migration and poor intrinsic stability under ambient air prevented successful commercialization of PSC.
nm were dispersed in chlorobenzene and spin-coated directly onto the perovskite layer to form a compact ETL (Figure 2.8(a)). The deposition process does not require annealing. The ZnO ETL acted as an excellent barrier against moisture and oxygen ingression and metal and iodine migration, enhancing the overall device operational stability. This helped retaining more than 90 % of the initial 16.1% PCE after storage in an ambient environment for 60 days (Figure 2.8(b)). In contrast, PSC using organic PEDOT:PSS as HTL and PCBM as ETL degraded rapidly to almost 0% PCE in just 5 days. Despite the enhanced stability, the all metal oxide CTL device suffered from hysteresis when compared to the device using PCBM as ETL (Figure 2.8(c)). It was suggested that the NPs do not have any synergetic interaction with the perovskite layer, while PCBM ability to passivate the perovskite|ETL interface eliminated the observed hysteresis.
Figure 2.8: (a) Cross-sectional SEM image of inverted PSC. (b) Device operation stability comparison of all metal oxide CTL vs. full organic CTL PSCs. (c) Current-voltage scans (forward and backwards) showing hysteresis behaviour (You et al., 2016).
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Najafi et al. (2018) added PCBM as an interlayer between ZnO NP ETL and perovskite layer (Figure 2.9(a)). This resulted in the elimination of hysteresis by PCBM passivation, while ZnO NPs acted as another ETL that shielded the PCBM and perovskite layers from moisture and oxygen interaction, at the same time suppressing Al migration from counter electrode. The ZnO ETL also enhanced electron injection into the Al counter electrode due to better band alignment with the WF of Al, as shown in Figure 2.9(b). This resulted in remarkably stable PSC that retained >80% PCE even after thermal stressing for 1000 h (Figure 2.9(c)). A maximum PCE of 18.6% was obtained on a solid indium tin oxide (ITO) substrate and 16.6% on a flexible PET|ITO substrate.
Figure 2.9: (a) Structure of inverted PSC with NiOx NPs as HTL and ZnO NPs|PCBM as ETL. (b) Band alignment of the PSC. (c) Performance parameters of PSC over time under different stressing temperature (Najafi et al., 2018).
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PVD processes such as sputtering or thermal evaporation are also a viable method to deposit metal oxide CTL on perovskite. Both methods also do not require annealing post-deposition, which made their incorporation process perovskite friendly. Zheng et al. (2018) thermally evaporated chromium oxide (Cr2O3) onto PCBM to enhance the overall device operational stability. By adjusting the concentration of oxygen in the thermal evaporation chamber, the vapour of the melting chromium in the evaporation chamber reacted with the oxygen to form Cr2O3 before deposited on PCBM encapsulated perovskite layer.
This approach provided additional benefits to the PSC performance and stability.
The Cr2O3 conformally coated the surface of PCBM and covered any pin-holes formed due to incomplete coverage by solution-processed PCBM. This effectively isolated the subsequent Ag counter electrode from directly attack perovskite layer beneath it, eliminated the shunt path between the Ag electrode and perovskite layer, and enhanced both the stability and performance of PSC.
This approach yielded PCE of 11.59%, which is higher than 9.86% for the device without Cr2O3. In terms of stability, the Cr2O3|PCBM device dropped gradually and stabilized at 50% of initial PCE in 21 days of storage under ambient air, while the PCBM only device dropped rapidly to 50% in just 2 days (Figure 2.10(a)). Although device stability enhancement was demonstrated by incorporation of Cr2O3, its inferior stability when compared to devices of You et al. (2016) and Najafi et al. (2018) may be due to the use of hygroscopic and acidic PEDOT:PSS organic HTL (Figure 2.10(b)) that damaged the perovskite layer over time. Hence, better long-term stability should be expected if metal oxide HTL is used instead.
Figure 2.10: (a) PSC with and without Cr2O3 ETL shielding PCBM and their ambient air storage stability over time. (b) Structure of PSC with Cr2O3 as ETL (Zheng et al., 2018).
It is also possible to eliminate the use of PCBM in the inverted PSC. Lei et al. (2019) used magnetron sputtering to deposit ZnO as ETL in inverted PSC.
In their work, ZnO was directly sputtered onto MAPbI3 layer to form inverted PSC with FTO|Cu:NiOx|MAPbI3|ZnO|Ag configuration. This approach yielded a PSC with negligible hysteresis PCE of 14.54% when compared to 15.23%
from PCBM counterpart (Figure 2.11(a)). While the PCE was slightly lower, the device benefited from better operational stability due to ZnO resisting moisture ingression. Under 30 °C and 60% RH storage condition, the PCBM device dropped to 0% in less than a month, whereas the sputtered ZnO decayed gradually to 14% after six months (Figure 2.11(b)). To further extend the stability, hydrophobic polytetrafluoroethylene (PTFE) deposited via thermal evaporation was used as an interlayer between ZnO and MAPbI3 layer (Figure 2.11(c)). While ZnO alone can suppress moisture ingression, its tendency to absorb moisture may lead to moisture to gradually traverse via its GBs into MAPbI3 layer. The inclusion of PTFE drastically increased the hydrophobicity of ZnO ETL (here referred as ZnO@PTFE), as shown in increased water contact angle from hydrophilic 22.5 ° for ZnO alone to hydrophobic (>90 °) 131.8 °for ZnO@PTFE (Figure 2.11(d)). While the inclusion of insulating PTFE further
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lowered the PCE to 13.62% due to increased charge resistance by PTFE, the ZnO@PTFE ETL gained improved the storage stability, retaining 76.8% of initial 13.62% PCE after six months.
