Novel back contact reflector for high efficiency and double‐graded Cu(In,Ga)Se₂ thin‐film solar cells

Abstract

For highest efficiency Cu(In,Ga)Se2 thin‐film solar cells, the V‐shaped bandgap grading can result in residual transmission through the absorber layer and parasitic absorbance in the Mo back contact. To improve the back contact reflectance, absorbers were grown in a multistage process on Mo/Al/InZnO substrates. Additionally, ultrathin layers of Mo and MoSeX were introduced at the InZnO‐absorber interface. Effects of the different back contacts on device performance, absorber morphology, and composition are characterized and discussed. Finally, a numerical model is proposed to understand the experimentally observed increase in device current density of 0.3 to 0.8 mAcm−2.

Challenges and opportunities for an efficiency boost of next generation Cu(In,Ga)Se2 solar cells: prospects for a paradigm shift

“…55 Once the non-radiative lifetime becomes higher than the radiative lifetime, a back reflector becomes necessary to further increase V OC . In theory, a very high reflectance is required 56 with no evidence of non-ohmic contacts on state-of-the-art CIGS solar cells. Back reflectors with very high reflectance were also integrated into ultrathin solar cells.…”

“…61 For CIGS, a comparable approach was never deemed attractive from the manufacturing perspective. Instead, combinations of TCO, metal and/or oxide layers have shown promising results, 56,57 suggesting that highly reflective and ohmic contacts may be implemented in high-performance devices. Another challenge is the availability of charge selective contacts.…”

Time-resolved photoluminescence on double graded Cu(In,Ga)Se2 – Impact of front surface recombination and its temperature dependence

“…The second term in the square brackets is the transmission probability for photons with an angle smaller than the critical angle. The reflection at the back contact has been neglected due to the small reflectivity of the CIGS/Mo interface [33,34].…”

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

“…Though Ag, Cu, Au, and Al are the most promising reflective metals to significantly improve light absorption in ultrathin CIGS solar cells, they are not compatible with the direct coevaporation of CIGS 12,19,20 . Up to now, only a few architectures of RBC that include a metallic mirror and are compatible with the CIGS deposition were reported, such as a metal/Al 2 O 3 bilayer with point contacts on Mo, 21 and metallic mirrors encapsulated by transparent conducting oxides (TCO) 22–24 . Such architectures with TCO‐based back contacts are promising as they should be compatible with low‐cost industrial processes, but also with back contact texturing strategies for additional light trapping in ultrathin CIGS layers 25 …”