Junta Kagae‡
,
Takaaki Yamanaka‡,
Shun Takahashi and
Kenichi Yamashita*
Faculty of Electrical Engineering and Electronics, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan. E-mail: yamasita@kit.ac.jp
First published on 20th November 2018
The fabrication method of a perovskite absorption layer on a [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) electron transport layer was investigated for application to perovskite/Si tandem photovoltaic devices. A dry/wet hybrid method that involves thermal evaporation of a PbI2 precursor followed by spin coating of an organic cation solution was found to be a useful means to form a perovskite layer without destruction of the underlying PCBM layer. To form the perovskite layer densely packed with crystals of large grain size and having smooth surface morphology, the rotational speed of the spin coating and the mixing ratio of organic cations were carefully modified. By using this modified absorption layer, a photovoltaic device fabricated under atmospheric exposure conditions showed a power conversion efficiency of ∼10.3% without any large hysteresis. These results have great importance to develop a production technology for high performance tandem solar cells.
Recent studies have shown that one of the key factors for high performance tandem devices is the constitution of intermediate layers between the absorption layers of top and bottom cells, which engage in extraction, transport, and recombination of photoexcited carriers.10,11 Fullerene derivatives such as [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) are useful candidates for an n-type carrier extraction layer that extracts electrons from the perovskite top cell and transports them to the recombination layer (e.g. a transparent intermediate electrode or the interface between highly doped p-type layer of the bottom cell and the n-type fullerene layer itself).11,14–17 This is because the PCBM can be deposited by a solution-based method at low temperature (<∼100 °C) without thermal damage of the Si bottom cells, which is in contrast with transparent conducting oxides requiring high temperature in their sintering procedure. In addition, a PCBM layer can extract efficiently photoexcited electrons from the perovskite absorption layer. Also its passivation effect on the perovskite layer causes a good electrical property without any significant hysteresis behavior.15–18 Therefore a fabrication scheme of the perovskite layer on the PCBM electron transport layer is important to realize tandem photovoltaic cells showing high PCE and stability.
To fabricate the perovskite layer on the PCBM layer successfully, a dry/wet hybrid method, that is thermal evaporation of lead iodide (PbI2) on PCBM (dry process) and following spin coating of methylammonium iodide (MAI) solution (wet process), has been used in recent studies.11,15,19,20 This hybrid method can avoid destructing the PCBM layer owing to the absence of direct exposure to perovskite precursor solutions. Therefore the PCBM layer can be introduced in the regular device configuration (i.e. an electron transport layer is at the bottom and a hole transport layer is at the top), which results in a high-quality perovskite layer as well as better electron extraction.15 Despite this large importance, however, there are few reports on systematic investigations of the film formation mechanism as well as on improvements in the fabrication recipe. In addition, fabrication under atmospheric exposure condition should also be investigated because this method would usually require sample transfers between the dry and wet processes.
In this study we studied on an improved dry/wet hybrid method for preparing a perovskite absorption layer on a planar PCBM electron transport layer. After deposition of PbI2 by thermal evaporation, we investigated the following wet process under atmospheric condition by comparing the spin coating method and dip coating one and by changing the rotational speed. Furthermore the impacts of formamidinium (FA) and MA cations mixing on the film formation were discussed in detail. Consequently a closely packed polycrystalline film, which was composed of grains in a ∼1 μm scale and showed a flat and pinhole-free surface morphology, was successfully fabricated. A planar-type photovoltaic device with this absorption layer showed an open circuit voltage as high as ∼1.0 V and a power conversion efficiency (PCE) of 10.3% without any large hysteresis.
