Towards Highly Efficient Perovskite/c-Si Monolithic Tandem Solar Cells (2018)

Solar photovoltaic technology based on crystalline silicon (c-Si) has dramatically reduced in cost in the last several years, to the point where the cost of solar electricity now rivals that from coal power plants in many locations around the world. Increasing the power conversion efficiency is the most promising method to further reduce the cost. Since the efficiency of c-Si cells is approaching its theoretical limit, alternative approaches are required to enable step changes in efficiency. The rapid development of perovskite cells provides an opportunity to fabricate highly efficient perovskite/c-Si tandem cells, with an efficiency substantially greater than that possible with c-Si. One potential difficulty to achieve a highly efficient perovskite top cell for such a tandem device is the hysteresis behaviour usually displayed by these cells. It is necessary to understand the root causes of hysteresis in order to assess whether and to what extent the underlying mechanisms responsible will limit the efficiency or stability of the cells. We show for the first time, that the transient changes in terminal voltage and luminescent intensity do not follow the relationship that would be predicted by the generalised Planck radiation law. Using numerical simulation, we demonstrate that due to the accumulation of mobile ions at interfaces and together with significant defect related interface recombination, a resistive barrier to majority carrier flow at the interfaces between the perovskite film and the electron or hole transport layer can result in decoupling of the internal quasi-Fermi level separation and the externally measured voltage. Additional to the perovskite work, we report a specially designed homojunction c-Si solar cell architecture which provides a wide window for the perovskite top cell processing temperature of up to 400 °C, and which features passivation on both sides of the c-Si substrate using conventional, industry standard homojunction (diffused junction) technology. With a modified vacuum flash assisted perovskite deposition method, we demonstrate a 1 cm2 monolithic perovskite/c-Si tandem cell with 22.5% stabilized efficiency, which is the highest efficiency reported to date with a homojunction c-Si substrate. The unique design presented in this work opens up a new approach for achieving highly efficient monolithic perovskite/c-Si tandem devices. In the final work, we carefully investigate the monolithic tandems based on HJT (heterojunction technology) and PERT (passivated emitter rear totally diffused) structures recently published. Based on the simulation results, for both structures, we show that a significant increase in efficiency can be achieved by simply reducing the resistivity of the c-Si wafer and changing the wafer from n-type to p-type without changing any process conditions for the entire monolithic tandem. Two new structures - LERL (localized emitter rear localized diffused) and rTOPCon (reversed tunnelling oxide passivating contact) are also proposed and simulated in this work to further improve the device efficiency while tolerating up to 400 oC processing temperature, which substantially increases the process flexibility for the perovskite cell. Importantly, the rTOPCon structure is industrially feasible for large scale production.