An efficacious multifunction codoping strategy on a room-temperature solution-processed hole transport layer for realizing high-performance perovskite solar cells

Abstract

A multifunctional carrier transport layer favoring outstanding carrier extraction, high-quality active layer formation, and a facile low-temperature process for efficient and large-scale perovskite solar cells (PSCs) are highly desirable. While co-doping approaches have recently become a hot topic in carrier transport layers to address the negative effects and limitations of typical single doping and further boost the carrier extraction properties and thus device performances, high-temperature, high power, and multi-steps processes/treatments are required which hinder their applications and potentially damage underneath structures particularly in emerging flexible electronics. In this work, we demonstrate the first kind of room-temperature solution-processed and post-treatment-free Li and Cu codoped NiO_x nanoparticle-based hole transport layer (HTL). Simultaneously, the Li and Cu codoped NiO_x HTLs show the interesting and critical features of (1) improved electrical conductivity and optical transmittance, (2) a high quality (pin-hole/crack free, compact and uniform) film morphology, (3) favoring large grain-size perovskite film formation, and (4) wide-range thermal stability up to 250 °C. With these interesting multiple functions, PSCs with Li and Cu codoped NiO_x HTLs achieve a PCE of 20.8% and 18.2% on rigid and flexible substrates, respectively. This work contributes to a promising route for realizing highly efficient and stable rigid and flexible PSCs using abundant low-cost inorganic HTLs.

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1. Introduction

The rapidly developed perovskite solar cells (PSCs) have aroused tremendous interest owing to their excellent optoelectronic properties (such as a broad absorption range, intense extinction coefficient, and high charge carrier mobility), long charge-carrier lifetime, low-temperature solution processability, and relatively low-cost fabrication.^1–4 The power conversion efficiency (PCE) of PSCs has been swiftly raised from ∼3.8% (ref. 5) to recent certified 25%,⁶ showing the great commercial potential of this cell technology. In PSCs, carrier transport layers (CTLs) are crucial not only for achieving better energy alignment and carrier transport but also for obtaining improved stability, and tremendous efforts have been devoted to exploring effective CTLs featuring low resistance, good optical transparency, superior charge extraction/transport ability, and excellent stability. Among various CTL materials, inorganic CTLs are of particular interest due to their decent carrier mobility, very low cost of raw materials and synthesis, and good stability.^7–15

Recently, the modification in carrier transport materials by codoping has become a hot and attractive topic for increasing the efficiency of photovoltaic devices due to the advantages of compensating for the negative effects of single doping or further enlarged the regulating range of carrier mobility, Fermi level, etc.^16–20 By combining the effect of Mg on the bandgap and Er on light absorption and emission, a co-doped Er, Mg TiO₂ electron transport layer (ETL) under 500 °C hydrothermal treatment effectively improves the device efficiency.¹⁷ A Sm³⁺, Eu³⁺ co-doped TiO₂ ETL was also demonstrated via a pulse laser deposition method and the PSCs exhibited a PCE of 19.01% with good ultraviolet (UV) light stability.²¹ The improvement of device performance was explained by the UV light utilization due to the down-conversion dopant Eu³⁺, which can be enhanced by further doping with Sm³⁺. By doping Zr and F in SnO₂ to increase the conduction band and electrical conductivity, the PSCs with suitable annealing and UV-ozone treatment could achieve over 19% PCE with low hysteresis.²² Moreover, co-doping also has been adopted in commonly used NiO_x hole transport layers (HTLs) for modifying the electrical properties by increasing the dopant solubility, the activation rate by lowering the ionization energy of Ni vacancies, and the carrier mobility.^20,23–25 For instance, high-temperature sol–gel processed Pb, Li and Ag, Li co-doped NiO_x thin films have been fabricated and used as HTLs in PSCs, which exhibited a PCE of 17.02% and 19.24%, respectively.^23–25 A Mg and Li co-doped NiO_x HTL formed by a spray technique also achieved a PCE of 18.3% in inverted PSCs.²⁴ However, all the above cases require high temperatures (>200 °C) during deposition and/or annealing treatment for realizing the expected electrical performance of co-doping materials, which not only increases the complexity and cost of device fabrication but also limits the possibility of PSC application on flexible plastic substrates. It is highly desirable to explore room-temperature solution-processed codoped NiO_x HTLs for flexible PSCs, featuring efficient hole extraction capabilities and uniform morphologies. To the best of our knowledge, there is no report on room-temperature solution-processed co-doped NiO_x HTLs for demonstrating highly efficient rigid and flexible PSCs.

