A cascade-type electron extraction design for efficient low-bandgap perovskite solar cells based on a conventional structure with suppressed open-circuit voltage loss

Abstract

The tandem architecture for perovskite solar cells has proven successful in promoting the development of such cells. A low-bandgap perovskite solar cell, which typically acts as a back cell, is one of the critical components for tandem perovskite solar cells. However, nowadays, highly efficient low-bandgap perovskite solar cells are mostly based on the inverted structure, which restricts the development of conventional perovskite tandem cells. Therefore, efficient low-bandgap perovskite solar cells based on the conventional structure need to be developed to further extend the availability of device architectures and interfacial materials for tandem cells. Here, by modifying the electron transport materials, we successfully demonstrated an efficient low-bandgap perovskite solar cell based on the conventional structure. A ZnO/SnO₂/C₆₀-SAM tri-layer was used to engineer the energy level alignment of electron transport layers to reduce the energy loss occurring at the interface and simultaneously suppress the interfacial recombination and improve the charge extraction, resulting in a reduced open-circuit voltage loss for the device. Finally, our low-bandgap perovskite solar cells achieved a power conversion efficiency of 13.8%, which is the record result for conventional device structures to date.

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Introduction

Since organic–inorganic hybrid perovskite solar cells (PVSCs) were first reported in 2009, the power conversion efficiency (PCE) of this kind of solar cell has skyrocketed to a certified record efficiency of 23.7%, approaching that of crystalline silicon solar cells.¹ The rapid development of PVSCs can be ascribed to their excellent photovoltaic properties and the choice of various compositions in the perovskite family.^2–5 However, the development of single-junction PVSCs has slowed down in recent years and a new device structure needs to be developed to overcome the bottleneck. As is well known, the tandem solar cell is a proven strategy to break through the Shockley–Queisser limit for single cells, thereby promoting the development of PVSCs. The two typical device structures for tandem PVSCs are the inverted structure and conventional structure. In inverted tandem PVSCs, the wide-bandgap front cell and low-bandgap back cell are in an inverted arrangement (ITO/hole transport layer (HTL)/perovskite/electron transport layer (ETL)/electrode, p–i–n structure) while in conventional tandem PVSCs, the front cell and back cell have a conventional arrangement (n–i–p structure). Therefore, both highly efficient wide-bandgap front cells and low-bandgap back cells are needed in both inverted and conventional structures, for different architectures of the tandem PVSCs. In the perovskite family, bandgaps ranging from around 1.2 to 1.9 eV can be easily tuned through composition engineering.^5,6 Nowadays, highly efficient wide-bandgap PVSCs (around 1.7 to 1.9 eV) based on inverted and conventional structures have been successfully demonstrated, whereas the low-bandgap PVSCs (around 1.2 eV) still suffer from inferior performance, particularly those with the conventional structure.^5,7–12 Therefore, it is an urgent target to develop efficient low-bandgap PVSCs based on the conventional structure to further extend the availability of device architectures and interfacial materials for tandem cells.

