We address here the need for a general strategy to control molecular assembly over multiple length scales. Efficient organic photovoltaics require an active layer comprised of a mesoscale interconnected networks of nanoscale aggregates of semiconductors. We demonstrate a method, using principles of molecular self-assembly and geometric packing, for controlled assembly of semiconductors at the nanoscale and mesoscale. Nanoparticles of poly(3-hexylthiophene) (P3HT) or [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) were fabricated with targeted sizes. Nanoparticles containing a blend of both P3HT and PCBM were also fabricated. The active layer morphology was tuned by the changing particle composition, particle radii, and the ratios of P3HT:PCBM particles. Photovoltaic devices were fabricated from these aqueous nanoparticle dispersions with comparable device performance to typical bulk-heterojunction devices. Our strategy opens a revolutionary pathway to study and tune the active layer morphology systematically while exercising control of the component assembly at multiple length scales.

With metal halide perovskite solar cells (PSCs) now reaching device efficiencies >23%, more emphasis must now shift toward addressing their device stability. Recently, a triarylamine-based organic hole-transport material (HTM) doped with its oxidized salt analogue (EH44/EH44-ox) led to unencapsulated PSCs with high stability in ambient conditions. Here we report criteria for triarylamine-based organic HTMs formulated with stable oxidized salts as hole-transport layer (HTL) for increased PSC thermal stability. The triarylamine-based dopants must contain at least two para-electron-donating groups for radical cation stabilization to prevent impurity formation that leads to reduced PSC performance. The stability of unencapsulated devices prepared using these new HTMs stressed under constant load and illumination far outperforms that of both EH44/EH44-ox and Li+-doped spiro-OMeTAD controls at 50 °C. Furthermore, the ability to mix and match these dopants with a nonidentical small-molecule-based HTL matrix broadens the design scope for highly stable and cost-effective PSCs without sacrificing performance.

For environmentally friendly and cost-effective manufacturing of organic photovoltaic (OPV) cells, it is highly desirable to replace haloarenes with water as the active layer fabrication solvent. Replacing an organic solvent with water requires retooling the device fabrication steps. The optimization studies were conducted using poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C 61 butyric acid methyl ester (PCBM) as active layer materials. These materials were dispersed in water as blend and separate nanoparticles using the miniemulsion method. Topologies of the active layers were investigated using atomic force microscopy and electron microscopy techniques. We have identified two essential steps to fabricate efficient OPVs from aqueous dispersions: (1) treatment of the hole-transport layer with UV-O 3 to make the surface hydrophilic and (2) the use of an electron-transporting buffer layer for efficient charge extraction. We have also identified relative humidity and substrate temperature as key fabrication parameters for obtaining uniform active layer films. The OPV devices were fabricated using PEDOT:PSS as the hole-transport layer and PCBM as electron-transport layer with Ca/Al as the counter electrode.Efficiencies of 2.15% with a fill factor over 66% were obtained; the efficiency and the fill-factor is the highest among all aqueous processing of P3HT-PCBM nanoparticle solar cells.

We report on the improved operational stability of unencapsulated perovskite solar cells (PSCs) aged in an ambient atmosphere at elevated temperatures (70 °C) for >1000 h under constant illumination and bias at 30− 50% relative humidity. We identify a previously unseen interfacial degradation mechanism concerning the use of a MoO x interlayer, which was originally added to increase operational stability. Specifically, the hole-transport layer/MoO x interface buckles under illumination at 70 °C, which leads to delamination and rapid losses of short-circuit current density corresponding to an average t 80 of ∼55 h. By judiciously evaluating various hole-transport layers, interlayers, and contacts, we find that replacing the MoO x with a VO x interlayer, regardless of the other components in the solar cell, alleviates this buckling issue due to its higher activation barrier toward crystallization, leading to significant gains in PSC operational stability. Unencapsulated devices aged in an ambient atmosphere with a VO x interlayer retain 71% of their initial PCE on average after constant illumination and bias at 70 °C for 1100 h (t 80 ∼ 645 h). Currently, this is the highest temperature reported for the operational stability of unencapsulated n-i-p PSCs aged in air. Identification of a new facet of the complex degradation mechanisms in PSCs will allow for targeted acceleration testing to speed the deployment of low-cost, long-lasting electricity generation under realistic operating temperatures.