Nonconjugated Polymer Poly(vinylpyrrolidone) as an Efficient

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Non-Conjugated Polymer Poly(vinylpyrrolidone) as an Efficient Interlayer Promoting Electron Transport for Perovskite Solar Cells Pengcheng Zhou, Zhimin Fang, Weiran Zhou, Qiquan Qiao, Mingtai Wang, Tao Chen, and Shangfeng Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12135 • Publication Date (Web): 07 Sep 2017 Downloaded from http://pubs.acs.org on September 8, 2017

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ACS Applied Materials & Interfaces

Non-Conjugated Polymer Poly(vinylpyrrolidone) as an Efficient Interlayer Promoting Electron Transport for Perovskite Solar Cells Pengcheng Zhou,a Zhimin Fang,a Weiran Zhou, a Qiquan Qiao,b Mingtai Wang,c Tao Chen,* a and Shangfeng Yang* a a

Hefei National Laboratory for Physical Sciences at Microscale, Key Laboratory of Materials

for Energy Conversion, Chinese Academy of Sciences, Department of Materials Science and Engineering, Synergetic Innovation Center of Quantum Information & Quantum Physics, University of Science and Technology of China (USTC), Hefei 230026, China b

Center for Advanced Photovoltaics, Department of Electrical Engineering and Computer Science, South Dakota State University, Brookings, SD 57007, USA

c

Institute of Applied Technology, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei 230088, China

* Corresponding Authors. E-mail: [email protected] (S.Y.); [email protected] (T. C.).

Keywords: Perovskite solar cells, poly(vinylpyrrolidone), interface engineering, electron transport layer, dipole layer.

ABSTRACT: The interfaces between perovskite layer and electrodes play a crucial role on efficient charge transport and extraction in perovskite solar cells (PSCs). Herein, for the first time we applied a low-cost non-conjugated polymer poly(vinylpyrrolidone) (PVP) as a new interlayer between PCBM electron transport layer (ETL) and Ag cathode for high-performance inverted planar heterojunction perovskite solar cells (iPSCs), leading to a 1

ACS Paragon Plus Environment


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dramatic efficiency enhancement. The CH3NH3PbI3-xClx-based iPSC device incorporating the PVP interlayer exhibited a power conversion efficiency (PCE) of 12.55%, which is enhanced by ~15.9% relative to that of the control device without PVP interlayer (10.83%). The mechanistic investigations based on morphological, optical and impedance spectroscopic characterizations reveal that incorporation of PVP interlayer promotes electron transport across the CH3NH3PbI3-xClx perovskite/Ag interface via PCBM ETL. Besides, PVP incorporation induces the formation of a dipole layer, which may enhance the built-in potential across the device, conjunctly promoting electron transport from PCBM to Ag cathode and consequently leading to significantly improved fill factor (FF) from 58.98% to 66.13%. Introduction Organic-inorganic hybrid perovskites, such as CH3NH3PbX3 (X = I, Br, Cl) have attracted increasing interests as light absorbing materials due to the highly achievable power conversion efficiency (PCE), low fabrication cost and rapid energy payback time. The high performance of perovskite solar cells originates from their large light absorption coefficient in visible region, small exciton binding energy, long carrier diffusion length and high ambipolar charge mobility.1-7 Previous investigations have indicated that, in order to achieve high PCE of perovskite solar cell, both active materials and interfaces between the functional layers/electrodes should be optimized simultaneously.8-16 In this regard, tremendous efforts have been put into improving crystallinity (grain size) and optimizing film thickness and morphology of perovskite active layer, which are critical for enhancing the charge transport, carrier diffusion length and lifetime within active layer.13-16 Moreover, to collect carriers from 2


