High-Performance Perovskite Solar Cells Engineered by an Ammonia

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High-performance Perovskite Solar Cells Engineered by an Ammonia Modified Graphene Oxide interfacial Layer Shanglei Feng, Yingguo Yang, Meng Li, Jinmiao Wang, Zhendong Cheng, Jihao Li, Gengwu Ji, Guangzhi Yin, Fei Song, Zhao-Kui Wang, Jingye Li, and Xingyu Gao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b02064 • Publication Date (Web): 27 May 2016 Downloaded from http://pubs.acs.org on May 27, 2016

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ACS Applied Materials & Interfaces

High-performance Perovskite Solar Cells Engineered by an Ammonia Modified Graphene Oxide interfacial Layer Shanglei Feng†,‡, Yingguo Yang*, †,‡ , Meng Li⊥, Jinmiao Wang⊥, Zhendong Cheng†,‡, Jihao Li*, †,‡, Gengwu Ji†,‡, Guangzhi Yin†,‡, Fei Song†,‡, Zhaokui Wang⊥, Jingye Li†,‡, Xingyu Gao*,†,‡,§, # †

Shanghai Institute of Applied Physics, Chinese Academy of Sciences, 2019 Jialuo Road, Shanghai 201800, China ‡

Shanghai Synchrotron Radiation Facility(SSRF), Shanghai Institute of Applied Physics, Chinese Academy of Sciences, 239 Zhangheng Road, Pudong New Area, Shanghai 201204, China

⊥

Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute of Functional Nano & Soft Materials (FUNSOM), Soochow University, 199 Ren’ai Road, Suzhou, 215123, China § #

Shanghai Institute of Materials Genome, Shanghai, 200444, China

Key Laboratory of Interfacial Physics and Technology,Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China

Abstract: The introduction of an ammonia modified graphene oxide (GO:NH3) layer into perovskite-based solar cells (PSCs) with a structure of indium-tin oxide (ITO)/poly(3,4-ethylene-dioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS)-GO: NH3/CH3NH3PbI3-xClx/phenyl C61-butyric acid methyl ester (PCBM)/(solution Bphen) sBphen/Ag improves their performance and perovskite structure stability significantly. The fabricated devices with a champion PCE up to 16.11 % are superior in all the performances in comparison with all the reference devices without the GO:NH3 layer. To understand the improved device performances, synchrotron-based grazing incidence X-ray diffraction (GIXRD), scanning electron microscopy (SEM), ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), and UV-visible absorption measurements have been conducted on perovskite films on different substrates. It was found that these improvements should be partially attributed to the improved crystallization and preferred orientation order of peovskite structure, partially to the improved morphology with nearly complete coverage, partially to the enhanced optical absorption caused by the PEDOT:PSS-GO:NH3 layer, and partially to the better matched energy-level-alignment at the perovskite interface. Furthermore, the device was shown to be more stable in the ambient condition, which is clearly associated with the improved peovskite structure stability by the GO:NH3 layer observed by the GIXRD measurements. All these achievements will promote more applications of chemically modified graphene oxide interfacial layer in the PSCs 1

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as well as other organic multilayer devices. Keywords: perovskite solar cells; ammonia modified graphene oxide; hole transfer layer; energy-level-match; perovskite structure; device performance.

Introduction Perovskite solar cells (PSCs) with an encouraging efficiency of over 20% are becoming important players in the field of photovoltaics, which can be easily prepared via various approaches including low-cost solution processes.1-5 It was found that the planar heterojunctions structure (PHJ) can also be fabricated via a simple low-temperature solution process3,

6-9

without involving the complexity of

mesoporous TiO2 or Al2O3 scaffolds widely used in the previous PSCs. To date, most groups are focused on processing the perovskite films and relevant material design.4, 7, 10-19

In a typical PHJ, the perovskite light-absorbing layer is actually sandwiched

between the hole- and electron-transporting layers (HTLs and ETLs).5,

20-26

It is

well-known that the charge carrier transport efficiency is also sensitive to both the morphology and the electronic structures of the various interfaces in PSCs, therefore, it is essential to manipulate them to facilitate the transport of charge carriers for better device performances.5, 7, 22, 27-31 As an example, PSC devices using phenyl C61-butyric acid methyl ester (PCBM) and poly(3,4-ethylene-dioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) as the carrier transporting layers were reported with efficiency of over 12%.21-22 Wang et al recently prepared a solution processed PEDOT:PSS-GeO2 composite films by simply incorporating the GeO2 aqueous solution into the PEDOT:PSS aqueous dispersion as a hole transport layer in planar PSCs which showed a best PCE of 15.15%.22 Yang et al modified interfaces between the transporting layers and the perovskite layer by injecting a proper amount of Yttrium-ion into TiO2 layer and Lithium-ion into 3,3'4,4'-Biphenyl tetracarboxylic acid dianhydride (Spiro-OMETAD) layer (an efficient carriers conductor) in the CH3NH3PbI3-xClx based planar cell with a remarkable increase of PCE from 14.5% to 19.3%.5 Although it is an efficient practice to modify the electronic properties of the transfer layers by incorporating particles or 2


