Abstract

Light absorption in semiconductors is a fundamental problem that has broad impact on a wide range of fields. However, it is intrinsically limited by the bandgap energy of the semiconductor. Herein, we study the enhancement of sub-bandgap light absorption in inorganic-organic hybrid perovskite semiconductor films via critical coupling. This is achieved at large incidence angles by balancing radiative and nonradiative decay rates in a planar multilayer structure. We found that a very small loss in the semiconductor layer can result in substantial light absorption. This simple but general method can be used to enhance the optical and optoelectronic responses of semiconductors below the bandgap energy.

© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

1. Introduction

Light absorption in semiconductors is a fundamental problem that has broad impacts on diverse disciplines, including energy harvesting, photodetectors, sensors, imaging, communication, and photocatalysis [1,2]. However, light absorption is intrinsically limited by the semiconductor bandgap energy; absorption coefficients drop rapidly near the band edge, and incident light cannot be efficiently absorbed below the bandgap energy. To overcome this problem, for example, heavy doping of impurities has been used to generate mid-gap energy levels and induce sub-bandgap light absorption [35]. This approach has been employed for intermediate bandgap solar cells, infrared photodetectors, and imaging sensors. More specifically, it was found that the incorporation of dopants at a concentration of the order of 1020 cm−3 in silicon increased light absorption by up to a few percentage points in the sub-bandgap region, and the optoelectronic response of hyperdoped silicon was extended to infrared wavelengths, at room temperature, to wavelengths as long as 2200 nm [6]. Moreover, laser-induced patterning of micrometer-scale surface features has been employed to achieve near-perfect absorption below the bandgap energy of silicon [7,8]. However, both high impurity densities and the presence of surface microstructures can create large numbers of nonradiative recombination centers, and consequently the electrical performance can be severely degraded, with very poor overall quantum efficiency as a result.

Here, we demonstrate light absorption enhancement in the sub-bandgap spectral region using a nano-optical approach. Specifically, we observed the enhancement of light absorption in the sub-bandgap region using solution-processed perovskite semiconductor films, specifically, inorganic–organic hybrid perovskite (methylammonium lead iodide, MAPbI3) films [9,10]. By adjusting the multilayer film geometry and incidence angle, we were able to substantially increase light absorption via critical coupling below the bandgap energy (approximately 1.6 eV). Absorption enhancement was achieved in planar films without surface patterning. In our experiments, we observed that in a perovskite layer a very small loss (k = Im[n] ≈ 0.001) can result in substantial light absorption, approximately 4.8%, in the sub-bandgap region around 1000 nm. We also measured several reference samples and compared experimental data to theoretical calculations. In our analysis, we considered both diffuse reflection (due to surface roughness) and side absorption in other layers (including a bottom metal layer). Our approach does not require heavy doping and thus will not degrade electrical performance. This simple but general method can be also applied to other semiconductors for potential applications in light harvesting and photon detection below the semiconductor bandgap energy.

Figure 1 explains a general idea on our approach. Light absorption in bulk semiconductors is determined by the absorption coefficient α, and the intensity of the incident light I(x) gradually attenuates as it propagates: I(x) = I0e-αx. Therefore, high optical losses or very thick materials are required to absorb a significant amount of incident power. However, when an absorbing material is integrated within an optical resonator, incident light can be strongly absorbed even in a low-loss film via critical coupling (Fig. 1). From the temporal coupled-mode theory [11,12], we find that light absorption in a resonant optical structure can be described by two parameters (the radiative and nonradiative damping rates γrad and γnonrad)

$$A(\omega ) = \frac{{4{\gamma _{rad}}{\gamma _{nonrad}}}}{{{{(\omega - {\omega _0})}^2} + {{({\gamma _{rad}} + {\gamma _{nonrad}})}^2}}},$$
where ω0 is the resonant frequency. When ω = ω0, we have the maximum absorption
$${A_m} = \frac{{4{\gamma _{rad}}{\gamma _{nonrad}}}}{{{{({\gamma _{rad}} + {\gamma _{nonrad}})}^2}}}.$$

 

Fig. 1. General idea for sub-bandgap absorption enhancement. (a) Schematic for light absorption in a planar multilayer film. (b) Critical coupling can be achieved by balancing radiative and nonradiative decay rates. Large absorption can be obtained even in a low-loss film at critical coupling.

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Therefore, when γrad = γnonrad, we can achieve total absorption of incident light, Am = 1. This corresponds to the critical coupling condition in an optical resonator [13,14]. When the two damping rates are not equal, incident light is partially reflected (corresponding to over-damping or under-damping cases). The radiative damping rate γrad is related to the rate of photon leakage from a resonant optical structure, while the nonradiative (or internal) damping rate γnonrad results from material losses. These two parameters can be also related to cavity quality (Q) factors; γrad and γnonrad are inversely proportional to a radiative Q factor, Qrad, and an absorption Q factor, Qabs, respectively.

In this work, we consider an approach based on planar perfect absorber structures that do not require sophisticated structural patterning. Planar perfect absorbers have been actively studied in recent years because they provide a means of enhancing light absorption over large areas, without the use of complicated lithographic patterning [1526]. In the structure we investigate here, a perovskite semiconductor layer is spincoated on a reflecting metal substrate, together with a non-absorbing phase controller, to allow the critical coupling condition to be generally satisfied at large incidence angles. The resulting sub-bandgap absorption is tunable over a broad spectral region via adjustment of the structure of the film. Such a multilayer film forms a planar Fabry–Perot–type resonator, and critical coupling is achieved by balancing the radiative and nonradiative decay rates. In this way, large-area enhancement of sub-bandgap absorption can be achieved without heavy impurity doping. Such critical coupling has been considered in graphene and other two-dimensional materials for enhancement of light absorption [2729], but it has not yet been investigated in the case of semiconductors in the sub-bandgap region, where the absorption coefficients are generally very small.

2. Sample preparation and characterization

To prepare MAPbI3 precursor solutions, a 0.77 : 1 molar ratio of methylammonium iodide (MAI) and PbI2 were dissolved in a mixture of dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) solvents (7 : 3 [v/v]). The concentration of the precursor solutions was carefully controlled to acquire a film of the desired thickness. The perovskite (MAPbI3) precursor solutions were spin-cast on precleaned substrates at 3500 rpm and 6500 rpm for 30 and 5 seconds, respectively. At the start of second step, the films were treated with a drop of chlorobenzene solvent to prevent fast crystallization. After spincoating, the films were annealed at 90 °C for 10 min. Poly(methyl methacrylate) (PMMA) solution was prepared with a concentration of 1 wt% in chlorobenzene. The solution was spin-cast onto the perovskite samples at 3000 rpm for 30 seconds to prevent the samples from degradation. The thicknesses of the perovskite films were measured using a surface profile (P6, KLA Tencor) and atomic force microscopy (AFM; DI-3100, Veeco). After making scratches on the film, we performed scanning over scratches on the surface and averaged the measured values to determine the final film thickness. The surface roughness of the perovskite films was also measured using AFM. The perovskite film on the quartz substrate had a root-mean-square (RMS) roughness of 11.9 nm, but after PMMA top-layer coating, the roughness was reduced to a value in the range of 4–5 nm.

The quality of the spincoated perovskite films was confirmed by x-ray diffraction (XRD) measurements (Fig. 2). Two strong X-ray diffraction peaks at 2θ = 14.2 and 28.5° correspond to the (110) and (220) planes of MAPbI3 tetragonal phase. Less dominant peaks at 2θ = 20.0, 23.6, 24.6, 32.0, 35.0, 40.5, 43.2, and 50.3° correspond to the (112), (211), (202), (222), (312), (224), (314), and (404) planes, respectively. As the proportion of MAI in the precursor solution—which is controlled to ensure the surface is smooth—is reduced, a sharp peak at 12.7° appears and begins to grow. This peak corresponds to the (001) plane of PbI2, and its presence in the XRD spectra implies that a PbI2 phase coexists with the MAPbI3 phase in the sample. Its relative intensity increases continuously as the volume of MAI in the precursor solutions decreases. Although two distinct phases are observed in the sample, the intense MAPbI3 peaks in the XRD spectra indicate that the quality of perovskite crystal is not significantly affected by the neighboring PbI2.

 

Fig. 2. (a) Schematic of the MAPbI3 crystal structure. (b) X-ray diffraction pattern of the perovskite (MAPbI3) film used in this work.

