4. MONOLITHIC PEROVSKITE / SILICON TANDEM SOLAR CELL
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Figure 4.1. (a) Schematic of monolithic PVK/Si tandem SC. Various techniques of tandemization, such as (b) current matching, (c) recombination layer, and (d) refractive index control for maximization of long-wavelength absorption.
Figure 4.2. Schematic of current matching for monolithic PVK/Si tandem SC, using a semi-transparent PVK SC with a tuned band gap and thickness of the PVK film.
Figure 4.3. J-V curve of the Si bottom SCs by using semi-transparent PVK SC filters with a (a) tuned band gap and (b) thickness of PVK film. EQE data of filtered Si bottom SC with different (c) tuned bandgaps and (d) thicknesses of PVK film.
Table 4.1. Summary of the device performance of the Si bottom cell, using semi-transparent PVK SC filters with a tuned band gap and thickness of PVK film.
PVK SC filter Jsc (mA/cm2) Voc (V) FF (%) Efficiency (%)
W/o Filter 23.94 0.571 73.69 10.07
1.4M Br 20% 9.24 0.548 73.62 3.73
1.4M Br 25% 9.36 0.548 73.61 3.78
1.4M Br 30% 9.63 0.549 73.64 3.89
1.2M Br 20% 9.33 0.548 73.63 3.76
1.3M Br 20% 9.26 0.548 73.61 3.73
Figure 4.4. Schematic of current matching for monolithic PVK/Si tandem SC, using the EQE measurement.
Figure 4.5. EQE data of monolithic PVK/Si tandem SC with (a) tuned band gap and (b) thickness of PVK film.
Figure 4.6. Schematic of recombination layer for monolithic PVK/Si tandem SC.
Figure 4.7. Transmittance data of ITO film with a thickness of 20 nm and various sputtering conditions:
(a) annealing temperature at 7 mTorr and (b) in-situ pressure at 250 °C.
Figure 4.8. XRD data of ITO film with 20 nm of thickness and various sputtering conditions: (a) annealing temperature at 7 mTorr and (b) in-situ pressure at 250 °C.
Figure 4.9. SEM image of ITO film with 20 nm of thickness and various sputtering conditions: (a–d) annealing temperature at 7 mTorr and (e–h) in-situ pressure at 250 °C.
Figure 4.10. AFM image of ITO film with a thickness of 20 nm, deposited in-situ at an annealing temperature of 250 ℃ and sputtering pressure of 7 mTorr.
Figure 4.11. Sheet resistance of ITO film with a thickness of 20 nm, deposited with a sputtering pressure of 7 mTorr as a function of annealing temperature.
Figure 4.12. (a) J-V curve and (b) EQE data of monolithic PVK/Si tandem SCs by using ITO film as a recombination layer with a thickness of 20 nm, deposited with a sputtering pressure of 7 mTorr as a function of annealing temperature.
Table 4.2. Summary of the device performance of monolithic PVK/Si tandem SCs using ITO film as a recombination layer with a thickness of 20 nm, deposited with a base pressure of 7 mTorr as a function of annealing temperature.
ITO 20 nm Jsc (mA/cm2) Voc (V) FF (%) Efficiency (%)
RT 14.51 1.69 67.74 16.61
in-situ 150 ℃ 14.53 1.71 75.92 18.86
in-situ 250 ℃ 14.57 1.71 76.01 18.94
in-situ 350 ℃ 14.55 1.71 75.86 18.87
Figure 4.13. Schematic of strategy to enhance absorption of long-wavelength region for a monolithic PVK/Si tandem SC by using graded refractive index matching.
Figure 4.14. Schematic of graded refractive index and (b) reflectance data of monolithic PVK/Si tandem SCs to enhance the absorption of the long-wavelength region, using a 66 nm SiNx layer and a refractive index of 2.84.
Figure 4.15. (a) J-V curve and (b) EQE data of monolithic PVK/Si tandem SCs to enhance the absorption of the long-wavelength region, using a 66 nm SiNx layer with a refractive index of 2.84.
Table 4.3. Summary of the device performance of monolithic PVK/Si tandem SCs to enhance the absorption of the long-wavelength region, using a 66 nm SiNx layer with a refractive index of 2.84.
Tandem SC Jsc (mA/cm2) Voc (V) FF (%) Efficiency (%)
Forward sweep 16.39 1.75 80.12 22.98
Reverse sweep 16.40 1.75 80.23 23.02
Figure 4.16. Total reflectance of monolithic PVK/Si tandem SCs to enhance the absorption of the long- wavelength region, using a 66 nm SiNx layer with a refractive index of 2.84.
