Build Your Own Dye-Sensitized Solar Cell

In 1991 Brian O’Regan and Michael Gratzel published a paper titled “A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO₂ films.” This paper is the foundation for an entire branch of solar energy conversion research known as dye-sensitized solar cells (DSSC).

The basic operation of a DSSC is summarized in the schematic below. In roughly a stepwise manner:

1. Light (hν) hits a light-absorbing molecule ( chromophore, C), causing it to enter an energetically excited state (C*).
2. The excited chromophore injects an electron (e^-) into the anode.
3. The iodine (I^-) in solution donates an e^- to the previously oxidized C and combines with I₂ to become I₃^- .
4. The high energy e^- from step 2 enters the external circuit where it can be used to perform work on a load (e.g., charge a battery, run a fan).
5. The low energy e^- then continues to the cathode where it catalytically reduces I₃^- to I^- and completes the circuit.

The 1991 Gratzel paper was groundbreaking because it introduced the use of a high surface area TiO₂ semiconductor electrode as the anode material. With a higher surface area, more chromophore can be loaded on the surface to increase light absorption and thus can generate more photocurrent. Using this basic architecture - with variations to its components - has allowed us to realizef efficiencies greater than 10% in the lab. Companies like Dysol and others are currently commercializing this technology.

As well as being groundbreaking the DSSC is relatively simple with four basic components: an anode acceptor material, a chromophore, a reversibly redox active electrolyte and a cathode material that can catalytically reduce the electrolyte. Given the availability of these components, Greg P. Smestad and Michael Gratzel later published a procedure in the Journal of Chemical Education that allows just about anyone to create their own DSSC. The Institute of Chemical Education (ICE), based out of the University of Wisconsin, has taken the educational utility of this paper one step further and  created a $45 Nanocrystalline Solar Cell Kit. The components of this kit can be seen in the figure below. It consists of 1) 10 x SnO₂ conductive glass slides, 2) 15 mL of I^-/I₂ ethylene glycol solution, 3) 25 g TiO2 powder, 4) a soft graphite pencil, 5) 10 x binder clips and 6) a variable resistor.

All that you need in addition to the kit is an oven/hot plate that can sinter the film at 450°C; a dye that can be easily obtained from raspberries, blue berries, black berries or other fruits; and a mortar/pestle to grind up the TiO₂ while adding acetic acid. The assembled solar cell is shown in the top left of the image. From the kit you can make up to five solar cells at a time and, because many of the components are reusable, the process can be repeated several times. The ICE manual that comes with the kit provides clear instructions for assembling, characterizing and cleaning the devices. It also includes background information, visual aids, graphing paper, teaching suggestions and other useful tips/hints for trouble-shooting the devices.

While DSSCs are currently being studied by graduate students and researchers all over the world, the concepts and components of these devices are so simple that they can be used for teaching activities in middle schools and undergraduate chemistry labs. In fact, last week I had the pleasure of demonstrating this kit/exercise for local high school and middle school science teachers through a program sponsored by the University of North Carolina at Chapel Hill’s Institute for the Environment.

I have been involved with solar cell technology as a researcher for several years, and I can say without a hesitation that I was blown away by how user-friendly the kits are – especially given the high level of science involved. For example, you can construct the circuit in the image to the right (top) using two $10 multimeters from Radio Shack, the variable resistor that comes with the kit and your fully assembled cell. While shining light on the cell and changing the variable resistor, the relationship between the current (I) and voltage (V) can be documented (The graph of voltage versus current is a common sight to anyone that has studied solar cells.)  An incredible amount of information can be obtained from these I/V curves, like open circuit voltage (V_oc), short circuit current (I_sc), power maximum (P_max), fill factor (FF), shunt resistance (R_SH) and series resistance (R_S). Also, if you know the power of incident light (P_inc = 800-1000 W/m², for daytime sunlight) you can calculate the device efficiency (h) by dividing P_max by P_inc.

The DSSC kits are exciting because of the various opportunities they provide to teach high level scientific concepts and troubleshooting through simple hands on activities. Middle school students can compare different dyes and their effect on the devices' efficiencies/current/voltages. Undergraduate inorganic chemistry students can synthesize and compare various dyes. These exercises can also bring together concepts in biology, chemistry, and physics classes. For example, students can prepare chlorophyll dye through enzymatic reactions in a biology lab,  fabricate and load the dye on the TiO₂ films in chemistry class, and then do the I/V characterization or measure the parallel versus series currents of several devices in physics class.

