2. O PTICAL S IMULATION TO O PTIMIZE P EROVSKITE /S ILICON T ANDEM S OLAR C ELLS
2.5. R EFERENCES
1. Eperon, G. E.; Hörantner, M. T.; Snaith, H. J., Metal halide perovskite tandem and multiple- junction photovoltaics. Nature Reviews Chemistry 2017, 1 (12), 1-18.
2. Bush, K. A.; Manzoor, S.; Frohna, K.; Yu, Z. J.; Raiford, J. A.; Palmstrom, A. F.;
Wang, H.-P.; Prasanna, R.; Bent, S. F.; Holman, Z. C.; McGehee, M. D., Minimizing Current and Voltage Losses to Reach 25% Efficient Monolithic Two-Terminal Perovskite–Silicon Tandem Solar Cells. ACS Energy Letters 2018, 3 (9), 2173-2180.
3. Lal, N. N.; Dkhissi, Y.; Li, W.; Hou, Q.; Cheng, Y.-B.; Bach, U., Perovskite Tandem Solar Cells. Advanced Energy Materials 2017, 7 (18), 1602761.
4. Leijtens, T.; Bush, K. A.; Prasanna, R.; McGehee, M. D., Opportunities and challenges for tandem solar cells using metal halide perovskite semiconductors. Nature Energy 2018, 3 (10), 828-838.
5. Bonnet-Eymard, M.; Boccard, M.; Bugnon, G.; Meillaud, F.; Despeisse, M.; Haug, F.; Ballif, C. In Current matching optimization in high-efficiency thin-film silicon tandem solar cells, 2013 IEEE 39th Photovoltaic Specialists Conference (PVSC), 16-21 June 2013; 2013; pp 0184-0187.
6. Fan, S.; Yu, Z. J.; Hool, R. D.; Dhingra, P.; Weigand, W.; Kim, M.; Ratta, E. D.;
Li, B. D.; Sun, Y.; Holman, Z. C.; Lee, M. L., Current-Matched III–V/Si Epitaxial Tandem Solar Cells with 25.0% Efficiency. Cell Reports Physical Science 2020, 1 (9), 100208.
7. Ren, Z.; Liu, H.; Liu, Z.; Tan, C. S.; Aberle, A. G.; Buonassisi, T.; Peters, I. M., The GaAs/GaAs/Si solar cell – Towards current matching in an integrated two terminal tandem. Solar Energy Materials and Solar Cells 2017, 160, 94-100.
8. Wei, J.; Shao, Z.; Pan, B.; Chen, S.; Hu, L.; Dai, S., Toward Current Matching in Tandem Dye-Sensitized Solar Cells. Materials 2020, 13 (13), 2936.
9. Köhnen, E.; Jošt, M.; Morales-Vilches, A. B.; Tockhorn, P.; Al-Ashouri, A.; Macco, B.; Kegelmann, L.; Korte, L.; Rech, B.; Schlatmann, R.; Stannowski, B.; Albrecht, S., Highly efficient monolithic perovskite silicon tandem solar cells: analyzing the influence of current mismatch on device performance. Sustainable Energy & Fuels 2019, 3 (8), 1995-2005.
10. Jiang, Y.; Almansouri, I.; Huang, S.; Young, T.; Li, Y.; Peng, Y.; Hou, Q.;
Spiccia, L.; Bach, U.; Cheng, Y.-B., Optical analysis of perovskite/silicon tandem solar cells. Journal of Materials Chemistry C 2016, 4 (24), 5679-5689.
11. Albrecht, S.; Saliba, M.; Correa Baena, J. P.; Lang, F.; Kegelmann, L.; Mews, M.;
Steier, L.; Abate, A.; Rappich, J.; Korte, L.; Schlatmann, R.; Nazeeruddin, M. K.; Hagfeldt, A.; Grätzel, M.; Rech, B., Monolithic perovskite/silicon-heterojunction tandem solar cells processed at low temperature. Energy & Environmental Science 2016, 9 (1), 81-88.
