Enhanced cation interaction in perovskites for efficient tandem solar cells with silicon.

To achieve the full potential of monolithic perovskite/silicon tandem solar cells, crystal defects and film inhomogeneities in the perovskite top cell must be minimized. We discuss the use of methylenediammonium dichloride as an additive to the perovskite precursor solution, resulting in the incorporation of in situ-formed tetrahydrotriazinium (THTZ-H+) into the perovskite lattice upon film crystallization. The cyclic nature of the THTZ-H+ cation enables a strong interaction with the lead octahedra of the perovskite lattice through the formation of hydrogen bonds with iodide in multiple directions. This structure improves the device power conversion efficiency (PCE) and phase stability of 1.68 electron volts perovskites under prolonged light and heat exposure under 1-sun illumination at 85°C. Monolithic perovskite/silicon tandems incorporating THTZ-H+ in the perovskite photo absorber reached a 33.7% independently certified PCE for a device area of 1 square centimeter.

Triiodide Formation Governs Oxidation Mechanism of Tin-Lead Perovskite Solar Cells via A-Site Choice.

Mixed tin-lead (Sn-Pb) halide perovskites stand out as promising materials for next-generation photovoltaics and near-infrared optoelectronics. However, their sensitivity to oxidative degradation remains a major hurdle toward their widespread deployment. A holistic understanding of their oxidation processes considering all their constituent ions is therefore essential to stabilize these materials. Herein, we reveal that A-site cation choice plays an inconspicuous yet crucial role in determining Sn-Pb perovskite stability toward oxidation. Comparing typical A-site compositions, we show that thin films and solar cells containing cesium are more resistant to oxidative stress relative to their methylammonium analogs. We identify degradation in these compositions to be closely linked to the presence of triiodide, a harmful species evolving from native I2 oxidants. We find that hydrogen bonding between methylammonium and I2 promotes triiodide formation, while the strong polarizing character of cesium limits this process by capturing I2. Inspired from these findings, we design two strategies to boost stability of sensitive methylammonium-based Sn-Pb perovskite films and devices against oxidation. Specifically, we modulate the polarizing character of surface A-sites in perovskite via CsI and RbI coatings, and we incorporate Na2S2O3 as an I2 scavenging additive. These crucial mechanistic insights will pave the way for the design of highly efficient and stable Sn-Pb perovskite optoelectronics.

Backbone-driven host-dopant miscibility modulates molecular doping in NDI conjugated polymers.

Molecular doping is the key to enabling organic electronic devices, however, the design strategies to maximize doping efficiency demands further clarity and comprehension. Previous reports focus on the effect of the side chains, but the role of the backbone is still not well understood. In this study, we synthesize a series of NDI-based copolymers with bithiophene, vinylene, and acetylenic moieties (P1G, P2G, and P3G, respectively), all containing branched triethylene glycol side chains. Using computational and experimental methods, we explore the impact of the conjugated backbone using three key parameters for doping in organic semiconductors: energy levels, microstructure, and miscibility. Our experimental results show that P1G undergoes the most efficient n-type doping owed primarily to its higher dipole moment, and better host-dopant miscibility with N-DMBI. In contrast, P2G and P3G possess more planar backbones than P1G, but the lack of long-range order, and poor host-dopant miscibility limit their doping efficiency. Our data suggest that backbone planarity alone is not enough to maximize the electrical conductivity (σ) of n-type doped organic semiconductors, and that backbone polarity also plays an important role in enhancing σ via host-dopant miscibility. Finally, the thermoelectric properties of doped P1G exhibit a power factor of 0.077 µW m-1 K-2, and ultra-low in-plane thermal conductivity of 0.13 W m-1K-1 at 5 mol% of N-DMBI, which is among the lowest thermal conductivity values reported for n-type doped conjugated polymers.

A Nonionic Alcohol Soluble Polymer Cathode Interlayer Enables Efficient Organic and Perovskite Solar Cells.

The choice of interfacial materials and their properties play a critical role in determining solar cell performance and stability. For compatibility with roll-to-roll printing, it is desirable to develop stable cathode interface layers (CILs) that can be processed over the photoactive layer using orthogonal solvents. In this study, an n-type naphthalene diimide core and oligo (ethylene glycol) side-chain-based conjugated polymer is reported as a universal, efficient CIL for organic and perovskite photovoltaics. Besides good thermal stability and easy processing in alcohol/water, the new CIL is found to possess electron transport properties with an electrical conductivity of 2.3 × 10-6 S cm-1, enabling its use as a CIL with a film thickness of up to ∼35(±2) nm. Utilizing the new CIL, 16% power conversion efficiency (PCE) is achieved for organic solar cells (OSCs) based on the PM6-Y6 photoactive layer (8.9% PCE for no CIL and 15.1% with state-of-the-art CIL, PDINO), and perovskite solar cells from methylammonium lead iodide yielded a PCE of 17.6%. Compared to the reference devices, the new CIL reduced trap-assisted carrier recombination and increased the built-in potential by 80 mV, simultaneously enhancing all photovoltaic parameters. Moreover, new CIL based devices had better photostability with no burn-in losses.