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Efficient wide-bandgap perovskite solar cells enabled by doping a bromine-rich molecule

  • Rui He , Tingting Chen , Zhipeng Xuan , Tianzhen Guo , Jincheng Luo , Yiting Jiang , Wenwu Wang , Jingquan Zhang , Xia Hao , Lili Wu , Ye Wang , Iordania Constantinou , Shengqiang Ren EMAIL logo and Dewei Zhao ORCID logo EMAIL logo
Published/Copyright: February 15, 2021
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Nanophotonics
From the journal Nanophotonics Volume 10 Issue 8

Abstract

Wide-bandgap (wide-E g , ∼1.7 eV or higher) perovskite solar cells (PSCs) have attracted extensive attention due to the great potential of fabricating high-performance perovskite-based tandem solar cells via combining with low-bandgap absorbers, which is considered promising to exceed the Shockley–Queisser efficiency limit. However, inverted wide-E g PSCs with a minimized open-circuit voltage (V oc) loss, which are more suitable to prepare all-perovskite tandem devices, are still lacking study. Here, we report a strategy of adding 1,3,5-tris (bromomethyl) benzene (TBB) into wide-E g perovskite absorber to passivate the perovskite film, leading to an enhanced average V oc. Incorporation of TBB prolongs carrier lifetimes in wide-E g perovskite due to reduction of defects in perovskites and makes a better energy level matching between perovskite absorber and electron transport layer. As a result, we achieve the power conversion efficiency of 17.12% for our inverted TBB-doped PSC with an enhanced V oc of 1.19 V, compared with that (16.14%) for the control one (1.14 V).

Keywords: 1,3,5-tris (bromomethyl) benzene; defect passivation; perovskite solar cells; wide-bandgap perovskite

1 Introduction

Power conversion efficiency (PCE) of perovskite solar cells (PSCs) has been boosted rapidly from 3.8% to certified 25.5% in the past decade [1], [2], gradually approaching that of crystalline silicon (c-Si) solar cells. However, the Shockley–Queisser (S–Q) radiative efficiency limits further performance enhancement of single-junction PSCs [3].

In order to exceed the S–Q limit, a promising approach is to fabricate tandem solar cells by combining a wide-bandgap (wide-E g , ∼1.7 eV or higher) perovskite as the top subcell absorber possessing a tunable bandgap with a low-E g c-Si [4], [5], copper indium gallium diselenide (CIGS) [6], or mixed tin–lead (Sn–Pb) perovskite [7], [8], [9], as the bottom subcell absorber. The semitransparent wide-E g top subcell absorbs short-wavelength light, while the low-E g bottom subcell harvests long-wavelength light, maximizing absorption of the solar spectrum [10], [11]. Up to now, remarkable progress in perovskite-based tandem cells has been made, with maximum PCE ranging from 29.1% for perovskite/c-Si tandems [12], to 25.6% for perovskite/perovskite tandems [13], and 24.17% for perovskite/CIGS tandems [12], indicating great promise.

The efficiency of perovskite-based tandem solar cells is still much lower than the theoretical efficiency limit [14], [15]. One of the determining factors influencing tandem cell performance originates from the unsatisfactory performance of wide-E g PSCs, in which the large open-circuit voltage (V oc) deficit defined as E g /q – V oc, where q is the elementary charge [16], [17], is a major obstacle restraining further performance improvement of wide-E g PSCs [18], [19]. The bandgap of wide-E g perovskites is tuned by the molar ratio of iodine (I) to bromine (Br), targeting an optimal one suitable for tandem cells [20]. High Br content may induce more defects in perovskite films, making it more challenging to prepare high-quality wide-E g perovskites [21], [22]. Moreover, Br-rich perovskites usually suffer from severe Hoke effect, leading to phase segregation of I-Br under illumination [23]. More traps will emerge in the films when I-rich and Br-rich phases are separated and the I-rich one with lower bandgap could capture carriers, restricting charge transport and deteriorating device performance, especially the undesirable V oc loss [24].

