Performance enhancement of mesoscopic perovskite solar cells with GQDs-doped TiO2 electron transport layer

Solar Energy Materials & Solar Cells 208 (2020) 110407

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Performance enhancement of mesoscopic perovskite solar cells with GQDs-doped TiO2 electron transport layer Marzieh Ebrahimi a, Ahmad Kermanpur a, *, Masoud Atapour a, Siavash Adhami a, Reyhaneh Haji Heidari a, Elahe Khorshidi b, Neda Irannejad b, Behzad Rezaie b a b

Department of Materials Engineering, Isfahan University of Technology, Isfahan, 84156–83111, Iran Department of Chemistry, Isfahan University of Technology, Isfahan, 84156–83111, Iran

A R T I C L E I N F O

A B S T R A C T

Keywords: Perovskite solar cell Mesoscopic titanium dioxide Graphene quantum dots Electron transfer layer Performance Stability

Electron transport layer (ETL) of perovskite solar cells (PSCs) plays an important role on transferring electrons from perovskite layer to transparent conductive oxide layer, strongly affecting PSC performance. In the present study, effects of adding graphene quantum dots (GQDs) as a dopant to the mesoscopic TiO2 (mp-TiO2) ETL on performance of a PSC were investigated. Different amounts (1.25, 2.5 and 5 vol%) of GQDs were directly added to the TiO2 precursor solution which was subsequently applied as the doped ETL by spin coating. The results showed that Jsc, Voc and FF of the 2.5 vol% GQDs-doped cell were 21.92 mA/cm2, 0.97 V and 0.63, respectively, corresponding to a PCE of 14.36% (champion cell), approximately 50% improvement compared to the un-doped cells (best PCE 9.55%). The perovskite film in the GQDs-doped cell was dense with fewer pinholes which facilitated electron extraction, and accelerated charge mobility in TiO2 layer, consequently promoting Jsc and Voc. Based on EIS results, GQDs doping into the TiO2 ETL significantly suppressed the recombination processes, resulting in a higher FF. Interestingly, the PSC based on 2.5 vol% GQDs-doped TiO2 ETL maintained ~88% of its initial PCE (champion cell), after 500 h under ambient conditions; whereas, the conventional PSC based on pure TiO2 ETL maintained only 61% of its initial PCE under the same conditions, suggesting a dramatic improvement in the device stability. The findings clearly showed that GQDs doping to TiO2 ETL could be a potential and confident approach for improving performance and stability of the mesoscopic PSCs.

1. Introduction Given the increasing energy demand, greenhouse gasses, and depleting fossil fuels, the development of renewable energies has gained considerable importance as one of the most promising options for mankind’s future sustainable development. Photovoltaic (PV) technol­ ogies in which sunlight is directly converted to electricity, can poten­ tially supply applicable devices to support clean energy for human civilization. During the past few years, among different generations of solar cells, the significant increase in efficiency of perovskite solar cells (PSCs), as a novel generation of thin-film solar cells, has shown them as desirable candidates for possible replacement of previous types of solar cells [1–5]. The promising behavior comes from the perovskite material characteristics such as excellent optical and electrical properties, direct band-gap, high light absorption coefficient, high charge carrier mobility, long charge carrier diffusion length, and long electron-hole recombi­ nation lifetime, all introducing this material as a suitable light harvester

for solar cells. That is why PSCs have emerged and rapidly progressed as one of the most promising PV technologies in a relatively short period of time [6–9]. In general, a mesoscopic PSC is prepared with a sandwich structure including a mesoscopic TiO2 (mp-TiO2) layer on transparent conductive oxide (TCO) as the electron transport layer (ETL), a perovskite layer as the light absorbing material, followed by capping with a hole transport layer (HTL) and a metal counter electrode [10]. The power conversion efficiency (PCE) of any PSC strongly depends on the electrical and physical properties of the ETL. Different metal oxides including titanium dioxide (TiO2), tin oxide (SnO2) and zinc oxide (ZnO) have been used as ETL to achieve high performance PSCs [11]. Among the transparent semiconducting metal oxides, TiO2 is widely employed as the ETL in PSCs due to its properties such as matched energy levels and good transparency. Despite all its benefits, it is known that although electron injection rates from the perovskite layer to TiO2 ETL are very fast, the low electron mobility of TiO2 limits the electron transport within the

* Corresponding author. E-mail address: [email protected] (A. Kermanpur). https://doi.org/10.1016/j.solmat.2020.110407 Received 24 June 2019; Received in revised form 24 November 2019; Accepted 12 January 2020 Available online 19 January 2020 0927-0248/© 2020 Elsevier B.V. All rights reserved.

