We present in-coupling gratings for improving the performance of thin film organic solar cells. The impact of the grating on the absorption in the active layer is modeled and explained using a standard cell architecture. An increase in absorption of 14.8% is predicted and is shown to be independent from the active material. The structure is then applied on blade-coated devices and yields an efficiency improvement of 12%. The angular behavior of the structures is measured showing superior performance for two dimensional gratings. By simulating the current generation for different angles and illumination conditions, we predict a total yearly increase of the generated current of 12% using an optimized grating. The fabrication of these structures, moreover, is compatible with roll-to-roll production techniques, thus making them an optimal solution for printed photovoltaics.
© 2015 Optical Society of America
Organic and printable photovoltaics represent the third generation of photovoltaic devices and offer a range of novel and unconventional applications, i.e. integrability into buildings, products and electric devices. State-of-the-art cells have recently exceeded efficiencies of 10% for polymer single junctions [1–4] and are already as high as 12% for small molecule vacuum processed multiple junction cells . Already today, pilot roll-to-roll production lines are able to print these solar cells on flexible substrates [6, 7] and a market of 87 million USD is predicted by 2023 . The device performance of organic photovoltaics (OPVs), however, is limited by a trade-off between good charge extraction and sufficient absorption. Thin absorber layers are electrically preferable, which in turn restricts the optical absorption of light in the device. For these reasons, OPV devices exhibit a high potential for light-management, i. e. additional optical structures which can increase light in-coupling independently from the active layer thickness . Recent achievements of record efficiencies for single-junction polymer solar cells are mainly based on additional optical light-management .
A rich variety of optical structures have been proposed to increase the light-harvesting in OPVs. On the one hand, different plasmonic nanostructures with strong field enhancement were extensively studied [10–15]. On the other hand, light-trapping structures were introduced to redirect the incoming light or to couple it into waveguided modes propagating in the absorbing layer [9, 16–21]. In this domain nanoimprint lithography was used to pattern either the absorber or the adjacent layers with periodic gratings [22–28], or more recently to create random and quasi-periodic structures [3, 29–31]. For efficient coupling, the optical structure in these approaches has to be introduced in the vicinity of the active region, i.e. within the functional layers of the OPV. Whereas for silicon photovoltaics, structures can be fabricated directly into the solid absorbing material, several complications arise for solution processed devices: i) the performance of the semiconductor blend of the active layer is very sensitive to changes in morphology, which in turn is strongly influenced by nano-scale patterning of the layers [32, 33]; ii) defects and high recombination rates are induced at an increased interface and if the homogeneity of the layer thickness is not preserved [9, 34]. For these reasons, the enhancement in absorption can then be partially lost by other mechanisms, thus restricting the overall performance.
To our knowledge, until now only little work has been carried out on light management structures for OPV that do not influence the device architecture and fabrication processes [31, 35–39]. For instance, anti-reflective structures can deliver a broadband, yet rather small enhancement . Moreover, refractive micro-structures showed enhanced device efficiencies up to 19% [36, 39], being promising candidates also for mass production. However, in contrast to refractive structures, gratings offer extended degrees of freedom in terms of optical design, since the efficiency of the various diffraction orders can be controlled. They thus allow for a stronger influence on the absorption enhancement in the complex optical system of the OPV multilayer stack. Furthermore, a grating can be functional even when embedded into a matrix, which provides protection against external influences .
In this work, we apply diffraction gratings as in-coupling light-management structures on the outside of the OPV to improve the light distribution in the device and hence its performance. Since the structures are added at the light-incident interface, they are fabricated independent from the OPV and exhibit high potential for integration into roll-to-roll production. The optical modeling is done using rigorous coupled wave analysis (RCWA) combined with a modified transfer matrix formalism . A very good agreement is obtained between the simulated absorption enhancement and the measured increase in external quantum efficiency (EQE). We find that a diffraction grating does not only increase the optical path-length, but moreover enables the light to reach an optimized distribution inside the OPV. Consequently, the resulting absorption enhancement induced by the grating is linked to the stack architecture and does not depend on the active material. Finally, the results of the angle dependent measurements are used to calculate the yearly current enhancement of an optimized structure, accounting for the position and illumination intensity of the sun at a given location.
