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 2016 we should have each of the layers in the structure designed specifically for the tandem and expect to have power conversion efficiency well above 20%.

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. We will announce that we have made substantial progress towards this goal at some point early in 2016.

One of our main goals is developing perovskite semiconductors with ideal bandgaps for tandems and we have submitted a manuscript on the subject for publication.
 

Polymer Bulk Heterojunction Solar Cells

Intercalation of PC71BM between pBTTT side chains.

The efficiency of polymer solar cells has improved rapidly over the last decade and is currently over 11%. Although our group strives to improve the efficiency of polymer solar cells, our primary goal is to understand in detail how the solar cells work. We take full advantage of the synchrotron x-ray source at the Stanford Linear Accelerator Laboratory and our strong collaboration with Mike Toney to use x-ray diffraction as a powerful tool for determining how molecules pack in solar cells. Using XRD along with solid-state NMR data and molecular mechanics simulations, we are in the process of learning precisely how electron accepting fullerenes touch the polymer backbone in various high performing systems and a collection of systems that mysteriously do not perform well even though they appear to have the properties that are needed for good power conversion.

The best polymer solar cells have a pure polymer domains, pure fullerene domains and domains where fullerenes are mixed into amorphous polymer. It appears that the energy for both electrons and holes is higher in the mixed domains and that there is consequently a favorable energetic gradient that separates most of the electrons and holes from each other. We are currently measuring the energetic gradient and determining how molecular packing influences it. We are performing kinetic Monte Carlo simulations to determine the probability of an electron and hole separating as a function of charge carrier mobility, electron transfer rates, the magnitude of the energetic gradient and the size of the mixed phase. We are also performing experiments to measure all of the parameters that go into the model. We hope to figure out precisely why the internal quantum efficiency is greater than 90 % for the highest performing materials and then design even better materials.

Schematic of a three-phase morphology


In summary, this sub-branch of the group combines structural characterization, electrical and optical characterization, and device modeling to understand how polymer solar cells work and make them better.
 

Studying Degradation in Solar Cells and Making Them More Stable

We have built one of the best facilities in the world for studying the long-term performance of air-sensitive solar cells. We have 6 lamps and 6 testing chambers with a controlled atmosphere. We are in the process of rewiring the chambers to increase the number of cells we can test from 96 to 2400 so that many materials can be tested at the same time.

We have been studying degradation in polymer solar cells for several years and have identified several different degradation modes. In some cells the efficiency drops by approximately 20% in the first couple of weeks as traps form in the polymer. We have not yet figured out why the traps form, but think that an impurity might be involved since this problem stops after a couple of weeks. When cells are made with polymers that have a low glass transition temperature, we observe minor degradation even if the cells are tested in the dark, but at a slightly elevated temperature. This problem can be avoided by using polymers with a glass transition temperature higher than 100 degrees Celsius. Finally, we see a slow degradation that occurs over a period of years that appears to involve a photochemical reaction.

This project involves building equipment for lifetime testing; seeing how cells are affected by light, heat, impurities and oxygen; working with the bulk heterojunction team to understand in detail how morphological changes and traps affect device performance and collaborating with chemists who can design more stable materials and improve purification techniques.

We are just starting lifetime tests for solar cells made with perovskite semiconductors.
 

 
Photo of 40 substrates, each with up to 10 solar cells, in an environmentally controlled chamber.   The sun never sets in the McGehee lab.

 

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.

A separate project on smart windows in the group focuses on using reversible metal electrodeposition to develop next-generation electrochromic devices. This strategy relies on the fact that most metals are highly reflective to visible and infrared light once they are greater than 20 nm thick. We are leveraging advanced electrochemical techniques such as electrochemical atomic layer deposition and pulsed electrodeposition to electrodeposit metals over large areas with a nanometer-level control of thickness. These techniques hopefully will allow us to construct robust smart windows that possess full modulation of window transparency.