New Organic Dye for Solar Cells

Title: An Organic D-π-A Dye for Record Efficiency Solid-State Sensitized Heterojunction Solar Cells

Authors: Ning Cai§, Soo-Jin Moon‡, Le Cevey-Ha‡, Thomas Moehl‡, Robin Humphry-Baker‡, Peng Wang†, Shaik M. Zakeeruddin‡, and Michael Gratzel‡

Affiliation: †State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, ‡Laboratory of Photonics and Interfaces, Institute of Chemical Sciences and Engineering, Ecole Polytechnique Federale de Lausanne, §Graduate School, Chinese Academy of Sciences

Journal: Nano Letters, 2011, ASAP

Depiction of a dye-sensitized solar cell made with C220

Dye-sensitized solar cells (DSCs) have the potential to be a cheap alternative to conventional semiconductor solar cells.  They use a wide band gap semiconductor such as TiO2 attached to a conducting material.  A molecular sensitizer (the dye) is then added to the surface.  When light excites the material, the sensitizer sends an electron into the conduction band of the semiconductor.  Many of the current molecular sensitizers are ruthenium complexes; however much work is being done to develop metal-free organic dyes.  These dyes can have a great deal of structural flexibility, high molar extinction coefficients, and lower toxicity to both humans and the environment.

The organic dyes consist of a molecule with donor and acceptor groups connected by a spacer.  The authors designed and synthesized a spacer, 4,4’-didodecyl-4H-cyclopenta[2,1-b:3,4-b’]dithiophene (CPDT), to connect an electron donor and acceptor, making a dye hereafter called C220.

(a) C220 (b) Z907

The researchers compared C220 to a standard ruthenium based dye Z907.  The maximum absorption coefficient of C220 was found to be almost five times higher than that of Z907.  This indicates that C220 absorbs far more light than Z907.  Additionally, the power conversion from solar power to electric power was found to be 6.08%, which is the highest ever reported for a device of this type.  The highest conversion from a Z907 ever reported was 5%.  The authors were also able to estimate the electron injection efficiency using two different methods with C220 providing efficiencies of 0.97 and 0.92 and Z907 providing efficiencies of 0.88 and 0.85.  This means that once C220 is excited it will inject almost all of its excited electrons into the conduction band of the semiconductor.

This dye has better light harvesting properties than the standard ruthenium based dye and nearly 100% electron injection efficiency.  Future work will be conducted on synthesizing more metal-free organic dyes with even greater molar extinction coefficients.

Just for fun – Using some pretty simple tools you can make your own dye-sensitized solar cell, instructions here.

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