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Abstract
A novel back-contacted solar cell based on a submicron copper indium gallium (di)selenide (CIGS) absorber is proposed and optically investigated. First, charge carrier collection feasibility is studied by band diagram analysis. Then, two back-contacted configurations are suggested and optimized for maximum current production. The results are compared with a reference front/back-contacted CIGS solar cell with a 750-nm-thick absorber. Current density production of 38.84 mA/cm2 is predicted according to our simulations for a realistic front-side texturing. This shows more than 38% improvement in optical performance compared to the reference cell and only 7.7% deviation from the theoretical Green absorption benchmark.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Reducing the absorber thickness of copper indium gallium (di)selenide (CIGS) solar cells to values below 1 μm is the key to more industrially viable thin-film solar cells [1-3]. The resulting optical losses can be compensated by advanced light management techniques. These techniques include: (i) light in-coupling [4-7], (ii) light scattering [8-10] and (iii) light trapping [3,11]. Even though the performance of ultra-thin CIGS solar cells can be improved by the abovementioned methods, there are still more challenges to overcome in this topic. Firstly, the parasitic absorption in the front layers of a submicron CIGS solar cell contributes to more than 10% of optical losses [4]. Secondly, for CIGS devices deposited on thin flexible foils [12], the metallic grid at the front side of a front/back-contacted (FBC) solar cell, blocks a significant part of the incident light from entering the cell (optical shading) [13].
In an interdigitated back-contacted (IBC) solar cell, both electron (e) and hole (h) contacts (e-contact and h-contact) are alternatively located at the rear side of the absorber, as demonstrated in a number of high efficiency c-Si IBC solar cells [14-22]. This way, the optical shading and parasitic absorption are eliminated and the high energy photons can reach the absorber bulk and contribute to charge carrier generation. In this work, we introduce an IBC CIGS solar cell with a submicron absorber thickness. The collection feasibility of both charge carriers is studied by band diagram analysis. Two light in-coupling configurations, namely, high aspect ratio front textures (see Fig. 1) and double-layer antireflection coating (ARC, see Fig. 2) will be investigated by means of rigorous three-dimensional (3D) optical simulations. In both configurations, the e-contact dimensions are optimized for maximum current generation. This study provides guidelines to CIGS research community about IBC CIGS solar cells, which to our knowledge has not been studied so far.
Fig. 1 Visual rendering of the back-contacted CIGS solar cell with high aspect ratio features at the front. WTCO and HTCO indicate, respectively, width and height of the TCO.
Fig. 2 Visual rendering of the back-contacted solar cell with natural CIGS morphology and optimized ARC. WTCO and HTCO indicate, respectively, width and height of the TCO.
2. Modelling platform
Sentaurus TCAD simulator [14,23,24] was used to compute the band diagram of the e- and h-contacts in equilibrium conditions. The physical parameters of each material are listed in Table 1.
Table 1. The model parameters used for band diagram modelling in TCAD software. Density of states of electrons and holes are indicated with eDOS and hDOS, respectively. Subscripts A and D stand for acceptor and donor, respectively.
The optical performance of the solar cell was modelled by using a 3D Maxwell’s equation solver, called Ansys HFSS. This finite element method-based tool allows for calculating the absorptance (A) and reflectance (R) spectra of complex thin-film structures with sub-wavelength features [4,28-31]. The structure is discretized (meshed) by tetrahedrons and the Maxwell’s equations are iteratively solved at each frequency until the solution reaches an acceptable level of convergence [31]. The optical constants of CIGS [28,32], Mo [28,33], ZnO:Ga (GZO) [26], MgF2 [34] and Al2O3 [27] are used as modelling inputs. Master and slave boundary conditions are deployed to model the periodic structure. More details about the modelling scheme can be found in [4,28]. The current density generated in the absorber (Jph-CIGS) or dissipated in the i-th layer of the structure (Jph-i) was calculated by integrating the product of wavelength-dependent absorptance spectrum (Ai) with AM1.5G photon flux (Φ(λ) [35]):
where q is the elementary charge and i refers to the i-th different layer. Here, Jph-CIGS represents the short circuit current density (Jsc) of the solar cell, assuming full charge carrier collection. It should be noted that both transverse electric (TE) and transverse magnetic (TM) polarizations are considered in the simulations and each absorptance spectrum is the average between the related spectra obtained from the two polarizations.Green absorption limit [36] is used as a benchmark with which the optical performance is compared. This parameter describes the maximum absorption spectrum of a randomly textured slab of a material (here, CIGS) for which (i) front reflectance is completely quenched, (ii) light propagating in it is completely randomized, and (iii) rear internal reflectance caused by a lossless metallic reflector is maximized. The Green limit is calculated as:
in which α is the absorption coefficient, d is thickness and n is refractive index of the absorbing material.The proposed solar cell structures are compared to a reference FBC cell with a CIGS thickness of 750 nm. From the light-facing window layers to the rear contact, the cell structure comprises the following layers: ZnO:Al / i-ZnO / CdS / CIGS / Mo. More details about the optical modelling of the reference cell can be found in [4] and [28].
