Core/Shell/Quasi-Shell Quantum Dot Absorbers: A Dual Sensitization Strategy for High Performance Photovoltaics

Sahasrabudhe, Atharva (2015) Core/Shell/Quasi-Shell Quantum Dot Absorbers: A Dual Sensitization Strategy for High Performance Photovoltaics. Masters thesis, Indian Institute of Science Education and Research Kolkata.

PDF (MS dissertation of Atharva Sahasrabudhe (10MS17)) Thesis_Atharva_Sahasrabudhe-10MS17.pdf - Submitted Version Restricted to Repository staff only Download (2MB)

Abstract

The current challenges in designing highly efficient quantum dot sensitized solar cells (QDSCs) are (i) expanding the light-harvesting range; (ii) improving charge separation and (iii) suppressing interfacial charge recombination at the TiO₂/quantum dot/electrolyte interface. Type-II quantum confined semiconductor heterostructures as sensitizers offer a unique way to address these issues, since the exciplex state of the type-II system widens the light absorption window while the staggered juxtaposition of the conduction and valence bands drives charge separation in such systems through compartmentalization of electrons and holes. This dissertation presents an optimized dual sensitization strategy for high performance, all-aqueous processed liquid junction QDSCs. Particularly, type-II core/shell/quasi-shell CdTe/CdS/CdS quantum dots (QDs) were used as absorbers and the QDSCs were fabricated by a 2 step approach which comprises of: (a) CdTe/CdS core/shell QD self assembly on porous TiO₂; (b) Deposition of an additional CdS layer through successive ionic layer adsorption and reaction (SILAR). At first, the QD surface coverage was optimized by systematic pH variation, whereby the device efficiency was improved from 2.04(±0.01) % (pH 11) to 3.696 (±0.005) % (pH 13). Secondly, the individual roles of the shell and the quasi-shell and their overall synergistic effect on device performance were critically analyzed. Various charge transport and recombination measurements revealed that while the epitaxial shell passivated the core surface traps, the non-epitaxial quasi-shell passivated the TiO₂ surface states and hence the two acted in tandem to increase the overall device performance. Thus while the conversion efficiency (η) was 1.45(±0.10) % and 3.62 (±0.40) % for core/shell and core/quasi-shell sensitized devices respectively, it reached an impressive 5.69 (±0.02) % with core/shell/quasi-shell architecture. Furthermore, the device performance was found to be extremely sensitive to the quasi-shell thickness. The origin of such dependence was traced back to increased interfacial recombination rate using various dynamical techniques such as impedance spectroscopy, open-circuit voltage decay and dark current measurements. While the light harvesting efficiency kept on increasing, the overall device performance dropped after a critical quasi-shell thickness (4 SILAR cycles), which highlights the detrimental effect of QD overloading on the device output. Having rationally optimized and established the roles of different components, the modified deposition technique allowed us to fabricate liquid junction devices with efficiencies as high as 6.41% (Jsc = 20.32 mA/cm², Voc = 0.61 V, FF= 51%) (champion cell) which, notably, is the highest for any all-aqueous processed QDSC.

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