The Effect of the Alkyl Chains of the Alkylammonium Pesudohalide Additives on the Performance of Dion Jacobson Perovskite Solar Cells.

Dion-Jacobson perovskite (DJP) films suffer from the high structural disorder and non-compact morphology, leading to inefficient and unstable solar cells (SCs). Here, how the alkyl chains of alkylammonium pseudohalide additives including methylammonium thiocyanate (MASCN) and ethylammonium thiocyanate (EASCN), and propylammonium thiocyanate (PASCN), impact the microstructures, optoelectronic properties and the performance of the solar cells is investigated. These additives substantially improve the structural order and the morphology of the DJP films, yielding more efficient and stable solar cells than the control device. They behave quite differently in modifying the morphological features. Particularly, EASCN outstands the additives in terms of the superior morphology, which is compact and uniform and consists of the largest flaky grains. Consequently, the corresponding device delivers a power conversion efficiency (PCE) of 15.27% and maintains ≈86% of the initial PCE after aging in the air for 182 h. Conversely, MASCN as an additive produces uneven DJP film and the device maintains only 46% of the initial PCE. PASCN as an additive produces the finest grains in the DJP film, and the corresponding device yields a PCE of 11.95%. From the economical point of view, it costs 0.0025 yuan per device for the EASCN additive, allowing for cost-effective perovskite solar cells.

Food safety risk-assessment systems utilized by China, Australia/New Zealand, Canada, and the United States.

Ensuring the chemical, physical, and microbial safety of food and ingredients underpins the international trade of food items and is integral to building consumer confidence. Achieving this requires effective systems to support the safety of food across the supply chain. Differing risk-assessment approaches are utilized globally for establishing food safety systems, and bench marking these approaches against international food safety standards can assist in the development of country-specific systems. This China-Australia collaborative review examined similarities and differences in the food safety risk-assessment systems of China, Australia/New Zealand, Canada, and the United States, with the view to identify areas that could support improvements to the Chinese system. Key differences include the level of cohesiveness among stakeholders and the level to which each country promotes the international harmonization of standards. The evidence highlights a need for greater capacity-building in risk assessment in China that may support greater stakeholders' cohesion, improve hazard identification, and allow regulators to more readily keep abreast of changes to international standards. This review may help the Chinese food industry to replicate the same level of food safety risk assessment currently applied by other key countries, and reflects the determination, government prioritization, and active strengthening of China's National Centre for Food Safety Risk Assessment currently underway.

Compositional Variation in FAPb1-xSnxI3 and Its Impact on the Electronic Structure: A Combined Density Functional Theory and Experimental Study.

Given their comparatively narrow band gap, mixed Pb-Sn iodide perovskites are interesting candidates for bottom cells in all-perovskite tandems or single junction solar cells, and their luminescence around 900 nm offers great potential for near-infrared optoelectronics. Here, we investigate mixed FAPb1-xSnxI3 offering the first accurate determination of the crystal structure over a temperature range from 293 to 100 K. We demonstrate that all compositions exhibit a cubic structure at room temperature and undergo at least two transitions to lower symmetry tetragonal phases upon cooling. Using density functional theory (DFT) calculations based on these structures, we subsequently reveal that the main impact on the band gap bowing is the different energy of the s and p orbital levels derived from Pb and Sn. In addition, this energy mismatch results in strongly composition-dependent luminescence characteristics. Whereas neat and Sn-rich compounds exhibit bright and narrow emission with a clean band gap, Sn-poor compounds intrinsically suffer from increased carrier recombination mediated by in-gap states, as evidenced by the appearance of pronounced low-energy photoluminescence upon cooling. This study is the first to link experimentally determined structures of FAPb1-xSnxI3 with the electronic properties, and we demonstrate that optoelectronic applications based on Pb-Sn iodide compounds should employ Sn-rich compositions.

Tin Halide Perovskites: From Fundamental Properties to Solar Cells.

Metal halide perovskites have unique optical and electrical properties, which make them an excellent class of materials for a broad spectrum of optoelectronic applications. However, it is with photovoltaic devices that this class of materials has reached the apotheosis of popularity. High power conversion efficiencies are achieved with lead-based compounds, which are toxic to the environment. Tin-based perovskites are the most promising alternative because of their bandgap close to the optimal value for photovoltaic applications, the strong optical absorption, and good charge carrier mobilities. Nevertheless, the low defect tolerance, the fast crystallization, and the oxidative instability of tin halide perovskites currently limit their efficiency. The aim of this review is to give a detailed overview of the crystallographic, photophysical, and optoelectronic properties of tin-based perovskite compounds in their multiple forms from 3D to low-dimensional structures. At the end, recent progress in tin-based perovskite solar cells are reviewed, mainly focusing on the detail of the strategies adopted to improve the device performances. For each subtopic, the current challenges and the outlook are discussed, with the aim to stimulate the community to address the most important issues in a concerted manner.

