Physics of Coloidal Quantum Dots

CQD =colloidal quantum dot

Material consideration. Absorption of light and extraction of photo charges

Once a photon enters into a solar cell (made up of a p-n junction) it can generate a pair of electron-hole carriers. This pair will be then spatially separated by the internal electric field of the junction. Therefore to reach a contact (on a side of the solar cell) a carrier will need to travel all the depletion region length and also to diffuse then through all the neutral region length. Because usually the diffusion length of charge carriers is usually smaller that the absorption length of photons, a compromise need to be made. (Usually the CQD layer electronic conduction characteristics can be described as a normal semiconductor having just the values of the carrier mobilities different).

Material Consideration. Variable-sized materials.

A variable size of the QCD means a variable band gap. If the dispersion of the size is too big, then the big band-gap of the small QCDs will be useless provided that it will be overcome by the small band-gap of the larger diameter QCDs. As trap densities are reduced, the band-gap (and thus the size of the QCD) begin to be an important factor in conduction. The bigger QCDs (having small band gap) can become thus “quantum traps”. Thus depending on the variation of QCD sizes, the nature of electronic transport in the QCD layer can change (from hopping from one trap to another, to free conduction.

Material consideration. Electronic traps (mobility and recombination)

There are two types of traps: shallow traps and deep traps. Deep traps act as recombination centers for electron and holes. Shallow traps impend on the free movement of charge carriers. Therefore to increase the mobility of the carriers it is necessary to decrease the concentration of traps. In fact in the electrical characteristics of a solar cell, a decrease in the trap concentration is equivalent to an increase of the carrier mobility. The concentration of traps is reduced by using passivization techniques of the surfaces.

Material consideration. Doping

The doping level of both the “p” and “n” parts of the junction is important in what it concerns the length of the depletion region that builds up in the adjacent layer. Thus if “p” doping is high the depletion length in the “n” layer will be bigger. Usually the QCD layer is of “p” type and high “p” type doping, results in a large depleted region into the transparent layer (ZnO) which harm the external quantum efficiency and the current density. If by contrary the “n” type doping (of the ZnO) is high, the depletion region in the “p” type layer (QCD) can extend over the actual physical size of the QCD.

Material considerations. Interfaces with electrodes

The interfaces with electrodes are important because here due to defects, traps can build up

resulting in recombination centers that lower the current carrier concentrations. Also because of

the different contact potential that electrodes different materials have different levels of forward

and backward currents can be engineered. In practise it is important to have an important

forward current provided by majority carriers while the backward current to be maintained at the

lowest possible values. A high value of the backward current reduces the shunt resistance of the

device, and thus is not desirable.

Describe the limitations and new materials insights gained by the community from CQD sensitized to Schottky to Depleted heterojunction devices.

CQD sensitized solar cells (CQD_SSD) are the earliest type of solar cells employing CQDs

(1998). They are made of columns of transparent ZnO (or quantum wires, coated with

layer of these quantum dots and immersed in an electrolyte. The holes are driven from the

junction into the electrolyte while the electrons are gathered by the ZnO material. Due to the the

large diameter of the CQDs used not all the surface of the ZnO quantum is covered. Thus the

shunt resistance is lowered, a negative fact (in theory the shunt resistance for a perfect solar cell

need to be infinite). Use of short ligands improve the surface covering factor up to 34%. Their

major limitation is the weaker absorption of light into the nanoporous transparent electrode

material (ZnO). Thus to absorb all available light, thicker materials need to be used, and thus the

serial resistance of the device is increased. Their biggest electrical efficiency as reported until

now is greater than 5% (2012 report).

Schottky solar cells are made up from two electrodes that have between then a thin film made up

of CQDs. Early structures (2005) used also a polymer organic semiconductor thin film as a hole

driving material inserted between the first transparent electrode and the QCD thin film. While

the upper transparent electrode was made of ITO (indium-tin-oxide), the back electrode was

reflective for light and usually made up of Mg (also Ag, Au or Al with weaker results have been

used). This device relies on the electric field that builds up over the QCD thin film due to the

electron affinity differences between the back and front electrodes. This electric field separates

the pairs of electron-hole produced by the incident light and drives them in different directions

(holes to front electrode, electrons to back electrode). The initial polymer layer was latter

removed in favor of pure CDQ films which were found to have greater electric efficiencies. The

CQD layer is made up of PbS or PbSe semiconductor materials. These structures (Schottky solar

cells) are simple and easy to fabricate but have one limitation. The Schottky built-in voltage is

generally small (when compared to the QCD band gap) and thus the “open circuit” voltage of the

entire device is small. The best achieved until now is an electrical efficiency greater than 4.5%

with an open voltage of about (as compared to the 1.6 eV CQD band gap) (2011 report).

The Depleted Heterojunction (DH) Solar Cells is a merger between the two previous types of solar cells (QCD-SSD and Schottky). This type of solar cell overcome the limitations in the absorption of light that QCD_SSD has and the limitation in the “open circuit” Voltage the Schottky solar cells have. The DH sollr cell is made up of a frontal transparent electrode (ITO), followed by a transparent layer of a large band-gap semiconductor ZnO or (as in the QCD-SSD). Folloing is a very thin layer of QCDs (50-300 nm) that makes with the first semiconductor a Schottky contact. Finally a metallic back reflector (Au) is used as back electrode. This structure allows large densities of current. Through various engineering techniques the size of the QCDs and thus the QCD layer band gap is modified between 0.9-1.3 eV. Also the doping of the layer can be modified and thus its band-gap can also be varied. The result are different Schottky barriers at the junction between the layer and the QCD layer. Thus the device can be fine tuned. Also the back contact is important because when it is deposited deep traps can build up because the QCD film is damaged by the metal thermal evaporation. To overcome this, a thin layer of is used to protect the QCDs layer from the metal deposition. The electrical efficiency of these DH solar cells is greater than 8.5% (the best attained until now for QCD devices).

What are the major limitations in tandem cells?

Tandem cells are QCDs solar cells made up from a series of sandwiched cells, each having a different band gap, and each absorbing light from different regions of visible spectra. Since they are a sandwich structure the current through all the series “pack” of cells is the same, while their open circuit Voltages ( add together. Between each “p-n” structure that makes an individual solar cell there is packed an additional recombination layer (made up for example of Au-gold). In theory this sandwich pack can achieve much higher electrical conversion efficiencies than a single “p-n junction” solar cell. However for this to be achieved careful engineering of the recombination layers and of matching of p and n type materials is necessary. (Each individual solar cell usually has a different current, and in practice in a “sandwich pack” these currents need to match).

There is only a single major limitation of the tandem cells, that is their power conversion efficiency (how much of the incoming light energy is transformed into electrical energy) achieved until now is the smallest from all of the possible QCD solar cell types. This maximum achieved efficiency for tandem cells is about 4.2% (as reported by an article from 2011).

How qualified are these authors to write this review?

From the two authors of this article, Ted Sargent is the senior and Illan Kramer is the junior.

Ted Sargen is a Physicist Engineer (B.Sc. Eng, Queen’s U., 1995) and a PhD in Electrical and Computer Engineering (U. of Toronto, 1988). At the present he is a full Professor at U. of Toronto and holds the Canada Research Chair in Nanotechnology.

Illan Kramer (the younger person) is an Electrical Engineer (B.A.Sc, U. of Waterloo, 2004) and an Electronics Engineer (M.Eng., MgGill U., 2006). Recently he became a PhD through his reaserch on QCDs (colloidal quantum dots) photovoltaic devices (solar cells).