Degradation behavior of planar heterojunction CH3NH3PbI3 perovskite solar cells

Synthetic Metals 227 (2017) 43–51

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Degradation behavior of planar heterojunction CH3NH3PbI3 perovskite solar cells Chunhua Wanga , Chujun Zhanga , Yulan Huanga , Sichao Tonga , Han Wua , Jian Zhangb , Yongli Gaoa,c , Junliang Yanga,* a b c

Hunan Key Laboratory for Super-Microstructure and Ultrafast Process, School of Physics and Electronics, Central South University, Changsha 410083, China School of Material Science and Engineering, Guilin University of Electronic Technology, Guilin 541004, China Department of Physics and Astronomy, University of Rochester, Rochester, NY 14627, USA

A R T I C L E I N F O

Article history: Received 10 November 2016 Received in revised form 7 February 2017 Accepted 27 February 2017 Available online xxx Keywords: Perovskite solar cells Planar heterojunction Stability Interface degradation

A B S T R A C T

The stability and degradation process of low-temperature, solution-processed planar heterojunction perovskite solar cells (PHJ-PSCs) with a structure of ITO/PEDOT:PSS/CH3NH3PbI3/PCBM/Al were studied in both nitrogen-filled glovebox and ambient environment (humidity 45%). The results suggested that PHJ-PSCs stored in glovebox without encapsulation showed good stability, and the power conversion efficiency (PCE) could be kept over 70% of original value even after 30 days. As compared, PHJ-PSCs stored in ambient environment without encapsulation showed an obvious degradation, and the PCE was about 35% of original value just after 1 h exposure in air. However, PHJ-PSCs fabricated with CH3NH3PbI3 thin films stored in ambient environment for the different times didn’t show obvious degradation and have the similar performance parameters, suggesting that the degradation of PHJ-PSCs mainly resulted from the interface issues rather than the attenuation of perovskite thin film itself. Furthermore, electrochemical impedance spectroscopy (EIS) characterization indicated that the degradation of photovoltaic performance parameters was prevailingly attributed to the interface degradation. The research provides good understanding to the stability and degradation process of low-temperature, solution-processed PHJ-PSCs, which would facilitate the stability improvement of PHJ-PSCs. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Organic-inorganic hybrid lead halide perovskite solar cells (PSCs) have been intensively studied as a hotspot in academia. Immense amounts of concrete research focused on high-performance PSCs were carried out, and awe-inspiring power conversion efficiency (PCE) has been acquired since the first report by Kojima et al. employed perovskite materials as the sensitizers [1–4]. In just a few years, the PCE has skyrocketed from 3.8% to state-of-the-art 22.1% [1,5]. The prominent and rapid-enhanced performance mainly attributed to the excellent characteristics of perovskite materials, such as ambipolar charge transport, direct band gap, large diffusion length, etc. [6–9]. The classified architectures of PSCs are consisted of mesoporous structure and planar heterojunction (PHJ) structure. The fabrication of mesoporous PSCs normally needs a high temperature

* Corresponding author. E-mail address: [email protected] (J. Yang). http://dx.doi.org/10.1016/j.synthmet.2017.02.022 0379-6779/© 2017 Elsevier B.V. All rights reserved.

sintering process, while the PHJ structure can be fabricated via low-temperature, solution-processing protocol, which is compatible with large scale, roll-to-roll (R2R) printing/coating process [10,11]. So far, considerable progress on high-performance PSCs have been made based on device architectures [12,13], interface engineering [14–17], compositional engineering [18], and so on. However, the stability of PSCs is still one of the biggest issues owing to the moisture-sensitive characteristic of perovskite materials. It is imperative to precisely understand the stability and degradation mechanism of PSCs, and some pioneering works have probed the stability of PSCs with via modifying the device structure or employing the interface material [19–24]. Herein, the stability and degradation process of low-temperature, solution-processed PHJ-PSCs with a structure of ITO/PEDOT: PSS/CH3NH3PbI3/PCBM/Al were studied under the conditions of nitrogen-filled glovebox and ambient environment (humidity 45%), respectively. The performance parameters, perovskite thin-film properties and electrochemical impedance spectroscopy (EIS) results suggested that the decay of PHJ-PSCs was mainly ascribed to the interface degradation. The research work conveys

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clear information to understand the stability and degradation process of low-temperature, solution-processed PHJ-PSCs, which is helpful to accelerate the development and commercialization of PHJ-PSCs.

