Nonstoichiometric Adduct Approach for High-Efficiency Perovskite

Oct 6, 2016 - ... De MarcoOliver LinChristopher S. ChoiBruce DunnJaekyung KohDino ... Anuj Goyal , Paul Kairys , Steven P. Harvey , Dong Hoe Kim , Kev...
0 downloads 0 Views 8MB Size
Download PDF
Forum Article pubs.acs.org/IC

Nonstoichiometric Adduct Approach for High-Efficiency Perovskite Solar Cells Nam-Gyu Park* School of Chemical Engineering, Sungkyunkwan University, Suwon 440-746, Republic of Korea ABSTRACT: Since the groundbreaking report on a solid-state perovskite solar cell employing a methylammonium lead iodide-sensitized mesoporous TiO2 film and an organic hole conducting layer in 2012 by our group, the swift surge of perovskite photovoltaics opens a new paradigm in solar-cell research. As a result, ca. 1300 peerreviewed research articles were published in 2015. In this Inorganic Chemistry Forum on Halide Perovskite, the researches with highlights of work on perovskite solar cells in my laboratory are reviewed. We have developed a size-controllable two-step spin-coating method and found that minimal nonradiative recombination in perovskite crystals could lead to high photovoltaic performance. A Lewis acid based adduct method and self-formed grain boundary process were developed for high-efficiency devices with reproducibility. A power conversion efficiency of 20.4% was achieved via grain boundary engineering based on a nonstoichiometric adduct approach. The incorporation of cesium in a formamidinium lead iodide perovskite was found to show better photostability and moisture-stability. A reduction in the dimensionality from a three-dimensitonal nanocrystal to a one-dimensional nanowire led to a hypsochromic shift of absorption and fluorescence. To enhance the charge-carrier transport and light-harvesting efficiency, a nanoarchitecture of oxide layers was proposed.

■

INTRODUCTION A perovskite solar cell has its roots in both liquid-junction and solid-state dye-sensitized solar cells developed by Grätzel et al. in 1991 and 1998,1,2 where a mesoporous TiO2 film whose surface is chemically covered by light-absorbing organic dye molecules is in contact with a liquid redox electrolyte for the liquid junction or a hole-transporting material of 2,2′,7,7′-tetrakis(N,N-p-dimethoxy-phenylamino)-9,9′-spirobifluorene (spiro-MeOTAD) for the solid-state junction. The logic behind the solid-state version of a dye-sensitized solar cell is in replacing the liquid electrolyte because of a leakage concern. However, a power conversion efficiency (PCE) from the solid-state dye-sensitized solar cell structure was not as competitive as that from the liquid-junction dye-sensitized solar cell because an increase in the TiO2 film thickness was restricted to 2−3 μm due to the limited hole diffusion length of spiro-MeOTAD. It is hard to improve solidstate dye-sensitized solar cells with low absorption coefficients because of the limited amount of dye adsorption in a very thin TiO2 film. Inorganic quantum dots thus received attention because a higher PCE was expected by replacing the organic dye with higher absorption coefficient inorganic quantum dots. Contrary to this expectation, most of the studied quantum dots could not reach over 5% mainly because of the much lower opencircuit voltage (Voc) than the theoretical value, associated with pronounced surface trap states,3 although a short-circuit current (Jsc) as high as 30 mA/cm2 was demonstrated.4 The dilemma of improving the PCE using a dye-sensitized solar cell is due to the limitation of both the dye (low absorption coefficient) and quantum dot (low Voc). Novel materials able to show high efficiency in a solid-state structure need to be developed. Organic−inorganic halide perovskite is the only solution that can be used to solve this problem at present. A detailed chronicle of © XXXX American Chemical Society

perovskite solar-cell development can be found in the review article in ref 5. In this Inorganic Chemistry Forum on Halide Perovskite, highlights of our work on perovskite solar cells are reviewed, along with the emergence of a perovskite solar cell.

■

EMERGENCE OF A PEROVSKITE SOLAR CELL The key materials for the recently spotlighted high-efficiency perovskite solar cells are composed of methylammonium lead iodide (MAPbI3) or formamidinium lead iodide (FAPbI3) or their mixture. Such organic−inorganic perovskites were first applied to dye-sensitized solar cell structures as alternatives to organic sensitizers in 2009, where a PCE of 3.8% was demonstrated with MAPbI3 adsorbed onto a nanocrystalline TiO2 surface.6 However, this work was not cited for 2 years probably because of the less reproducible recipe (the coating solution concentration was 8 wt %, and the TiO2 thickness was higher than 8 μm) and instability of perovskite in a polar liquid electrolyte. We have found that 8 wt % was too low for a deepcolored film to sufficiently absorb incoming light. We confirmed that a 10 wt % solution gave us a yellow color, but an increase in the concentration by up to 40 wt % lead to a dark-black-colored film (2-nm-sized MAPbI3 dots were sparsely deposited on the TiO2 surface, as confirmed by transmission electron microscopy) that delivered a PCE of 6.5% at a TiO2 film thickness of less than 4 μm.5 A thick film was not required because the absorption coefficient of MAPbI3 was measured to be 1 order of magnitude higher than the conventional dye N719.7 There were also no Special Issue: Halide Perovskites Received: July 5, 2016

