Once again, organometallic tri-halide perovskites

Pioneering work on alkali-metal lead and tin halides with the chemical formula CsPbX3 (X = Cl, Br or I) was performed in 1893 by Wells [1]. 94 years later, Weber et al. successfully replaced Cs by methylammonium cations (CH3NH3+) and studied various compositions of the first three-dimensional organic–inorganic hybrid perovskites, tuning their crystal structures and phases [2]. Of these, CH3NH3PbI3 perovskite demonstrates α, β and γ phases at >327.4 K, 162.2–327.4 K and <162.2 K respectively. These crystal structures also change with temperature and show cubic (Pm3m), tetragonal (I4/mcm) and orthorhombic (Pna2) systems. This p-type semiconducting material with a direct band gap of 1.55 eV (corresponding to an absorption onset at 800 nm) makes this material an excellent light harvester over the whole visible solar spectrum. The electrons and holes produced in CH3NH3PbI3 exhibit a small effective mass resulting in high carrier mobilities that range from 7.5 cm2 V−1 s−1 for electrons to 12.5–66 cm2 V−1 s−1 for holes. Further, the band gap can be tune by varying the composition and sub-halide groups [3].

Finally, after 21 years, CH3NH3PbI3 perovskite quantum-dots have once more been highlighted through work by Im et al. in 2011 via an ex situ method with 6.5% power conversion efficiency (PCE) [4]. However, these cells were not stable due to the fast degradation and decomposition of CH3NH3PbI3 into a liquid redox electrolyte. This problem was solved by Gratzel et al. in 2013 using solid state perovskite solar cells (PSCs) [5]. The CH3NH3PbI3 was synthesized by sequential deposition method (i.e. in situ solution process) and demonstrated a breakthrough 13% stable conversion efficiency.

In a typical PSC, an absorber layer (perovskite) is sandwiched between an electron transport layer (ETL) and a hole transport layer (HTL). The compact titanium oxide blocking layer (Bl-TiO2) – with or without mesoporous scaffold (TiO2 or Al2O3) – acts as an ETL, and the hole transporting material (HTM), that is spiro-MeOTAD (2,2′,7,7′-Tetrakis-(N,N-di-4-methoxyphenylamino)-9,9′-spirobifluorene), and the counter electrode (Au, Ag or Al) are the key components of the HTL. Delicate control of stoichiometric perovskite, the sub-halides and their uniform deposition, uniformity of the HTM layer, and the thickness of each layer are the key factors for achieving excellent PCE. Recently, the PCE of perovskite (MAPbX3, X = Cl, Br, I)-based thin film photovoltaic devices has risen steeply from 3.8% to more than 17% in just few years.

The problem of perovskite degradation has been nullified by using solid state spiro-MeOTAD [4]. After investigation of PSCs, methylammonium lead halide perovskite semiconductors have received great attention as a low cost solution processed photovoltaic technology within the last year. The PSC is typically boosted to 16.6% (certified) on average, with the highest efficiency of ∼19.3% (non-certified) in a planar geometry without antireflective coating [6].

On the other hand, Snaith et al. synthesized mixed halide perovskite CH3NH3PbI3−xClx using a dual-source vapor deposition technique for planar heterojunction PSCs [7]. The 15.4% PCE with 1.07 V open circuit voltage (VOC) was demonstrated for CH3NH3PbI3−xClx. The higher current density (JSC) of 21.5 mA cm−2 and the highest open circuit voltage was achieved due to the high diffusion length (LD) (1069 nm) [8] and pinhole free perovskite with uniform HTM layer [7]. The VOC can also be increased through the replacement of TiO2 with Al2O3 [9]. Low temperature flexible PSCs have also been demonstrated by Yang et al. using PCMB as a HTM material with 9.2% PCE. However, the optimization of PCBM thickness is critical factor for achieving high performance [10].

Therefore, solvent engineering processing has recently been adopted for high-performance PSCs. Sang Il Seok et al. remarkably improved the PCE up to 16.2% via toluene drip treatment [11]. This treatment is helpful for the formation of the intermediate MAI-PbI2-DMSO phase which retards the rapid reaction between PbI2 and MAI during the solvent evaporation results in a highly crystalline CH3NH3PbI3 perovskite layer.

This issue's cover image shows a field emission scanning electron micrograph (FESEM) of CH3NH3PbI3 perovskites deposited by a simple and cost-effective spin coating technique followed by covering with spiro-MeOTAD. The image was recorded using a FESEM (S-4700, Hitachi, Japan) operated at 15 keV. In the spin coating technique, spinning steps are atomized in such a way that the metal cations and halogen anions reacts with each other by evaporation solvent and dry on a hot plate to form pin-hole free highly crystalline tetragonal CH3NH3PbI3. However, various preparative conditions affect the quality of the samples deposited, such as the nature of the solvent (γ-butyrolactone, N,N-dimethylformamide, dimethylsulphoxide and N-methyl-2-pyrrolidone), spinning rotating rate (single-step, double-steps or multi-steps) and solvent processing (toluene dripping). The precursors containing methylamine, hydroiodic acid and lead iodide were used in a two-step reaction process for the synthesis of tetragonal CH3NH3PbI3 perovskite. The perovskite solution was prepared by dissolving equimolar amounts of CH3NH3I and PbI2 in anhydrous γ-butyrolactone. The clear filtered yellow solution was spin coated on top of the mp-TiO2 samples followed by heat treatment on the hot plate for 10 min to form dark-brown colored crystalline CH3NH3PbI3. The CH3NH3PbI3-sensitized TiO2 films were coated with spiro-MeOTAD solution using the spin-coating method at 3000 rpm for 30s [12]. The large size (∼5 μm) pin-hole free, compact micro-platelets of spiro-MeOTAD can form on top of the CH3NH3PbI3. The reaction process, deposition speed and drying temperature were optimized in order to get this morphology. The thickness and uniformity of HTM were optimized in such way to completely cover the pre-deposited perovskite structure. Further, these structures have been used for PSCs and our results show excellent solar cell performance. We are currently investigating the PSCs fabrication and testing based on low temperature processed one-dimensional (1D) and three-dimensional (3D) TiO2 nanostructures. [13]

This work was supported by the Priority Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (NRF-2009-0094055).

Further reading

1. H.L. Wells., Z. Anorg. Chem., 3 (1893), p. 195

2. A. Poglitsch, et al., J. Chem. Phys., 11 (1987), p. 87

3. B. Suarez, et al., J. Phys. Chem. Lett., 5 (2014), p. 1628

4. J.-H. Im, et al., Nanoscale, 3 (2011), p. 4088

5. J. Burschka, et al., Nature, 316 (2013), p. 499

6. H. Zhou, et al., Science, 345 (2014), p. 6196

7. M. Liu, et al., Nature, 501 (2013), p. 395

8. S.D. Stranks, et al., Science, 342 (2013), p. 341

9. M.M. Lee, et al., Science, 338 (2012), p. 643

10. J. You, et al., ACS Nano, 8 (2014), p. 1674

11. N.J. Jeon, et al., Nat. Mater., 13 (2014), p. 897

12. H.-S. Kim, et al., Nano Lett., 13 (2013), p. 2412

13. S.S. Mali, et al., Chem. Mater (2015) http://dx.doi.org/10.1021/cm504558g

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DOI: 10.1016/j.mattod.2015.02.011