Herein, we demonstrate a room-temperature solution-processed and post-treatment-free Li, Cu-codoped NiO_x (hereafter named (Li, Cu):NiO_x) nanoparticle (NP) based HTL for high-performance rigid and flexible inverted planar PSCs. The (Li, Cu):NiO_x film exhibits not only simultaneously enhanced electrical conduction and optical transmittance, but also improved morphology uniformity on various substrates such as rigid indium tin oxide (ITO)/glass and flexible ITO/polyethylene terephthalate (PET). Besides, the co-doped NiO_x HTL can promote large grain perovskite film formation. The simultaneous interesting features result in the improvement of PCE by 15% as compared to the case of pristine NiO_x to a value of 20.8% and 18.2% for inverted PSCs on the rigid and flexible substrates, respectively. While the effective co-doped (Li, Cu):NiO_x HTL is formed at room temperature, the high performance can sustain annealing temperature up to 250 °C. Moreover, the PSCs based on (Li, Cu):NiO_x showed decent stability, with 95% of the original efficiency remaining after 60 days of storage under inert conditions. This work provides a promising route to realize highly efficient and stable inverted planar PSCs both on rigid and flexible substrates using abundant and low-cost inorganic HTLs, which favor the development of large-scale perovskite optoelectronic devices.

2. Experimental section

2.1 Materials

All chemicals were of analytical reagent grade and used as received unless otherwise noted. Nickel(II) nitrate hexahydrate (Ni(NO₃)₂·6H₂O, 97%), lead(II) iodide (PbI₂, 99%), lead(II) chloride (PbCl₂, 99%), and zirconium(IV) acetylacetonate (ZrAcac, 98%) were purchased from Sigma-Aldrich. Lithium nitrate (LiNO₃, 99%) and copper(II) nitrate trihydrate (Cu(NO₃)₂·3H₂O, 98%) were purchased from JK Science. Dimethylformamide (DMF, extra dry, 99%), 1, 2-dichlorobenzene (DCB, extra dry, 98+%), chlorobenzene (CB, extra dry, 99.8%), and isopropanol (IPA, extra dry, 99.8%) were purchased from Acros Organics. CH₃NH₃I was purchased from Dyesol. PC₆₁BM and C₆₀ were purchased from Solarmer Energy, Inc. Ag (99.999%) was purchased from Alfa Aesar.

2.2 Synthesis of NiO_x NPs without/with doping

NiO_x NPs were synthesized by a chemical precipitation method according to previous reports.^11,26 Typically, Ni(NO₃)₂·6H₂O (0.5 mol) was dispersed in 100 mL deionized water to obtain a green solution. Under continuous magnetic stirring, the pH of the solution was adjusted to 10 by adding a NaOH aqueous solution (10 mol L⁻¹) drop by drop. After stirring for another 5 min, a green colloidal precipitation was collected by centrifugation and washed with DI water and ethanol two times, respectively. The obtained green powders were dried at 80 °C for 6 h and then calcined at 270 °C for 2 h to obtain a dark-black powder. For 5% Cu:NiO_x, 5% Li:NiO_x, and (Li, Cu):NiO_x NPs, 5 mol% Ni(NO₃)₂·6H₂O were respectively replaced with 5 mol% Cu(NO₃)₂·3H₂O, 5 mol% LiNO₃, and 5 mol% (Cu(NO₃)₂·3H₂O/LiNO₃ = 2 : 1), and all other procedures were identical to the synthesis of undoped NiO_x NPs. The as-prepared NiO_x and doped NiO_x NPs were dispersed in deionized water in a certain concentration range with ultrasonication for 10 min.

