MNE 15 (2022) 100128
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Micro and Nano Engineering
journal homepage: www.sciencedirect.com/journal/micro-and-nano-engineering
Down-shifting and antireflective effects of ZnO/PMMA thin films and their
influence on silicon solar cells performance
Alvaro Flores-Pacheco a, b, *, José Raúl Montes-Bojórquez b, Mario Enrique Álvarez-Ramos a,
Arturo A. Ayón b
a
b
Posgrado en Nanotecnología, Departamento de Física, Universidad de Sonora, C.P. 83000 Hermosillo, Sonora, Mexico
MEMS Research Lab, Department of Physics and Astronomy, University of Texas at San Antonio, San Antonio, TX 78249, USA
A R T I C L E I N F O
A B S T R A C T
Keywords:
Quantum dots
Down-shifting
Antireflective layers
Solar cells
The down-shifting effect of nanostructured II-VI semiconductors like zinc oxide (ZnO) is an attractive feature that
can be exploited for the performance enhancement of silicon solar cells. The UV-region of the solar spectrum can
be harvested more efficiently by the silicon solar cell after being absorbed by ZnO quantum dots (QDs) and reemitted in the visible range (centered ⁓510 nm). Additionally, the polymeric matrix (PMMA) used for the
fabrication of the ZnO/PMMA thin films can serve as an antireflective layer, enabling a better overall solar
radiation absorption. The present study discusses the synthesis and characterization of photoluminescent ZnO
QDs and their effect on the performance of in-house-fabricated single crystal silicon solar cells. The down-shifting
effect of the colloidal quantum dots was characterized by collecting and analyzing their absorption and pho­
toluminescence spectra. The structural characterization of the obtained ZnO QDs was performed employing Xray diffraction (XRD) and transmission electronic microscopy (TEM). Before the deployment of the ZnO QDs thin
film layers, the optimal thickness of the PMMA matrix was evaluated by ellipsometry seeking the optimal
antireflective effect. The performance characteristics of the solar cells before and after the application of the
ZnO/PMMA layers were determined from the J-V curves generated in a solar simulator and their spectral
response was evaluated by external quantum efficiency (EQE) measurements achieving a maximum relative PCE
increase above 19%.
1. Introduction
Single junction solar cells fabricated from single-crystal silicon are
still the preferred market choice due their relatively high efficiency,
mature processing techniques and the wide availability of silicon on the
Earth's crust. These are some of the reasons underlying the continuous
interest and development of silicon-based photovoltaics [1,2]. However,
it is still considered critical to enhance their performance to promote
their widespread utilization. When studying the losses associated with
the radiation spectrum, it is important to mention that in addition to the
transparency to wavelengths longer to 1100 nm there are other impor­
tant losses above the band gap energy. The excess energy of incoming
photons from the solar irradiation with energy above the band gap is lost
by the emission of phonons by the lattice in a process known as ther­
malization [3]. In addition to the excess energy lost by thermalization,
only a small fraction of UV-light range is converted in electron-hole pairs
due the low penetration depth for radiation within the 300–400 nm
wavelength range in silicon. A viable approach to enhance the energy
harvesting in the UV region has been the use of spectral-converting
materials like down-shifting quantum dots (QDs). These semi­
conductor nanocrystals exhibit three-dimensional quantum confine­
ment, having discrete and size-dependent energetic states for electrons,
holes and excitons. Additionally, the surface to volume ratio is
increased, enhancing their photoelectronic properties. Semiconductor
quantum dots (QDs) absorb high-energy photons and re-emit them at
wavelengths more suitable for absorption by a silicon solar cell [4].
Particularly, ZnO is a II-VI wide direct-bandgap semiconductor (3.37 eV)
[5] with multiple optoelectronic applications ranging from blue lasers to
solar cells [6,7]. Even though bulk ZnO exhibit excitonic radiative
transitions in the UV-range due to a strong exciton binding energy (60
meV) [8], the defect-related emissions in the visible range found on ZnO
nanocrystals [9,10] are better suited to be absorbed by crystalline sili­
con. To efficiently utilize the downshifting characteristics of ZnO
nanocrystals, a high optical absorption is needed, however, recent
* Corresponding author at: Posgrado en Nanotecnología, Departamento de Física, Universidad de Sonora, C.P. 83000 Hermosillo, Sonora, Mexico.
