Metal halide perovskites have attracted tremendous attention due to their excellent electronic properties. Recent advancements in device performance and stability of perovskite solar cells (PSCs) have been achieved with the application of self-assembled monolayers (SAMs), serving as stand-alone hole transport layers in the p-i-n architecture. Specifically, phosphonic acid SAMs, directly functionalizing indium–tin oxide (ITO), are presently adopted for highly efficient devices. Despite their successes, so far, little is known about the surface coverage of SAMs on ITO used in PSCs application, which can affect the device performance, as non-covered areas can result in shunting or low open-circuit voltage. In this study, we investigate the surface coverage of SAMs on ITO and observe that the SAM of MeO-2PACz ([2-(3,6-dimethoxy-9H-carbazol-9-yl)ethyl]phosphonic acid) inhomogeneously covers the ITO substrate. Instead, when adopting an intermediate layer of NiO between ITO and the SAM, the homogeneity, and hence the surface coverage of the SAM, improve. In this work, NiO is processed by plasma-assisted atomic layer deposition (ALD) with Ni(MeCp) 2 as the precursor and O 2 plasma as the co-reactant. Specifically, the presence of ALD NiO leads to a homogeneous distribution of SAM molecules on the metal oxide area, accompanied by a high shunt resistance in the devices with respect to those with SAM directly processed on ITO. At the same time, the SAM is key to the improvement of the open-circuit voltage of NiO + MeO-2PACz devices compared to those with NiO alone. Thus, the combination of NiO and SAM results in a narrower distribution of device performance reaching a more than 20% efficient champion device. The enhancement of SAM coverage in the presence of NiO is corroborated by several characterization techniques including advanced imaging by transmission electron microscopy (TEM), elemental composition quantification by Rutherford backscattering spectrometry (RBS), and conductive atomic force microscopy (c-AFM) mapping. We believe this finding will further promote the usage of phosphonic acid based SAM molecules in perovskite PV.

Interfaces between the photoactive and charge transport layers are crucial for the performance of perovskite solar cells. Surface passivation of SnO2 as electron transport layer (ETL) by fullerene derivatives is known to improve the performance of n–i–p devices, yet organic passivation layers are susceptible to removal during perovskite deposition. Understanding the nature of the passivation is important for further optimization of SnO2 ETLs. X‐ray photoelectron spectroscopy depth profiling is a convenient tool to monitor the fullerene concentration in passivation layers at a SnO2 interface. Through a comparative study using [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) and [6,6]‐phenyl‐C61‐butyric acid (PCBA) passivation layers, a direct correlation is established between the formation of interfacial chemical bonds and the retention of passivating fullerene molecules at the SnO2 interface that effectively reduces the number of defects and enhances electron mobility. Devices with only a PCBA‐monolayer‐passivated SnO2 ETL exhibit significantly improved performance and reproducibility, achieving an efficiency of 18.8%. Investigating thick and solvent‐resistant C60 and PCBM‐dimer layers demonstrates that the charge transport in the ETL is only improved by chemisorption of the fullerene at the SnO2 surface.

In the present investigation, we have synthesized a polypyrrole films by chemical polymerization technique for the development of ammonia sensor. The polypyrrole films were synthesized in an aqueous acidic medium on glass substrate with mild oxidation of ferric chloride at temperature 29-C. The concentrations (molar) of monomer (pyrrole), oxidant (ferric chloride), and dopant (polyvinyl sulfonate) have been optimized for the uniform and porous surface morphology of the synthesized polypyrrole film. The synthesized films were characterized by scanning electron microscopy, ultraviolet-visible, and Fourier transforms infrared spectroscopy. Ammonia gas sensing behavior of polypyrrole films was studied by using indigenously developed gas sensing chamber. The synthesized polypyrrole film with optimized process parameters shows excellent and reproducible response to low concentration (100 ppm) of ammonia gas.

Perovskite semiconductors hold a unique promise in developing multijunction solar cells with high-efficiency and low-cost. Besides design constraints to reduce optical and electrical losses, integrating several very different perovskite absorber layers in a multijunction cell imposes a great processing challenge. Here, we report a versatile two-step solution process for high-quality 1.73 eV wide-, 1.57 eV mid-, and 1.23 eV narrow-bandgap perovskite films. Based on the development of robust and low-resistivity interconnecting layers, we achieve power conversion efficiencies of above 19% for monolithic all-perovskite tandem solar cells with limited loss of potential energy and fill factor. In a combination of 1.73 eV, 1.57 eV, and 1.23 eV perovskite sub-cells, we further demonstrate a power conversion efficiency of 16.8% for monolithic all-perovskite triple-junction solar cells.

using single-junction solar cells. [1] Recent advances in metal-halide perovskites with a wide range of bandgaps have motivated their use in tandems with perovskite, crystalline silicon (c-Si), and copper indium gallium selenide (CIGS), among other PV technologies. [2] Of particular interest is the development of all-perovskite tandem solar cells that promise low-cost solution processing and high efficiencies. As a result, in less than a decade, gains made in material discovery and processing have led the PCE of all-perovskite tandems to a certified value of 26.4%, higher than 25.5% for single-junction perovskite solar cells (PSCs) and close to the 26.7% for state-of-the-art c-Si devices. [3] Device and optical simulations predict that monolithic, all-perovskite tandem solar cells can reach an empirical limit of 33.6% by coupling 1.77/1.22 eV absorbers. [4] For lead halide perovskites, with a nominal ABX 3 crystal composition, the wide-bandgap absorber is typically obtained by mixing Br − or Cl − with I − at the X-site, [5] whereas the narrow bandgap is achieved by alloying Pb 2+ with Sn 2+ at the B-site. [6] Closing the efficiency gap requires optimizing the performance of both sub-cells. [7] The wide-bandgap cell typically suffers from a large bandgap to open-circuit voltage (V oc ) deficit, originating from non-radiative recombination losses in the perovskite film and at perovskite/charge transport layer heterojunctions. [8,9] Additionally, light-induced halide segregation in I/Br-mixed perovskites raises concerns over their operational stability. [10] Mitigation strategies such as using additives and surface treatments to improve the film quality, and developing new charge transport materials to enable better energy alignment, have been investigated to address such issues encountered in wide-bandgap perovskites. [11][12][13] On the other hand, the narrow-bandgap cell is limited by a significant short-circuit current density (J sc ) loss, as a result of the low absorptivity of PbSn hybrid perovskite in the near-infrared (NIR) region and a reduced charge extraction efficiency during operation. [14,15] Meanwhile, uncontrolled hole doping due to Sn 2+ oxidation can drastically reduce the carrier diffusion length and impede the use of a thick absorbing layer. [16] Modifying such perovskite films with judiciously selected additives and posttreatments can help increase carrier lifetime and consequently device performance. [15,[17][18][19][20] Next to optimizing individual cells at each bandgap, electrical and optical constraints of a tandem configuration must also be considered. [7,21] Sub-cells are electrically and optically connected Perovskite-based multijunction solar cells are a potentially cost-effective technology that can help surpass the efficiency limits of single-junction devices. However, both mixed-halide wide-bandgap perovskites and lead-tin narrowbandgap perovskites suffer from non-radiative recombination due to the formation of bulk traps and interfacial recombination centers which limit the open-...