Effects of CsSnxPb1−xI3 Quantum Dots as Interfacial Layer on Photovoltaic Performance of Carbon-Based Perovskite Solar Cells

In this work, inorganic tin-doped perovskite quantum dots (PQDs) are incorporated into carbon-based perovskite solar cells (PSCs) to improve their photovoltaic performance. On the one hand, by controlling the content of Sn2+ doping, the energy level of the tin-doped PQDs can be adjusted, to realize optimized band alignment and enhanced separation of photogenerated electron–hole pairs. On the other hand, the incorporation of tin-doped PQDs provided with a relatively high acceptor concentration due to the self-p-type doping effect is able to reduce the width of the depletion region near the back surface of the perovskite, thereby enhancing the hole extraction. Particularly, after the addition of CsSn0.2Pb0.8I3 quantum dots (QDs), improvement of the power conversion efficiency (PCE) from 12.80 to 14.22% can be obtained, in comparison with the pristine device. Moreover, the experimental results are analyzed through the simulation of the one-dimensional perovskite/tin-doped PQDs heterojunction. Supplementary Information The online version contains supplementary material available at 10.1186/s11671-021-03533-y.


Introduction
In past few years, perovskite materials have been widely applied in solar cells due to their excellent electrical and optical properties, such as suitable bandgap width, large light absorption coefficient and good defect tolerance [1][2][3][4][5][6]. Interface engineering, as a strategy to modify the interface characteristics of thin film devices, has become one of the approaches to improve the performance of perovskite solar cells (PSCs) [7,8]. Recently, lead-based halide perovskite quantum dots (PQDs) in the form of APbX 3 (A = CH 3 NH 3 + (MA + ), Cs + ; X = Cl − , Br − , I − ) are often used as interfacial layers or additives for optimized band alignment thanks to their adjustable band structures [9][10][11][12][13][14][15]. The combination of perovskite absorbers and PQDs is regarded as an effective method for enhanced charge extraction and improved PSC properties.
It is worth noting that most of the relevant researches are based on PSCs with hole-transporting layers (HTLs). In recent years, carbon-based HTL-free PSCs with simple preparation processes and low costs have been given much attention [16][17][18]. Similarly, PQDs can also be used in this PSC structure. However, some other requirements besides band alignment should be taken into consideration. First, the lattice structures of lead-based PQDs are not very stable due to Pb 2+ with a large ionic radius reducing the tolerance factor. Therefore, lead-reduced PQDs are promising candidates. Second, because of the lack of HTLs, the hole transport performance is bound to be weakened. Consequently, the added PQDs are required to supply extra free holes, so that photogenerated holes can be smoothly transferred from the perovskite layer to the carbon electrode.
The ion exchange method using metal cations with smaller ionic radii (such as Cu 2+ , Zn 2+ , Sn 2+ , Cd 2+ ) to partially replace Pb 2+ has been proven to improve the lattice stability of PQDs [19][20][21]. Among these metal cations, Sn 2+ is easily to oxidize to Sn 4+ , which can introduce self-p-type doping effects to enhance hole transfer [22][23][24]. Particularly, Liu et al. synthesized CsSn 0.6 Pb 0.4 I 3 quantum dots (QDs) featuring a hole mobility of 40.12 cm 2 V −1 s −1 and good stability in the ambient air [25]. Xu and co-workers incorporated CsSnBr 3−x I x QDs between the CsPbBr 3 perovskite and the carbon electrode to promote charge extraction [26]. Very recently, Duan et al. found that MAPbI 3 /CsSnI 3 heterojunction as the lightharvester in the carbon-based HTL-free PSC could facilitate the hole transfer [27]. Inspired by these above, we propose that tin-doped PQDs with appropriate energy levels and self-p-type doping effects are able to function like HTLs to modify the injection and transport characteristics of holes.
In this work, tin-doped PQDs in the form of CsSn x Pb 1−x I 3 were incorporated between the MAPbI 3 perovskite and the carbon electrode to achieve optimized band alignment and improved hole transfer. An increment in power conversion efficiency (PCE) of 11.09%, from 12.80 to 14.22%, could be obtained after the addition of CsSn 0.2 Pb 0.8 I 3 QDs.

