Coloring Semitransparent Perovskite Solar Cells ... - ACS Publications

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Coloring Semitransparent Perovskite Solar Cells via Dielectric Mirrors César Omar Ramírez Quiroz, Carina Bronnbauer, Ievgen Levchuk, Yi Hou, Christoph J. Brabec, and Karen Forberich ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.6b00225 • Publication Date (Web): 12 Apr 2016 Downloaded from http://pubs.acs.org on April 13, 2016

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Coloring Semitransparent Perovskite Solar Cells via Dielectric Mirrors César Omar Ramírez Quiroz † a, Carina Bronnbauer† a,b, Ievgen Levchuka, Yi Hou a,b, Christoph J. Brabeca,c,, Karen Forbericha

a

Friedrich-Alexander University Erlangen-Nuremberg, Institute of Materials for Electronics and Energy Technology (I-MEET), Department of Materials Science and Engineering, Erlangen, Germany. b

Friedrich-Alexander University Erlangen-Nuremberg, Erlangen Graduate School in Advanced Optical Technologies (SAOT), Erlangen, Germany. c

Friedrich-Alexander University Erlangen-Nuremberg, Bavarian Center for Applied Energy Research (ZAE Bayern), Erlangen, Germany.

AUTHORS INFORMATION: C.O.R.Q.: [email protected]; I.L.: [email protected]; Y.H.: [email protected]; K.F.: [email protected]; C.J.B.: [email protected].

† This authors contributed equally to this manuscript.

Correspondence should be addressed to: Carina Bronnbauer Martensstr. 7 91058 Erlangen E-Mail: [email protected]. Telefon: ++49 (0)9131 85-27730 Fax: ++49 (0)9131 85-28495

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Mirror

25 mm

Eﬃciency % = 3.6 Transparency % = 44.3

4.1 15.6

4.0 4.9

4.3 9.6

4.2 31.4

Table of contents figure (ToC).


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Coloring Semitransparent Perovskite Solar Cells via Dielectric Mirrors César Omar Ramírez Quiroz † a, Carina Bronnbauer† a,b, Ievgen Levchuka, Yi Hou a,b, Christoph J. Brabeca,c,, Karen Forbericha

a

Friedrich-Alexander University Erlangen-Nuremberg, Institute of Materials for Electronics and Energy Technology (I-MEET), Department of Materials Science and Engineering, Erlangen, Germany. b

Friedrich-Alexander University Erlangen-Nuremberg, Erlangen Graduate School in Advanced Optical Technologies (SAOT), Erlangen, Germany. c

Friedrich-Alexander University Erlangen-Nuremberg, Bavarian Center for Applied Energy Research (ZAE Bayern), Erlangen, Germany.

AUTHORS INFORMATION: C.O.R.Q.: [email protected]; I.L.: [email protected]; Y.H.: [email protected]; K.F.: [email protected]; C.J.B.: [email protected].

† This authors contributed equally to this manuscript.

Correspondence should be addressed to: Carina Bronnbauer Martensstr. 7 91058 Erlangen E-Mail: [email protected]. Telefon: ++49 (0)9131 85-27730 Fax: ++49 (0)9131 85-28495


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ABSTRACT While perovskite-based semitransparent solar cells for window applications show competitive levels of transparency and efficiency compared to organic photovoltaics, the color perception of the perovskite films is highly restricted since bandgap engineering would result in losses in power conversion efficiencies. To overcome the limitation in visual aesthetics, we combined semitransparent perovskite solar cells with dielectric mirrors. This approach enables to tailor the device appearance to almost any desired color and simultaneously offers additional light harvesting for the solar cell. In the present work, opto-electrical effects are investigated through quantum efficiency and UV-to-visible spectroscopic measurements. Likewise, a detailed chromaticity analysis, featuring the transmissive and reflective color perception of the device including the mirror, from both sides and in different illumination conditions, is presented and analyzed. Photocurrent density enhancement of up to 21% along with overall device transparency values of up to 31% (4.2% efficiency) are demonstrated for cells showing a colored aesthetic appeal. Finally, a series of simulations emulating the device chromaticity, transparency and increased photocurrent density as a function of the photoactive layer thickness and the design wavelength of the dielectric mirror is presented. Our simulations and their experimental validation enabled us to establish the design rules that consider the color-efficiency-transparency interplay for real applications.

KEYWORDS Bragg mirror; dielectric mirror; semitransparent; perovskite photovoltaics; room-temperature crystallization.


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The integration of semitransparent solar cells into existing urban infrastructures that could contribute to self-sustainable buildings is one of the most exciting research directions for the third generation of photovoltaic technologies. Colors have been closely associated with human emotions. They can subconsciously influence purchasing decisions, perception of quality, cognitive performance and physiological stress levels 1–5. Accordingly, color is an essential design feature for establishing new products on a broad variety of markets, such as photovoltaics and architecture. A very successful concept for the integration of colored photovoltaics into architectural elements, e.g. household interiors and building facades, is window-integrated semitransparent photovoltaics. The utilization of this concept towards the realization of fully sustainable buildings represents an outstanding market opportunity that can potentially lead to an economical upswing in the photovoltaic industry.6,7 Predominantly, the color of semitransparent photovoltaics is mainly determined by the absorption spectrum of the photoactive material. Nevertheless, the overall appearance for the full photovoltaic stack can be modified by light management engineering.8–14 Color tuning by adjusting the absorption spectrum is especially known for organic photovoltaics as many different organic materials with various band gaps can be realized.15 In a theoretical study we previously reported that semitransparent organic photovoltaics can provide a broad diversity of color while showing power conversion efficiencies (PCEs) up to 12% at 30% transparency.16 However, despite the theoretical demonstration, experimental results do not reach the same efficiencies mainly due to a combination of energy loss mechanisms, which are not related to transparency.17,18 Ever since their discovery, and through a cumulative worldwide effort, perovskite-based, non-transparent photovoltaics have reached certified efficiencies up to 20.1% positioning them as the leading inorganic-organic photovoltaic technology which is based on