E-mail address: a215290120@correoa.uson.mx (A. Flores-Pacheco).
https://doi.org/10.1016/j.mne.2022.100128
Received 21 January 2021; Received in revised form 15 December 2021; Accepted 19 March 2022
Available online 21 March 2022
2590-0072/© 2022 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/bync-nd/4.0/).
A. Flores-Pacheco et al.
Micro and Nano Engineering 15 (2022) 100128
Fig. 1. a) Absorption and photoluminescent (PL) spectra of ZnO QDs. b) Band gap calculation of the ZnO QDs colloidal solution using Tauc's graphical method.
Fig. 2. a) Deconvolution of the defect-related PL emissions of ZnO QDs under UV illumination (λexc = 335 nm). b) Schematic diagram showing the radiative
transitions found in the ZnO QDs.
efforts in the incorporation of ZnO QDs in commercial silicon solar cells
have resulted in low power conversion efficiency (PCE) enhancements,
as the deposit of downshifting material layers damaged the original
antireflective properties of the cell [11]. To overcome this limitation, we
propose the utilization of ZnO/PMMA thin films as both, antireflective
coating, and luminescent downshifting material.
In the present work, the antireflective effect of PMMA layers of
different thicknesses was evaluated on c-Si substrates by ellipsometry.
After a suitable PMMA layer thickness was found, down-shifting ZnO
quantum dots (QDs) were embedded in the polymeric matrix and
deployed to the surface of fabricated c-Si solar cells. Finally, the influ­
ence of the ZnO/PMMA thin films on the power conversion efficiency
(PCE) was characterized.
dropwise under constant stirring until a pH value of 12 was achieved.
The Zn2+ ions available after the hydrolysis of zinc acetate react with the
OH− ions present in a basic chemical environment forming Zn(OH)2
[13].
Zn(CH3 COO)2 + (OH)− →HO − Zn − CH3 COO− + CH3 COO−
(1)
HO − ZnCH3 COO + (OH)− →Zn(OH)2 + CH3 COO−
(2)
Zn2+ + 2OH− →Zn(OH)2
(3)
The precursor solution was placed on an ultrasonic bath for 3 h. This
procedure allows the dehydration of Zn(OH)2 to produce ZnO due to the
microthermal heating generated by cavitational hot spots promoted by
the sonication energy [14].
2. Experimental
Zn(OH)2 →ZnO + H2 O
(4)
Afterward, a non-solvent (hexane) was added to the ZnO QDs solu­
tion in a volume ratio of 3:1 and allowed to stand for 24 h to allow the
ZnO nanoparticles to precipitate. Subsequently, the supernatant was
removed and the precipitated ZnO QDs were washed three times in
ethanol to remove any unreacted products and dispersed also in ethanol
for storage.
2.1. Synthesis of zinc oxide quantum dots
The ZnO QDs colloidal solution was synthesized employing a
controlled-precipitation method [12]. Starting with a 0.02 M solution of
Zinc Acetate, the pH value is adjusted adding a 0.1 M LiOH solution
2
A. Flores-Pacheco et al.
Micro and Nano Engineering 15 (2022) 100128
Fig. 3. a) Deconvolution of the X-ray diffraction pattern of ZnO QDs powders with crystallographic planes indicating wurtzite structure. b) HRTEM micrograph of
ZnO QDs. The inset shows the size distribution histogram of the obtained nanoparticles.
r.p.m. (60 s) and ending at 300 r.p.m. (10 s). The phosphorous (SOD)
solution was spin-cast on the opposite side of the live sample using the
previously mentioned parameters. Then, the sacrificial and live samples
were heated at 130 ◦ C for 10 min to remove the organic solvents. Sub­
sequently, the live c-Si sample was placed in a furnace with the pristine
side facing the surface of the sacrificial sample with the p-type film, both
samples were separated by small pieces of clean silicon wafers. The
samples were then annealed at 1000 ◦ C for 10 min to diffuse the dopants,
creating the p–n junction in the front of the solar cell and the n− back
surface field (BSF) in the back of the device. The heavily doped layer
formed due to the high annealing temperature was removed by
immersing the samples in a dilute HF aqueous solution (50:1 in volume)
for 120 s. The electric contacts of the device were created by thermal
evaporation using a VEECO thermal evaporator. 200 nm of aluminum
were deposited on each side of the device, a shadow mask was used on
the window side to produce a pattern of finger electrodes, while a
blanket deposition was performed on the backside. Finally, the samples
were annealed at 580 ◦ C for 10 min to reach the eutectic point of the
Al–Si system [17] and promote the formation of an ohmic Al–Si
contact.
