Elsevier

Synthetic Metals

Volume 281, November 2021, 116915
Synthetic Metals

On the photoconductivity behavior of emeraldine-salt polyaniline films

https://doi.org/10.1016/j.synthmet.2021.116915Get rights and content

Highlights

  • Polyaniline is synthesized in the emeraldine-salt phase as demonstrated by structural and optical characterization.

  • Photoluminescence shows optically active localized states and strong emissions related to π*→ PB and polaron bands PB*→ PB.

  • Photoconductivity profile exhibits a dependence of excitation intensities, and a rate equation system models it.

  • Shallow traps are responsible for negative photoconductivity, while deep traps increase response and recovery times.

Abstract

Deep and shallow traps play a fundamental role in the photoresponse profile of organic devices. Here we present an investigation on emeraldine-salt polyaniline films concerning structural, optical, and electrical properties by X-rays diffraction, optical and electron microscopies, and optical spectroscopies as Fourier Transform Infrared Spectroscopy (FTIR), absorption, and photoluminescence (PL). Photoconductivity measurements show signatures related to localized states near-edge absorption and the efficient transition between polaron bands. The photoconductive profile is dependent on the excitation intensity and presents negative photoconductivity (NPC) for higher intensities. A rate equations model attributes the NPC to shallow traps while the increase of response and recovery times are associated with deep traps.

Introduction

Low processing costs for wide-area devices, mechanical flexibility, and optimal wavelength selectivity are some key points that make organic photodetectors remarkable. Furthermore, conductive polymers present excellent carrier transport and high absorption of light, enabling potential applications as gas sensors, photovoltaic cells, optical pH sensors and photocatalytic devices [1], [2], [3], [4], [5]. Also, organic/inorganic composites present enhanced sensitive properties for optical-based sensors, as for example, determining acetic acid concentrations by ZnO/PANI [6]. Among many polymers, polyaniline (PANI) is one of the most promising due to the low monomer cost, environmental stability, higher conductivity, and porous nature, causing efficient photocatalytic properties [7], [8]. Depending on the synthesis process and the type of doping acid [9], different oxidative states provide the tunning of carrier concentrations between 1014 ~ 1022 cm−3 despite the small mobilities in the order of 0.1 ~ 3729 cm2V−1s−1 [10], [11], [12], [13], many orders of magnitude smaller than crystalline inorganic semiconductors [14].

Molecule deformations produce self-trapping potentials for the carriers depending on electron-lattice interactions. These interactions, which rely on the spatial conformation of the chain and charge distributions, can be long-ranged, producing a metallic behavior, or short-ranged, yielding a semiconducting behavior [15]. Fast response and recovery times are desirable for pragmatic photodetectors. Managing hybrid structures as oxide-PANI nanocomposites show an enhancement in photoresponse compared to pure materials due to the promotion of carrier separation in the interface band alignment, reducing the charge recombination [16], [17], [18], [19]. However, for PANI application in photoelectronic devices, its behavior under illumination and the dependence on intrinsic defect density are relevant. These defects create carrier traps in the electronic structure that play a significant role in increasing response and recovery times, decreasing photoresponse, modifying the transient photoresponse profile, and giving rise to anomalous effects as persistent photoconductivity and negative photoconductivity [20], [21].

To clarify the role of defects concerning their shallow and deep character in the electronic structure, this study analyses the electrical behavior of PANI in the emeraldine phase. Optical and structural measurements support the assumption of this oxidation state and enlighten the optical absorption and photoluminescence from near-infrared to near-ultraviolet optical range. Photoconductivity measurements in blue and ultraviolet excitations are equivalent and show a strong dependence on excitation intensities. For higher excitations, negative photoconductivity is observed in the plateau region under 442 nm illumination, and this effect also reflects in the recovery of the dark current level. A set of rate equations models the character of shallow and deep trap levels as a function of excitation intensity and shed light on the observed profile interpretation.

Section snippets

PANI film growth

The synthesis of emeraldine-salt polyaniline (ES-PANI) followed a chemical route resumed in Fig. 1. The precursors were aniline (from VETEC, Brazil, purified in Sugar Cane Brandy Physico-Chemical Analysis Laboratory at UFLA) as a monomer, dodecylbenzene sulfonic acid (DBSA- from Azul Quimica, Brazil) as a proton doping agent, and ammonium persulfate (APS – from Synth, Brazil, with 99.7% of purity) as an oxidizing agent based on the recipe developed by Silva et al.[22] with some modifications

Results and discussion

Fig. 2 exhibits the structural and morphological properties of typical synthesized PANI films. The assignments of main absorptions in the FTIR spectrum (Fig. 2(a)) are based on the data available from the literature [23], [24], [25]. Two regions are distinguished, above and below 2000 cm−1. The H2O and OH groups are responsible for the broadband centered at 3450 cm−1 [26], while the stretching vibration of C–H causes the absorption peaks between 2956 and 2950 cm−1 [22]. Besides, the presence of

Conclusions

The polyaniline (PANI) obtained by a chemical route presents the emeraldine-salt when DBSA e HCl are employed as reducing agents, doping by protonation with the presence of ions close to the polymeric chain. FTIR and optical absorbance analyses corroborate this phase. The morphology images exhibit a uniform film produced by casting technique. Photoluminescence shows optically active localized states and strong emissions related to π*→ PB and polaron bands PB*→PB. The investigation of

CRediT authorship contribution statement

A.B. de Paiva: Conceptualization, Methodology, Software, Formal analysis, Investigation, Data curation, Writing − original draft. G.I. Correr: Software, Data curation. J.C. Ugucioni: Conceptualization, Investigation, Resources, Validation. G.R.Carvalho: Investigation. R.G. Jasinevicius: Investigation, Data curation, Validation. M.P.F. de Godoy: Conceptualization, Methodology, Formal analysis, Resources, Investigation, Writing − review & editing, Supervision.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors thank the Laboratory of Electron Microscopy and Analysis of Ultrastructural Federal University of Lavras (http://www.prp.ufla.br/labs/microscopiaeletronica/). The fellowships support from the Brazilian agencies Conselho Nacional de Desenvolvimento Científico e Tecnológico - CNPq (141636/2018-1 and 309230/2020-9) and Fundação de Amparo à Pesquisa do Estado de São Paulo - FAPESP (18/25886-5) are also gratefully.

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