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The essential role of the poly(3-hexylthiophene) hole transport layer in perovskite solar cells
Journal of Power Sources 274 (2015) 1224e1230
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The essential role of the poly(3-hexylthiophene) hole transport layer in perovskite solar cells Yang Zhang*, Wenqiang Liu, Furui Tan, Yuzong Gu Institute of Physics in Microsystems, Department of Physics, Henan University, Kaifeng, Henan 475004, China
h i g h l i g h t s The oriented TiO2 films were fabricated by a solvothermal method. CH3NH3PbI3-xClx based solar cells without/with P3HT were fabricated. The charge collection efficiency of the device with P3HT is markedly enhanced.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 August 2014 Received in revised form 16 October 2014 Accepted 27 October 2014 Available online 29 October 2014
The compact and oriented TiO2 films are prepared by a solvothermal method, and used as electron transporting layers in perovskite CH3NH3PbI3-xClx based solar cells incorporating poly(3-hexylthiophene2,5-diyl) (P3HT) as the hole transporting material layer. The devices with P3HT exhibit a substantial increase in power conversion efficiency, open circuit voltage, and fill factor, compared with the reference device without P3HT. Impedance spectroscopy measurements demonstrate that the present P3HT layer decreases the internal resistance in solar cells and allows the interface between oriented TiO2 and CH3NH3PbI3-xClx to form more perfect in electronics. It is also found that the electron lifetime in the devices with P3HT is much longer than that of the device without P3HT. Thus, the charge collection efficiency of the device with P3HT is markedly enhanced, compared with the devices without P3HT. Analysis of the energy levels of the involved materials indicates that the P3HT film between the CH3NH3PbI3-xClx layer and the Au electrode provides a better energy level matching for efficient transporting holes to the anode. Meanwhile, the stability of such P3HT solar cells is enhanced because of the compact and oriented TiO2 film preventing the possible interaction between TiO2 and perovskite as time went on. © 2014 Elsevier B.V. All rights reserved.
Keywords: Perovskite solar cell Titanium dioxide Solvothermal synthesis P3HT Impedance spectroscopy
1. Introduction Recently, photovoltaic devices based on hybrid lead halide perovskites (methylammonium lead halide: CH3NH3PbX3) showed a fast and continuous increase in their performance [1,2]. More recently, the highest efficiency of perovskite based solar cells in a planar geometry is ~19.3% by the improvement of electron collection at the interface [3]. Generally, a typical device is composed of perovskite CH3NH3PbX3 as an absorber material, mesoporous ntype TiO2 as an electron transport layer, and the hole transport materials (HTMs). HTMs include organic hole conductors (spiroOMeTAD, poly(3-hexylthiophene-2,5-diyl) (P3HT), polyaniline (PANI) [4,5] and inorganic hole conductors such as CuI [6], NiO [7,8],
* Corresponding author. E-mail address: [email protected] (Y. Zhang). http://dx.doi.org/10.1016/j.jpowsour.2014.10.145 0378-7753/© 2014 Elsevier B.V. All rights reserved.
CuSCN [9]. In an ideal scenario, electrons are transferred through the mesoporous n-type TiO2 as electron transport layer to the FTO substrate. At the same time, holes transfer from the HTM to the Au or Ag electrode. Experimentally it was demonstrated that the proper HTM could efficiently inhibit back electron transfer, which would result in a solar cell with a higher fill factor (FF) and open circuit voltage (Voc) [10]. On the other hand, perovskite solar cells without HTMs have also been developed. That is, a hole conductor is not required at all in perovskite solar cells. In a certain case, Egar et al. first reported hole-conductor-free solar cells based on CH3NH3PbI3 perovskite, which acted as an absorber and HTM at the same time [11,12]. In our previous work, the electrical properties of TiO2/CH3NH3PbI3 heterojunction solar cells without HTM were analysed by the electron potential barrier height [13]. Nevertheless, the perovskite solar cells with HTMs can exhibit much higher efficiencies than that of corresponding without HTMs. Therefore, HTMs are crucial for fabricating efficient perovskite solar cells.
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Although various HTMs as one of the most crucial parts of perovskite solar cells are under intense investigation [14e19], the fundamental impact of the HTM on the charge transfer at the interfaces in devices and the charge collection remains poorly understood. In this work, the CH3NH3PbI3-xClx (¼MAPbI3-xClx) based solar cells employing P3HT hole transport layer and the compact, oriented TiO2 electron transporting layer were fabricated. The influence of P3HT on the performance of MAPbI3-xClx-based solar cells was investigated by photocurrent-voltage and impedance spectroscopy measurements. It is found that application of P3HT increases the Voc, FF, and PCE of the MAPbI3-xClx-based solar cells, which are attributed to the efficient charge collection. Furthermore, the P3HT devices are much more air stable compared with the reference device without P3HT. 2. Experimental 2.1. Materials synthesis Methylammonium iodide (CH3NH3I) was synthesized and purified using a modified process based on a previously reported method. The mixture of 10 mL of hydroiodic acid (57 wt.