Scanning probe microscopy of solar cells: From inorganic thin films to organic photovoltaics
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oduction Direct conversion of solar radiation into electricity with photovoltaics represents a major component of proposed renewable energy portfolios, yet currently comprises less than 0.2% of energy production in the United States1 and approximately 0.4% of global energy production,2 indicating tremendous opportunity for innovation in materials that more efficiently and inexpensively harness the abundance of solar insolation. Of existing photovoltaic technologies, crystalline silicon cells currently dominate production, with grid-connected electricity generation as the main end-use of these cells.3 The potential for lower production and materials costs make thin-film inorganic and organic solar cells leading candidates for ushering in widescale adoption of photovoltaics. Inorganic thin-film solar cells have demonstrated 20% efficiency, already making them competitive with crystalline silicon photovoltaics, which have exhibited 25% efficiency.4 It is estimated that organic photovoltaics will become cost competitive
with crystalline silicon cells at efficiencies between 5% and 10% and at lifetimes between 5 and 10 years;5 by comparison, crystalline silicon solar cells are typically sold with 20–25 year warranties. Remarkably, the efficiency of organic solar cells has increased from 3% to 10% in the last five years,4,6 and the upper practical efficiency limit has been estimated to be ∼20%.7 A better understanding of nanoscale phenomena in both organic and thin-film inorganic photovoltaics is needed for this progress to continue apace. The goal of this brief article is to survey scanning probe microscopy (SPM) studies of these fascinating materials. It is fair to say that SPM measurements continue to influence organic semiconductor development by forcing the materials science community to revise its assumptions about charge trapping and charge injection in these materials. For example, SPM has played a seminal role in disentangling and separately analyzing the processes of charge injection, charge transport, and charge trapping at the organic/metal and organic/ dielectric interfaces in organic transistors.8,9 While this article
James R. O’Dea, Cornell University; [email protected] Louisa M. Brown, Cornell University; [email protected] Nikolas Hoepker, Cornell University; [email protected] John A. Marohn, Cornell University; [email protected] Sascha Sadewasser, International Iberian Nanotechnology Laboratory, Braga, Portugal; [email protected] DOI: 10.1557/mrs.2012.143
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MRS BULLETIN • VOLUME 37 • JULY 2012 • www.mrs.org/bulletin
© 2012 Materials Research Society
SCANNING PROBE MICROSCOPY OF SOLAR CELLS: FROM INORGANIC THIN FILMS TO ORGANIC PHOTOVOLTAICS
Figure 1. The structures of representative inorganic and organic solar cells. The table enumerates general time and length scales of the fundamental processes contributing to the photovoltaic action in each device. Ovals represent excitons, circles represent mobile charges, and squares represent trapped charges. CIGSe, Cu(In,Ga)Se2.
focuses on scanning probe st
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