Nanoparticles as a Cathode Buffer Layer for ... - ACS Publications

Subscriber access provided by UNIV OF CALIFORNIA SAN DIEGO LIBRARIES

Article

Low-Temperature Solution-Processed SnO2 Nanoparticles as Cathode Buffer Layer for Inverted Organic Solar Cells Van-Huong Tran, Rohan B. Ambade, Swapnil B. Ambade, Soo-Hyoung Lee, and In-Hwan Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 16 Dec 2016 Downloaded from http://pubs.acs.org on December 16, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Low-Temperature Solution-Processed SnO2 Nanoparticles as Cathode Buffer Layer for Inverted Organic Solar Cells Van-Huong Tran,†,‡ Rohan B. Ambade, ‡ Swapnil B. Ambade, ‡ Soo-Hyoung Lee,*,‡ and InHwan Lee*,† †

School of Advanced Materials Engineering and Research Center of Advanced Materials Development, Chonbuk National University, 664-14, 1-ga Deokjin-dong, Deokjin-gu, Jeonju, Jeonbuk, 561-756, Republic of Korea.

‡

School of Semiconductor and Chemical Engineering, Chonbuk National University, 664-14, 1-ga Deokjin-dong, Deokjin-gu, Jeonju, Jeonbuk, 561-756, Republic of Korea.

1 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 30

ABSTRACT SnO2 recently has attracted particular attention as a powerful buffer layer for organic optoelectronic devices due to its outstanding properties like high electron mobility, suitable band alignment, and high optical-transparency. Here, we report on facile low-temperature solution-processed SnO2 nanoparticles (NPs) in application for cathode buffer layer (CBL) of inverted organic solar cells (iOSCs). The conduction band energy of SnO2 NPs estimated by ultraviolet photoelectron spectroscopy was 4.01 eV, a salient feature that is necessitated for appropriate CBL. Using SnO2 NPs as CBL derived from a 0.1 M precursor concentration, P3HT:PC60BM based iOSCs showed the best power conversion efficiency (PCE) of 2.9%. The iOSC devices using SnO2 NPs as CBL revealed excellent long-term device stabilities, and the PCE was retained ~95% of initial value after 10 weeks in ambient air. This solutionprocessed SnO2 NPs are considered to be suitable for the low-cost, high throughput roll-to-roll process on a flexible substrate for optoelectronic devices.

KEYWORDS SnO2 nanoparticles, solution-processed metal oxide, low-temperature synthesis, nanoparticle morphology, cathode buffer layer, inverted organic solar cells.


Page 3 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


INTRODUCTION OSCs have drawn considerable attention from researchers because of their outstanding advantages such as low-cost manufacturing, light-weight, high throughput; which could be further applied to the roll-to-roll process.1-5 Conventional OSC devices are typically build up with an active absorbing layer sandwiched between a transparent anode such as poly(3,4ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) deposited onto indium tin oxide glass (ITO) and a low work function (WF) metal cathodes such as Al, LiF/Al, Ca/Al, etc.6,7 Unfortunately, these metal cathodes are sensitive to moisture and oxygen in ambient air. Moreover, as the PEDOT:PSS layer has acidic as well as hygroscopic properties, it renders the ITO anode to be susceptible to corrosion, leading to the instability of conventional OSCs.813

To overcome these issues, an inverted electrode configurations without PEDOT:PSS layer

has been proposed with a reverse charge collection compared to conventional devices.14-16 In inverted devices, the ITO film modified with a transparent buffer layer is used as an alternative cathode to assemble electrons, whereas metals like Au, Ag (high WF) are functioned as air-stable anodes to collect holes. Therefore, inverted OSCs could eliminate the acidic problem of PEDOT:PSS, resulting in high efficiency along with great improvements in device stability.17 In iOSCs, the CBL is play an important role to its device performances,18-20 its optical transparency will affect the light absorption ability of the photoactive layers, whereas its energy level as well as charge-transporting ability will influence its electron extraction properties. As a result, CBL can directly affect various important parameters of a solar device like open voltage (VOC), current density (JSC), and fill factor (FF); consequently to the PCE.21 Among various materials, alkali metals, hybrid composites, and conjugated polymer electrolytes have been used as CBLs.22-28 In particular, metal oxides have attracted considerable attention as they possess good processability, transparency, charge transport 3 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

properties, as well as excellent stability.29 ZnO and TiOx are well-established metal oxide materials to use as CBLs in order to effectively promote charge collection in iOSCs.30-32 However, solar cells using metal oxide CBLs have suffered from technical issue of wellknown “light soaking” effect i.e., UV exposure is required for proper function.33-37 Notably, the need of light soaking appears to be a general phenomenon, regardless of the metal-oxidepreparation technique or device structure.37-39 Without the UV treatment, iOSCs exhibited strong S-shaped J-V characteristics that lead to a relatively low FF and poor PCE.35 As a result, looking for CBL metal oxides free of the light soaking issue is in high demand. Among various metal oxides, SnO2 is well-established as a wide-bandgap (3.6 eV), transparent semiconducting material. Recently, Riedl et al.40 reported on light-soaking-free SnO2 as electron extraction layer for OSCs, and Zhang et al.41 demonstrated SnO2 as an ideal buffer layer of long-term stable OSCs. With its excellent optical and electrical properties, SnO2 has attracted particular attention from researchers as a unique strategic functional material with a variety of technological applications, ranging from optoelectronic devices, gas sensors, to solar batteries, etc.42-47 So far, the SnO2 has predominantly been prepared by a costly thermal-evaporation method that requires a high vacuum and a high temperature (>450 o

C).41,48 Such a high temperature which could not be conducted on flexible substrates. The

latter procedure requires a low temperature (500 oC), leading to complicated process and high production cost.42,49-52 Here, we report a facile and effective preparation of SnO2 NPs to be used as CBL for iOSCs by solution-process at low temperature. To our knowledge, this work on the use of 4 ACS Paragon Plus Environment