In organic–inorganic hybrid perovskites, when using tin (Sn) to partially replace lead (Pb) in FA_xMA_1−xPbI₃ (0 ≤ x ≤ 1), where FA⁺ and MA⁺ represent formamidinium and methylammonium cations, respectively, the bandgap of perovskite can be easily tuned from around 1.6 to 1.2 eV, leading to an expansion of the absorption edge from around 775 to 1050 nm.¹³ The lowest-bandgap perovskites were achieved when the amounts of Sn and Pb were stoichiometrically identical in the perovskite lattice (i.e., FA_xMA_1−xSn_0.5Pb_0.5I₃).^5,11,13–16 In the past two years, a great effort has been made toward developing low-bandgap FA_xMA_1−xSn_0.5Pb_0.5I₃ PVSCs based on the inverted structure, leading to a substantial improvement in PCE from 10% to around 18%.^17–23 However, compared with the inverted structure, conventional FA_xMA_1−xSn_0.5Pb_0.5I₃ PVSCs have attracted less attention. In 2014, Hayase and co-workers for the first time demonstrated a low-bandgap PVSC based on the conventional structure with absorption up to 1060 nm by forming MASn_0.5Pb_0.5I₃.¹¹ Their device obtained a PCE of 4.18%, and this value was improved to 7.27% by Kanatzidis and co-workers several months later, which still stands as the best result for low-bandgap PVSCs based on the conventional structure to date, to the best of our knowledge.⁵ One reason for the inferior performances of conventional low-bandgap PVSCs was the high open-circuit voltage (V_oc) loss, which is defined by the difference between the V_oc of the PVSCs and the optical bandgap (E_g) of the active layer divided by the elementary charge (q) (i.e. E_g/q − V_oc).^24,25 The high V_oc loss probably resulted from the large energy level mismatching between the conduction band (CB) of the low-bandgap perovskites and TiO₂.¹¹ Therefore, a new ETL should be considered to substitute the high-temperature-processed TiO₂, in order to provide a better energy level alignment to suppress the V_oc loss and hence improve the device performance. Considering its electron mobility, light transparency and low-temperature processability, SnO₂, which is widely used for high-performance PVSCs, is a good candidate for TiO₂ substitution.^26–28 However, the CB mismatching between SnO₂ and perovskites could also cause a large V_oc loss, which therefore necessitates a modification of the energy level of SnO₂.

Herein, a self-assembled monolayer, C₆₀-SAM, was used to modify the energy level of the SnO₂ ETL. It has been reported that modification by C₆₀-SAM could up-shift the CB of SnO₂ from −4.5 eV to −4.1 eV, which matches well with the CB of low-bandgap FA_0.5MA_0.5Sn_0.5Pb_0.5I₃ perovskite (−4.0 eV).^29,30 In this case, the low-bandgap PVSCs obtained an improved V_oc from 0.35 V to 0.48 V, with an improved PCE from 5.7% to 9.0%. Subsequently, we inserted a layer of ZnO to form a cascade-type ETL of ZnO/SnO₂/C₆₀-SAM to further reduce the energy level mismatching between SnO₂/C₆₀-SAM and indium tin oxide (ITO) and to suppress the interfacial recombination as well, which hence further reduced the V_oc loss for the whole device. Eventually, we demonstrated a highly-efficient low-bandgap FA_0.5MA_0.5Sn_0.5Pb_0.5I₃ PVSC with PCE greater than 13%. To the best of our knowledge, this is the most efficient low-bandgap PVSC based on the conventional device structure to date.

Experimental

Precursor solution preparation

The FASnI₃ precursor solution was prepared by dissolving 372 mg of SnI₂, 172 mg of formamidinium iodide (FAI) and 15.6 mg of SnF₂ in 1 mL of mixed solvent of N,N-dimethylmethanamide (DMF) and dimethyl sulfoxide (DMSO), where the ratio of DMF : DMSO was 4 : 1. The MAPbI₃ precursor solution was prepared by dissolving 461 mg of PbI₂ and 159 mg of methylammonium iodide (MAI) in 0.7 mL of the aforementioned mixed solvent. The FA_0.5MA_0.5Sn_0.5Pb_0.5I₃ precursor solution was obtained by mixing stoichiometrically identical amounts of FASnI₃ and MAPbI₃ precursor solutions, followed by stirring for 12 hours.

Device fabrication

ITO glass substrates were sequentially washed with isopropanol, acetone, distilled water and isopropanol, then dried in an oven. For device fabrication, ITO glass was treated by plasma treatment, followed by the deposition of 10 nm-thick ZnO nanoparticles (purchased from Sigma Aldrich) through spin-coating. Subsequently, 40 nm-thick SnO₂ nanoparticles (purchased from Alfa Aesar) were spin-coated on the ITO/ZnO substrate, followed by annealing at 150 °C for 30 min. For SnO₂ modification, C₆₀-SAM solution (1.5 mg mL⁻¹ in chlorobenzene) was spin-coated above the SnO₂ layer at a spin speed of 4000 rpm for 30 s, and then spin-treated with chlorobenzene to wash away the residual C₆₀-SAM. From the X-ray photoelectron spectroscopy (XPS) and atomic force microscope (AFM) measurements shown in Fig. S1 (ESI†), we confirmed the existence of C₆₀-SAM above the SnO₂ layer via the appearance of the N 1s signal, and observed that the C₆₀-SAM-deposition process had no influence on the SnO₂ morphology. Finally, the FA_0.5MA_0.5Sn_0.5Pb_0.5I₃ precursor solution was spin-coated above the ITO/ZnO/SnO₂/C₆₀-SAM layer to form a 500 nm-thick perovskite film, followed by the deposition of a 40 nm-thick poly(3-hexylthiophene-2,5-diyl) (P3HT) layer, 10 nm-thick MoO₃ layer and 100 nm-thick Ag electrode.

