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p-type inorganic hole transport materials of Li, Cu-codoped NiOx films were deposited using a simple solution-based process. The as-prepared films were used as hole selective contacts for lead halide perovskite solar cell. An enhanced power conversion efficiency of 14.53% has been achieved due to the improved electrical conductivity and optical transmittance of the Li, Cu-codoped NiOx electrode interlayer.
© 2016 Optical Society of America
1. Introduction
Since they were firstly exploited as a semiconductor sensitizer in dye-sensitized solar cells in 2009 [1], the organic–inorganic halide perovskite materials of [CH3NH3(MA)PbX3, X = Br−, Cl−, or I− ] have shown great promise in the solar cell field [2–5]. Combining the appealing advantage of solution-phase processing with intense broad-band absorption, low exciton bonding energy and long charge diffusion length, the perovskite solar cell have therefore demonstrated rapid progress as a new photovoltaics (PV) technology [6–9]. The latest certified device efficiency of perovskite solar cell has exceeded 20% [10], which makes it promising in commercialization.
Generally, perovskite solar cells employ a methylammonium lead halide perovskite as the absorber layer, which was sandwiched between the n-type electron-transporting conductors and p-type hole-conducting interlayers [11–14]. The charge transport materials play an important role to finally determine the device performance and stability. For hole transport layers (HTLs), organic polymer or small molecule, such as Poly(3,4-ethylene dioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) and spiro-OMeTAD (2,2',7,7'-tetrakis (N,N-di-p-methoxyphenylamine) 9,9'-spirobifluorene) is widely used and have achieved promising efficiency [15,16]. However, from cost of fabrication and device for long-term operation points of view, the organic hole conductors were not better choice. On the other hand, inorganic p-type semiconductors demonstrate superior stability and tunable energy level, which provide potential method for maximizing hole extraction rates and minimizing energy losses [17–19]. Therefore, it is fundamental important to develop an appropriate inorganic hole-transport layer for further improving the photovoltaics performance and implementing future commercialization of perovskite solar cells.
Nickel oxide (NiOx) is a widely investigated p-type hole transporter material in organic photovoltaics (OPVs) [20–22]. Recently, there have been few reports of perovskite solar cells which used the NiOx as inorganic hole conductors [23–25]. However, it is known that stoichiometric NiOx is an insulator with high electrical resistivity over 1 kΩ cm [26]. Although its conductivity can be improved significantly by Cu doping, the transmittance of the electrode reduced simultaneously resulting from degradation in the photopermeability of NiOx films [27–29], which restricted the further improvement of device performance. On the other hand, the optical transparency of Li-doped NiOx thin films was reported as high as 80% [26,30], which inspired us to use Li, Cu-codoped NiOx films as potential HTLs in perovskite solar cells. To the best of our knowledge, it’s the first time that Li, Cu-codoped NiOx thin film was used as HTL in perovskite soalr cells. Compared to Cu-doped NiOx, both the conductivity of holes and transmittance of the electrode interlayer were enhanced by Li, Cu-codoping, the efficiency of perovskite solar cells was dramatically increased.
2. Experimental details
2.1 Materials
The nickel acetate hydrate (99.9%), copper acetate monohydrate (99.9%), and lithium acetate dehydrate (99.9%) were all supplied from Alfa Aesar. 2-Methoxyethanol (99%) was purchased from Aladdin. PbI2 (99%) and PCBM (99.9%) were obtained from Sigma Aldrich. CH3NH3I was synthesized according to a reported procedure [31]. Briefly, 30 mL of methylamine (33 wt% ethanol solution) and 32.3 mL of hydroiodic acid (57 wt% in water, Aldrich) were reacted in an ice bath for 2 h with stirring. The precipitate was collected, following by vacuum drying and cleaning with ethyl acetate.
2.2 Precursor solution preparation
The four different precursor solutions [pristine NiOx, 3 at% Cu-doped NiOx, 5 at% Cu-doped NiOx, 2 at% Li, 3 at% Cu-codoped NiOx] were prepared according the reported method [26]. Take 2 at% Li, 3 at% Cu-codoped NiOx solution for example, nickel acetate hydrate, copper acetate monohydrate and lithum acetate dihydrate were dissolved in 2-methoxyethanol together and stired at 70 °C for 2 h. The molar ratio of Ni2+: Cu2+: Li+ was 0.95: 0.03: 0.02 in the 0.3 M precursor solution.