In the following, we refer to the basic device configuration shown in Fig. 1(a) to examine the effects of light diffraction into OPVs, both theoretically and experimentally. The thickness of the layers is indicated in the schematic. In order to optimize the device, the active layer thickness was increased until the first absorption maximum was reached. A one- (1D) or two dimensional (2D) dielectric grating, sketched in Fig. 1(b), is placed on the light-incident side of the glass substrate. For the fabrication, we use a UV curable polymer with a refractive index n1 ≈ 1.5, matched with that of the glass, so that no refraction occurs at the polymer-glass interface. After the light has passed through the substrate it enters the OPV multilayer stack through the transparent indium tin oxide (ITO) electrode.
The presence of the dielectric diffraction grating will affect the light in-coupling and thus the generated photocurrent in the underlying OPV in two ways. On the one hand, being placed at the air-substrate interface, a subwavelength grating structure can act as an anti-reflection layer [42, 43]. On the other hand, the grating can couple the incident light into higher order modes propagating inside the substrate. For the general case of a 2D grating with period Λ, this means that for light incident from free space with wavelength λ0, instead of a single plane wave with wavevector k and intensity I ∝ |E|2, numerous waves are travelling inside the substrate [18,19], having wavevectors
The intensities In(λ) are defined by the grating geometry and the material properties and are calculated using RCWA . The propagating polar angles in the substrate θSj (j = mx,y) can be derived through geometrical considerations from the incident angle θI and Eq. (1). For the particular case of a 1D grating at air, however, this converts into the grating equation:
In the absence of any grating (Λ → ∞) the maximum angle after refraction at the planar air-glass interface is then given by the Snell’s law (θS,max = 41.8°), as sketched in Fig. 2(a). Increasing the incident angle in the simulation also increases the generated current. However, at high angles θI the surface reflection strongly increases and, thus, the absorption and the current in the active layer approaches zero. If the simulation is done for light propagating directly in the medium of the substrate [Fig. 2(b)], the surface reflections are avoided, and we observe much higher currents for angles θS > θS,max: a maximum value of 12.6 mA cm−2 is obtained at θS = 56°, which is 34% higher than for θI = 0° in the case of Fig. 2(a). This emphasizes, that it is desirable to guide light into these large angles using additional optical structures. In Fig. 2(c) a diffraction grating is included into the simulation and placed at the air-substrate interface. RCWA is used to obtain the diffraction efficiencies. More details on the simulations and the used grating parameters can be found in section A.2. According to Eq. (1), the incident light can now be coupled to higher order modes propagating into the glass substrate. Note that the angle dependent current generation has to be distinguished for the two azimuthal directions ϕ = 0° and ϕ = 90°, i.e. for a plane of incidence either perpendicular or parallel to the grating lines, respectively [indicated in Fig. 1(b)]. For light incident at θI = 0° we observe an increase in the maximum current density of 14.8% compared to the unstructured substrate (dotted line), since the light is now redirected by the grating into the angles shown in Fig. 2(b).
The strong angular dependent enhancement observed in Fig. 2(b) is usually attributed to an increased optical path length in the absorbing layer. For straight incidence, the light in the OPV stack undergoes a double pass through the multilayer structure, since the light is reflected at the metal back electrode. If the light is incident with an angle, the optical path length L in the absorbing layer with thickness d increases to L = 2d · cos(ϑA)−1, where ϑA is the propagation angle inside the active layer. For a perfectly reflecting back electrode, the attenuation Att in the active layer, mainly caused by absorption, will increase with L according to the Lambert-Beer lawFig. 3(a), which determines the distance in the medium, after which the light is attenuated to 1/e.
The effect of this path length enhancement [Eq. (3)] on the absorption in the active layer AA(λ, θS) is shown in Fig. 3(b), simulated for various propagation angles θS. In this case, interferences in the active layer were neglected in the transfer matrix calculation by treating the active layer as incoherent and keeping the layer thickness fixed at d = 90 nm. This figure thus reflects solely the consequences of the increased light path for different wavelengths. However, since OPVs are consisting of multiple thin layers, additional wavelength dependent interferences are present and will contribute to the absorption profile. If these interferences are taken into account, Eq. (3) is not any more enough to model the absorption [Fig. 3(c)]. Especially for larger θS > θS,max the absorption spectrum is dominated by the thin film interferences and also does not depend significantly on ℓA(λ). This is even more evident, if a wavelength independent absorber is chosen, with ℓA = 4d (see Fig. 8).
