Composition Engineering and Surface Modulation Towards Efficient and Stable Perovskite Solar Cells


Student thesis: Doctoral Thesis

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Award date2 Nov 2022


Perovskite solar cells (PVSCs) have drawn significant attention in the last decade due to their superior power conversion efficiencies (PCEs), low cost, solution processibility, and applications in wearable electronics, building-integrated photovoltaics (BIPVs), as well as multijunction solar cells. The champion PCE of single-junction PVSC has been approaching 26%, which is comparable to that of commercialized silicon solar cells and surpassing copper indium gallium selenide (CIGS) solar cells. Moreover, a certified PCE of 31.3% has been achieved for perovskite/silicon multijunction solar cells, making them promising candidates toward commercial deployment.

In spite of these efforts, the highest PCEs of PVSCs are still only ~75% of the Shockley–Queisser (S-Q) limit, there is still much room for further enhancement. More importantly, the long-term operational stability of PVSCs is still inferior to commercialized solar cells, which is the main factor that hinders their commercialization. Hence, to explore strategies that can simultaneously enhance the performance and stability of PVSCs is in urgent demand.

Generally, the efficiency and stability of PVSCs are greatly influenced by the quality of perovskite films. On one hand, the crystallinity of perovskite films plays a significant role in determining the performance and stability. The poor morphology and disordered orientation of perovskites will not only cause more non-radiative recombination centers, but also result in inferior stability as the pinholes could induce and accelerate perovskite film decomposition. On the other hand, unavoidable defects in the bulk and on the surface of perovskite films formed during the deposition process could also lead to non-radiative recombination and propelling ion migration, and thereby cause severe energy losses and performance decay under stress. Therefore, tailoring the crystallization and defects of perovskites could be an effective way to concurrently boost the efficiency and stability of PVSCs.

My work demonstrated in this thesis aims to achieve high-performance PVSCs with superior stability for different types of PVSCs. I firstly focus on the investigation of carbon-based all-inorganic PVSCs due to their excellent photo and thermal stability. Specifically, the crystal formation process of all-inorganic perovskites is controlled through a facile composition engineering strategy by bromide incorporation in CsPbI3 perovskite, which results in high-quality perovskite films with enlarged grain size, reduced grain boundaries, longer carrier lifetime and lower energy disorder, contributing to suppressed energy loss and enhanced device performance. Fabricated carbon-based HTM-free PVSCs with CsPbI2.33Br0.67 perovskite achieved a record PCE of 12.40% at that time. Moreover, unencapsulated device retains over 90% of its initial efficiency under continuous one sun illumination for 250 h in N2 atmosphere and keeps ~84% of its original value after stored in ambient environment (RH: 15–20%) for 200 h, suggesting the excellent long-term stability via developed strategy. This work provides a facile and effective way to realize high-performance and stable carbon-based all-inorganic PVSCs.

Then, based on the developed deposition method of all-inorganic perovskite in the first work, an all-inorganic perovskite/organic tandem solar cell (TSC) with a wide bandgap all-inorganic perovskite CsPbI2.1Br0.9 and a narrow bandgap organic photoactive layer (PM6:Y6) as top and bottom sub cells was fabricated. Phenmethylammonium chloride (PMACl) was selected to passivate the surface defects of all-inorganic perovskites, which enhanced the open-circuit voltage (VOC) and photostability of fabricated devices. TSC with as-fabricated all-inorganic PVSC sub cell demonstrated a remarkable PCE of 18.06%, with a VOC of 1.89 V, short-circuit current (JSC) of 12.77 mA cm−2, and fill factor (FF) of 74.81%. What’s more, benefiting from the outstanding UV and thermal stability of inorganic perovskites, as-prepared tandem devices exhibited excellent photo and thermal stability, with negligible degradation after 150 h of UV irradiation, 250 h of one sun illumination and 100 h of heating at 80 °C, respectively. This work proves that surface modulation could enable all-inorganic perovskites that are appropriate candidates for the realization of efficient and stable TSCs.

Next, in the third work, the fused-ring electron acceptor (FREA) molecules were employed to passivate defects of environmental-friendly lead-free Cs2AgBiBr6-based double perovskite. The strong binding of C≡N and N=C–S groups on FREA with Ag-exposed surface of Cs2AgBiBr6 efficiently reduced surface trap densities and considerably alleviated non-radiative recombination, contributing to a dramatically improved VOC from 1.079 V to 1.278 V and a record PCE up to 3.31%, which is the highest efficiency for double PVSCs to date. Additionally, the passivated devices showed superior long-term stability, which maintained 98.5% and 97.2% of the initial efficiency under continuous AM 1.5G illumination and 85 ℃ heating for 300 h, respectively. This work manifests the importance of the rational design of functional surface modulation molecules to improve the performance and stability of double PVSCs.

The last part of this thesis focuses on the development of pyridine-functionalized fullerene derivative material, fullerene-n-butyl-pyridine (C60-BPy), to modulate the energetic disorder, surface defects and energy level mismatch between perovskites and electron-transporting materials (ETMs) and thereby improve the PCE and lifespan of tin-based PVSCs. The C60-BPy can strongly anchor on the perovskite surface via coordination interactions between the pyridine moiety and the Sn2+ ion, which not only reinforces the passivation of the trap-state within the tin perovskite film, but also regulates the interface energy level alignment to reduce non-radiative recombination. Furthermore, the improved interface binding also contributed to superior device stability, the resultant devices realized a highest PCE of 14.14% with negligible hysteresis, maintained over 95% of their initial PCE under continuous one-sun illumination for 1000 hours.