Organolead Halide Perovskite: New Horizons in Solar Cell Research

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Organolead Halide Perovskite: New Horizons in Solar Cell Research Hui-Seon Kim,† Sang Hyuk Im,*,‡ and Nam-Gyu Park*,† †

School of Chemical Engineering and Department of Energy Science, Sungkyunkwan University, Suwon 440-746, Republic of Korea Department of Chemical Engineering, Kyung Hee University, Yongin 446-701, Republic of Korea

‡

ABSTRACT: Organolead-halide-perovskite-based solar cells have recently received significant attention due to their excellent photovoltaic performance and low cost. The general formula of this perovskite light harvester is RPbX3, where R and X stand for a monovalent organic cation and halide anion, respectively. Structures of the perovskite solar cell are designed based on the function of the perovskite. Organolead halide perovskites can be used either as sensitizers or n- or p-type light harvesters. Rapid progress has been made over the past year since the first report on long-term, durable, 9.7% efficiency perovskite solar cells based on CH3NH3PbI3-sensitized TiO2 in 2012. As a result, power conversion efficiencies as high as 16% have been achieved. Further improvement is expected from this material in terms of understanding charge accumulation and transport properties. Organolead halide perovskite is now regarded as a promising solar cell material, opening new horizons in solar cell research.

1. INTRODUCTION The development in 20121 of a highly efficient perovskite solar cell with long-term durability, following the first attempt at a perovskite sensitizer in 2009,2 represents a paradigm shift in solar cell technology. Methylammonium lead halides were confirmed as photoactive materials for the first time via a liquid-electrolytebased, dye-sensitized solar cell. The replacement of conventional organic dye with CH3NH3PbBr3 and CH3NH3PbI3 resulted in power conversion efficiencies (PCEs) of 3.1% and 3.8%, respectively, in a tri-iodide/iodine-based liquid electrolyte.2 The PCE of CH3NH3PbI3 was doubled to 6.5% in 2011 by carefully optimizing the perovskite coating procedure and electrolyte formulation.3 CH3NH3PbX3 (X = Br, I) was found to dissolve in a polar solvent, however, rendering the perovskite in a liquid-based solar cell unstable. This instability was eventually resolved by adapting solid hole conductors in place of the liquid redox electrolyte. A PCE as high as 9.7% was later achieved, even with a submicrometer-thick TiO2 layer.1 This superb photovoltaic property was due primarily to the high absorption coefficient of CH3NH3PbI3, which is estimated to be 10 times higher than a conventional organometallic dye.3 Mixed halide CH3NH3PbI3−xClx was developed and applied to a nonelectron injecting, mesoporous Al2O3 scaffold layer, the so-called mesosuperstructure perovskite solar cell, which demonstrated a PCE of 10.9%.4 This further elucidates the bifunctional role of perovskite light harvesters: harvesting light and transporting charge. In the mesoscopic, solid-state perovskite solar cells, pores in the oxide film were typically filled with a hole-transporting material (HTM); this was one of the most critical fabrication steps. The process of filling the holes with perovskite further improved the PCE to 12%, primarily due to increased contact between the HTM and perovskite overlayer, together with the high concentration of perovskite in the given volume.5 On the basis of this structural concept, a two-step procedure of CH3NH3I © 2014 American Chemical Society

intercalation following layered PbI2 deposition resulted in a PCE of 15% and a certified PCE of 14.1%.6 The planar heterojunction structure and perovskite layer, formed via a coevaporation technique, proposed for the meso-superstructure exhibited a PCE of 15.4%.7 Because CH3NH3PbI3−xClx has its own chargetransporting characteristics, electron injecting of an oxide such as TiO2 is no longer required, as confirmed by the Al2O3 system.4 The perovskite solar cell has recently been classified as a new solar cell type. The highest PCE obtained by a significant margin was confirmed from KRICT at 16.2%, according to the research cell efficiency record chart provided by the National Renewable Research Laboratory (NREL).8 Editors at the journal Science have chosen perovskite solar cell technology as one of the top ten biggest scientific breakthroughs of 2013.9 Perovskite solar cells hold promise as a low-cost and more efficient alternative to conventional semiconductor-based solar cells. In this paper, we review recent progress in perovskite solar cells. Progress in solid-state sensitized solar cells is briefly investigated as a background for the technology of perovskite solar cells. The structural and opto-electronic properties of organolead halide perovskite are described, and issues to be addressed regarding perovskite solar cells are also mentioned.

