A CdS/Bi2S3 bilayer and a poly(3,4-ethylenedioxythiophene)/S2− interface control quantum dot solar cell performance

Highlights: • Fabrication of a CdS/Bi2S3 quantum dot (QD) bilayer electrode for solar cells. • Electron transfer occurs from CdS to Bi2S3, hole transfer from CdS to S2− electrolyte. • Bi2S3 QDs have high intrinsic nanoscale electronic conductivity (0.06 S cm−1). • Bi2S3 QDs convert thermal energy into current, advantageous for solar cells. • PEDOT/S2− interface is chemically reduced, allows facile electron passage. -- Abstract: A CdS/Bi2S3 quantum dot (QD) bilayer was fabricated by chemical bath deposition of Bi2S3 quantum dots, followed by CdS deposition by successive ionic layer adsorption and desorption (SILAR) technique. High resolution transmission electron microscopy confirmed the assemblage of well-connected dots, as overlapping lattice fringes of cubic CdS and orthorhombic Bi2S3 were observed. Photoelectrochemical cells devoid of any wide gap oxide but based on CdS/Bi2S3 QD bilayer as photoanode and poly(3,4-ethylenedioxythiophene) or PEDOT as the counter electrode were constructed. Fluorescence quenching and emission life time studies showed that photogenerated electrons in CdS QDs are rapidly transferred to Bi2S3 QDs and electron–hole separation is further augmented by hole transfer from CdS to S2− electrolyte at the CdS/Bi2S3/S2− interface. Conducting-atomic force microscopy (C-AFM) and resistivity studies exemplified that Bi2S3 QDs are characterized by high intrinsic nanoscale electronic conductivity (0.06 S cm−1) and are also capable of converting thermal energy available at room temperature or under illumination into current. Impedance spectroscopy analyses also validated the advantage of Bi2S3 QDs in form of reduced electron transfer resistance, and higher diffusion coefficient for S2− ions. Kelvin probe force microscopy (KPFM) demonstrated that the PEDOT/S2− interface has a shallower workfunction than the traditional Pt counter electrode and Bi2S3 QDs have a deeper Fermi level than CdS. The energetics of the resulting CdS/Bi2S3–S2−–PEDOT cell is not only most favorable for efficient electron shuttling to the external circuit and hole scavenging at the CdS/Bi2S3/S2− interface but also for electron capture by PEDOT/S2− at the counter electrode. Fastest electron transfer rate and longest electron recombination life times were achieved for the CdS/Bi2S3 QD bilayer as opposed to neat CdS, which reflected the role of Bi2S3 QDs in ameliorating charge separation. The cell delivered a power conversion efficiency of 0.45% under 10% of one sun illumination, thus demonstrating that conducting Bi2S3 QDs and the PEDOT/S2− interface can be easily integrated with a plethora of visible light absorbing dots to realize significant improvements in current generation and charge propagation and as a consequence quantum dot solar cell efficiencies