A

DOI: 10.1021/acs.inorgchem.6b01294 Inorg. Chem. XXXX, XXX, XXX−XXX

Forum Article

Inorganic Chemistry

concentration and cross-sectional scanning electron microscopy (SEM) images of the first version of a solid-state perovskite solar cell comprising a mesoporous TiO2 film. It is noted that no degradation of the performance was observed from ex situ testing for 500 h, which may be due to the fact that moisture-sensitive MAPbI3 dots were fully wrapped with spiro-MeOTAD. Although the current structure of the perovskite solar cell exhibiting PCEs as high as 18−20% is not the same as our sensitized structure, the report on the first solid-state perovskite solar cell provided important information that a submicron-thick layer of MAPbI3 was sufficient to generate a high photocurrent density because of its high absorption coefficient and excellent charge separation between perovskite and spiro-MeOTAD. A swift surge of perovskite photovoltaics is followed by the report on a stable solid-state perovskite solar cell.8 Web of Science released about 1300 publications reporting on the perovskite solar cells in 2015, which is almost 3 times the number of publications in 2014. Publications related to perovskite photovoltaics are expected to continue based on the exponential increase profile, as can be seen in Figure 2. There are some issues

citations for our work, although the report in 2011 was selected as the most read article from Nanoscale. This is because MAPbI3 tends to dissolve quickly in a liquid electrolyte with a polar solvent such as acetonitrile. Nevertheless, the second report5 on a perovskite-sensitized liquid-junction solar cell gave important insight into perovskite solar-cell research and development because other researchers have utilized the experimental details for coating perovskite. We seventually olved the instability problem by introducing spiro-MeOTAD instead of a liquid electrolyte. First, we tested the solid-state device with a 2-μmthick mesoporous TiO2 film whose surface was deposited with MAPbI3 dots. However, it was difficult to achieve a high PCE from the 2-μm-thick film because of poor infiltration of spiroMeOTAD into the pores of the TiO2 film. We then recognized that a thin TiO2 film might be sufficient because a high absorption coefficient reduces the penetration depth of light, which motivated us to decrease the TiO2 layer thickness. We achieved a PCE of 9.7% from a MAPbI3-deposited 0.6-μm-thick TiO2 film, which was the first report on a solid-state perovskite solar cell in 2012.8 Figure 1 shows the dependence of the optical absorption and PCE on the perovskite coating solution

Figure 2. Publications on perovskite solar cells. Data were based on Web of Science using the keywords of “perovskite solar cell”.

to be solved and developed, although a PCE as high as 22% was certified from NREL,9 which provided a reproducible method for high-efficiency devices with less current−voltage hysteresis, material engineering, and encapsulation for long-term stability under continuous light soaking at 85 °C in an air atmosphere and a relative humidity of 85% and the treatment of the underlying toxicity of lead ion (Pb2+) that may result from decomposition of halide perovskite. In addition, the fundamentals of halide perovskites should also be unveiled and discovered.

■

CHARGE ACCUMULATION AND FERROELECTRIC PROPERTY We reported for the first time the charge accumulation property of MAPbI3 that was discovered from impedance spectroscopic studies. Figure 3 shows capacitance and current−voltage curves for MAPbI3 perovskite solar cells with three different layouts of “flat” (FTO/compact TiO2), “NS TiO2” (FTO/compact TiO2/ nanostructured TiO2), and “NS ZrO2” (FTO/compact TiO2/ nanostructured ZrO2), along with capacitance as a function of the applied voltage.10 All three configurations demonstrate photocurrent and voltage but different performances. It is interesting to see that the open-circuit voltage (Voc) of NS ZrO2 is almost the same as that of NS TiO2. It is also interesting that nonnegligible efficiency and photocurrent are obtained even for a Flat device, the first attempt for a planar heterojunction perovskite solar cell, which suggests that mesoporous oxide is not necessarily

