High-Performance Perovskite Solar Cells with a Non-doped Small

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Letter

High Performance Perovskite Solar Cells with a NonDoped Small Molecule Hole Transporting Layer Yang Li, Marcus D. Cole, Yige Gao, Todd Emrick, Zheng Xu, Yao Liu, and Thomas P. Russell ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00164 • Publication Date (Web): 22 Feb 2019 Downloaded from http://pubs.acs.org on February 22, 2019

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ACS Applied Energy Materials

High Performance Perovskite Solar Cells with a Non-Doped Small Molecule Hole Transporting Layer Yang Li1,3, Marcus D. Cole3, Yige Gao3, Todd Emrick3, Zheng Xu1*, Yao Liu2*, and Thomas P. Russell2,3*

1

Key Laboratory of Luminescence and Optical Information (Beijing Jiaotong University),

Ministry of Education, Beijing, 100044, China

2

Beijing Advanced Innovation Center for Soft Matter, Science and Engineering, Beijing

University of Chemical Technology, Beijing, 100029, China

3

Polymer Science and Engineering Department, University of Massachusetts Amherst,

120 Governors Drive, Amherst, MA 01003, USA

ACS Paragon Plus Environment

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* Authors to whom correspondence should be addressed. Email: [email protected], [email protected], [email protected]

Keywords: Inverted perovskite solar cells, hole transporting layer, small molecule, NPB, dopant-free, long-term stability

Abstract: The use of solution-processed hole transporting layers (HTLs), without the need of added dopants, is important for the advancement of high performance perovskite solar cells

(PSCs).

Here,

a

small

molecule,

N,N’-bis(naphthalen-1-yl)-N,N’-

bis(phenyl)benzidine (NPB), is utilized as an efficient, dopant-free HTL in inverted PSCs to replace the commonly used poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS). A best power conversion efficiency (PCE) of 19.96% is realized when using NPB as the HTL, along with a short circuit current density (JSC) of 22.92 mA/cm2, an open circuit voltage (VOC) of 1.11 V, and a fill factor (FF) of 78.4%. This VOC is one of the highest obtained values in CH3NH3PbI3-based inverted PSCs, and is comparable to devices with regular structure. The use of NPB as the HTL produced perovskite films with larger grain size and higher crystallinity than obtained with


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PEDOT:PSS. As a result of this improved perovskite film quality, recombination loss decreased dramatically, boosting JSC values. Furthermore, due to reduced recombination and better energy level alignment arising from the use of NPB, the VOC in the NPBcontaining device increased considerably. In addition, NPB-based PSCs exhibited longer term stability as a result of the hydrophobicity of NPB.

Organometallic trihalide perovskite solar cells (PSCs) are potential candidates for next-generation photovoltaic technology due to their superior optoelectronic properties including strong light absorption across the visible spectrum, long carrier diffusion length, tunable band gap, low cost, and solution processability1-3. PSCs have now reached a certified power conversion efficiency (PCE) value of 23.7%4. PSCs with a mesoporous metal oxide electron transporting layer (ETL) are highly efficient, but the high-temperature treatment (>450 °C) associated with their processing hinders the development of low-cost, flexible devices2, 5. In contrast, a simple planar structure of perovskite devices based on either a normal (n-i-p) or inverted (p-i-n) configuration has


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been developed6, 7. In particular, the inverted planar PSCs are attractive for their negligible hysteresis and simple device fabrication8, 9. In inverted PSC fabrication, the perovskite film is placed on top of a hole transporting layer (HTL), which typically consists of poly(3,4ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS)8-11. However, detrimental effects of the acidity and hydrophilicity of PEDOT:PSS on device performance and stability are now widely appreciated12. Furthermore, because of the low work function of PEDOT:PSS, inverted perovskite devices usually suffer from significant, unfavorable energy loss and thus their open circuit voltage (VOC) is typically smaller than PSCs fabricated with normal device geometry2, 9. Recently, efforts have been devoted to using HTLs which further increase the PCE of inverted PSCs. P-type inorganic semiconductors, such as copper compounds13, 14, nickel oxide (NiO)15, and vanadium oxide (V2O5)16, have been examined as HTLs in inverted PSCs due to their chemical stability, high mobility, and low cost. However, some inorganic HTLs require electrodeposition (copper thiocyanate, CuSCN)14 or chemical reaction (copper oxides, Cu2O)13 and others (V2O5 and NiO) require high-temperature annealing to maximize


