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

Facile Photochemical Synthesis of Graphene-Pt Nanoparticle Composite for Counter Electrode in Dye Sensitized Solar Cell

A low temperature route to synthesize graphene oxide–Pt nanoparticle hybrid composite by light assisted spontaneous coreduction of graphene oxide and chloroplatinic acid without reducing agent is demonstrated. Analysis indicates the importance of light as energy provider and ethanol as hole scavenger in the formation of small Pt nanoparticles (∼3 nm) on graphene oxide as well as graphene oxide reduction. Spray coating was used to deposit the hybrid material as a counter electrode in dye sensitized solar cells (DSCs). An efficiency of 6.77% for the hybrid graphene counter electrode has been obtained, higher than the control device made by low temperature sputtered Pt as counter electrode. Compatibility of the hybrid material with flexible plastic substrates was demonstrated yielding DSCs of an efficiency of 4.05%

Modulating Cationic Ratios for High-Performance Transparent Solution-Processed Electronics

Amorphous oxide semiconductors such as indium zinc tin oxide (IZTO) are considered favorites to serve as channel materials for thin film transistors (TFTs) because they combine high charge carrier mobility with high optical transmittance, allowing for the development of transparent electronics. Although the influence of relative cationic concentrations in determining the electronic properties have been studied in sputtered and PLD films, the development of printed transparent electronics hinges on such dependencies being explored for solution-processed systems. Here, we study solution-processed indium zinc tin oxide thin film transistors (TFTs) to investigate variation in their electrical properties with change in cationic composition. Charge transport mobility ranging from 0.3 to 20.3 cm²/(V s), subthreshold swing ranging from 1.2 to 8.4 V/dec, threshold voltage ranging from −50 to 5 V, and drain current on–off ratio ranging from 3 to 6 orders of magnitude were obtained by examining different compositions of the semiconductor films. Mobility was found to increase with the incorporation of large cations such as In³⁺ and Sn⁴⁺ due to the vast s-orbital overlap they can achieve when compared to the intercationic distance. Subthreshold swing decreased with an increase in Zn²⁺ concentration due to reduced interfacial state formation between the semiconductor and dielectric. The optimized transistor obtained at a compositional ratio of In/Zn/Sn = 1:1:1, exhibited a high field-effect mobility of 8.62 cm²/(V s), subthreshold swing of 1.75 V/dec, and current on–off ratio of 10⁶. Such impressive performances reaffirm the promise of amorphous metal oxide semiconductors for printed electronics

High Stability Bilayered Perovskites through Crystallization Driven Self-Assembly

In this manuscript we reveal the formation of bilayered hybrid perovskites of a new lower dimensional perovskite family, (CHMA)₂(MA)_<i>n</i>−1Pb_<i>n</i>I₃ with <i>n</i> = 1–5, with high ambient stability via its crystallization driven self-assembly process. The spun-coated perovskite solution tends to crystallize and undergo phase separation during annealing, resulting in the formation of 2D/3D bilayered hybrid perovskites. Remarkably, this 2D/3D hybrid perovskites possess striking moisture resistance and displays high ambient stability up to 65 days. The bilayered approach in combining 3D and 2D perovskites could lead to a new era of perovskite research for high-efficiency photovoltaics with outstanding stability, with the 3D perovskite providing excellent electronic properties while the 2D perovskite endows it moisture stability

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

Perovskite–Hematite Tandem Cells for Efficient Overall Solar Driven Water Splitting