Figure 1. (A) Effect of the coating solution concentration on (a) the color of the MAPbI3-coated 5.5-mm-thick TiO2 films, (b) UV−vis absorption spectra of 1.4-mm-thick TiO2 films, (c) photocurrent− voltage curves, and (d) EQE of the MAPbI3-coated 5.5-mm-thick TiO2 films. Solution concentrations of (1) 10 wt %, (2) 20 wt %, (3) 30 wt %, and (4) 40 wt %. Reprinted with permission from ref 7. Copyright 2011 John Wiley and Sons Inc. (B) Cross-sectional SEM images of a solidstate perovskite solar cell employing MAPbI3 nanodots deposited on a TiO2 surface, along with the current−voltage and dependence of photovoltaic parameters on time. Reprinted with permission from ref 8. Copyright 2012 Nature Publishing Group. B

DOI: 10.1021/acs.inorgchem.6b01294 Inorg. Chem. XXXX, XXX, XXX−XXX

Forum Article

Inorganic Chemistry

Figure 3. (A) Device configurations of “flat” (FTO/compact TiO2), “NS TiO2” (FTO/compact TiO2/nanostructured TiO2), and “NS ZrO2” (FTO/ compact TiO2/nanostructured ZrO2). (B) Current density (J)−voltage (V) curves of MAPbI3 perovskite solar cells for flat, NS TiO2, and NS ZrO2. (C) Capacitance analysis of flat and NS samples with and without perovskite: (left) capacitance of a flat sample with perovskite (PS) and NS TiO2 and ZrO2 samples without PS; (right) capacitance of NS TiO2 and ZrO2 samples with PS under illumination or in the dark. Reprinted with permission from ref 10. Copyright 2015 American Chemical Society.

required. Considering the conduction-band positions of oxides, electron injection is possible for TiO2 from MAPbI3 but not for ZrO2 because of the higher conduction-band position of ZrO2 than that of MAPbI3, which was confirmed by the lack of charge collection via compact ZrO2 instead of compact TiO2. Figure 3c compares the capacitances of Flat, NS TiO2, and NS ZrO2 with and without perovskite. Compared to the capacitance of the NS samples without perovskite, the capacitances for the NS TiO2 and ZrO2 with perovskite are higher regardless of the electrode materials, the NS layer thickness, and the illumination conditions. This is direct evidence of charge accumulation in perovskite. The ferroelectric behavior of a MAPbI3 crystal was detected by piezoresponse force microscopy (PFM).11 PFM phase images obtained without poling and with positive and negative poling in the dark showed spontaneous polarization in the absence of an electric field. Rotation of the dipoles was also observed for both positive and negative bias potentials. Ferroelectric polarization under illumination was found to be screened by photogenerated conducting electrons, which was, however, enhanced at positive poling. It was observed that ferroelectric polarization remained unchanged even after removal of the external electric field, where the retention of light-induced polarization was better for a larger MAPbI3 crystal than a smaller one. This ferroelectric property is regarded as one of factors causing current−voltage hysteresis.12 Because the grain size was found to affect the ferroelectric polarization and hysteresis, control of the perovskite grain size and crystallinity seem to play a critical role in the photovoltaic performance.

Figure 4. (A) Two-step spin-coating procedure for MAPbI3, where a PbI2 solution was spin-coated and dried at a temperature of less than 100 °C for a few minutes, and then a MAI solution was spin-coated. Heat treatment was performed to form perovskite. (B−D) SEM images for the perovskite films depending on the MAI concentration. Scale bar = 100 nm. Reprinted with permission from ref 13. Copyright 2014 Nature Publishing Group.

step spin coating is schematically illuminated, in which PbI2 is first coated on the substrate and then MAI is coated. We found that the MAPbI3 crystal size is significantly dependent on the MAI concentration. Figure 4B−D clearly shows that crystal size increases as the MAI concentration decreases. The 38 mM MAI leads to about 700-nm-sized cuboid MAPbI3, whereas a higher concentration of 63 mM results in about 100-nm-sized one. A crystal growth mechanism was elucidated based on a thermodynamic Gibbs free-energy change.14 The cuboid size (Y) was found to correlate with the concentration of MAI and temperature, which can be expressed by

■

SIZE-CONTROLLABLE TWO-STEP SPIN-COATING METHOD AND MICROPHOTOLUMINESCENCE To make a perovskite layer, a one- or two-step sequential coating is available. In a two-step method, we developed a sequential spin-coating process, where PbI2 was spin-coated and then methylammonium iodide (MAI) was coated (Figure 4A). We found that the MAPbI3 crystal size could be precisely controlled by modulation of the MAI concentration.13 In Figure 4A, twoC

DOI: 10.1021/acs.inorgchem.6b01294 Inorg. Chem. XXXX, XXX, XXX−XXX

Forum Article

Inorganic Chemistry 32 σsl 3

ln Y = 3kT

{

kT [ln Vm

2

}

X − ln C0(T )]

+ C′ (1)

where X is the MAI concentration, C0 the equilibrium concentration of MAI, σsl the average surface tension (equal to the change of the Gibbs free energy by the surface tension), Vm the volume of a solute particle, k the Boltzmann constant, and T the temperature. The measured crystal size is well fit with eq 1 (Figure 5). According to eq 1, the MAPbI3 crystal size is

Figure 6. (A−D) Effects of the MAPbI3 cuboid size on the photovoltaic parameters of Jsc, Voc, FF, and PCE. (E) LHE and (F) photo-CELIV transients. Reprinted with permission from ref 13. Copyright 2014 Nature Publishing Group.

radiative recombination is ascribed to the presence of nonradiative recombination. Therefore, a low photovoltaic performance for the micron size correlates to nonradiative recombination, associated with crystallinity, grain boundary, trap states, and surface defects of the perovskite film.