Figure 2.11: (a) PCE of device using ZnO, ZnO@PTFE and PCBM as ETL.
(b) Storage stability of device under 30 °C and 60% RH. (c) Schematic of PTFE deposition as an interlayer between ZnO and MAPbI3 layer. (d) Contact angle of water on glass slides with various coatings (Lei et al., 2019).
Later, Guo et al. (2020) used magnetron sputtering to further optimize the thickness of the Cu:NiOx HTL and ZnO ETL to 25 nm and 50 nm. This optimized CTLs further improved the PCE to 16.54% with negligible hysteresis, which is higher than previous PCBM device by Lei et al. (2019). Not only that, the device was able to maintain more than 90% of initial PCE after six months of storage under ambient air, as opposed to the devices that dropped to 30% and 0% with PEDOT:PSS HTL and PCBM ETL, respectively (Figure 2.12). Hence,
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magnetron sputtering is a viable method for metal oxide deposition on perovskite for the fabrication of a high stability PSC.
Figure 2.12: Ambient air stability of unencapsulated PSCs using different CTLs and their corresponding PCE decay over six months (Guo et al., 2020).
ALD is also another viable method to deposit metal oxides onto the perovskite layer. Raiford et al. (2019) employed ALD to deposit vanadium oxide (VOx) onto the perovskite layer. However, this approach required a thin organic 2,2’,7,7’-tetrakis-(N,N,-di-p-methylphenylamino)-9,9’-spirobifluorene (Spiro-TBB) HTL interlayer to protect perovskite layer from water damage as water is used as co-reagent in the VOx deposition process. This deposited a uniform 9 nm thickness VOx layer and created a VOx|Spiro-TBB bi-HTLs that can extract holes efficiently from the perovskite layer. Most importantly, this 9 nm thin VOx layer is crucial as it allowed transparent conductive ITO to be sputtered directly onto it, which prevented the high-power sputtering process damaging the Spiro-TBB and perovskite layers beneath it. This subsequently enabled the fabrication of semi-transparent PSC with ITO|SnO2-
C60|Cs0.17FA0.83Pb(Br0.17I0.83)3|Spiro-TBB|VOx|ITO|Ag structure, yielding 13.2%
efficiency. Because of double layers metal oxide encapsulation (VOx and ITO), the device benefited from superior thermal stability, enduring 1000 h of 85 °C thermal stressing without degradation (right of Figure 2.13(a)). Furthermore, a slight improvement in PCE was observed after the thermal stressing test (Figure 2.13(b)), presumably due to improved crystallinity by Ostwald ripening. Even if ITO was absent, the VOx alone can still significantly enhance the stability of the device, as it only showed slight degradation after the test (middle of Figure 2.13(a)). In contrast, the device using Spiro-OMeTAD degraded seriously after the thermal stressing test, as indicated by the prevalent yellow PbI2 region (left of Figure 2.13(a)).
Figure 2.13: (a) Thermal stability comparison of device with (left) Spiro- OMeTAD, (middle) VOx only, and (right) VOx|ITO HTL after thermal stressing at 85 °C for 1000 h. (b) J-V curve of device using VOx|ITO before and after the thermal stressing (Raiford et al., 2019).
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Similarly, Tepliakova et al. (2020) employed ALD to deposit 30–45 nm thick VOx on polytriarylamine (PTAA) interlayer. Different from Raiford et al.
(2019), no ITO was deposited on VOx. Also, the ALD-deposited VOx layer was significantly thicker at 30–45 nm instead of 9 nm. This yielded a conventional PSC device with ITO|ZnO|CsFAPbI3|PTAA|VOx|Al configuration (Figure 2.14(a)) that achieved 16.2% PCE. The ALD VOx, while together with PTAA served as bi-HTL, was also crucial in preventing the escape of volatile MAI from the MAPbI3 layer during thermal and light simultaneous stressing at prolonged 4500 h of 45–50 °C (Figure 2.14(b)). As shown in Figure 2.14(c), the thermal and light stability of the VOx|PTAA device was excellent as it only reported minor 10% PCE decay after the stressing test, indicating strong MAI containment and barrier against ion migration. A comparison was made against molybdenum oxide (MOx)|PTAA-based device that suffered more than 50%
decay. The serious decay observed in MOx based device may be assigned to unfavourable reaction that occurred between MOx and perovskite layer.
Figure 2.14: (a) Schematic of PSC with VOx on PTAA layer. (b) Light soaking test of PSCs at 45–50 °C in glovebox environment. (c) Stability comparison between PSCs using VOx|PTAA and MOx|PTAA as bi-HTL (Tepliakova et al., 2020).
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Figure 2.14: Continued.
2.4 Limitations of Current Methods for Metal Oxide CTL Deposition