Alkali-metal-ion-doping strategy to improve the photovoltaic properties of Ag2BiI5solar cells

Article abstract of DOI:10.1039/d1nj02627e

Alkali-metal-ion doping is an efficient strategy to improve the device performance of thin film solar cells. Though doping with Li⁺or Cs⁺doping has been reported in Ag-Bi-I solar cells, the influence of doping with other alkali metal ions on Ag-Bi-I solar cells has not been systematically studied. In this work, we investigate the effects of five alkali metal ions (Li⁺, Na⁺, K⁺, Rb⁺, and Cs⁺) on the structure, morphology, and optical and electrical properties of Ag₂BiI₅absorber layers, and also their influence on the device performance. All alkali-metal-doped Ag₂BiI₅thin films show high crystallinity and have similar suitable band gaps. Hall effect measurements demonstrate that the alkali-metal-doped Ag₂BiI₅films show an improved electrical performance compared with the pristine Ag₂BiI₅thin film, which is beneficial to achieving high-efficiency solar cells. Unexpectedly, Cs⁺doping can significantly increase the grain size of Ag₂BiI₅thin films, leading to a porous absorber that reduces the device efficiency. The device performance of Li⁺-, Na⁺-, K⁺- and Rb⁺-doped Ag₂BiI₅is significantly improved compared with that of pristine Ag₂BiI₅. It is found that the order of the power conversion efficiency promotion ability is K⁺> Rb⁺> Na⁺> Li⁺> Cs⁺under the same experimental conditions. The present approach can be extended to other Bi-based solar cells to improve their device performance, which is critical for applications in absorbers with poor electrical performance or low crystallinity.

Full text of DOI:10.1039/d1nj02627e

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Alkali-metal-ion-doping strategy to improve the
photovoltaic properties of Ag2BiI5 solar cells†
Cite this: New J. Chem., 2021,
45, 14158
Yuxiang Wang,ab Yue Liu,ab Xinan Shi,a Lijian Huang,a Junye Tong,a
ab
ab
Gang Wang
*
and Daocheng Pan
*
Alkali-metal-ion doping is an efficient strategy to improve the device performance of thin film solar
cells. Though doping with Li+ or Cs+ doping has been reported in Ag–Bi–I solar cells, the influence of
doping with other alkali metal ions on Ag–Bi–I solar cells has not been systematically studied. In this
work, we investigate the effects of five alkali metal ions (Li+, Na+, K+, Rb+, and Cs+) on the structure,
morphology, and optical and electrical properties of Ag2BiI5 absorber layers, and also their influence on
the device performance. All alkali-metal-doped Ag2BiI5 thin films show high crystallinity and have similar
suitable band gaps. Hall effect measurements demonstrate that the alkali-metal-doped Ag2BiI5 films
show an improved electrical performance compared with the pristine Ag2BiI5 thin film, which is benefi-
cial to achieving high-efficiency solar cells. Unexpectedly, Cs+ doping can significantly increase the grain
size of Ag2BiI5 thin films, leading to a porous absorber that reduces the device efficiency. The device
performance of Li+-, Na+-, K+- and Rb+-doped Ag2BiI5 is significantly improved compared with that of
Received 28th May 2021,
Accepted 30th June 2021
pristine Ag2BiI5. It is found that the order of the power conversion efficiency promotion ability is K+
4
Rb+ 4 Na+ 4 Li+ 4 Cs+ under the same experimental conditions. The present approach can be
extended to other Bi-based solar cells to improve their device performance, which is critical for applica-
tions in absorbers with poor electrical performance or low crystallinity.
DOI: 10.1039/d1nj02627e
rsc.li/njc
gaps of the absorber layers and improving the device performance.