Page 4 of 30

Page 5 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


low-temperature solution processed SnO2 NPs as CBL of bulk heterojunction (BHJ) OSCs is novel. In the synthesis of SnO2 NPs by solution-process, many input parameters can strongly influence the characteristics of NPs formation.53 Iversen et al. reported on the dependence of size and morphology of SnO2 NPs in hydrothermal synthesis on both reaction temperature and precursor concentration.54 Among these synthesis parameters, precursor concentration is considered to be the most important factor affecting the morphology, optical and electronic properties of SnO2 NPs. Yan et al.55 reported that the precursor concentration strongly affected the performance of perovskite solar cells, where SnO2 has utilized as electron-transporting layer, and the best precursor concentration was found to be 0.1 M. Additionally, other reports on the CBL based on solution-processed metal oxides such as ZnO (among other materials), revealed the precursor concentration for the best OSC device performance to be around 0.1 M.56 To this respect, we focus on the change of tin (SnCl2.2H2O) precursor concentration in absolute ethanol solvent in the range of 0.05 to 0.3 M, and studied on structural, optical, electronic, surface defect-related luminescence, surface morphology properties of the resulting SnO2 CBL as well as the relationship between precursor concentration of SnCl2.2H2O and its iOSC device performance. We successfully synthesized small SnO2 NPs (~3-5 nm), which exhibited excellent optical properties and morphologies. P3HT:PC60BM based iOSCs using SnO2 NPs as CBL showed the best PCE of 2.9% for 0.1 M precursor concentration. Moreover, our iOSCs revealed excellent long-term stabilities, with power conversion efficiencies maintained at 95% after storage for 10 weeks in ambient air. We believe that these results are comparable and even superior to the SnO2 prepared by various methods like sol-gel,44 RF magnetron sputtering, 41 and chemical vapor deposition.48 Furthermore, this facile and lowtemperature solution-processed SnO2 NPs can be applied for a low-cost, high throughput processing on flexible substrates for optoelectronic devices. 5 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 30

EXPERIMENTAL SECTION Synthesis of SnO2 NPs The facile synthesis of SnO2 NPs was carried out in a round bottom flask. Initially, precursor of tin (SnCl2.2H2O) was dissolved in absolute ethanol. In a 50 mL absolute ethanol, different amounts of SnCl2.2H2O (0.564, 1.128, 2.256g, and 3.385g) were added and completely dissolved to achieve different precursor concentrations of SnO2 NPs (0.05, 0.1, 0.2, and 0.3 M). Then, the resulting mixtures were uniformly stirred at 80 oC for 8 hours to yield a homogeneous solution. After cooling down, these solutions were taken out from the oil bath and then aged at room temperature for one day. Afterward, these solutions were used to prepare SnO2 NPs as CBL via spin-coating without any further purification. The mechanism of forming SnO2 NPs during the synthesis can be expressed as follows: SnCl2.2H2O + C2H5OH → Sn(OH)2 + C2H5Cl Sn(OH)2 → SnO + H2O SnO → SnO2

(1)

(2)

(3)

Characterizations of SnO2 NPs UV−vis absorption and transmission spectra for SnO2 NPs were achieved by Shimadzu UV2550 spectrophotometer machine. Structural properties were determined by X-ray diffraction (XRD) measurements on a PANalytical X-pert diffractometer (30kV, 20mA, Cu_Kα radiation with a wavelength λ= 1.5405 Ǻ).23 Room temperature photoluminescence (PL) studies were performed on a 325 nm He–Cd laser. The binding energies in SnO2 NPs was determined using X-ray photoelectron spectroscopy (XPS) (Shimadzu group “Kratos Analytical Probe”).23,57 Ultraviolet photoelectron spectroscopy (UPS) was conducted on AXIS-ULTRA DLD spectrometer (Kratos Analytical Ltd) equipped with a He I (21.2 eV) source.18,23 Morphology 6 ACS Paragon Plus Environment

Page 7 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


and structural properties were investigated with a field-emission scanning electron microscopy (FESEM, Hitachis-4700).23,58 The elemental mapping was obtained from Energy dispersive X-ray spectroscopy (EDX) attached to FESEM. Transmission electron microscopy (TEM), JOEL JEM-2010 was used to obtain particle sizes of SnO2. Tapping mode atomic force microscopy (AFM) was performed on a Multimode-8 (Bruker, USA) with the resonance frequency of 0.996Hz. Device fabrication SnO2 NPs as CBL were coated on precleared and UV-ozone treated ITO-glass substrate via spin-coating method at a speed of 3000 rpm for 40 s. After spin-coating, CBL samples were annealed on a hot plate at a temperature of 180 oC for 1 h in air. The photoactive BHJ of P3HT:PC60BM in 1,2-dichlorobenzene (ODCB) with a concentration of 25 mg.ml-1 was then spin coated on these CBL layers at a speed of 500 rpm for 40s in the N2 filled glovebox. Then, the photoactive-coated samples were placed in closed-disk containers for ~12h to achieve gradual solvent evaporation. iOSC devices were completed by evaporating in high vacuum, a thinner layer of hole transporting MoO3 (10 nm) and high work functional metal Ag (100 nm) to function as an anode. Current density-voltage (J−V) characteristics (under 1 sun illumination and dark) of iOSCs based on SnO2 NPs were measured on a Keithley 2400 source-measure (300 W, Newport, USA),59 on iOSCs having an active area of 0.36 cm2. The external quantum efficiency (EQE) of iOSC devices were tested by using Polaronix K3100 spectrometer under a monochromatic illumination.23



