1. Introduction

Nowadays, low production cost and high manufacturability or compatibility is the main orientations in the development of photovoltaic field. In the past decades, organic solar cells (OSCs), owing to their unique superiority such as abundant material source, low cost, large area production, good flexibility, etc., have attracted intensive attentions as a promising alternative effective photovoltaic technology compared to the conventional inorganic solar cells [1–4]. Recently, the improved power conversion efficiency (PCE) of OSCs (8~19%) [5–8] has been demonstrated in a few progress and industrial investment in high throughput manufacturing of OSCs with low cost has already emerged [9], leading to potential applications of OSCs including solar plants, portable electronic devices, etc. However, compared to their inorganic counterparts, the PCEs of organic OSCs are still significantly low due to the insufficient absorption in thin film OSCs devices with limited featured thickness of active layer (~100 nm). This limitation arises from the intrinsic disadvantageous properties of organic materials, i.e., relatively short exciton diffusion length and low carriers’ mobility of organic semiconductors [10,11].

For further improving the low absorption in OSCs, one key way is to enlarge the light harvesting efficiency in thin active layer to the hilt without increasing its thickness. Recently, a large amount of light trapping methods with different forms of nanostructures have been proposed for improving light absorption, including photonic crystals (PCs) [12–14], antireflection coatings [15], photonic fiber plates [16,17], and metallic nanostructures [18,19]. Among them, the utilization of nanostructures in single or tandem OSCs has been reported as a superior approach to improve the efficiency in devices performance [20,21], mainly attributed to the prolonged optical paths of the incident light in the active layer by scattering metallic nanoparticles (MNPs) [22,23], in addition to the enhanced electro-magnetic (EM) fields near the dielectric-metal interface due to the excitation of surface plasmons (SPs) and (or) photonic modes with the form of hybridization of cavity and Bloch modes. MNPs-incorporated OSCs has advantages in fabrication and diverse forms of light trapping components, however, embedding the solution-processed MNPs into the OSCs should inevitably encounter the possibility of exciton quenching effect in the active layer [24,25]. Also, this process usually involves large amount of solvents and needs to use surfactants to avoid aggregation among the MNPs, resulting in contamination problems from impurities and the hard handling in the control of distribution of MNPs during a spin coating process [21,26].

An alternative promising way, of interest here, relies on the strengthening of EM field intensity on top of the grating ridges and (or) within the grating grooves based on the excitation of SPPs by phase matching, leading to the manipulation of light in the subwavelength scale and the potential capability for effective light trapping in organic active material [27–31]. The absorption performance of OSCs with presence of metallic nanograting has been widely investigated theoretically and experimentally [32–36], owing to the development of nanofabrication technology for nanostructures such as nanoimprinting, cross-beam lithography and etching technology [37–40]. The distinct resonances of metal gratings-incorporated OSCs device can be tuned by adjusting the geometric parameters of gratings, such as the period, the depth of grating groove or the width of the grating ridge as well as the locating position of gratings [29,41,42]. On the basis of the properties of grating-incorporated nanostructures, many different designs have been proposed to realize the application of broadband absorption, such as multi-sized metal/dielectric/metal (MDM) structures [31], tapered metal-dielectric multilayer gratings, ultra-sharp convex metallic grooves [43], and nanowire arrays [44]. The absorption enhancement of these structures can be attributed to the combination of cavities modes and localized surface plasmons (LSPs), which can be tuned by changing the size of the metal strips. Meanwhile, structures with multi-sized patches in one unit cell (such as strips, circles, squares) have been utilized to realize broadband absorption with multi-resonances. Recently, gratings-incorporated OSCs device with the width of grooves a few nanometers were investigated to induce the complete absorption with a broad bandwidth [45,46]. Although, in short-pitched gratings the enhanced EM-filed corresponding to the excitation of LSPs, also known as hot spots, is much larger than that in the gratings with period in the scale of hundred nanometers and results in the enhancement of absorption, however, the manufacturing of short-pitched metallic grating still remains challenging.

