The organic materials D18, PM6, PYF-T-o, and L8-BO were sourced from Nanjing Zhiyan Technology Co., Ltd. (China). PTzBI-oF and PFN-Br were procured from Dongguan Fu’an Photoelectric Technology Inc., Ltd. (China). High-purity solvents, including chloroform (CF) and 1-chloronaphthalene (1-CN), along with PEDOT:PSS, were obtained from Sigma-Aldrich (USA).

The pre-patterned indium tin oxide (ITO)-coated glass substrates underwent a sequential cleaning process, beginning with detergent washing, followed by ultrasonic treatment in deionized water, acetone, and isopropyl alcohol (20 minutes for each solvent), and concluding with overnight drying in an oven^[36]. The AIL was deposited by spin-coating the precursor solution onto oxygen plasma-treated ITO substrates (5 min treatment) at 3000 rpm for 30 s, followed by thermal annealing at 150 °C for 15 min to achieve optimal film properties^[37]. For the PTzBI-oF:PYF-T-o blend system, the donor-acceptor weight ratio was maintained at 1:1.4 (w/w), with PTzBI-oF concentration fixed at 6 mg mL^-1. The active layer was fabricated by spin-coating the blend solution in CF (containing 0.5 vol% 1-CN as an additive) at 2000 rpm for 30 s, yielding uniform thin films with a thickness of approximately 120 nm. In the PM6:L8-BO photovoltaic system, the donor:acceptor ratio was optimized at 1:1.2 (w/w), with the PM6 concentration maintained at 5 mg mL^-1. The active layer was deposited by spin-coating the blend solution in CF (containing 0.5 vol% 1-CN as processing additive) at 4000 rpm for 30 s, resulting in uniform thin films with a thickness of approximately 100 nm. In the D18:L8-BO photovoltaic system, the donor:acceptor ratio was maintained at 1:1.2 (w/w), with the D18 concentration fixed at 4.3 mg mL^-1. The active layer was fabricated by spin-coating the blend solution in CF (containing 0.5 vol% 1-CN as processing additive) at 3500 rpm for 30 s, yielding uniform thin films with a thickness of approximately 100 nm. Subsequently, the films were subjected to thermal annealing at 100 °C for 5 min to optimize their morphological and electronic properties^[38]. A methanol solution of PFN-Br (0.5 mg mL^-1) was subsequently spin-coated onto the active layer at 2000 rpm for 30 s to form the cathode interfacial layer. Finally, a 100 nm-thick silver electrode was thermally evaporated under high vacuum conditions (< 1 × 10^-4 Pa) to complete the device fabrication^[39].

The hole-only PTzBI-oF:PYF-T-o device with the structure of ITO/(PEDOT:PSS or Ti₃C₂:PEDOT:PSS)/PTzBI-oF:PYF-T-o/MoO₃/Ag was fabricated to determine mobility μ by the formula J = (9/8)ε₀ε_rμV_eff²/d³, in which J, μ, ε₀, ε_r, d and V_eff delegate the current density, zero-field charge mobility at, free-space permittivity, material’s relative permittivity, the thickness of PTzBI-oF:PYF-T-o layer and effective voltage, respectively.

