Separating the two polarities of the POLO contacts of an 26.1%-efficient IBC solar cell

Abstract

This is because the high conductivity of the poly-Si does not impede a transport limitation. Therefore, a separation of the n-type POLO (nPOLO) and p-type POLO (pPOLO) contact fingers is required [25][26][27][28][29] .In this work we employ a POLO junction scheme that consists of an initially full area intrinsic poly-Si (i poly-Si) layer that is locally doped by ion implantation. From process leanness point of view an attractive option to avoid these pn junctions in the poly-Si is to leave an (i) poly-Si region between emitter and base fingers. This is expected to result in lateral p(i)n poly-Si junction at the rear side of the IBC cell, which is connected in parallel to the nPOLO/p-type c-Si junctions ( Fig. 1a). By applying the described junction scheme we demonstrated a solar cell with an efficiency of 26.1% 11,21 .This concept was previously tested by other groups on precursor level 30 achieving a V OC of 682 mV with an pFF of 80% and on the cell level 27 yielding an efficiency of 18.4%. Both devices showed high ideality factors pointing to a non-ideal recombination in the space charge region. To evaluate why our cell, in contrast, does not suffer from a high ideality factor we perform a systematic study of the device physics of the resulting p(i)n poly-Si diodes.We quantify the defect density in our poly-Si layers prior and after hydrogenation using time-dependent photoluminescence decay measurements in the picosecond regime.As the purpose of the (i) poly-Si regions is to suppress high recombination currents across the p(i)n diode a large (i) poly-Si region is desirable. On the other hand, we observed a poor passivation quality of the c-Si absorber by the iPOLO on full-area lifetime test structures (see below). This is due to the moderate chemical passivation of SiO x on p-type Si, which further degrades when forming the nanometer-sized pinholes in the interfacial oxide 31 . This reasoning favors small (i) poly-Si widths. To find an optimum between these counteracting requirements, we experimentally vary the width of the (i) poly-Si region from nominal d gap = 0 µm up to 380 µm.We experimentally find that a width of the initially i poly-Si layer of d gap = 30 µm together with a high annealing temperature of over 1000 °C enables a record cell efficiency of 26.1%. From measurements on full area test structures and numerical device simulations we know that the surface recombination velocity at the intrinsic poly-Si is above 2000 cm/s and would limit the device efficiency to 15%. We conclude that the nominally intrinsic poly-Si layer is no longer intrinsic after the full cell process.We therefore investigate an inter-diffusion of dopants from the n-type and p-type doped fingers into the initially intrinsic poly-Si region by a lateral Time of Flight Secondary Ion Mass Spectrometry (ToF-SIMS) measurement. Combining all three aspects, we are, for the first time, able to present a comprehensive understanding of the working principle of high-efficient POLO IBC cells with p(i)n poly-Si diodes. Scientific RepoRtS |(202...

Simulation-based roadmap for the integration of poly-silicon on oxide contacts into screen-printed crystalline silicon solar cells

“…In terms of technology, these structures add further challenges to those already mentioned for POLO IBC: the necessity to dope the poly-Si locally or at least single-sidedly, the necessity to contact p+-type poly-Si with high temperature screen-printing without alloying through it, and, for IBC, the necessity to electrically separate the n+-type and p+-type poly-Si regions 66 – 68 . For the first aspect, we proposed in Ref.…”

“…Since it could be a fundamental aspect that a certain amount of Al in the paste is required to form a low contact resistance to p+-type (poly-) Si 72 while the Al always yields a certain degradation of the passivation quality, alternative metallization concepts like plating or Physical Vapor Deposition (PVD) might become more attractive for structures than they used to be for PERC+ cells. For the third additional challenge (separation of n+-type and p+-type poly-Si for IBC), different approaches can be used: oxidation of the poly-Si between the fingers 73 , introduction of a trench between the fingers 67 , 74 – 77 , or introduction of an initially intrinsic poly-Si region 4 , 66 – 68 . To summarize, both concepts are likely to require more time for industrial integration than the POLO BJ and IBC concepts.…”

The influence of the bandgap on the photovoltaic‐thermoelectric hybrid system

“…The Current–voltage characteristic (IV) curves of the PV given by the experiment 29 and the SQ limit have been shown in Figure 2. The SQ limit is calculated by a thermodynamic model of the PV.…”

“…The IV curves of the photovoltaic given by the experiment 29 and the Shockley‐Queisser limit [Colour figure can be viewed at wileyonlinelibrary.com]…”

Modeling recombination and contact resistance of poly‐Si junctions

“…8 Experimentally, IBC cells with conversion efficiencies of 26.0% and 26.1% that used POLO or related junctions for both polarities have been demonstrated so far. [9][10][11][12] There are two main pictures of the current transport mechanism through these type of junctions: quantum mechanical tunneling models, as also previously applied for this junction scheme in the bipolar junction transistor community, 13 were successfully used by, for example, Steinkemper et al for the description of an electroncollecting n + type poly-Si/SiO x /c-Si junction with a thin interfacial oxide. 14 Since already the BJT community reports on an inconsistency of the tunnel model with respect to the description of electron collecting/emitting or hole collecting/emitting POLO junctions, 13 Peibst et al proposed an alternative picture, assuming that conduction through the interfacial oxide happens at places where the oxide is broken up locally (Pinholes).…”