Increased Efficiency for Perovskite Photovoltaics Based on Aluminum

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Increased Efficiency for Perovskite Photovoltaics Based on AluminumDoped Zinc Oxide Transparent Electrodes via Surface Modification Xiaoli Li, Wang Ye, Xianzhong Zhou, Feng Huang, and Dingyong Zhong J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 26 Apr 2017 Downloaded from http://pubs.acs.org on April 26, 2017

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The Journal of Physical Chemistry

Increased Efficiency for Perovskite Photovoltaics Based on Aluminum-Doped Zinc Oxide Transparent Electrodes via Surface Modification Xiaoli Li, † Wang Ye, † Xianzhong Zhou, † Feng Huang, ‡ Dingyong Zhong*’ † †

School of Physics and State Key Laboratory of Optoelectronic Materials and Technologies,

Sun Yat-sen University, Guangzhou 510275, China ‡

School of Materials Science and Engineering and State Key Laboratory of Optoelectronic

Materials and Technologies, Sun Yat-sen University, Guangzhou 510275, China

ACS Paragon Plus Environment

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ABSTRACT: A majority of perovskite solar cells use indium-tin oxide (ITO) or fluorine-doped

tin

oxide

(FTO)

as

the

transparent electrodes.

As

a

promising

transparent electrode material, aluminum-doped zinc oxide (AZO) possesses comparable transmittance and conductivity, and is environment-friendly and low-cost. Herein, we report the fabrication of perovskite solar cells using AZO as the transparent electrodes. A fullerene derivative layer was used to adjust the energy level alignment and to promote electron transport between the CH3NH3PbI3-xClx absorption layer and the AZO electrode. A power conversion efficiency of 13.07% and an open-circuit voltage of 1.02 V have been achieved. Our work demonstrates the potential in fabricating cost-effective solar cells on AZO with mild processing temperatures.


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INTRODUCTION Organo-metal halide perovskites have attracted great attention in photovoltaic community due to the promising optoelectronic properties such as appropriate and tunable band gap, high absorption coefficient, and excellent carrier transport behaviors. During the last six years, the power conversion efficiency of perovskite solar cells (PSCs) has reached 22.1%.1‒14 So far, much effort has been made to investigate the dependence of device performances on the perovskite layer as well as the electron- and hole-transport layers. Meantime, transparent conducting electrodes, one of the indispensable components in PSCs, affect not only the photon transmission but also the charge carrier collection. Fluorine-doped tin oxide (FTO) and indium-tin oxide (ITO), which exhibit relatively high visible light transmittance and low electrical resistance, have been widely used as transparent conducting electrodes in PSCs. Very impressive efficiency of PSCs based on FTO and ITO has been achieved with different device architectures and fabrication methods. However, the scarcity of both indium and tin in the Earth's crust is disadvantageous for reducing the producing cost of ITO/FTO based PSCs and therefore may limit their applications on a large scale in the long run. In order to reduce the potential cost related to the transparent electrodes, one approach is to recycle the FTO/glass substrates from degraded devices.15 Another way is to search for new types of transparent electrodes, such as graphenes,16,17 carbon nanotubes,18,19 oxidized Ni/Au films,20 Cu-Cu-lactate core-shell NWs,21 silver nanowires,22,23 gold nanowires,24 and others25‒27. For instance, Jeon and coworkers have studied the possibility to use direct- and dry-transferred aerosol single-walled carbon nanotubes as the electrodes in PSCs, obtaining an efficiency of 6.32%.28 Although these attempts above have been done, seeking a simple and low-cost


