An annealing-free aqueous-processed anatase TiO₂ compact layer for efficient planar heterojunction perovskite solar cells

Abstract

A facile aqueous-based fabrication scheme is developed for producing annealing-free anatase TiO₂ (AF-TiO₂) films that exhibit efficient electron transport properties in planar heterojunction perovskite solar cells (PSCs). AF-TiO₂ films are fabricated by spin coating on a substrate a colloidal solution of anatase TiO₂ nanoparticles (NPs) prepared via a low temperature hydrolytic sol–gel method. The resultant AF-TiO₂ films show low electrical resistance, high transmittance in the visible and near-infrared regions and facilitation of high-quality perovskite film formation, which can be attributed to their homogeneous surface morphology and nanocrystallinity. The AF-TiO₂ based PSCs achieve a power conversion efficiency (PCE) of 18.29 ± 0.18%, showing significant improvement compared to the control PSCs (PCE = 11.33 ± 0.32%) based on TiO₂ films made by high-temperature annealing of amorphous TiO₂ (HTA-TiO₂).

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Solution-processable perovskite solar cells (PSCs) have recently attracted significant attention as a promising candidate for making high efficiency and flexible photovoltaic devices.^1–6 To date, numerous PSCs using TiO₂ mesoporous structures have been shown to exhibit high power conversion efficiencies (PCEs) of over 20%.^7–10 However, mesoporous TiO₂ layers require a high-temperature annealing process (over 450 °C), which limits their use in flexible PSCs. As an alternative to mesoporous TiO₂ PSCs, planar heterojunction (PHJ) PSCs made via low-temperature (<150 °C) processing have been investigated extensively due to their simple device structures and their compatibility with flexible polymeric substrates as well as inverted device designs.^11–14 In general, the p–i–n type PHJ PSC structure consists of an electron transport layer (ETL), a perovskite absorber layer and a hole transport layer (HTL). The ETL and HTL are responsible for extracting photo-generated charge carriers from the perovskite layer and transporting these carriers to the designated electrodes. Various types of materials have been employed in carrier transport layers, such as PEDOT:PSS, NiO_x, CuSCN, poly(triaryl amine) (PTAA) and 2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (Spiro-OMeTAD) as hole transport layers, and ZnO, SnO₂, CsCO₃, PCBM and TiO₂ as electron transport layers.^7,15–21 Among these ETLs, TiO₂ is considered to be a promising electron transport material because of its environmentally friendly nature, wide band gap, high electron mobility and good stability.^22–24

Recently, several vacuum deposition approaches have been developed for fabricating low-temperature processable TiO₂ ETLs for PHJ PSCs, such as atomic layer deposition,²⁵ electron-beam evaporation,²⁶ and magnetron sputtering.¹⁴ Compared to vacuum methods, however, solution-based techniques are generally more cost-effective and scalable. Among them, sol–gel methods are arguably the most widely adopted routes for depositing TiO₂ films owing to ease in fabrication and low cost.^4,11,27–29 Nevertheless, the as-prepared TiO₂ films via the sol–gel methods are generally amorphous and therefore exhibit poor electrical conductivity, which incurs negative impacts on the performance of PSCs.^11,30 Strategies have been developed to enhance the electrical properties of TiO₂ films via optimising their morphology, doping and composition; these strategies include surface modification of TiO₂ with TiCl₄ treatment, ozone (O₃) treatment,^31,32 and doping modification of the TiO₂ film with an appropriate substitution of Li⁺, Mg²⁺, Al³⁺, Y³⁺, and Nb⁵⁺.^4,18,33–35 In particular, the formation of crystalline TiO₂ at a low temperature is considered to be one of the most direct and efficient strategies to improve the electrical characteristics for highly efficient PSCs.^29,30 In this context, considerable effort has been devoted to prepare TiO₂ nanocrystals (NCs) through a low-temperature non-hydrolytic sol–gel method for PSC applications.^16,32,36 However, it remains a challenge to synthesise high-quality anatase TiO₂ NCs via the low-temperature hydrolytic sol–gel method for efficient PSCs.