Photovoltaic devices studied here have a layered structure of SiO2/fluorinated tin oxide (FTO)/compact (c-) TiO2/PCBM/FAxMA1−xPbI3/P3HT/Au. The c-TiO2 layer was deposited onto the FTO by the supplier (Solaronix). The SiO2/FTO/c-TiO2 substrate was cleaned with acetone and ethanol in an ultrasonic bath. The substrate was dried at 80 °C for 5 minutes and exposed to UV light for 20 minutes under ozone atmosphere to increase the hydrophilicity. Next PCBM was spin coated at 2000 rpm for 30 seconds and then annealed at 100 °C for 10 minutes on a hot plate. The resultant thickness of PCBM layer was 100 nm. To fabricate the perovskite absorption layer, the PbI2 precursor layer was deposited by thermal deposition or spin coating. In the thermal deposition, the vacuum level was 2 × 10−5 Torr, and the deposition rate and time were in ranges of 2–3 Å s−1 and 15–20 minutes, respectively. The deposition rate was monitored with a crystal oscillator. The resultant thickness of deposited PbI2 was ∼300 nm. In the spin coating, the film of PbI2 precursor was fabricated at 3000 rpm for 30 seconds. The PbI2 film was annealed at 70 °C for 5 minutes. Thereafter a perovskite film was formed by either spin coating or dip coating. In the spin coating, the MAI precursor or the MAI/FAI precursor was spun on the PbI2 layer at 4000 or 1300 rpm for 30 seconds. The MAPbI3 film was annealed at 100 °C for 5 minutes whereas the FAxMA1−xPbI3 film were sequentially annealed; the annealing at 150 °C for 10 min followed by that at 100 °C for 40 min. In the dip coating, the sample, on which the PbI2 layer was deposited, was immersed into the MAI precursor solution for 1 minute and then annealed at 70 °C for 30 minutes. The thickness of perovskite absorption layer after the synthesis was ∼700 nm. Onto the perovskite layer, the P3HT hole transport layer with a thickness of 150 nm was fabricated by the spin coating at 3000 rpm for 30 seconds. The substrate was dried at room temperature for 30 minutes. Finally a gold top electrode with a thickness of 60 nm was deposited by vacuum evaporation.
A scanning electron microscope (SEM, S-5200, Hitachi) was employed for the observation of surface morphologies. X-ray diffraction measurements were also performed (Rigaku, RINT-2500). The Cu-Kα line was used. The current density–voltage (J–V) characteristics were measured using a source meter (Model 2400, Keythley) and a solar simulator (XES-40SI, SAN-EI). The J–V curves under both the forward and reverse voltage scanning directions were recorded.
We also investigated a dip coating method as the means to prepare the MAPbI3 perovskite layer from the evaporated PbI2 (see Fig. S2a in the ESI†). It is apparent that, as compared to the layer prepared by the spin coating (Fig. 1c), the grain size is smaller (130–200 nm) and the growth mode is more ‘three-dimensional’. This crystal growth mode would be due to the too rapid reaction of PbI2 with MAI. As shown in Fig. S2b in the ESI,† X-ray diffraction (XRD) intensity of the MAPbI3 prepared by the dip coating was as small as one half of that prepared by the spin coating. This result implies that the rapid reaction by the dip coating disturbs homogenous growth of the perovskite crystals. To make matter worse, the immersion time in the dip coating method was limited up to 1 minute because the peeling off of the film from the PCBM underlayer (soluble to the solvent) must be avoided. Namely, we can find that the MAI precursor volume and reaction rate are uncontrollable in the dip coating method.
When we introduced the method of MAI spin coating, on the other hand, the rotational speed was found to be an effective parameter to control them. As shown in a wide-area SEM image of Fig. 2a, the MAPbI3 surface after the spin coating at 4000 rpm still includes residual clustered PbI2. This would be due to the lack of MAI precursor and/or the too short reaction time. When the spin speed was reduced to 1300 rpm (see Fig. 2b), the volume of residual PbI2 was further decreased because of the further promotion in reaction and thus the grain size was also enlarged from ∼100 nm to ∼300 nm. By using this recipe, we were able to form a MAPbI3 film without significant degradation at the PCBM interface, which was confirmed in the photovoltaic performance as described later.
Next let us see cases of FAPbI3/MAPbI3-mixed perovskite films. We fabricated FAxMA1−xPbI3 films with x = 0, 0.2, 0.25, and 0.5 by the spin coating of FAI/MAI mixed precursor solution on the evaporated PbI2. As shown in Fig. S3 in the ESI,† the films only with x = 0 (MAPbI3) or 0.25 (FA0.25MA0.75PbI3) were successfully fabricated. The yellow and orange films at x = 0.2 or 0.5 are due to the bleaching of perovskite and the appearance of δ-phase perovskite, respectively.21 These results show an importance of molar fraction ratio to enhance perovskite stability,22–25 especially under the ambient condition. Fig. 2c and d show wide-area SEM images of the FA0.25MA0.75PbI3 films. It is apparent that the average grain size of crystals increases by introducing FAI, as reported in a previous paper.25 When the schemes of low-speed spin coating and FA/MA mixing were combined (see Fig. 2d), the grain size of crystal was enlarged further to ∼1 μm. The large grains lead to the closely-packed and almost pinhole-free perovskite film formation. The film quality obtained here is comparable with a case of dual-source thermal deposition method.26 A possible mechanism for the high-quality film formation by the FA/MA mixing is the formation of stable cubic perovskite.25 We also performed XRD measurement as shown in Fig. 3. The (110) diffraction peak intensity of the FA0.25MA0.75PbI3 was ∼1.2 times larger than that of MAPbI3, demonstrating that the crystallinity is also further improved by the FA/MA mixing.