2. SOLID-STATE SENSITIZED SOLAR CELLS AS BACKGROUND TECHNOLOGY FOR PEROVSKITE SOLAR CELLS Nanostructured photoelectrochemical solar cells based on organometallic dye and nanocrystalline anatse TiO2 were first introduced in 199110 and have received great attention for their semitransparency, a colorful appearance, and a low-cost, facile Received: September 9, 2013 Revised: January 15, 2014 Published: January 27, 2014 5615

dx.doi.org/10.1021/jp409025w | J. Phys. Chem. C 2014, 118, 5615−5625

The Journal of Physical Chemistry C

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fabrication. As a consequence of intensive research efforts to improve PCE over the last two decades, the initial efficiency of approximately 8% has been improved to 12.3%.11 However, this improved PCE is not sufficient to compete with traditional semiconductor-based solar cells with PCEs exceeding 20%.12 Moreover, concern over leakage of the liquid electrolyte has caused a bottleneck in development, motivating the design of solid-state sensitized solar cells to replace the liquid electrolyte with HTM. Molecular HTM of 2,2′,7,7′-tetrakis(N,N-p-dimethoxy-phenylamino)-9,9′-spirobifluorene (spiro-MeOTAD) was used as an HTM for the solid-sate N3 dye-sensitized solar cell, but this design displayed a PCE of less than 1% under one sun intensity (100 mW/cm2),13 garnering the first demonstrated solid-state device little attention due to its low performance. Attempts have been made to improve the photovoltaic performance of solidstate sensitized solar cells involving the addition of additives, 4-tert-butylpyridine (tBP) and lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI), into the pristine spiro-MeOTAD14 and synthesizing organic sensitizers with high molar extinction coefficients.15−21 A gradual increase in PCE from 1% to ∼4% occurred from 1998 to 2010. A poor fill factor in solid-state sensitized solar cells has been one of the obstacles for high efficiency. The conductivity of sprio-MeOTAD was increased by doping with tris(2-(1H-pyrazol-1-yl)pyridine)cobalt(III) (coded FK102), resulting in a higher fill factor (FF = 0.76).22 Polymeric materials such as poly(3-hexylthiophene) (P3HT) and poly(2,6(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)) (PCPDTBT) have also been tested as HTMs. The photovoltaic performance of solid-state cells containing polymeric HTMs was generally inferior to those containing molecular spiro-MeOTAD because of the difficult infiltration of the long-chain polymers into the mesopores.23−35 The highest PCE obtained was 7.2%, which was based on an organic sensitizer and spiro-MeOTAD in the solidstate sensitized structure.22 No significant improvement in solidstate sensitized solar cells is expected from the molecular-based organic sensitizers because most organic sensitizers have absorption coefficients of ∼103 cm−1, which are not sufficient in a limited oxide film thickness of approximately 2 μm.36 Higher PCEs may remain out of reach without finding light harvesters with absorption coefficients over 0.5 × 104 cm−1 (1/2 μm). The organolead iodide perovskite light harvester represents a significant leap in the solid-state device design due to its high absorption coefficient of 1.5 × 104 cm−1 at 550 nm.3 CH3NH3PbI3 nanodots that adsorbed 0.6 μm thick TiO2 film demonstrated a PCE of 9.7%, with a high photocurrent density of 17 mA/cm2 and voltage of ∼0.9 V under a simulated AM1.5G one sun illumination.1 This result has contributed to research activities on perovskite solar cells.

