Incorporation of Counter Ions in Organic Molecules: New Strategy in Developing Dopant-Free Hole Transport Materials for Efficient Mixed-Ion Perovskite Solar Cells

Hole transport matertial (HTM) as charge selective layer in perovskite solar cells (PSCs) plays an important role in achieving high power conversion efficiency (PCE). It is known that the dopants and additives are necessary in the HTM in order to improve the hole conductivity of the HTM as well as to obtain high efficiency in PSCs, but the additives can potentially induce device instability and poor device reproducibility. In this work a new strategy to design dopant‐free HTMs has been presented by modifying the HTM to include charged moieties which are accompanied with counter ions. The device based on this ionic HTM X44 dos not need any additional doping and the device shows an impressive PCE of 16.2%. Detailed characterization suggests that the incorporated counter ions in X44 can significantly affect the hole conductivity and the homogeneity of the formed HTM thin film. The superior photovoltaic performance for X44 is attributed to both efficient hole transport and effective interfacial hole transfer in the solar cell device. This work provides important insights as regards the future design of new and efficient dopant free HTMs for photovotaics or other optoelectronic applications.


Incorporation of Counter Ions in Organic Molecules: New Strategy in Developing Dopant-Free Hole Transport Materials for Efficient Mixed-Ion Perovskite Solar Cells
Organic-inorganic hybrid perovskites, with a formula of ABX 3 , (A: cations, B: Pb, X: halides), have recently attracted great attention in the solar-cell community for being efficient light absorbers.2][3] The perovskite-based solar cell (PSC) has demonstrated impressive performance by showing an increase of the power conversion efficiency (PCE) from 3% to a certified 22% during the past few years. 41][12] The motivation is justified by the fact that the preparation of the established state-of-the-art molecular HTM 2,2',7,7'-tetrakis-(N,N-di-p-methoxy-phenyl-amine) 9,9'spirobifluorene (spiro-OMeTAD) presents tedious synthetic steps; thus this material is very expensive with a price more than 10 times than that of Gold or Platinum. 13The high cost and relatively low hole conductivity of spiro-OMeTAD significantly limits its realistic large-scale application although it is so far the most commonly used HTM with outstanding performance in PSCs. 14The key function of the HTM layer in PSCs is to accept the holes from the photoexcited perovskite at the interfaces and to conduct the holes towards the contact; therefore, the HTM layer has a great impact on the charge extraction in the device. 15In this regards, high hole conductivity is one of the most primary parameters which should be considered in the design of the new HTMs.Generally organic small molecules have relatively low hole conductivities, for example ~10 -5 Scm -1 for undoped spiro-OMeTAD. 16During the operation of the PSC under illumination, a large amount of the photoinduced holes from the perovskite layer inject into the HTM layer after light excitation, followed by hole conduction through the HTM layer.During this process, rather high quantities of holes (oxidized HTM molecules) and negatively charged counter ions in HTM are highly desired in order to enhance the hole conductivity of the HTM.In this context, doping of the organic molecular HTM by chemical oxidants is an effective way to enhance the HTM hole conductivity.For example, Burschka, et al., used a cobalt-complex, coded as FK209, as dopant in order to oxidize spiro-OMeTAD for solid-state dye-sensitized solar cells (ssDSSCs), and an impressive efficiency of 7.2% was achieved. 13Currently the dopant FK209 together with LiTFSI [bis(trifluoromethane)sulfonimide lithium salt] and tert-butyl pyridine has been commonly applied in most of highefficiency PSCs.In addition, Xu, et al., developed AgTFSI as a new type of dopant and applied it successfully in the ssDSSCs and PSCs. 17Snaith, et al., demonstrated that LiTFSI can be a doping agent for the HTM in the presence of O 2 -containing atmosphere. 18In the doping process, the counter ions are required to compensate the positively charged HTM, such as spiro-OMeTAD + .In this respect, the salt LiTFSI is usually added in the HTM solution before the HTM deposition.However, it has been reported that the introduced additives in the HTM could cause severe device instability, 19 because some components, for instance LiTFSI, are hydrophilic and this can be the cause of the moistureinduced perovskite degradation. 13Besides, it is extremely challenging to precisely control the doping density in this poorly-defined oxidative process, and therefore difficulties arise with respect to device reproducibility. 20In this regard, developing dopant-free HTM is critical for making efficient and stable PSCs.To date, there are several reports on dopant-free HTMs for PSCs.For instance, Liao and coworkers designed a dopant-free polymer as HTM and a PCE of 12.3% was achieved.Fei et al., developed several star-shaped triphenylamine (TPA)-based dopant-free HTMs and the device based on the optimal of them exhibited a PCE of 15.4%. 213][24] Steck et al., developed A-D-A-type S, N-heteropentacene-based dopant-free HTM and obtained 10.3-11.4%. 25Liu et al., reported a tetrathiafulvalene derivative, TTF-1, as dopant-free HTM with a PCE of 11.0% was achieved. 26Recently, Kim et al., reported dopant-free polymeric HTM in PSC with a high PCE of 17.3%. 27Although different types of molecules or polymers have been designed to be used as dopant-free HTMs, the reported efficiencies of the PSCs based on them are still much lower than those with properly doped HTMs.Besides, there is lack of effective design strategies for guiding the future development of new dopant-free HTMs.