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the device, electrons and holes have to efficiently transport across several interfaces to the electrodes. Therefore, the interfaces between perovskite layer and electrodes play a crucial role on efficient charge transport and extraction, and numerous reports reveal that interface engineering is effective in promoting charge transport of perovskite solar cells.17-20 In general, interfacial engineering toward device performance improvement is through three mechanisms: (1) It introduces an intermediate energy level that facilitates charge transfer and impedes recombination loss. (2) The interfacial materials can direct the film growth of perovskite materials and lead to improved contact. (3) Through interface functionalization, the work function of metal electrode can be optimized for improved carrier collection.18-28 Interfacial engineering via a single interlayer is capable of achieving dual or multiple functionalities in some cases. For instance, for inverted (p-i-n) planar heterojunction perovskite solar cells (iPSCs), fullerene derivatives such as [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) have been commonly used as electron transport layers (ETLs) due to their strong electron-accepting abilities.19,20 Promoting the electron transport property of PCBM ETL further can be fulfilled by optimizing its film morphology via doping and/or tuning its energy level via surface modification.19-23 For the latter strategy, so far the reported modification layer incorporated between PCBM layer and metal cathode includes fullerene derivatives,24-28 low work-function metals like Ca,29 alkali metal compounds such as LiF,30 metal oxides such as TiOx,31 ZnO,32 small conjugated organic molecules such as bathocuproine

(BCP),33

conjugated

polymer 3


electrolytes

such

as


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poly[(9,9-bis(3’-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctyl-fluorene)] (PFN),34,35 and non-conjugated polymers such as an amino-functionalized polymer PN4N.36 In particular, such water/alcohol-soluble conjugated polymer electrolytes as PFN which have been commonly applied as cathode interlayers in polymer solar cells were not applicable for iPSCs as interlayers due to the damage of the perovskite films beneath by the processing highly polar solvent (typically methanol).36 To overcome such a solvent compatibility problem, it is necessary to develop novel non-conjugated polymers which are soluble in less polar solvents, while so far the reports on applying non-conjugated polymers as modification layer of PCBM ETL are quite limited.36,37 For example, Huang and Yip et al. synthesized a new amino-functionalized polymer PN4N which was soluble in isopropyl alcohol (IPA) and thus able to be incorporated atop of PCBM layer without destroying the perovskite films beneath, achieving a remarkable PCE enhancement owing to the improved electron transport.36 It is worth to mention that, however, the synthetic routes of such polymers are usually complicated together with tedious purification process, which in turn leads to the increase of device fabrication cost. Hence, it is desirable to search for new interlayers based on low-cost polymers which are readily available commercially. Herein, we reported for the first time the application of a commercially available non-conjugated polymer, poly(vinylpyrrolidone) (PVP), as an efficient interlayer promoting electron transport in iPSCs. PVP was incorporated between the PCBM ETL and Ag cathode via spin-coating in IPA solution, leading to a dramatic PCE enhancement from 10.83% to 4


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12.55% for CH3NH3PbI3-xClx-based iPSC devices. The PCE enhancement is primarily attributed to the increase of fill factor (FF), which results from promoted electron transport across the CH3NH3PbI3-xClx perovskite/Ag interface according to morphological, optical and impedance spectroscopic characterizations. Experimental Section Materials. The indium tin oxide (ITO) glass substrate with a sheet resistance of 10 Ω/sq was purchased from Shenzhen Nan Bo Group, China. PEDOT:PSS (Clevios P Al4083) was obtained from SCM Industrial Chemical Co., Ltd. CH3NH3I was bought from Xi'an Polymer Light Technology Corporation. PbCl2, dimethylsulfoxide (DMSO), γ-butyrolactone (GBL), isopropanol (IPA) and chlorobenzene were purchased from Alfa Aesar. PCBM was bought from Nichem Fine Technology Co., Ltd. PVP (K30, Mn = ca. 40000) was purchased from Sinopharm Chemical Reagent Co., Ltd. All materials were used as received without further purification. Device

fabrication.

Our

detailed

fabrication

procedure

of

the

control

CH3NH3PbI3-xClx-based iPSC devices has been reported previously.21 In brief, the patterned ITO-coated glass substrates was ultrasonicated in detergent, deionized water, acetone, and IPA for 15 min in consequence, and subsequently dried in an oven overnight, followed by UV ozone treatment for 15 min prior to use. PEDOT:PSS aqueous solution was first filtered by a 0.45 μm filter and then spin-coated onto the ITO substrates at 4000 rpm for 1 min. The as-prepared film was annealed at 150 oC for 15 min in air, affording a thin layer of around 40 5