All things considered, the high current enhancement shown in Fig. 2 can not only be attributed to an increased optical path-length, but it is enabled by the interplay with thin film interferences in the OPV stack. The latter effect provides particularly strong absorption for large propagation angles in the substrate, which are only accessible through the coupling to higher order modes given by the diffraction grating. Additional simulations with different absorbers confirm, that due to the observed behaviour, the device architecture determines the angular response of the OPV, since it determines the possible interference conditions (see Fig. 9). Moreover, we observed, that even for highly optimized device of both configurations (standard  and inverted ) an enhancement can be achieved using an in-coupling grating, which further demonstrates the general applicability of our approach.
A grating with the same parameters as used for the simulations in Fig. 2(c) was then replicated on glasses which served as substrates for the fabrication of 21 out of 40 OPV devices. Figure 4(a) shows a SEM picture of the replicated structure and in Fig. 4(b) the J-V characteristics of the two devices with the highest efficiency of the two sets are shown. For the device with the grating, a significantly higher current is obtained, while preserving the electronic properties of the solar cell, and thus yielding an increase in efficiency from 5.21% to 5.84% (+12%). The currents of all grating and non-grating samples, obtained by integrating the EQE curves, are compared in Fig. 4(c) and are summarized in Table 1. The best performing devices [Fig. 4(b)] are marked with a black cross. The average current of the grating cells, indicated by the black bar, is 4.8% higher than of the reference, and an improvement of 12% is obtained, if the two best devices are compared. The effect of the grating is further highlighted, if the groves are filled after the measurement with the same material, since the grating and the light diffraction are consequently lost. In this case, we observe an up to 14.1% higher current for a particular device equipped with the grating with respect to the current after filling. On the one hand, this confirms that the grating itself is responsible for this increase. On the other hand, the experimental values are in agreement with the value predicted by the simulations reported in Fig. 2(c). However, we observe that a second thick layer of the polymer gives rise to enhanced absorption in the UV and blue region of the spectrum. This is why slightly lower currents are obtained with respect to the references.
The EQE spectrum for the reference and the grating device is shown in Fig. 5(a). Considering the difference ΔEQE in Fig. 5(b), the impact of the in-coupling grating can be divided into two spectral regions: a strong increase is obtained in the blue (region I), while a broadband enhancement can be observed for longer wavelengths (region II). It is evident, that the enhancement is strongly wavelength dependent, since according to Eq. (2) blue light is propagating at smaller angles θS(λ) than red light. The grating-induced enhancement is thus determined by the wavelength dependent absorption AA(λ, θS(λ)), which is related to the OPV stack. When this is taken into account by our simulations, the measured ΔEQE is well reproduced by the predicted absorption increase ΔAA(λ) for straight incidence [Fig. 5(b)]. The two quantities are thereby related through the internal quantum efficiency IQE, which can be approximated to be constant over the absorption range of the organic semiconductor :
The spectral enhancement of this particular grating, hence, can be explained and retraced through the characteristic absorption AA(λ, θS), if it is combined with the diffraction angles θS(λ) and efficiencies of the grating. Figure 5(c) shows the change in absorption in the active layer ΔA(λ, θS) as a function of the angle in the substrate θS and the wavelength with respect to the absorption at θS = 0, which would apply without a grating:Eq. (2) is plotted showing the diffraction angles of the existing grating modes. In Fig. 5(d) the corresponding simulated and measured diffraction efficiencies ηn of the 1D grating are displayed.
The high values of ΔEQE in region I now can be explained by the light that is coupled into the second order diffraction at angles θS > θS,max, visible in position 1 in Figs. 5(c) and 5(d). At wavelengths λ ≈ 500 nm we observe that the enhancement is low because light is efficiently diffracted into the first order η1,−1 > 30%, which propagates at angles that do not enable an enhanced absorption (position 2). Finally, in region II, where the second order is extinct (position 3), the obtained enhancement can be attributed to the light, which propagates in the first order at angles θS > 25°, where absorption is slightly higher than for θI = 0.
Hence, the impact of a particular grating on the EQE enhancement in an OPV can be well explained through the angle dependent absorption in the active layer AA(λ, θS) and the grating diffraction. In the appendix we additionally show AA(λ, θS) for different device architectures and absorber materials. It gets evident, that for all cases the beneficial interference conditions enable a higher current, which can be maximized for a particular device architecture by optimizing the grating diffraction. From Fig. 9 it follows, that in this way the efficiency of the OPV can be enhanced by optical light management almost independently from the chosen semiconductor.