% in water) and 14 mL of methylamine (40% in methanol) was stirred in a round bottom flask at 0 C for 2 h. The resulting solution was evaporated at 90 C to remove the solvents. The yellowish product was obtained and recrystallized with methanol for five times. The collected white product (CH3NH3I) was dried at 60 C in a vacuum oven overnight. The perovskite precursor solution was prepared by mixing the as-synthesized CH3NH3I and lead (II) chloride (PbCl2) with a mole ratio of 3:1 in N,N-Dimethylformamide (DMF) at 60 C. The substrate of the device was FTO-coated glass (14 U/sq, Nippon Sheet Glass, Japan). A blocking layer was deposited on the FTO glass using a TiO2 colloid solution, following the procedure reported previously. The highly oriented TiO2 films on compact TiO2 coated FTO substrates were synthesized by an ethanoleHCl solvothermal process following a similar procedure reported previously [13]. In a typical process, 0.7 mL of tetrabutyl titanate was mixed with 20 mL of absolute ethanol and 20 mL 37 wt % HCl, and transferred to a Teflon-lined stainless steel autoclave (100 mL). FTO substrates coated with compact TiO2 layer were placed in an autoclave. The solvothermal reaction was carried out at 170 C for 1 h in a vacuum oven. Then, the autoclave was cooled to room temperature naturally. The samples were rinsed with ethanol and deionized water. After dried in air, they were heated in a furnace at 450 C and maintained for 1 h, subsequently raised to 550 C and maintained for 1 h. 2.2. Solar cell fabrication Perovskite precursor solution was spin-coated on the annealed TiO2 films at 2500 rpm for 30 s in an argon-filled glove box. The films were subsequently heated at 120 C for 30 min. Complete DSCs were fabricated with or without HTM. The HTM was deposited on the MAPbI3-xClx layer by spin-coating a P3HT solution in dichlorobenzene (10 mg/mL) at 1500 rpm for 60 s. Finally, a gold anode (~220 nm) was deposited on the top of MAPbI3-xClx layer or the P3HT layer through a shadow mask to give a device area of 0.08 cm2 under a vacuum level of 103 Pa. 2.3. Characterization The surface morphology and cross-sectional structure of the solar cells were examined by a scanning electron microscope (SEM)
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(JEOL, JSM-7001F, JEOL). The crystalline structure of TiO2 films prepared by solvothermal reaction were examined by X-ray diffraction (XRD) at room temperature with a Bruker D8-Advance diffractometer, employing Cu Ka radiation (l ¼ 1.5418 Å) in the range of 10 to 80 . The current densityevoltage (JeV) characteristics of the devices were measured by a Keithley 2440 Source Meter under 1 sun illumination (AM1.5) provided by a solar simulator (Oriel class AAA, Newport). The incident photon-tocurrent conversion efficiency (IPCE) measurements were performed with Qtest Station 500 (CROWNTECH Inc. USA) at room temperature in air. The electrochemical impedance measurements were performed using an electrochemical workstation (RST5200, Zhengzhou Shiruisi Instrument Co., Ltd.) with the frequency range from 10 Hz to 139 kHz under forward bias of 0.68 V in the dark. The magnitude of the alternative signal was 10 mV. The applied dc potential bias was changed by 50 mV steps from 850 to 0 mV. All device measurements were performed in air at room temperature. The photoluminescence (PL) measurements were performed on a Perkin Elmer LS55 Fluorescence spectrometer with an excitation wavelength of 480 nm at room temperature. 3. Results and discussion 3.1. Top-view SEM images of the compact TiO2, oriented rod-type TiO2, the perovskite films without/with P3HT, and cross-sectional SEM images of the devices The surface morphology and microstructure of perovskite plays a critical role in the performance of this type of solar cell. We took topview SEM images of the compact TiO2 film, oriented rod-type TiO2 film, the perovskite films without and with P3HT. The top-view SEM images of the compact and the oriented rod-type TiO2 films are shown in Fig. 1a and b, respectively. From Fig. 1a, it can be seen that the surface of the compact TiO2 film is smooth and flat. In the case of the oriented rod-type TiO2 film shown in Fig. 1b, the surface roughness is increased slightly, as compared with the compact TiO2 film. Furthermore, the grown TiO2 rods are aligned vertically, and formed into a dense film without any obvious pores and seams as compared with the traditional mesoporous TiO2 film. The top-view SEM images of the CH3NH3PbI3-xClx films without and with P3HT are shown in Fig. 1c and d, respectively. It can be seen that the surface coverage is not complete. Both surfaces are similar and composed of irregular grains with sizes of a few micrometres. This poor surface coverage is a common phenomenon in a single solution processed solid absorber layer [20]. Nevertheless, when P3HT is present, the coverage is increased, which would lead to the improvement of the performance of perovskite solar cells. Fig. 2a and b show cross-sectional SEM images of the full device architectures with and without P3HT hole transport layer. All the devices here were fabricated on cleaned fluorine doped tin oxide (FTO) coated glass. The TiO2 films in both samples prepared by an ethanol-HCl solvothermal process show the oriented rod-type TiO2 structure, indicating that TiO2 was densely formed on the compact TiO2 film. The thicknesses of the oriented TiO2 films in both devices are similar, ~2.3 mm. The XRD pattern of a typical a typical XRD pattern of TiO2 film prepared by solvothermal reaction is shown in Fig. 2c. We can see that there are seven diffraction peaks at 27.58 , 36.30 , 37.86 , 41.30 , 54.42 , 62.86 , and 65.66 , where the diffraction peaks at 27.58 , 36.30 , 41.30 , 54.42 , 62.86 , and 65.66 can be readily attributed to the (110), (101), (111), (211), (002), and (211) planes of rutile TiO2 (JCPDS Card No. 21-1276), respectively, while the peak at 37.86 can be attributed to the (004) plane of anatase TiO2 (JCPDS Card No. 21-1272). This suggests that rutile and anatase TiO2 phases coexist in the TiO2 film prepared by solvothermal reaction. Rutile phase is dominated. Meanwhile, the
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Fig. 1. Top-view SEM images of (a) the compact TiO2 film, the oriented rod-type TiO2 films, the CH3NH3PbI3-xClx films (c) without and (d) with P3HT hole transport layer.
intensity of the (002) peak is the strongest in all peaks, rather than the (110) peak of rutile TiO2. This result indicates that the growth of TiO2 is highly oriented along the (002) plane on FTO substrate during the solvothermal reaction. On the other hand, it should be noted that the MAPbI3-xClx film in Fig. 2a is made of large grains up
to a size of ~500 nm, while the MAPbI3-xClx film in Fig. 2b is compact with the thickness of ~200 nm, which is much smaller than that of the device without P3HT. The decrease in the thickness of MAPbI3-xClx after spin-coating P3HT might be due to high infiltration of the dissolved perovskite by the solvent of P3HT.
Fig. 2. Cross-sectional SEM images of the MAPbI3-xClx-based solar cells (a) without and (b) with P3HT hole transport layer. (c) XRD pattern of a typical TiO2 film prepared by solvothermal reaction, annealed at 550 C.
