Interface degradation of perovskite solar cells and its modification using an annealing-free TiO2 NPs layer

Organic Electronics 30 (2016) 30e35

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Interface degradation of perovskite solar cells and its modification using an annealing-free TiO2 NPs layer Jian Xiong a, b, c, Bingchu Yang a, b, *, Chenghao Cao a, b, d, Runsheng Wu a, b, Yulan Huang a, b, Jia Sun a, b, Jian Zhang c, Chengbin Liu d, Shaohua Tao a, b, Yongli Gao a, b, e, Junliang Yang a, b, ** a Institute of Super-microstructure and Ultrafast Process in Advanced Materials, School of Physics and Electronics, Central South University, Changsha 410083, Hunan, China b Hunan Key Laboratory for Super-microstructure and Ultrafast Process, School of Physics and Electronics, Central South University, Changsha 410083, Hunan, China c School of Material Science and Engineering, Guilin University of Electronic Technology, Guilin 541004, Guangxi, China d College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, China e Department of Physics and Astronomy, University of Rochester, Rochester 14627, NY, USA

a r t i c l e i n f o

a b s t r a c t

Article history: Received 24 June 2015 Received in revised form 26 October 2015 Accepted 11 December 2015 Available online xxx

Interface is one of the most important factors to influence the device stability, which directly determines the commercialization of perovskite solar cells (PSCs). The research disclosed the degradation process and mechanism of planar heterojunction (PHJ) PSCs with a structure of ITO/PEDOT:PSS/CH3NH3PbI3/ PCBM/Al using in-situ experiments. The degradation of PHJ-PSCs is mainly attributed to the interface decay of perovskite/cathode. Large amount of bubbles formed quickly at the interface and grew up as PHJ-PSCs exposed to air. The cathode electrode easily peeled off from the devices that led to lose the efficiency completely after only 1 h exposure to air. On the other hand, the degradation driven by intrinsic decomposition of perovskite itself under atmosphere (humidity ~ 45 RH%) was not obvious and the power conversion efficiency (PCE) could retain almost the same when only the perovskite layer was exposed to air for 200 h. Furthermore, annealing-free TiO2 nanocrystalline particles (TiO2 NPs) as an interface modification layer was inserted into PHJ-PSCs and dramatically improved the stability, of which the PCEs retained over 75% of its initial values after exposure to air for 200 h. The results provide important information to understand the degradation of PSCs and the improvement of the stability, which may potentially accelerate the development and commercialization of PSCs. © 2015 Elsevier B.V. All rights reserved.

Keywords: Perovskite solar cells Planar heterojunction Interface Degradation TiO2 NPs

1. Introduction Solar energy is attracting more and more attention and acts as one of the important candidates to solve the world's energy crisis [1e3]. Although conventional silicon solar cells have achieved reasonably high power conversion efficiencies (PCEs), they are still expensive which prevents their broad applications [1,4,5]. Lowcost, light-weight, solution-processed thin-film solar cells have been developed during the past two decades [6e10]. Recently,

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (B. Yang), [email protected] (J. Yang). http://dx.doi.org/10.1016/j.orgel.2015.12.010 1566-1199/© 2015 Elsevier B.V. All rights reserved.

there has been an unexpected breakthrough and rapid evolution of highly efficient solar cells based on organic-inorganic hybrid perovskite materials, and the PCEs have been improved remarkably from 3.8% to about 20% [11e13]. The low-temperature, solutionprocessed planar heterojunction perovskite solar cells (PHJ-PSCs) can be compatible with flexible substrates and roll-to-roll (R2R) techniques, offering low-cost and high-output production [14,15]. The perovskite morphology and interface are two of the most important factors to influence the performance of PHJ-PSCs [16e22]. The commercialization of PSCs is greatly dependent on the stability excepting for the PCEs. Normally perovskite materials are sensitive to the moisture, and PSCs show a serious degradation when exposedto air [23e27]. Because of the intrinsic hydrophilic property of perovskite materials and their distorted tetragonal crystal structure, perovskite materials also show decomposition in

