![\"Perovskite](\"http:\/\/cafe.foundation\/blog\/wp-content\/uploads\/2015\/11\/perovskite-NREL-Graph.jpg\")
<\/a>Perovskite efficiency has grown dramatically, now almost rivaling silicon solar cells<\/p><\/div>\n
Part of the attraction of perovskites cells has been their rapid increase in efficiency, nearly equaling the best silicon cells in about six years, as opposed to the 35 years needed for silicon cells to increase from 16 percent efficiency to a little over 25.<\/p>\n
Some factors get in the way, one being that perovskites react badly with moisture.\u00a0 Thus, their life spans are much shorter than the more expensive silicon alternatives.\u00a0 This raises the question of life cycle versus price \u2013 will the need to replace the low-cost solar panels that fade away in a few years be more expensive over time than another installation that lasts ten to 30 years?<\/p>\n
Another issue \u2013 that of viable cell size, has been an impediment to commercial adoption.\u00a0 In the laboratory, increasing output from solar cells is done regardless of aperture size, but if that aperture size is limited to bite-size chunks around 0.1 square centimeters (0.10155 square inches), production costs would overwhelm their commercial feasibility.\u00a0\u00a0 Apparently, scaling these cells to larger platforms causes them to lose stability and therefore reduces their lifespans.<\/p>\n
The National Renewable Energy Laboratory (NREL) <\/a>performs certification and characterization testing for all solar cells in this country, and is \u201cone of only two laboratories in the world to hold an International Organization for Standardization (ISO) 17025 accreditation for primary reference cell and secondary module calibration, in addition to accreditation for secondary reference cell calibration under American Society for Testing Materials (ASTM), and International Electrotechnical Commission (IEC) standards.\u201d\u00a0 Using standard size test cells would help enable a \u201clevel playing field\u201d for all testing.<\/p>\n Other standardized elements come into play.\u00a0 Matthew Gunther, writing in the Royal Chemistry Society\u2019s Chemistry World<\/a><\/em>, explains,\u201dIt is standard practice to test a cell by performing a current\u2013voltage measurement on it. The cell is irradiated with 1000 [Watts per square meter] of light at a temperature of 25\u00b0C, mimicking the conditions of the midday sun, and its power output is measured.\u201d<\/p>\n But, in the March 2, 2015 article, Gunther quotes Keith Emery, the manager of the NREL\u2019s photovoltaic cell and module performance characterization group taking a skeptical view.\u00a0 \u201cAll the exciting efficiencies and any energy claims that are associated with [perovskite solar cells] should be taken with a grain of salt,\u201d\u00a0 He felt the claims should be independently verified and held to a robust standard.\u201d\u00a0 At least two teams of researchers have stepped forward with proof of their own.<\/p>\n Two groups of researchers have been enlarging the size of perovskite cells and increasing efficiency. \u00a0Image: Brown University<\/p><\/div>\n One, a team of Swiss, Chinese and Japanese researchers <\/a>has found a way to scale up perovskite solar cells without a loss of stability, according to their paper in the journal Science<\/em>.\u00a0 Their results were verified by representatives from the National Institute of Advanced Industrial Science and Technology.<\/p>\n Their paper in the journal Science<\/em><\/a>, describes their approach.\u00a0 \u201cInstead of changing the perovskite, the researchers added a new element to a device that uses it, a thin film light collector that is placed over the cells. The film was constructed in two layers, one a positively charged cubic rocksalt semiconductor, the other a sheet of negatively charged titanium oxide. Arriving light causes the perovskite layer to be excited which results in liberated electrons moving through the\u00a0titanium oxide<\/a>\u00a0layer on one side of the film while holes are transported through the other. The result is a protective film covering that actually increases conductivity of the device.\u201d<\/p>\n The film allowed scaling cells to sizes allowing accreditation, boosted efficiency to over 15 percent, and protects the cells from humidity, a solar-powered hat trick.<\/p>\n Another team from Brown University, working with researchers at NREL,<\/a> increased cell sizes to larger than one square centimeter.\u00a0 Their findings were similar to those of the tri-national group\u2019s.<\/p>\n “The use of tiny cells for efficiency testing has prompted some to question comparison of perovskite solar cells with other established photovoltaic technologies,” said Nitin Padture, professor of engineering at Brown, director of Brown’s Institute for Molecular and Nanoscale Innovation, and one of the senior authors of the new research. “But here we have shown that it is feasible to obtain 15-percent efficiency on cells larger than a square centimeter through improved processing. This is real progress.”<\/p>\n Their process, though different, yielded similar outcomes.\u00a0 \u201cPerovskite precursors are dissolved in a solvent and coated onto a substrate. Then the substrate is bathed in a second solvent (called anti-solvent) that selectively grabs the precursor-solvent and whisks it away. What’s left is an ultra-smooth film of perovskite crystals.\u201d<\/p>\n The team developed a \u201ctrick\u201d to grow the perovskite crystals to a larger size. \u201cThe trick is to add excess organic precursor that initially “glues” the small perovskite crystals and helps them merge into larger ones during a heat-treatment, which then bakes away the excess precursor.\u201d\u00a0 This led to higher efficiency and fewer defects.\u00a0 Padture thinks he can make even larger cells with efficiencies of 20 to 25 percent.<\/p>\n Padture and colleagues at the University of Nebraska-Lincoln were recently awarded a $4-million grant by the National Science Foundation to expand their perovskite research.<\/p>\n Brown University\u2019s paper is published in Advanced Materials<\/em>.<\/a><\/p>\n<\/a>