Intrinsic light reflection of conjugated films towards high-performance organic solar cells

Domain: chemistry
Published: 2025-12-23
Editorial outcome: major_revision

Abstract
In organic solar cells (OSCs), which represent a quintessential application system for organic conjugated films, the intrinsic light reflection of these films significantly influences the optical performance of the devices through the Fabry–Pérot micro-cavity effect. However, this phenomenon has not been comprehensively investigated. This study demonstrates that the film’s intrinsic reflection arises from the light-matter interaction, which is mainly governed by the polarizability of delocalized electron cloud and the orientation of the conjugated backbone. Furthermore, the multi-level interference within the OSC micro-cavity lead to a dependence of both the reflection properties and the short-circuit current density (J SC) on the active layer thickness. Based on these insights, a strategy for synergistic optimization of device performance through precise tailoring of optical constants and the micro-cavity structure is proposed. By systematically analyzing the intrinsic reflection behavior of organic conjugated films, this study enhances the understanding of the role reflection plays in the performance of photovoltaic devices and provides theoretical support for further optical optimization of organic photovoltaics.

Introduction
Organic optoelectronic devices based on organic semiconductor materials have garnered widespread attention over the past decades due to their advantages of flexibility, solution processability, semi-transparency and low cost1–4, making them a crucial focus in the development of next-generation electronic devices. The rapid advancement of novel organic materials has propelled notable progress in device performance. However, to further exploit the full potential of the devices, fine-tuning the optical properties of organic semiconductor films is also a critical strategy and a variety of approaches have been proposed to enhance the light utilization in such devices. For instance, the Fabry–Perot micro-cavity effect based on multiple thin films5–7 is a particularly effective approach for achieving purer color characteristics and higher light-out-coupling efficiencies in organic optoelectronics, which is primarily attributed to the combination of two-beam or multi-beam interference effects. These effects have been extensively investigated in micro-cavity organic light-emitting diodes and other optoelectronics8–11. Generally, it occurs when the active layer with sub-micrometre thickness is sandwiched between a high-reflection metal electrode and a low-reflection metal electrode.

Organic solar cells (OSCs), as a class of prominent highly photosensitive organic optoelectronic devices, have achieved great development in materials designing and device optimization, boosting the power conversion efficiencies (PCEs) over 20%12–15 and edging closer industrialization expectations. However, according to Shockley–Queisser (S-Q) limit16,17, the PCEs of OSCs still far from the theoretical value. In addition to the widely discussed voltage losses, the photocurrent losses also have a significant impact on the PCEs. For example, in the D18:Z8:L8-BO based ternary OSCs, the maximum theoretical short-circuit current density (J SC, SQ) can reach 31.8 mA cm−2 with a band gap of 1.43 eV14, while the experimental J SC of the device is 27.2 mA cm−2, indicating a 15% loss of the incident photon, which mainly arises from the internal quantum efficiency (IQE) loss, the parasitic absorption loss in the interlayer and the electrodes, and the reflection loss from the devices14,18. Among the various losses, the reflection losses constitute the largest proportion and strongly correlated to the optical micro-cavity effect. Typically, the incident light first passes through the transparent electrode (e.g., ITO) and then penetrates the active layer to reach a thick highly reflective bottom metal electrode (e.g., Ag), forming an optical micro-cavity10. Such a micro-cavity is crucial for regulating the light absorption and reflection, thereby optimizing both the current and PCE of the devices.

Generally, the optical micro-cavity effect essentially arises from the interference behavior of multiple reflections, and the reflection at a planar interface is primarily determined by the difference in refractive index n between the two optical mediums. When the difference is minimal, the reflection is effectively suppressed. Conversely, a substantial refractive index difference could lead to a pronounced reflection19,20. Regarding organic semiconductor films, our previous work, together with other studies, have demonstrated that they exhibit wavelength-dependent refractive indices, resulting in a strong dispersion reflection at the interface formed with the air/metal21–23. Typically, the modulation for such reflection relies on empirical external strategies, such as morphology modulation, anti-reflection layer modulation24–26, etc. However, to our knowledge, there has been a lack of in-depth research on modulating the intrinsic reflection of the organic materials layers for device optimization. Early work by Pettersson et al.27 modeled the photocurrent action spectra in fullerene-based OSCs by incorporating exciton diffusion and interference effects, and established a connection between the optical field distribution and charge generation. However, these models may not be directly applicable to advanced non-fullerene acceptors (NFAs) due to their distinct optical constants (refractive index n and extinction coefficient k). Kerremans et al.20 reported a method for determining optical constants from transmission spectra using the Kramers–Kronig model, yielding promising computational results. Nevertheless, the relationship between the optical constants of NFAs and their molecular/aggregate structures remains unelucidated, which hinders a comprehensive understanding of the intrinsic reflective properties of the thin films, ultimately causing targeted manipulation of the optical micro-cavity to reach a bottleneck.

In this work, we systematically investigated the intrinsic reflection properties of organic semiconductor films. As demonstrated by the grazing incident wide-angle X-ray scattering (GIWAXS) experiments and time-dependent density functional theory (TD-DFT) calculation, the electronic polarization has an impact on the light-matter interaction in the conjugated films, leading to the intrinsically high reflection with pronounced chromatic dispersion simultaneously. Typically, the organic materials with higher reflection generally feature 2D-polarized molecular backbone and face-on molecular packing, and the film reflection can be well tuned by controlling the film thickness. Subsequently, the representative PM6:Y6 based OSCs were employed to reassess the rationality of active layer thickness optimization according to the characteristic reflection property of the PM6:Y6 blend films. The total absorption/reflection of the OSCs with varying active layer thicknesses evolves from stage I to IV: thin-film bottom reflection, interference transition, thick-film interference and thick-film surface reflection. Notably, the interference transition stage achieves the maximum total absorption and the largest J SC. However, with the film thickness increasing, the strong film top reflection subsequently become dominant, leading to a decline in both the total absorption and J SC. Based on the experimental and simulation results, we propose a strategy to synergistically enhance device performance parameters through precise modulation of optical constants and micro-cavity. The refractive index n can be appropriately elevated by increasing the molecular in-plane polarizability and enhancing face-on packing degree, thereby reducing the optimal thickness of the active layers and facilitating balanced V OC, J SC, and FF in devices, ultimately maximizing the PCE. Our study elucidates the origin of strong reflection in organic semiconductors, simultaneously providing a fundamental principle for designing light management in OSC devices.

Results
Origin of the high reflection of 1D- and 2D-polarized molecules
The typical conjugation polymer P3HT and the most efficient conjugation non-fullerene small molecule acceptor Y6 were selected as examples to explore the reflection properties, and the chemical structures are presented in Fig. 1a. A self-developed light reflection measurement platform was employed to collect the light reflection data23,28. The films of Y6 and P3HT were spin-coated onto the glass substrate, and based on the interference of top/bottom reflection light, the reflection intensity was optimized through precise control of film thickness. Additionally, our investigation revealed that both the film thickness and surface roughness significantly influenced the reflection phenomena. During the film-preparation processes, the choice of solvent emerges as a crucial factor. When chlorobenzene (CB) was used for film preparation, its slower volatilization rate led to markedly enhanced crystallinity of Y6 film, generating larger grains that increased diffuse reflection and consequently weakened specular reflection, as demonstrated in Supplementary Fig. 1. In contrast, when chloroform (CF) was applied, the faster solvent evaporation resulted in smoother film, thereby avoiding intense scattering. Therefore, all NFA films in this work were prepared using CF. As shown in the Supplementary Fig. 2, the maximum reflection intensity of Y6 film was achieved at a thickness of ~50 nm, and the experimental curves of film-thickness dependent reflection of Y6 film were well matched with calculated data based on transfer matrix method (TMM)27,29,30. As depicted in Fig. 1b, the reflection peaks of Y6 and P3HT were observed at 903 nm and 615 nm, respectively, with maximum reflection and chromatic dispersion index (R max/R min) of 52.9% and 7.03, 24.2% and 3.53. According to Fresnel effect, when a light beam incident from air (n air=1, k air=0) into a medium with refractive index n and extinction coefficient k, the reflection can be calculated by the following

R= ((n-1)^2+k^2)/((n+1)^2+k^2) (1)

As a result of the positive correlation between n and R, Y6 film exhibits significantly higher reflection intensity compared to P3HT, consistent with trends observed in their corresponding n and k (Fig. 1c). It is believed that the absorption is correlated to light-matter interaction, thus the distinct refraction and reflection properties of different materials should also be attributed to light-matter interactions31–33.

Additionally, the TD-DFT calculations using the Amsterdam Density Functional (ADF) package (ADF 2016.107) were furtherly performed to verify the experimental results, and the Perdew-Burke-Ernzerhof (PBE) exchange-correlations potential and double zeta polarized (DZP) Slater-type basis set were used in all the calculations34,35. The electronic polarizability was calculated by the AORESPONSE module of the ADF program package, and in this calculation, a damping parameter Γ is introduced to characterize the lifetime of the excited state36. The value of Γ was determined by fitting the absorption spectrum of the molecule and doesn’t vary significantly among molecules with similar structures37. Here, a value Γ = 0.004 a.u. was used in all calculations of dynamic polarizability, which was obtained by fitting the absorption spectrum of the Y6 and P3HT molecules. The molecular structures and the spatial coordinates of Y6 and P3HT (14) (degree of polymerization, DP = 14) employed in the TD-DFT calculations are shown in Fig. 1d, and the corresponding calculation results are presented in Fig. 1e. The solid lines represent the real part of the electronic polarizability (Re(α)), while the dashed lines denote its imaginary part (Im(α)). The polarizability α, as calculated by DFT, relates to optical constants via the dielectric constant ε. By neglecting the magnetic response of the material (i.e., assuming permeability μ ≈ 1), the square of the complex refractive index N is given by38

N^2 = 1 + 4πρα = ε (2)

where N = n + ik is the complex refractive index, α is the electric dipole polarizability, and ρ is the atomic density. The real part Re(α) and imaginary part Im(α) of the polarizability correspond to the dispersion (ε 1) and absorption (ε 2) components of the dielectric constant, respectively. From this equation, we observe that the variation of the refractive index real part (n) aligns with Re(α), while the imaginary part (k) is governed by Im(α). Moreover, an increase in Re(α) enhances n, whereas a larger Im(α) intensifies light absorption (higher k).

According to the electronic polarization mechanism, the incident light will resonate with the molecules when the photoelectric field vibrates at the same frequency as the characteristic electronic vibration frequency of the molecules. Besides, different vibrations are related to different frequencies, reflecting in different wavelengths of electronic polarizability peaks. For the electronic polarizability of the P3HT (14) films, the peaks in the real part along X direction are expected to be located at 564 nm with a shoulder peak at 509 nm according to the calculation. However, the experimental reflection peaks of P3HT are observed at 615 nm and 569 nm. The blue-shift of the calculated result is primarily attributed to the interference effects presented among the during experimentation, as well as the low DP of P3HT used in calculations. Notably, the maximum polarizability occurs along the X direction, while the polarizability in Y/Z direction shows no characteristic peaks (alkyl side chain), indicating that P3HT is 1D polarization along its conjugated chain. For the Y6 films, the peaks of the electronic polarizability in the calculated real part are located at 876 nm along X direction and located at 560 nm and 503 nm along the Y direction (different vibration frequency with X direction), demonstrating the 2D-polarization of the Y6 film. Therefore, compare to the P3HT, Y6 exhibits higher reflectivity, refractive index n and extinction coefficient k.

We introduce other 1D-polarized (J52, PBDB-T, PM6, FTAZ) or 2D-polarized (IEICO-4F, IT-4Cl) molecules for statistical analysis, and the corresponding plots of refractive index n and extinction coefficient k are provided in Supplementary Fig. 3. The n values of 1D-polarized polymer are commonly lower than 3, which are obviously lower than those of 2D-polarized small molecules with n values over 3.5. The k values for these materials follow the same trend as the n values as discussed above.

Furthermore, our observations indicate that both the maximum n peak and the maximum R intensity gradually increase while undergoing a red-shift in peak positions (Supplementary Fig. 4), which may be attributed to the polarization characteristics of the materials. As reported by Hou et al.39, enhancing the electron-withdrawing capacity of molecular end groups could increase the electrostatic potential (ESP) of the molecules. The ESP reflects the degree of electron delocalization in the molecular system, where a higher ESP amplifies the radial distribution of delocalized electron clouds, thereby increasing the polarizability, which could not only reduce the band gap and induce a redshift in absorption but also promote charge transport. Consequently, for organic semiconductors, a smaller band gap corresponds to a larger ESP and polarizability, resulting in greater optical constants, which are consistent with the statistical trend observed in our experimental measurements.

Effect of molecular packing and orientation on reflection intensity
For organic conjugated films, the ultra high reflection not only originates from molecular conjugation and film thickness, but also is closely related to molecular packing orientation. As shown in the GIWAXS results in Fig. 2a, the Y6 film exhibits a distinct face-on molecular orientation, with its conjugated plane parallel to the substrate and thus perpendicular to the incident light. Furthermore, the reduced broadening of the scattering signals for the (010), (200), and (300) planes after thermal annealing (TA) treatment generally indicate improved molecular orientation uniformity40,41, and the orientation changes of Y6 molecules can be quantified by analyzing the azimuthal variation of the scattering signals. Figure 2b displays the orientation ratios of the lamellar packing (300) plane for the Y6 film before and after TA treatment, where an azimuthal angle of 90° corresponds to the in-plane direction and 0° to the out-of-plane direction. After TA treatment, the increased ratio at larger azimuthal angles (closer to 90°) and the decreased ratio at smaller azimuthal angles (closer to 0°) demonstrate a higher degree of face-on orientation, revealing enhanced molecular order and crystallinity. As shown in Supplementary Fig. 5, the same phenomenon is observed in the GIWAXS results of other materials before and after TA treatment.

Molecular orientation changes result from rotation along different axes, which in turn lead to variations in light-matter interactions. As illustrated in Fig. 2c, for the Y6 molecular packing configuration after TA treatment, the rotation along the [100] axis induces a “face-on to end-on” orientation transition, while rotation along the [001] axis produces a “face-on to edge-on” transition. For 2D-polarized molecules like Y6, “face-on to end-on” rotation reduces the polarizability projected onto the X-Y plane, thereby weakening light-matter interactions and thus decreasing the reflection intensity. In contrast, for “face-on to edge-on” rotation, the reflection of the molecules is relatively less affected, though still influenced, because the polarizability along the Y-direction is much smaller than that along the X-direction. For 1D-polarized molecules like P3HT, “face-on to end-on” rotation also reduces the polarizability projected onto the X-Y plane, consequently diminishing light-matter interactions. However, due to their 1D-polarization characteristics, “face-on to edge-on” rotation has a negligible effect on light-matter interactions. Besides, regardless of 1D- or 2D-polarization, after TA treatment, most conjugated organic small molecules exhibited an enhanced reflection, and the corresponding reflection curves before and after TA treatment are plotted in Supplementary Fig. 6, where the TA treatment (160°C, and higher annealing temperature will affect the composition and flatness, as shown in Supplementary Fig. 7) helps suppress the molecular rotation. As shown in Fig. 2d, the maximum reflection of Y6 is increased from 46.7% to 52.9% after TA treatment, and the maximum refractive index n is also increased from 4.08 to 4.17 (Supplementary Fig. 8).

