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A solution processed CHNHPbI Cl perovskite based selfpowered ozone sensing element operated at room temperature George Kakavelakis, Emmanouil Gagaoudakis, Konstantinos Petridis, Valia Petromichelaki, Vassilis Binas, George Kiriakidis, and Emmanuel Kymakis ACS Sens., Just Accepted Manuscript • DOI: 10.1021/acssensors.7b00761 • Publication Date (Web): 01 Dec 2017 Downloaded from http://pubs.acs.org on December 1, 2017

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A solution processed CH3NH3PbI3-xClx perovskite based self-powered ozone sensing element operated at room temperature George Kakavelakis1,2,†, Emmanouil Gagaoudakis3,4,†, Konstantinos Petridis1,5,†,*, Valia Petromichelaki3, 4, Vassilis Binas3,4,6, George Kiriakidis3,4,6, and Emmanuel Kymakis1,* 1

Center of Materials Technology and Photonics & Electrical Engineering Department,

School of Applied Technology, Technological Educational Institute (TEI) of Crete Heraklion 71004, Crete, Greece 2

Department of Materials Science and Technology, University of Crete, Heraklion,

71003 Crete, Greece 3

University of Crete, Department of Physics, 710 03 Heraklion, Crete, Greece

4

Institute of Electronic Structure & Laser (IESL), Foundation for Research and

Technology (FORTH) Hellas, P.O. Box 1385, Heraklion 70013, Crete, Greece 5

Department of Electronic Engineering, Technological Educational Institute of Crete,

Romanou 3, 73100, Chania, Greece 6

Crete Center for Quantum Complexity and Nanotechnology, Department of Physics,

University of Crete, 71003 Heraklion, Greece † These

authors contributed equally to this work.

KEYWORDS. lead halide perovskites, self-powered ozone sensing element, Solution Processed, Room Temperature Operated, ultra-high sensitivity

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ABSTRACT. Hybrid lead halide spin coated perovskite films have been successfully tested as portable, flexible, operated at room temperature, self-powered and ultrasensitive ozone sensing elements. The electrical resistance of the hybrid lead mixed halide perovskite (CH3NH3PbI3−xClx) sensing element, was immediately decreased when exposed to an ozone (O3) environment and manage to recover its pristine electrical conductivity values within few seconds after the complete removal of ozone gas. The sensing measurements showed different response times at different gas concentrations, good repeatability, ultra-high sensitivity and fast recovery time. To the best of our knowledge, this is the first time that a lead halide perovskite semiconductor material is demonstrating its sensing properties in an ozone environment. This work shows the potential of hydrid lead halide based perovskites as reliable sensing elements, serving the objectives of environmental control, with important socio – economic impact.


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Gas sensing technologies are of a great importance in environmental monitoring, public security, air conditioning appliances / systems, commercial and domestic safety. 1,2,3,4 Ozone (O3) is one of the six principal pollutants considered harmful to the public health and the environment. It has been established that high concentrations of this powerful oxidizing gas in the ambient atmosphere are hazardous to human health.5,6,7 A high level of ozone gas in the atmosphere is harmful to the human respiratory system, causing inflammation and congestion of the respiratory tract. An individual who is exposed to 0.1 ppm O3 environment for two hours will sustain a loss of 20% breathing capacity while after remaining in 1 ppm O3 for six hours, will suffer an attack of bronchitis. A mouse kept in 10 ppm O3 will not survive.5,6,7 It is important to know that ozone, due to its relatively short half-life (minutes in confined spaces), does not instantly diffuse in air to create a uniform concentration for easy measurement.8 The increased level of ozone in the atmosphere is a result of the interaction between sunlight and various chemicals emitted into the environment by industrial means. Thus, ozone near the ground is a product of industrial and urban activity. In the last decade, considerable interest was shown in ecologically clean “ozone” technologies.9,10,11 Therefore, in order to control the ozone concentration in the atmosphere and in confined environments, it is necessary to develop both effective and inexpensive methods and devices for the continuous monitoring of ozone concentrations at multiple locations, including technological processes with incorporated ozone treatments. Experiments have shown that analytical techniques developed to solve these problems should be capable of measuring ozone concentrations in the range of < 0.01–10 ppm for these purposes11. For example, ambient air measuring at the ground requires instruments with a few ppbs accuracy in a range 20–


