||Inorganic P-type Nickel Oxide for High Efficiency Organo-metal Halide Perovskite solar cells
||Department of Photonics
heterojunction solar cells
porous Ni/Au counter electrode
此論文主要分成三大主題，分別為以多孔p型氧化鎳為主的p-i-n異質接面鈣鈦礦太陽能電池、鈣鈦礦材料與氧化鎳界面間的化學反應以及多孔鎳金對電極應用於鈣鈦礦太陽能電池。第一主題，我們提出利用多孔氧化鎳材料作為P型選擇性電極，並利用有效的n型選擇性電極PC61BM作為電子傳輸層，完成多孔的NiOnc/Perovskite/PC61BM異質鈣鈦礦太陽能電池。相較於平板型NiOx/Perovskite/PC61BM異質鈣鈦礦太陽能電池，引入多孔氧化鎳可以增加鈣鈦礦吸收層之厚度，進而改善光捕捉效率及外部光子轉換電子效率，此結構於AM1.5 G的光譜照射下，元件效率達到9.51%。為了優化元件效率和效率重複性以及未來量產可行性，我們改變阻擋層氧化鎳 (NiOx)沈積方式，從溶液沈積法改為濺鍍沈積法。使用低溫製程並在氬氣環境下濺鍍氧化鎳阻擋層,藉由控制濺鍍時間以準確控制阻擋層厚度,製作的多孔鈣鈦礦元件效率為10.7%。此外在濺鍍的過程中，我們摻雜不同的氧濃度以及利用濺鍍時間調變阻擋層氧化鎳之厚度，當氧摻雜濃度為10%和濺鍍時間為150秒時，阻擋層氧化鎳達到電性最佳化和高元件效率11.6%。若氧摻雜濃度提高至15%，過多的氧缺陷以及鎳空隙缺陷因此而形成，導致元件效率下降至8.1%。
第二主題，利用兩次沈積法 (sequential method) 將鈣鈦礦材料沈積於多孔氧化鎳材料過程中，從XPS分析發現PbI2與NiOnc之間會產生化學反應，生成新的化合物為PbO。接著將試片浸泡於MAI溶液時，PbO與MAI之間會產生化學反應，進而形成氧摻雜的鈣鈦礦材料 (CH3NH3PbI3-2δOδ)。此化合物的生成，界於MAPbI3與NiOnc之間，有助於電洞的傳輸，使短路電流 (Jsc)和填充率 (FF)有明顯地改善。
第三主題，我們將鎳金材料分別沈積於Al2O3上，接著經過500度高溫45分鐘後，即完成多孔鎳金對電極結構。SIMS和SEM的分析發現此多孔鎳金對電極主要由網狀形貌的金以及多孔性氧化鎳所組成。元件結構製作完成後，最後以二次沈積法將鈣鈦礦材料沈積於金屬氧化物中。由於鈣鈦礦材料必須經由多孔對電極，才能滲透於金屬氧化物，所以於鈣鈦礦製程中，我們分別探討PbI2 濃度變化、MAI濃度變化和時間變化以及再填充測試。利用適當的PbI2 濃度 (1M)和MAI濃度沈積 (10 mg/mL)，可以形成完全反應之鈣鈦礦材料，反之，會有部分的PbI2殘留於元件之。因此可以達到最高之元件效率10.3%。另外，由於環境會對元件中之鈣鈦礦材料造成衰退。我們利用DMF將鈣鈦礦材料去除於元件，接著再重複地沈積鈣鈦礦於元件中，鈣鈦礦太陽能電池經過數次地重複使用，元件效率相較於初始值，仍然可保持90%以上的初始元件效率。
We have the thesis divided into three topics, including mesoscopic p-i-n heterojunction for perovskite solar cells, interfacial redox reaction between NiOnc and MAPbI3 and porous Ni/Au counter electrode for perovskite solar cells. For the first topic, we propose a new paradigm for mesoscopic p-i-n based on perovskite solar cells using inorganic NiO as p-type selective electrode and [6,6]-phenyl C61-butyric acid methyl ester (PC61BM) as effective n-type selective electrode and implement mesoscopic NiOnc/Perovskite/PC61BM heterojunction perovskite solar cells. With the introduction of mesoscopic NiO, it is expected that the light harvesting efficiency and incident photo-current efficiency are improved, compared to planar NiOx/Perovskite/PC61BM heterojunction perovskite solar cells (PHJ), leading to a high efficiency of 9.51% achieved under AM 1.5G illumination. We change processing method for blocking layer of NiOx from solution method to sputtered method in order to improve the reproducibility of devices and the feasibility of mass production. NiOx is deposited by sputtered process under argon atmosphere at low temperature and the thickness of NiOx is fine controlled by sputtering time. The optimized device resulted in a power conversion efficiency of 10.7%. With oxygen doping of 10% and sputtering time of 150 s, the sputtered NiOx has the lowest resistance (49.5 Ω, measured by Hall effect) and the device achieves the best performance of 11.6%. With further increase of oxygen doping of 15%, the efficiency drops down to be 8.1% mainly owing to interstitial oxygen defect and nickel vacancies.
For the second topic, perovskite is deposited onto NiOnc using sequential method. We investigate the interfacial redox reaction at NiOnc/Perovskite heterojunction during the formation of the perovskite and confirmed from the XPS that PbO is formed by PbI2 reacting with NiOnc first and PbO subsequently reacts with MAI to form oxygen-doped perovskite CH3NH3PbI3-2δOδ when the PbI2-coated sample is immersed into MAI solution. This oxygen-doped perovskite (CH3NH3PbI3-2δOδ) matches energy level between perovskite and NiOnc and acts as the bridge between them to facilitate the hole transport and improve device performance in terms of Jsc and FF.
