PC61BM planar heterojunction perovskite solar cells

Organic Electronics 30 (2016) 281e288

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Organic Electronics journal homepage: www.elsevier.com/locate/orgel

Flexible, hole transporting layer-free and stable CH3NH3PbI3/PC61BM planar heterojunction perovskite solar cells Yong Zhang a, Xiaotian Hu a, Lie Chen a, b, Zengqi Huang a, Qingxia Fu a, Yawen Liu a, Lin Zhang a, Yiwang Chen a, b, * a b

College of Chemistry, Institute of Polymers, Nanchang University, 999 Xuefu Avenue, Nanchang 330031, China Jiangxi Provincial Key Laboratory of New Energy Chemistry, Nanchang University, 999 Xuefu Avenue, Nanchang 330031, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 7 December 2015 Received in revised form 31 December 2015 Accepted 2 January 2016 Available online xxx

Hole transporting layer (HTL)-free CH3NH3PbI3/PC61BM planar heterojunction perovskite solar cells were fabricated with the configuration of ITO/CH3NH3PbI3/PC61BM/Al. The devices present a remarkable power conversion efficiency (PCE) of 11.7% (12.5% best) under AM 1.5G 100 mW cm
2 illumination. Moreover, the HTL-free perovskite solar cells on flexible PET substrates are first demonstrated, achieving a power conversion efficiency of 9.7%. The element distribution in the HTL-free perovskite solar cell was further investigated. The results indicated that the PbI2 enriched near the PC61BM side for chlorobenzene treatment via the fast deposition crystallization method. Without using HTL on the ITO, the device is stable with comparison to that with poly(3,4-ethylenedioxylenethiophene): poly(styrene sulfonate) (PEDOT:PSS) as HTL. In addition, the fabricating time of the whole procedure from ITO substrate cleaning to device finishing fabrication only cost about 3 h for our mentioned devices, which is much more rapid than other structure devices containing other transporting layer. The high efficient and stable HTL-free CH3NH3PbI3/PC61BM planar heterojunction perovskite solar cells with the advantage of saving time and cost provide the potential for commercialization printing electronic devices. © 2016 Elsevier B.V. All rights reserved.

Keywords: Perovskites Solar cells Hole transporting layer-free Flexibility

1. Introduction Perovskite solar cells based on organic-inorganic lead halide have attracted considerable attention recently because of their excellent properties of low cost, lightweight and more importantly, high power conversion efficiency (PCE) [1e7]. The power conversion efficiency has rapidly increased from 3.8% to ~20.1% (certified) have been realized in just few years [8e10]. The mainly architectures of perovskite solar cells are mesoporous structure and planar structure distinguished by having or without the mesoporous layer. At present, the perovskite film can be fabricated by one-step solution processing, two-step sequential deposition and vacuumevaporation deposition methods. Consideration of the mass production of the perovskite solar cells, the one-step solution method would be the promising process. Despite of the structure, the high performance perovskite solar cells should be at least meeting the following two conditions: high-quality perovskite film and the well

* Corresponding author. College of Chemistry, Institute of Polymers, Nanchang University, 999 Xuefu Avenue, Nanchang 330031, China. E-mail address: [email protected] (Y. Chen). http://dx.doi.org/10.1016/j.orgel.2016.01.002 1566-1199/© 2016 Elsevier B.V. All rights reserved.

contact of the interface between each layer of devices. For the solution processing [9,11], recent studies confirm that high-quality perovskite films could be obtained by the solvent engineering process [12], fast deposition-crystallization (FDC) method [13], gasassisted dry [14], hot-casting technique [15], solvent annealing [16] or added addition in precursor solution [17,18], as a result to achieve high performance perovskite solar cells. Except the good morphology of the perovskite layer, most of the high performance perovskite solar cells were achieved by exclusively using electron-transporting layers (ETLs) and holetransporting layers (HTLs) for the critically necessary for achieving high open-circuit voltage (Voc), because of their well contact of the front and back electrode to promote effective carrier separations and charge recombination reduction. In conventional structure (neiep), the most common ETL and HTL materials reported in the literature are TiO2 and 2,20 ,7,70 -tetrakis-(N,Ndi-p-methoxyphenylamine)-9,90 -spirobifluorene (spiro-OMeTAD), respectively [9,19e21]. An impressive PCE of 19.3% has been realized in a planar, conventional perovskite solar cells with the use of yttrium doped compact TiO2 and spiro-OMeTAD via interfacial engineering [10]. However, the TiO2 layer requires high calcination

