Posts Tagged ‘thin-film solar cells’

8 August

Dr. Christopher Case (of Oxford PV) Discusses Game-Changing Future Transformer of Solar Power Generation-Perovskite Based Solar Cell Technology


Dear Friends, Visitors/Viewers/Readers,

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I  saved the most potential game-changing solar technology in the last interview spot, the update of Perovskite-based solar cell technology with the Chief Technology Officer of Oxford PV, Dr. Christopher Case, of this series of Intersolar North America 2015 interviews. Recalling one of our earlier posts, I wrote about Perovskite, a calcium titanium oxide mineral discovered in the Ural Mountains of Russia in 1839. This new old material is generating quite an explosive buzz because scientists have found, in recent years, that it is a great material to be used in solar absorption applications. It can be made simply and inexpensively by using common wet chemistry lab methods and low cost equipments instead of the expensive deposition equipments common in the semiconductor industry. To take a look at how this process is made cheap and accessible, I’m sharing the video below:

These solar (photovoltaic) cells are made in tandem (layer by layer) fashion on a specially coated glass support. In the video above:

  1. the glass is coated with a dense layer of titanium dioxide, by robotic arm, to prevent electrical charge generated by sunlight from leaking out of the cell.
  2. a less dense porous oxide layer covers the dense oxide layer (usually titanium dioxide, other oxides may also be used).
  3. a simple high speed spin coater deposits this layer from solution and spreads this coating evenly across the device.
  4. heating this glass/device in an oven conditions it for solar cell use.
  5. prepare the Perovskite material (which absorbs in the broad range of solar spectrum) by combining 2 precursor materials:   PbI2 (lead iodide) & CH6IN (methylammonium iodide)
  6. drip the liquid phase mixture (from 5.) onto the oxide coated device (from 4.)
  7.  spin the resulting device in 6 to assure even coating
  8.  applying halide solution
  9.  heating the device resulting from 8 on a hot plate–>spontaneously crystallizes precursors in freshly deposited liquid
  10.  color changes also result from crystallization process resulting from 9.

Such tandem product has the advantage of being able to be introduced into existing infrastructure of current silicon module manufacturing process, boosting its efficiency. With added few steps toward the end of the production line, the coating (equivalent to second solar cell) takes advantage of the blue portion of the solar spectrum and may improve the solar cell efficiency by 20-25% above the underlying silicon. The fact that Perovskite-based solar cell technology is of earth abundant material also insures its availability and low cost. Its high absorption in solar spectrum enables it to have comparable characteristics to that of gallium arsenide. Its ability to change its sensitivity to different band gaps in solar spectrum allows it to make different architectures in tandem solar cells. It can truly be considered as the Custom Solar Absorber! In short term,  Perovskite-based solar cell may boost the efficiency level of existing technology and in the long term, it may be a stand-alone technology with closer efficiency level to that of gallium arsenide but at a much lower cost. It may potentially be sprayed, ink-jet printed, dip-coated, etc. It is no wonder that Dr. Case commented, “the perovskite in solar application is the fastest increasing photovoltaic efficiency of any solar photovoltaic thin film material ever! In just a few years, it went from a lab efficiency of about 6% to well over 17%…the material is a very good solar absorber….bringing the material to 25% efficiency in a monolithic layer and 30%+ in a perovskite tandem layer….potentially the future replacement for silicon.”

The perovskite thin-film solar cells, is currently being developed by Oxford PV (a spin-out from the University of Oxford in 2009-2010 to commercialize this technology, which has exclusively licensed the intellectual property developed by Professor Henry Snaith and his team of 20 scientists). Below, Professor Henry Snaith will embellish upon the development of this solar technology:

Oxford PV plans on continuing to optimize this technology’s cell efficiency and accelerate the transfer of the technology into production. Furthermore, it aims to develop the range of substrates to which the cells can be applied. With its promising future, we, the solar enthusiasts and investors alike, should keep our eyes on Oxford PV in the coming years. In the next few years, we anticipate that Dr. Henry Snaith and his team of scientists will continue to tackle challenges in trap densities, doping densities, mobility, mechanisms for free carrier generations, etc., to further improve device performance. You will find that many in the solar industry share the optimism of Professor Henry Snaith and Dr. Christopher Case.