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200 ppb, whereas in industrial and commercial applications the precision is not so stringent and the expected concentrations are higher (up to 10 ppm).9,10,11 A number of analytical methods, such as UV absorbance, opto-chemical, optical, chemiluminescence, fluorescence, and electrochemical methods, have been developed to determine the ozone concentrations by sampling the atmospheric air.12,13 Not all of these methods offer a real time measurement and thus the requirement of development of low cost, portable, of high sensitivity real time ozone sensors is highly desired. In addition, most the conventional analytical methods do not provide enough sensitivity for the detection of small concentrations (few ppb) of ozone in many cases. In the literature, the ozone sensors have been proposed and based on its sensing principle can be categorized as (a) optical,14 (b) optochemical,17 (c) electrochemical,

15

and (d) conductometric.13 The operational

fundamental principle of a conductometric sensor is the change in resistance under the effect of reactions (adsorption, chemical reactions, diffusion and catalysis) that take place on the surface of the sensing layer. 16 They are fabricated using thin and thick film technologies by mainly metal oxides.17 Metal oxides employed as active layers in the conductometric based ozone sensors are among other, In2O3, SnO2, WO3, ZnO, MoO3 – TiO2, Fe2O3 and SmFeO3. However, important disadvantages for most of these systems are a) their need to operate at relatively higher temperatures (> 300oC) well above room temperature in order to demonstrate sensitivities down to few hundreds of ppb and b) in the majority of the cases they are not suitable for self-powered devices as they require to be switched on before start to detect the targeted gas molecules.17 The hybrid lead halide perovskites such as the mixed halide one studied here (CH3NH3PbI3−xClx), have recently attracted the scientific communities’ attention as one


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of the most promising light harvesting material due to its direct bang gap, long diffusion charge carrier lengths, large absorption coefficients, long carrier lifetime and large carrier mobility. 18 , 19 , 20 , 21 The power conversion efficiency of photovoltaics cells employing perovskite semiconductors have skyrocketed from 3.8% to higher than 20% over the last eight years. 22 These impressive results in solar cell technology, have attracted an intensive research interests towards the applications of perovskites beyond solar cells such as in lasers,23,24,25 in light emitting diodes,26,27,28,29,30,31 and photo-detectors.32, 33,34, 35 , 36 , 37

. However one of the key issues to be solved in order to boost the

commercialization of perovskite based devices is related to their stability.38 The latter is very sensitive to polar gases and vapors, as exposure to such elements deteriorates substantially and very fast the perovskite devices performance. Exploitation of another property, such as that of adsorption of gas molecules, could potentially lead to building the next generation of low cost, flexible gas sensors. Surprisingly only a very small number of publications has been released in the past dealing with perovskite semiconductors as gas sensors.39,40 Of these, selected works focused mainly on detecting ammonia NH339 and oxygen,40 were based in changes in the color or modulation of the film’s resistance. In this work, for the first time, we demonstrated a facile fabricated, low cost, of low temperature processed thin film of thin thickness ~ 300 nm based hybrid lead mixed halide perovskite (CH3NH3PbI3−xClx) planar ozone sensor. These films may be easily applied by spin coating or printable techniques on any substrate including flexible at low temperatures. The resulting sensing element may also operate at room temperature, be self-powered (no requirement of an external optical signal to be switched on prior to its


