Name: influence of hybrid perovskite fabrication methods on film formation, electronic structure, and solar cell performance
Uploaded: 2017-02-27T14:00:00.000+00:00
Duration: 11 min 38 s
Description: Watch this Scientific Journal Video about Influence of Hybrid Perovskite Fabrication Methods on Film Formation, Electronic Structure, and Solar Cell Performance at JoVE.com

The authors would like to acknowledge financial support by the state of North Rhine-Westphalia through the project PERO-BOOST (EFRE, project code NW-1-1-040a). Thanks go to Azar Jahanbakhsh and Ines Schmidt (both University of Cologne) for assistance with fabrication and characterization of the 2-step solution processed perovskite layers, Dr. J&#252;rgen Schelter (University of Cologne) for the synthesis of the MAI material as well as Prof. Dr. Riedl and Neda Pourdavoud (both University of Wuppertal) for the XRD measurements.

1. ITO Substrates
NOTE: For an improved sample contacting and to avoid shorting of devices, the ITO coated glass substrates have to be patterned using lithography and etching. The overlap of top and bottom electrode defines the active area of the produced solar cells.
<ol>
	<li>Cut the indium tin oxide (ITO) coated glass plate with a glass cutter to yield 2.5 cm by 2.5 cm substrates.</li>
	<li>Glue a circular-shaped label (diameter = 1.6 cm) in the center of an ITO covered substrate.</li>
	<li>Etch ITO by placing the substrates in a hydrochloric 1 M FeCl3 solution at 60 &#176;C for 10 min.</li>
	<li>Clean the substrates sequentially with chloroform, acetone and soap solution (2%) in an ultrasonic bath.</li>
	<li>Rinse once more with distilled water and dry under N2 flow.</li>
</ol>
2. Ozonation of the ITO Substrates
NOTE: In order to remove organic impurities, the ITO substrates have to be ozonized. This activates the oxidic surface and increases the wetting properties of the substrate, which is crucial for the reproducible deposition of the subsequent layers.
<ol>
	<li>Mount ITO substrates on a sample holder and place it in an ozonation chamber.</li>
	<li>Illuminate for 10 min under (atmospheric) oxygen with an ultraviolet lamp (20 W) that generates ozone.</li>
</ol>
3. Deposition of the PEDOT:PSS Hole-collecting Contact
NOTE: A film of poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) is deposited by spin coating from an aqueous suspension under ambient conditions (25 to 40% relative humidity). This layer is used as a hole collecting contact and features a high work function, reduces pinholes, and leads to increased reproducibility of devices.
<ol>
	<li>Place 150 &#181;L of the PEDOT:PSS in water suspension (1.5%) with a syringe on the substrate after filtering through a 0.45 &#181;m filter.</li>
	<li>Spin coat using a rotation speed of 2,500 rpm for 25 s directly followed by 4,000 rpm for 5 s with an acceleration of 4,000 rpm/s each. This procedure leads to 40 to 45 nm thick PEDOT:PSS films.</li>
	<li>Remove residual water from the film by thermally annealing the substrates on a hotplate at 150 &#176;C for 10 min in air.</li>
</ol>
4. Deposition of the Perovskite Layers
Caution: Lead iodide (PbI2) is highly toxic to humans. Even small amounts are extremely dangerous for the nervous, hematopoietic, renal, and hepatic systems. Handle lead containing solutions with care. 
NOTE: The perovskite films investigated throughout this paper are prepared by five different methods using PbI2 and CH3NH3I (MAI) as precursor materials. In order to avoid degradation15, fabrication of the films and devices and their characterization have to be performed under inert atmosphere, e.g. in a N2 filled glove box (solution processing) or under vacuum (vapor deposition).
<ol>
	<li>Solution processing
	<ol>
		<li>Sequential deposition
		<ol>
			<li>Place substrate on spin coater and drop 150 &#181;L PbI2 dissolved in N,N-dimethylformamide, DMF (400 mg/mL) with a pipette on the substrate.</li>
			<li>Spin coat immediately at 3,000 rpm for 30 s.</li>
			<li>Dip PbI2 films in MAI dissolved in isopropanol (10 mg/mL) for 40 s or drop MAI solution onto the dried PbI2 film and leave there for 40 s.</li>
			<li>Spin coat remaining MAI solution at 3,000 rpm for 30 s off the substrate.</li>
			<li>Heat for 15 min at 100 &#176;C by placing the substrate on a hotplate.</li>
		</ol>
		</li>
		<li>Co-solution:
		<ol>
			<li>Dissolve both PbI2 and MAI in the desired ratio (ideally 1:1 molar ratio) in DMF to form a precursor solution with a concentration of 250 mg/mL.</li>
			<li>Stir precursor solution at 50 &#176;C for at least 5 h.</li>
			<li>Preheat substrates covered with PEDOT:PSS at 50 &#176;C for 5 min on a hotplate.</li>
			<li>To guarantee an increased reproducibility throughout one device series, create a toluene atmosphere by dripping a small amount of the solvent (200 &#181;L) inside the spin coater bowl while spin coating the perovskite layer.</li>
			<li>Place 150 &#181;L precursor solution (from 4.1.2.1) on the substrate.</li>
			<li>Spin coat perovskite layers at 3,000 rpm for 30 s from this precursor solution. Spin coat one layer of perovskite material per substrate.</li>
			<li>Heat the resulting perovskite layers for 30 s at 110 &#176;C on a hotplate.</li>
		</ol>
		</li>
		<li>Molecular Additive:
		<ol>
			<li>Co-dissolve NH4Cl (18-20 mg/mL) in the spin coating solution (as described in 4.1.2.1) to ensure better film formation. Proceed as before. 
			NOTE: This is an alternative to the addition of the co-solution from 4.1.2.</li>
		</ol>
		</li>
	</ol>
	</li>
	<li>Vacuum deposition
	<ol>
		<li>General procedure
		<ol>
			<li>Load the cleaned substrates into the vacuum system and evacuate.</li>
			<li>After a pressure of 10-7 mbar is reached, transfer samples into the evaporation chamber and shield it off from the evaporations sources with a shutter.</li>
			<li>Heat the two sources that contain the precursor materials to approximately 330 &#176;C for PbI2 and 140 &#176;C for MAI. Be aware that the pressure in the chamber will rise to approximately 10-4 mbar due to the volatile nature of the MAI.</li>
			<li>Calibrate the evaporation rates for the two materials using one quartz crystal monitor (QCM) positioned close to the source while the other one is at the sample position. Heat the material to an evaporation temperature and simultaneously write down the respective thicknesses shown on both QCMs. Calculate the tooling factor by the ratio of the two values. For the thickness calculation use 6.16 and 1.23 g/cm-3 for PbI2 and MAI, respectively.</li>
		</ol>
		</li>
		<li>Co-evaporation
		<ol>
			<li>Adjust the rates of PbI2 to MAI to approximately 1:2 by increasing or decreasing the temperature of the sources to achieve a stoichiometrically correct perovskite film.</li>
			<li>Open the shutter in front of the sample to start the deposition. After the intended thickness is reached, close the shutter and turn of the heating of the sources.</li>
			<li>Heat the films for 1 h at 70 &#176;C inside the vacuum chamber to remove volatile compounds and complete film formation.</li>
		</ol>
		</li>
		<li>Sequential evaporation
		<ol>
			<li>Reduce the temperature of the MAI source again below the point of evaporation (approx. 100 &#176;C) and heat up the PbI2 source until the evaporation starts; deposit 50 nm of pure PbI2.</li>
			<li>Afterwards, cool down the PbI2 source and evaporate 50 nm of MAI likewise. For larger thicknesses repeat steps 4.2.3.1 and 4.2.3.2 alternatively.</li>
			<li>Heat the films for 1 h at 70 &#176;C inside the vacuum chamber to remove volatile compounds and complete film formation.</li>
		</ol>
		</li>
	</ol>
	</li>
</ol>
5. Preparation of Solar Cells
<ol>
	<li>Dissolve the acceptor phenyl-C60-butyric acid methyl ester (PC60BM) in chlorobenzene at a concentration of 20 mg/mL and stir for at least one day on a hotplate at 50 &#176;C.</li>
	<li>Prepare the perovskite layer (co-solution process with additive) as shown before (see 4.1.3.).</li>
	<li>Cool down the heated perovskite layers for 30 s on a metal plate to room temperature.</li>
	<li>Place 150 &#181;L of PC60BM solution on the substrate</li>
	<li>Spin coat PC60BM at 2,000 rpm for 30 s on top of the perovskite film resulting in 50 nm thick layers.</li>
	<li>Place fully coated substrates in a sample holder and cover with a shadow mask in order to evaporate contacts on top of the active layers.</li>
	<li>Scratch one of the contacts with a scalpel in order to establish contact to the ITO anode.</li>
	<li>Transfer into a vacuum chamber for the deposition of the cathode top contact.</li>
	<li>Deposit 10 nm aluminum at a maximum pressure of p = 3 x 10-6 mbar with a rate of 0.5 &#197;/s, as measured by a quartz crystal monitor. After the first 10 nm the rate can be increased up to 2.5 &#197;/s until a layer thickness of 100 nm is reached.</li>
	<li>Perform current density versus voltage (JV) measurements of the solar cells using a source measurement unit in the voltage range of -0.5 to 1.5 V (steps = 0.02 V). To ensure that no hysteresis is appearing in the JV characteristics, measure the reverse scan direction as well, by sweeping the bias from 1.5 to -0.5 V as well. Use a solar simulator (100 mW cm&#178;) calibrated using a certified silicon photodiode.</li>
</ol>

