Background

The solar energy has attracted much attention since it is a renewable and clean energy source [1–4]. In recent years, a large amount of research groups have focused on organic-inorganic lead halide perovskite solar cells as it have the advantages of a lower manufacturing cost and a simpler process compared with Si solar cells. Moreover, PSCs have a great potential for providing an alternative to conventional photovoltaic devices. The PCE of PSCs has increased from 3.8 to 22.1% in a few years [5–9]. However, the efficiency and stability of PSCs strongly depend on some crucial factors, for instance, the morphology of perovskite films and the preparation of electron/hole transport material [10–15].

Both electron transport layers (ETLs) and hole transport layers (HTLs), which can extract electrons and holes from the light harvesting layers, respectively, are essential for the high-efficiency PSCs. Most of the high performance, PSCs were accomplished using compact TiO2 layer or mesoporous TiO2 layer as the ETLs [8, 16]. However, the processes of both the compact TiO2 layer and the mesoporous TiO2 layer generally require a high sintering temperature (>450 °C), which is an obstacle for the stretchable device fabrication and the commercial development of PSCs [17, 18]. Previously, SnO2 has shown up as an effective electron transport layer in perovskite solar cells due to the wider band gap (about 3.6 eV) and higher mobility (100 to 200 cm2V−1s−1) compared with TiO2 [19–21]. Furthermore, the temperature of forming SnO2 thin films (<200 °C) is helpful for widespread implementation and large-scale commercialization of PSCs [22]. Therefore, SnO2 is a promising candidate for ETLs used in high-performance PSCs.

It has been reported that doping ETLs with metal aliovalent cations are an effective method to improve properties of both ETLs and ETLs/perovskite interfaces. Other groups have already doped Y3+ and Li+ in SnO2 to improve carrier transport and optical abilities [23, 24]. In addition, doping can also improve the conductivity of ETLs and optimize the energy level matching between ETLs and the perovskite film, resulting in the enhanced performance of device [23].

Here, we present a low-temperature synthesized Al-doped SnO2 as an ETL in the n-i-p structure perovskite solar cells. Al-doped SnO2 thin films prepared at a low temperature (190 °C) show a better charge transport and electron extraction behavior than the pristine SnO2. The enhanced electrical property of SnO2 improves the PCE, V OC, J SC, and fill factor (FF) of the PSCs. The champion cell based on Al-doped SnO2 reaches a PCE of 12.10% with a V OC of 1.03 V, a J SC of 19.4 mA/cm2, and a FF of 58%, while the PSCs based on undoped SnO2 achieves a PCE of 9.02% with a V OC of 1.00 V, a J SC of 16.8 mA/cm2, and a FF of 53%.

Methods

The fluorine-doped tin oxide (FTO) glass substrates were sequentially cleaned with acetone, ethyl alcohol, and deionized (DI) water in the ultrasonic bath for 15 min. Then the substrates were dried by a N2 flow and further cleaned by UV-ozone for 10 min before the deposition of SnO2.

SnO2 and Al-SnO2 ETLs were deposited by a spin-coating method. The solution was prepared by dissolving SnCl4•5H2O in isopropyl alcohol at a concentration of 0.075 M and subsequently stirred for 60 min at room temperature. For Al-doping, we dissolved AlCl3•6H2O in isopropyl alcohol. Then this aluminum precursor was added to the antecedent solution at a series of molar ratio and stirred until the solution became clear. Afterwards, the two different kinds of solution were separately deposited on cleaned FTO substrates at 3000 rpm for 30 s. The substrates were then pre-dried at 100 °C for 10 min and annealed at 190 °C for 1 h.

After the deposition of SnO2 and Al-SnO2 electron transport layers, the samples were treated by UV-ozone for 10 min again. The CH3NH3PbI3 film was fabricated by a one-step spin-coating method. The CH3NH3PbI3 solution (45 wt%) was deposited on the treated SnO2 at 5000 rpm for 30 s. 0.5 mL chlorobenzene was dropped onto the substrate when spin-coating the CH3NH3PbI3 solution. All the perovskite layers were annealed at 100 °C for 10 min. The hole transport layers were deposited by spin-coating the 2,29,7,79-tetrakis(N,Ndi-p-methoxyphenylamine)-9,9-spirobifluorene (spiro-OMeTAD) solution at 4000 rpm for 30 s. Finally, 100-nm-thick gold top electrode was deposited on the HTL via thermal evaporation.

