High-Performance Planar Perovskite Solar Cells with Negligible Hysteresis Using 2,2,2-Trifluoroethanol-Incorporated SnO2.

An efficient electron transport layer (ETL) between the perovskite absorber and the cathode plays a crucial role in obtaining high-performance planar perovskite solar cells (PSCs). Here, we incorporate 2,2,2-trifluoroethanol (TFE) in the commonly used tin oxide (SnO2) ETL, and it successfully improves the power conversation efficiency (PCE) and suppresses the hysteresis of the PSCs: the PCE is increased from 19.17% to 20.92%, and the hysteresis is largely reduced to be almost negligible. The origin of the enhancement is due to the improved electron mobility and optimized work function of the ETL, together with the reduced traps in the perovskite film. In addition, O2 plasma is employed to treat the surface of the TFE-incorporated SnO2 film, and the PCE is further increased to 21.68%. The concept here of incorporating organic small molecules in the ETL provides a strategy for enhancing the performance of the planar PSCs.

Introduction

Lead halide perovskite solar cells (PSCs) have attracted great attention for their high efficiency, high defect tolerance, and low cost (Wang et al., 2017a, Liu et al., 2015, Stranks et al., 2013, Chen et al., 2017a, Bush et al., 2016, Ono et al., 2017). Recently, the reported efficiency has exceeded 23% by optimizing the interface, perovskite thin film, and perovskite absorber materials (Jiang et al., 2019, Yang et al., 2018a, Jeon et al., 2015). Among the PSCs, planar ones are drawing more and more interest owing to their relatively simpler fabrication (in comparison with the mesoporous PSCs) (Jiang et al., 2019, Yang et al., 2018b).

In a typical planar PSC, the perovskite absorber is usually placed between the hole transport layer (HTL) and the electron transport layer (ETL). Generally, the commonly used HTLs are 2,2′,7′,7′-tetrakis-(N,N-di-4-methoxyphenylamine)-9,9′-spirobifluorene (Spiro-OMeTAD) and poly [bis (4-phenyl) (2,4,6-trimethylphenyl) amine] (PTAA), which have been proved to be efficient HTLs with high hole mobility and remarkable electron-blocking property (Heo et al., 2015). For ETLs, TiO2 is a typically used one, especially for high efficiency n-i-p type PSCs (Zhou et al., 2014, Jeon et al., 2018, Peng et al., 2017, Tan et al., 2017). However, the strong catalytic effect of TiO2 can damage the stability of the PSCs under light illumination (Shin et al., 2017, Luo et al., 2018). Thereby researchers are seeking for other n-type metal oxides for better choice. SnO2 can be processed into both compact and mesoporous films (Dong et al., 2015), and the films have high transparency in the visible region and good energy level alignment with the perovskite. Nowadays SnO2 ETLs are widely used in PSCs to achieve high power conversation efficiencies (PCEs) (Ke et al., 2015, Wang et al., 2016, Chen et al., 2017b, Jiang et al., 2016).

However, many reports have shown that the PSCs based on the pure SnO2 ETL still have serious hysteresis and unsatisfactory performance (Dong et al., 2017, Zhu et al., 2016, Bu et al., 2018, Wei et al., 2018). These problems are attributed to low electron mobility of the SnO2 ETL and high trap-state density in the perovskite device (Bai et al., 2017; Schulz et al., 2019, Xiong et al., 2018, Wang et al., 2018a, Xie et al., 2017). Thereby researchers are finding efficient ways of modifying the pure SnO2 layer to solve the problems. For examples, Ke et al. put a very thin PCBM layer on the SnO2 layer to promote electron transport and suppress interface carrier recombination (Ke et al., 2016), Yang et al. made EDTA-complexed SnO2 ETL to improve the electron mobility (Yang et al., 2018b), and several other groups used self-assembled monolayers (SAMs) to passivate the interfacial trap sites (Yang et al., 2017, Zuo et al., 2017). These methods all lead to enhanced performances.

