Preheating-assisted deposition of solution-processed perovskite layer for an efficiency-improved inverted planar composite heterojunction solar cell

Abstract

Planar heterojunction perovskite solar cells have received a lot of attention due to their great potential, such as low-cost and rapidly improving performance. However, the poor fill surface coverage and numerous pinholes usually exist in the organic–inorganic hybrid perovskite layers, leading to undesirable electron–hole recombination. Herein, inverted cell structures based on NiO_x/PCBM interfacial materials have been studied. A simple preheating method facilitates the formation of a relatively continuous and compact layer of well-crystallized CH₃NH₃PbI_3−xCl_x perovskite by a one-step solution process, which can minimize film defects and thus improve the charge extraction dissociation and transport abilities at the interface. As a result, synchronized improvements in photovoltage (V_oc), photocurrent (J_sc) and fill factor (FF) lead to ∼25% enhanced power conversion efficiency (PCE), compared to solar cells using conventional perovskite films. Our study highlights a simple and effective method to improve the perovskite film quality and the photovoltaic performance of inverted perovskite solar cells.

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1. Introduction

Solution-processed organic/inorganic hybrid metal halide perovskite (CH₃NH₃PbX₃, X = Cl, Br, I) solar cells emerging in recent years are increasingly becoming the focus of photovoltaic research due to many advantageous characteristics, such as direct optical band gap, strong light absorption ability, high extinction coefficients, long charge-carrier diffusion lengths and excellent ambipolar charge transport properties.^1–5 In only a few years, photoelectric conversion efficiency (PCE) values of perovskite solar cells increase from 4% to above 19%,^6–9 which make it the most competitive candidate in the next generation solar cells. Moreover, the features of easy fabrication, low fabrication cost and high reproducibility could further address the scalability and applications in the photovoltaic field.⁸

Currently, two types of architectures for perovskite solar cells have been employed, which depend on the relative positions of the electron and hole transport layers.^10–15 The conventional structure evolves from dye-sensitized solar cells, which can be obtained by coating the electron transport layer (such as TiO₂, ZnO, Al₂O₃) onto the bottom transparent conducting glass followed by the successive coating of the perovskite and hole transport layers. The design of conventional perovskite solar cells usually allows both compact and mesoporous TiO₂ layers as the electron-transport layers to be adopted. Recently, one promising approach involves removing the mesoporous scaffold to form planar heterojunction device architecture maintaining a high level PCE, which is especially helpful for simplifying the preparation process.^16,17 However, costly 2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD) is commonly used as the hole-transport layer of planar heterojunction structure, which hampers the commercialization of perovskite solar cells. Recently, inverted structure perovskite solar cells have been developed, in which the positions of the electron and hole transport layers are inverted. This structure allows the use of new hole selective interface materials instead of expensive spiro-OMeTAD, such as poly(3,4-ethylenedioxythiophene): polystyrene sulfonate (PEDOT:PSS) and p type NiO.^18–23 In many cases, the reliability and stability of PEDOT:PSS based devices are less than ideal due to its acidity, high water absorption, and incapacity to block electrons. As for the electron transport layer, a typical phenyl-C61-butyric acid methyl ester (PCBM) has been investigated extensively owing to its good band alignment with respect to perovskite layer.^24–29 Noted that high-temperature processing (>450 °C) is required for preparation of TiO₂ electron collector layer, while the PCBM can be fabricated at lower temperature (<150 °C). This will help to not only make it feasible for flexible/plastic substrates but also decrease the production cost from economy angle. In addition, the inverted device plays an important role in both fundamental research and practical application.¹⁹

Significant efforts have been expanded to investigate the deposition method of perovskite absorber, such as one-step spin-coating deposition, two-step sequential solution deposition and vapor deposition methods have been widely reported.^30–32 The complicated and rigorous fabrication procedure for two-step method may place constraints in practice. Furthermore, perovskite film with many pin-holes and non-homogeneous coverage via one-step spin-coating has usually been observed in planar perovskite solar cells,^12,33 which would influence the charge transport at the interface and thus lead to recombination losses. To further enhance the device performance and improve the long-term stability, many efforts have been made to control the morphology of perovskite thin films, such as the optimization of the synthesis conditions involved in annealing temperature and time, reaction solvents, and chemical compositions.^15,20,34,35 However, there is still considerable room for enhancement in device efficiency through further optimization of perovskite film morphologies.

