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WP2 - Implementation of novel materials as a multifunctional capping

layers serving both as selective contacts and as efficient light couplers

Team leader: Mgr. Jakub Holovský, Ph.D.

Team members:

POSTDOC1
POSTDOC2

Devices with efficiency close to theoretical maximum will be produced by silicon heterojunction technology with enhanced passivation of external surfaces and contacts. Novel materials (T. Polcar, B. Rezek) will be used as multifunctional capping layers serving as selective contact deposited on tunnel oxide passivated wafers as well as optical coupler (T. Markvart). Additionally, high level of photoexcitation in ultrathin silicon layers will be achieved by implementing a pioneering light harvesting scheme similar to solar energy capture in photosynthetic organisms. An important benefit of going to lower absorber thicknesses is the tailoring the dependence on weather conditions that has consequences in the losses or gains in the household system yield (P. Wolf) – Figure b).

When looking at the PV efficiency charts http://www.nrel.gov/pv/assets/images/efficiency-chart.png, there is a striking gap between a group of technologies centered at 10% level and a group of technologies reaching 20% and more. The crystalline silicon is the best-established technology achieving almost every year new efficiency record, now being at 26.3% [1], leading thus the group of high-efficient PV technologies. The competition is driven by the fact that higher efficiencies become more profitable - Figure a). As a rule, the record efficiencies are rated for conditions that are rarely met at real-life conditions. However, publicly frequently referred drawback of emerging standard crystalline silicon (called PERC = Passivated Emitter Rear Cell) technology is its temperature coefficient that can, in the worst case, reduce its nominal power by 20% for real-life operating temperatures, mainly due to a voltage drop. This temperature drop is strongly proportional to a difference between the bandgap and the open-circuit voltage, that itself already causes one of the main fundamental energy losses. Therefore, shrinking the difference between the bandgap and the open circuit voltage is the most desirable way to improvement. Recently, the technology of the standard high-temperature diffusion homojunction process turned out to be not anymore necessary. Instead, the heterojunction concept of the lightly doped wafer sandwiched between electron and hole selective transporting layers (nomenclature transferred from organic solar cells) is more and more experimentally validated - upon a sole condition of perfectly passivated interfaces. Moreover, with the recent photoluminescence methods, the passivation quality – interpreted as implied open-circuit voltage – can be quickly and easily imaged [2] allowing fast progress toward very low surface recombination velocities. The record technology [1] utilizes passivation by expensive plasma-enhanced chemical vapor deposited (PECVD) amorphous silicon to increase the voltage up to 750mV and thus reduce temperature coefficients twice [3]. Reaching such high voltage is not possible without going to wafer thickness below 100 µm to enhance carrier concentration – Figure c). This trend can be followed, while solar cells become flexible, until the fundamental limit of Auger recombination is reached [4]. Auger recombination is a severe efficiency limit for today’s devices designed to maximize energy production at midday in summer when the contributions to the total energy yield are the biggest. However, conversely, the future trends should optimize energy production in times when the consumption is high. For solar energy it means more attention to low light conditions. Fortunately, the Auger limit is relaxed at lower illumination, allowing us going further to thinner wafers. Once the well passivated wafer is obtained, the electron or hole transport layers can be made either from wide bandgap transparent oxides, e.g. MoOx [5] or organics, e.g. PEDOT:PSS [6]. The alternatives to expensive PECVD, utilize inexpensive chemically-prepared silicon oxide passivation tunneling layer that – in combination with doped polycrystalline silicon transport layers – reaches efficiencies over 24% [7]. Moreover, heterojunction-based solar modules have much lower temperature coefficients, yielding more generated energy over a year. Another benefit of the passivated contacts is the possibility to replace expensive Ag paste screen-printed contacts with inexpensive Ni-Cu electroplated contacts [8]. As the crystalline silicon technology is approaching its theoretical limits, the tandem structures, e.g. with inorganic-organic hybrid perovskites are becoming intensively studied as a way to go beyond 30% efficiency [9]. The technology of passivated contacts is better suited for the tandem structures than the standard technology.
figure_a figure_b figure_c
a) b) c)
Fig. a) shows that thanks to relatively high portion of cost of BOS (cables, supports, labor…) high efficient technologies become more profitable. Fig. b) shows time-energy diagram of performance of photovoltaic system with batteries and backup. Fig. c) dependence of voltage on light intensity and wafer thickness without effect of Auger recombination.

References

[1] http://www.kaneka.co.jp/kaneka-e/images/topics/1473811995/1473811995_101.pdf
[2] B. Hallam, B. Tjahjono, T. Trupke, S. Wenham, Photoluminescence imaging for determining the spatially resolved implied open circuit voltage of silicon solar cells, J. Appl. Phys. 115 (2014) 044901. doi:10.1063/1.4862957.
[3] T. Mishima, M. Taguchi, H. Sakata, E. Maruyama, Development status of high-efficiency HIT solar cells, Sol. Energy Mater. Sol. Cells. 95 (2011) 18–21. doi:10.1016/j.solmat.2010.04.030.
[4] A. Descoeudres, Z.C. Holman, L. Barraud, S. Morel, S. De Wolf, C. Ballif, >21% Efficient Silicon Heterojunction Solar Cells on n- and p-Type Wafers Compared, IEEE J. Photovolt. 3 (2013) 83–89. doi:10.1109/JPHOTOV.2012.2209407.
[5] C. Battaglia, S.M. de Nicolás, S. De Wolf, X. Yin, M. Zheng, C. Ballif, et al., Silicon heterojunction solar cell with passivated hole selective MoOx contact, Appl. Phys. Lett. 104 (2014) 113902. doi:10.1063/1.4868880.
[6] R. Liu, S.-T. Lee, B. Sun, 13.8% Efficiency Hybrid Si/Organic Heterojunction Solar Cells with MoO 3 Film as Antireflection and Inversion Induced Layer, Adv. Mater. 26 (2014) 6007–6012. doi:10.1002/adma.201402076.
[7] A. Moldovan, F. Feldmann, M. Zimmer, J. Rentsch, J. Benick, M. Hermle, Tunnel oxide passivated carrier-selective contacts based on ultra-thin SiO2 layers, Sol. Energy Mater. Sol. Cells. 142 (2015) 123–127. doi:10.1016/j.solmat.2015.06.048.
[8] J. Geissbuhler, S.D. Wolf, A. Faes, N. Badel, Q. Jeangros, A. Tomasi, et al., Silicon Heterojunction Solar Cells With Copper-Plated Grid Electrodes: Status and Comparison With Silver Thick-Film Techniques, IEEE J. Photovolt. 4 (2014) 1055–1062. doi:10.1109/JPHOTOV.2014.2321663.
[9] P. Loper, B. Niesen, S.-J. Moon, S. Martin de Nicolas, J. Holovsky, Z. Remes, et al., Organic-Inorganic Halide Perovskites: Perspectives for Silicon-Based Tandem Solar Cells, IEEE J. Photovolt. 4 (2014) 1545–1551. doi:10.1109/JPHOTOV.2014.2355421.