Organic photodiodes (OPDs) have gone through substantial research and development recently, owing to their merits  (higher absorption coefficients, cost-effective fabrication, and prospects in flexible electronics) which have the potential to overcome the limits of conventional light sensors. This renders this new class of photodiodes to be of significance in critical applications demanding high sensitivity or wearable and implantable devices such as for fluorescence imaging.

Deciphering the brain’s intricate neuronal circuits and related connections within a living system is requisite for understanding neurological diseases such as Alzheimer’s and Parkinson’s. Targeted optical stimulation (optogenetics) and optical read-out (fluorescence imaging) of neuronal activity have emerged as core technologies in this regard,[1],[2]   and photodetectors are at the heart of sensing these optical signals. The requirements of a light sensor for this application are narrowband detection, fabrication as a flexible device, a wider range of material choices, and no toxicity. In comparison to traditional photodiodes, OPDs can serve all these needs and hence are ideally suited for fluorescence imaging in vivo.

In order to read out neuronal activity from a large number of pixels with a high signal-to-noise ratio and high temporal precision, the OPDs need to detect not only the feeble fluorescence emission from neurons but also at a fast speed (at least in the MHz domain). These aspects require device architecture tuned to reach low dark currents and high response speed. Furthermore, an instrumental apparatus is needed for the characterization of OPDs at such feeble illuminations . 

Here, we developed an experimental setup that provides the ability to switch between blue, green, red or near-infrared light sources to illuminate the OPDs as desired. The steady-state light intensity is effectively maintained and can be varied in a controlled manner over 10 decades, allowing experimental measurement of a wide dynamic range from 10^-2 W to 10^-13 W. Due shielding is done to sufficiently mitigate national power grid frequency, which may otherwise get induced in the signal. Use of triaxial cables for recording OPD response ensured the ability to effectively measure current output, down to 10^-15 A (fA) domain.  We use this to present the noise spectrum of a bulk heterojunction-based OPD reaching the dark current of sub-picoampere and not interfered with by instrumental limits. Subsequent characterization of the OPD’s dynamic range presents over 80 dB of linearity in response. 

For augmenting the OPD’s response speed, we further discuss the intrinsic charge transport characteristics required and develop an analytical model for the same. It is shown that balanced electron and hole mobility is a critical criterion for fast-speed OPDs, which can be realized by tuning the composition ratio of the bulk heterojunction.[3] Subsequently, the speed can be more than quadrupled to MHz domain even without any externally applied voltage bias. 

Finally, the low dark currents and high-speed detection observed in our samples will bring our OPD’s photocurrent response to the range required for fluorescence imaging from optogenetically stimulated neurons.