Diffusion Photovoltage in Porous Semiconductors and Dielectrics

Abstract

Strong retardation of photovoltage transients has been observed in porous materials (meso-and nanoporous Si, sintered networks of TiO 2 nanoparticles, porous Al oxide) excited by short laser pulse. It has been shown that the photovoltage is mainly determined by the slow carriers diffusion with different diffusion coefficients for electrons and holes. The influence of particle dimension, sign of more mobile charge carriers, and surface conditioning on the diffusion photovoltage has been demonstrated. We propose the diffusion photovoltage method as a universal technique to study carrier diffusion in materials with very low conductivity.Introduction The electronic transport in porous semiconductors and dielectrics depends significantly on the complex network of the nanoporous structure and on the conditioning of the large internal surface. This makes them quite interesting for many applications as in sensors [1] or solar cells [2]. But, porous semiconductors and dielectrics belong to a class of materials whose drift mobilities, concentrations of charge carriers and conditions for electrical carrier injection vary over many orders of magnitude. For these reasons, the application and interpretation of standard methods of mobility measurements is rather complicated or impossible. For example, Hall measurements could be applied for mesoporous silicon (meso-PS), for which the drift mobility and the free carrier concentration are not too small (m e 30 cm/Vs, n % 10 13 cm ±3 [3]). In contrast, time-of-flight (TOF) measurements are applicable for dielectric materials. The electron drift mobilities are, for example, about 10 ±4 ±10 ±1 cm/Vs in nanoporous Si [4] or 10 ±4 ±10 ±6 cm/Vs in nanoporous TiO 2 [5]. However, neither Hall or TOF are applicable for studying systematically the electronic transport properties of porous semiconductors and dielectrics in a wide range of experimental parameters.The photovoltage (PV) is a property of solids which can be easily measured independent of ambience, drift mobilities or concentration of carriers. This makes it universal. In the past, photovoltage methods have been applied successfully to determine parameters of semiconductors as carrier lifetimes [6] or diffusion coefficients [7] and of semiconductor surfaces as surface potential [8] or surface state distribution [9±11]. Especially spectral photovoltage measurements are powerful to investigate states in the forbidden band gap [12]. Electrical charging phenomena in por-Si/c-Si structures were investigated by pulsed PV measurements [13,14].

MoS2/Zn3In2S6 composite photocatalysts for enhancement of visible light-driven hydrogen production from formic acid

“…Thus, the introduction of MoS2 with its low Fermi energy level enables the enrichment of photogenerated electrons from the surface of ZIS6. Specifically, when the laser irradiates the sample, the photogenerated charges are separated because of the action of the electric field in the space charge region, presenting the TPV photovoltaic signal in the range of 0-10 -6 s, and the diffused photovoltage signals are observed in a range exceeding 10 -4 s [64]. Because MoS2 can capture photogenerated electrons from the surface of ZIS6, photogenerated charges are separated rapidly in MoS2/ZIS6, and the separation rate is inherently higher than that of ZIS6.…”

Noble-metal-free CuS/CdS composites for photocatalytic H2 evolution and its photogenerated charge transfer properties

“…The positive photoelectric signal in 300e518 nm corresponds to band-to-band transition of CdS, which indicates that photoinduced electrons move toward the bulk and photoinduced holes move toward the surface [29]. It is a typical behavior for the surface photovoltage in n-type semiconductor where positive charges migrate to the surface in the surface space-charge region [37,38]. The negative photoelectric signal in 300e518 nm corresponds to the sub-band transition of CdS.…”

Enhanced Separation Efficiency of PtNix/g‐C3N4 for Photocatalytic Hydrogen Production

“…Furthermore, TPV measurements were employed to investigate the dynamic property of the photogenerated charge carriers at 355 nm laser wavelength on a logarithmic time scale (Figure ) . It is well known that two peaks exist for the TPV response; the first time scale at approximately 10 −6 –10 −7 s (P1 peak) reflects the separation process, and the second peak at approximately 10 −4 s (P2 peak) gives the time scale of the diffusion course . A positive photovoltage intensity can be seen for the pure g‐C 3 N 4 , which is associated with the result of SPV.…”