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 30

RESULTS AND DISCUSSION The UV-vis absorption properties of SnO2 NPs synthesized with varying precursor concentrations (0.05, 0.1, 0.2, and 0.3 M), spin-coated on glass substrates are shown in Figure 1 (a). The spectra reveal a strong ultraviolet absorption peak at 283.6, 282.6, 281.5, and 280.2 nm, that is, at energies of 4.37, 4.39, 4.40, and 4.42 eV for the respective precursor concentrations of 0.05, 0.1, 0.2, and 0.3 M. It is clear that the absorption peaks slightly shifted towards shorter wavelength with increasing concentration. We ascribe the blueshift of absorption peaks to the quantum-size-effect by a slight decrease of SnO2 NP sizes with increasing the precursor concentrations. The observations of UV-vis analyses are in good agreement with morphological differences as revealed by TEM analysis (see Figure S4). Figure 1(b) shows the light propagating properties (transmittance) of SnO2 NPs at different concentrations. All SnO2 NPs samples show good optical transparencies with average transmittance exceeding 93% in the visible range (380-750 nm). Although the transmittance slightly decreased with the increase of precursor concentration, these values were superior to the required transmittance of the transparent conductive thin film for solar cells, which is in general over 85%.60 With better optical properties, SnO2 NPs CBL could allow more photon to the photoactive layer, resulting in higher photocurrents in iOSCs. We note that the solutions of SnO2 NPs prepared at different precursor concentrations (Figure 1b, inset) are quite transparent and exhibit only slight changes in color. The PL emission spectra of SnO2 NPs coated on glass substrates are presented in Figure 2(a). The spectra corresponding to 0.05, 0.1, 0.2, and 0.3 M precursor concentration show strong peaks at 487, 493, 485, and 484 nm (that is, at energies 2.55, 2.52, 2.56, and 2.56 eV, respectively). In Figure 3(a), since the band gap of SnO2 NPs (3.94 eV) is higher than the excitation energy (3.82 eV), therefore the band edge emission was not seen. The distinctive 8 ACS Paragon Plus Environment

Page 9 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


blue emission has been ascribed to oxygen vacancy-related defects, Sn interstitial, and Sn vacancies that occur during the synthesis.61,62 Such defects in metal oxides act as recombination centers in solar cells and reduce power conversion efficiencies.63 From the PL spectra, therefore, it is evident that the SnO2 NPs synthesized with 0.1 M retain minimum defect density compared to other precursor concentrations. The structural properties of the SnO2 NPs coated on silicon substrates and annealed at a low temperature of 180 oC were determined by XRD (see Figure S5). The decreased diffraction intensity at 2θ = 33.05o with increasing precursor concentration is suggestive of increased coverage of SnO2 NPs. Also, the observed amorphous nature (from XRD) of SnO2 NPs is due to a low temperature annealing.40-42 The surface composition and chemical states of amorphous SnO2 NPs derived from 0.1 M precursor concentrations were investigated using XPS. As shown in Figure 2(b), no obvious impurity peaks can be detected in the XPS survey spectrum. The spectrum profile of Sn region in Figure 2(c) shows two prominent peaks at 486.55, and 495.0 eV that can be assigned to the binding Sn (3d5/2), and Sn (3d3/2) of SnOx, respectively. The Sn 3d5/2 peak of SnOx could be deconvoluted into three sub-peaks at 486.58, 485.88 and 484.38 eV representing constituents viz., Sn+4, Sn+2 and Sn0, respectively. Further, in SnOx, the peak related to O-Sn4+ and O-Sn2+ is known to be located at binding energies 530.37 and 529.77 eV, respectively.62,64,65 The appearance of O 1s peaks at 530.35 eV in our studies (Figure 2(d)) ascribed to binding energy of O-Sn4+ (530.37 eV) and the absence of peak related to O-Sn2+ bonding states provides clear evidence that the as-synthesized NPs are stoichiometrically pure SnO2 NPs. The morphology of CBL in iOSCs is crucial to its device performance. The surface of SnO2 NPs was investigated using FESEM and TEM. Figure S1(b-e) show the FESEM images 9 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 30

of SnO2 NPs for precursor concentrations of 0.05, 0.1, 0.2, and 0.3 M. Obviously, the SnO2 NPs stemmed from 0.05 M concentration is ultra-thin in nature and is composed of few voids, suggesting incomplete coverage of SnO2 NPs on the surface of ITO. With the increase of precursor concentration to 0.1 M, a compact and homogenous film of the SnO2 NPs was clearly seen. However, further precursor concentration increments to 0.2 and 0.3 M led to an overly dense layer of SnO2 NPs. The FESEM images of bare ITO-glass and the SnO2 NPs layer with higher precursor concentrations (0.4, 0.5, and 0.6 M) are also presented in Figure S1(f-h). Higher precursor concentrations gave rise to the formation of cracks on the surfaces of SnO2 NPs. In Figure S2(d-f), the EDX spectra and elemental mapping analysis of SnO2 NPs for the 0.1 M precursor concentration show the compositional distribution of Sn and O on the surface of SnO2 NPs. The band gap Eg of SnO2 NPs was computed based on its optical absorption profile using the Tauc plot calculation: (αhν)2 = A (hν − Eg), in which, A is a constant, hν is the photon energy, and α is the absorption coefficient. The Eg was then obtained with the extrapolation linear of the (αhν)2 and the hν-axis. Figure 3(a) shows the plot (αhν)2 vs. hν of the SnO2 NPs derived from 0.1 M precursor concentration. The Eg of SnO2 NPs was found out ~3.94 eV, this value is in good agreement with previously published results that the Eg of SnO2 NPs varying from 3.4 to 4.6 eV.66 The UPS measurements were carried out to verify energy level alignment of SnO2 NPs as CBL. The secondary electron cut-off edge (Ecutoff) of SnO2 NPs is found ~17.0 eV (Figure 3b) while its valence region (Eonset) is ~3.73 eV (Figure 3c). The valence-band-maximum (VBM) of SnO2 NPs was then computed by the formula below:67 VBM = hν – (Ecutoff – Eonset), where hν is the incident photon energy of He(I) source, which has a value of 21.22 eV. Using this equation, the valence band of the SnO2 NPs was found to be 7.95 eV. From the Eg and 10 ACS Paragon Plus Environment

