Optoelectronic Properties of (CH3NH3)3Sb2I9 Thin Films for

Jun 21, 2016 - Bi-perovskite-based solar cells exhibit conversion efficiencies of 1%(22) with a TiOx electron transport layer (ETL) and 0.1%(23) in pl...
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Optoelectronic Properties of (CH3NH3)3Sb2I9 Thin Films for Photovoltaic Applications Jan-Christoph Hebig,† Irina Kühn,† Jan Flohre,† and Thomas Kirchartz*,†,‡ †

IEK-5 Photovoltaik, Forschungszentrum Jülich GmbH, 52425 Jülich, Germany Faculty of Engineering and CENIDE, University of Duisburg-Essen, Carl-Benz-Straße 199, 47057 Duisburg, Germany

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S Supporting Information *

ABSTRACT: We present solution-based fabrication and characterization of the lead-free perovskite-related methylammonium antimony iodide (CH3NH3)3Sb2I9 compound. By photothermal deflection spectroscopy (PDS), we determined a peak absorption coefficient α ≈ 105 cm−1 and an optical band gap of 2.14 eV for amorphous films of (CH3NH3)3Sb2I9. Compared to the related Bi compound, the Sb-perovskite shows no exciton peak in its absorption spectrum. The photoluminescence emission (PL) is observed at 1.58 eV, and the Urbach tail energy of this amorphous compound is Eu = 62 meV, indicating a substantial amount of energetic disorder. We fabricate a planar heterojunction solar cell with a (CH3NH3)3Sb2I9 absorber layer that yields a power conversion efficiency of η ≈ 0.5%, already featuring a decent fill factor (FF) of 55% and open-circuit voltage of 890 mV but low photocurrent densities. The result of this basic study on (CH3NH3)3Sb2I9 shows that this compound is a possible starting point for further research into Sb-based lead-free perovskite solar cells.

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devices with pin-architectures where the exciton splitting at the interfaces is even more critical. In this work, we present the exchange of bismuth by antimony, which is less toxic than Pb and leads to the same 0D perovskite structure as that for Bi-perovskites but with lower exciton binding energies. While hybrid Sb-based 0D perovskites have been discussed previously from an experimental24−29 or theoretical point of view,30 there is so far little work on photovoltaic properties of these compounds. Photovoltaic performance of Cs3Sb2I9 has been recently reported in the context of a larger study31 on structural and optical properties of this material, but the results on solar cell performance were so far rather poor. Here, we report on the fabrication of a proof of principle planar heterojunction solar cell with a (CH3NH3)3Sb2I9 absorber material. While the performance of the device is still low (power conversion efficiency ≈ 0.5%), the result is better than that previously reported on Cs3Sb2I9.31 The investigated methylammonium antimony iodide (CH3NH3)3Sb2I9 thin films were fabricated via spin-coating of a precursor solution (SbI3 + CH3NH3I in a mixture of γbutyrolactone (GBL) and dimethyl sulfoxide (DMSO)), followed by low-temperature annealing (100−120 °C, 30 min) under a nitrogen atmosphere. Figure 1c shows an scanning electron microscopy (SEM) image of the obtained thin film by a one-step spin-coating process. The image shows

ead-based organic−inorganic hybrid semiconductors like CH3NH3PbI3 (MAPI) have shown high potential as efficient absorber materials for single-junction solar cells due to their outstanding optoelectronic properties1 and the easy and cheap fabrication methods. The power conversion efficiencies of these three-dimensional (3D) perovskite solar cells have increased rapidly up to more than 22% in 20162−5 after only 6 years of research starting from the first reported perovskite solar cell with an efficiency of 3.8%.6 However, the low chemical stability of this lead-based perovskite under ambient air and the toxicity of the heavy metal lead could be obstacles for commercialization.6−11 The tin analogous perovskite compound CH3NH3SnI3 has exhibited moderate conversion efficiencies of up to 6% but is even more unstable under air and moisture because of the rapid oxidation of the Sn2+ state to the Sn4+ state.12−14 Another promising group of materials for lead-free alternatives are the zero-dimensional (0D) Biperovskites A3Bi2X9 (e.g., A = Cs+, MA+; X = I−, Br−).15−17 These materials show high band gaps of Eg > 1.8 eV,18,19 which are suitable for tandem or triple solar cells.20,21 Bismuth is a nontoxic element, and the Bi-based perovskites showed a better chemical stability under ambient atmosphere than the MAPI perovskite.22 However, the Bi-perovskites also show the signature of excitons in their absorption spectrum that have binding energies in the range of 400 meV.19,22 This leads to relatively low short-circuit current densities due to insufficient exciton splitting and charge carrier extraction. Bi-perovskitebased solar cells exhibit conversion efficiencies of 1%22 with a TiOx electron transport layer (ETL) and 0.1%23 in planar © XXXX American Chemical Society

