SnO₂-Based Interfacial Engineering towards Improved Perovskite Solar Cells

Abstract

Interfacial engineering is of great concern in photovoltaic devices. Metal halide perovskite solar cells (PSCs) have garnered much attention due to their impressive development in power conversion efficiencies (PCEs). Benefiting from high electron mobility and good energy-level alignment with perovskite, aqueous SnO₂ as an electron transport layer has been widely used in n-i-p perovskite solar cells. However, the interfacial engineering of an aqueous SnO₂ layer on PSCs is still an obscure and confusing process. Herein, we proposed the preparation of n-i-p perovskite solar cells with different concentrations of SnO₂ as electron transport layers and achieved optimized PCE with an efficiency of 20.27%. I Interfacial engineering with regard to the SnO₂ layer is investigated by observing the surface morphology, space charge-limited current (SCLC) with the use of an electron-only device, and time-resolved photoluminescence (TRPL) of perovskite films.

1. Introduction

Named after mineral calcium titanate (CaTiO₃), metal halide perovskite originally possessed the same crystal structure as that of the former, with the chemical formula ABX₃. The A-site cation is coordinated to 12 X anions, forming a cuboctahedron, while the six-fold-coordinated B-site cation has an octahedral geometry [1,2,3]. Metal halide perovskite-based solar cells have garnered much attention worldwide due to the rapid progress they have facilitated in power conversion efficiency (PCE), and they have become a potentially strong competitor in the photovoltaic performance race [4,5,6]. Moreover, PSCs exhibit great promise in large-scale production and mainstream technology, owing to their low-cost, scalable, and simple solution processing techniques [7,8,9]. Despite the tremendous breakthroughs and rapid progress in the photovoltaic performance of perovskite solar cells, from 9.7% with pure MAPbI₃ in 2012 to 26%, the remaining issues in interfacial engineering that urgently need to be resolved constitute a multifaceted challenge [10,11,12].

The n-i-p PSC configuration is regarded as a normal device structure; the active perovskite layer is directly spin-coated on an n-type electron transport layer (ETL), such as SnO₂. The crystalline quality of the perovskite and the morphology of the buried interface are directly affected by the surface chemistry of the ETL [13,14,15]. Therefore, research on ETL has become one of the most relevant scientific subjects in the development of highly efficient and stable n-i-p PSCs. For instance, Yan et al. reported low-temperature solution-processed nanocrystalline SnO₂ as an excellent alternative ETL and demonstrated PSCs with an average efficiency of 16.02% [16]. Kim et al. studied the band alignment between La-doped BaSnO₃ (LBSO) and MAPbI₃ perovskite, demonstrating LBSO as the next-generation ETL, with its high mobility, photostability and structural stability [17]. Pang and co-workers designed a Cl-containing tin-based ETL, SnO_x-Cl, to realized a spontaneous ion exchange reaction at the interface of SnOx–Cl/MAPbI₃, producing PSCs with an efficiency of 20.32% [18]. Chlorine-capped TiO₂ colloidal nanocrystals were applied in a PSC to mitigate interfacial recombination and improve interface binding [19].

Aqueous SnO₂ has been widely employed in n-i-p perovskite solar cells due to its high electron mobility and good energy-level alignment with perovskite and electrodes [20,21,22,23,24]. A SnO₂ ETL can be obtained via the thermal oxidation of Sn(iv) isoprenoids, SnO₂ quantum dots, and ALD (atomic layer deposition) [25,26,27,28]. Theories on the formation and properties of crystalline interfaces have been developed [29,30]. An optimized SnO₂ electron transport layer showed great advantages, in terms of reduced interfacial recombination losses, controlled energy levels, and increased charge transport. SnO₂ purchased directly from the market requires concentration dilution before application in PSCs, but the optimization of SnO₂ concentrations is rarely reported. Moreover, researchers usually determine dilute SnO₂ concentration ratios experientially. For example, Yang and co-workers reported a SnO₂ precursor diluted in isopropanol and deionized water in a ratio of 1:3:2.5, achieving the same PCE as that of flexible PSC, reaching up to 18.71% [31]. Zhou et al. employed a 15 wt.% SnO₂ aqueous solution in H₂O as an ETL and realized a PCE of 21.92% [32]. However, those reports reached no consensus on the optimal SnO₂ dilution ratio. At the same time, fundamental knowledge is lacking for a thorough understanding of the key role of different concentrations of SnO₂ in crystallisation kinetics. Therefore, it is necessary to further explore the effects of concentrations of SnO₂ as an ETL on device performance.

