Enhancement of Stability of Inverted Flexible Perovskite Solar Cells by

Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Enhancement of Stability of Inverted Flexible Perovskite Solar Cells by Employing Graphene-Quantum-Dots Hole Transport Layer and Graphene Transparent Electrode Codoped with Gold Nanoparticles and Bis(trifluoromethanesulfonyl)amide Seung Hyun Shin,† Dong Hee Shin,† and Suk-Ho Choi*

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Department of Applied Physics and Institute of Natural Sciences, Kyung Hee University, Yongin 17104, Korea S Supporting Information *

ABSTRACT: We first report p-i-n-type perovskite solar cells (PSCs) using graphene quantum dots (GQDs) hole transport layer (HTL) and graphene transparent conductive electrode codoped with gold nanoparticles and bis(trifluoromethanesulfonyl)amide. The PSCs on rigid glass substrates show maximum power conversion efficiency (PCE) of 17.02/17.15% for forward/reverse scans, comparable to those (17.53/17.55%) of the control cells with poly(3,4ethylenedioxythiophene) (PEDOT:PSS) HTL. As 30 d elapsed in N2 atmosphere, only 28% of the PCE degradation is observed for the GQDs HTL, while the PCE is reduced by 42% for the PEDOT:PSS HTL. Flexible PSCs with GQDs HTL exhibit 13/15% PCE for forward/reverse scans, respectively, and maintain 70% of the original PCE value even after 3000 bending cycles at a curvature radius of 4 mm. KEYWORDS: Inverted perovskite solar cells, Graphene-quantum-dots, Au nanoparticles, (CF3SO2)2NH, Graphene, Flexibility

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INTRODUCTION In the last several years, organolead halide perovskite solar cells (PSCs) have consolidated their position as an attractive nextgeneration energy-harvesting device thanks to the large absorption coefficient, the weak binding strength/long diffusion length of excitons, and the balanced electron/hole mobilities.1−7 Especially, the PSCs are promising for an effective power source for portable and wearable appliances due to their large power conversion efficiency (PCE) and high flexibility.8−10 Typically, the PSCs are classified as n-i-p (normal) and p-i-n (inverted) types depending on the kind of the material on an electrode/substrate. Currently, on the one hand, n-i-p-type perovskite solar cells with mesoporous (mp)-TiO2 have shown a peak PCE of more than 22% based on the well-established design,11 but they are limited in the use of plastic substrates, because high-temperature (>450 °C) process is needed to form mp-TiO2 in the n-i-p type.12 On the other hand, p-i-n-type PSCs have received strong attention due to the small current density−voltage (J−V) hysteresis regardless of scan direction/speed and process temperature. Conventionally, a conductive polymer poly(3,4-ethylenediox© XXXX American Chemical Society

ythiophene) (PEDOT:PSS) has been used as a hole-extraction layer to enhance the hole collection in p-i-n-type PSCs, but the PEDOP:PSS deteriorates the long-term stability of the cells due to its acidity, hygroscopicity, and inhomogeneous electricity.13,14 It is therefore important to find innovative interfacial materials playing the role of charge transport layer for better performance and durability of PSCs. Recently, a graphene oxide hole conductor and a carbon nanotube hole contact were successfully employed for p-i-n-type PSCs,15,16 indicating that carbon-based materials have the potential to contribute significantly to enhancing the performance of the PSCs. Among carbon-based materials, graphene quantum dots (GQDs) have been widely applied to photovoltaic devices due to the inherently excellent optical and electronic properties together with the role of the p-type dopant for graphene (GR).17,18 If GQDs are used as a hole transport layer (HTL) for PSCs, it is believed that high PCE/long-term stability for Received: April 26, 2019 Revised: July 5, 2019 Published: July 8, 2019 A

DOI: 10.1021/acssuschemeng.9b02336 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

Figure 1. AFM topological images of (a) pristine GR and (b) Au NPs-, (c) TFSA-, and (d) Au NPs & TFSA-doped GR. (e) Raman, (f) XPS, and (g) transmittance spectra of pristine and doped GR. (h) Rs and (i) work function/carrier mobility of pristine and doped GR.

