Double electron transport layers for efficient and stable organic-inorganic hybrid perovskite solar cells

Organic Electronics 70 (2019) 292–299

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Double electron transport layers for efficient and stable organic-inorganic hybrid perovskite solar cells

T

Caixia Rena,1, Yue Hea,1, Shiqi Lia, Qinjun Suna, Yifan Liua, Yukun Wua, Yanxia Cuia,b, Zhanfeng Lia, Hua Wangb, Yuying Haoa,b,∗, Yucheng Wub,∗∗ a b

College of Physics and Optoelectronics, Key Lab of Advanced Transducers and Intelligent Control System, Taiyuan University of Technology, Taiyuan, 030024, China Key Laboratory of Interface Science and Engineering in Advanced Materials, Taiyuan University of Technology, Taiyuan, 030024, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Perovskite solar cells Double electron transport layer Power conversion efficiency Stability

Organic-inorganic hybrid perovskite solar cells (PSCs) have attracted tremendous attention recently because of their excellent photovoltaic performance. High power conversion efficiency (PCE) and long-term stability of PSCs mainly rely on high crystalline quality of perovskite film, good interface contact between functional layers and matching energy level alignment, which can be achieved by crystal growth regulation and interface engineering. Here, we demonstrate a high-performance planar heterojunction PSCs with PCBM/N2200 as double electron transport layers (ETLs). The N2200-based PSCs exhibit an average PCE of 15.50%, which obviously surpass the average PCE of 14.18% of conventional PSCs with only PCBM as ETLs. In comparison with the conventional PSCs, the PCBM/N2200 double ETLs increase built-in potential of devices and decrease interfacial energy barrier of MAPbI3/PCBM and then result in higher Voc. Moreover, non-fullerene material N2200 can efficiently inhibit the aggregation of PC61BM, promote perovskite surface grain growth and passivate perovskite interface defect. All these are beneficial for electron transfer and extraction, and thereby increase the shortcircuit current (Jsc) and the Fill Factor (FF) of N2200 PSCs. In addition, N2200 obviously improve the air- and illumination-stability of PSCs. These results could stem from the interaction between PbI2 as Lewis acid and N2200 as Lewis base. This work indicate that N2200/PC61BM as double ETL is an effective way to get high photovoltaic performance for the inverted planar heterojunction PSCs.

1. Introduction Organic-inorganic hybrid perovskites as photosensitive semiconductor materials have attracted great attentions due to their excellent intrinsic properties, such as highly perfect crystalline, like direct bandgap, high extinction coefficient, broad absorption spectra, high charge carrier mobility and long charge carrier diffusion length [1–4]. By efforts for less than a decade, power conversion efficiency (PCE) of perovskite solar cell (PSCs) has increased to 23.7% from 3.8% of the first PSC reported by Miyasaka and co-workers in 2009 [5,6]. The unprecedentedly rapid rise in PCE demonstrate that PSC could be a highly promising commercial technology for solar energy conversion. So far two major types of PSC structures, including the mesoporous [7–12] and planar heterojunction structures [13–16], have been developed and widely investigated. It had been found that the carbon

based hole-conductor-free mesoscopic PSCs possess good moisture- and oxygen-stability due to the presence of insulating oxide scaffolds and ultra-thick carbon electrodes [7–9]. Nevertheless, the inclusion of compact/mesoporous TiO2 and mesoporous-ZrO2 layer require high temperature annealing treatment, which is incompatible with low-cost fabrication and flexible devices. The oxygen vacancies on the surface of TiO2 seriously affect the stability of PSCs under UV light [10–12]. In addition, the current-voltage hysteresis still remains unsolved in this type of PSCs, especially for temperatures > 270 K [11]. In contrast, the inverted planar heterojunction structure provide good solution for these issues. However, the planar structure PSCs remains large challenges in efficiency and stability relative to mesoporous devices [15,16]. The carrier traps and defects both in the bulk material and at the interfaces are recognized as intrinsic factors that degrade the device performance and long-term stability. Adachi et al. reported that a large

∗ Corresponding author. College of Physics and Optoelectronics, Key Lab of Advanced Transducers and Intelligent Control System, Taiyuan University of Technology, No.79, West Yingze Street, Taiyuan, Shanxi, 030024, China. ∗∗ Corresponding author. E-mail addresses: [email protected] (Y. Hao), [email protected] (Y. Wu). 1 Authors provide equal contributions to this work.

https://doi.org/10.1016/j.orgel.2019.03.033 Received 14 January 2019; Received in revised form 7 March 2019; Accepted 18 March 2019 Available online 27 April 2019 1566-1199/ © 2019 Elsevier B.V. All rights reserved.

