Written by Stuart Thomson, August 2, 2021

Due to the attractive properties of perovskites, halide perovskite solar cells have become a research hotspot. High carrier mobility, large absorption coefficient, adjustable band gap and long carrier diffusion length. One of the challenges of any solar cell design is how to effectively remove charge carriers from the device. To help extract charge, electron and hole extraction layers are often incorporated into the device stack.

One promising material that has been studied as a hole extraction layer is vertically aligned carbon nanotubes (VACNT). The solar cell stack containing VACNT as the hole extraction layer is shown in Figure 1. VACNT is grown in a “tower” grid pattern on top of the ITO electrode to achieve improved charge extraction while maintaining high light transmission through the ITO/VACNT.

Figure 1 (a): The structure of the solar cell device stack using VACNT as the hole extraction layer and (b) the SEM image of the VACNT grown on the ITO substrate. Picture adapted from Ferguson et al.1

Photoluminescence (PL) is proportional to the number of charge carriers in the perovskite and is therefore sensitive to charge transfer to adjacent layers. This makes PL-based techniques very important for studying the performance of new extraction layers. In this application note, the hole transfer to the VACNT-based hole extraction layer is imaged using a steady-state and time-resolved confocal PL microscope and an Edinburgh Instruments RMS1000 confocal Raman and PL microscope.

Photothermal chemical vapor deposition (PTCVD) was used to grow a series of VACNT towers on an ITO-coated glass substrate, and a layer of mixed halide Cs0.05FA0.79MA0.16PbI2.4Br0.6 perovskite was spin-coated on the top. The full details of sample manufacturing can be found in previous publications. 1 Use the double-sided tape to mount the substrate on the microscope slide, and then fix it to the motorized stage of the RMS1000 confocal Raman and PL microscope. RMS1000 is equipped with 532 nm CW laser for spectrum acquisition, EPL-450 picosecond pulsed diode laser for time resolution, 600 gr/mm diffraction grating, back-illuminated CCD camera, time-correlated single photon counting (TCSPC) lifetime electronics Equipment and high-speed PMT life detector.

Figure 2: RMS1000 confocal Raman and PL microscope

In order to observe the transfer of holes from the perovskite to the VACNT tower, the surface of the ITO/VACNT/perovskite sample was imaged using PL intensity mapping. The reflected dark field image of the 500 μm x 500 μm area to be mapped is shown in Figure 3a. The VACNT tower array can be seen below the perovskite layer. Use 532 nm laser for light excitation and use 600 gr/mm diffraction grating and RMS1000 CCD camera to record the PL spectrum of each point to obtain a 100x100 point (5 μm resolution) PL map. Calculate the integrated intensity of each PL spectrum to create the PL intensity map shown in Figure 3b.

Figure 3: ITO/VACANT/perovskite surface imaged using (a) wide-field dark-field illumination, (b) confocal PL intensity mapping. The extracted spectra at points 1 and 2 of the PL graph are shown in figure (c).

The PL graph shows that the PL intensity at the top of the VACNT tower is reduced, which indicates the transfer of holes from the perovskite to the VACNT. In order for PL to occur, the photogenerated electrons and holes in the perovskite must recombine. The transfer of holes to VACNT will inhibit the electron-hole recombination in the perovskite layer and reduce the PL. The PL spectra extracted from the regions with and without VACNT are shown in Figure 3c, where the decrease in intensity and the change in the shape of the spectrum can be seen.

The PL intensity map provides strong evidence of hole transfer to the VACNT tower. However, this is not the only potential reason for the decrease in PL intensity. For example, a thinner perovskite layer deposited on top of VACNT will show a similar response. The RMS1000 can be equipped with a pulsed laser source and time-correlated single photon counting (TCSPC) electronics for PL life mapping. This provides supplementary information for the spectral PL mapping and can be used as a confirmatory measurement of the occurrence of hole transfer.

In order to obtain a PL life map, a 450 nm pulsed diode laser (EPL-450) was used to lightly excite the sample, and a high-speed PMT life detector of TCSPC and RMS1000 was used to measure the PL life. A 60 x 60 PL attenuation was obtained on the 150 μm x 150 μm area of ​​the sample to provide a life map with a resolution of 2.5 μm. Use the Ramacle® software of RMS1000 to fit the PL attenuation to the three-component exponential model (Equation 1-top), and calculate the strength-weighted average life of the component (Equation 2-bottom) to create the life diagram shown in Figure 4a .

The PL life chart shows that when the VACNT is under the perovskite, the average PL life of the perovskite decreases from ~100 ns to ~60 ns. Figure 4b shows examples of PL attenuation at locations with and without VACNT, highlighting the multi-exponential nature of the required fit. The reduction in the average lifespan of the VACNT tower top provides supporting evidence that hole transfer to VACNT does occur, because hole transfer is an additional way of rapid population reduction that will shorten the PL lifespan.

Figure 4: PL lifetime mapping of perovskite samples. (a) The average PL lifetime map of the perovskite surface and (b) the extracted PL attenuation (points) and three-component exponential fitting (solid line).

The RMS1000 confocal Raman and PL microscope were used to study the charge extraction characteristics of the VACNT hole extraction layer of the perovskite solar cell. RMS1000 can obtain the spectrum and lifetime of semiconductor samples, and use a combination of two PL mapping modes to confirm the transfer of holes to VACNT. Confocal PL microscopy is an ideal tool for visualizing charge extraction on the microscopic scale of perovskite solar cells to continuously optimize cell performance.

Written by Stuart Thomson, Senior Application Scientist, Edinburgh Instruments

We thank Dr. Victoria Ferguson from the Nanoelectronics Group at the University of Surrey for providing the perovskite samples used in this application note.

Tags: perovskite, photoluminescence mapping, VACNT, solar, spectroscopy, confocal, imaging, microscopy, Raman

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