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Enhancing stability for organic-inorganic perovskite solar cells by atomic layer deposited Al2O3 encapsulation
Solar Energy Materials and Solar Cells 188 (2018) 37–45
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Solar Energy Materials and Solar Cells journal homepage: www.elsevier.com/locate/solmat
Enhancing stability for organic-inorganic perovskite solar cells by atomic layer deposited Al2O3 encapsulation ⁎
Eun Young Choia, Jincheol Kimb, Sean Limc, Ekyu Hana, Anita W.Y. Ho-Baillieb, , Nochang Parka,
T ⁎
a
Electronic Convergence Material & Device Research Center, Korea Electronics Technology Institute, Seong-Nam, Republic of Korea School of Photovoltaic and Renewable Energy Engineering, University of New South Wales (UNSW), Sydney, NSW 2052, Australia c Electron Microscope Unit, University of New South Wales, Sydney 2052, Australia b
A R T I C LE I N FO
A B S T R A C T
Keywords: Solar Cells Perovskite Moisture barrier Stability Atomic layer deposition
In this work, we employ atomic layer deposition (ALD) to form Al2O3 layer as an encapsulant for perovskite solar cells (PSCs). Al2O3 layer deposited at temperature as low as 95 °C achieves water vapor transmission rate (WVTR) of 1.84 × 10−2 g m−2 d−1 at 45 °C–100%RH when thermal ALD is used. In order to test the moisture barrier capability of Al2O3 layer for PSCs, mesoporous perovskite devices, with spiro-OMeTAD or PTAA as hole transport layer (HTM) encapsulated by 50 nm Al2O3 film, are exposed to 65 °C–85%RH for 350 h and their stabilities are monitored. We find that the color of perovskite does not change after 350 h of exposure regardless of the type of HTM used. With regards to Th-ALD encapsulated devices, PTAA based PSCs experienced a smaller power conversion efficiency (PCE) drop than spiro-OMeTAD based PSCs after thermal stress at 65 °C. This is due to the presence of pinholes within spiro-OMeTAD layer after thermal stress which are not observed in PTAA. Finally, we successfully achieve excellent durability test results for mesoporous (HC(NH2)2PbI3)0.85(CH3NH3PbBr3)0.15/PTAA devices encapsulated by 50 nm Al2O3 with less than 4% drop in PCE after 7500 h (> 10 months) of exposure to 50%RH under room temperature.
1. Introduction Recently, organic–inorganic halide perovskite solar cells (PSCs) have emerged as a promising alternative to existing photovoltaic technologies [1] due to its ease of fabrication and excellent absorption and charge transport properties. These properties led to rapid improvement in power conversion efficiency [2–5] reaching over 22% in only a few years [6,7]. However, for PSCs to be commercially viable, instability needs to be overcome which is most severe at elevated temperatures or under the presence of moisture [8–11]. Han et al. reported that the maximum power output of methyl ammonium lead iodide (MAPbI3) with a spiro-OMeTAD of hole transfer materials (HTM) decreased by 50% in 70 h at 55 °C [10]. Conings et al. reported the intrinsic instability of perovskite layer exposed to annealing at 85 °C under a range of environmental conditions [12]. Divitini et al. claimed the thermal instability of perovskite through the in situ TEM observation [13]. However, for the most commonly used perovskite material ((HC(NH2)2PbI3)x(CH3NH3PbBr3)y ((FAPbI3)x(MAPbBr3)y) used in the state-of-the-art devices, the intrinsic instability issue under heat stress is mitigated [14]. With regards to the stability of carrier transport layers, You et al. showed that devices using organic charge transport layers
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degraded dramatically, with PCE drop to close to zero after only 5 days in an ambient environment [15]. Perovskite is highly prone to moisture which corrodes under moisture, resulting in decomposition [10,16–18]. Fortunately, methods to protect the PSCs from moisture have been demonstrated showing encouraging results under high humidity [19–23] such as the use of moisture barrier film [24,25]. Atomic layer deposition (ALD) is one of the effective techniques to fabricate moisture barrier layer which has been shown to improve the stability of various types of devices such as organic light emitting diodes (OLED), organic photovoltaic devices (OPV), and perovskite solar cell due to the dense and uniform film formation by ALD. Koushik et al. reported the use of ALD aluminum oxide (Al2O3) as an interface layer between the perovskite absorber and hole transport layer for planar MAPbI3 solar cells which provides moisture protection and tunneling contact. Un-encapsulated devices retaining about 60–70% of its initial PCE 70 days of humidity exposure (40–70% %RH at room temperature) [26]. Wang et al. reported the use of ALD to fabricate Al2O3/MgO which when coupled with solution processed polymer achieved high moisture barrier has a superior water vapor transmission ratio (WVTR) of 1.05 × 106 g m−2 d−1 at 60 °C–100%RH [27]. Nam et al. demonstrated WVTR of 2 × 10-3 g m−2 d−1 at 85 °C–85%RH using a composite layer of ALD
Corresponding authors. E-mail addresses: [email protected] (A.W.Y. Ho-Baillie), [email protected] (N. Park).
https://doi.org/10.1016/j.solmat.2018.08.016 Received 11 June 2018; Received in revised form 11 August 2018; Accepted 22 August 2018 0927-0248/ © 2018 Elsevier B.V. All rights reserved.
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Fig. 1. Cross-sectional images of complete (HC(NH2)2PbI3)0.85(CH3NH3PbBr3)0.15/Au or (FAPbI3)0.85(MAPbBr3)0.15/Au perovskite solar cells using (a) spiroOMeTAD, and (b) PTAA as HTM.
operating temperature on the both electrical properties and degradation of PSCs. In addition, we carried out stability test under 65 °C-85 RH % and 25 °C-50 RH%.