2.3 NiOx thin films fabrication
The transparent p-type hole transporter NiOx thin films were fabricated on FTO substrate. First of all, the substrates were cleaned by detergent, deionized water (DI water), acetone, 2-propanol, methanol and ethanol in sequence. Then the precursor solution was spin-coated onto FTO surface at 3000 rpm for 30s and pre-annealed at 150°C for 5 min in ambient air. This process was repeated for three times. Finally, the precursor thin films were annealed at 500°Cfor 30 min in ambient air.
2.4 Device fabrication and characterization
The preparation process of perovskite solar cells was conducted in the argon-filled glove box. The perovskite layer was synthesised by traditional two-step method [32]. Firstly the PbI2 solution (461mg ml−1) was spin-coated onto the UV zone-treated NiOx films and annealed at 75°C for 20 min, then 10 mg mL−1 of CH3NH3I was spin-coated onto the PbI2 layer, the as-prepared perovskite film was annealed at 90°C for 20 min. After cooling to room temperature, the PCBM (15 mg ml−1) was spin-coated onto the perovskite films. Finally, a 100 nm-thick silver electrode was thermally evaporated onto the PCBM layer to complete the fabrication of perovskite solar cells.
The scanning electron microscope (SEM) images were collected by using a Nova NanoSEM 450 and X-ray diffraction (XRD) patterns were performed by a Bruker D8 Advance X-ray diffractometer. Photocurrent density-voltage (PCE) curves (J–V curves) were recorded by a Keithley 2400 source meter, the light intensity was standardized to 100 mW cm2 by using Newport optical power meter (model 842-PE). The external quantum efficiency (EQE) curves were measured by using Zolix SCS100 QE system equipped with a 150-W xenon light source and a lock-in amplifier. The photoluminescence (PL) spectrum were carried out by JY HORIBA FluoroLog-3. The carrier density and carrier mobility were measured by Variable Temperature Hall effect Tester (ET9005-S). The Atomic force microscopy (AFM) images were obtained by atomic force scanning probe microscope (SPA - 400). The Electronic impedance spectroscopy was collected by electrochemical workstation Chen Hua CHI-660e.
3. Results and discussion
Planar perovskite solar cells with the device configuration of FTO/HTLs (pristine NiOx, 3 at% Cu-doped NiOx, 5 at% Cu-doped NiOx, 2 at% Li, 3 at% Cu-coped NiOx)/perovskite (MAPbI3)/PCBM/Ag were fabricated. Figure 1 shows XRD pattern of NiOx thin films with different doping parameters. As shown in Fig. 1, the three main diffraction peaks of the pristine NiOx were located at 37.3°, 43.3° and 63.1°, corresponding to the (111), (200), (220) plane of NiOx respectively. From Fig. 1 we can clearly see that the diffraction peaks of the doped NiOx films did not change dramatically, which could be interpreted as a small amount of doping of Cu and Li did not change the phase structure and the original cubic NaCl structure of NiOx was reserved [26].
Fig. 1 XRD patterns of pristine NiOx, 3 at% Cu-doped NiOx, 5 at% Cu-doped NiOx, 2 at% Li, 3 at% Cu-codoped NiOx.
The morphology of the NiOx electrode interlayer plays an important role in the film formation and surface morphology of the perovskite absorber layer, which finally determined the photovoltaic parameters of the ultimate solar cell devices. Atomic force microscopy (AFM) was used to characterize the surface morphology of the NiOx thin films. As presents in Fig. 2, high-quality NiOx nanocrystal films with smooth and crack-free surface were obtained after spin-coating and heating process. Uniform grains with size ranging from 20 to 50 nm aggregated densely together, which could effectively transfer hole and block electron through the NiOx film. Moreover, it is worth mention that the grain size of the NiOx nanocrystal films increased linearly with increasing doping concentration, resulting in the difference in root-mean-square (RMS) roughness and surface energy of the NiOx hole-transporting layer, which would affect the film formation and surface morphology of the active layer deposited on it and then yield disparity in short circuit current and fill factor of perovskite solar cells.
Fig. 2 AFM phase images of (a) pristine NiOx film, (b) 3 at% Cu-doped NiOx film, (c) 5 at% Cu-doped NiOx film, (d) 2 at% Li, 3 at% Cu-codoped NiOx film, the scale bar is 200 nm.
In order to investigate the extraction efficiency of photo-generated carriers from the perovskite absorber layer to the hole-transport layers, steady-state photoluminescence (PL) measurements were conducted. Figure 3 presents the PL quenching of MAPbI3 perovskite films on contact with different sublayers, from which it can be seen that the photoluminescence emission of perovskite layer was dramatically quenched by NiOx layers. Furthermore, with the increase of Cu contents from 0.03 to 0.05, stronger quenching effect was observed. The 2 at% Li, 3 at% Cu-codoped NiOx hole selective contact exhibits the highest quenching rates among all the hole conductor used in our experiment. As a universal method adopted in all organic solar cells, the PL quenching can effectively demonstrate efficient charge transfer from the photoactive layer to the transport layer. Our observation of enhanced PL quenching by introducing Li/Cu into NiOx indicates improved hole extraction and transport ability by doping.