4. Application examples
Until now, we focused on the performance enhancement under standard test conditions, which can readily be tested in the experiment. While we could understand the optical effects in the stack for straight incidence, in the final application, the device will be exposed to the continuously changing illumination conditions of the sun, i.e. changing θ. Angle dependent measurements are therefore performed for the OPV with the in-coupling grating, shown in Fig. 6(a). The azimuthal angle is ϕ = 0°, which corresponds to a plane of incidence perpendicular to the groves. We observe that the enhancement in the generated current persists with increasing angle up to θI = 40°, where the performance becomes comparable to the device without the grating.
Following the simulations shown in Fig. 2(c), we observe a large discrepancy in performance at different azimuthal angles ϕ. We therefore designed a two dimensional grating for the light in-coupling, in order to be more independent on the azimuthal angle ϕ. A SEM picture of the structure is shown in Fig. 6(b). Due to the fabrication and replication of the structure, the depth of the grating was smaller than for the 1D grating. This results in a lower diffraction efficiency, which directly affects the performance at θI = 0°, as can be seen in Fig. 6(a). However, the performance of the OPV with the 2D grating outperforms the reference for all angles of incidence and yields an enhancement as high as 20% at an angle of θI = 60°, much higher than for the 1D grating.
The good angular performance of the two dimensional grating is thus well suited for applied solar harvesting, in order to account for the changing solar angles throughout the year. As sketched in Fig. 7(a), the installation of an OPV is chosen to be parallel to the ground, which together with the location (Basel, Switzerland) defines the distribution of incident angles. The parameters of the 2D grating are then optimized in period, depth and width of the grooves (see Table 2) to yield a maximum average current at these angles at standard test conditions (air mass = 1.5). The depth of the grating was restricted to 350 nm in order to prevent problems in the fabrication. Finally, for every hour h in the year (24×365), the solar polar angle and the corresponding spectrum for the air-mass at ground is taken from the NREL database  and is used in the simulation of a maximum photocurrent Isc(h) in mA cm−2.
To estimate the value of the generated current Icorr(h) = IQE · Isc(h) the IQE is set to 90%. We then calculate the generated energy per year as1], these quantities can be assumed to be constant. We can then evaluate the enhancement factor f of our grating as Fig. 7(b). The obtained enhancement factor with this optimized 2D grating is f = 12.1% for the energy generated throughout one year. Assuming a constant fill factor of FF = 70% and Voc = 0.7 V, the yearly energy harvested by our reference device can be approximated to be 84.3 kWh m−2 y−1, which the grating could increase by an additional 10.7 kWh m−2 y−1.
In summary, we studied the influence of diffraction gratings on OPV devices as an in-coupling light-management structure. We found that thin film interferences caused by light at high propagation angles can lead to an enhanced efficiency. Dielectric gratings are thus used to couple the incident light into these higher order propagation modes and enhance the conversion efficiency. Consequently the approach is not dependent on the absorption spectrum of the organic semiconductor but only linked to the device architecture, i.e. the multilayer stack. Based on a good agreement of simulations and experiments, for which solar conditions throughout the year can be modeled for a fixed location, we could demonstrate an increase of the harvested energy of about 12%. We thus conclude that diffraction gratings provide a cheap and easily integrated method for light-management, which is independent on the OPV itself and whose opportunities in design and optimization are not exploited yet.
A.1. Experimental details
The nanostructures were fabricated via soft-embossing and UV casting. A quartz master is generated by interference holography and is then transferred into a Ni master by galvanic growth. The UV photocurable polymer Lumogen OVD Primer 301 (BASF) is then distributed between the Ni shim and the OPV substrate, which is pre-coated and structured with ITO on the opposite side. The polymer is cross-linked by UV illumination through the glass substrate. After de-moulding the substrate from the shim the nanostructure is replicated into the polymer, sticking on the glass side of the substrate. For the protection of the ITO layer throughout this process a protection foil is applied.
On the substrate side with the ITO on top, poly(3,4-ethylenedioxythiophene)/poly(styrene sulfonic acid) (PEDOT:PSS) and the photoactive polymer were deposited via doctor blading in air after plasma treatment. For the active polymer, we chose a blend of the commercial material Lisicon PV-D4610 (Merck Chemicals) and PC60BM dissolved in o-dichlorobenzene. Calcium and Aluminum layers were evaporated through a mask. The devices with and without the grating were fabricated using the same polymer solution and the same evaporation step. The EQE curves are recorded using a monochromator with a Xenon lamp and a Si photodiode as reference. For the J-V curves the devices are packed into an atmospheric box having a quartz window, through which the light is incident on the cells. The measurement was done with a solar simulator under 1 sun (AM 1.5G spectrum), where the illumination intensity was calibrated using a standard Si photodiode to adjust the power density to 1000 W/m2. Accurate short circuit currents were calculated from the EQE measurements. A fixed sample holder was built for angular dependent measurement so that the rotation axis is coincident with the sample position. The illuminated area was 0.04 cm2 masked with a metallic frame. The back electrode consisted of eight 3 mm wide fingers. The grating was always orientated perpendicular to these fingers.