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nm in thickness. The film was then transferred into a glove box (O2≤0.1 ppm; H2O≤0.1 ppm). For the fabrication of perovskite layer, a PbCl2/CH3NH3I mixture solution containing 0.8 M (222 mg) PbCl2 and 2.4 M (381 mg) CH3NH3I in 1 mL mixed solvents of DMSO:GBL (3:7, v/v) was prepared and stirred at 60 °C overnight. The mixture solution was then filtered and preheated at 80 °C for 5 min along with the substrate, followed by spin-coating at 5000 rpm for 30 s. 50 μL of anhydrous chlorobenzene was then quickly injected onto the film and stayed for 10 s, followed by spin-coating at 5000 rpm for 30 s again. The yellowish as-cast film was annealed at 100 °C for 30 min toward the formation of perovskite crystal film, followed by cooling down to room temperature naturally. PCBM (20 mg/mL) in chlorobenzene solution was spin-coated onto the perovskite layer at 1500 rpm for 30 s. Afterwards, an IPA solution of PVP with a concentration of 1 mg/mL was spin-coated onto PCBM at different spin-coating speeds (1000, 2000, 3000 rpm). To complete the device fabrication, the substrate was transferred into a vacuum chamber (~10−5 Torr), and an Ag electrode (ca. 70 nm in thickness) was thermally deposited through a shadow mask to define the effective active area of the devices (2 × 7 mm2). Measurements and Characterization. The current density-voltage (J-V) characterization of the devices was carried out by using a Keithley 2400 source measurement unit under simulated AM 1.5 irradiation (100 mW·cm−2) with a standard xenon-lamp-based solar simulator (Oriel Sol 3A, U.S.A.). The J-V measurement was carried out under reverse scan with a scan rate of 0.1 V/s. At the same time, a measurement under forward scan was also 6


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performed for the devices to check the hysteresis of the J-V curve. The solar simulator illumination intensity was calibrated by a monocrystalline silicon reference cell (Oriel P/N 91150 V, with KG-5 visible color filter) calibrated by the National Renewable Energy Laboratory (NREL). All of the measurements were carried out in air, and a mask with well-defined area size of 14.0 mm2 was attached onto the cell to define the effective area in order to ensure accurate measurement. Dozens of devices were fabricated and measured independently to obtain the statistical histograms of PCE devices. The best and average data were used in the following discussions. External quantum efficiency (EQE) was carried out with an ORIEL Intelligent Quantum Efficiency (IQE) 200TM Measurement system established with the tunable light source. The thickness of the film was measured by a KLA-Tencor P6 surface profilometer. X-ray diffraction (XRD) data was measured by a Rigaku-TTR III X-ray diffractometer with Cu-Kɑ radiation. UV-Vis spectroscopy was recorded on a UV-vis-NIR 3600 spectrometer (Shimadzu, Japan). The SEM image was obtained on a JSM-7401F instrument (JEOL, Japan). Atomic force microscopy (AFM) measurements were carried out on a XE-7 scanning probe microscope in non-contact mode (Park systems, Korea). Scanning Kelvin probe microscopy (SKPM) measurements were carried out on a Veeco DI-MultiMode V scanning probe microscope using SKPM mode. Impedance spectroscopic measurements were taken by using an electrochemical analyzer (Zahner Zennium, Germany). AC 5 mV perturbation was applied with a frequency from 1 MHz to 1 Hz. The obtained impedance spectra were fitted 7



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with ZView software (v2.8b, Scribner Associates, U.S.A.). Results and Discussion PVP is a commercially available water/alcohol-soluble non-conjugated polymer, which was previously applied as an effective cathode buffer layer of polymer solar cells facilitating electron extraction by Al cathode.38 More recently, PVP was doped into CH3NH3PbI3-xClx perovskite layer, resulting in decrease of the CH3NH3PbI3-xClx crystal size and consequently improved thermal stability of the perovskite film in air.39 Stimulated specially by the effectiveness of PVP as cathode buffer layer of polymer solar cells, in our present study we managed to apply PVP in iPSCs in a new manner as an interlayer other than a dopant in perovskite active layer. PVP was incorporated between the PCBM ETL and Ag cathode via spin-coating in IPA solution (Figure 1A). We first optimized the spin-coating speed of PVP by comparing the performance of the corresponding ITO/PEDOT:PSS/CH3NH3PbI3-xClx perovskite/PCBM/Ag iPSC devices, and the optimum spin-coating speed of PVP was determined to be 2000 rpm (see Supporting Information-Figure S1 and Table S1). Under this optimized condition, the current density-voltage (J-V) curve of the iPSC device incorporating PVP interlayer (~10.8 nm thick, see Supporting Information-Figure S2) is shown in Figure 1B, which includes also that of the control device for comparison. The measured photovoltaic parameters (short-circuit current (Jsc), open-circuit voltage (Voc), FF, and PCE) are summarized in Table 1. The control device without PVP interlayer exhibits a Voc of 0.95 V, a Jsc of 16.55 mA·cm-2, 8


