Seite - 121 - in Photovoltaic Materials and Electronic Devices

various finger heights for two different finger widths of 60µm (Figure 7a,b) and 100µm(Figure7c,d). Acomparisonbetweenascribingwidthof150µm(Figure7a,c) and350µm(Figure7b,d)arealsodisplayed. 6 A  grid  finger  height  of  10  μm  could  be  hard  to  accomplish  for  printed  lines  and  the  data  also  indicate  the  impact  of  lower  finger  heights  on  the  cell  efficiency  and  the  optimal  cell  length.  On  the  other  hand,  the  conductivity  of  the  finger  material  used  for  this  calculation  is  only  1/5  of  the  bulk  conductivity  of  copper.  Hence,  finger  material  improvement  can  further  increase  the  efficiency  [25].  At  present,  a  finger  width  of  20  μm  is  not  compatible  with  large  area  printing  technology.    For  this  reason,  wider  fingers  were  also  used  for  the  calculations  to  assess  the  impact  of  finger  width.  Figure  7  shows  the  efficiency  for  cell  lengths  up  to  20  mm,  various  finger  heights  for  two  different  finger  widths  of  60  μm  (Figure  7a,b)  and  100  μm  (Figure  7c,d).  A  comparison  between  a  scribing  width  of  150  μm  (Figure  7a,c   nd  350  μm  (Figure  7b,d)  are  also  displayed.  Figure  7.  Efficiency  of  solar  panels  as  a  function  of  the  individual  cell  length  for  different  grid  finger  heights  (HF,  in  μm)  for  a  scribe  width  of  150  μm  (a,c)  and  350  μm  (b,d).  The  finger  width  is  60  μm  (a,b)  and  100  μm  (c,d).  The  data  are  based  on  a  Voc  of  0.7  V.  Using  a  wider  finger  width  than  20  μm  decreases  the  efficiency  benefit  over  the  TCO  only  case.  Nevertheless,  for  the  presently  available  scribing  width  of  350  μm,  the  impact  is  still  considerable  and  worth  the  additional  manufacturing  step.  However,  reducing  the  scribe  width  to  150  μm  reduces  the  benefit  of  metallic  grids.  Figure 7. Efficiency of solar panels as a function of the individual cell length for different grid finger heights (HF, inµm) for a scribe width of 150µm (a,c) and 350µm (b,d). The finger width is 60µm (a,b) and 100µm (c,d). The data are based onaVocof0.7V. Using a wider finger w th than 20µm decreases the efficiency benefit over the TCOonlycase. Nevertheless, forthepresentlyavailablescribingwidthof350µm,the impact is still considerableandworth theadditionalmanufacturingstep. However, reducingthescribewidthto150µmreduces thebenefitofmetallicgrids. 2.4. Effect ofContactResistance Oneofthetopicsinthinfilmsolarcellsistheeffectofcontactresistance,although it isseldommentioned[26,27]. TheMo/CIGSspecificcontactresistancewasreported to be in the order of 0.08Ohmcm2 [28]. However, the specific contact resistance between TCO and Mo was found to be three orders of magnitude lower, in the range of 10´5Ω cm2 [29]. From the specific contact resistance (RSCR), the contribution of the contact resistance to the overall resistance in the cell can be estimated. We calculatedthecontact resistance fora1 cm2 cell. This wasdonefordifferent widths of overlap between the TCO and the Mo, as shown in the TCO/Mo contact width in Figure8a. Fora1cm2 solarcell, typical total series resistancesarebetween1and2 ohm. For two specific contact resistances (Rscr), the contact resistance was calculated 121