Jiyoon Park‡
a,
Jin Woo Choi‡b,
Woochul Kima,
Ryeri Leea,
Hee Chul Woob,
Jisoo Shinb,
Hyeonghun Kima,
Yeong Jun Sona,
Ji Young Joa,
Heon Leec,
Sooncheol Kwond,
Chang-Lyoul Lee*b and
Gun Young Jung*a
aSchool of Materials Science and Engineering (SMSE), Gwangju Institute of Science and Technology (GIST), 123 Cheomdangwagi-ro, Buk-gu, Gwangju 61005, Republic of Korea. E-mail: gyjung@gist.ac.kr
bAdvanced Photonics Research Institute (APRI), Gwangju Institute of Science and Technology (GIST), 123 Cheomdangwagi-ro, Buk-gu, Gwangju 61005, Republic of Korea. E-mail: vsepr@gist.ac.kr
cDepartment of Materials Science and Engineering, Korea University, Seoul 02841, Republic of Korea
dResearch Institute of Solar and Sustainable Energies (RISE), Gwangju Institute of Science and Technology (GIST), 123 Cheomdangwagi-ro, Buk-gu, Gwangju 61005, Republic of Korea
First published on 14th May 2019
As promising photo-absorbing materials for photovoltaics, organic–inorganic hybrid perovskite materials such as methylammonium lead iodide and formamidinium lead iodide, have attracted lots of attention from many researchers. Among the various factors to be considered for high power conversion efficiency (PCE) in perovskite solar cells (PSCs), increasing the grain size of perovskite is most important. However, it is difficult to obtain a highly crystalline perovskite film with large grain size by using the conventional hot-plate annealing method because heat is transferred unidirectionally from the bottom to the top. In this work, we presented radiative thermal annealing (RTA) to improve the structural and electrical properties of perovskite films. Owing to the omnidirectional heat transfer, swift and uniform nuclei formation was possible within the perovskite film. An average grain size of 500 nm was obtained, which is 5 times larger than that of the perovskite film annealed on a hot-plate. This perovskite film led to an enhancement of photovoltaic performance of PSCs. Both short-circuit current density and PCE of the PSCs prepared by RTA were improved by 10%, compared to those of PSCs prepared by hot-plate annealing.
Several factors, such as halide composition control for band-gap tuning of the perovskite photo-active layer,18 high crystallinity of the perovskite film with large grain size19 and optimization of energy band alignment in the device structure,20 must be improved to the satisfactory level to realize high PCE in PSCs. Among these factors, the crystallinity and grain size of perovskite, which are strongly related to exciton dissociation,21 charge transport22–24 and extraction efficiency,25,26 are extremely important to enhance the photovoltaic performance of PSCs. High crystallinity in a perovskite film signifies a low density of defects with less grain boundaries (GBs). It suppresses the energy band distortion induced by GBs of the perovskite film,27 resulting in efficient exciton dissociation by minimizing carrier recombination at the GBs. Perovskite films with larger grains have less GBs, which improve their charge transport and extraction efficiency26 by suppressing charge recombination and trapping.19
To realize a highly crystalline perovskite film with large grain size, various methods have been reported, including one-step spin-coating with anti-solvent dripping,28,29 deposition by an evaporation method under vacuum conditions13,30 and blade coating.31 In addition, control of the crystallinity and grain size of perovskite film through various post-annealing methods has been investigated.32,33 In general, as-deposited perovskite films are subsequently annealed on a hot-plate to form the crystalline perovskite by evaporating the solvent.10–17 However, this method has a critical limitation of unidirectional heat transfer from the bottom; the annealing temperature is inhomogeneous between the top and bottom sides of the perovskite film. Due to the lack of uniform heat distribution within the perovskite film, the hot-plate annealing method yields perovskite films with inferior crystallinity and smaller grain size compared to the other reported annealing methods and suffers from poor reproducibility. Therefore, there is a great need to develop new post-annealing methods to replace the hot-plate annealing method for obtaining highly crystalline perovskite films with large grains.
