Understanding Thermal Admittance Spectroscopy in Low-Mobility Semiconductors

Abstract

Thermal admittance spectroscopy (TAS) is frequently used to analyze the properties of trap states in semiconductor devices. We perform detailed simulations in combination with experiments to understand the effect of low carrier mobility on the analysis of trap states by TAS. We show that the apparent characteristic peak in the differential capacitance spectra is strongly dominated by the dielectric relaxation (DR) peak caused by low carrier mobilities for the case of shallow traps and low trap densities. The model for the DR dominated case is successfully applied to interpret the experimental results from poly(3-hexylthiophene-2,5diyl) (P3HT) based diodes. In contrast, for deep states with high density of states, we are able to properly estimate the energetic position, but the low carrier mobility affects the correct determination of the attempt-to-escape frequency as well as the capture cross section. Our results reveal that low carrier mobilities cause inherent obstacles in accurately determining the trap properties and thereby affect the analysis of the origin and nature of the trap states by admittance spectroscopy.

Reverse dark current in organic photodetectors and the major role of traps as source of noise

“…On the one hand, this enables the characterization of defects by a straightforward technique, such as IS. On the other hand, such a technique probes many different effects in the device, especially in an organic one, which can show identical capacitive spectra to those generated by traps, inspiring discussion in the community whether such a measurement can in fact reveal the trap characteristics 45 – 47 . Special caution should be taken when using this method in low-mobility materials and in devices where energetic transport barriers are present.…”

Utilization of Temperature-Sweeping Capacitive Techniques to Evaluate Band Gap Defect Densities in Photovoltaic Perovskites

“…). 3,[56][57]61 Last but not least, we remark that the apparent DOS peaks do not collapse in a unique curve in Figure 2b  and normalized to DOS units as Equation 4. This makes feasible for future works to compare results with other ways of obtaining DOS like thermally stimulated corrents (TSC) and optical techniques as photoluminescence decay.…”

“…The capacitive characterization procedures of trap levels within the band-gap in active semiconductors composing photovoltaic cells are well-established. [1][2][3] These techniques allow extracting relevant information about defect levels influencing solar cell operation and can be used to understand and optimize devices. Consequently, they have been recently applied to elucidate defect densities and related parameters as the activation energy in perovskite solar cells (PSCs), [4][5][6][7] which is maybe the most attractive photovoltaic technology during the last few years due to its high efficiency reports and easy manufacturing processes.…”

π-Conjugated small molecules enable efficient perovskite growth and charge-extraction for high-performance photovoltaic devices

“…As displayed in Fig. 5d, the PTAA:NPB-based device shows almost a one order of magnitude lower trap density compared to the reference device, which symbolizes that NPB doping in PTAA enormously reduces the overall trap states located above the valence band, and thus enhances VOC and FF [64,65].…”