Materials Chemistry A

View Article Online View Journal

Journal of

Materials Chemistry A Materials for energy and sustainability

Accepted Manuscript

This article can be cited before page numbers have been issued, to do this please use: M. Azam, S. Yue, R. XU, L. Kong, K. Ren, Y. sun, J. Liu, Z. Wang, S. Qu, Y. Lei and Z. Wang, J. Mater. Chem. A, 2018, DOI: 10.1039/C8TA03953D. Volume 4 Number 1 7 January 2016 Pages 1–330

Journal of

Materials Chemistry A Materials for energy and sustainability www.rsc.org/MaterialsA

This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the author guidelines.

ISSN 2050-7488

PAPER Kun Chang, Zhaorong Chang et al. Bubble-template-assisted synthesis of hollow fullerene-like MoS2 nanocages as a lithium ion battery anode material

Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the ethical guidelines, outlined in our author and reviewer resource centre, still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.

rsc.li/materials-a

Page 1 of 11

Pleaseof doMaterials not adjust margins A Journal Chemistry View Article Online

DOI: 10.1039/C8TA03953D

ARTICLE Highly efficient solar cells based on Cl incorporated tri-cation perovskite materials Received 00th January 20xx, Accepted 00th January 20xx

#, €

#, €

£

€

€

€

€

Muhammad Azam , Shizhong Yue , Rui Xu , Kong Liu , Kuankuan Ren , Yang Sun , Jun Liu , ,€ ,€ ,£ € Zhijie Wang* , Shengchun Qu* , Yong lei* , and Zhanguo Wang

Published on 18 June 2018. Downloaded on 6/22/2018 3:42:05 AM.

DOI: 10.1039/x0xx00000x www.rsc.org/

Though mixed cation hybrid organic-inorganic perovskite materials are of promises due to the high efficiency and longterm stability of the according devices, a fundamental understanding on the function of the mixed cation system is still unclear. Herein, we systematically investigate the roles of Cs cation and Cl anion in inverted structured perovskite solar cells based on CsxFA0.2MA0.8-xPb(I1-yCly)3. For the role of Cs, we observe that an appropriate amount of Cs in the film could improve the crystal quality of the film and optimize the energy band alignment of the device, thus reducing the nonradiative recombination and promoting the charge transport efficiently. The presence of Cl with a suitable concentration also possesses these functions. More importantly, the two limitations for the application of the devices, hysteresis and performance instability, could also be addressed in some sense. Thus a high power conversion efficiency of 20.31% has been realized and a universal method for constructing highly-efficient perovskite solar cells has been provided.

Introduction Hybrid organic-inorganic perovskite solar cells (PSCs) have attracted considerable attention in past few years. A power conversion efficiency (PCE) has reached 22.7% with a certified identification on single-junction PSCs, from the initial PCE of 1-2 3.8% in 2009. These great progresses are attributed to the intrinsic physical property of perovskite materials, such as high absorption coefficient, long charge-carrier diffusion length, tunable bandgap, low binding energy of exciton, and high charge-carrier mobility.3-5 Meanwhile tremendous efforts about the structural design of devices have been worked out, like developing electron or hole transport layers (ETL, HTL) with particular property and applying suitable buffer layers for modifying the interlayer, to facilitate the charge transport and collection by obtaining an ideal band alignment and reducing interfacial charge recombination.6-9 Furthermore, efficient 2 devices with large active area over 50 cm have been successfully realized by applying ETL/perovskite interface

engineering.10 After a period of rapid development, the photovoltaic efficiency of PSCs can compete with that of stateof-the-art crystalline silicon solar cells. To become the next candidate in the market of solar cells, the hybrid perovskite devices have to be of long-term stability. Generally, perovskite is one category of materials sharing similar crystal structure and chemical formula AMX3, where A+ + + is typically organic cations (methylammonium (MA ), or + + + 2+ 2+ 2+ formamidinium (FA )) or inorganic Cs , M is Pb or Sn , and X- is I-, Br- or Cl-. In the early years, researchers mainly focused on improving the performance of the devices with the active layer of methylammonium lead triiodide (CH3NH3PbI3 or 11-13 MAPbI3). However due to the instability for long-term operation under illumination or moisture environments and the limitations in harvesting near-infrared light in the solar spectrum, MAPbI3 is gradually substituted by formamidinium lead triiodide (CH(NH2)2PbI3 or FAPbI3). FAPbI3 presents significantly enhanced thermal stability and a suitable bandgap 12 (∼1.45 eV), which is close to the ideal bandgap (1.2−1.3 14 eV). Unfortunately, FAPbI3 shows an undesired phase transition, in which the photoactive black α-phase (only stable above 160 ℃) readily degrades into photo-inactive yellow δphase at room temperature. This seriously affects the practicability of the corresponding device, let alone the large15-16 scale application. To control the phase transition of FAPbI3, the strategy of diminishing the size of cation was developed by + + incorporating MA or Cs in the composition. Although the + + + + + + + MA /FA , Cs /FA , and Cs /MA /FA mixed-cation compositional modulation have been proposed as an effective 17 method to restrain the phase conversion, the PCE of the

J. Name., 2013, 00, 1-3 | 1

This journal is © The Royal Society of Chemistry 20xx

Please do not adjust margins

Journal of Materials Chemistry A Accepted Manuscript

Journal Name

Pleaseof doMaterials not adjust margins A Journal Chemistry

Page 2 of 11 View Article Online

DOI: 10.1039/C8TA03953D

Journal Name

corresponding device is slightly lower than that with pure MAPbI3. Recently Seok et al. found that (MAxFA1-x)Pb(IyBr1-y)3 can keep the photoactive phase stable at room temperature and the fabricated device presents a remarkable 18 performance. Hence, (MAxFA1-x)Pb(IyBr1-y)3 became a hotpot in mixed cation system. Based on this Br-doping composition, numerous efforts about cation-doping have been carried 17, 19-20 out. However, the reports rarely refer to cation-doping in the Cl-doping system, especially the effect of anion in the composition. The issue of the mutual function of cation-doping and Cl-doping thus deserves more attention. In this study, to realize a high PCE, we developed a novel composition for the perovskite active layer on the basis of CsxFA0.2MA0.8-xPb(I1-yCly)3 and systematically analyzed the influence of the combination of cation- and Cl- doping on the performance of PSCs. The influences of previously unexplored Cs ratio and Cl incorporation on photovoltaic properties of triple cation PSCs were investigated detailedly. More importantly, we obtained highly efficient, hysteresis free devices with PCE 20.31% by optimized composition of Cs (20%) and Cl (10%). The devices also present an improved stability in comparison with the device without Cl doping.