In addition, LiOH, NaOH, KOH, RbOH and CsOH have been
1. Introduction
Alkali-metal doping is an effective strategy to improve the device introduced into Sb-based precursor solutions to fabricate Sb2S3
performance of thin film solar cells, such as Cu(In,Ga)(S,Se)2 solar cells.9 It is worth noting that only heavy alkali metal ions
(CIGSSe),1–4 Cu2ZnSn(S,Se)4 (CZTSSe),2,5–8 Sb2(S,Se)39 and organic– (K+, Rb+, and Cs+) are beneficial for absorbers to grow into large
inorganic hybrid halide perovskite solar cells (PSCs).10–14 For grains and form a better morphology, which facilitate the improve-
instance, Li+, Na+, K+ and Rb+ doping can passivate surface or ment of the device performance of Sb2S3 solar cells. Moreover, the
grain-boundary defects, promote crystal growth, and improve the role of alkali metal ions in halide perovskite solar cells has also
electrical properties of CIGSSe3,4 and CZTSSe6–8 solar cells. Inter- been widely investigated.15–21 Low concentrations of Cs+ and Rb+
estingly, Li+ can alloy with the kesterite phase leading to a solid promote halide homogenization and enhance the charge-carrier
solution (LixCu1Àx)2ZnSn(S,Se)4 that offers a way of tuning the band lifetime, which plays a critical role in improving the photovoltaic
device performance.10 Alkali metal ions with a gradient size occupy
different positions in the crystal lattice of the perovskite, and they
play various roles, e.g. crystallinity adjustment, defect passivation
and ion migration.11 Based on the above discussion, we can infer
that the modes of alkali-metal doping include substitutional22 and
interstitial doping.23 In substitutional doping, the alkali metal can
replace ions with a similar radius and coordination mode, which
has a significant impact on the band gap and defects of the
a State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of
Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun,
Jilin 130022, China. E-mail: wsu@ciac.ac.cn, pan@ciac.ac.cn;
Fax: +86-431-85262544; Tel: +86-431-85262544
b University of Science and Technology of China, Hefei, Anhui 230026, China
† Electronic supplementary information (ESI) available: Top-view and cross-
sectional SEM images of as-prepared Ag–Bi–S–I films with alkali metal doping;
EDS mapping of the as-prepared undoped Ag2BiI5 film, the element contents of
the as-prepared Ag2BiI5 thin films doped with different alkali metal ions; XPS absorber layer.24,25 The interstitial doping is mainly to promote
spectra of Ag–Bi–S–I and Ag2BiI5 thin films; UPS spectra of the Ag2BiI5 thin film;
the growth of crystal grains, passivate the crystal surface and grain
statistical efficiency distribution of 30 devices with the K+-doped Ag2BiI5 absor-
boundaries, and improve the device performance.26 In the past few
ber; PV parameters of the Ag2BiI5 solar cells with and without alkali metal doping;
years, Bi-based materials, a new type of absorber, have aroused
widespread interest among researchers,27–33 but the performance
of Bi-based solar cells is far lower than those of CIGSSe, CZTSSe,
top-view and cross-sectional SEM images of 1% (a and b) and 2% (c and d) Cs+-
doped Ag2BiI5 thin films; and PV parameters of 1% and 2% Cs+-doped Ag2BiI5
solar cells (PTAA as the HTL). See DOI: 10.1039/d1nj02627e
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Sb2(S,Se)3 and PSCs. Therefore, alkali-metal doping has great (NaI, 99.5%), potassium iodide (KI, 99%), rubidium iodide
potential in improving the efficiency of Bi-based solar cells. (RbI, 99.9%), cesium iodide (CsI, 99.9%), tin chloride dihydrate
The Bi-based solar cell is a general term for a series of solar cells (SnCl2Á2H2O, 98%), n-butylamine (CH3(CH2)3NH2, 99%) and
containing the Bi element in the absorber layer, including Bi2S3,34,35 carbon disulfide (CS2, 99.9%) were purchased from Alad-
38,39
BiSI,36 AgBiS2,37 CuBiS2
and Bi-based halides (binary,40 ter- din. Poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA)
nary41–43 and quaternary44). AgaBibIa+3b is an important Bi-based was purchased from Luminescence Technology Corp. N,N-
absorber material, and a large number of encouraging results have Dimethylformamide (HCON(CH3)2, 99.8%) and chlorobenzene
been achieved.45–51 In the crystal structure of AgaBibIa+3b, [AgI6] and (C6H5Cl, 99.5%) were obtained from Alfa Aesar. Dimethyl
[BiI6] octahedra are connected through shared edges and further sulfoxide ((CH3)2SO, AR) and ethanol (CH3CH2OH, AR) were
increase the structural dimension.48 AgBiI4,49 AgBi2I7,52 Ag2BiI5,45 bought from Beijing Chemical Works. All chemicals were used
48
53
Ag3BiI6 and AgBi3I10 demonstrate an appropriate direct band directly without any further treatment.
