Comparative Study of Al₂O₃ and HfO₂ for Surface Passivation of Cu(In,Ga)Se₂ Thin Films: An Innovative Al₂O₃/HfO₂ Multistack Design

Abstract

In Cu(In,Ga)Se 2 (CIGS) thin-film solar cells, interface recombination is one of the most important limiting factors with respect to device performance. Herein, metal-insulator-semiconductor samples are used to investigate and compare the passivation effects of Al 2 O 3 and HfO 2 at the interface with CIGS. Capacitance-voltage-frequency measurements allow to qualitatively and quantitatively assess the existence of high negative charge density (Q f % À10 12 cm À2 ) and low interface-trap density (D it % 10 11 cm À2 eV À1 ). At the rear interface of CIGS solar cells, these, respectively, induce field-effect and chemical passivation. A trade-off is highlighted between stronger field-effect for HfO 2 and lower interface-trap density for Al 2 O 3 . This motivates the usage of Al 2 O 3 to induce chemical passivation at the front interface of CIGS solar cells but raises the issue of its processing compatibility with the buffer layer. Therefore, an innovative Al 2 O 3 /HfO 2 multistack design is proposed and investigated for the first time. Effective chemical passivation is similarly demonstrated for this novel design, suggesting potential decrease in recombination rate at the front interface in CIGS solar cells and increased efficiency. 300 C annealing in N 2 environment enable to enhance passivation effectiveness by reducing D it while surface cleaning may reveal useful for alternative CIGS processing methods.

Boosting Kesterite Solar Cell Performance with Tantalum Oxide Passivation Layer and In Situ Formed Point Contacts

“…[ 22 ] Based on these results, we conclude that both TaO x (on CZTSSe) and HfO 2 (on CIGS) are sustainable in the CBD solution. Therefore, TaO x –Al 2 O 3 or even TaO x –other insulating bilayers can be a viable route for engineering the passivation materials, which can protect Al 2 O 3 (or the chemically unstable insulating layer during the CBD process) from the attack of the chemical bath solution to maintain and even modify the interface quality of Al 2 O 3 (or the insulating layer)–CZTSSe, based on the HfO 2 ‐insulating bilayers proposed by Scaffidi et al [ 40 ] Moreover, TaO x is often connected with SiN x to reach its best passivation quality and carrier transport as an electron selective contact or passivation layer for Si solar cells. [ 23 ] Therefore, the passivation properties of TaO x may be improved by nitrogen doping, or annealed in a hydrogen atmosphere, which offers more opportunity for TaO x as a passivation layer for further improved efficiency.…”

“…This approach has been used for the characterization of the Q f and D it between CIGS and an oxide layer. [39,40] C-V measurements were conducted for the MOS structure of the kesterite film with 20 nm TaO x , depicted in Figure S11a, Supporting Information, to characterize the interface properties of TaO x -CZTSSe. A sufficient thick oxide layer (20 nm) was used here to prevent potential dielectric leakage or tunneling effect, which may significantly affect the analysis of C-V measurements.…”

Dielectric Front Passivation for Cu(In,Ga)Se2 Solar Cells: Status and Prospect

“…For CIGSe this has been done using AlO x and HfO x passivation layers. [ 22,24,25 ] The D it values determined from CV measurements were somewhere between 10 11 and 10 12 eV −1 cm −2 . This is a moderate reduction of interface defects, but not enough to reduce the effective lifetime to 100 cm s −1 .…”

“…Studies based on absorber/passivation structures have revealed passivating properties showing the potential of dielectric passivation of the front surface. [ 21–25 ] Though, to date no improvements in solar cells have been observed. [ 26–32 ] Often fill factor (FF) losses and problems with current extraction arise.…”

Implementation of Tunneling Junction Passivated Contact Concept in Flexible CIGS Solar Cells

“…[39,40] Moreover, several oxide materials, such as SiO x , GaO x , HfO 2 , and TiO 2 have been introduced to the CIGS thin film solar cells. [39][40][41][42][43][44][45][46][47] Among them, TiO 2 was one of the passivating contacts which has been widely applied to c-Si solar cells. [36,48] While for CIGS solar cells, TiO 2 was usually deposited on top of CdS layer as the capping layer to avoid shunt and sputtered damage, or replace the CdS layer as the buffer layer, and it has scarcely any reports that the interface of CdS/CIGS as the tunneling layer up till now.…”