Carbon quantum dot-incorporated nickel oxide for planar p-i-n type perovskite solar cells with enhanced efficiency and stability

Journal Pre-proof Carbon quantum dot-incorporated nickel oxide for planar p-i-n type perovskite solar cells with enhanced efficiency and stability Jung Kyu Kim, Duong Nguyen Nguyen, Jae-Hoon Lee, Seunghun Kang, Yunseok Kim, Seok-Soon Kim, Han-Ki Kim PII:

S0925-8388(19)34133-7

DOI:

https://doi.org/10.1016/j.jallcom.2019.152887

Reference:

JALCOM 152887

To appear in:

Journal of Alloys and Compounds

Received Date: 7 July 2019 Revised Date:

1 November 2019

Accepted Date: 1 November 2019

Please cite this article as: J.K. Kim, D.N. Nguyen, J.-H. Lee, S. Kang, Y. Kim, S.-S. Kim, H.-K. Kim, Carbon quantum dot-incorporated nickel oxide for planar p-i-n type perovskite solar cells with enhanced efficiency and stability, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/ j.jallcom.2019.152887. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

Carbon quantum dot-incorporated nickel oxide for planar pi-n type perovskite solar cells with enhanced efficiency and stability

Jung Kyu Kima, Duong Nguyen Nguyena, Jae-Hoon Leeb, Seunghun Kangb, Yunseok Kimb, Seok-Soon Kimc*, and Han-Ki Kimb* a

School of Chemical Engineering, Sungkyunkwan University, 2066 Seobu-ro, Jangan-gu, Suwon, 16419 Republic of Korea

b

School of Advanced Materials Science & Engineering, Sungkyunkwan University, 2066 Seobu-ro, Jangan-gu, Suwon, Gyeonggi-do 440-746, Republic of Korea

c

Department of Nano and Chemical Engineering, Kunsan National University, 558, Daehak-ro, Gunsansi, Jeollabuk-do, 54150, Republic of Korea

ABSTRACT Carbon quantum dots (CQDs) have attracted extreme interest as a promising nanocarbon platform for divergence optoelectronics due to their high stability, good dispersibility in solvents, and tunable optical and electronic properties. Herein, planar pi-n type perovskite solar cells (PSCs) with enhanced efficiency and long-term stability were developed by incorporating CQDs into a nickel oxide (NiO) hole transport layer (HTL). The incorporation of CQDs downshifts the band structure of NiO, leading to good alignment with the work-function of the tin-doped indium oxide (ITO) electrode and the band-edges of the perovskite. The efficient cascade charge transport achieved with the optimized incorporation ratio of CQDs resulted in an enhanced power conversion efficiency (PCE) of 17.02%, compared to that of the PSC fabricated with bare NiO (15.66%), even though they were fabricated in air. The suppressed charge recombination accompanied by restricted charge accumulation curtails the J-V hysteresis, with a reduction from 4.5% to less than 1%. Moreover, long-term stability under atmospheric conditions without any encapsulation was achieved with CQDincorporated NiO. More than 70% of the initial PCE was retained over 190 h. This work suggests a novel strategy for fabricating solution-processible metal oxide interlayers with highly efficient charge migration for divergence energy conversion devices. Keywords: Perovskite solar cells, Carbon quantum dots, Nickel oxide, Hole transport layer, Stability Electronic mail: H.-K. Kim ([email protected]), S.-S. Kim ([email protected])

2

1. Introduction Organic-inorganic hybrid metal halide perovskite solar cells (PSCs) have been considered as a promising candidate for next-generation photovoltaics due to their tunable bandgap, excellent carrier transport (carrier mobility ≈ 0.6 cm2 V−1s−1), high absorption coefficient (~5 × 104 cm−1 at 600 nm), long diffusion length (>1000 nm), and solution-based processability with high efficiencies [1-11]. In recent decades, two different device configurations (i.e., n-i-p and p-i-n) for planar PSCs have been extensively developed. For the n-i-p configuration, a compact titanium dioxide (TiO2) film with a thickness of less than 100 nm has been utilized as the electron transport layer (ETL) and a spiro-based organic thin film (thickness: 100−300 nm) was used as the hole transport layer (HTL) to achieve a PCE of over 20%[12, 13]. However, the high cost and poor long-term stability of the spiro-based HTL are regarded as the crucial limitations in the commercialization of PSCs[14]. Employing the p-i-n configuration is an alternative approach for fabricating cost-effective planar PSCs with long-term stability. For this purpose, inexpensive p-type metal oxides with high durability can be used for the HTL[15, 16]. Among the various p-type metal oxides, nickel oxide (NiO) has been widely exploited as an HTL in p-i-n PSCs due to its high optical transparency and solution-processability. Moreover, the favorable energy band structure with a valence band maximum (VBM) of 5.2−5.4 eV and work function (WF) of 4.9−5.1 eV can be well-aligned with the VBM of perovskites, which provide holes with a transport pathway from the perovskite to the transparent conductive oxide (TCO) substrate through the NiO HTL[17, 18]. However, the intrinsically low charge conductance of NiO can lead to accumulation of holes near the perovskite interface, which can induce undesired charge recombination and inefficient charge collection[6]. In addition, inefficient charge

