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Abstract
Perovskite solar cells (PSCs) have attracted much attention at home and abroad due to their excellent photoelectric properties. Defects in the electron transport layer (ETL) and ETL/perovskite interface greatly affect the power conversion efficiency (PCE) and stability of PSCs. In the paper, the surface of tin dioxide (SnO2) ETL was modified by an alkali metal salt (NaBr, KBr, and RbBr) solution to optimize electron transport and passivate SnO2/perovskite. The results show that the photovoltaic performance of the PSCs is significantly improved after interfacial modification, especially the KBr-modified PSC has the highest PCE, which is 7.8% higher than that of the unmodified device, and the open-circuit voltage, short-circuit current density and fill factor are all greatly improved. This improvement is attributed to the fact that interfacial modification reduces the trap density of the SnO2 films, increases the mobility of the SnO2 films film, effectively passivates defects, and significantly inhibits the recombination at the SnO2/perovskite interface. This method aims to use simple and low-cost inorganic materials for effective interface modification.
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1. Introduction
Organic-inorganic perovskite has a high optical absorption coefficient(104∼105cm-1), and excellent photoelectric characteristics such as tunable bandgap, long diffusion length and carrier lifetime, and high carrier mobility [1–4]. These characteristics have attracted wide attention in perovskite solar cells (PSCs), light-emitting diodes, photodetectors, sensors, and laser devices [5–10]. In 2009, perovskite materials were first applied to solar cells, and Miyasaka's group pioneered the use of MAPbI3 and others as sensitizers for dye-sensitized solar cells, obtaining power-conversion efficiency(PCE) of about 4% [11]. After more than ten years of development, the highest PCE has rapidly risen to 26.08% [12], showing great potential for commercial applications. Therefore, organic-inorganic perovskite has broad application prospects and will become an important research direction in new energy, optoelectronics, and other fields.
The quality of perovskite films is a crucial factor affecting the performance of PSCs [13]. The perovskite film morphology can directly affect light absorption, carrier diffusion length, and charge transport [14]. Perovskite films are mostly prepared by the solution method, and the solution-treated perovskite films are usually polycrystalline, with a high density of point defects on their surfaces and grain boundaries [1516]. The researchers have shown that these defects can decrease the efficiency and stability of PSCs. This is because the presence of defects may induce rapid charge recombination and increase voltage loss in the PSCs [17,18]. At the same time, the poor quality of the thin film crystals at the defects leads to a decrease in the surface coverage and compactness of the film, which in turn leads to inferior light absorption and higher leakage currents [19,20]. Furthermore, the grain size of the film also seriously affects the performance of PSCs. The reason is the presence of a large number of incompletely coordinated ions at the film grain boundaries and surfaces. These ions will form defect centers to capture carriers, impede carrier transfer at the interface, and impair the Jsc of PSCs [21]. In a PSC, in addition to the quality of the perovskite film, the interface between the perovskite film and electronic transporting layer (ETL) plays a key role in determining the overall photovoltaic performance of the device [22]. ETL services as an essential role in both the growth of perovskite and the extraction of charge carriers [23]. Therefore, the ETL/perovskite interfacial modification to passivate defects is crucial for further improving the efficiency and stability of PSCs [24,25].
Tin dioxide (SnO2) is widely used as the ETL in PSCs due to the superior electron mobility and suitable energy level alignment with perovskite [26]. Researchers have proposed many strategies for SnO2/perovskite interfacial modification to passivate perovskite defects. Several articles have reported SnO2/perovskite interfacial modification by some interlayer. For example, Wang For example, Zheng et al. adopted baclofen (BCF) as an interlayer between SnO2 and inorganic perovskite, and their study showed that the BCF can not only be beneficial for the growth of perovskite film but also act as a passivation layer for the SnO2/perovskite interface [27]. Bao et al. proposed a facile way to simultaneously passivate the defects at both the surface of SnO2 and the bottom surface of the perovskite layer by using an interlayer of 3-chlorothiophene-2-carboxylic acid (TCA-Cl) [28]. Chavan et. al. used novel azahomofullerene (AHF) as the interlayer between the SnO2/perovskite interface in planar n-i-p heterojunction PSCs, and validated that AHF interlayer can improve the quality of the perovskite film and reduces charge recombination in PSCs [29]. In addition, several articles have reported SnO2/perovskite interfacial modification by treating SnO2 with alkali halide salts. For example, Liu et al. used inorganic binary alkaline halides for low-temperature-treated planar PSCs, and their study showed that the residual KCl in the interfacial functional layer effectively passivated the perovskite/SnO2 interface defects [30]. Zheng et al. introduced KF into SnO2, improving the overall electron mobility and open-circuit voltage (Voc) of inorganic CsPbI2Br devices [31]. Zhuang et al. added RbF into SnO2 and deposited RbF at the perovskite/SnO2 interface, which improved the Voc value of the device and reduced the hysteresis [32]. Wang et al. introduced an alkali metal fluoride into the tin oxide SnO2, and they think F- ions can decrease the trap-state density and improves the electron mobility and the alkali metal ions (K+, Rb+, and Cs+) permeated into perovskite fill the organic cation vacancies and ameliorate the crystal quality of perovskite films [33]. Hoang et al. investigated the effect of alkali fluoride salts (KF, RbF, and CsF) on the properties of SnO2 and PSC performance. The results show different alkali have significant roles depending on their nature [34]. At present, the results on alkali cations’ effect on SnO2/perovskite interface for PSCs are not identical, and more research is necessary for this area.
