High coverage solution-processed planar perovskite solar cell grown based on the Stranski–Krastanov mechanism at low temperature and short time

Abstract

In this work, a full surface coverage CH₃NH₃PbI₃ layer is achieved by controlling the growth mechanism of crystals according to the Stranski–Krastanov mode. This pin-hole free perovskite layer is achieved from a one-step solution process at low temperature (100 °C) and short time (20 s) which provides the means for the fabrication of highly efficient flexible devices. As a result, perovskite solar cells (PSC) with 15.01% power conversion efficiency (PCE) on a glass substrate and 11.25% PCE on a flexible substrate (PET) are fabricated by the applied method.

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Introduction

In recent years, the demand for sustainable and clean energy resources has led to an intense growth in the development of solar cells. Photovoltaic technology is required to be efficient, stable, and cost effective in order to reach large scale industrial production. The most cost effective and convenient fabrication method for the production of solar cells is regarded as the solution process which provides roll-to-roll printing beneficially for large scale production.¹ Perovskite solar cell is one of the most promising technologies utilized for decreasing the manufacturing costs of solar cells. Organic–inorganic hybrid perovskites were first used as visible-light sensitizers for liquid-junction-sensitized solar cells in 2009. In these devices, the PCEs were around 3%.² The PCE of the perovskite-sensitized photocells reached 6.5% by optimizing the thickness of photoanode and modifying interfaces.³ The main problem in liquid-junction-sensitized devices was in relation to short lifetime (a few minutes), since the perovskite was easily dissolved in liquid electrolyte. This problem encouraged the researchers to concentrate on the solid-state hole transporting materials (HTM). The device with 9.7% PCE was generated by utilizing 2,2′,7,7′-tetrakis-(N,N-dimethoxyphenyl-amine-)-9,9′-spirobifluorene (spiro-MeOTAD) as a solid-state HTM.⁴ Subsequently, the PCE of perovskite based devices reached up to 20% by introducing two structures based on the mesoporous network and planar heterojunction.^5–9 The absorber in perovskite solar cells is an organic metal halide, which has the formula ABX₃, where A is an organic ammonium ion (e.g. CH₃NH₃⁺, HC(NH₂)₂⁺), B is a metallic ion (e.g. Pb²⁺, Sn²⁺), and X site is occupied by a halogen ion (I⁻, Cl⁻, Br⁻).¹⁰ Among the introduced absorbers, the CH₃NH₃PbI₃ is the most widely used material as a light absorber in perovskite solar cells. The structures of perovskite solar cells can be divided into three categories: perovskite sensitized mesoporous structure, planar heterojunction structure and inverted structure.¹¹ In all perovskite structures, after absorbing the incident photons by CH₃NH₃PbX₃ (X = Br, I), excitons with low binding energy are generated and dissociated into free charge carriers. In mesoporous devices, halide perovskite fills a mesoporous network and forms a thin cape layer on the top. The filled network is sandwiched between two contacts with different work functions. By utilizing the TiO₂ network, a large interface area between the absorber material and electron transporting layer material (ETM) is provided.¹²

Planar heterojunction configuration is similar to polymer solar cells. The strong s–p anti-bonding coupling in halide perovskites leads to small effective masses for electrons and holes which provides the means for the fabrication of an efficient solar cell with p–i–n configuration.¹³ In this architecture, a perovskite absorber layer is sandwiched between electron transporting layer (ETL) and hole transporting layer (HTL) without a mesoporous scaffold. Two heterojunctions are provided which are the junction between absorber and HTL, and the junction between the absorber and ETL.^14,15 The planar perovskite solar cells can be fabricated in an inverted manner which are more similar to polymer solar cells in comparison to normal planar devices. In contrast to the other types of perovskite cells, in invert devices, photogenerated electrons are collected in the cathode and photogenerated holes are collected in the anode. Poly[3,4(ethylenedioxythiophene)]:poly(styrenesulfonate) (PEDOT:PSS) and fullerene derivatives are commonly used as HTL and ETL, respectively.^16,17 The advantages of this structure are as follows; (1) TiO₂ compact layer is replaced by organic ETM which avoids the high temperature annealing process^18,19 and the device structure is simpler.²⁰ (2) The stability of the device is improved by removing TiO₂ layer which causes instability under UV light.⁶ (3) The materials and process method of this structure provide the fabrication of flexible perovskite solar cell.^1,21 (4) Due to the high cost associated with spiro-MeOTAD, it can be replaced by other organic materials.⁶