Here, we have introduced a new strategy for developing a new-type ionic organic molecule, X44 (see Figure 1), as dopant-free HTM for PSCs.To the best of our knowledge, this is for the first time the incorporation of counter ions directly in the organic molecules as HTM in PSCs is reported.The device based on X44 without any dopants and additives exhibited a promising PCE of 16.2% after aging for 15 days from an initial PCE of 15.2%, in comparison to two reference molecules AS37 (7.8%) and X41(<0.1%).We found that the introduced counter ions in the molecules play an important role in the hole conductivity property, HTM film formation, interfacial hole transfer yield, as well as device stability.We believe that this new strategy of designing ionic organic HTM promises to pave the way to the future efficient design of new dopant-free HTMs for PSCs.The chemical structures of AS37, X41 and X44 are depicted in Figure 1.The tail end of the alkyl chains shown in AS37 has been modified in X41 by attaching tertiary amine groups, and further in X44 to quaternary ammonium coupled with associated counter ions TFSI -.The detailed HTM synthesis (Scheme S1) and characterization (Figure S1-S6) are shown in Supporting Information.By this novel design, the anion TFSI -is successfully incorporated into the molecule X44.As discussed above, the incorporated counter ions may have a significant impact on the hole conduction process.Therefore, we investigated the possibility of using this quaternary ammonium salt as dopantfree HTM in PSCs and systematically studied the influence of the structural modulations in these three HTMs on their molecular properties and the photovoltaic performance in PSCs.
Firstly, the optoelectronic properties of three HTMs were characterized.The UV-vis absorption spectra of the three molecules dissolved in CH 2 Cl 2 are displayed in Figure S7 and summarized in Table 1.As shown, all three HTMs showed very similar maximum absorption peaks, although the λ max for X44 was slightly blue-shifted, at 381 nm, compared to X41 (λ max =383nm) and AS37 (λ max =385nm).It can be concluded that the alkyl chain in AS37 compared to the tertiary amine in X41 and quaternary ammonium in X44 has little influence on their optical absorption property.From the onset absorption wavelength we could extract the bandgaps of AS37, X41 and X44 at 2.95, 2.97 and 2.94 eV, respectively.
