Study On High-performance Small-molecule Organic Photovoltaic Cells

Organic photovoltaic cells （OPVs） have been considered as a promising option forrenewable energy due to their potential for flexible, low-cost, and large-scale production.High efficiency, long lifetime, and low cost are three key parameters forcommercialization. Although OPVs have approached commercialization, the high cost,short lifetime and low efficiency still need to be addressed. Therefore, the optimizationof device structure and understanding of device mechanism are important to overcomethese shortcomings. In this thesis, based on small-molecule organic photovoltaic cells,we systematically investigated device performance by focusing on increase of efficiency,reliability and device area.1. The effect of cathode buffer layer on the performance of OPV cells wassystematically investigated. Optical field distribution inside the OPV cell was simulatedto gain insight into the mechanism responsible for buffer layer used as an optical spacer.The influence of electron mobility and defect-state depth of buffer layers on chargecarriers transporting mechanism was also compared. The interface formed between C₆₀and Ag is the main limiting factor for OPV cells. By introducing bathophenanthroline（Bphen） and1,3,5-tris（2-N-phenylbenzimidazolyl） benzene （TPBi） as buffer layers, thecharge collection efficiency and absorption efficiency were increased due to their highmobility, wide bandgap and high reliability. Therefore, the efficiency and lifetime ofcopper phthalocyanin （CuPc）/C₆₀devices were increased by18%and162%,respectively.2. Through varying the annealing temperature of donor layer, the influence ofinterface morphology, absorption, crystallinity and hole mobility of CuPc films on theperformance of OPV cells was compared. Interdigitated bulk-heterojunction （BHJ）organic photovoltaic cells were fabricated by using thermal annealing-inducednanostructured CuPc as donor layer. By annealing the CuPc film at100°C, the verticalorientation of nanostructured CuPc can facilitate the separation of interfacialelectron-hole pairs and improve the charge carrier transport to electrodes. The maximumexternal quantum efficiency （EQE） of BHJ CuPc/C₆₀device at617nm was increasedby100%than that of planar device. Also, the efficiency was increased by133%.3. Aming at the S-shaped kink observed in the current-voltage curves of phosphorescent cells, two ways of using different hole transport layers to reduceinterfacial energy step and doping pentacene to improve charge carrier mobility wereused to investigate the origin of this kink. The result excludes interface dipoles as thefactor for the S-kink and showed that this anomalous feature is due to the presence oflarge interfacial energy step between the anode/donor interface and the low mobility ofdonor. By introducing N,N’-bis-（1-naphthyl）-N,N’-diphenyl-1,1’-biphenyl-4,4’-diamine（NPB） and1,1-bis（（di-4-tolylamino）phenyl） cyclohexane （TAPC） to decrease interfacialenergy step for （pbi）2Ir（acac） device, the S-shaped kink disappears.4. The large-area small-molecule OPV cells on substrates “snow-cleaned” with a jetof mixed-phase CO₂were fabricated. The effect of device area on photovoltaicparameters was studied using the analysis of Giebink theory. The influence ofparticulates on the indium-tin-oxide （ITO）-coated glass substrate was also investigated.We found the particulates on ITO substrate result in shunt paths across thedonor-acceptor heterojunction and reduce the device yield. Snow cleaning reducesparticulates （>20nm） on the ITO substrates, thereby reducing device shorts and shuntpaths. It can improve yield of1.44cm2boron-subphthalocyanine chloride （SubPc）/C₆₀OPV cells from zero for conventionally solvent-cleaned substrates to⁷0%. Thestandard deviation of photovoltaic parameters for snow-cleaned devices is≤4.0%. Thearea effect shows that the increase of series resistance and shunt path is the reason fordecrease of large-area device. By using a sub-electrode structure, the6.25cm2SubPc/C₆₀device has approximately82%efficiency of an analogous0.008cm2device,with the efficiency decrease due to series resistance of the ITO.5. The influence of intrinsic degradation of active layers and device structure onthe reliability of small-molecule OPV cells was studied. The photodegradation of SubPcand fullerene films aging in the dark and under AM1.5G illumination was presented.The initial burn-in deterioration and long term operational lifetime of encapsulateddevices were investigated. There are photobleaching and crystallization of SubPc film,and polymerization of C₆₀upon illumination under inert atmosphere. Thephoto-polymerization of C₆₀which causes the exciton-induced trap is responsible forthe observed burn-in loss in the planar SubPc/C₆₀device. By mixing SubPc with C₆₀,the presence of C₆₀in the SubPc:C₆₀film suppresses the formation of SubPccrystallization and slows the bleaching kinetics of SubPc film. The increasing ratio ofSubPc in the SubPc:C₆₀film can inhibit the dimerization of C₆₀, presumably by quenching the C₆₀excited state. Therefore, the mixed SubPc:C₆₀device has lowerburn-in loss and longer lifetime than the SubPc/C₆₀device. C70is more stable than C₆₀due to a much less extent of photo-polymerization. The devices based on C70show nodegradation of performance during the initial aging time. If we assume an exposure ofthe cell to an equivalent of5h of1sun, AM1.5G illumination per day, the long termlifetime of SubPc/C70device is973d.In summary, this work developed interface modification, large-area fabrication andaccelerated lifetime testing to address the three pillars of solar cell technology:efficiency, reliability, and scalability. It paves the way for transforming organicphotovoltaics into a fully practical energy solution.