Al2O3 and chemical-vapor-deposited (CVD) graphene [28]. However, for the disposition of Al2O3 in that work, high temperature was required, which could damage crystallinity and morphology of perovskite absorber layer [29,30] and commonly used hole transport materials (HTMs) such as spiro-OMeTAD [31,32]. Im et al. encapsulated PSCs with multilayer of pV3D3 and Al2O3 below 100 °C, whose WVTR was on the order of 10-4 g m−2 d−1 at 38 °C–90%RH [33]. They reported that PSCs experienced severe degradation during the ALD process even if temperature was below 100 °C when spiro-OMeTAD was used as a HTM. Therefore, it is essential to understand the degradation mechanism during ALD process. In addition, it is important to investigate the long-term stability of PSCs which are encapsulated by Al2O3 thin film. The objectives of the research reported here are: (1) to study the effect of ALD processing temperature on both PSC's properties and degradation; (2) examine the degradation mechanism during ALD process; (3) and to investigate the long-term stability of PSCs encapsulated by ALD Al2O3 moisture barrier film. For this, we fabricated two kinds of PSCs: (a) (HC(NH2)2PbI3)0.85(CH3NH3PbBr3)0.15/spiroOMeTAD/Au, and (b) (HC(NH2)2PbI3)0.85(CH3NH3PbBr3)0.15/PTAA/Au (Fig. 1). The details of I-V characteristics of device performance are given in Fig. 2. After fabricating solar cells, we performed 50 nm Al2O3 deposition on PSCs (Fig. 3), and then examined the effect of ALD
2. Experimental 2.1. Materials Perovskite absorber material: 15 g formamidine acetate and 30 mL HI (57 wt% in water) were dissolved in 100 mL ethanol at 0 °C for 2 h with stirring to obtain HC(NH2)2I. A solution of HC(NH2)2I was dried using rotary evaporator at 50 °C for 1 h. Following recrystallization from ethanol, white crystals were washed with diethyl ether. To synthesize CH3NH3Br, 11 mL methylamine (33 wt% in water) and 10 mL HBr (48 wt % in water) were mixed in 100 mL ethanol at 0 °C for 2 h with stirring. Prepared HC(NH2)2I (or CH3NH3Br) and PbI2 (or PbBr2) were dissolved at room temperature in dimethylformamide (DMF): dimethyl sulfoxide (DMSO) mixed solvent (1:0.25 (v/v)) to obtain 1.2 M HC(NH2)2PbI3 (or CH3NH3Br3) solution. To prepare the absorption layer precursor solution, HC(NH2)2PbI3 and CH3NH3Br3 solution were mixed with the specific volume ratio for (HC(NH2)2PbI3)0.85(CH3NH3PbBr3)0.15. And then extra PbI2 (5 mol% to HC(NH2)2PbI3) were dissolved in the synthesized (HC(NH2)2PbI3)0.85(CH3NH3PbBr3)0.15 solution by heating at 60 °C for
Fig. 2. Electrical properties of (FAPbI3)0.85(MAPbBr3)0.15/spiro-OMeTAD/Au, (b) (FAPbI3)0.85(MAPbBr3)0.15/PTAA/Au device performances.
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Fig. 3. (a) TEM image of a 50 nm ALD Al2O3 layer on a PSC and its corresponding (b) elemental mapping for Al, and (c) elemental mapping for O. Al2O3 deposition was performed at 95 °C.
generated with 100 W of RF power and Ar gas which was charge of improving generation of plasma were introduced for 3 s with 100 sccm, (4) Excess reactant was purged by Ar for 15 with 300 sccm. To investigate the effect of temperature on PSCs, three Th-ALD operating temperatures at 95 °C, 105 °C, 120 °C to deposit 50 nm of Al2O3 in the study. Once the desired temperature is chosen, e.g., at 95 °C, 50 nm thick Al2O3 were fabricated on PEN films using the two different ALD processes for the measurement of WVTR. The WVTR values were measured with AQUATRON Model 2 (Mocon Co., USA).
30 min. Materials used in this work were purchased from Alfa Aesar or Lumtec or Sigma-Aldrich and were not purified. 2.2. Cell fabrication To fabricated mesoporous solar cell for the experimental work, firstly, a compact TiO2 layer (c-TiO2) was deposited by spray pyrolysis (~ 50 nm) using 20 mM titanium diisopropoxide bis(acetylacetonate) solution at 450 °C on the clean FTO glass (TEC8). After the deposition of c-TiO2 layer, 150 mg/mL of mesoporous TiO2 paste (m-TiO2, Dyesol 30 NR-D) in ethanol was spin-coated at 5000 rpm (acceleration of 2000 rpm s-1) for 10 s. The deposited substrates were heated at 100 °C for 10 min followed by sintering at 500 °C for 30 min. The prepared perovskite solution was spun at 2000 rpm (acceleration of 200 rpm s-1) and 6000 rpm (acceleration of 2000 rpm s-1) for 10 s and 30 s. The antisolvent chlorobenzene was drop-casted (110 mL) during the last 20 s of the second spin-coating step. The coated perovskite film was dried on a hot plate at 100 °C for 20 min. For the deposition of hole transport layer, a solution containing 41.6 mg of spiro-OMeTAD, 7.5 µL of a 500 mg/mL lithium bis (trifluoromethylsulphonyl)-imide (Li-TFSI) in acetonitrile and 16.9 µL of 4-tert-butylpyridine (tBP) in 0.5 mL chlorobenzene was spin-coated respectively on the perovskite/m-TiO2/blTiO2/FTO substrate at 2000 rpm (acceleration of 1200 rpm s-1) for 20 s. Alternatively a solution containing 15 mg of PTAA, 15 µL of a 170 mg/ 1 mL Li-TFSI in acetonitrile and 7.5 µL of tBP in 1.5 mL toluene was deposited using the same conditions. All films on m-TiO2 were prepared in nitrogen filled glovebox. Finally, 100 nm of gold electrode was deposited by thermal evaporation.
2.4. Characterization and stability The J–V measurements were performed using a solar cell I–V testing system (LAB 200, McScence, Korea) under illumination power of 100 mW/cm2 by an AM1.5 G solar simulator (Oriel model 94023A) with 0.159 cm2 aperture and a scan rate of 1.2 V/s. X-ray diffraction (XRD) patterns were measured using a XRD-6100 (SHIMADZU, JAPAN) with a Cu Kα radiation source (λ = 0.1541 nm) at 30 kV and 30 mA. Reflectance (R) and transmittance (T) were measured using a Varian Cary UV–VIS–NIR spectrophotometer in the 300–900 nm wavelength range. Field emission scanning electron microscopy (ESEM–FEG XL30, FEI, Holland) was employed to investigate the effect of thermal stress on the morphology and structure of the perovskite. To carry out transmission electron microscopy (TEM) analysis, samples mounted on Cu girds were prepared using Focused Ion beam. The prepared sample was measured with JEM-2100F (JEOL LTD) at operating 200 kV equipped with EDS (TEM 250, Oxford Instruments). Stability tests were carried out in two kinds of conditions. These include: (a) 65 °C-85%RH (SE-CT-02, SUKSAN Tech. South Korea), and (b) ambient condition.
2.3. ALD process 3. Results and discussion Two kinds of ALD processes were employed in this work: 1) thermal ALD (Th-ALD), and plasma ALD (Pl-ALD). Th-ALD used Trimethylaluminum (TMA) and H2O (Classic, CN1, South Korea), and Pl-ALD employed TMA and O2 plasma as precursors (Lucida M200-PL, NCD, South Korea). Th-ALD procedure consists of 4 steps: (1) TMA was pulsed into reaction chamber for 0.1 s using N2 gas with 500 sccm after sample loading, (2) Excess TMA and methane reaction products were purged by N2 for 5 s with 500 sccm, (3) Water vapor was pulsed into reaction chamber for 0.1 s by N2 with 500 sccm, Finally (4) N2 was purged to remove excess water vapor for 5 s with 500 sccm. For the PlALD process, the procedure is similar to that of Th-ALD. Pl-ALD process consists of 4 steps: (1) TMA was injected and pulsed into reaction chamber for 0.2 s using Ar gas with 100 sccm after sample loading, (2) Excess TMA precursor was purged by Ar for 15 s with 300 sccm, (3) O2 reactant was injected for 2 s by Ar with 200 sccm and O2 plasma was
3.1. The effect of ALD temperature on PSCs To examine the effect of ALD temperature on the electrical property of PSCs, mesoporous (FAPbI3)0.85(MAPbBr3)0.15/spiro-OMeTAD cells were fabricated followed by Th-ALD of 50 nm of Al2O3. The PCE's of the cells before and after the ALD were measured as shown in Fig. 4. The PSC's with Al2O3 deposited at 120 °C experienced the most dramatic degradation, an average of 35% drop in PCE after the ALD process. As the ALD temperature decreases, so is the averaged PCE degradation that was reduced to 16% at 95 °C. Based on our previous finding [34], the crystallization of spiroOMeTAD is accelerated in high temperature, which results in deterioration of spiro-OMeTAD. We also found out that PTAA is more stable than spiro-OMeTAD at elevated temperature. We, therefore, repeated
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Fig. 4. Normalized averaged PCE of (FAPbI3)0.85(MAPbBr3)0.15/spiro-OMeTAD /Au/50 nm Al2O3 solar cells before and after ALD of Al2O3 at different temperatures.