In the paper, the PSCs were prepared by adding alkali bromide KBr, CsBr, and RbBr to SnO2 colloidal dispersion as interface modifiers. By introducing alkali bromide for interfacial modification, the crystallinity of PSCs is improved, defects are reduced and crystal quality is improved. The experimental results show that after interface modification, the photoelectric performance of PSCs has been improved significantly. In particular, the efficiency of the device modified by KBr is the highest, reaching 22.56%, which is 7.8% higher than that of the unmodified device. Meanwhile, the Voc, short-circuit current density (Jsc), and fill factor (FF) of PSCs are greatly improved. This study provides simple and feasible strategies for further improving the photoelectric conversion efficiency of PSCs and also provides technical support for the industrial application of PSCs.
2. Experimental section
2.1. Experimental materials
Dimethylformamide (DMF, 99.99%), dimethyl sulfoxide (DMSO, 99.99%), methylamine bromide (MABr 99.99%), chlorobenzene (CB, 99.99%), methylammonium chloride (MACl, 99.99%) and SnO2 colloid precursor (tin (IV) oxide, 15% in H2O colloidal dispersion) were purchased from Alfa Aesar. Lead iodide (PbI2, 99.99%) and lead bromide (PbBr2, 99.9%) were purchased from Tokyo Kasei Kogyo Co., Ltd. (TCI), Tokyo/Japan. Acetonitrile, bis(trifluoromethanesulfonyl)imide lithium (Li-TFSI, 99,99%), and tert-butylpyridine (TBP, 96%) were purchased from Sigma Aldrich. Formamidine iodide (FAI, 99.99%), tris[2-(1H-pyrazol-1-yl)-4-tert-butylpyridine]cobalt-(III) tris[bis(trifluoromethylsulfonyl) imide] (FK209), cesium iodide (CsI, 99.99%) and Spiro-OMeTAD (99.5%) were purchased from Xi'an Polymer Light Technology Corp, Xi'an/China. The alkali bromides NaBr, KBr, and RbBr were purchased from J&K Chemical Co., Ltd., Shanghai/China, with 99.5% purity. All materials above were used as received.
2.2. Preparation of ETL precursor
The SnO2 was mixed with deionized water at a volume ratio of 1:2, and then KBr, RbBr, and CsBr were dissolved at the rate of 2 mg/mL in the solution and set aside in separate centrifuge tubes.
2.3. Preparation of perovskite precursor
The perovskite precursor solution was made from lead iodide (PbI2 705.33 mg/mL), cesium iodide (CsI 38.97 mg/mL), formamidine hydroiodide (FAI 232.07 mg/mL), lead bromide (PbBr2 22.02 mg/mL), methylamine bromide (MABr 6.71 mg/mL), and the moderator methylamine chlorine (MACl 40.47 mg /mL) were made by dissolving in a solvent with a volume ratio of dimethylformamide (DMF) to dimethyl sulfoxide (DMSO) of 4:1 and stirred thoroughly for 3 hours.
2.4. Preparation of hole transport layer precursor
The Spiro-OMeTAD (90 mg/mL) was dissolved in chlorobenzene (CB) and stirred thoroughly for 30 min. Due to the weak charge transfer property of Spiro-OMeTAD itself, doping is needed to obtain a good device effect. On this basis, add 28 µL/mL 4-tert-butylpyridine (TBP) and stir thoroughly for 30 minutes. Then add 18 µL/mL lithium salt (Li-TFSI) 520 mg/mL dissolved in acetonitrile) and stir thoroughly for 30 minutes. Finally, 20 µL/mL FK209 was added and stirred well for 30 min, filtered, and reserved for use.