Because of the advantages of solution processing, the widely used method for perovskite deposition is in fact the solution process. In this method, the absorber is deposited via one-step or two-step processes. In the one-step process, a solution of mixture of methylammonium halide and lead iodide (PbI₂) is spin-coated on the substrate.¹⁵ In the two-step process, first a solution of PbI₂ is coated and then a solution of methylammonium halide is spun on it.⁶ There are some challenges about the fabrication of these devices via both one-step^17,22 and two-step processes.^6,23 However, the morphology of the film prepared by the one-step method is more uncontrollable than the two-step, the one-step possesses the most cost effective and straightforward technology for large-scale production. While low temperature spin-coating is the simplest methods to fabricate low-cost solar cell devices, it has been found very challenging to form continuous and uniform perovskite films by spin-coating of the directly mixed PbI₂ and methylammonium halide blend precursor. Up to now, some methods were used to achieve a high quality perovskite layer from one-step process.^16,24 Several researchers reported the perovskite film improvement quality by solvent engineering,^9,25–28 using additives,^7,29–31 and controlling the process condition.^32,33 Also, solution-based hot-casting technique was demonstrated to grow continuous thin films of organometallic perovskite with millimeter-scale crystalline grains for fabrication of perovskite solar cells.¹⁷ In this technique, processing at high temperature is a limitation for flexible substrates. Qiu et al. fabricated a high coverage perovskite film by developing a new precursor combination including Pb(CH₃CO₂)₂·3H₂O, PbCl₂ and CH₃NH₃I.⁷ A high coverage CH₃NH₃PbI_3−xCl_x film was fabricated with a one-step process followed by a 10 min annealing step at 130 °C.⁷ Using hole transporting materials (HTM) with desired surface properties is another approach for enhancing grain size of perovskite and achieving more uniform layers. As reported, non-wetting surface of HTL is more beneficial for grain growth. Morphology improvement of perovskite layer by using doped poly(triaryl amine) (PTAA)³⁴ and poly[N,N′-bis(4-butylphenyl)-N,N′-bisphenylbenzidine] (poly-TPD)³⁵ as HTLs was reported which led to device performance enhancement. Wang et al. achieved a perovskite film with 94.2% coverage by utilizing aligned perylene microstructure film as the underlayer.³⁶

Moisture sensitivity of halide perovskite is another challenge in this field which causes a complex effect on the perovskite film formation and degradation especially at higher temperatures.^37–39 Moisture can play a desired effect in the formation of perovskite film during crystallization. It also has a damaging effect on the formed perovskite crystals resulting in degradation.^40,41 As the favored atmosphere for device fabrication is ambient condition,⁴² the fabrication time of the device plays an important role in controlling the properties and reproducibility of the final products. Therefore, the exposure time of the perovskite layer to the air during spin-coating, annealing and other layers deposition should be reduced.

As stated, in one-step solution processing, achieving a uniform perovskite thin film at a short casting time and low temperature would create the opportunity to step towards the large-scale production of perovskite solar cells. Therefore, in this work, a new approach is reported to achieve a full surface coverage perovskite film at low temperature and short time for fabricating high efficient inverted planar perovskite solar cells on glass and flexible substrates.

