# Presentation: 260415

## Characterizing Electronic States that Control the Performance of CdTe Solar Cells

- Fred H. Seymour
- December 7, 2005

**Notes:**
Our presentation is on the oxidation of silicon.

## Contents

- Introduction and research goals
- Selected research results
- Admittance spectroscopy (AS) technique
- Computer simulations of AS
- Identification of deep electronic states
- Preliminary defect correlation to performance
- Conclusions

- Characterizing Electronic States that Control the Performance of CdTe Solar Cells

- December 7, 2005

## Solar Energy

- Earth irradiated with ~125,000 TW of solar energy
- 1 TW = 1012 W
- ~20 MW per person
- Average total global industrial energy consumption ~13 TW
- ~2 kW per person

- Characterizing Electronic States that Control the Performance of CdTe Solar Cells

- December 7, 2005

## Exponential Growth

- MW

- Elastic demand for PV
- Decreasing costs
- Economies of scale
- Increase energy conversion efficiency

- Characterizing Electronic States that Control the Performance of CdTe Solar Cells

- December 7, 2005

## Colorado School of Mines

- PV program since early 1990s
- Cell characterization, CdTe cell growth
- ~15 Ph.D.s/Collaborations with NREL
- PV ties into CSM 10 yr plan for all 4 focus areas
- Development of Earth’s resources
- Energy
- Advanced Materials
- Environment
- Training future engineers

- Characterizing Electronic States that Control the Performance of CdTe Solar Cells

- December 7, 2005

## Polycrystalline CdTe thin film solar cells

- Low cost – among simplest to manufacture
- Successful commercial production, First Solar scaling up to 75 MW/year capacity
- Production conversion efficiencies of ~10% leave plenty of room for improvement
- Must better understand electronic structures to optimize cell performance

- Characterizing Electronic States that Control the Performance of CdTe Solar Cells

- December 7, 2005

## CdTe Solar Cell Structure

- 3) HRT

- 5) CdTe  absorber (_**3 to 8 **__**μ**__**m**_)

- 2) TCO ( ~0.05 μm) (- Anode)

- 4) CdS window ( ~0.1 μm)

- 1) Glass superstrate (1000 μm)

- 6) Anneal with _**CdCl**__**2**_

- 7) Etch with NP or BrM

- 9) Back contact (+Cathode)

- Light

- 8) Anneal with  _**Copper**_

- Characterizing Electronic States that Control the Performance of CdTe Solar Cells

- December 7, 2005

## PN Junction

- External load for power extraction creates voltage potential
- Decreases band bending and depletion width
- Leads to recombination and current loss across PN junction

*Voltage potential, V = IR         Power extracted P=V*I*

*Photo generated current*

*Recombination*

- V = (EQfn – EQfp)/q

- Characterizing Electronic States that Control the Performance of CdTe Solar Cells

- December 7, 2005

## Deep Electronic States as  Recombination Centers 

- Semiconductor band gap
- Defects,  impurities, and GB’s cause allowed DES
- These states capture and emit electrons and holes from CB and VB
- Shockley-Read-Hall recombination degrades solar cell performance

- Characterizing Electronic States that Control the Performance of CdTe Solar Cells

- December 7, 2005

## Research 

- Goals
- Detect, identify, and characterize deep electronic states (DES) in polycrystalline CdTe solar cells
- Correlate DES to cell performance
- Recombination – degrades performance
- Doping – enhances performance
- Presentation of Selected Results
- Admittance Spectroscopy (AS) technique
- SCAPS simulations – variable cell thickness
- Experimental: yes/no Cu and yes/no CdCl2
- Effects of cell stressing on DES
- JSC performance versus T

- Characterizing Electronic States that Control the Performance of CdTe Solar Cells

- December 7, 2005

## Admittance Spectroscopytechnique for characterizing deep electronic states (DES) 

- Characterizing Electronic States that Control the Performance of CdTe Solar Cells

- December 7, 2005

## DES Emission and Capture Rates 

- Assumes no degeneracy and near equilibrium conditions
- Capture rate depends exponentially on Ef and T
- Emission rate depends exponentially on Et and T
- If Et = Ef, emission rate = capture rate, DES half full

- Characterizing Electronic States that Control the Performance of CdTe Solar Cells

