# Presentation: 016809

## High Throughput Processing of Stable CdTe/CdS Solar Cells

**Brian McCandless, Kevin Dobson, Steve Hegedus, Wayne Buchanan, Darshini Desai, Sergey Rykov, 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**

## Objectives

**Evaluate processing limits of CdTe/CdS thin-film solar cells**
**Growth rate target: equivalent of 2 ********m over 1 meter length in 1 min**
**Low-cost, commercially available substrates**
**Post deposition processing**
**Reduce halide treatment time, find safer alternative**
**Simplify contact preparation**
**Develop processing diagnostics**
**Quantify junction operation**
**Determine and overcome limits to performance**
**Voc > 900 mV**
**FF > 80%**

## Approach

**Vapor transport CdTe deposition (VTD)**
**T from 400****o****C to 600****o****C, adjustable translation speed, 10 x 10 cm samples**
**d(CdS) < 100 nm by chemical surface deposition**
**LOF TEC-15 TCO/SL glass**
**Vapor chloride post-deposition treatments**
**Diffusion reactor**
**New vapor transport reactor**
**Reduce processing time by increasing reaction temperature**
**Evaluate alternatives to CdCl****2**** ****such as**** ZnCl****2**
**Evaluate surface chemistry**
**Aniline, NaNO****2**** + H****3****PO****4**
**Oxidation of CdTe**
**Stress-induced changes at back contact (GIXRD, wetting angle)**
**Develop tools for probing junction operation**
**Bifacial device analysis using transparent contact**
**Electric field gradient microscopy of cross-sectioned devices**

## Vapor Transport Deposition (VTD): System

## Vapor Transport Deposition: Source

## Vapor Transport Deposition: Characteristics

***Vapor Transport Deposition: Characteristics***

**Source and substrate**
**Temperature-time profile**

***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**

## Vapor Transport Deposition: Film Properties

***Vapor Transport Deposition: Film Properties***

*********2****** vs E***

***T/(1-R) vs*********

***Thickness uniformity:***
***   ******± 5% direction of travel***
***   ******± 10% along each edge***

## Vapor Chloride Treatment System

**Saturation Pressures**

**Time to Reach Saturation**

**CdCl****2**
**400°C**
**Ar**

| T (oC) | psat (Torr)
CdCl2 | psat (Torr)
ZnCl2 |
| --- | --- | --- |
| 200 | 3.9 E-9 | 1.4 E-5 |
| 250 | 3.4 E-7 | 4.7 E-3 |
| 300 | 1.3 E-5 | 8.4 E-3 |
| 350 | 2.7 E-4 | 0.093 |
| 400 | 3.6 E-3 | 0.70 |
| 450 | 0.033 | 4.0 |

## Vapor Chloride Treatment System

**Reactor and Tower**

**CdCl****2**** Partial Pressure versus**
** ****Position in Packed Bed**

**400°C**
**Ar**

***Fixed bed depth***
***= 0.2 cm***

## Vapor Chloride Treatment Best Cell Device Results

| Chloride
Used | T CdTe
(oC) | time

(min) | Voc

(mV) | Jsc

(mA/cm2) | FF

(%) | 

(%) |
| --- | --- | --- | --- | --- | --- | --- |
| None | None | None | 570 | 15.9 | 46.9 | 4.2 |
|  |  |  |  |  |  |  |
| 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 |

***Mean efficiency for baseline process = 11.5%***

**CdTe deposited by VTD: 20 Torr, 2.5 cm/min, growth rate 10 ********m/min**

## Etching of CdTe: HNO3:H3PO4 (NP)

**Used to produce Te layer**
**autocatalytic reaction**
**significant conc. of autocatalyst required for reasonable reaction rate **
**>20 sec delay in bubbling on CdTe **
**autocatalyst is HNO****3****-related, though not identified**
**can catalyst be added to solution to increase etch rate**
**shorter immersion times?**
**Addition of NaNO****2 ****to replace HNO****3**
**decomposes in acid to give similar products (NO****(g)****, NO****2****(g)****, NO****+****, N****2****O****3****)**
**NaNO****2****-charged solution: bubbling on CdTe for ~1 sec immersion **
**may allow development of etches which avoid the use of corrosive HNO****3**

## 
Photo-treatment of CdTe in aniline to replace NP etch
Successful aniline treatment → reproducible light gray, crystalline Te surface
Achieved devices with  = 11.5% 

Critical control parameters for successful treatment:
pH, [aniline], [Cl-],  [O2]
light intensity

**Photo-treatment of CdTe in aniline to replace NP etch**
**Successful aniline treatment ****→**** reproducible light gray, crystalline Te surface**
**Achieved devices with ******** = 11.5% **
**Critical control parameters for successful treatment:**
**pH, [aniline], [Cl****-****],  [O****2****]**
**light intensity**

**VASE before and after aniline treatment**

***Etching of CdTe: Aniline Treatment***

**GIXRD following 60 min aniline treatment**

**No etch**

**60 min. etch**

**CdTe**
** ****|      |**

**Te**

## CdTe Device Stability: Humidity Stress

| Humidity Control Agent | Drierite | Sat. LiCl | Sat. NaBr | Sat. KBr | H2O | Room air (control,
~25C) |
| --- | --- | --- | --- | --- | --- | --- |
| Rel. Humid% | 0.0017 | 10.5 | 51.4 | 79 | 100 | 30 |
| Psat (Torr) | 0.006 | 37 | 182 | 280 | 355 |  |
| [H2O] (g/L) | 0.005 mg/L | 0.03 | 0.15 | 0.23 | 0.29 |  0.025 |