2.5. Fabrication of PSCs
Firstly, indium tin oxide (ITO) patterned glass substrates with a sheet resistance of about 15 Ω/square are cleaned with deionized water and organic solvents, and then exposed to UV–ozone ambiance for 10 min. Secondly, the above-mentioned ETL precursor is spin-coated at 3000 rpm for 30 s on a cleaned ITO glass substrate and then anneals at 150 °C for 30 min. Thirdly, after cooling to room temperature, ITO/ETL substrates are subjected to UV ozone treatment for 15 minutes to increase their wettability, and the above-mentioned perovskite precursors are spin-coated (3000 rpm for 15 s) onto the substrates and then the wet film was quickly transferred to a chamber connected to a vacuum pump, when the wet film changes color (∼20 s), then the vacuum condition is maintained (∼40 s), and the pump is turned off immediately. After vacuum extraction, the film was rapidly annealed at 150 °C for 20 min. Fourthly, the above-mentioned hole transport layer precursor is spin-coated at 3000 rpm for 30 s and put into a drying cabinet for 12 hours. Finally, a mask plate was used to vaporize 120 nm gold electrodes under 10−3 mbar.
3. Result and discussion
The schematic PSCs configuration ITO/MBr:SnO2/perovskite/Spiro-OMeTAD/Au and the band alignment are shown in Fig. 1(a) and Fig. 1(b), respectively. Here, alkali metal bromides were introduced into SnO2 (MBr:SnO2) and served as ETLs, where MBr is alkali metal bromide and M is K+, Rb+, and Cs + . Spiro-OMeTAD served as a hole transport layer. Perovskite thin films were prepared by vacuum quenching, with related preparation details provided in the Experimental Section. Pristine SnO2 is used as the ETL in the control device.
3.1 Effect of alkali metal bromide additive on the crystallinity of perovskite
To study the effect of alkali bromide on the crystal structure of perovskite films, X-ray diffraction (XRD) tests were conducted to determine the crystal structure, crystal orientation, and other structural information of perovskite films. The XRD patterns of perovskite films prepared on different SnO2 Layer are shown in Fig. 2. It was found that the typical perovskite peaks were observed in all films without new peaks, which indicates that the introduction of alkali bromide has little effect on the crystal structure of perovskite. Furthermore, it was observed that the crystallinity of the films was improved after the introduction of different alkali metal bromide in SnO2, which indicated that alkali metal bromide played an important role in promoting the crystallization of the inclusion crystal crystals. Compared with the diffraction peak of 13.83° on the (110) crystal plane of the control group, the (110) peak position of perovskite films prepared on KBr:SnO2 shifted to 13.97°. It shows that K+ enters the perovskite lattice, leading to the reduction of the lattice constant [35,36]. The decrease in the lattice constant may be attributed to the introduction of K+ (1.38 Å), with ionic radius smaller than that of FA+ (2.79 Å), which leads to a decrease in the volume of the cuboctahedron of the A-site cation surrounded by the angles shared by the eight PbI6 octahedra of the crystal cell, thus improving the stability of PSCs [37]. XRD also shows that the perovskite film prepared by the vacuum quenching process has good crystallinity and repeatability.
3.2 Influence of alkali metal bromide additive on the surface morphology of the film
To study the influence of alkali metal bromide additive on the surface morphology of the film, the surface morphology was investigated by SEM. As shown in Fig. 3(a-d), SEM images of the perovskite films show significantly larger grain size with alkali metal bromide, which may be due to the secondary growth of the grains on the surface of the films after alkali metal bromide modification. This phenomenon is also consistent with the research results in many other literatures, that is, after K+, Rb+, or Cs+ in the interfacial modification layer infiltrates into the perovskite film, the surface defects of the film are passivated and the grains become larger [38,39]. This means that the probability of composite loss of photogenerated carriers at the interface between the perovskite and ETL will be reduced, and more photogenerated electrons and holes will be collected by the electrodes on both sides, thus improving the Jsc of PSCs.
Fig. 3. Top-view SEM images of perovskite films prepared on different SnO2 Layer: (a) Control device, (b) CsBr:SnO2, (c) RbBr:SnO2, and (d) KBr:SnO2.
