| 1. |
- Antonio Cabas, Vidani, et al.
(author)
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Influence of the Rear Interface on Composition and Photoluminescence Yield of CZTSSe Absorbers: A Case for an Al2O3 Intermediate Layer
- 2021
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In: ACS Applied Materials and Interfaces. - : American Chemical Society (ACS). - 1944-8244 .- 1944-8252. ; 13:16, s. 19487-19496
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Journal article (peer-reviewed)abstract
- The rear interface of kesterite absorbers with Mo back contact represents one of the possible sources of nonradiative voltage losses (Delta V-oc,V-nrad) because of the reported decomposition reactions, an uncontrolled growth of MoSe2, or a nonoptimal electrical contact with high recombination. Several intermediate layers (IL), such as MoO3, TiN, and ZnO, have been tested to mitigate these issues, and efficiency improvements have been reported. However, the introduction of IL also triggers other effects such as changes in alkali diffusion, altered morphology, and modifications in the absorber composition, all factors that can also influence Delta V-oc,V-nrad. In this study, the different effects are decoupled by designing a special sample that directly compares four rear structures (SLG, SLG/Mo, SLG/Al2O3, and SLG/Mo/Al2O3) with a Na-doped kesterite absorber optimized for a device efficiency >10%. The IL of choice is Al2O3 because of its reported beneficial effect to reduce the surface recombination velocity at the rear interface of solar cell absorbers. Identical annealing conditions and alkali distribution in the kesterite absorber are preserved, as measured by time-of-flight secondary ion mass spectrometry and energy-dispersive X-ray spectroscopy. The lowest Delta V-oc,V-nrad of 290 mV is measured for kesterite grown on Mo, whereas the kesterite absorber on Al2O3 exhibits higher nonradiative losses up to 350 mV. The anticipated field-effect passivation from Al2O3 at the rear interface could not be observed for the kesterite absorbers prepared by the two-step process, further confirmed by an additional experiment with air annealing. Our results suggest that Mo with an in situ formed MoSe2 remains a suitable back contact for high-efficiency kesterite devices.
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| 2. |
- Marquez, Jose A., et al.
(author)
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High-temperature decomposition of Cu2BaSnS4 with Sn loss reveals newly identified compound Cu2Ba3Sn2S8
- 2020
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In: Journal of Materials Chemistry A. - : Royal Society of Chemistry (RSC). - 2050-7488 .- 2050-7496. ; 8:22, s. 11346-11353
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Journal article (peer-reviewed)abstract
- The earth-abundant quaternary compound Cu(2)BaSnS(4)is being currently studied as a candidate for photovoltaics and as a photocathode for water splitting. However, the chemical stability of this phase during synthesis is unclear. The synthesis of other quaternary tin-sulphur-based absorbers (e.g., Cu2ZnSnS4) involves an annealing step at high temperature under sulphur gas atmosphere, which can lead to decomposition into secondary phases involving Sn loss from the sample. As the presence of secondary phases can be detrimental for device performance, it is crucial to identify secondary phase chemical, structural and optoelectronic properties. Here we used a combination ofin situEDXRD/XRF and TEM to identify a decomposition pathway for Cu2BaSnS4. Our study reveals that, while Cu(2)BaSnS(4)remains stable at high sulphur partial pressure, the material decomposes at high temperatures into Cu(4)BaS(3)and the hitherto unknown compound Cu(2)Ba(3)Sn(2)S(8)if the synthesis is performed under low partial pressure of sulphur. The presence of Cu(4)BaS(3)in devices could be harmful due to its high conductivity and relatively lower band gap compared to Cu2BaSnS4. The analysis of powder diffraction data reveals that the newly identified compound Cu(2)Ba(3)Sn(2)S(8)crystallizes in the cubic system (space groupI4x304;3d) with a lattice parameter ofa= 14.53(1) angstrom. A yellow powder of Cu(2)Ba(3)Sn(2)S(8)has been synthesized, exhibiting an absorption onset at 2.19 eV.
