Solution-Derived NiO Hole Transport Layers on the PTB7:PC71BM Organic Solar Cells

Energy demands are increasing all over the world, and renewable energy resources are needed. Photovoltaics is one of the attractive choice. Recently, organic solar cells have attracted great attention because of light weight and potentially low cost from printing at low temperature on flexible substrates. Traditional bulk heterojunction polymer solar cells consist of a transparent indium tin oxide (ITO) anode, a hole transport layer (HTL), a photoactive layer, and a top cathode. HTLs must have high optical transparency, good chemical stability, a large ionization potential, and good electron blocking capability. In a typical polymer solar cell, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) has been used as an HTL, and has a work function (Φ) of 5.2 eV. However, The PEDOT:PSS HTLs caused inferior device performance and degrade long term reliability due to their acidity, tendency to absorb water, and inability to block electrons effectively. Accordingly, the oxides which do not have these problems have been considered to be used as potential HTLs. Nickel oxide (NiO) has recently attracted a lot of attention because it can replace a PEDOT :PSS of HTL in solar cells. On the other hand, bulk hetero-junction (BHJ) solar cells based on [4,8- biz[Poly[[4,8-bis[(2-ethylhexyl)oxy]benzo [1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2- ethylhexyl)carbonyl]thieno[3,4-b]-thiophenediyl] (PTB7) and phenyl-C₇₁-butyric-acid-methyl-ester (PC₇₁BM) on ITO-coated glass substrates have recently been reported to have the efficiency as high as 9.2%. However, solar cells with a NiO HTL based on PTB7 / PC₇₁BM have not been well investigated.

In this paper, we report on the performance of solar cells with a structure of glass/ITO/NiO/PTB7:PC₇₁BM/LiF/Al, and also report the effects of the NiO thickness on the performance of the solar cells. To synthesize the precursor solution, we dissolved nickel acetate tetrahydrate (Ni(OAc)2·H₂O) and mono ethanolamine (MEA) in 2-Methoxethanol. The NiO films were synthesized by spin-coating the NiO precursor followed by annealing on a hot plate in air at 350 °C.　The NiO thickness were optimized to realize better performance by changing concentration of nickel acetate tetrahydrate in 2-metxy ethanol solution used for spin coating deposition. The concentraton of Ni(OAc)2·H₂O was prepared from 0.1 M to 0.4 M. Resulting in, the organic solar cell with NiO HTL using 0.2 M precursor shows highest performance, the device with NiO HTL using 0.4 M precursor shows worst performance. Furthermore, The solar cells with the NiO HTL layers showed improved long term reliability compared to the devices with PEDOT：PSS. As a result, the maximum power conversion efficiency of 5.14% was realized at 0.2 M of the nickel acetate tetrahydrate concentration. The PCE degradation of the solar cells with the HTL was compared to that of the control solar cells with PEDOT:PSS as an HTL.