Full length articleAnnealing provoked modification of ZnPc photoactive nanolayer via self-assembling at SnO2 hybrid interface and its effect on layer electric properties towards application in hybrid photovoltaic devices
Graphical abstract
Annealing mobilized growth columnar nanostructure ZnPc film on SnO2 ETL via. self-assembly with improve charge transportation to ETL and photoresponse.
Introduction
In past few decades, large attention has been drawn by the new functional structures or hierarchical patterns with novel functionalities formed via inter-component interactions of components in a disordered system [1]. This has been closely related to the self-assembling properties. Self-assembly takes place at any length scale from atoms to molecules to full-size robots which lead to new shapes and functionalities that were not initially present in the disordered system. Self-assembly at molecular level called supramolecular self-assembly occurs via intermolecular forces including hydrogen bonds, π-π stacking and dipole interactions. In this regard, metallo-organic macrocyclic complexes (MOMC) have drawn lot of focus, as they possess self-assembling abilities because of strong attractive forces between the aromatic rings as a result of π-π stacking interactions [2,3].
Among MOMCs, the metal-phthalocyanines (MPcs) compounds are one of the most promising building blocks for organic thin-film electronics such as organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs), and organic photovoltaics (OPVs) [[4], [5], [6], [7], [8], [9], [10]]. These molecular units are highly versatile as they have unique optical, electronic, and photophysical properties and upon self-assembling among them their properties either enhances or transforms. In particular, they are suitable candidates in photoconversion [11] because of an excellent light harvesting ability till IR region [12,13], outstanding thermal & photochemical stability [14,15] and a potential ability to accept or release an electron due to its delocalized π systems in the macrocycles. MPcs are macrocyclic molecules characterized by an aromatic ligand carrying clouds of delocalised electrons and a central metal, that typically leads to molecular stacks in the form of H- and J-type aggregates [16,17]. Investigating elaborately the role of self-organization of these macrocycles to form different nanostructures is necessary to promote such compounds for advanced technological applications and for rationalize their performances.
In OPVs, different strategies were used for betterment of charge carrier's extraction from the photoactive layer in order to enhance its performance. The most important and frequently used strategies among them are I) Interface engineering of the active layer [18,19] and II) Inserting a charge extracting interfacial layer between the active layer and the electrode to facilitate efficient charge extraction [[20], [21], [22]]. The interface engineering of the active layer was explored by optimizing the morphology of the active layer i.e. ZnPc Langmuir Blodgett (LB) film. Methods generally applied for improving morphology either using post-production annealing [23,24] or by the use of different solvents [25,26]. In the present case, post-production annealing has been done to perform interface engineering in order to derive different functional nanostructures in the ZnPc active layer with help of self-assembling of ZnPc molecules at high temperature. LB technique supports in depositing supramolecular assemblies of molecular films of different functional materials [[27], [28], [29], [30], [31], [32], [33]]. Therefore, application of LB technique for deposition of ZnPc film further aids in having perfectly ordered thin film. The second method of using charge extraction interfacial layer for improvement of charge extraction from active layer was explored by using Tin Oxide (SnO2) film as Electron Transportation Layer. Among various transition metal oxides used as interfacial layer in OPVs for electron transport mostly TiO2 and ZnO find their application. However, the devices having TiO2 and ZnO as ETL suffered from external quantum efficiency (EQE) losses in the UV range because of higher parasitic absorption which lead to small current loss. Recently, it has been reported that within metal oxides SnO2 material shown to possess higher band gap, electron mobility and better stability than TiO2 and ZnO [34,35]. Wide band gap (3.8 eV) of SnO2 will lead to smaller ETL-induced current loss than TiO2 and ZnO. Ke et al. showed the best-performing planar perovskite solar cells using SnO2 as ETLs prepared from a solution of SnCl2.2H2O in ethanol spin-coated on FTO, having an average efficiency of 16.0% and higher JSC, VOC, than their TiO2-based counterparts [36]. We have used rutile phase of SnO2 also called cassiterite is having straight chain configuration and has attracted increasing interests owing to its outstanding electrical, optical, and electrochemical properties [[37], [38], [39], [40]].
In this article, we investigate mainly the role of self-assembling behavior of ZnPc molecules in the active layer to form different shape nanostructures with aid of heat treatment for the improvement of charge carrier's extraction from the active layer to the ETL. Best result was obtained for the 290 °C annealed sample having columnar shape nanostructure in the ZnPc active layer having mobility of 1.7 × 10−6 m2 V−1 s−1 and total trap carrier concentration 2 × 1026 m−3 and it also shows better response in presence of light.
Section snippets
Materials
All chemicals were of the highest purity available and were used as received without further purification. Zinc phthalocyanine (ZnPc) (97% dye content and Mw-577.91 g/mol) was purchased from Sigma Aldrich. Tin chloride hydrate (SnCl2.H2O), 2-Methoxy ethanol (CH3OC2H4OH), monoethanolaimne (NH2C2H4OH), trifluoroacetic acid (TFA), and chloroform (CHCl3) were purchased from Merck.
Preparation of SnO2 layer
The precursor material, SnCl2.H2O (8.374 g) was dissolved in 100 ml mixture of 2-methoxy ethanol (CH3OC2H4OH) and
Self-assembly and interaction study of active layer ZnPc with SnO2
The UV–Vis absorbance spectrum of ZnPc solution, virgin SnO2 spin coated film and ZnPc LB film on glass are shown in Fig. 2(a), Fig. 2(b) and Fig. 2(c), respectively. The UV–Vis absorption spectrum of ZnPc in solution exhibits characteristic features which is associated with the π-π* transitions of aromatic hydrocarbons with a well-defined vibronic structure [41]. In absorption spectra peaks related to π-π* transition i.e. Highest occupied molecular orbital (HOMO) - Lowest unoccupied molecular
Conclusions
In this article, the study elaborately explains the controlling of self-assembly of ZnPc molecules at molecular level with assistance of annealing to introduce surface morphology modification in the active layer and further its application on the improvement of transportation of charge carrier for enhancement of performance of organic based photovoltaic cells. The structure of ZnPc photoactive layer on SnO2 film taken as ETL can be considered a phase separated hybrid structure. Annealing
Acknowledgements
Dhrubojyoti Roy thanks DST-SERB for financial support for carrying out the research work by granting National Post-Doctoral Fellowship SERB File Number: PDF/2015/000111. We gratefully thank Dr. Mukul Gupta, UGC-DAE CSR, Indore for his support in taking XRD measurements and Dr. V. Ganesan, UGC-DAE CSR, Indore for providing AFM facility. We acknowledge UGC-DAE CSR, Indore for giving opportunity to carry out characterization of the samples under the CRS project.
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