1. Introduction

Organic-inorganic hybrid perovskites has emerged as destructive materials for solar cells with power conversion efficiency (PCE) exceeding 22% [1], owing to its lots of unique intrinsic properties such as high optical absorption coefficient [2, 3], large carrier mobility [4, 5] and extra-long diffusion lengths [6, 7]. The cutting edge PCE record of perovskite solar cells (PSC) is close to those of commercial CdTe and CIGS ones [8], and herein turns in a slower growth region due to the Schottky Queisser limit of single junction solar cells [9], The planar PSCs have attracted huge attentions owing to their simpler architecture and more facile process in contrast to the mesoporous. However, planar PSCs suffer from J-V hysteresis [10] and loss in carriers transport due to interface recombination and retard [11], which leads to inferior performance of the planar compared with the mesoporous. Therefore, as it already made huge effect in the commercialization of silicon and other photovoltaics, interface engineering is dispensable in the current and future evolution of PSCs techniques. Among them, the “inverted” planar PSCs constructed as “transparent conductive oxides (TCO)/hole transporting materials (HTM)/perovskite/electron transporting layer (ETL)/metal electrode” show extremely promising with as high performance as over 19% in PCE and eliminable hysteresis [12].

As a typical problem for the inverted planar PSCs, the mismatch at the interface between the Fermi level of electrodes (e.g., Ag, Au) and the lowest unoccupied molecular orbital (LUMO) of the organic material (e.g., Phenyl-C61-butyric acid methyl ester, PCBM) leads to poor performance of the perovskite solar cells [13]. Various functional materials were used to improve the ohmic contact at the cathode interface, such as LiF [14], bathocuproine [15], poly[(9,9-bis(3-(N,N-dimethylamion)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctyl)-fluorene [16], fullerene derivatives [17], ethoxylated polyethylenimine [18], Perylene-Diimide [19] and etc. Nevertheless, most of them are either sensitive to moisture, oxygen, organic solvents in process, or suffering from instability in thermal and light soaking environment [20, 21], which deviates them from the way-out for the mass production of PSCs due to their instability both in manufacturing process and hereafter utilization environment. Our group employed solution processed and highly stable poly (2-ethyl-2-oxazoline) (PEOz) as cathode interfacial layer to upgrade the performance of the inverted planar devices by near one fold from 8.73% to 17.20% in an averaged PCE along with the function of NiOx as holes transport layer (HTL), in contrast to the reference devices [22, 23]. The nitrogen element at the backbone of PEOz involved in the devices structure also acted as a barrier to block both the penetrating iodine ions and silver atoms, and hence enhanced the stability of devices to a large extent.

In this work, Ruthenium acetylacetonate (RuAcac) was adopted as cathode interfacial layer (CIL) to modulate the band alignment between silver electrode and PC₆₁BM in typical device architecture of “ITO/NiO_x/perovskite/PCBM/RuAcac/Ag”. The best photovoltaic performance of RuAcac based devices demonstrated a power conversion efficiency of 17.15% with an open circuit voltage (Voc) of 1.077 V, a short circuit current density (Jsc) of 21.28 mA/cm². In contrast to the only 12.74% in PCE of the control devices only without RuAcac, the remarkable performance enhancement is probably owing to minimum contact barriers at PCBM/RuAcac/Ag interface. Moreover, our devices demonstrated an excellent stability in both inert and some humidity environment, which may be attributed to the stable NiOx HTL and RuAcac CILs film.

2. Experiments

2.1 Materials

Unless stated otherwise, all materials are purchased from Sigma-Aldrich or Alfa-Aesar and used as received. CH₃NH₃I was purchased from Taiwan Lumtec Corp. PC₆₁BM (99.5%) were purchased from Shanghai MaterWin New Materials Co., Ltd.

2.2 NiO_x Nanocrystals (NCs) synthesis

NiO_x NCs were synthesized according to the procedures reported elsewhere with some modification [24]. Briefly, Ni(NO₃)₂·6H₂O (0.1 mol) was dispersed in 20 mL of deionized (DI) water to obtain a dark green solution. Then KOH solution (10mol/L) was slowly added into the solution until large amount of precipitation. After being stirred for 30 min, the colloidal precipitation was thoroughly washed with deionized water three times, and dried at 80 °C overnight. The obtained green powder was then calcined at 270 °C for 2 h to obtain a dark-black powder. The NiO_x NCs inks were prepared by dispersing the obtained NiO_x NCs in deionized water/isopropanol (3/1) to ca. 10mg/ml concentration.

