Sensitivity of Thin Film CdTe Device Operation to Processing by zrn20302

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									Sensitivity of Thin Film CdTe Device
 Operation to Processing Chemistry

     Brian McCandless, Steve Hegedus, Kevin Dobson,
         Darshini Desai, Michael Angelo, Meijun Lu


                   Institute of Energy Conversion
                       University of Delaware
                     Newark, Delaware 19716


              NREL sub-contract # ADJ-1-30630-12




Institute of Energy Conversion                           National CdTe Team Mtg 5-6 May 2005
             University Center of Excellence for Photovoltaic Research and Education
                                      Problem
  Understanding role of materials and processing chemistry on
    polycrystalline CdTe device operation with respect to high
    throughput deposition and post-deposition treatments



                                     Approach
Fabricate cells using baseline approach, analyze materials properties
  and devices using vapor transport and evaporated CdTe films




       Institute of Energy Conversion                           National CdTe Team Mtg 5-6 May 2005
                    University Center of Excellence for Photovoltaic Research and Education
                                 Topics

CdTe deposition:
       Thickness and growth rate
       O2 ambient

Post-deposition treatment:
       CdTe film oxidation with varying humidity
       Separating effects of CdCl2, Cu, O2
       Vapor ZnCl2 versus CdCl2




Institute of Energy Conversion                           National CdTe Team Mtg 5-6 May 2005
             University Center of Excellence for Photovoltaic Research and Education
                            Vapor Transport (VT) Deposition
                      BN Source Container

Perforated
Quartz Ampoule                              Radiative Shield
with CdTe Crystals




                                                 Heated Plate
   Heated Plate


     Substrate
                                             Direction of Travel


                     Heated Plate

            Exit slit located 1.5 cm
            above substrate surface




                                            Deposition rate from 1 to 85 mm/min at 550ºC
                      Institute of Energy Conversion                           National CdTe Team Mtg 5-6 May 2005
                                   University Center of Excellence for Photovoltaic Research and Education
                                   VT CdTe Deposition
Baseline Conditions:                                  Film Properties:
 Tsubstrate = 550ºC                                    Single phase CdTe
 99.9998% Alfa CdTe crystals                           Sharp band edge, Eg = 1.50 eV
 He/O2 Ambient                                         T/(1-R) > 60%
                                                                       8
 3 cm/min translation                                         1.0x10
 GR = 8 mm/min                                                              2 vs E
  5-7 mm thick
                                                                      0.8
Thickness uniformity (10 x 10 cm):
 ± 5% direction of travel                                             0.6




                                                             (cm )
                                                           -2
                                                                               IEC VT119
 ± 10% along each edge                                                         Eg = 1.50 eV
                     1.0




                                                           2
                                              IEC VT119
                           T/(1-R) vs l                               0.4
     T, R, T/(1-R)




                     0.8                   T/(1-R)
                     0.6
                     0.4                     T                        0.2
                     0.2
                                            R
                     0.0                                              0.0
                           400   600 800 1000 1200 1400                 1.30        1.40     1.50   1.60
                                  Wavelength (nm)                                    Energy (eV)

     Institute of Energy Conversion                           National CdTe Team Mtg 5-6 May 2005
                  University Center of Excellence for Photovoltaic Research and Education
 AFM 20 x 20 mm
                         VT CdTe Film Morphology
Substrate temp       500ºC                                               570ºC
            5 mm, 8 mm/min                                               5 mm, 8 mm/min



              Faceted morphology, grain size increases with substrate temperature

Film thickness      550ºC                                                 550ºC
           1 mm, 8 mm/min                                                 23 mm, 8 mm/min


                        Grain size increases proportional with CdTe thickness
Growth rate         550ºC                                                550ºC
           6 mm, 8 mm/min                                                6 mm, 81 mm/min



                      Grain size inversely proportional to CdTe growth rate
              Without O2 during growth: pinholes and discontinuous morphology → shunted cells

