Enhanced performance of planar perovskite solar cell by graphene

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Enhanced performance of planar perovskite solar cell by graphene quantum dot modification Jianjun Zhang, Tong Tong, Liuyang Zhang, Xiaohe Li, Haiyuan Zou, and Jiaguo Yu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00938 • Publication Date (Web): 07 Jun 2018 Downloaded from http://pubs.acs.org on June 7, 2018

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Enhanced performance of planar perovskite solar cell by graphene quantum dot modification

Jianjun Zhang,† Tong Tong,† Liuyang Zhang,† Xiaohe Li,† Haiyuan Zou,† and Jiaguo Yu*,†

†

State Key Laboratory of Advanced Technology for Materials Synthesis and

Processing, Wuhan University of Technology, 122 Luoshi Road, Wuhan, 430070, P. R. China. E-mail: [email protected]

E-mail: Jianjun Zhang: [email protected] Tong Tong: [email protected] Liuyang Zhang: [email protected] Xiaohe Li: [email protected] Haiyuan Zou: [email protected] Jiaguo Yu: [email protected]

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Abstract: In organic-inorganic halide perovskite solar cells (PSCs), the perovskite layer is the main source of photo-generated electron-hole pairs. Therefore, premier concern in PSCs is to improve the quality of perovskite film (PF). In the present research, graphene quantum dots (GQDs) were prepared and incorporated in perovskite precursor, and due to the merits of dangling bonds, quantum size, and excellent electronic conductivity of GQDs, PF of higher-quality with flat surface and pinhole-free hallmarks was garnered. It is delightful that the PFs with GQDs exhibit higher light absorption and faster charge extraction. Consequently, the power conversion efficiency (PCE) of PSCs incorporating GQDs achieves an improvement of 11% compared with the pristine ones. Our work confirms that incorporating GQDs is a viable approach to obtain high-quality PF with more efficient charge extraction for superior planar PSCs. Keywords: graphene quantum dots, perovskite film, planar heterojunction solar cell, charge extraction

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■ INTRODUCTION The development of renewable energy to supersede depleted traditional fossil fuels is exigent for the sustainable development of human society.1,2 Among all the protocols, converting solar energy directly to electrical energy by photovoltaics is a sustainable and thus promising way.3,4 Recently, organic-inorganic halide perovskites including MAPbI3 and FAxMA1-xPbI3 (MA = CH3NH3+, FA = CH(NH2)2+), have emerged as novel materials for perovskite solar cells (PSCs), and have drawn far-flung attention owing to their wide spectral absorption, tunable band gaps and easier charge transfer.5,6 The power conversion efficiency (PCE) of PSCs has soared like a shot from 3.8% to 22.7%.7,8 Therefore, PSCs are regarded as the most promising next-generation photovoltaics, and the great properties endow PSCs with potentials for various other applications.9 However, the relatively poor crystallization and charge recombination of perovskite create the bottlenecks for their performance advancement.10 The quality of PF is cardinal.11 The ideal PF to achieve superior PSCs should be single-crystalline, defect-free and cover the substrate completely.12 Thus, to fabricate desirable PSCs, it is vital to delicately tune the quality of the PF, manifested by the crystallinity and morphology, during the fabrication process.13 Some quality-control approaches have proved to be effective, such as using polymer additives to control the perovskite growth,14 employing solvent engineering to produce uniform domains,15 and incorporating substituent elements to improve film morphology.16 Besides, effective charge extraction is also critical for the enhancement of PSC performance. 3



Many efforts have been made to expedite charge transfer and suppress carrier recombination, including conductivity improvement of electron/hole transport layer,17 interlayer introduction for facilitated charge transfer,18 and band-gap adjustment of perovskite.19 To achieve outstanding PSCs, it is imperative and necessary to attain both aspects.20 It thus requires enormous effort to realize both features at the same time. Herein, we provide a simple approach to obtain highly crystallized and smooth PFs with superior charge extraction via incorporating graphene quantum dots (GQDs) into organic-inorganic halide perovskite (MAPbI3). GQDs are single, double or few layer(s) of graphene sheets whose sizes are no bigger than 10 nanometer,21 which possess the merits of both graphene and quantum dots, including high conductivity, excellent solubility, stable photoluminescence and quantum size effect.22 Thus, eminently efficient charge transport can be expected after the introduction of GQDs. In addition, the dangling bonds on GQDs (hydrophilic groups like −OH, −C=O and – COOH) contribute to excellent linkages with perovskite, attributing to denser perovskite grains possessing reduced pinholes and defects. In this research, we prove that incorporating GQDs into the precursor of perovskite can lead to an improved quality of the resultant PF as well as better charge extraction with enhanced light absorption. Consequently, the PCE of PSCs is obviously increased after GQD incorporation. Our work not only furnishes a feasible approach achieving high-quality PF, but also unveils the positive role of GQDs in charge extraction of PSCs.