Synthesis and Purification of Tin-Doped PQDs
We adopted a simple mixed-heating procedure to synthesize tin-doped PQDs. Briefly, Cs 2 CO 3 , SnI 2 and PbI 2 with a specific molar ratio (CsSn 0.1 Pb 0.9 I 3 QDs: 0.037 mmol Cs 2 CO 3 , 0.2 mmol PbI 2 , 0.15 mmol SnI 2 ; CsSn 0.2 Pb 0.8 I 3 QDs: 0.037 mmol Cs 2 CO 3 , 0.2 mmol PbI 2 , 0.2 mmol SnI 2 ; CsSn 0.3 Pb 0.7 I 3 QDs: 0.037 mmol Cs 2 CO 3 , 0.2 mmol PbI 2 , 0.25 mmol SnI 2 ) were mixed with 10 mL of ODE, 0.5 mL of OA, 0.5 mL of OAM and 0.5 mL of TOP in a 50-mL three-neck flask. OA, OAM and TOP were utilized to limit the particle size and to passivate the surface defects of tin-doped PQDs. Then, the mixture was stirred and heated at 100 °C for 30 min under nitrogen atmosphere to obtain a red solution, including nano-sized and micron-sized tin-doped perovskites. To extract and purify tin-doped PQDs, 10 mL of MeOAc was added into the red solution, followed by centrifuging at 7000 rpm for 5 min. The supernatant was discarded, and the brown black precipitate was dispersed in 5 mL of hexane. Finally, the brown black solution was centrifuged at 3000 rpm for 5 min, and the red supernatant contained only the tin-doped PQDs.

Device Fabrication
Fluorine-doped SnO 2 (FTO) glasses were washed with water, acetone, isopropanol and ethanol in sequence for 30 min each in an ultrasonic cleaner. After that, the FTO glasses were treated by ultraviolet (UV) for 20 min to remove residual organic solvents. The compact TiO 2 (c-TiO 2 ) layer was fabricated on the FTO layer by spincoating a solution of acetylacetonate (0.1 mL) diluted in ethanol (1.9 mL) with the speed of 4000 rpm for 30 s. Then, the glasses were annealed at 150 °C for 5 min and at 500 °C for 30 min. Subsequently, the mesoporous TiO 2 (m-TiO 2 ) layer was obtained by spin-coating a solution of TiO 2 paste diluted in ethanol onto the c-TiO 2 layer at 3500 rpm for 20 s and annealed at 500 °C for 30 min. The annealing process at 500 °C is to obtain TiO 2 layers with improved electron transport performance. Next, to prepare the MAPbI 3 precursor solution, PbI 2 (0.5 mmol) and MAI (0.5 mmol) were mixed with DMF (300 mg) and DMSO (39 mg). Afterward, the MAPbI 3 layer was fabricated by spin-coating the MAPbI 3 precursor solution (35 μL) onto the m-TiO 2 layer, with the speed of 1000 rpm for 10 s and 4000 rpm for 20 s, followed by heating at 100 °C for 10 min. After that, tin-doped PQDs dispersed in toluene (10 mg mL −1 ) were spin-coated onto the perovskite layer at 4000 rpm for 30 s and annealed at 90 °C for 5 min to remove the residual toluene. Finally, the carbon electrode paste was screen-printed on the device and annealed at 100 °C for 10 min.

Characterization
Transmission electron microscope (TEM) images, selected-area electron diffraction (SAED) views and energy-dispersive X-ray spectroscopy (EDS) analyses of tin-doped PQDs were obtained by a field emission high-resolution transmission electron microscope (JEM-2100F, JEOL, Japan) at an accelerating voltage of 200 kV. The valence band (VB) edges of different materials were acquired from an X-ray photoelectron spectrometer (ESCALAB 250Xi, Thermo Fisher Scientific, USA). Absorption and steady-state photoluminescence (PL) characteristics were collected via an UV-visible spectrophotometer (UV-3600, Shimadzu, Japan) and a fluorescence spectrometer (RF-6000, Shimadzu, Japan), respectively. The cross-sectional image of the PSC and the surface morphologies of perovskite films were obtained by a scanning electron microscope (Zeiss Ultra Plus, Zeiss, Germany). Curves of photocurrent density versus on voltage (J-V) were measured by a sourcemeter (2400, Keithley, USA) with a sunlight simulator (Oriel Sol3A, Newport, US), under AM 1.5G simulated illumination (100 mW cm −2 ). Monochromatic incident photon-to-electron conversion efficiency (IPCE) spectra and electrochemical impedance spectroscopies (EIS) were obtained from an electrochemical workstation (Zahner, Kronach, Germany). Finally, X-ray diffraction (XRD) patterns of perovskite films and PQDs were acquired from an X-ray diffractometer (Empyrean, PANalytical, Netherlands).