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temperature (about 400  C) to form high quality crystallization, which limits its printable applications [9]. Compared to conventional devices, inverted devices (peien) is a low-temperature process (100  C) based on organic transporting layer such as poly(3,4-ethylenedioxylenethiophene): poly(styrene sulfonate) (PEDOT:PSS) HTL and C60 ETL [11,16,22e25]. In fact, in consideration of mass production of flexible perovskite solar cells in the future, low-temperature fabrication and the simplified structure should be qualified to achieve reducing the cost and rapid fabrication. To further satisfying the high efficient perovskite solar cell in the mass production, several effects have been made to study that the HTL or ETL can be exempted in the perovskite solar cells [26e36]. Yan et al. fabricated a perovskite solar cell without the ETL using spiro-OMeTAD as HTL achieving a high PCE over 14% by making use of the ultravioleteozone (UVO) treatment to improve the coverage of perovskite film [32]. Jen et al. discovered that the work function of indium tin oxide (ITO) modified by perovskite leading to sufficient charge extraction efficiency at the ITO/perovskite interface [33]. Thus, the devices without ETL or HTL still have high efficiency. But the HTL-free perovskite are not applied on flexible substrate yet. In this work, a hole transporting layer-free (HTL-free) CH3NH3PbI3/PC61BM heterojunction perovskite solar cell was rapid fabricated with a simple structure of ITO/CH3NH3PbI3/PC61BM/Al without HTL (such as PEDOT:PSS). It should be noted that the HTLfree device is made of CH3NH3PbI3/PC61BM pen junction, and sandwiched with two electrodes (ITO and Al). The devices achieve an average PCE of 11.7% (maximum PCE of 12.5%) with a Voc of 0.99 V, a current-density (Jsc) of 16.1 mA cm
2 and fill factor (FF) of 74%. The HTL-free device was also favorable on flexible substrate with a best PCE as high as 9.7%. The element distribution for this HTL-free perovskite solar cell was further investigated. Interestingly, PbI2 is observed enrich near the PC61BM side. And we first discover that the underlayer perovskite pinhole by the crosssection transmission electron microscope (TEM) of the whole perovskite solar cell. The pinhole of the ITO/perovskite layer was main reason of the low Jsc and the hysteresis for the HTL-free perovskite solar cell. 2. Material and methods 2.1. Perovskite precursor preparation The perovskite CH3NH3PbI3 film was fabricated by the fast deposition-crystallization procedure as reported in literature [13]. The perovskite precursor solution was prepared by dissolved lead (II) iodide (PbI2, SigmaeAldrich, 99%) and Methylamine iodide (MAI, TCI, 99%) (molar ratio 1:1) in anhydrous N,Ndimethylformamide (DMF, SigmaeAldrich) with final concentrations of 40 wt% for the optimum thickness of 340 nm. The other thickness of the CH3NH3PbI3 perovskite is prepared in the same manner with the concentration of 25, 35 and 55 wt% for the thickness of 150 nm, 270 nm and 520 nm, respectively [13]. The solution was stirred at 60  C overnight in N2-glove box. The solution was filtered with a 0.45 mm polyvinylidene fluoride filter before use. 2.2. Device fabrication The perovskite solar cells were fabricated on indium-tin oxide (ITO) pattern glass substrates (Luminescence Technology Corp., <10 U) with the following device configuration: ITO/PEDOT:PSS/ CH3NH3PbI3/PC61BM/Al. First, the ITO glass substrates were cleaned by sequential ultrasonic treatment in detergent, acetone, deionized water, and isopropyl alcohol for 15 min each and then dried with a