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■ RESULTS AND DISCUSSION

Figure 1. (a) FTIR spectrum of GQDs. (b) The high-resolution XPS of C 1s of GQDs. (c) UV-vis absorption spectrum of GQDs. (d) PL spectrum of GQD suspension in water excited at 400 nm. Inset: the photograph of GQD suspension. (e) TEM image of GQDs. Insets: size distribution of GQDs and GQD suspension under 365 nm UV irradiation. 5



Experimental details can be found in the Supporting Information. The PFs, which were derived from precursor solutions with GQD mass ratios (relative to perovskite) of 0.05%, 0.1%, 0.2%, 0.5% and 1%, were designated as G0.05, G0.1, G0.2, G0.5, G1 (i.e., Gx, where x indicated the mass ratio of GQD in the precursos) and Pristine, respectively. Figure 1a shows the FTIR spectrum of GQDs. The absorption peaks at wavenumbers of 1097, 1353, 1579 and 3307 cm−1 indicate the bonds of C−O, C−H, C=C, and –OH group, respectively.23 Its high-resolution XPS of C 1s core level can be deconvoluted to three peaks assigned to C−C, C−O and C=O at 284.6, 285.5 and 288.3 eV, as shown in Figure 1b, confirming that the surface of GQDs is functionalized with plenty of hydroxyl and carboxylic acid groups.24 The UV-vis absorption spectrum of GQDs in Figure 1c exhibits obvious absorption in the whole visible region of the spectrum. Meanwhile, GQDs show a characteristic PL peak at around 500 nm (Figure 1d). It is noted that GQDs can be well dispersed in water, forming pale-yellow limpid solution (inset of Figure 1d), which further reveals the superior hydrophilicity of GQDs. From the above results, it is anticipated that combining GQDs with perovskite of MAPbI3 is feasible due to the hydrophilic groups on GQD surface, which is also favorable to reduce the grains of perovskite. Meanwhile, the visible-light–active feature of GQDs is beneficial for perovskite light harvesting if appropriate amounts of GQDs are introduced. The transmission electron microscopy (TEM) image and the size distribution (Figure 1e) show that the as-prepared GQDs measure less than 3 nm in each dimension with yellow 6


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fluorescence under 365 nm UV irradiation (insets of Figure 1e), suggesting that GQDs

are

zero-dimensional

nanostructured

material

with

stable

photoluminescence.25 Obviously, from these characterizations, it can be concluded that the desired GQDs have been successfully prepared.

Figure 2. SEM images depicting surfaces of PFs with diverse contents of GQD: (a) Pristine, (b) G0.05, (c) G0.1, (d) G0.2, (e) G0.5, (f) G1. Red circles indicate the pinholes. 7



In this research, a well-established one-step solution process was adopted to prepare pristine MAPbI3 film assisted by several drops of chlorobenzene onto the substrates during spin-coating. This method can make a relatively smooth MAPbI3 film with a complete coverage, but unfortunately also lead to small crystal size and pinhole defect.26 As reflected in Figure 2a, the pristine PF consists of numerous relatively small perovskite crystals with grain size ranging from 100 to 200 nm, and plenty of tiny crystal grains can also be found, whose boundaries are not in close contact testified by the existence of a few apparent pinholes. Once GQDs are uniformly dispersed in the precursors preparing perovskite, the morphological change of resultant PF, compared to the pristine one, is very evident. When the GQD content in perovskite is not beyond 0.1%, the size of the crystals grows larger and the surface of PF becomes smoother with fewer pinholes with the increment of GQD content (Figure 2a−2c). Distinctly, G0.1 PF has the maximum grain size of approximately 300 nm with a relatively smooth surface among all the prepared samples (Figure 2c). However, as shown in Figure 2d−2f, when the GQD content exceeds 0.1%, further increase GQD content results in smaller grains and more pinhole (highlighted in red circles). Especially in Figure 2f (G1), the number of pinholes are conspicuous. Thus, tentative conclusion can be drawn that GQD is the pivotal factor in the crystallization of PF. A moderate content of GQD contribute to better crystallized PF. The reason is that GQDs seize hydrophilic groups embracing −OH, −COOH and C=O, which can form strong bonds with the perovskite, ensuring closer contact with perovskite grains 8


