Room-temperature solution-processed hole transport layer design for high performance organic solar cells

Abstract

Due to the limited reserve of fossil fuel energy and accompanied environmental pollution issue, humans are now seeking for more sustainable and green energy sources. Solar energy, with its infinite reserve and environmental friendliness, is winning a wide attention. Solar cells, the technique which converts light energy into electrical energy, has been extensively investigated for the commercial production purpose. Among the various solar cells, inorganic solar cells are most developed. However, inorganic solar cells exhibit various disadvantages including fragility, high-cost and complex fabrication process. Until the discovery of organic solar cells (OSCs), researcher realized these drawbacks could be perfectly addressed. Due to the solubility of organic semiconductor materials in organic solvent, OSCs can be solution processed. Solution-processing techniques, including doctor blade and roll-to-roll (R2R), makes the continuous and massive production realizable. Besides, the plastic-like organic semiconductors are also more flexible and light-weight than inorganic semiconductors. Based on the understanding of working mechanism of OSCs, carrier transport layers (CTLs) play an important role in the working process of OSCs. The most commonly used hole transport layer (HTL) material, poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS), tends to corrode the electrodes and absorbs water in the air due to its acidic and hygroscopic properties. As result, inorganic oxides, including molybdenum oxide (MoOx), vanadium oxide (V2O5), tungsten oxide (WO3), nickel oxide (NiOx) and graphene oxide (GO), are promising substitutes of PEDOT:PSS. However, the existing preparation process of metal oxide based HTL still has several drawbacks. To solve these issues, we have conducted our research from the following aspects. 1) Efficient hole transport layers with widely tunable work function for deep-HOMO donor based organic solar cells Deep highest occupied molecular orbital (HOMO) energy level donor based OSCs has a potential to realize a large open-circuit voltage (VOC) value and result in high performance. This work focuses on HTL design for deep-HOMO donor based OSCs. By incorporating a small molecule, 2,3,4,5,6-pentafluorobenzylphosphonic acid (F5BnPA), into the HTL materials, the work function (WF) of HTL can be continuously tuned to match with the deep HOMO value of donor. 2) Pre- and post-treatments free NiOx based nanocomposite HTL for conventional OSCs. The developed NiOx still needs pre-treatments of underlying anodes due to the poor wettability of water on pristine indium tin oxide (ITO). This work demonstrates an ethanol dispersible 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) incorporated NiOx working as HTL in OSCs. Due to the good wettability of ethanol on pristine ITO, this NiOx:F4-TCNQ HTLs can be prepared without any pre- and post-treatments. This one-step prepared NiOx:F4-TCNQ based OSCs perform much better than one-step prepared PEDOT:PSS based OSCs. 3) NiOx NPs decorated graphene oxide (GO) nanosheets as HTL for inverted OSCs. Compared with conventional OSCs, inverted OSCs exhibit better stability. However, NiOx based HTLs have never been reported in inverted OSCs. The normally used evaporated MoOx is energy wasting and not compatible with solution-processed fabrication of OSCs. Here, we develop a NiOx NPs decorated GO nanosheets working as HTL in OSCs. Due to the good conductivity of NiOx, the thickness limit of GO has been addressed.