Received: May 25, 2016 Accepted: June 21, 2016

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Figure 1. (a) Crystal structure of (CH3NH3)3Sb2I9 (space group P63/mmc), (b) XRD pattern of (CH3NH3)3Sb2I9 thin films prepared by the one-step spin-coating process (upper panel) and after the toluene drop (lower panel), (c) SEM image of the (CH3NH3)3Sb2I9 thin film, showing hexagonal crystals after one-step spin-coating, and (d) SEM image of the thin film with additional an toluene drop during spincoating with the two-step process.

hexagonal-shaped crystals with more than 2 μm grain size but an incomplete coverage of the substrate surface. The hexagonal shape of the crystals is in agreement with the expected crystal structure of (CH3NH3)3Sb2I9 (space group P63/mmc) shown in Figure 1a. The structure is similar to the Cs3Sb2I9 and the (CH3NH3)3Bi2I9 perovskites. It consists of 0D octahedral anionic metal halide units (Sb2I9)3− surrounded by three (CH3NH3)+ or Cs+ cations and connected via a hydrogenbonding interaction.15,19,32 The X-ray diffraction pattern (XRD) in Figure 1b (upper panel) of the crystalline film shown in Figure 1c confirms the hexagonal structure of the (CH3NH3)3Sb2I9 but shows a strong preferential growth direction along the c-axis. In order to improve the homogeneity of the films and to prevent shunting, we used solvent engineering techniques. We used a two-step spin-coating (3000 rpm/5000 rpm) process and added a drop of toluene as antisolvent 10 s before the end of step 2. The resulting thin films shown in Figure 1d have a very flat and homogeneous surface but much smaller grains and seem to consist of amorphous material. The XRD measurement of the two-step process with the toluene drop shows no clear reflexes, emphasizing the amorphous character of the material (see Figure 1b, lower panel). Because a pinhole-free absorber layer is essential for the application in photovoltaic devices, we used the two-step spin-coating process with the toluene drop to fabricate the thin films for investigation of the optoelectronic properties of (CH3NH3)3Sb2I9. In order to determine the absorption properties of the (CH3NH3)3Sb2I9 thin film, photothermal deflection spectroscopy (PDS) measurements were performed on relatively amorphous films such as the one shown in Figure 1d. Therefore, the absorber layers were fabricated on quartz glass substrates. Because the PDS setup allows also a measurement of the transmitted and reflected light fraction, an absolute value for the absorption coefficient α was calculated. Figure 2a shows

Figure 2. (a) Comparison of the absorption coefficient of various Bi-based perovskites and (CH3NH3)3Sb2I9 determined by PDS measurements. (b) Normalized absorptance of (CH3NH3)3Sb2I9 (from PDS) with a calculated Urbach tail energy and corresponding room-temperature PL spectrum.

the absorption coefficient for Sb-perovskite ((CH3NH3)3Sb2I9) compared to various Bi-perovskite compounds with different cations [A = CH3NH3+ → (MA+), Cs+, CH(NH)2+ → (FA+)] over a large spectral range. Note that the Bi-based films shown here for comparison were prepared in the same way as the Sbbased films and are also fairly amorphous, as seen in the XRD and SEM data shown in the SI (Figures S1 and S2). While it is true that all of the analyzed compounds have relatively high band gaps, they show high absorption coefficients in the range of α > 105 cm−1, which is high enough for efficient light 310

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Figure 3. (a) Shockley Queisser limit41 of tandem solar cells depending on the band gaps of the top cell (higher band gap) and bottom cell (lower band gap), with dashed and dotted lines indicating the band gap of crystalline Si and of (CH3NH3)3Sb2I9. (b) Calculation of the maximum short-circuit current density Jsc of a (CH3NH3)3Sb2I9 and a (CH3NH3)PbI3 absorber embedded in the layer stack: ITO/ PEDOT:PSS (25 nm)/absorber/PC61BM (60 nm)/ZnO-NP (60 nm)/Al (150 nm). In addition, we show the absorber layer thicknessdependent Jsc normalized to the value at 700 nm absorber thickness to compare how quickly nearly full absorption is reached for the two absorber materials. Note that the line for (CH3NH3)3Sb2I9 belongs to both the absolute and the normalized Jsc axes.

