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Effect of Selective Contacts on Thermal Stability of Perovskite Solar Cell Xing Zhao, Hui-Seon Kim, Ja-Young Seo, and Nam-Gyu Park ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15673 • Publication Date (Web): 10 Feb 2017 Downloaded from http://pubs.acs.org on February 11, 2017

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ACS Applied Materials & Interfaces

Effect of Selective Contacts on Thermal Stability of Perovskite Solar Cell Xing Zhao,1† Hui-Seon Kim,1,2† Ja-Young Seo,1 Nam-Gyu Park1*

1

School of Chemical Engineering, Sungkyunkwan University, Suwon 440-746, Korea

2

Laboratory of Photomolecular Science, Institute of Chemical Sciences and Engineering, School of

Basic Sciences, Ecole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland.

ABSTRACT Thermal stability of CH3NH3PbI3 (MAPbI3)-based perovskite solar cells was investigated for the normal structure including mesoporous TiO2 layer and spiro-MeOTAD and the inverted structure with PCBM and NiO. MAPbI3 was found to be intrinsically stable from 85 oC up to 120 oC in the absence of moisture. However, fast degradation was observed for the encapsulated device including spiro-MeOTAD upon thermal stress at 85 oC. Photoluminescence (PL) intensity and time constant for charge separation increased with thermal exposure time, which is indicative of inhibition of charge separation from MAPbI3 into spiro-MeOTAD. A full recovery of photovoltaic performance was observed for the 85 oC-aged device after renewal with fresh spiro-MeOTAD, which clearly indicates that thermal instability of the normal structured device is mainly due to spiro-MeOTAD and MAPbI3 is proved to be thermally stable. The spiro-MeOTAD with additives was crystallized at 85 oC due to low glass transition temperature and hole mobility was significantly deteriorated, which was responsible for thermal instability. Thermal stability was significantly improved for the inverted structure with the NiO hole transporting layer, where the power conversion efficiency (PCE) maintained 74% of its initial PCE of 14.71% after 80th thermal cycle (one cycle: heating at 85 oC for 2 h and cooling at 25 oC for 2 h). This work implies that thermal stability of perovskite solar cell depends on selective contacts.

KEYWORDS: perovskite; solar cell; thermal-stability; glass transition temperature; selective contacts 1 ACS Paragon Plus Environment


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1. INTROUDCTION The first report on solid-state perovskite solar cells (PSCs) with an efficiency of 9.7% as well as 500 h long-term stability,1 following two pioneering reports on liquid junction PSCs in 2009 and 2011,2,3 triggered and casted light on perovskite photovoltaics. In the first solid-state PSC, methylammonium lead iodide perovskite (MAPbI3, MA=CH3NH3+) was deposited on the TiO2 surface in the form of nanodots. Long-term stability was observed for the device even without encapsulation since

perovskite

nanodots

were

fully

wrapped

with

2,2’,7,7’-tetrakis

(N,N-di-p-methoxyphenylamine)-9,9’-spirobifluorene (spiro-MeOTAD) hole transporting material (HTM). The great potential of the PSCs motivates the interests of researchers all over the world, and a fast development was achieved by improving the certified efficiency to 22.1% in 2016.4 For the commercialization of the perovskite photovoltaic devices, along with high efficiency equally important is stability.5,6 Factors affecting stability of perovskite materials and devices have been investigated. Usually, degradation of the PSCs mainly stems from three aspects: air (including O2 and H2O), UV-light and heat. Compositional engineering,7-9 advanced encapsulations10-12 and UV-filter13 technologies have been developed in order to protect the PSCs from degradation. Regardless of the influence of O2, H2O and light, it is expected to be difficult to control heat accumulation inside a device which can be conducted from outside environment or generated by solar cells assembly.14 In outdoor condition, the accumulated heat may lead to the temperature of a device as high as 85 oC assuming the outside temperature is 45 oC.15 Degradation of the PSCs is thus dependent on controlling the heat inside a device, which indicates that thermal stability of the PSCs is one of critical subjects. As selective contact materials in the PSCs, spiro-MeOTAD is commonly used for HTM and TiO2 for electron transporting material (ETM), where crystallization of spiro-MeOTAD at high temperature is likely to be expected due to a low glass transition temperature (Tg).16 Moreover, the utilization of the additives such as 4-tert-butylpyridine (tBP) and bis(trifluoromethane)sulfonimide lithium salt (Li-TFSI) can accelerate the crystallization of spiro-MeOTAD16 and corrode the perovskite materials as well.17 These additives are also used in polymeric HTMs such as poly(3-hexylthiophene-2,5-diyl) P3HT and poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) to enhance the photovoltaic performance in spite of the disadvantages of additives.18,19 PEDOT:PSS20-21and NiO22-24 were suggested as HTMs in an 2 ACS Paragon Plus Environment

