dev-phys-steve_hegedus

Document Sample
dev-phys-steve_hegedus Powered By Docstoc
					       Bifacial CdS/CdTe/ZnTe device characterization


                     Darshini Desai, Steven Hegedus
                            Institute of Energy Conversion
                                      University of Delaware
                                           Newark DE, USA


• Development of the transparent contact device and characterization
  techniques driven by need for better quantitative metrics and
  understanding of CdS/CdTe device operation




           Institute of Energy Conversion
           University of Delaware
           CdTe Team meeting NREL May 5-6, 2005                        1
                                       Motivation and Goals

• Motivation
  A transparent back contact made with controlled Cu doped ZnTe can
  be used as
  - A source of controlled Cu doping
  - A robust back contact
  - Transparent interconnect for tandem cell applications
  - A novel device characterization tool

• Goal
  Development of ZnTe:Cu based transparent, ohmic and stable contact
  for CdTe/CdS solar cells to enable bifacial device characterization and
  provide alternative source of Cu doping.




           Institute of Energy Conversion
           University of Delaware
           CdTe Team meeting NREL May 5-6, 2005                             2
Transparent (Semi) ZnTe:Cu Film Deposition

                                       •   Films grown by galvanic deposition
                                           with Zn as anode and substrate as
                                           cathode
                                       •   Source electrolytes ZnSO4, TeO2,
                                           CuSO4
                                       •   Deposition time 1.5 mins, pH=3 and
                                           T=68°C
                                       •    Triethanolamine(TEA) used to
                                           regulate amount of Cu in the solution.
                                           More TEA results in less free Cu and
                                           hence higher film transparency.
                                       •   Selected 20 drops[1E-8M] for device
                                           fabrication




Institute of Energy Conversion
University of Delaware
CdTe Team meeting NREL May 5-6, 2005                                                3
                                    JV results summary



Contact (Sample#)                       Light        Voc      Jsc       FF     Eff
                                      direction      (V)    (mA/cm2)   (%)    (%)
 ZnTe(VT149.4)                               front   0.82     20.3     68.0   11.3
 ZnTe(VT149.4)                               back    0.66      0.6     66.3   0.2
 Cu/Ni (VT149.1)                             front   0.82     20.8     68.7   11.8
 ZnTe (VT154.4)                              front   0.75     20.8     55.4   8.7
 ZnTe (VT154.4)                              back    0.64      0.9     68.2   0.4
 Cu/Ni (VT154.1)                             front   0.80     21.4     60.1   10.4




      Institute of Energy Conversion
      University of Delaware
      CdTe Team meeting NREL May 5-6, 2005                                           4
                                      JV data with Bifacial Illumination


              40                                                                    40
                      VT149.4-1                                                             VT154.4-2
              30                                                                    30
              20             Light, front                                           20             Light, front
                             Light, back
J (mA/cm 2)




                                                                                                   Light, back




                                                                      J (mA/cm 2)
              10                                                                    10
                             Dark                                                                  dark
               0                                                                     0

              -10                                                                   -10

              -20                                                                   -20

              -30                                                                   -30
                    -0.4 -0.2 0    0.2 0.4 0.6 0.8        1     1.2                       -0.4 -0.2 0   0.2 0.4 0.6 0.8   1   1.2
                                   V(Volts)                                                             V(Volts)


• VT 154.4.2 back-wall JV has no blocking barrier, unlike the dark and front-wall
  JV : Suggests photoconductive back contact
• Both devices: Higher fill-factor for back-wall illumination due to lower I2R loss


                         Institute of Energy Conversion
                         University of Delaware
                         CdTe Team meeting NREL May 5-6, 2005                                                                       5
     Spectral Response : Analysis and Modeling




•   Fit modeled [Phillips 1st WCPVEC 1994] to measured SR with W=2.5m and L= 0.8m
    Increasing L from 0.5m to 1.5m results in better collection for all wavelengths, for
    back wall SR
•    Front wall SR unaffected by L, back wall SR sensitive to L [400-800nm] and W[800-
    860nm]