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and a FF of 58.98%, corresponding to a PCE of 9.29%. This average PCE is comparable to the values reported for CH3NH3PbI3-xClx-based iPSC devices fabricated under similar conditions.21,25,37 Upon incorporation of PVP interlayer, the average PCE of the iPSC device increases dramatically to10.81% calculated from a Voc of 0.96 V, a Jsc of 16.96 mA·cm-2, and a FF of 66.13% (see Figure 1B and Table 1). Besides, the best PCE reaches 12.55%, which is enhanced by ~15.9% relative to that of the control device without PVP interlayer (10.83%). Among the three photovoltaic parameters, obviously such a PCE enhancement upon PVP incorporation is primarily due to the increase of FF (from 58.98% to 66.13%, ~12.1% enhancement), while both Jsc (from 16.55 to 16.96 mA/cm2) and Voc (from 0.95 to 0.96 V) exhibit negligible changes. This is confirmed further by comparing the statistical photovoltaic parameters (PCE, FF, Jsc and Voc) based on 50 devices fabricated independently with or without PVP incorporation, revealing that the PVP-incorporated device show apparent enhancement in the average FF (see Supporting Information-Figure S3 for the histograms and box plots of the photovoltaic parameters). Besides, the hysteresis of J-V curve was also checked for the devices with and without PVP interlayer, indicating that, similar to the control device, there was no detectable hysteresis of the J-V curve for the PVP-incorporated device (see Supporting Information-Figure S4). We further measured the stabilized power output under working conditions (at the maximum power point) and consequently the stabilized PCE, confirming that PCE can be stabilized at ~11.52% over the 250 s measurement period (see Supporting Information-Figure S5). 9



(B) Current density / (mA/cm2)

(A)

20

with PVP w/o PVP 15

10

5

0 0.0

0.2

0.4

0.6

0.8

1.0

Bias / V

(C)

(D)

18

with PVP w/o PVP

80

9 40 6 20

Current density / (mA/cm2)

12

Integrated Jsc / (mA/cm2)

60

3

0 300

400

500

600

700

100

with PVP w/o PVP

10

15

EQE / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1 0.1 0.01 1E-3 1E-4

0 800

1E-5 -1.0

-0.5

Wavelength / nm

0.0

0.5

1.0

1.5

Bias / V

Figure 1. (A) Schematic structure of the iPSC device and the chemical structure of PVP. (B) J-V curves of CH3NH3PbI3-xClx-based iPSC devices with and without PVP interlayer measured under illumination of an AM 1.5 solar simulator (100 mW·cm-2) in air. (C) EQE spectra along with the corresponding integrated Jsc curves of the devices with and without PVP interlayer measured in air. (D) Logarithmic plot of J-V characteristics of the devices measured in the dark. Table 1. Photovoltaic parameters of the ITO/PEDOT:PSS/CH3NH3PbI3-xClx/PCBM/Ag iPSC devices with and without PVP interlayer obtained from J-V curves under reverse scan. PCE (%)

b

b

Voc

Jsc

(V)

(mA/cm )

(%)

Average a

Best

(Ω·cm )

(Ω·cm )

w/o PVP

0.95±0.03

16.55±1.26

58.98±3.55

9.29±0.68

10.83

17.42

814.59

with PVP c

0.96±0.03

16.96±0.69

66.13±3.11

10.81±0.71

12.55

6.44

988.76

Interlayer

a

FF 2

Rs

2

Rsh

2

Averaged over 50 devices fabricated independently. b Rs and Rsh values are extracted from

