Effects of the additives n-propylammonium or n-butylammonium iodide on the performance of perovskite solar cells

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 (<100 C) 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 perovskite films 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
2][3][4][5][6] This 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.][9][10][11][12][13][14] To obtain high performance PSCs, it is important to produce a high-quality thin lm since the photoelectric characteristics and the surface topography are governed by the morphology.The crystallization behaviour of the perovskite thin lms controls the development of surface topography, which affects the charge separation, recombination mechanics, and diffusion-length of perovskite thin lms.3][24][25][26][27][28] PSCs with a PCE of more than 20% relied on process modication 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 thickness lm.Thus, it is necessary to have better charge transport with larger perovskite grains. 30The crystallization kinetics of the lms 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 crystallization 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 thin lms that leads to discontinuous lms with large size grains accompanied by many pores. 313][34][35][36][37][38][39][40][41][42][43][44][45] The addition of a small amount of chemical additives has been demonstrated as an efficient way to tailor the morphology of perovskite thin lm, including broad coverage and enhanced crystallinity, thereby enhancing the performance of the devices. 46,47Liang et al. observed that the addition of 1,8diiodooctane (DIO) as an additive 32 made the surface morphology of the perovskite thin lm denser; thus, the performance of the devices was improved.Recently, 2D 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 AE 0.76 to 10.2 AE 0.58%) and long term stability of the PSC.The EAI-derived device retained ca.80% of the PCEs under accelerated heating (65 C) 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. 51Recently, Snaith and coworkers have studied the incorporation of nbutylammonium 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. 52The embedding of C 3 H 7 NH 3 I (PAI) and C 4 H 9 NH 3 I (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 thin lm and form a at surface with less pinholes, thereby promoting the formation of the preferred surface topography that allows efficient free carrier transportation.

Materials and methods
All chemicals were purchased from Aldrich and used as received, unless otherwise specied.MAI, PAI, and BAI were synthesized according to previously reported techniques. 50N-Propylamine or n-butylamine (2.0 M in MeOH, Aldrich) and hydroiodic acid (57% w/w aq.soln, stabilized with 1.5% hypophosphorous acid, Alfa Aesar) were stirred at 0 C under N 2 for 2 h.The solvent was evaporated in a rotary evaporator under vacuum.The crude residue was dissolved in MeOH (5 mL) and poured into Et 2 O (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 thin lms with optimized C 3 H 7 NH 3 I and C 4 H 9 NH 3 I as additives.Upon adding 1% and 2.5% of C 3 H 7 NH 3 I and C 4 H 9 NH 3 I, absorption in the 300-400 nm region showed signicant enhancement.Thus, we infer that the perovskite thin lm surface morphology and coverage may become better with the addition of these two additives.Furthermore, according to grazing incidence wide angle X-ray diffraction (GIWAXS) analysis in Fig. 2, the addition of C 3 H 7 NH 3 I and C 4 H 9 NH 3 I 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 signicant 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 with l at the wavelength of 0.145 nm).The scattering peak appearing at the q of 10 nm À1 belongs to the scattering peak of the (110) plane of CH 3 NH 3 PbI 3Àx Cl x perovskite structure. 53We observed no characteristic peaks of MAI and PbCl 2 , and the peak for the precursor structure of perovskite at a value of q of 11 nm À1 disappears; 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 halfwidth 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 signicantly upon the addition of 1%, 2.5%, and 5% of C 3 H 7 NH 3 I, as well as 1% and 2.5% of C 4 H 9 NH 3 I, whereas the grain sizes became obviously smaller,  with 19.3 nm size, upon the addition of 5% of C 4 H 9 NH 3 I.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 critical 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 this lm.As shown in Fig. 3(a), (a-1) and (a-2), upon the addition of 1%, 2.5%, and 5% C 3 H 7 NH 3 I as additives, respectively, the pores on the lm surface were signicantly 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% C 4 H 9 NH 3 I resulted in increased coverage of upto 96.2%, 95.8%, and 95.7%, respectively.As a result, the addition of C 3 H 7 NH 3 I and C 4 H 9 NH 3 I as additives make the perovskite thin lm more at with reduction of size and number of the pores.As reected in their UV-vis spectra, the lms with a higher rate of coverage exhibited stronger absorption in the visible region.By adding 10% of C 3 H 7 NH 3 I and C 4 H 9 NH 3 I, 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 these lms.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 C 3 H 7 NH 3 I and C 4 H 9 NH 3 I helped the crystalline perovskite formation with higher coverage, i.e., reduction 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 BAIderived perovskite lms 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 C 3 H 7 NH 3 I and C 4 H 9 NH 3 I is summarized in Table 2.The pre-optimized PSC cells showed an average PCE of 8.4 AE 1.6% with the highest value of 9.7%, along with the value of J sc of 18.1 mA cm À2 , a value of open circuit voltage (V oc ) of 0.88 V and a ll factor (FF) value of 60.6%.The PCEs of devices doped with 1%, 2.5%, and 5% of C 3 H 7 NH 3 I were 10.2 AE 1.1%, 10.4 AE 0.9%, and 10.2 AE 0.5%, respectively (Fig. 4).The PCEs of C 4 H 9 NH 3 I-derived devices with concentration of 1%, 2.5%, and 5% of C 3 H 7 NH 3 I were 9.9 AE 0.3%, 10.0 AE 0.1%, and 9.3 AE 0.6%, respectively (Fig.

