Toward Stable Monolithic Perovskite/Silicon Tandem Photovoltaics: A Six-Month Outdoor Performance Study in a Hot and Humid Climate

Perovskite/silicon tandem solar cells are emerging as a high-efficiency and prospectively cost-effective solar technology with great promise for deployment at the utility scale. However, despite the remarkable performance progress reported lately, assuring sufficient device stability-particularly of the perovskite top cell-remains a challenge on the path to practical impact. In this work, we analyze the outdoor performance of encapsulated bifacial perovskite/silicon tandems, by carrying out field-testing in Saudi Arabia. Over a six month experiment, we find that the open circuit voltage retains its initial value, whereas the fill factor degrades, which is found to have two causes. A first degradation mechanism is linked with ion migration in the perovskite and is largely reversible overnight, though it does induce hysteretic behavior over time. A second, irreversible, mechanism is caused by corrosion of the silver metal top contact with the formation of silver iodide. These findings provide directions for the design of new and more stable perovskite/silicon tandems

Role of ions and interfaces for efficient and stable perovskite solar cells

Anand Agarwalla

Perovskite Solar Cells (PSCs) have grabbed global attention of the researchers due to their outstanding Photovoltaic (PV) performance. PSCs have the potential to be the future of the PV technology as they can generate power with performance being comparable with the leading Silicon solar cells, with the cost being lower than Silicon solar cells. The enormous potential of PSCs is evident from the fact that the efficiency of these cells has risen from 3.8% to 25.5% within a decade, and it is continuously rising to date. In my thesis, I investigate the impact of ions in the bulk of perovskite and also look at the ETL-perovskite interface to create a simpler, more efficient, more reproducible and more stable system for PSCs. At first, I managed to achieve high efficiency devices with the standard triple cation perovskite on both planar and mesoporous n-i-p ETL device architecture. Adding KI to the already established compositions showed a boost in performance of the solar cells while significantly reducing hysteresis. We then performed a systematic study of KI on various perovskite compositions comprised of FA, MA, Cs, Rb, I and Br ions. We demonstrated that KI reacts with the bromide ions in perovskite passivating the grains leading to a red shift hence and improvement in current density as well as efficiency.MAPbBr3 is generally used is small proportions along with FAPbI3 to stabilise the phase of FAPbI3. But the effect of Br- ions on the perovskite performance was relatively unknown. The introduction of Br- ions in the lattice causes a blue shift in the band gap pushing it further away from the ideal Shockley-Quessier limit. On top of this, the effect of MAPbBr3 on phase stability of FAPbI3 is similar to the effect of CsPbI3. This led me to examine the impact of reduced bromine concentration of the device performance. Interestingly, reducing the bromine concentration not only leads to a red shift in band gap, hence increasing the current density but also causes a reduced voltage loss resulting in much superior performance. Moreover, the Br- ions cause increased halide diffusion in the bulk leading to decreased stability with higher the bromide concentration.While perovskite composition plays an important role in defining the device characteristics, it could be argued that the interfaces play an equally important role in doing the same. With the rise in usage of SnO2 as viable substitute for TiO2 as ETL, I explore the effect of using TiO2 particles that exhibit similar properties to the SnO2 particles. These smaller size TiO2 nanoparticles can form a very thin, compact, uniform and pin-hole free layer with a morphology that follows the morphology of FTO. This results in a reduced recombination in the interface of TiO2 and perovskite causing the open circuit voltage and hence the device performance to improve significantly.

EPFL2021

Room Temperature as a Goldilocks Environment for CH3NH3PbI3 Perovskite Solar Cells: The Importance of Temperature on Device Performance

Juan-Pablo Correa-Baena, Ulf Anders Hagfeldt, Tor Jesper Jacobsson, Wolfgang Richard Tress

Terrestrial applications of solar cells during day-night cycling as well as operation in winter and summer involve substantial temperature variations, which influence the photophysics as well as the charge separation and transport properties in the various materials employed in a device. In this study, the optical absorption of methylammonium lead iodide (MAPbI(3)) and the device performance of MAPbI(3) solar cells have been investigated in an extended temperature range between -190 and 80 degrees C. The optical properties were found to change by only a small amount in that temperature range. The device performance did, however, show more dramatic changes and decreased in a reversible manner for temperatures both higher and lower than room temperature. For temperatures up to 80 degrees C and down to -80 degrees C, the drop in performance was up to 25% compared to the room temperature value. Given thermal stability and reversible device performance, this is probably not a showstopper for terrestrial applications of perovskite solar cells but should be considered when evaluating the total energy yield under outdoor operations. At temperatures of 100 degrees C and below, which are relevant for outer atmosphere and space applications, the performance decreases rather dramatically and approaches zero at even lower temperature. Irreversible changes set in for temperatures above 50 degrees C. In addition, the hysteresis decreases at reduced temperatures. As the effects for the absorption properties are minor, the decrease in performance can be attributed to a temperature induced limitation in the transport and extraction of the photogenerated charge carriers which is seen as a strong increase of the series resistance at reduced temperature. The drop of the photovoltage for temperatures below -100 degrees C might be related to reduced charge carrier separation in the perovskite due to excitonic effects and a lower dielectric constant.

American Chemical Society2016

Interface Play between Perovskite and Hole Selective Layer on the Performance and Stability of Perovskite Solar Cells

Laura Calio, Mohammad Khaja Nazeeruddin

Perovskite solar cells with variety of hole selective contacts such as 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (Spiro-OMeTAD), poly(3-hexylthiophene-2,5-diyl), poly[bis(4-phenyl)(2,5,6-trimentlyphenyl)amine], 5,10,15-trihexyl-3,8,13-tris (4-methoxyphenyl) -10,15-dihydro-SH-diindolo [3,2-a:3',2'-c]carbazole (HMPDI), and 2',7'-bis(bis(4-methoxyphenyl)-amino)spiro [cyclopenta[2,1-b:3,4-b']dithiophene-4,9'-fluorene] were employed to elucidate its role at the interface of perovskite and metallic cathode. Microscopy images revealed Spiro-OMeTAD and HMPDI produce smoother and intimate contact between perovskite/hole transporting materials (HTM) interfaces among others evaluated here. This morphological feature appears to be connected with three fiindamental facts: (1) hole injection to the HTM is much more efficient as evidenced by photoluminescence measurements, (2) recombination losses are less important as evidenced by intensity-modulated photovoltage spectroscopy and impedance spectroscopy measurements, and (3) fabricated solar cells are much more robust against degradation by moisture. Devices with higher open-circuit photovoltages are characterized by higher values of the recombination resistance extracted from the impedance data. The variation in device hysteresis behavior can be ascribed mainly due to the molecular interaction and the core of HTM employed. In all cases, this fact is related with a larger value of the low-frequency capacitance, which indicates that the HTM can induce specific slow processes of ion accumulation at the interface. Notably, these processes tend to slowly relax in time, as hysteresis is substantially reduced for aged devices.