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

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%.

Nanoscale Surface Analysis Reveals Origins of Enhanced Interface Passivation in RbF Post Deposition Treated CIGSe Solar Cells

“…Independent of the reported power conversion efficiencies, reports have shown that the near surface region undergoes an additional change in composition during cleaning. [35] The survey spectrum in Figure 1 underlines this result. Especially, a clearer Cu signal is visible after the NH 4 OH treatment, which highlights another compositional change of the near surface region upon surface cleaning.…”

“…Independent of the reported power conversion efficiencies, reports have shown that the near surface region undergoes an additional change in composition during cleaning. [ 35 ]…”

“…[13] In literature, there is not general consensus on whether a wet cleaning is indispensable for high efficiency CIGSe. Some reports show that an additional NH 4 OH / H 2 O cleaning is mandatory [35,36] whereas others could produce high efficiency solar cells without any additional treatment. [37] The necessity to clean the surface from Na and F precipitations seem to be linked to the type of buffer layer that is used to make the devices.…”

Investigation and Mitigation of Sputter Damage on Co‐Evaporated Cu(In,Ga)Se2 Absorbers for Photovoltaic Applications

“…Hereafter, only the ZnO power is stated, since ZnO is the parent material and MgO power can be determined accordingly. Immediately before entering the deposition chamber, CIGS absorbers have been rinsed in NH 4 OH for 1 min, [ 18 ] to remove alkali crystals and prepare the surface in a reproducible manner. The time between rinsing the absorber and introducing it to the deposition chamber is less than 10 min.…”

Electrical barriers and their elimination by tuning (Zn,Mg)O buffer composition in Cu(In,Ga)S2 solar cells: systematic approach to achieve over 14% power conversion efficiency

“…As a matter of fact, the record PCE of 15.5% has been achieved by replacing the CdS/i-ZnO buffer i-layer stack with a bilayer (Zn,Mg)O buffer stack [17]. While there have been numerous studies that explicitly study (Zn,Mg)O as a buffer layer partner for Cu(In,Ga)Se 2 [18][19][20][21], detailed study on (Zn,Mg)O as a buffer layer partner for CIGSu is still missing. Here, we investigate (Zn,Mg)O/Al:(Zn,Mg)O as a replacement to CdS/ZnO buffer and i-layer stack using qFLs and V OC measurements to better understand the losses, together with a model of how replacing these layers improves the device performance.…”