Structural and Energetic Disorder

Organic solar cell (OSC) research has made remarkable strides in the past decade, reaching power conversion efficiencies (PCEs) near 20%. Just ten years ago, this milestone seemed unattainable, and a decade earlier, even 10% PCE was considered overly ambitious. Two key advancements have driven these leaps: first, the adoption of bulk heterojunction (BHJ) morphology, and later, the emergence of new generation of non-fullerene acceptors (NFAs). However, the goal of commercialization remains elusive. Should OSCs achieve commercial viability, they hold immense promise, offering significantly lower costs than silicon-based technologies, along with light-weight, flexible, and versatile design possibilities. Yet, the path to commercialization is filled with challenges. OSCs still lag behind perovskite and silicon counterparts in efficiency due to their lower fill factors (FFs) and open-circuit voltages (V_OC), both hampered by high charge recombination rates and the inherently higher energetic disorder in organic materials. Extensive research is required to intricately link and understand the intrinsic properties of organic semiconductors, such as charge generation, recombination dynamics, transport in relation to FF, and V_OC which influence the overall performance of OSCs. This thesis investigates several key strategies aimed at improving OSC performance, with a focus on V_OC, FF from structural and energetic disorder perspective. One chapter compares the effect of the electron transport layers (ETLs), PDINN and PDINO, on the active layer PM6:Y6 blends. Electron paramagnetic resonance (EPR) and Hall effect measurements show that PDINN has higher doping levels compared to PDINO. Although both layers have ohmic contact with the active layer, PDINN, with its higher doping level, results in improved V_OC and FF. This improvement is linked to a charge spilling mechanism from the ETL to the active layer which is verified with Mott Schottky analysis. As a result, devices with PDINN ETL have reduced energetic disorder and subsequently suppressing the non-radiative charge recombination shown with the space charge limited current (SCLC), electroluminescence quantum yield (ELQY) and bias-assisted charge extraction (BACE) measurements. Another focus of this thesis examines all-small moledule (ASM) solar cells, which show potential for OSC commercialization due to their high solubility and simpler synthesis. Despite their smaller energetic disorders, which could theoretically lead to higher V_OCs, ASM blends often underperform compared to polymer: small molecule blends. To explore this, the thesis compares an ASM blend, ZR1:Y6, with a polymer: small molecule blend, PM7:Y6, both sharing the same non-fullerene acceptor (Y6) and similar energetic offsets measured with temperature dependent ELQY measurements. Morphological analysis shows that the ASM blend lacks the π-πstacking and intermixed morphology necessary for efficient charge generation and transport, limiting its performance. These observation was validated with time-delayed collection field (TDCF) and temperature dependent BACE measuremens. The final aspect of this thesis investigates the revolutionary NFA Y6, demonstrating that neat Y6 can achieve a PCE of 4.5% without the need for donor-acceptor blends, thanks to its ability to generate free charges. Combinig drfit-diffusion simulations with various measurements reveals that the π-π stacking in Y6 enables high mobility and spontaneous charge generation. If the efficiency of single-component solar cells improves, it could address two critical issues in OSCs: V_OC losses from charge transfer state formation, and the morphological problems arising from donor-acceptor incompatibility. Together, these findings highlight the importance of balancing doping, morphology, and energy offsets to optimize OSC performance, paving the way for further advancements and commercial viability in organic photovoltaics. One of the key takeaways from this thesis is that non-radiative recombination are closely tied to energetic disorder. However, as demonstrated in the PDINN study, these losses can be mitigated to a certain extent by the state-filling effect from the TL into the active layer, where higher doping levels reduce disorder. This leads to smaller recombination rates and FF, provided that an optimal morphology is maintained. However, ASM study highlights that in the absence of the π-π stacking, leads to suppressed charge generation and transport despite the lower energetic disorder, resulting in reduced FF. In the same cord, the ability of Y6 to generate bulk free charges, illustrates how morphology and quadrupole moments can play a decisive role in reducing performance losses typically associated with low-offset systems.