Y. Zhang et al. / Journal of Power Sources 274 (2015) 1224e1230
3.2. Current densityevoltage (JeV) characteristics Fig. 3a shows the JeV characteristics of the solar cells employing MAPbI3-xClx sensitizer with and without P3HT hole transport layer under AM 1.5 illumination. The extracted cell performance parameters for two kinds of solar cells are given in Table 1. The PCE of the P3HT device is 6.06%, resulting from the short circuit current density (Jsc) of 14.65 mA/cm2, Voc of 0.742 V, and FF of 55.8%. However, the reference device without P3HT has the Jsc of 13.64 mA/cm2, Voc of 0.663 V, and FF of 38.8%, a PCE of 3.51%. In comparison, there is a notable increase in the Voc (0.663 Ve0.742 V) with P3HT and a notable increase in FF from 38.8% to 55.8%. This leads to a substantial increase in PCE from 3.51% of the device without P3HT to 6.06% of the P3HT device. This result clearly implies that the P3HT hole transport layer has contributed to the enhancement of the charge collection. On the other hand, the best PCE of P3HT-incorporated device is 6.06%, which is lower than that of the reported device incorporating spiro-OMeTAD hole transporting layers in literature [21]. Fig. 3b shows the IPCE spectra of perovskite solar cells with and without P3HT HTM. As we can see, the IPCE is improved by P3HT HTM. The JSC values integrated from the IPCE of perovskite solar cells with and without P3HT HTM are 14.12 mA/cm2 and 9.13 mA/cm2, respectively, which are almost agreed with the obtained by JeV.
3.3. Impedance spectra of MAPbI3-xClx-based solar cells In order to further investigate the role of P3HT, impedance spectroscopy measurements were carried out in the dark. Fig. 4a shows the Nyquist plots of the MAPbI3-xClx-based solar cells with and without P3HT hole transport layer in the dark. The solid curves are the fitting results with a transmission line model, as shown in the inset in Fig. 4a. For the solar cells without P3HT hole transport layer, the data can be well fitted with a circuit consisting of a resistor Rs and two elements consisting of a charge transfer resistance (Rct) and a constant phase element (CPE) elements for the one interface between TiO2 and MAPbI3-xClx and another interface between MAPbI3-xClx and Au electrode. However, for the cells with P3HT hole transport layer, the data can be well fitted with a simple circuit (not shown here) consisting of a resistor Rs and one Rct and CPE element for the interface between TiO2 and MAPbI3-xClx. It is well known that Rct is associated with the interface charge transport process, defined by the charge transfer resistance, and CPE suggests a non-ideal behaviour of the capacitor. It is usually defined
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Table 1 Photovoltaic parameters of MAPbI3-xClx-based solar cells with and without P3HT hole transport layer under illumination of 100 mW/cm2. Cell (w/o P3HT)
Voc (V)
Jsc (mA/cm2)
FF (%)
PCE (%)
With P3HT Without P3HT
0.742 0.663
14.65 13.64
55.8 38.8
6.06 3.51
by two values, CPE-T and CPE-P. The value of CPE-P is related to an ideal capacitor [22,23]. The observed parameters of Rs, Rct and CPE values are presented in Table 2. It should be noted that the fitted value of Rs in the device with P3HT, representing the internal resistance, is 16.5 U cm2, smaller than that (22.8 U cm2) of the device without P3HT. This indicates that the present P3HT layer decreases the internal resistance of solar cells. For the P3HT device, the absence of another semicircle is likely due to the presence of HTM in this device structure. The single semicircle also indicates that the interface contacts well between MAPbI3-xClx and TiO2. This may be due to the contacts between Au and MAPbI3-xClx or FTO and TiO2 are Ohmic contacts, rather than the rectifying contact. On the other hand, the value of CPE-T in the device with P3HT is 5.6E-5, much larger than that in the device without P3HT. This shows that the interface capacitance between oriented TiO2 and MAPbI3-xClx is more perfect electrically than that in the device without P3HT. These results suggest that the charge collection efficiency of the P3HT device was markedly enhanced, compared with the device without P3HT. The high charge collection will lead to efficient photovoltaic performance with higher FF and Voc. It is well in agreement with JeV characteristics, as shown in Fig. 3. In addition, the electron lifetime (the recombination time, te) were obtained from the characteristic frequency of the Bode-phase plots, shown in Fig. 4b, using the equation of te ¼ 1/2pfmid, where fmid is the phase angle peak at the midfrequency peak [24,25]. It is found that a value of te (0.145 ms) in the device with P3HT is much longer than that (0.038 ms) of the device without P3HT. This result reveals that the use of P3HT in perovskite-base solar cells reduces the charge recombination and leads to the large output voltage (Voc) and large FF, obtained for these devices. 3.4. Photoluminescence quenching The PL measurements were performed to further confirm the effect of P3HT on charge collection efficiency. PL spectra of perovskite-based solar cells with and without P3HT HTM are
Fig. 3. (a) JeV characteristics of the MAPbI3-xClx-based solar cells with and without P3HT hole transport layer under AM 1.5 simulated sunlight (100 mW/cm2). (b) IPCE spectra of perovskite solar cells with and without P3HT HTM.
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Fig. 5. Photoluminescence spectra of perovskite films with and without P3HT on top TiO2 with an excitation wavelength of 480 nm irradiation at room temperature.
