The Study Of The Interfacial Modifications And Interfacial Processes In Organic Solar Cells

Organic solar cells are potential candidates for the new generation of photovoltaic industry because they have many advantages such as their low fabrication cost, variety organic materials, easy to carry, consume less power, light weight, and compatibility for large-scale manufacturing. Today, the low and unstable performance prevents the commercialization and practical application of organic solar cells. Many reports have shown that interface and interfical processes are the main reasons that affect the performance and stability of cells, but they are still not clear understood. In order to further enhance the power conversion effecienfy of organic solar cells, the study of interfacial modifications and interfacial processes becomes a hot topic. The design and optimization of interface, as well as the understand of interface processes have become effective ways to improve the performance of organic solar cells, attracting a lot of attention of researchers. The four issues including optimization of donor (D)/acceptor (A) interface, inserting an interface blocking layer into the active layer, optimization of buffer layer, and using double layer interfacial modification in organic solar cells have been studied in this work. The effect of these optimizations and designs on interfacial processes and device performance have been discussed in detail. The thesis is divided into the following five parts:1. D/A interface optimization to enhance the interfacial exciton dissociation in organic solar cellsPolymer is easy to aggregate in solution. Organic thin films that prepared from the aggregated polymer solution is not uniform, leading to low performance due to unfavourable exciton dissociation and charge collection. In this work, [6,6]-phenyl-C61-butyric acidmethyl ester (PCBM) and poly(3-hexylthiophene) (P3HT) are used as acceptor and donor, respectively, we systematically studied the effect of the dispersion degree of sol-gel P3HT:PCBM, which was characterized by the capillary rise height, on photovoltaic performance of bulk heterojunction (BHJ) organic solar cells. Without solvent and thermal annealing, the power conversion efficiency of devices fabricated from sol-gel blend solution with certain dispersion can reach 2.35%, 5.6 times higher than that of the devices from the same solution with complete dispersion. The optimal the capillary rise height is 3.2 cm. The 0.52 V of open circuit voltage,8.35 mA/cm2 of short circuit current and 54% of fill factor are achieved for the device prepared from P3HT:PCBM solution with the optimal capillary rise height. The effect of the dispersion degree of P3HT:PCBM solution has been investigated by various methods like I-V, UV-Vis, XRD, and EQE measurements. The results of UV-Vis absorption and XRD measurements indicate that the packing and the crystallinity of P3HT can be improved by using sol-gel P3HT:PCBM blend solution. Certain degree of dispersion is helpful for achieving better performance without any annealing due to the balance between good crystallinity of P3HT and good contact between P3HT and PCBM. The increased interfacial contact provide more exciton dissociation interface, improving the power conversion efficiency of organic solar cells.2. Blocking layer introduce into the active layer to enhance the performance of organic solar cellNPB as blocking layer is introduced into the active layer C60/F16ZnPc of n-n type solar cell in our work. On the one hand, NPB efficiently prevent the backflow recombination of electrons from C6o.On the other hand, NPB provide a dissociation interface of exciton from C60, increasing the interaction between holes in C60 and exciton from F16ZnPc. The improved photocurrent and open circuit voltage of the solar cell was then ascribed to this inerfacial modification. The EQE and I-V measurement show NPB blocking layer can efficiently prevent the backflow recombination of electrons. Our study shows that the main photocurrent is contributed from the interaction between exciton of C60 and exciton of F16ZnPc at C6o/F16ZnPc interface when the thickness of NPB blocking layer is less than 2.5 nm. Above 2.5 nm, the photocurrent is mainly contributed from the interaction between holes in C6o and excitons from F16ZnPc. The analysis on the EQE measured without and with bias light indicates that the hole-exciton interaction is increased with the NPB thickness increase, leading to an improved photocurrent. The transient photocurrent measurement further confirms the hole-exciton interaction existance in the device with NPB blocking layer.3. Interfacial buffer layer optimization to enhance the charge collection in solar cellWe systematically investigate the effect of the thickness of the PEIE interface layer on the performance of two types of polymer solar cells based on inverted P3HT:PCBM and thieno[3,4-b]thiophene/benzodithiophene (PTB7):[6,6]-phenyl C71-butyric acidmethyl ester (PC71BM). Maximum power conversion efficiencies of 4.18% and 7.40% are achieved at an optimized 5 nm-thick PEIE interface layer, for the two above-mentioned solar cell types, respectively. The optimized PEIE layer provides a strong enough dipole for the best charge collection while maintaining charge tunneling ability. The measured external quantum efficiencies for the devices with thick PEIE layer are quite similar to that of the optimized device, indicating the poor charge collection ability of thick PEIE layers. Aomic force microscopy measurements indicate the relative low performance of devices with a PEIE layer of thickness less than 5 nm is the result of weak dipole and partial coverage of the PEIE layer on ITO. The contact angle and the thickness of the active layer measurement show that the contact angle with different thickness PEIE layer is gradually decreased with the PEIE thickness layer increase and the active layer coated on the different thickness PEIE layer are almost unchange. Optical transmittance and active layer thickness measurements indicate that all PEIE films have the same high transmittance and similar thickness, ruling out the influence of the PEIE layer on these two parameters.4. Bilayer interfacial modification layer to efficiently suppress interfacial recombination in organic solar cellsA cathode bilayer buffer layer with higher charge transfer ability was fabricated by using PEIE covered ZnO nanoparticles (NPs) (ZnO/PEIE). High performance inverted P3HT:PCBM-based polymer solar cells without thermal annealing the active layer were realized using bilayer ZnO/PEIE as a cathode buffer layer. The power conversion efficiency without thermal annealing the active layer was 3.8% for the device with ZnO/PEIE buffer layer, with the 0.59 V of open circuit voltage,10.73 mA/cm2 of short circuit current and 60% of fill factor, much higher than that of the devices with only ZnO or PEIE buffer layer, respectively. Transient photovoltage/photocurrent and electrochemical impedance measurements confirm that the device with ZnO/PEIE bilayer has faster charge transfer ability and less interfacial charge recombination than devices with other cathode buffer layers. AFM characterization shows that ZnO NPs are uniformly covered by PEIE, which would be not only lower the work function of ZnO, but also be benefit for reducing defects caused by oxygen adsorption on ZnO, ultimate enhance the transporting of charge and decrease charge recombination at cathode interface.5. Summary and outlookThe work mentioned above are summarized in this chapter. The inadequate of our work is pointed out. The future working is proposed.