nitrogen stream. Then the precleaned ITO glass substrates were treated by ultraviolet (UV)-ozone for 20 min in UV chamber. For the hole-transporting layer free device, perovskite precursor solution was directly spin-coated on the ITO substrate at 5500 rpm in glove box. During the perovskite precursor solution spin-coating process, 150 mL chlorobenzene was quickly added on the surface of the substrate after a specific delay time of 6 s. For the inverted device, PEDOT:PSS was spin-coated onto ITO at 4000 rpm for 60 s dried at 140  C for 15 min in air and then spin-coating perovskite precursor solution. The PC61BM (American Dye Source, Inc., 99.5%) dissolving in CB, 2 wt% was then spin-coated onto the CH3NH3PbI3 at 2000 rpm for 30 s. Finally a 100 nm thick aluminum cathode (deposition rate of 1.0 Å/s) were deposited on the substrates through a shadow mask to give a device area of 0.04 cm2 under a vacuum level of 10
4 Pa. It will spend 1 h for the evaporation of Al electrode. The batch of devices can be fabricated within only 3 h from ITO clean to the end. Device fabrication was carried out in a N2-filled glove box. All devices measurements were performed in an ambient environment (below 25% humidity) at room temperature. 2.3. Flexible perovskite solar cell fabrication ITO-coated flexible PET substrates (35 U cm
2) were cleaned with detergent, deionized water, and isopropyl alcohol and dried by nitrogen flow followed by O2-plasma treatment for 20 min. After cleansing, CH3NH3PbI3, PC61BM and Al were fabricated on the substrate as the method mentioned on glass substrate. 2.4. Device characterization The illuminated current densityevoltage (JeV) characteristics were characterized using Keithley 2400. The currents were measured under 100 mW cm
2 simulated AM 1.5 G irradiation (Abet Solar Simulator Sun2000). The incident photon-to-current conversion efficiency (IPCE) spectrum was detected under monochromatic illumination (Oriel Cornerstone 260 1/4 m monochromator equipped with Oriel 70613NS QTH lamp), and the calibration of the incident light was performed with a monocrystalline silicon diode. Scanning electron microscopy (SEM) imaging was conducted on SU8020 scanning electron microscope operated at an acceleration voltage of 8 kV. Samples from specific locations on the cross-sections were prepared by focused ion beam (FIB). The transmission electron microscopy (TEM) images were taken using a JEOL 2100F microscope. The thicknesses of all the CH3NH3PbI3 layers were measured by surface profilometer (AlphaStep-IQ). X-ray diffraction (XRD) measurements were performed with a Rigaku D/Max-B X-ray diffractometer with Bragg-Brentano parafocusing geometry, a diffracted beam monochromator, and a conventional cobalt target X-ray tube set to 40 KV and 30 mA. The ultravioletevisible (UV) spectra of the samples were recorded on a PerkinElmer Lambda 750 spectrophotometer. The photoluminescence spectra were measured by photoluminescence spectroscopy (Hitachi F-7000). 3. Results and discussion To realize the HTL-free perovskite solar cells, the material structures of CH3NH3PbI3 and PC61BM are shown in Fig. 1a. The device architectures and the corresponding cross-section TEM image are demonstrated in Fig. 1b and c. Fig. 1d shows the energy diagram of the HTL-free planar perovskite solar cell. The lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) levels of CH3NH3PbI3 perovskite are
3.9 eV and
5.4 eV, respectively, and those of PC6BM

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Fig. 1. Materials structure, device structure and energy level diagram. (a) Structure of CH3NH3PbI3 and PC61BM. (b) Schematic view of the device configuration: glass/ITO/ CH3NH3PbI3/PC61BM/Al. (c) Cross-section transmission electron microscope image of ITO/CH3NH3PbI3/PC61BM/Al structure perovskite solar cell. (d) The energy diagram of the HTLfree planar perovskite solar cell.

are
4.2 eV and
6.0 eV [37]. After deposit a thin layer of PC61BM, CH3NH3PbI3 and PC61BM could be the ideal electron donor and acceptor, as a result to a CH3NH3PbI3/PC61BM heterojunction with electrons transporting to Al electrode and holes gathering to ITO electrode. The quality of the perovskite film is crucial influence the performance of perovskite solar cells [22,38,39]. High coverage and the high crystallization of the perovskite film lead to the good performance of the device. The morphology of CH3NH3PbI3 perovskite film was measured by scanning electron microscopy (SEM). As

shown in Fig. 2a, the CH3NH3PbI3 on bare ITO glass substrate was fully covered on the substrate without any pin-hole on the surface. The CH3NH3PbI3 crystals were expressed in a more plate-like shape (Fig. 2b) with diameters from 200 nm to 700 nm, suggesting that a high quality perovskite film was obtained. The results revealed the similar morphology to the perovskite film coated on ITO/ PEDOT:PSS substrate (Fig. S1). The crystal was confirmed by the Xray diffraction (XRD) pattern in Fig. 2c. Compared with perovskite film on ITO/PEDOT:PSS substrate, the perovskite film on bare ITO substrate showed the same diffraction peak at 14.1, 23.48 ,28.40 ,

