Snaith group

Abstract:

Inverted-structure metal halide perovskite solar cells (PSCs) have attractive advantages like low-temperature processability and outstanding device stability. The two-step sequential deposition method shows the benefits of easy fabrication and decent performance repeatability. Nevertheless, it is still challenging to achieve high-performance inverted PSCs with similar or equal power conversion efficiencies (PCEs) compared to the regular-structure counterparts via this deposition method. Here, an improved two-step sequential deposition technique is demonstrated via treating the bottom organic hole-selective layer with the binary modulation system composed of a polyelectrolyte and an ammonium salt. Such improved sequential deposition method leads to the spontaneous refinement of up and buried interfaces for the perovskite films, contributing to high film quality with significantly reduced defect density and better charge transportation. As a result, the optimized PSCs show a large enhancement in the open-circuit voltage by 100 mV and a dramatic lift in the PCE from 18.1% to 23.4%, delivering the current state-of-the-art performances for inverted PSCs. Moreover, good operational and thermal stability is achieved upon the improved inverted PSCs. This innovative strategy helps gain a deeper insight into the perovskite crystal growth and defect modulation in the inverted PSCs based on the two-step sequential deposition method.

Abstract:

Achieving the long-term stability of perovskite solar cells is arguably the most important challenge required to enable widespread commercialization. Understanding the perovskite crystallization process and its direct impact on device stability is critical to achieving this goal. The commonly employed dimethyl-formamide/dimethyl-sulfoxide solvent preparation method results in a poor crystal quality and microstructure of the polycrystalline perovskite films. In this work, we introduce a high-temperature dimethyl-sulfoxide-free processing method that utilizes dimethylammonium chloride as an additive to control the perovskite intermediate precursor phases. By controlling the crystallization sequence, we tune the grain size, texturing, orientation (corner-up versus face-up) and crystallinity of the formamidinium (FA)/caesium (FA)yCs1–yPb(IxBr1–x)3 perovskite system. A population of encapsulated devices showed improved operational stability, with a median T80 lifetime (the time over which the device power conversion efficiency decreases to 80% of its initial value) for the steady-state power conversion efficiency of 1,190 hours, and a champion device showed a T80 of 1,410 hours, under simulated sunlight at 65 °C in air, under open-circuit conditions. This work highlights the importance of material quality in achieving the long-term operational stability of perovskite optoelectronic devices.

Abstract:

Perovskite interfaces critically influence the final performance of the photovoltaic devices. Optimizing them by reducing the defect densities or improving the contact with the charge transporting material is key to further enhance the efficiency and stability of perovskite solar cells. Inverted (p-i-n) devices can particularly benefit here, as evident from various successful attempts. However, every reported strategy is adapted to specific cell structures and compositions, affecting their robustness and applicability by other researchers. In this work, we present the universality of perovskite top surface post-treatment with ethylenediammonium diiodide (EDAI₂) for p-i-n devices. To prove it, we compare devices bearing perovskite films of different composition, i.e., Sn-, Pb-, and mixed Sn-Pb-based devices, achieving efficiencies of up to 11.4, 22.0, and 22.9%, respectively. A careful optimization of the EDAI₂ thickness indicates a different tolerance for Pb- and Sn-based devices. The main benefit of this treatment is evident in the open-circuit voltage, with enhancements of up to 200 mV for some compositions. In addition, we prove that this treatment can be successfully applied by both wet (spin-coating) and dry (thermal evaporation) methods, regardless of the composition. The versatility of this treatment makes it highly appealing for industrial application, as it can be easily adapted to specific processing requirements. We present a detailed experimental protocol, aiming to provide the community with an easy, universal perovskite post-treatment method for reliably improving the device efficiency, highlighting the potential of interfaces for the field.

Abstract:

Metal halide perovskites are set to revolutionise photovoltaic energy harvesting owing to an unmatched combination of high efficiency and low fabrication costs. However, to improve the sustainability of this technology, replacing lead with less toxic tin is highly desired. Tin halide perovskites are approaching 15% in power conversion efficiency (PCE), mainly employing PEDOT:PSS as a hole-selective layer. Unfortunately, PEDOT:PSS is processed from an aqueous solution, which is hardly compatible with the strict anoxic requirements for processing tin halide perovskites due to tin's instability to oxidation. Here, we present a water-free PEDOT formulation for developing tin-based lead-free perovskite solar cells. We show that the main difference between the PCE of devices made from aqueous and water-free PEDOT is due to the marked hydrophobicity of the latter, which complicates the perovskite deposition. By modifying the surface of water-free PEDOT with a thin Al₂O₃ interlayer, we could achieve good perovskite morphology that enabled perovskite solar cells with a PCE of 7.5%.

Abstract:

While the strong interaction between the internal unit and the fullerene cage inside metallofullerenes is widely acknowledged, how the cage transformation interacts with the cluster configuration remains elusive. For this purpose, we herein synthesized three metallofullerene molecules with an easy-to-compare cluster configuration and cage arrangement, namely Lu₃N@C_s(17 490)-C₇₆, Lu₃N@C₂(22 010)-C₇₈, and Lu₃N@D_3h(5)-C₇₈. The three lutetium-based nitride clusterfullerenes (NCFs) with small C_76-78 carbon cages were synthesized by a modified arc-discharge method and their structures were unambiguously confirmed by X-ray crystallography. Notably, the cage transformation from C_s(17 490)-C₇₆ to C₂(22 010)-C₇₈via a simple C₂-unit insertion leads to a remarkable configuration change of the encapsulated Lu₃N cluster from an unusual asymmetric plane to a common symmetric one. This close correlation between the cluster configuration and cage transformation is further confirmed by the pyramidal Lu₃N cluster in Lu₃N@D_3h(5)-C₇₈ other than the symmetric planar Lu₃N unit in Lu₃N@C₂(22 010)-C₇₈, as a result of an even larger difference in the cage arrangement. Astonishingly, such a cluster shrinkage, accompanied by an increase in the cage size from C_s(17 490)-C₇₆ to D_3h(5)-C₇₈, is dramatically opposite to the cluster expansion with cage elongation found in La₂C₂- or Y₂C₂-based metallofullerenes.