**What happens to CdTe device performance and chemistry in presence of water vapor?**
** ****Monitor chemical and electrical changes in devices stressed at varying humidities**
**Humidity controlled with saturated salt solutions, Drierite pieces**
**Stress at 80********C ******** 5********C for 500 hrs.**
**In dark and under illumination, ********0.5 suns**
**Stressed IEC (Au/Cu/Te contacts)**

**Humidity conditions for each control agent at 80********C**

## Humid Stress (cont)

***Humid Stress (cont)***

**Visual changes of contacts**
**<50% RH dark ****→ ****cloudy, 0% RH light ****→ green**
**>50% RH dark ****→**** no change, >50% RH light ****→ no change**
**GIXRD characterization **
**always detect Cu****2-x****Te**
**TeO****2**** ****formed**** ****in contacts at low humidity in dark **
**No change at high humidity ******** amorphous hydroxides/hydrates?**
**0% RH light ****→ ****green CuTeO****3**** formed**** ****at expense of Cu****2-x****Te**

** ****J-V analysis**
** ****All devices show good stability **
** ****no rollover, R****oc**** stable**
** ****some V****oc**** decrease, 5-60 mV**
** ****Worse at high humidity, light**

**GIXRD of control,  0% humidity dark+light stress**

## Contact Wetting Angle: Reactivity of CdTe with Air

***M. Angelo, Senior Thesis, University of Delaware Chemical Engineering Department***

## Surface Energy: Reactivity of CdTe with Air

** ****Contact wetting angle of different fluids ****→ polar and dispersive energies**
**(harmonic mean method applied to Young’s equation)**

** ****As-deposited CdTe seeks to satisfy dangling bonds through reaction with ambient**
** ****Therefore ambient [H****2****O] and [O****2****] is critical during surface reconstruction**
** ****Other applications:**
** ********→ ****x-tal transformation **** in ITO, CdS and Cd****1-x****Zn****x****S films**
** ****formation of reaction products on CdCl****2**** treated CdTe**

| Condition | GIXRD | Ep
(dyn/cm) | Ed
(dyn/cm) | Es
(dyn/cm) |
| --- | --- | --- | --- | --- |
| As Deposited | CdTe | 47 | 28 | 75 |
| 2 mo <30% RH | CdTe + CdTeO3 | 39 | 26 | 65 |
| 2 mo >60% RH | CdTe + CdTe2O5 | 40 | 23 | 63 |

## Around 300K Voc is linear in -T, log in Intensity (I)
New data shows Voc saturates with T below 200K; observed in 3 types of CdTe
recombination and free carriers “frozen” out; Voc upper limit ~1 V (unstressed)
fundamental device property related to band structure, contact barriers, 
useful to identify impact of contacts and potential barriers

**Around 300K Voc is linear in -T, log in Intensity (I)**
**New data shows Voc saturates with T below 200K; observed in 3 types of CdTe**
**recombination and free carriers “frozen” out; Voc upper limit ~1 V (unstressed)**
**fundamental device property related to band structure, contact barriers, ******
**useful to identify impact of contacts and potential barriers**

## Transparent ZnTe:Cu allows bifacial JV, QE and tandem interconnect
Galvanic method with Cu2SO4; TEA complexing → Cu may be more controllable 
Bifacial JV: rear contact barrier and/or high R decreases with light
Bifacial QE: no QE due to back contact; bulk CdTe response depends on Cu 

**Transparent ZnTe:Cu allows bifacial JV, QE and tandem interconnect**
**Galvanic method with Cu****2****SO****4****; TEA complexing ****→**** Cu may be more controllable **
**Bifacial JV: rear contact barrier and/or high R decreases with light**
**Bifacial QE: no QE due to back contact; bulk CdTe response depends on Cu **

## Junction Operation: Electric Field Gradient

***0 V bias, alignment laser (632 nm) light bias***

- Morphology

- EFG

***CdS|CdTe***

***CdS|CdTe***

## Junction Operation: Electric Field Gradient

***0 V bias, alignment laser (632 nm) light bias***
***Can analyze microscopic local potential variations parallel to junction***

## Summary

**High throughput processing**
**CdTe vapor transport deposition demonstrated**
**Source operation well-correlated to source model**
**Deposit yield ~50% and 5% thickness uniformity routinely obtained**
**Cells with ******** > 13% achieved on thin CdS and low-cost moving substrate**
**Processing alternatives**
**Significantly reduced CdCl****2**** vapor treatment time**
**Utility of ZnCl****2**** vapor treatment demonstrated for CdTe/CdS**
**Use of aniline, or modified phosphoric etch shown**
**VTD cells with Cu****2****Te/Cu/Au contact are stable in humid ambient at 80****o****C **
**Processing and cell  diagnostic tools**
**Contact wetting angle of CdTe surface sensitive to oxidation**
**Electric field gradient microscopy of cross sectioned cells demonstrated**
**Bifacial device analysis with transparent contacts shows effect of Cu**
**Understanding device performance limits**
**Low T Voc measurements show 1V limit in CdTe/CdS**