Hot carriers are electrons and holes produced by photons with energies above the semiconductor bandgap. In bulk semiconductors, hot carriers quickly cool in a matter of just picoseconds, releasing phonons (vibrations of the crystal lattice, or heat). Indeed, such wasted heat can account for up to 50% of the energy losses in present-day solar cells.

The problem comes about because radiation from the Sun consists of photons that span an energy range from around 0.3 to 3 eV, so some of these photons have energies above many semiconductors' bandgaps. If the energy of hot carriers could be captured before it is converted into wasted heat, solar-to-electric power-conversion efficiencies could be greatly increased. Quantum dots, which are nanoparticles of semiconductor, could come into their own here.

Indeed, colloidal quantum-dot films (also known as QD solids) have recently been made into photodetectors, field-effect transistors and solar cells. In such devices, the QDs are placed quite closely together and are electronically coupled. The small interparticle distances result in overlapping wave functions – something that increases the mobility of charge carriers in the material to values that are high enough for real-world applications.

Lead chalcogenide films

The wavefunction overlap is especially large in lead chalcogenides thanks to the small effective masses of electrons and holes in these materials. Indeed, PbS and PbSe QD solids can be used to prepare solar cells with good efficiencies because the materials absorb light very efficiently. What is more, the colour of light that is absorbed can be tuned by varying the size of the quantum dots.

After absorbing light, the material generates charge carriers very efficiently too and the carriers themselves are highly mobile, which means that they can easily reach the electrodes of a photodetector or solar-cell device. What is more, multiple exciton generation results in more than one electron-hole being produced per absorbed photon. This increases the sensitivity of photodetectors and the power-conversion efficiency of solar cells.

However, there is still a problem in that researchers still do not really know how mobile charge carriers are photogenerated in such materials. They also have little information about carrier kinetics on ultrafast time scales. Such details would help greatly improve QD optoelectronic devices.

Carrier dynamics

Arjan Houtepen and colleagues have now studied how electron and holes move in PbSe QD solids. The researchers have particularly focused on the first few picoseconds after the material has been excited by light. They present their results in Nano Letters.

The Delft team has basically found two separate relaxation processes in the dots that occur just after photoexcitation: hot carrier relaxation from higher levels to lower electron and hole levels (also know as intraband cooling) and carrier relaxation that comes about thanks to "hopping" between different QDs (also known as spectral diffusion). According to the researchers, the intraband cooling seems to be faster in colloidal QD solids that have been made into conductive thin films than in QDs that have been dispersed in a solvent. The cooling times are around 0.25 picosesonds in the conductive films compared with 0.69 picoseconds in solvent. "We believe that the greater hot carrier cooling rate in QD solids assembled into conductive films could be a problem in solar cells made of these materials," Houtepen told nanotechweb.org.

"Efficient light absorption, charge generation, carrier multiplication, high-charge mobilities and band-like transport have all been demonstrated in QD solids," he said. "The next challenge is to increase the lifetime of charge carriers in these structures and key to this will be controlling the surface of the QDs."

Indeed, recent research by several groups in the US and Canada has shown that the surface of the dots can be coated with purely inorganic ligands that seem to kill two birds with one stone: they increase the dots' stability and the lifetime of the charge carriers as well. "An increased lifetime means that there is more time for the charge carriers to be extracted," explained Houtepen. Alternatively, thicker films can be made that absorb all the light that falls on the solar cell/photodetector.