Stabilizing Non‐Fullerene Organic Photodiodes through Interface Engineering Enabled by a Tin Ion‐Chelated Polymer

Abstract The recent emergence of non‐fullerene acceptors (NFAs) has energized the field of organic photodiodes (OPDs) and made major breakthroughs in their critical photoelectric characteristics. Yet, stabilizing inverted NF‐OPDs remains challenging because of the intrinsic degradation induced by improper interfaces. Herein, a tin ion‐chelated polyethyleneimine ethoxylated (denoted as PEIE‐Sn) is proposed as a generic cathode interfacial layer (CIL) of NF‐OPDs. The chelation between tin ions and nitrogen/oxygen atoms in PEIE‐Sn contributes to the interface compatibility with efficient NFAs. The PEIE‐Sn can effectively endow the devices with optimized cascade alignment and reduced interface defects. Consequently, the PEIE‐Sn‐OPD exhibits properties of anti‐environmental interference, suppressed dark current, and accelerated interfacial electron extraction and transmission. As a result, the unencapsulated PEIE‐Sn‐OPD delivers high specific detection and fast response speed and shows only slight attenuation in photoelectric performance after exposure to air, light, and heat. Its superior performance outperforms the incumbent typical counterparts (ZnO, SnO2, and PEIE as the CILs) from metrics of both stability and photoelectric characteristics. This finding suggests a promising strategy for stabilizing NF‐OPDs by designing appropriate interface layers.

The energy of the highest occupied molecular orbital (HOMO) of PEIE-Sn is calculated by Equation S1 HOMO = ℎ − ( cutoff −  onset ) (Equation S1) where hν is the energy of the incident photon from the He (I) source (21.22 eV), E cutoff , 17.05 eV as displayed in Figure 2g (right), is secondary electron cutoff edge, and E onset , 3.45eV as shown in Figure 2g (left), is the valence region onset.
An optical bandgap (E g,opt ) of 3.55 eV is estimated from the Tauc plot (Figure S1a).
Combining the HOMO of 7.62 eV, the LUMO of PEIE-Sn is estimated to be 4.07 eV.

The time-resolved photoluminescence (TRPL) curve fitting:
The TRPL decay characterization for the PBDB-T:ITIC-Th active layer deposited on various CILs were carried out to study the charge extraction.The PL decay time and amplitudes are fitted and estimated using the monoexponential function Equation S2 where τ 1 and A 1 are the decay times and the pre-exponential constants or decay amplitudes respectively; C is a constant for the baseline offset.

Paracrystalline disorder (g) calculations:
From 2D-( Grazing Incidence X-Ray Diffraction) GIXD data, the paracrystalline disorder parameter for the (010) peaks (g (010) ) can be calculated by using the single peak-width estimation method based on S3) where the Δq and q0 is the width and center position of the diffraction peak, respectively.And the paracrystalline disorder parameter for the (h00) peaks can be calculated from the slope (m) of δb -h 2 plot (Figure S15F), which is determined by where δb is the integral width of the diffraction peak, h is the order of diffraction and d is the domain spacing.

Defect density calculations:
The electron-only devices with the configurations of ITO/CILs (cathode interfacial layers) /PBDB-T:ITIC-Th/PFN-Br/Ag were prepared to calculate the defect density.In the space charge-limited current (SCLC) regime, the current is dominated by charge carriers injected from the contacts and the current-voltage characteristics become quadratic (I~V 2 ). Figure S17 shows the J-V curves of the fabricated devices on a double logarithmic scale, which comprises the Ohmic region, the trap-filling limit (TFL) region and the Child region.In the TFL region, the trap-state density (N t ) can be calculated by the following Equation S5 (Equation S5) where ε and ε 0 are the relative dielectric constant and vacuum permittivity, respectively.V TFL is the onset voltage of TFL region, q is elementary charge.L represents PBDB-T:ITIC-Th thin film thickness, which was calculated to 300 nm.The N t of OPDs with PEIE, ZnO, SnO 2 , PEIE-Zn, PEIE-Sn as CIL is 9.35×10 16 , 4.99×10 16 , 6.23×10 16 , 5.61×10 16 and 3.95×10 16 cm -3 , respectively The specific detectivity (D*) calculations: The where q, i d , and J d are the electron charge, dark current, and dark current density, respectively.

The linear dynamic range (LDR) calculations:
3 The linear weak-light response range is always characterized by the LDR, defined as an optical power margin within which the output photocurrent is linearly proportional to optical signal input: S8) where L upper and L lower are the upper and lower limits of the light intensity in a particular range.Figure S12b shows that as the film thickness increases, the device response with PEIE-Sn of 5:1 significantly decreases, which is consistent with the previously discussed sensitivity of PEIE to average thickness.In addition, the device with thick PEIE-Sn (51 nm) of 1:3 shows superior performance.
D* of a photodetector is one of the most important figure-of-merits that determines the sensitivity of a photodetector to optical signals, calculated by Equation S6 A, B, I n , and S n are responsivity, active device area, bandwidth, noise current and noise current spectral density, respectively.When the total noise of the device is dominated by the shot noise, D* can be obtained by Equation S7

Figure S8 .
Figure S8.Transmittance spectra of PEIE-Sn films with different M N /M Sn .

Figure S20 .
Figure S20.Dark J−V characteristics before and after illumination of ITO/CIL/Ag devices.(a) Visible light (650 nm).(b) Near-infrared (940 nm).(c) Dark J−V characteristics before (solid lines), after (short dashes) illumination, and leaving the ITO/ZnO or PEIE-Zn/Ag devices alone for a few hours (dotted lines).(d) Absorption spectra of CIL films on quartz.

Figure S22 .
Figure S22.Different positions of ITIC-Th'-were selected as the interaction points with PEIE-Sn or PEIE, and the structures of various systems were optimized by the DFT method and their binding energies were calculated.Molecular structures of (a) ITIC-Th'-, (b) PEIE, (c) PEIE-Sn.The DFT calculation of (d-f) PEIE and (g-i) PEIE-Sn with ITIC-Th'-.

Figure S25 .
Figure S25.Noise spectral density of PS-OPD at different biases.

Table S1 .
Organic element analysis test report of PEIE.

Table S2 .
The fitted parameters of time-resolved photoluminescence (TRPL) spectra (excitation at 405 nm and emission at 774 nm) of PBDB-T:ITIC-Th layer deposited on different CILs.