Abstract
Polyaniline, a low cost intrinsically conductive polymer, is stable under ambient conditions. Due to its rigid backbone structure, the solubility in common solvents is low and thus limits its commercial applications. After introducing sulfonic acid group into side chains, polyaniline can be water soluble and acid-doped, forming excess energy states between HOMO and LUMO, allowing ease for electron excitation from valence band to the created states. The conductivity of polyaniline changes from 10-9 S/cm to 10-1 S/cm after the doping. In this research, we demonstrate the performance of water soluble self-acid-doped polyaniline for use as hole transport layer (HTL) for polymer light emitting diode (PLED) and solar cell (PSC). Using MEHPPV as the active layer, we fabricate PLED device with the structure: ITO/HTL (20 nm)/MEHPPV (100 nm)/Ca (5 nm)/Al. As SPAN is used as HTL, the device exhibits the maximum brightness 20737 cd/m2 and maximum current efficiency 1.93 cd/A, which are comparable with the device utilizing commercialized PEDOT:PSS as HTL. However, compared to PEDOT:PSS, SPAN has lower fabrication cost and it has no phase separation. Thus, SPAN has potential to replace PEDOT:PSS as HTL for industrialization. Using PFO as the emitting layer, we fabricate PLED device with the structure: ITO/HTL (20 nm)/PFO (100 nm)/CsF (5 nm)/Al. As PANTCZ is used as HTL, the device exhibits the maximum brightness 2 cd/m2 and maximum current efficiency 10-4 cd/A, which are much worse than the device utilizing commercialized PEDOT:PSS as HTL. We suppose that di-tert-butylcarbazole side chains aggregate on ITO surface after spin-coating, leading hole injection barrier (0.5 eV) from ITO to polyaniline main chains. In order to verify the above presumption, we fabricate invert-PLED device with the structure: ITO/Al/PEO:Cs2CO3 (4:1, w/w) (12 nm)/PFO (80 nm)/HTL (20 nm)/Au (15 nm). As PANTCZ is used as HTL, the device exhibits the maximum brightness 3.8 cd/m2 and maximum current efficiency 3 x 10-3 cd/A, which are very poor indeed. However, if we compare both PLED device and invert-PLED device both with PANTCZ as HTL, we find that current efficiency has enhanced 30-fold, from 10-4 cd/A to 3 x 10-3 cd/A. The result means that we can change the alignment of main chains and side chains of PANTCZ by altering device structure. After adding dodecylbenzenesulfonic acid (DDBSA) into P3HT:PCBM system, we fabricate invert-PSC with the structure: ITO/ZnO/P3HT:PCBM with 2.5 % DDBSA (200 nm)/HTL (20 nm)/Ag. As SPAN is used as HTL, the device exhibits power conversion efficiency (PCE) 3.87 %. Comparing to the device with commercialized PEDOT:PSS as HTL (PCE = 3.35 %), SPAN device provides 0.52 % enhancement in PCE. We utilize the higher LUMO level material, ICBA, to replace PCBM as an acceptor in fabricating the invert-PSC device: ITO/ZnO/P3HT:ICBA with 2.5 % DDBSA (200 nm)/HTL (20 nm)/Ag. The result is similar to that of P3HT:PCBM system, when using SPAN as HTL, the device exhibits PCE 5.78 %. Comparing to the device with commercialized PEDOT:PSS as HTL (PCE 5 %), SPAN device provides 0.78 % enhancement in PCE. We fabricate hole-only and electron-only devices with the structures: ITO/PEDOT/Active layer/Au and ITO/Al/Ca/Active layer/Ca/Al, respectively. By using space charge limited current (SCLC) model, we can calculate hole mobility (µh) and electron mobility (µe). For P3HT:PCBM system, µe/µh decreases from 11.1 to 1.1 after DDBSA doping; for P3HT:ICBA system, µe/µh decreases from 8.1 to 1.3 after DDBSA doping. The results confirm that doping DDBSA into the active layer can help the balance between hole mobility and electron mobility, alleviating charge recombination due to intrinsically lower hole mobility.