- Email: [email protected]
bulk-heterojunction solar cells
Journal of Power Sources 440 (2019) 227151
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
10.34%-efficient integrated CsPbBr3/bulk-heterojunction solar cells Yuanyuan Zhao a, b, Hongzhe Xu a, Yudi Wang b, Xiya Yang b, Jialong Duan b, *, Qunwei Tang b, ** a b
School of Materials Science and Engineering, Ocean University of China, Qingdao, 266100, PR China Institute of New Energy Technology, College of Information Science and Technology, Jinan University, Guangzhou, 510632, PR China
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� Organic bulk-heterojunction photo active layer is integrated into inorganic CsPbBr3 PSCs. � Alkalis ion Rbþ has been doped into inorganic CsPbBr3 perovskite films. � The integrated CsPbBr3/bulk-hetero junction solar cell achieves a PCE of 10.34%. � The optimal device shows excellent long-term stability.
A R T I C L E I N F O
A B S T R A C T
Keywords: Inorganic perovskite solar cell Cesium lead bromide Wide-spectral absorption Photoactive layer Improved stability
Inorganic cesium lead bromide (CsPbBr3) perovskite solar cells (PSCs) have superior moisture- and thermalstability in comparison with organic-specie or/and I-containing devices. However, the narrow-spectra absorp tion (<550 nm) arising from their large bandgap of 2.3 eV for CsPbBr3 halide has markedly limited the further efficiency enhancement of corresponding inorganic device, therefore a great challenge is to broaden the light response range without sacrificing environmental tolerances. In this work, we constructively fabricate a CsPbBr3/bulk-heterojunction (organic J61-ITIC) photoactive layer to widen the optical absorption range of inorganic CsPbBr3 based interlayer-free device. Arising from the broadened light response wavelength from 550 to 780 nm and precisely optimized crystal lattice by incorporating Rbþ into CsPbBr3 film, the optimal device achieves a power conversion efficiency up to 10.34% under one sun illumination. Upon persistent attacks by heat of 80 � C (0% humidity) or 90% humidity (25 � C) over 40 days, the solar cell can still remain approximately 96% of initial efficiency, demonstrating the excellent stability for practical application.
1. Introduction Till now, the highest power conversion efficiency (PCE) of organicinorganic hybrid perovskite solar cells (PSCs) has been reported up to 25.2% [1], and the commercial application of PSCs has been put on the agenda. However, the intrinsic volatility of organic species in hybrid
perovskites is still a major impediment for this purpose under harsh operating environments. Therefore, forming all-inorganic perovskite halides (CsPbX3, X ¼ Br and I) as light-harvesters through substituting þ þ þ þ organic counterparts [CH3NHþ 3 (MA ) or HC(NH2)2 FA ] with Cs is considered a promising path for stale and highly efficient PSCs due to their non-volatile components, high absorption coefficient, high
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J. Duan), [email protected] (Q. Tang). https://doi.org/10.1016/j.jpowsour.2019.227151 Received 21 June 2019; Received in revised form 22 August 2019; Accepted 11 September 2019 Available online 17 September 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
Y. Zhao et al.
Journal of Power Sources 440 (2019) 227151
mobility and long carrier diffusion length. In recent two years, the PCEs of inorganic PSCs have achieved a relatively substantial breakthrough by optimizing fabrication strategy, modulating crystal lattice with im purity ions and stabilizing phase, with the certified PCE over 18% [2–11]. Among these inorganic perovskites, all-brominated CsPbBr3 halide demonstrates excellent tolerance to air atmosphere, whereas the PCEs of corresponding PSCs are much lower comparing to that of the state-of-the-art hybrid devices [12,13]. The reason behind this dragged PCE is mainly ascribed to intrinsic narrow-spectra response range (<550 nm) due to large bandgap (2.3 eV) of CsPbBr3 halide (Fig. S1). Although the bandgap of inorganic perovskites can be reduced by increasing iodide dosage or partially substituting Pb2þ with Sn2þ, the stability of the resultant perovskite structure will simultaneously suffer from a serious degradation owing to the boosted thermodynamics phase conversion and Sn2þ oxidation to Sn4þ [14]. Therefore, how to broaden the light-response range and utilize the solar light beyond 550 nm even up to near-infrared wavelength for CsPbBr3 based solar cell without sacrificing environmental stability is a persistent objective in this field. According to previous reports, several strategies such as fabricating perovskite/quantum dots system and tandem structures with two or more spectra-complementary sub-cells are promising to broaden spectra-response range [15–17]. However, the substantial intrinsic recombination within quantum dots limits the efficiency improvement although the theoretical efficiency of quantum dots based solar cell can be up to 44% [18]. On the other hand, to realize the maximal power output for series-connected tandem device, the currents generated by each sub-cells should be equal or similar to prevent the accumulation of photogenerated charges, and physical isolation of the sub-cells is also needed with a transparent recombination layer, leading to the device fabrication more difficult and complicated. To avoid those disadvan tages, interlayer-free parallel tandem devices are proved to be an attractive path to high-performance solar cells. For example, Yang’ group and Seok’ group have successfully integrated bulk-heterojunction (BHJ) with MAPbI3-xClx or Sb2S3 to efficiently harvest more solar light [19,20]. In this scenario, it is anticipated that the integrated CsPbBr3/BHJ solar cell will exhibit significantly improved PCE. Following this line of thought, in this study, we integrated the active layer J61-ITIC with CsPbBr3 to broaden the spectra-response range up to 780 nm. Upon further doping CsPbBr3 with Rbþ to decrease recombi nation loss with reduced grain boundaries and the energy barrier for carrier mobility, the final solar cell achieves a maximized PCE as high as 10.34% under standard air mass 1.5 global sunlight (100 mW cm 2 at room temperature) illumination.
deionized water was added into these powders after stirring for 15 min at 80 � C. When the solution became semi-transparent, the solu tion was transferred into autoclave to hydrothermal treatment at 200 � C for 12 h. Then, 0.4 g of commercial P25 was added and sonicated for 30 min and then heated at 200 � C for another 12 h. Finally, the resultant colloid was mixed with 0.8 g of PEG and 1 mL of OP, and eventually concentrated to 40 mL at 80 � C. 2.3. Fabrication of solar cells All the fabrication processes were operated under atmosphere con dition without humidity control. Firstly, FTO-coated glass was etched with Zinc powder and HCl (2 M) to obtain the desired pattern and then thoroughly rinsed in detergent, acetone, isopropanol, ethanol and deionized water. A compact TiO2 (c-TiO2) layer was deposited onto the glass by spin-coating an ethanol solution of titanium isopropoxide (0.5 M) and diethanol amine (0.5 M) at 7000 rpm for 30 s and annealed in air at 500 � C for 2 h. The mesoporous-TiO2 (m-TiO2) layer was sub sequently spin-coated on the c-TiO2 surface using the as-prepared TiO2 paste at 2000 rpm for 30 s and annealed in air at 450 � C for 30 min. Afterwards, the c-TiO2/m-TiO2 substrate was immersed in an aqueous solution of 0.04 M TiCl4 at 70 � C for 30 min, and then annealed in air for another 30 min at 450 � C. The perovskite film was fabricated by a multi-step solution-pro cessed technology in our previous report [13]. A DMF solution composing of 1 M PbBr2 and 0.09 M RbBr was spin-coated onto the c-TiO2/m-TiO2 surface at 2000 rpm for 30 s under 90 � C. After being dried at 90 � C for 1 h, 0.07 M CsBr methanol solution was spin-coated onto PbBr2/RbBr film at 2000 rpm for 30 s and continuingly heated at 250 � C for 5 min. This process was repeated for four times to obtain idea perovskite film. The obtained perovskite films were rinsed with iso propanol and dried at 250 � C again for 5 min. Finally, a carbon back-electrode with an average area of 0.09 cm2 was deposited on the perovskite film by a doctor-blade coating method. The fabrication pro cesses for perovskite/BHJ integrated devices were similar to that of pristine solar cells except for the spin-coating of J61-ITIC blend solution onto CsPbBr3 film at 2500 rpm for 30 s. 2.4. Characterizations Ultraviolet–visible (UV–vis) absorption spectra of various perovskite films were characterized employing a Meipuda UV-3200 spectropho tometer in the wavelength range of 350–800 nm. The PL spectra were measured on a FluoroMax-4 spectrofluorometer and the time-resolved PL decay characterizations were conducted with a Horiba spectrom eter excited by 500 nm laser. The morphology and element ratio of the perovskite films were obtained using a field-emission scanning electron microscope (SEM, SU8220, Hitachi). The X-ray diffraction (XRD) pat terns of the as-prepared films were recorded using a X-ray diffractometer (Bruker D8 ADVANCE) with Cu Kα (λ ¼ 1.5406 Å) radiation. The inci dent photon-to-electron conversion efficiency (IPCE) spectra were recorded by IPCE kit developed by Zolix Instruments Co., Ltd. The J-V curves were measured using a solar simulator (Newport, Oriel Class A, 91195A) under AM 1.5G simulated solar illumination (100 mW cm 2, calibrated by a standard silicon solar cell). The hole mobilities were measured with a space-charge limited current (SCLC) model using the Mott-Gurney law by constructing the hole-only device recorded from 1 to 7 V. UPS measurements were carried out using a Kratos AXIS ULTRA system with a helium discharge lamp, and with a concentric hemi spherical analyzer for photoexcited electron detection. The contact angle measurements are determined with a drop shape analyzer (JC2000DM). The thickness of different layers was measured by an L116 Ellipsometer from Gaertner Co., Ltd with He–Ne laser source.
