Strategy to Boost the Efficiency of Mixed-Ion Perovskite Solar Cells

Jun 15, 2016 - Organic Chemistry, Center of Molecular Devices, Department of Chemistry, Chemical Science and Engineering, KTH Royal Institute of Techn...
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Strategy to Boost the Efficiency of Mixed-Ion Perovskite Solar Cells: Changing Geometry of the Hole Transporting Material Jinbao Zhang,†,∥ Bo Xu,‡,∥ Malin B. Johansson, Nick Vlachopoulos,§ Gerrit Boschloo,† Licheng Sun,*,‡ Erik M. J. Johansson,*,† and Anders Hagfeldt*,§ †

Physical Chemistry, Center of Molecular Devices, Department of Chemistry−Ångström Laboratory, Uppsala University, SE-75120 Uppsala, Sweden ‡ Organic Chemistry, Center of Molecular Devices, Department of Chemistry, Chemical Science and Engineering, KTH Royal Institute of Technology, SE-10044 Stockholm, Sweden § Laboratory of Photomolecular Science, Institute of Chemical Sciences and Engineering, École Polytechnique Fédérale de Lausanne, EPFL-FSB-ISIC-LSPM, Chemin des Alambics, Station 6, CH-1015 Lausanne, Switzerland S Supporting Information *

ABSTRACT: The hole transporting material (HTM) is an essential component in perovskite solar cells (PSCs) for efficient extraction and collection of the photoinduced charges. Triphenylamine- and carbazole-based derivatives have extensively been explored as alternative and economical HTMs for PSCs. However, the improvement of their power conversion efficiency (PCE), as well as further investigation of the relationship between the chemical structure of the HTMs and the photovoltaic performance, is imperatively needed. In this respect, a simple carbazolebased HTM X25 was designed on the basis of a reference HTM, triphenylamine-based X2, by simply linking two neighboring phenyl groups in a triphenylamine unit through a carbon−carbon single bond. It was found that a lowered highest occupied molecular orbital (HOMO) energy level was obtained for X25 compared to that of X2. Besides, the carbazole moiety in X25 improved the molecular planarity as well as conductivity property in comparison with the triphenylamine unit in X2. Utilizing the HTM X25 in a solar cell with mixed-ion perovskite [HC(NH2)2]0.85(CH3NH3)0.15Pb(I0.85Br0.15)3, a highest reported PCE of 17.4% at 1 sun (18.9% under 0.46 sun) for carbazole-based HTM in PSCs was achieved, in comparison of a PCE of 14.7% for triphenylamine-based HTM X2. From the steady-state photoluminescence and transient photocurrent/photovoltage measurements, we conclude that (1) the lowered HOMO level for X25 compared to X2 favored a higher open-circuit voltage (Voc) in PSCs; (2) a more uniform formation of X25 capping layer than X2 on the surface of perovskite resulted in more efficient hole transport and charge extraction in the devices. In addition, the long-term stability of PSCs with X25 is significantly enhanced compared to X2 due to its good uniformity of HTM layer and thus complete coverage on the perovskite. The results provide important information to further develop simple and efficient small molecular HTMs applied in solar cells. KEYWORDS: mixed-ion perovskite, hole transporting materials, triphenylamine, carbazole, stability

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the solar energy industry. In high-performance PSCs, the free charges must be efficiently extracted and collected at the external circuit by using an electron conductor and hole transporting material (HTM) as interfacial contact materials, in order to impede the charge recombination in the device.10 In spite of the fact that a PCE of ∼10% has been reported for

rganic−inorganic hybrid perovskites (e.g., CH3NH3PbX3, X: halogen) were successfully applied as an excellent light absorber for perovskite solar cells (PSCs) in recent years.1−3 During the past several years, the power conversion efficiency (PCE) of PSCs has been improved from 3.9% to above 20%.4,5 The remarkable PCE is attributed to the remarkable nature of the perovskite crystals, for example, the tunable band gap, ambipolar charge transport, broad and intense light absorption, and high charge carrier mobility.6−9 The extraordinary photovoltaic performance of PSCs has attracted a great interest in the scientific community as well as © 2016 American Chemical Society

Received: April 11, 2016 Accepted: June 15, 2016 Published: June 15, 2016 6816

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Figure 1. Molecular structures of the HTM X2 (a) and X25 (b). (c) Normalized light absorption and emission curves for X2 and X25. (d) Cyclic voltammetry of X2 and X25 in dichloromethane solution. MM2 energy minimization of the HTM, X2 (e) and X25 (f) (hydrogens are not shown for clarity; carbon atoms have gray color, nitrogen atoms show blue color, and oxygen atoms presents in red color).

