# Presentation: 064597

## Alternative Junctions Subteam

- March 8, 2005

- Report by Jim Sites

- Participants:
-     Colorado State: Pam Johnson, Alex Pudov, Markus Gloeckler, Ana Kanevce, Jim Sites
-     NREL: Kannan Ramanathan, Miguel Contreras, Falah Hasoon, Hamda Al-Thani, Raghu Bhattacharya
-     SSI: Dale Tarrant, Florida: Don Morel; ISET (Vijay Kapur); EPV (Alan Delahoy)
-     International Collaborators: Tokio Nakada (Aoyama Gakuin), Hans Schock (Stuttgart/HMI)

- CIS Team Meeting

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## Subteam Goal (October 2001)

- March 8, 2005

- Alternative Junctions Subteam

- To fabricate several alternative junctions on CIGS substrates and quantitatively compare the resulting diode structures.  CBD CdS will be considered the standard for comparison purposes.

- Outline of Report

- Why do we want to look at alternatives?
- How does one choose specific alternatives?
- How well have they worked?

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## Possible Reasons for CdS Alternatives

- March 8, 2005

- Alternative Junction Subteam

- Basic Performance.
- 	Higher current from improved blue photon collection (generally realized), while maintaining equal voltage (generally not realized).

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## Best CIGS Performance

- March 8, 2005

- Alternative Junctions Subteam

- _Buffer Layer_			  _Efficiency_
-      CdS			19.5% (2004)
-  ZnS(O,OH)			18.6% (2003)
-   InS(O,OH)			16.2% (2003)
-     Cd-PE			15.7% (2003)
-       ZnO			15.0% (1999)
-   ZnIn2Se4			14.5% (1998)
-      ZnSe			14.2% (2000)
-   (Cd,Zn)S, (Zn,Mg)O, and others have also been investigated.

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## Cadmium Partial Electrolyte (Cd-PE)

- March 8, 2005

- Alternative Junctions Subteam

- NREL three-stage CIGS absorber (Eg near 1.15 eV)

- See K. Ramanathan et al, J. Phys Chem Solids 64, 1495 (2003)

- Current-Voltage 15.7%

- CdS Band Gap

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## Cd-PE and CdS Buffers (NREL Absorber)

- March 8, 2005

- Alternative Junctions Subteam

- Current-voltage		 Internal QE

- PK Johnson et al, PVSC 29 (2002)

- _Note_: Cd-PE buffer has consistently shown higher current, but (so far) lower voltage.

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## CdS and Cd-PE Buffers (SSI Absorbers)

- March 8, 2005

- Alternative Junctions Subteam

- Current-voltage		 Internal QE

- _Note_: With the selenized absorbers, efficiencies are lower, but the Cd-PE buffer still shows higher current and lower voltage.        _Also_ _note_: The CdS J-V curves show light/dark crossover, but the Cd-PE ones do not.  Not an accident!

- P.K. Johnson et al, EPVSEC (2001)

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## AGU ZnS(O,OH) Buffer on NREL CIGS

- March 8, 2005

- Alternative Junction Subteam

- Contreras, Nakada, Hongo, Pudov, and Sites WCPEC-3 (2003).
- Similar performance ZnS(O,OH)/CIGS cells reported by Bhattacharya et al..

- ZnS(O,OH)/CIGSCurrent-voltage

- Quantum Efficiency

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## Possible Reasons for CdS Alternatives

- March 8, 2005

- Alternative Junction Subteam

- Basic Performance.
- 	Higher current from improved blue photon collection (generally realized), while maintaining equal voltage (generally not realized).
- (2) Conduction-Band Offset Considerations
- 	(a) Better match to CIS, especially when no blue photons present (current issue).  (b) Better match to wide band-gap CIGS (voltage issue).

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## Red Light Distortion for CdS/CIS

- March 8, 2005

- Alternative Junctions Subteam

- The height of the conduction-band-offset barrier, relative to the quasi-Fermi level for electrons, can limit current for low-gap CIS, especially in forward bias and with no photons having energies above the CdS gap.

