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Novel Small Molecular Materials Based on Phenothiazine Core Unit for Efficient Bulk Hetero-junction Organic Solar Cells and Perovskite Solar Cells Ming Cheng, Cheng Chen, Xichuan Yang, Jing Huang, Fuguo Zhang, Bo Xu, and Licheng Sun Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b00001 • Publication Date (Web): 21 Feb 2015 Downloaded from http://pubs.acs.org on February 24, 2015

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Chemistry of Materials

Novel Small Molecular Materials Based on Phenothiazine Core Unit for Efficient Bulk Hetero-junction Organic Solar Cells and Perovskite Solar Cells Ming Cheng, ‡ a, b Cheng Chen, ‡ a Xichuan Yang,* a, c Jing Huang, d Fuguo Zhang, a Bo Xu, b Licheng Sun* a, b a

Institute of Artificial Photosynthesis, State Key Laboratory of Fine Chemicals, DUT–KTH Joint Education and Research Centre on Molecular Devices, Dalian University of Technology (DUT), 2 Linggong Rd, 116024 Dalian, China. b Department of Chemistry, KTH Royal Institute of Technology, SE-10044 Stockholm, Sweden. c Hebei Key Lab of Optic-electronic Information and Materials, College of Physics Science and Technology, Hebei University, Baoding 071002, P. R. China. d Department of Theoretical Chemistry & Biology, School of Biotechnology, Royal Institute of Technology (KTH), Se10691 Stockholm, Sweden. ABSTRACT: Two novel Acceptor-Donor-Acceptor (A-D-A) structured small molecular (SM-) materials POZ2 and POZ3 using an electron-rich phenothiazine (POZ) unit as core building block were designed and synthesized. Their unique characteristics, such as suitable energy levels, strong optical absorption in the visible region, high hole mobility and high conductivity, prompted us to use them both as p-type donor materials (DMs) in SM-bulk heterojunction organic solar cells (BHJ OSCs) and as hole transport materials (HTM) in (CH3NH3)PbI3-based perovskite solar cells (PSCs). The POZ2-based devices yielded promising power conversion efficiencies (PCEs) of 7.44% and 12.8% in BHJ OSCs and PSCs respectively, which were higher than the PCEs of 6.73% (BHJ-OSCs) and 11.5% (PSCs) obtained with the POZ3-based devices. Moreover, our results demonstrated that the POZ2 employing the electron-deficient benzothiazole (BTZ) as linker exhibited higher hole mobility and conductivity than that of the POZ3 using thiophene as linker, leading to better device performance both in BHJ-OSCs and PSCs. These results also provide guidance for the molecular design of high charge carrier mobility SM-materials for highly efficient BHJ OSCs and PSCs in the future.

INTRODUCTION As a promising photovoltaic device, solution-processed bulk heterojunction (BHJ) organic solar cells (OSCs) are attracting more and more attention all over the world due to their potential application in flexible, lightweight and cost-effective photovoltaic products.1-12 P-type donor materials (DMs), including both of small molecules and polymers, which are photosensitive in active layer, play important roles to achieve high efficiency. Small molecular (SM-) DMs have been considered as promising photosensitive materials for large area and efficient BHJ OSCs, due to their advantages of defined molecular structure, high purity, and less batch-to-batch variations compared with the polymer counterparts and attract lot of attention all over the world. 13-29 To date, power conversion efficiencies (PCEs) of over 9% have been achieved for SM-BHJ OSCs through the modification of the molecular structure of DMs, morphology control of the active layer and the improvement of device fabrication technology.30-32 For example, a series of SM-DMs employing dithieno(3,2-b;2’,3’-d)-silole (DTS) as the core blocking unit, together with thiophene (TP) and benzo-2, 1, 3-thiadiazole (BTZ) derivatives as π-conjugation were firstly reported by Bazan and Heeger et al.. 30, 33-39 Subsequently, the PCEs were further improved to 9.02% after the modification of the molecular structure and optimization of the device fabrication conditions. 30 Recently, Chen et al. and Yang et al. reported a series of small molecules with a ben-