HTM-free PSCs,11 a great effort has been devoted to interfacial engineering by designing and testing HTMs.12,13 Conventionally, the most efficient PSCs reported so far utilize 2,2,7,7tetrakis(N,N-di-p-methoxy-phenyl-amine)-9,9-spirobifluorene (spiro-OMeTAD) as HTM in PSCs,14 but spiro-OMeTAD possesses the disadvantages of high cost, tedious synthesis, and rather low hole conductivity.15 Therefore, it is very important to design efficient and inexpensive alternative HTMs for the improvement of the photovoltaic performance and the future application of PSCs. In this respect, many kinds of molecular HTMs were developed and tested in PSCs, e.g., triphenylamine,16 carbazole,17 phenoxazine,18 acene derivatives,19 azomethine,20 and triazatruxene.21 Triphenylamine and carbazole are widely studied structural motif among the reported HTMs, and they were often employed in photovoltaics due to their simple chemical structures, low cost and outstanding charge transport properties. For example, Meng and co-workers successfully designed a series of simple triphenylamine-based

HTMs for PSCs, showing a PCE of 12%.16 Ko et al., reported a PCE of 12.8% using substituted triphenylamine with ethylene unit as HTM in PSCs.22 After that, Ko and co-workers further developed planar triphenylamine hole conductors and produced a PCE of 13.6%.23 In addition, some symmetric triphenylmine-based oligomers were designed as HTMs for PSC and a maximal efficiency of 14.2% was obtained.24 In our recent work, we found the alkyl chains in the triphenylamine based HTMs play important roles in their hole transport property in PSCs.25 On the other hand, the recent interest in the carbazole derivatives as charge transporters has driven a handful of work on the carbazole-based HTMs for light emitting diodes,26 dye-sensitized solar cells,27 and PSCs.28 The HTMs with carbazole moiety have exhibited versatile features such as the low cost of carbazole precursor, easy functionalization to tune the optoelectronic properties, good photochemical stability. Sun et al.28 designed carbazole-based HTMs for PSCs and a PCE of ∼10% was obtained. Wu and co-workers reported 6817

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ACS Nano Table 1. Redox Potentials, Optical Properties, and the Conductivities of HTMs

a

HTM

λabs,max [nm]

λemission,max [nm]

Eredox−1 [V vs SHE]

E0−0 [eV]

EHOMO [eV]a

ELUMO [eV]b

conductivity [S cm−1]

X2 X25

370 335

430 420

0.69 0.76

3.05 3.20

−5.09 −5.16

−2.04 −1.96

1.3 × 10−4 2.8 × 10−4

HOMO is calculated by EHOMO = −4.4 eV − Eredox−1.35 bELUMO value calculated by ELUMO = EHOMO + E0−0

Figure 2. (a) Scheme of the PSC device structure. (b) Charge transport pathways and the energy levels of different components in PSCs. Considering the valence band of double-mixed perovskite is at −5.65 eV (vs vacuum).39,40 (c) Top view of mix-ion perovskite layer. (d) Top view of X25 on perovskite layer. (e, f) Top view of HTM X2 on top of the perovskite layer.

was obtained by utilizing a simple carbazole-based HTM in PSCs by Nazeeruddin et al.31 However, the reported solar cells with HTMs based on triphenylamine or carbazole moiety are still less efficient than the state-of-the-art PSCs based on spiroOMeTAD. Therefore, more effort has to be made to (a) develop high-performance HTMs based on simple chemical

a facile synthesis of simple carbazole molecules as HTMs and the device exhibited a PCE of more than 12%.29 Later, Kim et al. synthesized star shaped carbazole-based HTMs and a PCE of 13% was reported.30 Lee et al. applied three-arm-type carbazole derivatives in PSCs which exhibited a PCE of 14.8%.17 In the recent work, a promising efficiency of 16.9% 6818