over night at 60  C to ensure dissolve adequately. A 0.22 mm PVDF filter was employed to filter the perovskite precursor before deposition. All materials were used without further purification. 2.2. Device fabrication

2. Experiment details 2.1. Perovskite precursor preparation The perovskite precursor was prepared by dissolving methylammonium iodide (CH3NH3I, 99%, Jingge, Wuhan) and lead iodide (PbI2, 99%, Zhengpin, Shanghai) at a molar ratio of 1:1 in anhydrous N,N-Dimethylformamide (Super dry, DMF, J mg/ml. The fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM, Dye Source, American) was dissolved in anhydrous chlorobenzene (CB, J&K Seal) with a concentration of 15 mg/ml. The perovskite precursor solution and PCBM solution were energetically stirred

The PHJ-PSCs was fabricated with a structure of ITO/PEDOT: PSS/CH3NH3PbI3/PCBM/Al. The patterned indium tin oxide (ITO) glass substrate was ultrasonically cleaned using ultra-sonic cleaner in acetone, detergents, deionized water and isopropyl alcohol for 20 min in sequence. Then the glass substrate was dried by nitrogen flow and treated by UV-ozone for 20 min subsequently. The PEDOT: PSS solution was used as hole transport layer (HTL) material by spin-coating onto the ITO at a spinning speed of 3000 rpm for 30 s, and then was annealed at 150  C for 15 min. Overall devices were conducted in a N2-filled glovebox (both H2O and O2 < 1.0 ppm). Solvent-induced-fast-crystallization deposition method was

Fig. 1. (a) Typical J–V curves of CH3NH3PbI3-based PHJ-PSCs with forward and reverse measurement, respectively. (b–e) The degradation trend of statistical performance parameters (PCE, Voc, Jsc, FF) of eighteen PHJ-PSCs that stored in N2-filled glovebox for 30 days.

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employed to gain dense and smooth perovskite films [25,26]. The CH3NH3PbI3 precursor solution was dropped onto the PEDOT:PSSbased ITO substrate and spin-coated at 4000 rpm for 40 s. The anhydrous CB ( 65 ml) was promptly dropped onto the center of the substrate several seconds before the substrate color change from the yellow (the color of perovskite solution) to the white in order to accelerate the crystallization process, consequently the color of deposited samples transforms from the transparent to the light brown. Then the samples were treated on a hot plate at 100  C for 10 min and the red-brown perovskite thin film with a thickness of 290 nm could be obtained. The electron transport layer (ETL) was spin-coated with the PCBM solution at a speed of 3000 rpm for 30 s. Finally, a 100 nm Al electrode was deposited by thermal evaporation under a vacuum of about 8.0  10 6 mbar and the active area of PHJ-PSCs is 0.09 cm2. 2.3. Characterization The absorption spectra, crystallographic properties and morphologies of perovskite thin films that exposed to ambient environment (humidity 45%) for different time intervals were characterized by employing ultraviolet-visible spectrophotometer (UV-vis, Puxi, T9, China), X-ray diffractometer (XRD, Rigaku D, Max 2500, Japan) and scanning electron microscope (SEM, FEI Helios Nanolab 600i, America), respectively. Current density-voltage (J–V) characteristics of PHJ-PSCs devices were measured by digital Source Meter (Keithley, model 2420, USA). Standard silicon simulator was employed to calibrate the light intensity with a standard value 100 mW/cm2. The PCEs were measured using a solar simulator (Newport 91160s, AM 1.5G, USA). The thickness of thin films of PHJ-PSCs for each layer and the surface roughness of perovskite thin films were obtained by surface profilometer