3

extraction by NiO can lead to hysteresis of the photocurrent density–voltage (J–V), which lowers the maximum power output of device performance[19, 20]. Recently, introduction of carbon-based materials such as graphene oxide (GO) or reduced GO (rGO) sheets into the HTL has been studied as a means of improving the charge transport properties and device stability[21-24]. This GO-based modification can enhance the hole extraction capacity and enhance the charge conductance of HTL films. Moreover, the carbon-based materials can tune the surface properties of the HTL, such as the surface energy or surface morphology, which can interfere with crystallization of the perovskite film in the upper layer, thereby increasing the grain size of the perovskite film[24, 25]. Most recently, carbon quantum dots (CQDs) have been widely used for optoelectronic applications as emerging zero-dimensional quantum dots due to their tunable optoelectronic properties, low toxicity, and good dispersibility in solvents[26-32]. The incorporation of CQDs in the charge transport layers can enhance electronic coupling between the perovskite and metal oxide[33, 34]. Notably, the incorporated CQDs can downshift the band-edges of the metal oxide, which can influence the charge migration capacity in PSCs[34]. Herein, we demonstrate that the incorporation of CQDs into the NiO HTL enhances the long-term stability and efficiency of planar p-i-n type PSCs. The incorporated CQDs downshift the band structure and improve the mobility of NiO, which facilitates cascade hole migration from the perovskite layer to the tin-doped indium oxide (ITO) electrode. By optimizing the ratio of CQDs incorporated into the NiO film, efficient charge transport is achieved, resulting in an enhanced power conversion efficiency (PCE) of 17.02%, compared to that of the PSC fabricated with bare NiO (PCE = 15.66%). Moreover, J-V hysteresis is curtailed in the planar PSC employing CQD-incorporated

4

NiO (NiO:CQD), and the long-term stability is enhanced under atmospheric conditions without any encapsulation. The J-V hysteresis is reduced from 4.5% to less than 1% and a PCE exceeding 70% of the initial PCE is retained over 190 h.

2. Experimental 2.1 Synthesis of CQDs CQDs were synthesized via a facile bottom-up method using a hydrothermal reaction, as previously reported[35]. Briefly, the hydrothermal reaction was conducted with 10% fumaronitrile (TCI, Japan) aqueous solution by using a Teflon-lined stainless-steel autoclave at 200 °C for 20 min. After filtering with a polytetrafluoroethylene (PTFE) membrane (Sigma-Aldrich, 0.2 µm pore size), the resulting solution was further purified by repeated centrifugation and re-dispersion in water (10 times). The obtained CQDs were stored in a refrigerator for further use.

2.2 Fabrication of Perovskite Solar Cell Patterned ITO-coated glass substrates were cleaned with acetone, methanol, and deionized water for 3 min per cleaning agent in a sonication bath, dried with nitrogen gas, and treated with UV-ozone for 20 min to create a hydrophilic surface. For the NiO HTLs prepared by the sol-gel process, 0.1 M nickel acetate (Sigma-Aldrich) in ethanol was mixed with 6 vol.% ethanolamine. The as-synthesized CQDs were added to the NiO precursor solution in different mixing ratios (9.99:0.01, 9.95:0.05, and 9:1 volume ratio). NiO or NiO:CQD solution was spin-coated onto UV/ozone-treated ITO glass at 3,000 rpm for 40 s, followed by thermal-annealing at 350 ℃ for 30 min in air. The perovskite layer was spin-coated at 500 rpm for 5 s and 4000 rpm for 45 s using a 35 wt.% solution

5

of CH3NH3I and PbI2 with a 1:1 molar ratio in N,N-dimethylmethanamide (DMF) in air. During the second step of the spin-coating process (after 10 seconds of second step), 0.7 ml isopropyl acetate mixed with toluene (1 :1 vol %) was dropped as anti-solvent to obtain uniform and pinhole-free perovskite films.. After thermal annealing at 100 ℃ for 10 min in air, a high quality perovskite layer with a thickness of ~450 nm was obtained. Thereafter, [6,6]-Phenyl C61 butyric acid methyl ester (PCBM) solution (20 mg PCBM in 1 ml chlorobenzene) was spin-coated onto the perovskite films at 1000 rpm for 60 s. Finally, bathocuproine (BCP)/Ag (3 nm/80 nm) was deposited via thermal evaporation under vacuum at a pressure of 10-7 Torr with an active area of 4.64 mm2. Here, BCP was used to minimize the contact barrier between PCBM and the Ag electrode.

2.3 Characterization of Perovskite Solar Cell The electrical properties of the NiO:CQD buffer layer on the ITO-coated glass were measured with a Hall measurement system using the van der Pauw technique, performed under a magnetic field of 0.55 T (HMS-4000AM, Ecopia). The optical properties of the NiO:CQD buffer layer on ITO glass were investigated as a function of the incorporation ratio of the dispersed CQDs by using a UV-visible spectrometer (UV 540, Unicam). The absorption coefficient (α) of CQDs was estimated by using the equation: α=2.303A/t, where A is absorbance from raw UV-Vis spectra and t is path length of the cuvette in which liquid sample was kept. The band gap was evaluated by using the Tauc plot according to the following equation: αhν ∝ (hν-Eg)n/2 where α is the absorption coefficient, h is the Planck’s constant, ν is the light frequency, and n = 1 and 4 for direct and indirect band gap materials, respectively. In this study, n=4 was used because assynthesized CQD follows the indirect band gap behavior. The photoluminescence (PL)

6

spectra were recorded using a Varian FLR025 spectrometer (Agilent). Time-resolved PL (TRPL) measurements were performed to investigate the PL decay using an ultrafast detection system composed of a TCSPC module (PicoHarp 300, PicoQuant) with a photomultiplier tube (MCP-PMT, R3809U-59, Hamamatsu). A pulsed diode laser head (LDH-P-C-405, PicoQuant) coupled with a laser diode driver (PDL 800-B, PicoQuant) was used as the excitation source. The PL emission was spectrally resolved using collection optics and a monochromator (SP-2150i, Acton). X-ray photoelectron spectroscopy (XPS) was performed with an ESCALAB 250Xi (Thermo Scientific) spectrometer with an Al-Kα X-ray source (1486.6 eV); the spectra were corrected by the C1s line at 284.7 eV. High-resolution transition electron microscopy (HR-TEM) measurements were performed on either a JEM ARM 200F or JEM-2100F (JEOL) microscope. The morphology and work function of the NiO:CQD buffer layer were investigated by using a field-emission scanning electron microscope (FESEM: JSM7600F, JEOL) and a Kelvin probe force microscope (KPFM: NX-10, Park Systems). The contact angle was investigated using a contact angle analyzer (Phoenix-MT(A), SEO Co.), using deionized water with a constant volume of 3 ml. Under 100 mW cm−2 (AM 1.5 G) illumination, the system was calibrated with a Si-reference solar cell certified by the International System of Units (SI) (SRC 1000 TC KG5 N, VLSI Standards, Inc.). For analysis of the light intensity-dependent open circuit voltage (VOC) data, a Newport with neutral density (ND) filter wheel was used. The long-term stability of the device was evaluated by recording the change in the device performance as a function of the exposure time under atmospheric conditions with a humidity of 45±5% for 192 h without any encapsulation.