The diffraction efficiencies of the grating were measured on glass. The grating was UV casted on only one half of the substrate. The sample was installed with the grating facing a collimated light source and a cylindrical lens mounted on the glass backside, bonded with index matching gel. The sample and the light source were rotated by 180° and the transmitted light was collected by a spectrometer. After shifting the lens onto the substrate side without any grating structure, the reference measurement was recorded for the same angles. For plotting the data, data points with an intensity lower than a threshold were neglected. The recorded data of each order, which was distributed over 2–3 angular channels, was summed up and assigned to the central emerging angle. The efficiencies of positive and negative diffraction orders were averaged to obtain the data for emergent angles of 0 – 90°.
A.2. Optical simulations
In order to calculate the diffraction efficiencies of the grating, the illumination consists of a unpolarized white light spectrum with equal intensity for all wavelengths in the range of 300–900 nm and is incident on the grating once from the top and once from below. The diffraction efficiencies and the angular distribution behind the grating are calculated for both configurations using rigorous coupled wave analysis (RCWA) , for the respective grating parameters shown in Table 2. For any incident polar angle an intensity distribution I(θ, λ, ϕ) is derived, which for the yearly simulation was averaged over the three azimuthal angles ϕ = 0°, 45°, 90°, as the transfer matrix formalism can only handle θ ∈ [0 90°]. For the simulation of the current generation at ϕ = 90°, the averaging was not used for the light incident on the grating from air, but for the reflected light, which was incident from the glass substrate. The propagation of the diffracted orders through the subsequent incoherent polymer layer is assumed without absorption losses. The angle dependent light propagation, layer absorbance and reflection in the OPV stack are then calculated with a transfer matrix formalism , using the absorption module of the commercial software Setfos by Fluxim AG. Optical constants were provided through the software or obtained from ellipsometry measurements. For λ = 550 nm the complex index of refraction n = nR + inI reads: Lumogen OVD Primer 301 (1.51) / Glass (1.5) / ITO (1.9) / Al4083 (1.52) / PV-D4610:PC60BM (1.89+0.2i)/ Calcium (0.69+2i)/ Aluminum (0.77+5.6i). Detailed optical constants are also available from the authors on request. The maximum current density jsc is then derived by the program from a position dependent photon absorption rate G(z) in the active layer by
Figure 3 of the main text, shows how the angular dependent absorption spectrum of the used active material is changed, if interferences are taken into account. The effect gets even more pronounced when the simulation is done with an hypothetical absorber, which has a constant absorption coefficient and attenuation length ℓA = α−1, set to four times the active layer thickness d = 90 nm. The absorption in the active material in this case is independent on the wavelength, as shown in Fig. 8(a). With neglected interferences in the absorbing layer we observe a slight absorption enhancement when the angle in the substrate is increased towards 60–70° [Fig. 8(b)], as we would expect from an enlarged optical path in the absorbing layer. A slight wavelength dependence however is introduced by the other layers of the stack, in which interferences are considered. Figure 8(c) shows the absorption, if the stack with the homogeneous absorber is treated with all interferences. The absorption is not only increased for all angles, but also a strong wavelength dependency is introduced, although the absorption coefficient is equal for all λ. It gets apparent that the interferences at different λ in the layer stack influence the absorption even at straight incident light. Overall it leads to an enhanced absorption at increased angles for this particular stack (same configuration as in Fig. 3). If the layer configuration or the used materials for the device are changed, also the interference conditions will be affected and the angular dependent absorption spectrum will change as a consequence.
In Fig. 9 we compare three different active materials blends: a) the blend used in this work PV-D6410:PC60BM, b) poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b]dithiophene-co-3-fluorothieno[3,4-b]thiophene-2-carboxylate]:PC71BM (PTB7-Th:PC71BM) and c) the homogeneous absorber from the previous simulation. All three are modeled in two device configurations: standard (glass/ ITO/ PEDOT:PSS/ active/ Ca/ Al), [shown in Figs. 9(d)–9(f)] and inverted (glass/ ITO/ ZnO/ active/ PEDOT:PSS/ Ag), [shown in Figs. 9(g)–9(i)]. It gets apparent from the similarity of the pictures, that not the chosen semiconductor material, but the thin film multilayer system defines the angular dependent absorption. However, for both popular device configurations an enhancement can be achieved with the use of an incoupling grating, which redirects the light according to its period and diffraction efficiency.