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electrode material with high transparency and conductivity for perovskite solar cells is still a challenge. As a transparent conducting material, aluminum-doped zinc oxide (AZO) exhibits good transmittance and conductivity comparable to FTO and ITO.29 Furthermore, AZO is abundant of raw materials, friendly for environment, inexpensive, nontoxic, easy of processing, thermally and chemically stable,29 making it a promising candidate as conducting electrodes for optoelectronic devices. AZO has already been used as the electrodes in dye-sensitized30 and organic solar cells.31 As for PSCs, AZO has been used as modification layer between ZnO electron transport layer and perovskite layer in order to tune the energy band alignment.32 Compared with ZnO, the conducting band minimum (CBM) of AZO is higher, which will promote the electron transport and effectively suppress charge recombination at the ZnO/perovskite interface.33 AZO has been also used as the electron transport layer in FTO and ITO based PSCs.33,34 However, works on directly employing AZO as transparent electrodes in PSCs have been rarely reported.35,36 Herein, we study the fabrication of high-efficiency PSCs with AZO electrodes (glass/AZO/PC61BM/MAPbI3-xClx/P3HT/Au). A fullerene derivative, [6,6]-Phenyl C61 butyric acid methyl ester (PC61BM) is used as the electron transport layer to adjust the energy level alignment between the MAPbI3-xClx absorption layer and the AZO electrode. The detailed device fabrication procedure and characterization methods are described in the experimental section below. The best devices have reached an efficiency of 13.07%, with the open-circuit voltage of 1.02 V under standard AM 1.5 conditions. Our work provides a facile way to fabricate AZO-based high-efficiency PSCs by the low-temperature (< 100 ℃) solution process.


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EXPERIMENTAL SECTION Materials and Reagents. Unless otherwise stated, all materials were purchased from Sigma-Aldrich or Aladdin and used as received without any purification. AZO-coated glass was purchased from South China Xiangcheng Science and Technology Co., Ltd. (Shenzhen, China).

Solar cell fabrication. Photovoltaic devices were fabricated on AZO-coated glass. The laser-patterned AZO/glass substrates (sheet resistance < 10 Ω/sq, etching area is 20 mm × 8 mm and residual area is 20 mm × 12 mm) were sequentially cleaned by ultra-sonication in detergent, deionized water, acetone and isopropanol for 20 min. After drying with high purity nitrogen gas, oxygen plasma treatment (20 min, 70 W, 1.2 mbar) was conducted with a plasma cleaner (PDC-MG, Mingheng) to get rid of any remaining organic residues on the AZO surfaces. Further steps to fabricate the devices were implemented in the glove box with nitrogen atmosphere. First, AZO was preheated at 70 ℃ for 10 min on heating platform to remove the gasses

adsorbed on the surface. Then, PC61BM, perovskite and

poly(3-hexylthiophene-2,5-diyl) regioregular (P3HT) were spin-coated in sequence. All the precursor solutions were filtered by a 0.22 μm PTEF filter prior to the spin-coating. PC61BM as the electron transport layer was deposited on the AZO substrate at RT by spin-coating chlorobenzene solution with different concentrations at 2500 rpm for 40 s. And then, the substrates were immediately dried at 70 ℃ for 15 min. CH3NH3I and PbCl2 were dissolved in anhydrous DMF at a 3:1 molar ratio, to reach the stoichiometric I/Pb molar ratio to produce a 40 wt% MAPbI3-xClx perovskite precursor solution,37 which was spin-coated at 4000 rpm for 60 s, followed by drying at RT for 1 h and annealing at 80 ℃ for 35 min. After cooling down


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to RT, P3HT was deposited by spin-coating 15 mg/mL P3HT in chlorobenzene solution at 2500 rpm for 40 s on top of the perovskite film. Finally, 100 nm thick gold electrodes were deposited on top of the device by thermal evaporation at 1.0 × 10-6 mbar, through a shadow mask. The active area of the device was fixed to 4 mm × 4 mm defined by the overlapped square area of the gold and AZO electrodes. The as-prepared solar cells were stored for one night in air with relative humidity of 40% before measurement.

Measurements and Characterization. The surface morphology of the perovskite films and cross-sectional images of PSCs were characterized by scanning electron microscopy (SEM) (ZEISS Merlin). The film thickness of different layers was estimated with the cross-sectional SEM images. The reflection and absorption spectra of the samples were measured using a UV-vis spectrophotometer (Shimadzu UV-2600). Powder X-ray diffraction (XRD) was performed to investigate the structural phases of the samples, using a Bruker D8-Advance with a Cu Kα radiation (λ = 1.5406 Å). The chemical composition information, work function (WF) and valence band maximum (VBM) of Au, AZO, PC61BM and perovskite films were measured using a ThermoFisher Scientific ESCALAB 250Xi spectrometer. An Al Kα X-ray radiation source (1,486.60 eV) and a helium discharge ultraviolet light source (21.2 eV) were used for X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS), respectively. During the UPS measurement, a bias of 10 V was applied on the AZO, PC61BM and MAPbI3-xClx samples, which were connected with an Au plate by double-sided copper conductive adhesive tape to ensure the Fermi energy level of these films consistent with that of the Au plate. The


