Silver‐Promoted High‐Performance (Ag,Cu)(In,Ga)Se₂ Thin‐Film Solar Cells Grown at Very Low Temperature

Abstract

Achieving high power conversion efficiencies with Cu(In,Ga)Se2 (CIGS) solar cells grown at low temperature is challenging because of insufficient thermal energy for grain growth and defect annihilation, resulting in poor crystallinity, higher defect concentration, and degraded device performance. Herein, the possibilities for high‐performing devices produced at very low temperatures (≤450 °C) are explored. By alloying CIGS with Ag by the precursor layer method, (Ag,Cu)(In,Ga)Se2 (ACIGS) solar cells grown at about 450 °C reach an efficiency of 20.1%. Only a small efficiency degradation (0.5% and 1.6% absolute) is observed for ACIGS absorbers deposited at 60 and 110 °C lower substrate temperature. CIGS devices exhibit a stronger efficiency degradation, driven by a decrease in the open‐circuit voltage (VOC). The root cause of the VOC difference between ACIGS and CIGS devices is investigated by advanced characterization techniques, which show improved morphology, reduced tail states, and higher doping density in ACIGS absorbers. The proposed approach offers several benefits in view of depositions on temperature‐sensitive substrates. Increased Cu diffusion promoted by Ag allows end‐point detection in the three‐stage process at the substrate temperatures below 300 °C. The modified process requires minimal modification of existing processes and equipment and shows the potential for the use of different flexible substrates and device architectures.

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

“…[ 55,59,194,210,211 ] Nevertheless, ACIGS solar cells have been successfully developed in SLG substrates under low temperatures, down to a nominal substrate temperature of 253 ºC, albeit with a conversion efficiency value decrease as lower deposition temperatures are used. [ 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.…”

“…Although world record CIGS devices still depend on high‐deposition temperatures, ACIGS solar cells with a conversion efficiency value of 18.5% have recently been produced at low temperatures (≈303 °C) on SLG substrates, a disruptive result that paves the way for the metallic mirror integration. [ 67 ] However, although the pathway starts by implementing a metallic mirror, the light‐trapping ability has to go even further. As a perfect reflector only doubles the optical path length, light scattering from the rear interface is required.…”

“…[55,59,194,210,211] Nevertheless, ACIGS solar cells have been successfully developed in SLG substrates under low temperatures, down to a nominal substrate temperature of 253 ºC, albeit with a conversion efficiency value decrease as lower deposition temperatures are used. [67] Several works report an increased conversion efficiency when going from Mo to a • stands for SLG/Mo/CIGS/CdS/i-ZnO/AZO; b) ▴ stands for stainless steel (SS)/Mo/CIGS/CdS/i-ZnO/AZO; c) ▪ stands for SLG/Mo/CIGS/CdS; d) calculated from short circuit current (I sc ) value.…”

“…[ 66 ] Substrate temperatures lower than 350 ºC have been successfully used in a Ag‐based alloy (ACIGS) solar cell. [ 67 ] This expertise might be a key element to provide a wider portfolio of rear optical schemes to the CIGS technology. Thus, it is crucial to understand if nanophotonic and plasmonic architectures can be integrated in CIGS technology, to take full advantage of their light management benefits and access thin and ultrathin solar cells’ full potential.…”

Charge Carrier Lifetime Fluctuations and Performance Evaluation of Cu(In,Ga)Se2 Absorbers via Time‐Resolved‐Photoluminescence Microscopy

“…For instance, silver alloying is a promising route to increase the grain size and keep high material quality. [28,29]…”