2.3 Device fabrication

Firstly, the patterned ITO substrates were ultrasonic cleaned with DI water, acetone, toluene, acetone and isopropanol for 10 minutes, respectively. Then, NiOx NCs was deposited by spin coating (2000 r.p.m) on cleaned ITO substrate at room temperature without any post-treatment. The as-prepared perovskite precursor solution (PbI₂ and CH₃NH₃I with mole ratio of 1:1 with 1.1M in a mixture of DMF and DMSO (2:1 v/v)) was stirred at 70 °C for 3 hours and filtered using 0.45 µm PTFE syringe filter and coated onto the ITO/NiOx substrates at a speed of 3,000 r.p.m for 35 s. During the last 10 s of the spinning process, the substrates were treated gently by drop-casting 0.2 ml chlorobenzene solvent. After drying at low temperature for several minutes, the perovskite films were then annealed on a hot plate at 100 °C for 10 min, following by spin-coating a layer of PC₆₁BM (2 wt%, 1500 r.p.m) at its surface. Then, RuAcac with concentration of 1 mg/ml in methanol) were gently drop-casted (5000 r.p.m, 30s) on the top of PC₆₁BM films. Finally, a 100-nm-thick Ag electrode was deposited through a shadow mask by thermal evaporation to define the cell area 10 mm².

2.4 Device characterizations

The J−V curves were recorded using a Keithley 2400 Apparatus. Illumination was provided by a Newport Sol1A solar simulator with AM1.5G spectrum and light intensity of 100mW/cm², which was determined by a calibrated crystalline Si-cell. During device characterization, a shadow mask with an opening of 10 mm² was used. The EQE spectra were recorded with by an Enli Technology (Taiwan) EQE measurement system (QE-R), and the light intensity at each wavelength was calibrated with a standard single-crystal Si photovoltaic cell. Absorption and transmission spectra were recorded by a UV/Vis/NIR Spectrophotometer (LAMBDA 950, PerkinElmer). Room temperature photoluminescence (PL) and time resolved PL spectra were recorded by fluorescence spectrophotometer (FS5, Edinburgh Instruments). A 405 nm pulsed laser was selected as excitation source for the time resolved PL measurement. Top-view Scanning electron microscopy (SEM) images were characterized by MERLIN (Carl Zeiss AG, Germany) using an SE2 detector operating at an accelerating voltage of 5 kV. The cross-section Scanning electron microscopy (SEM) was prepared using a focused ion beam (FEI Helios Nanolab 600i) operating at 30 kV and subsequently imaged with the electron beam of the same instrument using an accelerating voltage of 5 kV. Atomic Force Microscope (AFM) (MFP-3D-Stand Alone, Asylum Research) was employed to investigate the surface potentials. Surface work function of silver with or without RuAcac layer were also measured by Ultraviolet photoelectron spectroscopy (UPS) (AXIS Ultra DLD, KRATOS Analytical).

3. Results and discussion

The effects of RuAcac CIL on band alignment at the cathode interface between cathode electrode and electron transport layer (ETL) were investigated by fabricating inverted planar devices with a configuration of “ITO/NiO_x/perovskite/PCBM/RuAcac/Ag”, as depicted in Fig. 1(a). The CH₃NH₃PbI₃ perovskite absorber was located between NiOx hole transport material (HTM) and PCBM ETL, where the holes and electrons were transported to the corresponding contacts via NiO_x and PCBM, respectively. NiO_x nanocrystals HTM were prepared according to the previous reports [22, 23, 25]. The chemical structure of the RuAcac CIL was shown in the insert at Fig. 1(b). The RuAcac CIL demonstrated two main absorption peaks at ca. 521 nm and 362 nm, respectively, as shown in Fig. 1(b), where the optical band gap of the RuAcac can be defined at ca 2.1 eV. According to the UPS spectra of the RuAcac film on silicon substrate (Fig. 1(c)), the valance band of the RuAcac is determined to be ca. 5.0 eV [26]. Herein, the conduction band of the RuAcac is ca. 2.9 eV. As a result, the energy level arrangements of various layers in the solar cell were drawn in Fig. 1(d). As we can see that the hole and electron can be effectively extracted via the NiOx HTM and PCBM/RuAcac ETM to the corresponding contacts, respectively [27]. In contrast to the reference device without RuAcac as CIL, RuAcac is believed to realize a quasi-ohmic contact between PCBM and Ag, and will be discussed in the following text.