2. Experimental section 2.1. Materials ITIC (98%) was purchased from Suna Tech Inc. J61 (98%) was purchased from Hunan Far Biological Technology Co. Ltd. FTO glass substrates and P25 powders were obtained from Yinkou Opvtech Co., Ltd. Titanium tetrabutanolate (98%), poly(ethylene glycol) (PEG, Mw ¼ 20, 000, 99%), OP emulsification agent (Triton X-100, 99%) were obtained from Sinopharm Chemical Reagent Co. Ltd. Unless stated otherwise, other materials and reagents such as PbBr2 (99.7%) and CsBr (99.9%) were purchased from Aladdin and used without further purification. 2.2. Synthesis of the TiO2 paste The TiO2 colloid were prepared by a hydrothermal-process method, following the previous report [21]. In details, 10 mL of titanium tetra butanolate was dropwise added into 100 mL of deionized water under vigorous agitation at room temperature for 30 min to obtain dehydrated filter powders. Subsequently, nitric acid (0.8 mL) and acetic acid (10 mL) were added slowly into the filter powders. Then, 160 mL of 2
Y. Zhao et al.
Journal of Power Sources 440 (2019) 227151
3. Results and discussion
while the spectra-response range is nearly unchanged (Fig. S4). It can be concluded that the individual organic molecule with great absorption in UV–vis–NIR region will not contribute photocurrent, which is accor dance with previous report [19]. However, the ITIC-individual tailored device demonstrates markedly increased Voc output, which may be mainly attributed to passivation effect through coordination between – N in ITIC and Pb species in perovskite film [22], and the – O or –C– –C– – detailed mechanism will be discussed in the following part. In this fashion, we can infer from enhanced photovoltaic performances that the combination of BHJ photoactive layer with CsPbBr3 provides new op portunities of broadening light absorption range and of increasing overall PCE of CsPbBr3 based solar cells. How does the thickness of J61-ITIC layer influence solar cell perfor mances? As shown in Fig. 2e, the thickness of active layer determined by an Ellipsomete increases from 130 to 170 nm (Table 2) by increasing the precursor concentration from 2.5 to 10 mg mL 1. In order to give a deep insight into this phenomenon, the relationship between absorbance and thickness is explored according to following relationship [23]:
As shown in Fig. 1a, compact and vertical CsPbBr3 layer is deposited onto c-TiO2/m-TiO2 surface through a multi-step spin-coating method developed by our group [13]. Subsequently, the J61 and ITIC blend is spin-coated onto CsPbBr3 film and heated at 100 � C for 10 min, forming a tightly-contacted CsPbBr3/bulk-heterojunction interface. Finally, the solar cell is covered by a carbon electrode with a blade coating tech nology. Fig. 1b shows the cross-sectional image of a typical PSC device with the average thickness of 200 nm, 300 nm, 150 nm and 15 μm for c-TiO2/m-TiO2, CsPbBr3, photoactive layer (J61-ITIC, the corresponding molecular structures are provided in Fig. 1c) and carbon electrode, respectively. How does the integrated solar cell improve the PCE? Prior to the photovoltaic characterizations, we have recorded the ultra violet–visible (UV–vis) absorption spectra of individual CsPbBr3 and CsPbBr3/J61-ITIC films, as shown in Fig. 2a. The perovskite-structured CsPbBr3 film has a characteristic absorption peak at 515 nm and weak absorption beyond 540 nm, which is mainly attributed to their large bandgap of 2.3 eV. Upon incorporating the BHJ layer onto the perov skite surface, the integrated film absorbs the light that has not been fully absorbed by perovskite ranging from 540 nm to 780 nm. This conclusion can be also cross-checked by the reduced transmittance spectra when incorporating BHJ into CsPbBr3 perovskite film as shown in Fig. S2. To better understand the effect of this active layer on photovoltaic perfor mances, the J-V curves of devices fabricated with different concentrated active layer precursors are shown in Fig. 2b and the corresponding photovoltaic parameters are summarized in Table 1. Obviously, the short-circuit current density (Jsc), open-circuit voltage (Voc), fill factor (FF) and PCE increase with increasing precursor concentration from 0 to 5 mg mL 1 and subsequently reduce beyond 5 mg mL 1 (Fig. S3), achieving the maximal Jsc, Voc, FF and PCE of 7.53 mA cm 2, 1.50 V, 80.9% and 9.14%, respectively. Under solar irradiation, the photocur rent should be produced in parallel by both perovskite layer and BHJ layer as described above for integrated devices, and the IPCE spectra are shown in Fig. 2d, confirming the validity of electricity generation beyond 540 nm. The proposed charge generation and transport mech anism of this integrated device is shown in Fig. 2c. The holes generated from perovskite can transport through the small organic molecule (J61) in BHJ film and then be collected by carbon electrode, while the electron generated from BHJ film can transport through electron acceptor ma terials (ITIC) to perovskite and then be collected by FTO/TiO2 electrode owing to the high electron mobility of perovskite film [19]. To further ensure the origination of photo-induced electric signal, the performance of the device integrated only with ITIC is also characterized, obviously showing improvement in photo-to-electrical conversion efficiency,
a(E) ¼ αd
(1)
where α is the absorption coefficient, E is specific photon energy, d is the thickness of photoactive layer. Since there is no backside mirror in this solar cell device, the reflection can be neglected and thereby the absorbance is proportional to absorption coefficient, as shown in equation (1). It is well-known that higher concentration of active layer precursor leads to a thicker BHJ layer, which is a crucial parameter for capturing enough solar light and realizing considerable photo-toelectrical conversion ability. However, excessive thickness will aggra vate the electron-hole recombination owing to the limited carrier diffusion length of around 145 nm in photoactive layer [24]. When the active layer is beyond 145 nm, the excessive interfaces may increase the charge-transfer resistance within active layer, resulting in the decreased carrier mobility. This conclusion can be identified by the SCLC tests in Fig. 2f and Fig. S5. The smallest trap-filled limit voltage (VTFL) for the hole-only device fabricated at a precursor concentration of 5 mg mL 1 demonstrates the lowest trap state density [25–27]. Taking the carrier diffusion length and defect trap state into considerations, it can be easily understood that the thickness of photoactive layer at around 145 nm is optimal to maximize power output. Although the performances are significantly enhanced by incorpo rating J61-ITIC photoactive layer into inorganic CsPbBr3 PSC, the PCE value is still lower than 10% arising from the serious charge recombina tion within perovskite film. Inspired by the previous work, we further optimized the film quality by doping Rbþ into CsPbBr3 crystal to realize Cs0.91Rb0.09PbBr3 [28–30]. It is well-known that the partial substitution of
Fig. 1. (a) Illustration of preparing integrated CsPbBr3/J61-ITIC device. (b) Cross-sectional SEM image of a whole device and (c) the molecule structures of J61 and ITIC. 3
