Defect passivation and electrical conductivity enhancement in perovskite solar cells using functionalized graphene quantum dots

Future perspectivesBased on their excellent optoelectronic properties, facile and low-temperature solution process, and low cost, organic-inorganic metal halide perovskite solar cells (PSCs) have become a rising star for the next-generation photovoltaics. A large number of advances have been achieved in the PSCs so far, however, the existence of various defects is still one of the big stumbling blocks for their rapid development. Here, we incorporated the graphene quantum dots (GQDs) with functional groups into the perovskite films to passivate their defect states. As a result, the defect densities in the perovskite films were reduced dramatically. Moreover, different from the polymeric passivator as reported previously, the GQDs also endowed the perovskite film with excellent electrical conductivity, which is beneficial for the carrier transport. In short, our findings provide a more effective strategy to design passivators and open the door for further improving the efficiency and stability of the PSCs.

1.   Introduction

Perovskite solar cells (PSCs) have aroused worldwide attentions due to the explosive growth rate of the power conversion efficiency (PCE) originating from their high extinction coefficient, low exciton binding energy and fast charge diffusion rate [1-3]. It is well known that the low-temperature solution process is one of the critical advantages for the PSCs when compared with other traditional semiconductor photovoltaic devices [4-9]. However, as the solution process is far from thermodynamic equilibrium, a large number of defect states inevitably exist within the polycrystalline perovskite film, such as anion/cation vacancies, antisite occupations, and interstitials [10-12]. These defects detrimentally act as nonradiative recombination sites, resulting in poor charge transport, severe ion migration, instability, and other shortcomings [13, 14]. In order to resolve these problems, tremendous efforts have been devoted, for instance, post-annealing treatment of MAPbI₃ perovskite films with methylamine effectively reduces the surface/interface defects [15]; the optimization of device structure could mitigate the defect induced ion migration [16]. The grain boundaries in perovskite film are currently considered to be responsible for causing recombination and trapping of charge carriers [17]. The importance of grain boundary passivation has been recognized, some effective approaches have been developed [18-20], such as adding excess PbI₂/MAI in the precursors [21, 22] or introducing polymer additives [23-27] into the perovskite films to passivate the trap states. These approaches, however, exhibited inferior reproducibility and electrical conductance because of the demanding experimental conditions and intrinsic non-conductive characteristics of the polymers. Therefore, it is imperative for us to explore new strategies to passivate the defect states in the perovskites and then enhance the PCEs and stability of the PSCs.

Graphene quantum dots (GQDs), an intriguing low-dimensional material, possess high conductivity, quantum size effect, superior optical properties, and other virtues [28, 29]. Moreover, the rich functionalized groups in GQDs deriving from the synthetic conditions, such as hydroxyl (-OH), carbonyl (-C=O), carboxylic (-COOH) and amine (-NH₂), endow them with various applications [30, 31]. Recently, carbonaceous materials including carbon nanodots [32, 33], graphdiyne [34] and fullerenes [35] were successfully employed in the PSCs as the passivators, which showed positive effects on reducing their nonradiative recombination rates and prolonging their carrier lifetime. Compared with these carbonaceous materials, GQDs show much better crystallinity and fewer defects due to their large homogeneous conjugated domains [36]. Moreover, the terminal functional groups in GQDs could passivate the trap states in the perovskite films, meanwhile the benign conductivity could facilitate the charge transport across the whole perovskite films. Following this line of thought, a perovskite composite film, incorporating GQDs into the MAPbI₃ perovskite, was fabricated by a dynamic two-step spin-coating process in this work. In order to avoid the aggregation, the concentrated GQDs solution was directly added into the PbI₂/N,N-Dimethylformamide (DMF) precursor. As expected, a low defect density was demonstrated in the GQD-modified MAPbI₃ perovskite film, and it also possessed a high electrical conductivity, pure phase structure and high-quality morphology. All of the advantages endowed the GQD-modified MAPbI₃ perovskite film with fast carrier separation and transport, long carrier lifetime, and low nonradiative recombination. As a result, the GQD-modified MAPbI₃-based PSC showed an improvement of more than 20% in the PCE when compared with the pristine MAPbI₃-based PSC. Moreover, this new PSC exhibited good long-term stability and resistibility against heat and moisture as well.