Fig. 6. Normalized averaged PCE change of (FAPbI3)0.85(MAPbBr3)0.15/PTAA/ Au cells as type of ALD. The Al2O3 was formed at 95 °C.
with previous researches [34–36]. In addition, we performed XRD on perovskite layer after exposure (for the same amount of time that the PSC would have been exposed to during ALD deposition) to elevated temperature at 95 °C. Results in Fig. 5(b) shows negligible changes. The 12.5° PbI2 peak was already in the (FAPbI3)0.85(MAPbBr3)0.15 film before the ALD process which had an excess (5 mol%) of PbI2 in the precursor for improving photovoltaic performance [34,37]. Due to the negligible change in the perovskite layer after the 95 °C ALD process, the observed PCE drop in Figs. 4, 5 originates from the failure of HTM or the associated interface rather than the intrinsic perovskite degradation induced by heat during the ALD process. To examine whether the type of ALD process will have an effect on PSC performance, Pl-ALD at 95 °C was used to deposit 50 nm of Al2O3 on mesoporous (FAPbI3)0.85(MAPbBr3)0.15/PTAA/Au cells. Fig. 6 shows that the PCE's before and after the two types of ALD processes. Very similar degradation is observed suggesting that the reactant (O2 or H2O) and the plasma used in the ALD process has less effect than the temperature used on the performance of PSCs. 3.2. The moisture barrier property of Al2O3 The WVTR values of 50 nm Al2O3 layer by Th-ALD was measured by which were 1.84 × 10-2 g m−2 d−1 at 45 °C–100%RH. Using the Eq. (1) of Ziegel et al.[38], the diffusion coefficient was calculated to be 5.84 × 10–16 cm2/s.
D = l 2/7.1999t0.5
(1)
where, l and t0.5 are the thickness of the film and the time required to reach a transmission rate equal to the half of the steady state value, respectively. t0.5 can be determined with time dependent WVTR as a function of time. As a comparison, Carcia et al. [39] determined that D = 1.4 × 10−17 cm2/s at 38 °C for diffusion through a Th-ALD Al2O3 barrier thin film that was 25 nm thick. They calculated the diffusion coefficient to be D = P/S. At 38 °C, the WVTR for the single layer Al2O3 ALD film corresponded to be a permeability of P = 7.33 × 10–19 g H2O/cm s atm. They used the solubility of sputtered Al2O3 of S = 0.029 g/cm3 atm [40]. We formed 50 nm Al2O3 on (FAPbI3)0.85(MAPbBr3)0.15/spiroOMeTAD/Au test device to investigate the moisture barrier property. The samples were stored in 45 °C–85% chamber for 72 h. We then investigated the stability of the films, in terms of color change. Fig. 7
Fig. 5. (a) Normalized averaged PCE of perovskite solar cells with different HTMs before and after 95 °C Th-ALD of Al2O3. (b) XRD spectra change of perovskite layer according to ALD temperature.
the 95 °C Th-ALD of 50 nm Al2O3 on cells that use PTAA instead of spiro-OMeTAD as HTM. Fig. 5(a) shows the change in PCE's for PSCs using different HTM before and after ALD. PTAA-based device is more stable than that with spiro-OMeTAD experiencing only 12% drop in PCE after the ALD process at 95 °C. The results are in good agreement 40
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Fig. 7. Photograph of perovskite/spiro-OMeTAD test devices (a) before and (b) after damp heat test (45 °C–85%RH) for 72 h. Photograph of perovskite/spiroOMeTAD/Al2O3 test devices (c) before and (d) after the same damp heat test.
3.3. The effectiveness of using Al2O3 as an encapsulant for perovskite test devices
shows the un-protected (FAPbI3)0.85(MAPbBr3)0.15/spiro-OMeTAD/Au test devices underwent color change suggesting degradation after damp heat exposure (cf. Fig. 7a and b) while the ALD 50 nm Al2O3 encapsulated test devices did not (cf. Fig. 7c and d) showing the effectiveness of the Al2O3 layer as a moisture barrier film even when it was deposited at a temperature as low as 95 °C. This indicates that 50 nm Al2O3 thin film can protect the sample from the moisture for 72 h under 45–85% condition. For obtaining further insight into the moisture barrier property of Al2O3, the absorption coefficients were plotted (Fig. 8) using the formula αeff = deff−1 ln(1 + A/Tspecular), for the negligible film-glass reflectance case [41,42]. The corresponding layer thickness deff = 500 nm and A/Tspecular were obtained by the relevant absorption spectrum. Firstly, the absorption coefficient curve of the test device without Al2O3 layer shows sharp shoulder before the damp heat test which represents perovskite layer is not damaged as a photovoltaic absorber. However, after the damp heat test, the blunt shoulder appeared and the absorption coefficient is decreased. Also, the slope of the absorption coefficients at the band edge, which also could be described as Urbach energy, was decreased when the damp heat test was applied on the test device without Al2O3 layer. These mean that perovskite layer was degraded under the damp heat test for the test sample without Al2O3 layer. On the other hand, as a similar result with Fig. 7, the sharp shoulder and the slope of band edge of absorption coefficient were maintained at the sample with Al2O3 layer. This indicates that 50 nm Al2O3 thin film can protect the sample from the moisture for 72 h under 45 °C–85%RH condition.
In order to examine the effectiveness of 50 nm ALD Al2O3 as an encapsulant for PSC's, mesoporous (FAPbI3)0.85(MAPbBr3)0.15/spiroOMeTAD/Au and (FAPbI3)0.85(MAPbBr3)0.15/PTAA/Au test devices that received Th-ALD of 50 nm Al2O3 deposition were stored in 65 °C and 65 °C-85%RH condition for 350 h. We then investigated the stability of the devices, in terms of photovoltaic parameter changes (Fig. 9). This shows that the degradation rate at 65 °C-85%RH is higher than that at 65 °C. This means that the device is slightly affected by moisture after 50 h. However, the influence of moisture is small since the WVTR of Al2O3 is low. The WVTR value is dependent on temperature [43]. Therefore, the lower the temperature, the lower the effect of moisture. We will show the stability result at room temperature in long-term stability section. Fig. 9 demonstrates that the deterioration rates on the all parameters were low when we used PTAA as an HTM. Interestingly, the PCE drop rate for both 65 °C–85%RH and 65 °C (in inert) condition showed similar results as shown in Fig. 9. This implies that 50 nm Al2O3 thin film protects moisture infiltration effectively, but the intrinsic thermal damage to PSCs still existed. As shown in Fig. 9b–d, while VOC of the test devices remains relatively stable, and JSC and FF drop causing PCE to drop at both stress conditions. The possible cause of the drop in JSC and FF is due to the degradation of perovskite/spiro-OMeTAD interface as discussed in our previous report [34]. Cross sectional SEM images were taken on mesoporous (FAPbI3)0.85(MAPbBr3)0.15/spiro-OMeTAD/Au and (FAPbI3)0.85
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Fig. 9. Evolution of (a) PCE, (b) VOC, (c) JSC, and (d) FF as function of time at 65 °C–85%RH or 65 °C (in inert) condition. Two kinds of PSCs were used in this experiment: (a) perovskite/spiro-OMeTAD/Au/Al2O3 and (b) perovskite/ PTAA/Au/Al2O3.
