2.1 Materials and solution preparation

Commercial PBTZT-stat-BDTT was purchased from Raynergy Tek and PC₆₁BM and phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) were obtained from NanoC. Poly [[6,7-difluoro[(2-hexyldecyl)oxy]-5,8-quinoxalinediyl]-2,5-thiophenediyl]] (PTQ10) with M_w of 49 kDa and M_n 17 kDa and poly[(2,6-(4,8-bis(5-(2-ethylhexyl-3-chloro)thiophen-2-yl)-benzo[1,2-b:4,5-b′]dithiophene))-alt-(5,5-(1′,3′-di-2-thienyl-5′,7′-bis(2-ethylhexyl)benzo[1′,2′-c:4′,5′-c′]dithiophene-4,8-dione)] (PM7) with M_w of 98 kDa and M_n 41 kDa were purchased from Brilliant Matters. O-xylene and methylnaphthalene (MN), diphenyl ether (DPE), and chloroform (CF) were obtained from Sigma-Aldrich and used as received. Commercial ETL inks formulations (SnO₂ and ZnO) were purchased from Avantama and used after 10 min horn sonication. Commercial copper(I) thiocyanate solution (CuSCN) was purchased from Sigma-Aldrich. The PC₆₁BM interlayer solution was prepared by dissolving 10 g/L PC₆₁BM powder either in CF or o-xylene. A 10 g/L of PEI (branched with M_w 25 kDa and M_n 10 kDa; Sigma-Aldrich) formulation was prepared in CF, whereas 10 g/L PS (with M_w of 35 kDa; Sigma-Aldrich) formulation was dissolved in o-xylene and MN with a ratio of 85:25 (w/w). Optimized PC₆₁BM:PEI formulation was obtained from 9 vol% of PEI in PC₆₁BM solution. Optimized PC₆₁BM:PS formulation was achieved from 11 vol% of PS in the PC₆₁BM solution. Poly(3,4-ethylenedioxythiophene): polystyrene sulfonate (PEDOT:PSS) (Clevios PH1000) was obtained from Heraeus GmbH. The pristine PH1000 was mixed with ethylene glycol (from Sigma-Aldrich) and Capstone FS-30 surfactant (Dupont) in a volume ratio of 93.5:6:0.5 (PH1000:EG:FS-30) and further diluted in deionized water in a 2:1 ratio (v:v, ink:water). A measure of 125 μm heat-stabilized polyethylene terephthalate (PET) Melinex ST505 (Tekra) was utilized as a substrate.

2.2 Device fabrication and electrical characterization

Fabrication of laminated OPV devices started from the slot-die coating (Solar X3; FOM Technologies) PEDOT:PSS on top of PET foils, followed by baking at 130°C for 15 min. The cathode side of the device was completed by spin-coating 32 nm SnO₂ and annealing at 115°C for 2 min in air. For reference AL/AL laminated devices, ALs were spin-coated on both cathode and anode stacks and were annealed at 80°C for 2 min in the glovebox. PBTZT-stat-BDTT and PC₆₁BM were blended in a 1:1.5 (w:w) ratio in o-xylene:methylnaphthalene (85:15, v:v) at 45 g/L total concentration. PTQ10:PC₆₁BM ink was prepared 1:1.5 (w:w) ratio in o-xylene:DPE (85:15, v:v) at 40 g/L total concentration.³³ PM7:PC₆₁BM ink was prepared 1:1.5 (w:w) ratio in o-xylene:DPE (85:15, v:v) at 45 g/L total concentration. For AL/acceptor-based interlayer laminated devices, PC₆₁BM, PC₆₁BM:PEI, or PC₆₁BM:PS solutions were spin-coated on the cathode stack on top of SnO₂ at 2000 r/min for 30 s and annealed at 80°C for 2 min in the glovebox. ALs were coated on the anode stack. The resulting stacks were prepatterned with a scalpel to isolate three active areas per substrate. The active area is subject to subjective errors of ±0.05 cm². The cathode and anode stacks were laminated in the air using a roll laminator (GSS DH-650S; Graphical Solutions Scandinavia AB) at a roll temperature of 115°C and a force of ∼50 N (measured with a force sensor FlexiForce A201; Tekscan). In the end, the laminated devices were placed between two glass slides to provide mechanical support for easier handling. Additionally, silver paint (Agar AGG302) was applied to the exposed PEDOT:PSS contacts. It is important to note that the devices were not encapsulated.

