Towards 28 %-efficient Si single-junction solar cells with better passivating POLO junctions and photonic crystals
Introduction
Lighthouses guide the way in a complex field. This is also the case in photovoltaics (PV), where many different absorber materials, their combinations, and, within these technologies, many different approaches and structures compete with each other with respect to efficiency improvement as a lever for further reduction of the Levelized Costs of Electricity. More specifically, an important emerging question is to what extent the current backbone technology of PV – crystalline (c-) Si single-junction cells with a market share >95% – can be optimized further before a transition to another technology – most likely perovskite/silicon tandem cells – is required. It is clear that there is still a difference of ∼6%/∼5.5%/∼5%/∼4.5% between the current median production efficiencies of the c-Si technologies PERC (Passivated Emitter and Rear)/TOPCon1/SHJ (H-rich amorphous Si/c-Si heterojunctions)/IBC (interdigitated back-junction cells with passivating contacts) and the theoretical limit of 29.56% for Lambertian light trapping and only intrinsic recombination [1]. However, this gap can hardly be closed completely due to extrinsic loss mechanisms that occur in every real solar cell. To what extend these extrinsic losses can be mitigated is difficult to quantify by simulation studies due to uncertainties in the input parameters.
Therefore, the guiding lighthouses in PV are often record-efficiencies on laboratory cells, demonstrating the potential of a technology and thus directing subsequent application-related developments to the most promising approaches.
The one sun record efficiencies for solar cells based on a single Si absorber have remained unchanged2 in the last ∼3 years at 26.7% [2,3] for c-Si cells with passivating contacts based on SHJ and at 26.1% for passivating contacts based on polycrystalline Si on oxide (POLO) junctions [4]. These values are very close to previously announced “practical limits” of 27.2% [2] and (26.0% [5]) 26.2% [6] for the respective contact schemes. By contrast, record efficiencies for small area perovskite/Si tandem cells have increased rapidly in the last years to 29.15% [7] or even 29.8% [8]. There is also impressive progress in upscaling [9,10] and stability [10] of perovskite (sub) cells. We nevertheless think that Si single-junction solar cells will stay competitive for a while, particularly if they succeed in further significant efficiency improvement.
In this work, we intend to depict pathways that allow an improvement of c-Si single-junction record efficiencies in order to revise the impression that the potential of Si is already exhausted. We argue that the practical limit, particularly for Si cells with POLO junctions and perhaps even the theoretical “intrinsic limit,” can be shifted upwards. The inspiration for the latter stems to a large extent from the theoretical work of Bhattacharya and John [11] who showed that one way to exceed the Lambertian light trapping and thus the corresponding the Auger & radiative efficiency [1,12] is to implement photonic crystals (PCs) [13,14] on the front side of the solar cell. The PCs, which are in this case just inverted pyramids with dimensions3 of ∼1–3 μm but arranged in a periodic pattern, improve the solar absorption due to the formation of optical modes propagating parallel to the surface with reduced group velocity [11]. Thus, the absorber thickness can be reduced, and intrinsic recombination can be mitigated. Based on this concept, Bhattacharya and John simulated a 15 μm-thin 31.1% efficient Si single-junction cell, including all losses [11].
We conduct a numerical device simulation study to evaluate to what extent this approach can be transferred to our current and future POLO2-IBC cells4 (Fig. 1) and to “standard” cell thicknesses that are compatible with industrial wafer handling. The experimental findings of our 26.1%-efficient record cell serve as a starting point [19]. We find that the current surface passivation quality, including our poly-Si on oxide (POLO) junctions, is not sufficient (i.e., not as good as assumed in Ref. [11]) and would prevent a large efficiency improvement by the PCs. Thus, we focus in the experimental part of this work on a significant improvement of our POLO junctions that is compatible with our cell process. We perform special two-terminal IV measurements on pads that are small enough to contact only the intact oxide area between pinholes [20]. Since the in this case dominating tunneling currents are very sensitive to the band alignment, it is possible to deduce the c-Si/SiOx interface state density Dit from a fit with our MarcoPOLO model [21]. A high Dit value of 2.9 × 1012 eV−1cm−2 is obtained. Consequently, we improve the hydrogenation process of our junctions. Using the reduced prefactors of the recombination current densities as input parameters, a simulation-based roadmap to efficiencies beyond 28% with PCs and beyond 27% without PCs is provided – for POLO2-IBC cells on a standard wafer thickness of 150 μm and even when subtracting perimeter losses of ∼0.3%abs.
