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Efficient Compact-Layer-Free, Hole-ConductorFree, Fully Printable Mesoscopic Perovskite Solar Cell Xixi Jiang, Yuli Xiong, Anyi Mei, Yaoguang Rong, Yue Hu, Li Hong, Yingxia Jin, Qingju Liu, and Hongwei Han J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b01815 • Publication Date (Web): 28 Sep 2016 Downloaded from http://pubs.acs.org on September 30, 2016

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The Journal of Physical Chemistry Letters

Efficient Compact-Layer-Free, Hole-Conductor-Free, Fully Printable Mesoscopic Perovskite Solar Cell Xixi Jiang,a, b Yuli Xiong,a Anyi Mei,a Yaoguang Rong,a Yue Hu,a Li Hong,a Yingxia Jin,b Qingju Liu*b and Hongwei Han*a a

Michael Grätzel Center for Mesoscopic Solar Cells, Wuhan National Laboratory for

Optoelectronics, School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan, Hubei 430074, P. R. China. b

Yunnan Key Laboratory for Micro/Nano Materials & Technology, School of Materials Science and Engineering, Yunnan University, Kunming 650091, Yunnan, P. R. China.

Author Information Corresponding Author * H. W. Han, E-mail: [email protected] * Q. J. Liu, Email: [email protected]

ACS Paragon Plus Environment

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Abstract A compact-layer-free, hole-conductor-free, fully printable mesoscopic perovskite solar cell presents a power conversion efficiency of over 13%, which is comparable to that of the device with a TiO2 compact layer. The different wettability of the perovskite precursor solution on the surface of FTO and TiO2 possess a significant effect on realizing efficient mesoscopic perovskite solar cell. This result shows a promising future in printable solar cells by further simplifying the fabrication process and lowering the preparation costs. TOC Graphics：：

Solar cells are devices that convert sunlight directly into electricity, which offer a practical and sustainable solution to the challenge of meeting the increasing global energy demand.1 Since the breaking report from T. Miyasaka group,2 organic-inorganic metal halide perovskite solar cell (PSC) has attracted strong scientific and technological interest due to its promise low-cost and simple process. Especially, the power conversation efficiency (PCE) of the PSC has boosted from 3.8% to the certified record of 22.1% within past few years3-6, presenting unprecedented development speed and application prospects in the history of solar cell. However, there are still


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some fundamental issues, which different from that exist in the traditional devices, such as charge transport and recombination require us to understand further. Generally, both a compact layer and a hole transport layer (HTL) such as spiro-OMeTAD or PTAA are requested for the recorded PSC5-7. For instance, the structure of the most representative mesoscopic PSC (MPSC) is FTO/compact TiO2 layer (c-TiO2)/mesoscopic TiO2 layer (meso-TiO2)/perovskite/HTL/Au. The compact layer acts as a hole-blocking layer within the device to avoid short-circuiting and loss of current through recombination due to the possible contact between the transparent conducting FTO (Fluorine doped tin oxide, SnO2:F) electrode and perovskite. According to this, in addition to the commonly used TiO2 compact layer5-7, significant efforts have been paid to search for an alternative compact layer to help collection of photogenerated electron and achieve high open-circuit voltage, such as SnO28, ZnO9, CsCO310, SrTiO311, and Nb2O5.12 Interestingly, some studies reported that a highly efficient PSC could also be obtained without a compact layer, which should be conducive to further simplify the preparation process. T. L. Kelly group has reported a compact-layer-free planar solar cell device with the structure of ITO/CH3NH3PbI3/Spiro-OMTAD/Ag showing a PCE of 13.5%, and they hold that the remnant PbI2 layer in a CH3NH3PbI3 film prepared by a two-step method may act as a hole-blocking layer.13 G. Fang group develops a planar solar cell with the structure of FTO/CH3NH3PbI3-xClx/Spiro-OMeTAD/Au achieving a PCE of 14.14%, and they presented that involving Cl in the synthesis process and applying ultraviolet-ozone (UVO) treatment to the FTO substrates are the two key factors for achieving high performance device without the compact layer.14 They found that the UVO treatment improves the perovskite coverage and Cl may segregate to and passivate FTO/CH3NH3PbI3-xClx interfaces and then improve its performance. However, the use of thermal evaporated noble metal (Au or Ag) back contact and expansive hole


