Programm

Metal halide perovskite films and devices are susceptible to degradation upon exposure to ambient, elevated temperature, illumination, and electrical bias. The degradation under illumination typically occurs due to electrochemical redox reactions initiated by photogenerated charge carriers. The oxidation of iodide and the generation of various oxidized species (interstitial iodide I_i, iodine I₂, triiodide I₃^-) starts a chain reaction of degradation since I₃^- can readily deprotonate organic cations such as methylammonium cation MA⁺ or formamidinium cation FA⁺.¹ While the electrochemical redox reactions are in principle reversible, the loss of volatile degradation products resulting from deprotonation of organic cations will cause irreversible degradation. The degradation processes in different environments are strongly dependent on the composition of the perovskite, as well as device architecture which affects the charge collection. Here we investigated the stability under illumination for three different perovskite compositions, namely commonly used CsFAMA mixed cation perovskite Cs_0.05(FA_0.87MA_0.13)Pb(I_0.87Br_0.13)₃,^2,3 low Br perovskite Cs_0.05(FA_0.98MA_0.02)Pb(I_0.98Br_0.02)₃,⁴ and MA-free perovskite Cs_0.1FA_0.9PbI_2.9Br_0.1.⁵ We have found that low-Br perovskite, while enabling high efficiency, exhibits poor stability with significant phase transformation to d-FAPbI₃ as well as decomposition to PbI₂ under a range of testing conditions. In comparison, CsFAMA films did not show the formation of d-FAPbI₃ phase, but demonstrated degradation to PbI₂. Among the three compositions considered, MA-free perovskite exhibited the best stability. In addition, surprisingly it exhibited higher sensitivity to exposure to moisture compared to oxygen under illumination. The stability of MA free perovskite could be significantly improved with additives, and the role of different additives in stability improvement is discussed.

 

Bibliography

1. J. N. Hu, Z. Xu, T. L. Murrey, I. Pelczer, A. Kahn, J. Schwartz, B. P. Rand, Adv. Mater. 2023, 35, 2303373.
2. M. Saliba, J. P. Correa-Baena, C. M. Wolff, M. Stolterfoht, N. Phung, S. Albrecht, D. Neher, A. Abate, Chem. Mater. 2018, 30, 4193−4201;
3. J. Y. Lin, Y. T. Wang, A. Khaleed, A. A. Syed, Y. L. He, C. C. S. Chan, Y. Li, K. Liu, G. Li, K. S. Wong, J. Popović, J. Fan, A. M. C. Ng, A. B. Djurišić,, ACS Appl. Mater. Interfaces 2023, 15, 24437-24447, 2023.
4. D. Li, Y. Huang, R. Ma, H. Liu, Q. Liang, Y. Han, Z. Ren, K. Liu, P. W. K. Fong, Z. Zhang, Q. Lian, X. Lu, C. Cheng, G. Li, Adv. Energy Mater. 2023, 13, 2204247.
5. Q. An, L. Vieler, K. P. Goetz, O. Telschow, Y. J. Hofstetter, R. Buschbeck, A. D. Taylor and Y. Vaynzof, Adv. Energy Sustainable Res., 2021, 2, 2100061.

With the increase of power conversion efficiency (PCE), the stability of organic solar cells becomes the biggest obstacle for their commercialization. Nanomorphology of the bulk-heterojunction layer (BHJ) plays an important role in determining the performance and stability of the cells. In addition to pure donor and acceptor phases, amorphous intermixing phases of the donor and acceptor molecules are also formed within the BHJ film. However, quantitatively determining the content and structure of the intermixing phase is quite challenging. In this presentation, we demonstrate a method to characterize the aggregation and composition of non-fullerene acceptors (NFA) in the intermixed phases. By measuring the absorption spectrum of the blend films with different NFA concentration, we found a red-shift of the maximum absorption wavelength with the increase of the NFA concentration, which is ascribed to the change of intermolecular interactions. With these results, we can identify the nanostructure of the BHJ films under different processing conditions, and the results are correlated to the device efficiencies. Also, we found that the structure of the intermixing amorphous phases is rather stable under light and thermal stress, demonstrating an excellent intrinsic stability of the organic solar cells.

