Tailoring the electronic properties of the SnO2 nanoparticle layer for n-i-p perovskite solar cells by Ti3C2TX MXene

Influence of Charge Transport Layers on Capacitance Measured in Halide Perovskite Solar Cells

Carrier Dynamics Engineering for High-Performance Electron-Transport-Layer-free Perovskite Photovoltaics

In perovskite solar cells, the interfaces between the perovskite and charge-transporting layers contain high concentrations of defects (about 100 times that within the perovskite layer), specifically, deep-level defects, which substantially reduce the power conversion efficiency of the devices1-3. Recent efforts to reduce these interfacial defects have focused mainly on surface passivation4-6. However, passivating the perovskite surface that interfaces with the electron-transporting layer is difficult, because the surface-treatment agents on the electron-transporting layer may dissolve while coating the perovskite thin film. Alternatively, interfacial defects may not be a concern if a coherent interface could be formed between the electron-transporting and perovskite layers. Here we report the formation of an interlayer between a SnO2 electron-transporting layer and a halide perovskite light-absorbing layer, achieved by coupling Cl-bonded SnO2 with a Cl-containing perovskite precursor. This interlayer has atomically coherent features, which enhance charge extraction and transport from the perovskite layer, and fewer interfacial defects. The existence of such a coherent interlayer allowed us to fabricate perovskite solar cells with a power conversion efficiency of 25.8 per cent (certified 25.5 percent)under standard illumination. Furthermore, unencapsulated devices maintained about 90 per cent of their initial efficiency even after continuous light exposure for 500 hours.Our findings provide guidelines for designing defect-minimizinginterfaces between metal halide perovskitesand electron-transporting layers.

Recently, it has been demonstrated that the use of SnO2 as the electron transport layer (ETL) of perovskite (PSK) solar cells (PSCs) yields high efficiency, which is comparable to that of the TiO2 layer with the same structure. At the same time, the SnO2-based PSCs show improved stability. Herein, the defects at the device interface are reduced and the efficiency of the planar PSCs is enhanced by improving the interface contact between the ETL and the perovskite (PSK) layer. As an essential amino acid, tyrosine (Tyr) is introduced into SnO2 to fill the oxygen vacancies in SnO2 films and improve the nucleation of PSK. From our analysis, it was found that the interface contact between the SnO2 ETL and the PSK layer was increased and the defects at the interface were reduced. In addition, it was demonstrated that the introduction of Tyr could effectively suppress the charge recombination and improve the electron extraction efficiency. As a result, a champion power conversion efficiency (PCE) of 22.17% was obtained from Tyr modified PSCs, owing to the enhanced PSK film quality and carrier extraction efficiency. On top of that, the Tyr-modified device still maintained 87% of the initial recorded PCE, which was stored in the ambient air (25 °C, 25% ± 5% RH) for 864 h without encapsulation.

In recent years, researchers have developed spray deposition technology to fabricate tin oxide electron transport layer (ETL) with the aim of manufacturing high‐efficiency, large‐area perovskite solar cell (PSC). However, the power conversion efficiency (PCE) of PSC based on sprayed SnO2 ETL remains inferior to that of the spin‐coated SnO2 ETL. Herein, the combined use of spray deposition and genetically engineered M13 bacteriophages for the deposition of M13‐SnO2 biohybrid ETL over large‐area (62.5 cm2) substrates is demonstrated. The spray‐deposited M13‐SnO2 ETLs exhibit mesoporous morphologies with >85% transmittance in UV–vis region. Through the use of M13‐SnO2 ETL, the sequential‐deposited PSCs achieve a maximum PCE of ≈22.1%. The improved performance of the PSC is attributable to the mesoporous morphology of M13‐SnO2 ETL that facilitates the growth of larger perovskite grains. The PSCs based on M13‐SnO2 ETLs also display highly consistent photovoltaic performance which manifests the excellent scalability of the spraying process. Furthermore, M13‐SnO2‐based PSCs exhibit higher ambient stability compared to the SnO2‐based PSCs, showing that the use of M13 bacteriophage is incredibly beneficial to both the efficiency and stability of PSCs.