gap of 1.6–1.86 eV. AgBi2I7 was first reported by Sargent et al. in
2016.52 The best power conversion efficiency (PCE) of the Ag–Bi–I-
2.2. Deposition of Ag2BiI5 thin films
based device was achieved by Pai et al. in 2018,54 by introducing
sulfide to partially substitute iodine. The band gap is decreased First, 2 mL of n-butylamine and 1.2 mL of carbon disulfide were
and the valence band edge is upshifted by sulfide doping, which slowly mixed in a DMF/DMSO (2.5 : 1.5 volume ratio) solution
contributes to improving the device performance. Recently, Li-TFSI55 with magnetic stirring at room temperature. Butylamine
and CsI56 were introduced into the precursor solutions to improve reacted with carbon disulfide and formed butyldithiocarbamic
the quality of AgBiI4 thin films and to enhance the PCEs of AgBiI4 acid. Then, 289.675 mg (1.25 mmol) of Ag2O, 325.025 mg
solar cells. However, the effects of other alkali metals being incor- (1.25 mmol) of Bi(OH)3 and 158.63 mg (0.625 mmol) of I2 were
porated into Ag–Bi–I absorbers have not been investigated.
dissolved in the solution by heating at 80 1C. For the respective
In this work, we investigate the effects of different alkali-metal alkali-metal-doping precursor solution, 0.1125 mmol of LiI,
doping on the performance of Ag2BiI5 solar cells by introducing NaI, KI, RbI or CsI was added to the precursor solution. The
LiI, NaI, KI, RbI and CsI into the Ag–Bi–S–I-based precursor molar ratio of alkali/(Ag + Bi) was set as 3%. After cooling down,
solutions. Compared with the traditional one-step deposition the precursor solution was centrifuged and spun at 5000 rpm
approach, our method can adjust the thickness of the as- for 30 s on the SnO2 electron-transport layers. The as-prepared
prepared films by repeating the spin-coating and annealing thin films were obtained after annealing at 230 1C for 1 min
processes. The as-prepared film is AgBiS2 and unknown Ag and the spin-coating/annealing process was repeated 5 times to
compounds. Interestingly, the XRD patterns demonstrate that obtain the required thickness (B300 nm) of the as-prepared
alkali-metal doping does not change the crystal structure of films. To obtain Ag2BiI5 thin films, the as-prepared thin films
Ag2BiI5, so the alkali-metal doping model is one of interstitial and 507.62 mg (2 mmol) of iodine were added to a weighing
doping. The Li+-, Na+-, K+- and Rb+-doped Ag2BiI5 films show bottle (Ø 60 Â 40 mm). Then, the bottle was placed on a 155 1C
dense and continuous surface morphologies. Unexpectedly, there heating plate for 40 min. After this iodization procedure, the
are a lot of pinholes in the Cs+-doped Ag2BiI5 film due to its larger Ag2BiI5 thin films were formed. Note that all procedures were
grain-size composition. All the absorption edges of Ag2BiI5 thin performed in the air.