Herein, we report a facile, efficient and environmentally friendly approach for preparing ultrathin anatase TiO₂ films via an aqueous-based and annealing-free fabrication scheme. The resultant annealing-free anatase TiO₂ (AF-TiO₂) films can act as an efficient electron transport layer for planar heterojunction PSCs. The nanocrystalline anatase TiO₂ nanoparticles (NPs) prepared by this low-temperature hydrolytic sol–gel method can form stable nanocolloidal solutions in aqueous media, enabling the preparation of a smooth film with high optical transmittance. The resultant AF-TiO₂ film shows better electron extraction ability than that of the conventional TiO₂ films prepared in organic solvents and treated via high-temperature annealing processes, abbreviated as HTA-TiO₂ below. Moreover, the AF-TiO₂ compact layer was also found to facilitate the formation of high-quality perovskite films.

A general drawback of the hydrolytic sol–gel method is the amorphous nature of the resultant TiO₂ NPs and thus the requirement of a post-synthetic high-temperature (over 450 °C) annealing step to induce crystallisation. In our case, the low-temperature hydrolysis of titanium(IV) isopropoxide (TTIP) is performed via a sol–gel method in the presence of HNO₃ as the hydrolysing agent, resulting in the formation of nanocrystalline anatase TiO₂ NPs (see the ESI† for detailed experimental procedures), consistent with other similar systems in the literature.³⁷ In this method, the fast initial nucleation step induces the formation of small electrostatically stabilised TiO₂ NPs. The subsequent ageing step at 80 °C triggers the formation of small nanocrystalline anatase domains within a TiO₂ NP. The mild reaction conditions (80 °C, 1 atmosphere pressure) and surfactant-free aqueous solvent systems make this method a promising way to produce high-quality anatase TiO₂ NPs for PSCs and other applications.

The as-synthesised anatase TiO₂ NPs can be readily dispersed in water to form nanocolloidal solutions that are stable for months (inset of Fig. 1a; concentration = 20 mg mL⁻¹). Powder X-ray diffraction (XRD) measurements of this sample reveal typical diffraction peaks of anatase TiO₂ (Fig. 1a). In particular, the diffraction peaks centred at 25.40°, 38.28°, 47.46°, 54.26°, and 62.68° were assigned to (101), (004), (200), (105) and (204) diffractions, respectively. This result verifies that the as-synthesised TiO₂ NPs indeed contain a significant amount of the anatase domains. Scherrer peak width analysis²² reveals the average size of the nanocrystalline domains to be approximately 6.1 nm. The inset of Fig. 1b shows a top-view scanning electron microscopy (SEM) image of an AF-TiO₂ film on a glass/ITO substrate prepared by spin coating. It was observed that AF-TiO₂ can form a dense, pinhole-free, and uniform continuous film compared to that of HTA-TiO₂ (ESI,† Fig. S1). Additionally, the resultant AF-TiO₂ film shows good optical transparency, with transmittance greater than 85% in the entire visible region (Fig. 1b). For comparison, the effects of anatase TiO₂ film thicknesses on transmittance performance are shown in Fig. S2 (ESI†).

Fig. 1 (a) Powder X-ray diffraction pattern of AF-TiO₂ nanoparticles. Diffraction peaks of each crystal plane are labelled. (b) Transmittance spectra of AF-TiO₂ films deposited on a glass substrate. The inset shows a representative SEM image of AF-TiO₂ deposited on a glass/ITO substrate, exhibiting a smooth morphology.

To gain further insights into the nanoscale morphology and nanocrystal structures, high-resolution transmission electron microscopy (HRTEM) has been performed. The HRTEM image (Fig. 2a) reveals that the TiO₂ NPs are around 15–25 nm in diameter. By analysing the diffraction-contrast image and the fast-Fourier transform FFT images of selected areas, individual crystal domains within a single NP are identified to have a domain size of 2.5–5.5 nm (Fig. 2b and c), consistent with that obtained by Scherrer peak width analysis. The selected area electron diffraction (SAED) pattern (Fig. 2a, inset) displayed typical electron diffraction rings corresponding to the (101), (004), (200), and (105) lattice planes of the anatase structure of TiO₂, consistent with the powder XRD results (Fig. 1a). These results indicate that the TiO₂ NPs indeed contain a high proportion of nanocrystalline anatase phase, implying that they have a reduced number of surface defect traps and could therefore enhance charge extraction on the interface between the TiO₂ layer and the perovskite layer.

