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									42 power forecasts
   the solar power industry is facing great change
   with the advent of thin-film technology but, as
   Nadya Anscombe discovers, there are still
   several paths for the sector to choose from

  a flexible future
  By 2020, solar cells will be                  semiconductors, are compli-          LEDs (OLEDs). The materials          that gave the longest lifetime.
  everywhere – and you may not                  cated to manufacture, and can        and manufacturing technology         Once the manufacture of these
  even be able to see them. They                be cumbersome. Thin, flexible        for both OLEDs and thin-film         cells has been scaled up – and the
  may be transparent, integrated                solar cells, will be made using      photovoltaics is remarkably          cells have been both encapsulated
  into your office window. They                 either polymers, a photosensi-       similar.                             to protect them against the
  may form a thin film on the roof              tive dye or organic small-              But these solar cells also        elements and integrated into
  of your car. Solar cells may be               molecule technology and              suffer from the same problems        commercially viable modules –
  integrated into your briefcase or             promise cheap, portable solar        that OLEDs had at the start. The     their efficiency is reduced by
  rucksack, charging up your                    power (see ‘technology               biggest issues are lifetime and                                      ,
                                                                                                                          50 per cent or more. In reality the
  mobile devices while you are                  backgrounder’ p44). They will be     efficiency.                          best performance that can be
  outside and even while you are                much simpler to manufacture                                               expected from a commercially-
  in your office. They may even be              and also will not use toxic,         Solar cell efficiency                available module – as opposed to a
  on the top of your television,                expensive materials.                 Today’s most efficient solar cells   cell – is currently less than 4 per
  reusing the light from your                      After more than 20 years of       have an efficiency of around         cent and closer to 1 per cent.
  living room bulb, enabling you                research and development these       15-20 per cent during the hottest       However, while flexible solar
  to leave your TV on standby all               cells are today coming to market     part of the day. While dye-          cells may not be as efficient as
  night.                                        and by 2020 are predicted to be a    sensitised and organic cells         current technology, they do have
     These applications will be                 prevalent technology. Effectively    cannot yet match this efficiency,    many other advantages.
  made possible because the solar               the reverse of an LED –              great strides have been made in         “Our polymer cells are afford-
  cells of tomorrow are not to                  converting light into electricity    recent years. Just two years ago,    able, flexible and able to perform
  made using the same technology                rather than electricity into light   the best efficiencies reported       in low or variable lighting condi-
  as today. Today’s solar cells,                – thin-film photovoltaic cells       were around 10 per cent and          tions,” says Stuart Spitzer, vice
  which are made using silicon or               have benefited hugely from the       4 per cent respectively. Today,      president of engineering at US
  other rare – often toxic –                    immense research in organic          these figures are closer to 12 per   company, Konarka. “Silicon
                                                                                     cent and 8 per cent respectively,    cells may be very efficient, but
   By 2020 flexible                                                                  with the most recent                 their efficiency drops off as the
   solar cells are                                                                   announcement coming from             sun moves across the sky. The
   predicted to be                                                                   US company Solarmer, a               efficiency of polymer solar cells
   a prevalent
                                                                                     manufacturer of polymer cells.       is more constant throughout the
                                                                                     The company announced in             day and the cells can pick up the
                                                                                     December last year that it           sun’s energy from lower angles.”
                                                                                     had achieved an efficiency of           It is for this reason that
                                                                                     7.9 per cent.                        polymer cells can be used in
                                                                                        Lifetimes are also increasing,    vertical surfaces such as walls
                                                                                     with companies estimating            and even windows. Konarka
                                                                                     lifetimes of up to 15 years for      recently installed a solar curtain
                                                                                     their products. While these          wall at Florida company Arch
                                                                                     figures look promising, it must      Aluminium & Glass. When fully
                                                                                     be pointed out that these are for    commissioned, this curtain wall
                                                                                     small cells performing under         is expected to produce 1.5kW of
                                                                                     laboratory conditions, and the       power. “The purpose of this
                                                                                     cell that gives the highest          project is to test the perform-
                                                                                     efficiency will not be the one       ance and robustness of our solar

  engineering & Technology [[[strING1]]]

