Enhancing light absorption for organic solar cells using front ITO nanograting and back ultrathin Al layer?

Li Zhang(張力) Wei-Ning Liu(劉衛寧) Yan-Zhou Wang(王艷周) Qi-Ming Liu(劉奇明)Jun-Shuai Li(栗軍帥) Ya-Li Li(李亞麗) and De-Yan He(賀德衍)

1Key Laboratory of Special Function Materials&Structure Design of the Ministry of Education,Lanzhou University,Lanzhou 730000,China

2School of Physical Science&Technology,Lanzhou University,Lanzhou 730000,China

3School of Materials&Energy,Lanzhou University,Lanzhou 730000,China

Keywords: light management structures,ITO nanograting,organic solar cells,thin film-based optoelectronic

1. Introduction

Organic solar cells (OSCs) have attracted a great deal of attention because of the potential advantages including low cost, light weight, mechanical flexibility, and easy-toimplement large-scale and roll-to-roll fabrication.[1–5]To date,power conversion efficiency (PCE) of single junction OSCs has surpassed 18%due to the massive efforts made in various respects from materials to device configurations.[6]However,the gap of the comprehensive performance including PCE,lifetime between OSCs and their inorganic counterparts such as silicon wafer solar cells is still evident.[7–9]

For most of OSCs, the discrepancy between limited carrier mobility and absorption coefficient is the main factor limiting their PCE improvement.[10]How to simultaneously ensure the effective collection of photogenerated carriers and efficient light absorption has become an important scientific issue, which therefore has attracted people to conduct the research in various aspects from the optimization of device configurations to the development of novel materials.[11,12]Among the diverse schemes,incorporating an appropriate light management structure is an effective way of improving light absorption without increasing the thickness of the photoactive layers, and thus has been developed into one of the frontiers. It has been suggested that various antireflection structures,[13–18]photonic crystals,[19–24]metallic nanoparticles,[25–30]and gratings[31–36]be used to enhance light absorption of the relevant devices. However, it is notable that the balance between light absorption enhancement and complexity of the light management structures should be seriously taken into consideration for solar cell applications.

In this study, a simple and high-performance light management structure consisting of an indium tin oxide (ITO)nanograting on the front surface and ultrathin Al layer inserted in between the photoactive layer and the electron transport layer (ETL) is proposed. The combination of antireflection and light scattering by the ITO nanograting leads the light absorption of the photoactive layer to increase remarkably.Moreover,the ultrathin Al layer can suppress the light absorption in the ETL and thus reduce the corresponding energy loss.The simulation results indicate that the short-circuit current density (Jsc) and PCE can increase 32.86% and 34.46%, respectively,after incorporating the optimized light management structure into the device with an 80-nm-thick P3HT:PC61BM blend active layer. Moreover, the light management structure exhibits good omnidirectional light management that is expected in solar cell applications.

2. Experiment

Figure 1(a) shows the schematic diagram of the threedimensional(3D)structure of the investigated device that consists of a front ITO nanograting of 160 nm in the total thickness (H1=H2= 80 nm), a 10-nm thick PEDOT:PSS hole transport layer, an 80-nm thick P3HT:PC61BM photoactive layer, an ultrathin Al layer, a 10-nm thick ZnO ETL, and a 100-nm thick bottom Al layer. As marked in Fig. 1, the ITO layers are arranged periodically(period ofP)into a nanograting, in which the width of bottom part (L1) is larger than the width of top part of the nanogratingL2(i.e.,L1>L2),and the thickness of the ultrathin Al layer is denoted asT. The optical properties of the device are investigated by solving the Maxwell’s equations through using a finite-difference timedomain method. Since the sunlight can be divided into any pair of mutually perpendicular linear polarization states without a fixed phase relationship,two polarization states with the electric field parallel and perpendicular to the nanograting are adopted in this investigation. Considering the bandgap energy of～1.9 eV for the P3HT:PC61BM blend[37]and the main energy region of the solar irradiation, the optical behaviors are studied in the spectral range of 300 nm–800 nm.

