Stable Efficiency Exceeding 20.6% for Inverted Perovskite Solar Cells

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Stable Efficiency Exceeding 20.6% for Inverted Perovskite Solar Cells through Polymer-Optimized PCBM Electron-Transport Layers Dong Yang, Xiaorong Zhang, Kai Wang, Congcong Wu, Ruixia Yang, Yuchen Hou, Yuanyuan Jiang, Shengzhong Liu, and Shashank Priya Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.9b00936 • Publication Date (Web): 15 Apr 2019 Downloaded from http://pubs.acs.org on April 15, 2019

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Stable Efficiency Exceeding 20.6% for Inverted Perovskite Solar Cells through Polymer-Optimized PCBM Electron-Transport Layers Dong Yang,† Xiaorong Zhang,§ Kai Wang,† Congcong Wu,† Ruixia Yang,§ Yuchen Hou,† Yuanyuan Jiang,† Shengzhong (Frank) Liu,*,‡,§ and Shashank Priya,* ,† †Materials

Science and Engineering, Pennsylvania State University, University Park,

Pennsylvania 16802, United States ‡Dalian

National Laboratory for Clean Energy, iChEM, Dalian Institute of Chemical Physics,

Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China §Key

Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education; Shaanxi

Engineering Lab for Advanced Energy Technology, School of Materials Science and Engineering, Shaanxi Normal University, Xi’an 710119, China

ACS Paragon Plus Environment

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ABSTRACT: Fullerene derivative, such as [6,6]-phenyl C61 butyric acid methyl ester (PCBM), is widely used as the electron-transport layer (ETL) in inverted perovskite solar cell (PSCs). However its low electron mobility, complexity in achieving quality film formation, and severe non-radiative recombination at perovskite/PCBM interface due to the large electron capture region, results in lower efficiency for inverted PSCs compared to the normal structures. Herein, we demonstrate an effective and practical strategy to overcome these challenges. Conjugated n-type polymeric materials were mixed together with PCBM to form a homogeneous bulk-mixed (HBM) continuous film with high electron mobility and suitable energy level. HBM film is found to completely cap the perovskite surface to enhance the electron extraction. The critical electron capture radius of the HBM decreases to 12.52 nm from 14.89 nm of PCBM due to the large relative permittivity, resulting in reduced nonradiative recombination at perovskite/HBM interface. The efficiency of inverted PSCs with HBM ETL exceeds 20.6% with a high fill factor of 0.82. Further, the stability of devices is improved owing to the high hydrophobicity of the HBM ETLs. Under ambient air condition after 45 days, the efficiency of inverted PSCs based on HBM display remains 80% of the initial value. This is significantly higher than the control devices which retain only 48% of the initial value under similar aging conditions. We believe these breakthroughs in improving efficiency and stability of inverted PSCs will expedite their transition.

KEYWORDS: perovskite, electron-transport, relative permittivity, recombination, fullerene

Organometallic halide perovskites have gained significant attention as promising energy harvesting materials for high efficiency low cost solar cells.[1-4] Within a very short period, the certified power conversion efficiency (PCE) of perovskite solar cells (PSCs) has increased from 3.8% to 23.7%.[5,

6]

This is due to the outstanding advantages offered by


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perovskites such as band gap, ultralow trap density, high charge carrier mobility, good absorption coefficient, long carrier lifetime, etc.[7-11] PSC device comprises of normal (n-i-p) and inverted (p-i-n) structures, decided by the type of transport layer on transparent electrode.[12] Although PSCs with normal structure have obtained exciting success demonstrating highest efficiency and outstanding stable output power, there are still some issues that needs to be resolved to ensure their transition. The fabrication process for commonly used TiO2 based electron-transport layers (ETLs) often requires high temperature sintering in order to achieve the best performance.[13] This results in increased energy cost and presents challenge in integration with low temperature flexible substrate materials. Although the SnO2 ETL can be fabricated at relatively low temperatures,[14, 15] the devices based upon this low temperature film often display severe current density-voltage (J-V) hysteresis because of carrier accumulation at the ETL/perovskite interface.[16] In order to suppress the hysteresis, it is often necessary to implement special processes to treat the SnO2 surface before the perovskite deposition, or adopt SnO2 in the form of nanoparticles and quantum dots.[4, 12, 17] In order to overcome the issues associated with normal structures, inverted PSCs with negligible hysteresis have been developed, which usually can be fabricated at low temperature.[18,

