Perovskite solar cells have garnered significant attention from research communities over the past decade, with the power conversion efficiency (PCE) increasing from 3.8% to 26.1%^[1⇓⇓-4]. It is anticipated that the PSCs are expected to develop as one of the third-generation solar cells in the future^[5]. Apart from optimizing the perovskite absorber layer, it is imperative to prepare an effective electron transport material that affects the charge extraction and transport at the interface^{[6⇓⇓⇓-10]}.

In the early stages of research on PSCs, most researchers used titanium dioxide (TiO₂) as the electron transport layer (ETL), which typically requires a high sintering temperature (>450 ℃) after spin-coating^[11⇓-13]. In addition, TiO₂ was found to have vigorous photocatalytic activity under ultraviolet light, inducing the decomposition of perovskite films, thereby hindering device stability and commercial application. In contrast, SnO₂ has better chemical stability, higher light transmittance and greater electron mobility^[14⇓-16]. Furthermore, the conduction band minimum of SnO₂ is conducive to electron extraction due to its proximity to the bottom of the conduction band of the perovskite^[17⇓-19]. Currently, spin-coating SnO₂ colloidal solution is commonly used to prepare SnO₂ films^[20⇓-22], but it yields nonuniform coverage on the rough FTO glass substrates. Chemical bath deposition (CBD) has attracted notable attention as an ideal method for preparing SnO₂ ETL under low-temperature conditions^[23-24]. For instance, a three-step CBD method was proposed to utilize SnO₂/TiO₂ as the ETL to enhance electron extraction and suppress carrier recombination at the SnO₂/perovskite interface^[25]. A low concentration of SnCl₂•2H₂O solution was used to repeat the CBD method three times to obtain a uniform ETL^[26]. Nevertheless, the CBD method relies on corrosive hydrochloric acid and toxic thioglycolic acid to prevent particle aggregation in the precursor solution, posing potential environmental pollution concerns. Hence, it is necessary to develop an environmentally friendly and facile method to prepare uniform and well-covered SnO₂ films on the FTO glass substrates.

In this work, a low-temperature, environmentally friendly, and in-situ growth (IG) method was employed to prepare a dense and uniform SnO₂ ETL. The IG SnO₂ film facilitates charge extraction at the buried interface and promotes the formation of large-sized perovskite grains, which holds great promise in achieving high-performance PSCs.

Dimethylsulfoxide (DMSO, 99.99%), acetonitrile (ACN, 99.8%), N, N-dimethylformamide (DMF, 99.8%), chlorobenzene (CB, 99.99%), 4-tert-butylpyridine (TBP, 98%) and bis(trifluoromethylsulfonyl)-imide lithium salt (LiTFSI, 99.9%) were purchased from Sigma-Aldrich. Phenethylammonium iodide (PEAI, 99.5%) and lead iodide (PbI₂, 99.99%) were purchased from Xi’an Yuri Solar. Methylammonium iodide (MAI, 99.9%) and formamidinium iodide (FAI, 99.9%) were purchased from Greatcell Solar Materials. SnCl₄•5H₂O (99.998%) was purchased from Macklin. Molybdenum trioxide (MoO₃, 99.95%) and methylammonium chloride (MACl, 98%) were purchased from Aladdin. Isopropanol (IPA, 99.9%) and tin (IV) oxide (SnO₂, 15% colloidal water dispersion) were purchased from Alfa Aesar. Fluorine-doped tin oxide (FTO, 14 Ω) glass was purchased from AGC. FK209 (Co(III) TFSI salt, 99%) and 2,2′,7,7′-tetrakis(N,N-p-dimethoxy-phenylamino)-9,9′- spirobifluorene (Spiro-OMeTAD, 99.8%) were purchased from Lumtec. All chemicals were used without further purification.

The FTO glass substrates were washed ultrasonically with aqueous detergent solution, deionized water, acetone, and ethanol for 6 h in sequence. Then, they were dried with an air gun. A commercial SnO₂ colloidal dispersion (Alfa Aesar) was spin-coated to prepare spin-coated SnO₂ (SC SnO₂) films. The SnO₂ colloidal solution was dissolved in deionized water at the volume ratio of 1 : 3 to prepare the SnO₂ precursor solution. Before spin-coating, the SnO₂ precursor solution and the FTO glass substrates were treated by a UV-Ozone cleaner for 15 min to remove residual organics. Subsequently, 100 μL of the SnO₂ precursor solution was dropped on the FTO glass substrates and spin-coated at 4000 r/min for 30 s. Then, the substrates were placed on a 150 ℃ hot plate for 30 min to afford the SC SnO₂ films.