Device Characterization

The J-V characteristics of perovskite solar cells were measured by Keithley 2400 source measuring unit under the AM 1.5 G solar-simulated light (Newport Oriel Solar 3 A Class AAA, 64023 A). The sun light (100 mw/cm2) was calibrated by a standard Si-solar cell (Oriel, VLSI standards). X-ray photoelectron spectroscopy (XPS) was measured using the Kratos XSAM 800 X-Ray Photoelectron Spectrometer.

Result and Discussion

In our work, SnO2 layers have been deposited on FTO substrates by a low-temperature solution method. Top view scanning electron micrographs (SEM) of the SnO2 and bare FTO are shown in Fig. 1a–b. A dense and pinhole-free film is formed by spin coating SnO2 ETLs solution on FTO substrates, suggesting that the FTO substrates have been fully covered. Dense ETLs are known as an essential element of high-performance PSCs. Thus, a compact SnO2 layer deposited on FTO can enhance the interfacial contact with perovskite layers and improve the performance of the solar cells [25].

Fig. 1
figure 1

a Top view SEM images of low-temperature solution-processed SnO2 on FTO and b bare FTO. c Cross-sectional SEM image of the planar perovskite solar cell based on Al-SnO2, d schematic of the PSCs grown on Al-SnO2, and e energy band diagram of the PSCs

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To examine the efficiency of PSCs based on the low-temperature solution-processed SnO2 and Al-SnO2 ETLs, we have fabricated the planar solar cells with the structure of FTO/(Al-)SnO2/MAPbI3/Spiro-OMeTAD/Au shown in Fig. 1d. In addition, Fig. 1c shows a cross-sectional SEM image of a PSC based on Al-doped SnO2 layer without the Au electrode, and the energy band diagram of PSCs is shown in Fig. 1e.

To confirm that the employment of Al-doping SnO2 as ETLs has no effect on the perovskite films, we measured the UV-vis absorbance spectra and the corresponding X-ray diffraction (XRD) of the MAPbI3 film on FTO/SnO2 and FTO/Al-SnO2 substrates. The results are shown in Fig. 2a and b, respectively. The MAPbI3 films deposited on SnO2 and Al-SnO2 ETLs exhibit almost identical absorbance spectra, suggesting that the absorption of the perovskite films is nearly not affected by Al-doping in SnO2 ETLs.

Fig. 2
figure 2

a UV-vis absorbance spectra and b XRD patterns of perovskite films grown on SnO2 and Al-SnO2 substrates. c EQE curves and integrated current density of perovskite solar cells based on SnO2/Al-SnO2 and d J-V curves of the best-performing PSC using SnO2/Al-SnO2 ETLs

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As to the XRD patterns, several strong peaks are located at 14.05, 23.44, 24.25, 28.18, 31.88, 34.93, and 40.16°. All these peaks can be assigned to orthorhombic crystal of the perovskite with high crystallization [26–28]. The XRD patterns show negligible difference between the samples of FTO/SnO2/MAPbI3 and FTO/Al-SnO2/MAPbI3, indicating the dopant of Al in SnO2 film does not affect on the structure property of MAPbI3 film. Furthermore, the main PbI2 peak is absent from the XRD patterns, which indicates PbI2 has sufficiently reacted with MAI.

As the evidences were obtained from the UV-vis absorbance spectra and XRD patterns of the devices, the dopant of Al in SnO2 does not cause any obvious changes on the structure and optical properties in the perovskite layers. Therefore, the performance enhancement induced by Al-doping in SnO2 as ETLs is most likely due to the improvement of the ETLs/perovskite interfacial properties. In other words, the charge transport and electron extraction are improved.

Figure 2c illustrates the external quantum efficiency (EQE) spectrum of the best-performance solar cells based on SnO2 and Al-SnO2. Obviously, the EQE of Al-doped device is higher than the device based on pristine SnO2 over the entire wavelength range. The higher EQE means superior electron extraction capability of the ETLs [29]. The calculated J SC (≈19.0 mA/cm2) based on Al-SnO2 from the EQE spectra is consistent with the measured value of the current density-voltage (J-V) curves measured under the one-sun light. As for undoped SnO2, the calculated J SC is approximately equal to 16.6 mA/cm2.

The J-V curves of the best-performance PSCs based on SnO2 and Al-SnO2 ETLs are shown in Fig. 2d. The PCE increases from 9.02 to 12.10% by doping SnO2 with Al. Al-doping may cause an improvement on the charge transport and electron extraction behavior of the SnO2, leading to the increment of the J SC (16.8 to 19.4 mA/cm2). Furthermore, the V OC of the best PSC based on Al-SnO2 (1.03 V) is a little higher than that of the best cell based on SnO2 (1.00 V), indicating less energy loss of electrons [30]. Therefore, the enhanced parameters mentioned above leads to the improvement of FF (53 to 58%).