3.3 Effect of alkali metal bromide additive on optical properties of the films
To study the effect of alkali bromide on the optical properties of perovskite films, the ultraviolet-visible (UV-Vis) absorption spectra and steady-state photoluminescence (PL) spectra of control the group and samples containing different alkali bromide were measured. As can be seen from Fig. 4(a), the wavelength corresponding to the absorption of the UV-Vis absorption spectrum did not change significantly, and all films showed high light absorption within the range of the test spectrum. Compared with the control group, after interface modification, the absorption capacity of perovskite thin film in the whole range of light absorption is improved, which is more conducive to the perovskite thin film to absorb sunlight, thus improving the photocurrent. In particular, the absorption capacity of KBr-modified perovskite films is significantly improved, thus ensuring a high flux of light, which is attributed to the increase in grain size and the decrease in reflectance. Therefore, by introducing alkali metal bromine, the light absorption capacity of the perovskite film can be improved, thus increasing the photovoltaic conversion efficiency of the solar cell.
Fig. 4. (a) UV-Vis absorption spectra and (b) steady-state PL spectra of perovskite films prepared using different alkali metal bromide in SnO2.
In addition, the charge transfer behavior between the ETL and the perovskite interface can be further investigated by analyzing the steady-state PL spectra. As can be seen from Fig. 4(b), compared with the control group, the PL excitation peak of the perovskite film modified with alkali metal bromide was significantly enhanced, indicating that the non-radiative recombination of the perovskite film was effectively inhibited. Among these three additives, the introduction of KBr has the most obvious enhancement effect on PL strength, that is, the most significant inhibition effect on the non-radiation compound. At the same time, it was found that the main position of the PL emission peak did not change significantly, which was consistent with the results of ultraviolet absorption spectrum characterization, indicating that the introduction of additives would not significantly change the band gap width of perovskite.
3.4 Effect of alkali metal bromide additive on photoelectric performance of devices
The PCE of solar cells is one of the most important properties. Eighteen PSC devices with an area of 0.07 cm2 were prepared in each group to verify the direct effect of the introduction of alkali metal bromide on efficiency. To determine the optimal doping amount of each alkali metal bromide, the PCE distribution values at each addition concentration were calculated. The statistical diagrams of PCE tested under AM 1.5G standard sunlight intensity are shown in Fig. 5. It can be found that the PCE of PSC modified with a certain amount of alkali metal bromide is significantly higher than that of the control device. The optimal concentration for the addition of alkali metal bromine is 2 mg/mL, and especially the PCE is highest in PSC with 2 mg/mL KBr added to SnO2. The J-V curves of PCSs based on control and 2 mg/mL MBr are shown in Fig. 5(d). The optimal PCE of PSCs with SnO2, KBr:SnO2, CsBr:SnO2, and RbBr:SnO2 ETL are 20.92%, 22.56%, 21.94%, and 21.72%, respectively, and the specific photovoltaic parameters of champion PSCs are shown in Table 1. The champion control device exhibits a Voc of 1.12 V, a Jsc of 23.21 mA/cm2, and an FF of 80.47%. After alkali metal bromide modification, the Voc, Jsc, and FF of PSCs are improved. The champion PSC at the optimal KBr concentration of 2 mg/mL exhibits a Voc of 1.13 V, a Jsc of 24.16 mA/cm2, an FF of 82.63%, and a highest PCE of 22.56%. The significant increase in photovoltaic parameters are related to the enhancement of defect passivation and charge transfer capability within the device.
Fig. 5. Statistical diagrams of PCE of PSCs prepared using different alkali metal bromide in SnO2: (a) RbBr, (b) CsBr, and (c) KBr. (d) J-V curve of PCSs based on control and 2 mg/mL MBr.
Table 1. Photovoltaic parameters of champion MBr-modified and control PSC device.
