Cathode Layers for Photovoltaics Based on poly(3,4-ethylenedioxythiophene)

Technology  

This invention is an inverted OPV that traps light to excite an organic electronic material, establishing a flow of electrons from an Ag anode to an ITO cathode. The electrons in this model flow in the opposite direction of a typical OPV. Inverted bilayer OPVs were constructed with layers in the following order from bottom to top: 1) glass, 2) ITO bottom cathode, 3) low work function  PEDOT cathode buffer layer treated with TDAE or Cs2CO3, 4) C60 electron acceptor (40 nm thick), 5) tetraphenyldibenzylperiflanthene (DBP) electron donor (30 nm thick), 6) MoO3 anode buffer layer, and 7) Ag top anode.  Work function measurements showed that TDAE treatment decreased the work function of PEDOT:PSS by 0.6 eV and the work function of CVD PEDOT by 0.8 eV. Cs2CO3 treatment decreased the work functions of both types of PEDOT by 0.3 eV. These low work function PEDOT layers increased the open-circuit voltage (Voc) to 0.9 V, which is comparable to conventional OPVs. In future iterations of this design, current flow through the cell could be could be increased by blending the electron acceptor and electron donor in the center of the structure into a bulk heterojunction or by choosing different materials for the electron acceptor and donor layers (layers 4 and 5).   

Problem Addressed 

OPVs with a conventional structural orientation are illuminated at the bottom through a transparent anode, such as indium tin oxide (ITO), and electrons are collected on a low work function metal cathode on top. Inverted OPVs use ITO as cathode material and low work function metal substances as anode material instead. Inverted OPVs have several benefits: they may 1) provide more degrees of freedom in designing OPV fabrication schemes, 2) allow for protection of the delicate organic semiconductor layers below the anodic buffer layers, and 3) be more stable by allowing use of higher work function metal top contacts. However, ITO has a high work function and makes a poor cathodes by itself. In order to increase the efficiency of an inverted OPV, a low work function (LWF) cathode buffer layer must be developed to provide a sufficient electric field throughout the device.   

Advantages

• More flexibility in the design of OPV fabrication schemes
• Protection of layers below anodic buffer layer
• Increased stability of OPVs
• Open-circuit voltage comparable to conventional OPVs

Publications

"Cathode Buffer Layers Based on Vacuum and Solution Deposited Poly(3,4-Ethylenedioxythiophene) for Efficient Inverted Organic Solar Cells." Applied Physics Letters, April 30, 2012.