2.   Methods

2.1   Synthesis and purification of GQDs

The single-crystalline GQDs were synthesized by a mild and green hydrothermal method. In brief, 2 g pyrene was nitrated into trinitropyrene in hot HNO₃ (160 ml) at 80 C and then stirred for 12 h. Then, the mixture was diluted with deionized water. 3 g resultant 1,3,6-trinitropyrene was dispersed in the NaOH solution and ultrasonicated for 3 h. The suspension was then transferred to a Teflon lined autoclave and kept at 200 C for 10 h. After cooling to room temperature, the water-soluble GQDs were further dialyzed in a dialysis bag for 2 d to remove impurities, such as sodium salts and unfused small molecules. The as-obtained GQDs aqueous dispersion showed a high concentration of about 10 mg ml^-1.

2.2   Electrode preparation

Fluorine doped tin oxide (FTO) glass was firstly etched by Zn powder and HCl solution, and then ultrasonically cleaned in detergent solution, deionized water and ethanol for 20, 15 and 15 min, respectively. A compact TiO₂ layer was deposited on the FTO substrate by spin coating at 4000 rpm for 30 s and calcined at 500 C for 30 min in air. Subsequently, a mesoporous TiO₂ layer was deposited on the compact TiO₂ layer by spin coating at 4000 rpm for 30 s. The layer was dried at 100 C for 10 min and then sintered at 500 C for 30 min in air again. Before each spin-coating step, the substrates were treated with Ultraviolet-O₃ for 30 min to remove organic residuals and increase surface wetting properties.

2.3   Fabrication of PSCs

The fabrication process of the PSCs has been described in our previous works [37-39]. Firstly, the GQDs were diluted in DMF with concentrations of 0.001, 0.005 and 0.01 g ml^-1, and 1.0 M PbI₂ was added into the DMF or DMF/GQDs to prepare the PbI₂ precursor solution. Meanwhile, 0.3 M methylammonium iodide (MAI) was added into isopropanol to prepare the MAI precursor solution. The MAPbI₃ perovskite layers were deposited onto the substrates by a specially designed dynamic two-step spin-coating method, which combines the merits of one-step antisolvent deposition and two-step sequential deposition. Specifically, 40 l PbI₂ or PbI₂/GQDs precursor solution was dripped on the substrate and then spin-coated at 3000 rpm for 30 s. During this step at 10 s, 50 l MAI precursor solution was immediately dripped on the PbI₂ or PbI₂/GQDs layer to form the perovskite layer. The perovskite layer was subsequently annealed at 70 C for 30 min and 150 C for 20 min, respectively. Spiro-OMeTAD solution was spin-coated on the perovskite film at 4000 rpm for 10 s as the hole transport layer. Finally, an 80 nm thick Au electrode was deposited by the thermal evaporation method through an aperture mask (0.16 cm² for each sub-cell) to pattern the electrodes.

3.   Results

GQDs were synthesized by a bottom-up hydrothermal method using the nitrated pyrene molecule as the precursor. Figure 1(a) shows the photographs of the as-synthesized GQD solution under different conditions, which reveals the GQDs disperse in water uniformly and exhibits green-yellow fluorescence under the illumination of 365 nm ultraviolet light. The steady state photoluminescence (PL) spectra of the GQDs/DMF solution (1 mg ml^-1) under different excitation wavelengths were investigated, as shown in figure S1. As the excitation wavelength increases from 350 to 450 nm, the emission peaks show a slightly red shift from 550 to 560 nm, which implies the GQDs possess defect-free single-crystalline structures and ideal electron donating functionalization at edge sites according to previous reports [40, 41]. This result is also echoed by the high-resolution transmission electron microscope (HRTEM) image, as shown the inset in figure 1(b), in which few misalignments of lattice structures can be found in such high resolution. Moreover, the HRTEM image also displays the distinct lattice fringe with a spacing of 0.21 nm, which corresponds to (100) plane of graphene. In order to identified the sizes of the GQDs, TEM and atomic force microscope (AFM) measurements were carried out. The TEM image in figure 1(b) shows that the lateral sizes of the GQDs distribute over a narrow range with an average lateral size of 2.5 nm. Meanwhile, the altitude intercept in the AFM image of 1.4 nm demonstrates that the as-prepared GQDs have the uniform thicknesses and the number of layers is 4-5, as shown in figure 1(c). Figure S2 displays the Fourier transform infrared spectroscopy spectrum of the GQD powder. The strong vibrations around 1584 cm^-1, 3353 cm^-1, and 1268 cm^-1 can be ascribed to the C=C bond, O-H bond, and C-O-H bond, respectively. Such functional groups can provide meaningful interaction with the perovskites, especially for their unbonded Pb²⁺ and I^- ions, which will be very helpful for passivating these defects.