J-V curves. c Under the optimized spin-coating speed of PVP of 2000 rpm. 10


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The photocurrent generation behavior of the devices with and without PVP interlayer were examined by external quantum efficiency (EQE) measurement (Figure 1C), indicating that both devices have a wide photoelectric response from visible to near-infrared range with an EQE value of 70-80% in the 450-600 nm range. The integrated photocurrent densities from the EQE are close to those measured from J-V response, verifying the device performance from different characterization. In addition, the EQE response onset at around 800 nm is consistent with the optical absorption onset of the perovskite film as shown in Figure 2. More importantly, it can be clearly seen that the incorporation of PVP interlayer has negligible influence on the overall EQE response, in line with the negligible change of Jsc as observed from J-V measurements. This is understandable because no detectable change on the optical absorption of perovskite film is observed upon PVP incorporation (see Figure 2), thus PVP incorporation contributes little to the photocurrent generation in the devices. It is intriguing to understand why PVP incorporation induces dramatic increase of FF only instead of Jsc and Voc. Compared to Jsc, FF is more sensitive to the photoactive layer/electrode interface, and is directly correlated with the series resistance (Rs) and shunt resistance (Rsh) of the device.40,41 In order to obtain a high-performance solar cell, the Rs value should be as low as possible while the Rsh value on the contrary should be as high as possible since a low Rs and a high Rsh value can facilitate electron transport and minimize the energy loss in the junctions and the interfaces between them. The extracted values of Rs and Rsh from J−V curves of the devices with and without PVP interlayer are compared in Table 1. The Rs and 11



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Rsh of the control device are 17.42 Ω·cm2 and 814.59 Ω·cm2, respectively. Upon PVP incorporation, the Rsh increases from 814.59 Ω·cm2 to 988.76 Ω·cm2 while the Rs decreases from 17.42 Ω·cm2 to 6.44 Ω·cm2, suggesting that PVP incorporation gives rise to a reduced contact resistance and improved interfacial contact between PCBM ETL and Ag cathode. To further probe the charge transport kinetics in the devices, we conducted dark J-V characterization for the devices with and without PVP interlayer. As shown in Figure 1D, the dark current density of the device with PVP interlayer in the reverse bias is relatively smaller than that of the control device, suggesting that the leakage current at the PCBM/Ag interface is reduced upon PVP incorporation. Besides, in the forward bias region of 0.9 ~ 1.5 V, the injected current density of the device with PVP interlayer is much higher than that of the control device, suggesting a lowered injection barrier between PCBM ETL and Ag cathode upon PVP incorporation.21,40 As a result, the electron transport from PCBM ETL to Ag cathode is promoted. This contributes to the increase of FF along with a reduced contact resistance discussed above. Whether PVP incorporation affects the optical absorption of the device is investigated by UV-Vis absorption spectroscopy as illustrated in Figure 2. The absorption onset of the CH3NH3PbI3-xClx perovskite layer is about 780 nm (curve a), corresponding to an optical bandgap of 1.6 eV, which is comparable to those reported in literatures for CH3NH3PbI3-xClx perovskite film prepared by one-step spin-coating method.25,37 After spin-coating the PCBM layer, in the UV region of 300-400 nm light absorption of perovskite/PCBM bilayer film 12


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increases obviously owing to the absorption of PCBM.21 In visible light region (400-800 nm), light absorption of the perovskite/PCBM bilayer film keeps almost same to that of the pristine perovskite film (curve b). Upon incorporation of PVP interlayer, negligible change on the overall UV-Vis absorption spectrum of the perovskite/PCBM film is observed, suggesting that PVP incorporation hardly affects the light absorption of perovskite/PCBM film. Because PVP as a non-conjugated polymer has no absorption in the UV and visible light regions, this result is quite understandable and coincides with the negligible change of Jsc revealed by J-V characterization.

b

3.0

PEDOT:PSS/Perovskite (a) PEDOT:PSS/Pervoskite/PCBM (b) PEDOT:PSS/Pervoskite/PCBM/PVP (c)

c 2.5

Absorbance / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.0

a 1.5

1.0

0.5

0.0 300

400

500

600

700

800

900

Wavelength / nm

Figure

2.