To evaluate the energy levels of the HTMs, cyclic voltammetry of HTMs in CH 2 Cl 2 was performed for three molecules, as displayed in Figure S7.The HOMO levels of the molecules were extracted from the first-electron redox potentials and listed in Table 1.A slightly down-shifted trend in E HOMO was demonstrated, from -4.99 eV, through -5.01 eV to -5.02 eV for AS37, X41 and X44, respectively.It can therefore be concluded that the change of the molecular structure does not significantly affect the band structures.The similar HOMO levels for three HTMs also indicate that comparable open-circuit voltage (V oc ) values are expected for the devices based on AS37, X41 and X44 if they present similar charge extraction and charge-recombination kinetics.As discussed above, the hole conductivity is a critical factor for determining the potential usability of these new HTMs in PSCs, since the hole conductivity can significantly affect the hole collection in the device.The hole conductivity measurements were performed by using a two-contact electrical conductivity setup. 16The obtained I-V curves are shown in Figure 2, and the calculated hole conductivities for different HTMs are summarized in Table 1.It can be seen that X44 exhibited a much higher hole conductivity (9.03×10 -4 S cm -1 ), more than 10 times higher than that of AS37 (7.65×10 -5 Scm -1 ) and X41 (8.95×10 -5 Scm -1 ).In comparison, the standard undoped spiro-OMeTAD has a conductivity of 8.67×10 -5 Scm -1 which was measured by using the same method. 16This demonstrates that the introduced counter ions in the HTM X44 can significantly enhance the hole conductivity, which will be highly beneficial to the device performance in terms of the charge extraction.The PSCs based on the three HTMs were fabricated with a standard device configuration of FTO/TiO 2 -underlayer/meso-TiO 2 /perovskite/HTM/Au.Figure 3a shows the J-V curves of the PSCs based on different HTMs measured under 100 mWcm -2 AM 1.5 illumination.Surprisingly, we found that the device with HTM X41 showed very low photovoltaic performance with a PCE of <0.1% because of very low short-circuit current (J sc ) of 0.58 mAcm -2 and fill factor (FF) of 0.15.In contrast, AS37-based PSC demonstrated a PCE of 7.8% with J sc of 20.05 mAcm -2 , V oc of 1.06 V, and FF of 0.37.The device based on X44 showed highest performance with a PCE of 15.2% (J sc =21.04 mAcm -2 , V oc =1.08 V, and FF=0.67).The detailed photovoltaic parameters of the corresponding PSCs are shown in Table 2.For comparison, undoped spiro-OMeTAD was used as HTM in the PSCs with same conditions, and the as-obtained device showed a low PCE of 7.5% (J sc =18.39 mAcm -2 , V oc =1.08 V, and FF=0.38; see Figure S8), as reported in a similar recent publication by Zhang et al. 28 In order to understand the reasons for the lower performance for AS37 and X41 compared to that for X44, the dark current curves of the devices were recorded, as shown in Figure 3b.Since the dark current is mainly determined by the energy-level differences between the two contacts (TiO 2 and HTM) as well as the series resistance of the components in the devices, one can get information from dark current data on these aspects.By considering the fact that the three HTMs showed similar E HOMO levels (see Table 1), the magnitude of the dark current could be mainly affected by the series resistance of HTM layer.As observed, X41-based device showed extremely low dark current compared to the PSCs with AS37 and X44.This is consistent with the fact that X41 has a low hole conductivity (8.95 ×10 -5 Scm -1 ), resulting in a high series resistance in the HTM layer which blocks the current flow through the X41-based capping layer in the device.AS37-based PSC exhibited a much higher dark current compared to that of X41 in spite of their similar hole conductivities, which could be due to the formed pinholes in the HTM-AS37 layer causing some current leakage, as observed in Figure 4b and 4c.As expected, X44-based device showed the highest dark current at high bias potentials which can be explained by its higher hole conductivity (9.03 ×10 - 5 Scm -1 ).Voltage / V V oc to J sc J sc to V oc

X44
Figure 3c shows the IPCE spectra of the devices with AS37 and X44.X44based device has higher IPCE values than that for AS37-based devices, consistent with the higher J sc obtained in X44-based PSCs. Figure 3d depicts the histogram of the performance of the obtained devices, confirming good reproducibility of PCEs.The current-voltage hysteresis behavior of the devices based on AS37 and X44 are shown in Figure 3e and 3f.The details of the parameters are shown in Table 2 and Table S1.It can be seen that the X44based device exhibited very little hysteresis, while the hysteresis for the AS37based device was more pronounced; a lower J sc but a higher FF was obtained at the scan direction from V oc to J sc than those under the reverse scan.These effects could be related to the low hole conductivity of AS37, as reported previously. 