1. Macroscopic versus Microscopic uniformity in Cu 2 ZnSnSe 4 absorber layers: the role of temperature and selenium supply Mehrnoush Mokhtarimehr Jose Marquez Prieto Dr. Ian Forbes Prof. Nicola Pearsall University of Northumbria 12 th Photovoltaic Science, Application and Technology Conference, PVSAT-12

2. This could be due to the formation of detrimental phases which are proper regions for SRH recombination, that reduce the open circuit voltage. 1.Investigation the influence of preheating at T<T m Se, Sn on the Macro and Micro structure of CZTSe absorbers prepared by Selenium cap layer. 2.Replacing the chalcogen supply method with Selenium pellets and evaluating its impact on Macro and Micro structure of absorbers. 3.Discussion about the advantages and disadvantages of using two different method of Selenium Supply. Strategies to improve the uniformity of the absorber layers Why non-uniformity of absorber layer is an issue? Introduction Substrate(Glass) Back contact (Mo) Absorber layer(CZTSe) Buffer layer (CdS) (i-ZnO/ITO) Front Contact(Al grids)

3. I. Using Selenium cap layer to supply the chalcogen Sputtering Cu, Zn, Sn Evaporation of Selenium Conversion, Thermal annealing SLG Cu-Zn-Sn SLG Cu-Zn-Sn Selenium SLG Cu 2 ZnSnSe 4 t Se cap layer ~ 1.2 µm Cu/(Zn+Sn) = 0.5 & Zn/Sn = 1.3 t Precursors = 0.8 µm Experimental

4. II. Using Se pellets to supply the chalcogen Sputtering Cu, Zn, Sn On Mo Layer Conversion, Thermal Annealing Selenium Source This approach allows us to have a comparison between the Macro and Micro uniformities of the CZTSe absorber with those absorbers made by selenium cap layer. Experimental

5. Annealing Process Sample D. Converted at 500°C for 15 min (the optimum condition). Cooled naturally. Device has been made Method I. Se cap layer Method II. Se pellets Sample C. Sample B. Sample A. Preheated at 160°C for 3 hrs. Cooled naturally. Preheated at 160°C for 3hrs. Heated up to 300°C for 30 min. Cooled naturally. Preheated at 160°C for 3hrs. Converted at 550°C for 30 min. Cooled naturally. Experimental

6. XRD measurements Intensity (arb. units) 2 Theta ( O ) Intensity (arb. units) 2 Theta ( O ) T<T m Se T m Se <T<T m Sn T > T mSe,Sn 300ºC 550ºC 225°C 160°C Results & Discussion 160 °C & 225 °C 300 °C 550 °C 500 °C T > T mSe,Sn

7. 100µm Background, Yellow areas : precursors Island structure, Blue regions: Selenium Binary Phases The Macro and Micro view of CZTSe absorber layers prepared by cap layer of Selenium 5 μm 50 μm 50µm 5 µm 500μm 50µm 50 μm Results & Discussion 4. Conversion at 300⁰C 3B. Preheating at T mSe <T<T mSn 225 ⁰C 5. Conversion at 550⁰C Phase Map After 160⁰C 30 mm 3A. Preheating at 160 ⁰C T<T mSe 2. Se evaporation 1.Sputtering

8. 5 μm 50 μm The Macro and Micro view of CZTSe absorber layers prepared by Selenium Pellets Results & Discussion 50 μm Sample D using Se Pellets

9. Making Device with the optimum annealing process  Using 4 Selenium Pellets in each graphite box  Conversion at 500ºC for 15 min under 300 mbar Argon,  CdS deposition with CBD & i-ZnO/ITO deposition by magnetron sputtering 1 Substrate(Glass) Back contact (Mo) Absorber layer(CZTSe) Buffer layer (CdS) (i-Zno/ITO) Front Contact(Al grids) light Results & Discussion 1. Márquez, Neuschitzer et al. 2016, ‘’Systematic compositional changes and their influence on lattice and optoelectronic properties of Cu2 ZnSnSe4 kesterite solar cells’’, J. Solar Energy Materials & Solar Cells 144 (2016) 579–585

10. Which strategy could be more reasonable to consider & which factors should be sacrificed… ↓ Se Partial Pressure Non-uniformities at microscopic One more vacuum step Less efficient usage of Se Uniform distribution Accurate control of Se amount Advantages No need a vacuum step ↑ Se partial pressure More efficient usage of Se Uniformities at microscopic level Non uniformities at Macro structure Lack of accurate control of Se amount Disadvantages Advantages Se cap layer Se pellets Conclusion