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ions into these layers to improve the PSCs, the stability of planar perovskite devices still remains a challenging issue. On the other hand, the implementation of innovative interfacial materials is another promising practice to maximize the device performance.22, 27, 32 In particular, it is known that graphene-based derivatives due to their excellent electronic properties are promising candidates for interfacial materials towards high-performance PSCs. It is reported that an ultrathin layer of graphene quantum dots (GQDs) between perovskite and TiO2 can significantly enhance the performance of PSCs due to the dramatically enhanced electron extraction by the inserted GQDs.23 Sun et. al. have reported that an intimate graphene derivative, graphene oxide (GO), introduced into the inverted planar PSC as a hole conductor can improve the device’s performance with a champion PCE of 12.4%.21 It was notable that GO layer not only can efficiently extract the holes out of the perovskite, but also improve the quality of perovskite film. In addition, GO has been reported to improve not only the device performance but also stability in PSCs.26 Even so, formation of a homogeneous uniform GO layer was shown to be difficult on indium tin oxide (ITO), which hinders improving further the hole transport due to the direct contact of perovskite active layer and ITO.26 It was reported that a moderate amount of amine introduced into GO solution can further regulate and control the conductivity of GO33-35, which could make GO more desirable as an interfacial material in PSCs. PEDOT:PSS is a commonly used hole transport layer (HTL) on ITO in conventional organic solar cells owing to its good conductivity, high work function, and solution process ability.20-22, 26-27, 33-37 Herein, a simply method was proposed to prepare the PEDOT:PSS-ammonia modified graphene oxide (GO:NH3) double-layer composite films by simply spin-coating the GO:NH3 solution onto the PEDOT:PSS layer on ITO as a composite HTL in the planar PSCs after the GO:NH3 solution was made by adding proper amount of ammonia into GO aqueous solution. The composite films were used as an anode interfacial layer in CH3NH3PbI3-xClx: PCBM based planar PSCs. Thus, it could combine the advantages of high conductivity of both PEDOT:PSS and GO as well as the merit of ambient stability of GO:NH3.22 Indeed, the fabricated PSC devices exhibit remarkable performances with a champion PCE of 3



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16.11%, open-circuit voltage of 1.03 V, short-circuit current density (Jsc) of 22.06 mA/cm2, and a fill factor (FF) of 71%. In fact, the present devices are in all the aspects superior to reference devices without the GO:NH3 films with a remarkable PCE increase. By using UV-visible absorption spectroscopy, synchrotron-based grazing incidence X-ray diffraction (GIXRD), scanning electron microscopy (SEM), and ultraviolet photoelectron spectroscopy (UPS), these remarkable performance improvements have been proven partially attributed to the improved crystallization and preferred orientation order of the perovskite film, partially to the improved morphology with nearly complete coverage, partially to the enhanced optical absorption due to using the PEDOT: PSS-GO:NH3 film, and partially to the better energy-level-match between the PEDOT: PSS-GO:NH3 HTL and perovskite active layer. Furthermore, the PEDOT: PSS-GO:NH3 composite film was found to improve the structural stability of the perovskite film leading to more stable PSCs in the ambient condition.

Experimental section Materials and sample preparation Methylammonium iodide (MAI) was synthesized according to the literature.36 GO was firstly prepared adopting a modified Hummer's method,37 which was reported in details elsewhere.38 Then, the GO:NH3 aqueous solutions (0.2 mg/ml) were obtained by adding different amounts of ammonia into the GO aqueous solution (four mixing ratio of GO solution : ammonia = 1:0, 1:0.3, 1:0.4, 1:0.5). Based on device performance measurements reported later, the mixing ratio of these two component in all the characterization experiments has been optimized to 1:0.3. PCBM and PEDOT:PSS (CLEVIOS Al 4083) were purchased from Solenne and Heraeus. To prepare the perovskite precursor solution, MAI and PbCl2 were dissolved in anhydrous dimethylformamide (DMF, amine free; 99.9%, Aldrich) with a molar ratio of 3:1. The perovskite precursor solution was stirred at 60 and filtered through PTFE filters (0.45µm) prior to use. Devices fabrication 4


overnight in a glovebox

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Figure 1.The flowchart of fabrication process for the present PSCs with the PEDOT:PSS-GO:NH3 layer.