5)
. We observed the highest PCE of 11.3% for the device with 2.5% of C 3 H 7 NH 3 I additive, with a J sc of 17.9 mA cm À2 , a value of V oc of 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 V oc and FF values signicantly increased aer doping with 1%, 2.5%, and 5% of C 3 H 7 NH 3 I; this indicated that the perovskite thin lm aer doping with C 3 H 7 NH 3 I 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 recombination or a low V oc .Moreover, the perovskite thin lm 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 efficiencythe 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, aer doping C 3 H 7 NH 3 I and C 4 H 9 NH 3 I into the devices, the power conversion efficiency was excellent.We calculated the series (R s ) and shunt (R sh ) resistances from the respective J-V curves.The values of R sh and R s of the C 3 H 7 NH 3 I-based devices are 471.5 and 9.3 U cm 2 , respectively, whereas for the C 4 H 9 NH 3 I-based devices, they are 401.7 and 15.2 U cm 2 , respectively.Moreover, for CH 3 NH 3 PbI 3Àx Cl x -based devices, they are 316.4 and 11.3 U cm 2 for R sh and R s values, respectively.The value of R s of a perovskite device is related to the resistance of the perovskite thin lm, the interfacial contact resistance for all layers, and the resistance of the electrode contacts.The resistance of the perovskite thin lm may relate to its morphology, grain size, and surface coverage.The corresponding greater values of R sh for the C 3 H 7 NH 3 I-based devices suggest lower leakage currents and fewer defects for charge recombination loss.Based upon FE-SEM study, the perovskite thin lm with C 3 H 7 NH 3 I as additives exhibited higher rate of coverage than those with C 4 H 9 NH 3 I doped and normal CH 3 -NH 3 PbI 3Àx Cl x .These devices with C 3 H 7 NH 3 I as additives showed delicate textures with less pin holes.It was worthy to note that delicate thin lm generally exhibited higher FF values and associated with larger R sh and smaller R s values.This notion was reected from the FF, R sh , and R s values in Table 2.The perovskite device with C 3 H 7 NH 3 I as additives showed relatively large (471.5 U cm 2 ) and smaller R s (9.3 U cm 2 ) and those with C 4 H 9 NH 3 I and CH 3 NH 3 PbI 3Àx Cl x exhibited R sh of 401.7 U cm 2 and R s of 15.2 U cm 2 as well as R sh of 316.4 U cm 2 and R s of 11.3 U cm 2 , respectively.The device with C 3 H 7 NH 3 I as additives showed best FF value (68.0%)-this value was greater Fig. 4 The J-V curve with doped C 3 H 7 NH 3 I devices.Fig. 5 The J-V curve with C 4 H 9 NH 3 I-doped devices.
This journal is © The Royal Society of Chemistry 2017 than those of the devices with C 4 H 9 NH 3 I (62.2%) and CH 3 -NH 3 PbI 3Àx Cl x (60.6%).We further investigated the device stability in the presence of various concentrations of C 3 H 7 NH 3 I and C 4 H 9 NH 3 I 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 ($25 C).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 C 3 H 7 NH 3 I 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 aer storage at room temperature under inert gas. 54The initial increase in PCE of C 3 H 7 NH 3 I 2.5%-based PSC device aer storage at room temperature inside N 2 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 C 3 H 7 NH 3 I in the devices maintain PCE at 88%, 93%, and 86% on the 49th day.This notion indicated that addition of C 3 H 7 NH 3 I as additives can improve the stability of the devices.Moreover, one typical doping with 2.5% of C 3 H 7 NH 3 I provided superior stability over the degradation test.Furthermore, as shown in Fig. 7, the addition of C 4 H 9 NH 3 I as additives helped device stability more than that upon the addition of C 3 H 7 NH 3 I.By addition of 1%, 2.5%, and 5% of C 4 H 9 NH 3 I as additives to the devices, the PCE of devices were maintained at 89%, 93%, and 98% efficiency, respectively, on the 49th day, which indicated that addition of C 4 H 9 NH 3 I could improve the stability of the device greatly.It is recently evidenced that Ruddlesden-Popper 2D perovskites have shown to appeal improved stability. 55Typically, 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 (C 3 H 7 NH 3 I and C 4 H 9 NH 3 I) 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 aer incorporation of additives.From the XRD and SEM analysis, it was proven that the addition of additives could make homogeneous nucleation and the crystallinity of perovskite thin lm 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

Fig. 1
Fig. 1 UV-vis absorption spectra for perovskite thin films obtained after doping with various concentration of C 3 H 7 NH 3 I and C 4 H 9 NH 3 I.

Fig. 2
Fig. 2 XRD pattern of the perovskite thin films obtained after doping with various concentrations of C 3 H 7 NH 3 I and C 4 H 9 NH 3 I.

Fig. 3
Fig. 3 FE-SEM images of CH 3 NH 3 PbI 3Àx Cl x perovskite thin films doped with different concentration of (a) to (a-3) C 3 H 7 NH 3 I and (b) to (b-3) C 4 H 9 NH 3 I precursors; magnification by 8000 times.
C 3 H 7 NH 3 I and C 4 H 9 NH 3 I-doped devices have the effect of improving the reproducibility and efficiency.Thus, C 3 H 7 NH 3 I-and C 4 H 9 NH 3 I-doped devices have excellent performance and can effectively improve the efficiency and stability of the PSCs.

Table 1
Variation of perovskite thin film grain size after doping with C 3 H 7 NH 3 I and C 4 H 9 NH 3 I

Table 2
PV parameters of PVSK devices upon doping C 3 H 7 NH 3 I and C 4 H 9 NH 3 I