Experimental section

Materials and device fabrication

In order to fabricate perovskite solar cell, indium tin oxide (ITO) coated glass (Lumtec, 15 Ω sq⁻¹) and ITO coated PET (Sigma-Aldrich, 60 Ω sq⁻¹) substrates are etched and cleaned with deionized water, acetone, n-hexane, and isopropanol, respectively. After drying of substrates at 100 °C for 30 min, a 50 nm-layer of PEDOT : PSS (Lumtec, 1 : 6 weight ratio) is spin-coated at 5000 rpm for 30 s and annealed for 20 min at 100 °C. The deposition of perovskite layer on the PEDOT:PSS layer is conducted using a one-step process at ambient condition with relative humidity of 27% ± 3. First, MAI and PbI₂ are separately dissolved in anhydrous N,N-dimethylformamide (DMF) at 1 M concentration, and kept at 70 °C. For achieving high coverage and low roughness perovskite films, these two precursors are mixed at various ratios and deposited at different temperatures. Before deposition, the substrates are kept at the selected temperature for 2 min and then quickly transferred to the spin coater chuck. 50 μl of precursor solution is poured on substrate and subsequently spin-coated at 4000 rpm for 10 s. After deposition, the substrates are quickly transferred to the hot plate and annealed for 10 s at the selected temperature for casting. The ETM is deposited on the perovskite layer by spin-coating of a 20 mg ml⁻¹ solution of phenyl-C₆₁-butyric acid methyl ester (PCBM) (Lumtec, 99.5%) in 1,2-dichlorobenzene. Finally, a 100 nm-thick Ag layer is deposited on the top of the PCBM film by thermal evaporator in vacuum condition (∼10⁻⁵ torr), yielding an active area of 0.1 cm².

Characterization

The morphology and structure of the films are characterized using Phenom scanning electron microscopy (SEM) (ProX model) and FESEM (TESCAN-LMU model). Besides, Veeco atomic force microscopy (CP-Research model) is used to take perovskite layer surface topography images. The coverage percentage of perovskite films is determined by ImageJ software. The absorption and transmission spectra of the device layers are recorded by an Avantes UV-Visible spectrophotometer (AvaSpec 2048 model). The photoluminescence (PL) spectra of perovskite films are taken on a Perkin-Elmer spectrofluorometer (Frontier model). For this analysis, organohalide perovskite layers are coated on glass substrate and excited at λ = 600 nm. In order to investigate the crystalline nature of perovskite films, X-ray diffraction (XRD) spectra are obtained on a Philips diffractometer (X'Pert MPD model) equipped with a proportional Xe filled detector, Cu tube (λ = 1.54056 Å). Current–voltage (I–V) characteristics of the fabricated devices are measured by an Ivium stat potentiostat (XRE model) under a calibrated AM 1.5 solar simulator at 100 mW cm⁻² light intensity (Sharif Solar 10-2 model).

Results and discussion

Since the crystal growth is strongly dependent on the temperature and concentration,^43,44 in this work, it is intended to control the crystallization mechanism, crystal size and coverage percentage by varying casting temperature and precursor concentration. However, the effect of centrifugal force during spin coating can not be ignored; thus, all samples are formed at constant spin coating condition. On the other hand, because of the moisture sensitivity of perovskite film, the importance of the solar cell flexibility and convenience of low-temperature process, we mainly focus on fabricating uniform films from low-temperature process at a short casting time. Fig. 1a and b schematically describe the device structure and the solution process method used in this research for achieving a pin-hole free perovskite layer from one-step method which takes a short time (at most 20 s). Fig. 2 shows the crystal structures obtained from various substrate temperatures (80 °C and 100 °C) and different precursor molar ratios summarized in Table 1. Here, we are concerned with two independent parameters; precursor concentration and temperature. It should be noted that, in films P1 and P2, the intermediate phase with light yellow color (Fig. S1 and S2†) is formed during spin coating, and the perovskite crystals are formed after annealing for 10 s at both 80 °C and 100 °C. However, in films P3 and P4, perovskite phase with dark color is formed during spin coating, before annealing for 10 s (Fig. S1†). It is perceived that with increase in MAI/PbI₂ molar ratio (at constant substrate temperature), the morphology type transforms from a packed layer to branched and ultimately, spherical islands are formed (Fig. 2a and b). The observed significant change in the film morphology is explained according to the three possible mode of crystal growth on the surface which are regarded as Volmer–Weber growth (or Island mode), Frank–Van der Merwe growth (or layer mode), and Stranski–Krastanov growth (or layer plus island), respectively.⁴⁵ In Island mode, small clusters are nucleated on the substrate and then growth into the island. The Island mode occurs when the molecules of the deposited film are strongly bound to each other than to the substrate.⁴⁵ In other words, the sum of the surface free energy of the deposited layer (γ_A) and the interfacial free energy (γ*) is larger than the surface free energy of the substrate (γ_B) (γ_A + γ* ≥ γ_B).⁴⁵ In the layer mode, the molecules of the deposited film have strong tendency to the substrate than to each other (γ_A + γ* ≤ γ_B). In layer plus island mode, islands are formed after forming the first monolayer as an intermediate layer. Any factor which disturbs the monotonic decrease in binding energy and characteristic of layer growth may be the possible reason for the occurrence of this mode.⁴⁵ As observed in Fig. 2, the crystals in perovskite films P2, P3 and P4 show the Volmer–Weber mode which was observed previously,⁴⁶ while the films P1 are in accordance to the Stranski–Krastanov mode with full coverage (Table 1). Among the films grown according to Volmer–Weber mode, films P2-80 and P2-100 show higher coverage percentage compared to films P3 and P4. As a result, the precursor molar ratio or concentration plays an important role in the crystallization mode and crystal shape and size of the perovskite films formed by one-step process. Therefore, when the MAI concentration increases, Volmer–Weber growth mode is appeared due to the reduction of the tendency of the perovskite crystals to the PEDOT:PSS layer.