Fig. 4. (a) Nyquist plots and (b) Bode phase plots of the MAPbI3-xClx-based solar cells with and without P3HT hole transport layer in the dark. The simulation result (solid curve) is fitted to experimental data (symbols) using the equivalent circuit in the insert of (a).
shown in Fig. 5. It is found that the PL peak from MAPbI3-xClx is at 793 nm, which is consistent with the reported previously [26]. It should be noted that the PL intensity is very sensitive to the P3HT, and almost entirely quenched by introducing P3HT HTM into the perovskite-based solar cell. The PL quenching indicates that the photon-generated carriers transfer to the anode from the perovskite layer. Therefore, when P3HT HTM was introduced into perovskite-based solar cell, charge collection efficiency was improved. 3.5. The energy levels of the materials involved in the MAPbI3-xClxbased solar cells To investigate the present P3HT interlayer concept, Fig. 6 shows the energy levels of the materials involved in the MAPbI3-xClxTable 2 Parameters employed for the fitting of the impedance spectra of MAPbI3-xClx-based solar cells with and without P3HT hole transport layer. HTM
Rs Rct1 (U cm2) CPE1-T CPE1-P Rct2 CPE2-T (F/cm2) (U cm2) (U cm2) (F/cm2)
Without 22.8 P3HT With 16.5 P3HT
280
7.7E-7
0.8
195.8
5.6E-5
0.655
180
CPE2-P
9.66E-8 0.858
based solar cells with and without P3HT hole transport layer. When light irradiates on the MAPbI3-xClx absorber in the devices through FTO electrode, the MAPbI3-xClx photoactive layer will absorb photons to produce excitons, and the excitons dissociate into electrons and holes. Subsequently, electrons flow to the conduction band bottom of the TiO2, and holes directly flow to the Au anode (shown as in Fig. 6a), or flow to the highest occupied molecular orbital (HOMO) of the P3HT, and then are collected at the Au anode. Since the HOMO level (5.2 eV) of P3HT is between the HOMO (5.4 eV) of MAPbI3-xClx and the work function (5.0 eV) of Au, as shown in Fig. 6b. Thus, the P3HT film between Au anode and perovskite layer provides a better energy level matching due to the well-aligned HOMO of P3HT with the HOMO of the MAPbI3-xClx. Holes can easily transport to Au anode through P3HT [27]. Therefore, the P3HT allows efficient hole collection and allows holes to transport from the photoactive perovskite to the Au anode. On the other hand, the highest occupied molecular orbital (LUMO) (3.0 eV) of P3HT prevents transferring electrons from MAPbI3-xClx to the Au electrode. Therefore, effective blocking of electron to the anode can efficiently reduce the charge recombination at the anode interface. Thus, P3HT has a band structure well suited for MAPbI3-xClx-based solar cells. The device with P3HT could exhibit higher charge collection efficiencies on both electrodes than those of the device without P3HT [28]. The P3HT layer plays an essential role in charge collection in perovskite-based cells. 3.6. Impact of the P3HT hole transport layer on device degradation As we all know that the stability of solar cells is very important to commercialized application. The long-term stability of perovskite solar cells is one issue that needs to be addressed. Park et al. first reported long-term stable perovskite solar cells for over 500 h [29]. Seok et al. reported the stability testing of perovskite solar cells based on a mixed halide of CH3NH3PbI3 and CH3NH3PbBr3 for 20 days, which confirmed that cubic phase cells had better stability than tetragonal phase devices [30]. H. J. Snaith and his colleagues demonstrated that the Al doping the mesoporous TiO2, or compact TiO2 films employed in perovskite cells could effectively suppress oxygen-induced defects in TiO2, which leads to the deterioration of ss-DSSC devices exposed to sunlight in an inert atmosphere [31]. Recently, Han et al. reported a triple-layer device exhibiting excellent long term stability over 1000 h in ambient air under full
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Fig. 6. The energy levels of the materials involved in the MAPbI3-xClx-based solar cells with and without P3HT hole transport layer. The arrows indicate hole or electron flows.
sunlight [32]. In this work, the air stability of devices with and without P3HT was investigated through the measuring of PCE maintenance of the device. The device stability was tested in a dark way. The typical devices without encapsulation were stored under ambient laboratory conditions. The P3HT devices and the reference devices without P3HT were stored in ambient environment without encapsulation for 14 days. The JeV curves of the fresh made devices and the devices stored in air for 14 days are shown in Fig. 7. The air stable properties of devices with and without P3HT are listed in Table 3. It can be seen that the PCE of the P3HT device still kept 81.2% of the initial value of the fresh made device, while the reference device without P3HT dropped down to only 2.20% after 14 days, 62.7% of the initial value of the fresh device. These results indicate that the P3HT device is more air stable compared with the reference device without P3HT. The relative stability of this P3HT device may be due to the encapsulation-like effect for the MAPbI3-xClx-based solar cells. More importantly, such devices employing the compact and oriented TiO2 film as the electron transporting layer is much more air stable in comparison with the devices based on multiporous TiO2. The compact and oriented TiO2 film could prevent the over infiltration of perovskite to avoid the possible interaction between TiO2 and perovskite as time went on.
Table 3 Air stability of the cells with and without P3HT hole transport layer. Device
Voc (V)
Jsc (mA/cm2)
FF (%)
PCE (%)
With P3HT (fresh) Without P3HT (fresh) With P3HT (after 14 days) Without P3HT (after 14 days)
0.742 0.663 0.739 0.617
14.65 13.64 12.33 9.57
55.8 38.8 54.0 37.2
6.06 3.51 4.92 2.20
4. Conclusions In conclusion, the function of a P3HT hole transport layer in perovskite MAPbI3-xClx-based solar cells was investigated systematically by JeV and impedance spectroscopy measurements. The P3HT device shows a substantial increase in PCE, Voc, and FF, compared with the reference device without P3HT. Impedance spectroscopy measurements demonstrated that the present P3HT layer decreases the internal resistance of solar cells and allows the interface between oriented TiO2 and MAPbI3-xClx to form more perfect electrically. It is also found that the electron lifetime in the device with P3HT is much longer than that of the device without P3HT. These results clearly suggest that the charge collection efficiency of the device with P3HT was markedly enhanced, compared with the device without P3HT. The analysis of the energy levels of the involved materials indicates that the P3HT film between Au anode and perovskite layer provides a better energy level matching for efficient transporting holes, and effectively prevents electron transfer back to the anode. Moreover, the P3HT layer can play a crucial role in the stability of such solar cells. Therefore, the proper hole transporting materials and the compact and oriented TiO2 electron transporting layer are essential in developing highly efficient and stable perovskite sensitized solar cells. Acknowledgements This work was supported by the Department of Education, Henan Province under contract No. 13A480062. References
Fig. 7. Degradation of JeV characteristics of the MAPbI3-xClx-based solar cells with and without P3HT hole transport layer exposed to ambient air for 14 days without encapsulation.