Fig. 2 (a) Bright-field HRTEM image of TiO₂ nanocrystals deposited on a carbon film. The inset shows a selected area electron diffraction of TiO₂ NPs. Diffraction rings of the (101), (004), (200) and (105) planes are observed. (b) Zoom-in of the region of interest denoted by the blue square in (a). Selected adjacent nanocrystal domains are highlighted and numbered. Domain size: 2.5–5.5 nm. (c) FFT images of selected nanocrystal domains in (b), verifying the difference in crystal orientation.

To investigate the performance of the AF-TiO₂ films as ETLs in PHJ PSCs, a complete PSC device has been fabricated. A cross-sectional SEM image of a typical device architecture is shown in Fig. 3a. The electrodes of indium tin oxide (ITO) and Ag were used as the bottom cathode and the top anode, respectively. The thicknesses of the ETL (TiO₂), the HTL (Spiro-OMeTAD) and the (FAPbI₃)_x(MAPbCl₃)_1−x perovskite absorber layer are approximately 60 nm, 260 nm, and 600 nm, respectively. It is noted that the smooth morphology of the AF-TiO₂ films can facilitate the formation of highly crystalline and compact perovskite films (see Fig. S1, S3–S5, ESI† for details).

Fig. 3 (a) Cross-sectional SEM image of a completed PSC device, with the structure of glass/ITO/TiO₂/(FAPbI₃)_x (MAPbCl₃)_1−x/Spiro-OMeTAD/Ag. (b) J–V curves of typical devices using AF-TiO₂ and HTA-TiO₂ as ETLs. The measurements were performed under simulated AM 1.5 G sunlight (100 mW cm⁻²). (c) EQE spectra of representative devices comprising AF-TiO₂ and HTA-TiO₂ as ETLs. (d) Forward-scan (from 0 V to 1.2 V) and reverse-scan (from 1.2 V to 0 V) J–V curves of a typical AF-TiO₂ based device.

Fig. 3b shows the current–density–voltage (J–V) curves of the perovskite solar cells based on AF-TiO₂ compared to devices made of HTA-TiO₂. The photovoltaic parameters of the devices are summarised in Table 1. Data in Table 1 show that the solar cell of HTA-TiO₂ exhibits considerably low short-circuit current density (J_sc), open-circuit voltage (V_oc), and fill factor (FF) values. In contrast, the overall PCE, including J_sc, V_oc, and FF, significantly increased when AF-TiO₂ NPs were used as the ETL. We propose that the notably high PCE of PSCs based on the AF-TiO₂ ETL is caused by the high content of the nanocrystalline anatase phase within the compact layer and the highly homogeneous morphology, which together favour electron extraction by reducing the rate of carrier recombination, compared to that of HTA-TiO₂.^11,21,32 In particular, the dramatic difference in series resistance (R_s) could be attributed to the presence of nanocrystalline TiO₂ domains that enhance electron transport, thereby improving the electrical characteristics.²² As a result, the devices of AF-TiO₂ exhibit a J_sc of 22.27 ± 0.19 mA cm⁻¹, a V_oc of 1.11 ± 0.01, and a FF of 73.63 ± 0.86%, corresponding to a PCE of 18.29 ± 0.18%, which shows significant improvement compared to that of control PSCs (PCE = 11.33 ± 0.32%) using HTA-TiO₂ films as the ETL. This result demonstrates that planar structure photovoltaic devices based on AF-TiO₂ can indeed achieve a highly efficient power conversion, which sheds light on the development of highly efficient and flexible PSCs without relying on mesoporous TiO₂ structures.

