Passivation of Bulk and Surface Defects in Inverted Single-junction and Tandem Perovskite Solar Cells

Abstract

As a representative of third-generation photovoltaic technology, perovskite solar cells (PSCs) have become a global research hotspot in the field of new energy due to their advantages of high power conversion efficiency (PCE), low cost, and solution preparation. Among them, inverted PSCs are regarded as the PV technology with the most commercialization potential by virtue of its low-temperature preparation process compatible with flexible devices and tandem technology, high stability and high PCE. However, perovskite thin films have defects on both the bulk phase and the surface, which are prone to form non-radiative composite centers, thus affecting the PCE of the devices; at the same time, they are prone to lead to problems such as ionic mobility, which reduces the operational stability of the devices. The prerequisite for realizing high-performance PSCs is the preparation of flat and dense perovskite films with good crystallinity and orientation, so the crystallization control of the light-absorbing layer of perovskite is one of the most effective strategies for the preparation of high-quality perovskite films, and the defects of the bulk of perovskite films can be passivated in this way. Meanwhile, the interfacial charge transfer between perovskite and the charge transport layer also directly affects the device performance, which can be effectively promoted by regulating the energy band alignment, passivating the defects on the surface of the perovskite, and other strategies. In addition, perovskite-based tandem devices have higher theoretical limiting efficiencies by improving photon utilization through complementary light absorption. Among them, perovskite/organic tandem solar cells have great potential for industrialization due to their low-temperature solution processability, lightweight flexibility, tunable subcell bandgap characteristics, and excellent operational stability and cost advantages. This thesis focuses on the passivation of bulk phase and surface defects in inverted single-junction and tandem perovskite photovoltaic devices, developing a new strategy of organic molecular defect passivation, which effectively improves the efficiency and stability of the devices, and mainly carries out the following three aspects of work:

(1) To address crystallization control and bulk passivation in inverted PSCs, a “top-down gradient regulation” strategy was developed, i.e., dripping chlorobenzene solution containing 1-acetylpiperazine (AP) onto the perovskite wet film (formed by inverted solvent extraction), followed by rapid high-speed spin coating and annealing. The in-situ treatment of perovskite wet films with AP utilizes gradient diffusion and coordination to synergistically optimize the crystalline quality, energy level alignment, and inhibit nonradiative recombination. The AP induces the formation of preferentially oriented (100) grains to enhance the crystallinity of perovskite, and the piperazines and acyls in the molecules can effectively passivate defects such as Pb, I, and FA vacancies, which can reduce the density of electron and hole defect states by about 22%. Through the gradient energy level bending, the interfacial barrier is lowered, thus realizing efficient carrier transport. The PCE of the inverted single-junction intermediate bandgap (1.55 eV) PSCs is improved from 24.02% to 26.42%. Meanwhile, the devices treated with this strategy show excellent light stability, maintaining 86.4% initial PCE after 1000 hours of continuous light exposure at 30 ℃ and 1-sun (AM 1.5G), while the control group devices not treated with this strategy only maintain 69.8% initial PCE under the same conditions.

(2) To solve surface defect passivation and perovskite/electron transport layer interface issues, a new method of fullerene C60 derivatization has been developed, i.e., two new 56π-electron fullerene derivatives (C₆₀-TFB and C₆₀-TFP) have been designed and synthesized as novel electron transport layers in inverted single-junction mid-bandgap (1.55 eV) PSCs. Rigid tert-butyl groups grafted onto fullerene cages suppressed molecular dimerization, enhanced intrinsic stability, inhibited iodine/silver migration, and improved environmental resistance via hydrophobicity. In addition, compared with the commonly used [6,6]-phenyl-C₆₁-butyric acid isomethyl ester (PCBM), the interfacial anchoring between C₆₀-TFB/TFP and the perovskite active layer is stronger, and the fluorine atoms and ester groups contained therein can effectively passivate the defects such as Pb, I, and FA vacancies of perovskite film. Their elevated LUMO levels optimized perovskite/ETL energy alignment, boosting electron transport. C₆₀-TFP-based devices achieved 25.93% PCE (PCBM: 24.08%) and retained 81.9% PCE after 1,000 hours of 55 °C/1-sun illumination (PCBM: 62.9%), demonstrating simultaneous efficiency and stability improvements.

(3) Although perovskite/organic tandem solar cells (PO-TSCs) theoretically surpass single-junction PSCs, their 1.85 eV wide-bandgap perovskite subcells suffer more severe open-circuit voltage losses as compared to the mid-bandgap (1.55 eV) PSCs, which mainly stems from severe nonradiative recombination caused by high concentration and multiple types of defects on its surface, which also accelerates perovskite phase separation and leads to device instability. To address this problem, we developed a mixed cation synergistic defect passivation strategy, i.e., mixing 4-trifluorophenethylammonium (CA) with ethylenediammonium (EDA) solution and spin-coating it onto the surface of wide-bandgap perovskite thin films. It was found that CA/EDA can be targeted to passivate specific types of defects on the perovskite surface, including uncoordinated Pb²⁺, halogen vacancies, and cationic defects. Based on this, the nonradiative recombination rate of 1.85 eV wide-bandgap perovskite was reduced from 0.80 ns^-1 to 0.11 ns^-1, while the interfacial charge transfer between the perovskite active layer and the electron transport layer was promoted, resulting in an open-circuit voltage of 1.35 V (the highest reported for this system at that time) for a single-junction wide-bandgap PSCs. Combined with quaternary organic solar cells, PO-TSCs reached 24.47% PCE (highest reported for this system at that time) and retained 90% PCE after 500 hours of maximum power point tracking.

Date of Award	9 Jun 2025
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Shangfeng Yang (External Supervisor), Qiyuan HE (Supervisor) & Zonglong ZHU (Co-supervisor)