IG SnO₂ films were prepared through an in-situ growth method. Firstly, 600 mg of SnCl₄•5H₂O was dissolved in 15 mL of deionized water and stirred for 10 min, and then 15 mL of water was added to the bottom of a hydrothermal reaction vessel, with a petri dish positioned atop a bracket. The FTO glass substrates were immersed in 15 mL of SnCl₄•5H₂O aqueous solution in the petri dish. Then, the hydrothermal reactor was placed in a 120 ℃ oven for 20 h. After the reactor cooled down, the FTO/SnO₂ films were put into deionized water and ethanol with ultrasonic treatment for 1 h, respectively. Following this treatment, the obtained dense SnO₂ layers on the FTO glass substrates were dried by an air gun without further annealing treatment.

PbI₂ precursor solution: 692 mg of PbI₂ was dissolved in the mixed solvent of 900 μL of DMF and 100 μL of DMSO and then placed on a 70 ℃ hot plate for 4 h, and filtered through a 0.45 μm filter before use.

Organic ammonium salt precursor solution: 9 mg of MACl, 90 mg of FAI, and 6.4 mg of MAI were dissolved in 1 mL of IPA and stirred overnight. PEAI precursor solution: 5 mg of PEAI was dissolved in 1 mL of IPA and stirred overnight.

Spiro-OMeTAD precursor solution: Spiro-OMeTAD (72.3 mg) was dissolved in 1 mL of CB. Then, 28.8 μL of TBP and 17.5 μL of LiTFSI solution (0.52 g of LiTFSI dissolved in 1 mL of ACN) were added. Subsequently, 29 μL of FK209 solution (0.30 g of FK209 dissolved in 1 mL of ACN) was added to oxidize Spiro-OMeTAD. The solution was stirred overnight and filtered through a 0.45 μm filter before use.

The FTO glass substrates were treated with a UV-Ozone cleaner for 15 min to enhance their wettability with the PbI₂ solution. They were then transferred into a dry-air glove box (relative humidity<15%) to spin-coat each functional layer. A two-step method was used to fabricate the photoactive perovskite layer. The PbI₂ precursor solution was preheated on a 70 ℃ hot plate for 10 min, and then 70 μL of that was spin-coated on the SnO₂ layer at 2000 r/min for 30 s. Then, the PbI₂ film was placed on a hot plate at 70 ℃ for 1 min. After the PbI₂ film cooled down, it was spin-coated with an organic ammonium salt solution. The spin-coating speed of the organic ammonium salt solution was the same as that of the PbI₂ solution. After that, the wet perovskite film was immediately transferred to a 150 ℃ hot plate for 15 min to evaporate the IPA solvent. After the perovskite film cooled to room temperature, 120 μL of PEAI precursor solution was spin-coated on the perovskite films at 3000 r/min for 30 s by dynamic spin-coating, followed by 15 min of vacuuming treatment. Next, 30 μL of Spiro-OMeTAD precursor solution was spin-coated on the PEAI passivation layer at 4000 r/min for 30 s. Finally, 7.5 nm MoO₃ and 100 nm Ag were prepared by thermal evaporation to afford the PSCs with the structure of FTO/ SnO₂/FA_0.92MA_0.08PbI₃/PEAI/Spiro-OMeTAD/MoO₃/Ag.

Fig. 1(a) shows the top-view scanning electron microscopy (SEM) image of FTO glass substrate for reference. As shown in Fig. 1(b-e), the cusp of the FTO glass substrate was not fully covered by SC SnO₂ film, while IG SnO₂ film completely covered the cusp of the FTO glass substrate. The IG SnO₂ film with a smaller particle size and a thickness of ~20 nm, thinner than the SC SnO₂ film (30 nm), is less prone to aggregation on the FTO glass substrate. Fig. 1(f) shows the transmittance spectra of SC SnO₂, IG SnO₂ film and FTO glass substrate. The transmittance of IG SnO₂ film is higher than that of SC SnO₂ film, particularly ranging from 300 to 700 nm. This is attributed to the anti-reflection property of IG SnO₂, facilitating more efficient light absorption of perovskite film. From the atomic force microscopy (AFM) measurements, the average roughness (R_a) of FTO glass substrate, SC SnO₂ film, and IG SnO₂ film is 26.43, 28.20, and 32.89 nm, respectively, as shown in Fig. 1(g-i). The conformal and uniform coverage of IG SnO₂ films on the FTO glass substrates reduces the carrier combination centers that arise from the direct contact between FTO and perovskite.