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| 3. |
- Phung, Nga, et al.
(author)
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Photoprotection in metal halide perovskites by ionic defect formation
- 2022
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In: Joule. - : Elsevier BV. - 2542-4351. ; 6:9, s. 2152-2174
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Journal article (peer-reviewed)abstract
- Photostability is critical for long-term solar cell operation. While light-triggered defects are usually reported as evidence of material degradation, we reveal that the formation of certain defects in metal halide perovskites is crucial for protection against intense or prolonged light exposure. We identify an inherent self-regulating cycle of formation and recovery of ionic defects under light exposure that mitigates the overheating of the lattice due to hot carrier cooling, which allows exposure to several thousand suns without degrading. The excess energy instead dissipates by forming defects, which in turn alters the optoelectronic properties of the absorber, resulting in a temporary reduction of photon absorption. Defects gradually recover to restore the original optoelectronic properties of the absorber. Photoprotection is a key feature for the photostability in plants. Thus, finding a protection mechanism in metal halide perovskites similar to those in nature is encouraging for the development of long-term sustainable solar cells.
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| 4. |
- Platzer Björkman, Charlotte, 1976-, et al.
(author)
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Back and front contacts in kesterite solar cells : state-of-the-art and open questions
- 2019
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In: Journal of Physics. - : Institute of Physics Publishing (IOPP). - 2515-7655. ; 1:4
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Journal article (peer-reviewed)abstract
- 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 Cd1−x Zn x S or Zn1−x Sn x O y , result in higher efficiencies than devices with CdS buffers when sulfur-rich 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.
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| 5. |
- Scragg, Jonathan, 1983-, et al.
(author)
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A low-temperature order-disorder transition in Cu2ZnSnS4 thin films
- 2014
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In: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 104:4, s. 041911-
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Journal article (peer-reviewed)abstract
- Cu2ZnSnS4(CZTS) is an interesting material for sustainable photovoltaics, but efficiencies are limitedby the low open-circuit voltage. A possible cause of this is disorder among the Cu and Zn cations, aphenomenon which is difficult to detect by standard techniques. We show that this issue can beovercome using near-resonant Raman scattering, which lets us estimate a critical temperature of 533±10 K for the transition between ordered and disordered CZTS. These findings have deepsignificance for the synthesis of high-quality material, and pave the way for quantitative investigationof the impact of disorder on the performance of CZTS-based solar cells.
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| 6. |
- Thomas, Sinju, et al.
(author)
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Effects of material properties of band-gap-graded Cu(In,Ga)Se-2 thin films on the onset of the quantum efficiency spectra of corresponding solar cells
- 2022
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In: Progress in Photovoltaics. - : John Wiley & Sons. - 1062-7995 .- 1099-159X. ; 30:10, s. 1238-1246
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Journal article (peer-reviewed)abstract
- Polycrystalline Cu(In,Ga)Se-2 (CIGSe) thin-film solar cells exhibit gradual onset in their external quantum efficiency (EQE) spectra whose shape can be affected by various CIGSe material properties. Apart from influences on the charge-carrier collection, a broadening of the EQE onset leads to enhanced radiative losses in open-circuit voltage (V-oc). In the present work, Gaussian broadening of parameters describing the EQE onset of thin-film solar cells, represented by the standard deviation, sigma(total), was evaluated to study the impacts of the effective band-gap energy, the electron diffusion length, and the Ga/In gradient in the CIGSe absorber. It is shown that sigma(total) can be disentangled into contributions of these material properties, in addition to a residual component sigma(residual). Effectively, sigma(total) depends only on a contribution related to the Ga/In gradient as well as on sigma(residual). The present work highlights the connection of this compositional gradient, the microstructure in the polycrystalline CIGSe absorber, and the luminescence emission with the residual component sigma(residual). It is demonstrated that a flat band-gap with no compositional gradient in the bulk of the CIGSe absorber is essential to obtain the lowest sigma(total) values and thus result in lower recombination losses and gains in V-oc.
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