Fig. 2. Morphology and the crystallinity of perovskites. (a) Low- and (b) high-magnification scanning electron microscope images of a CH3NH3PbI3 perovskite film on bare ITO substrate. (c) X-ray diffraction patterns of CH3NH3PbI3 perovskite film on ITO/PEDOT:PSS substrate and bare ITO substrate. (d) An atomic resolution transmission electron microscope image of a CH3NH3PbI3 grain, showing a pseudo-cubic lattice with interplanar spacing of ~0.31 nm. (e) The selected-area-diffraction pattern of perovskite. (f) Photoluminescence spectra of CH3NH3PbI3 perovskite film on bare glass, bare ITO substrate and ITO/PEDOT:PSS substrate.

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and 31.86 which could be respectively attributed to (110), (211) (220), and (310) lattice planes of CH3NH3PbI3 crystalline structure [9,40]. These perovskite diffraction peaks became significantly enhanced with the perovskite on bare ITO. A diffraction peak at 12.7 appeared near the (110) peak of the perovskite film, which was attributed to the (001) diffraction of PbI2 crystals. The residual PbI2 was not completely converted into CH3NH3PbI3. This CH3NH3PbI3 crystalline structure was further confirmed by interplanar spacing by a high-resolution TEM (Fig. 2d). Fig. 2d showed a high-resolution TEM image with interplanar spacing of ~0.31 nm, corresponding to (004) or (220) phanes, confirming the bCH3NH3PbI3 phase (lattice parameters: a ¼ b ¼ 8.849 Å, c ¼ 12.642 Å) [41]. The selected-area-diffraction (SAD) pattern in Fig. 2e also displays the tetragonal of CH3NH3PbI3 perovskite crystal. Fig. 2f presents the photoluminescence (PL) spectra of the CH3NH3PbI3 perovskite film on bare glass, bare ITO substrate and ITO/PEDOT:PSS substrate. The emission signal at 770 nm was observed. The CH3NH3PbI3 perovskite on glass substrate shows the strong intensity. However, the CH3NH3PbI3 perovskites on bare ITO and ITO/PEDOT:PSS substrate with comparable intensity show the reduced emission signal. We further investigated the morphology and surface roughness of the CH3NH3PbI3 perovskite film from the ITO substrate by atomic force microscopy (AFM). The height and phase images are shown in Fig. 3 and Fig. S2, respectively. The bare ITO substrate presents the root mean square (RMS) roughness of ~3.45 nm (Fig. 3a), and the PEDOT:PSS layer spin coated on ITO was much smooth with roughness of ~0.93 nm (Fig. 3b). The CH3NH3PbI3 film spin coated on bare ITO substrate and ITO/PEDOT:PSS substrate displays the similar roughness with the RMS roughness of 12.7 nm and 11.13 nm, respectively. The crystal size and morphology of CH3NH3PbI3 film on bare ITO substrate are consistent with the SEM results. The results show that the high quality CH3NH3PbI3 crystals fabricated by FDC method were obtained after treated by the chlorobenzene in spite of presence of PEDOT:PSS on the substrate.