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(Dektak 150, Veeco, USA) and atomic force microscope (AFM), respectively. The characteristics of carrier transport and recombination were evaluated by electrochemical impedance spectroscopy (EIS, CHI606D, Chenhua, Shanghai). All the measurements and tests were carried out in air with a controlled humidity of about 40%, and all the samples stored in glovebox or in air were under dark condition without encapsulation. 3. Results and discussion It is well recognized that PSCs are very sensitive to the humidity, hence the CH3NH3PbI3 perovskite films were fabricated under an inert atmosphere in a N2-filled glovebox (both H2O and O2 < 1.0 ppm) using spin-coating method. The quality of perovskite film plays a vital role in the performance of PSC devices. The uniform and dense CH3NH3PbI3 perovskite films could be fabricated using a solvent-induced-fast-crystallization deposition [25,26]. The carrier transfer and recombination dynamics of overall photovoltaic devices stored in glovebox and air (humidity 45%), respectively, as well as the individual perovskite film placed in air were carried out herein to explore the internal degradation mechanism, clearly disclosing whether the decay originates from the perovskite film itself or other factors such as interface degradation. The typical J-V curves of photovoltaic devices measured with forward and reverse scanning directions under AM 1.5 G illumination are exhibited in Fig. 1a. The typical forward measurement shows the PCE of 12.02% with short-circuit current (Jsc) of 21.17 mA/ cm2, open-circuit voltage (Voc) of 0.87 V and fill factor (FF) of 64.9%, while the reverse measurement shows the PCE of 11.58% with Jsc of 20.63 mA/cm2, Voc of 0.87 V and FF of 64.2%, suggesting that there is not obvious hysteresis for PHJ-PSCs. Fig. 1b–e are the statistical performance parameters (PCE, Jsc, Voc and FF) of eighteen

Fig. 2. Nyquist plot of CH3NH3PbI3 PHJ-PSCs stored in N2-filled glovebox and tested at DC bias of (a) V = Voc, (b) V = 0 under AM 1.5G illumination, symbols and lines are stand for experimental and the fitted data, respectively. (c) The equivalent circuit used to fit the impedance spectra for EIS analysis. (d) The statistic numerical values for Rsc and Rrec that tested at V = Voc and V = 0, respectively.

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CH3NH3PbI3-based PHJ-PSCs stored in glovebox for 30 days. Generally speaking, PSCs stored in inert environment should show excellent stability because the fatal factor moisture is removed. As the time goes on, the tendency of statistical data is generally compliant with the expectation. Indeed, there isn’t a big change occurred in the parameters of Jsc, Voc and FF, benefiting from the isolation of humidity. However, it should be noted that the PCE still collapsed by about 27% with the storage in glovebox for 30 days, resulting from the small drop in Jsc, Voc and FF, respectively. Obviously, there are two stages during the degradation process. Most of degradation happens at the first stage (the first ten days), and the performance parameters are almost the same at the second stage (the later twenty days). The first stage is mainly ascribed to the interface attenuation and the second stage is associated with the active layer materials, i.e., interface degradation and intrinsic oxidation driven degradation [23]. Owing to the storage of PHJ-PSCs in glovebox, the fatal factor moisture is removed, the degradation doesn’t show obvious drop during the second stage. In order to better understand the essence of degradation process, EIS was employed to study PHJ-PSCs. EIS is a forceful characterization method for exploring the carrier transfer performance and recombination dynamics, from which series resistance (Rs), contact resistance (Rsc) and recombination resistance (Rrec) can be extracted expediently [27], making it is possible to decouple physical processes at a certain time. The Nyquist plots of CH3NH3PbI3-based PHJ-PSCs stored in N2-filled glovebox at the DC bias voltages of V = Voc and V = 0 under AM 1.5G illumination are presented in Fig. 2a–b. The equivalent circuit is consisted of two RC circuits, i.e., ideal capacitors and constant phase elements (CPE), as shown in Fig. 2c. The fitted EIS pattern has the characteristics of

arcs or transmission line. Fig. 2a presents the EIS curves of PHJPSCs at V = Voc that stored in glovebox at the specific time. With the increase of storage time, the arc radii quickly become larger and larger. It reveals that Rsc has a skyrocketing, which is ascribed to the selective contacts as well as the interface between the CH3NH3PbI3 layer and its neighboring contacts [27–29], suggesting that the interface has an enormous effect on the stability of PHJ-PSCs. The EIS pattern extracted from the bias voltage V = 0 is exhibited Fig. 2b. It shows that the radius of transmission line become smaller with the storage time, which is directly related to the Rrec. The Rrec at a low frequency is mainly associated with perovskite layer and ETL or HTL, which is inversely proportion to the recombination ratio [30]. The statistic values of Rsc and Rrec that obtained with bias voltage at V = Voc and V = 0, respectively, are presented in Fig. 2d. The results indicated that the recombination occurred between the perovskite layer and its adjacent ETL or HTL increases severely with the storage time, and the interface issue mainly results in the attenuation of PHJ-PSCs. Furthermore, PHJ-PSCs were placed in air with a controlled humidity of 45% for monitoring the degradation process. Fig. 3 shows the average performance evolution as the function of exposure time in air for 0 min, 20 min, 40 min and 60 min, respectively. It is obvious that the exposure time in air greatly influence the performance parameters of PHJ-PSCs. The average PCEs downgrade fleetingly, resulting in about 70% decrease of original value after exposure in air for 60 min, which conveys a powerful message that the performance of PHJ-PSCs heavily defined by the humidity. Especially, the error bars of PCEs increase dramatically, indicating that the performance repeatability of PHJPSCs become worse as well with the exposure time in air. It is obvious that the degradation of PCEs mainly results from the

Fig. 3. The average performance parameters of six PHJ-PSC devices as the function of the exposure time in air with a controlled humidity of 45%. (a) PCE, (b) Voc, (c) Jsc and (d) FF.

