Interface Engineering toward Highly Efficient Polymer Light-Emitting Diodes and Solar Cells

Abstract

This dissertation focuses on two aspects including the active layer/cathode interface for polymer light-emitting diodes (PLEDs) and polymer solar cells (PSCs) (Chapter 2-4) as well as the electron donor/acceptor interface in the bulk heterojunction (BHJ) for nonfullerene PSCs (Chapter 5) In Chapter 2 an efficient PLED is demonstrated using octadecanamide (ODAD) as a solution-processed electron injection layer (EIL) The device with an ODAD/Al cathode exhibits the higher luminance efficiency (8 5 cd A-1) than that with a Ca/Al cathode (5 9 cd A-1) The ability for the EIL of ODAD to lower the electron injection barrier is investigated by the analysis of structural analogs photovoltaic measurements X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy The results verify the mechanism: the amide functional group forms the C-O-Al complex at the ODAD/Al interface to create the interfacial dipole for decreasing the work function (WF) of Al In Chapter 3 tetra-n-alkyl ammonium bromides (TAABs) are demonstrated as solution-processed electron extraction layers (EELs) in BHJ PSCs based on poly(3-hexylthiophene) (P3HT) and [6 6]-phenyl-C61 butyric acid methyl ester (PCBM) The EELs of TAABs lead to simultaneous increases in open-circuit voltage (VOC) short-circuit current density and fill factor enhancing the power conversion efficiency (PCE) of BHJ PSCs from 2 38% to 4 02-4 19% Interestingly the constant increase in VOC of ?0 14 V is obtained for devices with Al Ag and Au cathodes by using the EEL of tetraoctylammonium bromide (TOAB) The self-assembly of TOAB into the lamellar structure stacked upright atop P3HT:PCBM is corroborated by synchrotron X-ray diffraction (XRD) The underlying mechanism is inferred that the TAAB molecules introduce anisotropic dipoles toward P3HT:PCBM which decrease the WF of metals for the efficient electron extraction In Chapter 4 TOAB is employed as a model compound to investigate the significance of ions with an ordered arrangement for enhancing the electron injection in PLEDs and the electron extraction in PSCs The spin-coated films of TOAB are prepared on the active layers of both PLEDs and PSCs with the different thermal annealing processes Subsequently they are investigated by atomic force microscopy (AFM) and synchrotron XRD as well as simultaneously employed as the EIL or the EEL The results reveal that the ordered arrangement of ions in the TOAB crystals is essential to enhance electron injection/extraction In Chapter 5 it is shown how the molecular geometry of the polymer donors and the perylene diimide dimers (di-PDIs) influences the BHJ morphology in the nonfullerene PSCs The results reveal that the pseudo 2D conjugated poly[4 8-bis(5-(2-ethylhexyl)thiophen-2- yl)benzo[1 2-b;4 5-b′]dithiophene-2 6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3 4-b]thiophene-)-2-carboxylate-2-6-diyl)] (PTB7-Th) has better miscibility with both bay-linked di-PDI (B-di-PDI) and hydrazine-linked di-PDI (H-di-PDI) compared to its 1D analog poly[[4 8-bis[(2-ethylhexyl)oxy]benzo[1 2-b:4 5-b′]dithiophene-2 6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3 4-b]thiophenediyl]] (PTB7) to facilitate more efficient exciton dissociation in the BHJ films However the face-on oriented π-π stacking of PTB7-Th is severely disrupted by the B-di-PDI due to its more flexible structure On the contrary the face-on oriented π-π stacking is only slightly disrupted by the H-di-PDI which has a more rigid structure to provide suitable percolation pathways for charge transport As a result a very high PCE of 6 41% is achieved in the PTB7-Th:H-di-PDI derived device

Date of Award	2016 Jul 12
Original language	English
Supervisor	Ten-Chin Wen (Supervisor)