Effects of the additives propylammonium or
n-butylammonium iodide on the performance of
perovskite solar cells†
Cheng-Ming Hsieh,aYen-Lin Yu,bChih-Ping Chen *b
and Shih-Ching Chuang *a Organic–inorganic lead halide perovskite solar cells (PSCs) offer a promising low-cost manufactural solar technology as they are compatible with large-scale and low-temperature (<100C) solution processes. Through the optimization of perovskite active layer and phenyl-C61-butyric acid methyl ester (PC61BM) layer thickness, the normal cells showed 9.7% power conversion efficiency (PCE). Compared with the corresponding normal devices, we observed an improvement in PCE from 9.7% to 11.3% and 10.2% for the devices prepared using 1 vol% of C3H7NH3I and 1 vol% of C4H9NH3I as additives, respectively. Via analysis by ultraviolet-visible (UV-vis) spectroscopy, grazing incidence wide angle X-ray diffraction (GIWAXS), and field emission scanning electron microscopy (FE-SEM), we concluded that the morphological changes, absorption, and crystallinity of the perovskitefilms played an important role that influenced the performance of the PSC devices with various additives. The presence of 1% of C3H7NH3I or 1% of C4H9NH3I caused the CH3NH3PbI3 xClx films to grow uniformly with high coverage and continuous phase, as well as with higher absorption; this enabled the corresponding devices to display improved performance.
Introduction
Perovskite solar cells (PSCs) demonstrate many advantages, including low-cost solution process, thinlm, and high power conversion efficiencies (PCEs), and their PCE has been engi-neered to increase from 3.8% to 22% within only 8 years.1–6This technology has demonstrated considerable prospects in achieving efficiency comparable to or even better than those of other thin-lm solar cells (CIGS and CdTe). However, rendering PSCs toward commercialization with long-term stability is challenging since the perovskite materials are sensitive towards atmosphere, humidity, and temperature. In general, PSCs are actually fabricated at temperatures above 50 C, and the perovskites may degrade over time.7–14To obtain high perfor-mance PSCs, it is important to produce a high-quality thinlm since the photoelectric characteristics and the surface topog-raphy are governed by the morphology. The crystallization behaviour of the perovskite thinlms controls the development of surface topography, which affects the charge separation, recombination mechanics, and diffusion-length of perovskite
thin lms. Key factors, including deposition method,
surrounding, precursor composition, solvent,15–21 and the additives used, control the crystallization process.18,22–28 PSCs with a PCE of more than 20% relied on process modication and material engineering.29Using a simple perovskite solution deposition method, the mesoporous scaffold provides physical limitations on the size of the perovskite crystals to achieve the desired quality of the relative thicknesslm. Thus, it is neces-sary to have better charge transport with larger perovskite grains.30The crystallization kinetics of thelms grown on the planar structure are different from that of the mesoporous scaffolds because the lack of mesoporous scaffolds and the reduced surface energy of the perovskite precursors will result in less nucleation sites. The process of homogeneous crystalli-zation consists of two steps: nucleation and growth. Non-ideal surface energy can lead to Volmer–Weber growth, which is due to the rapid growth of perovskite thinlms that leads to discontinuous lms with large size grains accompanied by many pores.31Therefore, the key solution is the development of perovskite crystal manipulation that controls nucleation and growth effectively to achieve the best lm morphology and crystallinity.32–45 The addition of a small amount of chemical additives has been demonstrated as an efficient way to tailor the morphology of perovskite thinlm, including broad coverage and enhanced crystallinity, thereby enhancing the performance of the devices.46,47Liang et al. observed that the addition of
1,8-diiodooctane (DIO) as an additive32 made the surface
morphology of the perovskite thin lm denser; thus, the
performance of the devices was improved. Recently, 2D
aDepartment of Applied Chemistry, National Chiao Tung University, Hsinchu, Taiwan,
Republic of China. E-mail: jscchuang@faculty.nctu.edu.tw
bDepartment of Materials Engineering, Ming Chi University of Technology, New Taipei