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The authors have nothing to disclose.

We showed that the processing conditions have a significant influence on the film morphology and film coverage. This is the reason why a lot of research groups publish different results regarding solar cell performance and ionization potential for the same perovskite materials.
In order to ensure reproducibility, it is crucial for all the processing steps and characterization methods to be performed under inert atmosphere (or vacuum) to avoid degradation by humidity. Also the purity and vendor of the educts play an important role (not investigated herein). It is clear that vacuum deposited perovskite layers feature highly crystalline films; however, in comparison, solution processed films can be fabricated with a higher throughput.
In our study, using NH4Cl as an additive in the precursor solution and a toluene atmosphere in the spin coater bowl provided the most reproducible and smooth perovskite films. On the other hand, dip and drop coating processes lead to rather rough surfaces, and were not further considered for device application. Lastly, the vacuum processed layers feature overall smaller crystal sizes (~100 nm) but with a high degree of coverage through the whole film and smoother surfaces. From the sample series with varying precursor ratio, we learned that the composition has a significant influence on film formation as well. However, when investigating these layers with XRD (Figure 2) all films show a high degree of crystallinity and a similar tetragonal crystal structure indicated by reflections at 14.11&#176; and 28.14&#176; representing the (110) and (220) planes, while some of the layers seemed slightly more disordered, which can be seen by the appearance of weak (002) and (004) reflections. However, no significant broadening of the XRD diffraction peak is observed. Intriguingly, no sign of separate phases of PbI2 at 12.63&#176; are found, even for larger excess amounts of incorporated PbI2. This indicates that PbI2 is incorporated not as a separate phase or nanocrystalline islands but as dilute interstitials, which makes it undetectable by XRD. Thus, the use of XRD is limited.
On the other hand, XPS confirms that additional amounts of PbI2 or MAI (depending on the fabrication method) are present in the film, as evident from the variations in the lead to nitrogen ratio. These interstitials in turn have a significant influence on the electronic structure of the film as mentioned before. Now, combining the findings from XPS with the observation of differences in ionization energies revealed by UPS enables us to correlate these two phenomena. Figure 6 shows the combined plot where the measured IE value is plotted as a function of the film composition (lead to nitrogen ratio) of the corresponding film.
<img alt="Figure 6" src="/files/ftp_upload/55084/55084fig6.jpg" /> 
Figure 6: Extracted measurement points for the full data set of 40 perovskite &#64257;lms. Dependence of the ionization energy on the elemental ratio Rexp of the lead to nitrogen content (as determined by XPS); the solid line is a linear &#64257;t to the data and the dashed lines mark the standard deviation of &#177;0.12 eV. At the bottom, the residuals of the &#64257;t are shown. Reprinted with permission from reference16. Copyright 2015 Wiley-VCH. <a href="http://cloudflare.jove.com/files/ftp_upload/55084/55084fig6large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
We find a clear linear correlation between these two values. Our results, therefore, indicate that a perovskite film with MAI in excess exhibits a lower IE, while a PbI2 rich layer increases the IE. We find an IE of 6.05 &#177; 0.10 eV for the optimal molar ratio of Rexp = 1, which is considerably larger than the often published IE of 5.4 eV. This discrepancy is not likely to be due to processing conditions, as we find this value for a variety of differently prepared perovskite films. It is rather due to differences in data evaluation where the linear density of states slope used here results in higher readout values. An extensive discussion of this issue can be found in reference17. It is important to note that we find no change in the optical bandgap of these films (Eg = 1.60 &#177; 0.02 eV, data not shown), which means that it is not only a shift of IE with varying ratio, but the activation energy (EA) shifts simultaneously.
The maximum solar cell efficiency was found for a molar ratio R of 1.02 (PbI2 to MAI) with a power conversion efficiency of 9.6% which confirms findings from literature14 that slightly PbI2 rich films feature an improved carrier behavior by passivation of the perovskite film by PbI2 at the grain boundaries. A decrease in open-circuit voltage by 200 meV with increasing PbI2 content can be found. Since no change in the bandgap of the perovskite material occurs, the decrease of VOC cannot be explained by a concurrent decrease of the photovoltaic gap but rather by insufficient hole blocking at the interface of perovskite and PCBM (IEPCBM = 6.2 eV) due to the increase in perovskite IE. Simultaneously the FF for R &#62;1.05 decreases from 0.8 to 0.7 which supports these findings.
In conclusion, we presented an extensive study of perovskite films made by a variety of preparation methods and found that strong variations occur in film formation, electronic structure and device performance. Of particular interest is the possibility to adjust IE of perovskite by intentional incorporation of MAI or PbI2 interstitials which can be used for interface optimization in novel device architectures. Future studies will look at more advanced preparation techniques that aim towards larger device areas. These include methods like doctor blading, spray techniques, and large scale printing that are currently installed in our facility at the COPT.centre (COPT = center for organic production technologies).

Thin film photovoltaic technologies have attracted a significant attention in the research of solar cell applications due to their low material consumption and applicability on flexible substrates. Most notably, organic/inorganic halide perovskite materials have proven to be viable active layers in solar cell devices, leading to high efficiencies. Perovskites feature advantageous properties such as high absorption coefficient1, high charge carrier mobility2, and low exciton binding energy3. Perovskite layers can be produced by various solution or vapor phase based fabrication methods using low cost precursor materials like lead(II) iodide (PbI2) and methylammonium iodide (MAI). This way allows for an easy preparation of high crystallinity films using low fabrication temperatures compared to the commercially available silicon solar cells.
It has been shown that several parameters have a strong influence on the performance of perovskite solar cells, most notably film morphology, as it influences exciton diffusion length and charge carrier mobility. Nie et al. showed that by improving the morphology of perovskite films, regarding the coverage and average crystal size, the solar cell performance increases4,5. The morphology has been shown to be influenced by (i) choice of precursor material (e.g. the use of lead acetate6), (ii) molecular additives (like NH4Cl)7, (iii) choice of solvent, (iv) thermal annealing under solvent atmosphere (like toluene or chlorobenzene8), and particularly (v) the choice of preparation method9. Solution-based processes like one-step or two-step spin coating result in solar cells with efficiencies exceeding 17%4,10,11,12 whilst vacuum-deposited perovskite solar cells yield efficiencies of 15.4%13.
It has been shown that excess PbI2 in perovskite layers is advantageous for solar cell performance due to an improved carrier balance by passivation of the perovskite film by PbI2 at the grain boundaries14. However, little work has been done to understand the role of the effects of stoichiometry on perovskite film materials.
In this paper we present an extensive study on a wide range of differently prepared perovskite films and show how the preparation methods and precursor stoichiometry influence the morphology, crystallinity, density of states, film composition, and solar cell performance. A holistic overview is presented, ranging from fabrication to film characterization all the way to device performance.