Recently, two research groups reported new post-annealing methods using a light as an energy source and non-direct thermal heating; Jeon et al. introduced a near-infrared (NIR) laser annealing method and obtained a large grain size (∼300 nm) in perovskite film.32 F. Huang et al. reported a dual-flash annealing process, in which the perovskite film was treated via a 2-step annealing process involving a low-pressure flash evaporation stage followed by a high-temperature flash annealing step.33 The PSCs based on these films exhibited enhanced photovoltaic (PV) performances (∼19%). Although the abovementioned methods are significant for improving the crystallinity along with large grain size, there are limitations, such as long processing time and expensive equipment.
In addition, many studies have also been reported on annealing for as-deposited perovskite films by using rapid thermal annealing techniques. In general, the rapid thermal annealing methods have been widely used to improve the structural and electrical properties of semiconducting materials.34,35 These techniques can raise the temperature rapidly by using various heating sources such as intense pulsed light,32,36 flash light,33 infrared (IR) radiation,37,38 etc. Among them, the radiative annealing using the IR lamp is advantageous for transferring the heat omnidirectionally, allowing uniform heat distribution within the thin films but, it was barely used in PSCs. V. L. Pool et al. utilized the radiative thermal annealing (hereafter abbreviated as RTA) technique in perovskite film annealing stage to explain the decomposition kinetics of FAPbI3 depending on the RTA temperature and heating time.37 B. Dou et al. also used the RTA method for annealing as-deposited perovskite films to see the effect of processing conditions, i.e. anti-solvent, humidity, and annealing conditions, on the film quality of MAPbI3 through in situ XRD analysis.38
In this study, we applied a radiative thermal annealing (RTA) to improve the crystallinity and grain size of perovskite films. Differences in crystallinity and grain size of the perovskite films prepared by these two different post-annealing methods (RTA vs. hot plate annealing) and their effects on the device performance of PSCs were investigated. 20 PSCs were fabricated by each method and their PV performances were statistically characterized. In addition, the interfacial and optical (exciton) characteristics of perovskite films, such as charge transport, exciton binding energy and charge dissociation, were also investigated to verify the origin for the enhanced PV performance by the RTA.
The perovskite precursor solution was prepared by mixing 1.2 M PbI2 (99%, TCI), 1.02 M methylammonium iodide (MAI, 98%, Sigma Aldrich), 0.18 M formamidinium iodide (FAI, 99%, TCI) and 1.2 M DMSO (99%, Sigma Aldrich) in DMF (99%, Sigma Aldrich). The solution was then stirred at room temperature for 6 h. An approximately 300 nm-thick (MAPbI3)0.85(FAPbI3)0.15 layer was deposited by spin-coating the precursor solution at 3500 rpm for 40 s. Just before spin-coating was finished, 80 μL of toluene was dropped to instantaneously remove the DMF within the perovskite film. The as-deposited perovskite films were annealed in a radiative thermal annealing (RTA) system under N2 atmosphere. Fig. S1a (ESI†) shows the schematic presentation of the RTA steps. Chamber was heated up to 120 °C for 30 s with a ramping rate of 4 °C s−1. After reaching the set temperature, the film was annealed at 120 °C for 10 min. A spiro-OMeTAD layer as a hole-transport layer (HTL) was deposited onto the perovskite layer by spin-coating at 2000 rpm for 20 s. The spiro-OMeTAD solution was prepared by dissolving 72.3 mg of spiro-OMeTAD in 1 mL of chlorobenzene and then adding 28.8 μL of 4-tert-butylpyridine and 17.5 μL of a stock solution of lithium bis(trifluoromethylsulfonyl)imide (Li-TFSI 520 mg in 1 mL of acetonitrile). After spin-coating the HTL, the samples were stored in a desiccator for 24 h to oxidize the spiro-OMeTAD for improving the hole extraction and transport properties.39 Finally, an 80 nm-thick gold electrode was deposited by thermal evaporation. All processes for PSCs fabrication were performed in a glove box filled with high purity N2 gas except when transporting the sample to RTA apparatus. All procedures are illustrated in Fig. S1b (ESI†). A schematic and a cross-sectional scanning electron microscopy image of a perovskite solar cell are shown in Fig. S1c and S1d (ESI†).
