/images/rss.gif)
Dye-sensitized solar cells
The market for photovoltaic solar cells can be divided broadly into two categories with the largest share for large-area installations intended for power generation. Electric power can also be generated and used locally (off-grid sources to supply houses or run irrigation pumps in remote areas), or it can be used to feed to a national power grid. A smaller but growing share is based on small-area solar cells (sub-modules, typically < 100 cm2) for use in portable electronics such as laptop computers, solar bathroom balances, mobile-phone chargers, solar garden lamps and similar uses. Development work has been taking place in both areas.
An important point to note in photovoltaic cell studies is the size of the solar cells being examined. Nearly all fundamental and optimization studies in laboratories use small-area cells with illumination area of ≤ 1 cm2. The first stage of scaling up is to increase the active area by orders of magnitude in modules. Here the illumination area can be anywhere in the range of 25-100 cm2. Much larger-area installations for power generation use solar panels (1 m2 or larger). Panels are formed by combining several modules. Solar cells in general are sold in the form of sub-modules, modules and panels. The term solar array refers to using several large area solar panels in large-area field installations.
In this chapter, we highlight various product development studies on DSCs in different industrial laboratories of Asia and Europe. This chapter is complementary to the earlier chapter on packaging, scaling and commercialization of DSCs by Desilvestro & al.
Scaling up of laboratory cells to modules and panels
Development of modules is an important step in the advancement of any solar-cell device destined for practical applications. For all photovoltaic devices, the highest solar conversion efficiencies are obtained for small-area lab-test solar cells. When the illumination (charge generation and collection) area is made larger, invariably there is decrease (by a few percent) in the overall conversion efficiency per square centimeter. This is due to loss of some of the photogenerated charge carriers via recombination and/or trap processes. Losses in efficiency from lab-size cells to modules (several percent) have been noted in nearly all semiconductor-based PV systems (single crystal-Si, amorphous Si, CdTe, CIGS). Reducing substantially this scale-up loss is essential. For DSCs, some of the attempts in this regard involve optimization of the materials preparation; of the mode of deposition of the active oxide layers; of the counter-electrode design, and of the ways of inter-connecting small-area cells to modules. Efficiency of the solar cell has to be above a certain threshold before they are scaled up for pilot-level testing by major players of the photovoltaic (PV) industry. Based on cost-estimates, solar conversion efficiency of 10-12 % is considered the absolute minimum in order for DSCs to be competitive with alternate PV technologies.
Solar cells for portable electronics work invariably at ambient temperature and at a light flux less than the full solar level. Requirements for thermal stability is reduced for this type of application. Large-area solar panels employed out of doors (terrestrial or outer-space) have to sustain more stringent operating weather conditions. In tropical or high-altitude installations, panel temperatures during the year may from −10-60 °C with the solar cells exposed to high humidity (> 75 %). To be cost effective, they have to work reliably for long periods (product lifetime 15 years or longer). Depending on the location, the solar panel may be exposed to full solar radiation, or it may have to function under cloudy sky conditions for most of the year.
Constant improvements in the design and performance of portable electronic devices, thanks to technological advances, have resulted in shorter periods for the guaranteed cost-effective performance for portable electronics (10 years or even less). For dyesensitized solar cells, a product performance guarantee corresponding to a 20-year lifetime requires several million turnover cycles of the key components.
The most important parameter for cost reduction is the overall sunlight-to-electrical conversion efficiency. During the past decade, there has been growing sensitivity in the general public of industrialized economies about energy consumption. Increasingly, photovoltaic cells are integrated into building architecture, an area known as building-integrated photovoltaics (BIPV). As for large-area installations designed for power generation, higher efficiencies will reduce the area of the solar panel required. For certification on reliable performance, solar-cell modules have to be tested according to certain specifications. Some of the standards that apply to the testing of thin-film solar panels and photovoltaic modules used in DSC development include: IEC 61215 (terrestrial crystalline silicon photovoltaic (PV) modules, type suitability and type approval); IEC 61646 (terrestrial thin-film photovoltaic PV modules, type suitability and type approval, JIS C-8938 standard Japanese counterpart) and ASTM E 1171 (standard test method for photovoltaic modules in cyclic temperature and humidity environments).
The following thermal stability tests are part of these standards:
● thermal-cycling test: cycling between 85 °C and −40 °C at 100 °C/hour maximum (to assess the module’s ability to withstand exposure to several environmental conditions during transportation and/or storage);
● temperature-humidity cycling test: cycling between +85 ± 2 °C and −40 ± 3 °C at 85 ± 5 % RH, at 100 °C/hour maximum (to determine the deterioration level for use and/or storage in short time under conditions of temperature change in high relative humidity);
● light soaking: exposure of the solar cell to continuous illumination of solar radiation for 1000 hours.
Toyota/Aisin Seiki, Sharp, Shimane Institute of Technology and Fujikura are some of the DSC module developers who have already optimized their cell design and packaging to pass performance tests involving international and national standards, such as IEC 61646 “thin-film terrestrial photovoltaic (PV) modules” or Japan Industrial Standard C-8938. This underscores how DSC technology is rapidly evolving toward full commercial viability.
