PCBM is an organic CTL that has been demonstrated as a poor iodine migration barrier by Li et al. (2017). In their work, they investigated the effect of temperature and the thermal stability of fully fabricated PSC with FTO|NiOx|MAPbI3|PCBM|Ag configuration. When the silver (Ag) counter electrode is present, the PCE dropped from an initial 15.2% to 8.4% after 24 h of thermal stressing at 85 °C. However, if the device was thermal stressed the same followed by Ag counter electrode deposition to complete the PSC fabrication, a PCE of 14.8% was obtained, which was similar to the aforementioned initial 15.2% PCE. It was found that the presence of Ag counter electrode during thermal stressing facilitated the formation of silver iodide (AgI), which was caused by iodine migration from MAPbI3 layer through PCBM to Ag counter electrode. This was supported by the presence of AgI XRD and grazing incidence XRD (GIXRD) peaks on the peeled Ag electrode (Figure 2.2(a–b)), as well as excessive I− and I2−
signal accumulation at Ag electrode according to time of flight secondary ion mass spectroscopy (ToF- SIMS) analysis (Figure 2.2(c)). It was also revealed that MAI escaped from GBs during the iodine migration, forming poor conducting PbI2 grains that fused the
MAPbI3 grain, as demonstrated by the conductive atomic force microscopy (C- AFM) in Figure 2.2(d). This demonstrated PCBM was unable to act as a barrier to iodine migration, which caused mutual degradation on MAPbI3 active layer and Ag counter electrode. The mechanism of thermal degradation due to poor barrier properties of PCBM is shown in Figure 2.2(e).
Figure 2.2: (a) XRD and (b) GIXRD analyses on peeled Ag electrode post thermal stressing (85 °C, 24 h). (c) ToF-SIMS elemental depth profile with strong signal of I− and I2− signal near the Ag electrode. (d) C-AFM image of thermally stressed MAPbI3 film with the Ag electrode, with poor conducting PbI2 (bright region) surrounding the MAPbI3 grain. (e) Suggested thermal degradation mechanism involving loss of MAI from MAPbI3 layer through PCBM to Ag counter electrode at elevated temperature (Li et al., 2017).
To overcome the poor barrier properties of PCBM, Bi et al. (2017) mixed the PCBM with 2 wt.% functionalized graphene (G-PCBM). The graphene with its crystal lattice parameter (0.246 nm) smaller than iodine ions
(a) (c)
(d)
(e)
(b)
diameter (0.412 nm), does not permit the iodine to migrate through it, as illustrated in Figure 2.3(a). To reach the Ag electrode through the 150 nm G- PCBM layer, the iodine ions have to travel around the graphene sheet and overcome the diffusion length that was 10 times more than a direct diffusion distance through 50 nm PCBM. Their X-ray photoelectron spectroscopy (XPS) (Figure 2.3(b)) revealed the negligible presence of iodine signal at peeled Ag counter electrode after the PSC device was stressed at 100 °C for 90 h when G- PCBM was used as ETL in the device. Also, the energy dispersive X-ray (EDX) mapping confirmed the lack of iodine presence near the ETL|Ag interface (Figure 2.3(c)). With G-PCBM, they were able to fabricate a PSC with stabilized 15 % PCE over thermal stressing for 800 h at 85 °C, significantly more stable than Li et al. (2017) that observed degradation from 15.2% to 8.4%
in just 24 h.
Figure 2.3: (a) Illustration on graphene in PBCM blocking iodine ion migration through it. (b) Iodine signals from XPS analysis on peeled off Ag counter electrode after the device was stressed at 100 °C for 90 h. (c) EDX elemental mapping highlighting iodine signal presence at PCBM (50 nm)|Ag interface and absence in G-PCBM (150 nm)|Ag interface after the same thermal stressing process (Bi et al., 2017).
(a) (b)
Figure 2.3: Continued.