7

3. Results and discussion The transmission electron microscopy (TEM) image presented in Fig. 1 reveals that the CQDs were well dispersed, with an average particle size of approximately 3.2 nm. The high-resolution (HR) TEM images show that the CQDs exhibit an identical well-resolved lattice fringe spacing of 0.21 nm, corresponding to the (100) crystallographic planes of graphitic carbon. To investigate the composition of the CQDs, X-ray photoelectron spectroscopy (XPS) analysis was performed. The full spectra presented in Fig. 2a show a predominant graphitic C 1s peak at about 285 eV and an O 1s peak at about 533 eV. The O/C atomic ratio for the CQDs is about 24%, while the N/C atomic ratio was calculated to be about 7.4%, which is higher than that of CQDs doped ex-situ with nitrogen[36] due to the presence of nitrogen in the precursor. The C 1s XPS spectrum of the CQDs (Fig. 2b) showed four different peaks after spectral deconvolution, corresponding to sp2 C at a binding energy of 284.3 eV, sp3 C at 284.7 eV, a N-rich group (C-N, 285.6 eV), and an O-rich group (C-O, 286.9 eV). The N 1s band in Fig. 2c was deconvoluted into three peaks at 398.5, 399.9, and 401.0 eV, representing pyridinic N, pyrrolic N, and graphitic N, respectively. As shown in Fig. 2d, the O 1s spectrum comprised two peaks at 532.2 and 533.1 eV, corresponding to C–O and O–H, respectively. The chemical composition of the CQDs was also characterized by Fourier-transform infrared spectroscopy (FT-IR; Fig. S1, Supplementary Information). Therefore, it can be concluded that the CQDs developed herein mainly consisted of π-conjugated domains in their carbon cores and amorphous regions with oxygen- or nitrogen-based hydrophilic moieties on their edges and surfaces. The UV−vis absorption spectrum of the CQDs showed an absorption band at about 310 nm (Fig. 3a), which can be ascribed to the n-π* transition of the C–O/C–N bonds.

8

In addition, there was a weak shoulder peak at 350−450 nm from the carbene-like triplet states of the zig-zag edges of the CQDs[37, 38]. The bandgap of the prepared CQDs (~2.69 eV) was calculated from the Tauc plot (Fig. 3b). Under irradiation by a 365 nm UV-lamp, the CQDs emitted intense blue luminescence with an emission wavelength of about 455 nm. It has been reported that isolated sp2-domains with a size of about 3 nm within the carbon−oxygen matrix could tune the bandgap, consistent with the blue emission, due to localization of the electron−hole pairs (Fig. 3c)[39]. Moreover, the corresponding photoluminescence excitation (PLE) curve of the prepared sample showed two peaks (Fig. 3d): a minor peak at 250−300 nm, corresponding to π-π* transition of sp2 domain, and a major peak at ~380 nm attributed to the excitation of the edge-related states or n-π* transition[40, 41]. The PL behavior related to the edge state of CQDs is responsible to the nitrogen states such as pyrrole groups[42]. Thus, the use of CQDs with rich nitrogen moieties can be a promising strategy for divergence energy conversion applications because the N states in CQDs exhibit tunable optoelectronic properties and energy band levels[43]. The incorporated CQDs can modify the surface energy of the NiO film. In previous reports, the presence of ethanolamine increased the homogenous formation of nickel oxide from a Ni2+ solution[44]. In detail, nickel oxide is plausibly formed in situ due to the reduction of Ni2+ in the Ni(II) complex by hydrogen released from nucleophilic substitution of the -OH groups of ethanolamine. In addition, the hydrophilic groups on both NiO and the CQDs can provide the driving force to anchor the CQDs onto the NiO surface. Fig. 4a shows the contact angles and water droplet images for the NiO surfaces with different ratios of incorporated CQDs. The increase in the contact angle with increasing CQD incorporation indicates that the hydrophilic groups can effectively

9

anchor the CQDs on the surface of NiO. To evaluate the surface morphology and work function in detail, we examined the topography and work function of the NiO:CQD HTL by using a Kelvin probe force microscopy (KPFM) system. The work function of the NiO:CQD system was 4.91±0.02 eV, indicating that the energy levels of NiO:CQD would match well with the work function of ITO and the perovskite layer (Fig. 4b)[45, 46]. By incorporating CQDs into the nickel oxide layer, the nitrogen moieties in the CQDs allow systematic modification of the electronic structure through the effective orbital resonance of the nitrogen moieties with the energy levels of NiO. From the XPS valence spectra (see Fig. S2, Supplementary Information), the energy gap between the Fermi level (Ef) and the valence band maximum (VBM) of the CQD, NiO:CQD, and NiO samples was estimated to be 2.44, 1.61, and 0.59 eV, respectively[47]. These samples were coated onto ITO glass substrates. This trend demonstrates that CQD incorporation downshifts the energy band edges of NiO, which can improve charge transport by photogenerated holes in the planar PSC. As shown in Figure 4c (2D AFM images and surface potential), the root-mean-square (RMS) roughness of the NiO:CQD HTLs at mixing ratios of 9.99:0.01, 9.95:0.05, 9:1 and that for pure NiO was 1.8, 1.6, 2.4, and 2.6 nm, respectively. Both films types were largely uniform and crack-free, even with an increase in the mixing ratio of the CQDs. The effect of the CQDs in the NiO:CQD HTL on charge extraction from the photoactivated perovskite layer was further investigated by recording and analyzing the PL emission, PLE, and PL decay profiles as shown in Fig. 5. The strong emission at 460 nm without any significant shift under various excitation wavelengths is ascribed to the emission from the CQDs (Fig. 5a). There was a slight red-shift (5 nm) of the emission of the CQDs in the NiO:CQD (95:5) film compared with that of the bare CQDs. Fig. 5b