Effect of lanthanum doped SnO2 on the performance of mixed-cation mixed-halide perovskite layer

Incorporating chlorine into the SnO2 electron transport layer (ETL) has proven effective in enhancing the interfacial contact between SnO2 and perovskite in perovskite solar cells (PSCs). However, previous studies have primarily focused on the role of chlorine in passivating surface trap defects in SnO2, without considering its influence on the buried interface. Here, hydrochloric acid (HCl) is introduced as a chlorine source into commercial SnO2 to form Cl-capped SnO2 (Cl-SnO2) ETL, aiming to optimize the buried interface of the PSC. The incorporation of HCl into the SnO2 precursor solution works in two key ways. First, it converts the detrimental KOH stabilizer into KCl through an acid-base reaction. Second, it regulates the crystallization process of the perovskite, reducing PbI2 residues and voids at the buried interface. As a result, the efficiency of the PSC increases from 21.93% to 25.39%, with a certified efficiency of 25.69%, the highest efficiency reported for Cl-SnO2 ETL-based PSCs. Moreover, the target PSC exhibits excellent air stability, retaining 90% of its initial efficiency after 2900h of air exposure, compared to only 56.1% for the control PSC. This investigation highlights the effectiveness of HCl in the synergistic optimization of the buried interface in PSCs.

Introduction Currently, perovskite solar cells (PSCs) have received great curiosity from solar cell field, due to rapid improvement in their photoelectric conversion efficiencies (PCEs) from 9% to over 22% [1]. PSCs can be constructed with either invert or regular structure. In invert type, organic materials are employed as the hole transporting layer (HTL) and electron transporting layer (ETL). The PSCs with invert structure has reached PCE 20.15% [2]. The PSCs with regular structure typically employs both mesoporous and planar inorganic material of TiO2 to serve as ETL because the affinity of perovskite layers with TiO2 is generally lower than that with organic ETL. When considering the long stability, inorganic material would be better than organic material as ETL. The current best performing PSCs with mesoporous TiO2 as ETL perform PCE >22% because mesoporous TiO2 helps to support perovskite layer acting as a scaffold and make better connection with perovskite layer[1]. On the other hand, PSCs with planar TiO2 also shows PCE about 20.2%, which is lower than with mesoporous TiO2 but provides a more straightforward platform for fundamental investigations [3]. However, in planar structure, the quality of each layers is an important factor that affect overall cell performance, especially ETL [4]. However, a mechanism depending on ETL on cell performances should be investigated in detail. For ETL, various deposition methods are explored, such as spin coating, atomic layer deposition and sputtering, especially spray-coating has a great potential for the large-scale production due to the low cost and capability to obtain thin films in large areas for industrial application. The polycrystalline has grain boundaries. When having polycrystalline Si with more grain boundaries, Si solar cell shows lower performance because grain boundaries act as recombination centers for electrons and holes and become scattering centers of free carriers [5]. CuInSe2 (CIS) solar cell showed PCE about 20% without special grain boundaries passivation [6]. Similar to CIS solar cell, perovskite grain boundaries seem to have small effect on photoelectronic properties because even high efficient PSCs have smaller grain size less than 1 um. Therefore, the relationship between grain size and performance of PSCs have not been clarified clearly. Here, Kim et al. showed planar PSCs of FTO/compact TiO2/MAPbI3/spiro-OMeTAD/Au. They obtained dense MAPbI3 layers with different grain sizes ranging from ~100 to ~500 nm. With increasing grain size of perovskites, all the photovoltaic parameters were improved. As a result, the best performance was observed for the largest perovskite grain size (~500 nm) with PCE of 19.4% [7]. In this paper, we investigated the relationship between PSC performance and grain size distribution on the top of planar TiO2 by spray pyrolysis technique to construct more defined structure. SEM image from top view and cross section were observed and used to identify the grain size distribution. The surface roughness of TiO2 measured by AFM used for investigation of the relationship with perovskite grain size. These findings indicated that wide variations in the device efficiency also influence by perovskite grain size distribution in planar heterojunction structure. Experimental An around 80-100 nm thick compact-TiO2 layer was deposited on ITO by spray pyrolysis at 350, 450 and 550°C to investigate the of spray temperature (Ts) effect on compact-TiO2 surface state. Also, ethanol, propanol and 1-butanol were used as a precursor solvent with different boiling temperature (Tb) to find their effects on compact-TiO2 surface state. Next, samples were annealed at different temperatures (400, 450, 500 or 600°C) for 1 hr to study an effect of annealing temperature (Ta) as shown in Table 1. The device structure consists of; ITO/compact-TiO2/CH3NH3PbI3/Spiro-OMeTAD/Au with active area 0.5 cm2. Results and discussion In this work, planar PSCs were assembled with different ETL fabrication condition for comparison. The uniform TiO2 film without pin hole were successfully deposited by using 1-butanol as a solvent. However, the wide range variations of PCE values were observed even at the same fabrication condition. So, grain size distribution was measured and the results indicated that higher device efficiencies at the same ETL fabrication condition resulted from larger perovskite grain size distribution. The obtained dense MAPbI3 layers showed different grain sizes ranging from ~100 to ~300 nm. In addition, the relationship between roughness of TiO2 and perovskite grain size showed that the larger grain sizes were from lower roughness of TiO2 film. Therefore, higher PCEs were strongly achieved from larger perovskite grain size on planar TiO2. Figure 1