For the third topic, we fabricate recyclable and all-inorganic solar cell template. Ni and Au are sequentially deposited on the mesoporous substrate composed of FTO/c-TiO2/mp-TiO2/mp-Al2O3, followed by annealing at 500°C for 45 mins to form a porous counter electrode composed of network-like Au and mesoporous NiOx. Since the perovskite precursor requires penetrating the mesoporous counter electrode to the bottom of metal oxide, various perovskite process parameters using sequential method including the concentration of PbI2 and methylammonium iodide (MAI), dipping time of MAI and refilled perovskite are examined. When the template is spin-coated with 1 M PbI2 and dipping in MAI concentration of 10 mg/mL, the complete reaction between PbI2 and CH3NH3I within whole devices is achieved and the device shows the highest PCE of 10.3%. Removing the perovskite with DMF and reloading new perovskite can rejuvenate the perovskite device that allows us to reuse the substrate with all constituents except perovskite. The reused device delivers nearly 90% of initial efficiency.
Table of Contents VI
List of Figures IX
List of Tables XVI
1. Introduction 1
1.1 The origin of solar cells 1
1.2 Solar Characterization 2
1.2.1 Air mass and solar spectrum 2
1.2.2 Solar cell parameters 4
1.3 Motivation and master plan 7
1.3.1 p-i-n heterojunction PSC 7
1.3.2 PSC with nanoporous counter electrode 8
1.3.3 Overview of the thesis 8
2. The history of solar cells 10
2.1 The origin of perovskite solar cells 10
2.2 The history of n-i-p based on perovskite solar cells 11
2.3 The history of p-i-n heterojunction perovskite solar cells 20
2.3.1 Planar NiO based PSC 22
2.3.2 Mesoscopic NiO based PSC 25
2.4 Porous counter electrode based perovskite solar cells 26
3. Experimental section 28
3.1 Material preparation 28
3.1.1 NiOx solution 28
3.1.2 NiOx film 28
3.1.3 NiOnc paste 29
3.1.4 Al2O3 paste 29
3.1.5 CH3NH3I material 29
3.1.6 PbI2 solution 30
3.2 Device fabrication 30
3.2.1 ITO/NiOx (solution)/NiOnc/CH3NH3PbI3/PC61BM/BCP/Al 30
3.2.2 ITO/NiOx (sputtering)/NiOnc/CH3NH3PbI3/PC61BM/BCP/Al 31
3.2.3 FTO/c-TiO2/mp-TiO2/Al2O3/Ni-Au/CH3NH3PbI3 33
3.3 Measurement apparatus 35
3.3.1 Glancing angle X-ray diffraction (GA-XRD) 35
3.3.2 High resolution Field-Emission Scanning Electron Microscopy 35
3.3.3 Ultraviolet-Visible spectroscopy 35
3.3.4 X-ray Photoelectron spectroscopy 35
3.3.5 Mott-Schottky analysis 36
3.3.6 Photoluminescence spectroscopy 36
3.3.7 Photo-induced transient absorption analysis 36
3.4 Overview of Medicine 37
4. Solution-processed blocking layer as nickel oxide for Mesoscopic Perovskite NiO/CH3NH3PbI3 Heterojunction Solar Cells 38
4.1 Introduction 38
4.2 Results and discussion 40
4.2.1 Scanning Electron Microscopy 40
220.127.116.11 The effect of PbI2 solution deposited at different rotating speed on surface morphology 40
18.104.22.168 The effect of NiOnc deposited at different rotating speed on surface morphology 44
22.214.171.124 Device configuration confirmed by SEM 44
126.96.36.199 The comparison between planar and mesoscopic heterojunction Perovskite/ Fullerene solar cells 44
4.2.2 Photovoltaic characterization and IPCE response 46
4.2.3 IPCE spectrum and Optical characterization 48
4.2.4 Photoluminescence spectra 50
4.2.5 Photo-induced transient absorption spectra 51
4.2.6 Statistic histogram on photovoltaic parameters 53
4.3 Summary 54
5. Sputtered Nickel oxide as blocking layer for Mesoscopic Perovskite NiO/CH3NH3PbI3 Heterojunction Solar Cells 55
5.1 Introduction 55
5.2 Results and discussion 57
5.2.1 X-ray diffraction analysis 57
5.2.2 Scanning Electron Microscopy 58
5.2.3 UV-Vis spectroscopy 59
5.2.4 Mott-Schottky analysis 61
5.2.5 X-ray Photoelectron Spectroscopy 62
5.2.6 Photovoltaic characterization 64
188.8.131.52 J-V characterization on devices with different NiOnc layer thickness 64
184.108.40.206 J-V characterization on devices using compact NiOx film sputtered with different deposition time 65
220.127.116.11 J-V characterization of devices with NiOx films doped with different oxygen flow ratios 68
5.2.7 IPCE spectrum 70
5.2.8 Statistic histogram on photovoltaic parameters 73
5.2.9 Chemical oxide reaction at mesoscopic NiO/Perovskite heterojunction 74
18.104.22.168 Introduction 74
22.214.171.124 X-ray photoelectron spectroscopy 74
126.96.36.199 Near-Edge X-ray absorption fine structure 80
188.8.131.52 Chemical mapping of elemental distributions 83
184.108.40.206 Electronic structure 87
220.127.116.11 Ultraviolet photoelectron spectroscopy 90
5.3 Summary 91
6. Porous bilayer Ni/Au applied in perovskite solar cells 92
6.1 Introduction 92
6.2 Result and discussions 93
6.2.1 Scanning Electron Microscopy 93
18.104.22.168 Morphologies of bilayer Ni/Au thin film 93
22.214.171.124 Perovskite deposition fabricated by various MAI concentration 95
6.2.2 Secondary Ion Mass Spectrometry 98
126.96.36.199 Effect of annealing treatment on bilayer Ni/Au 98
6.2.3 X-ray diffraction 99
6.2.4 Photovoltaic characterization 100
188.8.131.52 J-V characterization on devices with different MAI solution concentration 100
184.108.40.206 J-V characterization on devices with different dipping time of MAI solution 101
220.127.116.11 J-V characterization on devices refilled by perovskite loading 103
18.104.22.168 J-V characterization on devices fabricated by different PbI2 solution concentration 104
6.2.5 Secondary Ion Mass Spectroscopy 105
6.3 Summary 106
7. Conclusion and Future work 107
7.1 Conclusion 107
7.2 Future work 108
Publication paper 126
 E. A. Gibson, A. L. Smeigh, L. Le Pleux, J. Fortage, G. Boschloo, E. Blart, et al., "A p-Type NiO-Based Dye-Sensitized Solar Cell with an Open-Circuit Voltage of 0.35 V," Angew. Chem. Int. Ed., vol. 48, pp. 4402-4405, 2009.
