Assessing the Blu-ray pattern for light trapping

The arrangement of nanostructures within a light-trapping layer dramatically affects how photon energy is coupled into the plasmonic and waveguide modes confined in the active layer of a solar cell20. Figure 1 shows the Fourier transforms of subwavelength features arranged in periodic, random, quasi-random and Blu-ray patterns. The red and blue circles in Fourier space mark the k-values required to couple incoming light into the waveguide modes at the red and blue ends of the solar spectrum, respectively. Periodic (Fig. 1a) and random (Fig. 1b) patterns yield wavevectors that are either too discretized or too diffuse, respectively. On the other hand, quasi-random patterns (Fig. 1c) can be optimized to yield Fourier spectra that are efficient in light trapping, but these patterns are typically prohibitively expensive to manufacture. In comparison, although still containing a periodic component, the pattern on a pre-written Blu-ray movie disc (Fig. 1d) produces a close-to-optimized distribution of k-values.

Figure 1: Fourier transforms of subwavelength features arranged in periodic, random, quasi-random and Blu-ray patterns. Real space (red scale bar, 2.5 μm) and Fourier space. The circles denote the largest (blue) and smallest k-values needed to couple the entire solar spectrum (315 nm to 2.5 μm) to surface plasmons at the interface between an example photoactive layer, PTB7:PC 71 BM and silver electrode. (a) A periodic and (b) a random pattern yield wavevectors that are either too discretized or too diffuse to lie between the two circles, in contrast to (c) an optimized quasi-random pattern. (d) A Blu-ray pattern, obtained by thresholding an AFM image of the recording layer of a movie disc (see Supplementary Fig. 1), produces surprisingly close-to-optimized distribution of k-values. Full size image

The Blu-ray disc (BD) standard21 was developed for high-density optical data storage, and has proven popular for distributing high-definition movies. The BD standard specifies that the track pitch and width are 320 and 130 nm, respectively, and a single bit would be 75 nm long. When recorded to a disc, the video signals are first compressed to a binary sequence with high entropy per bit (for example, according to the MPEG-4 format), whose local statistics is are very similar to those of a sequence of random, independent, unbiased coin tosses22. When reading data from the disc, however, very short runs of successive zeros or ones yield low signal amplitudes and very long runs are difficult to distinguish from noise due to scratches or fingerprints21. Therefore, an error-control modulation code is applied to the data so that the number of successive zeros or ones is between 2 and 7. When written to the disc, eventually a quasi-random pattern of islands and pits with lengths between 150 and 525 nm are generated (see a representative atomic force microscopy (AFM) image in Supplementary Fig. 1a), whose Fourier components are surprisingly well-suited for photon management over the broad visible and near-infrared portion of the spectrum, regardless of the movie content or area selected (Supplementary Fig. 2).

We numerically assessed the light-trapping effect of the Blu-ray structure on a typical polymer solar cell with a PTB7:PC 71 BM active layer23 (see Supplementary Fig. 3a for the device structure) by performing one-dimensional (1D) rigorous coupled-wave analysis24. We modelled the absorption both parallel and perpendicular to the track, as marked by the four lines on the AFM image shown in Supplementary Fig. 3b. We calculated the corresponding absorption spectra from 315 to 775 nm both under transverse-electric (Supplementary Fig. 3c) and transverse-magnetic (Supplementary Fig. 3d) polarizations in reference to the plane of the cross-section. When compared to a non-patterned solar cell, the spectra of Blu-ray-patterned devices display broadband enhancement under both polarization conditions16,25,26. Extremely high enhancement 113.9% can be found in the region between 700 to 800 nm for transverse-magnetic polarization, indicating the underlying importance of the light-trapping effect in the weak absorption region of the active layer. The overall broadband absorption enhancement of a Blu-ray-patterned device, 18.2%, was calculated by averaging over these simulation results, as shown in Supplementary Fig. 3e.

Photovoltaic device fabrication and characterization

Figure 2a illustrates the typical procedure for fabricating Blu-ray-patterned polymer solar cells. The BD was first delaminated to expose the pattern of pits and lands, which was replicated on a polydimethylsiloxane (PDMS) stamp. AFM image (Fig. 2b) of the resulting stamp confirms that it is a high-quality negative replica of the Blu-ray pattern with features as small as 150 nm across and 25 nm high. Next, the pattern was imprinted into a pre-fabricated polymer active layer by contact moulding using the stamp, followed by electrode deposition to complete the device. The final device structure is illustrated in Fig. 2c.

Figure 2: Processing of Blu-ray-patterned solar cells. (a) Schematic diagramming the process for delaminating a BD and casting a PDMS mould on the exposed recording layer; fabricating the nanopatterned solar cells; and imprinting the active layer using the nanopatterned PDMS mould and evaporating the MoO 3 /Ag electrode. (b) AFM image of the nanopatterned PDMS mould. (c) Schematic diagram of the solar cell architecture used in the numerical and experimental portions of this work. Full size image

The AFM images in Fig. 3a clearly show successful transfer of the Blu-ray pattern to the active layer after nanoimprinting, in stark contrast with a non-patterned one. Line scans (Fig. 3a) show that the feature sizes are highly consistent with those of both the stamp and the original BD. The transferred pattern (~1 cm2) displayed uniform iridescent reflection (Fig. 3b), demonstrating the reliability of this nanoimprinting process over large areas. We measured the reflection (R) spectra of both non-patterned and Blu-ray-patterned solar cells and plotted 1−R, that is, absorption in Fig. 3c. The absorption of the Blu-ray-imprinted cell is significantly enhanced by 21.8% over the entire absorption profile. Notably, the benefit of light trapping is most pronounced after 700 nm, reaching 49.0%, where the material absorbs weakly. Both observations are consistent with the simulation shown in Supplementary Fig. 3. The enhanced broadband absorption of Blu-ray-patterned solar cells indeed led to improved external quantum efficiencies (EQEs) (Fig. 3d). The overall EQE enhancement averaged over the entire absorption profile is 30.8%, while the averaged enhancement is 85.0% for wavelengths >700 nm. As a result, the patterned cells delivered 16.9% higher short-circuit current densities (J sc ), eventually leading to a power conversion efficiency enhancement of 11.9% (see Supplementary Fig. 4 and Supplementary Table 1 for detailed performance results). Although higher J sc values have been reported for PTB7:PC 71 BM solar cells with thicker active layers (~100 nm)23, in the current study, an active layer thickness of 50 nm was chosen to best demonstrate the light-trapping effect while avoiding shorting caused by imprinting with the 25-nm-deep pattern. However, the depth of the pits could be modified during the mould fabrication step (for example, via etching) to accommodate the needs of alternative thicknesses for device optimization.