Fig. 1a shows XRD patterns of perovskite films annealed by two different methods. The prominent XRD peaks in both films appeared at 14.08° and 28.38°, corresponding to the (110) and (220) planes of the perovskite, respectively.44 Compared to those in HPA-perovskite, the intensity of main (110) peak at 14.08° in RTA-perovskite was higher and the full-width at half-maximum (FWHM) value was smaller, indicating that the crystallinity of the RTA-perovskite was better than that of the HPA-perovskite. Interestingly, there was no PbI2 diffraction peak at 12.7° in the HPA-perovskite. We assumed that it resulted from the difference in heat transfer mechanisms between hot plate annealing and RTA system. In hot plate annealing system, the perovskite film received less energy from the heating source since the heat travelled through glass, fluorine-doped tin oxide (FTO), electron transporting layer (ETL) to reach the perovskite film. However, in RTA system, since the heat was transferred directly to the perovskite film, it received all energy from the heating source without any loss.38 As a result, we predict that the degree of methylammonium iodide (MAI) decomposition is different between hot plate annealing and RTA system, resulting in the formation of a small amount PbI2 only in RTA. Fig. 1b and c compare top-view SEM images of perovskite films prepared by two different post-annealing methods. The insets are low magnification SEM images of both perovskite films. The average grain size of RTA-perovskite was measured as approximately 500 nm, however, sub-100 nm grains were observed in the HPA-perovskite.
The grain size difference between the RTA- and HPA-perovskite was resulted from the difference in nucleation rate by heat transfer mechanism.47 In general, the grains are developed through three steps; (i) nucleation, (ii) subsequent crystal growth at the nucleus and (iii) merging of neighboring crystals. Of these steps, the nucleation process occurs when the embryo overcomes its activation energy (ΔG) for making nuclei.48 In the hot plate-annealing, the temperature gradient occurred within the as-deposited perovskite film since the thermal energy was delivered from the bottom to upper layer by unidirectional heat transfer via conduction (left scheme in Fig. 2a). Thus, the nuclei were initially formed only at the bottom side of perovskite film. Consequentially, a relatively longer time was required for nucleation in the whole perovskite film. Therefore, it shortened the time for the growth of crystals and their merging into grains during the limited annealing time, resulting in a smaller grain size.
Besides, the movement of precursor ions was largely dependent on their position within the as-deposited perovskite film due to the unidirectional heat transfer from bottom side. Compared to the bottom part of perovskite film, which received huge amount of thermal energy from hot-plate via conduction, the movement of precursor ions at the top side was relatively suppressed due to the less thermal energy. It reduced the chance of the precursor ions sticking to the embryos, rendering less chance for nuclei formation. Therefore, this thermal energy distribution within the film induced uneven nuclei formation and accordingly grain growth with different sizes along the thickness of perovskite film.47 These phenomena including slow nucleation and suppressed migration of ions generated perovskite grains of sub-100 nm size by hot-plate annealing (Fig. 1b).
However, in the RTA process, thermal energy distribution within the as-deposited perovskite film was much uniform than that of the hot-plate annealing process owing to the omnidirectional heat transfer via radiation (right scheme in Fig. 2a). Active movement of precursor ions produced uniform nuclei in a relatively short time within the as-deposited perovskite film regardless of the position. Therefore, more thermal energy can be used up for the crystal growth and their merging into a grain in the same annealing time, inducing larger perovskite grains of approximately 500 nm size (Fig. 1c).