It is known that the number of the absorbed photons is directly related to the absorber volume [37,38]. Therefore, the amount of absorber material in all of the configurations in this work is kept constant. Additionally, in order to account for the whole optically-active part of the absorber, the optical thickness is defined as the thickness from the top of the textures to the bottom of the absorber [38,39]. It should be noted that the optical thickness is different from the equivalent thickness (the ratio between the unit cell volume and its bottom area). Hence, the deviation of Jph from Green limit for a CIGS slab with the same optical thickness can be calculated as [38]:
where JGreen is the Green absorption limit integrated according to Eq. (1).3. Results and discussion
3.1 Design considerations
As mentioned in the introduction, light management applied to both front and back of the solar cell can improve the optical performance of ultra-thin CIGS solar cells [4]. However, the parasitic absorption of the front layers and the optical shading caused by the front contact still hinder full usage of high energy photons. In this part, we suggest an IBC configuration to overcome those challenges and step by step investigate the design considerations.
First of all, note that in order to prevent the charge carrier recombination, the contact distance should be smaller than twice the minority carrier diffusion length. In case of a sub-micron thick CIGS layer, in which [Ga]/([Ga] + [In]) ratio (GGI) is almost uniform [40], the diffusion length is close to 1 μm [41-44], which is considered in our design platform.
In the IBC solar cell, the CIGS front surface should be chemically and preferably electrically passivated. This can be achieved by deploying an Al2O3 layer, either by atomic layer deposition (ALD) or by plasma-enhanced chemical vapor deposition (PECVD). The latter allows for higher growth rates and thicker layers of Al2O3. In both cases, the surface passivation quality can be significantly improved by annealing at 400-450 °C [45,46].
The antireflection effect can be realized by evaporating a layer of MgF2 on the front side of the solar cell [47]. As the refractive index (n) of MgF2 is lower than that of Al2O3, the grading in n from CIGS to that of air leads to a wideband antireflection effect.
At the rear side, the e- and h-contacts should be separated by a dielectric to avoid electrical shunts. In this case, MgF2 with a high thermal resistivity [48] is chosen. A good chemical passivation will be guaranteed by the addition of a thin layer of Al2O3 onto the MgF2 layer. Optically, the low refractive index of this dielectric stack increases the internal rear reflection and therefore improves the light’s path length in the absorber [28].
A silver back reflector is placed at the rear side of the cell to ensure full internal rear reflection. This is merely to exclude the possible transmission losses from the optical performance in the model. Albeit, in practice, the cells can be fabricated on conventional soda lime glass [49,50] or flexible substrates [12,51] and a back reflector can be added to the rear side, if necessary.
Molybdenum is used for the h-contact, as it is proven to be inert, stands the high temperature fabrication process of CIGS and also forms with the absorber an ohmic contact for hole collection [52]. For the e-contact, an n-type transparent conductive oxide (TCO) should be selected to successfully collect the electrons. In our simulations, gallium-doped zinc oxide (GZO) type-a from [26] is used for this purpose. High carrier concentration, low free carrier absorption, transparency, high thermal stability and abundant constituting elements are the advantages of this material over other TCOs [26,31].
To ensure that charge carriers will be properly collected, an assessment of the band diagram of both contacts is necessary. Figure 3 shows the band diagram of the solar cell structure from the top Al2O3 layer to the e- and h-contacts. As can be seen in Fig. 3(b), at the front side, due to fixed negative charges in Al2O3 [53,54], an electric field, also known as front surface field [55] is formed in a way that allows accumulation of majority carriers, while repelling minority carriers at Al2O3/CIGS interface. The resulting electrical passivation prevents minority charge carriers’ recombination at the front side. At the rear side, high doping concentration makes GZO a degenerate semiconductor with a work function of about 4.26 eV. The resulting band bending inside CIGS forms a p-n junction in favor of electron collection and also hole repulsion. At the h-contact (Fig. 3(c)), Mo work-function (4.6 eV) enables an ohmic contact for CIGS majority carriers (holes) and therefore, their collection.
