Novel Plasma-Assisted Low-Temperature-Processed SnO₂ Thin Films for Efficient Flexible Perovskite Photovoltaics

The recent evolution of solution-processed hybrid organic–inorganic perovskite-based photovoltaic devices opens up the commercial avenue for high-throughput roll-to-roll manufacturing technology. To circumvent the thermal limitations that hinder the use of metal oxide charge transport layers on plastic flexible substrates in such technologies, we employed a relatively low-power nitrogen plasma treatment to achieve compact SnO₂ thin-film electrodes at near room temperature. The perovskite photovoltaic devices thus fabricated using N₂ plasma-treated SnO₂ performed on par with thermally annealed SnO₂ electrodes and resulted in a power conversion efficiency (PCE) of ca. 20.3% with stabilized power output (SPO) of ca. 19.1% on rigid substrates. Furthermore, the process is extended to realize flexible perovskite solar cells on indium tin oxide (ITO)-coated polyethylene terephthalate (PET) substrates with champion PCE of 18.1% (SPO ca. 17.1%), which retained ca. 90% of its initial performance after 1000 bending cycles. Our investigations reveal that deep ultraviolet irradiation associated with N₂ and N₂O plasma emission plays a major role in obtaining good quality metal oxide thin films at lower temperatures and offers promise toward facile integration of a wide variety of metal oxides on flexible substrates

Poor Photovoltaic Performance of Cs₃Bi₂I₉: An Insight through First-Principles Calculations

Bismuth-based halide perovskite derivatives have now attracted huge attention for photovoltaic (PV) applications after the unparalleled success of lead-based halide perovskites. However, the performances of PV devices based on these compounds are poor, despite theoretical predictions. In this Article, we have investigated the electronic structure and defect formation energies of Cs₃Bi₂I₉ using density functional theory (DFT) calculations. The calculated electronic bandstructure indicates an indirect bandgap and high carrier effective masses. Our calculations reveal a large stability region for this compound; however, deep level defects are quite prominent. Even the varying chemical potentials from the stoichiometric region do not eliminate the presence of deep defects, ultimately limiting photovoltaic efficiencies

Optimal Shell Thickness of Metal@Insulator Nanoparticles for Net Enhancement of Photogenerated Polarons in P3HT Films

Embedding metal nanoparticles in the active layer of organic solar cells has been explored as a route for improving charge carrier generation, with localized field enhancement as a proposed mechanism. However, embedded metal nanoparticles can also act as charge recombination sites. To suppress such recombination, the metal nanoparticles are commonly coated with a thin insulating shell. At the same time, this insulating shell limits the extent that the localized enhanced electric field influences charge generation in the organic medium. It is presumed that there is an optimal thickness which maximizes field enhancement effects while suppressing recombination. Atomic Layer Deposition (ALD) was used to deposit Al₂O₃ layers of different thicknesses onto silver nanoparticles (Ag NPs), in a thin film of P3HT. Photoinduced absorption (PIA) spectroscopy was used to study the dependence of the photogenerated P3HT⁺ polaron population on the Al₂O₃ thickness. The optimal thickness was found to be 3–5 nm. This knowledge can be further applied in the design of metal nanoparticle-enhanced solar cells

Highly Spin-Polarized Carrier Dynamics and Ultralarge Photoinduced Magnetization in CH₃NH₃PbI₃ Perovskite Thin Films