Fig. 8. Absorption coefficient of perovskite/spiro-OMeTAD test devices before and after damp heat test (45 °C–85%RH) on perovskite/spiro-OMeTAD/Au/ Al2O3 and (b) perovskite/PTAA/Au/Al2O3 test structure respectively.
shown in Fig. 11b and d. Therefore, thermal stress is a critical factor for the onset of degradation within the spiro-OMeTAD layer resulting in the failure of the rest of the device. This agrees with our results in Fig. 7 and results from our previous reports [34]. Interestingly, the pinhole did not collapse the spiro-OMeTAD layer even under 3 weeks (~ 500 h) thermal stress. It is reasonable that gold or Al2O3 layer could be decent in the presence of pinholes at spiro-OMeTAD. Therefore, we confirmed the presence of barrier such as Al2O3 effectively slows down moisture ingress even if there are pinholes present in the spiro-OMeTAD induced by heat.
(MAPbBr3)0.15/PTAA/Au test devices without Al2O3 before and after two different storage conditions to investigate different degradation processes. The conditions are: (a) ambient condition (75 ± 10%RH) at room temperature, (Fig. 10b and e) and (b) inert condition (in N2 filled glovebox) at 65 °C, (Fig. 10c and f). As seen in Fig. 10b and Fig. 10e, after 1 week of storage in ambient, a small amount of PbI2 is formed in the perovskite layer (circled by red dotted lines in Fig. 10b and e), which has been reported to adversely affect solar cell performance [44]. The presence of additive Li-TFSI salt in our HTMs which attracts and retains water is also believed to accelerate moisture ingress [45]. The formation of PbI2 is also observed for both types of test devices (Fig. 10c and f) after 1 week of thermal stress at 50–60 °C although unexpected as the thermal stress was conducted in inert condition. Yang et al. [46] has previously reported the formation of PbI2 under thermal stress at 65 °C even in ambient. The appearance of PbI2 in our case could be due to the intrinsic thermal instability of perovskite material as reported before [12]. Further investigation shows that pinholes were formed in spiroOMeTAD after 2 weeks of thermal stress at 65 °C in N2 (Fig. 11c). These pinholes further promote moisture ingress into perovskite layer which leads to rapid degradation [47]. On the other hand, pinhole is not observed when spiro-OMeTAD based test devices are stored in ambient only as shown in Fig. 11a. It should be noted that PTAA layers remained intact whether they went through 2 weeks of ambient storage (75 ± 10%RH) at room temperature or thermal stress (65 °C) in N2 as
3.4. The results of stability of PSCs Finally, complete devices encapsulated by Th-ALD Al2O3 were subjected to environmental tests to examine their stabilities in terms of changes in PCE. We fabricated full mesoporous (FAPbI3)0.85 (MAPbBr3)0.15/spiro-OMeTAD/Au/50 nm Al2O3 and (FAPbI3)0.85 (MAPbBr3)0.15/PTAA/Au/50 nm Al2O3 solar cells which were then stored at room temperature in 50%RH for 7500 h (~ 10 months). The color of PSCs did not change regardless of the type of HTM used. In the case of PTAA based devices, the drop in maximum power was below 4% (Fig. 12). Spiro-OMeTAD suffered 8% drop in maximum power under the same conditions. However, it is worth to note that these results shows better long-term stabilities even at 50%RH compared to our
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Fig. 10. Cross-sectional SEM images of spiro-OMeTAD based test devices a) as fabricated, (b) 1 week after exposure to ambient condition (75 ± 10%RH) at room temperature and c) 1 week after continuous thermal stress at 65 °C in inert condition (N2). d-f) Cross sectional SEM images of PTAA based test devices under the same conditions.
Fig. 11. Cross sectional SEM images of (a) spiro-OMeTAD and (b) PTAA based test devices 2 weeks after storage in ambient (75 ± 10%RH) at room temperature. Cross sectional SEM images of (c) spiro-OMeTAD and (d) PTAA based test devices after 2 weeks of continuous thermal stress at 65 °C in inert condition (N2).
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Fig. 12. PCE's of full mesoporous (FAPbI3)0.85(MAPbBr3)0.15/HTM /Au/50 nm Al2O3 PSCs during storage at room temperature (dark) in 50%RH for 7500 h (~ 10 months).
previous report [34] at room temperature under inert condition. This is likely caused by more stabilized FA-based perovskite as reported before [48]. These results indicate that the Al2O3 layer is effective in preventing moisture ingress improving the stability of PSC's.
4. Conclusions This work demonstrated the use of ALD 50 nm Al2O3 layer on top of PSCs as an encapsulant to enhance the stability of PSCs by preventing moisture ingress. ALD process temperatures were varied from 95 °C, to 105 °C, and to 120 °C. It was found that ALD process temperature of 95 °C induces the least damage to PSC's while still providing an effective moisture barrier. The WVTR values of 50 nm Al2O3 layers by optimized thermal ALD process at a temperature as low as 95 °C was measured to be 1.84 × 10-2 g m−2 d−1 at 45 °C–85%RH. Results of Al2O3 encapsulated perovskite/HTM/Au devices after 72 h of damp heat (45 °C–85%RH) demonstrate the effectiveness of Al2O3 as a good moisture barrier. In addition, we found out that PTAA based PSC's suffered a smaller drop in power conversion efficiency than spiroOMeTAD based PSC's after thermal ALD process. PTAA was also more stable than spiro-OMeTAD at high temperature. This is attributed to the pinholes formed within spiro-OMeTAD layer, under thermal stress (from ALD process or from environmental tests). These pinholes are not present in PTAA. Using the optimized Th-ALD process, 50 nm Al2O3 were used to encapsulate full mesoporous (FAPbI3)0.85(MAPbBr3)0.15/ HTM/Au/50 nm Al2O3 devices to examine their long-term stabilities. PTAA-based devices experienced less than 4% drop in PCE after 7500 h of 50%RH at room temperature. With further development in the ALD process to minimize thermal damage to perovskite solar cells, we propose that ALD process is an excellent approach to encapsulate perovskite solar cells.
Acknowledgements The authors in this paper, Eun Young Choi and Jincheol Kim contributed equally to this work. This research was supported by the Technology Development Program to Solve Climate Changes of the National Research Foundation of South Korea (NRF) funded by the Ministry of Science and ICT(NRF-2017M1A2A2048905). This research was also supported by the Australian Centre for Advanced Photovoltaics (ACAP) which encompasses the Australian-based activities of the Australia U.S. Institute for Advanced Photovoltaics (AUSIAPV) and is supported by the Australian Government through the Australian Renewable Energy Agency (ARENA). We thank the Analytical Centre at UNSW for their technical support.