12. Persson, N.-K.; Inganäs, O., Organic tandem solar cells—modelling and predictions. Solar energy materials and solar cells 2006, 90 (20), 3491-3507.
13. Gil-Escrig, L.; Roß, M.; Sutter, J.; Al-Ashouri, A.; Becker, C.; Albrecht, S., Fully Vacuum-Processed Perovskite Solar Cells on Pyramidal Microtextures. Solar RRL n/a (n/a), 2000553.
14. Hou, Y.; Aydin, E.; De Bastiani, M.; Xiao, C.; Isikgor, F. H.; Xue, D.-J.; Chen, B.;
Chen, H.; Bahrami, B.; Chowdhury, A. H.; Johnston, A.; Baek, S.-W.; Huang, Z.; Wei, M.;
Dong, Y.; Troughton, J.; Jalmood, R.; Mirabelli, A. J.; Allen, T. G.; Van Kerschaver, E.;
Saidaminov, M. I.; Baran, D.; Qiao, Q.; Zhu, K.; De Wolf, S.; Sargent, E. H., Efficient tandem solar cells with solution-processed perovskite on textured crystalline silicon. Science 2020, 367 (6482), 1135.
15. Chen, B.; Yu, Z. J.; Manzoor, S.; Wang, S.; Weigand, W.; Yu, Z.; Yang, G.; Ni, Z.; Dai, X.; Holman, Z. C.; Huang, J., Blade-Coated Perovskites on Textured Silicon for 26%- Efficient Monolithic Perovskite/Silicon Tandem Solar Cells. Joule 2020, 4 (4), 850-864.
16. Sahli, F.; Werner, J.; Kamino, B. A.; Bräuninger, M.; Monnard, R.; Paviet-Salomon, B.; Barraud, L.; Ding, L.; Diaz Leon, J. J.; Sacchetto, D.; Cattaneo, G.; Despeisse, M.;
Boccard, M.; Nicolay, S.; Jeangros, Q.; Niesen, B.; Ballif, C., Fully textured monolithic perovskite/silicon tandem solar cells with 25.2% power conversion efficiency. Nature Materials 2018, 17 (9), 820-826.
17. Hou, F.; Han, C.; Isabella, O.; Yan, L.; Shi, B.; Chen, J.; An, S.; Zhou, Z.;
Huang, W.; Ren, H.; Huang, Q.; Hou, G.; Chen, X.; Li, Y.; Ding, Y.; Wang, G.; Wei, C.; Zhang, D.; Zeman, M.; Zhao, Y.; Zhang, X., Inverted pyramidally-textured PDMS antireflective foils for perovskite/silicon tandem solar cells with flat top cell. Nano Energy 2019, 56, 234-240.
18. Kim, C. U.; Yu, J. C.; Jung, E. D.; Choi, I. Y.; Park, W.; Lee, H.; Kim, I.; Lee, D.-K.; Hong, K. K.; Song, M. H.; Choi, K. J., Optimization of device design for low cost and high efficiency planar monolithic perovskite/silicon tandem solar cells. Nano Energy 2019, 60, 213-221.
19. Eperon, G. E.; Stranks, S. D.; Menelaou, C.; Johnston, M. B.; Herz, L. M.; Snaith, H.
J., Formamidinium lead trihalide: a broadly tunable perovskite for efficient planar heterojunction solar cells. Energy & Environmental Science 2014, 7 (3), 982-988.
20. Deng, Y.; Peng, E.; Shao, Y.; Xiao, Z.; Dong, Q.; Huang, J., Scalable fabrication of efficient organolead trihalide perovskite solar cells with doctor-bladed active layers. Energy &
Environmental Science 2015, 8 (5), 1544-1550.