Figure 4.17. (a) Optical image and (b) schematic of glass-to-glass encapsulation for stability test.
Figure 4.18. Light soaking test at AM1.5G based on xenon lamp. The decay graph of PVK/Si tandem SC performance as function of time.
Figure 4.19. Thermal stress test at 85 °C in a N2 glove box. The decay graph of PVK/Si tandem SC performance as function of time.
Figure 4.20. Damp heat test at 85 °C/85 RH%. The decay graph of PVK/Si tandem SC performance as function of time.
Figure 4.21. PERC-cell-based tandem SC was fabricated by applying all of the tandem element technology, except graded refractive index matching.
5
S UMMARY AND S UGGESTIONS FOR F UTURE W ORKS
5.1.SUMMARY
This dissertation presents lossless monolithic PVK/Si tandem SC for the realization of clean and sustainable energy based on solar energy. The p-Si homojunction SC, which is currently the most popular in the SC market, was applied as the bottom cell, and the PVK top cell of the pin structure was applied to realize a lossless tandem SC.
In Chapter 2, we discuss the optical properties of a tandem SC with a top PVK SC based on triple cation and a p-Si homojunction bottom SC based on Al-BSF to realize tandem SC using the advantages of commercialization potential. In the study, the current matching point was determined by incorporating the PVK thickness and band gap as variables in the optical simulation. When the band gap and thickness of the PVK were 1.65 eV and 500 nm, respectively, the current density of the tandem SC was 16.84 mA/cm2. Additionally, we assumed the voltage deficit of each top and bottom cell to predict the efficiency of the tandem SC empirically; its fill factor was 80%, and the predicted maximum efficiency was 23.71%. The factors inducing optical loss—that affect the current density of our tandem device—are discussed based on the optical simulation. First, surface reflections occurred in the air and on the device surface. Second, the light absorption of the IZO layer was used as a transparent electrode.
Finally, light absorption in the short wavelength range in the PCBM was used as the ETL layer.
Therefore, it is necessary to develop an ARC film with a textured surface and gradually control the refractive index from the air to the device to reduce reflection on the device surface. Additionally, there is a need for a technology that can reduce sheet resistance while improving the transmittance of the IZO layer. Lastly, ETL technology is required to replace materials with light absorption in the short wavelength range, such as PCBM.
In Chapter 3, we discuss the design rules of a single cell for lossless monolithic tandem SCs; p–
i–n PVK top cell and p-Si homojunction bottom cell with Al-BSF layers were fabricated using cost- effective solution processes. To optimize PVK top cell, first, the wide bandgap PVK technology was introduced by controlling the composition of the PVK using cations and halide. Second, we found the optimal HTL considering the energy level for obtaining highly efficient PVK SCs. Third, to reduce
recombination from the grain boundary of the PVK film, MACl was introduced to increase the grain size and to ensure high quality of the PVK film. Fourth, to prevent recombination arising from the surface defects of the PVK thin film, BABr was introduced to form a 3D/2D PVK structure to minimize recombination caused by surface defects. Finally, an ETL layer was developed that minimized light absorption of PCBM, which was evenly coated even at a relatively lower thickness than in case of the existing PCBM by adding F8TBT additive to the PCBM. Through the inclusion of the F8TBT additive to PCBM, we developed an ETL layer that minimizes light absorption, thus enabling uniform coating even with a low PCBM thickness. The optimized opaque PVK top cell exhibited the highest PCE of 18.99%, with a JSC of 20.21 mA/cm2, VOC of 1.18 V, and FF of 79.63%. By applying this ETL to a semi- transparent PVK top cell, the efficiency of the semi-transparent PVK top cell was increased by reducing light absorption in the short wavelength range from the PCBM. The optimized semi-transparent PVK top cell exhibited the highest PCE of 15.83% with a JSC of 17.13 mA/cm2, VOC of 1.17 V, and FF of 78.97%. For the first time, in a Si bottom cell for tandem SC, the emitter doping technology through the RTA process was optimized based on a cost-effective solution. Second, the absorption of long wavelength was increased by introducing a textured structure on the back surface. Finally, to increase the efficiency of the tandem SC, SiNx, whose refractive index was adjusted to a value between the refractive index of PVK and Si, was inserted. This increased the absorption of long wavelengths, along with an increase in the voltage due to the passivation effect. The optimized Si bottom cell for the tandem SC exhibited the highest PCE of 13.17%, with a JSC of 29.52 mA/cm2, VOC of 0.607 V, and FF of 73.48%.
