Chapter 4. Transparent Conducting Electrode (TCE): Rational Design of AgNWs Cathode for
4.4. Photovoltaic Performance
41
Figure 4.14. AFM images of ZnO NPs film coated on the non-treated, the annealed, and the CIP-treated AgNW electrodes, respectively.
Table 4.1. Summary of physical properties of CIP-treated AgNW electrodes formed under different spin speed conditions.
Spin-speed [rpm]
Thickness [nm]
Sheet resistance [Ω/□]
Specular transmittance [%, at 550 nm]
Total transmittance [%, at 550 nm]
600_w/o 70.1. 49.3 91.6 94.3
600 55.9 20.7 91.5 94.8
800 35.3 28.4 93.6 96.1
1000 28.5 33.7 95.1 96.8
1200 23.8 42.6 95.1 97.4
42
Figure 4.16. The current-voltage (I-V) curves of the non-treated, annealed, and CIP-treated AgNW electrodes.
And the thermal annealed AgNWs have a higher resistance than the CIP electrode because the surface of the NWs can be oxidized during the heat treatment. The changes in the internal resistance are the FF of the directly manufactured IOSCs. The non-treated electrode and annealed electrode-based IOSCs showed the FF values of 53.5% ± 0.4% and 58.5% ± 1.5%, respectively, for the series resistance (Rseries) of 7.31 and 5.88 cm2, respectively (Table 4.3). However, the CIP electrode-based IOSCs showed a Rseries value of 4.98 cm2 and 63.5% ± 0.7% of FF. Figure 4.17 and Figure 4.18 show J-V curves and EQE spectra of three devices under AM 1.5G lighting conditions, respectively. Looking at the J-V curve, the IOSCs based on the CIP electrode show the JSC of 17.8 ± 0.2 mA cm-2, the VOC of 0.759 ± 0.016 V and a maximum PCE of 8.94%. This solar cell performance outperforms the other two devices. To demonstrate the applicability of the CIP electrodes in flexible IOSCs, the devices were fabricated on flexible PET substrates. Both devices on the glass substrate and the same structure on the PET substrate showed nearly identical results in the EQE spectra. However, PET-based devices have lower quantum efficiency than glass-based devices in the short wavelength range (300-350 nm) caused by the inherent UV absorption of PET. (Figure 4.19) Significant differences between the two devices have not been found about device performance, indicating that CIP-treated electrodes deposited on PET substrates will be ideal candidates for flexible devices.
43
Figure 4.17. The current-density-voltage (J-V) curves.
The performance of CIP-treated AgNW electrodes and heat-treated devices was investigated to confirm the superior flexibility properties of flexible IOSC using CIP-treated electrode base. (Figure 4.20) As shown in Figure 4.21, in both cases, the JSC and FF values start to change from a bend radius of R ≈ 6.8 mm. For the annealed electrodes, the JSC and FF values decreased to 5.0% and 4.4%, respectively, at an ultimate bending radius of R ≈ 1.5 mm. This slight reduction in JSC and FF values will be caused by imperfections in the mechanical stability of the AgNWs electrode. However, the CIP treated electrode showed a minimal decrease in JSC and FF values of 4.0% and 3.1%, respectively; It was considered that the AgNWs was well connected. Especially, the PCE of CIP-treated electrode-based devices decreased by only 2.4% in the ultimate bending radius of R ≈ 1.5 mm. Impressively, the CIP- based flexible IOSC efficiency change shows a stable value (95.5%) even after repeating 1,000 cycles for a bend test with R ≈ 4.8 mm. (Figure 4.22) The histogram shows the overall average efficiency and manufacturing yield of the device, as shown in Figure 4.23. Besides excellent flexibility, the CIP- treated electrode exhibited excellent reproducibility over the annealed electrode. Surprisingly, CIP- treated electrode-based devices showed a 100% fabrication yield and an average PCE of 8.58% without short circuit losses. This directly reflects the high reliability of the CIP electrode and shows much higher average efficiency and manufacturing yield than the annealed electrode-based device (75% yield, average PCE: 7.55%). Therefore, commercial use of a CIP electrode will not only aim to manufacture a highly efficient flexible IOSCs, but also provide an ideal platform in various related fields.
44
Figure 4.18. The EQE spectra of solar cells that used on glass and PET substrates.