21. Jošt, M.; Köhnen, E.; Morales-Vilches, A. B.; Lipovšek, B.; Jäger, K.; Macco, B.;
Al-Ashouri, A.; Krč, J.; Korte, L.; Rech, B.; Schlatmann, R.; Topič, M.; Stannowski, B.;
Albrecht, S., Textured interfaces in monolithic perovskite/silicon tandem solar cells: advanced light management for improved efficiency and energy yield. Energy & Environmental Science 2018, 11 (12), 3511-3523.
22. Günes, S.; Baran, D.; Günbas, G.; Durmus, A.; Fuchsbauer, A.; Sariciftci, N. S.;
Toppare, L., A green neutral state donor–acceptor copolymer for organic solar cells. Polymer Chemistry 2010, 1 (8), 1245-1251.
23. Lipovšek, B. CROWM. http://lpvo.fe.uni-lj.si/en/software/crowm/.
Table 2.1. Refractive indices (n, k), thickness, method of determination, and source.
Material/Layer Thickness (nm) Method Source
PDMS 100000 Adapted from literature Refractive index.info
IZO 150 Spectral ellipsometry This work
ZnO nps 30 Adapted from literature Refractive index.info
PCBM 30 Spectral ellipsometry This work
PVK 200~600 Adapted from literature PV lighthouse
PTAA 15 Spectral ellipsometry This work
ITO 25 Spectral ellipsometry This work
Si 525000 Adapted from literature Refractive index.info Al-BSF/Ag 1000 Adapted from literature Refractive index.info
Figure 2.1. Introduction of optical simulation for monolithic PVK/Si tandem SC.23
Figure 2.2. Schematic of monolithic PVK/Si tandem SC proposed in this dissertation.
Figure 2.3. Simulated EQE data of the monolithic PVK/Si tandem SC with various PVK band gaps and thicknesses.
Figure 2.4. Current matching point of monolithic PVK/Si tandem SC with various PVK band gaps and thicknesses based on simulation.
Figure 2.5. Absorption data of functional layer of monolithic PVK/Si tandem SC.
Table 2.2. Summary of current density loss due to absorption in functional layers.
Material/Layer Jsc (mA/cm2) Jsc relative (%)
Total reflection 2.43 5.22
Air 0.01 0.02
PDMS 0.76 1.63
IZO 5.53 11.88
ZnO nps 0.13 0.28
PCBM 1.85 3.98
PVK 16.90 36.32
PTAA 0.01 0.02
ITO 0.14 0.30
SiNx 0.01 0.02
Si 16.84 36.19
Al 1.92 4.13
Figure 2.6. Expected current matching point and efficiency of monolithic PVK/Si tandem SC with different PVK band gaps and thicknesses.
3
S INGLE C ELL F OR T ANDEM S OLAR C ELL :
P EROVSKITE T OP CELLS AND S ILICON B OTTOM
C ELLS
This chapter includes the following published contents:
Kim, C. U.; Yu. J. C.; Jung, E. D.; Choi, I. Y.; Park, W. J.; Lee, H. M.; Kim, I.; Lee, D. K.; Hong, K. K.; Song, M. H.; Choi, K. J. Nano Energy 2019, 60, 213–221.
DOI: 10.1016/j.nanoen.2019.03.056. Reproduced with permission of Elsevier.