To obtain a wide-angle broad band OSCs device covering the visible and near infrared bands, here we propose a one-dimensional OSCs structure based on the combination of SPs modes and cavity modes in the metallic nanogratings in the scale of hundred nanometers. The proposed structure consists of three grooves with different depth in one unit cell. The design principle of the broadband absorption performance arises from the decomposition of broadband absorption to multiple absorption bands, which correspond to the resonances of diverse grating grooves. In this work, a two-dimensional finite-difference time-domain (2D-FDTD) was adopted to investigate the absorption performance of the proposed structure. In particular, we focus on demonstrating how the grating parameters and the number of grooves influence the absorption performance. The physical mechanism of multiple-resonances under transverse-magnetic (TM) polarization was exhibited by solving the dispersion relationship of the system. Through the theoretical study, the results show that the proposed grating-incorporated OSCs device can maintain high enhancement factor of optical absorption within the large spectral region from 350 nm to 900 nm compared to either the planar control device or the structures with uniform grating grooves in identical period. In this work, we distinguish the respective contributions from the SPs modes and the cavity modes. The design and findings of this work can exploit further applications of metallic gratings in OSCs devices and we believe this will be of great importance in future photonics applications in energy harvesting or photon detecting.

2. Structure and method

The two-dimensional schematic configuration of a metal single-groove structure (SGS) is shown in Fig. 1(a). The PTB7:PC₇₀BM with 1:1.5 weight ratio acts as the active material and has two parts in the structure, including a homogenous layer with thickness denoted by t₁ and a grating structure with period, width, and depth denoted by p, w, d, respectively. Indium-tin-oxide (ITO) is used as anode and the adjacent PEDOT:PSS material is used as a hole transport layer. The thickness of these two materials are set as t₃ = 100 nm and t₂ = 50 nm, respectively, and they remain unchanged in the following calculations. Silver is selected for the counterpart of grating as light trapping elements as well as the back-reflecting cathode with whole thickness of 500 nm for preventing any transmission. In simulation, the wavelength-dependent complex optical constants of PTB7:PC₇₀BM and PEDOT:PSS are extracted from experiment results [46,47], as shown in Fig. 1(b). The maximum of extinction coefficient of the mixture can be observed around the wavelength of 670 nm, which results from the absorption properties of donor material (PTB7). Actually, the highest occupied and lowest unoccupied molecular orbital (HOMO and LUMO) energy levels of PTB7 are −5.15 eV and −3.31 eV, respectively [48]. The transition wavelength corresponding to the band gap (1.84 eV) of PTB7 locates at 674 nm. Therefore, the extinction coefficient of PTB7:PC₇₀BM shown in Fig. 1(b) can reach its maximum value at this wavelength and has a drastically drop with the increase of incident wavelength. The optical constants of ITO and silver were taken from Ref [49]. and [50], respectively.

 

Fig. 1 (a) Schematic diagram of gratings-incorporated OSCs device. (b) The refractive index (n) and extinction coefficient (κ) of the organic materials. The dotted box in x-y plane in (a) is a unit cell used in simulation.

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Numerical simulations of the absorption spectrum or distribution of EM-field were conducted using finite-difference time-domain (FDTD) method. This method has been widely adopted in the calculation of electromagnetic field distribution in optical and photoelectric devices [51,52]. The device was illuminated from the air and the perfect matched layers (PMLs) boundary conditions were used along the y direction. Since the device was modeled as a periodic structure along the x direction, indicated by the dotted box in Fig. 1(a), in all the calculations we used periodic boundary conditions in $\pm x$direction at normal incidence, while at oblique incidence the Bloch boundary conditions were adopted. A mesh size of 2 nm was used for homogenous part of active layer and larger mesh sizes were used for other areas. The incident light is a plane wave with the wavelength ranging from 350 to 900 nm. In calculation, we only investigated the performances of device at TM polarization (with the magnetic field polarized along z axis). The absorption efficiency in the active blend at a certain wavelength, denoted by Abs, can be directly extracted once the electric field was calculated. The calculation can be performed as