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exposure into an ozone environment) and may demonstrate high sensitivity (few ppbs) to ozone molecules. This work, envisions triggering a systematic study on the sensitivity of the perovskite films towards ozone and its applications as a gas sensor in general. The proposed sensor has demonstrated a quite fast response (i.e. 70s at 13 ppb) while requires much simpler fabrication steps and facilities compared to the laborious fabrication processes of other sensing element materials applied in ozone detection technologies such as pulsed laser deposition,41 molecular beam epitaxy,42 aqueous chemical growth,43 spray pyrolysis and sputtering44,45,46, 47,48,49,50,51. The proposed sensing element / active layer material may simply be deposited by spin coating on two predefined interdigitated platinum electrodes. The lower processing temperatures (~ 100oC) provide a compatibility with flexible and wearable sensing applications, two areas that are expected to play a significant role in the emerging sensing technologies in the near future. 52 Significantly, the similar effective electron and hole masses, recorded for this material allow this kind of sensors to demonstrate a bipolar conductivity and an ability to operate as a sensing element both for oxidizing and reducing gases. 53 The hereby-presented CH3NH3PbI3−xClx based ozone sensing material operation principle is based on changes of CH3NH3PbI3−xClx resistance induced by the ozone molecules presence. We found that the resistance of the CH3NH3PbI3−xClx perovskite film was repeatable decreased within seconds when the film was exposed to an ozone atmosphere while the film could return to its resistance state within seconds after the ozone atmosphere was removed. In addition, the said material was able to detect ultra-low ozone concentrations ranging from 2500 ppb to 5 ppb. This detection sensitivity is more than an order of magnitude below the 75 ppb mark imposed lately by International Agencies.54 Our work is believed to set


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the basis for a new conductometric, low cost, printable perovskite at ozone sensing element. This is of a paramount importance for health applications and will be proved very precious for a variety of automotive, air conditioning, food safety, water treatment, manufacturing and sterilization applications. Results CH3NH3PbI3-xClx thin-films characterization Atomic force microscopy (AFM) and scanning electron microscopy (SEM) were employed to the CH3NH3PbI3−xClx film in order to characterize the morphology of the proposed ozone sensing active material. The latter was porous and had a thickness of approximately 300 – 400 nm and an average roughness of 44 nm as shown by the AFM measurements (see Figure S1 a,b). Figure S2a depicts the acquired SEM image where the perovskite crystals can be seen and the film’s porosity is easily noticeable. It is believed that the long grain size (150 – 500 nm), as has been calculated from the SEM image (see figure S2b), encourages the fast diffusion of the gas molecules into the perovskite. Additionally the existence of the porous at the surface of the perovskite film provides the beneficial penetration and the evacuation pathways required. The successful crystallization of the spin coated perovskite film is shown in the acquired XRD image (see figure S3), where the crystal directions of the first (110) and the second (220) crystallographic plane can be seen, at ~14.2o and ~28.5o respectively, confirming the as expected cubic perovskite phase.55 This result is in accordance with our previous work. 56 To avoid any misconception, in the formation of the CH3NH3PbI3−xClx


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perovskite semiconductor, Cl, that was the lead source for the crystal formation (PbCl2), is not entered in the lattice of the perovskite, but acts as a crystallization inhibiter.57,58

CH3NH3PbI3-xClx-based ozone sensor preparation and characterization Afterwards, the CH3NH3PbI3−xClx films, spin-coated and crystalized on top of InterDigitated (IDT) transducer on glass substrates, were used for the electrical measurements (See Figure 1).

Figure 1. Image of the as prepared CH3NH3PbI3−xClx film on the prepatterned InterDigitated Pt electrodes glass substrate

Key issues characterizing the performance of the material such as sensitivity, response and recovery times as well as reversibility have been studied. The CH3NH3PbI3−xClx as sensing material, was placed inside a home-made gas testing chamber.59 The initial value of the current was approximately one µA. This value reached and stabilized within less than a minute. Such a high current is an indication of the excellent electrical properties as a result of the long grain’s size and high carrier’s diffusion length of the


