High-Performance CH3NH3PbI3-Inverted Planar Perovskite Solar

Research Article Cite This: ACS Appl. Mater. Interfaces 2017, 9, 35871-35879

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High-Performance CH3NH3PbI3‑Inverted Planar Perovskite Solar Cells with Fill Factor Over 83% via Excess Organic/Inorganic Halide Muhammad Jahandar,†,‡ Nasir Khan,‡,§ Hang Ken Lee,† Sang Kyu Lee,†,‡ Won Suk Shin,†,‡ Jong-Cheol Lee,†,‡ Chang Eun Song,*,‡,§ and Sang-Jin Moon*,†,‡ †

Advanced Materials Division, Korea Research Institute of Chemical Technology (KRICT), 141 Gajeongro, Yuseong, Daejeon 34114, Republic of Korea ‡ Advanced Materials and Chemical Engineering, University of Science and Technology (UST), 217 Gajeongro, Yuseong, Daejeon 34113, Republic of Korea § Center for Solar Energy Materials, Korea Research Institute of Chemical Technology (KRICT), 141 Gajeongro, Yuseong, Daejeon 34114, Republic of Korea S Supporting Information *

ABSTRACT: The reduction of charge carrier recombination and intrinsic defect density in organic−inorganic halide perovskite absorber materials is a prerequisite to achieving high-performance perovskite solar cells with good efficiency and stability. Here, we fabricated inverted planar perovskite solar cells by incorporation of a small amount of excess organic/inorganic halide (methylammonium iodide (CH3NH3I; MAI), formamidinium iodide (CH(NH2)2I; FAI), and cesium iodide (CsI)) in CH3NH3PbI3 perovskite film. Larger crystalline grains and enhanced crystallinity in CH3NH3PbI3 perovskite films with excess organic/inorganic halide reduce the charge carrier recombination and defect density, leading to enhanced device efficiency (MAI+: 14.49 ± 0.30%, FAI+: 16.22 ± 0.38% and CsI+: 17.52 ± 0.56%) compared to the efficiency of a control MAPbI3 device (MAI: 12.63 ± 0.64%) and device stability. Especially, the incorporation of a small amount of excess CsI in MAPbI3 perovskite film leads to a highly reproducible fill factor of over 83%, increased open-circuit voltage (from 0.946 to 1.042 V), and short-circuit current density (from 18.43 to 20.89 mA/cm2). KEYWORDS: CH3NH3PbI3, perovskite solar cells, organic/inorganic halides, grain growth, defect density

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INTRODUCTION Organic−inorganic halide perovskite solar cells (PeSCs) are the fastest growing solar cell technology due to their extraordinary intrinsic properties such as broad absorption spectra, high carrier mobility, long charge carrier diffusion length, low exciton binding energy, tunable band gap, and ability to be solution processed.1−3 The power conversion efficiency (PCE) of PeSCs has leaped from 3.8%4 to a certified world record efficiency of 22.1%5 in a few years. Tremendous developments in device architecture and compositional/interfacial engineering have made PeSCs a captivating device for future energy devices.6−9 Organic−inorganic halide perovskite materials have a simple AMX3 perovskite structure, normally composed of an organic/ inorganic cation (A = methylammonium (CH3NH3+; MA); formamidinium (CH(NH2)2+; FA); and cesium (Cs+; Cs)), a divalent metal (M = lead (Pb2+), tin (Sn2+)), and a halide anion (X = Cl−, Br−, and I−).10−12 The chemical flexibility of the organic−inorganic halide perovskite structure makes this material important for photovoltaic, light-emitting diode, and photodetector applications.13,14 © 2017 American Chemical Society

Pure methylammonium iodide (MAI) organic halide incorporating CH3NH3PbI3 (MAPbI3) PeSCs with stoichiometry ratio of MAI:PbI2 = 1:1 is the most widely studied perovskite structure, but device performance has never reached over 20%;15−17 however, pure formamidinium iodide (FAI) organic halide incorporating CH(NH2)2PbI3 (FAPbI3) PeSCs exhibits a low band gap as compared to MAPbI3 perovskite absorber that would allow higher solar light absorption.18−20 However, pure FAPbI3 perovskite shows structural instability at room temperature due to the formation of two crystalline phases, either a δ-phase (a yellow nonperovskite phase) or an α-phase (a black perovskite phase), which are sensitive to solvents or moisture.21−23 On the other hand, pure inorganic CsPbI3 perovskite with a suitable band gap of 1.73 eV and excellent thermal stability can be a suitable candidate for a perovskite absorber material, but the high temperature (above Received: July 27, 2017 Accepted: September 26, 2017 Published: September 26, 2017 35871