In our first attempt to implement the improved POLO junctions into solar cells (still w./o. PCs 5), we measured an improvement of the implied pseudo efficiency of 0.4%abs, which is consistent with the predictions from the simulations.
Section snippets
Device simulations
For the numerical device simulations, we use the Quokka2 implementation [22] of the Conductive Boundary Model [23] and the Free Energy Loss Analysis (FELA) concept [24]. We use our 26.1% efficient POLO2 IBC cell (on 290 μm thick wafers and with random pyramid front side texture) as a starting point, which has already been simulated by Hollemann et al. [19] based on experimentally determined input parameters. These parameters are listed in Table A1. One should remark that the surface
Deduction of optimization strategy for POLO junctions
We characterize the properties of the SiOx/c-Si interface from IV curves at room temperature of a sample that contacts a pinhole-free area fraction of the pPOLO junction. For that, we characterize the IV curves measured on the two-terminal test structures with a contacted junction area of (5 × 5) μm2. For this area fraction of the pPOLO junction, we expect a small number of 2–3 pinholes per pad from the deduced pinhole density with the etch-pit method [43]. One interesting finding is that none
Improved hydrogenation
Fig. 4 (a) shows the improvement of the passivation quality of our POLO junctions by our new rear side dielectric layer stack intended as a more efficient hydrogen donor. The effective lifetimes measured on the symmetric lifetime test structures with nPOLO, pPOLO and intrinsic poly-Si at an injection level of Δn = 1015 cm−3 are shown at different processing stages. The post-deposition anneals are performed cumulatively. The respective benchmark values for our previous passivation by
A simulation-based roadmap to efficiencies beyond 28%
Fig. 5 shows the main IV parameters efficiency η, open circuit voltage Voc, short circuit current density Jsc and fill factor FF as simulated for our current experimental POLO2-IBC cell as a starting point, as well as for different hypothetical solar cells including different improvements.
The potential path to directly implement photonic crystals with the previous surface passivation quality is indicated in blue squares. This approach – together with a reduction of the wafer thickness down to
Discussion
It is clear that these predictions from the simulations call for an experimental verification first. Nevertheless, it is worth discussing the perspectives: A 15 μm thin Si single-junction solar cell with an efficiency close to 30% would be very attractive. In particular, its mechanical flexibility could be utilized in special applications such as Vehicle-Integrated PV (VIPV). Energy yield modeling and measurements would be required to assess aspects like dependency on the angular distribution
Conclusion
In this work, we evaluated how far we can push the efficiencies of our POLO2-IBC Si single-junction solar cells under normal incident light. As an extension of previous completely idealized considerations [19], we attempt here to base our projection as much as possible on experimental input data, in particular for improved POLO junctions. Furthermore, we take into account the concept of photonic crystals [11] with excellent light trapping even for very thin wafers.
Our numerical device
Declaration of competing interest
Herby, I declare that I do not have any competing financial or non-financial interests.
Acknowledgments
We would like to thank H. Fischer, S. Spätlich, R. Winter, A. Raugewitz,G. Glowatzki and R. Zieseniβ for sample processing, M. Wolf, A. Dietrich, R. Reineke-Koch for discussion and support with the measurement systems, and Sajeev John for fruitful discussions. This work is funded by the German Ministry for Economic Affairs and Energy (grant FKZ 003EE1056A), by the federal state of Lower Saxony and by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany's
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