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Posted: July 24, 2008

Rosier future for solar energy conversion

(Nanowerk News) Recent progress in dye-sensitized solar cells (DSCs) research and development, which use innovative light-harvesting dye (the "sensitizer") to improve the optical absorption coefficient of the stained nanostructured electrodes, might color our dimming future energy security with a tint of rose, despite the looming depletion of fossil fuels. Costing only 10-20% of their silicon counterparts, the new devices might make it affordable for much more people to utilize solar energy, a handy renewable energy source.
A research group at the Chinese Academy of Sciences (CAS) Changchun Institute of Applied Chemistry (CIAC) headed by Prof. WANG Peng, in cooperation with a lab at the Swiss Federal Institute of Technology (EPFL) led by Prof. Michael Grätzel, reports in the Journal of American Chemical Society (JACS) two new dyes on July 22, which hit strikingly high energy conversion efficiencies and meanwhile demonstrate good stability under harsh thermal and light-soaking dual stress. This work successfully solves the dilemma between efficiency and stability that has bothered the scientists for a long time and therefore will remove a big hurdle to the commercialized application of DSCs, as anticipated by the researchers.
A flexible dye-sensitized solar cell
A flexible dye-sensitized solar cell with a solvent-free ionic liquid electrolyte. (Image: G24i Ltd.)
A preliminary test shows that one of the new dyes, coded C101, demonstrates a conversion efficiency up to 11.0~11.3%, when working along with an acetonitrile-based electrolyte and measured under the air mass 1.5 global (AM 1.5G) illumination. This performance keeps abreast with the record-holder sensitizers, the N719 and N749 invented by Grätzel's lab. Moreover, the C101 proves to remain stable and retain over 95% of its initial efficiency after soaking in intensive full sunlight for 1,000 hours at 60°C. In contrast, the N719 and N749 both fail to stand long-term thermal and light-soaking stress. So far, among the only three dyes in the world that hit the benchmark of 11% efficiency, the C101 is the most robust in terms of thermal and photochemical stability.
Actually, this productive team already initially solved the efficiency-stability dilemma about four months ago. On 4 April, the team reported in the Chemical Communications a novel heteroleptic ruthenium sensitizer that showed an overall conversion efficiency of 10.53% and kept stable under prolonged thermal and light-soaking stress if low-volatility electrolytes were used. More encouragingly, this dye could demonstrate an overall power conversion efficiencies over 10.7% if exposed to various lower light intensities.
Wang's team has also solved another dilemma by introducing a novel concept of eutectic melts in electrolyte development. High-performance DSCs often use volatile solvents as electrolytes. This makes it very impractical for outdoor application, because the solvents tend to evaporate very soon in the sunlight due to the heating, and might leak or permeate through cell encapsulations. This would counteract its low cost and relatively high efficiency because its potential commercial production would involve costly sealing. Later efforts to reach better thermal stability by replacing the solvents with ionic liquids, namely salts that melt at a room temperature, however, was frustrated by the low efficiency and poor optical stability of the new devices.
In this context, Wang and his colleagues creatively introduced the concept of using eutectic melts to produce solvent-free liquid redox electrolytes. Eutectic is a magical phenomenon in which compounds with different and relatively high melting points, melt together at a much lower temperature when mixed at certain molar ratios.
In their work published in Nature Material on June 29, the group mixed three solid imidazolium iodides, which are non-conductive solids at room temperature, in certain molar ratios. As a result they got a ternary melt with a melting point below 0ºC and strikingly high conductivity at ambient temperature. The ensuing experiments showed that DSCs based on a ternary melt reached excellent stability and an efficiency of 8.2% under AM 1.5G, which set a benchmark for solvent-free DSCs.
The team is still optimizing the cell parameters to explore the full potential of the new sensitizers. When asked about the potential maximum efficiency of his new inventions, like the heteroleptic ruthenium sensitizer reported in his ChemComm paper, Wang frankly remarks that what interests him the most is not the efficiency, though he gives a rough estimate, 13% under AM 1.5G. Compared with efficiency records achieved in experiments with toxic and volatile electrolytes containing acetonitrile, he emphasizes more on stability and production costs, which have more important practical implications.
Indeed, his invention of solvent-free ionic liquid electrolytes based on eutectic melts and high-performance metal-free sensitizers both target the practical needs in industrialized production. Earlier on 27 June, his team synthesized and reported in the JACS an organic sensitizer coded C203 with excellent stability. This marked a milestone because instability had been a major defect for most previously reported organic dyes. More encouragingly, an experimental DSC with 7% conversion rate was successfully formed by combining the C203 with the newly developed eutectic-based electrolyte, and tests showed that the output photocurrent was comparable to that of the DSCs based on volatile electrolytes.
The DSC is seen as a promising alternative to silicon solar cells because of its pretty high performance/cost ratio and better sensitivity to weak lights compared to the latter. And the discoveries published in the past few months bring it even closer to real application. "The conceptual DSC patent is out of protection since last April," comments Wang, "I am very confident that the DSC will enter the market very soon and may take over the market share currently held by silicon cells in time."
In the meantime, sources say that the G24 Innovations in UK, a large-scale manufacturer of solar cells, has been applying DSC technologies, and more excitingly, it has just got a big order for these flexible devices. This company is reported to be also interested in Wang's new inventions.
Despite the fact that currently DSCs present efficiencies only less than a half of what the best silicon cells, which is 24.7%, Wang is optimistic about the future market of DSCs. "It should be realized that, with this relatively low efficiencies, DSCs have already provided a fairly high practical output of electricity power, about 70~80% of silicon cells, according to an outdoor testing report by Aisin Seiki and Toyota Motor," Wang emphasizes. "This is because of their better sensitivity to weak lights," he explains: "and they can deal with diffusive sunlight very efficiently, due to their nanostructure. The best DSC under the diffusive sunlight has reached an overall efficiency of 14% in outdoor tests." In addition, under low light intensities, for example, under room light, a DSC exhibits much higher conversion efficiency than silicon cells, according to Wang.
Based on the above reasons, it is estimated that stable, low-cost, flexible and lightweight DSCs are attractive even if their solar conversion efficiencies are moderate, like between 5-9%.
So far, Wang and his co-workers have employed their new dyes in conjugation with their best electrolytes and state-of-the-art titania films to fabricate DSCs with enhanced comprehensive properties and performance. Their new ideas yield fantastic devices of long-term stability that hit a record efficiency of 10%. Another result is the first solvent-free DSC in the world that reaches a conversion rate of 9.1%. These inventions have got authorized patents and will be published very soon. "I believe," added Wang, "Overall, the DSC is the most promising technique which can compete with the sate-of-the-art photovoltaic cells in the future."
Source: Chinese Academy of Sciences