For building the structure for simulation and saving the calculation resource and time as well,periodic boundary conditions are applied to the unit,i.e.,the cuboid-shaped box with red edges as exhibited in Fig. 1(b), along thexandydirections. Meanwhile,the perfectly matched layer boundaries are imposed on the top and bottom bounds of the unit,and a monitor is placed above the top bound to determine the light reflection,R(λ). The absorption of the photoactive layer at a certain wavelength, denoted byA(λ), can be directly extracted from the following equation after the electric field has been calculated:[38]

whereωis the angular frequency of the incident light,εis the permittivity of vacuum,Eis the local electric-field strength in the photoactive layer,nandkrepresent the real and imaginary parts of the P3HT:PC61BM refractive index,respectively.Based on Eq.(1),the short-circuit current density(Jsc)of the devices from the following equation:

whereeis a unit charge,his the Planck constant,andIAM1.5G(λ)is the spectrum of the energy flux density at AM 1.5G illumination.And moreover,to make a quantitative comparison of light trapping capability among the devices,the integrated reflectance(Rin.)is calculated using the following equation:[39]

the integration range is from 300 nm to 652.6 nm,corresponding to the bandgap energy of the P3HT:PC61BM blend.

Fig.1. (a)Schematized 3D structure of investigated devices,and(b)simulated minimum period structure.

Furthermore,the current density(J)–voltage(V)relation is calculated to more visually evaluate the device performance.The open-circuit voltage(Voc)of the control device with a 160-nm-thick ITO planar layer and without the inserted Al layer is first estimated by the empirical formula[40]

wherekis the Boltzmann constant,Tis the absolute temperature,and 300 K is adopted in this study. Then theJ–Vcurves for the devices with the proposed light management structure can be plotted from the following equation:

3. Results and discussion

Figures 2(a),2(b),and 2(c)show the contour plots ofJscas a function ofL1andL2/L1,respectively,atPvalue of 200,320, and 450 nm. It is evident that excellent light confinement can be achieved in a broad parameter range,thus providing convenience for fabricating the related high-performance devices. It is notable that aJscvalue of～12.14 mA/cm2,i.e.,>72.0%absorption for the incident photons with energy larger than 1.9 eV, can be achieved for the device withPbeing 320 nm, andL1andL2/L1around 180 nm and 0.611, respectively,as exhibited in Fig.2(b)As a striking contrast,the control device with a 160-nm-thick flat ITO film only delivers theJscof～9.13 mA/cm2,indicating the excellent light management for the proposed optical structure. Furthermore,it is noted that for a fixedL1,Jscnormally decreases ifL2/L1is too large or small,and that theJsccontour shifts towards higherL1asPincreases.

Fig.2. Contour plots of Jsc as a function of L1 and L2/L1 for P value of(a)200,(b)320,and(c)450 nm,with thickness of the ultrathin Al layer being 5 nm.

To understand the mechanism of enhancingJscafter applying the ITO nanograting, the optical behaviors of the device with the ITO nanograting are compared with those of the control device with a 160-nm-thick flat ITO layer. Figure 3(a) shows the light reflection spectra andRin.(inset) for the devices after introducing the nanograting structures with aPvalue of 320 nm, anL2/L1of 0.611 and differentL1values of 80, 180, and 280 nm. It is evident that the light reflection is significantly suppressed especially in a spectral region from 400 nm to 650 nm as compared with the scenario of the control device. Accordingly, the light absorption of the photoactive layer in the corresponding spectral region (see Fig. 3(b))and thus theJsccan be enhanced for the ITO nanogratingincorporated device. Moreover,it is noted that for a fixedL1,the light reflection is observed to increase ifL2/L1is too large or too small due to the destroyed continuity of the refractive index of the ITO nanograting as demonstrated by the reflection spectra andRin.(inset) in Fig. 3(c) It thus leads to the better light absorption as exhibited in Fig.3(d)and the higherJsc(see Fig.2)of the photoactive layer for the modestL2/L1.

