46th IEEE Photovoltaic Specialists Conference (Chicago)

Jan Hascke, Angela Fioretti and Mathieu Boccard will be in Chicago from 16 to 21 of June 2019 to present their excellent results. Jan will present his work on the role of lateral transport of minority carrier. His talk his entitled "Injection-dependent lateral resistance in front-junction solar cells with nc-Si:H and a-Si:H hole selective contact".
Angela will advertise an advanced low temperature deposition method for enhanced cristalinity of thin p-type mc-Si:H contact layers. Finally, Mathieu will present how to reach high current densities in high-efficiency silicon heterojunction solar cells.

Abstract J. Hascke
We investigate the role of lateral transport of minority carriers in the wafer for front-junction solar cells. The study is based on two silicon heterojunction solar cells with p-type amorphous (a-Si), and nanocrystalline (nc-Si) silicon as hole selective layer respectively. The solar cells feature similar certified efficiencies (23.23% and 23.45%). Unexpectedly, FF and R_S at MPP of both solar cells are also similar. Fitting the JV curves in high-forward bias, and analytical calculations suggest that lateral transport is also taking place in the wafer at high injection, despite the front–junction configuration. The calculations reveal that junction-related R_S is lower with nc-Si(p) than with a-Si(p). The results underline the importance of analyzing JV curves over a wide voltage range to unravel different phenomena determining FF.


Abstract A. Fioretti:
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 mc-Si:H contact layers. With these layers, we demonstrate J_sc gains of 1 mA/cm², while reducing series resistance to 1 ohm cm², leading to cells with certified h=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 mc-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.