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decrease in FF, although the performance parameters Voc and Jsc also show slight decrease with the exposure time in air. Fig. 3d clearly indicates that the FF has a linear decrease leading to only about 50% of the origin value. Undoubtedly, it is a lethal shortcoming for the stability of PHJ-PSC devices. The details are discussed hereinafter based on impedance spectra. The efficient carrier extraction/injection and transport occurred at the interface plays an important role in determining the device performance. EIS was employed for PHJ-PSCs exposed in air for obtaining the information of the carrier transport and recombination. The impedance spectra gained at the bias voltage V = 0 and V = Voc under illuminated conditions are shown in Fig. 4, respectively. The graph gained from V = Voc shows the same tendency that the radii increase quickly alone with the exposure time, i.e., Rs and Rsc experience an obvious augment. The increasing of Rs is assigned to the interface contacts between the electrodes and ETL or HTL [27,28], resulting in the sharply decline of FF which perhaps is the fatal weakness for the interface degradation [31]. Furthermore, the soaring of Rsc is attributed to the selective contact and the interfaces of perovskite layer and ETL or HTL [30]. The transmission line extracted at V = 0 is exhibited in Fig. 4b. It shows that the radius become smaller over time, suggesting Rrec drop gradually. It means that the recombination rate grows largely. However, the Rrec within a range of low frequency mostly attributes to perovskite layer and ETL or HTL, resulting in the decline of Voc and Jsc, which are related to the interface to a great extent. The large Rsc shown in Fig. 4b is also observed over time from 0 min to 60 min. Fig. 4c depicts the statistic numerical values of Rsc and Rrec that measured at V = Voc and V = 0, respectively. The increase of Rs and Rsc, together with the decrease of Rrec, indicates that the interfaces have poisonous

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influence on the stability of PHJ-PSCs, i.e., the interface degradation dominates the damping of PHJ-PSCs. The SEM surface morphologies of perovskite films exposed in air for the different time were studied as well, as shown in Fig. 5a– d. It is clear that the perovskite films are still very smooth and uniform even placed in air for 60 min. They don’t show obvious degradation, indicating the humidity has little influence on individual perovskite films in 60 min. Thus the reason contributed to the degradation of photovoltaic performance should be associated with the interface. In addition, AFM was employed to characterize the surface roughness of perovskite films with the different exposure time, as shown in Fig. 5e–h, of which the root mean square roughness (RMS) are 2.98 nm, 3.05 nm, 2.93 nm and 3.05 nm, respectively. The results also demonstrate that perovskite film would not remarkably change as exposed in air, which further reflects that the intrinsic perovskite film degradation is not the determining factor leading to the sharp decay of PHJ-PSCs performance as exposed in air. XRD was employed to characterize crystallographic properties of perovskite film for checking the composition along with the exposure time in air. Fig. 6a shows the XRD patterns of perovskite films exposed to the specific humidity of about 45% at 20 min intervals. The perovskite diffraction peaks present at the same position for all films. The strong diffraction peaks at 14.25 , 28.56 , 32.00 , 40.77 and 43.31 can be ascribed to the perovskite crystal faces to (110), (220), (310), (224) and (314), respectively. As compared with the main perovskite diffraction peaks, the change of impurities peaks before and after exposure in air for 60 min are very faint and almost negligible, which indicates that perovskite films remained almost intact even after exposure in air for 60 min. Normally, the decomposition of perovskite films would present

Fig. 4. Nyquist plot of CH3NH3PbI3 PHJ-PSC devices exposed in air with a controlled humidity of 45% for the different time at DC bias of (a) V = Voc, (b) V = 0 under one-sun illumination. (c) The Rsc and Rrec values that obtained with the bias voltage at V = Voc and V = 0, respectively.