City, Republic of China. E-mail: cpchen@mail.mcut.edu.tw
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ra11286f
Cite this: RSC Adv., 2017, 7, 55986
Received 13th October 2017 Accepted 27th November 2017 DOI: 10.1039/c7ra11286f rsc.li/rsc-advances
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perovskites, obtained by inserting bulky alkylammonium cations in between anionic layers, have shown increased solar cell stability.48,49We have used ethylammonium iodide (EAI) as an additive in our previous study and observed an enhancement of PCE (9.4 0.76 to 10.2 0.58%) and long term stability of the PSC. The EAI-derived device retained ca. 80% of the PCEs under accelerated heating (65C) inside glove box for over 360 h.50In this study, we doped the perovskite layer with alkylammonium salts, i.e. n-propylammonium (0.24 nm) and n-butylammonium (0.26 nm) iodides, having greater ionic radius.51 Recently, Snaith and coworkers have studied the incorporation of n-butylammonium iodide into the caesium–formamidinium lead halide perovskite and observed plate-like crystallites standing up between the 3D perovskite grains, and this has dramatically enhanced the device stability.52 The embedding of C
3H7NH3I (PAI) and C4H9NH3I (BAI) as additives may lead to deformation and twisting of the lattice of perovskites; as a result, we may slow the rate of crystallization of perovskite thinlm and form
a at surface with less pinholes, thereby promoting the
formation of the preferred surface topography that allows effi-cient free carrier transportation.
Experimental
Materials and methodsAll chemicals were purchased from Aldrich and used as received, unless otherwise specied. MAI, PAI, and BAI were synthesized according to previously reported techniques.50 N-Propylamine or n-butylamine (2.0 M in MeOH, Aldrich) and hydroiodic acid (57% w/w aq. soln, stabilized with 1.5% hypo-phosphorous acid, Alfa Aesar) were stirred at 0C under N2for 2 h. The solvent was evaporated in a rotary evaporator under vacuum. The crude residue was dissolved in MeOH (5 mL) and poured into Et2O (200 mL). The precipitate was obtained and dried under vacuum to afford PAI and BAI as a white product. The detailed experimental and fabricated conditions are shown in the ESI.†
Results and discussion
From the UV-vis absorption spectra shown in Fig. 1, we observed enhancement of the absorption intensities for the perovskite thinlms with optimized C3H7NH3I and C4H9NH3I as additives. Upon adding 1% and 2.5% of C3H7NH3I and C4H9NH3I, absorption in the 300–400 nm region showed signicant enhancement. Thus, we infer that the perovskite thinlm surface morphology and coverage may become better with the addition of these two additives. Furthermore, accord-ing to grazaccord-ing incidence wide angle X-ray diffraction (GIWAXS) analysis in Fig. 2, the addition of C3H7NH3I and C4H9NH3I as additives does not give rise to extra diffraction peaks; this indicates that the addition of these two additives in these concentrations does not cause signicant changes in the perovskite structure. In this study, the GIWAXS images were plotted with the x-axial of the scattering vector q (equals to 4p sin q/l; herein, q refers to the half of the total scattering angle withl at the wavelength of 0.145 nm). The scattering peak
appearing at the q of 10 nm 1belongs to the scattering peak of the (110) plane of CH3NH3PbI3 xClxperovskite structure.53We observed no characteristic peaks of MAI and PbCl2, and the peak for the precursor structure of perovskite at a value of q of 11 nm 1disappears; this indicates the complete crystallization of perovskite for the studied condition. It was also found that the intensity of the (110) characteristic peaks varied with the addition of different additives; thus, it was further deduced that the crystal sizes of different additives could vary with the amount of additives, thereby affecting the grain nucleation and growth.
We used the GIWAXS data to calculate the individual half-width and then used the Scherrer formula to determine the grain size. As summarized in Table 1, it was found that the grain sizes did not change signicantly upon the addition of 1%, 2.5%, and 5% of C3H7NH3I, as well as 1% and 2.5% of C4H9NH3I, whereas the grain sizes became obviously smaller, Fig. 1 UV-vis absorption spectra for perovskite thinfilms obtained after doping with various concentration of C3H7NH3I and C4H9NH3I.