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device efficiency15, 16; these methods are more complicated and less environmentally friendly than PEDOT:PSS. Polymeric HTLs, as alternatives to PEDOT:PSS, have been studied to simplify film preparation and enhance device performance. For example, poly(N,N’-bis(4-butylphenyl)-N,N’-bis(phenyl)benzidine) (poly-TPD) and poly(bis(4phenyl)(2,4,6-trimethylphenyl)amine) (PTAA) were developed as efficient HTLs in inverted PSCs, where PCEs of 19.1%17 and 21.0%3 were realized, respectively. However, poly-TPD and PTAA are more expensive than PEDOT:PSS and most inorganic HTLs. Moreover, added dopants, such as 2,3,5,6-tetrafluoro-7,7,8,8tetracyanoquinodimethane (F4-TCNQ), are usually required to obtain desirable charge transport characteristics of these polymeric HTLs3, 17, which complicates the fabrication process. Compared to inorganic and polymeric HTLs, organic small molecules are promising for their well-defined structures, structural versatility, easy purification, and low cost18-20. For example, 1,4′-bis(4-(di-p-toyl)aminostyryl)benzene (TPASB)18, 4,4′cyclohexylidenebis[N,N-bis(4-methylphenyl) benzenamine] (TAPC)19, and TruxOMeTAD (a C3h Truxene core with arylamine terminal groups and n-hexyl side-chains)20


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have been reported as efficient, dopant-free HTLs in inverted PSCs with PCEs > 17%.

N,N’-bis(naphthalen-1-yl)-N,N’-bis(phenyl)benzidine (NPB), shown in Figure 1a, consists of triphenylamine (TPA) units and a highly π-conjugated structure, which has been used as interlayers in organic light emitting diodes (OLEDs)21. In addition to the above advantages of small molecule HTLs, the high hole mobility of NPB (reportedly (69)×10-4 cm2/Vs)22 facilitates efficient hole extraction and transport from perovskites. The highest occupied molecular orbital (HOMO) of NPB (5.20 eV) is situated between the work function of indium tin oxide (ITO) and the valence band (VB) of CH3NH3PbI3, implying that the potential energy loss at the interface could be smaller than PEDOT:PSS. Furthermore, the transmission spectra shown in Figure 1b indicates negligible absorption of NPB across the visible spectrum, except from 300 to 420 nm, which is beneficial for light harvesting of the perovskite layer. The superior electrical and optical properties of NPB underscore its potential as an HTL for inverted PSCs. This paper decribes the outstanding performance of inverted PSCs in which NPB was implemented as an efficient, dopant-free HTL. Devices with NPB as the interlayer produced a best PCE of 19.96% with a short circuit current density (JSC) of 22.92 mA/cm2,


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a VOC of 1.11 V, and a fill factor (FF) of 78.4%. A remarkably high VOC was achieved using NPB HTL, which is important for highly efficient inverted PSCs. This VOC of 1.11 V is among the highest values in CH3NH3PbI3-based inverted PSCs, and comparable to the normal geometry devices. Devices utilizing the NPB HTL also demonstrated superior long-term stability due to the hydrophobic of NPB. Morphological characterization confirmed that NPB-coated substrates afforded a pinhole-free perovskite film with larger grain size and better crystallinity than those possessing a PEDOT:PSS layer. The insertion of the NPB layer effectively reduced recombination losses, as measured by electrochemical impedance spectroscopy (EIS). Moreover, NPB layer more efficiently extracts and transports charge from perovskites than does PEDOT:PSS, leading to a significantly improved short circuit current density (JSC). Additionally, NPB affords a higher built-in potential in the devices, which is beneficial to reduce recombination loss and enhance VOC.


