The Role of Grain Boundaries in Perovskite Solar Cells

Grain boundaries (GBs) play an important role in most polycrystalline solar cells. In perovskite solar cells, the research community is just starting to understand their effects on performance and long‐term durability. In this essay, three important questions are explored: Do GBs affect: 1) recombination and thus open‐circuit voltage? Not dramatically, if at all; 2) current–voltage hysteresis? Most studies show that hysteresis is dominated by defects at GBs; and 3) long‐term durability? Yes, GBs definitely help increase the rate of perovskite degradation. In this essay, the latest reports are summarized and the authors' perspective on this very important subject is given.     

Oriented Grains with Preferred Low‐Angle Grain Boundaries in Halide Perovskite Films by Pressure‐Induced Crystallization

A general methodology is reported to create organic–inorganic hybrid metal halide perovskite films with enlarged and preferred‐orientation grains. Simply pressing polyurethane stamps with hexagonal nanodot arrays on partially dried perovskite intermediate films can cause pressure‐induced perovskite crystallization. This pressure‐induced crystallization allows to prepare highly efficient perovskite solar cells (PSCs) because the preferred‐orientation and enlarged grains with low‐angle grain boundaries in the perovskite films exhibit suppressed nonradiative recombination. Consequently, the photovoltaic response is dramatically improved by the uniaxial compression in both inverted‐planar PSCs and normal PSCs, leading to power conversion efficiencies of 19.16%.     

In Situ Grain Boundary Functionalization for Stable and Efficient Inorganic CsPbI2Br Perovskite Solar Cells

The phase instability and large energy loss are two obstacles to achieve stable and efficient inorganic‐CsPbI_3?xBr_x perovskite solar cells. In this work, stable cubic perovskite (α)‐phase CsPbI₂Br is successfully achieved by Pb(Ac)₂ functioning at the grain boundary under low temperature. Ac^? strongly coordinates with CsPbI₂Br to stabilize the α‐phase and also make the grain size smaller and film uniform by fast nucleation. PbO is formed in situ at the grain boundary by decomposing Pb(Ac)₂ at high‐temperature annealing. The semiconducting PbO effectively passivates the surface states, reduces the interface recombination, and promotes the charge transport in CsPbI₂Br perovskite solar cells. A 12% efficiency and good stability are obtained for in situ PbO‐passivated CsPbI₂Br solar cells, while Pb(Ac)₂‐passivated device exhibits 8.7% performance and the highest stability, much better than the control device with 8.5% performance and inferior stability. This article highlights the extrinsic ionic grain boundary functionalization to achieve stable and efficient inorganic CsPbI_3?xBr_x materials and the devices.     

Atomic‐Scale Observation of Oxygen Substitution and Its Correlation with Hole‐Transport Barriers in Cu2ZnSnSe4 Thin‐Film Solar Cells

Kesterite‐type Cu₂ZnSn(S,Se)₄ has been extensively studied over the past several years, with researchers searching for promising candidates for indium‐ and gallium‐free inexpensive absorbers in high‐efficiency thin‐film solar cells. Many notable experimental and theoretical studies have dealt with the effects of intrinsic point defects, Cu/Zn/Sn nonstoichiometry, and cation impurities on cell performance. However, there have been few systematic investigations elucidating the distribution of oxygen at an atomic scale and the correlation between oxygen substitution and charge transport despite unavoidable incorporation of oxygen from the ambient atmosphere during thin‐film fabrication. Using energy‐dispersive X‐ray spectroscopy, scanning transmission electron microscopy, and electron energy‐loss spectroscopy, the presence of nanoscale layers is directly demonstrated in which oxygen is substantially substituted for Se, near grain boundaries in polycrystalline Cu₂ZnSnSe₄ films. Density‐functional theory calculations also show that oxygen substitution remarkably lowers the valence band maximum and subsequently widens the overall bandgap. Consequently, anion modification by oxygen can make a major contribution to the formation of a robust barrier blocking the holes from bulk grains into grain boundaries, thereby efficiently attaining electron?hole separation. The findings provide crucial insights into achieving better energy conversion efficiency in kesterite‐based thin‐film solar cells through optimum control of oxidation during the fabrication process.     

Transfer Printed Flexible and Stretchable Thin Film Solar Cells Using a Water‐Soluble Sacrificial Layer

Recently, the rapid and significant progress in the development of various stretchable electronics has triggered intense research interest. Although the remarkable features of transfer printing processes have enabled the use of inorganic crystalline semiconductors in various types of stretchable devices, including solar cells, light‐emitting diodes, circuits, and photodetectors, there are few examples of stretchable electronics using thin film semiconductors. Transfer printing of inorganic amorphous thin film semiconductors remains a challenge because no suitable sacrificial layer is available. To meet this challenge, a water‐soluble germanium oxide sacrificial layer is developed. Stretchable inorganic amorphous thin film solar cells are produced using a transfer printing process with a water‐soluble sacrificial layer. This first attempt to fabricate stretchable solar cells with inorganic amorphous thin film semiconductors significantly broadens the scope of solar cell applications. Moreover, the germanium oxide sacrificial layer can be used in other thin film electronics applications.     