Figure 1 shows the flowchart of fabrication process for the PSCs with the PEDOT:PSS or PEDOT:PSS-GO:NH3 composite films. PSCs were fabricated on patterned ITO-coated glass substrates with a sheet resistance of 10 Ω/sq. Before usage, ITO was cleaned with a detergent, ultrasonicated in acetone and ethanol, and then blow dried with nitrogen. Subsequently, the substrates were treated with ultraviolet ozone plasma for 15 min. The PEDOT:PSS films were obtained by spin-coating the precursors on ITO surface at 4500 rpm/40 s and then were annealed at 120

for 10

min. The GO:NH3 aqueous solutions with different mixing ratios of GO solution and ammonia were spin-coated onto the PEDOT:PSS layer at 4000 rpm for 40 s in a N2 glovebox and subsequently annealed at 120

for 10 min. The thickness of the

GO:NH3 layer is about 2 nm, similar to that of GO layer (1~2 nm) reported previously.38 During the heat-treatment process, the ammonia physically absorbed on the surface should be removed completely, which is confirmed by N 1s X-ray photoelectron spectroscopy (XPS) spectrum reported in the following. A 30 wt% CH3NH3PbI3-xClx precursor solution was then spin-coated above at 4000 rpm for 40 s. After lying in a Petri dish at room temperature for 15 min, the perovskite films were annealed on a hot plate at 100

for 60 min, allowing the color of the films become

dark brown. After that, 20 mg/mL PCBM in chlorobenzene solution was spin-coated onto the perovskite layer at 2000 rpm for 40s in a N2 glovebox. At this stage, a 0.5 mg/mL sBphen in ethanol as interfacial layer was spin-coated without additional annealing.22, 30 Finally, the samples were transferred into a vacuum chamber for silver electrode evaporation, where 100 nm-thick Ag was thermally deposited onto the sBphen layer under a pressure of 2×10−6 Torr through a shadow mask, defining a device area of 7.25 mm2. Thus, the devices using the PEDOT:PSS-GO:NH3 HTL in 5



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this work

have

a structure of ITO/PEDOT:PSS-GO:NH3/CH3NH3PbI3-xClx/

PC61BM/sBphen /Ag. Characterization Current density-voltage (J-V) curves were measured in a glovebox under a Newport 94023 solar simulator equipped with a 300 W Xenon lamp and an air mass (AM) 1.5G filter generating a simulated AM 1.5G solar spectrum irradiation source. The irradiation intensity was 100 mW/cm2 calibrated by a Newport standard silicon solar cell 91150.21-22 Field-emission scanning electron microscopy (FE-SEM, Quanta 200 FEG, FEI Co.) were performed to evaluate the surface morphologies of the PEDOT:PSS, PEDOT:PSS-GO:NH3, and perovskite films. Optical characterization was conducted using a UV-Vis spectrophotometer (PerkinElmer Lambda 750). UPS spectra excited by an unfiltered He I (21.20 eV) gas discharge lamp were collected by a SPECS PHOIBOS 100 hemispherical analyzer. XPS spectra was measured using a monochromatic Al Kα (1486.61 eV) X-ray source and the same hemispherical analyzer. The fitting analysis of XPS spectra was performed using the program XPSPEAK ver. 4.1. The GIXRD measurements were performed at the BL14B1 beamline of Shanghai Synchrotron Radiation Facility (SSRF) using X-ray with a wavelength of 0.6887Å.21,

27

The two dimensional GIXRD (2D-GIXRD) patterns

were acquired by a MarCCD detector mounted vertically at a distance of around 560 mm from the sample with an exposure time of less than 40 sec at a grazing incidence angle of 0.2°. The 2D-GIXRD patterns were analyzed using software Fit 2D and displayed in scattering vector q coordinates with q = 4πsinθ/λ, where θ is half of the diffraction angle and λ is the wavelength of incident X-ray.21, 27

Results and discussion Figure 2 a) and b) show the SEM surface morphology of a pristine PEDOT:PSS film and that of a PEDOT:PSS-GO:NH3 ( GO aqueous solution: ammonia = 1: 0.3 ) film, respectively. As the formation of homogeneously dispersed GO solution is difficult and thus GO film cannot cover PEDOT:PSS film uniformly.26 Compared with the morphology of GO reported previously,38 the bright island-like particles observed on 6


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SEM image of ammonia decorated GO, as shown in Figure 2 b), may be related to GO aggregation in the presence of ammonia, leading to the conductivity change of the film surface. Notably, these particles, similar to those GeO2 particles existed on the PEDOT:PSS-GeO2 film as reported by Wang et al22, would affect strongly the growth and crystallization of CH3NH3PbI3-xClx film, as will be demonstrated in the following.

Figure 2. The SEM surface morphology of pristine PEDOT:PSS film a) and PEDOT:PSS- GO:NH3(1:0.3) b).