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fabrication by solution processeing.19–22 For this reason, along with the utilization of low temperatures for processing and inexpensive raw materials, perovskite materials have also gained momentum in the field of semitransparent photovoltaics.23–26 Recently, an overall transparency of ~44% combined with a PCE of 3.7% was reported by optimization of the semiconductor processing. In this work, energy loss mechanisms were minimized and internal quantum efficiencies of almost unity were achieved.26

Recent progress regarding aesthetics in perovskite-based photovoltaics has slightly extended the color rendering from brown-orange to red and yellow. Nonetheless, these changes in color, owed to band gap engineering, significantly influence the PCE (yellow perovskite ~ 4% PCE; brown perovskite ~11% PCE).27 Instead of changing the inherent properties of the perovskite semiconductor at cost of device performance, light management appears to be a more practical strategy for enabling full-device color tuning. Accordingly, variations of color appearance from green to blue, and even neutral colored devices could be realized.28–30 Within this scope, Wei Zhang et al. 9 achieved PCEs up to 8.8% for a blue appearing opaque device but only 4.5% for a red one. Here, the perovskite and its interface layers were directly spin-coated on top a series of dielectric mirrors (also known as Bragg reflectors, dichroic filters or 1-dimensional photonic crystals). These dielectric mirrors exhibit a wavelength-dependent reflectance as a function of their architecture and the refractive indices of the materials. Despite the very promising concept of improving the aesthetics of the cells by incorporating dielectric mirrors through this particular approach, the cells had to be illuminated from the photonic crystal side due to the evaporated, non-transparent back electrode. In this way, the amount of light reaching the cell is significantly


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decreased, and consequently, the cell performance is strongly dependent on the characteristics of the dielectric mirror. In this communication we introduce a unique combination of dielectric mirrors and highly transparent perovskite based solar cells. Our architectural design (Figure 1-a) allows us to exploit the aesthetic benefits of dielectric mirrors without jeopardizing the final cell performance. The perovskite active layers for our highly transparent devices were prepared at room temperature via solvent-solvent extraction and are fully solution-processed. Furthermore, the dielectric mirrors are fully printed and allow an easy customization to any desired wavelength (a representative micrograph of the utilized dielectric mirrors is shown in Figure S1-a). Due to illumination from the transparent-back electrode and not from the dielectric mirror side, our approach not only facilitates custom-made colors, but also enhances the PCE of the device. For example, if a light orange appearing cell is combined with a mirror, showing nearly 95% reflections at 650 nm, the color can be changed to blue-green while the short circuit photocurrent (JSC) is enhanced by 21.3%. To further underline the significance of our experimental results, we present a series of simulations emulating the device chromaticity, transparency and increased photocurrent density as a function of the photoactive layer thickness and the wavelength design of the dielectric mirror. Our findings demonstrate that almost every color within the visible range can be realized.


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RESULTS AND DISCUSSION

Photons impinging the surface of a solar cell at a given wavelength contribute only partially to the photocurrent generation. Assuming internal quantum efficiency (IQE) of 100%, that is, neglecting any recombination losses, this contribution is determined by the absorption spectrum and the thickness of the active layer. As a photon can either be absorbed or transmitted by the solar cell, there is an inherent tradeoff between PCE and transparency: the higher the PCE, the lower the transparency, and vice versa. Thus, the idea behind the incorporation of a dielectric mirror, with a well-defined reflection spectra, at the backside of a semitransparent solar cell is to reharvest unabsorbed photons while preserving high transparency or changing the color in a desired way.31 Essentially, a dielectric mirror is fabricated by alternating a high and a low refractive index material (HRI and LRI, respectively) (Figure S1-a). The optical response is controlled by the layer thicknesses, the amount of layers and the respective refractive indices. Within this work, we considered four fully printed different dielectric mirrors comprising ten layers (five LRI and five HRI). All of them satisfy the Bragg condition nHRI*dHRI=nLRI*dLRI=λ0/4, where n is the refractive index, d is the layer thickness, and λ0 is the wavelength with maximum reflection at incidence normal to the surface. A stack of ten alternating layers was chosen to achieve sufficiently low transmittance values ( ~5%) which are necessary to obtain distinct colors (see Figure S1-e) and maximum JSC enhancement for semitransparent solar cells.32 λ0 was set to 490 nm (color appearance: red), 570 nm (dark blue), 650 nm (light blue) and 750 nm (almost transparent) in order to cover a broad spectral range. Digital images of all bare dielectric mirrors are shown in Figure 1b. The respective transmittance, reflection and absorbance spectra are presented in Figure S1-b, Figure S1-c and


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Figure S1-d, respectively. The absorption spectra of the dielectric mirrors validate that the color perception is caused by thin film interference and not by material absorbance since no absorption is obtained in the visible region. The observed absorption in the UV region (mostly below 400 nm) is mainly due to nature of the building materials conforming the mirrors. For all dielectric mirrors, the minimum transmittance is close to zero and the maximum reflectance is almost 95%. Depending on λ0, the transparency of the bare mirror is 30.9% for λ0=490 nm, 13.4% for λ0=570 nm, 26.1% for λ0=650 nm and 71.6% for λ0=750 nm.

The semitransparent solar cell is based on the hysteresis free architecture:33,34 Glass / ITO (indium tin oxide, ~400 nm) / PEDOT:PSS (poly(3,4ethylenedioxythiophene):poly(styrenesulfonate), ~40 nm) / MHP (mixed halide perovskite; various thickness) / PC60BM ([6,6]-phenyl-C60-butryric acid methyl ester, ~50 nm) / ZnO (zinc oxide, ~50 nm ) / AgNWs (silver nanowires). In order to provide a detail color study, three different devices were fabricated having an average perovskite layer thickness of ~40 nm, ~130 nm, ~160 nm. These pristine devices exhibit transparencies of 44.3%, 14.6% and 9.4%, respectively. A digital image of the thinnest pristine cell with a perovskite layer thickness of ~40 nm is shown in Figure 1-b. In order to achieve this sub-50 nm thick perovskite for device fabrication preserving the photovoltaic performance such as full Voc (1030 mV) and relatively high fill factor (~67%) we use a simple solvent-solvent extraction protocol reported elsewhere.26 Our solvent-solvent extraction protocol basically consists on submerging the as casted semi-dry perovskite film into the extraction solvent. The significantly high miscibility between the precursor solvent and the extraction solvent allows spontaneous diffusion of the former onto the latter. This quick solvent diffusion leads to the spontaneous supersaturation and subsequent