DOI: 10.1021/acsami.7b11083 ACS Appl. Mater. Interfaces 2017, 9, 35871−35879

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) Cross-sectional TEM image of complete ITO/PEDOT:PSS/perovskite/PC61BM/LiF/Al inverted planar PeSCs, (b) energy band diagram, and (c) absorption spectra of control and excess OIH-incorporated MAPbI3 perovskite film.

300 °C) for the formation of the perovskite phase makes the processing of this material challenging.24 An inimitable property of organic−inorganic halide perovskite materials is that their formations comprise chemical reactions of organic/inorganic halide (MAX, FAX, CsX, where X = Cl, Br, I) and metal halide (PbX2, SnX2, where X = Cl, Br, I) precursors. This provides a playground for researchers to attain perovskite materials with improved thermal and structural stability.25−27 Furthermore, it has become an important design rule to mix organic/inorganic cations and halides in order to largely control the perovskite defects and crystalline grain size by changing the compositional ratios of the above materials in the perovskite precursor solution. Although these strategies meritoriously reduce the recombination losses and improve the charge carrier pathways within the perovskite absorber layer, solution-processed PeSCs generally have shown that undercoordinated ions at the perovskite crystal surface and at the grain boundaries have detrimental effects on the device performance of PeSCs, which are attributed as trap/ recombination centers.28,29 Wang et al.30 reported on a detailed study of the compositional excess of organic cation or halide anion in perovskite and the effect on the tuning of the elemental defects. On the other hand, Nam-Gyu Park et al.31 demonstrated a self-formed grain boundary healing layer at MAPbI3 perovskite grains by introducing excess MAI into the perovskite precursor solution. This grain boundary healing technique was found to play a crucial role in carrier lifetime improvement, suppression of nonradiative recombination at grain boundaries, and effective extraction of charge carriers at the interface between the perovskite and the selective contacts. Similarly, Marco et al.32 reported a passivation effect in a perovskite absorber layer via guanidinium-based additives; they achieved an extraordinarily enhanced charge carrier lifetime and higher open-circuit voltage by suppressing the nonradiative recombination at the perovskite grain boundaries. The optoelectronic and photovoltaic properties of the polycrystalline perovskite films highly depend on aspects of the morphology such as crystallinity and grain size.33 Recent reports have demonstrated that the diffusion length and the mobility of the charge carrier can be significantly improved in large perovskite grains.34,35 The improved electronic properties of perovskite film with large grains are normally ascribed to the decreasing grain boundary density, which might diminish the presence of traps and bring about a reduction of charge carrier recombination. However, large-scale crystal growth and high quality films with a minimum of pinholes are challenging to make using solution-processed PeSCs due to the low molecular kinetic energy that results from low-temperature processing. In this work, by adding a small amount of excess organic/ inorganic halide (OIH), we have established a simple

nonstoichiometric approach to reduce the charge carrier recombination and intrinsic defect density in MAPbI3 PeSCs. The improved perovskite grain size, crystallinity, and film morphology result in suppressed charge carrier recombination and reduced defect density in PeSCs, while simultaneously increasing the fill factor (FF), open-circuit voltage (VOC), and short-circuit current density (J SC). The charge carrier recombination and defect density of perovskite films play important roles in determining the power conversion efficiency (PCE, η) and device stability.