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2. Experimental

number of carrier traps are generated in solar cells with the perovskite CH3NH3PbI3 (MAPbI3) as a light absorber after operation at 85 °C due to phase transition, which are detrimental to the thermal stability. And they presented that the perovskite alloys do not have this phase transition, resulting in effective suppression of carrier traps and thus improved thermal stability of planar PSCs [13]. The defect engineering, including adding dopants in the precursor solutions and interface passivation also are very promising method for improving the efficiency and stability of PSCs. For example, the photochemically active additive benzoquinone (BQ) was introduced into a perovskite precursor solution to increases PCE through improved perovskite morphology and crystal quality while also significantly extending lifetime of planar PSCs by reducing the formation of carrier traps during solar irradiation [14]. An ultrathin poly (methyl methacrylate) (PMMA) films was inserted between the perovskite and carrier transport layer to improve significantly the photovoltaic performance and stability of PSCs by the excellent passivation effect of carbonyl (C]O) groups on the PMMA [17–19]. For the inverted planar heterojunction PSCs [6,6],-Phenyl-C61-butyric acid methyl ester (PCBM) was widely adopted as electron transport layer (ETL) due to its solubility in non-polar solvents, appropriate energy levels and good electron transport properties. Moreover, PCBM deposited on the top of the perovskites can effectively reduce the trap density and thus increase the PCE of MAPbI3 solar cells, and the photocurrent hysteresis was eliminated simultaneously by PCBM passivation effect [20]. However, the small molecule nature of PC61BM and its low viscosity in solution state easily lead to the incomplete coverage on the rough perovskite film and thus negatively impact on the efficiency and stability of devices. Moreover, PC61BM may not be able to completely passivate the Pb2+ cations of perovskite because of its Lewis acidic nature [21]. In order to solve this issue, various alternative materials as ETLs in PSCs had been explored [22–24]. Chen et al. employed a novel donor-acceptor (D-A) structured electron transport material in PSCs to achieve the impressive PCE of 19.2% with a negligible hysteresis, benefiting from the incorporating of a solution processable n-type dopant [22]. Wang et al. used a non-fullerene acceptor material poly{[N,N′-bis (2-octyldodecyl)-1,4,5,8-naphthalene diimide2,6-diyl]-alt-5,5′-(2,2′- bithiophene)} (N2200) as ETL in PSCs to obtain the PCE of 8.15%, which is slightly lower than the PCE of 8.51% for the reference with conventional PCBM as the ETL, mainly resulting from the increased recombination in the N2200 contained devices due to the lower electron mobility of N2200 than that of PCBM [23]. Zhu et al. tailor the electronic properties of N2200 with a polymeric additive poly [9, 9- bis (6′-(N, N-diethylamino) propyl)-fluorene-alt-9, 9-bis (3-ethyl (oxetane-3-ethyloxy-hexyl) fluorene] (PFN-Ox), which enhance PCE to 16.8% from 15% of the reference PSCs with only N2200 as ETL [24]. Although these polymer materials as ETLs are successful in PSCs, they highly depend on the utilization of dopant for increasing the electron mobility, which inevitably result in complexity of preparation procedure. In this work, N2200 as an interface modification layer was inserted into the perovskite/PC61BM interface to form double ETLs for overcoming the disadvantages of PC61BM, in the meanwhile remaining the properties of high electron mobility of PC61BM. The performance of N2200-based PSCs were investigated in detail. The results demonstrate that the incorporation of N2200 can efficiently inhibit the aggregation of PC61BM on the perovskite, promote perovskite surface grain growth and passivate the perovskite defect, simultaneously achieve better energy level alignment. Benefiting from the synergies of above advantages, the N2200-basd PSCs exhibit the average PCE of 15.50% with the enhancement of 9.30% relative to the PCE of 14.18% of the reference device. In addition, the hydrophobic of N2200 also improve the air- and illumination-stability of devices. This work provides a simple, effective strategy for achieving highly efficient and stable PSCs and other perovskite based devices.