Highly improved efficiency and stability of planar perovskite solar cells via bifunctional phytic acid dipotassium anchored SnO2 electron transport layer

The passivation of detrimental perovskite-based defects is critically acknowledged for fabricating highly effective perovskite solar cells (PSCs). The presence of a high-quality electron transport layer (ETL) is also considered a pivotal factor for effective charge extraction and transport dynamics. Herein, a simple small organic molecule, aminomethyl phosphonic acid (AMPA), is introduced as a multifunctional additive in the SnO2 ETL. The defects in the SnO2 ETL are effectively suppressed by passivating the oxygen vacancies upon the SnO2 surface. Simultaneously, the carrier mobility and crystallinity of SnO2 are enhanced, and the upward-regulated conduction band minimum (CBM) is beneficial for constructing a favorable energy level alignment with the perovskite layer. Notably, the introduced residuals on the SnO2 surface can function as crystalline seeds for growth of large perovskite grains, which can passivate the defects in the perovskite bulk phase, boundaries, as well as the SnO2/perovskite interface. Consequently, the power conversion efficiency (PCE) value of the AMPA-modified PSCs is enhanced from 19.91% to 24.22%. Most importantly, the unencapsulated PSCs with AMPA maintained 94.9% of the initial PCE during 720h of storage at a relative humidity of 10%, attributed to the improved hydrophobicity of both the SnO2 and perovskite layers after AMPA modification.

The passivation of detrimental perovskite‐based defects is critically acknowledged for fabricating highly effective perovskite solar cells (PSCs). The presence of a high‐quality electron transport layer (ETL) is also considered a pivotal factor for effective charge extraction and transport dynamics. Herein, a simple small organic molecule, aminomethyl phosphonic acid (AMPA), is introduced as a multifunctional additive in the SnO2 ETL. The defects in the SnO2 ETL are effectively suppressed by passivating the oxygen vacancies upon the SnO2 surface. Simultaneously, the carrier mobility and crystallinity of SnO2 are enhanced, and the upward‐regulated conduction band minimum (CBM) is beneficial for constructing a favorable energy level alignment with the perovskite layer. Notably, the introduced residuals on the SnO2 surface can function as crystalline seeds for growth of large perovskite grains, which can passivate the defects in the perovskite bulk phase, boundaries, as well as the SnO2/perovskite interface. Consequently, the power conversion efficiency (PCE) value of the AMPA‐modified PSCs is enhanced from 19.91% to 24.22%. Most importantly, the unencapsulated PSCs with AMPA maintained 94.9% of the initial PCE during 720 h of storage at a relative humidity of 10%, attributed to the improved hydrophobicity of both the SnO2 and perovskite layers after AMPA modification.

