The power conversion efficiency (PCE) of perovskite solar cells (PSCs) has exceeded 22% with mesoporous TiO 2 (mp‐TiO 2 ) as the electron transport material (ETM) on a transparent conducting oxide.1 Despite such high PCEs, poor long‐term stability of mp‐TiO 2 based PSCs is a problematic issue for commercialization. Photocatalytic chemical reaction with absorption of ultraviolet (UV) light by TiO 2 is considered to induce degradation of PSCs.2 This effect was significantly weakened by simply using UV‐cut filters during light soaking tests in TiO 2 ‐employed devices; however, the loss of power is inevitable as well. Introduction of luminescent photopolymers into PSCs for converting UV light to visible light is another solution, but is not cost effective.3 Imperfect interfaces, including charge recombination via deep traps at interfaces, are also known to induce the stability loss under illumination. Surface passivation of ETMs is considered as an effective method to prevent the degradation through this mechanism.4

Organic hole transport materials (HTMs) are also essential components for highly efficient mp‐TiO 2 based PSCs but are a major cause of poor long‐term stability,5 while HTM‐free PSCs have showed excellent long‐term operational stability.6 Unlike Spiro‐OMeTAD, poly(triarylamine) (PTAA) combined with a quadruple‐cation perovskite has showed excellent long‐term thermal stability under illumination (without UV light) at inert nitrogen condition.7 Under similar condition, PSCs with an inorganic HTM without any additives, CuSCN, showed long‐term stability as well as a high PCE above 20%.8 Interestingly, although CuSCN‐incorporated PSCs were quite stable at high temperatures, long‐term stability under light soaking could not be achieved without a barrier layer between CuSCN and Au.

It should be also noted that atom/ion migration from metal electrodes or the organometallic halide perovskites into the HTM is one of the leading causes of the degradation.9 Recently, it is reported that the iodine migration in the perovskites is enhanced by illumination, which can be associated with the decomposition of the perovskite.10 Thus, suppressing the intrinsic degradation factors such as ion migration as well as overcoming the extrinsic degradation factors such as humid air and UV light is currently challenging to realize a long‐term stability of PCSs including mp‐TiO 2 and organic HTMs under a practical operating condition.

We reason that operational stability of PSCs correlates with the external factors from the atmosphere because the stability of PSCs depends on encapsulations or the testing atmosphere.11, 12 In PSCs with wider bandgap oxides, such as SnO 2 , as ETMs, long‐term operational stability is not perfectly achieved.13 The long‐term operational stability of 1000 h in BaSnO 3 ‐based PSCs was realized only in a laminate cell. Even small amounts of oxygen can affect performances of PSCs since oxygen can easily react with halides.14, 15 It has also been demonstrated that oxygen induces photodegradation of methylammonium lead iodide (MAPbI 3 ).16, 17 Haque and co‐workers claimed that the degradation stems from the formation of reactive superoxide species in mediating halide vacancies in the perovskite materials.16, 18 In addition, organic semiconductors used as charge transport layers can also degrade when exposed to oxygen and light.19 Nevertheless, the effects of oxygen on the long‐term stability of PSCs have attracted only minor attention because it has been considered that the perovskites are much less susceptible to oxygen, rather than humidity and heat, even though oxygen is one of the most exposure species during the manufacture and the operation of PCSs. It is critical that the combination of oxygen and light as accelerated aging factors would cause the deterioration of the perovskites.

In this work, we investigate the effects of oxygen on the degradation of PSCs under illumination and reveal that charge transport from the perovskite to the HTM becomes ineffective by iodine migration before the degradation of bulk perovskite. Several papers have reported the degradation behavior of the devices and decomposition of perovskite materials by oxygen and ion migration; however, none of those have dealt with influences of oxygen on the operational stability of the devices. Most of them were carried out in ambient air including both moisture and oxygen; they could not specify the reason of the degradation and were more focused on the moisture rather than oxygen. Here, it is also demonstrated that mp‐TiO 2 based PSCs have long‐term stability under illumination including UV if devices are immune to oxygen and humidity. PSCs exceeding initial PCEs of 20% have a device structure composed of fluorine doped tin oxide (FTO)/d‐TiO 2 /mp‐TiO 2 /(FAPbI 3 ) 0.95 (MAPbI 3 ) 0.05 /PTAA/Au.