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
The essential role of the poly(3-hexylthiophene) hole transport layer in perovskite solar cells Yang Zhang*, Wenqiang Liu, Furui Tan, Yuzong Gu Institute of Physics in Microsystems, Department of Physics, Henan University, Kaifeng, Henan 475004, China
h i g h l i g h t s The oriented TiO2 films were fabricated by a solvothermal method. CH3NH3PbI3-xClx based solar cells without/with P3HT were fabricated. The charge collection efficiency of the device with P3HT is markedly enhanced.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 August 2014 Received in revised form 16 October 2014 Accepted 27 October 2014 Available online 29 October 2014
The compact and oriented TiO2 films are prepared by a solvothermal method, and used as electron transporting layers in perovskite CH3NH3PbI3-xClx based solar cells incorporating poly(3-hexylthiophene2,5-diyl) (P3HT) as the hole transporting material layer. The devices with P3HT exhibit a substantial increase in power conversion efficiency, open circuit voltage, and fill factor, compared with the reference device without P3HT. Impedance spectroscopy measurements demonstrate that the present P3HT layer decreases the internal resistance in solar cells and allows the interface between oriented TiO2 and CH3NH3PbI3-xClx to form more perfect in electronics. It is also found that the electron lifetime in the devices with P3HT is much longer than that of the device without P3HT. Thus, the charge collection efficiency of the device with P3HT is markedly enhanced, compared with the devices without P3HT. Analysis of the energy levels of the involved materials indicates that the P3HT film between the CH3NH3PbI3-xClx layer and the Au electrode provides a better energy level matching for efficient transporting holes to the anode. Meanwhile, the stability of such P3HT solar cells is enhanced because of the compact and oriented TiO2 film preventing the possible interaction between TiO2 and perovskite as time went on. © 2014 Elsevier B.V. All rights reserved.
Keywords: Perovskite solar cell Titanium dioxide Solvothermal synthesis P3HT Impedance spectroscopy
1. Introduction Recently, photovoltaic devices based on hybrid lead halide perovskites (methylammonium lead halide: CH3NH3PbX3) showed a fast and continuous increase in their performance [1,2]. More recently, the highest efficiency of perovskite based solar cells in a planar geometry is ~19.3% by the improvement of electron collection at the interface [3]. Generally, a typical device is composed of perovskite CH3NH3PbX3 as an absorber material, mesoporous ntype TiO2 as an electron transport layer, and the hole transport materials (HTMs). HTMs include organic hole conductors (spiroOMeTAD, poly(3-hexylthiophene-2,5-diyl) (P3HT), polyaniline (PANI) [4,5] and inorganic hole conductors such as CuI [6], NiO [7,8],
* Corresponding author. E-mail address: [email protected] (Y. Zhang). http://dx.doi.org/10.1016/j.jpowsour.2014.10.145 0378-7753/© 2014 Elsevier B.V. All rights reserved.
CuSCN [9]. In an ideal scenario, electrons are transferred through the mesoporous n-type TiO2 as electron transport layer to the FTO substrate. At the same time, holes transfer from the HTM to the Au or Ag electrode. Experimentally it was demonstrated that the proper HTM could efficiently inhibit back electron transfer, which would result in a solar cell with a higher fill factor (FF) and open circuit voltage (Voc) [10]. On the other hand, perovskite solar cells without HTMs have also been developed. That is, a hole conductor is not required at all in perovskite solar cells. In a certain case, Egar et al. first reported hole-conductor-free solar cells based on CH3NH3PbI3 perovskite, which acted as an absorber and HTM at the same time [11,12]. In our previous work, the electrical properties of TiO2/CH3NH3PbI3 heterojunction solar cells without HTM were analysed by the electron potential barrier height [13]. Nevertheless, the perovskite solar cells with HTMs can exhibit much higher efficiencies than that of corresponding without HTMs. Therefore, HTMs are crucial for fabricating efficient perovskite solar cells.
Y. Zhang et al. / Journal of Power Sources 274 (2015) 1224e1230
Although various HTMs as one of the most crucial parts of perovskite solar cells are under intense investigation [14e19], the fundamental impact of the HTM on the charge transfer at the interfaces in devices and the charge collection remains poorly understood. In this work, the CH3NH3PbI3-xClx (¼MAPbI3-xClx) based solar cells employing P3HT hole transport layer and the compact, oriented TiO2 electron transporting layer were fabricated. The influence of P3HT on the performance of MAPbI3-xClx-based solar cells was investigated by photocurrent-voltage and impedance spectroscopy measurements. It is found that application of P3HT increases the Voc, FF, and PCE of the MAPbI3-xClx-based solar cells, which are attributed to the efficient charge collection. Furthermore, the P3HT devices are much more air stable compared with the reference device without P3HT. 2. Experimental 2.1. Materials synthesis Methylammonium iodide (CH3NH3I) was synthesized and purified using a modified process based on a previously reported method. The mixture of 10 mL of hydroiodic acid (57 wt.% in water) and 14 mL of methylamine (40% in methanol) was stirred in a round bottom flask at 0 C for 2 h. The resulting solution was evaporated at 90 C to remove the solvents. The yellowish product was obtained and recrystallized with methanol for five times. The collected white product (CH3NH3I) was dried at 60 C in a vacuum oven overnight. The perovskite precursor solution was prepared by mixing the as-synthesized CH3NH3I and lead (II) chloride (PbCl2) with a mole ratio of 3:1 in N,N-Dimethylformamide (DMF) at 60 C. The substrate of the device was FTO-coated glass (14 U/sq, Nippon Sheet Glass, Japan). A blocking layer was deposited on the FTO glass using a TiO2 colloid solution, following the procedure reported previously. The highly oriented TiO2 films on compact TiO2 coated FTO substrates were synthesized by an ethanoleHCl solvothermal process following a similar procedure reported previously [13]. In a typical process, 0.7 mL of tetrabutyl titanate was mixed with 