Advanced research trends in dye-sensitized solar cells

Abstract

Dye-sensitized solar cells (DSSCs) are an efficient photovoltaic technology for powering electronic applications such as wireless sensors with indoor light. Their low cost and abundant materials, as well as their capability to be manufactured as thin and light-weight flexible solar modules highlight their potential for economic indoor photovoltaics. However, their fabrication methods must be scaled to industrial manufacturing with high photovoltaic efficiency and performance stability under typical indoor conditions. This paper reviews the recent progress in DSSC research towards this goal through the development of new device structures, alternative redox shuttles, solid-state hole conductors, TiO₂ photoelectrodes, catalyst materials, and sealing techniques. We discuss how each functional component of a DSSC has been improved with these new materials and fabrication techniques. In addition, we propose a scalable cell fabrication process that integrates these developments to a new monolithic cell design based on several features including inkjet and screen printing of the dye, a solid state hole conductor, PEDOT contact, compact TiO₂, mesoporous TiO₂, carbon nanotubes counter electrode, epoxy encapsulation layers and silver conductors. Finally, we discuss the need to design new stability testing protocols to assess the probable deployment of DSSCs in portable electronics and internet-of-things devices.

Fig. 1. Evolution of conversion efficiencies of DSSCs in recent years.

Fig. 2. Schematic illustration representing device structure and working principle of a dye-sensitized solar cell. CB = conduction band, E_{f TiO2} = fermi level of TiO₂, S = ground state of dye sensitizer molecule, S* = excited state of dye sensitizer molecule, S⁰ = oxidized dye, S⁺ = charge separation, I⁻ = iodide ion and I₃⁻ = triiodide ion.

Fig. 3. (a) Traditional DSSC use either a thermoplastic or porous insulating spacer to avoid short circuit between the mesoporous TiO₂ and the counter electrode. (b) Type II junction alignment of the band edges for the mesoporous TiO₂ film and a p-type semiconductor layer. The p-type semiconductor serves as an electron-blocking hole-selective charge collection layer. (c) The sensitized TiO₂ electrode and the PEDOT semiconductor-based counter electrode make direct contact via mechanically pressing and make a new DSSC embodiment. (d) In the DSSC with the contacted electrodes, the redox couple diffuses merely through the mesoscopic TiO₂ film (reproduced from reference with permission).

Fig. 4. Proposed process flow for producing advanced monolithic DSSCs with alternative Cu redox shuttles-based electrolytes and solid hole conductors.

Fig. 5. Energetics in DSSCs with respect to redox potentials of each redox couple (I⁻/I₃⁻, [Co(bpy)₃]^2+/3+ and [Cu(dmp)₂]^1+/2+) utilized in DSSCs.

Fig. 6. General characteristics of efficient semiconducting oxide layer in DSSCs.

Fig. 7. Rapidly sensitized photoelectrodes via inkjet printed dyes accelerates the staining process and can also be adopted to produce multicolour printed dyes based TiO₂ electrodes and for precise co-sensitization of the PEs (reproduced from ref. with permission).

Fig. 8. General characteristics of an efficient carbonaceous counter electrode in DSSCs.

Fig. 9. Illustration of the factors that affect DSSC devices and their possible consequences which hinder the photovoltaic performance.

Mikko Kokkonen

Parisa Talebi

Jin Zhou

Somayyeh Asgari

Sohail Ahmed Soomro

Farid Elsehrawy

Janne Halme

Shahzada Ahmad

Anders Hagfeldt

Syed Ghufran Hashmi