Chapter 4 Effect of APG-Au Nanoparticles on Hole Injection
4.3. Results and Discussion
86
87
Table 4.1 WF of ITO with 10 pulses of APG
Substrate WF [eV]
ITO/AuNP w/o UV 4.55
ITO/AuNP with UV 5.44
ITO/HPS/AuNPs w/o UV 5.15
ITO/HPS/AuNPs with UV 5.25
10 nm
(a) 3 pulses
(b) 10 pulses
88
Fig. 4.3 TEM images of APG-Au with different pulses
The current density versus voltage (J-V) characteristics of the HODs is measured with the semiconductor parameter analyzer (Agilent Technologies E5270B). Fig. 4.4 shows the current density versus voltage characteristics of devices under 3 pulses with different UV-ozone time. In the HODs without HPS, the J-V curves were very unstable on the voltage scans (Fig. 4.4 (a)). The initial current level was almost comparable to that of the reference HOD with no surface modification. However, the current increased abruptly at the fifth scan and kept high during next some scans, then decreased. This sort of unstable behavior was observed commonly in the HODs without the HPS layer.
On the other hand, HODs with HPS showed different behavior. Since HPS itself is an insulating material, the HODs with HPS layer showed very low current level when the UV-ozone treatment time was too short (Fig. 4.4(b)).
The slope of the J-V curves was more than five indicating very high injection barrier (Fig. 4.4(c) and (d)). After the UV-ozone treatment for 40 min, the current increased almost three orders of magnitude (Fig. 4.4(e)). The slope was close to two, indicating the space charge limited current (SCLC) mechanism3 and the substantial decrease of the carrier injection barrier.
Since the WF of the modified ITO increased by the treatment and came close to the
89
highest occupied molecular orbital (HOMO) level of NPB, 5.40 eV4, it is consistent that the barrier height was decreased by the treatment as well as the removal of the insulating HPS layer by the UV-ozone treatment. The J-V characteristics in Fig. 4.4(e) were very similar to those of the HOD with HPS-Au prepared by chemical reduction shown in Fig. 2.4(a). In the first scan, the J-V curve showed abrupt increase and then kept high value with the slop of two. The abrupt increase might be caused by the removal of cation from nanoparticles by the external electric field. The negatively charged HPS-Au induces a positive image charge accumulating at the inner surface of ITO, so an electric double layer is formed at the interface (Fig. 2.9).
In case of Fig. 4.4(e), similar process should take place. The chemical species formed during the UV-ozone treatment would release the counter cation during the first scan. At longer treatment time the J-V curve became unstable again probably due to weakened binding ability to AuNPs of HPS (Fig. 4.4(f)). HPS molecule has many DC end groups which can interact with gold specifically and binds with the AuNPs on the surface. Different from good repeatability of device with e-Au NPs but without HPS, device with APG-Au but no HPS performs very poor stability, which is speculated that e-Au NPs can fix on surface well, however, the unstable J-V behavior of the HODs without HPS layer is caused by the migration of the AuNPs due to the applied electric
90
field.
The HPS/APG-AuNPs on Si were examined by X-ray photoelectron spectroscopy (XPS). At the Au 4f signal, no oxidation was observed (Fig. 4.5). Interestingly it was observed in e-AuNPs after UV-ozone treatment (Fig. 3.3), which is due to that APG-AuNPs should have higher crystalline content than e-AuNPs.
91
10-9 10-7 10-5 10-3 10-1 101 103
0.1 1 10
1st scan 2nd scan 3rd scan 4th scan 5th scan 6th scan 7th scan 8th scan 9th scan 10th scan
C urr en t d ens ity (mA /cm
2)
Voltage (V)
10-9 10-7 10-5 10-3 10-1 101 103
0.1 1 10
1st scan 2nd scan 3rd scan 4th scan 5th scan 6th scan 7th scan 8th scan
C ur rent d en sity ( m A /c m
2)
Voltage (V)
(a) No HPS, 3 pulses, No UV-zone treatment
(b) HPS, 3 pulses, 5min UV-zone treatment
92
10
-910
-710
-510
-310
-110
110
30.1 1 10
1st scan 2nd scan 3rd scan 4th scan 5th scan 6th scan 7th scan 8th scan 9th scan 10th scan
C urre nt d en si ty (mA /cm
2)
Voltage (V)
10
-910
-710
-510
-310
-110
110
30.1 1 10
1st scan 2nd scan 3rd scan 4th scan 5th scan 6th scan 7th scan 8th scan 9th scan 10th scan
C ur re nt d ens ity (m A /c m
2)
Voltage (V)
(c) HPS, 3 pulses, 15min UV-zone treatment
(d) HPS, 3 pulses, 30min UV-zone treatment
93
10-9 10-7 10-5 10-3 10-1 101 103
0.1 1 10
1st scan 2nd scan 3rd scan 4th scan 5th scan 6th scan 7th scan 8th scan 9th scan 10th scan
Cur rent d en sity (m A /c m
2)
Voltage (V)
10-9 10-7 10-5 10-3 10-1 101 103
0.1 1 10
1st scan 2nd scan 3rd scan 4th scan 5th scan 6th scan 7th scan 8th scan
C urre nt d ens it y (m A /cm
2)
Voltage (V)
Fig. 4.4 current density-voltage characteristics with UV-ozone treatment time dependence
(e) HPS, 3 pulses, 40min UV-zone treatment
(f) HPS, 3 pulses, 50min UV-zone treatment
94
Fig. 4.5 XPS spectra of HPS/APG-AuNPs at 3 pulses before and after UV-ozone
treatment
93 91 89 87 85 83 81 79 77 75 73
Binding Energy (eV)
Before UV-ozone treatment
93 91 89 87 85 83 81 79 77 75 73
Binding Energy (eV)
After UV-ozone treatment
95
The effect of HPS thickness on current density is shown in Fig. 4.6, when the APG-Au is 10-pulse deposited. It can be seen that both devices have good performance in repeatability and identical level of current density even when the thickness of HPS is changed, which means small effect of thickness of HPS on device performance.