Fig. 1 (a) Schematic of the inverted perovskite device architecture, (b) schematic of the perovskite film deposition process.

Fig. 2 (a) SEM images of perovskite films deposited on 80 °C-substrate, and (b) on 100 °C-substrate with various precursor molar ratios, (c) topography images of perovskite films deposited on 100 °C-substrate with various precursor molar ratios, (scanning area: 20 μm × 20 μm) (d) schematic of crystal growth mode of deposited layers.

Table 1 Deposition conditions and characteristics of perovskite films

Sample	MAI/PbI2 molar ratio	Substrate temp.	Annealing temp. and time	Crystal growth mode	Coverage (%)	Thickness
P1-100	0.7 : 1	100 °C	100 °C, 10 s	Stranski–Krastanov	∼100	∼280 nm
P1-80	0.7 : 1	80 °C	80 °C, 10 s	Stranski–Krastanov	∼100	∼270 nm
P2-100	0.8 : 1	100 °C	100 °C, 10 s	Volmer–Weber	∼94	∼320 nm
P2-80	0.8 : 1	80 °C	80 °C, 10 s	Volmer–Weber	∼91	∼310 nm
P3-100	1 : 1	100 °C	100 °C, 10 s	Volmer–Weber	∼65	∼450 nm
P3-80	1 : 1	80 °C	80 °C, 10 s	Volmer–Weber	∼72	∼500 nm
P4-100	1.5 : 1	100 °C	100 °C, 10 s	Volmer–Weber	∼81	∼300 nm
P4-80	1.5 : 1	80 °C	80 °C, 10 s	Volmer–Weber	∼83	∼290 nm

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The strong dependency of the number and size of grains, and film coverage percentage on precursor solution composition is another point of Fig. 2a and b. The SEM images (Fig. 2 and S3†) and grain size distribution (Fig. 3a and S4†) of films P3 and P4 indicate that by increasing the MAI/PbI₂ molar ratio, additional number of nuclei contribute to crystal growth and thereby increasing the number of islands formed with smaller diameter (Fig. 3a and b). Due to enhanced nucleation, grain boundaries extend which can act as trapping sites for free charge carriers while, the coverage percentage increases as desired (Table 1). As demonstrated in Fig. 3b, nucleation and growth of islands are competing processes. Enhancement of the substrate temperature from 70 °C to 100 °C leads to the reduction of the number of the islands (fewer nuclei) and increment of the crystal size (further growth) (Fig. 3a and b and S5†). The reason is that, at high temperature, the critical free energy is higher due to the more equilibrium concentration of the chemical reaction.⁴⁴ As the number of nuclei (n) is inversely proportional to the critical free energy (G_c) (, where k is Boltzmann constant and T is the temperature⁴⁴), along with the increase of temperature, the number of the nuclei growing into the perovskite crystals declines, leading to grain size enhancement. In both one-step and two-step processes, the temperature shows its strong effect on grain size, not their shape.^17,44,47 It should be noted that, during MAI concentration variation, the observed competition between the nucleation and growth of perovskite grains in one-step process is in consistent with the reported dependency of two-step process.^44,48 But, in one-step process, the crystallization mode, grains shape and size are strongly dependent on the MAI concentration, while in two-step process it merely affects the grain size.^44,48

Fig. 3 (a) Grain size distribution of films P3 and P4, (b) number of grains and average grain size of films P3 and P4 as functions of temperature and MAI/PbI₂ molar ratio.