films are at the wavelength of B700 nm, which proves that the
doping mode of the alkali metal is interstitial. Hall effect mea-
surements indicate that alkali-metal doping can increase the
2.3. Fabrication of Ag2BiI5 solar cell devices
The Ag2BiI5 solar cell devices with an architecture of ITO/SnO2/
Ag2BiI5/PTAA/Ag were constructed. First, the patterned ITO
glass substrates (20 Â 20 Â 0.5 mm) were washed successively
with detergent, deionized water, acetone and isopropanol by
ultrasonic treatment for 20 min, dried under a fast nitrogen
flow, and then treated with UV-ozone for 15 min. Next,
415.3 mg (2 mmol) of SnCl2Á2H2O was dissolved in a mixture
of 2 mL of butyric acid and 2 mL of n-butylamine. Subse-
quently, the dissolved solution was diluted to a total volume
of 10 mL by using ethanol, and spun on the ITO glass
substrates at a speed of 3000 rpm for 30 s, followed by
thermal annealing at 180 1C for 30 min on the hot plate in
ambient air. The PTAA was dissolved in chlorobenzene at a
concentration of 10 mg mL-1 and was spun onto the
obtained ITO/SnO2/Ag2BiI5 substrates at 3000 rpm for
30 s. Finally, the top Ag electrodes were thermally deposited
using steel shadow masks, and devices with an area of
9 mm2 were achieved.
carrier concentration, thus leading to a high conductivity. Finally,
we fabricate Ag2BiI5 solar cells with the structure of glass/ITO/
SnO2/Ag2BiI5/PTAA/Ag. The device performance of Li+-, Na+-,
K+- and Rb+-doped Ag2BiI5 is significantly improved compared
with that of pristine Ag2BiI5. As a result, we achieve a PCE of
0.75% for the K+-doped Ag2BiI5 solar cell, with an open circuit
voltage (VOC) of 0.68 V, a short circuit current density (JSC) of
1.95 mA/cm2, and a fill factor (FF) of 56.20%, which is significantly
higher than that of other devices. Moreover, it is found that the
order of the power conversion efficiency promotion ability is K+ 4
Rb+ 4 Na+ 4 Li+ 4 Cs+ under the same experimental conditions.
2. Experimental section
2.1. Materials
Silver oxide (Ag2O, 99.7%), bismuth hydroxide (Bi(OH)3, 90%),
iodine (I2, 99.8%), lithium iodide (LiI, 99%), sodium iodide
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2.4. Characterization
should be some amorphous or low-crystallinity Ag compounds
(marked with the * sign) in the as-prepared film. However, we
cannot accurately determine the structure of the Ag compounds
by the XRD pattern. Moreover, the as-prepared thin film con-
sists of nanoparticles and has no penetrating holes from
the cross-sectional scanning electron microscopy (SEM) images
(Fig. 2b and c). Compared with the traditional one-step
approach, our deposition method can prepare high-quality thin
films with adjustable thicknesses, which are beneficial to
improve the morphology of absorber layers. Though the surface
of the as-prepared thin film is discontinuous, the subsequent
iodination process can convert small particles into large grains,
thereby eliminating these small pores. The top-view and cross-
sectional SEM images of the as-prepared Ag–Bi–S–I thin films
with alkali-metal doping are shown in Fig. S1 and S2 (ESI†), and
all films consist of small, cluster-like nanoparticles. There are
some holes present on the surface, but no penetrating holes
exist. In addition, some white components appear on the
surface, which may be amorphous or low-crystallinity Ag com-
pounds. Energy dispersive spectroscopy (EDS) mapping of the
as-prepared undoped Ag2BiI5 film is shown in Fig. S3 (ESI†),
and it shows that the composition in the prefabricated film is
uniform. Furthermore, to confirm the presence of the alkali-
metal elements in the Ag2BiI5 films, the compositions of the as-
prepared Ag2BiI5 films doped with different alkali-metal ions
were measured by surface EDS measurement. The values in
Table S1 (ESI†) represent the median and standard deviation of
each element’s atomic percentage, which was calculated from
five different points within each film. The EDS results confirm
the lateral uniformity and homogeneous nature of the absorber
layers.
