Interface engineering of ultrathin Cu(In,Ga)Se₂ solar cells on reflective back contacts

Abstract

Cu(In,Ga)Se2‐based (CIGS) solar cells with ultrathin (≤500 nm) absorber layers suffer from the low reflectivity of conventional Mo back contacts. Here, we design and investigate ohmic and reflective back contacts (RBC) made of multilayer stacks that are compatible with the direct deposition of CIGS at 500°C and above. Diffusion mechanisms and reactions at each interface and in the CIGS layer are carefully analyzed by energy dispersive X‐ray (EDX)/scanning transmission electron microscopy (STEM). It shows that the highly reflective silver mirror is efficiently encapsulated in ZnO:Al layers. The detrimental reaction between CIGS and the top In2O3:Sn (ITO) layer used for ohmic contact can be mitigated by adding a 3 nm thick Al2O3 layer and by decreasing the CIGS coevaporation temperature from 550°C to 500°C. It also improves the compositional grading of Ga toward the CIGS back interface, leading to increased open‐ circuit voltage and fill factor. The best ultrathin CIGS solar cell on RBC exhibits an efficiency of 13.5% (+1.0% as compared to our Mo reference) with a short‐circuit current density of 28.9 mA/cm2 (+2.6 mA/cm2) enabled by double‐pass absorption in the 510 nm thick CIGS absorber. RBC are easy to fabricate and could benefit other photovoltaic devices that require highly reflective and conductive contacts subject to high temperature processes.

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.…”

Exploiting the Optical Limits of Thin‐Film Solar Cells: A Review on Light Management Strategies in Cu(In,Ga)Se2

“…[ 67 ] Several works report an increased conversion efficiency when going from Mo to a mirror/TCO stack. [ 55,193–195 ] However, it is important to note that the CIGS deposition is changed from the state‐of‐the‐art process to answer the TCO requirement. Hence, even with the optical path length enhancement, Mo is still the most suitable candidate.…”

“…[42,181] Several approaches have been developed to replace the Mo rear contact by a more optically favorable configuration, ranging from 1) a direct metallic layer replacement; [179,[182][183][184][185][186] 2) addition of a nanostructured reflective passivation layer; [36,[187][188][189][190] 3) through the coupling of a metallic reflective layer with a point-contact passivation architecture; [191,192] or 4) different combinations of TCOs and metallic rear reflectors. [55,[193][194][195] Nonetheless, this is not a simple task, as a great variety of requirements must be fulfilled to attain a viable rear contact, such as thermal, chemical, and mechanical stability, low interface recombination, and the ability to achieve an ohmic contact. For an optimized optical performance, ideally, Au, Ag, copper (Cu), or aluminum (Al) should be used as rear reflectors, as they would allow for the highest rear-contact reflectance.…”

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 .…”