Here, we made 2,2,2-trifluoroethanol (TFE)-incorporated SnO2 (T-SnO2) ETL by incorporating TFE (Meng et al., 2018), a water-soluble organic small molecule with strong electron-withdrawing group (trifluoromethyl) into the SnO2 colloidal water solution. Consequently, the electron mobility in the SnO2 ETL is largely improved and trap states in the perovskite absorber is greatly reduced. As a result, the hysteresis is obviously eliminated and a high PCE of 20.92% is achieved. Furthermore, we employed O2 plasma to treat the surface of the T-SnO2 film, and a superior PCE of 21.68% is obtained.

Results and Discussion

Characterization of T-SnO2

The transmission electron microscopy images of the SnO2 and T-SnO2 nanoparticles are shown in Figure S1. For the pristine SnO2 particles, large-size clusters (50–70 nm) can be clearly seen; this is due to the aggregation of the SnO2 nanoparticles when they were in solution. For the T-SnO2 particles, the size is about 3–5 nm. The much smaller size is attributed to the strong electron-withdrawing property of the trifluoromethyl group in TFE, which greatly restricts the aggregation of the SnO2 nanoparticles in solution. X-ray photoelectron spectroscopy (XPS) was used to elucidate the state of the F and Sn in the SnO2 and T-SnO2 films coated on indium tin oxide (ITO) substrates. In Figure 1A, it is found that F 1s peak of the T-SnO2 film locates at ∼684.1 eV, which is consistent with the value in the literature (Wang et al., 2017b, Kim et al., 2018). In contrast, there is no associated peak for the SnO2 film. In Figure 1B, it is found that the Sn 3d peaks of the T-SnO2 film shift to lower binding energy by about 0.36 eV in contrast to that of the SnO2 film. Fourier transform infrared (FTIR) spectrum demonstrates that the T-SnO2 nanoparticles contain the characterization peaks of both TFE and SnO2 (Figure S2). All the aforementioned tests strongly indicate that the TFE is effectively incorporated in the SnO2 film.

Figure 1

The Characterizations of the ITO/SnO2 and ITO/T-SnO2 Films

(A and B) XPS spectra of F 1s peak (A) and Sn 3d peaks (B).

(C) AFM images of the SnO2 (left) and T-SnO2 (right) films.

(D) Schematic diagram of work functions of the ITO/SnO2 and ITO/T-SnO2 relative to the conduction band of the perovskite film.

(E) Optical transmission of the ITO, ITO/SnO2, and ITO/T-SnO2 on the glass substrates.

Atomic force microscopy (AFM) was performed to compare the roughness of the T-SnO2 and SnO2 films. As shown in Figure 1C, the roughness of the T-SnO2 film (root mean square [RMS]: 1.70 nm) is less than that of the SnO2 film (RMS: 2.17 nm). The smoother surface is beneficial for later perovskite film growth and a better contact with the T-SnO2 ETL. In addition, UV photoelectron spectroscopy (UPS) measurement was carried out to estimate the work function (WF) of the SnO2 and T-SnO2 films (Figure S3). Figure 1D shows the energy levels of the perovskite film and the two ETLs. It is seen that the WF of the T-SnO2 film is closer to the conduction band of the perovskite film (Figure S4) in comparison with that of the SnO2 film, which is beneficial for increasing Voc (Wang et al., 2018b, Yang et al., 2016, Yu et al., 2018).

Figure 1E compares the optical transmission spectra of the SnO2 and T-SnO2 films. It is seen that all the samples display good transparency in the visible region. In addition, the T-SnO2 film exhibits a higher electron mobility (6.17×10−3 cm2 V−1 s−1) than that of the SnO2 film (2.10×10−3 cm2 V−1 s−1), as measured by the space charge limited current (SCLC) method (Figure S5) (Jiang et al., 2016, Yu et al., 2018).