The shown topography images in Fig. 2c confirm the morphology obtained by SEM. The measured root mean square (RMS) for film P1-100, P2-100, P3-100 and P4-100 are 40 nm, 80 nm, 200 nm and 60 nm, respectively. Fig. 4a and b present the cross-sectional SEM images and height profile of perovskite layers acquired from AFM images (Fig. S6†). As observed, the lowest RMS and thickness are achieved for film P1-100 which has 100% coverage. The height profiles of the perovskite layers in Fig. 4b and c clearly demonstrate the formation of packed layer with low roughness in film P1-100 and the available holes in films P2-100, P3-100 and P4-100. Also, the high thickness and roughness of film P3 specify that the growth rate of the crystals along the c-axis direction is faster than the a- and b-axes directions (Fig. 4a and b). AFM images of films P3-100 and P4-100 reveal that the competition between nucleation and growth leads to a kinetically limited roughening of perovskite film at higher concentration of MAI. The analysis of the dependency of the crystallization mechanism and crystal structure on the precursor molar ratio and temperature indicates that precursor molar ratio is more dominant than temperature.

Fig. 4 (a) Cross-sectional images of glass/ITO/PEDOT:PSS/perovskite structures with perovskite layers P1-100, P2-100, P3-100 and P4-100, (b) height profiles of perovskite films obtained from AFM images, (c) schematics of the cross-section of the devices.

Fig. 5 shows the absorption and PL spectra of perovskite films. Comparison of the PbI₂ spectrum with perovskite films and the broad absorption across the UV-visible range confirm the formation of perovskite structure. As distinctly observed, films containing a large amount of PbI₂ have stronger absorbance in the UV range. Also, there is no significant difference between the PL spectra of samples. A small red-shift is observed for samples formed at 100 °C, compared to the samples formed at 80 °C. This shift may specify a higher electronic disorder in the films annealed at higher temperature which allows excitons to relax to lower energetic states or can be due to the significant difference between crystal size and structure.⁴⁹ Also, when MAPbI₃ crystallites grown in planar structure, organic cation has ordered arrangement. The interaction of organic cation with the inorganic cage and the displacement affect the electronic properties of the compound. Therefore, by changing the temperature, the ordering of the cation may be change which can cause the shift in PL.⁵⁰

Fig. 5 UV-visible and PL spectra of perovskite films excited at λ = 600 nm.

Fig. 6 illustrates the XRD patterns of perovskite structures formed in different condition. The major diffraction peaks at 14.2° and 28.6° corresponding to 1 1 0 and 2 2 0 planes reveal that the tetragonal perovskite structure is formed in all films.¹⁷ As shown, impurity peaks disappear merely in film P3-100 with precursor molar ratio 1 : 1, while PbI₂ peaks are observed in films with precursors containing higher PbI₂ molar percentage, particularly for the full covered film (P1-100). The presence of a low amount of PbI₂ in these films can promote the reduction of carrier recombination in the perovskite layer, as well as in the interfaces between perovskite and the carrier transport layer.⁵¹ Furthermore, MAI peaks appear in film P4-100 which its precursor is comprised of higher MAI molar percentage. The slight difference between the XRD pattern and absorbance spectra of the prepared samples indicates that the casting temperature and precursor ratio only affect the crystallization mode, grains dimension and film coverage.

Fig. 6 XRD patterns of the perovskite films deposited from precursor solutions with various molar ratios.