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the PSCs can be achieved, because GQDs were shown to be stable structurally, chemically, and optically in various device applications under ambient conditions.19−22 In addition, the PEDOT:PSS HTL is mainly used in p-i-n-type PSCs but can result in the decomposition of the perovskite film due to the acidic and hygroscopic properties.23−25 As another issue, it is very important to find a transparent conductive electrode (TCE), because fragile transparent conductive oxides are not suitable for flexible PSCs. Recently, a lot of studies have been done on flexible optoelectronic devices by using bare- or doped (mostly with a single dopant)GR TCEs as a transparent conductive anode instead of indium tin oxide (ITO),26−31 but the sheet resistance (Rs) of bare or singly doped GR is pretty larger than that of ITO. It is expected that the optimization of the GR TCEs can be achieved by reducing the Rs while maintaining the transmittance by codoping rather than single doping. Low Rs, high transmittance, and enhanced long-term stability of the GR TCEs can be simultaneously obtained by codoping with Au nanoparticles (NPs) and bis(trifluoromethanesulfonyl)-amide (TFSA) dopants, as shown in our recent report.32 Here, we first employ not only GQDs as an HTL but also codoped-GR as a TCE for p-i-n-type PSCs on rigid glass and flexible poly(ethylene terephthalate) (PET) substrates. We systematically compare the photovoltaic properties of the PSCs with GQDs and PEDOT HTLs. Furthermore, we monitor the stabilities of the PSCs with both HTLs as the time elapses for 30 d and check the bending stability of the flexible PSCs.

EXPERIMENTAL SECTION

Preparation of Graphene. Single-layer GR layer was fabricated by chemical vapor deposition at 1000 °C under flowing CH4 and H2 gas and was transferred onto glass and PET substrates through a generally known wet-transfer process.33 For the preparation of codoped GR, Au NPs were formed on the GR surface by sputtering for 3 s using radio frequency sputtering. Subsequently, the TFSA in nitromethane solution (20 mM) was dropped onto the surface of the Au NPs/GR and then spin-coated at 2500 rpm for 1 min and annealed at 100 °C for 1 min. Fabrication of Inverted Perovskite Solar Cells. GQDs solution was dropped on the entire surface of the GR, spin-coated at 1500 rpm for 1 min, and then annealed at 100 °C for 1 min. The coating was repeated up to 10 times to increase the density and thickness of the GQDs. PEDOT:PSS as an HTL for control samples was spin-coated on pristine and doped GR surface/glass or PET at 3000 rpm for 60 s and then annealed at 150 °C for 20 min. A 40 wt % CH3NH3PbI3 (MAPbI3) solution was spin-coated on the HTL layer by consecutive spin coatings at 1000 and 5000 rpm for 10 and 20 s, respectively. During the second spin-coating step, toluene was quickly dropped onto the rotating substrate and subsequently annealed on a hot plate at 100 °C for 5 min, thereby changing the color of the films to turn dark brown. Next, the phenyl C61 butyric acid methyl ester (PCBM) in chlorobenzene (2%) solution was spin-coated on the perovskite film at 2000 rpm for 60 s. Finally, the Al electrode was deposited by thermal evaporation in a vacuum system. Characterization. Topological images of pristine and doped GR were analyzed by noncontact-mode atomic force microscope (AFM, Park System XE-100). The size and distribution of GQDs were analyzed using a high-resolution transmission electron microscope B


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Table 1. Summary of Optical and Electrical Properties of Single-Layer GR TCEs for Various Doping Treatments dopant

Rq (nm)

I (G/2D)

transmittance at 550 λ (%)