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For our multilayer film ([PMMA/MAPbI3/SiO2/silver/quartz]), a thick silver film (≈ 200 nm) was first deposited on a quartz substrate (area: 1 inch × 1 inch) by electron-beam evaporation. Then, a silicon dioxide (SiO2) layer was deposited on top of the silver by plasma-enhanced chemical vapor deposition (PECVD). This oxide spacer layer works as a phase controller in our sample. The thickness of the oxide layer was varied in the range from 50 to 100 nm, and the thickness was measured using ellipsometry, after film deposition. We also prepared a reference sample without a perovskite layer ([PMMA/SiO2/silver/quartz]) to measure side absorptions in other layers. For near-normal incidence measurements (at an incident angle of 3.3° with respect to the surface normal), we used a spectrophotometer (Cary 5000, Varian) with an integrating sphere. In this way, we were able to measure both specular and diffuse components. For oblique incidence, we were able to measure only the specular reflection.

3. Results and discussion

Our key experimental results are presented in Fig. 3. We first measured a perovskite film (thickness ≈ 270 nm) on a quartz substrate at normal incidence [Fig. 3(a)]. Absorption A is obtained by taking A = 1 – RT, where R and T are the measured reflection and transmission, respectively. For all the near-normal incidence measurements, we obtained net absorption Anet in the sample, considering both the specular and diffuse components of light: Anet = 1 – RspecularRdiffuse. Light absorption in the perovskite film decreases rapidly at wavelengths longer than 750 nm because absorption coefficients drop rapidly near the band edge. However, our perovskite film still has small but non-zero absorption in the sub-bandgap region (850–1400 nm). For comparison, we also measured bare quartz [Appendix Fig. 8] and, in this case, absorption was nearly zero across the entire visible and near-IR region. The small sub-bandgap absorption in the perovskite film could originate from defect states in the mid-gap region that naturally arose during the solution-processing and crystallization of the perovskite layer [30,31]. This small but non-zero optical absorption can be further enhanced by critical coupling, as detailed below.

 

Fig. 3. Key experimental results: (a) Net absorption in normal incidence for [MAPbI3 (270 nm)/quartz]. The perovskite film on quartz has small but non-zero absorption (about 1.45% at 1130 nm). (b) Net absorption in normal incidence for [PMMA/MAPbI3/SiO2/silver/quartz]. (c) Nominal absorption (Anominal = 1 – Rspecular) in s-polarized oblique incidence for [PMMA/MAPbI3/SiO2/silver/quartz]. (d) and (e) Reference sample measurements in normal and s-polarized oblique incidences.

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We now consider a perovskite sample with a metal back layer. Because the metal layer is sufficiently thick, there is essentially no transmission (T ≈ 0). Therefore, strong absorption can be obtained by suppressing reflection via destructive interference. Our multilayer film includes an oxide spacer layer (50-nm thick) and a PMMA top layer (30-nm thick) together with a perovskite layer ([PMMA/MAPbI3/SiO2/silver/quartz]). The metal substrate with oxide spacer effectively works as a dispersive reflecting medium. The phase change from this reflecting medium combined with that from the absorbing semiconductor film can generate destructive interference of incident light [32]. We measured sub-bandgap absorption at both normal and oblique incidences. At normal incidence [Fig. 3(b)], resonance absorption is clearly visible (approx. 4% at 1100 nm).

At oblique incidence, we observe more drastic changes with incidence angle. Sub-bandgap absorption increases rapidly as the angle increases. Incidence angles were varied between 20° and 70°, and the measured s-polarized absorption spectra are shown in Fig. 3(c). A prominent sub-bandgap absorption peak occurs for large incidence angles, reaching about 20% absorption at 70°. This absorption peak gradually blueshifts with incidence angle, as expected for a Fabry–Perot resonance [33]. The measured p-polarized absorption spectra are given in Appendix Fig. 9. These also show angle-dependent behavior, but the absorption is generally smaller than the s-polarized light absorption.

For comparison, we also measured absorption spectra from a reference sample without a perovskite layer [Figs. 3(d) and 3(e)]. The sample structure is the same as the previously mentioned samples, except that it does not include a semiconductor layer ([PMMA/ SiO2/silver/quartz]). In this case, the thickness of the PMMA layer was further increased to approximately 590 nm, to ensure that the optical resonance remains in the same spectral region. Using this sample, we can observe the effects of side absorption in the metal and other layers. For both the normal and oblique incidences, we obtain some absorption features, but they are much smaller than those observed for the previous sample with the perovskite layer. This indicates that the drastic change in Fig. 3(c) is indeed related to absorption in the perovskite layer.

The surface roughness of the perovskite layer can cause diffuse reflection and affect our measurements. In the case of the normal incidence measurements, we were able to directly measure diffuse-reflection spectra using an integrating sphere. Figure 4 shows the measured diffuse-reflection spectra. The perovskite layer on quartz has an RMS roughness of approximately 11.9 nm, but we found that this bare perovskite layer has very weak diffuse reflection [Fig. 4(a)]. In case of the multilayer sample with a metal back layer (RMS surface roughness, approx. 6.08 nm), we measured a diffuse reflection level of 4.3% at 1040 nm [Fig. 4(b)]. Note that the RMS value in Fig. 4(b) was reduced by the PMMA top layer coating. The multilayer film in Fig. 4(b) supports a Fabry–Perot–type resonance centered at 1040 nm and scatters more light during multiple reflections in the film compared to the single layer film in Fig. 4(a). In Fig. 3(b) (normal incidence), we included both diffuse- and specular-reflection components, presenting the net absorption in our sample. However, in the case of oblique incidence, we were only able to measure specular reflection, and thus the spectrum in Fig. 3(c) is overestimated as it also includes diffuse reflection. Therefore, we denote the quantity in Fig. 3(c) as the nominal absorption: Anominal = 1 – Rspecular = Anet + Rdiffuse.

 

Fig. 4. Diffuse reflection measurements in normal incidence for (a) [MAPbI3/quartz] and (b) [PMMA/MAPbI3/SiO2/silver/quartz].

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In Fig. 5, measured (red lines) and simulated (dotted lines) spectra are compared, and the net absorption for oblique incidence is estimated. The optical constants for silver that we used for simulations are given in Appendix Fig. 10(a). Using data from the reference sample without the perovskite layer [Figs. 3(d) and 3(e)], we confirmed that our simulation agrees well the experimental spectra for both normal and oblique incidences [see Appendix Figs. 10(b) and 10(c)]. The refractive indices of the oxide and PMMA layers were determined from ellipsometry measurements. However, for the perovskite layer, it was not easy to accurately measure the extinction coefficient k (i.e., the imaginary part of the complex refractive index, Im[n] = k) in the sub-bandgap region, because ellipsometry fitting often tends to simply set the very small extinction coefficient below the bandgap energy to zero. Instead, we compared our experimental data with simulations and estimated the extinction coefficient k of the perovskite layer. For normal incidence, the net absorption measured in the sample was compared to the multilayer calculation and we found that using k ≈ 0.001 gave good agreement with the measured spectrum [Fig. 5(a)]. For oblique incidence, we calculated net absorption with k ≈ 0.001, and this was compared to the measured spectrum (Anominal = 1 – Rspecular) in Fig. 5(b). From our calculations, we obtained a net absorption value of about 11.5% at 1000 nm for an incidence angle of 70°. We suggest that the difference between the measured (red line) and calculated (black dotted line) spectra in Fig. 5(b) was caused by diffuse reflection from our sample. In fact, the perovskite extinction coefficient is very small in the sub-bandgap region and identifying its exact value is not straightforward. For example, when we compared the experimental data in Fig. 3(a) (bare perovskite on quartz) to calculation, we obtained a larger extinction coefficient, about k ≈ 0.0045, in the perovskite layer. Figure 11 in the Appendix includes more results related to this aspect.

 

Fig. 5. Comparison between measured (red line) and calculated (dotted line) spectra. (a) By comparing normal incidence measurement data with multilayer calculations, the extinction coefficient k of the perovskite layer is estimated to be k ∼ 0.001. (b) Calculated net absorption for oblique incidence is compared to the measured spectrum Anominal. We obtain net absorption about 11.5% at 1000 nm for the incidence angle of 70°. The difference between measured and calculated spectra is considered to be caused by diffuse reflection.

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The sub-bandgap absorption peak can be tuned over a broad spectral region by adjusting the thickness of the oxide spacer layer. Appendix Fig. 12 displays the measured spectra for the 75- and 100-nm oxide layers, where it can be seen that the resonance peaks shift to longer wavelengths (1100 and 1200 nm, respectively).