Fig. 3 XRD patterns of FA0.25MA0.75PbI3 and MAPbI3 films prepared by spin coating of precursor solutions at 1300 rpm. |
Finally we show photovoltaic characteristics of a solar cell device where the perovskite absorption layer was fabricated by the hybrid method described above. Fig. 4a and b shows the device structure and the appearance. PCBM and P3HT were used as the electron and hole transport layers (ETL and HTL), respectively. Fig. 4c shows current density–voltage (J–V) properties under dark condition and AM 1.5G illumination for the best performance device fabricated by the PbI2-thermal evaporation and the FAI/MAI-spin coating at 1300 rpm (corresponding to the sample of Fig. 2d). The J–V curve at the reverse scan shows a filling factor (FF) of 0.67 without pronounced hysteresis characteristics. The short-circuit current density Jsc and the open-circuit voltage Voc are 15.2 mA cm−2 and 1.02 V, respectively. As a result, the PCE of 10.3% is achieved. For comparison, we also measured J–V properties for the devices fabricated by the MAI-spin coating at 1300 rpm (corresponding to Fig. 2b) and that fabricated by the FAI/MAI-spin coating at 4000 rpm (corresponding to Fig. 2c). These results are shown in Fig. S4 in the ESI† and summarized in Table 1. The enhancement in Jsc by the FA/MA mixing (from 6.48 to 15.2 mA cm−2) is mainly due to the improvement of the perovskite crystallinity. Also, as compared Fig. 2c with 2d, the enhancement in Jsc by controlling the spin speed (from 2.29 to 15.2 mA cm−2) is caused by the enlargement of the average crystal grain size. Voc did not show significant variation among the samples, demonstrating the well stabilized recombination losses at the ETL/perovskite and perovskite/HTL interfaces. A stability test was performed for a device fabricated by FAI/MAI-spin coating at 1300 rpm as shown in Fig. S5 in the ESI.† The initial performance as large as ∼6% in PCE was improved to >10% after one day. A possible reason of this improvement is the slow evaporation of high boiling point solvents (CBz, DMF, and DMSO). After two days later, the device exhibited degradation in FF and increase in the hysteresis characteristic. While these negative features should be improved, the performance obtained in the reversed scan shows high stability even after 1 week (∼8% in PCE).
Jsc (mA cm−2) | Voc (V) | FF | PCE (%) | ||
---|---|---|---|---|---|
MAPbI3 (1300 rpm) | Forward | −7.28 ± 0.86 | 1.03 ± 0.01 | 0.51 ± 0.11 | 3.78 ± 0.29 |
Reverse | −8.47 ± 2.02 | 1.04 ± 0.02 | 0.61 ± 0.06 | 5.46 ± 0.75 | |
FA0.25MA0.75PbI3 (4000 rpm) | Forward | −2.23 ± 0.06 | 1.00 ± 0.02 | 0.48 ± 0.04 | 1.06 ± 0.03 |
Reverse | −2.15 ± 0.13 | 1.02 ± 0.01 | 0.57 ± 0.01 | 1.26 ± 0.10 | |
FA0.25MA0.75PbI3 (1300 rpm) | Forward | −14.94 ± 2.49 | 0.99 ± 0.03 | 0.51 ± 0.05 | 7.63 ± 1.29 |
Reverse | −16.18 ± 3.09 | 0.97 ± 0.05 | 0.62 ± 0.04 | 9.59 ± 1.32 | |
(Best sample) | Forward | −15.7 | 1.02 | 0.53 | 8.54 |
Reverse | −15.2 | 1.02 | 0.67 | 10.3 |
It should be noted that all the device fabrications in this study, except for the thermal evaporation, were intentionally processed under the atmospheric exposure conditions (Rh ∼ 40–60%). Furthermore the devices were not encapsulated. Even so, the PCE larger than 10% was found to be possible. This result would not be so bad because PCEs of devices with the similar compositional ratio (20–30% of FA) are reported to be in a range of 17–19% when the device were fabricated in a grove box filled with nitrogen and encapsulated in measurements.25 Furthermore this method would be improved further by optimizing the mixing schemes, e.g. hybridization with the caesium iodide and lead bromide as similar to a very recent study.27 Such the optimization will provide the enhanced device performance of the perovskite top cell together with the production feasibility under atmospheric exposure conditions, which is important to achieve high-performance tandem solar cell modules and to develop their mass production technologies.
Footnotes |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8ra08022d |
‡ J. K. and T. Y. contributed equally. |
This journal is © The Royal Society of Chemistry 2018 |