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structure; as a result, the possibility of charge recombination increases, strongly undermining the overall PSC performance. On the other hand, it is clear that the TiO2 ETL plays a crucial role in determination of the perovskite morphology and therefore the charge extraction process. Accordingly, many worldwide efforts have been made in order to improve the TiO2 performance as an ETL in the PSCs. Frankly; ion doping is one of the most interesting approaches that has been used to improve the performance of ETL of PSCs [12–17]. For instance, Chen et al. have reported that the Niobium-doped TiO2 used as compact ETL for mesoscopic PSCs led to the improvement of conductivity compared with pristine TiO2; as a result, the PCE increased from 14.9% to 16.3% [18]. Likewise, Giordano et al. have found out that the PSCs prepared using Lithium-doped TiO2 ETL exhibited efficiency up to over ~19% due to decreasing charge traps states and facilitating electron transfer [19]. It is reported that introduction of n-dopants to the mp-TiO2 ETL en­ hances charge transport properties, decreases electronic trap states and suppresses recombination processes, ultimately improving PCE of the PSCs [12-16,18and19]. Recently, carbon derivatives such as graphene, carbon quantum dots (CQDs), carbon nanotubes (CNTs), graphene quantum dots (GQDs) and fullerene have received a great attention as additive materials to the PSCs due to their unique properties including high conductivity, trans­ parency, flexibility and low cost [20–25]. Zhu et al. have demonstrated a PCE improvement from 8.81% to 10.15% through the deposition of an ultrathin GQDs layer between a single-cation perovskite and TiO2 in PSCs [26]. Moreover, Li et al. have observed that the use of CQDs/TiOx as efficient ETL led to the improvement of charge transport between the TiO2 and a single-cation, mixed halide perovskite layers, enhancing the PCE close to 19% [27]. Shen et al. have incorporated the GQDs on the surface of mp-TiO2 ETL, thereby leading to a PCE of 20.45%. They have found that GQDs can serve as an effective bridge for electron transport between a bi-cation, mixed halide perovskite film and TiO2 layer [28]. In another study, Ryu et al. have investigated the size effects of GQDs which were introduced onto the TiO2 blocking layer on the performance of planar type PSCs [29]. They have demonstrated that the band gap energy of GQDs strongly depended on their size; as a result, a 19.11% PCE was achieved by controlling the GQDs size. Previous studies have indicated that GQDs can improve the PSCs performance owing to their outstanding properties such as tunable band gap, ultrafast hole-electron extraction, long-term hole-electron lifetime and good chemical stability [29–31]. To the best of our knowledge, most investigations have focused on the fabrication of highly efficient PSCs applying metal ion-doped TiO2 ETL and there are few reports available on the modification of TiO2 ETL for PSCs using GQDs as dopants. Hence, in the present study, the effects of GQDs-doped TiO2 as mesoscopic ETL on the performance of triplecation, mixed-halide PSCs were investigated in details. The novelties of the present work include the GQDs synthesis method, the GQDs doping method, and study of the GQDs-doped cell stability. As a green, low cost and simple method, GQDs were herein synthesized from Fenugreek leaves using hydrothermal treatment. Furthermore, unlike other studies [28,29], in order to facilitate the cell fabrication process, GQDs were directly added to the TiO2 precursor solution before spin coating. This approach resulted in a homogeneous distribution of GQDs throughout the mesoscopic TiO2 ETL, enhancing the contribution of the GQDs to the performance of PSCs. The findings confirm that the 2.5 vol % GQDs doping to TiO2 ETL, as a simple, low cost and effective approach, not only enhanced the cell PCE as high as about 50%, but also led to a significant enhancement of the device stability after 500 h under ambient conditions.

>99.5%), cesium iodide (CsI, 99.9%), lead (II) iodide (PbI2, 99%), methylammonium bromide (MABr, >99.5%), lead (II) bromide (PbBr2, 99%), 2,20,7,70-tetrakis (N,N-di-p-methoxyphenylamine)-9,9-spirobifluorene) (Spiro-OMeTAD, 99%), 4tert-butylpyridine (TBP, 96%), bis (trifluoromethylsulphonyl)imide (LiTFSI, 97%), hydrochloric acid (HCl, 37%), anhydrous chlorobenzene (CB, 99.8%), anhydrous ethanol, anhydrous dimethyl sulfoxide (DMSO), anhydrous N,Ndimethylformamide (DMF), and Acetonitrile (ACN, 99.8%) were all supplied from Sigma-Aldrich Co. Fluorine doped tin oxide (FTO) and TiO2 paste were also purchased from Sharif Solar Co. 2.2. Layer fabrication 2.2.1. Synthesis of the GQDs Hydrothermal treatment as a simple and low cost process was used to synthesize the GQDs. In order to remove any contamination, Fenugreek leaves were washed several times with distilled water, followed by drying at room temperature. The dried sample was ground and then 2.0 g of the ground Fenugreek leaves was added to 100 mL of distilled water in a Teflon-lined autoclave and heated at 220 � C for 12 h. Then, the obtained solution was centrifuged (12000 rpm for 5 min) to separate unreacted components and the supernatant (containing GQDs) was purified using a centrifugal filter [32]. 2.2.2. Synthesis of the perovskite To prepare the “Cs0.05 (MA0.17FA0.83)0.95Pb(I0⋅83Br0.17)3 perovskite” precursor solution, FAI (1 M), PbI2 (1.1 M), MABr (0.2 M), and PbBr2 (0.22 M) were dissolved in anhydrous DMF: DMSO 4:1 (v:v). Then, a 1.5 M stock solution of CsI in DMSO, was added into perovskite precursor [33]. 2.3. Device fabrication The sandwich-type PSCs were fabricated on FTO coated glass sub­ strates with a size of 1.4 � 1.4 cm2. The FTO glasses were etched and patterned using Zn powder and a solution of 35.5 wt% HCl. The sub­ strates were subsequently cleaned by acetone, isopropanol and ethanol using an ultrasonic bath. A compact TiO2 (c-TiO2) layer was then deposited on the pre-treated FTO substrates by spin coating of a mixed solution at 1500 rpm for 50 s. To prepare the precursor solution for cTiO2 layer, 175 μL TTIP was diluted in 1.25 mL ethanol. In the following, 17 μL HCl diluted in 1.25 mL ethanol was gradually added to the latter solution. After that, the samples were first dried at 70 � C and then annealed at 450 � C for 30 min. In the next step, to deposit the mp-TiO2 layer, TiO2 precursor solutions with different TiO2 paste/ethanol ratios, introducing as TiO2-1:7, TiO2-2:7 and TiO2-3:7 (i.e. TiO2-X:Y, where X:Y indicated the TiO2 paste/ethanol ratio), spin-coated on the c-TiO2 under the same conditions, followed by sintering at 450 � C for 30 min. To investigate the effects of GQDs doping on the performance of PSCs, different amounts of GQDs (1.25, 2.5 and 5 vol%) as dopants were added to the mp-TiO2-2:7 precursor solution. The perovskite solution was spin coated over the doped and un-doped TiO2 layer in a N2-purged glove box at 1000 rpm for 10 s and 6000 rpm for 20 s, respectively. In the last 7 s of the latter, in order to completely transform PbI2 to perovskite, a few drops of chlorobenzene were dripped over the spinning substrate. The perovskite layer was annealed at 100 � C for 1 h. Afterward, SpiroOMeTAD solution as HTL was deposited by spin coating at 4000 rpm for 20 s. The HTL solution was prepared by dissolving 72.3 mg spiroOMeTAD, 28.8 mL TBP, and 17.5 mL of LiTFSI in acetonitrile (520 mg/mL) in 1 mL chlorobenzene. Finally, gold film with thickness of 50 nm for back contact was thermally evaporated on top of the devices using the DST3-A coater system (Nanostructured Coatings Co., Iran). The active area of all devices was 0.24 cm2. The top-view of the device is shown in Fig. 1(a).