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Survivor: Mechanisms (now accepting logo submissions)

I read an interesting article in May's issue of J. Chem. Ed. titled "Can Reaction Mechanisms Be Proven?" by Allen Buskirk and Hediyeh Baradaran of BYU.  Intriguing.  So I pop open the pdf and a Note from the Editor is boxed at the top of the page before the article starts.  It says:

"Can Reaction Mechanisms Be Proven?" generated spirited responses from its reviewers. The reviews were approximately evenly divided, and all were of very high quality. The authors agreed with the editor’s proposal that the reviewers convert their reviews into rebuttals or affirmations of the authors’ position for publication along with the article, which has been revised based on the reviews. Most agreed to such a process and their comments appear here. We hope that publication of this paper and well-reasoned rebuttals such as those provided here will initiate a wide-ranging discussion. JCE will provide an online forum for further discussion of the issue. Our hope is that both faculty and students will contribute their opinions and ideas to this discussion. -JWM

Huh.  You don't usually hear about that happening too often.  So now I had to read the article.  It's pretty fascinating, and I encourage you to read it all.  I'll summarize and give my thoughts below the jump

Read more »

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Nitrogen Triiodide Explosion [Photo]

A gorgeous photo of pressure sensitive nitrogen triiodide exploding at 1000 m/s is shown below.

by vastibadastoy from flickr photostream

From vastib

The photo was taken in the dark with the camera's shutter open, and a flash triggered by a pressure sensitive detector.

Apparently this was submitted as part of a manuscript to the Journal of Chemical Education for an undergraduate lab demo measuring how fast the explosion propagates through the material. Unfortunately, it was rejected as too dangerous.

The decomposition reaction is...

Chemical details on nitrogen triiodide can be found from the MOTM page contribution by Simon Cotton: Nitrogen Triiodide

Note 1: Yes, that is Latex embedded into the blog post.
Note 2: Originally from the Chemistry Reddit: Nitrogen Triiodide Photo

Mitch

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A Word on Research Misconduct

Dig out your dictionary and look up the word “hyperbole” (I know, it might be a while since you’ve last had English class)—exaggerated statement or claims not intended to be taken seriously.  I tend to hyperbolize a bit when I replay an incident that happened at the bar or in class, which I attribute to the fact that I’m a terrible storyteller.  I think we all do it to a certain extent.  I know I’ve once said something to the effect of, “It was the greatest movie, ever…in the history of humans.”  A hyperbole at its finest.

While most common vernacular is riddled with hyperboles, I’d argue that the majority of intellectual study makes an effort to stay away from gross exaggerations (with history being the exception).  In particular, science is the observation and study of the physical world, and it leaves no room for hyperboles.  Just facts.  For example, if you mix an aqueous solution of silver nitrate with an aqueous solution of sodium chloride it is a fact that a precipitate will form.  There are no equivocations about scientific facts.  Though, science sometimes falls short when making assumptions that connect two or more facts into one coherent theory or proposal.  Still, these assumptions, en route to a new theory, are usually reasonable if not simplistic (i.e. Occam’s razor).

What about bad data?  Of course, there are ways to make our raw data more “natural” without exaggerating.  In the event that we have to plot data points, for example, as scientists we can exclude data that “doesn’t belong.”  We call these anomalies “outliers” and there is statistical rationale as to why a stray might be “bounced” from the data set without any bias to the result.  But even in these cases, the data point is often so far away from the others that including it might be a detriment to a fact about Mother Nature.

What irritates me to no extent is a term I refer to as “hyperbolized research.”  We have all seen these situations before: yields that are bumped a good 5 to 10 to 50%, data that is fit just right, patent procedures that are not reproducible.  Why are these practices tolerated?  Contemporary science is themed “publish or perish,” which essentially means that if you are not producing enough results (nevermind quality) you will soon be unemployed.  I recall hearing stories about early 20^th century scientists who studied science without the proverbial gun to their respective heads and still made great findings.  A lot of these experiments were groundbreaking, marvelous and truly beautiful.

