Room-Temperature and Solution-Processable Cu-Doped Nickel ...

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Room-Temperature and Solution-Processable Cu-Doped Nickel Oxide. Nanoparticles for Efficient Hole-Transport Layers of Flexible large-area Perovskite.
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Room-Temperature and Solution-Processable Cu-Doped Nickel Oxide Nanoparticles for Efficient Hole-Transport Layers of Flexible Large-Area Perovskite Solar Cells Qiqi He,† Kai Yao,*,‡ Xiaofeng Wang,† Xuefeng Xia,† Shifeng Leng,‡ and Fan Li*,†,§ †

Department of Materials Science and Engineering and ‡Institute of Photovoltaics, Nanchang University, 999 Xuefu Avenue, Nanchang 330031, China § State Key Laboratory of Molecular Engineering of Polymer, Fudan University, Shanghai 200433, China S Supporting Information *

ABSTRACT: Flexible perovskite solar cells (PSCs) using plastic substrates have become one of the most attractive points in the field of thin-film solar cells. Low-temperature and solution-processable nanoparticles (NPs) enable the fabrication of semiconductor thin films in a simple and low-cost approach to function as charge-selective layers in flexible PSCs. Here, we synthesized phase-pure p-type Cu-doped NiOx NPs with good electrical properties, which can be processed to smooth, pinholefree, and efficient hole transport layers (HTLs) with large-area uniformity over a wide range of film thickness using a roomtemperature solution-processing technique. Such a high-quality inorganic HTL allows for the fabrication of flexible PSCs with an active area >1 cm2, which have a power conversion efficiency over 15.01% without hysteresis. Moreover, the Cu/NiOx NP-based flexible devices also demonstrate excellent air stability and mechanical stability compared to their counterpart fabricated on the pristine NiOx films. This work will contribute to the evolution of upscaling flexible PSCs with a simple fabrication process and high device performances. KEYWORDS: perovskite solar cells, Cu-doped NiOx, hole-transport layer, flexible solar cells, large area

1. INTRODUCTION In the past 5 years, perovskite solar cells (PSCs) have drawn extensive attention owing to the excellent properties of organic−inorganic hybrid perovskite-type semiconductors with high extinction coefficient, long carrier diffusion length, and ambipolar transport behavior.1−6 In particular, methylammoniun (CH3NH3PbI3, denoted as MAPbI3) and formamidinium lead iodide (NH2CHNH2PbI3, denoted as FAPbI3) have emerged as highly attractive solar light-harvesting materials.7−10 Moreover, their economically and practically feasible fabrication processes, including the use of low temperature and the roll-to-roll process, are desirable for constructing flexible photovoltaic (PV) cells on polymer substrates. Currently, there are two device architectures dominating in the field of PSCs: mesoporous type and planar heterojunction.11−18 Highest power conversion efficiencies (PCEs) of 22.1%19 and 21.6%20 have been achieved in PSCs © 2017 American Chemical Society

with mesoporous and planar architectures, respectively, based on the rigid glass. However, the fabrication of mesoporous layers in mesoporous PSCs commonly requires a high annealing temperature above 400 °C, which increases the production cost and hampers the development of flexible modules. As an alternative to mesoporous PSCs, the inverted (p−i−n) planar heterojunction design, which exploits low-temperature (typically 80%) in the visible region, even when the

J = (9/8)μ h ε0εr(V 2/L3)

where J is the current density, μh is the hole mobility, ε0 is the dielectric constant of the free space (ε0 = 8.85 × 10−12 F m−1), εr is the relative dielectric constant, V is the applied voltage, and L is the thickness of charge transport layers. The hole mobility increased from 3.05 × 10−3 cm2 V−1 s−1 for the pristine NiOx film to 1.05 × 10−2 cm2 V−1 s−1 for the Cu/NiOx film. We 41891

DOI: 10.1021/acsami.7b13621 ACS Appl. Mater. Interfaces 2017, 9, 41887−41897

Research Article

ACS Applied Materials & Interfaces

Figure 4. (a) Schematic structure of the PSC: glass/ITO/NiOx (or Cu/NiOx)/MAPbI3/PC71BM/BCP/Ag; (b) cross-sectional SEM image of a complete PSC; and (c) typical current−voltage (J−V) curves by reverse scan (R) and forward scan (F) (the measurement was carried out under AM 1.5G illumination at 100 mW cm−2 with an active area of 0.10 cm2). The inset shows the statistics on performance variations of the solar cells based on different HTL thicknesses (20 cells for each condition). (d) EQE of the inverted planar PSC using different HTLs.