For those of you interested in more details about Perovskite-based solar cell technology, please refer to the two videos below:

1. Introducing Perovskite Solar Cells to Undergraduates:

2. Perovskite Solar Cells: From Device Fabrication to Device Degradation-Timothy Kelly:

~have a bright and sunny day~
Gathered, written, edited, and posted by sunisthefuture-Susan Sun Nunamaker

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26 April

Multijunction Solar Cells May Exceed 50% Efficiency In The Near Future


Dear Friends, Visitors/Viewers/Readers,

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Just quickly sharing some of the things I’ve learned from a solar-cells course I’ve been taking recently at Stanford and from

In the past decade, the best lab examples of traditional silicon solar cells have efficiencies around 25%, while current world record of multi-junction/3-junction solar cells is 43.5% (based on former Professor/Dr. Jim Harris & his group  at Stanford and then Solar Junction ) . Commercial examples of tandem cells are widely available at 30% under one-sun illumination, and improve to around 40% under concentrated sunlight. However, this efficiency is gained at the cost of increased complexity and manufacturing price. To date, their higher price and lower price-to-performance ratio have limited their use to special roles,   such as in aerospace (where their high power-to-weight ratio is desirable).

Sun Seen From Space Station (Credit:STS-129 crew, NASA), available @ Sun Is The Future of

In earthly applications, these multijunction (MJ)  solar cells are used in concentrated photovoltaics (CPV) with operating plants all over the world.

Tandem techniques can also be used to improve the performance of existing cell designs, although there are strict limits in the choice of materials. In particular, the technique can be applied to thin-film solar cells using amorphous silicon to produce a cell with about 10% efficiency that is lightweight and flexible.

At this point, please allow me to share a graph developed by NREL (now available at, the Best Research-Cell Efficiencies, summarizing the results of various solar cell efficiencies from 1975-2013, below:

Best Research-Cell Efficiencies (by NREL, available at

Now, based on the collaboration of researchers from the California Institute of Technology in Pasadena; the National Institute of Standards and Technology in Gaithersburg, Maryland; the University of Maryland in College Park; and ,Boeing-Spectrolab Inc., in Sylmar, California. The team published a paper on their work in a recent issue of Applied Physics Letters.  These scientists have designed a new multijunction solar cell that can achieve an efficiency of 51.8%. This high performance exceeds the current goal of 50% efficiency in multijunction solar cell research as well as the current world record of 43.5% for a 3-junction solar cell. In multijunction solar cells each junction or subcell absorbs and converts sunlight from a specific region of the spectrum. The subcell can be stacked one on top of  another so that sunlight first strikes the highest bandgap subcell (tuned to light with shortest wavelengths or highest frequencies). The longer wavelengths pass through the first subcell and strike the lower band gap subcells. In order to approach its theoretical limit, multijunction solar cells not only need multiple subcells but also optimal semiconductor materials for the subcells to provide a combination of band gaps that cover as much of the solar spectrum as possible. To improve upon the current best 43.5% efficiency level of MJ solar cells, researchers focused on improving the current match between the different subcells, along with using a lattice-matched design. These two factors have previously limited MJ solar cell efficiency. Below is a video clip on new multijunction solar cell during a lecture given by Dr. Vijit Sabnis of Stanford University:


The lattice match corresponds to the matching between the crystal unit cells from the different subcells,” lead author Marina Leite, an energy researcher at the National Institute of Standards and Technology, told “By using subcells that are lattice-matched, we can minimize dislocations and other crystal defects that can significantly affect the performance of the device. A current match is required for two-terminal tandem configurations because in this case a single current passes through all the subcells and the voltages are added; therefore, if one subcell has less photocurrent it will limit the current generated by the entire device. The current match is desired so that each individual subcell works at its own maximum power point of operation.”

Solar energy world is definitely dynamic, with constant progress made beating the previous record ! We will look forward to the day when  a new Multijunction Solar Cell will be able to exceed 50% efficiency goal.

~have a bright and sunny day~

Gathered, written, and posted by sunisthefuture-Susan Sun Nunamaker

Any of your comments & suggestions are welcomed at


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