Many efforts have been devoted to mitigating this problem. Snaith and coworkers have firstly developed a mixed A-cation perovskite with formamidinium (FA) and cesium (Cs). The FA x Cs1−x Pb (I y Br1−y )3-based PSCs exhibit high PCEs with high V ocs [25]. Zhao, Yan and coworkers have explored the synergistic effects of solvent annealing and lead thiocyanate (Pb(SCN)2) on wide-E g perovskite formation and quality, reporting a high PCE exceeding 17% with a V oc of 1.25 V [26]. Tan and coworkers have incorporated methylammonium (MA) cation to heal the defects in 1.74 eV perovskite and achieved a relatively low V oc loss of 490 mV [27].

The n-i-p PSCs typically exhibit higher V ocs, while they also show non-negligible J-V hysteresis, and the device configuration is not suitable for all-perovskite tandem device construction. Chen et al. have developed a post-treatment approach with guanidinium bromide (GABr) on the perovskite surface, realizing a high V oc of 1.24 V in inverted p-i-n PSCs [28]. Very recently, they have further applied phenethylammonium iodide (PEAI) to retard the accumulation of ionic defects in 1.73 eV perovskite, leading to a V oc of 1.25 V [29]. However, research on inverted p-i-n wide-E g PSCs suitable for tandem solar cells is still lacking. A facile method to suppress the V oc loss and further promote device efficiency is highly desired.

A halide-rich nature with a benzene ring structure of 1,3,5-tris (bromomethyl) benzene (TBB) is typically used in chemical synthesis as an initiator or intermediate, by which high-quality CsPbBr3 perovskite nanocrystals have been prepared [30], [31], [32]. Wang et al. have employed TBB to modify the hole transport layer and achieved a high external quantum efficiency (EQE) of green perovskite nanocrystal light-emitting diodes [33]. Herein, we use a Br-rich small molecule TBB as an additive into perovskite precursor. The introduction of TBB effectively passivates the internal defects of wide-E g perovskite films and modulates the energy levels, leading to a better matching with charge transport layers. As a result, we achieve an improved PCE of 17.12% with a V oc approaching 1.2 V for 1.73 eV-perovskite-based inverted devices.

2 Experimental section

2.1 Materials

N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), chlorobenzene (CB) and Pb(SCN)2 were purchased from Sigma-Aldrich. Diethyl ether (DE) was purchased from Chengdu Chron Chemical Co., Ltd., Lead iodide (PbI2) and lead bromide (PbBr2) were purchased from TCI. Formamidinium iodide (FAI) was purchased from Greatcell Solar Company. Cesium iodide (CsI) and silver (Ag) were purchased from Alfa Aesar. Poly(bis(4-phenyl)(2,4,6-trimethylphenyl)amine) (PTAA) was purchased from Xi’an Polymer Light Technology Corporation. TBB was purchased from J&K reagent, C60 was purchased from Nano-C and bathocuproine (BCP) was purchased from Jilin OLED.

2.2 Device fabrication

Glass substrates coated with ITO (15 Ω sq−1) were ultrasonically cleaned with detergent, deionized water and ethanol for 15 min in sequence. The ITO glass substrates were dried with nitrogen flow and a treatment with ultraviolet-ozone for 15 min was conducted before the deposition of HTL. The ITO/glass substrates were transferred to a glove box filled with nitrogen. PTAA dissolved in CB (4 mg/mL) with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) (1 wt%) was spin-coated onto the ITO substrate at 4000 rpm for 30 s, followed by annealing at 100 °C for 10 min. The FA0.8Cs0.2PbI2.1Br0.9 perovskite precursor was prepared by dissolving the 0.88 mmol FAI, 0.22 mmol CsI, 0.605 mmol PbI2, 0.495 mmol PbBr2 and 0.017 mmol Pb(SCN)2 in 1 mL DMF:DMSO (3:1/v:v) and stirred at 60 °C for 3 h. For TBB-doped perovskite, 0.36, 1.08, 1.8, and 2.5 mg TBB was added into 1 mL, respectively, corresponding to varied doping ratio of 0.1, 0.3, 0.5, and 0.7 mol%, respectively. Before the deposition of perovskite absorber, 60 μL DMF was spin-coated on PTAA at 4000 rpm for 10 s to enhance the wettability. Then 70 μL precursor was dropped on PTAA and quickly spin-coated through a two-step process, i.e., 500 rpm for 2 s and 4000 rpm for 60 s, and the antisolvent of 700 μL DE was dripped at 25 s of the second step. Then the as-prepared perovskite film was annealed at 70 °C for 2 min and 110 °C for 10 min. After the films were cooled down to room temperature, all the samples were transferred to multisource evaporation chamber and 20 nm C60, 5 nm BCP and 100 nm Ag were sequentially evaporated at 5 × 10−4 Pa. The area of solar cells is defined by the overlapping of metal electrode and patterned ITO.