Results and Discussion
Three kinds of tin-doped PQDs were studied in this work, including CsSn 0.1 Pb 0.9 I 3 QDs, CsSn 0.2 Pb 0.8 I 3 QDs and CsSn 0.3 Pb 0.7 I 3 QDs. The actual atomic ratios of Sn/ (Sn + Pb) in these PQDs were estimated to be 13.03%, 22.12% and 32.57%, respectively (shown in Additional file 1: Fig. S1 and Tables S1-S3). As shown in Fig. 1, blue-shifts of the steady-state PL peak (673 nm, 669 nm and 656 nm in turn) and the edge of Tauc plot (1.79 eV, 1.80 eV and 1.81 eV in turn) were observed with the increase of Sn doping. For many bulk perovskite materials in the form of ABX 3 (A = Cs, MA, FA; B = Sn x Pb 1−x ; X = Cl, Br, I), the bandgap often exhibits a downward trend with the increase of the x value. That is because the bandgap is determined by the electronegativity of the B-site atom (Pb 2+ : χ = 1.6; Sn 2+ : χ = 1.7). However, when it comes to nanocrystals with quantum confinement, the impact of unit cell volume on bandgap matters more. The perovskite bandgap is known to increase with the decrease of the unit cell volume [19]. Therefore, more Sn 2+ substitution would further intensify the lattice contraction, which led to the augment of bandgap width, consistent with the reported research [28]. Meanwhile, the larger electronegativity of Sn atom might be the reason why the bandgap did not increase significantly.
The TEM images of these tin-doped PQDs are exhibited in Fig. 2a-c. These tin-doped PQDs were all square, consistent with the theoretical lattice structure of cubic phase. Besides, the average size of each of these three PQDs was about 15 nm, and there was no significant difference. That was because the size was mainly determined by the reaction temperature, which was kept at 100 °C for all the PQDs. Besides, SAED measurements  200) and (220) could be identified, which also indicated that these tin-doped PQDs were mostly composed of cubic nanocrystals (NCs) [20]. Moreover, enlarged TEM images shown in Fig. 2g-i are utilized to probe the crystal plane characteristics. The interplanar distances of (200) plane of these tin-doped PQDs were determined to be 0.308 nm, 0.303 nm and 0.296 nm, in turn, which demonstrated that increasing substitution of Pb 2+ by Sn 2+ led to the lattice shrinkage, in accordance with their optical characteristics mentioned above.
To further study the lattice structures of these tindoped PQDs, we performed normalized XRD measurements, shown in Fig. 3. According to the standard XRD data of CsPbI 3 in orthorhombic and cubic forms [20,29], the diffraction peaks associated with orthorhombic and cubic phases were marked with "*" and "#, " respectively. As the amount of Sn doping in PQDs increased, the diffraction angle of the peak corresponding to (200) plane slightly increased, implying that the interplanar distance of (200) plane was reduced, in line with the analysis above. Meanwhile, the intensity of the diffraction peak representing orthorhombic phase showed an increasing trend, which indicated that the phase transition process in the PQDs increased. This might be because the increase in the amount of Sn doping would intensify the oxidation reaction of the PQDs in the air, resulting in more Sn vacancies, which may make Pb refill these vacancies to form an unstable perovskite structure.