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X25 was higher compared to that in X2 could indicate that the transition upon excitation causes a significant change of the charge delocalization in the electronic conjugation.33 Meanwhile, it can be seen from Figure 1c that the emission spectra for X2 and X25 have maximum emission peak at 430 nm (similar to the result in ref 32) and 420 nm, respectively. From the intersection of the emission and the absorption curves, we can calculate the optical band gap values of 3.05 and 3.20 eV for X2 and X25, respectively, as listed in Table 1. The oxidation potentials of X2 and X25, corresponding to their HOMO levels, should be suitable relative to the perovskite energy levels in order to keep a high Voc of the device and at the same time efficient interfacial hole transfer kinetics. It is known that the HOMO of a carbazole-based donor is generally slightly lower than that of a triphenylaminebased donor.29 The HOMO levels of X2 and X25 were determined by cyclic voltammograms (CVs), which are exhibited in Figure 1d. The obtained oxidation potential values by CV are depicted in Table 1. X25 exhibited a more positive redox potential of 0.76 V/SHE as expected compared to 0.69 V/SHE for X2 (similar to what was previously measured for X2 in ref 32). The higher oxidation potential for X25 could be favorable for obtaining a high Voc in PSCs,34 while the relatively lower oxidation potential of X2 could result in a faster hole transfer between the perovskite layer and X2. In contrast, spiroOMeTAD presents a first-electron oxidation potential at 0.57 V/SHE, which is measured under the same condition (see Figure S5). Photovoltaic Performance. In order to test X2 and X25 as potential HTMs for PSCs, devices were fabricated with a configuration as monitored in Figure 2a. The energy level of the components in PSCs is listed in Figure 2b. The detailed device configuration can be viewed from the cross-sectional images in Figure S6. It is found that the ions in the perovskite composition (cations and anions) play important roles in the optoelectronic properties of perovskite crystals.25 The mixedion perovskite, 36−38 with an optimal combination of formamidinium lead iodide (HC(NH2)2PbI3) and methylammonium lead bromide (CH3NH3PbBr3), [HC(NH2)2]0.85(CH3NH3)0.15Pb(I0.85Br0.15)3, has recently been widely investigated due to its optimal band gap, promising photochemical stability and extraordinary photovoltaic performance.14 Therefore, the mixed-ion perovskite was used in this work and Figure 2c shows the top view morphology of the perovskite crystals, exhibiting an average grain size of 200−300 nm. The optimized thickness of the meso-TiO2/perovskite and perovskite capping layer was shown to be approximately 300− 400 nm, as shown in Figure S6. A uniform surface coverage of the perovskite crystals by HTM layer is very important for reducing interfacial charge recombination to obtain high charge collection efficiency and thus a high PCE of PSCs. The solubility of HTMs can significantly affect the solid HTM film formation. Therefore, the surface morphology of X2 and X25 on the perovskite film was investigated by SEM and is depicted in Figure 2d and e, respectively. It was found that the HTM X2 presented poor surface uniformity on the top of the perovskite film with flowerlike aggregates.25 However, X25 showed uniform thin film above the perovskite layer. The different HTM capping layer for X2 and X25 are expected to influence the charge transport properties in PSCs. The devices with the HTM X25 exhibited a high efficiency of 17.4% (Jsc = 22.5 mAcm−2, Voc = 1100 mV, and fill factor (FF)

structures, such as triphenylamine and carbazole; (b) reveal the fundamental relationship between the chemical structures of the HTMs and the device performance in order to obtain a basic guide for the future design of functional molecules for photovoltaics. In this work, a carbazole derivative (X25) developed from a triphenylamine molecule (X2) was designed and synthesized by simply linking two adjacent phenyl groups in triphenylamine through a carbon−carbon single bond. By this design, the aim is to study how the minor structural change from triphenylamine- to carbazole-based moiety influences the optoelectronic performance of the HTMs, and thereby the charge transport properties in PSCs. It was found that the formed carbazole structure from the triphenylmine core in X25 significantly influences the molecular planarity, the energy level and the charge transport property, compared to the reference HTM X2.32 The HTM X25 was investigated as HTM in PSCs and a highest reported PCE of 17.4% for carbazole-based HTMs was achieved by integrating X25 with the mixed-ion perovskite [HC(NH2)2]0.85(CH3NH3)0.15Pb(I0.85Br0.15)3 in PSCs. The origins of the dramatic improvement of PCEs from 14.7% (X2) to 17.4% (X25) were systematically investigated by morphological measurement, photoluminescence, transient photocurrent/photovoltage measurements. We concluded that the formed carbazole moiety in X25 gives a down-shift of the highest occupied molecular orbital (HOMO) level compared to X2. The lowered HOMO level for X25 was found to produce a higher open-circuit voltage (Voc) in devices but also a slower hole injection between the perovskite and the HTM layer due to the smaller energy difference. Additionally, the molecular solid film formation after spin coating was dramatically influenced by the structural change from triphenylamine to carbazole. The uniformity of the HTM X25 capping layer was significantly improved compared to X2, and a much faster hole transport was obtained in the bulk of HTM X25 than that in X2. The improved hole transport property and the downshifted HOMO level for X25 together contributed to a higher short-circuit current (Jsc), Voc and PCE in PSCs compared to X2. These results demonstrate that the minor structural difference from triphenylamine to carbazole significantly influences the molecular geometry and optoelectronic parameters as well as the hole conductivity of the HTMs. Thus, we believe that the present work will undoubtedly demonstrate an important guide for the further development of inexpensive and efficient molecular HTMs for perovskite based solar cells.