J. Xiong et al. / Organic Electronics 30 (2016) 30e35

the presence of water [26]. Meanwhile, there are pinholes in a perovskite film, which is a very important factor leading to the degradation of PSCs [28]. Hence, a buffer layer is inserted to protect the underlying perovskite, for example, Al2O3 [24] or organic semiconductor [29,30]. Furthermore, carbon electrode instead of other metal electrode could also slow the degradation rate of PSCs [31e33]. Although some progress has been made in improving the stability of PSCs, it is still not clear that which factors influence the performance degradation and how to influence the degradation process. Herein, we studied the degradation process and mechanism of PHJ-PSCs with a structure of ITO/PEDOT:PSS/CH3NH3PbI3/PCBM/Al. It was disclosed that the interface degradation between CH3NH3PbI3 and PCBM/Al is the main reason for the degradation of PHJ-PSCs, which was proved by in-situ experiments. While CH3NH3PbI3 itself stored in air at humidity of about 45 RH % didn't obvious influence the device performance of PHJ-PSCs. Furthermore, a solution-processed, annealing-free TiO2 nanoparticles (TiO2NPs) layer was inserted to improve the interface stability because it prevents the penetration of water from the Al electrode and the formation of Al bubbles. The relatively stable PHJ-PSC devices were fabricated using TiO2 NPs layer and the PCEs could retain 75% of their original values after exposure to air for 200 h. The research provides important information to understand the degradation of PSCs and its improvement.

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O2 < 1.0 ppm) and the solvent engineering method was introduced to get high-quality films. Firstly, CH3NH3PbI3 solution (50 ml) was first dropped onto a PEDOT:PSS coated ITO substrate, of which the size is 1.5 cm  1.5 cm. The substrate was then spun at 4000 rpm. When the precursor roll out, anhydrous chlorobenzene solvent (60 ml) was quickly dropped onto the center of the substrate. The color of just deposited CH3NH3PbI3 layer was changed from transparent to light brown. The samples were subsequently treated at 100
C for 10min. The 290 nm black perovskite film was formed finally. After cooling to room temperature, the PCBM solution was spin-coated onto the CH3NH3PbI3 layer with 3000 rpm for 30s. After that, the film was annealed at 100
C for 10min. For the interface modification devices, TiO2 NPs sol was spin-coated at 2500 rpm for 30s. Finally, a bar-like defined Al electrode (100 nm) by mask was deposited by thermal evaporation under the vacuum of ~4  106 mbar, resulting in an active area of 0.09 cm2. In order to measure the stability of CH3NH3PbI3 in air, the CH3NH3PbI3 films were deposited on ITO/PEDOT:PSS substrate and were exposed to air (humidity~45 RH%) for a specified time (0e200 h). Then the normal devices with a structure of ITO/PEDOT:PSS/CH3NH3PbI3/ PCBM/Al were prepared for testing the performance. For testing the stability of PSCs, the unencapsulated devices were exposed to air (humidity~45 RH %) and measured their performance at a specified time. 2.3. Characterization

2. Experiment section 2.1. Materials Organic-inorganic hybrid perovskite precursor solution was made with mixing 141 mg methylammonium iodide (CH3NH3I, 99%, Jingge, Wuhan) and 409 mg lead iodide (PbI2, 99%, Zhengpin, Shanghai) at a molar ratio of 1:1 in N,N-Dimethylformamide (Surper dry, DMF, J&K Seal). The precursor solution was vigorously stirred over night at 40e60
C, and was filtered with a 0.22 mm PVDF filter before the deposition. Fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) was purchased from American Dye Source, and was dissolved in anhydrous chlorobenzene (CB, J&K Seal) at 15 mg/ml. TiO2 NPs sol was synthesized by a facile solution-based method as reported [10]. Firstly, acidic solution was prepared by dissolving nitric acid (1.2 ml, HNO3) and acetic acid (4 ml, CH3COOH) in nbutanol (40 ml, CH3CH2CH2CH2OH). Nitric acid was introduced to provide a strong acidic environment for easier peptization, while acetic acid was used to improve dispersion of nanoparticles in the sol. Secondly, tetrabutyl titanate(24 ml, Ti(OBu)4) was added slowly into the above mixture under vigorously stirring. Then 7 ml H2O was added slowly into the above solution and the white gel was formed. The gel was diluted with H2O to be 0.4 M and stirred at 40
Cuntil becoming transparent sol again. Finally, the sol was stirred at 80
Cfor several hours. TiO2 NPs were achieved in such sol. At last, the sol was diluted in ethanol (Ti concentration was 0.016 M) and applied in PSCs. 2.2. Device fabrication The patterned indium tin oxide (ITO) coated glass was used as the substrate, which was ultrasonically cleaned in acetone, detergents, distilled water and IPA for 15 min, respectively, then dried by hot air and treated by UV-ozone for 15 min. The PEDOT: PSS layer (Baytron, PVP AI 4083) with a thickness of about 50 nm was spin-coated at 3000 rpm onto the patterned ITO substrate and dried on hot plate at 150
C for 15 min. The deposition of CH3NH3PbI3 layer was performed in a glove box (both H2O and

