The Influence of Work Function of Hybrid Carbon Electrodes on

Publication Date (Web): July 2, 2018 ... In this paper, we propose a hybrid carbon electrode based on a high-temperature mesoporous carbon (m-C) layer...
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The Influence of the Work Function of Hybrid Carbon Electrodes on Printable Mesoscopic Perovskite Solar Cells Pei Jiang,†,§ Yuli Xiong,†,§ Mi Xu,†,§ Anyi Mei,† Yusong Sheng,† Li Hong,† Timothy W. Jones,‡ Gregory J. Wilson,‡ Sixing Xiong,† Daiyu Li,† Yue Hu,† Yaoguang Rong,*,† and Hongwei Han*,† †

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Michael Grätzel Center for Mesoscopic Solar Cells, Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, Hubei, P. R. China ‡ CSIRO Energy, Mayfield West, NSW 2304, Australia S Supporting Information *

ABSTRACT: In printable mesoscopic perovskite solar cells (PSCs), carbon electrodes play a significant role in charge extraction and transport, influencing the overall device performance. The work function and electrical conductivity of the carbon electrodes mainly affect the open-circuit voltage (VOC) and series resistance (Rs) of the device. In this paper, we propose a hybrid carbon electrode based on a high-temperature mesoporous carbon (m-C) layer and a low-temperature highly conductive carbon (c-C) layer. The m-C layer has a high work function and large surface area and is mainly responsible for charge extraction. The c-C layer has a high conductivity and is responsible for charge transport. The work function of the m-C layer was tuned by adding different amounts of NiO, and at the same time, the conductivities of the hybrid carbon electrodes were maintained by the c-C layer. It was supposed that the increase of the work function of the carbon electrode can enhance the VOC of printable mesoscopic PSCs. Here, we found the VOC of the device based on hybrid carbon electrodes can be enhanced remarkably when the insulating layer has a relatively small thickness (500−1000 nm). An optimal improvement in VOC of up to 90 mV could be achieved when the work function of the m-C was increased from 4.94 to 5.04 eV. When the thickness of the insulating layer was increased to ∼3000 nm, the variation of VOC as the work function of m-C increased became less distinct.

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traditional PSCs. Besides, the inorganic mesoporous scaffolds of TiO2/ZrO2/carbon for hosting the perovskite absorber, especially the hydrophobic carbon layer, have enabled such devices with remarkable stability.19,20 The screen printing technique based fabrication process provides a prospect for upscaling and further mass-production.21,22 In printable mesoscopic PSCs, the carbon electrode which is cheap and stable extracts holes from the perovskite absorber and collects electrons from the external circuit.23,24 However, the Fermi levels (EF) of pristine carbon materials such as carbon black and graphite are usually far above the valence band (VB) of perovskite. The mismatched energy level alignment leads to inefficient hole extraction and low opencircuit voltage (VOC).25 Previously, we have investigated the effect of the morphology and corresponding electric properties of carbon electrodes on printable mesoscopic PSCs.26 Then, we attempted to increase the work function of the carbon electrode and improve the device performance.27 At the same time, other groups inserted an extra layer of NiO between the ZrO2 and carbon layer, constructing a p-i-n configuration.28 It was proposed that the NiO interlayer can enhance the device

rganic−inorganic hybrid perovskite solar cells (PSCs) have garnered recent interest in the scientific community due to their low cost of production via solution-processed fabrication techniques.1−5 In the past few years, the certified power conversion efficiency (PCE) of PSCs has reached 22.7%6 and is comparable to traditional commercialized solar cells (e.g., crystalline Si, CuInGaSe, and CdTe). The high performance of PSCs can be attributed to the outstanding optoelectronic properties of the hybrid perovskite materials (ABX3: A = CH3NH3+, HC(NH2)+, Cs+; B = Pb2+, Sn2+, Ge2+; X = Cl−, Br−, I−), including high absorption coefficient,7,8 high charge mobility,9,10 long balanced carrier diffusion length, and low exciton binding energy.10,11 As a photovoltaic technology, which needs to meet the requirements for working under outdoor conditions, PSCs currently suffer stability concerns for both materials and devices.12,13 Particularly, for the typical metal electrodes in PSCs, either Au, Ag, or Al may react with the perovskite absorber, leading to severe degradation of the device performance.14,15 Among different architectures for PSCs, printable mesoscopic PSCs based on carbon electrodes seem to be one of the most promising technical routes.16−18 In such devices, no hole transport materials (HTMs) are needed, and carbon materials are employed to replace the traditional metal electrodes. Thus, the material cost is much lower than © 2018 American Chemical Society