Photoelectrochemical water splitting half reactions on semiconducting photoelectrodes have received much attention but efficient overall water splitting driven by a single photoelectrode has remained elusive due to stringent electronic and thermodynamic property requirements. Utilizing a tandem configuration wherein the total photovoltage is generated by complementary optical absorption across different semiconducting electrodes is a possible pathway to unassisted overall light-induced water splitting. Because of the low photovoltages generated by conventional photovoltaic materials (e.g., Si, CIGS), such systems typically consist of triple junction design that increases the complexity due to optoelectrical trade-offs and are also not cost-effective. Here, we show that a single solution processed organic–inorganic halide perovskite (CH₃NH₃PbI₃) solar cell in tandem with a Fe₂O₃ photoanode can achieve overall unassisted water splitting with a solar-to-hydrogen conversion efficiency of 2.4%. Systematic electro-optical studies were performed to investigate the performance of tandem device. It was found that the overall efficiency was limited by the hematite’s photocurrent and onset potential. To understand these limitations, we have estimated the intrinsic solar to chemical conversion efficiency of the doped and undoped Fe₂O₃ photoanodes. The total photopotential generated by our tandem system (1.87 V) exceeds both the thermodynamic and kinetic requirements (1.6 V), resulting in overall water splitting without the assistance of an electrical bias

Interfacial Charge Transfer Anisotropy in Polycrystalline Lead Iodide Perovskite Films

Solar cells based on organic–inorganic lead iodide perovskite (CH₃NH₃PbI₃) exhibit remarkably high power conversion efficiency (PCE). One of the key issues in solution-processed films is that often the polycrystalline domain orientation is not well-defined, which makes it difficult to predict energy alignment and charge transfer efficiency. Here we combine ab initio calculations and photoelectron spectroscopy to unravel the electronic structure and charge redistribution at the interface between different surfaces of CH₃NH₃PbI₃ and typical organic hole acceptor Spiro-OMeTAD and electron acceptor PCBM. We find that both hole and electron interfacial transfer depend strongly on the CH₃NH₃PbI₃ surface orientation: while the (001) and (110) surfaces tend to favor hole injection to Spiro-OMeTAD, the (100) surface facilitates electron transfer to PCBM due to surface delocalized charges and hole/electron accumulation at the CH₃NH₃PbI₃/organic interfaces. Molecular dynamic simulations indicate that this is due to strong orbital interactions under thermal fluctuations at room temperature, suggesting the possibility to further improve charge separation and extraction in perovskite-based solar cells by controlling perovskite film crystallization and surface orientation

Low-Temperature Chemical Transformations for High-Performance Solution-Processed Oxide Transistors

The challenges associated with low-temperature solution-processed metal oxide network formation have hindered the realization of high-performance solution-based electronic circuitry at temperatures lower than 200 °C. Here, UV irradiation is embarked upon as a route to effectively transform the chemical precursors to semiconducting metal oxides with high electrical quality. High-performance UV-irradiated indium oxide (In₂O₃) and indium zinc oxide (IZO) thin film transistors with mobility greater than 30 cm²/(V s) have been obtained from nitrate-based precursors. The chemical transformation has been monitored by detailed spectroscopic studies, physical characterization, and temperature-dependent electrical transport measurements. In comparison to thermal annealing, UV annealing seems to result in higher M–O–M network formation (depicted by M–O bonds in XPS), better removal of chemical impurities (depicted by FTIR and XPS), and structural relaxation driven electron doping, transforming the oxygen vacancies to act as shallow donors (depicted by TFT characteristics, XPS, XRD, and Urbach studies). Our results provide new insight into how UV irradiation drives metal oxide network formation and passivates the subgap density of states (DOS)

Unravelling the Effects of Cl Addition in Single Step CH₃NH₃PbI₃ Perovskite Solar Cells

The effect of film morphology on the photovoltaic performances is illustrated. Pin holes free and uniform films could be formed by slowing down the crystallization rate through additional of excess CH₃NH₃⁺. Though better film coverage is obtained, the device series resistance increases as excess CH₃NH₃I increases. Thermogravimetric analysis confirms the presence of residual excess CH₃NH₃I in the final film. When chlorine is added to the CH₃NH₃⁺ rich system, CH₃NH₃Cl is formed and can easily sublime during annealing at 100 °C. A little trace of excess CH₃NH₃Cl can be detected from thermogravic analysis, and the residual chlorine atoms match with XPS reading

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