Increase the efficiency of a Perovskite-Silicon Tandem Solar Cell with a Full Electro-Optical Simulation
Solar cells have become ubiquitous in large parts of the world. In the last years, there has been a drastic reduction in the costs which allowed photovoltaic devices to produce fairly cheap, clean, and renewable energy. With the Levelized cost of energy being below 4 cents/kWh which is a more than 10 times reduction in cost over the last 10 years) solar power has even become the cheapest electricity source for a large part of the world.
The maximum efficiency for laboratory-scale silicon solar cells has not increased by much over the last 20 years. In these years, there were not so many groundbreaking achievements. The cost reduction was rather driven by improvements in the fabrication process, economies of scale, and the fact that the photovoltaic modules reached a similar efficiency than the solar cell at the lab-scale.
However, for further cost reduction, increasing efficiency is a required pathway. The highest efficiency silicon solar cells reach a power conversion efficiency (PCE) of 27.6%, which is already close to the theoretical maximum of 29% [Kowalczewski2016], but further developments are possible. Currently, one of the appealing solutions is to couple silicon with a semiconducting material that functionalizes its surface. This can be a perovskite structured compound, such as a hybrid organic-inorganic lead or tin halide-based material, which improves the light-harvesting properties of the solar cell.
Perovskites are receiving a lot of attention for their excellent optical and electrical properties, which makes them the new holy-grail of materials for photovoltaic applications. Optimized perovskite solar cells are reaching already an efficiency of 25.5% and the rate at which the community is improving this value is higher than anything that has been seen till now for the other photovoltaic technologies.
The advantage of perovskite solar cells is that they absorb light better than an indirect-band semiconductor, such as silicon. This is true especially in the visible part of the spectrum. When a perovskite-based solar cell is coupled with a device made out of crystalline silicon, the combined system can absorb the solar radiation efficiently in both the visible (where the perovskite plays a major role) and the near-infrared. This structure is commonly called a perovskite-silicon tandem solar cell. Fig 1a shows a common structure for such an innovative device. The absorption profiles of both the perovskite semi-device and the silicon-based cell are also reported in Fig. 1a.
The expected theoretical PCE for such a solar cell is around 40%. As shown in Figure 1b, up to now achieved efficiency in tandem solar cells is 29.15% [Al-Ashouri2020]. Despite the lower value with respect to the theoretical limit, the obtained results are already showing that this solution is promising and that further improvements will lead to easy-to-process photovoltaic devices with great potentials.
In this blog post, we will show how SETFOS can be used to simulate a perovskite-silicon tandem solar cell both optically and electrically. This simulation approach reveals bottlenecks in the optimization path and shows pathways for the improvement of these devices.
Figure 1 (a-b): (Left). Schematic of a perovskite-silicon tandem solar cell, togetehr with the absorption spectrum of both perovskite and silicon sub-cells. Courtesy of the PV-Lab at EPFL.
(Right): Solar cell efficiencies for 3 solar cell types and structures. Silicon single junction (blue), perovskite single junction (orange-yellow) and silicon-perovskite tandem solar cells (orange-blue). Single junction cells reach about 25-28% percent for both technologies. Tandem (dual-junction) silicon-perovskite PVs are shown to reach beyond 29% cell efficiency. (Adapted from NREL)
Optical Simulation
The optical model has been discussed already in an earlier blog post. There, the optical simulation has been used successfully to identify high-efficiency designs for tandems with fully textured silicon heterojunction bottom cells. The optical model consists of a transfer-matrix approach to compute the reflection and transmission of thin-film components, a ray-tracer to evaluate the (angular) scattering properties of the textured interfaces, and a net-radiation algorithm that uses this information to quantify the light propagation in the entire layer stack. The framework is implemented in Fluxim’s optoelectronic device simulation software SETFOS.
Figure 2: Multiscale optical simulation approach based on the combination of transfer matrix simulations for coherent thin film components with ray-tracing treatment of light scattering at large-scale textures, as implemented in the device simulation software SETFOS. [Altazin2018]
Electrical Simulation
For the electrical model (Fig. 3), the standard drift-diffusion-Poisson simulation framework of SETFOS is extended by drift-diffusion equations for mobile ions in the perovskite absorber, and by a hopping model for charge transfer at hetero-interfaces. Due to the current conservation inherent to the solution of the charge continuity equations, current matching is automatically enforced. Unbalanced optical generation, therefore, leads to increased recombination at steady-state operation.
Figure 3: Challenges for electrical simulation: The perovskite-silicon tandem device exhibits not only large disparity in the spatial extension of the individual component layers, but also several hetero-interfaces, with either intra- or interband charge transfer. The latter is simulated here by an interface hopping model similar to the Miller-Abrahams theory for thermally activated transport in disordered media.
Optical and electrical models are coupled in the standard way via the charge generation rate terms, and they are used together with an extended set of local and global optimization algorithms for the determination of the device configuration yielding the best performance.
Figure 4: Layer stack used in the electrical and optical simulation with their respective thicknesses (left) and energy levels (right).
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Results
Pure optical simulations have been done in Figure 5 to find the optimal layer structure (textured and coating). These simulations give an upper boundary for the maximum efficiency in our simulations. For the electrical simulations, the interfaces have been simplified to be planar as the electric stimulation is purely 1-dimensional.
Figure 5: Pure optical simulation to optimize short circuit current (Jsc) in various configurations. The 2-sided textured with anti-reflection coating (bottom right) yields the highest current.
An important aspect in the design of tandems is the question of how the sub-cell quality impacts the performance of the connected device. With almost no losses in current and voltage, we obviously have been looking at an ideal limit (Figure 6). We, therefore, consider now the presence of increasingly strong non-radiative recombination at defects located at the interfaces between the perovskite absorber and the charge transport layers.
Figure 6: Current-voltage characteristics from full opto-electronic simulations of perovskite (PSC) top- and silicon heterojunction (SHJ) bottom cells in single junction configuration (full line), with filtered optics (dashed line) as well as of the corresponding tandem device. For comparison, the approximation of the tandem JV-characteristics by the sum of the subcell JV curves is given as well (dashed line), revealing substantial disagreement close to MPP.
As expected, an increasing defect density leads to a reduction in Voc (Figure 7). Interestingly, however, the associated relative loss in efficiency is much larger in the tandem architecture (11.5 %) than in the single-junction perovskite absorber (6.9 %). Also, the optimum absorber layer thickness shifts away from the value inferred from optical simulation to larger values to compensate with generation for the loss of carriers.
Figure 7: Introducing defect states at the interface between the perovskite layer and the hole transport layer (NIO) and electron transport layer (C60), drastically reduces the open circuit voltage (left). This reduction in the open circuit voltage leads to a shift of the optimal layer thickness for the perovskite layer from 420 nm to 450 nm.
In conclusion, the experimentally validated multiscale electro-optical simulator SETFOS has been used to simulate high-performance, tandem solar cells. The coupled opto-electrical simulation has shown that depending on the quality of the recombination junction the optimal layer thicknesses shift. Therefore, explicit consideration of the recombination junction and of mobile ions enables the assessment of electrical losses in perovskite-silicon tandem solar cells and of device optimization at the maximum power point.
If we have sparked your interest to simulate and optimize silicon-perovskite multi-junction solar cells Setfos, get in contact with us or directly order a free of charge trial version of Setfos
This study has been presented at the EUPVSEC 2020 by our colleague Dr. Urs Aeberhard.