Since the seminal report of O’Regan and Grätzel in 1991 in the journal Nature, there have been numerous exploratory studies using a wide variety of dyes, redox mediators, electrolytes, oxide substrates, counter-electrodes and even component assembly modes. These studies have led to a systematic increase in the overall light conversion efficiency, currently around 12 % for small area lab-cells and 8 % for modules. The stability of dye-sensitized solar cells over extended photolysis periods has improved sufficiently to pass the international standards for thin-film PV modules. Encouraged by these developments, several laboratories have undertaken development of larger-area modules for practical applications, mainly over the past decade. Over one hundred industrial laboratories worldwide, big and small, are now engaged in the development of DSC-based photovoltaic power generation systems. Substantial amounts of money are being invested in new start-ups by venture investment agencies. In this chapter, we review the state of the art in some of the leading industrial laboratories, most of them located in Europe and Asia. The review intends to illustrate the challenges faced and the novel approaches taken by different industries to address them.
Module design considerations
It was mentioned earlier that fundamental studies to identify key elements of solar cell performance are done in small-area cells with a surface less than one square centimeter. Key parameters for optimization are incident monochromatic power conversion efficiency (IPCE), maximum photocurrent (Isc) and photovoltage (Voc) at optimum power point and the fill factor (ff). Overall solar-to-electrical conversion efficiency is based on these fundamental parameters. Solar cells size and conductivity of the substrates influence significantly the internal resistance of solar cells and consequently the fill factor and the conversion efficiency of the DSCs. Even for cases where the solar conversion efficiency is fairly high (> 8 %), simple scaling of area to modules of 10 × 10 cm deposited on TCO glass without any collector electrode yield power conversion of 1-2 % only. Printing silver finger as internal current collector electrode (reducing the charge carrier collection area to cm2 or less) and the manner in which the individual cell elements are connected thus are very important. For DSCs, three different types of connecting small area cells have been studied: (i) the “series-Z” design as used in the early studies of modules by Sustainable Technologies International STI/Dyesol of Australia; (ii) the parallel or “masterplate” design as used by the Dutch Energy research laboratory ECN; and (iii) the monolithic design as proposed initially by Andreas Kay of EPFL and developed by Aisin Seiki and others particularly in Japan. Depending on the module design and inter-connect of constituent cells, effective area for power generation (with respect to outer geometric area of the module) can be anywhere between 70-90 %.
Modules with series interconnections has provided the best route to industrial production. The Z-series inter-connect design (introduced by STI), can be used for glass/metal, plastic/metal, glass/plastic and plastic/plastic substrates, and is comprised of two opposing electrodes with the connection between cells consisting of a conducting medium. The advantage of this design is its high-voltage output with relatively small interconnect-resistance losses, and its facility for preand post-treatment of the working electrode. The disadvantage is the risk of a lower fill factor, which results from the series resistance of the interconnect electrode. STI selected this design after the invention of an interconnect design with low resistance. The working electrode and counter-electrode can be optimized separately, and there is no requirement to mask the counter-electrode or pre-seal the module when applying the dye. Consequently, the dye uptake can be more carefully controlled on the basis of manufacturing cost analysis and reproducibility. Research work at ECN led by Jan Kroon developed the ‘masterplate’ concept, and this has been used in several European projects. The monolithic module (or Kay cell named after the inventor Dr. Andreas Kay of EPFL) is now becoming the major design model for volume production and for relatively small cells.
DSC development studies in european laboratories
A large number of industrial laboratories in Europe have been studying both the fundamentals and scaling up of DSCs for nearly two decades. Some of the leading laboratories deserve mention: the Institute of Applied Photovoltaics (INAP, Gelsenkirchen, Germany); the Energy Research Center of Netherlands (ECN, Petten, Netherlands); IMRA-Europe (Sophia Antipolis, France); Solaronix (Aubonne, Switzerland); Fraunhofer Institute for Solar Energy (ISE, Stuttgart, Germany); SonyEurope Research Center (Stuttgart, Germany); G24Innovation (Cardiff, Wales, UK); Greatcell S.A. (now owned by DYESOL/STI of Australia); and Solterra Fotovoltaico S.A. (Chiasso, Switzerland and Dyesol Italia). Solterra was an industrial partner of the laboratory of Professor M. Grätzel until 2001, when they shifted their plans to focus on production of Si-solar cells. IMRA is the major research partner of Toyota and its research wing Aisin Seiki. Developments from Aisin Seiki are discussed later on in this chapter, along with the work of Sony.
Energy Research Centre of the Netherlands (ECN)
The Energy Research Centre of the Netherlands (ECN) was one of the first European Laboratories to take development work of DSC technology to sub-modules, with work beginning in 1995. They have developed the so-called masterplate design for DSC modules. In their collaborative efforts with several European partners, they have prepared DSCs in various colors.
