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

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, VOC was retained, while up to 14% absolute increase in FF was obtained with a mere 0.2 mA cm−2 loss in JSC. In the best performing cell, a 72.1% FF was achieved with a 0.7 mA cm−2 loss in JSC. 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±20mΩcm2 as a conservative estimate.