2.3 Device fabrication

ITO-coated glass substrates (∼15 Ω sq⁻¹) were cleaned by sequential ultra-sonication in detergent water, deionized water, acetone, and ethanol for 15 min, respectively, and then ultraviolet-ozone treated for 15 min. The HTLs were deposited by spin-coating the corresponding NiO_x or (Li, Cu):NiO_x aqueous solution (20 mg mL⁻¹) at 2000 rpm for 50 s. All of the above procedures were carried out at room temperature and without any post-treatment. The average thickness of NiO_x and doped NiO_x films was about 20 nm. After transferring samples into a nitrogen-filled glovebox, the MAPbI_3−xCl_x perovskite films were deposited onto the HTLs according to our previous report.¹² Afterward, a total concentration of 20 mg mL⁻¹ mixed solution of PC₆₁BM/C₆₀ (weight ratio, 2 : 3) in dichlorobenzene was spin-coated on top of the perovskite layer at 700 rpm for 60 s and then 4000 rpm for 60 s, followed by spin-coating ZrAcac (2 mg mL⁻¹ in isopropanol) at 4000 rpm. Finally, a 100 nm thick Ag electrode was thermally evaporated on top of the device under a base pressure of 10⁻⁴ Pa with a device area of 0.06 cm².

2.4 Characterization and measurement

The crystal phase of the samples was identified by X-ray powder diffraction with Cu Kα radiation at a λ of 1.54056 Å (XRD, Bruker D2 Phaser). The nanoparticle morphology and lattice spacing were examined by tapping mode atomic force microscopy (AFM, NT-MDT NTEGRA), transmission electron microscopy (TEM, Philips Tecnai G2 20 S-TWIN), and scanning electron microscopy (SEM, Hitachi S-4800, and LEO 1530 FEG), respectively. The I–V measurements of the sample films were performed by conductive atomic force microscopy (c-AFM). The chemical composition of sample films on ITO was verified by X-ray photoelectron spectroscopy (XPS), which was performed in an ultrahigh vacuum environment using a Physical Electronics PHI 5802 with a monochromatic Al Kα X-ray source. The thickness of the samples was measured by using a profilometer. The optical properties of the samples were determined under a dark ambient environment using spectroscopic ellipsometry (Woollam). Ultraviolet photoelectron spectroscopy (UPS) was performed by using a He discharged lamp (He I 21.22 eV, Kratos Analytical). A Keithley 2635 sourcemeter was used to determine the current density–voltage (J–V) characteristics and the solar cells were tested by using a Newport AM 1.5 G solar simulator under a light intensity of 100 mW cm⁻², calibrated with a standard silicon reference cell. The steady-state PCE, PCE (t), was recorded by setting the bias voltage to the V_MPP (obtained from the J–V curve) and then tracing the current density. Incident-photon-to-current conversion efficiency (IPCE) measurements were conducted by using a system combining a xenon lamp monochromatic chopper and a lock-in amplifier together with a calibrated silicon photodetector (Hamamatsu mono-Si cell). Time-resolved photoluminescence (PL) spectra were recorded on a PicoQuant FluoTime 300 instrument. A picosecond 375 nm pulse laser (LDH-P-C-75) with a pulse width of <40 ps and a repetition rate of 5 MHz was used to excite the samples. Electrochemical impedance spectroscopy (EIS) analysis was performed on a CHI660C electrochemical workstation (CH Instrument, Inc.) at 0.9 voltage in the frequency range from 10⁻¹ to 10⁶ Hz.

3. Results and discussion

The undoped NiO_x and (Li, Cu):NiO_x NPs have been newly synthesized by a modified chemical coprecipitation approach.^11,26 Particularly, a portion of Ni(NO₃)₂·6H₂O in the precursor was replaced by Cu(NO₃)₂·3H₂O and LiNO₃ with an appropriate mole ratio which can be used to low-temperature synthesize not only Cu doped and Li doped NiO_x NPs but also the targeted co-doped (Li, Cu):NiO_x NPs, as further detailed in the Experimental section.