In Chapter 4, we present the developed lossless tandem SC. First, we developed a current matching method for optimizing the current density of tandem SCs. Second, we determined the optimized conditions according to the sputtering temperature and pressure conditions of the ITO layer applied for recombination to drive the tandem SC. Third, we implemented a tandem SC based on an Al-BSF Si SC based on optical design with a tandem SC that minimized light loss by applying graded refractive index matching technology. JSC, VOC, FF, and efficiencies of the champion device with graded refractive index matching applied are 16.40 mA/cm2, 1.75 V, 80.23%, and 23.02%. Fourth, long-term stability of the tandem SCs was determined based on tests such as light soaking test, thermal stress test, and damp heat test by introducing glass to glass encapsulation technology. Most of the long-term stability tests maintained more than 90% of the initial efficiency. Finally, a tandem SC based on a PERC Si SC was fabricated among p-Si homojunction SCs that occupy the mainstream of the Si SC market.
Although graded refractive index matching technology was not applied, JSC, VOC, FF, and efficiencies of the champion device were 17.47 mA/cm2, 1.725 V, 79.34%, and 23.91%. In the case of PERC cells, since the long wavelength absorption is superior to the Al-BSF cell, there is a significant increase in efficiency, and it is expected that more than 25% efficiency when applied to graded refractive index matching in the future.
Overall, we i) established lossless PVK/Si tandem SC design by using optical simulation, ii) optimized PVK top cell and Si bottom cell for tandem SC, and iii) demonstrated lossless PVK/Si tandem SC. We highlight the importance of our lossless PVK/Si tandem SC based on the p-Si homojunction bottom cell, in both fundamental and commercialization aspects. When tandem SC is based on p-Si homojunction Si SC, which occupies the mainstream of the SC market, it has a very high possibility of commercialization. Therefore, the large area technology of PVK that can be implemented on a textured Si substrate with minimal optical loss in the future will receive considerable attention in the field of PVK/Si tandem SC research.
5.2.SUGGESTIONS FOR FUTURE WORKS
Recently, the measured efficiencies of 2T PVK/Si tandem SCs were 29.5%, and the area of large- scale PVK layer deposition process increased sufficiently to cover the size of commercial Si SC wafers.
Although the efficiency of 2T PVK/Si tandem SCs is higher than 29%, which is the theoretical limit of Si single-junction SCs, and the rapid development of large-scale deposition processes for PVK layers are promising, the cost of PVK/Si tandem SCs should be considered for commercialization. According to the recently reported cost analysis for PVK SCs and PVK/Si tandem SCs, the process equipment and maintenance costs of the evaporation process may be low with high throughput in mass production.
PVK SCs have generally reported high material cost issues rather than processing costs in cost analysis.
In particular, the cost barrier of PVK SCs is identified by expensive organic charge transport materials, such as spiro-OMeTAD, PTAA, and PC60BM.1-3 The cost of high-performance organic charge transport materials is usually expensive owing to the complex synthesis steps and additional cost for ultrahigh purity. As an alternative, inorganic charge transport materials, such as NiO, CuSCN, SnO2, and Nb2O5, are much cheaper than organic materials, and several large-scale compatible processed inorganic CTLs have been reported in PVK SCs.4-7
From the perspective of ecofriendly renewable energy, it is necessary to solve the toxicity and longevity of PVK/Si tandem SCs, which are the drawbacks of PVK, to gain an advantage over other energy sources. Most PVK SCs are based on toxic lead (Pb) because of its excellent optoelectronic properties. Although the total amount of Pb in PVK/Si tandem SCs is very low, studies have reported that the Pb in halide PVK is ten times more dangerous than the Pb that already exists on Earth.8 To lower its toxicity, Pb-free and less-Pb PVK based SCs have been researched using safe tin (Sn)-based PVKs.9-11 However, Sn-based PVK SCs have lower efficiencies and faster degradation than those of Pb-based PVK SCs due to phase instability and easy oxidation from Sn2+ to Sn4+. In addition, new approaches have been suggested to prevent Pb leakage in PVK solar modules by trapping Pb with cation-exchange resins containing abundant and inexpensive Ca2+ and Mg2+.12 SCs are exposed to