2.1. Materials Most of the reagents and solvents were purchased from Shanghai Aladdin Biological Technology Co., Ltd, including alcohol (99%), dimethyl formamide (DMF, ≥99.9%), dimethyl sulfoxide (DMSO, ≥99%), chlorobenzene (CB, 99.5%). The other reagents, like poly (3,4ethylenedioxythiophene):poly (styrenesulphonate) (PEDOT:PSS, Clevios PVP AI 4083), CH3NH3I (MAI, 99%), PbI2 (99.99%), phenylC61-butyric acid methyl ester (PC61BM, 99.5%) and 4,7-diphenyl-1,10phenanthroline (Bphen, 99%) were supplied by Xi'an Polymer Light Technology Corp. N2200 was purchased from Shanghai Han Feng Chemical Science and Technology Corp. (99.0%). All materials were used as received, without further purification. 2.2. Device fabrication The patterned indium tin oxide (ITO) glass substrates were cleaned sequentially in detergent, deionized water, absolute ethyl alcohol, acetone and isopropanol. Afterwards, the glass substrates were blown with N2 gas and then treated with a plasma cleaner for 5 min. Next, PEDOT:PSS aqueous solution was filtered by a 0.45 μm polyvinyl difluoride syringe filter and then spin-coated on the glass substrates at 6000 rpm for 30 s, following annealing treatment on a hot plate at 125 °C for 15 min in air. Subsequently, the photoactive perovskite layer was spin-coated on the PEDOT:PSS substrates under nitrogen ambient (O2 ≤ 0.1 ppm; H2O ≤ 0.1 ppm) by one step [25]. Further, N2200 dissolved in chlorobenzene with variable ratio (0, 0.5, 1.0, 1.5, 2.0 mg/ mL) was spun at 3000 rpm for 30 s in the glovebox, following annealing at 100 °C for 5 min. Then PC61BM (20 mg/mL, in chlorobenzene) was spun at 2700 rpm for 30 s, subsequently, Bphen (0.7 mg/mL in anhydrous ethanol) as an interfacial layer was spun at 6000 rpm for 30 s. Finally, 100 nm thickness silver film was deposited by vacuum thermal evaporation. Here, the precursor solution of perovskite MAPbI3 was prepared by dissolving PbI2 and MAI with a molar ratio of 1:1.095 in the mixture solution of DMF and DMSO with a volume ratio of 9:1. During spincoating the perovskite film, sec-butyl alcohol was added rapidly on the rotating perovskite film with a delay time of 7−9s to prompt the fast crystallization of perovskite. The specific delay time mainly depends on the concentration of the perovskite precursor solution. After that, the formed films were transferred on a hot plate, first annealed at 105 °C for 10–20 min in ambient air (real-time humidity of 30∼50%) and then annealed at 105 °C for 5–15 min in DMSO atmosphere. The solvent annealing time depends mainly on the humidity of the surrounding environment. Usually, the lower the humidity, the longer the annealing time. 2.3. Device characterization Film thickness was measured with a stylus profiler (Dektak XT, Bruker). Surface morphology was analyzed using a scanning electron microscopy (SEM, Jeol JSM-7100F), and atomic force microscopy (AFM, Park Systems NX10). Steady-state and transient-state photoluminescence (PL) spectra were measured using a transient fluorescence spectrometer (FLS980, Edinburgh Instruments, E I). X-ray diffraction (XRD) was recorded using a Rigaku D/Max-B X-ray diffractometer. Fourier transform infrared spectrum (FTIR) was recorded by Thermo SCIENTIFIC infrared spectrometer (NICOLET iS10). Contact angle was analyzed using a contact angle tester (AST Optima). Current density-voltage (J-V) characteristics were measured using a Keithley 2400 source meter under a simulated AM 1.5G solar irradiation (100 mW cm−2) with a standard xenon-lamp-based solar simulator (ABET Sun 3000). And solar simulator illumination intensity was calibrated by a silicon reference cell calibrated by National Renewable 293

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Fig. 1. (a) Schematic diagram of structure of the proposed PSCs and molecular structure of N2200 used in this work, (b) corresponding energy levels alignment.