20 mL of absolute ethanol and 20 mL 37 wt % HCl, and transferred to a Teflon-lined stainless steel autoclave (100 mL). FTO substrates coated with compact TiO2 layer were placed in an autoclave. The solvothermal reaction was carried out at 170 C for 1 h in a vacuum oven. Then, the autoclave was cooled to room temperature naturally. The samples were rinsed with ethanol and deionized water. After dried in air, they were heated in a furnace at 450 C and maintained for 1 h, subsequently raised to 550 C and maintained for 1 h. 2.2. Solar cell fabrication Perovskite precursor solution was spin-coated on the annealed TiO2 films at 2500 rpm for 30 s in an argon-filled glove box. The films were subsequently heated at 120 C for 30 min. Complete DSCs were fabricated with or without HTM. The HTM was deposited on the MAPbI3-xClx layer by spin-coating a P3HT solution in dichlorobenzene (10 mg/mL) at 1500 rpm for 60 s. Finally, a gold anode (~220 nm) was deposited on the top of MAPbI3-xClx layer or the P3HT layer through a shadow mask to give a device area of 0.08 cm2 under a vacuum level of 103 Pa. 2.3. Characterization The surface morphology and cross-sectional structure of the solar cells were examined by a scanning electron microscope (SEM)
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(JEOL, JSM-7001F, JEOL). The crystalline structure of TiO2 films prepared by solvothermal reaction were examined by X-ray diffraction (XRD) at room temperature with a Bruker D8-Advance diffractometer, employing Cu Ka radiation (l ¼ 1.5418 Å) in the range of 10 to 80 . The current densityevoltage (JeV) characteristics of the devices were measured by a Keithley 2440 Source Meter under 1 sun illumination (AM1.5) provided by a solar simulator (Oriel class AAA, Newport). The incident photon-tocurrent conversion efficiency (IPCE) measurements were performed with Qtest Station 500 (CROWNTECH Inc. USA) at room temperature in air. The electrochemical impedance measurements were performed using an electrochemical workstation (RST5200, Zhengzhou Shiruisi Instrument Co., Ltd.) with the frequency range from 10 Hz to 139 kHz under forward bias of 0.68 V in the dark. The magnitude of the alternative signal was 10 mV. The applied dc potential bias was changed by 50 mV steps from 850 to 0 mV. All device measurements were performed in air at room temperature. The photoluminescence (PL) measurements were performed on a Perkin Elmer LS55 Fluorescence spectrometer with an excitation wavelength of 480 nm at room temperature. 3. Results and discussion 3.1. Top-view SEM images of the compact TiO2, oriented rod-type TiO2, the perovskite films without/with P3HT, and cross-sectional SEM images of the devices The surface morphology and microstructure of perovskite plays a critical role in the performance of this type of solar cell. We took topview SEM images of the compact TiO2 film, oriented rod-type TiO2 film, the perovskite films without and with P3HT. The top-view SEM images of the compact and the oriented rod-type TiO2 films are shown in Fig. 1a and b, respectively. From Fig. 1a, it can be seen that the surface of the compact TiO2 film is smooth and flat. In the case of the oriented rod-type TiO2 film shown in Fig. 1b, the surface roughness is increased slightly, as compared with the compact TiO2 film. Furthermore, the grown TiO2 rods are aligned vertically, and formed into a dense film without any obvious pores and seams as compared with the traditional mesoporous TiO2 film. The top-view SEM images of the CH3NH3PbI3-xClx films without and with P3HT are shown in Fig. 1c and d, respectively. It can be seen that the surface coverage is not complete. Both surfaces are similar and composed of irregular grains with sizes of a few micrometres. This poor surface coverage is a common phenomenon in a single solution processed solid absorber layer [20]. Nevertheless, when P3HT is present, the coverage is increased, which would lead to the improvement of the performance of perovskite solar cells. Fig. 2a and b show cross-sectional SEM images of the full device architectures with and without P3HT hole transport layer. All the devices here were fabricated on cleaned fluorine doped tin oxide (FTO) coated glass. The TiO2 films in both samples prepared by an ethanol-HCl solvothermal process show the oriented rod-type TiO2 structure, indicating that TiO2 was densely formed on the compact TiO2 film. The thicknesses of the oriented TiO2 films in both devices are similar, ~2.3 mm. The XRD pattern of a typical a typical XRD pattern of TiO2 film prepared by solvothermal reaction is shown in Fig. 2c. We can see that there are seven diffraction peaks at 27.58 , 36.30 , 37.86 , 41.30 , 54.42 , 62.86 , and 65.66 , where the diffraction peaks at 27.58 , 36.30 , 41.30 , 54.42 , 62.86 , and 65.66 can be readily attributed to the (110), (101), (111), (211), (002), and (211) planes of rutile TiO2 (JCPDS Card No. 21-1276), respectively, while the peak at 37.86 can be attributed to the (004) plane of anatase TiO2 (JCPDS Card No. 21-1272). This suggests that rutile and anatase TiO2 phases coexist in the TiO2 film prepared by solvothermal reaction. Rutile phase is dominated. Meanwhile, the
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Fig. 1. Top-view SEM images of (a) the compact TiO2 film, the oriented rod-type TiO2 films, the CH3NH3PbI3-xClx films (c) without and (d) with P3HT hole transport layer.
intensity of the (002) peak is the strongest in all peaks, rather than the (110) peak of rutile TiO2. This result indicates that the growth of TiO2 is highly oriented along the (002) plane on FTO substrate during the solvothermal reaction. On the other hand, it should be noted that the MAPbI3-xClx film in Fig. 2a is made of large grains up
to a size of ~500 nm, while the MAPbI3-xClx film in Fig. 2b is compact with the thickness of ~200 nm, which is much smaller than that of the device without P3HT. The decrease in the thickness of MAPbI3-xClx after spin-coating P3HT might be due to high infiltration of the dissolved perovskite by the solvent of P3HT.
Fig. 2. Cross-sectional SEM images of the MAPbI3-xClx-based solar cells (a) without and (b) with P3HT hole transport layer. (c) XRD pattern of a typical TiO2 film prepared by solvothermal reaction, annealed at 550 C.
