Enhanced Performance of Planar Perovskite Solar Cells Using Low-Temperature Solution-Processed Al-Doped SnO2 as Electron Transport Layers
© The Author(s). 2017
Received: 21 February 2017
Accepted: 10 March 2017
Published: 31 March 2017
Lead halide perovskite solar cells (PSCs) appear to be the ideal future candidate for photovoltaic applications owing to the rapid development in recent years. The electron transport layers (ETLs) prepared by low-temperature process are essential for widespread implementation and large-scale commercialization of PSCs. Here, we report an effective approach for producing planar PSCs with Al3+ doped SnO2 ETLs prepared by using a low-temperature solution-processed method. The Al dopant in SnO2 enhanced the charge transport behavior of planar PSCs and increased the current density of the devices, compared with the undoped SnO2 ETLs. Moreover, the enhanced electrical property also improved the fill factors (FF) and power conversion efficiency (PCE) of the solar cells. This study has indicated that the low-temperature solution-processed Al-SnO2 is a promising ETL for commercialization of planar PSCs.
KeywordsPerovskite solar cells Electron transport layers Low-temperature solution-process Al-doped SnO2
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 . 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 .
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%.
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.
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
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.
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 . 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 . Therefore, the enhanced parameters mentioned above leads to the improvement of FF (53 to 58%).
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 . 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.
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+.
This work was supported by the National Natural Science Foundation of China under grant nos. 61421002, 61574029, and 61371046. This work was also partially supported by University of Kentucky.
HZ, DL, and FW designed and carried out the experiments. CW, TZ, PZ, and HS participated in the work to analyze the data and prepared the manuscript initially. SL and ZC gave equipment support. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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