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conductor materials such as Spiro-OMeTAD may not contribute to large-scale commercial applications of the photovoltaics. In this work, we demonstrated an efficient compact-layer-free, hole-conductor-free MPSC based on triple mesoscopic layer.15-17 The characteristic of the device is to print mesoscopic TiO2 nanocrystalline layer, mesoscopic ZrO2 insulating layer and the mesoscopic carbon electrode on the etched FTO glass layer by layer, and then filled with perovskite directly. The results indicate that this hole-transport-free MPSC without the compact layer presents a PCE of over 13% without hysteresis, which is comparable to that of the device with a compact layer. Interestingly, there are no obvious unreacted PbI2 between FTO/perovskite interfaces, and we also didn’t involve Cl in the synthesis process to passivate FTO/perovskite interfaces and apply UVO treatment to the FTO substrates. However, the different wettability of the perovskite precursor solution on the surface of FTO and TiO2 possess a significant effect on realizing efficient solar cell without a compact layer. The simpler fabrication process and lower preparation costs make these devices a very promising prospect for large-scale commercial applications of the low-cost and high-efficiency photovoltaics. The schematic structure of the fully printable MPSC is presented in Figure 1a. Mesoporous layers of TiO2 , ZrO2 and carbon are sceen-printed on a FTO glass substrate layer by layer. Perovskite is filled into the triple mesoscopic layers directly with a simple solution-processed one-step method. The cross-sectional image of the FTO/TiO2/ZrO2/carbon layers with perovskite is showed in Figure 1b. It is clear that the mesoporous TiO2 , ZrO2 and carbon layers have a thickness of ~0.5 µm, ~3 µm and ~10 µm, respectively. Meanwhile, the perovskite homogeneously distributes in the mesoporous layers. In this work the 5-aminovaleric acid (5AVA) cations was introduced into the perovskite with a mixture of MA and 5-AVA in the


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precursor solution that maintained a 1:1 molar ratio of organic ammonium cations and PbI2,and the molar ratio of ammonium cations in (5AVA)x(MA)1-xPbI3 was fixed at 9:200. The x-ray diffraction (XRD) patterns of neat MAPbI3, (5AVA)x(MA)1-xPbI3, MAI and PbI2 deposited on

Figure 1. (a) The schematic structure, (b) the actual cross-sectional SEM image and (c) the energy level diagram of compact-layer-free, hole-conductor-free fully printable MPSC. the FTO substrates are compared in Figure S1. It could be found that there are no obvious unreacted PbI2 and MAI at the FTO/perovskite interfaces. Figure 1c presents their energy band alignment. Obviously, the perovskite absorbs sunlight and then will generate electrons and holes. Due to the matching band structure of MPSC, in the ideal case, the electrons should be injected into TiO2 conduct band and the holes will transported into carbon electrode. However, although the TiO2 nanocrystalline layer and carbon electrode is cut off by the ZrO2 insulating layer and the electron injected interfaces between FTO-TiO2 and perovskite, espically at FTO/perovskite serious charge recombination will be occurred and lead to poor photoelectric properties, since the FTO layer is a rich electronic area and not blocked by an additional compact layer, which is


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normally used in MPSC.

Figure 2. (a) Photovoltaic characteristics for 20 randomly select (5-AVA)x(MA)1-xPbI3 based MPSC with or without TiO2 compact layer on a 10cm×10cm FTO glass; (b) J-V curves under AM 1.5G illumination, (c) IPCE curves, (d) J-V curves recorded in reverse (from Voc to Jsc) and forward (from Jsc to Voc); and (e) Maximal steady-state photocurrent output and corresponding power output for the champion (5-AVA)x(MA)1-xPbI3 based MPSC with or without TiO2 compact layer.