Perovskite film characterization

Ultraviolet-visible light absorption spectra were recorded on an HP8453 spectrophotometer. Scanning electron microscope (SEM) images were obtained by a Zeiss EVO 18 SEM. The crystalline structure of the perovskite films was investigated by an X-ray diffractometer (PANalytical X’pert PRO) equipped with a Cu-Kα X-ray tube. The XPS data were obtained by using ESCALAB 250Xi equipment. Ultraviolet photoelectron spectroscopy (UPS) data were obtained using a K-ALPHA+ instrument. AFM images were obtained by a Bruker Multimode 8 scanning probe microscope.

Perovskite solar cell characterization

The current density–voltage (J–V) characteristics for the devices were measured under conditions of 100 mW cm⁻² air mass, 1.5 global (AM 1.5G) illumination with a solar simulator (Taiwan, Enlitech, SS-F5). The light intensity was calibrated with a National Renewable Energy Laboratory-calibrated silicon photodiode using a KG5 filter. The external quantum efficiency (EQE) of the devices was measured on a commercial QE measurement system (QE-R3011, Enlitech). For the transient photovoltage (TPV) measurements, the device was serially connected to a digital oscilloscope (Tektronix TDS 3052C), and the oscilloscope's input impedance was set to 1 MΩ to form open-circuit conditions. The TPV was measured under 0.3 sun illumination. An attenuated laser pulse (0.54 μJ cm⁻²; 530 nm) was used as a small perturbation to the device's background illumination. The laser pulses were generated from an optical parametric amplifier (TOPAS-Prime) pumped by a mode-locked Ti:sapphire oscillator-seeded regenerative amplifier, with a pulse energy of 1.3 mJ at 800 nm and a repetition rate of 1 kHz (SpectraPhysics Spitfire Ace). The time resolution of the overall measurement was approximately 1 ns. The light-dependent V_oc measurements were performed under the J–V measurement system, and the light intensity was modulated from 0.1 sun to 1 sun. The transient photocurrent (TPC) and electrochemical impedance spectroscopy (EIS) measurements were carried out using a Paios 4.0 Measurement Instrument (FLUXiM AG, Switzerland).

Results and discussion

Although SnO₂ has been successfully applied in medium-bandgap PVSCs, there has been no report of low-bandgap PVSCs based on a SnO₂ ETL in a conventional structure. In our study, the low-bandgap perovskite of FA_0.5MA_0.5Sn_0.5Pb_0.5I₃ was chosen as the active layer to absorb light up to around 1050 nm. The valence band (VB) of FA_0.5MA_0.5Sn_0.5Pb_0.5I₃ perovskite was measured as −5.2 eV by UPS as shown in Fig. S2 (ESI†), and the CB of −4.0 eV was confirmed by its optical bandgap. The large energy offset of CBs between SnO₂ (−4.5 eV) and FA_0.5MA_0.5Sn_0.5Pb_0.5I₃ perovskite (−4.0 eV) can be expected to cause a large V_oc loss in the device.^31–34 Therefore, energy level modification of SnO₂ is needed in this case.

