# Presentation: 016811

## Sensitivity of Thin Film CdTe Device Operation to Processing Chemistry

- Darshini Desai, Michael Angelo, Meijun Lu
- Institute of Energy Conversion
- University of Delaware
- Newark, Delaware  19716
- NREL sub-contract # ADJ-1-30630-12*** ***

***Sensitivity of Thin Film CdTe Device ******Operation to Processing Chemistry***

## 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

## Topics

- 	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

***Topics***

## Vapor Transport (VT) Deposition

**Exit slit located 1.5 cm**
**above substrate surface**

***Deposition rate from 1 to 85 ************m/min at 550******o******C***

## VT CdTe Deposition

- Baseline Conditions:
-    Tsubstrate = 550oC
-    99.9998% Alfa CdTe crystals
-    He/O2 Ambient
-    3 cm/min translation
-    GR = 8 m/min
-     5-7 m thick
- Thickness uniformity (10 x 10 cm):
-    ± 5% direction of travel
-    ± 10% along each edge

- Film Properties:
-   Single phase CdTe
-   Sharp band edge, Eg = 1.50 eV
-   T/(1-R) > 60%

## VT CdTe Film Morphology

**500oC**
**5 ********m, 8 ********m/min**

**570oC**
**5 ********m, 8 ********m/min**

**550oC**
**1 ********m****, 8 ********m/min**

**550oC**
**6 ********m, ****8 ********m/min**

**550oC**
**23 ********m****, 8 ********m/min**

**550oC**
**6 ********m, ****81 ********m/min**

***Without O******2****** during growth: pinholes and discontinuous morphology ******→****** shunted cells***

**AFM 20 x 20 ********m**

***Substrate temp***

***Film thickness***

***Growth rate***

***Faceted morphology, grain size increases with substrate temperature***

***Grain size increases proportional with CdTe thickness***

***Grain size inversely proportional to CdTe growth rate***

## VT CdTe Devices: TEC15/30 nm Ga2O3/80 nm CdS

| Run | CdTe
thk/rate
(m/
m/min) | Voc
(mV) | Jsc
(mA/
cm2) | FF
(%) | Eff
(%) |
| --- | --- | --- | --- | --- | --- |
| 149.2 | 7 / 9 | 842 | 24 | 64 | 13.0 |
| 145.1 | 5 / 6 | 804 | 24 | 60 | 11.5 |
| 150.5 | 2 / 9 | 774 | 23 | 53 | 9.0 |
| 144.1 | 1.5 / 2 | 807 | 23 | 57 | 10.5 |
| 151.3 | 1.4 / 13 | 766 | 23 | 54 | 9.5 |
| 139.1 | 1.4 / 18 | 752 | 23 | 58 | 10.0 |

***VT CdTe Devices: TEC15/30 nm Ga******2******O******3******/80 nm CdS***

***Effect of CdTe thickness and growth rate on cell performance***

**Vapor CdCl****2**
**415****o****C, 25 min**
**Cu/Ni**

**Vapor CdCl****2**
**400****o****C, 20 min**
**CuI/Ni**

## VT CdTe Devices: TEC15/30 nm Ga2O3/80 nm CdS

| Cell | CdTe
Thk
(m) | Voc
(mV) | Jsc
(mA/
cm2) | FF
(%) | Eff
(%) |
| --- | --- | --- | --- | --- | --- |
| 144.1-2 | 1.45 | 807 | 23 | 57 | 10.5 |
| 144.1-4 | 1.25 | 789 | 23 | 58 | 10.5 |
| 144.1-6 | 1.10 | 743 | 22 | 54 | 8.5 |
| 144.1-8 | 0.75 | 723 | 21 | 50 | 7.5 |

***VT CdTe Devices: TEC15/30 nm Ga******2******O******3******/80 nm CdS***

***Effect of CdTe thickness on cell performance – single piece***

***2X thickness reduction ******→ ******~10% reduction in Voc, Jsc, FF***

***Final CdS thickness estimates:***
***60 nm for 1.45 ************m CdTe***
***45 nm for 0.75 ************m CdTe***

***CdTe band edge shifted 5 nm to***
***lower energy for 0.75 mm CdTe***

***Thinner CdTe, smaller grain size, enhanced CdS consumption***

## Post-Deposition Treatment: Interface Diffusion

***GIXRD with ***_***in situ***_*** heating, 400******o******C, ************ = 5******o***

***CdTe******0.95******S******0.05****** and CdS******0.95******Te******0.05***
***CdTe******0.96******S******0.04****** and mixed CdSTe***
***CdTe and CdS***