2.3 Film and device characterization

Wide-E g perovskites with various doping ratio of TBB were characterized by X-ray diffraction (XRD, Shimazu XRD-6100) with Cu-Kα (λ = 0.154 nm) radiation at 40 kV and 30 mA excitation. Scanning electron microscope (SEM) images were taken by Hitachi S-5200 microscope. Absorbance spectra of perovskite films were measured by ultraviolet–visible (UV–vis) spectrophotometer (PerkinElmer Lambda 950). Photoluminescence (PL) and time-resolved photoluminescence (TRPL) were performed by FLS980 (Edinburgh Inc.) PL measurements were carried out using a 532-nm Xeon lamp with a monochromator and TRPL were measured using a 655-nm picosecond pulsed laser. X-ray photoelectron spectra (XPS) and ultraviolet photoemission spectroscopy (UPS) were measured using a photoelectron spectrometer (ESCALAB 250Xi, Thermo Fisher Scientific). Current density-voltage (J-V) curves were measured using Keysight B2901A source meter under AM1.5G (100 mW cm−2) illumination (Enlitech, SS-F5) in an N2-filled glove box. All J-V measurements were performed via an aperture with an active area of 0.078 cm2. The spectral response was recorded by a solar cell quantum efficiency measurement system (QEX10, PV Measurements, Inc.). Transient photovoltaic (TPV), transient photocurrent (TPC), and electrochemical impedance spectroscopy (EIS) were measured by the all-in-one characterization system (PAIOS, Switzerland).

3 Results and discussion

Figure 1A shows the chemical structure of TBB and the XRD patterns of the perovskite samples doped with different concentrations of TBB are shown in Figure 1B and their magnified spectra Figure 1C and D. The diffraction peaks at 14.27°, 20.2°, 28.76°, 32.24°, and 41.04° correspond to the (100), (101), (020), (201), and (022) crystal planes of FA0.8Cs0.2PbI2.1Br0.9 cubic phase, respectively. The peak positions do not show obvious shift which indicates that TBB does not enter the perovskite lattice. With the increase of doping molar ratio, the peaks at 20.2° and 41.04° reach the highest intensity and narrowest full width at half maximum (FWHM) of 0.167° and 0.208° at the doping concentration of 0.3%, respectively, suggesting an enhancement in crystallinity and average grain size in terms of Debye–Scherrer formula. Further, excessive TBB may reduce the quality of perovskite films in terms of peak intensity and FWHM value at the doping ratio of 0.5 and 0.7%.

Figure 1: Diagram of 1,3,5-tris (bromomethyl) benzene (TBB)-doping, X-ray diffraction (XRD) patterns and optical properties of perovskite samples doped with different concentrations of TBB. (A) Chemical structure of TBB. (B) Full range of XRD patterns. (C, D) Magnified spectra of different 2θ ranges. (E) Tauc plots.
Figure 1:

Diagram of 1,3,5-tris (bromomethyl) benzene (TBB)-doping, X-ray diffraction (XRD) patterns and optical properties of perovskite samples doped with different concentrations of TBB.

(A) Chemical structure of TBB. (B) Full range of XRD patterns. (C, D) Magnified spectra of different 2θ ranges. (E) Tauc plots.

The optical properties of the perovskite samples doped with different concentrations of TBB have been characterized. The Tauc plots are displayed in Figure 1E, exhibiting that the incorporation of a small amount of TBB does not change optical E g much, and only slightly increases the E g from 1.73 to 1.74 eV upon excessive TBB (0.5 and 0.7%).

We further investigated the effect of TBB doping concentration on the morphology of the perovskite film by performing SEM. Figure 2A–E shows the top-view SEM images of FA0.8Cs0.2PbI2.1Br0.9 films with varied doping concentration (0, 0.1, 0.3, 0.5, and 0.7%) and Figure 2F summarizes the distribution of average grain size of these perovskite films. The grain size in 0.1 and 0.3% TBB-doped films slightly increases compared to that in pristine perovskite film, which is more likely to reduce the defects located at grain boundaries and thus the nonradiative recombination. Meanwhile, the enlarged crystal grains are consistent with the results of XRD.