- EV

- EFp

- EFn

- EC

- Dark

- White Light

- Red Light

- Result is reduced photo- and forward-current under “red” light (and in the dark).

**Important Area**

- Current-Voltage  Curves

- CIS

- CdS

- Band Diagram Under Illumination

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## Basic Big-Spike Rule

- March 8, 2005

- Alternative Junctions Subteam

- Assume current is limited by available electrons: J = qnvth
- If we assume sufficient electrons to support a typical photocurrent (J = 32 mA/cm2), then

- at the top of the spike with

- Hence,

- This is a general rule, independent of materials, carrier densities, defect densities, thicknesses, photoconductivity, or bias.  It does change numerically with temperature or light intensity. [Kanevce et al, upcoming MRS]
- Red or dark J-V distortion simply means that the rule holds in reverse bias,   but fails in forward bias.

- Also

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## When is Distortion Present or Absent?

- March 8, 2005

- Alternative Junctions Subteam

- Photoconductivity can shift conduction band and change big-spike inequality from above 0.48 to below.

- CdS defect density and thickness can also determine whether 0.48-eV rule holds.

- Confirmed by both experiments and simulations.

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## Big-Spike Rule with Alternative Junction

- March 8, 2005

- Alternative Junctions Subteam

- Cells with two types of junction were fabricated at NREL on nominally identical CIGS (1.15 eV) absorbers.
-         Modest Spike      			Bigger Spike

- Pudov et al, E-MRS (2005)

- Here the 0.48 eV rule holds for the CdS/CIGS junction at all biases, but fails for the alternative junction in forward bias.  Note small amount of residual distortion in white-light curve.

- [Buffer from AGU]

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## Progression from “Spike” to “Cliff”

- March 8, 2005

- Alternative Junctions Subteam

- Expansion of band gap with Ga-content occurs almost exclusively in conduction band.
- Distortion goes away as more gallium is added and 0.48-eV rule is satisfied at all biases.

- Pudov, Hasoon et al, JAP, in press.

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## Effect of Interfacial Recombination on VOC

- Effect of Interfacial Recombination on VOC

- March 8, 2005

- Alternative Junction Team

- CdS Window

- Expand Eg  to vary EC (simulated)
- Gloeckler and Sites, E-MRS (2004).
- Lack of spike allows significant interfacial recombination
- Several groups have discussed effect of EC at constant Eg

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## Effect of Offset on Efficiency

- March 8, 2005

- Alternative Junction Subteam

- Vary the offset independently of Eg

- Any single buffer is well matched to CIGS over an approximate 0.3 eV range.

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## Choosing the Window Material

- March 8, 2005

- Alternative Junction Subteam

- Big Spike

- Small Spike or Cliff

- Offset Values from Zhang,Wei, and Zunger, JAP 83, 3192 (1998)

- Strategy is to match absorber and window materials so EC is in the optimal range

- Add Mg

- Add Zn

- Add S

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## Possible Reasons for CdS Alternatives

- March 8, 2005

- Alternative Junction Subteam

- Basic Performance.
- 	Higher current from improved blue photon collection (generally realized), while maintaining equal voltage (generally not realized).
- (2) Conduction-Band Offset Considerations
- 	(a) Better match to CIS, especially when no blue photons present (current issue).  (b) Better match to wide band-gap CIGS (voltage issue).
- (3) Other Considerations
- 	(a) More robust junction when front contact deposition requires elevated temperature.  (b) “Cadmium-free” could assist marketing.

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## Conclusions and Recommendations

- March 8, 2005

- Alternative Junctions Subteam

- Several alternative junctions with typical-band-gap CIGS (1.1 to 1.15 eV) have produced very respectable solar-cell efficiencies (higher current, lower voltage).
- Reduced conduction-band offset for CIS absorbers could be advantageous when blue photons are not present.
- Increased conduction-band offset for wide-gap CIGS, or other absorbers with gaps above 1.3 eV, should enhance voltage (not demonstrated).
- Priority for future alternative-junction research should be discussed by the team.
- Stability testing at the cell level should be an important part of future alternative-junction research.

**Notes:**
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