Figure 1. ABX3 perovskite structure showing (left) BX6 octahedral and (right) AX12 cuboctahedral geometry.

magnetic properties.37,38 Perovskite formation can be estimated simply using the Goldschmidt tolerance factor (t),39 t = (rA + rX)/[21/2(rB + rx)], where rA, rB, and rX are the effective ionic radii for the ions in the A, B, and X sites, respectively. It has been generally accepted that perovskites can be stabilized when t is in a range between 0.76 and 1.13.40 However, it has also been argued that perovskite structures are not stable, even in the most favorable range of 0.8 < t < 0.9,41 indicating that perovskite stability cannot be predicted solely based on a tolerance factor. An additional consideration for perovskite formability is the octahedral factor (μ), μ = rB/rX.42 In the case of the alkali metal halide perovskite, formability was determined from the t−μ mapping, where the perovskite was stabilized for a tolerance factor ranging between 0.813 and 1.107 and an octahedral factor ranging from 0.442 and 0.895.42 In Table 1, the ionic radii of rA in APbX3 perovskite are estimated based on effective ionic radii,43−45 where the A cation radii Table 1. Estimation of the A Cation Radii in ABX3 (B = Pb2+ and X = Cl−, Br−, or I−) Ba

Xa

rAb (pm) at t = 0.8

rAb (pm) at t = 1.0

Pb2+ (rB = 119 pm)

Cl− (rX = 181 pm) Br− (rX = 196 pm) I− (rX = 220 pm)

158.4 160.4 163.5

243.3 249.5 259.4

Effective ionic radii for 6 coordination number. brA = t × 21/2 (rB + rX) − rX. a

range from ∼160 picometers (pm) to ∼250 pm, corresponding to one or two C−C (150 pm) or C−N (148 pm) bonds. Tolerance factors for CH3NH3PbX3 are calculated at 0.85, 0.84, and 0.83 for X = Cl, Br, and I, respectively, based on the radii of CH3NH3+ = 180 pm,46 Pb2+ = 119 pm, Cl− = 181 pm, Br− = 196 pm, and I− = 220 pm. It was found that the t of most cubic perovskites is between 0.8 and 0.9;47 thus, methylammonium lead halide perovskites are expected to have a cubic structure. The tetragonal phase of the iodide perovskite is stabilized at ambient temperature with lattice parameters of a = 8.855 Å and c = 12.659 Å because the cubic phase is stable above 54.4 °C. Figure 2 shows the 4 × 4 × 4 supercell of the cubic CH3NH3PbI3 perovskite.48 Since the report on semiconductor-to-metal transition in organotin iodide in 1994,49 organometal halide perovskite has attracted significant attention due to its interesting electrical properties. The layered perovskite (C4H9NH3)2SnI4 is an insulator and can obtain metallic properties via an increase in the number of corner-sharing, 2-dimensional SnI6 octahedral layers. The insertion of CH3NH3+ ions between SnI6 layers was used as a method to increase the number of 2-dimensional SnI6 octahedral

3. ORGANOMETAL HALIDE PEROVSKITE: STRUCTURE AND PROPERTIES Perovskite has a general ABX3 (X = oxygen, carbon, nitrogen, or halogen) formula, where A and B cations coordinate with 12 and 6 X anions, forming cuboctahedral and octahedral geometry, respectively. The A cation is larger than the B cation and is bulky when it occupies the cuboctahedral site (Figure 1). Although most materials with a perovskite structure are oxides, halide perovskites will be the focus of this review. In the case of the halide perovskite structure, ABX3 (where X is F, Cl, Br, or I), monovalent and divalent cations are usually stabilized in the A and B sites, respectively. Halide perovskite structures have been known to exhibit a number of interesting optical, electrical, and 5616