Here, we propose a simple and effective strategy to adjust the concentration of SnO₂ and preliminarily validate the optimal dilution ratio of SnO₂ in water. In this work, the efficiency of the device prepared by using the optimal concentration ratio (SnO₂ 2.4%) reached 20.27%. This work provides a comprehensive understanding of SnO₂ concentrations and of how to realize efficient perovskite photovoltaic devices with optimized SnO₂ layers.

2. Experimental Section

Materials. All chemicals and materials were purchased from Aladin and directly used in the reactions and depositions without further purification. SnO₂ hydrocolloidal dispersion solution (SnO₂, 99.99%), leaching iodide (PbI₂, 99.99%), lead bromide (PbBr2, 99.99%), formamidine hydroiodide (FAI, 99.99%), formamidine bromide (FABr, 99.99%), cesium iodide (CsI, 99.99%), methylammonium iodide (MAI, 99.99%), methylammonium chloride (MACl, 99.99%), n, n-dimethylformamide (DMF, 99.99%), dimethyl sulfoxide (DMSO, 99.99%), ethyl acetate (EA, 99.99%), Spiro-OMeTAD (99.99%), LiTFSi (99.99%), and 4-tert-butylpyridine (tBP, 99.99%) were used.

Device Fabrication. The FTO substrate was pre-washed with pure water, acetone, and isopropanol in an ultrasound bath for 15 min, followed by undergoing UV zone treatment for 20 min. We prepared four kinds of perovskite solar cell devices with different concentrations of SnO₂. SnO₂ was spin-coated at a speed of 3000 r/30 s, followed by undergoing annealing at 200 °C for 40 min. The perovskite film was deposited onto an electron transport layer, followed by the preparation of a hole transport layer (HTL) and a Ag metal electrode. Finally, devices with a structure of FTO/ETL/Perovskite/HTL/Ag were fabricated.

Measurement. The current density–voltage (J–V) characteristics of the perovskite solar cells were measured using an integrated solar simulator (JIS C 8942 Class MA). Solar cell performance was characterized under illumination using a standard amorphous Si photodetector (BS 520 S/N 007, Bunko Keiki, Tokyo, Japan), an air mass 1.5 global (AM 1.5 G) solar simulator with an irradiation intensity of 100 mw/cm². Furthermore, 0.1 cm² size apertures made of thin metal were attached to each cell before measurement. Scanning electron microscopy (SEM) was performed using JSM-7800F to analyze the surface morphology of the perovskite thin films.

3. Results and Discussion

A plane structure diagram of an n-i-p perovskite solar cell composed of FTO/ETL/Perovskite/Spiro-OMeTAD/Ag is shown in Figure 1, and the detailed fabrication process is displayed accordingly. It is obvious that the n-i-p perovskite solar cell consists of the conductive substrate FTO, a SnO₂ electron transport layer, a perovskite active layer, a Spiro-OMeTAD hole transport layer, and a conductive metal electrode made of Ag. The SnO₂ ETL was employed to transport electrons that were generated in the active perovskite to the circuit. Therefore, systematic studies of SnO₂ electron transport layers are of critical importance for the development of perovskite solar cells.

In this work, the original aqueous SnO₂ (12%) purchased from the aforementioned company was employed after further processing. In detail, the original SnO₂ was in the form of nanoparticles, which were subjected to dilution with water, forming SnO₂ concentrations of 4%, 3%, 2.4%, and 2%. The SnO₂ solutions with varying degrees of dilution showed varied surface characteristics, directly affecting perovskite crystallization. Based on this, the surface morphology evolution and conductivity of the perovskite layer deposited on SnO₂ thin films with different concentrations and the performance variation of the PSCs based on different SnO₂ concentrations were studied. Interfacial engineering based on SnO₂ ETLs was discussed systematically, and this is expected to provide in-depth understanding of ETL dynamics and give guidance on enhanced PSC performance.

The SnO₂ hydrocolloidal dispersion solution purchased from the aforementioned company generally exists as a SnO₂ quantum dot aqueous solution, with an original concentration of 12%. The original SnO₂ solution may emerge in a cluster state; thus, further dilution of the original solution is necessary. Bonding issues are very important factors determining the quality of deposited perovskite films. As shown in Figure 2, through scanning electron microscopy (SEM), the morphological evolution of the perovskite films deposited on the different SnO₂ concentrations is evident. When the SnO₂ concentration was 4%, the grain size of the perovskite film deposited on it was small, with the average grain size being about 340 nm. As the SnO₂ concentration was decreased to 3%, the grain size of perovskite films increased to 401 nm; the grain size further increased to 439 nm as the concentration was decreased to 2.4%. At this point, the perovskite layer showed a more compact grain boundary and a smooth surface topography, indicating that SnO₂ at this concentration is more suitable as an ETL for PSC device fabrication. The further dilution (2% SnO₂) resulted in a rough surface and inferior perovskite deposition. The surface morphologies of different SnO₂ ETLs were compared, and the differences were not obvious (Supporting Information Figure S1).