absorption in thin film photovoltaic devices. The band gap energy of (CH3NH3)3Sb2I9 is calculated by using a Tauc plot of the absorption coefficient measured by PDS (see Figure S3). Assuming a direct band gap, we calculated a value of Eg = 2.14 eV. The different Bi compounds show strong pronounced preedge absorption peaks at around 2.4−2.5 eV, which have been explained by the existence of excitons in the material.32 The binding energy of these excitons is in the range of 400 meV and therefore leads to inefficient exciton dissociation and consequently low photocurrent generation. Compared to the Bi compounds, the (CH3NH3)3Sb2I9 shows no significant exciton peak in its absorption spectrum. In addition, the Sbperovskite shows lower sub-band-gap absorption as compared to Cs3Bi2I9, MA3Bi2I9, and FA3Bi2I9, indicating a lower density of defect states in the material than that for the Bi-perovskites. To further investigate the optical properties of the Sbperovskite, the room-temperature photoluminescence (PL) was measured. Figure 2b shows the determined PL spectrum of the Sb-perovskite compared to the absorptance of the material. The measured PL signal is relatively weak and broad. Similar to what has been previously observed for Bi-based perovskites,21 the maximum of the PL is shifted substantially relative to the absorption onset. Here we observe the PL peak at 1.58 eV, which is therefore shifted 560 meV to lower energies relative to the band edge at 2.14 eV. This could imply that the luminescence in both Bi- and Sb-based 0D perovskites originates from radiative recombination involving subgap states. Luminescence from subgap states is also present in other amorphous or microcrystalline solar cell materials like amorphous and microcrystalline Si,33 and in these cases, the radiative recombination via these states makes up only a tiny fraction of the total amount of recombination. This means that the presence of radiative subgap transitions suggests that the films suffer from a much larger amount of nonradiative recombination processes via these subgap states. In addition to the subgap states that cause the PL peak, the absorption onset can be analyzed in terms of its steepness, which is typically measured by fitting an exponential of the form exp(E/Eu) to the exponential part of the absorption onset measured with PDS. Here Eu is called the Urbach energy34 and is related to the density of tail states in the material. We obtain a value of Eu ≈ 62 meV, which is four times higher than that for

MAPI35,36 but in the same range as that for the Bi compounds for which we found Urbach energies of 58 meV ((CH 3 NH 3 ) 3 Bi 2 I 9 ), 57 meV (Cs 3 Bi 2 I 9 ), and 69 meV (FA3Bi2I9) (see Figure S3). These relatively high Urbach energies for all of the Bi/Sb-perovskites indicate a higher degree of energetic disorder as compared to that for Pb-based 3D perovskites. The higher energetic disorder leads to additional tail states that are expected to be an additional source of nonradiative recombination leading to lower open-circuit voltages.37−40 In addition, a second PL peak with much smaller intensity is observed at 2.05 eV. This second peak could originate from direct band to band recombination. To put the optical properties and the band gap in perspective, Figure 3a shows a colormap of the efficiencies that could be obtained in the Shockley Queisser limit41 for twoterminal tandem solar cells as a function of the two band gaps. For typical low-band-gap materials such as crystalline Si (c-Si) or Cu(In,Ga)Se2, high-band-gap materials in the range around Eg2 = 1.75 eV are ideally suited. Due to the requirement of current matching, there is a sharp drop in efficiency for higher band gaps, showing that for applications in tandem solar cells, the band gap of the compounds shown in Figure 2a would have to be further reduced. Figure 3b shows how the short-circuit current density Jsc roughly depends on thickness assuming this device stack: ITO/PEDOT:PSS (25 nm)/absorber/PC61BM (60 nm)/ZnO-NP (60 nm)/Al (150 nm). It is clear by comparison between the Sb-based perovskite and the Pb-based perovskite that there is a substantial decrease in the achievable photocurrent under solar illumination when going from a band gap of around 1.6 eV to a band gap of 2.14 eV. However, when normalizing the short-circuit current density Jsc to the value at a thickness of 700 nm, it becomes clear that the Sb-based material reaches “full” absorption nearly as fast as the Pb-based material, which is due to the fact that the absorption coefficient reaches high values very close in energy to the absorption onset. To test the applicability of the Sb-perovskite as an absorber material for solar cells, a planar heterojunction device was fabricated, again using the two-step method with the toluene drop to obtain amorphous perovskite films. As a first test, the device was fabricated in the inverted pin-structure known from MAPI solar cells with the layer stack ITO (120 nm)/ PEDOT:PSS (25 nm)/(CH3NH3)3Sb2I9 (300 nm)/PC61BM 311

