Highly Efficient Inverted Perovskite Solar Cells With Sulfonated Lignin

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Highly Efficient Inverted Perovskite Solar Cells With Sulfonated Lignin Doped PEDOT as Hole Extract Layer Ying Wu,†,‡,∥ Junyi Wang,§,∥ Xueqing Qiu,*,†,‡ Renqiang Yang,§ Hongming Lou,†,‡ Xichang Bao,*,§ and Yuan Li*,†,‡ †

School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, China State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou, China § CAS Key Laboratory of Bio-based Materials, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, China ‡

S Supporting Information *

ABSTRACT: Sulfonated-acetone-formaldehyde (SAF) was grafted with alkali lignin (AL) to prepare grafted sulfonated-acetone-formaldehyde lignin (GSL). Considering the rich phenolic hydroxyl groups in GSL, we detected a hole mobility of 2.27 × 10−6 cm2 V−1 s−1 with GSL as a hole transport material by space-charge-limited current model. Compared with nonconjugated poly(styrene sulfonic acid), GSL was applied as p-type semiconductive dopant for PEDOT to prepare water-dispersed PEDOT:GSL. PEDOT:GSL shows enhanced conductivity compared with that of PEDOT:PSS. Simultaneously, the enhanced open-circuit voltage, short-circuit current density, and fill factor are achieved using PEDOT:GSL as a hole extract layer (HEL) in sandwich-structure inverted perovskite solar cells. The power conversion efficiency is increased to 14.94% compared with 12.6% of PEDOT:PSS-based devices. Our results show that amorphous GSL is a good candidate as dopant of PEDOT, and we provide a novel prospective for the design of HEL based on lignin, a renewable biomass and phenol derivatives. KEYWORDS: lignosulfonate, PEDOT:PSS, hole transport material interface engineering, organic electronic, phenol radical

1. INTRODUCTION With the rapid development of high performance perovskite solar cells (PSCs),1−4 the anode interface engineering is of critical importance to boost their performance.5 The power conversion efficiency (PCE) of organic solar cells (OSCs) has been achieved over 10% by modification of cathode. To name a few, use of solution-processed conjugated polymer as cathode buffer layer has significantly improved PCE in single layer BHJSCs for the first time by group of Cao.6 However, for the anode, poly(3,4-ethylene dioxythiophene):poly(styrene sulfonic acid) (PEDOT:PSS) is one usually used water-soluble conductive polymer with good performance. The highest occupied molecular orbital (HOMO) energy level and conductivity are the most important factors to determine the performance of PEDOT:PSS in devices. For the control of HOMO energy level of PEDOT: PSS, application of perfluorinated ionomer (PFI) doped PEDOT also improved the device performance due to its HOMO energy level compared with PEDOT:PSS.7 For the conductivity of PEDOT:PSS, the regular structure of PSS dopant contributes to the excellent conductivity of PEDOT:PSS.8 However, PSS is nonconjugated and produces electrical and microstructural inhomogeneities to make PEDOT:PSS an undesired candidate of hole injection layer (HIL) . Lots of researchers have focused on the modification of PEDOT:PSS, such as the addition of addictive treatment,9 to change the conductivity. However, the © XXXX American Chemical Society

alternative of PSS has been rarely reported except for our recent reports such as semiconductive lignosulfonate,10,11 and sulfonated phenol-formaldehyde resin.12 Lignosulfonate (LS) is the product of sulfonation of lignin. From the view of green economics, the potential production of aromatic derivatives as chemicals has been pursued by chemists for many years.13 In contrast with lignin, lignosulfonate has practical application in the high-value application of a surfactant and dispersant in industrial field. From the view of material scientist, we noticed that, due to its aromatic structure and electron transfer property of lignin,10,11 we propose that lignin might act as a transfer mediate for the conversion of photonic and chemical energy during the growth of plants. The research to apply lignin and lignosulfonate as materiasl will be an interesting field in future. As we know, annually, numerous industrial lignin is produced; however, most of the industrial lignin is not applied as value added products. Currently, for the conventional applications of industry, lignin and lignosulfonate act as cheap additives, binders, dispersants, and surfactants. The high-value-added application is very important and prospective but rarely reported. Can sustainable LS replace expensive PSS as dopant for PEDOT? Received: January 4, 2016 Accepted: April 28, 2016

A

DOI: 10.1021/acsami.6b00084 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) Chemical structure of grafted sulfonated-acetone-formaldehyde lignin (SAF). (b) Proposed branched chemical structure of GSL and oxidation of GSL. (c) Polymerization of EDOT in the presence of GSL as dopant and proposed chemical structure of PEDOT:GSL.