3.1.INTRODUCTION
Recently, the efficiency of Si SCs has reached 26.6%, which is very close to the theoretical limit.1 One of the ways to overcome the theoretical efficiency limit of a single-junction SC is to utilize multi- junction tandem structures.2 Among these, PVK/Si tandem SCs have a high potential for commercialization due to the high efficiency and excellent price competitiveness of PVK and Si SC technologies.3, 4 Tandem SCs can be fabricated into many different structures, such as mechanically stacked 4-terminal tandem and monolithic 2-terminal tandem structures. Compared to mechanically stacked 4-terminal tandem SCs, monolithic 2-terminal tandem SCs have the advantage of a simple monolithic one-body structure and low parasitic light absorption but require sophisticated current matching techniques.4, 5
Although the efficiency of stand-alone Si heterojunction (SHJ) cells is already more than 23%
and the efficiency enhancement via tandemization is only marginal, most monolithic PVK/Si tandem cells use SHJ cells.6-10 In addition, SHJ cells are expensive owing to complicated processes, and even if they are successfully fabricated as tandem cells, the possibility of commercialization might be very low.11, 12 Furthermore, many previous studies have demonstrated the importance of minimizing parasitic absorption in each layer to achieve higher current densities and efficiencies.13, 14 To reduce parasitic light absorption, physical vapor deposition techniques, such as atomic layer deposition, sputtering, or thermal evaporation, have been widely adopted, which require expensive equipment and thus are not suitable for commercialization.12
Over the past few decades, p-type c-Si SCs have accounted for more than 90% of the global market share, and more than 70% of p-type c-Si SCs are based on homojunctions with Al-BSFs.15
Therefore, if a p-type Al-BSF homojunction Si SC is used as the bottom cell for a PVK/Si tandem cell, then not only can the current production facilities be utilized, but also the efficiency enhancement due to tandemization can be maximized. However, there are few reports on tandem cells based on p-type Al-BSF homojunction Si SCs.
In this chapter, we present the design rules of a single cell for lossless monolithic tandem SCs;
p–i–n PVK and p-Si homojunction bottom cells with Al-BSF layers were fabricated using cost-effective solution processes. 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 the recombination from the grain boundary of the PVK film, MACl was introduced to increase the grain size and high quality of the PVK film. Fourth, to prevent recombination arising from the surface defects of the PVK thin film, n-BABr was introduced to form a 3D/2D PVK structure to minimize recombination caused by surface defects.
Finally, an ETL layer was developed that minimizes the light absorption of PCBM, which is evenly coated even at a relatively lower thickness than 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 the light absorption, thus enabling uniform coating even with a low PCBM thickness. By applying this ETL to a semi-transparent PVK SC, the efficiency of the semi-transparent PVK SC was increased by reducing light absorption in the short wavelength range from the PCBM. For the first time in 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 monolithic PVK/Si tandem SC, SiNx, whose refractive index was adjusted to a value between the refractive index of the PVK and Si, was inserted. This increased the absorption of long wavelengths, along with an increase in the VOC due to the passivation effect.
3.2.EXPERIMENTAL METHODS
3.2.1. Experimental Details of Perovskite Top Cell
The PVK SCs were fabricated in a standard arrangement of ITO/HTL/PVK/PC60BM/ZnO/Ag.
The HTLs, such as PEDOT:PSS (AI4083) and PTAA (6 mg/ml in chlorobenzene (CB)), were spin- coated onto the ITO substrate at 5000 rpm and annealed at 140 °C and 100 °C for 10 min, respectively.
The NiOx layer was spin-coated following the reported method.16 The PVK precursor solution of (FAPbI3)1-x(MAPbBr3)x was prepared by dissolving FAI, MAI, MABr, PbI2, and PbBr2 in a DMF/DMSO (4:1 v/v) co-solvent, and the mixed solution was prepared using MAPbI3 (1.2 M), FAPbI3
(1.2 M), and MAPbBr3 (1.2 M) precursor solutions. In the case of Br 25%, the PVK precursor solution of Cs0.05FA0.80MA0.15Pb(I0.75Br0.25)3 was prepared by dissolving 0.07 mmol CsI (TCI), 0.28 mmol MABr
DMF/DMSO (4:1 v/v). MACl at 10 mg/mL was added to the PVK precursor solution. The PVK solution was spin-coated in a one-step program at 5000 rpm at ramping times of 1.7 and 30 s. During this period, 300 μL of ethyl acetate was poured onto the spinning substrate 20 s prior to the end of the program. The substrates were then annealed at 100 °C for 1 h in a nitrogen-filled glove box. BABr at 1 mg/mL was prepared by dissolving isopropanol. As a 2D PVK layer, BABr was deposited by spin- coating (5000 rpm for 30 s) with dynamic dropping and annealed at 100 °C for 10 min in a nitrogen- filled glove box. PC60BM (1-Material):F8TBT (Lumtec) = 1:0.22 solution dispersed in CB (10 mg/ml) and ZnO nanoparticle solution (Avantama) were sequentially spin-coated onto the PVK layer at 2000 and 4000 rpm for 45 and 20 s, respectively. After ZnO coating, the sample was annealed at 100 °C for 1 min in a nitrogen-filled glove box. The SCs were completely fabricated by thermal deposition of 100 nm Ag with a device area of 0.16 cm2.
