Laser firing in silicon heterojunction interdigitated back contact architecture for low contact resistance

Abstract

This work reports a laser firing technique applied to completed silicon heterojunction interdigitated back contact solar cells in order to lower contact resistance. Previously, the implementation of a-Si:H(i) at the electron contact of polycrystalline silicon solar cells on glass substrates led to an increase in series resistance. The cell architecture with the current record efficiency of 14.2 % (with illumination through glass) utilizes only an a-Si:H(n+) layer and 2 −2.9 mA cm −2 of short circuit current density is lost due to electrical shading under the electron contact [1],[2]. The goal of implementing an a-Si:H(i) layer and laser firing at this contact is to achieve low contact resistance at fired spots while preserving a-Si:H(i) passivation in unfired regions. After the laser firing, V OC was retained, while up to 14 % absolute increase in FF was obtained with a mere 0.2 mA cm −2 loss in J SC. In the best performing cell, a 72.1 % FF was achieved with a 0.7 mA cm −2 loss in J SC. Two laser sources were used to first ablate a part of the silver contact metal, and then to laser fire through the Si(n)/a-Si:H(i/n+)/ITO/Ag contact. The optimal laser fluence was found to be 1.1 −0.5 J cm −2 (355 nm, picosecond pulse duration) and 4.4 −5.2 J cm −2 (532 nm, nanosecond pulse duration), respectively. The upper limit on specific contact resistance in the laser fired spots was calculated to be 38 ± 20 mΩ cm 2 as a conservative estimate.

Toward High Solar Cell Efficiency with Low Material Usage: 15% Efficiency with 14 μm Polycrystalline Silicon on Glass

“…V OC was not significantly changed in any cell. These trends are consistent with those observed previously . The highest V OC observed in these experiments was 658 mV.…”

“…Around 55–90% of the total series resistance comes from the electron contact due to the small widths used (30–90 μm) . Therefore, a laser firing approach developed recently is crucial in achieving high fill factors (FFs).…”

“…Therefore, a laser firing approach developed recently is crucial in achieving high fill factors (FFs). The fired spots show low contact resistance due to the local removal of a‐Si:H(i) and the formation of a Ag–Si ohmic contact . They are spaced apart such that a‐Si:H(i) passivation is preserved in 10 μm spaces between consecutive fired spots (Figure d) .…”

“…The fired spots show low contact resistance due to the local removal of a‐Si:H(i) and the formation of a Ag–Si ohmic contact . They are spaced apart such that a‐Si:H(i) passivation is preserved in 10 μm spaces between consecutive fired spots (Figure d) . Increased deposition times were also used for this contact, namely 48% for a‐Si(i) (followed by 120 s hydrogen plasma treatment) and 77 s for a‐Si(n+), respectively.…”

“…This led to 83 laser fired spots each on eight electron contact fingers, with an impact diameter of 50 μm for the picosecond laser and 33 μm for the nanosecond laser. More details on this approach can be found in a previous study . The laser sources and XYZ stage were from Rofin laser.…”