2. EXPERIMENTAL SECTION

It is well-known that PEDOT:PSS acts as a famous and efficient HIL for the modification of indium−tin oxide (ITO) anode in organic light-emitting diodes (OLEDs).14 However, PEDOT:PSS still showed some drawbacks, for instance, it tends to corrode ITO and causes reduced stability of organic electronic devices due to its relatively high acidity (pH: ∼ 1.5).15,16 Moreover, PSS is an insulated polymer and will cause electronic inhomogeneities of PEDOT:PSS film. Keeping in mind that there are still several drawbacks of PSS, we applied GSL, a modified lignin, as dispersant and dopant for PEDOT as GSL showed the following advantages. First, grafted sulfonatedacetone-formaldehyde lignin (GSL) was prepared with graft of sulfonated acetone-formaldehyde (SAF) to alkali lignin(AL). As we all know, SAF has been applied in industrial fields as an good dispersant.17 Consequently, because of the long and linear aliphatic chain and plentiful sulfonic groups, GSL was used as efficient dispersants of cement water reducing agent. Second, based on our experimental results, GSL also can be used as an excellent dispersant of PEDOT. It makes the high-value-added application of GSL very realizable. Moreover, considering the molecular structure of lignin, the old chemistry and our recent work also reported that lignin and lignosulfonate can act as a hole transport material with similar hole mobility like conventional conjugated polymers in organic electronic devices.10,18,19 In this report, the preparation and characterization of PEDOT:GSL was reported in details. Water-dispersed PEDOT:GSL was successfully utilized as a hole extraction layer in PSCs, and it showed highly improved performance compared with PEDOT:PSS. The underlying mechanism was also investigated and discussed in details based on our recent work on lignin- or phenol-based organic electronics.10,11

2.1. Preparation of GSL and PEDOT:GSL. The detailed synthesis condition, purification, and procedure are provided in the Supporting Information. 2.2. Fabrication of PSCs and Characterization. Methylammonium iodide (CH3NH3I) was prepared according to previously reported work. Phenyl-C61-butyric acid methyl ester (PCBM), PbI2, and ITO with sheet resistance of 15 Ω/sq were obtained from American Dye Sources (Canada), Aladdin Reagent (China), and Shenzhen Display (China), respectively. After an ultrasonic bath, the samples were washed using acetone, toluene, methanol, and isopropyl alcohol, sequentially. After oxygen plasma treatment of ITO glass for 6 min, the glass was spin-coated with a 40 nm PEDOT:PSS layer or 40 nm PEDOT:GSL layer. After that, the substrates were baked at 150 °C for 20 min or 100 °C for 10 min, respectively. The HIL-coated ITO glass substrate was transferred into a N2-filled glovebox. The CH3NH3PbI3 precursor solution was spin-coated to form a 300 nm thick perovskite layer on the modified ITO coated glass. After thermal treat at 100 °C for 10 min, a layer of PCBM (∼40 nm) was spin-cast on the perovskite layer. The devices were prepared after aluminum cathode deposition at a pressure of 4 × 10−4 Pa. The device area was 0.1 cm2 for each cell defined by shadow mask Bruker D8 ADVANCE was applied to record the X-ray diffraction (XRD) pattern for perovskite film. Surface morphology obtained on an Agilent 5400 atomic force microscope (AFM). Film thicknesses were measured by Veeco Dektak150 surface profiler. Fluoromax 4 spectrometer (HORIBA Jobin Yvon) was used to obtain steady-state photoluminescence (PL) spectra under a photoexcitation at 507 nm. Current voltage curves (J−V) of the devices with the step voltage of 10 mV were measured with a Keithley 2420 under a Newport solar simulator. Light intensity was calibrated with a standard silicon solar cell. The external quantum efficiency (EQE) of solar cell was analyzed using a certified Newport incident photon conversion efficiency (IPCE) measurement system. The PL lifetime was measured by timecorrelated single-photon counting (TCSPC) in air using a HORIBAB

DOI: 10.1021/acsami.6b00084 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 2. (a) Cyclic voltammogram of GSL film in 0.1 M Bu4NPF6 in dry CH2Cl2 solution. (b) J−V curves of hole-only devices with SCLC fitting from GSL (the structure of hole-only devices inserted in the picture). (c) Cyclic voltammogram of PEDOT:GSL film in 0.1 M Bu4NPF6 in dry CH2Cl2 solution. (d) HOMO levels of PEDOT:PSS and PEDOT:GSLs in PSCs. fm-2015. The solar cells were stored in the N2-filled glovebox at room temperature to test the lifetime.