zo[1,2b:4,5b’]-dithiophene (BDT) unit as the core blocking unit, TP derivatives as π- conjugation and end-capped with electron-withdrawing units (dicyanovinyl, alkyl cyanoacetate or 3-alkyl-rodanine). 31, 32, 40-43 A record PCE of 9.95% was achieved using BDT based SM-DM (DR3TSBDT) for single junction solar cells. 31 However, the onerous synthesis of BDT and DTS core blocking units require four steps from commercially available materials under harsh reaction conditions. 43-46 In addition, the raw materials are not cheap. These factors result in the extremely high cost of BDT and DTS core blocking units, limiting their scale-up application in BHJ OSCs. Therefore, the development of next generation core blocking unit with low-cost, facile synthesis, thermally and electrochemically robust for efficient SM-BHJ OSCs is highly desired. Moreover, some SM-DMs exhibit high hole mobility and conductivity, which also have potential to be used as hole transport materials (HTMs) 24, 47, 48 in efficient perovskite solar cells (PSCs), which have recently emerged as promising photovoltaic devices. 49-53 Phenothiazine (POZ) unit is nonplanar fused ring systems with dihedral angles of ∼169°, and has been extensively used to construct functional materials for the application in dye sensitized solar cells (DSSCs) and organic light-emitting diodes (OLEDs), presenting unique electronic and optical properties. 54-59 However, it is rather surprising that POZ has been scarcely explored as core blocking unit to construct SM-DMs

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for SM-BHJ OSCs or HTMs for PSCs, in spite of being stronger electron donors. Herein, we designed and synthesized two novel acceptor–donor–acceptor (A–D–A) conjugated small molecules POZ2 and POZ3 employing POZ as a core building block, BTZ or TP as π-conjugations and dicyanovinyl as electron-withdrawing end-group (see Figure 1). With their unique properties, such as suitable energy levels, strong optical absorption in the visible region and high hole mobility, POZ2 and POZ3 have been used both as SM-DMs in BHJ OSCs and as HTMs in CH3NH3PbI3 based PSCs. Promising PCE of 7.44% under 100 mW·cm-2 irradiation was achieved using POZ2 as SM-DM in BHJ OSCs, which is higher than that of POZ3 based ones. When applied in PSCs, POZ2 in which containing two BTZ units as π-conjugations also perform better than POZ3, producing efficiency of 12.8%.

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the absorption peaks (λmax) are observed at 519 nm (molar extinction coefficients ε = 36900 L·mol−1·cm−1). The redshifted spectrum of POZ2 (λmax = 564 nm, ε = 35900 L·mol−1·cm−1) compared to that of POZ3 can be mainly attributed to the introduction of electron-deficient BTZ unit, considering the similar molecular structure. Spin-coated as a thin film, obvious red-shifts of 39 nm and 34 nm were detected for POZ2 and POZ3, respectively. These significant redshifts indicate strong intermolecular π-π stacking of POZ2 and POZ3 in the presumably sterically more restricted film. Again, such effects are expected to promote a higher hole mobility and thus lead to better photovoltaic performance in BHJ OSCs and PSCs. Besides, compared with POZ3, POZ2 exhibits wider absorption which is beneficial to photovoltaic performance.

RESULTS AND DISCUSSION The structure of the small molecular materials POZ2 and POZ3 are shown in Figure 1. The POZ central unit was functionalized with π-bridge (BTZ or TP) in order to generate a low band gap material. The incorporated electron-deficient πconjugation BTZ can be treated as an electron trap, which facilitating the electron transfer from the donor part (POZ unit here) to the acceptor part (dicyanovinyl unit). The symmetrical structure is expected to enhance π−π stacking interactions, which could be beneficial for promoting a high hole mobility and also to enhance the lifetime of a charge-separated excited state. The dicyanovinyl unit was introduced for adjusting the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of POZ2 and POZ3. The aim of the introduction of the long alkyl chains is to increase the solubility of POZ2 and POZ3 in organic solvent.