The current–density–voltage (J–V) curves were obtained by using a Keithley 2400 SourceMeter while the devices were under cool LED irradiation at a temperature ranging from 20°C to 25°C. The LED source's emission spectra and irradiance (Supporting Information: Figure S1) were measured using a high-precision fiber optics spectrometer (QE-Pro; Ocean Optics) and a Hamamatsu silicon photodiode S1133-01. Integrating the corresponding emission spectrum obtained³⁴ from the specific device location, the illuminance, power density, and current density were calculated as 552 lx, 148 µW/cm², and 67 µA/cm², respectively.

2.3 Photoelectron spectroscopy (ultraviolet photoelectron spectroscopy [UPS])

The substrates used for UPS and NR were cleaned via sonication in detergent, followed by sequential washing in deionized water, acetone, and 2-propanol. The films (18–30 nm) of fullerene-based interlayers were spin-coated and annealed in the glovebox according to the previously mentioned fabrication methods. The samples were transferred to the load lock chamber of the ultrahigh vacuum system. The UPS experiments were performed in a home-designed spectrometer. The excitation source was monochromatic He I radiation with a photon energy of 21.22 eV. The W_F was derived from the secondary electron cutoff and the vertical ionization potential from the frontier edge of the occupied density of states with an error margin of 0.05 eV. All the measurements were executed with a base pressure lower than 1 × 10⁻⁹ mbar.

2.4 External quantum efficiency (EQE)

The Solar Cell Spectral Response Measurement System QE-R3011 from Enli Technology Co., Ltd. was employed for EQE. To obtain the EQE_EL (external quantum efficiency—electroluminescence) values, an in-house built system was utilized. This system consisted of a Hamamatsu silicon photodiode 1010B, a Keithley 2400 SourceMeter for voltage supply and recording injected currents, and a Keithley 485 picoammeter to measure the intensity of emitted light.

2.5 Fourier transform photocurrent spectroscopy-EQE (FTPS-EQE)

For FTPS-EQE measurements, a Bruker Optics Vertex 70 spectrometer was employed. The spectrometer was equipped with a quartz tungsten halogen lamp, a quartz beam splitter, and an external detector option. To amplify the photocurrent produced when the photovoltaic devices were illuminated with modulated light from a Fourier transform infrared spectroscopy (FTIR), a low-noise current amplifier (SR570) was used. The output voltage from the current amplifier was then fed back into the external detector port of the FTIR, enabling the FTIR's software to collect the photocurrent spectrum.

2.6 NR

The NR experiments were carried out using the reflectometer MORPHEUS located at SINQ (Paul Scherer Institut). During NR experiments, a neutron beam was directed at the sample surface under a small angle (θ), reflected at each composition interface, and detected. The samples were mounted vertically, and the reflected beam was detected using a He-3 detector. The intensity of the reflected beam was measured as a function of the scattering vector normal to the surface Q_z = 4π(sin θ)/λ, with λ being the neutron wavelength. The wavelength used in the experiments was 4.8 Å. Samples were measured from 0° to 3° in 2θ.

2.7 Atomic force microscopy (AFM)

AFM measurements were performed with a dimension 3100 system using antimony-doped silicon cantilevers in tapping mode NR.

3.1 Elimination of V_OC losses by the utilization of PC₆₁BM:PEI and PC₆₁BM:PS cathodic acceptor-based interlayers

Studies on perovskite solar cells with fullerenes as an interlayer have shown that fullerenes can result in nonradiative recombination losses. It was shown that the critical loss process in high-performing p–i–n-type solar cells is a surface-mediated process that occurs in the first monolayer of fullerenes.³⁵ There have been studies where PCBM interlayers were modified with insulating molecules (e.g., PEI or PS) to achieve high-quality pinhole-free films, resulting in passivation of the trap states at the perovskite surface and a reduction of nonradiative recombination losses.^{31, 32} These studies provide a strategy to mitigate the V_OC losses across the perovskite–interlayer interface.