Fig. 3 a) Simplified layer stack of the e- and h-contacts in the envisioned IBC solar cell. From top to bottom: MgF2, Al2O3, CIGS, GZO (e-contact on left-hand side) and Mo (h-contact on the right-hand side). Band diagrams in equilibrium b) and c) refer to e- and h-contact, respectively. EF, Ev and Ec are Fermi, valance band edge and conduction band edge energies, respectively.
3.2 IBC solar cell with antireflective front textures
Figure 1 shows a visual rendering of the IBC CIGS solar cell. The presence of steep grooves with height to width ratio of 2 at the front side promotes light in-coupling and, hence, minimal reflection losses. As mentioned earlier, a thin layer of Al2O3 improves the chemical and electrical passivation of the front surface.
The height and width of GZO (HTCO and WTCO, respectively) were optimized for the best current density production. WTCO was varied between 400 and 1000 nm in steps of 100 nm. HTCO took values between 240 (30 nm thicker than Mo to ensure a gap between Mo and Ag) and 330 nm in steps of 30 nm. Figure 4(a) shows Jph-CIGS as a function of these parameters. Maximal Jph-CIGS can be obtained for WTCO > 800 nm and HTCO > 290 nm. Even though the optical performance might be better for larger WTCO, we limited its range to values below the diffusion length to ensure good electrical performance as well. As a result, we selected 1000 nm and 320 nm as the optimal values for WTCO and HTCO, respectively. The absorptance and reflectance (1-R) spectra of the resulting configuration compared to the absorptance spectrum of the reference FBC solar cell is presented in Fig. 4(b). Owing to the elimination of front layers’ parasitic absorption and the low reflectance, almost all of the incident high energy photons are absorbed by the CIGS layer. This leads to an improvement of Jph from 28.04 mA/cm2 for the reference cell to 39.69 mA/cm2 for the IBC cell with optimal TCO (41.55% improvement). Using Eq. (3) to calculate ΔGreen for an optical thickness of 1120 nm (the peak-to-valley height of the textures plus the bulk thickness) shows that the IBC solar cell deviates from the benchmark by only 6.65%.
Fig. 4 a) Implied photocurrent density in CIGS layer (Jph-CIGS) as a function of width and height of TCO. b) absorptance and 1-R spectra of the IBC solar cell when WTCO = 1000 nm and HTCO = 320 nm.
Even though the development of high aspect ratio textures on a CIGS layer is proven to be possible [56], this approach would need the partial removal of the absorber by ion bombardment [57] or wet etching [58]. This is in contrast with the photovoltaic (PV) market goal of increasing industrial throughput of CIGS PV technology by reducing material consumption [59-61]. In the following section, the development of the back-contacted solar cell with natural CIGS morphology is studied.
3.3 IBC solar cell with as-grown absorber morphology
According to our atomic force microscopy measurements for CIGS samples made at TNO [50], the lateral correlation length of the as-grown grains is about 330 nm [4,28]. This was included in the structure of a back-contacted CIGS solar cell model without antireflective textures (see Fig. 2). The natural roughness was modelled by introducing periodic truncated pyramids on the absorber bulk. Even though this is a simplification of the realistic device, the calibrated external quantum efficiency (EQE) and reflectance spectra sufficiently match the measured counterparts, confirming the validity of the assumption [28]. Note that the absorber volume in Figs. 1 and 2 are equal for fair comparison. In the absence of steep features at the front side for high light in-coupling, an alternative approach was employed. The thickness of passivating Al2O3 and antireflective MgF2 layers in a reference solar cell were optimized for minimal reflection. The optimization algorithm is discussed in more details in [4]. The resulting thicknesses for Al2O3 and MgF2 are 80 and 85 nm, respectively.
Performing the same TCO optimization procedure as in the previous session leads to the results shown in Fig. 5. Jph-CIGS values higher than 38.8 mA/cm2 can be achieved for WTCO > 950 nm and HTCO > 300 nm. The optimal combination of these parameters (WTCO = 1000 nm and HTCO = 320 nm) resulted in the absorptance and 1-R spectra plotted in Fig. 5(b). As it can be seen in such a figure, a small drop in absorptance occurs at short wavelength region due to higher reflection from the front side of the cell. In total, Jph-CIGS = 38.84 mA/cm2 is expected from the optimized cell, showing less than 0.9 mA/cm2 decrease in Jph-CIGS compared to the previous design. Considering cheaper and easier fabrication process of this approach, the drop in Jph-CIGS appears negligible. In this case, where the optical thickness is 750 nm, JGreen = 42.11 mA/cm2, and therefore, according to Eq. (3), ΔGreen will be 7.7%. This value is still very small compared to the case of the reference cell for which ΔGreen = 33.4%. The Jph-CIGS obtained in this work is comparable to the Jsc of world record efficiency CIGS solar cell [62], [63], in which parasitic absorption is still not addressed and the absorber thickness is considerably thicker than that of our design.