Figure 2: Morphological characterizations of the perovskite samples doped with different concentrations of 1,3,5-tris (bromomethyl) benzene (TBB). (A–E) Scanning electron microscope (SEM) images. (F) Distribution of average grain size of these perovskite films.
Figure 2:

Morphological characterizations of the perovskite samples doped with different concentrations of 1,3,5-tris (bromomethyl) benzene (TBB).

(A–E) Scanning electron microscope (SEM) images. (F) Distribution of average grain size of these perovskite films.

We then performed PL and TRPL measurements to assess the interaction between TBB additives and the wide-E g FA0.8Cs0.2PbI2.1Br0.9 perovskite absorber. As the TBB doping concentration increases, the PL emission intensity of wide-E g perovskites first increases remarkably until TBB doping level reaches 0.3% and then decreases when the doping concentration is beyond 0.5% (Figure 3A), suggesting that the nonradiative recombination is effectively suppressed and meanwhile the defects are induced by excessive TBB. A slight blue-shift is also observed compared with the pristine film. Figure 3B shows the PL decay spectra fitted with a biexponential decay function defined as i = A1 exp(−t/τ 1)+ A2 exp(−t/τ 2), and the fitting data summarized in Table 1. The τ 1 value corresponds to fast decay time which is attributed to rapid process of interfacial charge extraction and evidently it is not dominant in our case. The second decay process τ 2 with larger weight factor (A2) reflects a rather longer lifetime due to bulk and interface recombination and thus dominates the carrier lifetime [34]. All TBB-incorporated films have prolonged average carrier lifetimes compared with the pristine one, in which the perovskite with 0.3% TBB exhibits the longest carrier lifetime of 668 ns, 1.5 times longer than the control one (414 ns). These results indicate that TBB molecules effectively passivate the defects at the grain boundaries of wide-E g perovskite due to the large amount of grains, leading to suppressed nonradiative recombination loss [33].

Figure 3: Photoluminescence (PL) characterizations of perovskite samples doped with different concentrations of 1,3,5-tris (bromomethyl) benzene (TBB). (A) PL spectra. (B) Time-resolved photoluminescence (TRPL) spectra.
Figure 3:

Photoluminescence (PL) characterizations of perovskite samples doped with different concentrations of 1,3,5-tris (bromomethyl) benzene (TBB).

(A) PL spectra. (B) Time-resolved photoluminescence (TRPL) spectra.

Table 1:

TRPL fitting data of perovskite samples doped with different concentrations of TBB.

Samples τ1 (ns) A1 (%) τ2 (ns) A2 (%) τave (ns)
w/o TBB 0.45 3.32 414.5 96.68 414.48
0.1% TBB 0.49 1.48 536 98.52 535.99
0.3% TBB 0.57 0.38 668 99.62 667.99
0.5% TBB 0.55 1.02 628 98.98 627.99
0.7% TBB 0.55 1.31 566 98.69 565.99

  1. TBB, 1,3,5-tris (bromomethyl) benzene; TRPL, time-resolved photoluminescence.

To clarify the effects of TBB on the electrical properties and chemical states of the perovskite absorber, we carried out XPS measurement to evaluate the interaction between TBB and other elements. Figure 4A plots the elemental survey spectra of the pristine film and 0.3% TBB-incorporated film and Figure 4B–E shows the high-resolution result of Pb 4f, Br 3d, I 3d5 and Cs 3d5 in pristine films and 0.3%TBB-doped films. It is evident that the binding energy peaks of Pb 4f and Br 3d at 68.3 eV (Br 3d5/2) and 69.35 eV (Br 3d3/2) for the perovskite with 0.3% TBB shift to lower positions, while the binding energy peaks of I 3d5 and Cs 3d5 remain almost unchanged. We attribute the shifts to the doping of TBB into the wide-E g FA0.8Cs0.2PbI2.1Br0.9 perovskite and also more electronegative Br− in TBB molecules [33], [35]. Moreover, Br− in TBB molecules may partially compensate the I− vacancies arising from the evaporation during the thermal annealing process. Stronger Pb-Br bonding with less polarity could prolong the carrier lifetime, providing better passivation effect [36], [37].