Figure 6(a) shows the absorption colormap as a function of wavelength and incidence angle for s-polarized incident light, obtained from transfer matrix method (TMM) calculations. In our measurements, the range of incidence angles was limited to a maximum of 70°. However, in the TMM calculations, we considered the full range of incidence angles (0–90°). Enhanced absorption band in the sub-bandgap region is apparent in Fig. 6(a); the peak blueshifts with increasing angle, in agreement with our measured spectra [Fig. 3(c)]. Moreover, we obtain very strong absorption at a large incidence angles (>85°). We also investigated the fraction of incident light energy that is actually absorbed within the perovskite layer. To this end, we calculated the absorbed power profile of our multilayer film ([PMMA/MAPbI3/SiO2/silver/quartz]) using finite-difference time domain (FDTD) simulations. The absorbed power per unit volume (Pabs) can be determined from the divergence of the Poynting vector, and we obtained the following relation [34]:

$${P_{abs}} = \frac{1}{2}\omega {|{E(\omega )} |^2}{\mathop{\textrm {Im}}\nolimits} [{\varepsilon _0}\varepsilon (\omega )].$$
Figure 6(b) shows the absorbed power profile for incidence angles of 0 and 70°, respectively, assuming an extinction coefficient k ≈ 0.001 for perovskite. By integrating the absorbed power density in each layer, we can obtain the fraction of light absorption for each layer. We found that about 42% of the incident light is absorbed in the perovskite layer at 70°, with the remaining absorption occurring in the metal layer. From Figs. 5(b) and 6(b), we estimate that about 4.8% of the light incident upon our multilayer sample was absorbed in the perovskite layer at 1000 nm. This represents enhancement by a factor of 3.3 compared to the absorption of bare perovskite on quartz [Fig. 3(a)]. In fact, the fraction of light absorbed in the perovskite layer rapidly increases with the extinction coefficient. In Appendix Fig. 11, we present the calculated absorbed power profile for k ≈ 0.0045, and can be seen that the fraction absorbed in the perovskite layer reaches 76.4% at 70° and increases up to 77.31% at 88°. In this case, the sub-bandgap absorption in the perovskite layer increases correspondingly.

 

Fig. 6. (a) Absorption colormap as a function of wavelength and incidence angle for s-polarization. (b) Absorbed power profile in our multilayer film for incidence angles of 0 ° and 70°. By integrating the absorbed power density in each layer, the fraction of light absorption in each layer can be obtained.

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To explicitly assess the critical coupling condition in our multilayer film, we directly compared the radiative and nonradiative damping rates. Figure 7(a) shows the simulated absorption spectra for different incidence angles, assuming a perovskite extinction coefficient of k ≈ 0.001. We used these spectra to extract radiative and nonradiative damping rates. Absorption spectra were fitted to a Lorentzian function to determine the resonance linewidth, 2γtotal. Using Eq. (1) and the total damping rate (γtotal = γrad + γnonrad), we obtained the two damping rates γrad and γnonrad. Figure 7(b) displays the extracted damping rates; it can be seen that the radiative damping rate gradually decreases with incident angle, while the nonradiative one remains almost constant. At large angles of incidence (>85°), the two rates approach the same value, and thus we can obtain strong absorption even in a low-loss film. For a film with higher optical losses, the nonradiative damping rate is greater, and an exact crossing of two damping rates would occur at a smaller incidence angle. This critical coupling is a very general phenomenon, and therefore we expect this approach could be applied to other semiconductors. Although a large incidence angle is required in the low-loss sub-bandgap region, it could be achieved with appropriate mirror configurations [35].

 

Fig. 7. (a) Simulated absorption spectra for s-polarization. (b) Extracted radiative and nonradiative damping rates for different incidence angles. Two damping rates are getting very close at large incidence angles and thus strong absorption can be achieved in the low-loss, sub-bandgap region.

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4. Conclusion

In conclusion, we described a simple but general approach to obtain light absorption enhancement via critical coupling at wavelengths that correspond to energies below the semiconductor bandgap. Such critical coupling can be achieved at a large incidence angle by balancing the radiative and nonradiative decay rates in planar semiconductor films. Planar multilayer structures are easily fabricated, and straightforwardly integrated with other device components. Specifically, we demonstrated sub-bandgap absorption enhancement in inorganic–organic hybrid perovskite semiconductor films. In our analysis, we considered the effect of surface roughness and side absorptions and identified the net sub-bandgap absorption in the semiconductor layer. Our approach does not require heavy impurity doping and thus will not degrade electrical performance. Furthermore, it can also be applied to other semiconductors, to enhance optical and optoelectronic responses below the semiconductor bandgap energy.

Appendix

Here, we provide additional data for reference measurements and simulations (Figs. 812).

 

Fig. 8. Net absorption spectrum for bare quartz.

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Fig. 9. p-polarized nominal absorption spectra for [PMMA/MAPbI3/SiO2/silver/quartz].

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Fig. 10. (a) The optical constants for silver used in simulations. (b) Simulations match well the experimental spectra from the reference sample for both (b) normal and (c) oblique (70°) incidences.

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Fig. 11. (a) Estimation of perovskite extinction coefficient k in the sub-bandgap region from [MAPbI3/quartz]. We find a larger extinction coefficient about k ∼ 0.0045 in the perovskite layer. (b) Extracted radiative and nonradiative damping rates (c) Absorption color map. (d) Absorbed power profile for k ∼ 0.0045. The absorbed fraction in the perovskite reaches 76.4% at the incidence angle of 70°.

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Fig. 12. Measured spectra for 75 nm and 100 nm oxides. The resonance peaks shift to longer wavelengths (1100 nm and 1200 nm, respectively). Sub-bandgap absorption can be tuned over a broad spectral region by adjusting the spacer layer thickness.

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Funding

National Research Foundation of Korea (NRF-2017R1A2B3010049, NRF-2018R1E1A2A02086050, NRF-2019R1A2C1008330); Ulsan National Institute of Science and Technology (1.190109.01).

Acknowledgments

We thank Prof. Kyoung Jin Choi and Chan Ul Kim for the help of film deposition.

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30. D. W. Miller, G. E. Eperon, E. T. Roe, C. W. Warren, H. J. Snaith, and M. C. Lonergan, “Defect states in perovskite solar cells associated with hysteresis and performance,” Appl. Phys. Lett. 109(15), 153902 (2016). [CrossRef]  

31. C. M. Sutter-Fella, D. W. Miller, Q. P. Ngo, E. T. Roe, F. M. Toma, I. D. Sharp, M. C. Lonergan, and A. Javey, “Band tailing and deep defect states in CH3NH3Pb(I1−xBrx)3 perovskites as revealed by sub-bandgap photocurrent,” ACS Energy Lett. 2(3), 709–715 (2017). [CrossRef]  

32. G.-H. Jung, S. J. Yoo, J.-S. Kim, and Q.-H. Park, “Maximal visible light energy transfer to ultrathin semiconductor films enabled by dispersion control,” Adv. Opt. Mater. 7(7), 1801229 (2019). [CrossRef]  

33. B. E. A. Saleh and M. C. Teich, Fundamentals of Photonics (Wiley, 2007), Chap. 10.

34. J. D. Jackson, Classical Electrodynamics (Wiley, 1999), Chap. 6.

35. S. J. Kim, G. Y. Margulis, S. Rim, M. L. Brongersma, M. D. McGehee, and P. Peumans, “Geometric light trapping with a V-trap for efficient organic solar cells,” Opt. Express 21(S3), A305–A312 (2013). [CrossRef]  