10-9 10-7 10-5 10-3 10-1 101 103
0.1 1 10
1st scan 2nd scan 3rd scan 4th scan 5th scan 6th scan 7th scan 8th scan 9th scan 10th scan
C ur re nt d ens ity (m A /c m
2)
Voltage (V)
(a) 2000rpm
96
10
-910
-710
-510
-310
-110
110
30.1 1 10
1st scan 2nd scan 3rd scan 4th scan 5th scan 6th scan
Cur rent d ens ity (m A /c m
2)
Voltage (V)
10
-310
-210
-110
010
110
210
30.1 1 10
2000rpm 3000rpm
C ur re nt d ens it y (mA /cm
2)
Voltage (V)
Fig. 4.6 current density-voltage characteristics with different HPS thickness
(c)
(b) 3000rpm
97
From comparing the current density versus voltage characteristics of devices with different times of pulse evaporation (Fig. 4.4(e) and Fig. 4.6(a)), it can be found that when the pulse is increased from 3 pulses to 10 pulses, the value of current density in increased from 43mA/cm2 to 155mA/cm2 at 6V, which proves that the higher current density could cause higher hole injection.
It is also found that comparing with evaporated Au and HPS-Au, the level of enhancement of current density by 10 pulses APG-Au is not as high as the level of number density increase. One of possible reasons is that during the process of deposition, the particles with high speed may have damage of HPS surface (エラー! 参 照元が見つかりません。). Moreover, the energy barrier is not small enough to have efficient hole injection, because the WF is just 5.25eV for 10 pulses APG-AuNPs which is 0.25eV smaller than evaporated gold.
98
Fig. 4.7 SEM image of 3-pulse APG-Au on ITO surface
Fujita lab also applied this APG-Au in OSCs5. Fig. 4.8 and
Table 4.2 summarize the J-V characteristics and device parameter of OSCs with various anode buffer layers (APG-AuNPs, HPS-Au, e-AuNPs, PEDOT:PSS, and none).
The AuNPs buffer including APG-AuNPs gave comparable performance to the representative buffer layer, PEDOT:PSS, suggesting that the AuNPs can establish quasi-ohmic junction between ITO and the active layer even at the very low surface coverage. It is reported that the PEDOT:PSS can damage the electrodes in OSC and shorted the device life span6. The AuNPs do not contain strong acid or hygroscopic moieties like PEDOT:PSS or MoOx. So the AuNPs can be good candidates for the anode buffer of OSCs.
99
Fig. 4.8 J-V characteristics of OSCs with the structure of ITO/X/P3HT:PCBM/LiF/Al (——)X=AGP-AuNP (1 pulse, 40 min UV); (---)X= PEDOT: PSS; (•••)X=e-AuNP,
(—·—·—·)X=HPS-Au; (———)X=none
Table 4.2 The performance of devices with different modification layer
X PCE[%] VOC[V] JSC[mA/cm2] FF[%]
HPS-Au 2.7 0.58 7.9 59
HPS/APG-AuNP(1pulse) 2.4 0.55 7.3 60
HPS/e-AuNP 2.4 0.55 7.1 63
PEDOT:PSS 2.5 0.58 7.6 56
none 1.8 0.46 7.1 54
PCE: power conversion efficiency, Voc: open circuit voltage, Jsc: short circuit current density, FF: fill factor
-10 -8 -6 -4 -2 0 2
-0.1 0 0.1 0.2 0.3 0.4 0.5 0.6
C u rren t D ensity (mA/ c m
2)
Voltage (V)
100