Task 1: The polymer screening process has been conducted, and among all the tested polymers (PM6, D18, D18-Cl, PTQ10, and JD40-BDD20), D18-Cl emerged as the most suitable for our purpose based on the following observations:
1. GIWAXS analysis revealed that D18-Cl exhibits superior π-π stacking, facilitating the formation of a compact and dense thin film.
2. The HOMO of D18-Cl aligns well with the valence band maximum (VBM) of perovskite and the HOMO of home-grown hole-transporting layer (HTL), spiro-OMeTAD, resulting in improved charge extraction.
3. Steady-state photoluminescence (PL) and time-resolved photoluminescence (TRPL) measurements confirmed that D18-Cl enhances hole extraction efficiency.
Building on our OPV experience, we successfully incorporated D18-Cl on the perovskite surface to passivate interface trap states. While the one-step spin-coating process with antisolvent pipetting is widely used to control perovskite nucleation and crystallization, surface defects often lead to poor interfaces with charge-transport layers. To address this, we introduced D18-Cl into the antisolvent as a template agent, achieving high PCE even without additives in spiro-OMeTAD. This approach effectively modifies the perovskite/spiro-OMeTAD interface, enabling a simple and efficient antisolvent engineering technique.
Achievement 1: Using the anti-solvent film deposition method, we fabricated perovskite solar cells with the structure: ITO/SnO2/perovskite/OAI/spiro-OMeTAD/gold. After testing concentrations from 0.5 to 2.5 mg/ml, 1 mg/ml of D18-Cl was found optimal. The device performance (Figure 1) were compared for two antisolvent conditions: (1) pure CB (control) and (2) CB with 1 mg/ml D18-Cl. Both used identical HTL conditions without additives. Perovskite (D18-Cl)-based devices showed a 0.07 V increase in voltage, reducing recombination losses, and an fill factor rise from 75% to over 81%, improving charge extraction. This approach increased the efficiency from below 19% to over 23%.
Task 2: It is crucial to investigate the role of D18-Cl in the perovskite antisolvent. This includes performing various surface analyses, such as XRD, XPS, UPS, ToF-SIMS, FTIR, and SEM, alongside optical and electrical characterizations, to evaluate any changes in the perovskite's properties and surface. A key aspect of this work is determining whether the polymer remains on the perovskite surface or penetrates into its bulk. To explore this further, we also conducted X-ray reflectivity (XRR) and Neutron reflectivity (NR) measurements.
Achievement 2: From XRR and NR measurements (Figure 2), we identified an ultra-thin (1.2 nm) pure polymer layer on top and a 10 nm mixed region (D18-Cl and perovskite) underneath. These layers support proper energy level alignment and charge transport. UPS analysis (Figure 3) revealed that without the polymer, strong charge transfer (dipole size: 0.52 eV) occurs at the perovskite/spiro-OMeTAD interface, potentially leading to spiro-OMeTAD dedoping. With the polymer, weaker charge transfer (dipole size: 0.15 eV) occurs, reducing dedoping. The potential gradient in the D18-Cl layer promotes hole transport from the perovskite to the anode while blocking electrons, suggesting that D18-Cl acts as an efficient hole transport or electron blocking layer. Photoluminescence quantum yield (PLQY) measurements (Figure 4) revealed that optimal energy level alignment between perovskite/D18-Cl/spiro-OMeTAD reduces interfacial energy loss and improves charge transfer. The ITO/SnO2/perovskite(D18-Cl)/OAI stack showed a PLQY of 1.3%, lower than the control perovskite (CB) stack (3.34%), indicating that charge extraction dominates at the perovskite/D18-Cl interface rather than chemical passivation. However, after adding spiro-OMeTAD, the perovskite (D18-Cl) device exhibited a higher PLQY than the control, suggesting that D18-Cl reduces recombination across the interface rather than directly passivating the perovskite surface. This results in reduced interfacial losses due to well-matched band alignment in the perovskite/D18-Cl/spiro-OMeTAD stack.