Fig. 2b shows the external quantum efficiency (EQE) spectra and the integrated current density from the measured EQE for the reference and the optimized PSCs. From Fig. 2b, it can be seen that the EQE of N2200 PSC are superior to that of the reference over the entire spectrum region. The corresponding integrated current densities are 18.43 mA cm−2 and 20.37 mA cm−2, respectively, which are slightly lower than the test JSC of 19.19 mA cm−2 and 20.69 mA cm−2 extracted from the corresponding J-V curves due to the fact that the light spot size is slightly smaller than the effective area of the cells for EQE measurements. To further clarify the improvement of the performance of the N2200 PSCs, the forward and reverse scan J-V curves of the reference and the optimal N2200 PSCs were measured, as displayed in Fig. 2c. It can be seen that the reverse and forward scan PCE of the reference PSC are 14.61% and 14.36%, while the reverse and forward scan PCE of the N2200 PSC are 15.90% and 15.66%, respectively. The hysteresis factor is defined by formula (1):

Energy Laboratory (NREL). The active area of devices is defined as 0.04 cm2 by a black mask in J-V measurement. External quantum efficiency (EQE) was measured using a power source (Zolix Sirius-SS) with monochromator (Zolix Omni-λ). All measurements were performed in air environment without any encapsulation.

3. Results and discussion In order to study the effect of N2200 interface layer, the inverted planar PSCs were fabricated with the architecture of ITO/PEDOT:PSS/ MAPbI3/N2200/PC61BM/Bphen/Ag, as shown in Fig. 1a. The corresponding energy levels alignment present in Fig. 1b. Here, PEDOT:PSS deposited on ITO anode serves as hole transport layer (HTL). The MAPbI3 is used as photoactive layer. PC61BM and Bphen work as ETL and cathode buffer layer, respectively. In particular, a thin layer of polymer N2200 is employed to form double ETLs for making up the shortage of PC61BM. The molecular structure of N2200 is shown in Fig. 1a as well. The Ag film serves as cathode. The photovoltaic parameters of PSCs made with different concentration of N2200 were summarized in Table 1, where the performance parameters of reference devices without N2200 were also listed for a comparison. The statistical distributions of photovoltaic parameters as a function of N2200 concentrations were shown in Fig. S1 (support information). Fig. 2a presents the J-V curves of the best PSCs for each concentration of N2200. Fig. S2 display the histograms of PCE measured for thirty reference and optimized devices, respectively. One can find from all these investigations that N2200 devices exhibit better photovoltaic performance than the reference device. Under simulated one sun illumination, the reference devices have an average open circuit voltage (Voc) of 0.956 V, an average short-circuit current density (Jsc) of 19.35 mA cm−2, and an average fill factor (FF) of 76.67% and then an average PCE of 14.18%. While N2200-based PSCs have slightly higher Voc and obviously higher Jsc and FF, and thus exhibit better PCE. Particularly, when N2200 reaches an optimal concentration of 1.0 mg/ mL, the corresponding PSCs obtain an average Voc of 0.974 V, an average Jsc of 20.42 mA cm−2 and an average FF of 77.97%, and then an average PCE of 15.50%, which is increased by 9.30% compared with reference one. But as the concentration of N2200 further increase, the performance of the PSCs gradually decrease. The reason could be the excess N2200 hindering electron transport and reducing electron collection in the cathode due to the lower electron mobility of N2200 than that of PC61BM.

Hysteresis index =

PCEreverse − PCEforward PCEreverse

(1)

The hysteresis index of the reference and N2200 PSCs are 0.017 and 0.015, respectively. The slightly lower hysteresis index of the N2200 device indicates the weaker photocurrent hysteresis, which could be the reason of better passivation effect of N2200 on perovskite layer than PCBM. In order to further prove the reliability of the J-V measurement, we performed the steady-state output measurement of Jsc and PCE for the PSCs at maximum power output point with a fixed bias voltage of 0.82 V under continuous light illumination of 400 s. The N2200 PSC exhibits a continuous and stable output with the steady-state PCE of 16.26%, being higher than 14.72% PCE of the reference one, which is well consistent with the results of J-V measurement. In order to clarify the role of N2200 in PSCs, we conducted a series of effective measurements for MAPbI3 active layer deposited on ITO/ PEDOT:PSS substrate with and without N2200 coating. Fig. 3a shows the absorption spectra of pristine perovskite film and perovskite/N2200 film. It can be seen that the absorption intensity of the perovskite at 550–800 nm is slightly enhanced after depositing N2200. The enhancement of absorption of photosensitive layer means produce more electron-hole pairs and then contribute to the enhanced photocurrent. Fig. S3 shows the absorption enhancement spectra of perovskite/N2200 film relative to pristine perovskite film, which was not consistent with the feature of absorption spectrum of N2200, indicating that the absorption enhancement of perovskite/N2200 film not mainly originate