The FA_0.92MA_0.08PbI₃ perovskite film was prepared on the SnO₂ film by a two-step spin-coating method. The cross-sectional SEM images (Fig. 2(a, b)) show that the perovskite film obtained on the IG SnO₂ film has more regular columnar grains compared to the SC SnO₂ film. As shown in Fig. 2(c), the steady-state photoluminescence (PL) spectrum of the perovskite film deposited on the IG SnO₂ film shows a weaker PL intensity, which suggests that the photogenerated electrons can be extracted more efficiently at the IG SnO₂/perovskite interface. This kind of morphology promotes effective charge extraction and transport^[27]. The top-view SEM images of the perovskite surface are shown in Fig. 2(d, e). The grains of the perovskite film on the IG SnO₂ film are larger than those on the SC SnO₂ film. Based on the SC SnO₂ film, as shown in Fig. 2(f), the average grain size of the perovskite film is 700 nm (accounting for 53.4%), while on the IG SnO₂ film, the average grain size increases to 800 nm (accounting for 60.0%). This indicates that IG SnO₂ is beneficial for the formation of high-quality perovskite films.

To investigate the influence of IG SnO₂ on device performance, SC SnO₂ and IG SnO₂ films were used as electron transport layers to fabricate n-i-p structure PSCs. Fig. 3(a) shows J-V curves of PSCs with different SnO₂ films. It is evident that IG SnO₂ device exhibits higher open-circuit voltage (V_OC) and fill factor (FF) than SC SnO₂ device, which is attributed to the reduction of recombination centers at the buried interface. The IG SnO₂ device shows the highest PCE (21.97%) with short-circuit current density (J_SC) of 25.14 mA·cm^-2, V_OC of 1.131 V and FF of 77.25%. In contrast, the SC SnO₂ device exhibits a lower PCE of 20.93%, with J_SC of 24.81 mA·cm^-2, V_OC of 1.110 V and FF of 76.04%. Table S1 presents the statistical data of twenty individual devices fabricated with SnO₂ films by spin-coating and in-situ growth method. Compared to those of SC SnO₂ devices, IG SnO₂ devices show improvements in photovoltaic parameters. The average V_OC increases from 1.095 V to 1.117 V, the average J_SC increases from 24.91 mA·cm^-2 to 25.06 mA·cm^-2, the average FF increases from 73.65% to 76.65%, and the average PCE increases from 20.08% to 21.45%. The incident photon-to-electron conversion efficiency (IPCE) spectra are shown in Fig. 3(b), and the integrated current density (J_{SC, IPCE}) of the SC SnO₂ device is 23.88 mA·cm^-2, while that of the IG SnO₂ device increases to 24.53 mA·cm^-2 due to the anti-reflection property of the IG SnO₂ layer. As shown in Fig. 3(c-f), V_OC, J_SC, FF and PCE distributions of the SC SnO₂ and IG SnO₂ devices were tested under a standard solar simulator (AM 1.5 G, 100 mW·cm^-2). V_OC, FF, and J_SC of the IG SnO₂ devices were enhanced, resulting in an increment of PCE. Transient photocurrent (TPC) and transient photovoltage (TPV) measurements were conducted to investigate the photogenerated electron extraction and transfer. The exponential function was used to fit the TPV and TPC decay curves, and the function used is shown as follows:

Where τ represents carrier lifetime of perovskite film (μs) and A is amplitude coefficient. The calculation formula of the average carrier lifetime τ_avg is given as follows:

The carrier lifetime τ, derived from the TPC decay curves (using a single exponential function), decreases from 7.45 μs to 4.23 μs (Fig. 3(g) and Table S2). This implies that IG SnO₂ enhances charge extraction, indicating a faster transfer rate of carriers from the perovskite layer to the SnO₂ layer. Table S2 presents the carrier lifetime τ obtained by fitting the TPV decay curves using biexponential functions, which displays two distinct electron lifetimes: one characterized by fast decay component τ₁ attributed to electron transport and another characterized by slow decay component τ₂ associated with trap depopulation kinetics. The prolonged τ_avg observed in the IG SnO₂ device (Fig. 3(h)) suggests a decrease of trap density and significant suppression of carrier recombination (1.199 ms for the SC SnO₂ device and 1.307 ms for the IG SnO₂ device)^[28]. Generally, a short carrier recombination lifetime of perovskite film and a fast carrier transfer rate at the buried interface ensure the efficient utilization of the carriers.