Figure 5. Plot of the theoretically derived MAPbI3 grain size (Y) as a function of the MAI concentration (X). The filled squares and line represent the measured data and bet-fit result, respectively. Reprinted with permission from ref 14. Copyright 2015 Royal Society of Chemistry.

■

LEWIS ACID−BASE ADDUCT APPROACH FOR HIGH-EFFICIENCY PEROVSKITE SOLAR CELLS As one of the effective methods, a Lewis acid−base adduct approach was proposed to prepare a high-quality MAPbI3 perovskite film for high-PCE perovskite solar cells.16 The precursor chemicals PbI2 and I− from MAI are Lewis acid and base, respectively. Polar aprotic solvents such as dimethyl sulfoxide (DMSO), thiourea, and N-methyl-2-pyrrolidone to dissolve the precursor chemicals are good Lewis bases because of the lone pairs on oxygen, sulfur, and nitrogen. Lewis acid−base reaction can lead to an adduct.17 We mixed equimolar PbI2, MAI, and DMSO in N,N-dimethylformamide (DMF), which was deposited on the substrate to prepare an intermediate MAI·PbI2· DMSO adduct in order to control the crystal growth kinetics. The interaction between DMSO as a Lewis base and PbI2 as a Lewis acid led to a transparent adduct film, which was converted to MAPbI3 by removing DMSO at mild heat treatment. In Figure 7, a procedure of the adduct approach is schematically illustrated. While the stoichiometric mixture of PbI2, MAI, and DMSO in DMF is spin-coated, diethyl ether is dripped in to remove DMF and thereby to form close to a 1:1:1 adduct film. Diethyl ether contributes to nucleation. The increased nucleation sites and reduced Ostwald ripening may improve film coverage in this route. A shift to lower wavenumber of the SO stretching vibration of DMSO in Fourier transform infrared is good evidence of adduct formation.17 Thermal removal of DMSO from the adduct film is believed to kinetically control MAPbI3 growth. The charge-carrier mobility of adduct-induced MAPbI3 was determined to be 3.9 × 10−3 cm2/(V s) from photo-CELIV, which was found to be 1 order of magnitude higher than that [3.2 × 10−4 cm2/(V s)] of the MAPbI3 film prepared without the adduct process. In addition, charge-extraction characteristics were improved by the adduct method because nonradiative

controlled not only by the concentration of MAI but also by the temperature. At a given concentration of 50 mM, the grain size increased as the temperature increased from −10 to +50 °C. This increase of the crystal size at higher temperature is attributed to the increased critical free energy and thereby the decreased number of nuclei. Photovoltaic parameters are significantly influenced by the MAPbI3 cuboid size. The short-circuit photocurrent density (Jsc) increases with increasing size (decreasing MAI concentration), Voc is highest for the intermediate size (50 mM), and the fill factor (FF) is better for large sizes (>200 nm) than for small ones of ∼100 nm (Figure 6A−D). As a result, a higher average PCE of about 17% is achieved for MAPbI3 larger than 200 nm, while a closed-packed small size of about 100 nm shows a relatively low PCE of around 14%. High Jsc from large size is due to better lightharvesting efficiency (LHE) at long wavelength (Figure 6E), being related to the enhanced internal light scattering in the gap between crystals, and the charge extraction ability as measured by photo-CELIV (charge extraction by linearly increasing voltage; Figure 6F). Relatively low Voc for the 800-nm-sized MAPbI3 is probably due to slow charge extraction, which becomes fast for an intermediate size of about 200 nm. A slow charge mobility may increase the chance for recombination, which is one of the reasons for the low Voc. The fast charge mobility of the intermediate size (∼200 nm from 50 mM) is responsible for the highest Voc. Higher Jsc may be expected for the much larger size because Jsc tends to increase with increasing MAPbI3 size. A few micron sizes could be prepared by further lowering the MAI concentration to 32 mM, which, however, lead to Jsc values as low as 12.8 mA/ cm2.15 We observed from microphotoluminescence measurements that the crystals made by 32 mM exhibited substantially lower radiative recombination than the smaller crystals. Low D

DOI: 10.1021/acs.inorgchem.6b01294 Inorg. Chem. XXXX, XXX, XXX−XXX

Forum Article

Inorganic Chemistry

Figure 7. (A) Schematic procedure of the adduct method, where the transparent MAI·PbI2·DMSO adduct film was converted to a mirror-like darkbrown MAPbI3 film upon removal of DMSO. (B) Current−voltage curve. (C) EQE spectrum and accumulated Jsc of the perovskite solar cell based on adduct-induced MAPbI3. Reprinted with permission from ref 16. Copyright 2015 Amercian Chemical Society.

recombination was minimized. The best PCE of 19.7% was achieved along with excellent external quantum efficiency (EQE) profile.