3. Results and discussion

In order to study the influence of the basic parameters of the grating to the absorption performance, the OSCs device with grating consisting of one groove in one unit cell was investigated firstly at normal incidence. In this calculation, the thickness of the homogenous part of active layer was set to be 20 nm. The other parameters of the grating were selected as follows: p = 390 nm, w = 100 nm, and d = 160 nm. The calculated results of the single groove structure (SGS) are shown in Fig. 2(a). In this absorption spectrum, four obvious absorption peaks can be found: The first absorption peak at 432 nm with relatively strong efficiency is mainly caused by excitation of the propagating surface plasmons (SPs) modes on the interface between the Ag grating ridges and the homogeneous active layer, as shown in Fig. 2(c). This excitation of SPs modes at normal incidence reveals that the dispersion diagram of SPs modes for this SGS lies within the light cone and satisfies the momentum matching condition, enabling the trapping of incident light mostly into the homogenous part of active layer. The second absorption peak locates at wavelengths around 530 nm, which arises from both the propagating SPs modes close to the upper corners of grating groove and the cavity modes formed by the coupling of SPs modes inside the grating groove, as shown in Fig. 2(d). The third absorption peak around 680 nm is caused by the propagating SPs modes, and the enhanced EM fields at this resonance distribute in a thin region of homogeneous active layer, as shown in Fig. 2(e), which leads to the limited enhancement of the absorption efficiency compared to that of the nearby wavelengths. The last absorption peak located at 746 nm is almost attributed to the excitation of cavity mode in grating groove constructed by the interference between the SPs modes propagating along the –y direction and the reflected one from the bottom of grating groove, as shown in Fig. 2(f). Moreover, one absorption valley appears at 713 nm with relatively less absorption efficiency (only 9.3%). The distribution of magnetic field at 713 nm shown in Fig. 2(a) is mainly concentrated on the PTB7:PC₇₀BM/Ag interface and partly extends to PEDOT:PSS, meanwhile the extinction coefficient of blend decreases drastically, so that the light is prevented from approaching the groove bottom and the decline in the absorption spectrum of active layer can be found.

 

Fig. 2 (a) Absorption spectrum of SGS with p = 390 nm, w = 100 nm, d = 160 nm. (b) Absorption performance of s SGS as a function of the groove depth (b). The magnetic field distribution at 713 nm is shown in the inset of (a). (c)-(f) are electric field distributions corresponding to four absorption peaks of the SGS-based device at 432 nm, 530 nm, 680 nm, and 746 nm, respectively.

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The distinct resonance peaks in the spectrum can be tuned by adjusting the grating parameters, meanwhile, the properties of cavity modes supported the grating groove corresponding to the second and fourth peaks are dependent on the length of the cavity. Therefore, the absorption performance of the SGS as a function of the groove depth ranging from 5 nm to 300 nm was calculated to investigate the shift mechanism of resonance location and the results are shown in Fig. 2(b). It can be found that the strong absorption is chiefly distributed within the wavelength range of 350 nm to 700 nm in spite of the variation of groove depth. In particular, the strong absorption efficiency located between 400 nm to 450 nm remains unchanged for the whole groove depth investigated. This is because that the matching condition of propagating SPs mode determined by the period of the grating cell keeps unchanged in simulation. In contrary, when the groove depth is increased, the wavelengths of the second and fourth absorption peaks mainly associated with the cavity modes exhibit simultaneously a general property of red shifting from 450 nm to 600 nm and 720 nm to 820 nm, respectively. This sifting feature of the single-groove metal grating is consistent with the previous work [53], meanwhile, in addition, it demonstrates periodicity when the groove depth increases. In detail, the second absorption peak shows relatively strong efficiency when the groove depth changes from 10 nm to 90 nm, and the absorption efficiency decreases when this resonance undergoes other shifting routes. However, different behavior can be observed in the shifting routes of the fourth absorption peak compared to that of the second one. The wavelength of this absorption peak shows linear dependence on the groove depth and the relatively strong absorption efficiency appears with the groove depth in the range of 140 nm to 210 nm, which is sufficiently large to support the cavity mode with multiple entities of half-wavelengths distributing inside the groove. These resonances can lead to high absorption values, although their role in the enhancement of local EM field is limited, leading to an enhancement factor of 2~4 times when compared to the 10-folds increase due to the excitation of SPs modes [54]. The nature thought arises with respect to the above results and following considerations. For improving the absorption efficiency over the investigated wavelength range (especially in the range from 650 to 700 nm) with broad bandwidth, it is a promising way to construct an grating-incorporated OSCs device containing more than one groove with diverse depth in one grating cell in order to taking advantage of multiple resonances, such as the SPs mode or the cavity mode.