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nucleation and crystallization of the perovskite film. Henceforth we obtained surprisingly smooth perovskite layers showing root mean square (RMS) roughness of 7 nm with remarkable welldistributed crystal domains on the range of 100 nm in diameter (Figure 2-a). Additionally, the XRD analysis of the bare film shows the typical peaks at 14.18º and 28.46º, corresponding to the Miller indices (110) and (220) of the tetragonal phases of perovskite. Noteworthy, these devices showed outstanding shelf lifetimes exceeding 1700 hours under nitrogen atmosphere and without being encapsulated (Figure S5). The thickest device with active layer of ~160 nm is displayed in Figure S3-a. It has to be mentioned that the digital images of the dielectric mirrors as well as the pristine cells were taken with a white light-source placed beneath. This is necessary in order to demonstrate the windowintegrated case in which sunlight is shining through the complete stack (daytime condition -see Figure S2). In the following paragraph we mainly refer to this illumination condition. Subsequently, we will also discuss the color appearance without having a light source behind the stack. This scenario refers to night conditions: dark outside while the building interior is illuminated (see Figure S2). It is necessary to distinguish between these two cases, as color appearance can be either triggered by reflection, transmission or a combination of both. Furthermore, the stack architecture (dielectric mirror or solar cell first) has to be considered as well. Both, daytime and nighttime conditions and the alignment of the stack are schematically presented in Figure S2.

The semitransparent solar cell and the dielectric mirror are combined with each other by simply placing them together while both upper layers (AgNWs and HRI material) are facing each other.24 Notably, we observed a strong change in color for all cases. Throughout our discussion


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we will refer to any change and perception of color by using adjectives describing the color itself. All numerical quantifiable representations, as measured by the CIE 1931 metric system, of these color changes and descriptions can be found in Figure 4. The initially orange/brown very thin pristine device (active layer thickness of 40 nm) became red when using the mirror with λ0=490 nm, dark blue for λ0=570 nm and blue-green for λ0=650 nm (see Figure 1-b). However, we were unable to see any significant color change for the cell combined with a dielectric mirror having λ0=760 nm. By rotating the stack 180° with respect to the white light-source, the optical response remains unchanged (Figure 1-b view from cell, view from mirror). When using this architecture for window integrated photovoltaics applications, the window would have the same color outside and inside the building during daytime (assuming that the illumination in the visible region form outside and inside is comparable). The same experiment was also performed for a solar cell having an active layer thickness of 130 nm. The resulting digital images are presented in Figure S3-a. Here, the dark orange pristine cell, again, turns into red (for λ0=490 nm), dark blue (for λ0=570 nm), blue-green (for λ0=650 nm) or even retains its color (for λ0=760 nm). Concluding, an increase in active layer thickness leads to the same change in color but to a more intense color appearance of the full device (compare Figure 1-a with Figure S3-a). The corresponding wavelength dependent transmittance spectra of the thin pristine device with and without the dielectric mirrors are presented in Figure 1-c. At λ0 position of the dielectric mirror, the transmittance value of the overall stack reaches almost 0%. Hence, the overall transparency is influenced and changes from 44.3% for the pristine cell to 15.6% for the full device stack using mirror with λ0=490 nm, to 4.9% for λ0=570 nm, to 9.6% λ0=650 nm and to 31.4% for λ0=750 nm. The optical effect, in terms of transmittance, of the full stack with solar cells having thicker active layers (130 nm and 160 nm) is shown in Figure S4-a and d. In


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addition a full collection of all key parameters is presented in Table S1. As expected, an increase in active layer thickness impacts the transmittance spectrum of the pristine cell and reduces the overall transparency. Nevertheless, the spectral shape of the full device stack remains the same for all device thicknesses. Hence, for devices having the same architecture and the same active layer material, the color appearance is not only a function of λ0 but also a function of active layer thickness. Likewise, considering no back illumination, the color response of the full device stack changes significantly (Figure S-2a and b). In this case, the color of the device stack is mainly determined by the reflection profile and not by the transmission. It is worth mentioning that the case of reflective color rendering is similar to the color tuning method described by Wei Zhang et al. for opaque devices.9 A collection of digital images for the thin pristine cell (active layer with and without the dielectric mirrors for both front and back view is presented in Figure S3-b. As depicted by Figure S3-b, the pristine cell appears light gray. When combining the pristine cell with a dielectric mirror, when light is incident from the mirror side (Figure S2-a) the color appearance of the full stack is mainly determined by the reflection of the mirror resulting in intense colors (red for λ0=490 nm, dark blue for λ0=570 nm, light-blue for λ0=650 nm and gray for λ0=760 nm). The underlying six solar cells are hardly visible (Figure S3-b). In contrast, when having the light coming from the solar cell (Figure S2-b) the size of each device is clearly visible. In addition, the color of the dielectric mirror is much less pronounced. This observation can be explained by looking at the reflection spectra (Figure 1-d,e). When light passes through the cell first, the reflection spectra are more flat and broad compared to the situation on which the dielectric mirror is placed first. Thus, the colors appear less intense. The same observation is valid for solar cells with thicker active layers.


