Fazal E. Subhan*ab,
Aimal Daud Khancd,
Adnan Daud Khana,
Najeeb Ullaha,
Muhammad Imrane and
Muhammad Nomana
aUS-Pakistan Center for Advanced Studies in Energy, University of Engineering & Technology, Peshawar, 25000, Pakistan
bArizona State University, Tempe, Arizona 85287, USA. E-mail: fsubhan@asu.edu
cCollege of Energy, Soochow Institute for Energy and Materials Innovations (SIEMIS), Soochow University, Suzhou 215006, China
dKey Laboratory of Advanced Carbon Materials and Wearable Energy Technology of Jiangsu Province, Key Laboratory of Modern Optical Technologies of Ministry of Education, Suzhou 215006, China
eDepartment of Electrical Engineering, Military College of Signals, National University of Sciences and Technology (NUST), Islamabad, 46000, Pakistan
First published on 16th July 2020
Tandem configuration-containing perovskite and silicon solar cells are promising candidates for realizing a high power conversion efficiency of 30% at reasonable costs. Silicon solar cells with planar front surfaces used in tandem devices cause high optical losses, which significantly affects their efficiency. Moreover, some studies have explored the fabrication of perovskites on textured silicon cells. However, due to improper texturing, light trapping is not ideal in these devices, which reduces the efficiency. In this work, we optimized the pyramid height of textured silicon cells and efficiently characterized them to achieve enhanced light trapping. Two different kinds of perovskites, namely, Cs0.17FA0.6Pb(Br0.17I0.7)3 and Cs0.25FA0.6Pb(Br0.20I0.7)3 with wide bandgaps were conformally deposited on textured silicon cells, and the performance of these flat and fully textured tandem devices was numerically analyzed. The thickness of each layer in the tandem cell was optimized in a way to ensure a perfect current match between the top perovskite and bottom silicon subcells. The results indicated that the textured tandem configuration enhances light absorption over a broad spectral range due to the optimized pyramid height compared to flat surfaces. Eventually, the photovoltaic parameters of the proposed tandem cell were compared with the already existing structures, and our design supports large values of open circuit voltage (Voc) = 1.78 V, short circuit current density (Jsc) = 20.09 mA cm−2, fill factor (FF) = 79.01%, and efficiency (η) = 28.20% compared to other kinds of tandem solar cells.
Selecting a top cell partner with a wide band gap to reduce thermal losses and enhance solar spectral utilization is an effective way to achieve an efficiency value beyond the Auger efficiency limit for silicon solar cells. In recent times, a dual junction combining III–V materials and Si reached a power conversion efficiency (PCE) of 32.8%.5 However, due to expensive deposition techniques, III–V material-based solar cells have not yet attracted attention in the PV market. Recently, organic–inorganic metal halide perovskites such as cesium–formamidinium-based CsyFA1−yPb(BrxI1−x)3 with wide bandgaps of 1.63 eV and 1.68 eV have been recognized as unique light-absorbing materials that have passed the record efficiency.6,7 The perovskite material is found to be the best partner for Si solar cells due to their long carrier diffusion lengths, sharp optical absorption edge, low exciton binding energies, and excellent defect tolerance.8–12 These characteristics theoretically allow the perovskite/silicon tandem solar cells to grasp efficiencies above 30% at reasonable production costs.13,14 Experimentally, power conversion efficiencies over 26% and 27% have been achieved for perovskite–silicon monolithic two-terminal and mechanically stacked four-terminal arrangements, respectively.15–17
The two-terminal perovskite–silicon tandem cell has achieved greater attention due to extensive use in industries with the least number of processing steps, substrates, and interconnection requirements.18 Some research groups have investigated both cells with planar front surfaces19 and those with textured surfaces.20 The textured silicon solar cell is found to be better due to efficient light trapping in both silicon and top subcell perovskites. The textured solar cell with randomly placed pyramids of various size distributions (i) helps in the reduction of reflectance by a flat surface from over 30% to less than 10%, (ii) leads to enhanced absorption nearer the forward-facing collecting junction, with the inclined ray tracing inside the absorbing layer, and (iii) contributes to efficient light trapping via total internal reflection on the surface owing to the geometric texture.21 It is noteworthy to mention that the reflectance is high for randomly placed pyramids with low size distribution, while pyramids with large heights of typically 3–10 μm size distribution hinder the subsequent processing stages and the conformal deposition of perovskites.22,23 R. Pandey et al. have numerically demonstrated the hysteresis of moisture-free tandem perovskite–Si solar cells with flat surfaces. However, due to poor light trapping, they were able to achieve an efficiency of only 23.1%.24 K. A. Bush et al. have fabricated a 23.6% efficient single-side-textured perovskite–silicon tandem cell with the front side polished.25 However, due to reflection losses from the top surface, they did not succeed in attaining high efficiencies. E. Köhnen et al. have fine-tuned the perovskite absorber and the indium zinc oxide (IZO) flat front electrode to achieve photocurrents well above 19 mA cm−2 with a stabilized efficiency of 26.0%.26 However, they did not consider double-side texturing for improved light trapping to achieve high matched short-circuit current for further efficiency enhancement. Thus, the polished surface leads to strong front surface reflection losses, poor match short circuit current and inefficient light trapping. Therefore, a c-Si bottom cell with a heterojunction configuration needs better light management, in particular in the infrared part of the solar spectrum, where it absorbs weakly. It has been found that by introducing double-side texturing, the current match can be potentially improved up to 2–4 mA cm−2.27–29 F. Sahli et al., have demonstrated 25.2% efficient double-side-textured monolithic two-terminal perovskite/silicon tandem cells with improved optical modelling.30 However, the optimization of pyramid height, charge transport and transparent conductive oxide (TCO) layer thickness is still necessary for high-matched short-circuit current and conformal deposition of perovskites on textured surfaces. Recently, B. Chen et al., have achieved a perovskite/silicon tandem cell with an efficiency of 26% on textured silicon.31 However, improvement in efficiency is still required to get close to the Auger limit.