While Bi et al. (2017) showed G-PCBM can suppress iodine migration, it does not address the intrinsic instability of PCBM under oxygen and moisture exposure. Earlier work by Bao et al. (2014) has demonstrated the degradation of PCBM exposed under oxygen and moisture. The presence of oxygen shifted the work function (WF) and ionization potential (IP, also known as valance band) of PCBM by a fixed value after some exposure time, as shown in Figure 2.4(a).
Fortunately, this oxygen interaction can be reversed by heating. However, this also meant that PSC will be subjected to an elevated temperature to remove oxygen interaction and may lead to iodine migration problem as mentioned previously (see Figure 2.2). Upon exposure to water vapor, PCBM underwent irreversible changes in WF and IP (Figure 2.4(b)) that did not stop with exposure time. Its ultraviolet photoelectron spectroscopy (UPS) spectral feature also broadened, suggesting strong water interaction and PCBM degradation.
G-PCBM (150 nm) PCBM (50 nm)
(c)
Hence, caution is required when using PCBM in PSC as exposure to ambient air in a short duration may degrade the device. Ideally, the device should be encapsulated in inert atmosphere before exposure to ambient air. This, however, will increase the complexity and cost of encapsulation process.
Figure 2.4: UPS spectral evolution of PCBM on aluminium (Al) substrate under (a) oxygen exposure and (b) water vapour exposure (Bao et al., 2014).
Kato et al. (2015) demonstrated the intrinsic instability of Li+ doped Spiro-OMeTAD. In conventional PSC, Spiro-OMeTAD is often used as HTL.
WF IP
Broadened spectral feature (a)
(b)
The pristine form Spiro-OMeTAD has low hole-mobility, requiring hygroscopic Li+ dopant to enhance the mobility that is essential to achieve high PCE. However, the hygroscopic dopant also leads to chained degradation events that lead to all three MAPbI3, Spiro-OMeTAD, and Ag layers to degrade. As shown in Figure 2.5, the dopant in Spiro-OMeTAD reacted with moisture to form pin-holes that furthered the moisture penetration into the MAPbI3 layer.
The moisture then degrades the MAPbI3 layer and releases volatile iodine compounds. The iodine compounds then reacted with Ag electrode through the pin-holes and irreversibly reacted with Ag to form AgI. All these lead to the degradation of Spiro-OMeTAD, reduced amount of MAPbI3 phase for charge generation, and efficiency loss at Ag counter electrode due to increase in resistance associated with AgI formation. This highlights the disadvantage of Spiro-OMeTAD under ambient air, which are intrinsic moisture instability that leads to self-degradation, and poor barrier properties that lead to moisture and ion migration-induced degradation.
Figure 2.5: Degradation mechanism of Spiro-OMeTAD, followed by perovskite and Ag counter electrode under ambient air (Kato et al., 2015).
Despite without the presence of moisture, Spiro-OMeTAD is also a poor ion migration barrier similar to PCBM. Domanski et al. (2016) investigated the thermal stability of FA0.83MA0.17Pb(I0.83Br0.17)3 PSC, with gold (Au) as counter electrode and Spiro-OMeTAD as HTL. They subjected their PSC to a thermal stressing test inside a glovebox environment to eliminate moisture-induced degradation. After a 15 h thermal stressing test at 75 °C, ToF-SIMS analysis revealed increased Au elemental signal in the perovskite layer, as shown in Figure 2.6(a–b). This demonstrated Au migrated into the perovskite layer at elevated temperature, causing mutual degradation to perovskite and Au layers.
As a result, the PCE irreversibly degraded from an initial 19.4% to just 4.9%.
Such PCE deterioration was not observed when the Au was deposited after the thermal stressing test. When chromium interlayer was deposited in between the Au CE and Spiro-OMeTAD HTL, the Au metal-induced migration was mitigated, with the device PCE stabilized above 80% of its initial efficiency.
This suggests Spiro-OMeTAD is a poor barrier layer against Au migration, requiring a special Cr interlayer to suppress it. Despite that, the device suffered a loss in PCE due to the mismatching between work functions of Cr and Au, and the introduction of Cr interlayer reduced the otherwise obtainable 19.4%
PCE to just 13% PCE.
