Efficiency droop, i.e. the loss of efficiency at high operating current, afflicts nitride‐based light‐emitting diodes (LEDs). The droop phenomenon is currently the subject of intense research, as it retards the advancement of solid‐state lighting which is just starting to supplant fluorescent as well as incandescent lighting. Although the technical community does not yet have consented to a single cause of droop, this article provides a summary of the present state of droop research, reviews currently discussed droop mechanisms, and presents a recently developed theoretical model for the efficiency droop. In the theoretical model, carrier leakage out of the active region caused by the asymmetry of the pn junction, specifically the disparity between electron and hole concentrations and mobilities, is discussed in detail. The model is in agreement with the droop's key behaviors not only for GaInN LEDs but also for AlGaInP LEDs.

We report for the first time highly conductive poly(3,4-ethylenedioxythiophene): poly(4-styrenesulfonate) (PEDOT:PSS)/graphene composites fabricated by in situ polymerization and their applications in a thermoelectric device and a platinum (Pt)-free dye-sensitized solar cell (DSSC) as energy harvesting systems. Graphene was dispersed in a solution of poly(4-styrenesulfonate) (PSS) and polymerization was directly carried out by addition of 3,4-ethylenedioxythiophene (EDOT) monomer to the dispersion. The content of the graphene was varied and optimized to give the highest electrical conductivity. The composite solution was ready to use without any reduction process because reduced graphene oxide was used. The fabricated film had a conductivity of 637 S·cm⁻¹, corresponding to an enhancement of 41%, after the introduction of 3 wt.% graphene without any further complicated reduction processes of graphene being required. The highly conductive composite films were employed in an organic thermoelectric device, and the device showed a power factor of 45.7 μW·m⁻¹K⁻² which is 93% higher than a device based on pristine PEDOT:PSS. In addition, the highly conductive composite films were used in Pt-free DSSCs, showing an energy conversion efficiency of 5.4%, which is 21% higher than that of a DSSC based on PEDOT:PSS.

Tungsten trioxide/bismuth vanadate heterojunction is one of the best pairs for solar water splitting, but its photocurrent densities are insufficient. Here we investigate the advantages of using helical nanostructures in photoelectrochemical solar water splitting. A helical tungsten trioxide array is fabricated on a fluorine-doped tin oxide substrate, followed by subsequent coating with bismuth vanadate/catalyst. A maximum photocurrent density of ~5.35±0.15 mA cm⁻² is achieved at 1.23 V versus the reversible hydrogen electrode, and related hydrogen and oxygen evolution is also observed from this heterojunction. Theoretical simulations and analyses are performed to verify the advantages of this helical structure. The combination of effective light scattering, improved charge separation and transportation, and an enlarged contact surface area with electrolytes due to the use of the bismuth vanadate-decorated tungsten trioxide helical nanostructures leads to the highest reported photocurrent density to date at 1.23 V versus the reversible hydrogen electrode, to the best of our knowledge.

We report that a simple chemical dedoping treatment of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) nanofilms enhances the thermoelectric properties of the polymer nanofilms. The dedoping process was done by over-coating a mixture of dimethyl sulfoxide (DMSO) and hydrazine (HZ), a strong chemical reducing agent, onto the PEDOT:PSS nanofilms. This additional step led to the removal of excess PSS chains and the formation of neutral states of PEDOT chains, resulting in an improvement in the Seebeck coefficient, from 30 μV K⁻¹ to 142 μV K⁻¹, and a decrease in the electrical conductivity from 726 S cm⁻¹ to 2 S cm⁻¹. By controlling the concentration of HZ, we obtained an optimized power factor of 112 μW m⁻¹ K⁻² at 0.0175 wt% of HZ in DMSO at room temperature. The corresponding electrical conductivity and Seebeck coefficient under optimized conditions were 578 S cm⁻¹ and 67 μV K⁻¹, respectively. We expect that this simple dedoping process can be applied to general thermoelectric nanofilms based on chemically doped polymers in order to enhance the power factor.