Self-assembled monolayers are widely used as modifying agents not only to modulate the surface tension, surface energy and energy level of metal oxides, but also to passivate the surface traps of metal oxides.^29,35–38 Among all of the known modifiers, C₆₀-SAM, whose molecular structure is shown in Fig. 1b, has proven to be one of the most successful, up-shifting the CB of SnO₂ to −4.1 eV, and thereby reducing the CB offset between the ETL and FA_0.5MA_0.5Sn_0.5Pb_0.5I₃ perovskite.²⁹ The carboxyl end-groups in C₆₀-SAM can strongly bond with SnO₂ to form a self-assembled monolayer, leaving the C₆₀ bodies exposed on the exterior, which can affect the morphologies and crystallinity of the upper perovskite layer.^38,39

Fig. 1 (a) SEM images of FA_0.5MA_0.5Sn_0.5Pb_0.5I₃ film above SnO₂ and SnO₂/C₆₀-SAM substrates. (b) XRD pattern of FA_0.5MA_0.5Sn_0.5Pb_0.5I₃ film above SnO₂ and SnO₂/C₆₀-SAM substrates. The molecular structure of C₆₀-SAM is also shown.

Morphologies and crystal qualities

Fig. 1a shows top-view SEM images of FA_0.5MA_0.5Sn_0.5Pb_0.5I₃ perovskite films prepared on SnO₂ and SnO₂/C₆₀-SAM substrates, respectively. In both cases full-coverage films with similar morphologies were obtained, suggesting that no negative impact on the perovskite film morphology was caused by the modification with C₆₀-SAM. Subsequently, X-ray diffraction (XRD) measurements were performed to reveal the crystal quality of the perovskite films. As can be seen in Fig. 1b, compared with the perovskite film fabricated above the SnO₂ substrate, the perovskite film fabricated above the SnO₂/C₆₀-SAM substrate obtained better crystallinity, which in turn improved the absorption of light by the FA_0.5MA_0.5Sn_0.5Pb_0.5I₃ film, as shown in Fig. S3 (ESI†). We attributed the better crystallinity of the perovskite film to the passivation effect of C₆₀-SAM toward the SnO₂ surface, which has been widely reported previously.^30,39,40 The removal of defects on the SnO₂ surface facilitated a better growth of the upper perovskite layer.^33,38

To further confirm this, we estimated the trap density of the ETL/perovskite film by studying an electron-only device based on the device structure of ITO/ETL/perovskite/PCBM/Ag. As shown in Fig. 2, the overall trap density of SnO₂/C₆₀-SAM/FA_0.5MA_0.5Sn_0.5Pb_0.5I₃ film is 2.0 × 10¹⁵ cm⁻³, which is lower than that of the SnO₂/FA_0.5MA_0.5Sn_0.5Pb_0.5I₃ film (3.1 × 10¹⁵ cm⁻³). This result can be attributed to the reduced trap density on the SnO₂ surface owing to passivation by C₆₀-SAM and the improved perovskite crystal quality. Films with better absorption and lower trap density can be expected to be more suitable for photovoltaic applications.

Fig. 2 Trap density evaluation by dark current–voltage measurements of the electron-only device with the device structure of ITO/ETL/perovskite/PCBM/Ag, where ETL is SnO₂ in (a) and SnO₂/C₆₀-SAM in (b). The pink lines represent the Ohmic regime of each case, and the red lines indicate the trap-filled limit (TFL) regime with the onset voltage (V_TFL). The trap density (N_t) of FA_0.5MA_0.5Sn_0.5Pb_0.5I₃ thin films with different ETLs can be extracted by the equation displayed in (a), where ε₀, q and L represent vacuum permittivity, elementary charge and thickness of the perovskite film, respectively; ε_r is the average relative dielectric constant, which is generally assigned as 25.⁴¹