2. Materials and experimental procedures 2.1. Materials Titanium (IV) isopropoxide (TTIP 97%), formamidinium iodide (FAI, 2

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2.4. Materials and device characterizations

3. Results and discussion

The size and morphology of the synthesized GQDs was characterized by Philips CM30 Transmission electron microscopy (TEM) operating at an acceleration voltage of 200 kV. Dynamic light scattering (DLS, Mal­ vern ZEN3600 A, UK) was used to evaluate the particle size distribution of the bulk synthesized GQDs. The Fourier transform infrared (FT-IR) spectrum of the synthesized GQDs (at 4000-400 cm 1, KBr plate) was recorded using Jasco 680-plus spectrophotometer. The ultra­ violet–visible (UV–Vis) absorption spectra of the different layers were measured using an AVANTES-2048 ULS spectrophotometer. The surface morphology of thin films and the structure of the PSCs were investigated by Quanta 450 FEG (FEI Co.) field emission scanning electron micro­ scopy (FE-SEM) at an acceleration voltage of 25 kV. Grazing incidence Xray diffraction (GIXRD) analysis was employed to characterize the thin layers using Asenware AW/XDM 300 diffractometer by Cu Kα radiation (λ ¼ 1.542 Å) under operation condition of 40 kV and 30 mA. The PV characteristics of the PSCs were measured by IV-28 potentiostat under the illumination of a simulated sunlight (AM 1.5, 100 mW cm 2) pro­ vided by a solar light simulator (500 W Xe lamp, SIM-1030). The PV data for all devices were acquired by using a non-reflective metal aperture of 0.09 cm2 to define the active area. Electrochemical impedance spectra of the PSCs were obtained in the dark conditions using the Eco-Chemie Autolab PGSTAT 302 N electrochemical workstation controlled with NOVA software in the frequency range from 0.01 Hz to 100 kHz under 100 mWcm 2 illumination at Voc.

3.1. Device structure The schematic device structure of PSC and cross sectional FE-SEM image of the fabricated PSC consisting of FTO/c-TiO2/GQDs-doped mp-TiO2/perovskite/spiro-OMeTAD/Au are shown in Fig. 1 (b) and (c), respectively. The cross sectional FE-SEM image of the PSC indicates that different layers are deposited closely to each other and the thickness of perovskite film was around 250 nm. Fig. 1 (d) illustrates the energy level of each layer of the GQDs-doped TiO2 ETL based PSC. 3.2. Structural properties The TEM image along with different DLS, XRD, FT-IR, and UV–vis patterns of the synthesized GQDs are shown in Fig. 2(a–f), respectively, confirming the formation of the GQDs. Interestingly, a very good agreement is achieved between the average size of the GQDs measured by the DLS technique shown in Fig. 2(c) (~6 nm) and determined by the image analysis presented in Fig. 2(b) (~5.8 nm). A relatively narrow size distribution can also be seen from both methods. The FT-IR spectra shows the stretching vibrations of O–H at 3350 cm 1, C–H at 1450 cm 1 and oxygen-containing functional groups such as vibrational absorption – O) at 1750 cm 1 and epoxy (-C-O-C-) at 1100 band of carbonyl (-C– cm 1. The XRD patterns of FTO, pure TiO2-2:7 ETL, 2.5 vol% GQDs-doped TiO2-2:7 ETL and perovskite film that was deposited on 2.5 vol% GQDsdoped TiO2-2:7 ETL are shown in Fig. 3. Compared to XRD pattern of pure TiO2-2:7 ETL, the appearance of new peaks at 26.5� and 46.3� in