The morphology was characterized by atomic force microscopy (Agilent Technologies5500 AFM/SPM System, USA) with tappingmode. Optical microscopy images of electrode and the in-situ movie for the evolution of electrode bubbles were taken using an optical microscope (OlympusIX 71, Japan). The surface and crosssectional images was also measured by scanning electron microscopy (SEM, FEI Helios Nanolab 600i, USA). Absorption properties of CH3NH3PbI3 films were characterized by UVevis spectrophotometer (UVevis, TV-1800PC, Pgeneral). Crystallographic properties of CH3NH3PbI3 films were characterized by X-ray diffraction (XRD, Rigaku D, Max 2500, Japan). Current densityevoltage (JeV) characteristics of PHJ-PSCs were measured by digital Source Meter (Keithley, model 2420, USA). A solar simulator (Newport 91160s, AM 1.5G, USA) was used for the PCE test. Light intensity was 100 mW/cm2 calibrated by a standard silicon solar cell. The thickness of CH3NH3PbI3 and PCBM films were measured with surface profiles (Dektak 150, Veeco, USA) and AFM. 3. Results and discussion As reported in our previous work, very uniform and flat CH3NH3PbI3 films were prepared by a solvent engineering method, and the tetragonal structure perovskite crystalline was confirmed by X-ray diffraction (XRD) [34]. The surface morphology of CH3NH3PbI3 coated on ITO/PEDOT:PSS substrate is shown in Fig. S1a. The CH3NH3PbI3 film is very uniform and exhibits a rootmean-square (RMS) roughness of about 2.1 nm for an area of 10 mm  10 mm, and the CH3NH3PbI3 crystal size is ranged from 100 to 500 nm. It is obvious that the pinhole-free CH3NH3PbI3 film fully covers the substrate, which is beneficial to form a high-quality interface contact and enhance the performance of PHJ-PSCs. With a deposition of PCBM layer on the CH3NH3PbI3 film, the RMS does not show obvious change and is still about 2.1 nm (Fig. S1b). The film was further used to fabricate PHJ-PSCs with a deposition of Al electrode. Fig. 1a is the typical current density-voltage (JV) characteristics of PHJ-PSC devices under AM 1.5 G one-sun illumination. The device shows a PCE of 12.7% with a shortcircuit-current density (Jsc) of 20.52 mA/cm2, an open-circuit

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Fig. 1. (a) A typical current density-voltage (J-V) characteristics of PHJ-PSC device with a structure of ITO/PEDOT:PSS/CH3NH3PbI3/PCBM/Al under AM 1.5 G one-sun illumination. (b) The average degradation behavior of 10 PHJ-PSCs devices with the exposure time in air. (c) The J-V curves of PHJ-PSC devices with ITO/PEDOT:PSS/CH3NH3PbI3 films exposed to air for a specified time. (d) The average degradation performance of PHJ-PSC devices with ITO/PEDOT:PSS/CH3NH3PbI3 film exposed to air during 200 h.

