We report the optoelectronic properties occurring in single-walled carbon nanotubes (SWNTs)—conjugated polymer, poly(3-octylthiophene) composites. Composite films were drop or spin cast from a solution on indium–tin oxide (ITO) and quartz substrates and studied using absorption spectroscopy and electrical characterization methods. Diodes (Al/polymer-nanotube composite/ITO) with a low nanotube concentration (<1%) show photovoltaic behavior, with an open circuit voltage of 0.7–0.9 V. The short circuit current is increased by two orders of magnitude compared with the pristine polymer diodes and the fill factor also increases from 0.3 to 0.4 for the nanotube/polymer cells. It is proposed that the main reason for this increase is the photoinduced electron transfer at the polymer/nanotube interface. The results show that the conjugated polymer-SWNTs composite represents an alternative class of organic semiconducting material that is promising for organic photovoltaic cells with improved performance.

Single-wall carbon nanotube/conjugated polymer photovoltaic devices

The power conversion efficiency of organic photovoltaic cells has increased with the introduction of the donor–acceptor heterojunction that serves to dissociate strongly bound photogenerated excitons1. Further efficiency increases have been achieved in both polymer2,3 and small-molecular-mass4 organic photovoltaic cells through the use of the bulk heterojunction (BHJ), where the distance an exciton must diffuse from its generation to its dissociation site is reduced in an interpenetrating network of the donor and acceptor materials. However, the random distribution of donor and acceptor materials in such structures can lead to charge trapping at bottlenecks and cul-de-sacs in the conducting pathways to the electrodes. Here, we present a method for growing crystalline organic films into a controlled bulk heterojunction; that is, the positions and orientations of donor and acceptor materials are determined during growth by organic vapour-phase deposition (OVPD5), eliminating contorted and resistive conducting pathways while maximizing the interface area. This results in a substantial increase in power conversion efficiency compared with the best values obtained by 'random' small-molecular-weight BHJ solar cells formed by high-temperature annealing, or planar double heterojunction photovoltaic cells using the same archetypal materials systems.

http://fulltext.calis.edu.cn/nature/nmat/4/1/nmat1285.pdf

Controlled growth of a molecular bulk heterojunction photovoltaic cell

The flash-photolysis time-resolved microwave conductivity technique (FP-TRMC) has been used to study photoinduced charge separation in bilayers consisting of a smooth, transparent, 80 nm thick layer of anatase TiO2 onto which poly(3-hexylthiophene) (P3HT) sensitizer layers have been spin-coated. Interfacial charge separation, resulting from excitation of the polymer in the visible, is found to persist well into the millisecond time domain. Photoconductivity action spectra have been measured between 420 and 700 nm for P3HT layer thicknesses, L, from ∼2 to 200 nm. Using this electrodeless technique, the bilayers could be irradiated from either the polymer (“front”) or semiconductor (“back”) side. On front-side irradiation at 540 nm (close to the absorption maximum of the polymer), the efficiency of charge separation per incident photon (IPCSE) initially increased to a maximum value of 0.8% for L ≈ 10 nm. For thicker layers the IPCSE gradually decreased, eventually to 0.1% for L ≈ 170 nm. On back-side irradi...

Contactless determination of the photoconductivity action spectrum, exciton diffusion length and charge separation efficiency in polythiophene-sensitized TiO2 bilayers

A n‐ZnO∕p‐Si thin film heterojunction has been fabricated by a low cost sol-gel technique. The wavelength dependent photoresponse properties of the heterojunction is investigated in detail by studying the effect of light illumination on current-voltage (I‐V) characteristics, photocurrent, and photocapacitance spectra at room temperature. It shows good diode characteristics with IF∕IR=3.4×103 at 4V and reverse leakage current density of 7.6×10−5Acm−2 at −5V. From the photocurrent spectra, it is observed that the visible photons are absorbed in the depleted p‐Si under reverse bias conditions, while ultraviolet (UV) photons are absorbed in the depleted n‐ZnO under positive bias conditions. This indicates that such a sol-gel n‐ZnO∕p‐Si thin film heterojunction can be used to sense both UV and visible photons though the photoresponse for UV is much slower than that of visible. The photocapacitance measurements suggest the presence of a shallow defect level in the sol-gel derived ZnO film which acts as an elect...

Ultraviolet and visible photoresponse properties of n‐ZnO∕p‐Si heterojunction

Enhanced device performance using different carbon nanotube types in polymer photovoltaic devices

Electrical and impedance spectral characterisation of ITO/DAG/In device

Studies on electrical and photoelectrical behaviour of ITO/ArV/In Schottky barrier device

Current–voltage, impedance spectroscopy and capacitance–voltage characteristics have been carried out on indium tin oxide (ITO)/poly(phenyl azo methane thiophene) (PPAT)/In schottky barrier device. In this device, injection and transport is expected to be dominated by holes. The conduction mechanism in low field region is ohmic whereas the space charge limited current theory with an exponential distribution of traps is in extremely good agreement with at higher voltage region. The characteristic energy ( E t ) of the exponential traps distribution is 0.13 eV from the valence band edge at room temperature but this shifts towards the valence band edge with the increase in temperature. Electrical impedance measurements on PPAT schottky barrier diodes in the frequency range 40 Hz–100 kHz were also reported. The device having configuration ITO/PPAT/In display the schottky behaviour, which can be modelled by a simple equivalent circuit of two RC elements in series, representing a bulk and a junction region. Low frequency device capacitance shows a pronounced voltage dependence and from a detailed analysis, the ionised acceptor concentration, potential barrier height and width of depletion layer have been calculated and discussed in detail.

M.S. Roy

Papers

Electrical and impedance spectral characterisation of ITO/DAG/In device

Studies on electrical and photoelectrical behaviour of ITO/ArV/In Schottky barrier device

Investigation of schottky barrier of poly(phenyl azo methane thiophene) using current–voltage and impedance spectroscopy

Dark, photoelectrical properties and impedance analysis of organic semiconductor based donor/acceptor device

Photocarriers generation process and photovoltaic effect in PPHT thin film Schottky barrier devices