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However in this case, the hole-conductor-free fully printable MPSC with or without a compact layer presents similar performance. J-V characteristics of such printable MPSC with or without a TiO2 compact layer were measured under illumination of AM1.5G 100 mW cm-2. Figure 2a and Table S1 summarized the statistical data concerning the numerical distribution of the key photovoltical parameters for a random selection of 20 (5-AVA)x(MA)1-xPbI3 based photovoltaic devices, which were screen-printed on a 10cm×10cm FTO glass as shown in Figure S2. It could be found that both of the MPSC with or without compact layer present similar distribution in the value of the key photovoltical parameters. Noted that only a small deviation were observed for these devices, which indicates the perfect reproductivity of these devices. The (5-AVA)x(MA)1-xPbI3 based devices without the compact layer exhibits an average PCE value of 12.64%, Jsc of 22.87 mA cm-2, Voc of 890 mV, FF of 0.62, and the devices with the compact layer exhibite an average PCE value of 12.94%, Jsc of 22.66 mA cm-2, Voc of 910 mV, FF of 0.63 as shown in Table S2. Meanwhile, as shown in Figure 2b, the champion device with the compact layer gives Jsc of 22.78 mA cm-2, Voc of 920 mV, FF of 0.64 and PCE of 13.43%. However, when the compact layer is eliminated, the champion (5-AVA)x(MA)1-xPbI3 based device gives Jsc of 22.95 mA cm-2, Voc of 900 mV, FF of 0.63, and PCE of 13.10%. Compared to the devices with the TiO2 compact layer, the Jsc of compact-layer-free MPSCs increased a little and Voc decreased around 20 mV. Thereby, the efficiency is only 0.3% lower than the c-TiO2-based devices when the TiO2 compact layer is eliminated. Interestingly, when the (5-AVA)x(MA)1-xPbI3 is replaced by pristine MAPbI3, the MPSCs with or without the compact layer present the same pattern, as shown in Figure S5, 6 and Table S3, 4. This indicated that the compact layer in holeconductor-free MPSC might not be indispensable for achieving a high efficiency, and the 5-AVA cation doesn’t play an important role to get an efficient MPSC without the compact layer.


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Figure 2c shows the incident photo-to-current conversation efficiency (IPCE) curves taken with monochromatic light for the champion devices with and without c-TiO2 over the spectral range from 350 to 820 nm. It shows that the perovskite solar cell without TiO2 compact layer has a slight higher IPCE values than that with c-TiO2. This is in accordance with the improved Jsc after the removal of c-TiO2. To further investigate the photoresponse behavior of the MPSCs, the hysteresis effect scans in both the reverse and forward directions were taken to compare the photoresponse behavior of the MPSCs, as shown in Figure 2d and Figure S3. For both devices under 1 sun irradiation with the scanning rate of 50 mv/s, there were almost no substantial hysteresis effect. Under forward and reverse scan, The MPSC without c-TiO2 gives a PCE of 12.90% in forward scan and 13.09% in reverse scan, respectively. The hysteresis index is 0.014 as calculated according to equation (1) in supporting information.18 The device with c-TiO2 shows a hysteresis index of 0.023, which is slightly higher than that without c-TiO2. This is probably due to the elimination of the c-TiO2. Consistently, Miyasaka and his co-workers also found that the interfaces between FTO and the c-TiO2 can be one of the origins to hysteresis.

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The maximal steady-state photocurrent output and their corresponding power output were taken as shown in Figure 2e and Figure S4. Which alongside the current-voltage scan derived power conversation efficiency, because of the existences of hysteresis in the devices.20 Compact-layerfree based device delivers a current density of 19.54 mA cm-2 at 670 mV, corresponding to a PCE of 13.09%, which is well in accordance with the J-V results. For the c-TiO2 based device, a PCE of 13.40% was determined. This results gives a solid evidence for the comparable efficiency between MPSCs with and without the compact layer. In order to understand the principle of such compact-layer-free device, photoluminescence (PL) spectroscopy was used to compare the efficiency of electron-hole extraction from the


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Figure 3. (a) Time-resolved and (b) Steady-state PL spectra of perovskite films：Glass/ (5AVA)x(MA)1-xPbI3 (black), Glass/FTO/ (5-AVA)x(MA)1-xPbI3 (red), Glass/c-TiO2/ (5AVA)x(MA)1-xPbI3 (blue). perovskite. Figure 3 shows the PL spectrum of the (5-AVA)x(MA)1-xPbI3 film in a variety of bilayer configurations, including Glass/perovskite, Glass/FTO/perovskite and Glass/c-TiO2/ perovskite. In Figure 3a, the time-resolved PL spectra indicates that the PL decay of Glass/cTiO2/perovskite and Glass/FTO/ perovskite exhibites a similar time constant of 16.57 ns and 18.49 ns, respectivley. This performance is consisitant with the steady-state PL spectra in Figure 3b, the intensity of PL quenching in the Glass/FTO/perovskite and Glass/c-TiO2/perovskite samples is also very similar. Therefore, it can be concluded that the MPSCs with or without cTiO2 have similar rates of electron transfer at the interfaces and the elimination of c-TiO2 is a feasible sheme in the structure of fully printable triple layer MPSC. Electrochemical impedance spectroscopy (EIS) measurements were conducted to investigate the charge-transport and interfacial charge-transfer processes of MPSCs. Figure 4a shows the Nyquist plots and Figure 4b shows the corresponding Bode phase plots in the frequency range