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Solar Energy Materials and Solar Cells journal homepage: www.elsevier.com/locate/solmat
Enhancing stability for organic-inorganic perovskite solar cells by atomic layer deposited Al2O3 encapsulation ⁎
Eun Young Choia, Jincheol Kimb, Sean Limc, Ekyu Hana, Anita W.Y. Ho-Baillieb, , Nochang Parka,
T ⁎
a
Electronic Convergence Material & Device Research Center, Korea Electronics Technology Institute, Seong-Nam, Republic of Korea School of Photovoltaic and Renewable Energy Engineering, University of New South Wales (UNSW), Sydney, NSW 2052, Australia c Electron Microscope Unit, University of New South Wales, Sydney 2052, Australia b
A R T I C LE I N FO
A B S T R A C T
Keywords: Solar Cells Perovskite Moisture barrier Stability Atomic layer deposition
In this work, we employ atomic layer deposition (ALD) to form Al2O3 layer as an encapsulant for perovskite solar cells (PSCs). Al2O3 layer deposited at temperature as low as 95 °C achieves water vapor transmission rate (WVTR) of 1.84 × 10−2 g m−2 d−1 at 45 °C–100%RH when thermal ALD is used. In order to test the moisture barrier capability of Al2O3 layer for PSCs, mesoporous perovskite devices, with spiro-OMeTAD or PTAA as hole transport layer (HTM) encapsulated by 50 nm Al2O3 film, are exposed to 65 °C–85%RH for 350 h and their stabilities are monitored. We find that the color of perovskite does not change after 350 h of exposure regardless of the type of HTM used. With regards to Th-ALD encapsulated devices, PTAA based PSCs experienced a smaller power conversion efficiency (PCE) drop than spiro-OMeTAD based PSCs after thermal stress at 65 °C. This is due to the presence of pinholes within spiro-OMeTAD layer after thermal stress which are not observed in PTAA. Finally, we successfully achieve excellent durability test results for mesoporous (HC(NH2)2PbI3)0.85(CH3NH3PbBr3)0.15/PTAA devices encapsulated by 50 nm Al2O3 with less than 4% drop in PCE after 7500 h (> 10 months) of exposure to 50%RH under room temperature.
1. Introduction Recently, organic–inorganic halide perovskite solar cells (PSCs) have emerged as a promising alternative to existing photovoltaic technologies [1] due to its ease of fabrication and excellent absorption and charge transport properties. These properties led to rapid improvement in power conversion efficiency [2–5] reaching over 22% in only a few years [6,7]. However, for PSCs to be commercially viable, instability needs to be overcome which is most severe at elevated temperatures or under the presence of moisture [8–11]. Han et al. reported that the maximum power output of methyl ammonium lead iodide (MAPbI3) with a spiro-OMeTAD of hole transfer materials (HTM) decreased by 50% in 70 h at 55 °C [10]. Conings et al. reported the intrinsic instability of perovskite layer exposed to annealing at 85 °C under a range of environmental conditions [12]. Divitini et al. claimed the thermal instability of perovskite through the in situ TEM observation [13]. However, for the most commonly used perovskite material ((HC(NH2)2PbI3)x(CH3NH3PbBr3)y ((FAPbI3)x(MAPbBr3)y) used in the state-of-the-art devices, the intrinsic instability issue under heat stress is mitigated [14]. With regards to the stability of carrier transport layers, You et al. showed that devices using organic charge transport layers
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degraded dramatically, with PCE drop to close to zero after only 5 days in an ambient environment [15]. Perovskite is highly prone to moisture which corrodes under moisture, resulting in decomposition [10,16–18]. Fortunately, methods to protect the PSCs from moisture have been demonstrated showing encouraging results under high humidity [19–23] such as the use of moisture barrier film [24,25]. Atomic layer deposition (ALD) is one of the effective techniques to fabricate moisture barrier layer which has been shown to improve the stability of various types of devices such as organic light emitting diodes (OLED), organic photovoltaic devices (OPV), and perovskite solar cell due to the dense and uniform film formation by ALD. Koushik et al. reported the use of ALD aluminum oxide (Al2O3) as an interface layer between the perovskite absorber and hole transport layer for planar MAPbI3 solar cells which provides moisture protection and tunneling contact. Un-encapsulated devices retaining about 60–70% of its initial PCE 70 days of humidity exposure (40–70% %RH at room temperature) [26]. Wang et al. reported the use of ALD to fabricate Al2O3/MgO which when coupled with solution processed polymer achieved high moisture barrier has a superior water vapor transmission ratio (WVTR) of 1.05 × 106 g m−2 d−1 at 60 °C–100%RH [27]. Nam et al. demonstrated WVTR of 2 × 10-3 g m−2 d−1 at 85 °C–85%RH using a composite layer of ALD
Corresponding authors. E-mail addresses: [email protected] (A.W.Y. Ho-Baillie), [email protected] (N. Park).
https://doi.org/10.1016/j.solmat.2018.08.016 Received 11 June 2018; Received in revised form 11 August 2018; Accepted 22 August 2018 0927-0248/ © 2018 Elsevier B.V. All rights reserved.
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Fig. 1. Cross-sectional images of complete (HC(NH2)2PbI3)0.85(CH3NH3PbBr3)0.15/Au or (FAPbI3)0.85(MAPbBr3)0.15/Au perovskite solar cells using (a) spiroOMeTAD, and (b) PTAA as HTM.
operating temperature on the both electrical properties and degradation of PSCs. In addition, we carried out stability test under 65 °C-85 RH % and 25 °C-50 RH%.
Al2O3 and chemical-vapor-deposited (CVD) graphene [28]. However, for the disposition of Al2O3 in that work, high temperature was required, which could damage crystallinity and morphology of perovskite absorber layer [29,30] and commonly used hole transport materials (HTMs) such as spiro-OMeTAD [31,32]. Im et al. encapsulated PSCs with multilayer of pV3D3 and Al2O3 below 100 °C, whose WVTR was on the order of 10-4 g m−2 d−1 at 38 °C–90%RH [33]. They reported that PSCs experienced severe degradation during the ALD process even if temperature was below 100 °C when spiro-OMeTAD was used as a HTM. Therefore, it is essential to understand the degradation mechanism during ALD process. In addition, it is important to investigate the long-term stability of PSCs which are encapsulated by Al2O3 thin film. The objectives of the research reported here are: (1) to study the effect of ALD processing temperature on both PSC's properties and degradation; (2) examine the degradation mechanism during ALD process; (3) and to investigate the long-term stability of PSCs encapsulated by ALD Al2O3 moisture barrier film. For this, we fabricated two kinds of PSCs: (a) (HC(NH2)2PbI3)0.85(CH3NH3PbBr3)0.15/spiroOMeTAD/Au, and (b) (HC(NH2)2PbI3)0.85(CH3NH3PbBr3)0.15/PTAA/Au (Fig. 1). The details of I-V characteristics of device performance are given in Fig. 2. After fabricating solar cells, we performed 50 nm Al2O3 deposition on PSCs (Fig. 3), and then examined the effect of ALD
2. Experimental 2.1. Materials Perovskite absorber material: 15 g formamidine acetate and 30 mL HI (57 wt% in water) were dissolved in 100 mL ethanol at 0 °C for 2 h with stirring to obtain HC(NH2)2I. A solution of HC(NH2)2I was dried using rotary evaporator at 50 °C for 1 h. Following recrystallization from ethanol, white crystals were washed with diethyl ether. To synthesize CH3NH3Br, 11 mL methylamine (33 wt% in water) and 10 mL HBr (48 wt % in water) were mixed in 100 mL ethanol at 0 °C for 2 h with stirring. Prepared HC(NH2)2I (or CH3NH3Br) and PbI2 (or PbBr2) were dissolved at room temperature in dimethylformamide (DMF): dimethyl sulfoxide (DMSO) mixed solvent (1:0.25 (v/v)) to obtain 1.2 M HC(NH2)2PbI3 (or CH3NH3Br3) solution. To prepare the absorption layer precursor solution, HC(NH2)2PbI3 and CH3NH3Br3 solution were mixed with the specific volume ratio for (HC(NH2)2PbI3)0.85(CH3NH3PbBr3)0.15. And then extra PbI2 (5 mol% to HC(NH2)2PbI3) were dissolved in the synthesized (HC(NH2)2PbI3)0.85(CH3NH3PbBr3)0.15 solution by heating at 60 °C for
Fig. 2. Electrical properties of (FAPbI3)0.85(MAPbBr3)0.15/spiro-OMeTAD/Au, (b) (FAPbI3)0.85(MAPbBr3)0.15/PTAA/Au device performances.
