McGehee Group

Research

Professor McGehee's primary interests and areas of expertise are organic electronics, patterning materials at the nanometer length scale and developing materials for renewable energy and sustainability applications. His research group's projects are briefly described here and thoroughly described in our publications, which are listed elsewhere on this website.
 

Structure-Property Relationships in Semiconducting Polymers

	Intercalation of PC71BM between pBTTT side chains.

We use techniques such as x-ray diffraction, spectroscopy, and atomic force microscopy to determine how molecules pack in polymer solar cells and how the packing affects properties such as exciton diffusion, electron transfer and charge transport. Several years ago we discovered that the molecular weight of a polymer is one of the most important factors that determines its charge carrier mobility (Macromolecules, 38 (2005) 3312). We showed that the size of the molecules affects the rate at which crystals form and that this determines whether crystals nucleate off the substrate or in the bulk of the film. When crystals nucleate off the substrate, they tend to be well aligned. Consequently the insulating sidechains do not impede charge transport between neighboring crystals. In 2008 we discovered that one of the most important properties of a semiconducting polymer for photovoltaic applications is the amount of space that is available between the sidechains. We found that the fullerene derivatives that are commonly used to make bulk heterojunction solar cells can often intercalate between the sidechains (Advanced Functional Materials, 19 (2009) 1173). When this happens, exciton splitting is extremely efficient because the polymer and fullerene are not phase separated. On the other hand, we believe that recombination is faster since the polymer and fullerene are mixed at the molecular scale. We find that when intercalation occurs, one must use substantially more fullerene to optimize the performance of the blend because one needs enough fullerene to fill in the spaces between the polymer sidechains and create a pure fullerene phase that can carry electrons out of the solar cell.

Our x-ray diffraction measurements are performed in collaboration with Michael Toney (SSRL). The molecules we investigate are usually made by the research groups of Jean Frechet (Berkeley), Zhenan Bao (Stanford), Alan Sellinger (Stanford), Iain McCulloch (Imperial), and Martin Heeney (Queen Mary).
 

Solar Cell Device Physics

Polymer based organic solar cells have been continuously improving in performance over the last 15 years most recently achieving power conversion efficiencies of over 6%. To reach efficiencies of 10% and beyond will require not only the development of new polymers and acceptor materials but also the establishment of effective screening procedures, and the prioritization of characterization techniques to enable a targeted approach to optimizing and improving devices. We are working closely with synthetic chemists in the Bao and Sellinger groups to speed up the evaluation feedback cycle of the new polymers and accepter molecules that they are developing.

We probe the physical processes that occur in organic and dye-sensitized solar cells. The major processes of interest include photoexcitation, charge generation and migration, and recombination. We use techniques such as photogenerated charge extraction with linearly increasing voltage (photo-CELIV) and space-charge limited current techniques to measure charge carrier mobilities. We use time-resolved photoluminescence as well as time-resolved and steady-state photoinduced absorption techniques to measure the lifetimes of excited states and free charges. We model optical fields in devices to predict optimal device geometries as well as to separate active layer absorption from parasitic absorption, which we use to calculate internal quantum efficiencies of our devices from measurements of external quantum efficiency. We use capacitance-voltage measurements to quantify the doping density in our materials, which is useful for predicting the electric field strength within the device. We collaborate with the Salleo group, who can take sensitive absorption measurements that sub-bandgap states using photothermal deflection spectroscopy as well as the constant photocurrent method (CPM). We also develop new techniques as our needs change.

Dye Sensitized Solar Cells and Organic-Inorganic Hybrid Solar Cells

Dye-sensitized solar cells (DSCs) have excellent charge collection efficiencies, high open circuit voltages (800-850mV), and good fill factors (0.70-0.75). However, DSCs do not completely absorb all of the photons from the visible and near infrared domain and consequently have lower short circuit photocurrent densities (<21 mA/cm2) compared to inorganic photovoltaic devices. We have recently demonstrated a new design where high energy photons are absorbed by highly photoluminescent dyes unattached to the titania and undergo Förster resonant energy transfer (FRET) to the sensitizing dye (see scheme below) (Nature Photonics, 3 (2009) 406). This novel architecture allows for broader spectral absorption, an increase in dye loading, and relaxes the design requirements for the sensitizing dye. We demonstrate a 26% increase in power conversion efficiency when using an energy relay dye (PTCDI) with an organic sensitizing dye (TT1). This architecture can provide a viable means to create highly efficient DSCs.

We are currently developing new near-infrared sensitizing dyes necessary to increase the short-circuit photocurrent density with Zhenan Bao (Stanford) and Jean Frechet (UC-Berkeley). We have also recently begun studying the structure/electronic property relationships of hole conductors in solid-state DSC to improve efficiency and have begun making tandem devices with Peter Peumans at Stanford. We have a strong DSC collaboration with Prof. Michael Graetzel at Ecole Polytechnique Fédérale de Lausanne through the Center for Advanced Molecular Photovoltaics.

DSC schematic representation of a dye-sensitized solar cell with energy relay dyes. The right side of the figure shows the typical absorption process for lower energy (red) photons in DSCs: light is absorbed by the sensitizing dye (1), transferring an electron into the titania and a hole is transported to the back contact through the electrolyte. The energy relay dye process is similar except that, higher energy (blue) photons are first absorbed by the energy relay dye that undergoes Förster energy transfer (2) to the sensitizing dye.
 

Carbon Nanotube Networks as Transparent Electrodes

	A flexible solar cell made in Mike McGehee's lab

We have demonstrated that it is possible to make extremely flexible transparent electrodes with networks of carbon nanotubes and use them to replace brittle ITO in polymer solar cells (Applied Physics Letters, 88 (2006), 233506). The conductivity of these films is approximately 10 times less than that of ITO. We are trying to understand what limits the performance of the films so that we can enable our collaborators (George Gruner, Zhenan Bao) to increase the conductivity. Back-of-the-envelope calculations show that it should be possible to make films that are approximately 10 times better than ITO. Our investigation involves using electric force microscopy to create maps of the resistance in films and computer simulations to determine what the conductivity should be as a function of how many tubes there are, how long the tubes are, what the conductivity of the tubes is, what the junction resistance is, and whether the tubes are metallic or semiconducting.
 

Reliability and Lifetime Program

Stanford is currently developing a reliability and lifetime program for organic PV. The program will focus on both identifying the key modes of degradation within OPV devices and extending lifetimes through novel device architectures.

The device testing program will begin with the ability to test 32 devices in parallel, with plans to quickly expand this capability to 64 devices. Automated systems will monitor and control the temperature and voltage of each device, take period light intensity measurements and have the ability to take I-V curves on a predetermined schedule. Additional characterization techniques will be used to measure mobilities, local current patterns, morphology and compositional changes as the devices are aged.
 

Silicon nanowire solar cells with photon-harvesting shells

We are developing a new class of nanowire solar cells (Journal of Applied Physics, 105 (2009) 124509). In this architecture, organic molecules which absorb strongly in the near infrared (IR) where silicon absorbs weakly, are coupled to silicon nanowires (SiNWs) with radial p-n junctions. This enables significantly shorter SiNWs to absorb a large fraction of the light above the bandgap of silicon. This is very important since nanowire solar cells suffer from large surface recombination problems due to the large surface area to volume ratio. To date detailed modeling has been performed and a proof of concept has been done on a simple electronic structure.

Current efforts are being made to develop an full array of nanowires with organic photon-harvesting shells.