Fig. 1(a) shows the XRD patterns of NiO_x and (Li, Cu):NiO_x powders. All samples show three prominent characteristic diffraction peaks that can be respectively assigned to the (111), (200), and (220) planes of the NiO phase according to JCPDS (22-1189). The similar diffraction peaks for undoped and doped NiO_x illustrate that light doping of Cu and Li could hardly change the phase structure of NiO_x.^27,28 Notably, the peak shift from 62.43° to 62.60° indicates a slight increase of the lattice parameter after successfully co-doping Cu and Li into NiO_x, which is similar to other doped NiO_x.^29,30 Moreover, the high-resolution TEM image and selective area electron diffraction pattern (SAED) of the (Li, Cu):NiO_x nanocrystal (Fig. 1(b)) further verified that codoped NiO_x NPs possess good crystalline properties and a pure NiO phase. TEM was applied to confirm the crystallite sizes of NiO_x and (Li, Cu):NiO_x NPs as shown in Fig. 1(c) and S1.† Compared with pristine NiO_x (5.62 nm on average), the smaller grain size of (Li, Cu):NiO_x (4.28 nm on average, Fig. 1(d)) is beneficial for the formation of thin films featuring a densely packed and smooth morphology. The smaller size may be attributed to the smaller size of hydroxide intermediates.

Fig. 1 (a) XRD patterns of the as-synthesized of NiO_x and (Li, Cu):NiO_x NPs. (b) HRTEM image with the indexed SAED pattern of the as-synthesized (Li, Cu):NiO_x NPs. (c) TEM image and (d) size distribution of the as-synthesized (Li, Cu):NiO_x NPs.

The XPS spectra of pristine NiO_x and (Li, Cu):NiO_x samples are shown in Fig. S2.† Both pristine and codoped NiO_x films have typical peaks including the main peak at ∼854.1 eV, a shoulder at ∼855.5 eV, and a shake-up satellite peak at ∼861.2 eV.^31,32 The main peak corresponds to Ni²⁺ in the standard Ni–O octahedral bonding configuration in cubic rock salt NiO_x.^33,34 The shoulder peak is ascribed to the Ni-vacancy induced Ni³⁺ ion,^34–37 nickel hydroxides and oxyhydroxides,^34,35 and nonlocal screening and surface states.³¹ Notably, in Fig. S2(b) and (c),† XPS peaks corresponding to Cu 2p_2/3 (933.6 eV) and Li 1 s (55.9 eV) are clearly observed for the (Li, Cu):NiO_x film, which are absent in the undoped NiO_x film, revealing that Cu and Li are successfully incorporated into NiO_x.³⁸ Due to the Li and Cu doping, the obtained Ni³⁺/Ni²⁺ ratio of the (Li, Cu):NiO_x film (1.68) is larger than that of undoped NiO_x (1.57) which contributes to the increase of hole conductivity as further studied in a later section.

The surface morphologies of undoped NiO_x and (Li, Cu):NiO_x thin films on ITO glass were determined by top-view SEM and AFM as shown in Fig. 2(a)–(d). Notably, the (Li, Cu):NiO_x film consists of small-sized crystalline grains and is uniform without pin holes (Fig. 2(a)). Moreover, 3D AFM images further confirm the closely packed and crack-free morphology of the (Li, Cu):NiO_x film with a smaller root-mean-square roughness (RMS) of 3.09 nm (Fig. 2(c)) compared to the pristine NiO_x film (4.00 nm; Fig. 2(d)). It is worth noting that the smooth and uniform surface of the (Li, Cu):NiO_x film should be beneficial to the formation of high-quality perovskite films for realizing high-performance devices. Thus, the morphologies of perovskite layers on NiO_x and the (Li, Cu):NiO_x thin film were characterized by SEM as shown in Fig. S3(a) and (b).† It is found that all the perovskite films are homogeneous and densely packed. Notably, the perovskite crystal grains formed on (Li, Cu):NiO_x (511 nm) show a larger grain size than those grown on NiO_x (440 nm), as displayed in Fig. S3(c) and (d).† That is because the slightly larger contact angle (inset in Fig. S3(a) and (b)†) provides a non-wetting surface, favoring the suppression of heterogeneous nucleation³⁹ and the promotion of the grain growth.^12,40,41 Consequently, the (Li, Cu):NiO_x based HTL favors the formation of a good quality perovskite film, which is one of the interesting properties for obtaining highly efficient PSCs.

Fig. 2 SEM images of (a) (Li, Cu):NiO_x and (b) NiO_x films on ITO glass. 3D AFM images: (c) (Li, Cu):NiO_x and (d) NiO_x films. (e) The transmittance spectra of NiO_x and (Li, Cu):NiO_x films on ITO glass. (f) I–V conducting curves of NiO_x and (Li, Cu):NiO_x films measured in the c-AFM mode.

