Transient Photocurrent Measurements

For further reviews of experimental techniques, see Dolezalek (1976), Seanor (1982), Joshi (1990), and Melnyk and Pai (1993). 

The standard method for measuring mobilities in materials with long dielectric relaxation times is the time-of-flight photocurrent technique, first described by Haynes and Shockley (1951) and Lawrance and Gibson (1952). This method was developed in considerable detail during the 1950s and 1960s by Brown 

A schematic of a typical time-of-flight arrangement is illustrated in Fig. 3. A sample, typically 10 to 50 Jim in thickness, is placed between two electrodes, 

During the past two decades, time-of-flight photocurrent measurements have become the conventional method for studies of transport phenomena in organic solids. The limitations of the technique are that it is difficult to measure mobilities in the presence of trapping and thfc need for relatively thick samples. 

---

Transient photocurrent measurements canlbe performed under a wide range of experimental conditions. The measurements can also be made under conditions such that the photocurrents are significantly less ithan the dark currents. The basic limitation of the technique, as with all techniques based on current measurements, is that it is difficult to determine pihotogeneration... 

Figure 42 Photogeneration efficiencies for a double-layer photoreceptor comprised of a MSQ generation layer determined by photoacoustic and transient photocurrent measurements. The transport layer was a hydrazone doped polymer.

In semiconductor device physics, transient photocurrent measurements are known as time-of-flight measurements and have been used to measure the transit time of... 

For the transient photocurrent measurements a Dye laser pumped by 500 ps pulsed Nitrogen laser was used for the optical excitation. The incident light was linearly polarized parallel and perpendicular to the stretch-direction. The bias field was applied parallel to the c-axis, therefore the photocurrent reflects the transport properties along the chains. Details of the photoconductivity apparatus can be found in Ref. 5. 

Li, Z. and McNeill, C.R. (2011) Transient photocurrent measurements of PCDTBT PC70BM and PCPDTBTPCyoBM solar cells evidence for charge trapping in efficient polymer/fullerene hlends./. Appl. Phys., 109, 074513. 

Figure 9.27 shows the transient photocurrent, measured at a photon energy of hv = 2.0 eV and at room temperature, in a C60 film for different levels of oxygen contamination [43]. Measurements are performed in a 0.5 pm thick film, with top coplanar Au electrodes, at an applied electric field of 10 V/cm, using laser pulses having a width of 20-30 ps and an intensity of 2—3 pJ. The overall temporal resolution of the measurements is about 50 ps. Each decay in Figure 9.27 consists of a short and a long-lived component. 

Figure 9.27. Transient photocurrent measured at T= 300 K and hv = 2.0 eV, in a film at different levels of oxygen contamination. The dark current /j and relaxation times x and T2 (see text) for each curve are as follows. Squares oxygen-ffee film, /d = 3.6 nA, t, =693 ps, T2 = 7.2 ns circles /a = 0.00567 nA, ti =640 ps, T2 = 5.0 ns triangles U = 0.00024 nA, T =463 ps, i2 = 2.2 ns diamonds < 0.00001 nA, T = 238 ps T2 = 1.0 ns. Inset shows the normalized transient photocurrent of oxygen-ffee C6o at different laser intensities (from top to bottom) 2.7 x lO", l.I x lO, and 5.4 x lO photons/cm. (Reproduced by permission of the American Physical Society from ref. 43.)...

Figure 9.28. Temperature dependence of the peak transient photocurrents measured at hv = 2.0 eV in oxygen-free 50 (circles), partially oxygen-exposed C o (triangles) and fully oxygenated 50 (squares). The inset compares the transient photoconductivity at the peak (circles) and at 2 ns after excitation (triangles). (Reproduced by permission of the American Physical Society from ref 43.)...

Figure 19 Experimental arrangement for the photoinduced transient photocurrents measurements.

---