19]

It is well known that ETL plays key role in improving performance of

inverted PSCs. The compound [6,6]-phenyl C61 butyric acid methyl ester (PCBM) is the most commonly used ETL in inverted PSC.[20-23] However, it suffers from several drawbacks, resulting in inferior efficiency of inverted PSCs. First, the surface morphology of PCBM ETL is largely dependent on the underlying perovskite morphology. The rough surface over large perovskite grains may lead to fragmented PCBM coating, resulting in severe non-radiative recombination.[24] To resolve this problem, thermal evaporation of lithium fluoride and C60/bathocuproine has been employed to cap the PCBM film surface,[25, 26] adding not only


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cost but also complications in fabrication process. It is therefore imperative to develop a reliable one-step process. Second, low electron mobility and imperfect energy level impede the electron transport. Recently there have been reports on non-fullerene materials and polymers to replace, dope or modify PCBM in inverted perovskite cells in order to improve the electron mobility and Fermi level.[27-30] However, the small relative permittivity of PCBM generated a large electron capture region, which leads to severe recombination at the interface. Herein, we demonstrate a homogeneous bulk-mixed (HBM) film as an ETL, which coupled with PCBM and conjugated with n-type polymer poly[(9,9-dioctyluorene)-2,7-diylalt-(4,7-bis(3-hexylthien-5-yl)-2,1,3-benzothiadiazole)-2′,2″-diyl] (F8TBT). The HBM ETL has higher electron mobility and suitable energy level compared to pristine PCBM ETLs. More importantly, the favorable film forming property of the polymer enables the full coverage of perovskite film, facilitating higher effective electron extraction. The relative permittivity of HBM film increases to 4.73 from 3.82 for PCBM, leading to smaller electron capture region, which can significantly reduce the non-radiative recombination at interface between ETL and perovskite. As a result, for the inverted methylammonium (MA)-based PSCs, the PCE of 20.60% with a high fill factor (FF) of 0.82 is achieved. This is 19% higher value in comparison to the devices based on pristine PCBM. In addition, the inverted PSCs with HBM show good stability due to the high hydrophobicity, with the PCE retaining 80% of the initial efficiency when the unsealed devices were exposed to the ambient air in dark for 45 days. In comparison, the devices with pristine PCBM only retain 48% of the initial efficiency after being stored under the similar conditions. Results and Discussion. The surface coverage of perovskite by ETLs is crucial in achieving high performance of the inverted PSCs. Commonly-used PCBM ETL often displays


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inhomogeneous film formation, leading to the openings or very thin regions between the ETL and perovskite.[24] When the electrode is fabricated, the moving metal atoms would permeate into these openings, resulting in a direct contact between the perovskite and electrode. In this study, n-type F8TBT polymer with good film formation[31] was employed as a dopant and introduced into PCBM to form HBM continuous films, achieving complete coverage on the perovskite layer. Figure S1 shows the molecular structure of F8TBT and PCBM. Scanning electron microscopy (SEM) images of pristine PCBM and HBM deposited on perovskite films are shown in Figure S2. There are some defects on the perovskite surface because PCBM could not form a continuous thin film. In comparison, the perovskite is fully covered by HBM thin film owing to the significantly improved film formation ability.

Figure 1. Illustration of the morphology of various ETLs and architecture of device. (a) AFM height image of PCBM, and (b) its corresponding phase image. (c) AFM height image of HBM, and (d) its corresponding phase image. (e) Structure of the inverted PSC with HBM as an ETL. The atomic force microscopy (AFM) height images reveal that pristine PCBM thin layer consists of structural defects, as shown in Figure 1a. The HBM layer forms a continuous film


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over the complete perovskite surface (Figure 1b), in agreement with SEM results. Corresponding phase images show that the pristine PCBM layer displays single phase (Figure 1c), and HBM thin film exhibits homogeneous phase separation (Figure 1d). The scale of this phase separation is about 15 nm, which is beneficial towards carrier transfer, considering that the carrier transport distance in organic materials is less than 10 nm.[32] Given the improved film formation and the advantageous phase separation, we have investigated HBM films as ETLs in the inverted PSCs with the structure glass/FTO/PEDOT:PSS/MAPbI3/HBM/Ag, as shown in Figure 1e. Here, FTO and 100 nm-thick silver are employed as the anode and cathode, respectively, 25nm-thick poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) is used as hole-transport layer, and ca. 60 nm-thick PCBM and HBM ETLs are sandwiched in between electrode and the perovskite MAPbI3 absorber (ca. 560 nm).