UV-vis

absorption

spectra

of

PEDOT:PSS/perovskite

(a),

PEDOT:PSS/perovskite/PCBM (b) and PEDOT:PSS/perovskite/PCBM/PVP (c) films. It is known that the crystallinity and surface morphology of perovskite layer directly determine the performance of a perovskite solar cell.13-16 According to XRD characterization, the CH3NH3PbI3-xClx perovskite shows good crystallinity similar to those reported in 13



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literatures (see Supporting Information-Figure S6A).21,39 The effect of PVP incorporation on the surface morphology of perovskite/PCBM bilayer film was investigated by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The surface topographic SEM image of the pristine CH3NH3PbI3-xClx perovskite film on ITO/PEDOT:PSS substrate and the cross-sectional SEM image of the entire device are illustrated in Figures 3A and 3B, respectively, indicating that the perovskite layer with a thickness of ca. 150 nm is very compact and dense, and it is hard to observe PVP interlayer atop the PCBM layer (ca. 40 nm) due to its small thickness (~10.8 nm, determined by a surface profilometer, see Supporting Information-Figure S2). The influence of PVP incorporation on the surface morphology of perovskite/PCBM bilayer film is further studied by AFM in tapping mode. Figures 3C-D compare the AFM topographic images of perovskite/PCBM bilayer film with and without PVP interlayer. While the AFM image of the single perovskite layer shows an inhomogeneous film with a very large root-mean-square (RMS) roughness of 14.92 nm (see Supporting Information Figure S6B), a more uniform and smoother film is observed for perovskite/PCBM bilayer as reflected by the dramatically decreased RMS roughness of 8.35 nm (Figure 3C). Upon incorporation of a thin PVP interlayer atop PCBM layer, negligible change on the overall image including the RMS roughness (from 8.35 nm to 8.57 nm) is observed (Figure 3D). This is understandable since the PVP interlayer is so thin (~10.8 nm) that its influence on the underneath perovskite/PCBM bilayer film is negligible.

14


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(B) (B)

(A)

PCBM/PVP

40 nm 150 nm 40 nm

PEDOT:PSS

200 nm

1 μm

(C) RMS=8.35 nm

nm 20

(D) RMS=8.57 nm

0

-20

-40 0

5 μm

5 μm

0

Figure 3. (A) Surface topographic SEM image of the CH3NH3PbI3-xClx perovskite film on ITO/PEDOT:PSS substrate. (B) Cross-sectional SEM image of the device without Ag cathode. (C) AFM image of the perovskite/PCBM bilayer film. (D) AFM image of the perovskite/PCBM/PVP film. In order to investigate the effect of PVP incorporation on the interface between perovskite/PCBM layer and Ag cathode, we measured the electrochemical impedance spectroscopy (EIS) of the devices, which is an effective tool to investigate the interfacial charge transport properties of perovskite solar cell devices.40,41 EIS was measured in the dark at an applied voltage of 0.9 V (close to Voc) with a frequency ranging from 1 Hz to 1 MHz, and Figure 4 compares the Nyquist plots of devices with and without PVP interlayer, which 15



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includes also the corresponding fitted curves using an equivalent circuit based on a transmission line model generally used in literatures (see inset of Figure 4).21,40,41 The fitted parameters from Nyquist plots are summarized in Table 2, including Rs which is correlated to internal series resistance, Rct which represents the charge transfer resistance, and a constant phase element (CPE) which is a non-ideal capacitor defined by CPE-T and CPE-P correlated with the interface capacitor and an ideal capacitor, respectively.21,40 Since the structures and the fabrication conditions of devices with and without PVP interlayer are identical, their differences in Rs and Rct arise solely from the interfacial resistance at PCBM/Ag and PCBM/PVP/Ag interfaces. Upon PVP incorporation, the Rs value exhibits negligible change, whereas the Rct value drops dramatically from 38.01 Ω·cm2 to 5.81 Ω·cm2 , indicating a reduced interfacial resistance and an improved interfacial contact between PCBM and Ag cathode. This improvement consequently promotes electron transport from PCBM to Ag cathode and suppresses electron-hole recombination. These results are consistent with the decreased series resistance (from 17.42 Ω·cm2 to 6.44 Ω·cm2) and increased shunt resistance (from 814.59 Ω·cm2 to 988.76 Ω·cm2) extracted from J-V curves, confirming the promoted electron transport, which contributes directly to the increase of FF.

