The WITec system

The WITec alpha300 S system was used to perform reflectance mappings and photocurrent mappings on a silicon thin film solar cell (Section 4.1), as well as PL mappings and time-dependent PL intensity measurements on CH3NH3PbI3 perovskite films (Chapter 5).

The WITec alpha300 S system combines the advantages of Scanning Near-field Optical Microscopy (SNOM), Confocal Microscopy and Atomic Force Microscopy (AFM) in a single instrument. It comes with a single photon-counting Si avalanche photodiode (APD) detector. The highly-linear, piezo-driven, and feedback-controlled scan stage

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provides high accuracy and stability and eliminates the hysteresis, creep and non- linearity that may arise from multiple scanning. The objective lenses used in this system are an Olympus MPlanFL N 10× and an MPlanFL N 100×. The focus was adjusted to give the highest PL signal, rather than the smallest spot size during the measurements. Thus, the focus was adjusted in the same way when measuring the laser spot (laser spot profiles are shown in Figure 3.4).

Figure 3.4 Cross-section plots of the Gaussian beam captured by CCD camera using (a) Olympus MPlanFL N 10× objective lens and (b) Olympus MPlanFL N 100× objective lens.

37 There are two laser sources in this lab: a Fianium Whitelase Supercontinuum laser (Model SC400-2) and a continuous wave 532 nm diode laser. An Acousto-Optic Tunable Filter (AOTF) is connected to the output of Supercontinuum laser to select up to 8 simultaneous tunable wavelengths from the laser source. The AOTF crystals in this lab cover the spectral range from 400 nm to 1100 nm.

Figure 3.5 The schematic diagram of measurements using WITec alpha300 S system. Adapted from WITec alpha300 S user manual [97].

The experimental methods of reflectance mapping, photocurrent mapping, PL mapping and time-dependent PL measurement are described below.

Reflectance mapping

In reflectance mapping, the 100× objective lens is used to extract high resolution image. The reflected light is collected by the APD detector and then the signal is transmitted to a computer through the microscope controller to generate the image (shown in Figure 3.5). Figure 3.6 shows an example of the output reflectance image.

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Figure 3.6 Reflectance image of a Silicon thin film solar cell coated with silver nanoparticles extracted by WITec system (bright areas correspond to higher reflectance). The integration time of each pixel is 3 ms and there are 10 pixels per nanometre.

Photocurrent mapping

As demonstrated in Figure 3.5, white light generated by the Fianium Whitelase Supercontinuum laser source is filtered to a narrow band (~8-10nm) source by the AOTF. A square wave created by Function/Arbitrary Waveform Generator (model Agilent 33220A) is used to modulate the AOTF output (on/off) through a channel modulator which is connected to the corresponding channel of the AOTF output. At the scan stage, two needle contacts are connected to the anode and cathode of the solar cell. The photogenerated current is amplified by a Low-Noise Current Pre- amplifier (model SR570) and then fed into a DSP Lock-in Amplifier (model SR850). In the meantime, the function generator provides the reference signal to the lock-in amplifier for synchronising the photocurrent signal when the laser is turned on in order to remove noise signal. The modulation frequency of the function generator is 333.3 Hz (3 ms period) and the lock-in time constant is 30 µs. The output photocurrent image is shown in Figure 3.7.

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Figure 3.7 Photocurrent image of a Silicon thin film solar cell coated with silver nanoparticles extracted by WITec system (bright areas correspond to higher photocurrent). The integration time of each pixel is 3 ms and there are 10 pixels per nanometre.

Since the optical signal of the reflectance and the electrical signal of the photocurrent are detected independently, this system can collect the reflectance and photocurrent data during a single scan. As shown in Figure 3.6 and Figure 3.7, the same area of the silicon thin film solar cell is scanned, and the reflectance mapping and photocurrent mapping are generated simultaneously.

PL mapping

The configuration of PL mapping is similar to the reflectance mapping. However, a long-pass filter with a cut-off edge larger than the laser wavelength is inserted in front of the detector (Filter Holder in Figure 3.5) to block the reflected excitation light and to transmit the emitted photoluminescence. Figure 3.8 shows a PL image of a large crystal perovskite film.

Figure 3.8 PL image of a large crystal perovskite film extracted by WITec system. The integration time of each pixel is 1 ms and there are 10 pixels per nanometre.

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Time-dependent PL intensity

Due to the instability of perovskite films, the PL intensity changes with time. To quantify such phenomenon, time-dependent PL intensity is used to track the change of a single point. The real-time signal is sampled by an avalanche photodiode detector (APD) with millisecond resolution in the WITec system. The experimental result shown in Figure 3.9 illustrates the stability of perovskite films.

Figure 3.9 Time-dependent PL intensity of a perovskite film measured by WITec system. Laser wavelength is 532 nm and laser intensity is 150 W/cm2_.

The measurement demonstrated in Figure 3.9 can successfully trace the time- dependent change in PL for a single point. Thus, combining both PL mapping and time- dependent PL intensity measurement, a continuous scan across an area is employed to visualise the change of the scan area (Figure 3.10).

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Figure 3.10 Continuous PL mapping of the same area on a perovskite film. (a) is the first scan, (b) is the second scan, (c) is the third scan, and (d) is the fourth scan. Each image requires 2 min to be extracted. The integration time of each pixel is 1 ms and there are 10 pixels per nanometre.