Light coupling to quasi-guided modes in nanoimprinted perovskite solar cells

https://doi.org/10.1016/j.solmat.2019.110080Get rights and content

Highlights

  • Nanoimprinted perovskite solar cells increase the PCE by 2% compared to their planar reference devices.

  • Incident light couples to quasi-guided modes near the bandgap of the perovskite absorber.

  • The electrical properties of the nanoimprinted perovskite solar cells remain unchanged.

  • An increase in JSC by up to 9% is proposed for semi-transparent nanophotonic perovskite solar cells.

Abstract

Perovskite photovoltaics has emerged as a promising technology for highly efficient and low-cost solar cells. Further advances in the power conversion efficiencies are awaited by improved light harvesting concepts. One promising route to improve the current-generation in the perovskite solar cells employs nanophotonic perovskite layers. This work reports on a facile route to fabricate nanophotonic perovskite solar cells with enhanced current generation by employing thermal nanoimprint lithography. The nanoimprinted perovskite solar cells show a relative increase in power conversion efficiency by 2% with respect to their planar reference devices. The enhancement in the external quantum efficiency near the bandgap of the nanoimprinted perovskite solar cells by the coupling of the incident light to quasi-guided modes is analyzed in detail. As an outlook, the potential of the nanoimprinting route for semi-transparent perovskite solar cells is discussed via numerical simulations with relative enhancements in short-circuit current density of up to 9%. This increase is particularly promising for nanophotonic perovskite-based multi-junction solar cells.

Introduction

Within only a few years of research, perovskite photovoltaics (PV) emerged as a promising technology for highly efficient and low-cost solar cells, demonstrating high power conversion efficiencies (PCE) exceeding 24% on a laboratory scale [1]. Organic-inorganic perovskite materials combine excellent optoelectronic properties, such as high absorption coefficients and high carrier mobilities, with low material costs and a wide range of potential deposition techniques [2]. Moreover, via compositional engineering of the halide anion in the crystal structure, their bandgap can be easily tuned, making this material a prime candidate for multi-junction PV [[3], [4], [5], [6]]. Thereby, for the first time, a versatile and inexpensive thin-film technology for multi-junction PV is at hand that promises to exceed the Shockley-Queisser limit of the market-dominating single-junction crystalline silicon solar cells [7,8].

The recent advances in perovskite solar cells have been largely underpinned by advances in the composition [[9], [10], [11]] and morphology [12] of the perovskite absorber layer as well as progress in device architectures [13], which employ custom passivation [14,15], hole- and electron-transport layers [[16], [17], [18]]. Furthermore, extensive research has been directed to advance the light harvesting by light-incoupling concepts like random pyramids [19], micro lens arrays [20], nanophotonic front electrodes [21], and fiber array-based front electrodes [22]. Moreover, improved light trapping has been demonstrated via nano- and micro-patterned charge transport layers [[23], [24], [25], [26], [27], [28], [29]], nanophotonic back electrodes [30], rough back electrodes [31], and corrugated substrates for single [32] and multi-junction [33,34] perovskite PV. In addition, nano-patterning the perovskite layer with periodic textures was proposed [[34], [35], [36], [37], [38], [39]] and simulations have predicted an improved short-circuit current density (JSC) in such nanophotonic perovskite solar cells [40,41]. In our recent study, we have shown that patterning the perovskite absorber layer enhances the absorption by coupling incident sunlight to quasi-guided modes [40]. Enhancing the absorption in the weakly absorbing regimes close to the bandgap of the perovskite is essential to maximize the PCE of both opaque as well as of semi-transparent perovskite solar cells [42]. Various nano-patterning techniques have been proposed for the perovskite layer: (1) focused ion beam lithography [43,44], (2) electron beam etching [45,46], recrystallization through phase transformation [47], and (3) thermal nanoimprint lithography (TNIL) [37,38,[48], [49], [50], [51]]. From these, TNIL appears particularly promising as it allows for the patterning of nanostructures at both large scale and high throughput, which is crucial for upscalable fabrication technologies such as roll-to-roll processing [52,53]. In the case of perovskite PV, the applied heat and pressure during the TNIL process is reported to trigger the recrystallization of the perovskite absorber. As shown in the work Mayer et al., the grains of the multicrystalline perovskite thin-film increase during an imprint step [50]. Using a textured mold to directly pattern the perovskite solid-state film with well-designed periodic nanostructures improves its absorption properties and can yield a better crystal structure that exhibits fewer surface defects [49].

