Downscaling the Sample Thickness to Sub-Micrometers by Employing Organic Photovoltaic Materials as a Charge-Generation Layer in the Time-of-Flight Measurement

Abstract

Time-of-flight (TOF) measurements typically require a sample thickness of several micrometers for determining the carrier mobility, thus rendering the applicability inefficient and unreliable because the sample thicknesses are orders of magnitude higher than those in real optoelectronic devices. Here, we use subphthalocyanine (SubPc):C70 as a charge-generation layer (CGL) in the TOF measurement and a commonly hole-transporting layer, N,N’-diphenyl-N,N’-bis(1,1’-biphenyl)-4,4’-diamine (NPB), as a standard material under test. When the NPB thickness is reduced from 2 to 0.3 μm and with a thin 10-nm CGL, the hole transient signal still shows non-dispersive properties under various applied fields, and thus the hole mobility is determined accordingly. Only 1-μm NPB is required for determining the electron mobility by using the proposed CGL. Both the thicknesses are the thinnest value reported to data. In addition, the flexibility of fabrication process of small molecules can deposit the proposed CGL underneath and atop the material under test. Therefore, this technique is applicable to small-molecule and polymeric materials. We also propose a new approach to design the TOF sample using an optical simulation. These results strongly demonstrate that the proposed technique is valuable tool in determining the carrier mobility and may spur additional research in this field.

Electron mobility of non-fullerene acceptors using a time of flight method

“…11 This technique has several benefits such as selective charge polarity, sensitivity to charge transport perturbations, 12 and avoids the necessity of Ohmic contacts required by methods such as space-charge limited current (SCLC). 13,14 The mobility of the charges is measured perpendicular to the substrate and is therefore more relevant to the charge transport behavior in working OPV devices, in contrast to the in-plane mobility measured by the field-effect transistor (OFET) method. In TOF a thick layer of semiconductor is sandwiched between two electrodes and a pulse of light is used to excite a thin layer of excitons.…”

Determining Out-of-Plane Hole Mobility in CuSCN via the Time-of-Flight Technique To Elucidate Its Function in Perovskite Solar Cells

“…CuSCN exhibits an absorption onset of 320 nm and therefore does not absorb the laser used for ToF measurements (355 nm wavelength), thus requiring the use of a sensitizer layer. Ideally, a small absorption depth and high charge-generation efficiency are needed for ToF . MAPbI 3 exhibits an absorption onset of ∼770 nm and serves as the source of photogenerated charge carriers.…”

“…Ideally, a small absorption depth and high charge-generation efficiency are needed for ToF. 43 MAPbI 3 exhibits an absorption onset of ∼770 nm and serves as the source of photogenerated charge carriers. Second, smoother films of CuSCN form on MAPbI 3 , reducing the thickness error during the final measurement.…”

On the Donor: Acceptor Features for Poly(3-hexylthiophene): TiO2 Quantum Dots Hybrid Materials Obtained via Water Vapor Flow Assisted Sol-Gel Growth

“…Frischknecht et al have shown that P3HT:TiO 2 hybrid solar cells present a relevant dependence of the photocurrent on the incident light wavelength, exhibiting a particularly strong photocurrent enhancement upon UV monochromatic illumination due to the filling of shallow traps that become donor sites with an n-doping effect improving the titania electron mobilities [ 21 ]. It has been demonstrated that the use of time-of-flight (TOF) measurements are quite appropriate for observing the effects of the molecular structures, trap states, scattering centers, and dispersivity on hole/electron carrier transport [ 22 , 23 , 24 , 25 , 26 ]. In one of the later reports, bulk heterojunctions are studied in thick (>1 μm) devices showing that electron transport occurs mainly by diffusion in the bulk of the active layer [ 24 ].…”