**PV Program Contract Summary Report**

**Thin-film PV Partnership Program**

**SUBCONTRACT NO. ADJ -- 1 -- 30630-12**

**"Processing, Materials, Devices and Diagnostics for Thin-film
Photovoltaics:**

**Fundamental and Manufacturability Issues"**

**Contract Period: 9/5/02 -- 9/5/03**

**P.I.: Robert W. Birkmire**

**Institute of Energy Conversion, University of Delaware, Newark, DE
19716**

**Ph. 302-831-6220, Fax 302-831-6226, e-mail: rwb@udel.edu**

##### CdTe/CdS based solar cells A baseline process for high throughput fabrication of CdTe cells was developed using chemical surface deposition (CSD) of CdS, vapor transport (VT) CdTe deposition onto moving substrates and vapor CdCl~2~ treatment. CdS films deposited by CSD on crystallized ITO, In~2~O~3~, and Ga~2~O~3~ exhibit conformal growth with \>80% utilization of solvated Cd species. CdTe films 4 μm thick were deposited at 550ºC on 10 x 10 cm TEC15/CdS superstrates at a static equivalent growth rate of \> 5 μm/min. CdTe/CdS structures were thermally annealed then exposed to CdCl~2~ vapor at 400-430°C for 20-30 minutes. The ambient humidity and CdCl~2~-CdTe plate spacing during CdCl~2~ treatment changes the surface oxide composition and has a strong effect on performance. Contact processing consisted of a BDH etch and Cu/Au or Cu/graphite contacts. The best VT device had 11.2% efficiency. Investigation of the etch dynamics, back contact formation and ambient during stress on cell stability was carried out using glancing incidence XRD and variable angle spectroscopic ellipsometry (VASE). It showed transformation of the Te layer from amorphous to crystalline and subsequent oxidation of the layer. The best stability was found for cells with thicker Te layers. The chemistry of Cu with Te-rich CdTe surfaces, prepared by different etch treatments, showed different Cu-Te products depending on Te thickness and chemical state of Cu species. Thermal stressing of cells and constituent layers in the dark in conditions of varying humidity showed that TeO~2~ and CdTeO~3~ were formed at lower humidity on CdTe, while the formation of amorphous hydroxide, oxide or hydrate phases are more likely at higher humidity. The IEC Cu-Te back contact appears to be stable at higher humidity. Cu-Te phases are susceptible to oxidation. The robustness of the IEC back contacts to humidity and thermal stress may be due to the thicker Te film in the back contact.

**CuInSe~2~-based Solar Cells**

Development of the roll-to-roll deposition of CIGS on polyimide web
focused on film cracking and low adhesion. Mo films were deposited onto
two types of Upilex (S and SG) with and without oxygen plasma treatment
and at different temperatures and gas pressures but CIGS film cracking
and adhesion were insensitive to these variations. Changes were made to
the CIGS roll-to-roll deposition system to improve its robustness and
reproducibility. A new Se source and shielding of the thermocouple now
allows the control of the surface temperature of the liquid Se source.
Work on CIS-based devices with increased V~oc~ focused on adhesion and
material and device performance of Al containing alloys, and on S
diffusion into CIS and CIGS films. Al alloy films have been deposited by
1-step (constant flux), 2-step (Al-rich flux followed by Cu-free flux),
and 3-step (Cu-free flux at low temperatures followed by only Cu flux
and then Cu-free flux) at higher temperature processes. Going from
1-step to 3-step processes increased grain size. Film adhesion was
similar for 1-step and 2-step processes, but much worse for the 3-step
process. Efficiency was similar for 1-step and 2-step processes,
decreasing with increasing Al content. Films prepared by the 3-step
process could not be made into devices except for the lowest Al content
films due to the poor adhesion. QE data showed increasing voltage
dependent collection with increasing Al content, i.e. increasing
bandgap. Alternative emitter layer materials and processes were
evaluated. Air annealed chemical bath deposited ZnS films to form mixed
sulfide-oxide layers were characterized. While bandgap decreases with
air anneal intensity, glancing angle incidence XRD does not indicate any
sulfide-oxide alloy formation. Devices made with the ZnS window layer do
not show any correlation between efficiency and air-anneal time. CdS
layers formed by CSD are highly desirable from a manufacturing point of
view, since it has high utilization rate and low waste. Devices with CSD
CdS and efficiency \>14% were demonstrated. Optical constants of
CuIn~1-x~Ga~x~Se~2~ alloy thin films with Ga/(Ga+In) ratios from 0 to 1
have been determined by VASE over an energy range from 0.75 to 4.6eV.
Due to high surface roughness, a new method was developed in which VASE
measurements were carried out on the reverse side of the films
immediately after peeling them from the Mo-coated glass substrates.