For the seek of unveiling the mechanism of strong and broad absorption in the OSCs device based on the metallic grating with diverse grooves in one cell, the structure consisting of double grooves of different depth should be analyzed first. The schematic of this double-grooves structure (DGS) is depicted in Fig. 3(a), the depths of two grooves are denoted as d₁ and d₂, respectively, and the distance of the grooves is denoted by g, which represents the thickness of the Ag ridge between the two grooves in essence. The depths of grooves are set as d₁ = 160 nm, d₂ = 200 nm, and the initial value of g is set as 30 nm. The other parameters are the same as in Fig. 2 and the absorption spectrum for DGS (black squares) is shown in Fig. 3(b). It can be found that the absorption peak around 421 nm is corresponded to the propagating SPs mode, and the three other peaks around 520 nm, 740 nm and 810 nm are mostly related to the cavity mode. The absorption with SGS with same period are also shown in Fig. 3(b) by red and blue curves corresponding to d = 160 nm and d = 200 nm, respectively. It can be seen from the curve shape that the SPs mode associated to the unchanged period of the cell can be excited consistently at same short wavelength for three structures. The cavity mode around 520 nm for DGS is a combination of cavity modes supported in two single grooves, leading to an increase of the width of this peak. The last two peaks of DGS are separated with respect to the cavity modes in two single grooves. That means the spectrum of DGS can be seen as the overlap of the absorption spectrum of the SGS with the same period. Figure 3(c) shows the absorption efficiency with varied groove gap g (the distance between the right side of the first groove and the left side of the second groove). Similar to Fig. 2(b), two strong absorption bands from 400 nm to 570 nm are observed clearly, and the second absorption band shows a feature of slight red-shifting when the gap beyond 20 nm. In OSCs device based on SGS, the groove gap should keep a certain thickness for avoiding the field-penetration through the metal wall between the grooves, otherwise the two grooves act as a single one [55]. However, the effect of changing gap on absorption efficiency is negligible in a short wavelength range. In addition to this, an absorption peak between 750 nm and 800 nm generates a slightly blue shift when g is within 20 nm, and the narrow absorption band around 810 nm becomes strong when the gap beyond 25 nm. As a result, the resonance of cavity modes in different grooves can coexist in a combined structure at different wavelength, and the interaction between grooves can be neglected with suitable geometric parameters. When g is within 20 nm, the grating structure of can be modeled as blend/Ag/blend structure, similar to the insulator/metal/insulator (IMI) structure in Ref [56]. It can be found that, with the increase of metal layer thickness, the even mode will gradually approach the mode of a single interface from the dispersion relation of SPs coupling in IMI structure. When the plasmon wave vector corresponding to lower incident light frequency is the same, the structure with a higher Ag thickness leads to a higher incident frequency, then the coupled SPs mode will gradually shift from long wavelength to shorter ones. As the Ag thickness continues to increase, the coupled SPs mode is closer to the mode of a single interface and the dispersion curve of IMI structures moves close to the light line. This reveals the reason for the blue shift of the spectrum with the increase of thickness of g from 5 nm to 20 nm. And the resonance generated by a coupled-microcavity effect between 700 nm and 800 nm is similar with Ref [57].

 

Fig. 3 (a) Schematic diagram of the DGS-based OSCs device. (b) Absorption efficiency of active material in DGS with d₁ = 160 nm, d₂ = 200 nm, g = 30 nm (black squares), the absorption efficiency of the SGS with d = 160 nm (red circles), and d = 200 nm (blue upper triangular), respectively. (c) Absorption efficiency of DGS with varied grating gap g. Other parameters are the same as in Fig. 2(a).

