Spectral Dependence of Degradation under Ultraviolet Light in

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Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 21985−21990

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Spectral Dependence of Degradation under Ultraviolet Light in Perovskite Solar Cells Amjad Farooq,*,† Ihteaz M. Hossain,†,‡ Somayeh Moghadamzadeh,‡ Jonas A. Schwenzer,‡ Tobias Abzieher,‡ Bryce S. Richards,†,‡ Efthymios Klampaftis,† and Ulrich W. Paetzold*,†,‡ †

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Institute of Microstructure Technology, Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany ‡ Light Technology Institute, Karlsruhe Institute of Technology, Engesserstrasse 13, 76131 Karlsruhe, Germany S Supporting Information *

ABSTRACT: Perovskite solar cells (PSCs) demonstrate excellent power conversion efficiencies (PCEs) but face severe stability challenges. One key degradation mechanism is exposure to ultraviolet (UV) light. However, the impact of different UV bands is not yet well established. Here, we systematically study the stability of PSCs on the basis of a methylammonium lead iodide (CH3NH3PbI3) absorber exposed to (i) 310−317 (UV-B range) and (ii) 360−380 nm (UV-A range), under accelerated conditions. We demonstrate that the investigated UV-B band is detrimental to the stability of PSCs, resulting in PCE degradation by more than 50% after an exposure period >1700 sun-hours. This finding is valid for architectures with a range of electron transport layers, including SnO2, compact-TiO2, electron-beam TiO2, and nanoparticle-TiO2. We also show that photodegradation is apparent for high, as well as for low illumination intensities of UV-B light, but not for illumination with UV-A wavelengths. Finally, we show that degradation of PSCs is preventable at the cost of a small fraction of photocurrent by using UV-filtering or luminescent downshifting layers. KEYWORDS: perovskite solar cells, UV degradation, spectral dependence, electron transport layers, luminescent downshifting layers, stability

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through encapsulation,8 light-induced degradation remains as a key challenge that needs to be tackled in order to realize more stable devices. Reports discussing the light-induced instability of PSCs emphasize different causes of light induced degradation so far, however the majority conclude that ultraviolet (UV) radiation is the most damaging region of the solar spectrum.9,10 In particular, the most commonly used electron transport layer (ETL), titanium dioxide (TiO2), is reported to reduce device stability under UV illumination in two ways. First, the photocatalytic effect at the surface of the titanium dioxide surface is reported to decompose the methylammonium lead iodide (CH3NH3PbI3) perovskite to PbI2 in an irreversible process.11−13 Second, upon UV exposure, deep trapping at oxygen vacancies on the TiO2 surface is reported to strongly enhance recombination and, thus, decreases performance of the PSC. The oxygen vacancies originate from holes in the valence band which recombine with adsorbed oxygen causing desorption and leaving positively charged trap states of long lifetime.14,15 To overcome these problems various approaches have been suggested:9,16−18 (i) replacing TiO2 with another ETL material; (ii) insertion of interfacial layers between the

INTRODUCTION Perovskite solar cells (PSCs) are a potential disruptive technology in the field of photovoltaics (PV) because of their excellent power conversion efficiencies (PCEs), and promise of low costs due to the ease of solution processing.1 However, any PV manufacturer today that wishes to be competitive in large-scale solar power generation needs to guarantee that their PV module will produce more than 80% of its original rated power after a period of 25 years.2 This is the key aspect of PSCs in which they fall far behind their inorganic market-dominating counterpartssuch as silicon (Si), copper indium gallium diselenide (CIGS), or cadmium telluride (CdTe)because of the intrinsic material instability at elevated temperatures3 and their vulnerability to degrade during prolonged light exposure,4 humidity,5 and oxygen.6 The underlying degradation processes occur not only within the perovskite absorber itself, but rather they are cumulative phenomena originating from other layers and interfaces as well. For instance, the most commonly used hole transport layer (HTL) 2,2′,7,7′-Tetrakis[N,N-di(4-methoxyphenyl)amino]9,9′-spirobifluorene (C81H68N4O8, or commonly known as spiro-OMeTAD) is neither moisture stable nor can it withstand elevated temperatures.7 Since exposure to moisture and oxygen can be effectively prevented via proper encapsulation5 as other successful technologies (e.g., CIGS solar cells) have overcome susceptibility to water vapors © 2018 American Chemical Society