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inverted structure. The device using PEDOT:PSS showed poor stability due to its hygroscopic nature,25 whereas NiO-based device demonstrated good air-stability.24-26 [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) is suitable for ETM in the inverted structure27. Although intensive studies on stability have started recently, little investigation on thermal stability has been carried out. In this report, thermal stability of MAPbI3 and PSCs is systematically investigated under the thermal cycling condition as suggested by IEC 61646. Thermal stability of MAPbI3 film is investigated at temperature ranging from 85 oC to 120 oC for 10 days. No degradation is observed in the absence of moisture from UV-vis spectra, which confirms that material itself is intrinsically thermally stable. Thermal cycling test (one cycle: heating a device at 85 oC for 2 h and at room temperature for 2 h) is performed

for

the

devices

(FTO/bl-TiO2/mp-TiO2/MAPbI3/spiro-MeOTAD/Au)

with

the

and

the

normal

structure

inverted

structure

(FTO/NiO/MAPbI3/PCBM/BCP/Au). Fast degradation of the normal structure with spiro-MeOTAD is observed, while little degradation for 80 thermal cycles is monitored for the inverted structure with NiO. Photoluminescence is measured as a function of thermal cycle to study interfacial charge separation. The thermally aged device with spiro-MeOTAD is renewed by replacing with a fresh spiro-MeOTAD, which is direct evidence of thermal stability of perovskite material itself.

2. EXPERIMENTAL SECTION CH3NH3I (MAI) was synthesized by reacting 27.8 ml methylamine (40% in methanol, TCI) with 30 ml hydroiodic acid (57 wt% in water, Aldrich) in round bottomed flask at 0 oC for 2 h. The precipitate was collected by rotary evaporation at 50 oC for 1 h, followed by washing with diethyl ether 4 times. Finally, MAI was dried in vacuum oven at 60 oC for 24 h. 54 wt% MAPbI3 precursor solution was prepared according to the method reported elsewhere.28 MAI, PbI2 and N,N-dimethyl sulfoxide (DMSO)

were

dissolved

in

N,N-dimethylformamide

(DMF)

solvent

(molar

ratio

of

MAI:PbI2:DMSO=1:1:1). Fluorine doped tin oxide (FTO) or indium tin oxide (ITO) glasses (Pilkington, TEC-8, 8 Ω sq-1) were cleaned by detergent, followed by ultrasonic treatment in ethanol for 20 min and then UV/Ozone 3 ACS Paragon Plus Environment


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treatment for 15 min. For the normal structure, the blocking TiO2 (bl-TiO2) layer was first deposited by spin-coating a 0.1 M titanium diisopropoxide bis(acetylacetonate) (75 wt% in isopropanol, Sigma) in 1-butanol (0.1 g/2.06 ml ) on the cleaned FTO glass at 2000 rpm for 20 s, followed by annealing at 125 o

C for 5min, which was repeated for three times. Home-made nanocrystalline TiO2 (average particle

size of about 40 nm) paste was diluted in 1-butanol (1 g/10 ml), which was spin-coated on the bl-TiO2 layer at 2000 rpm for 20 s to form mesoporous TiO2 (mp-TiO2) layer. The spin-coated mp-TiO2 layer was annealed at 125 oC for 5 min on hotplate and then at 550 oC for 1 h in furnace. The mp-TiO2 film was post-treated with 20 mM TiCl4 aqueous solution at 90 oC for 10 min, followed by annealing at 500 o