The powder XRD patterns were measured using a Bruker D8
FOCUS X-ray diffractometer. Scanning electron microscopy
(SEM) images were recorded using a Hitachi S-4800 electron
microscope equipped with an energy dispersive X-ray (EDX)
analyzer (Bruker AXS XFlash detector 4010). X-ray photoelec-
tron spectroscopy (XPS) spectra were collected using a Thermo
ESCALAB 250 X-ray photoelectron spectrometer with an Al Ka
X-ray source. The binding energy was calibrated using the C 1s
peak at 284.6 eV. UV–vis absorption spectra were recorded
using a Shimadzu UV-3600 spectrometer. Ultraviolet photoelec-
tron spectroscopy (UPS) was carried out using an Axis Ultra
DLD, with an emission line of HeI (21.22eV). The thicknesses of
the thin films were measured using a step profiler (AMBIOS,
XP-100). Current density–voltage (J–V) measurements were con-
ducted under standard AM 1.5G, 100 mW cmÀ2 illumination
conditions using a Keithley 2400 source meter. External quan-
tum efficiency (EQE) spectra were acquired using a Zolix
SCS100 QE system. Hall effect measurements were obtained
using an ET9000 Hall effect system (Eastchanging Technologies
Inc.) via the van der Pauw method.
3. Results and discussion
Herein, Ag2O, Bi(OH)3 and iodine were dissolved together in
butyldithiocarbamic acid solution.57,58 It should be noted that
the traditional silver bismuth iodide precursor solution can
only be deposited once,45,47,49,50 and it is hard to fabricate high-
quality Ag–Bi–I thin films with adjustable thicknesses. In this
work, the thicknesses of the as-prepared films were easily
adjusted by repeating the spin-coating and annealing pro-
cesses. Subsequently, we use the iodization process to convert
S2À in the as-prepared film to IÀ. By choosing 155 1C and
40 min as the iodization conditions, large-grain Ag2BiI5 thin
films were obtained. The procedure for the deposition of
Ag2BiI5 thin films is summarized in Fig. 1.
After the iodization process, the diffraction peak intensities
increase markedly (Fig. 3), which demonstrates the crystallinity
enhancement. Hexagonal Ag2BiI5 (PDF #35-1025) with the
%
space group R3m is formed after iodization. The strongest
diffraction peak is located at 12.81, corresponding to the (003)
plane, and indicating that the Ag2BiI5 thin film is highly
orientated along the c-axis of the unit cell. It should be noted
that the two peaks with a similar intensity at 41.51 and 42.21
from the (110) and (108) planes confirm the existence of Ag2BiI5
without other Ag–Bi–I phases.59 By introducing 3 mol% LiI,
NaI, KI, RbI, or CsI into the precursor solution, the almost
constant XRD peak positions demonstrate that the doping
alkali metal ions do not change the crystal structure of Ag2BiI5,
and the alkali-metal doping model is one of interstitial doping
(Fig. 3a). In previously reported studies, the Li+ and Cs+ ions
undergo substitutional doping. Differently, the alkali metal
ions in our work undergo interstitial doping, which may be
attributed to the difference in the preparation method of the
absorber layers. Furthermore, the enhanced XRD peak inten-
sities of the alkali-metal-doped samples indicate that the dop-
ing can increase the crystallinity of the Ag2BiI5 thin films,
especially for the main peak (003), following the sequence of
Cs+ 4 K+ 4 Rb+ 4 Na+ 4 Li+ (Fig. 3b).
The XRD pattern (Fig. 2a) demonstrates that the as-prepared
film is mainly AgBiS2 (PDF #21-1178). It should be noted that
since the ratio of Ag to Bi in the precursor solution is 2 : 1, there
Moreover, XPS spectra are also provided to check whether
the as-prepared Ag–Bi–S–I film converts to Ag2BiI5 completely
Fig. 1 Schematic illustration of the fabrication of Ag2BiI5 films.
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Fig. 2 (a) XRD pattern, and (b) top-view and (c) cross-sectional SEM images of as-prepared Ag–Bi–S–I films.