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Fig. 3. (a) TEM image of a 50 nm ALD Al2O3 layer on a PSC and its corresponding (b) elemental mapping for Al, and (c) elemental mapping for O. Al2O3 deposition was performed at 95 °C.
generated with 100 W of RF power and Ar gas which was charge of improving generation of plasma were introduced for 3 s with 100 sccm, (4) Excess reactant was purged by Ar for 15 with 300 sccm. To investigate the effect of temperature on PSCs, three Th-ALD operating temperatures at 95 °C, 105 °C, 120 °C to deposit 50 nm of Al2O3 in the study. Once the desired temperature is chosen, e.g., at 95 °C, 50 nm thick Al2O3 were fabricated on PEN films using the two different ALD processes for the measurement of WVTR. The WVTR values were measured with AQUATRON Model 2 (Mocon Co., USA).
30 min. Materials used in this work were purchased from Alfa Aesar or Lumtec or Sigma-Aldrich and were not purified. 2.2. Cell fabrication To fabricated mesoporous solar cell for the experimental work, firstly, a compact TiO2 layer (c-TiO2) was deposited by spray pyrolysis (~ 50 nm) using 20 mM titanium diisopropoxide bis(acetylacetonate) solution at 450 °C on the clean FTO glass (TEC8). After the deposition of c-TiO2 layer, 150 mg/mL of mesoporous TiO2 paste (m-TiO2, Dyesol 30 NR-D) in ethanol was spin-coated at 5000 rpm (acceleration of 2000 rpm s-1) for 10 s. The deposited substrates were heated at 100 °C for 10 min followed by sintering at 500 °C for 30 min. The prepared perovskite solution was spun at 2000 rpm (acceleration of 200 rpm s-1) and 6000 rpm (acceleration of 2000 rpm s-1) for 10 s and 30 s. The antisolvent chlorobenzene was drop-casted (110 mL) during the last 20 s of the second spin-coating step. The coated perovskite film was dried on a hot plate at 100 °C for 20 min. For the deposition of hole transport layer, a solution containing 41.6 mg of spiro-OMeTAD, 7.5 µL of a 500 mg/mL lithium bis (trifluoromethylsulphonyl)-imide (Li-TFSI) in acetonitrile and 16.9 µL of 4-tert-butylpyridine (tBP) in 0.5 mL chlorobenzene was spin-coated respectively on the perovskite/m-TiO2/blTiO2/FTO substrate at 2000 rpm (acceleration of 1200 rpm s-1) for 20 s. Alternatively a solution containing 15 mg of PTAA, 15 µL of a 170 mg/ 1 mL Li-TFSI in acetonitrile and 7.5 µL of tBP in 1.5 mL toluene was deposited using the same conditions. All films on m-TiO2 were prepared in nitrogen filled glovebox. Finally, 100 nm of gold electrode was deposited by thermal evaporation.
2.4. Characterization and stability The J–V measurements were performed using a solar cell I–V testing system (LAB 200, McScence, Korea) under illumination power of 100 mW/cm2 by an AM1.5 G solar simulator (Oriel model 94023A) with 0.159 cm2 aperture and a scan rate of 1.2 V/s. X-ray diffraction (XRD) patterns were measured using a XRD-6100 (SHIMADZU, JAPAN) with a Cu Kα radiation source (λ = 0.1541 nm) at 30 kV and 30 mA. Reflectance (R) and transmittance (T) were measured using a Varian Cary UV–VIS–NIR spectrophotometer in the 300–900 nm wavelength range. Field emission scanning electron microscopy (ESEM–FEG XL30, FEI, Holland) was employed to investigate the effect of thermal stress on the morphology and structure of the perovskite. To carry out transmission electron microscopy (TEM) analysis, samples mounted on Cu girds were prepared using Focused Ion beam. The prepared sample was measured with JEM-2100F (JEOL LTD) at operating 200 kV equipped with EDS (TEM 250, Oxford Instruments). Stability tests were carried out in two kinds of conditions. These include: (a) 65 °C-85%RH (SE-CT-02, SUKSAN Tech. South Korea), and (b) ambient condition.
2.3. ALD process 3. Results and discussion Two kinds of ALD processes were employed in this work: 1) thermal ALD (Th-ALD), and plasma ALD (Pl-ALD). Th-ALD used Trimethylaluminum (TMA) and H2O (Classic, CN1, South Korea), and Pl-ALD employed TMA and O2 plasma as precursors (Lucida M200-PL, NCD, South Korea). Th-ALD procedure consists of 4 steps: (1) TMA was pulsed into reaction chamber for 0.1 s using N2 gas with 500 sccm after sample loading, (2) Excess TMA and methane reaction products were purged by N2 for 5 s with 500 sccm, (3) Water vapor was pulsed into reaction chamber for 0.1 s by N2 with 500 sccm, Finally (4) N2 was purged to remove excess water vapor for 5 s with 500 sccm. For the PlALD process, the procedure is similar to that of Th-ALD. Pl-ALD process consists of 4 steps: (1) TMA was injected and pulsed into reaction chamber for 0.2 s using Ar gas with 100 sccm after sample loading, (2) Excess TMA precursor was purged by Ar for 15 s with 300 sccm, (3) O2 reactant was injected for 2 s by Ar with 200 sccm and O2 plasma was
3.1. The effect of ALD temperature on PSCs To examine the effect of ALD temperature on the electrical property of PSCs, mesoporous (FAPbI3)0.85(MAPbBr3)0.15/spiro-OMeTAD cells were fabricated followed by Th-ALD of 50 nm of Al2O3. The PCE's of the cells before and after the ALD were measured as shown in Fig. 4. The PSC's with Al2O3 deposited at 120 °C experienced the most dramatic degradation, an average of 35% drop in PCE after the ALD process. As the ALD temperature decreases, so is the averaged PCE degradation that was reduced to 16% at 95 °C. Based on our previous finding [34], the crystallization of spiroOMeTAD is accelerated in high temperature, which results in deterioration of spiro-OMeTAD. We also found out that PTAA is more stable than spiro-OMeTAD at elevated temperature. We, therefore, repeated
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Fig. 4. Normalized averaged PCE of (FAPbI3)0.85(MAPbBr3)0.15/spiro-OMeTAD /Au/50 nm Al2O3 solar cells before and after ALD of Al2O3 at different temperatures.