Table 1
ZnO QDs size calculations from X-ray diffraction peaks.
2θ (degrees)
(hkl)
β (radians)
D (nm)
31.4616
34.3528
36.1973
47.5237
56.5555
62.7230
(100)
(002)
(101)
(102)
(110)
(013)
0.03518
0.02944
0.03426
0.03633
0.03716
0.03478
4.10
4.93
4.26
4.17
4.23
4.67
2.2. Fabrication of single-crystal silicon solar cells and ZnO QDs
application
Single-crystal silicon solar cells were fabricated employing n-type,
〈100〉 silicon wafers. All the samples were cleaned using the standard 3step RCA cleaning process [15]. Boron and phosphorous spin-on dopant
(SOD) solutions were prepared by the sol-gel method [16]. The boron
(SOD) solution was spin-cast on the polished side of a sacrificial p-type,
〈111〉 silicon sample at 300 r.p.m. (10 s) ramping to a final speed of 1000
Fig. 4. a) Reflectance from the surface of single-crystal silicon samples with PMMA thin films spin-cast at various speeds compared with a reference bare silicon
sample. b) Reflectance from the surface of PMMA-only and 0.5 mg/ml ZnO/PMMA thin films deposited at 1000 R.P.M (ZnO QDs absorption spectrum included
as reference).
3
A. Flores-Pacheco et al.
Micro and Nano Engineering 15 (2022) 100128
Fig. 5. a) AM1.5G irradiance spectrum. b) Power reflected from the AM1.5G illumination source incident on the surface of single-crystal silicon samples with PMMA thin films spin-cast at various speeds and a reference
bare silicon sample. c) Power reflected from the AM1.5G illumination source incident on the surface of PMMA-only and 0.5 mg/ml ZnO/PMMA thin films deposited at 1000 R.P.M.
Table 2
Weighted reflectance and total power reflected of single-crystal silicon samples
with PMMA thin films spin-cast at various speeds.
PMMA thickness (nm)
Weighted reflectance (%)
Total power reflected (W/m2)
0 (Bare silicon)
75
80
83
85
88
93
18.18
7.75
7.11
7.13
6.64
6.97
6.21
146.46
63.24
57.31
57.45
53.5
56.13
50.06
Table 3
Weighted reflectance and total power reflected of single-crystal silicon samples
with PMMA-only and 0.5 mg/ml ZnO/PMMA thin films deposited at 1000 R.P.
M.
ZnO concentration (mg/
ml)
Weighted Reflectance
(%)
Total power reflected (W/
m2)
0 (Pure PMMA)
0.5
6.21
6.09
50.06
49.09
The ZnO QDs storage solutions were dried for 2 h at 50 ◦ C to obtain a
solid precipitate that was crushed and grinded into fine powders
employing a mortar. The ZnO QDs powders were dispersed in a polymer
matrix. Polymethylmethacrylate (PMMA) was selected due to its high
transparency to wavelengths in the UV–visible range [18], high weather
and UV resistance and excellent thermal insulation. Solutions with tar­
geted ZnO QDs powder concentrations of 0.5, 0.25 and 0.125 mg/ml
were prepared and spin-cast on the window side of the fabricated solar
cells. Three solar cell sets conformed each by three solar cells were
fabricated and characterized before and after the application of ZnO/
PMMA layers.
