Page - 117 - in Photovoltaic Materials and Electronic Devices

contact interface (Figure 1c,d). Alternatively, a metal busbar can be used for this interconnectionpurpose (Figure1e,f) [20]. The isolationof thebackcontactcanbe filled with a dedicated insulator material [21]. This approach was mentioned to have more design freedom. In addition, the metal can function both as an interconnection and as a top contact enhancer (Figure 1g,h). The contact surface area between the frontandthebackcontact, as indicatedbythewhitedashedbox, isnotchangedby these different layouts. In the case of the metal interconnect, the fingers on the TCO will increase the total contact surface area between themetal and the TCO, which is an important feature,aswillbediscussed inSection2.4. A modest cell efficiency of 19% was chosen, as this has been reported for different thinfilmmaterialswithvariousbandgaps,whichresult indifferentopen circuitvoltages. Three I-Vcurveswerechosenwithanefficiencyof19%andopen circuit voltages (Voc) of 0.7, 0.9 and 1.1 V, as to represent typical values for thin film CIGS, CdTe and perovskite solar cells, respectively. The curves are shown in Figure2a. Moredetailsof the IVcurvescanbefoundintheExperimentalSection. 3   Figure  1.  Schematic  representations  (not  to  scale)  of  different  interconnection  and  cell  layouts  with  a  side  view  (a,c,e,g)  and  a  top  view  (b,d,f,h).  The  top  image  shows  the  front  contact  (in  green),  the  absorber  material  (in  blue)  and  the  back  contact  (in  grey).  In  addition,  the  separation  and  interconn ction  lay ut  between  two  adjac nt  cells  is  shown.  The  urface  area  of  the  TCO/back  contact  material  interface  is  indicated  by  the  white  dashed  box.  The  flow  of  current  is  depicted  by  the  arrows.  The  second  highest  image  shows  the  case  where  the  front  contact  is  supplemented  by  a  metal  grid  (in  orange),  whereas  the  right  image  displays  the  area  covered  by  the  metal  (not  to  scale).  The  third  image  shows  the  case  of  the  metal  interconnect,  for  which  two  material  interfaces  are  important:  the  metal  back  contact  area  represented  by  the  white  dashed  box  and  the  metal/TCO  contact  areas  represented  by  the  blue  dashed  box.    A  modest  cell  efficiency  of  19%  was  chosen,  as  this  has  been  reported  for  different  thin  film  materials  with  various  band  gaps,  which  result  in  different  open  circuit  voltages.  Three  I‐V  curves  were  chosen  with  an  efficiency  of  19%  and  open  circuit  voltages  (Voc)  of  0.7,  0.9  and  1.1  V,  as  to  represent  typical  values  for  thin  film  CIGS,  CdTe  and  perovskite  solar  cells,  respectively.  The  curves  are  shown  in  Figure  2a.  More  details  of  the  IV  curves  can  be  found  in  the  Experimental  Section.    Figure  2.  I‐V  characteristics  used  for  the  study  (a)  cells  of  19%  efficiency  with  different  open  circuit  voltages  (in  V,  see  legend);  and  (b)  cell  with  a  Voc  of  0.7  V  for  different  light  intensities  (see  legend)  in  which  one  sun  is  equivalent  to  1000  W/m2).  For  the  curve  with  a  Voc  of  0.7  V,  the  illumination  intensity  was  varied  and  its  effect  on  the  IV  curve  is  shown  in  Figure  2b.  As  the  light  induced  current  density  goes  down,  so  do  the  Voc  and  the  Figure 2. I-V characteristics used for the study (a) cells of 19% efficiency with different open circ it voltages (in V, see legend); and (b) cell with a Voc of 0.7 V for different light intensities(seelegend)inwhichonesunisequivalentto1000W/m2). For thecurvewithaVocof0.7V, the illumination intensitywasvariedandits effectontheIVcurveisshowninFigure2b. Asthelight inducedcurrentdensitygoes down,sodotheVocandthefill factor. Thesecurveswereusedinthemodelingto represent reference small cell without interconnection of front contact related losses. 2.2. CellswithaTCOFrontContact The typical trade-off between transmittance and sheet resistance of the TCO, as usedfor themodeling, is showninFigure3. Below10Ω/sq, the transmittancedrops with reduced sheet resistance. Figure 4a shows the efficiency as a function of the cell lengthfordifferentTCOsheet resistances. Thedetailsof theTCOcanbefound intheExperimentalSection. Thecell efficiencyshowsamaximumwithcell length. 117