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operating pressure in the analysis chamber was maintained at approximately 10-8 mbar, and Ar+ ion etching was performed for 2 min with 4000 eV in the cluster mode before measurement. Survey spectrum with the pass energy of 100 eV and core level spectra with the pass energy of 20 eV were acquired by XPS. The chemical valent state and relative elemental content were calculated using Gaussian functions based on integrated counts of respective peaks fitted. For comparison of different samples, all spectra were normalized to Au4f signal at 84.8 eV. The J-V characteristics were conducted in the ambient atmosphere condition under simulated AM 1.5G sunlight (ABET Technologies Sun 3000) at 100 mW/cm2 with a Keithley model 2400 source meter with the accuracy of 1 µV for voltage and 10 pA for current. The light intensity of Xenon lamp was calibrated with an NREL-calibrated KG5 filtered silicon reference with a solar mismatch factor of 1.0. The voltage scan rate was 5 mV/s in the reverse direction (from 1.2 V to -0.3 V). No device preconditioning, such as prolonged light soaking or forward voltage bias, was applied before starting the measurement for all the devices. Except for the efficiencies showed in statistical histogram, all the efficiencies mentioned are the best efficiencies.

RESULTS AND DISCUSSION Figure 1a depicts the geometry of our PSCs with chlorine-doped methylammonium lead iodide (MAPbI3-xClx) as the light absorption layer and AZO and Au as the electrodes. Compared with MAPbI3, it has been reported that MAPbI3-xClx exhibits much longer excition diffusion length.37‒39 In order to promote the charge carrier separation, PC61BM is adopted as


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the electron transport layer between the perovskite layer and AZO electrode. Figure 1b is a cross-sectional SEM image illustrating the architecture of our solar cell. At the bottom, there is a uniform AZO film with a thickness of ~1 μm coated on the glass. In the middle, the perovskite layer is about 430 nm thick and consists of compact microcrystals with the grain size of several hundred nanometers. In addition, a top Au layer deposited on the perovskite layer shows a thickness of ~100 nm. The PC61BM (~30 nm) and P3HT (~40 nm) layers, which are located between the perovskite layer and the electrodes, are too thin to be distinguished here (See Supporting Information, Figures S1 and S2). In order to directly evaluate the optical absorption properties of perovskite films in our cells, we acquired the transmission spectra of different types of films on the AZO/glass substrate, as shown in Figure 1c. For the clean AZO/glass substrate, a light transmittance above 80% was obtained in the range of wavelength from 350 nm to 1000 nm, which is comparable with ITO and FTO. After the orange-yellow PC61BM layer was deposited on the AZO substrate by spin-coating, the light transmittance slightly decreased to ~60%. The transmission spectra of the perovskite films with or without PC61BM exhibited very low transmittance in the visible light region (400 nm to 800 nm), indicating that the perovskite layers are thick enough for sufficient light absorption. An absorption edge was observed at 755 nm, corresponding to an optical gap of 1.6 eV for our MAPbI3-xClx films. Compared with the samples without PC61BM layer, the perovskite films coated on PC61BM layers exhibit better crystallization and uniformity. As shown in the top-view SEM image (Figure 1d), the perovskite film on the 20 mg/ml PC61BM-coated AZO/glass substrate is uniform and compact with well-crystallized grains.


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Figure 1. (a) Schematic illustration of solar cell structure with AZO electrode. (b) Cross-sectional SEM image of the solar cell. (c) Transmission Spectrum of different types of films on AZO/glass substrates. (d) Top-view SEM image of perovskite film on PC61BM-deposited AZO (Inset, high resolution SEM).

Figure 2. XRD patterns of perovskite film on PC61BM-deposited AZO and AZO-coated glass substrate.