10

shows the PLE spectra. The minor peak at 250−300 nm in the spectrum of the bare CQDs (Fig. 3d) disappeared in the PLE spectrum of the NiO:CQD (95:5) film. The results indicate that the defect states associated with the functional groups in the CQD had no effect on the photoluminescence. Hence, this observation suggests that the functional groups in the CQDs play a key role in the electrostatic interaction with NiO, rather than presenting any emission upon irradiation. Additionally, no significant changes in the optical transmittance and absorbance were observed for NiO and NiO:CQD (95:5) coated on ITO glass (Fig. S3, Supplementary Information). Here, the ITO glass was used as a reference for obtaining the baseline. The incorporation of CQDs into the NiO HTL did not affect the transmittance at 400−800 nm (Table S1, Supplementary Information). The time-resolved PL (TRPL) spectra of perovskitecoated bare NiO and perovskite-coated NiO:CQD (95:5) deposited directly on the glass substrate are shown in Fig. 5c, with the parameters listed in Table S1 (Supplementary Information). The excitation and detection wavelengths were 514 and 780 nm, respectively. The average PL lifetime was reduced from 9.6 ns for bare NiO to 7.1 ns for the NiO:CQD, indicating faster transfer of photoexcited charge carriers. This implies that the CQD-incorporated NiO film provides an effective charge transport pathway for the

photogenerated

holes

in

the

perovskite

layer

and

suppresses

carrier

recombination[48, 49]. In addition, the electrical properties of the NiO:CQD HTL on ITO glass were investigated by Hall measurement (Fig. 4 and Table S2, Supplementary Information). The sheet resistance parameters of the HTL on ITO were 10.1 Ohm sq−1 for bare NiO and 11.8 Ohm sq−1 for NiO:CQD. The carrier mobility of the NiO:CQDcoated HTL on ITO was higher (24.9 cm2 V−1s−1) than that of the NiO-coated ITO without CQDs (19.4 cm2 V−1 s−1). Although carrier mobility of the NiO:CQD-coated

11

ITO is slightly higher than that of the NiO-coated ITO film, reduced carrier concentration led to an increase in sheet resistance of the NiO:CQD HTL. However, both sheet resistance of NiO:CQD and pure NiO HTL are fairly low which acceptable in fabrication of PSC with low leakage current [50]. The influence of NiO:CQD on the photovoltaic performance of a PSC with the p-i-n

configuration

was

investigated.

PSCs

with

the

ITO/HTL/Perovskite/PCBM/BCP/Ag structure were fabricated; the energy level diagram is illustrated in Figure 6. Cross-sectional bright-field (BF) high-resolution (HR) TEM analysis was employed to compare the microstructures of the PSCs with the bare NiO (10:0) and NiO:CQD (95:5) HTL (Fig. 6b−d). The layers were 200 nm (ITO), 25 nm (NiO or NiO:CQD), 450 nm (CH3NH3PbI3), 80 nm (PCBM), and 100 nm (BCP/Ag) thick, respectively. The enlarged images of the interfacial region in the ITO/HTL/Perovskite (Fig. 6c and 6d) structure show that the ITO electrode was crystalline with a columnar structure, and partially crystallized NiO was present in the HTL. Microcrystalline NiO particles were randomly embedded in the amorphous matrix, as shown in the enlarged TEM images. No significant impurities or residuals at the interface between ITO and the HTL were observed. The average values and standard deviation of the photovoltaic parameters, including the open circuit voltage (VOC), short circuit current density (JSC), fill-factor (FF), and PCE are summarized in Table 1. The PSCs with bare NiO had a VOC of 1.08±0.006 V, JSC of 19.11±0.768 mA cm−2, FF of 75.77±1.221%, and PCE of 15.64±0.615% on average. Here, the FF and PCE of ~77% and 16% are reasonable values considering the composition of the perovskite active materials and device structures processed in air. The distributions of the PCE, JSC, FF, and VOC are also summarized in Fig. S5 12

(Supplementary Information). By using the optimized NiO:CQD with the ratio of 95:5, a higher PCE of 16.42±0.392% was obtained. Figure 7a shows the J-V curves of representative devices with the bare NiO HTL and NiO:CQD HTL. The performance parameters are summarized in Table 2. Notably, the J-V hysteresis was reduced from 4.5% to less than 1%. The PCE values obtained from the J-V curves with respect to the forward and reverse scan directions in Fig. 7a were 15.34% and 16.03% for bare NiO, and 16.75% and 16.91% for NiO:CQD, respectively. The series resistance (RS) of the PSCs was 6.12 Ω cm2 for NiO and 4.25 Ω cm2 for NiO:CQD. Although the pure NiO HTL on ITO electrode has slightly lower sheet resistance than NiO:CQD HTL on ITO electrode, the PSC with NiO:CQD HTL showed lower series resistance than PCS with pure NiO HTL due to well-matched work function of NiO:CQD. In order to understand trap-assisted recombination in the PSC, the light intensity-dependent VOC was measured (Fig. 7b) [51]. The decrease in the slopes of the related plots from 3.99 kT sq−1 for the PSC with bare NiO to 1.57 kT sq−1 for the PSC with NiO:CQD indicates that the trapassisted recombination was significantly reduced by using NiO:CQD[52, 53]. To investigate the impact of the CQDs on the stability and feasibility of PSCs, the longterm stability of the PSCs in air was evaluated (Fig. 7c). All devices were kept in air at room temperature without any encapsulation. The PCE of the PSC with bare NiO declined gradually, whereas the PSC with NiO:CQD retained over 70% of its initial PCE after 192 h. The enhanced stability might result from efficient charge extraction, which prevents the accumulated holes from deteriorating the perovskite layer. Moreover, effective anchoring of the CQDs on NiO can suppress the diffusion of moisture or heavy metal components from the HTL or ITO[54].

