Chapter 5 Advanced Measurements
5.3 Voltage response –Mott-Schottky(M-S)
5.3 Voltage response – Mott-Schottky (M-S)
Mott-Schottky analysis [45-47] is an useful technique to examine the characteristics of thin films and junctions. It can provide us information about the flat-band potential and carrier concentration in devices by examined its related capacitance response to voltage under appropriate frequency range. The flat-band potential of TiO2 electrode and CuSCN film was obtained in the previously proposed paper [48], and the correlate energy band model was constructed with the Mott-Schottky results. The flat-band potential can be obtained according to the M-S equation: 1
C2 = εε 2
oeNA A2�−V0 + Vfb −kTe �, where C is the space-charge capacitance, ε is the dielectric constant of Cu2O, εo is the permittivity of free space, NA in the density of acceptors in Cu2O, A is the electrode area, V is the bias potential, k is the Boltzmann constant, T is the absolute temperature and e is the electronic charge; and by extrapolating the linear part to the potential axis in the C-2 vs V plot, the flat-band potential can be obtained.
In the final part of the thesis, EIS measurement was conducted under positive and negative bias to investigate the voltage response of the junction. With the extraction of the EIS data, we try to acquire the information about the variation of capacitance with different voltage bias and obtain the Mott-Schottky result. Fig.5.7 (a) and (b) shows the phase and Nyquist plot of junctions under positive voltages from 0V to +0.6V, it is obvious that the trends of peak frequency shift in EIS phase and impedance circle change in Nyquist plot were similar. Base on the results investigated, it seems that there were some similar characteristics of junctions under forward bias in dark condition and under illumination conditions; and further researches were needed to explain the characteristics.
Fig.5.7 (c) is the Mott-Schottly plot extracted from the EIS results. With linear fitting in the negative bias part, flat-band potential of -0.23V was acquired; but the acquired
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flat-band potential value is not correct since it must be a positive value. The deviation of flat-band potential is due to incorrect frequency region of extracted EIS results. The Mott-Schottky plot is acquired by extracting the circle related to the middle frequency region, which corresponded to recombination mechanism but not the junction capacitance.
Fig.5.8 (a) is the EIS phase of junction under 0.2V bias, the high frequency region of phase plot was not complete since the limitation of our equipment, which the highest frequency value is about 1M Hz. So frequency response of junctions beyond 1M Hz cannot be measured, and the response measured near limited frequency value was not accurate. The equipment limitation restricts our research of frequency response. Fig.5.8 (b) shows the Nyquist plot of junctions under the same bias condition, the incomplete circle related to high frequency region is observed. It seemed that if we can extract the capacitance values from high frequency region, we may acquire the more correct flat-band potential value.
In an attempt to prove our guess, we extracted the capacitance values from the high frequency region in Nyquist plot, although the EIS result is not complete, and compared it to that extracted from middle frequency region. The Mott-Schottky results extracted from different frequency ranges are shown in Fig.5.9 (a) and (b). The flat-band potential in Mott-Schottky plot extracted from middle frequency is -0.207V, and the one extracted from high frequency region is -0.176V. Although it was still a negative value, the positive shift accorded to our speculation. Noted that the impedance magnitude near the high frequency region was not accurate, hence the Mott-Schottky result was not completely correct; but the trend of positive shift still existed. From the results we investigated, it is believed that with high frequency range, we can acquire more complete information from the EIS results. Therefore, we can obtain the more accurate Mott-Schottky plot and construct the energy band model.
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Fig.5.1 Forward and reverse current to voltage characteristics of junctions with elevated temperatures: (1) 298K, (2) 308K, (3) 318K, (4) 328K, and (3) 338K.
-800 -600 -400 -200 0 200 400 600 800 0.1
1 10 100 1000
C ur re nt de ns it y , l og IJ I (µ A/ c m 2 )
Voltage , V (mV)
298 K 308 K 318 K 328 K 338 K
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(a)
(b)
Fig.5.2 LogJ to reciprocal temperature plot of junctions in (a) forward bias, and (b) reverse bias region with elevated temperatures: (1) 298K, (2) 308K, (3) 318K, (4) 328K, and (3) 338K.
0.0029 0.0030 0.0031 0.0032 0.0033 0.0034 1
10 100
Current density , logIJI (uA/cm2 )
Temperature-1 , 1/T (1/K)
-0.1V -0.5V -0.2V -0.6V -0.3V -0.7V -0.4V -0.8V
0.0029 0.0030 0.0031 0.0032 0.0033 0.0034 0.01
Current density , logJ (µA/cm2 )
Temperature-1 , 1/T (1/K)
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(a)
(b)
Fig.5.3 (a) EIS phase and (b) Nyquist plot of junctions with elevated temperatures: (1) 298K, (2) 308K, (3) 318K, (4) 328K, and (3) 338K.
-100k 0 100k 200k 300k 400k 500k 600k 700k 0.0
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Fig.5.4 Photocurrent to voltage characteristics of junctions under varied illumination intensities.
Table 5.1 The ND filters we used and its transmittances with relative neutral density.
