Atomic Force Photovoltaic Microscopy (27148)
INVENTOR: Mark Hersam and Benjamin Leever
INVENTION: 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 ηp of functioning photovoltaic devices.
ADVANTAGES: Unlike techniques that characterize only photovoltaic films, AFPM examines functioning photovoltaic devices, providing standard figures of merit such as power conversion efficiency. The flexibility of AFPM suggests applicability to nanoscale characterization of a wide range of opto-electronic materials and devices.
SUMMARY: 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 <10 µm. Examination of scans over a truncated current scale exhibit a transient response with Isc decaying ~40% during the ~ 1 min probe contact interval (Figure 2). AFPM in conjunction with cAFM, which can measure current-voltage characteristics of individually addressed solar cells, provide powerful tools to explore the structural and design aspects of OPVs, including defect density and power conversion efficiency as a function of device area, and other performance-spatial inhomogeneiety relationships in photovoltaic cells.
Mark Hersam and Benjamin Leever
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