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For further improving the integrate absorption efficiency of OSCs device and realizing the broadband absorption spectrum with multiple resonances, a structure including three grooves in one period is designed with considering of complexity of nanostructure. The configuration is shown in Fig. 4(a). The geometrical parameters of the proposed triple-grooves structure (TGS) is p = 390 nm, w = 100 nm, and the thickness of homogenous part of the active material is still 20 nm. Considering the wavelength and width of the peaks, all grooves are equally separated with g = 30 nm and the depths of three grooves are chosen from Fig. 2(b): d₁ = 60 nm, d₂ = 160 nm, and d₃ = 200 nm. Four absorption peaks can be observed clearly in the spectrum of TGS-based device (black squares) and they are labeled in Fig. 4(b) as λ₁ = 410 nm, λ₂ = 570 nm, λ₃ = 707 nm, and λ₄ = 790 nm, respectively. Since the absorption performance of active material in TGS (black squares) can be divided into two parts involving the absorption in homogeneous layer and that inside the grating grooves, the contribution of these two parts should be distinguished and the results are also shown in Fig. 4(b). It can be found that the absorption peaks of the grating grooves (blue up triangles) are approximately consistent with that of TGS except the first absorption peak around λ₁, which results from the combination of absorption resonances of the grating grooves and homogeneous layer (red circles). Besides, the absorption in grating grooves plays a dominant role for the whole wavelength range of TGS compared to that of homogeneous layer. In order to compare the absorption efficiencies of the proposed TGS and SGS and reveal the location-mechanism of the absorption peaks, the calculated spectra of SGS-based device with d = 60 nm (red circles), d = 160 nm (blue up triangles), and d = 200 nm (green down triangles) are respectively shown in Fig. 4(c). The other parameters are the same as the situation in the absorption spectrum of TGS (black squares) in this panel. It can be seen from the results that the locations of resonances for TGS at short wavelength are almost consistent with the absorption peaks of the SGS, elucidating the fact that the SPs modes in SGS are maintained in the hybridized structure. This resonance can be confirmed from the distributions of magnetic and electric field in the structure shown in Figs. 5(a) and 5(a′), respectively. Moreover, the coupled SPs modes around 570 nm in each single groove contribute simultaneously to the performance of the TGS at this resonance, which can be confirmed from the magnetic and electric field distribution at λ₂ in Figs. 5(b) and 5(b′), respectively. Apart from the SPs modes excited on top of the grating ridges, the magnetic field penetrates into the bottom and vertical side of each grating groove at this resonance. In spite of the maximum electric field concentrated in the vicinity of the upper corners of grooves, the enhanced electric field can be found inside the grooves with larger area and have the formation of a cavity mode compared to Fig. 5(a′), which leads to the increase of absorption efficiency of TGS. In particular, the electric field in the groove with d = 60 nm has the formation of a standing wave with minimum value at the bottom and maximum value at the opening.

 

Fig. 4 Schematic diagram of the TGS-based OSCs device (a) and absorption efficiency of active material TGS (black squares) divided into two parts (b): the Abs in homogeneous layer (red circles) and Abs in the grating grooves (blue up triangles), respectively. (c) Absorption efficiency of active material (black squares) and Ag (pink diamonds) in TGS-based device, and the absorption efficiency in SGS-based device with d = 60 nm (red circles), 160 nm (blue up triangles), and 200 nm (green down triangles), respectively. (d) Absorption efficiency of active material in proposed TGS (black squares) and that in TGS with equal depth of grooves: d = 60 nm (red circles),160 nm (blue up triangles), 200 nm (green down triangles), respectively. Other parameters are set as p = 390 nm, w = 100 nm, t₁ = 20 nm, g = 30 nm. The inset in (d) is the electric field distribution of a TGS with equal grooves depth of 200 nm at wavelength of 790 nm.

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Fig. 5 Normalized magnetic field distribution at wavelength of: (a) 410 nm, (b) 570 nm, (c) 707 nm, (d) 740 nm, (e) 790 nm and (f) 840 nm, respectively. Normalized electric field distribution at wavelength of: (a’) 410 nm, (b’) 570 nm, (c’) 707 nm, (d’) 740 nm, (e’) 790 nm, and (f’) 840 nm, respectively. The parameters are set as in Fig. 4(b).