Inspired by the developments in perovskite solar cells, we have implemented these strategies to investigate the effects of these insulating molecules mixed with PC₆₁BM on the performance of the laminated OPVs. Devices with anode stacks with BHJ AL laminated to cathode stacks with PC₆₁BM:PEI (9 vol% PEI) show an increased V_OC of 0.62 V and FF of 0.68, and interlayer PC₆₁BM:PS (11 vol% PS) display an improved V_OC of 0.60 V and FF of 0.74, compared to 0.49 V V_OC and 0.60 FF for pristine PC₆₁BM-based interlayer (Figure 2D). Modified cathodic PC₆₁BM interlayer-based devices exhibit similar V_OC as BHJ AL/AL laminated devices (Table 1). These results strongly suggest that thermalization loss at the pure phase of PC₆₁BM on the cathode electrode could result in the loss of V_OC. By diluting the cathodic PC₆₁BM with either PEI or PS, we speculate that the thermalization losses are eliminated and conditions for a better energetic alignment are created. To investigate the sensitivity of PS and PEI to PC₆₁BM vol%, three stoichiometries were evaluated, all with a similar increase of V_OC compared to pure PC₆₁BM cathode (Supporting Information: Figures S4 and S5).

To examine the generality of the energy loss across the interface of polymer:PC₆₁BM and interlayer PC₆₁BM, we have investigated two more donor materials, PTQ10 and PM7 with PC₆₁BM as the acceptor. The molecular structures of the materials used in the investigation are shown in Supporting Information: Figure S6A,B. We find that the previously observed voltage loss when PC₆₁BM was used as an interlayer and performance enhancement with PC₆₁BM interlayer is applicable to all the tested donor materials (Supporting Information: Figure S7A,B).

To gain further insights on V_OC loss in these interlayers, we utilized FTPS-EQE and EQE_EL and analyzed the contributions of radiative and nonradiative charge recombination (Supporting Information: Figure S8). Additionally, we determined the energy gap (E_g) by examining the derivative of the EQE edge,^{36, 37} which was found to be 1.81 eV despite the variations in the interlayer. The radiative loss, calculated using the formula $${V}_{\text{OC},\text{rad}}=({k}_{{\rm{B}}}T/e)\phantom{\rule{}{0ex}}\mathrm{ln}(({J}_{\mathrm{ph}}/{J}_{0})+1)$$, remains constant (1.00 eV) despite changes in the interlayer. This suggests that radiative recombination makes negligible contributions to the difference of the V_OC loss. The nonradiative loss, calculated using the formula $${V}_{\text{OC},\text{nonrad}}=({k}_{{\rm{B}}}T/e)\mathrm{ln}({\text{EQE}}_{\text{EL}})$$, yields a value of 0.41 V for the PC₆₁BM interlayer, while PC₆₁BM:PEI and PC₆₁BM:PS interlayer-based devices show a value of 0.36 V, largely suppressing the V_OC loss compared to pristine PC₆₁BM interlayer-based devices. In summary, our findings indicate that V_OC loss in fullerene-based interlayers is not primarily influenced by radiative recombination, but rather by nonradiative recombination losses. Introducing insulating molecules into the PC₆₁BM interlayer effectively mitigates these nonradiative losses at the interfaces between the absorbing layer and cathodic acceptor interlayers.