Device properties

To estimate the influence of SnO₂ modification by C₆₀-SAM on the photovoltaic properties, PVSCs based on the device architecture of ITO/ETL/FA_0.5MA_0.5Sn_0.5Pb_0.5I₃/P3HT/MoO₃/Ag were fabricated, as shown in Fig. 3a. Firstly, we compared the ETLs of SnO₂ and SnO₂/C₆₀-SAM. The J–V curves of the PVSCs with different ETLs are shown in Fig. 3b and their performances are summarised in Table 1. The EQEs of both cases are shown in Fig. S4 (ESI†) and the deviations between the short-circuit current density (J_sc) measured by the solar simulator and the integrated J_sc calculated from the EQE spectra are less than 5%. The PVSC based on the bare SnO₂ ETL suffered from an inferior PCE of 5.7% with an ultra-poor V_oc of 0.35 V and small fill factor (FF) of 59%, resulting in a high V_oc loss of 0.85 V. However, after the SnO₂ ETL was modified by C₆₀-SAM, the PVSC based on the SnO₂/C₆₀-SAM ETL obtained an improved V_oc of 0.48 V and an improved FF of 65.5%. We attributed this result to three possible reasons. First, the C₆₀-SAM passivated the surface traps on the SnO₂ surface, which weakened the trap-assisted recombination at the interface between the ETL and perovskite, as we will discuss later. Second, the perovskite film based on the SnO₂/C₆₀-SAM substrate obtained better crystal quality. Third, the SnO₂/C₆₀-SAM obtained a higher CB (−4.1 eV) than that of bare SnO₂ (−4.5 eV), which reduced the energy offset between the ETL and perovskite and hence suppressed the energy loss at the interface.

Fig. 3 (a) Device architecture and energy level of each layer in the device. (b) J–V curves of PVSCs with different ETLs. (c) Statistical performances of 36 PVSCs with different ETLs. (d) Stability measurements of PVSCs with different ETLs, stored in an N₂-filled glovebox.

Table 1 Performances of PVSCs with the structure ITO/ETL/FA_0.5MA_0.5Sn_0.5Pb_0.5I₃/P3HT/MoO₃/Ag

ETL	V oc (V)	J sc (mA cm^−2)	FF (%)	PCE (%)	V oc loss (V)
SnO2	0.35	27.6	59.0	5.7	0.85
SnO2/C60-SAM	0.48	28.7	65.6	9.0	0.72
ZnO/SnO2/C60-SAM	0.66	29.0	72.5	13.8	0.54

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However, as seen in the energy level diagram in Fig. 3a, there was still a large energy offset between the SnO₂/C₆₀-SAM and ITO layers, which must be the main reason why the device still suffered from a large V_oc loss of 0.72 V. Clearly, it can be expected that if one extra ETL with a CB around −4.4 eV could be inserted between the SnO₂/C₆₀-SAM and ITO layers, this would further reduce the V_oc loss for the overall device. Among all of the widely used ETLs, ZnO nanoparticles, which are low-temperature solution-processable, are one of the most appropriate because of their high electron mobility and satisfactory CB of −4.4 eV.^35,42 We therefore inserted a ZnO layer between ITO and SnO₂/C₆₀-SAM to fabricate a conventional PVSC based on the device structure of ITO/ZnO/SnO₂/C₆₀-SAM/FA_0.5MA_0.5Sn_0.5Pb_0.5I₃/P3HT/MoO₃/Ag. As shown in Fig. 3b and Table 1, we found that the formation of a cascade-type ETL of ZnO/SnO₂/C₆₀-SAM suppressed the V_oc loss notably further to 0.54 V, resulting in a better FF of 72.5% and a dramatically improved PCE of 13.8%. To the best of our knowledge, this is the best result so far for low-bandgap PVSCs based on the conventional device structure. Moreover, as seen in Fig. 3c and d, the device based on the ETL of SnO₂/C₆₀-SAM and ZnO/SnO₂/C₆₀-SAM obtained better reproducibility and stability compared with the device based on the ETL of bare SnO₂. This can again be attributed to the reduced number of trap states on the SnO₂ surface and the better crystal quality of the perovskite film. The best device maintained almost 90% of its initial performance when stored in an N₂-filled glovebox for 1 month.

