２８
２９ 4.2 Results and Discussion.
In this work, we used a vacuum-assisted NIL to produce the pattern of the active layer without heating. The schematic structure of OPVs that have a dual-patterned gold electrode is shown in Figure 4.1.
Figure 4.1｜Schematic image of OPV structure with dual patterned Au electrode.
The morphologies of the active layers were investigated. For comparison, AFM images and the heights of the flat, grating, and dual-patterned active layer are also shown in Figure. 4.2.
Figure 4.2｜AFM images and heights of flat (left), grating (middle) and dual pattern (right) of active
３０ layer.
Compared with the grating patterned active layer, the nano-holes and cones with grating pattern are clearly observed in the AFM image of the dual-patterned active layer. The dual pattern has a grating pattern with holes in various sizes and shapes. When the gold (Au) is subsequently evaporated on the nanostructured active layer, the Au layer interface will closely follow the surface profile of the active layer, hence the nano-patterned Au layer will be present.
The current density-voltage (J-V) characteristics and parameters of different patterned OPVs are shown in Figure 4.3 and Table 4.1
Figure 4.3｜J-V characteristics under air mass 1.5 illumination.
Table 4.1｜Device characteristics of OPVs based on flat, grating and dual pattern.
0.0 0.2 0.4 0.6 0.8
-20 -16 -12 -8 -4 0
Flat Pattern Grating Pattern Dual Pattern
Current density (mA cm-2 )
Voltage (V)
-2 -1 0 1 2
10-2 100 102
J (mA cm-2 )
Voltage (V)
OPV configuration Jsc(mA cm-2) Voc(V) FF (%) η (%) Flat Pattern 17.3±0.14 0.74±0.01 50.67±0.18 6.46±0.01 Grating Pattern 18.6±0.04 0.75±0.01 50.85±0.10 7.10±0.01 Dual Pattern 19.3±0.10 0.75±0.01 53.35±0.15 7.79±0.08
３１
In order to understand the mechanisms of the performance improvement of OPVs with the dual- patterned electrode, we first fabricated OPVs with a single grating electrode. We then investigated the effect of the dual pattern on the performance improvement. For Au-dual pattern OPVs, it was clearly seen that PCE obviously improved from 6.46 % (flat OPVs) to 7.79 % (dual OPVs), while the PCE of Au-grating-only OPVs is 7.10 % mainly due to the enhancement of the short circuit current density (Jsc) near 7.5 % and 11.6 % for the grating pattern and dual pattern, respectively. The improvement of Jsc from the dual pattern was more than twice that of the grating pattern. These parameters were achieved for five identical devices. To elucidate the light-trapping nature of our dual pattern, we extracted incident photon-to-electron conversion efficiency (IPCE) as shown in Figure 4.4. To further clarify our data, we calculated the enhancement factors of the extracted IPCE of the devices. The IPCEs of Au-dual-pattern OPVs and Au-grating-only OPVs showed a region of enhancement from 300 nm to 900 nm wavelength, which is the entire range of visible light. In addition, the intensity of IPCE enhancement of the Au-dual pattern OPVs was much higher than Au-grating-only OPVs. This indicates that more photons from light are converted to excitons by employing a dual pattern electrode, which contributes to the improvement of Jsc. Furthermore, the absorption characteristic of different active layer patterns with Au (30nm of thickness) (Figure 4.5a) and without Au (Figure 4.5b) showed the role of the Au pattern. To elucidate the light-trapping nature of our dual pattern, we also extracted the UV-Vis absorption spectrum of the active layers. Figure 4.5a shows the absorption of the active layer without the Au layer, showing the different patterns. It shows that the characteristics of the dual patterned active layer and the grating patterned active layer show similar characteristics. In addition, differences of absorptions between the patterned active layers and flat active layer were also insufficient to elucidate the IPCE characteristic. As enhancements of IPCEs are introduced from different patterns of Au electrodes, we evaporated a semitransparent Au layer. The thickness of the Au layer was 30 