When the devices are exposed to oxygen, PCE is reduced, and this degradation is accelerated by illumination. In order to clarify the effects of oxygen, the devices were illuminated in a gas‐tight purging chamber with O 2 (purity >99.999%) flow as described in Figure 1a. After devices were exposed to oxygen with 1 sun illumination, PCE reduced in half within an hour, (Figure 1b) which is much faster than the degradation in bulk properties such as absorption and photoluminescence of perovskite films in literature. While all photovoltaic parameters (J sc , V oc , and fill factor, FF) decreased with increasing the duration time of the O 2 –light exposure, FF was affected the most among the performance factors. The large decrease in FF indicates that transport of charge carriers becomes inefficient and/or the nonradiative recombination of photo‐excited carriers is enhanced. In addition, hysteresis in J–V curves increased because the drop of PCE for a forward voltage scan was larger than that for a reverse voltage scan. The broader hysteresis after the exposure is likely to be caused by more unbalanced charge transport between electrons and holes near perovskite/ETM or HTM interfaces.20 The performance of the degraded PSC was not recovered after aging the device in dark and in ambient air. Such degradation could be observed regardless of the types of perovskites, ETMs, and HTMs as well as device structure (Figures S1–S3, Supporting Information).

Figure 1 Open in figure viewer PowerPoint Variation of performances of perovskite solar cells (PSCs) with a device stack of FTO/d‐TiO 2 /mp‐TiO 2 /(FAPbI 3 ) 0.95 (MAPbI 3 ) 0.05 /poly(triarylamine) (PTAA) /Au. a) A schematic of the test system for the O 2 –light exposure. b) Current density versus voltage (J–V) plots under 1 sun illumination of pristine devices (black), and devices after 1 h of light soaking in O 2 (red), and after 1 day of aging under dark and ambient air conditions (blue). Plots of c) PCE, d) short‐circuit current density (J SC ), e) open‐circuit voltage (V OC ), and f) fill‐factor (FF) as a function of the duration of the O 2 –light exposure.

On the other hand, when PSC devices were exposed to Ar atmosphere under illumination or to O 2 atmosphere in the dark for an hour, a little degradation in V oc and FF was observed, but their performances were completely recovered after the 1 day aging in the dark (Figure S4, Supporting Information). However, even under dark conditions, the higher pressure of O 2 induced the permanent degradation of PCSs (Figure S5, Supporting Information). Consequently, oxygen ingress into PSCs is a main cause of the device degradation, and illumination seems to accelerate the degradation in the presence of O 2 . Considering the results of Ar‐light exposure, we can also exclude photocatalytic effect of TiO 2 with UV light as one of the many causes of the photo‐induced degradation of the devices.

In order to elucidate the cause of the degradation of PSCs by oxygen, we investigated the change in time‐resolved photoluminescence (TRPL) of the perovskite thin film with and without charge transport layers according to the O 2 exposure (Figure 2). When a perovskite single layer and a mp‐TiO 2 /perovskite bilayer were exposed to O 2 and light, photoluminescence (PL) decay curves were slightly extended (Figure 2a). The PL enhancement in the perovskite under illumination in O 2 atmosphere is explained by the curing of trapping sites as claimed in literature.15 The transient behavior in the short‐time range (<100 ns) for the mp‐TiO 2 /perovskite double layers, which involves with the electron transport from the perovskite to the TiO 2 , did not change much by the O 2 –light exposure (Figure 2b). However, the PL decay curves for the perovskite/PTAA bilayer were largely extended after the O 2 –light exposure. (Figure 2c) After O 2 exposure under dark, only the perovskite/PTAA sample showed slightly extended PL decay. These results indicate that the degradation of PSC devices by oxygen stems from the interruption of the transport of photo‐excited holes from the perovskite into PTAA.

Figure 2 Open in figure viewer PowerPoint Time‐resolved photoluminescence (TRPL) decay curves of a) the perovskite film, b) perovskite film on TiO 2 (d‐TiO 2 /mp‐TiO 2 ), and c) perovskite film covered with PTAA. All samples are coated on FTO‐coated glass substrates. d) TRPL decay curves of the perovskite (black) and PTAA‐coated perovskite (red) on FTO substrates, FTO/perovskite/PTAA stacked samples applied with the following consecutive sequences: after 1 h of O 2 –light exposure (blue), then after removal of the PTAA layer (perovskite exposed to air again, magenta), and then after coating a new PTAA layer (green). e) J–V curves for a device before (black) and after 1 h of the O 2 –light exposure (red), and after coating a new PTAA and Au on the device in which the degraded PTAA was removed (blue). f) Nyquist plots at V OC and g) electrical resistance corresponding to semicircles in Nyquist plots as a function of the duration of the O 2 –light exposure. Inset: An equivalent circuit for plotting the Nyquist plots (R_low represents a Warburg impedance).