The absorption and photoluminescence (PL) spectra of PTzBI-oF, PYF-T-o, PTzBI-oF:PYF-T-o, PEDOT:PSS and Ti₃C₂:PEDOT:PSS substances were analyzed by utilizing an absorption photoluminescence spectral analysis system (APLS-AS), specifically model APL-AS II. The current density-voltage (J-V) curves of PTzBI-oF:PYF-T-o (1:1.4) devices, both with and without Ti₃C₂ incorporated in the PEDOT:PSS interlayer, were measured using a computer-controlled Keithley 2450 SourceMeter. These measurements were conducted under AM 1.5G irradiation provided by a solar simulator (Sirius-SS150A, 150W AAA grade, manufactured by Zolix Instruments). The external quantum efficiency (EQE) spectra of PTzBI-oF:PYF-T-o (1:1) devices with (or without) Ti₃C₂ in the PEDOT:PSS interlayer were measured with a self-built external quantum measurement system (Zolix Instruments) calibrated with standard silicon cells. Impedance spectrum of PTzBI-oF:PYF-T-o (1:1) devices with (or without) Ti₃C₂ in the PEDOT:PSS interlayer was measured by utilizing an electrochemical workstation (interface 1010). Transient absorption (TA) spectra were obtained by comparing the probe light spectra with and without pump light excitation (an 805 nm seed light with a single pulse energy was used). The transient photocurrent (TPC) and transient photovoltage (TPV) characteristics of devices were measured by the TPC/photovoltage test system (CEL-TPV2000). Atomic force microscopy (AFM) graphics of PEDOT:PSS and Ti₃C₂:PEDOT:PSS substances were captured by utilizing a Bruker Multimode 8 Microscope. Energy dispersive spectrometer (EDS) images were performed using the Czech TESCAN MIRA LMS scanning electron microscope. The transmission electron microscopy (TEM) images of a few-layer Ti₃C₂ nanosheet were performed using the JEM-F200 instrument from Japan Electronics. X-ray diffraction (XRD) of a few-layer Ti₃C₂ nanosheet was performed by using an X-ray diffuser (Bruker, D8 Advance, λ = 0.15418 nm). Grazing incidence wide-angle X-ray scattering (GIWAXS) of sample films was performed by using an Incidence Wide-Angle X-ray diffuser (Xeuss 2.0, λ = 1.54189 Å). X-ray photoelectron spectroscopy (XPS) analysis of PEDOT:PSS and Ti₃C₂:PEDOT:PSS substances was conducted using the Thermo Fischer Nexsa system. The water and oil contact angle (CA) θ of PTzBI-oF, PYF-T-o, PEDOT:PSS and Ti₃C₂:PEDOT:PSS substances were obtained by a Dataphysics OCA40 Micro surface CA analyzer.

The block Ti₃AlC₂ material was etched with hydrofluoric acid produced in situ by hydrochloric acid and lithium fluoride [Figure 1A]. The specific experimental process was as follows: 20 mL 9 mol/L hydrochloric acid was put into a polytetrafluoroethylene beaker, slowly added 1.6 g LiF, and stirred for 5 min. 1 g of 400 mesh Ti₃AlC₂ powder was slowly added to the above mixture and continuously stirred at 36 °C for 24 h. After the etching, 5 mL of liquid was added to 30 mL of deionized water, centrifuged at 3500 r/min for 5 min, and the cleaning was finished when the pH value of the supernatant was 6 to 7. The precipitated substance obtained after centrifugation is freeze-dried, and the powder obtained after freeze-drying is multilayer Ti₃C₂. 500 mg of multilayer Ti₃C₂ was put into 30 mL of deionized water, protected by Ar gas, and sonicated in an ice water bath for 30 min. After sonication, the solution was centrifuged at 3500 r/min for 1 h, and the supernatant was taken as few-layer Ti₃C₂ nanosheets.

A TEM image for a few-layer Ti₃C₂ nanosheet solution is shown in Figure 1B and C, where its surface is relatively flat and smooth, its edge is clear and dry, and there are no obvious holes on the sheet, which proves that this nanosheet obtained by this method has less defects. From the XRD pattern of Ti₃C₂ films [Figure 1D], not only the obvious (002) characteristic peak at 5.9° was observed, but also the (004), (006), (008), (010) and (012) crystal faces were observed in the range of 5° ≤ 2θ ≤ 60°, demonstrating that Ti₃C₂ has been successfully etched and the layers are well separated, forming highly ordered crystal structure.

