Mikas
Remeika
,
Sonia Ruiz
Raga
,
Shijin
Zhang
and
Yabing
Qi
*
Energy Materials and Surface Sciences Unit (EMSS), Okinawa Institute of Science and Technology Graduate University (OIST), 1919-1 Tancha, Onna-son, Okinawa, Japan. E-mail: Yabing.Qi@OIST.jp
First published on 9th February 2017
Ultrasonic spray coating is a promising pathway to scaling-up of perovskite solar cell production that can be implemented on any scale – from table-top to mass production. However, unlike spin-coating, spray coating processes are not easily described by a set of machine-independent parameters. In this work, in situ measurement and modeling of wet film thickness and evaporation rate are presented as a machine-independent description of the ultrasonic spray coating process, and applied to fabrication process optimization for high-performing perovskite solar cells. Optimization based on physical wet film parameters instead of machine settings leads to better understanding of the key factors affecting film quality and enables process transfer to another fabrication environment. Spray coated PbI2 film morphology is analyzed under a range of coating conditions and strong correlation is observed between spray coating parameters and PbI2 film uniformity. Premature precipitation and sparse nucleation are suggested as causes of film non-uniformity, and optimal process parameters are identified. Device fabrication based on the optimized process is demonstrated under ambient conditions with a relative humidity of 50%, achieving a power conversion efficiency of 13% in 1 cm2 area devices, with negligible hysteresis.
As a new technology reaches a stage where mass production is considered, development focus shifts from champion device performance to reproducibility. Reproducibility between different locations on the substrate must be considered when device size is increased, and reproducibility between different batches must be considered when production volume is scaled up. For devices based on thin film coating, film uniformity is strongly connected to process reproducibility. In this study we develop the strategy to achieve uniform spray-coated films as device area is increased above 1 cm2. We demonstrate that the mm-scale uniformity of spray coated PbI2 films is determined by spray coating conditions. Furthermore, we address the problem of reproducing the result in a different environment and on equipment where current machine settings may not be easily translated. Through a combination of direct measurement and modeling we accurately determine wet film thickness and evaporation rate that correspond to optimal coating quality. By real-time monitoring of laser light interference and scattering by a wet film, we are able to measure in situ the wet film thickness and evaporation rate. Armed with accurate data for a single solvent, we are able to compute an evaporative mass transfer coefficient that is characteristic of the process chamber, and thereon predict evaporation rates for any solvent, based on data available in the literature.
Evaporation of a thin liquid film is a complicated process that is strongly affected by the properties of the evaporating material, air flow pattern in the process chamber, and the geometry of the evaporating film. In this work we demonstrate that with solvent properties available in the literature, air flow pattern can be characterized by in situ measurement of film evaporation rate, and spray pattern geometry can be accounted for by spatially resolved computation using the COMSOL Multiphysics software suite.
By quantitatively characterizing the wet film under various coating conditions we can advance from intuition to quantitative prediction of coated film quality. The method of in situ wet film characterization that we present requires minimal modification to machinery and can be easily adapted to another spray coating system to determine evaporative mass transfer coefficient and wet film thickness as a function of specific machine settings. Once these parameters are known, process recipes become transferrable between the two machines. Even differences in spray patterns due to different sample sizes can be accounted for by local evaporation rate modeling described in this work.
The immediate goal of this work is optimization of the spray coating process to achieve the best uniformity of the PbI2 precursor for the MAPbI3 perovskite film fabrication. In the sequential perovskite formation process,9,10 the final perovskite film thickness is determined by the thickness of the PbI2 layer, as methyl ammonium iodide (MAI) is supplied from solution as much as is necessary to react with PbI2. While the size and quality of perovskite crystal grains is driven by the concentration of the MAI solution16 (see the ESI†), we show that the large scale (mm scale) uniformity of the perovskite films is determined by the uniformity of the PbI2 layer. Therefore, fabrication processes capable of achieving high uniformity of PbI2 film are crucial to scale-up of perovskite solar cell production.
ID | T/°C | DMF evap. rate/mol m−2 s−1 | Ink pump/mL min−1 | Wet film thick./μm |
---|---|---|---|---|
A | 30 | 0.77 | 7.0 | 4.2 |
B | 50 | 4.2 | 7.0 | 4.2 |
C | 70 | 13 | 7.0 | 4.2 |
D | 30 | 0.77 | 3.5 | 2.1 |
E | 50 | 4.2 | 3.5 | 2.1 |
F | 70 | 13 | 3.5 | 2.1 |
G | 30 | 0.77 | 2.3 | 1.4 |
H | 50 | 4.2 | 2.3 | 1.4 |
I | 70 | 13 | 2.3 | 1.4 |
Evaporation rate by mass change measurement was obtained by multiplying a directly measured rate of wet film weight change by a factor that adjusts for different sample geometries (computed by modeling the local evaporation rate using COMSOL Multiphysics, see Section 3.5).
