Photoelectron Spectroscopy of the Contact between the Cathode and the Active Layers in Plastic Solar Cells: The Role of LiF

Abstract

The surfaces and electrode interfaces of a polymer blend used in prototype solar cells have been characterized with photoelectron spectroscopy. The polymer blend in question is a 1 : 4 mixture of APFO-3 : PCBM. Based on surface analysis of the pristine film we can conclude that the surface of the blend is a 1 : 1 mixture of APFO-3 and PCBM. The electrode systems studied are the widely used Al and Al/LiF contacts. LiF prevents formation at the Al/organic interface of Al-organic complexes that destroy the -conjugation. In addition to this, there are two other beneficial, thickness dependent, effects. Decomposition of LiF occurs for thin enough layers in which the LiF species are in contact with both the organic film and the Al atoms, which creates a low workfunction contact. For thicker (multi)layers, the dipole formed at the LiF/organic interface is retained as no decomposition of the LiF occurs upon Al deposition.

Mixed interlayers at the interface between PEDOT:PSS and conjugated polymers provide charge transport control

“…The previously published IE was 5.8 eV. 56 These measurements indicate that the Fermi energy is within or very close to the LUMO band. This result is inconsistent with a typical understanding of how organic semiconductors operate and is thus most likely a measurement artifact.…”

Energy-Level Alignment at Metal–Organic and Organic–Organic Interfaces in Bulk-Heterojunction Solar Cells

“…The work function of metallic aluminum is ~4.1 eV. Note that LiFmodified Al contacts are often used for a variety of reasons and the resulting work function of LiF-modified Al can take a range of values depending on deposition order and materials involved [31][32][33][34][35]. In all cases, however, the resulting work function will be smaller than that for clean Al, and as we will show, from an interface energetics standpoint, the work function of clean Al already is low enough to pin the PCBM/Al contact.…”

“…Note that reducing the effective work function of the Al contact by a LiF sandwich layer will not change this, only increase the interface dipole at the metal contact. The LiF layer can effect the contact in other ways, however, including preventing covalent bonding between Al and PCBM and possible defects resulting thereof [32][33][34].…”

Influence of solvent treatment with fluoro compounds on the properties of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) polymer as a hole transport layer in polymer solar cells

“…The interface layer modification on BHJ active layer/electrodes has been comprehensively carried out for the improvement of charge injection or collection, especially those due to mismatched energy levels between polymer materials and metal electrodes. [20][21][22] In order to enhance the PCE, several buffer layers have been reported to modify the interface between the active layer and metal electrodes. Many materials such as poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), molybdenum trioxide (MoO 3 ), vanadium pentoxide (V 2 O 5 ), nickel oxide (NiO 2 ), and tungsten oxide (WO 3 ) have been used as hole transport buffer layers, [23][24][25][26] and titania (TiO x ), lithium fluoride (LiF), cesium carbonate (CsCO 3 ), and zinc oxide (ZnO) have been reported for electron extraction layers [27][28][29][30] to improve the device performance.…”