3.2 Interfacial energy loss dependence on energy levels of the device

To investigate why the mixture of insulating molecules with PC₆₁BM can improve the device V_OC compared to the pristine PC₆₁BM, UPS measurement was performed on fullerene-based interface layers (Supporting Information: Figure S9). It was found that the W_f of the pristine PC₆₁BM varies from 4.40 to 4.45 eV depending on the processing solvent (Figure 3A). The energy diagram of the devices with and without modification of PC₆₁BM interlayer is illustrated in Figure 3B. The addition of PEI into PC₆₁BM lowers the W_f to 4.33 eV, while PC₆₁BM:PS also shows a reduced W_f of 4.27 eV. One would assume that the energy level alignment was intuitively well matched from the pristine PC₆₁BM interface layer to polymer:PC₆₁BM-based AL. However, we observed a significant V_OC loss as shown in Figure 2D. The reduced W_f due to the addition of either PEI or PS to PC₆₁BM resulted in an improved V_OC compared to pristine PC₆₁BM-based devices. For PC₆₁BM:PEI interlayer compared to pristine PC₆₁BM, we see a V_OC difference of 0.13 V and a W_f difference of 0.12 eV, while for PC₆₁BM:PS interlayer compared to pristine PC₆₁BM, the difference in V_OC is 0.11 V and in W_f is 0.13 eV, resulting in a change in W_f and V_OC of similar magnitudes. This indicates that reduced W_f leads to a better energetic alignment across the AL and acceptor-based interlayer, increasing the device V_OC.^{38, 39} Possible mechanisms for the reduced W_f of the insulating molecule incorporated acceptor interlayers are discussed in the following section.

3.3 Interfacial energy loss dependence on the vertical distribution of the acceptor-based interlayers

Since the addition of PEI or PS in PC₆₁BM film helps to reduce the V_OC loss, we are motivated to understand the mechanism behind the device V_OC increase and W_f reduction. Therefore, we move forward to investigate the interaction between PC₆₁BM and PEI or PC₆₁BM and PS, especially considering that microstructure-related aspects like vertical phase separation could be very important. In the field of OPVs, NR has been employed as a technique for exploring the vertical stratification of BHJ ALs.^{40, 41} In this study, we have employed NR as a means of investigating the influence of interlayer's vertical arrangement on the performance of OPV devices. NR was performed to determine component distribution normal to the surface, enabling us to see the influence of PEI or PS on PC₆₁BM. The PC₆₁BM:PEI in CF or PC₆₁BM:PS in o-xylene were deposited on SnO₂/SiO_x/Si substrate to ensure the corresponding cathode interface properties of AL/cathodic PC₆₁BM laminated devices. The experimentally measured reflectivity curves were fitted using GenX (details in Supporting Information: Figure S10) to obtain the neutron scattering length density (SLD) distributions. The corresponding SLD depth profiles derived from fitting are shown in Figure 4A–C. Fitted SLDs of SnO₂, PC₆₁BM from o-xylene, PEI, and PS were found to be 3 × 10⁻⁶ Å⁻², 3.8 × 10⁻⁶ Å⁻², 2 × 10⁻⁶ Å⁻², and 1.2 × 10⁻⁶ Å⁻², respectively. Note that the SLD of the interface between the Si substrate and SiO_x is shown by the abrupt increase of SLD located at 0 Å in the depth profile. On top of the SiO_x, the thickness of the SnO₂ layer ranged from 320 to 450 Å. Since the gradual change in SLD from one layer to the next represents the roughness of the film, the comparison of Figure 4A–C indicates that the roughness of the PC₆₁BM layer at the air interface is decreased upon the addition of PEI or PS, consistent with the results obtained using AFM in Supporting Information: Figure S11.