3.5 Passivation defect effect of alkali metal bromide additive
To experimentally verify the passivated perovskite, we evaluated the defect density (Ntrap) by the electron-only devices (ITO/SnO2/Perovskite (PVK)/C60/Au and ITO/KBr:SnO2/Perovskite (PVK)/C60/Au) using the space charge limited current (SCLC) measurement, as shown in Fig. 6(a-b). The defect density was determined by the trap-filled limit voltage (VTFL) using the following equation [40]:
where ɛ0 is the dielectric constant of vacuum, ɛ is the dielectric constant of perovskite film, VTFL is the trap filling limit voltage, L is the thickness of perovskite, and q is the basic charge. The calculation results show that the trap density of the modified perovskite film is significantly reduced compared with the control device. The Ntrap values of SnO2 and KBr:SnO2 as ETL perovskite films are 9.92 × 1015 cm-3 and 8.15 × 1015 cm-3, respectively. This indicates that adding alkali metal bromide, especially when using KBr, effectively reduces the defect state density in perovskite films, which has positive significance for increasing the Voc of devices.Fig. 6. Dark state J-V curve of the electron-only devices (a) ITO/SnO2/Perovskite (PVK)/C60/Au and (b) ITO/KBr:SnO2/Perovskite (PVK)/C60/Au.
Transient fluorescence spectra (TRPL) were measured to investigate the effect of adding alkali metal bromide to the ETL of PSCs on charge carrier transport dynamics. The role of introducing alkali metal bromides in passivating interfacial defect states and suppressing carrier complexation can be more intuitively demonstrated by TRPL. To avoid the influence of the transport layer on the extraction or transfer of photogenerated carriers, quartz glass was used as a substrate for the deposition of perovskite films, with a control structure of glass/SnO2/perovskite and a superior-performing KBr experimental group of glass/KBr:SnO2/perovskite. In this way, the photogenerated carriers produced within the perovskite films can only release energy through competitive radiative recombination and non-radiative recombination forms. The TRPL of the test film is shown in Fig. 7, and the fluorescence lifetime of the carriers can be quantified using a double-exponential decay model as shown in Eq. (2).
.
Table 2 gives TRPL fitting parameters, and τavg is the average photocarrier lifetime. The τavg of the control device is 417.21 ns, and the τavg of the KBr-modified device is 554.70 ns. The significant increase of τavg indicates that the additional introduction of alkali metal bromides has led to the suppression of the nonradiative recombination process of photogenerated carriers, which is caused by the reduction of the density of the interfacial defect states that act as the center of the recombination, allowing for a longer charge transport distance.
Table 2. TRPL fitting parameters of KBr modified perovskite film and control group
3.6 Effect of alkali metal bromide additives on the PSCs device stability
Finally, the long-term stability of the device in the environment is tested. The device was placed in an environment of 25°C with a relative humidity of 40%, and the change of and the change in PCE of the PSC was recorded versus time, as shown in Fig. 8. It can be found that the control device decays rapidly after 150 hours, while normalized PCE of the KBr-modified device remains at 89% after 500 hours. Meanwhile, the normalized PCE of the control device only maintains about 60%. The results of the stability tests show that the modification of SnO2 with KBr can significantly delay the efficiency degradation of the PSCs device. This is due to the fact that the alkali metal cation K+ can migrate from the SnO2 layer to the perovskite layer, passivate the defects within the perovskite film, and inhibit ion migration and non-radiative recombination, thus improving the stability of the device.
4. Conclusion
In summary, three alkali metal salts, including KBr, CsBr, and RbBr, are introduced into the SnO2 precursor, and the influence of the SnO2 doped with different alkali metal salts used as ETL on the properties of perovskite films and performance of PSCs are studied. It is found that SnO2 doped with different alkali metal salts has a great effect on grain size, crystallization, UV-Vis absorption spectra, steady-state PL spectra, defect density, and fluorescence lifetime of perovskite film. The PSCs based on KBr:SnO2, CsBr:SnO2, and RbBr:SnO2 all showed better performance compared to the control device, especially the KBr:SnO2-based PSCs had the most excellent performance. The PSC device based on 2 mg/mL KBr shows a superior PCE of 22.56% and retains 89% of its initial efficiency after 500 hours, which is due to the K+ cation can migrate into the upper perovskite layer, which subsequently helps reduce defects and thus recombination in the perovskite layer. These experimental results show that alkali metal cations can effectively passivate the ETL/perovskite interface defect, improve the crystalline quality of perovskite films, and enhance the efficiency and stability of PSCs. This work presents a simple and efficient interfacial modification strategy, which will help to further improve the performance of PSC by conducting more research.
Funding
Xi'an University Institutes Talent Service Enterprise Program (23GXFW0089); Natural Science Foundation of Shaanxi Provincial Department of Education (2022JC057); Key Research and Development Projects of Shaanxi Province (2022GY-210, 2023-YBGY-256).
Disclosures
The authors declare no conflicts of interest.
Data availability
All data in the main text are available from the corresponding author upon reasonable request.
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