The UPS spectra of a PEDOT:PSS film, a PEDOT:PSS-GO film, and a PEDOT:PSS-GO:NH3 (1:0.3) film on ITO substrates are shown in Figure 3 a), where the numbers marked in Figure 3 a) are the corresponding low kinetic energy cutoffs of these spectra in binding energy scale.22 Thus, the work functions of PEDOT:PSS-GO and PEDOT:PSS-GO:NH3 films can be deduced to be 5.19 and 5.37 eV, respectively, which are higher than that of the pristine PEDOT:PSS (5.10 eV) and can result in a better energy alignment between ITO (~4.80 eV) and the perovskite active layer (HOMO~5.40 eV) leading to high performance PSCs.5, 22 The work function of GO increased by ammonia should be caused by the chemical bonds formed on the GO surface due to the chemical reaction of ammonia with the GO, which will be demonstrated by XPS results in the following. Thus, the energy diagram of each layer in the device using PEDOT:PSS-GO:NH3 layer can be drawn in Figure 3 b) without considering the sBphen and Ag layer, where better energy-level-match between PEDOT:PSS (5.10 eV) /GO:NH3 (5.37 eV) and perovskite layer (HOMO: 5.40 eV) could result in more efficient charge transport and collection to the ITO electrode.5, 22, 26

7



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Figure

3. a) UPS spectra of three films: PEDOT:PSS (black line), PEDOT:PSS-GO (red line), and

PEDOT:PSS-GO:NH3 (blue line). The numbers indicates the second-electron cutoffs of these films. b) The energy level diagram of the devices using the PEDOT:PSS-GO:NH3 layer without considering the sBphen and Ag layer. c) and d) XPS spectra in the C 1s and N 1s regions of PEDOT:PSS-GO:NH3 (1:0.3), respectively. In d), N 1s spectrum of PEDOT:PSS/GO is also shown as a reference.

Figure 3 c) reports the C 1s spectrum of GO modified by ammonia at an optimized doping ratio of 1:0.3, where the fitting results resolve into four different components. Among them, the highest peak at 284.4 eV comes from sp2 carbon (C-C bond), proving that most conjugated carbon bonds are intact even after ammonia modification38. The peak at 286.4 eV is from the C-O bonds in epoxy and alkoxy groups while that one at 288.1 eV originates from the C=O bonds in carbonyl groups38. It is notable that the peak at 285.4 eV is attributed to the C-N bonds, which must be formed by the chemical reaction of ammonia with the GO.34,35 The existence of C-N bonds is also supported by Figure 3 d), where the fitting of N 1s spectrum of GO modified by ammonia indicates one small peak from C-NH2 bonds at 399.72 eV34,35 besides the main peak at 401.59 eV from N-N bonds.33 To verify the origins of 8


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these two N components, N 1s spectrum of pristine GO is also shown in Figure 3 d) as a reference. It is clear that the small peak in the spectrum of modified GO is absent in that of pristine GO whereas the main peak in both spectra are identical. Thus, the main peak is most likely from nitrogen physically absorbed on the GO surfaces whereas the small peak has to come from C-N bonds due to the chemical reaction of ammonia with the GO. All the XPS results clearly point out a chemical reaction between the ammonia with the GO surface, which should be responsible for the work function change reported earlier. In addition, N 1s results indicate that there should be no residual ammonia on GO surface, which must be removed completely during the heat-treatment process.

Figure 4. UV-Vis absorption spectra of CH3NH3PbI3-xClx perovskites films on three different substrates: ITO/PEDOT:PSS, ITO/PEDOT:PSS-GO, and ITO/PEDOT:PSS-GO:NH3.

Figure 4 shows the UV-vis absorption spectra of CH3NH3PbI3-xClx perovskite films on three different substrates: ITO/PEDOT:PSS, ITO/PEDOT:PSS-GO and ITO/PEDOT:PSS-GO:NH3 (1:0.3). All the perovskite films for these measurements were prepared under the same conditions. As shown in Figure 4, the shapes of all the UV-vis spectra are similar to those of CH3NH3PbI3-xClx films reported on different HTLs with a shoulder band around 480 nm and an onset at ~780 nm correlated to the optical band gap of ~1.50 eV of perovskite.19, 39-40 In fact, the absorption of all the three HTL layers have been measured to be less than 10% of the total absorption, which is similar to the reported highly transparent HTL layers (GO, GO/PEDOT:PSS, PEDOT:PSS) by Lee et al.26 In Figure 4, the perovskite film on the 9