Figure  1.  (a) Typical digital pictures of the GQD solution under the illumination of visible and 365 nm ultraviolet light, respectively. (b) TEM and HRTEM images of the GQDs. The inset shows the lateral size distribution. (c) AFM image of the GQDs. The inset shows the corresponding height profile. Digital pictures of (d) DMF solution and (e) PbI₂/DMF solution with and without GQDs, respectively. (f) Fabrication process of GQD-modified perovskite films using the dynamic two-step spin-coating method.

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In order to demonstrate the effects of the GQDs on the perovskite materials, MAPbI₃, GQD-modified MAPbI₃ perovskite films with different concentrations of GQDs were prepared, as shown in figure 1(f). The fabrication process was described in the Methods. The GQD-modified MAPbI₃ perovskite films are hereafter referred to as MAPbI₃/xGQDs, where x is the concentration of the GQD solution. In order to verify the incorporation of GQDs in the MAPbI₃ perovskite films successfully, Raman spectra of the MAPbI₃ and MAPbI₃/xGQDs (x = 0.001, 0.005, and 0.01) perovskite films were measured, as shown in figure 2(a). No peaks can be found in the MAPbI₃ film, while the MAPbI₃/xGQDs (x = 0.001, 0.005, and 0.01) films display obvious two peaks at 1373 and 1576 cm^-1, corresponding to the disordered D-band and the crystalline G-band of GQDs, respectively, which demonstrates that the GQDs were incorporated into the MAPbI₃ perovskite films successfully [42, 43]. Moreover, the intensities of G-bands are much stronger than those of the D-bands in the MAPbI₃/xGQDs (x = 0.001, 0.005, and 0.01) perovskite films, which suggests that the crystallinities of the GQDs in these perovskite films are high. In order to examine the interaction between MAPbI₃ and GQDs, the x-ray photoelectron spectroscopy (XPS) spectra of the MAPbI₃ and MAPbI₃/xGQDs (x = 0.001, 0.005, and 0.01) perovskite films were performed. Figure 2(b) shows wide scan XPS survey, which demonstrates that all of signals of the Pb 4f, C 1s, N 1s, O 1s, and I 3d peaks can be found in the four kinds of perovskite films. Figures 2(c) and (d) further show the high-resolution XPS spectra of the Pb 4f and I 3d, respectively. The Pb 4f core level displays two main peaks corresponding to Pb 4f_7/2 and 4f_5/2 due to the spin-orbital splitting. The peaks in MAPbI₃ film are located at 137.1 and 142.0eV, which show around 0.2 eV higher than those in MAPbI₃/xGQDs (x = 0.001, 0.005, and 0.01) perovskite films. These shifts can be ascribed to the changes of chemical bonding. Therefore, it is deduced that the incorporated GQDs generate an increased electron cloud density around the Pb and I atoms due to the strong interaction between GQDs and perovskites [33].

Figure  2.  (a) Raman spectra, (b) XPS survey spectra, high-resolution XPS spectra of (c) Pb 4f and (d) I 3d peaks, (e) XRD patterns, and (f) absorbance spectra of the MAPbI₃ and MAPbI₃/xGQDs (x = 0.001, 0.005, and 0.01) perovskite films.