voltage (Voc) of 0.91 V and a fill factor of (FF) of 68%, which are typical performance parameters for PHJ-PSCs with a structure of ITO/PEDOT:PSS/CH3NH3PbI3/PCBM/Al. One should notice that this kind of PHJ-PSCs show an obvious degradation as exposed to air. Fig. 1b is the average degradation behavior of 10 PHJ-PSC devices with the time when they are exposed to air. It can be found that the deviation of PCEs is very large at the exposure time from 10 min to 30 min even though the original performance deviation is very small at 0 min. Especially, the PCEs dramatically degrade from 11.40% to 0.53% after only 1 h exposure to air, i.e., PHJ-PSC devices almost completely lose the efficiency. Previous reports showed that perovskite materials were sensitive to the moisture [22e26]. In order to confirm the stability of CH3NH3PbI3 film itself with exposure to air (RH ~ 45%), we prepared the CH3NH3PbI3 films on ITO/PEDOT:PSS substrates and exposed them to air for a specified time. The color of the CH3NH3PbI3 films didn't show obvious change as observed with our naked eyes (Fig. S2a), which is probably ascribed to the absence of pin-holes in the CH3NH3PbI3 films [28]. UVevis absorption and XRD were further used to analyses the change of the CH3NH3PbI3 films and the results are shown in Fig. S2b and S2c. The absorbance spectra are almost the same. However, it is worth to notice that the diffraction peak of PbI2 in XRD pattern becomes more and more obvious, confirming that some CH3NH3PbI3 film on the surface was decomposed into PbI2. In order to understand the influence of CH3NH3PbI3 films exposed to air on the performance of their PHJPSC devices, a PCBM layer was spin-coated on the CH3NH3PbI3film and an Al electrode was deposited subsequently to finish the fabrication of PHJ-PSC devices. Fig. 1c shows the J-V curves of the typical PHJ-PSC devices with the CH3NH3PbI3 films exposed to air for 0, 100 h and 200 h, respectively. The PHJ-PSC device prepared

with the pristine CH3NH3PbI3 film shows a PCE of 12.43%, Jsc of 19.54 mA/cm2, Voc of 0.98 V and FF of 65%. For the CH3NH3PbI3 film exposed to air for 100 h, the PHJ-PSC device exhibits a PCE of 10.52% with Jsc of 17.63 mA/cm2, Voc of 0.98 V and FF of 61%. For the CH3NH3PbI3 film exposed to air for 200 h, the performance parameters of PHJ-PSC are almost the same as that for 100 h, with the PCE of 11.33%, Jsc of 18.18 mA/cm2, Voc of 0.97 V, and FF of 64%. Fig. 1d shows the degradation trend of PHJ-PSC devices prepared from the CH3NH3PbI3 films exposed to air for different time. Every PCE point is a statistic result from 6 PHJ-PSC devices, and the error bars show the standard deviation. It is clearly seen that the performance degradation of PHJ-PSC devices based on the CH3NH3PbI3 films exposed to air, even with 200 h, is not obvious, which is totally different to the quick degradation of PHJ-PSC devices exposed to air (Fig. 1b). The exact reason why the CH3NH3PbI3 films exposed to air does not obviously degrade the performance of PHJ-PSCs is not clear and needs a further study to understand the mechanism. The reported results showed that perovskite films with a little excess PbI2 or inadequate PbI2 used as the active layer in PSCs could still have the PCEs of over 10% [35]. The above results obviously suggest that the CH3NH3PbI3 films exposed to air do not significantly influence the performance of PHJ-PSC devices. In other words, the CH3NH3PbI3 film itself is not the main reason to contribute the degradation of PHJ-PSC devices. It has been proved that the degradation of PHJ organic solar cells (OSCs) can be classified into two kinds of modes, i.e., interface degradation and intrinsic oxidation driven degradation [36]. Generally speaking, the interface degradation leads to the initial decay, while the intrinsic oxidation driven degradation leads to a relatively slow decay at a longer time scale. Hence the quick degradation in PHJ-PSCs is likely related to the interface. The in-situ

J. Xiong et al. / Organic Electronics 30 (2016) 30e35

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Fig. 2. The morphology of the Al electrode bubbles formed on the surface of perovskite layer. (a) At the initial stage, (b) after 1 h exposure to air. (c) Cross-section SEM image of the Al electrode bubbles.

