Perovskite solar cells (PSCs) have been developing rapidly since they were discovered, and their excellent photoelectric properties have attracted wide attention from researchers. The compact layer is an important part of PSCs, which can transport electrons and block holes. SnO2 is an excellent and commonly used electron transport layer (ETL) material, and doping modification is an effective way to improve performance. Here, Zn with a similar radius to Sn has been introduced to the doping of the SnO2 compact layer to achieve the purposes of conductivity enhancement of the compact layer and followed photoelectric performance improvement of the device. Zn-SnO2 compact layers with different doping concentrations were prepared and applied to mesoporous architecture PSCs. When the doping content was 5%, the power conversion efficiency (PCE) of the device based on the Zn-SnO2 compact layer has increased from 9.08% to 10.21%, with an increase of 12.44%. The doping of SnO2 promotes its application in low-cost PSCs.

1. Introduction

Organic-inorganic lead hybrid perovskite materials have become the research focus of researchers all over the world due to their advantages of low cost, solution-based preparation method, strong light absorption capacity, adjustable band gap, and long carrier diffusion length. It is also the ideal light-absorbing material for solar cells [14]. The first perovskite solar cells (PSCs) came out in 2009, and in a few years, the power conversion efficiency (PCE) of solar cells based on perovskite light-absorbing materials has increased from 3.8% to the current 25.5% [516].

PSCs are usually composed of conductive substrate, electron transport layer (ETL), perovskite photosensitive layer, hole transport layer (HTL), and counterelectrode. Perovskite materials have the bipolar properties of electron and hole transport [1719]. In the development of PSCs, in order to reduce the device cost and simplify the preparation process, devices without HTL and electron free transport layer structure have appeared [20, 21], and the current PCE of the device based on this structure has reached 15.7% [2225]. The compact layer has the function of blocking holes and transporting electrons and largely determines the performance of the device. An excellent compact layer requires the following characteristics: high light transmission, uniform and dense film formation, few surface defects, an energy band structure that is more compatible with perovskite materials, high electron mobility, and good hole-blocking ability. As an n-type semiconductor, SnO2 has high electron mobility (~240 cm2 V-1 s-1), good UV stability, and relatively low conduction band bottom, which make it an ideal compact layer material [2628]. There are many preparation methods of the SnO2 layer, such as atomic layer deposition, chemical bath deposition, and spray pyrolysis [2937]. It is also necessary to functionalize the compact layer with optimized surface contact and electrical properties for achieving superior band alignment and charge transportation in PSCs. The common optimization methods are surface treatment, metal cation doping, and composite with other materials. Rao et al. improved PCE of the device from 6.5% to 14.6% by treating the surface of the SnO2 compact layer with aqueous TiCl4 solution [38]. Park et al. used Li-doped SnO2 to improve the electrical conductivity of the SnO2 compact layer and reduce the conduction band energy of SnO2, thereby facilitating the transfer of electrons from the perovskite layer to the compact layer, and finally achieved PCE of 18.2% [39]. Bai et al. doped the SnO2 compact layer with Sb, which enhanced the electrical conductivity of SnO2 and increased its Fermi energy level, allowing it to transfer more charges while effectively inhibiting the recombination of carriers, thus increasing the PCE from 15.7% to 17.2% [40]. Ren et al. prepared Nb-doped SnO2 compact layers using a low-temperature solution-processed method to enhance the electron mobility and electrical conductivity of SnO2, resulting in improved open-circuit voltage () and fill factor (FF) of the devices and an increase of PCE from 15.3% to 17.57% [41]. Liu et al. used TiO2 and Mg-doped SnO2 to form a composite compact layer, which effectively utilized the advantages of the two materials to obtain a compact layer with excellent performance, and the PCE of the device was increased from 11.85% to 13.01% [42]. Successful metal cation doping can improve the coverage of the compact layer on the surface of the substrate, adjust the energy band of the compact layer to make the energy level between the compact layer and the perovskite material more match, and improve the interface contact between the compact layer and the perovskite light-absorbing layer. Besides, it can enhance the conductivity of the thin film, thus improving the performance of the device.

