1. Introduction
Solar photovoltaic (PV) is one of the world’s best renewable energy sources, having experienced exponential growth over the last several years [
1], with an expected installed capacity of 1 terawatt (TW) by 2022–2023 [
2]. One prominent emerging type of solar cells is perovskite solar cells. Even though perovskites had a weak start in photovoltaic applications in 2009, with power conversion efficiency as little as 3.8% [
3], their efficiencies have skyrocketed to 14% in 2013 and to ~23% in 2018 [
4]. Additionally, as of June 2018, thin-film perovskite technologies have exceeded all other thin-film solar cell technologies, including cadmium telluride (CdTe) and Copper Indium Gallium Selenide (CIGS), proving to be a strong candidate to the future of solar PV.
One of the most promising perovskite materials is methylammonium lead triiodide (MAPbI
3 or MAPI), exhibiting many outstanding and desirable properties. Its electrical properties include: a direct bandgap [
5] with a value around 1.6 eV with shallow defect levels [
6], tunable bandgap (up to 2.25 eV) [
7], a large open circuit voltage of 1.07 [
8], and a relatively high short-circuit current density. Not only does MAPbI
3 excel electrically, but it also possesses some appealing optical properties, such as high carrier mobilities [
9], high absorption coefficient [
10,
11], and long diffusion lengths [
12,
13,
14]. All these exceptional and advantageous properties are important metrics for high performance solar cells [
15], making MAPbI
3 one of the top contenders in perovskite solar cell technology.
One of the ways to enhance the performance of perovskite solar cells is by the addition of plasmonic NPs to the cell. Plasmonic NPs, known for their very useful and interesting properties, have become the subject of many scientific applications over the past decade [
16]. More importantly, metallic NPs have attracted the most attention due to their optical properties, allowing their applications to vary from biology, to photothermal imaging and therapy, to solar cells and much more. Many metallic NPs exhibit high absorption in the visible electromagnetic spectrum range, but only Au, Ag, and copper (Cu) NPs exhibit surface plasmon resonance (SPR) effect in that visible region too [
16]. SPR is a phenomenon in which polarized light hits a metallic film at the interface of media with different refractive indices which causes the collective oscillation of free electrons, known as surface plasmons. These plasmons can be set to resonate with light, resulting in high absorption of light [
17]. This SPR effect is particularly effective in solar cell applications due to the great absorption of light NPs have to offer, especially Au and Ag. This improvement in absorption can lead to an improved power conversion efficiency which we aspire to achieve. Over the last few years, there have been some recent experimental works of embedding plasmonic nanostructures, such as both Ag and Au NPs, within perovskite solar cells to enhance the power conversion efficiency (PCE). The experimental synthesized plasmonic-perovskite cell improved PCE from 15.4% up to 17.3% when adding both blended Ag and Au nanocrystals of 1 vol.%. However, the design technique is still dependent on blending the plasmonic nanocrystals within the perovskite layer of the cell [
18,
19,
20].
In this paper, the enhancement of perovskite solar cells through the addition of Au and Ag NPs is studied. This enhancement is analyzed both optically and electrically. Our aim is to demonstrate the effect of these NPs to the optical and electrical characteristics of the cell through different simulations. It is shown that plasmonic NPs are able to compensate for the low absorption of MAPbI
3 in the visible spectrum by adding NPs with resonance frequencies close to that spectrum [
15]. Optimal diameters of Au and Ag NPs are found and simulated, and their impact on different parameters such as transmission, absorption, reflection, external quantum efficiency (EQE), open circuit voltage (
Voc), short-circuit current density (
Jsc), fill factor (FF), and most importantly, efficiency (η) of the perovskite solar cell, is investigated. Through our work, the plasmonic layer of NPs is added to the rear side of the solar cell. That configuration is proven to be more effective in enhancing the photocurrent [
21,
22].
3. Case Study
The cell under study is a semitransparent perovskite solar cell with an n-i-p architecture: TCO/ETL/Perovskite/HTL/TCO. For the transparent conductive oxide (TCO), a 140 nm thick indium–tin oxide (ITO)-coated glass substrate as front electrode and 50 nm thick ITO as the back electrode. The electron transport layer (ETL) consists of 5 nm thick SnO
2 combined with 15 nm thick phenyl-C61-butyric acid methyl ester (PCBM) bilayer. The perovskite material used in this study is a 372 nm thick MAPbI
3 followed by a 250 nm thick 2,2’,7,7’-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9’-spirobifluorene (Spiro-OMeTAD) as hole transport layer (HTL). Roughness layers for front ITO, MaPbI
3, and rear ITO were added to simulate experimental irregularities with thicknesses of 4 nm, 50 nm, and 10 nm, respectively. Both Au and Ag NPs will be applied to the rear of the perovskite solar cell, and their effect on the performance of the solar cell will be studied both optically and electrically. The model of the cell is illustrated in the schematic in
Figure 1.
