Designing Planar Silicon Solar Cells for Singlet Fission Sensitization

Abstract

Singlet fission (SF)-sensitized silicon (Si) solar cells offer a path towards surpassing the Shockley-Queisser efficiency limit for single-junction solar cells. However, realizing efficient charge transfer from the SF material to Si remains a significant challenge that requires careful interface engineering. Prior work showed that Si microwire cells sensitized with tetracene (Tc) and a zinc phthalocyanine (ZnPc) donor layer can boost photocurrent and external quantum efficiency (EQE). Planar devices are simpler to fabricate than microwire devices and reproduce the planar geometry of optical test samples to connect studies of the interface to device performance. This thesis integrates modeling and experimental approaches to guide the design of planar SF-sensitized Si solar cells. We developed a fabrication process for planar cells comparing varied oxide passivation layer growth conditions and surface treatments, Si(100) versus Si(111) orientation, and junctions formed by diffusion doping versus ion implantation. Complementary surface photovoltage (SPV) measurements on matching optical stacks show evidence of an illumination-induced transient positive charge density at the Tc/ZnPc/oxide/Si interface, consistent with increased field effect passivation. We find that SPV responses on AlOx/n-Si are dominated by substrate band bending; consequently, SiOx is the preferred passivation to suppress the background and isolate the SPV signals driven by the organics. A drift–diffusion model shows that the diffusion doping (exponential) emitters reduce surface recombination rates compared to ion implantation (Gaussian) emitters. We also show that a positive fixed charge density at the surface enhances short wavelength EQE, with the effect strongest for Gaussian emitters. Together, these results provide practical design rules for planar SF-sensitized Si cells and the study of charge transfer at organic-Si interfaces.