Low-temperature solution-processed organic–inorganic halide perovskite CH₃NH₃PbI₃ has demonstrated great potential for photovoltaics and light-emitting devices. Recent discoveries of long ambipolar carrier diffusion lengths and the prediction of the Rashba effect in CH₃NH₃PbI₃, that possesses large spin–orbit coupling, also point to a novel semiconductor system with highly promising properties for spin-based applications. Through circular pump–probe measurements, we demonstrate that highly polarized electrons of total angular momentum (<i>J</i>) with an initial degree of polarization <i>P</i>_ini ∼ 90% (i.e., −30% degree of electron spin polarization) can be photogenerated in perovskites. Time-resolved Faraday rotation measurements reveal photoinduced Faraday rotation as large as 10°/μm at 200 K (at wavelength λ = 750 nm) from an ultrathin 70 nm film. These spin polarized carrier populations generated within the polycrystalline perovskite films, relax via intraband carrier spin-flip through the Elliot-Yafet mechanism. Through a simple two-level model, we elucidate the electron spin relaxation lifetime to be ∼7 ps and that of the hole is ∼1 ps. Our work highlights the potential of CH₃NH₃PbI₃ as a new candidate for ultrafast spin switches in spintronics applications

Impact of Anionic Br^– Substitution on Open Circuit Voltage in Lead Free Perovskite (CsSnI_3‑xBr_<i>x</i>) Solar Cells

Replacement of lead in the hybrid organic–inorganic perovskite solar cells invokes the need for non-toxic materials such as Sn. Although solution processed CsSnI₃ has been demonstrated as a lead-free halide perovskite which can function as a light absorber with high photocurrent densities, the power conversion efficiencies were bottlenecked by low open circuit voltages. In this work, the open circuit voltages are modulated by chemical doping of CsSnI₃ with Br leading to formation of CsSnI_3‑<i>x</i>Br_<i>x</i> (0 ≤ <i>x</i> ≤ 3) perovskites. The beneficial effect of Br incorporation for <i>V</i>_oc improvement is evident for CsSnI₃ system even without the addition of SnF₂. There is an evolution of the crystal structure of CsSnI₃ from orthorhombic to cubic for CsSnBr₃ accompanied by changes in its optical properties with a blue shift of the absorption and IPCE onset, as the Br^– doping is increased. The <i>V</i>_oc enhancement is attributed to the decrease in Sn vacancies which is reflected by the lower charge carrier densities of 10¹⁵ cm^–3 and a high resistance to charge recombination in case of Br rich CsSnI_3‑<i>x</i>Br_<i>x</i> perovskite. By the addition of SnF₂ to CsSnI_3‑<i>x</i>Br_<i>x</i> perovskite, the current densities are improved significantly

Recovery of Shallow Charge-Trapping Defects in CsPbX₃ Nanocrystals through Specific Binding and Encapsulation with Amino-Functionalized Silanes

We report a facile methodology to restore photoluminescence (PL) of CsPbBr₃ nanocrystals (NCs) based on their postsynthetic modification with 3-aminopropyltriethoxysilane (APTES). By this methodology, a stark PL recovery factor of near 2-fold was obtained compared to their uncoated counterparts. ¹H NMR studies confirmed the presence of APTES on the NCs shell and provided more insight into the nature of the alkoxysilane passivation mechanisms. We further highlight that, contrary to expectations, preferential attachment of APTES does not take place through their amine terminal groups. The proposed surface-repair strategy can be extended to other halide compositions, yielding similarly effective 4-fold and 2-fold PL enhancements for CsPbCl₃ and CsPbI₃ NCs, respectively. Our work thus exemplifies that careful management of the perovskite NC interfaces and surface engineering is one of the most important frontiers in this emerging class of optoelectronic materials

Inorganic Halide Perovskites for Efficient Light-Emitting Diodes

Lead-halide perovskites have transcended photovoltaics. Perovskite light-emitting diodes (PeLEDs) emerge as a new field to leverage on these fascinating semiconductors. Here, we report the first use of completely inorganic CsPbBr₃ thin films for enhanced light emission through controlled modulation of the trap density by varying the CsBr-PbBr₂ precursor concentration. Although pure CsPbBr₃ films can be deposited from equimolar CsBr-PbBr₂ and CsBr-rich solutions, strikingly narrow emission line (17 nm), accompanied by elongated radiative lifetimes (3.9 ns) and increased photoluminescence quantum yield (16%), was achieved with the latter. This is translated into the enhanced performance of the resulting PeLED devices, with lower turn-on voltage (3 V), narrow electroluminescence spectra (18 nm) and higher electroluminescence intensity (407 Cd/m²) achieved from the CsBr-rich solutions