Figure 1. Structure of the colourful small molecular materials POZ2 and POZ3 The UV-Vis absorption spectra of POZ2 and POZ3 are shown in Figure 2 and the corresponding data are summarized in Table 1. Both POZ2 and POZ3 showed broad and strong optical absorption in the visible region in dichloromethane (CH2Cl2) solution and in thin film state. For POZ3, in CH2Cl2,

Figure 2. UV-Vis absorption spectra of POZ2 and POZ3 in CH2Cl2 solution and in the form of a thin film on a glass substrate Figure 3 shows the energy level diagram of the corresponding components in SM-BHJ OSCs and PSCs. The energy levels of POZ2 and POZ3 were determined by cyclic voltammetry (CV). The HOMO level and the LUMO level of POZ3 are -5.33 eV and -3.66 eV vs. vacuum level, respectively. Compared with the energy level of POZ3, with the introduction of electron-deficient π-conjugation BTZ unit, the HOMO level of POZ2 negatively shifts 30 mV, while the LUMO level negatively shifts 110 mV, correspondingly, a much narrower band gap (1.59 eV) SM-material was obtained by introduction of electron-deficient π-conjugation unit. For SM-BHJ OSCs, in principle, the open circuit voltage (Voc) is usually determined by the gap between LUMO of acceptor material (PC70BM) and HOMO of DM (POZ2 or POZ3 here), as well as the work function difference between anode and cathode. So, much higher Voc can be expected for POZ2 based BHJ OSCs. Also, the energy surplus between the LUMO levels of DMs and PC70BM (-3.91 eV) are large enough for exciton dissociation. On the other hand, for PSCs, the HOMO levels of POZ2 and POZ3 are more positive than that of CH3NH3PbI3 (-5.43 eV), which indicates that POZ2 and POZ3 are energetically favorable for hole transfer. Moreover, the LUMO levels of POZ2 and POZ3 are more positive than that of CH3NH3PbI3 (-3.92 eV). Thus, POZ2 or POZ3 can not only act as hole transporting layer, but also play as an electron blocking layer in the PSCs, leading to more efficient charge extraction in the devices.


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Chemistry of Materials Table 1. Optical and electrochemical data of the POZ2 and POZ3

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λmax solutiona / nm

ɛ / M·cm-1

λmax filmb / nm

E0-0 c/ eV

HOMOc / eV

LUMOc / eV

Hole mobility / cm2·V-1·S-1

Conductivity / S·cm-1

1

POZ2

564

35900

603

1.59

-5.36

-3.77

5.98×10-4

2.01×10-4

POZ3

519

36900

553

1.67

-5.33

-3.66

4.46×10-4

9.36×10-5

a)

Absorption spectra were recorded in CH2Cl2 solution (2×10 -5 M). b) Films were prepared by spin-coating an o-dichlorobenzene solution (20 mg/mL) of POZ2 or POZ3 onto glass slides at a spin speed of 1500 rpm. c) CV measurements were carried out in CH2Cl2 solutions with TBAPF6 (0.1 M) as supporting electrolyte, ferrocene/ferrocenium (Fc/Fc+) as internal reference, and converted to the vacuum scale according to the formula of EHOMO = ‒ (Eox+5.1) (eV) and ELUMO= ‒ (Ered+5.1) (eV)

mized geometries of POZ2 shows a little larger dihedral angles between the BTZ and the POZ or TP units (12° and 15°, respectively), that is, POZ2 is more twisty compared with POZ3, providing evidence that the red-shifted absorption of the POZ2 is partly due to the introduction of electrondeficient BTZ unit as π-conjugation. Both these two materials can realize effective charge separation at HOMO and LUMO, which facilitates to the separation of strongly bound frenkel excitons on p-n interface.

Figure 3. Energy level diagram of the corresponding components in BHJ OSCs (a) and PSCs (b)

Figure 4. Geometrical configuration and electron distribution of small molecule POZ2 and POZ3 Molecular simulation with density functional theory (DFT) was carried out to gain insight into the structure-property relationship of these two materials. In particular, the HOMO and LUMO levels and related electron distributions were calculated (see Figure 4). From the calculate results we can see that POZ3 shows a nearly planar configuration. The opti-

Figure 5. a) J-V Plots of the hole-only devices based on POZ2 and POZ3, b) conductivity characters of devices based on POZ2 and POZ3 Hole mobility and conductivity are important parameters for both DMs and HTMs. The hole mobility of POZ2 and POZ3 were tested by using the space-charge-limited current (SCLC) method with the device structure of ITO/MoO3/POZ2 or POZ3/ MoO3/Ag and the test results are shown in Figure 5. The conductivity of POZ2 and POZ3 were determined by using a two-contact electrical conductivity set-up. 54, 60 The extracted average hole mobility of POZ2 is 5.98×10-4 cm2·V-