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EXPERIMENTAL SECTION

Materials and Preparation of Perovskite Precursor Solution. Lead iodide (99.999% trace metal basis), cesium iodide (99.999% trace metals basis), γ-butyrolactone (GBL), dimethyl sulfoxide (DMSO), and chlorobenzene (CB) were purchased from Sigma-Aldrich. Methylammonium iodide (MAI) and formamidinium iodide (FAI) were purchased from 4Chem and Dyesol, respectively, and used as received without any further purification. To prepare the perovskite precursor solution, we mixed MAI (159 mg) powder and PbI2 (461 mg) (1:1 molar ratio) in 1 mL of mixed GBL:DMSO (0.7:0.3) solvent for the reference perovskite precursor solution, whereas for the excess OIH-incorporated MAPbI3 perovskite solution 0.05 M MAI (7.95 mg), FAI (8.59 mg), and CsI (12.99 mg) were added in the reference perovskite precursor solution. All perovskite precursor solutions were kept for stirring at 70 °C overnight before use. Device Fabrication. For inverted planar perovskite solar cell device fabrication, first, the patterned ITO/glass substrates were cleaned with DI water, acetone, and isopropanol and dried in an oven at 140 °C overnight. PEDOT:PSS (Clevios AI4083) was spin-coated on UV-ozone-treated ITO/glass substrates at 4000 rpm for 60 s in air and dried at 150 °C for 20 min. Then, the samples were transferred to the N2-filled glovebox for a further device fabrication process. Perovskite precursor solution without and with OIH was spin coated in a N2-filled glovebox at 4000 rpm for 60 s, followed by a step of 1000 rpm for 20 s. During the second step of 4000 rpm for 60 s, a chlorobenzene (CB) solution (200 μL) was dropped on the substrate during spin coating after 40 s and continued the spin for a further 20 s. The important point to be noted here is that the CB dripping time during the second spin-coating step was further delayed approximately 5 to 10 s for OIH-containing perovskite precursor solutions as compared to reference solution. Then, the samples were dried on a hot plate at 100 °C for 5 min. The PC61BM (EM Index) ETL layer was deposited on the ITO/PEDOT:PSS/perovskite substrate by spin coating PC61BM (20 mg/1 mL in CB) solution at 1200 rpm for 60 s. After PC61BM spin coating, the samples were left in a glovebox at room temperature to dry overnight before electrode deposition. Finally, the LiF/Al (0.5 nm/100 nm) counter electrode was deposited by thermal evaporation. Materials Characterization. Absorption spectra of perovskite films on ITO glass substrates were recorded on a Shimadzu UV-2550 spectrophotometer. XRD was performed using a 2 kW Rigaku SmartLab X-ray diffractometer in reflection mode at 9 kW (45 kV, 200 35872


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Figure 2. (a−d) FE-SEM images, (e) XRD spectra, and (f−i) 2D GIWAXS patterns of control and excess OIH-incorporated MAPbI3 perovskite films. mA, Cu, λ = 1.5409 Å, rotating anode) with scintillation counter detector. FE-SEM was performed on Tescan Mira 3 LMU FEG operated at 20 kV. AFM images were obtained from Bruker (Nanoscope) with tapping mode. Film thicknesses were measured with a surface profile-meter (KLA Tencor). 2D-GIWAXS measurements were recorded on the PLS−II in vacuum at the Pohang Accelerator Laboratory in South Korea using the 3C beamline. Device Characterization. The current density−voltage (J−V) curves were measured by a solar simulator (Polaronix K201 Solar Simulator LAB50, McScience) with a source meter (KEITHLEY 2400) under illumination of 1 sun (100 mW/cm2 AM 1.5G) which is calibrated with a Si-reference cell (Model# RCSiG5, Serial# PVM977) certificated by PV measurements Inc., USA. The scan rate of J−V curves was set to 100−500 ms per 0.01 V. The EQE was measured by using a K3100 − Spectral IPCE measurement system (McScience). The C−V characteristics were performed by an Agilent 4284A LCR Meter.