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excitons. The CH3NH3PbX3 (X = Br, I) perovskites, having relatively higher dielectric constants, will produce Wannier-type excitons, and thus the free charge carriers or the weakly bound excitons will be created by the illumination of light because the exciton binding energies are comparable to thermal energy (kBT ∼ 26 meV, where kB is the Boltzmann constant and T is the temperature) at room temperature.58 Accordingly, the perovskite-based solar cells could achieve high open-circuit voltages as a result of the generation of Wannier-type excitons. When a material is subjected to a direct current (DC) electric field, ε is unchanged; however, the value of ε is dependent on the frequency and therefore changes with an applied alternating current (AC) electric field. The dielectric constant is complex and takes the form ε = ε′ + iε″, where ε′ is the real part which is responsible for the increase in capacitance in a capacitor, and ε″ is an imaginary part responsible for loss. The dielectric constant is closely related to the polarization of a material in an electric field, and consequently it is greatly dependent on the frequency of the applied electric field. In general, the polarization is dipolar, ionic (atomic), and electronic as it goes through higher-frequency regions, as shown in Figure 3.59 The dipolar orientational Figure 2. 4 × 4 × 4 supercell of cubic CH3NH3PbI3 perovskite. Large dark gray: Pb2+; purple: I−; brown: carbon; small light gray: nitrogen; small light brown: hydrogen. Reprinted from ref 48.

sheets by forming an inner CH3NH3SnI3 perovskite layer. Initial interest in the metallic or superconducting properties of organometal halide perovskite has shifted to optical properties because organic light-emitting diodes (OLEDs) attracted attention around the same time.50 Since organometal halide perovskite contains both organic and inorganic parts, electronic excitation can lead to Frenkel-type excitons with a high exciton binding energy and small Bohr exciton radius and Wannier-type excitons with a low exciton binding energy and large Bohr exciton radius. Frenkel excitons arise from organic molecules, whereas Wannier excitons originate from the inorganic part of organometal halide perovskite, usually the metal halide octahedral.51 The application of organometal halide perovskite to LEDs via intercalation of the organic chromophore has been studied.52 Unlike the intensive studies on tin(II)-based halide perovskite, lead(II)-based halide perovskites have received less attention due to undesirable optoelectronic properties with respect to use in superconductor or LED applications. The optical properties of CH3NH3PbBr3 and CH3NH3PbI3 were investigated, where the exciton binding energy and exciton Bohr radius were determined to be 20 Å and 76 meV, respectively, for bromide, and 22 Å and 50 meV, respectively, for iodide.53 In reference 53, dielectric constants (ε) were estimated to be 4.8 and 6.5 for bromide and iodide, respectively. Since the Bohr radii are relatively large and the exciton binding energies small, the excitons from CH3NH3PbX3 (X = Br, I) will be Wannier-type. Contrary to the 3-dimensional structure, low-dimensional structures have a higher exciton binding energy of more than 200 meV;54−56 therefore, organic photovoltaic cells (OPVs) with a lower dielectric constant (ε < 4) generate tightly bonded Frenkel-type excitons under illumination from light and the generated excitons diffused to the pn heterojunction interface. At the heterojunction, excitons are separated into charge carriers by consuming binding energy (>0.25 eV) under an applied electric field (E > 106 V/cm);57 therefore, the thickness of the active layer (absorption layer) is restricted to ∼100 nm, and the diffusion length of excitons is very short, ∼10 nm, due to the directionless and random diffusion of neutral, Frenkel-type

Figure 3. Frequency-dependent dielectric constant of the real part (ε′). Reprinted from ref 59.