***scan acquisition: 5 min each***

## CdTe Film Oxidation with Varying Humidity

- 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: 500******o******C, RH = 0% and 40%***

***CdTe Film Oxidation with Varying Humidity***

## CdTe Film Oxidation

***GIXRD with ***_***in situ***_*** heating,***
********* ******= 1******o***

***Humid ambient: initially TeO******2****** then CdTeO******3****** ***
***CdTe intensity ******↓ as ******oxide intensity ******↑***
***Oxidation rate enhanced***

***ambient sparged onto film surface***
***scan acquisition: 10 min each***

***Dry ambient: ************-CdTeO******3****** ***
***CdTe intensity ******↓ as****** oxide intensity ******↑***

## Estimated Oxide Thickness vs Time: 500oC

***Calculated from attenuation of CdTe (111) GIXRD peak intensity***

## Oxidized CdTe Films

***Oxidized CdTe Films***

**PVD Films, 500****o****C, 100 min**
**XPS PHI 5000**
**Al K******** **
**200 W, 15 kV**

***RH = 0%***

***RH = 40%***

***Etched***
***180 min***

***Etched***
***260 min***

**Ar****+**** etched**
**2 kV, 25 mA**
**P = 0.02 Pa**
**~1 nm/min**

***Oxidized***

***Oxidized***

## Oxidation vs Depth

***PVD Films, 500******o******C***
***XPS PHI 5000, Al K************ ***

***Surface oxide and penetrating oxide***
***Transition from Te-O to Te-Cd bonding with sputter depth***
***Oxidation enhanced in humid ambient***

***RH = 0%***

***RH = 40%***

***          ******Etch Time***
***             ******(min)***
***               ******0***
***180***

- Binding Energy (eV)

## CdTe Film Oxidation: VT CdTe Devices

***Exposure to humidity prior to CdCl******2****** HT detrimental to device operation***
***Suspect stoichiometric change; preferential Te removal from CdTe lattice***
***CdCl******2****** treatment doesn’t “heal” film***

| Piece | Condition
prior to vapor CdCl2 | Voc
(mV) | Jsc
(mA/
cm2) | FF
(%) | Eff
(%) |
| --- | --- | --- | --- | --- | --- |
| 161.2 | Vacuum Stored
25oC, 1 week | 812 | 23 | 64 | 12.0 |
| 161.1 | Stored in air 40oC, 16H
(~0.1 g/L) | 
764 | 
22.5 | 
56 | 
9.5 |
| 161.4 | HT in air 400oC, 5 min
(5 g/L) | 
778 | 
22 | 
45 | 
7.5 |

***Pieces cut from adjacent locations on VT 161: 6 ************m CdTe, Cu/Ni contact***

## Separating CdCl2, Cu, O2

| Piece | CdCl2 
temp, time | Ambient | Cu
Thk
(nm) | Voc
(mV) | Jsc
mA/cm2 | FF
(%) | Eff
(%) | Comment |
| --- | --- | --- | --- | --- | --- | --- | --- | --- |
| 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 |  |

***Without CdCl******2****** HT, little Cu benefit – eliminates rollover***
***CdCl******2****** with no O******2****** improves current collection, Cu improves Voc***
***Cu and O independently influence Voc and FF ***

***Separating CdCl******2******, Cu, O******2***

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

**Notes:**
Selected cells sent to Fred Seymour (CSM)

## Separating Effects: halide vapor, Cu

| Piece
 | Halide
Vapor
 | Ambient | Cu
Thk
(nm) | Voc
(mV) | Jsc
mA/cm2 | FF
(%) | Eff
(%) | Comment |
| --- | --- | --- | --- | --- | --- | --- | --- | --- |
| 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 |  |

***Separating Effects: halide vapor, Cu***

***Cu in contact can significantly influence Voc and FF***
***ZnCl******2****** treatment with Cu/Ni contact similar to CdCl******2****** treatment with Ni only***

## 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 CdCl******2******, etching and contact***
***CdTe film “storage” prior to CdCl******2****** HT can contribute to variability***
***in batch-processed pieces: suspect stoichiometric changes***
***Grain boundaries likely decorated by oxides***
***CdCl******2****** primary effect on minority carrier collection***
***Cu, O primary effect on junction (mixed results for Cu – buffer role?)***
***ZnCl******2****** shows promise as alternative to CdCl******2***