In fact, based on their molecular structures, polymer donor materials such as PM6 and PBDB-T possess a certain degree of 2D-polarizability. However, their optical constants are nearly identical in magnitude to those of 1D-polarized molecules like P3HT. This phenomenon can be partly attributed to their weaker 2D-polarization characteristics compared to molecules with extended 2D-conjugation, such as Y6. Additionally, these polymers tend to adopt edge-on/isotropic molecular packing orientations, where the molecular y-axis remains perpendicular to the electric field vector of incident light. Consequently, polarization along the y-direction remains inactive, resulting in optical properties that more closely resemble those of 1D-polarized molecules.

The correlation between molecular packing within the film and the film’s absorption/reflection is illustrated in Fig. 2e. For 2D-polarized small molecules like Y6, the high maximum reflection primarily stems from their highly ordered face-on orientation. As mentioned earlier, edge-on rotation causes a slight reduction in the film’s reflection, while end-on rotation leads to a more pronounced decrease. The S 2D parameter, calculated using the formula S 2D = 2<cos^2γ> - 132, can be employed to quantify the degree of out-of-plane orientation from GIWAXS measurements, where γ represents the angle between the molecular backbone plane and the substrate plane normal. An S 2D value between -1 and 0 indicates a preferred face-on orientation, a value between 0 and 1 indicates a preferred edge-on orientation, and S 2D = 0 represents an isotropic orientation. We selected the lamellar packing scattering signals, specifically the (100) plane or its higher-order diffraction for S 2D calculation for the following reasons: (1) The lamellar packing signals are strong and highly reliable, whereas the scattering signals from the (001) plane, which directly indicates end-on orientation, are generally difficult to observe; (2) Improved lamellar packing uniformity leads to a reduction in isotropic molecules, which then infers to a suppression of the end-on oriented molecules that are the primary factor in reflection degradation, thereby resulting in the enhancement of film reflection40,42. (3) For molecules like Y6 that feature a three-dimensional network-like packing structure, rotation of the lamellar packing plane also implies an increase in the proportion of “end-on” oriented molecules41. We systematically investigated a series of 1D- and 2D-polarized molecules by comparing their S 2D parameters and maximum reflection before and after TA, as shown in Fig. 2f. The TA treatment increases the absolute S 2D value for all molecules, indicating enhanced orientation uniformity, and the suppression of isotropic and end-on molecular rotation, consequently leading to improved maximum reflection. The representative 2D-polarized molecules, including Y6, IT-4Cl, and IEICO-4F, exhibit maximum reflection exceeding 40% with highly face-on orientation characteristics. In contrast, the other molecules shown in Fig. 2f demonstrate reflection below 35%, primarily due to their 1D-polarization optical properties. These results are in strong agreement with refractive index measurements.

Film-thickness-dependent OSC device performances
Inspired by the dependence of reflection on film-thickness and wavelength, we further investigate the correlation between film-thickness-dependent reflection and device absorption in an optical micro-cavity utilizing the device architecture of ITO/PEDOT:PSS/PM6:Y6/PNDIT-F3N/Ag (Fig. 3a). To this end, a series of PM6:Y6 blend films with thicknesses ranging from 20 nm to 200 nm were fabricated using chloroform as the solvent and subsequently subjected to TA treatment at 100°C for 5 mins, and the photovoltaic performances with the thickness of 53 nm, 75 nm, 135 nm and 212 nm are presented in Fig. 3b and Table 1. Interestingly, the J SC of the four devices show an undulation behavior. The device with a 53 nm thickness of the active layer shows the lowest J SC (24.99 mA cm−2) due to its unsaturated light absorption, which is understandable and generally admissive. However, the device with a 135 nm thickness unexpectedly exhibits a lower J SC (25.43 mA cm−2) than those of 75 nm thickness (27.16 mA cm−2) and 212 nm thickness (26.77 mA cm−2) based devices simultaneously. Additionally, the integrated J SC calculated from the external quantum efficiency (EQE) curves are summarized in Table 1 and plotted in Fig. 3c, d, which can well support this phenomenon.

As observed, when the active layer thickness is too thin, the insufficient light absorption will result in a lower J SC. Conversely, when the thickness exceeds 200 nm, the J SC typically decrease as well due to the diminished charge dissociation and poorer charge collection properties43,44. Generally, as the active layer thickness increasing, the J SC is expected to rise initially before decreasing, and the optimized thickness anticipated to be in the range of 80–120 nm. Furthermore, the photocurrent density (J ph) versus effective voltage (V eff) plots were measured to explore the charge dynamics in devices with various thickness of the PM6:Y6 blend films45. J ph is calculated as J ph = J L - J D, where J L and J D represent the photocurrent densities under light and dark conditions, respectively. V eff = V 0 - V, where V 0 is the voltage at which J ph = 0 and V is the external applied voltage. The probabilities of the charge dissociation (P diss) and charge collection (P coll) were calculated as the ratios of J ph to the saturated photocurrent density (J sat) under short-circuit and maximum output conditions, respectively. As shown in Fig. 3e, for the devices with thickness of 53 nm, 75 nm, 135 nm and 212 nm, the P diss values are 95.6%, 95.8%, 93.4% and 93.4%, respectively, indicating efficient charge dissociation in the four devices. In other words, the charge dissociation is not significantly affected by active layer thickness. However, the P coll values are 86.7%, 87.0%, 83.5% and 80.1% for the respective devices, demonstrating a strong dependence of charge collection on the active layer thickness. Notably, carrier recombination and transport significantly influence the FF, reflected in P coll and current density at maximum power (J Pmax). As the thickness increasing from 75 nm to 135 nm, the P coll decreases slightly, and further thickening to 212 nm causes a pronounced reduction in P coll, indicating the recombination exacerbated with the increased film thickness, thereby degrading FF. The variations of J SC, J Pmax, and J sat with active layer thickness are extracted from J ph-V eff curves and plotted in Fig. 3f. The similar trends of J sat and J SC provide compelling evidence that the fluctuations of experimental J SC are not attributed to charge dissociation (IQE or light utilization efficiency) but rather arise from the difference light absorption and reflection.

Charge transfer and recombination kinetics and film-depth-dependent probing
To further estimate the effect of charge transfer on carrier collection, the well-recognized ultrafast spectral transfer kinetic characterization, femto-second transient absorption (fs-TA) was subsequently conducted to gain deeper insights46. As shown in the contour maps of Fig. 4a, the negative signals at ~630 nm and ~807 nm correspond to the ground state bleaching (GSB) signals of PM6 and Y6, respectively, and the positive signal at ~675 nm is assigned to the excited state absorption (ESA)47. The 75 nm-thick film presents the highest ΔA signal intensity, indicating the strong absorption of the ground state PM6 and Y6, while the 135 nm-thick film shows the lowest ΔA signal intensity. Besides, the variation in absorption intensity at ~750 nm can be attributed to fluctuations in film-thickness dependent reflection, resulting in the absorption enhancement or deterioration. However, no significant differences in exciton lifetime are evident in the contour maps. Therefore, a fitting of exciton lifetimes was subsequently performed to elucidate the ultrafast kinetics of TA spectra, as illustrated in Fig. 4b and Supplementary Fig. 9. According to the triexponential function, three lifetimes (τ) can be derived for each decay, where τ 1 represents the ultrafast exciton dissociation at the PM6/Y6 interface, τ 2 is related to the exciton diffusion time toward the interfaces before dissociation, and τ 3 reflects the reflection of the dynamics of free charges48. The similar τ 1 values for films of varying thickness at both 630 and 807 nm corroborate the similar ultrafast hole/electron charge transfer at the interfaces, except that of 53 nm films at 630 nm, which is slightly shorter than others, indicating the more efficient hole transfer of 53 nm films. Comparing to the other film, the prolonged τ 2 for the 75 nm film at both 630 and 807 nm can be attributed to the extended exciton diffusion length, which is beneficial for suppressing charge recombination and enhancing the J SC and FF49. However, the strong built-in voltage under short-circuits conditions (the sufficiently large V eff) is capable to weaken the effect of exciton diffusion length50,51, while the FF corresponds to the maximum output power conditions is more sensitive to the exciton diffusion distance. In addition, τ 3 values for all the films of different thicknesses fall within the same order of magnitude, suggesting minimal impact on device current density, especially under short-circuits conditions.

Moreover, the transient photovoltage (TPV) and transient photocurrent (TPC) are subsequently measured to further illustrate the charge recombination kinetics52,53, and the corresponding lifetime simulation results are provided in Fig. 4c, d. Note that device with 212 nm active layer thickness shows shorter photovoltage decay lifetime (2.47 μs) and longer photocurrent decay lifetime (1.26 μs) than those of other devices with thinner active layer thicknesses, suggesting the enhanced recombination in the 212 nm thickness-based device, thus resulting in the reduced FF values. On the other hand, the device with the thinnest film thickness of 53 nm shows the longest photovoltage decay lifetime (9.67 μs) and relatively short photocurrent decay lifetime (0.65 μs), indicating that the weakest charge recombination in 53 nm thick devices50. The shorter photocurrent lifetime (0.48 μs) observed in the 75 nm film compared to the 135 nm film (0.78 μs) arises from reduced carrier drift distance due to the thinner active layer, enabling faster charge collection. Moreover, the carrier mobility is another possible affecting factor on experimental J SC, therefore the photo-induced charge carrier extraction in linearly increasing voltage (photo-CELIV) measurement was afterwards performed54,55. As shown in Supplementary Fig. 10, the calculated carrier mobilities of the devices with 53 nm, 75 nm, 135 nm and 212 nm active layer thickness are 2.29×10−5 cm−2 V−1 s−1, 6.12×10−5 cm−2 V−1 s−1, 1.59×10−4 cm−2 V−1 s−1 and 1.44×10-4 cm−2 V−1 s−1, respectively. Therefore, the slightly prolonged photocurrent lifetime in the 53 nm film primarily stems from a notable decline in carrier mobility.

However, under short-circuit conditions, the built-in electric field is sufficiently strong to substantially suppress carrier recombination. When recombination’s impact on J SC is negligible, minor variations in carrier mobility exert limited influence on collection efficiency. The dominant factor affecting J SC remains the P diss, which is ranging between 93% and 96%, leading to minimal influence on the observed trends in J SC and J sat. In summary, the subtle differences in carrier dynamics minimally impact J SC, as confirmed by the experimentally observed thickness-dependent trends of J SC and J sat. This further underscores that the variation in J SC predominantly originates from optical absorption differences, given the comparable P diss across thicknesses.

Additionally, we observed that the series resistance (R s) increases linearly with film thickness beyond 75 nm under dark conditions, indicating that contact resistance is insensitive to film thickness and morphology (0.41±0.08 Ω·cm²)56, as shown in Fig. S11 and Table S1. Concurrently, a monotonic increase in shunt resistance (R sh) with thickness was observed, demonstrating that thicker films effectively suppress pinhole effects, thereby reducing device leakage current. Generally, Higher R sh and lower R s could both lead to a reduction in J SC, and these resistive characteristics confirm that the 75 nm-thick PM6:Y6 device achieves the optimal balance of R s and R sh, which aligns with its highest PCE. Critically, interfacial contact is not the primary cause of the reduced J SC in the 135-nm device.

Furthermore, vertical phase separation may also affect the J SC of the device by adjusting the distribution of the optical electric field. To isolate this effect, we investigated the influence of vertical phase separation on the J SC of the PM6:Y6 device using the film-depth-dependent light absorption spectroscopy (FLAS) technology57,58. The calculated vertical component distribution was plotted in Supplementary Fig. 12. The distribution of 212 nm films is the most uniform while the others present analogous n-i-p configuration, which is more favorable for conventional devices. The contour maps of film-depth-dependent exciton generation rates (G)59 are drawn in Fig. 4e, and the corresponding distributions of photoelectric filed (|E|2)60,61 of different devices are provided in Supplementary Fig. 13, which is simulated via TMM from FLAS results. The exciton generation rates of 53 nm and 75 nm thickness films reach their maximum at the middle of the films, which possess the relatively short carrier diffusion distance and thus are beneficial to carrier collection62, while those of 135 nm and 212 nm films are concentrated at the active layer/ITO interfaces with enlarged carrier diffusion distance. This discrepancy is mainly due to the multi-interferences between reflections at the interfaces of different optical mediums. Based on the exciton generation rates calculation, the integrated total G of the devices is plotted in Supplementary Fig. 14, along with the simulated J SC values for devices with different active layer thickness. As illustrated, the integrated G is identical to the simulated J SC, corroborating the trends observed in the experimental J SC variation with active layer thickness.

Principle of film-thickness dependent reflection/absorption in cell micro-cavity
Both J ph-V eff results and ultrafast transfer/recombination kinetics characterization are insufficient to explain the observed fluctuations in J SC related to film-thickness. Additionally, the surface roughness of PM6:Y6 thin films exhibits minimal variation (at least two orders of magnitude smaller than the wavelength) across different active layer thicknesses and is also not the primary factor contributing to fluctuations in J SC (Supplementary Fig. 15). However, the simulations based on FLAS measurement reveal trends similar to those of the experimental J SC. Consequently, we believe that the variation in total reflection, resulting from the micro-cavity effect, is the primary factor influencing the J SC-thickness dependence. As demonstrated in Fig. 5a, the incident light experiences multi-level reflection, resulting in the interference device total reflection. Generally, the reflection intensity is mainly corelated with the difference in refractive index n between the two optical medium (specular reflection)63, with a larger difference typically leading to stronger reflection. Consequently, due to the significant differences in the complex refractive index (n, k values) of the active layer compared to the Ag electrode, ITO electrode, HTL, and ETL, as provided in Supplementary Fig. 16, the strongest reflection should occur at either HTL/active layer interface (R 1, top reflection) or ETL/Ag bottom interface (R 2, bottom reflection), and in this case, reflections at other layer interfaces are hidden in order to a comprehensive illustration. In addition, the R 2 will sequentially pass through the whole device again, contributing to the transmitted light (T 2)22,23. Given the high reflection property of the thick Ag electrode, the total transmission intensity is close to zero, implying that the total absorption of the device can be obtained by subtracting the total reflection from the incident light (I in - R total). Therefore, the first transmitted light intensity (T 1) can be considered equivalent to the bottom reflection intensity (R 2). Thus, the competition and interference between R 1 and T 2 are the critical factors that determine the total reflection of OSCs devices and thus the absorption and J SC.

Based on experimentally measured refractive indices of the different materials, the total reflection R total of OSCs devices with different active layer thicknesses was simulated using TMM, and the thicknesses of ITO, PEDOT:PSS, PNDIT-F3N, and Ag electrode layers are strictly kept consistent with the experimental condition in the simulation. As depicted in Fig. 5b, c, the R total significantly reduced as the active layer increases from 20 nm, suggesting that more incident light is absorbed. When the active layer thickness reaches ~75 nm, the strong interference effects become apparent, leading to the wavelength-dependent reflection fluctuation, and thus the R total intensity is hard to be distinguished. Upon refining the evolution of R total with active layer thickness, it can be categorized into four distinct stages (Fig. 5d):

Stage I: Thin-film (<70 nm) bottom reflection (Ag/active layer interface). During this stage, although the bottom reflection makes the incident light pass through the whole film again, the light absorption is still not close to saturation. The strong bottom reflection induced secondary transmission (T 2) with a shape of reversed film absorption caused by unsaturated light absorption is significantly larger than the active layer top reflection (T 2 >> R 1). In this case, T 2 plays a dominating role on the interference with T 2 and R 1, and consequently the R total at this stage will continuously become weaker, resulting in the increased device total absorption (A total) as well as J SC of the device.