and poly(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS) as a hole transport layer (HTL), respectively. The PeSC device structure with PC 61 BM ETL and PEDOT:PSS HTL provides well-balanced electron and hole fluxes in this device architecture, leading to high efficiency without significant current density−voltage (J−V) hysteresis under forward and reverse scan directions.17 The UV−visible absorption spectra of the control and of the excess OIH (MAI, FAI and CsI) MAPbI3 perovskite film are shown in Figure 1(c). All the perovskite films were prepared under the same conditions of film thickness of around 350 nm. Compared to the control MAPbI3 perovskite film, perovskite films with excess OIH show slightly strong absorption, whereas a slight red-shift and a slight blue-shift of ∼2 nm were observed for the FAI and CsI incorporated perovskite samples, respectively. To investigate the role of excess OIH in crystalline grain growth and in the morphology of perovskite films, we prepared perovskite films on the ITO/PEDOT:PSS substrate under optimum conditions with film thickness of around 350 nm; films were then analyzed using scanning electron microscopy (SEM) and atomic force microscopy (AFM), with results as shown in Figure 2(a−d) and Figure S1, respectively. With the incorporation of excess OIH, the perovskite grain size increased compared to that of the control MAPbI3 perovskite film, whereas the root-mean-squared (RMS) roughness of the perovskite film was reduced (MAI: 17.1 nm, MAI+: 15.6 nm, FAI+: 14.5 nm, and CsI+: 13.4 nm). The roughness of the excess OIH incorporating perovskite films was mainly reduced due to the increase of the perovskite grain size and the reduced grain boundary density.36 The increases in the perovskite grain size and crystallinity were further investigated by X-ray diffraction (XRD) analysis, with results as shown in Figure 2(e). Compared to the control MAPbI3 perovskite film, the

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RESULTS AND DISCUSSION MAPbI 3 perovskite films with excess OIH in the γbutyrolactone (GBL) and dimethyl sulfoxide (DMSO) mixed solvent system were prepared by the solvent dripping technique. The preparation of the perovskite precursor solution is described in detail in the Experimental Section. Figure 1(a) provides cross-sectional transmission electron microscope (TEM) images of MAPbI3 PeSCs composed of ITO/ PEDOT:PSS/perovskite/PC61BM/LiF/Al, whereas Figure 1(b) shows the energy band diagram. For ease of description, the labels used for figures and tables in this manuscript are MAI, MAI+, FAI+, and CsI+ for control and 0.05 M excess MAI, FAI, and CsI incorporating MAPbI3 perovskite samples. Under light illumination, the electrons and holes generated in the perovskite active layer can efficiently transport to the respective electrode through [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) as an electron transport layer (ETL) 35873


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Figure 3. (a−d) J−V curves of the control and excess OIH-incorporated MAPbI3 inverted planar PeSCs under 1 sun light illumination with respect to scan direction, and the reverse and forward scan rate was set to 100 ms per 0.01 V, and (e−h) corresponding external quantum efficiency graph with integrated current density.

Table 1. Summary of Device Performance of Control and Excess OIH-Incorporated MAPbI3-Inverted Planar PeSCs perovskite

scan direction

VOC (V)

JSC (mA/cm2)

FF (%)

η (%)a

avg. η (%)b

MAI

reverse forward reverse forward reverse forward reverse forward

0.946 0.943 0.963 0.963 0.974 0.980 1.042 1.039

18.43 18.28 18.73 18.42 20.50 20.16 20.89 20.66

77.93 76.29 82.81 81.45 83.99 83.32 83.84 83.56

13.58 13.15 14.93 14.46 16.77 16.45 18.24 17.94

12.63 ± 0.64

MAI+ FAI+ CsI+ a

14.49 ± 0.30 16.22 ± 0.38 17.52 ± 0.56

Best device performance. bAverage device performance of 30 independent devices.

organic/inorganic halide and attained the J−V characteristics under one sun (100 mW/cm2 AM 1.5G) light illumination (Figure S3). Relevant device performance is summarized in Table S1. Figure 3(a−d) shows the J−V characteristics of MAPbI3 PeSCs with control and optimum amounts of excess incorporation of MAI, FAI, and CsI contents with active area of 10 mm2. The device performance of control and with excess OIH is summarized in Table 1. The control device had VOC = 0.946 V, JSC = 18.43 mA/cm2, FF = 77.93%, and η = 13.58% under the reverse scan directions, whereas under forward scan direction, the values were VOC = 0.943 V, JSC = 18.28 mA/cm2, FF = 76.29%, and η = 13.15% with slight J−V hysteresis under forward and reverse scan directions. Among the MAPbI3 PeSCs with excess MAI, FAI, and CsI, the highest power conversion efficiency was obtained with excess CsI incorporation (VOC: 1.042 V, JSC: 20.89 mA/cm2, FF: 83.84%, and η: 18.24% under reverse scan direction and VOC: 1.039 V, JSC: 20.66 mA/cm2, FF: 83.56%, and η: 17.94% under forward scan direction) with reduced J−V hysteresis. On the other hand, the photovoltaic properties when employing excess MAI and FAI incorporation were (VOC: 0.963 V, JSC: 18.73 mA/cm2, FF: 82.81%, and η: 14.93% under reverse scan direction and VOC: 0.963 V, JSC: 18.42 mA/cm2, FF: 81.45%, and η: 14.46% under forward scan direction) and (VOC: 0.974 V, JSC: 20.50 mA/cm2, FF: 83.99%, and η: 16.77% under reverse scan direction and VOC: 0.980 V, JSC: 20.16 mA/cm2, FF: 83.32%, and η: 16.45% under forward scan direction), respectively. The enhanced device performance of the MAPbI3 PeSCs with excess MAI, FAI, and CsI was attributed to the improved VOC, JSC, and FF. The larger crystal grains, preferential crystal orientation, and high quality

