Development of nanoimprint processes for photovoltaic applications

Abstract

Abstract. Due to its high resolution and applicability for large area patterning, nanoimprint lithography (NIL) is a promising technology for photovoltaic (PV) applications. However, a successful industrial application of NIL processes is only possible if large-area processing on thin, brittle, and potentially rough substrates can be achieved in a high-throughput process. The development of NIL processes using the SmartNIL technology from EV Group with a focus on PV applications is described. The authors applied this tooling to realize a honeycomb texture (8 μm period) on the front side of multicrystalline silicon solar cells, leading to an improvement in optical efficiency of 7% relative and a total efficiency gain of 0.5% absolute compared to the industrial standard texture (isotexture). On the rear side of monocrystalline silicon solar cells, the authors realized diffraction gratings to make use of light trapping effects. An absorption enhancement of up to 35% absolute at a wavelength of 1100 nm is demonstrated. Furthermore, photolithography was combined with NIL processes to introduce features for metal contacts into honeycomb master structures, which were initially realized using interference lithography. As a final application, the authors investigated the realization of very fine contact fingers with prismatic shape in order to minimize reflection losses.

Efficiency increase of crystalline silicon solar cells with nanoimprinted rear side gratings for enhanced light trapping

“…It was found that the NIL process did not lead to problems concerning wafer breakage. In [36] it is shown that this even holds for wafers as thin as 50 mm. The reference cells featured no amorphous silicon layer but a 100 nm thick SiO 2 layer instead.…”

“…Behind this passivation layer, a sputtered amorphous Si layer is structured by nanoimprint lithography and subsequent plasma-etching. The photoresist is applied via spin coating and patterned using the smart-Nil technology by EVG with a soft PDMS stamp [36]. The reactive ion etching process for the pattern transfer to the amorphous silicon is conducted at an Oxford Plasmalab 133 with SF 6 and O 2 as etching gases [30,35].…”

“…In terms of reaching highest voltages while also reaching high current densities, this concept might be promising to reach ultimate efficiencies in the long term. In contrast to the sphere grating concept presented in [34], which follows a "bottom-up" approach for the structure fabrication, the concept presented in this paper is a "topdown" approach that requires a mastering but enables the realization of a large variety of structures including additional features, for example for contact formation as presented in [36]. In addition to the results published in [35], we also combine this diffractive rear side structure with a pyramidal front texture (Section 3.2) and investigate the effect of the grating also for thinner wafer based solar cells (150 and 100 mm cell thickness, Section 3.1), where the light trapping effect is more pronounced and where the optimum thickness for ultimately efficient silicon solar cells lies [37].…”

Nanoparticle Scattering for Multijunction Solar Cells: The Tradeoff Between Absorption Enhancement and Transmission Loss

“…Double-layer MgF 2 /TaO x ARCs with Al NP arrays embedded within the TaO x layer were then fabricated. The NP arrays were realized over the whole solar cell area by nanoimprint lithography [19].The mastering was done using interference lithography as described in Ref. [20].…”

Large area patterning using interference and nanoimprint lithography

“…The successful transfer of such concepts requires upscalable high-resolution patterning techniques. We are working on nanoimprint lithography (NIL) processes in order to open up an industrially feasible pattern replication technology [11].…”