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Figure 2(e) shows the x-ray diffraction patterns of the MAPbI₃ and MAPbI₃/xGQDs (x = 0.001, 0.005, and 0.01) perovskite films. The diffraction peaks at 14.2, 19.9, 23.4, 24.5, 28.4, 31.9, 40.6 and 43.2 are correlated to the (110), (112), (211), (202), (220), (310), (224) and (314) planes of the tetragonal MAPbI₃, respectively. As compared with the MAPbI₃ film, no peak shift is observed for the MAPbI₃/xGQDs (x= 0.001, 0.005, and 0.01) perovskite films, implying that the incorporated GQDs would not affect the lattice structure of the MAPbI₃ film [44]. It is well known that PbI₂ residues usually exist in the perovskite films obtained from the two-step deposition process [21]. However, the MAPbI₃/0.001GQDs perovskite film does not show any PbI₂ residue, indicating that appropriate GQDs are beneficial for the reaction between the PbI₂ and MAI, and then resulting in the best MAPbI₃ film with the purest phase structure. Following, the crystallite sizes of the MAPbI₃ and MAPbI₃/xGQDs (x = 0.001, 0.005, and 0.01) perovskite films were evaluated by the Scherrer formula based on their full width at half maximums of (110) peaks. The MAPbI₃ film exhibits the crystallite size of about 58.6 nm, whereas the crystallite size of the MAPbI₃/0.001GQDs film is determined to be 60.62 nm. This result indicates that the incorporation of the GQDs into the MAPbI₃ precursor solution will also beneficial for the crystal growth. Figure 2(f) shows the UV-Vis absorption spectra of the MAPbI₃ and MAPbI₃/xGQDs (x = 0.001, 0.005, and 0.01) perovskite films. Clearly, the four kinds of films exhibit similar absorption edge at around 785 nm, indicating the bandgap of MAPbI₃ perovskite (about 1.58 eV) will not be affected by the incorporation of the GQDs. Intriguingly, the MAPbI₃/0.001GQDs perovskite film has a slightly enhanced absorbance in the visible light range, which can be attributed to its relatively purer phase structure and larger crystallite size.

In order to investigate the effects of GQDs on the morphology of the MAPbI₃ perovskite films, scanning electron microscope (SEM) images of the MAPbI₃ and MAPbI₃/xGQDs (x = 0.001, 0.005, and 0.01) perovskite films were characterized, as shown in figure 3. The left column in figure 3 illustrates the top-view SEM images of the four samples. Clearly, the morphologies of the four samples are very alike, which are uniform, dense-packed, and without any pinholes. Despite the GQDs cannot be observed by SEM, we consider that the GQDs mainly locate at the grain boundaries of perovskites. In order to find the GQDs at the grain boundaries, we tried to increase the concentration of the GQDs to 0.1 mg ml^-1 [45], however, it is still difficult for us to distinguish the GQDs from the perovskite grains boundaries (figure S3), which can be ascribed to the still low concentration of the GQD, the small size of the GQD, and a good dispersity of the GQD in the whole film. In theory, because the bonding between the GQDs and PbI₂ is weaker than that between MAI and PbI₂, the GQDs will be repelled during the formation process of MAPbI₃ [32]. Thus, we speculated that the GQDs finally aggregate at the grain boundaries when the perovskite grain growth is completed. The statistical average grain size of the MAPbI₃/0.001GQDs perovskite film is around 580 nm, which is larger than that of the MAPbI₃, MAPbI₃/0.005GQDs, and MAPbI₃/0.01GQDs perovskite films (570, 520, and 510 nm). Such difference of the grain sizes could be attributed to the following two reasons. On one hand, the lone unpaired electrons of carbonyl (-C=O) and carboxylic (-COOH) on GQDs may interact with PbI₂ and thus decelerate the crystallization rate [46, 47]. On the other hand, the existent of the trace water in the GQDs solution would prompt the crystallization and improve the surface coverage [48-50]. Additionally, the right column in figure 3 displays the cross-sectional SEM images of PSCs based on the four kinds of perovskite films, which demonstrate the electron transport layers of the mesoporous TiO₂ and the hole transport layers of the spiro-OMeTAD contact with the perovskite layers smoothly. Obviously, the MAPbI₃/0.001GQDs perovskite layer in figure 3(d) grows perpendicular to the substrate and almost no horizontal grain boundaries can be found, which will be beneficial for the carrier transport. In short, all of the results obtained from figure 3 imply that the MAPbI₃/0.001GQDs perovskite film shows the best morphology for the applications in PSCs among the four kinds of perovskite films.

