NCAFM2023 Programme Booklet

Tuesday 0940 - 1000

TIME RESOLVED KELVIN PROBE FORCE MICROSCOPY TO UNDERSTAND CHARGE DYNAMICS IN OPTOELECTRONIC MATERIALS AND DEVICES

Sascha Sadewasser, Nicoleta Nicoara, Marcel Claro

INL – International Iberian Nanotechnology Laboratory, 4715-330 Braga, Portugal Email: sascha.sadewasser@inl.int

Kelvin probe force microscopy (KPFM) has extensively been applied to develop understanding of nanoscale electronic properties in optoelectronic materials and devices. Gaining knowledge about the dynamics of electronic processes requires high temporal resolution, for which standard KPFM is usually considered to too slow. Nevertheless, several emerging time-resolved (TR) KPFM techniques have recently been developed that permit time resolution down to the picosecond range. Here, we apply illumination-modulated KPFM, where a sequence of light pulses is used to induce changes in the charge carrier distribution. The resulting modification of the contact potential difference (CPD), too fast for the KPFM controller to be followed directly, is measured as an average surface photovoltage (SPV), which can be analyzed to extract information about the charge carrier dynamics. Specifically, we show here two case studies. First, for a fast photodetector based on the two-dimensional (2D) material β -In 2 Se 3 , we measure TR-KPFM of bare β -In 2 Se 3 material, obtaining a fast exponential decay or the TR-SPV with a time constant of ~14 ms (Fig. a) [1]. The good agreement with the response time of a photodetector device (7 ms) suggests that the release of carriers from trap states in the β -In 2 Se 3 material governs the time scale, suggesting that photoconduction is the main mechanism, rather than photogating, which is often due to charge accumulation on defective interfaces and leads to response times in the range of seconds [2]. Furthermore, a sample with mixed β - and -In 2 Se 3 phases shows a significant variation (~130 mV) of the CPD between the phases. A statistical analysis of the SPV dependence with laser wavelength shows SPV values ranging from 50 to 275 mV. On the other hand, TR-KPFM shows a homogeneous distribution of relaxation times of ~25 ms, without significant differences between the β - and -In 2 Se 3 phases. Second, for the solar cell material CuInSe 2 , samples grown with copper excess (leading to poor solar cell performance) and without copper excess (leading to good performance) exhibit time constants of the TR-SPV decay of ~0.8 n s and ~6 n s, respectively (Fig. c) [3]. The shorter decay time of the Cu-rich material is interpreted as a signature of the faster recombination activity, while the lack of the full recovery of the dark CPD signal after the illumination sequence (Fig. b) reflects the existence of long-lived trap states. Both phenomena corroborate the poor performance of related solar cell devices [3].

Fig. (a) TR-KPFM on β -In 2 Se 3 showing a main relaxation time of ~14 ms. (b) TR-SPV on Cu-rich (blue) and Cu-poor (red) CuInSe2 solar cell material exhibiting different relaxation time constants.

References [1] M.S. Claro, …, S. Sadewasser, Adv. Opt. Mat., 2020, 9, 2001034. [2] M. Buscema, et al., Chem. Soc. Rev., 2015, 44 , 3691. [3] D. Colombara, …, S. Sadewasser, S. Siebentritt, Nat. Communications, 2020, 11 , 3634.

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