Table 1 Parameters of the PSCs, showing TiO₂ layer thickness, short-circuit current density (J_sc), open-circuit voltage (V_oc), fill factor (FF), power conversion efficiency (PCE) and series resistance (R_S). Statistical analysis (average ± standard deviation) is based on measurements of 10 individual devices for both AF-TiO₂ and HTA-TiO₂ films. “Champion” refers to the measurement data of the device with the highest PCE

Compact layer (Annealing temperature)	Thickness^a (nm)		J sc (mA cm^−2)	V oc (V)	FF (%)	PCE (%)	R S (Ω cm^2)
a Thickness of the TiO2 films was measured using a surface profilometer, consistent with the thickness determined by SEM.
AF-TiO2 (25 °C)	58 ± 2	Champion	22.41	1.10	74.87	18.50	4.49
		Average	22.27 ± 0.19	1.11 ± 0.01	73.63 ± 0.86	18.29 ± 0.18	4.79 ± 0.20
HTA-TiO2 (500 °C)	61 ± 2	Champion	18.85	0.99	62.74	11.70	11.38
		Average	18.75 ± 0.11	0.98 ± 0.01	61.47 ± 1.44	11.33 ± 0.32	12.56 ± 1.34

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Typical external quantum efficiency (EQE) data of the devices are shown in Fig. 3c. The integrated J_sc values of the devices calculated from the EQE data were 21.63 mA cm⁻² and 17.45 mA cm⁻² for AF-TiO₂ and HTA-TiO₂, respectively, which match well with the J–V measurement data (Table 1). The enhancement of EQE from 500 nm to 800 nm can be attributed to the smaller R_s value and the improved morphology and crystallisation of the perovskite film facilitated by the AF-TiO₂ ETL (Fig. S1, S3–S5, ESI†).

The J–V curves and parameters of the AF-TiO₂ device measured for reverse and forward scan directions are shown in Fig. 3d and Table S2 (ESI†). Notably, the device does not exhibit distinct hysteresis, which is an indicator of inefficient charge transfer at the TiO₂/perovskite interface, ferroelectric effects, ionic displacement and the trapping/detrapping of the charge carrier at the perovskite interface.^38–41 In an attempt to optimise the performance of TiO₂ ETLs, we have investigated the effects of the ETL thickness and that of the additional low-temperature annealing step after the deposition of anatase TiO₂ NPs by spin coating. Device-level measurement data based on 10 PSCs per configuration are presented in the ESI† (Fig. S2, S6–S12 and Tables S1–S4).

Electrical impedance spectroscopy (EIS) was performed to further study the interfacial charge transport properties of the PSCs.^14,42 Nyquist plots of the devices were measured at a DC bias of 0–0.8 V in the dark and the equivalent circuit model for the perovskite solar cells is shown in Fig. 4a (see Fig. S12 (ESI†) for details). Compared to HTA-TiO₂-based devices, the AF-TiO₂ based devices exhibit significantly smaller R_s and larger recombination resistance (R_rec), indicating more efficient extraction and transport of electrons at the AF-TiO₂/perovskite interface, thus resulting in larger J_sc, V_oc and FF values for these solar cells.

Fig. 4 (a) Nyquist plots of PSCs based on AF-TiO₂ and HTA-TiO₂ in the dark at a DC bias of 0 V. A 20 mV AC signal was applied with a frequency range of 0.01 Hz to 100 kHz. The inset shows the equivalent circuit adopted. (b) Nyquist plots of the high frequency region. (c) R_s as a function of bias. (d) R_rec as a function of bias.

In conclusion, we have developed a facile and environmentally friendly method for fabricating nanocrystalline anatase TiO₂ films as compact electron transport layers for PHJ PSCs. TiO₂ NPs with small (2.5–5.5 nm) anatase domains were successfully prepared via a low temperature hydrolytic sol–gel method. The aqueous TiO₂ nanocolloids can form via spin coating continuous thin films that show superior electron extraction capability and high optical transmittance and can facilitate the formation of high-quality perovskite films. The PSC based on AF-TiO₂ achieves a PCE of 18.29 ± 0.18%. This powerful and generic strategy offers a novel way towards green, efficient and flexible PSC designs and beyond.

The research was funded by the Fundamental Research Funds for the Central Universities (2013121031) and the UCL BEAMS Future Leader Award through the EPSRC 2016 Institutional Sponsorship Award (EP/P511262/1). DY is grateful to the China Scholarship Council for funding. Nanfeng Zheng is acknowledged for his assistance in the IPCE measurements. Suyuan Xie is also gratefully acknowledged for his assistance in the J–V characteristic measurements.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details including synthesis, experimental procedure and supporting data. See DOI: 10.1039/c7cc01104k

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