increased perovskite peak intensity and reduced full width at half-maximum (fwhm) of these films clearly indicate the improved perovskite grain size and crystallinity due to incorporation of excess OIH. The increased perovskite grain size and reduced surface roughness could reduce the trap sites, resulting in improved values of VOC, FF, and JSC, hence enhancing the PCE.37 In order to further investigate the crystalline nature and orientation of the perovskite nanostructure on the substrate, 2D scattering patterns were obtained with synchrotron-based grazing incidence wide-angle X-ray scattering (GIWAXS), which has become an important technique for characterizing organic semiconductor and perovskite thin films.38−40 The recorded 2D-GIWAXS patterns of the control and of the excess OIH incorporating MAPbI3 perovskite films are shown in Figure 2(f−i). The higher intensity of the (110) ring around q = 0.98 Å−1 in the 2D-GIWAXS data shows that the excess OIH incorporating MAPbI3 perovskite films exhibit preferential crystal orientation, with the c-axis perpendicular to the substrate. The crystallites of the excess OIH incorporating MAPbI3 perovskite samples with the (110) plane oriented to the out-of-plane direction can be identified by a clear diffraction spot in the qz direction compared to the control MAPbI3 perovskite film. This is further verified by the azimuthal integration around q = 0.98 Å−1 (Figure S2), which clearly shows that the superiority of the excess OIH incorporating MAPbI3 perovskite samples exhibits the stronger degree of preferential orientation. To estimate the appropriate amount of excess OIH in MAPbI3 PeSCs for the optimization of device performance, we fabricated inverted planar PeSCs with different contents of 35874


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Figure 4. Average photovoltaic parameter of (a) power conversion efficiency (PCE), (b) open-circuit voltage (VOC), (c) short-circuit current density (JSC), and (d) fill factor (FF) for 30 devices of control and excess OIH incorporating MAPbI3-inverted planar PeSCs.

Figure 5. (a−d) VOC, JSC, FF, PCE, and corresponding RS as a function of light intensity for control and excess OIH-incorporated MAPbI3-inverted planar PeSCs. (e−h) Current density−voltage characteristics of device with ITO/perovskite/Au configuration for estimating the defect density in perovskite films.

perovskite film formation effectively enhanced the values of VOC, JSC, and FF of the PeSCs that incorporated excess OIH as compared to those values of the control device. The deviation of the values of VOC, JSC, FF, and η of the 30 PeSCs are shown in Figure 4. The deviation in efficiency was 12.63 ± 0.64, 14.49 ± 0.30, 16.22 ± 0.38, and 17.52 ± 0.56% for the control and excess MAI, FAI, and CsI incorporating MAPbI3 perovskite solar cells, respectively. The discernibly reproducible enhanced values of VOC, JSC, and FF of the PeSCs processed with excess OIH have facilitated the achievement of consistently high PCEs compared to those values of the control device. The external quantum efficiency (EQE) spectrum and integrated current density of each device are shown in Figure 3(e−h); the tendency of this value can be seen to match well with the JSC value of the J−V curves under 1 Sun illumination. The EQE is a product of the light-harvesting efficiency, charge separation efficiency, and charge collection efficiency.41 The EQE spectrum also confirmed that the excess OIH incorporating PeSCs had increased JSC compared to that of the control cell. The main reason for this phenomenon can be