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Besides manipulating the color, the impact of the dielectric mirrors on the device performance is very important too and will be discussed in detail in the following paragraph. The photocurrent density-voltage (J-V) characteristics of each pristine cell are displayed in Figure 3-d. Additionally a complete collection of key parameters for device performance and transparency are listed in Table 1 and Table S1. As expected, with increasing the active layer thickness the short circuit photocurrent (JSC), and consequently the PCE, increases as well. The Jsc is 4.38 mA cm-2, 12.37 mA cm-2, to 13.78 mA cm-2 corresponding to the average active layer thickness of ~40 nm, ~130 nm and ~160 nm, respectively. In order to better understand the opto-electrical contribution of the dielectric mirrors on the photocurrent density generation, EQE studies were performed. For all measurements, the EQE was measured with the light source impinging on the solar cell first (Figure 1-a). The results for all active layer thicknesses are presented in Figure 3a, Figure 3-b and Figure 3-c. For all spectra, the black line refers to the pristine cell, the orange solid line corresponds to the device stack using the λ0=490 nm dielectric mirror, the purple solid line for λ0=570 nm, the dark blue solid line for λ0=650 nm and the light blue solid line for λ0=750 nm. Additionally, the relative increase of the EQE is presented below in the same color code. As is evident by the EQE spectra, the photocurrent enhancement becomes more pronounced in the vicinity of λ0 of each mirror. Looking at the impact of the active layer thickness on the percentage EQE enhancement, it is obvious that with increasing the active layer thickness, this percentage is increasing. While the maximum increase in EQE is only ~ 5% for a 40 nm device, it is ~16% with a 160 nm device. This observation can be attributed to a simple mathematical consideration containing the transmittance, absorption and reflection characteristics. Perovskite solar cells exhibit an IQE of ~100%,26,35,36 meaning that roughly every photon absorbed by the photoactive material is creating a charge carrier, which is effectively


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being transported to its corresponding electrode. Thus, the final EQE of the solar cells with dielectric mirrors (EQEwith DM) is estimated for each wavelength by: EQEwith DM=EQEwithout DM + (RDM*Twithout DM*EQEwithout DM) where EQEwithout DM is the EQE of the pristine cell, RDM is the reflection of the dielectric mirror and Twithout DM is the transmittance of the pristine device. Hence, due to the high EQE values of a thick film compared to those from a thin film, the product (RDM *Twithout DM*EQEwithout DM) rises with increasing the active layer thickness. This increment becomes less significant for very thick films as Twithout DM becomes small. However, comparing the percentage JSC enhancement induced by dielectric mirrors for all active layer thicknesses (Table S1 and Figure 3-f), the ones with λ0 =650 nm and 750 nm, are the most effective ones. Up to 21.3% increase in JSC is observed. Nevertheless, a clear trend between JSC enhancement and thickness of active layer cannot be drawn yet. In general, the thicker the SC, the lower should be the percentage JSC enhancement. Finally, in order to demonstrate the unprecedented opportunity in terms of color tuning and efficiency enhancement for ST-perovskite solar cells in combination with DMs, simulations have been performed, showing which colors can be created in general and how this impacts the increase in JSC and the overall transparency of the device. The transmittance spectra of perovskite solar cells with and without dielectric mirror were simulated using the transfer matrix formalism.37 Thus, homogeneous and perfectly smooth layers had to be assumed. The same device structure, which was used for the experiments (Figure 1-a), is also used for the calculations. Moreover, all refractive indices are set to previously experimentally determined values allowing a direct comparison between theory and experiment later on. While keeping the interfacial layers constant in thickness, the active layer thickness was


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varied between 0 and 500 nm and λ0 from 350 nm to 750 nm. Then, the color coordinates as well as the transparencies were calculated by using the simulated transmittance spectra. The JSC was determined by convoluting the absorption spectra with the AM1.5G solar spectrum. Finally, transparency (Figure 4-a) and JSC (Figure 4-b) are plotted as a function of the CIE 1931 coordinates. Accordingly, the simulations reveal that by incorporating dielectric mirrors to perovskite-based semitransparent devices, the device stack can adopt almost every color in the visible region, except bright green (CIE y > 0.5). Furthermore, it becomes evident that the JSC and consequently the efficiency are related to the color. In terms of device optimization, transparency as well as efficiency should be optimized. For example, dark blue or purple appearing devices (y CIE ~0.15-0.20 and x CIE ~ 0.20-0.50) show high JSC on the one hand, but on the other hand they suffer from low transparencies due to the large overlap with the maximum of the human-eye sensitivity curve. In contrast, light-green, -blue and –orange looking like (y CIE ~0.40-0.45 and x CIE ~ 0.20-0.50) devices simultaneously provide, high efficiency and high transparencies. Figure 4-c to Figure 4-f show the CIE 1931 color space chromaticity diagram including the simulated (solid line) as well as the experimentally determined (symbols) CIE coordinates for the dielectric mirrors and the active layer thicknesses which were previously discussed in the experimental section. Figure 4-c shows the results for λ0=490 nm, Figure 4-d for λ0=570 nm, Figure 4-e for λ0=650 nm and Figure 4-f for λ0=750 nm. In addition the CIE coordinates for the pristine cells is displayed in every graph. The agreement between simulations and experiment is very good. The appearance color of the stack (from digital images) as well as the CIE coordinates are almost identical and thus further confirm the simulations shown in Figure 4-a and Figure 4-b.


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CONCLUSIONS In summary, we developed a systematic approach that allows tailoring the color appearance as well as the power conversion efficiency of semitransparent perovskite solar cells. This approach is based on the incorporation of dielectric mirrors into the device architecture. As the dielectric mirror and the solar cell are not electrically connected, but instead mechanically stacked on top each other, different combinations can be easily realized. Chromaticity and color perception under different illumination conditions and mirror-cell orientation with respect to the observer or light source were investigated. This provides a detailed insight into the relevant factors to be considered for the realization of perovskite-based semitransparent into building integrated applications. Regarding device performances, our results showed an increase in JSC of up to 21.3% for devices having an active layer thickness of ~40 nm incorporated to a dielectric mirror (λ0=650 nm), resulting on a blue/green appearance for full-device stack. Additionally, by customizing the mirror in the near-IR region (λ0=750 nm), JSC was increased by 18.7% having a light blue appearance, 31.4% overall device transparency and 4.2% efficiency. In addition, theoretical studies considering active layer thickness of 0 nm to 500 nm and λ0 of 350 nm to 750 nm predict the realization of a broad range of colors except for bright green (CIE y > 0.5). Our simulations show that the best combination of efficiency and transparency is given for the colors of light green, light blue and light orange (y CIE ~0.40-0.45 and x CIE ~ 0.20-0.50) for full stack. The close agreement between experimental and simulated results further validates our approach enabling its possible application on different systems other than perovskites. By successfully overcoming the intrinsic drawback of semitransparent perovskite-based photovoltaics of lack of aesthetic appeal with fully solution processable materials, this study leads the way for the potential realization of perovskite-based building integrated photovoltaics.