Ti₃C₂ nanosheets have good electrical conductivity and water solubility, and the addition of Ti₃C₂ nanosheets to PEDOT:PSS can form a uniform solution without precipitation. Supplementary Figure 1 displays the preparation process of the Ti₃C₂:PEDOT:PSS interlayer. The device structure of OPVs with Ti₃C₂:PEDOT:PSS hybrid AIL was shown in Figure 2A, and the chemical structures for PEDOT:PSS, Ti₃C₂, PTzBI-oF, PYF-T-o and PFN-Br were displayed in Figure 2B and C, Supplementary Figure 2, respectively. As shown in XPS of Ti₃C₂:PEDOT:PSS hybrid AIL [Figure 2D], the characteristic S 2p peak around 168 eV corresponded to S element in PEDOT:PSS, and O 1s peak located at about 531 eV was attributed to the O element in PEDOT:PSS. The Ti 2p peak located at about 453 eV in the XPS of Ti₃C₂:PEDOT:PSS hybrid AIL, certificating the few-layer Ti₃C₂ was successfully embedded into the PEDOT:PSS component. As shown in the EDS images [Supplementary Figure 3], Ti element was evenly distributed in PEDOT:PSS interlayer, further indicating that Ti₃C₂ nanosheets exhibit good dispersibility in the PEDOT:PSS interlayer.

Figure 2E illustrates the transmittance variation of PEDOT:PSS films when Ti₃C₂ is incorporated. The luminance of the PEDOT:PSS film increased by 2% in the wavelength range of 300-1000 nm. This enhancement may facilitate the light absorption capacity of the active layer, thereby boosting the photocurrent in OPVs. To confirm our hypothesis, we systematically analyzed the absorption spectra of PTzBI-oF, PYF-T-o, and PTzBI-oF:PYF-T-o membranes that were spin-coated onto PEDOT:PSS films, both with and without the incorporation of Ti₃C₂ nanosheets [Supplementary Figure 4]. As Ti₃C₂ nanosheets added into the PEDOT:PSS interlayer, PTzBI-oF and PYF-T-o films exhibited intensive absorption bands in the range of 450-700 nm and 700-900 nm [Supplementary Figure 4A], respectively. Such strengthened absorption in neat PTzBI-oF (or PYF-T-o) membranes was well retained in PTzBI-oF:PYF-T-o blends [Supplementary Figure 4B], facilitating the photons acquisition in PTzBI-oF:PYF-T-o active layer. Supplementary Figure 5 exhibited the ultraviolet photoelectron spectroscopy (UPS) curves of PEDOT:PSS and Ti₃C₂:PEDOT:PSS interlayer. Interestingly, the measured work function of PEDOT:PSS interlayer increased from 5.00 eV to 5.09 eV with the addition of 2% Ti₃C₂ nanosheet [Supplementary Table 1], which may be related to the charge redistribution caused by Ti₃C₂ dopant induced structural and aggregate state transitions of PEDOT molecules. The energy level variation in PEDOT:PSS induced by Ti₃C₂ mixing and the energy level structure diagram of each functional layer of OPV devices are shown in Figure 2F. The energy level structure of AIL has an important influence on the efficiency of hole extraction, and the more matched the energy level structure, the higher the extraction efficiency^[40]. As 2% Ti₃C₂ nanosheet introduced into PEDOT:PSS, its work function (-5.09 eV) is more matched with the highest occupied molecular orbital (HOMO) energy level of PTzBI-oF (-5.25 eV), which is conducive to improving hole extraction efficiency of the device. Therefore, we believe that the OPVs constructed with a more matched hole extraction layer are expected to improve hole extraction efficiency and inhibit the electron-hole recombination, so as to improve the overall performance of the device.

