Quantum Dot Solar Cells: Toggling the Band Gap

Overview

Recent advancements in solar cell research have begun to question whether it is possible to engineer the conductive properties of solar cells in a manner that is somewhat independent of the chemical compound used. Whereas our last blog post focused on a particularly auspicious arrangement of ions within crystalline structures, this blog post will examine the effect of individual particle size on a material’s energy band gap—the central concern of quantum dot (QD) solar cells (QDCs).

Theory

While the research into QDs is in its infancy, a theory behind considerations of the near-maximum efficiency of a photovoltaic cell is based on the ability to control the band gap of the material in use. What makes such a thing possible comes from understanding of the Pauli Exclusion Principle and the Bohr radius (average distance from electron to proton in hydrogen atom in ground state). The exclusion principle states that no two electrons within an atom can have the same quantum numbers (the same energy level, momentum, orientation, and spin). This results in theformation of closely-spaced energy levels within a material (a band). To account for the Exciton Bohr Radius (used when we want to look at other atoms), the formula is applied:

exciton-bohr

Other than constants ε (permittivity of free space), ℏ (reduced Planck’s constant), and e (elementary charge), two important variables, me and mh, are the effective mass of electrons in the material and the effective mass of electron holes in the material.

rB shows not only that the radius of electron excitation is dependent on the ratio of charge densities, but that reducing the size of a particle to less than the the Exciton Bohr Radius results in a discontinuity in its energy levels, therefore putting it in a quantum state. The result? Changing the size of these quantum dots below the radius enables one to control the available energy levels within a material, “tuning” its band gap.

Screenshot from 2014-04-23 08:36:29.png

Available bands are controlled by the “room” for the excited electron, or exciton. Its properties are modified when the size of the individual quantum dot is restrictive. This presentation more thoroughly covers the subject matter. Note above that α = rB. Thus, as a result of controlling for rB, a quantum dot solar cell can be made receptive to radiation of various energies (and various wavelengths). This holds important value in capturing infrared radiation, a large segment of available light that has yet to be capitalized on by conventional solar technology, yielding unheard-of efficiencies.

Stoichiometric Considerations

However, achieving ideal experimental results that are consistent with theory has been complicated by the fact that stoichiometry at the quantum dot level is less predictable than at the macroscopic level. Put another way, research has found it difficult to maintain the same ratio of elements within a material at smaller sizes. This buildup of impurities devastates the efficiency of the solar cell by hindering electron movement between quantum dots.

Above: impure quantum dots, where the stoichiometry is different from the overall stoichiometry of the material. This leads to localized electrons (in red) that are trapped in the quantum dot.

In lead(II) sulfide (PbS)—the typical semiconductor used in solar cells—expected results differ from experimental results because the 1:1 ratio between lead and sulfur is not constant throughout the material. Yet instead of trying to achieve perfection for every dot in the cell, the solution is to pair these “dangling bonds” in the compound with additional particles.

Plasma-Enhanced Chemical Vapor Deposition

Researchers at the Tokyo Institute of Technology have developed a method of combating this problem using PECVD with silicon quantum dot cells. This technique involves the reaction between a substrate and ionized gas in a gradual process under high temperatures (193°C in this experiment).

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The benefit of this method ensures the purity of the product that precipitates on the interface of the substrate (c). Reactant gases SiH4, monomethylsilane (MMS), and CO2 contribute to the deposition of hydrogenated amorphous silicon oxycarbonate onto the quartz (SiO2) substrate. Afterwards, the product is treated with hydrogen plasma in a process known as plasma treatment, frequently employed in the smoothing of surfaces of manufactured plastics. This is done under even greater thermal conditions (200°C – 600°C). Hydrogen particles react with any oxidized particles on the substrate, restructuring the compound:

Screenshot from 2014-04-23 13:26:02.png

The removal of these impurities is most effective at higher temperatures. However, some  undesired structural breakage occurs around the surface due to the weakness of the Si-Si bonds.

Wrapping Up

Advancements in quantum dot manufacturing have promised greater efficiencies across more wavelengths of the EM spectrum. While consumer cells range in efficiency from 5% to 19%, current QDCs have already achieved efficiencies of 14%, but researchers propose the maximum theoretical efficiency to be as high as 63%. Such developments foretell a great future for the world of solar power.

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