INTEGRAL TIME-OF-FLIGHT AND CHARGE EXTRACTION BY LINEARLY INCREASING VOLTAGE TECHNIQUES FOR DETERMINING CARRIER MOBILITY
Open Access
- Author:
- Berger, Alan
- Graduate Program:
- Engineering Science
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- July 06, 2009
- Committee Members:
- S Ashok, Thesis Advisor/Co-Advisor
S Ashok, Thesis Advisor/Co-Advisor
Jian Xu, Thesis Advisor/Co-Advisor - Keywords:
- organic solar cell
charge carrier mobility
CELIV
time-of-flight
nanocrystal hybrid solar cell - Abstract:
- Charge transport properties of thin film organic solar cells are investigated using both integral time-of-flight (I-TOF) and charge extraction by linearly increasing voltage (CELIV) measurement techniques. Test devices are constructed with active layers consisting of blended of poly-3-hexylthiophene (P3HT) and [6,6]-phenyl–C61–butyric acid methyl ester (PCBM) in a glass/ITO/PEDOT:PSS/P3HT:PCBM/Al structure. Samples are tested using voltages varying from 0 to 15V and light intensities from 2.5 to 600mW. Characteristic voltage transients are obtained with each technique. From experimental data, electron mobility in the P3HT:PCBM solar cell devices is found to be on the order of μn = 5x10-4 cm2/Vs using I-TOF and μn = 2x10-5 cm2/Vs using CELIV. Both results are close to published data, with our I-TOF data being somewhere in the middle, and the CELIV results being slightly lower [1-5]. Integral TOF mobility results are probed as a function of collection capacitance, voltage, and light intensity. No significant variation is seen for changing the capacitance or the light intensity, but changing voltage yields the intriguing negative mobility dependence which has been reported in literature [6]. Comparison between the two techniques is made. The dielectric relaxation time is found to be fast while carrier lifetime is short (both relative to the transit time of carriers across the sample). These indicate redistribution of the electric field and concentration near the contact regions, a violation which will cause I-TOF measurements to be overestimated [7]. This may explain why I-TOF values are higher than their CELIV counterparts.