               Institute of Energy Conversion                           National CdTe Team Mtg 5-6 May 2005
                            University Center of Excellence for Photovoltaic Research and Education
VT CdTe Devices: TEC15/30 nm Ga2O3/80 nm CdS
                 Effect of CdTe thickness and growth rate on cell performance

                        Run             CdTe          Voc        Jsc       FF        Eff
                                       thk/rate      (mV)       (mA/       (%)      (%)
                                         (mm/                   cm2)
                                       mm/min)
 Vapor CdCl2           149.2             7/9          842        24        64       13.0
415ºC, 25 min
        Cu/Ni          145.1             5/6          804        24        60       11.5

                       150.5             2/9          774        23        53        9.0

 Vapor CdCl2
                       144.1            1.5 / 2       807        23        57       10.5
400ºC, 20 min
        CuI/Ni         151.3           1.4 / 13       766        23        54        9.5

                       139.1           1.4 / 18       752        23        58       10.0



            Institute of Energy Conversion                           National CdTe Team Mtg 5-6 May 2005
                         University Center of Excellence for Photovoltaic Research and Education
VT CdTe Devices: TEC15/30 nm Ga2O3/80 nm CdS
                 Effect of CdTe thickness on cell performance – single piece

    Cell          CdTe          Voc       Jsc       FF         Eff           1
                                                                                 VT144.1
                   Thk         (mV)      (mA/       (%)       (%)
                  (mm)                   cm2)                              0.8


                                                                           0.6
  144.1-2          1.45         807        23        57      10.5




                                                                      QE
                                                                           0.4
  144.1-4          1.25         789        23        58      10.5                         Cell 8 (0.75 µm)
                                                                           0.2            Cell 2 (1.45 µm)

  144.1-6          1.10         743        22        54       8.5
                                                                             0
                                                                             400    500      600     700     800   900
  144.1-8          0.75         723        21        50       7.5                          wavelength (nm)
                                                                           Final CdS thickness estimates:
                                                                           60 nm for 1.45 mm CdTe
2X thickness reduction → ~10% reduction in Voc, Jsc, FF                    45 nm for 0.75 mm CdTe

                                                                           CdTe band edge shifted 5 nm to
                                                                           lower energy for 0.75 mm CdTe
            Thinner CdTe, smaller grain size, enhanced CdS consumption

            Institute of Energy Conversion                           National CdTe Team Mtg 5-6 May 2005
                         University Center of Excellence for Photovoltaic Research and Education
                 Post-Deposition Treatment: Interface Diffusion
          GIXRD with in situ heating, 400ºC, W = 5º

    W                   100 nm CdCl2
                                                                     2q                     2.4                          CdTe1-x Sx       7.2




                                                                                                                                                lattice parameter (Å)
                        1 mm VT CdTe                                                        2.2
                                                                                                                    co
                      80 nm CSD CdS                                                         2.0




                                                                                  Eg (eV)
                                                                                                                                          6.8

                           1 mm SnO2                                                        1.8
                                                                                                         ao
                                                                                                                                          6.4
                                Glass            Heater                                     1.6

                                                                                            1.4
                              CdTe             CdS                        SnO2                                                            6.0
                                                                                                  0    0.2    0.4        0.6   0.8    1
                              (111)           (100)                       (110)
                                                                                                                x (EDS)


                  5
                 10
Intensity (AU)




                                         scan acquisition: 5 min each
                                                        t = 40 min
                                                                                                  CdTe0.95S0.05 and CdS0.95Te0.05
                  4
                 10
                                                        t = 20 min
                                                                                                  CdTe0.96S0.04 and mixed CdSTe
                  3
                 10

                                                          t=0                                     CdTe and CdS
                      23           24              25                26                     27
                                           2-Theta (deg)
                           Institute of Energy Conversion                           National CdTe Team Mtg 5-6 May 2005
                                        University Center of Excellence for Photovoltaic Research and Education
CdTe Film Oxidation with Varying Humidity
Do oxidation/humidity in post-deposition processing significantly enahnce
CdTe film oxidation?

