2.5 Low-Temperature Solution-Processed Metal Oxide as CTL
2.5.3 CeO x
Figure 2.17: (a) Inverted PSC with TiOx as an interlayer between PCBM and Al counter electrode. (b) Illustration of formation of a continuous sol- gel network of TiOx on PCBM (Hong et al., 2020).
Figure 2.18: (a) Configuration of CeOx ETL-based PSC and (b) corresponding band alignments of each layer of the PSC (X. Wang et al., 2017).
CeOx can also be annealed directly on top of the perovskite layer, replacing the role of PCBM as ETL in inverted PSC. Yang et al. (2019) had fabricated an inverted PSC with NiOx as HTL, CeOx as ETL, and CsPbIBr2 as perovskite layer. The CeOx on top of the perovskite layer was deposited by spin- coating Ce(acac)3 dissolved in chlorobenzene, followed by annealing at 100 °C for 10 min. This all inorganic CsPbIBr2-based PSC yielded champion PCE of 5.60%, with 90% of initial efficiency after storage in ambient air and at elevated storage temperature condition at 40–45 °C.
2.5.4 VOx
VOx had been employed in inverted solar cells (perovskite or organic) by Tan et al. (2012), Guo et al. (2018), and Cong et al. (2017). In an inverted structure, the VOx sol (vanadyl acetylacetonate (VO(acac)2) in alcohol) is spin- coated onto the substrate and annealed to form VOx HTL. This VOx sol requires annealing at 150 °C to break the acetylacetonate rings for VOx formation, as
(a) (b)
shown in Figure 2.19. Several studies have used this VOx sol as HTL in inverted perovskite or organic solar cell. Guo et al. (2018) annealed the same VOx sol for 20 min to fabricate inverted PSC with PCE of 14.5%. In earlier work by Tan et al. (2012), a shorter 10 min of annealing time was employed for their organic solar cell, achieving PCE of 6.35%. This PCE was comparable with PEDOT:PSS counterpart. Further shortening of annealing time to 3 min was reported by Cong et al. (2017) in their organic solar cell, although it required aid from UV-Ozone exposure to break the acetylacetonate rings effectively.
Using this approach, they obtained PCE of 8.11% with VOx as HTL, which is superior to PEDOT:PSS counterpart (7.67%).
Figure 2.19: Annealing of VO(acac)2 at 150 °C to break the acetylacetonate rings and formation of amorphous VOx (Tan et al., 2012).
Other VOx precursors have also been employed to deposit VOx HTL in their inverted organic solar cell. For example, Zilberberg et al. (2011) employed vanadium triisopropoxide dissolved in isopropanol to prepare V2O5 sol-gel. The precursor solution required only an annealing temperature of 110 °C for 1 h.
Although the V2O5 sol-gel is amorphous, it yielded a similar work function as thermally evaporated V2O5 (WF = 5.3 eV vs. 5.4 eV). An optimum 10 nm thick
V2O5 thick HTL layer yielded a device with 3% PCE, which is higher than PEDOT:PSS counterpart (2.7%). The high chemical stability V2O5 also proven to be more stable than acidic PEDOT;PSS, retaining 80% of initial PCE after 400 h storage time, as shown in Figure 2.20. Whereas the PEDOT:PSS device completely failed at the same duration.
Figure 2.20: Comparison of stability of organic solar cell using V2O5 and PEDOT:PSS as HTL (Zilberberg et al., 2011).
In the work from Alsulami et al. (2016), the V2O5 HTL prepared from the same vanadium triisopropoxide in isopropanol was annealed at even lower temperature at 100 °C for 30 min on ITO substrate. This yielded inverted organic solar cell with 6.0% PCE. At high annealing temperatures (300 and 400 °C), the PCE deteriorated to ≤5.6% owing to the increased series resistance of the V2O5 layer associated with the change of oxidation state. Hence, low- temperature processed V2O5 provided competitive performance against high- temperature counterparts.
2.5.5 ZnO
Low-temperature ZnO has been used as ETL in PSC. In the work from Mahmud et al. (2017), ZnO with 140 °C processing temperature was used as ETL in conventional PSC. The ZnO was derived from zinc acetate dihydrate in 2-methoxyethanol with ethanolamine as an additive. The precursor sol-gel was spin-coated onto the ITO substrate and annealed for 60 min to obtain optimum ZnO ETL thickness (45 nm). Their best PCE was 8.77%.
F. Yang et al. (2017) used the same approach to deposit ZnO. Their annealing time was shortened significantly to just 10 min, but with slightly higher thermal annealing temperature of 160 °C. It was found that the deposition method for MAPbI3 under ambient air had a drastic effect on the performance of PSC. Under ambient air and with the aid of moisture, the statically spin- coated MAI can react and etch off the ZnO layer, revealing the ITO layer beneath, as shown in Figure 2.21(a). However, this was not the case for MAI spin-coated on ZnO in a glovebox environment due to the lack of moisture (Figure 2.21(b)). Based on this observation, they adopted dynamic spin-coating and optimized the anti-solvent dripping process to minimize the reaction window duration between MAI from MAPbI3 precursor solution and ZnO ETL under ambient air, which prevented the etching of ZnO and yielded champion PCE of 11.23%. While ZnO has proven to be capable to function as an ETL with low processing temperature, its tendency to react with MAPbI3 requires one to practice caution when using it in PSC.
Figure 2.21: State of ZnO (a) when the MAI solution was statically spin- coated onto ZnO under ambient air vs. (b) when MAI was spin-coated statically in moisture free glovebox environment (F. Yang et al., 2017).