Enhanced Performance and Stability of Planar Heterojunction Solar Cells via Hole Transport Layer Engineering and Low-Cost Fabrication

Abstract

Global energy demand has grown extensively in recent decades, and it has continued to rely on fossil fuels that are accompanied with environmental concerns. This has intensified the research for renewable energy alternatives with solar power standing out as a leading candidate due to its abundance, scalability and rapidly declining costs. As photovoltaic (PV) technologies have evolved significantly, their widespread adoption continues to face barriers in efficiency, stability and manufacturing costs. While inorganic semiconductors such as crystalline silicon remain dominant due to their favorable band gap and long-term stability, hybrid solar cells such as organic–inorganic heterojunction solar cells have gained increasing attention for their ability to combine the tunability and ease of processing of organics with the superior charge transport and stability of inorganics. In this work, we have investigated the stabilization of planar heterojunction solar cells through the incorporation of dimethyl sulfoxide (DMSO) into PEDOT:PSS-based hole transport layers (HTLs). The acidic and hygroscopic nature of PEDOT:PSS is a well-known source of device instability, leading to accelerated degradation under ambient conditions. By employing DMSO as a cosolvent alongside ethylene glycol and methanol, this work demonstrates that optimized modification enhances electrical conductivity, reduces recombination, and markedly improves stability. Devices incorporating DMSO-treated PEDOT:PSS films retain nearly 89.4% of their initial efficiency after 72 hours of ambient storage, in contrast to the sharp decline seen in control devices without DMSO. Atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) confirm improved surface morphology and a favorable redistribution of conductive domains. The findings establish DMSO modification as a practical, cost-effective strategy for producing more inherently resilient heterojunction solar cells. Following these insights, we introduce dimethyl sulfone (DMSO₂) as a solid-state additive for PEDOT:PSS films. Unlike liquid cosolvents, DMSO₂ crystallizes upon drying, inducing a unique reorganization of polymer microstructures that enhances phase separation and alignment of conductive PEDOT chains. The resulting films exhibit superior conductivity, improved charge transport, and greater stability against moisture induced degradation. Devices fabricated with DMSO₂-doped PEDOT:PSS achieve efficiencies up to 15.5% (EMD2) and an average T80 of ∼913 h of ambient storage (ED2), a substantial improvement over conventional treatment. Through a combination of external quantum efficiency (EQE), AFM, and conductivity analyses, this work highlights the ability of DMSO₂ to simultaneously enhance efficiency and extend ambient storage longevity, offering an environmentally benign and scalable pathway for advancing PEDOT:PSS-based solar technologies. We also address the challenge of electrode optimization by introducing a rapid and low-cost method of shadow mask fabrication by desktop 3D printing. While electrode geometry is critical to current collection efficiency, series resistance reduction, and overall photovoltaic performance, traditional fabrication techniques are expensive, time-consuming, and inflexible. By employing polyethylene terephthalate glycol-modified (PETG) filaments for 3D printing, this study demonstrates a streamlined approach to fabricating custom shadow masks for top electrode manufacturing in hours rather than weeks. Comparative testing of three geometries (comb-like busbar, central busbar, and crossed busbar) shows that the central busbar design achieves superior efficiency enhancement by 21.62% and improves the fill factor by reducing resistive losses and balancing optical transparency. This work illustrates how low-cost additive manufacturing can democratize device prototyping, accelerate design iterations, and lower research and production costs without compromising performance. In summary, this dissertation presents a cohesive exploration of strategies to improve efficiency, stability, and fabrication simplicity of planar heterojunction solar cells. Through targeted material modifications and innovative fabrication methods, the studies collectively highlight pathways to bridge laboratory innovation with commercial feasibility. Together, these contributions underscore the critical role of polymer modification and accessible fabrication in the evolution of next-generation solar cells, with the ultimate goal of advancing the prospects of clean, scalable, and sustainable energy technologies.