We further investigated the steady-state photoluminescence (PL) spectra and time-resolved photoluminescence (TRPL) of perovskite films grown on different concentrations of SnO₂. The quenched luminous intensity suggested that the electron transport layer had an enhanced capacity to extract and collect charge carriers generated by the perovskite layer. When the concentration of SnO₂ was 4%, the highest PL intensity indicated carrier accumulation and aggregation in the perovskite film. This also implied that the perovskite layer was not conducive to good carrier transport performance, as confirmed by the conductivity analysis in Figure 3b. As SnO₂ concentration decreased (3%), the carrier transportation that occurred in the electron transport layer was improved, and the conductivity of the perovskite layer was correspondingly enhanced. As the SnO₂ was diluted to 2%, the carriers produced by the perovskite film became more favorably absorbed by the electron transport layer, resulting in the lowest corresponding luminous spectral intensity. The quenching efficiencies of PL emission at the interface between the perovskite and ETL were in the order of 2.4% > 3% > 2% > 4%, which indicated more effective electron extraction from the perovskite/diluted SnO₂ (2.4%).

According to the formula for conductivity (conductivity = current/voltage = 1/resistance), it is evident that a steeper slope corresponds to higher conductivity and lower resistance in the electron transport layer, indicating improved carrier transport efficiency. When the SnO₂ concentration is 4%, it can be clearly seen that the slope is the lowest, and the value is about 0.13, which indicates that the large resistance generated by SnO₂ hinders carrier transport. With a decreased SnO₂ concentration, the perovskite conductivity is obviously increased. When the SnO₂ concentration is 2.4%, the resulting perovskite film exhibits the highest electrical conductivity (approximately 0.23), confirming minimal resistance and enhanced charge carrier absorption from the perovskite layer. The electron-transport properties of diluted SnO₂ layers were evaluated using the space charge-limited current (SCLC) measurement taken with the electron-only device, as indicated in Figure 3d. The evaluated trap-filled limit voltages (V_TFLs) of diluted SnO₂ with the original, 4%, 3% and 2.4% concentrations are 0.18, 0.1, 0.08 and 0.14 V, respectively. The lower V_TFL for SnO₂ of 2.4% indicates a lower trap density.

To more accurately quantify the impact of the SnO₂ layer, the detailed distribution of the performance parameters extracted from more cells is shown in Figure 4. In detail, when the SnO₂ concentration was 4%, the device exhibited an average PCE of only 17.40%, a Voc of 1.02 V, an FF of 73.26%, and a J_SC of 23.29 mA/cm². This can be attributed to the thick SnO₂ thin film and its inferior perovskite conductivity. When the concentration increased to 3% and 2.4%, the device performance became gradually enhanced. In particular, for the device with an SnO₂ concentration of 2.4%, it showed the highest average PCE of 20.27%. The further addition of water in SnO₂ solution (2%) resulted in the inferior performance of device, with an average PCE of only 18.28%. To investigate the effect of different concentrations of SnO₂ on the performance of perovskite solar cells, the activities of different perovskite solar cells are summarized in

Table 1. The performance of perovskite solar cells with different SnO₂ concentrations.

Concentration Ratios	PCE (%)	VOC (V)	FF (%)	JSC (mA/cm2)
SnO2 (4%)	17.40	1.02	73.26	23.29
SnO2 (3%)	19.34	1.06	76.54	23.84
SnO2 (2.4%)	20.27	1.07	79.9	24.29
SnO2 (2%)	18.28	1.04	72.82	24.14

.

The corresponding external efficiency (EQE) spectra and integrated current density (integrated J_sc) values of different devices were recorded; the results indicated that the integrated J_sc values were in accordance with the J_sc values from the J–V curves (Supporting Information Figure S2). The enhanced photovoltaic performance can be ascribed to the following factors: (1) dense and uniform coverage of the SnO₂ layer on the substrate; (2) the smooth interface and controlled defects originating from uniformly dispersed SnO₂ colloids in the aqueous solution; and (3) improved perovskite crystallinity and conductivity.

4. Conclusions

In summary, aqueous SnO₂ solutions with different concentrations were employed in perovskite solar cells. Decreased concentrations facilitated perovskite crystallinity and conductivity, and improved the performance of the devices. The PSC with 2.4% SnO₂ showed an efficiency of 20.27%. However, the further dilution of SnO₂ with a 2% concentration resulted in reduced performance. A moderate SnO₂ concentration is conducive to dense and uniform coverage, conductivity, and the electron transport of the perovskite layer. An appropriate SnO₂ concentration is essential for efficient perovskite solar cells.