Figure 2. The properties of different ETLs and schematic diagram for carrier recombination. (a) UPS data of PCBM and HBM films. (b) Energy level diagram between different ETLs and perovskite materials. (c) Electron mobility of PCBM and HBM deposited on perovskite


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surface. (d) Diagram for the mechanism of carrier recombination at interface between perovskite and ETL. (e) Capacitance versus frequency at room temperature for pristine PCBM and HBM. Figure 2a shows the ultraviolet photoelectron spectroscopy (UPS) data of the PCBM and HBM films. The binding energy of HBM is located at 17.16 eV, which is shift of 0.27 eV compared to PCBM layer. As a result, the Fermi level of HBM increases to -4.06 eV from 4.33 eV (see calculation in the supporting information). Figure 2b describes the energy level alignment between perovskite and different ETLs. It is apparent that the Fermi level of HBM film is closer to the conduction band of perovskite, leading to facile electron transfer from perovskite to ETL. The electron mobility of different ETLs is analyzed by the space-chargelimited-current (SCLC) method using electron-only devices.[40] Figure 2c shows the dark J-V curves of the electron-only devices. The detailed experimental process and calculations are shown in the supporting information. The electron mobility of the PCBM and HBM are 9.27 × 10-5 cm2/V s and 2.56 × 10-4 cm2/V s, respectively. The higher electron mobility of HBM compared to pristine PCBM is attributed to the phase separation, which is beneficial in generating larger photocurrent in inverted PSCs.[34] The non-radiative recombination at perovskite/ETL interface is decided by the electron capture region, which is a function of critical radius (Rc).[35, 36] In general, the electron in the capture region could be trapped by positive trap states at perovskite/ETL interface due to the Coulombic interactions, as shown in Figure 2d. Thus, if the electron capture region is reduced, the recombination can be effectively suppressed. The Rc can be calculated by following equation (1): 𝑞2

𝑅𝑐 = 4𝜋𝑘𝑇𝜀0𝜀𝑟

(1)


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where q is the elementary charge, k is the Boltzmann constant, T is the temperature, ε0 and εr are vacuum permittivity and the relative permittivity of the ETL, respectively. Increasing εr can reduce the Rc, resulting in smaller electron capture region. The relative permittivity is determined by capacitance, so the capacitance of different ETLs was measured using parallel plate capacitor configuration. Details on these calculations can be seen in the supporting information. Figure 2e shows the capacitance versus frequency curves. HBM shows larger relative permittivity of 4.73 compared to that of PCBM (3.82), accordingly yielding the Rc of 12.52 nm and 14.89 nm, respectively. Therefore, the electron capture region, which is determined by 4πRc3/3, decreases to 1.4 × 10-17 cm-3 from 8.3 × 10-18 cm-3, reduced by about one order of magnitude. The smaller electron capture region for the HBM would effectively suppress the trap assisted recombination at interface, leading to high FF in the perovskite devices. The outstanding properties of HBM film, including good film formation, high electron mobility, suitable energy level and reduced trap assistant recombination, provide encouragement for fabrication of the inverted PSCs using HBM as the ETLs. The top-view SEM and x-ray diffraction (XRD) of MAPbI3 film are shown in Figure S3a and S3b respectively indicating that the pinhole-free perovskite film with full coverage and good crystallinity is obtained. The J-V curves of inverted perovskite devices based on different F8TBT weight ratio with PCBM is next investigated. Figure S4a shows the J-V curves of the inverted PSCs with various ETLs, and the average key parameters are listed in Table S1. The devices with F8TBT display the lowest PCE because of the mismatched energy level between ETL and perovskite. The lowest unoccupied molecular orbital (LUMO) of F8TBT is located at -3.15 eV,

[37]

which is larger than the conduction band of perovskites, leading to poor

electron transfer.


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The inverted PSCs with pristine PCBM demonstrate lower efficiency than that of the devices with HBM ETLs, due to inferior coverage of the PCBM ETLs that causes direct contact between perovskite and cathode,[38, 39] leading to severe charge recombination at the interface. The PCE is enhanced with increasing the weight ratio between F8TBT and PCBM, and the optimal value occurring at 0.22. Beyond the threshold of 0.22, the PCE sharply decreases as F8TBT does not have proper band alignment. Consequently, inverted PSCs based on the HBM ETLs with a weight ratio of 0.22 between F8TBT and PCBM were adopted to study the PSC devices performance. Figure S3c and S3d show the cross-sectional SEM images of complete devices based on PCBM and HBM ETLs. It is apparent that the perovskite could not be completely capped by pristine PCBM due to its bad film formation property, while the HBM exhibits full coverage, in good accordance with morphology characterizations (Figure 1 and S2).