29ble 2. Photovoltaic parameters of the devices based on undoped HTMs AS37, X41, X44 and spiro-OMeTAD.In order to obtain insights into the origins of the different photovoltaic performance for the devices based on the three HTMs, the morphological properties of the solid-state HTM films on the surface of perovskite were characterized in detail.As shown in Figure 4a, high-quality and pinhole-free mixed-ion perovskite ([HC(NH 2 ) 2 ] 0.85 (CH 3 NH 3 ) 0.15 Pb(I 0.85 Br 0.15 ) 3 ) films are used in the devices.The perovskite film showed uniform grain size distribution with a diameter of ~200 nm. Figure S9 exhibits the elemental analysis in the perovskite film, showing that the lead and halide ions (Br and I) are homogeneously dispersed in the whole perovskite film.Figure 4b and 4c show the top view of the HTM AS37 layer deposited on top of the perovskite layer.The AS37 layer showed very rough and island-like aggregates.This may be attributed to its low glass-transition temperature, as observed in the previous work. 30The ease of crystallization of AS37 results to its capping layer with poor uniformity and with many pinholes, which can affect the hole transport property as well as the perovskite protection from humidity. 31Besides, the formed pinholes in AS37 film could allow some current flow, which could be the reason for the obtained higher dark current in the AS37-based device (see Figure 3b) as well as the higher photovoltaic performance than that for X41-based device.However, the pinholes could enable some direct contact between Au and the perovskite, thereby resulting to a low shunt resistance and thus fast charge recombination in the device, as confirmed by the S-shaped J-V curve, and low FF of 0.37 for the AS37-based devices.In contrast, the X41 and X44 showed a much more uniform HTM capping layer.Both the capping layer compactness and low hole conductivity of X41 lead to a strong charge-conduction blocking effect and thus to poor solar-cell performance.Thus, it can be learnt that the molecular structural change from the alkyl chain to tertiary amine, and quaternary ammonium has a great impact on the uniformity of HTM capping layer.As illustrated in Figure 5a, after light excitation of the perovskite-based light absorber, the hole-transfer process at the perovskite/HTM interface and charge transport in the bulk of HTM play a critical role in the charge collection and thus to the device performance.Since the only difference in the devices investigated here is the HTM layer, the device performance can be directly linked to the HTM properties.Figure 5b shows the photoluminescence (PL) spectra of samples containing FTO/TiO 2 /perovskite with and without HTM layer.The intensity of the PL can on the one hand give useful information on the yield of the radiative recombination and on the other hand reflects the efficiency of the charge transfer at the interfaces between perovskite and charge-selective layers.The substrate without the HTM layer demonstrated a very strong PL signal, meaning that the photogenerated charges recombine in the form of radiation.In contrast, after deposition of the HTM layer on perovskite, the PL intensity significantly

Wavelength / nm
No HTM decreased, illustrating that the photo-induced charge carriers transfer to the contact materials so as to reduce the radiative recombination.Specifically, the X44-based substrate showed a very efficient PL quenching compared to that for AS37 or X41.Therefore, it can be concluded that a more efficient hole transfer at the perovskite/HTM interface occurs for X44 than AS37 and X41.The less efficient hole transfer for X41 compared to the other two HTMs could be one of the main reasons for the lower performance of the X41-based device.We suggest that the low hole-transfer yield for the X41-based device could be related to the strong electrostatic interaction between perovskite and the tertiary amines in X41, which may lead to a barrier for the interfacial hole transfer, as similarly found in one previous study that the tertiary amines and quaternary ammonium showed very different electrostatic binding property in biological applications. 32In order to study the charge transport properties in the PSCs, transient photocurrent decay (TPCD) measurements were performed at the complete devices by applying a small-modulation light on top of a constant light intensity (100 mWcm -2 in this work) illumination on the device at the shortcircuit condition.By having this perturbation of light the photo-induced current rise and decay processes can be recorded with time, as shown in Figure 5c.By normalizing the TPCD curves, a qualitative comparison of the charge-extraction kinetics for the three HTMs is feasible.As shown in Figure 5d, the X44-based PSC showed faster photocurrent decay than the devices based on AS37 and X41, demonstrating that the charge extraction for X44 is far more efficient compared to the other two.The more efficient charge extraction in the X44-based device can explain the obtained higher J sc and FF in PSCs with X44, as shown in Figure 3a.It can be suggested that this efficient charge extraction in the X44based device is attributed to the high hole conductivity and uniform cappinglayer formation capability of X44.
