Chapter 7: Vacuum Deposited PbI 2 Film Grown at Elevated Temperature for Improved
7.3. Results and Discussion
7.3.1. Morphology Analysis
For the development of large-area Pe solar cells for commercialization, it is essential to explore alternative techniques different from the most explored spin coating method for the Pe deposition.
Here, the light-absorbing Pe active layer was deposited by a modified two-step vacuum-solution deposition method without the help of the spin coating technique. Fig. 7.1 shows a schematic illustration of the two-step vacuum-solution deposition process for the growth of the CH3NH3PbI3
Pe layer. First, the PbI2 layer was deposited by vacuum thermal evaporation technique on top of PEDOT:PSS coated FTO substrates. The substrate temperature was varied (30°C, 50°C, 100°C and 150°C) for different samples for the deposition of PbI2 layer. Note that in thermal evaporation, the thickness of the film and substrate temperature can be controlled precisely, which are very difficult in the spin coating method.13, 14 After the deposition of the PbI2 layer, the substrates were dipped into the IPA solution containing CH3NH3I (10 mg/ml) for 5 min, as shown in Fig. 7.1.
Next, the substrates were annealed on a hotplate at 90 °C for 10 min to form the CH3NH3PbI3 Pe layer. In the process of dipping, the CH3NH3I solution diffuses into the PbI2 layer and reacts with it through the grain boundaries of PbI2 crystals, and crystallization occurs during the annealing.15 The inset of Fig. 7.1 shows the digital photographs of the PbI2 and Pe films. After dipping in CH3NH3I/IPA solution and annealing, the shining yellow color of vapor-deposited PbI2 film turns into a shining black color confirming the formation of CH3NH3PbI3. Note that in our vapor- solution deposition technique for the Pe film growth, no toxic solvents are used, which is in stark contrast to the spin coating method that involves the use of different toxic organic volatile solvents, such as DMF or DMSO. Thus, we adopted a relatively green approach for the improvement of solar cell performance, and it can be a good alternative for the deposition/fabrication of Pe thin film-based solar cells.
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The morphology and structural properties of the vapor-deposited PbI2 layer can be controlled by varying the substrate temperatures during the deposition. Here, we have deposited PbI2 layer at Ts = 30°C, 50°C, 100 °C, and 150°C, at a constant deposition rate. These vapor- deposited PbI2 layers were used for the growth of Pe film used in the fabrication of solar cells. Fig.
7.2(a) shows the FESEM image of the PbI2 layer deposited at Ts =30°C, while Fig. 7.2(b) shows the FESEM image of the PbI2 layer deposited at Ts = 150°C.
Fig. 7.2: (a) FESEM image of the vapor-deposited PbI2 layer grown at 30 °C of substrate temperature. The inset shows a higher magnification image of the surface morphology. (b) FESEM image of the vapor-deposited PbI2 layer grown at 150 °C (substrate temperature). The inset shows a higher magnification image of the surface morphology. (c) AFM micrograph of the PbI230 film, and (d) AFM micrograph of PbI2150 film. RMS roughness is indicated in each case.
The insets of Fig. 7.2(a) and Fig. 7.2(b) show the higher magnification image depicting the surface morphology of PbI2 layers. As shown in Fig. 7.2(a), the morphology of the PbI2 layer deposited at Ts = 30°C is very rough with pinholes. On the other hand, the PbI2 layer deposited at Ts = 150°C is smooth, compact, and pinhole-free, as shown in Fig. 7.2(b). It is due to the fact that the higher substrate temperature gives extra thermal energy for oriented crystal growth of PbI2 molecules,
which result in highly crystalline smooth and pinhole-free compact PbI2 films and superior device performance.16-18 In the case of deposition at low substrate temperature, the movement/ alignment of the molecules is minimal due to insufficient thermal energy, and this results in the growth of Pe film with pinholes and finally poor device performance. Fig. 7.2(c) and Fig. 7.2(d) show the AFM topography images of PbI2 films deposited at Ts = 30°C and 150°C, respectively. As evident from Fig. 7.2(c), PbI2 film deposited at Ts = 30°C contains a high density of pinholes with high roughness, while the film deposited at Ts = 150°C is compact and nearly pinhole-free (see Fig.
7.2(d)). The corresponding RMS roughness of PbI2 films is observed to be 41 nm and 23 nm at Ts
=30°C and 150°C, respectively. Thus, the RMS roughness is relatively lower for deposition at elevated temperature.
Fig. 7.3(a) and 7.3(b) show the FESEM image of the morphology of the Pe films Pe30 and Pe150, respectively.
Fig 7.3: (a) FESEM image of CH3NH3PbI3 perovskite film using PbI230 film showing a lot of pinholes. (b) FESEM image of CH3NH3PbI3 perovskite film using PbI2150 film showing pinhole-free morphology.
It is evident that the Pe30 sample contains a lot of pinholes, and the surface is very rough compared to that of Pe150. The high roughness along with many pinholes in the Pe30 layer is due to the higher roughness of the corresponding PbI2 layer deposited at Ts = 30°C. In contrast, the Pe150 is relatively smooth and compact with no visible pinholes due to the nearly pinhole-free PbI2 layer deposited at Ts = 150°C. This high-quality Pe layer is responsible for the high optical absorbance of the film (discussed later). Due to the smooth and compact nature of the Pe film with large grains, the recombination of photogenerated carriers is reduced (discussed later). Thus, improved separation and transport of carriers in the Pe150 film leads to the superior performance of the corresponding solar cell.
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