Rational screening low-cost counter electrodes for dye-sensitized solar cells

Abstract

Dye-sensitized solar cells have attracted intense research attention owing to their ease of fabrication, cost-effectiveness and high efficiency in converting solar energy. Noble platinum is generally used as catalytic counter electrode for redox mediators in electrolyte solution. Unfortunately, platinum is expensive and non-sustainable for long-term applications. Therefore, researchers are facing with the challenge of develo** low-cost and earth-abundant alternatives. So far, rational screening of non-platinum counter electrodes has been hamstrung by the lack of understanding about the electrocatalytic process of redox mediators on various counter electrodes. Here, using first-principle quantum chemical calculations, we studied the electrocatalytic process of redox mediators and predicted electrocatalytic activity of potential semiconductor counter electrodes. On the basis of theoretical predictions, we successfully used rust (α-Fe₂O₃) as a new counter electrode catalyst, which demonstrates promising electrocatalytic activity towards triiodide reduction at a rate comparable to platinum.

Introduction

As one of the most abundant and yet least collected sources of renewable energy, solar energy has attracted considerable interest in scientific research and industrial applications. Dye-sensitized solar cells (DSCs) have stood out among various photovoltaic devices because of their low cost, relative high energy-conversion efficiency and simple production procedure^1,2,3,4,5. In this rapidly develo** field, finding active counter electrode (CE) material for triiodide reduction reaction (iodine reduction reaction) is of great importance for the promotion of DSCs. It has been confirmed that platinum (Pt) is a superior CE material with excellent catalytic activity, high electrical conductivity and stability. Unfortunately, the disadvantages such as the high cost and low abundance greatly restrict the large-scale application of Pt in DSCs^6,7,8,9. Thus, several Pt-free alternative materials have been explored as CEs in DSCs, such as carbon materials^8,10, conductive organic polymers^6,11 and inorganic semiconductor materials including metal sulfides^12,13, metal nitrides^14,15, metal carbides^16,17, metal oxides¹⁸ and copper zinc tin sulfide^{Material characterization}

The morphology and structure of the samples were characterized by high-resolution transmission electron microscopy (JEOL JEM-2010F, F20, 200 kV), field emission scanning electron microscopy (Hitachi S4800) and X-ray diffraction (Bruker D8 Advanced Diffractometer; Cu K_α radiation; 40 kV). The current-voltage tests of DSCs were performed under one sun condition using a solar light simulator (Oriel 91160; air mass 1.5 G). The power of the simulated light was calibrated to 100 mW cm⁻² using a Newport Oriel PV reference cell system (model 91150 V). The EIS experiments and Tafel-polarization curves were measured with dummy cells in the dark by using an electrochemical workstation (Parstat 2273; Princeton). The frequency range of EIS experiments was from 100 MHz to 1 MHz with an AC modulation signal of 10 mV and bias DC voltage of 0.60 V. The curves were fitted by the ZSimpWin software. Tafel-polarization measurements were carried out with a scan rate of 50 mV s⁻¹. Cyclic voltammetry was conducted in a three-electrode system in an acetonitrile solution of 0.1 M LiClO₄, 10 mM LiI and 1 mM I₂ at a scan rate of 20 mV s⁻¹ by using a BAS 100B/W electrochemical analyser. Pt served as a CE, and the Ag/Ag⁺ couple was used as a reference electrode.