The exploration of enhanced photocurrent with Zn-perylene metal organic frameworks thin film and bodipy via triplet triplet annihilation upconversion

Highly crystalline surface supported metal organic framework thin film has been used to generate enhanced photocurrent in the photoelectrochemical cells. The combination of Zn-perylene SURMOF and pyridine functionalized Bodipy has ~ two times higher photocurrent due to triplet triplet annihilation upconversion than its own parts. This experimentally determined data shows that MOF thin film material is well suited for overcoming the energy loss due to Shockley-Queisser limit for dye sensitized solar cell technology.


Introduction
The exploration of highly efficient materials for dye sensitized solar cell technology is the one of the challenges of this century. The highly oriented and crystalline Zn-perylene SURMOFs as acceptor/emitter [1] and pyridine functionalized Bodipy photosensitizer [2] has recently been been used to generate the enhanced photocurrent. There are cost and performance reasons to use the Bodipy as photosensitizer inside the photoelectrochemical cell.
It has been reported that due to large molar absorption coefficients, relatively long excited-state lifetimes, excellent photo-stability, facile preparation, strong visible light absorption, and most importantly low cost, it is believed that the Bodipy [3,4] is suitable for reducing the cost and improved performance which follows TTA UC mechanism shown in Figure 1. Moreover, the triplet triplet annihilation upconversion is the wavelength shift methodology where the photons having the high wavelength is upconverted into the photons of lower wavelength [2,[4][5][6][7][8].

Experimental Materials
All chemical reagents were purchased were purchased from Sigma-Aldrich. The TiO 2 18-NRT is purchased from Heptachroma Company.
Preparation of FTO-TiO 2 substrate: All the FTO glasses were cleaned with acetone, isopropyl alcohol, ethanol and deionized water for half hour, respectively. Then, the FTO glasses were treated with plasma (Diener FEMTO SR CE, 70 W) under O 2 (0.3 mbar) for 3 min. After that, TiO 2 thin films were prepared with TiO 2 nanoparticles (mixture of TiO 2 and EtOH, 1 g: 4 g) by spin coating (2000 r/s, 30 s). Subsequently, the TiO 2 coated films were dried at 70°C for one hour and annealed at 500°C for 12 hours.
Preparation of Zn-perylene SURMOF: Liquid phase epitaxy technique are used for the preparation of the Zn-Perylene SURMOFs on top of FTO/TiO 2 substrates. We prepared a concentration zinc acetate ethanolic solution (1 mM) on top of FTO-TiO 2 which was sprayed for 15 s. After 30 s wait 3,9 perylene dicarboxylic acid ethanolic solution was sprayed (40 µM; spray time: 20 s, waiting time: 30 s). This alternate spray process of Zn-acetate as metal linker and 3,9 perylene dicarboxylic acid as organic linker supported the formation of highly crystalline metal organic framework thin film which can be found somewhere in the literature [2]. The pure ethanol was used for rinsing to get rid of the unreacted zinc acetates and perylene molecules from the surface.

XRD characterization of Zn-perylene SURMOF:
The as-prepared Zn-perylene SURMOF thin film showed (100) and weak (200) peaks  [1,2] observed with out-of-plane XRD pattern suggesting that the fabricated Zn-perylene SURMOF can be grown exclusively along with (100) direction on TiO 2 surface, which is accordance with the simulated XRD diffractogram with preferred (100) orientation [9]. Moreover, the XRD pattern with 2θ=5.8° corresponds to a d value of 1.5 nm which suggest the same length of of 3,9-perylenedicarboxylic acid and Zn paddle-wheel structure. It has been inferred that the Zn-perylene SURMOF shows a similar structure similar to SURMOF 2 analogues [9] having the perpendicular layers to the substrates which is comprising 1D channels with a diameter of ~1.5 nm, and a layer distance of ~0.58 nm [1].

The infrared characterizations:
The infrared spectra of perylene powder is being compared with the Zn-perylene MOF thin film. It has been inferred that the C=O stretching in free carboxylic groups was found at the 1686 cm -1 for perylene dicarboxylic acid, whereas 1589 cm -1 and 1552 cm -1 for Zn-perylene SURMOF which are attributed to the asymmetric and symmetric stretching of COO-groups, respectively [10].

Scanning Electron Microscope (SEM):
The morphology of the Zn-perylene SURMOF films prepared with LPE method on TiO 2 substrate was characterized with scanning electron microscope (SEM) as displayed in Figure 2. which exhibits a homogeneous and compact surface with a thickness of ~600 nm (60 LPE cycles) [11].

Results and discussions
The UV visible is measured by dipping the Zn-perylene MOF thin film into the acetonitrile Bodipy solution. It demonstrates that the strong absorption at ~550 nm is the signature absorption of the Bodipy chromophore whereas the Zn-perylene SURMOFs showed strong absorption at about 420 nm. The combined system for of Bodipy plus Zn-perylene SURMOFs showed both of these characteristics shown in Figure 3.
The process of triplet triplet annihilation upconversion is absorption of low energy and upconvert it into high energy using the sensitizer acceptor pair [3,5,6,[12][13][14]. On the basis of UV-vis profile it is idealized that TiO 2 -perylene SURMOF+Bodipy [2] can be an effective architecture to facilitate the triplet energy transfer and the enhancement of energy via triplet-triplet within a photoelectrochemical cell. Moreover,

Photoelectrochemical experiments
The chronoamperometric experiments were performed in a standard electrochemical cell using TiO 2 -Zn-perylene+Bodipy or TiO 2 -Zn-perylene, or TiO 2 +Bodipy as working electrodes, Ag/AgNO 3 as reference electrode, and platinum wire as counter electrode with an external potential (0.2 V). During this process, the electrochemical cell was irradiated by using simulated solar light (AM1.5 solar) passing through a 532 nm long pass filter coupled with an automatic shutter control the light irradiation i.e. light on and light off.
The intensity dependent experiments are one of standard and basic experiments for the confirmation of TTA UC mechanisms. The important intensity dependent experiments were carried out with 532 nm green light source to observe the phenomenon of TTA UC. Raising the light power from low energy to high energy displays the linear behavior from TiO 2 -Zn-perylene/Bodipy which is consistent with the TTC UC [3] which is demonstrated in Figure 5.
Summing up, the MOF thin film material is very efficient to upconvert the low energy into high energy due to its highly crystalline, porous and versatile nature. It has been analyzed that the two times higher performance of our observed system is due to triplet triplet annihilation upconversion (TTA UC). The studied demonstration can be used for future dye sensitized solar cell technology. The limitation of the results can be overcome and optimized by using the more efficient electrocatalyst and conducting the detail investigations of mechanism.
Copyright: ©2018 Ahmad S. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.