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  17. J. Park, J.-H. Kang, A. P. Vasudev, D. T. Schoen, H. Kim, E. Hasman, and M. L. Brongersma, “Omnidirectional near-unity absorption in an ultrathin planar semiconductor layer on a metal substrate,” ACS Photonics 1(9), 812–821 (2014).
    [Crossref]
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    [Crossref]
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    [Crossref]
  23. S. S. Mirshafieyan, T. S. Luk, and J. Guo, “Zeroth order Fabry-Perot resonance enabled ultra-thin perfect light absorber using percolation aluminum and silicon nanofilms,” Opt. Mater. Express 6(4), 1032–1042 (2016).
    [Crossref]
  24. B. H. Woo, I. C. Seo, E. Lee, S. Y. Kim, T. Y. Kim, S. C. Lim, H. Y. Jeong, C. K. Hwangbo, and Y. C. Jun, “Dispersion control of excitonic thin films for tailored super-absorption in the visible region,” ACS Photonics 4(5), 1138–1145 (2017).
    [Crossref]
  25. B. H. Woo, I. C. Seo, E. Lee, S.-C. An, H. Y. Jeong, and Y. C. Jun, “Angle-dependent optical perfect absorption and enhanced photoluminescence in excitonic thin films,” Opt. Express 25(23), 28619–28629 (2017).
    [Crossref]
  26. E. Lee, B. H. Woo, I. C. Seo, S.-C. An, and Y. C. Jun, “Surface bound waves and optical interactions in excitonic thin films,” Opt. Mater. Express 8(9), 2687–2701 (2018).
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  27. M. Pu, P. Chen, Y. Wang, Z. Zhao, C. Wang, C. Huang, C. Hu, and X. Luo, “Strong enhancement of light absorption and highly directive thermal emission in graphene,” Opt. Express 21(10), 11618–11627 (2013).
    [Crossref]
  28. L. Zhu, F. Liu, H. Lin, J. Hu, Z. Yu, X. Wang, and S. Fan, “Angle-selective perfect absorption with two-dimensional materials,” Light: Sci. Appl. 5(3), e16052 (2016).
    [Crossref]
  29. D. Liu and Q. Li, “Sub-nanometer planar solar absorber,” Nano Energy 34, 172–180 (2017).
    [Crossref]
  30. D. W. Miller, G. E. Eperon, E. T. Roe, C. W. Warren, H. J. Snaith, and M. C. Lonergan, “Defect states in perovskite solar cells associated with hysteresis and performance,” Appl. Phys. Lett. 109(15), 153902 (2016).
    [Crossref]
  31. C. M. Sutter-Fella, D. W. Miller, Q. P. Ngo, E. T. Roe, F. M. Toma, I. D. Sharp, M. C. Lonergan, and A. Javey, “Band tailing and deep defect states in CH3NH3Pb(I1−xBrx)3 perovskites as revealed by sub-bandgap photocurrent,” ACS Energy Lett. 2(3), 709–715 (2017).
    [Crossref]
  32. G.-H. Jung, S. J. Yoo, J.-S. Kim, and Q.-H. Park, “Maximal visible light energy transfer to ultrathin semiconductor films enabled by dispersion control,” Adv. Opt. Mater. 7(7), 1801229 (2019).
    [Crossref]
  33. B. E. A. Saleh and M. C. Teich, Fundamentals of Photonics (Wiley, 2007), Chap. 10.
  34. J. D. Jackson, Classical Electrodynamics (Wiley, 1999), Chap. 6.
  35. S. J. Kim, G. Y. Margulis, S. Rim, M. L. Brongersma, M. D. McGehee, and P. Peumans, “Geometric light trapping with a V-trap for efficient organic solar cells,” Opt. Express 21(S3), A305–A312 (2013).
    [Crossref]

2019 (1)

G.-H. Jung, S. J. Yoo, J.-S. Kim, and Q.-H. Park, “Maximal visible light energy transfer to ultrathin semiconductor films enabled by dispersion control,” Adv. Opt. Mater. 7(7), 1801229 (2019).
[Crossref]

2018 (1)

2017 (5)

B. H. Woo, I. C. Seo, E. Lee, S. Y. Kim, T. Y. Kim, S. C. Lim, H. Y. Jeong, C. K. Hwangbo, and Y. C. Jun, “Dispersion control of excitonic thin films for tailored super-absorption in the visible region,” ACS Photonics 4(5), 1138–1145 (2017).
[Crossref]

B. H. Woo, I. C. Seo, E. Lee, S.-C. An, H. Y. Jeong, and Y. C. Jun, “Angle-dependent optical perfect absorption and enhanced photoluminescence in excitonic thin films,” Opt. Express 25(23), 28619–28629 (2017).
[Crossref]

D. Liu and Q. Li, “Sub-nanometer planar solar absorber,” Nano Energy 34, 172–180 (2017).
[Crossref]

C. M. Sutter-Fella, D. W. Miller, Q. P. Ngo, E. T. Roe, F. M. Toma, I. D. Sharp, M. C. Lonergan, and A. Javey, “Band tailing and deep defect states in CH3NH3Pb(I1−xBrx)3 perovskites as revealed by sub-bandgap photocurrent,” ACS Energy Lett. 2(3), 709–715 (2017).
[Crossref]

J. Huang, Y. Yuan, Y. Shao, and Y. Yan, “Understanding the physical properties of hybrid perovskites for photovoltaic applications,” Nat. Rev. Mater. 2(7), 17042 (2017).
[Crossref]

2016 (5)

M. Shirayama, H. Kadowaki, T. Miyadera, T. Sugita, M. Tamakoshi, M. Kato, T. Fujiseki, D. Murata, S. Hara, T. N. Murakami, S. Fujimoto, M. Chikamatsu, and H. Fujiwara, “Optical transitions in hybrid perovskite solar cells: ellipsometry, density functional theory, and quantum efficiency analyses for CH3NH3PbI3,” Phys. Rev. Appl. 5(1), 014012 (2016).
[Crossref]

L. Zhu, F. Liu, H. Lin, J. Hu, Z. Yu, X. Wang, and S. Fan, “Angle-selective perfect absorption with two-dimensional materials,” Light: Sci. Appl. 5(3), e16052 (2016).
[Crossref]

D. W. Miller, G. E. Eperon, E. T. Roe, C. W. Warren, H. J. Snaith, and M. C. Lonergan, “Defect states in perovskite solar cells associated with hysteresis and performance,” Appl. Phys. Lett. 109(15), 153902 (2016).
[Crossref]

M. A. Kats and F. Capasso, “Optical absorbers based on strong interference in ultra-thin films,” Laser Photonics Rev. 10(5), 735–749 (2016).
[Crossref]

S. S. Mirshafieyan, T. S. Luk, and J. Guo, “Zeroth order Fabry-Perot resonance enabled ultra-thin perfect light absorber using percolation aluminum and silicon nanofilms,” Opt. Mater. Express 6(4), 1032–1042 (2016).
[Crossref]

2015 (4)

J. Yoon, M. Zhou, M. A. Badsha, T. Y. Kim, Y. C. Jun, and C. K. Hwangbo, “Broadband epsilon-near-zero perfect absorption in the near-infrared,” Sci. Rep. 5(1), 12788 (2015).
[Crossref]

Z. Li, S. Butun, and K. Aydin, “Large-area, lithography-free super absorbers and color filters at visible frequencies using ultrathin metallic films,” ACS Photonics 2(2), 183–188 (2015).
[Crossref]

B. Franta, D. Pastor, H. H. Gandhi, P. H. Rekemeyer, S. Gradečak, M. J. Aziz, and E. Mazur, “Simultaneous high crystallinity and sub-bandgap optical absorptance in hyperdoped black silicon using nanosecond laser annealing,” J. Appl. Phys. 118(22), 225303 (2015).
[Crossref]

Y. Okada, N. J. Ekins-Daukes, T. Kita, R. Tamaki, M. Yoshida, A. Pusch, O. Hess, C. C. Phillips, D. J. Farrell, K. Yoshida, N. Ahsan, Y. Shoji, T. Sogabe, and J.-F. Guillemoles, “Intermediate band solar cells: recent progress and future directions,” Appl. Phys. Rev. 2(2), 021302 (2015).
[Crossref]

2014 (6)

J. P. Mailoa, A. J. Akey, C. B. Simmons, D. Hutchinson, J. Mathews, J. T. Sullivan, D. Recht, M. T. Winkler, J. S. Williams, J. M. Warrender, P. D. Persans, M. J. Aziz, and T. Buonassisi, “Room-temperature sub-band gap optoelectronic response of hyperdoped silicon,” Nat. Commun. 5(1), 3011 (2014).
[Crossref]

S. Collin, “Nanostructure arrays in free-space: optical properties and applications,” Rep. Prog. Phys. 77(12), 126402 (2014).
[Crossref]

B. Ding, M. Qiu, and R. J. Blaikie, “Manipulating light absorption in dye-doped dielectric films on reflecting surfaces,” Opt. Express 22(21), 25965–25975 (2014).
[Crossref]

J. Park, J.-H. Kang, A. P. Vasudev, D. T. Schoen, H. Kim, E. Hasman, and M. L. Brongersma, “Omnidirectional near-unity absorption in an ultrathin planar semiconductor layer on a metal substrate,” ACS Photonics 1(9), 812–821 (2014).
[Crossref]