Fig. 6. Normalized averaged PCE change of (FAPbI3)0.85(MAPbBr3)0.15/PTAA/ Au cells as type of ALD. The Al2O3 was formed at 95 °C.
with previous researches [34–36]. In addition, we performed XRD on perovskite layer after exposure (for the same amount of time that the PSC would have been exposed to during ALD deposition) to elevated temperature at 95 °C. Results in Fig. 5(b) shows negligible changes. The 12.5° PbI2 peak was already in the (FAPbI3)0.85(MAPbBr3)0.15 film before the ALD process which had an excess (5 mol%) of PbI2 in the precursor for improving photovoltaic performance [34,37]. Due to the negligible change in the perovskite layer after the 95 °C ALD process, the observed PCE drop in Figs. 4, 5 originates from the failure of HTM or the associated interface rather than the intrinsic perovskite degradation induced by heat during the ALD process. To examine whether the type of ALD process will have an effect on PSC performance, Pl-ALD at 95 °C was used to deposit 50 nm of Al2O3 on mesoporous (FAPbI3)0.85(MAPbBr3)0.15/PTAA/Au cells. Fig. 6 shows that the PCE's before and after the two types of ALD processes. Very similar degradation is observed suggesting that the reactant (O2 or H2O) and the plasma used in the ALD process has less effect than the temperature used on the performance of PSCs. 3.2. The moisture barrier property of Al2O3 The WVTR values of 50 nm Al2O3 layer by Th-ALD was measured by which were 1.84 × 10-2 g m−2 d−1 at 45 °C–100%RH. Using the Eq. (1) of Ziegel et al.[38], the diffusion coefficient was calculated to be 5.84 × 10–16 cm2/s.
D = l 2/7.1999t0.5
(1)
where, l and t0.5 are the thickness of the film and the time required to reach a transmission rate equal to the half of the steady state value, respectively. t0.5 can be determined with time dependent WVTR as a function of time. As a comparison, Carcia et al. [39] determined that D = 1.4 × 10−17 cm2/s at 38 °C for diffusion through a Th-ALD Al2O3 barrier thin film that was 25 nm thick. They calculated the diffusion coefficient to be D = P/S. At 38 °C, the WVTR for the single layer Al2O3 ALD film corresponded to be a permeability of P = 7.33 × 10–19 g H2O/cm s atm. They used the solubility of sputtered Al2O3 of S = 0.029 g/cm3 atm [40]. We formed 50 nm Al2O3 on (FAPbI3)0.85(MAPbBr3)0.15/spiroOMeTAD/Au test device to investigate the moisture barrier property. The samples were stored in 45 °C–85% chamber for 72 h. We then investigated the stability of the films, in terms of color change. Fig. 7
Fig. 5. (a) Normalized averaged PCE of perovskite solar cells with different HTMs before and after 95 °C Th-ALD of Al2O3. (b) XRD spectra change of perovskite layer according to ALD temperature.
the 95 °C Th-ALD of 50 nm Al2O3 on cells that use PTAA instead of spiro-OMeTAD as HTM. Fig. 5(a) shows the change in PCE's for PSCs using different HTM before and after ALD. PTAA-based device is more stable than that with spiro-OMeTAD experiencing only 12% drop in PCE after the ALD process at 95 °C. The results are in good agreement 40
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Fig. 7. Photograph of perovskite/spiro-OMeTAD test devices (a) before and (b) after damp heat test (45 °C–85%RH) for 72 h. Photograph of perovskite/spiroOMeTAD/Al2O3 test devices (c) before and (d) after the same damp heat test.
3.3. The effectiveness of using Al2O3 as an encapsulant for perovskite test devices
shows the un-protected (FAPbI3)0.85(MAPbBr3)0.15/spiro-OMeTAD/Au test devices underwent color change suggesting degradation after damp heat exposure (cf. Fig. 7a and b) while the ALD 50 nm Al2O3 encapsulated test devices did not (cf. Fig. 7c and d) showing the effectiveness of the Al2O3 layer as a moisture barrier film even when it was deposited at a temperature as low as 95 °C. This indicates that 50 nm Al2O3 thin film can protect the sample from the moisture for 72 h under 45–85% condition. For obtaining further insight into the moisture barrier property of Al2O3, the absorption coefficients were plotted (Fig. 8) using the formula αeff = deff−1 ln(1 + A/Tspecular), for the negligible film-glass reflectance case [41,42]. The corresponding layer thickness deff = 500 nm and A/Tspecular were obtained by the relevant absorption spectrum. Firstly, the absorption coefficient curve of the test device without Al2O3 layer shows sharp shoulder before the damp heat test which represents perovskite layer is not damaged as a photovoltaic absorber. However, after the damp heat test, the blunt shoulder appeared and the absorption coefficient is decreased. Also, the slope of the absorption coefficients at the band edge, which also could be described as Urbach energy, was decreased when the damp heat test was applied on the test device without Al2O3 layer. These mean that perovskite layer was degraded under the damp heat test for the test sample without Al2O3 layer. On the other hand, as a similar result with Fig. 7, the sharp shoulder and the slope of band edge of absorption coefficient were maintained at the sample with Al2O3 layer. This indicates that 50 nm Al2O3 thin film can protect the sample from the moisture for 72 h under 45 °C–85%RH condition.
In order to examine the effectiveness of 50 nm ALD Al2O3 as an encapsulant for PSC's, mesoporous (FAPbI3)0.85(MAPbBr3)0.15/spiroOMeTAD/Au and (FAPbI3)0.85(MAPbBr3)0.15/PTAA/Au test devices that received Th-ALD of 50 nm Al2O3 deposition were stored in 65 °C and 65 °C-85%RH condition for 350 h. We then investigated the stability of the devices, in terms of photovoltaic parameter changes (Fig. 9). This shows that the degradation rate at 65 °C-85%RH is higher than that at 65 °C. This means that the device is slightly affected by moisture after 50 h. However, the influence of moisture is small since the WVTR of Al2O3 is low. The WVTR value is dependent on temperature [43]. Therefore, the lower the temperature, the lower the effect of moisture. We will show the stability result at room temperature in long-term stability section. Fig. 9 demonstrates that the deterioration rates on the all parameters were low when we used PTAA as an HTM. Interestingly, the PCE drop rate for both 65 °C–85%RH and 65 °C (in inert) condition showed similar results as shown in Fig. 9. This implies that 50 nm Al2O3 thin film protects moisture infiltration effectively, but the intrinsic thermal damage to PSCs still existed. As shown in Fig. 9b–d, while VOC of the test devices remains relatively stable, and JSC and FF drop causing PCE to drop at both stress conditions. The possible cause of the drop in JSC and FF is due to the degradation of perovskite/spiro-OMeTAD interface as discussed in our previous report [34]. Cross sectional SEM images were taken on mesoporous (FAPbI3)0.85(MAPbBr3)0.15/spiro-OMeTAD/Au and (FAPbI3)0.85
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Fig. 9. Evolution of (a) PCE, (b) VOC, (c) JSC, and (d) FF as function of time at 65 °C–85%RH or 65 °C (in inert) condition. Two kinds of PSCs were used in this experiment: (a) perovskite/spiro-OMeTAD/Au/Al2O3 and (b) perovskite/ PTAA/Au/Al2O3.