In this paper, we report that the Zn-doped SnO2 compact layer can effectively enhance the electron extraction and transfer capability, which are confirmed by steady-state photoluminescence (PL) and electrochemical impedance spectroscopy (EIS) tests. The application of the Zn-doped SnO2 compact layer in mesoporous structured PSCs with carbon electrode and without HTL has been demonstrated to effectively improve the device performance as compared to the devices with a pure SnO2 compact layer. The Zn-doped SnO2 compact layer-based PSCs have achieved a PCE of 10.21%, short-circuit photocurrent current density () of 19.57 mA/cm2, FF of 51.58%, and of 1.01 V.

2. Materials and Methods

2.1. Material

Lead iodide (PbI2, 99.99%) and TiO2 paste (solid concentration: 20%) were purchased from Xi’an Polymer Light Technology Corp. SnCl2·2H2O (98%) and ZnCl2 (98%) were purchased from Macklin. Titanium diisopropoxide bis (acetylacetonate, 75%), dimethylsulfoxide (DMSO, 99.7%), and N,N-dimethylformamide (DMF, 99.8%) were purchased from Sigma-Aldrich. Methylammonium iodide (MAI, 99.5%) and FTO (7 Ω/sq) were purchased from Yingkou Youxuan. ZrO2 paste (solid concentration: 20%) was acquired from Shanghai MaterWin New Material. Carbon ink (10 Ω/cm2) was purchased from Jujo Printing Supplies & Technology (Pinghu) Co., Ltd. All chemicals were used without further purification.

2.2. Preparation of Zn-SnO2 Precursor Solution

The Zn-doped SnO2 compact layer precursor solution was prepared as follows: 0.136 g ZnCl2 dissolved in 10 mL absolute ethanol, stirred, and fully dissolved to get 0.1 mol/L ZnCl2 ethanol solution. The above ZnCl2 solution was mixed with 0.1 mol/L SnCl2·2H2O according to the volume ratio of 0 : 100, 2.5 : 97.5, 5 : 95, 7.5 : 92.5, and 10 : 90; the precursor solution with 0%, 2.5%, 5%, 7.5%, and 10% mole ratio Zn doping was obtained.

2.3. Device Fabrication

Mesoporous structured PSCs were prepared by the method we reported before [4345]. The area of the etched glass/FTO substrate is , and the etched area is . The etched glass/FTO substrates were precleaned by immersion in acetone, isopropanol, and anhydrous ethanol in an ultrasonic box, each followed by 30 min of ultrasonication. The substrates were treated with UVO for 20 min before preparation. The Zn-doped SnO2 compact layer precursor solution was spin-coated on the FTO glass substrate (4000 rpm, 30 s), heated at 180°C for 10 min, and sintered at 500°C for 30 min. The mesoporous TiO2 (mp-TiO2) layer was deposited on top of the compact film via spin-coating of the precursor of the TiO2 mesoporous layer (mass ratio 1 : 5 TiO2 paste to alcohol) at 3500 rpm for 20 s, followed by heating at 150°C for 10 min then sintering at 500°C for 30 min. After cooling to room temperature, the ZrO2 layer was deposited on the device via spin-coating of the precursor of the ZrO2 mesoporous layer (mass ratio 1 : 5 ZrO2 paste to alcohol) at 5000 rpm for 20 s, followed by heating at 150°C for 10 min then sintering at 500°C for 30 min. The addition of ZrO2 enhances the stability of the mesoporous structured calcium titanate solar cells [46]. The devices were transferred to the glove box and applied by one-step spin-coating using perovskite precursor solution (462 mg PbI2 and 159 mg MAI mixed in 600 mg DMF and 78 mg DMSO) at 1000 rpm for 10 s and 4000 rpm for 30 s [46]. At the 15th second of spin-coating, 160 μL toluene was added and then heated at 100°C for 10 min. Finally, the device was placed under the screen printing plate, and the carbon electrode with an area of was scraped onto the sample and then heated at 100°C for 10 min. At this point, the whole preparation process was completed.