<table><tbody><tr><td>ITO</td><td>Rose</td><td></td><td>&amp;lt; 15 &amp;Omega;/sq</td></tr><tr><td>PEDOT:PSS</td><td>Heraeus Clevios</td><td>P VP .Al&amp;nbsp; 4083</td><td></td></tr><tr><td>MAI</td><td></td><td></td><td>Synthesized as found in literature</td></tr><tr><td>PbI&lt;sub&gt;2&lt;/sub&gt;</td><td>Alfa Aesar</td><td>44314</td><td>99.999% trace metals basis, -10 mesh beads</td></tr><tr><td>NH&lt;sub&gt;4&lt;/sub&gt;Cl</td><td>Suprapure</td><td>101143</td><td>99.995%</td></tr><tr><td>PCBM</td><td>Nano C</td><td></td><td>99.9%</td></tr><tr><td>Chlorobenzene</td><td>Sigma Aldrich</td><td>270644</td><td>Chromasolv for HPLC (99.9%)</td></tr><tr><td>&lt;em&gt;N,N&lt;/em&gt;-Dimethylformamide</td><td>Acros Chemicals</td><td>&amp;nbsp;348430010</td><td>Extra dry, stored over molecular sieves (99.8%)</td></tr><tr><td>Toluene</td><td>Sigma Aldrich</td><td>244511</td><td>anhydrous</td></tr></tbody></table>

influence of hybrid perovskite fabrication methods on film formation, electronic structure, and solar cell performance

Hybrid organic/inorganic halide perovskites have lately been a topic of great interest in the field of solar cell applications, with the potential to achieve device efficiencies exceeding other thin film device technologies. Yet, large variations in device efficiency and basic physical properties are reported. This is due to unintentional variations during film processing, which have not been sufficiently investigated so far. We therefore conducted an extensive study of the morphology and electronic structure of a large number of CH3NH3PbI3 perovskite where we show how the preparation method as well as the mixing ratio of educts methylammonium iodide and lead(II) iodide impact properties like film formation, crystal structure, density of states, energy levels, and ultimately the solar cell performance.

In order to obtain a holistic view over the different fabrication methods for perovskite films, it is important to combine structural, electronic, and device characterization. Scanning electron microscopy (SEM) gives a good impression of morphology. Therefore, all films produced by the different fabrication methods were investigated. A representative subset of perovskite thin films is shown in Figure 1, which visualizes the significant influence of the preparation method on the film morphology.
Ideally, a smooth and pin-hole free film is desired for devices. As can be seen, this is the case for the vapor-deposited films (f, g), the dip coated ones (d, e), and the films prepared from co-solution with the additive NH4Cl and toluene atmosphere (a1 to a5) with a varying ratio R of PbI2 to MAI of 0.6 to 1.4. By contrast, the films without additive (h), as well as the dip-coated (d, e) and drop-coated ones (b, c) show large voids, needle like structures, or large surface roughness and are therefore not useful for device applications.
<img alt="Figure 1" src="/files/ftp_upload/55084/55084fig1.jpg" /> 
Figure 1: SEM images of perovskite films prepared by the different processing methods. (a1 - a5) Co-solution with additive and toluene atmosphere with different ratio of PbI2 to MAI (R), (b) drop-coating with 40 s loading time, (c) drop-coating with 120 s loading time, (d) dip coating (10 s loading time), (e) dip coating (3,600 s loading time), (f) co-evaporation, (g) sequential evaporation, (h) co-solution without additive. The scale bars indicate a length of 1 &#181;m. <a href="http://cloudflare.jove.com/files/ftp_upload/55084/55084fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
Finally, Figure 1 a1-a5 show variations in mixing ratio R of PbI2 to MAI from 0.6 to 1.4 using the co-solution preparation with additive and toluene atmosphere. The variations in surface coverage and crystal size show that here as well different film morphologies appear.
Although SEM is a good tool to visualize morphology and coverage of thin films and to get an impression on film roughness, however no structural information is given.
Therefore, in order to further characterize the perovskite films, X-ray diffraction (XRD) was used. With this technique using a Cu K&#945; anode (&#955; = 1.54056 &#197;) crystal phases in the range of 2&#952; between 10&#176; and 40&#176; (step size of 0.00836&#176;) were monitored and characterized. In many publications XRD is used to determine the quality of perovskite films.
Figure 2 shows XRD measurements of a subset of samples having six different mixing ratios of PbI2 to MAI, and therefore corresponds to the SEM images in Figure 1a1-a5. In addition, the spectrum of pure PbI2 is shown. From this, the influence of stoichiometry in the precursor solution on the quality of the perovskite layers, as such as the appearance of different phases, i.e. the incorporation of additional phases of PbI2 and MAI, is investigated. The spectra show a tetragonal crystal structure, and the reflections are indexed with their corresponding crystal planes. Surprisingly, no additional phases of MAI or PbI2 were observed in the off-stoichiometric films.
<img alt="Figure 2" src="/files/ftp_upload/55084/55084fig2.jpg" /> 
Figure 2: XRD pattern of pure PbI2 as well as perovskite samples prepared by the co-solution method (with NH4Cl and toluene atmosphere) using various molar ratios of the precursors (ratios given by the numbers on the right). For comparability, the curves are normalized to the peak at 14.11&#176; and shifted vertically. Reprinted with permission from reference16. Copyright 2015 Wiley-VCH. <a href="http://cloudflare.jove.com/files/ftp_upload/55084/55084fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