5. Results and Discussion
With the simulator calibrated, J–V curves were produced for our semitransparent cell with and without the optimally sized Au and Ag NPs.
Figure 8 show a comparison between the three J–V curves along with a summary of the extracted electrical parameters in
Table 3. It is clear that the added NPs have a direct effect on the enhancement of the
Jsc, with an increase of 1.45 mA/cm
2 and 1.58 mA/cm
2 for Au and Ag NPs, respectively. This may be due to the improvement in the EQE of the cell. However, this had minimal-to-negligible effect on
Voc; which almost remained constant. The FF, however, decreased, as expected, by ~1% with the addition of the Au and Ag NPs at the rear side. This can be explained due to the increased value of the cell’s sheet resistance. Overall, an improvement of ~1% on the conversion efficiency was achieved by adding either Au or Ag NPs at the rear-side of a semitransparent MAPbI
3 perovskite solar cell with Ag slightly outperforming Au.
To further investigate the reason behind the slight improvement in performance of Ag NP over the Au NP, and to gain a clear-cut understanding, three optical analyses are considered in this paper:
5.1. Solar-Weighted Reflectance
The solar-weighted reflectance (SWR) is a measure of reflectivity over the entire solar spectrum weighted by the intensity of sunlight of each wavelength. Since the reflection plots produced were a function of wavelength, it is sometimes difficult to determine which reflection curve results in lower or higher overall reflection. The SWR provides a more tangible measure of reflectance by providing a single numerical expression for the reflectance rather than a function of wavelength. It is expressed in Equation (5) by the following expression [
44]:
where
is the wavelength,
R(
) is the reflection of the cell, and
is the AM1.5G solar spectrum. Integrations were performed from 300 nm to 800 nm, which is the interval of interest in our perovskite solar cell. A summary of the results is provided in
Table 4. With the SWR calculations, it can be seen that the addition of the NPs has a clear and significant impact on the overall reflection of the cell, resulting in an increase of ~3.5% with the Au NPs and ~4% with the Ag NPs. This may cause photon recycling, and therefore, increase the probability of absorption in the perovskite layer which causes an increase in the external quantum efficiency of the solar cell. As a result, a slight increase in
Jsc. with Ag NPs over Au NPs can be seen.
5.2. Solar Absorptance Enhancement
Similar to SWR, the solar absorptance enhancement (SAE) also provides a better understanding and interpretation of one of the most important optical parameters, absorption. The SAE translates the absorption vs. wavelength into a numeric ratio which indicates the degree in absorption enhancement. The SAE is expressed in Equation (6) by the following expression [
45]:
where A is the total absorption of the cell (i.e.,
),
represents the absorption of the semitransparent cell with no NPs, whereas
represents the absorption of the cell with the added NPs. A value of SAE > 1 implies a positive effect or an enhancement and SAE < 1 implies a negative effect or a deterioration, whereas SAE = 1 implies no enhancement or deterioration in absorption. Integrations were performed from 300 nm to 800 nm, which is the interval of interest in our perovskite solar cell. Calculations were performed on three different kinds of absorptions: the total cell absorption, perovskite absorption, and parasitic absorption. A summary of the results is provided in
Table 5.
With the SAE calculations, it can be seen that the addition of the plasmonic NPs has a clear and significant impact on the overall absorption enhancement of the cell, with Au NPs resulting in just a minor fraction more total absorption enhancement than the Ag NPs. However, this extra absorption is being squandered on parasitic absorption, as the NPs resulted in an SAE value of 1.956 and 1.83 for Au and Ag NPs, respectively. Thus meaning, the Ag NPs contributed more to the perovskite absorption (i.e., EQE), with an SAE value of 1.084 as opposed by 1.077 for the Au NPs, resulting in higher photocurrent; see
Figure 9.