Fig. 1. Schematic representation of the (a) device top-view and (b) device structure; (c) Cross sectional FE-SEM image with a structure of FTO/c-TiO2/GQDs-doped mp-TiO2/perovskite/spiro-OMeTAD/Au and (d) the energy level of each layer of the GQDs-doped TiO2 ETL based PSC. 3

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Fig. 2. (a) TEM image and (b) particle size distribution of the TEM image shown in (a) for the GQDs; (c) DLS, (d) XRD, (e) FT-IR and (f) UV–vis patterns of the synthesized GQDs.

XRD pattern of 2.5 vol% GQDs-doped TiO2-2:7 ETL, belonging to GQDs, confirms GQDs doping to TiO2 layer (see Fig. 3 (b) and (c)) [38]. As shown in Fig. 3 (c), the diffraction peaks located at 14� , 20� , 24.5� , 28.5� , 31.9� , 38� , 40.5� , 43� and 51.5� indicated that the perovskite film successfully formed on TiO2 layer [33]. Moreover, it can be claimed that the absence of PbI2 peaks in the XRD pattern of the perovskite film suggests that PbI2 was completely converted to perovskite. Raman spectroscopy was used to phase evaluation of doped and undoped TiO2 ETLs. Fig. 4 shows the Raman spectra of pure TiO2-2:7 and 2.5 vol% GQDs-doped TiO2-2:7 ETLs. It was reported that peaks located at 145, 380, 540 and 680 cm 1 are corresponding to Eg, B1g, A1g and Eg of anatase TiO2 phase, respectively. In fact, the Eg peak appeared due to symmetric stretching vibration of O–Ti–O band and B1g and A1g peaks can be attributed to symmetric banding vibration of O–Ti–O and anti­ symmetric banding vibration of Ti–O–Ti, respectively [39]. As can be seen in Fig. 4, compared to Raman spectrum of pure TiO2-2:7 ETL, two new peaks appeared at around 1280 and 1560 cm 1 for 2.5 vol% GQDs-doped TiO2-2:7 ETL, which can be assigned to D-band and G-band of GQDs, respectively. D-band is related to the vibration of carbon atom

Fig. 3. XRD patterns of (a) FTO, (b) pure TiO2-2:7 ETL, (c) 2.5 vol% GQDsdoped TiO2-2:7 ETL and (d) perovskite film deposited on 2.5 vol% GQDsdoped TiO2-2:7 ETL. 4

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samples exhibited good absorbance at UV region. Meanwhile, the lack of the peaks in visible region indicated that the TiO2 ETLs had less absor­ bance in this region. These results confirm that prepared TiO2 can be served as a favorable ETL for PSCs. In order to improve the TiO2 ETL performance, GQDs as dopants were added to the TiO2 precursor pre­ pared with TiO2 paste to ethanol ratio of 2:7 (optimum concentration). Fig. 6 (b) shows the effect of GQDs doping on the light absorption of the mp-TiO2, confirming that GQDs doping to TiO2 layer led to an increase of the absorbance at UV region (from 0.85 to 0.92) and a decrease of absorbance at visible region when compared to the un-doped one, bringing us to the conclusion that GQDs improve the TiO2 optical properties. On the other hand, it is proved that perovskite layer as the light absorbing layer plays a vital role to achieve superior PSCs. Therefore, it is necessary to improve the quality of the perovskite layer, in terms of its crystallinity and morphology, using different methods [25,34and35]. Various studies have concluded that the modification of TiO2 layer can be beneficial for improving the performance of perovskite layer [19,36]. The corresponding UV–Vis absorption spectra of perovskite films deposited on the doped and un-doped TiO2 ETLs are depicted in Fig. 7. No significant changes identified between the spectra of the perovskite layers based on pure TiO2 prepared with different TiO2 paste/ethanol ratios as shown in Fig. 7(a). However, it is shown in Fig. 7 (b), that although GQDs doping to TiO2 layer leads to a decrease in the light absorption of perovskite in the visible region, there is a favorably sig­ nificant decrease in the light absorption of perovskite in the UV region as well, which can ultimately result in a significant improvement of PSCs stability [37]. Indeed, UV–Vis measurements disclose that GQDs doping not only promotes the TiO2 optical performance but also can enhance the perovskite film stability.

Fig. 4. Raman spectra of pure TiO2-2:7 and 2.5 vol% GQDs-doped TiO22:7 ETLs.

with dangling bond in disorder structure and G-band is due to the E2g phonon mode of vibrations of sp2 C atoms in the 2D hexagonal lattice of a graphite cluster [40]. Moreover, the D band to G band intensity ratio ( ID IG ) of 2.5 vol% GQDs-doped TiO2-2:7 ETL is around 0.85. Fig. 5 (a) and (b) show the surface morphology and grain size dis­ tribution of the perovskite films deposited on doped and un-doped TiO2 ETLs. The perovskite film deposited on 2.5 vol% GQDs-doped TiO2-2:7 ETL formed a capping layer with a total thickness of about 300 nm and exhibited a dense structure with low defects/pinholes. Based on the FESEM images, thanks to GQDs doping, it can be observed that the defects/ pinholes of perovskite films dramatically decreased. This significant defects decreasing of perovskite film can suppress the charge trapping and recombination processes and facilitate the photogenerated carriers transport, ultimately improving the PSCs performance. Interestingly, GQDs doping to TiO2 layer improved the quality of perovskite film, in terms of defect states and structure density, as a useful parameter to improve the cell performance, suggesting that the efficacy of the perovskite film can be influenced by the TiO2 ETL [41].