2.4. Characterization

The crystal structure of the devices was analyzed by an X-ray diffractometer (XRD, D8, Advance, AXS, Germany). X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Fisher Scientific, US) was used to analyze the element composition and chemical bonding of the devices. Cross-sectional images of the devices and morphology of nanoparticles were observed by a scanning electron microscope (SEM, JSM-IT300, JEOL, Japan). The morphology of the SnO2 nanoparticles was analyzed by a transmission electron microscope (TEM, JEM-2100F, JEOL, Japan). Surface morphology and roughness were measured by atomic force microscopy (AFM, Nanoscope IV, VEECO, US). Steady-state photoluminescence spectroscopy (RF-6000, Shimadzu, Japan) spectra were obtained by exciting the devices deposited on the compact layer at 500 nm. The photocurrent density-voltage () curves of the PSCs were measured by a source meter (2400, Keithley, US) and a sunlight simulator (Oriel Sol3A, Newport Corporation, US) under AM 1.5 G and light intensity of 100 mW/cm2. The external quantum efficiency (EQE) of the devices was obtained by using the monochromatic incident photon-to-electron conversion efficiency spectrometer (IPCE, Newport Corporation, US). Electrochemical impedance spectroscopy measurements were performed on an electrochemical workstation (Zahner Company, Kronach, Germany) at frequencies of 0.01 MHz~4 MHz with bias voltage of 0.8 V and AM 1.5 G spectrum and light intensity of 100 mW/cm2.

3. Results

The XRD spectrum of the Zn-doped SnO2 compact layer shown in Figure 1 was obtained to investigate the crystal structure of the doped SnO2 compact layer (c-doped-SnO2). It shows that the SnO2 compact layer still has good crystallinity after a small amount of Zn doping. There are strong diffraction peaks at 26.6°, 33.9°, 37.9°, 51.9°, 54.8°, 62.6°, and 65.6°, corresponding to (110, 101, 200, 211, 220, 221, and 301) of the tetragonal rutile phase SnO2 (standard JCPDS card: 41-1445); due to the low concentration of elemental doping, no obvious diffraction peaks related to Zn2+ were found in the XRD pattern.

To further investigate the elemental composition and chemical bond formation in the Zn-doped SnO2 compact layer, XPS was used to characterize the 5% Zn-SnO2 compact layer, and the corresponding XPS spectra are shown in Figure 2. Figures 2(a) and 2(b) show the full XPS spectrum of the Zn-SnO2 compact layer in the range of electron binding energies from 0 eV to 1350 eV, in which the characteristic peaks corresponding to Sn, O, and C elements as well as the Auger peak of Sn can be observed. Two smaller characteristic peaks appear at 1022 eV and 1045 eV, corresponding to the binding energy peak position of Zn 2p [4749]. To observe more details of the elemental composition in the SnO2 compact layer after doping, Figures 2(b)2(d) show the XPS binding energy peaks of Sn 3d, O 1s, and Zn 2p. The binding energies of Sn 3d5/2 and Sn 3d3/2 appear at 485.2 eV and 493.6 eV, and the spin-orbit coupling is 8.4 eV, indicating that the Sn element exists in the form of Sn4+ in the compact layer. The binding energy of O 1s appears at 530 eV, which corresponds to O-2. The binding energies of Zn 2p3/2 and Zn 2p appear at 1022 eV and 1045 eV, and the spin-orbit coupling of 22 eV corresponds to the Zn2+ valence state of the Zn element. The above details indicate that Zn is presented in the compact layer of SnO2 after doping.The structural diagram and physical cross-sectional view of the Zn-SnO2-based compact layer-based PSCs without the thick carbon counterelectrode prepared by screen printing are shown in Figure 3. The thickness of each layer in the PSC structure is FTO conductive glass ~500 nm, SnO2 compact layer ~40 nm, the mixed layer of mesoporous TiO2 and CH3NH3PbI3 ~150 nm, the mixed layer of mesoporous ZrO2 and CH3NH3PbI3 ~150 nm, and CH3NH3PbI3 cap layer over the mixed layers ~300 nm.