Fig. 5 a) Implied photocurrent density (Jph) as a function of width and height of TCO. b) absorptance and 1-R spectra of the IBC solar cell when WTCO = 1000 nm and HTCO = 320 nm.
We analyzed the optical performance of the IBC cell in more details for wavelengths longer than 900 nm. Figure 6(a) shows a close-up of Fig. 5(b) overlaid by TM and TE components of the absorptance spectrum. As can be seen in this figure, the wavelength and amplitude of the average CIGS absorptance in low-energy part of the spectrum is mainly dominated by TM polarization. Figures 6(b)-6(d) present the electric field magnitude () at wavelengths corresponding to three local maxima in the TM-polarized absorptance spectrum. These maxima are located at wavelengths 1020, 1070 and 1110 nm, indicated with red arrows in Fig. 6(a). It should be noted that each local maximum in CIGS absorptance is correlated with a local minimum in total reflectance. This means that as more photons are trapped in the solar cell bulk, less photons escape from the cell. At 1020 nm (Fig. 6(b)), the Fabry-Perót modes are the main components of the electric field, causing the light to mainly propagate in perpendicular direction with respect to the plane of incidence [64]. As the wavelength increases, the diffraction modes triggered by the rear dielectric/metal IBC arrangement (GZO / MgF2 / Mo / MgF2) and coupled with waveguide modes inside the absorber bulk outweigh the Fabry-Perót modes. This phenomenon is clearly visible in Figs. 6(c) and 6(d), where waveguide modes are the dominant components of the electric field.
Fig. 6 a) CIGS absorption for TM (blue line), TE (black line) and average of TM and TE (brown area) polarizations and average 1-R (yellow area) spectra. The graphs correspond to the optimized IBC solar cell with as-grown CIGS grains. b) – d) Electric field magnitude at wavelength b) 1020 nm, c) 1070 nm and d) 1110 nm for TM polarization corresponding to local maxima in CIGS absorption, also indicated with red arrows in a).
Even though the CIGS absorptance might have a slightly different shape in the two IBC designs in the long-wavelength regime (see Fig. 4(b) and Fig. 5(b)), the total generated Jph in that part is not very different. For instance, for wavelengths between 900 and 1200 nm, the obtained Jph’s from the first and the second IBC structures are 7.09 and 6.62 mA/cm2, respectively. This is due to the limited capability of the absorber in absorbing close-to-bandgap photons. Therefore, the higher intensity of the electric field in Fig. 6(d) does not necessarily mean that the absorber will be capable of absorbing all of the trapped photons.
It should be noted that we also considered ITO as e-contact material in our models. In that case, due to the lower transparency of ITO material [26], less Jph-CIGS can be expected by the optimized structure. More importantly, the presence of indium in ITO contradicts our goal of decreasing indium usage and thus fabricating cost-effective CIGS solar cells. Hence, we excluded ITO from our proposed IBC structure.
4. Conclusions
CIGS with a low bandgap and high absorption coefficient, is a great candidate for thin-film solar cell applications. High parasitic absorption in front/back-contacted solar cell architectures hinders the full utilization of CIGS’s optical potential. In this work a back-contacted CIGS solar cell was proposed. The assessment of electron- and hole-contacts’ band diagram shows well-functioning structure from electrical point of view. An IBC cell with antireflective textures and optimized TCO dimensions showed a potential 41.55% improvement in implied photocurrent density (Jph-CIGS) compared to a front/back-contacted reference cell. A simpler structure with as-grown CIGS morphology and optimized flat antireflection coating revealed a maximum Jph-CIGS value of 38.84 mA/cm2. This value deviates from the theoretical Green limit by only 7.7% and is comparable to the short circuit current density of the world record CIGS solar cell, albeit with significantly thicker absorber.
Even though the fabrication of our proposed IBC structure in the current design might be complicated, eventually including a number of etching and lithography steps, we believe that this is an important step towards the development of high efficiency and cost-effective CIGS solar cells. More theoretical and experimental studies are needed to reach a balance between the cost and efficiency of such devices.