Stage II: Interference-transition (70–90 nm). As active layer thickness further increasing, the A total gradually approaches saturation. Consequently, the interference between T 2 and R 1 enters a transition step, therefore, the R total exhibits obvious interference characteristics. Since the optical path difference (OPD) between R 1 and T 2 at this circumstance is approximately an odd-multiple of half-wavelength (at PM6:Y6 absorption peak), namely the phase difference also becomes an odd-multiple of π, resulting in the destructive interference between R 1 and T 2. Due to the similar intensity of both beams (R 1 ≈ T 2) and the destructive interference, the R total intensity at this film thickness range is minimized, and the A total and J SC approach their maximum values.

Stage III: Thick-film interference (90–150 nm). As the OPD of R 1 and T 2 continues to increase, closing to the half-wavelength integer multiple at the absorption peak, the two light beams will produce constructive interference, leading to a gradual increase in R total. Additionally, as the thickness of the active layer increases, the intensity of T 2 is further reduced (R 1 >> T 2), which increases the weight of R 1 in the interference process, and results in an increased R total.

Stage IV: Thick-film (>150 nm) surface reflection (ITO/active layer interface). The thick active layer is able to absorb all the incident light (I in – R 1), and the secondary transmission light T 2 can be considered as zero (T 2 ≈ 0), leading to the surface dominating reflection. In this case, after experiencing a small period of constructive interference, the R total approaches a fixed value, which is equal to the active layer surface reflection.

The experimental evolution of the total light loss factor with variation active layer thicknesses is provided in Fig. 5e and Supplementary Fig. 17, alongside the calculated results, with the corresponding four evolution stages emphasized. At an optimized active layer thickness of ~75 nm, the total light loss reaches ~8.7%. According to Zhu’s observation14, a state-of-the-art device fabricated by the most advanced photovoltaic materials with ~20% PCEs still experiences ~15% light loss. Our experimental and theorical findings further demonstrate that more than half of the light loss is attributed to the reflection loss, which is as a result of intrinsic chromatic dispersed light reflection property of organic conjugated films.

In addition to conventional devices, we also conducted identical studies on inverted devices with different transport layers and electrodes, as well as flexible devices with different substrates, using the PM6:Y6 system as an example, in order to verify the effect of conjugated film intrinsic light reflection on device performance. The device performance test results are shown in Supplementary Fig. S18 and Supplementary Table 3. It can be observed that both inverted and flexible devices exhibit a similar trend of J SC variation with thicknesses as conventional devices (Fig. 5), indicating that minor differences in the optical constants of the transport layers, electrodes, and substrates do not significantly affect the optical performance of the devices (Supplementary Fig. S19). This conclusion is further supported by the simulated light reflection results of PM6:Y6 inverted devices with different thicknesses (Supplementary Fig. 20).

Indeed, this optical micro-cavity effect, which depends on the reflection properties of the active layer, also holds instructive significance for organic photodetectors. The responsivity of a photodetector to light of different wavelengths reflects its applicability under irradiation at various wavelengths and can likewise be modulated by adjusting the active layer thicknesses. By converting the EQE curves of PM6:Y6-based OSC devices, the photoresponsivity (R) curves of the organic photodetectors can be obtained64,65. As shown in Supplementary Fig. 21, these curves exhibit a fluctuation trend highly similar to that of the device’s reflection light (Supplementary Fig. 17). This not only confirm the decisive role of reflection light in device absorption but also offers a strategy for wavelength-selective optimization.

Correlation between film-thickness dependent reflection and device absorption
In order to simulate the device absorption under real conditions, we carefully calculate the integrated A total under AM1.5G illumination (Abs AM1.5G). Since the total transmission is close to zero, the A total can be ascribed as 1-R total, so the Abs AM1.5G can be calculated using the following

Abs AM1.5G = ∫ 893nm 382nm A total × AM1.5G dλ (3)

where the A total is simulated via TMM, and the AM1.5G is the standard reference solar radiation spectrum. The reason for choosing this wavelength range of 382–893 nm is that PM6:Y6 film only absorbs in this wavelength range, and the apparent absorption greater than this wavelength range is actually interference from the reflection light of Y6. The active layer thickness-dependent Abs AM1.5G calculation results and the experimental J SC results (J-Exp and J-EQE) are normalized and plotted in Fig. 6a, with corresponding values lists in Supplementary Table 2, and the experimental total reflection of devices with different active layer thicknesses are provided in Supplementary Fig. 18. All the curves in Fig. 6a experience a fluctuating process and reach a maximum in the range of 70–90 nm, which aligns with our R total calculations. Their high coincidence among these results means that our simulation model is reasonable and reliable.

To further prove the universality of this micro-cavity effect, two more series of devices were prepared using other state-of-the-art active layer materials D18:N366,67 and PM6:L8-BO68,69, and the corresponding J-V curves are provided in Supplementary Fig. 17. As shown in Fig. 6b, the J Exp SC and J EQE SC were extracted from J-V measurements, and the solid lines are our simulation results of Abs AM1.5G based on TMM. Notably, The J SC depressions also appeared and located at ~160 nm and ~170 nm for D18:N3 and PM6:L8-BO, respectively.

Although the configurations of OSCs devices is composed by packing multiple functional layers, the optical model of micro-cavity can be simplified as the equal thickness interference due to the large refractive indices difference between organic conjugated films and other interface layers (ITO, ETL and HTL). According to the equal thickness interference, the following equation can be used to predict the interference behaviors (when the incident angle equals to 0°)

2nd = mλ (m=1, 2, ...) Contructive
2nd = (m+0.5)λ (m=1, 2, ...) Destructive (4)

where λ is wavelength, n is the refractive index, d is the film thickness, m is an integer variable. Using this relationship, the thickness corresponding to the destructive interference of different binary blends with diverse n values at difference absorption peaks can be calculated as equal to λ/4n, as shown in Fig. 6c.

The n values of PM6:L8-BO and D18:N3 blend films are measured by ellipsometer and provided in Fig. 6d, and the calculated corresponding thicknesses to the destructive interference are plotted in Fig. 6e. As we can see, for the two systems, the refractive index n of the acceptor absorption region is similar (~800 nm), with a maximum of around 3; however, the refractive index n of the donor absorption region varies greatly. The refractive peaks of PM6 and D18 are around 638 nm and 598 nm with the maximum refractive indices of 2.03 and 2.49, respectively.

Experimental results from PM6:Y6, PM6:L8-BO, and D18:N3 systems, combined with calculations based on the equal-thickness interference model, indicate that an increase in the n values lead to a reduction in both the optimal thickness (d opt) for J SC and the thickness threshold at which J SC begins to decline. However, the impact of non absolute absorption intensity remains unclear. To address this issue, we simulated the Abs AM1.5G of PM6:Y6 by independently varying either n or k, as shown in Fig. 6f, g. The results demonstrate that the k has negligible influence on d opt for J SC. However, deviations in k (either higher or lower) may reduce the maximum absorption intensity, which is likely because reflectance is jointly determined by n and k. In contrast, n plays a more dominant role: increasing n monotonically decreases d opt for J SC, which may facilitate synergistic optimization of J SC, FF, and V OC. Interestingly, reducing n not only increases d opt for J SC but also diminishes the maximum absorption intensity. Based on these experimental and simulation findings, we believe that a moderate increasing in n can enhance the J SC and simultaneously minifying the optimal thicknesses for J SC.

Discussion on optimizing refractive index to enhance optical device performance
The microcavity effect enables the J SC to attain an optimal thickness d opt. However, the optimal thicknesses for FF and V OC generally reside at lower values. For instance, in the PM6:Y6 system mentioned above, optimal thickness for FF is ~45 nm, for V OC is ~35 nm, and for J SC is ~75 nm (Fig. 7a). However, this thickness mismatch between electrical parameters (FF/V OC) and optical parameter (J SC) fundamentally limits device efficiency optimization.

For this dilemma situation, based on our measurements of optical constants for the PM6:Y6, PM6:L8-BO, and D18:N3 systems, combined with thickness-dependent J SC and TMM-based simulations, we conclude that larger optical constants correlate with a reduced d opt, which approaches the optimal thickness for maximizing FF and V OC. Building on these findings, we propose the following two material design strategies to reconcile this thickness divergence, as illustrated in Fig. 7b:

(1) Increased in-plane molecular polarizability: Strengthening electron-withdrawing capabilities at molecular terminals increases electrostatic potential, which promotes charge dissociation/transport by delocalizing electron clouds. This molecular engineering approach could shift the optimal thicknesses of FF/V OC toward higher values.

(2) Enhanced face-on orientation: Preferential face-on packing aligns molecular polarization with the optical electric field vector. This configuration simultaneously improves charge transport and reduces the optimal thickness of J SC through enhancing light-matter interaction.

It is important to clarify that these two strategies are not competitive but complementary. Enhancing the in-plane polarizability of materials (for instance, through fluorinated end groups or asymmetric molecular structures) simultaneously promotes charge generation/transport and improves face-on molecular stacking70,71. This dual effect optimizes the thickness corresponding to peak performance in J SC/FF/V OC, thereby collectively elevating the device’s PCE. This paradigm provides critical guidelines for next-generation OPV material development.

Methods
Materials
Unless stated otherwise, materials and solvents were commercially obtained. PM6, Y6, D18, N3, L8-BO, P3HT and other conjugated organic small molecules and polymers were purchased from Solmer Materials Inc. All materials were used as received without further purification.

Reflection measurement
1.5 cm × 1.5 cm size quartz was pre-cleaned with ultrasonic in deionized water, acetone and isopropyl alcohol for three times, 10 min each, then dried with Argon blow. The active layer was spin-coated in air condition from a chloroform (CF) or o-dichlorobenzene (o-DCB) solution of Y6(P3HT) which was prepared with the concentration of 10 mg/ml. The thickness of the Y6(P3HT) active layer was optimized as 40 nm (60 nm). The thermal annealing step was carried out in air, and the annealing temperatures of Y6 and P3HT were 160°C and 200°C, respectively. The reflection measurement was performed by a self-built setup which was composed with a Y-shape optical fiber and a grating spectrometer. Finally, the reflection results were revised by a standard Aluminum mirror. To accelerate the speed of the spectrometer, a fast CCD detector is used, which guaranteed that the optical reflection at each wavelength was simultaneously recorded. For each spectrum, the capture time could be tuned towards less than a few milliseconds.

Refraction measurement
Spectral data below 1.7 µm were measured using an RC2 ellipsometer to obtain the ellipsometric parameters Ψ and Δ. The measurements were performed at angles ranging from 45° to 60° in 5° increments. The dielectric function was extracted by fitting the ellipsometric parameters using a B-Spline oscillator model. For wavelengths above 1.7 µm, an IR-VASE Mark II ellipsometer was used, with measurements taken at 60° and 70° at a resolution of 8 cm−1. The dielectric function was obtained by fitting the ellipsometric parameters (Ψ and Δ) using three Lorentz oscillators. The sample preparation for the ellipsometer measurement are the same as for reflection measurement.

Device preparation
The conventional devices were fabricated with a structure of glass/ITO/PEDOT:PSS/active layers/PNDIT-F3N/Ag. The ITO-coated glass substrates were cleaned by ultrasonic treatment in detergent, deionized water, acetone, and isopropyl alcohol under ultrasonication for 15 min each and subsequently dried by a nitrogen blow. A thin layer (~20 nm) of PEDOT:PSS (Clevios P VP AI 4083, filtered at 0.45 μm) was spin-coated at 4000 rpm onto ITO surface. After baked at 150°C for 20 min, the substrates were transferred into a nitrogen-filled glovebox. Subsequently, the blend solution of PM6:Y6 (1:1.2 w/w, from CF with 0.5% CN), PM6:L8-BO (1:1.2 w/w, from CF with 0.25% DIO) or D18:N3 (1:1 w/w, from CF) was spin-coated to form the active layers with different thicknesses. Then the substrates were TA treatment with 100°C for 5 min. After that, The ETL solution PNDIT-F3N was spin-coated on the top of active layers at 3000 rpm. Finally, 120 nm Ag layer were deposited under high vacuum (<1.5×10−4 Pa). The effective areas of cells were ~4 mm2 defined by shallow masks.

The inverted devices were fabricated with a structure of glass/ITO/ZnO/active layer/MoO 3/Ag. A ZnO sol-gel solution was deposited onto pre-treated ITO-coated glass substrates via spin-coating, followed by annealing on a hot plate at 200°C for 30 min to form an ~30 nm-thick electron transport layer. Subsequently, a CF solution of the prepared PM6:Y6 blend active layer was spin-coated onto the ZnO layer. By adjusting the concentration and spin speed, thin films with different thicknesses were fabricated, which were then annealed on a hotplate at 100°C for 5 min. The sample was then transferred into a vacuum deposition chamber, where ~8 nm of MoO 3 and 120 nm of Ag metal electrodes were thermally evaporated sequentially under high vacuum conditions (<1.5×10−4 Pa). The effective areas of cells were ~4 mm2 defined by shallow masks.

The fabrication process for flexible PM6:Y6 devices was consistent with that of conventional devices, except that the substrate was replaced with ITO-coated PET substrates.

Device characterization
The J–V curves of all devices were recorded by a Keithley 2400 source meter and a simulated AM 1.5G (100 mW/cm−2) solar spectrum generating from LSS-55 solar simulator (50×50 mm2 spot size) of Jinzhu Technology CO., LTD.

The EQE spectral were recorded utilizing spectral response measurement system LST-QE of Jinzhu Technology CO., LTD.

Ultrafast recombination characterization
Femtosecond transient absorption (fs-TA) spectroscopy (Coherent Legend) was conducted with a wavelength of 800 nm and a laser of 2840 mW (80 MHz, 120 fs) emitted from a seed source through a titanium sapphire amplifier. The laser wavelength is still 800 nm, the pulse is 1000 Hz, the time interval is 120 fs. 30% of the beam is used as the probe enters the transient absorption spectrometer, and the remaining 70% is converted to 400 nm by BBO crystal, which is pumped into the sample as excitation light (400 nm, 1000 Hz, 100 mW). The test section is the visible light region (450–800 nm).

Transient photovoltage (TPV)/transient photocurrent (TPC)
The TPV and TPC data were acquired with the all-in-one characterization platform Paios (Fluxim AG, Switzerland). In the TPV measurement, the light intensity is in the range of 0.10% to 80.0%, the relative intensity is 20.0%, the stabilization time is 30.0 ms, and the pulse length is 5.0 ms. The TPC of the device was tested under short-circuit conditions without bias light.

