Zn-Doped SnO2 Compact Layer for Enhancing Performance of Perovskite Solar Cells

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.


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 [1][2][3][4]. 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% [5][6][7][8][9][10][11][12][13][14][15][16].
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 [17][18][19]. 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% [22][23][24][25]. 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, SnO 2 has high electron mobility (~240 cm 2 V -1 s -1 ), good UV stability, and relatively low conduction band bottom, which make it an ideal compact layer material [26][27][28]. There are many preparation methods of the SnO 2 layer, such as atomic layer deposition, chemical bath deposition, and spray pyrolysis [29][30][31][32][33][34][35][36][37]. 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 [40]. Ren et al. prepared Nb-doped SnO 2 compact layers using a low-temperature solution-processed method to enhance the electron mobility and electrical conductivity of SnO 2 , resulting in improved open-circuit voltage (V OC ) and fill factor (FF) of the devices and an increase of PCE from 15.3% to 17.57% [41]. Liu et al. used TiO 2 and Mg-doped SnO 2 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 lightabsorbing 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 SnO 2 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 SnO 2 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 SnO 2 compact layer. The Zndoped SnO 2 compact layer-based PSCs have achieved a PCE of 10.21%, short-circuit photocurrent current density (J SC ) of 19.57 mA/cm 2 , FF of 51.58%, and V OC of 1.01 V.

Device Fabrication.
Mesoporous structured PSCs were prepared by the method we reported before [43][44][45]. The area of the etched glass/FTO substrate is 1:5 cm * 1:5 cm, and the etched area is 0:5 cm * 1:5 cm. 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 Zndoped SnO 2 compact layer precursor solution was spin-  International Journal of Photoenergy 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 TiO 2 (mp-TiO 2 ) layer was deposited on top of the compact film via spin-coating of the precursor of the TiO 2 mesoporous layer (mass ratio 1 : 5 TiO 2 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 ZrO 2 layer was deposited on the device via spin-coating of the precursor of the ZrO 2 mesoporous layer (mass ratio 1 : 5 ZrO 2 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 ZrO 2 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 PbI 2 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 2 mm * 8 mm was scraped onto the sample and then heated at 100°C for 10 min. At this point, the whole preparation process was completed.

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 SnO 2 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

Results
The XRD spectrum of the Zn-doped SnO 2 compact layer shown in Figure 1 was obtained to investigate the crystal structure of the doped SnO 2 compact layer (c-doped-SnO 2 ). It shows that the SnO 2 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 SnO 2 (standard JCPDS card: 41-1445); due to the low concentration of elemental doping, no obvious diffraction peaks related to Zn 2+ were found in the XRD pattern.
To further investigate the elemental composition and chemical bond formation in the Zn-doped SnO 2 compact layer, XPS was used to characterize the 5% Zn-SnO 2 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-SnO 2 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 [47][48][49]. To observe more details of the elemental composition in the SnO 2 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 Sn 4+ 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 Zn 2+ valence state of the Zn element. The above details indicate that Zn is presented in the compact layer of SnO 2 after doping.The structural diagram and physical cross-sectional view of the Zn-SnO 2 -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, SnO 2 compact layer~40 nm, the mixed layer of mesoporous TiO 2 and CH 3 NH 3 PbI 3~1 50 nm, the mixed layer of mesoporous ZrO 2 and CH 3 NH 3 PbI 3 150 nm, and CH 3 NH 3 PbI 3 cap layer over the mixed layers 300 nm. The PL spectrum was used to characterize the photoluminescence of CH 3 NH 3 PbI 3 grown on a glass substrate with   International Journal of Photoenergy Zn-SnO 2 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 CH 3 NH 3 PbI 3 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 SnO 2 compact layer with 5% Zn doping, indicating the highest electron extraction rate. Figure 5 shows the surface morphology of undoped and Zn-doped SnO 2 compact layers. Root-mean-squared roughness (Rq) decreases after the doping of Zn; the Rq of the undoped SnO 2 film is 20.8 nm; after Zn doping, it decreased to 15.4 nm. Combined with Figure 4, it is shown that Zndoped SnO 2 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 SnO 2 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, SnO 2 nanoparticles with a lattice spacing of 3.36 Å correspond to the crystal planes of the SnO 2 crystal (110). Under the irradiation of simulated standard sunlight (AM 1.5 G, 100 mW/cm 2 ), 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 J-V curves of PSCs based on different doping levels of Zn-SnO 2 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 J SC , FF, and PCE of Zn-SnO 2 compact layer-based PSCs increase with the increase of Zn 2+ doping concentration and reach the largest value at 5% Zn 2+ doping concentration. After the Zn 2+ doping SnO 2 compact layer, J SC of PSCs based on the Zn-SnO 2 compact layer increased from 19.11 mA/cm 2 to 19.57    5 International Journal of Photoenergy mA/cm 2 , the FF increased from 48.36% to 51.58%, and the PCE increased from 9.08% to 10.21%. As the Zn 2+ doping concentration continued to increase, the performance of the PSCs based on the Zn-SnO 2 compact layer decreases. The reason is that excessive doping will affect the crystallization of SnO 2 and provide nonradiative carrier recombination centers, resulting in the decrease of J SC , FF, and PCE. It is noticed that V OC was almost unchanged. V OC 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 SnO 2 energy band. The performance of PSCs is optimal when the Zn 2+ doping concentration is 5%, and the device performance is improved by 12.4% compared to the PSCs based on the SnO 2 compact layer without doping.
The IPCE of the pure SnO 2 compact layer and 5% Zn-SnO 2 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-SnO 2 compact layer-based device was higher than that of the pure SnO 2 compact layer-based device, indicating that the carrier extraction and transfer capability is enhanced by the doping of Zn 2+ . Besides, the integration of the IPCE curve from the 5% Zn-SnO 2 compact layer and pure SnO 2 compact layer devices is obtained to be 19.46 mA/cm 2 and 19.08 mA/cm 2 , which are consistent with the J SC values measured in the J-V curves.  International Journal of Photoenergy The EIS of PSCs based on different concentrations of Zn 2+ -doped SnO 2 compact layer and pure SnO 2 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 (R tr ) 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 (R rec ) 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 R rec value of the device based on the 5% Zn 2+ -doped SnO 2 compact layer is the largest, which indicates that the Zn-SnO 2 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 R s value in 5% Zn 2+ -doped SnO 2 compact layer device is the smallest, which means that the device has higher FF.  Generally speaking, the contact resistance R s value of the pure SnO 2 compact layer is the largest, which is also consistent with the conclusion drawn from the J-V 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 R tr values are close.

Conclusions
In the study, Zn 2+ (~74 pm), which has a similar ionic radius to Sn 4+ (~72 pm), was introduced to dope into the SnO 2 compact layer. By controlling the proportion of Zn 2+ in the SnCl 2 ·2H 2 O precursor solution, Zn-SnO 2 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 SnO 2 compact layer. PL and EIS spectra of PSCs based on the Zn-SnO 2 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 Zn 2+ 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 R rec value of devices based on the Zn-SnO 2 compact layer is higher than that of devices based on the pure SnO 2 compact layer, indicating that it has good conductivity and can effectively transfer electrons, thereby reducing the carrier recombination. The J-V curves of PSCs based on the SnO 2 compact layer with different Zn 2+ doping concentrations were tested and characterized. Compared with the devices without the doping SnO 2 compact layer, when the Zn 2+ doping concentration was 5%, the performance of the Zn-SnO 2 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, J-V, IPCE, and EIS data used to support the findings of this study are included within the article.