■

GRAIN BOUNDARY ENGINEERING Because solution-processed MAPbI3 forms grains, a grain boundary is inevitably created. It was reported that the grain boundaries showed less pronounced photoluminescence intensity and faster nonradiative decay than the grains in the same perovskite film.18 This indicates that grain boundary engineering by removal of any nonradiative pathways could further improve the photovoltaic performance. We proposed a grain boundary healing process to remove any unwanted drawbacks affecting the photovoltaic parameters, where excess MAI in a precursor solution enabled self-formed grain boundaries comprising a thin MAI layer.19 The adduct approach is again a very useful method because the intermediate can control the well-defined final composition. Excess MAI would be placed in the intermediate adduct film, forming (MAI)1+x·PbI2·DMSO, which was converted to highly crystalline MAPbI3 grains whose surface was in situ passivated with MAI. SEM measurements confirm that the stepped grains from a stoichiometric precursor disappeared for the excess MAI case (Figure 8A), which suggests that excess MAI changes the MAPbI3 grains and is accompanied by grain boundary modification. Conductive atomic force microscopy (c-AFM) images are compared under a bias voltage of 2 V, where more conducting current flows through grain boundaries in spite of insulating MAI on the grain boundary (Figure 8B). The grain boundaries are considered as charge-transporting channels because the brighter contrast indicates that the grain boundary carries the current more efficiently. Time-resolved photoluminescence and transient absorption spectroscopic studies reveled that the self-formed grain boundary healing process showed more efficient carrier extraction and much longer carrier lifetimes. The addition of 6 mol % excess MAI led to the best PCE of 20.4% (reverse-scanned PCE = 20.6% and forward-scanned PCE = 20.2%), as can be seen in Figure 9A, along with an EQE of about 90% in almost the entire wavelength ranging from 400 to

Figure 8. (A) SEM images and (B) c-AFM before and after a self-formed grain boundary by 6 mol % excess MAI in a precursor solution with MAI, PbI2, and DMSO. Reprinted with permission from ref 19. Copyright 2016 Macmillan Publishers Ltd.

760 nm (Figure 9B). The accumulated photocurrent density estimated from EQE values is close to the Jsc measured under white-light 1 sun illumination. I−V hysteresis was found to be very sensitive to the fabrication temperature and humidity, where temperatures lower than 20 °C and relative humidities of less than 25% gave rise to hysteresis-less characteristics (Figure 9C). We fabricated more than 700 cells, among them 50 cells prepared under these conditions showed an average PCE of 20.1%.

■

MOISTURE-STABLE AND PHOTOSTABLE FAPBI3 BY MIXED CATIONS Stability is one of the critical issues to be addressed in perovskite solar cells. Higher performance is expected from FAPbI3 than from MAPbI3 because of its lower band gap. However, FAPbI3 is more moisture-sensitive than MAPbI3 because the FA cation is less stable than the MA cation in the cuboctahedral cage. We reported first that partial substitution of the inorganic cesium (Cs) cation in the FA site of FAPbI3 induced moisture-stability and photostability of FAPbI3.20 The nominal composition of E

DOI: 10.1021/acs.inorgchem.6b01294 Inorg. Chem. XXXX, XXX, XXX−XXX

Forum Article

Inorganic Chemistry

Figure 9. (A) Current−voltage curves of the perovskite solar cell employing MAPbI3 prepared from excess MAI, measured at reverse and forward scans at a scan rate of 6 mV/s. (B) EQE spectrum and integrated Jsc based on the EQE data. (C) Effect of the temperature and humidity in my laboratory on the hysteresis of perovskite solar cells employing MAPbI3 prepared from excess MAI, showing negligible hysteresis of perovskite films prepared under temperatures lower than 20 °C and relative humidities of less than 25%. Reprinted with permission from ref 19. Copyright 2016 Macmillan Publishers Ltd.

has an influence on the nanowire growth. Figure 11 clearly shows the solubility effect, where DMSO is able to produce a nanowire,