16


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160

w/o PVP w/o PVP (fit) with PVP with PVP (fit)

140 120 100

-Z'' / ohm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80 60 40 20 0 0

50

100

150

200

250

300

Z' / ohm

Figure 4. Nyquist plots of the CH3NH3PbI3-xClx-based iPSC devices with and without PVP interlayer measured in the dark. Inset: the equivalent circuit model employed for the fitting of the impedance spectra. Table 2. Parameters employed for the fitting of the impedance spectra of the CH3NH3PbI3-xClx-based iPSC devices with and without PVP interlayer. Rs Interlayer

Rct 2

CPE-T 2

CPE-P

(Ω·cm )

2

(Ω·cm )

(F/cm )

w/o PVP

3.15

38.01

1.47E-7

0.87

with PVP

3.29

5.81

1.46E-7

0.98

The enhancement mechanism induced by PVP interlayer was further studied from the viewpoint of energy level alignment. We carried out Scanning Kelvin probe microscopy (SKPM) measurement to probe the change of the work function of PCBM layer upon PVP incorporation, which is linearly correlated with its surface potential.38,42,43 Upon incorporation 17



of PVP interlayer, the surface potential of PCBM layer exhibits a ~300 mV increase (see Supporting Information-Figure S7). Such a positive change in surface potential suggests that a microscopic electric dipole moment is formed in the PVP interlayer, with negative charge end pointing towards PCBM photoactive layer and positive charge end pointing towards Ag electrode.38.43 According to the energy level diagram shown in Scheme 1 based on the energy level data available in literatures,25,44 the formation of the PVP dipole layer may enhance the built-in potential across the device, consequently promoting electron transport from PCBM to Ag cathode.38.43 A plausible explanation is that PVP has a large dipole moment (4.07 D)45 induced by the side chain of pyrrolidone containing oxygen atoms which can readily coordinate with Ag atoms,38 resulting in the formation of the dipole layer. -2.0

e

-3.0 -3.7 Energy (eV)

-3.9

-4.0

-4.8 -5.0

ITO

- +

CH3NH3PbI3-xClx

-5.0

PCBM

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 24

PEDOT:PSS -5.3

-6.0

h

-6.0

-4.3 Ag

- +

- + - + PVP

-7.0

Scheme 1. Energy level diagram for the corresponding materials used in the CH3NH3PbI3-xClx-based iPSC devices with PVP interlayer. e and h represent electron and hole, respectively. 18


Page 19 of 24

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Conclusions In summary, a commercially available low-cost non-conjugated polymer PVP was incorporated into iPSC devices for the first time as a new interlayer between PCBM ETL and Ag cathode, resulting in a dramatic efficiency enhancement. The CH3NH3PbI3-xClx-based iPSC device incorporating the PVP interlayer exhibited a PCE of 12.55% under the optimized spin-coating condition, which is enhanced by ~15.9% relative to that of the control device without PVP interlayer. The PCE enhancement is primarily due to the increase of FF. According to the morphological, optical and impedance spectroscopic characterizations, incorporation of PVP interlayer leads to a reduced interfacial resistance and an improved interfacial contact between PCBM and Ag cathode. This improvement consequently promotes electron transport across the perovskite/Ag interface via PCBM ETL and suppresses electron-hole recombination. Besides, PVP incorporation induces the formation of a dipole layer, which may enhance the built-in potential across the device, conjunctly promoting electron transport from PCBM to Ag cathode. Our study offers a cost-effective and facile interfacial engineering strategy towards efficiency enhancement of high-performance perovskite solar cells. Acknowledgements. This work was partially supported by the National Key Research and Development Program of China (2017YFA0402802), National Natural Science Foundation of China (21371164, 51572254, 11474286), the Major/Innovative Program of Development Foundation of Hefei Center for Physical Science and Technology (No. 2016FXZY003), and 19



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Page 20 of 24

the Fundamental Research Funds for the Central Universities (WK3430000002, WK2060140023).

Supporting Information Available:

Optimization of the spin-coating speed of PVP/IPA solution, Determinaton of the thickness of PVP interlayer, Histograms and Box plots of photovoltaic parameters, hysteresis, stabilized photocurrent density and power output of the device, XRD pattern and AFM image of CH3NH3PbI3-xClx perovskite film, and SKPM data of perovskite/PCBM and perovskite/PCBM/PVP films, etc. This material is available free of charge via the Internet at http://pubs.acs.org.

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23



Table of Contents (TOC) image

20

with PVP w/o PVP

Current Density (mA/cm2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 24

15

10

5

0 0.0

0.2

0.4

0.6

0.8

Bias (V)

24


1.0

Nonconjugated Polymer Poly(vinylpyrrolidone) as an Efficient

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