Physical, Chemical, and Electrical Properties at the Interfaces of P3HT:PCBM Based Organic Solar Cells
Date Issued
2012
Date
2012
Author(s)
Tseng, Wei-Hsuan
Abstract
In this dissertation, the polymer/fullerene bulk heterojunction (BHJ) solar cells based on poly(3-hexylthiophene) (P3HT) mixed with [6,6]-phenyl C61-butyric acid methyl ester (PCBM) are studied through surface analysis and device characterization. Via ultraviolet and x-ray photoemission spectroscopy (UPS and XPS), the electronic structures and interfacial chemical reactions are investigated. The results are directly compared with the performance of photovoltaics, discovering the relationship between surface modifications and device characteristics.
The first topic of this dissertation investigates the mechanisms leading to the power conversion efficiency improvement in P3HT:PCBM based organic solar cells using calcium (Ca) in cathode structures. Both XPS and UPS results show that chemical reactions occur at P3HT/Ca interface, pulling down the energy levels of P3HT. The presence of Ca thus enlarges the energy difference between the highest occupied molecular orbital (HOMO) of P3HT and the lowest unoccupied molecular orbital (LUMO) of PCBM at the cathode interfaces, resulting in the increase of open circuit voltage and the enhancement of device efficiency.
In addition to the contacts between active layers and electrodes, the second topic focuses on the phase segregations and the evolutions of energy levels in overall solar cells during annealing. Upon aluminum (Al) deposition onto top of the P3HT:PCBM layer, the HOMO level of P3HT exhibits a large downward shift after annealing, resulting in the larger Voc in devices. Furthermore, both UPS and XPS results illustrate the annealing treatment will induce the out-diffusion of PCBM toward cathode, which can provide more ideal hetero-structures for electron extractions.
The third and fourth topics investigate the effects of cathode buffer layers on the efficiency of solar cells. In the case of the device using bathocuproine (BCP) as buffer layer, photoemission results shows that no reaction occurs at the active layer/BCP interface and only 2 nm of BCP could effectively suppress the chemical reactions between Al and active layers. Besides, Atomic force microscope images indicate that BCP layers can provide smoother contact surfaces with Al cathodes and suppress the generation of leakage current. On the other hand, for using PCBM as the buffer layer, the device performance shows a 20% improvement in power conversion efficiency as compared to the conventional devices without extra evaporated-PCBM layers. Results of surface analysis indicate that thermal annealing treatments lead to better mixture structures of PCBM layers with P3HT at the blended surfaces, providing more ideal hetero-structures and vertically graded PCBM distribution to optimize the carrier extraction efficiency.
Subjects
Polymer solar cell
P3HT
PCBM
photoemission spectroscopy
surface analysis
energy level
SDGs
Type
thesis
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