3.4. Electronic properties Determination of flat-band potential (Efb) of the TiO2 as an ETL in PSCs is necessary for elucidating the position of the quasi-Fermi level of TiO2. In addition, donor density as a determinant parameter can also show the charge transport in TiO2 layer [42]. To this end, Mott-Schottky analysis was used to investigate the effects of GQDs doping on the Efb and donor density of mp-TiO2 layer. In the Mott-Schottky analysis, it is predicted that there is a linear relationship between the inverse square of the capacitance and applied potential. Accordingly, the space charge capacity (CSC) as a function of applied potential (E) can be calculated by the Mott-Schottky relationship as follows [43]: � 2ðE Efb kTÞ e 1 ¼ (1) 2 eε0 εND CSC

3.3. Optical properties UV–Vis measurement was employed to analyze the light absorption of the pure mp-TiO2 layers. Fig. 6(a) shows the absorbance spectra of the pure TiO2 prepared with different TiO2 paste/ethanol ratios. While no difference is seen between the spectra, a sharp peak can be specified at around 345 nm with absorbance of 0.85 (<1), demonstrating that these

where ε, ε0, ND and e are the dielectric constant of TiO2, the dielectric

Fig. 5. Top-view FE-SEM images of perovskite layer deposited on (a) un-doped and (b) GQDs-doped TiO2 ETLs (defects are marked by red circles). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 5

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Fig. 6. UV–Vis absorption spectra of (a) pure TiO2 prepared with different TiO2 paste/ethanol ratios and (b) TiO2 modified with different amounts of GQDs.

Fig. 7. UV–Vis absorption spectra of perovskite layer based on (a) pure TiO2 prepared with different TiO2 paste/ethanol ratios and (b) TiO2 modified with different amounts of GQDs.

constant of vacuum, the charge carrier density and the electronic charge, respectively, k is the Boltzmann constant and T is the absolute temper­ ature [42,43]. Mott-Schottky plots of the doped and un-doped TiO2 layers were drawn and the obtained results are summarized in Table 1. According to Table 1, it can be observed that, among the pure TiO2 ETLs, the TiO2-2:7 ETL showed the highest VOC. It was known that mpTiO2 layer act as a conducting pathway to facilitate electron transfer from perovskite to FTO. Therefore, it is essential that the mp-TiO2 layer completely covers the substrate. Accordingly, it can be suggested that lower VOC of the TiO2-1:7 ETL than the TiO2-2:7 ETL is attributed to the inappropriate TiO2 paste/ethanol ratio in this sample in which the cTiO2 surface was not completely covered by mp-TiO2 layer; as a result, the electron transfer was limited. On the other hand, it was

demonstrated that structural properties of the mp-TiO2 layer such as thickness and porosity content play an important role in improving the ETL performance. Porosity content of mp-TiO2 layer can be controlled by optimizing the TiO2 paste amount during the preparation process of TiO2 [44]. The poor performance of TiO2-3:7 ETL compared with TiO2-2:7 ETL can be related to the decreasing of porosity content of this sample with increasing the TiO2 paste/ethanol ratio (see Fig. 8). Based on the results presented in Table 1, it can be concluded that the Efb of GQDs-doped TiO2 ETLs in comparison with pure TiO2 ETLs shifted to more negative values, indicating that quasi-Fermi level of the GQDs-doped TiO2 ETLs is higher than that of the pure TiO2 ETLs. Therefore, one could conclude that GQDs in the role of the TiO2 modifier lead to an increase in the VOC of the PSCs.

Table 1 Mott-Schottky characteristics of PSCs fabricated using different TiO2 ETLs.

3.5. Electrochemical properties

ETL material

Voc (V)

Slope ( � 10þ13)

Intercept ( � 10þ13)

Efb (V vs. Ag/AgCl)

TiO2-1:7 TiO2-2:7 TiO2-3:7 1.25% GQDs/ TiO2-2:7 2.5%GQDs/ TiO2-2:7 5%GQDs/ TiO2-2:7

0.83 0.88 0.86 1.00

8 4 8 3

5.7 3.4 6.2 3.0

0.74 0.88 0.80 1.03

3.1 6.3 3.2 8.4

0.97

1

0.9

0.98

25.0

0.93

2

1.8

0.93

12.5

Electrochemical impedance spectroscopy (EIS) analysis was employed to investigate the charge carrier transport and recombination process in the doped and un-doped TiO2 ETL based-PSCs. Fig. 9(a) and (b) show the Nyquist plots of devices under dark condition at open circuit voltage. In all EIS spectra, the starting point value of Nyquist plot at high frequency represents the series resistance (Rs) which can be attributed to all electron transfer processes in the external circuit, in particular the resistance of FTO substrate. Moreover, two semicircles were observed where the high frequency semicircle corresponds to the resistance of hole transporting at HTL/perovskite interface (Rct) and the low

Nd (£10þ14 cm¡2)

6

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Fig. 8. Schematic illustration of the effect of TiO2 precursor solution with different TiO2 paste/ethanol ratios on the ETL performance.

