# Presentation: 334305

## CHOOSING DEFECT PROPERTIES FOR 

- NREL CdTe Team Meeting May 5-6, 2005

**Alan Fahrenbruch**
**Colorado State University, Dept. of Physics,**
** ****Fort Collins, CO 83052**

- The author acknowledges the support by NREL through Colorado State University, and the encouragement of Jim Sites and the CdTe Team members.  Modeling results obtained using AMPS-1D (Version 1,0,0,1), written by S. Fonash, Pennsylvania State Univ., with EPRI support.

**CHOOSING DEFECT PROPERTIES FOR **
**DEVICE MODELING**

**Notes:**
I appreciate the support by NREL through CSU.

## DEFECT PROPERTIES             CELL EFFICIENCY?

- NREL CdTe Team Meeting May 5-6, 2005

**SOLUTION REQUIRES DEFECTS’: ****	**
**	****	****ENERGY LEVEL**
**	****	****NUMBER DENSITY**
**	****	****e’ AND h****o ****CAPTURE CROSS SECTIONS**
**	****	****CHARGE STATE**

**DEFECT PROPERTIES             CELL EFFICIENCY?**

**SRH**
**Equation**

**Poisson’s**
**Equation**

**Continuity**
**Equation**

**Recombination**
**Rate**

**Notes:**
We want to establish a more direct connection between defect properties and cell efficiency.

The recombination loss in the bulk CdTe and at the interfaces has a major role in determining the cell efficiency.  AMPS simulations suggest that the major loss is in the bulk CdTe near the junction at voltages of Vmax or below.

Calculating  the recombination loss requires simultaneous solution of these equations and...
it also requires information about the defects such as Ea.....

## MEASURING PROPERTIES OF DEFECT LEVELS

- NREL CdTe Team Meeting May 5-6, 2005

**MEASURING PROPERTIES OF DEFECT LEVELS**
**	****P****HOTOCONDUCTIVITY**
**	****T****HERMALLY ****S****TIMULATED ****C****URRENT**
**	****D****EEP LEVEL ****T****RANSIENT ****S****PECTROSCOPY**
**	****P****HOTO-****I****NDUCED ****C****APACITANCE ****S****PECTROSCOPY**
**	****P****HOTO-****I****NDUCED ****T****RANSIENT ****S****PECTROSCOPY**
**	****P****HOTO-****L****UMINESCENCE**
**	****D****RIVE ****L****EVEL ****C****APACITANCE ****P****ROFILING**
**	****SUB-BAND-GAP OPTICAL ABSORPTION**
**	****POSITRON LIFETIME/DOPPLER BROADENING**
**	****E****LECTRON ****P****ARAMAGNETIC ****R****ESONANCE**
**	****.........****and more**

**Notes:**
One of the parts of establishing the connection is characterization of the defects.  
Many methods exist and it seems their number is increasing each year. 

In addition, there are more indirect methods, e.g., measuring minority carrier diffusion length by SR or EBIC.

## PROBLEM AREAS:

- NREL CdTe Team Meeting May 5-6, 2005

**PROBLEM AREAS:**
**ATTRIBUTION IS AMBIGUOUS**
**  ****e.g., THE ENERGY LEVELS OF  V****Cd****-****  HAVE BEEN REPORTED AS:  0.05, 0.13, 0.21, and 0.40 eV**
**INDIVIDUALITY OF CdTe SAMPLES**
**RELATIVELY LITTLE WORK ON PX CdTe**
**INTERPRETATION OF EXPERIMENTS IS DIFFICULT**

- R.E. Kemer, M.C. Arikan, J.C. Abele, and J.S. Blakemore, J. Appl. Phys. 62, 2431 and 2438  (1987)
- “Laplace-transform deep-level spectroscopy**,” **L.** **Dobaczewski et al., J. Appl. Phys., 4689 (2004)

## NREL CdTe Team Meeting May 5-6, 2005

**Notes:**
Perhaps the most powerful and orderly method of obtaining defect energy levels is to calculate them. 
Then, at least you know the identity of the defects. 
These are some results by Wei et al. at NREL
To the best of my knowledge, these methods don’t give information about the capture cross sections, however.

## NREL CdTe Team Meeting May 5-6, 2005

**Notes:**
Most of the experimental work has been done on SX, and a large part of that is on compensated, insulating detector material.
Castaldini has several excellent papers on SX CdTe, some of which compare various measurement methods.  