C for 30 min in furnace. For the inverted structure having a NiO layer, 0.1 M nickel acetate

tetrahydrate (Ni(CH3COO)2·4H2O, Alfa Aesar, 98%) and monoethanolamine (MEA = NH2CH2CH2OH, Aldrich, 99%) were dissolved in ethanol at 70 oC for 4 h, where the molar ratio of Ni2+:MEA was maintained at 1:1 in solution.29 Finally, the precursor solution was spin-coated at 6000 rpm for 40 s, followed by annealed at 450 oC for 30 min to form a NiO layer. The prepared perovskite precursor solution was spin-coated on the mp-TiO2 or the NiO layer at 4000 rpm for 25 s. Diethyl ether was dripped while spin coating to form MAI·PbI2·DMSO adduct.25 The adduct film was heated at 65 oC for 1 min to eliminate DMSO from the adduct and then 100 oC for 30 min to form perovskite phase. For the normal structure, a spiro-MeOTAD solution was prepared by dissolving 72.3 mg spiro-MeOTAD, 28.8 µl of tBP and 17.5 µl of Li-TFSI (Li-TFSI solution: 520 mg Li-TSFI in 1 ml acetonitrile) in 1 ml of chlorobenzene (CB). 25 µl of the spiro-MeOTAD solution was spin-coated on the perovskite layer at 3000 rpm for 30 s. For the inverted structure, PCBM) solution (20 mg/ml in CB) was spin-coated at 5000 rpm for 30s, followed by spin-coating a bathocuproine (BCP) solution (0.5 mg/ml in ethanol) at 6000 rpm for 30 s. Finally, Au electrode with a thickness of ca. 100 nm was deposited on top of the spiro-MeOTAD or BCP layer. All the devices were stored in the inert atmosphere overnight and then encapsulated in glove box filled with N2. The devices were covered with the concave glass by epoxy (3124L(MS), Three Bond) and put under the UV-lamp for 90s to cure epoxy. While UV-curing the active area of solar cell was protected by putting black tape atop the galss and the side area was only exposed to UV for curing the epoxy, not all area of solar cell. 4 ACS Paragon Plus Environment

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The quantity of epoxy was controlled by the micro-computer controlled electronic-pneumatic system (SUPER ∑CMII-V5, Musashi) with a pressure of 0.7 MPa. For the thermal stability test of perovskite film, PMMA (0.1g/ml in CB) was spin-coated at 4000 rpm 25 s on perovskite films. Photocurrent-voltage (J-V) curves were measured by a Keithley 2400 source meter using a solar simulator (Oriel, 3A class) equipped with a 450 W Xenon lamp (Newport 6279NS). During J-V measurement, the device was covered with metal mask with an aperture area of 0.125 cm2 and measured at voltage settling time of 200 ms, corresponding to scan rate of 112 mV/s (voltage range: -0.1~1.02 V, data points: 50 points). The UV-vis absorbance spectra of the perovskite films were recorded by a UV-vis spectrophotometer (Agilent 8453). Photoluminescence (PL) was measured by Quantaurus-tau (HAMAMATSU, C11367) with an emission light of 464 nm. Polarized optical microscopy was measured by optical microscope with polaroid (Nikon, Eclipse). Differential scanning calorimetry (DSC) measurement was conducted at heating rate of 10 oC/min using DSC7020 (SEICO Instrument).

3. RESULTS AND DISCUSSION The two PSC structures are schematically represented in Figure 1. For a normal n-i-p structure, n-type TiO2 and p-type organic molecular spiro-MeOTAD are used as ETM and HTM, respectively. Whereas, in an inverted p-i-n configuration, NiO and PCBM are used as HTM and ETM, respectively. Thermal stability of the encapsulated devices is carried out at 85 oC in dark condition according to the IEC 61646 standard.30 Encapsulation is reqired to eliminate moisture effect while thermal stability testing.

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Figure 1. The schematic device structure of (a) a normal and (b) an inverted FTO/bl-TiO2/mp-TiO2/MAPbI3/spiro-MeOTAD/Au FTO/NiO/MAPbI3/PCBM/BCP/Au.