Fig. 9. Nyquist plots and equivalent circuit of PSCs fabricated using (a) pure TiO2 ETL prepared with different TiO2 paste/ethanol ratios and (b) TiO2-2:7 ETL doped with different amounts of GQDs.

frequency semicircle is related to the recombination resistance (Rrec) at the TiO2/perovskite interface [45]. Based on EIS results, it can be seen that high frequency semicircle did not appear that can be due to the higher magnitude of Rrec than Rct. EIS results indicated that the fabri­ cated PSCs with different ETLs were very different in terms of recom­ bination resistance. According to semicircles radius, the TiO2-2:7 ETL based PSC, in comparison to other un-doped cells, have exhibited highest recombination resistance. It is worth noting that higher Rrec can be evidence of an increase in the electron lifetime, accelerating the charge transfer and reducing the charge recombination processes [45]. As previously stated, this PSCs performance improvement can be related to the optimization of the TiO2 paste/ethanol ratio which leads to (i) the formation of uniform perovskite film on TiO2 layer, and (ii) sufficient density of TiO2 layer. As can be seen in Fig. 9(b), GQDs doping led to the increasing of semicircle radius at low frequency, demonstrating the enhancement of recombination resistance which significantly contributes to improve the PSCs performance. The improvement of PSCs performance can be due to the presence of GQDs which can accelerate the charge mobility in TiO2 layer and improve charge transfer processes in cell structure via an in­ crease in charge lifetime and a decrease in charge trap states density. On the other hand, by converting UV light to visible photons, GQDs can lead to an enhancement of excitation efficiency within the perovskite layer [46]. As a result, the increase in the number of electrons injected into TiO2 layer in the presence of GQDs, followed by the enrichment of the trapping sites can be another reason for reducing the recombination

processes. Nyquist plots show that 2.5 vol% GQDs-doped TiO2-2:7 ETL based PSC provided 3 times higher Rrec than un-doped TiO2-2:7 ETL based PSCs. Indeed, 2.5 vol% GQDs-doped TiO2-2:7 ETL based PSC exhibited an Rrec of 15.09 kΩ, whereas un-doped TiO2-2:7 ETL based PSC exhibited an Rrec of 5.03 kΩ which confirms the effective role of GQDs doping in improving the PSCs performance. However, according to Fig. 9(b), it can be observed that with increasing the GQDs amount from 2.5% to 5%, the Rrec dramatically decreased in comparison with other GQDs-TiO2 ETL based PSCs. It can be related to the excessive amounts of GQDs that could form new charge recombination centers [25]. In summary, it can be concluded that the selection of the appro­ priate amount of GQDs can potentially be an effective approach to modify the TiO2 ETL. 3.6. Photovoltaic performance of PSCs For assessing PV performance of PSCs, standard photocurrent density-voltage (J-V) measurements were conducted at ambient condi­ tions under an AM1.5G solar simulator with a light intensity of 100 mWcm 2. Fig. 10(a) illustrates J-V curves of the PSCs based on TiO2 ETL prepared with different TiO2 paste/ethanol ratios. The PV parameters of these cells are also summarized in Table 2. As can be seen in Table 2, having increased the TiO2 paste/ethanol ratio from 1:7 to 2:7, the shortcircuit current density (Jsc), open-circuit voltage (Voc) and fill factor (FF) were all boosted; as a result, the TiO2-2:7 ETL based PSC provided a better PCE of 9.55% that is approximately 21% higher than that of TiO27

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Fig. 10. J-V curves of PSCs fabricated using (a) pure TiO2 prepared with different TiO2 paste/ethanol ratios and (b) TiO2 doped with different amounts of GQDs, as ETL. (c) Incident-photon-to-current-efficiency (IPCE) spectra of PSCs fabricated using different TiO2 ETLs.

further increasing of the TiO2 paste/ethanol ratio from 2:7 to 3:7, the JSC, VOC, FF and consequently PCE of PSCs decreased. This performance degradation may be ascribed to the increasing of trap states and electron recombination rate in the mp-TiO2 film. Based on the J-V results, the Jsc, Voc and FF of the TiO2-2:7 ETL based PSC, as the champion cell, were 18.16 mA cm 2, 0.88 V and 0.60, respectively, corresponding to a PCE of 9.55%. The J-V curves of PSCs fabricated using TiO2-2:7 and GQDs-doped TiO2-2:7 with different percentages of GQDs as ETL are shown in Fig. 10(b). The PV parameters of these cells are also summarized in Table 2. The findings reveal that GQDs doping to TiO2 layer led to a dramatic improvement of PSCs PV performance. The mp-TiO2 layer not only is the path to transfer electrons from perovskite to FTO but also serves as the scaffold of the perovskite [48and49]. It can be suggested that GQDs can promote the infiltration of perovskite into the mp-TiO2 and improve the TiO2/perovskite interface due to the increasing of the specific surface area of mp-TiO2 which leads to the formation of uniform perovskite film on TiO2. Compared to TiO2-2:7 ETL based PSC, as a champion PSC fabricated with pure TiO2 layer, the PCE of PSCs based on GQDs-doped TiO2 ETL with 1.5, 2.5 and 5 vol% GQDs increased 34.7% (from 9.55 to 12.87), 50.3% (from 9.55 to 14.36) and 16.2% (from 9.55 to 11.1), respectively. It was previously stated that since GQDs possess outstanding properties such as high conductivity, excellent solubility, stable photoluminescence, and quantum size effect [29–31], it can be expected that GQDs doping to the mp-TiO2 layer can result in the improvement of PSCs performance. Another unique feature of GQDs, as luminescent down shifting materials, is the conversion of UV light, which can degrade the perovskite material and limit the stability of PSCs to visible photons [46]. Therefore, GQDs doping, as an effective way, can improve the PSCs stability and increase the light harvesting effi­ ciency of PSCs. On the other hand, it can be claimed that GQDs not only