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CH3NH3PbI3−xClx perovskite layer employed as the sensor’s active layer. Sensing responses were monitored without the need of any external assistance such as UV irradiation or heating in order to bring it in a steady state prior to its exposure in the targeted ozone molecules in contrast to what other sensing elements require for an effective operation.41,43,44,45,46,48,49,50,64 This is a very important feature of our set-up from the perspective of its energy consumption and portability. Our choice for a photoreduction process under vacuum rather in ambient conditions was deliberate since we wanted to demonstrate sensitivity responses in ozone concentrations much lower than those in ambient air. Subsequently, the CH3NH3PbI3−xClx sensing elements were exposed for five minutes to ozone at a constant flow of 500 sccm (standard cubic centimeters per minute), while the pressure in the chamber was kept constant at 120 mbar, leading to an increase of the current. Interrupting ozone flow leads to the recovery of the prior sensing element characteristics. The ozone concentrations that the detector was exposed were 2500, 1700, 500, 180, 13 and 5 ppb. Additionally, as an indication of our perovskite based sensing element capability to show different responses at different environments we are providing its different response when exposed to O3 and the synthetic air. (see Figure S4). Furthermore as the need for low temperature gas sensors is becoming a necessity, all tests were performed at room temperature thus sacrificing part of the response signal obtained at elevated temperatures. All the measurements were recorded until the current through the sample had reached its maximum value. This process duration was for approximately five minutes after which no further substantial changes were observed. Sensor sensitivity (S) was defined as the ratio of (Imax)/Imin), where Imax denotes the maximum value of the current across the sample in the presence of ozone and


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Imin the minimum value of the current in the applied synthetic air (absence of ozone). Tests have shown that the magnitude of response to different ozone concentrations was, as expected, decreasing (following a characteristic exponential growth form) with decreasing ozone presence, while remained distinctively detectable even ultra-low ozone concentrations (13 ppb). The detection limit of 5 ppb is the lowest reported in the open literature regarding spin coated ozone sensors by more than an order of magnitude and much below the minimum safety protocols requirements.5,6,7 The high sensitivity of the particular sensor under ozone exposure in combination with its high initial conductivity provides the confidence that even much lower ozone concentrations in the range of a few ppb or ppt can be detected. Figure 2 depicts the film response (I-t curve) after exposure to ozone concentrations varying more than two orders of magnitude i.e. from 2400 to 5 ppb.

Figure 2. Hybrid lead mixed halide ozone sensing element electrical response under various ozone concentrations. The duration of the measurement (~ 90 min) and its


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repeatability during the 2nd cycle demonstrates the stability of the sensor under severe conditions.

Figure 2 helped us to characterize the proposed ozone sensor by calculating its sensitivity, responsivity and recovery times. The sensitivity of the CH3NH3PbI3−xClx film was very much affected by the ozone concentration; it was increased with the ozone concentration. The response time (tr), as calculated at the time takes for the current to reach the 90% of its maximum value, was between 188 and 225 seconds. It was noticed that the tr was getting lower values as the concentration of the O3 was increased. This tendency was observed from 2500 ppbs until 180 ppbs. However, for concentrations below 180 ppbs this behavior changed in a persistent way (the tr were getting values of the order of hundreds seconds). Further investigation is needed to be paid in order to explain this physical process and how the tr is linked with the concentration of the targeted gas. The recovery time (trec) was between 40 and 60 seconds. The reproducibility of the results obtained increases the credibility of our sensing element performance and reliability. The demonstrated response times are superior to those reported i.e. by self – powered Si-ZnO gas sensors (~1200 sec). 60 Differences in the response times (estimated at t90 point) and sensitivity during the oxidation process under various ozone concentrations are indicative of the ability of these CH3NH3PbI3−xClx sensing films to detect ultra-low ozone concentrations. The long exposure times (more than 90 minutes) of the CH3NH3PbI3−xClx sensing film under ozone environments did not affect the repeatability of the sensor showing an extremely stable behavior and lifetime. All the experimental results and analysis are summarized in Table 1.