It’s no surprise that this issue of “publish or perish” rears its ugly head in science.  Society is incredibly fast-paced, and science is certainly trying to keep up.  But, it’s really hard to do so with a tiny, bankrupt research group (where most if not all members are teaching) versus a behemoth firm with hundreds of years of experience and millions of dollars of materials to use. 

So, what do groups do to keep pace (or at least appease the boss)?  “How did you do with that reaction you couldn’t get to work last week?”  “Um…I got 98% yield with 95% ee.”  “Great, let’s write up a manuscript and submit to JACS.”  I’ve heard stories of “big name” research groups who’s members purposely inflate their yields to keep “the man” happy.  In these cases, researchers keep two sets of lab notebooks: the real one (usually under lock and key with the actual experimental results) and the boss’ one (usually kept in the open, so the boss can see how his researcher got a 90% yield on chemistry that is next to impossible to reproduce).  The bottom line is that papers get published, lectures are given and proposals are funded—criminality is rewarded.  How is this right?  Furthermore, how is it fair to another researcher who needs to repeat the results?

Have we not learned anything from the Bell Lab incident?  For those not familiar, Hendrik Schön was a groundbreaking physicist working for Bell Labs in the late 1990’s.  He was purportedly on par to win a Nobel Prize with his creation of an “organic molecular transistor.”  The papers describing this work were met with criticism in the scientific community and at some point (c. 2001), Bell Labs launched an internal investigatory committee to examine Schön’s work.  Their final report ultimately alleged 24 accounts of misconduct that were essentially fit into three categories: “Substitution of data,” “unrealistic precision of data,” “Results that contradict known physics.”  In the end, he was ultimately stripped of his doctoral degree.  But think about the repercussions of not investigating Schön’s findings.  Had Schön’s work not been policed, potentially millions of dollars would’ve been invested into falsified research.  While I’m aware that Bell Labs was recently closed, without insinuating anything, it makes me wonder if this Schön incident had any weight in the lab’s termination.

Rex Dalton covered the aftermath of this incident along with several other examples of research misconduct (Nature 2002, 420, 728-729).  He ultimately offered up the following observation:

“Science may be self-correcting, but sometimes it is a painfully slow process.”

Perhaps he’s right.  Sure, several papers are going to be questioned in the future.  And of those papers, a few might be blatant lies.  How much time is it going to take to correct these mistakes?  According to Corey: "Occasionally, blatantly wrong science is published, and to the credit of synthetic chemistry, the corrections usually come quickly and cleanly.”  Case in point?  The hexacyclinol incident that was excellently covered by C&EN and by a couple of fellow bloggers: Derek Lowe and Paul Docherty.  In this case, there was a rapid turn around (possibly due to public interest).  However, this case might be the exception.  It could be years before a questionable project is proven incorrect.

I know…you want me to provide a solution.  Maybe there isn’t an immediate, reasonable answer.  But, alas, here’s what I’ve uncovered: there are a few wonderful articles in J. Chem. Ed. about scientific misconduct, which both hover around the LBNL and Bell Lab incidents (see: J. Chem. Ed. 2002, 79, 1391; ibid. 2005, 82, 1521).  The authors’ messages (albeit bluntly or implied) were that ethics and empathy should be at the forefront in the early years of scientific training.  Some people cannot discern between right and wrong and teachers should do their jobs by teaching students about the rights and responsibilities of being a scientist.  While I did not receive formal training on scientific misconduct, I was given a lambasting for bordering on plagiarism my freshman year of college.   I learned my lesson early—you and your lab partner need to keep separate lab notebooks.  Perhaps this experience has formed me into the scientist that I am today (I’m anal-retentive about my lab notebook). 

I guess there is a remaining question still looming.  What sparked this rant about “doing the right thing”?  I’ve been repeating experiments for the past couple of months that were reported to be exceptionally clean (requiring no chromatography) and high yielding.  Most of these reactions have tanked—miserably—even with exceptional preparation and precision.  So, I’m painstakingly re-optimizing experimental procedures so someone else doesn’t have to.  It’s taking a while—much longer than it reasonably should.  But, hey, “sometimes (correcting science) is a painfully slow process.” 

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