FA0.9Cs0.1PbI3 showed a single phase, as confirmed by X-ray diffraction measurement, in which the unit cell volume was decreased from 761.2263 Å3 (FAPbI3) to 749.4836 Å3 because the lattice constants were shrunk as a result of the smaller ionic radius of the Cs cation (1.81 Å) than that of the FA cation (2.79 Å). The improved stability was therefore attributed to reinforced interaction between FA cations and iodide anions by shrinkage of the lattice constant. The photovoltaic active back phase of FAPbI3 usually forms at a high temperature of around 150 °C.21 However, we discovered that the back phase is stabilized even at room temperature in the presence of CsI in the precursor solution (Figure 10), which indicates that the incorporation of Cs

Figure 11. Plane-view SEM images of MAPbI3 formed by a two-step spin coating method using a MAI solution (35 mg/5 mL) including different additives of (A) 50 μL DMF, (B) 50 μL of DMSO, and (C) 50 μL of GBL. Scale bars represent 1 μm. Reprinted with permission from ref 22. Copyright 2015 Amercian Chemical Society.

although not perfectly, but γ-butyrolactone (GBL) cannot make a nanowire because of the less soluble property of PbI2 in GBL. This proves that MAPbI3 nanowire growth is related to the liquid catalyst cluster model. The optical property of the nanowire was found to be somewhat different from that of the cuboid nanocrystals. The hypsochromic shift of both absorption and fluorescence spectra is the main feature for the nanowire compared to the cuboid nanocrystals, which is likely to be attributed to more localized exciton states in nanowires.

Figure 10. Photographs of (A) FAPbI3 and (B) FA0.9Cs0.1PbI3 films heat-treated at different temperatures for 5 min, showing a Csincorporation-stabilized FA ion in the perovskite lattice even at room temperature. Reprinted with permission from ref 20. Copyright 2015 Scrivener Publishing LLC.

■

NANOROD ZNO AND MOTH-EYE TIO2 LAYERS We have investigated several nanostructures for perovskite solar cells. The perovskite solar cell based on nanorod ZnO was compared with that based on nanorod TiO2.24 The ZnOnanorod-based device showed slower recombination kinetics than the TiO2-nanorod-based one, which is beneficial to higher Voc. Compared to the TiO2 nanorod, the ZnO nanorod system showed faster saturation in charge collection, which indicates that the ZnO nanorod is an effective charge-collecting oxide in the perovskite solar cell. During the preparation of the ZnO nanorod on conductive oxide substrates, the morphology and coverage of a seed layer played critical roles in the final morphology of the nanorod and thereby the photovoltaic parameters, especially photovoltage.25 A compact seed layer fully covering the FTO surface led to vertically aligned ZnO nanorods, which delivered a PCE of 14.35% after suppression of the recombination at the ZnO nanorod/perovskite interface by surface modification with (NH4)2TiF6.

strongly stabilizes FA ions in the perovskite lattice. When FAPbI3 and FA0.9Cs0.1PbI3 were exposed to continuous illumination (100 mA/cm2) and a relative humidity of 85%, the former degraded more rapidly than the latter with Cs.

■

ONE-DIMENSIONAL (1D) MAPBI3 NANOWIRE A 1D MAPbI3 nanowire was prepared for the first time by means of a liquid catalyst approach and applied to the perovskite solar cell.22 For the coating of MAI in a two-step spin-coating procedure, adding a small amount of aprotic solvent in isopropyl alcohol can grow MAPbI3 into a nanowire. The locally dissolved PbI2 by a small amount of DMF during the second spinning step can serve as a preferential site for reacting with MAI to grow a 1D structure, like a liquid catalyst cluster model.23 It cannot be ruled out that the 1D MAPbI3 nanowire might be caused by the intermediate compound MAPbI3·DMF, which is likely to serve as the parent in one dimension and then be converted to a perovskite. Thus, the solubility of PbI2 in a polar aprotic solvent F

DOI: 10.1021/acs.inorgchem.6b01294 Inorg. Chem. XXXX, XXX, XXX−XXX

Forum Article

Inorganic Chemistry

Figure 12. Spatial profile of the optical absorption per unit volume with respect to the xz plane (the z axis is normal to the FTO substrate) at wavelengths of 550, 620, and 700 nm for the perovskite layer with (A) the flat TiO2 layer and (B) the moth-eye TiO2 layer. The data were obtained by a threedimensional finite-difference time-domain method. Reprinted with permission from ref 26. Copyright 2016 Wiley-VCH Verlag GmbH & Co. KGaA.

density and nonradiative recombination portion of the perovskite is one way to achieve such a high voltage. Equally important is interfacial engineering to minimize recombination at opencircuit conditions, where electron- and hole-selective layers are expected to play a critical role in the interfaces.