13

4. Conclusions In summary, enhanced photovoltaic performance and stability of planar p-i-n type PSCs by incorporating CQDs into a NiO-based p-type buffer layer was demonstrated. The incorporation of CQDs with sufficient hydrophilic moieties including oxygen and nitrogen related functional groups at the edge sites induces the systematic modification of the electronic structure on NiO. Incorporation of the CQDs downshifted the band structure of NiO, providing a good match with the work-function of the ITO electrode and the band-edges of the perovskite layer. Furthermore, incorporation of the CQDs improved the electrical properties of the HTL, which contributed to the enhanced charge conductance. Consequently, the efficient cascade charge transport due to introduction of the CQDs into the NiO HTL resulted in enhanced efficiency and ameliorated the J-V hysteresis of the planar PSC. Moreover, long-term stability of the PSCs was achieved with CQD-incorporated NiO. Without any encapsulation, under atmospheric conditions, the PSCs with CQD-incorporated NiO retained ~70% of the initial efficiency after 192 h. These results suggest that the novel approach of incorporating CQDs into the p-type metal oxide buffer layer fabricated by a facile solution process is potentially beneficial for enhancing the durability of PSCs and preventing performance degradation.

Acknowledgements This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) (No. 2018R1A2B2003826) and partially supported by Korea Electric Power Corporation (KEPCO, CX72170049)

14

References [1] L. Xu, X. Chen, J. Jin, W. Liu, B. Dong, X. Bai, H. Song, P. Reiss, Inverted perovskite solar cells employing doped NiO hole transport layers: A review, Nano Energy. 63 (2019) 103860. [2] R. Wang, M. Mujahid, Y. Duan, Z. K. Wang, J. Xue, Y. Yang, A review of perovskites solar cell stbility, Adv. Funct. Mater. (2019) 1808843. [3] W. Xiang, W. Tress, Review on recent progress of all-inorganic metal halide perovskites and solar cells, Adv. Mater. (2019) 1902851. [4] J. Kim, A. Ho-Baillie, S. Huang, Review of novel passivation techniques for efficient and stable perovskite solar cells, Sol. RRL 3 (2019) 1800302. [5] S. Bai, P. Da, C. Li, Z. Wang, Z. Yuan, F. Fu, M. Kawecki, X. Liu, N. Sakai, J. T. W. Wang, S. Huettner, S. Buecheler, M. Fahlman, F. Gao, H. J. Snaith, Planar perovskite solar cells with long-term stability using ionic liquid additives, Nature, 571 (2019) 245-250. [6] M. Zhang, Q. Chen, R. Xue, Y. Zhan, C. Wang, J. Lai, J. Yang, H. Lin, J. Yao, Y. Li, L. Chen, Y. Li, Reconfiguration of interfacial energy band structure for highperformance inverted structure perovskite solar cells, Nat. Commun. 10 (2019) 4593. [7] D. Zheng, G. Wang, W. Huang, B. Wang, W. Ke, J. L. Logsdon, H. Wang, Z. Wang, W. Zhu, J. Yu, M. R. Wasielewski, M. G. Kanatzidis, T. J. Marks, A. Facchetti, Combustion synthesized zinc oxide electron-transport layers for efficienct and stable perovskite solar cells, Adv. Funct. Mater. 29 (2019) 1900265. [8] S. Yang, S. Chen, E. Mosconi, Y. Fang, X. Xiao, C. Wang, Y. Zhou, Z. Yu, J. Zhao, Y. Gao, F. D. Angelis, J. Huang, Stabilizing halide perovskite surfaces for solar cell operation with wide-bandgpa lead oxysalts, Science 365 (2019) 473-478. [9] W. Q. Wu, Z. Yang, P. N. Rudd, Y. Shao, X. Dai, H. Wei, J. Zhao, Y. Fang, Q. Wang, Y. Liu, Y. Deng, X. Xiao, Y. Feng, J. Huang, Bilateral alkylamine for suppressing charge recombination and improving stability in blade-coated perovskite solar cells, Sci. Adv. 5 (2019) eaav8925. [10] Z. Wu, Z. Liu, Z. Hu, Z. Hawash, L. Qiu, Y. Jiang, L. K. Ono, Y. Qi, Highly efficient and stable perovskite solar cells via modification of energy levels at the perovskite/carbon electrode interface, Adv. Mater. 31 (2019) 1804284. [11] C. Liu, W. Kong, W. Li, H. Chen, D. Li, W. Wang, B. Xu, C. Cheng, A. K. Y. Jen, Enhanced stability and photovoltage for inverted perovskite solar cells via precusor engineering, J. Mater. Chem. A 7 (2019) 15880-15886. [12] J. Y. Seo, R. Uchida, H. S. Kim, Y. Saygili, J. Luo, C. Moore, J. Kerrod, A. 15