Background:
 CdTe surfaces can be diffusion sources (e.g., Cu, Cl)
 CdTe thin films oxidize in room air: CdTeO3, CdTe2O5
 CdTe crystals oxidize via TeO2 and CdO formation

          PVD CdTe Films                                  VT CdTe Films

          RH = 0, 20, 40%                                 RH = 40%
          T = 400ºC, 500ºC, 550ºC                         T = 500ºC

Bottled “dry air”: RH = 0%
Bubbler: RH ~ 20%
Room Air: RH ~ 40%         PVD data shown: 500ºC, RH = 0% and 40%



     Institute of Energy Conversion                           National CdTe Team Mtg 5-6 May 2005
                  University Center of Excellence for Photovoltaic Research and Education
                                CdTe Film Oxidation
GIXRD with in situ heating,
                                              PVD CdTe                               ambient sparged onto film surface
       W = 1º                                   Glass                                scan acquisition: 10 min each

                                                 Heater
      Dry Air: (RH = 0%, <0.001 g/L H2O)                                      Humid Air: (RH ~ 40%, ~ 0.1 g/L H2O)
                        CdTe                                                                CdTe
                        (111)                                                               (111)                  10000
                                              10000
                                                                                                         500ºC
                                   500ºC




                                                                                                                           Intensity (A.U.)
                                                      Intensity (A.U.)
                                                                                                                   1000
                                              1000                                                          TeO2

                                                                                                                   100
                                              100


                                                                                                                   10 min
                                              10 min
                                                                                                               50 min
                                        50 min                                                              100 min
                                     100 min                             20 21 22 23 24 25 26 27 28 29 30
    20 21 22 23 24 25 26 27 28 29 30
               2-Theta (deg)
                                                                                    2-Theta (deg)
                                                                                                               b-CdTeO3
                          -CdTeO3                                        b-CdTeO3                  -CdTeO3
       -CdTeO3
   •Dry ambient: -CdTeO3                             •Humid ambient: initially TeO2 then CdTeO3
   •CdTe intensity ↓ as oxide intensity ↑             •CdTe intensity ↓ as oxide intensity ↑
                                                      •Oxidation rate enhanced

             Institute of Energy Conversion                           National CdTe Team Mtg 5-6 May 2005
                          University Center of Excellence for Photovoltaic Research and Education
                          Estimated Oxide Thickness vs Time: 500ºC
                           Calculated from attenuation of CdTe (111) GIXRD peak intensity
                       1.20E-01
                                                                       RH = 0%
                                                                       RH ~ 20%
                                                                       RH ~ 40%
                       1.00E-01
Oxide Thickness (um)




                       8.00E-02



                       6.00E-02



                       4.00E-02



                       2.00E-02



                       0.00E+00
                                  0            20            40           60            80           100           120
                                                                      time (min)

                                  Institute of Energy Conversion                           National CdTe Team Mtg 5-6 May 2005
                                               University Center of Excellence for Photovoltaic Research and Education
                                                         Oxidized CdTe Films
PVD Films, 500ºC, 100 min
XPS PHI 5000               RH = 0%
                         Survey before etching
                                                                                                                       80
                                                                                                                                                 RH = 40%
                                                                                                                                                Survey before etching


Al K
              60 Oxidized          Te 3d5/2                                                                                 Oxidized                        Te 3d5/2
200 W, 15 kV                                                                                                           70


                                                        Te 3d3/2           Cd 3d5/2                                         Auger Te                Te 3d3/2            Cd 3d5/2
                                                                                                                       60                                        Cd 3d3/2
                         50   Auger Te                              Cd 3d3/2
                                                                                                                                                 Cd 3p3/2
                                                 Cd 3p3/2                                                                     Auger O Te 3p3/2                 O 1s
                                Auger O Te 3p3/2                                                                                 Te 3p1/2    Cd 3p1/2
                  kC/s




                                                                                                                kC/s
                                               Cd 3p1/2   O 1s                                                         50
                                   Te 3p1/2

