Bulk heterojuntion organic solar cells (BHJ OSCs) have gained much attraction in recent years. These solar cells have layered geometric structure and optimization of each layer is essential to achieve the overall performance of the device. Active layer consisting of donor and acceptor material is sandwiched between two electrodes. In order to improve the device stability and efficiency, often interfacial layers, namely electron and hole transport layers, between the active layer and electrodes are introduced. Poly (3.4-ethylenedioxythiophene): Poly (styrenesulfonate) (PEDOT:PSS) is most commonly applied hole transport layer (HTL) in OSCs. However, it limits the device performance due to its highly hygroscopic and acidic nature which arise reliability issues while reducing the cell life drastically. Metal oxides have been proved to be a good alternative to it. These metal oxides improve the device efficiency comparable with standard HTL and at the same time they substantially enhance cell stability. The aim of this work is to optimize the hole transport layer by using different metal oxides such as vanadium pentaoxide (VO) by improving its structural, optical, and electrical properties. VO has been extensively studied as a promising candidate for charge extraction and transportation because of its exceptional electronic properties. It proved to be more stable due to its good transparency, low resistance and better adhesions to the active layer. The novelty of our study is to develop a highly controllable, reproducible and cost effective fabrication technique to build our device with poly [N-9’-heptadecanyl-2,7-carbazole-alt-5,5-(4’,7’-di-2-thienyl-2’,1’,3’-benzothiadiazole)] (PCDTBT), and (6,6)-Phenyl C71 butyric acid methyl ester (PC71BM) based active layer yielding high stability and efficiency. This work is focused on improving the device stability along with the efficiency which has not been explored much to date.

In this work, we demonstrate the fabrication of PCDTBT:PC71BM based bulk heterojunction solar cells with two variants of hole transport layer (HTL) in different fabrication atmospheres. Device with standard PEDOT:PSS hole transport layer has been compared with its organic-inorganic (VO in PEDOT:PSS) hybrid variant in terms of efficiency and life time stability when they are fabricated in controlled mode in the nitrogen filled atmosphere and an ambient mode in the presence of oxygen and 65% relative humidity (RH). Vanadium pentaoxide (VO) nanoparticles, synthesized by co precipitation method, are mixed in PEDOT:PSS layer to form an organic-inorganic hybrid HTL. Both variant of hole transport layer are further characterised for their structural and optical properties to optimise these fundamental properties to ensure highly stable and efficient solar cell. Normalized efficiencies are calculated as a function of time for life time stability tests of both types of devices fabricated in both atmospheric modes. The best performance is achieved for a device with hybrid HTL fabricated in controlled mode where the life time stability has improved from 70% to 94% over one week period and from 65% to 90% over four week time when they are compared with their standard PEDOT:PSS HTL counterpart. Our results confirm that fabrication environment play a key role in device performance in terms of stability and efficiency along with the improvement brought by addition of VO nanoparticles in the pristine PEDOT:PSS hole transport layer.

Keywords: Hole transport layer; Organic solar cells; PEDOT:PSS; Stability; VO


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