Fig. 3 (a) XRD patterns of Ag2BiI5 films with and without alkali metal doping and (b) enlarged peaks at approximately 12.81.
after the iodization process. As shown in Fig. S4a (ESI†), both the of Ag2BiI5 (Fig. S4c, ESI†) are obviously enhanced compared with
as-prepared film and the Ag2BiI5 thin film exhibit two peaks of Ag the as-prepared film. The I 3d XPS peaks can be fitted into two
3d3/2 and 3d5/2, and the corresponding binding energies are nearly individual peaks at 630.5 eV and 619.1 eV, corresponding to the
consistent. The peaks located at approximately 374.1 eV and binding energies of I 3d3/2 and I 3d5/2, which are the characteristic
368.1 eV indicate the existence of Ag+ cations. In addition, the peaks of IÀ. In addition, the S 2s peak at 225.4 eV (Fig. S4d, ESI†)
two peaks at 164.1 eV and 158.8 eV, as shown in Fig. S4b (ESI†), completely disappears after iodization, which illustrates that S2À
can be attributed to Bi 4f5/2 and Bi 4f7/2, respectively, which is removed. As a result, Ag+, Bi3+, IÀ and S2À are confirmed to
confirm that Bi3+ remains unchanged during the iodization coexist in the as-prepared film before iodization. After the iodiza-
process. It is worth noting that the intensities of the I 3d peaks tion process, S2À is completely converted to IÀ.
Fig. 4 Top-view and cross-sectional SEM images of the (a1 and a2) pristine Ag2BiI5 thin film; (b1 and b2) Li+-doped Ag2BiI5 thin film; (c1 and c2) Na+-
doped Ag2BiI5 thin film; (d1 and d2) K+-doped Ag2BiI5 thin film; (e1 and e2) Rb+-doped Ag2BiI5 thin film; and (f1 and f2) Cs+-doped Ag2BiI5 thin film.
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The absorber morphology is of vital importance to the device
Paper
Table 1 Hall effect measurements of Ag2BiI5 thin films with and without
alkali-metal doping
performance of Ag2BiI5 solar cells. In Fig. 4, the obtained films
after iodization are composed of large grains with a size of
nearly 1 mm, and the average thickness of the Ag2BiI5 thin films
is nearly 300 nm. The pristine Ag2BiI5 and the Li+-, Na+-, K+-,
Doping
ion
Carrier concen-
Electrical resis-
Hall mobility
tration (1014 cmÀ3
)
tivity (103 OÁcm)
cm2 (VÁs)À1
w/o
1.33
2.02
4.36
6.51
4.39
5.48
7.87
7.71
5.70
3.74
5.62
4.37
4.87
3.93
2.61
1.71
2.51
3.80
Rb+-doped Ag2BiI5 thin films present dense morphologies. Li+
Na+
Nevertheless, the Cs+-doped Ag2BiI5 thin film shows pinholes,
K+
Rb+
which may be attributed to a larger grain size than for the other
Ag2BiI5 films, which is detrimental to the device performance. Cs+
To examine the optical properties of the Ag2BiI5 thin films,
the UV–vis absorption spectra and Tauc plots of alkali-metal-
doped and pristine Ag2BiI5 thin films are shown in Fig. 5. All 6.51 Â 1014 cmÀ3, which is 4.9 times that of pristine Ag2BiI5
thin films show absorption edges at the wavelength around thin films. At the same time, the electrical resistivity of the
700 nm (Fig. 5a). Analysis of the Tauc plots (Fig. 5b) indicates K+-doped Ag2BiI5 thin film shows the lowest value of 3.74 Â
the direct band gap characteristic of Ag2BiI5. From a linear fit 103 OÁcm, indicating the best conductivity. However, the high
to the Tauc plots, the band gap values are evaluated to be carrier concentration increases the carrier scattering and
1.78–1.81 eV, close to those in previously reported studies.47,48 decreases the carrier mobility. Therefore, the Hall mobility of
In the previous studies reported in the literature, similar the K+-doped Ag2BiI5 thin film is only 1.71 cm2 (VÁs)À1, lower
doping concentrations can significantly affect the band gap than that of the Li+-, Na+-, Rb+-, Cs+-doped and pristine Ag2BiI5
values, so the alkali metal should undergo interstitial doping in films. Therefore, we speculate that doping K+ in the precursor
this work. To further examine the position of the energy level, solution is the most conducive approach to improve the
ultraviolet photoelectron spectroscopy (UPS) spectra of the performance of the Ag2BiI5 solar cell. In addition, Xin et al.