Y. Long, R. Su, Q. Wang, L. Shen, B. Li, and W. Zheng, “Deducing critical coupling condition to achieve perfect absorption for thin-film absorbers and identifying key characteristics of absorbing materials needed for perfect absorption,” Appl. Phys. Lett. 104(9), 091109 (2014).
[Crossref]

J.-B. You, W.-J. Lee, D. Won, and K. Yu, “Multiband perfect absorbers using metal-dielectric films with optically dense medium for angle and polarization insensitive operation,” Opt. Express 22(7), 8339–8348 (2014).
[Crossref]

2013 (2)

2012 (2)

M. A. Kats, D. Sharma, Z. Yang, and F. Capasso, “Ultra-thin perfect absorber employing a tunable phase change material,” Appl. Phys. Lett. 101(22), 221101 (2012).
[Crossref]

A. Luque, A. Martí, and C. Stanley, “Understanding intermediate-band solar cells,” Nat. Photonics 6(3), 146–152 (2012).
[Crossref]

2011 (1)

S. H. Pan, D. Recht, S. Charnvanichborikarn, J. S. Williams, and M. J. Aziz, “Enhanced visible and near-infrared optical absorption in silicon supersaturated with chalcogens,” Appl. Phys. Lett. 98(12), 121913 (2011).
[Crossref]

2006 (1)

2003 (1)

S. Fan, W. Suh, and J. D. Joannopoulos, “Temporal coupled mode theory for fano resonance in optical resonators,” J. Opt. Soc. Am. A 20(3), 567–572 (2003).
[Crossref]

2002 (1)

A. Yariv, “Critical coupling and its control in optical waveguide-ring resonator systems,” IEEE Photonics Technol. Lett. 14(4), 483–485 (2002).
[Crossref]

2001 (1)

C. Wu, C. H. Crouch, L. Zhao, J. E. Carey, R. Younkin, J. A. Levinson, E. Mazur, R. M. Farrell, P. Gothoskar, and A. Karger, “Near-unity below-band-gap absorption by microstructured silicon,” Appl. Phys. Lett. 78(13), 1850–1852 (2001).
[Crossref]

Ahsan, N.

Y. Okada, N. J. Ekins-Daukes, T. Kita, R. Tamaki, M. Yoshida, A. Pusch, O. Hess, C. C. Phillips, D. J. Farrell, K. Yoshida, N. Ahsan, Y. Shoji, T. Sogabe, and J.-F. Guillemoles, “Intermediate band solar cells: recent progress and future directions,” Appl. Phys. Rev. 2(2), 021302 (2015).
[Crossref]

Akey, A. J.

J. P. Mailoa, A. J. Akey, C. B. Simmons, D. Hutchinson, J. Mathews, J. T. Sullivan, D. Recht, M. T. Winkler, J. S. Williams, J. M. Warrender, P. D. Persans, M. J. Aziz, and T. Buonassisi, “Room-temperature sub-band gap optoelectronic response of hyperdoped silicon,” Nat. Commun. 5(1), 3011 (2014).
[Crossref]

An, S.-C.

Aydin, K.

Z. Li, S. Butun, and K. Aydin, “Large-area, lithography-free super absorbers and color filters at visible frequencies using ultrathin metallic films,” ACS Photonics 2(2), 183–188 (2015).
[Crossref]

Aziz, M. J.

B. Franta, D. Pastor, H. H. Gandhi, P. H. Rekemeyer, S. Gradečak, M. J. Aziz, and E. Mazur, “Simultaneous high crystallinity and sub-bandgap optical absorptance in hyperdoped black silicon using nanosecond laser annealing,” J. Appl. Phys. 118(22), 225303 (2015).
[Crossref]

J. P. Mailoa, A. J. Akey, C. B. Simmons, D. Hutchinson, J. Mathews, J. T. Sullivan, D. Recht, M. T. Winkler, J. S. Williams, J. M. Warrender, P. D. Persans, M. J. Aziz, and T. Buonassisi, “Room-temperature sub-band gap optoelectronic response of hyperdoped silicon,” Nat. Commun. 5(1), 3011 (2014).
[Crossref]

S. H. Pan, D. Recht, S. Charnvanichborikarn, J. S. Williams, and M. J. Aziz, “Enhanced visible and near-infrared optical absorption in silicon supersaturated with chalcogens,” Appl. Phys. Lett. 98(12), 121913 (2011).
[Crossref]

Badsha, M. A.

J. Yoon, M. Zhou, M. A. Badsha, T. Y. Kim, Y. C. Jun, and C. K. Hwangbo, “Broadband epsilon-near-zero perfect absorption in the near-infrared,” Sci. Rep. 5(1), 12788 (2015).
[Crossref]

Blaikie, R. J.

Bradley, M. S.

Brongersma, M. L.

J. Park, J.-H. Kang, A. P. Vasudev, D. T. Schoen, H. Kim, E. Hasman, and M. L. Brongersma, “Omnidirectional near-unity absorption in an ultrathin planar semiconductor layer on a metal substrate,” ACS Photonics 1(9), 812–821 (2014).
[Crossref]

S. J. Kim, G. Y. Margulis, S. Rim, M. L. Brongersma, M. D. McGehee, and P. Peumans, “Geometric light trapping with a V-trap for efficient organic solar cells,” Opt. Express 21(S3), A305–A312 (2013).
[Crossref]

Bulovic, V.

Buonassisi, T.

J. P. Mailoa, A. J. Akey, C. B. Simmons, D. Hutchinson, J. Mathews, J. T. Sullivan, D. Recht, M. T. Winkler, J. S. Williams, J. M. Warrender, P. D. Persans, M. J. Aziz, and T. Buonassisi, “Room-temperature sub-band gap optoelectronic response of hyperdoped silicon,” Nat. Commun. 5(1), 3011 (2014).
[Crossref]

Butun, S.

Z. Li, S. Butun, and K. Aydin, “Large-area, lithography-free super absorbers and color filters at visible frequencies using ultrathin metallic films,” ACS Photonics 2(2), 183–188 (2015).
[Crossref]

Capasso, F.

M. A. Kats and F. Capasso, “Optical absorbers based on strong interference in ultra-thin films,” Laser Photonics Rev. 10(5), 735–749 (2016).
[Crossref]

M. A. Kats, D. Sharma, Z. Yang, and F. Capasso, “Ultra-thin perfect absorber employing a tunable phase change material,” Appl. Phys. Lett. 101(22), 221101 (2012).
[Crossref]

Carey, J. E.

C. Wu, C. H. Crouch, L. Zhao, J. E. Carey, R. Younkin, J. A. Levinson, E. Mazur, R. M. Farrell, P. Gothoskar, and A. Karger, “Near-unity below-band-gap absorption by microstructured silicon,” Appl. Phys. Lett. 78(13), 1850–1852 (2001).
[Crossref]

Charnvanichborikarn, S.

S. H. Pan, D. Recht, S. Charnvanichborikarn, J. S. Williams, and M. J. Aziz, “Enhanced visible and near-infrared optical absorption in silicon supersaturated with chalcogens,” Appl. Phys. Lett. 98(12), 121913 (2011).
[Crossref]

Chen, P.

Chikamatsu, M.

M. Shirayama, H. Kadowaki, T. Miyadera, T. Sugita, M. Tamakoshi, M. Kato, T. Fujiseki, D. Murata, S. Hara, T. N. Murakami, S. Fujimoto, M. Chikamatsu, and H. Fujiwara, “Optical transitions in hybrid perovskite solar cells: ellipsometry, density functional theory, and quantum efficiency analyses for CH3NH3PbI3,” Phys. Rev. Appl. 5(1), 014012 (2016).
[Crossref]

Collin, S.

S. Collin, “Nanostructure arrays in free-space: optical properties and applications,” Rep. Prog. Phys. 77(12), 126402 (2014).
[Crossref]

Crouch, C. H.

C. Wu, C. H. Crouch, L. Zhao, J. E. Carey, R. Younkin, J. A. Levinson, E. Mazur, R. M. Farrell, P. Gothoskar, and A. Karger, “Near-unity below-band-gap absorption by microstructured silicon,” Appl. Phys. Lett. 78(13), 1850–1852 (2001).
[Crossref]

Ding, B.

Ekins-Daukes, N. J.