Electrical and optical properties are particularly important for realizing efficient HTLs. Firstly, we investigated the optical transmittance of NiO_x and (Li, Cu):NiO_x films coated on ITO glass as shown in Fig. 2(e). Interestingly, for the (Li, Cu):NiO_x thin film, the transmittance reaches 87% at 460 nm, and shows an improvement over a large wavelength range from 500 to 800 nm than that of pristine NiO_x. Furthermore, while light Li only doping does not change the transparency, the Cu doping will reduce the doping transmittance decreases as shown in Fig. S4† due to the d–d transition of Cu²⁺ in the crystalline environment of NiO_x, which agrees with previous reports.^34,42,43

We further study the effect of Li and Cu co-doping on the electrical conductivity of NiO_x films by performing c-AFM measurements, as described in Fig. 2(f). From the I–V curves, a significantly higher current in the (Li, Cu):NiO_x film (14.6 nA at a bias of 0.5 V) is clearly observed, indicating a dramatically increased conductivity compared to pristine NiO_x (8 nA at a bias of 0.5 V). The increased ratio of Ni³⁺/Ni²⁺ supports the successful doping of Li and Cu as well as the improved hole conductivity, which is in good agreement with other reports (Fig. S5†).^{24,29,34,43–45} These results indicate the Li and Cu codoping strategy on NiO_x could achieve significantly enhanced conductivity and keep high transparency, making it more suitable as an efficient HTL for achieving high-performance PSCs.

To explore the function of the (Li, Cu):NiO_x film as the HTL, inverted PSCs with a structure of ITO/NiO_x (or (Li, Cu):NiO_x)/perovskite/PC₆₁BM : C₆₀/ZrAcac/Ag have been fabricated, where the corresponding energy level diagram is depicted in Fig. 3(a). The energy levels of ITO, perovskite, PC₆₁BM : C₆₀, ZrAcac, and Ag are collected from the literature,^12,13,46 and NiO_x or (Li, Cu):NiO_x are obtained from UPS. It should be noted that zirconium acetylacetonate (ZrAcac) is inserted between the PC₆₀BM : C₆₀ and Ag cathode for efficiently extracting and transporting electrons. The cross-sectional view SEM image of the device configuration is shown in Fig. 3(b). Fig. 3(c) depicts the J–V curves of the champion devices based on pristine NiO_x and (Li, Cu):NiO_x under AM 1.5 G illumination. The (Li, Cu):NiO_x based PSCs with an optimized thickness of 20 nm exhibited a short circuit current density (J_SC) of 23.156 mA cm⁻², an open-circuit voltage (V_OC) of 1.11 V, and a fill factor (FF) of 0.81 to yield the best PCE of 20.83%, clearly outperforming the PSCs based on pristine NiO_x as the HTL. The best-performing reference cell using pristine NiO_x as the HTL gave a J_SC of 22.02 mA cm⁻², V_OC of 1.09 V, FF of 0.75, and PCE of 17.91%. These values are comparable with those of a previous report on devices with the same structure.⁴⁷ Thanks to the simultaneously enlarged transmittance and conductivity of the codoped HTL and the high-quality perovskite film with a larger grain size, the inverted PSCs displayed significantly enhanced J_SC, FF, and PCE, which are also the highest among the reported NiO_x based inverted PSCs, as shown in Table S1.† Additionally, the PSCs based on (Li, Cu):NiO_x HTLs showed negligible J–V hysteresis in Fig. 3(d), which is better than that of NiO_x based PSCs (Fig. S6†). The narrow distributions of the statistical photovoltaic parameters (J_SC, V_OC, FF, and PCEs) from 30 devices further confirm the reliability of performance improvement of (Li, Cu):NiO_x-based PSCs (Fig. S7†). Fig. 3(e) shows the IPCE spectra of the champion devices based on NiO_x and (Li, Cu):NiO_x, where the integrated J_SC from the IPCE over an AM 1.5 G spectrum approximately agrees with those obtained from J–V curves. In addition, the steady-state photocurrent output at the maximum power point over 200 s has been measured by applying a bias of 0.97, as shown in Fig. 3(f). There is no performance degradation under long illumination and a stable PCE of 20.24% for (Li, Cu):NiO_x based PSCs was obtained, which is consistent with that of J–V measurement.