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Figure 4. (a) Illuminated J−V curve of (CH3NH3)3Sb2I9 solar cell measured with “up” and “down” sweep with a rate of 0.1 V/s. (b) EQE measurement of the (CH3NH3)3Sb2I9 solar cell compared to the reference device (ITO (120 nm)/PEDOT/PCBM/ZnO-NP/Al).

a much lower exciton binding energy than that in the Bi compounds. The weak PL signal was shifted 560 meV away from the band edge, indicating radiative recombination involving sub-band-gap states. This is also emphasized by the relatively high Urbach tail energy of 62 meV, which indicates a much higher degree of disorder in the absorber material than that for the 3D MAPI. To prove the potential of (CH3NH3)3Sb2I9 as an absorber material for photovoltaic applications, a planar heterojunction device was fabricated reaching a power conversion efficiency of 0.5%. By optimizing the contact layers and the morphology of the Sb-perovskite, much higher power conversion efficiencies might be possible.

(60 nm)/ZnO-NP (60 nm)/Al (150 nm) that was already used for the simulations in Figure 3b. Figure 4a shows the J−V curve of the Sb-perovskite under illumination, showing only little hysteresis at a scan speed of 0.1 V/s. The values for Jsc are nearly unaffected by the sweep direction and agree reasonably well with the values obtained from integrating the external quantum efficiency (EQE), as shown in Figure 4b. The fill factor (FF) slightly changes from 52 (up direction) to 55% (down direction), and the open-circuit voltage Voc changes from 885 (up) to 896 mV (down). Compared to the band gap of 2.14 eV, Voc ≈ 0.9 V is relatively low but still slightly higher when compared to the highest value (0.85 V for Cs3Bi2I9 with a band gap of 2.2 eV) obtained for Bi-based perovskites so far.22 The gap between the band gap and Voc could be explained by the high defect density of the obtained material (see PDS and PL). By improving the material quality and optimizing the device architecture with suitable contact layers, it might be possible to increase Voc significantly. To check how much the contact layer contributes to the photocurrent and therefore to the performance of the device, we prepared a heterojunction without the (CH3NH3)3Sb2I9 absorber layer. The determined short-circuit current of the reference device from EQE measurements (0.1 mA/cm2) is more than 1 order of magnitude smaller than that for the complete Sb-perovskite device, which reached Jsc = 1.1 mA/cm2 (see Figure 4b). Nevertheless, the maximum of the EQE of the Sb-perovskite-based cell is around 12%. Up to now, this value was very low compared to the high absorption coefficient of α = 105 cm−1, indicating that charge carrier collection is inefficient in this kind of device. This suggests that future work has to focus on either improving mobility-lifetime products in the material42,43 or finding ways of still extracting the charge carriers. Measures to improve charge extraction could be mesoporous scaffolds, which are often used with lead-free perovskites,44−47 or bulk-heterojunction-like absorber geometries that have been proven successful in achieving high FFs also in Pb-based perovskite devices.48 In summary, we introduced the (CH3NH3)3Sb2I9 0D perovskite as a possible candidate for lead-free perovskite solar cells. By using solvent engineering techniques including a toluene drop during the spin-coating process, a homogeneous and compact but amorphous thin film could be prepared. The absorption spectrum determined from PDS measurements showed a high peak absorption coefficient of α > 105 cm−1 and a high band gap of 2.14 eV. In contrast to the related Biperovskite compounds, amorphous films of the Sb-perovskite showed no exciton peak in the absorption spectrum, indicating

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.6b00170. Absorber and device fabrication process, additional data on optoelectronic properties, information on measurement setups (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +49 2461 61 96500. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank Oliver Thimm and Josef Klomfaß for the support on PDS measurements and Florian Köhler for performing the XRD measurements. The authors acknowledge financial support from the Bavarian Ministry for Economics, Media, Energy and Technology via the project “Recombination in lead-free perovskite solar cells”. The authors thank Uwe Rau for continuous support.

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REFERENCES

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DOI: 10.1021/acsenergylett.6b00170 ACS Energy Lett. 2016, 1, 309−314

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DOI: 10.1021/acsenergylett.6b00170 ACS Energy Lett. 2016, 1, 309−314