Judges’ Queries and Presenter’s Replies

  • Icon for: Qiaobing Xu

    Qiaobing Xu

    Faculty
    May 20, 2013 | 11:19 p.m.

    nice work. what’s the light conversion efficiency of your device? how could you possibly improve it?

  • Icon for: Kevin Brew

    Kevin Brew

    Presenter
    May 21, 2013 | 11:19 a.m.

    Great question! To my knowledge, the Purdue Solar Lab holds the nanocrystalline based solar cell world record efficiencies at 9.2% and 14.3% for CZTSSe and CIGSSe based devices respectively. We also hold the record for highest CZTGeSSe nanocrystalline based devices, which is also above 9% conversion efficiency. Apart from better materials processing, there are several ways to improve our devices. For CZTSSe, higher sulfur content would help expand the band gap and increase the open circuit voltage. Some groups are looking into extrinsic dopants such as sodium for improving device performance as well. Our group has a strong focus on electrical characterization; by ascertaining the flaws of our devices, we hope to overcome any efficiency barriers for our materials.

  • Icon for: Caleb Miskin

    Caleb Miskin

    Co-Presenter
    May 22, 2013 | 08:08 a.m.

    To add to Kevin’s comments, the holy grail of many thin-film film technologies is determining the fundamental cause of the relatively low open-circuit voltage (Voc) compared to the devices’ built-in voltage (Vbi) or bandgap. This inherent limitation has vexed many reserach groups including our own. Through materials characterization and device modeling/simulation we are working to understand this limitation and improve the materials synthesis and fabrication to eliminate it.

  • Icon for: Aparna Baskaran

    Aparna Baskaran

    Faculty
    May 21, 2013 | 03:45 p.m.

    Dear Kevin, I am naive when it comes to context of solar cell device physics. Hence here are some rather simple clarification questions. The stated goals of the work are improved materials and improved device fabrication to get high efficiency photovoltaics. Your poster tells me about fabrication studies of specific materials (and not probing the phase space of possible materials for photovoltaics). Is there a simple reason why these are the best choice? Also, does the size dispersion/asperity of the post sintering material affect efficiency?

  • Icon for: Kevin Brew

    Kevin Brew

    Presenter
    May 21, 2013 | 06:22 p.m.

    While we have researchers working towards synthesizing novel photovoltaic materials, many members are focusing on CIGSSe and CZTSSe at Purdue and CdTe at UTEP. CIGSSe is an attractive material because it is commercially produced and has surpassed 20% power conversion efficiency on a lab scale. However; this efficiency was achieved using physical deposition processes. Our study on CIGSSe with a nanocrystalline approach has the opportunity to produce the same thin film for a much lower cost. As for CZTSSe, the use of earth-abundant materials over indium and gallium have made it an attractive alternative. CdTe is another high efficiency material that is commercially produced; however, we believe higher efficiencies can be achieved by tuning the device structure on a nanoscale. The final grain size, composition and structure of the solar absorber material are some of the greatest factors in dictating the final solar performance. By using gradient concentrations of gallium through CIGS, researchers have been able to control the bandgap through the material and improve performance.

  • Icon for: Caleb Miskin

    Caleb Miskin

    Co-Presenter
    May 22, 2013 | 08:31 a.m.

    To comment on phase space probing, our group at Purdue is divided into three subgroups (new materials, CIGSSe, and CZTSSe). The new materials group works under the hypothesis that certain families of crystal structure result in good solar performance. We then take a structure, look at the periodic table, and imagine what substitutions might be made. We then consider how such substitutions should theoretically affect the bandgap of the material and position of the conduction and valence bands. If an attractive potential material is identified, we try to make it and characterize its properties. In this manner new absorber materials can be found.

    An added level of difficulty is then pairing the material with an appropriate buffer layer to form the device. CdS has been the default for many years, but may not have the required band alignment for new materials. Thus, the hunt begins for an appropriate buffer layer.

    To add to the last question about the post-sintered material, a common trend we observe is that device performance increases as the sintered grain size increases. Thus, the sintering conditions are chosen to promote large grains (>1 micron), while limiting unwanted reactions at the back contact of the device (the formation of MoSe2) and the loss of volatile material from the film surface (such as SnSe2).

  • May 21, 2013 | 05:49 p.m.

    Kevin, in the abstract of your poster you wrote that through these studies you obtain “crucial insight into fundamental physics of these solar devices”.. Can you, please, tell more about what is the most interesting fundamental problem here, and how your results help to solve it. Thank you

  • Icon for: Kevin Brew

    Kevin Brew

    Presenter
    May 22, 2013 | 02:12 p.m.

    Great question. The most interesting fundamental problem is with the kesterites; CZTSSe has an incredibly low open circuit voltage (around 0.42 V) for its bandgap (around 1.05 eV). All the kesterites suffer from very low open circuit voltages. Whether this dearth of performance is due to the absorber material or due to the overall solar diode architecture is still debated. We employ a plethora of electrical characterization techniques coupled with device simulations that are helping us elucidate why this material is falling short of expectations.