experiments were observed with an optical microscope. Fig. 2a is an optical microscope image of the Al electrode morphology for PHJ-PSC devices exposed to air at the initial stage. It is obvious that some bubbles form quickly, as indicated by the red arrows. After only 1 h, the size of the formed bubbles becomes larger and larger. Meanwhile, more and more bubbles form as well (Fig. 2b). One should notice that the bubbles could be formed as PHJ-PSC devices are exposed to air, no matter the perovskite film exposes to air or not before completing the fabrication of PHJ-PSC devices. The detailed dynamic transition process was followed with a video of an in-situ optical microscope, as shown in the video of the support information (The video with a time of 100 min was compressed to a 45 s one for reducing the file size). Actually, the large bubbles could be observed when PHJ-PSC devices are exposed to air for a few minutes. Thus, the Al electrode becomes rough quickly, and the roughening process could be observed by naked eyes on the back side of the devices, as shown in Fig. S3. Supplementary data related to this article can be found online at http://dx.doi.org/10.1016/j.orgel.2015.12.010. As shown in Fig. 2c, cross-sectional SEM image confirms that the bubbles are hollow. It can also be proved by puncturing the bubbles with a needle guided by an optical microscopy, resulting in collapsed bubbles as shown in Fig. S4a. Meanwhile, the bubbles would leak and collapse by themselves after the PHJ-PSC devices are exposed to air for a relatively longer time (Fig. S 4b). Our experiments further proved that lots of Al electrode bubbles formed quickly when the Al electrode deposited directly on the CH3NH3PbI3 film and all the bubbles seem to be leakage within about only 3 min exposure to air (Fig. S5). However, no bubble formed when the devices stored in the N2 atmosphere. Meanwhile, It is well known that there are not bubbles formed in OSCs with a structure of ITO/PEDOT:PSS/organic active layer (e.g., P3HT:PCBM)/ Al exposed to air. Therefore, the formation of Al bubbles is closely related to the CH3NH3PbI3 material and its exposure to air. The CH3NH3PbI3 material exposed to air would react with the water at the presence of light and result in the production of H2 [24].The detail reaction processes are shown below as the Eqs. (1)e(4).

H2 O

CH3 NH3 PbI3 ƒƒƒƒƒƒ! ƒƒƒƒƒƒ CH3 NH3 I þ PbI2

(1)

CH3 NH3 I4CH3 NH2 þ HI

(2)

4HI þ O2 42I2 þ 2H2 O

(3)

hn

2HIƒ! ƒ H2 [ þ I2

(4)

The CH3NH3PbI3 can react with the water and decomposes to CH3NH3I and PbI2. The product PbI2 was detected by XRD measurement, as shown in Fig. S2c. The CH3NH3I can further decompose to HI, which ultimately produces H2 under the light. The H2 can penetrate and pass through the ultrathin PCBM layer accelerating the formation of the Al bubbles. No bubble could be observed if without depositing the Al electrode, as shown in Fig S6. According to the above statements, the results obviously show that the interface issue between the CH3NH3PbI3/PCBM (or CH3NH3PbI3) and the Al cathode is the main reason that results in the performance degradation of PHJ-PSCs. In order to further confirm the degradation mechanism, PHJ-PSC devices with Ag as the cathode were fabricated as well. The PHJ-PSC devices with Ag electrode degraded with a slower speed than that with Al electrode, as shown in Fig. 1b and Fig. S7 in support information. The degradation trend is similar for both PHJ-PSC devices with Ag and Al electrodes. There are also some bubbles formed on the surface of Ag electrode when PHJ-PSC devices exposed to air (Fig S8), although the electrode bubbles are less than PHJ-PSC device with Al electrode, and the size of electrode bubbles is smaller as well. The formation of bubbles should be related to the quality of the electrode layer upon perovskite films. The existence of pinholes is not helpful to form the bubbles. It is probably that the Ag film is not as dense as the Al film, and some of the gas produced from the decomposition of perovskite film flows into the air directly, which would result in less bubbles in Ag film as compared with the bubbles formed in dense Al film. It is the reason that the degradation rate of PHJ-PSCs with Ag electrode is slower than that with

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J. Xiong et al. / Organic Electronics 30 (2016) 30e35