pristine Ag2BiI5 thin film were obtained, as shown in Fig. S5 have demonstrated that lithium doping in CZTSSe can change
(ESI†). As can be observed from the cut-off region, the cutoff the polarity of the electric field at the grain boundary (GB) such
energy (Ecutoff) is 15.9 eV. Therefore, the Fermi energy (EF) is that minority carrier electrons are repelled from the GB.60
estimated to be 5.32 eV from the equation EF = 21.22 eV (He I) À Therefore, we speculate that alkali-metal doping can play a
Ecutoff. Combined with the onset linear extrapolation, the Eonset similar role for Ag2BiI5 films.
value is 0.85 eV, which is the value of (EVBMÀEF), where the
Considering the locations of EVBM and ECBM of the Ag2BiI5
EVBM represents the valence band maximum energy. Therefore, absorber layer, we construct Ag2BiI5 solar cells with an n–i–p
the EVBM is regarded as being 6.17 eV. With the band gap of the planar structure using SnO2 and PTAA as the electron-
pristine Ag2BiI5 film of 1.81 eV, the conduction band minimum transport and hole-transport layers, respectively (Fig. 6a). It
energy (ECBM) is calculated to be 4.36 eV.
should be noted that all procedures are conducted in air. The
In addition, we examine the electrical properties of these energy level alignment of the pristine Ag2BiI5 solar cell is
Ag2BiI5 thin films to investigate the influence of alkali-metal illustrated in Fig. 6b. The Ag2BiI5 absorber is deposited on
doping. Hall effect measurements indicate that after doping the 30-nm-thick SnO2 layer. Then, PTAA and silver are
with alkali metal ions, the concentration of free carriers is sequentially deposited by spin-coating and thermal evapora-
increased, and thus a higher conductivity is achieved. The tion, respectively. The devices are tested under standard AM
higher carrier-density levels can be attributed to the better 1.5G illumination. The current density–voltage ( J–V) curves
crystallinity. As can be observed in Table 1, K+-doping is the of the pristine and different alkali-metal-doped Ag2BiI5 solar
most effective strategy to enhance the carrier concentration. cells are presented in Fig. 6c. The detailed photovoltaic
The carrier concentration value of K+-doped Ag2BiI5 is parameters are summarized in Table S2 (ESI†). The PCEs of
Fig. 5 UV–vis absorption spectra (a) and Tauc plots (b) of Ag2BiI5 films with and without alkali-metal doping.
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Fig. 6 Schematic diagram (a) and the energy level alignment (b) of the pristine Ag2BiI5 solar cell; J–V curves (c) and EQE spectra (d) of the Ag2BiI5 solar
cells with and without alkali-metal doping.
the devices are increased to different extents by doping 4. Conclusion
with Li+, Na+, K+, and Rb+, respectively. Due to the
In summary, an effective alkali-metal (Li+, Na+, K+, Rb+, or
enhancement of crystallinity, all alkali metal ions, includ-
Cs+) doping strategy is applied to improve the photovoltaic
performance of Ag2BiI5 solar cells. The introduction of
alkali metal ions improves the film crystallinity and carrier
concentration, which leads to an increase of the JSC of the
ing Cs+, increase the JSC value of the devices. The increased
integrated JSC in the external quantum efficiency (EQE) spectra
(Fig. 6d) matches well with the enhanced JSC results in the J–V
curves. However, doping with Cs+ cannot improve the device
corresponding solar cells. The band gaps remain practically
performance due to the poor morphology in the SEM images.