Figure 2.6: (a) ToF-SIMS depth profiles of the PSC showing the presence of Au (yellow line) inside the perovskite layer (brown region) after the device was thermally stressed. (b) 3D mapping of the elements of interest obtained from depth profile (Domanski et al., 2016).
By further excluding moisture and thermal interactions, the Spiro- OMeTAD can still fail to suppress ion migration. Cacovich et al. (2017) demonstrated a mere light soaking of the MAPbI3 PSC at room temperature and in a moisture-free glovebox environment could induce ion migration-induced degradation. When their PSC was biased at maximum power point (MPP) as one of the methods to assess PCE over time, they found the PCE drop rapidly from 15.89% to 0.37% in just 100 h of white LED light illumination. Through high angular annular dark field (HAADF) scanning transmission electron microscopy (STEM) analysis (Figure 2.7(a–b)), it was revealed that the Au from
(a)
(b)
counter electrode migrated and accumulated inside MAPbI3 and TiO2 layer after light soaking for 200 h. Further EDX depth analysis (Figure 2.7(c)) and Au signal mapping (Figure 2.7(d)) showed the bright spots from HAADF STEM image of the light-soaked device were indeed Au. The presence of Au at the TiO2 and MAPbI3 layer created recombination defects and prevented efficient electron injection into TiO2 ETL. The EDX analysis also revealed the ratio of I/Pb in both the layers to be 2.75 in the light-soaked device, which was less than 3 for a stoichiometrically correct value for MAPbI3. Only when Spiro- OMeTAD HTL was included together with the MAPbI3 and TiO2 layer, the I/Pb value restored to 3. This indicates iodine migration into the Spiro-OMeTAD layer. Hence, this work provided further evidence on poor ion migration barrier properties of Spiro-OMeTAD.
Figure 2.7: (a) HAADF STEM cross-sectional morphology of freshly prepared MAPbI3 PSC. (b) 200 h light-soaked PSC, with bright Au spots migrated to the MAPbI3 and TiO2 layers. The red boxed region was subjected to (c) EDX depth elemental analysis. (d) The corresponding highlight of Au signal in the MAPbI3 andTiO2 layers (Cacovich et al., 2017).
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(b)
(c) (d)
In addition to poor barrier properties, Spiro-OMeTAD is also a thermally unstable HTL. Jena et al. (2018) investigated the short term (1 h) thermal stability of the PSC with Spiro-OMeTAD as HTL. It was observed that the device heated with Spiro-OMeTAD (without Au counter electrode) reduced the PCE significantly, specifically due to reduction in fill factor (FF). When the MAPbI3 film was stressed without Spiro-OMeTAD, the subsequent reassembly to PSC yielded similar PCE to the freshly prepared device, suggesting the inferior PCE was not caused by thermal degradation of MAPbI3 but rather Spiro-OMeTAD. Hence, the decreased FF may be due to chemical modification induced by Spiro-OMeTAD at the interface. To verify this, they recycled the MAPbI3 film first by removing Spiro-OMeTAD using chlorobenzene, and then replenished MAPbI3 phase by MAI immersion, and finally reassembling the PSC. However, these recycled PSCs PCE were always inferior to the freshly prepared device. The failure of recycling in restoring the PCE, together with a drastic reduction in FF, suggested the Spiro-OMeTAD permanently degraded the MAPbI3|Spiro-OMeTAD interface. Considering the PSC will often reach elevated temperature during operation, it will be realistic to assume that the thermal degradation of Spiro-OMeTAD is unavoidable. Hence, consideration to substitute Spiro-OMeTAD with other stable materials with proper barrier properties is required. Additional criteria such as ease of preparation at ambient air and low-cost will also accelerate the commercialization of PSC.
To summarize, both PCBM and Spiro-OMeTAD are poor CTLs for PSC.
In additional to prohibitive cost, their poor barrier properties against ion
migration and poor intrinsic stability under ambient air prevented successful commercialization of PSC.