Model curves combine forced convection and free convection models, as described in the ESI.† Forced convection mass transfer constant was obtained from the direct measurement of mass change in the chamber for a specific solvent. Free convection mass transfer coefficient was calculated as described in ESI,† and multiplied by a geometry correction factor. It is important to note that the value of only one fitting parameter was used to fine tune the model for all solvents and conditions. The geometry correction factor for forced convection was determined completely through modeling of the local evaporation rate using COMSOL Multiphysics, and the ratio between correction factors for narrow and broad spray patterns was also determined from the local evaporation rate model. For IPA, evaporative cooling was taken into account by using reduced liquid surface temperature in the model, for DMF evaporative cooling was deemed negligible (see the ESI†).
Film discontinuity occurs when ink droplets impinging on the substrate do not form a flat wet film, but instead dry as separate droplets. A flat wet film will not form when the ink wetting of the substrate is poor (i.e., with a high liquid contact angle) or when the ink dries faster than the time it takes for the droplets to merge. The chief method of ensuring a good contact angle is to select an ink solvent with surface energy lower than that of the substrate. Wetting can also be improved by adding surfactants to the ink, or by oxygen plasma treatment of the substrate (to increase the surface energy).
The coffee stain effect is a term that is applied to describe accumulation of material at the edges of wet films or droplets. It occurs due to preferential evaporation from a pinned edge of a wet film,11 and can cause an overwhelming fraction of the solids to be deposited at the edges of wet films. The coffee stain effect can be counteracted by inducing Marangoni flow which acts in the opposite direction, for example by addition of a co-solvent with a suitable evaporation rate and surface tension.12 It can also be minimized by creating a wet film with a uniform thickness which will minimize the effective “edge” area. Increasing the substrate temperature will accelerate evaporation, reducing the time available for ink redistribution, however it is not suitable for suppressing the accumulation of ink solids at film edges as it accelerates the coffee stain effect.13
The Marangoni effect is a flow that occurs on the surface of a liquid in the presence of a gradient of surface tension. A flow will occur from the low surface tension region to the high surface tension region. Generally this will occur in cases of a mixture of liquids, and can be used to engineer a flow that counteracts the coffee stain effect,12 however it can also occur in droplets of single solvent and cause accumulation of material in the center of the droplet.14
Another source of non-uniformity that is important to consider is sparse nucleation of precipitate crystals. For salts such as PbI2 deposition will occur preferentially at sites where nucleation has already occurred. Sparse/slow nucleation and a long drying time will result in accumulation of solids in sparse nucleation sites. To achieve the best film uniformity, nucleation rate/density needs to be maximized, and the time during which ink solids are mobile (drying time) needs to be minimized.
To counteract the coffee stain effect by a co-solvent system12 a solvent with a lower surface tension and a lower evaporation rate is required. The surface tensions of DMF, DMSO, and NMP are 34.4 dyn cm−1, 42.9 dyn cm−1, and 40 dyn cm−1, respectively. The boiling temperatures of DMF, DMSO, and NMP are 153 °C, 189 °C, and 202 °C respectively. In this case, low evaporation rate (usually corresponding to a solvent with a high boiling temperature) solvents have a higher surface tension as well, so a co-solvent mixture to counteract the coffee stain effect cannot be prepared from these solvents, and other solvents that dissolve PbI2 are not readily available. Therefore, the remaining pathway to suppressing the coffee stain effect is to minimize the wet film thickness to achieve a faster drying time without increasing the temperature.
Keeping the above guidelines in mind, it is intuitive that the best dry film uniformity would be achieved at a low temperature and a low wet film thickness. Fig. 1 shows transmission light microscope images of spray coated PbI2 and resulting MAPbI3 films fabricated at varying substrate temperatures and wet film thicknesses. The ink concentration is adjusted to maintain the same amount of PbI2 per area in all samples. Table 1 gives the spray coating parameters for each sample in Fig. 1, as well as the calculated wet film thickness and DMF evaporation rate. It is observed that at elevated temperatures (samples B, C, E, F, H and I), thin wet films result in dry films with mm-scale non-uniformity, indicating that precipitation occurred before droplets could form a flat wet film. At the process temperature of 30 °C, films appear uniform in optical images. Detailed examination by AFM (Fig. 2a–c) reveals that perovskite films formed from PbI2 films sprayed at high wet film thicknesses have reduced surface coverage, possibly due to sparse nucleation of PbI2 during film drying and resulting in microscopically uneven films. To quantify this observation, solar cell samples (fabricated as described in Materials and methods) were imaged in AFM after MAPbI3 film formation, before the deposition of the hole transport layer and metal electrode. Fig. 2f shows a clear negative correlation between wet film thickness of the PbI2 spray coating process and surface coverage fraction by MAPbI3.