Wagstaff, M. Eklund, R. Mclntyre, N. Pellet, S. M. Zakeeruddin, A. Hagfeldt, M. Grätzel, Boosting the efficiency of perovskite solar cells with CsBr-modified mesoporous TiO2 beads as electron-selective contact, Adv. Funct. Mater. 28 (2018) 1705763. [13] J. K. Kim, S. U. Chai, Y. Ji, B. L. Wendt, S. H. Kim, Y. Yi, T. F. Heinz, J. K. Nørskov, J. H. Park, X. Zheng, Resolving hysteresis in perovskite solar cells with rapid flame-processed cobalt-doped TiO2, Adv. Energy Mater. 8 (2018) 1801717. [14] Z. Yu, L. Sun, Recent progress on hole-transporting materials for emerging organometal halide perovskite solar cells, Adv. Energy Mater. 5 (2015) 1500213. [15] J. H. Park, J. Seo, S. Park, S. S. Shin, Y. C. Kim, N. J. Jeon, H. W. Shin, T. K. Ahn, J. H. Noh, S. C. Yoon, C. S. Hwang, S. I. Seok, Efficient CH3NH3PbI3 perovskite solar cells employing nanostructured p-type NiO electrode formed by a pulsed laser deposition, Adv. Mater. 27 (2015) 4013-4019. [16] N. Li, Z. Zhu, C. C. Chueh, H. Liu, B. Peng, A. Petrone, X. Li, L. Wang, A. K. Y. Jen, Mixed cation FAxPEA1-xPbI3 with enhanced phase and ambient stability toward high-performance perovskite solar cells, Adv. Energy Mater. 7 (2017) 1601307. [17] X. Yin, Z. Yao, Q. Luo, X. Dai, Y. Zhou, Y. Zhang, Y. Zhou, S. Luo, J. Li, N. Wang, H. Lin, High efficiency inverted planar perovskite solar cells with solutionprocessed NiOx hole contact, ACS Appl. Mater. Interfaces 9 (2017) 2439-2448. [18] Z, Liu, J. Chang, Z. Lin, L. Zhou, Z. Yang, D. Chen, C. Zhang, S. Liu, Y. Hao, High-performance planar perovskite solar cells using low temperature, solutioncombustion-based nickel oxide hole transporting layer with efficiency exceeding 20%, Adv. Energy Mater. 8 (2018) 1703432. [19] L. J. Tang, X. Chen, T. Y. Wen, S. Yang, J. J. Zhao, H. W. Qiao, Y. Hou, H. G. Yang, A solution-processed transparent NiO hole-extraction layer for highperformance inverted perovskite solar cells, Chem. Eur. J. 24 (2018) 2845-2849. [20] Y. H. Seo, E. C. Kim, S. P. Cho, S. S. Kim, S. I. Na, Hysteresis data of planar perovskite solar cells fabricated with different solvents, Data in Brief 16 (2018) 418-422. [21] J. S. Yeo, R. Kang, S. Lee, Y. J. Jeon, N. Myoung, C. L. Lee, D. Y. Kim, J. M. Yun, Y. H. Seo, S. S. Kim, S. I. Na, Highly efficient and stable planar perovskite solar cells with reduced graphene oxide nanosheets as electrode interlayer, Nano Energy, 12 (2015) 96-104. [22] S. Erten-Ela, H. Chen, A. Kratzer, A. Hirsch, C. J. Brabec, Perovskite solar cells fabricated using dicarboxylic fullerene derivatives, New J. Chem. 40 (2016) 2829-

16

2834. [23] E. Nouri, M. R. Mohammadi, P. Lianos, Improving the stability of inverted perovskite solar cells under ambient conditions with graphene-based inorganic charge transporting layers, Carbon 126 (2018) 208-214. [24] F. Zhang, M. M. Mezgeb, W. Guo, H. Xu, X. Liu, An efficient and simple dual effect by under-layer abduction design for highly flexible NiOx-based perovskite solar cells, J. Power Sources 399 (2018) 246-253. [25] J. V. Milić, N. Arora, M. I. Dar, S. M. Zakeeruddin, M. Grätzel, Reduced graphene oxide as a stabilizing agent in perovskite solar cells, Adv. Mater. Interfaces 5 (2018) 1800416. [26] J. R. Manders, S. W. Tsang, M. J. Hartel, T. H. Lai, S. Chen, C. M. Amb, J. R. Reynolds, F. So, Solution-processed nickel oxide hole transport layers in high efficiency polymer photovoltaic cells, Adv. Funct. Mater. 23 (2013) 2993-3001. [27] J. K. Kim, S. Bae, Y. Yi, M. J. Park, S. J. Kim, M. Nosoung, C. L. Lee, B. H. Hong, J. H. Park, Origin of white electroluminescence in graphene quantum dots embedded host/guest polymer light emitting diodes, Sci. Rep. 5 (2015) 11032. [28] W. Chen, Y. Zhou, L. Wang, Y. Wu, B. Tu, B. Yu, F. Liu, H. W. Tam, G. Wang, A. B. Djurišić, L. Huang, Z. He, Molecule-doped nickel oxide: verified charge transfer and planar inverted mixed cation perovskite solar cell, Adv. Mater. 30 (2018) 1800515. [29] J. W. Jung, C. C. Chueh, A. K. Y. Jen, A low-temperature, solution-processable, cu-doped nickel oxide hole-transporting layer via the combustion method for highperformance thin-film perovskite solar cells, Adv. Mater. 27 (2015) 7874-7880. [30] Z. Zhu, J. Ma, Z. Wang, C. Mu, Z. Fan, L. Du, Y. Bai, L. Fan, H. Yan, D. L. Phillips, S. Yang, Efficiency enhancement of perovskite solar cells through fast electron extraction: the role of graphene quantum dots, J. Am. Chem. Soc. 136 (2014) 3760-3763. [31] S. Park, R. S. Ruoff, Chemical methods for the production of graphenes, Nat. Nanotechnol. 4 (2009) 217-224. [32] W. Chen, F. Z. Liu, X. Y. Feng, A. B. Djurišić, W. K. Chan, Z. B. He, Cesium doped NiOx as an efficient hole extraction layer for inverted planar perovskite solar cells, Adv. Energy Mater. 7 (2017) 1700722. [33] J. Ryu, J. W. Lee, H. Yu, J. Yun, K. Lee, J. Lee, D. Hwang, J. Kang, S. K. Kim, J. Jang, Size effects of a graphene quantum dot modified-blocking TiO2 layer for efficient planar perovskite solar cells, J. Mater. Chem. A 5 (2017) 16834-16842. [34] H, Li, W. Shi, W. Huang, E. P. Yao, J. Han, Z. Chen, S. Liu, Y. Shen, M. Wang,