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MATERIALS AND METHODS Materials and preparation: Unless stated otherwise, all materials were used as received and were purchased by Merck or Aldrich. Lead (III) Chloride ultra dried 99.99% ampouled under Argon was purchased from Alfa Aesar and CH3NH3I was provided by Dyenano. Neutral-PEDOT:PSS (Clevios SCA 228) was provided by Heraeus. Nanograde provided ZnO nanoparticle ink as well as the low and high refractive index ink for the mirror fabrication. The DMF-perovskite precursor solution was made by adding PbCl2 and CH3NH3I powders with a molar ratio of 1:3 and a concentration of 40 wt% and 30 wt% to a vial and mixed with anhydrous Dimethylformamide. The solution was then stirred for 30 minutes at 60 ºC and filtered prior to deposition. The GBL-perovskite precursor was made by adding PbI2 and CH3NH3I powders with a molar ratio of approximately 1:1 and a concentration of 40 wt% and 20 wt% to a vial and mixed with anhydrous gamma-Butyrolactone. The solution was then stirred for 30 minutes at 60 ºC and filtered prior to deposition.

The solar cell fabrication was performed using protocols reported elsewhere.24 The solar cells with the thinnest active layer of 40 nm is produced via solvent-solvent extraction while the thicker ones (130 nm and 160 nm) are prepared with the utilization of the fast crystallization deposition combined with gas quenching. The dielectric mirrors were fabricated at 40 °C on a microscopy glass (7.5*2.5 cm²) by doctor blading the low and the high refractive ink alternatingly on top of each other. After each deposition step, either UV curing for the high refractive index ink or heat treatment of 80 °C for three minutes each had to be applied. For more details, see our previous work. 32

Device characterization: Layer thicknesses were determined via profilometer (Tencor Alpha Step). Optical measurements, such as transmission and reflection were performed with an UVVis-NIR spectrometer (Lambda 950 from Perkin) equipped with an integrating sphere. Thus, in


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the present manuscript the use of the term transmission or reflection always refers to total transmission or total reflection as diffuse scattering is included. All average transmittance (AVT) values are calculated by averaging the transmittance recorded between 400 nm and 800 nm. The transparency values were determined by convoluting the human eye sensitivity curve with the transmittance spectrum. For device characterization and description of photovoltaic performance we followed the photovoltaic checklist as recently established by Nature Materials (http://www.nature.com/authors/policies/solarchecklist.pdf). Photocurrent density-voltage (J-V) characteristics were recorded at room temperature in the glove box using a measurement unit from BoTest. The voltage scan conditions are as follows: forward direction, speed: 1mV/ms, dwell time 8 ms. The devices were illuminated with an AM1.5 G spectrum at 0.1W/cm², which was determined by a calibrated single-crystal standard silicon cell. A shadow mask was used for all J-V characterizations in order to avoid current contribution from adjacent area during the measurements. The size of the mask is identical with the size of the solar cell with an area of 10.44 mm². We previously demonstrated the statistical stability of the protocols used for solar cell fabrication elsewhere.26 Hence, the results presented are derived from a statistical population of 6 solar cells. External quantum efficiencies (EQE) were measured at room temperature in air atmosphere with an Enli Technology QE-R, also calibrated with a standard single-crystal Silicon cell. The EQE measurements were performed with a mask with dimensions of 4 mm by 1 mm. Both, J-V and EQE were measured by illuminating the device from the ITO side. Cross section micrographs were acquired by means of a scanning electron microscopy (SEM, Ultra FE-SEM Gemini Ultra 55, Carl Zeiss Jena).


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Reflection (incidence angle of 8°) and transmission (normal to the sample) of the deposited layers are measured and implemented to the software NIKA38 in order to obtain the wavelength dependent complex refractive index (wavelength region: 320 nm to 900 nm). Digital images: All digital images presented in this work were taken using the following settings: aperture f/8, shutter 1/100 s and ISO 100. A white flat lamp (FlatLine Leuchtplatten) with 15 mW cm-² is used for back illumination.

Supporting Information The supportive information is available free of charge on the ACS Publication website, at DOI:

ACKNOWLEDGEMENTS The authors would like to gratefully acknowledge the funding of the Erlangen Graduate School in Advanced Optical Technologies (SAOT) at the University of Erlangen-Nürnberg which is funded by the German Research Foundation (DFG) within the framework of its “Excellence Initiative”. The work was further supported by the service and facilities of the Energie Campus Nürnberg (EnCn) and financial support through the “Aufbruch Bayern” initiative of the state of Bavaria and the Cluster of Excellence “Engineering of Advanced Materials” (EAM) at the Universität Erlangen-Nürnberg. C.O.R.Q would like to gratefully acknowledge the financial support from The Mexican National Council for Science and Technology (CONACYT). C.B. and K.F would like to acknowledge the EU-project SOLPROCEL (“Solution Processed high performance transparent organic photovoltaic cells”, Grant no. 604506).


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Table 1. Key metrics for perovskite solar cell (AL = ~40 nm) with the implementation of dielectric mirrors. ΔJsc (%)a

Voc (V)

FF (%)

PCEb

5.40

-

1.03

65.6

3.0/3.6

44.3

46.1

5.06

6.08

15.5

1.03

65.6

3.4/4.1

15.6

28.4

λ0 = 570 nm;

4.94

5.96

12.8

1.03

65.6

3.3/4.0

4.9

17.2

λ0 = 650 nm;

5.31

6.33

21.2

1.03

65.6

3.6/4.3

9.6

12.3

λ0 = 750 nm;

5.20

6.23

18.7

1.03

65.6

3.5/4.2

31.4

16.8

ID

* a

EQE

Sol. Sim.

Jsc (mA cm-2)

Jsc (mA cm-2)

Pristine Cell;

4.38

λ0 = 490 nm;

T (%)c

AVT (%)d

All values derived from J-V and EQE characterization represent the arithmetic mean along 6 samples.