XRD patterns of AZO electrode and perovskite film on the 20 mg/ml PC61BM-coated AZO/glass substrate are shown in Figure 2. The diffraction peaks (2θ) of the clean AZO substrates located at 34.5°and 72.6°are assigned to the (002) and (004) plane of hexagonal phase with predominant c-axis orientation perpendicular to the surface.40 Such preferential orientation is beneficial to the transport of electrons along the c-axis (across the PC61BM


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film), due to the reduced scattering at the grain boundaries.41 For the perovskite film, a set of peaks at 14.3°, 28.6°and 43.4°are assigned to the (110), (220), and (330) diffraction peaks of the tetragonal crystal structure, respectively, similar with the result reported in the literature.42 The narrow (110) and (220) peaks indicate the good crystallization highly oriented along the [110] direction. The XPS survey spectrum reveals that the perovskite film mainly contains C, N, Pb, I, and observable trace of Cl as well as a little of C and O from external contamination (Figure S3). The peak positions of Pb4f7, I3d5, and Cl2p are 138.7, 619.0, and 199.0 eV, respectively, consistent with previous work.43 The core-level binding energy (BE) values observed for I, Pb, and N in the XPS spectra of Cl-containing perovskite are very similar to those of Cl-free perovskites.44 Judging from the XPS core level spectra, the perovskite film is MAPbI3-xClx with an atomic ratio of Cl in the total halide approximately equals to ~3%, similar to the previously reported values (2-5%) acquired by EDS, XPS and ion chromatographic analysis.39,43,45 In addition, the atomic ratio of Al (~3%) in AZO was characterized by XPS and showed in Figure S4. We have further investigated the energy level configuration of each layer in the solar cell by UPS, which plays a key role in exciton dissociation and charge transport. To ensure efficient electron extraction, low WF materials are generally adopted as cathodes.43,46,47 However, low WF metals such as Al (‒4.3 eV) or Ca (‒2.9 eV) are usually vulnerable to oxidation in ambient conditions, leading to poor device stability. On the contrary, AZO is quite stable and has excellent visible light transmittance. To acquire the WFs of the AZO and


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gold electrodes, the secondary cutoff edges have been measured by UPS, as showed in Figure 3a. The BE value of the Fermi edges of the gold film was set to zero and used as a reference and the WFs can be written as: WF = hν – BEcutoff, where hν = 21.2 eV is the incident photon energy and BEcutoff is the binding energy at the secondary electron cutoff edge. The measured WF of the AZO film is 4.1 eV, implying that AZO is a suitable candidate as cathode. In comparison, the values of the PC61BM layer, perovskite layer and gold electrode are 4.2, 4.5 and 5.1 eV, respectively. Figure 3b depicts the valence band spectra of PC61BM and perovskite layers, showing a VBM of 1.8 and 1.0 eV below the Fermi energy, respectively. In Figure 3c, the energy level diagram of our AZO-based PSCs is summarized according to above UPS measurements. Here, the vacuum level energy is set to zero. The highest occupied molecular orbital (HOMO) of PC61BM (‒3.9 eV) and the lowest unoccupied molecular orbital (LUMO) of P3HT (‒5.1 eV) are from the literature.48 The MAPbI3-xClx absorption layer possesses a conduction band minimum (CBM) and the VBM at ‒3.9 and ‒5.5 eV, respectively. On the electron transport side, the LUMO level of PC61BM layer is located at ‒3.9 eV, which is equal to the CBM of MAPbI3‒xClx and 0.2 eV higher than the Fermi level of the AZO electrode, allowing the efficient electron transfer from MAPbI3-xClx to PC61BM and subsequent collection by the AZO electrode. On the other hand, the HOMO level of PC61BM (‒6.0 eV) is 0.5 eV lower than that of MAPbI3‒xClx, so that holes can be blocked on this side. The appropriate energy level position of PC61BM makes it a suitable electron transport material for effectively reducing charge carrier recombination and shunt currents in our solar cells. On the hole transport side, the LUMO (‒3.2 eV) and HOMO (‒5.1 eV) levels of the P3HT layer are 0.7 and 0.4 eV higher than the CBM and VBM of MAPbI3‒xClx, respectively,


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suitable for hole transport and subsequent collection on the gold electrode with the Fermi level at ‒5.1 eV.