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and better crystallization. However, excess GQD incorporation is adverse, causing more pinholes and dividing perovskite grains. The formation of perovskite crystallization contains two steps: nucleation and crystal growth. Excess GQDs can serve as the heterogeneous nucleation centers of perovskite.27 Perovskite crystals tend to grow independently due to massive nucleation sites, leading to more crystals with smaller sized grains. However, excess GQDs lead to aggregation, generating more pinholes inside the PF and inducing substandard PFs.

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Figure 3. (a−f) AFM images for pristine PF and PFs with different GQD contents.

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Figure 4. (a) Line segments and (b) depth distribution with the average height (Hav). (c, d) Arithmetic mean deviation (Ra) and root mean square deviation (Rq) of pristine and GQD-revamped PFs and, respectively.

Figure 3 illustrates indiscernible alteration among images for the surface profiles of the films from Pristine to G0.5. When the content of GQD reaches 1%, the disparity occurs, evidenced by relatively huge pinholes and height fluctuation (marked by red circle). This AFM result is different from that obtained from SEM investigation (Figure 2), probably due to the relatively lower magnification of AFM images, which may hide some valuable details. Therefore, the details of surface profile of these PFs are further analyzed by the line segments (Figure 4a) and depth distribution (Figure 4b). Among all the samples, the surface of G0.1 PF is relatively smooth with narrow 11



depth distribution. Meanwhile, Ra and Rq to describe the surface profile are exhibited as well in Figure 4c and 4d, respectively. The Ra and Rq of PFs firstly decline and then rise with an increasing amount of GQD. And G0.1 PF has the minimum Ra and Rq, which also validates its smoothest surface. These foregoing results are accordant with the SEM investigation. From all the surface investigations, it is verified that 0.1% is the optimum ratio of GQDs to achieve denser perovskite grains and fewer defects/pinholes.

Figure 5. XRD patterns of PFs on FTO glass with diverse amounts of GQDs.

XRD analysis unfolds and acquires the impact of GQDs. As shown in Figure 5, strong diffraction peaks centered at 14.0°, 28.5°, 31.9° and weak peaks centered at 20.0°, 23.5° correspond to the {110}, {220}, {310} and {112}, {211} crystal facets of perovskite (MAPbI3), respectively, demonstrating that all the PFs are tetragonal.28 It is 12


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noted that the intensities of the first three peaks are the strongest when the GQD content is 0.1% (G0.1), suggesting that a moderate amount of GQD makes perovskite crystal grow along the (001) direction with the best crystallinity.29 A further increase in GQD content leads to reduced crystallinity of PFs compared with G0.1, because excess GQDs act as the heterogeneous nuclei,30 which is detrimental to the perovskite crystallization for PSCs, as illustrated in the SEM (Figure 2) and AFM (Figure 3 and Figure 4) results.

Figure 6. (a) Absorption and (b) photoluminescence spectra of pristine PFs and GQDs/PFs on the compact-TiO2/FTO substrates (excited at 480 nm).

UV-vis absorption spectra of PFs with and without GQDs (Figure 6a) explicate enhanced adsoption was manifested by the ones incorporating GQDs from G0.05 to G0.2. The boosted film quality should be accredited. However, as has been stated, excessive GQDs have adverse effect via acting as the heterogeneous nuclei, resulting in grain shrinking. Besides, the aggregation of excess GQDs leads to more cracks and pinholes in the film, and excess GQDs can shield the light that PF can utilize effectively. These two factors account for the decreased UV-vis absorption of G0.5 13



and G1 films compared with G0.2.31 Steady-state PL quenching spectra were taken to investigate the charge extraction efficiency of the PFs. The sharing characteristic PL peaks at around 780 nm match well with the common UV-vis absorption edge at the same wavelength, which also implies that the as-prepared well-crystallized perovskite is deemed as a direct band-gap semiconductor. Figure 6b clearly demonstrates that the PL intensity weakens after GQD incorporation, which means that GQDs are favorable to quicken the electron extraction from PF to TiO2 compact layer. Moreover, the PL peaks of G0.5 and G1 films are visually blue-shifted. This is because the PL peak for bare GQD reside at ca. 500 nm. If the amount of GQDs is appropriate, they serve as electron acceptor, analogous to the most commonly used fullerenes in PSCs.32 Besides, GQDs can also bridge the electron transport and injection from perovskite to TiO2, contributing to the effective separation of active electrons and holes.33

