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J. Eur. Opt. Society-Rapid Publ. 22, 38( 2026)
Figure 3. Differential absorption efficiency of the nanograting PSC as a function of both the ratio height / width of the stripes and the unit cell size or period. The width of the stripes has been fixed to a value of w = 100 nm. The maximum( 91.14 %) is shown at 190 nm period and a ratio of 0.7, represented against the reference( 84.05 %).
wavelengths of indoor LEDs, as shown in Figure 1b. Furthermore, the size of the unit cell( p) and the remaining thickness of the planar section of the HTL( h PEDOT: PSS) are also included. For the discretization, auto non-uniform mesh( mesh accuracy 4) was selected. To improve the convergence of the results, a refinement mesh in the grating was used with a 5 nm size along the x-, y- andz-axis.
We parametrically investigated the nanograting’ sgeo- metrical characteristics( like shape and size) to find the optimal design for enhancing light absorption within specific spectral ranges. To do this, we defined an applicationoriented absorption efficiency that specifically weights the absorption of the cell by the actual measured indoor LED light spectrum( as shown in Fig. 1b). This weighting, formalized in equation( 2), ensures that the calculated efficiency reflects the true performance of the device in indoor real-world conditions.
g ¼
Z kf
k
Z
0 kf
k 0
AbsðkÞIrrðkÞdk
IrrðkÞdk
: ð2Þ
Quantification of this absorption enhancement is accomplished through the introduction of a Figure of Merit( FOM) formally defined as the differential absorption efficiency( in percentage) as shown in equation( 3):
FOM ¼ g � g 0 g 0
100: ð3Þ
Being g and g 0 the absorption efficiency with and without the integration of the nanograting structure, respectively.
Figure 3 shows a 3D plot of the percentage differential absorption efficiency versus the geometric dimensions of
Figure 4. Active layer absorbance for the planar and optimized structures is presented as a function of incident wavelength( k). The characterized LED emission spectrum is superimposed to assess the resulting performance enhancement under the specified illumination.
the structure. To facilitate the concise presentation and clarification of the data, the derived results are graphically represented as a function of the following geometric parameters: the aspect ratio of the stripes, defined as the height-width ratio and the period of the modeled unit cell( see Fig. 2). The plot demonstrates a continuous consistent response, showing that the magnitude of the defined FOMthe differential absorption efficiency-is positively correlated with the increasing period in the studied interval( 100 nm – 200 nm) except for a short range close to 200 nm at low aspect ratio, while it first increases, and then decreases with decreasing ratio. A peak value of 7.09 percentage points constitutes the maximum observed, in a ratio of h / w = 0.7 and a period of 190 nm. This optimum case, highlighted in the Figure 3, particularly considers w = 100 nm, being the width for which the best result has been achieved. Moreover, it is worth mentioning that this result is specific to the active layer, indicating a potential similar improvement in the generated photocurrent.
The active layer absorbance spectrum for both the planar reference structure and the proposed nanograting structure integrated into the HTL is shown in Figure 4. Nearfield profiles obtained resolve a field redistribution around the nanograting that increases the light intensity in the active layer, and thus, absorption. The nanograting primarily improves the overall absorbance for incident wavelengths( k) less than 675 nm, although it causes a degradation in performance for k > 675 nm. Crucially, the performance of the PSC is significantly improved within the specific spectral region of interest under LED illumination, particularly due to a pronounced increase in the absorption in the active layer within the k interval of 520 nm to 620 nm. This region directly aligns with a major LED emission peak at 600 nm. Although x-polarized light shows a better response, y-polarized one also slightly enhances the planar case around similar wavelength intervals, with an increment of 4.59 percentage points. Thus,