GIWAXS measurement
GIWAXS data were collected at beamline BL14B1 of the Shanghai Synchrotron Radiation Facility (SSRF). The wavelength was 1.2398 Å and the incident angle was 0.12o.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Shen and coworkers systematically investigate the intrinsic light reflection characteristics of organic conjugated thin films and their impact on the performance of organic photovoltaic devices. Through GIWAXS experiments and TD-DFT theoretical calculations, they found that the electronic polarizability and molecular stacking orientation of organic semiconductor materials are the key factors determining their high reflectivity (up to 50%). Further researches show that by regulating film thickness and molecular orientation, microcavity effects can be optimized, thereby improving the performance of OSCs. Through analyzing the four stages of the effect of film thickness on the reflection situation, the author clarified the differences in reflection sources and light loss levels under different film thicknesses, and based on this, proposed a solution to synergistically enhance material polarizability and face-on orientation to improve the material’s refractive index. In addition, the authors verified their strategy in different material systems (PM6:Y6, D18:N3). This work focuses on the important but rarely discussed issue of light reflection in organic photovoltaic devices, and further analyzes the sources, effects, and optimization methods of light reflection, which are crucial for the further development of OSCs. However, whether light reflection has such a significant effect still requires more evidence to support it; therefore, I recommend its publication in Nature Communications after major revision. Here are my comments:\n1.The author explored the reflectivity of 1D and 2D materials separately. What is the classification basis for them? Electronic polarizability or other factors? Actually, the polymers such as PM6 were designed based on the method of 2D conjugate extension.\n2.In the first part of the high reflection of 1D- and 2D-conjugated molecules, very high reflection values were found for Y6 and P3HT-based films. However, in the device characterization part, the reflection values are missing. To illustrate the real effect of reflection on device performance, the author should comprehensively analyze light utilization in devices, including the interrelationships between EQE, reflection, and transmission, to quantitatively explain the impact of reflection.\n3.The effect of conjugated film intrinsic light reflection on device performance should be universally applicable in different device architectures, such as inverted, flexible, and LBL devices. The corresponding characterization and analysis should be conducted.\n4.This manuscript is titled “Intrinsic Light Reflection of Conjugated 1 Films towards Organic Optoelectronics”, while only OSC material and device are involved and discussed. How about OLED and OPD? Are the conclusions still universally valid?\n5.There are several minor errors throughout the manuscript, such as the Y6 structure in Fig. 1. Please review and revise accordingly.", "reviewer_2": "The work by Shen et al. studied the impact of active layer thickness and optical constant on the light management of organic photovoltaics. The four stages they proposed rationalized the thickness-dependent Jsc of OPVs in a straightforward manner, providing effective strategies for future performance optimization. While many previous works claimed that thick-film OPVs are required to achieve full absorption of the solar spectrum, this work pointed out that a thin active layer is more desirable in NFA-based OPVs, as the 2D conjugation of NFAs leads to a much higher refractive index compared to traditional 1D homopolymers like P3HT. Therefore, I think this work is suitable for publication in Nature Communications after revising the following issues.\n1. In lines 222-225 of the manuscript, the authors assigned the in-plane diffraction peaks of Y6 at 0.28 Å-1 and 0.42 Å-1 to the (001) and (100) peaks, corresponding to periodic packings along the backbone (the long axis of conjugated backbone) and the lamellar (the short axis of conjugated backbone) directions, respectively, as illustrated in Fig. 2. Such assignment is incorrect. In previous work by Xiao et al. [Journal of Materials Chemistry A, 2021, 9(31): 17030-17038.], both peaks were assigned to the second and third order diffraction along the backbone direction. In two other works [Nature communications, 2020, 11(1): 3943.] and [Advanced Energy Materials, 2020, 10(18): 1904234.], those two peaks were also assigned to periodic packings along the backbone direction of Y6 molecules, although they were referred to as ‘lamellar stackings’. Therefore, while it is reasonable to claim that the molecular orientation order improves upon thermal annealing, the detailed peak assignment should be revised.\n2. In lines 259-262 of the manuscript, the author defined the 2D orientation parameter S2D as 2cos(2γ)-1. However, based on the paper they referred to [Chemistry of Materials, 2020, 32(14): 6043-6054], S2D should be defined as 2cos2(γ)-1. Please check.\n3. In lines 368-372 of the manuscript, the authors claimed that the shunt resistance (Rsh) of the device peaks at 75 nm and decreases as the thickness is further increased to 220 nm. This is unexpected as thicker devices typically show higher Rsh. It should be noted that Rsh should be determined from the inverse differential of the dark J-V curve at 0 V [Advanced Energy Materials, 2016, 6(11): 1502250.]. On the other hand, Rsh extracted from the light J-V curve takes into account the transport resistance of photo-generated charge carriers [Nature Communications, 2015, 6(1): 6951], which no longer reflects the true shunt resistance of the device. Similarly, series resistance should also be obtained from the dark J-V curve. The authors should include the experimental procedure they employed to measure Rs and Rsh to avoid misinterpretation.\n4. In lines 414-417 of the manuscript, the authors claimed that the reflection primarily takes place at the HTL/active layer interface and the ETL/Ag interface, where the difference in refractive index (n) is the largest. The authors should include the wavelength-dependent values of n for all layers involved in OPVs for better illustration.\n5. It should be noted that the optical length difference between R1 and T2 is 2nd rather than nd. Therefore, the condition of the first destructive interference, as noted in Fig. 6c of the manuscript, should be d = λ/4n rather than d = λ/2n.", "reviewer_3": "In this study, the intrinsic reflection characteristics of organic conjugated films are systematically investigated to elucidate their impact on the optical properties and performance of organic solar cells (OSCs). The authors reveal that the polarizability of 2D conjugated molecules and their face-on orientation determine the intrinsic reflectance of organic films (up to 52 %). Due to the microcavity interference effect, the PM6:Y6-based devices exhibit a maximum short-circuit current density (JSC) at a thickness of approximately 75 nm. Furthermore, the authors experimentally and computationally demonstrate that the microcavity effect and optical interference are key factors driving variations in device performance such as JSC, and they propose a performance optimization strategy based on molecular polarizability and backbone orientation. However, several points need to be addressed to improve the quality of the manuscript and to provide a more comprehensive perspective.\n1. The manuscript lacks a systematic analysis of how device performance and film properties vary under different solvent systems (e.g., chloroform, chlorobenzene) and thermal annealing conditions. It would strengthen the study if the authors could justify their choice of specific solvent and annealing parameters or discuss whether alternative conditions were considered.\n2. The manuscript proposes two key strategies-enhancing molecular polarizability and maximizing face-on packing-for improving device performance. Could the authors clarify whether these two strategies can be simultaneously achieved in practice, and whether there are any potential trade-offs or conflicting aspects between them?\n3. While the study focuses on optical optimization for enhanced device performance, it would be valuable to discuss how such optimization strategies might affect the long-term durability and stability of the devices. Could the authors comment on any potential implications in this regard?", "reviewer_4": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts."}, "author_response": "Response to Reviewers\nReviewer 1\nComments: Shen and coworkers systematically investigate the intrinsic light reflection characteristics of organic conjugated thin films and their impact on the performance of organic photovoltaic devices. Through GIWAXS experiments and TD-DFT theoretical calculations, they found that the electronic polarizability and molecular stacking orientation of organic semiconductor materials are the key factors determining their high reflectivity (up to 50%). Further researches show that by regulating film thickness and molecular orientation, microcavity effects can be optimized, thereby improving the performance of OSCs. Through analyzing the four stages of the effect of film thickness on the reflection situation, the author clarified the differences in reflection sources and light loss levels under different film thicknesses, and based on this, proposed a solution to synergistically enhance material polarizability and face-on orientation to improve the material’s refractive index. In addition, the authors verified their strategy in different material systems (PM6:Y6, D18:N3). This work focuses on the important but rarely discussed issue of light reflection in organic photovoltaic devices, and further analyzes the sources, effects, and optimization methods of light reflection, which are crucial for the further development of OSCs. However, whether light reflection has such a significant effect still requires more evidence to support it; therefore, I recommend its publication in Nature Communications after major revision.\nResponse: We sincerely appreciate your encouraging feedback on our work. Your constructive comments and insightful suggestions have been invaluable in strengthening this study, and we have carefully addressed each comment point-by-point in the revised manuscript.\nHere are my comments:\n1.The author explored the reflectivity of 1D and 2D materials separately. What is the classification basis for them? Electronic polarizability or other factors? Actually, the polymers such as PM6 were designed based on the method of 2D conjugate extension.\nResponse: We thank the reviewer for highlighting this important issue. The 1D- and 2D- polarized materials were distinguished according to the polarizability direction of the materials films. Although donor polymers like PM6 possess 2D conjugated molecular segments, they exhibit quasi-1D polarized characteristics due to superior electron delocalization along the polymer backbone direction. Furthermore, the predominantly edge-on/isotropic molecular stacking orientation of PM6-type polymer donors aligns their yy-polarization direction perpendicular to the substrate plane—orthogonal to the electric field vector of incident light (https://doi.org/10.1002/adfm.202509532). Consequently, yy-direction polarization remains largely unexcited under standard illumination conditions, thereby exhibiting optical behavior analogous to 1D- polarized systems. This is experimentally evidenced by their comparable maximum reflectivity to 1D- polarized polymers (e.g., P3HT), which remains significantly lower than that of 2D- polarized molecules (e.g., Y6) featuring face-on stacking orientations.\nWe have added “In fact, polymer donor materials such as PM6 and PBDB-T possess a certain degree of two-dimensional polarizability based on their molecular structures. Nevertheless, they exhibit optical constants nearly identical in magnitude to those of one-dimensionally polarized molecules like P3HT. This phenomenon arises partly because their two-dimensional polarizability is less pronounced than that of molecules with extensive two-dimensional conjugation, such as Y6. Additionally, they tend to adopt an edge-on/isotropic molecular packing orientation, where the molecular y-axis is perpendicular to the electric field vector of incident light. Consequently, polarization along the y-direction remains inactivated. This results in optical characteristics more closely resembling those of one-dimensionally polarized molecules.” on page 9 to explain how we classify 1D and 2D conjugated materials.\n2.In the first part of the high reflection of 1D- and 2D-conjugated molecules, very high reflection values were found for Y6 and P3HT-based films. However, in the device characterization part, the reflection values are missing. To illustrate the real effect of reflection on device performance, the author should comprehensively analyze light utilization in devices, including the interrelationships between EQE, reflection, and transmission, to quantitatively explain the impact of reflection.\nResponse: Thank you for pointing this out. As shown in Supplementary Fig. 17, we measured the total reflection spectra of PM6:Y6 devices with varying active-layer thicknesses and convolved these spectra with the AM1.5G solar irradiance profile to evaluate the light loss fraction, which aligns well with our simulation results. As evidenced by Supplementary Fig. 17 and Fig. 5b, the overall device reflection is substantially lower than that measured from the bare active-layer film, which primarily stems from the optical behavior at the R interface (defined in Fig. 5a). In OSCs, R 1 1 corresponds to the active-layer/HTL interface, where the refractive index contrast is smaller (n ≈ 1.5, Supplementary Fig. 16). Conversely, in bare active-layer HTL measurements, R represents the active-layer/air interface with a large difference (n ≈ 1 air 1), resulting in significantly stronger reflection. Furthermore, through strategic thickness optimization, the Y6 film exhibiting maximum reflection was engineered at ~50 nm. At this thickness, constructive interference enhances the T component (i.e., 2 the second-pass reflected light), thereby amplifying the overall reflected intensity. Additionally, we provide an in-depth discussion correlating the interplay among EQE, reflection, and transmission on page 19, Supplementary Information.\n“Theoretically, due to the presence of the Ag electrode, OSC devices are opaque (T = 0). Therefore, the wavelength-dependent characteristics of the device reflection spectra should correspond to the EQE measurement results: stronger reflection light at a specific wavelength leads to a weaker EQE signal, and vice versa. Accordingly, we compared the reflection spectra with the EQE for devices of different thicknesses (Fig. 3c and Supplementary Fig. 17). It can be observed that the two indeed exhibit a trade-off relationship. For example, at wavelengths of 450 nm and 850 nm, the 212 nm-thick device shows the lowest reflection and the highest EQE, while near 550 nm and 700 nm, the 53 nm-thick device exhibits the highest reflection and the lowest EQE. For the 75 nm-thick device, two peaks in EQE appear at 500 nm and 700 nm, which correspond precisely to two dips in the reflection spectrum. Thus, the correspondence between the EQE and the device reflection spectra indicates that optical reflection is a decisive factor in shaping the EQE curve, which in turn determines the J obtained by SC integrating the EQE curve.”\nFig. R2 (Fig. S17) Experimental total reflection result of PM6:Y6 devices with different thickness. (a) Curves for all tested devices in the range from 21 nm to 212 nm; (b) Comparison of reflectance curves for 53 nm, 75 nm, 135 nm, and 212 nm devices; (c-f) Interrelationships between EQE, reflection, and parasitic absorption/IQE for devices with 53 nm, 75 nm, 135 nm and 212 nm active layer thickness, respectively, to quantitatively explain the impact of reflection.\n3.The effect of conjugated film intrinsic light reflection on device performance should be universally applicable in different device architectures, such as inverted, flexible, and LBL devices. The corresponding characterization and analysis should be conducted.\nResponse: Thank you for your valuable suggestion. We agree that it is necessary to verify the observed phenomena and the proposed theory in devices with different structures. However, the reported layer-by-layer (LBL) processed devices still possess a BHJ region at the donor/acceptor interface, and the vertical phase separation observed in the BHJ region can influence optical absorption and may significantly interfere with the thickness-dependent behavior of J . Additionally, during the sequential coating SC process of the active layer (especially spin-coating), due to the use of incompletely orthogonal solvents, the second coating step will cause mass loss in the initially deposited layer, resulting in uncertainty in the donor-acceptor composition. Compared to LBL devices, we believe that inverted and flexible BHJ devices are sufficient to validate the observed phenomena and the reliability of the proposed optical model. Therefore, we conducted the same investigations on inverted and flexible devices as on conventional devices, and the results are shown in Supplementary Figs. 18-20. We have also added the following explanation on page 20 of the main text.