Fig. 2 XRD pattern of the perovskite thinfilms obtained after doping with various concentrations of C3H7NH3I and C4H9NH3I.
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with 19.3 nm size, upon the addition of 5% of C4H9NH3I. This indicates that the addition of more large ionic species as additives affects the grain sizes; thus, the perovskite precursors affect the formation of the perovskite layer.
For an optimized perovskite layer, the high coverage is crit-ical to determine the performance of PSC. Fig. S1† displays the
SEM image of the normal perovskite thin lm; it can be
observed that the surface morphology contains irregular sizes of hole with coverage of about 93.1%. We also observed the discontinuity of the grain boundaries for thislm. As shown in Fig. 3(a), (a-1) and (a-2), upon the addition of 1%, 2.5%, and 5% C3H7NH3I as additives, respectively, the pores on the lm surface were signicantly reduced, and the grain boundary became more continuous with coverage increased up to 97.4%, 97.6%, and 97.5%. As shown in Fig. 3(b), (b-1) and (b-2), the addition of 1%, 2.5%, and 5% C4H9NH3I resulted in increased coverage of upto 96.2%, 95.8%, and 95.7%, respectively. As a result, the addition of C3H7NH3I and C4H9NH3I as additives make the perovskite thinlm more at with reduction of size and number of the pores. As reected in their UV-vis spectra,
the lms with a higher rate of coverage exhibited stronger absorption in the visible region. By adding 10% of C3H7NH3I and C4H9NH3I, the excess additives result in a change of the surface topographies; as shown in Fig. 3(a-3) and (b-3), we observed large amount of pores for theselms. This indicated that limited light harvesting and irregular grain may impede the free carrier transport in the interface. Based on the SEM images and UV-vis and XRD analyses, we concluded that the addition of optimized ratios of C3H7NH3I and C4H9NH3I helped the crys-talline perovskite formation with higher coverage, i.e., reduc-tion of pores, and made the surface morphology more at. Thus, more efficient electron and hole transport may occur and enhance the performance of the devices. An optimized annealing time for our normal (no PAI or BAI) and PAI and BAI-derived perovskitelms was 2.5 and 3.5 h, respectively. Due to the greater ionic radius of propyl and butyl cation when compared with that of methyl-based ammonium iodide, the incorporation of these additives slowed down the formation of the perovskite structure. Alternatively, the embedding of bulky ammonium cations may form the 2D perovskites, and these structures can form a self-assembly structure, and typically, a smoother surface is exhibited.49Based on these reasons, we observed the perovskites growing into a at surface with a higher coverage as well as higher absorbance.
The performance of the PSC devices doped with C3H7NH3I and C4H9NH3I is summarized in Table 2. The pre-optimized PSC cells showed an average PCE of 8.4 1.6% with the high-est value of 9.7%, along with the value of Jscof 18.1 mA cm 2, a value of open circuit voltage (Voc) of 0.88 V and all factor (FF) value of 60.6%. The PCEs of devices doped with 1%, 2.5%, and 5% of C3H7NH3I were 10.2 1.1%, 10.4 0.9%, and 10.2 0.5%, respectively (Fig. 4). The PCEs of C4H9NH3I-derived devices with concentration of 1%, 2.5%, and 5% of C3H7NH3I were 9.9 0.3%, 10.0 0.1%, and 9.3 0.6%, respectively (Fig. Table 1 Variation of perovskite thinfilm grain size after doping with
C3H7NH3I and C4H9NH3I
Doping
Full width
at half maximum Grain size
CH3NH3PbI3 xClx 0.38 20.8 nm C3H7NH3I 1% 0.38 20.8 nm C3H7NH3I 2.5% 0.38 20.8 nm C3H7NH3I 5% 0.39 20.8 nm C4H9NH3I 1% 0.39 20.8 nm C4H9NH3I 2.5% 0.39 20.8 nm C4H9NH3I 5% 0.41 19.3 nm
Fig. 3 FE-SEM images of CH3NH3PbI3 xClxperovskite thinfilms doped with different concentration of (a) to (a-3) C3H7NH3I and (b) to (b-3) C4H9NH3I precursors; magnification by 8000 times.