 

Fig. 1 a) Device architecture sketch of the p-i-n planar perovskte solar cells; b) Absorption spectrum of the RuAcac CIL thin film on quartz (insert is the chemical structure of RuAcac); c) UPS spectrum of the RuAcac thin film deposited on silicon substrate. The corresponding cutoff and onset energy values were shown as well; d) energy level diagram of different layers used in the device; e) X-ray diffraction (XRD) patterns of perovskite layer on NiOx/ITO substrates; f) scanning electron microscopy (SEM) characteristic of the perovskite films; g) Cross-section SEM morphology of an intact perovskite solar cell.

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The perovskite absorber layer was fabricated through one-step solvent engineering method (see more details in the experimental section). Figure 1(e) shows good crystallinity and purity of CH₃NH₃PbI₃ films on NiO_x HTM, with the diffraction peaks at 14.02°, 19.98°, 23.38°, 24.42°, 28.38°, 31.83°, 34.92°, 40.53°, and 43.08° corresponding to the reflections from (110), (112), (211), (202), (220), (310), (312), (224), and (314) crystal planes of the tetragonal phase of CH₃NH₃PbI₃. The uniformity and grain size of the perovskite absorber layer were clearly demonstrated by the SEM image in Fig. 1(f). The compact, highly crystalline film morphology are a prerequisite for high performance, since a higher density of larger grains might have less grain boundary and positively suppress charge recombination due to the located less density of charge traps in these regions [28]. The cross section profile of an intact perovskite solar cell was prepared by focus ion beam (FIB) protocol and followed by SEM imaging, as shown in Fig. 1(g). Each layer of the device can be clearly distinguished, which is confirmed as a typical planar p-i-n device architecture. The thickness of the perovskite film is about 320 nm and thick enough to absorb solar spectra while a 20-30nm thin and compact layer of NiOx HTM enables holes transport from perovskite to ITO and blocking of electrons owing to high conduction band level of NiOx [23]. At the cathode side, a ca. 90 nm layer of PCBM fully covering the surface of perovskite with dense and uniform morphology effectively isolate perovskite from electrodes to avoid the direct contact between them, which is critical to both performance and stability of planar PSCs [29].

It’s worthy to note that RuAcac itself with low carrier mobility is not a suitable ETL to replace directly PCBM [26]. However, the small ionization potential of RuAcac allows for holes direct recombining with electrons at the suitable valance band (5.0 eV) of RuAcac [30]. Another merits of RuAcac with high pyrolysis temperature and high hydrolysis threshold owing to that Acac group has strong chelate bonding to Ru (III) ions definitely lead to reliable process and stable photovoltaics devices [31]. The chemical component of RuAcac powder and spin coated RuAcac thin film on quartz substrate was analyzed by X-ray photoelectron spectroscopy (XPS) as shown in Fig. 2. The binding energies (BE) obtained in the XPS analysis are rectified for specimen charge by referencing the C 1 s peak to 284.8 eV. As shown in Fig. 2(a), the survey scan for both powder and thin film samples in the range from 0 to 1100 eV BE show characteristic peaks of the elements Ru, O, and C. The high resolution XPS spectra of the core level of Ru 3d (Fig. 2(b)) for the powder sample and thin films demonstrated very similar characters, showing a strong spin–orbit doublet due to Ru 3d_5/2 at 181.5 eV and Ru 3d_3/2 at 185.9 eV [32, 33]. The C 1s and O 1 s XPS characteristics have no obvious variation for the powder and thin film samples, as shown in Fig. 2(c) and 2(d). The XPS results confirm that no structure change turns up in the spin coated thin film in comparison with the source powder.

 

Fig. 2 XPS analysis of the RuAcac powder and the corresponding films coated on silicon substrates. a) Survey scan; b) Ru 3d; c) C 1s; d) O 1s.