Results and Discussion Fig. 1a displays the schematic representation of a standard inverted planar perovskite solar cell in this study，where NiOx : 5%Cu is used as HTL and PC61BM/ZrAcac is used as ETL. The perovskite film in the device was prepared by using one-step fast crystallization method with anti-solvent treatment by 21 tuluene. Fig. 1b shows the corresponding cross-sectional scanning electron microscopy (SEM) image of the best performing device. The multi-layered structure can be observed clearly. The thickness of the active layer was gauged as 340 nm which could absorb sufficient sunlight (Fig. S1) and diffuse charge carriers to the respective transport layer efficiently. By adopting the above method and structure, the PSCs with the active layer of Cs0.2FA0.2MA0.6Pb(I0.9Cl0.1)3 present a decent photovoltaic property. Fig. 1c illustrates current density versus voltage (J-V) curves of the best performing device (reverse and forward scan) based on Cs0.2FA0.2MA0.6Pb(I0.9Cl0.1)3 perovskite. The device presents a promising PCE of 20.31% measured in the forward scan direction, including a Voc of 1.13 V, a Jsc of 23.72 mAcm-2, and a FF of 75%. Additionally, the PCE is 20.24% when measured in the reverse scan direction, with a Voc of 1.13 V, a Jsc of 23.67 mAcm-2, and a FF of 75%. This suggests that the hysteresis in the device is negligible. Incident photon-to-current efficiency (IPCE) spectra with integrated current density were measured (Fig. 1d). The device shows the remarkable capacity of converting solar energy into electricity in the range of 300-800 nm and the value of the integrated Jsc is close to the Jsc from J- V curve. Further, to explore the effect of Cs to the photovoltaic property of device, systematic

Fig. 1 (a) The schematic representation of the perovskite photovoltaic device. (b) The cross-sectional scanning electron microscopy (SEM) image of the planar inverted perovskite photovoltaic devices. (c) J-V characteristics of best performing device (Reverse and Forward scan) based on Cs0.2FA0.2MA0.6Pb(I0.9Cl0.1)3 perovskite. (d) The IPCE spectrum of the corresponding device. The integrated Jsc from the IPCE spectrum is also shown. experiments were executed. Fig. 2a displays the J–V curves for the devices based on CsxFA0.2MA0.8-xPb(I0.9Cl0.1)3 (x = 0, 0.1, 0.2, 0.3, 0.4) and the detailed parameters and statistical analysis are shown in Table 1 and Fig. S2 respectively. The acceptable derivation of the integrated Jsc from IPCE spectra with the Jsc from J-V curves is discussed in Supporting Information (Section 2). The device without Cs exhibits a low PCE of 15.5 ± 1.5% and the efficiency of the devices with the perovskite film spincoated from the precursor containing 10% Cs is enhanced to 17.5 ± 0.6%. When the ratio of Cs precursor increases to 20%, an optimized PCE of 20 ± 0.31% is obtained. Interestingly, as further increasing the Cs ratio in precursor solution, the device performance is deteriorated and PCE decreases to 14.2 ± 0.8% for the device with the film spin-coated from the precursor containing 30% Cs. The IPCE spectra are shown in Fig. 2b. The value of IPCE for the devices with the active layer spin-coated from the 20% Cs containing precursor is remarkably higher than that of devices without Cs incorporation. With the concentration of Cs precursor increasing from 0 to 20%, the value of IPCE as well as integrated current density both increase accordingly, but the IPCE value decreases with further increase in Cs ratio in Table 1. Photovoltaic device parameters based CsxFA0.2MA0.8-xPb(I0.9Cl0.1)3 (x = 0, 0.1, 0.2, 0.3, 0.4). x 0 0.1 0.2 0.3 0.4

Jsc [mAcm-2] 20 ± 0.8 21 ± 0.65 23.0 ± 0.7 17.2 ± 1.1 14.30 ± 0.5

2 | J. Name., 2012, 00, 1-3

Voc [V] 1.11 ± 0.02 1.12 ± 0.01 1.13 ± 0.01 1.14 ± 0.01 1.15 ± 0.01

FF [%] 69 ± 1 71 ± 2 75 ± 2 71± 1 67± 3

PCE [%] 15.5 ± 1.5 17.5 ± 0.6 20.1 ± 0.3 14.2 ± 0.8 11.3± 1.55

on

Jsc (IPCE) [mAcm-2] 17.18 19.29 20.24 12.9 11.07





ARTICLE

Page 3 of 11


DOI: 10.1039/C8TA03953D

ARTICLE

Fig. 2 (a) J-V characteristics of the photovoltaic devices based on CsxFA0.2MA0.8-xPb(I0.9Cl0.1)3 (x = 0, 0.1, 0.2, 0.3, 0.4) perovskite under AM 1.5. (b) Corresponding IPCE spectra. (c) XRD spectra of corresponding perovskite films on ITO glass. (d) TRPL decay curves of corresponding films made on glass. precursor solution, in well consistency with the variational tendency of Jsc obtained from J-V curves. This indicates that an optimum ratio of Cs is beneficial for enhancing the photoelectric conversion efficiency of the devices. To reveal the effect of Cs on the crystal structure of perovskite films, Xray diffraction (XRD) measurements were carried out. As shown in Fig. 2c, the series of diffraction peaks at 2θ values of 13.93ᵒ, 20ᵒ, 28.07ᵒ and 40.11ᵒ can be assigned to the standard (101), (012), (202), and (024) lattice planes of the black 22-23 CsxFA0.2MA0.8-x phase, respectively. A small peak at 9.8ᵒ 24 corresponds to the peak of δ-CsPbI3 and its intensity strengthens with the concentration of Cs increasing, indicative of that there could be a phase separation between main black 25 CsxFA0.2MA0.8-x and δ-CsPbI3. Considering that the δ-CsPbI3 phase needs a high temperature for transforming into 26-27 photoactive α-phase, there would be excessively unnecessary photoinactive δ-phase of CsPbI3 in the film, resulting in a detrimental effect to the performance of PSCs.28 Fig. S3 shows the color variation of the corresponding perovskite films after annealing at 100 ℃ for 10 min. When the ratio of Cs is over 40%, the film cannot completely turn into black, suggesting that the film is not suitable for absorbing visible light. X-ray photoelectron spectra (XPS) of the series of perovskite films indicate that all the films have an acceptable purity without any elemental contamination. Meanwhile, as shown in Fig. S4, with the Cs ratio increasing in precursor, the gradual enhancement in Cs3d peaks intensity and atomic percentage is obtained. To understand the effect of Cs to nonradiative recombination behaviours of photo-excited carriers