Received: February 20, 2018 Accepted: June 11, 2018 Published: June 11, 2018 21985

DOI: 10.1021/acsami.8b03024 ACS Appl. Mater. Interfaces 2018, 10, 21985−21990

Research Article

ACS Applied Materials & Interfaces TiO2 ETL and perovskite absorber; and (iii) the use of UVabsorbing filters and introduction of luminescent downshifting (LDS) layers to improve UV stability. Despite the general understanding of the degradation mechanism at the TiO2 surface upon UV illumination, the literature remains inconclusive in defining the harmfulness of UV degradation for PSCs.19−21 This is due to the multitude of coexisting degradation mechanisms occurring across different layers and interfaces of the PSC, including metal infiltration from top contact to perovskite,4 deterioration of chemical bonding between HTL and gold (Au) electrode,10 photoinduced oxidation of spiro-OMeTAD22 and the dependence of perovskite film thickness variation on photoinduced performance degradation.23 In addition, there is a lack of standardized long-term illumination test conditions for PSC. Even the “class A” spectral match of the standardized test conditions (STC) for solar simulators does not provide an intensity specification for UV light of wavelengths shorter than 400 nm.24 As a consequence, the literature reports use of different UV spectra, ranging from studies with light stress of one sun to studies using only UV light stress.9,14,16 Moreover, various illumination levels are used ranging from 1000 W m−2 of a 325 nm UV laser light (>250 suns) to simulated one-sun and 100-suns spectra without specifying in detail the UV content.18,25,26 The existing standardized test with regard to UV stress is the International Electrotechnical Commission (IEC) qualification test 61215, which uses the air-mass 1.5 global (AM 1.5G) spectrum, but this test does not ensure that a PV module will withstand >20 years of UV radiation.27 Thus, a comparative study of the spectral dependence of the UV photostability of PSCs with different device architectures is missing, as it is not clear which UV wavelengths and at what intensities are detrimental to device stability.

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MATERIALS AND CHARACTERIZATION

In this Research Article, we present an extensive study of different ETLs under different UV spectra to ascertain the spectral dependence of UV degradation mechanisms within PSCs. Out of wide range of ETLs used in PSCs28,29 (TiO2, SnO2, ZnO, ZrO2, BaSnO3, CdSe); the studied ETLs are selected on the basis of their usage frequency in PSC research and provided the fact that they bear the potential of low temperature processability and high device stability (for example SnO2) or yield the record-high efficiency (for example most widely used TiO2).28,29 The devices used in this study are based on solutionprocessed methylammonium lead iodide (CH3NH3PbI3 or MAPbI3) employing five different kinds of ETLs: (i) tin oxide (SnO2); plus four made from TiO2: (ii) post annealed at 500 °C compact (c-TiO2) deposited via spin coating; (iii) nanoparticles (np-TiO2) grown via wet-chemical synthesis; (iv) TiO2 deposited via electron-beam evaporation (e-TiO2); and (v) post annealed at 450 °C meso-TiO2 on top of compact TiO2. We have provided further details on device fabrication in the Supporting Information (SI). The two different UV spectra used are 310−317 nm (emitted from a narrowband UV−B fluorescent Philips lamp, hereafter referred to as 311 nm UV) and 360−380 nm (emitted from an actinic Philips UV lamp in the UV-A region, hereafter referred to as 370 nm), shown in Figure 1a. The choice of these UV lamps is justified as the scarcity of the UV sources with broad spectral emission limits such degradation studies to span over many UV spectral bands. Initial experiments are “‘accelerated”’ such that the UV light intensities are significantly greater than the respective UV content in AM 1.5G spectrum (112 times for 311 nm and 57 times for 370 nm UV light). However, in a later control experiment, we also verify the findings for lower intensities (11 suns for 311 nm). In order to isolate only the effects from light-induced degradation, all experiments are carried out in nitrogen (N2) filled

Figure 1. (a) Schematic illustration of perovskite solar cells under UV illumination. UV spectral ranges are highlighted in red and blue along with AM 1.5G spectrum; (b) short-circuit current density (JSC) values for ITO/SnO2/MAPbI3 based solar cells exposed to 311 nm UV (left) and 370 nm UV (right); (c) Stabilized PCEs of the reference and UV-exposed devices with different ETLs illuminated with 311 nm UV (left) and 370 nm UV (right). glovebox. A schematic illustration of the UV exposure experiments is presented in Figure S1.