Table 1 Performance parameters of PSCs made with different concentration N2200, the average parameters were obtained based on thirty devices for each kind. Concentration (mg/mL) 0 0.5 1.0 1.5 2.0

Voc (V) 0.956 0.967 0.974 0.971 0.967

± ± ± ± ±

0.015 0.012 0.009 0.010 0.011

Jsc (mA/cm2)

FF (%)

19.35 19.83 20.42 20.07 19.73

76.67 77.75 77.97 77.77 77.33

± ± ± ± ±

0.53 0.42 0.31 0.37 0.40

294

± ± ± ± ±

PCE (average, %) 1.74 1.10 0.93 1.27 1.03

14.18 14.90 15.50 15.16 14.74

± ± ± ± ±

0.31 0.35 0.28 0.35 0.24

Rs (Ω) 4.70 3.71 3.50 3.62 4.03

± ± ± ± ±

PCE (best, %) 2.16 1.19 1.03 1.15 1.68

14.72 15.46 16.26 15.73 15.10

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Fig. 2. (a) J–V curves of PSCs made with different concentration of N2200. (b) EQE spectra and (c) reverse and forward scan J–V curves of the reference and the optimized N2200 PSCs. (d) Steadystate output measurement of Jsc and PCE for the reference and optimized PSCs at a fixed bias voltage of 0.82 V under continuous light illumination.

from the absorption of N2200 film. Fig. 3b displayed the X-ray diffraction (XRD) patterns of the corresponding films. It is found that the main XRD peaks of perovskite MAPbI3 located at 14.20°, 28.60° are obviously strengthened after adding N2200, indicating that N2200 can effectively improve the crystallization of the perovskite film. This conclusion can be further proved by the scanning electron microscopy (SEM) measurement, as shown in Fig. 3c and d. From top-view SEM image, one can see that spin-coating N2200 on perovskite layer induce further growth of perovskite crystal grain and thus the average size of perovskite grains increase from about 400 nm to 440 nm (see Fig. S3). These results can be well understood. After the chlorobenzene solution of N2200 was spin-coated on the perovskite layer, an annealing step was followed at 100 °C for 5 min. During the annealing process, the chlorobenzene vapor atmosphere diffused into the MAPbI3 films, and then dissolved residual DMSO/DMF solvent and helped residual DMSO/ DMF solvent evaporate, and thus promoted the small grains to merge

together to form larger grains, which is well consistent with the reports in literature [26,27]. Therefore, we suggest that the enhanced light absorption of perovskite/N2200 film mainly stem from better crystallization of perovskite. Fig. 4a and b shows the atomic force microscopy (AFM) images of perovskite/PC61BM films with and without N2200 at their interface, respectively. The results indicates that the insertion of N2200 make the average surface roughness of perovskite/PC61BM films decrease to 1.132 nm from 1.527 nm of the pristine film, meaning the surface of PC61BM become smoother or the converage of PC61BM on perovskite layer is improved. It is well known that for PSCs energy level matching is very important to charge extraction. The surface potential energy of perovskite/PC61BM with and without N2200 were measured by the Kelvin probe force microscopy (KPFM), and the corresponding KPFM mapping were displayed in Fig. 4c and d, respectively. It is obtained that the average surface potential energy of perovskite/N2200/PC61BM

Fig. 3. (a) Absorption spectra, (b) XRD patterns of perovskite film and perovskite/N2200 film with optimized concentration of N2200. (c) and (d) SEM images of perovskite film and perovskite/N2200 film with optimized concentration of N2200, respectively. 295

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Fig. 4. (a) and (b) AFM images, (c) and (d) surface potential mapping of perovskite/PC61BM, (a) and (c) without N2200, (b) and (d) with N2200.