Figure 4: X-ray photoelectron spectra (XPS) and ultraviolet photoemission spectroscopy (UPS) characterizations of perovskite samples doped without 1,3,5-tris (bromomethyl) benzene (TBB) and with 0.3% TBB. (A) Elemental survey spectra of these two samples. (B–E) High-resolution results of Pb 4f, Br 3d, I 3d5 and Cs 3d5. (F) Secondary electron cut-off edge. (G) Valence band. (H) Energy level diagram of the entire device.
Figure 4:

X-ray photoelectron spectra (XPS) and ultraviolet photoemission spectroscopy (UPS) characterizations of perovskite samples doped without 1,3,5-tris (bromomethyl) benzene (TBB) and with 0.3% TBB.

(A) Elemental survey spectra of these two samples. (B–E) High-resolution results of Pb 4f, Br 3d, I 3d5 and Cs 3d5. (F) Secondary electron cut-off edge. (G) Valence band. (H) Energy level diagram of the entire device.

To get a further insight into the energy level evolution in FA0.8Cs0.2PbI2.1Br0.9 perovskite after the addition of TBB, we conducted UPS characterization to analyze the change of valence band maximum (VBM) and Fermi level (E F ). According to Einstein’s photo-emission law, the E F can be obtained as E F  = hν  − E cutoff, where the hν is photon energy (21.22 eV here) and the E cutoff represents the secondary electron cut-off edge showed in Figure 4F. Figure 4G shows the “energy distance” from EF to VBM, and thereby we can acquire the position of valence band edge. To be specific, in this case, the E F of pristine film and 0.3% TBB-doped film are calculated to be 3.84 and 4.23 eV, while the VBM values are determined at −5.55 and −5.72 eV, respectively. Combined with their 1.73 eV E g ’s, the energy level diagram of the entire device is plotted in Figure 4H. This result suggests that the addition of TBB increases the E F and thus makes the energy levels of perovskite shift downward, leading to the decrease in energy barrier between perovskite absorber and C60 electron transport layer, as well as allowing a better transportation of electrons.

We then fabricated wide-E g PSCs with perovskite absorbers doped with different concentrations of TBB. Figure 6A shows the cross-sectional SEM image of our wide-E g device with the architecture of ITO/PTAA/wide-E g perovskite/C60/BCP/Ag. The main photovoltaic parameters of these devices are summarized in Table 2. The statistic parameters are shown in Figure 5. The short-circuit current density (J sc) and fill factor (FF) of these wide-E g PSCs remain almost the same as the doping ratio increases from 0 to 0.3%, however, the V oc is significantly enhanced. Further increase in TBB doping ratio to 0.7% remarkably reduces the J sc and FF, while the V oc does not change much at a saturation level. Finally, the device performs the best at the doping ratio of 0.3%. The increase in average V oc (from 1.14 to 1.19 V) (Table 2) drives the performance improvement benefiting from incorporating a small amount of TBB in the precursor solution. However, excessive TBB in films may induce more defects and inhibit charge transport, resulting in tremendous increase in series resistance and a subsequent reduction in both FF and PCE.

Figure 5: Statistic photovoltaic parameters of perovskite solar cells (PSCs) doped with various concentration of 1,3,5-tris (bromomethyl) benzene (TBB). (A) Open-circuit voltage (V oc). (B) Short-circuit current density (J sc). (C) Fill factor (FF). (D) Power conversion efficiency (PCE).
Figure 5:

Statistic photovoltaic parameters of perovskite solar cells (PSCs) doped with various concentration of 1,3,5-tris (bromomethyl) benzene (TBB).

(A) Open-circuit voltage (V oc). (B) Short-circuit current density (J sc). (C) Fill factor (FF). (D) Power conversion efficiency (PCE).

Figure 6: Device structure and performance. (A) Cross-sectional image of our wide-E g perovskite device. (B) J-V characteristics and (C) external quantum efficiency (EQE) spectra of the best-performing device without 1,3,5-tris (bromomethyl) benzene (TBB) and with 0.3% TBB under forward voltage scan. (D) J-V curves of the 0.3% TBB device under reverse and forward voltage scans. (E) Steady-state power conversion efficiency (PCE) of the 0.3% TBB device. (F) Dependence of the V oc on light intensity for the control device and the 0.3% TBB device.
Figure 6:

Device structure and performance.