attributed to the formation of large crystal grains, the improved perovskite crystallinity, and high quality perovskite film.42 These factors effectively reduced the charge carrier recombination and improved the charge separation, as well as improved the charge collection efficiency. On the other hand, there was no significant improvement in the light-harvesting efficiency, as can be seen in the UV−visible absorption spectra provided in Figure 1(c). To investigate the photogenerated charge recombination during operation of solar cells under light illumination, we measured VOC and JSC as a function of light intensity for the MAPbI3 PeSC devices with/without excess OIH. The relevant J−V characteristics of the control and of the excess incorporating OIH MAPbI3 PeSCs with respect to change in light intensity are shown in Figure S4. By linearly fitting VOC versus light intensity in Figure 5(a), we found a slope of 1.33 kBT/q and 1.27 kBT/q (where kB is the Boltzmann constant, T is absolute temperature, and q is elementary charge) for the control and for the excess MAI-incorporating MAPbI3-inverted PeSCs devices, respectively. Similarly, slopes of 1.18kBT/q and 35875


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MAPbI3 perovskite films were obtained on glass substrates as shown in Figure S5. The PL intensity of the excess OIH incorporating MAPbI3 perovskite film is higher than that of the control MAPbI3 perovskite film which shows that nonradiative decay is suppressed in excess OIH incorporating MAPbI3 perovskite films.51−53 To investigate the impact of excess OIH incorporation on the electrical properties of the MAPbI3 PeSCs, we applied Mott−Schottky analysis by measuring the capacitance−voltage (C−V) characteristics of complete devices having device architectures of ITO/PEDOT:PSS/perovskite/PC61BM/LiF/ Al. The C−V measurement of the complete PeSCs was performed under dark at room-temperature conditions. The doping density N of the PeSCs can be estimated by calculating the slope of 1/C2 versus the applied voltage and using the following equation54,55

1.14kBT/q were determined for the excess FAI- and CsIincorporating MAPbI3 PeSC devices, respectively, which indicate that bimolecular recombination dominates over the trap-assisted recombination process, which the control and the excess MAI-incorporating MAPbI3 PeSCs employed during device operation.43 In the power law dependence of JSC on light intensity, the deviation of the slope from unity implies that bimolecular recombination has taken place. Figure 5(b) shows the relation of JSC versus light intensity in a double logarithmic scale with slopes of 0.95, 0.96, 0.96, and 0.98 for the MAI (control cell), MAI+, FAI+, and CsI+ PeSCs, respectively, indicating that the excess CsI-incorporating MAPbI3 PeSCs, compared to other MAPbI3 PeSCs, show the reduced bimolecular recombination.44 Furthermore, the FF dependence of the control and excess OIH incorporating MAPbI3 PeSCs on incident light intensity is shown in Figure 5(c). The FF of the excess OIH incorporating MAPbI3 device appears to be steady and almost below the error limit as the light intensity decreases. However, the FF of the control MAPbI3 PeSC decreases significantly with reduced light intensity. The change in FF with respect to change in light intensity can be attributed to the series resistance (RS) and to bimolecular recombination of the PeSCs in operational conditions.45 The RS values of the control and excess OIH incorporating MAPbI3 PeSCs, as a function of the light intensity, are shown in Figure 5(d). To calculate RS under light condition, we adopted a method based on computation of the area under an I−V curve.46 The lack of dependence of the values of FF and RS of the excess OIH incorporating MAPbI3 PeSCs on the light intensity indicates that a high quality perovskite film has been fabricated in contrast to the case of the control device. This phenomenon shows that the parasitical leakage current is sufficiently low in the excess OIH-incorporated perovskite devices.47,48 The reduced charge carrier recombination in the excess OIH incorporating MAPbI3 PeSCs is mainly due to the large crystal grains and high quality perovskite film formation, both of which play important roles in improving the charge separation and charge collection efficiency. Furthermore, to quantitatively evaluate the density of defects, we fabricated devices with ITO/perovskite/Au device architectures. The evolution of the space-charge-limited current (SCLC) for different biases was characterized as shown in Figure 5(e−h). The sharp rise of the dark J−V curve is associated with a trap-filled limit, where all the defects are occupied by charge carriers. The defect density is calculated according to the following equation49,50 Ndefects = 2εε0VTFL /qL2