Y. Zhao et al.
Journal of Power Sources 440 (2019) 227151
Fig. 2. (a) The absorption spectra of CsPbBr3 film with and without J61-ITIC layer. (b) Characteristic J-V curves of various devices made with different concentrated precursors measured at a standard AM 1.5G solar illumination (100 mW cm 2). (c) Charge generation and transport mechanism of the integrated CsPbBr3/J61-ITIC device. (d) The IPCE spectra of FTO/c-TiO2/m-TiO2/CsPbBr3/carbon and FTO/c-TiO2/m-TiO2/CsPbBr3/J61-ITIC/carbon devices. (e) The plots of J61-ITIC thickness as a function of precursor concentration. (f) SCLC curves of various hole-only devices recorded from 1 to 7 V. Table 1 Photovoltaic data for corresponding solar cells. Devices
Concentrations (mg mL 1)
Jsc (mA cm 2)
Voc (V)
FF (%)
PCE (%)
CsPbBr3 CsPbBr3 /J61ITIC
0 2.5 5.0 7.5 10.0
6.51 7.25 7.53 7.03 6.97
1.41 1.40 1.50 1.48 1.38
76.2 78.1 80.9 80.5 76.7
7.0 7.93 9.14 8.38 7.38
Table 2 Thickness of photoactive layer measured at different points. Concentrations
2.5 mg mL
Thickness (nm)
128.3 132.4 135.5 136.0 123.1 131.1
Average thickness (nm)
1
5.0 mg mL 143.8 145.7 146.7 144.7 148.2 145.8
1
7.5 mg mL 164.0 165.2 168.7 158.3 161.5 163.6
1
10.0 mg mL
1
176.7 159.6 172.1 170.1 172.3 170.2
A-site cation in ABX3 halide by smaller cations can bring promoting effects on perovskite crystals such as lattice shrink, enlarged crystalline grain and reduced bandgap. According to our previous reports, all the structure deviations can be confirmed by XRD characterization, optical measure ment and SEM images, as shown in Fig. S6 and Fig. S7, demonstrating the successful incorporation of Rb into CsPbBr3 lattice owing to the shift of characteristic peaks in XRD spectra, reduced bandgap to 2.24 eV as well as obviously enlarged grain size. Meanwhile, the mapping images in Fig. 3 demonstrate a homogeneous distribution of Rb element throughout the film. These changes are beneficial to suppress the charge recombination and to boost charge transfer dynamics. Finally, a significantly enhanced
Fig. 3. EDS mapping images for Rb-doped CsPbBr3 film.
PCE as high as 10.34% along with Jsc of 8.18 mA cm 2, Voc of 1.58 V and FF of 80.0% is obtained for the integrated PSC with FTO/c-TiO2/m- TiO2/Cs0.91Rb0.09PbBr3/J61-ITIC/carbon configuration. The PCE 4
Y. Zhao et al.
Journal of Power Sources 440 (2019) 227151
enhancement mainly originates from the increased Jsc and Voc outputs in Fig. 4a, and the corresponding steady-state PCE plots under maximum power points are provided in Fig. S8. From the IPCE spectra in Fig. S9, one can find the optimized device has increased IPCE values ranging from 300 to 540 nm and broadened light response in a spectral range of 550–780 nm. To further study the reproducibility of the integrated solar cell, 30 individual devices have been fabricated and the photovoltaic data distributions are depicted in Fig. S10, presenting an excellent reproduc ibility for the device made using the method reported here. Comparing to the control device, the coupling of Cs0.91Rb0.09PbBr3 perovskite with J61-ITIC photoactive layer is positive to enhance overall performances of inorganic PSC devices. To better understand the mechanism behind this phenomenon, we further study the charge transfer pathways and recom bination behaviors in solar cells. As shown in Fig. 4b, the average carrier lifetime of CsPbBr3 is 5.83 ns and it reduces to 2.11 ns for CsPbBr3/J61-ITIC, demonstrating the promoted carrier extraction and transportation [31], which can be also cross-checked by the rapid quenching of PL intensity, as shown in Fig. S11. Furthermore, a slight increased lifetime is determined for Cs0.91Rb0.09PbBr3/J61-ITIC in com parison with CsPbBr3/J61-ITIC film, arising from optimized crystallinity, reduced grain boundaries and suppressed recombination. Substantial charge-carrier recombination in perovskite film including non-radiative losses and radiative recombination limits the overall power output of a monolithic solar cell, especially with the presence of active layer (more interfaces) in the physical integrated solar cells [32]. Therefore, understanding recombination mechanism is a prerequisite to realize maximum power output. The charge transfer dynamics and recombination behavior are investigated by plotting Jsc and Voc versus light intensity (I) according to the following equations [33,34]: Jsc ∝ Iδ (δ � 1)
Voc ¼ nkTln(I)/e þ constant
(3)
where δ is a factor related to bimolecular recombination, n is an ideal parameter related to monomolecular recombination, the k is the Boltz mann constant, T is absolute temperature, e is a coulomb charge. As shown in Fig. 4c, the plots of log(Jsc) versus log(I) is fitted linearly with increased δ value closer to unity after introducing J61-ITIC layer and Cs0.91Rb0.09PbBr3, suggesting a reduced bi-molecular radiative recom bination. Furthermore, the trap-dominated recombination (n ¼ 2) asso ciated with trap state density is significantly eliminated under opencircuit condition according to equation (3) and Fig. 4d. The moleculeperovskite interaction should be responsible for this suppressed – O) or recombination losses. In details, the interaction between (C– – N) in Lewis base (ITIC in the current work) and Pb2þ ions can cyano (C– – passivate vacancies created by under-coordinated Pb atoms, while the energy offset between the semiconducting molecules and the perovskite can easily form much shallower trap states and thereby boosting carrier transport [22]. Remarkably, the recombination can be further elimi nated upon doping Rbþ into CsPbBr3 lattice, mainly attributing to the optimization of grain size and modulation of energy levels (Fig. S7 and Fig. S12). High PCE and improved stability are two crucial criterias to evaluate the practical application of a solar cell. The normalized photovoltaic data including PCE, Jsc, Voc and FF of control (CsPbBr3) and optimal (Cs0.91Rb0.09PbBr3/J61-ITIC) devices are shown in Fig. 5a by exposing the devices in 80 � C and 0% RH without any encapsulation. It can be seen that the optimal PSC remains 96% of initial efficiency over 40 days storage, which is comparable to 95.7% for the control CsPbBr3 PSC. The high thermal tolerance is attributed to relatively high decomposition temperature of J61 (300 � C) and CsPbBr3 beyond 200 � C [35–37]. When persistently attacked by 90% RH at 25 � C for 40 days, as shown in Fig. 5b, the FTO/c-TiO2/m-TiO2/Cs0.91Rb0.09PbBr3/J61-ITIC/carbon
(2)
Fig. 4. (a) J-V curves of the CsPbBr3/J61-ITIC and Cs0.91Rb0.09PbBr3/J61-ITIC tailored devices. (b) The time-resolved PL decay curves of various films. (c) Jsc and (d) Voc plots versus incident light intensity. 5
Y. Zhao et al.
Journal of Power Sources 440 (2019) 227151
Fig. 5. (a) The PCE stability of the control and optimal devices in 80 � C and 0% RH. (b) The PCE stability of the control and optimal devices upon persistent attack by 90% RH at room temperature.
PSC still remains 97% of initial efficiency. The improved moisture tolerance benefits to the hydrophobicity of methyl groups in J61 and ITIC because of contact angles of 75� and 101� for CsPbBr3 and Cs0.91Rb0.09PbBr3/J61-ITIC films, respectively [38].