For the semi-transparent device, the IZO layer (150 nm) was grown on a ZnO surface by a RF magnetron sputter at room temperature with a sputtering power of 80 W. Then, the Ag metal front electrode was deposited by e-beam evaporation. Finally, as ARC film, the LiF antireflection layer (150 nm) was deposited by thermal evaporation or textured PDMS ARC film was attached on the cell. The devices area, which was defined by shadow mask, was 0.25 cm2.
3.2.2. Experimental Details of Silicon Bottom Cell
Monocrystalline Si wafers (1–5 Ω cm, p-type, CZ, 525 µm) were cleaned using the RCA cleaning procedure, and oxide was removed using a buffered oxide etchant solution. In case of the back-side pyramidal textured silicon SC, silicon surface was etched by KOH/IPA solution. The phosphorus SOD (Filmtronics SOD P507) was spin-coated on the c-Si wafers, and on the other side, Al (2 µm) was evaporated. During heat treatment for 2 min using a RTA system to control the doping level, the n+
emitter was formed with a sheet resistance of 100 Ω/sq on the front side and p+ Al BSF was formed on the rear side of the Si wafers. The residual phosphorous silicate glass was removed by hydrogen fluoride.
In case of local contact design, a 66 nm-SiNx passivating layer which has refractive index of 2.84 at the wavelength of 750 nm was deposited by plasma enhanced chemical vapor deposition (PEH-600, SORONA) at 350℃. To make current path, local contact was patterned through photolithography. And then, contact area was etched by ICP etch. The Ti/Pd/Ag was deposited in the opening area.
Subsequently, the Ag metal back electrodes (1 µm) with the insertion of a Ti buffer layer (20 nm) were deposited by e-beam evaporation. Finally, the ITO recombination layer was grown on the top surface using a RF magnetron sputter at room temperature with a sputtering power of 120 W.
3.2.3. Characterization Methods
The J-V characteristics of the SCs were determined using a Keithley 4200 source measuring unit under simulated AM 1.5G light (Oriel Sol3A Solar Simulator) at 100 mW cm−2, with a metal aperture
and a scan rate of 5 mV s-1 in the direction from the open-circuit voltage to the short-circuit current.
The light intensity was calibrated using a KG-5 Si diode. The bias voltage for the steady-state measurements was chosen as the average of the maximum power point voltage of the J-V measurement.
The EQE of the cells was measured using an incident-photon-to-current conversion measurement system Quantx-300 (Newport) at a chopping frequency of 4 Hz. The EQE response was calibrated using certified reference cells for the 300–1200 nm wavelength regions. To measure the EQE of the perovskite sub cell, the silicon sub cell was saturated using a white bias. To maintain short circuit conditions, a bias voltage of 0.3 V was applied. The silicon sub cell was measured by saturating the perovskite sub cell with white bias with 650 nm cut off filter and applying a bias voltage of 1 V. All J-V and EQE measurements were conducted in air without encapsulation. SE-mode scanning electron microscopy measurements were performed using a cold field-emission scanning electron microscope (Hitachi SU- 8200) with an accelerating voltage of 1 kV. The sheet resistances were measured using a four-point probe (CMT-2000N, AIT). The absorption, reflectance and transmittances were measured using an UV-visible-near-infrared spectrometer (Cary 5000, Agilent). Time-resolved and steady-state photoluminescence spectra were measured using a time-correlated single-photon counting setup (FluoTime 300). The crystal structure of samples was identified using X-ray diffraction patterns. A step size of 0.01° was used, and the acquisition rate was as high as 5 min deg-1 using an X-ray diffractometer (Rigaku D/Max 2500V/PC) equipped with a Cu Kα source. The effective lifetime was characterized using photoconductance decay or photoluminescence techniques and measured using a Sinton WCT‐ 120 in the transient mode.