Figure 3. (a) Cutoff edges of Au, AZO, PC61BM and perovskite obtained by UPS. (b) Valence band of PC61BM and MAPbI3-xClx obtained by XPS. The binding energy (BE) is referred to the Fermi levels. (c) The energy band diagram of the AZO-based solar cell. The energy is referred to the vacuum level.

Figure 4. (a) J-V curves for champion AZO-based PSCs without electron transport layer or with PC61BM layer spin-coated with different solution concentrations. For comparison, the curves for FTO-based cells without or with a PC61BM layer are displayed. (b) PCE of champion AZO-based solar cells as a function of PC61BM solution concentration. (c) JSC and VOC of champion AZO-based solar cells as a function of PC61BM solution concentration. (d) Statistical histograms of PCE for AZO-based PSCs without electron transport layer or with PC61BM layer spin-coated with different solution concentrations.


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PSCs with AZO as the electrode have been fabricated and characterized. In order to investigate the effect of PC61BM thickness on the device performance, PC61BM solutions with different concentrations, i.e., 10, 15, 20, and 25 mg/ml in chlorobenzene were used for depositing the PC61BM layer by spin-coating, resulting in the estimated thicknesses of 40 nm for 25 mg/ml by SEM (Figure S1). For comparison, electron-transport layer free PSCs based on AZO have been also fabricated. J-V characteristic curves of champion cells and performance statistics histograms of 150 cells in total are showed in Figure 4a-d. The best device without PC61BM reached a PCE of 7.05% with a Voc of 0.96 V and a Jsc of 9.16 mA/cm2. Although a quite high fill factor (FF) of 80.06% was obtained, the efficiency is depressed by the low short-circuit current, probably due to the ineffective electron extraction and hole blocking at the AZO/perovskite interface. On the other hand, the device performance was effectively improved for the cells with a PC61BM electron-transport layer and the champion device in an optimal thickness (20 mg/ml solution) exhibits a significant increase of PCE reaching to 13.07%, which is mainly contributed from the increase of short circuit current density Jsc (18.75 mA/cm2). Nevertheless, the FF, 68.60%, is relatively low compared with the PC61BM-free cells, probably due to the defects and pinholes of the perovskite films resulting from the poor wettability of DMF on the PC61BM surface. Decreasing or increasing the thickness of PC61BM layer results in the worse device performance. The thinner PC61BM films exhibit an uncompleted coverage on the AZO surface and the thicker films exhibit much larger roughness. In both cases the homogeneity and smoothness of the perovskite film deposited subsequently are reduced, with increasing density of defects and pinholes.


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We have also fabricated the solar cells with FTO as the transparent electrode. A rather lower PCE of 6.4% for the device based on bare FTO was obtained. When an electron-transport layer was prepared on the FTO surface by spin-coating 20 mg/ml PC61BM solution, an efficiency of 8.09% has been achieved for the champion device, lower than the AZO-based cells. Although a quite large short-circuit density (18.61 mA/cm2) was obtained, the FTO-based cell exhibited a lower open-circuit voltage (0.82 V), which may originate from the mismatched energy levels between the Fermi energy of FTO (‒4.4 eV) and the LUMO of PC61BM (‒3.9 eV).

Figure 5. The device stability of AZO and FTO based PSCs with a PC61BM layer (20 mg/ml).

In addition, as shown in Figure 5, the stability of the AZO-based device with a PC61BM layer (20 mg/ml) was investigated. In a period of 18 days, we measured every day the J-V characteristics of the as-prepared cells, which were stored under ambient conditions at 25 ℃ with a humidity of 60%~80%. The efficiency was slightly increased at the beginning, from 12.08% to 12.48% measured one day later, and gradually decreased during the following days. The efficiencies were 9.68% after a week and only 1.14% after 18 days, respectively.


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Although a similar tendency for the efficiency evolution was observed, the FTO-based PSCs exhibited a better stability performance than AZO-based cells. An efficiency of nearly 5% was left after 18 days. Further work should be carried out to reveal the degradation mechanism of AZO-based perovskite solar cells.

