Solution-processable hole-transporting inorganic semiconductors for electronic applications

Abstract

This thesis presents the development of solution-processable hole-transporting inorganic semiconductors for electronic applications. As the improvement of electron-transporting oxides, such as ZnO, In2O3, and SnO2, has continued to advance, the progress on the hole-transporting counterparts is still largely lagging behind. Copper(I) oxide (Cu2O) is one of a handful oxide materials that shows hole transport. This thesis shows the first demonstration of p-channel TFTs from Cu2O thin films processed from the spray pyrolysis technique. The field-effect hole mobility (µ_FE) is in the range of 10^–4 to 10^–3 cm^2/(V.s) while the on-to-off channel current ratio (I_D^on-off) is on the order of 10^3. The work presented here emphasises the versatility of the spray pyrolysis, which has previously been employed to successfully produce n-type oxides and dielectrics. Another copper(I)-based inorganic compound, copper(I) thiocyanate (CuSCN), has a unique combination of good hole-transporting characteristics and excellent optical transparency. This thesis presents extensive characterisation results of CuSCN thin films from their chemical, electronic, optical, morphological, to structural properties and demonstrates, for the first time, p-channel TFTs with a truly transparent active layer based on a solution-processed wide-band-gap inorganic semiconductor. µ_FE in the range of 0.01–0.1 cm^2/(V.s) has been achieved whereas I_D^on-off is on the order of 10^4. A p-channel unipolar voltage inverter has also been constructed from CuSCN TFTs, showing the possibility of realising transparent electronics. CuSCN-based TFTs are also studied in more details by analysing their transfer characteristics for the distribution of localised states and examining their temperature dependence for the hole transport modes. Furthermore, due to its novel electronic and optical properties, CuSCN is also offered as a replacement of PEDOT:PSS as a hole-transporting layer in bulk-heterojunction organic photovoltaics (OPVs) and organic light-emitting diodes (OLEDs). Owing to its superior optical transparency, CuSCN-based solar cells consistently exhibit higher power conversion efficiency (PCE) than PEDOT:PSS-based cells, especially in the case of cells employing active layers that absorb light in the near-infrared spectral range. Specifically, cells based on PDPP-2T-TT:PC71BM show PCE of ~8% with CuSCN HTL compared to ~6.2% with PEDOT:PSS HTL. In addition, the energy levels of CuSCN lead to a lower hole injection barrier as well as effective electron blocking property, yielding OLEDs with a low turn-on voltage and low leakage current. OLEDs employing an active layer of (PPy)2Ir(acac) in 26DCzPy:TCTA with CuSCN HTL show higher values of highest achievable efficacies (51 cd/A and 55 lm/W at 1 cd/m^2) compared to those with PEDOT:PSS HTL (38 cd/A at 2142 cd/m^2 and 14 lm/W at 1710 cd/m^2).