Bifacial Cu(In,Ga)Se2 solar cells using hydrogen‐doped In2O3 films as a transparent back contact

Keller, Jan; Chen, Weichao; Riekehr, Lars; Kubart, Tomáš; Törndahl, Tobias; Edoff, Marika

doi:10.1002/pip.3025

Cited by 44 publications

(68 citation statements)

References 38 publications

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“…In addition to the use of different passivation layers, the question of Na supply needs to be addressed. TCO layers have often much lower permeability for Na than the standard Mo back contact [104] and differences in the Na concentration further complicate comparison of the device performance.…”

Section: Transparent Back Contactsmentioning

confidence: 99%

Back and front contacts in kesterite solar cells: state-of-the-art and open questions

et al. 2019

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We review the present state-of-the-art within back and front contacts in kesterite thin film solar cells, as well as the current challenges. At the back contact, molybdenum (Mo) is generally used, and thick Mo(S, Se) 2 films of up to several hundred nanometers are seen in record devices, in particular for selenium-rich kesterite. The electrical properties of Mo(S, Se) 2 can vary strongly depending on orientation and indiffusion of elements from the device stack, and there are indications that the back contact properties are less ideal in the sulfide as compared to the selenide case. However, the electronic interface structure of this contact is generally not well-studied and thus poorly understood, and more measurements are needed for a conclusive statement. Transparent back contacts is a relatively new topic attracting attention as crucial component in bifacial and multijunction solar cells. Front illuminated efficiencies of up to 6% have so far been achieved by adding interlayers that are not always fully transparent. For the front contact, a favorable energy level alignment at the kesterite/CdS interface can be confirmed for kesterite absorbers with an intermediate [S]/([S]+[Se]) composition. This agrees with the fact that kesterite absorbers of this composition reach highest efficiencies when CdS buffer layers are employed, while alternative buffer materials with larger band gap, such as Cd 1−x Zn x S or Zn 1−x Sn x O y , result in higher efficiencies than devices with CdS buffers when sulfurrich kesterite absorbers are used. Etching of the kesterite absorber surface, and annealing in air or inert atmosphere before or after buffer layer deposition, has shown strong impact on device performance. Heterojunction annealing to promote interdiffusion was used for the highest performing sulfide kesterite device and air-annealing was reported important for selenium-rich record solar cells. Cu 2 ZnSnS 4 (CZTS) absorbers for solar cell applications was first reported by Ito and Nakazawa [1]. Early results on CZTS device performance were reported by Friedlmeier et al [2], Seol et al [3], and Katagiri et al [4]. Later a group at IBM reached record performances with power conversion efficiencies (PCEs) above 10% for CZTSSe devices in 2010 [5] and the current record PCE of 12.6% in 2013 [6]. In 2018, researchers from DGIST reached the same performance, 12.6%, but for a larger device area [7]. The device structure used for kesterite solar cells was originally copied from that of Cu(In, Ga)Se 2 , (CIGSe) TFSCs, and is formed by sequential deposition, upon a soda lime glass substrate, of a Mo back contact, the absorber, a CdS buffer layer, and a ZnO/ZnO:Al bi-layer window (i.e. transparent top contact). This structure, schematically shown for CZTSSe in figure 1, is not necessarily ideal for kesterite solar cells, and extensive work has been invested into studies of alternative backand front contacts and related deposition processes. In this paper, the state-of-the-art and current open questions related to the back and front c...

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Section: Transparent Back Contactsmentioning

confidence: 99%

Back and front contacts in kesterite solar cells: state-of-the-art and open questions

et al. 2019

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“…In order to investigate the effects and influence of back surface passivation in CIGS solar cells, there are studies that used bi-facial thin CIGS solar cells [28][29][30]. For bi-facial solar cells, all characterizations are made from both the front and back side of the solar cell.…”

Section: Approaches Used To Add the Passivation Layermentioning

confidence: 99%

Dielectric-Based Rear Surface Passivation Approaches for Cu(In,Ga)Se2 Solar Cells—A Review

et al. 2019

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This review summarizes all studies which used dielectric-based materials as a passivation layer at the rear surface of copper indium gallium (di)selenide, Cu(In,Ga)Se2, (CIGS)-based thin film solar cells, up to 2019. The results regarding the kind of dielectric materials, the deposition techniques, contacting approaches, the existence of additional treatments, and current–voltage characteristics (J–V) of passivated devices are emphasized by a detailed table. The techniques used to implement the passivation layer, the contacting approach for the realization of the current flow between rear contact and absorber layer, additional light management techniques if applicable, the solar simulator results, and further characterization techniques, i.e., external quantum efficiency (EQE) and photoluminescence (PL), are shared and discussed. Three graphs show the difference between the reference and passivated devices in terms of open-circuit voltage (Voc), short-circuit current (Jsc), and efficiency (η), with respect to the thicknesses of the absorber layer. The effects of the passivation layer at the rear surface are discussed based on these three graphs. Furthermore, an additional section is dedicated to the theoretical aspects of the passivation mechanism.

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“… the formation of a detrimental Ga oxide layer at the CIGS/TCO back interface, 26–30 which is promoted when an external supply of Na is used 31,32 ; the diffusion of metallic elements from the back contact to the absorber 21,24 …”

Section: Introductionmentioning

confidence: 99%

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

Gouillart

Cattoni

Chen

et al. 2020

Progress in Photovoltaics

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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.

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Bifacial Cu(In,Ga)Se₂ solar cells using hydrogen‐doped In₂O₃ films as a transparent back contact

Cited by 44 publications

References 38 publications

Back and front contacts in kesterite solar cells: state-of-the-art and open questions

Back and front contacts in kesterite solar cells: state-of-the-art and open questions

Dielectric-Based Rear Surface Passivation Approaches for Cu(In,Ga)Se2 Solar Cells—A Review

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

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Bifacial Cu(In,Ga)Se2 solar cells using hydrogen‐doped In2O3 films as a transparent back contact

Cited by 44 publications

References 38 publications

Back and front contacts in kesterite solar cells: state-of-the-art and open questions

Back and front contacts in kesterite solar cells: state-of-the-art and open questions

Dielectric-Based Rear Surface Passivation Approaches for Cu(In,Ga)Se2 Solar Cells—A Review

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

Contact Info

Product

Resources

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Bifacial Cu(In,Ga)Se₂ solar cells using hydrogen‐doped In₂O₃ films as a transparent back contact

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