ECN Researchers reported results in 2003 on their semi-automated system for reproducible manufacturing of DSCs on sizes up to 100 cm2. Manufacture of two types of glass-glass DSCs were examined: small-area cells (< 5 cm2) with a conversion efficiency of 5.9 %. These cells had an active area/ total area ratio of 0.68, translating into a device efficiency of 4.3 %. Batch production of 27 cells per day was successful (26 of 27 cells with satisfactory yield and 22 out of 27 with good reproducibility). Average I-V parameters for the 26 cells (with active area of 68 cm2) was found to be: Voc = 0.68V, Isc = 10 mA/cm2, ff = 0.62, and the efficiency was measured to be 4.3 % (active area).
During the two-year period 2002 to 2004, a consortium of four European universities (EPFL, Imperial College, Cracow University, Materials Research Center of Freiburg), three research institutes (ECN Solar Energy of Netherlands, Fraunhofer ISE of Germany, IVF Industrial Research and Development Corporation) and one industrial partner (Greatcell Solar S.A.) cooperated under a European project called NANOMAX. The goal of the NANOMAX was to test new strategies for DSC cell design, cell materials and fabrication protocols with the aim to increase the efficiency to above 12 % under standard test conditions (AM1.5, 1000 W/m2) with good longterm stability. In addition, cost analyses were made to demonstrate the potential of DSCs as a low-cost thin-film PV technology.
The combined research efforts have led to the following technical achievements:
● New ruthenium-containing sensitizing dyes with enhanced optical absorption in the visible part of the spectrum have become available and have been successfully applied in DSCs;
● Protocols for making metal-oxide blocking layers on TiO2 result in the retardation of recombination dynamics and improvement of the photovoltage of the device. New concepts, such as the TCO-less design, have been developed and introduced. New scatter phenomena in TiO2 films have been discovered.
● A maximum power conversion efficiency under full sunlight of 11 % for areas of approximately 1 cm2 has been achieved.
● A cell with efficiency exceeding 8 % and that retains over 98 % of its initial performance after 1000 hours of accelerated testing, and under thermal stress at 80 °C in the dark, has been demonstrated. Negligible device degradation was observed for 1000-h visible-light soaking at 60 °C. Long term stability at elevated temperature has been achieved using hydrophobic Ru-dyes with pendant alkyl chains, such as Z90 and K19.
● Advanced techniques have been developed for in situ characterization of dyesensitized photovoltaic devices under operation, including transient optical studies covering all key steps of interfacial charge separation and recombination dynamics. Also, transient photovoltage and photocurrent studies of transport dynamics have been conducted. These experiments have been correlated with I-V data in the dark and under illumination, as well as with a numerical model of these I-V data based upon the non-ideal diode equation. These methodologies have been employed to characterize fundamental loss factors in both standard devices and in a range of innovative device concepts as developed in this proposal. For standard devices the primary loss mechanism is confirmed to be charge recombination of electrons in the metal oxide to the oxidized redox couple.
● Modules of different designs (Z-type, current-collecting and monolithic) and sizes (from 100 up to 900 cm2) have been demonstrated in the existing processing baselines with maximum active area efficiencies of 5.5 % under conditions of 1 sun and 6.5 % at 0.1 sun.
A European consortium financed under the Joule program (LOTS-DSC, JOR3CT98-0261) has confirmed the cell-photocurrent stability during 10,000 hours of light soaking at 2.5 suns, corresponding to an approximate 56 million turnovers of the dye without any significant degradation. A more difficult task has been to reach stability under prolonged stress at higher temperatures, i.e., from 80-85 °C. Accelerated longterm tests for a number of single cells containing different dye and electrolyte combinations have been performed at different temperatures up to 85 °C in the dark for periods up to 1000 hours, as well as under simulator sunlight. The ageing experiments that were conducted to determine the stability of the DSCs on an intrinsic, molecular level. With the solar conversion efficiency of liquid-electrolyte dye cells reaching an efficiency of 10 % and above in the laboratory (surfaces of about 1 cm2), ECN has set its project goal to develop a solid-state dye-sensitized solar cell with an efficiency of 10 % and a stability of 10 years.
Extracted from Dye-Sensitized Solar Cells Edited by K. Kalyanasundaram with forewords by Michael Grätzel and Shozo Yanagida Published by EPFL Press
Si vous avez apprécié cet article, s'il vous plaît, prenez le temps de laisser un commentaire ou de souscrire au flux afin de recevoir les futurs articles directement dans votre lecteur de flux.
Commentaires
I have not checked out your site since summer and I am astonished thatI noticed a lot of new things. The content is great and the videos and pictures just wonderful! I can not take my eyes from your blog… Keep it up!
homemadesolarheating…
You write important, the topics are cool. I like this web site. Depending on how long have you been blogging? How much time do you dedicate to it? Hopefully that I can make use of several of your texts on my web site….
/images/thumb1.gif)
/images/thumb2.gif)
/images/thumb3.gif)
There are a number of reasons to choose ultrahigh efficiency photovoltaic cells (like DSC) over other solar cells.