Applied Science and Convergence Technology 2022; 31(6): 167-170
Published online November 30, 2022
https://doi.org/10.5757/ASCT.2022.31.6.167
Copyright © The Korean Vacuum Society.
Seungsun Choi , Woojin Shin , Jaewon Oh , Mee-Yi Ryu , and Hyunbok Lee*
Department of Physics and Institute of Quantum Convergence Technology, Kangwon National University, Chuncheon 24341, Republic of Korea
Correspondence to:hyunbok@kangwon.ac.kr
As perovskite solar cells (PSCs) are composed of multilayer structures, the formation of an appropriate interface between the perovskite lightabsorbing layer and charge transport layer is necessary to achieve a high power conversion efficiency (PCE). [6,6]-phenyl C61 butyric acid methyl ester (PCBM) is a popular organic material for the electron transport layer (ETL) in PSCs owing to its solution processability and excellent n-type properties. However, the annealing effects of the PCBM ETL on PSC performance are not adequately understood. Therefore, we fabricated p-i-n structured PSCs with annealed and unannealed PCBM ETL, and their device performances were compared. The PCE of the PSCs was markedly increased by annealing of the PCBM. The improved device performance resulted from the (1) enhanced electron transport ability of PCBM and (2) reduced non-radiative recombination at the interface between the perovskite and PCBM. These results show that annealing of PCBM is an efficient method to attain a high PCE of PSCs.
Keywords: Perovskite solar cell, PCBM, Electron transport layer, Annealing process
Recently, perovskitesolar cells (PSCs) have been extensively studied owing to their high light absorbance, tunable band gap, and low fabrication cost [1–4]. Since the first report of solid-state devices in 2009, the power conversion efficiency (PCE) of PSCs has increased dramatically [5,6]. In particular, the PCE of PSCs can be significantly improved by adopting a multilayer structure. The basic structure of PSCs consists of a cathode, an electron transport layer (ETL), a perovskite light-absorbing layer, a hole transport layer (HTL), and an anode. The ETL/HTL facilitates electron/hole transport toward the cathode and anode, respectively, after the dissociation of the photogenerated exciton. For efficient electron/hole transport in devices, the proper formation of an interface between the perovskite layer and ETL/HTL is essential. This implies that the interface should have wellaligned charge transport levels, absence of pinholes, and high wettability [7–9]. Such a multilayer structure is a universal strategy applied in various emerging optoelectronic devices including organic lightemitting diodes, organic solar cells (OSCs), and quantum-dot lightemitting diodes [10–12].
Various organic and inorganic materials have been employed as ETL and HTL to achieve efficient PSCs [13–16]. Among them, [6,6]-phenyl C61 butyric acid methyl ester (PCBM) is a representative organic ETL because of its excellent
In this study, we compared the device performance of
Tin-doped indium oxide (ITO)-coated glass substrates (AMG, South Korea) were sequentially cleaned via ultrasonication in deionized (DI) water, detergent, acetone, methanol, and DI water. Subsequently, the ITO was dried under N2 gas flow and treated with ultraviolet-ozone (UV-O3) at 100 °C for 15 min using a PSDP-UV4T-UV-O3 cleaner (Novascan Technology Inc., United States). Poly[bis(4-phenyl)(2,4,6- trimethylphenyl)amine] (PTAA,
The current density (
Figure 1(a) shows the device structure of the fabricated
Table 1 . Solar cell parameters of PSCs without and with annealing. Statistics were evaluated from five devices..