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Table 1. Performance Characteristics of the (CH3NH3PbI3−xClx) based ozone sensor O3 in ppb

Imax (µΑ)

Ιmin (µΑ)

Sensitivity, S

Response time, tres (s)

2500

10.86

1.12

9.69

188.55

1700

8.66

1.22

7.09

190.52

500

3.87

1.07

3.61

219.53

180

2.4

0.8

3

225.03

13

1.13

0.67

1.68

-

5

0.87

0.57

1.52

-

Figure 3 demonstrates the sensitivity of the proposed ozone sensing material as a function of the ozone concentration. From the above measurements it is quite obvious that the minimum current especially for the ozone concentrations in the range of 2500 – 500 ppb is quite stable. On the other hand, the recorded maximum current value increases with the ozone concentration. The detection response times of the CH3NH3PbI3−xClx based ozone sensing elements to the various concentrations was quite short compared to the response time of the several minutes for commercially used ozone sensors operating at even much higher ozone concentrations (> 8 ppm).

Differences in the exponential decay (see

response and recovery times in figure 2) during the oxidation process under various ozone concentrations are indicative of the ability of these perovskite films to resolve ultra-low ozone concentrations.


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Figure 3. Hybrid lead halide ozone sensor’s sensitivity as a function of the ozone concentration

To explain the CH3NH3PbI3−xClx sensing properties towards ozone when the perovskite film interacts with ozone, we have considered that the gas molecules are adsorbed within the perovskite lattice (close to the surface) and passivate the traps (unpaired Pb+2 ions) in CH3NH3PbI3−xClx; as a result the sensing films resistance is reduced. It is the uncoordinated Pb ions that cause the recorded CH3NH3PbI3−xClx sensitivity to the environmental gases. The increase of the current through the perovskite film during its exposure to ozone indicates the charge transfer from the O3 to the Pb2+ cation and neutralize the excess positive charges and therefore drastically modulate the surface recombination rate in perovskite sensing film. This is in agreement with previous reported extensive work and theoretical models have been developed in metal oxide ozone sensors.61 It is therefore believed that the ozone gas passivates the surface traps and plays an important role in the CH3NH3PbI3−xClx sensing capability. This results in


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the enhancement, due to lower hole – electron carrier trapping, of the CH3NH3PbI3−xClx film conductivity as a result of donation of an electron from the ozone to under coordinated Pb+2 ions in the (CH3NH3PbI3−xClx) surface. Such change in the conductivity is a function of the amount of ozone concentration molecules adsorbed onto the (CH3NH3PbI3−xClx) surface and the surface morphology of the perovskite film (grain size, porosity, roughness). This process is reversible through two cycles made for each ozone concentration tested in this work. During the de-oxidizing phase (absence of ozone environment) the CH3NH3PbI3−xClx restores its pristine electrical conductivity (high resistance state) since the ozone molecules are quickly desorbed. 62 The short response times of the pristine conductivity after the ozone flow stops is surprisingly small for the adsorption and de-adsorption processes that usually require several minutes. It is believed that these processes are very much accelerated due to the porous size of the CH3NH3PbI3−xClx film. The porosity and the average grain size of the latter facilitate the ozone molecules to be introduced and be released through the CH3NH3PbI3−xClx sensing elements’ surface. This morphology cannot be easily implemented with established MOS materials, as they need grain sizes below their Debye lengths (ca. 6 - 30nm) to achieve sufficiently strong electrical transduction of the gas reactions on their surfaces.63 The fact that when the ozone flow stops the current through sensor returns almost to its initial level (see figure 2), is a clear indication that the CH3NH3PbI3−xClx phase does not undergo any change as previous publications indicated i.e. in the presence of ammonia.40 Moreover, this is in contrast with MOS sensing materials. For example, the conductivity of InOx has been reported to reduce after UV (photo-reduction) irradiation (with known exceptions such as the commercial


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available mics2614 ozone sensor) and oxidize in the presence of ozone, remaining constant in the absence of the gas almost in all gas concentrations. Prolonged exposure to ozone turns the sensing element back to its initial state.64 This hypothesis has been reconfirmed by the photoluminescence (PL) measurements taken before and after the ozone exposure in bare CH3NH3PbI3−xClx films. In Figure 4a are presented the PL spectra of the CH3NH3PbI3−xClx film before and after its exposure in simulated one sun irradiation, a typical procedure performed in such film for the improvement of its electronic quality, 65 (See Methods for more details), while Figure 4b depicts the substantially enhancement of the PL intensity after the exposure of the same film in ozone environment. This enhancement is mainly attributed to the ozone exposure that passivates the surface traps in the perovskite films and thus increases the intensity of the radiative recombination.66 Additionally no translation in the peak PL wavelength is noticed, indicating no substantial crystal phase change during the ozone exposure.

