Interfacial Engineering for Organic/Inorganic Hybrid Bulk－heterojunction and the Application in Photovoltaics
Date Issued
2015
Date
2015
Author(s)
Lin, Jhih-Fong
Abstract
Since the good thermal stability of conjugated polymer/inorganic nanocrystal hybrids in the photovoltaic application, such organic/inorganic bulk‒heterojunction has attracted much attention in recent years. However, the limited power conversion efficiency is the main issue for further development and commercialization. As a result, adequate approaches to improve the photovoltaic performance are urgently required. The objective of this dissertation is to demonstrate different methodologies of interfacial engineering as well as morphology control between polymer and inorganic nanocrystals. The effects of each approach were evaluated by the properties of treated polymer/nanocrystal hybrid thin film and their corresponding photovoltaic performance. To have better morphology control in polymer/nanocrystal hybrid thin film, the rod‒coil block copolymers P3HT‒b‒P2VP with different molecular designs were used as donor materials. The loading limit of TiO2 nanoparticles is found highly correlated to the weight percentage of P2VP segment in the self‒assembled block copolymer (10 wt%, 20 wt% and 40 wt% accommodation limit of TiO¬2 nanoparticles for lamella (57 wt% P2VP), cylindrical (75 wt% P2VP) and spherical (86 wt% P2VP) ordered structures, respectively). In addition, since the good miscibility of block copolymer P3HT‒b‒P2VP to both conjugated homopolymer P3HT (P3HT segment) and TiO2 nanorods (P2VP segment), different small amounts of P3HT‒b‒P2VP was incorporated into the P3HT/TiO2 hybrid thin film as the processing additive for better morphology control. After adding 1.50 wt% P3HT‒b‒P2VP, the reduced P3HT crystalline domain size (from 88.21 Å to 85.47 Å) and low extinction intensity of photoluminescence in P3HT/TiO2/P3HT‒b‒P2VP ternary blend thin films indicate the aggregation of P3HT donor is reduced and the miscibility of TiO2 acceptor in P3HT is improved. On the other hand, techniques aim to improve the crystallinity, electrical properties as well as the surface characteristics of nanocrystals were also adopted in this dissertation. After different treatments such as post‒ripening process and heterogeneous doping of TiO2 nanorods, the electron mobility of corresponding treated nanorods are both significantly enhanced (5.27×10-4 and 2.33×10-4cm2·V-1·s-1 for boron‒doped and ripened nanorod, respectively) compared to the as‒synthesized nanorod (6.21×10-5 cm2·V-1·s-1). Moreover, capping conjugated modifier onto nanocrystal seems to be another feasible approach to cover the pristine surface defects. The existence of surface defect may trap charge carrier during the charge separation and transport. Accordingly, we adopted two-stage ligand exchange process on the surface of TiO2 nanorods: three different pyridine derivatives such as pyridine, 2, 6‒Lutidine (Lut) and 4‒tert‒butylpyridine (tBP) were applied as dispersion solvent in first stage surface modification to create distinct surface characteristics of TiO¬2 nanorod and elucidate the anchoring behavior of conjugated modifier on these surfaces in second stage modification. Additionally, the quantitative studies for category and anchoring amount of ligand on as‒synthesized and three different modified TiO2 nanorods were obtained by elemental analysis, in which the tBP and Lut were proven as the most effective solvent for oleic acid (OA) removal and ligand-anchoring during the ligand exchange process, respectively. After the second stage surface modification using conjugated modifier of 2‒cyano‒3‒(5‒(7‒(thiophen‒2‒yl)‒benzothiadiazol‒4‒yl)thiophen‒2‒yl)acrylic acid (W4), the capping amount is correlated to the number of unbounded sites on the surface of TiO2 nanorods. As a result, the TiO2‒tBP with the lowest amount of anchored ligands shows the highest attaching amount of W4, the bonded W4 number can be reached up to 0.62 mol% compared to those of other two modified TiO2 nanorods (0.38 and 0.19 mol% for TiO2‒Pyridine‒W4 and TiO2‒Lut‒W4, respectively). Additionally, the anchored W4 surface modifier on TiO2 nanorods would also improve the charge carrier transport in TiO2 percolated domains due to its conjugated nature. The enhanced electron mobility (i.e. electron is transported by TiO2 interpenetrating network in P3HT/TiO2 hybrid solar cell) in either solution‒cast TiO2 or hybrid thin film is observed in the sequence of μe, TiO2‒tBP‒W4 > μe, TiO2‒Pyridine‒W4 > μe, TiO2‒Lut‒W4 > μe, TiO2‒OA with the decreasing amount of anchored W4 modifiers. Last, the engineered P3HT/TiO2 nanorod hybrid film was fabricated into solar cell and then its photovoltaic performance was assessed. Compared to the reference hybrid thin film without any treatment (P3HT/TiO2‒Pyridine with about 0.40% of power conversion efficiency), the power conversion efficiency of P3HT/TiO2 nanorod hybrid solar cell are increased by 186%, 31%, 79% and 240% after the incorporation of P3HT‒P2VP additive, as well as ripened, B‒doped, and tBP‒W4 modified TiO2, respectively. The results in this work demonstrate the importance of interfacial engineering for polymer/nanocrystal hybrid thin film in photovoltaic application, and which are useful to boost the technological innovation for relevant commercial exploitation.
Subjects
interfacial engineering
polymer
block copolymer
inorganic nanocrystals
photovoltaic
morphology control
P3HT
P3HT?b?P2VP
TiO2
crystallinity
electron mobility
pyridine
2, 6?Lutidine (Lut)
4?tert?butylpyridine (tBP)
oleic acid (OA)
2?cyano?3?(5?(7?(thiophen?2?yl)?benzothiadiazol?4?yl)thiophen?2?yl)acrylic acid (W4)
surface modification
SDGs
Type
thesis
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