Fabrication And Charge Transport Mechanism Of Interfacial Layers For Organic Solar Cells

In the past few years, organic solar cells(OSCs) have been studied extensively because of many competitive advantages including their versatility for large-scale fabrication through roll-to-roll printing technique, flexibility, portability, lightweight, and low cost. Effective interfacial layer can form ohmic contacts, which can effectively collect and transport charges at the interfaces. PEDOT:PSS is the widely used buffer layer on an indium tin oxide(ITO) electrode. However, PEDOT:PSS has a side effect on the stability of OSCs due to its high acidity and hygroscopic nature. The device performance and stability can be improved through introducing other air-stable interfacial materials.In this work, vanadium oxide(V2O5) prepared via a simple O2 plasma processing was used as anode buffer layer on indium tin oxide(ITO) coated glass for polymer solar cells(PSCs). The V2O5 layer with high transmittance, good electrical and interfacial properties was prepared by spin coating vanadium(V) triisopropoxide oxide alcohol solution on ITO and then O2 plasma treatment. PSCs based on P3HT:PC61BM and PBDTTT-C:PC71BM using V2O5(O2 plasma) as anode buffer layer show high power conversion efficiencies(PCEs) of 4.47% and 7.54% under the illumination of AM 1.5G at 100 mW/cm2. Compared to the control device with PBDTTT-C:PC71BM as active layer and PEDOT:PSS(PCE of 6.52%) or thermal annealed V2O5(PCE of 6.27%) as anode buffer layer, the PCE was improved by 15.6% and 20.2% with the introduction of V2O5(O2 plasma) anode buffer layer. The improved PCE is ascribed to the greatly improved fill factor and enhanced short circuit current density of the devices, which is benefited from the change of work function of V2O5, the surface with lots of dangling bonds for better interfacial contact, and excellent charge transport property of V2O5(O2 plasma) layer. The results indicate that O2 plasma processed V2O5 film is an efficient and economical anode buffer layer for high-performance PSCs. It also provides an attractive choice for low cost fabrication of organic electronics.As a promising two-dimensional nanomaterial, graphene(G) shows significant potential for applications in many areas, such as energy, material, electronic information technology and bio-medicine industry, due to its outstanding physical and chemical properties, including high surface area, high thermal conductivity, high mechanical strength and high electron mobility. In addition, graphene can be used for flexible electronic device and functional composite materials. A low-cost and oxygen functionalized graphene(FG) was synthesized via a simple two-step method for applications in OSCs as anode buffer layer. The FG shows excellent dispersion in aqueous solution and great process compatibility with spin coating process. PCE of FG-based devices(4.13%, 4.49%, and 7.11% for P3HT:PC61BM, P3HT:PC71BM and PBDTTT-C:P71BM, respectively) outperform PEDOT:PSS-based devices(3.67%, 4.17%, and 6.46%, respectively). Moreover, the stability of the devices was improved with FG as anode buffer layer compared to PEDOT:PSS. The PCE of FG based devices maintain ratios at 85% after 10 days. The results indicate that simple synthesized FG is a promising solution-processed anode buffer layer material for high-efficiency and stable OSCs.Zinc oxide(ZnO) and aluminum-doped ZnO(AZO) were synthesized and investigated as electron transport layer(ETL) for inverted polymer solar cells(IPSCs). The PBDTTT-C:PC71BM based cells with AZO ETL demonstrate high power conversion efficiency of 7.36%. Compared to the cells with ZnO(PCE of 6.85%), the PCE is improved by 7.45% with the introduction of AZO layer. The improved PCE is ascribed to the enhanced short circuit current density, which results from electron transport property of AZO layer. Moreover, AZO is a more stable interfacial layer than aZnO. The PCE of AZO-based solar cells remain 85% of the original value after storage for 120 days, superior to 39% of ZnO-based cells. The results above indicate that simple low-temperature solution-processed AZO film is an efficient and economical ETL for high-performance IPSCs.Perovskite solar cell is a new type of solar cell. We applied a high-mobility diketopyrrolopyrrole-based copolymer(P) in compact layer free CH3NH3PbI3 perovskite solar cells as hole-transporting layer(HTL). By using the P-HTL, the 6.62% device efficiency with conventional poly-3-hexylthiophene was increased to 10.80% in the simple device configuration(ITO/ CH3NH3PbI3/HTL/MoO3/Ag). With improved short circuit current density, open circuit voltage and fill factor, the higher power conversion efficiency of P-HTL device is ascribed to the higher carrier mobility, more suitable energy level, and lower interfacial charge recombination.The research of this work will contribute to industrialization production and commercial applications for solar cell with low cost, low temperature solution processing technology.