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Figure 7. (a−b) Dynamic polarization data (phase and modulation) and (c−d) the distributions of lifetime for Pristine/TiO2 and G0.1/TiO2 films (excited at 480 nm and measured at 780 nm).

Table 1: Fluorescence decay parameters for Pristine/TiO2 and G0.1/TiO2 films. Samples

τ1 (ns)

Pristine G0.1

α1 (%)

τ2 (ns)

α2 (%)

τave (ns)

111

97.3

12.2

2.7

111

55.2

91.7

11.7

8.3

54.4

To further characterize the photo-induced charge transfer between the PF and TiO2 film,

time-resolved

fluorescence

measurements

were

conducted

by

using

frequency-domain fluorometry. Figure 7a and 7b demonstrate the experimental data 15



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and the fitting results of phase delay and modulation ratio of the pristine and G0.1 perovskite/TiO2 films, respectively. The abscissa of the intersection of the phase delay curve and modulation ratio curve indicates the value of frequency fi, which is inversely proportional to the PL lifetime. The frequency f1 (for Pristine/TiO2) is smaller than f2 (for G0.1/TiO2) (Figure 7a and 7b), suggesting that the G0.1/TiO2 film possesses shorter PL lifetime than the Pristine/TiO2 film. Thus the G0.1/TiO2 film has a lower electron recombination than the Pristine/TiO2 film. Besides, the distributions of lifetime for Pristine/TiO2 and G0.1/TiO2 films are exhibited in Figure 7c−d, respectively. Average recombination lifetime (τave) is calculated by PL decay time (τ1,

τ2) and the amplitudes of the samples (α1, α2) according to the following equation:34

τ =

α τ α τ α τ α τ

(1)

where the slow decay component (τ1) and the ultrafast decay component (τ2) are correlated

with

radiative

recombination

and

trap-mediated

non-radiative

recombination, respectively.35 A shorter average recombination lifetime corresponds to more efficient charge transport between TiO2 compact layer and perovskite layer. As shown in Table 1, for Pristine/TiO2 film, τave is 111 ns. After the incorporation of 0.1% GQD into the perovskite, τave of the film reduces to 55.2 ns, suggesting more efficient and rapid electron transport from PF to TiO2 compact film. Therefore, the results of time-resolved fluorescence measurement also reveal that GQDs can serve as the electron transport bridge for faster charge extraction.

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Figure 8. (a) Schematic illustration of the structure of our planar PSCs. (b) Cross-section SEM image of G0.1 PSCs. (c) The energy band position (relative to vacuum) of each layer in GQD-containing PSCs.

Figure 8a shows our n-i-p type planar heterojunction PSCs with a schematic layered

structure

of

FTO

glass/compact-TiO2/MAPbI3

(with

GQDs)/Spiro-OMeTAD/Au. Each layer is distinctive in Figure 8b. Figure 8c exhibits the energy level of each layer of the PSC. The conduction band edge position for FTO, TiO2 and perovskite are −4.4, −4.1 and −3.93 eV vs. vacuum, respectively. Moreover, the valence band edges of perovskite, spiro-OMeTAD and the work function of Au are −5.4, −5.2 and −5.1 eV, respectively. All of the energy band alignments allow the 17



perovskite photo-generated carriers to be separated effectively to flow through the whole PSC device. From Figure 8c, it is inferred that compared to the pristine perovskite-based PCS, in the GQD-incorporated perovskite, GQDs can serve as the electron trapping and transport agents between perovskite and compact TiO2 simultaneously. Thus the photo-generated electrons can be easily injected from perovskite to TiO2, contributing to more effective separation of photo-generated electron-hole pairs in the PF.

Figure 9. (a) Current density-voltage characteristics for the pristine and 18


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GQD-incorporated

PSCs.