Fig. 3 Photoluminescence of perovskite which based on glass, pristine NiOx, 3 at% Cu-doped NiOx, 5 at% Cu-doped NiOx, 2 at% Li, 3 at% Cu-codoped NiOx.
The surface morphology of perovskite film plays an important role in ultimate device performance. Scanning electron microscope (SEM) image of MAPbI3 film fabricated by two-step spin-coating method is shown in Fig. 4(a). From which it is evident that large grain size of MAPbI3 films formed and the obtained perovskite thin film is homogeneous and densely packed. This demonstrates the advantage of NiOx interface on perovskite crystallization. X-ray diffraction (XRD) pattern of MAPbI3 thin film is shown in Fig. 4(b). The as-prepared MAPbI3 film displays a comparatively strong diffraction peak at 14.1°, which corresponds to the tetragonal structured perovskite. At the same time, a small PbI2 diffraction peak located at 12.6° is observed, indicating partial PbI2 was not converted into perovskite.
Fig. 4 (a) SEM and (b) XRD of MAPbI3 film fabricated by a two-step spin-coating process onto NiOx-based HTLs.(Diamond stands for FTO, triangle stands for NiOx, star stands for PbI2).
Encouraged by the enhanced photo-generated carrier extraction efficiency of NiOx thin films and the fine surface morphology of the perovskite absorber layer grown on them, perovskite solar cells using the pristine and doped NiOx thin films described above as HTLs were fabricated. Figure 5 shows the cross-sectional-view SEM image of the final perovskite photovoltaic devices, which clearly exhibits the inverted planar heterojunction device configuration consisted of glass/FTO/HTLs/perovskite (MAPbI3)/PCBM/Ag. In this picture, PCBM and Ag act as the electron transport layers and negative electrode, respectively. The light irradiated from the NiOx transparent conductive electrode and was absorbed by the MAPbI3. Charge carriers generated in the perovskite layer were subsequently collected by the respective selective contact.
Figure 6(a) illustrates the illuminated (AM 1.5) current-voltage (J–V) curves for the best perovskite solar cells with different NiOx films. The J–V curves were obtained by reverse scan (1.1 V→﹣0.1 V, step 0.08 V, delay time 500 ms). Photovoltaic devices with pristine NiOx HTLs exhibited the highest open-circuit voltage (VOC) of 0.995V, which was resulted from the minimal energy loss at the NiOx/CH3NH3PbI3 perovskite heterojunction interfaces because of favorable energy level alignment [29]. But the power conversion efficiency (PCE) of pristine NiOx-based solar cell was the lowest in the listed four different kinds of devices, owing to the low short circuit current density (JSC) and fill factor (FF). 3 at% Cu-doped NiOx substantially improved the photovoltaic performance, giving a VOC = 0.957 V, a JSC = 17.68 mA/cm2, and a FF = 69%, corresponding to a PCE of 11.67%. However, with the increase of Cu content to 5%, the PCE of device decreased to 9.48%. The perovskite solar cell with 2 at% Li, 3 at% Cu-codoped NiOx film delivered the best device performance, achieving a VOC of 0.961 V, JSC of 20.8 mA/cm2 and FF of 72.71%, leading to an overall device efficiency of 14.53%. The average value of VOC, JSC, FF and PCE from ten perovskite solar cells with different NiOx HTLs are shown in Table 1. External quantum efficiency (EQE) spectra of the photovoltaic devices were also examined and presented in Fig. 6(b). The perovskite solar cell exhibits a broad spectral response from the visible to near-infrared (300 to 800 nm) region. The integrated photocurrent from the EQE over an AM 1.5G spectrum is consistent well with corresponding J–V measurement. In addition, the hysteresis of NiOx-based solar cell was examined. As presented in Fig. 7, the device exhibited negligible hysteresis, which demonstrated that the NiOx HTLs could effectively mitigate hysteresis in perovskite solar cells [29].
Table 1. Summarized characteristics of perovskite solar cells based on different HTLs
Fig. 7 Typical J–V curves of perovskite solar cells based on different HTLs with reverse scan (1.1 V→﹣0.1 V, step 0.08 V, delay time 500 ms) and forward scan (– 0.1V→1.1V, step 0.08 V, delay time 500 ms): (a) pristine NiOx, (b) 5 at% Cu-doped NiOx, (c) 3 at% Cu-doped NiOx, (d) 2 at% Li, 3 at% Cu-codoped NiOx.