Received: March 5, 2018 Revised: June 28, 2018 Published: July 2, 2018 16481

DOI: 10.1021/acs.jpcc.8b02163 J. Phys. Chem. C 2018, 122, 16481−16487

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The Journal of Physical Chemistry C

Figure 1. Schematic configuration and energy band diagram of printable mesoscopic PSCs with a single m-C layer (a, c) and a hybrid m-C/c-C layer (b, d) as the counter electrode. Efn and Efp are the electron and hole quasi-Fermi levels, respectively.

the conductive band (CB) of the TiO2, and the holes can be collected by the carbon electrode and arrive at the external circuit. The working principles of such printable mesoscopic PSCs are shown in Figure 1c and d.30 The VOC is mainly determined by the difference between the Efn (electron quasiFermi level) and Efp (hole quasi-Fermi level).31 In printable mesoscopic PSCs, Efn is determined by the interaction between the electron transfer layer (e.g., TiO2) and the perovskite layer, while Efp is determined by the interaction between the perovskite layer and the carbon electrode. Lower EF for the carbon electrode can help increase the VOC and PCE. When the single carbon layer is replaced by a hybrid carbon layer, the Efp position may drop due to the lower EF of the p-type metal oxides modified m-C in the hybrid carbon layer. As a result, the VOC of the devices may increase. Table S1 presents the work function of m-C layers with NiO, MoO3, Co2O3, and CuO. When p-type metal oxides were mixed with the m-C layer, the work function of the m-C layer increased from 4.94 to 4.97 eV for m-C/10% NiO, 5.15 eV for m-C/10% MoO3, and 4.98 eV for m-C/10% Co2O3. Table S2 summarizes the photovoltaic performance of printable mesoscopic PSCs based on different hybrid carbon electrodes. Unfortunately, only the devices using a NiO modified m-C electrode showed a tiny enhancement in performance, while the devices using MoO3, Co2O3, and CuO showed much decreased efficiencies. Figure 2 shows the photograph, crosssectional scanning electron microscope (SEM) images, and energy dispersive X-ray (EDX) spectroscopy of the devices. It can be observed that the printable mesoscopic PSC based on a hybrid carbon electrode with MoO3 (Figure 2a) shows very poor perovskite pore filling compared with NiO based devices (Figure 2b and c), which is the cause of the extremely low short-circuit current density (JSC) of 1.54 ± 0.02 mA cm−2. For the devices using Co2O3 and CuO as the modifiers, it was found that the elements of Co and Cu also existed in the

performance by extracting holes efficiently and can enlarge the difference between the electron/hole EF of selective materials. Herein, we designed a hybrid carbon electrode based on a mesoporous carbon (m-C) layer and a compact and highly conductive carbon (c-C) layer and investigated the effect of their properties on the performance, especially the VOC, of printable mesoscopic PSCs. The m-C layer was modified with p-type metal oxides and possessed a high work function. Due to the mesoporous microstructure, the m-C layer has quite a large surface area and thus can further facilitate the contact and charge extraction between the perovskite absorber and the carbon counter electrode. The c-C layer has a high conductivity29 and can minimize the influence of the variation of the conductivity of the carbon electrodes after mixing with the metal oxides. It was found that the work function of the mC layer can be effectively enhanced from 4.94 to 5.04 eV by incorporating NiO. When the ZrO2 layer is relatively thin (500−1000 nm), through varying the ratio of NiO in the m-C layer and the thickness of the ZrO2 layer, the enhancement in VOC of the devices can be up to 90 mV. When the thickness of the ZrO2 layer increased to ∼3000 nm, the enhancement in the VOC as the work function increased became less distinct. Parts a and b of Figure 1 show the schematic configurations of printable mesoscopic PSCs based on a typical single m-C layer and a hybrid m-C/c-C layer as the counter electrode. For the device based on a single m-C layer, the compact TiO2 layer, mesoporous TiO2 layer, ZrO2 layer, and carbon layer are used as the hole blocking layer, the electron transport layer (ETL), the insulating layer, and the counter electrode, respectively. The perovskite is infiltrated in the mesoporous triple-layer of TiO2/ZrO2/carbon. For the device based on a hybrid carbon layer, after the infiltration of the perovskite absorber in the m-C layer, a c-C layer is screen printed on the top of m-C/perovskite. Due to the ambipolar perovskite and energy-level alignment, the electrons can efficiently inject into 16482