Herein, we consider anisotropic etching to get optimized random pyramids with the 〈111〉 facets in order to improve the optical response of the cell in the near infrared region via a light scattering effect. Different c-Si pyramid heights and qualities of the textured c-Si surface are characterized using a scanning electron microscope (SEM) and an atomic force microscope (AFM), respectively to create a guide to maximize the current and matching in perovskite/silicon tandem cells and provide a way aimed at conformal top cell deposition. Two kinds of perovskite materials such as Cs0.17FA0.6Pb(Br0.17I0.7)3 and Cs0.25FA0.6Pb(Br0.20I0.7)3 having bandgaps of 1.63 and 1.68 eV, also referred to as Cs17/Br17 and Cs25/Br20 through their Cs and Br contents, were combined with textured silicon along with essential optimized functional layers to make efficient monolithic perovskite–silicon tandem solar cells. The proposed cells are found to exhibit good optical properties compared to flat top surfaces. Moreover, the highest efficiency of 28.20% is achieved in case of Cs25/Br20–Si tandem cells.
Fig. 1 (a–f) SEM images of the textured silicon sample to analyze the surface topography of different pyramid heights under 5000 resolution, and (g) AFM image of textured silicon. |
For evaluating the optical response of the textured silicon sample, we used an ARTA and a spectrophotometer. Fig. 2(a) shows the angular reflectance of the textured sample obtained at different pyramid heights ranging from 3 to 0.9 μm size. These ARTA results were obtained from an angle in the range of −90 to 90 degree for an entire front surface coverage. The results indicated that H1 of 3 μm was not a suitable option for solar cell applications because it supports a maximum reflection of 0.91 at −24° with a base angle of 51°. Similarly, the H2, H3 and H4 heights were not good options because they exhibited maximum reflection peaks. The height H6 appeared to provide very low reflection; however, at large angles, the reflection gradually maximized. Therefore, the height H5 of 1.2 μm was comparatively suitable in our case. Moreover, the scattering effect, which is significant for light trapping, usually comes into picture when the pyramid height is equal or closer to silicon bandgap energy.32 In our case, the scattering effect is only supported by heights H5 and H6 at a wavelength in the range of 400–800 nm, as shown in Fig. 2(b). Therefore, we have chosen a pyramid height between 0.9 and 1.2 μm for our further analyses of perovskite/silicon tandem development in the simulation environment.
Fig. 2 (a) Angular resolved reflectance of textured c-Si with different pyramid heights, and (b) reflectance of textured c-Si with different pyramid heights (extracted from AFM). |
Fig. 3 Perovskite/silicon tandem solar cells with (a) front side polished and (b) double sides textured. |
As discussed, for analysis, we have considered two different kinds of wide bandgap perovskite materials such as Cs17/Br17 having a bandgap of 1.63 eV and Cs25/Br20 having a bandgap 1.67 eV, respectively. The thicknesses of each layer of both kinds of perovskites (Cs17/Br17, Cs25/Br20) in a tandem configuration were optimized, which are displayed in Table 1. The optimization is performed by matching currents between perovskite and silicon subcells. Thus, in this way, a thickness of 500 nm was obtained for Cs17/Br17 and 650 nm for Cs25/Br20, and all other thicknesses remained constant. Moreover, the short-circuit current density (Jsc) was evaluated by taking integration of the product of the absorption spectrum as a function of wavelength under AM1.5G spectral irradiance over the wavelength range of 300–800 nm for Cs17/Br17, while the spectral range was 300–760 nm for Cs25/Br20. Table 2 indicates the thickness and corresponding current density values of each layer in a tandem configuration. A perfect match was obtained between the current density of perovskites and silicon subcells.