Enhancement of light harvesting is one of the effective methods to further increase the photovoltaic performance. We have successfully fabricated a moth-eye structure on the FTO substrate using soft-nanoimprinting lithography and poly(dimethylsiloxane) stamping methods.26 Figure 12 clearly demonstrates that the absorption density for the flat TiO2 case is concentrated only on the perovskite thin layer near the TiO2 layer, while the absorption is enhanced at both the boundary and center for the moth-eye TiO2 structure. This explained the observed higher absorption densities of the moth-eye structure compared to the flat one over nearly the entire visible spectrum. Structural modification at the mesoporous layer improved the PCE from 15.74% (flat) to 17.48% (moth-eye) because of enhanced LHE. This research provides insight into optical manipulation, which is equally important as high-quality perovskite layer fabrication.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The author declares no competing financial interest.

■

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 NRF2012M3A6A7054861 (Global Frontier R&D Program on Center for Multiscale Energy System), NRF2015M1A2A2053004 (Climate Change Management Program), and NRF-2012M3A7B4049986 (Nano Material Technology Development Program). This was also supported, in part, by Contracts NRF-2016M3D1A1027663 and NRF2016M3D1A1027664 (Future Materials Discovery Program).

■

SUMMARY AND OUTLOOK The highlights of research works in my laboratory were described in this special issue. We developed for the first time a 9.7% efficiency solid-state perovskite solar cell in 2012 that demonstrated long-term stability for 500 h. We discovered charge accumulation ability in MAPbI3. A capacitive current causing I−V hysteresis might be related to charge accumulation characteristics. A Lewis acid−base adduct method was developed to prepare high-efficiency perovskite films, which delivered ∼19% from a stoichiometric precursor and over 20% from a nonstoichiometric precursor having excess MAI. The grain boundary was found to play an important role in charge conductance. A nanostructured perovskite or oxide scaffold modulated the optical absorption and light harvesting. We believe that more than 25% is able to be experimentally obtained from a single-junction perovskite solar cell in the case that the open-circuit voltage is higher than 1.25 V. Minimizing the trap

■

REFERENCES

(1) O’Regan, B.; Graetzel, M. A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO2 films. Nature 1991, 353, 737− 740. (2) Gratzel, M.; Bach, U.; Lupo, D.; Comte, P.; Moser, J. E.; Weissortel, F.; Salbeck, J.; Spreitzer, H. Solid-state dye-sensitized mesoporous TiO2 solar cells with high photon-to-electron conversion efficiencies. Nature 1998, 395, 583−585. (3) Mora-sero, I.; Gimenez, S.; Fabregat-santiago, F.; Gomez, R.; Shen, Q.; Toyoda, T.; Bisquert, J. Recombination in quantum dot sensitized solar cells. Acc. Chem. Res. 2009, 42, 1848−1857. G

