Four X-shaped quinoxaline-based organic dyes, PQx (1), TQx, (2), PQxD (3), and TQxD (4) (D = dye sensitizers) are developed and served as p-type selfassemble monolayer (SAM) for tin perovskite solar cells (TPSC). Thermal, optical, and electrochemical properties of these SAMs are thoroughly investigated and characterized. Tin perovskite layers are successfully deposited on these four SAM surfaces according to a two-step approach and the devices exhibit power conversion efficiency in the order of TQxD (8.3%) > TQx (8.0%) > PQxD (7.1%) > PQx (6.1%). With thiophene π-extended conjugation unit in SAM structure, TQxD (4) exhibits the highest hole extraction rates, greatest hole mobilities, and slowest charge recombination to achieve great device performance of 8.3%, which is the current best result for SAM-based TPSC ever reported. Furthermore, all devices except PQx shows great enduring stability for the performance retaining ≈90% of their original values for shelf storage over 1600 h.
The traditional way to stabilize α‐phase formamidinium lead triiodide (FAPbI3) perovskite often involves considerable additions of methylammonium (MA) and bromide into the perovskite lattice, leading to an enlarged bandgap and reduced thermal stability. This work shows a seed‐assisted growth strategy to induce a bottom‐up crystallization of MA‐free perovskite, by introducing a small amount of α‐CsPbBr3/DMSO (5%) as seeds into the pristine FAPbI3 system. During the initial crystalization period, the typical hexagonal α‐FAPbI3 crystals (containing α‐CsPbBr3 seeds) are directly formed even at ambient temperature, as observed by laser scanning confocal microscopy. It indicates that these seeds can promote the formation and stabilization of α‐FAPbI3 below the thermodynamic phase‐transition temperature. After annealing not beyond 100 °C, CsPbBr3 seeds homogeneously diffused into the entire perovskite layer via an ions exchange process. This work demonstrates an efficiency of 22% with hysteresis‐free inverted perovskite solar cells (PSCs), one of the highest performances for MA‐free inverted PSCs. Despite absented passivation processes, open‐circuit voltage is improved by 100 millivolts compared to the control devices with the same stoichiometry, and long‐term operational stability retained 92% under continuous full sun illumination. Going MA‐free and low‐temperature processes are a new insight for compatibility with tandems or flexible PSCs.
Imidazolium (IM) and cesium (Cs) were treated as A-site cationic additives for a triiodide tin perovskite solar cell (FASnI 3 ; FA, formamidinium) with sulfamic acid (SA) as a bifunctional additive in varied proportions. IM has a tight aromatic structure that is feasible to passivate the crystal defects and help in perovskite crystallization. Cs can stabilize the crystal structure and decrease trap state densities. TOF-SIMS measurements show the passivation effect of both IM and SA occurring on the surface of the film. The bifunctional SA additive can occupy the iodine vacancies in tin perovskites to passivate the uncoordinated Sn atoms and to passivate the surface defects effectively. Furthermore, SA has the effect of reducing Sn 4+ back to Sn 2+ via its ammonia/ acid form converted from its zwitterionic form upon irradiation, as observed from in situ XPS measurements. The hybrid device was optimized to attain a PCE value of 12.5% for the perovskite structure Cs 0.02 IM 0.1 FA 0.88 SnI 3 +1% SA with superior, long-lasting stability.
A new set of pyrrolopyrrole‐based (PPr) polymers incorporated with thioalkylated/alkylated bithiophene (SBT/BT) is synthesized and explored as hole‐transporting materials (HTMs) for Sn‐based perovskite solar cells (TPSCs). Three bithiophenyl spacers bearing the thioalkylated hexyl (SBT‐6), thioalkylated tetradecyl (SBT‐14), and tetradecyl (BT‐14) chains are utilized to examine the effect of the alkyl chain lengths. Among them, the TPSCs are fabricated using PPr‐SBT‐14 as HTMs through a two‐step approach by attaining a power conversion efficiency (PCE) of 7.6% with a remarkable long‐term stability beyond 6000 h, which has not been reported elsewhere for a non‐PEDOT:PSS‐based TPSC. The PPr‐SBT‐14 device is stable under light irradiation for 5 h in air (50% relative humidity) at the maximum power point (MPP). The highly planar structure, strong intramolecular S(alkyl)···S(thiophene) interactions, and extended π‐conjugation of SBT enable the PPr‐SBT‐14 device to outperform the standard poly(3‐hexylthiophene,‐2,5‐diyl (P3HT) and other devices. The longer thio‐tetradecyl chain in SBT‐14 restricts molecular rotation and strongly affects the molecular conformation, solubility, and film wettability over other polymers. Thus, the present study makes a promising dopant‐free polymeric HTM model for the future design of highly efficient and stable TPSCs.
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