Methods and Apparatus for Spatially Resolved Photocurrent Mapping of Operating Photovoltaic Devices Using Atomic Force Photovoltaic Microscopy

Mark Hersam (Inventor)

Research output: Patent

Abstract

An atomic force photovoltaic microscope (AFPM) has been developed to characterize spatially localized inhomogeneities in organic photovoltaic (OPV) devices. A biased conductive atomic force microscopy (cAFM) probe is raster-scanned over an array of illuminated solar cells enabling the determination of short-circuit current Isc, open-circuit voltage Voc, fill factor and power conversion efficiency ?p of functioning photovoltaic devices. Abstract Performance of OPV devices is generally characterized by the power conversion efficiency. However, this does not provide insight into local photocurrent spatial variations within these devices. Bulk-heterojunction (BHJ) devices may exhibit local efficiency variations from defects or the phase-separated nature of the microstructure, as established by SEM, TEM and other methods. Correlations between electrical properties and morphology in OPV films have been demonstrated by the scanning probe techniques (1) time resolved electrostatic force microscopy (trEFM) and (2) photoconductive AFM (pcAFM). However, only photovoltaic films are characterized by these techniques. This invention provides methods for quantitative characterization of variations in fully operational photovoltaic devices by scanning across an array of solar cells that include the metal cathodes. The AFPM affords standard photovoltaic figures of merit near actual operating conditions, and minimizes substrate-cAFM tip contact effects. AFPM was applied to elucidate variations within the BHJ organic solar cells of poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM). Simultaneous AFPM topography-current maps from P3HT:PCBM OPVs are shown in Figure 1. The current maps in Figures 1b and 1c collected sequentially from the same set scan area, with the sample bias at 0.0V and -0.5V respectively. Similar device-to-device variations are observed, including variations in Isc up to ~ 25% between OPVs separated by
Original languageEnglish
Patent number8099792
StatePublished - Jan 17 2012

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