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structure structure

of of

Figure 2 (a)-(d) shows dependence of photovoltaic parameters on number of thermal cycles, where the interval of 85 oC for 2 h and room temperature (25 oC) for 2 h is defined to be one thermal cycle as presented in inset in Figure 2(a). The devices were heated at 85 oC for 2 h and then cooled down to room temperature. The device was kept for 2 h at room temperature and J-V was measured at the end of one thermal cycle. For the normal structure with TiO2 and spiro-MeOTAD, PCE is dramatically decreased from 14.62% to 2.36% after 9th thermal cycle due to significant decrease in short-circuit current density (Jsc) and fill factor (FF). However, little change in open-circuit voltage (Voc) is observed. The changes in photovoltaic parameters indicate that there is problem in charge collection and interfacial resistance, but perovskite material itself does not seem to be damaged. The inverted structure with NiO and PCBM selective contacts, exhibiting no J-V hysteresis (Figure S1 in Supporting Information), shows much better thermal stability. Jsc and Voc remain almost unchanged even after 80th thermal cycle, where initial Jsc of 21.743 mA/cm2 and Voc of 0.901 V are changed to to 22.93 mA/cm2 and 0.889 V, respectively, after 80th thermal cycle. Due to gradual decrease in FF from 75.1% to 53.2%, the device maintains 74% of its initial PCE of 14.71%. Figure 2(e) and (f) compares J-V curves of as-prepared and thermally-aged devices for the normal and inverted configurations. The data shown here underline that thermal stability is dependent on selective contacts.


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Figure 2. Thermal stability of a normal structure of FTO/bl-TiO2/mp-TiO2/MAPbI3/spiro-MeOTAD/Au (black) and an inverted structure of FTO/NiO/MAPbI3/PCBM/BCP/Au (red) device with glass encapsulation at 85 oC in dark condition. Photovoltaic parameters of (a) short-circuit current density (Jsc), (b) open-circuit voltage (Voc), (c) fill factor (FF) and (d) PCE as a function of thermal cycles. Insert in (a) shows one thermal cycle comprising 85 oC for 2 h and room temperature for 2 h. J-V curves of the as-prepared and thermally aged devices for (e) the normal structure and (f) the inverted structure. All the parameters are collected under AM 1.5G one sun illumination.

Regarding thermal stability of perovskite layer, environmental conditions such as moisture and oxygen or electron transporting layers such as TiO2 and ZnO were reported to induce thermal decomposition of perovskite layer.31,32 We first investigate thermal stability of perovskite layer in contact with spiro-MeOTAD and NiO, in which the cells are encapsulated to avoid moisture and oxygen effect while testing at elevated temperature. Figure 3(a) shows the image of the real 7 ACS Paragon Plus Environment


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encapsulated devices along with a cross-sectional schematic structure, where devices are encapsulated with glass using epoxy in N2 atmosphere in glove box. Since the normal structure with spiro-MeOTAD demonstrated dramatic decrease in photovoltaic performance under thermal stress whereas the inverted structure with NiO showed excellent thermal stability, change in absorbance of perovskite films is investigated with a glass/MAPbI3/spiro-MeOTAD and a FTO/NiO/MAPbI3 semi-cell encapsulated with glass, in order to figure out whether the degradation is caused by perovskite decomposition or selective layers. Figure 3(b) and (c) compares absorbance of perovskite films, where UV-vis spectra are measured for the as-prepared and 10th thermally cycled (85 oC-20 h-aged) semi-cells because photovoltaic performance for the normal structure with spiro-MeOTAD is significantly degraded after 10th thermal cycle. However, no change in absorbance for both semi-cells is observed, which confirms that the thermal degradation of the normal structure is not due to decomposition of perovskite layer. We also investigate the thermal stability of perovskite films with a FTO/bl-TiO2/mp-TiO2/MAPbI3 and a FTO/bl-TiO2/mp-TiO2/MAPbI3/spiro-MeOTAD covered with PMMA at temperature of 85 oC, 100 oC and 120 oC. According to the absorbance spectra and color of the films, the perovskite layers show thermal stability after heat-treatment for 10 days (Figure S2 and S3 in Supporting Information). This indicates that perovskite film is quite stable at elevated temperature, which is contrary to previous report on decomposition at 100 oC.33 For the FTO/NiO/MAPbI3 case, little degradation is shown from the absorbance spectra. Hence, we come to the conclusion that the fast thermal degradation of the normal structure and the thermally stable inverted structure are related to selective contact layers rather than the perovskite itself.


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Figure 3. (a) An encapsulated real device along with its cross-sectional schematic layout. All the devices were encapsulated by glass using epoxy under N2 atmosphere in glove box. Absorbance of the as-prepared and thermally aged perovskite films in contact with (b) spiro-MeOTAD and (c) NiO.