A.4. State-of-the-art devices
To demonstrate that the presented in-coupling approach is also able to improve highly optimized organic solar cells, we choose to model two recently reported high performing OPV stacks, that are using PTB7-Th as a photoactive material. For the standard architecture, the cell reported by Zhang et al. is one of the very few reports of solution processed cells reaching an efficiency of 9% in this configuration . We performed an optical optimization for the unknown thickness of ITO and the active layer to make sure it is the best possible device. With IQE set to 0.9 we obtain a maximum photocurrent density of 16.6 mA cm−2, which is reasonable compared to the measured 16.9 mA cm−2 for this device . By testing different two dimensional in-coupling gratings the simulated absorption in this device can be improved to reach a current of 18.8 mA cm−2 (+12.8%). While the reported cell had an efficiency of 9%, this absorption enhancement could yield an efficiency exceeding 10%, which would be the highest value for a single junction solar cell in the ’standard’ configuration.
As stated before, the angle dependent enhancement is strongly dependent on the multilayer thin film stack of the OPV. We therefore consider another state-of-the art organic solar cell, having an inverted device structure. Kong et al. reported an efficiency of 9.74% which is among the record values reached for the PTB7-Th polymer . Our simulation yields a short circuit current of 18.8 mA cm−2 for this device, comparable to the measured 18.4 mA cm−2. Even for this highly optimized record cell, our simulations predict an increase in the photocurrent of about 11.7%, reaching 21.0 mA cm−2. Translated to the device efficiency, this enhancement through light management would yield a value close to 11%, which has not yet been achieved in single junctions. It is noteworthing that even in these highly optimized state-of-the-art devices of different architecture, an enhanced current is predicted by redirecting light into steep angles propagating in the substrate.
The work was performed in the framework of the EU project SUNFLOWER (Grant number 287594) and the INTERREG IV Upper Rhine project number C25: Rhin-Solar, supported by the European Fund for Regional Development (FEDER). The authors thank the U.S. Department of Energy (DOE)/NREL/ALLIANCE for providing the calculation of the position dependent illumination conditions online, T. Gasser for the diffraction efficiency measurement and Stéphane Altazin and Beat Ruhstaller from Fluxim AG for the support in adapting the optical model. Further, J.M. thanks F. Lütolf for helpful discussions and C. Schönenberger and M. Calame of the University of Basel for additional supervision of the work.
References and links
1. Z. He, B. Xiao, F. Liu, H. Wu, Y. Yang, S. Xiao, C. Wang, T. P. Russell, and Y. Cao, “Single-junction polymer solar cells with high efficiency and photovoltage,” Nat. Photonics 9, 174–179 (2015). [CrossRef]
2. L. K. Jagadamma, M. Al-Senani, A. El-Labban, I. Gereige, G. O. N. Ndjawa, J. C. D. Faria, T. Kim, K. Zhao, F. Cruciani, D. H. Anjum, M. A. McLachlan, P. M. Beaujuge, and A. Amassian, “Polymer solar cells with efficiency >10% enabled via a facile solution-processed al-doped ZnO electron transporting layer,” Adv. Energy Mater.5, n/a–n/a (2015).
3. J.-D. Chen, C. Cui, Y.-Q. Li, L. Zhou, Q.-D. Ou, C. Li, Y. Li, and J.-X. Tang, “Single-junction polymer solar cells exceeding 10% power conversion efficiency,” Adv. Mater. 27, 1035–1041 (2014). [CrossRef]
4. Y. Liu, J. Zhao, Z. Li, C. Mu, W. Ma, H. Hu, K. Jiang, H. Lin, H. Ade, and H. Yan, “Aggregation and morphology control enables multiple cases of high-efficiency polymer solar cells,” Nat. Commun. 5, 5293 (2014). [CrossRef] [PubMed]
5. Heliatek GmbH, “Heliatek consolidates its technology leadership by establishing a new world record for organic solar technology with a cell efficiency of 12%,” http://www.heliatek.com/en/press/press-releases/details/ (2013). Accessed: July, 2015.
