McGehee Group

Stanford University | Stanford Materials Science & Engineering |

 

Research

The McGehee group is currently doing research on perovskite and organic solar cells as well as smart windows. Historically Prof. McGehee has investigated organic light-emitting diodes and lasers, light extraction from LEDs, light trapping in solar cells, charge transport in organic semiconductors, nanopatterning techniques, dye-sensitized solar cells and semi-transparent electrodes based on meshes of carbon nanotubes or silver nanowires. The group is inspired to develop technology that can provide humanity with clean energy and solve environmental problems. Although all of our projects have a clear application and we interact closely with companies, we take a very scientific approach, rather than an Edisonian trial and error approach, that involves sophisticated characterization and advanced modeling. Each of our current projects is described below. Our latest results are always posted promptly on the publications page.
 

Polycrystalline Tandem Solar Cells

Many believe that solar cells will need to have a power conversion efficiency around 25% and a cost below $0.5/W to revolutionize how the world's population obtains its electricity. We believe that the most promising approach to reaching this goal is to make tandem solar cells with a high band gap solar cell harvesting the high-energy photons and a lower bandgap solar cell harvesting the low-energy photons. This approach has already been used to make solar cells with power conversion efficiency greater than 40%. The challenge is to figure out how to make the tandems at low cost. Much of the research in the McGehee group is focused on finding ways to deposit high bandgap solar cells on top of either silicon or copper indium gallium diselenide, which have an ideal bandgap for the bottom cell. We think the recently developed hybrid perovskite semiconductors could be used to boost the efficiency of silicon solar cells to 26%. The hybrid tandem project provides an overarching goal for the entire research group. Various branches of the group work on polymer solar cells, perovskite solar cells, transparent electrodes and the long-term reliability of the solar cells.

A schematic of a hybrid tandem solar cell.

 

Hybrid Perovskite Solar Cells

In the last few years hybrid perovskite semiconductors have emerged as one of the most promising materials for solar cells. The power conversion efficiency has soared from a few percent to over 20 %. In this fast-moving highly competitive field, we have picked a few areas of focus in which we think we can have a big impact.

Our primary technological goal is to develop semitransparent perovskite solar cells with a bandgap around 1.8 eV that can be deposited on top of silicon solar cells to make tandems. Our group and our collaborators were the first to demonstrate mechanically stacked and monolithic two-terminal perovskite-silicon solar cells. In June 2016 we achieved an NREL certified record 23.6 % power conversion efficiency for monolithic perovskite-silicon tandems. We also developed a new 1.2 eV band gap perovskite with Henry Snaith's group and used it to make an all-perovskite tandem with > 20 % efficiency in the summer of 2016. We are currently working hard to improve both of these tandems and take the efficiency towards 30%.

Our group was one of the first to discover that perovskite solar cells often have hysteretic current-voltage curves and was the first to realize that perovskites that contain both iodine and bromine undergo phase separation under illumination. These discoveries led to the conclusion that halogens are mobile in perovskite films. One of our research priorities is understanding the implications of this complexity that is not found in most semiconductors. This research involves nanocharacterization, spectroscopy and device modeling. With this fundamental research we hope to figure out how to push the efficiency of perovskite solar cells towards their theoretical limit.

Since perovskite solar cells already have the efficiency that is needed for commercialization and can almost certainly be manufactured at a highly attractive cost, the primary barrier to commercialization is going to be obtaining long-term stability. The challenge appears to be that the films are highly reactive with water and have a tendency to emit methylammonium iodide. We are developing a comprehensive program to study the mechanisms of degradation and make highly stable devices. By replacing methylammonium with a combination of cesium and formamidinium, replacing metal electrodes with indium tin oxide and packaging the solar cells, our group was able to pass the industry standard 1000 hour 85C 85% humidity damp heat test during the summer of 2016. We have also passed aggressive thermal shock and ultraviolet radiation tests.
 

Superior Smart Windows

Smart windows utilize electrochromic materials, which change their transmission characteristics upon application of a voltage, to modulate the transparency of a window towards visible and/or infrared light. The widespread implementation of affordable and reliable smart windows in buildings would result in substantial energy savings by minimizing lighting and heating costs. Currently, however, several shortcomings have confined electrochromic windows to niche applications such as in luxury car and airplane windows.

Schematic of an electrochromic device


An electrochromic device consists of two transparent conducting electrodes on opposite sides of an electrolyte. One or both of the electrodes contain an electrochromic material, which changes its transparency upon application of a voltage. In one of the group's projects, we are developing new composite materials that allow for the independent modulation of the visible and near-infrared portions of the electromagnetic spectrum. These materials combine polymers, which are visibly electrochromic, with transparent conducting oxide nanoparticles, which operate in the near-infrared through plasmonic absorption. At three distinct voltages, electrochromic devices containing these composites exhibit three different states: "bright and warm," "bright and cool," and "dark and cool" modes.