# Presentation: 274544

## Steady State 

**Photocapacitance (PHCAP) **
**Study of **
**CdS/CdTe PV Devices**

- R. A. Enzenroth, T. Takamiya, K.L. Barth,
- and W.S. Sampath

- National CdTe R&D Team Meeting
- May 5-6/05 NREL

## Presentation Outline

**Introduction to steady state photocapacitance** (**PHCAP) **
**     ****and experimental details**
**PHCAP** **results for 5 cells: **
**	****	****With varied CdCl****2**** treatment both with and without a **
**      ****	****Cu back contact [1]**
**PHCAP result on one cell with different initial occupancy**
**Refined estimates of optical activation energies taking**
**     ****account of the general form of optical cross sections**
**Preliminary results for DLTS on two no Cu cells**
**Discussion of comparison of PHCAP and DLTS  **
**Conclusions**

## Introduction

**Steady State Photocapacitance (PHCAP) ****[2]**

-  ** ****From clear ****Δ****C can obtain estimate of**
**   ****ionization threshold energies**

-  **Levels above midgap exchange**
**  ****carriers with both bands **

**In general :**

** ****Can obtain a lower bound estimate of N****t **

** ****One possible model of the optical cross **
**  ****section for holes and electrons **

**[3,4,5,6]**

## Experimental

** **_**Cool cell to 77 K**_** so that: thermal emission << optical emission**
** **_**Apply sub band gap monochromatic light**_** on cell to stimulate**
**  ****optical emission **
** ****Vary incident light energy in **_**0.05 eV increments**_** from 0.62 eV to **
**  ****1.46 eV; **_**allow 1 minute to come to steady state**_
** ****Measure capacitance changes (10 kHz, 0.020 V OSC)**
** ****Sign of capacitance change depends on initial trap occupancy**
**  ****and trap type (minority, majority) **
** ****Initial 5 cells with +1V forward bias during cool down: assume**
**  ****n(0) and p(0) = Nt. However, initial occupancy is indeterminate**
**  ****due to uncertainty in carrier capture and thermal emission**
** ****One cell with -0.5 V during cool down => n(0) and p(0) = 0**
** ****Variation in incident flux characterized with InGaAs photodiode**
** ****Normalized PHCAP = (light cap – dark cap) / (dark cap ***
**  ****photodiode current detected ) {energy range from 0.73 eV to 1.38 eV **}

## Steady State Photocapacitance Spectrums

** ****poor treatment increases trap density over non treated sample**
** ****optimum treatment decreases trap density over non treated sample**

**[1]**

-  *+0.4 V during PHCAP [7] ; +1 V bias at room temp and during cool down *

**Variation in CdCl****2**** treatment**

##  Cu contact application decreases defect density over non Cu sample

** ****Cu contact application decreases defect density over non Cu sample**
** ****Unable to quantify Na change in bulk due to Cu by CV measurement **
** ****Clear change in initial dark capacitance** (**depletion width) with Cu **
**  ****application, but magnitude does not match decrease in signal peaks**** **

**Variation in Cu back contact process**

**[1]**

-  *+0.4 V during PHCAP [7] ; +1 V bias at room temp and during cool down *

## New Initial Trap Occupancy

**Condition**

-  By cooling sample to 77K
-   thermal emission << optical emission

_**Capacitance change with optical emission**_

_**Initial trap occupancy **_** ****[5]**

- Example of singly ionized minority level in p-type

-  Majority trap would have opposite delta C

## Identification of trap type

**Identification of trap type**

** ****Cell held at - 0.5 V reverse bias for 1 hr at room temp**
** ****Cooled to 77 K at - 0.5 V bias => n(0) = 0  p(0) =0**
** ****PHCAP measurement at - 0.5 V reverse bias**
** ****Sign of delta C for reverse bias condition indicates electron traps**

- 12745
- cells
- poor
- CdCl2
- treatment

## PHCAP Results

**Due to the general form of optical cross sections steady state PHCAP gives clear **
**ionization thresholds for one type of carrier only (holes in this case), ionization **
**threshold of opposite carrier (electrons) can be masked by maxima in cross section of **
**first type of carrier (hole) [4].**

- * Using Eg = 1.6 eV at 77 K

**Table1: PHCAP  preliminary energy level **
**estimates**

- * Model used as example of optical
-   cross section, levels in study may
-   not fit this model

| Level | Epo(hv)
[eV] | Ec – Et
[eV] |
| --- | --- | --- |
| E1 | 0.69 | 0.91 |
| E2 | 0.91 | 0.69 |
| E3 | 1.05 | 0.55 |
| E4  | 1.22 | 0.38 |
| E5 | 1.35 | 0.25 |

## DLTS Spectrums

-  **Steve Johnston**
**  ****NREL DLTS**
**  ****Optical excitation hv > Eg**
**  ****(quantification has uncertainties,**
**  ****peaks not resolved)**

**Poor Cd chloride treatment**

**Optimum Cd chloride treatment**

-  **No minority peaks in 12745 sample**

-  DLTS: +ΔC is minority peak; -ΔC is majority peak

## Discussion

## Conclusions

** ****Steady state PHCAP has demonstrated good potential as a** **defect **
**    ****characterization method in thin depleted CdTe devices, however**
**    ****more work is needed. **
** ****The CdCl****2**** treatment has an effect on the densities of trap levels **
**    ****in the upper half of the band gap [1]**
**   ****Poor CdCl****2**** treatment increases all observed trap densities **
**    ****over untreated samples [1]**
**   ****Optimum CdCl****2**** treatment decreases all observed trap densities **
**    ****over untreated samples [1, also shown by Duke et al. 8]**
** ****The Cu back contact** **application decreases all trap densities [1]**
** ****Based on initial trap occupancy traps in study identified as **
**    ****minority type  **
** ****Energy estimates for optical ionization thresholds refined => **** **
**    ****5 ionization thresholds for hole emission and trap activation **
**    ****energies quantified**

## References

- 1.“Characterization of deep level states in CdS/CdTe thin-film solar cells
- by Steady State Photocapacitance method”, Toru Takamiya., masters thesis,
- Colorado State University, 2005.
- 2.”Direct observation of the multiplicity for impurity charge states in semiconductors from
- low temperature high-frequency photocapacitance”, C.T. Sah, et al. App. Phys. Lett.,
- (15) 10, 1969, 316 - 318
- 3. “The electrical characterization of semiconductors: majority carriers and electron states”,
- P. Blood and J. W. Orton, Academic Press, 1992, ISBN 0-12-528627-9, Chapt. 13
- 4.”Photocapacitance and defect levels in gallium-doped zinc selenide”, A.A. Qidwai and
- J. Woods, J. Phys. C: Solid State Phys., 16 (1983) 6789-6797
- 5.“Capacitance Transient Spectroscopy”, G. L. Miller, D.V. Lang, and L. C. Kimerling,
- Ann. Rev. Mater. Sci. 1977, 377-448
- 6.“Deep Level Impurities in Semiconductors”,H. G. Grimmeiss, Ann. Rev. Mater. Sci.
- 1977, 341-376
- 7. “Defect levels in electrodeposited n-type CdTe thin films”, S. M. So et al., J. Appl. Phys,
- 61 (6), 1987, 2234-2243
- 8.”Characterization of in-situ thermally evaporated Cds/CdTe thin film solar cells with
- Ni-P back contacts”, S. Duke, et al., J. Cry. Gr., 159, (1996) pp 916-919

- Acknowledgments

-  Toru Takamiya for PHCAP for much of the work in this presentation
-  Steve Johnson of NREL for DLTS results
-  The NREL thin-film partnership program for support