17

Y. Yang, Carbon quantum dots/TiOx electron transport layer boosts efficiency of planar heterojunction perovskite solar cells to 19%, Nano Lett. 17 (2017) 23282335. [35] B. J. Moon, Y. Oh, D. H. Shin, S. J. Kim, S. H. Lee, T. W. Kim, M. Park, S. Bae, Facile and purification-free synthesis of nitrogenated amphiphilic graphitic carbon dots, Chem. Mater. 28 (2016) 1481-1488. [36] Y. Li, Y. Zhao, H. Cheng, Y. Hu, G. Shi, L. Dai, L. Qu, Nitrogen-doped graphene quantum dots with oxygen-rich functional groups, J. Am. Chem. Soc. 134 (2012) 15-18. [37] A. Ananthanarayanan, X. Wang, P. Routh, B. Sana, S. Lim, D. H. Kim, K. H. Lim, J. Li, P. Chen, Facile synthesis of graphene quantum dots from 3D graphene and their application for Fe3+ sensing, Adv. Funct. Mater. 24 (2014) 3021-3026. [38] W. Li, S. Wu, H. Zhang, X. Zhang, J. Zhuang, C. Hu, Y. Liu, B. Lei, L. Ma, X. Wang, Enhanced biological photosynthetic efficiency using light-harvesting engineering with dual-emissive carbon dots, Adv. Funct. Mater. 28 (2018) 1804004. [39] G. Eda, Y. Y. Lin, C. Mattevi, H. Yamaguchi, H. A. Chen, I. S. Chen, C. W. Chen, M. Chhowalla, Blue photoluminescence from chemically derived graphene oxide, Adv. Mater. 22 (2010) 505-509. [40] Z. Wang, H. Zeng, L. Sun, Graphene quantum dots: versatile photoluminescence for energy, biomedical, and environmental applications, J. Mater. Chem. C 3 (2015) 1157-1165. [41] F. Liu, M. H. Jang, H. D. Ha, J. H. Kim, Y. H. Cho, T. S. Seo, Facile synthetic method for pristine graphene quantum dots and graphene oxide quantum dots: origin of blue and green luminescence, Adv. Mater. 25 (2013) 3657-3662. [42] D. Qu, M. Zheng, J. Li, Z. Xie, Z. Sun, Tailoring color emissions from N-doped graphene quantum dots for bioimaging applications, Light Sci. Appl. 4 (2015) e364. [43] H. Tetsuka, A. Nagoya, T. Fukusumi, T. Matsui, Molecularly designed, nitrogenfunctionalized graphene quantum dots for optoelectronic devices, Adv. Mater. 28 (2016) 4632-4638. [44] M. S. Bazarjani, H. J. Kleebe, M. M. Müller, C. Fasel, M. B. Yazdi, A. Gurlo, R. Riedel, Nanoporous silicon oxycarbonitride ceramics derived from polysilazanes in situ modified with nickel nanoparticles, Chem. Mater. 23 (2011) 4112-4123. [45] P. V. Kumar, M. Bernardi, J. C. Grossman, The impact of functionalization on the stability, work function, and photoluminescence of reduced graphene oxide, ACS

18

Nano 7 (2013) 1638-1645. [46] Z. Ding, Z. Hao, B. Meng, Z. Xie, J. Liu, L. Dai, Few-layered graphene quantum dots as efficient hole-extraction layer for high-performance polymer solar cells, Nano Energy 15 (2015) 186-192. [47] J. Zheng, L. Hu, J. S. Yun, M. Zhang, C. F. J. Lau, J. Bing, X. Deng, Q. Ma, Y. Cho, W. Fu, C. Chen, M. A. Green, S. Huang, A. W. Y. Ho-Baillie, Solutionprocessed, silver-doped NiOx as hole transporting layer for high-efficiency inverted perovskite solar cells, Acs Appl. Energy Mater. 1 (2018) 561-570. [48] Q. Wang, Z. Jin, D. Chen, D. Bai, H. Bian, J. Sun, G. Zhu, G. Wang, S. Liu, µgraphene crosslinked CsPbI3 quantum dots for high efficiency solar cells with much improved stability, Adv. Energy Mater. 8 (2018) 1800007. [49] A. Privitera, M. Righetto, D. Mosconi, F. Lorandi, A. A. Isse, A. Moretto, R. Bozio C. Ferrante, L. Franco, Boosting carbon quantum dots/fullerene electron transfer via surface group engineering, Phys. Chem. Chem. Phys. 18 (2016) 31286-31295. [50] D. Raeyani, S. Shojaei, S. A. Kandjani, W. Wlodarski, Synthesizing graphene quantum dots for gas sensing applications, Procedia Eng. 168 (2016) 1312-1316. [51] M. Kuik, H. T. Nicolai, M. Lenes, G. J. A. H. Wetzelaer, M. Lu, P. W. M. Blom, Determination of the trap-assisted recombination strength in polymer light emitting diodes, Appl. Phys. Lett. 98 (2011) 093301. [52] Y. Zheng, G. Wang, D. Huang, J. Kong, T. Goh, W. Huang, J. Yu, A. D. Taylor, Binary solvent additives treatment boosts the efficiency of PTB7:PCBM polymer solar cells to over 9.5%, Solar. RRL 2 (2018) 1700144. [53] T. S. Sherkar, C. Momblona, L. Gil-Escrig, J. Ávila, M. Sessolo, H. J. Bolink, L. J. A. Koster, Recombination in perovskite solar cells: significance of grain boundaries, interface traps, and defect ions, ACS Energy Lett. 2 (2017) 1214-1222. [54] M. M. Tavakoli, R. Tavakoli, P. Yadav, J. Kong, A graphene/ ZnO electron transfer layer together with perovskite passivation enables highly efficient and stable perovskite solar cells, J. Mater. Chem. A 7 (2019) 679-686.

