Enhancing Crystallinity in p-type Microcrystalline Silicon Carrier Selective Contacts for Silicon Heterojunction Solar Cells
Angela N. Fioretti, Mathieu Boccard, Raphaël Monnard, Christophe Ballif
Photovoltaics and Thin Film Electronics Laboratory, Ecole Polytechnique Fédérale de Lausanne, Neuchâtel, Switzerland

Heterojunction-based silicon solar cells have reached record-breaking efficiency, particularly when implemented in all-back-contacted architectures. Classical, two-side contacted silicon heterojunction (SHJ) solar cells suffer from parasitic absorption and series resistance losses in the amorphous silicon contacts. An alternative to doped amorphous silicon is to use doped microcrystalline silicon instead, which exhibits improved transparency and charge extraction while still providing the superior passivation quality of all-silicon contact stacks. However, depositing films with high crystalline volume fraction thin enough to maintain improved transparency has remained a challenge until recently. In this work, we combine the successful pretreatment method of previous studies with lower deposition temperature to achieve enhanced crystallinity in thin p-type μc-Si:H contact layers. With these layers, we demonstrate Jsc gains of 1 mA/cm2, while reducing series resistance to 1 Ohm cm2, leading to cells with certified η=23.5%. Raman spectroscopy determined that deposition temperature below 200 °C leads to an increase in crystalline volume fraction from 35% to 55% for p-type films, whereas n-type film crystallinity remains constant. External quantum efficiency measurements revealed that enhanced p-layer crystallinity results in short wavelength efficiency gains, consistent with reduced parasitic absorption. Additionally, ellipsometry of representative p-type μc-Si:H deposited at varying temperature showed that E04 shifts to higher energy for lower temperature layers, suggesting a widening bandgap. Dark conductivity measurements showed that despite the increase in crystallinity, both activation energy and conductivity are similar among all films, no matter the deposition temperature. Using PC1D illuminated band diagrams, we explain these results as due to increased band bending at the c-Si interface with the p-type contact, which is a direct result of the wider bandgap for higher crystallinity, low-temperature layers. These findings provide a method to improve SHJ solar cells performance, while offering insight into the importance of band bending considerations when optimizing heterojunction designs.