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Boosting Photovoltaic Performance by the Synergistic Interaction of Halide Perovskites and Semiconductor Quantum Dots

Periodic Reporting for period 2 - No-LIMIT (Boosting Photovoltaic Performance by the Synergistic Interaction of Halide Perovskites and Semiconductor Quantum Dots)

Reporting period: 2019-03-01 to 2020-08-31

Solar energy is not only a clean source of renewable energy but also the most abundant one, as just in one hour the Earth receives from the Sun the same amount of energy that all humankind consumes in a year. The maximum theoretical efficiency of a single absorber photovoltaic device is defined by the so-called Shockley-Queisser (SQ) limit. Current record monocrystalline Si and GaAs single absorber solar cells have reach SQ limit in terms of photocurrent and are very close to this maximum in terms of efficiency, see Fig. 1a. Even polycrystalline devices as the promising Perovskite Solar Cells (PSCs) based on Halide Perovskites (HPs) have practically attained SQ limit in terms of photocurrent and also present a significant efficiency, see Fig. 1b.
Despite the significant efficiencies obtained with polycrystalline halide perovskite, Si, CdTe and CuInGaSe (CIGS), their performance lay below the monocrystalline Si and thin film GaAs. Conversely, in the last years the plummeting of the cost of Si solar modules has changed the traditional scenario. Nowadays, with cost per W as low as 0.6 $ for Si, Fig. 1b, modules cost issue is no longer the main problem to make this technology more competitive. Greater competitiveness should emerge from a higher efficiency. However, as already stated, single absorber solar cells are very close to their theoretical maximum. Consequently, a real breakthrough on photovoltaic technology implies to exceed the SQ limit of 33% (at 1 sun illumination). Intermediate bandgap Solar Cells (IBSCs) can move theoretically the photoconversion efficiencies even significantly higher, see Fig. 1c. The main scope of No-LIMIT is to bring a breakthrough on photovoltaic energy. The breakthrough comes from the development of an original Intermediate Bandgap Solar Cells able to exceed the Shockley-Queisser (SQ) photoconversion limit.
No-LIMIT proposes a completely novel implementation of IBSCs using the synergistic interaction between Halide Perovskites (HP) and colloidal Quantum Dots (QDs). The final objective is the development of IBSCs. In order to reach this ambitious objective first it will be needed to determine and model the unique properties of HP and QD interaction from theoretical and experimental points of view. Demonstrate the light absorption with external quantum efficiency of HP-QD systems at wavelengths lower than the BG of both HP and QD. We will tailor the properties of HP, by the appropriated choice of constituent elements of HP with general formula ABX3, and QDs, by the choice of adequate compound and size, to optimize the different BGs and band alignment. Preparation of QD layers embedded in HP matrix with low non-radiative recombination, high long wavelength absorption and excellent transport properties will allow the development of an IBSCs.
The results will undoubtedly produce a great scientific impact in the photovoltaic field Moreover, these achievements will be produced at low cost with efficiencies potentially higher than those obtained from conventional devices based on monocrystalline materials. There is a global interest in the potentiality of HP, and No-LIMIT will help to ensure a leading role of Europe in this research field, proposing a completely new breakthrough technology. However, No-LIMIT also foresees the transition from lab to industry, that will lead to the development of technologies that would have a considerable impact on diversifying the regional industry and on the creation of jobs.
It is worth to highlight that collateral benefits for the development of other applications as transistors, sensors, photocatalysis, LEDs and laser could take benefit from this research. The characterization of these materials will undoubtedly benefit the physico-chemical knowledge, thus allowing being transferred to other systems and materials.
An important objective of the No-LIMIT project is the determination and modelling of the unique properties of halide perovskite and semiconductor quantum dots from theoretical and experimental points of view. With ultrafast spectroscopy we observed the interaction between MAPbI3 (MA= methylammonium) perovskite and PbS QDs, causing and interfacial recombination that must be reduced to take full advantage of the interaction. This configuration also allowed to determine the diffusion length in perovskite thin films. We have observed that MAPbI3 films with embedded PbS QDs in small concentration improves the performance of the perovskite solar cells. However, this concentration is not high enough to take advantage of the sub-bandgap absorption. We have investigated FAPbI3 (FA=formamidinium) without a decrease of the PSCs but an increase in solar cells performance and significantly longer solar cell stability. This fact is significant as FAPbI3 is more interesting than MAPbI3 for solar cell fabrication due to its narrower bandgap, closer to the Schockley-Queisser limit. Beyond the experimental study we carried out a theoretical analysis at three levels: i) Mechano-quantic macroscopic calculations, with k·p model; ii) Density Functional Theory (DFT), see Figure 2; iii) the linear elastic theory in continuous media.
The project has also produce important results in the optimization of benchmark solar cells and on the modeling of these devices. Fundamental working mechanisms of perovskite solar cells remain an elusive topic of research. Impedance spectroscopy has been a key factor to characterizing photovoltaic devices; however, its application to perovskite solar cells has generated misleading outputs. We provide an equivalent circuit to evaluate the impedance spectroscopy of perovskite solar cells in terms of the processes directly affecting their performance, a tool for further development of perovskite photovoltaics.
Important results have been also obtained regarding the fabrication of perovskite quantum dots and their use in optoelectronic devices showing the collateral benefits of the No-LIMIT project in other areas as the photocatalysis or LEDs. We have also show important properties of halide perovskite quantum dots. We show that there is a reduced phase segregation in CsPbBr3−xIx with size lower than 46 nm. Furthermore, through Kelvin probe force microscopy, we confirmed the correlation between the phase segregation and the reversible halide ion migration among grain centers and boundaries. These results open a way to achieve segregation-free mixed halide perovskites and improve their performances in optoelectronic devices.
In order to reach the ambitious final objective of the No-LIMIT in the second part of the project we will focus on the demonstration of the light absorption and external quantum efficiency of devices based on the halide perovskite-quantum dot systems at wavelengths lower than the bandgap of both HP and QDs. We will tailor the properties of HP, by the appropriated choice of constituent elements of halide perovskite with general formula ABX3, and QDs, by the choice of adequate compound and size, to optimize the different bandgaps and band alignment. We will also study additional synergies arising from the interaction between HP and QDs as the phase stability and the increase of the device long term stability. The preparation of QD layers embedded in halide perovskite matrix with low non-radiative recombination, high long wavelength absorption and excellent transport properties will allow the development of IBSC.