For none, one, or two polarities—How do POLO junctions fit best into industrial Si solar cells?

Abstract

We present a systematic study on the benefit of the implementation of poly-Si on oxide (POLO) or related junctions into p-type industrial Si solar cells as compared with the benchmark of Passivated Emitter and Rear Cell (PERC). We assess three aspects: (a) the simulated efficiency potential of representative structures with POLO junctions for none (=PERC+), one, and for two polarities; (b) possible lean process flows for their fabrication; and (c) experimental results on major building blocks. Synergistic efficiency gain analysis reveals that the exclusive suppression of the contact recombination for one polarity by POLO only yields moderate efficiency improvements between 0.23% abs and 0.41% abs as compared with PERC+ because of the remaining recombination paths. This problem is solved in a structure that includes POLO junctions for both polarities (POLO 2 ), for whose realization we propose a lean process flow, and for which we experimentally demonstrate the most important building blocks. However, two experimental challenges-alignment tolerances and screen-print metallization of p+ poly-Si-are unsolved so far and reduced the efficiency of the "real" POLO 2 cell as compared with an idealized scenario. As an intermediate step, we therefore work on a POLO IBC cell with POLO junctions for one polarity. It avoids the abovementioned challenges of the POLO 2 structure, can be realized within a lean process flow, and has an efficiency benefit of 1.59% abs as compared with PERC-because not only contact recombination is suppressed but also the entire phosphorus emitter is replaced by an n+ POLO junction.This is an open access article under the terms of the Creative Commons Attribution-NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes.

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

“…Technological challenges are, besides aspects like homogeneity of Si layer thickness and (if present) in-situ dopant distribution, the absence of blistering and in particular the mitigation of wrap-around—also for nominal single-side deposition techniques like PE-CVD and AP-CVD. Alternatively, the process sequence can be designed in a way that it tolerates or even utilizes double-sided Si deposition 11 . For the post deposition high-temperature treatment, quartz tube furnace anneals (either in inert or oxidizing ambient or within a 44 or diffusion 45 ) are currently standard.…”

“…Laser processing, e.g. laser oxidation 11 , might avoid the latter aspect and even the necessity to deposit or to print an etch barrier. The most attractive option would be to deposit the poly-Si already structured, e.g.…”

“…It should be noted that some values differ from the ones used in our recently published paper in Ref. 11 , because they were either just recently measured or a publication exists for a similar value, which makes it easier for the reader to check the experimental details of the input parameters.…”

“…3 along with two reference structures. (a) the reference PERC+ cell, (b) the reference TOPCon cell based on n-type Si with a diffused boron emitter and a full-area n-type POLO junction on the rear, (c) the PERC+ POLO structure, which is a PERC+ cell with n-type POLO under the front contacts, (d) the POLO BJ structure, which features Al-alloyed (Al-p+) contacts on the front and a full-area n-type POLO on the rear side 10 , (e) the POLO IBC structure, which is an IBC concept featuring Al-p+ and n-type POLO junctions 10 , 22 , (f) the BJ structure, which features local p-type POLO junctions on the front side and a full-area n-type POLO emitter on the rear 11 , (g) the IBC structure, which is an IBC concept with POLO junctions for both polarities 4 and (h) the n-Si BJ structure, which is the same structure as (f) but with reversed polarities. The six cell structures (c) through (f) feature either only one n-type poly-Si contact and an Al-p+ contact (POLO cell) or two POLO junctions ( cells) in either front junction (FJ), back junction (BJ) or interdigitated back contact (IBC) configuration.…”

“…in Ref. 11 . Here, we focus on good comparability of the simulated efficiency between all analyzed cell structures and discuss the main processing challenges to give an impression of the expected process complexity.…”