Optimisation of rear reflectance in ultra-thin CIGS solar cells towards>20% efficiency

• In order to decrease their cost and the use of rare metal elements, thin film solar cell thicknesses are con-tinuously reduced at the expense of their efficiency, due to a lack of absorption for long wavelengths.Optimisation of cells rear reflectance (Rb) thus becomes meaningful to provide non-absorbed light a sec-ond chance to be harvested by the active cell layer. In this sense, we present a way to keep the rear reflec-tance in advanced Cu(In, Ga) Se2(CIGS) cell as high as possible while keeping in mind the progressalready done regarding the rear passivation techniques. We show that introducing a stack of thin Al2O3 and aluminium between the CIGS layer and the rear molybdenum electrode increases Rbup to92% in the long wavelength 800–1100 nm range. Several other stacks, using MgF2, SiO2or TiO2, are opti-mised in order to investigate the best trade-off between passivation, material consumption and perfor-mances, resulting in Rbranging from 42% (moderate case) to 99% in the best case. Those CIGS rearinterface reflectance optimisations were performed by using a standard transfer matrix method (TMM)in the long wavelength range. Seven interesting stacks are then analysed for solar cell performances usingSCAPS simulation software to understand the impact of rear reflectance on short circuit current density(Jsc) and eventually on the cell efficiency (g), for ultra-thin CIGS absorber thicknesses (<1 lm). Based onthese results, we propose Rboptimisation to achieve Jsc> 40 mA/cm2and g > 20% with a 500 nm-thickCIGS absorber film using CIGS-Al2O3-Mo stack, where the Al2O3thickness can be chosen in between104 and 139 nm. This way, we can ensure good rear reflectance (Rb= 65%) and reduced interface recom-bination while being industrially feasible with present technologies.

Subject headings

TEKNIK OCH TEKNOLOGIER  -- Elektroteknik och elektronik (hsv//swe)

ENGINEERING AND TECHNOLOGY  -- Electrical Engineering, Electronic Engineering, Information Engineering (hsv//eng)

TEKNIK OCH TEKNOLOGIER  -- Nanoteknik (hsv//swe)

ENGINEERING AND TECHNOLOGY  -- Nano-technology (hsv//eng)

Publication and Content Type

ref (subject category)

art (subject category)