[6] C.F. Lau, M. Zhang, X.F. Deng, J.H. Zheng, J.M. Bing, Q.S. Ma, J. Kim, L. Hu, M. A. Green, S.J. Huang, A. Ho-Baillie, Strontium-doped low-temperature-processed CsPbI2Br perovskite solar cells, ACS Energy Lett 2 (2017) 2319–2344. [7] H. Bian, D.L. Bai, Z.W. Jin, K. Wang, L. Liang, H.R. Wang, J.R. Zhang, Q. Wang, S.Z. ( Frank) Liu, Graded bandgap CsPbI2þxBr1-x perovskite solar cells with a stabilized efficiency of 14.4%, Joule 2 (2018) 1500–1510. [8] Q. Wang, X.P. Zheng, Y.H. Deng, J.J. Zhao, Z.L. Chen, J.S. Huang, Stabilizing the α-phase of CsPbI3 perovskite by sulfobetaine zwitterions in one-step spin-coating films, Joule 1 (2017) 371–382. [9] T.Y. Zhang, M.I. Dar, G. Li, F. Xu, N.J. Guo, M. Gr€ atzel, Y.X. Zhao, Bication lead iodide 2D perovskite component to stabilize inorganic α-CsPbI3 perovskite phase for high-efficiency solar cells, Sci. Adv. 3 (2017) e1700841. [10] Y. Wang, T.Y. Zhang, M. Kan, Y.X. Zhao, Bifunctional stabilization of all-inorganic α-CsPbI3 perovskite for 17% efficiency photovoltaics, J. Am. Chem. Soc. (2018), https://doi.org/10.1021/jacs.8b07927. [11] Y. Wang, M.I. Dar, L.K. Ono, T. Zhang, M. Kan, Y. Li, L. Zhang, X. Wang, Y. Yang, X. Gao, Y. Qi, M. Gr€ atzel, Y. Zhao, Thermodynamically stabilized β-CsPbI3-based perovskite solar cells with efficiencies >18%, Science 365 (2019) 591–595. [12] I. Poli, J. Baker, J. McGettrick, F.D. Rossi, S. Eslava, T. Watson, P.J. Cameron, Screen printed carbon CsPbBr3 solar cells with high open-circuit photovoltage, J. Mater. Chem. A 6 (2018) 18677–18686. [13] J.L. Duan, Y.Y. Zhao, B.L. He, Q.W. Tang, High-purity inorganic perovskite films for 9.72%-efficiency solar cells, Angew. Chem. Int. Ed. 57 (2018) 3787–3791. [14] F. Wang, J.L. Ma, F.Y. Xie, L.K. Li, J. Chen, J. Fan, N. Zhao, Organic cationdependent degradation mechanism of organotin halide perovskites, Adv. Funct. Mater. 26 (2016) 3417–3423. [15] J. Han, S. Luo, X. Yin, Y. Zhou, H. Nan, J. Li, X. Li, D. Oron, H. Shen, H. Lin, Hybrid PbS quantum-dot-in-perovskite for high-efficiency perovskite solar cell, Small 14 (2018) 1801016. [16] A. Karani, L. Yang, S. Bai, M.H. Futscher, H.J. Snaith, B. Ehrler, N.C. Greenham, D. Di, Perovskite/colloidal quantum dot tandem solar cells: theoretical modeling and monolithic structure, ACS Energy Lett 3 (2018) 869–874. [17] Q. Guo, H. Liu, Z. Shi, F. Wang, E. Zhou, X. Bian, B. Zhang, A. Alsaedi, T. Hayat, Z. Tan, Efficient perovskite/organic integrated solar cells with extended photoresponse to 930 nm and enhanced near-infrared external quantum efficiency of over 50%, Nanoscale 10 (2018) 3245–3253. [18] J. Duan, H. Zhang, Q. Tang, B. He, L. Yu, Recent advances in critical materials for quantum dot-sensitized solar cells: a review, J. Mater. Chem. A 3 (2015) 17497–17510. [19] Y.S. Liu, Z.R. Hong, Q. Chen, W. Chang, H.P. Zhou, T. Song, E. Young, Y. Yang, J. You, G. Li, Integrated perovskite/bulk-heterojunction toward efficient solar cells, Nano Lett. 15 (2015) 662–668. [20] J.A. Chang, S.H. Im, Y.H. Lee, H.J. Kim, C.S. Lim, J.H. Heo, S.I. Seok, Panchromatic photon-harvesting by hole-conducting materials in inorganic-organic heterojunction sensitized-solar cell through the formation of nanostructured electron channels, Nano Lett. 12 (2012) 1863–1867. [21] Y.Y. Duan, Q.W. Tang, Z.H. Chen, B.L. He, H.Y. Chen, Enhanced dye illumination in dye-sensitized solar cells using TiO2/GeO2 photo-anodes, J. Mater. Chem. A 2 (2014) 12459–12465. [22] T.Q. Niu, J. Lu, R. Munir, J.B. Li, D. Barrit, X. Zhang, H.L. Hu, Z. Yang, A. Amassian, K. Zhao, S.Z.( Frank) Liu, Stable high-performance perovskite solar cells via grain boundary passivation, Adv. Mater. 30 (2018) 1706576. [23] J. Bisquert, Theory of the impedance of electron diffusion and recombination in a thin layer, J. Phys. Chem. B 106 (2016) 325–333. [24] Z.G. Zhang, Y.K. Yang, J. Yao, L.W. Xue, S.S. Chen, X.J. Li, W. Morrison, C. Yang, Y. F. Li, Constructing a strongly absorbing low-bandgap polymer acceptor for highperformance all-polymer solar cells, Angew. Chem. Int. Ed. 56 (2017) 13503–13507. [25] Q.F. Dong, Y.J. Fang, Y.C. Shao, P. Mulligan, J. Qiu, L. Cao, J.S. Huang, Electronhole diffusion lengths > 175 m in solution-grown CH3NH3PbI3 single crystals, Science 347 (2015) 967–970.
4. Conclusions In summary, we have demonstrated the successfully coupling of wide-spectral photoactive layer with perovskite to broaden the light response of inorganic CsPbBr3 solar cells. By fabricating integrated de vice with FTO/c-TiO2/m-TiO2/Cs0.91Rb0.09PbBr3/J61-ITIC/carbon configuration, an unprecedented efficiency of 10.34% and wide-spectral absorption to 780 nm is determined using the method reported here. Due to the thermal stability and hydrophobicity of J61-ITIC film, the optimized solar cell has improved long-term stability even under persistent attacks by temperature of 80 � C and 90% RH. Although the PCE is still lower than those of organic-inorganic hybrid PSCs, it is ex pected to further enhance the PCE of CsPbBr3 solar cells by discovering other advanced photoactive layers with longer wavelength absorbance and more matching energy levels. Acknowledgments This work was supported by the National Natural Science Foundation of China (61774139, 21503202, 61604143), the Fundamental Research Funds for the Central Universities (11618409, 21619311), and China Postdoctoral Science Foundation (55350315). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227151. References [1] https://www.nrel.gov/pv/assets/pdfs/best-research-cell-efficiencies.20190802. pdf. [2] D. Bai, H. Bian, Z. Jin, H. Wang, L. Meng, Q. Wang, S.( Frank) Liu, Temperatureassisted crystallization for inorganic CsPbI2Br perovskite solar cells to attain high stabilized efficiency 14.81%, Nano Energy 52 (2018) 408–415. [3] D.L. Bai, J.R. Zhang, Z.W. Jin, D.L. Bai, J.R. Zhang, Z.W. Jin, H. Bian, K. Wang, H. R. Wang, L. Liang, Q. Wang, S.Z.( Frank) Liu, Interstitial Mn2þ-driven high-aspectratio grain growth for low-trap-density microcrystalline film for record efficiency inorganic CsPbI2Br solar cells, ACS Energy Lett 3 (2018) 970–978. [4] Y.Q. Hu, F. Bai, X.B. Liu, Y.Q. Hu, F. Bai, X.B. Liu, Q.M. Ji, X.L. Miao, T. Qiu, S. F. Zhang, Bismuth incorporation stabilized α-CsPbI3 for fully inorganic perovskite solar cells, ACS Energy Lett 2 (2017) 2219–2227. [5] C.Y. Chen, H.Y. Lin, K.M. Chiang, W.L. Tsai, Y.C. Huang, C.S. Tsao, H.W. Lin, Allvacuum-deposited stoichiometrically balanced inorganic cesium lead halide perovskite solar cells with stabilized efficiency exceeding 11, Adv. Mater. 29 (2017) 1605290.