DOI: 10.1021/acs.inorgchem.6b01294 Inorg. Chem. XXXX, XXX, XXX−XXX

Forum Article

Inorganic Chemistry (4) Lee, J.-W.; Son, D.-Y.; Ahn, T. K.; Shin, H.-W.; Kim, I. Y.; Hwang, S.-J.; Ko, M. J.; Sul, S.; Han, H.; Park, N.-G. Quantum-dot-sensitized solar cell with unprecedentedly high photocurrent. Sci. Rep. 2013, 3, 1050. (5) Park, N.-G. Organometal perovskite light absorbers toward a 20% efficiency low-cost solid-state mesoscopic solar cell. J. Phys. Chem. Lett. 2013, 4, 2423−2429. (6) Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. Organometal halide perovskites as visible-light sensitizers for photovoltaic cells. J. Am. Chem. Soc. 2009, 131, 6050−6051. (7) Im, J.-H.; Lee, C.-R.; Lee, J.-W.; Park, S.-W.; Park, N.-G. 6.5% efficient perovskite quantum-dot-sensitized solar cell. Nanoscale 2011, 3, 4088−4093. (8) Kim, H.-S.; Lee, C.-R.; Im, J.-H.; Lee, K.-B.; Moehl, T.; Marchioro, A.; Moon, S.-J.; Humphry-Baker, R.; Yum, J.-H.; Moser, J. E.; Grätzel, M.; Park, N.-G. Lead iodide perovskite sensitized all-solid-state submicron thin film mesoscopic solar cell with efficiency exceeding 9%. Sci. Rep. 2012, 2, 591. (9) http://www.nrel.gov/ncpv/images/efficiency_chart.jpg. (10) Kim, H.-S.; Mora-Sero, I.; Gonzalez-Pedro, V.; Fabregat-Santiago, F.; Juarez-Perez, E. J.; Park, N.-G.; Bisquert, J. Mechanism of carrier accumulation in perovskite thin-absorber solar cells. Nat. Commun. 2013, 4, 2242. (11) Kim, H.-S.; Kim, S. K.; Kim, B. J.; Shin, K.-S.; Gupta, M. K.; Jung, H. S.; Kim, S.-W.; Park, N.-G. Ferroelectric polarization in CH3NH3PbI3 perovskite. J. Phys. Chem. Lett. 2015, 6, 1729−1735. (12) Kim, H.-S.; Park, N.-G. Parameters affecting I−V hysteresis of CH3NH3PbI3 perovskite solar cells: effects of perovskite crystal size and mesoporous TiO2 layer. J. Phys. Chem. Lett. 2014, 5, 2927−2934. (13) Im, J.-H.; Jang, I.-H.; Pellet, N.; Grätzel, M.; Park, N.-G. Growth of CH3NH3PbI3 cuboids with controlled size for high-efficiency perovskite solar cells. Nat. Nanotechnol. 2014, 9, 927−932. (14) Ahn, N.; Kang, S. M.; Lee, J.-W.; Choi, M.; Park, N.-G. Thermodynamic regulation of CH3NH3PbI3 crystal growth and its effect on photovoltaic performance of perovskite solar cells. J. Mater. Chem. A 2015, 3, 19901−19906. (15) Mastroianni, S.; Heinz, F. D.; Im, J.-H.; Veurman, W.; Padilla, M.; Schubert, M. C.; Würfel, U.; Grätzel, M.; Park, N.-G.; Hinsch, A. Analysing the effect of crystal size and structure in highly efficient CH3NH3PbI3 perovskite solar cells by spatially resolved photo-and electroluminescence imaging. Nanoscale 2015, 7, 19653−19662. (16) Ahn, N.; Son, D.-Y.; Jang, I.-H.; Kang, S. M.; Choi, M.; Park, N.-G. Highly reproducible perovskite solar cells with average efficiency of 18.3% and best efficiency of 19.7% fabricated via Lewis base adduct of lead (II) iodide. J. Am. Chem. Soc. 2015, 137, 8696−8699. (17) Wharf, I.; Gramstad, T.; Makhija, R.; Onyszchuk, M. Synthesis and vibrational spectra of some lead (II) halide adducts with O-, S-, and N-donor atom ligands. Can. J. Chem. 1976, 54, 3430−3438. (18) deQuilettes, D. W.; Vorpahl, S. M.; Stranks, S. D.; Nagaoka, H.; Eperon, G. E.; Ziffer, M. E.; Snaith, H. J.; Ginger, D. S. Impact of microstructure on local carrier lifetime in perovskite solar cells. Science 2015, 348, 683−686. (19) Son, D.-Y.; et al. Self-formed grain boundary healing layer for highly efficient CH3NH3PbI3 perovskite solar cells. Nat. Energy 2016, 1, 16081. (20) Lee, J.-W.; Kim, D.-H.; Kim, H.-S.; Seo, S.-W.; Cho, S. M.; Park, N.-G. Formamidinium and Cesium Hybridization for Photo-and Moisture-Stable Perovskite Solar Cell. Adv. Energy Mater. 2015, 5, 1501310. (21) Lee, J.-W.; Seol, D.-J.; Cho, A.-N.; Park, N.-G. High-Efficiency Perovskite Solar Cells Based on the Black Polymorph of HC(NH2)2PbI3. Adv. Mater. 2014, 26, 4991−4998. (22) Im, J.-H.; Luo, J.; Franckevicius, M.; Pellet, N.; Gao, P.; Moehl, T.; Zakeeruddin, S. M.; Nazeeruddin, M. K.; Grätzel, M.; Park, N.-G. Nanowire perovskite solar cell. Nano Lett. 2015, 15, 2120−2126. (23) Hu, J.; Odom, T. W.; Lieber, C. Chemistry and physics in one dimension: synthesis and properties of nanowires and nanotubes. Acc. Chem. Res. 1999, 32, 435−445.

(24) Son, D.-Y.; Im, J.-H.; Kim, H.-S.; Park, N.-G. 11% efficient perovskite solar cell based on ZnO nanorods: an effective charge collection system. J. Phys. Chem. C 2014, 118, 16567−16573. (25) Son, D.-Y.; Bae, K.-H.; Kim, H.-S.; Park, N.-G. Effects of seed layer on growth of ZnO nanorod and performance of perovskite solar cell. J. Phys. Chem. C 2015, 119, 10321−10328. (26) Kang, S. M.; Jang, S.; Lee, J.-K.; Yoon, J.; Yoo, D.-E.; Lee, J.-W.; Choi, M.; Park, N.-G. Moth-Eye TiO2 Layer for Improving Light Harvesting Efficiency in Perovskite Solar Cells. Small 2016, 12, 2443− 2449.

H

DOI: 10.1021/acs.inorgchem.6b01294 Inorg. Chem. XXXX, XXX, XXX−XXX