 A. Nattestad, A. J. Mozer, M. K. R. Fischer, Y. B. Cheng, A. Mishra, P. Bauerle, et al., "Highly efficient photocathodes for dye-sensitized tandem solar cells," Nat Mater, vol. 9, pp. 31-35, 2010.
 A. Garcia, G. C. Welch, E. L. Ratcliff, D. S. Ginley, G. C. Bazan, and D. C. Olson, "Improvement of Interfacial Contacts for New Small- Molecule Bulk-Heterojunction Organic Photovoltaics," Adv. Mater., vol. 24, pp. 5368-5373, 2012.
 J. R. Manders, S.-W. Tsang, M. J. Hartel, T.-H. Lai, S. Chen, C. M. Amb, et al., "Solution-Processed Nickel Oxide Hole Transport Layers in High Efficiency Polymer Photovoltaic Cells," Adv. Funct. Mater., vol. 23, pp. 2993-3001, 2013.
 S. Powar, T. Daeneke, M. T. Ma, D. Fu, N. W. Duffy, G. Götz, et al., "Highly Efficient p-Type Dye-Sensitized Solar Cells based on Tris(1,2-diaminoethane)Cobalt(II)/(III) Electrolytes," Angew. Chem. Int. Ed., vol. 52, pp. 602-605, 2013.
 E. L. Ratcliff, A. Garcia, S. A. Paniagua, S. R. Cowan, A. J. Giordano, D. S. Ginley, et al., "Investigating the Influence of Interfacial Contact Properties on Open Circuit Voltages in Organic Photovoltaic Performance: Work Function Versus Selectivity," Adv. Energy Mater., vol. 3, pp. 647-656, 2013.
 M. D. Irwin, D. B. Buchholz, A. W. Hains, R. P. H. Chang, and T. J. Marks, "p-Type semiconducting nickel oxide as an efficiency-enhancing anode interfacial layer in polymer bulk-heterojunction solar cells," Proceedings of the National Academy of Sciences, vol. 105, pp. 2783-2787, 2008.
 F. Odobel and Y. Pellegrin, "Recent Advances in the Sensitization of Wide-Band-Gap Nanostructured p-Type Semiconductors. Photovoltaic and Photocatalytic Applications," The Journal of Physical Chemistry Letters, vol. 4, pp. 2551-2564, 2013.
 D. Weber, "CH3NH3PbX3, a Pb(II)-System with Cubic Perovskite Structure Z. Naturforsch," J. Chem. Sci, vol. 33, pp. 1443-1445, 1978.
 R. J. Cava, B. Batlogg, G. P. Espinosa, A. P. Ramirez, J. J. Krajewski, W. F. Peck, et al., "Superconductivity at 3.5 K in BaPb0.75Sb0.25O3: why is Tc so low?," Nature, vol. 339, pp. 291-293, 1989.
 R. J. Cava, B. Batlogg, J. J. Krajewski, R. Farrow, L. W. Rupp, A. E. White, et al., "Superconductivity near 30 K without copper: the Ba0.6K0.4BiO3 perovskite," Nature, vol. 332, pp. 814-816, 1988.
 T. Ishihara, T. Ogawa, and Y. Kanemitsu, Optical Properties of Low-dimensional Materials: World Scientific, 1995.
 S. Wang, D. B. Mitzi, C. A. Feild, and A. Guloy, "Synthesis and Characterization of [NH2C(I):NH2]3MI5 (M = Sn, Pb): Stereochemical Activity in Divalent Tin and Lead Halides Containing Single .ltbbrac.110.rtbbrac. Perovskite Sheets," Journal of the American Chemical Society, vol. 117, pp. 5297-5302, 1995.
 T. Ishihara, J. Takahashi, and T. Goto, "Optical properties due to electronic transitions in two-dimensional semiconductors (CnH2n+1NH3)2PbI4," Physical Review B, vol. 42, pp. 11099-11107, 1990.
 M. Era, S. Morimoto, T. Tsutsui, and S. Saito, "Organic‐inorganic heterostructure electroluminescent device using a layered perovskite semiconductor (C6H5C2H4NH3)2PbI4," Appl. Phys. Lett., vol. 65, p. 676, 1994.
 K. Shibuya, M. Koshimizu, Y. Takeoka, and K. Asai, "Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms," Nucl. Instrum. Methods B, vol. 194, p. 207, 2002.
 J. Ishi, H. Kunugita, K. Ema, T. Ban, and T. Kondo, "Influence of exciton-exciton interactions on frequency-mixing signals in a stable exciton-biexciton system," Physical Review B, vol. 63, p. 073303, 2001.
 T. Kondo, S. Iwamoto, S. Hayase, K. Tanaka, J. Ishi, M. Mizuno, et al., "Resonant third-order optical nonlinearity in the layered perovskite-type material (C6H13NH3)2PbI4," Solid State Communications, vol. 105, pp. 503-506, 1998.
 I. B. Koutselas, L. Ducasse, and G. C. Papavassiliou, "Electronic properties of three- and low-dimensional semiconducting materials with Pb halide and Sn halide units," J. Phys.:Condens. Matter vol. 8, p. 1217, 1996.
 T. Ishihara, "Optical properties of PbI-based perovskite structures," Journal of Luminescence, vol. 60–61, pp. 269-274, 1994.
 M. Hirasawa, T. Ishihara, T. Goto, K. Uchida, and N. Miura, "Magnetoabsorption of the lowest exciton in perovskite-type compound (CH3NH3)PbI3," Physica B: Condensed Matter, vol. 201, pp. 427-430, 1994.