As XRD did not provide information on the composition of the film, X-ray photoelectron spectroscopy (XPS) is used, which is able to directly measure film stoichiometry. For this measurement a Mg K&#945; excitation source (h&#957; = 1252.6 eV) with a pass energy of 10 eV (Energy resolution = 800 meV) is used. The relative sensitivity factors (RSF) must be taken into account for each individually measured element. As such, it is important to calibrate the RSFs for our measurement system (Mg K&#945; source, angle between X-ray source and analyzer 50.0&#176;). We used a number of small molecules to calibrate the iodine peak with Tris-(4-iodphenyl)-amine (C18H12I3N), and calibrated Pb via PbI2. Carbon is used as a reference, as such with an RSF(C1s) = 1, therefore, the RSF factors for the individual elements are; RSF(N1s) = 1.8, RSF(I3d5/2) = 32.8, and RSF(Pb4f7/2) = 16.5.
Figure 3 shows a representative XPS spectrum of a vapor deposited film, with the characteristic core level peaks indicated in upper sub-figure. In the lower subfigures I3d5/2 (619.6 eV), N1s (402.7 eV), C1s (286.6eV), and Pb4f7/2 (138.6 eV) peaks are shown. All signals can be fitted by a single mixed Gaussian/Lorentzian peak, only in the case of iodine a small feature at higher binding energies is typically observed which is however a shake-up peak and therefore not related to an actual additional bonding state. We are able to extract the relative film composition of all prepared perovskite layers by integrating over the signal intensity and normalizing it by their respective RSF16. In some of the films, large deviations were found from the ideal film stoichiometry C:N:Pb:I of 1:1:1:3; e.g. the lead to nitrogen ratio varied between 0.4 an d 1.5. This was especially true for the vapor deposited films, where the co-evaporation is difficult to control and reproduce. For the solution processed samples, on the other hand, the actual and intended mixing ratios of the precursor agreed very well with the final film composition as determined from XPS.
<img alt="Figure 3" src="/files/ftp_upload/55084/55084fig3.jpg" /> 
Figure 3: Representative XPS spectra. Above a full XPS scan is shown, below the close-up XPS measurements of the integrated peaks are shown. <a href="http://cloudflare.jove.com/files/ftp_upload/55084/55084fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
In order to investigate how these variations in film composition influence the density of states, we turned to UV photoelectron spectroscopy (UPS). To perform UPS measurements, a helium discharge lamp (He I at 21.22 eV, sample bias -8 V) is used, with a 2 eV pass energy, and a 110 meV energy resolution (as determined from the Fermi edge width). For all samples, a full spectrum was first measured, then using a higher analyzer aperture to increase the signal to noise ratio, a detailed higher resolution scan of the VB region was performed. In the UPS spectra, specifically the VB region scan, satellite peaks resulting from the polychromatic He I radiation were corrected for numerically during data analysis.
Figure 4 shows UPS curves of the complete dataset of all investigated samples, covering both co-and sequential evaporation (light red) as well as the different solution processing (dark red) methods. We only want to stress that significant variations in the ionization energies (IE) are observed, notable from the variations of the high binding energy cutoff position in the left hand plot of Figure 4. These changes are brought about by variations in processing and film composition and lead to a tunability of IE between 5.67 and 6.4 eV. For a more detailed discussion, refer to reference16.
<img alt="Figure 4" src="/files/ftp_upload/55084/55084fig4.jpg" /> 
Figure 4: UPS scans of a representative subset of the investigated samples. The left panels show the high binding energy cutoff (HBEC) and the valence band region, while the right-hand side shows the high resolution close up of the VB onset for vapor-deposited (light red lines) and solution-processed (dark red lines) perovskite films. All curves have been shifted along the x -axis in order to be aligned to the feature at around 3 eV. Reprinted with permission from reference16. Copyright 2015 Wiley-VCH. <a href="http://cloudflare.jove.com/files/ftp_upload/55084/55084fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
As we learned from our previous experiments that perovskite films prepared from different precursor mixing ratios lead to variations in electronic structure without disrupting the crystal structure of the film, we wanted to investigate the effect of precursor ratio on the solar cell performance. Therefore, ITO/PEDOT:PSS/perovskite/PC60BM/Al solar cells were prepared using co-solution processed perovskite layers (with additive and toluene atmosphere) using ratios of PbI2 to MAI from 0.7 to 1.2. Figure 5 shows the impact of film stoichiometry (intended ratio R) on the solar cell characteristics power conversion efficiency (PCE), short-circuit current (JSC), open-circuit voltage (VOC) and fill factor (FF). The highest efficiency of 9.6% is found for an intended molar ratio of 1.02, i.e. close to the ideal perovskite composition.
<img alt="Figure 5" src="/files/ftp_upload/55084/55084fig5.jpg" /> 
Figure 5: Characteristic values of PCE, JSC, VOC, and FF. These values were extracted from the measurements of the solar cell devices independent of the intended mixing ratio Rint of PbI2 to MAI used for the film preparation. Reprinted with permission from reference16. Copyright 2015 Wiley-VCH. <a href="http://cloudflare.jove.com/files/ftp_upload/55084/55084fig5large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Influence of Hybrid Perovskite Fabrication Methods on Film Formation, Electronic Structure, and Solar Cell Performance at JoVE.com

