Seite - 178 - in Photovoltaic Materials and Electronic Devices

Er3+ ions tobehaveas opticallyactivecenters forup-conversionemissionswith the mutual contribution of green light in addition to the low red emission [15,19]. In addition, the conductivity of the REDC NPs is measured to be 77µS/cm, which is foundexperimentally tobe~22%higher thantheconductivityofnormalEDCNPs, 63µS/cm. Materials  2015,  8,  page–page  the  efficiency  of  Er3+  ions  to  behave  as  optically  active  centers  for  up‐conversion  emissions  with  the  mutual  contribution  of  green  light  in  addition  to  the  low  red  emission  [15,19].  In  addition,    the  conductivity  of  the  REDC  NPs  is  measured  to  be  77  μS/cm,  which  is  found  experimentally  to  be  ~22%  higher  than  the  conductivity  of  normal  EDC  NPs,  63  μS/cm.    Figure  3.  Emission  spectrum  of  REDC  NPs  under  simultaneous  excitations  of  both  near‐UV    (430  nm)  and  IR  (780  nm)  excitations.  The  surface  profile  of  the  coated  cell  is  shown  in  Figure  4,  with  focus  on  the  region  between  the  electrode  and  the  coated  edge.  It  could  be  observed  that  the  mean  thickness  of  the  coating  is  around  20  nm  with  quite  a  non‐uniform  distribution  of  the  coating,  as  shown  in  Figure  4b  regarding  the  intensity  imaging,  which  may  be  due  to  the  spin  coating  technique  itself.  This  coating  technique  could  be  considered  as  a  trade‐off  between  surface  uniformity  and  simplicity.  However,  other  coating  techniques  may  lead  us  to  miss  the  conductivity  of  the  nanostructures  due  to  missing  oxygen  vacancies  with  the  conversion  of  Ce3+  to  Ce+4.    (a) Figure3. EmissionspectrumofREDCNPsundersimultaneousexcitationsof near-UV(430nm)andIR(780nm)excitations. The surface profile of the coated cell is shown in Figure 4, with focus on the region between the electrode and the coated edge. It could be observed that the meanthicknessofthecoatingisaround20nmwithquiteanon-uniformdistribution ofthecoating,asshowninFigure4bregardingtheintensityimaging,whichmaybe dueto thespincoatingtechnique itself. Thiscoatingtechniquecouldbeconsidered as a trade-off between surface uniformity and simplicity. However, other coating techniques may lead us to miss the conductivity of the nanostructures due to missingoxygenvacancieswith theconversionofCe3+ toCe+4. 2.2. CoatedSolarCellCharacterization Asinvestigatedintheprevioussections, coatingthebacksideofasiliconsolar cell with REDC NPs has the advantages of improving multi-optical conversions, leading to the conversion of some UV and IR wavelengths that solar cells cannot absorb to visible light wavelengths which can be absorbed. Figure 5a,b show the improvement in P–V and I–V curves, respectively, after coating the cell with REDC NPs. The promising comparison between coated and uncoated cells was shown in Table 1, and it clearly shows that power conversion efficiency (PEC) has been improved from 15.1% to 16.7%, which is about a 10.8% improvement of cell efficiency due to coating compared to uncoated cells. As can be noticed from 178