The J–V characteristic of the fabricated devices using presented perovskite layers (Table 1) are presented in Fig. 7a. It should be noted that each device is named according to the name of the used perovskite layer. The extracted photovoltaic characteristics are summarized in Table 2. The evaluation of devices performance is in consistent with the observed trends in SEM and AFM images. As seen, the highest performance (15.01%) is achieved for the device including pin-hole free perovskite layer (P1-100). Removing holes from the layer by controlling the growth mode according to Stranski–Krastanov mechanism leads to reduce the recombination rate and enables the achievement of 70%-Fill Factor (FF) device. Full covering of perovskite film prevents direct contact of HTM and ETM (Fig. 4c) and also enhances the light absorbance. Among all devices, cell P3-80 and cell P3-100 which have the lowest coverage and the highest thicknesses of perovskite layer, show the lowest PCEs, 4.35% and 4.48%, respectively. Considering that, the hysteresis appears in photovoltaic performance of the perovskite solar cells which showed strong dependency on device processing method, the J–V characteristics of the best device P1-100 scanned from different direction and by applying various voltage scan-rates are exhibited in Fig. S7 and S8.† When the device is scanned from reverse bias to forward bias, it shows a short-current density (J_SC) of 22.5 mA cm⁻², open-circuit voltage (V_OC) of 0.94 V and FF of 0.71, leading to the 15.01% PCE. When scanning is from forward bias to reverse bias, the device shows a J_SC of 22.8 mA cm⁻², V_OC of 0.9 V, FF of 0.7, and a PCE of 14.4%. Thus, by fabricating a uniform perovskite layer and reducing trap densities,⁵² a slight hysteresis in device performance is observed (Fig. S7 and S8†). Resulting uniform and reproducible perovskite layer provides the fabrication of high performance devices with the average efficiency of 14.25% ± 0.8%, measured from 50 devices.

Fig. 7 J–V characteristics of the fabricated devices with different perovskite layer compositions.

Table 2 Photovoltaic performance of the fabricated devices

Device	VOC (V)	JSC (mA cm^−2)	FF (%)	PCE (%)	The best PCE (%)
P1-100	0.97 ± 0.05	21 ± 2	70 ± 2	14.25	15.01
P1-80	0.9 ± 0.05	17 ± 2	62 ± 3	9.48	10.02
P2-100	0.9 ± 0.05	17 ± 2	56 ± 2	8.56	9.37
P2-80	0.8 ± 0.1	16 ± 2	55 ± 2	7.04	8.24
P3-100	0.72 ± 0.05	12 ± 2	46 ± 2	3.97	4.48
P3-80	0.7 ± 0.05	12 ± 1	45 ± 1	3.78	4.35
P4-100	0.95 ± 0.05	17 ± 1	58 ± 3	9.36	9.8
P4-80	0.9 ± 0.05	14 ± 2	57 ± 2	7.18	8.41

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The proposed low-temperature process for perovskite fabrication offers the fabrication of flexible devices on polymeric substrates. Fig. 8a shows the J–V characteristics of the best devices with perovskite layer P1-100 fabricated on the glass/ITO and flexible substrates. It indicates that the high level of uniformity, low temperature and short-time processing introduced the proposed method as a desired technique for fabricating high performance rigid and flexible solar cells. Fig. 8b shows the variation of current density of the best rigid and flexible devices at the maximum power point condition with a light soaking time. As observed, both cells show stable saturated current density under continuous illumination. Furthermore, the fabricated inverted MAPbI₃ devices show high stability under exposure to ambient condition with 27 ± 3% relative humidity which will report in our future work.

Fig. 8 (a) The J–V characteristics of the best devices fabricated with perovskite layer P1-100 on the glass/ITO and flexible (PET/ITO) substrates, (b) measured photocurrent output at the maximum power point of the best devices on rigid and flexible substrates.

Conclusions

In summary, regulation of the perovskite crystallization through the control of temperature and composition led to growth of a full-covered perovskite layer based on Stranski–Krastanov mode. In this method, a perovskite film was fabricated by one-step process at low temperature (100 °C) and short time (20 s) which provided the fabrication of high efficient planar flexible solar cells at ambient condition. Regarding the crystallization of perovskite layer, it was found that the impact of precursor solution concentration is dominant in comparison to the effect of temperature. Furthermore, high performance devices with 15.01% PCE and 11.25% PCE were fabricated on glass/ITO and PET/ITO, respectively.

Acknowledgements

The authors would like to thank Tarbiat Modares University for the financial support of this work and the equipment services from Nano-optoelectronic Laboratory.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra23074a

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