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Figure 1. (a) Chemical structure of NPB; (b) transmission spectra of ITO/NPB with ITO glass as the reference; (c) inverted PSC device structure; (d) schematic energy level diagram of the fabricated device. Inverted PSCs with a device structure consisting of ITO/HTL/CH3NH3PbI3/[6,6]-phenyl C61 butyric acid methyl ester (PC61BM)/perylene diimide-based polyelectrolyte (PDIBr)/silver (Ag) was fabricated as shown in Figure 1c. Previously, we showed that PDIbased interlayers preferentially modify the work function of Ag, facilitating electron transport and enhancing device performance in organic solar cells23. Based on those


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results, PDI-Br (shown in Figure S1) was selected as an interfacial modification layer at the silver electrode in this work. PEDOT:PSS or NPB was spin-coated onto ITO substrates to serve as the HTL; for NPB, a solution in a mixture of chlorobenzene and tetrahydrofuran was employed, and the morphological characteristics (Figure S2) indicated NPB had good coverage on ITO substrate. The perovskite precursor, a 1:1 molar ratio of lead iodide (PbI2) and methylammonium iodide (MAI), was applied by spincoating onto the as-cast ITO/HTL substrates, and chlorobenzene was dripped onto the substrate during the spin-coating process. After annealing the perovskite film, PC61BM was applied as the ETL by spin-coating from a chlorobenzene solution. Finally, a 100 nm thick Ag electrode was deposited by thermal evaporation. As shown in Figure 1d, the HOMO of NPB is closer to the VB of CH3NH3PbI3 than is the work function of PEDOT:PSS, which reduces the potential energy loss at the HTL/CH3NH3PbI3 interface and increases device VOC. Meanwhile, the lowest unoccupied molecular orbital (LUMO) of NPB is much higher than the conduction band (CB) of CH3NH3PbI3, which would block electron transport to the anode and decrease electron-hole recombination at the anode.


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Figure 2. (a) Current density–voltage (J-V) characteristics of solar cells based on PEDOT:PSS and NPB HTLs; (b) 459 hour stability measurements of unencapsulated devices (four devices of each type) stored in an inert atmosphere glove box; (c) normalized current density output of devices employing PEDOT:PSS vs NPB as a


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function of time under maximum power point (MPP) tracking tests; (d) J-V characteristics of the best PSC measured under reverse scan; (e) corresponding external quantum efficiency (EQE) profile of the best device; (f) PCE histogram of 40 devices containing NPB as the HTL (measured under reverse scan). The current density-voltage (J-V) characteristics of the devices with PEDOT:PSS or NPB as the HTL under 100 mW/cm2 illumination (AM1.5G) are shown in Figure 2a. Combining the key photovoltaic parameters listed in Table S2, PSCs utilizing a conventional PEDOT:PSS HTL gave a maximum PCE of 14.35% under reverse scan. When PEDOT:PSS was replaced with NPB, a significant improvement in the photovoltaic performance was observed, with the PCE reaching 19.33%. Compared to devices containing a PEDOT:PSS HTL, the NPB interlayer boosts the JSC and VOC significantly. Meanwhile, devices containing the NPB interlayer exhibited lower hysteresis than those with PEDOT:PSS. The dependence of device performance on NPB thickness was probed by using NPB solutions with different concentrations (shown in Figure S3 and Table S3), noting a plateau in PCE at a thickness of ~27 nm. A thin NPB layer apparently provides insufficient hole extraction and transport, while thick NPB films likely increase series


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resistance. Long-term device stability was tested by storing four separate devices of each type (PEDOT:PSS and NPB) in the inert atmosphere glove box without encapsulation, as presented in Figure 2b. The devices with NPB HTLs showed superior stability over those containing PEDOT:PSS interlayers. For example, the NPB devices maintained ~92% of their initial PCE after 459 hours, whereas the PCE of the devices with a PEDOT:PSS HTL decreased below ~52% over the same timeframe. Longer time scale stability measurements (Figure S4) show that NPB containing device retained a PCE of 9.22% after 126 days in golve box and 7 days in ambient air while the efficiency of the PEDOT:PSS containing device degraded completely. Since the only difference between these two types of PSCs is the HTL, these results confirm that the NPB layer effectively suppresses PSC degradation. The results of maximum power point (MPP) tracking tests, shown in Figure 2c, also indicate that PSCs with NPB interlayers produce more stable current density output at MPP relative to devices containing PEDOT:PSS HTL (Vmax = 0.79 V for PEDOT:PSS-based device; Vmax = 0.86 V for NPB-based device). As shown in Figure 2d, even without dopants in the NPB layer, a best PCE of 19.96% was achieved, along with a JSC of 22.92 mA/cm2, a VOC of 1.11 V, a FF of 78.4%. Figure 2e shows the