(b)

IPCE

spectra

of

the

pristine

and

0.1%

GQD-incorporated PSCs. J-V curves of (c) pristine and (e) G0.1 PSCs under reverse and forward scan. Stabilized photocurrent measurement and power output at a bis voltage of (d) 0.81 V for pristine PSC and (f) 0.87 V for G0.1 PSC at maximum power output.

Figure 10. Photovoltaic parameters of PSCs. Average performance parameters were obtained based on 40 tested cells.

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Table 2. Photovoltaic parameters of PSCs Average performance parameters were obtained based on 40 cells for each type. Samples

Voc (V)

Jsc (mA cm-2)

FF (%)

PCE (%)

Pristine

1.07 ± 0.02

23.18 ± 0.29

68.1 ± 1.9

17.0 ± 0.9

G0.05

1.09 ± 0.02

23.67 ± 0.32

68.6 ± 2.3

17.7 ± 0.7

G0.1

1.13 ± 0.01

23.86 ± 0.31

70.0 ± 1.6

18.9 ± 0.8

G0.2

1.07 ± 0.01

23.30 ± 0.27

69.8 ± 1.7

17.4 ± 0.7

G0.5

1.06 ± 0.03

23.72 ± 0.35

64.8 ± 1.6

16.3 ± 0.7

G1

1.03 ± 0.03

23.60 ± 0.34

60.5 ± 2.2

14.7 ± 0.9

The GQD effect on photo-electron conversion performance of our PSCs was investigated and exhibited in Figure 9, and the corresponding photovoltaic parameters are shown in Figure 10 and Table 2. The device with the pristine perovskite has an average power conversion of 17.0%, with an open-circuit voltage (Voc) of 1.07 V, a short-current density (Jsc) of 23.18 mA cm−2 and a fill factor (FF) of 68.1%. When a moderate amount of GQD was incorporated into the PF (G0.05 and G0.1), Voc and Jsc are significantly enhanced. Specifically, the device with G0.1 perovskite unfolds the best performance and achieves an average PCE of 18.9% (the champion one can achieve a PCE of 19.7%), corresponding to 11% improvement compared with that of the pristine perovskite-based device, which is mainly attributed to the higher Voc (1.13 V), Jsc (23.86 mA cm-2) and FF (70.0%). With GQD incorporation, the increase of open-circuit voltage mainly due to two reasons: more effective photo-generated 20


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electron-hole pairs and suppressed charge recombination. As aforementioned in the UV-vis absorption spectra (Figure 6a), the PFs with moderate GQD incorporation exhibit enhanced light absorption, contributing to growing amount of photo-generated electrons and holes. In the meanwhile, moderate GQD modified PFs process fewer flaws and pin-holes, so the number of charge recombination center is significantly decreased. Moreover, the PSC with G0.1 exhibits higher IPCE (Figure 9b) than the pristine one, which is consistent with the highest Jsc shown in Figure 10. Figure 9c and 9e reflect the J-V curves under reverse and forward scan of the pristine and G0.1 PSCs.

The

degree

of

current

voltage

hysteresis

is

defined

as

H=

100%×(PCErev−PCEfor)/PCErev where PCErev and PCEfor denote the PCE for reverse scan and forward scan, respectively. The hysteresis factor H is suppressed from 32% (PCEfor= 11.6% and PCErev= 17.0%) for the pristine PSC to 14% (PCEfor= 16.3% and PCErev= 18.9%) for G0.1 PSC. Meanwhile, G0.1 PSC is more stable than the pristine PSC as shown in Figure 9d and 9f. These results again corroborate the merits of GQDs which contribute to the better crystallized perovskite and charge carrier transport by serving as the electron collector. However, with excess GQD incorporation from 0.2% to 1%, the PCEs of the devices gradually decrease. The culprit is the low-quality PF and inefficient separation of photo-generated electron-hole pairs as discussed before.

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Figure 11. (a) Nyquist plots and (b) Tafel plots of pristine and GQD-incorporated PSCs under dark condition.