Charge recombination and extraction properties

Besides an appropriate energy level alignment of each layer, the recombination properties in PVSCs also strongly affects the total V_oc loss of the device.⁴³ In a PVSC, typically, trap-assisted recombination dominates the monomolecular recombination and causes a severe V_oc loss in the device.⁴⁴ Such recombination becomes severe at perovskite crystal surfaces and the interface between the perovskite and ETL/HTL.^45,46 Therefore, it is important to passivate the surface traps in perovskites as well as in ETL/HTL materials, and to spatially separate the photo-generated holes and electrons into different materials, aiming to prolong the recombination time. The overall monomolecular recombination of the device can be measured via the light-intensity-dependent V_oc of the device (the relationship between V_oc and the natural logarithm of light intensity, i.e. ln(I)).⁴⁷ Generally, if the slope of this relationship has a value around 1 k_bT/q, this represents that the recombination process is dominated by bimolecular recombination (where k_b is the Boltzmann constant and T is the temperature), while the slope tends to be 2 k_bT/q when monomolecular recombination (or trap-assisted recombination) is involved.⁴⁸ As seen in Fig. 4a, the device with the ZnO/SnO₂/C₆₀-SAM ETL obtained the smallest slope, tending to 1 k_bT/q, compared with the larger slopes of the devices with ETLs of SnO₂/C₆₀-SAM (slope of 1.37 k_bT/q) and SnO₂ (slope of 1.52 k_bT/q). This result illustrated that the device with the bare SnO₂ ETL suffered from the most severe monomolecular recombination, while such recombination was suppressed after the C₆₀-SAM modification and an even better suppression was achieved by using the cascade-type ETL of ZnO/SnO₂/C₆₀-SAM.

Fig. 4 Charge recombination and extraction characterization of FA_0.5MA_0.5Sn_0.5Pb_0.5I₃ solar cells with different ETLs: (a) photovoltage as a function of light intensity and (b) TPV measurements. (c) TPC measurements and (d) EIS measurements. For the devices with bare SnO₂, SnO₂/C₆₀-SAM and ZnO/SnO₂/C₆₀-SAM ETLs, the extracted R_tran values are 1.9 kΩ, 1.0 kΩ and 0.8 kΩ, respectively, while the extracted R_rec values are 11.2 kΩ, 15.8 kΩ and 19.5 kΩ, respectively. (e) Schematic diagrams of the charge recombination situations in cases of different ETLs.

To provide a deeper understanding of the recombination properties of PVSCs with different ETLs, TPV measurements were also conducted. The decay of photovoltage results from the recombination of photo-excited electrons and holes, and therefore a longer photovoltage decay time reflects a slower charge recombination in the device.⁴⁹Fig. 4b shows the normalized photovoltage as a function of time in PVSCs with different ETLs, and the lifetime τ of each case was extracted by single exponential fitting. The device with the bare SnO₂ ETL obtained the shortest lifetime of 11.4 μs, while longer lifetimes of 22.8 μs and 36.6 μs were achieved in the SnO₂/C₆₀-SAM and ZnO/SnO₂/C₆₀-SAM ETL cases, respectively. The fact that the longest lifetime was achieved by the ZnO/SnO₂/C₆₀-SAM ETL illustrated that the photogenerated charge carriers in this case had the most favourable opportunity to be transported to the electrodes before they recombined with each other. Both the weak monomolecular recombination and the small charge recombination rate strongly support the significant V_oc improvement in the device.

Charge extraction properties are also important for a high-performance PVSC to reach low V_oc loss, because a faster charge extraction also assists the suppression of charge recombination. TPC is a suitable technique to investigate the charge extraction properties within the whole device. Fig. 4c shows the TPC results of PVSCs with different ETLs. It is obvious that a faster decay of photocurrent occurred in the device with the ZnO/SnO₂/C₆₀-SAM ETL compared with the other two cases. These results suggest that the cascade-type ETL is efficient for charge extraction in the device, which can be attributed to the better energy-level alignment and the suppressed interfacial recombination. To cross check these results, EIS was conducted to study the interfacial properties of the low-bandgap PVSCs, including charge extraction and charge recombination properties. Fig. 4d shows the Nyquist plots from the impedance spectra results for the devices with different ETLs, and the circuit model we used for data analysis. The extracted transport resistance (R_tran) values are 1.9 kΩ, 1.0 kΩ and 0.8 kΩ for the device with bare SnO₂, SnO₂/C₆₀-SAM and ZnO/SnO₂/C₆₀-SAM ETLs, respectively. A smaller R_tran value reflects fewer barriers for charge transport, therefore, the device with ZnO/SnO₂/C₆₀-SAM ETL obtained the best charge transport property, which was also consistent with the TPC results. In addition, the extracted recombination resistance (R_rec) values are 11.2 kΩ, 15.8 kΩ and 19.5 kΩ for the device with bare SnO₂, SnO₂/C₆₀-SAM and ZnO/SnO₂/C₆₀-SAM ETLs, respectively. A larger R_rec value reflects larger barriers for charge recombination, therefore, the photoexcited charge carriers are the most difficult to recombine in the device with the ZnO/SnO₂/C₆₀-SAM ETL, compared with the other two cases.

