Research

My recent research activity has been focused on four different solid-state energy conversion technologies. My PhD dissertation and the thesis defense presentation can be found at:

n      ..\publications\Mukul_Agrawal_PhD_Thesis.pdf

n      ..\publications\Presentations\PhD Thesis Defense.pdf

List of our publications on these topics can be found at:

n      http://www.stanford.edu/~mukul/publications/

In the following you can find brief summary of our current research on these topics:

n      Organic Light Emitting Diodes (OLEDs)

n      Organic and Inorganic Thin Film Photovoltaic (PV Cells)

n      Thermolectric Supperlatices and

n      Thermo-Photovoltaic (TPV) and Solar-Thermal Collectors

Contact me to find out more.

 

Optical Design for Maximal Light Outcoupling From an OLED

Problem Description

The efficiency of organic light-emitting devices (OLEDs) has increased steadily since Ching Tang's landmark paper in 1987. The internal quantum and voltage efficiencies now both approach 100%. However, large efficiency gains still remain to be made by increasing the efficiency at which photons are extracted from the device structure, i.e. the outcoupling efficiency.

Numerous approaches to improve the outcoupling efficiency have been proposed and demonstrated. However, none of these methods are applicable to displays either because they introduce angular color/intensity dependence, or because they rely on scattering which leads to pixel blurring. The method we developed has none of these disadvantages.

Our Approach

We have developed a method that modifies only the substrate with a 2-component dielectric stack that adjust the local photon density of states (LPDOS) in the emissive layer to suppress emission into unwanted modes and enhance emission into desired modes. As a result, a given OLED can appear up to 2.5 times brighter using this approach, for the same power consumption, as shown in Fig. 1.

efficiency and cie

Fig. 1: (a) Relation between achievable apparent outcoupling efficiency and viewing cone for which a Lambertian response is desired. For a 60-degree viewing cone (sufficient for many small displays), an effective brightness that is 2.5 times that of a conventional OLED can be achieved. If a Lambertian response is not required, overall outcoupling efficiencies of 73% (polymer OLED) and 45% (small-molecule OLED) can be achieved. (b) CIE coordinates vs angle for a 60-degree cone and 120-degree cone design, compared to a standard OLED.

We are currently extending our work to metal-dielectric media in 1D, 2D and 3D structures. Contact us if you want to find out more.

Publications

Journal Publications

Conference Talks

Patents

Project info

 

Coherent Light Trapping for Photovoltaics

Problem Description

For efficient photovoltaic energy conversion one wants to absorb as much light in the active material as possible. Increasing the thickness of active layer to achieve so is not desirable because of many reasons – decreasing efficiency of charge separation/collection, cost of active material, manufacturability etc. Light trapping provides a method to increase absorption without increasing the thickness. 1n 1980’s Yablonovitch and co-workers proposed a light trapping scheme based of statistical geometric optics principles. This scheme can enhance the absorption probability by 4n2 in low absorption limit when thickness of absorbing layer is much larger than the wavelength. As a concrete example, this scheme leads to about 25% increase in the short circuit current from 250nm thick a:Si based PV cell.  In last 3 decades or so there is no explicit evidence that any other structure, even in principle, can absorb more light in same volume.

Our Approach

We have develop to different schemes for low refractive index materials such as organic materials (light trapping into radiation modes) and for high index materials as silicon or a:Si (light trapping into confined modes).

For Low Index Materials

From prior work on resonant cavity enhanced (RCE) photodetectors it is well known that over narrow range of wavelengths and angles 100% absorption can be achieved through use of high reflectivity mirrors (DBR). On the other hand anti-reflection coating (ARC) are common in PV cells. Is there an optimum between the two extremes?

We first locate the spectral properties of ideally desired and physically realizable mirrors that would provide maximum absorption in the active layer. Desired mirrors are neither DBR nor ARC! Result obtained here significant for any PV technology as this tells us how one can do better than an ARC which often seems to be a no-brainer obvious choice. We then show how numerical optimization techniques can be used to obtain two-component few layer (5-6) planar mirrors that improves short circuit current in organic PV cells by >50%.

Figure 1 (a), (b) and (c) show the ideally desired mirror characteristics. DBR’s would be ideally desired if target bandwidth is very narrow (photodetector). For PV applications, ideally desired and physically realizable (causal) mirrors are those which show low reflectance in high absorbance spectral region and show high reflectance and anomalous phase dispersion in low absorbance spectral region.

Fig. 1. (a) Absorbance as a function of wavelength is shown for a device with 15nm thick active layer (control structure) without a multilayer mirror (dashed line). Also shown are the spectrally-selective upper bound of absorbance (solid line) and the absorbance of devices with optimal causal top mirror such that the AM 1.5 weighted average EQE over different target wavelength bandwidths (BW) is maximized (squares: BW=20nm, circles: BW=60nm, triangles: BW=150nm, stars: BW=400nm). (b) shows the corresponding amplitude of reflectance of the top mirror and (c) shows the corresponding phase of reflectance of the top mirror to achieve the spectrally selective-upper bound (solid line). The desired amplitude and phase of reflectance for causal top mirrors designed for broadband (BW=400nm, stars) and narrowband (BW=20nm, squares) response are also shown.

