Date Published: January 06, 2018
Publisher: John Wiley and Sons Inc.
Author(s): Yayun Zhu, Kaimo Deng, Haoxuan Sun, Bangkai Gu, Hao Lu, Fengren Cao, Jie Xiong, Liang Li.
In the planar perovskite solar cells (PSCs), the electron transport layer (ETL) plays a critical role in electron extraction and transport. Widely utilized TiO2 ETLs suffer from the low conductivity and high surface defect density. To address these problems, for the first time, two types of ETLs based on TiO2 phase junction are designed and fabricated distributed in the opposite space, namely anatase/rutile and rutile/anatase. The champion efficiency of PSCs based on phase junction ETL is over 15%, which is much higher than that of cells with single anatase (9.8%) and rutile (11.8%) TiO2 as ETL. The phase junction based PSCs also demonstrated obviously reduced hysteresis. The enhanced performance is discussed and mainly ascribed to the excellent capability of carrier extraction, defect passivation, and reduced recombination at the ETL/perovskite interface. This work opens a new phase junction ETL strategy toward interfacial energy band manipulation for improved PSC performance.
Organic–inorganic lead halide perovskites have attracted increasing attention as light absorbers of solar cells. Since the first report in 2009, the power conversion efficiencies (PCEs) of perovskite solar cells (PSCs) have increased rapidly from 3.8% to over 22.1%.1, 2, 3, 4, 5, 6 Perovskite solar cells have good prospect due to their advantage, such as low‐cost technology, high light absorption, long carrier diffusion length, high carrier mobility, and engineered energy band.7, 8, 9, 10 PSCs have different device architecture including mesoscopic, planar, and inverted structure.11, 12, 13 No matter what kind of cells, the uniform coverage of perovskite and perfect interface properties is crucial to device performance.14 Traditional PSCs consist of transparent anode layer, electron transport layer (ETL), light absorption layer, hole transport layer (HTL), and metal electrode (Ag or Au). As for planar PSCs, ETL plays an important role in carrier extraction and transport, which suppresses the recombination of electrons with holes generated in the perovskite.15, 16, 17 Until now, TiO2 is still widely considered to be the preferred ETL for PSCs because of its matched conduction band (CB) with perovskite, and thus the high ability of electron injection and collection is obtained. However, the application of TiO2 ETL in PSCs is also limited owing to some disadvantage such as low conductivity and a large amount of defects, leading to the unavoidable carrier recombination.18, 19
Figure1a,b shows the X‐ray diffraction (XRD) pattern of TiO2 layers on fluorine‐doped tin oxide (FTO) substrates fabricated by atomic layer deposition (ALD) and water bath reaction in the TiCl4 solution, respectively. For the ALD TiO2 based substrate, the film thickness is about 10 nm and a weak diffraction peak appears at 26°, which is ascribed to the (101) plane of anatase phase TiO2 (JCPDS 21–1272). Raman spectra in Figure S1a (Supporting Information) further prove the formation of anatase TiO2. The XRD pattern (Figure 1b) of film synthesized in the water bath demonstrates the characteristic of rutile TiO2 (JCPDS 21–1276). The above results indicate that the anatase and rutile TiO2 films can be fabricated by ALD and TiCl4 solution reaction, respectively. Consequently, we can fabricate anatase/rutile (simplified as AR) and rutile/anatase (simplified as RA) phase junction films on FTO substrates by adjusting the deposition order of ALD and water bath reaction. As shown in Figure S2 (Supporting Information), the diffraction peaks corresponding to the rutile TiO2 phase and anatase TiO2 phase appear in the XRD pattern, which confirms the existence of phase junction. The top‐view scanning electron microscopy (SEM) images of FTO with AR and RA films are given in Figure 1c,d, indicating the TiO2 layers are evenly coated on the FTO surface compared with the SEM image of bare FTO shown in Figure S1b (Supporting Information). SEM images of the single layered films are shown in Figure S1c,d (Supporting Information). The morphology of the layers indicates no obvious difference, which ensures these two ETLs will have negligible effects on the formation of perovskite films. Both the AR and RA TiO2 coated FTO substrates remain transparent (Figure S1e, Supporting Information) and the transmittance of AR structure is almost identical with bare FTO substrate (Figure S1f, Supporting Information), which is beneficial for device performance due to the great utilization of incident light. As for RA films, the values of transmission decline obviously and the utilization rate of incident light is lower than AR films, but the higher device performance is found in the RA structure. This phenomenon implies superior electronic transport in the RA structure, which will be discussed in the next section.
In summary, TiO2 phase junctions with type‐II energy band structure have been successfully obtained by combing the ALD and water bath reaction. Employing the TiO2 phase junction as ETL, two types of planar perovskite cells with different phase order are fabricated. Compared with single phase TiO2 based ETL, TiO2 phase junction based PSCs demonstrate improved FF, Jsc, and PCE with reduced hysteresis. The optimized rutile/anatase and anatase/rutile based devices yield the champion PCEs of 15.33% and 15.11%, respectively. The outstanding performance of TiO2 phase junction ETL based devices is mainly ascribed to the passivation of trap states, enhanced carrier extraction capability as well as reduced recombination rate. This work may open up new opportunities for fabricating high‐efficiency planar perovskite solar cells based on interfacial energy band control of ETLs.
Preparation of AR and RA ETLs on FTO Substrates: The FTO substrates were patterned by etching with Zn powder and diluted HCl, and then cleaned by sequential ultrasonic treatment in acetone, alcohol, and deionized (DI) water each for 20 min. The AR ETL was grown on FTO substrate by ALD followed by a water bath reaction process. ALD anatase TiO2 with different thickness (5, 10, and 15 nm) was deposited on the FTO substrate and then immersed in the 0.1 m TiCl4 aqueous solution at 343 K for 60 min to obtain rutile TiO2. Meanwhile, the 10 nm ALD TiO2/FTO substrates without TiCl4 treatment served as the reference. The RA ETL was grown on FTO substrate by first immersing in the 0.1 m TiCl4 solution at 343 K for 60 min and then different thicknesses (10, 15, 20, 25, and 30 nm) of ALD TiO2 films were deposited. The TiCl4 treated FTO substrates without ALD were also prepared as reference. Finally, after washing with DI water, the substrates with TiO2 films were annealed at 773 K in air for 120 min.
The authors declare no conflict of interest.