Figure 4. Photoluminescence spectra of (a) the pristine (black line) and Solar A.M. 1.5G exposed (red line) and (b) the ozone exposed CH3NH3PbI3-xClx perovskite films.


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However the sensor’s exposures to the ozone for prolong periods has some detrimental effects that need further investigation. It has been visually observed after the termination of the testing that perovskite material has been ablated. Figure 5 depicts the uniform drop of the UV-Vis absorption spectra intensity after the exposure of the CH3NH3PbI3−xClx sensing element material under 2500 ppb ozone concentration is a clear indication of partial material loss after 60 minutes of continuous exposure. However, since the absorption onset of the CH3NH3PbI3−xClx film before and after its exposure in such an extreme environment of ozone (75 ppb is the safety limit that International Agencies have imposed),54 lies at the same position (~770 nm)56, we can conclude that CH3NH3PbI3−xClx perovskite based ozone sensing element has passed an extremely hard stability test (it is not expected for an Ozone sensor to be exposed in a such extreme environment for a such a long time).5,6,7

Figure 5. A uniform drop in the UV-VIS absorption confirms this color modification, after film’s exposure to an ozone gas.

It is believed that this material loss during the measurement is due to the ozone reaction activity with the perovskite semiconductor material. Further investigation regarding the


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sensor’s lifetime under ozone environment as a function of its composition, deposition technique, solvents employed, applied voltage and ozone concentration, will be performed.

Conclusions These results are very promising as we demonstrated for the first time a fast, solutionbased in low temperature processed flexible substrate compatible CH3NH3PbI3−xClx ozone sensing material, working at room temperature, without the need for an external source for recovery. It’s sensing properties are based on changes of its electrical properties. Further investigations should be implemented in order to further investigate the sensing properties (sensitivity, response time, kinetics and stability) of CH3NH3PbI3−xClx ozone sensing material with its crystal grain size, morphology and thickness. Additionally, more work has to be done in order to prolong the operational lifetime of this type of sensors. It has been observed the material’s ablation after the exposure in high levels of ozone (~2500 bbps) for 60 minutes take place. It is believed that the reaction of ozone with the perovskite semiconductor film is responsible for this material degradation. More research should be implemented in order to investigate deeper this issue. The testing of cesium based perovskite sensors probably is an option. The fabrication of lead free perovskite sensors, is also very important regarding the demonstration of high performance, disposable and environmental friendly ozone sensors. Among other future objectives is the testing of these devices ageing as a function of the storage conditions & material constitution. The CH3NH3PbI3−xClx platform can be realized as a new breakthrough in the field of wearable optoelectronics and its sensing applications can be applied in fields like personalized non-invasive medicine and defense


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purposes. Also more work has to be done in order to demonstrate selectivity i.e. the ability of such sensors to be able to distinguish more than two gases at the same time. Further understanding and theoretical modeling on how perovskite (CH3NH3PbI3−xClx) interacts with the surrounding environment is of great significance to improve the stability of perovskite solar cells. Our work could initiate additional to improve even more the long-term stability and to shorten the response time of the CH3NH3PbI3−xClx ozone sensing even more. The CH3NH3PbI3−xClx sensing properties & stability in collaboration with other innovative materials such as graphene, other 2D materials should be examined. Among our future plans is the design of a new gas chamber that will permit the simultaneous flow of more than one gasses including O3, like nitrogen dioxide etc. and humidity tests. Finally to further understand, the trap passivation kinetics in CH3NH3PbI3-xClx films or other metal halide perovskite based sensing elements under ozone exposure time resolved PL and absorption measurements will be taken.