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PEDOT:PSS-GO:NH3 film overall shows stronger absorption than the other two films especially within 360-640 nm, where the obvious increase of the two absorption features belong to CH3NH3PbIxCl3-x perovskite crystal at ~380 nm and ~480 nm implying that the crystallines of CH3NH3PbI3-xClx perovskite should be improved due to the modified interface between the ITO and the perovskite film by the PEDOT:PSS-GO:NH3 layer.41 In order to investigate the crystallographic structures, Figure 5 a), b), and c) show the 2D-GIXRD profiles of the perovskite films on three different substrates: ITO/PEDOT:PSS, ITO/PEDOT:PSS-GO and ITO/PEDOT:PSS-GO:NH3 (1:0.3). Apparently, similar diffraction rings with spotty patterns were obtained, indicating highly textured crystal domains formed on all these HTLs.42-43 It is also seen that the scattered rings at q ≈ 10, 20, 22.2 nm-1 ((110), (220) and (310) diffraction of tetragonal perovskite crystal, respectively) show up weakly preferred in-plane and out-of-plane orientations in Figure 5 a), indicating relatively weak crystallographic structures of perovskite formed on the ITO/PEDOT:PSS substrate. Notably, these diffraction peaks of perovskite crystal become much more enhanced especially along the

out-of-plane

direction

on

ITO/PEDOT:PSS-GO

(Figure

5

b)

and

ITO/PEDOT:PSS-GO:NH3 (Figure 5 c), indicating the successful formation of highly ordered perovskite grains formed in these films.21 In fact, they are more bright and sharper on ITO/PEDOT:PSS-GO:NH3 than those on other substrates indicating the best crystallization on this substrate. 21, 43

10


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Figure 5. The 2D-GIXRD profile of perovskite films on three different substrates: a) ITO/PEDOT:PSS,

b)

ITO/PEDOT:PSS-GO, and c) ITO/PEDOT:PSS-GO:NH3, respectively.

Figure 6. a) The radially integrated intensity plots of the GIXRD patterns along the ring at q ≈ 10 nm-1 corresponding to the (110) plane of the CH3NH3PbI3-xClx perovskite on different substrates; b) the azimuthally integrated intensity plots of the GIXRD patterns from the perovskites films on different substrates.

It is known that the crystallographic orientations of different structural domains in all directions can be examined in details by radially integrating the corresponding scattered ring.43 In the following, the radially integrated intensity plots along the ring corresponding to the typical (110) perovskite crystalline plane at q ≈10 nm-1 for the three films are plotted as functions of azimuth angle φ in Figure 6 a).21,

43

It

demonstrates clearly that the (110) planes in all the three films have a preferential in-plane orientation leading to their sharpest peaks at a azimuth angle of 90° in Figure 6 a). Among them, the peak height of the film on PEDOT:PSS-GO:NH3 is the highest and that on ITO/PEDOT:PSS-GO is slightly lower, which are all dramatically several times sharper than that on ITO/PEDOT:PSS, indicating that the successful formation of highly ordered perovskite structure with a preferential in-plane orientation 11



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especially on PEDOT:PSS-GO:NH3 substrates.21, 44 In addition, Figure 6 b) shows the azimuthally

integrated

intensity

plots

of

the

GIXRD

patterns

of

these

CH3NH3PbIxCl3-x films on different substrates, which reflects their degree of crystallinity. It is notable that the (110) diffraction peak of the CH3NH3PbIxCl3-x films on the PEDOT:PSS-GO:NH3 film is the highest and sharpest among all the three curves in Figure 6 b). Especially, its full width at half maximum is shown to be the smallest indicating the size of the perovskite crystallines of this film on PEDOT:PSS-GO:NH3 is the largest among all the three films.21, 27, 43 All these results show clearly highly ordered crystalline perovskite structure preferentially oriented along in-plane (110) perovskite crystal has been formed due to the introduction of the GO:NH3 film. The improved perovskite film grown on the PEDOT:PSS-GO:NH3 would be in favor of forming more charge transport channels.27

Figure

7. The SEM images of perovskite films on three different substrates: a) ITO/PEDOT:PSS,

b)

ITO/PEDOT:PSS-GO, and c) ITO/PEDOT:PSS-GO:NH3, respectively.

To study the film morphology, Figure 7 compares SEM images of perovskite films on three different substrates: a) ITO/PEDOT:PSS, b) ITO/PEDOT:PSS-GO, and c) ITO/PEDOT:PSS-GO:NH3. It is seen that the perovskite film formed on the pristine PEDOT:PSS substrate has a quite rough surface with random grains 12


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overlapping each other, consistent with those reported in literatures.21 On ITO/PEDOT:PSS-GO, the grain size increases significantly and becomes more regular but with some pores. Notably, the perovskite films develop large textured domains on PEDOT:PSS-GO:NH3 with a nearly complete coverage. Thus, the high quality CH3NH3PbI3−xClx films is formed on the PEDOT:PSS-GO:NH3 film with highly ordered structure and a nearly complete coverage, which should be caused by the morphology and the electronic property changes at the interface due to the ammonia modified GO.

Figure

8. a) J−V curves of PSC champion devices with a PEDOT:PSS-GO:NH3 HTL using different

amounts of ammonia into GO aqueous solution (ratio of GO solution: ammonia = 1:0, 1:0.3, 1:0.4, 1:0.5) during the preparation. For a comparison, the J-V curve of the champion device with only a PEDOT:PSS layer is also reported. b) Incident photon-to-electron conversion efficiency (IPCE) curve of the best device.