19

Fig. 1 (a) Schematic Illustration of synthesis of CQDs. (b) TEM image showing distribution of CQDs. (c) HR-TEM image of CQD with the lattice fringes.

20

Fig. 2 (a) XPS survey spectra of CQDs. High-resolution (b) C 1s, (c) N 1s, and (d) O 1s spectra of prepared CQDs.

21

Fig. 3 (a) UV-Vis absorbance spectra, (b) Tauc plot, (c) PL emission, and (d) PL excitation spectra of prepared CQDs.

22

(a)

(b)

NiO

(c)

99:1

NiO

95:5

NiO

90:10

99:1

5 µm by 5 µm

99:1

95:5

90:10

5 µm by 5 µm

0.5 V

0.5 V

0V

0V

265 ± 17 mV

Rq: 1.8 nm 95:5

289 ± 16 mV

Rq: 1.6 nm

4.90 ± 0.02 eV

4.88 ± 0.02 eV

90:10 0.5 V

0.5 V

5 um by 5 um

0V

0V

222 ± 16 mV

253 ± 16 mV

Rq: 2.4 nm

4.92 ± 0.02 eV

Rq: 2.6 nm

4.95 ± 0.02 eV

Fig. 4 (a) Contact angle values and water droplet images, (b) work function parameters of HTL surface with different ratios of incorporated NiO on ITO glass, and (c) 2D surface AFM images (Topography and surface potential). Here, RMS indicates rootmean-square.

23

Fig. 5 (a) PL emission and (b) PL excitation spectra of prepared NiO:CQDs (95:5). (c) Comparison of typical PL decay profiles of perovskite-coated pristine NiO and NiO:CQDs (95:5). Here, the average PL decay is calculated with the bi-exponential equation: τavg = f1τ1 + f2τ2, where f1 and f2 are the fractional intensities and τ1 and τ2 are the lifetime components.

24

Fig. 6 (a) Schematic illustration and (b) cross-sectional SEM images of planar PSCs with p-i-n configuration and ITO/HTL/Perovskite/PCBM/BCP/Ag cell structure. (c) Energy level diagram of PSCs with NiO and NiO:CQD HTLs. Typical cross-sectional high-resolution TEM image of the p-i-n PSCs at the layer of (d) ITO/NiO/Perovskite and (e) ITO/NiO:CQD/Perovskite.

25

Fig. 7 (a) J-V curves with respect to the forward and reverse scan directions, (b) dependence of open circuit voltage on light intensity, and (c) steady-state PCE measurement. As a default, NiO:CQD with 5 % CQD was used for characterization.

26

Table 1. Device parameters for PSCs with different CQD concentrations in NiO VOC FF PCE JSC NiO:GQD [%] [%] [V] [ mA/cm2 ] 100:0

1.08 ( ± 0.006 )

19.11 ( ± 0.768 )

75.77 ( ± 1.221 )

15.64 ( ± 0.615 )

99:1

1.06 ( ± 0.019)

18.56 ( ± 0.531 )

73.03 ( ± 5.417 )

14.42 ( ± 1.517 )

95:5

1.08 ( ± 0.004 )

19.57 ( ± 0.333 )

77.48 ( ± 0.821 )

16.42 ( ± 0.392 )

90:10

1.08 ( ± 0.006 )

18.99 ( ± 0.361 )

76.88 ( ± 0.817 )

15.75 ( ± 0.346 )

27

Table 2. Summary of device performance for PSCs with respect to forward and reverse scans NiO:GQD

VOC

JSC

FF

PCE

[V]

[ mA/cm2 ]

[%]

[%]

Forward

1.08

18.28

77.43

15.34

Reverse

1.09

20.49

70.02

16.03

Forward

1.10

20.87

73.13

16.75

Reverse

1.08

20.22

77.15

16.91

Scan

100:0

95:5

28

Research Highlight Carbon quantum dot-incorporated nickel oxide for planar pi-n type perovskite solar cells with enhanced efficiency and stability

Jung Kyu Kima, Duong Nguyen Nguyena, Jae-Hoon Leeb, Seunghun Kangb, Yunseok Kimb, Seok-Soon Kimc*, and Han-Ki Kimb*

a

School of Chemical Engineering, Sungkyunkwan University, 2066 Seobu-ro, Jangan-gu, Suwon, 16419 Republic of Korea b School of Advanced Materials Science & Engineering, Sungkyunkwan University, 2066 Seobu-ro, Jangan-gu, Suwon, Gyeonggi-do 440-746, Republic of Korea c Department of Nano and Chemical Engineering, Kunsan National University, 558, Daehak-ro, Gunsansi, Jeollabuk-do, 54150, Republic of Korea

Carbon quantum dots (CQDs) incorporated in NiO hole transport layer (HTL). Perovskite solar cells with enhanced efficiency and long-term stability by CQDs incorporated NiO HTL. Good match with the work-function of the ITO electrode

Declaration of interests ☒The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

We declare that we have no known competing financial interest or personal relationships that could have appeared to influence the work reported in this paper Professor Han-Ki Kim