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ratio of NiO in the m-C layer was further increased to 30 and 50% for comparison. Figure 3a shows the X-ray diffraction (XRD) spectra of m-C modified with different amounts of NiO. The peak at 2θ = 54.79° can be indexed as the (004) crystal planes of carbon black and graphite. The peaks at 2θ = 37.14, 43.29, and 62.80° can be indexed as the (111), (200), and (220) crystal planes of NiO.32 As the ratio of NiO increased from 10 to 30 to 50%, the intensity of these peaks correspondingly increased. The increased ratio of NiO in the m-C layer was also confirmed by X-ray photoelectron spectroscopy (XPS) measurements (Figure S3). As the ratio of NiO in the m-C layer increased, the peak of Ni 2p at 532 eV continuously increased, while the peak of C 1s at 284 eV decreased, which is consistent with the results of XRD. Figure 3b shows the variation of the conductivity of the m-C layer with different amounts of NiO. As the ratio of NiO increased, the sheet resistance (R□) increased from 26.76 Ω □−1 for the pristine m-C layer to 114.78 Ω □−1 for the m-C/ 10% NiO layer. When the ratio of NiO further increased to 30 and 50%, the R□ of the modified m-C layer increased to 141.74 and 617.00 Ω □−1. To avoid the negative effect caused by the increased resistance on device performance, we employed a c-C layer on the top of the m-C layer. The R□ of the m-C/c-C hybrid carbon electrodes is only around 8 Ω □−1, and it stayed almost unchanged when they were modified with NiO (Table S3). The work function of the NiO modified m-C layers was characterized by Kelvin probe measurements, and the values are summarized in Table S4. As shown in Figure 3c, the work function of the m-C layers continuously increased from 4.94 to 5.04 eV, as the ratio of NiO increased from 0 to 50%. Figure 3d shows the time-resolved photoluminescence (TRPL) spectra of the perovskite layer in the m-C layers

Figure 2. (a) Photograph of printable mesoscopic PSCs based on hybrid carbon electrodes with NiO, MoO3, Co2O3, and CuO, respectively. (b, c) SEM cross-section images and EDX elements mapping of printable mesoscopic PSCs based on a hybrid carbon electrode with NiO and MoO3.

TiO2/perovskite and ZrO2/perovskite layers (Figure S2). This indicates that Co2O3 and CuO may react or interact with the perovskite absorber, and thus cause a negative effect on the device performance. The devices based on m-C/10% NiO showed slightly improved performance with an average JSC of 23.43 mA cm−2, VOC of 0.92 V, fill factor (FF) of 0.65, and PCE of 13.94%. The

Figure 3. (a) XRD patterns of the pristine m-C layer and m-C layers with 10, 30, and 50% NiO. (b) Sheet resistance of the pristine m-C layer and m-C layers with 10, 30, and 50% NiO (the thickness is ∼10 μm). For the average and s.d. values, the results were obtained with five samples prepared with the same condition. (c) Work function of the pristine m-C layer and m-C layers. For the average and s.d. values, the results were obtained with eight samples prepared under the same conditions. (d) Time-resolved photoluminescence (TRPL) spectra of the samples of perovskite/m-C, perovskite/(m-C/10% NiO), perovskite/(m-C/30% NiO), and perovskite/(m-C/50% NiO). 16483

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Figure 4. Photovoltaic parameters of devices based on hybrid carbon electrodes with 10, 30, and 50% NiO. The thickness of the insulating ZrO2 layer varies from 500 nm to 3 μm. For the average and s.d. values, the results were obtained from five devices prepared under the same conditions.

