# Presentation: 413442

## Thin CdTe Solar Cells with High Throughput Processing

**Brian McCandless, Kevin Dobson, Steve Hegedus, Wayne Buchanan, Darshini Desai, Robert Birkmire**
**Institute of Energy Conversion**
**University of Delaware**
**Newark, Delaware  19716**
**NREL Center of Excellence**
**For Photovoltaic Research and Education**
** ****Subcontract ADJ-130630-12**

## Goal and Objectives

**R&D to Reduce $/Wp for CdTe Module Fabrication**
**Target < $1/Wp**
**Target >11% module efficiency**
**Close gap between cell and module efficiency**
**Reduce $**
**Reduce CdTe thickness: ~1 ********m (absorption limit)**
**Reduce processing steps and time/step ~1 min**
**Reduce/Eliminate CdCl****2**
**Increase Wp**
**Voc >900 mV**
**FF >75%**

## Experimental Approach

**Rapid CdTe/CdS Deposition**
**Vapor Transport (VT) CdTe Deposition with**
**Substrate T > 500****o****C**
**Adjustable translation speed, ambient composition**
**10 x 10 cm samples**
**Chemical surface deposition CdS < 100 nm on LOF TEC-15**
**Vapor chloride post-deposition treatments**
**Diffusion reactor**
**Reduce processing time by increasing reaction temperature**
**Evaluate alternatives to CdCl****2**** ****such as**** ZnCl****2**
**Back Contact Surface Chemistry**
**CdTe oxidation**
**Alternative etchants**
**Bifacial device analysis using transparent contact**

## Vapor Transport (VT) Deposition

**Min sat zone L vs carrier gas v**

**Saturate carrier gas, transport to substrate, de-saturate, film growth**

**Vapor concentration vs source L**

## Vapor Transport Deposition (VTD): System

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

## Vapor Transport Deposition: Sources

-  Aremco Machinable Ceramic
-  Exposed Kanthal-A Filament

-  Boron Nitride Manifold
-  Confined Ta Filament

## Vapor Transport Deposition: Characteristics

***Vapor Transport Deposition: Characteristics***

**Temperature-time profile**
**BN source, TEC-15 substrate**

***Substrate translation***

**Source Effusion**

**Effusion and Deposition**

**Source Rate**

**Deposition Rate**

**Baseline run:**
**T****sou**** = 800oC**
**T****sub**** = 550oC**
**2.5 cm/min**
**20 Torr He/O****2**

*** ******Source model verified***
*** ******Batch mode utilization = 50%***
*** ******Similar operation from 10-100 Torr***

**Modeled Rate**

**20 torr**

## VT CdTe Film Morphology: Tsub, Thk, Growth Rate

**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***

## Baseline Cell Fabrication

**glass superstrate**
**500 nm SnO****2**** (TEC15)**
**60 nm Ga****2****O****3**
**50 nm CdS**
**5-7 ********m CdTe**
**Cu/Ni contact**

- Post Deposition Processing
-  Treat in CdCl2 vapor at 415oC for 25 min
-  Etch surface: BDH procedure
-  Deposit 15 nm Cu/70 nm Ni contact (area = 0.4 cm2)
-  HT 180oC for 30 min

## Baseline Cell Results (450 cells)

**Voc:  790 - 840 mV**
**Jsc****:  22.5 - 23.9 mA/cm****2**
**FF:       55 - 72%**
**Eff:      10 - 13%**

## Reduced VT CdTe Thickness 

| CdTe
Thk
(m) | Voc

(mV) | Jsc
(mA/
cm2) | FF

(%) | Eff

(%) |
| --- | --- | --- | --- | --- |
| 7 | 842 | 24.1 | 64 | 13.0 |
| 6 | 808 | 23.8 | 69 | 13.3 |
| 5 | 792 | 23.4 | 67 | 12.3 |
| 2 | 780 | 21.5 | 64 | 10.7 |
| 1.4 | 807 | 23.0 | 57 | 10.5 |
| 1.1 | 743 | 22.0 | 52 | 8.5 |
| 0.75 | 723 | 21.0 | 50 | 7.5 |

***Reduced VT CdTe Thickness ***

**For fixed CdCl****2**** treatment, drop in junction quality as thk decreases**

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

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

**CdTe deposited on moving substrates: growth rate > 8 ********m/min**

## Reduced VT CdTe Thickness

***Reduced VT CdTe Thickness***

**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***

**For sub-micron CdTe, need to separate effects**

| 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 |

## Post-Deposition Treatment: Interface Diffusion

## Increased VT CdTe Deposition Rate 

| CdTe
Rate
(m/min) | Voc

(mV) | Jsc
(mA/
cm2) | FF

(%) | Eff

(%) |
| --- | --- | --- | --- | --- |
| 9 | 842 | 24.1 | 64 | 13.0 |
| 81 | 736 | 24.8 | 53 | 11.0 |

***Increased VT CdTe Deposition Rate ***

**~10X increase in growth rate:**
** ****11% efficiency obtained**
** ****Similar reduction in junction quality as for reduced CdTe thk **