Upon the addition of PEI to PC₆₁BM, the surface of the film (air interface) is significantly enriched with a layer of ~5 nm PEI with an SLD of 2 × 10⁻⁶ Å⁻², depicting strong surface segregation of PEI from PC₆₁BM (Figure 4B). This is different from pristine PC₆₁BM (Figure 4A), which is composed of one layer with a thickness of 17 nm. We find that PC₆₁BM on top of SnO₂ is characterized by an SLD of 4.1 × 10⁻⁶ Å⁻², implying that the blend has a composition of around 23 nm pure PC₆₁BM. The SLD value of pure phase PC₆₁BM in the PC₆₁BM:PEI blend shows a different value from the SLD of the pristine PC₆₁BM (Figure 4A) due to the processing from different solvents. The results indicate total segregation of PEI from PC₆₁BM. We believe that the phase segregation of PEI on the top surface originates from the surface energy and chemical structure difference. With the NR results on the vertical phase separation, we can now rationalize why the PEI addition enhances the V_OC. The reduced V_OC observed in pure acceptor interlayer devices we tentatively ascribe to the ambipolar nature of PC₆₁BM,^{42, 43} which facilitates the transfer of both electrons and holes. This characteristic increases the likelihood of hole (minority carrier) back-transfer from the BHJ polymer HOMO to the HOMO of the acceptor interlayer, leading to recombination losses in the pure cathodic PC₆₁BM phase. Conversely, the presence of self-arranged PEI between the AL and acceptor interlayer potentially eliminates hole transfer from the AL blend to the pure acceptor interlayer, resulting in an increased V_OC and decreased W_f. The increased PC₆₁BM content has a strong influence on the device V_OC and FF_, which could be translated to longer extraction times and more likelihood of recombination events (Supporting Information: Figure S12).

In the case of PS-added PC₆₁BM, the vertical phase separation is different from PEI-added PC₆₁BM, although we also observed W_f reduction from the UPS measurements. In this case, a gradient film is observed on top of the SnO₂ layer. To quantify the vertical distribution of the components, one can estimate the volume fraction of PC₆₁BM and PS. A mass conversion equation was used to calculate $${\phi }_{\text{PCBM}}$$ and $${\phi }_{\text{PS}}$$: $$\rho (z)=\phantom{\rule{}{0ex}}{\phi }_{\text{PCBM}}(z)\,{\rho }_{\text{PCBM}}+{\phi }_{\text{PS}}(z)\,{\rho }_{\text{PS}},$$ where $${\phi }_{\text{PS}}=1-{\phi }_{\text{PCBM}}$$ is the volume fraction of PS. The PC₆₁BM:PS gradient film exhibits three regions with PC₆₁BM ratios of 53 vol%, 68 vol%, and 40 vol% at the SnO₂ interface, in the middle part, and at the air interface (Figure 4D). Overall, PC₆₁BM:PS mixture at the bottom, PC₆₁BM enrichment in the middle, and PS enrichment at the top surface area interpreted by the NR fitting. Therefore, the PS addition has a different mechanism for improving the device V_OC compared to the self-assembled layer of PC₆₁BM:PEI as stated above. The mixture between PS and PC₆₁BM enhances the structural ordering PC₆₁BM, resulting in a narrower width of the LUMO, and thereby decreasing the thermalization losses. This reduction in the thermalization losses leads to a decrease in the negative pinning energy and a subsequent decrease in the W_f at the contact.

Based on the observed changes in vertical arrangement, W_f, and pinning energy, it is speculated that the electron affinity (EA) of PC₆₁BM interlayers also undergoes modifications when using insulating molecules. This phenomenon can be attributed to two potential reasons. First, the introduction of a PS “wetting layer” at the interface between PC₆₁BM and the cathode may result in a decoupling effect.¹⁰ This implies that the screening of electrons in the PC₆₁BM LUMO by the cathode is reduced, making the interface EA more closely resemble the bulk EA. Consequently, thermalization losses decrease.

The second possibility is that the introduction of PS induces changes in the film morphology, thereby affecting the stacking of PC₆₁BM. This alteration may lead to a sharper edge in the LUMO density of states at the interface. Consequently, the density of states does not extend as far into the energy gap, resulting in reduced thermalization losses. Both scenarios ultimately contribute to an improved energy level matching at the interface of the BHJ AL–cathodic acceptor interlayer, as discussed in our manuscript.

Our findings demonstrate the potential to significantly reduce V_OC loss across the AL and cathodic interlayer interface. This paves the way for the development of materials with improved interface properties and decreased energy losses. These results open new avenues for the design and optimization of indoor OPVs for industrial applications. Further investigations and optimizations are warranted to fully explore the potential of these advancements in enhancing device performance and efficiency.