Figure 11a exhibits their Nyquist plots measured at near Voc under dark conditions, and the inset shows the equivalent circuit diagram. The equivalent circuit diagram is simulated by series resistance (Rs), contact resistance (Rco) and recombination resistance (Rrec). In the Nyquist plots, the arc at higher frequencies corresponding to the hole transport and extraction in the cathode (Au) is too small to recognize, and the pronounced semicircle at lower frequencies corresponds to the charge recombination resistance (Rrec), is obvious. As shown in Figure 11a, with moderate GQD incorporation, the diameter of the arc turns larger and that of G0.1-based device ranks first, which indicates that the recombination of electrons and holes is reduced after GQD incorporation, confirming the roles of GQD as the electron accumulator and electron transport bridge in PSC devices. Nevertheless, when the GQD content is excess, i.e. from 0.2% to 1%, the arc becomes smaller again due to the poor crystallization of the PF. The excessive GQD in poorly crystallized PF will aggregate, thus leading to more grain boundaries, pinholes and flaws that act as the charge 22


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recombination centers. Meanwhile, Tafel polarization measurements are adopted to decipher the leakage current inside PSCs. As shown in Figure 11b, the dark current densities of all the devices with GQD-containing perovskite are lower than that of the device with pristine perovskite. Considering that a lower dark current density corresponds to a lower leakage current, it can be concluded that the leakage current of devices incorporating GQDs is greatly suppressed, indicating the retarded charge recombination and reduced defects of PF accredited to GQDs. Notably, the optimum content of GQD (G0.1) in perovskite makes the PSC possess the lowest dark current, which is believed to be one of the contributing factors to its best photovoltaic performance. Thus, the EIS and Tafel results under dark condition are in consistent with the features of GQDs under light illumination as discussed in PL analysis.

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Figure 12. The work function of (a) perovskite MAPbI3, (b) GQD and (c) TiO2. (d) Energy levels of TiO2, GQD and perovskite relative to vacuum level, with illustrations of the corresponding crystal structures. The schematic diagrams of electron transport between perovskite and TiO2 with (e) moderate content of GQDs and (f) excess GQDs.

The work function, which is a vital parameter for energy levels, is defined by the following equation and simulated: Φ = Evac − EF

(2)

where Evac stands for the energy of stationary electron in the vacuum near the surface, and EF represents electron potential inside the surface. The work functions of MAPbI3, GQDs and TiO2 are calculated and illustrated in Figure 12a−12c. The work functions of perovskite, GQDs and TiO2 are 2.54, 4.11 and 6.45 eV, respectively. The Fermi level can be obtained from Equation 3: EF = −Φ

(3)

On the basis of Equations 2 and 3, the Fermi levels of MAPbI3, GQDs and TiO2 are −2.54, −4.11 and −6.45 eV (Figure 12d), respectively. These Fermi levels demonstrate that the electrons tend to flow from perovskite to GQDs, and subsequently to TiO2, so 24


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GQDs can serve as a highway for effective electron transport from PF to TiO2 film. All the aforementioned DFT calculations emphasize the positive role that GQDs play in electron collection and electron transport, which is clearly illustrated in Figure 12e. Nevertheless, as is shown in Figure 12f, excess GQDs will aggregate and cause more defects and pinholes. And they become the recombination center of photo-generated electron-hole pairs, thus the available photo-generated electrons tend to be trapped by defects and therefore the charge extraction efficiency is greatly reduced.

■ CONCLUSIONS Overall, an effective method has been put forward for high-quality perovskite layer preparation by GQD incorporation. GQDs contribute to a better crystallized PF with fewer defects in grain boundaries, which greatly enhances the light harvesting of perovskite. Higher charge extraction efficiency and smaller leakage current are obtained as well by incorporating GQDs. It is implied that GQDs can serve not only as an electron collector but also an electron transport bridge, which lead to efficient separation of photo-generated electron-hole pairs. The perovskite solar cell with 0.1% GQDs exhibits the best performance with a 11% enhancement in PCE relative to the pristine devices, which is believed to be due to the more efficient charge extraction. The introduction of GQDs in the perovskite crystallization paves a way for the preparation of higher-quality PF in PSCs and other energy-conversion related devices.

■ ASSOCIATED CONTENT 25



Supporting Information The Supporting Information is available free of charge on the ACS Publications website.

■ AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Notes The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (51320105001, 21433007 and U1705251), the Natural Science Foundation of Hubei Province of China (2015CFA001) and Innovative Research Funds of SKLWUT (2017-ZD-4).

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■ Graphical Abstract

Incorporating GQDs is a viable approach to obtain high-quality perovskite film with efficient charge extraction in planar PSCs for green and sustainable energy conversion.

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Enhanced performance of planar perovskite solar cell by graphene

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