4. MONOLITHIC PEROVSKITE / SILICON TANDEM SOLAR CELL
4.3. R ESULTS AND D ISCUSSION
4.3.1. Current Matching of Tandem Solar Cell
As shown in Figure 4.1a, PVK/Si tandem SCs were fabricated. To implement a tandem SC, current matching technology was first introduced, as shown in Figure 4.1b; then, a recombination layer was implemented, as shown in Figure 4.1c. Finally, graded refractive index matching technology was utilized, as shown in Figure 4.1d.
Figure 4.2 illustrates a method for predicting the current matching point of a tandem SC by analyzing the change in the current density value of the Si bottom SC, using a semi-transparent PVK SC as a filter. Figure 4.2a and Figure 4.2b present the J-V curves of the Si bottom SC when a semi- transparent PVK SC, comprising a PVK bandgap and varying thickness, is used as a filter. As can be seen from Table 4.1, among the parameters of the Si bottom SC before and after the implementation of the filter, the JSC and VOC vary the most. This is because the light transmitted through the semi- transparent PVK SC, rather than the spectrum of AM 1.5G, is transmitted to the Si bottom SC, such that a smaller amount of light is transmitted. In addition, the current density of the Si bottom SC slightly differs, depending on the band gap and thickness of the PVK. In Figure 4.3c and Figure 4.3d, the change in the EQE in the Si bottom SC according to the change in the PVK band gap and thickness was confirmed. However, determining the optimal current matching point in an actual monolithic PVK/Si tandem SC by using the filter method is not easy. Owing to the difference in refractive index due to the layer of air between the semi-transparent PVK SC filter and the Si bottom SC, serious reflection loss occurs,29-33 making it impossible to accurately predict the current matching point of the tandem SC.
Therefore, the most accurate method of performing current matching through EQE analysis is to apply the band gap and thickness of the PVK to the actual device in the range predicted through optical design, as shown in Figure 4.4. In Figure 4.5a and Figure 4.5b, the current matching point of the tandem SC was determined through EQE analysis based on the change in the band gap and thickness of the PVK applied to the ITO-finished Si bottom SC. In this study, the current matching method through EQE analysis was applied to tandem SCs to determine the optimal points.
4.3.2. Recombination Layers of Tandem Solar Cell
As shown in Figure 4.6, an ITO layer was used as the recombination layer. Basically, in the case of homojunction Si SC, the n+ emitter layer and ITO layer generated on the p-Si Si form a Schottky barrier, causing the recombination of the top and bottom cells of the tandem SC to drive the device.10,
11, 34, 35 For optimal recombination layer formation, we obtained high transmittance and optimal sheet resistance of ITO thin film conditions by controlling the sputtering temperature and pressure parameters of the ITO layer, as shown in Figure 4.7a and Figure 4.7b. As shown in Figure 4.8a, when the ITO is 20 nm and the sputter annealing temperature is 250 ℃ or higher, the crystallinity of (222) of the ITO
stages of growth.36 In addition, this orientation can be attributed to the crystal structure of the indium metal. Indium is known as a face-centered tetragonal structure, in which the (111) plane is the densest and the lowest energy plane.37 Similarly, when the sputter heat treatment temperature is fixed at 250 ℃ and a pressure variable is given, as shown in Figure 4.8b, the crystallinity of (222) appears for the same reason. However, it was confirmed that the structure was amorphous at temperatures of 150 ℃ or less.
Figure 4.9 is a SEM image of the surface of ITO with a thickness of 20 nm for each sputtering condition. On the surface in each SEM image, ITO particles of approximately 50 nm generated during the sputtering process can be observed. If these particles are present on the surface, the HTL thickness is approximately 10 nm when the tandem SC is fabricated; thus, there is a possibility that the device may be shorted, causing the presence of a minimal number of particles. In addition, Figure 4.10 shows the surface of the optimized ITO thin film as an atomic force microscopy (AFM) image, in which the roughness is notably low at 0.554 nm. This roughness can be uniformly coated with a thin HTL without problem when tandem SC is produced.
