Chapter 2. Background and Literature Survey
3.4. Photovoltaic Performance
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photoactive materials, than AgNWs. The reason for this is that the PFN interlayer is located on the surface of the AgNWs, and the work function can be turned on by charge alignment from the photoactive due to the permanent interface dipole formation. Also, it was confirmed that ZnO used as a typical n-type material has a value of 4.31 eV like PFN/AgNWs film together with AgNWs.
Figure 3.7. Ultra-violet photoelectron spectroscopy (UPS) spectra of AgNWs (black line), ZnO/AgNWs (red line), and PFN/AgNWs (blue line)
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Figure 3.8. J-V curves of IOSCs fabricated on PFN/ITO/glass or PFN/AgNWs/PET.
However, in our structure without annealing treatment, there is a unique advantage that it is possible to save energy by forming a PFN interlayer only from a simplest spin casting process without considering heat-problem. Comparing PET/AgNWs/PFN film based IOSCs with glass/ITO/PFN film based IOSCs within the 300-800 nm range of the EQE curves, I can see a photo-response gap of 12.3%
at 670 nm, which is similar to the JSC of J-V characteristics. It can be seen clearly that the result of the tendency is shown.
Figure 3.9. External quantum efficiency (EQE) spectra of IOSCs fabricated on PFN/ITO/glass or PFN/AgNWs/PET.
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Figure 3.10. Measured changes in the resistances of the electrodes based on ITO/PET and PFN/AgNWs/PET films as a function of the bending radius. Inset shows an optical image of the PET/ITO film after bending with a radius of 4.5 mm.
Figure 3.10 shows the resistance change of electrodes using commercially available ITO and PFN/AgNWs films on PET from arbitrarily defined bending radius. The bending test to confirm the compressive stresses of films can be expressed as a percentage of the resistance change of flexible TCEs.
The value can be expressed as ΔΩ · Ω0-1 for two factors, ΔΩ, which is the change after the actual bending, and initial resistance, Ω0. The PFN/AgNWs films showed high mechanical flexibility with slight changes and were able to obtain ΔΩ · Ω0-1 value (~ 7.51% ± 2.3%) at R 0.43 mm, which is the bending radius of the extreme, because of amorphous ZnO films/AgNWs. On the other hand, the commercial ITO increased sharply from R 4.8 mm and showed ΔΩ · Ω0-1 value (~ 90.24% ± 1.4%) at R 0.43 mm, confirming the comparative evaluation with PFN/AgNWs films. Furthermore, Figure 3.11 shows the actual state of a flexible device made of solar cells using PFN/AgNWs on a PET substrate. In Figure 3.12, the various characteristics of the AgNWs-based IOSC are compared with the ITO-based COSC at different bending radii. The most noticeable change in bending radius is FF as shown in Figure 3.12 (a). ITO-based COSC FFs decrease rapidly with increasing bend radius, while PFN/AgNW-based IOSCs exhibit only a relatively slightly decrease in FF. It allows the crystal structure of the ITO film to mitigate the mechanical stress during bending test by the generation of cracks and defects. However, as a result of the fact that the film is divided into several pieces, resulting in high contact resistance to electron movement, the resistance of the ITO electrode steady increases with the bending test, which in turn increases the series resistance and reduces the FF inside the device. AgNWs,
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on the other hand, exhibits the excellent performance of stress relief with little change in resistance even at low bending radii. The excellent bending stability of the IOSC based on PFN/AgNWs allows the original FF to be maintained. Like FF, the JSC of the ITO-based COSC is inversely proportional to the change in the bending radius and shows a reduction of about 34% at R 1.5 mm compared to the initial value. (Figure 3.12 (b))
Figure 3.11. Practically implemented flexible solar cells.
Conversely, the JSC of the PFN/AgNWs based IOSC does not change up to bending radius R 4.8 mm, but only 7% at R 1.5 mm compared to the initial value. The reduction is lower than ITO-based COSC, but at R ≤ 1.5 mm there is more change than FF reduction (3%). This change is considered to be a problem at the anode (Ag metal) of the IOCS, rather than the instability of the cathode (PFN/AgNWs). It is because the separation of electron-hole pairs generally does not occur properly due to the high exciton binding energy in the active layer. The destruction of the anode electrode by extreme bending generates the independent part. (Figure 3.13) That is, all carriers built in this separate region are degraded in JSC because the anode cannot collect smoothly. The VOC of the IOSC is not significantly affected by the bend radius, as shown in Figure 3.12 (c). Though cracks occur in the ITO electrode or Ag electrode, it is considered that the influence of the electrodes is relatively less because the inside of the photoactive layer heavily influences it. Figure 3.12 (d) shows the efficiency variation of ITO-based COSC and PFN/AgNWs based IOCS with various bending radii and shows that the bending radius maintains 96% of efficiency below R ≤ 5 mm. At an extreme bending radius of R ≤ 1.5 mm, it still retains 90% of efficiency. This reduction in PCE may be due to an increase in resistance due to cracking of the Ag electrode, which is also a significant motive for the decrease in JSC and FF. Consequently, it is not a variation of PFN/AgNWs, but an increase in the area where carriers cannot be collected. With the need to find a way to protect the metal film anode better, it is now under study to develop a very flexible and stable anode structure. Furthermore, I have found excellent bending property from changing PCEs of PET/AgNWs/PFN based IOSCs that are repeatedly bent at the R 4.8 mm radius.
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(Figure 3.14) Even after repeating 1000 cycles, the PCE maintained at ~93.8% is very close to the initial bending result of ~96.2%.
Table 3.2 Average performance parameters of IOSCs under AM 1.5G illumination (100 mW/cm2).
Values in the brackets represent the champion device.
JSC [mA/cm2] VOC [V] FF [%] PCE (best) [%]
Glass/ITO/PFN 16.5 0.753 66.6 8.22 ± 0.11 (8.27)
PET/AgNWs/PFN 12.8 0.740 65.3 6.13 ± 0.09 (6.17)
Figure 3.12. Parameter values for flexible conventional organic solar cells based on ITO/PET film and IOSCs based on PFN/AgNWs/PET as a function of their bending radius during compressive bending.
Figure 3.13. Silver film anode before and after bending.
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Figure 3.14. Change in efficiency by bending cycles of the radius of 4.8 mm.