CONCLUSIONS In summary, we have fabricated perovskite solar cells based on AZO transparent electrodes with gold as the counter electrodes. As investigated by UPS, the AZO substrate possesses a Fermi level located at ‒4.1 eV (referred to the vacuum level), which is 0.2 eV below the conduction band minimum of MAPbI3-xClx (x = ~3%), making it an appropriate cathode for perovskite solar cells. An energy conversion efficiency of 7.05% has been obtained for the electron-transport layer free perovskite solar cells with P3HT as the hole-transport layer. By further adopting a PC61BM layer as the electron transport layer, the LUMO of which is aligned with the CBM of the perovskite layer and the HOMO located 0.5 eV higher than the VBM, the charge extraction process in the solar cells has been promoted with a significantly increased short-circuit current density (18.75 mA/cm2). As a result, a higher energy conversion efficiency of 13.07% has been achieved. Our work demonstrates the potential in fabricating cost-effective solar cells on AZO with mild processing temperatures.

ASSOCIATED CONTENT Supporting Information Cross-sectional SEM images of the PC61BM film and the device； XPS of the perovskite film and AZO.


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AUTHOR INFORMATION Corresponding Author *Tel: +86-020-84110755. E-mail: [email protected] (D. Zhong).

Notes The authors declare no competing financial interest.

Acknowledgements The work was financially supported by National Natural Science Foundation of China (11574403, 11374374) and Guangzhou Science and Technology Project (201607020023).

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(23) Yang, K. Y.; Li, F. S.; Zhang, J. H.; Veeramalai, C. P.; Guo, T. L. All-solution Processed Semi-transparent Perovskite Solar Cells with Silver Nanowires Electrode. Nanotechnology 2016, 27, 095202 (24) Bao, C.; Zhu, W.; Yang, J.; Li, F.; Gu, S.; Wang, Y.; Yu, T.; Zhu, J.; Zhou, Y.; Zou, Z. Highly Flexible Self-powered Organolead Trihalide Perovskite Photodetectors with Gold Nanowire Networks as Transparent Electrodes. ACS Appl. Mater. Interfaces 2016, 8, 23868-23875. (25) Bryant, D.; Greenwood, P.; Troughton, J.; Wijdekop, M.; Carnie, M.; Davies, M.; Wojciechowski, K.; Snaith, H. J.; Watson, T.; Worsley, D. A Transparent Conductive Adhesive Laminate Electrode for High-efficiency Organic-inorganic Lead Halide Perovskite Solar Cells. Adv. Mater. 2014, 26, 7499-504. (26) Ou, X.-L.; Xu, M.; Feng, J.; Sun, H.-B. Flexible and Efficient ITO-free Semitransparent Perovskite Solar Cells. Sol. Energy Mater. Sol. Cells 2016, 157, 660-665. (27) Makha, M.; Fernandes, S. L.; Jenatsch, S.; Offermans, T.; Schleuniger, J.; Tisserant, J. N.; Veron, A. C.; Hany, R. A Transparent, Solvent-free Laminated Top Electrode for Perovskite Solar Cells. Sci. Technol. Adv. Mater. 2016, 17, 260-266. (28) Jeon, I.; Chiba, T.; Delacou, C.; Guo, Y.; Kaskela, A.; Reynaud, O.; Kauppinen, E. I.; Maruyama, S.; Matsuo, Y. Single-walled Carbon Nanotube Film as Electrode in Indium-free

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Investigation

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Electron-blocking Layers and Dopants. Nano lett. 2015, 15, 6665-6671. (29) Bernède, J. C.; Cattin, L.; Morsli, M.; Berredjem, Y. Ultra-thin Metal Layer Passivation of the Transparent Conductive Anode in Organic Solar Cells. Sol. Energy Mater. Sol. Cells 2008, 92, 1508-1515. (30) Lee, S. H.; Han, S. H.; Jung, H. S.; Shin, H.; Lee, J.; Noh, J. H.; Lee, S.; Cho, I. S.; Lee, J. K.; Kim, J. et al. Al-Doped ZnO Thin Film: A New Transparent Conducting