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The morphology of the perovskite/PCBM film before and after RuAcac CIL modification has been investigated by surface SEM. As shown in Figs. 3(a) and 3(c), there is no morphology variation for the PCBM film after RuAcac CIL deposition, indicating the very thin and uniform RuAcac layer formed on top of PCBM, which further been confirmed by the energy dispersive X-ray spectrometry (EDS). The comparison in the EDS mapping for Ru elements between the perovskite/PCBM film (Fig. 3(b)) and the perovskite/PCBM/ RuAcac film (Fig. 3(d)), it clearly show a homogeneous and uniform layer of RuAcac film covering on the top of PCBM, which is crucial to achieve high performance devices.

 

Fig. 3 Surface SEM characteristics of the perovskite/PCBM (a) and perovskite/PCBM/RuAcac (c); Energy-dispersive X-ray spectroscopy (EDS) mapping of Ru for the perovskite/PCBM (b) and perovskite/PCBM/RuAcac (d).

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Figure 4(a) shows current density-voltage (J-V) curves of the inverted planar PSCs with and without RuAcac CIL under the 1 sun illumination (100 mW/cm²). The corresponding device performance parameters are summarized in Table 1. The reference cell with pure Ag contact gives a PCE of 12.74% under backward scanning, with an open-circuit voltage (V_oc) of 1.038 V, a short-circuit current density (J_sc) of 19.45 mA/cm², and a fill factor (FF) of 63.1%. After inserting a thin layer of RuAcac between PCBM and Ag, all photovoltaic parameters demonstrate apparent improvement, achieving an optimal PCE of 17.15% (Table 1). We attribute the enhanced J_sc and FF to the reduced contact barrier and enhanced charge carrier extraction efficiency at the PCBM/Ag interface after integrating the RuAcac CIL, which have been interpreted by previous reports and our following experimental results [34, 35].

 

Fig. 4 a) J-V characteristics of our perovskite solar cells with and without RuAcac CIL under 100 mW/cm² illumination at forward and backward scanning mode; b) stable photocurrent density output for the Ag reference and RuAcac/Ag optimal device recorded at maximum power point (0.88V); c) EQE spectra and the corresponding integrated short circuit current density spectra of our perovskite solar cells with and without RuAcac CIL.

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Table 1. Summary of the photovoltaic parameters for the inverted planar perovskite solar cells with and without various RuAcac interfacial layers under AM 1.5G simulated sun light illumination (100 mW/cm²).

View Table

On the other hand, the device performances show negligible hysteresis behaviors for all devices when measuring the J-V curves with different scan directions, which are well consistent with previous report (Fig. 4(a)) [22, 23, 28, 36]. Additionally, as shown in Fig. 4(b), the reference and RuAcac CIL devices show an instant photocurrent response during illumination and demonstrated a highly stability of photocurrent at maximum power point with no decay in 500 seconds light soaking. In order to further investigate the Jsc improvement for RuAcac based devices in contrast to the reference, the external quantum efficiency (EQE) spectra of them were characterized, as shown in Fig. 4(c). The integrated Jsc profiles from EQE spectra for both devices were displayed at Fig. 4(c) as well. As observed from the EQE spectra, the RuAcac based device shows more efficient photon responsibility in the entire visible region than that for the reference. These photon-physics results confirm again that the interfacial modification with RuAcac is an effective way to improve device performance by reducing the contact barrier, improving electron extraction efficiency at the cathode.