Al-SnO2 films deposited by a low-temperature solution-processed were further investigated by X-ray photoemission spectroscopy (XPS). Figure 3a displays the full XPS spectrum, which shows the presence of O, C, and Sn. The binding energies of 487.3 and 495.8 eV shown in Fig. 3b corresponds to Sn 3d5/2 and Sn 3d3/2, respectively. The main binding energy of 531.0 eV shown in Fig. 3c corresponds to O 1 s, which reveals the O2− state in SnO2 [21]. The absence of Al in the full XPS spectrum can be attributed to the low concentration (5% molar ratio), while Al 2p peak can be observed in Fig. 3d with a relatively low content, indicating that the doping is truly practicable. Identically, the Cl 2p peak missed in the full XPS spectrum can also be observed in Fig. 2e. The low-content Cl suggests that both most of SnCl4 and AlCl3 have been oxidized.

Fig. 3
figure 3

XPS spectra of a survey, b Sn 3d, c O 1 s, d Al 2p, and e Cl 2p peaks for Al-SnO2 film deposited on the FTO substrate. f Normalized time-resolved PL decay curves of MAPbI3 films deposited on SnO2 (black line) and Al-SnO2 (red line) coated substrates

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To identify the reasons for the enhanced performance due to the Al-doping, we carried out the time-resolved photoluminescence (TRPL) to study the electron-extraction behavior of different ETLs. The TRPL decay curves, shown in Fig. 3f, are exponentially fitted, where τ1and τ2 represent the bulk recombination in perovskite bulk films and the delayed recombination of trapped charges, respectively [31]. For the FTO/SnO2/MAPbI3 sample, τ1 is 1.07 ns and τ2 is 7.98 ns, while τ1 is 1.32 ns and τ2 is 5.13 ns for the doped SnO2 sample (1% Al-doping content). Apparently, the perovskite film deposited on the Al-SnO2 ETL has a lower τ2 with a lower ratio of τ2/τ1, indicating a better change transfer from perovskite to ETLs and more efficient extraction of the photo-induced electrons between the perovskite and ETLs, as compared to the film deposited on the pristine SnO2 ETL [32, 33]. In addition, the decay curves also confirm the remarkable enhancement of the electron extraction and charge transport induced by Al-doping in SnO2. These properties result in the improvement of current density and the power conversion efficiency.

We also compared four different parameters of the cell performance with a series of doping concentration. From the box charts in Fig. 4a, it is obvious that the PCE of the cells is strongly influenced by Al doping. The average PCE of the cells is improved with the increment of Al content before the concentration of 1%, while the average PCE is reduced with the higher Al content (3 and 5%). The change of J SC of these solar cells is shown in Fig. 4b, and the variation tendency is like the trend of PCE. The highest J SC is 23.82 mA/cm2, which confirms a good charge transportation of the cells. Regarding the change of V OC, Fig. 4c shows the variation of V OC, value is smallest with 1% Al-doping content. The results demonstrate that the solar cells with 1% Al-doping exhibit the best repeatability. As exhibited in Fig. 4d, the change tendency of FF is analogous to the trend of PCE. In addition, the average FF of the solar cells doped with 0.5 and 1% Al3+ content is higher than the undoped solar cells impressively. The results mentioned above reveal that a suitable Al-doping is beneficial for the performance of the perovskite solar cells based on SnO2.

Fig. 4
figure 4

Histograms of the different Al content on photovoltaic parameters including a PCE, b JSC, c VOC, and d FF

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Conclusions

In summary, we studied the effect of Al-doping on SnO2 as ETLs for planar perovskite solar cells. According to the results of UV-vis absorbance spectra and XRD patterns of perovskite films deposited on Al-SnO2 and SnO2, the Al dopant in SnO2 does not influence the structure and optical properties of the perovskite layers. Based on the TRPL test, the Al-dopant in SnO2 enhances the charge transport and electron extraction behavior of the PSCs and then the J SC of the devices is improved. The champion cell based on Al-SnO2 exhibited a higher efficiency of 12.10% than that using SnO2 (9.02%) as ETLs. Our results suggest that efficient planar perovskite solar cells based on SnO2 can be fabricated by doping SnO2 with Al3+.