Figure 5. (a) Current density−voltage (J−V) characteristics measured from forward bias to reverse bias under simulated AM 1.5 G illumination (100 mW cm−2) conditions and in the dark of the representative devices based on hybrid carbon electrodes with 10, 30, and 50% NiO and with a 500 nm ZrO2 layer. (b) Impedance spectroscopy characterizations of the devices; the Nyqusit plots are collected in the frequency range of 50 mHz to 4 MHz under illumination at 10 mW cm−2 with applied bias of 0.5 V. The solid lines are the fitted curves. (c, d) Charge transport lifetime (τt) and recombination time (τr) obtained by fitting the transient photocurrent/photovoltage decay curves for devices based on hybrid carbon electrodes with 10, 30, and 50% NiO.

with or without NiO. It was found that the samples of perovskite/(m-C + 10%/30%/50% NiO) showed a much stronger PL quenching effect compared with the sample of perovskite/m-C. The perovskite/m-C interface yielded a much longer lifetime of 15.95 ns, while the perovskite/(m-C/10% NiO), perovskite/(m-C/30% NiO), and perovskite/(m-C/ 50% NiO) yielded shorter lifetimes of 2.77, 1.65, and 2.59 ns

(Table S5). The results indicate that the incorporation of NiO in the m-C layers can effectively enhance the hole extraction ability of the m-C electrodes. Notably, the m-C/30% NiO layer showed a slightly higher quenching efficiency than m-C/50% NiO. This might be related to variation of the conductivity of the m-C layer, for which incorporating 50% NiO in the m-C layer caused a severe decrease of the conductivity. 16484

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recombination time constant (τr) were derived from the IMPS and IMVS responses. As illustrated in Figure 5c and d, the devices based on m-C with 30% NiO exhibited the shortest τt, while the devices based on m-C with 50% NiO exhibited the longest τt. For τr, the devices based on m-C with 50% NiO showed the largest values, indicating a significantly suppressed recombination process. A more efficient hole extraction due to the lower Efp of the m-C would enhance the electron−hole separation at the interface of perovskite infiltrated in the ZrO2 layer and carbon layer, hence suppressing the charge recombination. These results are consistent with the EIS data as discussed above. According to the above discussions, we propose that increasing the work function of the carbon electrodes can effectively improve the VOC without sacrificing the JSC and FF of the devices on the condition that the ZrO2 possesses a relatively small thickness. However, it should be noted that the devices also achieved a relatively low VOC, even with NiO in the m-C layer. This indicated that the insulating effect of the ZrO2 layer was weakened as the thickness decreased. The TiO2 layer and carbon layer may contact directly and lead to severe charge recombination. Increasing the thickness of ZrO2 is an effective method to completely separate the TiO2 layer and the carbon layer. However, the correspondingly increased charge diffusion length will also aggravate the recombination process and dominate the VOC. Besides, the quality of the perovskite absorber in the mesoporous scaffolds and corresponding interfaces also plays an essential role in the recombination process for the device performance.33,34 When NiO is incorporated in the m-C layer, the microstructure of the scaffold may also change, especially for the high ratio of NiO, influencing the crystallization process and quality of the perovskite absorber. The NiO can enhance the extraction of holes from the perovskite. At the same time, NiO may help separate the TiO2 and carbon layer when the ZrO2 layer is too thin. Thus, in the next step, to further improve the performance of printable mesoscopic PSCs, it is urgent to develop counter electrodes with higher work function to facilitate the charge extraction and explore more efficient insulating layers to minimize the charge diffusion length. In conclusion, a hybrid carbon electrode was developed for printable mesoscopic PSCs via incorporating a m-C layer with a c-C layer. As the work function of m-C is increased from 4.94 to 5.04 eV, the VOC of the devices can be enhanced without sacrificing the JSC and FF, when the ZrO2 layer possesses a relatively small thickness. The more efficient hole extraction ability of the m-C layer can effectively suppress the charge recombination in the device. When the thickness of ZrO2 increased to ∼3 μm, the VOC of the devices was mainly dominated by the recombination process due to the longer charge diffusion length. This work has revealed how the work function of hybrid carbon electrodes influences the performance of printable mesoscopic PSCs and provided theoretical support to further break the performance bottlenecks of printable mesoscopic PSCs.

