Contactless analysis of surface passivation and charge transfer at the TiO2-Si interface.

Transition metal oxides are pivotal in enhancing surface passivation and facilitating charge transfer (CT) in silicon based photonic devices, improving their efficacy and affordability through interfacial engineering. This study investigates TiO2/Si heterojunctions prepared by atomic layer deposition (ALD) with different pre-ALD chemical and post-ALD thermal treatments, exploring their influence on the surface passivation and the correlation with the CT at the TiO2-Si interface. Surface passivation quality is evaluated by the photoconductance decay method to study the effective carrier lifetime, while CT from Si to TiO2 is examined by transient reflectance spectroscopy. Surprisingly, the as-deposited TiO2 on HF-treated n-Si (without interfacial SiOx) demonstrates superior surface passivation with an effective lifetime of 1.23 ms, twice that of TiO2/SiOx/n-Si, and a short characteristic CT time of 200 ps, tenfold faster than that of TiO2/SiOx/n-Si. Post-ALD annealing at temperatures approaching the TiO2 crystallization onset re-introduces the SiOx layers in HF-treated samples and induces chemical and structural changes in all the samples which decrease passivation and prolong the CT time and are hence detrimental to the photonic device performance.

Transient Absorption Spectroscopy of Films: Impact of Refractive Index.

Transient absorption spectroscopy is a powerful technique to study the photoinduced phenomena in a wide range of states from solutions to solid film samples. It was designed and developed based on photoinduced absorption changes or that photoexcitation triggers a chain of reactions with intermediate states or reaction steps with presumably different absorption spectra. However, according to general electromagnetic theory, any change in the absorption properties of a medium is accompanied by a change in the refractive properties. Although this photoinduced change in refractive index has a negligible effect on solution measurements, it may significantly affect the measured response of thin films. In this Perspective paper, we examine why and how the measured responses of films differ from their expected "pure" absorption responses. The effect of photoinduced refractive index change can be concluded and studied by comparing the transmitted and reflected probe light responses. Another discussed aspect is the effect of light interference on thin films. Finally, new opportunities of monitoring the photocarrier migration in films and studying nontransparent samples using the reflected probe light response are discussed. Most of the examples provided in this article focus on studies involving perovskite, TiO2, and graphene-based films, but the general discussion and conclusions can be applicable to a wide range of semiconductor and thin metallic films.

Effect of Single-Crystal TiO2/Perovskite Band Alignment on the Kinetics of Electron Extraction.

The kinetics of electron extraction at the electron transfer layer/perovskite interface strongly affects the efficiency of a perovskite solar cell. By combining transient absorption and time-resolved photoluminescence spectroscopy, the electron extraction process between FA0.83Cs0.17Pb(I0.83Br0.17)3 and TiO2 single crystals with different orientations of (100), (110), and (111) were probed from subpicosecond to several hundred nanoseconds. It was revealed that the band alignment between the constituents influenced the relative electron extraction process. TiO2(100) showed the fastest overall and hot electron transfer, owing to the largest conduction band and Fermi level offset compared to FA0.83Cs0.17Pb(I0.83Br0.17)3. It was found that an early electron accumulation in these systems can have an influence on the following electron extraction on the several nanosecond time scale. Furthermore, the existence of a potential barrier at the TiO2/perovskite interface was also revealed by performing excitation fluence-dependent measurements.

Monitoring Charge Carrier Diffusion across a Perovskite Film with Transient Absorption Spectroscopy.

We have developed a new noninvasive optical method for monitoring charge carrier diffusion and mobility in semiconductor thin films in the direction perpendicular to the surface which is most relevant for devices. The method is based on standard transient absorption measurements carried out in reflectance and transmittance modes at wavelengths below the band gap where the transient response is mainly determined by the change in refractive index, which in turn depends on the distribution of photogenerated carriers across the film. This distribution is initially inhomogeneous because of absorption at the excitation wavelength and becomes uniform over time via diffusion. By modeling these phenomena we can determine the diffusion constant and respective mobility. Applying the method to a 500 nm thick triple cation FAMACs perovskite film revealed that homogeneous carrier distribution is established in few hundred picoseconds, which is consistent with mobility of 66 cm2 (V s)-1.

Refractive index change dominates the transient absorption response of metal halide perovskite thin films in the near infrared.

Perovskites have lately attracted a lot of attention as promising materials for the next-generation of efficient, low-cost, and solution processable optoelectronics. Their complex transient photophysics, in time scales ranging from femtoseconds to seconds, have been widely investigated. However, in most of the reported works the spectral window of ultrafast transient absorption (TA) spectroscopy of perovskite films is limited to the visible region, hence missing crucial information coming from the near-infrared (NIR). Furthermore, the measured TA responses are affected by light interference in a thin perovskite layer making data interpretation a challenge even in the visible part of the spectrum. Here, we demonstrate a method that allows us to separately obtain the changes in absorption and refractive index from conventional transmission and reflection pump-probe measurements. We show that the contribution of the absorption change to the response of metal halide perovskite thin films in the NIR is much smaller than that of the refractive index change. Furthermore, the spectral shape of TA responses in the NIR range is predominantly determined by perovskite layer thickness and its refractive index. However, the time profile of the responses bears important information on the carrier dynamics and makes the NIR a useful range to study perovskite photophysics.