  • Icon for: Nathaniel Carter

    Nathaniel Carter

    Co-Presenter
    May 22, 2013 | 03:58 p.m.

    Indeed, the low Voc (compared to the bandgap) reported for CZTSSe throughout literature is arguably the most interesting fundamental problem we are working to tackle in our group. This problem is particularly interesting since CIGSSe – which is in many ways very similar to CZTSSe – does not generally exhibit as detrimental a Voc deficiency. The characterization techniques Kevin mentioned include:

    - temperature-dependent current-voltage measurements, which provide insight into advanced diode characteristics such as the ideality factor, dark saturation current, and series and parallel resistances (given an assumed equivalent circuit model);
    - temperature-dependent capacitance-voltage and capacitance-frequency measurements, from which we can calculate energies and densities associated with defects and the free carriers in our absorber materials;
    - optoelectronic measurements such as time-resolved photoluminescence and temperature-dependent, steady-state emission photoluminescence, which provide carrier lifetimes and information about the dominant radiative recombination mechanisms present in our absorbers.

    Many issues we see for CZTSSe were reported for CIGSSe when it was a “young” material, but have since been resolved to a large degree. By comparing measurements taken for both CIGSSe and CZTSSe devices, we hope to learn about what limits our CZTSSe cells. Once we understand the fundamental factors inhibiting device performance, we can modify our fabrication processes to overcome these limitations and improve our solar cell efficiencies.

  • Icon for: Qi-Huo Wei

    Qi-Huo Wei

    Faculty
    May 21, 2013 | 10:19 p.m.

    Kevin: can you summarize what you have learned from your studies, in another word, what new physics or new technology your work can tell us?

  • Icon for: Kevin Brew

    Kevin Brew

    Presenter
    May 22, 2013 | 02:35 p.m.

    If I have learned one thing from working with solar cells, it is that there are an infinite number of ways to make a device perform worse. Each step is crucial towards the final performance. All the way from the glass you use, to the grids you lay, everything you do is important. This process takes roughly 3 days start to finish, and the best results are when the full fabrication is done quickly.

    From my earlier studies, I have learned that the environment in which you dry a coated film, and the medium in which you disperse your nanoparticles to form the ink, are crucial towards the final device performance.

    I’ve learned that by varying the sulfur:selenium content, we can tune the bandgap of our materials and increase the open circuit voltage. Chuck Hage’s research has also shown that other elements like germanium effectively adjust the band gap as well. His CZTGeSSe devices are our newest technology.

    We still have a long way to go to achieve our goal of 15% PCE for kesterite materials, but with our approach of focusing on the physics of these materials, we hope to pinpoint the fundamental flaws in CZTS so we can rectify them.

  • Icon for: Hyunjoon Kong

    Hyunjoon Kong

    Faculty
    May 21, 2013 | 11:04 p.m.

    Very good work. What was the media used to prepare nanocrystal ink?

  • Icon for: Caleb Miskin

    Caleb Miskin

    Co-Presenter
    May 22, 2013 | 08:12 a.m.

    Thank you. We suspend our nanocrystals in 1-hexanethiol for doctor blade coatings. The balance is in finding a solvent that will suspend the particles and uniformly wet the substrate. On the other hand, spray coatings are often done with toluene, though other solvents may be used.

  • Further posting is closed as the competition has ended.

Poster Discussion

  • May 22, 2013 | 11:12 p.m.

    Awesome project! Great job outlining the steps of production. What is the prospect for cheap mass production of this kind of PV?

  • Icon for: Caleb Miskin

    Caleb Miskin

    Co-Presenter
    May 23, 2013 | 09:21 a.m.

    The future is very promising. Several techonologies exist for mass production of thin films, but a key constraint will be the efficiency of module scale films, as most work with CZTS-based materials is currently on a lab scale. Scaleup usually takes a toll on device efficiency, but we are confident this can be overcome as it has to a large extent with other PV technologies.

  • Icon for: Samson Lai

    Samson Lai

    Trainee
    May 22, 2013 | 11:35 p.m.

    Wow, lots of awesome materials science processing going on here. I also approve of Anamanaguchi’s music.

  • Icon for: Kevin Brew

    Kevin Brew

    Presenter
    May 23, 2013 | 01:18 p.m.

    Thanks! After their AMA on Reddit, I was pretty sold on using their music, and Helix just fit so well with what I wanted to do.

  • Icon for: Robert Opila

    Robert Opila

    Faculty
    May 24, 2013 | 12:38 a.m.

    Is the particular advantage of the nanoparticles in making printable inks? Your cells seem to have really good efficiencies — any performance advantages?

  • Icon for: Nathaniel Carter

    Nathaniel Carter

    Co-Presenter
    May 24, 2013 | 12:31 p.m.

