Organic photodetectors (OPDs) have gained increasing interest as they offer cost-effective fabrication methods using low temperature processes, making them particularly attractive for large area image detectors on lightweight flexible plastic substrates. Moreover, their photophysical and optoelectronic properties can be tuned both at a material and device level. Visible-light OPDs are proposed for use in indirect-conversion X-ray detectors, fingerprint scanners, and intelligent surfaces for gesture recognition. Near-infrared OPDs find applications in biomedical imaging and optical communications. For most applications, minimizing the OPD dark current density (J d ) is crucial to improve important figures of merits such as the signal-to-noise ratio, the linear dynamic range, and the specific detectivity (D*). Here, a quantitative analysis of the intrinsic dark current processes shows that charge injection from the electrodes is the dominant contribution to J d in OPDs. J d reduction is typically addressed by fine-tuning the active layer energetics and stratification or by using charge blocking layers. Yet, most experimental J d values are higher than the calculated intrinsic limit. Possible reasons for this deviation are discussed, including extrinsic defects in the photoactive layer and the presence of trap states. This provides the reader with guidelines to improve the OPD performances in view of imaging applications.

Minimizing the reverse bias dark current while retaining external quantum efficiency is crucial if the light detection sensitivity of organic photodiodes (OPDs) is to compete with inorganic photodetectors. However, a quantitative relationship between the magnitude of the dark current density under reverse bias (  Jd) and the properties of the bulk heterojunction (BHJ) active layer has so far not been established. Here, a systematic analysis of Jd in state‐of‐the‐art BHJ OPDs using five polymers with a range of energy levels and charge transport characteristics is presented. The magnitude and activation energy of Jd are explained using a model that assumes charge injection from the metal contacts into an energetically disordered semiconductor. By relating Jd to material parameters, insights into the origin of Jd are obtained that enable the future selection of successful OPD materials.

Photovoltaic retinal prostheses show great potential to restore sight in patients suffering from degenerative eye diseases by electrical stimulation of the surviving neurons in the retinal network. Herein, organic photodiodes (OPDs) sensitive to near‐infrared (NIR) light are evaluated as photovoltaic pixels for future application in retinal prostheses. Single‐junction and tandem OPDs are compared. In the latter, two nominally identical single‐junction cells are processed on top of each other, effectively doubling the open‐circuit voltage (V OC). Both single‐junction and tandem OPD micropixels can deliver the required charge to stimulate neurons under pulsed NIR light at physiologically safe intensities when connected to stimulating microelectrodes in a physiological saline solution. However, only tandem OPD pixels can cover the entire charge per pulse neural stimulation window due to their higher V OC (≈1.4 V). This demonstrates the viability of high‐resolution retinal prostheses based on flexible OPD arrays.

NIR) spectral region, to estimate the ratio of oxyhemoglobin and deoxyhemoglobin in arterial blood. Transmission pulse oximeters restrict the sensing location to tissues that can be transilluminated, generally fingertips or ear lobes. To overcome this limitation, pulse oximeters should be used in reflective mode, [7] that is, sensing light that slightly penetrates in the tissue and is then reflected. This opens the possibility to monitor pulse rate and blood oxygenation beyond the traditional sensing locations (e.g., fingertip and earlob).Recent developments in the field of organic electronics have led to reflectance oximeters based on organic photodiodes (OPDs). [8][9][10][11] The potential for large-area manufacturing using industrially scalable coating techniques, [12] combined with the wide absorption spectrum and high color selectivity, [13] makes OPDs attractive for this class of optoelectronic sensors. Organic reflectance oximeters on light-weight flexible substrates [14] can easily adapt to complex shapes of the body, thereby potentially providing a versatile alternative to rigid conventional designs. Furthermore, the signal-to-noise ratio (SNR) of flexible oximeters is enhanced by the formation of a conformal sensor-skin interface with lower effective impedance, [15][16][17] which reduces the electronic noise during PPG acquisition. Lochner et al. [8] first combined organic light emitting diodes (OLEDs) with two OPD pixels in an all-organic pulse oximeter. This sensor successfully measured pulse rate and blood oxygenation level within an experimental error of 1% and 2%, respectively. In 2018, Khan et al. [9] presented a reflectance oximeter array composed of four red and four NIR OLEDs, and eight OPDs. Such configuration introduces the functionality of 2D oxygenation mapping capability. The sensor was used to measure oxygen saturation on the forehead with 1.1% error and to create 2D oxygenation maps of adult forearms under under normal and ischemic conditions.Here, we develop an NIR sensitive OPD array of 16 × 16 pixels and demonstrate its potential in reflectance PPG. Each OPD pixel exhibits NIR sensitivity up to ≈ 950 nm together with a low dark current density in the order of 10 −6 mA cm −2 . As the OPD array is operated in reflective mode, a thin semi-transparent top electrode in each pixel is needed not to hinder light that reaches the fingertip and is then reflected. At the same time, the top electrode must form a continuous layer to ensure low series resistance. To fulfill both requirements, we use a 10 nm Ag top electrode. Notably, we use opaque bottom electrodes in combination with the semi-transparent non-patterned top electrode, so that only reflected light is detected by the OPD array.Reflectance oximeters based on organic photodiode (OPD) arrays offer the potential to map blood pulsation and oxygenation via photoplethysmography (PPG) over a large area and beyond the traditional sensing locations. Here, an organic reflectance PPG array based on 16 × 16 OPD pixels is developed. The individual ...

The interconnection layer (ICL) that connects adjacent subcells electrically and optically in solution‐processed multi‐junction polymer solar cells must meet functional requirements in terms of work functions, conductivity, and transparency, but also be compatible with the multiple layer stack in terms of processing and deposition conditions. Using a combination of poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate, diluted in near azeotropic water/n‐propanol dispersions as hole transport layer, and ZnO nanoparticles, dispersed in isoamyl alcohol as electron transport layer, a novel, versatile ICL has been developed for solution‐processed tandem and triple‐junction solar cells in an n‐i‐p architecture. The ICL has been incorporated in six different tandem cells and three different triple‐junction solar cells, employing a range of different polymer‐fullerene photoactive layers. The new ICL provided an essentially lossless contact in each case, without the need of adjusting the formulations or deposition conditions. The approach permitted realizing complex devices in good yields, providing a power conversion efficiency up to 10%.