Performance Optimization Of PTB7:PC₇₀BM-based Organic Solar Cells By Incorporating Ag Nanocubes

Recently, organic solar cells（OSCs） have attracted wide attentions due to abundant raw materials, low-cost fabrication process, flexibility and other unique advantages. However, the performance of OSCs is still less satisfactory compared with their inorganic counterparts. One of the challenges is the limited light absorption due to the mismatch between optical absorption length and charge transport scale. In other words, the active layer is too thick which is not conducive to the carrier’s dissociation, transport and collection. Therefore, it is necessary to find ways to enhance light absorption effectively in the active layer without increasing its thickness. For this aim, the method of incorporating metallic nanostructures in OSCs has been actively explored. In particular, the use of metal nanoparticles（NPs） for improving the performance of OSCs has several advantages in terms of their controllable optical properties, low-cost and process compatibility with OSC fabrication. Two effects are mainly responsible for the improved PCE of noble metal NPs-contained OSCs, i.e., the plasmonic near field enhancement and the far field scattering effects.In this work, the effect of Ag nanocubes, with and without the silica shell, i.e., Ag NCs@Si O2 and Ag NCs that were incorporated at the bottom of active layer, on the performance of OSCs was investigated and wherein the physical mechanism was also analyzed in detail. The contents and results of this work are shown as follows:1. Firstly, we synthesized the Ag NCs by the liquid chemical synthesis. Then the Ag NCs@Si O2 was prepared according to the St?ber method. And these nanoparticles have been characterized by UV-vis spectrophotometer, field emission scanning electron microscope and high resolution transmission electron microscopy. The results showed that the bare cuboidal shaped Ag NCs have an edge length of ca. 40-50 nm and the Ag NCs@Si O2 are covered completely by an 8-15 nm thick Si O2 shell. The resonance absorption peaks of Ag NCs were located at 351 nm, 384 nm and 440 nm, and the resonance absorption peaks of Ag NCs Si O2 were located at 356 nm,386 nm and 451 nm. Compared with main absorption peak of Ag NCs, Ag NCs@Si O2 had a redshift of about 11 nm.2. We designed the structure of OSCs. Then the reference and with Ag NPs in PTB7:PC₇₀BM-based OSCs were optimized, and the photovoltaic performance and stability were also characterized. The combination of these advantages result in an average power conversion efficiency（PCE） of 7.84% for cells with Ag NCs@Si O2, and an average PCE of 7.53% for cells with Ag NCs, leading to a 13.8% and 9.13% increase in PCE respectively over that of the structurally identical control cell without Ag NPs. In addition, we found that it could enhance the durability of OSCs when introduced the Ag NPs.3. We analyzed the effect mechanism of Ag NPs on photovoltaic performance by studied the effect of Ag NPs on surface morphology of active layer and the effect of Ag NPs on exciton generation, exciton dissociation and the carrier mobility. And we also clarified the function of Si O2 shell in Ag NCs@Si O2 by compared the photovoltaic performance of the devices doping Ag NCs and Ag NCs@Si O2. The results indicated that doping the Ag NPs increased the rate of exciton generation duo to it improved the light absorption capacity of the active layer, and increased the rate of exciton dissociation and the carrier transfer and collection due to it improved the phase separation of the active layer. It changed the optical field distribution in the active layer, which let the region of exciton generation and dissociation move to the cathode, reduced the transport distance of low mobility of electrons, reduced the accumulation of space charge within the active layer and then increased the carrier transmission and collection, at the same time it improved the stability of the devices. And compared to Ag NCs, Ag NCs@Si O2 stimulated a stronger surface plasmon resonance, and let the Ag NPs deeper into the interior of the active layer, thereby rendered a stronger ability to capture light and induced a higher efficiency of transport and collection of carriers.