##### Si-Based Solar Cells

The grain size enhancement of HWCVD Si films deposited on Al layers has
been investigated in two temperature regimes (430 and 600°C), which are
below and above the Si-Al eutectic point (575°C). Si layers were
deposited by HWCVD onto Al coated glass substrates at growth rates of
1.0 and 5.0 μm/hr, with thicknesses of 2 and 5 μm. Si films deposited on
Al at 430°C had random orientation and higher degree of crystallization
compared to films deposited on glass. For 2 μm thick films grown at 1
μm/hr, the average grain size and crystalline fraction increases with
decreasing Si/Al ratio. As the growth rate increased from 1 to 5 μm/hr,
the average grain size and crystalline fraction decreases at a constant
Si/Al ratio. TEM analysis found heterogeneous structure; several large
grains but mostly nano-crystalline regions. The bulk of the films
contain Al. Al layer inversion is incomplete, perhaps due to
insufficient interaction time between the Si and the Al layer. Smaller
grain size for 5 μm Si films suggests a limiting thickness beyond which
the Al ceases to influence the growth process. Si films deposited at
600°C exhibit larger average grain size. However, all 600°C films were
non-homogeneous in terms of morphology and composition. The crystalline
volume fraction of films deposited at 600°C is similar to those
deposited at 430°C.

##### Diagnostics

Contact wetting angle, VASE, and GIXRD techniques are being used to
routinely investigate changes in surface properties, which occur during
processing CdTe and CIGS solar cells. For CdTe cells, correlations have
been found for changes in surface energy of CdS due to crystallization
and of CdTe due to oxidation during post deposition storage and
thermo-chemical processing such as CdCl~2~ treatment and etching. For
CIGS cells, diagnostic evaluation is being carried out with respect to
CIGS film composition and Mo substrate stress and surface energy.

##### Publications

1.  \"Ionizing Radiation Effects in Copper Indium Gallium Diselenide
    Thin-Film Solar Cells,\" J. Tringe, J. Nocerino, R. Tallon, W.
    Kemp, W. Shafarman, D. Marvin, *Journal of Applied Physics* **7**
    (2002).

2.  \"Phase Behavior in the CdTe-CdS Pseudobinary System,\" B.
    McCandless, G. Hanket, D. Jensen, R. Birkmire, *Journal Vac. Soc.
    Technology A* **20**, 4 (2002) p. 1462-1467.

3.  \"Analysis of Quantum Efficiency and Optical Enhancement in
    Amorphous Si p-i-n Solar Cells,\" S. Hegedus, R. Kaplan, *Progress
    in Photovoltaics,* 10 (2002) p. 257-269.

4.  \"Deposition and Structural Characterization of Poly-Si Thin Films
    on Al Coated Glass Substrates using Hot-Wire CVD,\" O. Ebil, R.
    Aparicio, S. Hazra, R. Birkmire, E. Sutter, *Thin Solid Films,*
    **430** (2002) p. 120-124.

5.  \"Improved Fill Factors in Amorphous Silicon Solar Cells on Zinc
    Oxide by Insertion of A Ge Layer,\" G. Ganguly, D. Carlson, S.
    Hegedus, R. Gordon, R. Reedy, submitted to *Applied Physics Letters*
    (2003).

6.  \"Optical Characterization of CuIn ~1-x~ Ga x Se~2~ Alloy Thin Films
    by Spectroscopic Ellipsometry,\" P. Paulson, R. Birkmire, W.
    Shafarman, *Journal of Applied Physics* **94**, 2 (2003) p. 879-885.

7.  \"Determination of Activation Barriers for the Diffusion of Sodium
    Through CIGS Thin-Film Solar Cells,\" M. Zellner, R. Birkmire, E.
    Eser, W. Shafarman, J. Chen, *Progress in Photovoltaics* **11**,
    8 (2003) p. 543-548.

8.  \"Correlation of Surface Phases with Electrical Behavior in
    Thin-Film CdTe Devices,\" B. McCandless, S. Hegedus, R. Birkmire, D.
    Cunningham, *Thin Solid Films* **431-432** (2003) p. 249-256.

9.  \"Distinguishing Metastable Changes in Bulk CIGS Defect Densities
    for Interface Effects,\" J. Heath, J. Cohen, W. Shafarman, *Thin
    Solid Films* **431-432** (2003) p. 426-430.