FF (%) | PCE (%) | |||
---|---|---|---|---|
w/o annealing | 18.55 (18.44±0.70) | 1044.4 (1035.8±9.3) | 54.3 (54.7±0.9) | 10.52 (10.45±0.26) |
w/annealing | 18.28 (18.17±0.33) | 1008.4 (1018.7±24.4) | 65.1 (62.2±2.8) | 12.01 (11.52±0.75) |
The device performance was evaluated using a reverse-direction bias sweep. Without annealing, the short-circuit current density (
The changes in the electron transport properties with PCBM annealing were additionally studied by characterizing the electron-only devices. Figure 2 shows the
To study the morphological changes of the PCBM ETL by annealing, top-view and cross-sectional-view SEM images of the ITO/PTAA/ PMMA/MAPbI3/PCBM sample were obtained, as shown in Fig. 3. Figures 3(a) and 3(b) show the cross-sectional SEM images of the samples with and without annealing. The PTAA and PMMA layers on the ITO were very thin and thus not clearly observable. In the MAPbI3 layer, large perovskite crystals without pinholes were observed due to the enhanced wettability of PMMA. The thickness of the MAPbI3 layer was estimated to be approximately 360 nm in both the cases. In both the cases, The PCBM ETL was uniformly deposited on the MAPbI3 layer. The thickness of the annealed PCBM ETL is 70 nm, whereas that of the unannealed PCBM ETL is 75 nm. This might be attributed to the dense packing of the PCBM molecules by annealing. Figures 3(c) and 3(d) show the top-view SEM images of the samples. In both the cases, PCBM clusters were observed. However, no noticeable changes were observed during the annealing. Therefore, changes in the morphology of the samples after annealing were not noticeable, except for a slight difference in the thickness of the PCBM layer.
PL analysis was performed to investigate the exciton and carrier dynamics. Figure 4(a) shows the SSPL spectra of glass/MAPbI3 and glass/MAPbI3/PCBM, with and without annealing. The characteristic PL peak of MAPbI3 was observed at 772 nm (i.e., a band gap of 1.61 eV). With PCBM layer deposition, the peak intensity significantly decreased, indicating exciton dissociation and electron transport to the PCBM. However, the peak intensity of the annealed sample was higher than that of the unannealed sample. This is attributed to the reduction in non-radiative recombination at the interface of the MAPbI3 and PCBM layers. Figure 3(b) shows the TRPL spectra of glass/MAPbI3 and glass/MAPbI3/PCBM with and without annealing. The PL decay curves were fitted using a bi-exponential function:
where
The fitting parameters are presented in Table II. Without the PCBM ETL, the PL decay was ascribed primarily to radiative recombination (~ 95 %). Consequently, τavg was as high as 137.01 ns. However, with the PCBM ETL, considerable non-radiative decay was observed. This fast decay is resulted from electron transport and interface defect trapping phenomena. For the unannealed PCBM, the evaluated A1, τ1, A2, and τ2 were 38.40, 2.05 ns, 61.60, and 5.08 ns, respectively. The resultant τavg was 4.47 ns. However, in the case of the annealed PCBM, the fast decay component decreased; the A1, τ1, A2, and τ2 were 35.29, 2.73 ns, 64.71, and 6.95 ns, respectively, yielding a τavg of 6.21 ns. This result is in good agreement with the changes observed in the SSPL spectra. When considering the PL results together with the device results, it can be concluded that the increased τavg originates from the reduced interface defects. The passivation of interfacial defects plays a decisive role in increasing the PCE of PSCs [24,25]. Therefore, the reduction in defect trapping between MAPbI3 and PCBM is also a cause of the enhanced PCE with annealing.
Table 2 . Fitting parameters of TRPL decay curve..
A1 | τ1 (ns) | A2 | τ2 (ns) | τavg (ns) | |
---|---|---|---|---|---|
w/o PCBM | 4.75 | 30.76 | 95.25 | 138.19 | 137.01 |
w/o annealing | 38.40 | 2.05 | 61.60 | 5.08 | 4.47 |
w/ annealing | 35.29 | 2.73 | 64.71 | 6.95 | 6.21 |
In this study, we investigated the changes in the device performance of PSCs by annealing the PCBM ETL. The annealing process resulted in a conspicuous increase in PCE from 10.52 to 12.01 %. In particular, the FF increased from 54.3 to 65.1 %, and the S-shaped kink was removed. The
This study was supported by the National Research Foundation of Korea (NRF-2021R1A2C1009324 and 2018R1A6A1A03025582).
The authors declare no conflicts of interest.