Novel porous P-doped graphitic carbon nitride (g-C3N4) nanosheets were for the first time fabricated by combining P doping and thermal exfoliation strategies. The as-prepared P-doped g-C3N4 nanosheets show a high visible-light photocatalytic H2-production activity of 1596 μmol h−1 g−1 and an apparent quantum efficiency of 3.56% at 420 nm, representing one of the most highly active metal-free g-C3N4 nanosheet photocatalysts. This outstanding photocatalytic performance originates from the P-doped conjugated system and novel macroporous nanosheet morphology. Particularly, the empty midgap states (−0.16 V vs. standard hydrogen electrode) created by P doping are for the first time found to greatly extend the light-responsive region up to 557 nm by density functional theory and experimental studies, whilst the novel macroporous structure promotes the mass-transfer process and enhances light harvesting. Our study not only demonstrates a facile, eco-friendly and scalable strategy to synthesize highly efficient porous g-C3N4 nanosheet photocatalysts, but also paves a new avenue for the rational design and synthesis of advanced photocatalysts by harnessing the strong synergistic effects through simultaneously tuning and optimizing the electronic, crystallographic, surface and textural structures.

Porous P-doped graphitic carbon nitride nanosheets for synergistically enhanced visible-light photocatalytic H2 production

Bandtail states in disordered semiconductor materials result in losses in open-circuit voltage (Voc) and inhibit carrier transport in photovoltaics. For colloidal quantum dot (CQD) films that promise low-cost, large-area, air-stable photovoltaics, bandtails are determined by CQD synthetic polydispersity and inhomogeneous aggregation during the ligand-exchange process. Here we introduce a new method for the synthesis of solution-phase ligand-exchanged CQD inks that enable a flat energy landscape and an advantageously high packing density. In the solid state, these materials exhibit a sharper bandtail and reduced energy funnelling compared with the previous best CQD thin films for photovoltaics. Consequently, we demonstrate solar cells with higher Voc and more efficient charge injection into the electron acceptor, allowing the use of a closer-to-optimum bandgap to absorb more light. These enable the fabrication of CQD solar cells made via a solution-phase ligand exchange, with a certified power conversion efficiency of 11.28%. The devices are stable when stored in air, unencapsulated, for over 1,000 h.

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Hybrid organic–inorganic inks flatten the energy landscape in colloidal quantum dot solids

An elementary empirical model for the distribution of electronic states of an amorphous semiconductor is presented. Using this model, we determine the functional form of the optical absorption spectrum, focusing our analysis on the joint density of states function, which dominates the absorption spectrum over the range of photon energies we consider. Applying our optical absorption results, we then determine how the empirical measures commonly used to characterize the absorption edge of an amorphous semiconductor, such as the Tauc gap and the absorption tail breadth, are related to the parameters that characterize the underlying distribution of electronic states. We, thus, provide the experimentalist with a quantitative means of interpreting the physical significance of their optical absorption data.

The relationship between the distribution of electronic states and the optical absorption spectrum of an amorphous semiconductor: An empirical analysis

With the development of new generations of optoelectronic devices that combine high performance and novel functionalities (e.g., flexibility/bendability, adaptability, semi or full transparency), several classes of transparent electrodes have been developed in recent years. These range from optimized transparent conductive oxides (TCOs), which are historically the most commonly used transparent electrodes, to new electrodes made from nano- and 2D materials (e.g., metal nanowire networks and graphene), and to hybrid electrodes that integrate TCOs or dielectrics with nanowires, metal grids, or ultrathin metal films. Here, the most relevant transparent electrodes developed to date are introduced, their fundamental properties are described, and their materials are classified according to specific application requirements in high efficiency solar cells and flexible organic light-emitting diodes (OLEDs). This information serves as a guideline for selecting and developing appropriate transparent electrodes according to intended application requirements and functionality.

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Transparent Electrodes for Efficient Optoelectronics

Room temperature direct gap photoluminescence (PL) was observed from n-type tensile-strained epitaxial Ge-on-Si. The PL intensity increases with n-type doping due to a higher electron population in the direct Γ valley as a result of increased Fermi level. The direct gap emission also increases with temperature due to thermal excitation of electrons into the direct Γ valley, exhibiting robustness to heating effects. These unique properties of direct gap emission in an indirect gap material agree with our theoretical model and make Ge a promising light emitting material in 1550 nm communication band.

Direct gap photoluminescence of n-type tensile-strained Ge-on-Si

The density of states $\ensuremath{\rho}(E)$ in the tail for an electron in a correlated Gaussian random potential in three dimensions is constructed from first principles by means of a simple physical argument. This yields a linear exponential dependence of $\ensuremath{\rho}$ on $E$ which, for reasonable values of the rms potential fluctuation and correlation length, spans many decades, and occupies most of the experimentally observable energy range. This is suggested as the origin of the fundamental Urbach optical-absorption edge.

Sajeev John

Papers

Theory of electron band tails and the Urbach optical-absorption edge.