PL spectroscopy was employed to investigate the influence of Ti₃C₂ nanosheets on charge transfer dynamics in OPV devices. The PL spectra of PTzBI-oF:PYF-T-o films deposited on both pristine PEDOT:PSS and Ti₃C₂-doped PEDOT:PSS AIL were systematically analyzed [Figure 2G and Supplementary Figure 6]. Notably, the PTzBI-oF:PYF-T-o layer spin-coated on the Ti₃C₂:PEDOT:PSS AIL exhibited significantly enhanced PL quenching efficiency for both PTzBI-oF and PYF-T-o components, indicating improved charge transfer characteristics that facilitate more efficient hole and electron extraction in the corresponding OPVs^[41,42]. The Time-resolved photoluminescence (TRPL) measurements of Ti₃C₂:PEDOT:PSS/PTzBI-oF:PYF-T-o bilayer display a longer decay time of 0.21 ns compared to 0.18 ns for PEDOT:PSS/PTzBI-oF:PYF-T-o bilayer, further confirming efficient hole transfer from active layer to Ti₃C₂:PEDOT:PSS interlayer [Figure 2H]. Electrical conductivity represents a crucial parameter for assessing the performance of interfacial layers. The conductivity measurements of both pristine PEDOT:PSS and Ti₃C₂-incorporated PEDOT:PSS interlayers were conducted using a two-point probe configuration with an ITO/AIL/Ag device structure^[43]. The current (I)-voltage (V) characteristics between Ag and ITO strips were measured [Figure 2I]. The electrical conductivity of AIL is derived from Pouillet’s law as follows: V = Id/(σlh), where V is the voltage, I is the current passing through AIL film, d is the distance between two ITO strips, l is the length of ITO strip, h is the thickness of AIL film, and σ is the transverse conductivity of the AIL film^[44]. The electrical conductivity measurements revealed significant enhancement upon Ti₃C₂ incorporation, with values of 3.20 × 10^-3 S m^-1 for pristine PEDOT:PSS and 5.47 × 10^-3 S m^-1 for Ti₃C₂-modified PEDOT:PSS interlayers, respectively [Supplementary Table 2]. To further confirm Ti₃C₂ nanosheets induced the increased conductivity of PEDOT:PSS interlayer, we used the space charge limited current model (SCLC) to calculate the hole mobility (μ_h) of hole-only devices (ITO/AIL/PTzBI-oF:PYF-T-o/MoO₃/Ag) with and without Ti₃C₂ nanosheets in the PEDOT:PSS interlayer [Supplementary Figure 7]. Compared with PTzBI-oF:PYF-T-o devices employing PEDOT:PSS interlayer (1.43 × 10^-3 cm² V^-1 s^-1), PTzBI-oF:PYF-T-o devices employing Ti₃C₂:PEDOT:PSS interlayer displayed dramatically increased μ_h values of 1.58 × 10^-3 cm² V^-1 s^-1, further confirming Ti₃C₂ nanosheets induced the increased conductivity of PEDOT:PSS interlayer that can facilitate fast hole extraction from PTzBI-oF to Ti₃C₂:PEDOT:PSS interlayer [Supplementary Table 3].

To investigate the influence of Ti₃C₂ incorporation on photovoltaic performance, we fabricated OPV devices employing Ti₃C₂-doped PEDOT:PSS as the AIL in a conventional device architecture: ITO/AIL/PTzBI-oF:PYF-T-o/PFN-Br/Ag. The photovoltaic performance for PTzBI-oF:PYF-T-o devices with different Ti₃C₂ doping ratios are shown in Supplementary Figure 8 and Supplementary Table 4, from which it can be seen that 2% volume ratio has the optimal doping effect. Figure 3A and Table 1 show the J-V characteristic curve and corresponding photovoltaic performance of PTzBI-oF:PYF-T-o devices with (or without) optimal (2% volume ratio) Ti₃C₂ in PEDOT:PSS interlayer, respectively. The results of device optimization show that Ti₃C₂ doping PEDOT:PSS can significantly improve the performance in PTzBI-oF:PYF-T-o devices, and these improvements are due to the improvement of short-circuit current density (J_SC) and fill factor (FF). It was found that the PTzBI-oF:PYF-T-o devices based on Ti₃C₂ doped PEDOT:PSS showed an optimal PCE of 16.83%, which was higher than that of devices based on PEDOT:PSS (15.61%). Among them, J_SC and FF of Ti₃C₂-doped PEDOT:PSS devices can reach 25.48±0.49 mA cm^-2 and 76.23±0.82%, respectively. Notably, PTzBI-oF:PYF-T-o devices based on Ti₃C₂ doped PEDOT:PSS exhibited a smaller series resistance (R_S = 10.27 Ω cm^-2) and larger parallel resistance (R_P = 8,745.09 Ω cm^-2) than those of devices based on PEDOT:PSS (R_S = 10.46 Ω cm^-2, R_P = 4,410.98 Ω cm^-2), indicating Ti₃C₂ nanosheets promote the Ohmic contact between the PEDOT:PSS and PTzBI-oF:PYF-T-o layer, thereby boosting the FF of the device. EQE spectra for PTzBI-oF:PYF-T-o devices with (or without) Ti₃C₂ in the PEDOT:PSS interlayer were illustrated in Figure 3B. The PTzBI-oF:PYF-T-o devices based on Ti₃C₂-doped PEDOT:PSS demonstrated a distinctly improved EQE response in the range of 340-850 nm than that of PTzBI-oF:PYF-T-o devices based on PEDOT:PSS, mainly derived from the transmittance enhancement of Ti₃C₂:PEDOT:PSS interlayer. The J_SC values obtained from J-V curves were well consistent with those (J_{SC, EQE}) integrated from EQE spectrum (with a deviation less than 3%). After 160 h of thermal aging at 60 °C [Figure 3C], PTzBI-oF:PYF-T-o devices using Ti₃C₂:PEDOT:PSS as interlayer retained a PCE of 5.1%, while PTzBI-oF:PYF-T-o devices using PEDOT:PSS as interlayer only retained a PCE of 4.1%, implying the Ti₃C₂ nanosheets can improve the thermal stability of PTzBI-oF:PYF-T-o devices.