Optimized band alignment is crucial for enhancing the extraction of photogenerated carriers and suppressing non-radiative recombination [30][31][32][33]. Figure 4 shows the band structures of various materials including FTO, TiO 2 , MAPbI 3 , CsSn 0.1 Pb 0.9 I 3 QDs, CsSn 0.2 Pb 0.8 I 3 QDs, CsSn 0.3 Pb 0.7 I 3 QDs and carbon. Corresponding UPS data and Tauc plots are shown in Additional file 1: Fig. S2. It is clear that the valence band (VB) edge of CsSn 0.1 Pb 0.9 I 3 QDs (− 5.53 eV) or CsSn 0.2 Pb 0.8 I 3 QDs (− 5.50 eV) was higher than that of MAPbI 3 (− 5.54 eV), satisfying the band alignment requirement. It was able to eliminate the large Schottky barrier formed by the MAPbI 3 /carbon junction, thus enhancing the hole extraction ability (discussed later) [31]. Furthermore, the higher conduction band (CB) edges of these tin-doped PQDs were expected to hinder the flow of electrons from MAPbI 3 to the carbon electrode. However, the VB edge of CsSn 0.3 Pb 0.7 I 3 QDs (− 5.58 eV) was lower than that of MAPbI 3 , which would block the hole injection, leading to more charge recombination at the interface between MAPbI 3 and the PQDs.
Moreover, the VB edge originates from the interactions between Pb (6s) and I (5p) orbitals, which are also determined by the Sn doping amount. On the one hand, the substitution of Pb 2+ by Sn 2+ will shrink the lattice structure, leading to shorter Pb-I bonds and stronger interactions between Pb and I orbitals, so that the VB tends to shift to a higher energy position [19]. On the other hand, more lattice distortions (transformation from cubic NCs to orthorhombic NCs) will be introduced into the PQDs with excessive Sn 2+ substitution, resulting in expanded volume of [PbI 6 ] octahedra and weaker Pb-I interactions, thus moving the VB to a lower energy position [21]. As a result, the VB edge does not vary linearly with the Sn doping of the PQDs. A reasonable Sn doping content is the key to obtaining an appropriate band structure.
Unlike ordinary lead-based PQDs, tin-doped PQDs will partially undergo oxidation in air due to the presence of Sn 2+ , described by (1) 2CsSn x Pb 1−x I 3 + xO 2 → xCs 2 SnI 6 + (2 − 2x)CsPbI 3 + xSnO 2 .  CsSn x Pb 1−x I 3 can be assumed to be the combination of CsSnI 3 and CsPbI 3 with a certain molar ratio. Among the compounds, only CsSnI 3 participates in the oxidation reaction. Then, this process can be simplified to In reaction (2), the transformation from CsSnI 3 to Cs 2 SnI 6 is regarded as breaking the connections between [SnI 6 ] octahedra. The reason is that CsSnI 3 is formed by corner sharing [SnI 6 ] octahedra, while Cs 2 SnI 6 is made up of isolated [SnI 6 ] octahedra [22]. Therefore, these half of Sn atoms do not leave the perovskite lattice. However, the other half of the Sn atoms are oxidized to SnO 2 , leaving a lot of Sn vacancies in the lattice, which will accept electrons (or supply holes) and act as p-type dopants. It can be described by Eq. (3) as follows: That is the reason for the self-p-type doping effects of tin-doped PQDs. Accordingly, under the premise that the lattice structure of tin-doped PQDs can be stabilized, the acceptor concentration of the PQDs will increase with the Sn doping amount.
The cross-sectional image of the PSC is shown in Fig. 5a. The widths of FTO layer, m-TiO 2 layer and MAPbI 3 layer were about 400 nm, 200 nm and 800 nm, respectively. Because of the low concentration of the PQD solution (10 mg mL −1 ), it was hard to observe a PQD layer that could be distinguished from the underlying MAPbI 3 film. To prove the existence of PQDs on MAPbI 3 , we performed XPS measurement on the film with the structure of FTO/c-TiO 2 /m-TiO 2 /MAPbI 3 / PQDs. The XPS results are shown in Additional file 1: Fig. S3. The elements including Cs, I, Sn and Pb were all detected, demonstrating that there was a PQD layer (2) 2CsSnI 3 + O 2 → Cs 2 SnI 6 + SnO 2 .