] Therefore, at first the stability of the devices based on X44 and AS37 at the maximum power point (MPP) condition was monitored, as shown in Figure 6a, 6b, 6c.It can be seen that the device with X44 shows very stable parameters of V mpp , I mpp and P mpp and remain >90% of the initial performance after 1500s at MPP conditions (light intensity: 100 mWcm -2 ).In contrast, the AS37-based device showed a fast degradation in all of V mpp , I mpp and P mpp , so that >40% of the initial performance is lost under the same conditions.In addition, the long-term stability of the device was investigated with controlled aging conditions (room temperature and <20% humidity in N 2 atmosphere in dark without encapsulation).AS37-based device maintained the photovoltaic performance by showing a PCE of 7.4% after 15 days compared to the initial PCE of 7.8%, but with improved FF and lower J sc .The detailed parameters are listed in the inset table of Figure 6d.In comparison, the device with X44 showed a slight increase of efficiency from the initial 15.2% to 16.2% after 15 days with both improved J sc and FF (see Figure 6e).It can be concluded that the ionic HTM X44 with counter ions showed promising stability in PSCs and holds a great potential for future applications.In conclusion, we have developed a new type of ionic organic molecule as dopant-free HTM for PSCs.Neither the light absorption properties nor energy level positioning of the molecules is not affected after the incorporation the ions in the molecules.However, by accompanying counter ions in the quaternary ammonium based molecule X44, the hole conductivity of X44 has been significantly improved (9.03×10 -4 Scm -1 ) compared to a tertiary amine-based reference molecule X41(8.95×10-5 Scm -1 ) and its alkyl-chain based analogue Voltage / V AS37 (7.65×10 -5 Scm -1 ).In contrast to the low performance of X41 and AS37 based devices with PCEs of 0.08% and 7.8%, respectively, this ionic molecule X44 has been successfully employed as dopant-free HTM in mixed-ion PSCs by exhibiting an impressive PCE of 16.2% after 15 days aging from an initial PCE of 15.2%.It has been observed that the counter ions in the molecule X44 presented great contributions to the more charge extraction in the device and also to the higher hole transfer yield at the interfaces.Besides, the device with HTM X44 demonstrated an impressive stability at MPP condition by maintaining more than 90% of initial performance.In addition, the PSC with X44 showed improved J sc and FF after 15-days aging under controlled humidity without encapsulation.We believe this new strategy of designing ionic HTM via incorporation of counter ions will open one new promising way for developing efficient dopant-free HTMs for PSCs in the future.

Figure 3 .
Figure 3. (a) J-V curves under light intensity of 100 mWcm -2 , (b) J-V curves in dark, (c) Incident Photon-to-current-conversion efficiency (IPCE) spectra, (d) device efficiency statistics for the devices based on HTM AS37, X41 and X44.(e) J-V curves at different scan directions of the devices based on AS37.(f) J-V curves at different scan directions of the devices based on X44.

Figure 5 .
Figure 5. (a) Pathways of interfacial charge transfer and charge transport in the perovskite solar cells; (b) photoluminescence spectra of the electrode FTO/ZrO 2 /perovskite with and without HTMs; (c) transient photocurrent rise and decay curves and (d) normalized transient photocurrent decay spectra.

Figure 6 .
Figure 6.The device stability test under the maximum power point conditions by showing the change of V mpp (a), J mpp (b) and P mpp (c).Long-term stability test of the PSCs in a controlled humidity (<20%) in dark without encapsulation after 15 days for AS37 (d) and X44 (e).

Table 1 .
Summary of the optical, electrochemical and photoelectric properties of HTMs UV-vis absorption is conducted in CH 2 Cl 2 solution.b) E 0-0 is extracted from the onset absorption wavelength.c) E HOMO calculated from the first-electron redox potentials, with an equation E HOMO =-4.4-E redox-1 .The redox potentials were calibrated vs. standard hydrogen electrode (SHE) with E (Fc/Fc+) = 624 mV vs SHE as conversion.d) E LUMO =E HOMO +E 0-0 .