6
Y. Zhao et al.
Journal of Power Sources 440 (2019) 227151
[26] Q. Lin, A. Armin, R.C.R. Nagiri, P.L. Burn, P. Meredith, Electro-optics of perovskite solar cells, Nat. Photonics 9 (2015) 106–112. [27] S.Y. Ye, H.X. Rao, W.B. Yan, Y.L. Li, W.H. Sun, H.T. Peng, Z.W. Liu, Z.Q. Bian, Y. F. Li, C.H. Huang, A strategy to simplify the preparation process of perovskite solar cells by co-deposition of a hole-conductor and a perovskite layer, Adv. Mater. 28 (2016) 9648–9654. [28] P. Yadav, M.I. Dar, N. Arora, E.A. Alharbi, F.G.S. Zakeeruddin, M. Gr€ atzel, The role of Rubidium in multiple-cation-based high-efficiency perovskite solar cells, Adv. Mater. 29 (2017) 1701077. [29] M. Zhang, J.S. Yun, Q. Ma, J.H. Zheng, C.F.J. Lau, X.F. Deng, J.C. Kim, D. Kim, J. Seidel, M. Green, S.J. Huang, A. Ho-Baillie, High efficiency rubidium incorporated perovskite solar cells by gas quenching, ACS Energy Lett 2 (2017) 438–444. [30] Y.N. Li, J.L. Duan, H.W. Yuan, Y.Y. Zhao, B.L. He, Q.W. Tang, Lattice modulation of alkali metal cations doped Cs1-xRxPbBr3 halides for inorganic perovskite solar cells, Sol. RRL 2 (2018) 1800164. [31] W.J. Chen, J.W. Zhang, G.Y. Xu, R.M. Xue, Y.W. Li, Y.H. Zhou, J.H. Hou, Y.F. Li, A semitransparent inorganic perovskite film for overcoming ultraviolet light instability of organic solar cells and achieving 14.03% efficiency, Adv. Mater. 30 (2018) 1800855. [32] S.D. Stranks, V.M. Burlakov, T. Leijtens, J.M. Ball, A. Goriely, H.J. Snaith, Recombination kinetics in organic-inorganic perovskites: excitons, free charge, and subgap states, Phys. Rev. Appl. 2 (2014) 034007.
[33] K.Y. Lu, Y.J. Wang, J.Y. Yuan, Z.Q. Cui, G.Z. Shi, S.H. Shi, Lu Han, S. Chen, Y. N. Zhang, X.F. Ling, Z.K. Liu, L.F. Chi, J. Fan, W.L. Ma, Efficient PbS quantum dot solar cells employing a conventional structure, J. Mater. Chem. A 5 (2017) 23960–23966. [34] F.L. Cai, J.L. Cai, L.Y. Yang, W. Li, R.S. Gurney, H.N. Yi, A. Iraqi, D. Liu, T. Wang, Molecular engineering of conjugated polymers for efficient hole transport and defect passivation in perovskite solar cells, Nano Energy 45 (2017) 28–36. [35] H.J. Bin, Z.G. Zhang, G. Liang, S.S. Chen, L.C.L. Zhong, L.W. Xue, C. Yang, Y.F. Li, Non-fullerene polymer solar cells based on alkylthio and fluorine substituted 2Dconjugated polymers reach 9.5% efficiency, J. Am. Chem. Soc. 138 (2016) 4657–4680. [36] C. Liu, W.Z. Li, J.H. Chen, J.D. Fan, Y.H. Mai, R.E.I. Schropp, Ultra-thin MoOx as cathode buffer layer for the improvement of all-inorganic CsPbIBr2 perovskite solar cells, Nano Energy 41 (2017) 75–83. [37] Z. Wang, Z.J. Shi, T.T. Li, Y.H. Chen, W. Huang, Stability of perovskite solar cells: a prospective on the substitution of the A cation and X anion, Angew. Chem. Int. Ed. 56 (2017) 1190–1212. [38] S. Rodroguez-Hermida, M.Y. Tsang, C. Vignatti, K.C. Stylianou, V. Guillerm, J. P. Carvajal, F. Teixidor, C. Vinas, D. Choquesillo-Lazarte, C. Verdugo-Escamilla, I. Peral, J. Juanhuix, A. Verdaguer, I. Imaz, D. Maspoch, J.G. Planas, Switchable surface hydrophobicity-hydrophilicity of a metal-organic framework, Angew. Chem. Int. Ed. 55 (2016) 16049–16053.
7
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
10.34%-efficient integrated CsPbBr3/bulk-heterojunction solar cells Yuanyuan Zhao a, b, Hongzhe Xu a, Yudi Wang b, Xiya Yang b, Jialong Duan b, *, Qunwei Tang b, ** a b
School of Materials Science and Engineering, Ocean University of China, Qingdao, 266100, PR China Institute of New Energy Technology, College of Information Science and Technology, Jinan University, Guangzhou, 510632, PR China
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� Organic bulk-heterojunction photo active layer is integrated into inorganic CsPbBr3 PSCs. � Alkalis ion Rbþ has been doped into inorganic CsPbBr3 perovskite films. � The integrated CsPbBr3/bulk-hetero junction solar cell achieves a PCE of 10.34%. � The optimal device shows excellent long-term stability.
A R T I C L E I N F O
A B S T R A C T
Keywords: Inorganic perovskite solar cell Cesium lead bromide Wide-spectral absorption Photoactive layer Improved stability
Inorganic cesium lead bromide (CsPbBr3) perovskite solar cells (PSCs) have superior moisture- and thermalstability in comparison with organic-specie or/and I-containing devices. However, the narrow-spectra absorp tion (<550 nm) arising from their large bandgap of 2.3 eV for CsPbBr3 halide has markedly limited the further efficiency enhancement of corresponding inorganic device, therefore a great challenge is to broaden the light response range without sacrificing environmental tolerances. In this work, we constructively fabricate a CsPbBr3/bulk-heterojunction (organic J61-ITIC) photoactive layer to widen the optical absorption range of inorganic CsPbBr3 based interlayer-free device. Arising from the broadened light response wavelength from 550 to 780 nm and precisely optimized crystal lattice by incorporating Rbþ into CsPbBr3 film, the optimal device achieves a power conversion efficiency up to 10.34% under one sun illumination. Upon persistent attacks by heat of 80 � C (0% humidity) or 90% humidity (25 � C) over 40 days, the solar cell can still remain approximately 96% of initial efficiency, demonstrating the excellent stability for practical application.
1. Introduction Till now, the highest power conversion efficiency (PCE) of organicinorganic hybrid perovskite solar cells (PSCs) has been reported up to 25.2% [1], and the commercial application of PSCs has been put on the agenda. However, the intrinsic volatility of organic species in hybrid
perovskites is still a major impediment for this purpose under harsh operating environments. Therefore, forming all-inorganic perovskite halides (CsPbX3, X ¼ Br and I) as light-harvesters through substituting þ þ þ þ organic counterparts [CH3NHþ 3 (MA ) or HC(NH2)2 FA ] with Cs is considered a promising path for stale and highly efficient PSCs due to their non-volatile components, high absorption coefficient, high
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J. Duan), [email protected] (Q. Tang). https://doi.org/10.1016/j.jpowsour.2019.227151 Received 21 June 2019; Received in revised form 22 August 2019; Accepted 11 September 2019 Available online 17 September 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
Y. Zhao et al.
Journal of Power Sources 440 (2019) 227151
mobility and long carrier diffusion length. In recent two years, the PCEs of inorganic PSCs have achieved a relatively substantial breakthrough by optimizing fabrication strategy, modulating crystal lattice with im purity ions and stabilizing phase, with the certified PCE over 18% [2–11]. Among these inorganic perovskites, all-brominated CsPbBr3 halide demonstrates excellent tolerance to air atmosphere, whereas the PCEs of corresponding PSCs are much lower comparing to that of the state-of-the-art hybrid devices [12,13]. The reason behind this dragged PCE is mainly ascribed to intrinsic narrow-spectra response range (<550 nm) due to large bandgap (2.3 eV) of CsPbBr3 halide (Fig. S1). Although the bandgap of inorganic perovskites can be reduced by increasing iodide dosage or partially substituting Pb2þ with Sn2þ, the stability of the resultant perovskite structure will simultaneously suffer from a serious degradation owing to the boosted thermodynamics phase conversion and Sn2þ oxidation to Sn4þ [14]. Therefore, how to broaden the light-response range and utilize the solar light beyond 550 nm even up to near-infrared wavelength for CsPbBr3 based solar cell without sacrificing environmental stability is a persistent objective in this field. According to previous reports, several strategies such as fabricating perovskite/quantum dots system and tandem structures with two or more spectra-complementary sub-cells are promising to broaden spectra-response range [15–17]. However, the substantial intrinsic recombination within quantum dots limits the efficiency improvement although the theoretical efficiency of quantum dots based solar cell can be up to 44% [18]. On the other hand, to realize the maximal power output for series-connected tandem device, the currents generated by each sub-cells should be equal or similar to prevent the accumulation of photogenerated charges, and physical isolation of the sub-cells is also needed with a transparent recombination layer, leading to the device fabrication more difficult and complicated. To avoid those disadvan tages, interlayer-free parallel tandem devices are proved to be an attractive path to high-performance solar cells. For example, Yang’ group and Seok’ group have successfully integrated bulk-heterojunction (BHJ) with MAPbI3-xClx or Sb2S3 to efficiently harvest more solar light [19,20]. In this scenario, it is anticipated that the integrated CsPbBr3/BHJ solar cell will exhibit significantly improved PCE. Following this line of thought, in this study, we integrated the active layer J61-ITIC with CsPbBr3 to broaden the spectra-response range up to 780 nm. Upon further doping CsPbBr3 with Rbþ to decrease recombi nation loss with reduced grain boundaries and the energy barrier for carrier mobility, the final solar cell achieves a maximized PCE as high as 10.34% under standard air mass 1.5 global sunlight (100 mW cm 2 at room temperature) illumination.