            Institute of Energy Conversion
            University of Delaware
            CdTe Team meeting NREL May 5-6, 2005                                         6
            Voc vs. T vs. Intensity: A fundamental characterization
                                       tool
     •   J-V at low T often distorted by blocking diode and/or very high Rs
     •   Cannot analyze J-V curve with single diode model
     •   J=O at Voc so no blocking, Rs

                                                                       1.6

                  b AkT J 00 
           Voc        ln                                                                           a-Si
                 q    q     J L                                    1.2




                                                         VOC (Volts)
                                                                                                         CdTe

     •   Voc:linear in -T, log in I
                                                                       0.8
     •   Voc upper limit is fundamental parameter
     •   Slope dV/dT indicative of recombination mech.

                                                                                 Cu(InGa)Se 2
     •   JL depends on V field driven collection                      0.4           CuInSe 2
           – JL(V)=JL0  (V)
                                                                             0        100        200    300
                                                                                                T (K)




                  Institute of Energy Conversion
                  University of Delaware
                  CdTe Team meeting NREL May 5-6, 2005                                                          7
Voc-T: linear fit and extrapolation above 220K

                             Voc(0K)~1.5 V indep. of intensity
                                      FS D040 10440C2-23
                                            initial
                1.1
                                                                100% light
                                                                60% light
                   1                                            40% light
                                                                6% light
                0.9
    Voc(V)




                0.8

                                 y   = 1.52   -   0 .0 023 9x   R= 0.99 8
                0.7              y   = 1.53   -   0 .0 025 2x   R= 0.99 8
                                 y   = 1.53   -   0 .0 025 8x   R= 0.99 9
                                 y   = 1.52   -   0 .0 026 2x   R= 0.99 8
                0.6
                  200          220        240        260 280            300   320
                                                     T(K)


             Institute of Energy Conversion
             University of Delaware
             CdTe Team meeting NREL May 5-6, 2005                                   8
                                        Voc saturation below 200K

                                                                           •       After 5 days stress
         •     Initial results
                                                                           •       Voc(300K) decreased 0.03V
         •     Saturation: independent of T, L
                                                                           •       Voc(150K) decreased 0.06V
         •     No recombination?
             1.1                                                           1.1
                                  FS D040 10440C2-23                                         FS D040 10440C2-12
                                        initial
              1                                                                1


             0.9                                                           0.9
Voc(V)




                                                                  Voc(V)
             0.8                                                           0.8
                        Voc(100% light)                                                  100% light
                        Voc(~60% light)                                                  46% light
             0.7        Voc(~40% light)                                    0.7           22% light
                        Voc(~6% light)                                                   10% light
             0.6                                                           0.6
               100    150      200   250          300       350              100       150     200   250   300    350
                                  T(K)                                                            T(K)



                     Institute of Energy Conversion
                     University of Delaware
                     CdTe Team meeting NREL May 5-6, 2005                                                               9
                                         Bifacial Voc vs. T at 100, 10% light
                     Voc for front (CdS ) and back (ZnTe)                               Voc for Front (CdS) and Back (ZnTe)
                     illumination (100, 10%): IEC VT149                                   illumination (100, 10%): FS /IEC
              1000                                                               1000

                                                F, 100%
               900                                                                900                            F, 100%

                                                                                                      F, 10%




                                                                      Voc (mV)
   Voc (mV)




               800                    F, 10%                                      800


               700                                                                700
                                                                                                     B, 10%
                                              B, 100%
               600                                                                600
                                              B, 10%                                                  B, 100%

               500                                                                500
                 100        150         200         250         300                 100        150      200      250      300
                                       T (K)                                                           T (K)

•Front illumination : Voc independent of temperature and light intensity below 220K
•Back illumination : Voc continues to increase irrespective of front-wall Voc
•Transient increase in Voc ~mins observed at temperatures below 110K from dark to 100%
This behavior seen in ALL CdTe cells irrespective of the back contact or the manufacturer