(A) Cross-sectional image of our wide-E g perovskite device. (B) J-V characteristics and (C) external quantum efficiency (EQE) spectra of the best-performing device without 1,3,5-tris (bromomethyl) benzene (TBB) and with 0.3% TBB under forward voltage scan. (D) J-V curves of the 0.3% TBB device under reverse and forward voltage scans. (E) Steady-state power conversion efficiency (PCE) of the 0.3% TBB device. (F) Dependence of the V oc on light intensity for the control device and the 0.3% TBB device.

Table 2:

Summary on main photovoltaic parameters of PSCs doped with different concentrations of TBB.

TBB doping ratio (%) V oc (V) J sc (mA·cm−2) FF (%) PCE (%)
0 (champion device) 1.14 ± 0.01 (1.14) 17.88 ± 0.11 (17.81) 78.62 ± 0.63 (79.51) 16.10 ± 0.17 (16.14)
0.1 (champion device) 1.17 ± 0.01 (1.17) 17.92 ± 0.13 (17.83) 78.16 ± 1.05 (78.7) 16.33 ± 0.21 (16.41)
0.3 (champion device) 1.19 ± 0.01 (1.19) 17.93 ± 0.16 (17.90) 78.58 ± 0.80 (80.22) 16.72 ± 0.25 (17.12)
0.5 (champion device) 1.18 ± 0.01 (1.18) 17.78 ± 0.15 (17.71) 75.68 ± 1.36 (76.73) 15.94 ± 0.30 (16.03)
0.7 (champion device) 1.17 ± 0.01 (1.17) 17.44 ± 0.22 (17.49) 74.07 ± 0.91 (75.59) 15.22 ± 0.23 (15.47)

  1. FF, fill factor; J sc , short-circuit current density; PCE, power conversion efficiency; PSC, perovskite solar cells; TBB, 1,3,5-tris (bromomethyl) benzene; V oc, open-circuit voltage.

Figure 6B shows the J-V characteristics of the best-performing device without TBB and the one with 0.3% TBB under reverse voltage scan. The control device without TBB exhibits a PCE of 16.14% with a V oc of 1.14 V, a J sc of 17.81 mA⋅cm−2, and a FF of 79.51%. The champion device with 0.3% TBB achieves an enhanced PCE of 17.12% with largely increased V oc of 1.19 V and FF of 80.22%, as well as a J sc of 17.9 mA⋅cm−2. The V oc deficit in our device with 1.73 eV E g absorber is reduced from 590 to 540 mV upon the incorporation of 0.3% TBB, indicating the effective passivation effect of TBB.

The integrated J sc’s over the AM 1.5G solar spectrum for the control device and 0.3% TBB obtained from the EQE measurements shown in Figure 6C are 17.37 and 17.63 mA cm−2, respectively, and are in good agreement with the J sc values obtained from the J-V curves. It is found that the spectral response of the 0.3% TBB device shows an obvious improvement from 630 to 700 nm, compared with that of the control device. We attribute this to the reduced energy barrier between the perovskite absorber and the C60 ETL as aforementioned.

Figure 6D shows the J-V hysteresis behavior of the TBB-doped device under reverse and forward scans suggesting a negligible hysteresis. A steady-state PCE of 17.08% of 0.3% TBB device is obtained under the duration of 300 s (Figure 6E).

To further understand the charge recombination mechanism, we compared the J-V characteristics of the control device and the 0.3% TBB device under different illumination light intensities (I light). Figure 6F shows the dependence of the V oc on light intensity for the control device and the 0.3% TBB device. The V oc has a monotonic increase with logarithmic light intensity, implying the trap-assisted Shockley–Read–Hall recombination dominates in these devices [38], [39]. Their ideality factor n value of 1.563 for the 0.3% TBB device, derived from V oc = nkTln(I light)/q, where k is Boltzmann constant, T is kelvin temperature, and q is elementary charge, is reduced, compared with that of 2.108 for the control device, confirming the effective passivation of TBB in the perovskite, in agreement with the prolonged carrier lifetime and crystallinity of the perovskite absorber [7].