polarization is associated with the orientational polarization of molecules, while the others are related to resonances of the atoms; therefore, the dielectric constant decreases gradually with increasing frequency due to the freezing out of each polarization mode, eventually resulting in the dielectric constant of vacuum (ε0 = 1). In principle, the ionic compound has a permanent electric dipole moment and thus exhibits a high dielectric constant with great dependence on the frequency of the applied electric field. The dielectric constant of CH3NH3PbX3 (X = Br, I) perovskites is therefore relatively high because they are composed of a CH3NH3+ cation and PbX3− anion. This implies that the perovskites have a high capacitance owing to the linear relationship between the dielectric constant and capacitance. Recently, Kim et al.60 reported that the CH3NH3PbI3 perovskite can accumulate charge carriers within itself, like a capacitor, via impedance spectroscopy analysis. This result also supports that the Wanniertype excitons are generated through the illumination of light, and consequently the free charge carriers or weakly bound excitons are created, building a high open-circuit voltage. 5617



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Table 2. Temperature-Dependent Structural Data of CH3NH3PbX3 (X = Br, I)61 phase CH3NH3PbBr3 α β γ δ CH3NH3PbI3 α β γ a

temperature (K)

crystal structure

>236.9 155.1−236.9 149.5−155.1 327.4 162.2−327.4 55%), due to the dissolution of disordered CH3NH3I. One-dimensional nanostructures were also combined with the perovskite light harvester. CH3NH3PbI3-deposited rutile TiO2 nanorods (∼0.6 μm long) resulted in a PCE value of 9.4%.70 An increase in the length of the nanorod from 0.6 to 1.6 μm resulted in decreased photovoltaic performance associated with the structure of the nanorod film. The number of the tilted nanorods increased with nanorod length, which led to problems filling pores with spiro-MeOTAD. These may be addressed via a change from nanodot perovskite deposition to a pillared perovskite structure. Rectangular nanorod TiO2 was prepared, and its surface was decorated with two separate perovskite materials. A Br−I mixed perovskite CH3NH3PbI2Br exhibited a PCE of 4.87%, which is slightly higher than CH3NH3PbI3 due to its higher Voc. This was interpreted with an injection model associated with a larger driving force for the mixed halide perovskite.71 CH3NH3PbBr3 was also applied to a mesoporous TiO2 layer, and this bromide perovskite-sensitized TiO2 was placed in contact with poly[N-9-hepta-decanyl-2,7-carbazole-alt-3,6-bis(thiophen-5-yl)-2,5-dioctyl-2,5-dihydropyrrolo[3,4-]pyrrole1,4-dione] (PCBTDPP), resulting in a surprisingly high Voc value of approximately 1.2 eV.72 When P3HT was used as a HTM, however, a significantly lower Voc of approximately 0.5 V was observed. When considering electron injection from the bromide perovskite to the TiO2, the Voc will be determined based on the difference between the Fermi level of TiO2 and the HOMO level of the HTM (PCBTDPP: 5.4 eV and P3HT: 5.2 eV). Despite small differences in HOMO levels, the large difference in Voc between PCBTDPP and P3HT was ascribed to the degree of chemical interaction and the strength of the light filtering effect.72 Weaker light filtering effects, along with stronger interaction in the PCBTDDP-based device, resulted in a significant suppression of charge recombination and an upward shift in Fermi level, leading to a high Voc. The measured electron lifetime for P3HT was 1 order of magnitude shorter than that for the spiroMeOTAD in TiO2/CH3NH3PbI3/HTM, due to the significantly faster recombination.73 The above results suggest that factors beyond the energy levels must be carefully considered in selecting an HTM. 5.2. Meso-Superstructured PSCs Based on Nonelectron Injecting Oxides. A perovskite solar cell comprised of a mesoporous Al2O3 thin film surface coated with a thin layer of mixed halide perovskite CH3NH3PbI2Cl showed a PCE of 10.9%.4 This cell was called a “meso-superstructured solar cell” because the Al2O3 merely acted as a scaffold layer. Photoexcited electrons are assumed to be transported through the thin, continuous perovskite layer because electrons cannot be injected into Al2O3. CH3NH3PbI2Cl formed via spin-coating the N,Ndimethylformamide solution containing 3 M CH3NH3I and 1 M PbCl2 was argued to be relatively more stable in an air 5620