 T. Ishihara, "Optical properties of PbI-based perovskite structures," J. Lumin., vol. 60-61, p. 294, 1994.
 N. Kitazawa, Y. Watanabe, and Y. Nakamura, "Optical properties of CH3NH3PbX3 (X = halogen) and their mixed-halide crystals," Journal of Materials Science, vol. 37, pp. 3585-3587, 2002.
 M. Hirasawa, T. Ishihara, and T. Goto, "Exciton Features in 0-, 2-, and 3-Dimensional Networks of [PbI6]4- Octahedra," Journal of the Physical Society of Japan, vol. 63, pp. 3870-3879, 1994.
 K. Tanaka, T. Takahashi, T. Ban, T. Kondo, K. Uchida, and N. Miura, "Comparative study on the excitons in lead-halide-based perovskite-type crystals CH3NH3PbBr3 CH3NH3PbI3," Solid State Communications, vol. 127, pp. 619-623, 2003.
 A. Kojima, K. Teshima, Y. Shirai, and T. Miyasaka, "Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells," J. Am. Chem. Soc., vol. 131, pp. 6050-6051, 2009.
 J.-H. Im, C.-R. Lee, J.-W. Lee, S.-W. Park, and N.-G. Park, "6.5% efficient perovskite quantum-dot-sensitized solar cell," Nanoscale, vol. 3, pp. 4088-4093, 2011.
 W. Li, J. Li, L. Wang, G. Niu, R. Gao, and Y. Qiu, "Post modification of perovskite sensitized solar cells by aluminum oxide for enhanced performance," Journal of Materials Chemistry A, vol. 1, pp. 11735-11740, 2013.
 H.-S. Kim, C.-R. Lee, J.-H. Im, K.-B. Lee, T. Moehl, A. Marchioro, et al., "Lead Iodide Perovskite Sensitized All-Solid-State Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%," Sci. Rep., vol. 2, p. 591, 2012.
 L. Etgar, P. Gao, Z. Xue, Q. Peng, A. K. Chandiran, B. Liu, et al., "Mesoscopic CH3NH3PbI3/TiO2 Heterojunction Solar Cells," Journal of the American Chemical Society, vol. 134, pp. 17396-17399, 2012.
 W. A. Laban and L. Etgar, "Depleted hole conductor-free lead halide iodide heterojunction solar cells," Energy & Environmental Science, vol. 6, pp. 3249-3253, 2013.
 M. M. Lee, J. Teuscher, T. Miyasaka, T. N. Murakami, and H. J. Snaith, "Efficient Hybrid Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites," Science, vol. 338, pp. 643-647, 2012.
 G. Rothenberger, D. Fitzmaurice, and M. Graetzel, "Spectroscopy of conduction band electrons in transparent metal oxide semiconductor films: optical determination of the flatband potential of colloidal titanium dioxide films," The Journal of Physical Chemistry, vol. 96, pp. 5983-5986, 1992.
 J. S. Henry, H.-B. Robin, C. Peter, C. Ilkay, M. Z. Shaik, and G. Michael, "Charge collection and pore filling in solid-state dye-sensitized solar cells," Nanotechnology, vol. 19, p. 424003, 2008.
 G. Boschloo and A. Hagfeldt, "Photoinduced absorption spectroscopy as a tool in the study of dye-sensitized solar cells," Inorganica Chimica Acta, vol. 361, pp. 729-734, 2008.
 J. Burschka, N. Pellet, S.-J. Moon, R. Humphry-Baker, P. Gao, M. K. Nazeeruddin, et al., "Sequential deposition as a route to high-performance perovskite-sensitized solar cells," Nature, vol. 499, pp. 316-319, 2013.
 J. H. Heo, S. H. Im, J. H. Noh, T. N. Mandal, C.-S. Lim, J. A. Chang, et al., "Efficient inorganic-organic hybrid heterojunction solar cells containing perovskite compound and polymeric hole conductors," Nature Photon., vol. 7, pp. 487-492, 2013.
 N. J. Jeon, J. H. Noh, Y. C. Kim, W. S. Yang, S. Ryu, and S. I. Seok, "Solvent engineering for high-performance inorganic-organic hybrid perovskite solar cells," Nat Mater, vol. 13, pp. 897-903, 2014.
 S. D. Stranks, G. E. Eperon, G. Grancini, C. Menelaou, M. J. P. Alcocer, T. Leijtens, et al., "Electron-Hole Diffusion Lengths Exceeding 1 Micrometer in an Organometal Trihalide Perovskite Absorber," Science, vol. 342, pp. 341-344, 2013.
 G. Xing, N. Mathews, S. Sun, S. S. Lim, Y. M. Lam, M. Gratzel, et al., "Long-range balanced electron- and hole-transport lengths in organic-inorganic CH3NH3PbI3," Science, vol. 342, pp. 344-7, 2013.
 M.-H. Li, P.-S. Shen, K.-C. Wang, T.-F. Guo, and P. Chen, "Inorganic p-type contact materials for perovskite-based solar cells," Journal of Materials Chemistry A, vol. 3, pp. 9011-9019, 2015.
 J.-Y. Jeng, K.-C. Chen, T.-Y. Chiang, P.-Y. Lin, T.-D. Tsai, Y.-C. Chang, et al., "Nickel Oxide Electrode Interlayer in CH3NH3PbI3 Perovskite/PCBM Planar-Heterojunction Hybrid Solar Cells," Advanced Materials, vol. 26, pp. 4107-4113, 2014.
 L. Wei-Chih, L. Kun-Wei, G. Tzung-Fang, and L. Jung, "Perovskite-Based Solar Cells With Nickel-Oxidized Nickel Oxide Hole Transfer Layer," Electron Devices, IEEE Transactions on, vol. 62, pp. 1590-1595, 2015.
 W.-C. Lai, K.-W. Lin, Y.-T. Wang, T.-Y. Chiang, P. Chen, and T.-F. Guo, "Oxidized Ni/Au Transparent Electrode in Efficient CH3NH3PbI3 Perovskite/Fullerene Planar Heterojunction Hybrid Solar Cells," Advanced Materials, vol. 28, pp. 3290-3297, 2016.