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corresponding external quantum efficiency (EQE) profile, where broad photoresponses with high values were obtained and the calculated current density (Jcal) based on EQE is 21.51 mA/cm2. EQE profile for the control device based on PEDOT:PSS was shown in Figure S5, and Jcal for the PEDOT:PSS device is 18.42 mA/cm2. A comparison between

JSC and Jcal was made, as shown in Table S4. The relatively low Jcal arises primarily from the lack of an EQE profile from 300 to 400 nm, which could not be obtained due to instrumental limitations. Figure 2f shows the PCE distribution histograms of 40 devices with NPB HTL, the NPB devices producing an average PCE of 18.86% with a fairly narrow distribution, suggesting reliable and reproducible device performance when using NPB as the HTL.


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Figure 3. Top-view scanning electron microscopy (SEM) images of perovskite film on (a) ITO/PEDOT:PSS and (b) ITO/NPB (scale bar are 1 μm); (c) and (d) are the grain size distributions of the perovskite films; (e) X-ray diffraction (XRD) patterns and (f) UVvis absorption spectra of perovskite films on ITO/PEDOT:PSS and ITO/NPB substrates.


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To understand the significant improvement in device performance using NPB interlayers, the as-prepared perovskite films of the type used in the PSCs were characterized further. Figure 3a and b show top-view scanning electron microscopy (SEM) images of perovskite films fabricated on ITO/PEDOT:PSS and ITO/NPB substrates; the corresponding grain size distributions are shown in Figure 3d and e. The perovskite films fully covered both PEDOT:PSS and NPB, without evidence of pinholes, but with NPB as the underlying material, larger perovskite grains (average grain size ~300 nm) were produced relative to PEDOT:PSS (~200 nm). A larger grain size of the perovskite on NPB is also seen in the cross-sectional SEM images shown in Figure S6. The lower grain boundary density for the NPB case corresponds to fewer grain boundary defects, which benefits device performance. X-ray diffraction (XRD) patterns of the perovskite films on different HTLs were then recorded, as shown in Figure 3e, with no significant change in diffraction for the different samples, indicating similar crystal orientation of the perovskite films, irrespective of the interlayer composition. However, the full width at half maximum (FWHM) varied significantly, as shown in Table S5, in which the perovskite film fabricated on NPB exhibited narrower peaks relative to


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PEDOT:PSS, indicating greater perovskite crystallinity for the NPB interlayer. Notably, the signal at 12.7°, associated with PbI2, is suppressed significantly in the perovskite film deposited on NPB, which indicates a more complete reaction between PbI2 and MAI during the formation of CH3NH3PbI3 crystallites19. It is reported that PEDOT:PSS is partly dissolved by the solvent used in perovskite fabrication, which damages the perovskite film24. Meanwhile, the pH value, as well as the wetting properties of the selected HTLs, also affect the quality of perovskite crystallinity25, 26. Therefore, NPB’s insolubility in the perovskite precursor solvent and pH neutrality (Figure S7), as well as the hydrophobicity (Figure S9) all combine to promote the complete formation and enhanced crystallinity of CH3NH3PbI3. As shown in Figure 3f, perovskite films associated with either HTL absorb as anticipated, with NPB affording a slightly stronger absorption from 575 to 850 nm.


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Figure 4. (a) Nyquist plot, and (b) plot of ln(JSC-J) versus (V+RSJ) and the linear fitting curves of PSCs with a PEDOT:PSS vs NPB HTL; (c) time-resolved photoluminescence (TRPL) decay curves of perovskite film on glass/PEDOT:PSS or glass/NPB substrate; (d) Mott-Schottky plot of PSCs with PEDOT:PSS or NPB HTL. Electrochemical impedance spectroscopy (EIS) was performed to understand recombination losses and charge transport properties in devices with different HTLs. Impedance measurements were carried out at 20 mV AC voltage and a scanning