Page 11 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


VBM, the conduction-band-minimum (CBM) of SnO2 NPs was 4.01 eV, this is considered as a good value that next to the lowest-unoccupied-molecular-orbital (LUMO) of PC60BM (~3.7 eV). The reduction in electronic energy can help electrons easily move to the cathode from PC60BM through SnO2 CBL in the OSCs (see Figure 4(b)). To investigate the capabilities of SnO2 NPs as CBL, iOSCs with a configuration of ITOglass/SnO2 NPs/P3HT:PC60BM/MoO3/Ag was fabricated. Figure 4(a,b) show the iOSC device architecture based on SnO2 CBL along with energy levels of each component. To optimize the best conditions for CBL, we first study the effect of annealing temperature (from 130~300 oC for 1h) for SnO2 NPs CBL on its device performance. The J-V characteristics of the iOSCs using SnO2 NPs derived from 0.1 M precursor concentration at different annealing temperatures are given in Figure 5(a). The corresponding photovoltaic characteristics are given in Table S1. Obviously, the iOSCs annealed at 180 oC show the best performance with high Voc, good fill factors, and high PCEs. Note that the annealing temperature can directly affect morphology as well as properties of SnO2 CBL, therefore is considered as the most crucial parameters to achieve a good performance of iOSC device. Taking 180 oC as the optimized annealing temperature, we then fabricated iOSCs by using SnO2 NPs as CBL with different precursor concentrations. Figure 5(b) shows the J-V characteristics of iOSCs derived from different precursor concentrations, and their detail photovoltaic parameters are recorded in Table 1. The J-V curve of the devices fabricated with 0.05 M precursor concentration yielded a strong S-shaped, resulting in a low PCE of 1.52%. The poor device performance for a low precursor concentration can be attributed to incomplete coverage of SnO2 NPs on the ITO surface. However, with the increase of the precursor concentration to 0.1 M, the S-shaped disappeared and resulted in the best PCE of 2.9% (Voc = 0.58 V, Jsc = 9.29 mA cm-2, and FF = 0.54). Further precursor concentration increments to 0.2 and 0.3 M resulted in a lower PCE of 2.06%. The impressive device 11 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

performances at a 0.1 M precursor concentration were expected from the low defect concentration in SnO2 NPs as evidenced from PL studies (Figure 2a) and the good surface morphology (Figure S1(c)) in comparison with other precursor concentrations. Figure 5(c) and (d) show the dark current and EQE. The iOSC device using SnO2 NPs with 0.1 M precursor concentration shows the lowest leakage current and the highest EQE of 65.90 %. The lower leakage current should result in the increased Jsc owing to higher electron collection to the cathode, while the higher EQE results in a higher conversion ratio of phototo-electron in the iOSC devices. To verify the effect of SnO2 CBLs with different precursor concentration on electron transporting properties in the iOSCs, the electron only device was fabricated with a configuration of ITO-glass/SnO2/P3HT:PC60BM/LiF/Al for measuring electron mobility (µe). Figure 6 shows the J-V characteristic curves in the dark of these electron only devices. Based on the space-charge-limited-current (SCLC) model,68-70 and the linear fitting for J-V electron only curves; the µe was computed using the Mott-Gurney expression:68,70 9 V = ( ) 8 L Where JSCLC is the current density, V is stand for the applied voltage potential to the electrononly devices and L is the thickness of the photoactive layer (P3HT:PC60BM), ɛr is the relativedielectric-constant of the photoactive layer, most organic semiconductors have dielectric constants in the range 3.0ɛ0 < ɛ < 4.0ɛ0.71 Typically a relative dielectric constant of 3.5 is assumed,70-72 and ɛ0 is the permittivity of free space which is physical constant with a value of 8.85 x 10-12 C V-1 s-1. The calculated µe values are recorded in Table S3. The µe in the SnO2 based device derived from 0.1 M precursor concentration is 1.77 x 10-4 cm2 V-1 s-1, is higher than 1.13 x 10-4, 9.31 x 10-5, and 9.27 x 10-5 cm2 V-1 s-1, corresponding to 0.05, 0.2, and 0.3 M precursor concentration. It can be anticipated that the iOSC devices based on SnO2 derived from 0.1 M precursor concentration should exhibit a higher electron charge-transporting 12 ACS Paragon Plus Environment

Page 12 of 30

Page 13 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


abilities upon other precursor concentrations, as the reason that elucidates the increase of Jsc. To investigate the morphology of CBLs as well as photoactive layers, AFM were conducted on our samples. Figure 7 presents the 2D AFM of SnO2 NPs coated on ITO-glass substrates and the P3HT:PC60BM photoactive coated on the SnO2 NPs CBL. The surface roughness that is represented by root-mean-square (RMS) of SnO2 CBLs for 0.05, 0.1, 0.2, and 0.3 M precursor concentration are shown in Figure 7(a), (b), (c), and (d) are 1.27, 0.898, 0.646, and 0.62 nm, respectively. Obviously, the RMS surface roughness of SnO2 NPs is drastically decreased when increasing precursor concentration from 0.05 to 0.1 M, but it does not vary much for further increments (from 0.1 to 0.3 M). However, all these RMS values of SnO2 NPs coated on ITO-glasses are lower than the RMS surface roughness value for bare ITO-glass (1.54 nm) (see Figure S7), indicating that an excellent CBL morphology can be achieved with the SnO2 NPs. Figure 7(e-h) show AFM images of P3HT:PC60BM active layers coated on SnO2 CBL/ITO-glasses. One can observe that the P3HT:PC60BM active layer is quite sensitive to the CBL/ITO-glass surface topography.56 The lowest RMS value (8.73 nm) was observed in Figure 7(f) for 0.1 M precursor concentration. In addition to RMS values, the AFM images of P3HT and PC60BM active layer coated on SnO2 CBL prepared with 0.1 M precursor concentration clearly demonstrates the morphology with good phase separation along with proper domain size, which strongly affects charge separation for effective p/n junctions. Apart from high efficiency, long-term device stability is another critical requirement for possible commercialization of OSCs. Hence, we have studied the stability of iOSC devices with SnO2 NPs as CBL for over a period of 10 weeks. Figure 8 shows the photovoltaic performance of iOSC devices derived from 0.1 M precursor concentration recorded with storing time (weeks) in air conditions. Our record results (see Table S4) indicate that the solar 13 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 30

cell device performance tested at a regular interval until 10 weeks remains almost unchanged upon a time. CONCLUSIONS We have demonstrated that facile low-temperature solution-processed SnO2 NPs is an excellent CBL for iOSCs. By dissolving SnCl2.2H2O as a precursor in absolute ethanol, SnO2 NPs with small sizes around 3-5 nm were achieved. Our SnO2 NPs films possessed excellent properties such as high transparencies, good morphologies, and a suitable conduction band energy level of -4.01 eV i.e., very close to the LUMO energy level of PC60BM acceptor. The iOSCs using SnO2 NPs as CBL with a precursor concentration of 0.1 M based P3HT:PC60BM achieved the best PCE of 2.9%. Moreover, iOSC devices based on SnO2 NPs as CBL revealed excellent long-term stability with PCE maintained at 95% after stored for 10 weeks in ambient air. We believe that SnO2 NPs prepared by this facile low-temperature solution process can also be used as an ideal CBL for organic as well as colloidal quantum dot light emitting diodes, or other optoelectronic devices on flexible substrates.