deionized water was added into these powders after stirring for 15 min at 80 � C. When the solution became semi-transparent, the solu tion was transferred into autoclave to hydrothermal treatment at 200 � C for 12 h. Then, 0.4 g of commercial P25 was added and sonicated for 30 min and then heated at 200 � C for another 12 h. Finally, the resultant colloid was mixed with 0.8 g of PEG and 1 mL of OP, and eventually concentrated to 40 mL at 80 � C. 2.3. Fabrication of solar cells All the fabrication processes were operated under atmosphere con dition without humidity control. Firstly, FTO-coated glass was etched with Zinc powder and HCl (2 M) to obtain the desired pattern and then thoroughly rinsed in detergent, acetone, isopropanol, ethanol and deionized water. A compact TiO2 (c-TiO2) layer was deposited onto the glass by spin-coating an ethanol solution of titanium isopropoxide (0.5 M) and diethanol amine (0.5 M) at 7000 rpm for 30 s and annealed in air at 500 � C for 2 h. The mesoporous-TiO2 (m-TiO2) layer was sub sequently spin-coated on the c-TiO2 surface using the as-prepared TiO2 paste at 2000 rpm for 30 s and annealed in air at 450 � C for 30 min. Afterwards, the c-TiO2/m-TiO2 substrate was immersed in an aqueous solution of 0.04 M TiCl4 at 70 � C for 30 min, and then annealed in air for another 30 min at 450 � C. The perovskite film was fabricated by a multi-step solution-pro cessed technology in our previous report [13]. A DMF solution composing of 1 M PbBr2 and 0.09 M RbBr was spin-coated onto the c-TiO2/m-TiO2 surface at 2000 rpm for 30 s under 90 � C. After being dried at 90 � C for 1 h, 0.07 M CsBr methanol solution was spin-coated onto PbBr2/RbBr film at 2000 rpm for 30 s and continuingly heated at 250 � C for 5 min. This process was repeated for four times to obtain idea perovskite film. The obtained perovskite films were rinsed with iso propanol and dried at 250 � C again for 5 min. Finally, a carbon back-electrode with an average area of 0.09 cm2 was deposited on the perovskite film by a doctor-blade coating method. The fabrication pro cesses for perovskite/BHJ integrated devices were similar to that of pristine solar cells except for the spin-coating of J61-ITIC blend solution onto CsPbBr3 film at 2500 rpm for 30 s. 2.4. Characterizations Ultraviolet–visible (UV–vis) absorption spectra of various perovskite films were characterized employing a Meipuda UV-3200 spectropho tometer in the wavelength range of 350–800 nm. The PL spectra were measured on a FluoroMax-4 spectrofluorometer and the time-resolved PL decay characterizations were conducted with a Horiba spectrom eter excited by 500 nm laser. The morphology and element ratio of the perovskite films were obtained using a field-emission scanning electron microscope (SEM, SU8220, Hitachi). The X-ray diffraction (XRD) pat terns of the as-prepared films were recorded using a X-ray diffractometer (Bruker D8 ADVANCE) with Cu Kα (λ ¼ 1.5406 Å) radiation. The inci dent photon-to-electron conversion efficiency (IPCE) spectra were recorded by IPCE kit developed by Zolix Instruments Co., Ltd. The J-V curves were measured using a solar simulator (Newport, Oriel Class A, 91195A) under AM 1.5G simulated solar illumination (100 mW cm 2, calibrated by a standard silicon solar cell). The hole mobilities were measured with a space-charge limited current (SCLC) model using the Mott-Gurney law by constructing the hole-only device recorded from 1 to 7 V. UPS measurements were carried out using a Kratos AXIS ULTRA system with a helium discharge lamp, and with a concentric hemi spherical analyzer for photoexcited electron detection. The contact angle measurements are determined with a drop shape analyzer (JC2000DM). The thickness of different layers was measured by an L116 Ellipsometer from Gaertner Co., Ltd with He–Ne laser source.
2. Experimental section 2.1. Materials ITIC (98%) was purchased from Suna Tech Inc. J61 (98%) was purchased from Hunan Far Biological Technology Co. Ltd. FTO glass substrates and P25 powders were obtained from Yinkou Opvtech Co., Ltd. Titanium tetrabutanolate (98%), poly(ethylene glycol) (PEG, Mw ¼ 20, 000, 99%), OP emulsification agent (Triton X-100, 99%) were obtained from Sinopharm Chemical Reagent Co. Ltd. Unless stated otherwise, other materials and reagents such as PbBr2 (99.7%) and CsBr (99.9%) were purchased from Aladdin and used without further purification. 2.2. Synthesis of the TiO2 paste The TiO2 colloid were prepared by a hydrothermal-process method, following the previous report [21]. In details, 10 mL of titanium tetra butanolate was dropwise added into 100 mL of deionized water under vigorous agitation at room temperature for 30 min to obtain dehydrated filter powders. Subsequently, nitric acid (0.8 mL) and acetic acid (10 mL) were added slowly into the filter powders. Then, 160 mL of 2
