Abstract
We attempt to reconcile experimental and theoretical methodologies for the determination of the energy gap, which is essential to properly characterize a series of key phenomena related to the applications of organic semiconductors.
Highlights
It classifies the gaps in two major groups:[66] (i) The first group is formed by the techniques that quantifies the optical gaps (Eopt). It corresponds to the experimental gaps given by Reflection Electron Energy Loss Spectroscopy (REELS) and UV Vis and the theoretical gap calculated by TD-Density Functional Theory (DFT) (OECGalc). The methods in this group are characterized by very fast electronic transitions so that the geometry of the molecules is essentially frozen during the gap quantification. (ii) The second group is constituted by the techniques that evaluate the fundamental gap (Efund)
Different assessments of the gap are a source of divergences and even controversies in the literature which may eventually lead to misleading conclusions
The situation can be specially confusing when comparing gaps obtained by methods that involves adiabatic processes with the ones obtained involving fast electronic transitions
Summary
For a couple of decades, organic semiconductors (OS) have been extensively investigated as candidate materials for optoelectronic devices such as field-effect transistors (OFETs),[1,2,3,4] light-emitting devices (OLEDs),[5,6,7,8,9,10] photovoltaics (OPVs)[11,12,13,14,15,16,17] and more recently for solar-fuel production[18,19,20,21,22,23] and energy storage such as Lithium-ion batteries (LIB).[24,25,26,27,28,29] OS are very appealing thanks to several intrinsic characteristics of organic materials viz. processability from solution, mechanical flexibility enabling integration to curved or flexible substrates, easy functionalization leading to the possibility to fine-tune their optoelectronic properties. Paper possibility of adjusting the energy levels which, to some extent, will control the charge transfer mechanism in the redox-like reactions as well as the optical absorption. This tailoring can be done by combining electron-rich and electron-withdrawing moieties forming the so-called Donor–Acceptor (D–A) molecules or copolymers.[30,31,32,33] This strategy was one of the driving forces for the impressive development of OPVs, which are reaching the market as an alternative for clean energy generation. In CV measurements both the reduction and oxidation processes are accessible so that the energy gap will be defined as the difference between the reduction and oxidation potentials multiplied by the fundamental electron charge. The energy gap can be evaluated by spectroscopic measurements if one has access to ultraviolet photoelectron spectroscopy (UPS)[43] to directly measure the HOMO energy and inverse photoemission spectrocopy[44,45] to probe the LUMO energy ( the energy gap is determined by the HOMO–LUMO difference)
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