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5.3. Method for Improving Fill Factor. Perovskite solar cells typically suffer from a low fill factor for several reasons. The low conductivity of HTM leads to low fill factor and can be remedied via the addition of a codopant to the HTM. The addition of a lithium salt such as Li-TFSI to spiro-MeOTAD was found to increase hole conductivity.20 The basis for such an increase in conductivity was studied, where photoelectron spectroscopy combined with absorbance measurements confirmed that the Fermi level in spiro-MeOTAD was shifted toward the HOMO, and up to 24% of the spiro-MeOTAD molecules were oxidized in the presence of Li-TFSI.79 This observation indicates that the p-doping effect is induced by the addition of Li-TFSI. p-type dopants based on cobalt complexes were designed to enhance the conductivity of spiro-MeOTAD and were found to increase the fill factor of solid-state, organic, and dye-sensitized solar cells.80 The addition of p-dopant coded FK209 (tris(2-(1Hpyrazol-1-yl)-4-tert-butylpyridine)-cobalt(III)-tris(bis(trifuoromethylsulfonyl)imide)) to the spiro-MeOTAD, together with Li-TFSI and tBP, improved the photovoltaic performance of the CH3NH3PbI3-based perovskite solar cell due to the increased fill factor and Voc.81 Recently, protic ionic liquid was proposed as an efficient p-dopant for molecular and polymeric HTMs.82 It was observed that bis(trifluoromethanesulfonyl)imide (H-TFSI) was an effective single electron oxidation p-dopant, and as a consequence, the conductivity of spiro-MeOTAD was significantly improved by 3 orders of magnitude following the addition of H-TFSI. This can be compared to Li-TFSI, and H-TFSI contained spiro-MeOTAD, which showed higher photovoltaic performance owing to the improved fill factor and Voc. A defect- or pinhole-free perovskite layer has been proposed to improve the fill factor and increase HTM conductivity because it is expected to reduce series resistance. Perfect pn contact is required between the perovskite and the HTM to improve the fill factor, for which chemical interaction should be considered to induce better contact between the organic layer from the HTM and the inorganic from the perovskite. Since Jsc and Voc approach 22 mA/cm2 and 1.1 V, PCE will reach 20% if a fill factor of 0.8 is realized. Thus, determining a method to improve the fill factor will be an important topic for perovskite solar cells. 5.4. Planar Heterojunction-Structured PSCs. Due to the bifunctional property (light harvesting and charge transporting) of perovskite, mesoporous oxide may not be necessarily required. CH3NH3PbI3−xCl3 was deposited directly onto FTO with a thin, compact TiO2 layer via the coevaporation of CH3NH3I and PbCl2 using a thermal evaporator.7 Vapor-deposited perovskite showed a maximum PCE of 15.4%, whereas the solutionprocessed perovskite layer in the planar heterojunction structure showed lower performance. The higher PCE of the vapordeposited perovskite is due to the formation of a homogeneous, flat surface and a pinhole-free perovskite layer, which implies that the quality of the perovskite layer is important. It is noted that a flat CH3NH3PbI3−xClx layer can easily be formed from solution casting, while it is difficult to form a flat CH3NH3PbI3 layer using the solution casting method. This may be due to the fact that apical chloride ions strengthen the 2-dimensioanl nature of the PbI4 planes in the PbI4Cl2 octahedra, leading to a flat layer. In addition, the chloride in CH3NH3PbI3−xClx plays a role in the improvement of charge transport,83 which is in part associated with high performance. 5.5. Balanced Electron- and Hole-Transporting Properties. To demonstrate highly efficient solar cells, the electron- and hole-diffusion length of the active materials must be checked, as the optimal device architecture is determined from this property.

