Interface state contribution to the photovoltaic effect in organic phototransistors: Photocapacitance measurements and optical sensing

Abstract

We report the results of an investigation into the contribution that trapping in interface states makes to the photovoltaic effect observed in organic phototransistors. To isolate this effect from other processes that occur in the transistor structure when under illumination, we focus attention on the photo-response of metal-insulatorsemiconductor (MIS) capacitors-the core structure of transistors. The capacitors comprised poly(3-hexylthiophene), (P3HT), as the active semiconductor in combination with one of three insulators, namely, poly (amide-imide), (PAI), SU-8 photoresist and polysilsesquioxane (PSQ). Following initial characterization in the dark, the capacitor response was measured both during and after irradiation with light in the wavelength range 400-700 nm. Three different approaches were employed to study the photo-response, each providing a different insight into the processes occurring. Capacitance-voltage sweeps before, during and after illumination provided direct evidence supporting the view that the photovoltaic effect occurred as a result of electron trapping in interface states of density up to ∼2 × 10 12 cm −2 in the P3HT/PAI combination but lower for SU-8 and PSQ. The dynamic photo-response, in which device capacitance was held constant by changing the applied bias, showed a fast component related to optically induced photoconduction in the semiconductor and a slower component reflecting the dynamics of interface electron trapping. Finally, photo-induced capacitance changes occurring with constant applied voltage were used to demonstrate a simple 3 × 3 imaging array.

Photo-induced effects in organic thin film transistors based on dinaphtho [2,3-b:2′,3′-f] Thieno[3,2-b′] thiophene (DNTT)

“…One possibility is that in deriving equation 6 The results obtained after applying PBS and NBS during illumination (Figures 9-12) are readily explained as well in terms of interface charge trapping. Under illumination, the electron quasi Fermi level, EFe, rises in the bandgap [1,16] resulting in more interface electron traps becoming active. The differences in band bending at the interface cause EFe to rise closer to the lowest unoccupied molecular orbital (LUMO) of DNTT under PBS than NBS.…”

“…integrating OTFTs into large scale imaging arrays. Metal-insulator-semiconductor (MIS) capacitors, the core structure of OTFTs, have also been investigated as photodetectors but have been used mainly for obtaining specific information on optically induced processes occurring both in the semiconductor [15] and at the semiconductor-insulator interface [16].…”

“…Electrons trapped near the source electrode will reduce the injection barrier there, encouraging further hole injection into the semiconductor [3,4,14] resulting in a positive shift of the threshold voltage, VT. Electrons trapped in interface or insulator states will cause a positive shift, VFB, in the flat-band voltage resulting in an identical shift in VT which also increases the device current [6,9,12,16,[18][19][20][21][22]. Optically induced shifts in VT are identified with the so-called photovoltaic effect in OTFTs.…”

“…However, when a device is biased into saturation, |VD| > |VG -VT|, the electric field in the insulator at the drain end of the channel reverses, thus encouraging interface electron trapping while discouraging hole trapping there. Furthermore, under illumination the thermal equilibrium Fermi level, EF, splits into two quasi-Fermi levels, EFe for electrons and EFh for holes [1,16]. In the accumulation channel where the hole concentration is high, EFh will shift only slightly below EF.…”

“…In the accumulation channel where the hole concentration is high, EFh will shift only slightly below EF. Conversely, owing to the paucity of thermally generated electrons, EFe will rise significantly above EF so that electrons photo-generated within a diffusion length of the interface will experience a higher trapping probability than in the dark as interface trap states lying between EF and EFe become active [16]. It is possible, therefore, for photo-generated electrons to become trapped in interface states even when the device is turned on and the electrostatic conditions appear unfavourable.…”