film is −145.6 mV, which is obviously higher than that of perovskite/ PC61BM film (−223.9 mV). The increase of surface potential reflects the Fermi level is shifted toward the vacuum level or the corresponding work function become lower [28]. This measurement result is consistent with the LUMO energy level alignment displayed in Fig. 1b. One can see from Fig. 1b that there is an energy loss value of 0.2 eV for electron transfer from perovskite to PC61BM ETL. After spin-coating the N2200, the energy offset is reduced due to the more matching LUMO energy level of N2200 with perovskite. The higher LUMO of N2200 could induce an enhanced built-in electric field in PSCs, which could be the reason of the slightly increased Voc for N2200-based PSCs. Moreover, the enhancement of built-in electric field also is more beneficial to electron transfer and extraction, and thus reduce the recombination probability of electrons and holes, and then contribute to the enhanced Jsc and FF of N2200-based PSCs. Fig. 5a and b presents the steady state PL spectra and the timeresolved PL decay spectra for four kinds of films of MAPbI3, MAPbI3/ PC61BM, MAPbI3/N2200 and MAPbI3/N2200/PC61BM deposited on

ITO/PEDOT:PSS substrate. It is observed from Fig. 5a that relative to the pristine MAPbI3 film, the intensity of PL peak of MAPbI3 is greatly quenched by PC61BM or N2200 coating due to the electron extraction from perovskite layer to ETL. It is worth noting that the addition of N2200 at MAPbI3/PC61BM interface result in a larger scale PL quenching, meaning more effective electron extraction for MAPbI3/ N2200/PC61BM interface, which could originate from better trap-state passivation effect of N2200 on the perovskite than PCBM and alleviated aggregation of PCBM (see AFM), in addition more matching energy level alignment. Time-resolved PL decay spectra reflected in Fig. 5b were fitted by a bi-exponential decay function of I(t) = A1exp (-t/τ1) +A2exp (-t/τ2), where τ1 and τ2 are fast and slow decay lifetimes, A1 and A2 are corresponding weight fractions, respectively [15,29]. The fitted results are summarized in Table S1. One can observe that the charge carrier lifetimes of MAPbI3/PC61BM or MAPbI3/N2200 films is much shorter than that of pristine perovskite film, while that of MAPbI3/N2200/PC61BM is further shortened by the insertion of N2200. All of these further confirm the fact that N2200 insert layer improve the

Fig. 5. (a) Steady-state PL spectra and (b) time-resolved PL spectra of perovskite, perovskite/PC61BM, perovskite/N2200, perovskite/N2200/PC61BM films, respectively. 296

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Fig. 6. (a) Electrochemical impedance spectroscopy (EIS) and (b) J-V curves under dark condition for the reference and optimized PSC made with the optimized concentration of N2200. (c) J-V curves of single-electron devices with the structures of ITO/ TiO2/perovskite/N2200/PC61BM/Ag as the experimental device and ITO/TiO2/perovskite/PC61BM/ Ag as reference device.

contact between perovskite layer and ETL and thus increase electron transfer and extraction. Electrochemical impedance spectroscopy (EIS) is usually used to characterize the charge transfer and recombination properties in PSCs by the analysis of some performance parameters including series resistance (Rs), charge transmission resistance (Rtr), recombination resistance (Rrec) etc. [16,17,30,31]. This measurement was operated at the bias voltage of 0.8 V (near the Voc) in the frequency range of 0.1 Hz∼1 MHz under dark state. The measured Nyquist spectra for the reference PSC and the proposed PSC made with 1.0 mg/mL N2200 are shown in Fig. 6a. It can be observed that the Nyquist curves of two kind of devices are quite different. The fitted electrical property data using the equivalent circuit shown in the inset of Fig. 6a for each kind of device are summarized in Table S2. The results indicate that the Rs and Rtr decrease from 8.97 Ω to 4.27 Ω and from 865.3 Ω to 805.5 Ω, respectively, in the meanwhile, the Rrec increases from 2962 Ω to 4221 Ω after the addition of N2200, which reflect the enhanced charge transfer and reduced charge recombination loss within N2200 PSCs. It is well known that the dark J-V characteristics at the low voltage are primarily determined by the shunt resistance Rsh and that at high voltages by Rs [18,32]. It can be clearly seen from Fig. 6b that the leakage current of PSC at the low voltage region is obviously reduced after adding N2200 and the curve slope at high voltages region become steeper, meaning a higher shunt resistance Rsh and a lower series resistance Rs. All of these results are well consistent with the conclusion of the electrochemical impedance spectrum shown in Fig. 6a. Further, the single-electron devices with the structure of the ITO/TiO2/MAPbI3/N2200/PC61BM/Ag was fabricated, as shown in the set of Fig. 6c. The reference singleelectron device without N2200 in MAPbI3/PC61BM interface also was made for a comparison. Their J-V curves were displayed in Fig. 6c. According to the trap-filled limited current (TFLSCLC) method, one can find an onset voltage from the linear J-V region with an ohmic response (A line in Fig. 6c) to the trap-filled limit (TFL) region with square-law (B line in Fig. 6c), which is termed as the trap-filled limit voltage (VTFL). The VTFL values of 0.343 V and 0.247 V were obtained for the singleelectron device without and with N2200, respectively. Trap-state density (nt) can be calculated by VTFL as follows [17]:

