Fabrication of high temperature carrier-selective passivating contact solar cells for application in monolithic perovskite/c-Si tandem devices

Abstract

Crystalline silicon (c-Si) homojunction architectures like aluminum back-surface field (Al-BSF) and passivated emitter and rear cell (PERC) dominated the solar cell market for the last decades. Recently, carrier-selective passivating contact (CSPC) designs have shown that they can effectively reduce recombination losses and exhibit efficiencies above 25 % which approaches the Shockley-Queisser limit of single-junction solar cells. To overcome this intrinsic efficiency limit, research and industry have started to investigate multijunction devices which utilize two different absorber layers in order to reduce the spectral mismatch losses. In this project, the focus lies on the fabrication of a high temperature CSPC bottom cell with polycrystalline silicon oxide (poly-SiOx) for the application in a 2-terminal perovskite/c-Si tandem device. Poly-SiOx features a higher transparency due to a wider band gap which can decrease the free-carrier absorption (FCA) and induce larger band bending in comparison to the classic polycrystalline silicon (poly-Si) contacts.
To fabricate a c-Si solar cell with high temperature carrier-selective passivating contacts, the passivation of the contacts was optimised using symmetric lifetime samples. It was found that the optimal processing conditions for the dry thermal oxidation process and the high temperature annealing step are 675 °C for 3 minutes and 900 °C for 15 minutes, respectively. This resulted in an implied open-circuit voltage (iV_OC) for the n-doped and p-doped symmetric samples after hydrogenation of 745 mV and 674 mV, respectively. Those optimised processing conditions were adopted for the fabrication of a single-junction n-type front-side polished and p-type rear-side textured CSPC solar cell. This resulted in a solar cell with an efficiency of 16.67 %. By lowering the thermal budget of the recovery annealing conditions after the sputtering of the transparent conductive oxide (TCO), the passivation properties are restored more effectively. This resulted in an enhanced solar cell with a final efficiency of 18.76 %. Other techniques to improve the performance like the modification of the p-layer thickness, the incorporation of an additional annealing step after the a-Si deposition, or the introduction of indium tungsten oxide (IWO) as the TCO did not result in an improvement of the final solar cell efficiency. Optical simulations were performed in GenPro4 to investigate the optical performance of the solar cell. Finally, a 2-terminal perovskite/c-Si tandem device with a high temperature carrier-selective passivating c-Si bottom cell was fabricated in cooperation with TU Eindhoven. The final tandem solar cell exhibits an efficiency of 23.10 % with a fill factor of 74.00 %, a V_OC of 1.76 V and a J_SC of 17.81 mA/cm².