High Efficiency Solid-State Sensitized Solar Cell-Based on Submicrometer Rutile TiO₂ Nanorod and CH₃NH₃PbI₃ Perovskite Sensitizer

We report a highly efficient solar cell based on a submicrometer (∼0.6 μm) rutile TiO₂ nanorod sensitized with CH₃NH₃PbI₃ perovskite nanodots. Rutile nanorods were grown hydrothermally and their lengths were varied through the control of the reaction time. Infiltration of spiro-MeOTAD hole transport material into the perovskite-sensitized nanorod films demonstrated photocurrent density of 15.6 mA/cm², voltage of 955 mV, and fill factor of 0.63, leading to a power conversion efficiency (PCE) of 9.4% under the simulated AM 1.5G one sun illumination. Photovoltaic performance was significantly dependent on the length of the nanorods, where both photocurrent and voltage decreased with increasing nanorod lengths. A continuous drop of voltage with increasing nanorod length correlated with charge generation efficiency rather than recombination kinetics with impedance spectroscopic characterization displaying similar recombination regardless of the nanorod length

Over 20% Efficient CIGS–Perovskite Tandem Solar Cells

The development of high efficiency semitransparent perovskite solar cells is necessary for application in integrated photovoltaics and tandem solar cells. However, perovskite’s sensitivity to temperature and solvents impose a restriction on following processes, thus favoring physical vapor deposition for the transparent contacts. Protection may be necessary, especially for high energy sputtering and a transparent buffer layer providing good electrode adhesion and conductivity is desired. Here we evaluate Ag and MoO_<i>x</i> buffer layers in pursuit of high efficiency tandem solar cells. The usage of thin Ag as a buffer layer demonstrated indium tin oxide (ITO) contacts that were resistant to delamination and yielded a 16.0% efficiency of semitransparent perovskite solar cell with average transparency of 12% in visible range and >50% in near-infrared. Further application in tandem with Cu­(In,Ga)Se showed an overall efficiency of 20.7% in a 4-terminal (4T) configuration, exceeding the individual efficiencies of the subcells

Reduced Graphene Oxide Conjugated Cu₂O Nanowire Mesocrystals for High-Performance NO₂ Gas Sensor

Reduced graphene oxide (rGO)-conjugated Cu₂O nanowire mesocrystals were formed by nonclassical crystallization in the presence of GO and <i>o</i>-anisidine under hydrothermal conditions. The resultant mesocrystals are comprised of highly anisotropic nanowires as building blocks and possess a distinct octahedral morphology with eight {111} equivalent crystal faces. The mechanisms underlying the sequential formation of the mesocrystals are as follows: first, GO-promoted agglomeration of amorphous spherical Cu₂O nanoparticles at the initial stage, leading to the transition of growth mechanism from conventional ion-by-ion growth to particle-mediated crystallization; second, the evolution of the amorphous microspheres into hierarchical structure, and finally to nanowire mesocrystals through mesoscale transformation, where Ostwald ripening is responsible for the growth of the nanowire building blocks; third, large-scale self-organization of the mesocrystals and the reduction of GO (at high GO concentration) occur simultaneously, resulting in an integrated hybrid architecture where porous three-dimensional (3D) framework structures interspersed among two-dimensional (2D) rGO sheets. Interestingly, “super-mesocrystals” formed by 3D oriented attachment of mesocrystals are also formed judging from the voided Sierpinski polyhedrons observed. Furthermore, the interior nanowire architecture of these mesocrystals can be kinetically controlled by careful variation of growth conditions. Owing to high specific surface area and improved conductivity, the rGO-Cu₂O mesocrystals achieved a higher sensitivity toward NO₂ at room temperature, surpassing the performance of standalone systems of Cu₂O nanowires networks and rGO sheets. The unique characteristics of rGO-Cu₂O mesocrystal point to its promising applications in ultrasensitive environmental sensors