3.3.RESULTS AND DISCUSSION
3.3.1. Wide Band Gap Engineering of Top Cells
As shown in Figure. 3.1a, one of the typical PVK SC technologies to be used as the top cell in tandem SCs is PVK bandgap engineering. This is accomplished along with suitable energy leveling, as shown in Figure. 3.1b.12
To fabricate PVK films with a higher Br to I ratio without deteriorating the quality of the films, we adjusted the composition of Cs0.05FA0.80MA0.15Pb(I1-xBrx)3. The addition of CsI to (FAPbI3)1- x(MAPbBr3)x is known to help alleviate yellow phase, thereby favoring the formation of more pure and uniform PVK phases.17-19 Figure 3.8a shows the UV-vis absorption spectra of the PVK. From the Tauc plots, the band gap energy of these PVK films was confirmed to increase from 1.65 to 1.71 eV as the Br composition increased from 20 to 30%. Figure 3.8b shows the J–V curves of the PVK SC fabricated with band gap-tuned Cs0.05FA0.80MA0.15Pb(I1-xBrx)3 PVK films. The detailed photovoltaic performances are summarized in Table 3.6. The photovoltaic performances of the tandem cells based on
and then began to deteriorate at the composition of 0.3. In (FAPbI3)1-x(MAPbBr3)x, when the concentration of Br increases, a transition from trigonal to cubic structure occurs, causing phase instability, which in turn leads to a yellow phase. By adding Cs cations, lattice instability was reduced, and the device was uniformly driven even when the Br concentration was 0.2 to 0.3. The top-view SEM images also showed uniform PVK film morphologies at compositions (x = 0.20, 0.25, 0.30) in the Cs0.05FA0.80MA0.15Pb(I1-xBrx)3 PVK layers, as shown in Figure 3.9.
In the SEM image of Cs0.05FA0.80MA0.15Pb(I1-xBrx)3, most of the grain sizes of PVK are small. It is known that ion migration at the grain boundary of PVK is significantly faster than that of bulk ion migration.20, 21 In addition, it is known that defects and impurities are concentrated in the grain boundary of the PVK film.22, 23 Therefore, the device efficiency is degraded, owing to the trap-assisted recombination of the grain boundary of the PVK thin film.24 Various methods have been proposed to increase the grain size. Subsequently, it was confirmed that the grain size increased when chloride was added in MA- and FA-based PVKs. Therefore, Various concentrations of MACl were added to Cs0.05FA0.80MA0.15Pb(I1-xBrx)3 to increase the grain size. Figure 3.10a shows their UV-vis absorption spectra. From the Tauc plots, the band gap energy of these PVK films was confirmed to shift slightly by ~0.01 eV as the MACl concentration increased from 0 to 20 mg/mL. Figure 3.10b shows the J–V curves of PVK SC fabricated with the MACl concentration increased from 0 to 20 mg/mL. The detailed photovoltaic performances are summarized in Table 3.7. As shown in Figure 3.11a, we used XRD to analyze the crystallinity of the MACl-doped PVK thin film. The typical peak for PVKs of all compositions is known to be ~14°.25 As the concentration of MACl gradually increased, the crystallinity of the PVK improved. In addition, it was confirmed that the shift of the XRD pattern of the PVK thin film did not appear even when MACl was added. However, as a result of observing the peak position of (110) in detail as shown in the Figure 3.11b, a blue shift occurred as the concentration of MACl increased. As the concentration of MACl increases, the ratio of the cation composition of CS/FA/MA changes. Moreover, the composition ratio of the halide composition of I/Br changes as the amount of Cl increases. Therefore, when the concentration of MACl increases, the (110) peak of the PVK containing MA is shifted, owing to a slight change in the composition ratio of the PVK. As shown in Figure 3.12, as the amount of MACl increased, the grain size of the PVK continued to increase.