N = (2/qεε0A2 )dV /d(1/C 2)

(2)

where N is the doping density; q is the elementary charge; ε is the dielectric constant of the material; ε0 is the permittivity of free space (8.854 × 10−14 F/cm); A is the area of the device; and C is the capacitance. Figure S6 shows the 1/C2 versus applied voltage graphs of the control and of the excess OIH incorporating MAPbI3 perovskite devices. Wang et al.30 reported compositional-dependent self-doping in MAPbI3 perovskite; they found that the MAI-rich and PbI2-rich MAPbI3 perovskite films are of p-type and n-type, respectively. Therefore, the composition of the perovskite materials is directly related with the doping density. As the slopes in the linear region have negative values, all the perovskite films fabricated in this experiment were identified as p-type. The doping density values calculated by C−V profiling without and with excess MAI, FAI, and CsI contents are 5.17 × 1016, 6.05 × 1016, 8.45 × 1016, and 1.14 × 1017 cm−3, respectively. The doping density of the control device is comparable values found in previous reports using MAPbI3-based PeSCs.56−58 It is well-known that perovskite solar cells show current− voltage hysteresis at the certain voltage scanning rate or forward and reverse scan directions. The current−voltage hysteresis in the perovskite solar cells can occur due to the charge traps of low quality perovskite film, an unbalanced electron and hole flux, or the interphase defects.59,60 Therefore, the PCE of the perovskite solar cells with the issue of J−V hysteresis may not truly represent the actual device performance. To verify highperformance perovskite solar cells with the high value of FF, the J−V characteristics of control and excess organic/inorganic halide incorporated MAPbI3 perovskite solar cells with different very low scan rates (100 ms per 0.01 V to 500 ms per 0.01 V) with respect to scan direction are examined (Figure S7). In the case of organic/inorganic halide incorporated MAPbI3 perovskite solar cells, no significant J−V hysteresis was exhibited with different very low scan rates, whereas a clear J−V hysteresis can be seen for reference MAPbI3 perovskite solar cells. These results suggests that the J−V hysteresis is more likely due to the defects on the perovskite film and the interface of PEDOT:PSS/perovskite/PC61BM, which can be eliminated partly by using a high quality perovskite film prepared by the excess organic/inorganic halide incorporation method. As we have revealed, experimentally a high quality perovskite film can be prepared with reduced charge carrier recombination and trap density by simply adding excess organic/inorganic halides in MAPbI3 perovskite precursor. To further confirm the excess OIH-incorporated device with the high FF value, we measured

(1)

where Ndefects is the defect density; ε is the dielectric constant of the perovskite material; ε0 is the permittivity of free space (8.854 × 10−14 F/cm); q is the elementary charge; and L is the thickness of the perovskite film. The values of defect density (Ndefects), calculated for the control and for the excess MAI, FAI, and CsI incorporating MAPbI3 perovskite films, were 3.98 × 1016, 1.96 × 1016, 1.20 × 1016, and 7.36 × 1015 cm−3, respectively. The defect density in the excess CsI-incorporating MAPbI3 perovskite film has been significantly reduced to ∼1/5 the value of the control MAPbI3 perovskite film. The defect density of the control device is comparable to that of previous devices using MAPbI3-based PeSCs mentioned in the literature.49 To further explain the mechanisms of the charge carrier recombination and defect density, steady-state PL spectra of the control and the excess OIH incorporating 35876


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Figure 6. Normalized (a) efficiency (η), (b) open-circuit voltage (VOC), (c) current-density (JSC), and (d) fill factor (FF), of control and excess OIH incorporating MAPbI3-inverted planar PeSCs with respect to time.

more effectively passivate undercoordinated ion species at the perovskite crystal surface and at the grain boundaries.

