Solution-processed inorganic solar cells based on colloidal semiconducting quantum dots and nanocrystals show much promise because they can absorb light over a wide spectrum of wavelengths thanks to the fact that the bandgaps in quantum dots can be tuned over a large energy range. They are also comparatively cheap to produce. However, only a limited number of materials have been exploited for this type of solar cell – two common examples being lead or cadmium-based crystals – because charge carriers in these compounds last a fairly long time. A long carrier lifetime is important in solar-cell materials because it allows photogenerated electrons and holes to travel through the device and produce useful current before they recombine.

“Unfortunately, not all materials are as obliging in this respect,” said team leader Gerasimos Konstantatos of the Institut de Ciències Fotoniques in Barcelona. “However, lead- and cadmium-based quantum dots are based on toxic elements so we researchers are actively looking for other, safer, materials, even if their optoelectronic properties are poorer – but then we need a device structure to accommodate them in a useful way.”

Reducing recombination

Konstantatos’s team chose to create a bulk nano-heterojunction in a solar-cell device consisting of an electron acceptor and donor material. The two materials were mixed in such a way that, when exposed to sunlight, photogenerated electron-hole pairs were then able to separate at the nanoscale and travel along the device via two very different nano-paths, something that reduced the chances of them recombining.

The team then made bilayer p-n junction devices by combining the n-type Bi2Si3 nanocrystals with p-type PbS quantum dots. One device the researchers fabricated was composed of p-type PbS quantum dots and n-type Bi2Si3 nanocrystals forming a planar p–n heterojunction. A second bulk nano-heterojunction device consisted of a nanocomposite comprising PbS quantum dots and Bi2Si3 nanocrystals sandwiched between a hole-blocking/electron-transporting layer of neat Bi2Si3 nanocrystals and an electron-blocking/hole-transporting layer of PbS quantum dots. The power conversion efficiency of the bulk nano-heterojunction devices was found to be around 4.8%, a value that is three times higher than that of a bilayer p–n junction made of the same materials.

Increased charge carrier lifetimes

To work out the reason for this improved efficiency, team member Arup Rath and colleagues took up the relay and measured the lifetimes of charge carriers in the devices while exposing the cells to varying optical intensities. Although both devices show long lifetimes at low light intensity, at higher intensities (comparable to light intensities from the Sun), the bilayer device contains carriers with shorter lifetimes because electron and holes combine at a faster rate here. Carriers in the bulk heterojunction device, on the other hand, appear to last three times longer than in the bilayer structure since electrons and holes recombine at a significantly slower rate.

“Although the power conversion efficiency of our cells is still a bit lower than record efficiency devices based on PbS quantum dots and titania n-type electrodes, it does demonstrate the proof-of-principle,” Konstantatos tols “What is more, unlike previous studies that relied on either sputtered oxide electron acceptors or high-temperature sintering at 500 °C, our technique works using fully solution-based process and at low temperatures of less than 100 °C – non-negligible advantages for low-cost roll-to-roll manufacturing, for example.”

The results were published in Nature Photonics.