Layer | Perovskite Cs17/Br17 thickness (nm) | Perovskite Cs25/Br20 thickness (nm) |
---|---|---|
Front ITO | 120 | 120 |
SnO2 | 15 | 15 |
C60 | 15 | 15 |
Perovskite | 500 | 650 |
NiOx | 20 | 20 |
Rear ITO | 20 | 20 |
Glass | 1000 | 1000 |
Ag | 300 | 300 |
Layer | Perovskite/silicon tandem layer thickness (nm) | Jsc (mA cm−2) |
---|---|---|
MgF2 | 100 | 0.065 |
ITO front | 120 | 1.653 |
SnO2 | 15 | 0.256 |
C60 | 15 | 1.214 |
Perovskite (Cs17/Br17/Cs25/Br20) | 500/650 | 20.34 |
NiOx | 20 | 0.325 |
Intermediate ITO | 20 | 0.418 |
a-Si:H(n) | 8 | 0.005 |
a-Si:H(i) | 8 | 0.002 |
c-Si | 250000 | 20.09 |
a-Si:H(i) | 8 | 0.000 |
a-Si:H(p) | 11 | 0.000 |
Rear ITO | 180 | 0.431 |
Ag | 300 | 0.225 |
In order to analyze the performance of the two types of tandem cells (Cs17/Br17–Si, and Cs25/Br20–Si), we have simulated the cells with flat and textured top perovskite subcells. It has to be noted that the bottom silicon subcell is textured in both kinds of tandem cells. Fig. 4(a) shows the absorption and reflectance spectra of a Cs17/Br17–Si tandem configuration with flat perovskite top subcells. It appears that the reflectance indicated by the gray curve is relatively large at long wavelengths, which essentially reduces the current density in this spectral region. Moreover, the current mismatching was high in this case; perovskite exhibited 20.34 mA cm−2, while silicon supported 18.44 mA cm−2. This current mismatch between the cells will lead to low efficiency and eventually reduce the life time of the cell. In Fig. 4(b), the absorption and reflectance spectra were calculated for the double-side-textured surface. In this case, the absorption in the long wavelength region is significantly improved due to scattering effect induced by the textured surfaces. The difference in the current density values reduced from 1.86 mA cm−2 (flat perovskite) to 0.25 mA cm−2, which is quite acceptable in tandem configuration.26 Fig. 4(c) shows the absorption and reflectance spectra of a Cs25/Br20–Si tandem configuration with flat perovskite top subcells. The absorption characteristics in this case slightly changed compared to the previous case. Herein, the current mismatch was 1.65 mA cm−2, which was somewhat lower than that of the previous case. However, by introducing double-side texturing (Fig. 4(d)), the difference in the current matching appears the same as calculated in Fig. 4(b). Furthermore, the tandem models were tested with planar and double-side-textured silicon surfaces, and it was found that the planar structure limits the photocurrent value to ∼18.44 mA cm−2, a value that is ∼2 mA cm−2 lesser in comparison to a double-side-optimized texture design. Finally, in order to show which of the tandem configurations is better, we calculated the photovoltaic parameters including short current density (Jsc), open circuit voltage (Voc), fill factor (FF), and efficiency (η), as shown in Table 3. It becomes clear that the tandem cell of type Cs25/Br20–Si is the best option because of improved photovoltaic parameters compared to Cs17/Br17–Si. The highest conversion efficiency obtained was about 28.20%, which indicated that the proposed cell may be used for commercial applications.
Perovskite–silicon tandem cell | Voc (V) | Jsc (mA cm−2) | FF (%) | η (%) |
---|---|---|---|---|
Cs17/Br17–Si | 1.72 | 20.09 | 77.43 | 26.75 |
Cs25/Br20–Si | 1.78 | 20.09 | 79.01 | 28.20 |
We finally compared the proposed efficient tandem solar cell with the existing tandem structures, as shown in Table 4. Our cell supports relatively large values of Voc, Jsc, FF, and η, respectively, which indicates that it can be used for practical applications.
Reference | Structure | Voc (V) | Jsc (mA cm−2) | FF (%) | η (%) |
---|---|---|---|---|---|
24 | Monolithic single side textured perovskite–c-Si HJ | 1.75 | 16.89 | 74.30 | 21.93 |
25 | Monolithic planar perovskite–c-Si HJ | 1.69 | 18.04 | 75.43 | 23.08 |
30 | Monolithic fully textured perovskite–c-Si HJ | 1.78 | 19.5 | 73.10 | 25.20 |
26 | Monolithic planar perovskite–c-Si HJ | 1.76 | 19.2 | 76.50 | 26.00 |
17 | 4-Terminal planar perovskite–c-Si HJ | — | — | — | 27.0 |
31 | Monolithic fully textured perovskite–c-Si HJ | 1.82 | 19.3 | 74.4 | 26.0 |
Current study | Proposed monolithic perovskite–c-Si HJ | 1.78 | 20.09 | 79.01 | 28.20 |
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