The light absorbance of the CH3NH3PbI3 perovskite on ITO/ PEDOT:PSS and bare ITO was investigated by UVevis spectra. As shown in Fig. S3, the spectra of the CH3NH3PbI3 perovskite film on ITO/PEDOT:PSS and bare ITO substrates show the same absorbance of the light in 500e800 nm wavelength passing through the substrate. However, the CH3NH3PbI3 perovskite film on bare ITO shows slightly enhanced absorbance of the light in 400e500 nm, due to a lack of a 30e40 nm thickness PEDOT:PSS layer. The thickness of the light-absorbing layer plays a crucial role in determining the performance of thin-film solar cells [42]. If the absorber layer is too thick, the photogenerated carriers cannot be collected effectively because they must travel through the absorber to reach the carrier collecting layers before they recombine. The performance about different thickness of CH3NH3PbI3 layer in TLfree perovskite solar cells was explored. The thickness of the CH3NH3PbI3 layer was controlled by the concentration of the CH3NH3PbI3 perovskite precursor (the detail is shown in experimental section). The performance of the perovskite solar cells was shown in Fig. 4. The corresponding averaged device parameters based on different thickness of perovskite films on ITO substrate were shown in Table 1. The JeV curves were measured under reverse voltage scans at AM 1.5G illumination with the scan rate of 100 mV step. As shown in Fig. 4, the 150 nm thick CH3NH3PbI3 layer shows the poor performance with the PCE of 6.4% for the reason of the low absorbance of the light. As the thickness of the CH3NH3PbI3 perovskite increased, the Jsc, Voc and FF were first increased and then decreased when the thickness of CH3NH3PbI3 layer exceed 340 nm. The optimum perovskite layer thickness ~340 nm perovskite solar cell achieves the PCE of 11.7%, with the Jsc of 16.1 mA cm
2, Voc of 0.99 V and FF of 74%. To investigate the universality for different substrates, we fabricated the HTL-free perovskite solar cells on the flexible polyethylene terephthalate (PET) substrate. The device fabrication method is provided in the experimental section. The JeV curve and the photographical picture of the flexible perovskite solar cell are

Fig. 3. Tapping-mode atomic force microscopy height images of (a) bare ITO substrate, (b) PEDOT:PSS on ITO substrate, (c) CH3NH3PbI3 on bare ITO substrate, (d) CH3NH3PbI3 on ITO/PEDOT:PSS substrate with an area of 5 mm  5 mm.

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Fig. 4. JeV curves of the TL-free perovskite solar cells with the different thickness CH3NH3PbI3 layer from 150 nm to 520 nm.

showing in Fig. 5. As shown in Fig. 5a, the flexible perovskite solar cells achieved a best PCE of 9.7%, with a Jsc of 14.8 mA cm
2, a Voc of 0.96 V and a FF of 68.1%, indicating that the structure was applicable on the flexible substrate. Fig. 5b shows the corresponding incident photon-to-current conversion efficiency (IPCE) spectrum of the device, which is used to calculate Jsc as 13.6 mA cm
2. The main reason of the photovoltaic parameters of comparably low Jsc, Voc and FF of PET based devices relative to glass based devices may ascribe to the high square resistance of the ITO on PET substrate. In order to understand the stabilized power output, the steady-state photocurrent was measured by hold at a forward bias of 0.82 V Fig. 5d shows the steady-state photocurrent as a function of time. The photocurrent stabilizes within seconds to approximately 11 mA cm
2, achieved a stabilized PCE of 9.1%. The best performance HTL-free perovskite with optimum thickness of the CH3NH3PbI3 layer was chosen to investigate the element distribution. Fig. 6a is a cross-section transmission electron microscopy (TEM) image of the whole CH3NH3PbI3 perovskite solar cell with the structure of ITO (180 nm)/CH3NH3PbI3 (340 nm)/ PC61BM (30 nm)/Al (100 nm). Fig. 6bed is the energy dispersive spectroscopy (EDS) elemental maps defining the cross-section of the perovskite solar cell. Fig. S4 presents the elemental maps of In, Al, N and O in HTL-free perovskite. The TEM results demonstrated well contact with each layer except that some pinhole between CH3NH3PbI3 layer and the ITO substrate, indicating that the underlayer of CH3NH3PbI3 covered incompletely the ITO substrate. This pinhole under the CH3NH3PbI3 could block the photogenerated carriers to the ITO leading to a relatively low Jsc in HTLfree perovskite solar cell with comparison to the inverted perovskite solar cell with PEDOT:PSS as HTL (Fig. S5). Furthermore, as show in Fig. 6bed, the EDS elemental maps of the cross-sectional

Table 1 Device parametersa for HTL-free perovskite solar cell with different thickness of CH3NH3PbI3 perovskite layer.b Thickness (nm)

Jsc (mA/cm2)

150 270 340 520

12.7 14.7 16.1 15.8

a

± ± ± ±

2.5 2.1 1.8 1.5

Voc (V) 0.83 0.97 0.99 0.97

± ± ± ±

FF (%) 0.08 0.06 0.05 0.03

61 72 74 72

± ± ± ±

5 3 6 5

PCE (%) 6.4 10.3 11.7 11.0

± ± ± ±

1.5 0.6 0.7 0.4

The above data is obtained measurements from reverse voltage scan. The data of the device is average from 40 devices for 340 nm and 10 devices for others. b