Fig. 3 (i) shows the V_OC dependence of the light intensity with a slope equal to nkT/q, where n (ideal factor) reflects the degree of trap-assisted recombination, k is the Boltzmann constant (1.38×10^-23 J·K^-1), T is the thermodynamic temperature (K) and q is the elementary charge (1.60×10^-19 C). When n is close to 2, trap-assisted recombination (Shockley-Read-Hall) dominates^[29]. The slopes of SC SnO₂ device and IG SnO₂ device are 1.99kT/q and 1.68kT/q, respectively. n of the IG SnO₂ device is smaller than that of the SC SnO₂ device, which indicates that its trap-assisted recombination is reduced.

Based on the above analysis, a possible mechanism for perovskite layer fabrication on SC SnO₂ and IG SnO₂ films is proposed (Fig. 4). When the spin-coating method is used to prepare SnO₂ films, the rough surface of the FTO glass substrates with an undulation of approximately 200 nm leads to an incomplete coverage and nonuniform thickness of the SnO₂ layers. However, the in-situ growth method not only enables the uniform coverage of SnO₂ films on the FTO glass substrates, but also helps to form large-sized perovskite grains.

In this study, a simple and environmentally friendly in-situ growth method for preparing a conformal SnO₂ ETL was developed. The perovskite films grown on this kind of SnO₂ layer exhibit large grain sizes. Furthermore, the IG SnO₂ films facilitate charge extraction at the buried interface and suppress trap-assisted recombination of perovskite film. Consequently, PSCs fabricated with IG SnO₂ films achieved an enhanced PCE of 21.97%, compared to 20.93% of PSCs with SC SnO₂ films. Furthermore, as the in-situ growth method is acid-free and avoids corrosion of the ITO substrates, this method can be extended to flexible PSCs.

Supporting materials related to this article can be found at https://doi.org/10.15541/jim20240202.

In-situ Growth of Conformal SnO₂ Layers for Efficient Perovskite Solar Cells

LIU Suolan^1,2, LUAN Fuyuan^1,3, WU Zihua^3,4, SHOU Chunhui⁵, XIE Huaqing^3,4, YANG Songwang^1,2

(1. CAS Key Laboratory of Materials for Energy Conversion, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201899, China; 2. Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, China; 3. School of Energy and Materials, Shanghai Polytechnic University, Shanghai 201209, China; 4. Shanghai Engineering Research Center of Advanced Thermal Functional Materials, Shanghai 201209, China; 5. Zhejiang Baima Lake Laboratory Co. Ltd., Hangzhou 310000, China)

Characterization

Scanning electron microscopy was performed using a Thermo Scientific G4 UC. The transmittance spectra were measured with a U-2800 UV-Vis spectrometer. Atomic force microscopy images were obtained using an NTEGRA instrument. A Yamashita Denso YSS-150A solar simulator was used to measure the PCE of the PSCs under one sun (100 mW·cm^-2) and calibrated using a reference silicon photodiode with a BS-520BK filter (Bunkoukeiki, Japan). Steady-state photoluminescence spectra were characterized using a FluoroMax-4 steady-state fluorescence test system with an excitation wavelength of 532 nm. Electrochemical measurements were performed by a Zahner GIMPS electrochemical workstation (1 Hz-1 MHz). The dependence of V_OC on light intensity was investigated using an LED light source (LSW-2, 4300 K, 100 mW·cm^-2). The incident photon-to-electron conversion efficiency spectra were measured by a Bunkoh-Keiki CEP-1500 instrument with a step length of 10 nm. For the transient photocurrent and transient photovoltage measurements, the record time of transient photocurrent and transient photovoltage was 10 ms, with a potentiostat setting time of 0.2 µs. During the 2 s light exposure, 10% of the data was captured before triggering, while 90% was captured after triggering. When recording the photovoltage decay process, the potentiostat was off-state. When recording the photocurrent decay process, it was necessary to turn on the potentiostat and apply a voltage of 0 V.