We demonstrate a new device concept for organic photodetectors to overcome the lack of organic materials efficiently absorbing in the near-infrared spectral region. We exploit the properties of the weakly absorbing charge transfer state formed at the donor-acceptor interface in a bulk hetero junction photovoltaic device implemented into a specially designed optical microcavity . This enables us to spectrally selective detect light with wavelengths up to 1700 nm. Based on this technological platform, we build a miniaturized 16-channel spectrometer without the need for further optical elements such as gratings or prisms for food screening applications and liquid analysis.
With very high and competitive detectivities, we are entering the Indium-Gallium-Arsenide dominated world and show a route towards low-cost applications and novel devices.
 B. Siegmund et al. Nature Communications 8, 15421 (2017)
Organic optoelectronic materials and devices are nowadays found in many devices such as smart phone displays or solar panels. The advantages of this already mature technology are clear: The large oscillator strengths of the materials enable bright and colorful, low-power displays with extremely high contrast or relatively high photo conversion efficiencies in solar cells while the fabrication costs tend to further decrease.
However, organic optical sensors are not common in application although they would benefit from similar advantages but cannot compete with silicon detectors in the visible spectral range. However, in the near infrared (NIR) spectral region the situation is different: The standard inorganic material for detectors and LEDs is Indium-Gallium-Arsenide (InGaAs), which has many drawbacks: They are expensive due to epitaxial growth and contain reasonable amounts of highly toxic arsenic.
We developed a new type of organic photovoltaic detector based on blends of electron accepting and donating molecules. We exploit the properties of the weakly absorbing charge transfer state formed at the donor-acceptor interface and combine the device with an optical microcavity. This allows us to spectrally-selective detect light up to 1600 nm. We show a prototype of a miniaturized spectrometer with basic calibration allowing for simple food screening applications and liquid analysis.