Figure  3.  Top-view SEM images of the (a) MAPbI₃, (c) MAPbI₃/0.001GQDs, (e) MAPbI₃/0.005GQDs, and (g) MAPbI₃/0.01GQDs perovskite films. Cross-sectional SEM images of the PSCs based on the (b) MAPbI₃, (d) MAPbI₃/0.001GQDs, (f) MAPbI₃/0.005GQDs, and (h) MAPbI₃/0.01GQDs perovskite films.

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To investigate the distribution of the GQDs in the MAPbI₃/xGQDs (x = 0.001, 0.005, and 0.01) perovskite films, fluorescence images were captured using a confocal laser scanning microscope. The MAPbI₃/0.01GQDs perovskite film was chose to test here because of its high concentration of the GQDs. The optical profile of this perovskite is clearly observed from the bright field image (figure 4(a)), which shows the same surface morphology with that in figure 3(g). The collected emissions from GQDs were set in the range of 500 nm to 580 nm to avoid the disturbance caused by PbI₂, as shown the green dots in figure 4(b). It can be found that the GQDs are distributed homogeneously in the MAPbI₃/0.01GQDs film, which suggests that the charge diffusion will be uniform and smooth in the MAPbI₃/xGQDs (x = 0.001, 0.005, and 0.01) perovskite films.

Figure  4.  CLSM images of the MAPbI₃/0.01GQDs perovskite films: (a) bright field image, (b) fluorescence image and (c) merged image. (d) Steady-state PL and (e) TRPL spectra of the MAPbI₃ and MAPbI₃/xGQDs (x = 0.001, 0.005, and 0.01) perovskite films on glass substrates, respectively.

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The steady-state (PL spectra of the MAPbI₃ and MAPbI₃/xGQDs (x = 0.001, 0.005, and 0.01) perovskite films on glass substrates were measured, as shown in figure 4(d). Clearly, the emission peaks for the four samples are at around 772 nm, indicating that the incorporation of GQDs does not affect the bandgap of the MAPbI₃ perovskite film, which is in accordance with the results obtained from figure 2(f). Moreover, the PL intensity of the MAPbI₃/xGQDs (x = 0.001, 0.005, and 0.01) perovskite film is much stronger than that of the MAPbI₃ perovskite films, and MAPbI₃/0.001GQDs perovskite film shows the strongest PL intensity among them. It is well known that the steady-state PL intensity depends on the recombination of photo-induced charge carriers [51]. Therefore, the stronger PL intensity indicates lower electron-hole recombination rate and lower trap states in the MAPbI₃/xGQDs perovskite films. To further gain more insight into the effects of the GQDs on the carrier lifetime of the MAPbI₃ perovskite film, time-resolved PL spectra were performed, as shown in figure 4(e). A bi-exponential decay function was used to fit the PL decay time of the samples. Generally, the fast decay lifetime ₁ is ascribed to non-radiative trap assisted recombination and the slow decay lifetime ₂ is assigned as exciton recombination [52, 53]. For the MAPbI₃ perovskite film, the PL decay lifetime of ₁ and ₂ are 3.3 ns and 100.1 ns, respectively, while they are increased to 4.3 ns and 460.5 ns, respectively, for the MAPbI₃/0.001GQDs perovskite film. Such enhanced lifetime further demonstrates that the GQDs reduce the defect states effectively in the MAPbI₃/0.001GQDs perovskite film [54].

Figure 5(a) shows the current density-voltage (J-V) curves of the PSCs based on the MAPbI₃ and MAPbI₃/xGQDs (x = 0.001, 0.005, and 0.01) perovskite films. The corresponding photovoltaic parameters are listed in table 1. As a control sample, the PSC based on the MAPbI₃ film shows an open-circuit voltage (V_OC) of 1.072 V, a short-circuit current density (J_SC) of 22.25 mA cm^-2, a fill factor (FF) of 67.3%, and a PCE of 16.05%. After incorporating the GQDs into the MAPbI₃ films, the PSCs based on the MAPbI₃/xGQDs (x = 0.001, 0.005, and 0.01) films display clear increases in all parameters. Particularly, the PSC based on the MAPbI₃/0.001GQDs film exhibits the highest PCE of 19.59% with a J_SC of 23.36 mA cm^-2, a V_OC of 1.158 V, and a FF of 72.4%, respectively. Moreover, a smaller concentration of 0.0005 mg ml^-1 GQDs was also investigated, but the obtained PCE of 18.28% and FF of 71% were still lower than those of PSCs based on the MAPbI₃/0.001GQDs films (figure S4 and table S1). Compared with the control sample, such large increases in the PSC based on the MAPbI₃/0.001GQDs film can be ascribed to the pure phase structure, high-quality morphology, low defect density, and high electrical conductivity caused by the incorporation of the GQDs. Figure 5(b) shows the incident photon-to-charge conversion efficiency (IPCE) spectra of the four PSCs. Obviously, the PSC based on the MAPbI₃/0.001GQDs film shows an obvious enhanced IPCE in the entire wavelength region, especially in the wavelength range from 600 to 800 nm. The improved response in the long wavelength range means that the photo-electrons in the deeper region of the perovskite film could be collected more effectively after incorporating GQDs, which further confirms that GQDs interact with perovskite and form a bulk heterojunction [34]. The integrated J_SC calculated from IPCE is also displayed in figure 5(b), which is close to the J-V scan results.

