Abstract
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)Se2(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%.
Highlights
Prior to the recent record cells [1], the best performing thin film solar cells with chalcopyrite absorbers (Cu(In,Ga) (S,Se)2) have employed chemical bath deposited (CDB) CdS as the buffer layer [2,3,4] over alternative buffer materials or deposition methods [5,6,7]
Zn1ÀxMgxO thin films were deposited by atomic layer deposition (ALD) as buffer layers on co-evaporated Cu(In,Ga)Se2 (CIGS) absorbers
We investigate the influence of different absorber surface treatments before Zn1ÀxMgxO deposition on the final device properties, notably KCN, ammonia and variants inspired by the successful CdS-CBD deposition process
Summary
Prior to the recent record cells [1], the best performing thin film solar cells with chalcopyrite absorbers (Cu(In,Ga) (S,Se)2) have employed chemical bath deposited (CDB) CdS as the buffer layer [2,3,4] over alternative buffer materials or deposition methods [5,6,7]. This work focuses on the substitution of CBD-CdS with a dry deposited high bandgap material and the effects of surface modifications before the growth of the buffer layer. The properties of the ALD deposited Zn1ÀxMgxO thin films are analysed in terms of thickness, composition, carrier density, mobility and sheet resistance This high Eg material allows modification of the conduction band minimum [41,42] in the vicinity of the ones of CIS and CGS [21], which is used to match the conduction band of the absorber. We investigate the influence of different absorber surface treatments before Zn1ÀxMgxO deposition on the final device properties, notably KCN, ammonia and variants inspired by the successful CdS-CBD deposition process. The effects of wet and dry absorber treatments before the buffer deposition are discussed and resulting cells are compared to CdS-CBD references
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