on the perovskite film. Besides, as shown in Fig. 5b-e, there were many small-sized white PbI 2 particles on the original perovskite film, caused by the partial decomposition of the perovskite in the air. After adding tin-doped PQDs, the number of white particles decreased, and the perovskite films exhibited slightly better grain uniformity and compactness than the pristine sample. However, the morphology difference between the various perovskite films was still not obvious. In order to further distinguish their surface characteristics, we performed grazing incidence XRD (GIXRD) patterns of perovskite films with different tin-doped PQDs, exhibited in Fig. 6. The diffraction peak at about 12.7° is associated with PbI 2 [34]. After the modification of tin-doped PQDs, the diffraction intensity ratio of PbI 2 :(110) plane was decreased, suggesting that the decomposition process of the perovskite film was suppressed.   Table 1. The values of J sc , V oc , FF and PCE of the PSC without modification by tin-doped PQDs were 22.69 mA cm −2 , 0.99 V, 56.78% and 12.80%, respectively. For the CsSn 0.1 Pb 0.9 I 3 QDs-added PSC, various parameters were improved. However, the improvement was not optimal, which might ascribe to the relatively low Sn doping of the PQDs. In contrast, with the incorporation of CsSn 0.2 Pb 0.8 I 3 QDs, a J sc of 23.30 mA cm −2 , a V oc of 1.05 V, a FF of 57.90% and a PCE of 14.22% could be obtained. The significant increase in each parameter indicated the reduction of non-radiative recombination and the effective extraction of photogenerated holes. Besides, as shown in Additional file 1: Fig. S4 [29,[35][36][37], thus seriously impeding the transport process of photogenerated carriers.  As described in Fig. 7b, the IPCE spectra in a wavelength range from 350 to 800 nm increased in the order of CsSn 0.3 Pb 0.7 I 3 QDs-added device < control device < CsSn 0.1 Pb 0.9 I 3 QDs-added device < CsSn 0.2 Pb 0.8 I 3 QDs-added device, in agreement with the corresponding trend of J sc acquired from the J-V curves. It is clear that the difference of these IPCE curves was mainly reflected in the wavelength range from 550 to 800 nm. Tin-doped PQDs added onto the perovskite film would significantly affect the built-in electric field near the back surface of the perovskite (analyzed in detail later). At the same time, the longwavelength photons were mainly absorbed by the perovskite near the back surface due to their low energy. When these photons were transformed to carriers, their transport properties would be more easily changed by the above-mentioned built-in electric field than those carriers converted from short-wavelength photons.
In addition, EIS measurements, in a frequency range from 4 to 0.2 MHz at a bias of 0.8 V under simulated AM 1.5G radiation, were performed to analyze the charge transport resistance (R CT ) and the barrier capacitance (C T ) near the carbon electrode, described in Fig. 7c. Corresponding EIS parameters are also shown in Table 1. With the addition of CsSn 0.2 Pb 0.8 I 3 QDs, the R CT value was reduced, exhibiting promoted hole extraction and decreased energy loss on the back surface of MAPbI 3 . Furthermore, compared with the pristine and the CsSn 0.1 Pb 0.9 I 3 QDs-added PSCs, the value of C T increased, so that a shorter depletion width near the back surface of MAPbI 3 (W D ) could be deduced based on the following formulas, suggesting facilitated hole transfer.
where A is the active area and d QD is the width of the PQD layer. It is worth noting that the contact between MAPbI 3 and the PQDs would form a hole depletion region in MAPbI 3 . Then, the contact between the PQDs and the carbon electrode would generate a Schottky barrier, which led to a hole depletion region in the PQD layer. Both the depletion regions in MAPbI 3 and the PQDs contributed to the barrier capacitance value. For the PSC in the presence of CsSn 0.3 Pb 0.7 I 3 QDs, the lower VB edge of the PQDs allowed more holes to migrate from (4) the PQD layer to the MAPbI 3 film. These holes gradually moved away from the MAPbI 3 /PQDs interface under the isotype heterojunction electric field, thereby increasing the W D . This might be the reason for the low C T value of CsSn 0.3 Pb 0.7 I 3 QDs-added device.
