Article pubs.acs.org/accounts
Rational Strategies for Efficient Perovskite Solar Cells Published as part of the Accounts of Chemical Research special issue “Lead Halide Perovskites for Solar Energy Conversion”. Jangwon Seo,† Jun Hong Noh,† and Sang Il Seok*,†,‡ †
Division of Advanced Materials, Korea Research Institute of Chemical Technology, 141 Gajeong-Ro, Yuseong-Gu, Daejeon 305-600, Republic of Korea ‡ School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), 50 UNIST-gil, Eonyang-eup, Ulju-gun, Ulsan 689-798, Korea
CONSPECTUS: A long-standing dream in the large scale application of solar energy conversion is the fabrication of solar cells with high-efficiency and long-term stability at low cost. The realization of such practical goals depends on the architecture, process and key materials because solar cells are typically constructed from multilayer heterostructures of light harvesters, with electron and hole transporting layers as a major component. Recently, inorganic−organic hybrid lead halide perovskites have attracted significant attention as light absorbers for the fabrication of low-cost and high-efficiency solar cells via a solution process. This mainly stems from long-range ambipolar charge transport properties, low exciton binding energies, and suitable band gap tuning by managing the chemical composition. In our pioneering work, a new photovoltaic platform for efficient perovskite solar cells (PSCs) was proposed, which yielded a high power conversion efficiency (PCE) of 12%. The platform consisted of a pillared architecture of a three-dimensional nanocomposite of perovskites fully infiltrating mesoporous TiO2, resulting in the formation of continuous phases and perovskite domains overlaid with a polymeric hole conductor. Since then, the PCE of our PSCs has been rapidly increased from 3% to over 20% certified efficiency. The unprecedented increase in the PCE can be attributed to the effective integration of the advantageous attributes of the refined bicontinuous architecture, deposition process, and composition of perovskite materials. Specifically, the bicontinuous architectures used in the high efficiency comprise a layer of perovskite sandwiched between mesoporous metal−oxide layer, which is a very thinner than that of used in conventional dye-sensitized solar cells, and hole-conducting contact materials with a metal back contact. The mesoporous scaffold can affect the hysteresis under different scan direction in measurements of PSCs. The hysteresis also greatly depends on the cell architecture and perovskite composition. In this Account, we will describe what we do with major aspects including (1) the film morphology through the development of intermediate chemistry retarding the rapid reaction between methylammonium or formamidinium iodide and lead halide (PbI2) for improved perovskite film formation; (2) the phase stability and band gap tuning of the perovskite layer through the materials engineering; (3) the development of electron and hole transporting materials for carrierselective contacting layers; and (4) the adoption of p−i−n and n−i−p architectures depending on the position of the electron or hole conducting layer in front of incident light. Finally, we will summarize the recent incredible achievements in PSCs, and finally provide challenges facing the future development and commercialization of PSCs.
1. INTRODUCTION Since the seminal work reported by Miyasaka and co-workers in 2009,1 attempts to improve the durability of perovskite materials in perovskite-sensitized liquid-electrolyte solar cells brought them to an unprecedented success in terms of both efficiency and stability.2−4 Then, a new class of photovoltaics called perovskite solar cells (PSCs) was created and emerged as next-generation solar cells. The efficiency of PSCs have increased at an unexpected rate from 3.8% to 21% over the past 5 years, approaching the highest efficiency of Cu(In,Ga)© XXXX American Chemical Society
Se2 and exceeding that of both conventional organic photovoltaics (OPVs) and dye-sensitized solar cells (DSSCs).5,6 To date, we have achieved record efficiencies in PSCs by integrating the key features of innovative architectures, chemical composition, and deposition process for perovskite materials with newly designed electron and hole transporting layers. PSCs can be fabricated with a n−i−p or p−i−n Received: September 30, 2015
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DOI: 10.1021/acs.accounts.5b00444 Acc. Chem. Res. XXXX, XXX, XXX−XXX
Article
Accounts of Chemical Research
Figure 1. Device structure and key materials in each layer of a perovskite cell: (a) pillared PSC. (a) Reproduced with permission from ref 4. Copyright 2013 Nature Publishing Group, (b) n−i−p and p−i−n cell architectures, and (c) schematic energy level diagram of perovskite materials with electron (n-type)/hole (p-type) transporting materials.