As the work function of the m-C layers significantly increased when NiO was incorporated, it was expected the VOC and overall PCE of the devices can also be improved.25,27 However, when we fabricated devices with m-C/30% NiO and m-C/50% NiO, the VOC stayed almost unchanged (Figure 4a and Table S6), while the PCE slightly increased (Figure 4b). For printable mesoscopic PSCs, the photogenerated holes have to travel through the perovskite absorber in the entire ZrO2 layer and reach the carbon electrode. Considering the ZrO2 layer has a thickness of 3 μm, the recombination due to the relatively long charge diffusion length might dominate the VOC of the device and weaken the influence from the counter electrode. To gain a deeper insight into how the work function of the carbon electrode influences the photovoltaic performance of printable mesoscopic PSCs, we reduced the thickness of the insulating ZrO2 layer to 1 μm and 500 nm. When the thickness of the ZrO2 layer was reduced to 1 μm, an enhancement in the VOC of ∼60 mV could be observed when NiO was incorporated in the m-C layer. When an ultrathin ZrO2 layer of 500 nm was employed, the enhancement could further increase to ∼90 mV, as the ratio of NiO varied from 0 to 50% in the m-C layer. At the same time, the FF of the devices based on carbon electrodes with different amounts of NiO were around 0.64−0.65 (Figure 4c). The JSC of the devices slightly increased when 10% NiO was incorporated and then stayed unchanged for higher NiO ratios (Figure 4d). Thus, the values of the overall PCE of the devices were mainly determined by the variations of the VOC. Figure 5a shows the current density−voltage (J−V) curves of the representative devices based on hybrid carbon electrodes with 10, 30, and 50% NiO and with a 500 nm thick ZrO2 layer. The devices showed similar JSC and FF and varied VOC. We mainly focused on the charge extraction and recombination process in the devices. It was expected that a more efficient charge extraction of the m-C layer due to a higher ratio of NiO can suppress charge recombination in the device, which was characterized by impedance spectroscopy (IS) measurements. As shown in Figure 5b, the RC response in the high-frequency region is assigned to the charge exchange process at the interface of perovskite/m-C, whereas the low frequency region represents the interface of perovskite/TiO2. Rs is the series resistance, mainly including the sheet resistance of FTO, the contact resistance, and so on. Charge-transfer resistance (Rct) is the charge transfer resistance corresponding to the charge transport through the interface of perovskite/m-C. Chargerecombination resistance (Rrec) is the charge transfer resistance of the recombination process between electrons in the TiO2 layer and holes in the perovskite. The impedance data were fitted by three RC circuit models, as shown in Figure S4, and all capacitor elements were replaced by constant phase elements. In Table S7, Rct of devices based on m-C with 30% NiO was found to be considerably lower than those of 10 and 50%, which supported the higher charge transfer rate at the perovskite/m-C interface. The Rrec of m-C with 50% NiO was found to be considerably higher than those of 30 and 10%, which supported the lower recombination rate at the perovskite/TiO2 interface. The photoexcited charge-carrier dynamics in these devices were further investigated by intensity modulated photocurrent spectroscopy (IMPS) and intensity modulated photovoltage spectroscopy (IMVS) measurements, which can be used to reveal the charge transport and recombination processes in PSCs. The charge transport time constant (τ t ) and

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b02163. The experimental details, the SEM cross-section and EDX elements mapping images (Figure S1), the XPS 16485

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spectra (Figure S2), the equivalent circuit diagram (Figure S3), the scheme of the fabrication process of the devices (Figure S4), and the photovoltaic parameters of the devices (Tables S1−S9) (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Yaoguang Rong: 0000-0003-4794-8213 Hongwei Han: 0000-0002-5259-7027 Author Contributions §

P.J., Y.X., and M.X. contributed equally in this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge financial support from the National Natural Science Foundation of China (Grant Nos. 51502141, 91733301, 91433203, 61474049, and 21702069), the Ministry of Science and Technology of China (863, 2015AA034601), the Fundamental Research Funds for the Central Universities, the Science and Technology Department, the Science and Technology Department of Hubei Province (No. 2017AAA190), the 111 Project (No. B07038), the China Postdoctoral Science Foundation (2017M612452), and Special financial aid to postdoctor research fellow (2017T100548). We thank the Analytical and Testing Center of Huazhong University of Science and Technology (HUST) for performing various characterizations and measurements.

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DOI: 10.1021/acs.jpcc.8b02163 J. Phys. Chem. C 2018, 122, 16481−16487

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DOI: 10.1021/acs.jpcc.8b02163 J. Phys. Chem. C 2018, 122, 16481−16487