SnO2 film is one of the most widely used electron transport layers (ETL) in perovskite solar cells (PSCs). However, the inherent surface defect states in SnO2 film and mismatch of the energy level alignment with perovskite limit the photovoltaic performance of PSCs. It is of great interesting to modify SnO2 ETL with additive, aiming to decrease the surface defect states and obtain well aligned energy level with perovskite. In this paper, anhydrous copper chloride (CuCl2) was employed to modify the SnO2 ETL. It is found that the adding of a small amount of CuCl2 into the SnO2 ETL can improve the proportion of Sn4+ in SnO2, passivate oxygen vacancies at the surface of SnO2 nanocrystals, improve the hydrophobicity and conductivity of ETL, and obtain a good energy level alignment with perovskite. As a result, both the photoelectric conversion efficiency (PCE) and stability of the PSCs based on SnO2 ETLs modified with CuCl2 (SnO2-CuCl2) is improved in comparison with that of the PSCs on pristine SnO2 ETLs. The optimal PSC based on SnO2-CuCl2 ETL exhibits a much higher PCE of 20.31% as compared to the control device (18.15%). The unencapsulated PSCs with CuCl2 modification maintain 89.3% of their initial PCE after exposing for 16 d under ambient conditions with a relative humidity of 35%. Cu(NO3)2 was also employed to modify the SnO2 ETL and achieved a similar effect as that of CuCl2, indicating that the cation Cu2+ plays the main role in SnO2 ETL modification.

Charge extraction by electron transport layers (ETLs) plays a vital role in improving the performance of perovskite solar cells (PSCs). Here, PSCs with four different types of ETLs, such as SnO2, amorphous‐Zn2SnO4 (am‐ZTO), am‐ZTO/SnO2, and SnO2/am‐ZTO, are successfully synthesized. The interface recombination behavior and the charge transport properties of the devices affected by four types of ETLs are systematically investigated. For dual am‐ZTO/SnO2 ETLs, compact am‐ZTO ETL prepared by the pulsed laser deposition method provides a dense physical contact with FTO than the spin coating films, decreasing leakage current and improving charge collection at the interface of ETL/FTO. Moreover, dual am‐ZTO/SnO2 ETLs lead to large free energy difference (ΔG), improving electron injection from perovskite to ETLs. One additional electron pathway from perovskite to am‐ZTO is formed, which can also improve electron injection efficiency. A power conversion efficiency of 20.04% and a stabilized efficiency of 19.17% are achieved for the device based on dual am‐ZTO/SnO2 ETLs. Most importantly, the devices are fabricated at a low temperature of 150 °C, which offers a potential method for large‐scale production of PSCs, and paves the way for the development of flexible PSCs. It is believed that this work provides a strategy to design ETLs via controlling ΔG and interface contact to improve the performance of PSCs.

Buried modification with tetramethylammonium chloride to enhance the performance of perovskite solar cells with n-i-p structure

Inorganic CsPbI3 perovskite solar cells (pero-SCs) have attracted great attention in photovoltaic research in recent years. Thermal annealing treatment is important for device optimization of the pero-SCs, nevertheless, traditional annealing process leads to residual strain owing to the difference of thermal expansion coefficients between perovskite active layer and SnO2 electron transporting layer (ETL) in n-i-p pero-SCs. Additionally, abundant defects and mismatched energy levels restrict the application of SnO2 as ETL in the CsPbI3 n-i-p pero-SCs. Here, by applying a molten mixture of Li salts during the fabrication of the ETL, the residual strain is eliminated and the energy level alignment is optimized. The molten salt (MS) treatment improves the quality of the SnO2 ETL, facilitates better crystallization of the perovskite active layer, and reduces lattice strain. As a result, the n-i-p pero-SCs based on CsPbI3 with SnO2 ETL show a notable increase in power conversion efficiency, from 17.46% for the device without the MS treatment to 20.49% for that with the MS treatment, and enhanced stability of the pero-SCs. This approach addresses key challenges in the pero-SCs, demonstrating that optimizing the interface between the ETL and perovskite active layer can greatly improve the photovoltaic performance of the CsPbI3-based n-i-p pero-SCs.