Rs sheet resistance (ohm/sq)

prisitne Au NPs TFSA codoping

0.332 1.834 0.395 2.215

0.43 0.76 0.59 1.35

97.6 94.3 97.2 93.5

735 ± 32 162 ± 9 212 ± 7 83 ± 6

(TEM, JEOL JEM-2100F) with electronic energy loss spectroscopy (EELS) mapping capabilities. The cross-sectional structure of the cells was checked using a field emission scanning electron microscope (FESEM) (Carl Zeiss, model LEO SUPRA 55). The atomic bonding states of the pristine or doped GR were characterized by X-ray photoelectron spectroscopy (XPS) using Al Kα line of 1486.6 eV. The Rs, transmittance, and work function of GR were measured by fourprobe van der Pauw method (Dasol eng, model FPP-HS8−40K), ultraviolet (UV)-visible-near-infrared optical spectroscopy (Agilent Varian, model cary 5000), and Kelvin probe force microscopy (Park systems, model XE 100), respectively. Photoluminescence (PL), timeresolved PL (TRPL), and Raman spectra were recorded with excitation at 325, 470, and 532 nm, respectively. Current density− voltage (J−V) characteristics of the cells were measured using a Keithley 2400 source meter at a scan rate of 200 ms/10 mV. The active area of the cells was defined as 0.1 cm2. Photovoltaic characterizations were done using a solar simulator (McScinece K201) under illumination of 1 Sun (100 mW cm−2 AM 1.5G) and a calibrated Si reference cell. External quantum efficiency (EQE) was measured by a power source (450 W xenon lamp, Oriel Apex Illuminator, Newport) with a monochromator (Cornerstone 260, Newport). The bending tests were conducted at a 0.5 Hz bending frequency.

work function (eV)

mobility (cm2/(V s))

± ± ± ±

2150 ± 127 1137 ± 84 1352 ± 93 954 ± 31

4.57 4.74 4.88 4.93

0.032 0.017 0.015 0.011

σDC/σOP 20 39 63 67

± ± ± ±

6 5 4 3

with Au NPs, TFSA, and Au NPs & TFSA, respectively, as shown in Figure 1h. The suitability of the TCE is evaluated by the ratio of direct-current (DC) conductivity/optical conductivity (σDC/σop) based on the following equation: T = {1 + (Z0/2Rs) (σop/σDC)}−2, where Zo is the impedance of free space.35 The σDC/σop values of pristine GR and Au NPs, TFSA, and codoped GR TCEs are ∼20, 39, 63, and 67, respectively, as summarized Table 1, indicating the codoped GR TCE is the best for PSCs. As shown in Figure 1i, the average work function/carrier mobility values of the GR TCE, measured for randomly oriented 10 sample points, are 4.62 eV/2150 cm2/Vs before doping, and they increase/decrease to 4.74/1137, 4.88/1352, and 4.93 eV/954 cm2/V-s after doping with Au NPs, TFSA, and Au NPs/TFSA, respectively. These results suggest that the GR TCE becomes strongly p-type by the codoping with Au NPs/TFSA, judging from the significant increase in the work function. Figure 2a,b shows low- and high-magnification TEM images of the GQDs, indicating the size distribution of the GQDs in

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RESULTS AND DISCUSSION Surface morphologies of pristine and doped GR TCEs were analyzed by AFM, as shown in Figure 1a−d. The square rootmean-square roughness (Rq) of the pristine GR TCE is ∼0.332 nm, consistent with previously reported results.34 The Rq of the Au NPs, TFSA, and codoped GR TCEs is 1.834, 0.395, and 2.215 nm, respectively, all of which are greater than that of the pristine GR due to the dopants. Figure 1e shows Raman spectra of the GR TCEs, where the G and 2D bands associated with GR are clearly visible. The G/2D peaks of the Au NPs, TFSA, and codoped GR layers are shifted by 6/4, 10/9, and 16/18 cm−1 with respect to those of the pristine GR, respectively. The intensity ratios of G and 2D peaks, another parameter for evaluating the doping strength of GR, were 0.43, 0.76, 0.59, and 1.35 for pristine GR and Au NPs, TFSA, and codoped GR TCEs, respectively, originating from the electron configuration change of GR by doping, consistent with the previous literature.32 Figure 1f shows XPS spectra of the undoped and doped GR TCEs. The Au NPs, TFSA, and codoped GR TCEs clearly exhibit the XPS peaks associated with Au 4f or/and N 1s core levels, respectively, indicating successful doping of the GR. The transmittance spectra of the pristine and doped GR TCEs on quartz substrates are shown in Figure 1g. By the single doping or codoping, the transmittance is reduced in the range of wavelength from 300 to 900 nm due to the impurities adsorbed on the surface. In particular, the transmittance at 550 nm is ∼97.6% before doping, and it decreases to 94.3, 97.2, and 93.5% after doping with Au NPs, TFSA, and Au NPs/ TFSA, respectively. Note that the transmittance of the codoped GR is maintained at over 90% in the visible region. The Rs of pristine GR is in the range of 700−760 Ω/sq, and the average Rs is reduced to 162, 212, and 83 Ω/sq by doping