References
1. B. Vermang, J. T. Wätjen, V. Fjällström, F. Rostvall, M. Edoff, R. Gunnarsson, I. Pilch, U. Helmersson, R. Kotipalli, F. Henry, and D. Flandre, “Highly reflective rear surface passivation design for ultra-thin Cu(In,Ga)Se2 solar cells,” Thin Solid Films 582, 300–303 (2015). [CrossRef]
2. L. M. Mansfield, A. Kanevce, S. P. Harvey, K. Bowers, C. Beall, S. Glynn, and I. L. Repins, “Efficiency increased to 15.2% for ultra-thin Cu(In,Ga)Se2 solar cells,” Prog. Photovolt. Res. Appl. 26(11), 949–954 (2018). [CrossRef]
3. G. Yin, P. Manley, and M. Schmid, “Light trapping in ultrathin CuIn1-xGaxSe2 solar cells by dielectric nanoparticles,” Sol. Energy 163, 443–452 (2018). [CrossRef]
4. N. Rezaei, O. Isabella, Z. A. E. P. Vroon, and M. Zeman, “Optical optimization of a multi-layer wideband anti-reflection coating using porous MgF2 for sub-micron-thick CIGS solar cells,” Sol. Energy 177, 59–67 (2019).
5. S. HwangJ.-H. Jang, “3D simulations for the optimization of antireflection subwavelength structures in CIGS solar cells,” in Photovoltaic Specialists Conference (PVSC), 2012 38th IEEE, 864–867 IEEE, (2012). [CrossRef]
6. L. Aé, D. Kieven, J. Chen, R. Klenk, T. Rissom, Y. Tang, and M. C. Lux-Steiner, “ZnO nanorod arrays as an antireflective coating for Cu (In, Ga) Se2 thin film solar cells,” Prog. Photovolt. Res. Appl. 18(3), 209–213 (2010). [CrossRef]
7. J. Krc, M. Sever, A. Campa, Z. Lokar, B. Lipovsek, and M. Topic, “Optical confinement in chalcopyrite based solar cells,” Thin Solid Films 633, 193–201 (2016).
8. M. Schmid, P. Manley, A. Ott, M. Song, and G. Yin, “Nanoparticles for light management in ultrathin chalcopyrite solar cells,” J. Mater. Res. 31(21), 3273–3289 (2016).
9. B. Vermang, J. T. Wätjen, V. Fjällström, F. Rostvall, M. Edoff, R. Gunnarsson, I. Pilch, U. Helmersson, R. Kotipalli, F. Henry, and D. Flandre, “Highly reflective rear surface passivation design for ultra-thin Cu (In, Ga) Se2 solar cells,” Thin Solid Films 582, 300–303 (2015). [CrossRef]
10. C. Onwudinanti, R. Vismara, O. Isabella, L. Grenet, F. Emieux, and M. Zeman, “Advanced light management based on periodic textures for Cu(In,Ga)Se2 thin-film solar cells,” Opt. Express 24(6), A693–A707 (2016). [CrossRef] [PubMed]
11. J. Goffard, C. Colin, F. Mollica, A. Cattoni, C. Sauvan, P. Lalanne, J.-F. Guillemoles, N. Naghavi, and S. Collin, “Light trapping in ultrathin CIGS solar cells with nanostructured back mirrors,” IEEE J. Photovolt. 7(5), 1433–1441 (2017). [CrossRef]
12. R. Kaczynski, J. Lee, J. van Alsburg, B. Sang, U. Schoop, J. Britt, “In-line Potassium Fluoride Treatment of CIGS Absorbers Deposited on Flexible Substrates in a Production-Scale Process Tool,” in IEEE PVSC, 1455–1458 IEEE, (2017).