Y. Zhao et al.
Journal of Power Sources 440 (2019) 227151
3. Results and discussion
while the spectra-response range is nearly unchanged (Fig. S4). It can be concluded that the individual organic molecule with great absorption in UV–vis–NIR region will not contribute photocurrent, which is accor dance with previous report [19]. However, the ITIC-individual tailored device demonstrates markedly increased Voc output, which may be mainly attributed to passivation effect through coordination between – N in ITIC and Pb species in perovskite film [22], and the – O or –C– –C– – detailed mechanism will be discussed in the following part. In this fashion, we can infer from enhanced photovoltaic performances that the combination of BHJ photoactive layer with CsPbBr3 provides new op portunities of broadening light absorption range and of increasing overall PCE of CsPbBr3 based solar cells. How does the thickness of J61-ITIC layer influence solar cell perfor mances? As shown in Fig. 2e, the thickness of active layer determined by an Ellipsomete increases from 130 to 170 nm (Table 2) by increasing the precursor concentration from 2.5 to 10 mg mL 1. In order to give a deep insight into this phenomenon, the relationship between absorbance and thickness is explored according to following relationship [23]:
As shown in Fig. 1a, compact and vertical CsPbBr3 layer is deposited onto c-TiO2/m-TiO2 surface through a multi-step spin-coating method developed by our group [13]. Subsequently, the J61 and ITIC blend is spin-coated onto CsPbBr3 film and heated at 100 � C for 10 min, forming a tightly-contacted CsPbBr3/bulk-heterojunction interface. Finally, the solar cell is covered by a carbon electrode with a blade coating tech nology. Fig. 1b shows the cross-sectional image of a typical PSC device with the average thickness of 200 nm, 300 nm, 150 nm and 15 μm for c-TiO2/m-TiO2, CsPbBr3, photoactive layer (J61-ITIC, the corresponding molecular structures are provided in Fig. 1c) and carbon electrode, respectively. How does the integrated solar cell improve the PCE? Prior to the photovoltaic characterizations, we have recorded the ultra violet–visible (UV–vis) absorption spectra of individual CsPbBr3 and CsPbBr3/J61-ITIC films, as shown in Fig. 2a. The perovskite-structured CsPbBr3 film has a characteristic absorption peak at 515 nm and weak absorption beyond 540 nm, which is mainly attributed to their large bandgap of 2.3 eV. Upon incorporating the BHJ layer onto the perov skite surface, the integrated film absorbs the light that has not been fully absorbed by perovskite ranging from 540 nm to 780 nm. This conclusion can be also cross-checked by the reduced transmittance spectra when incorporating BHJ into CsPbBr3 perovskite film as shown in Fig. S2. To better understand the effect of this active layer on photovoltaic perfor mances, the J-V curves of devices fabricated with different concentrated active layer precursors are shown in Fig. 2b and the corresponding photovoltaic parameters are summarized in Table 1. Obviously, the short-circuit current density (Jsc), open-circuit voltage (Voc), fill factor (FF) and PCE increase with increasing precursor concentration from 0 to 5 mg mL 1 and subsequently reduce beyond 5 mg mL 1 (Fig. S3), achieving the maximal Jsc, Voc, FF and PCE of 7.53 mA cm 2, 1.50 V, 80.9% and 9.14%, respectively. Under solar irradiation, the photocur rent should be produced in parallel by both perovskite layer and BHJ layer as described above for integrated devices, and the IPCE spectra are shown in Fig. 2d, confirming the validity of electricity generation beyond 540 nm. The proposed charge generation and transport mech anism of this integrated device is shown in Fig. 2c. The holes generated from perovskite can transport through the small organic molecule (J61) in BHJ film and then be collected by carbon electrode, while the electron generated from BHJ film can transport through electron acceptor ma terials (ITIC) to perovskite and then be collected by FTO/TiO2 electrode owing to the high electron mobility of perovskite film [19]. To further ensure the origination of photo-induced electric signal, the performance of the device integrated only with ITIC is also characterized, obviously showing improvement in photo-to-electrical conversion efficiency,
a(E) ¼ αd
(1)
where α is the absorption coefficient, E is specific photon energy, d is the thickness of photoactive layer. Since there is no backside mirror in this solar cell device, the reflection can be neglected and thereby the absorbance is proportional to absorption coefficient, as shown in equation (1). It is well-known that higher concentration of active layer precursor leads to a thicker BHJ layer, which is a crucial parameter for capturing enough solar light and realizing considerable photo-toelectrical conversion ability. However, excessive thickness will aggra vate the electron-hole recombination owing to the limited carrier diffusion length of around 145 nm in photoactive layer [24]. When the active layer is beyond 145 nm, the excessive interfaces may increase the charge-transfer resistance within active layer, resulting in the decreased carrier mobility. This conclusion can be identified by the SCLC tests in Fig. 2f and Fig. S5. The smallest trap-filled limit voltage (VTFL) for the hole-only device fabricated at a precursor concentration of 5 mg mL 1 demonstrates the lowest trap state density [25–27]. Taking the carrier diffusion length and defect trap state into considerations, it can be easily understood that the thickness of photoactive layer at around 145 nm is optimal to maximize power output. Although the performances are significantly enhanced by incorpo rating J61-ITIC photoactive layer into inorganic CsPbBr3 PSC, the PCE value is still lower than 10% arising from the serious charge recombina tion within perovskite film. Inspired by the previous work, we further optimized the film quality by doping Rbþ into CsPbBr3 crystal to realize Cs0.91Rb0.09PbBr3 [28–30]. It is well-known that the partial substitution of
Fig. 1. (a) Illustration of preparing integrated CsPbBr3/J61-ITIC device. (b) Cross-sectional SEM image of a whole device and (c) the molecule structures of J61 and ITIC. 3
Y. Zhao et al.
Journal of Power Sources 440 (2019) 227151
Fig. 2. (a) The absorption spectra of CsPbBr3 film with and without J61-ITIC layer. (b) Characteristic J-V curves of various devices made with different concentrated precursors measured at a standard AM 1.5G solar illumination (100 mW cm 2). (c) Charge generation and transport mechanism of the integrated CsPbBr3/J61-ITIC device. (d) The IPCE spectra of FTO/c-TiO2/m-TiO2/CsPbBr3/carbon and FTO/c-TiO2/m-TiO2/CsPbBr3/J61-ITIC/carbon devices. (e) The plots of J61-ITIC thickness as a function of precursor concentration. (f) SCLC curves of various hole-only devices recorded from 1 to 7 V. Table 1 Photovoltaic data for corresponding solar cells. Devices
Concentrations (mg mL 1)
Jsc (mA cm 2)
Voc (V)
FF (%)
PCE (%)
CsPbBr3 CsPbBr3 /J61ITIC
0 2.5 5.0 7.5 10.0
6.51 7.25 7.53 7.03 6.97
1.41 1.40 1.50 1.48 1.38
76.2 78.1 80.9 80.5 76.7
7.0 7.93 9.14 8.38 7.38
Table 2 Thickness of photoactive layer measured at different points. Concentrations
2.5 mg mL
Thickness (nm)
128.3 132.4 135.5 136.0 123.1 131.1
Average thickness (nm)
1
5.0 mg mL 143.8 145.7 146.7 144.7 148.2 145.8
1
7.5 mg mL 164.0 165.2 168.7 158.3 161.5 163.6
1
10.0 mg mL
1
176.7 159.6 172.1 170.1 172.3 170.2
A-site cation in ABX3 halide by smaller cations can bring promoting effects on perovskite crystals such as lattice shrink, enlarged crystalline grain and reduced bandgap. According to our previous reports, all the structure deviations can be confirmed by XRD characterization, optical measure ment and SEM images, as shown in Fig. S6 and Fig. S7, demonstrating the successful incorporation of Rb into CsPbBr3 lattice owing to the shift of characteristic peaks in XRD spectra, reduced bandgap to 2.24 eV as well as obviously enlarged grain size. Meanwhile, the mapping images in Fig. 3 demonstrate a homogeneous distribution of Rb element throughout the film. These changes are beneficial to suppress the charge recombination and to boost charge transfer dynamics. Finally, a significantly enhanced
Fig. 3. EDS mapping images for Rb-doped CsPbBr3 film.
PCE as high as 10.34% along with Jsc of 8.18 mA cm 2, Voc of 1.58 V and FF of 80.0% is obtained for the integrated PSC with FTO/c-TiO2/m- TiO2/Cs0.91Rb0.09PbBr3/J61-ITIC/carbon configuration. The PCE 4
Y. Zhao et al.
Journal of Power Sources 440 (2019) 227151
enhancement mainly originates from the increased Jsc and Voc outputs in Fig. 4a, and the corresponding steady-state PCE plots under maximum power points are provided in Fig. S8. From the IPCE spectra in Fig. S9, one can find the optimized device has increased IPCE values ranging from 300 to 540 nm and broadened light response in a spectral range of 550–780 nm. To further study the reproducibility of the integrated solar cell, 30 individual devices have been fabricated and the photovoltaic data distributions are depicted in Fig. S10, presenting an excellent reproduc ibility for the device made using the method reported here. Comparing to the control device, the coupling of Cs0.91Rb0.09PbBr3 perovskite with J61-ITIC photoactive layer is positive to enhance overall performances of inorganic PSC devices. To better understand the mechanism behind this phenomenon, we further study the charge transfer pathways and recom bination behaviors in solar cells. As shown in Fig. 4b, the average carrier lifetime of CsPbBr3 is 5.83 ns and it reduces to 2.11 ns for CsPbBr3/J61-ITIC, demonstrating the promoted carrier extraction and transportation [31], which can be also cross-checked by the rapid quenching of PL intensity, as shown in Fig. S11. Furthermore, a slight increased lifetime is determined for Cs0.91Rb0.09PbBr3/J61-ITIC in com parison with CsPbBr3/J61-ITIC film, arising from optimized crystallinity, reduced grain boundaries and suppressed recombination. Substantial charge-carrier recombination in perovskite film including non-radiative losses and radiative recombination limits the overall power output of a monolithic solar cell, especially with the presence of active layer (more interfaces) in the physical integrated solar cells [32]. Therefore, understanding recombination mechanism is a prerequisite to realize maximum power output. The charge transfer dynamics and recombination behavior are investigated by plotting Jsc and Voc versus light intensity (I) according to the following equations [33,34]: Jsc ∝ Iδ (δ � 1)
Voc ¼ nkTln(I)/e þ constant
(3)
where δ is a factor related to bimolecular recombination, n is an ideal parameter related to monomolecular recombination, the k is the Boltz mann constant, T is absolute temperature, e is a coulomb charge. As shown in Fig. 4c, the plots of log(Jsc) versus log(I) is fitted linearly with increased δ value closer to unity after introducing J61-ITIC layer and Cs0.91Rb0.09PbBr3, suggesting a reduced bi-molecular radiative recom bination. Furthermore, the trap-dominated recombination (n ¼ 2) asso ciated with trap state density is significantly eliminated under opencircuit condition according to equation (3) and Fig. 4d. The moleculeperovskite interaction should be responsible for this suppressed – O) or recombination losses. In details, the interaction between (C– – N) in Lewis base (ITIC in the current work) and Pb2þ ions can cyano (C– – passivate vacancies created by under-coordinated Pb atoms, while the energy offset between the semiconducting molecules and the perovskite can easily form much shallower trap states and thereby boosting carrier transport [22]. Remarkably, the recombination can be further elimi nated upon doping Rbþ into CsPbBr3 lattice, mainly attributing to the optimization of grain size and modulation of energy levels (Fig. S7 and Fig. S12). High PCE and improved stability are two crucial criterias to evaluate the practical application of a solar cell. The normalized photovoltaic data including PCE, Jsc, Voc and FF of control (CsPbBr3) and optimal (Cs0.91Rb0.09PbBr3/J61-ITIC) devices are shown in Fig. 5a by exposing the devices in 80 � C and 0% RH without any encapsulation. It can be seen that the optimal PSC remains 96% of initial efficiency over 40 days storage, which is comparable to 95.7% for the control CsPbBr3 PSC. The high thermal tolerance is attributed to relatively high decomposition temperature of J61 (300 � C) and CsPbBr3 beyond 200 � C [35–37]. When persistently attacked by 90% RH at 25 � C for 40 days, as shown in Fig. 5b, the FTO/c-TiO2/m-TiO2/Cs0.91Rb0.09PbBr3/J61-ITIC/carbon
(2)
Fig. 4. (a) J-V curves of the CsPbBr3/J61-ITIC and Cs0.91Rb0.09PbBr3/J61-ITIC tailored devices. (b) The time-resolved PL decay curves of various films. (c) Jsc and (d) Voc plots versus incident light intensity. 5