Figure 4(a) and (b) shows the photoluminescence (PL) of perovskite in contact with different p-type selective layers. For the perovskite contacting spiro-MeOTAD, PL intensity is significantly reduced without heating due to a strong quenching effect, which is indicative of efficient charge separation. PL intensity is gradually increased with period of heat treatment at 85 oC, which means that charge transfer from perovskite to spiro-MeOTAD occurs ineffectively. Tg of pure spiro-MeOTAD without additives was reported to be 124 oC.16 We also measure differential scanning calorimetry (DSC) and confirm that Tg of pure spiro-MeOTAD is detected at 125 oC in Figure S4 in Supporting Information, which is however lowered to 85 oC in the presence of additives. In addition, heat-treatment at 85 oC for 20 h decreases further Tg to 65 oC for the spiro-MeOTAD with additives, whereas little change in Tg is observed upon heat-treatment for pure spiro-MeOTAD. A sample of FTO/sprio-MeOTAD/Au is prepared and exposed at 85 oC for 15 h, which leads to crystallization of spiro-MeOTAD as observed by polarized microscopic image in Figure S5 in Supporting Information. Hole mobility of the additive-contained spiro-MeOTAD, measured by space-charge limited current (SCLC) in the dark, is significantly reduced from 3.2×10-3 cm2/Vs to 2.2×10-7 cm2/Vs after 9 ACS Paragon Plus Environment


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heat-treatment at 85 oC for 20 h in Figure S6 in Supporting Information. Thus, inhibition of charge separation is ascribed to the crystallization of spiro-MeOTAD and the significant decrease in hole mobility. PL intensity is marginally increased after 5 h heat treatment at 85 oC for the inverted structure with NiO, which however ceases even for longer period of heat treatment. Time-resolved PL is measured to investigate charge separation kinetics before and after thermal treatment at 85 oC. Longer time is required for charge separation in case of MAPbI3/spiro-MeOTAD after heat-treatment at 85 oC for 20 h (Figure 4(c)), whereas little dependence of charge separation kinetics on thermal stress is observed for NiO/MAPbI3 case (Figure 4(d)). This indicates that charge separation at NiO/MAPbI3 occurs efficiently regardless of heat treatment, while slower charge separation after long-period heat-treatment in spiro-MeOTAD based semi-cell is indicative of problem at MAPbI3/sipro-MeOTAD interface. Since organic PCBM is used in the NiO-based inverted structure, thermal stability may be influenced by PCBM. However, thermal stability observed from the inverted structure might be due to the higher Tg (~130 oC) of pristine PCBM than the spiro-MeOTAD with additives (~85 oC).


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Figure 4. (a, b) Steady-state and (c, d) time-resolved photoluminescence (PL) of the MAPbI3 perovskite films in contact with (a, c) spiro-MeOTAD and (b, d) NiO as a function of exposed time at 85 oC. Devices for PL measurement were covered with glass and heat treated at 85 oC in dark condition.

In order to confirm that the thermally aged spiro-MeOTAD is responsible for the fast degradation rather than perovskite itself, we have renewed the thermally aged devices by replacing the aged spiro-MeOTAD with new spiro-MeOTAD and investigated the photovoltaic performance of the renewed devices. A renewal process is shown in Figure 5. The encapsulation glass covered on FTO substrate is first removed mechanically. For sample 1 aged at 85 oC for 20 h in Figure 5(a), only the aged Au electrode is removed by 3M tape, which is followed by depositing a new Au electrode on top of the thermally aged spiro-MeOTAD layer. For sample 2 aged at 85 oC for 18 h (2 h difference from sample 1 is just to show different bath) in Figure 5(b), both the thermally aged spiro-MeOTAD and Au 11 ACS Paragon Plus Environment


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are removed (spiro-MeOTAD is cleansed by chlorobenzene), which is followed by spin-coating a new spiro-MeOATD layer and depositing of a new Au on top of the new spiro-MeOTAD layer.

