Photovoltaic magic of quantum dots unraveled

The efficiency of a solar cell is determined by the number of electrons that are released by light.

An important drawback of current solar cells is that only one electron is generated per absorbed photon. It was always assumed that in semiconductor nanocrystals, quantum dots, multiple electrons per photon could be generated. Researchers of the FOM Institute AMOLF in Amsterdam, the university of Lille (France), and the Ben-Gurion University in Israel have now showed that, contrary to all expectations, more electrons per photon are generated in bulk than in quantum dots. This observation has far-reaching implications for quantum dot solar cells. The FOM researchers published their findings in the 6 September edition of Nature Physics.

Solar cells are based on the photovoltaic effect, in which absorption of photons in a material leads to free electrons that can generate electrical current. The efficiency of solar cells is largely determined by the fact that only one electron is generated per absorbed photon. This is a limiting factor because the energy of visible photons is in principle more than enough to generate two or three electrons. The unused photon energy is now lost as heat.

Carrier Multiplication in Quantum Dots
There is a process, however, in which hot electrons can use their excess energy: ‘Carrier Multiplication’. In this process, an excited electron transfers its excess energy to a second electron, hereby enabling this electron to contribute to the electrical current. Unfortunately, in most materials Carrier Multiplication is not very efficient. Therefore researchers have been trying for years to find new materials in which Carrier Multiplication can occur in an efficient way. One strategy that seemed promising was the use of Quantum Dots (QDs). In various publications exceptionally high Carrier Multiplication efficiencies of up to seven electrons per photon were reported. These findings were explained by special nano effects, resulting from the confinement of electrons in the small QD volume. Hence, QDs seemed ideal to make solar cells more efficient: higher photo currents in QD solar cells using the same input of light.

Controversy
Soon, however, a controversy arose: Carrier Multiplication in QDs did not seem to be as efficient as initially thought, and it was not clear what the underlying physical mechanism was. The controversy increased to such extent that the scientific journal Science published an editorial on this topic in December 2008. After fierce objections of the involved scientists, this editorial was partially corrected. The core of the controversy was that it was not clear if, and if so why, Carrier Multiplication was more efficient in QDs than in bulk material. Until now, it was always assumed that nano effects played an important role: QDs are special because electrons are confined in the QD nanocrystal, resulting in much stronger interactions than in the according bulk material. This was supposed to result in more efficient Carrier Multiplication. For the investigated QD materials, lead sulfide (PbS) and lead selenide (PbSe), the bulk Carrier Multiplication efficiencies were unknown, however. As a result, there was no definitive proof that nano effects in QDs were really leading to higher Carrier Multiplication efficiencies.

New insights
Researchers of the FOM Institute AMOLF in Amsterdam have now determined the Carrier Multiplication efficiency in bulk PbS and PbSe. Employing a unique experimental method, they determined the photocurrent, picoseconds after exciting the material with light of a specific wavelength. By correlating the photocurrent (which is proportional to the number of free electrons) with the number of absorbed photons, it was possible to determine the number of free electrons per absorbed photon for different colors of light.

The result was surprising: Carrier Multiplication appeared more efficient in bulk than in QDs of the same material. A collaboration with French theoretical physicists resulted in an explanation for this finding: although electrons in QDs interact more strongly with other electrons, this effect is counterbalanced by a second effect, i.e. the special energy structure in QDs. In QDs, there are much fewer energy levels present, as a result of which it becomes less probable that the recombination energy of a hot electron is exactly the same as the energy that is needed to excite an electron from the ground state. Hereby, it becomes more difficult to fulfill the law of energy conservation, and the probability of Carrier Multiplication in QDs decreases.

As a result of this research, the expectations of efficient Carrier Multiplication in QDs have to be adjusted drastically. It is highly unlikely that Carrier Multiplication can still contribute to efficient QD solar cells. QDs are also interesting for solar cells for other reasons. Hence, the outcome of this research does not necessarily mean that QDs have no potential as future solar cell materials.

The research was co-financed by the Joint Solar Programme (JSP) of the Foundation for Fundamental Research on Matter (FOM), supported by the Netherlands Organisation for Scientific Research (NWO). The JSP is co-financed by the area Chemical Sciences of NWO and Shell Research. The research is partly financed by the Israel Science Foundation.

Reference:
'Assessment of carrier-multiplication efficiency in bulk PbSe and PbS', J.J.H. Pijpers, R. Ulbricht, K.J. Tielrooij, A. Osherov, Y. Golan, C. Delerue, G. Allan and M. Bonn, Nature Physics, 6 September 2009.

Contact
J. (Joep) Pijpers Msc. FOM Institute AMOLF (020) 608 12 34
Prof.dr. M. (Mischa) Bonn, FOM Institute AMOLF (020) 608 12 34

Figure 1. Carrier Multiplication. In the Carrier Multiplication process, the energy of one high-energy photon (blue wave) can be used to generate not one, but several free electrons (red spheres) in a material. In a solar cell this results in a higher photocurrent and hence a higher efficiency.

Figure 2. Carrier Multiplication efficiency for bulk PbSe (black line) and Quantum Dots (open circles). By determining the photocurrents with very high time resolution, the number of generated free electrons per absorbed photon could be assessed for excitation with different light colors. The most reliable QD data points lie below the line of the bulk data.