in the perovskite films, time-resolved photoluminescence (TRPL) measurements were carried out on the perovskite film coated on glass and the results are shown in Fig. 2d. When we increase the concentration of Cs precursor from 0 to 20%, the PL lifetime of the resulting film presents an increasing tendency (207 to 352 ns). Since longer lifetime indicates that the photo-excited carriers possess a lower probability of non29-30 this result indicates the presence radiative recombination, of Cs in the perovskite film is beneficial for reducing the probability of non-radiative recombination. As the concentration of Cs in precursor is increased further, however, the lifetime of the film decays. As revealed by the XRD spectrum, overloaded Cs in the film would cause the serious phase separation and the phase boundary would behave as the recombination center for charge recombination. To get more information, the morphology of perovskite films with the different Cs/MA ratios was investigated by SEM shown in Fig. 3. Interestingly, the film spin-coated from the 20% Cs containing precursor, whose photovoltaic performance outperforms comparing with the others, shows a clear and smooth morphology without pinholes. By decreasing the Cs ratio in precursor, we can still obtain a compact film, but many small crystals begin growing and attach on the surface. 31 As revealed by our previous work, the small size of crystal grains would increase the probability of non-radiative recombination and create an adverse effect on the PCE. Alternatively, by increasing the ratio of Cs, lots of pinholes appear on the surface of the film. These pinholes as the center of non-radiative recombination would promote the recombination process of photo-excited carriers and the efficiency of corresponding devices would seriously decay. These results can support our former analysis in TRPL again. The optimum doping of Cs in perovskite is helpful for obtaining well crystalline films, which can promote the separation and collection processes of photo-excited carriers. To directly obtain the charge transport information of the devices, we have calculated the charge transfer resistance by impedance spectra (Fig. S5). Nyquist plots of the corresponding devices were measured at 1.1 V bias voltage under dark condition and the respective fitted values of resistance are listed in Table S1. Considering the diameter of semicircle is proportional to the 32 charge transfer resistance, the device with the film spincoated from the 20% Cs precursor shows the smallest semicircle, indicating that the charge transport resistance is the lowest in comparison with other devices. This result illustrates that optimized Cs ratio is helpful for device performance in the aspect of charge transport. Changes of the Cs ratio (0-40%) in perovskite precursor may also effect the position of energy levels of the resulting perovskite film and thus the photovoltaic performance of the device by altering the charge transport efficiency. The energy levels of relevant photo-active layers, as characterized by ultraviolet (UV)

J. Name., 2013, 00, 1-3 | 3





Journal Name



DOI: 10.1039/C8TA03953D


ARTICLE

Fig. 3 Planar SEM images of perovskite films spin-coated from (a) 0% Cs, (b) 10% Cs, (c) 20% Cs, (d) 30% Cs, (e) 40% Cs incorporated precursor solution. (f) Comparative band energy diagram of the CsxFA0.2MA0.8-xPb(I0.9Cl0.1)3 (x = 0, 0.1, 0.2, 0.3, 0.4) perovskite compounds. The values of these energy band positions are referred to vacuum level.

photoemission spectroscopy (UPS) and UV-visible absorption spectroscopy measurements (Fig. S6), are shown in Fig. 3f. Specifically, the Fermi level (Ef) as well as valence band maximum (VBM) can be determined from the cut-off of UPS spectra in Fig. S7. Ef is determined by the position of lower kinetic energy cut-off and the position of lower binding energy cut-off represents the gap between Ef and VBM, with inset showing the exact cut-off position in lower binding energy side. The conduction band minimum (CBM) is determined by subtracting the bandgap energy (Eg) from the corresponding VBM. It is obviously concluded from the energy band diagram that the VBM upshifts from 4.40 eV to 3.88 eV with Cs ratio increasing from 0 to 20%. Further increase in Cs ratio (30-40%) shifts the VBM downwards from 3.88 eV to 4.32 eV. On the other hand, CBM shows the similar tendency as the corresponding variation in Cs ratio. In comparison with the 33 CBM of PC61BM (4.00 eV ), only the perovskite film spincoated from the precursor with 20% Cs presents the CBM that is higher than 4.00 eV and this alignment is energetically favourable for transporting the photo-generated electrons from perovskite to PC61BM. The other perovskite films all have

a lower CBM than PC61BM and such alignment would deteriorate the electron transport due to the formed energy barrier. Alternatively, for facilitating the collection of the photo-generated holes in the perovskite, the VBM of the perovskite films should be lower than that of NiOx and a smaller distance of the VBM of these two component usually indicates a lower energy loss for hole transport. In this case, the perovskite film from 20% Cs precursor also exhibits a superior capability in transporting the holes to NiOx with a low energy loss in comparison with the other films. In addition, we have also focused on optimizing the Cl incorporation in perovskite material based on Cs0.2FA0.2MA0.6PbI3 by choosing champion ratio (20%) of Cs for fabricating the film. Although some groups have investigated the Cl doping and its influence on the performance of MAPbI3 34-36 devices, as well as perovskite devices based on double 37-38 cation FA/MA, there is still no reports on Cl addition in triple cation perovskite. Here we have investigated the effect of Cl ratio on photovoltaic, structural and opto-electrical properties of Cs/FA/MA-based perovskite. Fig. 4a shows the JV curves to examine the influence of Cl incorporation on