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from 2 MHz to 10 mHz for devices with and without c-TiO2 in the dark with a bias of -0.7V. The frequency analysis shows two seperated semicrcles in the Nyquist diagram and two peaks in the

Figure 4. (a) Nyquist plots and (b) Bode phase plots for the (5-AVA)x(MA)1-xPbI3 based MPSC with or without a TiO2 compact layer as measured in the dark at -0.7V. Bode plots for devices with c-TiO2 and without c-TiO2. As shown in Figure 4a, the data in the Nyquist plot are fitted by a ralatively simple equivalent circuit consisting of a parallel RC and RCPE element connect in series, along with an additional contribution from series resistance (Rs). According to the previous report,21 the high-frequency RC element is ascribed to contact resistance (Rct) at either the ETL/perovskite or perovskite/counter electrode (CE), while the lower frequency element is assosiated with the recombination resistance (Rret) and chemical capacitance of the system. In comparing the Nyquist plots for the devices with and without the cTiO2, it can be found that both arcs have been a little different. Since the other interface is identical in both cases, we ascribe the different feature of the contact impedance is associated with the FTO/perovskite and c-TiO2/perovskite. The values of Rs, Rct, and Rrec are tabulated in Table S5. As expected, the compact-layer-free based devices exhibit a less Rs. This is consisitant well with the higher Jsc of the devices without a TiO2 compact layer. Besides, the Rrec of the


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devices with or without c-TiO2 are in the same order of magnitude, which translated the slightly lower Voc of MPSC without TiO2 compact layer.

Figure 5. (a) Optical images of contact-angle measurements at ambient condition for the (5AVA)x(MA)1-xPbI3 perovskite precursor solution on FTO, mp-TiO2 films, c-TiO2 films; (b) diagram of the capillary phenomenon in the MPSCs with or without a c-TiO2. Optical images of the contact angle were measured to characterize wettability of the perovskite precursor solution on the FTO substrate, mp-TiO2 film and c-TiO2 film, respectively. As shown in Figure 5a, the contact angle of the perovskite precursor solution on the FTO substrate has been maintained at 47° from 0 s to 2 s. However, the contact angle of the perovskite precursor solution on the mp-TiO2 film or c-TiO2 film decreased from 14°to 3°or from 16°to 6°within the same time. The detail information of optical images of contact-angle measurements at ambient condition for both of the (5AVA)x(MA)1-xPbI3 and MAPbI3 precursor solution on the different films are shown in Figure S7 and S8. It was demonstrated that the perovskite precursor solution showed a significantly different wettability on the FTO substrate and mp-TiO2 film, but a similar wettability on the c-TiO2 and mp-TiO2. According to the


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capillary phenomenon, when an object that containing many minor cracks contacts with a liquid, the liquid will rise along or penetrate to the slit, if it shows good wettability to the object. Figure 5b shows the diagrammatic sketch of the capillary phenomenon in the MPSCs with and without a c-TiO2. Because of the wettability of the perovskite precursor solution on the mp-TiO2 film is similar to the c-TiO2 film, the perovskite precursor solution will rise along the holes in the mpTiO2 layer and penetrate to the slit in the c-TiO2 layer at the same time, which will not lead to air cavities on the interfaces between FTO and perovskite active layer, and lead to a good contact between the TiO2 compact layer and perovskite. However, the wettability of the perovskite precursor solution on the mp-TiO2 is much better than FTO substrate, the perovskite precursor solution will rise along the pin holes in the mp-TiO2 layer, which may result in high resistance between FTO and perovskite active layer for the poor contact. This phenomenon agrees well with the Photoluminescence and impedance spectroscopy results, and is a good explanation for the comparable performance for the hole-conductor-free MPSC with or without the compact layer. In summary, we have demonstrated a compact-layer-free, hole-conductor-free printable MPSC showing an efficiency of over 13%, which is comparable to the device with a TiO2 compact layer. Photoluminescence and impedance spectroscopy results suggest that the elimination of the TiO2 compact layer is a feasible sheme in the structure of fully printable triple layer MPSC. The contact angle measurement reveals that the different wettability of the perovskite active layer on the surface of FTO and TiO2 possess a significant effect on realizing an efficient MPSC, which lead to poor contact and then block the charge recombination between the perovskite and FTO substrate when the TiO2 compact layer is not coated on the FTO surface. As a result, the compact-layer-free, hole-conductor-free, fully printable MPSC offer a simpler


