As silicon solar panels reach their fundamental efficiency limit, demand for increasingly efficient and economical photovoltaics must be met by emerging technologies. One promising branch of nascent solar technology, quantum dot (QD) solar cells have progressed substantially over the past five years, from less than 4% efficiency in 2010, to 9% in 2015, although not yet commercialized. In time, QD cells may more than double the efficiency of silicon panels, with lower manufacturing costs and comparable stability. 

 

Quantum dot solar cells utilize nanoscale semiconductor crystals to absorb solar radiation. The sizes and shapes of these crystals, which can be carefully controlled, correspond directly to their bandgaps. This is desirable because the bandgap is perhaps the most important parameter in photovoltaic engineering: it determines what spectrum of solar energy can be absorbed by a material. Semiconductors do not absorb photons with energies below their bandgap, and photons with excess energy are largely wasted by traditional solar cells. The adjustable bandgap of QDs allow for tailored solar energy conversion, with a theoretical maximum efficiency of 86%. Beyond tunable bandgaps, QDs are capable of enabling novel mechanisms for increased efficiencies and unique applications.

 

Before they are incorporated into a solar cell, QDs must first be created. While there are multiple techniques to accomplish this, colloidal quantum dot (CQD) synthesis is the primary method considered in the realm of solar cells. Within that category there are yet several methods of synthesis, but they all begin with precursor compounds dissolved in solution. In simplified terms, the CQDs are formed within a solution by the addition of heat. Carefully controlling this growth process is essential, as the performance of CQD cells is greatly impacted by deviations from desired specifications. 

 

Though challenges involved in improving CQD quality are non-trivial, their synthesis entails less expense and complexity compared to other QD production methods, significantly less compared to crystal silicon production. Minimal specialized equipment is required, and the temperatures achieved during production vary little from room temperature. CQDs are furthermore suited for production in large batches, and once produced, they are easily stored.

 

The process of integrating CQDs into the body of solar cells is generally simple. CQDs suspended in a solvent are deposited on an underlying cell substrate, such as glass. The solvent subsequently evaporates and leaves behind a uniform layer of CQDs. While the simplest deposition method involves no more than dipping the substrate in QD solution, cells can also be created autonomously by spray-coating. This is possible, in part, because QDs are stable and resilient once formed: making them more compatible, at this stage, with mass production than other emerging light absorbers such as perovskites. 

 

Quantum dots are innovative light absorbers, but solar cells must both absorb light, and convert the absorbed energy into useful current and voltage. In silicon photovoltaics this is accomplished by means of the p-n junction (the letters p and n indicate whether a semiconductor has a higher concentration of electrons or electron holes). In a p-n junction, the interaction between these two types creates an internal electric field at the junction between them. This internal field allows solar cells to generate current when electrons are energized by light.

 

Currently the predominant CQD cells are based on p-n heterojunctions, which are much the same as standard p-n junctions. Until recently, p type CQDs were being used in combination with standard n type materials. However, there is inherent inefficiency in using different materials for each side of these junctions, so researchers have sought n type CQDs to create what is known as a quantum junction. CQD cells based on quantum junctions are not only more efficient, but also easier to form, since the processes involved in creating the layers are the same.

 

Methods of layer formation in solar cells are an integral part of the manufacturing process. The most efficient existing solar cells are multi-junction cells which, as the name suggests, employ multiple junctions layered atop one another. Each junction features a distinct bandgap, thus enabling absorption from broader swathe of the solar spectrum. Unfortunately, creating these cells has traditionally been difficult and expensive. Both the difficulty and expense are rooted in the necessity of perfectly matching crystal lattices among layers. With their highly tunable bandgaps, CQDs are well suited for layering in multi-junction cells, and they do not require the same crystal matching as traditional multi-junction cells. Yet while the efficacy of CQD multi-junction cells is promising, the most exciting aspect of QD solar is the potential for taking better energetic advantage of high energy light, through a process known as multiple exciton generation (MEG).

 

In a traditional solar cell, photons impart energy to electrons on a one photon to one electron basis, with no regard to how much energy was used in the process. But due to the quantum-confinement of CQDs, it is possible to excite multiple electrons with a single photon. The magnitude of this efficiency gain is so great that a single junction CQD cell could reach efficiencies above 60%, as compared to silicon photovoltaics, which are fundamentally limited around 33%. The first MEG CQD cell demonstrating this concept was created in 2011, and work continues to make the concept a commercial reality.

 

Despite the many advantages of quantum dot solar cells, it will be some time before they are capable of competing with silicon photovoltaics in the marketplace. However, their progress has been significant and steady. The union of affordability, efficiency, stability, and scalability, make them a technology to watch over the coming years.  

 

Taylor Hubbard works at the University of Boston with expertise in Materials Engineering and partners with The European Energy Centre (EEC) on their training programs. The EEC works with the United Nations Environment Programme and major universities promoting best practice in renewable energy and energy efficiency.

 

For renewable energy professional development seminars, visit www.euenergycentre.org