James C. Blakesley

Bio: James C. Blakesley is an academic researcher from National Physical Laboratory. The author has contributed to research in topics: Charge carrier & Organic semiconductor. The author has an hindex of 22, co-authored 43 publications receiving 3009 citations. Previous affiliations of James C. Blakesley include University of Cambridge & Zhejiang University.

Solution-processed ultraviolet photodetectors based on colloidal ZnO nanoparticles.

Abstract: A “visible-blind” solution-processed UV photodetector is realized on the basis of colloidal ZnO nanoparticles. The devices exhibit low dark currents with a resistance >1 TΩ and high UV photocurrent efficiencies with a responsivity of 61 A/W at an average intensity of 1.06 mW/cm2 illumination at 370 nm. The characteristic times for the rise and fall of the photocurrent are <0.1 s and about 1 s, respectively. The photocurrent of the device is associated with a light-induced desorption of oxygen from the nanoparticle surfaces, thus removing electron traps and increasing the free carrier density which in turn reduces the Schottky barrier between contacts and ZnO nanoparticles for electron injection. The devices are promising for use in large-area UV photodetector applications.

Relationship between energetic disorder and open-circuit voltage in bulk heterojunction organic solar cells

Abstract: We simulate organic bulk heterojunction solar cells. The effects of energetic disorder are incorporated through a Gaussian or exponential model of density of states. Analytical models of open-circuit voltage (${V}_{\mathrm{OC}}$) are derived from the splitting of quasi-Fermi potentials. Their predictions are backed up by more complex numerical device simulations including effects such as carrier-density--dependent charge-carrier mobilities. It is predicted that the V${}_{\mathrm{OC}}$ depends on: (1) the donor-acceptor energy gap; (2) charge-carrier recombination rates; (3) illumination intensity; (4) the contact work functions (if not in the pinning regime); and (5) the amount of energetic disorder. A large degree of energetic disorder, or a high density of traps, is found to cause significant reductions in ${V}_{\mathrm{OC}}$. This can explain why ${V}_{\mathrm{OC}}$ is often less than expected in real devices. Energetic disorder also explains the nonideal temperature and intensity dependence of ${V}_{\mathrm{OC}}$ and the superbimolecular recombination rates observed in many real bulk heterojunction solar cells.

Moderate doping leads to high performance of semiconductor/insulator polymer blend transistors

Abstract: Polymer transistors are being intensively developed for next-generation flexible electronics. Blends comprising a small amount of semiconducting polymer mixed into an insulating polymer matrix have simultaneously shown superior performance and environmental stability in organic field-effect transistors compared with the neat semiconductor. Here we show that such blends actually perform very poorly in the undoped state, and that mobility and on/off ratio are improved dramatically upon moderate doping. Structural investigations show that these blend layers feature nanometre-scale semiconductor domains and a vertical composition gradient. This particular morphology enables a quasi three-dimensional spatial distribution of semiconductor pathways within the insulating matrix, in which charge accumulation and depletion via a gate bias is substantially different from neat semiconductor, and where high on-current and low off-current are simultaneously realized in the stable doped state. Adding only 5 wt% of a semiconducting polymer to a polystyrene matrix, we realized an environmentally stable inverter with gain up to 60.

Photogeneration and Recombination in P3HT/PCBM Solar Cells Probed by Time-Delayed Collection Field Experiments

Abstract: Time-delayed collection field (TDCF) experiments are performed on bulk heterojunctionsolarcellscomprisedofablendofpoly(3-hexylthiophene)and(6,6)-phenylC71 butyric acid methyl ester. TDCF is analogous to a pump-probe experiment using optical excitation and an electrical probe with a resolution of <100 ns. The number of free charge carriers extracted after a short delay is found to be independent of the electric field during illumination. Also, experiments performed with a variable delay between the optical excitation andtheelectricalprobedonotrevealanyevidenceforthegenerationofchargevia field-assisted dissociation of bound long-lived polaron pairs. Photocurrent transients are well fitted by computational drift diffusion simulations with only direct generation of free charge carriers. Withincreasingdelaytimesbetweenpumpandprobe,twolossmechanismsareidentified; first, charge-carriers areswept outofthe deviceby theinternalelectric field,and second,bimolecular recombination of the remaining carriers takes place with a reduced recombination coefficient.

Solution-processed hybrid perovskite photodetectors with high detectivity

Abstract: Organic–inorganic hybrid perovskite materials are attracting great interest for their applications in photovoltaics where they have demonstrated excellent efficiency. Here, Dou et al. demonstrate room temperature, solution-processed hybrid perovskite photodetectors with fast response and high detectivity.

π-Conjugated Polymers for Organic Electronics and Photovoltaic Cell Applications†

Abstract: The optoelectronic properties of polymeric semiconductor materials can be utilized for the fabrication of organic electronic and photonic devices. When key structural requirements are met, these materials exhibit unique properties such as solution processability, large charge transporting capabilities, and/or broad optical absorption. In this review recent developments in the area of π-conjugated polymeric semiconductors for organic thin-film (or field-effect) transistors (OTFTs or OFETs) and bulk-heterojunction photovoltaic (or solar) cell (BHJ-OPV or OSC) applications are summarized and analyzed.

Single-junction polymer solar cells with high efficiency and photovoltage

Abstract: Organic solar cells with efficiency greater than 10% are fabricated by incorporating a semiconductor polymer with a deepened valence energy level. Polymer solar cells are an exciting class of next-generation photovoltaics, because they hold promise for the realization of mechanically flexible, lightweight, large-area devices that can be fabricated by room-temperature solution processing1,2. High power conversion efficiencies of ∼10% have already been reported in tandem polymer solar cells3. Here, we report that similar efficiencies are achievable in single-junction devices by reducing the tail state density below the conduction band of the electron acceptor in a high-performance photoactive layer made from a newly developed semiconducting polymer with a deepened valence energy level. Control over band tailing is realized through changes in the composition of the active layer and the structure order of the blend, both of which are known to be important factors in cell operation4,5,6. The approach yields cells with high power conversion efficiencies (∼9.94% certified) and enhanced photovoltage.

Single-photon detectors for optical quantum information applications

Abstract: This review highlights the recent progress which has been made towards improved single-photon detector technologies and the impact these developments will have on quantum optics and quantum information science.

Invited review article: Single-photon sources and detectors

Abstract: We review the current status of single-photon-source and single-photon-detector technologies operating at wavelengths from the ultraviolet to the infrared. We discuss applications of these technologies to quantum communication, a field currently driving much of the development of single-photon sources and detectors.