Colloidal semiconductor quantum dots (QDs) are solution-processed nanocrystals in the size range of a few nanometers that exhibit swift charge transport and size-tunable strong optical absorption. QDs are promising enablers of low-cost optoelectronics and have been actively pursued over the last 15 years.

Traditionally, lead chalcogenide QDs have been the center of attention as they possess good charge transport and efficient splitting of photogenerated electron-hole pairs upon light absorption when used as a solar cell absorber layer. Although high photocurrents are possible from lead chalcogenide QDs, they show low photovoltages (V_OC) as a result of charge carrier trap states and prevalent QD-QD fusion (dimerization). These issues ultimately limit their power conversion efficiencies (PCEs). Now a research group from the National Renewable Energy Laboratory (NREL), Nankai University, and Warren Wilson College has reported solar cells with a record-high PCE of 17.4%.

Led by Joseph M. Luther of NREL, the research team built from Luther’s previous work in 2016 where his group reported QDs made of inorganic perovskites with significantly enhanced environmental- and phase-stability, typically absent for the bulk perovskite phase. Cesium lead triiodide (CsPbI₃) QDs were used by the group to demonstrate stable and high PCE solar cells. Importantly, notably high V_OCs are possible when these QDs are used as solar absorbers.

Now Luther’s team has developed a new device architecture that improves carrier separation and extraction, thereby improving the remaining two device parameters crucial to device performance, the short-circuit current and the fill-factor, resulting in PCE of 17.4%. Short-circuit current and fill-factor are parameters that define a solar cell’s ability to generate charge carriers upon light absorption, their migration toward and extraction at the respective electrodes. Reporting in a recent issue of Nature Communications, the researchers demonstrate the concept of perovskite QD (PQD) heterostructures and their utilization as an absorber layer in devices.

Success of this strategy lies in the ability of PQDs to be deposited in a layer-by-layer (LbL) fashion. In the colloidal state, the PQDs are surface-protected with long-chain organic ligands that provide colloidal stability. However, solids formed from these PQDs show relatively poor charge transport due to the organic surface ligands. A brief soak in methyl acetate (MeOAc) removes the ligand and brings the PQDs closer, resulting in a conductive, coupled solid. Importantly, this process renders the CsPbI₃ PQD film insoluble to subsequent PQD film deposition.

The possibility of forming PQD film stacks in an LbL fashion is a major advantage over bulk perovskite thin films that use polar solvents and get re-dissolved if a subsequent layer is to be coated. “One can use solution processing to deposit PQDs on existing perovskite films, whether that existing perovskite film be composed of QDs or of thin film perovskites. We can now process multiple layers of films; unique device designs can be realized which impart gradient compositions throughout the film to better control energy flow within the film,” says Luther on the potential of the LbL processing scheme of PQD films.  

The researchers next deposited a slightly modified PQD layer, where the modification was done by introducing another cation, formadinium (FA⁺), besides Cs⁺, which resulted in a modified band structure of the PQDs. The heterostructure therefore had an interfacial energy band offset that would result in enhanced charge separation during device operation. The optimized heterostructure consisted of Cs_0.25FA_0.75PbI₃ PQD and CsPbI₃ PQD films with a thickness ratio of 1:3, and had the optimal energy band offset for enhanced photogenerated carrier separation. Photoelectron spectroscopy was utilized to quantify the energy band offset between the two films. Improved carrier separation was confirmed by transient absorption spectroscopy.

Maria A. Loi, a professor at the University of Groningen, the Netherlands, and an expert in the physics of optoelectronic devices, says this significant demonstration by the NREL team opens ways to more complex and efficient optoelectronic device structures. “The devices reported by the [researchers] show very limited hysteresis, which highlights the stability of these heterostructures under the given conditions. This exciting finding gives opportunities for reflection on the actual role and condition for ion migration in this materials class,” Loi says.    

It is possible that this heterostructure strategy is generic and can be utilized in related research fields that aim to employ PQDs for displays and electronic devices. “It will be interesting to see how this concept could play out in other devices, perhaps PV devices with the absorber containing thin films with nanoparticles modifying an interface, or even other technologies like LEDs [light-emitting diodes], transistors, or others,” Luther says.

Read the article in Nature Communications.