Top electrode and active layer design for achieving fully solution-processed high-efficiency organic solar cells

Abstract

Solar energy plays a critical role concerning current energy shortage and environment pollution. Solar cell technologies including crystalline silicon solar cells, thin-film solar cells, perovskite solar cells and organic solar cells (OSCs) that transfer solar energy into electricity have attracted tremendous attention in the past decades. Among them, OSCs with their non-toxic nature, lightweight, flexibility and scalability have made a huge success that the power conversion efficiency (PCE) of the state-of-the-art OSC have achieved over 18%, comparable to conventional silicon-based solar cells. To realize the practical application of OSCs, it is essential to fabricate the device by fully solution process because of its high materials utility and low energy consumption. However, solution-processed top electrode is challenging in the device fabrication due to the detrimental solvents in the precursor solution or post-treatments required are likely to cause irreversible damage to the underlying functional layers. Meanwhile, the morphology of active layer has huge influence on device performance. The donor/acceptor shall gradiently enrich near anode/cathode to facilitate charge transportation and collection. Unfortunately, since the active layer are normally deposited from a mixed solution of donor and acceptor, it is difficult to achieve a favorable vertical distribution. To solve these issues and promote the practical application of the fully solution-processed high-efficiency OSCs, we have conducted the following researches. 1. Propose a self-sintering strategy to connect isolated silver nanoparticles into conductive top electrode After immersing into silver nitrate solution, the isolated silver nanoparticles will merge with each other with a crystal orientation of 21° and to form continuous Ag film. The sintered film presents high purity, excellent conductivity and surface flatness, which is applicable as top electrode in OSCs. With this strategy, OSCs either with inverted or conventional structures are fabricated. Since the self-sintering process completes in seconds and the employed solvent is chemically compatible to the bottom layers, the devices display efficient charge extraction and transportation properties, the corresponding photovoltaic performance is comparable to its evaporated counterpart. 2. Demonstrate non-immersive and dry sintering of silver nanoparticles in OSCs with negligible influence on underlying layers The solvent issue and detrimental post-treatments are critical during the solution deposition of top electrode as it may cause severe material dissolution, corruption and poor interlayer contact. A dry layer of hydrogen intercalated molybdenum oxide is adopted to sinter the silver nanoparticles that enables great interfacial contact and negligible influence on underlying layers. The sintering mechanism is systematically uncovered and the fabricated evaporation-free device exhibits a high PCE of 15%. 3. Uncover the additive induced vertical distribution of organic donor and acceptor in active layer to enhance device performance The additive 1-chloronaphthalene has been widely applied in organic solar cell to tune the aggregation behavior of the organic components. We reveal CN can optimize the vertical distribution of donor and acceptor to make the acceptor accumulate on top. Such structure is beneficial for the charge dissociation and collection. When applied in the high-efficiency OSCs system, D18-Cl:PC71BM:Y6, a PCE of 18% is achieved, which is one of the top efficiencies nowadays.