Structure and Performance of PSCs

We then fabricated planar PSCs with the T-SnO2 and the SnO2 ETLs in an architecture of glass/ITO/ETL/perovskite/Spiro-OMeTAD/Au. The cross-sectional scanning electron microscopy (SEM) of the T-SnO2 device is shown in Figure 2A, in which each layer is clearly seen. The ∼680-nm-thick perovskite layer is coated on the ETL substrates using the widely adopted two-step method (more details are shown in Transparent Methods) (Jiang et al., 2016, Wang et al., 2018b). SEM top view of the perovskite films grown on the two ETLs are provided in Figure S6; it is seen that both films are pinhole-free and uniform and contain similar crystal grains. The grain sizes are ∼750 nm. X-ray diffraction (XRD) measurements (Figure S7) also give very similar results for the two perovskite films (Wang et al., 2018b, Jiang et al., 2017, Chen et al., 2014, Kim et al., 2016). These studies confirm that TFE has a negligible effect on the perovskite crystallization.

Figure 2

Structure and Performance of the PSCs

(A) Cross-sectional SEM of the T-SnO2 device.

(B) J-V curves at both forward (solid square) and reverse (solid circle) scans of the best SnO2 and T-SnO2 devices.

(C) PCE performance distribution of 50 SnO2 or 50 T-SnO2 devices.

(D) EQE spectrum of the T-SnO2 device.

The device performance is optimized by varying the TFE volume and the annealing temperature for the ETL. The PCE reaches a maximum when the TFE volume increases to 350 μL (Figure S8 and Table S1). The optimal annealing temperature is 130°C for the T-SnO2 film (Figure S9). Figure 2B indicates the J-V curves of the best SnO2 and T-SnO2 devices, and the device parameters are shown in Table 1. The SnO2 device displays quite obvious hysteresis: under reverse scan direction, it has a PCE of 19.17% (Voc: 1.10 V, Jsc: 23.12 mA cm−2, and FF: 0.755); under forward scan direction, it has a PCE of 16.47% (Voc: 1.06 V, Jsc: 23.03 mA cm−2 and FF: 0.674). In contrast, the T-SnO2 device presents negligible hysteresis: under reverse scan, it has a PCE of 20.92% (Voc: 1.12 V, Jsc: 23.91 mA cm−2 and FF: 0.780); under forward scan, it has a PCE of 20.62% (Voc: 1.11 V, Jsc: 23.87 mA cm−2 and FF: 0.777). We conducted statistical studies for the SnO2 and T-SnO2 devices. Fifty PSCs were made for each, and the results are given in Figure S10. For the 50 SnO2 devices, the average PCE, Voc, Jsc, and FF are 18.38%, 1.09 V, 22.91 mA cm−2, and 0.734, respectively, whereas for the 50 T-SnO2 devices, the corresponding values are 20.12%, 1.11 V, 23.66 mA cm−2, and 0.764. The larger Voc can be attributed to the better-aligned energy levels of the T-SnO2 and perovskite layers. The higher FF and Jsc are likely due to the improved electron mobility. In addition, the T-SnO2 devices exhibit a narrower distribution of PCE (19%–21% versus 17%–20%), indicating their excellent reproducibility (Figure 2C). All the above-mentioned statistical results confirm the advantage of the T-SnO2 ETL. The external quantum efficiency (EQE) spectra of the T-SnO2 device is exhibited in Figure 2D, and the integrated Jsc for the T-SnO2 device is 23.48 mA cm−2, which is consistent with the Jsc of 23.91 mA cm−2 obtained from the J-V result (within 2% deviation).

Table 1

Performances of the SnO2 and T-SnO2 Devices

ETL	Scan Direction	Voc (V)	Jsc (mA cm−2)	FF (−)	PCE (%)
SnO2	Reverse	1.10	23.12	0.755	19.17
	Forward	1.06	23.03	0.674	16.47
T-SnO2	Reverse	1.12	23.91	0.780	20.92
	Forward	1.11	23.87	0.777	20.62

