# Presentation: 738727

## Solid-State Lighting:A Case Study in Science and Technology Evolution

**Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy’s National Security Administration under Contract DE-AC04-94AL85000.**

***Jeff Tsao***
***Sandia***
***National***
***Laboratories***

**Notes:**
In this talk, I’d like to take a step back, and say a few words about the evolution of science and technology (S&T) as a whole, using solid-state lighting as an interesting and timely case study.  Solid-state lighting is interesting because it has so much potential for impact on the global economy.  And it is timely because it is growing so fast and shows every indication that it may achieve that potential.
This figure illustrates the growth of the S&T infrastructure underling SSL.  It’s a time series of the numbers of journal articles published, and U.S. patents awarded, over the past 30 years or so, in the broad area of electroluminescent materials and phenomena.  The blue data points are for inorganic electroluminescent materials and phenomena, and the green data points are for organic electroluminescent materials and phenomena.  In both cases the filled circles represent journal articles and the open circles represent U.S. patents.
To put this growth in perspective, the journal and patent literatures, averaged over all fields, have both historically been growing at a rate of 5-10%.  Instead, inorganic SSL S&T has been growing at about double that rate, about 10-15% per year, while organic SSL S&T has been growing at quadruple that rate, about 20-30% per year.  So, we can say that SSL S&T has been growing very fast.
The question I would like to explore today is: what are the driving forces for this growth, and in what directions are these forces driving growth?

## Driving Forcesfor Science & Technology Evolution

**A****1**

**A****2**

**A****3**

**A****4**

**Science &**
**Technology**

**Applications**

**Notes:**
To start with, what do I mean by driving force for S&T growth?  What I mean is illustrated here.
This lower plane represents S&T.  Each of these nodes is meant to represent some chunk of S&T knowledge, and each of these links is meant to represent an interdependency between two chunks of S&T knowledge.  As times goes on, S&T evolves through the creation, strengthening, and interlinking of these nodes.
What is it that drives this evolution?  In the usual way of thinking, there are two types of drivers.
The first is intrinsic to S&T (that is, in the plane):  it is our intrinsic human desire to solve challenging puzzles and to be recognized for our solutions. We can think of this desire as a driving force that acts preferentially on those areas of S&T that are ripe for progress, regardless of whether it can be used by any particular application.
The second type is extrinsic to S&T (that is, out of the plane):  the use to which the S&T can be put by applications.  Each application, A1, for example, uses and depends on, some mix of S&T.  But that mix differs from application to application.  So some areas of S&T will by chance not be in the mix of any application, and will have a small extrinsic driving force.  But some will by chance be in the mix of many applications, and these will have a large extrinsic driving force.
So we have these two types of drivers: one type directed by intrinsic ripeness for progress, but largely random with respect to usefulness to applications; and the other type directed by applications extrinsic to S&T, but largely random with respect to intrinsic ripeness for progress.  Both drivers are interesting and important, but in this talk I’ll focus on the second type, the applications extrinsic to S&T, for the specific case of solid-state lighting.

## Extrinsic Applications Drivers for SSL S&T

**Notes:**
For solid-state lighting there are of course many many applications, ranging from calculators in the early years, to traffic lights, cell phone backlights and flashes, mobile LCD screen backlights, non-mobile LCD backlights and, before too long, automotive headlights.  Each application uses and depends on some mix of S&T, but, as I just mentioned, that mix differs from application to application.
One way to make this more concrete is to represent each application by a mix of technology performance metrics.  For solid-state lighting, these include, e.g.:  shock threshold, source size, color gamut, etendu, turn-on time, color-rendering index, luminous efficacy, or cost of light.  Moreover, each application will weight these technology performance metrics differently – I’ve indicated this schematically with xx’s for a strong weighting, x’s for a medium weighting and a blank for a low weighting.
The extrinsic part of the evolution of solid-state lighting S&T can be thought of as being driven by all of these applications, each investing in improving those technology performance metrics it cares most about.
Of course, some applications will have greater influence than others.
Applications with large markets can have large influence, since they have the most resources to invest in R&D.  So LCD backlights for mobile appliances, with a more than US$1B per year market size, have driven a continuing increase in luminous efficacy, which determines battery life, though at the expense of color rendering index, which isn’t as important for direct-view applications.
But also early applications, regardless of size, can have large influence, because they determine core technology paradigms that become established early and become difficult to overturn later.  So traffic signals, which were one of the first major applications, have driven a continuing decrease in the cost of light, though again at the expense of color rendering index.

