Gas flow sputtering of Cu(In,Ga)Se₂ with extra selenium supply

Sammanfattning:

In this thesis CIGS absorber layers have been deposited by gas flow sputtering with an extra supply of selenium, a method that displays promise for large scale production because of its one-step sputtering route which deposits low energy particles in a high deposition rate.

In this thesis a method was developed where selenium was added to the sputtering process inside the sputter chamber in a controllable manner and in larger amount than done in previous projects. A total of five samples were manufactured with altered evaporation temperatures and an extra supply of selenium which then were finalized to solar cells using the standard baseline process of the Ångström solar center.

The characteristics of the CIGS layer and solar cells were analyzed by XRF, IV- and QE measurements. A cell with a conversion efficiency of 11.6 %, J_sc of 27.9 mA/cm², V_oc of 0.63 V and fill factor of 66.2 % was obtained on a 0.5 cm² cell area without an antireflective coating.

All samples contained cells with obtained efficiencies above 10 %, but over the whole samples the efficiencies varied considerably. The samples that were deposited with moderately large selenium evaporation provided the highest efficiencies with a relatively good homogeneity over the substrate. Results show a deficiency of copper in the CIGS films compared to the target composition. The copper content was lower than 70 % expressed in Cu/(Ga+In), which probably resulted in a low diffusion length for electrons, leading to limited cell efficiencies. 

Through the duration of the thesis issues that concerned the power supply- and the controls to the substrate heaters as well as the control of the evaporation temperature during the depositions arose that required problem solving and needs to be resolved for the future progression of this work.

The conclusions drawn from this thesis are that it is possible to vary the temperature of the selenium source and thereby control the amount of selenium that evaporates during the deposition process even though there is a lot of additional heating in the sputter chamber from both the substrate heaters and the sputter source which could affect the ability to control the amount of selenium being evaporated. That the most likely reason for the limited efficiencies is due to the low copper content in the CIGS films and that a larger amount of evaporated selenium compared to previous work did not result in higher obtained efficiencies.