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and more effective photovoltaic technology that combines low cost and high efficiency. Acknowledgment The authors acknowledge financial support from the China National High-tech R&D Program (863, 2015AA034601)，the National Natural Science Foundation of China (91433203, 61474049), and the Science and Technology Department of Hubei Province (2013BAA090). We also thank the Analytical and Testing Center of Huazhong University of Science and Technology (HUST) for FESEM testing. Supporting Information. Experimental Section; Table S1-S5; Figure S1-S8. References (1) Polman, A.; Knight, M.; Garnett, E. C.; Ehrler, B.; Sinke, W. C. Photovoltaic Materials: Present Efficiencies and Future Challenges. Science 2016,352, 307-319. (2) 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. (3) Lee, M. M.; Teuscher, J.; Miyasaka, T.; Murakami, T. N.; Snaith, H. J. Efficient Hybrid Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites. Science 2012, 338, 643-647. (4) Kim, H. S.; Lee, C. R.; Im, J. H.; Lee, K. B.; Moehl, T.; Marchioro, A.; Moon, S. J.; Humphry-Baker, R.; Yum, J. H.; Moser, J. E. et.al. Lead Iodide Perovskite Sensitized AllSolid-State Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%. Sci. Rep.2012, 2,1-7. (5) Yang, W. S.; Noh, J. H.; Jeon, N. J.; Kim, Y. C.; Ryu, S.; Seo, J.; Seok, S. I. HighPerformance Photovoltaic Perovskite Layers Fabricated through Intramolecular Exchange. Science 2015, 348,1234-1237. (6) Li, X.; Bi, D. Q.; Yi, C. Y.; Decoppet, J. D.; Luo, J. S.; Zakeeruddin, S. M.; Hagfeldt, A.; Gratzel, M. A Vacuum Flash-Assisted Solution Process for High-Efficiency Large-Area


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TOC GRAPHICS 55x50mm (300 x 300 DPI)


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Figure 1. (a) The schematic structure, (b) the actual cross-sectional SEM image and (c) the energy level diagram of compact-layer-free, hole-conductor-free fully printable MPSC. 70x60mm (300 x 300 DPI)



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Figure 2. (a) Photovoltaic characteristics for 20 randomly select (5-AVA)x(MA)1-xPbI3 based MPSC with or without TiO2 compact layer on a 10cm×10cm FTO glass; (b) J-V curves under AM 1.5G illumination, (c) IPCE curves, (d) J-V curves recorded in reverse (from Voc to Jsc) and forward (from Jsc to Voc); and (e) Maximal steady-state photocurrent output and corresponding power output for the champion (5AVA)x(MA)1-xPbI3 based MPSC with or without TiO2 compact layer. 174x170mm (300 x 300 DPI)


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Figure 3. (a) Time-resolved and (b) Steady-state PL spectra of perovskite films：Glass/ (5-AVA)x(MA)1xPbI3 (black), Glass/FTO/ (5-AVA)x(MA)1-xPbI3 (red), Glass/c-TiO2/ (5-AVA)x(MA)1-xPbI3 (blue). 33x13mm (300 x 300 DPI)



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Figure 4. (a) Nyquist plots and (b) Bode phase plots for the (5-AVA)x(MA)1-xPbI3 based MPSC with or without a TiO2 compact layer as measured in the dark at -0.7V. 32x12mm (300 x 300 DPI)


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Figure 5. (a) Optical images of contact-angle measurements at ambient condition for the (5-AVA)x(MA)1xPbI3 perovskite precursor solution on FTO, mp-TiO2 films, c-TiO2 films; (b) diagram of the capillary phenomenon in the MPSCs with or without a c-TiO2. 50x30mm (300 x 300 DPI)


Efficient Compact-Layer-Free, Hole-Conductor ... - ACS Publications

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