The exciton dissociation and charge collection efficiency of the device were studied by plotting the relationship between photocurrent (J_ph) and effective applied voltage (V_eff) (J_ph-V_eff). The devices were characterized under different conditions, and the results are shown in Figure 3D. The dissociation efficiency of excitons can be characterized by normalized optical flux density (η_diss = J_ph/J_sat) when OPVs are in the short-circuit state. At low V_eff (< 2V), the J_ph of all OPVs reaches saturation^[45,46]. Quantitative analysis of exciton dissociation efficiency revealed η_diss values of 0.980 and 0.985 for OPVs utilizing pristine PEDOT:PSS and TiO₂-doped PEDOT:PSS interlayers, respectively. The maximum exciton generation rate was calculated through the relationship G_max = J_sat/(qL), where q is the elementary charge constant (1.602 × 10^-19 C) and L represents the measured thickness of the PTzBI-oF:PYF-T-o photoactive layer^[47]. Compared to the reference OPVs with pristine PEDOT:PSS as the AIL, PTzBI-oF:PYF-T-o-based OPVs incorporating Ti₃C₂-doped PEDOT:PSS exhibited significantly enhanced G_max [Supplementary Table 5]. This improvement suggests that Ti₃C₂ modification of the PEDOT:PSS interlayer effectively promotes exciton dissociation in the PTzBI-oF:PYF-T-o active layer, consequently leading to enhanced J_SC and FF in the photovoltaic devices.

To gain a deeper understanding of the reasons for the improved photovoltaic performance, carrier recombination dynamics in these devices can be explored by measuring the functional relationship between open-circuit voltage (V_OC), J_SC and light intensity (P_light). The relationship between the P_light and V_OC can be expressed as: V_OC ∝ nkT/[qln(P_light)], where k is the Boltzmann constant, T is the absolute temperature, and q is the fundamental charge^[48]. By measuring the V_OC of the device under different light intensities, the results shown in Figure 3E were obtained through linear fitting. Analysis of the slops revealed comparable values of n = 1.79 and n = 1.78 for PTzBI-oF:PYF-T-o photovoltaic devices fabricated with conventional PEDOT:PSS and Ti₃C₂-doped PEDOT:PSS interlayers, respectively, suggesting similar recombination characteristics in both device configurations. The light intensity dependence of J_SC conforms to the power law relationship: J_SC ∝ (P_light)^S, where the S value is an exponential factor, and the closer the value is to 1, the charge recombination mechanism of the device tends to be bimolecular recombination mechanism^[49]. By measuring the J_SC of PTzBI-oF:PYF-T-o device under different light intensities, the results shown in Figure 3F were obtained through linear fitting. Analysis of the recombination parameter (S) revealed values of 0.970 and 0.972 for PTzBI-oF:PYF-T-o photovoltaic devices fabricated with pristine PEDOT:PSS and Ti₃C₂-incorporated PEDOT:PSS interlayers, respectively. The proximity of these S-values to unity strongly indicates that bimolecular recombination is the dominant loss mechanism in both device architectures, while trap-assisted recombination processes are effectively suppressed. Among them, the S value of Ti₃C₂:PEDOT:PSS-based devices is closer to 1 compared with PEDOT:PSS-based devices, which helps to enhance J_SC.