Influence of Hybrid Perovskite Fabrication Methods on Film Formation, Electronic Structure, and Solar Cell Performance

We present an extensive study on the effects of different fabrication methods for organic/inorganic perovskite thin films by comparing crystal structures, density of states, energy levels, and ultimately the solar cell performance.

Hybrid organic/inorganic halide perovskites have lately been a topic of great interest in the field of solar cell applications, with the potential to ...

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Nanyang Technological University

Research

JoVE Journal

Chemistry

18.4K Views.  University of Cologne. We present an extensive study on the effects of different fabrication methods for organic/inorganic perovskite thin films by comparing crystal structures, density of states, energy levels, and ultimately the solar cell performance.

Video: Influence of Hybrid Perovskite Fabrication Methods on Film Formation, Electronic Structure, and Solar Cell Performance

Printing Fabrication of Bulk Heterojunction Solar Cells and In Situ Morphology Characterization

Polymer-based materials hold promise as low-cost, flexible efficient photovoltaic devices. Most laboratory efforts to achieve high performance devices have used devices prepared by spin coating, a process that is not amenable to large-scale fabrication. This mismatch in device fabrication makes it difficult to translate quantitative results obtained in the laboratory to the commercial level, making optimization difficult. Using a mini-slot die coater, this mismatch can be resolved by translating the commercial process to the laboratory and characterizing the structure formation in the active layer of the device in real time and in situ as films are coated onto a substrate. The evolution of the morphology was characterized under different conditions, allowing us to propose a mechanism by which the structures form and grow. This mini-slot die coater offers a simple, convenient, material efficient route by which the morphology in the active layer can be optimized under industrially relevant conditions. The goal of this protocol is to show experimental details of how a solar cell device is fabricated using a mini-slot die coater and technical details of running in situ structure characterization using the mini-slot die coater.