the steady-state current density and efficiency at maximum power point (MMP) under continuous 1 sun illumination and for 100 s. Figure S8 shows that the control MAPbI3 and excess OIH-incorporated MAPbI3 perovskite solar cells exhibit steadystate PCE of MAI (13.2%), MAI+ (14.4%), FAI+ (16.2%), and CsI+ (17.9%) under continuous illumination and constant bias voltage. The excess OIH-incorporated MAPbI3 perovskites solar cells show good stability in photocurrent, which was ∼5% less than the JSC of transient J−V measurement. However, the control MAPbI3 perovskite device without incorporation of excess OIH shows a relative drop in current density with course of time. Based on these results, we conclude that the approach of incorporation of excess OIH in MAPbI3 perovskite is capable of producing stable device performance with high value of FF. The larger crystal grain sizes and improved perovskite film morphology of the excess OIH incorporating MAPbI3 perovskite film, compared to that of the control cell, significantly reduced the trap density. The reduction in trap density, in which traps are actually defect sites at which recombination occurs, can improve the device stability. The stability of the control and the excess OIH incorporating MAPbI3-inverted planar PeSCs are compared in Figure 6. All the PeSCs were kept in air for 4 weeks after encapsulation. The PeSCs were encapsulated to avoid the degradation of the LiF/Al contact because the LiF/Al metal contact is highly sensitive to moisture and can degrade quickly without proper encapsulation. After 4 weeks, the excess OIH incorporating MAPbI3 PeSCs showed an approximately 10% drop in efficiency, whereas the control MAPbI3 device showed a drop in efficiency of around 30%. The drop in efficiency of the control device was mainly due to a drop in FF and JSC which could possibly be due to increased trap density. Among the OIH incorporating PeSCs, the MAPbI3 perovskite solar cells employing excess CsI showed the best PCE (18.24%). These results indicate fabrication of high quality perovskite films with large-scale crystal growth, which led to a reduced charge carrier recombination and suppressed the defect density. As Cs is substantially smaller in size (1.81 Å) than the MA (2.70 Å) and FA (2.79 Å) cations,27 we believe that it can

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CONCLUSIONS In conclusion, we have demonstrated excess organic/inorganic halide incorporating MAPbI3 PeSCs. The incorporation of a small amount of excess MAI, FAI, and CsI in MAPbI3 perovskite absorbers forms larger crystalline grains and improves the crystallinity of the perovskite films, which result in reduced charge carrier recombination and defect density. The suppressed charge carrier recombination and reduced defect density in excess OIH incorporating MAPbI3 PeSCs led simultaneously to enhanced photovoltaic properties. Consequently, the excess OIH incorporating MAPbI3 PeSCs exhibited greatly improved device efficiency (best PCE = MAI+: 14.93%, FAI+: 16.77%, and CsI+: 18.24%) compared to the efficiency of the control MAPbI3 device (best PCE = 13.58%) and showed no significant J−V hysteresis under forward and reverse scan directions. Overall, our promising strategy of incorporation of excess OIH in MAPbI3-based perovskite absorbers could enable reproducible manufacturing of large and uniform crystalline perovskite films which would further improve the device performance.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b11083. AFM images, GIWAXS data, J−V characteristics of optimized performance, summary of device performance, J−V characteristics with respect to change in light intensity, PL spectra, and Mott−Schottky analysis graph of the control and of the excess organic/inorganic halide incorporated MAPbI3 perovskite devices (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S.-J. Moon). *E-mail: [email protected] (C. E. Song). 35877


Research Article

ACS Applied Materials & Interfaces ORCID

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Muhammad Jahandar: 0000-0002-2058-263X Nasir Khan: 0000-0003-3119-8702 Hang Ken Lee: 0000-0002-2104-5334 Sang Kyu Lee: 0000-0002-7568-2902 Won Suk Shin: 0000-0001-7151-519X Jong-Cheol Lee: 0000-0002-2892-2213 Chang Eun Song: 0000-0001-6910-8755 Sang-Jin Moon: 0000-0002-0451-8558 Author Contributions

M.J. and C.E.S. wrote the manuscript. M.J. and N.K. convinced of the experiments and prepared the sample. M.J. and H.K.L. performed device fabrication and analyzed the data. N.K. and C.E.S. performed the GIWAXS measurements and data analysis. S.K.L., W.S.S. and J.-C.L. suggested experimental advice. All authors have given approval to the final version of the manuscript. C.E.S. and S.-J.M. supervised the experiment and manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Technology Development Program to Solve Climate Changes of the National Research Foundation (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2015M1A2A2055631),by the KRICT core project (KK1702-A00) funded by the National Research Council of Science & Technology and by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (No. 20173010012960).

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High-Performance CH3NH3PbI3-Inverted Planar Perovskite Solar

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