In order to describe the effect of Ti₃C₂ dopant on the charge transport characteristics of the device, R_S at the interface of the device was measured by impedance spectrum. The electrochemical impedance characteristics of PTzBI-oF:PYF-T-o photovoltaic devices fabricated with PEDOT:PSS and Ti₃C₂-doped PEDOT:PSS interlayers are presented in the Nyquist plots shown in Figure 3G. The experimental data were fitted using an equivalent circuit model to quantitatively determine the charge transport and recombination parameters in both device configurations. A smaller R_S value indicates that Ti₃C₂:PEDOT:PSS has a smaller interface resistance than PEDOT:PSS, which means that the double-anode interface layer has fewer interface trap locations that can effectively reduce the trap-assisted recombination at the interface, resulting in a reduction of non-radiative energy loss^[50,51]. The dark current behavior of PTzBI-oF:PYF-T-o photovoltaic devices, fabricated with conventional PEDOT:PSS and Ti₃C₂-incorporated PEDOT:PSS AIL, is presented in the dark J-V characteristics shown in Figure 3H. The leakage current density in PTzBI-oF:PYF-T-o device employing Ti₃C₂:PEDOT:PSS as AIL slightly decreased compared with that of PTzBI-oF:PYF-T-o device employing PEDOT:PSS as AIL, indicating that Ti₃C₂ dopant promoted the ohmic contact between the PEDOT:PSS and PTzBI-oF:PYF-T-o layer, and thus boosting the hole extraction of the device.

TPV and TPC were used to probe electron extraction and recombination dynamics [Figure 4A and B]^[52,53]. Fitting TPV signal by utilizing the formula y = Aexp(-x/τ), PTzBI-oF:PYF-T-o devices employing Ti₃C₂:PEDOT:PSS as AIL deliver a longer decay time (τ = 0.61 μs) than that of PTzBI-oF:PYF-T-o devices using PEDOT:PSS as AIL (τ = 0.51 μs), providing a longer carrier lifetime for hole extraction and then collected by cathode. The hole extraction time from TPC was estimated to be 1.45 μs and 1.37 μs for PTzBI-oF:PYF-T-o devices using PEDOT:PSS and Ti₃C₂:PEDOT:PSS interlayers, respectively, indicative of the accelerated hole extraction property of PEDOT:PSS interlayer after inserting Ti₃C₂ nanosheets. TA spectra of PEDOT:PSS/PTzBI-oF:PYF-T-o and Ti₃C₂:PEDOT:PSS/PTzBI-oF:PYF-T-o bilayers were measured to study the hole transfer kinetics at the PEDOT:PSS/PTzBI-oF:PYF-T-o interface. As shown in the two-dimensional color diagram [Figure 4C and D], TA spectra [Figure 4E and F] and trace kinetic of ground state bleaching (GSB) signals [Figure 4G and H] of PEDOT:PSS/PTzBI-oF:PYF-T-o and Ti₃C₂:PEDOT:PSS/PTzBI-oF:PYF-T-o bilayers, the observed bleaching peak at 600-690 nm and 790-850 nm represented GSB for PTzBI-oF and PYF-T-o, respectively. The GSB signal of Ti₃C₂:PEDOT:PSS/PTzBI-oF:PYF-T-o bilayer decayed rapidly, indicating that the incorporation of Ti₃C₂ nanosheets into PEDOT:PSS interlayer can enhance the hole transfer at PEDOT:PSS/PTzBI-oF:PYF-T-o interface.