Printing Fabrication of Bulk Heterojunction Solar Cells and In Situ Morphology Characterization

Here, we present a protocol to fabricate organic thin film solar cells using a mini-slot die coater and related in-line structure characterizations using synchrotron scattering techniques.

Polymer-based materials hold promise as low-cost, flexible efficient photovoltaic devices. Most laboratory efforts to achieve high performance devices ...

Engineering

Fabrication of Fully Solution Processed Inorganic Nanocrystal Photovoltaic Devices

We demonstrate a method for the preparation of fully solution processed inorganic solar cells from a spin and spray coating deposition of nanocrystal inks. For the photoactive absorber layer, colloidal CdTe and CdSe nanocrystals (3-5 nm) are synthesized using an inert hot injection technique and cleaned with precipitations to remove excess starting reagents. Similarly, gold nanocrystals (3-5 nm) are synthesized under ambient conditions and dissolved in organic solvents. In addition, precursor solutions for transparent conductive indium tin oxide (ITO) films are prepared from solutions of indium and tin salts paired with a reactive oxidizer. Layer-by-layer, these solutions are deposited onto a glass substrate following annealing (200-400 &#176;C) to build the nanocrystal solar cell (glass/ITO/CdSe/CdTe/Au). Pre-annealing ligand exchange is required for CdSe and CdTe nanocrystals where films are dipped in NH4Cl:methanol to replace long-chain native ligands with small inorganic Cl- anions. NH4Cl(s) was found to act as a catalyst for the sintering reaction (as a non-toxic alternative to the conventional CdCl2(s) treatment) leading to grain growth (136&#177;39 nm) during heating. The thickness and roughness of the prepared films are characterized with SEM and optical profilometry. FTIR is used to determine the degree of ligand exchange prior to sintering, and XRD is used to verify the crystallinity and phase of each material. UV/Vis spectra show high visible light transmission through the ITO layer and a red shift in the absorbance of the cadmium chalcogenide nanocrystals after thermal annealing. Current-voltage curves of completed devices are measured under simulated one sun illumination. Small differences in deposition techniques and reagents employed during ligand exchange have been shown to have a profound influence on the device properties. Here, we examine the effects of chemical (sintering and ligand exchange agents) and physical treatments (solution concentration, spray-pressure, annealing time and annealing temperature) on photovoltaic device performance.

This protocol describes the synthesis and solution deposition of inorganic nanocrystals layer by layer to produce thin film electronics on non-conductive surfaces. Solvent-stabilized inks can produce complete photovoltaic devices on glass substrates via spin and spray coating following post-deposition ligand exchange and sintering.

We demonstrate a method for the preparation of fully solution processed inorganic solar cells from a spin and spray coating deposition of nanocrystal ...

Low Pressure Vapor-assisted Solution Process for Tunable Band Gap Pinhole-free Methylammonium Lead Halide Perovskite Films

Organo-lead halide perovskites have recently attracted great interest for potential applications in thin-film photovoltaics and optoelectronics. Herein, we present a protocol for the fabrication of this material via the low-pressure vapor assisted solution process (LP-VASP) method, which yields ~19% power conversion efficiency in planar heterojunction perovskite solar cells. First, we report the synthesis of methylammonium iodide (CH3NH3I) and methylammonium bromide (CH3NH3Br) from methylamine and the corresponding halide acid (HI or HBr). Then, we describe the fabrication of pinhole-free, continuous methylammonium-lead halide perovskite (CH3NH3PbX3 with X = I, Br, Cl and their mixture) films with the LP-VASP. This process is based on two steps: i) spin-coating of a homogenous layer of lead halide precursor onto a substrate, and ii) conversion of this layer to CH3NH3PbI3-xBrx by exposing the substrate to vapors of a mixture of CH3NH3I and CH3NH3Br at reduced pressure and 120 &#176;C. Through slow diffusion of the methylammonium halide vapor into the lead halide precursor, we achieve slow and controlled growth of a continuous, pinhole-free perovskite film. The LP-VASP allows synthetic access to the full halide composition space in CH3NH3PbI3-xBrx with 0&#160;&#8804; x&#160;&#8804; 3. Depending on the composition of the vapor phase, the bandgap can be tuned between 1.6 eV&#160;&#8804; Eg&#160;&#8804; 2.3 eV. In addition, by varying the composition of the halide precursor and of the vapor phase, we can also obtain CH3NH3PbI3-xClx. Films obtained from the LP-VASP are reproducible, phase pure as confirmed by X-ray diffraction measurements, and show high photoluminescence quantum yield. The process does not require the use of a glovebox.

Here, we present a protocol for the synthesis of CH3NH3I and CH3NH3Br precursors and the subsequent formation of pinhole-free, continuous CH3NH3PbI3-xBrx thin films for the application in high efficiency solar cells and other optoelectronic devices.

Organo-lead halide perovskites have recently attracted great interest for potential applications in thin-film photovoltaics and optoelectronics. Herein, ...

Morphology Control for Fully Printable Organic&#8211;Inorganic Bulk-heterojunction Solar Cells Based on a Ti-alkoxide and Semiconducting Polymer

The photoactive layer of a typical organic thin-film bulk-heterojunction (BHJ) solar cell commonly uses fullerene derivatives as the electron-accepting material. However, fullerene derivatives are air-sensitive; therefore, air-stable material is needed as an alternative. In the present study, we propose and describe the properties of Ti-alkoxide as an alternative electron-accepting material to fullerene derivatives to create highly air-stable BHJ solar cells. It is well-known that controlling the morphology in the photoactive layer, which is constructed with fullerene derivatives as the electron acceptor, is important for obtaining a high overall efficiency through the solvent method. The conventional solvent method is useful for high-solubility materials, such as fullerene derivatives. However, for Ti-alkoxides, the conventional solvent method is insufficient, because they only dissolve in specific solvents. Here, we demonstrate a new approach to morphology control that uses the molecular bulkiness of Ti-alkoxides without the conventional solvent method. That is, this method is one approach to obtain highly efficient, air-stable, organic-inorganic bulk-heterojunction solar cells.