VTFL =

eNtrap L2 2ε0 ε

(2)

Where L is the thickness of perovskite film (about 600 nm), ε is relative dielectric constant of MAPbI3 (ε = 32), ε0 represents vacuum permittivity, and e is electron charge. The result indicates that the trapstate density is reduced from 3.38 × 1015 cm−3 of the pristine MAPbI3/ PC61BM film to 2.43 × 1015 cm−3 of MAPbI3/N2200/PC61BM film. In addition, the electron mobility in electron-only device was estimated by the space charge limited current (SCLC) method. The current-voltage of trap-free SCLC regime (C line in Fig. 6c) can be well fitted by the MottGurney law [20,33]:

J=

9ε0 εμe Vb2 8L3

(3)

We found that the addition of N2200 make the electron mobility enhance from the 9.52 × 10−5 to 9.86 × 10−4 cm2 V−1 s−1. All of these more directly proves that N2200/PC61BM as ETL is more excellent than only PC61BM in passivating the defect of perovskite layer and forming good contact between perovskite and ETL, and thereby increase electron transfer and extraction, which is consistent with the above PL measurement. Based on the abovementioned effective investigation, the incorporation of N2200 into PSCs produce several beneficial effects: (1) promoting grain growth of perovskite surface and passivating the perovskite defect; (2) efficiently inhibiting the aggregation of PC61BM and then increasing the converage of PC61BM on the perovskite; (3) improving the energy level alignment at perovskite/ETL interface and then increasing the built-in potential within PSCs. All these contribute to the improvement of the electron transfer and extraction ability and thus the enhancement of PCE and the reduction of J–V hysteresis for N2200 PSCs. Stability is a common issue for PSCs and thus limits their applications [21,34]. Next, we explore the effect of N2200 on the stability of PSCs. It is known that metal halide perovskites are very sensitive to moisture, oxygen and light etc. Fig. 7a and b shows hydrophobicity tests for perovskite film with and without N2200 coating, respectively. One can observed that the water contact angle of perovskite/N2200 film is about 101°, which far more than that of bare perovskite film 297

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Fig. 7. (a) and (b) Water contact angle measurement of perovskite film without and with N2200, respectively. PCEs attenuation as function of storage times for the reference and the PSCs without unencapsulation stored at room temperature (c) in air with the humidity of about 30%–50% and (d) in nitrogen environment with the alternant of 10 h dark and 14 h illumination (under fluorescent lamps of about 1000 lux), respectively. All measuements were carried out in air environment (about 30% RH).

lux), as shown in Fig. 7d. After 30 days of storage, the PCE of reference device dropped from 14.72% to 10.47%, retaining approximately 71.19% of initial value. While the PCE of N2200 device dropped from 16.27% to 14.22%, retaining approximately 87.40% of initial value. Corresponding surface morphology evolution of Ag cathode after the storage of 30 days are reflected in the pictures i-k (reference PSC) and jn (N2200 PSC) inserted in Fig. 7d. It is clear that the surface of Ag electrode of reference PSCs had begun degradation due to partial decomposition of perovskite. While the surface of Ag electrode of the N2200 PSC was still in a good condition, implying the better illumination stability of N2200 PSCs. To confirm the effect mechanism of N2200 on the performance of PSCs, we conducted Fourier transform infrared (FTIR) spectroscopy measurements on perovskite, N2200, and perovskite/N2200 films on the glass substrates, respectively. For the neat N2200 film (see Fig. 8), the peak 1706 cm−1 is assigned to the stretching vibrations of C]O bonds (ν(C]O)). To distinguish differences between N2200 and perovskite/N2200 easily, we normalized their FTIR maximum peak (1665 cm−1) assigned to stretching vibrations of C]C bonds (aromatic nucleus), which is thus insensitive to Pb ions. One can find that for FTIR spectroscopy of the perovskite/N2200 films the vibration of ν(C]O) is strengthened by 64.70%, confirming the presence of interaction between perovskite and C]O of N2200, which is consistent with the report of literature [22,35].

