Indoor photovoltaics (IPV) is increasing the scientific interest due to the expected internet of things (IoT) market increase, doubling its size by 2030. IPV can be used as a sustainable energy supply for sensors and low consumption applications, as an alternative to batteries. Thus, reducing the need of replacement and the amount of waste generated along the lifetime of the devices. Research is ongoing to improve relevant materials and devices for this application, focusing in indoor (inside of buildings) environments. In this sense, organic photovoltaics (OPV) is an interesting alternative due to the bandgap tuning of these materials, their high absorption in thin layers and their good performances (>30%[1,2]) already demonstrated under low illumination. Furthermore, the possibility of using more sustainable processes: printing in air with short high temperature steps; meaning lower energy consumption and cost than other PV technologies. Positioning IOPV as an alternative for this application.

IOPV creates a new field of application with strong differences compared to outdoor applications. First, illumination in indoor is based on fluorescent or LED lamps with different color temperatures, so absorption needs to be tuned to the visible region considering the emission spectra of those different lamps. Second, devices need to be performant at low illumination (200-1000lux). These requirements need device optimizations using material choices that reduce defect-driven recombination in the different layers of the devices. Meaning that, in order to improve their performance (PCE) at low illuminances, a high open-circuit current and low shunt resistance are required. Further, a good compromise of low series resistance (depending on the device behavior at low illumination) is required. Finally, stress factors in indoor operation conditions are milder than outdoor, being a favorable scenario for the use of OPV devices. However, research is still needed to investigate the stability of indoor cells and modules [3].

In the IOPV-Lab, the common laboratory between CINaM, IM2NP and the company Dracula Technologies we are working on an OPV blend system based in non-halogenated solvents processed in air with PCEs over 20% at low illumination. We are also studying the ageing behavior of this system, trying to understand the impact of cosolvent and additives utilization on their stability under combined indoor conditions. This study may allow us to understand if the devices are stable under operation for IoT applications in the long term. First tests seem to show that the most performant formulation is not stable after ageing, so a detailed study is ongoing to proof the formulation variability on the stability of the device. Opening up an alternative non-halogenated formulation that represents a suitable option for IOPV devices

IOPV samples have been fabricated in air by doctor-blade coating (with a final evaporation step), as scalable technique for industrial transfer. Samples have been characterized in different indoor conditions and illumination ranges before and after encapsulation. Then, an “indoor ageing test”, which includes temperature, humidity and indoor lighting conditions, is ongoing to check the interaction of the different stress-factors under real degradation conditions. Further, different optical (such as absorbance), opto-electronical (IV, EQE, LBIC, …) and morphological characterization methods are used at different times of the ageing test to understand the stability behavior of the different sample types. Results are expected to give us an overview of the best combination of solvent/cosolvent + additive in terms of stability.

The oral presentation expects to give an overview about indoor applications field and to present the best results of the studied system. It will include the stability using different non-halogenated formulations and using scalable techniques. Thus, providing valuable information of the real feasibility of IOPV devices use for indoor-PV applications.