Figure 2. (MAPbI3)1−x(MAPbBr3)x mixed perovskite: (a) Tetragonal and cubic perovskite structure of MAPbI3 and MAPbBr3 at room temperature, respectively. Red, polyhederon [PbX6/2]− (X = I, Br); green sphere, CH3NH3(= MA). (b) The conduction band minimum (CBM) and valence band maximum (VBM) of MAPbI3, MAPbBr3, and TiO2 are represented in eV. (c) Photographs of mp-TiO2: (MAPbI3)1−x(MAPbBr3)x nanocomposites bilayer on FTO glass substrates and the quadratic relationship of the band gaps of the mixed perovskites as a function of x. (d) PCE variation of the (MAPbI3)1−x(MAPbBr3)x PSCs (x = 0, 0.06, 0.20, 0.29) with time stored in air at room temperature without encapsulation. (a−d) Reproduced with permission from ref 7. Copyright 2013 American Chemical Society.
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DOI: 10.1021/acs.accounts.5b00444 Acc. Chem. Res. XXXX, XXX, XXX−XXX
Article
Accounts of Chemical Research
Figure 3. (FAPbI3)1−x(MAPbBr3)x mixed perovskite: (a) Polyhedral representations of the perovskite and nonperovskite polymorphs of FAPbI3. (a) Reproduced with permission from ref 14. Copyright 2014 American Chemical Society. (b) TG-DSC curves of the as-prepared nonperovskite FAPbI3 powder under an Ar atmosphere. The red and green lines represent the DSC results for prepared powders at x = 0 and 0.15, respectively. (c) XRD spectra of solvent-engineering-processed FAPbI3, (FAPbI3)1−x(MAPbI3)x, (FAPbI3)1−x(FAPbBr3)x, and (FAPbI3)1−x(MAPbBr3)x films with x = 0.15. Symbols α, *, and # denote the identified diffraction peaks corresponding to the perovskite and nonperovskite polymorphs of FAPbI3 and FTO, respectively. (d) fwhm of the X-ray diffracted (−111) peak for (FAPbI3)1−x(MAPbBr3)x films as a function of x from XRD spectra of solventengineering-processed films. (e) SEM plane view images of (FAPbI3)1−x(MAPbBr3)x films with x = 0, 0.05, and 0.15. (b−e) Reproduced with permission from ref 4. Copyright 2015 Nature Publishing Group.
conducting layer in front of incident light. To fabricate efficient
2. MANIPULATION OF PEROVSKITE MATERIALS AS LIGHT HARVESTERS
PSCs, several basic components including the fabrication
2.1. Compositional Engineering
process, key materials, and architecture should be optimized.
Inorganic−organic hybrid AMX3 perovskite materials (A, organic cation, methylammonium (MA), or formamidinium (FA); M, metal; X, halide anion, Br, I) exhibit beneficial properties for high-performance photovoltaic systems such as a suitable band gap (1.5−1.4 eV),7,8 high absorption coefficient (104−105 cm−1),9 low exciton binding energy ( 175 μm in solution-grown CH3NH3PbI3 single crystals. Science 2015, 347, 967−970. (12) Amat, A.; Mosconi, E.; Ronca, E.; Quarti, C.; Umari, P.; Nazeeruddin, M. K.; Grätzel, M.; De Angelis, F. Cation-induced bandgap tuning in organohalide perovskites: Interplay of spin−orbit coupling and octahedra tilting. Nano Lett. 2014, 14, 3608−3616. (13) Stoumpos, C. C.; Malliakas, C. D.; Kanatzidis, M. G. Semiconducting tin and lead iodide perovskites with organic cations: phase transitions, high mobilities, and near-infrared photoluminescent properties. Inorg. Chem. 2013, 52, 9019−9038. (14) Koh, T. M.; Fu, K.; Fang, Y.; Chen, S.; Sum, T.; Mathews, N.; Mhaisalkar, S. G.; Boix, P. P.; Baikie, T. Formamidinium-containing metal-halide: an alternative material for near-IR absorption perovskite solar cells. J. Phys. Chem. C 2014, 118, 16458−16462. (15) Jeon, N. J.; Noh, J. H.; Yang, W. S.; Kim, Y. C.; Ryu, S.; Seo, J.; Seok, S. I. Compositional engineering of perovskite materials for highperformance solar cells. Nature 2015, 517, 476−480. (16) Yang, W. S.; Noh, J. H.; Jeon, N. J.; Kim, Y. C.; Ryu, S.; Seo, J.; Seok, S. I. High-performance photovoltaic perovskite layers fabricated through intramolecular exchange. Science 2015, 348, 1234−1237. (17) Shin, S. S.; Yang, W. S.; Noh, J. H.; Suk, J. H.; Jeon, N. J.; Park, J. H.; Kim, J. S.; Seong, W. M.; Seok, S. I. High-performance flexible perovskite solar cells exploiting Zn2SnO4 prepared in solution below 100 °C. Nat. Commun. 2015, 6, 7410. K
DOI: 10.1021/acs.accounts.5b00444 Acc. Chem. Res. XXXX, XXX, XXX−XXX
