TiO2 Electron Transport Layer with p-n Homojunctions for Efficient and Stable Perovskite Solar Cells

doi:10.1007/s40820-024-01407-3

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Abstract

Low-temperature processed electron transport layer (ETL) of TiO₂ that is widely used in planar perovskite solar cells (PSCs) has inherent low carrier mobility, resulting in insufficient photogenerated electron transport and thus recombination loss at buried interface. Herein, we demonstrate an effective strategy of laser embedding of p-n homojunctions in the TiO₂ ETL to accelerate electron transport in PSCs, through localized build-in electric fields that enables boosted electron mobility by two orders of magnitude. Such embedding is found significantly helpful for not only the enhanced crystallization quality of TiO₂ ETL, but the fabrication of perovskite films with larger-grain and the less-trap-states. The embedded p-n homojunction enables also the modulation of interfacial energy level between perovskite layers and ETLs, favoring for the reduced voltage deficit of PSCs. Benefiting from these merits, the formamidinium lead iodide (FAPbI₃) PSCs employing such ETLs deliver a champion efficiency of 25.50%, along with much-improved device stability under harsh conditions, i.e., maintain over 95% of their initial efficiency after operation at maximum power point under continuous heat and illumination for 500 h, as well as mixed-cation PSCs with a champion efficiency of 22.02% and over 3000 h of ambient storage under humidity stability of 40%. Present study offers new possibilities of regulating charge transport layers via p-n homojunction embedding for high performance optoelectronics.

Key words： Electron transport layer p-n homojunction Electron mobility Buried interface Perovskite solar cells

Received: 22 January 2024 Published: 03 May 2024

Corresponding Authors: Pengfei Guo, Hongqiang Wang

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	Wenhao Zhao
	Pengfei Guo
	Jiahao Wu
	Deyou Lin
	Ning Jia
	Zhiyu Fang
	Chong Liu
	Qian Ye
	Jijun Zou
	Yuanyuan Zhou
	Hongqiang Wang

Cite this article:

Wenhao Zhao,Pengfei Guo,Jiahao Wu, et al. TiO₂ Electron Transport Layer with p-n Homojunctions for Efficient and Stable Perovskite Solar Cells[J]. Nano-Micro Letters, 2024, 16(1): 191-.

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https://www.qk.sjtu.edu.cn/nml/EN/10.1007/s40820-024-01407-3 OR https://www.qk.sjtu.edu.cn/nml/EN/Y2024/V16/I1/191

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Scheme 1 Schematic illustration of the effects of embedding coherent p-n homojunctions on carrier dynamics at buried interface compared with those of conventional works [<xref ref-type="bibr" rid="b7">7</xref>,<xref ref-type="bibr" rid="b8">8</xref>,<xref ref-type="bibr" rid="b16">16</xref>]

Fig. 1 a TEM image of Ti0.936O2 nanocrystals (inset: nanocrystals size distribution diagram and Mie-scattering image of colloids). b HRTEM and corresponding FFT of Ti0.936O2 nanocrystals. c SEM images (inset: AFM images) of pristine TiO2 and 6%-target TiO2 films. d XRD patterns of different ETLs. e Dark I-V measurement of the electron-only devices (inset) displaying VTFL kink point related to the trap density. f Electron mobilities of different ETLs using the SCLC model, the inset shows the device structure of ITO/Al/ETLs/Al. g Mott-Schottky plots of Ti0.936O2@TiO2 matrix. h UPS results of Ti0.936O2 nanocrystals (inset: band gap and energy level of Ti0.936O2 nanocrystals). i Different TiO2 films with UPS Fermi edge (left) and the cut-off energy (right). Scale bar: a 50 nm; b 1 nm; c 1 μm (inset: 1 μm)

Fig. 2 SEM top-view images of perovskite films based on a pristine TiO2 and b 6%-target TiO2 films (inset: contact angles of different ETLs dropped by perovskite precursor). Steady-state c and time-resolved d PL spectra of CsFAMA perovskite films spin-coated on different TiO2 layers. e Arrhenius plots of the characteristic transition frequencies. f Trap state density (NT) of the perovskite photovoltaics measured at 300 K. Dependence of g Jsc and h VOC on the irradiation intensity of the devices based on different TiO2 ETLs. i Nyquist plots of the devices based on different ETLs measured in the dark at a bias of 0.8 V. Scale bar: a 500 nm; b 500 nm

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Fig. 3 a J-V curves of CsFAMA devices with different TiO2 layers (inset: schematic illustration of device structure). b J-V plots of CsFAMA champion devices containing the pristine TiO2 and 6%-target TiO2 layers measured both in reverse scan and forward scan, the insets show stabilized power output at MPP tracking. c J-V curves of FAPbI3 devices employing different TiO2 layers with stabilized power output at MPP tracking. d EQE spectra of FAPbI3 champion devices upon the pristine TiO2 and 6%-target TiO2 layers, respectively. e PCE distribution of 50 individual CsFAMA and FAPbI3 devices. f Comparison of efficiency of PSCs employing TiO2 as ETLs. g Energy level diagram for each component of devices upon different TiO2 layers. The energy level structures of Spiro-OMeTAD and Au refer to the literature [<xref ref-type="bibr" rid="b23">23</xref>,<xref ref-type="bibr" rid="b24">24</xref>]

Table 1

Photovoltaic parameters of the CsFAMA and FAPbI₃ type PSCs upon different TiO₂ ETLs

Fig. 4 a Humidity stability and b operational stability of different CsFAMA-based devices. c Humidity stability and d operational stability of different FAPbI3-based devices. Normalized PL spectra of perovskite films grown on e pristine TiO2 and f 6%-target TiO2 layers under continuous 254 nm UV irradiation (50 mW cm−2) for 20 h. The error bars represent the standard deviation for 20 devices

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