2.3. Characterization
The absorption spectrum of the colloidal ZnO QDs solution was ob­
tained with an Ocean Optics Flame UV-VIS spectrometer. The photo­
luminescent emission of the ZnO QDs colloidal solution was
characterized at room temperature using the fluorescence mode of the
same spectrometer connected by an optical fiber to a Newport Oriel
74,100 Cornerstone 1/4 m UV-VIS monochromator illuminated by a
250-W Xenon lamp, a 335 nm wavelength was selected for excitation of
the sample (λexc). The X-ray diffraction pattern was obtained on a Rigaku
Ultima IV diffractometer operating at 40 keV and the HRTEM micro­
graphs were obtained on a JEOL JEM-2010F field emission transmission
electron microscope (TEM) operating at 200 kV. The reflectance spectra
and thickness of the finished solar cells were obtained in a J.A. Woollam
spectroscopic ellipsometer VB-400 VASE. The current density-voltage
(J–V) performance curves of the solar cells before and after the appli­
cation of the ZnO QD layers was characterized using an Oriel Sol2A solar
simulator under AM1.5G illumination at standard testing conditions, i.
e., 1000 W/m2 at room temperature. The external quantum efficiency
(EQE) characterization was performed with a Newport External Quan­
tum Efficiency Measurement System to study the spectral response of the
fabricated solar cells before and after the application of the ZnO QDs
layers.
3. Results and discussion
Fig. 1a shows the normalized absorption and emission spectra of the
ZnO QDs. The absorption intensity of the ZnO QDs quickly increases in
wavelengths below 350 nm with a maximum value around 320 nm. The
photoluminescence (PL) maximum of the ZnO QDs is centered at 510
nm. The large Stokes-shift of the ZnO QDs (~160 nm) largely decreases
4
A. Flores-Pacheco et al.
5
Micro and Nano Engineering 15 (2022) 100128
Fig. 6. J-V characteristics for c-Si solar cells before (continuous lines) and after (dashed lines) application of ZnO/PMMA layers from solutions with three different ZnO QDs powders concentrations: a) 0.125 mg/ml, b)
0.25 mg/ml and c) 0.5 mg/ml. d) J-V characteristics for c-Si collar cells before (continuous lines) and after (dashed lines) application of PMMA-only layers.
A. Flores-Pacheco et al.
Micro and Nano Engineering 15 (2022) 100128
indicative of the formation of nanostructured materials, being in this
particular case related to spherical nanoparticles of ZnO [25].
Fig. 3b shows the HRTEM micrograph and the size distribution his­
togram for the ZnO QDs, with an average particle size of 4.42 ± 0.65
nm.
Furthermore, the X-ray diffraction pattern can also be utilized to
calculate the particle size by examining the diffraction peaks and
applying the Scherrer Eq. [26].
Table 4
Average silicon solar cell performance parameters before and after application
of 0.125 mg/ml, 0.25 mg/ml, 0.5 mg/ml ZnO/PMMA and PMMA-only layers.
Sample
Voc
(mV)
Jsc
(mA/
cm2)
Jsc
from
EQE
FF
(%)
PCE
(%)
Solar cell set 1
Solar cell set 1
+ 0.125 mg/
ml ZnO
Solar cell set 2
Solar cell set 2
+ 0.25 mg/
ml ZnO
Solar cell set 3
Solar cell set 3
+ 0.5 mg/ml
ZnO
Solar cell set 4
Solar cell set 4
+ PMMA
541.40
29.09
28.96
59.52
9.37
552.55
530.20
32.59
29.49
32.51
28.33
57.47
61.71
10.35
9.64
543.05
540.43
34.94
28.42
34.92
27.57
58.22
62.32
11.04
9.57
549.26
489.92
36.21
25.43
36.25
24.63
57.38
62.10
11.41
7.74
496.36
28.78
27.84
59.90
8.55
ΔPCE
(%)
10.44
± 0.14
D=
14.50
± 0.05
Kλ
βcosθ
(7)
where D is the crystallite size in nanometers, K is a dimensionless shape
factor with a typical value of 0.9 [27], λ is the radiation wavelength of
the X-ray source (typically 1.5406 Å from Cu K-α), β is the full width at
half-maximum intensity (FWHM) in radians and θ is given by the
angular position of the diffraction peak.
The results of the evaluation of Eq. (7) with the information
extracted from the deconvolution of Fig. 3a is summarized in Table 1.