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Layer for ZnO Nanowire-Based Dye-Sensitized Solar Cells. J. Phys. Chem. C 2010, 114, 7185-7189. (31) Chauhan, R. N.; Anand, R. S.; Kumar, J. RF-sputtered Al-doped ZnO Thin Films: Optoelectrical Properties and Application in Photovoltaic Devices. Phys. Status Solidi A 2014, 211, 2514-2522. (32) Dong, J.; Zhao, Y.; Shi, J.; Wei, H.; Xiao, J.; Xu, X.; Luo, J.; Xu, J.; Li, D.; Luo, Y.; Meng, Q. Impressive Enhancement in the Cell Performance of ZnO Nanorod-based Perovskite Solar Cells with Al-doped ZnO Interfacial Modification. Chem. Commun. 2014, 50, 13381-13384. (33) Mahmood, K.; Swain, B. S.; Jung, H. S. Controlling the Surface Nanostructure of ZnO and Al-doped ZnO Thin Films Using Electrostatic Spraying for Their Application in 12% Efficient Perovskite Solar Cells. Nanoscale 2014, 6, 9127-9138. (34) AitDads, H.; Bouzit, S.; Nkhaili, L.; Elkissani, A.; Outzourhit, A. Structural, Optical and Electrical Properties of Planar Mixed Perovskite Halides/Al-doped Zinc Oxide Solar Cells. Sol. Energy Mater. Sol. Cells 2016, 148, 30-33. (35) Zhao, X.; Shen, H.; Zhang, Y.; Li, X.; Zhao, X.; Tai, M.; Li, J.; Li, J.; Li, X.; Lin, H. Aluminum-doped Zinc Oxide as Highly Stable Electron Collection Layer for Perovskite Solar Cells. ACS Appl. Mater. Interfaces 2016, 8, 7826-7833. (36) Wang, H.; Yan, L.; Liu, J.; Li, J.; Wang, H. Fabrication of Well-aligned ZnO Nanorod Photoanodes for Perovskite Solar Cells. J. Mater. Sci. Mater. Electron. 2016, 27, 6872-6880. (37) Wehrenfennig, C.; Eperon, G. E.; Johnston, M. B.; Snaith, H. J.; Herz, L. M. High Charge Carrier Mobilities and Lifetimes in Organolead Trihalide Perovskites. Adv. Mater. 2014, 2, 1584-1589. (38) Stranks, S. D.; Eperon, G. E.; Grancini, G.; Menelaou, C.; Alcocer, M. J.; Leijtens, T.; Herz, L. M.; Petrozza, A.; Snaith, H. J. Electron-hole Diffusion Lengths


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Exceeding 1 Micrometer in an Organometal Trihalide Perovskite Absorber. Science 2013, 342, 341-344. (39) Colella, S.; Mosconi, E.; Fedeli, P.; Listorti, A.; Gazza, F.; Orlandi, F.; Ferro, P.; Besagni, T.; Rizzo, A.; Calestani, G.; Gigli, G.; De Angelis, F.; Mosca, R. MAPbI3-xClx Mixed Halide Perovskite for Hybrid Solar Cells: the Role of Chloride as Dopant on the Transport and Structural Properties. Chem. Mater. 2013, 25, 4613-4618. (40) Gu, J. H.; Long, L.; Lu, Z.; Zhong, Z. Y. Optical, Electrical and Structural Properties of Aluminum-doped Nano-Zinc Oxide Thin Films Deposited by Magnetron Sputtering. J. Mater. Sci. Mater. Electron. 2014, 26, 734-741. (41) Liang, L.; Huang, Z.; Cai, L.; Chen, W.; Wang, B.; Chen, K.; Bai, H.; Tian, Q.; Fan, B. Magnetron Sputtered Zinc Oxide Nanorods as Thickness-Insensitive Cathode Interlayer for Perovskite Planar-Heterojunction Solar Cells. ACS Appl. Mater. Interfaces 2014, 6, 20585-20589. (42) Conings, B.; Baeten, L.; De Dobbelaere, C.; D'Haen, J.; Manca, J.; Boyen, H. G. Perovskite-based Hybrid Solar Cells Exceeding 10% Efficiency with High Reproducibility Using a Thin Film Sandwich Approach. Adv. Mater. 2014, 26, 2041-2046. (43) You, J.; Hong, Z.; Yang, Y. (Michael); Qi Chen, M.C.; Song, T.-B.; Chen, Ch.-Ch.; Lu, S.; Liu, Y.; Zhou, H.; Yang, Y. Low-temperature Solution-Processed Perovskite Solar Cells with High Efficiency and Flexibility. ACS Nano 2014, 8, 1674-1680. (44) Pellegrino, G.; Colella, S.; Deretzis, I.; Condorelli, G. G.; Smecca, E.; Gigli, G.; La Magna, A.; Alberti, A. Texture of MAPbI3 Layers Assisted by Chloride on Flat TiO2 Substrates. J. Phys. Chem. C 2015, 119, 19808-19816.