Y. Okada, N. J. Ekins-Daukes, T. Kita, R. Tamaki, M. Yoshida, A. Pusch, O. Hess, C. C. Phillips, D. J. Farrell, K. Yoshida, N. Ahsan, Y. Shoji, T. Sogabe, and J.-F. Guillemoles, “Intermediate band solar cells: recent progress and future directions,” Appl. Phys. Rev. 2(2), 021302 (2015).
[Crossref]

Eperon, G. E.

D. W. Miller, G. E. Eperon, E. T. Roe, C. W. Warren, H. J. Snaith, and M. C. Lonergan, “Defect states in perovskite solar cells associated with hysteresis and performance,” Appl. Phys. Lett. 109(15), 153902 (2016).
[Crossref]

Fan, S.

L. Zhu, F. Liu, H. Lin, J. Hu, Z. Yu, X. Wang, and S. Fan, “Angle-selective perfect absorption with two-dimensional materials,” Light: Sci. Appl. 5(3), e16052 (2016).
[Crossref]

S. Fan, W. Suh, and J. D. Joannopoulos, “Temporal coupled mode theory for fano resonance in optical resonators,” J. Opt. Soc. Am. A 20(3), 567–572 (2003).
[Crossref]

Farrell, D. J.

Y. Okada, N. J. Ekins-Daukes, T. Kita, R. Tamaki, M. Yoshida, A. Pusch, O. Hess, C. C. Phillips, D. J. Farrell, K. Yoshida, N. Ahsan, Y. Shoji, T. Sogabe, and J.-F. Guillemoles, “Intermediate band solar cells: recent progress and future directions,” Appl. Phys. Rev. 2(2), 021302 (2015).
[Crossref]

Farrell, R. M.

C. Wu, C. H. Crouch, L. Zhao, J. E. Carey, R. Younkin, J. A. Levinson, E. Mazur, R. M. Farrell, P. Gothoskar, and A. Karger, “Near-unity below-band-gap absorption by microstructured silicon,” Appl. Phys. Lett. 78(13), 1850–1852 (2001).
[Crossref]

Franta, B.

B. Franta, D. Pastor, H. H. Gandhi, P. H. Rekemeyer, S. Gradečak, M. J. Aziz, and E. Mazur, “Simultaneous high crystallinity and sub-bandgap optical absorptance in hyperdoped black silicon using nanosecond laser annealing,” J. Appl. Phys. 118(22), 225303 (2015).
[Crossref]

Fujimoto, S.

M. Shirayama, H. Kadowaki, T. Miyadera, T. Sugita, M. Tamakoshi, M. Kato, T. Fujiseki, D. Murata, S. Hara, T. N. Murakami, S. Fujimoto, M. Chikamatsu, and H. Fujiwara, “Optical transitions in hybrid perovskite solar cells: ellipsometry, density functional theory, and quantum efficiency analyses for CH3NH3PbI3,” Phys. Rev. Appl. 5(1), 014012 (2016).
[Crossref]

Fujiseki, T.

M. Shirayama, H. Kadowaki, T. Miyadera, T. Sugita, M. Tamakoshi, M. Kato, T. Fujiseki, D. Murata, S. Hara, T. N. Murakami, S. Fujimoto, M. Chikamatsu, and H. Fujiwara, “Optical transitions in hybrid perovskite solar cells: ellipsometry, density functional theory, and quantum efficiency analyses for CH3NH3PbI3,” Phys. Rev. Appl. 5(1), 014012 (2016).
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Fujiwara, H.

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Sogabe, T.

Y. Okada, N. J. Ekins-Daukes, T. Kita, R. Tamaki, M. Yoshida, A. Pusch, O. Hess, C. C. Phillips, D. J. Farrell, K. Yoshida, N. Ahsan, Y. Shoji, T. Sogabe, and J.-F. Guillemoles, “Intermediate band solar cells: recent progress and future directions,” Appl. Phys. Rev. 2(2), 021302 (2015).
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Stanley, C.

A. Luque, A. Martí, and C. Stanley, “Understanding intermediate-band solar cells,” Nat. Photonics 6(3), 146–152 (2012).
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Y. Long, R. Su, Q. Wang, L. Shen, B. Li, and W. Zheng, “Deducing critical coupling condition to achieve perfect absorption for thin-film absorbers and identifying key characteristics of absorbing materials needed for perfect absorption,” Appl. Phys. Lett. 104(9), 091109 (2014).
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Sugita, T.

M. Shirayama, H. Kadowaki, T. Miyadera, T. Sugita, M. Tamakoshi, M. Kato, T. Fujiseki, D. Murata, S. Hara, T. N. Murakami, S. Fujimoto, M. Chikamatsu, and H. Fujiwara, “Optical transitions in hybrid perovskite solar cells: ellipsometry, density functional theory, and quantum efficiency analyses for CH3NH3PbI3,” Phys. Rev. Appl. 5(1), 014012 (2016).
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J. P. Mailoa, A. J. Akey, C. B. Simmons, D. Hutchinson, J. Mathews, J. T. Sullivan, D. Recht, M. T. Winkler, J. S. Williams, J. M. Warrender, P. D. Persans, M. J. Aziz, and T. Buonassisi, “Room-temperature sub-band gap optoelectronic response of hyperdoped silicon,” Nat. Commun. 5(1), 3011 (2014).
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C. M. Sutter-Fella, D. W. Miller, Q. P. Ngo, E. T. Roe, F. M. Toma, I. D. Sharp, M. C. Lonergan, and A. Javey, “Band tailing and deep defect states in CH3NH3Pb(I1−xBrx)3 perovskites as revealed by sub-bandgap photocurrent,” ACS Energy Lett. 2(3), 709–715 (2017).
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Y. Okada, N. J. Ekins-Daukes, T. Kita, R. Tamaki, M. Yoshida, A. Pusch, O. Hess, C. C. Phillips, D. J. Farrell, K. Yoshida, N. Ahsan, Y. Shoji, T. Sogabe, and J.-F. Guillemoles, “Intermediate band solar cells: recent progress and future directions,” Appl. Phys. Rev. 2(2), 021302 (2015).
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M. Shirayama, H. Kadowaki, T. Miyadera, T. Sugita, M. Tamakoshi, M. Kato, T. Fujiseki, D. Murata, S. Hara, T. N. Murakami, S. Fujimoto, M. Chikamatsu, and H. Fujiwara, “Optical transitions in hybrid perovskite solar cells: ellipsometry, density functional theory, and quantum efficiency analyses for CH3NH3PbI3,” Phys. Rev. Appl. 5(1), 014012 (2016).
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C. M. Sutter-Fella, D. W. Miller, Q. P. Ngo, E. T. Roe, F. M. Toma, I. D. Sharp, M. C. Lonergan, and A. Javey, “Band tailing and deep defect states in CH3NH3Pb(I1−xBrx)3 perovskites as revealed by sub-bandgap photocurrent,” ACS Energy Lett. 2(3), 709–715 (2017).
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J. Park, J.-H. Kang, A. P. Vasudev, D. T. Schoen, H. Kim, E. Hasman, and M. L. Brongersma, “Omnidirectional near-unity absorption in an ultrathin planar semiconductor layer on a metal substrate,” ACS Photonics 1(9), 812–821 (2014).
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Wang, C.

Wang, Q.

Y. Long, R. Su, Q. Wang, L. Shen, B. Li, and W. Zheng, “Deducing critical coupling condition to achieve perfect absorption for thin-film absorbers and identifying key characteristics of absorbing materials needed for perfect absorption,” Appl. Phys. Lett. 104(9), 091109 (2014).
[Crossref]

Wang, X.

L. Zhu, F. Liu, H. Lin, J. Hu, Z. Yu, X. Wang, and S. Fan, “Angle-selective perfect absorption with two-dimensional materials,” Light: Sci. Appl. 5(3), e16052 (2016).
[Crossref]

Wang, Y.

Warren, C. W.

D. W. Miller, G. E. Eperon, E. T. Roe, C. W. Warren, H. J. Snaith, and M. C. Lonergan, “Defect states in perovskite solar cells associated with hysteresis and performance,” Appl. Phys. Lett. 109(15), 153902 (2016).
[Crossref]

Warrender, J. M.

J. P. Mailoa, A. J. Akey, C. B. Simmons, D. Hutchinson, J. Mathews, J. T. Sullivan, D. Recht, M. T. Winkler, J. S. Williams, J. M. Warrender, P. D. Persans, M. J. Aziz, and T. Buonassisi, “Room-temperature sub-band gap optoelectronic response of hyperdoped silicon,” Nat. Commun. 5(1), 3011 (2014).
[Crossref]

Williams, J. S.