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RESULTS AND DISCUSSION The schematic illustration of a perovskite solar cell under UV illumination, highlighting the two different UV illumination spectra tested in this study, along with the AM 1.5G spectrum are shown in Figure 1a. All the devices are exposed to UV light from the glass/ETL side, while the light exposure times are recorded in terms of sun-hours. The transmittance of the ETL coated substrates varies in the two wavelength ranges of interest (310−317 nm and 360−380 nm) as shown in Figure S2. It should be noted that average transmission for all ETLs in the former wavelength range is low (∼20% on average for all ETLs) because of absorption by the soda lime glass substrate, while in the longer wavelength range the transmission is >70% on average. The stress tests are performed on a number of devices always including two kinds of reference configurations: 21986

DOI: 10.1021/acsami.8b03024 ACS Appl. Mater. Interfaces 2018, 10, 21985−21990

Research Article

ACS Applied Materials & Interfaces

dark and under UV filters. It should be noted that the devices with np-TiO2 ETL appear intrinsically less stable with regard to PCE-5 min, although the PCE from voltage−current scan is not degraded (see Figure S3). Furthermore, all TiO2 ETL based devices exhibit similar performance degradation under 311 nm UV. Thus, it can be concluded that devices fabricated using any of the ETLs investigated here exhibit similar degradation trends when exposed to 311 nm UV irradiance, while the reference devices retain their initial PCEs-5 min. Having demonstrated a strong spectral dependence of the UV degradation for PSCs with TiO2 and SnO2 ETLs for high UV exposure intensities at 311 nm, the impact of UV intensity is investigated. Since the intensities of both UV light sources used in this study are high, ranging from 50 to 100 times more than respective UV content in the AM 1.5G spectrum, it is important to understand whether the observed degradation is also present at lower intensities. To demonstrate that this degradation is solely caused by the photon energy instead of the high UV light intensity, polymer based UV long-pass filters were used allowing only the 10% of incoming 311 nm UV light (11 times of 311 nm UV content in AM 1.5G spectrum) to be transmitted toward the devices under test. As demonstrated in Figure 2, the degradation trend remains the same under 90%

(i) the dark reference devices (6 in total), which are wrapped with aluminum foil so that they experience the same heat stress as other samples, but are not exposed to any light; and (ii) devices covered with optical long-pass filters (4 in total), which cutoff wavelength of 400 nm. The third set of devices (6 in total) are the ones exposed to UV light for degradation testing. Both types of references (dark and UV filters covered) are used here to isolate the temperature effects (if any) as some studies report the decreased device performance due to adverse effect of temperature cycles (20−70 °C) on spiro-OMeTAD/ perovskite interface.30 The results of the experiments demonstrate that the devices exposed to 311 nm UV experience severe decrease in shortcircuit current density (JSC). In contrast, devices exposed to 370 nm UV experience no degradation in general and a light soaking effect in the case of the c-TiO2 devices is prominent. Light soaking generally describes the improved performance as a result of photoinduced structural transformation due to trap filling by photogenerated carriers upon prolonged light exposure.31 Figure 1(b) shows JSC measured for devices with SnO2. The reference devices do not exhibit any decrease in JSC, while devices exposed to 311 nm UV experience a significant decrease in JSC of up to 64% over the 1750 sun-hour testing period. We attribute this reduction to the decomposition of the perovskite absorber layer.32 Charge generation and extraction remains unaltered for the devices exposed to 370 nm UV wavelength for equal number of sun-hours. The observed loss in JSC at 311 nm UV resulted in reduced PCEs, measured by tracking the power output of the PSC for 5 min at constant voltage close to the maximum power point (hereafter referred to as PCE-5 min). The open circuit voltage (VOC) is less affected as compared to JSC, while the fill factor (FF) decreased due to the decrease in JSC as shown in Figures S3−S5. Additional data on the devices, like RS, RSh, and cross-sectional scanning electron microscope (SEM) images are presented in Figures S6−S8. The initial and final values of PCE-5 min for all the devices exposed to both UV spectra are given in Table S1. A similar trend of decreased JSC under 311 nm UV light is observed for all other ETLs (c-TiO2, e-TiO2, np-TiO2) decreasing 70.3%, 72.2% and 81.8% respectively, as shown in Figure S3. This underlines the fact that degradation under UV−B light is present for all types of ETLs. Interestingly among all the studied ETLs, SnO2 appeared to be 43% more stable in terms of PCE-5 min as compared to np-TiO2 (the least stable). The findings reported here are also applicable in devices based on meso-TiO2 as ETL which do not have state of the art PCE as shown in Figure S9. The trend in JSC degradation for the devices based on SnO2 and tested under 311 nm UV radiation translates into reduction of PCE as illustrated in Figure 1c. The PCE-5 min decrease is proportional to the JSC decrease for the same devices. All devices with other ETLs (c-TiO2, e-TiO2, np-TiO2, meso-TiO2) also show a decrease in stabilized PCE-5 min with increasing exposure times when illuminated with 311 nm UV light. In contrast, all the devices exposed to 370 nm UV show no degradation in PCEs-5 min when compared to the reference devices kept in dark or stored under UV filters. For the PSCs prepared with SnO2, e-TiO2, and c-TiO2, the initial PCE-5 min is maintained throughout the testing period. A degradation in PCE-5 min is only observed for PSCs with np-TiO2 ETL and stressed under 370 nm UV, as shown in Figure 1c. However, this decrease is not caused by the UV stress, as it is also apparent from the reference devices kept in