                             Institute of Energy Conversion
                             University of Delaware
                             CdTe Team meeting NREL May 5-6, 2005                                                               10
                        Bifacial Jsc vs. T : Intensity and Spectral effects
                                    VT149-4-1 cell5                                               VT149-4-1 cell5
                   25
                            100%                                                                                     100%
                            46%                                                       8                              46%
                            22%                                                                                      22%
                   20       10%                                                                                      10%
                            RED                                                                                      RED
                            BLUE                                                                                     BLUE
                                                                                      6




                                                                       Jsc(mA/cm2 )
                   15
Jsc(mA/cm2 )




                                                                                      4
                   10



                    5                                                                 2


                    0
                                                                                      0
                    100       150        200          250        300
                                                                                      100   150       200      250     300
                                           T(K)
                                                                                                       T(K)

               •    Front illumination Jsc decreases below 220K. Virtually no change above 220K
               •    Back illumination Jsc increases below 220K . Opposite effect
               •    Mobility increases at lower temperature ?
               •    Temperature dependence of field/depletion region which causes better collection
                    from back but reduces field in front?
                                       Similar temperature behavior seen in FS samples

                          Institute of Energy Conversion
                          University of Delaware
                          CdTe Team meeting NREL May 5-6, 2005                                                               11
Maximum achievable Voc : Band diagram model
2
         SnO2 CdS/CdTexS1-x                     CdTe
                                                                       Electron fermi level pinned
                                                             Ec
1                                                                      at SnO2 interface
                                                             Efn       Ec - 0.3eV

0                                                            Efp
                                                                       Hole fermi level pinned
                                                             Ev        at Cu defect
                                                                       Ev + 0.3eV
-1                                                     Ec

                                                       Ev

-2                                                     Efn

                                                       Efp

                                                       Ev+0.33
-3
     0               1             2       3           4           5
                                   X(microns)

         Institute of Energy Conversion
         University of Delaware
         CdTe Team meeting NREL May 5-6, 2005                                                    12
                                               Observations

• Voc saturation below 220K observed for > 30 samples
• Is the Voc @ 100K the “Built-in voltage”– Maximum achievable limit ??
    - Unclear whether back-wall Voc really saturates due to system limitations
• Fermi-level pinning at SnO2 interface alters the quasi-fermi level profile
• dEg/dT= (- 1.70 mV/K) so CdTe bandgap at 100K ~ 1.7eV. If this hypothesis
  valid then the maximum achievable Voc is
                              1.7 - 0.3- 0.3 = 1.1 eV
• Typically @ 100K maximum “Saturated Voc” ~ 0.95-1.05V achieved
• Other issues with low temperature measurement
   - Blocking contact at lower temperatures
   - Defect/Trap energy level positions not known
   - Slow transient increase after light exposure
• Interpretation of Voc complicated by large Jsc temperature dependence



        Institute of Energy Conversion
        University of Delaware
        CdTe Team meeting NREL May 5-6, 2005                                13
                             Conclusions and Future Work

• Bifacial device characterization enhances conventional characterization
  tools and helps to isolate properties of front and back junction
• Temperature dependent Voc and Jsc measurements fundamental to
  quantify transport in CdTe/CdS solar cells
• No spectral dependence of Voc or Jsc
• Bifacial spectral response model fit to data with W = 2.5m and L= 0.8m
  the back-wall spectral response sensitive to L and W
• In future intend to make cells with thinner CdTe to evaluate drift and
  diffusion limited transport
• We intend do Voc-T measurement on cells with CdZnTe window layer to
  further investigate the possibility of interface recombination limited
  transport




           Institute of Energy Conversion
           University of Delaware
           CdTe Team meeting NREL May 5-6, 2005                              14

				
DOCUMENT INFO
Shared By:
Categories:
Stats:
views:11
posted:3/27/2011
language:English
pages:14