The work demonstrates the use of semiconductor quantum dots (nanoparticles that exhibit size-dependent optical and electronic properties) as light harvesting materials for solar cells. Many research laboratories are involved in the research related to synthesis and characterization of semiconductor quantum dots. Yet, their use in solar cells is lacking.

“In order to meet the demand for clean energy, it is important to seek new ways to harvest renewable sources such as solar energy.”

In this paper we have assembled Cadmium Selenide (CdSe) quantum dots on a nanostructured TiO₂ film using bifunctional linker molecules. These films are robust and deliver maximum photoconversion efficiency of 12-15%. The CdSe quantum dots absorb visible light and inject electrons into TiO₂ which are then collected at a conducting electrode to produce photocurrent.

Use of quantum dots in solar cells is significant because it opens up new opportunities to design next-generation solar cells with the capability of tuning photoresponse by controlling particle size. Many researchers find this approach very attractive and promising.

The principle of sensitization of large bandgap semiconductors (e.g., TiO₂, ZnO) with dyes and short bandgap semiconductors is well known. Effective coupling of two semiconductor particles is crucial in achieving efficient electron transfer under light irradiation. CdSe quantum dots are assembled on mesoscopic TiO₂ films via linker molecules such as mercaptopropionic acid. The binding of carboxylic acid group to TiO₂ and that of the thiol group to CdSe facilitates creation of an ordered assembly of semiconductor nanoparticles. Such an approach is likely to be useful to assemble other light-harvesting assemblies on electrode surfaces.

In order to meet the demand for clean energy, it is important to seek new ways to harvest renewable sources such as solar energy. Silicon solar cells which are currently employed in photovoltaic panels can deliver maximum power at a rate of >30%. New nanomaterials such as semiconductor quantum dots can overcome this limitation by generating multiple electron-hole pairs and thus can deliver power at greater efficiency.

Our work is a first step in this direction. By attaching cadmium selenide quantum dots to TiO₂ particles, we were able to harvest light energy in terms of electrical energy. Further improvement in efficiency is necessary before one could use them in practical solar cells.

We have been engaged in solar photochemistry research for more than two decades. The continued funding by the US Department of Energy Basic Energy Sciences (BES) program has enabled us to look into the fundamental problems associated with photoinduced charge separation in semiconductor particulate systems.

A multiprong approach of employing spectroscopy tools and electrochemistry techniques has enabled us to unravel various electron transfer events that occur in the subpicosecond-to-millisecond time scale. Assembly of semiconductor quantum dots on electrode surfaces in a robust way and minimizing charge recombination at the grain boundaries are the major challenges in developing quantum dot solar cells

We are currently working towards the development of rainbow solar cells. These next-generation solar cells will consist of different size quantum dots assembled in an orderly fashion. Smaller size particles in the outer edge absorb blue light while the red light gets transmitted through the film and is absorbed by the larger size particles present in the inner layer.

By carefully creating a gradient based on the size of quantum dots, we can maximize the light harvesting efficiency and improve the photoconversion efficiency of such solar cells. We are also currently exploring tubular TiO₂ architecture in order to collect and transport injected electrons efficiently towards the collecting electrode.

Solar cells will have a significant role in providing clean energy during the coming decades. The placement of solar cells on every roof in the US can fulfill the domestic need of electricity. New technologies that can produce safe and cheap solar cells can pave the way towards achieving this goal.

Another potential application of quantum dot solar cells may be in the development of colored windows that can incorporate quantum dots as light absorbers. While the color of these windows can be tuned by varying the particle size of quantum dots, the absorbed light could be converted into electricity!