Researchers at Uppsala University in Sweden have fabricated a silver alloy copper, indium, gallium and selenium (ACIGS) thin film solar cell that relies on indium tin oxide (ITO) transparent back contacts or hydrogen-doped indium oxide (ACIGS) IOH).
The scientists built the cells using a three-stage co-evaporation technique and a high-temperature absorber deposition process. They also used cadmium sulfide (CdS) as a buffer layer, ITO and IOH contacts with thicknesses of 180/310 nm, and silver alloying to increase the absorber bandgap energy.
As a conventional transparent back contact (TBC), the ITO contact may lead to the formation of gallium oxide (GaO x ) at the TBC/absorber interface, which is detrimental to the cell's fill factor. Keller emphasizes that ITO forms a similar amount of GaOx as IOH. It was also found that fine-tuning of sodium fluoride (NaF) supply, as well as the use of stoichiometric absorbent compositions, can achieve high fill factor values and most likely mitigate GaOx formation.
According to glow discharge optical emission spectroscopy (GDOES) measurements, the ITO film used in this study effectively blocked sodium (Na) diffusion from the substrate. The researchers point out that the reason why ACIGS solar energy improves sodium sensitivity in cells remains unclear.
The ACIGS cell with a bandgap of 1.44 eV and ITO contacts achieves a power conversion efficiency of 10.4%, an open-circuit voltage of 817 mV, a short-circuit current of 19.3 mA cm, and a fill factor of 63.4%. The same device with IOH contacts has an efficiency of 12.0%, an open-circuit voltage of 835 mV, a short-circuit current of 22.1 mA cm, and a fill factor of 65.2%.
As a comparison, a silver-free reference CIGS cell with ITO contacts has a power conversion efficiency of 11.2%, an open-circuit voltage of 828 mV, a short-circuit current of 19.8 mA cm, and a fill factor of 68.0%. The same device with IOH contacts has an efficiency of 10.5%, an open-circuit voltage of 828 mV, a short-circuit current of 18.6 mA cm, and a fill factor of 67.9%.
The best cells with IOH back contacts can achieve efficiencies of 12%, the academics said. This is the highest reported value for a wide gap chalcopyrite solar cell on TBC. Furthermore, very high infrared transparency can be achieved due to the negligible free charge carrier absorption (FCA) of the IOH (compare Figure 6) and the absence of an interlayer to improve the electronic properties of the contacts.
Future attempts to make these devices in superstructures are planned, which would require the use of thermally stable buffer layers. In this way, only one glass plate is required, which will reduce the cost of the final module.