To get insight on the carrier transfer process, the steady-state PL spectra for the MAPbI 3 films with and without tin-doped PQDs were measured. As shown in Fig. 7d, the PL peak intensity at about 775 nm was obviously decreased after the incorporation of CsSn 0.1 Pb 0.9 I 3 QDs or CsSn 0.2 Pb 0.8 I 3 QDs. There are two explanations for the weakening of PL intensity: First, the PQDs cause additional non-radiative pathways to capture photogenerated carriers; second, the higher VB edge of PQDs allows more photogenerated holes to migrate to the PQD layer; thus, the number of carriers participating in direct recombination is reduced. However, after adding CsSn 0.3 Pb 0.7 I 3 QDs with more orthorhombic by-products and lower VB edge, the PL intensity increased, which showed that more carriers were limited in the perovskite film without being trapped by defects. Therefore, the PL quenching of the perovskite film with CsSn 0.1 Pb 0.9 I 3 QDs or CsSn 0.2 Pb 0.8 I 3 QDs was caused by the optimized band alignment promoting the hole extraction, instead of interfacial trap-assisted recombination.
In order to further understand the effects of tin-doped PQDs on the hole transport in MAPbI 3 films, a onedimensional MAPbI 3 /tin-doped PQDs heterojunction model was constructed, shown in Fig. 8a. To simplify the analysis, this structure was regarded as a mutant isotype heterojunction, and MAPbI 3 and tin-doped PQDs were determined to be p-type semiconductors. Theoretically, MAPbI 3 is a kind of intrinsic semiconductor with low doping concentration. However, in the carbon-based perovskite PSCs with no HTLs, the perovskite layer needs to undergo p-type doping treatment. A small amount of DMSO was added in the precursor of perovskite to form a complex with PbI 2 , so that there were Pb vacancies in the perovskite, which made the perovskite become a p-type semiconductor. Moreover, Laban and Etgar utilized Mott-Schottky analysis to find that the acceptor concentration of MAPbI 3 was 2.14 × 10 17 cm −3 , belonging to the doping level of p-type materials [38]. The contact of two semiconductors with different Fermi levels would form an electric field from the one with a high Fermi level to the another one with a low Fermi level. Consequently, the p-p isotype heterojunction energy band diagram under the equilibrium condition could be obtained, shown in Fig. 8b. According to the Poisson's equation, the field continuity condition and the depletion approximation [39], barrier distributions of the isotype heterojunction were expressed by the following equations: where q is the elementary charge and ε QD and N A_QD are the dielectric coefficient and the acceptor concentration for tin-doped PQDs, respectively. V D_MAPbI3 and V D_QD are the potential difference in MAPbI 3 and tin-doped PQDs in turn. E Fermi_MAPbI3 and E Fermi_QD stand for the Fermi levels of MAPbI 3 and tin-doped PQDs, respectively. k B is the Boltzmann constant and T is the room temperature. N C and N V are the effective density of states of electrons in conduction band and the effective density of states of holes in valence band, respectively. N a is the acceptor concentration, n i is the intrinsic carrier concentration and W D is the depletion width in MAPbI 3 . The simulation results are exhibited in Fig. 8c

Conclusions
In summary, tin-doped PQDs were added between MAPbI 3 and the carbon electrode for enhanced PSC performance, due to their flexible energy levels and self-ptype doping effects. Particularly, with the incorporation of CsSn 0.2 Pb 0.8 I 3 QDs, the PCE value could be improved from 12.80 to 14.22%, in comparison with the pristine device. It was attributed to the band alignment and the appropriate Sn 2+ doping content of the PQDs facilitating the hole extraction. This work is prospected to provide a direction for the interface optimization of carbon-based PSCs based on PQDs.

Supplementary Information
The online version contains supplementary material available at https:// doi. org/ 10. 1186/ s11671-021-03533-y. Then, the conduction band (CB) edges were calculated by E CB = E VB + E g . Fig. S3. XPS measurement on the film with the structure of FTO/c-TiO 2 /m-TiO 2 /MAPbI 3 /PQDs. Fig. S4. Bar charts indicating photovoltaic parameters of ten CsSn 0.2 Pb 0.8 I 3 QDs-added devices at the PQD concentration of 10 mg mL −1 . Fig. S5. Error bars of photovoltaic parameters for different PSCs. Fig. S6. Normalized PCE values for CsSn 0.2 Pb 0.8 I 3 QDs-added and the pristine devices at 60% humidity in room temperature. Table S1. EDS chemical composition analysis for CsSn 0.1 Pb 0.9 I 3 QDs.