Page 15 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ASSOCIATED CONTENT Supporting Information FESEM images, EDX spectra and elemental mapping, TEM images, XRD, AFM, and iOSC devices performances as noted in the main text of the paper. AUTHOR INFORMATION Corresponding Authors Prof. Soo-Hyoung Lee, Email: [email protected], Tel: +82 63-270-2435, Fax: +82 63-2702306. Prof. In-Hwan Lee, Email: [email protected], Tel: +82 63-270-2292, Fax: +82 63-2702305. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This research was supported by National Research Foundation of Korea (NRF) funded by Ministry of Science, ICT & Future Planning (2015042417). This research was also supported by Basic Science Research Program through NRF funded by Ministry of Science, ICT & Future Planning (2015R1A2A2A01004404).



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

REFERENCES (1)

(2) (3) (4)

(5) (6)

(7) (8)

(9)

(10)

(11)

(12) (13) (14)

(15)

(16)

Kaltenbrunner, M.; White, M. S.; Głowacki, E. D.; Sekitani, T.; Someya, T.; Sariciftci, N. S.; Bauer, S. Ultrathin and Lightweight Organic Solar Cells with High Flexibility. Nat. Commun. 2012, 3, 770. Forrest, S. R. The Path to Ubiquitous and Low-cost Organic Electronic Appliances on Plastic. Nature 2004, 428, 911–918. Søndergaard, R.; Hösel, M.; Angmo, D.; Larsen-Olsen, T. T.; Krebs, F. C. Roll-to-Roll Fabrication of Polymer Solar Cells. Mater. Today 2012, 15, 36–49. Krebs, F. C.; Gevorgyan, S. A.; Alstrup, J. A Roll-to-Roll Process to Flexible Polymer Solar Cells: Model Studies, Manufacture and Operational Stability Studies. J. Mater. Chem. 2009, 19, 5442–5451. Krebs, F. C.; Tromholt, T.; Jorgenson, M. Upscaling of Polymer Solar Cell Fabrication Using Full Roll-to-Roll Processing. Nanoscale 2010, 2, 873–886. Liang, Y.; Xu, Z.; Xia, J.; Tsai, S.-T.; Wu, Y.; Li, G.; Ray, C.; Yu, L. For the Bright Future−Bulk Heterojunction Polymer Solar Cells with Power Conversion Efficiency of 7.4%. Adv. Mater. 2010, 22, E135−E138. Zhou, H.; Yang, L.; You, W. Rational Design of High Performance Conjugated Polymers for Organic Solar Cells. Macromolecules 2012, 45, 607−632. Kawano, K.; Pacios, R.; Poplavskyy, D.; Nelson, J.; Bradley, D. D. C.; Durrant, J. R. Degradation of Organic Solar Cells due to Air Exposure. Sol. Energy. Mater. Sol. Cells 2006, 90, 3520–3530. Wong, K.; Yip, H.; Luo, Y.; Wong, K.; Lau, W.; Low, K.; Chow, H.; Gao, Z.; Yeung, W.; Chang, C. Blocking Reactions Between Indium-tin Oxide and Poly (3, 4-ethylene dioxythiophene): Poly (styrene sulphonate) with a Self-Assembly Monolayer. Appl. Phys. Lett. 2002, 80, 2788−2790. De Jong, M. P.; Van IJzendoorn, L. J.; De Voigt, M. J. A. Stability of the Interface between Indium-Tin-Oxide and Poly (3, 4-ethylene dioxythiophene): Poly (styrene sulphonate) in Polymer Light-Emitting Diodes. Appl. Phys. Lett. 2000, 77, 2255–2257. Cao, H.; He, W.; Mao, Y.; Lin, X.; Ishikawa, K.; Dickerson, J. H.; Hess, W. P. Recent Progress in Degradation and Stabilization of Organic Solar Cells. J. Power Sources 2014, 264, 168−183. Jørgensen, M.; Norrman, K.; Krebs, F. C. Stability/Degradation of Polymer Solar Cells. Sol. Energy. Mater. Sol. Cells 2008, 92, 686–714. Grossiord, N.; Kroon, J. M.; Andriessen, R.; Blom, P. W. M. Degradation Mechanisms in Organic Photovoltaic Devices. Org. Electron. 2012, 13, 432–456. White, M. S.; Olson, D. C.; Shaheen, S. E.; Kopidakis, N.; Ginley, D. S. Inverted BulkHeterojunction Organic Photovoltaic Device Using a Solution-Derived ZnO Underlayer. Appl. Phys. Lett. 2006, 89, 143517. Zhang, F.; Xu, X.; Tang, W.; Zhang, J.; Zhuo, Z.; Wang, J.; Wang, J.; Xu, Z.; Wang, Y. Recent Development of the Inverted Configuration Organic Solar Cells. Sol. Energy. Mater. Sol. Cells 2011, 95, 1785–1799. Kyaw, a. K. K.; Sun, X. W.; Jiang, C. Y.; Lo, G. Q.; Zhao, D. W.; Kwong, D. L. An Inverted Organic Solar Cell Employing a Sol-Gel Derived ZnO Electron Selective Layer 16 ACS Paragon Plus Environment