Al electrode. For some PHJ-PSCs with Ag electrode, the large collapsed bubbles could be found as well, as shown in support information Fig. S8f. All above statements suggest that the interface degradation, attributing to the perovskite decomposition, is the mainly reason for the performance degradation of PHJ-PSC devices with Al electrode or Ag electrode. It is well known that some interface materials could efficiently prevent water diffusing into the active layer in OSCs. In our previous study, the stability of OSCs could be enhanced dramatically using annealing-free TiO2 NPs as the interface modification layer [8]. The annealing-free TiO2 NPs is also introduced for hopefully enhancing the stability of PHJ-PSCs. The oxygen deficiencies-rich TiO2 can prevent the penetration of oxygen and humidity into the electronically active layer and leading the lifetime of OSCs enhanced nearly two orders of that without TiO2 interface, which confirmed by X-ray photoemission spectroscopy and degradation test [37,38]. Fig. 3 is the degradation curve of the PCEs for PHJ-PSCs with and without TiO2 NPs interface layer. It is obvious that PJH-PSC devices with a TiO2 NPs layer show much more stable than that without a TiO2 NPs layer. After exposure to air for 200 h, the PCE could retain 75% of its initial value for the former one; while the PCE quickly drops to 44% and 2% after only 10 min and 1 h, respectively, for the latter one. Fig. S9 is the typical J-V curves for PHJ-PSC devices with and without TiO2 NPs interface layer stored in air for a specified time, and the detailed parameters are summarized in Table 1. The results also suggest that PHJ-PSC devices with a TiO2NPs layer show a better stability than that without a TiO2 NPs layer. The effect of ethanol, acting as the solvent for TiO2 NPs, was excluded and analyzed in detail in the supporting information (Fig. S10, Fig. S11 and Table S1). The results state that the ethanol does not evidently influence the performance of PHJ-PSC devices, while the TiO2 NPs layer can enhance the performance, especially the stability, and the reproducibility of PHJ-PSC devices. The above discussion confirms that TiO2 NPs interface layer could act as a shielding and scavenging layer to prevent the penetration of oxygen and humidity into the electronically active layer since their oxygen deficiencies [8]. Thus it restrains the reaction between the CH3NH3PbI3 and H2O and prevents the production of H2 and the formation of the Al bubbles, resulting in the stable PHJ-PSCs as exposed to air.

Table 1 Performance parameters of typical PHJ-PSC devices with and without TiO2 NPs interface layer after exposure to air for a specified time. cathode configuration

Time

Voc (V)

Jsc(mA/cm2)

FF (%)

PCE (%)

PCBM/Al

0 min 10 min 60 min 0h 120 h 200 h

0.93 0.86 0.73 0.99 0.96 0.98

17.84 13.66 1.99 17.40 15.60 16.28

68 43 16 66 58 54

11.40 5.04 0.23 11.42 8.66 8.59

PCBM/TiO2 NPs/Al

4. Conclusion Interface degradation is the important factor to influence the performance stability of PHJ-PSC devices. The interface degradation process was disclosed using in-situ experiments for PHJ-PSC devices based on the structure of ITO/PEDOT:PSS/CH3NH3PbI3/PCBM/ Al. The PHJ-PSC devices almost completely lost their PCEs with only 1 h exposure to air, resulting from the formation of the Al bubbles between the CH3NH3PbI3/PCBM layer and the Al electrode. In contrast, the device degradation resulted from the decomposition of the CH3NH3PbI3film itself under atmosphere (~45 RH%) was proved to be not evident, and the PCEs could retain over 10% for the CH3NH3PbI3 films exposed to air for 200 h. More importantly, an annealing-free TiO2 NPs interface layer was inserted between the PCBM layer and the Al electrode for dramatically enhancing the stability of PHJ-PSCs, and the PCEs retained 75% of their initial values for the devices exposed to air for 200 h. Meanwhile, the PCEs also showed a little improvement for the PHJ-PSC devices with TiO2 NPs interface layer as compared with the devices without TiO2 NPs interface layer. The research provides the important information to better understand the degradation of PSCs and the improvement of the stability, showing greatly potential applications in fabricating efficient and stable PHJ-PSCs with large-area, high-output printing techniques. Acknowledgments This work was supported by the National Natural Science Foundation of China (51203192, 11334014), the Program for New Century Excellent Talents in University (NCET-13-0598), and Hunan Provincial Natural Science Foundation of China (2015JJ1015). Y. L. Gao acknowledges the support from NSF CBET-1437656. J. Xiong acknowledges the support from the PhD research startup foundation of Guilin University of Electronic Technology (UF15016Y). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.orgel.2015.12.010. References

Fig. 3. The degradation trend of PCEs for PHJ-PSC devices with and without TiO2 NPs interface layer.

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