The hole transport can be hindered most probably due to the barrier formation at the interface between the perovskite and the HTM or the degradation of the HTM from reducing electrical conductivity. We designed experiments to reveal where the hole extraction is hindered: 1) only the PTAA layer in the degraded device was washed out by using toluene, and 2) then, a new PTAA layer was recoated via spin‐coating on top of the perovskite film in the degraded device and a new Au electrode was deposited on top. TRPL measurements and J–V measurements were conducted before and after the O 2 –light exposure and the removal and recoating of the PTAA layer (Figure 2d,e).

Interestingly, the device in which the PTAA layer was removed showed a similar PL decay curve to the degraded device by the O 2 –light exposure. It is likely that the PTAA in the degraded device cannot extracts holes. When a new PTAA layer was coated on the sample, the PL decay curve was shortened but was still much longer than that of the pristine sample (denoted by Perov./PTAA). Replacing the PTAA layer restored the performance of the PSC partly, but it was still not perfect as well. The PCE of the devices rose from 11.0% to 16.2% for reverse scans by replacing the degraded PTAA with a new one but was lower than the initial PCE of 20.3%. While J sc was fully recovered, FF increased from 51% to 67%; neither V oc nor hysteresis was improved. Removing the PTAA and coating a new PTAA layer on a pristine sample have little effect on the performance of the device (Figure S6, Supporting Information). The incomplete recovery of the PL decay curves and J–V curves illustrate that the O 2 –light exposure induces degradation of the perovskite near the interface with the PTAA as well as degradation of the PTAA layer.

The interface between the PTAA and the Au electrodes does not contribute to the degradation by the O 2 –light exposure. When Au electrodes on the degraded device were replaced by newly deposited Au electrodes after removing old ones, the performance of the device was not recovered (Figure S7, Supporting Information).

We also observed that interfacial adhesion between the perovskite and the PTAA strengthened by the O 2 –light exposure in this experiment. In order to remove Au electrodes from devices, a tape detaching method was employed. The PTAA layer on the degraded device was not completely removed by detaching tape, while the PTAA layer in pristine device was removed clearly (Figure S8, Supporting Information). This result can be an evidence on chemical interaction between the perovskite and the PTAA.

The hindered hole transport leads to increase of series resistance of the devices. The increase of series resistance can be directly measured by analyzing impedance spectra (Figure 2f,g). Pristine PSC devices show two semicircles in Nyquist plots. The first semicircle at high frequencies corresponding to series resistance of the devices was significantly enlarged with increasing the duration of the O 2 –light exposure. Such a similar observation was reported in a previous work where the device with MAPbI 3 and Spiro‐OMeTAD was exposed to light under ambient air condition.12 Even though a humid air atmosphere, this also supported our finding that the O 2 –light exposure could cause a deterioration of the perovskite/HTM interface of the device.

It should be noted that the optical absorbance, surface morphology, and crystallinity of the perovskite film were not varied after the O 2 –light exposure for 2 h. (Figures S9 and S10, Supporting Information). In addition, absorbance and conductance of PTAA on a thin layer of the perovskite do not change by the O 2 –light exposure (Figure S11, Supporting Information). These results indicate that rather than the bulk properties of the perovskite and PTAA, the interfacial properties are more likely to be affected by the exposure.

Next, depth profiles of time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) for the degraded PSC devices were compared to that of a pristine device (Figure 3a,b). In the degraded devices, interestingly, we observed no change in the oxygen profile, whereas we found a large variation in the iodine profile. Iodine concentration in the PTAA layer was increased after the O 2 –light exposure. The accumulation of iodine at the Au/PTAA interface could also be identified. Larger amounts of iodine within PTAA and the interface were observed in devices with longer duration of the O 2 –light exposure (Figure 3c).