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5). We observed the highest PCE of 11.3% for the device with 2.5% of C3H7NH3I additive, with a Jscof 17.9 mA cm 2, a value of Vocof 0.99 V, and an FF value of 63.8%, which was increased by 16.5% when compared with that of the pre-optimized normal devices. It was observed that Voc and FF values signicantly increased aer doping with 1%, 2.5%, and 5% of C3H7NH3I; this indicated that the perovskite thin lm aer doping with C3H7NH3I produced a good surface morphology because the reduction of holes could effectively reduce the chance of contact between the hole transport layer and the electron transport layer and thus avoid the occurrence of a free carrier recombi-nation or a low Voc. Moreover, the perovskite thinlm has good contact with the hole transport layer (PEDTO:PSS) and the electron transport layer (PCBM), producing better charge separation and transfer effect; thus, the PCE of the PSC device is improved. It was found that the addition of these additives not only increased the device performance but also improved the reproducibility of the device efficiency – the variation in the PCE was small. Via observation from the UV-vis measurement and FE-SEM coverage, the absorption intensity and coverage are relatively high with doped devices. Therefore, aer doping C3H7NH3I and C4H9NH3I into the devices, the power conversion efficiency was excellent.
We calculated the series (Rs) and shunt (Rsh) resistances from the respective J–V curves. The values of Rsh and Rs of the
C3H7NH3I-based devices are 471.5 and 9.3U cm2, respectively, whereas for the C4H9NH3I-based devices, they are 401.7 and 15.2U cm2, respectively. Moreover, for CH
3NH3PbI3 xClx-based devices, they are 316.4 and 11.3U cm2 for R
shand Rs values, respectively. The value of Rsof a perovskite device is related to the resistance of the perovskite thinlm, the interfacial contact resistance for all layers, and the resistance of the electrode contacts. The resistance of the perovskite thinlm may relate to its morphology, grain size, and surface coverage. The corre-sponding greater values of Rshfor the C3H7NH3I-based devices suggest lower leakage currents and fewer defects for charge recombination loss. Based upon FE-SEM study, the perovskite thinlm with C3H7NH3I as additives exhibited higher rate of coverage than those with C4H9NH3I doped and normal CH3 -NH3PbI3 xClx. These devices with C3H7NH3I as additives showed delicate textures with less pin holes. It was worthy to note that delicate thinlm generally exhibited higher FF values and associated with larger Rsh and smaller Rs values. This notion was reected from the FF, Rsh, and Rsvalues in Table 2. The perovskite device with C3H7NH3I as additives showed relatively large (471.5 U cm2) and smaller Rs (9.3U cm2) and those with C4H9NH3I and CH3NH3PbI3 xClx exhibited Rshof 401.7U cm2and Rsof 15.2U cm2as well as Rshof 316.4U cm2 and Rsof 11.3U cm2, respectively. The device with C3H7NH3I as additives showed best FF value (68.0%)—this value was greater
Fig. 4 The J–V curve with doped C3H7NH3I devices.
Table 2 PV parameters of PVSK devices upon doping C3H7NH3I and C4H9NH3I
Doping Jsc(mA cm 2) Voc(V) FF (%) PCE (%) Best PCE (%) Rsh(U cm2) Rs(U cm2)
CH3NH3PbI3 xClx 18.1 0.88 60.6 8.4 1.6 9.7 316.4 11.3 C3H7NH3I 1% 18.7 0.89 68.0 10.2 1.1 11.3 471.5 9.3 C3H7NH3I 2.5% 17.9 0.99 63.8 10.4 0.9 11.3 428.9 13.5 C3H7NH3I 5% 17.8 0.92 64.8 10.2 0.5 10.6 443.2 13.4 C4H9NH3I 1% 18.3 0.92 60.8 9.9 0.3 10.2 316.3 11.1 C4H9NH3I 2.5% 17.4 0.93 62.2 10.0 0.1 10.1 401.7 15.2 C4H9NH3I 5% 17.6 0.93 61.1 9.3 0.6 10.0 318.8 13.9
Fig. 5 The J–V curve with C4H9NH3I-doped devices.