For instance, if the diffusion length of charge carriers is shorter than the depth of absorption, the meso-structured devices will favor fully absorbing solar light. If not, the planar-structured device will be more desirable because the charge carriers generated by the fully absorbed light can be effectively transported without significant recombination. Very recently, the transport properties of CH3NH3PbI3 and CH3NH3PbI3−xClx were reported simultaneously by Xing et al.84 and Stranks et al.85 By measuring the transient PL and transient absorption spectroscopy, it was determined that the diffusion lengths of the electron and hole are ∼129 nm (130 nm) and ∼105 nm (90 nm), respectively, in CH3NH3PbI3 and ∼1069 nm and ∼1213 nm, respectively, in CH3NH3PbI3−xClx.84,85 Therefore, the meso-structured cells for CH3NH3PbI3 and the planar cells for CH3NH3PbI3−xClx will be desirable to attain high-power conversion efficiency because the absorption depth of each perovskite is ∼600 nm. In addition, the generation of free-charge carriers or weakly bound excitons via the illumination of light in the CH3NH3PbI3−xClx perovskite has been indirectly confirmed based on the comparable decay time of photoinduced absorption at ∼550 nm in the transient absorption spectroscopy and transient PL. From the dielectric constant, impedance spectroscopy, transient absorption spectroscopy, and transient PL data, we believe that Wannier-type excitons can be generated in perovskite materials under light illumination. 5.6. Hybrid Perovskite Solar Cells. Concepts or components in organic polymer photovoltaics (OPV) were recently adapted to the perovskite solar cell. In OPV, P3HT and PCBM have been used as donors and acceptors, respectively. Replacement of P3HT with CH3NH3PbI3 in the planar heterojunction structure resulted in a PCE of 3.9%.86 The PEDOT:PSS-coated ITO substrate provided better wettability with the DMF solution than with the γ-butyrolactone solution while coating the halide perovskite. Molecular-level engineering on the TiO2 surface with a self-assembled monolayer of C60 (C60SAM) was found to significantly improve the photovoltaic performance of the TiO2/ CH3NH3PbI3−xClx/P3HT structure, in which the PCE was improved from 3.8% without C60SAM to 6.7% with C60SAM. This change is due to a significant increase in both Jsc and Voc.87 The improvement was interpreted based on the C60SAM function that inhibits electron injection from the perovskite to TiO2. Recently, a perovskite was hybridized in a planar organic solar cell. When a CH3NH3PbI3 layer (285 nm) was sandwiched between the electron-accepting PCBM layer (10 nm) and the hole-accepting poly(N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine) (polyTPD) layer (10 nm), a PCE of 12% was achieved.88 To fabricate this device, charge-transporting organic layers were spin-coated from chlorobenzene solutions, whereas CH3NH3PbI3 was vacuum-deposited by heating CH3NH3I to 70 °C and PbI2 to 250 °C. 5.7. Flexible Perovskite Solar Cells. By taking advantage of the ability to process perovskite solar cells at low temperatures, flexible solar cells were attempted using a perovskite light absorber. CH3NH3PbI3−xClx was used as a light harvester in regular and inverted OPV structures employing PEDOT:PSS and PCBM, where a solution-processed device based on an ITOcoated PET substrate showed 6.4%.89 A higher PCE of 10.2% was achieved from flexible perovskite solar cells based on an ITO/ZnO (25 nm)/CH3NH3PbI3/spiro-MeOTAD/Ag planar structure, where the device was fabricated via a roomtemperature solution processing technique.90 5621



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6. ISSUES TO BE ADDRESSED Organolead halide perovskites are typically unstable in humid conditions. Thus, fabrication must be carefully carried out in a moisture-free environment. Perovskite can last for some time when wrapped in HTM; however, without encapsulation the device will be unstable, since most of the hole-conducting molecules and polymers are not completely waterproof, and effective methods to protect against moisture will be required. The UV instability of perovskite solar cells was recently studied, where the origin of such instability was attributed to TiO2 and not perovskite decomposition.91 When comparing the long-term performance of TiO2-based perovskite solar cells with TiO2-free perovskite solar cells, the latter configuration showed better longterm stability. This indicates that effective methods to protect against UV instability should be also developed.

working on perovskite-based solid-state solar cells under the supervision of Prof. Nam-Gyu Park at the Next Generation Photovoltics Laboratory in SKKU.