Y. Zhang et al. / Journal of Power Sources 274 (2015) 1224e1230
3.2. Current densityevoltage (JeV) characteristics Fig. 3a shows the JeV characteristics of the solar cells employing MAPbI3-xClx sensitizer with and without P3HT hole transport layer under AM 1.5 illumination. The extracted cell performance parameters for two kinds of solar cells are given in Table 1. The PCE of the P3HT device is 6.06%, resulting from the short circuit current density (Jsc) of 14.65 mA/cm2, Voc of 0.742 V, and FF of 55.8%. However, the reference device without P3HT has the Jsc of 13.64 mA/cm2, Voc of 0.663 V, and FF of 38.8%, a PCE of 3.51%. In comparison, there is a notable increase in the Voc (0.663 Ve0.742 V) with P3HT and a notable increase in FF from 38.8% to 55.8%. This leads to a substantial increase in PCE from 3.51% of the device without P3HT to 6.06% of the P3HT device. This result clearly implies that the P3HT hole transport layer has contributed to the enhancement of the charge collection. On the other hand, the best PCE of P3HT-incorporated device is 6.06%, which is lower than that of the reported device incorporating spiro-OMeTAD hole transporting layers in literature [21]. Fig. 3b shows the IPCE spectra of perovskite solar cells with and without P3HT HTM. As we can see, the IPCE is improved by P3HT HTM. The JSC values integrated from the IPCE of perovskite solar cells with and without P3HT HTM are 14.12 mA/cm2 and 9.13 mA/cm2, respectively, which are almost agreed with the obtained by JeV.
3.3. Impedance spectra of MAPbI3-xClx-based solar cells In order to further investigate the role of P3HT, impedance spectroscopy measurements were carried out in the dark. Fig. 4a shows the Nyquist plots of the MAPbI3-xClx-based solar cells with and without P3HT hole transport layer in the dark. The solid curves are the fitting results with a transmission line model, as shown in the inset in Fig. 4a. For the solar cells without P3HT hole transport layer, the data can be well fitted with a circuit consisting of a resistor Rs and two elements consisting of a charge transfer resistance (Rct) and a constant phase element (CPE) elements for the one interface between TiO2 and MAPbI3-xClx and another interface between MAPbI3-xClx and Au electrode. However, for the cells with P3HT hole transport layer, the data can be well fitted with a simple circuit (not shown here) consisting of a resistor Rs and one Rct and CPE element for the interface between TiO2 and MAPbI3-xClx. It is well known that Rct is associated with the interface charge transport process, defined by the charge transfer resistance, and CPE suggests a non-ideal behaviour of the capacitor. It is usually defined
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Table 1 Photovoltaic parameters of MAPbI3-xClx-based solar cells with and without P3HT hole transport layer under illumination of 100 mW/cm2. Cell (w/o P3HT)
Voc (V)
Jsc (mA/cm2)
FF (%)
PCE (%)
With P3HT Without P3HT
0.742 0.663
14.65 13.64
55.8 38.8
6.06 3.51
by two values, CPE-T and CPE-P. The value of CPE-P is related to an ideal capacitor [22,23]. The observed parameters of Rs, Rct and CPE values are presented in Table 2. It should be noted that the fitted value of Rs in the device with P3HT, representing the internal resistance, is 16.5 U cm2, smaller than that (22.8 U cm2) of the device without P3HT. This indicates that the present P3HT layer decreases the internal resistance of solar cells. For the P3HT device, the absence of another semicircle is likely due to the presence of HTM in this device structure. The single semicircle also indicates that the interface contacts well between MAPbI3-xClx and TiO2. This may be due to the contacts between Au and MAPbI3-xClx or FTO and TiO2 are Ohmic contacts, rather than the rectifying contact. On the other hand, the value of CPE-T in the device with P3HT is 5.6E-5, much larger than that in the device without P3HT. This shows that the interface capacitance between oriented TiO2 and MAPbI3-xClx is more perfect electrically than that in the device without P3HT. These results suggest that the charge collection efficiency of the P3HT device was markedly enhanced, compared with the device without P3HT. The high charge collection will lead to efficient photovoltaic performance with higher FF and Voc. It is well in agreement with JeV characteristics, as shown in Fig. 3. In addition, the electron lifetime (the recombination time, te) were obtained from the characteristic frequency of the Bode-phase plots, shown in Fig. 4b, using the equation of te ¼ 1/2pfmid, where fmid is the phase angle peak at the midfrequency peak [24,25]. It is found that a value of te (0.145 ms) in the device with P3HT is much longer than that (0.038 ms) of the device without P3HT. This result reveals that the use of P3HT in perovskite-base solar cells reduces the charge recombination and leads to the large output voltage (Voc) and large FF, obtained for these devices. 3.4. Photoluminescence quenching The PL measurements were performed to further confirm the effect of P3HT on charge collection efficiency. PL spectra of perovskite-based solar cells with and without P3HT HTM are
Fig. 3. (a) JeV characteristics of the MAPbI3-xClx-based solar cells with and without P3HT hole transport layer under AM 1.5 simulated sunlight (100 mW/cm2). (b) IPCE spectra of perovskite solar cells with and without P3HT HTM.
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Fig. 5. Photoluminescence spectra of perovskite films with and without P3HT on top TiO2 with an excitation wavelength of 480 nm irradiation at room temperature.