ΔJsc (%) is extracted from the increase on EQE-Jsc as a result of incorporating dielectric mirrors to the device architecture. b

PCE values are calculated using short circuit photocurrent extracted from EQE characterization, followed by the value calculated with the short circuit photocurrent extracted from J-V characterization AM 1.5 irradiation at 0.1 W/cm2 illumination. c Transparency convoluted with human sensitivity curve for full devices including dielectric mirror. d Average visible transmittance in the 400 to 800 nm regime for full devices including dielectric mirror.


22

600 hv 700

Full Stack Reflection (%) Full Stack Reflection (%)

Wavelenght (nm) AgNW

60 60

PC60BM

λ0 = 570 nm

400

EQE (%)

20

30

700

100 100 90 90

400

800

50 50 500 600 40 40 20 20 10 10

400

500

600

700

λ = 490 nm

500

600

700

700

800

25 mm

e 100 100 90 90

30

80 80 70 70

20 800

0 300

50 50 40 40

400

Wavelenght (nm)

Wavelength (nm) Wavelenght (nm) Wavelenght (nm)

Wavelength (nm)

Pristine Cell Pristine Cell λ0λ0 = 490 nm = 490 nm λ0λ0 = 570 nm = 570 nm λ0λ0 = 650 nm = 650 nm λ0λ0 = 750 nm = 760 nm

60 60

10

30 30 500 600 20 20

700

800

10 10

0 0 Wavelenght (nm) 300 400 500 500 600 600 700 700 800 800 300 400 800

Wavelenght (nm) Wavelenght (nm)

Active Area

λ = 570 nm

0 0 0 0 300 400 400 500 500 600 600 700 700 800 800 300

Wavelenght(nm) (nm) Wavelenght

40

80 80

650 nm

λ020 =20 650 nm

λ0 = 570 nm λ0 λ0 = 650 nm nm = 650 λ0 λ0 = 750 nm nm = 760

30 30

10 0 300

Pristine Cell Pristine Cell λ0λ0 = 490 nm = 490 nm λ0λ0 = 570 nm = 570 nm λ0 = 650 nm λ0 = 760 nm

d

0 60 300 60 400

0 300

LLB λ0 =

30 30 10 10

70 70

10

40

20 500 600

10

25_018

40 40 nm 570

Pristine Cell (nm) Wavelenght Wavelenght (nm) λ0 = 490 nm

30 20

24_018

50 50

300 400 400 500 500 600 600 700 700 800 800 300

Reflection DM First (%) Reflection D. Mirror First (%)

0 300

Transmittance (%)

10

30

λ0 = 570 nm

23_018

60 60

0 0 300 400 400 500 500 600 600 700 700 800 800 300

Wavelength(nm) (nm) Wavelenght (nm) Wavelenght Wavelenght (nm)

Figure70 1. Schematic representation of device architecture (a). Digital images (aperture f/8, shutter speed of Bare Cell 23_018 1/100 s, ISO 100 and 15 mW cm-2 back illumination) for pristine cell, active layer (AL) thickness of ~40 nm), 60 24_018 25_018 bare dielectric mirror and full device stack comprising perovskite solar cell and dielectric mirror, including 50 LLB view from the solar cell and form the mirror (b). Total transmittance spectra of pristine cell and full stack 40 including dielectric mirrors (c). Total reflection spectra of pristine cell and full stack on which light-front 30 impinges the dielectric mirror first (d). Total reflection spectra of pristine cell and full stack on which light-front 20 impinges the cell first (e). Transmittance (%)

EQE (%)

20

50

EQE (%)

40

λ0 =

70 70

λ0 0 = 0 650 nm

40

c 60

ΔEQE (%)

View from mirror 10 10

hvi

30

λ0 = 490 nm

20 20

ITO

Pristine Cell Pristine Cell λ0λ0 = 490 nm = 490 nm λ0λ0 = 570 nm = 570 nm λ0λ0 = 650 nm = 650 nm λ0λ0 = 750 nm = 760 nm

λ0 = 490 nm

30 30

λ0 = 490 nm

70

LLB

40 40

MHP

PEDOT:PSS

40

80 80

LLB

View from cell 50 50

25_018 LLB

24_018

= ~40 nm 70 70 AL25_018

24_018

Wavelenght (nm)

25_018

80 80

ZnO

23_018

700

b

800 hv i

i

600

Reflection Cell First (%) Reflection Cell First (%)

500

500

Reflection Cell First (%) Reflection Cell First (%)

400

Dielectric mirror

6 800 4 2 Wavelenght (nm) 400 500 Pristine 600 700 800 Cell λ0 = 490 nm 0 λ0 = 570 nm λ0 = 650 nm λ0 = 750 nm -2 Wavelenght (nm) 23_018 100 300 400 100 500 600 700 800 100 100 + Bare DM 24_018 90 90 23_018 90 90 400

EQE (%)

a

6 4 2 0 -2 300

6 4 2 0 -2 300

ΔEQE (%)

ΔEQE (%)

6 4 2 0 -2 300

ACS Nano

- +

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ΔEQE (%)

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10 0 300

400

500

600

700

800

Wavelenght (nm)


23

λ0 = 650 nm

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d

Light on Light on

Camera

10

20

30

40

Dielectric mirror

Light on

Solar cell

Light off

Dielectric mirror

Camera

Solar Cell

Light on Light off

Daytime condition

c Dielectric mirror

Camera

Solar cell

Light on

b

Solar Cell

Nighttime condition

a

XRD intensity (a.u.)

SSE-TE SSE-DEE

Dielectric mirror

Light on

Camera

50

60

Position (º2 Theta)

a

b

20

10

10

5

0

0 -5

-10 -20 -25

5 µm

500 nm

SSE-TE

SSE Perovskite ITO-PEDOT:PSS ITO

(220)

25 nm

(110)

15 nm

XRD intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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-10 -15 10

20

30

40

50

60

Position (º2 Theta)

Figure 2. Topography of ultra-thin perovskite film as measured by intermittent contact atomic force microscopy. The ~40 nm thick perovskite film was fabricated with the solvent-solvent extraction method using toluene as extraction solvent. The left-hand side figure shows 20 μm x 20 μm while the right-hand side shows a 2 μm x 2 μm micrograph. Calculated roughness was estimated to be RMS = 7 nm (a). ~130 nm (b). XRD spectra of bare perovskite film.