Accordingly, the recombination caused by the small grain boundary of the triple cation-based PVK is curbed by MACl. It forms a large grain boundary, thereby reducing recombination and improving the efficiency of the device. However, recombination losses due to defects present on the device surface persist. The subsequent chapter will discuss how this is overcome.
3.3.2. Interfacial layer Engineering of Top Cells
Generally, it is known that numerous defects exist on the surface of 3D PVK.26-29 Therefore,
although the recombination occurring at the grain boundary was minimized by using MACl, the recombination due to defects still present on the 3D PVK surface causes a voltage drop in the device.30 To prevent this, various studies related to passivation, such as ALD passivation,31, 32 ETL side passivation,33, 34 organic passivation,35 polymer passivation,36 organic halide treatment,37 and bulk organic halide passivation38 applied to the PVK surface were conducted. We introduced n-BABr to realize 3D/2D PVK and obtained a uniform thin film using the dynamic dropping method. Figure 3.13 illustrates the application of a 2D material, n-BABr, to Cs0.05FA0.80MA0.15Pb(I0.8Br0.2)3. A partial proton transfer process occurred at the A sites of the top 3D PVK lattice, leading to the formation of 2D BAy(CsxFA1−x)1−yPb2(I0.8Br0.2)7 in some regions. This was accomplished by introducing alkylammonium cation acting as a spacer. Figure 3.14a shows their UV-vis absorption spectra. From the Tauc plots, the band gap energy of these PVK films was confirmed to become stable as the n-BABr concentration increased from 0.5 to 2 mg/mL. Figure 3.14b shows the J–V curves of the PVK SC fabricated with the n-BABr, whose concentration increased from 0.5 to 2 mg/mL. The detailed photovoltaic performances are summarized in Table 3.8. Figure 3.15a shows the typical crystal plane diffraction of 3D PVK in XRD analysis data (100), (110), (111), and (200). The new peaks marked with a star are ascribed to BABr. 2D PVK is not apparent in the overview XRD scan for the very thin interlayer prepared from a low‐concentration BABr solution of 1 mg/mL. Because XRD data are obtained from diffraction in the superlattice of 2D PVK, the peak is determined according to the number of stacked 2D PVKs formed according to the concentration of BABr. In the case of 1 mg/mL optimized as shown in Figure 3.15b, a 3D/2D PVK film with a thin layer of 2D PVK has a peak with a slight difference compared to the 3D PVK thin film. As shown in Figure 3.16a and Figure 3.16b, 3D/2D PVK reduces surface defects, owing to the passivation effect compared to 3D PVKs, thereby reducing voltage loss due to recombination. PL and TRPL measurements were performed to analyze the increased carrier lifetime attributed to passivation. When 2D PVK was applied, surface defects of 3D PVK were reduced, owing to the passivation effect, and the overall PL intensity increased compared to that of 3D PVK. There was an increase in TRPL lifetime at an optimized 2D PVK concentration of 1 mg/mL. This reduced the nonradiative recombination, owing to the passivation effect by stacking 2D PVK on the surface defects of the 3D PVK thin film. Figure 3.17 shows that as the amount of BABr increases, the 2D structure of PVK thickens on the surface of the PVK. BABr reacts with unstable PVK due to defects on the 3D surface to create a 3D/2D PVK structure. This reduces the recombination loss caused by defects to maximize device efficiency. Because of its thickness, it acts as a series resistance in the device, causing a sharp drop in the device efficiency.
Optical simulation of the existence of short wavelength absorption of the PCBM layer used as the ETL is already known.39, 40 Therefore, one of the simple ways to reduce the absorption of short wavelengths is to reduce the thickness by adjusting the PCBM concentration. If the thickness reduces,