 J. Cui, F. Meng, H. Zhang, K. Cao, H. Yuan, Y. Cheng, et al., "CH3NH3PbI3-Based Planar Solar Cells with Magnetron-Sputtered Nickel Oxide," ACS Applied Materials & Interfaces, vol. 6, pp. 22862-22870, 2014.
 I. J. Park, M. A. Park, D. H. Kim, G. D. Park, B. J. Kim, H. J. Son, et al., "New Hybrid Hole Extraction Layer of Perovskite Solar Cells with a Planar p–i–n Geometry," The Journal of Physical Chemistry C, vol. 119, pp. 27285-27290, 2015.
 Y. Li, S. Ye, W. Sun, W. Yan, Y. Li, Z. Bian, et al., "Hole-conductor-free planar perovskite solar cells with 16.0% efficiency," Journal of Materials Chemistry A, vol. 3, pp. 18389-18394, 2015.
 J. H. Kim, P.-W. Liang, S. T. Williams, N. Cho, C.-C. Chueh, M. S. Glaz, et al., "High-Performance and Environmentally Stable Planar Heterojunction Perovskite Solar Cells Based on a Solution-Processed Copper-Doped Nickel Oxide Hole-Transporting Layer," Advanced Materials, vol. 27, pp. 695-701, 2015.
 J. W. Jung, C.-C. Chueh, and A. K. Y. Jen, "A Low-Temperature, Solution-Processable, Cu-Doped Nickel Oxide Hole-Transporting Layer via the Combustion Method for High-Performance Thin-Film Perovskite Solar Cells," Advanced Materials, vol. 27, pp. 7874-7880, 2015.
 J. H. Park, J. Seo, S. Park, S. S. Shin, Y. C. Kim, N. J. Jeon, et al., "Efficient CH3NH3PbI3 Perovskite Solar Cells Employing Nanostructured p-Type NiO Electrode Formed by a Pulsed Laser Deposition," Advanced Materials, vol. 27, pp. 4013-4019, 2015.
 X. Yin, M. Que, Y. Xing, and W. Que, "High efficiency hysteresis-less inverted planar heterojunction perovskite solar cells with a solution-derived NiOx hole contact layer," Journal of Materials Chemistry A, vol. 3, pp. 24495-24503, 2015.
 H. Zhang, J. Cheng, F. Lin, H. He, J. Mao, K. S. Wong, et al., "Pinhole-Free and Surface-Nanostructured NiOx Film by Room-Temperature Solution Process for High-Performance Flexible Perovskite Solar Cells with Good Stability and Reproducibility," ACS Nano, vol. 10, pp. 1503-1511, 2015.
 J. You, L. Meng, T.-B. Song, T.-F. Guo, Y. Yang, W.-H. Chang, et al., "Improved air stability of perovskite solar cells via solution-processed metal oxide transport layers," Nat Nano, vol. 11, pp. 75-81, 2016.
 K.-C. Wang, J.-Y. Jeng, P.-S. Shen, Y.-C. Chang, E. W.-G. Diau, C.-H. Tsai, et al., "p-type Mesoscopic Nickel Oxide/Organometallic Perovskite Heterojunction Solar Cells," Scientific Reports, vol. 4, p. 4756, 2014.
 H. Tian, B. Xu, H. Chen, E. M. J. Johansson, and G. Boschloo, "Solid-State Perovskite-Sensitized p-Type Mesoporous Nickel Oxide Solar Cells," ChemSusChem, vol. 7, pp. 2150-2153, 2014.
 Z. Zhu, Y. Bai, T. Zhang, Z. Liu, X. Long, Z. Wei, et al., "High-Performance Hole-Extraction Layer of Sol–Gel-Processed NiO Nanocrystals for Inverted Planar Perovskite Solar Cells," Angewandte Chemie International Edition, vol. 53, pp. 12571-12575, 2014.
 W. Chen, Y. Wu, J. Liu, C. Qin, X. Yang, A. Islam, et al., "Hybrid interfacial layer leads to solid performance improvement of inverted perovskite solar cells," Energy & Environmental Science, vol. 8, pp. 629-640, 2015.
 M.-H. Li, J.-H. Yum, S.-J. Moon, and P. Chen, "Inorganic p-Type Semiconductors: Their Applications and Progress in Dye-Sensitized Solar Cells and Perovskite Solar Cells," Energies, vol. 9, 2016.
 Z. Ku, Y. Rong, M. Xu, T. Liu, and H. Han, "Full printable processed mesoscopic CH3NH3PbI3/TiO2 heterojunction solar cells with carbon counter electrode," Sci Rep, vol. 3, p. 3132, 2013.
 Y. Rong, Z. Ku, A. Mei, T. Liu, M. Xu, S. Ko, et al., "Hole-Conductor-Free Mesoscopic TiO2/CH3NH3PbI3 Heterojunction Solar Cells Based on Anatase Nanosheets and Carbon Counter Electrodes," J Phys Chem Lett, vol. 5, pp. 2160-2164, 2014.
 A. Mei, X. Li, L. Liu, Z. Ku, T. Liu, Y. Rong, et al., "A hole-conductor-free, fully printable mesoscopic perovskite solar cell with high stability," Science, vol. 345, pp. 295-298, 2014.
 L. Zhang, T. Liu, L. Liu, M. Hu, Y. Yang, A. Mei, et al., "The effect of carbon counter electrodes on fully printable mesoscopic perovskite solar cells," J. Mater. Chem. A, vol. 3, pp. 9165-9170, 2015.
 T. Liu, L. Liu, M. Hu, Y. Yang, L. Zhang, A. Mei, et al., "Critical parameters in TiO2/ZrO2/Carbon-based mesoscopic perovskite solar cell," Journal of Power Sources, vol. 293, pp. 533-538, 2015.