                         40
                                                                                                                       40
                                                                                                                                                                                                Cd 4d
                                                                                   C 1s          Te 4dCd 4d                                                                    C 1s         Te 4d
                                                                                                                       30
                         30


                                                                                                                       20
                                1000          800           600           400             200            0                    1000        800           600              400          200            0
                                                    Survey etching 178 mins
                                                     Binding Energy (ev)                                                                      Binding Energy (ev)
                                                                                                                                            Survey etching 260 mins
                                                                                                                       90
 Ar+ etched              70   Etched                                                                                        Etched
                                                                                                                                                              Te 3d5/2
 2 kV, 25 mA                  180 min                         Te 3d5/2                                                 80
                                                                                                                            260 min
 P = 0.02 Pa             60
                                                        Te 3d3/2              Cd 3d5/2
                                                                                                                       70
                                                                                                                            Auger Te
                                                                                                                                                   Te 3d3/2
                                                                                                                                                                           Cd 3d5/2
 ~1 nm/min                                                                                                             60                       Cd 3p3/2
                                                                                                                                                                Cd 3d3/2
                         50   Auger Te                                                                                                Te 3p3/2
                 kC/s




                                                                   Cd 3d3/2
                                                                                                                kC/s
                                                                                                                                             Cd 3p1/2
                                                      Cd 3p3/2
                                         Te 3p3/2                                                                      50        Te 3p1/2
                         40        Te 3p1/2     Cd 3p1/2

                                                                                                                       40
                                                                                                                                                               O 1s
                                                                                                        Cd 4d                                                                                       Cd 4d
                         30                                                                     Te 4d                                                                                       Te 4d
                                                                                                                       30


                         20                                                                                            20
                                1000          800           600          400              200            0                    1000        800           600              400          200            0
                                                     Binding Energy (ev)                                                                         Binding Energy (ev)

                 Institute of Energy Conversion                           National CdTe Team Mtg 5-6 May 2005
                              University Center of Excellence for Photovoltaic Research and Education
PVD Films, 500ºC                                Oxidation vs Depth
XPS PHI 5000, Al K

                                      RH = 0%                                               RH = 40%
                                                              250000                                                                250000

                                                                                                                                    200000




                                                                                                                                                Y Axis
                                                              200000




                                                                             Y Axis
                                                              150000                                                                150000

                                                              100000                                                                100000

                                                              50000                                                                 50000
                                                                                                                                        Etch Time
                                                              0                                                                     0     (min)
                                                                                                                                    B      0
                                                                                                                                 B
                                                                                                                                B
                                                                                                                             B
                                                                                                                            B
                                                                                                                        B
                                                                                                                        B
        Te-O Cd-Te           Te-O Cd-Te                                Te-O Cd-Te           Te-O Cd-Te
                                                                                                                    B
                               Te 3d5/2                                 Te 3d3/2             Te 3d5/2
         Te 3d3/2                                                                                                   B
                                                                                                                B
                                                                                                                B
                                                                                                                            180
                                                 Binding Energy (eV)
  595       590       585       580       575     570       595        590            585   580     575   570

                            •Surface oxide and penetrating oxide
                             X Axis                                X Axis

                            •Transition from Te-O to Te-Cd bonding with sputter depth
                            •Oxidation enhanced in humid ambient

                     Institute of Energy Conversion                           National CdTe Team Mtg 5-6 May 2005
                                  University Center of Excellence for Photovoltaic Research and Education
         CdTe Film Oxidation: VT CdTe Devices
        Pieces cut from adjacent locations on VT 161: 6 mm CdTe, Cu/Ni contact

Piece       Condition           Voc       Jsc       FF         Eff                              45
          prior to vapor       (mV)      (mA/       (%)       (%)                               40