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At long wavelength range in Fig. 4(c), there are discrepancies of location between the resonance peaks (λ₃ and λ₄) and that of SGS with d = 160 nm and d = 200 nm, respectively. At the wavelength of λ₃, the normalized magnetic field and the electric field mainly distributed inthe grating grooves as shown in Figs. 5(c) and 5(c′), respectively. The electric field intensity in the second groove is the greater than that in the other two grooves, which illustrates that a cavity-like mode is supported in the second groove with two nodes and two antinodes. This excitation of the cavity-like modes leads to the enhancement of electric field inside the groove as well as that near the grating upper corners. In addition, the extinction coefficient of the blend is relatively large at this wavelength, therefore, the absorption peak can be found obviously at this wavelength, as shown in Fig. 4(c). In Figs. 5(d) and 5(d′), we also found that at the wavelength of 740 nm (close to the resonance wavelength of SGS with d = 160 nm) the enhanced EM fields in TGS mostly distribute in the second groove and the electric field oscillates like a standing wave obviously with the formation of a cavity mode. However, the enhancements of electric field resulting in the absorption in the other two grooves are relatively small and the extinction coefficient of blend decrease drastically at this wavelength. Then, the deviation of the absorption peak with respect to the cavity resonance in TGS from that in SGS with d = 160 nm can be observed in Fig. 4(c). For further study, the EM field distribution of the fourth resonance peak at λ_4, are shown in Figs. 5(e) and 5(e′). It can be seen that the enhanced eclectic field inside the second groove still exhibit the properties of a cavity mode, and the enhanced electric fields (10-folds) induced by the localized SPs modes distributed in the vicinity of the outside corners of the second groove. That means the EM field in the TGS at this wavelength is not a cavity mode but a non-resonance condition [37]. Therefore, the relatively large absorption efficiency of TGS at this resonance wavelength can be promised by the extremely enhanced electric field at the corners of the ridge and the absorption in Ag (pink diamonds) can be improved at the same wavelength, which deviates from the resonance location (green down triangles) of the SGS-based device with d = 200 nm, as shown in Fig. 4(c). In fact, the cavity mode resonance in the third groove occurs at the wavelength around 840 nm, which can be confirmed from standing wave formation of the magnetic and electric field distribution in Figs. 5(f) and 5(f′), respectively.

For comparison, we also calculated the absorption spectrum of TGS-based device with equal depth of grooves: d_1,2,3 = 60 nm, 160 nm, 200 nm, respectively as shown in Fig. 4(d). It can be found that the absorption efficiency of TGS with the d_1,2,3 = 60 nm (green lower triangles) was higher in the longer wavelength and the absorption efficiency of TGS with the d_1,2,3 = 160 nm (blue circles) behaves in the opposite way. The absorption efficiency of TGS with the d_1,2,3 = 200 nm (red up triangles) has a broadband in the range of 350 nm to 750 nm, but the absorption efficiency of blend was limited in the infrared range due to the low electric field intensity inside the grooves, which can be confirmed by the electric field distribution at 790 nm, as shown in the inset of Fig. 4(d). However, the proposed TGS with different groove depths not only maintains the broad absorption in the visible wavelengths, but also improves the absorption efficiency of the blend in the infrared wavelengths, and further broadens the absorption band of the TGS as shown in Fig. 4(d) (black squares). In conclusion, the multiple resonances of TGS could be utilized in the OSC device to obtain broadband absorption spectrum and relatively large efficiency. During the research process, adding a hole-blocking layer (i.e. BCP) to the structure is also considered. It can be found that when adding the hole-blocking layer with the different thickness, the absorption enhancement is only 2% at most, which is slight and can be neglected. Besides, the position of resonance peaks in the absorption spectrum is almost unchanged. Thus, in the next study including the physical mechanism analysis and numerical simulation, the existence of hole-blocking layer was not considered in he proposed structure.