panel solution for a curtain wall     process runs at room temperature       Konarka is able to
application,” says Spitzer.           and does not need a clean room,        produce large areas
“Before we installed the panels,      unlike today’s silicon cell manufac-   of cells very quickly
the concrete vertical wall was        turing process. “Due to available
not generating any power at           equipment capacity in the printing
all, so even with our lower                    ,
                                      industry capital costs of polymer
efficiency – which is better than     photovoltaics production line are
no power at all – this wall will      between 5 per cent and 10 per cent
enable us to show developers          that of a silicon production line,”
and architects what our               says Spitzer.
technology can do and will also
enable us to test out our             MoBile applicationS
lifetimes and other performance       As well as building-integrated
data on a real application.”          applications, Spitzer sees many
   Konarka makes its cells using a    mobile applications for his
modified printing process, and is     company’s products, including
therefore able to produce large       applications in developing
areas of cells very quickly “We
                           .          countries where small amounts
can make cells up to 150cm-wide       of electricity are needed after
at 0.5m/s and we plan to run faster   dark to power lights, for
in the future,” says Spitzer. The     example. “A sheet of cells about       the size of an A4 piece of paper           been invested into dye-sensitised
                                                                             can charge two AA batteries in                          ,
                                                                                                                        technology we can exceed the
Konarka recently                                                             two hours,” he says. “This could           performance of dye-sensitised
installed a solar                                                            be used to power a light to enable         cells.”
curtain wall at US                                                           a child to do his or her school               That may be the case, but
company arch
aluminum & Glass                                                             work, for example.”                                   ,
                                                                                                                        currently dye-sensitised
                                                                                While Konarka has made great            technology is several strides
                                                                             strides in commercialising                 ahead of polymer technology –
                                                                             polymer solar cells, it started out        both in performance and
                                                                             developing dye-sensitised solar            commercial products. Konarka
                                                                             cells, a flexible technology that          sold its manufacturing
                                                                             works on a different principle to          technology to UK company G24
                                                                             polymer cells. “In 2004, we                Innovations in 2004, and at the
                                                                             decided to move to polymer                 end of last year, G24 announced
                                                                             technology for several reasons,”           the first ever commercial
                                                                             says Spitzer. “The main reason             shipment of dye-sensitised
                                                                             was the liquid electrolyte. It was a       photovoltaic modules to Hong
                                                                             challenging material to work               Kong-based consumer electronics
                                                                             with. We had a great efficiency  ,         bag manufacturer, Mascotte
                                                                             but I would speculate that once            Industrial Associates.
                                                                             companies such as Konarka put                 “We deliver 150 × 200mm
                                                                             in the same man years into                 modules to Mascotte and the
                                                                             polymer technology that have               company integrates them into E

                                                                                            [[[strING1]]] engineering & Technology
44 power forecasts
  ‘a sheet of cells
  about the size of
  an a4 piece of
  paper can charge
  two aa batteries
  in two hours’
  Stuart Spitzer, Konarka

   techNoloGy backGrouNder
   The electrons that make a flexible            not involved in charge transport. It    polymer cell
   solar cell work can come from three           just absorbs light and generates a
   different sources – a polymer, a dye          charge.
   or small-molecule organic
   semiconductors.                               polymer systems                                        Active layers
                                                 In polymer systems, the conjugated
   Dye-sensitised systems                        polymer acts as the electron donor
   The current leader is the dye-                with another material, such as a
   sensitised cell, which was invented           fullerene, acting as an electron
   by Michael Grätzel from the Swiss             acceptor. The beauty of the
   Federal Institute of Technology               polymer system is that the right
   (EPFL) in Lausanne and now bares              combination of polymer and
   his name – the Grätzel cell. The              acceptor can simply be blended
   cells have a simple structure that            together in organic solvents and
   consists of two electrodes and an             then printed or spin-coated onto a                                                                         Encapsulation
   iodide-containing electrolyte. One            substrate. While progress in the last
   electrode is dye-absorbed highly              few years has been staggering,               Printed substrate
   porous nanocrystalline titanium               there are still many challenges
   dioxide (nc-TiO2) deposited on a              including increasing the absorption
   transparent electrically conducting           range, the photon-electron              Materials                                        cells may catch up someday but a
   substrate. Both these electrodes              conversion rate and the carrier         One of the keys to efficient solar cells         substantial materials breakthrough
   are made using common, non-toxic              mobilities of the light-harvesting      is the electron mobility of the                  will be required. In that time, the
   materials and the cells have been             polymers.                               materials used to make the cells. The            dye-sensitised world will not sit still
                                                                                         electron mobility of polymers are                either and, in theory, dye-sensitised
   Dye-sensitised cell                                                                   lower that those of dye-sensitised               cells can reach higher efficiency than
                                                                                         cells, however, polymer materials                silicon cells.
                                                                                         have the advantage of being a solid                  It remains to be seen if these
         Glass                                                                           while dye-sensitised cells require a             efficiencies can ever be realised in
                                                                                         liquid electrolyte which is difficult to         a commercially viable photovoltaic
                                                                                         handle. The efficiency of polymer                module.

                                                                   Electrolyte           Small-molecule
   Graphite                                                                              organic cell

                                                                                                                 Cheap transparent foil
                                                                                                                  p          +
                                      Dye             Cathode               (current)
                                                                                                                      Heliatek absorber

                                                                                                                                                              Organic vacuum
   compared to photosynthesis                    Small-molecule systems
                                                                                                                                                              deposited layers
   because they use the redox                    The small-molecule system works                                  p          +
   reaction of the electrolyte. When             on a similar principle to the
   light falls onto the dye-sensitised           polymer design except that smaller
   solar cell it is absorbed by the dye.         molecules, called oligomers, act as                                                                          Total thickness:
   The electrons that are excited, due           the photon absorbers. In Heliatek’s                            Heliatek / BASF absorber                      0.2 μm
   to the extra energy the light                 design, in order to cover more of
   provides, can escape from the dye             the sun’s spectrum, two different
   and into the TiO2 and diffuse                 oligomer blends are used: one to
   through the TiO2 to the electrode.            absorb blue and green light, and                                                                             Only 0.5 grams
   They are eventually returned to the           another to absorb yellow and red                                                                             organic material
                                                                                                                                                              per m2
   dye through the electrolyte. This is          light. Unlike polymer technology,                                      Metal contact
   similar to photosynthesis because             this material system is layered
   the dye, like chlorophyll in plants, is       using vacuum deposition.