Fig. 3 (a) Energy band level diagram of the various layers in inverted PSCs. (b) Cross-sectional SEM image of a complete perovskite device with (Li, Cu):NiO_x films as the HTL. (c) J–V characteristics of the best NiO_x and (Li, Cu):NiO_x based PSCs measured under AM 1.5 G 100 mW cm⁻² illumination. (d) J–V characteristics of (Li, Cu):NiO_x based devices extracted from forward and reverse sweeping. (e) IPCE spectra and integrated J_SC curves of the best NiO_x and (Li, Cu):NiO_x based PSCs. (f) Steady-state photocurrent output at the maximum power point of (Li, Cu):NiO_x-based PSCs under continuous simulated AM 1.5 G illumination for 200 s.

In order to explore the reasons for the improved performance by codoping with Li and Cu, we first compared the work function (WF) of NiO_x and (Li, Cu):NiO_x films by Kelvin-probe measurement. For each sample, 500 points are collected, as shown in Fig. S8.† The measured WF values are collected in Table S2.† The WF of the pristine NiO_x film is determined to be 4.99 eV and shifted to 5.12 eV for (Li, Cu):NiO_x. From the UPS and the corresponding energy level diagram in Fig. 4(a) and (b), Cu and Li codoping can cause energy band shifts and increase the work function,⁴⁸ resulting in a favorable energy level alignment with the perovskite layer (−5.4 eV, Fig. 3(a)) and an enhanced hole extraction ability. In addition, the valence band (VB) shifted by about 0.02 eV closer to the Fermi energy level after Li and Cu codoping (Fig. 4(b)), showing enhanced p-type properties and improved film conductivity, which is consistent with the previous discussion.

Fig. 4 (a) UPS of the NiO_x and (Li, Cu):NiO_x films on ITO glass. (b) Energy level diagrams of the NiO_x and (Li, Cu):NiO_x films. (c) PL spectra for perovskite films on different substrates: glass (orange), pristine NiO_x (black), and (Li, Cu):NiO_x (red), respectively. (d) Time-resolved PL spectra of the perovskite film on NiO_x and (Li, Cu):NiO_x, respectively. (e) Nyquist plots of EIS spectra for the device with NiO_x and (Li, Cu):NiO_x as HTLs, respectively.

To verify the charge extraction ability at the interface of HTL/perovskite, PL (Fig. 4(c)) and time-resolved PL (Fig. 4(d)) measurements of the perovskite layers deposited on NiO_x, and (Li, Cu):NiO_x films were conducted, respectively. It can clearly be observed that the (Li, Cu):NiO_x HTL exhibits efficient PL quenching compared to that of pristine NiO_x, illustrating a more favorable ohmic contact with the perovskite layer and obvious improvement of the hole extraction efficiency. Meanwhile, the TRPL profiles are shown in Fig. 4(d), where the PL lifetime is fitted by two-exponential decay.^29,49 There is a decrease in the average PL lifetime for (Li, Cu):NiO_x/perovskite (13 ns) as compared to pristine NiO_x/perovskite (20 ns), which further validated a faster and more efficient hole extraction and transport ability at the (Li, Cu):NiO_x/perovskite interface. Compared to NiO_x, the more effective hole extraction and transport by the (Li, Cu):NiO_x HTL from the perovskite active layer could be ascribed to the higher conductivity and increased work function, which would contribute to high J_SC and FF values for (Li, Cu):NiO_x based PSCs.

Charge transport and recombination ability can also be studied by EIS measurements, which are performed at a bias voltage of 0.9 V under dark conditions (Fig. 4(e)). The only one semicircle in the obtained Nyquist plot will be more likely related to the recombination resistance at the interface of the HTL/perovskite layer, where the larger the diameter is, the less the hole recombination at the interface. Compared to that of pristine NiO_x based devices (1.3 KΩ), (Li, Cu):NiO_x HTL based PSCs exhibit a larger charge recombination resistance (2.6 KΩ), illustrating less carrier recombination, which results in increased J_SC and enhanced FF in the (Li, Cu):NiO_x HTL based PSCs. As shown in the dark current–voltage curves of Fig. S9,† the lower dark current under negative bias in the (Li, Cu):NiO_x HTL based PSCs illustrates that those PSCs based on the (Li, Cu):NiO_x HTL exhibited a lower leakage current, reducing leakage-induced recombination that would improve the V_OC.