7. SUMMARY AND OUTLOOK Recent breakthroughs and progress in the area of organolead halide perovskite solar cells were reviewed. Triiodide perovskite CH3NH3PbI3 and mixed halide perovskite CH3NH3PbI3−xClx have been proven key materials in the fabrication of highefficiency perovskite solar cells. PCEs greater than 15% have been achieved from both mesoscopic and planar heterojunction structures. Perovskite light harvesters were determined to be capable of transporting electrons and holes, a unique property of perovskite. Since perovskites can be prepared via solution casting or low-temperature vapor deposition, diverse applications such as flexible devices and hybrid solar cells are possible. Perovskite solar cells were selected as one of the biggest scientific breakthroughs of 2013. A further increase in PCE to 20% is possible from a single-junction perovskite solar cell,92,93 and over 29% is achievable from a tandem structure with a silicon solar cell.93 On the basis of these advances, organolead halide perovskites are opening new horizons for solar cell research.

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Sang Hyuk Im is an associate professor in the department of Chemical Engineering at Kyung Hee University and currently leads the Group of Nano Energy Convergence System (http://necs.khu.ac.kr). He received his Ph.D. degree in Chemical & Biomolecular Engineering from the Korean Advanced Institute of Science and Technology (KAIST) in 2003. He worked as a postdoctoral fellow in Professor Younan Xia’s Chemistry group at the University of Washington until 2005. He has worked in the LG Chemicals Research Park (∼2009) and Korean Research Institute of Chemical Technology (KRICT) until 2013 as a senior research scientist before moving to Kyung Hee University. His research interests include the structure and morphology design of nanoenergy materials and the development of energy convergence systems, including organic−inorganic (including colloidal quantum dots) hybrid solar cells.

AUTHOR INFORMATION

Corresponding Authors

*Tel.: 82-31-201-5274. Fax: 82-31- 204-8114. E-mail: imromy@ khu.ac.kr (S.H.I.). *Tel.: 82-31-290-7241. Fax: 82-31-290-7272. E-mail: npark@ skku.edu (N.G.P.). Notes

The authors declare no competing financial interest. Biographies

Nam-Gyu Park is a professor at the School of Chemical Engineering and adjunct professor at Department of Energy Science, Sungkyunkwan University (SKKU), where he leads the Group of Next Generation Photovoltaics. He received his Ph.D. in Inorganic Chemistry from Seoul National University in 1995. He worked at ICMCB-CNRS, France, from 1996 to 1997 and at the National Renewable Energy Laboratory (NREL), USA, from 1997 to 1999 as a postdoctoral researcher. He worked as Director at the Solar Cell Research Center at the Korea Institute of Science and Technology (KIST) from 2005 to 2009 and as a principal scientist at the Electronics and Telecommunications Research Institute from 2000 to 2005 before joining SKKU in 2009. His research has been focused on mesoscopic sensitized solar cells since 1997. Additional information can be found at http://ngplab.skku.edu/.

Hui-Seon Kim is a Ph.D. student in the Department of Energy Science at Sungkyunkwan University (SKKU), Suwon, Korea. She graduated from the School of Chemical Engineering, SKKU in 2011. She has been 5622



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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) grants funded by the Ministry of Science, ICT & Future Planning (MSIP) of Korea under contracts No. NRF2010-0014992, NRF-2012M1A2A2671721, NRF2013R1A2A2A01067999 (S.H.I.), NRF-2012M3A7B4049986 (Nano Material Technology Development Program), and NRF2012M3A6A7054861 (Global Frontier R&D Program on Center for Multiscale Energy System).

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