Light Induced Electron Paramagnetic Resonance

MEH-PPV

Light induced electron paramagnetic resonance measurements were performed on MEH-PPV to ensure that literature results could be replicated [80, 81].

The X-band CW light induced EPR signal is shown in Figure 6.20, and the pulsed spectra at X-band and Q-band are shown in Figure 6.21. The spectra show one line, which does not vary significantly with changing frequency, and no dark signal was seen. The temperature dependence of the MEH-PPV signal is shown in Figure 6.22. The characteristics of a single line, and decreasing signal intensity with the temperature increasing from 20 K to 110 K were also reported by Kuroda

Figure 6.20: CW light induced EPR signal of MEH-PPV.

Figure 6.22: Temperature dependence of CW light induced EPR signal of MEH- PPV.

MEH-PPV: PCBM

A blend of MEH-PPV to PCBM in the ratio 1: 4 was investigated. This ratio was chosen as it is used to fabricate solar cells. The dark, illuminated, and light induced spectra are shown in Figure 6.23. These spectra are averages of twenty scans.

Figure 6.24 shows the behaviour of the signals at 338.4 mT and 338.9 mT with time. Both signals rise to their maximum almost immediately after they are illuminated and decay away after the light has been removed. On the time scale studied, both signals show a persistent component after the light has ceased. Due to the smaller size of the low field peak and the incomplete separation of the two peaks with field, the uncertainties of the peak-to-peak height for the low field peak are quite significant.

Figure 6.25 presents the power dependence of the two signals. The low field peak height does not vary significantly with microwave power, whilst the high field

Figure 6.23: Dark, illuminated, and light induced spectra for MEH-PPV: PCBM.

Figure 6.24: Time dependence of the two signals generated in a blend of MEH-PPV and PCBM.

peak grows with power before beginning to saturate at around 0.5 mW.

Figure 6.25: Power dependence of the illuminated signal in a blend of MEH-PPV and PCBM.

By comparing theg values of the peaks in these spectra with previous spectra of MEH-PPV and also with spectra of other polymer blends with PCBM (presented in Chapter 8), the low field peak can be attributed to MEH-PPV and the high field peak to PCBM. The time dependence of the signals and the differences in power dependence between the two lines reproduce many of the trends seen in other polymer: PCBM blends [105].

the dark, illuminated, and light induced spectra are shown in Figure 6.26. At Q-band the lines due to MEH-PPV and PCBM were able to be separated, how- ever to resolve the g anisotropy in these materials W-band measurements may be necessary [106]. The low field line is due to a positive polaron on MEH-PPV, and the high field line is due to a radical anion on the PCBM.

Figure 6.26: Dark, illuminated, and light induced spectra of MEH-PPV: PCBM taken at Q-band.

The signal in the blend was much stronger than that from pure MEH-PPV, indicating that charge separation is occurring. This supports the quenching of the photoluminescence discussed in Section 6.2.1.

6.3

Electrically Detected Magnetic Resonance

Electrically detected magnetic resonance (EDMR) is an extremely interesting tech- nique whereby conductivity measurements on a macroscopic level are combined with EPR techniques which examine the microscopic properties of a material. In the past EDMR has been limited to CW measurements and it is only recently

that pulsed techniques (pEDMR) have been developed [90]. Using pEDMR it is possible to observe coherent spin propagation, which allows the measurement of properties such as coherence times [84].

In order to perform pEDMR measurements a device must be placed inside a microwave resonator where the magnetic fields are generated. The conducting contacts on the device need to be very carefully designed so that they do not alter the eigenmodes of the microwave cavity and hence cause the magnetic field to become inhomogeneous. If this occurs then observation of Rabi oscillations is impossible [107]. These constraints were taken into account when designing device structures for organic solar cells for pEDMR measurements.

Previous cwEDMR studies of organic semiconductor devices have used con- ventional device designs [88, 94] with contacting occurring very close to the active area of the device. This design is unsuitable for pEDMR experiments because the contacts would cause the magnetic field to be inhomogeneous.

The substrate dimensions chosen were 50 mm by ∼2.8 mm. The width was chosen so that the device would fit inside an X-band EPR tube with inner diameter of 3 mm. The length of the device was chosen so that the top of the device, where contacting occurred, was outside the resonator.

The device structure used was glass/ ITO/ PEDOT/ MEH-PPV or MEH-PPV: PCBM/ Al. This structure and its constituent layers are shown in Figure 6.27. The materials used are identical to those used in cwEDMR measurements by Schar- ber et al. [108] except for the choice of PPV derivative. However, unlike in the structures of Scharber et al. the contacts have thicknesses below the microwave penetration depth, and hence the eigenmodes of the cavity are not distorted. Fur- thermore, the use of thin strips of ITO and aluminium running up the sides of the glass substrate allows contacting well outside the resonance cavity whilst the active area of the device remains at the centre of the cavity.

The steps used in the device fabrication were based on those described in Section 5.3.1 for standard organic solar cells, but were adapted to the particular substrate size and device design. These steps will be briefly outlined below.

Figure 6.27: The fabrication steps for a device for EDMR. All dimensions are in millimetres. The top panel shows the ITO coated area of the glass, the second panel shows the area on which PEDOT and MEH-PPV are deposited, the third panel shows the area on which aluminium is evaporated, and the bottom panel shows the contacting of the com- pleted device. The cross hatched area shows the active area of the device.