PEDOT:GSL with a mass ratio of 1:5 (PEDOT:GSL) was prepared, and structure characterization was investigated using FT-IR, UV−vis adsorption, and transmittance measurement. The FT-IR spectra of EDOT and PEDOT:GSL showed the signals of EDOT monomer all disappeared in the PEDOT:GSL UV−vis absorption spectrum of PEDOT:GSLs aqueous solution, as shown in Figure S3. The result showed the presence of an obvious absorption band at around 800 nm. On the basis of typical UV−vis absorption spectrum of PEDOT:PSS, the UV−vis absorption spectrum of PEDOT:GSL verified that PEDOT:GSL was successfully prepared in our work. Transmittance spectra of PEDOT:GSL showed its good transmittance to UV−vis light (Figure S4). Furthermore, the particle size was crucial for film-forming property of PEDOT:GSL and dynamic light scattering (DLS) measurements of the PEDOT:GSL aqueous solution were investigated. The particle size distribution of PEDOT:GSL was shown in Figure S5. The average size is about 160 nm, which is even lower than that of PEDOT:PSS.10 This will ensure the good film-forming property of PEDOT:GSL for device fabrication. In order to study the differences of film-forming properties between PEDOT:PSS and PEDOT:GSL, X-ray photoelectron spectroscopy (XPS) measurement of spin-coated PEDOT:PSS film and PEDOT:GSL film were conducted to study the surface element distribution. As shown in Figure S6, the surface XPS spectra exhibited that two types of S 2p signals were found on both of the two samples. The peak around 168 eV was ascribed to sulfur atoms in PSS or GSL. The peaks between 163 and 166 eV could be assigned to the sulfur atom in PEDOT. Comparing the two samples’ integrated S 2p peak area ratio, we can observed that the macroscopic phase separation of PEDOT:PSS led to a higher sulfonic group content on its film surface than that of PEDOT:GSL. Cyclic voltammogram (CV) were applied to study the HOMO energy levels of GSL and PEDOT:GSL. The CV test

3. RESULTS AND DISCUSSIONS 3.1. Preparation and Characterization of GSL. The detailed procedure for the preparation of GSL was reported in previously published work,17 using AL, sodium sulfite, acetone, and formaldehyde as raw materials. A proposed chemical structure of GSL was provided in Figure 1. Gel-permeation chromatography (GPC) was used to detect the molecular weight (Mw) of AL and GSL and the fundamental structural properties of them was further determined by functional group content measurement. As can be seen in Figure S1, greatly increase of Mw has been made through polymerization of AL using formaldehyde. The results of Mw distribution of AL and GSL were listed in Table S1. The Mw of GSL (Mw = 24900 Da, Mw/Mn = 2.74) achieved a factor of 12-fold of AL (Mw = 2100 Da, Mw/Mn = 1.91). The dispersion capability of GSL is related with the following factors including water-solubility, functional phenolic hydroxyl group and sulfonic group contents. GSL shows a sulfonic group content of 2.54 mmol/g which is a relatively high sulfonation degree among reported lignosulfonates (Table S1). For our previous work, a sulfonic group content of 1.81 mmol/g was achieved from sulfomethylated lignin (SL) via conventional sulfomethylation method.10 The significantly enhanced Mw and water-solubility of GSL ensure its excellent dispersion properties. Moreover, it should be noted that phenolic hydroxyl group content was slightly decreased from 2.11 mmol/g of AL to 1.63 mmol/g of GSL. The phenolic hydroxyl groups of GSL contribute to the hole transport property as reported in our previous work.10,11 3.2. Preparation and Characterization of PEDOT:GSL. GSL was always applied as water reducing agents and showed good performance in our previous study.17 In this work, GSL was used to dope and disperse PEDOT in aqueous solution. C