To demonstrate how the present findings influence the PSCs, CH3NH3PbI3-xClx solar cells using a PEDOT:PSS-GO:NH3 layer with varied doping ammonia ratio were fabricated and Figure 8 a) shows the photovoltaic performances of their champion devices under 100 mW/cm2 AM 1.5 illumination. For a comparison, solar cells with only a PEDOT:PSS layer were fabricated with the J-V curve of their champion device also reported in Figure 8 a). The dark current density has been subtracted in Figure 8 a). It is obvious that all the devices with a double layer are better than that with only the PEDOT:PSS layer. The performances of these PSCs with the double layer are clearly dependent on the doping ratio of ammonia into GO aqueous solution and the champion device using a doping ratio of 1:0.3 (GO:NH3) exhibits the best performances among all the devices in Figure 8 a). On the GO 13



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surface, both the number of chemical bonds formed and the number of the induced island-like particles should depend on the doping ratio of ammonia into GO aqueous solution, which therefore will influence the interfacial electronic structures and the perovskite film quality leading to different the device performances. The external quantum efficiency (EQE) spectrum of this champion device is shown in Figure 8 b). Integrating the overlap of the EQE spectrum in Figure 8 b) under the AM 1.5G solar photon flux gives a current density of 21.02 mA/cm2, which is close to the measured short circuit photocurrent density of 22.06 mA/cm2 obtained in Figure 8 a).21-22 Table 1 Electrical output characters of two types of devices. Device 1: ITO/PEDOT:PSS /CH3NH3PbI3-xClx/ PC61BM/sBphen /Ag and Device 2-5: ITO/PEDOT:PSS-GO:NH3/ CH3NH3PbI3-xClx/PC61BM/sBphen/Ag. Data were collected from 36 devices of each type. Numbers in bold are the best recorded values and those in italic are the average values obtained from all the same type devices.

JSC (mA/cm2)

VOC (V)

FF (%)

PCE (%)

21.86

0.89

64.0

12.50

(21.33±0.10)

(0.89±0.02)

(67.0±0.02)

(12.27±0.17)

21.80

0.96

69.0

14.42

(21.33±0.33)

(0.98±0.01)

(68.0±1.0)

(14.18±0.17)

3 PEDOT:PSS-GO:NH3

22.06

1.03

71.0

16.11

(1:0.3)

(21.27±0.55)

(1.02±0.01)

(72.0±0.01)

(15.90±0.17)

4 PEDOT:PSS-GO:NH3

20.90

1.01

69.0

14.49

(1:0.4)

(20.76±0.10)

(1.00±0.01)

(69.0±1.0)

(14.35±0.10)

5 PEDOT:PSS-GO:NH3

19.63

0.96

69.0

12.95

(1:0.5)

(19.62±0.12)

(0.95±0.02)

(68.0±1.01)

(12.80±0.18)

Devices

1 PEDOT:PSS

2 PEDOT:PSS-GO

Table 1 summarizes the key output cell parameters of the fabricated devices using the ITO/PEDOT:PSS-GO:NH3 layer with varied doping ammonia ratio measured under AM 1.5G solar illumination of 100 mW/cm2. For a comparison, the reference device using only the PEDOT:PSS layer is also reported with a best PCE of 12.50% and an average PCE of 12.27%. As mentioned, it is noteworthy that photovoltaic devices using the PEDOT:PSS-GO:NH3 (1:0.3) layer demonstrate the best performances among all the devices with short circuit current density (JSC) of 14


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22.06 mA/cm2, open circuit voltage 1.03 V, and fill factor (FF) of 71%, yielding the highest PCE of 16.11 %. In comparison with the reference devices with only the PEDOT:PSS layer, the increased FF from 64% to 71% suggests that more balanced charge carriers are obtained in this PSC.22, 30 In the meanwhile, Voc is increased from 0.89 V to 1.03V, which could be related to the improved band alignments of different HTL materials based on the UPS measurements in Figure 3 a).22 Thus, the remarkable about 30% increase of PCE of the present device can be mainly attributed to its much bigger Voc and partially to its slightly higher FF and Jsc. Based on the previous study, the overall remarkable improved device performances might be contributed by four possible factors: (1) the improved crystallization and preferred orientation order of perovskite crystallines due to the modified interfaces by the GO:NH3 layer;11, 21 (2) more carriers can be easily collected due to the modified interface between the HTL and the perovskite film with better energy-level-alignment;22 (3) the enhanced optical absorption due to the PEDOT:PSS-GO:NH3 layer; (4) the improved morphology with nearly complete coverage of perovskite film.7

Figure 9. The reverse (1.2 V→0 V) and forward scan (0 V→1.2 V) J-V curves of three perovskite solar cells under 100 mW/cm2 AM 1.5G illumination. Table 2 The photovoltaic parameters of the three devices derived from the forward and reverse scans in Figure 9.