further analyzed the surface composition change of the films to probe the reason for the electrical conductivity increase upon the Cu inclusion. The decomposition of the Ni 2p3/2 spectrum could be well-fitted by two different oxidation states (Ni2+ and Ni3+) using Gaussian function (Figure S7). The XPS results revealed that the Cu doping could increase the relative content of Ni3+ acceptors in the Cu/NiOx film. The concentration ratio for NiOOH, Ni2O3, and NiO three composition in the Ni 2p3/2 spectra increased from 0.70:0.95:1 to 0.86:1.58:1 by Cu doping (2%). The effect of Cu ion doping in NiOx was similar to the ozone treatment.34 It was known that the Ni3+ energy level in NiOx was located above the top of VB and extract electron transfer from VB, acting as a p-type doping. To explore the function of the Cu/NiOx film as a HTL, PSCs have been prepared and characterized with the following architecture: glass/ITO/Cu/NiO x (or NiO x )/MAPbI 3 / PC71BM/BCP/Ag (Figure 4a). The cross-sectional SEM image of this architecture based on Cu/NiOx is shown in Figure 4b. Clearly, the perovskite film was pinhole-free and had a thickness of about 380 nm. Figure 4c shows the J−V curves of the champion devices based on NiOx and Cu/NiOx HTLs in both reverse and forward directions. The Cu/NiOx NP-based PSCs with an optimized thickness of 20 nm showed a JSC of 20.74 mA cm−2, a VOC of 1.11 V, and a fill factor (FF) of 0.81 to yield a best PCE of 18.58%. Detailed parameters are listed in Table 2. As compared with those of the cell based on the pristine NiOx, the PCE of the Cu/NiOx-based device was significantly increased and the degree of J−V hysteresis was reduced. The major improvement relied on the simultaneously increased JSC and FF, which could be ascribed to the improved energy level alignment and hole extraction/collection of the Cu-doped NiOx. Particularly, the high FF of 81.2% and PCE of 18.6% were one of the highest values reported for lowtemperature-processed inverted planar PSCs. External quantum efficiency (EQE) spectra of the champion devices are shown in Figure 4d, where integral photocurrents as a function of wavelength are also presented. Using Cu-doped HTLs, the

Table 2. Performance of Champion PSCs Scanned with 10 mV Voltage Steps and 10 ms Delay Times under Standard AM 1.5G Illumination at Small-Size Area (0.10 cm2) and Large-Size Area (1.08 cm2) in Different Scan directionsa scan direction

JSC (mA cm−2)

VOC (V)

FF

PCE (%)

glass/ITO/NiOxb

forward reverse average

1.10 1.10 1.10

0.73 0.76 0.75

15.06 15.87 15.47

glass/ITO/Cu/NiOxb

forward reverse average

1.11 1.11 1.11

0.80 0.81 0.81

18.50 18.66 18.58

PEN/ITO/NiOxb

forward reverse average forward reverse average forward reverse average forward reverse average

18.75 18.98 18.87 (18.47)d 20.71 20.76 20.74 (19.95)d 18.79 18.98 18.89 17.15 17.41 17.28 19.79 19.87 19.83 18.81 18.88 18.82

1.08 1.09 1.09 1.06 1.07 1.07 1.11 1.11 1.11 1.09 1.10 1.09

0.65 0.69 0.67 0.59 0.66 0.63 0.76 0.78 0.77 0.71 0.74 0.73

13.19 14.27 13.73 10.73 12.29 11.51 16.76 17.16 16.96 14.60 15.42 15.01

substrate/HTL

PEN/ITO/NiOxc

PEN/ITO/Cu/NiOxb

PEN/ITO/Cu/NiOxc

a

The fabrication procedure was similar for all devices in the optimized condition with an average thickness of (Cu)NiOx 20 nm. bSmall-size area: 0.10 cm2. cLarge-size area: 1.08 cm2. dCalculated from the EQE spectra.

devices exhibited overweighed EQE in the whole photoresponsive region. The integral JSC values from the EQE spectra were roughly comparable with those data from the J−V curves. In our experiment, we noticed that the performance of the pristine NiOx-based devices was sensitive to the HTL thickness. Therefore, we examined the influence of the thickness of the 41892