nm. The absorbance data of the active layer with a thickness of Au electrode of 30 nm, which clarifies the intensity improvement, are shown in Figure 4.5b. The figure shows that the clear enhancement of dual-patterned active layer compared to the grating active layer owing to plasmonic effects and scattering by Au layer. To understand the absorbance and its enhancements, we constructed theoretical models of the OPVs and calculated the extinction spectra using the discrete dipole approximation (DDA) method. We calculated the extinction spectra of flat and grating models and dual pattern effect models (Figure 4.6). Figure 4.6a clearly shows that the grating enhances extinction spectra at a wide range of wavelength (450nm~900nm). Most high values of enhancements appeared at around the 535nm wavelength. Figure 4.6b shows the extinction spectra and their enhancement between dual pattern and non-dual pattern effect model. Compared with the non-dual pattern, the dual pattern showed significantly improved average extinction spectra based on three
３２
types of dual patterns. Especially, the highest value of extinction enhancement by dual pattern is near 16% at a 565nm wavelength. This is a similar phenomenon to that where dual pattern enhances the UV-vis absorption spectra at a wide range of wavelength. Also, most high values of extinction enhancements were appeared around 500 ~ 650nm wavelength in dual pattern compared with grating pattern. Therefore, we can predict that the wavelength enhances the extinction spectra using a dual pattern effect model. Electric field contour plots of flat model (Figure 4.7a), grating model (Figure 4.7b) and dual pattern effect model (Figure 4.7c) are also shown. In contrast to the electric field of the flat model, the grating model showed high enhancement at the interface between Au and PBDTTT-C-T:PC[70]BM. This means that the grating pattern increases the electric field intensity of the interface and enhances the extinction spectra for OPVs efficiency.
Figure 4.4｜EQEs and its enhancement compare with flat, grating and dual pattern 300 400 500 600 700 800 900 0
20 40 60 80 100
Wavelength(nm)
Enhancement(%)
Flat Pattern Grating Pattern Dual Pattern
IPCE(%)
100 110 120 130
３３
Figure 4.5｜Absorbance and enhancement of flat, grating and dual patterned active layer without Au layer (a), and with 30nm thickness of Au layer (b).
500 600 700 800 900
0.5 0.6 0.7 0.8
Wavelength (nm)
Absorbance
95 100 105 110 115 120 125 A
bsorbance Enhancement (%)
a
b
500 600 700 800 900
0.0 0.1 0.2 0.3
Wavelength (nm)
Absorbance
85 90 95 100 105 110 115 120 125
Flat Pattern Grating Pattern Dual Pattern
３４
Figure 4.6｜simulated extinction spectra of grating and flat models with 80nm thickness Au layer (a), and dual pattern and non-dual pattern effect models (b).
a
b
500 600 700 800 900
0.5 1.0 1.5 2.0
Flat Dual Pattern
Wavelength (nm)
Extinction
100 104 108 112 116
Enhancement (%)
500 600 700 800 900
1.75 1.80 1.85 1.90 1.95 2.00
Wavelength (nm)
Extinction
100 101 102
Enhancement (%)
３５
Figure 4.7｜Electric field contour plot of flat (a) and grating (b) model at 715nm wavelength.
Electric field contour plot of dual(c) model at 565nm wavelength.
３６ 4.3 Conclusion
In summary, we demonstrate high-performance OPVs using dual patterned gold electrode as the back reflector. The change of electrode pattern leads to the enhancement of electric field distribution, giving rise to significant differences in light absorption and scattering within the device architecture.
The incorporation of dual pattern based OPV remarkably improved device performance in PCE of 7.79 % via additional light absorption and scattering effect over the broad range of 400 to 750 nm.
This is the first report on introducing a dual pattern in OPVs for enhancing the device.
３７
Chapter 4. Surface plasmon resonance of dual patterned gold back reflector electrode:
versatility in organic photodiodes 4.1 Experimental
Fabrication of OPDs
The photodiodes were fabricated with the structure of glass/ITO/PEDOT:PSS/P3HT:PC[60]BM /Al.