From the measured J–V curves, it is obvious that doping NiOx electrode interlayer with Li and Cu is an effective way to improve the photon-to-electron output of organometal halide perovskite-based solar cells, especially the JSC and FF. The enhanced JSC and FF from Li, Cu-codoped NiOx-based device could stem from two factors. The first is reduced resistivity by doping, resulting from creating nickel vacancies and forming interstitial oxygen atoms in NiOx crystallites [29]. Carrier concentration and carrier mobility of the pristine NiOx and Li, Cu-codoped NiOx film were measured via Hall effect, the result is plotted in Fig. 8. The positive carrier concentration demonstrates NiOx films exhibit p-type conduction. Although there was little change in the carrier concentration, the carrier mobility of the doped NiOx films significantly increased, especially for the Li, Cu-codoped NiOx film. Compared to 5 at% Cu-doped NiOx thin film, the carrier mobility of 2 at% Li, 3 at% Cu-codoped NiOx film increased from 100.9 cm2 V−1 s−1 to 165.9 cm2 V−1 s−1. Therefore, the electrical conductivity of the Li, Cu-codoped NiOx film improved, which would increase the charge extraction efficiency of the hole transport layers, resulting in enhanced JSC and FF. To further investigate the internal charge transport and recombination of the perovskite solar cells, Electronic impedance spectroscopy (EIS) was performed from 1 Hz to 1 MHz under illuminated condition of 1 sun (see Fig. 9). The EIS demonstrated two obvious arcs at high frequency and lower frequency for all cells. According to recent literature [33], the high frequency arc is associated with the charge transfer resistance at NiOx/MAPbI3 interface (RHTM), and the lower frequency arc is assigned to the recombination resistance of the perovskite film. One can see that the cell based on Li, Cu-codoped NiOx HTL exhibited lower charge transfer resistance compared to other three samples, which represented better charge transport capacity for device. The detailed data of RHTM for each device is shown in Table 2. As RHTM has contributed to the series resistor, JSC and FF of perovskite solar cells would be improved with the decrease of RHTM, which is conformed to our research results (see Table 1).
Fig. 8 The carrier concentration and carrier mobility of four different HTLs: (Z1 stands for pristine NiOx film, Z2 stands for 3 at% Cu-doped NiOx film, Z3 stands for 5 at% Cu-doped NiOx film, Z4 stands for 2 at% Li, 3 at% Cu-codoped NiOx film).
Table 2. EIS data of four different perovskite solar cells
Another important factor is the transmittance of HTLs, which could save the incident light for the perovskite absorber. Figure 10 shows the transmission spectrum of the pristine NiOx and Li, Cu-codoped NiOx films. The pristine NiOx displayed the best transmittance of about 90% in the visible region. The transmittance decreased with increasing Cu content in the Cu-doped NiOx thin films, which can be ascribed to the incident light being scattered and reflected by emerging grain boundaries and Cu clusters separately [28]. Accordingly, lower photovoltaic performance was observed for the 5 at% Cu-doped NiOx compared to 3 at% Cu-doped NiOx. However, as shown in Fig. 8, doping with Li can improve the transmittance of Cu-doped NiOx film, the transmittance of 2 at% Li, 3 at% Cu-codoped NiOx films is almost 85% in visible region. As a result, the solar cell based on Li, Cu-codoped NiOx film yielded the best device performance.
4. Conclusion
In summary, Li, Cu-codoped NiOx films were fabricated by a simple solution-based process. The pristine and doping NiOx films were employed as HTLs in planar heterojunction perovskite (MAPbI3) solar cells. The results demonstrated that the electrical conductivity of NiOx films could be significantly improved by Cu doping, whereas the transmittance of the NiOx films decreased. By further introducing Li in the Cu-doped NiOx thin films, inorganic p-type metal oxide electrode interlayer with high transmittance and low resistivity were obtained. The perovskite photovoltaic device fabricated using 2 at% Li, 3 at% Cu-codoped NiOx hole acceptors exhibits a decent efficiency of 14.53%. This work offers a reliable and effective approach to enhance the optical and electrical properties of p-type NiOx hole collection layer, which could be applicable to fabricate many other high quality optoelectronic devices.
Funding
National Natural Science Foundation of China (21271064 and 61306016); China Postdoctoral Science Foundation (2015M582179); The Program for Changjiang Scholars and Innovative Research Team in University (PCS IRT1126) of Henan University.
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