DOI: 10.1021/acsami.6b00084 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces of GSL and PEDOT:GSL film were conducted in 0.1 M Bu4NPF6 (in dichloromethane) solution (Figure 2a,c). We could calculate the HOMO energy levels of GSL and PEDOT: GSL from cyclic voltammogram with the values of −5.5 eV and −5.16 eV, respectively. It is interesting that the HOMO level of PEDOT:GSL was lower than −4.9 eV of PEDOT:PSS. Moreover, the conductivities of PEDOT:PSS and PEDOT:GSL were tested as 1.22 × 10−2 and 2.64 × 10−2 S/cm, respectively. It is interesting that PEDOT:GSL shows higher conductivity comparing with that of PEDOT:PSS. As we all know that insulated PSS will aggregate on top of spin-coated film layer and it will decrease the conductivity of the PEDOT:PSS layer. For PEDOT:GSL, the high conductivity make it a potential good hole extract material. Furermore, the hole transport properties of GSL was also measured by testing the dark current−voltage curves of hole-only devices by using the spacecharge-limited current (SCLC) method with a device structure of ITO/PEDOT:PSS/GSL/MoO3/Al (Figure 2b).10 The equation is as follows: J = (9/8)εrε0μ(V 2/d3)

Table 1. Photovoltaic Performances of PSCs with PEDOT:PSS and PEDOT:GLS as HEL in Devices ITO/ HTL/CH3NH3PbI3/PC61BM/Al anode modifier

Voc (V)

Jsc (mA/cm2)

FF (%)

PCE (%)

none PEDOT:PSS PEDOT:GSL PEDOT:GSLa reverse PEDOT:GSLa forward

0.97 1.02 0.98 1.031 1.026

15.76 18.12 19.21 20.1 19.93

69.5 68.5 74.7 72.08 72.45

10.66 12.62 14.10 14.94 14.82

Device with GSL was optimized by the control of perovskite film quality. a

(Voc) of 0.97 V, and fill factor (FF) of 69.5%, which is in line with our previous report.20 Furthermore, these devices without HEL also show good reproducibility if the devices with highquality photoactive layers. The low PCE is mainly due to its low Jsc that exists interface barrier between the ITO and perovskite layers. For the PSCs with traditional PEDOT:PSS as HEL. The PCE is increased to 12.62%, with an improved Jsc of 18.12 mA/ cm2. The insert of PEDOT:PSS between the ITO and perovskite layers can reduce the barriers, and the carrier can be collected easily. As can be seen in Figure 3, the PCE of PSCs with PEDOT:GSL as HEL reached PCE of 14.10% with improved Jsc of 19.21 mA/cm2 and FF of 74.7%. The results show that GSL is a good dopant of PEDOT, which gives positive effect on the hole extraction properties. Recently, a new group of devices was prepared to record the corresponding IPCE spectra. The PEDOT:GSL-based device showed higher PCE of 14.94% (Figure 4). The average PCE is 14.66%, and the derivation is 1.2%. Simultaneously, enhanced Jsc and FF of

(1)

where, ε0 is the permittivity of free space, εr is the relative permittivity, μ is the hole mobility, and d is film thickness. As can be seen from Figure 2b, the thickness of GSL is 90 nm and the mobility value of 2.27 × 10−6 cm2 V−1 s−1 was calculated. The result indicates that GSL can act as hole extract materials. 3.3. Performances of PEDOT:GSL as HEL in PSCs. To evaluate the effect of PEDOT:GSL as HEL in photovoltaic performance, we fabricated the sandwich-structure inverted PSCs with the configuration of glass/ITO/HEL/ CH3NH3PbI3/PC61BM/Al (Figure 3); the HEL is PE-

Figure 3. Device architecture of PSC and J−V curves of PSCs with PEDOT:PSS and PEDOT:GSLs as HEL, respectively.

DOT:GSL, PEDOT:PSS, or without any HEL, respectively. Film thickness of the photoactive layer is around 300 nm, which is adequate for the application in PSC. In our previous report, 40 nm PC61BM can be fully covered the perovskite films due to the smooth surface of perovskite film.20,21 The typical J− V characteristics of the PSCs were obtained under AM 1.5G, 100 mW/cm2, as shown in Figure 3. The related device parameters are also given in Table 1. Solar cells on ITO-coated glass without HEL exhibit a PCE of 10.66%, with short-circuit current density (Jsc) of 15.76 mA/cm2, open-circuit voltage

Figure 4. EQE with integrated short-circuit current−density curve and J−V curves of PSCs using PEDOT:GSLs as HEL. D

