Surface photovoltage spectroscopy (SPS) is a non-contact, highly sensitive technique for investigating electronic properties at semiconductor surfaces and interfaces. By measuring light-induced changes in surface potential, SPS provides critical insights into band bending, surface states, and carrier dynamics without requiring electrical contacts. This method is particularly valuable for studying emerging materials like perovskites and traditional semiconductors like silicon, where interfacial properties significantly influence device performance.

The principle of SPS relies on the photovoltage effect, where illumination generates electron-hole pairs that modify the surface potential. When light with energy exceeding the semiconductor’s bandgap is incident on the surface, photoexcited carriers separate due to built-in electric fields, altering the surface charge distribution. A Kelvin probe or capacitive coupling measures these potential changes as a function of photon energy, generating a spectrum that reveals electronic transitions, defect states, and band alignment.

A typical SPS setup includes a tunable light source, a vibrating Kelvin probe or metal-insulator-semiconductor (MIS) capacitor, and a lock-in amplifier for noise reduction. The light source scans across a range of wavelengths, while the probe detects the surface potential changes synchronously. For quantitative analysis, the photovoltage signal is often normalized to the incident photon flux to account for variations in light intensity. Key parameters extracted from SPS include the surface band bending, density of surface states, and minority carrier diffusion length.

Band bending is deduced from the onset of the photovoltage signal, which corresponds to the energy required to excite carriers across the bandgap. The magnitude of the photovoltage reflects the strength of the built-in field. For example, studies on methylammonium lead iodide (MAPbI3) perovskites have shown SPS-determined band bending of 200-300 meV at grain boundaries, indicating charge accumulation and potential recombination sites. In silicon, SPS reveals oxidation-induced band bending shifts of up to 500 meV, highlighting the sensitivity of surface electronic structure to chemical treatments.

Surface states within the bandgap are identified as sub-bandgap features in the SPS spectrum. These states act as traps or recombination centers, influencing carrier lifetime and device efficiency. Research on silicon surfaces has demonstrated that SPS can detect defect states with densities as low as 10^10 cm^-2 eV^-1, crucial for optimizing passivation layers in solar cells. Similarly, in perovskites, SPS has revealed mid-gap states attributed to halide vacancies, which correlate with non-radiative recombination losses.

Carrier diffusion length is another critical parameter accessible via SPS. By analyzing the photovoltage as a function of incident light modulation frequency, the diffusion length of minority carriers can be extracted. For instance, SPS measurements on single-crystal MAPbI3 have reported diffusion lengths exceeding 1 μm, while polycrystalline films show values around 300-500 nm due to grain boundary scattering. In silicon, diffusion lengths vary from tens to hundreds of micrometers depending on doping and defect concentration.

Applications of SPS extend beyond basic material characterization. In perovskite solar cells, SPS has been used to optimize interfacial layers by quantifying band alignment and charge extraction efficiency. Studies comparing TiO2 and SnO2 electron transport layers revealed that SnO2 induces less band bending, reducing interfacial recombination. For silicon heterojunction solar cells, SPS has identified optimal surface passivation schemes by monitoring defect state reduction after atomic layer deposition of Al2O3.

In conclusion, surface photovoltage spectroscopy is a powerful tool for probing semiconductor interfaces with high precision. Its ability to non-invasively measure band bending, surface states, and carrier diffusion makes it indispensable for advancing both conventional and emerging materials. By bridging the gap between fundamental surface science and device engineering, SPS continues to play a pivotal role in the development of next-generation optoelectronic technologies.