A method for fully printable, fullerene-free, highly air-stable, bulk-heterojunction solar cells based on Ti alkoxides as the electron acceptor and the electron-donating polymer fabrication is described here. Moreover, a method for controlling the morphology of the photoactive layer through the molecular bulkiness of the Ti-alkoxide units is reported.

The photoactive layer of a typical organic thin-film bulk-heterojunction (BHJ) solar cell commonly uses fullerene derivatives as the electron-accepting ...

Monovalent Cation Doping of CH3NH3PbI3 for Efficient Perovskite Solar Cells

Here, we demonstrate the incorporation of monovalent cation additives into CH3NH3PbI3 perovskite in order to adjust the optical, excitonic, and electrical properties. The possibility of doping was investigated by adding monovalent cation halides with similar ionic radii to Pb2+, including Cu+, Na+, and Ag+. A shift in the Fermi level and a remarkable decrease of sub-bandgap optical absorption, along with a lower energetic disorder in the perovskite, was achieved. An order-of-magnitude enhancement in the bulk hole mobility and a significant reduction of transport activation energy within an additive-based perovskite device was attained. The confluence of the aforementioned improved properties in the presence of these cations led to an enhancement in the photovoltaic parameters of the perovskite solar cell. An increase of 70 mV in open circuit voltage for AgI and a 2 mA/cm2 improvement in photocurrent density for NaI- and CuBr-based solar cells were achieved compared to the pristine device. Our work paves the way for further improvements in the optoelectronic quality of CH3NH3PbI3 perovskite and subsequent devices. It highlights a new avenue for investigations on the role of dopant impurities in crystallization and controls the electronic defect density in perovskite structures.

Monovalent Cation Doping of CH3NH3PbI3 for Efficient Perovskite Solar Cells

Here, we present a protocol to adjust the properties of solution-processed CH3NH3PbI3 through the incorporation of monovalent cation additives in order to achieve highly efficient perovskite solar cells.

Here, we demonstrate the incorporation of monovalent cation additives into CH3NH3PbI3 perovskite in order to adjust the optical, excitonic, and electrical ...

Flash Infrared Annealing for Perovskite Solar Cell Processing

Organic&#8211;inorganic perovskites have an impressive potential for the design of next generation solar cells and are currently considered for upscaling and commercialization. Currently, perovskite solar cells rely on spin-coating which is neither practical for large areas nor environmentally friendly. Indeed, one of the conventional and most effective lab-scale methods to induce perovskite crystallization, the antisolvent method, requires an amount of toxic solvent that is difficult to apply on larger surfaces. To solve this problem, an antisolvent-free and rapid thermal annealing process called flash infrared annealing (FIRA) can be used to produce highly crystalline perovskite films. The FIRA oven is composed of an array of near-infrared halogen lamps with an illumination power of 3,000 kW/m2. A hollow aluminum body enables an effective water-cooling system. The FIRA method allows the synthesis of perovskite films in less than 2 s, achieving efficiencies &#62;20%. FIRA has a unique potential for the industry because it can be adapted to continuous processing, is antisolvent-free, and does not require lengthy, hour-long annealing steps.

We describe a flash infrared annealing method used for the synthesis of perovskite and mesoscopic-TiO2 films. Annealing parameters are varied and optimized for processing on fluorine-doped tin oxide (FTO) glass and indium tin oxide-coated polyethylene terephthalate (ITO PET), subsequently giving devices power conversion efficiencies &#62;20%.

Organic&#8211;inorganic perovskites have an impressive potential for the design of next generation solar cells and are currently considered for upscaling ...

Environment

Key Factors Affecting the Performance of Sb2S3-sensitized Solar Cells During an Sb2S3 Deposition via SbCl3-thiourea Complex Solution-processing

Sb2S3 is considered as one of the emerging light absorbers that can be applied to next-generation solar cells because of its unique optical and electrical properties. Recently, we demonstrated its potential as next-generation solar cells by achieving a high photovoltaic efficiency of &#62; 6% in Sb2S3-sensitized solar cells using a simple thiourea (TU)-based complex solution method. Here, we describe the key experimental procedures for the deposition of Sb2S3 on a mesoporous TiO2 (mp-TiO2) layer using a SbCl3-TU complex solution in the fabrication of solar cells. First, the SbCl3-TU solution is synthesized by dissolving SbCl3 and TU in N,N-dimethylformamide at different molar ratios of SbCl3:TU. Then, the solution is deposited on as-prepared substrates consisting of mp-TiO2/TiO2-blocking layer/F-doped SnO2 glass by spin coating. Finally, to form crystalline Sb2S3, the samples are annealed in an N2-filled glove box at 300 &#176;C. The effects of the experimental parameters on the photovoltaic device performance are also discussed.

Key Factors Affecting the Performance of Sb2S3-sensitized Solar Cells During an Sb2S3 Deposition via SbCl3-thiourea Complex Solution-processing

This work provides a detailed experimental procedure for the deposition of Sb2S3 on a mesoporous TiO2 layer using a SbCl3-thiourea complex solution for applications in Sb2S3-sensitized solar cells. This article also determines key factors governing the deposition process.

Sb2S3 is considered as one of the emerging light absorbers that can be applied to next-generation solar cells because of its unique optical and electrical ...