\n“In addition to conventional forward devices, we also conducted identical studies on inverted devices with different transport layers and electrodes, as well as flexible devices with different substrates, using the PM6:Y6 system as an example. The device performance test results are shown in Supplementary Fig. 18 and Supplementary Table 3. It can be observed that both inverted and flexible devices exhibit a similar trend of J variation with thickness as conventional devices (Fig. 5), indicating that minor SC differences in the optical constants of the transport layers, electrodes, and substrates do not significantly affect the optical performance of the devices (Supplementary Fig. 19). This conclusion is further supported by the experimental and simulated light absorption results of PM6:Y6 inverted devices with different thicknesses (Supplementary Fig. 20).”\nFig. R3 (Fig. S18) Photovoltaic performance of PM6:Y6 devices with different structures. Inverted devices: (a) J-V curves; (b) Comparison of J for devices with different thicknesses; (c) EQE curves SC for devices with different thicknesses. Flexible devices: (d) J-V curves; (e) Comparison of J for SC devices with different thicknesses; (f) EQE curves for devices with different thicknesses.\nFig. R4 (Fig. S19) The optical constants of the electrodes and transport layers used in inverted PM6:Y6 devices. (a) Wavelength-dependent refractive indices; (b) Wavelength-dependent extinction coefficients.\na\nd\n)2-mc Am( ytisneD\ntnerruC\n)2-mc Am( ytisneD\ntnerruC\n0 -10\n-20\n-30\n0 -10\n-20\n-30\nIn v e rte d P M 6 :Y 6 D e vice s 53 nm 75 nm 135 nm 212 nm\n0.0 0.2 0.4 0.6\nV o lta g e (V )\nF le x ib le P M 6 :Y 6 D e vice s 53 nm 75 nm 135 nm 212 nm\n0.0 0.2 0.4 0.6\nV o lta g e (V )\na\n2.5\n2.0\nxed\n1.5 nI\nevi\n1.0 tcarfeR\n0.5\n0.0\n300 400 500W\n0.8\n0.8\n600\na v e le n g\nb\nE Q E E xp n J J 25 e sc sc D\n53 nm : 22.70 23.89 tn 24 e 75 nm : 26.50 27.53 rru 135 nm : 25.55 25.91 23 C\n212 nm : 25.95 27.03\n2240\n80 120 160 200\nA ctive L a ye r T h ickn e ss (n m )\ne\nE Q E E xp n J J 25 e sc sc D\n2 5 .0 3 2 5 .1 1 53 nm : tn 24 e 2 7 .1 1 2 7 .5 4 75 nm : rru 2 5 .9 2 2 6 .0 8 135 nm : 23 C\n2 6 .4 2 2 7 .2 1 212 nm :\n2240\n80 120 160 200\nA ctive L a ye r T h ickn e ss (n m )\nb\n10\n1 tneiciff\n0.1 eoC\nn ITO no\n0.01 n it ZnO cn\nn i Ag tx\nn E 0.001 M oO3\n1E -4\n700 800 900 1000 300 400 500W\nth (n m )\nc\nf\na\nEQE\n40\n53 nm 75 nm 20 135 nm\n212 nm\n0300\n400 500 600 700 800\nW a ve le n g th (n m )\nEQE\n40\n53 nm 75 nm 20 135 nm\n212 nm\n0300\n400 500 600 700 800\nW a ve le n g th (n m )\nk ITO\nk ZnO\nk Ag\nk M oO3\n600 700 800 900 1000\nv e le n g th (n m )\n900\n900\n30\n25 20\n15\n10 5\n0\n1000\n30\n25 20\n15\n10 5\n0\n1000\n)2-mc Am( ytisneD\ntnerruC\n)2-mc Am( ytisneD\ntnerruC\nFig. R5 (Fig. S20) Simulation of optical loss in inverted-configuration PM6:Y6 devices. (a) Simulated reflection spectra of PM6:Y6 devices based on TMM; (b) Thickness-dependent light loss curves obtained from convolution of simulated reflection spectra and AM1.5G spectrum, and their comparison with experimentally measured J .\nSC\n4.This manuscript is titled “Intrinsic Light Reflection of Conjugated Films towards Organic Optoelectronics”, while only OSC material and device are involved and discussed. How about OLED and OPD? Are the conclusions still universally valid?\nResponse: Thank you very much for your valuable feedback. You rightly pointed out the discrepancy between the scope of the title and the content discussed in the manuscript, which is a very pertinent observation. We fully agree with your view.\nAs you noted, the experimental data and discussions in this paper primarily focus on OSCs. Therefore, to more accurately reflect the research scope of this study and avoid misleading the readers, we have revised the title according to your suggestion to:\n“Intrinsic Light Reflection of Conjugated Films towards High-Performance Organic Solar Cells”. We believe the modified title more precisely summarizes the contribution of this work.\nRegarding the universality of the conclusions for OLEDs and OPDs that you mentioned, we would like to provide the following explanations:\nFor OLEDs, the optical micro-cavity effects have indeed been extensively and deeply studied. A substantial body of literature has demonstrated the modulation of emission color and efficiency through precise control of the thickness of various layers, including the organic layers (such as [Chem. Eng. J. 2024, 501, 157552], [J. Mater. Chem. C 2022, 10(8), 3241-3247], and [J. Mater. Chem. C 2019, 7(39), 12114-12120]). Given the maturity of research in this field, and considering that OLED performance is also\na\n)%(\nnoitcelfeR\n6\n5\n4\n3\n2\n1\n0\n0\n0\n0\n0\n0\n0\n4 0 0\nF ro m 4 0\n5 0 0\nn m (y e llo w ) to 2 5 0 n m\nIn v e rte d P M 6 :Y 6 d e v\n6 0 0 7 0 0\nW a v e le n g th (n m )\n(p u rp\nic e s\n8 0 0\nle )\n9 0 0\nb\n)%(\nssol\thgiL\n0\n0\n0\n0\n0\n.3\n.2\n.2\n.1\n.1\n0\n5\n0\n5\n0\n5 0\nA c tiv e\nS im u la te d lig h t lo s s\nJ E x p e rim e n ta l SC\nP M 6 :Y 6\nIn v e rte d c o n fig u ra tio n\n1 0 0 1 5 0 2 0 0\nla y e r th ic k n e s s (n m )\n2\n2\n2\n2\n2\n2\n2\n2\n2\n3\n4\n5\n6\n7\n8\n9\n)2-mc\nAm(\nJ\nCS\nstrongly influenced by numerous complex factors such as carrier injection, transport, and exciton quenching, adding a discussion on OLEDs in this paper might divert attention from the core findings of our study. Hence, we chose to focus on the system where our research offers the most novelty.\nOn the other hand, for OPDs, the device structures of many OPDs are indeed highly similar to those of OSCs. Since the photoresponsivity (R) can be derived from the EQE curve, we found that it is also determined by the intrinsic reflection of the active layer, as shown in Supplementary Fig. 21. We have included further discussion in the main text regarding the relationship between EQE and the device's reflection on page 19, Supplementary Information,\n“Theoretically, due to the presence of the Ag electrode, OSC devices are opaque (T = 0). Therefore, the wavelength-dependent characteristics of the device reflection spectra should correspond to the EQE measurement results: stronger reflection light at a specific wavelength leads to a weaker EQE signal, and vice versa. Accordingly, we compared the reflection spectra with the EQE for devices of different thicknesses (Fig. 3c and Supplementary Fig. 17). It can be observed that the two indeed exhibit a trade-off relationship. For example, at wavelengths of 450 nm and 850 nm, the 212 nm-thick device shows the lowest reflection and the highest EQE, while near 550 nm and 700 nm, the 53 nm-thick device exhibits the highest reflection and the lowest EQE. For the 75 nm-thick device, two peaks in EQE appear at 500 nm and 700 nm, which correspond precisely to two dips in the reflection spectrum. Thus, the correspondence between the EQE and the device reflection spectra indicates that optical reflection is a decisive factor in shaping the EQE curve, which in turn determines the J obtained by SC integrating the EQE curve.”\nand have also elaborated on the impact of intrinsic reflection on OPDs on page 20.\n“Indeed, this optical micro-cavity effect, which depends on the reflection properties of the active layer, also holds instructive significance for organic photodetectors. The responsivity of a photodetector to light of different wavelengths reflects its applicability under irradiation at various wavelengths and can likewise be modulated by adjusting the active layer thickness. By converting the EQE curves of PM6:Y6-based OSC devices, the photoresponsivity (R) curves of the organic photodetectors can be obtained[63,64]. As shown in Supplementary Fig. 21, these curves exhibit a fluctuation trend highly similar to that of the device’s reflection light (Supplementary Fig. 17). This not only confirms the decisive role of reflection light in device absorption but also offers a strategy for wavelength-selective optimization.”\nThis aims to demonstrate the potential applicability of the proposed mechanism in devices with similar structures and to provide direction for future research.\nOnce again, we sincerely appreciate your review. Your comments have significantly enhanced the rigor and quality of this manuscript. We hope that the revisions and explanations provided above meet with your approval.\nFig. R6 (Fig. S21) The photoresponsivity (R) curves of organic photodetectors with different PM6:Y6 layer thicknesses, obtained by converting the EQE curves of OSC devices.\n5.There are several minor errors throughout the manuscript, such as the Y6 structure in Fig. 1. Please review and revise accordingly.\nResponse: Thank you for pointing this out, and we have already revised the Y6 structure in Fig. 1. We have also reviewed the whole manuscript to avoid other errors.\nReviewer 2\nComments: The work by Shen et al. studied the impact of active layer thickness and optical constant on the light management of organic photovoltaics. The four stages they proposed rationalized the thickness-dependent Jsc of OPVs in a straightforward manner, providing effective strategies for future performance optimization. While many previous works claimed that thick-film OPVs are required to achieve full absorption of the solar spectrum, this work pointed out that a thin active layer is more desirable in NFA-based OPVs, as the 2D conjugation of NFAs leads to a much higher refractive index compared to traditional 1D homopolymers like P3HT. Therefore, I think this work is suitable for publication in Nature Communications after revising the following issues.\nResponse: Thank you for your review of our manuscript, and thank you for your recognition of our work’s significance. We have carefully revised our manuscript based on your feedback.\n1. In lines 222-225 of the manuscript, the authors assigned the in-plane diffraction peaks of Y6 at 0.28 Å-1 and 0.42 Å-1 to the (001) and (100) peaks, corresponding to periodic packings along the backbone (the long axis of conjugated backbone) and the lamellar (the short axis of conjugated backbone) directions, respectively, as illustrated in Fig. 2. Such assignment is incorrect. In previous work by Xiao et al. [Journal of Materials Chemistry A, 2021, 9(31): 17030-17038.], both peaks were assigned to the second and third order diffraction along the backbone direction. In two other works [Nature communications, 2020, 11(1): 3943.] and [Advanced Energy Materials, 2020, 10(18): 1904234.], those two peaks were also assigned to periodic packings along the backbone direction of Y6 molecules, although they were referred to as ‘lamellar stackings’.\nTherefore, while it is reasonable to claim that the molecular orientation order improves upon thermal annealing, the detailed peak assignment should be revised.\nResponse: We sincerely appreciate the reviewer's meticulous critique. Based on the canonical literature (Xiao et al., J. Mater. Chem. A 2021, 9, 17030-17038; Yuan et al., Nat. Commun. 2020, 11, 3943), we confirm that the peaks at 0.28 Å⁻¹ and 0.42 Å⁻¹ both originate from periodic packing along the backbone direction (long molecular axis).\nThe original text has been revised to “Additionally, the broadening of the (010), (200), and (300) diffraction peaks is reduced following thermal annealing (TA), indicating improved molecular orientation uniformity” on page 9 to ensure accurate assignment of the diffraction peaks.\n2. In lines 259-262 of the manuscript, the author defined the 2D orientation parameter S2D as 2cos(2γ)-1. However, based on the paper they referred to [Chemistry of Materials, 2020, 32(14): 6043-6054], S2D should be defined as 2cos2(γ)-1. Please check.\nResponse: Thank you for bringing this error to our attention. Upon our verification, we have determined that this discrepancy may have occurred during the document editing process. We have corrected “2<cos2γ>-1” to “2<cos2γ>-1” in the original text accordingly.\n3. In lines 368-372 of the manuscript, the authors claimed that the shunt resistance (Rsh) of the device peaks at 75 nm and decreases as the thickness is further increased to 220 nm. This is unexpected as thicker devices typically show higher Rsh. It should be noted that Rsh should be determined from the inverse differential of the dark J-V curve at 0 V [Advanced Energy Materials, 2016, 6(11): 1502250.]. On the other hand, Rsh extracted from the light J-V curve takes into account the transport resistance of photo-generated charge carriers [Nature Communications, 2015, 6(1): 6951], which no longer reflects the true shunt resistance of the device. Similarly, series resistance should also be obtained from the dark J-V curve. The authors should include the experimental procedure they employed to measure Rs and Rsh to avoid misinterpretation.\nResponse: We sincerely appreciate your insightful observation regarding the necessity of measuring series resistance (R ) and shunt resistance (R ) under dark conditions, as s sh we fully concur that photocurrent generation fundamentally alters carrier transport behavior, whereas dark-state measurements reflect the intrinsic resistive properties of the device.\nTo address this, we performed equivalent circuit analysis on dark-state J-V curves following the methodology from Advanced Energy Materials, 2016, 6, 1502250, with results presented in Supplementary Fig. S11, and we added the statement “The monotonic increase of R with thickness is consistent with reduced shunt paths, as sh thicker films dilute pinhole defects and extend carrier transit paths, thereby suppressing direct electrode leakage. However, we observed a non-monotonic relationship between Rs and film thickness, attributed to the competition between contact resistance and bulk resistance. At 53 nm, elevated contact resistance dominates due to interfacial defects and non-ideal electrode coverage; at 75 nm, optimized contact and minimal bulk resistance yield the lowest R , indicating optimal interfacial conditions likely resulting s from balanced energy alignment and film continuity. Beyond 75 nm, R increases s linearly with thickness, confirming bulk resistance dominance as charge transport distance lengthens. This minimum R at 75 nm critically correlates with the highest s power conversion efficiency observed at this thickness, as it minimizes resistive losses while maintaining efficient charge extraction.” In Supplementary Information elucidate the impact of R and R on the J of devices, including their thickness-dependent s sh SC characteristics.\nWe also added “Additionally, we observed that the series resistance (R ) increases s linearly with film thickness beyond 75 nm under dark conditions, indicating that contact resistance is insensitive to film thickness and morphology (0.41 ± 0.08 Ω·cm²), as shown in Figure S11 and Table S1. Concurrently, a monotonic increase in shunt resistance (R ) with thickness was observed, demonstrating that thicker films sh effectively suppress pinhole effects, thereby reducing device leakage current. Generally, Higher R and lower R could both lead to a reduction in J , and these resistive s sh SC characteristics confirm that the 75-nm-thick PM6:Y6 device achieves the optimal balance of R and R , which aligns with its highest PCE. Critically, interfacial contact s sh is not the primary cause of the reduced J in the 135-nm device.” on page 16 to discuss SC the contribution of resistive effects to the observed J degradation in devices with a SC 135 nm active layer.\na b\n3.0 1500\nR\nLi\ns\nn\no\ne\nf\na\nP\nr F\nM\nit\n6\no\n:Y\nf\nd\ns\nevices 1\n1\n4\n3\n0\n0\n0\nsh\nof PM6:Y6 devices\n2.5 E P q lo u t ation y = a A A + b*x 1200\n2m ) W I S R n l e t o e e s p i r i g c e d h e u t p a t l Sum of Squares 0. 0 0 . 1 4 N 0 0 5 o 9 4 0 6 W . 5 ± 0 e 0 ± 5 i 2 g . 0 1 4 h . 0 6 0 ti 7 7 n 5 7 g 1 3 E 4 -4 2m )1 1 1 0 0 0 0 0\nc W 2.0 P R A e d -S a j. q r R s u o - a S n re ' q s u ( r C ar O e D) 0 0 0 . . . 9 9 9 9 9 9 8 7 5 8 6 3 3 6 2 c W 900\nR ( s R ( hs 8 7 0 0 0 0\n1.5 600\nR C (>75 nm) = 0.41 ± 0.08 W cm2 500\nR (53 nm) = 0.89 ± 0.08 W cm2 400\nC\n1.0 300\n40 80 120 160 200 240 40 80 120 160 200\nActive layer thickness (nm) Active layer thickness (nm)\nFig. R7 (Fig. S11) Relationship between dark-state R, R , and active layer thickness in PM6:Y6 s sh devices. (a) Linear correlation between R and active layer thickness; (b) monotonic relationship s between R and active layer thickness.\nsh\n4. In lines 414-417 of the manuscript, the authors claimed that the reflection primarily takes place at the HTL/active layer interface and the ETL/Ag interface, where the difference in refractive index (n) is the largest. The authors should include the wavelength-dependent values of n for all layers involved in OPVs for better illustration.\nResponse: We sincerely appreciate your thoughtful suggestion. We have provided the wavelength-dependent values of n and k for all layers involved in OPVs, and we also added some discussion to clearly demonstrate our viewpoint on page 18.\n“Consequently, due to the significant differences in the complex refractive index (n, k values) of the active layer compared to the Ag electrode, ITO electrode, HTL, and ETL, as provided in Supplementary Fig. 16, the strongest reflection should occur at either HTL/active layer interface (R , bottom reflection) or at ETL/Ag top interface (R , top 1 2 reflection)”\nFig. R8 (Fig. S16) The optical constants of the electrodes and transport layers used in OSC devices. (a) Wavelength-dependent refractive indices; (b) Wavelength-dependent extinction coefficients.\n5. It should be noted that the optical length difference between R1 and T2 is 2nd rather than nd. Therefore, the condition of the first destructive interference, as noted in Fig. 6c of the manuscript, should be d = λ/4n rather than d = λ/2n.