Quantum Dot Sensitized Solar Cells

As the drive to seek alternative energy sources significantly increases across the world, there is a growing interest in low cost, easily manufactured and efficient energy sources. Among all alternative energy sources (renewable energy) such as wind, water, biomass or solar energy; the present work is focused on solar energy. A solar cell is a solid state electronic device that converts the solar energy directly into electricity by Photovoltaic (PV effect). Ever since the French scientist Alexandre Edmund Becquerel discovered the PV effect in 1839, the research in the field of solar cells is continuously progressing for and now is the era of third generation solar cells in which Quantum Dot Sensitized Solar Cells (QDSSCs) have emerged out to be a potential candidate in converting solar energy directly into electricity.

QDSSCs are a promising low cost alternative to existing PV technologies such as crystalline silicon (Si) and thin inorganic films. A QDSSC make use of Quantum Dots (QDs) as light absorbing material. The absorption spectrum of QDs can be tailored by controlling their size. So, different sized QDs of varying wavelengths having absorption maximum peaks in the entire visible spectrum can be used in QDSSCs to harness solar energy efficiently. This could boost the power conversion efficiency above the Schokley Queisser limit of 31% for conventional Si based solar cells. QDSSC is composed of four main components: (i) a 2–20 μm thick, porous, wide bandgap semiconductor film (typically TiO₂ or ZnO) composed of crystalline nanoparticles abutting one another on Transparent Conducting Oxide (TCO) glass; (ii) QDs adsorbed onto these nanoparticles; (iii) a hole conductor electrolyte interpenetrating the nanocrystalline semiconductor network; and (iv) a counter electrode. The present review focuses on the fabrication and characterization of ZnO based QDSSCs using CdS as sensitizers. ZnO was chosen because of its high stability against photo corrosion upon optical excitation. ZnO is one of the II-VI compound semiconductors having wide bandgap of ∼3.37 eV and large exciton binding energy of 60 meV with electrical conductivity of the order ∼ 103 ohm⁻¹ cm⁻¹ at room temperature. The large bandgap of ZnO is advantageous in QDSSCs for the transparency of front electrode in the large portion of the visible spectrum. So, ZnO acts as a TCO due to its high optical transparency (> 80%) in the visible region and high electrical conductivity.

The QD layer also plays very important role in performance of QDSSCs. The interfacial properties of the QDs should be such that it gets properly adsorbed onto the wide bandgap semiconductor surface and that it should absorb light in the entire visible range. CdS is II-VI compound semiconductor having a direct bandgap of 2.42 eV and was used as sensitizer in the present work. The unique properties of II-VI semiconductor materials such as direct bandgap, tunable size and hence bandgap, broad absorption spectrum, narrow emission spectrum, high temperature stability and large extinction coefficient make them attractive for solar cell applications.

CdS QDs were synthesized using chemical route and then used to fabricate QDSSC. Stability issues of QDSSC are also discussed. The performance of QDSSC is depicted on the basis of power conversion efficiency and emphasis is given on the importance of energy level alignment of the system to increase light to electric power conversion efficiency.