In order to further evaluate the passivation effect of TBB, we made the TPV and TPC measurements to probe the charge recombination and charge transport process in the devices. As shown in Figure 7A, the charge recombination lifetime is estimated to be 6.5 and 8.3 μs for the control and 0.3% TBB devices, respectively. This result of TPV implies that the TBB-doped devices possess a prolonged carrier lifetime [40], in agreement with the TRPL results. The TPC result in Figure 7B exhibit charge-extraction time of 0.74 and 0.64 μs for the control and target group, respectively. Faster charge-extraction time in TBB-doped PSCs allowed efficient carrier transport process, which can be attributed to a better matched energy level along with decreased trap state in TBB-doped devices. EIS has also been carried out to further study the charge recombination and transport mechanism. The Nyquist plots of wide-E g PSCs with 0.3% TBB and without TBB are shown in Figure 7C, measured at a bias voltage of 0.9 V under dark condition with a frequency range from 10 MHz to 10 Hz. Apparently, the 0.3% TBB-doped device has a smaller value of series resistance (R s) at high frequency and a larger value of recombination resistance (R rec) at low frequency. These results further support that the incorporation of 0.3% TBB will promote the charge transfer process and restrict the charge recombination.

Figure 7: Characterizations of the best-performing wide-E g device without 1,3,5-tris (bromomethyl) benzene (TBB) and with 0.3% TBB. (A) Transient photovoltaic (TPV) curves. (B) Transient photocurrent (TPC) curves. (C) Nyquist plots. (D) Device stability.
Figure 7:

Characterizations of the best-performing wide-E g device without 1,3,5-tris (bromomethyl) benzene (TBB) and with 0.3% TBB.

(A) Transient photovoltaic (TPV) curves. (B) Transient photocurrent (TPC) curves. (C) Nyquist plots. (D) Device stability.

We then compared the stability of the device without TBB and with 0.3% TBB to judge the effect of TBB on the photostability of the Br-rich wide-E g PSCs (Figure 7D). Apparently, the device with 0.3% TBB-incorporated exhibits excellent stability, maintaining 93% of the initial PCE after storage in N2-filled glove box over a period of 30 days, while the control one declines to 69%. This result implies an enhanced stability of our TBB-doped device.

4 Conclusion

We have demonstrated the performance enhancement of wide-E g PSCs by adding a small amount of Br-rich small molecule TBB as an additive to passivate wide-E g perovskite film and reduce defect density. Proper incorporation of TBB leads to prolonged carrier lifetimes in wide-E g perovskites and also modulates energy levels of wide-E g perovskite with a better energy level matching with that of charge transport layer. As a result, an improved V oc approaching 1.2 V with an enhanced FF of 80.22% has been obtained in inverted devices. The V oc deficit is significantly reduced by 50 mV, compared to that of the control one. We achieve a champion PCE of 17.12% and a steady PCE of 17.08%. Our work offers a facile approach to reduce the nonradiative recombination losses in wide-E g PSCs and further understanding of how Br-rich molecules influence perovskite materials, beneficial for development of all-perovskite tandem solar cells.

Corresponding authors: Shengqiang Ren and Dewei Zhao, College of Materials Science and Engineering & Institute of New Energy and Low-Carbon Technology, Sichuan University, Chengdu 610065, China; and Engineering Research Center of Alternative Energy Materials & Devices, Ministry of Education, Chengdu 610065, China, E-mail: ,

Funding source: Science and Technology Program of Sichuan Province

Award Identifier / Grant number: 2020YFH0079

Award Identifier / Grant number: 2020JDJQ0030

Award Identifier / Grant number: 2019KJT0120-2019ZDZX0015

Funding source: National Natural Science Foundation of China

Award Identifier / Grant number: U1804132

Funding source: National Key Research and Development Program of China

Award Identifier / Grant number: 2019YFE0120000

Funding source: the Fundamental Research Funds for the Central Universities

Award Identifier / Grant number: YJ201722

Award Identifier / Grant number: YJ201955

Funding source: the Ministry of Science and Culture (MWK) of Lower Saxony, Germany within the framework of the SMART BIOTECS alliance between the Technische Universität Braunschweig and the Leibniz Universität Hannover

  1. Author contributions: All the authors have accepted responsibility for the entire content of this submitted manuscript and approved submission.

  2. Research funding: This work was financially supported by the Science and Technology Program of Sichuan Province (nos. 2020YFH0079, 2020JDJQ0030, and 2019KJT0120-2019ZDZX0015), the National Key Research and Development Program of China (No. 2019YFE0120000), the Fundamental Research Funds for the Central Universities (nos. YJ201722 and YJ201955), and the National Natural Science Foundation of China (Grant No. U1804132), the Ministry of Science and Culture (MWK) of Lower Saxony, Germany within the framework of the SMART BIOTECS alliance between the Technische Universität Braunschweig and the Leibniz Universität Hannover.