## NREL CdTe Team Meeting May 5-6, 2005

**Notes:**
Another method of choice is to ask a trusted colleague---Scotty Gilmore chose this set for his thesis work.

## NREL CdTe Team Meeting May 5-6, 2005

**Notes:**
Putting the sets together produces a rather daunting array, with some reasonable agreements, but also some outliers.  In addition, this listing is far from complete.

Except for a few levels, the theoretical calculations correspond quite well with experimental values in SX

Some results on PX thin film solar cell material  by Balcioglu and Gilmore are shown here in red.  
The PX CdTe has several of its own of energy levels which don’t correspond with any of the SX levels and may arise from grain boundaries.  Fortunately, the 0.45 eV acceptor, which has been seen by a number of workers, is present in both PX and SX, although it has no theoretical counterpart.

## NREL CdTe Team Meeting May 5-6, 2005

## MODELING CONSIDERATIONS

- NREL CdTe Team Meeting May 5-6, 2005

**MODELING CONSIDERATIONS**
**Three dominant defect levels – maximum for modeling**
**Beyond that it becomes too complex, unless other simplifications can be made.**
**Focus on more general defect properties in context of devices**

**Notes:**
AMPS models are understandable with a maximum of about 3 dominant defect levels, 
Beyond that it becomes too complex, at least for me.  

Focusing on the function of more general defect properties in the context of a solar cell may be helpful.

## NREL CdTe Team Meeting May 5-6, 2005

**Notes:**
An example of that is shown here.  Once a model is set up, a Voc-ff plot can be used to determine the effect of each defect property by itself.

For example, increasing the back contact barrier height reduces Voc substantially, by using up some of the junction potential.
This is a three defect model, but now adding 1e17 shallow donors and acceptors in equal numbers has a relatively small effect.  So they might be treated more like a perturbation.

In this way one could at least put bounds on the properties of the defect groups.

## -	DETERMINATION OF THE DEFECT PROPERTIES USING MODELING TOGETHER WITH EXPERIMENTS (J-V-T, C-V, C-f, SR(V)) MIGHT BE POSSIBLE.
-	WILL ONLY WORK IF ALL MEASUREMENTS ARE MADE ON ONE SET OF DEVICES.
-	MANY OF THE MEASUREMENT METHODS LISTED EARLY IN THE TALK ASSUME ONE-DIMENSIONAL MODELS.

**-****	****DETERMINATION OF THE DEFECT PROPERTIES USING MODELING TOGETHER WITH EXPERIMENTS (J-V-T, C-V, C-f, SR(V)) MIGHT BE POSSIBLE.**
**-****	****WILL ONLY WORK IF ALL MEASUREMENTS ARE MADE ON ONE SET OF DEVICES.**
**-****	****MANY OF THE MEASUREMENT METHODS LISTED EARLY IN THE TALK ASSUME ONE-DIMENSIONAL MODELS.**

_**IF**_** THE CELL CAN BE REPRESENTED BY A ONE-DIMENSIONAL MODEL:**

**Notes:**
If the cell can be represented by a one dimensional model, it may be possible to actually determine the defect properties using a combination of modeling and experiments such as current-voltage-temperature, ....., C-V (light and dark), SR(V, with and without bias light).  

However, this can only work if all the measurements are made on one device.

It’s worthwhile pointing out that most of the measurement  methods listed early in the talk assume one-dimensional models.

## CONCLUSIONS

**CONCLUSIONS**

**DETERMINING DEFECT PROPERTIES BY MODELING EXTENSIVE EXPERIMENTAL DATA ON ONE DEVICE MIGHT BE POSSIBLE.**
**IN EFFECT, THAT WOULD BE REPLACING E THDLTS WITH ANOTHER SET OF MEASUREMENT TOOLS, MORE SUITED TO SOLAR CELLS.**
**THE ASSUMPTION THAT THE CELL CAN BE ADEQUATELY MODELED IN ONE DIMENSION FOR THIS PURPOSE MUST BE QUESTIONED.**

**Notes:**
DETERMINING DEFECT PROPERTIES BY MODELING EXTENSIVE EXPERIMENTAL DATA ON ONE DEVICE MIGHT BE POSSIBLE.

IN EFFECT, THAT WOULD BE REPLACING THE DLTS WITH ANOTHER SET OF MEASUREMENT TOOLS, PERHAPS MORE SUITED TO SOLAR CELLS.

OF COURSE, THE ASSUMPTION THAT THE CELL CAN BE ADEQUATELY MODELED IN ONE DIMENSION FOR THIS PURPOSE CAN BE QUESTIONED.

Thank you for your attention.