DOI: 10.1021/acsami.7b13621 ACS Appl. Mater. Interfaces 2017, 9, 41887−41897

Research Article

ACS Applied Materials & Interfaces

Figure 5. (a) PL spectra of the MAPbI3 perovskite films deposited on the NiOx and Cu/NiOx films; (b) normalized transient PL decay profiles (logarithmic plot) of MAPbI3 layers on the NiOx and Cu/NiOx films; (c) Mott−Schottky plot of capacitance−voltage measurements of devices based on different HTLs; and (d) VOC dependence upon different light intensities.

significantly contribute to the high JSC and FF values for the Cu/NiOx-based devices. Moreover, we used capacitance−voltage (C−V) measurements to evaluate the Cu-doping effects occurring at the photoactive bulk and the interfaces within PV devices. Because measured doping densities (N) will be related to the bulk properties, flat-band potential values (Vfb) will depend on energy equilibration at the contacts.60 A plot of C−2 (V) for the pristine and doped NiOx NP-based devices measured by Mott− Schottky impedance analysis is presented in Figure 5c. The capacitance variation could be due to the depletion of the HTL at the ITO interface while accumulation or depletion within the perovskite at the perovskite/HTL interface might also contribute. The measured Vfb and N for the pristine NiOx NP-based device increased after Cu doping, with a Vfb from 1.13 to 1.16 eV and an N from 3.86 × 1016 to 6.75 × 1016 cm−3. This indicated that Cu doping could evidently reduce the charge accumulation and improve the doping density. The internal series resistance plays a vital role in the PV parameters, especially in the FF. We further studied the relation between VOC and the light intensity to probe the recombination variation, as shown in Figure 5d. The reduced ideality factor of PSCs with Cu/NiOx (n = 1.18) compared to those with NiOx (n = 1.39) was consistent with the suppressed recombination due to the more efficient charge extraction at the HTL interface. Besides the influence on charge-transfer kinetics, doping of NiOx NPs could induce composite and nanostructure change of the film surface, which would affect the process of above perovskite crystallization. The bulk quality of perovskite films on various substrates was then systemically investigated. However, SEM images (Figure 6a−d) showed similar polycrystalline morphologies of uniform pinhole-free perovskite films with a comparable grain size (∼300 nm) on both NiOx and Cu/NiOx films. Furthermore, XRD (Figure S9) and absorption spectra (Figure S10) confirmed the same tendency as SEM images presented. They indicated that the surface

NiOx and Cu/NiOx HTLs on the performance of solar cells in detail. J−V characteristics are presented in Figure S8, and average performances of 20 devices in each condition are plotted in the inset of Figure 4c. Their corresponding JSC, VOC, FF, and PCE values are shown in Table S1. When the pristine NiOx was used as the HTL, the optimized thickness was ∼20 nm, yielding an average PCE of 14.53%. However, the average PCE dropped to 12.87% and its corresponding series resistance increased to 7.84 Ω cm2 when the NiOx thickness slightly increased to 35 nm. In comparison, the Cu/NiOx-based device achieved an average PCE of 18.06% at an optimized Cu/NiOx thickness of ∼20 nm and the PCE remained above 17.77%, even for a relatively thicker (35 nm) Cu/NiOx film. Although further increase in the Cu/NiOx thickness to 48 nm would gradually drop the device performance, the Cu/NiOx-based device performance exhibited a weak dependency on the HTL thickness because of the minimized resistive losses. The wide thickness-processing window of the Cu/NiOx HTLs can be easily translatable to blade-coating or roll-to-roll processing, which is attractive for future scalable device fabrication. To obtain further insights into the performance enhancement resulting from the use of our Cu-doped NiOx NPs, we investigated the charge extraction property and recombination behaviors that influence the PV performance of PSCs. We conducted PL measurements of the perovskite layers deposited on different HTLs. As illustrated in Figure 5a, the MAPbI3 film exhibited more efficient PL quenching by replacing the pristine NiOx film with the Cu/NiOx film, indicating the better hole extraction capability of the Cu/NiOx film from the perovskite film. Meanwhile, the time-resolved photoluminescence (TRPL) profiles in Figure 5b show that the PL decay rate of MAPbI3 was also higher for Cu/NiOx. The average PL lifetime (τavg) of MAPbI3 on the pristine NiOx was estimated to be 7.1 ns, and it decreased to 5.2 ns when deposited on top of Cu/NiOx. These results validated that faster and more efficient hole extraction was achieved at the Cu/NiOx/MAPbI3 interface, which would 41893