DOI: 10.1021/acsami.6b00084 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Second, the XRD pattern of the as prepared perovskite layers on PEDOT:GSL and PEDOT:PSS modified substrate were given in Figure 6. It is clear that there are three intense

PSCs were achieved using PEDOT:GSL as hole extract layer. Moreover,Voc of 1.03 or 1.04 V for PSCs was achieved using PEDOT: GSL as HEL in our reproduced devices (Figure 4 and Table S2). The HOMO energy level of PEDOT: GSL is slightly lower than PEDOT:PSS, which gives an good interprataion about the changes of Voc. The IPCE from 600 to 800 nm is higher than that of the PSC,which is in good agreement with the enhanced performance of PEDOT:GSL. Compared with PEDOT:PSS, the relatively higher conductivity and lower HOMO level of PEDOT:GSL might contribite to the enhanced performances of PSCs. 3.4. Investigation of Mechanism for Photovoltaic Performance of PEDOT:GSL. On the basis of the highly enhanced photovoltaic performance using PEDOT:GSLs as HELs in PSC devices, we studied the possible mechanism carefully from the following four aspects: First, the surface of PEDOT:GSL- and PEDOT:PSSmodified ITO glass and its perovskite layer were measured by AFM (Figure 5). The root means square (RMS) values of

Figure 6. XRD pattern of the as-prepared perovskite films on PEDOT:GSL and PEDOT:PSS modified ITO glass substrate, respectively.

diffraction peaks at 2θ of 14.17, 28.49, and 31.90°, corresponding to (110), (220), and (312) crystal planes, respectively, which confirm the formation of an orthorhombic structure.23 The results show that the two film samples have same phases even with different HELs. However, the average crystallite size on PEDOT:GSL- and PEDOT:PSS-modified substrate is 67 and 61 nm, respectively, determined by Debby− Scherrer formula, considering the (110) peak. Large grains could improve the current density of related devices.22 Furthermore, to further investigate the ability of the hole extraction of the two different HELs the steady-state photoluminescence (PL) spectra are given in Figure 7. There is a PL

Figure 5. AFM images of (A) PEDOT:PSS, (B) PEDOT:PSS/Pero, (C) PEDOT:GSL, (D) PEDOT:GSL/Pero, respectively (images show 5 × 5 μm films).

PEDOT:GSL- and PEDOT:PSS-modified ITO coated glass are 4.01 and 2.31 nm, respectively. It is well-known that filmforming property could affect the hole transport property of HELs. To a large degree, the film-forming property was influenced by particle size, so dynamic light scattering (DLS) showed the particle size decreased and was smaller than that of PEDOT:PSS. It indicated that the dispersion property of GSL is excellent for conductive polymer PEDOT. The AFM images of PEDOT:GSL films were also provided, as shown in Figure 5. The surface of PEDOT:GSL films was quite homogeneous, and more obvious nanoparticles were detected. Compact and uniform films were obtained, which further illustrated that GSL shows excellent dispersion property in PEDOT. All of this facilitates charge transport and contributes the unexpected good results in devices. The formed perovskite layers are composed with compact grain sizes. However, the average grain sizes of the film on PEDOT:GSL modified ITO glass are slightly larger than that of on PEDOT:PSS modified ITO glass. According to the previous report, the larger average grain sizes would have higher current density.22 As a result, the large compact grain sizes on PEDOT:GSL have good carrier transport properties than the films with small sizes of PEDOT:PSS.

Figure 7. Steady-state photoluminescence (PL) spectra of perovskite layers with PEDOT:PSS- and PEDOT:GSL-modified ITO glass, respectively.

spectrum at 770 nm for the film, which is in good agreement with previous reports.1−4 The quenching effect for the perovskite layer on PEDOT:GSL-modified ITO glass was very obvious. The result shows that the holes can be effectively collected by PEDOT:GSL-modified ITO glass and further confirms that the device performance with PEDOT:GSL as HEL can be improved. In the other two cases of ITO/pero and PEDOT:PSS/pero, we also investigated PL quenching efficiency, as shown in Figure 7. The largest PL intensity was found on the perovskite on glass. The results also show that the best PL quenching efficiency is the sample based on E

DOI: 10.1021/acsami.6b00084 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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hole transport material from biomass, is an interesting hole transport material, which is very different from previous polymeric semiconductors.

