R ESULTS AND D ISCUSSION

3. S INGLE CELL FOR TANDEM SOLAR CELL : PEROVSKITE TOP CELLS AND SILICON

3.3. R ESULTS AND D ISCUSSION

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,

(Figure 3.20b). It causes the device performance to deteriorate. To overcome this, F8TBT was added.

As shown in the molecular structure of Figure 3.18., F8TBT is a conjugated n-type polymeric material, and when used as an additive to PCBM, it has high electron mobility and uniformly covers the entire PVK surface to improve electron extraction. By exploiting this advantage, we optimized the efficiency of an opaque PVK SC by applying the PCBM:F8TBT ratio in a single device, as shown in Figure 3.19.

The detailed photovoltaic performances are summarized in Table 3.9. As mentioned above, the reason for the smooth operation of the device is believed to be possible to form a uniform thin film, and it was confirmed using the SEM image in Figure 3.20 that a uniform thin film was formed. Figure 3.21a shows the structure of a semi-transparent PVK SC and the light irradiation direction. Figure 3.21b shows the device J–V curves under the same top illumination as the light irradiation direction of the tandem SC. Unlike an opaque PVK SC, because light comes in from the opposite direction, the order of the light path is in the order of a transparent electrode IZO/PCBM/PVK/PTAA/ITO glass. Therefore, the absorption of light in the short wavelength region of PCBM in the semi-transparent PVK and tandem SCs affects the absorption of the PVK. Therefore, as shown in the transmittance and absorbance data in Figure 3.22a and Figure 3.22b, the introduction of PCBM:F8TBT minimizes light absorption in the short wavelength region and increases device efficiency. The detailed photovoltaic performances are summarized in Table 3.10. and Table 3.11.

3.3.3. Silicon Bottom Cells

In this chapter, we will discuss the Si bottom cell technology for tandem SCs as shown in Figure 3.23a. First, as shown in Figure 3.23b, the doping process to form a p–n junction of a Si SC is discussed.

Second, as shown in Figure 3.23c, the process of introducing the back surface texture process to increase the absorption of long wavelengths of Si SCs is elucidated. Finally, as shown in Figure 3.23d, we propose the process of introducing SiNx with an adjustable refractive index for graded refractive index matching in tandem SCs. To manufacture the bottom cell for tandem SCs, the SOD method was introduced as a cost effective alternative. Aluminum was deposited using an E-beam on the back surface of the Si wafer, phosphorus coating was applied using a spin-on dopant on the front surface, and the emitter and BSF were simultaneously constructed by co-firing through a RTA process (Figure 3.24a).

The initial temperature at which the phosphorus doping occurs is 800 ℃. It involves heat treatment for 2 min in a nitrogen atmosphere, and the temperature variables are set to 800, 850, 900, and 950 ℃.

Meanwhile, the sheet resistances are set to 160, 100, 60, and 40 ohm/sq, respectively. The corresponding values were obtained and shown in Figure 3.24b. As a result of fabricating the Si bottom cell on the basis of this condition, the J–V curve and EQE data as shown in Figure 3.25a and Figure 3.25b were obtained. The detailed photovoltaic performances are summarized in Table 3.12.

In addition, as shown in Figure 3.26a and Figure 3.26b, an Si bottom cell with a textured back

surface was introduced to maximize the long wavelength absorption. The rear reflection occurred from the texture of the back surface. This increases the absorption of long wavelengths, while increasing the current density of the Si bottom cell by 0.33 mA/cm². The detailed photovoltaic performances are summarized in Table 3.13. The highest PCE of the Si bottom SC for the tandem SC is 10.07%, with a JSC of 23.94 mA/cm², VOC of 0.571 V, and FF of 73.69%.

In particular, optical loss appears large, owing to the difference in the refractive indices of PVK and Si.¹⁴ To compensate for this, we introduced a SiNx layer with a controlled refractive index (Figure 3.27a). The optimum refractive index of SiNx for minimizing the optical loss was obtained from the following equation:

𝐧SiNx≅ √(𝐧PVK× 𝐧Si)

The refractive index of the PVK is 2.4 and that of Si is 3.7. If this is applied to the above equation, the refractive index of Si nitride is approximately 2.98. However, the maximum value of the refractive index that can be realized using PECVD in our laboratory is 2.84. Therefore, this value is substituted into the following equation⁴¹:

thickness⁡of⁡SiNx≅ 𝝀 𝟒𝒏⁄ SiNx

It was calculated that 66 nm is the thickness required to minimize the reflectance at 750 nm when introducing the SiNx refractive index of 2.84 (Figure 3.27b). Considering that the refractive index of air is 1, this reflectance data appears slightly higher for Si nitride (refractive index of 2.84) than Si nitride (refractive index of 2.1). SiNx layer is composed of dielectric materials and requires a current path for the tandem SC. Therefore, we introduced a local contact design to construct the current path, as shown in Figure 3.28. Figure 3.29a displays the J–V curves of the Si bottom cell with the local contact design as the difference of contact area ratio. Figure 3.29b shows the device EQE of the Si bottom cell with the SiNx layer (n = 2.84, 66 nm).