Density and mobility effects of the majority carriers in organic semiconductors under light excitation

Abstract

This study demonstrates that the effect of light excitation on the density and the mobility of the majority carriers can be explored in organic semiconductors by modulated photocurrent spectroscopy. The spectra of phase and amplitude of the modulated photocurrent of pentacene films indicate a significant increase in the density of the photogenerated mobile holes (majority carriers). This increase is accompanied by a comparatively much smaller increase of the steady state photocurrent response which can be reconciled with a decrease in the mobility (μ) of holes. The decrease of μ is supported from an unusual increase of the Y/μ ratio of the out-of-phase modulated photocurrent (Y) signal to the mobility under light excitation. It is proposed that the mobile holes, which are generated from the dissociation of the light-created excitons more likely near the pentacene-substrate interface by electron trapping, populate grain boundaries charging them and producing a downward band bending. As a result, potential energy barriers are build up which limit the transport of holes interacting through trapping-detrapping with deep partially occupied traps in the charged grain boundaries. On the other hand, the transport of holes interacting through trapping-detrapping with empty traps is found unaffected.

Organic Optoelectronic Materials: Mechanisms and Applications

“…Frequency-dependent photocurrent under cw illumination, measured using lock-in detection, enables measurements of low-level signals and introduces another tunable parameter, the frequency (typically in the 0.001–10 kHz range), in the system. In these experiments, the lock-in amplifier is locked either to a chopper (or other modulator, such as an acousto-optic modulator) frequency when the light intensity is modulated or to a frequency of an ac electric field applied to the sample. , For example, the latter measurement was employed to obtain trap distributions, to establish a charge photogeneration mechanism, and to separately determine photoexcited carrier concentration and mobility . For a recent discussion of photocurrent spectroscopy and microscopy, see ref .…”

“…They were able to model the electric field dependence and light intensity dependence of photocurrent dynamics with one set of parameters and quantified changes in charge generation and transport characteristics depending on the acceptor and on the film morphology and crystallinity. , In the subsequent study, Paudel et al used a similar approach to establish that ultrafast charge generation efficiency (η SSC in Table ) in ADT-based small-molecule D:A blends was promoted by crystallinity. In addition to the time domain, modeling of frequency-dependent (photo)­conductivity has been carried out, providing information on trap DOS, , carrier concentration-dependent mobility, and recombination rates …”

“…For example, Nasrallah et al exposed diF TES-ADT FETs to ozone and observed oxidation via quinone and carboxyl formation; the former was found to serve as traps responsible for the FET hysteresis. Additionally, traps can be induced by bias stress, photoexcitation, or thermal annealing. ,,− In PR polymer composites that use a small concentration (<2 wt %) of a sensitizer (e.g., C 60 , PCBM, and TNFM; see Table ), the accumulation of sensitizer anions (e.g., C 60 – ) under illumination and bias has been directly linked to formation of hole traps . It is possible that traps of this origin also exist in OPVs with morphologies with small acceptor domains.…”

Band Bending and Trap Distribution along the Channel of Organic Field-Effect Transistors from Frequency-Resolved Scanning Photocurrent Microscopy

“…In the present work, FR-SPCM measurements are employed to elucidate the microscopic processes governing the frequency dependence of photocurrents in pentacene FETs. For the analysis and interpretation of these measurements, the frequency dependence of the photocurrents of organic two-terminal devices studied previously is considered very useful to interpret the present results [14,15]. To this end, a detailed comparison of the FR-SPCM with the photocurrent spectra of the two-terminal devices is made.…”

New insight into the modulated photocurrent technique using 2D full numerical simulations

“…It has been widely used to study disordered semiconductors such as hydrogenated amorphous silicon (a‐Si:H) but not only. The MPC has also been applied to a large variety of materials such as (i) lead sulfide (PbS) quantum dot arrays (), (ii) antimony sulfide (SbS) (), (iii) copper gallium selenium compounds (CGSe) (), (iv) phase change alloys like germanium telluride (GeTe) () or germanium tin telluride (GeSnTe) (), (v) cadmium telluride (CdTe) (), and (vi) organic semiconductors like pentacene films .…”