To further check the nucleation and grain growth at the film surface in initial annealing state of RTA process, grazing-incidence XRD (GIXRD) measurements were conducted with the HPA- and RTA-treated films with an incidence angle of 0.1°. In general, as the GIXRD uses a small incidence angle for incoming X-ray, the penetration depth of X-ray is limited to the surface and most scattering signals mainly comes from it.49 The heat treatment of perovskite films was performed for only 30 s to check the initial nuclei creation at the surface region. Fig. 2b shows the GIXRD patterns of HPA- and RTA-perovskite in initial annealing stage. The intensity of (110) main peak of RTA-perovskite was higher than that of HPA-perovskite, demonstrating that crystallization process including nucleation and crystal growth occurred more actively at the surface by the RTA. The swift nucleation and crystal growth consequentially contributed to larger grains.
Fig. 3 shows the PV performances of PSCs. Fig. 3a compares representative current density–voltage (J–V) curves, external quantum efficiency (EQE) spectra and the corresponding integrated current density of the PSCs produced by two different post-annealing methods. The best and average PV parameters, including the open-circuit voltage (Voc), short-circuit current density (Jsc), fill factor (FF) and PCE, are listed in Table 1. The PSCs using the HPA-perovskite (HPA-PSCs) demonstrated average PV parameter values, which were obtained from 20 PSCs (the histograms of Jsc and PCE are shown in Fig. 3b); Voc of 1.04 V, Jsc of 20.26 mA cm−2, FF of 0.65 and PCE of 13.74%. In comparison, the PSCs using RTA-perovskite (RTA-PSCs) demonstrated enhanced PV performances with a Voc of 1.06 V, a Jsc of 21.94 mA cm−2, a FF of 0.67, a PCE of 15.49% and improved EQE in visible wavelengths. As explained earlier, the small amount of PbI2 acted as a passivation layer, contributing to the improvement in Jsc in the RTA-PSCs. Compared with the HPA-PSCs, both Jsc and PCE were enhanced by 10%. In addition, the RTA-PSCs demonstrated a lower series resistance of 99.479 Ω than the HPA-PSCs (106.679 Ω) measured under illumination conditions, suggesting that the high crystallinity and large perovskite grains allowed the transport of photogenerated carriers with minimal energy loss along the interfaces, which is advantageous for the higher FF.
Voc (V) | Jsc (mA cm−2) | FF | Rs (Ω) | PCE (%) | |
---|---|---|---|---|---|
HPA-PSCs | 1.04 (1.04 ± 0.03) | 23.20 (20.26 ± 1.58) | 0.67 (0.65 ± 0.03) | 106.68 | 16.02 (13.74 ± 1.31) |
RTA-PSCs | 1.06 (1.06 ± 0.02) | 24.33 (21.94 ± 0.97) | 0.68 (0.67 ± 0.02) | 99.48 | 17.37 (15.49 ± 0.95) |
The interfacial characteristics of the two PSCs prepared by different post-annealing methods were investigated by electrochemical impedance spectroscopy (EIS). EIS was performed at Voc of each PSC under dark. Fig. 4a and b show the Nyquist plots with the equivalent circuit model for interpreting EIS data. The equivalent circuit is shown in the inset of Fig. 4b. The Nyquist plot is comprised of two semi-circles; the large semi-circle in the low-frequency region below 10 kHz and the small semi-circle in the high-frequency region above 10 kHz. The large semi-circle in Fig. 4a is associated with the recombination resistance. Meanwhile, the small semi-circle magnified in Fig. 4b relates to the charge-transport resistance.50,51 Herein, Rc represents the contact resistance between the FTO electrode and counter electrode, Rrec and Rct are defined as the recombination resistance and charge transport resistance at the interfaces, respectively. The values of Rc, Rrec and Rct are summarized in Table 2. The Rrec measured in the RTA-PSCs (312 Ω) was higher than that measured in the HPA-PSCs (166 Ω). On the other hand, the RTA-PSCs had a lower Rct (32.5 Ω) compared to the HPA-PSCs (37.6 Ω). The higher Rrec and lower Rct of the RTA-PSCs suggested that charge recombination was greatly suppressed at the interfaces of the HTL and/or electron-transport layer (ETL) with the perovskite film.