- December 7, 2005

## Band Diagram with DES

- E valence

- E conduction

- E DES

- E Fermi

- E acceptor

- Depletion Region

- One Sided PN Junction

- P type

*At equilibrium DES*
*are charged just up*
*to the fermi level*

- Characterizing Electronic States that Control the Performance of CdTe Solar Cells

- December 7, 2005

## Band Diagram with Small AC Signal 

- E valence

- E conduction

- E DES

- Oscillating Fermi level

- E acceptor

- Depletion Region

- One Sided PN Junction

- P type

- Band Diagram with Small AC Signal

*DES fill/empty in response to oscillating Fermi level *

*Small AC signal*
*Oscillates the Fermi level*

- Characterizing Electronic States that Control the Performance of CdTe Solar Cells

- December 7, 2005

## Determining Characteristic Frequency 

- If AC freq < capture/emission rate, DES charge near  Fermi level crossing point follows the oscillation.
- If AC freq > capture/emission rate, DES charge does not follow the oscillation.
- Measure over a range of AC frequencies to determine capture/emission rate characteristic frequency
- Measure over range of T to determine DES      as a function of T

- Characterizing Electronic States that Control the Performance of CdTe Solar Cells

- December 7, 2005

## Capacitance vs Frequency

- Decreasing Cp with frequency

- dCp/dln(ω) minima indicate ωt

- CdTe Solar Cell from NREL

- Characterizing Electronic States that Control the Performance of CdTe Solar Cells

- December 7, 2005

## Capacitance vs Temperature

- 0

- 2

- 4

- 6

- 8

- Temperature ( C )

- Cp (nF/cm2)

*1MHz line*

*Each line is at a fixed AC signal frequency *

*1kHz line*

*Decreasing C**p** with T indicates steep DES concentration gradient *

*Majority carrier DES*
*∆**Cp indicates trap concentration*

- Characterizing Electronic States that Control the Performance of CdTe Solar Cells

- December 7, 2005

## SCAPS Simulation

- SCAPS software models 1D solar cell based on Poisson’s equation, continuity equations, and boundary conditions
- Examples illustrate
- Cell thickness impact on capacitance amplitude (8μm vs 3μm)
- Fermi level pinning from high concentration defects distorts measured Ea values
- Concentration gradient can cause decreasing capacitance with increasing temperature.

- Characterizing Electronic States that Control the Performance of CdTe Solar Cells

- December 7, 2005

## SCAPS Simulated CdTe Solar Cell, -170oC

- SCAPS Simulated CdTe Solar Cell, -170oC

- Distance from back contact *μ**m*

- eV

- eV

- Lateral extent of Fermi level (blue line) pinning by trap level (red/green line) is greater with thick CdTe layer

- Cell A
- 8μm CdTe layer

- Cell B
- 3μm CdTe layer

- Simulated DES, (Ea=0.15 eV, σa=2 X 10-14 cm2 , Nt=3 X 1014 cm-3)
- Negligible shallow level acceptor concentration Na=1012 cm-3

- EC

- EV

- Ef

- Characterizing Electronic States that Control the Performance of CdTe Solar Cells

- December 7, 2005

## SCAPS Simulated CdTe Solar Cell, -170oC

- eV   0.15eV DES Relative to Fermi

- Distance from back contact μm

- Fermi
- 8 μm cell
- 3 μm cell

- SCAPS Simulated CdTe Solar Cell, -170oC

- 1

- 102

- 104

- 106

- cp  8μm cell
- ep  8μm cell
- cp  3μm cell
- ep  3μm cell

- s-1   Hole Capture/Emission Rates

- Characterizing Electronic States that Control the Performance of CdTe Solar Cells

- December 7, 2005

## SCAPS Simulated CdTe Solar Cell, -170oC

- Distance from back contact μm

- 1012

- 1013

- 1014

- 1015

*n**t*  8μm cell
*n**t*  3μm cell

- cm-3   Ionized acceptor DES Density *n**t*

- SCAPS Simulated CdTe Solar Cell, -170oC

- 1010

- p  8μm cell
- p  3μm cell

- cm-3  Free Carrier Density (holes)

- 1011

- 1012

- 1013

- Characterizing Electronic States that Control the Performance of CdTe Solar Cells