The average crystallite size of 4.39 nm extracted from Table 1 has
good correspondence to the 4.54 nm diameter calculated using Brus's
model and the average particle size of 4.42 nm measured with TEM.
PMMA has an approximate refractive index value of 1.5 within the
400 to 500 nm wavelength range [28] that can be used as antireflective
layer. The thickness d1 of the antireflective layer of a dielectric material
like PMMA with a refractive index η1 is one-quarter of the wavelength of
the incoming radiation λi
19.22
± 0.06
10.59
± 0.18
self-absorption, making the ZnO QDs good candidates as spectral con­
verters for silicon solar cells.
During the investigation of the optical and electronic properties of
germanium, Tauc et al [19] proposed a graphical method for band gap
determination using the absorption data plotted in terms of energy.
Based on Tauc's work, Davis and Mott [20] described how the absorp­
tion intensity depends on the difference between the incident photon
energy hv and the semiconductor band gap Eg
)
(
(5)
(αhv)1/n = A hv − Eg
d1 =
λi
4η1
(8)
The thickness of the PMMA thin films and therefore, the wavelength
range where the antireflective effect occurs can be controlled by the spin
casting speed of the polymeric solution applied to the silicon surface.
The thickness d1 of a spin-cast thin film is proportional to the inverse of
the angular velocity ω squared.
where h is Planck's constant, v is the photon's frequency, α is the ab­
sorption coefficient and A is a proportionality constant. The value of the
exponent denotes the nature of the electronic transition. ZnO is a
semiconductor with direct allowed transitions having n = 1/2. The plot
of the absorption data of the ZnO QDs processed by Eq. (5) is shown in
Fig. 1b. When the energy of the incident photons is near the band gap
value, the absorption becomes more pronounced and exhibits a region of
linearity in the Tauc Plot. The extrapolation of the linear zone of Fig. 1b
renders a band gap value of 3.63 eV.
The energy dependence on particle size described by the Brus model
[21] can be used to extract the approximate radius value R of the ZnO
particles using the following equation
(
)
ℏ2 π2 1
1
1.8e2
Eg = Eg bulk + 2
−
+
(6)
2R m*e m*h
4πεR
1
d1 ∝√̅̅̅̅
(9)
ω
Fig. 4a shows the reflectance spectra of PMMA thin films with
thicknesses from 75 to 93 nm controlled by the spin-cast speed of the
film. A significant shift in the maximum antireflective effect from
around 420 nm to 500 nm was observed. The solar irradiation with
wavelengths near to 500 nm can be more efficiently absorbed by the
surface of the solar cell due to the reduced light reflection in this
wavelength range. After obtaining the optimal deposition parameters, a
PMMA solution with ZnO QDs powder concentration of 0.5 mg/ml was
deposited at 1000 R.P.M. and compared against a PMMA-only thin film.
Fig. 4b shows similar reflectance features for the samples given the
minimal impact of the embedded ZnO QDs in the refractive index of the
PMMA film due to the relatively low concentration of this nanoparticles.
It is important to mention that there is a small decrease in reflectance
from the PMMA-only to the ZnO/PMMA layer within the 300–400 nm
region that could be related to the absorption spectrum of the ZnO
nanoparticles.
To obtain quantitative values of the antireflective effect of the PMMA
and ZnO PMMA layers under solar illumination (Fig. 5a), the weighted
reflectance 〈R〉 and total power reflected PReflected values were calculated
from the reflectance spectra shown in Fig. 4 within the wavelength
range of interest (300–1100 nm) using the following expressions:
∫ 1100
R(λ)I(λ)dλ
(10)
〈R〉 = 300
∫ 1100
I(λ)dλ
300
where ℏ is the reduced Planck's constant, Eg = 3.37 eV is the bulk band
gap value for ZnO, me* = 0.26m0 and mh* = 0.59m0 are the effective
electron and hole masses respectively and ε = 8.5ε0 [22] is the
permittivity value of ZnO. The resulting estimate diameter of the ZnO
QDs is 4.54 nm (R = 2.27 nm).