In this paper, we report an inverted type planar perovskite solar cell with the device configuration of FTO/NiO_x/CH₃NH₃PbI_3−xCl_x/PCBM/Au, in which high quality CH₃NH₃PbI_3−xCl_x perovskite film as light absorbing layer can be obtained by means of preheating modified one-step spin-coating deposition method. It is found that preheating treatment process not only improves the surface status of films, but also diminishes the charge recombination in the perovskite layer. These advantages yield a higher average PCE value of 6.4% than average PCE 5.1% for reference cell employing the conventional perovskite layer under standard AM 1.5G 100 mW cm⁻² illuminations. Furthermore, the best device performance is obtained with short current (J_sc) of 19.21 mA cm⁻², open voltage (V_oc) of 0.80 V, and fill factor (FF) of 45%, corresponding to a maximum PCE value of 7.05%. We believe that this facile preheating modified solution-processed method can be used in other perovskite materials for obtaining a continuous and compact perovskite layer with high crystallizing degree, which will further promote the development of this hot solar cell.

2. Experimental section

2.1 Fabrication of compact NiO_x films

The methanol solution containing 0.4 M nickel formate dihydrate (Alfa Aesar) with molar equivalents of ethylenediamine (Aldrich) was stirred overnight and then filtered with 0.45 μm nylon filters. Subsequently, the obtained NiO_x precursor solution was spun on top of the cleaned FTO-coated glass substrate at 4000 rpm for 30 s, followed by annealing at 340 °C for 60 min. When naturally cooled to room temperature, the preparation of NiO_x was repeated once on the obtained NiO_x film. It was noted that the cleaned FTO substrate and the resulting NiO_x were under UV-ozone treatment for 15 min and 2 min respectively before use.

2.2 Fabrication of perovskite films

CH₃NH₃PbI₃ perovskite films were prepared in a nitrogen-filled glove box using one step solvent extraction method. First, methylamine iodide (MAI) was prepared by reacting methylamine with hydroiodic acid (HI), during which HI was added dropwise while stirring. A white powder was formed after heating at 100 °C, followed by drying treatment overnight in a vacuum oven before use. Next, MAI and lead(II) chloride (PbCl₂) with a molar ratio about 3 : 1 were dissolved in anhydrous N,N-dimethylformamide (DMF) solvent, subsequently stirred overnight to obtain a transparent yellow solution. Then, the obtained PbCl₂/MAI mixing solution were preheated to 60 °C and immediately spin coated onto the compact NiO_x film surface at a rotation speed of 3000 rpm for 50 s. For comparison, in the conventional scheme, the mixture of PbCl₂ and MAI was spin-coated at room temperature. Finally, both deposited perovskite layers were post-annealed for 90 min on a hot plate maintained at 100 °C. During this period, the color of the film gradually turned from light yellow to dark brown.

2.3 Fabrication of perovskite solar cells

After the perovskite film was cooled to room temperature, a 15 mg mL⁻¹ chlorobenzene solution of PCBM ([6,6]-phenyl-C61-butyric acid methyl ester) was spin-coated at 2000 rpm for 30 s. Finally, one batch of films was quickly transferred to the evaporator and then 200 nm Au counter electrodes were thermally evaporated on PCBM layers under vacuum condition. The area of each device was 0.06 mm² determined by a shadow mask.

2.4 Characterization

The surface morphologies of perovskite films were characterized via field emission scanning electron microscopy (FE-SEM, JSM-7500F, JEOL, Japan). X-ray diffractometer (XRD) patterns were recorded using SHIMADZU XRD-7000 with Cu Kα at voltage of 40 kV and a current of 30 mA. The absorbance spectra were obtained by UV/VIS/NIR Spectrophotometer (Lambda 950, Perkin Elmer, USA). Time resolved photoluminescence (TRPL) for the corresponding perovskite films was measured by the fluorescence spectrometer (FLS920, Edinburgh Instruments, UK) with an excitation at 404.5 nm. Photocurrent–voltage (J–V) curves were measured using a digital source meter (2400, Keithley) under a solar simulator equipped with a 450 W xenon lamp (Newport 69920). The simulated AM 1.5 solar illumination intensity of 100 mW cm⁻² was calibrated with a silicon solar cell. The incident-photon-to-current efficiency (IPCE) was measured using QEX10 PV equipment with a monochromatic incident light in DC mode.