Besides the structural parameters of the ITO nanograting and the thickness of the ultrathin Al layer,Tis found to have an evident influence on light absorption of the photoactive layer. As summarized in Table 1 for the device withPof 320 nm, andL1andL2/L1of 180 nm and 0.611, the maximumJsccan be predicted to be 12.14 mA/cm2that is 14.21%higher than that in the case without this thin Al layer, whenTis around 5 nm. The further investigation indicates that the reduced light reflection (see Fig. 4) in the spectral region of 500 nm–600 nm in which the photon density is relatively high for the P3HT:PC61BM blend can improveJscfor the device withTof 5 nm as compared with the scenarios in other cases.

To more comprehensively understand light management of the proposed structure,influences of the optical structures,i.e.the ITO nanograting and 5-nm-thick ultrathin Al layer on the electric-field strength in the devices are investigated. Figure 5 shows the comparison between the distributions of the electric-field strength in the cross-section of the devices at two representative wavelengths of 420 nm and 580 nm. It is first noted that the electric-field strength in the photoactive layer(see Figs. 5(a), 5(b), 5(e), and 5(f)) is evidently enhanced for the nanograting-incorporated structure(P=320 nm,L1=180 nm,L2/L1=0.611) because of the scattering effect on the incident light compared with the counterparts for the device without the nanograting (see Figs. 5(c), 5(d), 5(g), and 5(h)). Moreover,after introducing the 5-nm-thick Al layer between the photoactive layer and the electron transport layer,i.e.ZnO, the intensity of the electric field in ZnO is significantly suppressed as more clearly demonstrated in Fig.6,thus reducing the corresponding energy loss.

Fig. 3. (a) Reflection spectra and Rin. (inset) for devices after introducing nanograting structures with P of 320 nm, L2/L1 of 0.611 and different L1 values of 80, 180, and 280 nm, and (b) corresponding absorption spectra of photoactive layers, (c) reflection spectra and Rin.(inset)for devices after introducing nanograting structures with P of 180 nm and L2/L1 values of 0.278,0.611,and 1,and(d)corresponding absorption spectra of photoactive layers with corresponding data of the device with a 160-nm thick ITO film serving as reference.

Table 1. Jsc of devices with ultrathin Al layers with different thicknesses in between photoactive layer and ZnO ETL.

Fig.4. Reflection spectra of devices with ITO nanograting of P=320 nm,L1 =180 nm, L2/L1 =0.611, and varying ultrathin Al layer thicknesses,with inset showing high-resolution reflection spectra between 500 nm and 600 nm.

For an excellent light management structure,it is vital to have efficient absorption even under oblique incidence. Owing to the reduced symmetry, the proposed structure should show better sensitivity to the incident plane. Accordingly,the absorption spectra at the incident angles of 0°, 30°, and 60°are exhibited for two representative cases with the incident planexz(Fig. 7(a)) and incident planeyz(Fig. 7(b)). The ITO nanograting ofP=320 nm,L1=180 nm,L2/L1=0.611 and the 5-nm-thick Al layer are adopted. It is evident that the light absorption with the incident planexzis more sensitive to the incident angle as compared with that with the incident planeyz. Anyway, for both cases, the reduction of light absorption with the incident angle in a broad range of 0°–60°is limited,thus demonstrating the excellent light confinement for the proposed light management structure. The calculatedJscat the incident angles of 0°, 30°, and 60°is respectively,12.14,10.07,and 5.45 mA/cm2,showing limited reduction as compared with the corresponding value of 16.95, 14.68, and 8.47 mA/cm2for an ideal photoactive layer(100%absorption to the incident light). Significantly,the light utilization rate is still 64% even at 60°, which is much higher than 54% under normal incidence for the planar control device.

Fig. 5. Distributions of electric-field strength in cross-section of devices respectively [(a), (e)] with nanograting and 5-nm-thick Al layer in between photoactive layer and electron transport layer of ZnO at wavelength 420 nm and 580 nm,respectively,[(b),(f)]with nanograting but without 5-nm-thick Al layer at wavelength 420 nm and 580 nm,respectively,((c),(g))with the 160-nm-thick ITO planar layer and 5-nm-thick Al layer at wavelength 420 nm and 580 nm,respectively,[(d)(h)]with only 160-nm-thick ITO planar layer at wavelength 420 nm and 580 nm,P,L1,and L2/L1 of 320 nm,180 nm,and 0.611,respectively.

