Extremely reduced dielectric confinement in two-dimensional hybrid perovskites with large polar organics

Two dimensional inorganic–organic hybrid perovskites (2D perovskites) suffer from not only quantum confinement, but also dielectric confinement, hindering their application perspective in devices involving the conversion of an optical input into current. In this report, we theoretically predict that an extremely low exciton binding energy can be achieved in 2D perovskites by using high dielectric-constant organic components. We demonstrate that in (HOCH2CH2NH3)2PbI4, whose organic material has a high dielectric constant of 37, the dielectric confinement is largely reduced, and the exciton binding energy is 20-times smaller than that in conventional 2D perovskites. As a result, the photo-induced excitons can be thermally dissociated efficiently at room temperature, as clearly indicated from femtosecond transient absorption measurements. In addition, the mobility is largely improved due to the strong screening effect on charge impurities. Such low dielectric-confined 2D perovskites show excellent carrier extraction efficiency, and outstanding humidity resistance compared to conventional 2D perovskites.Two-dimensional inorganic–organic hybrid perovskites are expected to play an important role in photovoltaic devices but suffer from issues related to dielectric confinement. The authors theoretically outline a method and experimentally succeed to overcome this issue by using materials with large dielectric constants.

and low carrier mobility, and thus hinder their application perspective on the devices converting an optical input into current 1,2,5 . In this report, we theoretically predicted that extremely low exciton binding energy can be achieved by using high dielectricconstant organic groups, and demonstrated that in (HOCH2CH2NH3) 2 PbI 4 , whose organic groups have dielectric constant of 37, the dielectric confinement is largely reduced and thus an exciton binding energy of 13 meV has been obtained which is 20 times smaller than that in conventional 2D perovskites and also smaller than thermal activation energy at room temperature.
As a result, the photo-induced excitons can be thermally dissociated efficiently at room temperature, as clearly indicated from femtosecond transient absorption (fs-TA) measurements. In addition, the mobility is largely improved due to the strong screening effect on charge impurities. Our low dielectric-confined 2D perovskites show excellent carrier extraction efficiency, and outstanding humidity resistance compared to conventional 2D perovskites.
Due to high absorption coefficient, intense photoluminescence, low electron-hole recombination rate and long carrier diffusion length, 3D perovskites are regarded as one of the leading candidates of next-generation photovoltaics (PVs) [7][8][9][10] . Recently, two dimensional layered organic-inorganic halide perovskites (2D perovskites) have been attracting attentions as an alternative to 3D perovskites in PV devices due to their improved stability and moisture resistance [1][2][3] . In addition, owing to the weak Van der Waals force between the functional organic groups, 2D perovskite nanosheets can be produced by solution method 4,5,11 and mechanical exfoliation 12 to fit limited geometric requirements, making them a new platform for lowdimensional nano-electronics/optoelectronics. 3 However, following the general trend of quantum confinement 13 , the exciton binding energy of 2D perovskites is greatly increased compared to its 3D counterpart. Moreover, since the conventional organic linker used in 2D perovskites, such as C 6 H 5 (CH 2 ) 2 NH 3 + (PEA) and C 4 H 9 NH 3 + (BA), have a small dielectric constant, the dielectric confinement emerges and makes the exciton binding energy even larger [13][14][15][16] . Such large exciton binding energy makes excitons difficult to form free carriers via thermal activation.
Accordingly, the strong combined effect of quantum and dielectric confinement leads to carrier mobility in 2D perovskites two orders of magnitude lower than that in 3D perovskites 17 . In order to fully utilize the optoelectronic properties of 2D perovskites for photoelectric applications, intrinsic methods to lower the confinement in 2D perovskites are urgently required. Quantum confinement can be reduced by increasing the width of the inorganic semiconductor layer in 2D perovskites (see supplementary information ), but even when the width of inorganic layer is enlarged by five times, the binding energy is still up to 200 meV 18 . Therefore, it is necessary to develop an alternative approach to "weaken" the dielectric confinement in 2D perovskites. However, the current method of reducing dielectric confinement by atom intercalation 19 can only lower the exciton binding energy by tens of meV, which is not enough for thermal dissociation of excitons at room temperature.
In this report, we conceived that by using large dielectric constant organic groups the dielectric confinement in 2D perovskite can be largely reduced. Then we demonstrated that (HOCH 2 CH 2 NH 3 ) 2 PbI 4 (2D_EA perovskites) 20 , in which the organic groups have dielectric constant of 37 21 ( Fig. 1a), has an twenty times smaller exciton binding energy and dozens of times larger carrier mobility compared to the high dielectric-confined 2D perovskites. It demonstrates that reducing dielectric confinement by using large dielectric constant organic groups is much more efficient than reducing quantum confinement and reducing dielectric confinement by atom 4 intercalation, leading to the significant improvement of carrier separation efficiency in 2D perovskites. Finally, we found that these low dielectricconfined 2D perovskites are highly stable under humidity, due to the strong dipole-dipole force between the organic groups.