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Figure 3. Performance of the inverted PSCs. (a) J-V curves of the inverted PSCs with PCBM and HBM ETLs under different scan directions. (b) EQE and corresponding integrated photocurrent density of the inverted PSCs based on different ETLs. (c) Stable photocurrent density and calculated efficiency established as a function of time for inverted PSCs with PCBM and HBM biased at 0.90 V, and 0.96 V, respectively. (d) The PCE statistics distribution of the inverted PSCs with various ETLs. Table 1. Parameters for the champion inverted PSCs based on PCBM and HBM ETLs under both reverse and forward scan directions.

ETL

Scan direction

Jsc (mA/cm2)

Voc (V)

FF

PCE (%)

RS

21.18

1.06

0.77

17.29

FS

21.08

1.06

0.75

16.76

RS

22.43

1.12

0.82

20.60

FS

22.38

1.12

0.82

20.55

PCBM

HBM

Figure 3a shows the J-V curves of champion inverted PSCs based on PCBM and HBM ETLs in both forward and reverse scan directions under AM 1.5G irradiation, and the parameters are summarized in Table 1. The PCE of inverted PSCs based on pristine PCBM displays is found to be 17.29% with a short-circuit current density (Jsc) of 21.18 mA/cm2, open-circuit voltage (Voc) of 1.06 V and FF of 0.77, respectively. After employing the HBM as ETL, all key J-V parameters were improved, leading to the PCE of 20.60%, one of the highest efficiency reported for inverted PSCs to date. The large Jsc and FF are attributed to high electron mobility of HBM ETLs due to the perfect phase separation (Figure 1d and Figure 2c). Especially, the FF, one of the highest values for PSCs, is contributed by the reduced non-radiative recombination, that originates from the narrowed electron capture


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region (Figure 2d). The Voc also increases to 1.12 V from 1.06 V, due to the less energy loss due to adjacent energy levels of HBM and perovskite absorber (Figure 2b). The J-V curves of inverted PSCs based on pristine PCBM exhibits small hysteresis under forward and reverse scan directions, which is attributed to the carrier accumulation at the interface due to low electron mobility of PCBM and severe trap assistant recombination.[4, 40-42] Interestingly, the devices with HBM ETL display negligible hysteresis (Figure 3a and Table 1), due to the high electron mobility and effective suppressing of trap related recombination. Figure 3b shows the external quantum efficiency (EQE) and integrated photocurrent density of inverted perovskite devices based on various ETLs. It is clear that the EQE of device with HBM is enhanced in the entire absorption region in contrast to that of device based on pristine PCBM. The high EQE is ascribed to excellent properties of HBM ETLs. The obvious increase in EQE in wavelength of 500-630 nm is observed in HBM RTLs, which may be attributed to F8TBT absorption (Figure S5), indicating the successful doping of the polymer. The integrated Jsc of inverted PSCs with pristine PCBM and HBM are 20.72 mA/cm2 and 21.90 mA/cm2, in good agreement with J-V results. The stable photocurrent density of inverted PSCs with different ETLs was investigated at the maximum power point, as shown in Figure 3c. The stable photocurrent density of devices using pristine PCBM and HBM are found to be 19.14 mA/cm2 and 21.53 mA/cm2, yielding the stable PCEs to be 17.23% and 20.67%, respectively. Figure 3d shows the PCE statistics distribution of 30 individual inverted devices with various ETLs, and the parameters are summarized in Table S2 and S3. Both devices exhibit small standard deviation, indicating good reproducibility. The carrier recombination in the device was investigated by tracing the change of Jsc and Voc with the decrease of illumination intensity. Figure 4a shows the Jsc versus light intensity


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curve. The devices based on pristine PCBM and HBM ETLs exhibit linear correlation and both slopes are close to 1, indicating the negligible bimolecular recombination.[4, 43] Figure 4b shows the Voc versus light intensity plot. The trap assistant recombination is reflected in the deviation between the slope and kT/q.[44, 45] The slope of the device based on pristine PCBM is 1.42 kT/q, which decreases to 1.12 kT/q when using HBM ETL, implying negligible trap assistant recombination, in agreement with above analysis.