unchanged because the alkali-metal-doping model is one of
Furthermore, since the band gaps are nearly unchanged by doping
interstitial doping. Large-grain and high-quality Ag2BiI5
with alkali metal ions, the VOC remains practically unchanged for
the Li+-, Na+-, K+- and Rb+-doped devices. The best performance is
thin films are obtained using our new deposition approach.
achieved by the introduction of K+, which is consistent with the
It should be noted that different alkali metal ions can
improve the crystallinity and increase the carrier concen-
previous evaluation of electrical properties. The obtained PCE is
tration to various degrees. For instance, the Cs+-doped
Ag2BiI5 thin film shows pinholes in the top-view SEM
images, which may be attributed to the larger grain size
compared with the other alkali-metal-doped films. In addi-
tion, the carrier concentration value of K+-doped Ag2BiI5 is
higher than that of the other alkali-metal-doped films,
which indicates the most conducive tendency of K+ to
improve the device performance. Subsequently, devices
with the structure of ITO/SnO2/Ag2BiI5/PTAA/Ag were fabri-
cated. The highest PCE of 0.75% is achieved after introdu-
cing K+. The order of the power conversion efficiency
promotion ability is K+ 4 Rb+ 4 Na+ 4 Li+ 4 Cs+ under
the same experimental conditions. Based on a systematic
understanding of the effects of doping with different alkali
metals, our work provides a feasible avenue to improve the
photovoltaic performance of Ag2BiI5 solar cells.
0.75% with a VOC of 0.68 V, a JSC of 1.95 mA/cm2, and an FF of
56.20%. Moreover, Fig. S6 (ESI†) shows the statistical efficiency
distribution of 30 devices with the Ag2BiI5 absorber doped with K+,
which indicates a good reproducibility of efficiency. Besides, there
are some pinholes on the surface of the Cs+-doped Ag2BiI5 film, and
this may damage the device performance. Therefore, the doping
concentrations of 1% and 2% for Cs+-doped Ag2BiI5 films are used
to investigate the effect of doping concentration on the film proper-
ties (Fig. S7, S8 and Table S3, ESI†). As shown in Fig. S7 (ESI†), the
1% Cs+-doped Ag2BiI5 film shows a dense and continuous morphol-
ogy. In addition, some pinholes are still presented on the surface of
the 2% Cs+-doped Ag2BiI5 film. Furthermore, the J–V curves and PV
parameters of the 1% and 2% Cs+-doped Ag2BiI5 solar cells are
shown in Fig. S8 (ESI†) and Table S3 (ESI†), and the 1% Cs+-doped
Ag2BiI5 solar cell achieves an improved device performance (PCE =
0.49%).
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Author contributions
Yuxiang Wang contributed to the investigation, data curation,
formal analysis, visualization, and writing – original draft. Yue
Liu contributed to the investigation. Xinan Shi contributed to
the investigation and resources. Lijian Huang contributed to
the investigation and resources. Junye Tong contributed to the
investigation. Gang Wang contributed to the conceptualization,
project administration, and writing – review and editing. Dao-
cheng Pan contributed to the conceptualization, funding acqui-
sition, resources, supervision, and writing – review and editing.
Conflicts of interest
There are no conflicts to declare.
11 A. Kausar, A. Sattar, C. Xu, S. Zhang, Z. Kang and Y. Zhang,
Advent of alkali metal doping: a roadmap for the evolution
of perovskite solar cells, Chem. Soc. Rev., 2021, 50,
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H. Wang, W. Gao, W. J. Fan, W.-S. Li and Q. Zhang,
Improving the fill factor of perovskite solar cells by employ-
ing an amine-tethered diketopyrrolopyrrole-based polymer
as the dopant-free hole transport layer, ACS Appl. Energy
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Acknowledgements
The authors are very thankful to Prof. Nripan Mathews and Dr
Biplab Ghosh of Nanyang Technological University for discus-
sions on the crystal structure of Ag2BiI5. This work was sup-
ported by the National Natural Science Foundation of China
(Grant No. 61974145 and 51672267) and the Jilin Provincial
Science and Technology Development Program (20180101
186JC).
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