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Improved Stability in Perovskite Tandem Solar Cells

Source: Solid State, CC BY-SA 3.0

Halide perovskites are generally regarded as materials which have high performance potential while reducing production costs. The name is derived from the mineral calcium titanium oxide CaTiO 3 (XIIA2+VIB4+X2−3) and is also used for the class of compounds which have the same crystal structure, called the perovskite structure. Perovskite solar cells have witnessed a rapid development in recent years, with efficiencies increasing to over 25%. However, four main challenges remain which have to be tackled for perovskite technologies to be successfully launched onto the market, including stability and durability, large-scale power conversion efficiency, scaling-up production as well as finding investors.

Tandem solar cells are either individual cells or connected in series. Cells connected in series are easier to produce. The most common arrangement for tandem cells is to grow them monolithically which means that all the cells have the form of layers on a substrate with tunnel junctions connecting the individual cells. All-perovskite tandem solar cells, which are characterized by the absence of a substrate, hold the promise of surpassing the efficiency limits of common solar cells; however, until now, even those all-perovskite tandem solar cells with the best performance have exhibited lower efficiencies than single-junction perovskite solar cells.

Now (2022), scientists at the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) have designed a tin-lead perovskite cell which can overcome the problems concerning stability and improves efficiency. The new cell, which consisted of a tandem design with two layers of perovskites, measured a 25.5% efficiency. The new NREL cell retained 80% of its maximum efficiency after 1,500 hours of continuous operation, or more than 62 days.

The new solar cell is based on the results of an earlier research in 2019, where they constructed a tin-lead tandem perovskite cell with an efficiency of 23.1%. The scientists were able to eliminate any problems caused by tin by adding the chemical compound guanidinium thiocyanate, which resulted in improved structural and optoelectronic properties of the cell. Solar cells create electricity when sunlight triggers the movement of electrons. A longer carrier lifetime which is caused by this movement improves the efficiency of the cell. The addition of guanidinium thiocyanate increased the carrier lifetime from less than 200 nanoseconds (each nanosecond is a billionth of a second) to 1 microsecond (or a millionth of a second).

Improving the earlier experimental setup, the scientists added phenethylammonium iodide along with guanidinium thiocyanate. The improved tin-lead perovskite showed an increased carrier lifetime of about 9 microseconds. The combined additives also reduced the defect density caused by tin oxidation to an unprecedented level and similar to the values for lead-only perovskites. The new cell also showed an enhancement in the voltage generated, at 2.1142 volts. In comparison, the best certified tandem device registered 2.048 volts.

Scientists have long sought to improve efficiencies of perovskite tandem solar cells. In 2015, scientists discovered an effective strategy to analyse what role the extrinsic ion played for optoelectronic properties. The chlorine incorporation was found to mainly improve the carrier transport across the heterojunction interfaces, rather than within the perovskite crystals. Further optimisation with this strategy led to solar cells reaching a power conversion efficiency of 17.91%.

Image: Perovskite film fabrication. Schematic illustration of different approaches to incorporate chlorine in the perovskite films. The Reference sample was obtained by a modified two-step solution process, where PbI2 and CH3NH3I were sequentially deposited via spin-coating and subsequently annealed. Sample 1 was obtained via the same procedure, except that a solution of CH3NH3Cl and CH3NH3I (for example, 1:10 in weight) was used instead of CH3NH3I solution to introduce chlorine during the film growth. Sample 2 was obtained by a proper treatment on the Reference sample with CH3NH3Cl solution

Source: Qi Chen, Yihao Fang, Adam Z Stieg, Huanping Zhou/ The optoelectronic role of chlorine in CH3NH3PbI3(Cl)-based perovskite solar cells/ Nature Communications 6(1):7269, June 2015/ DOI:10.1038/ncomms8269/ Open Source This article is licensed under a
Creative Commons Attribution 4.0 International

In 2021, scientists performed simulations to optimise the parameters of a lead-free perovskite/silicon tandem solar cell for the improved efficiency and stability of commercial devices. The top sub-cell was lodged on a lead-free perovskite with a large bandgap of 1.8 eV, an electron transport layer of tin(IV) oxide/ the fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester and a hole transport layer of nickel(II) oxide to improve stability, whereas the bottom sub-cell was lodged on n-type silicon to increase the efficiency of the whole cell. First, the two sub-cells were simulated under standalone conditions for calibration purposes. Then, the current matching condition was achieved by optimising the thicknesses of the absorber layers of both sub-cells and the doping concentration of the back surface field (BSF) layer of the silicon sub-cell. As a result of this optimisation phase, a large open-circuit voltage of 1.76 V and a power conversion efficiency of 24.4% for the whole cell was gained. Finally, the effect of the working temperature was evaluated. The results showed that the high performance of lead-free perovskite sub-cells was affected to a lesser extent by an increase in temperature compared to lead-based solar cells, such as those based on CH3NH3PbI3 perovskite.

Image: (a) The 3D contour plot of Jsc of the tandem cell at different thicknesses of the absorber layers of both sub-cells. (b) Jsc for top and bottom sub-cells as a function of both absorber thicknesses. The optimum pairing is highlighted by the red square

Source: Khaoula Amri, Michel Aillerie, Rached Gharbi, Rabeb Belghouthi/ Device Optimization of a Lead-Free Perovskite/Silicon Tandem Solar Cell with 24.4% Power Conversion Efficiency/ Energies 14(12):3383, June 2021/ DOI:10.3390/en14123383/ Open Source This article is licensed under a
Creative Commons Attribution 4.0 International

Using perovskite tandem solar cells for harnessing sunlight has several potential advantages: one of them is that tandem solar cells make better use of sunlight. A perovskite tandem solar cell can combine substances with different bandgaps and thus efficiently convert a larger share of the incident solar energy to electricity. The current world record holder for most efficient solar cell, for example, is a four-junction solar cell with a power conversion efficiency (PCE) of 46%. However, tandem solar cells have not yet reached full commercialisation because production costs are high. New tandem solar cell technologies might be able to help tackle this issue as they have fewer fabrication processes and also need lower energy for recycling at the end of their lifecycle, which saves money. Therefore, they create revenue much faster than the conventional panels.

Despite constant improvement of the perovskite tandem solar technology, which has led to an enormous increase in their efficiencies, it has not yet found widespread application, mainly due to the high costs involved in production. There is hope that the with the aid of the current research it will finally help it gain a foothold in the international market.