Ultrathin CIGSe Solar Cells with Integrated Structured Back Reflector

Abstract

Reducing the thickness of thin-film solar cells reduces the total cost of ownership of solar module production. Theoretically, a 500 nm Cu(In,Ga)Se 2 (CIGSe) solar cell allows more than 20% efficiency. [1-4] However, this requires a very low back contact recombination velocity and advanced optical light management. The standard molybdenum back contact is generally attributed with a high recombination velocity [5-7] and has a poor optical reflectivity. [8] In ref. [9], a ZrN layer on top of molybdenum having a reflectivity of 60% was used with the result of increased quantum efficiency at long wavelengths. An even higher reflectivity and higher gain in photocurrent could be achieved by an Au back reflector applied by post-deposition recontacting after removing the Mo layer. [10] In addition to cost aspects, Au is not suitable for direct deposition of CIGSe thin films at high temperatures due to the chemical reactivity with the chalcogen species during CIGSe deposition. The latter is also true for other highly reflective metals such as Ag, Cu, and Al. [11] Instead, these metals have to be encapsulated by a transparent and conductive diffusion barrier layer. Bissig et al. investigated an Al/InZnO back contact for solar cells with an absorber thickness of 2.1 μm and were able to increase the short-circuit density up to 1.4 mA cm À2 compared with a sample with Mo back contact, despite the relatively high absorber thickness. [12] According to former work on transparent back contacts, the transparent conductor indium-tin-oxide (ITO) is a good candidate as a transparent back contact [13] and as a diffusion barrier. [14] Recently, ref. [15] showed an experimental gain in short-circuit current density of 4.9 mA cm À2 in a structure Mo/Ag/ITO/0.5 μm CIGSe with respect to the reference structure of Mo/0.5 μm CIGSe. Al/ITO-based solar cells can be prepared up to at least 600 C without Al diffusion inside the absorber while providing high reflectivity at long optical wavelengths. [16] This raises the question of the electronic properties of the ITO/CIGSe contact. Keller et al. investigated 650 nm CIGSe bifacial solar cells with a hydrogen-doped In 2 O 3 back contact [17] and quantified the back-contact recombination velocity to the range of 10 7 cm s À1. This value dropped by application of an Al 2 O 3 þ NaF precursor layer stack as confirmed by a strongly increased rear-side external quantum efficiency (EQE) at short wavelengths. Unfortunately, the 5 nm Al 2 O 3 conformal layer reduces the fill factor (FF). [17] Although a flat optical reflector enhances the long-wavelength EQE upon front-side illumination to some extent, light scattering is required in addition to approach the Yablonovitch limit. [18] Yin et al. used ITO as transparent back contact and applied both dielectric SiO 2 nanoparticles on top of ITO and an external Ag mirror behind the substrate glass achieving a very high short-circuit current density of 32.4 mA cm À2 with an absorber thickness of only 390 nm. [19] This was an important step toward a structured an...

Tailored Light Scattering through Hyperuniform Disorder in Self‐Organized Arrays of High‐Index Nanodisks

“…Micro‐ and nanostructured interfaces for electromagnetic scattering continue to play a major role in photonics due to their particularly wide range of existing as well as prospective applications, for example, for light management in solar cells [ 1–3 ] and solid‐state lighting devices, [ 4,5 ] spectral molecular fingerprint detection, [ 6,7 ] or colored surfaces in architecture and design. [ 8–10 ] The key challenge in ongoing research is the ability to tailor the optical response from these interfaces in a quite sophisticated manner to meet the demands of specific applications.…”

Ultrathin Cu(In,Ga)Se2 Solar Cells with Ag/AlOx Passivating Back Reflector

“…In combination with a metal, higher internal reflection is calculated and thus higher current generation [8]. Practically, current improvements are observed upon applying structured SiO x islands [9,10], using metallic mirrors [10][11][12], TCO layers [13] and encapsulated Au nanoparticles [14]. These methods are based on advanced nano-lithography techniques, or rather thick metallic layers, making it expensive or difficult to upscale the methods.…”

Comparison of Mo and ITO back contacts in CIGSe solar cells: Vanishing of the main capacitance step

“…Transparent conductive oxides like ITO serve as back contacts for bifacial solar cells 8–12 . Recently, they also became of interest for light management applications 9,11‐17 . CIGSe solar cells with different absorber layer thicknesses and with ITO or Mo back contacts were studied recently in order to determine the back contact recombination velocity at the CIGSe/ITO interface from bifacial measurements combined with systematic simulations 18 .…”