Electrode Interface Engineering For High-efficiency And Stable Organic Solar Cells

Organic solar cells（OSCs）have become a promising photovoltaic technology because of its unique advantages such as light weight,flexibility and large area solution processing.With the development of photovoltaic materials,the design of device structure and the optimization of electrode interface,the power conversion efficiency（PCE）of OSCs is improved rapidly.However,stability is still an important barrier hindering the commercialization of OSCs at present.Electrode interface engineering based on cathode interface layer and anode interface layer is an important strategy to improve the efficiency and stability of OSCs.In this thesis,we aim to improve the efficiency and stability of OSCs by developing new organic and inorganic interface materials as electrode interface layer.The specific work is mainly divided into two parts:one is the high-efficiency and stable OSCs based on the cross-linked polymer PFOPy cathode interface layer;the other is the high-efficiency and stable conventional and inverted OSCs based on the solution processing MoO_x anode interface layer.First,a novel cross-linkable and alcohol-soluble pyridine-incorporated polyfluorene derivative,denoted as PFOPy,is used as a cathode interface layer in OSCs.For PFOPy,the pendant epoxy group can be effectively cross linked through cationic polymerization under thermal treatment and the pendant pyridine group can offer good alcohol solubility.Optical absorption tests of PFOPy films before/after washing by chloroform demonstrate the excellent solvent-resistance property for the cross-linked PFOPy film.Compared with the typical ITO modified by the Zn O cathode interface layer（ITO/Zn O）,the ITO modified by the cross-linked PFOPy cathode interface layer（ITO/PFOPy）also has a significantly lower work function,which can form a good energy level match with most acceptors.Utilizing an inverted device structure and a typical active layer of PM6:Y6,OSCs based on ITO/Zn O display an optimal power conversion efficiency（PCE）of 15.83%while OSCs ITO/PFOPy exhibit superior photovoltaic performance with an optimal PCE of 16.20%.Moreover,OSCs based on ITO/Zn O and ITO/PFOPy separately maintain 89%and 90%of the corresponding initial PCE after 12 h of illumination,indicating similarly excellent photostability.More importantly,after 26 complete thermal cycles,OSCs based on ITO/Zn O only maintain 81%of the initial PCE while OSCs ITO/PFOPy retain 92%of the initial PCE and exhibit obviously better thermal cycling stability,indicating that the cross-linked PFOPy cathode interface layer should offer stronger interface robustness against thermal cycling stress due to the viscoelastic and cross-linked characteristics of PFOPy.The impressive results indicate that the cross-linked PFOPy cathode interface layer would be a very promising cathode interface layer in OSCs.Second,a simple,low-cost and environment-friendly method was introduced to synthesize molybdenum oxide（MoO_x）nanoparticle alcohol solution,and the solution-processed MoO_x（s-MoO_x）anode interface layer was prepared by low temperature annealing process for conventional and inverted OSCs.The UPS test shows that the anode interface layer based on s-MoO_x can effectively improve the work function of ITO and form a good energy level match with most donors.Using PM6:Y6 as active layer,conventional and inverted OSCs based on s-MoO_x as anode interface layer were prepared respectively.At the same time,the conventional OSCs based on the typical PEDOT:PSS anode interface layer and the inverted OSCs based on evaporated MoO₃（e-MoO₃）are prepared as reference devices.The efficiency of the two reference devices based on PEDOT:PSS and e-MoO₃anode interface layer is 16.94%and 16.03%respectively,while the efficiency of conventional and inverted devices based on s-MoO_x is as high as 17.09%and 16.28%respectively,which is equivalent to or even slightly better than the corresponding reference devices.In addition,the storage stability of unpackaged inverted devices based on s-MoO_xanode interface layer in air is investigated.After 240 h,the efficiency of the device based on the e-MoO₃ anode interface layer decreased to 46%of the initial efficiency,while the device based on the s-MoO_x anode interface layer retained 42%of the initial efficiency under the same experimental conditions,indicating that the storage stability of the devices based on s-MoO_x in the air is comparable to that of the devices based on the dense e-MoO₃ anode interface layer.In a word,it has great potential to construct efficient and stable OSCs based on s-MoO_x anode interface layer.