The deconvolution of the PL spectrum of the ZnO QDs under UV
illumination (335 nm) shown in Fig. 2a identifies the defect-related
emissions of the ZnO quantum dots that can be exploited to enhance
the solar cell spectral response. The different transition mechanisms are
summarized in Fig. 2b. The radiative recombination processes are
attributed to electron transitions from a shallow donor, related with Zni
(interstitial Zn) defects to different acceptor levels: shallow acceptor
level due to zinc vacancies (VZn) at 445 nm, oxygen vacancies (VO) at
492 nm, VZn deep acceptor at 530 nm and interstitial oxygen (Oi) at 587
nm [23].
The X-ray diffraction pattern of the ZnO QDs powders shown in
Fig. 3a exhibit the diffraction peaks corresponding to the hexagonal
wurtzite crystal structure of ZnO indexed in the Crystallography Open
Database (COD) entry # 9011662 [24]. The wide diffraction peaks are
∫
1100
PReflected =
R(λ)I(λ)dλ
300
6
(11)
A. Flores-Pacheco et al.
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Micro and Nano Engineering 15 (2022) 100128
Fig. 7. Spectral response (EQE) and calculated short circuit current from EQE data for c-Si solar cells before (continuous lines) and after (dashed lines) application of ZnO/PMMA layers from solutions with three different
ZnO QDs concentrations: a) 0.125 mg/ml, b) 0.25 mg/ml and c) 0.5 mg/ml. d) EQE of c-Si collar cells before (continuous lines) and after (dashed lines) application of PMMA-only layers.
A. Flores-Pacheco et al.
Micro and Nano Engineering 15 (2022) 100128
Finally, for the 0.5 mg/ml ZnO QDs concentration (Fig. 7c) a noticeable
improvement within the 300–400 nm range wavelengths by the downshifting effects of the ZnO QDs, which in addition to the antireflective
properties of the PMMA layer greatly enhances photocurrent generation.
To separate the antireflective and down-shifting effects, an additional
solar cell set EQE was evaluated with PMMA-only layers. The EQE
improvement is given within the 400–1000 nm range in concordance of
the antireflective effect shown in Fig. 4a without any sign of enhance­
ment in the 300–400 nm range.
The EQE values can be utilized to determine Jsc without the need of
measuring the cell's irradiated area and can be more accurate than the
standard J-V characterization due to the elimination of measurement
errors of said irradiated area. The value of Jsc can be calculated
combining the photon flux bs(λ) given by the AM1.5G irradiance spec­
trum, in combination with the EQE values and integrating across all the
relevant wavelengths, namely,
∫ λ2
Jsc = − q
bs (λ) EQE(λ)dλ
(12)
λ1
The calculated Jsc values obtained with Eq. (12) for the solar cells
before and after application of ZnO/PMMA and PMMA-only layers were
plotted in Fig. 7. Since EQE is employed to provide an independent
measurement of the current density, it has been incorporated in the
performance characteristics of the solar cells evaluated in the present
work and summarized in Table 4 as an indicator of the consistency of the
collected values.
The data of Table 4 shows an interesting behavior for all the PMMAZnO and PMMA-only coated samples, which is the reduction of the fill
factor FF. Under high irradiance operative conditions (comparable to
one sun), the fill factor is dominated by the resistive losses mainly due to
the series resistance [29]. These losses are proportional to the series
resistance RS and the short circuit current Jsc of the solar cell and
inversely proportional to the open circuit voltage VOC [30].
Fig. 8. ZnO QDs down-shifting (Stokes shift) mechanism (blue lines). Antire­
flective effect (black lines) and silicon solar cell EQE response effect (red lines)
of the 0.5 mg/ml PMMA/ZnO layer. The AM1.5G spectral irradiance spectrum
is included as reference (dark yellow line). (For interpretation of the references
to colour in this figure legend, the reader is referred to the web version of
this article.)
where R(λ) is the reflectance of the sample and I(λ) the AM 1.5G irra­
diance spectrum.
An important decrease in power lost by reflection is evident in Fig. 5b
from the bare silicon sample to the PMMA coated silicon samples. The
numerical values of this effect (weighted reflectance and total power
reflected) calculated using Eqs. (10,11) are summarized in Table 2 and
shows a weighted reflectance decrease of 11.97% for the 93 nm layer
compared against the bare silicon sample. The reflectance decrease
allowed the recovery of around 96.4 W/m2 of total power reflected from
the AM1.5G solar spectrum (1000 W/m2) by antireflective effect in the
silicon sample with a 93 nm PMMA thin film. Similarly to the previous
section, the weighted reflectance and total power reflected of a ZnO/
PMMA layer was compared with the best PMMA-only sample with
minimal changes in the weighed reflectance (0.12%) and total power
reflected (0.97 W/m2). (See Table 3.)