Figure 5. A renewal process of the thermally aged PSCs with FTO/bl-TiO2/mp-TiO2/MAPbI3/spiro-MeOTAD/Au layout. (a) Removal of only the aged Au electrode (sample 1) and (b) removal of both the aged spiro-MeOTAD and Au electrode (sample 2). Au electrode was removed by 3M tape and spiro-MeOTAD was removed by cleansing with chlorobenzene. (1) through (5) represent removal of encapsulation glass, removal of the aged Au electrode, deposition of a new Au electrode, removal of both the aged Au and spiro-MeOTAD and deposition of a new spiro-MeOTAD and Au.

Figure 6(a) and (b) shows the J-V curves of the devices (sample 1 and sample 2 in Figure 5) before and after the refurbishment. Thermally aged devices show poor performance. A new Au layer deposited on the aged spiro-MeOTAD layer (Figure 6(a)) cannot recover the photovoltaic parameters except for a slight increase in FF, which indicates that Au is not responsible for thermal instability. The slight increase in FF might be due to the improvement of interfacial contact between spiro-MeOTAD and Au electrode. Interestingly, a complete recovery of photovoltaic performance is observed by renewing both spiro-MeOTAD and Au electrode (Figure 6(b)), which shows direct evidence that thermal instability is mainly due to the transition to crystallization of spiro-MeOTAD at 85 oC as shown in Figure S5. The dramatic decrease in the hole mobility leads to inhabitation of the charge carriers transferring from MAPbI3 to spiro-MeOTAD. At the same time, the glassy state of spiro-MeOTAD tends to de-laminate the aged spiro-MeOTAD layer from perovskite layer and Au 12 ACS Paragon Plus Environment

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electrode, also causing charge transfer problems in PSCs, which is consistent with the gradual increase in PL as observed in Figure 4. The renewed device demonstrates a similar Jsc but higher FF and Voc than the as-prepared one. The improved FF and Voc are likely to be attributed to better interfacial contact by refurbishment.

Figure 6. J-V curves of as-prepared and thermally aged devices. Thermally aged devices were renewed by (a) removing only the aged Au electrode and re-deposition of a new Au electrode on the aged spiro-MeOTAD and (b) removing both Au and spiro-MeOTAD from the aged device and re-deposition of a new spiro-MeOTAD layer and an Au electrode. 4. CONLUSIONS MAPbI3 itself was found to be thermally stable at temperature ranging from 85 oC to 120 oC in dark condition

in

absence

of

moisture.

Nevertheless,

a

device

with

FTO/bl-TiO2/mp-TiO2/MAPbI3/spiro-MeOTAD/Au configuration showed fast degradation after exposure to 85 oC. Such degradation was mainly due to the transition of spiro-MeOTAD to the glassy state at high temperature and thereby significant decrease in hole mobility. Photovoltaic performance of the thermally aged device was fully recovered by replacement of the thermally aged spiro-MeOTAD with a fresh one, which indicates that spiro-MeOTAD was responsible for the thermal instability. On the contrary, the device was thermally stable when selecting NiO as HTM and PCBM as ETM because these selective contacts are thermally stable. This study is expected to provide important insight into 13 ACS Paragon Plus Environment


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design thermally stable perovskite solar cell.

ASSOCIATED CONTENT Supporting Information Supporting information is available Figures providing forward and reverse scanned J-V curve for NiO-based cell, UV-vis absorbance spectra with respect to temperature and exposed time at given temperature, DSC curves, glassy state of spiro-MeOTAD and hole mobility of spiro-MeOTAD.

AUTHOR INFORMATION Corresponding Author *E-mail:[email protected]; Tel: +82-31-290-7241; Fax: +82-31-290-7272

Authors Contributions † Xing Zhao and Hui-Seon Kim contributed equally to this work.

Funding sources This work was supported by the National Research Foundation of Korea (NRF) grants funded by the Ministry of Science,

ICT & Future Planning (MSIP) of Korea under contracts No.

NRF-2012M3A6A7054861 (Global Frontier R&D Program on Center for Multiscale Energy System), NRF-2015M1A2A2053004 (Climate Change Management Program), and NRF-2012M3A7B4049986 (Nano Material Technology Development Program). This was also supported in part by NRF-2016M3D1A1027663 and NRF-2016M3D1A1027664 (Future Materials Discovery Program).

Notes The authors declare no competing financial interest.


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Effect of Selective Contacts on the Thermal ... - ACS Publications

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