The surface morphology of PEDOT:PSS interlayer with (or without) Ti₃C₂ was measured by AFM. As shown in Figure 5A and B, the height image of PEDOT:PSS clearly shows many crystal particles clustered together and fine particles are distributed on the polymer chain as Ti₃C₂ doped into PEDOT:PSS. The PEDOT:PSS grains and the PEDOT enrichment domain increase significantly after Ti₃C₂ doping, which also contributes to the improvement of the conductivity after Ti₃C₂ doping. The roughness of PEDOT:PSS is 1.59 nm, which is reduced to 1.47 nm with Ti₃C₂ adulterating into PEDOT:PSS interlayer, mainly due to the enrichment PEDOT on PEDOT:PSS surface. The reduced surface roughness facilitates improved interfacial contact formation with the subsequent active layer while effectively preventing the diffusion of active layer materials into the PEDOT:PSS matrix. This optimized interface morphology contributes to the enhanced J_SC and FF observed in the photovoltaic devices. As shown in Figure 5C and D, the phase image of PEDOT:PSS clearly shows PEDOT was slightly separated from PSS as Ti₃C₂ doped into PEDOT:PSS, which may be a key factor in the increased mobility^[54]. The phase separation of PEDOT and PSS often results in changes in the surface composition of PEDOT:PSS films. Therefore, the degree of phase separation between PEDOT and PSS can be determined by the analysis of the surface composition of PEDOT:PSS films.

The effect of Ti₃C₂ nanosheets on the crystalline packing of PEDOT:PSS was studied using GIWAXS, with the scattering images and averaged I-q curves in the out-of-plane (OOP) directions displayed in Figure 5E-G. The significantly pronounced π-π stacking reflections in the OOP direction for Ti₃C₂:PEDOT:PSS films indicate that Ti₃C₂ nanosheets induce a preferential face-on orientation of PEDOT:PSS interlayer [Supplementary Table 6], which improve the conductivity and facilitate fast hole transport capacity in the PEDOT:PSS interlayer.

Surface wettability characteristics of PTzBI-oF, PYF-T-o, PEDOT:PSS, and Ti₃C₂-doped PEDOT:PSS films were evaluated through CA measurements using both water and ethylene glycol as probe liquids [Figure 5H]. These measurements provide critical insights into the interfacial compatibility between the donor/acceptor materials (PTzBI-oF/PYF-T-o) and the respective AIL (PEDOT:PSS/Ti₃C₂:PEDOT:PSS)^[55]. Surface energy (γ) calculations were performed based on the modified Young’s equation (1 + cosθ)γ_p1 = 2(γ_s^dγ_p1^d₎^1/2 + 2(γ_s^pγ_p1^p₎^1/2, where θ is the measured CA, γ_s and γ_p1 correspond to the total surface energies of the solid sample and testing liquid, respectively^[56]. Quantitative analysis of solubility parameters was performed according to the equation δ = Kγ_s^1⁄2, where δ represents the solubility parameter, K is the proportionality constant (116 × 10³ m^-1⁄2)^[57,58]. This calculation was applied to characterize the solubility properties of PTzBI-oF, PYF-T-o, pristine PEDOT:PSS, and Ti₃C₂-incorporated PEDOT:PSS materials. Detailed CA, δ, and γ parameters of PTzBI-oF, PYF-T-o, PEDOT:PSS, and Ti₃C₂:PEDOT:PSS were listed in Table 2. The Flory-Huggins interaction parameter (χ) that determined by the formula χ_ab = V₀(δ_a-δ_b)²/RT, (where χ_ab, V₀, R, and T represent the interaction parameters between a and b, average molar volumes, gas constant, and Kelvin temperature, respectively)^[59], can be used to describe the interaction between a and b. The mutual force between PTzBI-oF (or PYF-T-o) and PEDOT:PSS significantly changed as Ti₃C₂ doping into PEDOT:PSS interlayer, proving that Ti₃C₂ nanosheets can indeed lead to changes in the surface composition of PEDOT:PSS films.

