Dear Ram,

Most of the publications describe the use of both methods (XPS and UPS) since each one gives different characteristics of the material tested.

For example, in the attached study entitled "Photoemission Spectroscopy and Atomic Force Microscopy Investigation of Vapor Phase CoDeposited Silver/Poly(3-hexylthiophene) Composites " by  Scudiero et al., the XPS probes the core level binding energies allowing us to determine the chemical reactions at the vast

interfaces between Ag and P3HT. UPS characterization provides insightful interfacial electronic information of the metal/polymer composite matrices.

I have copied the abstract and some important paragraphs for quick view:

ABSTRACT

Nanocomposite matrices of silver/poly(3-hexylthiophene) (P3HT) were prepared in ultrahigh vacuum through vapor-phase co-deposition. Change in microstructure, chemical nature and electronic properties with increasing filler (Ag) content were investigated using in-situ XPS and UPS, and ambient AFM. At least two chemical binding states occur between Ag nanoparticles and sulfur in P3HT at the immediate contact layer but no evidence of interaction between Ag and carbon (in P3HT) was found. AFM images reveal a change in Ag nanoparticles size with concentration which modifies the microstructure and the average roughness of the surface. Under co-deposition, P3HT largely retains its conjugated structures, which is evidenced by the similar XPS and UPS spectra to those of P3HT films deposited on other substrates. We demonstrate here that the magnitude of the barrier height for hole injection ( and the position of the highest occupied band edge (HOB) with respect to the Fermi level of Ag can be controlled and changed by adjusting the metal (Ag) content in the composite. Furthermore, UPS reveals distinct features related to the C 2p (-states) in the 5-12 eV regions, indicating the presence of ordered P3HT which is different from solution processed films.

KEYWORDS

Polythiophene, silver, vapor phase co-deposition, nanocomposites, photoelectron

spectroscopy

INTRODUCTION

The nanoengineering of hybrid polymer-metal, 1-3 -metal oxide, 4 -semiconductors,5, 6 -polymers,7, 8 and –fullerenes,9 thin films is a fast developing field of nanotechnology. These new composite materials show promise for a variety of applications in novel organic optoelectronics such as solid state electronics, 10 , 11 electroluminescent devices and photovoltaics, 12 - 14 and photodetectors. 15 In particular, poly (3-hexylthiophene) (P3HT), owing to its high drift mobility 16 (up to 0.1 cm2 V-1 s-1 ),10 has been widely adopted as a hole transporting agent in developing plastic electronics.10, 12 The functionality and performance of these devices are largely dependent on the charge transfer process across the interfacial structure between organic semiconductors (polymer) and filler materials. The offsets of the energy levels of the two materials result in the formation of a heterojunction with the polymer being the electron donor and the filler the electron acceptor. The charge transfer process is a function of the relative energy levels of the HOMO and LUMO derived molecular orbitals in the valence band and conduction band regions, respectively. Studies of electronic structures using ultra-violet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy

(XPS) have primarily focused on thin films in the layered configuration. However, the electronic properties of nanoparticle-loaded polymers could be very different from those of the extended interfaces due to distinct structural morphology. Most of polymeric devices are solution processed in order to incorporate nanocrystal fillers into the matrix. However, one of the limiting factors for device performance using this process is inefficient charge transport. The presence of solvent adsorbed on the surface of the nanoparticles impedes the interaction between polymer and nanoparticles and the transfer of charges between the two materials and the transport of electron from nanocrystal to nanocrystal. 17 , 18 Vapor phase co-deposition is an alternative route to make metal/polymer blend. This process of producing nanocomposites is solvent free and has been used by Grytsenko,3

Takele2 and others. In the present work, we demonstrate for the first time that P3HT and silver can be simultaneously thermally evaporated to form nanocomposite materials. The codeposition of metal (Ag) and P3HT from two independent sources was achieved to produce polymeric films with different metal contents. The composite materials were investigated by photoemission spectroscopy (XPS and UPS) and atomic force microscopy (AFM). The surface sensitive analysis techniques are utilized to investigate the chemical nature and electronic states of the newly formed composite. XPS probes the core level binding energies allowing us to determine the chemical reactions at the vast interfaces between Ag and P3HT. UPS characterization provides insightful interfacial electronic information of the metal/polymer composite matrices. Our findings are compared with the layered structures (P3HT/Ag). Finally, AFM images provide morphological information such as surface roughness and Ag nanoparticle size as a function of Ag loading.

Hoping this will be helpful,

Rafik

Dear Ram Kumar Balasubramanyam Sir , Do I get it right ? It is found that the mobility of carrier injected from the Schottky contact whose barrier height is less than 0.4 eV can be correctly determined by analyzing the frequency dependence of conductance of OLEDs (mobility can also be determined by analyzing the frequency dependence of capacitance in case where the barrier height is less than 0.2 eV.).