\nResponse: We sincerely thank the reviewer for identifying the inconsistency in our original manuscript, where the constructive interference condition at d = λ/(2n) was mistakenly labeled as destructive interference in the text. This error has been corrected on page 22 (Fig. 6c) to accurately reflect that the thickness d = λ/(4n) corresponds to destructive interference, consistent with the fundamental principle that light traversing the active layer twice yields a path difference of 2nd. The revision ensures alignment between the textual description and the labeled interference extrema in the figure. We appreciate this insightful correction, which strengthens the optical analysis in our work.\na\nxednI\nevitcarfeR\n2\n2\n1\n1\n0\n0\n.5\n.0\n.5\n.0\n.5\n.0\n3 0 0 4 0 0 5 0 0W\na\n6 0 0\nv e le n g\nn ITO\nn\nPED O T:PSS\nn\nAg\nn PN D IT-F3N\n7 0 0 8 0 0\nth (n m )\n9 0 0 1 0 0 0\nb\ntneiciffeoC\nnoitcnitxE\n1 0\n1\n0 .1\n0 .0 1\n0 .0 0 1\n0 0 4 0 0 5 0 0W\na\n6 0 0\nv e le n g\n7 0 0\nth (n m\n8 0)\nk\nk\nk\nk\nPED O T:PSS\nAg\nPN D IT-F3N\n9 0 0 1 0 0 0\nReviewer 3\nComments: In this study, the intrinsic reflection characteristics of organic conjugated films are systematically investigated to elucidate their impact on the optical properties and performance of organic solar cells (OSCs). The authors reveal that the polarizability of 2D conjugated molecules and their face-on orientation determine the intrinsic reflectance of organic films (up to 52 %). Due to the microcavity interference effect, the PM6:Y6-based devices exhibit a maximum short-circuit current density (JSC) at a thickness of approximately 75 nm. Furthermore, the authors experimentally and computationally demonstrate that the microcavity effect and optical interference are key factors driving variations in device performance such as JSC, and they propose a performance optimization strategy based on molecular polarizability and backbone orientation. However, several points need to be addressed to improve the quality of the manuscript and to provide a more comprehensive perspective.\nResponse: We sincerely appreciate your thorough evaluation and constructive feedback. Below are our point-by-point responses to the key concerns raised.\n1. The manuscript lacks a systematic analysis of how device performance and film properties vary under different solvent systems (e.g., chloroform, chlorobenzene) and thermal annealing conditions. It would strengthen the study if the authors could justify their choice of specific solvent and annealing parameters or discuss whether alternative conditions were considered.\nResponse: We appreciate this important observation. Regarding the systems discussed in the manuscript, taking PM6:Y6 as an example, our preparation method largely follows most reported literature protocols: spin-coating from chloroform (CF) solution at room temperature followed by thermal annealing at 100℃ for 5 minutes (literature typically reports annealing between 90-110℃). Firstly, CF was selected as the solvent because chlorobenzene (CB) has a higher boiling point and slower evaporation rate, which promotes the formation of larger crystalline domains of Y6 (and its derivatives) in the film. This coarsened morphology restricts the formation of a nanoscale interpenetrating network, thereby reducing charge generation efficiency. In contrast, chloroform’s lower boiling point enables faster solvent evaporation, limiting further crystal growth of Y6 and facilitating the development of an interpenetrating donor-acceptor nanomorphology. Our early work attempted CB as solvent but yielded unsatisfactory performance [Adv. Mater. Interfaces 2020, 7, 2000577]. However, an alternative approach using CB is possible through hot spin-coating. For D18, which exhibits extremely poor solubility in CF but dissolves well in hot CB solutions, hot spin-coating with CB at elevated temperatures allows rapid solvent evaporation due to the high temperature, thereby restricting excessive growth of NFA crystals. The D18:Y6 devices in this study were fabricated via hot spin-coating at 80℃ using CB as solvent. Similarly, excessively high annealing temperatures can also promote further growth of Y6 crystals. For instance, annealing above 200℃ leads to micron-scale crystal formation, severely disrupting the film morphology. Even annealing at 160°C, while not causing significant micron-scale phase separation, still damages the nanoscale interpenetrating network. Device performance is highly sensitive to such morphological degradation. Therefore, following common literature practice, our study employs 100 ℃ thermal annealing.\nWe have added a brief explanation “To this end, a series of PM6:Y6 blend films with thicknesses ranging from 20 nm to 200 nm were fabricated using chloroform as the solvent and subsequently subjected to TA treatment at 100℃ for 5 minutes” on Page 12 to clarify our rationale for selecting chloroform and 100℃ annealing as the preparation conditions for PM6:Y6 devices.\n2. The manuscript proposes two key strategies-enhancing molecular polarizability and maximizing face-on packing-for improving device performance. Could the authors clarify whether these two strategies can be simultaneously achieved in practice, and whether there are any potential trade-offs or conflicting aspects between them?\nResponse: We sincerely appreciate the reviewer's insightful comments, which prompted us to provide an in-depth explanation of the synergistic feasibility and potential trade-offs between the two core strategies proposed in this study—enhancing molecular polarizability and maximizing face-on packing—in practical applications. Based on experimental evidence and theoretical analysis presented in the manuscript, we confirm that these two strategies can be simultaneously implemented in practical devices and mutually reinforce each other.\nThe enhancement of molecular polarizability is primarily achieved through molecular design, such as introducing electron-withdrawing end groups or asymmetric structures to improve the in-plane electronic polarizability of conjugated molecules. This increases the material’s refractive index and extinction coefficient, which not only reduces the optimal absorption thickness of the film but also enhances charge generation and transport efficiency. For example, asymmetric acceptor materials represented by BO-5Cl achieve high polarizability through their unique dual-interface conformation, optimizing charge generation kinetics while reducing non-radiative voltage loss to 0.178 eV [Nat. Commun. 2022, 13, 2598]. Additionally, Hou et al. reported that introducing electron-withdrawing groups into the P3HT backbone significantly increases its electrostatic potential (ESP), which is a manifestation of enhanced molecular polarizability [Chem 2023, 9, 2518-2529]. Greater electrostatic potential energy strengthens intermolecular interactions, thereby facilitating charge generation and transport [Angew. Chem. Int. Ed. 2021, 60, 15988-15994].\nSimultaneously, the strategy of maximizing face-on packing focuses on optimizing molecular alignment orientation. Through methods such as thermal annealing or solvent engineering, molecules can be induced to preferentially adopt an arrangement where the conjugated plane is parallel to the substrate. This orientation not only promotes out-of-plane charge transport efficiency but also aligns the molecular polarization direction precisely with the electric field vector of incident light, thereby enhancing light-matter interactions. Our GIWAXS data clearly demonstrate the synergistic enhancement of optical constants and face-on packing degree in Y6 after thermal annealing. In the reported ternary system PM6:BO-4Cl:BO-5Cl [Nat. Commun. 2022, 13, 2598], the high polarizability and face-on characteristics of BO-5Cl (verified by both single-crystal and GIWAXS analyses) further drove the certified efficiency to exceed 18.2%, providing strong evidence for the synergy of these strategies.\nIt is worth exploring the potential trade-offs between these two strategies. We found that enhancing polarizability (e.g., through fluorination or chlorination of end groups) is often accompanied by strengthened intermolecular interactions, which promote π-π stacking formation and thereby facilitate face-on packing. Hou et al. also utilized solid additives with strong electrostatic interactions to induce better face-on stacking of NFAs [Chem 2023, 9, 2518-2529]. Furthermore, Table 1 in [J. Am. Chem. Soc. 2024, 146, 14724−14733] reveals a key pattern: an increase in exciton polarizability (Δp) correlates significantly with longer π-π coherence lengths (L ) and higher face-on c orientation ratios, collectively driving improvements in charge transfer rates and internal quantum efficiency (IQE). Although enhancing polarizability and optimizing face-on packing are inherently synergistic rather than conflicting, fine-tuned control remains essential during implementation. For instance, excessive thermal annealing may induce molecular rotation toward edge-on or end-on orientations, slightly reducing in-plane polarizability, but this risk can be effectively mitigated by optimizing process parameters.\nIn summary, enhancing molecular polarizability and maximizing face-on packing are fundamentally compatible and mutually reinforcing. Any potential trade-offs mainly arise in specific scenarios such as thickness optimization and can be fully resolved through molecular design and process control. Our proposed co-optimization framework (Fig. 7b) leverages the synergistic effect of these two strategies to reduce the optimal active layer thickness, balance device parameters, and ultimately maximize efficiency. Future research will focus on scalable solutions such as designing asymmetric acceptors that integrate polarizability with self-assembled face-on characteristics, thereby advancing the industrial application of this strategy. We thank the reviewer again for their valuable comments, as this clarification further confirms the robustness and universal applicability of our proposed strategies.\nWe have added a brief explanation “It is important to clarify that these two strategies are not competitive but complementary. Enhancing the in-plane polarizability of materials (for instance, through fluorinated end groups or asymmetric molecular structures) simultaneously promotes charge generation/transport and improves face-on molecular stacking. This dual effect optimizes the thickness corresponding to peak performance in J /FF/V , thereby collectively elevating the device’s PCE. This SC OC paradigm provides critical guidelines for next-generation OPV material development” on Page 25 to further clarify the synergistic relationship between the two strategies for readers.\n3. While the study focuses on optical optimization for enhanced device performance, it would be valuable to discuss how such optimization strategies might affect the long-term durability and stability of the devices. Could the authors comment on any potential implications in this regard?\nResponse:\nWe appreciate your valuable suggestions. These not only contribute to our deeper understanding of the influence of molecular polarizability on device performance but also provide valuable direction for our future research. Prior to this, we had not fully considered the potential impact of our proposed optimization strategies on device stability.\nIn our view, the factors influencing device stability are multifaceted. Due to considerations of manuscript length and focus, we do not intend to discuss this aspect further within the current study, to avoid confusing the reader. However, based on reported literature and our current understanding, we can derive hypotheses regarding the potential influence of molecular polarizability and face-on oriented packing on the stability of OPV devices.\nFirstly, an increase in molecular polarizability may promote the internal ordering of molecular packing within the film. This could potentially enhance the mechanical stability of the device, which might be beneficial for flexible OPV devices. Secondly, higher molecular polarizability can strengthen intermolecular dipole-dipole interactions, thereby elevating the glass transition temperature and consequently improving the device’s thermal stability. Furthermore, regarding photostability, literature reports indicate that enhancing the electron-withdrawing capability of the end groups in NFAs can effectively improve device photostability. This improvement is attributed to denser molecular packing suppressing triplet state generation and inhibiting molecular isomerization [Adv. Energy Mater. 2022, 12, 2201267]. Additionally, increasing the polarizability of small-molecule acceptors may foster stronger interactions between donor and acceptor materials, leading to the formation of a highly intermixed stable blend morphology and enabling efficient excited-state quenching.\nSimilarly, methods such as thermal annealing or solvent engineering that enhance the face-on oriented molecular packing within the film can also lead to tighter intermolecular π-π interactions, thereby stabilizing the structure. In summary, we hypothesize that the two strategies we propose (enhancing polarizability and face-on orientation) may be beneficial for improving device stability. However, given that current research on device stability is not yet fully comprehensive, or due to limitations in our current understanding, there is a possibility of unintended adverse effects, such as vertical phase separation induced by differences in surface energy. We plan to investigate this aspect in future work, and we sincerely thank you again for your insightful comments.\nReviewer 4\nComments: I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts.\nResponse: Thank you and the co-reviewer for the time and expertise dedicated to reviewing our manuscript. We have addressed all reviewers’ comments point-by-point and thoroughly revised the manuscript accordingly.\nResponse to Reviewers\nResponse to Reviewer #1\nReviewer Comments: The manuscript has been revised according to the reviewer's suggestions. and I have no further comments.\nResponse: We sincerely thank the reviewer for the positive feedback regarding our revised manuscript. We are delighted to hear that the revisions made in response to the reviewer's constructive comments have been met with approval.\nResponse to Reviewer #2\nReviewer Comments: The authors have satisfactorily addressed my comments 1, 3, and 4. However, comments 2 and 5 were partially resolved and require further clarification.\nResponse: We extend our sincere gratitude to the reviewer for their insightful feedback. While we are delighted that our responses to Comments 1, 3, and 4 gained their approval, we regret that our initial revisions for Comments 2 and 5 fell short of expectations, being somewhat perfunctory and lacking in depth. In light of the reviewer’s valid critique, a dedicated re-examination was undertaken. The substantially revised responses below are consequently provided, and it is our sincere hope that they will prove satisfactory.\nComment 1: In comment 2, I mentioned that the 2D orientation parameter S2D should be defined as 2cos^2(γ)-1 according to the reference 32 [Chemistry of Materials, 2020, 32(14): 6043-6054]. However, I noted that the S2D is still defined as 2cos(2γ)-1 in line 268 of the revised manuscript. The author is recommended to carefully read ref 32 before adopting this method.\nResponse: We are grateful to the reviewer for identifying the error in our manuscript. We acknowledge that the formula was incorrectly stated and should read “S = 2⟨cos2γ⟩ 2D - 1 (Page 10, line 265)”. The necessary correction has been made in the text, and we sincerely apologize for this mistake.\nComment 2: In Fig. 2f of the manuscript, the author obtained S2D for a range of materials and defined the physical meaning of S2D as ‘S2D value in the range of -1 to 0 represents preferred face-on orientation while in the range of 0 to 1.’ The author should clarify which peak they used to obtain S2D and how the value of γ is defined (whether γ=0 refers to the in-plane direction or the out-of-plane direction). For instance, if the pole figure of P3HT lamellar peak is used to obtain S2D, then the value of S2D can be used to distinguish between face-on and edge-on oriented P3HT crystallites, whereas if the backbone peak of Y6 is used to obtain S2D, then its value distinguishes between face-on and end-on oriented Y6 crystallites. Since the authors claimed in their manuscript that for both 1-D and 2-D molecules, it is the face-on to end-on rotation that determines the light-matter interaction, I think the S2D of all materials included in Fig. 2f should be obtained from the pole figure of the backbone peak in order to facilitate a fair comparison. I understand that this may be difficult, as not all molecules show backbone peaks in their GIWAXS patterns. Nonetheless, in its current form, Fig. 2f does not provide any valuable information that can support the author’s statement.\nResponse: We sincerely thank the reviewer for this insightful comment, which has been most helpful. Firstly, when calculating the orientation proportion, we defined the in-plane orientation as the expected orientation, thus setting the in-plane direction as 0° and the out-of-plane direction as 90°. However, when calculating S , we strictly 2D followed the coordinate system in the reference (Chem. Mater., 2020, 32, 6043-6054), defining the tilt angle of the in-plane direction as 90° and that of the out-of-plane direction as 0°, which essentially treats the edge-on orientation as the expected orientation. We did not provide further clarification on this in the original text, leading to confusion between the definitions of the two coordinate systems. To enhance clarity, we unified the coordinate system for the tilt angle and continued to adopt the coordinate system from the reference, defining the in-plane tilt as 90°. This adjustment allows our S results to be more intuitively validated by the reference. Due to this change in the 2D definition, we have correspondingly modified the horizontal axis of Fig. 2b (Page 11).