Figure 4.11 shows the amount of change in sheet resistance according to the heat treatment conditions of the ITO thin film at 7 mTorr. The J-V and EQE of a tandem device including the ITO conditions are shown in Figure 4.12a and Figure 4.12b. To determine the current matching point of the tandem SC, the current limit was applied to the PVK top SC, with the band gap and thickness of the adjusted PVK applied, and the current density of the tandem SC appeared to be fixed. The following device characteristics were confirmed (Table 4.2). In the case of ITO deposited at room temperature, the sheet resistance was approximately 800 ohm/sq, and the J-V characteristics of the tandem SC were the lowest, with VOC and FF reduced. This means that an ITO thin film with low sheet resistance is needed to make a minimum current path for recombination. Therefore, for the ITO thin film formed through annealing, it has a sheet resistance of 480 ohm/sq or less at 150 ℃, through which the tandem SC was smoothly driven. We obtained optimal ITO thin film conditions for the tandem SC at 250 °C and 7 mTorr.
4.3.3. Graded Refractive Index Matching Designs for Tandem Solar Cell
Figure 4.13 shows the structure that minimizes the optical loss by using graded refractive index matching for the PVK/Si tandem SC. As shown in Figure 4.14a, to minimize the optical loss in the 750–1200 nm wavelength band, where the Si bottom SC starts to absorb light, the refractive index of each functional layer of the tandem SC is considered by obtaining the indices of the PVK top SC and Si bottom SC. The refractive index of PVK at 750 nm is 2.4, and that of Si is 3.7. In the previous chapter, the refractive index and thickness of the SiNx layer, which can be realized through similar calculations, were 2.84 and 66 nm, respectively. Figure 4.14b shows the reflectance of the Si substrate and the
reflectance when the refractive index and thickness of the SiNx layer applied to the tandem SC are applied to the Si substrate. The long-wavelength reflectance is considerably lower but is not close to zero. In this case, since the refractive index of the air is 1, the refractive index of the SiNx layer is 2.84, resulting in a reflection loss due to the difference in refractive index. However, when the tandem SC was applied, the light path originated from the air to the SiNx layer through the PVK top SC; thus, it was confirmed that the device efficiency was improved by minimizing the reflection loss occurring at this time (Figure 4.15a and Figure 4.15b). Table 4.9 shows that the JSC, VOC, FF, and efficiency of the champion device with graded refractive index matching applied are 16.40 mA/cm2, 1.75 V, 80.23%, and 23.02%, respectively. This represents the highest efficiency among the p-Si homojunction tandem SC efficiencies reported so far. The graded refractive index was controlled between the top cell and bottom cell by inserting a SiNx layer. The total reflectance of tandem SCs decreased to 16.85% without the SiNx layer, 15.78% with the SiNx layer, and 6.41% with the application of the ARC film (Figure 4.16).
4.3.4. Stability Test
To evaluate the stability of the PVK/Si tandem SC, glass-to-glass encapsulation was applied, as shown in Figure 4.17a. As shown in Figure 4.17b, XNR5570 material with 2.00 10-4 g/m2/day of epoxy applied was used. Figure 4.18 indicates that when the light soaking test was conducted in the air using the AM 1.5G xenon lamp, more than 90% of the initial efficiency was maintained after 1000 h, confirming once more that the light stability of the PVK thin film due to the addition of Cs increased, and halide segregation was prevented. Figure 4.19 shows that 92% of the initial efficiency was maintained, even after 1000 h of heat treatment at 85 ℃ in a glove box in a nitrogen atmosphere.
Likewise, this was the result of the increased stability of the PVK structure due to the addition of Cs.
However, in the damp heat test, as shown in Figure 4.20, the device performance rapidly deteriorated after 200 h because of the intrusion of moisture arising from the problem of the glass-to-glass encapsulation structure. With glass-to-glass encapsulation, moisture penetrates through the gap as the wire comes out; thus, we plan to introduce a new encapsulation design that extends the moisture penetration path or improves the wiring method. If penetration is prevented, more than 90% of the initial efficiency is expected to be maintained, even after 1000 h.