Table 2 Backward (BW) scan photovoltaic characteristics of PSCs fabricated using different TiO2 ETLs. ETL material

JSC (mA cm¡2)

VOC (V)

FF

Champion PCE %

Rs (Ω cm2)

Rsh (Ω cm2)

TiO2-1:7 TiO2-2:7 TiO2-3:7 1.25% GQDs/ TiO2-2:7 2.5%GQDs/ TiO2-2:7 5%GQDs/ TiO2-2:7

16.98 18.16 17.52 19.76

0.83 0.88 0.86 1.00

0.55 0.60 0.58 0.65

7.85 9.55 8.84 12.87

14 12.2 13.4 10.3

151.5 214.1 175.7 285.7

21.92

0.97

0.67

14.36

8.9

303.0

18.81

0.93

0.63

11.10

11.5

261.8

1:7 ETL based PSC. PSCs performance improvement can be due to the fact that with increasing the TiO2 paste amount in TiO2 precursor so­ lution, donor density improved, demonstrating that electron transfer as a crucial parameter for enhancing the PSCs performance improved, which is in agreement with the Mott-Schottky analysis results. TiO2 porosity is an effective factor for improving the perovskite/TiO2 inter­ face (i.e. the formation of uniform perovskite film) and the TiO2/TiO2 interfaces (i.e. the improvement of interconnectivity of the TiO2 layer), affecting the electrons injection from the perovskite layer and their transportation to the FTO layer [47]. Therefore, as another reason for the improvement of PSCs performance, with increasing the amount of TiO2 paste in TiO2 precursor solution, the porosity content of TiO2 layer is reduced, which subsequently resulted in an increase of the electron transport property and facilitated the formation of a more uniform coating of perovskite on the TiO2. However, it is observed that with 8

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can decrease the trap states due to its quantum size which is effective to reduce recombination process, but also improves the interface contact between TiO2 and perovskite and increase electron injection that can ultimately promote the PSCs PV performance. In addition, due to the appropriate LUMO level of the GQDs [25and26], electrons transferred rapidly in the TiO2 layer through the GQDs, increasing the electrons lifetime and suppressing the charge recombination process. The Mott-Schottky analysis also indicated that GQDs doping to TiO2 layer increased the donor density that consequently can lead to enhancing the Jsc. According to the previous results, it can be observed that GQDs doping to TiO2 layer can result in the formation of a uniform perovskite film on TiO2 layer and improve TiO2/perovskite interface which can be beneficial for enhancing the electrons transfer and suppressing the recombination process. These positive effects can contribute to a decrease in the series resistance (RS) and an increase in the shunt resistance (RSH), successfully improving the fill factor of devices. How­ ever, it can be observed that when the GQDs amount increased from 2.5% to 5%, the JSC and average PCE of devices decreased. It is probable that with excessive increase in GQDs amount, GQDs can act as new charge recombination centers which are detrimental to PSCs perfor­ mance [25]. Therefore, it can be concluded that the best PV performance of PSCs was achieved through 2.5 vol% GQDs doping to TiO2. The 2.5 vol% GQDs-doped TiO2-2:7 ETL based PSC, as a champion cell, shows a PCE of 14.36% with JSC and VOC, FF of 21.9 mA cm 2, 0.97 V and 0.63, respectively. Fig. 10(c) IPCE spectra of the 2.5 vol% GQDs-doped TiO2-2:7 ETL

based PSC in comparison to the un-doped TiO2-2:7 ETL based PSC. As can be observed, in the presence of GQDs, as TiO2 ETL modifier, an IPCE of ~85% was achieved at the peak maximum, which can be attributed to superior properties of GQDs. As it can be seen, the obtained JSC values from the IPCE spectra are well matched with the obtained JSC values from the J-V curves. In order to ensure reproducibility of properties of the PSCs and analyze the statistical distributions of PV parameters, 15 devices were fabricated for each condition. The corresponding PV parameters were presented in the form of a standard box plot as shown in Fig. 11. A reasonable standard deviation can be observed for the PV parameters. It can be seen that the 2.5 vol% GQDs-TiO2-2:7 ETL based cells have the highest JSC and FF, resulting in the highest PCE. 3.7. Stability of PSCs The above-mentioned results demonstrated that GQDs doping to TiO2 ETL improved the efficiency of PSCs. Concerns, however, remain in terms of the long-term stability of the PSCs. Despite the rapid increasing of PCSs efficiency in recent years, the commercialization of PSCs is still facing a lot of limitations such as material toxicity, device instability and reproducibility. As known, long-term stability of PSCs is one of the major issues which limits the outdoor device applications [50,51]. Therefore, in this study, the effects of GQDs doping on the PSCs stability after 500 h storage in an ambient environment were monitored and corresponding results are shown in Fig. 12. It can be seen that the PSCs based on