J. Name., 2013, 00, 1-3 | 4




Journal Name

Page 5 of 11


DOI: 10.1039/C8TA03953D


ARTICLE

Fig. 4 (a) J-V characteristics of the photovoltaic devices based on Cs0.2FA0.2MA0.6Pb(I1-yCly)3 (y = 0, 0.1, 0.15) perovskite under AM 1.5. (b) The corresponding IPCE spectra and the integrated Jsc. (c) XRD spectra of corresponding perovskite films on ITO glass. (d) TRPL decay curves of corresponding films made on glass. photovoltaic performance of devices and relevant detailed parameters are listed in Table 2 and the corresponding statistical analysis is shown in Fig. S8. The device without Cl presents a PCE of 15.5 ± 0.5%, a Jsc of 19.0 ± 0.8 mAcm-2 , a Voc of 1.09 ± 0.01 V and a FF of 69 ± 3%, in consistency with the 39 reported values. Substituting 10% of the iodide by Cl in precursor significantly improves the PCE further to a value of -2 20.0 ± 0.31%, with a Jsc of 23.0 ± 0.7 mAcm , a Voc of 1.13 ± 0.01 V and a FF of 75 ± 2%. While further increasing the Cl ratio to 15%, the device performance begins deteriorating. These findings imply that with the incorporation of 10% Cl in the precursor, substantial improvement is simultaneously -2 observed in Jsc from 19.0 to 23.0 mAcm , in Voc from 1.09 to 1.13 V and in FF from 69 to 75%, thus suggesting that photovoltaic properties are exceptionally effected by tiny amount of Cl inclusion in perovskite. To get more information, the IPCE spectra and the corresponding integrated Jsc of the series of devices were measured and displayed in the Fig. 4b. The IPCE peaks blueshift with higher concentration of Cl incorporation which is consistent with the previous report.38 The integrated Jsc values obtained from IPCE spectra are approximately in agreement with those from the J-V curves. Fig. 4c shows the XRD patterns of Cs0.2FA0.2MA0.6Pb(I1-yCly)3 on ITO glass prepared from precursor solutions containing different ratios of Cl. It is interesting to note that (110) peak at

about 14ᵒ increases slightly in intensity with the amount of 10% Cl incorporation in the precursor, which may suggest an 40 increase in crystallinity in perovskites with 10% Cl ratio. A conspicuous systematic shift in peak position (101) at 14ᵒ toward higher 2θ values could be observed with the increase of the Cl% (Fig. 4c inset), these results correspond to the gradual reduction in lattice parameter as well as lattice cell 41 volume, because of the gradual replacement of the iodine 38, 42 anions by the smaller chlorine anions. The small peak of PbI2 at 12.54ᵒ can be attributed to the Cl incorporation in 33 perovskite solution (10-15% Cl) . The remnant tiny amount of PbI2 in perovskite film has been reported on showing somehow positive effect on device performance.33, 43-44 To confirm the presence of Cl and elemental impurity in perovskite films, XPS was measured. Fig. S9a reveals the XPS spectra of the designated compositions and Fig. S9b-c demonstrate that the gradual increase in the Cl2p peaks intensities, as we increase the Cl concentration in the precursor. To get more understanding on the effect of Cl incorporation, we performed TRPL measurement on perovskite films spin-coated from the precursors containing 0, 10 and 15% Cl respectively. As shown in Fig. 4f, the average PL lifetime of the 10% Cl precursor spin-coated film (352 ns) is much longer than that without Cl (50.65 ns). This indicates that a suitable amount of Cl in the perovskite film is beneficial to have a low possibility of non-radiative recombination. An overloaded of Cl in the film could cause a fast decay of the lifetime. We have further studied the impact of Cl incorporation on charge transport resistance of the photovoltaic devices by impedance spectroscopy. As revealed by Fig. S10 and Table S2, the device with the optimized ratio of Cl shows the lowest interfacial charge transfer resistance. Fig. 5a-c show the planar SEM images of perovskite films spin-coated from the precursor containing 0, 10 and 15% Cl respectively. The reference film without Cl has a non-uniform morphology with a large number of pinholes and the grains pack together irregularly with rather Table 2. Photovoltaic device parameters Cs0.2FA0.2MA0.6Pb (I1-yCly)3 (y = 0, 0.1, 0.15). Cl [%]

Jsc [mAcm-2]

Voc [V]

FF [%]

based

PCE [%]

on

Jsc (IPCE) -2 [mAcm ]

0

19 ± 0.8

1.09 ± 0.01

69 ± 3

15.5 ± 0.5

16.27

10

23.0 ± 0.7

1.13 ± 0.01

75 ± 2

20.1 ± 0.3

20.24

15

20.5 ± 0.8

1.11 ± 0.01

72 ± 3

17.2± 1.4

17.45

J. Name., 2013, 00, 1-3 | 5




Journal Name



DOI: 10.1039/C8TA03953D

Journal Name

Fig. 5 Planar SEM images of perovskite films (a) 0 Mol.% Cl (b)10 Mol.% Cl (c) 15 Mol.% Cl based on Cs0.2FA0.2MA0.6Pb(I1-yCly)3 (y = 0, 0.1, 0.15). (d) Comparative band energy diagram of the corresponding perovskite films. The values of these energy band positions are referred to vacuum level (e) J–V curves of the electron-only devices with the structure (ITO/Al/(0-15% Cl) perovskite/PC61BM/Al). (f) Hole-only device with the structure (ITO/ NiOx+5% Cu/(0-15% Cl) perovskite/Au) measured under dark conditions. sharp grain boundaries. The addition of 10% Cl to the precursor strongly effects the morphology of the film, and a uniform and pinhole-free film could be obtained. However, when the concentration of Cl in precursor increases up to 15%, the density of grain boundaries raises in the perovskite film as compared to the optimized condition, making the morphology adverse for ideal performance of PSCs. Interestingly, the variation of PCE exactly follows the tendency on the changes of the morphology to the response of Cl variation. To get better understanding on the significance of Cl to the charge transport dynamics, we have examined the energy band structure alignment as the function of Cl ratio as shown in the Fig. 5d, which is determined from the UVabsorption spectra (Fig. S11) and UPS spectra (Fig. S12). The VBM could be obtained as 5.77, 5.52 and 5.35 eV for the films spin-coated from the 0, 10 and 15% Cl incorporated precursors, whereas, the CBM positions are obtained as 4.15, 3.88 and 3.72 eV for respective perovskite films. In comparison with the CBM of PC61BM, the perovskite films without Cl have a lower CBM and this alignment is not favourable for transporting the excited electrons from the perovskite to PC61BM. Alternatively, the perovskite film spin-coated from the precursor with 10% Cl shows an upward shifted CBM to 3.88 eV, such value is beneficial for transporting the electrons to PC61BM. However, overloaded Cl in the film would move the CBM upwards more. Though the electron transport is favourable, this would cause more energy loss in the transport.

