Department of Chemical Engineering, Stanford University , Stanford, California 94303, United States.
Department of Photo-Molecular Science, Institute for Molecular Science , Myodaiji, Okazaki, Aichi 444-8585, Japan.
ACS Appl Mater Interfaces. 2018 Jan 10;10(1):1340-1346. doi: 10.1021/acsami.7b14034. Epub 2017 Dec 20.
Efficient n-doping of organic semiconductors requires electron-donating molecules with small ionization energies, making such n-dopants usually sensitive to degradation under air exposure. A workaround consists in the usage of air-stable precursor molecules containing the actual n-doping species. Here, we systematically analyze the doping mechanism of the small-molecule precursor o-MeO-DMBI-Cl, which releases a highly reducing o-MeO-DMBI radical upon thermal evaporation. n-Doping of N,N-bis(fluoren-2-yl)-naphthalene tetracarboxylic diimide yields air-stable and highly conductive films suitable for application as electron transport layer in organic solar cells. By photoelectron spectroscopy, we determine a reduced doping efficiency at high doping concentrations. We attribute this reduction to a change of the precursor decomposition mechanism with rising crucible temperature, yielding an undesired demethylation at high evaporation rates. Our results do not only show the possibility of efficient and air-stable n-doping, but also support the design of novel air-stable precursor molecules of strong n-dopants.
高效的有机半导体 n 型掺杂需要具有小电离能的供电子分子，这使得此类 n 型掺杂剂通常在暴露于空气中时容易降解。一种解决方法是使用含有实际 n 型掺杂剂的空气稳定前体分子。在这里，我们系统地分析了小分子前体 o-MeO-DMBI-Cl 的掺杂机制，该前体在热蒸发时会释放出高度还原的 o-MeO-DMBI 自由基。N,N-双（芴-2-基）-萘二酰亚胺的 n 型掺杂可得到空气稳定且高导电性的薄膜，适用于有机太阳能电池中的电子传输层。通过光电子能谱，我们确定在高掺杂浓度下掺杂效率降低。我们将这种降低归因于坩埚温度升高导致前体分解机制的变化，在高蒸发速率下产生不期望的脱甲基化。我们的结果不仅展示了高效且空气稳定的 n 型掺杂的可能性，还支持了新型强 n 型掺杂剂的空气稳定前体分子的设计。