\n0.20\nY6 (300) - as cast\n0.18\nY6 (300) - TA\no\nita 0.16\nr\nn0.14\no\nita0.12\ntn\ne\nir0.10\nO\n0.08\n0.06\n85 80 75 70 65 60 55 50 45\nAngle (o)\nFig. 2b (300) orientation ratio in the range of 85-45° azimuth angle.\nAdditionally, regarding the suggestion that the S values for all materials should be 2D derived from the pole figure of the backbone peak to ensure a consistent comparison basis, we fully appreciate the rationale. However, as the reviewer correctly noted, obtaining a clear backbone peak signal is challenging for most molecules in our study. The lamellar diffraction peak, however, provides a strong and reliable signal across the dataset.\nConsequently, we made the deliberate choice to use the lamellar peak for the S 2D calculation for all molecules, including Y6, to assess the tilt behavior of the molecular assemblies (Fig. 2f). We believe this extrapolation is justified for the following reasons:\n1. The lamellar packing signals are strong and highly reliable, whereas the scattering signals from the (001) plane, which directly indicates end-on orientation, are generally difficult to observe.\n2. An S value closer to 0 indicates a more isotropic molecular packing (lower 2D crystallinity), which inherently leads to a greater degree of backbone tilting (J. Mater. Chem. A 2023, 11, 26499-26507; Adv. Funct. Mater. 2011, 21, 3697-3705), especially when such changes in S are related to thermal annealing treatment, which is widely 2D recognized as a method that enhances crystallinity.\n3. In the case of Y6, which maintains a three-dimensional network morphology in films (Nat. Commun. 2020, 11, 3943) similar to its single crystal, any out-of-plane tilt of the lamellar stacks inherently necessitates a proportional out-of-plane tilt (i.e., towards end-on) of the constituent molecular backbones.\nWe have thoroughly revised the section “Effect of molecular packing and orientation on reflection intensity” as the following and hope this elaboration meets your approval.\nOn page 9, “For organic conjugated films, the ultrahigh reflection not only originates from molecular conjugation and film thickness, but also is closely related to molecular packing orientation. As shown in the GIWAXS results in Fig. 2a, the Y6 film exhibits a distinct face-on molecular orientation, with its conjugated plane parallel to the substrate and thus perpendicular to the incident light. Furthermore, the reduced broadening of the scattering signals for the (010), (200), and (300) planes after thermal annealing (TA) treatment generally indicate improved molecular orientation uniformity40,41, and the orientation changes of Y6 molecules can be quantified by analysing the azimuthal variation of the scattering signals. Fig. 2b displays the orientation ratios of the lamellar packing (300) plane for the Y6 film before and after TA treatment, where an azimuthal angle of 90° corresponds to the in-plane direction and 0° to the out-of-plane direction. After TA treatment, the increased ratio at larger azimuthal angles (closer to 90°) and the decreased ratio at smaller azimuthal angles (closer to 0°) demonstrate a higher degree of face-on orientation, revealing enhanced molecular order and crystallinity. As shown in Supplementary Fig. 5, the same phenomenon is observed in the GIWAXS results of other materials before and after TA treatment.\nMolecular orientation changes result from rotation along different axes, which in turn leads to variations in light-matter interactions. As illustrated in Fig. 2c, for the Y6 molecular packing configuration after TA treatment, the rotation along the [100] axis induces a “face-on to end-on” orientation transition, while rotation along the [001] axis produces a “face-on to edge-on” transition. For 2D-polarized molecules like Y6, “face-on to end-on” rotation reduces the polarizability projected onto the X-Y plane, thereby weakening light-matter interactions and thus decreasing the reflection intensity. In contrast, for “face-on to edge-on” rotation, the reflection of the molecules is relatively less affected, though still influenced, because the polarizability along the Y-direction is much smaller than that along the X-direction. For 1D-polarized molecules like P3HT, “face-on to end-on” rotation also reduces the polarizability projected onto the X-Y plane, consequently diminishing light-matter interactions. However, due to their 1D-polarization characteristics, “face-on to edge-on” rotation has a negligible effect on light-matter interactions. Besides, regardless of 1D- or 2D-polarization, after TA treatment, most conjugated organic small molecules exhibited an enhanced reflection, and the corresponding reflection curves before and after TA treatment are plotted in Supplementary Fig. 6, where the TA treatment (160 ℃, and higher annealing temperature will affect the composition and flatness, as shown in Supplementary Fig. 7) helps suppress the molecular rotation. As shown in Fig. 2d, the maximum reflection of Y6 is increased from 46.7% to 52.9% after TA treatment, and the maximum refractive index n is also increased from 4.08 to 4.17 (Supplementary Fig. 8).\nIn fact, based on their molecular structures, polymer donor materials such as PM6 and PBDB-T possess a certain degree of 2D-polarizability. However, their optical constants are nearly identical in magnitude to those of 1D-polarized molecules like P3HT. This phenomenon can be partly attributed to their weaker 2D-polarization characteristics compared to molecules with extended 2D-conjugation, such as Y6. Additionally, these polymers tend to adopt edge-on/isotropic molecular packing orientations, where the molecular y-axis remains perpendicular to the electric field vector of incident light. Consequently, polarization along the y-direction remains inactive, resulting in optical properties that more closely resemble those of 1D-polarized molecules.\nThe correlation between molecular packing within the film and the film’s absorption/reflection is illustrated in Fig. 2e. For 2D-polarized small molecules like Y6, the high maximum reflection primarily stems from their highly ordered face-on orientation. As mentioned earlier, edge-on rotation causes a slight reduction in the film’s reflection, while end-on rotation leads to a more pronounced decrease. The S 2D parameter, calculated using the formula S = 2⟨cos2γ⟩ − 1 32, can be employed to 2D quantify the degree of out-of-plane orientation from GIWAXS measurements, where γ represents the angle between the molecular backbone plane and the substrate plane normal. An S value between -1 and 0 indicates a preferred face-on orientation, a value 2D between 0 and 1 indicates a preferred edge-on orientation, and S = 0 represents an 2D isotropic orientation. We selected the lamellar packing scattering signals, specifically the (100) plane or its higher-order diffraction for S calculation for the following 2D reasons: (1) The lamellar packing signals are strong and highly reliable, whereas the scattering signals from the (001) plane, which directly indicates end-on orientation, are generally difficult to observe; (2) Improved lamellar packing uniformity leads to a reduction in isotropic molecules, which then infers to a suppression of the end-on oriented molecules that are the primary factor in reflection degradation, thereby resulting in the enhancement of film reflection.40,42; (3) For molecules like Y6 that feature a three-dimensional network-like packing structure, rotation of the lamellar packing plane also implies an increase in the proportion of “end-on” oriented molecules41. We systematically investigated a series of 1D- and 2D-polarized molecules by comparing their S parameters and maximum reflection before and after 2D TA, as shown in Fig. 2f. The TA treatment increases the absolute S value for all 2D molecules, indicating enhanced orientation uniformity, and the suppression of isotropic and end-on molecular rotation, consequently leading to improved maximum reflection. The representative 2D-polarized molecules, including Y6, IT-4Cl, and IEICO-4F, exhibit maximum reflection exceeding 40% with highly face-on orientation characteristics. In contrast, the other molecules shown in Fig. 2f demonstrate reflection below 35%, primarily due to their 1D-polarization optical properties. These results are in strong agreement with refractive index measurements.”\nComment 3: In comment 5, the authors have corrected the \"nd\" typo in the revised manuscript. However, equation 4 in the main text should also be updated accordingly, as the destructive interference should correspond to 2nd = (m-0.5)λ, where m can be any positive integer.\nResponse: We extend our sincere appreciation to the reviewer for highlighting this inaccuracy. The error occurred due to an omitted factor of 1/2 during the drafting process; our original intention was to express the condition as “2nd = (1/2)(2m - 1)λ”. We confirm that this is fully equivalent to the reviewer's correct formulation, “2nd = (m - 0.5)λ (Page 23, line 552)”, and we have therefore revised Equation 4 to reflect this. We deeply regret this lack of attention to detail, which has imposed an additional burden on the reviewer, and we unreservedly apologize for this mistake.\nAdditional Comments: In addition, there are some more typos in the main text that should be corrected.\nComment 1: In line 406 of the revised manuscript, the author stated that ‘This discrepancy is mainly due to the multi-conferences between reflections at the interfaces of different optical mediums.’ I think the author meant to say ‘multi-interference’ instead of ‘multi-conferences’. Please check.\nComment 2: In line 451 of the revised manuscript, the author stated that ‘The strong top-reflection induced secondary transmission (T2) with a shape of reversed film absorption caused by unsaturated light absorption is significantly larger than the active layer top reflection (T2 >> R1).’ Based on the definition mentioned in previous paragraphs, R1 should be the bottom reflection instead of the top reflection. Please check.\nResponse: We are truly grateful that you have caught these typos in our text. We would like to extend our profound apologies for these careless mistakes—the incorrect use of “multiple-conference” instead of “multiple-interference (line 418)” and “top reflection” instead of “bottom reflection (line 462)”, which escaped our attention during proofreading. Accordingly, we have thoroughly scrutinized the full text again during this revision round and have made further corrections as listed below. We once again extend our utmost appreciation to the reviewer and the editor for your remarkable patience and diligence, and for the invaluable time and effort dedicated to improving our work.\nThrough multiple rounds of proofreading the manuscript, we have also identified and corrected the following errors.\n1. On page 3, line 67, “highly reflective bottom metal electrode”.\n2. On page 4, line 105, “The total absorption/reflection of the OSCs with varying active layer thicknesses evolve from stage I to IV: thin-film bottom reflection, interference transition, thick-film interference and thick-film surface reflection.”\n3. On page 5, line 141, “(n = 1, k = 0)”.\nair air\n4. On pages 19 and 20, all confusion regarding “top reflection” and “bottom reflection” were corrected.\n5. On pages 19 and 20, we have redefined the film thickness ranges for the four stages of reflection evolution. Compared to the original definition, the new ranges now cover the full thickness range.\n“Stage I: Thin-film (< 70 nm) bottom reflection (Ag/active layer interface).\nStage II: Interference-transition (70 - 90 nm).\nStage III: Thick-film interference (90 - 150 nm).\nStage IV: Thick-film (> 150 nm) surface reflection (ITO/active layer interface).”\n6. On pages 19 and 23, we provide the full form when an abbreviation first appears.\n“the total absorption of the device can be obtained by subtracting the total reflection from the incident light (I - R ).”\nin total\n“device total absorption (A )”\ntotal\n“optimal thickness (d )”\nopt\n7. On page 1, we have undertaken a revision of the manuscript’s abstract to ensure greater coherence and improved clarity.\n“In organic solar cells (OSCs), which represent a quintessential application system for organic conjugated films, the intrinsic light reflection of these films significantly influences the optical performance of the devices through the Fabry–Pérot micro-cavity effect. However, this phenomenon has not been comprehensively investigated. This study demonstrates that the film’s intrinsic reflection arises from the light-matter interaction, which is mainly governed by the polarizability of delocalized electron cloud and the orientation of the conjugated backbone. Furthermore, the multi-level interference within the OSC micro-cavity leads to a dependence of both the reflection properties and the short-circuit current density (J ) on the active layer thickness. Based SC on these insights, a strategy for synergistic optimization of device performance through precise tailoring of optical constants and the micro-cavity structure is proposed. By systematically analyzing the intrinsic reflection behavior of organic conjugated films, this study enhances the understanding of the role reflection plays in the performance of photovoltaic devices and provides theoretical support for further optical optimization of organic photovoltaics.”\nResponse to Reviewer #3\nReviewer Comments: The manuscript is well revised by following comments. So, it is good enough to be published in this journal.\nResponse: We sincerely appreciate your expert review, which has significantly improved our manuscript. Thank you for the considerable time and effort you have invested, as well as for your endorsement of our work.\nResponse to Reviewer #4\nReviewer Comments: I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts.\nResponse: We extend our sincere appreciation to you and the co-reviewer for the time and dedicated effort you have devoted to providing valuable insights on our manuscript."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The manuscript has been revised according to the reviewer's suggestions. and I have no further comments.", "reviewer_2": "The authors have satisfactorily addressed my comments 1, 3, and 4. However, comments 2 and 5 were partially resolved and require further clarification.\n1. In comment 2, I mentioned that the 2D orientation parameter S2D should be defined as 2cos^2(γ)-1 according to the reference 32 [Chemistry of Materials, 2020, 32(14): 6043-6054]. However, I noted that the S2D is still defined as 2cos(2γ)-1 in line 268 of the revised manuscript. The author is recommended to carefully read ref 32 before adopting this method.\n2. In Fig. 2f of the manuscript, the author obtained S2D for a range of materials and defined the physical meaning of S2D as ‘S2D value in the range of -1 to 0 represents preferred face-on orientation while in the range of 0 to 1.’ The author should clarify which peak they used to obtain S2D and how the value of γ is defined (whether γ=0 refers to the in-plane direction or the out-of-plane direction). For instance, if the pole figure of P3HT lamellar peak is used to obtain S2D, then the value of S2D can be used to distinguish between face-on and edge-on oriented P3HT crystallites, whereas if the backbone peak of Y6 is used to obtain S2D, then its value distinguishes between face-on and end-on oriented Y6 crystallites. Since the authors claimed in their manuscript that for both 1-D and 2-D molecules, it is the face-on to end-on rotation that determines the light-matter interaction, I think the S2D of all materials included in Fig. 2f should be obtained from the pole figure of the backbone peak in order to facilitate a fair comparison. I understand that this may be difficult, as not all molecules show backbone peaks in their GIWAXS patterns. Nonetheless, in its current form, Fig. 2f does not provide any valuable information that can support the author’s statement.\n3. In comment 5, the authors have corrected the \"nd\" typo in the revised manuscript. However, equation 4 in the main text should also be updated accordingly, as the destructive interference should correspond to 2nd = (m-0.5)λ, where m can be any positive integer.\nIn addition, there are some more typos in the main text that should be corrected.\n1. In line 406 of the revised manuscript, the author stated that ‘This discrepancy is mainly due to the multi-conferences between reflections at the interfaces of different optical mediums.’ I think the author meant to say ‘multi-interference’ instead of ‘multi-conferences’. Please check.\n2. In line 451 of the revised manuscript, the author stated that ‘The strong top-reflection induced secondary transmission (T2) with a shape of reversed film absorption caused by unsaturated light absorption is significantly larger than the active layer top reflection (T2 >> R1).’ Based on the definition mentioned in previous paragraphs, R1 should be the bottom reflection instead of the top reflection. Please check.", "reviewer_3": "The manuscript is well revised by following comments. So, it is good enough to be published in this journal.", "reviewer_4": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts."}, "author_response": "Response to Reviewers\nResponse to Reviewer #1\nReviewer Comments: The manuscript has been revised according to the reviewer's suggestions. and I have no further comments.\nResponse: We sincerely thank the reviewer for the positive feedback regarding our revised manuscript. We are delighted to hear that the revisions made in response to the reviewer's constructive comments have been met with approval.\nResponse to Reviewer #2\nReviewer Comments: The authors have satisfactorily addressed my comments 1, 3, and 4. However, comments 2 and 5 were partially resolved and require further clarification.