For the hole transport to NiOx, the VBM positions of the films spin-coated from the precursors with 0 and 10% Cl are all lower than that of NiOx and this is beneficial for the hole transport. However, the later one possesses a VBM that is closer to the VBM of NiOx than the former one and this enables the later film with a lower energy loss in the hole transport. An overloaded of Cl moves the VBM upwards more, even higher than the VBM of NiOx. In this case, the hole transport is not favourable. The accuracy of the measurement of energy levels is good. Fig. S13 shows three sets of UPS spectra of the CsxFA0.2MA0.8-xPb(I1-yCly)3 film with 20% Cs and Cl 10%. Each sample was prepared and measured at different time. The results demonstrate that the energy levels of CsxFA0.2MA0.8-xPb(I1-yCly)3 film with 20% Cs and Cl 10% for different samples are in good accuracy and the according deviation could be ignorable in comparison with the difference in energy levels of the films with different Cs or Cl doping concentrations. The effect of Cl incorporation in Cs/FA/MA perovskite has also been investigated via measuring electron and hole mobilities (μe, μh) as well as electron and hole trap state densities (nt) in the perovskite device by the space-charge-limited current 45 (SCLC) technique. Fig. 5e and f display the J-V characteristics for the electron and hole-only devices respectively fitted with the Mott–Gurney law, where three regions could be clearly observed. These three regions can be identified according to the different values of the exponent n (J ∝ Vn relation): n = 1 is

6 | J. Name., 2012, 00, 1-3





ARTICLE

Page 7 of 11


DOI: 10.1039/C8TA03953D


ARTICLE

Fig. 6 (a) Energy band diagram of best performing device based on Cs0.2FA0.2MA0.6Pb(I0.9Cl0.1)3 perovskite. (b) Hysteresis behaviour as the function of Cl ratio in perovskite. (c) Stability test of the devices with and without Cl incorporation in perovskite measured under same conditions. XRD spectra of perovskite measured after different intervals of time, stored under ambient conditions (RH > 50%) (d) whithout Cl (e) with 10% Cl (f). Absorption profile of perovskite/ITO/glass films with and without Cl incorporation (the value of absorbance intensity was taken at 600 nm wavelength after different intervals of time). the Ohmic region, n = 2 is the SCLC region, and in between is the trap-filled limited region. At kink point where the current increases abruptly, the voltage is termed as the trap-filled limit voltage (VTFL). From this region, the trap density nt could be 46 calculated using the following relation : ࢂࢀࡲࡸ ൌ

ࢋ࢔࢚

ࡸ૛

૛ࢿࢿ࢕

(1)

where L is the thickness of the perovskite films, ε is the relative dielectric constant of perovskite (taken as the value of 32 for 47 MAPbI3 from a previous report ), and εo is the vacuum permittivity. The VTFL values, including the calculated densities of electron and hole trap-state for the devices with the perovskite films fabricated from the precursors with different ratios of Cl, are listed in the Table S3 and S4 respectively. In comparison with the either reference device (with 0% Cl) or with 15% Cl, the optimized device (10% Cl) presents an obvious decrease in trap-state density. This reduction in trap density is from the high quality crystallized film, possible passivation effect with the inclusion of tiny amount of Cl in perovskite film, and the optimized bandgap alignment of the device.

Furthermore, we have calculated the carrier mobilities (μe, μ h) 48 at trap free regime (n = 2) by applying the Mott−Gurney law ࡶࡰ ൌ

ૢࣆࢿࢿ࢕ ࢂ૛ ૡࡸ૜

(2)

where JD is the dark current, μ is the carrier mobility, ε is the relative dielectric constant of perovskite, εo is the vacuum permittivity, V is applied voltage and L is the thickness of active layer. The corresponding J1/2-V curves are given in the Fig. S14 and S15 for electron and hole only devices respectively. A 1/2 higher slop of J -V curve results in a larger mobility and the relevant fitted values are given in Table S5 and S6. Expectably, the device with film spin-coated from the 10% Cl precursor presents the largest electron and hole mobilities, 1.68×10-3cm2 -1 -1 -3 -2 -1 -1 V s and 5.90×10 cm V s respectively, which are almost 4 folds higher than the device without Cl modification. Either a lower or higher concentration than 10% of Cl in precursor would induce a deterioration in charge carriers mobilities. Fig. 6a represents the bandgap alignment of the whole device. The CBM and VBM for NiOx+5%Cu, PC61BM and ZrAcac were 33 obtained from our previous research. Here we have studied