Fig. 6 shows the J-V curves of the fabricated solar cell sets evaluated
before and after the application of the ZnO/PMMA layers obtained from
solutions with three different concentrations of ZnO QDs powders
(0.125, 0.25 and 0.5 mg/ml) and a solar cell set with PMMA-only layers.
Taking as reference the J-V curves of the PMMA-only solar cell set, an
increasing beneficial effect in current generation with higher ZnO con­
centrations is noticeable from Figs. 6a through 6c. The average silicon
solar cell performance parameters before and after application ZnO/
PMMA layers summarized in Table 4 shown average peak PCE im­
provements of 10.44, 14.50 and 19.22% for the 0.125, 0.25 and 0.5 mg/
ml concentrations respectively and 10.59% for the reference PMMAonly layers. This indicates that a 0.125 mg/ml of ZnO QDs concentra­
tion in the PMMA layers does not have significative impact in photo­
generation and will be practically the same of PMMA-only layers.
The external quantum efficiency (EQE) was measured to study the
spectral response of the fabricated c-Si solar cells with and without the
deployed ZnO QDs films. At the lowest ZnO QDs concentration of 0.125
mg/ml (Fig. 7a), the expected increase in EQE within the 300–400 nm
range due to the down-shifting effect cannot be detected. Meanwhile,
the improvement in EQE located within the 400–1000 nm wavelength
range and can be attributed mainly to the anti-reflective effect of PMMA
[28]. With the ZnO QDs concentration increased to 0.25 mg/ml (Fig. 7b)
the spectral response starts to experience a better performance below
450 nm wavelength, which after being absorbed by the ZnO QDs can
promote defect-related radiative transitions in the visible range [23].
ΔFF ≈ −
RS Jsc
Voc
(13)
The increment in open circuit voltage Voc is not linearly related with
the increment in Jsc. From the ideal diode equation in the superposition
approximation (J = Jsc − Jdark) [31].
( qV
)
J = Jsc − J0 eKT − 1
(14)
were J is the net current density of the cell V is the solar cell voltage, J0 is
related to the recombination rate constant, K is Boltzmann's constant, T
the temperature and q the elemental charge. When the cell is in the open
circuit condition J = 0
)
(
KT
Jsc
Voc =
ln
− 1
(15)
q
J0
Equation (15) shows that the open circuit voltage changes logarith­
mically with the short circuit current, promoting a reduction in fill factor
with an increase in photogeneration (higher Jsc values).
Fig. 8 shows the simultaneous phenomena that occur under solar
illumination and permits a maximum average PCE improvement of
19.22% in the solar cell set with 0.5 mg/ml ZnO/PMMA layers. This
performance enhancement can be attributed to the accumulative
contribution of the antireflective effect of the PMMA matrix and downshifting effect (Stokes shift) of the ZnO QDs, that converts the UV region
(300–400 nm) of the solar irradiance spectrum to the visible range
(centered around 510 nm) on which the silicon solar cell has a better
spectral response (described by EQE).
4. Conclusions
The silicon solar cells with optimized ZnO/PMMA layers exhibit
8
A. Flores-Pacheco et al.
Micro and Nano Engineering 15 (2022) 100128
significant performance enhancements. The maximum average PCE
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the PMMA matrix and the down-shifting effects of the ZnO QDs. The
experimental results of the present work show the feasibility of the
application of ZnO/PMMA layers on the surface of single-crystal silicon
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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.
The authors declare the following financial interests/personal re­
lationships which may be considered as potential competing interests.
Acknowledgments
The authors would like to acknowledge the U.S. Army Research
Office (Grant W911NF-13-1-0110), NSF (Grant 1650571), CONACYT,
the Department of Physics and Astronomy at the University of Texas at
San Antonio and the Physics Department of the University of Sonora, for
the financial support provided for this research project.
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