Page 16 of 30

Page 17 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


and Thermal Evaporated MoO3 Hole Selective Layer. Appl. Phys. Lett. 2008, 93, 221107. (17) Yin, Z.; Zheng, Q.; Chen, S.C.; Cai, D.; Ma, Y. Controllable ZnMgO Electron‐ Transporting Layers for Long‐Term Stable Organic Solar Cells with 8.06% Efficiency after One‐Year Storage. Adv. Energy Mater. 2015, 1501493. (18) Ambade, R. B.; Ambade, S. B.; Mane, R. S.; Lee, S.-H. Interfacial Engineering Importance of Bilayered ZnO Cathode Buffer on the Photovoltaic Performance of Inverted Organic Solar Cells. ACS Appl. Mater. Interfaces 2015, 7, 7951–7960. (19) Ma, H.; Yip, H. L.; Huang, F.; Jen, A. K. Y. Interface Engineering for Organic Electronics. Adv. Funct. Mater. 2010, 20, 1371–1388. (20) Po, R.; Carbonera, C.; Bernardi, A.; Camaioni, N. The Role of Buffer Layers in Polymer Solar Cells. Energy Environ. Sci. 2011, 4, 285–310. (21) Chen, S.; Manders, J. R.; Tsang, S.-W.; So, F. Metal Oxides for Interface Engineering in Polymer Solar Cells. J. Mater. Chem. 2012, 22, 24202–24212. (22) Yang, L.; Xu, H.; Tian, H.; Yin, S.; Zhang, F. Effect of Cathode Buffer Layer on the Stability of Polymer Bulk Heterojunction Solar Cells. Sol. Energy Mater. Sol. Cells 2010, 94, 1831–1834. (23) Ambade, S. B.; Ambade, R. B.; Eom, S.-H.; Baek, M.-J.; Bagde, S. S.; Mane, R. S.; Lee, S.-H. Co-Functionalized Organic/Inorganic Hybrid ZnO Nanorods as Electron Transporting Layers for Inverted Organic Solar Cells. Nanoscale 2016, 8, 5024–5036. (24) Bhande, S. S.; Shinde, D. V.; Shaikh, S. F.; Ambade, S. B.; Ambade, R. B.; Mane, R. S.; Inamuddin; Naushad, M.; Han, S.-H. Low-Temperature Solution-Processed Zn-Doped SnO2 Photoanodes: Enhancements in Charge Collection Efficiency and Mobility. RSC Adv. 2014, 4, 20527–20530. (25) Lee, E.-J.; Heo, S.-W.; Han, Y.-W.; Moon, D.-K. An Organic-Inorganic Hybrid Interlayer for Improved Electron Extraction in Inverted Polymer Solar Cells. J. Mater. Chem. C 2016, 4, 2463–2469. (26) Jin, W.-Y.; Ginting, R.-T.; Jin, S.-H.; Kang, J.-W. Highly Stable and Efficient Inverted Organic Solar Cells based on Low-Temperature Solution-Processed PEIE and ZnO Bilayers. J. Mater. Chem. A 2016, 4, 3784–3791. (27) Chang, Y.-M.; Leu, C.-Y. Conjugated Polyelectrolyte and Zinc Oxide Stacked Structure as an Interlayer in Highly Efficient and Stable Organic Photovoltaic Cells. J. Mater. Chem. A 2013, 1, 6446–6451. (28) Yuan, T.; Zhu, X.; Zhou, L.; Zhang, J.; Tu, G. Efficient Inverted Polymer Solar Cells Based on Conjugated Polyelectrolyte and Zinc Oxide Modified ITO Electrode. Appl. Phys. Lett. 2015, 106, 083302. (29) Zilberberg, K.; Meyer, J.; Riedl, T. Solution Processed Metal-Oxides for Organic Electronic Devices. J. Mater. Chem. C 2013, 1, 4796–4815. (30) Ambade, S. B.; Ambade, R. B.; Lee, W.; Mane, R. S.; Yoon, S. C.; Lee, S.-H. Development of Highly Transparent Seedless ZnO Nanorods Engineered for Inverted Polymer Solar Cells. Nanoscale 2014, 6, 12130–12141. (31) Liang, Z.; Zhang, Q.; Jiang, L.; Cao, G. ZnO Cathode Buffer Layers for Inverted Polymer Solar Cells. Energy Environ. Sci. 2015, 8, 3442–3476. (32) Waldauf, C.; Morana, M.; Denk, P.; Schilinsky, P.; Coakley, K.; Choulis, S. A.; Brabec, C. J. Highly Efficient Inverted Organic Photovoltaics Using Solution Based Titanium Oxide 17 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 30

as Electron Selective Contact. Appl. Phys. Lett. 2006, 89, 233517. (33) Schmidt, H.; Zilberberg, K.; Schmale, S.; Flügge, H.; Riedl, T.; Kowalsky, W. Transient Characteristics of Inverted Polymer Solar Cells Using Titanium Oxide Interlayers. Appl. Phys. Lett. 2010, 96, 243305. (34) Cowan, S. R.; Schulz, P.; Giordano, A. J.; Garcia, A.; MacLeod, B. A.; Marder, S. R.; Kahn, A.; Ginley, D. S.; Ratcliff, E. L.; Olson, D. C. Chemically Controlled Reversible and Irreversible Extraction Barriers Via Stable Interface Modification of Zinc Oxide Electron Collection Layer in Polycarbazole‐based Organic Solar Cells. Adv. Funct. Mater. 2014, 24, 4671–4680. (35) Kim, J.; Kim, G.; Choi, Y.; Lee, J.; Heum Park, S.; Lee, K. Light Soaking Issue in Polymer Solar Cells: Photoinduced Energy Level Alignment at the Sol-Gel Processed Metal Oxide and Indium Tin Oxide Interface. J. Appl. Phys. 2012, 111, 114511-1– 114511-9. (36) Steim, R.; Choulis, S. a.; Schilinsky, P.; Brabec, C. J. Interface Modification for Highly Efficient Organic Photovoltaics. Appl. Phys. Lett. 2008, 92, 093303. (37) Trost, S.; Zilberberg, K.; Behrendt, A.; Polywka, A.; Görrn, P.; Reckers, P.; Maibach, J.; Mayer, T.; Riedl, T. Overcoming the “Light‐Soaking” Issue in Inverted Organic Solar Cells by the Use of Al: ZnO Electron Extraction Layers. Adv. Energy Mater. 2013, 3, 1437–1444. (38) Sista, S.; Park, M.-H.; Hong, Z.; Wu, Y.; Hou, J.; Kwan, W. L.; Li, G.; Yang, Y. Highly Efficient Tandem Polymer Photovoltaic Cells. Adv. Mater. 2010, 22, 380–383. (39) Gilot, J.; Wienk, M. M.; Janssen, R. A. J. Double and Triple Junction Polymer Solar Cells Processed from Solution. Appl. Phys. Lett. 2007, 90, 143512. (40) Trost, S.; Behrendt, A.; Becker, T.; Polywka, A.; Görrn, P.; Riedl, T. Tin Oxide (SnOx) as Universal “Light‐Soaking” Free Electron Extraction Material for Organic Solar Cells. Adv. Energy Mater. 2015, 5, 1500277. (41) Yu, S.; Yang, W., Li, L.; Zhang, W. Improved Chemical Stability of ITO Transparent Anodes with a SnO2 Buffer Layer for Organic Solar Cells. Sol. Energy Mater. Sol. Cells 2016, 144, 652–656. (42) Lee, H.; Kang, C. M.; Park, M.; Kwak, J.; Lee, C. Improved Efficiency of Inverted Organic Light-Emitting Diodes using Tin Dioxide Nanoparticles as an Electron Injection Layer. ACS Appl. Mater. Interfaces 2013, 5, 1977–1981. (43) Lu, L.P.; Finlayson, C.E.; Friend, R.H. A Study of Tin Oxide as an Election Injection Layer in Hybrid Polymer Light-Emitting Diodes. Semicond. Sci. Technol. 2014, 29, 125002. (44) Bob, B.; Song, T.-B.; Chen, C.-C.; Xu, Z.; Yang, Y. Nanoscale Dispersions of Gelled SnO2: Material Properties and Device Applications. Chem. Mater. 2013, 25, 4725–4730. (45) Trost, S.; Zilberberg, K.; Behrendt, A.; Riedl, T. Room-Temperature Solution Processed SnOx as an Electron Extraction Layer for Inverted Organic Solar Cells with Superior Thermal Stability. J. Mater. Chem. 2012, 22, 16224–16229. (46) Chen, Z.; Pan, D.; Li, Z.; Jiao, Z.; Wu, M.; Shek, C.-H.; Wu, C. L.; Lai, J. K. Recent Advances in Tin Dioxide Materials: Some Developments in Thin Films, Nanowires, and Nanorods. Chem. Rev. 2014, 114, 7442–7486. (47) Bhande, S. S.; Ambade, R. B.; Shinde, D. V.; Ambade, S. B.; Patil, S. A.; Naushad, Mu.; 18 ACS Paragon Plus Environment