Regarding the stability and reversibility of PSCs, the PCE of (Li, Cu):NiO_x HTL based PSCs was characterized after one-month storage in a nitrogen-filled glovebox, as portrayed in Fig. 5. It is found that (Li, Cu):NiO_x HTL-based devices exhibited excellent stable performance, retained 95% of initial PCE after 60 days of storage, implying the beneficial role of Cu and Li in preventing the undesired oxidation process on the NiO_x surface.

Fig. 5 Photovoltaic performance parameter (J_SC, V_OC, FF, and PCE) of (Li, Cu):NiO_x based PSCs as a function of storage time under an inert environment for almost 60 days. All devices were encapsulated and stored in a glove box.

To demonstrate the feasibility of post-treatment free and room-temperature processed (Li, Cu):NiO_x HTLs for application in flexible devices, we deposited a (Li, Cu):NiO_x film onto an ITO-PET substrate to fabricate flexible PSCs. A photograph of a typical flexible device with an area of 0.08 cm² is shown in Fig. S10(a).† Fig. S10(b)† illustrates the J–V curves of the champion flexible PSCs with a (Li, Cu):NiO_x HTL (under AM 1.5 G illumination). The device exhibited a J_SC of 22.02 mA cm⁻², a V_OC of 1.06, and a FF of 0.78, corresponding to the best PCE of 18.16%, which are comparable to those of the reported best flexible PSCs with inverted structures (Fig. S10(c)).† To the best of our knowledge, 18.2% is the highest PCE value attained for NiO_x based flexible inverted PSCs. This result suggests that the (Li, Cu):NiO_x HTL without any high-temperature treatment is compatible with the flexible substrate and shows a broad application prospect for the roll-to-roll scale-up fabrication process.

Another interesting feature of the (Li, Cu):NiO_x films is that while only room temperature conditions are needed for forming efficiency HTLs, the films can tolerate a wide temperature range for the practical application of PSCs in a temperature changing environment. The J–V characteristics of PSCs using a (Li, Cu):NiO_x HTL with different annealing temperatures are collected in Table S3.† Our results show that all the devices show similar performances for the co-doped (Li, Cu):NiO_x HTL with annealing temperature up to 250 °C. With the annealing temperature further increasing to 300 °C, the PCE decreases, which is attributed to the decreased J_SC and V_OC. Our results demonstrate that the (Li, Cu):NiO_x HTL in this work can function as an effective HTL without any post-treatment and offer wide temperature stability from room temperature up to 250 °C, which can contribute to the device stability during the fabrication and application of PSCs.

4. Conclusions

In this work, we have successfully demonstrated efficient HTLs by a simple room-temperature solution process without any treatment for high performance rigid and flexible PSCs. Particularly, a co-doped (Li, Cu):NiO_x film simultaneously exhibits several interesting features of (1) improved electrical conductivity, (2) enhanced optical transmittance, (3) high quality (compact and uniform), and (4) favoring large grain-size perovskite film formation. The PCE of PSCs based on (Li, Cu):NiO_x HTLs improved by 15% as compared to the case of pristine NiO_x and reached a promising value of 20.8% on rigid substrates and 18.2% on flexible substrates. While an effective co-doped (Li, Cu):NiO_x HTL is formed at room temperature, the high performance can sustain annealing temperature up to 250 °C. Meanwhile, the PSCs exhibit excellent stability, with 95% of the original efficiency remaining after 1000 h of storage under ambient conditions. This work provides a promising route to realize highly efficient and stable rigid and flexible PSCs using abundant low-cost inorganic HTLs.

Conflicts of interest

The authors declare no competing financial interests.

Acknowledgements

This work was supported by the University Grant Council of the University of Hong Kong (Grant# 201811159147, and Research Equipment Fund), the General Research Fund (Grant 17204117, 17200518, 17201819, and 17211220), and the Collaborative Research Fund (C5037-18G) from the Research Grants Council (RGC), and the Environment and Conservation Fund (ECF Project 64/2018) of Hong Kong Special Administrative Region, China.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0ta05873d

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