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frequency range of 40 Hz - 1 MHz under AM1.5G illumination. The resultant Nyquist plots are shown in Figure 4a, in which each type of device produced two semicircles associated with different charge transport regimes. The equivalent circuit model composed of the series resistance (Rs), recombination resistance (Rrec), and charge transfer resistance (Rct) is shown in Figure S827. The Nyquist plots can be divided into the high frequency semicircle (closest to the origin), predominately reflecting the electronic transport and recombination kinetics, and the low frequency regime arising from slow ion relaxation/diffusion28. The parameters of the equivalent circuit with PEDOT:PSS and NPB HTL are listed in Table S6. The NPB-containing device shows a higher Rrec value relative to the device using PEDOT:PSS, which helps suppress recombination loss. Since the interface of perovskite/ETL is identical in both devices, the Rct can be ascribed to the HTL/perovskite or ITO/HTL interface. As a result of using NPB as the HTL, device resistance (Rct) decreased significantly, allowing more efficient charge extraction and transport from the perovskite film to ITO. To further study recombination loss, we calculated the reverse saturation current density (J0), assuming that the planar-structured PSCs reflect a single heterojunction


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diode29. In heterojunction solar cells, J0 correlates with carrier recombination, and a lower J0 value corresponds to less recombination loss. As shown in Figure 4b, J0 in the PSC with the NPB HTL is ~2 orders of magnitude lower than the PSC containing PEDOT:PSS HTL. Therefore, the recombination loss in inverted PSCs is greatly reduced when using NPB, in agreement with EIS results. Note that the plots shown in Figure 4b were extracted from the J-V curves in Figure 2a (reverse scan). Time-resolved photoluminescence (TRPL) measurements, presented in Figure 4c, were also performed to characterize the hole-extraction efficiency at the HTL/perovskite interface. From biexponential fitting of the dynamic TRPL data, the lifetime of the carriers was obtained and is summarized in Table S7. The results show that the perovskite film fabricated on NPB exhibits a shorter lifetime of 78.33 ns, whereas the lifetime of perovskite film on PEDOT:PSS is 105.64 ns. The shorter carrier lifetime for the NPB indicates more efficient hole extraction from perovskite layer into the NPB layer. In addition, the hole mobility of PEDOT:PSS and NPB were characterized. The results shown in Figure S9 indicate a higher mobility in NPB relative to PEDOT:PSS, which means that NPB affords more efficient carrier transport.


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Based on these results, the NPB HTL promotes a higher quality perovskite film with larger grain size and better crystallinity, thus significantly reducing carrier recombination losses in the device. Moreover, the NPB HTL affords more efficient hole extraction and transport from the perovskite layer to the HTL. All of these advantages of the NPB interlayer contributed to the outstanding enhancement of crucial device parameters, such as JSC and FF. At the same time, we note that the increase of VOC in these NPBcontaining devices is remarkable. Since the primary difference in these PSCs is the HTL, we speculate that the origins of the improved VOC also correlate with the properties of the HTL itself and the HTL/perovskite interface. Mott-Schottky analysis of HTL/perovskite heterojunction further illustrated the effect of 1

different HTLs on VOC of inverted PSCs. The Mott-Schottky model is described as 𝐶2 = 2 𝑒𝜀𝑆𝑁(𝑉𝑏𝑖

―𝑉 ―

2𝐾𝐵𝑇 𝑒

), where 𝜀𝑆 is the dielectric constant of CH3NH3PbI3, N is the doping

density, 𝑉𝑏𝑖 is the built-in potential, e is elementary charge, V is the applied bias, KB is the Boltzmann constant, and T is absolute temperature30. According to Figure 4d, the value of 𝑉𝑏𝑖 increased from 0.77 V for PEDOT:PSS device to 0.84 V for the PSC with


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NPB HTL, likely due to the lower HOMO of NPB relative to the work function of PEDOT:PSS. Therefore, we conclude that the remarkable VOC achieved in NPB-based PSCs mainly arises from the better energy level alignment between the HOMO of NPB and the VB of CH3NH3PbI3. However, we also noticed that the difference in 𝑉𝑏𝑖 (~0.07 V) between PEDOT:PSS and NPB devices is less than the VOC difference (~0.12 V) (Figure 2a). The recombination loss in solar cells is also important for determining VOC, 𝐴𝐾𝐵𝑇

according to 𝑉𝑂𝐶 = (

𝑒

𝐽𝑆𝐶

)ln ( 𝐽0 +1)29. As discussed above, the NPB-containing devices

produced much lower J0 relative to the PEDOT:PSS case, and a lower J0 yields a larger