The PL spectrum was used to characterize the photoluminescence of CH3NH3PbI3 grown on a glass substrate with Zn-SnO2 compact layers deposited from different doping concentrations of precursors under an excitation wavelength of 500 nm. As seen from Figure 4, the peak of the PL spectra occurs at 775 nm, when the fluorescence intensity is strongest. The lower peak intensity of the PL spectra means that the photoinduced charge carriers from CH3NH3PbI3 were extracted and quickly transferred to the compact layer, resulting in a quenching effect that reduces the peak intensity. As the doping amount of Zn in the compact layer continues to increase, the emission peak intensity rises again, and the lowest emission of the perovskite layer is obtained on the SnO2 compact layer with 5% Zn doping, indicating the highest electron extraction rate. Figure 5 shows the surface morphology of undoped and Zn-doped SnO2 compact layers. Root-mean-squared roughness (Rq) decreases after the doping of Zn; the Rq of the undoped SnO2 film is 20.8 nm; after Zn doping, it decreased to 15.4 nm. Combined with Figure 4, it is shown that Zn-doped SnO2 provides a smoother surface and better electrical properties, which is the best ETL for the perovskite layer. Figure 6 shows nanoparticles of the compact layer. It shows that the dispersion of SnO2 nanoparticles in ethylene glycol is not good and there are agglomerations. At the edge, it can be seen that the size of the nanoparticles is very uniform and the particle size is about 4.5 nm. As shown in Figure 6, SnO2 nanoparticles with a lattice spacing of 3.36 Å correspond to the crystal planes of the SnO2 crystal (110).

Under the irradiation of simulated standard sunlight (AM 1.5 G, 100 mW/cm2), the photovoltaic characteristics of PSCs were tested and characterized with a digital source meter at a rate of 0.15 V/s in the room ambient condition. The curves of PSCs based on different doping levels of Zn-SnO2 compact layers are shown in Figure 7, and the photovoltaic parameters are shown in Table 1. Figure 8 is the corresponding distribution of PCE distribution. According to Figure 7 and Table 1, we can conclude that , FF, and PCE of Zn-SnO2 compact layer-based PSCs increase with the increase of Zn2+ doping concentration and reach the largest value at 5% Zn2+ doping concentration. After the Zn2+ doping SnO2 compact layer, of PSCs based on the Zn-SnO2 compact layer increased from 19.11 mA/cm2 to 19.57 mA/cm2, the FF increased from 48.36% to 51.58%, and the PCE increased from 9.08% to 10.21%. As the Zn2+ doping concentration continued to increase, the performance of the PSCs based on the Zn-SnO2 compact layer decreases. The reason is that excessive doping will affect the crystallization of SnO2 and provide nonradiative carrier recombination centers, resulting in the decrease of , FF, and PCE. It is noticed that was almost unchanged. is related to the energy level matching among each layer, thus indicating that the moderate amount of doping does not have a large impact on the SnO2 energy band. The performance of PSCs is optimal when the Zn2+ doping concentration is 5%, and the device performance is improved by 12.4% compared to the PSCs based on the SnO2 compact layer without doping.

The IPCE of the pure SnO2 compact layer and 5% Zn-SnO2 compact layer PSCs were measured and characterized using a quantum efficiency measurement system. The IPCE curves and matching integral current curves are shown in Figure 9. It is observed that the IPCE curves of the two PSCs show a similar trend, and the IPCE value in the visible range of the 5% Zn-SnO2 compact layer-based device was higher than that of the pure SnO2 compact layer-based device, indicating that the carrier extraction and transfer capability is enhanced by the doping of Zn2+. Besides, the integration of the IPCE curve from the 5% Zn-SnO2 compact layer and pure SnO2 compact layer devices is obtained to be 19.46 mA/cm2 and 19.08 mA/cm2, which are consistent with the values measured in the curves.