Figure 3 Open in figure viewer PowerPoint Depth profile of elements in PSCs from time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) measurements a) before and b) after 1 h of O 2 –light exposure. c) Variation of depth profiles with increasing exposure duration. Diffusion of iodine ions within the PTAA especially increases with longer exposure times to O 2 and light. d) Schematic to describe the activation of iodine diffusion by oxygen and light.

The incorporation of iodine into PTAA seems to inhibit the hole transport. The existence of iodine in PTAA stems from the diffusion of iodine from the perovskite to the PTAA layer by O 2 and light. Photo‐excited holes bind with iodine ions, thus forming neutral iodine.10 The photo‐excited neutral iodine with high activity might tend to diffuse into PTAA in which iodine activity is much lower. In addition, several experimental results show that O 2 provides enhanced iodine activity and creates iodine vacancies in the perovskite,21 thereby activating the diffusion of iodine out of the perovskite. The diffusion of iodine into PTAA was also observed in the degraded device by exposing to high‐pressure O 2 in the dark. If iodine is irreversibly removed by a condensed matter sink such as the HTM, the perovskite is eventually degraded into a nonstoichiometric phase.

To address the solution to prevent photo‐induced degradation in oxygen‐contained atmosphere, we can consider two approaches: 1) suppression of the diffusion of iodine and 2) isolation of devices from oxygen. For the first solution, a np‐Al 2 O 3 layer was inserted between the perovskite and PTAA as a diffusion barrier. This device showed good stability against the O 2 –light exposure (Figure 4a). The PCE of the devices was slightly decreased just after the exposure, but it was recovered after aging for a day under dark in ambient air conditions. The suppression of the iodine diffusion from the perovskite into PTAA layer was apparently confirmed by ToF‐SIMS depth profiling (Figure S12, Supporting Information). When the np‐Al 2 O 3 layer was inserted not between the perovskite and PTAA but between PTAA and Au, it could not inhibit the degradation induced by the O 2 –light exposure (Figure S13, Supporting Information). These findings imply that the np‐Al 2 O 3 layer is not enough as a barrier layer to prevent the outside oxygen from penetrating into the device but is still effective to suppress iodine diffusion from the perovskite to the PTAA layer. The improved long‐term stability of PSCs with adding of a np‐Al 2 O 3 buffer in a literature seems to attribute to this effect.22

Figure 4 Open in figure viewer PowerPoint Suppression of the degradation induced from exposure to O 2 and long‐term device stability of encapsulated perovskite solar cells. J–V scans of devices a) with a np‐Al 2 O 3 layer between the perovskite and PTAA layers and b) with encapsulation. c) Shelf test results of the encapsulated device under 85 °C and 85% RH conditions and d) long‐term operational stability of the encapsulated devices under simulated real operating conditions (maximum power point tracking under 1 sun AM1.5G illumination including UV without controlling temperature in ambient air).

For the second solution, we have encapsulated PSC devices by laminating face‐sealing adhesive sheets on the top of the devices. In order to remove trapped air during the lamination and to improve the adhesion between the devices and the adhesive sheets, the encapsulated devices were aged in an autoclave. Then, the devices were completely protected against the ingress of oxygen and moisture. As shown in Figure 4b, the device is not degraded at all after 2 h of the O 2 –light exposure.

We also examined the long‐term stability of the encapsulated PSCs. First, we conducted a shelf test under 85 °C and 85% relative humidity (RH) conditions to evaluate encapsulation efficiency (Figure 4c). PCE of the device was maintained up to 96% of its initial PCE after 1000 h (from 20.9% to 20.0%). In this case, only 0.9% efficiency decrease was found in early burn‐in stage due to slight decrease of J SC and FF. There is no additional drop in the device performance. This assures that the encapsulation of the device is fairly solid.

The long‐term operational stability under real operating conditions was also investigated with the encapsulated device (Figure 4d). Surprisingly, the PCE retained 96% of its initial value (20.6%) after 1000 h operation (maximum power point tracking, MPPT) under 1 sun AM1.5G illumination of a xenon lamp including UV despite using mp‐TiO 2 as the ETM. The final PCE from a J−V sweep was 20.0%. Even though PSCs were encapsulated with the face‐sealing adhesive sheets, the long‐term stability of the PSCs was not guaranteed without the aging in the autoclave. As a result, a successful long‐term stability of PSCs can be realized by a solid encapsulation to completely block oxygen and even moisture in the atmosphere, thereby impeding the ion movement as well as the sublimation of volatile components from the perovskite.