Origin of high solar cell efficiency in hybrid perovskites

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Laboratory researchers and a collaborator have discovered why solution-processed, large-grain hybrid (organic/inorganic) perovskites produce solar cells with exceptional (approximately 18%) power conversion efficiencies. This class of materials offers low-cost solution processing preparation together with favorable intrinsic properties for optoelectronic applications. The journal Advanced Functional Materials published the findings.

The team discovered that charge carriers created by light are not free electrons and holes. Instead, they are manifested as “large” polarons, a more massive entity resulting from long-range interactions between photo-excited charges and polar molecules of the hybrid perovskite lattice. The large polaron has interesting physical properties that result in effective “shielding” from optical phonons and electronic impurities that are otherwise detrimental for efficient transport in semiconducting materials. This explains the superior transport properties of hybrid-perovskites compared with other solution-processed semiconducting materials, such as polymers and nanoparticles.

This is the first study that carefully addresses the intrinsic optoelectronic properties of large grain perovskites under solar cell operation conditions. The team used confocal time-resolved photoluminescence spectroscopy to examine the dynamics of photoexcited charge carriers in the organic-inorganic perovskite thin films. They found that the devices illuminated with sunlight operate in a bimolecular regime, where the free recombination of two spatially separated opposite charge carriers is the only relaxation pathway. The bimolecular recombination process is surprisingly efficient and comparable with that of high quality, defect-free direct bandgap IIIV semiconductors, like gallium arsenide, which are produced through more costly fabrication methods. The recombination is much slower than the transient time of charges to electrodes in devices that capture photocurrent, making overall charge extraction much more efficient than recombination.

The findings bridge the gap between the microscale optical properties and macroscale solar cell performance of crystalline methylammonium halide thin films. The results demonstrate that devices based on solution processed thin film semiconductors could be operated in regimes free of impurities that degrade transport and alter optical properties. This discovery is an important step toward creation of solution-processed semiconductors that have properties comparable to materials grown by traditional high temperature methods. The hybrid nature of these materials leads to new physical phenomena involving charge carriers that are not observed in either organic or inorganic systems alone. Hybrid perovskite materials have excellent optoelectronic properties that enable a wide variety of potential device applications. The research gives the team insight to develop this material for U.S. energy and global security applications, such as solar cells and sensors.

Reference: “The Effects of Electronic Impurities and Electron-hole Recombination Dynamics on Large-grain Organic-inorganic Perovskite Photovoltaic Efficiencies,” Advanced Functional Materials 26, 4283 (2016); doi: 10.1002/adfm.201505324. Authors: Jean-Christophe Blancon and Jared Crochet (Physical Chemistry and Applied Spectroscopy, C-PCS), Wanyi Nie, Amanda Neukirch, Gautam Gupta, and Aditya Mohite (Materials Synthesis and Integrated Devices, MPA- 11); Sergei Tretiak (Physics and Chemistry of Materials, T-1); and Laurent Cognet (Université de Bordeaux).

The Laboratory Directed Research and Development (LDRD) program and the U.S. DOE, Office of Science, Energy Frontier Research Center for Advanced Solar Photophysics at LANL funded different aspects of the Los Alamos work, and Laboratory Institutional Computing provided computational resources. The work supports the Lab’s Energy Security and Global Security mission areas and the Materials for the Future science pillar. Technical contact: Jared Crochet

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Figure 2. (Left): Illustration of the optical experiments conducted to discover the nature of charge carriers in hybrid perovskite thin films. (Right): A schematic of the electronic bands in the hybrid perovskite material with relevant relaxation processes. The rate of charge recombination in the material is much less than rate of charge extraction in a device.

 

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Figure 3. Comparison of the properties of a large polaron (left) and an electron (right). The velocity of a large polaron is much less than that of a free charge, and its momentum is much larger than that of a free charge. This type of excitation enables material defects to be less important in transport processes and allows for efficient charge extraction under the applied electrical fields that are found in optoelectronic devices.