The EIS of PSCs based on different concentrations of Zn2+-doped SnO2 compact layer and pure SnO2 compact layer were tested and characterized to further investigate the carrier transport characteristics. The EIS spectrum and equivalent circuit diagram are shown in Figure 10. There are two semicircles in the Nyquist diagram. The left semicircle corresponds to the high-frequency region, which reflects the interface contact between the perovskite light-absorbing layer and the carbon electrode and is shown in the form of transfer resistance () in the equivalent circuit. The right semicircle corresponds to the low-frequency region, reflecting the interface contact between the perovskite light-absorbing layer and the ETL, and is expressed in the form of composite resistance () in the equivalent circuit [50]. The values of the individual resistances in the equivalent circuit fitted from the Nyquist plot are shown in Table 2. From the data in the table, it can conclude that the value of the device based on the 5% Zn2+-doped SnO2 compact layer is the largest, which indicates that the Zn-SnO2 compact layer has a good ability of electron extraction and transfer and can quickly transfer the electrons generated by perovskite materials to the substrate electrode. The contact resistance of value in 5% Zn2+-doped SnO2 compact layer device is the smallest, which means that the device has higher FF. Generally speaking, the contact resistance value of the pure SnO2 compact layer is the largest, which is also consistent with the conclusion drawn from the curves. Because the upper surface of perovskite materials of the two devices is deposited with carbon electrode, a strict preparation process will not lead to a great difference in carbon electrode; the transfer resistance values are close.

4. Conclusions

In the study, Zn2+ (~74 pm), which has a similar ionic radius to Sn4+ (~72 pm), was introduced to dope into the SnO2 compact layer. By controlling the proportion of Zn2+ in the SnCl2·2H2O precursor solution, Zn-SnO2 compact layers with different doping concentrations were prepared and applied to PSCs. XRD, XPS, SEM, EIS, and PL were used to characterize and analyze the surface morphology, crystal structure and crystallinity, element composition, and chemical bond formation of the SnO2 compact layer. PL and EIS spectra of PSCs based on the Zn-SnO2 compact layer with different doping concentrations were tested and analyzed to explain the change of electron extraction ability of the compact layer after doping. The peak intensity of the PL spectrum decreased after Zn2+ doping and reached the lowest when the doping concentration was 5%. According to the fitting data from the Nyquist diagram measured by EIS, the composite resistance value of devices based on the Zn-SnO2 compact layer is higher than that of devices based on the pure SnO2 compact layer, indicating that it has good conductivity and can effectively transfer electrons, thereby reducing the carrier recombination. The curves of PSCs based on the SnO2 compact layer with different Zn2+ doping concentrations were tested and characterized. Compared with the devices without the doping SnO2 compact layer, when the Zn2+ doping concentration was 5%, the performance of the Zn-SnO2 compact layer device was the best. The PCE increased from 9.08% to 10.21%, with an increase of 12.4%.

Data Availability

The XRD, XPS, SEM, PL, AFM, , IPCE, and EIS data used to support the findings of this study are included within the article.

Conflicts of Interest

The authors declare no conflict of interest.

Authors’ Contributions

Conceptualization was handled by M.C. and C.Y.; experiment was taken care of by C.Y., M.C., and J.W.; data curation was managed by C.Y., J.W., and H.L.; writing (original draft) was performed by C.Y. and M.C.; and writing (review and editing) was made by C.Y., M.C., J. W., and H.L.


This research was funded by the National innovation and entrepreneurship training program for college students (202010497048) and the Natural Science Foundation of China (NSFC) (11704293 and 11974266).