10. \"Near-Surface Defect Distributions in Cu(InGa)Se~2~,\" A.
    Rockett, D. Liao, J. Heath, J. Cohen, Y. Strzhemechny, L.
    Brillson, K. Ramanathan, W. Shafarman, *Thin Solid Films*
    **431-432** (2003) p. 301-306.

11. \"New Cd-Free Buffer Layer Deposited by PVD: In ~2~ S ~3~ Containing
    NA Compounds,\" N. Bernede, S. Marsillac, C. Amoy, W. Shafarman,
    *Thin Solid Films* **431-432** (2003) p. 326-329.

12. \"Novel Electroactive Polymers and Ogligomers for Flexible
    Photovoltaic Cells\" M. Galvin, R. Birkmire, Univ. R&D for Future
    Gen. Solar Elec. Tech. (2003).

13. \"Cadmium Telluride Solar Cells,\" Ch14 in Handbook of Photovoltaic
    Science and Engineering, B. McCandless, J. Sites, Wiley and Sons,
    Chichester (2003) p. 617-662.

14. \"Cu(InGa)Se~2~ Solar Cells,\" Ch 13 in Handbook of Photovoltaic
    Science and Engineering, (A. Luque and S. Hegedus, Eds) Wiley and
    Sons, Chichester (2003) p. 567-616.

15. \"Status, Trends, Challenges, and the Bright Future of Solar
    Electricity from Photovoltaics,\" in Handbook of Photovoltaic
    Science and Engineering, S. Hegedus, A. Luque Wiley and Sons,
    Chichester (2003) 1, 43.

> **[NCPV Denver, February (2003)]{.underline}**

16. \"Growth and Characterization of HWCVD Si Films on Al Coated
    Glass,\" O. Ebil, R. Aparicio, S. Hegedus, R. Birkmire,

17. \"Role of Process Chemistry and Stability on CdTe-Based Solar Cell
    Performance,\" B. McCandless, K. Dobson, S. Hegedus, P. Paulson.

18. \"The Solar Electric Future: Linking Science, Engineering, Invention
    and Manufacturing,\" R. Birkmire.

19. \"Advances in CuInSe~2~-based Solar Cells: From Fundamental to
    Processing,\" W. Shafarman, J. Titus, M. Haimbodi, M. Gossla, G.
    Hanket, S. Marsillac, T. Minemoto, P. Paulson, B. Sang, U. Singh, E.
    Eser, R. Birkmire

> **[Proc. Mat. Res. Soc. Symp. (2003)]{.underline}**

20. \"Defects in Copper Indium Aluminum Diselenide Films and their
    Impact on Photovoltaic Device Performance,\" J. Heath, J. Cohen, W.
    Shafarman, **763,** p. 441-446.

21. \"New Junction Capacitance Methods for the Study of Defect
    Distributions and Carrier Properties in the Copper Indium Diselenide
    Alloys,\" J. Cohen, J.. Heath, and W. Shafarman. **763** (2003) p.
    429-439.

22. \"Photoconductive CdS: How Does It Affect CdTe/CdS Solar Cell
    Performance?\" S. Hegedus, D. Ryan, K. Dobson, B. McCandless, D.
    Desai, **763** (2003) p. 447-452.

23. \"Sputtered ZnTe:N and ZnO:Al for Solar Cell Electrodes and
    Recombination Junctions,\" J. Drayton, V. Parikh, G. Rich, A.
    Gupta, T. Osborn, R.. Bohn, A. Compaan, B. McCandless, P. Paulson,
    **763** (2003) p. 353-358.

24. \"The Dynamics of CdTe Etching,\" K. Dobson, P. Paulson, B.
    McCandless, R. Birkmire, **763** (2003) p. 107-118.

> **[WCPEC-3 Osaka, Japan (2003)]{.underline}**

25. \"Critical Issues for Cu(InAl)Se~2~, Thin Film Solar Cells\" W.
    Shafarman, S. Marsillac, T. Minemoto, P. Paulson, R. Birkmire.

26. \"Deposition and Characterization of Cu(InAl)Se~2~ Thin Films and
    Solar Cells,\" W. Shafarman, S. Marsillac, T. Minemoto, P.
    Paulson, R. Birkmire.

27. \"Chemical Surface Deposition of Ultra-Thin Cadmium Sulfide Films
    for High Performance and High Cadmium Utilization,\" B.
    McCandless, W. Shafarman.