Figure 2. UV degradation of perovskite solar cells based on a SnO2 as ETL under ∼11 suns UV irradiation. Reference devices covered with optical long-pass filters (left) are compared with the devices exposed to UV light (right).

less intense 311 nm UV radiation, indicating that the UV light intensity itself is not playing a major role here. Assuming a linear degradation rate with respect to intensity, a PSC will degrade at a rate of 1.27% /100 sun-hours of exposure time to 311 nm UV light. Due to their impact, these UV−B photons cannot be ignored, since sunlight contains ∼1.6 W m−2 in the UV−B band of 300−320 nm. This observation suggests that this degradation trend may be the cause of instability of PSCs under simulated one-sun solar irradiance. The remedies suggested to prevent this 311 nm UVstimulated degradation include the employment of a UV filter or a LDS layer. The former approach is already commonly employed in PV applications. For example, the front glass sheet of PV modules in extra-terrestrial applications is doped with trivalent cerium (Ce3+) ions to minimize the impact of high energy radiation,33 while for terrestrial applications UV absorbers are used in commercial PV encapsulants like ethylene vinyl acetate (EVA).34 However, while UV filtering will protect devices from photodegradation, but the energy content of the UV photons is not harnessed. The addition of a LDS layer will also protect the devices assuming total absorption of the harmful photons. However, it 21987

DOI: 10.1021/acsami.8b03024 ACS Appl. Mater. Interfaces 2018, 10, 21985−21990

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CONCLUSIONS In summary, spectral dependent UV degradation of PSCs is studied for two kinds of UV light spectra: (i) a 310−317 nm band, referred as 311 nm UV and (ii) a 360−380 nm band, referred as 370 nm UV. We find that 370 nm UV radiation only causes 64% degradation in stabilized PCE-5 min in PSCs based on np-TiO2 ETLs. For other devices using SnO2 or TiO2 (c-TiO2 and e-TiO2) as ETLs no performance degradation is observed. In contrast, 311 nm UV radiation heavily deteriorates the JSC of all PSCs−regardless of whether SnO2 or TiO2 ETLs are utilized within the devices−already after 500 h. The overall quantification of degradation in PCE-5 min for devices based on SnO2, c-TiO2, e-TiO2 and np-TiO2 as ETLs is 50%, 69%, 83%, and 88%, respectively, after an exposure period of >1700 sun-hours. To establish a global reference for photostability studies of PSCs, the intensities of UV light are translated to AM 1.5G spectral content for the respective UV radiation bands. The degradation rate slightly depends on the intensity of photon dose but, in general, similar stability issue persists even with 10 times less intense UV radiations. To address the UV light induced degradation of PSCs, the most simple solution is to filter the UV photons with UV−B wavelengths at the cost of a small fraction (0.53 mA cm−2) of photocurrent. However, the implementation of LDS layer can ideally recover photocurrent of up to 0.3 mA cm−2.