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Fig. 5. SEM and AFM morphology images of perovskite thin films that exposure to air for (a and e) 0 min, (b and f) 20 min, (c and g) 40 min and (d and h) 60 min, respectively, in which AFM morphology images is 3  3 mm2.

notable signal not only from internal element distribution but also macroscopic change of peaks by XRD analysis [32]. Hence, it may

draw a conclusion that the deterioration of PHJ-PSCs does not mainly ascribe to the perovskite film influenced by the humidity.

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Fig. 6. (a) XRD and (b) absorbance spectra of CH3NH3PbI3 films exposed to air range from 0 min to 60 min at the interval of 20 min.

The absorption spectrum of perovskite films exposed to air for specific time were investigated, as shown in Fig. 6b. It turns out that almost identical absorbance spectra from 350 nm to 850 nm appeared for all thin films. Meanwhile, there is no distinct difference in absorption intensity. There should be a reaction threshold for the degradation of perovskite films [33], thus it could be inferred that the material composition shows no distinct variation and the perovskite material doesn’t degrade apparently within this timeframe. Further experiments were carried out in air within the same timeframe in order to understand the relationship between perovskite film and device decay. Fig. 7 presents the average performance parameters of PHJ-PSC devices for the PCE, Voc, Jsc and FF, respectively, of which perovskite films were exposed to air for 0 min, 20 min, 40 min and 60 min in sequence and the fabrication

process of all PHJ-PSC devices are identical except for the exposure time. It obviously shows that the statistic PCEs just have small fluctuations within a narrow range, which could be powerful evidence that the damping of PHJ-PSCs doesn’t hinge on perovskite film itself but the interface issues. Furthermore, PHJ-PSC devices exposed to air were also followed by in-situ optical microscope. As shown in Fig. 8, it is obvious that with increasing the exposure time the bubbles gradually forms on surface of the top electrode Al, and the bubbles become larger and larger. The formation of bubbles is associated with the gas resulted from the degradation of perovskite which destroys the Al electrode, i.e., the gas can penetrate and pass through the ultrathin PCBM layer and Al electrode accelerating the formation of the Al bubbles. The degradation of perovskite would produce hydrogen gas and accelerate the formation of Al bubbles [22,23]. However, as

Fig. 7. The average photovoltaic parameters of (a) PCE, (b) Voc, (c) Jsc and (d) FF that obtained from four PHJ-PSC devices, of which perovskite films exposed in air with controllable humidity of 45% for 0 min, 20 min, 40 min and 60 min, respectively.

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Fig. 8. The morphology image of the Al electrode bubbles formed on the surface of perovskite layer for PHJ-PSC devices exposed to air for 60 min at the time interval of 20 min. (a) 0 min, (b) 20 min, (c) 40 min, (d) 60 min. The arrows marked in the images represent the formed Al electrode bubbles.

perovskite films exposed to air, the morphology, XRD and absorption spectrum analysis all showed that there are hardly obvious changes, resulting in the similar PCEs of PHJ-PSCs fabricated from perovskite films that exposed to air for the different time. The Al electrode bubbles formed fleetly as PHJ-PSC devices exposed to air, leading to the quick drop of PCEs. Together with the EIS analysis, they obviously suggest that the degradation of PHJ-PSCs photovoltaic devices mainly comes from the interface rather than the perovskite film itself.

This work was supported by the National Natural Science Foundation of China (51673214), the Program for New Century Excellent Talents in University (NCET-13-0598), the Hunan Provincial Natural Science Foundation of China (2015JJ1015), and the Project of Innovation-driven Plan in Central South University (2015CXS036). Y.L.G. acknowledges the support by National Science Foundation CBET-1437656.

4. Conclusion

References

The stability of PHJ-PSCs is an essential asset that must be taken into consideration for making it possibly transform from laboratory-scale to commercially large-scale application. We demonstrated that the degradation of CH3NH3PbI3-based PHJPSCs with the structure of ITO/PEDOT:PSS/CH3NH3PbI3/PCBM/Al mainly result from the interface but not determined by perovskite film itself via studying perovskite film and the damping of photovoltaic performance by means of UV-vis, XRD, SEM, AFM and EIS. The results of perovskite film itself manifested that there are scarcely obvious changes with exposure in air for 60 min. The degradation of CH3NH3PbI3-based PHJ-PSCs stored in N2-filled glovebox and air by EIS technique shown that interface is the determining factor contributed to quick degradation of PHJ-PSCs, especially for the FF. The research work further uncovered the degradation mechanism of PHJ-PSCs and potentially accelerates the development of PSCs and its industrialization.

Acknowledgments

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