J. Name., 2013, 00, 1-3 | 7




Journal Name



DOI: 10.1039/C8TA03953D

Journal Name

the Cs+ and Cl- compositional engineering to get a suitable band alignment with NiOx+5% Cu based HTL and PC61BM/ZrAcac based ETL (Fig. 3f, 5d and 6a). These aligned energy levels of multiple layers suggest the efficient extraction of photo-generated carriers without generating excessive interface recombination, thus facilitating the carriers transport to the respective extraction layer by reducing the current leakage as shown in Fig. S16. Besides improvement in PCEs with 10% Cl incorporation in the precursor, the most interesting phenomenon is the elimination of common hysteresis in fabricated devices. Fig. 6b shows the hysteresis behaviour of the devices prepared with 0-15% Cl precursors and device parameters with forward and backward scan are shown in Fig. S17 and Table S7 respectively. Since the J-V hysteresis is related to the native defects in the perovskite 49 layer and we have proved that the defects or trap in the devices with champion Cl ratio are extensively reduced than other devices, it is understandable that the device with the film spin-coated from the 10% Cl precursor shows an ignorable hysteresis effect. One major concern is the durability of PSCs in ambient environment. We therefore monitored the stability of the devices based on the perovskite film with or without Cl incorporation. Both of the devices were stored in N2 environment at room temperature without encapsulation and photovoltaic measurements were performed after different interval of time for over 1000 hours and the corresponding normalized PCEs are shown in Fig. 6c. These results indicate that Cl based device shows a moderate decrease in PCE during the stability test and over a period of 1000 h it has maintained about 90% of its initial efficiency. Under the same storage and fabricated conditions, the device without Cl incorporation shows a rapid decrease in PCE and maintained about 40% of its initial PCE after 1000 h. To explore the reason behind the fast decaying performance, the XRD measurements were taken on perovskite films without and with 10% Cl incorporation in the precursor as shown in the Fig. 6d-e. The samples were stored under ambient conditions (RH > 50%) without encapsulation and measurements were taken after different intervals of time. Previously, it was proved that the ratio of PbI2 peaks to perovskite peaks quantified the degree of perovskite 50 degradation. It can be observed that the PbI2 peak intensity at 12.6ᵒ increases with aging time more rapidly for film without Cl as compared to the film spin-coated from the 10% Cl precursor. Whereas, the perovskite peak intensities for the film without Cl incorporation at 13.93ᵒ, 20ᵒ, 28.07ᵒ and 40.11ᵒ were decreasing with the storage time and disappeared after 30 days (dark brownish film turns into yellow). However, the films made from 10% Cl precursor maintained the perovskite phase even after 30 days of aging time in ambient conditions, suggesting that the films show a significantly improved phase stability. Furthermore, we have examined absorption capability of the films with or without Cl incorporation, stored under ambient conditions (RH > 50%). Fig. 6f shows the absorption spectra of corresponding films after regular

intervals of time. The film without Cl shows a rapid degradation, because the dark brownish color turns into pale yellow due to moisture sensitive nature of perovskites. The absorptivity of the perovskite film decreases quickly as compared to the perovskite film made from the precursor with 10% Cl. These results suggest that the addition of Cl into Cs/FA/MA based perovskite material improves the device stability. Fig. S18 shows the transient photocurrent and photovoltage curves of the devices. In comparison with devices with perovskite films that are not doped by Cs or Cl, the champion device presents the fastest photocurrent decay and lowest photovoltage decay. These results demonstrate that the best-performed device is of decent capability in extracting the photo-generated charge carriers efficiently and the charge recombination possibility is the lowest among the devices herein.

Conclusions In summary, we have investigated the function of Cs cation and Cl anion in perovskite solar cells with inverted structure systematically. Using a combination of techniques, we found that 20% Cs and 10% Cl incorporation to the precursor could improve the morphology quality of the perovskite film, regulate the corresponding alignment of energy levels of the photovoltaic device, and facilitate the transport of charge carriers by improving the electron and hole mobility and reducing the density of respective trap state of the device. Most interestingly, the stability of the devices could also be enhanced with the addition of Cl. Thus, an impressive PCE about 20.31% with ignorable hysteresis has been realized. With detailed working mechanism discussed in a fundamental way, this designed method provides a novel and universal strategy to improve the efficiencies and stability of perovskite photovoltaic devices.

Experimental Section Materials and solution Preparation: Formamidinium iodide (FAI) Methylammonium iodide (MAI; >99.5%), lead chloride (PbCl2) and lead iodide (PbI2; >99.99%) were purchased from Xi’an Polymer Light Technology Corp. Caesium iodide (CsI), [6, 6]-Phenyl C61butyric acid methyl ester (PC61BM; >99.9%) and anhydrous N, N-dimethylformamide (DMF; 99.8%) were purchased from Sigma-Aldrich. Nickel acetate tetrahydrate (Ni(CH3COO)2•4H2O) and cupric acetate monohydrate (Cu(CH3COO)2•H2O) were got from Sinopharm Chemical Reagent Co.,Ltd. Zirconium(IV) acetylacetonate (ZrC20H28O8) was provided by Alfa Aesar. All the solvents were purchased from Beijing Chemical Works. Unless otherwise stated, all chemicals were used as received. For the function investigation of Cl in triple cations perovskite, FAI, CsI, and MAI were mixed with a fixed ratio (0.2: 0.2: 0.6) and the molar ratio of PbI2/PbCl2 was varied (1:0, 0.9:0.1, 0.85:0.15) in anhydrous DMF. To study the effect of Cs,

8 | J. Name., 2012, 00, 1-3





ARTICLE

Page 9 of 11


DOI: 10.1039/C8TA03953D

ARTICLE

CsI/MAI ratio was varied (0:0.8, 0.1:0.7, 0.2:0.6, 0.3:0.5, 0.4:0.4), whereas FAI, PbI2 and PbCl2 were mixed with fixed ratio (0.2:0.9:0.1). The concentration of the perovskite precursor was 1.2 M. The mixture was then stirred overnight at 60 °C in the nitrogen glove box. The final solution of the perovskite precursor was aged in a glove box for 2 to 3 days, and filtrated with 0.45 μm PTFE filters before use. For hole transporting layer Ni(CH3COO)2•4H2O and Cu(CH3COO)2•H2O were dissolved separately in anhydrous ethanol (0.1 M). Cu doped NiOx films were prepared by the volume ratio of the solution (Ni: Cu, 95:5). PC61BM was dissolved in -1 cholorobenzene (20 mgmL ). For electron buffer layer, ZrC20H28O8 (ZrAcac) was dissolved in anhydrous ethanol (2 -1 mgmL ). Device Fabrication: FTO patterned glasses were cleaned sequentially with detergent, isopropanol, acetone and then ethanol. Cu doped NiOx film was spin coated at 3000 rpm for 30 s and annealed for 1 h at 340 ℃ in ambient conditions. When the temperature decreased to 100 ℃ the substrates were immediately transferred into the nitrogen glove box (H2O and O2 < 1 ppm). To deposit the active layer, perovskite solution (25 μL) was dropped onto the center of substrates and then spun at 6000 rpm for 30 s, and anhydrous toluene was quickly dropped onto the centre of substrate after desired interval of time. The obtained dark brownish films were annealed at 100 °C for 10 min on the hot plate. After cooling down to room temperature, PC61BM solution was subsequently spin coated on top of the perovskite layer at 1000 rpm for 30 s. The final solution for buffer layer ZrAcac was deposited on top of the PC61BM layer at 2000 rpm for 30 s. To complete the device, Ag (100 nm) was deposited by thermal evaporation under high vacuum. Characterization: The current Density-voltage (J-V) characteristics were measured using a Keithley 2450 source measure unit under AM 1.5G illumination with an intensity of 100 mWcm-1. The XRD spectra were obtained using a Rigaku V2500 X-ray diffractometer. IPCE measurements were obtained at QEPVSI-B Measurement System (Newport). The cross-sectional and planar scanning electron microscope (SEM) image of the device structure was measured by JSM-7401F. Absorbance spectrum measurements were performed on ultraviolet-visible spectrophotometer (UV-vis, Persee TU-1950) in absorption mode. PL measurements were performed with time correlated single photon counting (TCSPC) system (Edinburgh F900) and the samples were excited by a pulsed laser with the wavelength of 485 nm.