                                                                     Current Density(mA/cm2)
                                                                                                35
              CdCl2                      cm2)                                                   30
                                                                                                25
                                                                                                20
161.2    Vacuum Stored          812        23        64      12.0                               15
          25ºC, 1 week                                                                          10
                                                                                                 5
161.1     Stored in air                                                                          0
           40ºC, 16H            764      22.5        56       9.5                               -5
           (~0.1 g/L)                                                                          -10
                                                                                               -15
161.4   HT in air 400ºC,                                                                       -20
                                                                                               -25
             5 min              778        22        45       7.5                              -30
            (5 g/L)                                                                                  -0.5   0.0   0.5      1.0
                                                                                                             Voltage (V)
 •Exposure to humidity prior to CdCl2 HT detrimental to device operation
 •Suspect stoichiometric change; preferential Te removal from CdTe lattice
 •CdCl2 treatment doesn’t “heal” film


           Institute of Energy Conversion                           National CdTe Team Mtg 5-6 May 2005
                        University Center of Excellence for Photovoltaic Research and Education
                         Separating CdCl2, Cu, O2
Piece       CdCl2            Ambient         Cu       Voc        Jsc          FF       Eff      Comment
          temp, time                         Thk     (mV)       mA/cm2        (%)      (%)
                                            (nm)
125.41      None                 -           0       584          11.5         45       3.0     LDX, Roll
125.42      None                 -           15      606          10.5         55       3.5        LDX
125.61   415ºC, 25 min           Ar          0       603           23          63       8.7     LDX, Roll
125.62   415ºC, 25 min           Ar          15      773           23          61      10.8
125.11   415ºC, 25 min     20% O2 30 s       0       704          22.5         65      10.3        Hyst
125.12   415ºC, 25 min     20% O2 30 s       15      755           23          65      11.3
125.51   415ºC, 25 min        20% O2         0       795           23          61      11.1     Hyst, LDX
125.52   415ºC, 25 min        20% O2         15      788           23          59      10.7

  LDX = light-dark crossover on JV plot
  Roll = “rollover” in JV characteristic at V > Voc
  Hyst = hysteresis between forward and reverse JV sweeps

                 •Without CdCl2 HT, little Cu benefit – eliminates rollover
                 •CdCl2 with no O2 improves current collection, Cu improves Voc
                 •Cu and O independently influence Voc and FF

            Institute of Energy Conversion                           National CdTe Team Mtg 5-6 May 2005
                         University Center of Excellence for Photovoltaic Research and Education
             Separating Effects: halide vapor, Cu

Piece       Halide            Ambient        Cu       Voc        Jsc          FF        Eff      Comment
            Vapor                            Thk     (mV)       mA/cm2        (%)       (%)
                                            (nm)
127.6    5 mTorr CdCl2        20% O2          0       749          23          62      10.5      LDX, Roll
127.4    5 mTorr CdCl2        20% O2         10       791          23          68      12.5
127.5    5 mTorr CdCl2        20% O2         15       801          23          71      13.0
127.2    5 mTorr ZnCl2        20% O2         15       746          23          62      10.5




        •Cu in contact can significantly influence Voc and FF
        •ZnCl2 treatment with Cu/Ni contact similar to CdCl2 treatment with Ni only




             Institute of Energy Conversion                           National CdTe Team Mtg 5-6 May 2005
                          University Center of Excellence for Photovoltaic Research and Education
                                  Conclusions
VT CdTe: grain size increases with substrate temperature, proportional
   to film thickness, inversely proportional to growth rate

Thin CdTe: smaller grains, enhanced CdS-CdTe diffusion, promising
   results with modified CdCl2, etching and contact

CdTe film “storage” prior to CdCl2 HT can contribute to variability
in batch-processed pieces: suspect stoichiometric changes

Grain boundaries likely decorated by oxides

CdCl2 primary effect on minority carrier collection

Cu, O primary effect on junction (mixed results for Cu – buffer role?)

ZnCl2 shows promise as alternative to CdCl2




       Institute of Energy Conversion                           National CdTe Team Mtg 5-6 May 2005
                    University Center of Excellence for Photovoltaic Research and Education

								
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