Crystalline acenes are promising organic p-type semiconductors for future electronics. However, despite their importance to emerging and future electronics and in distinct contrast to the large technological care that is devoted to control the impurity levels in inorganic semiconductors, organic impurities in crystalline organic electronic components were left largely unexplored. The impurities in commercially available crystalline anthracene, tetracene and pentacene were analyzed and compared to crystalline organics that were obtained by two methods: solution (flux) growth, where the solvent was naphthalene, solid at room temperature but liquid in the crystal growth temperature range and by physical vapor transport. The impurities were identified by different methodologies: GC-EI-MS, a gas chromatograph GC equipped with an electron ionization ion source and a single quadrupole mass analyzer; GC-SMB-EI-QQQ-MS, a GC equipped with a low temperature electron ionization ion source (supersonic molecular beam, SMB), and a triple quadruple mass analyzer; high resolution MALDI-TOF-TOF-MS, matrix assisted laser desorption with two time of flight analyzers in series; and ESI-Q-TOF-MS, electrospray ionization mass spectrometers equipped with a quadruple mass analyzer and time of flight analyzers. Regular GC-EI-MS exhibited poor impurity identification power. The other three techniques provided complementary pictures of the various impurities present in these semiconductors with a significant overlap between identified impurities by the three instrumental approaches. GC-SMB-EI-QQQ-MS was the most revealing instrumental technique especially when combined with ChromatoProbe™ thermal desorption injector since the latter helped minimize degradation of pentacene during sample preparation. In all three organic semiconductors (OSs), the analyses clearly revealed a significant cleansing of the impurities by the physical vapor transport technique. The naphthalene flux crystallization method introduced a significant amount of naphthalene and benzothiofuran impurities, the latter being a known naphthalene impurity.
Bibliographical noteFunding Information:
This research was supported by the Singapore National Research Foundation under CREATE programme: Nanomaterials for Energy and Water Management, and by the Israel Ministry of Science. We thank the Russian Foundation for Basic Research (Grants 11-03-00551 and 11-03-92478). The authors thank Dr. Vitaly Lipik for his help in laser desorption TOF measurements.
- Crystalline organic semiconductors
- Supersonic molecular beam (SMB)