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TOC Graphic


22

Page 23 of 27

(a)

Au P3HT

(b)

Au

MAPbI3-xClx

MAPbI3-xClx

PC61BM

AZO 1 μm

AZO 100

Transmittance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41


(c)

(d)

80

1 μm

60 AZO AZO/PC61BM AZO/MAPbI3-xClx

40

AZO/PC61BM/MAPbI3-xClx

20

300

2 μm 450

600

750

900

Wavelengh (nm)

Figure 1. (a) Schematic illustration of solar cell structure and (b) Cross-sectional image of the solar cell. (c) Transmission Spectrum of different types of films on AZO/glass substrates. (d) Top-view SEM image of perovskite film on a PC61BMdeposited AZO (Inset, high resolution SEM) ACS Paragon Plus Environment

20

30

0

34.5

40

72.6

50

60

0

(004) AZO

(330)

(002) AZO

(220)

AZO

(002) AZO

10

Page 24 of 27

MAPbI3-xClx

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

(110)


70

2Theta

Figure 2. XRD patterns of perovskite film on PC61BM-deposited AZO and AZO-coated glass substrate.


Page 25 of 27

(b)

Energy (eV)

1.02

16.0

-4.1

Au

1.8

-3.9 -3.9

P3HT

Au 17.1 AZO 17.0 PC61BM 16.7 MAPbI3-xClx

-3.2

(c)

MAPbI3-xClx

PC61BM MAPbI3-xClx

PC61BM

Intensity(a.u.)

(a)

AZO

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41


-5.1 -5.1

-5.5 -6.0 20

19

18

17

BE (eV)

16

15

5

4

3

2 1 BE(eV)

0

-1

Figure 3. (a) Cutoff edges of Au, AZO, PC61BM and perovskite obtained by UPS; (b) Valence band of PC61BM and MAPbI3-xClx obtained by XPS. The binding energy (BE) is referred to the Fermi levels; (c) The energy band diagram of the AZO-based solar cell. The energy is referred to the vacuum level.


2

(a)

15


(d)

10 5

10 mg/ml

0 0.0

0.5 Voltage(V)

1.0

(b) 60

Eta (%)

10

Eta FF

5

20 mg/ml

1.0

VOC (V)

10

30

(c)

2

JSC (mA/cm )

20

15 mg/ml

0

0

Jsc

25 mg/ml

0.8

VOC 0 5 10 15 20 25 Concentration of PC61BM（ mg/ml）


0

2

Page 26 of 27

0 mg/ml

FF (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

J (mA/cm )

20

0 mg/ml 20 mg/ml 10 mg/ml 15 mg/ml 25 mg/ml FTO FTO/PCBM

4

6 8 10 12 14 Eta(%)

Figure 4. (a) J-V curves of champion AZO-based PSCs without electron transport layer or with PC61BM layer spin-coated with different solution concentrations. For comparison, the curves for FTObased cells without or with a PC61BM layer are displayed. (b) PCE and FF of the champion AZObasedsolar cells as a function of PC61BM solution concentration (c) JSC and VOC(c) of the champion AZO-basedsolar cells as a function of PC61BM solution concentrationand. (d) Statistical histograms of PCE for AZO-based PSCs without electron transport layer or with PC61BM layer spi-coated with different solution concentrations.

Page 27 of 27

14 12

AZO FTO

10

Efficiency (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41


8

Figure 5. The device stability of AZO and FTO based PSCs with a PC61BM layer (20 mg/ml ).

6 4 2 0 0

2

4

6

8 10 12 14 16 18 Days


Increased Efficiency for Perovskite Photovoltaics Based on Aluminum

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