Figure 2 (a) and (b) show that >50% improvement in short circuit current in achievable for a thin film bilayer or bulk hetrojunction organic PV cell.

Fig. 2. (a) AM1.5-weighted average EQE for the control BHJ device (open triangles) and a BHJ device with an optimal causal top mirror (filled triangles) for different charge collection lengths (LC=20nm and 40nm) as a function of the thickness of active layer. Also shown is the broadband improvement achieved through coherent light trapping as a function of the active layer thickness. (b) Optimal causal limit of absorbance averaged over the spectral range l=425nm-825nm that is achievable as a function of the targeted bandwidth for a device with 15nm-thick active layer. Inset: Example of an absorbance spectrum without stack (open squares) and with stack (solid squares) optimized for a bandwidth of 150nm.

 

For High Index Materials

In high index materials, the density of states (DOS) of guided modes which is proportional to n3 is significantly higher than that in freespace. This means that if light could be coupled to all confined modes than a very high improvement in absorption is possible. We explore – 1) how accurate is Yablonovitch’s 4n2 improvement factor when effects of optical coherence as well as the effect of smaller DOS in thin films compared to bulk material are included. We find that for 250nm thick films Yablonovitch’s limit over estimates absorption by about 15%. 2) We explore what is the maximum limit of absorption enhancement in arbitrarily nano-structured optical cell. We find that it is possible, in principle, to achieve 70% higher absorption as compared to Yablonovitch limit for a:Si cells.

Fig. 1 (a) and (b) shows two designs that provide absorption enhancement very close to that predicted by idealized/extrapolated Yablonovitch limit for 250nm thick a:Si cell.

Publications

Journal Publications

Conference Talks

Project Info

 

Engineering Phonon Transport

Problem Description

We're interested in lowering the thermal conductivity by building specialized phonon mirrors. This would allow us to make better thermoelectric materials that convert a larger fraction of the heat flux you push through them into electrical power.

Our Approach

Our approach is to take the knowledge we've gained working on specialized multilayer optical structures for OLED outcoupling, PV incoupling, thermophotovoltaics and solar thermal collectors, and turn it into a tool that can be used to build the best possible phonon mirrors. The best structures we've designed are predicted to lower the thermal conductivity by approximately a factor of 3 compared to superlattices.

Fig. 1 shows thermal conductivity of one of the specially design thermoelectric superlatice.

Publications

Conference Talks

·        M. Agrawal and P. Peumans, " Stopping Heat Flow: Aperiodic Superlattices for Thermoelectric Energy Conversion," ICEM 2008, Sydney, Australia, Jul 28-Aug 01, 2008.

Project Info

 

 

TPV and Solar Thermal

Thermophotovoltaics (TPV) and solar thermal are approaches to solar energy conversion with efficiencies that are not bound by the Shockley-Queisser limit. We're interested in how engineered photonic media might be used to improve the efficiency of these approaches.

Thermophotovoltaics

In this approach, sunlight (or a terrestrial heat source) is used to heat an intermediate body. The intermediate's glow is captured by a solar cell to convert it into electrical power. In principle, if the intermediate's absorption and emission characteristics are perfectly tuned, overall system efficiencies approaching the thermodynamic limit (85%) are possible.

Our research focuses on realizing optimal coatings that have the desired spectral characteristics, that are manufacturable using thin-film processing, and that are stable at typical operation temperatures of TPV intermediates (~1800K).

Figure 1 is an example of the projected performance of metal (W, Ta or Mo)-dielectric (MgO) stacks we've designed for use with a Si and GaSb solar cell. You notice that the stacks are emissive above the bandgap of the solar cell, but much darker for photon energies below the solar cell's bandgap.

emitters

Fig. 1: Hemispherically integrated emissivity for W/MgO, Ta/MgO and Mo/MgO stacks matched to a Si (left) and GaSb (right) solar cell. The improvement in characteristics compared to a Nb/Al2O3 periodic stack is evident. In particular at longer wavelengths.

Solar Thermal

This approach uses sunlight to heat up a working fluid that is used to power a rotary engine (turbine or Stirling engine) which in turn drives a generator. One such system uses long pipes through which the fluid flows and parabolic trough collectors to focus sunlight onto the pipe. We are interested in how photonic media can be applied to the pipe to maximize the absorption of sunlight and minimize losses by thermal emission.

Figure 2 shows the projected performance of an optimized metal-dielectric stack. This stack is very absorptive in the visible, capturing 94% of the incident solar power. At the same time, the stack is very reflective (i.e. not emissive) in the infrared, limiting losses by thermal radiation to a minimum.

absorber

Fig. 2: Hemispherically integrated reflectivity of a W/MgO stack designed for solar thermal applications.

Publications

Conference Talks

Project Info