Impact of the Hole Transport Layer on the Charge Extraction of Ruddlesden-Popper Perovskite Solar Cells.

Recent works demonstrate that polyelectrolytes as a hole transport layer (HTL) offers superior performance in Ruddlesden-Popper perovskite solar cells (RPPSCs) compared to poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS). The factors contributing to such improvement need to be systematically investigated. To achieve this, we have systematically investigated how the two HTLs affect the morphology, crystallinity, and orientation of the Ruddlesden-Popper perovskite (RPP) films as well as the charge extraction of the RPPSCs. PEDOT:PSS as a HTL leads to RPP films of low crystallinity and with a number of large pinholes. These factors lead to poor charge carrier extraction and significant charge recombination in the RPPSCs. Conversely, a PCP-Na HTL gives rise to highly crystalline and pinhole-free RPPSC films. Moreover, a PCP-Na HTL provides a better energy alignment at the perovskite/HTL interface because of its higher work function compared to PEDOT:PSS. Consequently, devices using PCP-Na as HTLs are more efficient in extracting charge carriers.

Enhancing the Performance of the Half Tin and Half Lead Perovskite Solar Cells by Suppression of the Bulk and Interfacial Charge Recombination.

In this article it is investigated how the hole extraction layer (HEL) influence the charge recombination and performance in half tin and half lead (FASn0.5 Pb0.5 I3 ) based solar cells (HPSCs). FASn0.5 Pb0.5 I3 film grown on PEDOT:PSS displays a large number of pin-holes and open grain boundaries, resulting in a high defect density and shunts in the perovskite film causing significant bulk and interfacial charge recombination in the HPSCs. By contrast, FASn0.5 Pb0.5 I3 films grown on PCP-Na, an anionic conjugated polymer, show compact and pin-hole free morphology over a large area, which effectively eliminates the shunts and trap states. Moreover, PCP-Na is characterized by a higher work function, which determines a favorable energy alignment at the anode interface, enhancing the charge extraction. Consequently, both the interfacial and bulk charge recombination in devices using PCP-Na HEL are considerably reduced giving rise to an overall improvement of all the device parameters. The HPSCs fabricated with this HEL display power conversion efficiency up to 16.27%, which is 40% higher than the efficiency of the control devices using PEDOT:PSS HEL (11.60%). Furthermore, PCP-Na as HEL offers superior performance in larger area devices compared to PEDOT:PSS.

Long-lived hot-carrier light emission and large blue shift in formamidinium tin triiodide perovskites.

A long-lived hot carrier population is critical in order to develop working hot carrier photovoltaic devices with efficiencies exceeding the Shockley-Queisser limit. Here, we report photoluminescence from hot-carriers with unexpectedly long lifetime (a few ns) in formamidinium tin triiodide. An unusual large blue shift of the time-integrated photoluminescence with increasing excitation power (150 meV at 24 K and 75 meV at 293 K) is displayed. On the basis of the analysis of energy-resolved and time-resolved photoluminescence, we posit that these phenomena are associated with slow hot carrier relaxation and state-filling of band edge states. These observations are both important for our understanding of lead-free hybrid perovskites and for an eventual future development of efficient lead-free perovskite photovoltaics.

Printable highly conductive conjugated polymer sensitized ZnO NCs as cathode interfacial layer for efficient polymer solar cells.

We report a facile way to produce printable highly conductive cathode interfacial layer (CIL) for efficient polymer solar cells (PSCs) by sensitizing ZnO nanocrystals (NCs) with a blue fluorescent conjugated polymer, poly(9, 9-bis-(6'-diethoxylphosphorylhexyl) fluorene) (PFEP). Herein, PFEP plays dual distinctive roles in the composite. Firstly, PFEP chains can effectively block the aggregation of ZnO NCs, leading to uniform and smooth film during solution processing via assembly on ZnO NC surfaces through their pending phosphonate groups. Secondly, PFEP can greatly improve the conductivity of ZnO NCs by charge transfer doping, that is the charge transfer from the sensitizer driven by electron-chemical potential equilibrium, which could be even more pronounced under light illumination because of light excitation of PFEP sensitizer. The increased conductivity in ZnO-PFEP layer renders more efficient electron transport and extraction compared to pristine ZnO layer. This ZnO-PFEP CIL was successfully applied to PSCs based on three polymer donor systems with different band-gaps, and efficiency enhancements from 44 to 70% were observed compared to those PSCs with pristine ZnO CIL. The highest efficiency of 7.56% was achieved in P(IID-DTC):PC70BM-based PSCs by using ZnO-PFEP film as CIL. Moreover, the enhanced conductivity due to the charge-transfer doping effect allows thick ZnO-PFEP film to be used as CIL in high-performance PSCs. Both the high conductivity and good film-forming properties of ZnO-PFEP CIL are favorable for large-scale printable PSCs, which is also verified by high-efficiency PSCs with ZnO-PFEP CIL fabricated using doctor-blading, a large-scale processing technique. The work provides an efficient printable cathode interfacial material for efficient PSCs.