First, for gaining the insight into the relations between exciton binding energy and dielectric constant of the organic layers in 2D perovskites, we adopted the image charge model 13 . According to our simulation (see supplementary materials), when the dielectric constant of organic layers is small, such as PEA whose dielectric constant is of ~3.3 14 (Fig. 1b), Bohr radius describing the mean distance between the electron and hole in the exciton will decrease and accordingly the exciton binding energy will be largely enhanced (Fig. 1c), indicating a strong dielectric confinement.
Conversely, as the dielectric constant of the organic layers increases, the dielectric screening effect will be enhanced, so that the Coulomb force between electron and hole in excitons will decrease, leading to increased Bohr radius and decreased exciton binding energy. If the dielectric constant of organic layer is sufficiently large, we can even achieve an exciton binding energy similar to that of 3D perovskites.
To experimentally achieve low dielectric-confined 2D perovskites, we used Ethanolamine (EA for a brief, and EA = HOCH 2 CH 2 NH 3 + ) to form the organic layers (right inset of Fig. 1c). EA has a dielectric constant as high as 37, because of the charge dipole induced by hydroxy group and short carbon chain (Fig. 1a). The bulk single crystal of 2D_EA perovskite was confirmed by single crystal X-ray diffraction (XRD) ( perovskites drops rapidly as the temperature increases from 50 K to 160 K 5 ( Fig. 2a), indicating weak attracting force between electron and hole induced by the screening effect (inset of Fig. 2a). Meanwhile the PL intensity of 2D_PEA perovskites shows a much slower decreasing rate as the temperature increases (Fig. 2b), indicating a more stable bound state for the excitons (inset of Fig. 2b). Integrated PL intensity was calculated (Fig. S1) and Arrhenius equation fitting (see supplementary materials) was used to get thermal dissociation ratio (Fig. 2c), obtaining a binding energy for 2D_EA perovskites of about 13 meV and for 2D_PEA perovskites about 250 meV.
The absorption was taken on the micron-size flakes of those two 2D perovskites (see supplementary materials), and in 2D_PEA sample we can see the exciton peak while in 2D_EA sample the exciton peak is absent ( Fig.   2d), indicating that the exciton binding energy in 2D_EA perovskite is much lower than that in 2D_PEA perovskite, which agrees with the temperature dependent measurement. Such exciton binding energy of the 2D_EA perovskites is as low as that of 3D perovskites, and one order-of-magnitude smaller than the lowest binding energy of conventional 2D perovskites reported 19 , making thermally activated exciton dissociation to free carriers much more effective than that for conventional 2D perovskites 24 .
To explore the exciton dissociation and free carrier generation processes in  Carrier mobility is another important factor that affects the device performance of perovskites. In conventional 2D perovskites, the strong quantum confinement is known to be associated with the carrier mobility two 7 order-of-magnitude lower than that of 3D perovskites 17 . But it is rarely discussed that dielectric confinement is another important origin of such low mobility: a low dielectric constant of organic layer will reduce the screening effect of charge impurities, leading to a large scattering cross section and thus short relaxation time for charge carriers 28  perovskites is similar to that in conventional 2D perovskites (Table. S1), so we attribute the improved mobility in 2D_EA perovskites to the increased relaxation time caused by enhanced screening effect resulting from the high dielectric constant of the organic layers 28 .
Thanks to low binding energy and high carrier mobility, 2D_EA perovskites are expected to present much better photo-excited carrier extraction efficiency compared to conventional 2D perovskites in terms of responsivity, photo gain, and response time. As shown in Fig. 4b, in the metalsemiconductor-metal (MSM) structure based on 2D-EA perovskites, we achieve ~10 times higher photoresponsivity than that 2D_PEA over a wide range of incident light wavelength from 360 nm to 600 nm (Fig. 4b). As shown in Fig. 4c, the photo gain of 2D_EA perovskites is also one order-ofmagnitude higher than that of 2D_PEA perovskites (we chose incident light wavelength 550 nm for 2D_EA perovskites, and 530 nm for 2D_PEA perovskites to achieve their best photo gain, respectively). Note that the 8 MSM junctions were done within one hour in vacuum after the perovskite crystal was prepared and we finished the measurement in two hours.