In order to provide further evidence of the surface composition changes of PEDOT:PSS, the S elements on the surface of PEDOT:PSS and Ti₃C₂:PEDOT:PSS films were analyzed by XPS. As shown in Figure 6A, there are two types of peaks in the S 2p signal. The sulfur atoms in PSS correspond to bands ranging from 166-172 eV, while the sulfur atoms in PEDOT correspond to bands ranging from 162-166 eV. The S 2p peak at 164.5 eV can reflect the crystal configuration of PEDOT. It can be seen from Figure 6A that the S 2p peak moves from 164.5 eV to low-energy 163.7 eV, indicative of the enhanced crystal configuration of PEDOT, which induces part of the resonance structure in the PEDOT polymer chain changing from spiral benzoyl structure to linear quinone structure^[60]. PEDOT molecular chains in spiral benzoyl state structure have stronger interchain interaction than spiral benzoyl state structure, which is conducive to the formation of PEDOT conductive nanocrystals with large grain size and tight packing, and larger conductive nanocrystals help to enhance the charge transfer between molecular chains. In addition, conductive Ti₃C₂ nanosheets connect discontinuous conductive nanocrystals to construct additional carrier transport channels^[61]. Due to the highly conductive Ti₃C₂ nanosheets and interconnected conductive nanonetworks, the conductivity of the composite hole extraction layer greatly improved.

The peak area of PEDOT and PSS can be calculated by integration, and the relative components of PEDOT and PSS on the membrane surface can be represented by the ratio of PEDOT to PSS. The results showed that the PEDOT/PSS ratio of pure PEDOT:PSS films [Figure 6B] was 0.10. This is consistent with the common belief that the surface of PEDOT:PSS films contains more PSS. For Ti₃C₂:PEDOT:PSS films, the PEDOT/PSS ratio is estimated to be 0.14 [Figure 6C and Supplementary Table 7], which indicates that more conductive PEDOT components are enriched on the surface of the films after Ti₃C₂ doping into PEDOT:PSS. The increased conductivity of Ti₃C₂:PEDOT:PSS is well in accordance with the boosting trend of PEDOT/PSS ratio, which further confirms that the improved conductivity of Ti₃C₂:PEDOT:PSS is due to the redistribution of PEDOT and PSS chains. The chain rearrangement is mainly attributed to the fact that the Ti₃C₂ dopant is beneficial to the chain segment movement of PEDOT, which weakens the Coulomb force interaction between PEDOT and PSS.

Based on the above AFM, CA and XPS results, we propose the following mechanism: the size of PEDOT:PSS grains and PEDOT enrichment domains increase significantly after Ti3C2 doping. With the doping of Ti₃C₂, PEDOT changed from light bulb shape thin fiber to thick fiber, which represents the conformational change of PEDOT chain from spiral benzoyl structure with low electrical conductivity to linear quinone structure with strong electrical conductivity, and the nanophase separation of the film was enhanced [Figure 6D]. The grain size and crystallinity of PEDOT:PSS increase with the doping of Ti₃C₂, which is one of the main reasons for the high conductivity of Ti₃C₂:PEDOT:PSS.

To demonstrate the universal applicability of the Ti₃C₂ doping strategy in PEDOT:PSS-based systems, we fabricated OPV devices employing two distinct bulk heterojunction systems: D18:L8-BO and PM6:L8-BO, utilizing both pristine PEDOT:PSS and Ti₃C₂-doped PEDOT:PSS as AIL. Figure 7A-C displayed the molecular structures of D18, L8-BO and PM6, respectively. The J-V curves of D18:L8-BO and PM6:L8-BO devices with (or without) Ti₃C₂ in the PEDOT:PSS interlayer were measured under simulated AM 1.5G illumination [Figure 7D and E], with the homologous photovoltaic performance summarized in Supplementary Table 8. The incorporation of Ti₃C₂ into the PEDOT:PSS interlayer resulted in a significant enhancement of photovoltaic performance, with the D18:L8-BO-based device achieving a maximum PCE of 19.05%, a boosted J_SC of 27.37 mA cm^-2, and improved FF of 76.94%; the maximum PCE of the PM6:L8-BO device is increased to 17.33%, the J_SC is increased from 26.71 mA cm^-2 to 28.31 mA cm^-2, and the FF is increased from 66.48% to 69.23%. The substantial enhancement in PCE observed in both D18:L8-BO and PM6:L8-BO-based OPVs conclusively demonstrates the universal applicability of the Ti₃C₂ doping strategy in PEDOT:PSS AIL. This innovative approach is anticipated to provide valuable guidance for the development of high-performance, universally applicable AIL in OPVs.