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ACS Nano 800

λ0 = 490 nm λ0 = 570 nm λ0 = 650 nm λ0 = 760 nm

6 4 2 0 -2 300

18

Pristine λ0 = 490 nm λ0 = 570 nm λ0 = 650 nm λ0 = 760 nm Pristine

6 4 2 0 -2 300

15 16 10 14 5

ΔEQE (%)

600 Pristine 700

ΔEQE 2) (%) Integrated Jsc (mA/cm

6 500

400

ΔEQE (%)

0 -2 300

ΔEQE (%)

ΔEQE (%

4

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20 15 10 5 0 -5 300

ΔEQE (%) Integrated Jsc (mA/cm2)

ΔEQE (%)

ΔEQE (%)

ΔEQE (%)

Integrated Jsc (mA/cm2)

16 1 5 Wavelenght (nm) 2 14 0 15 12 4 -5 23_018 3 10 12 400 500 600 700 800 400 10 500 400 600 500 700 600 800 700 300 800 5 24_018 4 10 3 0 Wavelenght (nm) 8 Wavelenght (nm) Wavelenght (nm) 25_018 5 -5 8 LLB 6 6 2 300 400 500 600 700 800 6 23_018 23_018 λ0 = 490 nm 7 Wavelenght (nm) 4 λ0 = 570 nm 24_018 24_018 4 1 λ0 = 650 nm 8 25_018 25_018 λ0 = 760 nm 2 2 9 λ0 = 490 nm LLB 0 λ0 = 490 nm λ0 = 570 nm λ0 = 650 nm LLB λ0 = 570 nm 0 0 10 300 400 500 600 700 800 λ0 = 650700 nm 300 400 500 600 800 300 6 400 500 600 700 800 6 6 λ0 = 760 nm 11 4 Wavelenght (nm) 4 4 Wavelenght (nm) Wavelenght (nm) 2 12 2 2 0 λ0 = 490 nm λ0 = 570 nm λ0 =0 650 nm λ0 = 490 nm λ0 = 570 nm 13 0 -2 -2 b a c -2 14 300 400 500 600 700 800 80 80 300 400 500 600 700 800 80 300 400 500 600 700 800 40 Pristine Cell Pristine ; AL 40 nm PristineCell Cell 15 λ0 = 490 nm λ λ0 = 490 nm = 490 nm Wavelenght (nm) = 570 nm Wavelenght (nm) λ λ0 = 570 nm (nm) 570 λ0 = 570 nm 60 Wavelenght 650 60 60 16 30 λ0 = 650 nm λ λ0 = 650 nm = 650 nm λ = 490 nm λ = 570 nm λ = 650 nm λ0 = 760 nm 0 0 0 λ λ0 = 750 nm = 760 nm 17 23_018 23_018 25 23 24 23_018 40 Pristine Cell Pristine Cell 40 40 20 18 40 40 24_018 λ0 = 490 nm λ0 = 490 nm red dark blue dark light blue ; AL160 nm Pristine Cell ; AL130 nm Pristine Cell 24_018 Pristine Cell Pristine Cell 24_018 λ0 = 570 nm λ0 = 570 nm λ λ0 = 490 nm λ λ0 = 490 nm = 490 nm = 490 nm 25_018 19 10 0 nm 20 λ0 = 650 nm λ0 = 650 nm 25_018 20 20 λ = 570 nm λ λ0 = 570 nm λ0 = 570 nm 25_018 = 570 nm 0 nm 30 30 λ0 = 760 nm λ0 = 760 nm λ λ0 = 650 nm λ λ0 = 650 nm = 650 nm = 650 nm LLB 20 0 nm LLB λ = 750 nm LLB λ λ0 = 750 nm λ0 = 760 nm = 760 nm 0 nm 20 0 0 20 0 20 0 21 16 16 20 20 300 400 700 800 15 300 400 500 600 700 800 300 400 500 600 700 800 15 Pristine 500 300 400 500 15 800 600 1000! 5 600 λ0700 22 10 = 490 nm 10 10 14 14 (nm) 10 λ0 Wavelenght = 570 nm 5 Wavelenght (nm) Wavelenght (nm) 5 5 10 23 100! λ0 = 650 nm 0 12 0 λ0 = 490 nm 12 λ0 = 760 nm λ = 490 nm λnm λ0 = 570 nm λ0 -5 =0 650 nm 0 = 490 nm λ0 = 570 nm λ = 650 0 -5 24 0 0 -5 0 0 300 400 500 600 700 800 10! 10 300 400 500 600 700 800 10 300 400 500 600 700 800 25 300 400 500 600 700 800 300 400 500 600 700 800 Wavelenght (nm) 8 1! Wavelenght (nm) -5 26 8 Wavelenght (nm) Wavelength (nm) Wavelength (nm) Wavelenght (nm) Wavelength (nm) 27 6 6 0.1! λ0 = 490 nm λ0 = 490 nm 28 600 700 800 d -10 e 4 f λ0 = 490 nm Dev. 2 λ0 = 570 nm 500 4 λ0 = 570 nm λ0 = 570 nm 20 2540 16 1000! λ0 = 650 nm 40 10 Dev. 3 Pristine 29 0.01! 40 λ0 = 650 nm Cell Pristine Cell Pristine Cell λ0 = 650 nm AL ~160 nm Pristine Cell 15 Pristine Cell λ λ0 = 490 nm λ0 = 760 nm =nm 490 nm λ = 490 nm avelenght (nm) λ0 = 760 2 = 490 nm λ0 = 760 nm Dev. 4 2 λ λ0 = 490 nm = 490 nm Wavelenght (nm) Wavelenght (nm) λ λ0 = 570 nm 10 14 100! λ λ0 30 = 570 nm -15 = 570 nm λ0 = 570 nm 30 λ λ0 = 570 nm = 570 nm Dev.nm 5 λ = 650 nm 0.001! 30 λ0 = 650 nm 5 AL ~140 5 30 λ λ0 = 650 nm 20 = 650 nm λ λ0 = 650 nm = 650 nm 0 λ λ0 = 750 nm 31 0 = 760 nm Dev. 6 λ = 750 nm 12 0 λ0 = 760 nm λ λ0 = 750 nm = 760 nm 10! 300 400 500 600 700 800 300 400 500 600 700 800 -5 20 0.0001! 32 20 0 -20 20 10 300 400 500 600 700 800 -0.5 0 0.5 1 1.5 15 Wavelenght (nm) -0.2 0 Wavelenght 0.2 0.4 0.6 1! -1 (nm) 0.8 1 33 Pristine Cell