Cesium Formamidinium Lead halide (CsFA) is one of the promising perovskites from high performance and stability^1,2 standpoint. However, works on fabricating this CsFA perovskite in relatively high humidity level, especially with carbon electrodes for economical reasons and ease of scaling up are still scarce. In this talk, I will present an environmentally friendly approach to fabricate high-quality perovskite via gas-quenching of urea-perovskite precursor in 40% RH humidity condition. This recipe lengthens annealing window and yield perovskite with 1 micrometers grain size with high compactness³. Additionally, we showcase various surface defect passivation, especially organic molecules and polymers to not only improve the efficiency and stability of perovskite solar cells⁴ but also enable ambient fabrication of hole transport layer alternatives to Spiro-OMETAD. This includes ambient fabrication of Copper(I) thiocyanate and its solvent modification to improve their stability. Additionally, development of transferable carbon electrode will be discussed to complete the full PSCs totally in ambient.

References

1.      R. Cheacharoen, N. Rolston, D. Harwood, K.A. Bush, R.H. Dauskardt, M.D. McGehee*, “Design and understanding of encapsulated perovskite solar cells to withstand temperature cycling”, Energy & Environmental Science, 11, 144-150 (2018).

2.      R. Cheacharoen, C.C. Boyd, G.F. Burkhard, T. Leijtens, J.A. Raiford, K.A. Bush, S.F. Bent, M.D. McGehee*, “Encapsulating perovskite solar cells to withstand damp heat and thermal cycling”, Sustainable Energy & Fuels, 2, 2398-2406 (2018).

3.      C. Harnmanasvate, R. Chanajaree, N. Rujisamphan, Y. Rong, R. Cheacharoen*, “Ambient Gas-Quenching Fabrication of MA-Free Perovskite Solar Cells Enabled by an Eco-Friendly Urea Additive”, ACS Applied Energy Materials, 6,20, 10665-10673 (2023)

4.      M.Hu, Y. Zhu, Z. Zhou, M. Hao, C. Harnmanasvate, J.Waiyawat, Y. Wang, J. Lu, Q. An, X. Li, T. Zhang, Y. Zhou*, R. Cheacharoen*, Y. Rong*, “Post‐Treatment of Metal Halide Perovskites: From Morphology Control, Defect Passivation to Band Alignment and Construction of Heterostructures”, Advanced Energy Materials, 2301888 (2023)

Talk will provide an update on the NREL perspective on metal halide perovskite (MHP) based photovoltaics (PVs).  This will include an argument of why MHP-PV technologies are critical and how the need to develop these technologies drives research needs in stability and reliability.  Results from across multiple studies undertaken at NREL will be summarized to identify both technical success and failures or missed opportunities to advance the stability of MHP and MHP enabled PV technologies.  Connections to device efficiency and manufacturing which are intimately linked to stability and reliability will also be highlighted.

Organic solar cells have recently reached power conversion efficiencies of over 19%, highlighting the stability as their last remaining weak point. Their organic nature makes them strongly influenced by stresses such as oxygen, light, heat and humidity, which can be commonly found in their working environment. 

Incorporation of stabilizing additives (antioxidants, radical scavengers, hydroperoxide decomposers, UV absorbers) in active layers of organic solar cells is an attractive approach for inhibiting degradation as it is both inexpensive and easily upscalable, and it does not introduce further complexity into the device architecture.

Here we present our recent results on long-term stability improvement using naturally occurring antioxidants, such as vitamin C and beta-carotene, that act as singlet oxygen quenchers and radical scavenging compounds, as well as explore the synergistic effects of such compounds on the mechanical properties. The reported results and methods indicate a desirable route for mitigating degradation in organic solar cells.

References

1. Atajanov R, Turkovic V et al.  The mechanisms of degradation and stabilization of high-performing non-fullerene acceptor based organic solar cells. (in preparation)

2. Balasubramanian S, Turkovic V et al. Vitamin C for Photo-Stable Non-fullerene-acceptor-Based Organic Solar Cells. ACS Appl Mater Interfaces 2021; dx.doi.org/10.1021/acsami.3c06321

3. Prete M, Turkovic V et al. Synergistic effect of carotenoid and silicone-based additives for photooxidatively stable organic solar cells with enhanced elasticity. J Mater Chem C 2021; dx.doi.org/10.1039/D1TC01544C 

4. Turkovic V et al. Biomimetic Approach to Inhibition of Photooxidation in Organic Solar Cells Using Beta-Carotene as an Additive. ACS Appl Mater Interfaces 2019; dx.doi.org/10.1021/acsami.9b13085

5. Bregnhøj M, Turkovic V et al. Oxygen-dependent photophysics and photochemistry of prototypical compounds for organic photovoltaics: inhibiting degradation initiated by singlet oxygen at a molecular level. Methods Appl Fluoresc 2019; dx.doi.org/10.1088/2050-6120/ab4edc