Effects of Patterning Polymer Electrode/Blend Interface of Bulk Heterojunction Solar Cells

The highest efficiencies reported in the field of organic solar cells are now in the 5-6% range. Charge extraction is of fundamental importance for their efficient operation. The external quantum efficiency (EQE) (which is the current obtained outside the device per incoming photon) of many organic and hybrid organic-inorganic devices are still limited by low exciton harvesting efficiencies, most apparent in planar heterostructures that suffer from direct trade-off between light absorption and exciton diffusion, prior to their dissociation at the donor-acceptor junction.

This project aims to improve the efficiency of organic solar cells, which could potentially be manufactured at low cost in roll-to-roll coating machines. The approach is to pattern the polymer (PEDOT:PSS) between the photoactive layer and the ITO anode at the nanometre length scale. This will provide an interpenetrating interface and shorter pathway for separated charge carriers to be extracted faster at the electrode, as well as increase light absorption by increasing scattering and total internal reflections.

The project involves: making PDMS (rubber) stamps with diffraction gratings using available and cheap DVD plates and establishing the right treatment for hydrophilicity; finding the right conditions for imprinting PEDOT:PSS using the put down process of soft lithography; fabrication/characterization of devices on patterned PEDOT:PSS; understanding the physics behind the improvements due to patterning by conductivity, photoluminescence and absorption measurements; making master stamps using ion beam lithography with 100nm depth features for PDMS stamp preparations.

 In the last year we have made progress in establishing the tools required for the project. In particular, we have adopted a unique method for making the PDMS stamps from readily available DVDs. We have also made progress in demonstrating a nanoscale patterning of PEDOT:PSS with depths of 15nm and 700nm period.

We fabricated and characterised devices on patterned PEDOT:PSS and evidence efficiencies which are twice of their counterpart reference devices.