13. M. D. Lammert and R. J. Schwartz, “The interdigitated back contact solar cell: a silicon solar cell for use in concentrated sunlight,” IEEE Trans. Electron Dev. 24(4), 337–342 (1977). [CrossRef]
14. P. Procel, A. Ingenito, R. De Rose, S. Pierro, F. Crupi, M. Lanuzza, G. Cocorullo, O. Isabella, and M. Zeman, “Opto-electrical modelling and optimization study of a novel IBC c-Si solar cell,” Prog. Photovolt. Res. Appl. 25(6), 452–469 (2017). [CrossRef]
15. G. Yang, P. Guo, P. Procel, G. Limodio, A. Weeber, O. Isabella, and M. Zeman, “High-efficiency black IBC c-Si solar cells with poly-Si as carrier-selective passivating contacts,” Sol. Energy Mater. Sol. Cells 186, 9–13 (2018). [CrossRef]
16. D. D. Smith, G. Reich, M. Baldrias, M. Reich, N. Boitnott, G. Bunea, “Silicon solar cells with total area efficiency above 25%,” in Photovoltaic Specialists Conference (PVSC), 2016 IEEE 43rd, 3351–3355 IEEE, (2016). [CrossRef]
17. P. Wagner, J. C. Stang, M. Mews, A. B. Morales-Vilches, B. Stannowski, B. Stegemann, and L. Korte, “Interdigitated back contact silicon heterojunction solar cells: Towards an industrially applicable structuring method,” AIP Conf. Proc. 1999, 60001 (2018). [CrossRef]
18. A. Tomasi, B. Paviet-Salomon, Q. Jeangros, J. Haschke, G. Christmann, L. Barraud, A. Descoeudres, J. P. Seif, S. Nicolay, M. Despeisse, S. De Wolf, and C. Ballif, “Simple processing of back-contacted silicon heterojunction solar cells using selective-area crystalline growth,” Nat. Energy 2(5), 17062 (2017). [CrossRef]
19. K. Yoshikawa, H. Kawasaki, W. Yoshida, T. Irie, K. Konishi, K. Nakano, T. Uto, D. Adachi, M. Kanematsu, H. Uzu, and K. Yamamoto, “Silicon heterojunction solar cell with interdigitated back contacts for a photoconversion efficiency over 26%,” Nat. Energy 2(5), 17032 (2017). [CrossRef]
20. F. Haase, C. Hollemann, S. Schäfer, A. Merkle, M. Rienäcker, J. Krügener, R. Brendel, and R. Peibst, “Laser contact openings for local poly-Si-metal contacts enabling 26.1%-efficient POLO-IBC solar cells,” Sol. Energy Mater. Sol. Cells 186, 184–193 (2018). [CrossRef]
21. K. Masuko, M. Shigematsu, T. Hashiguchi, D. Fujishima, M. Kai, N. Yoshimura, T. Yamaguchi, Y. Ichihashi, T. Mishima, N. Matsubara, T. Yamanishi, T. Takahama, M. Taguchi, E. Maruyama, and S. Okamoto, “Achievement of more than 25% conversion efficiency with crystalline silicon heterojunction solar cell,” IEEE J. Photovolt. 4(6), 1433–1435 (2014). [CrossRef]
22. J. Nakamura, N. Asano, T. Hieda, C. Okamoto, H. Katayama, and K. Nakamura, “Development of heterojunction back contact Si solar cells,” IEEE J. Photovolt. 4(6), 1491–1495 (2014). [CrossRef]
23. Synopsys, Sentaurus Device User (2014).
24. P. Procel, G. Yang, O. Isabella, and M. Zeman, “Theoretical evaluation of contact stack for high efficiency IBC-SHJ solar cells,” Sol. Energy Mater. Sol. Cells 186(May), 66–77 (2018). [CrossRef]
25. J. Pettersson, C. Platzer-Björkman, U. Zimmermann, and M. Edoff, “Baseline model of graded-absorber Cu(In,Ga)Se2 solar cells applied to cells with Zn1−xMgxO buffer layers,” Thin Solid Films 519(21), 7476–7480 (2011). [CrossRef]
26. H. Fujiwara and M. Kondo, “Effects of carrier concentration on the dielectric function of ZnO:Ga and In2O3:Sn studied by spectroscopic ellipsometry: Analysis of free-carrier and band-edge absorption,” Phys. Rev. B Condens. Matter Mater. Phys. 71(7), 75109 (2005). [CrossRef]
27. G. Dingemans and W. M. M. Kessels, “Status and prospects of Al2O3-based surface passivation schemes for silicon solar cells,” J. Vac. Sci. Technol. A 30(4), 040802 (2012).
28. N. Rezaei, O. Isabella, Z. Vroon, and M. Zeman, “Quenching Mo optical losses in CIGS solar cells by a point contacted dual-layer dielectric spacer: a 3-D optical study,” Opt. Express 26(2), A39–A53 (2018). [CrossRef] [PubMed]
29. R. Vismara, O. Isabella, and M. Zeman, “Back-contacted BaSi2 solar cells: an optical study,” Opt. Express 25(8), A402–A408 (2017). [CrossRef] [PubMed]
30. O. Isabella, S. Solntsev, D. Caratelli, and M. Zeman, “3-D optical modeling of thin-film silicon solar cells on diffraction gratings,” Prog. Photovolt. Res. Appl. 21(1), 94–108 (2013). [CrossRef]
31. O. Isabella, H. Sai, M. Kondo, and M. Zeman, “Full-wave optoelectrical modeling of optimized flattened light-scattering substrate for high efficiency thin-film silicon solar cells,” Prog. Photovolt. Res. Appl. 22(6), 671–689 (2014). [CrossRef]
32. O. Lundberg, M. Bodegård, J. Malmström, and L. Stolt, “Influence of the Cu(In,Ga)Se2 thickness and Ga grading on solar cell performance,” Prog. Photovolt. Res. Appl. 11(2), 77–88 (2003). [CrossRef]
33. C. Onwudinanti, R. Vismara, O. Isabella, L. Grenet, F. Emieux, and M. Zeman, “Advanced light management based on periodic textures for Cu(In,Ga)Se2 thin-film solar cells,” Opt. Express 24(6), A693–A707 (2016). [CrossRef] [PubMed]
