Homogeneously Large Polarons in Aromatic Passivators Improves Charge Transport between Perovskite Grains for >24 % Efficiency in Photovoltaics

Abstract

Aromatic passivators, such as porphyrin, with large π-backbones have attracted considerable attention to boost the charge carrier in polycrystalline perovskite films, thus enabling the fabrication of efficient and stable perovskite solar cells (PSCs). However, they often selfassemble into supramolecules that probably influence the charge-transfer process in the perovskite grain boundary. Here, by doping a monoamine Cu porphyrin into perovskite films, two porphyrin-based self-assembled supramolecules were successfully prepared between perovskite grains. Crystal structures and theoretical analyses reveal the presence of a stronger interaction between the amine units and the central Cu ions of neighbouring porphyrins in one of the supramolecules. This has a modified effect on the dipole direction of the porphyrins to be quantized as homogeneously large polarons (HLPs) in a periodic lattice. The porphyrin supramolecules can stabilize perovskite grain boundaries to greatly improve the stability of PSCs, while the HLPs-featured supramolecule facilitates hole transport across perovskite grains to remarkably increase the cell performance to as high as 24.2 %. This work proves that the modulation of the intermolecular interaction of aromatic passivators to yield HLPs is crucial for the cascaded acceleration of charge transport between perovskite grains. Organic-inorganic hybrid perovskite solar cells (PSCs)have attracted enormous attention because of their excellent power conversion efficiencies (PCEs) and low cost. [1][2][3][4] In recent years, the best PCE of PSCs has developed rapidly and has exceeded 25 %, approaching the single-junction Shockley-Queisser (SQ) limit. [5][6][7][8] A polycrystalline perovskite film fabricated by low-temperature solution-processed growth inevitably forms a large number of grain boundaries with a high defect density. [9][10][11][12] They are highly reactive to water and oxygen, thus inducing the degradation of the perovskite film and reducing the stability of PSCs. [13][14][15] These defects at grain boundaries acting as the recombination and trap-state centers are detrimental for the cell performance. [16][17][18] Thus, effective management of perovskite grain boundaries is vitally important for maximizing the performance and stability of PSCs. [19][20][21] A series of functional organic small molecules has been employed to passivate the perovskite defects and increase the performance and stability of PSCs. [22][23][24] However, these molecules have a high degree of volatility and high diffusion coefficients, resulting in poor stability under harsh environments, such as strong light, high temperature, and electric fields. [17,25] Stable aromatic passivators should be potential candidates to decorate perovskite films to promote the efficient charge transport across perovskite grains, [26][27][28][29][30][31] and/ or contribute to the surface band edges to influence the charge carrier dynamics, [32] thus leading to the fabrication of efficient and stable PSCs. It is worth noting that it is...

Polarons in perovskite solar cells: effects on photovoltaic performance and stability

“…Moreover, polarons can lower HC relaxation, enabling the possibility to extract high-energy carriers from perovskite materials [69][70][71]. In addition, large polarons were also observed in conjugated organic materials, which can be used to facilitate charge extraction in PSCs as grain boundary passivators [34] or a charge transport layer [33]. Therefore, the modulation of polarons in perovskites and organic charge-transport materials is expected to eliminate energy losses in PSCs.…”

“…The strong electron-phonon interaction within polarons generates pseudo-free dressed carriers, which can screen the carriers from defect states to prevent nonradiative recombination and trapping, leading to a prolonged charge-carrier lifetime and diffusion length [30][31][32]. Moreover, polarons were also found in conjugated organic semiconductors such as charge-transport materials in PSCs, which can promote charge extraction from the device and hence improve device performance [33,34]. In addition, the formation of polarons has been employed to explain some stability issues in PSCs owing to the generated polaronic gradients and metastable defects, which majorly affect the phase stability and charge transport [35,36].…”

Spontaneous Formation of Heterostructured Perovskite Films for Photovoltaic Application

“…The organic cations in perovskites protonate the MP molecules in supramolecules, further forming the ammonium porphyrins (MPÀ H +) anchored in perovskite surfaces. Moreover, the CuP dopants enable redesign of the self-assembly supramolecules through strong interaction between the amine components and the central Cu ions, [25] generating the homogeneously large polarons to promote the hole transport (Figure 1d-l). The champion PSCs based on CuP delivered a significantly improved PCE of 24.2 %.…”

“…Hole transmission path images of i) CuP‐S1‐ and j) CuP‐S2‐systems. k) Transient photoluminescence and l) hole trap‐state densities of samples without and with CuP‐S1 and CuP‐S2 [25] . Reproduced with permission from ref.…”

“…Hole transmission path images of i) CuP-S1-and j) CuP-S2-systems. k) Transient photoluminescence and l) hole trap-state densities of samples without and with CuP-S1 and CuP-S2 [25]. Reproduced with permission from ref [25]…”