Chemistry of Materials 1

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·s-1, which is slightly higher than that of POZ3 (4.46×10-4 cm2·V-1·s-1). Also, POZ2 shows higher conductivity (2.01×104 S·cm-1) than that of POZ3 (9.36×10-5 S·cm-1). Therefore, it is a realistic hypothesis that hole mobility and conductivity of PDM or HTM can be effectively adjusted by molecular engineering and BTZ unit seems have positive impacts on hole mobility and conductivity.

Figure 6. a) Current–voltage characteristics and b) EQE spectra of SM-BHJ OSCs that employ POZ2 or POZ3 as SMDMs, c) histogram of average efficiencies of 50 devices Firstly, we investigated the photovoltaic properties of POZ2 and POZ3 as SM-DMs in SM-BHJ OSCs. The devices were fabricated with an inverted device structure of ITO/ZnO/polyethylenimine (PEIE)/POZ2 or POZ3:PC70BM/MoO3/Ag. The current-voltage (J-V) characteristics of the best performing SM-BHJ OSCs that employ

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POZ2 or POZ3 as the SM-DMs are shown in Figure 6a and the related data (Voc, short circuit current density (Jsc) and fill factor (FF), PCE) is summarized in Table 2. With a series of testing, an optimal donor/acceptor (D/A) weight ratios were obtained as 1:1.1 for POZ2:PC70BM and 1:0.9 for POZ3:PC70BM, and the PCEs of the champion devices with POZ2 and POZ3 as SM-DMs are 7.44% and 6.73%, respectively. The statistical data based on fifty identical devices using POZ2 or POZ3 as SM-DMs are collected in Figure 6c, giving average PCEs of 7.25±0.16% (Voc = 876±7.7 mV, Jsc = 12.1±0.4 mA·cm-2 and FF = 68.4±1.4 %) and 6.56±0.14% (Voc = 913±11.4 mV, Jsc = 10.5±0.5 mA·cm-2 and FF = 68.0±2.7 %), respectively. It is clear that for POZ2 based SM-BHJ OSCs, the short-circuit currents (Jsc) values are much higher than that of POZ3 based ones. This can mainly be ascribed to wider response to solar spectrum for POZ2 based SM-BHJ OSCs, as will be discussed later. In addition, POZ2 based devices exhibited higher Voc, which mainly results from 30 meV negative HOMO level of POZ2 compare with that of POZ3. The external quantum efficiency (EQE) spectra of the best performing devices using POZ2 and POZ3 as SM-DMs are shown in Figure 6b. The EQE curve of POZ2:PC70BM (w/w, 1:1.1) exhibits efficient conversion efficiency in the range of 300 to 800 nm, with the highest EQE value reaching 69.7% at 580 nm. The calculated Jsc integrated from the EQE spectrum for POZ2 based device is 12.2 mA·cm−2, which matched well with the Jsc recorded from J−V measurement. In comparison, the EQE values of the device based on POZ3 are much higher in short wavelength region (330 to 500 nm), while it shows relatively narrower EQE response than that of POZ2, resulting in a lower Jsc value for the POZ3 based device, as mentioned above. These results are echoing the results of their UV absorption and prove that the incorporation of electron-deficient BTZ unit as π-conjugation could indeed improve significantly the PCEs and broaden the response range.

Figure 7. TEM images of BHJ films based on POZ2 and POZ3 In order to better understand the effects of molecular structures on device photovoltaic performance, we conducted a morphology study of the active layer by using transmission electron microscopy (TEM), and the test results are shown in Figure 7. The bright regions can be attributed to POZ2 or POZ3-rich domains, while the black regions can be attributed to PC71BM-rich domains. From the TEM tests we can see that POZ2: PC71BM blend film exhibits much better features of phase separation relatively smaller domains, matching well with photovoltaic performance stated above. Therefore we conclude that incorporation of electron-deficient BTZ unit into molecular structure may can induce better phase separation in the BHJ films, correspondingly, enhances charge generation.