6. T. R. Andersen, H. F. Dam, M. Hösel, M. Helgesen, J. E. Carlé, T. T. Larsen-Olsen, S. A. Gevorgyan, J. W. Andreasen, J. Adams, N. Li, F. Machui, G. D. Spyropoulos, T. Ameri, N. Lemaître, M. Legros, A. Scheel, D. Gaiser, K. Kreul, S. Berny, O. R. Lozman, S. Nordman, M. Välimäki, M. Vilkman, R. R. Søndergaard, M. Jørgensen, C. J. Brabec, and F. C. Krebs, “Scalable, ambient atmosphere roll-to-roll manufacture of encapsulated large area, flexible organic tandem solar cell modules,” Energy Environ. Sci. 7, 2925 (2014). [CrossRef]
7. CSEM Brazil, “New phase for printed organic photovoltaics with productionscale facility in brazil,” http://www.csem.ch/site/card.asp?pId=27160 (2014). Accessed: July, 2015.
8. H. Zervos, R. Das, and K. Ghaffarzadeh, “Organic photovoltaics (opv) 2013–2023: Technologies, markets, players,” http://www.idtechex.com/research/reports/organic-photovoltaics-opv-2013-2023-technologies-markets-players-000349.asp (2014). Accessed: August, 2015.
9. Z. Tang, W. Tress, and O. Inganäs, “Light trapping in thin film organic solar cells,” Mater. Today 17, 389–396 (2014). [CrossRef]
10. K. Tvingstedt, N.-K. Persson, O. Inganas, A. Rahachou, and I. V. Zozoulenko, “Surface plasmon increase absorption in polymer photovoltaic cells,” Appl. Phys. Lett. 91, 113514 (2007). [CrossRef]
12. M.-G. Kang, T. Xu, H. J. Park, X. Luo, and L. J. Guo, “Efficiency enhancement of organic solar cells using transparent plasmonic ag nanowire electrodes,” Adv. Mater. 22, 4378–4383 (2010). [CrossRef] [PubMed]
15. X. Li, X. Ren, F. Xie, Y. Zhang, T. Xu, B. Wei, and W. C. H. Choy, “High-performance organic solar cells with broadband absorption enhancement and reliable reproducibility enabled by collective plasmonic effects,” Adv. Opt. Mater. 3, 1220–1231 (2015). [CrossRef]
16. K. R. Catchpole, “A conceptual model of the diffuse transmittance of lamellar diffraction gratings on solar cells,” J. Appl. Phys. 102, 013102 (2007). [CrossRef]
19. S. Mokkapati and K. R. Catchpole, “Nanophotonic light trapping in solar cells,” J. Appl. Phys. 112, 101101 (2012). [CrossRef]
20. C. Wang, S. Yu, W. Chen, and C. Sun, “Highly efficient light-trapping structure design inspired by natural evolution,” Sci. Rep.3 (2013). [CrossRef]
21. A. Peer and R. Biswas, “Nanophotonic organic solar cell architecture for advanced light trapping with dual photonic crystals,” ACS Photonics 1, 840–847 (2014). [CrossRef]
22. L. Stolz Roman, O. Ingans, T. Granlund, T. Nyberg, M. Svensson, M. R. Andersson, and J. C. Hummelen, “Trapping light in polymer photodiodes with soft embossed gratings,” Adv. Mater. 12, 189–195 (2000). [CrossRef]
23. M. Niggemann, M. Glatthaar, A. Gombert, A. Hinsch, and V. Wittwer, “Diffraction gratings and buried nano-electrodes—architectures for organic solar cells,” Thin Solid Films 451–452, 619–623 (2004). [CrossRef]
24. S.-I. Na, S.-S. Kim, J. Jo, S.-H. Oh, J. Kim, and D.-Y. Kim, “Efficient polymer solar cells with surface relief gratings fabricated by simple soft lithography,” Adv. Funct. Mater. 18, 3956–3963 (2008). [CrossRef]
25. J. R. Tumbleston, D.-H. Ko, E. T. Samulski, and R. Lopez, “Absorption and quasiguided mode analysis of organic solar cells with photonic crystal photoactive layers,” Opt. Express 17, 7670 (2009). [CrossRef] [PubMed]
26. X. Zhu, W. C. Choy, F. Xie, C. Duan, C. Wang, W. He, F. Huang, and Y. Cao, “A study of optical properties enhancement in low-bandgap polymer solar cells with embedded PEDOT:PSS gratings,” Sol. Energy Mater. Sol. Cells 99, 327–332 (2012). [CrossRef]
