Tockhorn, Philipp: Minimising Optical and Electrical Losses in Perovskite Solar Cells - From Single-Junctions to Advanced Tandem Designs. , Technische Universität Berlin, 2020
Open Access Version
Abstract:
Hybrid organic-inorganic perovskites are considered a promising material class for the application in single- and multi-junction solar cells. Prior to the commercialisation of such solar cells, further understanding of the device physics has to be gained. This dissertation addresses the optimisation of PSC single-junction devices for the application in perovskite/silicon tandem solar cells. For an efficient solar cell it is important that both, the absorption of light and the conversion of photogenerated charge carriers to usable electricity, are optimised. Despite its high absorption coefficients, which enable high quantum yields up to the band edge, PSC still suffer from optical losses caused by Fresnel and thin-film interference reflections. A study presented in this thesis demonstrates that these losses can be largely reduced by the implementation of a NaF antireflective layer and nanotextured glass substrates into p-i-n PSC. The employed nanotextures are compatible with perovskite layers deposited atop by spin coating, which is confirmed by a morphological and optoelectronic analysis. With this combined light trapping approach, the PCE of nanotextured devices reaches 19.7%, thus constituting an improvement by 1.0%abs in comparison to planar reference solar cells. The further optoelectronic characterisation of nanotextured PSC demonstrates that the gain in PCE can be largely attributed to reduced reflection losses resulting in an increase in JSC of up to 1.0 mA/cm2. The such optimised PSC reach 93.6% of their attainable current density, which is clearly surpassing existing literature reports of high-efficiency PSC. The demonstration of nanotextured PSC based on solution processing also paves the way to perovskite-based tandem solar cells in which the improved light trapping is expected to be of even higher relevance as all subcells can benefit. Tin oxide (SnO2) is an often-used ETL in nip PSC enabling high efficiencies at low deposition temperatures. However, PSC employing SnO2 often suffer from electrical collection losses at its interface with perovskite. Here, two studies on the influence of SnO2 on the device performance of PSC are presented. In the first study, three different processing routes for the fabrication of SnO2 layers are investigated and it is found that strong variations in the performance of n-i-p PSC occur. In particular, the VOC is strongly varying between 0.88 and 1.15 V for the different SnO2 fabrication routes. The second study investigates the effect of SnO2 surface treatments. It is found that the application of O2 plasma leads to a significant improvement of device performance, enabling a PCE of 19.2% in current-density/voltage (J-V) characterisation (18.2% MPP-tracked). In addition, the O2 plasma treatment narrows down the variation in device performance.[…] This study demonstrates that for the application of SnO2 in tandem solar cells, a precise control over the fabrication process has to be ensured to achieve a high PCE. Finally, the PSC layer stack employing SnO2 is transferred into a novel perovskite/ilicon IBC 3T tandem solar cell. This monolithic tandem design, employing an IBC SHJ as a bottom cell, allows the independent operation of both subcells. Thereby, it combines the specific advantages of the more common two- (2T) and four-terminal (4T) tandem architectures. The fabricated proof-of-concept device achieves a combined PCE of 17.1% when both subcells are operated at their individual MPP. For the further understanding, drift-diffusion simulations are employed and indicate efficient charge carrier transport throughout the whole device. […] In a last step, we discuss steps to optimise the experimentally realised cell efficiency and examine the advantages and disadvantages of the different tandem architectures with regard to a commercial application.