J. P. Mailoa, A. J. Akey, C. B. Simmons, D. Hutchinson, J. Mathews, J. T. Sullivan, D. Recht, M. T. Winkler, J. S. Williams, J. M. Warrender, P. D. Persans, M. J. Aziz, and T. Buonassisi, “Room-temperature sub-band gap optoelectronic response of hyperdoped silicon,” Nat. Commun. 5(1), 3011 (2014).
[Crossref]

S. H. Pan, D. Recht, S. Charnvanichborikarn, J. S. Williams, and M. J. Aziz, “Enhanced visible and near-infrared optical absorption in silicon supersaturated with chalcogens,” Appl. Phys. Lett. 98(12), 121913 (2011).
[Crossref]

Winkler, M. T.

J. P. Mailoa, A. J. Akey, C. B. Simmons, D. Hutchinson, J. Mathews, J. T. Sullivan, D. Recht, M. T. Winkler, J. S. Williams, J. M. Warrender, P. D. Persans, M. J. Aziz, and T. Buonassisi, “Room-temperature sub-band gap optoelectronic response of hyperdoped silicon,” Nat. Commun. 5(1), 3011 (2014).
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J. Huang, Y. Yuan, Y. Shao, and Y. Yan, “Understanding the physical properties of hybrid perovskites for photovoltaic applications,” Nat. Rev. Mater. 2(7), 17042 (2017).
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Yang, Z.

M. A. Kats, D. Sharma, Z. Yang, and F. Capasso, “Ultra-thin perfect absorber employing a tunable phase change material,” Appl. Phys. Lett. 101(22), 221101 (2012).
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A. Yariv, “Critical coupling and its control in optical waveguide-ring resonator systems,” IEEE Photonics Technol. Lett. 14(4), 483–485 (2002).
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G.-H. Jung, S. J. Yoo, J.-S. Kim, and Q.-H. Park, “Maximal visible light energy transfer to ultrathin semiconductor films enabled by dispersion control,” Adv. Opt. Mater. 7(7), 1801229 (2019).
[Crossref]

Yoon, J.

J. Yoon, M. Zhou, M. A. Badsha, T. Y. Kim, Y. C. Jun, and C. K. Hwangbo, “Broadband epsilon-near-zero perfect absorption in the near-infrared,” Sci. Rep. 5(1), 12788 (2015).
[Crossref]

Yoshida, K.

Y. Okada, N. J. Ekins-Daukes, T. Kita, R. Tamaki, M. Yoshida, A. Pusch, O. Hess, C. C. Phillips, D. J. Farrell, K. Yoshida, N. Ahsan, Y. Shoji, T. Sogabe, and J.-F. Guillemoles, “Intermediate band solar cells: recent progress and future directions,” Appl. Phys. Rev. 2(2), 021302 (2015).
[Crossref]

Yoshida, M.

Y. Okada, N. J. Ekins-Daukes, T. Kita, R. Tamaki, M. Yoshida, A. Pusch, O. Hess, C. C. Phillips, D. J. Farrell, K. Yoshida, N. Ahsan, Y. Shoji, T. Sogabe, and J.-F. Guillemoles, “Intermediate band solar cells: recent progress and future directions,” Appl. Phys. Rev. 2(2), 021302 (2015).
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Younkin, R.

C. Wu, C. H. Crouch, L. Zhao, J. E. Carey, R. Younkin, J. A. Levinson, E. Mazur, R. M. Farrell, P. Gothoskar, and A. Karger, “Near-unity below-band-gap absorption by microstructured silicon,” Appl. Phys. Lett. 78(13), 1850–1852 (2001).
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Yu, Z.

L. Zhu, F. Liu, H. Lin, J. Hu, Z. Yu, X. Wang, and S. Fan, “Angle-selective perfect absorption with two-dimensional materials,” Light: Sci. Appl. 5(3), e16052 (2016).
[Crossref]

Yuan, Y.

J. Huang, Y. Yuan, Y. Shao, and Y. Yan, “Understanding the physical properties of hybrid perovskites for photovoltaic applications,” Nat. Rev. Mater. 2(7), 17042 (2017).
[Crossref]

Zhao, L.

C. Wu, C. H. Crouch, L. Zhao, J. E. Carey, R. Younkin, J. A. Levinson, E. Mazur, R. M. Farrell, P. Gothoskar, and A. Karger, “Near-unity below-band-gap absorption by microstructured silicon,” Appl. Phys. Lett. 78(13), 1850–1852 (2001).
[Crossref]

Zhao, Z.

Zheng, W.

Y. Long, R. Su, Q. Wang, L. Shen, B. Li, and W. Zheng, “Deducing critical coupling condition to achieve perfect absorption for thin-film absorbers and identifying key characteristics of absorbing materials needed for perfect absorption,” Appl. Phys. Lett. 104(9), 091109 (2014).
[Crossref]

Zhou, M.

J. Yoon, M. Zhou, M. A. Badsha, T. Y. Kim, Y. C. Jun, and C. K. Hwangbo, “Broadband epsilon-near-zero perfect absorption in the near-infrared,” Sci. Rep. 5(1), 12788 (2015).
[Crossref]

Zhu, L.

L. Zhu, F. Liu, H. Lin, J. Hu, Z. Yu, X. Wang, and S. Fan, “Angle-selective perfect absorption with two-dimensional materials,” Light: Sci. Appl. 5(3), e16052 (2016).
[Crossref]

ACS Energy Lett. (1)

C. M. Sutter-Fella, D. W. Miller, Q. P. Ngo, E. T. Roe, F. M. Toma, I. D. Sharp, M. C. Lonergan, and A. Javey, “Band tailing and deep defect states in CH3NH3Pb(I1−xBrx)3 perovskites as revealed by sub-bandgap photocurrent,” ACS Energy Lett. 2(3), 709–715 (2017).
[Crossref]

ACS Photonics (3)

J. Park, J.-H. Kang, A. P. Vasudev, D. T. Schoen, H. Kim, E. Hasman, and M. L. Brongersma, “Omnidirectional near-unity absorption in an ultrathin planar semiconductor layer on a metal substrate,” ACS Photonics 1(9), 812–821 (2014).
[Crossref]

Z. Li, S. Butun, and K. Aydin, “Large-area, lithography-free super absorbers and color filters at visible frequencies using ultrathin metallic films,” ACS Photonics 2(2), 183–188 (2015).
[Crossref]

B. H. Woo, I. C. Seo, E. Lee, S. Y. Kim, T. Y. Kim, S. C. Lim, H. Y. Jeong, C. K. Hwangbo, and Y. C. Jun, “Dispersion control of excitonic thin films for tailored super-absorption in the visible region,” ACS Photonics 4(5), 1138–1145 (2017).
[Crossref]

Adv. Opt. Mater. (1)

G.-H. Jung, S. J. Yoo, J.-S. Kim, and Q.-H. Park, “Maximal visible light energy transfer to ultrathin semiconductor films enabled by dispersion control,” Adv. Opt. Mater. 7(7), 1801229 (2019).
[Crossref]

Appl. Phys. Lett. (5)

D. W. Miller, G. E. Eperon, E. T. Roe, C. W. Warren, H. J. Snaith, and M. C. Lonergan, “Defect states in perovskite solar cells associated with hysteresis and performance,” Appl. Phys. Lett. 109(15), 153902 (2016).
[Crossref]

Y. Long, R. Su, Q. Wang, L. Shen, B. Li, and W. Zheng, “Deducing critical coupling condition to achieve perfect absorption for thin-film absorbers and identifying key characteristics of absorbing materials needed for perfect absorption,” Appl. Phys. Lett. 104(9), 091109 (2014).
[Crossref]

C. Wu, C. H. Crouch, L. Zhao, J. E. Carey, R. Younkin, J. A. Levinson, E. Mazur, R. M. Farrell, P. Gothoskar, and A. Karger, “Near-unity below-band-gap absorption by microstructured silicon,” Appl. Phys. Lett. 78(13), 1850–1852 (2001).
[Crossref]

S. H. Pan, D. Recht, S. Charnvanichborikarn, J. S. Williams, and M. J. Aziz, “Enhanced visible and near-infrared optical absorption in silicon supersaturated with chalcogens,” Appl. Phys. Lett. 98(12), 121913 (2011).
[Crossref]