3. Results and discussion

Fig. 1(a) and (b) show the architecture and energy band diagram of the inverted type planar perovskite solar cell, respectively. A schematic diagram of the device structure reveals the cell consisting of glass/FTO/NiO_x/CH₃NH₃PbI_3−xCl_x/PCBM/Au (Fig. 1(a)). p type NiO_x and PCBM are the hole and electron transport layers, respectively. For this architecture, the formation of well-crystallized and homogeneous CH₃NH₃PbI_3−xCl_x perovskite layer is extremely important for enhancing the photovoltaic performance. Preheating assisted solvent-engineering technology is developed in this work as an effective method for growing high quality perovskite layer. Fig. 1(b) illustrates the energy level diagram for a planar-structured perovskite solar cell. The highest occupied molecular orbital (HOMO) (5.4 eV) and the lowest unoccupied molecular orbital (LUMO) (3.75 eV) levels¹⁷ of the CH₃NH₃PbI_3−xCl_x perovskite are well-matched with those of PCBM (HOMO: 5.9 eV, LUMO: 3.9 eV),³⁶ leading to efficient exciton dissociation and charge extraction.

Fig. 1 (a) Device architecture and (b) energy band diagram (relative to vacuum) of the inverted planar structure perovskite solar cell (glass/FTO/NiO_x/CH₃NH₃PbI_3−xCl_x/PCBM/Au).

The SEM top images of the perovskite films fabricated via the conventional and preheating method are shown in Fig. 2, respectively. The conventional film exhibits a large number of internal and external connected pin-holes with the size of 1–2 μm, as shown in Fig. 2(a) and (c). These bulk defects make it difficult for PCBM to completely cover on top of the perovskite layer, which hinders the charge transportation and further contributes to charge recombination at interface. Fig. 2(b) and (d) show a remarkable improvement in the film morphology with good coverage and interconnectivity when using the preheating modified deposition process. The cross section SEM images are shown in Fig. 3. There is no obvious thickness distinction with the SEM images of Fig. 3(a) and (b). The thickness of NiO_x layer is about 73 nm, meanwhile the thickness of both perovskite films is about 360 nm. Moreover, the preheated perovskite film is more uniform and compact as shown in the images. Furthermore, the amount and the size of the pinholes existing in the optimized perovskite film is much less and smaller. Different temperature-treated perovskite films are shown in Fig. 4. As shown in Fig. 4, with the preheating temperature further increasing, the perovskite films become less uniform, some large grain assembles and the basal substrate NiO_x can be observed which result in uneven surface and short-circuit in the performance of our devices. We compare different temperature-treated perovskite films and find out a most uniform layer and promising experimental condition and finally we set 60 °C as the preheating temperature to fabricate our solar cells. A significant decrease in the numbers of pin-holes improves the charge carrier mobility in perovskite films, thus leading to the improved photovoltaic performance of the devices.

Fig. 2 SEM top views of the deposited perovskite films under different conditions. (a) and (c) using conventional method. (b) and (d) using preheating assisted deposition method.

Fig. 3 Crosssection SEM images of different temperature treated perovskite films. (a) Without preheating method (b) preheated to 60 °C.

Fig. 4 SEM images of different temperature treated perovskite films coated on NiO_x (a) without preheating (b) 60 °C (c) 100 °C (d) 140 °C.

The recent studies reveal that the grain size and crystallinity of perovskite layer remain key scientific challenges for the realization of high device performance.³⁷ Hence, X-ray diffraction (XRD) spectra are shown in Fig. 5, in which the strong diffraction peaks at 14.1°, 28.4°, 43.3° can be assigned to crystal planes (110), (220) and (330) of the MAPbI_3−xCl_x respectively,⁸ implying the formation of perovskite structure. The sharp and strong perovskite (110) peak indicates a highly oriented crystal structure, which is found in good agreement with the previous report.³⁸ Although there were no differences in the position of XRD peaks of both films, the preheating modified perovskite films exhibit higher crystallinity due to sharper main peak of plane (110), consistent with more uniform and compact film observed from SEM images, compared to the conventional perovskite films. In order to further characterize the effect of preheating method on the crystal growth of perovskite film, the average grain size is estimated from Scherrer's equation

D = 0.89λ/β cos θ (1)

where D is the grain size, 0.89 corresponds to the shape factor of a grain, β is the corrected half width of the diffraction peak, and θ is the diffraction angle. It is obvious that a smaller β for the preheating modified perovskite film indicates a larger average grain size. We also calculated the crystallite size of values in different prepared perovskite films and the results are shown in Table 1. The values of D prepared by conventional method and preheating method is about 54 nm and 81 nm. This result presented us the tendency of enlarged crystallite size with preheating method. The preheating treatment of precursor solution would contribute to promoting the solubility of perovskite components in DMF and thus facilitate a better surface coverage on the compact NiO_x layer. Moreover, the completely dissolved PbCl₂/MAI mixture ensures that no solid precipitation occurs during post annealing, which can be attributed to the ideal component proportion. These advantages would allow for homogeneous morphology and better crystallization. As the crystalline grains increase, there is a significant decrease in the volume fraction of grain boundaries, which could improve charge carrier mobility and diminish the charge recombination. As for the convention process, however, the relatively slow nucleation rate of MAPbI_3−xCl_x layer has a negative effect on the crystallization during spin-coating owning to the high boiling point of DMF (153 °C)³⁹ (Table 1).