Fig.6. Distributions of electric-field strength in cross-section of devices in the vicinity of ultrathin Al layer or interface between photoactive layer and ZnO:[(a),(e)]with nanograting and 5-nm-thick Al layer in-between photoactive layer and electron transport layer of ZnO at wavelength 420 nm and 580 nm, respectively, [(b), (f)] with nanograting but without 5-nm-thick Al layer at 420 and 580 nm, respectively, [(c),(g)]with wavelength 160-nm ITO planar layer and 5-nm-thick Al layer at wavelength 420 nm and 580 nm, respectively, [(d), (h)]with only 160-nm-thick ITO planar layer at wavelength of 420 nm and 580 nm,P,L1,and L2/L1 are 320 nm,180 nm,and 0.611,respectively.

Table 2. Device parameters extracted from the J–V curves in Fig.8.

Fig.7. Light absorptions of photoactive layer in device with nanograting of P=320 nm,L1=180 nm,L2/L1=0.611 and 5-nm-thick ultrathin Al layer for light with (a) incident plane of xz and (b) incident plane yz at incident angles of 0°,30°,and 60°,respectively.

Figure 8 exhibits the calculatedJ–Vcurves at AM 1.5G illumination for the devices with only the ITO nanograting(P=320 nm,L1=180 nm,L2/L1=0.611), and with both ITO nanograting and 5-nm-thick Al layer in between the photoactive layer and the ZnO, together with a control device with a 160-nm-thick ITO layer, and the device with an ideal P3HT:PC61BM absorber,i.e.100%light absorption. The key device parameters,includingJsc,Voc,fill factor(FF)PCE extracted from theJ–Vcurves, and the PCE estimated assuming FF=60%[44,45]are summarized in Table 2. It is observed that the PCE of the device with an optimal light management structure is significantly improved compared with that of the control device because of the enhancedJsc.

Finally, the PTB7:PC71BM layer[46,47]is employed to evaluate the feasibilities of the proposed light management structure for other photoactive layers. Figure 9 shows the light absorption spectra of the 100-nm-thick PTB7:PC71BM layers for the device with and without the light management structure,i.e., the ITO nanograting (P=320 nm,L1=180 nm,andL2/L1= 0.611) and the 5-nm-thick ultrathin Al, under the normal incidence. It is evident that comparing with the control device, the light absorption of the photoactive layer is remarkably enhanced after incorporating the light management structure,leading to 22.47%increment ofJsc,i.e.,from 15.13 mA/cm2to 18.53 mA/cm2.

Fig.8. Calculated J–V curves at AM 1.5G illumination.

Fig. 9. Light absorption of 100-nm-thick PTB7:PC71BM layer for the device with and without optimal light management structure.

4. Conclusions

In this paper, we introduce a simple light management structure consisting of a front ITO nanograting and ultrathin Al layer inserted in between the photoactive layer and the ETL,which is applicable to thin film-based optoelectronic devices.Owing to the synergetic effects including antireflection and light scattering by the nanograting and suppressed light absorption of the ETL by the inserted Al layer,the light absorption of the photoactive layer can be remarkably enhanced in a broad parameter range of the nanograting, and thus provides the convenience for fabricating the relevant devices. Furthermore,it is found that compared with the control device with a planar configuration of ITO/PEDOT:PSS/P3HT:PC61BM(80-nm thick)/ZnO/Al,Jscand PCE can be improved by 32.86%and 34.46% after introducing the optimized light management structure,i.e., the ITO nanograting ofP= 320 nm,L1=180 nm,L2/L1=0.611, and 5-nm-thick Al layer. In addition,the proposed device structure exhibits good omnidirectional light management and feasibility for other photoactive materials. Owing to the simple structure and excellent performance, we believe that this work provides valuable exploration of light management structures for thin film-based optoelectronic devices.