# Presentation: 266962

##  Bifacial CdS/CdTe/ZnTe device characterization

**Darshini Desai, Steven Hegedus**
**Institute of Energy Conversion**
**University of Delaware**
**Newark DE, USA**
**Development of the transparent contact device and characterization techniques driven by need for better quantitative metrics and understanding of CdS/CdTe device operation **

## Motivation and Goals

**Motivation**
**     ****A transparent back contact made with controlled Cu doped ZnTe can be used as**
**     ****-  A source of controlled Cu doping**
**     ****-  A robust back contact**
**     ****-  Transparent interconnect for tandem cell applications**
**     ****-  A novel device characterization tool**
**Goal**
**     ****Development of ZnTe:Cu based transparent, ohmic and stable contact for CdTe/CdS solar cells to enable bifacial device characterization and provide alternative source of Cu doping.**

## Transparent (Semi) ZnTe:Cu Film Deposition

** **Films grown by galvanic deposition with **Zn** as **anode **and **substrate** as **cathode**
-  Source electrolytes  ZnSO4, TeO2, CuSO4
-  Deposition time **1.5 mins, pH=3 and T=68°C**
-  **Triethanolamine(TEA**) used to regulate amount of Cu in the solution. More TEA results in less free Cu and hence higher film transparency.
- Selected 20 drops[1E-8M]  for device fabrication

## JV results summary

| Contact (Sample#) | Light
direction | Voc
(V) | Jsc
(mA/cm2) | FF
(%) | Eff
(%) |
| --- | --- | --- | --- | --- | --- |
| ZnTe(VT149.4)
ZnTe(VT149.4) | front
back | 0.82
0.66 | 20.3
0.6 | 68.0
66.3 | 11.3
0.2 |
| Cu/Ni (VT149.1) | front | 0.82 | 20.8 | 68.7 | 11.8 |
| ZnTe (VT154.4)
ZnTe (VT154.4) | front
back | 0.75
0.64 | 20.8
0.9 | 55.4
68.2 | 8.7
0.4 |
| Cu/Ni (VT154.1) | front | 0.80 | 21.4 | 60.1 | 10.4 |

## JV data with Bifacial Illumination

-  **VT 154.4.2 back-wall JV has no blocking barrier, unlike the dark and front-wall **
**  ****JV : Suggests photoconductive back contact**
** ****Both devices: Higher  fill-factor for back-wall   illumination due to lower  I****2****R loss**
**             **

## Spectral Response : Analysis and Modeling

**Fit modeled [Phillips 1st WCPVEC 1994] to measured SR  with W=2.5********m and L= 0.8********m Increasing L from 0.5********m to 1.5********m results in better  collection for all wavelengths, for back wall SR**
** ****Front wall SR unaffected by L, back wall SR sensitive to L [400-800nm] and W[800-860nm]**

##  Voc vs. T vs. Intensity: A fundamental characterization tool  

**J-V at low T often distorted by blocking diode and/or very high Rs**
**Cannot analyze J-V curve with single diode model**
**J=O at Voc so no blocking, Rs**
**Voc:linear in -T, log in I**
**Voc upper limit is fundamental parameter**
**Slope dV/dT indicative of recombination mech.**
**J****L**** depends on V******** field driven collection**
**J****L****(V)=J****L0**** ******** (V)**

## Voc-T: linear fit and extrapolation above 220K

**Voc(0K)~1.5 V indep. of intensity**

##  Voc saturation below 200K

**Initial results**
**Saturation: independent of T, L**
**No recombination?**

**After 5 days stress **
**Voc(300K) decreased 0.03V**
**Voc(150K) decreased 0.06V**

## Bifacial Voc vs. T at 100, 10% light

**Front illumination : Voc independent of temperature and light intensity below 220K**
**Back illumination  : Voc continues to increase  irrespective of front-wall Voc**
**Transient increase in Voc ~mins observed at temperatures below 110K from dark to 100%**
***This behavior seen in  ALL CdTe cells irrespective of the back contact or  the manufacturer***

## Bifacial Jsc vs. T : Intensity and Spectral effects

**Front illumination Jsc **_***decreases***_** below 220K.  Virtually no change above 220K**
**Back illumination Jsc **_***increases ***_** below 220K . Opposite effect**
**Mobility increases at lower temperature ?**
**Temperature dependence of field/depletion region which causes better collection from back but reduces field in front?**
**Similar temperature behavior seen in FS samples**** **

**Jsc(mA/cm****2****)**

**Jsc(mA/cm****2****)**

## Maximum achievable Voc : Band diagram model

**Hole fermi level pinned **
**at Cu defect**
_**Ev + 0.3eV**_

**Maximum achievable Voc : Band diagram model**

**SnO****2**

**CdS/CdTe****x****S****1-x**

**CdTe**

**Electron fermi level pinned **
**at SnO2 interface **
_**Ec - 0.3eV**_

## Observations

**Voc saturation below 220K observed for > 30 samples**
**Is the Voc @ 100K the “Built-in voltage”– Maximum achievable limit ??**
**       ****- Unclear whether back-wall Voc really saturates due to system limitations**
**Fermi-level pinning at SnO****2  ****interface alters the quasi-fermi level profile**
**dEg/dT= (- 1.70 mV/K) so CdTe bandgap at 100K ~ 1.7eV.  If this hypothesis valid then the maximum achievable Voc is **
**1.7 - 0.3- 0.3**** ****= 1.1 eV**
**Typically @ 100K maximum “Saturated Voc” ~ 0.95-1.05V achieved**
** ****Other issues with low temperature measurement**
**      ****-  Blocking contact at lower temperatures**
**      ****-  Defect/Trap energy level positions not  known**
**      ****-  Slow transient increase after light exposure **
**Interpretation of Voc complicated by large Jsc temperature dependence**

## Conclusions and Future Work

**Bifacial device characterization enhances conventional characterization tools and helps to isolate properties of front and back junction**
**Temperature dependent Voc and Jsc measurements fundamental to quantify transport in CdTe/CdS solar cells**
**No spectral dependence of Voc or Jsc**
**Bifacial spectral response model fit to data with W = 2.5********m and L= 0.8********m the back-wall spectral response sensitive to L and W**
**In future intend to make cells with thinner CdTe to evaluate drift and diffusion limited transport**
**We intend do Voc-T measurement on cells with CdZnTe window layer to further investigate the possibility of interface recombination limited transport**