In order to clarify the physical mechanism of multiple-resonances under TM polarization, the grating grooves should be modeled as a waveguide with finite length between two silver layers. In the groove, the plasmon waves excited by the incident light propagate along vertical side of the waveguide. At the bottom end, the reflection wave has a π-phase shift and plasmonic standing wave occurs along the waveguide. That means the electric field is always zero at the metal end and maximal at the opening in such a semi-closed cavity. The positions of resonances are related to the cavity length, i.e., the grating groove depth d, and can be derived from the boundary conditions. Then, the plasmon wavevector can be solved by

 

Fig. 6 The light line in air (solid line) and the dispersion relations of the SPs resonance in the grating grooves obtained from simulation data (red scatters), compared to those analytically calculated results for SPs resonance in Ag/blend/Ag model with gap of width 100 nm (red dashed line), SPs resonance in blend/Ag/blend model with gap of 30 nm (green dash-dotted line), and SP resonance at a single Ag/blend interface (blue dotted line), respectively.

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In the above part of this paper, we mainly studied the effect of parameters of grating grooves on the absorption performance of the TGS. Here, we discussed the effect of the homogeneous layer thickness (t₁) on the absorption efficiency of the OSCs device. The absorption performance of TGS-based OSCs device with different thickness of the homogeneous active layer was investigated and the results are shown in Fig. 7. As we can see from Fig. 7(a) that the absorption efficiency of TGS can be improved along in short wavelength range with the increase of the homogeneous active material thickness, which is caused by the increase in amount of active material in the homogeneous layer. Besides, we find that the absorption efficiency of TGS around 625 nm (denoted with black dashed line) keeps almost unchanged with the variation of homogeneous layer thickness. The detail is shown in Fig. 7(b). The main reason for this scenario is that the reflection from ITO is prominent at this wavelength and the electric-field penetrating into the TGS is mostly concentrated in grating grooves.

 

Fig. 7 (a) Absorption spectrum of TGS-based OSCs device as a function of the thickness of homogeneous active layer. (b) Absorption efficiency of TGS-based OSCs device at wavelength of 625 nm with varied homogeneous layer thickness. Other parameters are the same as in Fig. 4(b).

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In order to study the influence of the incident angle (θ) on the absorption performance of TGS-based OSCs device, the absorption spectrum at varied incident angle for the TGS-based and the planar control OSCs was simulated respectively, and results are displayed in Fig. 8(a). From the middle panel of Fig. 8(a), it can be seen obviously that the absorption band of the TGS-based device is broadened, compared to that for the equivalent planar device shown in the left panel of Fig. 8(a). In particular, the absorption band remains stable between 350 nm and 800 nm as the incident angle changes from 0 to 50 degree. In the wavelength range of 460 nm to 560 nm, the absorption efficiency of the planar structure is always lower as the angel incident light changes from 0 to 65 degree; however, the absorption efficiency of TGS is improved within this band as the incident angle changes from 0 to 30 degree. Besides, the long wavelength band of the absorption of TGS extends to 850 nm as the incident angle increases and that for the planar control device strops at wavelength of 760 nm, confirming the validity of the multiple depth of grooves in the OSCs device for improving the absorption in near-infrared region. Taking into account of the unpolarized sunlight, the absorption efficiency of TGS-based device should be performed under TE polarization, and the results are displayed in the right panel of Fig. 8(a). This panel shows that the absorption band of the TGS-based device is broad in the region of visible, compared to that for the equivalent planar device shown in the left panel of Fig. 8(a). In the range of incidence angles from 0 to 40 degrees, the absorption efficiency of the active layer in the range of from 450 nm to 600 nm is significantly better than that of the flat plate structure and TGS under the TM polarizationmode, which is helpful to improve the absorption efficiency of TGS in this band range under the hybrid polarization mode. In addition, we also calculate the integrated absorption efficiency from Eq. (2) by varying the incident angle for both the TGS and the planar structure with regardless of the weight of AM 1.5 G solar spectrum. The results are shown in Fig. 8(b), and it can be seen that the integrate absorption efficiency for TGS under TM polarization is always higher than that of the planar control cell as the incident angel varies from 0 to 65 degree. As the incident angle increases, the integrate absorption efficiency of the two devices decreases and the difference between these two values reduces gradually. In particular, the maximum value of Abs_int for TGS-based device reaches 53.7% at normal incidence and is increased by 28.5% compared with the absorption efficiency of the equivalent planar device (41.8%). Although the integrate absorption efficiency of the active layer in TE polarization mode is lower than that of the planar structure when the incident angle is greater than 30 degrees, the integrate absorption efficiency of TGS is still higher than that of the planar structure under the hybrid mode with the incident angle of less than 50 degrees, confirming the enhancement of absorption efficiency for the TGS-based device. In considering of the weight of AM 1.5 G solar spectrum, the value of Abs_int for TGS-based device under TM polarization is 57.4% at normal incidence, which is improved by 13.4% with respect to that of the equivalent planar device (50.6%). In the wavelength range of 350 to 800 nm, the value of Abs_int of proposed TGS-based device under TM mode is 66.3%, which is improved by 15.5% compared to the results in the previous work using the same active material [46]. In addition, the possible limitations of the device should be considered. When the depth of grating groove increases, the probability of carrier recombination increases, which will affect the efficiency of carrier migration and the IQE of the cell consequently. However, it can be observed that the main absorption portion arises from the SPs effect inside the first two grating grooves, while the third grating groove mainly affects the long wavelength region in the absorption spectrum, which has relatively small absorption efficiency compared to the short wavelength region. Organic solar cells with active layer thickness of about 60 nm and 160 nm have been widely studied [41,58], and the specific influence of the depth of the third groove on the TGS-based device performance will be discussed and studied in experimental process in our future works.