Figure 2. (a) Low- and (b) high-magnification TEM images of GQDs. (c) Carbon K-edge EELS spectrum of GQDs. (d) UV−Visible absorption and PL spectra of GQDs in solution. (inset) Photograph of the GQDs aqueous solution in light yellow color.

the range of 3.8−7.8 nm, resulting in the average diameter of 5 nm. Carbon-K edge EELS spectrum of the GQDs is peaked at ∼285 and ∼291 eV, corresponding to the π* and σ* states, respectively, as shown in Figure 1c, further identifying the GQDs. Figure 1d shows absorbance and PL spectra of the GQDs, peaked at 356 and 450 nm, respectively, indicating a down-conversion behavior that the light is absorbed in the UV C


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Figure 3. (a) Schematic device architecture and (b) corresponding band diagram of a typical planar p-i-n-type PCS. (c) Cross-sectional FE-SEM image of a typical PCS.

region and emitted in the visible region, consistent with previously reported results.18,36 The PSCs were fabricated based on a structure of GR/ GQDs/MAPbI3/PCBM/Al, where the GR is pristine or doped single layer, as shown in Figure 3a. The GQDs were prepared on the GR surface in the same way as reported previously.18 The AFM image demonstrated uniform distribution of the GQDs (see Supporting Information, Figure S1). Figure 3b shows schematic band diagram of a typical PSC. Here, the highest unoccupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of the GQDs are described according to the previously reported size-dependent values.19 As shown in Figure 3b, the large band offset at the MAPbI3/GQDs interface could prevent the holes from transporting smoothly toward the GR TCE. However, it is well-known that oxygen or water molecules are adsorbed on the surface of the GQDs in the atmosphere,37,38 resulting in the formation of defect energy levels (the dashed lines in the band gap of the GQDs) close to the HOMO level,39 thereby making the GQDs p-type.18 Therefore, the holes can transport through the defect levels of the GQDs to the GR TCE by tunneling between the GQDs rather than by overcoming the barrier to the HOMO level. Figure 3c shows cross-sectional scanning electron microscope (SEM) image of a typical inverted MAPbI3 PSCs, suggesting that all the layers are wellorganized, as expected. Figure 4a shows typical J−V curves under dark and illumination for PSCs with pristine and doped GR TCEs. The pristine GR PSCs show lowest PCE due to the relatively high Rs despite the high transmittance/mobility. In contrast, the doped GR TCEs exhibit significantly high photovoltaic performance due to the increased conductivity regardless of the type of dopant. Especially, the codoped GR PSCs showed largest photovoltaic parameters: 17.02/17.15% PCE, 1.08/1.08 V Voc, 20.42/20.55 mA/cm2 Jsc, 77.15/77.29% fill factor (FF) for forward/reverse scans, respectively, as summarized in Table 2. The codoped GR PSCs also showed highest PCE of 15.10 ± 1.92% by statistical evaluation of the average PCE for 16 PSCs (see Supporting Information, Figure S2 and Table 2). As a control sample, the PSCs employing PEDOT:PSS conventionally used as an HTL were fabricated for comparing the photovoltaic performance (see Supporting Information, Figure S3). The codoped GR PSCs with PEDOT:PSS exhibited 1.08/ 1.08 V Voc, 20.86/20.83 mA·cm−2 Jsc, 77.81/78.02% FF, and 17.53/17.55% PCE for the forward/reverse scans, respectively. (For undoped and singly doped GR TCEs, see Supporting Information, Table S1.) Note that the PCE of the GQDs-HTL PSCs is comparable to that of the PEDOT:PSS-HTL ones.