Figure  5.  (a) J-V curves and (b) IPCE spectra and integrated J_SC of the PSCs based on the MAPbI₃ and MAPbI₃/xGQDs (x = 0.001, 0.005, and 0.01) perovskite films, respectively. (c) J-V curves of the PSCs based on the MAPbI₃ and MAPbI₃/0.001GQDs perovskite films under reverse and forward scan modes, respectively. (d) Steady-state output PCEs at the maximum power point. (e) Box statistical diagram of the PCEs. (f) Nyquist plots of the PSCs based on the MAPbI₃ and MAPbI₃/0.001GQDs perovskite films, respectively. The inset displays the equivalent circuit.

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Table  1.  Photovoltaic parameters of the PSCs based on the MAPbI₃ and MAPbI₃/xGQDs (x = 0.001, 0.005, and 0.01) perovskite films.

Sample		Voc (V)	Jsc (mA cm-2)	FF (%)	PCE (%)
MAPbI3	Best	1.072	22.25	67.3	16.05
	Average	1.073 0.003	21.72 0.26	66.0 1.3	15.39 0.38
MAPbI3/0.001GQDs	Best	1.158	23.36	72.4	19.59
	Average	1.153 0.004	22.96 0.64	69.5 1.0	18.40 0.64
MAPbI3/0.005GQDs	Best	1.150	22.90	70.3	18.51
	Average	1.149 0.006	22.37 0.62	68.2 1.1	17.54 0.55
MAPbI3/0.01GQDs	Best	1.122	22.49	70.4	17.78
	Average	1.132 0.006	22.10 0.50	67.4 1.1	16.86 0.51

 | Show Table

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The anomalous hysteresis effects usually exist in PSCs when they are tested under forward and reverse scan modes, which can be attributed to the ion migrations, ferroelectric polarizations and trap states in perovskites and interfaces [55, 56]. Figure 5(c) shows the J-V curves of the PSCs based on the MAPbI₃ and MAPbI₃/0.001GQDs films under the forward and reverse scan modes, respectively. The PSC based on the MAPbI₃ film displays a very large hysteresis, while the PSC based on the MAPbI₃/0.001GQDs film reveals almost negligible hysteresis. The hysteresis indexes (HIs) of the two kinds of PSCs can be calculated by the equation of (PCE_reverse-PCE_forward)/PCE_reverse, where the PCE_reverse and PCE_forward represent the PCEs tested under the reverse and forward scan modes, respectively. As a result, the HIs for the PSCs based on the MAPbI₃ and MAPbI₃/0.001GQDs films were calculated to be 0.133 and 0.007 respectively. The decreased HI can be ascribed to the low defect density and low MA⁺/I^- migration because of the introduction of the GQDs according to previous reports [57]. The steady-state output PCEs were measured to confirm the operation status of PSCs. As shown in figure 5(d), the MAPbI₃/0.001GQDs PSC maintains a stabilized output PCE of 19.5% at the maximum power point (V_MPP = 0.922 V), which is consistent with that attained from reverse scan. In contrast, the MAPbI₃ PSC shows a declined PCE of 14.7% at the V_MPP = 0.797 V. The statistical distribution of the PCEs based on 25 devices for each kind of PSCs is shown in figure 5(e). The MAPbI₃/xGQDs PSCs show apparently enhanced PCE as compared with the MAPbI₃ PSCs. Electrochemical impedance spectroscopy (EIS) was conducted to further study the effects of the GQDs on the charge transport and recombination behaviors in the MAPbI₃ films [58, 59]. Figure 5(f) shows the Nyquist plots of the PSCs based on the MAPbI₃ and MAPbI₃/0.001GQDs films in the dark condition with a bias voltage of 0.9 V, where the inset image is the equivalent circuit. The primary arc reflects the charge recombination (R_rec) process within the PSCs. The PSC based on the MAPbI₃/0.001GQDs film exhibits a much larger R_rec than the PSC based on the MAPbI₃ film (2888 vs. 1772 ), which indicates the charge recombination in the PSC based on the MAPbI₃/0.001GQDs film was effectively diminished due to the low defect density caused by the incorporation of the GQDs.