The interest in nano-patterned perovskite layers is broad within optoelectronics, having been employed to demonstrate optically-pumped lasing [37,44,49], light-emitting diodes [47] and nanostructured photodetectors [48]. Moreover, recent studies reported an improved JSC for nanoimprinted perovskite solar cells along with a broadband enhancement of the external quantum efficiency [51,54]. Whereas Kim et al. identified the uniaxial compression leading to an enhanced crystal quality and Wang et al. described the nanophotonic light trapping by diffraction of incident light at the textured perovskite interface. However, the enhanced absorption via the coupling of incident light to distinct quasi-guided modes was not examined in detail. The present work builds up on these studies and provides a detailed optical analysis of light coupling to quasi-guided modes in the nanoimprinted perovskite solar cells. An enhanced JSC of the nanoimprinted perovskite solar cells by 2% relative with respect to their planar references is demonstrated. The improvement in PCE is a result of the enhanced external quantum efficiency close to the bandgap of the nanoimprinted perovskite solar cells. Moreover, this study experimentally and numerically discusses the limited enhancement in opaque perovskite solar cells by a nanophotonic absorber layer and reveals the potential of nanophotonic semi-transparent perovskite PV for multi-junction solar cells by a relative enhancement in JSC of 9% compared to their planar references.

Section snippets

Methods

Device fabrication: Pre-patterned indium tin oxide (ITO) substrates on glass (Luminescence Technology) were cleaned in ultrasonic baths of detergent, deionized water, acetone and isopropyl alcohol for 10 min each. Then the tin oxide (SnO2) electron transport layer was spin-coated at a speed of 4000 rpm for 30 s. Therefore, a 15% aqueous colloidal dispersion of SnO2 (Alfa Aesar) is diluted to a final concentration of 2%. The spin-coated SnO2 layer was then annealed in air at 250 °C for 30 min.

Nanoimprinted perovskite solar cells

In order to enhance the JSC in perovskite solar cells, the perovskite layer is periodically textured by TNIL. The textured perovskite enhances the absorption by coupling incident light to quasi-guided modes. The TNIL is performed directly on the triple cation perovskite (Cs0.1(MA0.17FA0.83)0.9Pb(I0.83Br0.17)3) layer, which is solution deposited on the glass /indium tin oxide (ITO) /tin oxide (SnO2–np) superstrate. Prior to the TNIL, the triple cation perovskite layer is annealed at 100 °C for

Conclusion

This work presents a facile route to fabricate nanophotonic perovskite solar cells and demonstrate a 2% improved power conversion efficiency of nanoimprinted perovskite solar cells with respect to their planar references. The enhanced absorption by coupling incident light to quasi-guided modes in the perovskite absorber layer is achieved by texturing the perovskite layer using thermal nanoimprint lithography. The enhanced absorption increases the short-circuit current density of the

Competing financial interest

The authors declare no competing financial interest.

Acknowledgements

The authors would like to thank S. Moghadamzadeh and S. Geisert for conducting the XRD measurements, M. Worgull and M. Schneider for providing the nanoimprint infrastructure, L. Hahn and A. Bacher for conducting the electron-beam lithography, and M. Guttmann for support during the fabrication of the molds. Further, the authors would like to gratefully acknowledged the financial support by the Helmholtz Association through the program “Science and Technology of Nanosystems” (STN), the HYIG of

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