# Presentation: 209191

## CdS:Cu and “Buffer” Layers

**K. Barri, S. Bapanapalli, S. Gayam, M. Jayabal, L. Nemani, Y. Venkatraman, H. Zhao, D. L. Morel and C. S. Ferekides**
**Clean Energy Research Center**
**Department of Electrical Engineering**
**University of South Florida, Tampa, FL 33620**
**SIMS and AES by S. Asher, J. W. Pankow, and M. R. Young**
**National Renewable Energy Laboratory, Golden, CO, 80401**
_**Work performed under NREL Sponsorship**_

## OUTLINE – Three Parts

_**CSS-CdS:Cu**_
_**“**__**what is the effect of “interfacial” Cu?”**_
**Cu in Back Contact vs. Cu in CdS**
**CdS by CSS (He/O****2****)**
**Aq. soln. of CuCl (optim. range 10****-9****-10****-7****M)**
**“****Cu-free” electrodes:  Plain graphite,**
**                         ****and Sb****2****Te****3****/Mo**
_**SnO**__**2**__**:F/SnO**__**2**__**/CdS(CSS)/CdTe(CSS)**_

_**VK’s Buffer vs Cu Experiment**_
**Two buffers:  CVD and ****sputtered SnO****2**
**Two CdS dep. Proc.: CBD vs ****CSS**
**Two back contacts:  Graphite vs. ****Mo**

_**Zn-Sn-O Buffers**_
**Films are co-sputtered from ZnO and SnO****2**
**Varied of Zn/Sn ratio and studied its effect on:**
**Film Composition**
**Film Resistivity**
**Cell Performance**

## SOLUTION CONCENTRATION & TREATMENT TIME

| CuCl Concentration [M] | VOC
[mV] | FF
[%] |
| --- | --- | --- |
| 6.0 x 10-8 | 830 | 67.4 |

_***Results suggest that Cu in CdS is not necessarily detrimental IF the “optimum” concentrations are achieved***_

| Time
[min] | VOC
[mV] | FF
[%] |
| --- | --- | --- |
| 10 | 830 | 66.3 |
| 15 | 830 | 65.3 |

## SPECTRAL RESPONSE

**CdS thickness variations not intentional; most likely due to CdS source “aging”**
**Collection losses exhibited by device with “largest” amount of Cu**
**Behavior similar to devices fabricated with sputtered Cu – and interpreted to be due to “excess” Cu**

## SIMS/AES ANALYSIS(S. Asher and colleagues)

**Cu-levels correlate well with solution concentrations**
**Cu levels in lowest concentration sample below detection limits of AES**
**Cu-concentration correlates with O****2**** suggesting that CdS:O may be critical (?)**

**CdS treated in CuCl:**
**    ****6 x 10****-9****,10****-8****, 10****-7**** M**
**Max. Cu at the surface:**
**1 x 10****19**** – 1 x 10****20**** cm****-3**
**Cu penetrates to almost 2x depth for highest concentration – must be further investigated**

## Sb2Te3/Mo CONTACTS

| Treatment Time | Cell # | VOC
[mV] | FF
[%] | JSC
[mA/cm2] | Eff.
[%] |
| --- | --- | --- | --- | --- | --- |
| 5min | 862B1a | 0.760 | 62.30% |  | 10.42% |
|  | B | 0.760 | 64.00% | 20.81 | 10.12% |
|   | C | 0.750 | 61.60% | 21.00 | 9.70% |
|   | D | 0.750 | 57.50% | 21.00 | 9.06% |
| 10min | 871A1a | 0.730 | 60.60% | 21.00 | 9.29% |
|  | B | 0.770 | 71.10% | 20.37 | 11.15% |
|   | C | 0.780 | 69.10% | 21.00 | 11.32% |
|   | D | 0.780 | 67.00% | 21.00 | 10.97% |
| 15min | 861B1a | 0.750 | 53.80% | 23.00 | 9.28% |
|  | B | 0.740 | 61.20% | 23.00 | 10.42% |
|   | C | 0.720 | 64.70% | 19.04 | 8.87% |
|   | D | 0.660 | 51.70% | 23.00 | 7.85% |
| 20min | 871B1a | 0.750 | 49.90% | 22.00 | 8.23% |
|  | b | 0.780 | 55.00% | 20.83 | 8.94% |
|   | c | 0.780 | 54.40% | 21.00 | 8.91% |
|   | d | 0.650 | 41.80% | 20.00 | 5.43% |

_**Previous Work with Sb**__**2**__**Te**__**3**__**:**_
**V****OC****~ 600’s mV**
**FF ~low 60’s **
**Process “noisy”**

## OUTLINE – Part II

_**CSS-CdS:Cu**_
_**“**__**what is the effect of “interfacial” Cu?”**_
**Cu in Back Contact vs. Cu in CdS**
**CdS by CSS (He/O****2****)**
**Aq. soln. of CuCl (optim. range 10****-9****-10****-7****M)**
**“****Cu-free” electrodes:  Plain graphite,**
**                         ****and Sb****2****Te****3****/Mo**
_**SnO**__**2**__**:F/SnO**__**2**__**/CdS(CSS)/CdTe(CSS)**_

_**VK’s Buffer vs. Cu Experiment**_
**Two buffers:  CVD and ****sputtered SnO****2**
**Two CdS dep. Proc.: CBD vs ****CSS**
**Two back contacts:  Graphite vs. ****Mo**

_**Zn-Sn-O Buffers**_
**Films are co-sputtered from ZnO and SnO****2**
**Varied of Zn/Sn ratio and studied its effect on:**
**Film Composition**
**Film Resistivity**
**Cell Performance**