5.3. Current Density Loss (Jloss) Analysis
Reflection, transmission, and absorption are essential parameters that optically characterize a solar cell. They can provide a definite indication on the performance of the cell and ultimately translate into optical current. In order to calculate the current density loss due to reflection, transmission, and parasitic absorption, current loss analysis was carried out to investigate the effect of Au and Ag NPs on the performance of the semitransparent perovskite solar cell, by using TMM simulations. At the device front, part of the light is lost due to external reflection. These reflection losses (i.e.,
J reflection-loss) are calculated by integrating the reflection spectrum on the front side of the cell according to Equation (7) [
15]. Moreover, at the device rear, part of the light is lost due to transmission, where long wavelength light is transmitted through the transparent rear ITO. This external loss is calculated by integrating the transmission curve of the cell according to Equation (8) [
15]. Finally, the current loss due to parasitic absorption (i.e.,
Jparasitic-loss) is calculated using Equation (9) [
15], by integrating the area between the EQE and the absorbance curves over the AM1.5G solar spectrum for the wavelength region of interest.
Furthermore, the total
Jsc which can be produced by the solar cell for each specific NP can be expressed in Equation (10) as:
where q is the elementary charge, h is Planck’s constant, c is the speed of the light, R is the reflection of the cell, T is the transmission of the cell, and A is the absorption of the cell.
A summary of the results can be found in
Table 6. It can be shown that
for all structures, which is approximately 27.3 mA/cm
2, the Shockley-Quiesser
Jsc limit for a 1.55 eV bandgap material [
46]. For the semitransparent cell, much of that current is lost due to transmission losses and only a little is lost due to parasitic absorption and reflection. Adding the Au and Ag seems to mitigate these transmission losses and improve the
Jsc by 7.76% and 8.44%, respectively; however, the parasitic and reflection losses have also increased unfavorably as depicted by the SWR and SAE calculations. The
Jreflection increased by ~1 mA/cm
2 and ~1.1 mA/cm
2 while the
Jparasitic increased by 1.73 mA/cm
2 and 1.5 mA/cm
2 with the addition of Au and Ag NPs, respectively. Overall, the addition of these NPs resulted in a positive impact.
Jsc also had a direct improvement due to the increase in the EQE of the cell. It should be noted that silver resulted in better overall characteristics than gold with higher absorption, EQE and reflection, along with lower transmission and parasitic absorption. An illustration of these optical parameters can be shown in
Figure 10. The magnitude of each parameter is illustrated by the area it covers at its pertinent wavelength region. It can be noticed from
Figure 10 that the EQE was also enhanced at wavelength range close to the plasmonic resonance frequency. This enhancement is due to the improvement in the absorption and the entrapment of the reflected photoelectrons [
15]. However, the overall parasitic absorption also increased absorption and reflection at the resonance frequency.
6. Conclusions
This work shows the enhancement of both optical and electrical properties of a semitransparent perovskite solar cell by the addition of either Au or Ag NPs of different diameters. Optical simulations show that these nanoparticles have improved the external quantum efficiency of the perovskite solar cell at certain parts of the spectrum and in turn, the short-circuit current density. Jsc has increased by 1.45 mA/cm2 (7.76%) and 1.58 mA/cm2 (8.44%), with the addition of the optimally sized Au and Ag NPs, respectively. On the other hand, electrical simulations showed little-to-no changes in both Voc and FF, but an improvement in the efficiency due to the enhancement of the Jsc. The efficiency, η, has increased by ~0.98% and ~1.07% with the addition of the optimally sized Au and Ag NPs, respectively.
Additional optical analysis tools and methods were also considered including: Solar-Weighted Reflectance (SWR), Solar Absorptance Enhancement (SAE), and Current Density Loss (Jloss) analysis. SWR calculations show that the overall reflection of the cell increased by ~3.5% with the Au NPs and ~4% with the Ag NPs. SAE calculations show that the Au NPs have increased the total absorption slightly more than the Ag NPs, but this increase was mostly wasted on parasitic absorption rather than the perovskite, unlike the Ag NPs. Jloss analysis showed the exact distribution of light in terms of current density. Due to the increase in reflection and parasitic absorption in the cell, Jreflection and Jparasitic have also increased. The Jreflection increased by ~1 mA/cm2 and ~1.1 mA/cm2 while the Jparasitic increased by 1.73 mA/cm2 and 1.5 mA/cm2 with the addition of Au and Ag NPs, respectively. Jtransmission underwent the biggest improvement, dropping from 4.24 mA/cm2, down to 0.09 mA/cm2 and 0.07 mA/cm2 for Au and Ag NPs, respectively.