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CH3NH3PbI3 showed that the Pb element enriched near the PC61BM side with accompanying of deficiency near the ITO side, so does the I element. CH3 NH3 þ near ITO may form hydrogen bonding with the ITO surface resulting in a slight increased work function (WF) of ITO [33]. The increased WF makes the ITO to afford the high performance of perovskite due to the align energy level. To investigate whether the enrichment of PbI2 is conducive for device performance, ultraviolet photoelectron spectroscopy (UPS) was measured to HOMO level of the PbI2 enrichments perovskite. In Fig. S6, the perovskite with or without HTL show the same HOMO level at around 5.35 eV. The results are lower than the date reported in the literature (5.4 eV) [27], as a result to match the ITO work function (pristine 4.9 eV) for the efficient transporting hole. As shown the absorbance spectrum in Fig. S3, the onset of the spectrum appears at around 782 nm, the results show the bandgap around 1.58 eV. So, the LUMO level of the PbI2 enrichment would be 3.77 eV. It should be noted that in the inverted perovskite solar cell with PEDOT:PSS as HTL, although the elemental distributed might be the same with the above results, the element distribution cause the energy level align leading to an efficient hole collection at ITO electrode. The parameters of perovskite solar cells are normally different from batch to batch. To check the reproducibility of the performance of the HTL-free perovskite solar cell, the parameters of 40 devices with optimum thickness perovskites were adopted from different batch. As shown in Fig. S7, most of the devices showed the PCE higher than 10% indicating high reproducibility of the HTL-free perovskite. The JeV curves of the best-performance HTL-free perovskite solar cell on glass/ITO under AM 1.5G 100 mW cm
2 illumination are shown in Fig. 7a. The HTL-free perovskite solar cell achieved a best PCE of 12.5%, with the Jsc of 17.8 mA cm
2, Voc of 0.98 V and FF of 71.8% under the reverse voltage scan with the scan rate of 100 mV step. Fig. 7b shows the IPCE spectrum of the bestperformance perovskite solar cell. The IPCE spectrum exhibited exceeding 60% in the 400e750 nm wavelength range, which was consistent with the UVevis absorbance of the perovskite CH3NH3PbI3 film. The calculated Jsc of 16.5 mA cm
2 from IPCE is also consistent with that of the JeV curve. The hysteresis behavior in HTL-free perovskite solar cell was observed. As shown in Fig. 7c, the PCE by forward scan (from Jsc to Voc) was 10.2%, which was lower than the PCE of 12.5% by reverse scan (from Voc to Jsc). However, the well fabricated perovskite solar cell with the structure ITO/PEDOT:PSS/CH3NH3PbI3/PC61BM/Al behaved the lesser hysteresis (Fig. S8). The reason of the hysteresis could be attributed to the pinhole between the CH3NH3PbI3 perovskite and bare ITO, leading an imbalance rate of hole and electron transporting [43]. Moreover, the electrons can transport easily to the Al electrode while the hole could not efficiently transport to ITO electrode in presence of the pinhole. It is noted that the PCE of reference devices (Fig. S5) with PEDOT:PSS as HTL reported here are comparable or higher than the similar devices reported in the previous literature (<10%) with several additional ETLs on PC61BM (LiF, TiOx, ZnO, C60 or BCP) [44]. To investigate the device stability of the HTL-free perovskite solar cells storage in ambient condition. The stability of the HTLfree perovskite solar cells is compared with reference device with the configuration of glass/ITO/PEDOT:PSS/CH3NH3PbI3/PC61BM/Al. The PCE and IPCE of reference devices are provided in Fig. S5. Fig. S9 display the cross-sectional SEM image of the structure of glass/ITO/ PEDOT:PSS/CH3NH3PbI3. The PCE of these two kinds of unencapsulated devices was measured under ambient air condition. The average testing temperature and humidity were 20 ± 5  C and 30 ± 10%. Fig. 7d showed the normalized PCE of PEDOT:PSS HTL and HTL-free perovskite solar cells as a function of exposure time. The devices with PEDOT:PSS as HTL showed a reduction rate of 99%

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Fig. 5. (a) JeV curve of the best-performance of the flexible perovskite solar cell. (b) Corresponding incident photon-to-current conversion efficiency spectrum of the flexible HTLfree perovskite solar cell. (c)The photograph of the bended flexible HTL-free perovskite solar cell. (d) Photocurrent density and PCE as a function of time for the same cell held at a forward bias of 0.82 V.