- December 7, 2005

## SCAPS Simulated CdTe Solar Cell, -170oC

- 102

- 103

- 104

- 105

- 106

- DES Characteristic Frequency *f**t*

*f**t*  8μm cell
*f**t*  3μm cell

- 1017

- 1018

- 1019

- 8μm cell
- 3μm cell

- Capture/emission rate per unit volume

- Distance from back contact μm

- SCAPS Simulated CdTe Solar Cell, -170oC

- s-1 cm-3

- s-1

- Characterizing Electronic States that Control the Performance of CdTe Solar Cells

- December 7, 2005

## SCAPS Simulated CdTe Solar Cell, -170oC

- 0

- 1

- 2

- 3

- nF cm-2              Capacitance

- Cp 8μm cell
- Cp 3μm cell

- nF cm-2              dCp/dln(ω)

- 0.0

- -0.5

- -0.4

- -0.3

- -0.2

- -0.1

-   0.1

- 8μm cell
- 3μm cell

- Simulated LCR meter frequency (Hz)

*f**t*

*f**t*

- SCAPS Simulated CdTe Solar Cell, -170oC

- Non-exponential behavior

- Characterizing Electronic States that Control the Performance of CdTe Solar Cells

- December 7, 2005

## SCAPS Simulated CdTe Solar Cell

- 0.1

- 1.0

- 10.0

- 100.0

- 1000.0

- 1/τT2 (s-1 K-2)     Arrhenius Regression,
-                            Simulated DES, (Ea=0.15 eV, σa=2 X 10-14 cm2 , Nt=3 X 1014 cm-3)

- 8 μm cell     Ea=0.10 eV
-                     σa=2 X 10-15 cm2
-                                β=0.45

- 3 μm cell     Ea=0.15 eV
-                     σa=2 X 10-14 cm2
-                                β=0.94

- -170 oC

- Characterizing Electronic States that Control the Performance of CdTe Solar Cells

- December 7, 2005

## SCAPS Simulated CdTe Solar Cell

- Fermi level pinning can cause AS signal intensity dependency on cell thickness
- Anything that impacts Fermi/trap proximity impacts signal intensity
- Back contact Schottky barrier height
- Shallow acceptor concentration
- Because ωt depends on Et and Ef, band bending can distort Ea and σa.
- Thus, ranges of Ea values can represent the same discrete energy defect level.

- Characterizing Electronic States that Control the Performance of CdTe Solar Cells

- December 7, 2005

## Decreasing Cp with increasing T

- Three criteria for decreasing Cp with increasing T
- 	High concentration DES (1014 cm-3)
- 	Back contact barrier (0.5eV)
- 	Concentration gradient near back contact (1019 cm-3 to 1014 cm-3)

- Characterizing Electronic States that Control the Performance of CdTe Solar Cells

- December 7, 2005

## Decreasing Cp with increasing T

- -190

- -170

- -150

- -130

- -110

- -90

- 0

- 1

- 2

- 3

- 4

- Cp (nF cm-2)

- Temperature (oC)

- 1 MHz

- 1 kHz

- Decreasing *C**p* with increasing *T*

- Characterizing Electronic States that Control the Performance of CdTe Solar Cells

- December 7, 2005

## Identification of DES

- Characterizing Electronic States that Control the Performance of CdTe Solar Cells

- December 7, 2005

## NREL cells Cu/CdCl2 study

- NREL cells Cu/CdCl2 study

- Characterizing Electronic States that Control the Performance of CdTe Solar Cells

- December 7, 2005

## H1

***NREL CdTe cell***
***Yes Cu, Yes CdCl******2***
***Before stress***

***NREL CdTe cell***
***No Cu, Yes CdCl******2***
***Before stress***

***NREL CdTe cell***
***Yes Cu, No CdCl******2***
***Before stress***

***NREL CdTe cell***
***No Cu, No CdCl******2***
***Before stress***

- Temperature (Celsius)

- Temperature (Celsius)

- H2

- H2

- H3

- H3

*Top line 1KHz, Bottom line 1MHz*

*Top line 1KHz, Bottom line 1MHz*

*Top line 1KHz, Bottom line 1MHz*

*Top line 1KHz, Bottom line 1MHz*

- Characterizing Electronic States that Control the Performance of CdTe Solar Cells

- December 7, 2005

## Arrhenius Plot

- 10-2

- 10-1

- 100

- 101

- 102

- 103

- 1/(τ T2)

- H1
- Ea~0.13eV

- H2
- Ea~0.30eV

- H3
- Ea~0.47eV

- Characterizing Electronic States that Control the Performance of CdTe Solar Cells

- December 7, 2005

## 	H1 Signature

- H1 attributed to       and A-center
- Ea=0.13eV consistent with reported values
- Higher concentration with addition of CdCl2
- Lower concentration with addition of Cu because of           displacing
- NREL cells had a nitric phosphoric acid (NP) etch that creates a tellurium rich layer at the back contact and a        concentration gradient.

