Low‐Temperature In Situ Amino Functionalization of TiO₂ Nanoparticles Sharpens Electron Management Achieving over 21% Efficient Planar Perovskite Solar Cells

Abstract

Titanium oxide (TiO2) has been commonly used as an electron transport layer (ETL) of regular‐structure perovskite solar cells (PSCs), and so far the reported PSC devices with power conversion efficiencies (PCEs) over 21% are mostly based on mesoporous structures containing an indispensable mesoporous TiO2 layer. However, a high temperature annealing (over 450 °C) treatment is mandatory, which is incompatible with low‐cost fabrication and flexible devices. Herein, a facile one‐step, low‐temperature, nonhydrolytic approach to in situ synthesizing amino‐functionalized TiO2 nanoparticles (abbreviated as NH2‐TiO2 NPs) is developed by chemical bonding of amino (‐NH2) groups, via TiN bonds, onto the surface of TiO2 NPs. NH2‐TiO2 NPs are then incorporated as an efficient ETL in n‐i‐p planar heterojunction (PHJ) PSCs, affording PCE over 21%. Cs0.05FA0.83MA0.12PbI2.55Br0.45 (abbreviated as CsFAMA) PHJ PSC devices based on NH2‐TiO2 ETL exhibit the best PCE of 21.33%, which is significantly higher than that of the devices based on the pristine TiO2 ETL (19.82%) and is close to the record PCE for devices with similar structures and fabrication procedures. Besides, due to the passivation of the surface trap states of perovskite film, the hysteresis of current–voltage response is significantly suppressed, and the ambient stability of devices is improved upon amino functionalization.

Interfacial Modification through a Multifunctional Molecule for Inorganic Perovskite Solar Cells with over 18% Efficiency

“…For the pristine TiO 2 thin film, the O 1s core level could be deconvoluted into two peaks centered at 530.3 and 532.6 eV, respectively. The former peak (530.3 eV) was assigned to be the O 2− anion bonded with the Ti 4+ cation (TiO) in the stoichiometric TiO 2 structure, and the latter one (532.6 eV) represented the hydroxyl groups on the surface of TiO 2 film (TiOH) . After TiO 2 substrate was treated with ATPA, the peak intensity of TiOH was significantly reduced and a new peak appeared at 532.0 eV, indicating the formation of COTi bonds .…”

“…These results implied that the esterification between the hydroxyl groups on TiO 2 surface and the carboxylic acid groups on ATPA occurred. Furthermore, the peaks of Ti 2 p 1/2 (459.2 eV) and Ti 2 p 3/2 (464.9 eV) changed little before and after the ATPA treatment (Figure S7, Supporting Information), indicating negligible influence of the reaction between TiO 2 and ATPA on the valence state of Ti. Considering that XPS could only probe the chemical states of top surface and the ATPA layer was sandwiched between TiO 2 and CsPbI 3 , we could only probe the interaction between the CsPbI 3 film and ATPA by spin‐coating a thin layer of ATPA onto the CsPbI 3 film and measured with XPS.…”

“…Therefore, ATPA should offer two types of chemical linkages to anchor the CsPbI 3 layer on top and TiO 2 substrate simultaneously, as shown in Figure b. It has been reported that the surface of TiO 2 layer is rich in dangling bonds such as hydroxyl groups, which might form deep trap states to induce charge recombination . The carboxylic acid group on ATPA would react with hydroxyl groups on TiO 2 to result in reduced surface defects .…”

“…The light intensity‐dependent V OC is shown in Figure a. The PVSC based on TiO 2 /ATPA showed smaller slope (1.26 k B T / e ) compared with that based on TiO 2 (1.71 k B T / e ) ( k B is the Boltzmann constant and T is the absolute temperature), indicating that the trap‐assisted recombination was effectively inhibited by the ATPA because of the reduced trap states . Meanwhile, the transient photovoltage (TPV) decay curve of the PVSC based on TiO 2 /ATPA showed longer lifetime than that based on TiO 2 (Figure S14, Supporting Information), indicating obviously suppressed charge‐carrier recombination.…”

A low-temperature TiO2/SnO2 electron transport layer for high-performance planar perovskite solar cells

“…To solve these problems, doping and surface modifications have been widely researched in recent years. Hu et al [22] reported that upon applying NH 2 -TiO 2 nanoparticles as a novel ETL in the low-temperature process, the regular-structure PSCs exhibited a significant PCE improvement to over 21%. Liu and coworkers [23] introduced zinc ions into a compact TiO 2 lattice by a simple low-temperature solution process, which improved the interfacial carrier extraction between the ETL and perovskite, obtaining a PCE of 19.04% with 4.5% Zn-doped ETL PSCs.…”