Fig. 8. FTIR spectra of the perovskite, N2200 and perovskite/N2200 films.

(approximately 40°). This indicates that the hydrophobic N2200 could protect the perovskite layer from the erosion of moisture and oxygen and thereby improve the stability of PSCs, which is demonstrated by tracking the PCE evolution of unencapsulated PSC with and without N2200 stored at room temperature in air with the humidity is about 30%–50%, as shown in Fig. 7c. It is found that the PCE of reference device dropped from 14.62% to 5.70%, retaining approximately 38.98% of initial value. While the PCE of N2200 device dropped from 16.07% to 9.61%, retaining approximately of 59.80% of initial value. Correspondingly, one can see from the pictures e-g (reference PSC) and f-h (N2200 PSC) shown in set of Fig. 7c that the surface of Ag electrode of reference PSC was corroded more seriously compared with that of N2200 PSC. This indicates that the perovskite film could be decomposed more severely. This can be confirmed by the XRD evolution of perovskite film with and without N2200 after 30 day storage, as shown in Fig. S5. For the degraded perovskite, one can observe an additional diffraction peak at 12.44°, which reflect the appearance of PbI2. But the diffraction peak of PbI2 for N2200 coated perovskite film is weaker than that of bare perovskite film. This imply that the hydrophobicity and Levis acid nature of N2200 are favorable for hindering the intrusion of water and oxygen into perovskite layer and suppressing the degradation of MAPbI3. In order to investigate light soaking effect on PSC, the evolution of PCE of PSCs with and without N2200 stored in the glove box (humidity and oxygen content are controlled below 0.01 ppm) were investigated during 30 days at room temperature environment with the alternant of 10 h dark and 14 h illumination (under fluorescent lamps of about 1000

4. Conclusions In conclusion, we demonstrated higher performance PSCs with N2200/PC61BM as double ETL compared with the traditional PSC with PC61BM as ETL. The traditional PSCs have an average PCE of 14.18% and a maximum PCE 14.72 %with Voc of 0.974 V, an Jsc of 19.51 mA cm−2, and a FF of 77.43%. While the optimized PSCs made with 1.0 mg/mL N2200 yielded an average PCE of 15.50% and a maximum PCE of 16.26% with a Voc of 0.986 V, an Jsc of 20.69 mA cm−2, and a FF of 79.69%, which obviously surpass those of traditional devices. The enhanced performance of N2200 devices can be attributed to (1) N2200 promoting perovskite surface grain growth and passivating the perovskite defect; (2) efficiently inhibiting the aggregation of PC61BM and then increasing the converage of PC61BM on the perovskite; (3) improving the energy level alignment at perovskite/ 298

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ETL interface and then increasing the built-in potential within PSCs. Moreover, the addition of N2200 improve the air- and illuminationstability. All these could stem from the presence of interaction between PbI2 as Lewis acid and of N2200 as Lewis base. This work indicate that the combination of N2200 and PC61BM as double ETLs is a feasible and effective way to improve the photovoltaic performance of the inverted planar heterojunction PSCs.

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Acknowledgements

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This research work was supported by NSFC-Shanxi Joint Foundation Project of Shanxi Coal-Based Low-Carbon (U1710115, U1810204), Key Research and Development (International Cooperation) Program of Shanxi (201603D421042), Platform and Base Special Project of Shanxi (201605D131038), and National Natural Scientific Foundation program of China (61274056, 61571317, and 61475109).

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Appendix A. Supplementary data

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Supplementary data to this article can be found online at https:// doi.org/10.1016/j.orgel.2019.03.033.

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