Figure 4. Carrier transfer dynamics in inverted PSCs. (a) Jsc versus light intensity and (b) Voc versus light intensity of the inverted PSCs with PCBM and HBM. (c) Steady-state PL and (d) TRPL spectra when PCBM and HBM deposited on perovskite surface. Figure 4c shows the steady state photoluminescence (PL) of the perovskite covered with different ETLs. The PL intensity of perovskite capped with HBM is significantly quenched


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compared to that of PCBM. The strong PL quenching demonstrates the efficient electron transfer form perovskite to HBM ETL. The normalized time-resolved PL (TRPL) spectra of the perovskite coated with various ETLs were measured to investigate the radiative recombination, as shown in Figure 4d, and the fitting parameters are shown in Table S4. In general, the fast decay lifetime (τ1) is caused by the non-radiative recombination at interface, and the slow decay lifetime (τ2) originates from the radiative recombination in the bulk perovskite.[46,

47]

The fast decay lifetime dominates the TRPL in both HBM and PCBM,

indicating that non-radiative recombination is the main factor. When the HBM is deposited on perovskite, τ2 significantly decreases to 1.02 from 1.40, implying effective electron extraction. In addition, the electrical impedance spectroscopy (EIS) shows the smaller circle (Figure S6) when using HBM ETLs, indicating the low carrier transfer resistance. These results demonstrate that HBM is efficient ETL for high performance inverted PSCs.

Figure 5. Environmental stability of the inverted PSCs. (a) Long term stability results of the


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inverted perovskite devices without encapsulation stored in the dark under ambient air for 45 days. (b) The light stability of inverted PSCs placed in continuous illumination for 80 hours. (c) The water contact angle of PCBM (left bottom) and HBM (right bottom) deposited on a perovskite film surface. The stability of inverted PSCs with pristine and HBM ETLs was also investigated under dark and continuous illumination conditions. Figure 5a shows the stability measurements of the unsealed devices stored in dark. The PCE of device based on HBM maintains about 80% of its initial efficiency after being stored in the dark under ambient air for 45 days, while the PCE of device with pristine PCBM degrades by 52% of its initial value after being stored in same conditions. Under continuous illumination, the device with HBM retains about 82% of its initial efficiency after 80 hours, while the PSC based on PCBM keeps only 51% of its initial value under the same test condition, as shown in Figure 5b. Figure 5c provides the water contact angle of different ETLs deposited on perovskite films. The contact angle sharply increased to 105.38° from 92.37° when the perovskite surface is covered by HBM. The high hydrophobicity of the HBM films effectively suppresses the moisture permeation into the perovskite devices, leading to good long term stability of the inverted PSCs. In conclusion, the inverted PSCs based on efficient HBM ETLs have been developed, and the efficiency was found to increase to 20.60% with the high FF of 0.82. This excellent performance is attributed to outstanding properties of HBM ETLs that includes good quality film formation, suitable energy level, high electron mobility due to the perfect phase separation, and reduced non-radiative recombination at the interface owing to small electron capture region. In addition, the unsealed devices exhibit good environmental stability due to the hydrophobicity of HBM ETL. This practical and relevant development of ETL will expedite the transition of perovskite solar cells.


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Method. Synthesis of MAI Materials. MAI was synthesized according to the process reported elsewhere.[48] The weight ratio of 33% methylamine absolute ethanol solution of 48 mL was added into round bottom flask, and then hydroiodic acid aqueous solution in the amount of 20 mL corresponding to the weight ratio of 57% was dropped in above solution. The mixed solution with continuous stirring was kept at 0 °C for 4 hours. After that, a rotary evaporator was used to remove solvent at 60 °C, and the impure yellow MAI was precipitated out. Next recrystallization of the impure MAI was conducted by using absolute ethanol and diethyl ether several times until the white color appeared. Finally, the pure MAI product were dried at 60 °C for 24 hours in the vacuum oven. Solar

cell

fabrication.

The

inverted

structure

device

given

as

glass/FTO/PEDOT:PSS/MAPbI3/PCBM/Ag were fabricated. The preparation process is described below. The area of 2.5 cm × 2.5 cm glass/FTO was cleaned by acetone and isopropyl alcohol successively using ultrasonic bath for 10 min, and dried in the vacuum oven at 80 °C for 30 min. The cleaned glass/FTO substrates were treated by ozone for 15 min in

the

ultraviolet-ozone

chamber.