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than those of the devices with C4H9NH3I (62.2%) and CH3 -NH3PbI3 xClx(60.6%).
We further investigated the device stability in the presence of various concentrations of C3H7NH3I and C4H9NH3I and compared it to that of the normal devices. To eliminate the factor of the uncertainty of encapsulation, we determined the stability of the device inside the glove box at room temperature (25C). As shown in Fig. 6, the performance of normal device
on the seventh day reduced to 95% or less as compared to the
PCE on the rst day of the measurement. The addition of
C3H7NH3I as additives helped in the stability of devices. In a previous report, Boschlo and coworkers used XRD to demonstrate the transformation of perovskite lm from less order structure to more crystalline aer storage at room temperature under inert gas.54 The initial increase in PCE of C3H7NH3I 2.5%-based PSC device aer storage at room temperature inside N2 lled glove box might arise from the enhancement of the crystallinity of the perovskite lm.53The
PCE of normal devices measured on the 49th day has been reduced to 80%; however, those with 1%, 2.5%, and 5% of C3H7NH3I in the devices maintain PCE at 88%, 93%, and 86% on the 49th day. This notion indicated that addition of C3H7NH3I as additives can improve the stability of the devices. Moreover, one typical doping with 2.5% of C3H7NH3I provided superior stability over the degradation test. Furthermore, as shown in Fig. 7, the addition of C4H9NH3I as additives helped device stability more than that upon the addition of C3H7NH3I. By addition of 1%, 2.5%, and 5% of C4H9NH3I as additives to the devices, the PCE of devices were maintained at 89%, 93%, and 98% efficiency, respectively, on the 49th day, which indi-cated that addition of C4H9NH3I could improve the stability of the device greatly. It is recently evidenced that Ruddlesden– Popper 2D perovskites have shown to appeal improved stability.55 Typically, 2D perovskites are formed through the embedding of large and bulky alkylammonium cations.49,52It is possible that a partial 2D perovskite structure may appear in the PAI and BAI-doped perovskite and cause the enhancement of the stability.
Conclusion
We have demonstrated that small amounts of alkylammonium iodides (C3H7NH3I and C4H9NH3I) in the perovskite precursor solution can improve the surface morphology and crystallinity of the perovskite thin lms. It could effectively increase the efficiency of the PSC devices from 9.7% to 11.3% and 10.2%. This can be clearly observed in the UV-vis spectra since the change of the surface morphology can increase the absorption intensity of perovskite, indicating that perovskites exhibit good surface morphology aer incorporation of additives. From the XRD and SEM analysis, it was proven that the addition of additives could make homogeneous nucleation and the crys-tallinity of perovskite thinlm more continuous with decreased porosity. By enhancing the crystallinity and the improvement of the surface morphology, the charge transfer efficiency between the perovskite layer and the charge transport layers can be improved. Both C3H7NH3I and C4H9NH3I-doped devices have the effect of improving the reproducibility and efficiency. Thus, C3H7NH3I- and C4H9NH3I-doped devices have excellent perfor-mance and can effectively improve the efficiency and stability of the PSCs.
Con
flicts of interest
There are no conicts to declare.
Acknowledgements
We thank the Ministry of Science and Technology of Taiwan (MOST 106-2113-M-131-001-MY2; MOST 105-2221-E-131-033-) for providing thenancial support.
Fig. 6 The stability of devices with C3H7NH3I doping.
Fig. 7 The stability of devices with C4H9NH3I doping.
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