Figure 4. (a) J−V curves of the best cells with pristine GR and Au NPs, TFSA, and codoped GR TCEs under forward and reverse scans. (b) EQE spectra/corresponding integrated Jsc curves and (c) dark J− V curves of PSCs for pristine and doped GR TCEs.

The EQE spectra of the PSCs before and after various doping treatments are shown in Figure 4b. The EQE increases in the range of wavelength from 300 to 900 nm after doping, irrespective of the doping type. The Jsc values obtained by integrating the EQE spectra are 17.18, 20.25, 20.50, and 19.98 mA cm−2 for pristine GR and Au NPs, TFSA, and codoped GR D


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Table 2. Photovoltaic Parameters of Perovskite Solar Cells with GQDs HTL for Various Doping Treatments, Measured in the Forward and Reverse Scans electrode pristine Au NPs TFSA Au NPs & TFSA

scan direction

Voc (V)

Jsc (mA/cm2)

FF (%)

PCE (%)

average PCE (%)

forward reverse forward reverse forward reverse forward reverse

1.01 1.02 1.05 1.05 1.06 1.06 1.08 1.08

17.72 17.84 20.74 20.85 20.84 20.84 20.42 20.55

57.24 60.92 71.50 72.31 69.82 70.93 77.15 77.29

10.24 11.09 15.57 15.83 15.43 15.67 17.02 17.15

8.22 ± 2.02

TCEs, respectively, and they are only ∼3% different from the measured Jsc ones, as summarized in Table 2. Figure 4c shows dark J−V curves representing the rectifying behaviors. By the nonideal diode equation,40 the diode ideality factor (n) is calculated by a linear fit to the J−V plot. The n is estimated to be 4.08, 3.30, 3.12, and 2.97 for pristine GR and Au NPs, TFSA, and codoped GR TCEs, respectively. These results suggest that the codoped GR TCE together with the p-type GQDs layer effectively reduce the charge recombination loss at the GR TCE/GQDs/MAPbI3 interface. We measured steady-state PL and TRPL to evaluate how the GQDs layer acts for extracting photogenerated holes from MAPbI3. The hole-extraction process prevents the radiative charge recombination in the absorber material, resulting in the PL quenching. As shown in Figure 5a, the addition of the GQDs instead of PEDOT:PSS between MAPbI3 and the GR TCE suppresses the PL emission from the MAPbI3 layer, indicating that the GQDs are more effective for the extraction of the holes than PEDOT:PSS. Figure 5b shows PL decay curves, well-fitted by an exponential function. The PL shows faster decay for the GQDs (2.52 ns) than for the PEDOT:PSS (3.01 ns), in good agreement with the behavior of the PL intensity. This further suggests that the extraction of the photoinduced carriers toward the GR TCE becomes more efficient than their recombination by using the GQDs HTL. To compare the hole transport in both HTLs, we studied their electrical behaviors in a sandwich structure of codoped GR/ HTL/Al, as shown in the inset of Figure 5c. From the I−V characteristics of the sandwich devices, the conductivity (σ0) was calculated by the following equation: I = σ0Ad−1V,41 where d (43 nm for GQDs and 38 nm for PEDOT:PSS, obtained from the AFM height profiles, see Supporting Information, Figure S4) and A (0.1 cm2) are the thickness of the HTL layer and the area of the device, respectively. The resulting σ0 values are 1.41 and 0.99 μS-cm−1 for GQDs and PEDOT:PSS, respectively, indicating comparable transport properties of both HTLs. We monitored the PCE with time to evaluate the long-term stability of the PSCs with PEDOT:PSS and GQDs HTLs under N2 atmosphere for 30 d, as shown in Figure 6 (for J−V curves, see Supporting Information, Figure S5). The GQDs and PEDOT:PSS PSCs maintained ∼72 and 58% of their initial PCEs (absolutely from 16.98 and 17.46% to 12.16 and 9.56%) after 30 d, respectively. The GQDs-based PSCs are more stable than the cells with PEDOT:PSS, because the perovskite films are decomposed by the acid and hydrophobic PEDOT:PSS,23−25 but the GQDs are structurally and chemically stable in air,19−22 resulting in the reduction of the decomposition. These results suggest that the GQDs layer