10 ~40 nm active layer λ0 = 490 nm 10 8 -5 Wavelenght (nm) 10 34 Voltage [V] Voltage (V) λ0 = 570 nm 0.1! 10 λ0 = 650 nm Dev. 2 6 35 0 λ0 =-10 490 nm λ0 = 570 nm 0 λ0 = 650 nm λ0 = 760 nm 80 0 Dev. 3 80 0.01! AL ~40 nm 300 400 500 600 700 800 λ0 = 490 nm ive layer 23 300 0 nm act 36 400 24500 4 600 700 800 ~13 300 400 500 25 600 700 800 Dev. 4 λ0 = 570 nm 5 red dark blue dark light blue λ0 = 650 nm Dev. 5 layer 37 -15 0.001! 60 ~160 nm active 60 2 λ0 = 760 nm Dev. 6 38 0.0001! 2.00 -20 0 Pristine Cell 40 0 Pristine Cell 40 39 -1 -0.5 0.5 1 1.5 device 20 40 60 80 100 120 140 160 180 =0 490 nm -0.2 0 0.2 Perinstine 0.6 0.8 1 λ0 0.4 = 490 nm 300 400 500 λ0600 700 800 λ0 = 570 nm λ0 = 570 nm 40 600 700 800 1.00 500 λ0 = 650 nm [V] 20 Voltage λ0 = 650 nm (nm) (nm) 20 ThicknessWavelenght of Active Layer Voltage (V) Wavelenght (nm) Wavelength (nm) Wavelenght (nm) λ0 = 760 nm Wavelenght (nm) λ0 = 760 nm 41 (nm) 0.00 avelenght 42 0 0 -1.00 300 of400 500 600 700 and 800 Figure 300 3. External Quantum Efficiency spectra perovskite solar, with the addition of different dielectric 400 500 600 700 800 43 80 -2.00 Wavelenght Wavelenght (nm) layer (AL) is: ~40 nm mirrors, the thickness of active (a), ~130(nm) nm (b) and ~160 nm (c). Photocurrent 44 2.00 -3.00 60 45 Perinstine device density-voltage curves under AM 1.5 irradiation at 0.1 W/cm2 illumination for devices comprising three 1.00 46 -4.00 thicknesses showing the contribution of the for the case of the solar cell with the thinner Pristine Cell 40 dielectric mirrors 0.00 47 λ0 = 490 nm -5.00 active layer (d). Integrated photocurrent density extracted external quantum efficiency measurement λ0 = 570from nm 48 -1.00 λ0 = 650 nm 20 λ0 = 760 nm -6.00 showing contribution of the dielectric mirrors (e). Photocurrent density contribution relative to pristine film as 49 -2.00 -0.2 0 0.2 0.4 0.6 0.8 1 0 and horizontal solid-line whiskers represent standard deviation (f). a function of active layer thickness, vertical 50 Voltage (V) -3.00 300 400 500 600 700 800 51 500 600 700 800 -4.00 Wavelenght (nm) 52 velenght (nm) -5.00 53 54 -6.00 λ0 = 490 nm -0.2 0 0.2 0.4 0.6 0.8 1 55 λ0 = 570 nm nm Voltage (V) 56 λ0λ0 == 650 760 nm 57 58 59 60

400

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λ0 = 650 nm

0 0

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EQE (%)

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ΔEQE (%) Jsc Increase (%) EQE (%)

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EQE (%)

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2 ΔEQE 2 (%) EQE (%) Integrated Jsc (mA/cm ) 2Integrated Jsc (mA/cm Current Density (mA/cm2) ) Current Density (mA/cm )

EQE (%)

0

0 0 0 0

EQE (%)

0

EQE (%)

EQE (%)

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sc

2 Current Density (mA/cm 2) 2 2) Current) Density (mA/cm2EQE ) (%) Current Density J(mA/cm Current Density (mA/cm ΔEQE Integrated (mA/cm ) (%)

EQE (%)

EQE (%)

0


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λ0 = 570 nm

ACS Nano

b

0.50 0.45

50

0.25

30

0.20

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0.15

CIE Y

0.3

0.4

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0.25 0.20

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0.8

0.8

590 4 3

610 700

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550

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CIE X

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0.4 0.3

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λ0 = 650 nm Pristine Cell

4

0.7

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Simulated; 1-500 nm

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570

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Simulated; 1-500 nm

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570

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CIE Y

CIE Y

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0.7

0.5

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f

Simulated; 1-500 nm

550

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530

0.6

590

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570

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550

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λ0 = 490 nm 1 = Pristine DM 2 = AL 40 nm 3 = AL 130 nm 4 = AL 160 nm Pristine Cell

530

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CIE Y

CIE Y

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0.30

Simulated; 1-500 nm

0.9 0.8

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CIE Y

a

Transparency (%)

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570

0.5 2

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590 610 700

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CIE X

CIE X

Figure 4. Simulation of transparency as a function of color coordinates (a). Simulation of photocurrent density as a function of color coordinates (b). Chromaticity of pristine perovskite solar cell with active layer of ~40 nm (blue solid diamonds) along with the full stack comprising solar cell and dielectric mirror; λ0 = 490 nm (empty squares) (c), λ0 = 570 nm (empty circles) (d), λ0 = 650 nm (empty up triangles) (e), λ0 = 750 nm (empty down triangles) (f).


26

Coloring Semitransparent Perovskite Solar Cells ... - ACS Publications

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