Fig. 5 XRD patterns of the CH₃NH₃PbI_3−xCl_x perovskite films deposited via conventional and preheating methods.

Table 1 The XRD data of perovskite films with different preparing methods

Sample type	D	2T	β	λ (nm)
Conventional perovskite^39	54 nm	13.84	0.14	0.154
Preheating perovskite	81 nm	14.23	0.098	0.154

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In order to estimate the photovoltaic performance of inverted planar device based on perovskite layer with and without preheating methods, IPCE spectra and ultraviolet-visible absorbance spectra were investigated, respectively. The IPCE spectra in Fig. 6(a) exhibit an onset of around 800 nm, indicating that the absorption is dominated by MAPbI_3−xCl_x perovskite material. In addition, both devices show broad IPCE curves throughout the visible spectrum to the near IR wavelength region (300–800 nm). Preheating modified perovskite solar cell leads to an increased IPCE in the whole conversion region, which may be attributed to increased UV-vis light absorption (Fig. 6(b)). This demonstrates an enhancement of short current density (J_sc) obtained from the integrated current density, which will be discussed later in this paper. It's worth noting that the maximum value of IPCE can reach up to 60% at around 540 nm for the preheating device while the device based on conventional perovskite active layer exhibits an IPCE below 50%. As shown in Fig. 6(b), there is no significant peak shift for the two kinds of perovskite films, both of which display broad absorption ranging from the visible to near-IR region. The reduced absorbance of conventional perovskite device could be attributed to the poor coverage and a large number of pin-holes, leading to a considerable leakage of light.

Fig. 6 (a) Incident photon-to-electron conversion efficiency (IPCE) specta of the inverted planar solar cells based on perovskite materials prepared at different deposition methods and (b) ultraviolet-visible absorbance spectra.

In order to seek indications regarding the extent of charge diffusion in the hole-transport layers, we performed the TRPL measurements with excitation at 404.5 nm for the compact NiO_x film interfaced with perovskite layers deposited by the different methods, as shown in Fig. 7. The PL decay curves are fitted with a biexponential function²²

Y = A₁ exp(t/τ₁) + A₂ exp(t/τ₂) (2)

Fig. 7 Normalized TRPL profiles (excitation at 404.5 nm) of the compact NiO_x layers interfaced with perovskite films prepared by preheating and conventional methods.

In this function, the small constant τ₁ reflects the initial stage for photogenerated excitons diffusing into defects and the large time constant τ₂ is closely related to the exciton lifetime in the perovskite layer. A smaller τ₂ indicates a faster hole diffusion process. According to the fitting result, the preheating process gives lifetimes of approximately 11 ns, whereas the conventional method increases the lifetime to 22 ns, indicating that the efficiencies of hole extraction and transportation process are improved due to the number of defects reduced at the interface of NiO_x and perovskite layer with preheating treatment.

The photocurrent density versus voltage (J–V) characteristics under AM 1.5G irradiation (100 mW cm⁻²) of the inverted perovskite solar cells with conventional and preheating treatment are shown in Fig. 8(a). Compared with the conventional method, it is clear that the cell with preheating treatment achieves significantly improved J_sc, open-circuit voltage (V_oc) and fill factor (FF), thereby increasing PCE of the device. The detailed average performance parameters of the two kinds of perovskite solar cells are summarized in Table 2, in which the J_sc are in good agreement with the integrated IPCE curves within 10% error. A significant 25% increase in PCEs can be observed over the conventional perovskite solar cell, which further confirms the poor performance arising from less light absorption and more chances of electron–hole recombination owing to imperfect surface coverage and lots of pin-holes.

Fig. 8 (a) Current density–voltage (J–V) characteristics of the inverted solar cell employing the perovskite films deposited via conventional and preheating methods under AM 1.5G illumination. (b) Plots of (−dJ/dV) versus (J_sc − J)⁻¹ and the corresponding linear fitting straight. (c) Plots of ln(J_sc − J) versus V + R_sJ and the corresponding linear fitting straight. (d) J–V curve for the best inverted planar cell with preheating modified perovskite layer.