Conflicts of interest There are no conflicts to declare.

Acknowledgements This work was mostly supported by the National Basic Research Program of China (Grant No. 2014CB643503), the National Key Research and Development Program of China (Grant No.

2017YFA0206600), Key Research Program of Frontier Science, CAS (Grant No. QYZDB-SSW-SLH006), National Natural Science Foundation of China (Contract Nos. 61674141, 61504134 and 21503209), Beijing Natural Science Foundation (2162042), European Research Council (ThreeDsurface, 240144 and HiNaPc, 737616), Federal Ministry of Education and Research in Germany (BMBF, ZIK- 3DNanoDevice, 03Z1MN11), German Research Foundation (DFG: LE 2249_4-1 and LE 2249/5-1), Z. W. appreciates the support from Hundred-Talent Program (Chinese Academy of Sciences).

References 1. A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, J. Am. Chem Soc. 2009, 131, 6050. 2. Best Research-Cell Efficiencies, National Renewable Energy Laboratory (NREL), https://www.nrel.gov/pv/ (Accessed on December 2017) 3. M. M. Lee, J. Teuscher, T. Miyasaka, T. N. Murakami, H. J. Snaith, Science 2012, 338, 643.4. M. I. Dar, G. Jacopin, S. Meloni, A. Mattoni, N. Arora, A. Boziki, S. M. Zakeeruddin, U. Rothlisberger, M. Gratzel, Sci. Adv. 2016, 2, e1601156. 5. H. Yu, K. Ren, Q. Wu, J. Wang, J. Lin, Z. Wang, J. Xu, R. F. Oulton, S. Qu, P. Jin, Nanoscale 2016, 8, 19536. 6. J. H. Kim, P. W. Liang, S. T. Williams, N. Cho, C. C. Chueh, M. S. Glaz, D. S. Ginger, A. K. Jen, Adv. Mater. 2015, 27, 695. 7. Q. Wang, C. Bi, J. Huang, Nano Energy 2015, 15, 275. 8. F. Giordano, A. Abate, J. P. Correa Baena, M. Saliba, T. Matsui, S. H. Im, S. M. Zakeeruddin, M. K. Nazeeruddin, A. Hagfeldt, M. Graetzel, Nat. Commun. 2016, 7, 10379. 9. S. Yue, S. Lu, K. Ren, K. Liu, M. Azam, D. Cao, Z. Wang, Y. Lei, S. Qu, Z. Wang, Small 2017, 13, 1700007. 10. A. Agresti, S. Pescetelli, A. L. Palma, A.E. Del Rio Castillo, D. Konios, G. Kakavelakis, S. Razza, L. Cinà, E. Kymakis, F. Bonaccorso, A. D. Carlo, ACS Energy Lett. 2017, 2, 279. 11. S. D. Stranks, H. J. Snaith, Nat. Nanotechnol. 2015, 10, 391. 12. N. J. Jeon, J. H. Noh, W. S. Yang, Y. C. Kim, S. Ryu, J. Seo, S. I. Seok, Nature 2015, 517, 476. 13. Y. Zhou, K. Zhu, ACS Energy Letters 2016, 1, 64. 14. F. Meillaud, A. Shah, C. Droz, E. Vallat-Sauvain, C. Miazza, Sol. Energy Mater. Sol. Cells 2006, 90, 2952. 15. J. W. Lee, D. J. Seol, A. N. Cho, N. G. Park, Adv. Mater. 2014, 26, 4991. 16. G. E. Eperon, S. D. Stranks, C. Menelaou, M. B. Johnston, L. M. Herz, H. J. Snaith, Energy Environ. Sci. 2014, 7, 982. 17. D. P. McMeekin, G. Sadoughi, W. Rehman, G. E. Eperon, M. Saliba, M. T. Horantner, A. Haghighirad, N. Sakai, L. Korte, B. Rech, M. B. Johnston, L. M. Herz, H. J. Snaith, Science 2016, 351, 151. 18. W. S. Yang, B. W. Park, E. H. Jung, N. J. Jeon, Y. C. Kim, D. U. Lee, S. S. Shin, J. Seo, E. K. Kim, J. H. Noh, S. I. Seok, Science 2017, 356, 1376. 19. C. Yi, J. Luo, S. Meloni, A. Boziki, N. Ashari-Astani, C. Grätzel, S. M. Zakeeruddin, U. Röthlisberger, M. Grätzel, Energy Environ. Sci. 2016, 9, 656. 20. D. P. McMeekin, Z. Wang, W. Rehman, F. Pulvirenti, J. B. Patel, N. K. Noel, M. B. Johnston, S. R. Marder, L. M. Herz, H. J. Snaith, Adv. Mater. 2017, 29, 1607039. 21. N. J. Jeon, J. H. Noh, Y. C. Kim, W. S. Yang, S. Ryu, S. I. Seok, Nat. Mater. 2014, 13, 897. 22. M. Saliba, T. Matsui, J. Y. Seo, K. Domanski, J. P. CorreaBaena, M. K. Nazeeruddin, S. M. Zakeeruddin, W. Tress, A. Abate, A. Hagfeldt, M. Gratzel, Energy Environ. Sci. 2016, 9, 1989.

