Organic Solar Cells - Characterizing Photo- and Electroluminescence Using Fluorescence Spectrometry

Solar energy can be used converted to electricity using organic solar cells, which are known for their adjustable chemical configuration and are ease of processing when producing large area photovoltaic panels. Organic/polymer blends often use organic semiconductors with a band-gap from 1.4 to 3 eV so these materials tend to absorb only a small part of the solar spectrum.

This restraint can be overcome by stacking cells of varied band-gaps together. However, to improve the efficiency of solar cells a better knowledge of their diffusion dynamics is required. Other factors that have limited the development of highly efficient organic solar cells are their poor stability, which persists even after the device is covered to protect it from oxidation.

In this article, steady-state and time-resolved spectroscopy is used to characterize the well-researched organic solar cell blend Poly(3-hexyl)thiophene-2,5-diyl: [6,6]-phenyl C61 butyric acid methyl ester (P3HT: PCBM).

Methods and Materials

A FLS980 fluorescence spectrometer fitted with double excitation and emission monochromators and a 450W Xe lamp was used to measure the sample's emission, photoluminescence excitation, and electroluminescence spectra. At the excitation and emission arms gratings of 250 nm and 750 nm were used respectively. Long wave-pass filters included in the FLS980 spectrometer were used to filter higher diffraction orders. Hamamatsu’s R928P photomultiplier tube detector and a R5509-72 NIR-PMT from Hamamatsu were employed.

A EPL-445, 438.40 nm, 5 mW picosecond pulsed diode laser and a Ti-Sapphire laser of Coherent Verdi G10, 10 W and Mira 900, 200 fs, 76M Hz were used for time-resolved measurements. The output from the Ti-Sapphire was passed through a pulse picker which led to a repetition rate of 4.75 MHz and this was followed by increasing the frequency two-fold to deliver an excitation at 445 nm. Hamamatsu’s R38094-50 micro-channel PMT detector was used to detect the fluorescence emission. The three ports available in the FLS980 spectrometer were fitted with all three detectors simultaneously.

Poly(3-hexyl)thiophene-2,5-diyl (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) were dissolved in a ratio of 1:15 in 17 mg/ml solution of o-dichlorobenzene (Sigma-Aldrich, 240598) and stirred for a period of 12 hours at a temperature of 50 °C. This type of solvent was used because of its high boiling point at 180 °C, to support structural ordering on drying and also to further increase the charge transport.

Spin coating was carried out on the solution on quartz disks for at 1000 rpm for a period of 30 seconds, and the solution was left to cool in a culture dish for 2 hours. This was subsequently annealed at a temperature of 150°C. The coated films were covered with UV-cured epoxy on a quartz slide to prevent exposure to atmospheric air. The process of solution preparation, spin coating, sample annealing and encapsulation were performed within a glove box containing 0.1 ppm H2O and O2.

Results – Discussion

The normalized excitation and emission spectra of the air-exposed, encapsulated P3HT: PCBM solution samples are shown in Figure 1. It was observed that the emission spectra before and after encapsulation are similar, showing peaks at 720 nm. The air-exposed sample tracked at 720 nm had excitation spectra, displaying the main peak at 600 nm, a shoulder peak at 450 nm and extra peaks at 220 nm and 280 nm for the covered sample arising from the epoxy used. It was seen that the excitation spectra is in agreement with the absorbance spectra with absorption from PCBM at 200-400 nm and absorption from P3HT at 400-600 nm.

Excitation and emission spectra of air-exposed and encapsulated P3HT: PCBM solar cells.

Figure 1. Excitation and emission spectra of air-exposed and encapsulated P3HT: PCBM solar cells.

Figure 2 illustrates the fluorescence decays of the P3HT: PCBM samples. The decay of the sample that was exposed to air was fitted in an exponential of lifetime 46.84 ps which is much lower than that of the encapsulated sample (647.20ps). The lifetime of the covered sample is in agreement with samples of pure P3HT which implies that in this sample the polymer was not quenched by PCBM.

Fluorescence decays of air-exposed and encapsulated P3HT: PCBM solar cells

Figure 2. Fluorescence decays of air-exposed and encapsulated P3HT: PCBM solar cells. The inset displays a photo of the encapsulated solar cell.

Figure 3 displays the normalized electroluminescence (EL) spectra of the encapsulated sample at varied driving currents. A blue-shift of the spectrum is detected for increasing injection current, linked to an augmented population of higher energy states.

Electroluminescence spectra of encapsulated P3HT: PCBM device

Figure 3. Electroluminescence spectra of encapsulated P3HT: PCBM device. In the inset, a photo of the encapsulated device is shown

Conclusion

The characterization of P3HT: PCBM solar cells was performed using photoluminescence and electroluminescence spectroscopy. The photoluminescence excitation spectra are found to agree with the absorption spectra.

The quenching of the fullerene by the polymer was shown by making time-resolved measurements in the picosecond range. Such an approach is important to develop organic solar cells using novel materials. Further, a blue-shift of the electroluminescence spectrum is also seen for increasing injection current.

This information has been sourced, reviewed and adapted from materials provided by Edinburgh Instruments.

For more information on this source, please visit Edinburgh Instruments.

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