Solution-Processed "Silver-Bismuth-Iodine" Ternary Thin Films for Lead-Free Photovoltaic Absorbers

Bismuth-based hybrid perovskites are regarded as promising photo-active semiconductors for environment-friendly and air-stable solar cell applications. However, poor surface morphologies and relatively high bandgap energies have limited their potential. Silver-bismuth-iodine (Ag-Bi-I) is a promising semiconductor for optoelectronic devices. Therefore, we demonstrate the fabrication of Ag-Bi-I ternary thin films using material solution processing. The resulting thin films exhibit controlled surface morphologies and optical bandgaps according to their thermal annealing temperatures. In addition, it has been reported that Ag-Bi-I ternary systems crystallize to AgBi2I7, Ag2BiI5, etc. according to the ratio of the precursor chemicals. The solution-processed AgBi2I7 thin films exhibit a cubic-phase crystal structure, dense, pinhole-free surface morphologies with grains ranging in size from 200 to 800 nm, and an indirect bandgap of 1.87 eV. The resultant AgBi2I7 thin films show good air stability and energy band diagrams, as well as surface morphologies and optical bandgaps suitable for lead-free and air-stable single-junction solar cells. Very recently, a solar cell with 4.3% power conversion efficiency was obtained by optimizing the Ag-Bi-I crystal compositions and solar cell device architectures.

Herein, we present detailed protocols for solution-processed silver-bismuth-iodine (Ag-Bi-I) ternary semiconductor thin films fabricated on TiO2-coated transparent electrodes and their potential application as air-stable and lead-free optoelectronic devices.

Bismuth-based hybrid perovskites are regarded as promising photo-active semiconductors for environment-friendly and air-stable solar cell applications. ...

Inkjet Printing All Inorganic Halide Perovskite Inks for Photovoltaic Applications

A method for synthesizing photoactive inorganic&#160;perovskite quantum dot inks and an inkjet printer deposition method, using the synthesized inks, are demonstrated. The ink synthesis is based on a simple wet chemical reaction and the inkjet printing protocol is a facile step by step method. The inkjet printed thin films have been characterized by X-ray diffraction, optical absorption spectroscopy, photoluminescent spectroscopy, and electronic transport measurements. X-ray diffraction of the printed quantum dot films indicates a crystal structure consistent with an orthorhombic room temperature phase with (001) orientation. In conjunction with other characterization methods, the X-ray diffraction&#160;measurements show high quality films can be obtained through the inkjet printing method.

A protocol for synthesizing inorganic-lead-halide hybrid perovskite quantum dot inks for inkjet printing and the protocol for preparing and printing the quantum dot inks in an inkjet printer with post characterization techniques are presented.

A method for synthesizing photoactive inorganic&#160;perovskite quantum dot inks and an inkjet printer deposition method, using the synthesized inks, are ...

Facile Synthesis of Colloidal Lead Halide Perovskite Nanoplatelets via Ligand-Assisted Reprecipitation

In this work, we demonstrate a facile method for colloidal lead halide perovskite nanoplatelet synthesis (Chemical formula: L2[ABX3]n-1BX4, L: butylammonium and octylammonium, A: methylammonium or formamidinium, B: lead, X: bromide and iodide, n: number of [BX6]4- octahedral layers in the direction of nanoplatelet thickness) via ligand-assisted reprecipitation. Individual perovskite precursor solutions are prepared by dissolving each nanoplatelet constituent salt in N,N-dimethylformamide (DMF), which is a polar organic solvent, and then mixing in specific ratios for targeted nanoplatelet thickness and composition. Once the mixed precursor solution is dropped into nonpolar toluene, the abrupt change in the solubility induces the instantaneous crystallization of nanoplatelets with surface-bound alkylammonium halide ligands providing colloidal stability. Photoluminescence and absorption spectra reveal emissive and strongly quantum-confined features. X-ray diffraction and transmission electron microscopy confirm the two-dimensional structure of the nanoplatelets. Furthermore, we demonstrate that the band gap of perovskite nanoplatelets can be continuously tuned in the visible range by varying the stoichiometry of the halide ion(s). Lastly, we demonstrate the flexibility of the ligand-assisted reprecipitation method by introducing multiple species as surface-capping ligands. This methodology represents a simple procedure for preparing dispersions of emissive 2D colloidal semiconductors.

This work demonstrates facile room-temperature synthesis of colloidal quantum-confined lead halide perovskite nanoplatelets by ligand-assisted reprecipitation method. Synthesized nanoplatelets show spectrally narrow optical features and continuous spectral tunability throughout the visible range by varying the composition and thicknesses.

Influence of Hybrid Perovskite Fabrication Methods on Film Formation, Electronic Structure, and Solar Cell Performance

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Chapters in this video

Printing Fabrication of Bulk Heterojunction Solar Cells and In Situ Morphology Characterization

Fabrication of Fully Solution Processed Inorganic Nanocrystal Photovoltaic Devices

Low Pressure Vapor-assisted Solution Process for Tunable Band Gap Pinhole-free Methylammonium Lead Halide Perovskite Films

Morphology Control for Fully Printable Organic–Inorganic Bulk-heterojunction Solar Cells Based on a Ti-alkoxide and Semiconducting Polymer

Monovalent Cation Doping of CH₃NH₃PbI₃ for Efficient Perovskite Solar Cells

Flash Infrared Annealing for Perovskite Solar Cell Processing

Key Factors Affecting the Performance of Sb₂S₃-sensitized Solar Cells During an Sb₂S₃ Deposition via SbCl₃-thiourea Complex Solution-processing

Solution-Processed "Silver-Bismuth-Iodine" Ternary Thin Films for Lead-Free Photovoltaic Absorbers

Inkjet Printing All Inorganic Halide Perovskite Inks for Photovoltaic Applications

Facile Synthesis of Colloidal Lead Halide Perovskite Nanoplatelets via Ligand-Assisted Reprecipitation