 Z. Liu, M. Zhang, X. Xu, L. Bu, W. Zhang, W. Li, et al., "p-Type mesoscopic NiO as an active interfacial layer for carbon counter electrode based perovskite solar cells," Dalton Transactions, vol. 44, pp. 3967-3973, 2015.
 X. Xu, Z. Liu, Z. Zuo, M. Zhang, Z. Zhao, Y. Shen, et al., "Hole Selective NiO Contact for Efficient Perovskite Solar Cells with Carbon Electrode," Nano Letters, vol. 15, pp. 2402-2408, 2015.
 K. Cao, Z. Zuo, J. Cui, Y. Shen, T. Moehl, S. M. Zakeeruddin, et al., "Efficient screen printed perovskite solar cells based on mesoscopic TiO2/Al2O3/NiO/carbon architecture," Nano Energy, vol. 17, pp. 171-179, 2015.
 Z. Ku, X. Xia, H. Shen, N. H. Tiep, and H. J. Fan, "A mesoporous nickel counter electrode for printable and reusable perovskite solar cells," Nanoscale, vol. 7, pp. 13363-13368, 2015.
 X. Zhou, C. Bao, F. Li, H. Gao, T. Yu, J. Yang, et al., "Hole-transport-material-free perovskite solar cells based on nanoporous gold back electrode," RSC Adv., vol. 5, pp. 58543-58548, 2015.
 S. Powar, T. Daeneke, M. T. Ma, D. Fu, N. W. Duffy, G. Gotz, et al., "Highly efficient p-type dye-sensitized solar cells based on tris(1,2-diaminoethane)cobalt(II)/(III) electrolytes," Angew Chem Int Ed Engl, vol. 52, pp. 602-605, 2013.
 M. M. Wienk, M. Turbiez, J. Gilot, and R. A. J. Janssen, "Narrow‐Bandgap Diketo‐Pyrrolo‐Pyrrole Polymer Solar Cells: The Effect of Processing on the Performance," Advanced Materials, vol. 20, pp. 2556-2560, 2008.
 D. Mühlbacher, M. Scharber, M. Morana, Z. Zhu, D. Waller, R. Gaudiana, et al., "High Photovoltaic Performance of a Low-Bandgap Polymer," Advanced Materials, vol. 18, pp. 2884-2889, 2006.
 Y. Liang, Y. Wu, D. Feng, S.-T. Tsai, H.-J. Son, G. Li, et al., "Development of New Semiconducting Polymers for High Performance Solar Cells," Journal of the American Chemical Society, vol. 131, pp. 56-57, 2009.
 F. Huang, K.-S. Chen, H.-L. Yip, S. K. Hau, O. Acton, Y. Zhang, et al., "Development of New Conjugated Polymers with Donor−π-Bridge−Acceptor Side Chains for High Performance Solar Cells," Journal of the American Chemical Society, vol. 131, pp. 13886-13887, 2009.
 Y. Bai, Q. Dong, Y. Shao, Y. Deng, Q. Wang, L. Shen, et al., "Enhancing stability and efficiency of perovskite solar cells with crosslinkable silane-functionalized and doped fullerene," Nat Commun, vol. 7, p. 12806, 2016.
 P. Docampo, J. M. Ball, M. Darwich, G. E. Eperon, and H. J. Snaith, "Efficient organometal trihalide perovskite planar-heterojunction solar cells on flexible polymer substrates," Nat Commun, vol. 4, p. 2761, 2013.
 B. Conings, L. Baeten, C. De Dobbelaere, J. D'Haen, J. Manca, and H. G. Boyen, "Perovskite-based hybrid solar cells exceeding 10% efficiency with high reproducibility using a thin film sandwich approach," Adv Mater, vol. 26, pp. 2041-2046, 2014.
 G. Boschloo and A. Hagfeldt, "Spectroelectrochemistry of Nanostructured NiO," The Journal of Physical Chemistry B, vol. 105, pp. 3039-3044, 2001.
 S. Sun, T. Salim, N. Mathews, M. Duchamp, C. Boothroyd, G. Xing, et al., "The origin of high efficiency in low-temperature solution-processable bilayer organometal halide hybrid solar cells," Energy & Environmental Science, vol. 7, pp. 399-407, 2014.
 M. Guziewicz, J. Grochowski, M. Borysiewicz, E. Kaminska, J. Z. Domagala, W. Rzodkiewicz, et al., "Electrical and optical properties of NiO films deposited by magnetron sputtering," Opt. Appl., vol. 41, pp. 431-440, 2011.
 J. D. Christian and W. P. Gilbreath, "Defect structure of NiO and rates and mechanisms of formation from atomic oxygen and nickel," Oxidation of Metals, vol. 9, pp. 1-25, 1975.
 J. Burschka, N. Pellet, S.-J. Moon, R. Humphry-Baker, P. Gao, M. K. Nazeeruddin, et al., "Sequential deposition as a route to high-performance perovskite-sensitized solar cells," Nature, vol. 499, pp. 316-319, 2013.
 B. Peng, G. Jungmann, C. Jäger, D. Haarer, H.-W. Schmidt, and M. Thelakkat, "Systematic investigation of the role of compact TiO2 layer in solid state dye-sensitized TiO2 solar cells," Coordination Chemistry Reviews, vol. 248, pp. 1479-1489, 2004.
 S. Nandy, B. Saha, M. K. Mitra, and K. K. Chattopadhyay, "Effect of oxygen partial pressure on the electrical and optical properties of highly (200) oriented p-type Ni1−x O films by DC sputtering," Journal of Materials Science, vol. 42, pp. 5766-5772, 2007.
 M. Guziewicz, J. Grochowski, M. Borysiewicz, E. Kaminska, J. Z. Domagala, W. Rzodkiewicz, et al., "Electrical and optical properties of NiO films deposited by magnetron sputtering," vol. 41, pp. 431-440, 2011.
 Y. A. K. Reddy, A. M. Reddy, A. S. Reddy, and P. S. Reddy, "Preparation and Characterization of NiO Thin Films by DC Reactive Magnetron Sputtering," J. Nano-Electron. Phys., vol. 4, p. 04002, 2012.