34. M. J. Dodge, “Refractive properties of magnesium fluoride,” Appl. Opt. 23(12), 1980–1985 (1984). [CrossRef] [PubMed]
35. NREL, “Reference solar spectral irradiance: air mass 1.5,” <https://www.nrel.gov/grid/solar-resource/spectra.html>.
36. M. A. Green, “Lambertian light trapping in textured solar cells and light-emitting diodes: Analytical solutions,” Prog. Photovolt. Res. Appl. 10(4), 235–241 (2002). [CrossRef]
37. K. X. Wang, Z. Yu, V. Liu, Y. Cui, and S. Fan, “Absorption enhancement in ultrathin crystalline silicon solar cells with antireflection and light-trapping nanocone gratings,” Nano Lett. 12(3), 1616–1619 (2012). [CrossRef] [PubMed]
38. O. Isabella, R. Vismara, D. N. P. Linssen, K. X. Wang, S. Fan, and M. Zeman, “Advanced light trapping scheme in decoupled front and rear textured thin-film silicon solar cells,” Sol. Energy 162, 344–356 (2018). [CrossRef]
39. C. S. Schuster, A. Bozzola, L. C. Andreani, and T. F. Krauss, “How to assess light trapping structures versus a Lambertian Scatterer for solar cells?” Opt. Express 22(S2Suppl 2), A542–A551 (2014). [CrossRef] [PubMed]
40. E. Jarzembowski, M. Maiberg, F. Obereigner, K. Kaufmann, S. Krause, and R. Scheer, “Optical and electrical characterization of Cu (In, Ga) Se2 thin film solar cells with varied absorber layer thickness,” Thin Solid Films 576, 75–80 (2015). [CrossRef]
41. G. Brown, V. Faifer, A. Pudov, S. Anikeev, E. Bykov, M. Contreras, and J. Wu, “Determination of the minority carrier diffusion length in compositionally graded Cu(In, Ga)Se2 solar cells using electron beam induced current,” Appl. Phys. Lett. 96(2), 22104 (2010). [CrossRef]
42. T. Hara, T. Maekawa, S. Minoura, Y. Sago, S. Niki, and H. Fujiwara, “Quantitative Assessment of Optical Gain and Loss in Submicron-Textured CuIn 1−x Ga x Se2 Solar Cells Fabricated by Three-Stage Coevaporation,” Phys. Rev. Appl. 2(3), 34012 (2014). [CrossRef]
43. O. Lundberg, M. Edoff, and L. Stolt, “The effect of Ga-grading in CIGS thin film solar cells,” Thin Solid Films 480, 520–525 (2005). [CrossRef]
44. R. Kniese, M. Powalla, and U. Rau, “Evaluation of electron beam induced current profiles of Cu(In,Ga)Se2 solar cells with different Ga-contents,” Thin Solid Films 517(7), 2357–2359 (2009). [CrossRef]
45. B. Macco, “Atomic layer deposition of metal oxide thin films for Si heterojunction solar cells,” Technische Universiteit Eindhoven (2016).
46. G. Dingemans, M. C. M. van de Sanden, and W. M. M. Kessels, “Influence of the Deposition Temperature on the c-Si Surface Passivation by Al2O3 Films Synthesized by ALD and PECVD,” Electrochem. Solid-State Lett. 13(3), H76 (2010). [CrossRef]
47. P. Jackson, R. Wuerz, D. Hariskos, E. Lotter, W. Witte, and M. Powalla, “Effects of heavy alkali elements in Cu(In,Ga)Se2 solar cells with efficiencies up to 22.6%,” Phys. Status Solidi-R 10(8), 583–586, Wiley Online Library (2016).