The ITO glass was cleaned by sequential ultrasonic treatment in detergent, deionized water, acetone, and isopropanol. The pre-cleaned ITO substrate was treated with UV-Ozone. After treatment, the poly(3,4-ethylenedioxythiophene):polystyrene sulfonic acid (PEDOT:PSS, Heraeus) was spin coated on ITO at 5000rpm for 40s and then dried at 140°C for 30min. Subsequently, the dichlorobenzene solution of poly(3-hexylthiophene) (P3HT, EM Index, Mw = 85000 g mol-1, ~95% regioregularity) and [6,6]-phenyl-C61 butyric acid methyl ester (PC[60]BM, EM Index) (1:1 w/w, polymer concentration of 10 mg/mL) was spin coated at 1000 rpm and then annealed at 140℃ for 10min. During the annealing process, 3D patterned PDMS was placed on the active layer with pressure (35000 Pa). After the annealing process, the substrate is cooled on the hot plate and the PDMS stamp is then lifted off from the substrate. After removal of the PDMS stamp, 100nm Al was thermally deposited on the photo active layer under a base pressure of 4×10-4 Pa. The area of the Al electrode defines the active area of the device as 11 mm2.
Characterization of OPDs
Current–voltage characteristics of the devices and the photo-responses upon on-and-off switching of light were measured in an N2-filled glove box using a Keithley 4200-SCS semiconductor parametric analyzer. Intensity of light is 461µW/cm2
３８ 4.2 Results and Discussion.
In this work, we used NIL to produce an active layer pattern with heating. The schematic structure of OPDs that has a dual patterned aluminum (Al) electrode is shown in Figure 5.1.
The performances of the OPDs were measured under 532 nm green light with an intensity of 461µWcm-2 and electrode width of 8.35mm2. Typical I-V curves of the photodiodes as different patterns of Al electrodes are shown in Figure 5.2a. To further clarify our data, we focused the I-V curve as a range of bias from 0V to 1V as shown in Figure. 5.2b
Figure 5.1｜Schematic image of OPD structure with dual patterned Al electrode.
３９
Figure 5.2｜(a) J-V characteristics under 532nm wavelength (461 µW cm-2), (b) magnified graph.
The figure clearly shows the enhancement of the photocurrent by employing dual-patterned Al electrode, while the effect of Al-grating-only OPD is small. At 1V, the photocurrent was improved from 37.5 μA (flat OPDs) to 40 μA (Al-dual pattern OPDs), while the Al-grating-only OPDs was 3.83 μA. Figure 5.3 presents the on-off photo-switching characteristics (at a bias of 1V with 30 sec duration) of OPDs.
0.0 0.2 0.4 0.6 0.8 1.0
1x1002 2x102 3.6x104 3.7x104 3.8x104 3.9x104 4x104
Light Off
Current (nA)
Bias (V)
Light On
-0.4 0.0 0.4 0.8 1.2 1.6 -4x104
-2x104 0 2x104 4x104
Current (nA)
Bias (V)
Flat Pattern Grating Pattern Dual Pattern
Light Off Light On
a
b
４０
Figure 5.3｜Photo-switching of OPDs with flat, grating and dual pattern.
It can be clearly seen that all the devices can be switched on and off repeatedly. The enhanced performance of the optimized photodiode was stable during the light on and off cycling process.
In optoelectronic devices, it is well known that light response is closely related to the responsivity (R, a parameter characterizing the sensitivity of the device to light). R is defined by the following equation (5.1).
light dark ill
I I
R P
= -
(5.1)
Where ILight is the current under the illumination, Idark is the dark current, and Pill is the incident illumination power on the effective area.
