Organic-inorganic hybrid perovskite solar cells (PSCs) are a promising photovoltaic technology due to their low cost and high efficiency potential (exceeding 25%). In n-i-p PSCs, hole transport materials (HTMs) are crucial for efficient hole extraction and transport, and also act as a barrier against moisture-induced perovskite degradation. Doped 2,2',7,7'-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9-spirobifluorene (Spiro-OMeTAD) is commonly used, but its hygroscopicity and dopant migration lead to instability issues. Therefore, dopant-free HTMs are highly desirable. While various organic conjugated materials have been explored as dopant-free HTMs, achieving both high efficiency and stability comparable to doped Spiro-OMeTAD remains a challenge. Poly(3-hexylthiophene) (P3HT), a readily available p-type semiconductor with high hole mobility, presents an attractive alternative. However, its "edge-on" stacking arrangement results in poor contact with the perovskite and increased non-radiative recombination, limiting PCE to around 16%. Interfacial engineering strategies, such as defect passivation or P3HT mobility modification, have been attempted but haven't fully addressed the poor contact issue. This research introduces a molecular bridge to improve the perovskite/P3HT interface and enhance PSC performance and stability.

Several studies have explored dopant-free HTMs for PSCs, including small molecules like D-A-π-A-D-type DTP-C6Th, 1,10-phenanthroline (YZ22), DTB-FL, and Ni phthalocyanine, and polymers like 2DP-TDB, phenanthrocarbazole 6 (PC6), and Mes-TABT. While some report high PCEs (over 21%), none have successfully replaced doped Spiro-OMeTAD due to factors such as complex synthesis, high cost, and poor reproducibility. Solvent-annealing assisted thermal evaporation of undoped Spiro-OMeTAD has shown promise (around 20% PCE), but the technique's complexity and reproducibility require further investigation. The use of P3HT as a HTM is another approach, but its "edge-on" packing leads to poor interfacial contact and low PCE. Previous attempts to improve P3HT-based PSCs involved interfacial engineering techniques such as defect passivation using materials like BTCIC-4CPP or CuSCN, and modifying P3HT packing using HTAB or doping with Ga(acac)3, achieving efficiencies up to 24%. However, the poor contact at the perovskite/P3HT interface remained a significant challenge.

The study designed and synthesized a molecular bridge molecule, 2-((7-(4-(bis(4-methoxyphenyl)amino)phenyl)-10-(2-(2-ethoxyethoxy)ethyl)-10H-phenoxazin-3-yl)methylene)malononitrile (MDN), and a control molecule, RDN. MDN features a malononitrile group to anchor the perovskite and a triphenylamine (TPA) group to facilitate π-π stacking with P3HT. The optical and electrochemical properties of MDN and RDN were characterized using UV-vis absorption spectroscopy and cyclic voltammetry. MDN and RDN were incorporated into P3HT solutions (M-P3HT and R-P3HT, respectively) and their packing structures were investigated using grazing-incidence wide-angle X-ray scattering (GIWAXS). Hole mobility was measured using the space charge limited current (SCLC) method. n-i-p type PSC devices (FTO/SnO₂/Cs₀.₀₅FA₀.₈₅MA₀.₁₀Pb(Br₀.₀₃I₀.₉₇)₃/HTM/Ag) were fabricated using P3HT, M-P3HT, and R-P3HT as HTMs. The photovoltaic performance was evaluated using J-V curves, external quantum efficiency (EQE) measurements, and steady-state PCE testing. X-ray photoelectron spectroscopy (XPS) was used to analyze the perovskite surface after modification. Steady-state and time-resolved photoluminescence (PL and TRPL) spectroscopy investigated the charge transfer kinetics at the perovskite/HTM interface. Density functional theory (DFT) calculations explored the interactions between MDN/RDN, P3HT, and perovskite. Electrical impedance spectroscopy (EIS) and light intensity dependence of Voc were used to analyze charge transport dynamics and recombination behavior. Finally, the long-term stability of unencapsulated devices was assessed under high relative humidity conditions.

GIWAXS showed that MDN and RDN did not significantly alter the "edge-on" packing of P3HT. SCLC measurements showed slightly lower hole mobility for M-P3HT and R-P3HT compared to P3HT. PSCs with M-P3HT exhibited a significantly higher PCE (22.87%) than those with P3HT (12.48%) or R-P3HT (16.66%). The improvement was attributed to enhanced Voc, Jsc, and FF. M-P3HT also showed significantly reduced hysteresis. XPS revealed a change in the electronic environment of the perovskite surface upon MDN modification, suggesting the formation of an electrostatic coupling between MDN's malononitrile group and Pb in the perovskite. PL and TRPL measurements indicated faster carrier transfer and reduced non-radiative recombination at the perovskite/M-P3HT interface. The introduction of a thin PS layer between the perovskite and M-P3HT significantly reduced the device efficiency, confirming the importance of direct MDN-perovskite interaction. DFT calculations showed strong interaction between MDN's electron-withdrawing groups and the perovskite surface, and efficient π-π stacking between MDN's TPA group and P3HT. EIS and light intensity-Voc analysis indicated reduced transfer resistance and increased recombination resistance in M-P3HT-based devices. The unencapsulated M-P3HT-based device exhibited superior long-term stability under high humidity conditions, maintaining 92% of its initial efficiency after three months.

The significantly improved PCE and stability of PSCs with M-P3HT are attributed to the formation of a molecular bridge by MDN. MDN effectively links the perovskite and P3HT, facilitating efficient charge transport and minimizing recombination losses. The malononitrile group's interaction with the perovskite surface passivates defects, while the TPA group's π-π stacking with P3HT enhances the interfacial contact. The reduced hysteresis in M-P3HT-based devices suggests improved charge extraction and reduced interfacial charge accumulation. The long-term stability under high humidity is likely due to the improved hydrophobicity of M-P3HT and the reduction in perovskite surface defects. This approach offers a simple and effective strategy to enhance the performance and stability of P3HT-based PSCs.

This study demonstrates the successful construction of a molecular bridge using MDN to improve the performance and stability of P3HT-based PSCs. The resulting device achieves a high PCE of 22.87% and shows remarkable long-term stability. This molecular bridge approach offers a promising pathway for developing low-cost, stable, and efficient PSCs using readily available materials. Future research could explore variations of the MDN molecule to further optimize the charge transport and defect passivation properties, as well as exploring other polymer-based systems.

The study primarily focuses on a specific perovskite composition and device architecture. The generalizability of the findings to other perovskite materials and device structures requires further investigation. While the long-term stability is significantly improved, the stability under harsher conditions (e.g., higher temperatures) still needs to be thoroughly evaluated. The synthesis of MDN might be a limiting factor for large-scale production, requiring optimization to reduce cost and complexity.