 

Fig. 8 (a) Absorption spectra calculated for the OSCs device with planar structure (left panel) and the proposed TGS-based OSCs device under the TM and TE polarization mode (center panel and right panel). (b) Angular resolved integrated absorption efficiency for TGS-based device under the different polarization modes and that of the planar control device.

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4. Conclusion

In conclusion, our work has theoretically evaluated the influence of the groove depth and the combination way in the metallic grating-based OSCs device. The numerical study for investigating the enhancement of absorption efficiency over a broadband wavelength range in the active material (PTB7:PC₇₀BM) was performed using 3D-FDTD method under TM polarization. We have found that the propagating SP mode on the grating ridges and the cavity mode originating from vertical SP mode along the groove sides, leading to the enhanced E-field in the vicinity of grooves corners or inside the grooves, both have strong effects on the enhancement of absorption efficiency of the OSCs device. The triple-grooves grating structure was proposed to realize enhanced absorption efficiency with broadband by taking advantage of multiple-resonances of cavity modes that have the red-shift property with increasing the groove depth. The magnetic and electric distributions have been investigated to seek the origin of the absorption enhancement in the OSCs device. Our results demonstrated that an enhancement of absorption efficiency can be observed by tuning the depth of grooves in one unit cell of grating, at the same time, tuning the gap between neighboring grooves in the range of 25~35 nm. The integrated absorption efficiency can be obtained as 53.7% (57.4%), which is increased by 28.5% (13.4%) without (with) with respect to that of the equivalent planar device considering of the weight of AM 1.5 G solar spectrum in the wavelength range from 350 to 900 nm. More importantly, the long wavelength band edge of the absorption spectrum of the proposed TGS-based device extends to 850 nm in comparison with the band edge of 800 nm for grating structures consisting of three grooves with equal depth (60, 160, 200 nm). Meanwhile, the wide-angle (0 to 65 degree) absorption for the TGS-based OSCs device can be achieved. For manufacturing the sample of the TGS-based OSCs device, the vacuum evaporation deposition technique is a widely used method. The process can be performed as follows: the silver film can be prepared on the template with the method of vacuum evaporation deposition or sputtering method. Three grating grooves with width of 100 nm and different depths can be obtained by etching method, such as ion beam etching technology. Then, the grating grooves can be filled with hole-blocking layer and active layer by spin-coating method. The buffer layer was deposited by the same method, and finally the ITO conductive layer can be added on top of the whole device. Although this work is based on numerical simulations performed using sophisticated physical models, the findings from our results can still provide important guidelines for designing novel grating-based composite OSCs devices with significantly enhanced absorption efficiency and the multiple-resonance induced spectrum broadening has potential applications with broadband for absorbers in infrared acquisition, thermal photovoltaic, and other applications. Besides, the similar physical mechanism of multiple-resonances structure has potential in application of light-emitting devices [59].