  engineering & Technology [[[strING1]]]
                                          Mascotte integrates
                                          150mm × 200mm
                                          modules into the bags

the G24i                                                               not competing with grid applica-        and start-up companies devel-
modules deliver                                                        tions, but rather that the              oping flexible solar cells have
0.6W of power                                                          technology can reduce the               sprung up all over the world.
                                                                       power draw on the grid. “8 per          German company Heliatek
                                                                       cent of the average UK                  recently received $27m in a
                                                                       electricity bill is due to standby      second round of financing
                                                                       power,” says Bedlow. “Our               from a group of investors
                                                                       technology cannot power a TV     ,      that include electronics giant
                                                                       but it can certainly power a            Bosch and chemicals company
                                                                       device in standby mode. We              BASF.
                                                                       cannot change consumer habits,             Heliatek’s technology is
                                                                       but if all consumer devices had         fundamentally different to
                                                                       dye-sensitised solar cell               dye-sensitised and polymer
                                                                       technology integrated into              technology in that it uses
                                                                       them, we can save the UK 8 per          vacuum deposition as a
                                                                       cent of its energy bills by             manufacturing process. “Just as
F the bags,” explains Roy Bedlow,   products matches those of the      powering the standby mode on            small-molecule technology has
chief marketing officer at G24i.    intended application. So for a     devices using energy from               won the race in the OLED
“The modules deliver 0.6W of        consumer product, this lifetime    indoor lighting during the              industry, we believe that small-
power, which is enough to           might be in the region of five     day. That’s a considerable              molecule organic solar cell
extend the talk-time of a mobile    years.                             saving.”                                technology will be the
phone, or recharge a mobile            Like most manufacturers of         Applications such as these           technology of choice in the
device in a day.”                   flexible solar cells, Bedlow       have captured the imagination           future,” says Andreas
  The lifetime of G24i’s            admits that G24i’s technology is   of the investment community             Rückemann, CEO of Heliatek. E

                                                                                   [[[strING1]]] engineering & Technology
46 power forecasts
  ‘after more than
  20 years of
  research and
  these cells are
  today coming
  to market’
  F “While printing solar cells is
  an intriguing technology, we
                                                the efficiencies reported for
                                                its cells.
                                                                                  flexIble solar: Is It worth It?
  believe these companies                          When it comes to talking       Flexible solar cells may be much         sceptical. He points out that even if
  will have problems in the                     about applications, Rückemann     cheaper to manufacture than              the photovoltaic element is installed
  future.”                                      is more cautious than other       silicon solar cells, but whether their   at zero extra cost, each module has
     Heliatek has achieved                      proponents of organic photo-      installation makes financial sense       to be electrically wired throughout
  efficiencies greater than 6 per               voltaic technology.               depends on many factors. Nigel           the building, connected to a DC/AC
  cent by employing a tandem                       “We are really pleased         Mason, from UK consultancy firm          inverter, attached to the grid supply
  technology – stacking cells on                that companies are getting        PV Consulting, believes today’s          and contain all the appropriate legis-
  top of each other in order to                 flexible solar cells into         flexible solar cell technology has       lated safety equipment. The system
  cover a large range of the solar              consumer applications, as this    not yet reached a level of efficiency    also needs to be maintained. “The
  spectrum. “We believe tandem                  raises awareness of the           to make them useful in large-scale       cost of this ‘free’ electricity can be
  technology is the way forward,                technology,” he told E&T          power applications. He told E&T          much more than that from a high-
  and this is very difficult to do              magazine. “But we are aiming      magazine: “In my view, low-              efficiency photovoltaic system with
  when printing cells without                   at the large-area applications    efficiency/low-lifetime PV products      25 year power warranty,” says
  dissolving the bottom layer,”                 and may even, by 2020, have       might find application in consumer       Mason. “Or put another way, this is
  says Rückemann.                               some grid applications,           products to trickle-charge battery-      all extra cost that has to be recov-
     Heliatek has reported 4.4 per              something we did not think        powered devices or applications          ered from the limited electricity that
  cent efficiency for a module –                possible when we started work     that can be expensive to connect         the device generates. Modules with
  the largest reported efficiency               on this technology. Whatever      to the grid such as parking meters       only 1 per cent efficiency will not
  for any organic technology.                   applications prevail, they must   and road signage.”                       generate sufficient electricity to
  The effective area efficiency                 be financially viable, and this       However, with regard to building     justify the cost of connecting to
  of that module is 5.3 per cent,               must be calculated very           walls or windows, he is more             the grid.”
  only 10 per cent different to                 carefully.” L

  engineering & Technology [[[strING1]]]

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