Data in Fig. 1 demonstrate that the mm-scale uniformity of coated PbI2 films is strongly correlated with spray coating conditions, and that the uniformity pattern of PbI2 films is reflected in the resulting MAPbI3 films. Fig. 2f indicates that a larger wet film thickness results in poorer surface coverage by MAPbI3. A low wet film thickness and a low evaporation rate are optimal for film uniformity. A high evaporation rate results in the mm-scale non-uniformity (i.e., macroscopic non-uniformity), whereas a high wet film thickness results in reduced surface coverage on the μm scale (i.e., microscopic non-uniformity).
Fig. 3a shows the principle of operation for a wet film measurement system that allows in situ measurement of wet film thickness in a spray coating system. A laser beam is set up to be reflected from the substrate that is being spray coated. The substrate must be at least partially reflective, a requirement that is readily satisfied by transparent glass-FTO, or glass-ITO substrates typically used for perovskite solar cell fabrication. When a film of pure solvent is sprayed onto the substrate, a fraction of the light is scattered or absorbed, reducing the total amount reaching the detector. Additionally, the wet film creates a second partially reflective surface. The laser beams reflected from the liquid film surface and the substrate surface will interfere constructively or destructively depending on the thickness of the wet film (principle of operation of anti-reflective coatings). If the intensity is tracked in real time, the reflected beam intensity will oscillate. The reflected beam intensity will go through a complete oscillation when the path difference between the beams reflected from the liquid and from the substrate will change by one wavelength. Note that scattering and absorption do not mask the interference as long as they do not change on exactly the same time-scale. If the change of film thickness in one complete oscillation is denoted as Δh then
(1) |
This method assumes that during the majority of the drying time the evaporation rate of the solvent film is constant. This is a good assumption as long as the film thickness does not limit the heat flow available for evaporation. Experimental observation of evenly spaced reflected intensity oscillations supports the assumption that the evaporation rate of a flat wet film is constant once the film is stabilized (see Fig. 3b). Fig. 3c–e show that the drying time depends linearly on the amount of dispensed ink, further supporting the assumption that evaporation rate is independent of film thickness. It is notable that the best accuracy wet film drying time and evaporation rate measurement are obtained when the total drying time is longer than 20 s. This time frame ensures that the measurement time is much longer than the spray coating time (about 0.5 s). Fig. 3c–e show that at temperatures above 40 °C drying time can become less than 10 s. To accurately estimate wet film evaporation rate in this regime modeling is employed as described in Sections 3.4 and 3.5.
To further validate evaporation rates determined from laser reflection and interference, the evaporation rates of DMF and IPA were determined by directly measuring the rates of mass change of a liquid film (with a known area) in the process chamber. Rates measured by laser reflection and direct mass change are shown in Fig. 3f (see Materials and methods for details).
The method based on laser light reflection is effective if the reflectivity of the surfaces is constant and the wet film evaporates completely (no precipitation). This means that ink properties have to be inferred from pure solvent properties. This assumption is valid if the solvent viscosity and evaporation rate are not strongly affected by the presence of the solute. For non-polymer materials such as PbI2 this is a good assumption as even at a saturated concentration, the solvent viscosity is not strongly affected. The evaporation rate will be slightly affected by the presence of the solute in the ink; if necessary, this can be accounted for by Raoult's law.15 It can also be noted that in the presence of the precipitate, after most of the solvent has evaporated, solvent evaporation rate may be substantially reduced if it is absorbed into a porous precipitate and is no longer accurately described by a thin liquid film on a flat surface. Our model addresses only the condition where a continuous liquid surface is present, as in this regime ink solids are highly mobile. We do not address the condition of solvent absorbed in a porous precipitate because in that condition ink solids are no longer mobile over macroscopic (mm-scale) distances.