Devices

Scan type

JSC (mA/cm2)

VOC (V)

FF (%)

PCE (%)

PEDOT:PSS

Forward

19.25

0.88

67.5

11.38

15



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Page 16 of 25

Reverse

20.10

0.87

69.1

12.11

Forward

21.78

0.96

70.6

14.71

Reverse

21.00

0.95

71.2

14.18

Forward

21.71

1.03

70.5

16.10

Reverse

22.02

1.02

71.8

16.09

PEDOT:PSS-GO

PEDOT:PSS-GO:NH3(1:0.3)

It is known that the PSC device performance is often overestimated by the hysteresis effect,49 another three devices using PEDOT:PSS, PEDOT:PSS-GO, and PEDOT:PSS-GO:NH3 (1:0.3) HTL have been fabricated following the same processes described before for the study of the hysteresis behavior. As shown in Figure 9, the differences between the reverse and forward current-voltage scans of all the three devices are overall small indicating relatively small hysteresis effect for all the three devices, which is also consistent with the report by Wu et al.50 Table 2 summarizes the key output cell parameters of the fabricated devices derived from both the reverse (1.2 V→0 V) and forward scans (0 V→1.2 V) under 100 mW/cm2 AM 1.5G illumination. From Figure 9 and Table 2, it is clear that the PEDOT:PSS-based device shows relatively larger hysteresis than the other two devices and that the PEDOT:PSS-GO:NH3-based device shows the smallest hysteresis. In fact, the PCE values

derived

from

both

the

reverse

and

forward

scans

of

the

PEDOT:PSS-GO:NH3-based device are identical to that of previous device 3 in Table 1. The decrease of hysteresis effect caused by the ammonia modified GO could be partially related to the improved crystallization and preferred orientation order of perovskite structure, partially to the improved morphology with nearly complete coverage, and partially to the better matched energy-level-alignment at the perovskite interface, which in turn can affect charge carrier collection efficiency and recombination behavior in PSC devices.49-52 It is noteworthy that these three devices fabricated in a different batch from those reported in Figure 8 reproduced almost the same performance improvements brought by the ammonia modified GO.

16


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Figure 10. a) The normalized PCE of a CH3NH3PbIxCl3-x solar cell with a PEDOT:PSS-GO:NH3 HTL as a function of exposing time to air and that of another one with only a PEDOT layer, respectively. b) the GIXRD patterns in a small area around the perovskite (110) out-of-plane diffraction peak of these two CH3NH3PbIxCl3-x devices before and after exposure to air for 96 hours; c) the perovskite (110) diffraction peaks obtained by azimuthally integrating intensity in the GIXRD patterns of the perovskites device with a PEDOT:PSS-GO:NH3 HTL before and after exposure to air for 96 hours.

Thus, it is clear that PSCs can be improved remarkably by the introduction of the GO:NH3 layer. To further investigate if the device stability is also improved, Figure 17



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10 a) shows the stability test results of a CH3NH3PbIxCl3-x solar cell with a PEDOT:PSS-GO:NH3 (1:0.3) layer and another one with only a PEDOT:PSS layer in the ambient condition without any encapsulation. After 96 h, the PCE of the device using the double-layer only decreased to 72.2% of its initial value, whereas the one using only the PEDOT:PSS layer drops to 65.6% of its initial value. Obviously, the GO:NH3 layer has improved the device stability of PSCs slightly, which is probably due to the fact that the highly ordered perovskite structure formed on the GO:NH3 layer could be more stable in the ambient condition. To further explain the improved stability, Figure 10 b) presents the GIXRD patterns in a small area around the perovskite (110) out-of-plane diffraction peak of these two devices before and after exposure to air for 96 hours. Before exposure to air, both devices show clearly bright diffraction features similar to those reported in Figure 5. After the exposure of 96 hours, the PbI2 (001) diffraction peak at q ~8.4 nm-1

9,26

obviously appeared in the

device with only the PEDOT:PSS layer due to the decomposition of the perovskites possibly by the reactions with the water molecules. Notably, there is no such PbI2 diffraction peak observed in that device with the double layer even after the same exposure of 96 hours, which illustrates that the degradation of perovskite into PbI2 did not occur there. Thus, the different behaviors of perovskite films in these two devices in the ambient condition indicate immediately that the perovskite structure on the double layer is more robust due to the formation of highly ordered crystallines as expected. This obviously leads to the slightly different device stability of these two devices as the degradation of perovskite will jeopardize the performances of PSCs with a quicker PCE decrease of PSC with only the PEDOT:PSS layer. Even though the PSC on the PEDOT:PSS-GO:NH3 layer is more stable, its PCE still drops quite quickly in Figure 10 a). To explain this quick drop and to study the perovskite structural changes more quantitatively upon exposure to air, Figure 10 c) reports the perovskite (110) diffraction peak obtained by the azimuthally integrated intensity of the GIXRD patterns from the device with the double layer before and after exposure to air for 96 hours. It is clear that the diffraction peak intensity for the device with the double layer still drops clearly by ~34% after 96 hours in the ambient condition, 18