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Table 2. Current–voltage characteristics of SM-BHJ OSCs that employ POZ2 or POZ3 as SM-DMs DM

Voc / mV

Jsc / mA·cm-2

FF / %

η/%

integral Jsc / mA·cm-2

POZ2

907

11.9

68.9

7.44

12.2

POZ3

885

10.5

72.4

6.73

10.7

Figure 8. a) Current–voltage characteristics and b) EQE spectra of PSCs that employ POZ2 or POZ3 as HTM, c) histogram of average efficiencies of 50 devices Subsequently, we also used POZ2 and POZ3 as dopant free HTM in PSCs, with the device structure of FTO/compact TiO2/mesoporous TiO2 (mp-TiO2)/CH3NH3PbI3/POZ2 or POZ3/Ag. The CH3NH3PbI3 was fabricated by two-steps deposition method and the crystalline structure was characterized using X-ray diffraction (XRD) as shown in Figure S3. For comparison, the devices with well-known doped SpiroOMeTAD as HTM were also fabricated and tested. The J-V

characteristics of PSCs that employ dopant-free POZ2, POZ3 or doped Spiro-OMeTAD as HTM are shown in Figure 8a and the relevant data is summarized in Table 3. The highest PCEs of the POZ2 or POZ3 based devices are 12.8% and 11.5%, respectively. The statistical data based on fifty identical devices containing POZ2 or POZ3 as dopant-free HTM are collected in Figure 8c, giving average PCEs of 11.0 ± 1.1% (Voc = 949±31 mV, Jsc = 17.2±1.2 mA·cm-2 and FF = 67.7±3.4 %) and 9.85±1.0% (Voc = 928±41 mV, Jsc = 16.5±1.5 mA·cm-2 and FF = 64.3±3.8 %), respectively. Comparing the devices containing POZ2 and POZ3 as HTM, the following observations can be made. By using POZ2 as HTM, higher Voc can be obtained, an effect expected because of the calculationally indicated deeper HOMO energy level of POZ2. Beyond that, the slightly higher PCEs, as compared to devices containing POZ3 as HTM, can also be linked to the higher Jsc and FF. The improvement in FF can mainly attributed to much higher conductivity of POZ2, which is beneficial to reduce the series resistance of PSCs. From EQE spectra (see Figure 8b) we can deduce that both PSCs containing POZ2 or POZ3 as HTM displayed wide response to the solar spectrum with a long wavelength limit at around 800 nm. Particularly, POZ2 based devices showed more effective photoelectric conversion in the range of 400 to 700 nm than that of POZ3 based ones; correspondingly, the Jsc was greatly improved. The integrated current densities from the IPCE spectra for devices containing POZ2 or POZ3 as HTM were 16.9 and 16.2 mA·cm-2, respectively, which agreed well with the measured Jsc. The PSC based on doped spiro-OMeTAD achieved higher Jsc, Voc and FF values, showing Jsc of 19.6 mA·cm-2, Voc of 922 mV, FF of 70.6%, thus yielded a higher efficiency of 12.8%, which is also the highest efficiency can be achieved in our lab. Histogram of average efficiencies based on fifty devices is shown in Figure 7c, giving an average PCE of 11.7%. Comparing with the state-of-the-art HTM spiro-OMeTAD, dopantfree HTM POZ2 and POZ3 demonstrated a little poorer photovoltaic performance, which is mainly due to distinct lower Jsc, considering a little higher Voc and FF values. The lower Jsc results from the strong absorption of POZ2 or POZ3 in the visible range, which will compete with that of the perovksite film. Table 3. Current–voltage characteristics of PSCs that employ POZ2 or POZ3 as HTM HTM

Voc / mV

Jsc / mA·cm-2

FF / %

η/%

integral Jsc / mA·cm-2

POZ2

971

17.8

74.2

12.8

16.9

POZ3

947

16.8

72.3

11.5

16.2

SpiroOMeT AD

922

19.6

70.6

12.8

19.8



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prompted us to use them both as SM-DMs in BHJ OSCs and as HTM in CH3NH3PbI3-based PSCs. Applied in SM-BHJ OSCs, through optimization, a PCE of 7.44% was obtained for POZ2 based device, using a device structure of ITO/ZnO/PEIE/POZ2:PC70BM (1:1.1)/MoO3/Ag, which is higher than that of POZ3 based ones. Moreover, POZ2 also showed better device performance than POZ3 when used as dopant free HTMs in PSCs. Our results showed that the incorporation of electron-deficient BTZ unit as π-conjugation seems can improve the hole mobility and conductivity of functional materials and broaden the response range to solar spectrum. We also have great confidence that much higher efficiency can be achieved by molecular engineering of these POZ based small-molecule materials. The present results offer a new design strategy towards the development of low-cost and highly efficient photovoltaic devices in the future