- Characterizing Electronic States that Control the Performance of CdTe Solar Cells

- December 7, 2005

## 	Cells from IEC

- 	Cells from IEC

- H1

- H1

- H2

- H3

- H3

- H2

- IEC (Institute for Energy Conversion) cells had a bromine methanol etch that has less cadmium leaching and creates fewer VCd.
- No VCd gradient (no decreasing Cp with increasing temperature)
- H1 not detected in non-CdCl2 treated cells.

- Characterizing Electronic States that Control the Performance of CdTe Solar Cells

- December 7, 2005

## 	H2, H3 Signature

- H2 Attributed to
- Ea=0.30eV consistent with reported values
- Only detected in Cu treated cells
- H3 possibly      or
- Only detected with non-Cu treated cells
- Assignment is tentative

- Characterizing Electronic States that Control the Performance of CdTe Solar Cells

- December 7, 2005

## 	NREL CELL Yes Cu, Yes CdCl2Stressing

- 	NREL CELL Yes Cu, Yes CdCl2Stressing

- VCd concentration decreases
- CuCd concentration increases
- VCd gradient essentially not detectable

- Characterizing Electronic States that Control the Performance of CdTe Solar Cells

- December 7, 2005

## correlation to

- Characterizing Electronic States that Control the Performance of CdTe Solar Cells

- December 7, 2005

## NREL CELL J-V-T Analysis

-    yes-Cu/yes-CdCl2
-    20oC to   -40oC
-   -50oC to -110oC
- -120oC to -180oC

- Characterizing Electronic States that Control the Performance of CdTe Solar Cells

- December 7, 2005

## Jsc versus T for NREL cells

- Sharp carrier transport freeze out for CdCl2 cells
-   Corresponds to Fermi energy Ef=0.16eV
- Indicates that A-center                      impacts Jsc current collection

- JSC

- Characterizing Electronic States that Control the Performance of CdTe Solar Cells

- December 7, 2005

## Jsc versus T for IEC cells

- Jsc versus T for IEC cells

- Sharp carrier transport freeze out for CdCl2 cells
-   Corresponds to Fermi energy Ef=0.14eV
- Indicates that A-center                      impacts Jsc current collection

- Characterizing Electronic States that Control the Performance of CdTe Solar Cells

- December 7, 2005

## Conclusions

- Applied admittance spectroscopy to detect, identify, and characterize DES in CdTe solar cells
- Identified         , A-center                 and         defects
- Observed       concentration gradient in NP etched cells (NREL) but not in BrM etched cells (IEC)
- Detected migration/mutation of DES with stressing
- SCAPS simulations have shown how high concentration DES can distort AS results
- Indications that                  enhances current collection in CdCl2 treated cells.

- Characterizing Electronic States that Control the Performance of CdTe Solar Cells

- December 7, 2005

## Future Work

- Determine if                   really is the dopant in CdCl2 treated cells.  Ramifications for cell optimization.
- Continue characterizing cells from different sources/process treatments to better understand their DES
- Design and develop higher efficiency cell growth processes based on results from admittance spectroscopy and other characterization techniques

- Characterizing Electronic States that Control the Performance of CdTe Solar Cells

- December 7, 2005

## Acknowledgements

- Thesis Advisors - Tim Ohno, Victor Kaydanov
- Dave Albin for supplying NREL cells and stressing, and Brian McCandless for supplying IEC cells
- CSM/NREL/CdTe National Team for numerous discussions
- Dave Albin, Joe Beach, Reuben Collins, Sam Demtsu, Al Enzenroth, Alan Fahrenbruch, Scott Feldman, Steve Hegedus, Steve Johnston, Tim Gessert, Brian McCandless, Dennis Readey, Jim Sites, Xuanzhi Wu
- NREL/DOE sponsors for funding this research
*NREL subcontract ADJ-2-30630-05*

- Characterizing Electronic States that Control the Performance of CdTe Solar Cells

- December 7, 2005

## Questions ?

- Characterizing Electronic States that Control the Performance of CdTe Solar Cells

- December 7, 2005