DOI: 10.1021/acsami.7b13621 ACS Appl. Mater. Interfaces 2017, 9, 41887−41897

Research Article

ACS Applied Materials & Interfaces

flexible PEN substrate with a low glass-transition temperature (140 °C). To verify that our new HTL is also efficient for flexible substrates, we first explored the morphology of the perovskite film on the PEN substrate. As expected, the SEM images (insets of Figure 6) demonstrated the densely packed morphologies with high film uniformity, independent of the substrates. Then, we attempted to fabricate the flexible PSCs at a small-size area (0.10 cm2) (inset of Figure 7a). Noticeably, the average PCE of the champion flexible Cu/NiOx device from different scan directions was obtained to be 16.96% with a JSC of 19.82 mA cm−2, a VOC of 1.11 V, and a FF of 0.77 upon one sun illumination at 100 mW cm−2 (Figure 7c and Table 2). It should be noted here that the flexible Cu/NiOx device also exhibited competitive efficiency compared to the control flexible devices based on the pristine NiOx (Figure 7b and Table 2). Incorporation of Cu dopants into the NiOx NPs evidently increased the JSC and FF of flexible PSCs. Moreover, flexible Cu/NiOx devices in small size barely exhibited J−V hysteresis with respect to forward and reverse scan directions. As mentioned above, the smaller size of Cu/NiOx NPs favored the fabrication of HTLs with a uniform morphology because of the reduction of pinholes and cracks over large areas. Therefore, we further prepared PSCs with an active area of 1.08 cm2 under the same experimental conditions (Figure 7a). Maximum PCEs of 14.60 and 15.42% were obtained during reverse and forward voltage sweeps, respectively, and the average PCE was 15.01% (Table 2). To the best of our knowledge, 15.01% is the highest PCE value attained for largearea (over 1 cm2) flexible inverted PSCs. In comparison, the corresponding value of 11.51% was obtained for the pristine NiOx flexible device with a large-size area (Table 2). Similarly, the PV parameters and the degree of hysteresis presented a substantial change as the active area of the pristine NiOx-based

Figure 6. SEM images of MAPbI3 perovskite films deposited on (a) the pristine NiOx; (b) Cu/NiOx (1%); (c) Cu/NiOx (2%); and (d) Cu/NiOx (4%) using rigid ITO/glass substrates (insets: top-view SEM images of the corresponding films on flexible ITO/PEN substrates).

chemistry of the films fabricated from Cu/NiOx NPs with low Cu content was similar to that of the pristine NiOx film, which would hardly influence the nucleation behavior of above perovskite. This similarity together with the hole extraction results implied that interfacial charge transfer, rather than the perovskite quality, was the main reason for the solar cell performance enhancement by Cu/NiOx. Because the fabrication of pinhole-free HTLs from Cu/NiOx NPs is a very simple processing technique and irrelevant to substrates, this enables the fabrication of PV devices on a

Figure 7. (a) Photograph of a complete large-area Cu/NiOx-based flexible device. The inset shows the photograph of a small-area Cu/NiOx-based flexible device; J−V curves in reverse and forward scans for the best performing (b) NiOx-based and (c) Cu/NiOx-based flexible cells with active areas of 0.10 and 1.08 cm2; (d) histograms comparing the difference in the average PCEs determined for 60 flexible PSC devices; (e) PCE values as a function of bending cycles at a fixed bending radius of 8 mm for flexible PSCs with various active areas; and (f) the air stability of the unencapsulated flexible devices using NiOx (Cu/NiOx) or PEDOT/PSS as HTLs. 41894

