Over 100 mV V_OC Improvement for Rear Passivated ACIGS Ultra‐Thin Solar Cells

Abstract

A decentralized energy system requires photovoltaic solutions to meet new aesthetic paradigms, such as lightness, flexibility, and new form factors. Notwithstanding, the materials shortage in the Green Transition is a concern gaining momentum due to their foreseen continuous demand. A fruitful strategy to shrink the absorber thickness, meeting aesthetic and shortage materials consumption targets, arises from interface passivation. However, a deep understanding of passivated systems is required to close the efficiency gap between ultra‐thin and thin film devices, and to mono‐Si. Herein, a (Ag,Cu)(In,Ga)Se2 ultra‐thin solar cell, with 92% passivated rear interface area, is compared with a conventional nonpassivated counterpart. A thin MoSe2 layer, for a quasi‐ohmic contact, is present in the two architectures at the contacts, despite the passivated device narrow line scheme. The devices present striking differences in charge carrier dynamics. Electrical and optoelectronic analysis combined with SCAPS modelling suggest a lower recombination rate for the passivated device, through a reduction on the rear surface recombination velocity and overall defects, comparing with the reference solar cell. The new architecture allows for a 2% efficiency improvement on a 640 nm ultra‐thin device, from 11% to 13%, stemming from an open circuit voltage increase of 108 mV.

Development of a Plasmonic Light Management Architecture Integrated within an Interface Passivation Scheme for Ultrathin Solar Cells

“…With respect to the contacting approach, a high‐passivating area is desired for a more substantial decrease in the active recombination mechanisms, while allowing for the utilization of the significant passivation area for effective light trapping through the encapsulation of metallic NPs. [ 20,23,57 ] Nevertheless, the high passivation area needs to be counterbalanced with a contacting area which does not compromise the charge extraction, as passivation areas higher than 90% have shown an impact on ultrathin solar cells’ series resistance. [ 9,20,23 ] Therefore, in this work, a line contact pattern with a nominal passivation area of 75% was chosen to attain an effective passivation and allow a substantial dielectric coverage to couple with light trapping, without hindering charge carrier collection.…”

“…[ 20,23,57 ] Nevertheless, the high passivation area needs to be counterbalanced with a contacting area which does not compromise the charge extraction, as passivation areas higher than 90% have shown an impact on ultrathin solar cells’ series resistance. [ 9,20,23 ] Therefore, in this work, a line contact pattern with a nominal passivation area of 75% was chosen to attain an effective passivation and allow a substantial dielectric coverage to couple with light trapping, without hindering charge carrier collection. The dielectric encapsulation layer thickness is as well critical, since the encapsulation of the metallic nanostructures needs to be ensured.…”

“…[ 16–18 ] Nonetheless, there is a significant efficiency drop following the pursuit of ultrathin configurations, mostly due to: 1) increased rear interface recombination and 2) incomplete light absorption. [ 10,16,17,19–21 ] To address the rear interface recombination, passivation schemes based on patterned dielectric layers, such as SiO 2 , [ 20,22–25 ] Al 2 O 3 , [ 26–30 ] HfO 2 , [ 31,32 ] and TiO 2 , [ 33,34 ] have been efficiently introduced, yielding promising enhancements in open‐circuit voltage ( V OC ), with improvements over 100 mV compared with a nonpassivated device. [ 20 ] However, to tackle the challenges imposed through the absorber thickness reduction, the electrical gains provided by dielectric passivation contact schemes need to be coupled with significant optical gains, as thinning down the absorber from ≈2000 to 500 nm leads to losses in the short‐circuit current density ( J SC ) that can be as high as 4 mA cm −2 .…”

“…[ 10,16,17,19–21 ] To address the rear interface recombination, passivation schemes based on patterned dielectric layers, such as SiO 2 , [ 20,22–25 ] Al 2 O 3 , [ 26–30 ] HfO 2 , [ 31,32 ] and TiO 2 , [ 33,34 ] have been efficiently introduced, yielding promising enhancements in open‐circuit voltage ( V OC ), with improvements over 100 mV compared with a nonpassivated device. [ 20 ] However, to tackle the challenges imposed through the absorber thickness reduction, the electrical gains provided by dielectric passivation contact schemes need to be coupled with significant optical gains, as thinning down the absorber from ≈2000 to 500 nm leads to losses in the short‐circuit current density ( J SC ) that can be as high as 4 mA cm −2 . [ 17 ] The dielectric rear interface passivation architectures already allow for a rear optical reflectance improvement, usually around 0.1–0.9 mA cm −2 , [ 20,22,35,36 ] due to a refractive index mismatch between the absorber and the dielectric layer with a low refractive index.…”

“…[ 20 ] However, to tackle the challenges imposed through the absorber thickness reduction, the electrical gains provided by dielectric passivation contact schemes need to be coupled with significant optical gains, as thinning down the absorber from ≈2000 to 500 nm leads to losses in the short‐circuit current density ( J SC ) that can be as high as 4 mA cm −2 . [ 17 ] The dielectric rear interface passivation architectures already allow for a rear optical reflectance improvement, usually around 0.1–0.9 mA cm −2 , [ 20,22,35,36 ] due to a refractive index mismatch between the absorber and the dielectric layer with a low refractive index. [ 19,36 ] Nevertheless, considering a CIGS‐based absorber with a bandgap energy value of 1.16 eV and assuming an ideal charge carrier collection, even if a perfect metallic mirror is integrated at the rear of an ultrathin device with a 500 nm absorber, the simulated J SC value is still 2 mA cm −2 lower than the one of the conventional architecture with a 2 μm absorber.…”