Abstract:
Anode interlayers (AILs) are important for enhancing the efficiency of organic solar cells (OSCs). AILs are used in OSC devices as charge transportation layers, and to supress the charge recombination process. The challenge of producing low-cost, solution processed AILs with high environmental stability and good charge carrier mobility has gained attention in the recent years. Solution-processing is particularly attractive for easy fabrication of mass produced devices. However, apart from poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) and a few metal oxides, there are not many AIL materials being studied, and fewer still have been experimented with in OSC devices. This thesis focuses on producing different types of AILs and evaluating their performance when included within standard OSC structures. Electrospray deposition, an unconventional solution processing technique, has been trialled. All the AILs used in this thesis are solution processable, deposited from alcohol solvents and fabricated with simple procedures at low annealing temperatures. Firstly, a non-conjugated polyvinylpyrrolidone (PVP) polymer was used to modify molybdenum trioxide (PVP-MoO3) and vanadium pentoxide (PVP-V2O5) AILs. Interestingly, the surface properties of these metal oxides were transformed from planar to nanotextured. Both PVP-MoO3 and PVP-V2O5 nanocomposites had good optical properties, and the formation of metal oxides from the precursor was confirmed with X-ray photoelectron spectroscopy (XPS) analysis. Contact angle measurements showed that, compared to PVP-MoO3, PVP-V2O5 has higher hydrophobicity which leads to the smooth spread of the active layer deposited from anon-polar solvent. The resultant OSC device performance with these modified AILs is outlined with stability studies. Overall, among PVP-metal oxide AILs, PVP-V2O5 showed the best PCEof 7.10 % retaining 91 % of the initial PCE after 96 hours. V2O5 AILs are known to produce stable OSC devices with good efficiency, but solution processing has previously been confined to lab-scale spin coating procedures. We used the electrospray deposition technique to deposit V2O5 AILs (ESD-V2O5) and compare them with spin coated AILs (SC-V2O5). ESD-V2O5 thin films are optimized in terms of different flowrates, the nozzle to substrate distance, and also composition of the V2O5 precursor. The ESDV2O5 devices had transmittance and surface morphology comparable to the SC-V2O5 interlayers used in high-efficiency OSC devices. XPS indicated that the obtained ESD-V2O5 surfaces had a favourable chemical composition. Hole mobility of ESD-V2O5 was four times higher than SCV2O5 with very low charge transport resistance. The enhanced charge transportation properties, which can reduce charge recombination, can be attributed to the advantageous surface and interfacial properties of ESD-V2O5 AILs. Results indicate that optimized ESD-V2O5 had unperturbed diode characteristics compared to SC-V2O5, with 7.61 % PCE.Finally, a new carbazole derived small molecule named 7H-dibenzo(c,g)carbazole (DBCz) wasinvestigated as an AIL in OSC devices. This AIL is highly soluble in alcohol and showedremarkable thermal stability with excellent optical properties. DBCz AILs had nanotextured surfaces with higher surface roughness than PEDOT:PSS, and a similar trend was observed for the PTB7:PCBM active layer coated on these AILs. DBCz also formed hydrophobic surfaces, which could be beneficial for active layers deposited from non-polar solvents. Because of these qualities, DBCz AIL based OSC devices showed a 8 % higher short-circuit current (JSC) with good fill factor (FF) when compared with conventional PEDOT:PSS AILs. However, DBCz has a misaligned energy level with the donor. This leads to low open-circuit voltage values, andless efficiency compared to PEDOT:PSS. Apart from this issue, DBCz also showed good charge transportation properties, with similar hole mobility and conductivity to PEDOT:PSS, as well as to other reports of organic and inorganic AILs. DBCz could be chemically modified to fine-tune its energy alignment with the active layers.