title: Organic nanoparticle inks for OPV interface layers
Creators: name: Xu, Junyi
Publisher: name: Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU)
Year: 2024

description: Organic solar cells (OSCs) offer potential as flexible, lightweight, and cost-effective sources of renewable energy. Key considerations for their success include efficiency, long lifetime, and low cost. While recent progress has boosted power conversation efficiency (PCE), the focus is now shifting to extending the lifetime of OSCs. Beyond optimizing active layers, critical to both efficiency and stability is interface tailoring. A significant challenge is the absence of a high-performance and stable hole transporting layer (HTL) between the non-fullerene active layer and the top electrode. This challenge is particularly pronounced in the pursuit of all-solution-processed OSCs, emphasizing the need for solution-processed HTLs. The p-type semiconductors from the hulk heterojunction meet the criteria for HTLs with varying energy levels. However, their use as HTLs is impeded by their mutual solubility with active layer materials, facing a challenge for processing multiple layers. Our approach involves developing ligand-free p-type organic nanoparticle (NP) dispersions in alcohols using a simple, rapid precipitation process. The NPs can be doped by forming a pre-assembled structure in the precursor solution, allowing for the tuning of electrical properties and enhancement of charge transport. Specifically, OSCs utilizing doped P3HT nanoparticles (D-P3HTnp) as the HTL for P3HT:O-IDTBR achieve a champion efficiency of 6.7%. Additionally, PM6:Y6 OSCs with doped Spiro-OMeTAD nanoparticles (D-Spiro-OMeTADnp) exhibit a PCE exceeding 12%. By leveraging the wetting properties of a mesoporous NP surface, we have employed organic nanoparticles to fabricate a mesoscopic interface layer at low temperatures. Through the optimization of sequential doping with a novel class of organic salt dopants, we achieved the creation of homogeneously doped interfaces. The choice of solvent in the sequential doping process plays a critical role in forming a donor-rich and uniformly doped top interface in the n-i-p architecture. This results in high-efficiency and stable OSCs on both rigid and flexible substrates. Moreover, unencapsulated devices demonstrate remarkable shelf-life stability in air for 22000 h and exhibit long-term operational stability under 1 sun illumination. Sequential doping, especially for acceptors with good organic solubility, risks washing out too much of the acceptor, leading to poor device performance. In the last part, we propose a more general polymer bilayer HTL approach for non-fullerene acceptor based OSCs. The bilayer HTL, composed of doped PTAA nanoparticle (D-PTAAnp) and PEDOT:PSS without additional surfactants, provides ideal energy level alignment and forms a quasi-Ohmic hole contact to the donor material. Inverted OSCs fabricated using this approach achieved a PCE of 17.1%. These devices exhibit superior operational stability, maintaining 94% of their initial performance after 1800 h of operation under metal-halide lamp illumination at 60 °C. Finally, all-solution-processed OSCs with Ag nanowire electrodes also demonstrate remarkable stability.