RESULTS AND DISCUSSION Optoelectronic Performance. The aim is to develop HTMs which have similar chemical structure but different properties in this work in order to reveal the basic principles for the future HTM design. The chemical structures of X2 and X25 are exhibited in Figure 1a and b. The detailed synthesis process and the material characterization can be found in Figures S1− S3. It can be seen that there is a carbon−carbon single bond between two phenyl groups in the central triphenylamine core in X25, compared to X2. The formed carbazole structure in X25 gives a more planar structure due to the covalent bond between two phenyl groups. The normalized UV−vis absorption and emission spectra of X2 and X25 dissolved in dichloromethane are shown in Figure 1c, while the determined molecular extinction coefficient is listed in Table 1 and Figure S4. The maximal absorption peaks of X2 and X25 were located at 370 and 335 nm, respectively. The extinction coefficient in 6819

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Figure 3. J−V curves of PSCs based on X2 (a) and X25 (b). IPCE curves of the PSCs based on X2 (c) and X25 (d). Inset diagram: Histogram of devices efficiencies based on X2 (c) and X25 (d).

= 70%, (1.0 sun: 100 mW cm−2, at AM 1.5G condition), which is the highest reported PCE for carbazole-based HTMs in PSCs. A PCE of 18.9% was achieved for X25 at a low light intensity of 46 mW cm−2. For comparison, X2-based PSCs show a PCE of 14.7% (Jsc = 21.7 mAcm−2, Voc = 1055 mV, and FF = 0.64). The current−voltage characteristics of PSCs with X2 and X25 can be seen in Figure 3a and b. The current− voltage curves with different scan directions for the devices with X2 and X25 are shown in Figure S7. The presented high current−voltage hysteresis for X2 and X25 can be because of the low conductivity of the electron conductor TiO2 (without lithium doping) used in this work, as explained in the recent work by Gratezel and co-workers.41 In addition, the relatively larger J−V hysteresis in the devices with X2 than that for X25 imply that the lower conductivity of X2 could be another reason for the high J−V hysteresis. The incident photon-tocurrent conversion efficiency (IPCE) curves of the devices based on X2 and X25 are shown in Figure 3c and d. The PSCs based on X25 showed a higher IPCE than that of X2, in good agreement with the higher Jsc for X25 based PSCs compared to X2. The photovoltaic performance of the devices with X2 and X25 at various light intensities is listed in Table 2, and the light intensity dependence of the Jsc and Voc are depicted in Figure S8. In addition, the histograms of PCEs of X2 and X25 based PSCs are exhibited as inset diagrams in Figure 3c and d. It is noteworthy that the Voc (1100 mV) of X25 based PSC in this study was achieved compared to a Voc (1055 mV) for devices with X2, which can be explained by the lower HOMO level of X25 in relative to that of X2. Additionally, X25-based PSC presented higher Jsc and FF compared to those of X2. These higher photovoltaic parameters obtained for X25 than X2 could

Table 2. Photovoltaic Performance of PSCs with the HTMs X2 and X25, Measured at Different Light Intensities HTM X2

X25

light intensity [mW cm−2]

PCE [%]

Voc [mV]

Jsc [mA cm−2]

FF

100 85 46 32 100 85 46 32

14.7 14.9 16.4 15.6 17.4 17.1 18.9 18.4

1055 1040 1030 1010 1100 1070 1070 1040

21.7 19.3 10.9 7.48 22.5 19.6 11.4 7.7

0.64 0.63 0.67 0.66 0.70 0.69 0.72 0.73

be related to the hole collection ability through HTMs since the same electron conductor was used for both cases. Charge Transfer and Transport Properties. Figure 4 illustrates the photovoltaic parameters of the optimal PSCs based on X2 and X25. It can be seen that the Voc, Jsc, and FF for devices with X25 are all higher than those for devices based on X2. These together contributed to a higher PCE in X25-based PSC compared to X2. To further analyze the higher photovotaic performance of PSCs based on the carbazole derivative X25 compared to the triphenylamine-based HTM X2, different charge transport pathways will be discussed later, including hole injection from the perovskite layer to the HTMs after light excitation, the charge carriers (holes and electrons) recombination in the PSCs, and the hole hopping in the X2 or X25 capping layer.25 These three pathways play important roles in the charge extraction in the PSCs and thereby the photovoltaic performance of the PSCs. 6820

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Figure 4. Box charts of the photovoltaic parameters of the optimal PSCs based on X2 and X25.

