This technology can be applied to develop high-efficiency photodetectors -- devices that convert light into electrical signals -- as well as low-noise, high sensitivity polymer-based chemical sensors.
With existing single-layer photoconductor technology, designers are constrained by the need to select a single material that satisfies both optical and electronic requirements for the device. This has limited the quantum efficiencies achievable by single-layer photoconductor designs. This invention describes a multi-layer device architecture that addresses this problem by decoupling the optical and electronic properties of the device.
A conventional fluorescent polymer sensor used to detect small concentrations of chemicals consists of a chemically sensitive fluorescent polymer film, a light source to excite the fluorescent layer, a photodetector to measure changes in fluorescence levels, and an optical filter to separate the light from the source from the fluorescent signal. Not only does this complexity drive costs up, the need for filtering also results in high optical losses and a low signal-to-noise ratio that hampers the detection of chemicals present in extremely small concentration. When adapted for chemical detection, the multi-layer photoconductor described in this invention eliminates the need for a photodetector and filter, thereby significantly improving sensitivity.
This invention describes a photoconductor device made up of an electron generation layer (EGL) and a charge transport layer (CTL), separated by a charge separation layer (CSL). When the device is exposed to light, paired electron-hole pairs known as excitons are generated in the EGL. Subsequently, these excitons diffuse to the EGL-CTL interface and dissociate, resulting in an electron in the CTL and a hole in the EGL, which allows a current to be extracted from the device. The presence of a CSL is not strictly necessary for exciton dissociation to take place, but it improves the longevity of the mobile charge carriers by delaying recombination and improves charge transfer efficiency, thereby increasing the device’s photoresponse.
In this design, light-induced generation of excitons and electron transport take place in different layers. This physical separation allows independent optimization of the device’s optical and electronic characteristics to achieve higher internal quantum efficiencies than those attainable by conventional single-layer photoconductors. For example, an amplified fluorescent polymer sensitive to a particular analyte could be used as the EGL to build a chemosensing photoconductor. Binding of the analyte to the EGL alters the rate of electron generation, measurable as a change in current extracted from the device. This multilayer device architecture allows designers to avoid the need to functionalize a charge transport material to the analyte, which often degrades its charge transport characteristics.
Decoupled electron generation and charge transport allows the optical and electronic properties of the device to be optimized separately
Gain inherent to the device design allows high internal quantum efficiencies to be obtained
- Electrical amplification characteristics allow the use of thinner chemosensing layers which allows diffusion of the analyte throughout the film and reduces noise from the unquenched polymer