Table 2 Device performances of perovskite films with the same thicknesses prepared by conventional and preheating methods

Sample type	Voc^a (mV)	Jsc^b (mA cm^−2)	FF^c (%)	PCE^d (%)
a Open-circuit photovoltage.b Short-circuit photocurrent.c Fill factor.d Power conversion efficiency.
Conventional perovskite cell	706 ± 36	16.35 ± 1.2	35 ± 4	5.1 ± 0.5
Preheating perovskite cell	798 ± 30	18.23 ± 0.8	47 ± 3	6.4 ± 0.5

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In order to further investigate the intrinsic relationship between these photovoltaic parameters of these devices, the J–V characteristics are analyzed by using the following expression, which is deduced from a single junction diode behaving like a planer structured perovskite solar cell with a large shunt resistance^29,40,41

J = J_ph − J₀[exp(q(V + JR_s)/(Ak_BT)) − 1] (3)

where J is the density of the current passing through the external circuit, J_ph is the light induced current density, which is considered equal to the J_sc, J₀ is the dark saturate current density, q is the elementary charge, V is the applied voltage, R_s is the series resistance, A is the ideality factor (typically 1 < A < 2), k_B is the Boltzmann constant, and T is the absolute temperature.

According to eqn (3), Shockley equation is deduced when the J is zero, which can be used to predict the variation of V_oc.²⁹

V_oc = Ak_BT(ln(J_ph/J₀) + 1)/q (4)

An improvement of V_oc value of the device with preheating method can be attributed to the significantly lower J₀, as shown in the dark J–V curves.

Eqn (3) can be transformed into the following expressions^42,43

ln(J_ph − J) = q(V + JR_s)/Ak_BT + ln J₀ (6)

The data from J–V curves shown in Fig. 6(a) are used for analysis by combining eqn (5) and (6). The plots of versus (J_ph − J)⁻¹ and ln(J_ph − J) versus V + R_sJ are exhibited in Fig. 6(b) and (c), respectively. Linear plot fitting of Fig. 6(b) can give the R_s values of 0.2 Ω cm² and 3.6 Ω cm² for the conventional method and preheating method, respectively. J₀ can be obtained by linearly fitting the ln(J_ph − J) versus V + R_sJ, as shown in Fig. 6(c). In our case, J₀ for the device based on the conventional method is 3.1 × 10⁻⁴ A cm⁻², which is more than twice as large as that based on preheating method of 1.3 × 10⁻⁴ A cm⁻², which is in agreement with the J–V measurements under dark condition. Typically, the J₀ is closely associated with the electron–hole recombination rate. The preheating method prepared devices showed the reduced bulk defects, which allow for enhanced interfacial contact and reduced voids at both interfaces of NiO_x/perovskite and perovskite/PCBM, and hence reduce the recombination loss.

It is worth noting that the best performance achieved through our preheating process exhibits V_oc of 800 mV, J_sc of 19.21 mA cm⁻² and FF of 0.45, leading to a PCE of 7.05%, demonstrating the importance of the reduced bulk defects and good coverage of perovskite films to the improvement of photovoltage performance.

4. Conclusion

In summary, we have developed a preheating assisted deposition technology to optimize the morphology of CH₃NH₃PbI_3−xCl_x perovskite layer based on the one-step deposition method. A significantly promotion of average PCE value from 5.1% to 6.4% of the inverted planar perovskite solar cells under simulated AM 1.5G irradiation (100 mW cm⁻²) has been achieved. The preheating method enables the formation of a highly uniform and dense perovskite film surface. The improved morphology and enhanced crystallization increases the light absorption efficiency. Furthermore, high quality interfaces at NiO_x/perovskite and perovskite/PCBM facilitate the charge transfer process and charge collection, thereby greatly inhibiting the recombination channels and improve exciton dissociation efficiency. In our case, the highest PCE value can reach up to 7.05%. As for the conventional spin-coating method, however, a large number of voids and bulk defects can be observed, contributing to the poor coverage and significantly increased surface roughness. This work provides a promising route to achieve high quality perovskite polycrystalline films with superior optoelectronic properties, which can pave the way towards further enhancing PCE.

Acknowledgements

This work was financially supported by China-Japan International Cooperation Program Funds (No. 2010DFA61410 and 2011DFA50530), National Natural Science Foundations of China (No. 51272037, 51272126, 51303116 and 51472043) and Program for New Century Excellent Talents in University (No. NCET-12-0097).

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Footnote

† These authors contributed equally to this work.

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