Figure 5. (a) Photoluminesence curves of perovskite film with and without HTMs. (b) Normalized photovoltage decay at the open-circuit condition, and (c) normalized photocurrent decay at the short-circuit condition, for PSCs with X2 and X25 measured under a small modulation (10 mV) on a simulated light intensity of 100 mW cm−2. (d) Normalized photocurrent decay at the short-circuit condition for PSCs with X2 and X25 at different light intensities.

which is presented in Figure 5a. The devices prepared for this measurement have the structure: glass substrate/ZrO2/perovskite/HTM.

First, in order to investigate the effects of molecular structure from triphenylamine to carbazole on the hole transfer at the interface, steady state photoluminescence (PL) was measured, 6821

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properties, interfacial charge recombination and thus in charge collection efficiency the PSCs. In addition, the long-term stability of PSCs is essential to reach the requirements for the future commercialization of PSCs. The resistance to water ingress for HTM is important to prevent the degradation of perovskite crystals in the humid condition.43 Therefore, the stability test of PSCs with X2 and X25 under a controlled humidity of 10% was performed, as shown in the Figure 6. It can be seen that a significant loss

As shown in Figure 5a, the device with bare perovskite layer (no HTMs) presented a high light emission intensity caused by the radiative recombination between the photogenerated electrons and holes. Since an incident light of 500 nm was employed in the PL measurements to excite the perovskite layer, there is no light absorption or emission from HTMs X2 and X25 at this wavelength, as shown in Figure 2c. The perovskite shows a stronger PL compared to in our recent publication, and the reason is related to a higher quality of the perovskite film, with a smoother surface.25 A significant reduction of the emission signal was obtained after a layer of the HTM was deposited on the perovskite film. The different PL quenching efficiencies for X2 and X25 indicate a more efficient hole transfer yield (96%) between the perovskite and the HTM X2 compared to that between the perovskite and X25 (92%). Again this result can be explained by the lower oxidation potential of X2 than X25, providing a larger driving force for the hole transfer from the perovskite to the HTM layer. Therefore, from triphenylamine to carbazole unity in the HTM, a slightly less favorable interfacial hole transfer was obtained due to the increased HOMO level although it in turn contributed an increased Voc in PSCs. In order to further confirm the origin of the higher Voc in X25 based PSCs compared to X2, time-resolved photovoltage decay at an opencircuit condition was measured via introducing a perturbation light on top of the simulated 1 sun light intensity (100 mW cm−2).25 This measurement can provide important information on the charge recombination kinetics in the devices. It can be seen from Figure 5b that both devices showed very similar trend of the transient photovoltage decay, demonstrating a similar electron lifetime in the PSCs based on X2 and X25. This can further indicate that the Voc difference obtained for X2 and X25 was attributed to the different HOMO level of two HTMs, rather than the interfacial charge recombination kinetics. Moreover, the hole conductivity of X2 and X25 layer is another key parameter affecting the charge extraction efficiency in PSCs. To analyze this, transient photocurrent measurements were conducted as shown in Figure 5c. This measurement was also conducted via introducing a perturbation light on top of a light intensity of 100 mW cm−2 to the PSCs at a short-circuit condition.25 Because the only difference in the two PSCs is the HTM layer, the different photocurrent decay time observed for two PSCs are mainly because of the different hole conductivity of the HTMs. Additionally, the derived time constant is around milliseconds. However, the charge transport should be faster in the perovskite film or the mesoporous TiO2.25 Thus, the difference in the decay kinetics is related to the different hole conductivities in X2 and X25.42 The devices with X25 showed a faster decay than that in PSCs with X2, demonstrating a faster charge transport property in devices. This results agree well with the lower hole conductivity of X2 (1.3 × 10−4 S cm−1) than that for X25 (2.8 × 10−4 S cm−1), as shown in Table 1 and Figure S9. Figure 5d monitors the charge transport time of PSCs at different light intensities and the X25-based PSCs showed a much faster charge transport behavior than that for X2. This could point to higher resistance in the X2 compared to that in the X25 films, which may be caused by the uneven, islandlike X2 capping layer on perovskite. Poor hole transport property for X2 could be one of the origins of the low Jsc and FF in the corresponding PSCs. Therefore, the minor structural change from triphenylamine to carbazole in the HTM can cause a significant improvement in the charge transport

Figure 6. Stability test of PSCs based on X2 (a) and X25 (b) HTMs under room temperature in the dark. The solar cells are stored in a box with a relative humidity of 10% filled with the N2 gas.

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