Direct Measurement of Density for Evaporated Thin Films

A simple and elegant method is reported for direct measurement of the density of vapor-deposited thin films using a combination of profilometry, microscopy, and high-vacuum thermogravimetric analysis (TGA). Density affects fabrication control, optical properties, charge transport, and mechanical properties of thin film devices. Accurate density determination is essential for optimizing device design to ensure film stability, charge balance, and light-matter interactions. Exact mass of the vapor-deposited thin films with suitable mass and aspect ratio is determined by high-vacuum TGA. Combined with precise measurements of thickness and area for each film, the true density of individual films is captured as deposited. This method avoids common universalizing assumptions about the degree of crystallinity in a film. Furthermore, this method does not depend on bulk optical or electrical parameters that may vary as a result of the unique nanoscale properties of thin films. Density values are reported for a range of small molecule semiconductors commonly found in organic electronics such as organic light emitting diodes (OLEDs), organic photovoltaics (OPVs), and organic thin film transistors (OTFTs), and compared to values calculated by other means. It is anticipated that this technique can be used to measure the density of a wide range of vacuum-stable thin film materials.