Page 19 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(48) (49)

(50) (51)

(52)

(53) (54)

(55)

(56)

(57)

(58)

(59)

(60)

(61)

Mane, R. S.; Alothman, Z. A.; Lee, S.-H.; Han, S.-H. Improved Photoelectrochemical Cell Performance of Tin Oxide with Functionalized Multiwalled Carbon Nanotubes– Cadmium Selenide Sensitizer. ACS Appl. Mater. Interfaces 2015, 7, 25094–25104. Hatem, D.; Belkaid, M.S. SnO2 Thin Films Prepared by APCVD for Organic Solar Cells Application. Adv. Mater. Res. 2013, 685, 166–173. Song, J.; Zheng, E.; Bian, J.; Wang, X.-F.; Tian, W.; Sanehira, Y.; Miyasaka, T. LowTemperature SnO2-based Electron Selective Contact for Efficient and Stable Perovskite Solar Cells. J. Mater. Chem. A 2015, 3, 10837–10844. Aziz, M.; Abbas, S. S.; Baharom, W. R. Size-Controlled Synthesis of SnO2 Nanoparticles by Sol−Gel Method. Mater. Lett. 2013, 91, 31–34. Ahn, H.-J.; Choi, H.-C.; Park, K.-W.; Kim, S.-B.; Sung, Y.-E. Investigation of the Structural and Electrochemical Properties of Size Controlled SnO2 Nanoparticles. J. Phys. Chem. B 2004, 108, 9815–9820. Gu, F.; Wang, S. F.; Lu, M. K.; Zhou, G. J.; Xu, D.; Yuan, D. R. Photoluminescence Properties of SnO2 Nanoparticles Synthesized by Sol-Gel Method. J. Phys. Chem. B 2004, 108, 8119–8123. Byrappa, K.; Adschiri, T. Hydrothermal Technology for Nanotechnology. Prog. Cryst. Growth Charact. Mater. 2007, 53, 117–166. Jensen, K. M. Ø.; Christensen, M.; Juhás, P.; Tyrsted, C.; Bøjesen, E. D.; Lock, N.; Billinge, S. J. L.; Iversen, B. B. Revealing the Mechanisms behind SnO2 Nanoparticle Formation and Growth during Hydrothermal Synthesis: An In Situ Total Scattering Study. J. Am. Chem. Soc. 2012, 134, 6785–6792. Ke, W.; Fang, G.; Liu, Q.; Xiong, L.; Qin, P.; Tao, H.; Wang, J.; Lei, H.; Li, B.; Wan, J.; Yang, G.; Yan, Y. Low-Temperature Solution Processed Tin Oxide as an Alternative Electron Transporting Layer for Efficient Perovskite Solar Cells. J. Am. Chem. Soc. 2015, 137, 6730–6733. Liang, Z. Q.; Zhang, Q. F.; Wiranwetchayan, O.; Xi, J. T.; Yang, Z.; Park, K.; Li, C. D.; Cao, G. Z. Effects of the Morphology of a ZnO Buffer Layer on the Photovoltaic Performance of Inverted Polymer Solar Cells. Adv. Funct. Mater. 2012, 22, 2194–2201. Ambade, R. B.; Ambade, S. B.; Salunkhe, R. R.; Malgras, V.; Jin, S. H.; Yamauchi, Y.; Lee, S. H. Flexible-Wire Shaped All-Solid-State Supercapacitors Based on Facile Electropolymerization of Polythiophene with Ultra-High Energy Density. J. Mater. Chem. A 2016, 4, 7406–7415. Ambade, R. B.; Ambade, S. B.; Shrestha, N. K.; Nah, Y. C.; Han, S. H.; Lee, W.; Lee, S. H. Polythiophene Infiltrated TiO2 Nanotubes as High-Performance Supercapacitor Electrodes. Chem. Commun. 2013, 49, 2308–2310. Jang, L. W.; Park, H.; Lee, S. H.; Polyakov, A. Y.; Khan, R.; Yang, J. K.; Lee, I. H. Device Performance of Inverted Polymer Solar Cells with AgSiO2 Nanoparticles in Active Layer. Opt. Express 2015, 23, 211–218. Lee, S. C.; Lee, J. H.; Oh, T. S.; Kim, Y. H. Fabrication of Tin Oxide Film by Sol−Gel Method for Photovoltaic Solar Cell System. Sol. Energy Mater. Sol. Cells 2003, 75, 481– 487. Bonu, V.; Das, A.; Amirthapandian, S.; Dhara, S.; Tyagi, A.K. Photoluminescence of Oxygen Vacancies and Hydroxyl Group Surface Functionalized SnO2 Nanoparticles. 19 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 30