Charge Transport Studies

To study the trap density, the SCLC model was adopted with the electron-only devices (ITO/ETL/perovskite/PCBM/Ag) (Chen et al., 2017b). Figure 3A shows the dark I-V curves of the two devices. Generally, at low bias voltage, the I-V curve shows linear ohmic-type response. With the increase of the bias voltage, the current starts to increase nonlinearly, indicating the trap filling process is triggered. The kink point between the linear region and the nonlinear region is defined as trap-filled limit voltage (VTFL), and the trap density (Nt) can be calculated using the following Equation 1:where ɛ0, ɛ, e, and L are permittivity of vacuum, relative dielectric constant, elementary charge, and perovskite film thickness, respectively. The calculated trap density of the perovskite film on the T-SnO2 ETL is about 8.94×1015 cm−3, much lower than that of the perovskite film deposited on the SnO2 ETL (1.95×1016 cm−3).

Figure 3

Charge Transport Properties Studies

(A) Dark I-V measurement of the electron-only devices based on SnO2 and T-SnO2 ETLs (inserted picture).

(B and C) PL (B) and TRPL (C) spectra of the perovskite films coated on Glass, SnO2, and T-SnO2, respectively.

(D) EIS spectra of the SnO2 and T-SnO2 devices in dark with a bias of −1 V.

Figure 3B shows the steady-state photoluminescence (PL) spectra of the perovskite films coated on bare glass and the SnO2 and T-SnO2 ETLs. It is seen that the perovskite film on the T-SnO2 ETL gives most significant PL quenching, indicating very efficient electron transfer from the perovskite film to the T-SnO2 ETL. This is due to the reduced trap density in the perovskite film and enhanced electron mobility in the T-SnO2 ETL. Time-resolved photoluminescence (TRPL) of the three samples are given in Figure 3C, from which carrier lifetime can be calculated. The carrier lifetime of the Glass/perovskite, SnO2/perovskite, and T-SnO2/perovskite samples are ∼763, 147, and 52 ns, respectively (Table S2). The significantly reduced carrier lifetime of the perovskite/T-SnO2 sample strongly indicates a fast electron transfer from the perovskite film into the T-SnO2 film, hence carrier recombination can be greatly suppressed (Zhu et al., 2014, Liang et al., 2014).

Figure 3D shows the Nyquist plots of the impedance spectroscopy (EIS) for the SnO2 and T-SnO2 devices; the equivalent circuit is also shown. Rtr is charge transfer resistance; Rrec is recombination resistance (Yang et al., 2015, Li et al., 2015, Jin et al., 2016). The extracted Rtr and Rrec are listed in Table S3. It is seen that the T-SnO2 device has larger Rrec (650.5 vs 294.2), meaning a weaker carrier recombination in the perovskite film. The T-SnO2 device has a smaller Rtr (36.8 vs 56.4), meaning a more efficient electron transfer process from the perovskite film to the ETL. The EIS results are consistent with the above-mentioned trap density, PL, and TRPL analysis. All the results presented in Figure 3 are in good agreement with the enhanced PCE and the remarkable negligible hysteresis of the T-SnO2 device.