Fig. 11. The statistical PV parameters of PSCs fabricated using different TiO2 ETLs: (a) VOC, (b) JSC, (c) FF and (d) PCE. 9

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Solar Energy Materials and Solar Cells 208 (2020) 110407

GQDs-doped TiO2-2:7 ETL exhibited ~88% of the initial efficiency even after 500 h; whereas, the PSCs based on pure TiO2-2:7 ETL exhibited only 61% of the initial efficiency after the same time period. It should be noticed that perovskite material, as the light absorbing material for PSCs, degrades when exposed to moisture, UV light and high temperature [52–54]. It has been reported that, as one of the most accepted theories for decomposition of a perovskite material (including þ CH3NHþ 3 , Pb and I ), TiO2 can extract electrons from I , leading to the formation of I2 at the TiO2/perovskite interface. This mechanism will eventually result in the decomposition of the perovskite materials. It should be noted that UV light illumination accelerates this perovskite degradation [54,55]. On the other hand, it is known that degradation of TiO2 performance is mainly due to its sensitivity to the UV light illu­ mination and the presence of surface defects such as oxygen vacancies [56,57]. According to the above explanations, it is believed that the GQDs can prevent the degradation of PSCs in two ways. Firstly, GQDs, as luminescent down shifting materials, absorb UV light and convert it into visible light, which can be a promising factor to improve durability of perovskite film. Secondly, the presence of GQDs in the TiO2 layer can reduce the interaction of TiO2 to UV light absorption, thereby leading to the improvement of PSCs performance. It seems that both effects could make a delay in cell degradation. XRD analysis was also employed to characterize the doped and undoped perovskite at different times, as shown in Fig. 13. As can be seen, a new peak is appeared around 12.7� after 500 h in the XRD pat­ terns of both doped and un-doped ETL substrates, which belongs to PbI2. This shows that the perovskite material decomposes to PbI2 over time, which eventually leads to the degradation of PSCs performance. Inter­ estingly, however, it can be observed that intensity of the PbI2 peak in the XRD pattern of the GQDs doped-TiO2 based perovskite is far less than that of the un-doped one. This clearly confirms that in the presence of GQDs, the rate of perovskite degradation dramatically decreased, which can be attributed to the superior ability of GQDs to decrease the harmful effects of UV light illumination. Based on the results, by employing the GQDs as TiO2 ETL modifier, the device efficiency and stability of PSCs significantly improved.

Fig. 12. The PCE stability of the PSCs after 500 h stored under ambient con­ ditions. The insets are the top-view optical micrographs of the cells.

4. Conclusions Different structural, optical, electronic, and electrochemical prop­ erties and PV performance of the GQDs-doped TiO2 ETL based meso­ scopic PSCs were investigated. It was found that in the presence of GQDs, the performance of PSCs, in terms of PCE and long term stability, significantly improved thanks to facilitating the charge transfer and suppressing the recombination processes. Indeed, the optimized GQDs doping led to the enhancing of carrier density and photo-generated electron injection as well as electrons transfer at TiO2/perovskite interface, contributing to improve the PSCs performance. This is mainly attributed to the GQDs outstanding properties including high conduc­ tivity, light harvesting abilities, excellent solubility, and quantum size effect. Accordingly, the 2.5 vol% GQDs-doped TiO2 ETL based PSC, as a champion cell, exhibited the Jsc, Voc and FF of 21.9 mA/cm2, 0.97 V and 0.63, respectively, corresponding to a PCE of 14.36% which is approx­ imately 50% higher than that of pure TiO2 ETL based PSC. Meanwhile, the PSC based on 2.5 vol% GQDs-doped TiO2 ETL was able to maintain ~88% of its initial PCE, after 500 h in ambient conditions; whereas, the conventional PSC based on pure TiO2 ETL was able to maintain only 61% of its initial PCE under the same conditions. This work clearly shows that doping of GQDs, as a modifier, to the TiO2 layer can be a beneficial approach to improve the PSC’s performance and stability.

Fig. 13. XRD patterns of the perovskite layer coated on the un-doped and GQDs-doped ETL at different times.

curation, Writing - review & editing Reyhaneh Haji Heidari: Visuali­ zation Elahe Khorshidi: Data curation, Validation Neda Irannejad: Methodology, Data curation, Software Behzad Rezaie: Resources. Declaration of competing interest 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. Acknowledgements The authors greatly acknowledge Isfahan University of Technology (IUT), Iran National Science Foundation (INSF), and Iran’s National Elites Foundation (INEF) for their financial support. Grateful thanks are also extended to the Surface Analysis Laboratory (SAL), Department of Mechanical Engineering Sciences, University of Surrey, UK and specif­ ically Dr. Steven Hinder for providing the opportunity to do the XPS analyses.

Author contribution Marzieh Ebrahimi: Methodology, Investigation, Writing- original draft preparation, Ahmad Kermanpur: Conceptualization, Supervision, Masoud Atapour: Supervision, Siavash Adhami: Resources, Data 10

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