Quantifying and modeling the impact of interconnection failures on the electrical performance of crystalline silicon photovoltaic modules

Failures in the metallic interconnections are among the main degradation modes for photovoltaic modules. Fatigue accumulation due to thermomechanical stresses can result in deterioration of the solder joints or in broken ribbons. In this paper, we first quantify experimentally how the module performance is affected when one or more cell interconnect ribbons are cut or disconnected. For this purpose, we manufactured a set of minimodules, composed by six monocrystalline silicon cells with three bus bars connected in series. Cells were encapsulated in a glass/backsheet construction, employing a polymeric (ETFE) backsheet that can be easily opened. We then sequentially cut one or more ribbons. We observe that the power loss strongly depends on the ribbon's position with respect to the cell (external or central ribbon). In a second step, we implemented an electrical model in LT‐SPICE where the solar cell is composed by three subcells (as the number of cell bus bars) and show that this model is able to reproduce the experimental results with a good accuracy. We then use the model to demonstrate that these results are directly transferable to the case of large‐area modules composed of 60 or 72 cells. Finally, we analyze the case when the disconnections are randomly distributed in the module. As a first approximation, a module with 10% of disconnections has a Pmax variation between −1.34% and −2.75% in average, while 20% of disconnections lead to a Pmax variation in the range of −2.83% and −5.64%.

Perovskite/Silicon Tandem Solar Cells: Toward Affordable Ultra-High Efficiency Photovoltaics ?

Jérémie Werner

Nowadays the photovoltaics market is dominated by crystalline silicon solar cells, which are approaching their theoretical efficiency limit. Novel solutions have to be found for photovoltaics to further enhance its competitiveness with other energy sources. One of the most promising approaches lies in combining market-proven silicon solar cell technology, having a low optical band gap, with an efficient near-infrared transparent wide-band gap top cell to form a tandem cell. Organic-inorganic halide perovskite solar cells are promising candidates for top cells, showing high efficiencies with simple and cost-effective device fabrication. We first develop perovskite solar cells specifically for tandem applications. We develop and optimize a hybrid sequential deposition method combining thermal evaporation and solution processing to fabricate perovskite absorber materials with specifically tailored optoelectronic properties. These materials are systematically investigated for their optical, structural and electronic properties, and then applied in perovskite solar cells both in n-i-p and p-i-n configurations, for which we show several charge transport materials combinations. As a replacement for the standard metal opaque rear electrode of perovskite cells, we develop a TCO-based transparent electrode, for which sputtered IZO is presented as a good candidate thanks to its high carrier mobility and broadband transparency. Sputtering-induced damages are reduced by the introduction of a buffer layer: MoOx for n-i-p cells and SnO2 for p-i-n cells. The parasitic absorption losses in the MoOx are described in details. We then discuss the parasitic absorption losses in charge transport layers and TCOs, with experimental comparison of various materials. The optimisation of the perovskite absorber deposition method and the improvement of the charge transport layers and transparent electrode allows the fabrication 1cm2 area semitransparent perovskite solar cells with >16% efficiency. We then integrate the semitransparent perovskite cells in mechanically stacked 4-terminal tandem solar cells. The challenges in reducing the strong parasitic absorption and reflection losses are first discussed. We then demonstrate 4-terminal tandem measurements with >25% total efficiency with a small area top cell. With larger 1cm2 area top cells, we fabricate integrated 4-terminal tandem devices with both subcells having similar size and total efficiency >23%. The integration of perovskite solar cells in 2-terminal monolithically connected tandem solar cells with silicon heterojunction bottom cells is finally presented. First, we show the development of a TCO-based recombination layer and the important reflection losses and interference effects observed in all-flat devices. The origins of parasitic absorption losses in monolithic tandems are then explained and new architectures and materials are investigated supported by optical simulations. We then replace the polished wafers by fully textured silicon bottom cells for better light management. The development of a tandem device with the top cell conformally coated onto the textured bottom cell is explained, leading to >25% certified power conversion efficiency. The end of the thesis presents preliminary results on the up-scalability and light soaking stability of the developed textured tandems, as well as a proof-of-concept of a first perovskite/perovskite/silicon triple junction solar cell on textured wafers.

EPFL2018

Electrical Losses Mitigation in Silicon Heterojunction Solar Cells

Laurie-Lou Senaud

To overcome the worldwide challenges of climate change, photovoltaics is foreseen to play a significant role in the world electricity production. Nowadays, single junction crystalline silicon (c-Si) based solar cells hold the largest share of the global photovoltaic market. To achieve their maximum energy conversion efficiency, carrier-selective passivating contacts have been demonstrated to be the most promising technologies. Among them, silicon heterojunction (SHJ) solar cells are expected to further reduce the cost of photovoltaics. Indeed, they have achieved record efficiency thanks to high c-Si bulk quality and excellent full area surface passivation. However, despite remarkable performance, the material layers used to build SHJ cells are at the origin of significant trade-offs between electrical and optical properties. In particular, the properties coupling arising between the layers can cause significant electrical transport losses, strongly capping the SHJ device performance. The aim of this thesis is to accurately describe, quantify, and mitigate the electrical losses occurring in SHJ solar cells by means of advanced methodologies and characterization methods.Key outcomes of this work are fourfold. First, we introduce a generalized and unambiguous description of the concept of contact using the terminology of shell. Then, we propose a characterization methodology using two new approaches named top-down and bottom-up, designed to independently track the properties of the material layers used to build the shells, as well as the performance of the solar cells incorporating the shells. This was shown to allow one to accurately investigate and mitigate the electrical and optical losses affecting solar cells. Secondly, we present transfer length method (TLM) measurements under variable illumination as a novel advanced characterization method to study the electrical transport losses in SHJ devices. We demonstrated (i) that illumination, i.e., the injected carrier density inside the c-Si bulk, has a strong impact on the contact resistivity (rc), (ii) the importance of measuring rc under maximum power point conditions for a relevant characterization of transport losses, and (iii) how the dependence of the rc on the injected carrier density enables the comparison of the illumination response of different SHJ shells. In addition, we conducted preliminary investigations of the applicability of TLM under illumination to measure p-type shell parts on n-type c-Si wafers. Thirdly, we engineered various material layers and studied their properties. Multilayering the thin hydrogenated silicon and the TCO layers demonstrated a high potential for electrical losses mitigation by (i) improving the properties coupling in the shell, (ii) addressing the constraints of the device architecture, and (iii) mitigating the trade-offs impacting solar cell performance. Finally, the last outcome is the integration of the most promising shells in actual baseline solar cells. This resulted in fill-factors up to 83.2%, for 6-inch SHJ cells, and in efficiencies up to 25.45% for SHJ IBC cells. Finally, combining optimal electrical and optical shell properties, an impressive efficiency of 24.24% was achieved using an aluminium-doped zinc oxide as rear TCO. The advanced methodologies and characterization methods presented in this work are foreseen to drive the understanding and optimization of other solar cell technologies such as, e.g. TOPCon or tandem solar cells.

EPFL2021

Perovskite/Silicon Tandem Solar Cells: Toward Affordable Ultra-High Efficiency Photovoltaics ?

Jérémie Werner

EPFL2018

Electrical Losses Mitigation in Silicon Heterojunction Solar Cells

Laurie-Lou Senaud

EPFL2021