**CdTe deposition rate varied by adjusting source temperature**
**6 ********m thick CdTe**

## Effect of Humidity Prior to CdCl2 HT

- Low performance correlated with post-deposition batch processing:
-  Plates stored in humid air prior to CdCl2 HT → CdTe2O5, CdTeO3 (XRD, XPS)
-  CdCl2 source degradation: CdO formation, sintering reduce pCdCl2

| Condition
prior to vapor CdCl2 | Voc

(mV) | Jsc

(mA/
cm2) | FF

(%) | Eff

(%) |
| --- | --- | --- | --- | --- |
| Vacuum Stored
25oC, 1 week | 812 | 23 | 64 | 12.0 |
| Stored in air 40oC, 16H
(~0.1 g/L) | 
764 | 
22.5 | 
56 | 
9.5 |
| HT in air 400oC, 5 min
(5 g/L) | 
778 | 
22 | 
45 | 
7.5 |

## Shorter, Safer Vapor Chloride Treatment

| Chloride
Used | T CdTe
(oC) | time

(min) | Voc

(mV) | Jsc

(mA/cm2) | FF

(%) | h

(%) |
| --- | --- | --- | --- | --- | --- | --- |
| None | None | None | 570 | 15.9 | 46.9 | 4.2 |
| CdCl2 | 415 | 25 | 801 | 23.0 | 71.7 | 13.2 |
| CdCl2 | 410 | 20 | 808 | 23.8 | 69.2 | 13.3 |
| CdCl2 | 440 | 2 | 634 | 20.4 | 60.2 | 7.8 |
| CdCl2 | 465 | 2 | 755 | 24.2 | 60.2 | 11.0 |
|  |  |  |  |  |  |  |
| ZnCl2 | 400 | 15 | 780 | 23.9 | 59.7 | 11.1 |

**Note: Mean efficiency for CdCl****2**** baseline process = 11.5%**

**Reduced CdCl****2**** treatment time by 10X:**
** ****Lower junction quality**
** ****Compensate by increasing treatment temperature**

## Separating Effects: Chloride Vapor, Cu

| Chloride
Vapor
 | Cu
Thk
(nm) | Voc

(mV) | Jsc
(mA/
cm2) | FF

(%) | Eff

(%) | Comment |
| --- | --- | --- | --- | --- | --- | --- |
| 5 mTorr CdCl2 | 0 | 749 | 23 | 62 | 10.5 | LDX, Roll |
| 5 mTorr CdCl2 | 10 | 791 | 23 | 68 | 12.5 |  |
| 5 mTorr CdCl2 | 15 | 801 | 23 | 71 | 13.0 |  |
| 5 mTorr ZnCl2 | 0 | 656 | 23 | 52 | 7.8 | LDX, Roll |
| 5 mTorr ZnCl2 | 15 | 746 | 23 | 62 | 10.5 |  |

***Separating Effects: Chloride Vapor, Cu***

** ****Cu significantly increases Voc and FF, independent of chloride**

**LDX = light-dark crossover on JV plot**
**Roll = “rollover” in JV characteristic at V > Voc**

## Aniline-Treated Devices

** ****Cu–based back contacts **
** ****Best devices with short times**
** ****30’ light on CdTe, 60’ light on glass**
** ****Voc ********800 mV, FF>65%, Eff. ********12% **
** ****similar Te surfaces generated**

***Aniline-Treated Devices***

**Aniline with light on CdTe (********,********), and on glass (********) and 10s in 0.05% Br****2****/methanol**** ****(********)**** **

**  ****cf. Br****2**** etch, **
** ****poorer devices, increased R****s**
**  ****Longer aniline treatment times**
** ****decreasing performance**
** ****grain boundary etching **
** ****but not as harsh as other ****	****CdTe etches **

## Transparent ZnTe:Cu allows bifacial JV, QE and tandem interconnect
Deposited on Aniline and Br2 etched surfaces
Galvanic method with Cu2SO4; TEA complexing → control [Cu] 
Bifacial JV: light decreases blocking behavior:
Increase in CdTe conductivity?
Reduction of back contact barrier? 
Bifacial QE: 
No QE contribution from back contact
Bulk CdTe response depends on Cu 

**Transparent ZnTe:Cu allows bifacial JV, QE and tandem interconnect**
**Deposited on Aniline and Br****2**** etched surfaces**
**Galvanic method with Cu****2****SO****4****; TEA complexing ****→**** control [Cu]**** **
**Bifacial JV: light decreases blocking behavior:**
**Increase in CdTe conductivity?**
**Reduction of back contact barrier? **
**Bifacial QE: **
**No QE contribution from back contact**
**Bulk CdTe response depends on Cu **

## Summary

**CdTe/CdS Cells with Vapor Transport CdTe**
**CdTe vapor transport deposition demonstrated**
**Source operation well-correlated to source model**
**Deposit yield ~50% and 5% thickness uniformity routinely obtained**
**Baseline ******** > 11% for 5-7 ********m CdTe deposited at 8-10 ********m/min**
****** ****> 13% achieved on thin CdS and low-cost moving substrate**
**High Throughput Processing Tolerance**
****** ****= 11% achieved for:**
**1.4 ********m thick CdTe deposited at 10 ********m/min**
**6 ********m thick CdTe deposited at > 80 ********m/min**
**2 minute CdCl****2**** vapor treatment**
**15 minute ZnCl****2**** vapor treatment**
**Transparent contact**
**No detectable contribution to QE from ZnTe**
**CdTe QE influenced by [Cu]**
**Using to probe effect of device thk on Voc**