Oxygen Plasma Treatment

Oxygen plasma is usually used to treat the ITO or (FTO) surface in fabrication of planar PSCs (Minarik and Vana, 2015, Dao et al., 2015, Tang et al., 2018, Huang et al., 2017). Here, we use O2 plasma to treat the surface of the T-SnO2 ETL. The O2 gas flow rate is fixed at 0.05 L h−1, and the power varies. The treated ETL is named p-T-SnO2 ETL. Figure 4A shows the structure of the p-T-SnO2 device (ITO/p-T-SnO2/Perovskite/Spiro-OMeTAD/Au), and a cross-section SEM is exhibited in Figure S11. The FTIR and XPS results show that the F mainly exists in TFE state in the p-T-SnO2 ETL (Figure S12). Figure 4B shows the J-V curves of the p-T-SnO2 devices (with different plasma powers); the device parameters are shown in Table S4. The champion device exhibits a PCE of 21.68% (Voc: 1.12 V, Jsc: 24.06 mA cm−2, and FF: 0.802), which is higher than the best T-SnO2 device. The improvement can be attributed to the smoother surface of the p-T-SnO2 film, which is indicated by AFM measurements (Figure S13 and Table S5): the roughness of the p-T-SnO2 ETL is 1.13 nm and that of the T-SnO2 ETL is 1.67 nm. The very high FF of larger than 0.80 should have resulted from the improved interface between the p-T-SnO2 ETL and the perovskite film. As depicted in Figure 4B, Voc decreases from 1.12 to 1.07 V as the power increases from 60 to 140 W; this can be explained by the plasma-caused WF change of the T-SnO2 ETL (Figure S14 and Table S6): the WF is going down (from 4.21 to 4.40 eV) away from the conduction band of the perovskite film (4.18 eV). Figure 4C shows the EQE and integrated Jsc (23.61 mA cm−2) of the p-T-SnO2 device, which is consistent with the Jsc of 24.06 mA cm−2 obtained from J-V measurement (with 2% deviation). Statistical study was conducted for 50 p-T-SnO2 devices, and the PCE distribution is shown in Figure 4D. The PCE ranges from 20% to 22%, and most of the devices are among the >21% range, indicating very good reproducibility of the p-T-SnO2 devices.

Figure 4

Oxygen Plasma Treatment

(A) Schematic illustrations of the architecture of the p-T-SnO2 device.

(B) J-V curves of the p-T-SnO2 devices with different plasma power.

(C) EQE spectrum of the 60-W plasma-treated devices.

(D) PCE distribution for the 60-W plasma-treated devices.

Stability Tests

The long-term stabilities of the SnO2, T-SnO2, and p-T-SnO2 devices were measured without any encapsulation. Figure 5A shows PCE vs time in 720-h dark condition storage (RH 30%–40%); it is seen that the SnO2 device maintains only 76% of its initial PCE, whereas the T-SnO2 and p-T-SnO2 devices retain over 90% of their initial PCEs. The enhanced stability is due to improved interface binding strength induced by the TFE modification and plasma treatment (Tress et al., 2016, Tan et al., 2017). Figure 5B shows the steady-state efficiency of the devices under MPP (maximum power point) conditions. It is seen that the SnO2 device takes about 50 s to reach the maximum photocurrent, likely owing to a trap-filling process or ion migration (Wei et al., 2018, deQuilettes et al., 2016). Its PCE then stabilizes at about 18.6%, whereas the PCEs of the T-SnO2 and p-T-SnO2 devices immediately stabilize at about 20.1% and 21.0%, respectively. The stability test clearly indicates the advantage of the T-SnO2 and p-T-SnO2 ETLs.

Figure 5

Stability Tests for the SnO2, T-SnO2, p-T-SnO2 Devices without any Encapsulation

(A) Air stability (30%–40% RH).

(B) Maximum power point tracking.

Conclusion

In conclusion, we present a simple, low-cost method by introducing TFE in the SnO2 colloidal solution and achieved an effective ETL. The T-SnO2 ETL exhibits improved electron mobility, suitable energy levels that aligned well with that of the perovskite film. The ETL also shows a very smooth surface, which allows high-quality perovskite film growth and ensures a good ETL/perovskite interface. As a result, the trap density at the interface and inside the perovskite absorber is greatly reduced, leading to largely suppressed carrier recombination. As a result, the device displays an improved PCE of 20.92% with negligible hysteresis. In addition, the surface of T-SnO2 film is further optimized by O2 plasma treatment, and a higher PCE of 21.68% is obtained, together with a very high FF of larger than 0.80. Moreover, the devices with T-SnO2 ETLs exhibit excellent stability. The simple and economical method provides an insightful strategy for preparing efficient ETLs for future PSCs.

Limitations of the Study

In this study, we found that the fluorine was incorporated into the perovskite crystal at T-SnO2/perovskite interface (Figure S15), which might contribute to the defects passivation. However, we do not have convincing evidence for this. More studies are needed to reveal the role of fluorine at the ETL/perovskite interface.

Methods

All methods can be found in the accompanying Transparent Methods supplemental file.