Research: Next-Generation Photovoltaic Technologies

A new understanding of organic semiconductor junctions

The foundation of almost all conventional inorganic semiconductor devices is the p-n junction, defined by the interface between an electron rich (n-type) and an electron poor (p-type) semiconductor. First developed in 1949, the Shockley ideal diode equation comprehensively describes the current-voltage characteristics of inorganic semiconductor p-n junctions, providing both physical insight and a quantitative analytical tool that has aided our understanding of the most fundamental properties of semiconductor devices, and in particular, solar cells, over the past six decades.

Organic, or ‘plastic’ electronics, are a relatively new technology that holds the prospect of providing ultra-cheap, lightweight, and flexible electronic applications. Indeed, organic solar cells (OSCs) provide a particularly useful and compelling application of organic electronics. These devices operate on the basis of a heterojunction formed between ‘donor’ and ‘acceptor’ organic semiconductors, often viewed as analogs to p and n-type inorganic semiconductors, respectively. As a result, the Shockley Equation has often been applied to analyze OSCs since their inception, but the inherently different physics of organic semiconductors has limited its ability to make useful predictions and direct improvements for both materials and device architectures. In particular, organic semiconductors are characterized by hopping transport on the nanoscale and tightly bound, localized exciton states that require significant energy to dissociate into free charge carriers as opposed to the delocalized nature of charge carriers in inorganic semiconductors.

Recently, scientists at the Center for Nanoscale Materials collaborated with scientists from the University of Michigan and Northwestern University to develop and test an ideal diode equation for organic semiconductor junctions. The work focuses on the dynamics of bound charge carrier pairs at the heterojunction and results in an equation that is analogous to the Shockley Equation (see figure, below). It predicts the temperature and light intensity dependence of solar cell parameters such as the dark current, open circuit voltage (V_oc) and short-circuit current (J_sc), particularly in situations where Shockley-based models break down. Furthermore, the analysis predicts the maximum V_oc attainable for a given heterojunction material pair, in agreement with the empirically-based conclusions of experimental studies.

Schematic illustrating the current-voltage characteristics and energetics of Coulombically bound charge carrier pairs at the heterojunction in organic semiconductor junctions.

The model is successfully applied to two archetype, planar heterojunction organic photovoltaic cells composed of copper phthalocyanine (CuPc) and boron subphthalocyanine chloride (SubPc) donors, and a fullerene (C₆₀) acceptor. These results should have a significant impact on both the understanding and further development of this exciting new class of solar cells.

The researchers on this project at Argonne National Laboratory were Noel Giebink (George Wells Beadle Postdoctoral Fellow) and Gary Wiederrecht. The University of Michigan scientists were Stephen Forrest and Brian Lassiter. The Northwestern University scientist was Michael Wasielewski, who also holds a Senior Scientist appointment at Argonne National Laboratory.

References

October 2010