Cu2‐xS Surface Phases and Their Impact on the Electronic Structure of CuInS2 Thin Films – A Hidden Parameter in Solar Cell Optimization

The surface properties of CuInS₂ (CIS) thin‐film solar cell absorbers are investigated by a combination of electron and soft X‐ray spectroscopies. Spatially separated regions of varying colors are observed and identified to be dominated by either CuS or Cu₂S surface phases. After their removal by KCN etching, the samples cannot be distinguished by eye and the CIS surface is found to be Cu‐deficient in both regions. However, a significantly more pronounced off‐stoichiometry in the region initially covered by Cu₂S can be identified. In this region, the resulting surface band gap is also significantly larger than the E_g^Surf of the initially CuS‐terminated region. Such variations may represent a hidden parameter which, if overlooked, induces irreproducibility and thus prevents systematic optimization efforts.     

Complex Interplay between Absorber Composition and Alkali Doping in High‐Efficiency Kesterite Solar Cells

Sodium treatment of kesterite layers is a widely used and efficient method to boost solar cell efficiency. However, first experiments employing other alkali elements cause confusion as reported results contradict each other. In this comprehensive investigation, the effects of absorber composition, alkali element, and concentration on optoelectronic properties and device performance are investigated. Experimental results show that in the row Li–Na–K–Rb–Cs the nominal Sn content should be reduced by more than 20% (relative) to achieve the highest conversion efficiency. The alkali concentration resulting in highest device efficiencies is lower by an order of magnitude for the heavy alkali elements (Rb, Cs) compared to the lighter ones (Li, Na, K). Utilization of a wide range of characterization techniques helps to unveil the complex interplay between absorber composition and alkali doping. A ranking of alkali for best device performances, when employing alkali treatment, resulted in the order of Li > Na > K > Rb > Cs based on the statistics of more than 700 individual cells. Finally, a champion device with 11.5% efficiency (12.3% active area) is achieved using a high Li concentration with an optimized Sn content.     

Subcellular localization of cadmium in the root cells of<Emphasis Type="Italic">Allium cepa</Emphasis> by electron energy loss spectroscopy and cytochemistry

The ultrastructural investigation of the root cells ofAllium cepa L. exposed to 1 mM and 10 mM cadmium (Cd) for 48 and 72 h was carried out. The results indicated that Cd induced several obvious ultrastructural changes such as increased vacuolation, condensed cytoplasm with increased density of the matrix, reduction of mitochondrial cristae, severe plasmolysis and highly condensed nuclear chromatin. Electron dense granules appeared between the cell wall and plasmalemma. In vacuoles, electron dense granules encircled by the membrane were aggregated and formed into larger precipitates, which increase in number and volume as a consequence of excessive Cd exposure. Data from electron energy loss spectroscopy (EELS) confirmed that these granules contained Cd and showed that significantly higher level of Cd in vacuoles existed in the vacuolar precipitates of meristematic or cortical parenchyma cells of the differentiating and mature roots treated with 1 mM and 10 mM Cd. High levels of Cd were also observed in the crowded electron dense granules of nucleoli. However, no Cd was found in cell walls or in cells of the vascular cylinder. A positive Gomori-Swift reaction showed that small metallic silver grains were abundantly localized in the vesicles, which were distributed in the cytoplasm along the cell wall.     

Energetics and Energy Loss in 2D Ruddlesden–Popper Perovskite Solar Cells

2D Ruddlesden–Popper perovskites (RPPs) are emerging as potential challengers to their 3D counterpart due to superior stability and competitive efficiency. However, the fundamental questions on energetics of the 2D RPPs are not well understood. Here, the energetics at (PEA)₂(MA)_n?1Pb_nI_3n+1/[6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) interfaces with varying n values of 1, 3, 5, 40, and ∞ are systematically investigated. It is found that n–n junctions form at the 2D RPP interfaces (n = 3, 5, and 40), instead of p–n junctions in the pure 2D and 3D scenarios (n = 1 and ∞). The potential gradient across phenethylammonium iodide ligands that significantly decreases surface work function, promotes separation of the photogenerated charge carriers with electron transferring from perovskite crystal to ligand at the interface, reducing charge recombination, which contributes to the smallest energy loss and the highest open‐circuit voltage (V_oc) in the perovskite solar cells (PSCs) based on the 2D RPP (n = 5)/PCBM. The mechanism is further verified by inserting a thin 2D RPP capping layer between pure 3D perovskite and PCBM in PSCs, causing the V_oc to evidently increase by 94 mV. Capacitance–voltage measurements with Mott–Schottky analysis demonstrate that such V_oc improvement is attributed to the enhanced potential at the interface.     

Multifaceted applications of nanomaterials in cell engineering and therapy

Nanomaterials with superior physiochemical properties have been rapidly developed and integrated in every aspect of cell engineering and therapy for translating their great promise to clinical success. Here we demonstrate the multifaceted roles played by innovatively-designed nanomaterials in addressing key challenges in cell engineering and therapy such as cell isolation from heterogeneous cell population, cell instruction in vitro to enable desired functionalities, and targeted cell delivery to therapeutic sites for prompting tissue repair. The emerging trends in this interdisciplinary and dynamic field are also highlighted, where the nanomaterial-engineered cells constitute the basis for establishing in vitro disease model; and nanomaterial-based in situ cell engineering are accomplished directly within the native tissue in vivo. We will witness the increasing importance of nanomaterials in revolutionizing the concept and toolset of cell engineering and therapy which will enrich our scientific understanding of diseases and ultimately fulfill the therapeutic demand in clinical medicine.