 S. Seo, I. J. Park, M. Kim, S. Lee, C. Bae, H. S. Jung, et al., "An ultra-thin, un-doped NiO hole transporting layer of highly efficient (16.4%) organic-inorganic hybrid perovskite solar cells," Nanoscale, vol. 8, pp. 11403-11412, 2016.
 V. Trifiletti, V. Roiati, S. Colella, R. Giannuzzi, L. De Marco, A. Rizzo, et al., "NiO/MAPbI3-xClx/PCBM: A Model Case for an Improved Understanding of Inverted Mesoscopic Solar Cells," ACS Applied Materials & Interfaces, vol. 7, pp. 4283-4289, 2015.
 K.-C. Wang, P.-S. Shen, M.-H. Li, S. Chen, M.-W. Lin, P. Chen, et al., "Low-Temperature Sputtered Nickel Oxide Compact Thin Film as Effective Electron Blocking Layer for Mesoscopic NiO/CH3NH3PbI3 Perovskite Heterojunction Solar Cells," ACS Applied Materials & Interfaces, vol. 6, pp. 11851-11858, 2014.
 G. A. Sepalage, S. Meyer, A. Pascoe, A. D. Scully, F. Huang, U. Bach, et al., "Copper(I) Iodide as Hole-Conductor in Planar Perovskite Solar Cells: Probing the Origin of J–V Hysteresis," Advanced Functional Materials, vol. 25, pp. 5650-5661, 2015.
 W.-Y. Chen, L.-L. Deng, S.-M. Dai, X. Wang, C.-B. Tian, X.-X. Zhan, et al., "Low-cost solution-processed copper iodide as an alternative to PEDOT:PSS hole transport layer for efficient and stable inverted planar heterojunction perovskite solar cells," Journal of Materials Chemistry A, vol. 3, pp. 19353-19359, 2015.
 J. A. Christians, R. C. M. Fung, and P. V. Kamat, "An Inorganic Hole Conductor for Organo-Lead Halide Perovskite Solar Cells. Improved Hole Conductivity with Copper Iodide," Journal of the American Chemical Society, vol. 136, pp. 758-764, 2014.
 W. Yu, F. Li, H. Wang, E. Alarousu, Y. Chen, B. Lin, et al., "Ultrathin Cu2O as an efficient inorganic hole transporting material for perovskite solar cells," Nanoscale, vol. 8, pp. 6173-6179, 2016.
 B. A. Nejand, V. Ahmadi, S. Gharibzadeh, and H. R. Shahverdi, "Cuprous Oxide as a Potential Low-Cost Hole-Transport Material for Stable Perovskite Solar Cells," ChemSusChem, vol. 9, pp. 302-313, 2016.
 F. Igbari, M. Li, Y. Hu, Z.-K. Wang, and L.-S. Liao, "A room-temperature CuAlO2 hole interfacial layer for efficient and stable planar perovskite solar cells," Journal of Materials Chemistry A, vol. 4, pp. 1326-1335, 2016.
 S. Chatterjee and A. J. Pal, "Introducing Cu2O Thin Films as a Hole-Transport Layer in Efficient Planar Perovskite Solar Cell Structures," The Journal of Physical Chemistry C, vol. 120, pp. 1428-1437, 2016.
 C. Zuo and L. Ding, "Solution-Processed Cu2O and CuO as Hole Transport Materials for Efficient Perovskite Solar Cells," Small, vol. 11, pp. 5528-5532, 2015.
 M. I. Hossain, F. H. Alharbi, and N. Tabet, "Copper oxide as inorganic hole transport material for lead halide perovskite based solar cells," Solar Energy, vol. 120, pp. 370-380, 2015.
 K. Zhao, R. Munir, B. Yan, Y. Yang, T. Kim, and A. Amassian, "Solution-processed inorganic copper(i) thiocyanate (CuSCN) hole transporting layers for efficient p-i-n perovskite solar cells," Journal of Materials Chemistry A, vol. 3, pp. 20554-20559, 2015.
 S. Ye, W. Sun, Y. Li, W. Yan, H. Peng, Z. Bian, et al., "CuSCN-Based Inverted Planar Perovskite Solar Cell with an Average PCE of 15.6%," Nano Letters, vol. 15, pp. 3723-3728, 2015.
 W. Nilushi and D. A. Thomas, "Copper(I) thiocyanate (CuSCN) as a hole-transport material for large-area opto/electronics," Semiconductor Science and Technology, vol. 30, p. 104002, 2015.
 S. Chavhan, O. Miguel, H.-J. Grande, V. Gonzalez-Pedro, R. S. Sanchez, E. M. Barea, et al., "Organo-metal halide perovskite-based solar cells with CuSCN as the inorganic hole selective contact," Journal of Materials Chemistry A, vol. 2, pp. 12754-12760, 2014.
 D.-Y. Lee, S.-I. Na, and S.-S. Kim, "Graphene oxide/PEDOT:PSS composite hole transport layer for efficient and stable planar heterojunction perovskite solar cells," Nanoscale, vol. 8, pp. 1513-1522, 2016.
 H. Wang, A. D. Sheikh, Q. Feng, F. Li, Y. Chen, W. Yu, et al., "Facile Synthesis and High Performance of a New Carbazole-Based Hole-Transporting Material for Hybrid Perovskite Solar Cells," ACS Photonics, vol. 2, pp. 849-855, 2015.
 R. Lindblad, D. Bi, B.-w. Park, J. Oscarsson, M. Gorgoi, H. Siegbahn, et al., "Electronic Structure of TiO2/CH3NH3PbI3 Perovskite Solar Cell Interfaces," The Journal of Physical Chemistry Letters, vol. 5, pp. 648-653, 2014.
 A. Roy, D. D. Sarma, and A. K. Sood, "Spectroscopic studies on quantum dots of PbI2," Spectrochimica Acta Part A: Molecular Spectroscopy, vol. 48, pp. 1779-1787, 1992.