HPA-perovskite | RTA-perovskite | |
---|---|---|
Contact resistance, Rc (Ω) | 30.1 | 33.0 |
Recombination resistance, Rrec (Ω) | 166 | 312 |
Charge transport resistance, Rct (Ω) | 37.6 | 32.5 |
Hole and electron mobilities of the two perovskite films were measured with the hole- and electron-only devices; ITO/PEDOT:PSS/perovskite/spiro-OMeTAD/Ag and fluorine-doped tin oxide (FTO)/TiO2 blocking layer/mesoporous TiO2 (mp-TiO2)/perovskite/PC60BM/Al, respectively. The hole and electron mobilities can be calculated in the SCLC regime, where the current depends on only the charge carrier mobility and J–V characteristics become quadratic, following the Mott–Gurney law:
(1) |
The photocurrent of PSCs strongly depends on the exciton dissociation efficiency in the perovskite film and this exciton dissociation efficiency is determined by the exciton binding energy. Fig. 5 shows the integrated photoluminescence (PL) intensity of HPA- and RTA-perovskite films at the temperatures from 60 K to 200 K, which were integrated from the PL spectra in Fig. S3 (ESI†). The exciton binding energy of perovskite film can be estimated by fitting temperature-dependent integrated PL intensity with following Arrhenius equation:
(2) |
Fig. 5 Temperature dependence of integrated PL intensity of perovskite films; (a) RTA-perovskite and (b) HPA-perovskite. The lines are fitted by Arrhenius equation. |
The transient PL characteristics of perovskite films were also measured to evaluate the exciton dissociation efficiency. The PL decay curves (Fig. 6) were monitored at 775 nm, which is the PL emission peak of perovskite film (inset of Fig. 6). The PL decay curves were fitted by bi-exponential function to extract the lifetimes (τ) and fractional intensities (f) for fast-decay (τ1, f1) and slow-decay (τ2, f2) components and the intensity-weighted average exciton lifetime (τave) (see Table 3). Compared to the τave (226 ns) of HPA-perovskite, the τave of RTA-perovskite was significantly shortened to 136 ns, implying that the generated excitons were efficiently dissociated in the RTA-perovskite, enabling more electrons to contribute to external current in PSCs. The excitons with less binding energy can dissociate efficiently, resulting in a short exciton lifetime.
Fig. 6 Time-resolved PL decay curves of the RTA-perovskite and HPA-perovskite prepared on a mp-TiO2/glass substrate. The inset shows the steady-state PL spectra. |
τ1 (f1) (ns) | τ2 (f2) (ns) | χ2 | τave (ns) | |
---|---|---|---|---|
a The monitored PL emission peak of both perovskite films was λ = 775 nm. The PL decay curves were fitted with a bi-exponential function to calculate the lifetime of perovskite. χ2 is the reduced chi-squared value. Intensity-weighted average lifetime (τave) is defined as f1τ1 + f2τ2, in which f1 and f2 are fractional intensities and τ1 and τ2 are lifetimes. | ||||
HPA-PSCs | 74 (0.23) | 271 (0.77) | 1.146 | 226 |
RTA-PSCs | 55 (0.30) | 170 (0.70) | 1.106 | 136 |
Footnotes |
† Electronic supplementary information (ESI) available: The structural (SEM, XRD) characterization of RTA-perovskite films, photovoltaic performance of RTA-PSCs at different annealing times, temperature-dependence of PL spectra of perovskite films prepared by hot-plate annealing and RTA, and schematic of PSC fabrication via RTA. See DOI: 10.1039/c9ra01309a |
‡ These authors equally contributed to this work. |
This journal is © The Royal Society of Chemistry 2019 |