Fig. 4. (a) Nyquist plots and (b) Bode phase plots of the MAPbI3-xClx-based solar cells with and without P3HT hole transport layer in the dark. The simulation result (solid curve) is fitted to experimental data (symbols) using the equivalent circuit in the insert of (a).
shown in Fig. 5. It is found that the PL peak from MAPbI3-xClx is at 793 nm, which is consistent with the reported previously [26]. It should be noted that the PL intensity is very sensitive to the P3HT, and almost entirely quenched by introducing P3HT HTM into the perovskite-based solar cell. The PL quenching indicates that the photon-generated carriers transfer to the anode from the perovskite layer. Therefore, when P3HT HTM was introduced into perovskite-based solar cell, charge collection efficiency was improved. 3.5. The energy levels of the materials involved in the MAPbI3-xClxbased solar cells To investigate the present P3HT interlayer concept, Fig. 6 shows the energy levels of the materials involved in the MAPbI3-xClxTable 2 Parameters employed for the fitting of the impedance spectra of MAPbI3-xClx-based solar cells with and without P3HT hole transport layer. HTM
Rs Rct1 (U cm2) CPE1-T CPE1-P Rct2 CPE2-T (F/cm2) (U cm2) (U cm2) (F/cm2)
Without 22.8 P3HT With 16.5 P3HT
280
7.7E-7
0.8
195.8
5.6E-5
0.655
180
CPE2-P
9.66E-8 0.858
based solar cells with and without P3HT hole transport layer. When light irradiates on the MAPbI3-xClx absorber in the devices through FTO electrode, the MAPbI3-xClx photoactive layer will absorb photons to produce excitons, and the excitons dissociate into electrons and holes. Subsequently, electrons flow to the conduction band bottom of the TiO2, and holes directly flow to the Au anode (shown as in Fig. 6a), or flow to the highest occupied molecular orbital (HOMO) of the P3HT, and then are collected at the Au anode. Since the HOMO level (5.2 eV) of P3HT is between the HOMO (5.4 eV) of MAPbI3-xClx and the work function (5.0 eV) of Au, as shown in Fig. 6b. Thus, the P3HT film between Au anode and perovskite layer provides a better energy level matching due to the well-aligned HOMO of P3HT with the HOMO of the MAPbI3-xClx. Holes can easily transport to Au anode through P3HT [27]. Therefore, the P3HT allows efficient hole collection and allows holes to transport from the photoactive perovskite to the Au anode. On the other hand, the highest occupied molecular orbital (LUMO) (3.0 eV) of P3HT prevents transferring electrons from MAPbI3-xClx to the Au electrode. Therefore, effective blocking of electron to the anode can efficiently reduce the charge recombination at the anode interface. Thus, P3HT has a band structure well suited for MAPbI3-xClx-based solar cells. The device with P3HT could exhibit higher charge collection efficiencies on both electrodes than those of the device without P3HT [28]. The P3HT layer plays an essential role in charge collection in perovskite-based cells. 3.6. Impact of the P3HT hole transport layer on device degradation As we all know that the stability of solar cells is very important to commercialized application. The long-term stability of perovskite solar cells is one issue that needs to be addressed. Park et al. first reported long-term stable perovskite solar cells for over 500 h [29]. Seok et al. reported the stability testing of perovskite solar cells based on a mixed halide of CH3NH3PbI3 and CH3NH3PbBr3 for 20 days, which confirmed that cubic phase cells had better stability than tetragonal phase devices [30]. H. J. Snaith and his colleagues demonstrated that the Al doping the mesoporous TiO2, or compact TiO2 films employed in perovskite cells could effectively suppress oxygen-induced defects in TiO2, which leads to the deterioration of ss-DSSC devices exposed to sunlight in an inert atmosphere [31]. Recently, Han et al. reported a triple-layer device exhibiting excellent long term stability over 1000 h in ambient air under full
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Fig. 6. The energy levels of the materials involved in the MAPbI3-xClx-based solar cells with and without P3HT hole transport layer. The arrows indicate hole or electron flows.
sunlight [32]. In this work, the air stability of devices with and without P3HT was investigated through the measuring of PCE maintenance of the device. The device stability was tested in a dark way. The typical devices without encapsulation were stored under ambient laboratory conditions. The P3HT devices and the reference devices without P3HT were stored in ambient environment without encapsulation for 14 days. The JeV curves of the fresh made devices and the devices stored in air for 14 days are shown in Fig. 7. The air stable properties of devices with and without P3HT are listed in Table 3. It can be seen that the PCE of the P3HT device still kept 81.2% of the initial value of the fresh made device, while the reference device without P3HT dropped down to only 2.20% after 14 days, 62.7% of the initial value of the fresh device. These results indicate that the P3HT device is more air stable compared with the reference device without P3HT. The relative stability of this P3HT device may be due to the encapsulation-like effect for the MAPbI3-xClx-based solar cells. More importantly, such devices employing the compact and oriented TiO2 film as the electron transporting layer is much more air stable in comparison with the devices based on multiporous TiO2. The compact and oriented TiO2 film could prevent the over infiltration of perovskite to avoid the possible interaction between TiO2 and perovskite as time went on.
Table 3 Air stability of the cells with and without P3HT hole transport layer. Device
Voc (V)
Jsc (mA/cm2)
FF (%)
PCE (%)
With P3HT (fresh) Without P3HT (fresh) With P3HT (after 14 days) Without P3HT (after 14 days)
0.742 0.663 0.739 0.617
14.65 13.64 12.33 9.57
55.8 38.8 54.0 37.2
6.06 3.51 4.92 2.20
4. Conclusions In conclusion, the function of a P3HT hole transport layer in perovskite MAPbI3-xClx-based solar cells was investigated systematically by JeV and impedance spectroscopy measurements. The P3HT device shows a substantial increase in PCE, Voc, and FF, compared with the reference device without P3HT. Impedance spectroscopy measurements demonstrated that the present P3HT layer decreases the internal resistance of solar cells and allows the interface between oriented TiO2 and MAPbI3-xClx to form more perfect electrically. It is also found that the electron lifetime in the device with P3HT is much longer than that of the device without P3HT. These results clearly suggest that the charge collection efficiency of the device with P3HT was markedly enhanced, compared with the device without P3HT. The analysis of the energy levels of the involved materials indicates that the P3HT film between Au anode and perovskite layer provides a better energy level matching for efficient transporting holes, and effectively prevents electron transfer back to the anode. Moreover, the P3HT layer can play a crucial role in the stability of such solar cells. Therefore, the proper hole transporting materials and the compact and oriented TiO2 electron transporting layer are essential in developing highly efficient and stable perovskite sensitized solar cells. Acknowledgements This work was supported by the Department of Education, Henan Province under contract No. 13A480062. References
Fig. 7. Degradation of JeV characteristics of the MAPbI3-xClx-based solar cells with and without P3HT hole transport layer exposed to ambient air for 14 days without encapsulation.
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