Seite - 120 - in Photovoltaic Materials and Electronic Devices

2.3. CellswithMetallicGrid For cells with a metallic grid on top of the TCO, it was found that a TCO of 50Ω/sq is preferable over a large range of finger widths [16]. Therefore, Figure 6 shows the efficiency as a function of the cell length for cells with a 50Ω/sq TCO suppliedwithametallicfingergridwithvariousfingerheights (HF). Wealsoshow thevalues forcellswitha10Ohm/sqTCOfrontcontact (black line). Forascribingwithof150µm(seeFigure6a), theefficiency increases fromjust below 17% to 17.8% when a high finger grid is used. For lower finger grid, the efficiency issomewhat lowerandthecell length isalsosmaller. Nevertheless, even for a finger height of 1µm, the increase in efficiency is 0.5 absolute %. This gain increases when a wider scribing area of 350µm is taken into account. This is logical, because a TCO only approach cannot accommodate as long cells as compared to TCO supplemented with a finger grid, which show optimal cell lengths that are about twice that of the TCO only configuration. Therefore, the scribe area forms a lower proportion of the total area for longer cells and scribe related losses are proportionallyreduced.Materials  2016,  9,  96  Figure  6.  Efficiency  of  solar  panels  as  function  of  the  individual  cell  length  for  TCO‐plus‐grid  front  contact  with  different  finger  heights  (HF,  in  μm)  for  a  scribe  width  of  150  μm  (a)  and  350  μm  (b).    The  cell  was  based  on  a  Voc  of  0.7  V  and  the  finger  width  is  20  μm.  The  TCO  in  the  legend  refers  to  calculations  with  a  cell  with  a  TCO  of  10  Ω/sq.  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)  and  350  μm  (Figure  7b,d)  are  also  displayed.  Figure 6. Efficiency of solar panels as function of the individual cell length for CO-plus-gridfrontcontactwithdifferentfingerheights (HF, i µm)forascribe width f150µm(a)and350µm(b). ThecellwasbasedonaVocof0.7Vandthe finger width is 20µm. The TCO in the legend refers to calculations with a cell with aTCOof10Ω/sq. A grid finger height of 10µm 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 thi calculation is only 1/5 of the bulk conductivity of copp r. Hence, finger material improvementcanfurther increase theefficiency[25]. Atpresent,afingerwidthof20µmisnotcompatiblewith largeareaprinting technology. Forthisreason,widerfingerswerealsousedforthecalculationstoassess the impactoffingerwidth. Figure7showstheefficiencyforcell lengthsupto20mm, 120