Stable Carbon-Based Perovskite Solar Cells without Hole-transporting Layer using Tetraethyl Orthosilicate

Perovskite solar cells (PSCs) have emerged as an attractive technology in the photovoltaic field, due to their exceptional power conversion efficiency (PCE), solution processability and low cost. Among them, the carbon-based PSC without hole transport layer has shown great potential for application by simplifying the device structure, reducing material costs and improving environmental resistance. In the past decade, although the PCE of carbon-based PSC has been rapidly increased, the defects in perovskite materials can lead to severe carrier recombination and the reduction of carrier mobility and lifetime, thus negatively affecting the performance improvement of such devices. In addition, the inherent poor stability of perovskite thin films to external environments is not conducive to the long-term, stable, and efficient operation of devices. To address these issues, various strategies, including component engineering, additive engineering, interface engineering and device packaging technology, have been adopted to optimize the device's efficiency and stability. Among them, interface engineering has shown great potential due to its direct effect on defect enriched interface areas. In this article, tetraethyl orthosilicate (TEOS) was introduced onto the surface of perovskite. The silicon oxygen oligomers formed by the reaction between TEOS and water in air effectively passivate the surface defects of perovskite and reduce the probability of carrier recombination. Meanwhile, the protective film formed by silicon oxygen oligomers effectively prevents the penetration of moisture and oxygen from air, effectively improving the PCE and operational stability of the carbon-based PSC without hole transport layer.

TEOS/isopropanol solutions with different concentrations were spin coated onto the surface of perovskite films at a speed of 5000 r·min⁻¹ for 30 s, followed by heating at 60 ℃ for 5 min to form a passivation layer. Then, a carbon-based PSC without hole transport layer was prepared. The microstructure of the sample was observed by using a scanning electron microscope (SEM). The chemical interaction was analyzed by using Fourier transform infrared spectrometer (FTIR). To obtain the crystal structure of the sample, an X-ray diffractometer (XRD) was used. By using a fluorescence spectrometer to test steady-state fluorescence spectra (PL) and transient fluorescence spectra (TRPL), the photoexcitation characteristics and carrier behavior of the sample were studied. J-V curve was measured and calculated using AM 1.5 G and a digital source instrument.

The perovskite prepared by airflow assisted extraction method exhibits a uniform and dense surface morphology. From the PL and TRPL results, the perovskite film modified with TEOS exhibits stronger fluorescence and longer carrier lifetime (enhanced from 93.51 ns to 123.38 ns), indicating suppressed carrier recombination. In FTIR spectra, the shift of the peak clearly indicates that, after TEOS interface modification, the surface modified material undergoes bonding with perovskite. The champion PCE of the TEOS modified PSC increased from 14.52% to 16.33%. From XRD pattern, the TEOS modified perovskite film aged for 35 days at humidity ≈30% and temperature ≈25 ℃ did not show the characteristic peak of PbI₂, while the unmodified sample clearly showed the PbI₂ peak, indicating the good moisture stability of the TEOS modified perovskite film. Heated at 65 ℃ for 4 h, the unmodified sample showed that the characteristic peak of PbI₂ began to appear, while the modified samples showed no such peak, indicating the good thermal stability of the TEOS modified perovskite film. Under atmospheric conditions (humidity ≈30%, temperature ≈25 ℃), the TEOS modified devices exhibit excellent stability, with their PCE to be maintained 80% of the initial value on the 100th day.

With TEOS modification, Si-O oligomers were formed on the surface of perovskite thin films due to the reaction of TEOS and water, passivating surface defects and improving the carrier transport performance at the perovskite/carbon interface. Meanwhile, Si-O oligomers located on the surface of perovskite can greatly prevent the damage of the water and oxygen and enhance the stability of perovskite thin films and PSC devices during preparation and service. The strong interaction between Si-O oligomers and perovskite enhances the integrity and resistance to decomposition of the perovskite structure. Also, the Si-O oligomer coating could avoid the volatilization of organic components. This work is expected to provide effective support and technical guarantee for the practical application and commercial promotion of carbon-based PSC.