VOC in the device. Thus, a combination of energy-level alignment and reduced recombination loss explains the observed VOC enhancement in NPB-containing devices. Apart from the significant improvement of device efficiency, the NPB-containing devices exhibited impressive stability (Figure 2b and c). As shown in Figure S10, contact angle measurements showed that the surface wettability of PEDOT:PSS and NPB differed dramatically: 9.8° for PEDOT:PSS and 74.5° for NPB. The hydrophilicity of PEDOT:PSS has been noted to accelerate device degradation12. Therefore, the


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hydrophobic property of NPB may improve device stability significantly. As shown in Figure S11, PEDOT:PSS induced more rapid perovskite degradation than NPB, irrespective of the perovskite side vs glass side. Thus, we conclude that the large enhancement of device stability is attributed to the hydrophobic property of NPB. In summary, the π-conjugated small molecule NPB was utilized as an efficient, dopant-free HTL in inverted PSCs. A maximum PCE of 19.96% was achieved when using NPB as the HTL, along with a substantially improved stability relative to devices containing a PEDOT:PSS HTL. NPB contact with the perovskite active layer leads to larger grain size and better crystallinity, and significantly reduced recombination loss. Moreover, relative to PEDOT:PSS, NPB exhibits more efficient hole extraction and transport, which further enhances JSC. Due to the reduction of recombination and better energy level alignment with NPB, the VOC values in these NPB containing devices increased dramatically, being comparable to normal devices. In addition, NPB-based PSCs also showed much longer stability, owing in part to the hydrophobicity of NPB. This work highlights the potential of NPB to serve as an efficient, dopant-free HTL in


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inverted PSCs, and represents a simple and effective method to further advance the photovoltaic performance of inverted devices.

ASSOCIATED CONTENT

Supporting Information

Details of experimental section, properties of NPB, chemical structure of PDI-Br interlayer, morphological characteristics of NPB film on an ITO substrate, summary of key parameters for devices with and without NPB, J-V curves of devices based on NPB with different concentration and corresponding devices parameters, stability of PSCs with different HTLs, J-V curve of the device with PEDOT:PSS HTL and corresponding EQE profile, comparison of JSC and Jcal, FWHM values of perovskite films at different diffraction peaks, cross-sectional SEM images, NPB in the solvent of perovskite precursor, comparison of pH value, J-V curves of hole-only devices, equivalent circuit model for EIS analysis, EIS model fit parameters, parameters of TRPL measurements, contact angles of ITO/PEDOT:PSS and ITO/NPB surfaces, photograph of the aging perovskite films based on different HTLs.


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AUTHOR INFORMATION

Corresponding Author * Email: [email protected]

* Email: [email protected]

* Email: [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT Y. Li acknowledges the support of the Fundamental Research Funds for the Central Universities (No. 2018YJS182), the National Natural Science Foundation of China (No. 61575019). Y.Liu (in part), Y.G. (in part) and T.P.R. were supported by the Office of


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Naval Research, under Contract N00014-15-1-2244. Y.Liu also acknowledges the support from the China Scholarship Council (CSC). T.E. and M.C. acknowledge the support of NSF-CHE1506839 (T.E.) and the W.M. Keck Electron Microscopy Facility for SEM. Device fabrication and characterization were performed in the Laboratory for Electronic Materials and Devices at UMASS. The authors thank Dr. Juan Meng from Key Laboratory of Luminescence and Optical Information, Beijing Jiaotong University for help with TRPL measurements.

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(2) Jeon, N. J.; Na, H.; Jung, E. H.; Yang, T.-Y.; Lee, Y. G.; Kim, G.; Shin, H.-W.; Il Seok, S.; Lee, J.; Seo, J., A Fluorene-Terminated Hole-Transporting Material for Highly Efficient and Stable Perovskite Solar Cells. Nat. Energy 2018, 3, 682-689.


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High-Performance Perovskite Solar Cells with a Non-doped Small

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