Fig. 8. Absorption coefficient of perovskite/spiro-OMeTAD test devices before and after damp heat test (45 °C–85%RH) on perovskite/spiro-OMeTAD/Au/ Al2O3 and (b) perovskite/PTAA/Au/Al2O3 test structure respectively.
shown in Fig. 11b and d. Therefore, thermal stress is a critical factor for the onset of degradation within the spiro-OMeTAD layer resulting in the failure of the rest of the device. This agrees with our results in Fig. 7 and results from our previous reports [34]. Interestingly, the pinhole did not collapse the spiro-OMeTAD layer even under 3 weeks (~ 500 h) thermal stress. It is reasonable that gold or Al2O3 layer could be decent in the presence of pinholes at spiro-OMeTAD. Therefore, we confirmed the presence of barrier such as Al2O3 effectively slows down moisture ingress even if there are pinholes present in the spiro-OMeTAD induced by heat.
(MAPbBr3)0.15/PTAA/Au test devices without Al2O3 before and after two different storage conditions to investigate different degradation processes. The conditions are: (a) ambient condition (75 ± 10%RH) at room temperature, (Fig. 10b and e) and (b) inert condition (in N2 filled glovebox) at 65 °C, (Fig. 10c and f). As seen in Fig. 10b and Fig. 10e, after 1 week of storage in ambient, a small amount of PbI2 is formed in the perovskite layer (circled by red dotted lines in Fig. 10b and e), which has been reported to adversely affect solar cell performance [44]. The presence of additive Li-TFSI salt in our HTMs which attracts and retains water is also believed to accelerate moisture ingress [45]. The formation of PbI2 is also observed for both types of test devices (Fig. 10c and f) after 1 week of thermal stress at 50–60 °C although unexpected as the thermal stress was conducted in inert condition. Yang et al. [46] has previously reported the formation of PbI2 under thermal stress at 65 °C even in ambient. The appearance of PbI2 in our case could be due to the intrinsic thermal instability of perovskite material as reported before [12]. Further investigation shows that pinholes were formed in spiroOMeTAD after 2 weeks of thermal stress at 65 °C in N2 (Fig. 11c). These pinholes further promote moisture ingress into perovskite layer which leads to rapid degradation [47]. On the other hand, pinhole is not observed when spiro-OMeTAD based test devices are stored in ambient only as shown in Fig. 11a. It should be noted that PTAA layers remained intact whether they went through 2 weeks of ambient storage (75 ± 10%RH) at room temperature or thermal stress (65 °C) in N2 as
3.4. The results of stability of PSCs Finally, complete devices encapsulated by Th-ALD Al2O3 were subjected to environmental tests to examine their stabilities in terms of changes in PCE. We fabricated full mesoporous (FAPbI3)0.85 (MAPbBr3)0.15/spiro-OMeTAD/Au/50 nm Al2O3 and (FAPbI3)0.85 (MAPbBr3)0.15/PTAA/Au/50 nm Al2O3 solar cells which were then stored at room temperature in 50%RH for 7500 h (~ 10 months). The color of PSCs did not change regardless of the type of HTM used. In the case of PTAA based devices, the drop in maximum power was below 4% (Fig. 12). Spiro-OMeTAD suffered 8% drop in maximum power under the same conditions. However, it is worth to note that these results shows better long-term stabilities even at 50%RH compared to our
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Fig. 10. Cross-sectional SEM images of spiro-OMeTAD based test devices a) as fabricated, (b) 1 week after exposure to ambient condition (75 ± 10%RH) at room temperature and c) 1 week after continuous thermal stress at 65 °C in inert condition (N2). d-f) Cross sectional SEM images of PTAA based test devices under the same conditions.
Fig. 11. Cross sectional SEM images of (a) spiro-OMeTAD and (b) PTAA based test devices 2 weeks after storage in ambient (75 ± 10%RH) at room temperature. Cross sectional SEM images of (c) spiro-OMeTAD and (d) PTAA based test devices after 2 weeks of continuous thermal stress at 65 °C in inert condition (N2).
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Fig. 12. PCE's of full mesoporous (FAPbI3)0.85(MAPbBr3)0.15/HTM /Au/50 nm Al2O3 PSCs during storage at room temperature (dark) in 50%RH for 7500 h (~ 10 months).
previous report [34] at room temperature under inert condition. This is likely caused by more stabilized FA-based perovskite as reported before [48]. These results indicate that the Al2O3 layer is effective in preventing moisture ingress improving the stability of PSC's.
4. Conclusions This work demonstrated the use of ALD 50 nm Al2O3 layer on top of PSCs as an encapsulant to enhance the stability of PSCs by preventing moisture ingress. ALD process temperatures were varied from 95 °C, to 105 °C, and to 120 °C. It was found that ALD process temperature of 95 °C induces the least damage to PSC's while still providing an effective moisture barrier. The WVTR values of 50 nm Al2O3 layers by optimized thermal ALD process at a temperature as low as 95 °C was measured to be 1.84 × 10-2 g m−2 d−1 at 45 °C–85%RH. Results of Al2O3 encapsulated perovskite/HTM/Au devices after 72 h of damp heat (45 °C–85%RH) demonstrate the effectiveness of Al2O3 as a good moisture barrier. In addition, we found out that PTAA based PSC's suffered a smaller drop in power conversion efficiency than spiroOMeTAD based PSC's after thermal ALD process. PTAA was also more stable than spiro-OMeTAD at high temperature. This is attributed to the pinholes formed within spiro-OMeTAD layer, under thermal stress (from ALD process or from environmental tests). These pinholes are not present in PTAA. Using the optimized Th-ALD process, 50 nm Al2O3 were used to encapsulate full mesoporous (FAPbI3)0.85(MAPbBr3)0.15/ HTM/Au/50 nm Al2O3 devices to examine their long-term stabilities. PTAA-based devices experienced less than 4% drop in PCE after 7500 h of 50%RH at room temperature. With further development in the ALD process to minimize thermal damage to perovskite solar cells, we propose that ALD process is an excellent approach to encapsulate perovskite solar cells.
Acknowledgements The authors in this paper, Eun Young Choi and Jincheol Kim contributed equally to this work. This research was supported by the Technology Development Program to Solve Climate Changes of the National Research Foundation of South Korea (NRF) funded by the Ministry of Science and ICT(NRF-2017M1A2A2048905). This research was also supported by the Australian Centre for Advanced Photovoltaics (ACAP) which encompasses the Australian-based activities of the Australia U.S. Institute for Advanced Photovoltaics (AUSIAPV) and is supported by the Australian Government through the Australian Renewable Energy Agency (ARENA). We thank the Analytical Centre at UNSW for their technical support.
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