20]  
A. Ghazy et al.                                                                                                                                                                                                                                  
Solar Energy Materials and Solar Cells 230 (2021) 111234
11
CRediT authorship contribution statement 
Amr Ghazy: Conceptualization, Visualization, Writing – original 
draft. Muhammad Safdar: Writing – review & editing. Mika Lastu-
saari: Writing – original draft. Hele Savin: Writing – original draft. 
Maarit Karppinen: Conceptualization, Supervision, Funding acquisi-
tion, Writing – original draft. 
Declaration of competing interest 
The authors declare that they have no known competing financial 
interests or personal relationships that could have appeared to influence 
the work reported in this paper. 
Acknowledgements 
Funding was received from the Academy of Finland (Profi 3), and the 
Flagship Programme, Photonics Research and Innovation (PREIN). Also, 
Amr Ghazy acknowledges Jenny & Antti Wihuri Foundation for finan-
cial support. 
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