The

poly(3,4-ethylenedioxythiophene)

poly(styrenesulfonate) (PEDOT:PSS, AI4083, Sigma Aldrich) hole transport layer was spincoated on glass/FTO surface at 4500 rpm for 30 s, and then annealed at 150 °C for 30 min in ambient air. After cooling, the samples were transferred into glovebox to deposit MAPbI3 perovskite absorbers. 646.8 mg PbI2 (99.9985%, Alfa Aesar) and 222.6 mg pure MAI were dissolved in a mixed solvent, which consisted of N,N-Dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) (v:v = 4:1). This solution was spin-coated on PEDOT:PSS by anti-solvent method. The spin-coating process was divided into two steps. The spin speed of first and second step was 1000 rpm and 4000 rpm and the spin time was 20 s and 45 s, respectively. 200 μL chlorobenzene was added into the sample at the begin of the second step. Subsequently, the samples were annealed at 95 °C for 6 min in the glovebox. After that, 10


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mg/mL PCBM chlorobenzene solution and different weight ratio of PCBM and F8TBT mixed solution were spin-coated on perovskite films. Next, the samples were annealed at 100 °C for 10 min. Similar thickness of ETLs was obtained by controlling the spin-casting rate. Lastly, 100 nm of silver was thermally evaporated on top of PCBM surface using metal mask. Characterization. Scanning electron microscopy (SEM) results were obtained using field emission scanning electron microscopy (SU-8020). Atomic force microscopy (AFM) measurements were performed on a Multimode 8 atomic force microscope using tapping mode. The charge transfer dynamics of the perovskite films was tested by steady-state photoluminescence (PL) and time-resolved PL (TRPL) using the Edinburgh Instruments FLS920 fluorescence spectrometer. The external quantum efficiency (EQE) was acquired on the QE system (Crowntech. Inc., USA). J-V characteristics of the inverted perovskite photovoltaics was measured using Keithley 2400 system at 100 mW/cm2 light illumination (AM 1.5G), provided by solar simulator (150 W Sol 2ATM, Oriel). The light intensity was calibrated by a silicon reference cell (filtered by NREL-traceable KG5). The scan rates in both reverse (from 1.2 V to -0.1 V) and forward (from -0.1 V to 1.2 V) direction were 0.1 V/s, and the scan step was 0.02 V. Area of 0.09 cm2 was used for metal aperture during measurement process.

ASSOCIATED CONTENT The Supporting Information is available free of charge on the Internet at http://pubs.acs.org. The details of calculation on electron mobility, energy level and relative permittivity for PCBM and HBM ETLs. Top-view SEM of perovskite, PCBM and HBM films. XRD of


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perovskite deposited on PEDOT:PSS substrates. Cross-sectional SEM and EIS of completed perovskite devices. UV-Vis spectra of PCBM and HBM films. J-V curves of perovskite cells based on pristine PCBM, pure F8TBT and different ratios of PCBM and F8TBT ETLs. The parameters of inverted PSCs measured under different conditions. Fitting data from TRPL spectra. AUTHOR INFORMATION Corresponding Author *E-mail: (S.Liu) [email protected]. *E-mail: (S.Priya) [email protected]. Author Contributions D.Y. designed the experimental procedures and conducted the experiments. D.Y. drafted the first version of manuscript, and all authors contributed to discussion of the results and writing. D.Y., S.L. and S.P. supervised the overall project, and contributed to the revisions on the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT S.L. acknowledges the National Key Research Program of China under award number: 2016YFA0202403, and the National Natural Science Foundation of China under award number: 61604090. D.Y., Y.H. and K.W. acknowledge the financial support from Air Force Office of Scientific Research through award number FA9550-18-1-0233. S.P. acknowledges


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the financial support from Air Force office of Scientific Research under award number: FA9550-17-1-0341. Y.J. acknowledges the financial support from NSF I/UCRC: Center for Energy Harvesting Materials and Systems (CEHMS). C.W. acknowledges the support from STTR program (Nanosonic).

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Table of contents (TOC)

Stable Efficiency Exceeding 20.6% for Inverted Perovskite Solar Cells through PolymerOptimized PCBM Electron-Transport Layers


Stable Efficiency Exceeding 20.6% for Inverted Perovskite Solar Cells

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