Basically, the photo gain is proportional to Շ 1 /Շ t , where Շ 1 and Շ t are carrier lifetime and carrier transit time, respectively 29  to the carrier mobility 30 , which is only about ~1 order-of-magnitude different in those two perovskites. As a result, the photo-excited carrier extraction time in 2D_EA perovskite/metal interface is much faster than that in 2D_PEA perovskite/metal interface. Here, based on the results of responsivity, photo gain, and response time we conclude that due to low binding energy and high carrier mobility 2D_EA perovskite show the signficantly improved carrier extraction efficiency, which is expected to have huge impact on the optoelectronic devies particularly for solar cells and photodetectors. For implementing 2D_EA perovskite in the devices, more device fabrication optimization is further needed and under investigation. 9 Our method of reducing dielectric constant in 2D perovskites can be generalized in several aspects. For example, if the halogen I is replaced by Br to synthesize EA 2 PbBr 4 , a similarly low exciton binding energy can be achieved (Fig. S4). As shown in Fig. S5, the ratio of I and Br can even be tuned to achieve color-tuning photoluminescence. Furthermore, as discussed earlier, the dielectric confinement reduction by inserting high dielectricconstant organic layers can combine with quantum confinement reduction by increasing the thickness of inorganic PbI layer in 2D_EA perovskites for achieving further decrease in exciton binding energy and increase in mobility 17,28 .
In addition to optoelectronic properties, stability under humidity is crucial metric for PVs and one of the major obstacles for commercial adoption of perovskite PV technologies 1,2 . A variety of technological methods have been proposed to address the instability problem, including "intrinsic" method adopting the hydrophobic properties of organic group for preventing the perovskite framework from the direct exposure to moisture, and "extrinsic" method done by the encapsulated technic where the polycrystalline thin film can be protected by transportation layers 2 . In this report, we focus on single crystals without any transportation layer so that we can get insight into the "intrinsic" humidity stability related to the organic groups only. 2D_EA and 2D_PEA perovskite single crystals were exposed to an environment of 60% Relative humidity (RH) and 20 o C temperature, and time-dependent photoresponsivity of MSM junction based on those crytals were measured. As shown in Fig. 5b, photoresponsivity of 2D_PEA MSM junction started to decrease in a few hours (e.x. a 30% decrease after 48 hours). The 2D_EA MSM junction show a much better stability and remain at ~90% photoresponsivity after five days relative to its starting value. Our results indicate that 2D_EA perovskites can lead to a much better humidity resistance than conventional 2D perovskites. 10 To prove the enhanced humidity stability of 2D_EA single crystal, timedependent XRD (TDXRD) was measured. The TDXRD of 2D_EA single crystal does not show any indication of degradation after 20 days exposed in 60% RH, 20 o C environment. On the other hand, the TDXRD of 2D_PEA shows a new peak which can be attribed to PbI 2 only after one day exposed in 60% RH, 20 o C environment (Fig. S6). To gain further insight into the excellent stability of 2D_EA perovskites, time-dependent X-ray photoelectron spectroscopy (XPS) of the Pb4f region was measured on 2D_EA single crystals to test the degradation as a function of exposure time in 60% RH, 20 o C environment. In Fig. S7, the peaks at 137.9 eV and 142.5 eV, corresponding to the binding energy of Pb 4f 7/2 and Pb 4f 5/2 of Pb-I in 2D_EA single crystal, do not show any indication of degradation after six days. To evaluate the chemical composition of the surface, we recorded XPS at 360 eV incidence photon energy, corresponding to a mean free path (for the outgoing ~220 eV photoelectrons) of less than 1 nm. 2D_EA perovskites (Fig.   S8) was unchanged over six days without any indication of a PbI 2 or Pb 0 peak 31 . These XRD and XPS results confirmed the outstanding humidity stability of 2D_EA perovskites as shown in time-dependent photoresponsivity results.
We attribute the remarkable enhancement of stability in 2D_EA perovskites to the strong hydrogen bonding between the organic layers ( Fig. 5a) compared to that in 2D_PEA crystal (London dispersion force), so that our hydrophilic organic groups can restrict the penetration of water and oxygen more effectively.
In summary, the effect of organic layer with different dielectric constant on the exciton binding energy and carrier mobility in 2D perovskites was investigated. We found that the dielectric confinement effect can be significantly tuned by inserting different dielectric-constant organic layers between inorganic Pb-I units. To investigate this concept, we compared two different kind of 2D perovksite, one is highly dielectric-confined and the other one has a extremely low dielectric confinement, showing that in the