## General Illumination Drivers for SSL S&T

**Notes:**
Now, there is, of course, one special application: white light for general illumination.  This application is special because there is a societal benefit to solid-state lighting being used by these applications over and above that reflected by the markets.
As you all know, general illumination consumes about 8% of all energy worldwide, and hence contributes both to international tensions associated with supplying that energy, and to environment degradation through global warming.  So, apart from how market forces will drive SSL S&T, we as a society have an additional non-market incentive for driving SSL S&T.
So one question we might ask is: are these two driving forces, the market and the non-market, aligned are not?
At first blush, perhaps not, since the market force would tend to weight CRI and CoL more heavily, while the non-market societal force would tend to weight luminous efficacy more heavily.
But at second blush, perhaps, since there is clearly a relationship between luminous efficacy and CoL.  So let’s spend a few minutes exploring that relationship.

## Cost of Light Evolution

**Cost  of        Operating       Capital**
**Light             Cost                 Cost**

***Electricity***

***Luminous***
***Efficacy***

***Power***
***Delivery***
***Cost***

***Lifetime***

***Mainte-nance Cost***

**=                       +**

**Notes:**
Here, I’ve broken down the cost of light into its two sub-costs.  There’s operating cost, which is the cost of electricity divided by the luminous efficacy.  Then there’s purchase cost, which is something that might be called the power delivery cost, divided by luminous efficacy and lifetime, along with a small piece having to do with maintenance.  The electricity and maintenance costs we can’t control much, and lifetime is already high enough for general illumination, so for our purpose here we won’t worry about either of these.
That leaves these two:  the power delivery cost, which is how much it costs you to sink a certain amount of power at the chip, and the luminous efficacy, the lumens you can create given the power you’re sinking at the chip.  Each of these can be improved, and as each is improved, one can calculate the impact on the cost of light.
That’s shown on this diagram, which shows contours of constant cost of light for various power delivery costs and luminous efficacies.
You can see that for high power delivery costs, over on the right side of the diagram, the contours are relatively steep, so decreasing power delivery cost decreases the cost of light by a lot.
But for low power delivery costs, over on the left side of the diagram, the contours are relatively flat, so decreasing power delivery cost doesn’t decrease the cost of light by much.  Instead, increasing luminous efficacy is what will decrease the cost of light.
So where are we now?
These data points, again, show progress in commercial high-CRI devices from 2004 through 2006.  Empirically, it looks like during those years power delivery cost has been driven harder than luminous efficacy – power delivery cost has decreased by 3x while luminous efficacy has hardly increased.  That’s reasonable, since in 2004 and earlier we were in a zone where decreasing power delivery cost could decrease the cost of light by a lot.
But it also looks like we are just now reaching the point of diminishing returns, where decreasing power delivery cost further won’t make as large a difference.  Instead, as we try to approach the frontier of possibility, this point up here, it will be more important to push luminous efficacy.

## Summary

**Science &**
**Technology**

**Applications**

**CRI**
**CoL**
**  ****$/W****in**
**  ****lm/W****in**

**lm/W**

**Traffic**
**Signals**

**LCD**
**Backlights**

**General**
**Illumination**

**General**
**Illumination**

**Market**
**(profit)**

**Non-market**
**(energy savings)**

**Driver**

**Notes:**
So, now we’re in a position to say something about the alignment between the driving forces for general illumination.
Consider again this illustration, where now I’ve filled in some of the market driven applications (traffic signals, LCD backlights, general illumination), but I’ve listed one application, general illumination, twice, because it also has an important non-market societal driver, energy savings.
For the non-market (energy savings) driver, luminous efficacy is the primary technology performance metric of importance.
For the market (profit) driver, CRI and CoL are important, while luminous efficacy is only important insofar as it contributes to CoL.  If there are other routes to influencing CoL, routes that don’t involve improving luminous efficacy, e.g., by improving power delivery cost, then the application will be just as happy with those routes.
In the recent past, that is precisely what has been happening.  In other words, the market and non-market drivers for general illumination have been less well aligned in the past.
In the future, though, we are rapidly approaching the point where power delivery cost cannot further influence the cost of light by much, and will likely see a renewed emphasis on luminous efficacy.  Then, the market and non-market drivers are likely to become better aligned.
With that, let me thank you for your attention.