M. A. Kats, D. Sharma, Z. Yang, and F. Capasso, “Ultra-thin perfect absorber employing a tunable phase change material,” Appl. Phys. Lett. 101(22), 221101 (2012).
[Crossref]

Appl. Phys. Rev. (1)

Y. Okada, N. J. Ekins-Daukes, T. Kita, R. Tamaki, M. Yoshida, A. Pusch, O. Hess, C. C. Phillips, D. J. Farrell, K. Yoshida, N. Ahsan, Y. Shoji, T. Sogabe, and J.-F. Guillemoles, “Intermediate band solar cells: recent progress and future directions,” Appl. Phys. Rev. 2(2), 021302 (2015).
[Crossref]

IEEE Photonics Technol. Lett. (1)

A. Yariv, “Critical coupling and its control in optical waveguide-ring resonator systems,” IEEE Photonics Technol. Lett. 14(4), 483–485 (2002).
[Crossref]

J. Appl. Phys. (1)

B. Franta, D. Pastor, H. H. Gandhi, P. H. Rekemeyer, S. Gradečak, M. J. Aziz, and E. Mazur, “Simultaneous high crystallinity and sub-bandgap optical absorptance in hyperdoped black silicon using nanosecond laser annealing,” J. Appl. Phys. 118(22), 225303 (2015).
[Crossref]

J. Opt. Soc. Am. A (1)

S. Fan, W. Suh, and J. D. Joannopoulos, “Temporal coupled mode theory for fano resonance in optical resonators,” J. Opt. Soc. Am. A 20(3), 567–572 (2003).
[Crossref]

Laser Photonics Rev. (1)

M. A. Kats and F. Capasso, “Optical absorbers based on strong interference in ultra-thin films,” Laser Photonics Rev. 10(5), 735–749 (2016).
[Crossref]

Light: Sci. Appl. (1)

L. Zhu, F. Liu, H. Lin, J. Hu, Z. Yu, X. Wang, and S. Fan, “Angle-selective perfect absorption with two-dimensional materials,” Light: Sci. Appl. 5(3), e16052 (2016).
[Crossref]

Nano Energy (1)

D. Liu and Q. Li, “Sub-nanometer planar solar absorber,” Nano Energy 34, 172–180 (2017).
[Crossref]

Nat. Commun. (1)

J. P. Mailoa, A. J. Akey, C. B. Simmons, D. Hutchinson, J. Mathews, J. T. Sullivan, D. Recht, M. T. Winkler, J. S. Williams, J. M. Warrender, P. D. Persans, M. J. Aziz, and T. Buonassisi, “Room-temperature sub-band gap optoelectronic response of hyperdoped silicon,” Nat. Commun. 5(1), 3011 (2014).
[Crossref]

Nat. Photonics (1)

A. Luque, A. Martí, and C. Stanley, “Understanding intermediate-band solar cells,” Nat. Photonics 6(3), 146–152 (2012).
[Crossref]

Nat. Rev. Mater. (1)

J. Huang, Y. Yuan, Y. Shao, and Y. Yan, “Understanding the physical properties of hybrid perovskites for photovoltaic applications,” Nat. Rev. Mater. 2(7), 17042 (2017).
[Crossref]

Opt. Express (5)

Opt. Lett. (1)

Opt. Mater. Express (2)

Phys. Rev. Appl. (1)

M. Shirayama, H. Kadowaki, T. Miyadera, T. Sugita, M. Tamakoshi, M. Kato, T. Fujiseki, D. Murata, S. Hara, T. N. Murakami, S. Fujimoto, M. Chikamatsu, and H. Fujiwara, “Optical transitions in hybrid perovskite solar cells: ellipsometry, density functional theory, and quantum efficiency analyses for CH3NH3PbI3,” Phys. Rev. Appl. 5(1), 014012 (2016).
[Crossref]

Rep. Prog. Phys. (1)

S. Collin, “Nanostructure arrays in free-space: optical properties and applications,” Rep. Prog. Phys. 77(12), 126402 (2014).
[Crossref]

Sci. Rep. (1)

J. Yoon, M. Zhou, M. A. Badsha, T. Y. Kim, Y. C. Jun, and C. K. Hwangbo, “Broadband epsilon-near-zero perfect absorption in the near-infrared,” Sci. Rep. 5(1), 12788 (2015).
[Crossref]

Other (4)

B. E. A. Saleh and M. C. Teich, Fundamentals of Photonics (Wiley, 2007), Chap. 10.

J. D. Jackson, Classical Electrodynamics (Wiley, 1999), Chap. 6.

C. F. Klingshirn, Semiconductor optics (Springer, 2012).

J. Singh, Semiconductor optoelectronics: Physics and Technology (McGraw-Hill, 1995).

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Figures (12)

Fig. 1.
Fig. 1. General idea for sub-bandgap absorption enhancement. (a) Schematic for light absorption in a planar multilayer film. (b) Critical coupling can be achieved by balancing radiative and nonradiative decay rates. Large absorption can be obtained even in a low-loss film at critical coupling.
Fig. 2.
Fig. 2. (a) Schematic of the MAPbI3 crystal structure. (b) X-ray diffraction pattern of the perovskite (MAPbI3) film used in this work.
Fig. 3.
Fig. 3. Key experimental results: (a) Net absorption in normal incidence for [MAPbI3 (270 nm)/quartz]. The perovskite film on quartz has small but non-zero absorption (about 1.45% at 1130 nm). (b) Net absorption in normal incidence for [PMMA/MAPbI3/SiO2/silver/quartz]. (c) Nominal absorption (Anominal = 1 – Rspecular) in s-polarized oblique incidence for [PMMA/MAPbI3/SiO2/silver/quartz]. (d) and (e) Reference sample measurements in normal and s-polarized oblique incidences.
Fig. 4.
Fig. 4. Diffuse reflection measurements in normal incidence for (a) [MAPbI3/quartz] and (b) [PMMA/MAPbI3/SiO2/silver/quartz].
Fig. 5.
Fig. 5. Comparison between measured (red line) and calculated (dotted line) spectra. (a) By comparing normal incidence measurement data with multilayer calculations, the extinction coefficient k of the perovskite layer is estimated to be k ∼ 0.001. (b) Calculated net absorption for oblique incidence is compared to the measured spectrum Anominal. We obtain net absorption about 11.5% at 1000 nm for the incidence angle of 70°. The difference between measured and calculated spectra is considered to be caused by diffuse reflection.
Fig. 6.
Fig. 6. (a) Absorption colormap as a function of wavelength and incidence angle for s-polarization. (b) Absorbed power profile in our multilayer film for incidence angles of 0 ° and 70°. By integrating the absorbed power density in each layer, the fraction of light absorption in each layer can be obtained.
Fig. 7.
Fig. 7. (a) Simulated absorption spectra for s-polarization. (b) Extracted radiative and nonradiative damping rates for different incidence angles. Two damping rates are getting very close at large incidence angles and thus strong absorption can be achieved in the low-loss, sub-bandgap region.
Fig. 8.
Fig. 8. Net absorption spectrum for bare quartz.
Fig. 9.
Fig. 9. p-polarized nominal absorption spectra for [PMMA/MAPbI3/SiO2/silver/quartz].
Fig. 10.
Fig. 10. (a) The optical constants for silver used in simulations. (b) Simulations match well the experimental spectra from the reference sample for both (b) normal and (c) oblique (70°) incidences.
Fig. 11.
Fig. 11. (a) Estimation of perovskite extinction coefficient k in the sub-bandgap region from [MAPbI3/quartz]. We find a larger extinction coefficient about k ∼ 0.0045 in the perovskite layer. (b) Extracted radiative and nonradiative damping rates (c) Absorption color map. (d) Absorbed power profile for k ∼ 0.0045. The absorbed fraction in the perovskite reaches 76.4% at the incidence angle of 70°.
Fig. 12.
Fig. 12. Measured spectra for 75 nm and 100 nm oxides. The resonance peaks shift to longer wavelengths (1100 nm and 1200 nm, respectively). Sub-bandgap absorption can be tuned over a broad spectral region by adjusting the spacer layer thickness.

Equations (3)

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A ( ω ) = 4 γ r a d γ n o n r a d ( ω ω 0 ) 2 + ( γ r a d + γ n o n r a d ) 2 ,
A m = 4 γ r a d γ n o n r a d ( γ r a d + γ n o n r a d ) 2 .
P a b s = 1 2 ω | E ( ω ) | 2 Im [ ε 0 ε ( ω ) ] .

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