\nResponse: We extend our sincere gratitude to the reviewer for their insightful feedback. While we are delighted that our responses to Comments 1, 3, and 4 gained their approval, we regret that our initial revisions for Comments 2 and 5 fell short of expectations, being somewhat perfunctory and lacking in depth. In light of the reviewer’s valid critique, a dedicated re-examination was undertaken. The substantially revised responses below are consequently provided, and it is our sincere hope that they will prove satisfactory.\nComment 1: In comment 2, I mentioned that the 2D orientation parameter S2D should be defined as 2cos^2(γ)-1 according to the reference 32 [Chemistry of Materials, 2020, 32(14): 6043-6054]. However, I noted that the S2D is still defined as 2cos(2γ)-1 in line 268 of the revised manuscript. The author is recommended to carefully read ref 32 before adopting this method.\nResponse: We are grateful to the reviewer for identifying the error in our manuscript. We acknowledge that the formula was incorrectly stated and should read “S = 2⟨cos2γ⟩ 2D - 1 (Page 10, line 265)”. The necessary correction has been made in the text, and we sincerely apologize for this mistake.\nComment 2: In Fig. 2f of the manuscript, the author obtained S2D for a range of materials and defined the physical meaning of S2D as ‘S2D value in the range of -1 to 0 represents preferred face-on orientation while in the range of 0 to 1.’ The author should clarify which peak they used to obtain S2D and how the value of γ is defined (whether γ=0 refers to the in-plane direction or the out-of-plane direction). For instance, if the pole figure of P3HT lamellar peak is used to obtain S2D, then the value of S2D can be used to distinguish between face-on and edge-on oriented P3HT crystallites, whereas if the backbone peak of Y6 is used to obtain S2D, then its value distinguishes between face-on and end-on oriented Y6 crystallites. Since the authors claimed in their manuscript that for both 1-D and 2-D molecules, it is the face-on to end-on rotation that determines the light-matter interaction, I think the S2D of all materials included in Fig. 2f should be obtained from the pole figure of the backbone peak in order to facilitate a fair comparison. I understand that this may be difficult, as not all molecules show backbone peaks in their GIWAXS patterns. Nonetheless, in its current form, Fig. 2f does not provide any valuable information that can support the author’s statement.\nResponse: We sincerely thank the reviewer for this insightful comment, which has been most helpful. Firstly, when calculating the orientation proportion, we defined the in-plane orientation as the expected orientation, thus setting the in-plane direction as 0° and the out-of-plane direction as 90°. However, when calculating S , we strictly 2D followed the coordinate system in the reference (Chem. Mater., 2020, 32, 6043-6054), defining the tilt angle of the in-plane direction as 90° and that of the out-of-plane direction as 0°, which essentially treats the edge-on orientation as the expected orientation. We did not provide further clarification on this in the original text, leading to confusion between the definitions of the two coordinate systems. To enhance clarity, we unified the coordinate system for the tilt angle and continued to adopt the coordinate system from the reference, defining the in-plane tilt as 90°. This adjustment allows our S results to be more intuitively validated by the reference. Due to this change in the 2D definition, we have correspondingly modified the horizontal axis of Fig. 2b (Page 11).\n0.20\nY6 (300) - as cast\n0.18\nY6 (300) - TA\no\nita 0.16\nr\nn0.14\no\nita0.12\ntn\ne\nir0.10\nO\n0.08\n0.06\n85 80 75 70 65 60 55 50 45\nAngle (o)\nFig. 2b (300) orientation ratio in the range of 85-45° azimuth angle.\nAdditionally, regarding the suggestion that the S values for all materials should be 2D derived from the pole figure of the backbone peak to ensure a consistent comparison basis, we fully appreciate the rationale. However, as the reviewer correctly noted, obtaining a clear backbone peak signal is challenging for most molecules in our study. The lamellar diffraction peak, however, provides a strong and reliable signal across the dataset.\nConsequently, we made the deliberate choice to use the lamellar peak for the S 2D calculation for all molecules, including Y6, to assess the tilt behavior of the molecular assemblies (Fig. 2f). We believe this extrapolation is justified for the following reasons:\n1. The lamellar packing signals are strong and highly reliable, whereas the scattering signals from the (001) plane, which directly indicates end-on orientation, are generally difficult to observe.\n2. An S value closer to 0 indicates a more isotropic molecular packing (lower 2D crystallinity), which inherently leads to a greater degree of backbone tilting (J. Mater. Chem. A 2023, 11, 26499-26507; Adv. Funct. Mater. 2011, 21, 3697-3705), especially when such changes in S are related to thermal annealing treatment, which is widely 2D recognized as a method that enhances crystallinity.\n3. In the case of Y6, which maintains a three-dimensional network morphology in films (Nat. Commun. 2020, 11, 3943) similar to its single crystal, any out-of-plane tilt of the lamellar stacks inherently necessitates a proportional out-of-plane tilt (i.e., towards end-on) of the constituent molecular backbones.\nWe have thoroughly revised the section “Effect of molecular packing and orientation on reflection intensity” as the following and hope this elaboration meets your approval.\nOn page 9, “For organic conjugated films, the ultrahigh reflection not only originates from molecular conjugation and film thickness, but also is closely related to molecular packing orientation. As shown in the GIWAXS results in Fig. 2a, the Y6 film exhibits a distinct face-on molecular orientation, with its conjugated plane parallel to the substrate and thus perpendicular to the incident light. Furthermore, the reduced broadening of the scattering signals for the (010), (200), and (300) planes after thermal annealing (TA) treatment generally indicate improved molecular orientation uniformity40,41, and the orientation changes of Y6 molecules can be quantified by analysing the azimuthal variation of the scattering signals. Fig. 2b displays the orientation ratios of the lamellar packing (300) plane for the Y6 film before and after TA treatment, where an azimuthal angle of 90° corresponds to the in-plane direction and 0° to the out-of-plane direction. After TA treatment, the increased ratio at larger azimuthal angles (closer to 90°) and the decreased ratio at smaller azimuthal angles (closer to 0°) demonstrate a higher degree of face-on orientation, revealing enhanced molecular order and crystallinity. As shown in Supplementary Fig. 5, the same phenomenon is observed in the GIWAXS results of other materials before and after TA treatment.\nMolecular orientation changes result from rotation along different axes, which in turn leads to variations in light-matter interactions. As illustrated in Fig. 2c, for the Y6 molecular packing configuration after TA treatment, the rotation along the [100] axis induces a “face-on to end-on” orientation transition, while rotation along the [001] axis produces a “face-on to edge-on” transition. For 2D-polarized molecules like Y6, “face-on to end-on” rotation reduces the polarizability projected onto the X-Y plane, thereby weakening light-matter interactions and thus decreasing the reflection intensity. In contrast, for “face-on to edge-on” rotation, the reflection of the molecules is relatively less affected, though still influenced, because the polarizability along the Y-direction is much smaller than that along the X-direction. For 1D-polarized molecules like P3HT, “face-on to end-on” rotation also reduces the polarizability projected onto the X-Y plane, consequently diminishing light-matter interactions. However, due to their 1D-polarization characteristics, “face-on to edge-on” rotation has a negligible effect on light-matter interactions. Besides, regardless of 1D- or 2D-polarization, after TA treatment, most conjugated organic small molecules exhibited an enhanced reflection, and the corresponding reflection curves before and after TA treatment are plotted in Supplementary Fig. 6, where the TA treatment (160 ℃, and higher annealing temperature will affect the composition and flatness, as shown in Supplementary Fig. 7) helps suppress the molecular rotation. As shown in Fig. 2d, the maximum reflection of Y6 is increased from 46.7% to 52.9% after TA treatment, and the maximum refractive index n is also increased from 4.08 to 4.17 (Supplementary Fig. 8).\nIn fact, based on their molecular structures, polymer donor materials such as PM6 and PBDB-T possess a certain degree of 2D-polarizability. However, their optical constants are nearly identical in magnitude to those of 1D-polarized molecules like P3HT. This phenomenon can be partly attributed to their weaker 2D-polarization characteristics compared to molecules with extended 2D-conjugation, such as Y6. Additionally, these polymers tend to adopt edge-on/isotropic molecular packing orientations, where the molecular y-axis remains perpendicular to the electric field vector of incident light. Consequently, polarization along the y-direction remains inactive, resulting in optical properties that more closely resemble those of 1D-polarized molecules.\nThe correlation between molecular packing within the film and the film’s absorption/reflection is illustrated in Fig. 2e. For 2D-polarized small molecules like Y6, the high maximum reflection primarily stems from their highly ordered face-on orientation. As mentioned earlier, edge-on rotation causes a slight reduction in the film’s reflection, while end-on rotation leads to a more pronounced decrease. The S 2D parameter, calculated using the formula S = 2⟨cos2γ⟩ − 1 32, can be employed to 2D quantify the degree of out-of-plane orientation from GIWAXS measurements, where γ represents the angle between the molecular backbone plane and the substrate plane normal. An S value between -1 and 0 indicates a preferred face-on orientation, a value 2D between 0 and 1 indicates a preferred edge-on orientation, and S = 0 represents an 2D isotropic orientation. We selected the lamellar packing scattering signals, specifically the (100) plane or its higher-order diffraction for S calculation for the following 2D reasons: (1) The lamellar packing signals are strong and highly reliable, whereas the scattering signals from the (001) plane, which directly indicates end-on orientation, are generally difficult to observe; (2) Improved lamellar packing uniformity leads to a reduction in isotropic molecules, which then infers to a suppression of the end-on oriented molecules that are the primary factor in reflection degradation, thereby resulting in the enhancement of film reflection.40,42; (3) For molecules like Y6 that feature a three-dimensional network-like packing structure, rotation of the lamellar packing plane also implies an increase in the proportion of “end-on” oriented molecules41. We systematically investigated a series of 1D- and 2D-polarized molecules by comparing their S parameters and maximum reflection before and after 2D TA, as shown in Fig. 2f. The TA treatment increases the absolute S value for all 2D molecules, indicating enhanced orientation uniformity, and the suppression of isotropic and end-on molecular rotation, consequently leading to improved maximum reflection. The representative 2D-polarized molecules, including Y6, IT-4Cl, and IEICO-4F, exhibit maximum reflection exceeding 40% with highly face-on orientation characteristics. In contrast, the other molecules shown in Fig. 2f demonstrate reflection below 35%, primarily due to their 1D-polarization optical properties. These results are in strong agreement with refractive index measurements.”\nComment 3: In comment 5, the authors have corrected the \"nd\" typo in the revised manuscript. However, equation 4 in the main text should also be updated accordingly, as the destructive interference should correspond to 2nd = (m-0.5)λ, where m can be any positive integer.\nResponse: We extend our sincere appreciation to the reviewer for highlighting this inaccuracy. The error occurred due to an omitted factor of 1/2 during the drafting process; our original intention was to express the condition as “2nd = (1/2)(2m - 1)λ”. We confirm that this is fully equivalent to the reviewer's correct formulation, “2nd = (m - 0.5)λ (Page 23, line 552)”, and we have therefore revised Equation 4 to reflect this. We deeply regret this lack of attention to detail, which has imposed an additional burden on the reviewer, and we unreservedly apologize for this mistake.\nAdditional Comments: In addition, there are some more typos in the main text that should be corrected.\nComment 1: In line 406 of the revised manuscript, the author stated that ‘This discrepancy is mainly due to the multi-conferences between reflections at the interfaces of different optical mediums.’ I think the author meant to say ‘multi-interference’ instead of ‘multi-conferences’. Please check.\nComment 2: In line 451 of the revised manuscript, the author stated that ‘The strong top-reflection induced secondary transmission (T2) with a shape of reversed film absorption caused by unsaturated light absorption is significantly larger than the active layer top reflection (T2 >> R1).’ Based on the definition mentioned in previous paragraphs, R1 should be the bottom reflection instead of the top reflection. Please check.\nResponse: We are truly grateful that you have caught these typos in our text. We would like to extend our profound apologies for these careless mistakes—the incorrect use of “multiple-conference” instead of “multiple-interference (line 418)” and “top reflection” instead of “bottom reflection (line 462)”, which escaped our attention during proofreading. Accordingly, we have thoroughly scrutinized the full text again during this revision round and have made further corrections as listed below. We once again extend our utmost appreciation to the reviewer and the editor for your remarkable patience and diligence, and for the invaluable time and effort dedicated to improving our work.\nThrough multiple rounds of proofreading the manuscript, we have also identified and corrected the following errors.\n1. On page 3, line 67, “highly reflective bottom metal electrode”.\n2. On page 4, line 105, “The total absorption/reflection of the OSCs with varying active layer thicknesses evolve from stage I to IV: thin-film bottom reflection, interference transition, thick-film interference and thick-film surface reflection.”\n3. On page 5, line 141, “(n = 1, k = 0)”.\nair air\n4. On pages 19 and 20, all confusion regarding “top reflection” and “bottom reflection” were corrected.\n5. On pages 19 and 20, we have redefined the film thickness ranges for the four stages of reflection evolution. Compared to the original definition, the new ranges now cover the full thickness range.\n“Stage I: Thin-film (< 70 nm) bottom reflection (Ag/active layer interface).\nStage II: Interference-transition (70 - 90 nm).\nStage III: Thick-film interference (90 - 150 nm).\nStage IV: Thick-film (> 150 nm) surface reflection (ITO/active layer interface).”\n6. On pages 19 and 23, we provide the full form when an abbreviation first appears.\n“the total absorption of the device can be obtained by subtracting the total reflection from the incident light (I - R ).”\nin total\n“device total absorption (A )”\ntotal\n“optimal thickness (d )”\nopt\n7. On page 1, we have undertaken a revision of the manuscript’s abstract to ensure greater coherence and improved clarity.\n“In organic solar cells (OSCs), which represent a quintessential application system for organic conjugated films, the intrinsic light reflection of these films significantly influences the optical performance of the devices through the Fabry–Pérot micro-cavity effect. However, this phenomenon has not been comprehensively investigated. This study demonstrates that the film’s intrinsic reflection arises from the light-matter interaction, which is mainly governed by the polarizability of delocalized electron cloud and the orientation of the conjugated backbone. Furthermore, the multi-level interference within the OSC micro-cavity leads to a dependence of both the reflection properties and the short-circuit current density (J ) on the active layer thickness. Based SC on these insights, a strategy for synergistic optimization of device performance through precise tailoring of optical constants and the micro-cavity structure is proposed. By systematically analyzing the intrinsic reflection behavior of organic conjugated films, this study enhances the understanding of the role reflection plays in the performance of photovoltaic devices and provides theoretical support for further optical optimization of organic photovoltaics.”\nResponse to Reviewer #3\nReviewer Comments: The manuscript is well revised by following comments. So, it is good enough to be published in this journal.\nResponse: We sincerely appreciate your expert review, which has significantly improved our manuscript. Thank you for the considerable time and effort you have invested, as well as for your endorsement of our work.\nResponse to Reviewer #4\nReviewer Comments: I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts.\nResponse: We extend our sincere appreciation to you and the co-reviewer for the time and dedicated effort you have devoted to providing valuable insights on our manuscript."}, {"round": 3, "reviewer_comments": {"reviewer_2": "The authors have addressed my comments.", "reviewer_4": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts."}, "author_response": ""}]

Decision letter