It’s well known that the interfacial band alignment are extreme significant to form free barrier contact between each layers in organic and perovskite solar cell [34, 35]. Due to the mismatch between LUMO level of PCBM and work function of Ag, the energy barriers at PCBM/Ag interface are relative large and hence hinder charge extraction and transfer at the cathode interface. Therefore, the V_oc, J_sc and FF for an inverted planar PSC are likely to be suppressed, and hence the high efficient device can hardly be achieved. In our work, after the introduction of a metal acetylacetonate (RuAcac CIL) at this interface, the energy barrier at PCBM/Ag interface can be effectively reduced to form a quasi-ohmic contact, which improves the charge extraction efficiency and thereafter the final device power conversion efficiency. To investigate how RuAcac CIL impacts the PCBM/Ag interface, scanning kelvin probe microscopy (SKPM) and UPS were employed to explore the surface work function variation of Ag contact with and without RuAcac CIL. As shown in Fig. 5(a) and 5(b), the surface potential of Ag contact was largely reduced after integrating a thin layer of RuAcac CIL. the work function values can be deduced from the equation ${\text{φ}}_{\text{Ag}}={\text{φ}}_{\text{t}}-{\text{e V}}_{\text{sp}}$, where ${\text{φ}}_{\text{t}}$ is the work function of conductive tip and ${\text{V}}_{\text{sp}}$ is the measured surface potential determined from the surface potential profiles [29]. The work function ${\text{φ}}_{\text{Ag}}$ for RuAcac modified Ag contact was determined to be ca. 4.37 eV according to the surface potential profiles (Fig. 5(d)). Furthermore, from the UPS spectra (Fig. 5(d)), the work function of RuAcac modified Ag sample was calculated to be ca. 4.38 eV, which is identical to that from SKPM analysis. That demonstrates the effective dropping of the work function of Ag modified by RuAcac CIL. It’s already recognized that the reduced work function of Ag electrode by RuAcac CILs are derived from the dipoles formed along the interfaces [22, 37–39]. which leads to the formation of quasi-ohmic contact between PCBM and Ag, and hence “barrier-free” electron extraction from PCBM to Ag electrode.

 

Fig. 5 SKPM images for the pure Ag (a) and RuAcac modified Ag (b) films coated on silicon substrates; c) Surface potential profiles (V_sp) extracted from the SKPM images at (a) and (b); d) UPS spectra of the pure Ag and RuAcac coated Ag films coated on silicon substrates. The work function of Ag was reduced after RuAcac modification.

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Moreover, interfacial charge-carrier characteristics between various functional layers in the inverted planar PSCs were explored by steady state photoluminescence (PL) spectroscopy and PL decay dynamics measurement (Fig. 6). Steady state PL spectra for the perovskite films correlated to various interfaces were presented at Fig. 6(a). The photon excited charge carriers can be effectively quenched after coating PCBM on top of perovskite layer due to the charge transfer from perovskite film to PCBM. More efficient quenching was observed after the RuAcac CIL integrating into the PCBM/Ag interfaces, indicating more efficient charge carrier transfer occurred at those interfaces. PL decay is a comprehensive protocol to discover charge carriers’ dynamics at semiconductor interfaces. Figure 6(b) shows the PL decay dynamics by exciting the devices with and without RuAcac by a pulsed laser of 405 nm. The PL lifetime of ITO/Perovskite, ITO/NiOx/Perovskite/PCBM/Ag, ITO/NiOx/Perovskite/PCBM/RuAcac/Ag were determined to be 50.6 ns, 15.3 ns and 13.2 ns, respectively [6]. Because both non-radiative recombination and energy transfer process of the charge carriers were involved in PL decay experiment, a faster PL decay meant more efficient energy transfer process (charge transfer here) was involved if constant non-radiative recombination rate was assumed. The PL decay comparison shows the RuAcac based device exhibit the shortest lifetime and hereafter the fast carriers transport to ITO electrode, in contrast to the PCBM only sample and pure perovskite one. The conclusion of those PL data is in a line with the above and confirm further the charge carrier transfer enhancement of RuAcac CIL in the inverted planar PSCs

 

Fig. 6 a) Room temperature photoluminescence (PL) spectra of the pure perovskite, perovskite/PCBM and perovskite/PCBM/RuAcac samples; b) PL decay curves of the corresponding samples presented at (a).

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4. Conclusion

In summary, RuAcac, with small ionization potential, high thermal stability and high hydrolysis threshold, was successfully adopted as a reliable and stable cathode interfacial layer to improve the inverted planar PSCs. The power conversion efficiency of the optimal devices was enhanced from 12.74% for the control device without RuAcac, to 17.15% for the RuAcac based devices along with obvious improvement in the corresponding Voc, Jsc and FF, respectively. A series of photo physics and microscopy protocols, including EQE, UPS, XPS, PL, SKPM and EFM, were used to discover the function of RuAcac as CIL in the inverted planar PSCs. The evidences draw a consistent conclusion that RuAcac enables the formation of quasi-ohmic contact at the cathode side by eliminating the barrier between the LUMO of PCBM and Fermi level of silver electrode. The low temperature and facile processed Ruthenium acetylacetonate in this work, with high stability in chemical, thermal, optical and electrical fields, definitely offer us a robust interface-engineering way for the perovskite solar cells and also their commercialization.