Methods The hybrid lead mixed halide perovskite (CH3NH3PbI3−xClx) precursor solution was prepared by mixing methylammonium iodide (MAI, Dyesol) with lead (II) chloride (PbCl2, 99.999% Sigma Aldrich), at a 3:1 molar ratio of CH3NH3I to PbCl2, in anhydrous N,Ndimethylformamide (DMF). The final concentration was 40 wt% and the prepared solution was stirred overnight at 70 °C. Before the preparation of the CH3NH3PbI3−xClx thin film, its precursor solution was filtered through a 0.45µm PTFE filter. The substrates, where the sensors were fabricated were purchased pre-patterned from DropSens and were composed of two interdigitated electrodes with two connection tracks, all made of platinum, on a glass substrate. The space between the interdigitated


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electrodes was 5µm. Before the deposition of the CH3NH3PbI3−xClx precursor solution, the substrates were placed inside an ultraviolet ozone cleaner for the removal of the organic contamination and increase of their surface hydrophilicity. Afterwards, the as filtered precursor solution was cooled down at room temperature and 20 µl of it was spread over the pre-patterned substrate. Subsequently, the precursor solution was spincoated at 4000 rpm for 45 second followed by a thermal annealing at 100 °C for 75 minutes to form the CH3NH3PbI3−xClx hybrid perovskite semiconductor. The entire processing was conducted inside a nitrogen filled glove box with O2 and H2O concentration below 0.1 ppm. Before the ozone sensing measurements the glass/Pt/CH3NH3PbI3−xClx sandwich devices were exposed in simulated solar light (A.M 1.5G at 100mW/cm2) under ambient conditions (~45% relative humidity) for the improvement of the CH3NH3PbI3−xClx film electronic quality. The topography and roughness of the CH3NH3PbI3−xClx film was examined by means of atomic force microscopy (AFM), by employing a Park XE-7 instrument in tapping mode. The AFM measurement was performed with a scan rate of 0.3 Hz, while the total area of each scan was 50x50µm. Photoluminescence measurements were performed using a 532 nm continuous wave diode laser as an excitation source with an intensity of 1 mW. UV-vis absorption spectra were recorded using a Shimadzu UV-2401 PC spectrophotometer over the wavelength range of 300-850 nm. The size distribution and the morphology of the asfabricated CH3NH3PbI3-xClx was characterized by scanning electron microscope (SEM JEOL JSM- 7000F). The crystallographic properties of CH3NH3PbI3-xClx film on the Glass substrate was investigated using a D/MAX-2000 X-ray diffractometer under monochromated Cu Kα irradiation (λ=1.5418 Å) at a scan rate of 4° min−1. The


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electrical characterization of the as prepared sensors was performed inside a home-made gas test chamber under dark conditions. Mechanical pump initially evacuated the chamber to a pressure of 1.6 mbar. A constant potential difference of one Volt was applied between the two electrodes of the transducer and the current across the CH3NH3PbI3−xClx film was measured by a Keithley 6517A electrometer. All tests were performed at room temperature. The CH3NH3PbI3−xClx sensors were exposed for five minutes to ozone at a constant flow of 500 sccm (standard cubic centimeters per minute), while the pressure in the chamber was kept constant at 120 mbar, leading to an increase of current. The ozone concentrations that the detector was exposed were 2500, 1700, 500, 180, 13 and 5 ppb. The CH3NH3PbI3−xClx sensor was exposed for approximately five minutes at each concentration, while another five minutes was given to the sensor to relax to its steady state conditions. Corresponding Author *Address correspondence to [email protected] and [email protected] ASSOSIATED CONTENT Supporting Information Supporting Information is available: The following files are available free of charge. AFM characterization, SEM images and grain size histogram, XRD pattern of CH3NH3PbI3-xClx film and electrical characterization of the CH3NH3PbI3-xClx based sensing element under various ozone concentrations and exposure at full synthetic air conditions. Notes The authors declare no competing financial interest.


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