The rise of the Internet of Things (IoT) has brought a growing need for reliable, sustainable ways to power connected devices. Currently, most of them rely on batteries, which come with challenges such as frequent replacement, maintenance, toxicity concerns, and recycling issues. Organic photovoltaics (OPVs) are a promising alternative thanks to their energy-efficient production, lower environmental impact, and design flexibility. One of the key strengths of OPVs lies in the ability to tailor the chemistry of organic semiconductors to fine-tune their optoelectronic properties for optimal performance. This makes them highly appealing for powering IoT sensors and devices, especially in indoor environments.

Indoor OPVs work by converting ambient light into electricity through an active layer made of a blend of two organic semiconductors, placed between two electrodes. While newly developed polymer donor:non-fullerene acceptor (NFA) indoor OPVs have demonstrated impressive power conversion efficiencies (PCE) of up to 30% under warm LED lighting, a challenge remains: designing devices that are both efficient and compatible with upscaling. The transition from academic, laboratory-scale devices to scalable manufacturing processes is still limited, and the key criteria for successful rollto- roll (R2R) production have yet to be fully achieved to enable commercial impact in the IoT market.

This thesis explores scalable device architectures using a lamination-based fabrication method developed at Epishine, where I carried out my PhD research. In this approach, the cathode and anode sides are processed separately using solution-based techniques and then laminated under heat and pressure to form a functioning indoor OPV. Unlike conventional lab-scale methods, this strategy allows for novel device architectures and provides flexibility to explore new materials. It also offers a valuable platform for investigating various failure modes that can emerge during the transition to scalable processing.

Although NFAs have pushed the efficiency limits of OPVs, fullerene-based systems remain widely used in industrial contexts due to their stability, established processing, and reproducibility. While fullerenes are not typically used as electron transport layers (ETLs), the lamination process provides a unique opportunity to explore solution-processed fullerene interlayers at the active layer (AL)/cathode interface. In theory, such a structure should offer favorable energy alignment and efficient charge extraction; however, we observed unexpected voltage losses in laminated polymer:fullerene devices using a fullerene interlayer. Various solution-processable fullerene-based interlayers were therefore investigated, including blends with additives such as polyethyleneimine (PEI) or polystyrene (PS). When incorporated, these additives led to a reduction in the cathode work function, improving energy alignment and reducing voltage losses. Neutron reflectivity studies revealed that PEI tends to accumulate at the interface, which could block the movement of positive charges (holes). In contrast, PS creates a gradual layering effect through the film thickness, with its concentration changing from one side to the other. This smooth variation, called vertical gradient, may help reduce energy losses. These findings highlight how the structure and composition at the interface play a crucial role in improving charge extraction in indoor OPVs.

One of the key issues investigated in this thesis was related to performance anomalies in laminated, thick, semi-transparent indoor OPVs. These devices exhibit unusual current density-voltage (J-V) characteristics, where performance is significantly better under cathode illumination compared to anode illumination. X-ray and neutron reflectivity measurements revealed vertical stratification in the active layer coated on the cathode side, a non-uniform distribution of materials through the film’s thickness, with the polymer component accumulating near the top surface. In contrast, the active layer deposited on the anode side was uniform. When the two layers were laminated together, the polymer-rich surface from the cathode side ended up in the center of the full active layer stack, resulting in a layered structure: active layer (anode side) / polymer-rich region / active layer (cathode side). Electrical simulations confirmed that this polymer-rich region was the primary cause of the J-V asymmetry. A model was proposed to describe this behavior, showing that electron extraction is hindered during anode illumination. Furthermore, laminated devices were developed with improved cathode/anode balance, achieving efficient and air-stable indoor OPV performance.

A key priority in scaling up high-performing active-layer-based OPVs is transitioning to air processing with green solvents, while ensuring compatibility with upscaling requirements such as material cost, batch-to-batch variation, and suitability for R2R processing, all while maintaining high performance. We screened various wide-bandgap polymers and NFAs for indoor applications and developed a high-performing system that meets the requirements for large-scale printing. The PTQ10:FCC-Cl system demonstrates a PCE up to 25% under 500 lux, 4000K LED light. We also investigated why certain wide-gap system combinations failed during early screening. The results suggest that these systems struggle with charge generation, leading to lower PCE compared to the champion system.

Organic photovoltaics (OPVs) offer a promising solution for indoor energy harvesting. However, fundamental investigations to understand and optimize industrial processes such as roll-to-roll lamination for upscaling remain limited. This study investigates a critical failure mode in the upscaling of OPVs. One major challenge for thick semitransparent laminated OPV devices is current-voltage (J-V) asymmetry, where performance under cathode-side illumination exceeds that under anode-side illumination. X-ray reflectivity, neutron reflectivity, and drift-diffusion simulations reveal that a vertically stratified polymer-rich region within the bulk of photoactive layers is the main cause of asymmetric J-V characteristics. Based on this fundamental understanding, a model is proposed to explain the mechanism, wherein electron extraction is hindered by the polymer-rich region during anode illumination. By exploring upscaling-compatible blends, cathode/anode-balanced, high-performing, and air-stable semitransparent laminated OPVs are developed for indoor applications using commercially available PV-X-plus material. These findings provide valuable guidance for designing OPVs with balanced performance, facilitating roll-to-roll adoption and commercialization.

Harvesting indoor light to power electronic devices for the Internet of Things has become an application scenario for emerging photovoltaics, especially utilizing organic photovoltaics (OPVs). Combined liquid- and solid-state processing, such as printing and lamination used in industry for developing indoor OPVs, also provides a new opportunity to investigate the device structure, which is otherwise hardly possible based on the conventional approach due to solvent orthogonality. This study investigates the impact of fullerene-based acceptor interlayer on the performance of conjugated polymer-fullerene-based laminated OPVs for indoor applications. We observe open-circuit voltage (V-OC) loss across the interface despite this arrangement being presumed to be ideal for optimal device performance. Incorporating insulating organic components such as polyethyleneimine (PEI) or polystyrene (PS) into fullerene interlayers decreases the work function of the cathode, leading to better energy level alignment with the active layer (AL) and reducing the V-OC loss across the interface. Neutron reflectivity studies further uncover two different mechanisms behind the V-OC increase upon the incorporation of these insulating organic components. The self-organized PEI layer could hinder the transfer of holes from the AL to the acceptor interlayer, while the gradient distribution of the PS-incorporated fullerene interlayer eliminates the thermalization losses. This work highlights the importance of structural dynamics near the extraction interfaces in OPVs and provides experimental demonstrations of interface investigation between solution-processed cathodic fullerene layer and bulk heterojunction AL.