0 100 200 300 400 500
Current density , J (µA/cm2 )
Voltage , V (mV)
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(a)
(b)
Fig.5.5 (a) EIS phase and (b) Nyquist plot of junctions under varied illumination intensities.
10-2 10-1 100 101 102 103 104 105
0.0 50.0k 100.0k 150.0k 200.0k 250.0k 300.0k 0
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(a)
(b)
(c)
Fig.5.6 Photocurrent to voltage characteristics of junctions with different Cu2O deposition time of (a) 8 hours, (b) 6 hours, and (c) 4 hours under varied Current density , J (µA/cm2 )
Voltage , V (mV) Current density , J (µA/cm2 )
Voltage , V (mV) Current density , J (µA/cm2 )
Voltage , V (mV)
61
(a)
(b)
(c)
Fig.5.7 (a) EIS phase and (b) Nyquist plot of junctions with positive voltages from 0V to +0.6V, and (c) Mott-Schottky plot extracted from the EIS results.
10-2 10-1 100 101 102 103 104 105
0.0 50.0k 100.0k 150.0k 200.0k 250.0k 300.0k 0.0
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(a)
(b)
Fig.5.8 (a) EIS phase and (b) Nyquist plot of junctions under 0.2V bias.
10-3 10-2 10-1 100 101 102 103 104 105 106 0.0
0.2 0.4 0.6 0.8 1.0 1.2 1.4
Phase (Rad)
Frequency , f (Hz)
-100k 0 100k 200k 300k 400k 500k 600k 700k 800k 0.0
50.0k 100.0k 150.0k 200.0k
Im(Z) (Ω)
Re(Z) (Ω)
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(a)
(b)
Fig.5.9 Mott-Schottky plot of junctions extracted from (a) middle frequency region, and (b) high frequency region.
-0.4 -0.2 0.0 0.2 0.4
0.000 0.005 0.010 0.015 0.020 0.025
0.030 M-S
Capacitance-2 , C-2 (1016 F-2 )
Voltage , V (mV)
-0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0
20 40
M-S
Capacitance-2 , C-2 (1016 F-2 )
Voltage , V (mV)
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Chapter 6
Summary and Outlook
Dye sensitized solar cell, fabricated with inexpensive methods and materials, is an attractive candidate for photovoltaic device. It has shorter payback time in comparison with conventional solar cells since the demand of the purity of materials used in DSSC is less.
Although there are many advantages, the main problem of DSSC is the long time stability.
Generally speaking, the degradation on the sealant is hard to avoid and will cause the leakage of electrolyte. In order to improve the drawback, it seems necessary to replace the electrolyte with solid-state materials. In this thesis, Cu2O was adapted as a p-type material to replace the electrolyte layer and the basic characteristics of Cu2O/TiO2 bulk hetero-junction were investigated.
At the beginning of the experiment, two kinds of deposition methods of electrochemical deposition are examined, including galvanostatic method and potentialstatic method. After the choice of appropriate method, variety conditions of Cu2O deposition are examined, such as bath pH value, and current density. Besides, the deposition condition is related to the surface properties of matter on which we deposited. With the analyses of XPS and XRD results, correct composition and best condition was found. By adapting the best deposition condition, a complete Cu2O/TiO2 bulk hetero-junction was formed successfully.
Next, the basic understanding of the junction is constructed by current to voltage measurement and electrical impedance spectroscopy measurement. There are many similar characteristics between the bulk hetero-junction and conventional p-n junction. Besides, it is found that inhibition of recombination centers of the junction is a critical issue to improve the optical performance. Therefore, many conditions are applied to the junctions and the characteristics of junctions are examined. With different conditions and the measurement
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results, we can advanced characterized the bulk hetero-junction.
Afterward, many advanced measurements are performed to help us to comprehend the temperature dependence, optical condition, and voltage response of the Cu2O/TiO2 bulk hetero-junction. Furthermore, the voltage response of the junction can let us acquire the Mott-Schottky plot to calculate the flat-band potential and carrier concentration and even construct the energy band model. Unfortunately, the frequency range of EIS measurement we used is not higher enough now to measure the higher frequency response, which is related to the junction itself, of the junction.
In conclusion, the Cu2O/TiO2 bulk hetero-junction is formed successfully by electrochemical deposition method in the first time. Before applying to DSSC, the recombination centers of junction should be eliminated first. Since the deposition condition is dependent on the TiO2 surface, the conditions of Cu2O deposition on dyed-TiO2 surface may be different from now. As a result, further researches are needed to find the best condition of Cu2O deposition on the dyed-TiO2 surface. Additionally, Cu2O film can also used in nano-tube structure DSSC, and apparently, the deposition condition must be different again since the surface properties of nano-tube structure is not the same as the nano-porous structure.
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簡歷
姓名：許文朋 性別：男
出生：民國74年3月22日 籍貫：台灣省台北縣
學歷：國立中興大學電機工程學系 [ 92年9月 – 96年6月 ]
國立交通大學電子研究所碩士班 [ 96年9月 – 98年8月 ]
碩士論文題目：
對於電化學沉積的氧化亞銅/二氧化鈦塊材異質接面特性的研究