affords the advantage of not wasting the energy content of the UV photons, but instead downshifting these via photoluminescence to longer wavelengths, hence harvesting a fraction of the available photocurrent.9,35 The ideal case here would be a LDS material with strong absorption in the UV region of the spectrum and emission near the peak external quantum efficiency (EQE) of the solar cells, in the range 600− 800 nm. However, it must be noted that the introduction of a LDS layer also results in new loss mechanisms.36 The most important are those arising from a subunity photoluminescence quantum yield (PLQY)defined as the number of lower-energy photons emitted per the number of higher-energy incident photonsand emission of light to directions other than that of the solar cell (from the top and the edges of the LDS layer). Additionally, the LDS materials can either be made of organic or inorganic materials, and thus exhibit their own UV stability issues. Thus, a trade-off between current gain and increased device stability versus added complexity due to a newly introduced layer and cost arises.36 To examine the potential of LDS layers for mitigating JSC losses compared to UV filtering, a detailed calculation of total loss in JSC and the potential regain of this lost JSC with downshifting is done. It should be noted that this is an estimation considering the favorable assumptions to realize the potential use of LDS layers in Perovskite PV. A series of experiments is required to explore the real practical gain achieved from these luminescent layers. The estimated loss and potential regain of the lost JSC is presented in Figure 3 showing the JSC that will be lost by filtering the UV photons with wavelength ranging from 300 to 400 nm with a step of 10 nm.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b03024. Materials and device fabrication methods; transmittance of all used ETLs; schematic of UV exposure setup; initial and final PCE values of all devices; plots of device parameters like JSC, VOC, FF, RS, RSh; cross sectional SEM images; details of the calculation of JSC loss and gain using UV filtering and downshifting (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Figure 3. Loss and gain in integrated JSC by filtering out specific UV wavelengths, maximum recoverable and the realistically recoverable JSC via LDS materials.

ORCID

Amjad Farooq: 0000-0002-8604-8536 Ihteaz M. Hossain: 0000-0001-6533-1757 Jonas A. Schwenzer: 0000-0001-8795-4875 Tobias Abzieher: 0000-0002-2733-0136 Bryce S. Richards: 0000-0001-5469-048X

These results show that if we cut off the photons of wavelengths shorter than 360 nm, then the total lost JSC, maximum recoverable JSC and realistically recoverable JSC are 0.53, 0.45, and 0.22 mA cm−2, respectively. Hence, it is concluded that the LDS technology can afford the protection from UV light and at the same time afford a small gain in terms of photocurrent. This, however, is less than 0.3 mA cm−2 as denoted by the realistic scenario in Figure 3, which still includes favorable assumptions. Here, it should also be noted that up-to-date there are no luminescent materials that will exhibit 100% PLQY for the long period of time that a PV module is expected to withstand sunlight exposure in the field (>20 years). Hence, it is concluded that the approach of filtering the UV light appears to be the simplest, leading to protection of the devices at the cost of a very small fraction of the available photocurrent.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge financial support of the German Federal Ministry for Education and Research (BMBF project PeroSol FKZ 03SF0483B); the Helmholtz Association for the Young Investigator Group of Dr. U. W. Paetzold; Recruitment Initiative of Prof. B. S. Richards; the Helmholtz Energy Materials Foundry (HEMF); and the Science and Technology of Nanostructures research programme); as well as the Karlsruhe School of Optics and Photonics (KSOP).The authors express their gratitude to the great spirit of the “KIT perovskite PV taskforce” and the KIT Young Investigator 21988

DOI: 10.1021/acsami.8b03024 ACS Appl. Mater. Interfaces 2018, 10, 21985−21990

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Network. Mr. Amjad Farooq thankfully acknowledges the financial support from DAAD (Deutscher Akademischer Austauschdienst/German academic exchange service) for his doctoral research work under personal reference number 91604868.

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DOI: 10.1021/acsami.8b03024 ACS Appl. Mater. Interfaces 2018, 10, 21985−21990