27. K. Q. Le, A. Abass, B. Maes, P. Bienstman, and A. Alù, “Comparing plasmonic and dielectric gratings for absorption enhancement in thin-film organic solar cells,” Opt. Express 20, A39 (2011). [CrossRef]
28. J.-Y. Chen, M.-H. Yu, C.-Y. Chang, Y.-H. Chao, K. W. Sun, and C.-S. Hsu, “Enhanced performance of organic thin film solar cells using electrodes with nanoimprinted light-diffraction and light-diffusion structures,” ACS Appl. Mater. Interfaces 6, 6164–6169 (2014). [CrossRef] [PubMed]
29. J.-D. Chen, L. Zhou, Q.-D. Ou, Y.-Q. Li, S. Shen, S.-T. Lee, and J.-X. Tang, “Enhanced light harvesting in organic solar cells featuring a biomimetic active layer and a self-cleaning antireflective coating,” Adv. Energy Mater.4, n/a–n/a (2014). [CrossRef]
30. A. J. Smith, C. Wang, D. Guo, C. Sun, and J. Huang, “Repurposing blu-ray movie discs as quasi-random nanoimprinting templates for photon management,” Nat. Commun. 5, 5517 (2014). [CrossRef] [PubMed]
31. L. Zhou, Q.-D. Ou, J.-D. Chen, S. Shen, J.-X. Tang, Y.-Q. Li, and S.-T. Lee, “Light manipulation for organic optoelectronics using bio-inspired moth’s eye nanostructures,” Sci. Rep. 4, 4040 (2014).
35. C. Cho, H. Kim, S. Jeong, S.-W. Baek, J.-W. Seo, D. Han, K. Kim, Y. Park, S. Yoo, and J.-Y. Lee, “Random and v-groove texturing for efficient light trapping in organic photovoltaic cells,” Solar Energy Materials and Solar Cells 115, 36–41 (2013). [CrossRef]
36. J. D. Myers, W. Cao, V. Cassidy, S.-H. Eom, R. Zhou, L. Yang, W. You, and J. Xue, “A universal optical approach to enhancing efficiency of organic-based photovoltaic devices,” Energy Environ. Sci. 5, 6900 (2012). [CrossRef]
37. Y. Chen, M. Elshobaki, Z. Ye, J.-M. Park, M. A. Noack, K.-M. Ho, and S. Chaudhary, “Microlens array induced light absorption enhancement in polymer solar cells,” Phys. Chem. Chem. Phys. 15, 4297 (2013). [CrossRef] [PubMed]
38. K. Forberich, G. Dennler, M. C. Scharber, K. Hingerl, T. Fromherz, and C. J. Brabec, “Performance improvement of organic solar cells with moth eye anti-reflection coating,” Thin Solid Films 516, 7167–7170 (2008). [CrossRef]
39. S. Esiner, T. Bus, M. M. Wienk, K. Hermans, and R. A. J. Janssen, “Quantification and validation of the efficiency enhancement reached by application of a retroreflective light trapping texture on a polymer solar cell,” Adv. Energy Mater. 3, 1013–1017 (2013). [CrossRef]
41. M. G. Moharam and T. K. Gaylord, “Rigorous coupled-wave analysis of planar-grating diffraction,” J. Opt. Soc. Am. 71, 811 (1981). [CrossRef]
43. J.-Q. Xi, M. F. Schubert, J. K. Kim, E. F. Schubert, M. Chen, S.-Y. Lin, W. Liu, and J. A. Smart, “Optical thin-film materials with low refractive index for broadband elimination of fresnel reflection,” Nat. Photonics 1, 176–179 (2007).
44. S. Zhang, L. Ye, W. Zhao, D. Liu, H. Yao, and J. Hou, “Side chain selection for designing highly efficient photovoltaic polymers with 2d-conjugated structure,” Macromolecules 47, 4653–4659 (2014). [CrossRef]
45. M. R. Lenze, T. E. Umbach, C. Lentjes, and K. Meerholz, “Determination of the optical constants of bulk heterojunction active layers from standard solar cell measurements,” Org. Electron. 15, 3584–3589 (2014). [CrossRef]
46. U.S. Department of Energy (DOE)/NREL/ALLIANCE, “Reference solar spectral irradiance: Air mass 1.5; bird simple spectral model:spctral2,” http://rredc.nrel.gov/solar/. Accessed: July, 2015.
47. T. Lanz, B. Ruhstaller, C. Battaglia, and C. Ballif, “Extended light scattering model incorporating coherence for thin-film silicon solar cells,” J. Appl. Phys. 110, 033111 (2011). [CrossRef]