Enhanced performance of inverted polymer solar cells by using poly(ethylene oxide)-modified ZnO as an electron transport layer.

In this paper, we report enhanced performance of inverted polymer solar cells composed of poly[2,3-bis-(3-octyloxyphenyl)quinoxaline-5,8-diyl-alt-thiophene-2,5-diyl] (TQ1):[6,6]-phenyl-C(71)-butyric acid methyl ester (PC(71)BM) blends by using poly(ethylene oxide) (PEO)-modified ZnO as an electron transport layer. It is found that PEO modification to the ZnO nanoparticle surface can effectively passivate the surface traps of ZnO, suppress the recombination loss of carriers, reduce the series resistance, and improve the electrical coupling of ZnO/active layer. Consequently, both the short-circuit current (J(SC)) and the fill factor (FF) of the inverted solar cells are considerably improved. The resulting power conversion efficiency (PCE) is improved to 5.64% as compared to 4.5% of the reference device using a ZnO electron transport layer. Moreover, this approach can also successfully improve the J(SC) and FF of anther inverted solar cell composed of poly[N-9″-hepta-decanyl-2,7-carbazole-alt-5,5-(4',7'-dithienyl-2',1',3'-benzothiadiazole)] (PCDTBT):PC(71)BM blends. The PCE of the device based on the PEO-modified ZnO layer is increased to 6.59% from 5.39% of the reference device based on the ZnO layer.

Fast monolayer adsorption and slow energy transfer in CdSe quantum dot sensitized ZnO nanowires.

A method for CdSe quantum dot (QD) sensitization of ZnO nanowires (NW) with fast adsorption rate is applied. Photoinduced excited state dynamics of the quantum dots in the case of more than monolayer coverage of the nanowires is studied. Transient absorption kinetics reveals an excitation depopulation process of indirectly attached quantum dots with a lifetime of ~4 ns. Photoluminescence and incident photon-to-electron conversion efficiency show that this process consists of both radiative e-h recombination and nonradiative excitation-to-charge conversion. We argue that the latter occurs via interdot energy transfer from the indirectly attached QDs to the dots with direct contact to the nanowires. From the latter, fast electron injection into ZnO occurs. The energy transfer time constant is found to be around 5 ns.

Interface-induced crystalline ordering and favorable morphology for efficient annealing-free poly(3-hexylthiophene): fullerene derivative solar cells.

A simple approach to fabricate high-efficiency annealing-free poly(3-hexylthiophene): [6,6]-phenyl C(61)-butyric acid methyl ester (P3HT:PCBM) solar cells is reported by using p-type CuI to substitute PEDOT:PSS as anode buffer layer. It is found that the P3HT:PCBM blend films deposited on CuI surface show different orientation of crystalline P3HT domains and phase separation from those deposited on PEDOT:PSS surface. A nanoscale phase separation of P3HT and PCBM with domain sizes about 10-30 nm is formed for the P3HT:PCBM blend films deposited on CuI surface. Absorption and grazing incidence X-ray diffraction (GIXRD) experiments indicate that the CuI layer not only induces the self-organization of P3HT chains into well-ordered structure but also results in the vertical orientation of π-π stacking planes of P3HT with respect to the substrate which is favorable for the hole collection in polymer solar cells. Hole-transport investigation discloses that hole mobility of the as-spincast P3HT:PCBM blend film on CuI surface is increased with 3 orders of magnitude compared to the P3HT:PCBM film deposited on PEDOT:PSS. A power conversion efficiency of 3.1% for the as-spincast P3HT:PCBM solar cell with CuI buffer layer is about 4-fold enhancement compared to 0.83% of the control device with PEDOT:PSS, and is comparable to the reported P3HT:PCBM solar cells subjected to post thermal treatments. This work implies that interfacial engineering is a promising approach for manipulating morphology of active layer and can potentially simplify the process and shorten the fabrication time of polymer solar cells in low-cost roll-to-roll manufacturing.

High-efficiency inverted polymer solar cells with transparent and work-function tunable MoO(3)-Al composite film as cathode buffer layer.

High-efficiency inverted polymer solar cells based on PCDTBT:PC(70)BM blend with the MoO(3)-Al composite film as the cathode buffer layer and the MoO(3)/Al as the anode have been demonstrated. A V(OC) of 0.88 V, a J(SC) of 10.88 mA cm(-2), a FF of 70.7% and a PCE of 6.77% are achieved. The MoO(3)-Al composite films are highly transparent with adjustable work functions which can be fine tuned based on the Al content in the composite, thus allowing us to optimize the interfacial property at cathode buffer layer/BHJ interfaces to reduce recombination loss and to improve the photovoltaic performance. This new approach has simplified the device fabrication and will render economizing in large scale applications.