Phys. Chem. Chem. Phys. 2015, 17, 9794–9801. (62) Her, Y. C.; Wu, J. Y.; Lin, Y. R.; Tsai, S. Y. Low-Temperature Growth and Blue Luminescence of SnO2 Nanoblades. Appl. Phys. Lett. 2006, 89 (43115), 1–3. (63) Lai, T.-H.; Tsang, S.-W.; Manders, J. R.; Chen, S.; So, F. Properties of Interlayer for Organic Photovoltaics. Mater. Today 2013, 16, 424–432. (64) Szuber, J.; Czempik, G.; Larciprete, R.; Koziej, D.; Adamowicz, B. XPS Study of the LCVD Deposited SnO2 Thin Films Exposed to Oxygen and Hydrogen. Thin Solid Films 2001, 391, 198–203. (65) Barreca, D.; Garon, S.; Tondello, E.; Zanella, P. SnO2 Nanocrystalline Thin Films by XPS. Surf. Sci. Spectra 2000, 7, 81–85. (66) Akgul, F. A.; Gumus, C.; Er, A. O.; Farha, A. H.; Akhul, G.; Ufuktepe, Y.; Liu, Z. Structural and Electronic Properties of SnO2. J. Alloys Compd. 2013, 579, 50–56. (67) Meyer, J.; Hamwi, S.; Kroger, M.; Kowalsky, W.; Riedl, T.; Kahn, A. Transition Metal Oxides for Organic Electronics: Energetics, Device Physics and Applications. Adv. Mater. 2012, 24, 5408–5427. (68) Mihailetchi, V. D.; Wildeman, J.; Blom, P. W. M. Space-Charge Limited Photocurrent. Phys. Rev. Lett. 2005, 94, 126602. (69) Azimi, H.; Senes, A.; Scharber, M. C.; Hingerl, K.; Brabec, C. J. Charge Transport and Recombination in Low‐Bandgap Bulk Heterojunction Solar Cell Using Bis‐adduct Fullerene. Adv. Energy Mater. 2011, 1, 1162–1168. (70) Chen, X.; Liu, Q.; Wu, Q.; Du, P.; Zhu, J.; Dai, S.; Yang, S. Incorporating Graphitic Carbon Nitride (g‐C3N4) Quantum Dots into Bulk‐Heterojunction Polymer Solar Cells Leads to Efficiency Enhancement. Adv. Funct. Mater. 2016, 26, 1719–1728. (71) Blakesley, J. C.; Castro, F. A.; Kylberg, W.; Dibb, G. F.; Arantes, C.; Valaski, R.; ...; Kim, J. S. Towards Reliable Charge-Mobility Benchmark Measurements for Organic Semiconductors. Org. Electron. 2014, 15, 1263–1272. (72) Zhen, J.; Liu, Q.; Chen, X.; Li, D.; Qiao, Q.; Lu, Y.; Yang, S. An EthanolamineFunctionalized Fullerene as an Efficient Electron Transport Layer for High-Efficiency Inverted Polymer Solar Cells. J. Mater. Chem. A 2016, 4, 8072–8079.


Page 21 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. (a) UV-vis absorption, (b) optical transmission spectra of SnO2 NPs coated on glass substrates.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 30

Figure 2. (a) Room temperature PL spectra of SnO2 NPs coated on glass substrates. XPS spectra: (b) survey, (c) Sn 3d, and (d) O 1s.


Page 23 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. (αhν)2 vs. photon energy (a), and UPS spectra for secondary electron cutoff (b), valence region (c) of SnO2 NPs.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 30

Figure 4. (a) Schematic device structure and (b) energy level diagram of the inverted P3HT:PC60BM based OSCs using SnO2 NPs as cathode buffer layer.


Page 25 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. Device performance of the iOSCs using SnO2 NPs as cathode buffer layer: (a) J-V characteristics of 0.1 M precursor concentration at different annealing temperatures, (b) J-V characteristics under AM 1.5 G irradiation (100 mW cm-2), (c) dark-current for different precursor concentrations, and (d) EQE.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 30

Figure 6. (a) J-V and (b) J0.5-V characteristics in the dark of the electron-only devices based on different precursor concentrations of SnO2 CBL.


Page 27 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 7. AFM images of SnO2 NPs coated on ITO-glasses and of P3HT:PC60BM coated on corresponding SnO2 NPs: (a,e) 0.05, (b,f) 0.1, (c,g) 0.2, and (d,h) 0.3 M precursor concentration, respectively.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 30

Figure 8. Stability of the inverted P3HT:PC60BM OSC devices using SnO2 NPs as cathode buffer layer for 0.1 M precursor concentration: (a) PCE, (b) Jsc, (c) Voc, and (d) FF.


Page 29 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1. Photovoltaic performance of iOSCs using SnO2 NPs as cathode buffer layer

Cathode buffer layer

Jsc (mA/cm2)

Voc (V)

FF (%)

PCE (%)

EQE (%)

SnO2 NPs (0.05 M)

8.55

0.53

33.81

1.52

61.59

SnO2 NPs (0.1 M)

9.29

0.58

53.82

2.89

65.90

SnO2 NPs (0.2 M)

7.65

0.57

47.04

2.06

64.20

SnO2 NPs (0.3 M)

8.07

0.58

44.01

2.06

63.50



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 30

Table of Contents (TOC) Graphic

Low-Temperature Solution-Processed SnO2 Nanoparticles as Cathode Buffer Layer for Inverted Organic Solar Cells Van-Huong Tran,†,‡ Rohan B. Ambade, ‡ Swapnil B. Ambade, ‡ Soo-Hyoung Lee,*,‡ and InHwan Lee*,†


Nanoparticles as a Cathode Buffer Layer for ... - ACS Publications

Recommend Documents