According to the above analyses, a credible mechanism of the MAPbI₃ perovskites with the GQDs is depicted in figure 6. It is well known that there are many kinds of defects in the MAPbI₃ perovskites, like Pb vacancies, I vacancies, and so on, all of which will lead to severe carrier recombination [60, 61]. Such severe recombination will decrease the carrier extraction efficiency, and then lead to lower current and voltage. We suppose that these disadvantages will be weakened after the incorporation of the GQDs in this work. Figure 6(d) shows that the functional groups in GQDs, like carbonyl and carboxylic, could coordinate with the unsaturated Pb²⁺ ions in the MAPbI₃ perovskites, which will reduce the defect density and then inhibit the charge recombination effectively. Meanwhile, the hydroxyl functional groups in GQDs would link with I^- ions in the MAPbI₃ perovskites via hydrogen bonding, which will inhibit ion migrations and then increase the structure stability of the frameworks. Besides, the single-crystalline GQDs possess good conductivities, therefore, the incorporation of GQDs will improve the electrical conductivity of the MAPbI₃/xGQDs (x = 0.001, 0.005, and 0.01) films, which is beneficial for the carrier transport in them, as shown in figures 6(b) and (c). The last but not the least, because the conduction band minimum (CBM) of the GQDs is relatively higher than TiO₂ electron transport layer (ETL) [62], gradient energy levels are formed in the PSCs based on the MAPbI₃/xGQDs (x = 0.001, 0.005, and 0.01) films, which will lead to a high-speed electron transport from the CBM of the perovskite layer to that of the TiO₂ layer, as shown in figure S5.

Figure  6.  (a) Schematic diagram of the PSC based on the MAPbI₃/0.001GQDs film. Schematic diagrams of the charge transport in the (b) MAPbI₃ and (c) MAPbI₃/0.001GQDs perovskite films. (d) Crystal structure of the MAPbI₃/0.001GQDs perovskite.

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To investigate the trap densities and hole mobilities of the MAPbI₃ and MAPbI₃/xGQDs (x = 0.001, 0.005, and 0.01) perovskite films experimentally, the hole-only devices with the structure of FTO/PEDOT:PSS/perovskite/spiro-OMeTAD/Au were fabricated. As shown in figure 7(a), three regions can be identified at different voltage range: ohmic region, trap-filled limited (TFL) region, and space-charge-limited current (SCLC) region. The linear relationship at the low bias voltage represents the ohmic response of the PSCs. When the voltage exceeds the kink point (V_TFL), the current nonlinearly increases, indicating that the trap states are completely filled. The trap density (n_t) can be calculated by the following formula: V_TFL = en_td²/2₀, where e is the elementary charge, d is the thickness of the film, is the relative permittivity of MAPbI₃ (= 32), ₀ is the vacuum dielectric constant, and V_TFL is the trap-filled limit voltage [63]. As shown in table S2, compared with the MAPbI₃ perovskite (1.57 10¹⁶ cm^-3), the trap density of the MAPbI₃/0.001GQDs perovskite is reduced to be 7.08 10¹⁵ cm^-3, which confirms our supposition in figure 6 that the incorporation of the GQDs will reduce the trap density in the MAPbI₃ perovskite. Moreover, the mobility () of the various perovskite films can also be calculated by the Mott-Gurney law in the SCLC region by the following formula: J= 9₀V_b²/8d³, where J is the current density, and V_b is the applied voltage [64]. The hole mobilities of the MAPbI₃ and MAPbI₃/xGQDs (x = 0.001, 0.005, and 0.01) perovskite films derived from figure 7(b) are also summarized in table S2. The mobility of MAPbI₃/0.001GQDs perovskite is 8.56 10^-3 cm² V^-1s^-1, which is much larger than the MAPbI₃ perovskite (3.84 10^-3 cm² V^-1 s^-1). The improved charge mobility is a main reason for the enhanced photovoltaic performance as well.