Except for R, the other two figures of merit for evaluating a photodiode are the signal to noise ratio (SNR) and External quantum efficiency (EQE), (a parameter characterizing the creation of an electron-hole pair on absorption of a photon)
0 100 200 300 400
2x10
24x10
23x10
44x10
45x10
4Grating Dual
Flat
Off
On
C u rr en t (n A )
Time (s)
Off
On
Off
On
Off Off
On
On
４１ SNR is defined by the following equation (5.2),
light dark dark
I I
SNR I
= -
(5.2)EQE is given by the following equation (5.3).
ph ill
EQE I hc
eP A l
=
(5.3)where h is the Planck constant, c is the speed of light, e is the charge of electron, A is the active area, and λ is the wavelength. Based on equations (1)-(3), we calculated R, SNR, and the EQE values of the photodiodes based on the non-pattern, grating pattern, and dual pattern at 1V bias, as shown in Figure 5.4. In EQEs, compared with flat and grating, it was clearly shown the enhancements by employing dual pattern as well as R. Furthermore, Au-dual-patterned OPDs showed a dramatically increased SNR of 574 compared with flat OPDs (225), while grating OPDs showed an SNR of 464. Moreover, the higher enhancements shown for all parameters by employing the dual pattern compared with the grating pattern showed that dual patterns can be utilized in OPDs by substituting conventional grating patterns. This clearly showed that employing dual–patterned Al electrodes led to higher enhancements than employing grating Al electrodes. We calculated the extinction spectra of flat and grating models (Figure 5.5a), and dual pattern effect models (Figure 5.5b). In Figure 5.5a, the grating shows slightly enhanced extinction spectra at a wide range of wavelength (450 ~ 900nm). Most high values of enhancements appeared at around 850 ~ 900nm wavelength. Figure 5.5b shows a comparison of the extinction spectra and their enhancement between dual pattern and non-dual pattern effect models.
The dual pattern, compared with the flat pattern, shows enhanced average extinction spectra based on three types of dual patterns. Especially, the highest value of extinction enhancement by dual pattern was near 2% at a 455nm wavelength, and most high values of extinction enhancements appeared at around a short range wavelength (~450nm). We can predict the enhancement should be improved at that wavelength if dual patterns are incised on the grating. Electric field contour plots of the flat model (Figure 5.6a), grating model (Figure 5.6b), and dual pattern effect model (Figure 5.6c) are also shown. The electric field of the grating model, compared with the flat model, showed high enhancement at the interface between Al and P3HT:PC[60]BM. Especially, the bottom of the grating pattern was more enhanced than the top. In the flat and grating models, the electric field of the Al
４２
layer did not spread over the entire layer due to the thickness of the Al layer. Also, the electric field of the dual pattern effect model showed strong enhancements at the interfaces of patterns.
Figure 5.4｜Device characteristics of OPDs under 532nm wavelength. (461 µW cm-2)
EQE, R (a), and SNR (b), of the OPDs with flat, grating, and dual patterned electrode (Averages EQE, R, and SNR) were achieved for five identical devices. (Error bars indicate standard deviations of devices.)
４３
Figure 5.5｜Simulated extinction spectra of grating and flat models with 100nm thickness Al layer (a), and dual pattern and non-dual pattern effect models (b).
.
500 600 700 800 900
1.995 1.996 1.997 1.998 1.999 2.000
Wavelength (nm)
Extinction
100.00 100.01 100.02 100.03 100.04 100.05 100.06 100.07 100.08
Enhancement (%)
500 600 700 800 900
1.76 1.78 1.80 1.82 1.84 1.86 1.88
Flat Dual Pattern
Wavelength (nm)
Extinction
100 101 102 103
Enhancement (%)
b
a
４４
Figure 5.6｜Electric field contour plot of flat (a) and grating (b) model and dual(c) model at 532nm wavelength.
４５ 5.3 Conclusion
In summary, we demonstrate high-performance OPDs using a dual patterned aluminum electrode as the back reflector. The change of electrode pattern leads to enhancement of electric field distribution, giving rise to significant differences in light absorption and scattering within the device architecture.
The incorporation of dual pattern based OPD, compared with the flat model, remarkably improved device performance in EQE (~20%) and SNR (~150%) via additional light absorption and scattering effect at 532 nm. This is the first report on introducing dual pattern in OPDs for enhancing an optoelectronic device.
４６