Fig. 6. (a) Cross-section transmission electron microscope image of ITO/CH3NH3PbI3/PC61BM/Al structure perovskite solar cell. The corresponding elemental energy dispersive spectroscopy maps of the distribution of (b) Pb, (c) C and (d) I, respectively.

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Fig. 7. (a) The JeV curve of the best-performing HTL-free perovskite solar cell with the structure of glass/ITO/(340 nm) CH3NH3PbI3/PC61BM/Al. (b) Incident photon-to-current conversion efficiency spectrum of the best-performing HTL-free perovskite solar cell. (c) Forward (from Jsc to Voc) and reverse (from Voc to Jsc) scan current densityevoltage curves for the HTL-free perovskite solar cells with the structure of glass/ITO/(340 nm)CH3NH3PbI3/PC61BM/Al. (d) Device stability of the HTL-free and PEDOT:PSS based HTL perovskite solar cells under ambient condition.

after exposure in air for 50 min. However, the reduction 99% of HTLfree perovskite solar cell need exposure for nearly 300 min. PEDOT:PSS was an acidic hygroscopic polymer that can corrode the ITO substrate as a result to make the device suffer from degradation in organic photovoltaic (OPV) and perovskite solar cells [45e47]. The deteriorate degradation might be that the acidic nature of PEDOT:PSS accelerates corrosion to the ITO substrate [47]. Without using PEDOT:PSS on ITO, the degradation could be depressed. In addition, the simple structure (ITO/CH3NH3PbI3/PC61BM/Al) was fabricated by only the two electrodes (ITO and Al) sandwiched with the CH3NH3PbI3/PC61BM planar heterojunction without using hole transporting layers. This simple structure leads to the simplified fabrication method. The reference perovskite solar cells [25,48] with high efficiency structure were repeated to compare fabricating time of the whole device. As shown in Fig. S10 and Table S1, the fabricating time of the whole procedure from ITO substrate cleaning to device finishing fabrication only cost about 3 h for our mentioned devices. The reference structures cost at least 5 h for fabrication. This structure greatly decreases the cost and promotes the work efficiency for the commercial production. Though the performance of the devices is not as high as that of the PEDOT:PSS based devices, the HTL-free structures of perovskite solar cells are easily fabricated and more stable. The simply and easily operated method to achieve high efficient and stable perovskite solar cells is anticipated in the future.

4. Conclusions In summary, a HTL-free CH3NH3PbI3/PC61BM planar heterojunction perovskite solar cell was rapid fabricated with the simple structure of ITO/CH3NH3PbI3/PC61BM/Al. The devices achieve an average PCE of 11.7% (maximum 12.5%) with a Voc of 0.99 V, a Jsc of 16.1 mA cm
2 and FF of 73.9% when measured under AM 1.5G

100 mW cm
2 illumination. The HTL-free device was also fabricated on flexible substrate with a best PCE as high as 9.7%. The element distribution in the HTL-free perovskite solar cell was further investigated. The results indicated that the PbI2 enriched near the PC61BM side for chlorobenzene treatment via the fast deposition crystallization method. The underlayer perovskite pinholes close to ITO discovered by the cross-sectional TEM of the whole perovskite solar cells were revealed for the generation of low Jsc and the hysteresis of the HTL-free perovskite solar cells. Moreover, the HTL-free perovskite solar cell is more stable than the device using PEDOT:PSS on ITO without encapsulation storage in ambient air condition. Without using hole transporting layer, the HTL-free perovskite solar cells are rapid fabricated in comparison with the device containing other transporting layer. The simple structure and simplified prepared method of perovskite solar cells provide a potential for commercialization printing electronic devices. Acknowledgments This work was financially supported by the National Science Fund for Distinguished Young Scholars (51425304), and National Basic Research Program of China (973 Program 2014CB260409). Yong Zhang, Xiaotian Hu, and Lie Chen contributed equally to this work. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.orgel.2016.01.002. References [1] G. Xing, N. Mathews, S. Sun, S.S. Lim, Y.M. Lam, M. Gr€ atzel, S. Mhaisalkar,

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