## HR TO vs. Cu

**Device Structure:**
**SnO****2****:F/****SnO****2**** (sput. 500 Å)**
**CdS by CSS**
**Cu by CuCl dip of CdS (concentrations in the 10****-8****M range) **
**Back Contact: ****Mo**** (by sputtering; approx. 0.6-0.7 ****μ****m)**
**Devices ****w Buffer and No Cu**** exhibit V****OC****’s and FF in excess of 820 mV and 75%!**
**In all cases a back barrier (roll over) is present**
**The dark currents are not consistent with V****OC****s**
**Collection losses in No HR/No Cu device**

## SR and C-V

**Blue QE in the 30-50% range**
**Smaller CdS thicknesses lead to loss in performance**
**Collection Losses for “No HR/No Cu” device**

**No HR/No Cu device appears fully depleted**
**ONLY HR devices exhibit “normal” C-V behavior; V****BI****≈0.9-1.0 V**
**“****No Cu”: 3.0 & “Cu” 2.3 x 10****14**** cm****-3**

## OUTLINE – Three Parts

_**CSS-CdS:Cu**_
_**“**__**what is the effect of “interfacial” Cu?”**_
**Cu in Back Contact vs. Cu in CdS**
**CdS by CSS (He/O****2****)**
**Aq. soln. of CuCl (optim. range 10****-9****-10****-7****M)**
**“****Cu-free” electrodes:  Plain graphite,**
**                         ****and Sb****2****Te****3****/Mo**
_**SnO**__**2**__**:F/SnO**__**2**__**/CdS(CSS)/CdTe(CSS)**_

_**VK’s Buffer vs Cu Experiment**_
**Two buffers:  CVD and ****sputtered SnO****2**
**Two CdS dep. Proc.: CBD vs ****CSS**
**Two back contacts:  Graphite vs. ****Mo**

_**Zn-Sn-O Buffers**_
**Films are co-sputtered from ZnO and SnO****2**
**Varied of Zn/Sn ratio and studied its effect on:**
**Film Composition**
**Film Resistivity**
**Cell Performance**

## Zn-Sn-O

| Zn-Sn-O | VOC
[mV] | FF
[%] | JSC
[mA/cm2] | RS
[Ω-cm2] | RSH
[Ω-cm2] |
| --- | --- | --- | --- | --- | --- |
| AD | 730 | 58.0 | 23.40 | 2.05 | 1100 |
| HT 550 oC | 730 | 55.0 | 22.00 | 2.10 | 1000 |
| HT 575 oC | 820 | 68.0 | 21.40 | 1.55 | 1170 |
| HT 600 oC | 830 | 69.0 | 22.00 | 0.96 | 900 |

**Substrates:  SnO****2****:F/Zn-Sn-O**
**Strong peaks due to substrate**
**Zn****2****SnO****4**** phase begins to form at 575°C**
**Amorphous Zn-Sn-O films lead to increased CdS “consumption” and poor cell performance**

## Zn-Sn-O ... TDEP=400 °C

| Zn2SnO4 | ZnSnO3 | ZnO2 |
| --- | --- | --- |
| (311) | (006) | (211) |
| (311) | (012); (006) | (211) |

| Thick. [Å] | VOC [mV] | FF [%] | JSC [mA/cm2] | RS
[Ω-cm2] | RSH
[Ω-cm2] |
| --- | --- | --- | --- | --- | --- |
| 500  | 840 | 68.2 | 24.40 | 1.64 | 1700 |
| 500 | 830 | 71.0 | 23.80 | 1.44 | 1700 |
| 500 | 830 | 69.3 | 24.74 | 1.33 | 1900 |
| 750 | 830 | 68.2 | 24.84 | 1.75 | 1900 |
| 1250 | 820 | 69.1 | 24.39 | 1.47 | 2000 |

**At high T****DEP**** the ZnSnO****3**** phase dominates (orthorhombic)**
**Highest blue QE-V****OC****-FF combination!**

## SUMMARY

**CdS:Cu**
**From a performance point of view it appears that Cu in CdS is required; results based on CVD SnO****2****:F/SnO****2**
**A Cu concentration in the range of 10****19**** cm****-3**** was found to yield optimum performance**
**Oxygen may also be important (based on correlation with Cu in AES analysis)**
**It may be possible to eliminate the use of Cu during back contact fabrication – a low temperature process - leading to a more “robust” device**
**HR Buffer vs. Cu**
**Sputtered SnO****2**** HR buffer was found to be more effective than CVD SnO****2**
**The use of sput. SnO****2**** buffer influences the back contact and CdTe characteristics (roll over and apparent doping levels change with the presence of this buffer)**
**Zn-Sn-O Buffers**
**The cubic spinel structure of ZTO (Zn****2****SnO****4****) yielded poor solar cell characteristics (for thin CdS)**
**A mixture dominated by orhtorombic ZnSnO****3**** (Zn****2****SnO****4****, ZnO****2****) resulted in the best blue QE, in the range of 70%, without losses in V****OC**** or FF**
**The various “buffer” layers considered have different properties that can influence the device characteristics; the entire CdTe process must be optimized depending on the type of buffer, Cu-concentration, back contact etc.**
**IN ALL INSTANCES WE NEED TO ESTABLISH THE FINAL Cu CONCETRATION (profile) THROUGH THE ENTIRE DEVICE (post fabrication)**

## Additional

**Device Structure:**
**SnO****2****:F/SnO****2****/CdS/CdTe**
**CuCl-treated CdS/Plain Graphite**
**No-CuCl/Plain Graphite**
**No-CuCl/Doped Graphite(HgTe:Cu)**

## AdditionalLight Soaking @ VOC for 1000 Hrs