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- Kotipalli, Ratan, et al.
(author)
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Addressing the impact of rear surface passivation mechanisms on ultra-thin Cu(In,Ga)Se2 solar cell performances using SCAPS 1-D model
- 2017
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In: Solar Energy. - : Elsevier. - 0038-092X .- 1471-1257. ; 157, s. 603-613
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Journal article (peer-reviewed)abstract
- We present a (1-D) SCAPS device model to address the following: (i) the surface passivation mechanisms (i.e.field-effect and chemical), (ii) their impact on the CIGS solar cell performance for varying CIGS absorberthickness, (iii) the importance of fixed charge type (+/−) and densities of fixed and interface trap charges, and(iv) the reasons for discrete gains in the experimental cell efficiencies (previously reported) for varying CIGSabsorber thickness. First, to obtain a reliable device model, the proposed set of parameters is validated for bothfield-effect (due to fixed charges) and chemical passivation (due to interface traps) using a simple M-I-S teststructure and experimentally extracted values (previously reported) into the SCAPS simulator. Next, we providefigures of merits without any significant loss in the solar cell performances for minimum net −Qf and maximumacceptable limit for Dit, found to be ∼5 × 1012 cm−2 and ∼1 × 1013 cm−2 eV−1 respectively. We next showthat the influence of negative fixed charges in the rear passivation layer (i.e. field-effect passivation) is morepredominant than that of the positive fixed charges (i.e. counter-field effect) especially while considering ultrathin(<0.5 μm) absorber layers. Furthermore, we show the importance of rear reflectance on the short-circuitphotocurrent densities while scaling down the CIGS absorber layers below 0.5 μm under interface chemical andfield-effect passivation mechanisms. Finally, we provide the optimal rear passivation layer parameters for efficienciesgreater than 20% with ultra-thin CIGS absorber thickness (<0.5 μm). Based on these simulation results,we confirm that a negatively charged rear surface passivation with nano-point contact approach is efficient forthe enhancement of cell performances, especially while scaling down the absorber thickness below 0.5 μm.
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| 2. |
- Poncelet, Olivier, et al.
(author)
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Optimisation of rear reflectance in ultra-thin CIGS solar cells towards>20% efficiency
- 2017
-
In: Solar Energy. - : Elsevier BV. - 0038-092X .- 1471-1257. ; 146, s. 443-452
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Journal article (peer-reviewed)abstract
- In order to decrease their cost and the use of rare metal elements, thin film solar cell thicknesses are con-tinuously reduced at the expense of their efficiency, due to a lack of absorption for long wavelengths.Optimisation of cells rear reflectance (Rb) thus becomes meaningful to provide non-absorbed light a sec-ond chance to be harvested by the active cell layer. In this sense, we present a way to keep the rear reflec-tance in advanced Cu(In, Ga) Se2(CIGS) cell as high as possible while keeping in mind the progressalready done regarding the rear passivation techniques. We show that introducing a stack of thin Al2O3 and aluminium between the CIGS layer and the rear molybdenum electrode increases Rbup to92% in the long wavelength 800–1100 nm range. Several other stacks, using MgF2, SiO2or TiO2, are opti-mised in order to investigate the best trade-off between passivation, material consumption and perfor-mances, resulting in Rbranging from 42% (moderate case) to 99% in the best case. Those CIGS rearinterface reflectance optimisations were performed by using a standard transfer matrix method (TMM)in the long wavelength range. Seven interesting stacks are then analysed for solar cell performances usingSCAPS simulation software to understand the impact of rear reflectance on short circuit current density(Jsc) and eventually on the cell efficiency (g), for ultra-thin CIGS absorber thicknesses (<1 lm). Based onthese results, we propose Rboptimisation to achieve Jsc> 40 mA/cm2and g > 20% with a 500 nm-thickCIGS absorber film using CIGS-Al2O3-Mo stack, where the Al2O3thickness can be chosen in between104 and 139 nm. This way, we can ensure good rear reflectance (Rb= 65%) and reduced interface recom-bination while being industrially feasible with present technologies.
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