Thibaud Hildebrandt

Towards a better understanding of the use of additives in Zn(S,O,OH) deposition bath for high efficiency Cu(In,Ga)Se2-based solar cells

CBD-Zn(S,O,OH) remains one the most studied and promising Cd-free buffer layer for Cu(In,Ga)Se2-based solar cells, and has already demonstrated its potential to lead to high-efficiency solar cells. However Zn(S,O,OH) deposition time and metastable behavior of the final devices remain critical to outperform CdS-based devices. The aim of this work is to study and understand the influence of additives such as H2O2 on the deposition bath and on the surface of the absorber. These results will be related with final performances of the devices. A new promising additive, persulfate S2O82-, will be presented and could be the key to go beyond CdS-based solar cell records.

Electrodeposition of ZnO-doped films as window layer for Cd-free CIGS-based solar cells

The Cu(In,Ga)Se2 (CIGS) thin film solar cell technology has made a steady progress within the last decade reaching efficiency up to 22.3% on laboratory scale, thus overpassing the highest efficiency for polycrystalline silicon solar cells. High efficiency CIGS modules employ a so-called buffer layer of cadmium sulfide CdS deposited by Chemical Bath Deposition (CBD), which presence and Cd-containing waste present some environmental concerns. A second potential bottleneck for CIGS technology is its window layer made of i-ZnO/ZnO:Al, which is deposited by sputtering requiring expensive vacuum equipment. A non-vacuum deposition of transparent conductive oxide (TCO) relying on simpler equipment with lower investment costs will be more economically attractive, and could increase competitiveness of CIGS-based modules with the mainstream silicon-based technologies. In the frame of Novazolar project, we have developed a low-cost aqueous solution photo assisted electrodeposition process of the ZnO-based window layer for high efficiency CIGS-based solar cells. The window layer deposition have been first optimized on classical CdS buffer layer leading to cells with efficiencies similar to those measured with the sputtered references on the same absorber (15%). The the optimized ZnO doped layer has been adapted to cadmium free devices where the CdS is replaced by chemical bath deposited zinc oxysulfide Zn(S,O) buffer layer. The effect of different growth parameters has been studied on CBD-Zn(S,O)-plated co-evaporated Cu(In,Ga)Se2 substrates provided by the Zentrum für Sonnenenergie-und Wasserstoff-Forschung (ZSW). This optimization of the electrodeposition of ZnO:Cl on CIGS/Zn(S,O) stacks led to record efficiency of 14%, while the reference cell with a sputtered (Zn,Mg)O/ZnO:Al window layer has an efficiency of 15.2%.

Search for new bath formulations Of Zn(S, O, OH) buffer layer to outperform record performances Of CdS-based CIGSe solar cells

Zn(S, O, OH) represents the most studied Cd-free material for replacing chemical bath deposited (CBD)-CdS buffer layers in Cu(In, Ga)Se2-based solar cells. However, the record performances remain lower than the CdS. The aim of this work is to study new bath compositions for CBD-Zn(S, O, OH), by introducing new complexing and non-complexing reactants for higher efficiencies, and to analyze their effects on growth mechanisms. Promising bath compositions based on the combined used of H2O2 and tri-sodium citrate have been developed, and could be a new avenue to outperform the CdS-based solar cells.

Photochemical deposition of ZnS buffer layers for Cu(In, Ga)Se 2 thin film solar cells via reusable solutions

A new bath composition for ZnS buffer layer deposition is presented allowing its photochemical growth on CIGSe absorbers. The main advantages of this solution compared to classical CBD-Zn(S, O) bath are: the deposition occurs at room temperature, the concentration of chemical precursors is 10 times lower than classical CBD-Zn(S, O), the use of a complexing agent such as ammonia is avoided and the same bath can be reused for several depositions. The same bath can be re-used for at least 4 times, leading to efficiencies between 13 and 14%, which is similar to what is obtained with CdS-buffered references.A new bath composition for ZnS buffer layer deposition is presented allowing its photochemical growth on CIGSe absorbers. The main advantages of this solution compared to classical CBD-Zn(S,O) bath are: the deposition occurs at room temperature, the concentration of chemical precursors is 10 times lower than classical CBD-Zn(S,O), the use of a complexing agent such as ammonia is avoided and the same bath can be reused for several depositions. The same bath can be re-used for at least 4 times, leading to efficiencies between 13 and 14%, which is similar to what is obtained with CdS-buffered references.

Impact of the deposition conditions of buffer and windows layers on lowering the metastability effects in Cu(In,Ga)Se2/Zn(S,O) based solar cell

The highest and most reproducible (Cu(In,Ga)Se2 (CIGSe) based solar-cell efficiencies are obtained by use of a very thin n-type CdS layer deposited by chemical bath deposition (CBD). However because of both Cadmium’s adverse environmental impact and the narrow bandgap of CdS (2.4–2.5 eV) one of the major objectives in the field of CIGSe technology remains the development and implementation in the production line of Cd-free buffer layers. The CBDZn( S,O) remains one the most studied buffer layer for replacing the CdS in Cu(In,Ga)Se2-based solar cells and has already demonstrated its potential to lead to high-efficiency solar cells up to 22.3%. However one of the key issue to implement a CBD-Zn(S,O) process in a CIGSe production line is the cells stability, which depends both on the deposition conditions of CBD-Zn(S,O) and on a good band alignment between CIGSe/Zn(S,O)/windows layers. The most common window layers applied in CIGSe solar cells consist of two layers : a thin (50–100 nm) and highly resistive i-ZnO layer deposited by magnetron sputtering and a transparent conducting 300–500 nm ZnO:Al layer. In the case of CBD-Zn(S,O) buffer layer, the nature and deposition conditions of both Zn(S,O) and the undoped window layer can strongly influence the performance and stability of cells. The present contribution will be specially focused on the effect of condition growth of CBD-Zn(S,O) buffer layers and the impact of the composition and deposition conditions of the undoped window layers such as ZnxMgyO or ZnxSnyO on the stability and performance of these solar cells.