  3. Conflict of interest statement: The authors declare no conflicts of interest regarding this article.

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Received: 2020-11-30
Accepted: 2021-01-28
Published Online: 2021-02-15

© 2021 Rui He et al., published by De Gruyter, Berlin/Boston

This work is licensed under the Creative Commons Attribution 4.0 International License.

Articles in the same Issue

  1. Frontmatter
  2. Editorial
  3. Photonics for enhanced perovskite optoelectronics
  4. Perspective
  5. Prospects of light management in perovskite/silicon tandem solar cells
  6. Reviews
  7. Ultrafast dynamics of photoexcited carriers in perovskite semiconductor nanocrystals
  8. Silicon heterojunction-based tandem solar cells: past, status, and future prospects
  9. Photon recycling in perovskite solar cells and its impact on device design
  10. Advances in Dion-Jacobson phase two-dimensional metal halide perovskite solar cells
  11. Recent advancements and perspectives on light management and high performance in perovskite light-emitting diodes
  12. Quasi-2D lead halide perovskite gain materials toward electrical pumping laser
  13. Lead-free metal-halide double perovskites: from optoelectronic properties to applications
  14. Lead-free halide perovskite photodetectors spanning from near-infrared to X-ray range: a review
  15. Research Articles
  16. Ligand-modulated electron transfer rates from CsPbBr3 nanocrystals to titanium dioxide
  17. Exploring the physics of cesium lead halide perovskite quantum dots via Bayesian inference of the photoluminescence spectra in automated experiment
  18. Comparing optical performance of a wide range of perovskite/silicon tandem architectures under real-world conditions
  19. Efficient wide-bandgap perovskite solar cells enabled by doping a bromine-rich molecule
  20. Two-dimensional perovskites with alternating cations in the interlayer space for stable light-emitting diodes
  21. Hard and soft Lewis-base behavior for efficient and stable CsPbBr3 perovskite light-emitting diodes
  22. Tailoring the electron and hole dimensionality to achieve efficient and stable metal halide perovskite scintillators
  23. Remarkable photoluminescence enhancement of CsPbBr3 perovskite quantum dots assisted by metallic thin films
https://doi.org/10.1515/nanoph-2020-0634
Keywords for this article
1,3,5-tris (bromomethyl) benzene; defect passivation; perovskite solar cells; wide-bandgap perovskite
Creative Commons
BY 4.0

Articles in the same Issue

  1. Frontmatter
  2. Editorial
  3. Photonics for enhanced perovskite optoelectronics
  4. Perspective
  5. Prospects of light management in perovskite/silicon tandem solar cells
  6. Reviews
  7. Ultrafast dynamics of photoexcited carriers in perovskite semiconductor nanocrystals
  8. Silicon heterojunction-based tandem solar cells: past, status, and future prospects
  9. Photon recycling in perovskite solar cells and its impact on device design
  10. Advances in Dion-Jacobson phase two-dimensional metal halide perovskite solar cells
  11. Recent advancements and perspectives on light management and high performance in perovskite light-emitting diodes
  12. Quasi-2D lead halide perovskite gain materials toward electrical pumping laser
  13. Lead-free metal-halide double perovskites: from optoelectronic properties to applications
  14. Lead-free halide perovskite photodetectors spanning from near-infrared to X-ray range: a review
  15. Research Articles
  16. Ligand-modulated electron transfer rates from CsPbBr3 nanocrystals to titanium dioxide
  17. Exploring the physics of cesium lead halide perovskite quantum dots via Bayesian inference of the photoluminescence spectra in automated experiment
  18. Comparing optical performance of a wide range of perovskite/silicon tandem architectures under real-world conditions
  19. Efficient wide-bandgap perovskite solar cells enabled by doping a bromine-rich molecule
  20. Two-dimensional perovskites with alternating cations in the interlayer space for stable light-emitting diodes
  21. Hard and soft Lewis-base behavior for efficient and stable CsPbBr3 perovskite light-emitting diodes
  22. Tailoring the electron and hole dimensionality to achieve efficient and stable metal halide perovskite scintillators
  23. Remarkable photoluminescence enhancement of CsPbBr3 perovskite quantum dots assisted by metallic thin films
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