Abstract:Cu(In,Ga)Se2
based solar cells have reached efficiencies close to 23%. Further knowledge-driven improvements require accurate determination of the material properties. Here, we present refractive indices for all layers in Cu(In,Ga)Se2 solar cells with high efficiency. The optical bandgap of Cu(In,Ga)Se2 does not depend on the Cu content in the explored composition range, while the absorption coefficient value is primarily determined by the Cu content. An expression for the absorption spectrum is proposed, with… Show more
“…It should be noted that the activation energies for the strongly graded cells (BG2 + BG3) lie above the bandgap determined from the EQE. We assume that all EQE bandgaps are measured slightly lower because of the influence of tail states on the optical absorption [24]. …”
Section: Resultsmentioning
confidence: 99%
“…To do so we modeled the light propagation in a multilayer solar cell by using the transfer matrix method formalism following the approach of Ref. [24], which notably takes into account reflectance and parasitic absorption in window layers. The compositional gradings of Figure 1 were discretized in 25 nm thick slices and the EQE was computed as the cumulated absorption in each of the CIGS slices, assuming perfect collection of the charge carriers.…”
Multi-junction solar cells show the highest photovoltaic energy conversion efficiencies, but the current technologies based on wafers and epitaxial growth of multiple layers are very costly. Therefore, there is a high interest in realizing multi-junction tandem devices based on cost-effective thin film technologies. While the efficiency of such devices has been limited so far because of the rather low efficiency of semitransparent wide bandgap top cells, the recent rise of wide bandgap perovskite solar cells has inspired the development of new thin film tandem solar devices. In order to realize monolithic, and therefore current-matched thin film tandem solar cells, a bottom cell with narrow bandgap (~1 eV) and high efficiency is necessary. In this work, we present Cu(In,Ga)Se2 with a bandgap of 1.00 eV and a maximum power conversion efficiency of 16.1%. This is achieved by implementing a gallium grading towards the back contact into a CuInSe2 base material. We show that this modification significantly improves the open circuit voltage but does not reduce the spectral response range of these devices. Therefore, efficient cells with narrow bandgap absorbers are obtained, yielding the high current density necessary for thin film multi-junction solar cells.
“…It should be noted that the activation energies for the strongly graded cells (BG2 + BG3) lie above the bandgap determined from the EQE. We assume that all EQE bandgaps are measured slightly lower because of the influence of tail states on the optical absorption [24]. …”
Section: Resultsmentioning
confidence: 99%
“…To do so we modeled the light propagation in a multilayer solar cell by using the transfer matrix method formalism following the approach of Ref. [24], which notably takes into account reflectance and parasitic absorption in window layers. The compositional gradings of Figure 1 were discretized in 25 nm thick slices and the EQE was computed as the cumulated absorption in each of the CIGS slices, assuming perfect collection of the charge carriers.…”
Multi-junction solar cells show the highest photovoltaic energy conversion efficiencies, but the current technologies based on wafers and epitaxial growth of multiple layers are very costly. Therefore, there is a high interest in realizing multi-junction tandem devices based on cost-effective thin film technologies. While the efficiency of such devices has been limited so far because of the rather low efficiency of semitransparent wide bandgap top cells, the recent rise of wide bandgap perovskite solar cells has inspired the development of new thin film tandem solar devices. In order to realize monolithic, and therefore current-matched thin film tandem solar cells, a bottom cell with narrow bandgap (~1 eV) and high efficiency is necessary. In this work, we present Cu(In,Ga)Se2 with a bandgap of 1.00 eV and a maximum power conversion efficiency of 16.1%. This is achieved by implementing a gallium grading towards the back contact into a CuInSe2 base material. We show that this modification significantly improves the open circuit voltage but does not reduce the spectral response range of these devices. Therefore, efficient cells with narrow bandgap absorbers are obtained, yielding the high current density necessary for thin film multi-junction solar cells.
“…Illuminated J-V characteristics (AM1.5G) were calculated using input parameters shown in the supporting information (Table 1 supp.). The employed optical properties were previously measured on CIGS samples with different compositions and on all additional layers by transmittance-reflectance spectroscopy and spectroscopic ellipsometry (details published elsewhere [22]). At the voids’ internal surfaces, surface recombination velocities were varied between 2 · 10 3 cm/s and 1 · 10 5 cm/s.10.1080/14686996.2018.1536679-F0009Figure 9.Structures employed in three-dimensional void simulations.…”
Section: Discussionmentioning
confidence: 99%
“…Details can be found in previous publications (standard calibration in the publication by Carron et al [22]).…”
Section: Methodsmentioning
confidence: 99%
“…This was necessary due to an over-estimation of the In signal in the quantification of STEM-EDX at.% maps. We consider the XRF values more reliable than the STEM/EDX quantification, after a careful calibration of the XRF-based quantification method by inductively coupled plasma optical emission spectrometry (ICP-OES), as described in an earlier publication [22]. …”
Structural defects such as voids and compositional inhomogeneities may affect the performance of Cu(In,Ga)Se2 (CIGS) solar cells. We analyzed the morphology and elemental distributions in co-evaporated CIGS thin films at the different stages of the CIGS growth by energy-dispersive x-ray spectroscopy in a transmission electron microscope. Accumulation of Cu-Se phases was found at crevices and at grain boundaries after the Cu-rich intermediate stage of the CIGS deposition sequence. It was found, that voids are caused by Cu out-diffusion from crevices and GBs during the final deposition stage. The Cu inhomogeneities lead to non-uniform diffusivities of In and Ga, resulting in lateral inhomogeneities of the In and Ga distribution. Two and three-dimensional simulations were used to investigate the impact of the inhomogeneities and voids on the solar cell performance. A significant impact of voids was found, indicating that the unpassivated voids reduce the open-circuit voltage and fill factor due to the introduction of free surfaces with high recombination velocities close to the CIGS/CdS junction. We thus suggest that voids, and possibly inhomogeneities, limit the efficiency of solar cells based on three-stage co-evaporated CIGS thin films. Passivation of the voids’ internal surface may reduce their detrimental effects.
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.
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