ASSOCIATED CONTENT Supporting Information. The synthesis of POZ2 and POZ3, cyclic voltammograms of POZ2 and POZ3 and fabrication of solar cells supplied as Supporting Information. This materials are available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * Xichuan Yang, E-mail: [email protected] * Licheng Sun, E-mail: [email protected]

Figure 9. a) Recombination resistance and b) transport resistance of HTMs in the PSC devices containing POZ2 or POZ3 as HTMs under one sun illumination obtained from a fitted electrochemical model to EIS data Electrochemical impedance spectroscopy (EIS) measurements were performed to characterize both hole transportation in the HTM and the recombination process. We measured the representative PSCs with dopant-free POZ2, POZ3 or doped Spiro-OMeTAD as HTMs under illumination. The resulting spectra were fitted according to a previously reported equivalent circuit. 61, 62 The recombination resistance (Rrec, see Figure 9a) presents almost identical behavior for POZ2 and POZ3 based devices. As a result, the differences in Voc follow a similar trend to the HOMO level positions of POZ2 and POZ3. The devices containing Spiro-OMeTAD as HTM present slightly lower recombination resistances than those of POZ2 or POZ3 based devices. Additionally, the HOMO energy levels of POZ2 and POZ3 are more negative than that of Spiro-OMeTAD (-5.22 eV).57 Both these factors result in the higher Voc observed for the devices containing POZ2 or POZ3. The slight variations in the FF can be explained from the series resistance of the different HTM-based cells (RHTM). The RHTM value extracted from the fittings are plotted in Figure 8b. The lowest values obtained for POZ2 based PSCs are in agreement with the highest FF values obtained.

Conclusions In summary, we have designed and synthesized two A-D-A structured small-molecule materials POZ2 and POZ3, containing an electron-rich POZ unit as core building block, flanked by BTZ and TP units, and end-capped with electronwithdrawing dicyanovinyl unit. Their unique characteristics, such as suitable energy levels, strong optical absorption in the visible region, high hole mobility and high conductivity,

Author Contributions The manuscript was written through contributions of all authors. ‡ These authors contributed equally.

ACKNOWLEDGMENT We gratefully acknowledge the financial support of this work from China Natural Science Foundation (Grant 21276044, 21120102036, 91233201), the National Basic Research Program of China (Grant No. 2014CB239402), the Swedish Energy Agency, the Knut and Alice Wallenberg Foundation.

ABBREVIATIONS A-D-A, Acceptor-Donor-Acceptor; BHJ OSCs, bulk heterojunction organic solar cells; HTM, hole transport materials; PSCs, perovskite solar cells; PCEs, power conversion efficiencies; POZ, phenothiazine; BTZ, benzothiazole; DMs, donor materials; OLED, organic light-emitting diode; DSSCs, dye sensitized solar cells; HOMO, the highest occupied molecular orbital; LUMO, lowest unoccupied molecular orbital; CV, cyclic voltammetry; DFT, density functional theory; SCLC, space-charge-limited current; Voc, open circuit voltage; Jsc short circuit current density; FF, fill factor; EQE, external quantum efficiency; EIS, Electrochemical impedance spectroscopy.

REFERENCES 1. Cao, W.; Xue, J., Recent progress in organic photovoltaics: device architecture and optical design. Energy Environ. Sci. 2014, 7, 2123. 2. Diao, Y.; Shaw, L.; Bao, Z.; Mannsfeld, S. C. B., Morphology control strategies for solution-processed organic semiconductor thin films. Energy Environ. Sci. 2014, 7, 2145. 3. Huang, Y.; Kramer, E. J.; Heeger, A. J.; Bazan, G. C., Bulk heterojunction solar cells: morphology and performance relationships. Chem. Rev. 2014, 114, 7006.


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