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which illustrates the decrease of the ordered crystallines in the perovskite film leading to the quick drop of PCE in this PSC. It is reported that degradation of the PSCs is mainly caused by the decomposition of the perovskite active layer due to the reactions with moisture intruding into the perovskite active layer through the degraded top electrode,47-48 which lead to the leakage current caused by the formation of hot spots in bulk or at the interface. Further work is necessary to develop more innovative interfacial materials between perovskite film and top electrode to effectively avoid the moisture and further maximize the device stability.53 Our work proves that graphene derivatives such as GO are promising candidates as effective HTL towards stable high performance PSCs.

Conclusion Solution-processed PEDOT:PSS-GO:NH3 film were demonstrated as an effective HTL to modify the interface between the ITO and the perovskite film in planar PSCs. The introduction of ammonia modified GO layer into PSC devices improves their performance and perovskite structure stability significantly. The improvements, especially the remarkable over 30% enhancement of PCE and 16% increased of Voc, should be partially attributed to the improved crystallization and preferred orientation order of peovskite structure, partially to the improved morphology with almost complete coverage, partially to the enhanced optical absorption due to using the PEDOT:PSS-GO:NH3 layer, and partially to the better energy alignment between the PEDOT:PSS-GO:NH3 layer and the perovskite active layer. Moreover, the GO:NH3 interfacial layer was shown clearly to improve the structure stability in the PSC with more stable device performance in the ambient condition. These findings will promote the applications of chemically modified graphene oxide interfacial layer in the developments of PSCs as well as other solar cell devices. AUTHOR INFORMATION Corresponding Author *X.Y.G. Tel: (86)-21-3393-3199. E-mail: [email protected]. *J.H.L. Tel: (86)-21-3393-3023. E-mail: [email protected]. 19



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*Y.G.Y. Tel: (86)-21-3393-3023. E-mail: [email protected]. Author Contributions Shanglei Feng, Yingguo Yang and Meng Li contributed equally to this paper. Shanglei Feng and Yingguo Yang carried out the SEM, UPS, XPS, UV-visible absorption, GIXRD measurements experimental work and all the experimental data analysis. Yingguo Yang, Meng Li, Jinmiao Wang and Zhaokui Wang performed the device fabrication and characterization. Fei Song, Gengwu Ji Zhendong Cheng and Guangzhi Yin participated in the UPS and XPS experimental; Meng Li, Jihao Li and Jingye Li participated in the preparation of GO and GO:NH3 solutions. Xingyu Gao and Yingguo Yang mainly wrote this paper, all authors contributed to the writing of the paper. The project was conceived, planned, and supervised by Xingyu Gao. The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work is supported by the National Natural Science Foundation of China (Grant Nos. 11175239, 11405253, 11405257, 11505270 and U1332205), Strategic Priority Research Program of the Chinese Academy of Sciences with Grant No.XDA02040200, the research grant (No.14DZ2261200) from the Science and Technology Commission of Shanghai Municipality, Shanghai Municipal Commission for Science and Technology (No.15ZR1448300), and One Hundred Person Project of the Chinese Academy of Sciences. The authors thank beamline BL14B1 and BL16B1 at Shanghai Synchrotron Radiation Facility (SSRF) for providing the beam time.

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Lett. 2014, 5, 1511-1515. (50) Wu, Y.; Wang, J.; Qiu, X.; Yang, R.; Lou, H.; Bao, X.; Li, Y. Highly Efficient Inverted Perovskite Solar Cells With Sulfonated Lignin Doped PEDOT as Hole Extract Layer. ACS Appl. Mater. Interfaces 2016, DOI:10.1021/acsami.6b00084. (51) Correa-Baena, J. P., Anaya, M., Lozano, G., Tress, W., Domanski, K., Saliba, M., Grätzel, M. Unbroken Perovskite: Interplay of Morphology, Electro-Optical Properties, and Ionic Movement. Adv. Mater. 2016, DOI: 10.1002/adma.201600624. (52) Tress, W., Marinova, N., Moehl, T., Zakeeruddin, S. M., Nazeeruddin, M. K., Grätzel, M. Understanding the Rate-Dependent J-V Hysteresis, Slow Time Component, and Aging in CH3NH3 PbI3 Perovskite Solar Cells: the Role of a Compensated Electric Field. Energ. Environ. Sci. 2015, 8(3), 995-1004. (53) Steim, R.; Choulis, S. A.; Schilinsky, P.; Lemmer, U.; Brabec, C. J. Formation and Impact of Hot Spots on the Performance of Organic Photovoltaic Cells. Appl. Phys. Lett. 2009, 94 (4), 043304.

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High-Performance Perovskite Solar Cells Engineered by an Ammonia

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