The evaporation rate of a single solvent can be computed as follows:
(2) |
The evaporative mass transfer coefficient without external heating is dominated by the air flow due to process chamber exhaust. Computation of the mass transfer coefficient due to a forced air flow requires detailed knowledge of the air flow rate above the evaporating surface, which is very difficult to determine. Instead, the evaporative mass transfer coefficient at room temperature was directly measured by tracking the weight change of the sample placed on a precision scale in the spray coating chamber. Because the evaporation rate is significantly elevated at sample edges, the average evaporation rate depends on the evaporating surface geometry. Therefore, to determine the evaporation rate of a spray-coated wet film, the local evaporation rate was determined by modeling in COMSOL Multiphysics (see details below).
At elevated temperatures buoyant convection dominates the air flow. The mass transfer coefficient for buoyant convection can be computed from the sample geometry, air properties, and heated surface temperature15
(3) |
Local evaporation rate modeling can also be used to compute the evaporation rate adjustment when spray patterns are changed, as would be expected to happen when scaling-up to higher volume production. Fig. 3f shows evaporation rates computed for a broad DMF spray pattern (100 mm) and a narrow spray pattern (40 mm).
To summarize, results of three complementary approaches to determining the evaporation rate of a film are shown in Fig. 3f. The film evaporation rate and film thickness can be completely determined immediately following spray coating by interference and scattering of a reflected laser beam. Once the laser reflection measurement is accomplished for a single solvent and several temperatures, the evaporative mass transfer coefficient of the process chamber can be computed and applied to predict evaporation rates of different solvents and at temperatures where the film evaporation rate is too fast for accurate measurement. The laser reflection measurement and modeling are verified by a direct measurement of mass change of a wet film of a known area. The set of process parameters to achieve optimal PbI2 film uniformity are: temperature = 30 °C and ink pump rate = 3.5 mL min−1, corresponding to wet film thickness 2.1 μm and DMF evaporation rate of 0.77 mmol s−1 m−2.
To demonstrate complete devices incorporating a spray coated PbI2 layer, we chose a commonly used perovskite solar cell architecture that is known to be compatible with ambient processing.10Fig. 5a shows the layer structure of the cell and a scanning electron microscopy (SEM) cross-section image of the optimized design. The spray coating process for PbI2 films was selected to match the film with the best uniformity (sample D, temperature = 30 °C and ink pump rate = 3.5 mL min−1). Device performance data for non-optimal wet film thickness can be found in ESI Fig. S8.† As expected, the perovskite solar cells fabricated using the optimal parameters show not only the highest PCE, Voc, Jsc, and FF, but also the lowest standard deviations, strongly suggesting the significantly improved device reproducibility under the optimal coating conditions.
Fig. 5c shows the XRD diffractogram of a typical device. Peaks at 2Θ = 14.1° corresponding to MAPbI3 and 2Θ = 12.7° corresponding to unreacted PbI2 are observed. The external quantum efficiency (EQE) spectrum shown in Fig. 5d shows slightly greater EQE at shorter wavelengths, also suggesting the presence of unreacted PbI2. In several recently published studies it has been suggested that residual PbI2 may be difficult to eliminate in a 2-step process, and may not necessarily lead to substantial performance loss. Ko et al.10 implemented a 2-step perovskite formation process under ambient conditions and found that optimal performance was obtained with PbI2 peak in the XRD pattern around 50% as high as MAPbI3 peak. Kim et al.17 discussed in detail the formation process of the MAPbI3 in a 2-step process, and concluded that excess PbI2 cannot be eliminated by simple optimization of MAI solution concentration and immersion time, without compromising the performance. On the other hand, it is expected that further enhancement of MAPbI3 crystallinity can lead to increased performance.
Table 2 shows performance statistics for devices incorporating a spray-coated PbI2 layer. Maximum power conversion efficiency (PCE) of 13.0% is achieved with the average PCE of 10.2%. Device performance is on par with the best performance achieved for spray coated devices with an active area of 1 cm2.8 We also observed that these devices could be operated at the maximum power point for over a 100 hours preserving 60% of initial PCE. See Fig. S7 in the ESI† for steady state measurement of device performance. Thus, we can confirm that the PbI2 films for which uniformity was optimized are suitable for high-performance perovskite solar cell fabrication.
Parameter | Average | St. dev. | Best |
---|---|---|---|
PCE (%) | 10.2 | 1.70 | 13.0 |
V oc (V) | 0.96 | 0.034 | 1.02 |
J sc (mA cm−2) | 16.8 | 1.43 | 18.8 |
FF (%) | 63.1 | 5.41 | 71.9 |
Hysteresis | 0.13 | 0.10 | 0.006 |
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ta09922j |
This journal is © The Royal Society of Chemistry 2017 |