Considering together the passivation effect of C₆₀-SAM, the cascade-type ETL, the charge recombination and extraction properties of the devices, we schematically illustrate in Fig. 4e the factors affecting recombination, and their consequences for device performance, in the devices with different ETLs. In the device with the bare SnO₂ ETL, the surface defects of SnO₂ become traps at the interface between the SnO₂ and perovskite. The photo-excited electrons in the perovskite or the transferred electrons in SnO₂ can be trapped by such surface trap states, leading to severe monomolecular recombination and fast recombination rate for the photo-generated charge carriers. However, in the device with the SnO₂/C₆₀-SAM ETL, the surface traps of SnO₂ have been passivated by C₆₀-SAM. The removal of trap states at the interface allows successful electron transfer from the perovskite to SnO₂, separating the photo-excited electrons and holes, and therefore suppressing the monomolecular recombination and slows down the recombination rate. Finally, in the device with the ZnO/SnO₂/C₆₀-SAM ETL, photo-excited electrons are easily transferred from the perovskite to the ZnO layer because of the gradient energy level alignment. The spatial separation of photo-excited electrons and holes dramatically reduces their recombination at the interface, leading to a small V_oc loss. A similar phenomenon was previously observed while using PCBM/C₆₀ as the ETL to form a spike structure that dramatically suppressed the interfacial recombination through the spatial separation of photo-excited electrons and holes.²⁰

Conclusions

Using a cascade-type tri-layer of ZnO/SnO₂/C₆₀-SAM as the ETL, we successfully demonstrated a highly efficient low-bandgap PVSC with a PCE of 13.8%. To the best of our knowledge, this is the best result thus far for any conventional low-bandgap PVSC, being almost two-times higher than the previous highest published PCE to date, and provides the possibility of an efficient conventional tandem PVSC. We consider the extremely large V_oc loss to be one of the main causes of the inferior performance of low-bandgap PVSCs based on the conventional structure, while our findings prove that the utilization of a cascade-type ETL (i.e. ZnO/SnO₂/C₆₀-SAM) is an efficient strategy to reduce the V_oc loss. First, in the context of energy level alignment, the C₆₀-SAM up-shifted the energy level of SnO₂ from −4.5 eV to −4.1 eV, which minimized the offset of CBs between SnO₂ and perovskite, and hence reduced the energy loss. In addition, the larger energy offset between ITO and SnO₂/C₆₀-SAM was also reduced by the insertion of ZnO. Such a cascade-type ETL provided a smooth pathway for the electrons, facilitating a better electron extraction and a low V_oc loss in the device. Second, in the context of recombination suppression, the C₆₀-SAM passivated the surface traps of SnO₂ and consequently reduced the trap-assisted recombination at the interface. Moreover, the fast electron transfer from the perovskite to the additional ZnO layer spatially separated the photo-excited electrons and holes, leading to a further suppression of interfacial recombination, which also facilitated a low V_oc loss in the device. However, the V_oc loss of 0.54 V is still higher than those in pure Pb-based PVSCs (∼0.45 V).⁵⁰ Therefore, there is still room to further develop low-bandgap PVSCs based on the conventional structure.

Conflicts of interest

The authors declare no competing interests.

Acknowledgements

This study was financially supported by the Ministry of Science and Technology of the People's Republic of China (No. 2017YFA0206600), the Science and Technology Program of Guangzhou, China (No. 201607020010) and the National Natural Science Foundation of China (No. 21761132001, 51573057 and 91733302).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8qm00620b

‡ These authors contributed equally to this work.

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