J. Name., 2013, 00, 1-3 | 9





Journal Name



DOI: 10.1039/C8TA03953D

Journal Name

23. T. Bu, X. Liu, Y. Zhou, J. Yi, X. Huang, L. Luo, J. Xiao, Z. Ku, Y. Peng, F. Huang, Y.-B. Cheng, J. Zhong, Energy Environ. Sci. 2017, 10, 2509. 24. T. Zhang, M. I. Dar, G. Li, F. Xu, N. Guo, M. Gratzel, Y. Zhao, Sci. Adv. 2017, 3, e1700841. 25. R. E. Beal, D. J. Slotcavage, T. Leijtens, A. R. Bowring, R. A. Belisle, W. H. Nguyen, G. F. Burkhard, E. T. Hoke, M. D. McGehee, J. Phys. Chem. Lett. 2016, 7, 746. 26. Z. Li, M. Yang, J.-S. Park, S.-H. Wei, J. J. Berry, K. Zhu, Chem. Mater. 2015, 28, 284. 27. A. Swarnkar, A. R. Marshall, E. M. Sanehira, B. D. Chernomordik, D. T. Moore, J. A. Christians, T. Chakrabarti, J. M. Luther, Science 2016, 354, 92. 28. G. E. Eperon, G. M. Paternò, R. J. Sutton, A. Zampetti, A. A. Haghighirad, F. Cacialli, H. J. Snaith, J. Mater. Chem. A 2015, 3, 19688. 29. D. W. deQuilettes, S. M. Vorpahl, S. D. Stranks, H. Nagaoka, G. E. Eperon, M. E. Ziffer, H. J. Snaith, D. S. Ginger, Science 2015, 348, 683. 30. D. Shi, V. Adinolfi, R. Comin, M. Yuan, E. Alarousu, A. Buin, Y. Chen, S. Hoogland, A. Rothenberger, K. Katsiev, Y. Losovyj, X. Zhang, P. A. Dowben, O. F. Mohammed, E. H. Sargent, O. M. Bakr, Science 2015, 347, 519. 31. M. Azam, S. Yue, K. Liu, Y. Sun, J. Liu, K. Ren, Z. Wang, S. Qu, Z. Wang, J. Alloy. Comp. 2018, 731, 375. 32. S. Lu, K. Liu, D. Chi, S. Yue, Y. Li, Y. Kou, X. Lin, Z. Wang, S. Qu, Z. Wang, J. Power Sources 2015, 300, 238. 33. S. Yue, K. Liu, R. Xu, M. Li, M. Azam, K. Ren, J. Liu, Y. Sun, Z. Wang, D. Cao, X. Yan, S. Qu, Y. Lei, Z. Wang, Energy Environ. Sci. 2017, 10, 2570. 34. J. Chae, Q. Dong, J. Huang, A. Centrone, Nano Lett. 2015, 15, 8114.

35. F. Jiang, Y. Rong, H. Liu, T. Liu, L. Mao, W. Meng, F. Qin, Y. Jiang, B. Luo, S. Xiong, J. Tong, Y. Liu, Z. Li, H. Han, Y. Zhou, Adv. Funct. Mater. 2016, 26, 8119. 36. C. Cao, C. Zhang, J. Yang, J. Sun, S. Pang, H. Wu, R. Wu, Y. Gao, C. Liu, Chem. Mater. 2016, 28, 2742. 37. F. Xie, C.-C. Chen, Y. Wu, X. Li, M. Cai, X. Liu, X. Yang, L. Han, Energy Environ. Sci. 2017, 10, 1942. 38. C. Mu, J. Pan, S. Feng, Q. Li, D. Xu, Adv. Energy Mater. 2017, 7, 1601297. 39. P. Luo, S. Zhou, Y. Zhou, W. Xia, L. Sun, J. Cheng, C. Xu, Y. Lu, ACS Appl. Mater. Interfaces 2017, 9, 42708. 40. D. Luo, L. Zhao, J. Wu, Q. Hu, Y. Zhang, Z. Xu, Y. Liu, T. Liu, K. Chen, W. Yang, W. Zhang, R. Zhu, Q. Gong, Adv. Mater. 2017, 29, 1604758. 41. Y. Li, W. Sun, W. Yan, S. Ye, H. Peng, Z. Liu, Z. Bian, C. Huang, Adv. Funct. Mater. 2015, 25, 4867. 42. J. H. Noh, S. H. Im, J. H. Heo, T. N. Mandal, S. I. Seok, Nano Lett. 2013, 13, 1764. 43. Q. Jiang, Z. Chu, P. Wang, X. Yang, H. Liu, Y. Wang, Z. Yin, J. Wu, X. Zhang, J. You, Adv. Mater. 2017, 29, 1703852. 44. Q. Jiang, L. Zhang, H. Wang, X. Yang, J. Meng, H. Liu, Z. Yin, J. Wu, X. Zhang, J. You, Nature Energy 2016, 2, 16177. 45. A. Rose, Phy. Rev. 1955, 97, 1538. 46. M. A. Lampert, Phy. Rev. 1956, 103, 1648. 47. X. Zeng, T. Zhou, C. Leng, Z. Zang, M. Wang, W. Hu, X. Tang, S. Lu, L. Fang, M. Zhou, J. Mater. Chem. A 2017, 5, 17499. 48. Q. Dong, Y. Fang, Y. Shao, P. Mulligan, J. Qiu, L. Cao, J. Huang, Science 2015, 347, 967. 49. J. W. Lee, S. G. Kim, S. H. Bae, D. K. Lee, O. Lin, Y. Yang, N. G. Park, Nano Lett. 2017, 17, 4270. 50. J. Xu, O. Voznyy, R. Comin, X. Gong, G. Walters, M. Liu, P. Kanjanaboos, X. Lan, E. H. Sargent, Adv. Mater. 2016, 28, 2807.

10 | J. Name., 2012, 00, 1-3





ARTICLE

Page 11 of 11

Journal of Materials Chemistry A View Article Online

DOI: 10.1039/C8TA03953D

Appropriate amounts of Cs and Cl incorporation to the pervoskite precursor could improve the stability of corresponding devices. A high power conversion efficiency of 20.31% without


hysteresis is realized.

1


Table of Contents