48. J. D. Bass, C. Boissiere, L. Nicole, D. Grosso, and C. Sanchez, “Thermally Induced Porosity in CSD MgF2-Based Optical Coatings: An Easy Method to Tune the Refractive Index,” Chem. Mater. 20(17), 5550–5556 (2008). [CrossRef]
49. I. Repins, M. A. Contreras, B. Egaas, C. DeHart, J. Scharf, C. L. Perkins, B. To, and R. Noufi, “19· 9%‐efficient ZnO/CdS/CuInGaSe2 solar cell with 81· 2% fill factor,” Prog. Photovolt. Res. Appl. 16(3), 235–239 (2008). [CrossRef]
50. A. J. Blanker, P. Berendsen, N. Phung, Z. Vroon, M. Zeman, and A. H. M. Smets, “Advanced light management techniques for two-terminal hybrid tandem solar cells,” Sol. Energy Mater. Sol. Cells 181, 77–82 (2018). [CrossRef]
51. D. J. Hwang, S. Kuk, Z. Wang, S. Fu, T. Zhang, G. Kim, W. M. Kim, and J.-H. Jeong, “Laser scribing of CIGS thin-film solar cell on flexible substrate,” Appl. Phys. A 123(1), 55 (2017). [CrossRef]
52. K. Orgassa, H. W. Schock, and J. H. Werner, “Alternative back contact materials for thin film Cu (In, Ga) Se2 solar cells,” Thin Solid Films 431, 387–391 (2003). [CrossRef]
53. R. Kotipalli, B. Vermang, J. Joel, R. Rajkumar, M. Edoff, and D. Flandre, “Investigating the electronic properties of Al2O3 Cu(In,Ga)Se2 interface,” AIP Adv. 5(10), 107101 (2015). [CrossRef]
54. B. Macco, B. W. H. van de Loo, and W. M. M. Kessels, “Atomic Layer Deposition for High Efficiency Crystalline Silicon Solar Cells,” in Atomic Layer Deposition in Energy Conversion Applications, pp. 41–99, Wiley-VCH Verlag (2017).
55. A. Ingenito, O. Isabella, and M. Zeman, “Nano‐cones on micro‐pyramids: modulated surface textures for maximal spectral response and high‐efficiency solar cells,” Prog. Photovolt. Res. Appl. 23(11), 1649–1659 (2015). [CrossRef]
56. S. R. Thomas, C.-W. Chen, M. Date, Y.-C. Wang, H.-W. Tsai, Z. M. Wang, and Y.-L. Chueh, “Recent developments in the synthesis of nanostructured chalcopyrite materials and their applications: a review,” RSC Advances 6(65), 60643–60656 (2016). [CrossRef]
57. C. H. Liu, C. H. Chen, S. Y. Chen, Y. T. Yen, W. C. Kuo, Y. K. Liao, J. Y. Juang, H. C. Kuo, C. H. Lai, L. J. Chen, and Y. L. Chueh, “Large scale single-crystal Cu(In,Ga)Se2 nanotip arrays for high efficiency solar cell,” Nano Lett. 11(10), 4443–4448 (2011). [CrossRef] [PubMed]
58. W. N. Shafarman, R. S. Huang, and S. H. Stephens, “Characterization of Cu(InGa)Se2 solar cells using etched absorber layers,” Conf. Rec. 2006 IEEE 4th World Conf. Photovolt. Energy Conversion, WCPEC-4 1, 420–423, IEEE (2007).
59. F. Mollica, J. Goffard, M. Jubault, F. Donsanti, S. Collin, A. Cattoni, L. Lombez, N. Naghavi, R. Edf, I. Umr, D. Renaissance, “Comparative study of patterned TiO 2 and Al 2 O 3 layers as passivated back-contact for ultra-thin Cu (In, Ga)Se2 solar cells,” in Photovoltaic Specialists Conference (PVSC), 2016 IEEE 43rd, 6–10 IEEE, (2016).
60. Z. Jehl, F. Erfurth, N. Naghavi, L. Lombez, I. Gerard, M. Bouttemy, P. Tran-Van, A. Etcheberry, G. Voorwinden, B. Dimmler, W. Wischmann, M. Powalla, J. F. Guillemoles, and D. Lincot, “Thinning of CIGS solar cells: Part II: Cell characterizations,” Thin Solid Films 519(21), 7212–7215 (2011). [CrossRef]
61. M. Schmid, “Review on light management by nanostructures in chalcopyrite solar cells,” Semicond. Sci. Technol. 32(4), 43003 (2017). [CrossRef]
62. “Solar Frontier Achieves World Record Thin-Film Solar Cell Efficiency of 22.9%,” 2017, <http://www.solar-frontier.com/eng/news/2017/1220_press.html>.
63. Y. Hishikawa, E. D. Dunlop, M. A. Green, J. Hohl, E. Anita, W. Y. H. Baillie, and D. H. Levi, “Solar cell efficiency tables (version 52),” 427–436 (2018).
64. R. Vismara, O. Isabella, A. Ingenito, F. T. Si, and M. Zeman, “Geometrical optimisation of core-shell nanowire array for enhanced absorption in thin crystalline silicon heterojunction solar cells,” Beilstein J. Nanotechnol. 10, 322-331 (2019).
