RESEARCH AREAS

RESEARCH AREAS

Interface Science of Organic Photovoltaics

The Organic Photovoltaics Subgroup is focused on the chemistry, physics and device performance gains possible at the bulk-heterojunction organic photovoltaic device interface. Of primary focus is the interface between the organic light-absorbing layer (P3HT:PCBM or MDMO-PPV:PCBM) and the anode layer, typically tin-doped indium oxide. An interlayer is introduced between these two layers to induce the proper energy level for an ohmic contact, or a contact of reduced resistance to majority charge carriers. Additionally, the interlayer has energy levels well out of alignment with minority charge carriers (i.e., electrons at the anode) to prevent their collections.

Due to the nature of bulk-heterojunction design, both donor and acceptor species are in direct contact with both electrodes, allowing charges to travel to the "wrong" electrode and erode device efficiency. Literature-based devices were modified with an interfacial hole-transport/electron-blocking layer (HTL/EBL) on the anode side of the device with the intention of preventing “leakage” flow of electrons to the anode.

Thus far, two materials have helped to accomplish this goal: NiO¹ by sputtering, and solution-processable TPDSi₂:TFB.² NiO deposited by pulsed-laser deposition yields an increase in light to electricity conversion efficiency of 3% to 5%. TPDSi₂:TFB is a spin-coated EBL consisting of a blend of the small molecule TPDSi₂ and the hole-transporting polymer TFB. This layer can be used in conjunction with the commonly-used interfacial layer PEDOT:PSS, or as a non-corrosive PEDOT:PSS replacement that increases device thermal stability. The use of the TPDSi₂:TFB blend significantly increases solar cell performance compared to devices made with PEDOT:PSS, with the overall power conversion efficiency increasing from 1.73% to 2.21%.

Currently, further improvements to these interfacial layers are being explored that could yield even larger gains in device efficiency. On-going research is also looking into how exactly the improvements occur, using techniques such as impedance spectroscopy and conductive atomic force microscopy. This work is being done in conjunction with Professors Mark Hersam and Joseph Hupp.

Footnotes

1. "p-Type semiconducting nickel oxide as an efficiency-enhancing anode interfacial layer in polymer bulk-heterojunction solar cells," M. D. Irwin, D. B. Buchholz, A. W. Hains, R. P. H. Chang, T. J. Marks, Proceedings of the National Academy of Sciences of the United States of America, 105(8), 2783-2787 (2008).

2. "High-efficiency hole-extraction/electron-blocking layer to replace poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) in bulk-heterojunction polymer solar cells," A. W. Hains, T. J. Marks, Applied Physics Letters, 92(2), 023504-1-3 (2008).

April 2010