Figure 4.19. The PET substrate shows a higher absorption than that of the glass substrate at a wavelength of 300-400 nm region.
45
Figure 4.20. Images of the bending test (from R ≈ initial to R ≈ 1.5 mm) of flexible IOSCs.
Figure 4.21. Measured efficiency parameter for flexible IOSCs based on annealed and CIP-treated AgNW electrodes as a function of their bending radius during compressive bending.
46
Figure 4.22. Change in normalized PCE with bending to a radius of 4.8 mm.
Figure 4.23. A histogram of comparison with the different PCEs extracted from the annealed and CIP- treated AgNW electrodes.
47 Table 4.2. Summary of recent progress in TCEs.
Electrode Deposition and treatment Electrical and
optical properties Flexibility Reference
SWCNT SDS-dispersed SWCNTs in water FoM = 25.3 No data Chem. Phys.
Lett., 200864 PEDOT:PSS Dropping H2SO4 solution on a
PEDOT:PSS film
T550nm = 87%
Rsh = 67 Ω sq−1
FoM = 39 No data Adv. Mater., 201265
P-welded AgNWs NWs spray-coated on plastic wrap by plasmonic welding;
T550nm = 80%
Rsh = 10 Ω sq−1 FoM = 159.6
No data Nat. Mater., 201266
Laser nano-welded AgNWs
Long NWs-collected on a Teflon filter to form a uniform film; then transferred to the target substrates by
applying uniform vacuum suction pressure; NWs thermal annealing (250 °C for 2 hours) or laser-welded
T550nm = 96%
Rsh = 186 Ω sq−1 FoM = 49.1
During 10,000 bending cycles (2 mm bending
radius)
Nanoscale, 201267
AgNWs/graphene NWs spin-coated; graphene hot- pressed at 500 psi and 130 °C
T550nm = 85.6%
Rsh = 13.2 Ω sq−1
FoM = 176.6 No data
ACS Appl.
Mater.
Interfaces, 201368 Joule heating-
welded AgNWs
Output direct current power supply with 25 V compliance voltage was used to apply current to the AgNWs
network
T550nm = 86.7%
Rsh = 19.7 Ω sq−1
FoM = 129.3 No data ACS Nano, 201469
AgNWs/Al2O3/ZnO
NWs Mayer rod coated and post- annealed 150 °C @ 3min, then dipped into DI water for 10 min and
again annealed at 165 °C @ 8min;
Al2O3 thin film via ALD @ 100 °C followed by ZnO via ALD @ 100 °C
T550nm = 87%
Rsh = 10 Ω sq−1 FoM = 250
Withstands 5000 bending cycles to 3 mm
radius
Adv. Funct.
Mater., 201570
AgNWs/MoOX
NWs bar-coated at 50 °C substrate;
MoOx spin -coated from ammonium heptamolybdate solution and
annealed at 100 °C
T550nm = 90%
Rsh = 29.8 Ω sq−1 FoM = 116.9
Tensile bending to 3 mm radius for
3000 cycles;
Rsh doubled after the 1st tape peeling test, low adhesion
Nanoscale, 201571
CIP-AgNWs NWs spin-coated on substrate and then NWs water pressure of 50 MPa
for 60 s by CIP equipment
T550nm = 94.8%
Rsh = 20.7 Ω sq−1 FoM = 336.5
During compressive bending (1.5 mm bending radius) and 1,000 bending cycles (4.8 mm bending radius)
Our work
48
Table 4.3. Device performance of IOSCs with different flexible TCEs.
Device Jsc [mA cm-2] Voc [V] FF [%] Rseries [Ω cm2] PCE (best) [%]
Non-treated
AgNWs on glass 16.0 ± 0.3 0.759 ± 0.012 53.5 ± 0.4 7.31 6.50(6.77) Annealed
AgNWs on glass 17.0 ± 0.3 0.760 ± 0.010 58.5 ± 1.5 5.88 7.55(7.99) CIP-AgNWs on
glass 17.8 ± 0.2 0.759 ± 0.016 63.5 ± 0.7 4.98 8.58(8.94) CIP-AgNWs on
PET 17.4 ± 0.5 0.764 ± 0.005 64.2 ± 0.5 4.13 8.56(8.75)