 U. St, C. Scharfschwerdt, M. Neumann, G. Illing, and H. J. Freund, "The influence of defects on the Ni 2p and O 1s XPS of NiO," Journal of Physics: Condensed Matter, vol. 4, p. 7973, 1992.
 A. R. González-Elipe, R. Alvarez, J. P. Holgado, J. P. Espinos, G. Munuera, and J. M. Sanz, "An XPS study of the Ar+-induced reduction of Ni2+ in NiO and Ni-Si oxide systems," Applied Surface Science, vol. 51, pp. 19-26, 1991.
 S. Oswald and W. Brückner, "XPS depth profile analysis of non-stoichiometric NiO films," Surface and Interface Analysis, vol. 36, pp. 17-22, 2004.
 E. L. Ratcliff, J. Meyer, K. X. Steirer, A. Garcia, J. J. Berry, D. S. Ginley, et al., "Evidence for near-Surface NiOOH Species in Solution-Processed NiOx Selective Interlayer Materials: Impact on Energetics and the Performance of Polymer Bulk Heterojunction Photovoltaics," Chemistry of Materials, vol. 23, pp. 4988-5000, 2011.
 C. C. Oey, A. B. Djurišić, C. Y. Kwong, C. H. Cheung, W. K. Chan, J. M. Nunzi, et al., "Nanocomposite hole injection layer for organic device applications," Thin Solid Films, vol. 492, pp. 253-258, 2005.
 M.-W. Lin, K.-C. Wang, J.-H. Wang, M.-H. Li, Y.-L. Lai, T. Ohigashi, et al., "Improve Hole Collection by Interfacial Chemical Redox Reaction at a Mesoscopic NiO/CH3NH3PbI3 Heterojunction for Efficient Photovoltaic Cells," Advanced Materials Interfaces, vol. 3, p. 1600135, 2016.
 L. A. Grunes, R. D. Leapman, C. N. Wilker, R. Hoffmann, and A. B. Kunz, "Oxygen K near-edge fine structure: An electron-energy-loss investigation with comparisons to new theory for selected 3d Transition-metal oxides," Physical Review B, vol. 25, pp. 7157-7173, 1982.
 I. Davoli, A. Marcelli, A. Bianconi, M. Tomellini, and M. Fanfoni, "Multielectron configurations in the x-ray-absorption near-edge structure of NiO at the oxygen K threshold," Physical Review B, vol. 33, pp. 2979-2982, 1986.
 S.-i. Nakai, T. Mitsuishi, H. Sugawara, H. Maezawa, T. Matsukawa, S. Mitani, et al., "Oxygen K x-ray-absorption near-edge structure of alkaline-earth-metal and 3d-transition-metal oxides," Physical Review B, vol. 36, pp. 9241-9246, 1987.
 S. L. Dudarev, G. A. Botton, S. Y. Savrasov, C. J. Humphreys, and A. P. Sutton, "Electron-energy-loss spectra and the structural stability of nickel oxide: An LSDA+U study," Physical Review B, vol. 57, pp. 1505-1509, 1998.
 P. Kuiper, G. Kruizinga, J. Ghijsen, G. A. Sawatzky, and H. Verweij, "Character of Holes in LixNi1-xOand Their Magnetic Behavior," Physical Review Letters, vol. 62, pp. 221-224, 1989.
 M. Finazzi and N. B. Brookes, "Resonant Auger spectroscopy at the O K edge of NiO," Physical Review B, vol. 60, pp. 5354-5358, 1999.
 S. L. Dudarev, M. R. Castell, G. A. Botton, S. Y. Savrasov, C. Muggelberg, G. A. D. Briggs, et al., "Understanding STM images and EELS spectra of oxides with strongly correlated electrons: a comparison of nickel and uranium oxides," Micron, vol. 31, pp. 363-372, 2000.
 F. M. F. de Groot, M. Grioni, J. C. Fuggle, J. Ghijsen, G. A. Sawatzky, and H. Petersen, "Oxygen 1s x-ray-absorption edges of transition-metal oxides," Physical Review B, vol. 40, pp. 5715-5723, 1989.
 H. Kurata, E. Lefèvre, C. Colliex, and R. Brydson, "Electron-energy-loss near-edge structures in the oxygen K-edge spectra of transition-metal oxides," Physical Review B, vol. 47, pp. 13763-13768, 1993.
 Z. Y. Wu, S. Gota, F. Jollet, M. Pollak, M. Gautier-Soyer, and C. R. Natoli, "Characterization of iron oxides by x-ray absorption at the oxygen K edge using a full multiple-scattering approach," Physical Review B, vol. 55, pp. 2570-2577, 1997.
 C. Colliex, T. Manoubi, and C. Ortiz, "Electron-energy-loss-spectroscopy near-edge fine structures in the iron-oxygen system," Physical Review B, vol. 44, pp. 11402-11411, 1991.
 W.-J. Yin, T. Shi, and Y. Yan, "Unusual defect physics in CH3NH3PbI3 perovskite solar cell absorber," Applied Physics Letters, vol. 104, p. 063903, 2014.
 Y. Zhao and K. Zhu, "Charge Transport and Recombination in Perovskite (CH3NH3)PbI3 Sensitized TiO2 Solar Cells," The Journal of Physical Chemistry Letters, vol. 4, pp. 2880-2884, 2013.
 M. H. Kumar, N. Yantara, S. Dharani, M. Graetzel, S. Mhaisalkar, P. P. Boix, et al., "Flexible, low-temperature, solution processed ZnO-based perovskite solid state solar cells," Chemical Communications, vol. 49, pp. 11089-11091, 2013.
 F. Brivio, A. B. Walker, and A. Walsh, "Structural and electronic properties of hybrid perovskites for high-efficiency thin-film photovoltaics from first-principles," APL Materials, vol. 1, p. 042111, 2013.
 Y. Wang, T. Gould, J. F. Dobson, H. Zhang, H. Yang, X. Yao, et al., "Density functional theory analysis of structural and electronic properties of orthorhombic perovskite CH3NH3PbI3," Physical Chemistry Chemical Physics, vol. 16, pp. 1424-1429, 2014.