13.24 ± 2.33 13.24 ± 2.19 15.10 ± 1.92

Figure 5. (a) Steady-state PL spectra and (b) TRPL of bare perovskite film, and perovskite films on PEDOT:PSS and GQDs. (c) I−V characteristics of the codoped GR/HTL/Al sandwich structures.

does not only play a role as an HTL but also enhances the long-term stability. On the basis of the preparation conditions for the highest PCE on rigid substrates, flexible PSCs were fabricated. Figure 7a shows J−V curves of a typical flexible PSC under dark and E



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CONCLUSION The GR codoped with Au NPs and TFSA exhibited excellent TCE properties such as high transmittance in visible region (∼93.5%), low Rs (83 Ω/sq), and high work function (4.93 eV), and it was therefore successfully employed for PSCs with GQDs HTL. These PSCs showed 17.02/17.15% PCE on rigid glass substrates for forward/reverse scans due to the improved charge separation and retarded charge recombination, comparable to those (17.53/17.55%) of the control cells with PEOT:PSS HTL. The GQDs cells showed relatively small degradation (∼28% decrease of the PCE) during 30 d in N2 atmosphere. In contrast, the PEDOT:PSS cells exhibited 48% reduction in the PCE for the same period. The flexible PSCs exhibited the best PCE of 15.12/15.38% for forward/reverse scans, and they maintained ∼70% of their original PCE values even after 3000 bending cycles at R = 4 mm.

Figure 6. Stability characteristics of the PSCs on rigid substrates with PEDOT:PSS or GQDs HTLs under N2 atmosphere for 30 d.

illumination. The flexible PSC exhibits 1.06/1.06 V Voc, 19.35/ 19.47 mA/cm2 Jsc, 73.71/74.50% FF, and 15.12/15.38% PCE for forward/reverse scans, respectively. Figure 7b shows the EQE spectrum and the integrated Jsc value (18.77 mA/cm2), consistent with the measured one, as shown in Figure 7a. To examine the durability of the flexible PSC, we evaluated the operational stability of the J−V curves under repeated bending at a curvature radius (R) of 4 mm, as shown in Figure 7c. Figure 7d shows the PCE maintained at 70% of its initial value even after 3000 bending cycles (for the variation of all the photovoltaic parameters at each bending cycle, see Supporting Information, Table S2), indicating excellent flexibilities of the PSCs. The relative Rs (ΔRs/Rs) value of the codoped GR TCE/PET was measured under repeated bending tests at R = 4 mm (see Supporting Information, Figure S6a). The initial Rs before bending was 83 ± 6 Ω/sq and increased to 1.32 times as the original value due to the delamination/cracking of the GR TCE after 3000 bending cycles, as proved by the SEM images (see Supporting Information, Figure S6b,c). These results suggest that the PCE was degraded by 30% due to the increased Rs of the GR TCE after the repeated bending.

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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/acssuschemeng.9b02336.

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AFM images, average photovoltaic parameters, dark and photocurrent density−voltage curves, AFM image/ height profiles, evolution of current density−voltage curves with time, and evolution of sheet resistance of graphene TCE under repeated bending (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Suk-Ho Choi: 0000-0001-7539-9106

Figure 7. Photovoltaic properties of flexible PSCs with GQDs HTL and codoped GR TCEs. (a) Dark and photo J−V curves. (inset) A photograph of a typical flexible PSC. (b) EQE spectrum and calculated Jsc curves. (c) Evolution of J−V curves throughout 1000 bending cycles at d = 4 mm. (d) Normalized PCE as a function of bending cycle at a fixed bending radius of 4 mm. Here, the data are presented with error bars of standard deviation after being averaged for four different cells. F


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ACS Sustainable Chemistry & Engineering Author Contributions

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†

These authors contributed equally to this study. S.H.S. and D.H.S. performed device design, device fabrication, and characterization for the solar cells. D.H.S. wrote the paper. S.-H.C. initiated, supervised the work, and wrote/corrected the paper. All authors discussed the results and commented on the manuscript. Funding

This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2017R1A2B3006054). Notes

The authors declare no competing financial interest.

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