Figure  7.  (a) Dark current-voltage curves of the hole-only devices with the structure of FTO/PEDOT:PSS/perovskite/spiro-OMeTAD/Au. The perovskite layers are the MAPbI₃ and MAPbI₃/xGQDs (x = 0.001, 0.005, and 0.01) perovskite films respectively. (b) Hole mobility measurement using the SCLC model. (c) TGA curves of the MAPbI₃ and MAPbI₃/xGQDs (x = 0.001, 0.005, and 0.01) perovskite films. (d) Stability of the unsealed PSCs based on the MAPbI₃ and MAPbI₃/0.001GQDs films in the ambient atmosphere with a humidity of 35%-45%. The inset photographs are the corresponding initial and 200 h-stored PSCs.

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The water contact angles of the perovskite films were measured to evaluate the hydrophilic performances of the MAPbI₃ and MAPbI₃/xGQDs (x = 0.001, 0.005, and 0.01) perovskite films, as shown in figure S6. The contact angles of the MAPbI₃/xGQDs (x = 0.001, 0.005, and 0.01) perovskite films have slight increases when compared with that of the MAPbI₃ film, which can be ascribed to the hydrophobic characteristic of graphene. It is well known that the contact angle evolution could reflect the decomposition of perovskite films when they are exposed to the water droplets. Therefore, the incorporation of GQDs in the MAPbI₃ perovskite will provide better water-repellent property for the MAPbI₃/xGQDs (x = 0.001, 0.005, and 0.01) perovskite films. In order to study the thermal stabilities of the MAPbI₃ and MAPbI₃/xGQDs (x = 0.001, 0.005, and 0.01) perovskite films, the thermogravimetric analyses were performed on the four samples in the argon atmosphere, as shown in figure 7(c). There are two main steps of the weight loss at 200 C-300 C and 450 C-550 C, which are correlated with the sublimation of MAI and PbI₂ respectively [65]. For the MAPbI₃ perovskite, the weight loss starts at around 195 C, while the temperature gradually increases to 230 C in the MAPbI₃/xGQDs (x = 0.001, 0.005, and 0.01) perovskites, which suggests that the thermal stability of the MAPbI₃ perovskite is improved dramatically after incorporating the GQDs. Finally, the environmental stability of the unsealed devices was also tested in the ambient atmosphere with a humidity of 35%-45%. Figure 7(d) shows the normalized PCE decays versus the storage time of the PSCs based on the MAPbI₃ and MAPbI₃/0.001GQDs perovskite films, respectively. Remarkably, the PSC based on the MAPbI₃/0.001GQDs perovskite film retains 91.2% of its initial PCE after around 200 h, while the PSC based on the MAPbI₃ film retains only 75.8% of its initial PCE, indicating the incorporation of the GQDs can improve the stability of the MAPbI₃ perovskites in air effectively.

4.   Conclusion

In summary, the MAPbI₃/0.001GQDs composite film was successfully prepared by a dynamic two-step spin-coating process. After incorporating the GQDs into the MAPbI₃, this perovskite exhibited a pure phase structure and high-quality morphology, which provided a smooth path for the charge transport in the MAPbI₃/0.001GQDs perovskite film. More importantly, the MAPbI₃/0.001GQDs perovskite film also displayed low trap density and high electrical conductivity, improving the carrier separation and transport ability, prolonging the carrier lifetime and reducing the nonradiative recombination rate. As a result, the PSCs based on the MAPbI₃/0.001GQDs films showed a dramatically enhanced PCE compared to the PSCs based on the bare MAPbI₃ films. Moreover, the new PSCs were demonstrated to have good long-term stability and resistibility against heat and moisture. This study not only reveals the underlying factors contributing to the perovskite research community, but also opens the door for further exploring novel approaches to enhance the quality of perovskite films.