    The primary advantage of the nanoparticle approach is the ability to make printable inks. For the CIGSSe material system, nanocrystal-based methods fail to reach the record efficiencies achieved via vacuum deposition processes (~15% vs. 20%). Currently, the world record CZTSSe device efficiency (11%) belongs to a solution-processed solar cell; our champion nanocrystal-based CZTSSe efficiency of 9.2% is comparable to the best values reported for vacuum-deposited CZTSSe cells. If there is a slight performance advantage to nanocrystal-based or solution-processed CZTSSe devices over vacuum-deposited cells, it may be due to marginally better control of compositional uniformity throughout the CZTSSe film afforded by these methods. This advantage could be irrelevant for CIGSSe, a ternary material system, since ensuring compositional uniformity is arguably less difficult for CIGSSe than for quaternary CZTSSe.

  • Further posting is closed as the competition has ended.

  1. Kevin Brew
  2. /igert2013/to_client?target=http%3A%2F%2Fwww.igert.org%2Fprofiles%2F3820
  3. Project Associate
  4. Presenter’s IGERT
  5. /igert2013/to_client?target=http%3A%2F%2Fwww.igert.org%2Fprojects%2F235
  6. Purdue
  1. Brandon Aguirre
  2. /igert2013/to_client?target=http%3A%2F%2Fwww.igert.org%2Fprofiles%2F4815
  3. Graduate Student
  4. Presenter’s IGERT
  5. /igert2013/to_client?target=http%3A%2F%2Fwww.igert.org%2Fprojects%2F235
  6. Purdue
  1. Nathaniel Carter
  2. /igert2013/to_client?target=http%3A%2F%2Fwww.igert.org%2Fprofiles%2F3798
  3. Graduate Student
  4. Presenter’s IGERT
  5. /igert2013/to_client?target=http%3A%2F%2Fwww.igert.org%2Fprojects%2F235
  6. Purdue
  1. Chuck Hages
  2. /igert2013/to_client?target=http%3A%2F%2Fwww.igert.org%2Fprofiles%2F3828
  3. Project Associate
  4. Presenter’s IGERT
  5. /igert2013/to_client?target=http%3A%2F%2Fwww.igert.org%2Fprojects%2F235
  6. Purdue
  1. Mark Koeper
  2. /igert2013/to_client?target=http%3A%2F%2Fwww.igert.org%2Fprofiles%2F5410
  3. Graduate Student
  4. Presenter’s IGERT
  5. /igert2013/to_client?target=http%3A%2F%2Fwww.igert.org%2Fprojects%2F235
  6. Purdue
  1. Damian Marrufo
  2. /igert2013/to_client?target=http%3A%2F%2Fwww.igert.org%2Fprofiles%2F5450
  3. Graduate Student
  4. Presenter’s IGERT
  5. /igert2013/to_client?target=http%3A%2F%2Fwww.igert.org%2Fprojects%2F235
  6. Purdue
  1. Caleb Miskin
  2. /igert2013/to_client?target=http%3A%2F%2Fwww.igert.org%2Fprofiles%2F5327
  3. Graduate Student
  4. Presenter’s IGERT
  5. /igert2013/to_client?target=http%3A%2F%2Fwww.igert.org%2Fprojects%2F235
  6. Purdue
  1. Wei-Chang Yang
  2. /igert2013/to_client?target=http%3A%2F%2Fwww.igert.org%2Fprofiles%2F3172
  3. Project Associate
  4. Presenter’s IGERT
  5. /igert2013/to_client?target=http%3A%2F%2Fwww.igert.org%2Fprojects%2F235
  6. Purdue

Nanotechnology Based Thin-film Photovoltaics

Since their development in the 1970s, thin film photovoltaics have been an attractive low material cost technology well suited for producing clean and sustainable energy. Through the SEIGERT program, Purdue University, in collaboration with the University of Texas at El Paso, is engineering thin film photovoltaic devices, with a focus on high efficiency absorbers like cadmium telluride (CdTe) and copper indium gallium sulfoselenide (CIGSSe), as well as earth-abundant copper zinc tin sulfoselenide (CZTSSe). Close space sublimation allows the controlled growth of graded ZnCdTe with mono-crystalline CdTe pillars. These nanostructures act as a buffer to reduce defects between the CdTe absorber and its window layers, improving device performance. Synthesis of nanocrystalline CIGSSe and CZTSSe solar absorbers via highly scalable solution-based processes produces versatile nanoparticle inks that can be uniformly coated on almost any surface—including flexible substrates. These coatings can be applied by many techniques including doctor blading, spray coating, and inkjet printing. By improving the nanocrystalline solution based process, CIGSSe and CZTSSe have reached efficiencies of 14.3% and 9.2% respectively. Interdisciplinary electrical, material, and optical characterization of completed devices have enhanced our physical understanding of these thin film photovoltaic absorbers, allowing for a holistic approach to developing this technology.