DOI: 10.1021/acsami.7b13621 ACS Appl. Mater. Interfaces 2017, 9, 41887−41897

Research Article

ACS Applied Materials & Interfaces flexible devices is increased, whereas the Cu/NiOx flexible device exhibited slight performance drop and hysteresisless behavior. Compared with the PSCs on glass/ITO, the flexible devices exhibited slightly lower JSC and FF, which should be attributed to the lower optical transmission and higher sheet resistance of ITO/PEN electrodes compared to those of ITO/ glass electrodes (Figure S11). In addition, the large resistance would affect the extraction of charge carriers, which was correlated with the hysteresis behavior. Histograms for the PCEs of the large-size and small-size flexible devices are shown in Figure 7d and indicated the good reproducibility of the highefficiency flexible devices. Bending tests were further conducted to evaluate the flexible device performance against mechanical bending, which is of great importance for practical applications.61 In our device architecture, all layers were highly flexible and hence the mechanical durability of the devices could be retained for a bending radius (r) of up to 8 mm (the inset of Figure 7e). As shown in Figure 7e, the small-area flexible PSC showed no significant decrease in efficiency during the first 500 bending cycles, retaining over 96% of its initial PCE. For the large-size one, 92% of its initial PCE remained after 400 cycles. However, the performance of both devices on PEN/ITO substrates showed an obvious deteriorated trend after ∼500 bending cycles. It was consistent with previous reports that the series resistance of PEN/ITO electrodes abruptly increased because of the fracture in the ITO polycrystalline structure. Nevertheless, mechanical properties might be further improved by substituting the ITO transparent conducting films with highly bendable conducting materials, such as amorphous transparent conducting oxides, metal grid/conducting polymers, and graphene electrodes. The related experiments are underway in our group. Besides the performance, the device stability is always another major concern, which restricts its practical PV applications. Therefore, we performed an initial investigation of air stability on our unencapsulated devices in an ambient environment at 25 °C with about 40% humidity by recording the device efficiency as a function of storage time (Figure 7f). For PEDOT:PSS-based devices, the flexible solar cells deteriorated quickly and showed nonPV characteristics only after 1 week. The acidic and hygroscopic characteristics of PEDOT/PSS would damage the ITO electrodes and the adjacent perovskite layers.62 For the NiOx NP-based devices, the flexible cell stored in ambient atmosphere was relatively stable at room temperature. After 1 month, the Cu/NiOx device and NiOx device retained 86 and 75% of their initial performance, respectively. The relative long-term stability of the Cu/NiOx devices was attributed to the compact and dense morphology of the Cu/NiOx layer as well as its depressed charge accumulation at the HTL/perovskite interface.63,64 Figure S12 shows the XRD patterns and images of MAPbI3/ PC71BM films deposited on various HTLs as a function of storage time, including PEDOT/PSS, NiOx, and Cu/NiOx (2%). For the perovskite film deposited on the Cu/NiOx layer, the appearance of a small PbI2 peak in the XRD pattern, derived from the decomposition of perovskite, supports the long-term stability of Cu/NiOx-based devices. In comparison, the XRD pattern of the MAPbI3 film on PEDOT/PSS had severely deteriorated and most of the black film had decomposed into PbI2 and turned yellow.

4. CONCLUSIONS In this work, we have demonstrated a simple chemical coprecipitation method for the direct preparation of highquality and phase-pure Cu/NiOx NPs. The effects of Cu doping on the physical properties of NiOx NPs are systematically investigated, and the content of Cu dopants is optimized. Particularly, the Cu/NiO x NPs with enhanced p-type conductivity can be easily processed to effective HTLs with large-area uniformity for PSCs over a wide range of film thickness via a room-temperature solution-processing technique, providing the opportunity for the fabrication of large-area flexible devices. Consequently, highly efficient flexible PSCs are successfully fabricated on PEN/ITO substrates. The PCE of such flexible devices can reach a promising value with negligible hystereses of 16.96 and 15.01% in small area and large area, respectively. Moreover, the Cu/NiOx NP-based flexible PSCs exhibit excellent air stability and mechanical stability, which can contribute to the realization of commercial wearable solar cells. The wide thickness window and excellent performance are attractive for future scalable device fabrication.



ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b13621. Photographs of the (Cu)NiOx NP aqueous dispersion solutions; wide survey XPS spectra and enlarged XRD spectra of the (Cu)NiOx NPs; 3D AFM images, XPS results, and Tauc plot of (Cu)NiOx films; energy level diagram of the PSC device; J−V curves of PSCs fabricated from different thicknesses of HTLs; performance of perovskite devices fabricated from different thicknesses of HTLs; XRD patterns and UV−vis absorption spectra of MAPbI3 perovskite films deposited on various HTLs; optical transmission spectra of ITO/ glass and ITO/PEN substrates; and degradation profile of MAPbI3/PC71BM films deposited on various HTLs as a function of storage time (PDF)



AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (K.Y.). *E-mail: [email protected] (F.L.). ORCID

Fan Li: 0000-0001-8972-4715 Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS

The authors thank financial support from the National Natural Science Foundation of China (61664006, 61464006, and 61504053) and Natural Science Foundation of Jiangxi Province, C h i n a ( 2 01 5 1 B A B 21 7 0 2 3, 2 0 16 1 A CB 2 1 0 0 4, a n d 20171ACB21010). F.L. thanks the generous support from Jiangxi Province Young Scientist Project (20142BCB23002) and State Key Laboratory of Molecular Engineering of Polymers (Fudan University, K2015-24). 41895

DOI: 10.1021/acsami.7b13621 ACS Appl. Mater. Interfaces 2017, 9, 41887−41897

Research Article

ACS Applied Materials & Interfaces



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ACS Applied Materials & Interfaces

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