ALD-ZnMgO and absorber surface modifications to substitute CdS buffer layers in co-evaporated CIGSe solar cells

High efficiency chalcopyrite thin film solar cells generally use chemical bath deposited CdS as buffer layer. The transition to Cd-free buffer layers, ideally by dry deposition methods is required to decrease Cd waste, enable all vacuum processing and circumvent optical parasitic absorption losses. In this study, Zn1-xMgxO thin films were deposited by atomic layer deposition (ALD) as buffer layers on co-evaporated Cu(In,Ga)Se-2 (CIGS) absorbers. A specific composition range was identified for a suitable conduction band alignment with the absorber surface. We elucidate the critical role of the CIGS surface preparation prior to the dry ALD process. Wet chemical surface treatments with potassium cyanide, ammonium hydroxide and thiourea prior to buffer layer deposition improved the device performances. Additional in-situ surface reducing treatments conducted immediately prior to Zn1-xMgxO deposition improved device performance and reproducibility. Devices were characterised by (temperature dependant) current-voltage and quantum efficiency measurements with and without light soaking treatment. The highest efficiency was measured to be 18%.

Automated summary

The study successfully deposited Zn1-xMgxO thin films as buffer layers on co-evaporated CIGS absorbers using atomic layer deposition (ALD), improving device performance and reproducibility through wet chemical surface treatments and in-situ surface reducing treatments prior to the ALD process. The highest efficiency of 18% was achieved through these optimizations.