PEDOT:GSL-modified ITO-coated glass, which is consistent with our photovoltaic results. As discussed above, one more factor can be ascribed to the suitable HOMO level of PEDOT:GSL. Compared with the work function between −4.9 and −5.1 eV reported for PEDOT:PSS, PEDOT:GSL has slightly lower HOMO energy level. Moreover, conductivity of PEDOT:GSL film is higher than that of PEDOT:PSS. Last but not least, on the basis of our previous result, the electrochemical property and hole transport properties of lignosulfonate, GSL might act as hole collection materials in the PEDOT:GSL system. In contrast, PSS is totally nonconjugated. GSL will reduce the electronic inhomogeneities comparing with PEDOT:PSS. The time-resolved PL decay profile of the perovskite layer on PEDOT:GSL- and PEDOT:PSS-modified ITO glass was given in Figure 8. The hole transfer are very quickly for both samples, and the hole transfer rate from perovskite to PEDOT:GSL (19.7 ns) is slightly faster than that of PEDOT:PSS (24.3 ns).

4. CONCLUSION In conclusion, water-dispersed PEDOT:GSL was prepared using GSL as novel dopant of PEDOT and applied as HEL in PSCs. Enhanced JSC, FF, and VOC, of PSCs were achieved using PEDOT:GSL as HEL. A highly improved PCE of 14.94% was achieved. Our results show that GSL is one of promising materials as dopant of semiconductive polymers such as PEDOT. More importantly, the idea that PEDOT:GSL can act as HEL to fabricate PSCs provides a new view for future study on precursors of radical-containing polyaromatichydrocarbons,24 and lignin- or phenol-based derivatives based on our recent work.10,11,25,26 We believe it will lead to highperformance hole transport materials based on phenol derivatives and more in-depth study of phenol-based organic electronics in future.27

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b00084. Detailed preparation and characterization of PEDOT:GSL, molecular weight distribution of alkali lignin and GSL, FT-IR spectra, UV−vis absorption, device repeatability data, XPS of PEDOT:PSS and PEDOT:GSL, transmittance spectra and particle size distribution of PEDOT:GSL and PEDOT:PSS. (PDF)

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AUTHOR INFORMATION

Corresponding Authors

Figure 8. Time-resolved PL decay profile of the perovskite layer on PEDOT:GSL and PEDOT:PSS modified ITO glass.

*E-mail: [email protected]. Tel.: +86-20-87114033. *E-mail: [email protected]. Tel. +86-532-80662701. *E-mail: [email protected]. Tel. +86-87114722.

The lifetimes of the two devices was also studied and are given in Figure 9. The solar cells were stored in the N2-filled

Author Contributions ∥

These authors contributed equally to this work

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge financial support from the National Natural Science Foundation of China (21402054, 21436004, 21274134, 51573205), the National Basic Research Program of China 973 (2012CB215302), and the International S&T Cooperation Program of China (2013DFA41670). The authors thank Prof. Xuefei Wang from University of Chinese of Academy Sciences for PL lifetime measurements and discussion. X.B. thanks the Youth Innovation Promotion Association CAS for financial support (2016194).

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Figure 9. Lifetime of PSCs with PEDOT:PSS and PEDOT:GSL as HEL, respectively.

REFERENCES

(1) Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells. J. Am. Chem. Soc. 2009, 131, 6050−6051. (2) Zhou, H. P.; Chen, Q.; Li, G.; Luo, S.; Song, T. B.; Duan, H. S.; Hong, Z. R.; You, J. B.; Liu, Y. S.; Yang, Y. Interface Engineering of Highly Efficient Perovskite Solar Cells. Science 2014, 345, 542−546. (3) Stranks, S. D.; Eperon, G. E.; Grancini, G.; Menelaou, C.; Alcocer, M. J. P.; Leijtens, T.; Herz, L. M.; Petrozza, A.; Snaith, H. J. Electron-Hole Diffusion Lengths Exceeding 1 Micrometer in an

glovebox at room temperature for the test of their lifetime. Both showed similar stability during the test time. After 50 days, the PCE was still higher than 10%. The similar hygroscopic nature of PEDOT:PSS and PEDOT:PSS is one of the factor that degrade the perovskite by accelerating the absorption of water during device operation. GSL, as a novel F

DOI: 10.1021/acsami.6b00084 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

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DOI: 10.1021/acsami.6b00084 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX