卤化钙钛矿太阳能电池的缺陷容忍及缺陷钝化研究进展

缺陷在钙钛矿太阳能电池的快速发展中起着至关重要的作用。缺陷容忍性,即金属卤化钙钛矿的主导缺陷是浅能级缺陷,它们不会成为强非辐射复合中心,这被认为是金属卤化钙钛矿的独特特性,是其具有高光电转换效率的主要原因。然而,要进一步提高金属卤化钙钛矿的光电转换效率,就需要消除一些可作为非辐射复合中心并严重影响器件性能的少量深能级缺陷,包括点缺陷、晶界、表面和界面等。本文综述了缺陷容忍的研究进展,包括软声子模式和极化子效应。此外,还总结了缺陷钝化的策略,包括通过阳离子或阴离子来钝化离子键,以及通过路易斯酸或路易斯碱来钝化配位键等。     

Quasiparticle Band Structures of Defects in Anatase TiO2 Bulk

Quasiparticle band structures of the defective anatase TiO₂ bulk with O vacancy, Ti interstitial and H interstitial are investigated by the GW method within many-body Green's function theory. The computed direct band gap of the perfect anatase bulk is 4.3 eV, far larger than the experimental optical absorption edge (3.2 eV). We found that this can be ascribed to the inherent defects in anatase which drag the conduction band (CB) edge down. The occupied band-gap states induced by these defects locate close to the CB edge, excluding the possible contribution of these bulk defects to the deep band-gap state below CB as observed in experiments.     

Structural and Size Effects on the Spectroscopic and Redox Properties of CdSe Nanocrystals in Solution: The Role of Defect States

Two series of CdSe quantum dots (QDs) with different diameters are prepared, according to frequently used protocols of the same synthetic procedure. For each sample the photophysical properties and the potentials for the first reduction and oxidation processes in organic solution are determined. The band gap obtained from electrochemical experiments is compared with that determined from the absorption and luminescence spectra. While the optical band gap decreases upon increasing the nanocrystal diameter, as expected on the basis of quantum confinement, the redox potentials and the electrochemical band gap are not monotonously related to the QD size. For both series, the smallest and largest QDs are both easier to oxidize and reduce than mid‐sized QDs. In fact, the latter samples exhibit very broad voltammetric profiles, which suggests that the heterogeneous electron‐transfer processes from/to the electrode are kinetically hindered. Conversely, the electrochemical band gap for the smallest and largest particles of each series is somewhat smaller than the optical band gap. These results indicate that, while the optical band gap depends on the actual electron–hole recombination within the nanocrystal, and therefore follows the size dependence expected from the particle‐in‐a‐box model, the electrochemical processes of these QDs are strongly affected by other factors, such as the presence of surface defects. The investigations suggest that the influence of these defects on the potential values is more important for the smallest and largest QDs of each series, as confirmed by the respective luminescence bands and quantum yields. An interpretation for the size‐dependent evolution of the surface defects in these nanocrystals is proposed based on the mechanism of their formation and growth.     

Improving the stability of metal halide perovskite solar cells from material to structure

Metal halide perovskites(MHPs) are promising photovoltaic(PV) materials owing to their advantages such as high carrier mobility, excellent absorption coefficient, bandgap tenability, long diffusion length,and low material cost. These qualities have increased the efficiency of MHP solar cells to 23.3%. However,MHPs are hindered by a lack of stability. In addition, the applications of MHP solar cells are restricted by the instability of perovskite materials and devices. In this article, the most urgent stability problems faced by perovskite solar cells are identified, and recent progresses in MHPs are enumerated. The factors affecting the stability of perovskite materials and devices are also discussed. We analyzed the thermal and humid stability of perovskite materials in terms of transporting materials and their interface. In view of these recent advances, future works should focus on the large-scale application of MHP solar cells.     

Fluorine substituted conjugated polymer of medium band gap yields 7% efficiency in polymer-fullerene solar cells

Recent research advances on conjugated polymers for photovoltaic devices have focused on creating low band gap materials, but a suitable band gap is only one of many performance criteria required for a successful conjugated polymer. This work focuses on the design of two medium band gap (~2.0 eV) copolymers for use in photovoltaic cells which are designed to possess a high hole mobility and low highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels. The resulting fluorinated polymer PBnDT-FTAZ exhibits efficiencies above 7% when blended with [6,6]-phenyl C(61)-butyric acid methyl ester in a typical bulk heterojunction, and efficiencies above 6% are still maintained at an active layer thicknesses of 1 μm. PBnDT-FTAZ outperforms poly(3-hexylthiophene), the current medium band gap polymer of choice, and thus is a viable candidate for use in highly efficient tandem cells. PBnDT-FTAZ also highlights other performance criteria which contribute to high photovoltaic efficiency, besides a low band gap.     

DFT+U study of defects in bulk rutile TiO(2)

We present a systematic study of electronic gap states in defected titania using our implementation of the Hubbard-U approximation in the grid-based projector-augmented wave density functional theory code, GPAW. The defects considered are Ti interstitials, O vacancies, and H dopants in the rutile phase of bulk titanium dioxide. We find that by applying a sufficiently large value for the Hubbard-U parameter of the Ti 3d states, the excess electrons localize spatially at the Ti sites and appear as states in the band gap. At U=2.5?eV, the position in energy of these gap states are in fair agreement with the experimental observations. In calculations with several excess electrons and U=2.5?eV, all of these end up in gap states that are spatially localized around specific Ti atoms, thus effectively creating one Ti(3+) ion per excess electron. An important result of this investigation is that regardless of which structural defect is the origin of the gap states, at U=2.5?eV, these states are found to have their mean energies within a few hundredths of an eV from 0.94 eV below the conduction band minimum.     

Hybrid polypyrrole/copper/graphene oxide nanocomposite films synthesized via potentiostatic deposition with enhanced photovoltaic properties

Novel ternary nanocomposites films of Polypyrrole/copper/graphene oxide (PPy/Cu/GO) showed enhanced optical and electronic properties. In this study, PPy/Cu/GO films were synthesized with different GO load (0.0, 0.4, 0.6, and 0.8 wt%) using electrochemical deposition technique. The structural, optical and electrical properties of the composites were evaluated using X-Ray Diffraction (XRD) spectroscopy, UV–visible spectroscopy, Scanning electron microscopy (SEM), Energy dispersive X-ray spectroscopy (EDX), and four-point probe methods. XRD results reveal that the GO was completely intercalated and dispersed uniformly in the nanocomposites. The results also revealed that the nanocomposite films are crystalline in nature, with distinct peaks corresponding to indexed miller indices. UV-visible analysis revealed that all of the nanocomposites showed good UV absorbance which was significant in the UV–Vis region of ≈450 nm. The energy band gap decreased with increase in GO load and was found within 3.46 to 2.25 eV, across the range of GO load which fall within the range of energy band gap for photovoltaic applications. The SEM results revealed that the nanocomposite films showed unevenly shaped structures with porous surface which increases with increasing GO loading, while the EDX result revealed the presence of carbon, oxygen nitrogen and copper as fundamental elements deposited. The nanocomposites' four-point probe analysis revealed slight increase in conductivity with low GO content. The incorporation of Cu and GO nanoparticles in PPy matrix provides a better balance and thus improved the photovoltaic properties of PPy/Cu/GO making them suitable for photovoltaic applications.     

Tuning the band gap in hybrid tin iodide perovskite semiconductors using structural templating

Structural distortions within the extensive family of organic/inorganic hybrid tin iodide perovskite semiconductors are correlated with their experimental exciton energies and calculated band gaps. The extent of the in- and out-of-plane angular distortion of the SnI4(2-) perovskite sheets is largely determined by the relative charge density and steric requirements of the organic cations. Variation of the in-plane Sn-I-Sn bond angle was demonstrated to have the greatest impact on the tuning of the band gap, and the equatorial Sn-I bond distances have a significant secondary influence. Extended Hückel tight-binding band calculations are employed to decipher the crystal orbital origins of the structural effects that fine-tune the band structure. The calculations suggest that it may be possible to tune the band gap by as much as 1 eV using the templating influence of the organic cation.     

STUDY OF THE DEFECTIVE STRUCTURE OF LITHIUM NIOBATE CRYSTALS OF DIFFERENT COMPOSITIONS AND THEIR INFLUENCE ON THE OPTICAL AND ELECTRICAL PROPERTIES

Journal of Structural Chemistry - The work considers the influence of various types of equilibrium defects on photovoltaic and diffusion fields and on the band gap of nominally pure and zinc-doped...     

Bandgap Engineering of Lead‐Free Double Perovskite Cs2AgBiBr6 through Trivalent Metal Alloying

The double perovskite family, A₂M^IM^IIIX₆, is a promising route to overcome the lead toxicity issue confronting the current photovoltaic (PV) standout, CH₃NH₃PbI₃. Given the generally large indirect band gap within most known double perovskites, band‐gap engineering provides an important approach for targeting outstanding PV performance within this family. Using Cs₂AgBiBr₆ as host, band‐gap engineering through alloying of In^III/Sb^III has been demonstrated in the current work. Cs₂Ag(Bi_1−x M_x )Br₆ (M=In, Sb) accommodates up to 75 % In^III with increased band gap, and up to 37.5 % Sb^III with reduced band gap; that is, enabling ca. 0.41 eV band gap modulation through introduction of the two metals, with smallest value of 1.86 eV for Cs₂Ag(Bi_0.625Sb_0.375)Br₆. Band structure calculations indicate that opposite band gap shift directions associated with Sb/In substitution arise from different atomic configurations for these atoms. Associated photoluminescence and environmental stability of the three‐metal systems are also assessed.     

Probing the electronic structure and band gap evolution of titanium oxide clusters (TiO(2))(n)(-) (n = 1-10) using photoelectron spectroscopy

TiO2 is a wide-band-gap semiconductor, and it is an important material for photocatalysis. Here we report an experimental investigation of the electronic structure of (TiO2)n clusters and how their band gap evolves as a function of size using anion photoelectron spectroscopy (PES). PES spectra of (TiO2)n- clusters for n = 1-10 have been obtained at 193 nm (6.424 eV) and 157 nm (7.866 eV). The high photon energy at 157 nm allows the band gap of the TiO2 clusters to be clearly revealed up to n = 10. The band gap is observed to be strongly size-dependent for n < 7, but it rapidly approaches the bulk limit at n = 7 and remains constant up to n = 10. All PES features are observed to be very broad, suggesting large geometry changes between the anions and the neutral clusters due to the localized nature of the extra electron in the anions. The measured electron affinities and the energy gaps are compared with available theoretical calculations. The extra electron in the (TiO2)n- clusters for n > 1 appears to be localized in a tricoordinated Ti atom, creating a single Ti3+ site and making these clusters ideal molecular models for mechanistic understanding of TiO2 surface defects and photocatalytic properties.     

Ce and La single- and double-substitutional defects in yttrium aluminum garnet: first-principles study

The atomistic structure, energetics, and electronic structure of single-substitutional Ce and La defects and double-substitutional Ce-La defects in Ce,La-codoped yttrium aluminum garnet (YAG) Y(3)Al(5)O(12) have been studied by means of first-principles periodic boundary conditions density functional theory calculations. Single substitution of Y by Ce or by La produces atomistic expansions around the impurities, which are significantly smaller than the ionic radii mismatches and the overall lattice distortions are found to be confined within their second coordination spheres. In double-substitutional defects, the impurities tend to be as close as possible. La-codoping Ce:YAG provokes an anisotropic expansion around Ce defects. The Ce impurity introduces 4f occupied states in the 5.0 eV computed gap of YAG, peaking 0.25 eV above the top of the valence band, and empty 4f, 5d, and 6s states starting at 3.8 eV in the gap and spreading over the conduction band. La-codoping produces very small effects on the electronic structure of Ce:YAG, the most visible one being the decrease in covalent bonding with one of the oxygen atoms, which shifts 0.05 ? away from Ce and gets 0.04 ? closer to La in the most stable Ce-La double-substitutional defect.     

基于非富勒烯受体的溶液加工型全小分子太阳能电池研究进展

有机太阳能电池(OPV),具有质量轻、可成本低制备等优势,是一种具有实际应用潜力的光伏技术。有机太阳能电池活性层可以由共轭聚合物或溶液可加工的小分子材料(给体与受体)共混组成。由于小分子材料具有明确的分子结构,纯度可控及无批次差别影响的特点;并结合近年来非富勒烯小分子受体的快速发展,使得非富勒烯全小分子(NF-SM-OPV)电池研究受到广泛关注。由于大部分A-D-A型非富勒烯受体分子具有各向异性的特点,这使激子解离和电荷传输,很大程度上受分子间堆积方式的影响,导致非富勒烯全小分子电池活性层形貌调控更加复杂。虽然非富勒烯小分子太阳能电池具有非富勒烯受体材料和小分子材料的双重优势,但高效率非富勒烯小分子太阳能电池的制备,仍具有很大挑战。因此,本文总结近年来高性能非富勒烯小分子太阳能电池的相关进展。着重介绍针对非富勒烯受体的给体小分子材料设计工作,并在此基础上近一步讨论非富勒烯小分子太阳能电池面临的挑战与展望。     

First-Principles Study on Electronic and Optical Properties of Graphene-Like Boron Phosphide Sheets

Two-dimensional semiconducting materials with moderate band gap and high carrier mobil-ity have a wide range of applications for electronics and optoelectronics in nanoscale. On the basis of first-principles calculations, we perform a comprehensive study on the electronics and optical properties of graphene-like boron phosphide (BP) sheets. The global structure search and first-principles based molecular dynamic simulation indicate that two-dimensional BP sheet has a graphene-like global minimum structure with high stability. BP monolayer is semiconductor with a direct band gap of 1.37 eV, which reduces with the number of layers. Moreover, the band gaps of BP sheets are insensitive to the applied uniaxial strain.= The calculated mobility of electrons in BP monolayer is as high as 10⁶ cm²/(V·s). Lastly, the MoS₂/BP van der Waals heterobilayers are investigated for photovoltaic applications, and their power conversion efficiencies are estimated to be in the range of 17.7%－19.7%. This study implies the potential applications of graphene-like BP sheets for electronic and optoelectronic devices in nanoscale.     

给电子基团对吲哚染料电子结构和吸收光谱的影响

利用密度泛函理论(DFT)和含时密度泛函理论(TD-DFT), 分别基于B3LYP和PBE1PBE方法研究了一系列含有不同给电子基团的吲哚染料分子(ID1-ID3)的电子结构和吸收光谱性质. 重点比较了不同电子给体对染料的分子结构、吸收光谱以及其在电池中的光伏性能的影响. 结果表明从ID1、ID2到ID3, 随着电子给体中苯环数目的增加, 吲哚分子上的共轭效应逐渐增大, 导致吲哚分子最高占据分子轨道-最低未占据分子轨道(HOMO-LUMO)之间的能隙变窄, 分子的吸收光谱发生红移. 染料分子的吸收光谱和LUMO能级分别影响染料的吸光效率和光电转化过程中电子的注入过程, 从而使其二者成为决定电池光伏性能的重要参数. 综合考虑上述两个参数对电池性能的贡献, 通过理论研究证实, 在ID1-ID3系列染料中, ID3具有较长的吸收谱带、较大的分子消光系数和合适的LUMO能级, 从而表现出最为优越的光伏性能, 这与实验得出的结论很好地吻合.     

Bandgap Engineering of Lead-Free Double Perovskite Cs2AgBiBr6 through Trivalent Metal Alloying

The double perovskite family, A₂M^IM^IIIX₆, is a promising route to overcome the lead toxicity issue confronting the current photovoltaic (PV) standout, CH₃NH₃PbI₃. Given the generally large indirect band gap within most known double perovskites, band-gap engineering provides an important approach for targeting outstanding PV performance within this family. Using Cs₂AgBiBr₆ as host, band-gap engineering through alloying of In^III/Sb^III has been demonstrated in the current work. Cs₂Ag(Bi_1−xM_x)Br₆ (M=In, Sb) accommodates up to 75 % In^III with increased band gap, and up to 37.5 % Sb^III with reduced band gap; that is, enabling ca. 0.41 eV band gap modulation through introduction of the two metals, with smallest value of 1.86 eV for Cs₂Ag(Bi_0.625Sb_0.375)Br₆. Band structure calculations indicate that opposite band gap shift directions associated with Sb/In substitution arise from different atomic configurations for these atoms. Associated photoluminescence and environmental stability of the three-metal systems are also assessed.     

Recent Progress and Emerging Applications of Rare Earth Doped Phosphor Materials for Dye‐Sensitized and Perovskite Solar Cells: A Review

Although the efficiency of Dye‐sensitized and Perovskite solar cell is still below the performance level of market dominance silicon solar cells, in last few years they have grabbed significant attention because of their fabrication ease using low‐cost materials, and henceforth these cells are considered as a promising alternative to commercial photovoltaic devices. However, third generation solar cells have significant absorption in the visible region of solar spectrum, which confines their power conversion efficiency. Subsequently, the performance of current photovoltaics is significantly hampered by the transmission loss of sub‐band‐gap photons. To overcome these issues, rare earth doped luminescent materials is the favorable route followed to convert these transmitted sub‐band‐gap photons into above‐band‐gap light, where solar cells typically have significant light‐scattering effects. Moreover, the rare earth based down/up conversion material facilitates the improvement in sensitization, light‐scattering and device stability of these devices. This review provides insight into the application of various down/up conversion materials for Dye‐sensitized and perovskite solar cell applications. Additionally, the paper discusses the techniques to improve the photovoltaic performance in terms of current density and photo voltage in detail.     

Electronic Properties of Vinylene-Linked Heterocyclic Conducting Polymers: Predictive Design and Rational Guidance from DFT Calculations

The band structure and electronic properties in a series of vinylene-linked heterocyclic conducting polymers are investigated using density functional theory (DFT). In order to accurately calculate electronic band gaps, we utilize hybrid functionals with fully periodic boundary conditions to understand the effect of chemical functionalization on the electronic structure of these materials. The use of predictive first-principles calculations coupled with simple chemical arguments highlights the critical role that aromaticity plays in obtaining a low band gap polymer. Contrary to some approaches which erroneously attempt to lower the band gap by increasing the aromaticity of the polymer backbone, we show that being aromatic (or quinoidal) in itself does not ensure a low band gap. Rather, an iterative approach which destabilizes the ground state of the parent polymer toward the aromatic ? quinoidal level crossing on the potential energy surface is a more effective way of lowering the band gap in these conjugated systems. Our results highlight the use of predictive calculations guided by rational chemical intuition for designing low band gap polymers in photovoltaic materials.     

Effect of nitrogen and intrinsic defect complexes on conversion efficiency of ZnO for hydrogen generation from water

Band gap narrowing is important for applications of ZnO, especially for photoelectrochemical water splitting. In this work, we carried out first-principles electronic structure calculations with a hybrid density functional on defected ZnO. It is found that nitrogen substitutional doping alone cannot explain the largely enhanced conversion efficiency observed in nitrogen doped ZnO. Instead, complex defects formed by substitutional nitrogen and intrinsic defects play an important role in the band gap narrowing, in agreement with recent experimental results. We propose ZnO fabricated in a Zn-rich environment with heavy nitrogen doping as a photocatalyst for hydrogen generation from water splitting. A method for controlling the band gap of ZnO is also proposed.     

Isolating the effect of torsional defects on mobility and band gap in conjugated polymers

Conjugated polymers represent a promising class of organic semiconductors with potential applications in a variety of molecular devices. Poly(3-alkylthiophene)s, in particular, are garnering interest due to their large charge carrier mobility and band gap in the visible region of the spectrum. Defects play a pivotal role in determining the performance of polymer electronics, and yet the function of specific types of defects is still largely unknown. Density functional theory calculations of alkyl-substituted oligothiophenes are used to isolate the effect of static inter-ring torsion defects on key parameters such as electronic coupling between rings and band gap. Results have potential implications both for the fundamental understanding of intramolecular charge transport and for improving processing in organic devices.