Study of organic solar cells based on functional materials

Abstract

Organic-inorganic hybrid solar cells (HSCs) and organic solar cells (OSCs) have many advantages compared to traditional silicon solar cells, such as light weight, mechanical flexibility and low cost, because the processing of the devices from solution allows for using technologies such as printing or roll-to-roll techniques. In order to increase the efficiency and flexibility of solar cells, many advanced functional materials have been studied as the alternative for conventional inorganic materials used in HSCs and OSCs. For example, in order to improve the flexibility and stability, reduce the cost of OSCs, graphene and its derivatives were employed to replace indium tin oxide (ITO) transparent electrodes. OSCs with graphene electrodes show higher mechanical flexibility and stability than those with ITO under bending condition. In this thesis, the contents mainly focused on the study of HSCs and OSCs, including four parts. Part 1 is for the study of HSCs with novel oxides. Bi-based oxides, including BiFeO₃ (BFO), Bi₂WO₆ (BWO) and BiVO₄ (BVO), were used as n-type semiconductors in inorganic/organic HSCs with conjugated polymer Poly(3-hexylthiophene) (P3HT) as donor. The devices based on BVO show the best photovoltaic performance, including the highest open circuit voltage (Voc), Short Circuit Current Density (Jsc) and Power Conversion Efficiency (PCE). Compared with typical metal oxides, such as ZnO and TiO₂, Bi-based oxides show smaller band gap and higher light absorbance to visible light in short wavelength region. It is notable that the devices based on BWO and BVO show better photovoltaic performance than those based on TiO₂ or ZnO, which indicate that the Bi-based oxides are potential candidates for application in HSCs. Part 2 is focusing on the study of semitransparent OSCs with graphene transparent electrodes. Compared to conventional silicon photovoltaic devices, another advantage of OSCs is the possibility to realize semitransparent devices for special needs. The semitransparent OSCs based on P3HT and Phenyl-C₆₁-butyric acid methyl ester (PCBM) were fabricated with single-layer graphene as the top electrode. The conductance of the graphene electrode was improved by doping with Au nanoparticles and poly(3,4-ethylenedioxythiophene):poly(styrene sulfonic acid) (PEDOT:PSS) solution, which resulted in an increase of conductance for more than 400%. The performance of the organic solar cell was optimized by using the doped graphene electrode and the efficiency up to 2.7% was obtained in the devices with the area of 20 mm². Both the efficiency and the active area of the devices are much better than the reported results for the OSCs with graphene electrodes. Then the effect of active area on the device performance was studied. The efficiency decreased with the increase of the active area due to the increased series resistance and the decreased edge effect in the device. In addition, it is interesting to find that the semitransparent OSCs showed higher efficiency illuminated from graphene than from ITO side due to better transmittance of the graphene electrode. Part 3 is for the fabrication of flexible OSCs by using graphene electrodes and flexible plastic substrates. Package-free flexible OSCs are fabricated on polyimide (PI) substrates with doped multilayer graphene as top electrodes and Ag films as bottom electrodes. The device with the double-layer graphene electrode shows the maximum PCE of 3.2% and excellent bending stability. It exhibits the relative degradation of PCE for about 8% after 1000 times bending cycles. More importantly, it was demonstrate that two or more layers of graphene top electrodes can protect the OSCs very well from the contamination of air because multilayer graphene films are impermeable to air. Therefore, stable OSCs with graphene top electrodes can be fabricated without package, which may simplify the device fabrication, enhance the flexibility and decrease the cost of the devices. These results indicate that graphene is an excellent material for transparent electrodes of flexible OSCs as well as other organic devices especially some air sensitive ones. Finally, graphene doped by PEG with different molecular weights and concentrations has been systematically investigated in part 4. PEG drop-coated on graphene surface could induce significant n-type doping, and the conductivity of monolayer graphene was increased for more than 12 times after PEG doping. The n-type unipolar graphene filed effect transistors (GFETs) were created by doping part of the graphene channels with PEG. It is believed that PEG is an effective dopant to modulate graphene properties for practical applications.