Y. Zhao et al.
Journal of Power Sources 440 (2019) 227151
Fig. 5. (a) The PCE stability of the control and optimal devices in 80 � C and 0% RH. (b) The PCE stability of the control and optimal devices upon persistent attack by 90% RH at room temperature.
PSC still remains 97% of initial efficiency. The improved moisture tolerance benefits to the hydrophobicity of methyl groups in J61 and ITIC because of contact angles of 75� and 101� for CsPbBr3 and Cs0.91Rb0.09PbBr3/J61-ITIC films, respectively [38].
[6] C.F. Lau, M. Zhang, X.F. Deng, J.H. Zheng, J.M. Bing, Q.S. Ma, J. Kim, L. Hu, M. A. Green, S.J. Huang, A. Ho-Baillie, Strontium-doped low-temperature-processed CsPbI2Br perovskite solar cells, ACS Energy Lett 2 (2017) 2319–2344. [7] H. Bian, D.L. Bai, Z.W. Jin, K. Wang, L. Liang, H.R. Wang, J.R. Zhang, Q. Wang, S.Z. ( Frank) Liu, Graded bandgap CsPbI2þxBr1-x perovskite solar cells with a stabilized efficiency of 14.4%, Joule 2 (2018) 1500–1510. [8] Q. Wang, X.P. Zheng, Y.H. Deng, J.J. Zhao, Z.L. Chen, J.S. Huang, Stabilizing the α-phase of CsPbI3 perovskite by sulfobetaine zwitterions in one-step spin-coating films, Joule 1 (2017) 371–382. [9] T.Y. Zhang, M.I. Dar, G. Li, F. Xu, N.J. Guo, M. Gr€ atzel, Y.X. Zhao, Bication lead iodide 2D perovskite component to stabilize inorganic α-CsPbI3 perovskite phase for high-efficiency solar cells, Sci. Adv. 3 (2017) e1700841. [10] Y. Wang, T.Y. Zhang, M. Kan, Y.X. Zhao, Bifunctional stabilization of all-inorganic α-CsPbI3 perovskite for 17% efficiency photovoltaics, J. Am. Chem. Soc. (2018), https://doi.org/10.1021/jacs.8b07927. [11] Y. Wang, M.I. Dar, L.K. Ono, T. Zhang, M. Kan, Y. Li, L. Zhang, X. Wang, Y. Yang, X. Gao, Y. Qi, M. Gr€ atzel, Y. Zhao, Thermodynamically stabilized β-CsPbI3-based perovskite solar cells with efficiencies >18%, Science 365 (2019) 591–595. [12] I. Poli, J. Baker, J. McGettrick, F.D. Rossi, S. Eslava, T. Watson, P.J. Cameron, Screen printed carbon CsPbBr3 solar cells with high open-circuit photovoltage, J. Mater. Chem. A 6 (2018) 18677–18686. [13] J.L. Duan, Y.Y. Zhao, B.L. He, Q.W. Tang, High-purity inorganic perovskite films for 9.72%-efficiency solar cells, Angew. Chem. Int. Ed. 57 (2018) 3787–3791. [14] F. Wang, J.L. Ma, F.Y. Xie, L.K. Li, J. Chen, J. Fan, N. Zhao, Organic cationdependent degradation mechanism of organotin halide perovskites, Adv. Funct. Mater. 26 (2016) 3417–3423. [15] J. Han, S. Luo, X. Yin, Y. Zhou, H. Nan, J. Li, X. Li, D. Oron, H. Shen, H. Lin, Hybrid PbS quantum-dot-in-perovskite for high-efficiency perovskite solar cell, Small 14 (2018) 1801016. [16] A. Karani, L. Yang, S. Bai, M.H. Futscher, H.J. Snaith, B. Ehrler, N.C. Greenham, D. Di, Perovskite/colloidal quantum dot tandem solar cells: theoretical modeling and monolithic structure, ACS Energy Lett 3 (2018) 869–874. [17] Q. Guo, H. Liu, Z. Shi, F. Wang, E. Zhou, X. Bian, B. Zhang, A. Alsaedi, T. Hayat, Z. Tan, Efficient perovskite/organic integrated solar cells with extended photoresponse to 930 nm and enhanced near-infrared external quantum efficiency of over 50%, Nanoscale 10 (2018) 3245–3253. [18] J. Duan, H. Zhang, Q. Tang, B. He, L. Yu, Recent advances in critical materials for quantum dot-sensitized solar cells: a review, J. Mater. Chem. A 3 (2015) 17497–17510. [19] Y.S. Liu, Z.R. Hong, Q. Chen, W. Chang, H.P. Zhou, T. Song, E. Young, Y. Yang, J. You, G. Li, Integrated perovskite/bulk-heterojunction toward efficient solar cells, Nano Lett. 15 (2015) 662–668. [20] J.A. Chang, S.H. Im, Y.H. Lee, H.J. Kim, C.S. Lim, J.H. Heo, S.I. Seok, Panchromatic photon-harvesting by hole-conducting materials in inorganic-organic heterojunction sensitized-solar cell through the formation of nanostructured electron channels, Nano Lett. 12 (2012) 1863–1867. [21] Y.Y. Duan, Q.W. Tang, Z.H. Chen, B.L. He, H.Y. Chen, Enhanced dye illumination in dye-sensitized solar cells using TiO2/GeO2 photo-anodes, J. Mater. Chem. A 2 (2014) 12459–12465. [22] T.Q. Niu, J. Lu, R. Munir, J.B. Li, D. Barrit, X. Zhang, H.L. Hu, Z. Yang, A. Amassian, K. Zhao, S.Z.( Frank) Liu, Stable high-performance perovskite solar cells via grain boundary passivation, Adv. Mater. 30 (2018) 1706576. [23] J. Bisquert, Theory of the impedance of electron diffusion and recombination in a thin layer, J. Phys. Chem. B 106 (2016) 325–333. [24] Z.G. Zhang, Y.K. Yang, J. Yao, L.W. Xue, S.S. Chen, X.J. Li, W. Morrison, C. Yang, Y. F. Li, Constructing a strongly absorbing low-bandgap polymer acceptor for highperformance all-polymer solar cells, Angew. Chem. Int. Ed. 56 (2017) 13503–13507. [25] Q.F. Dong, Y.J. Fang, Y.C. Shao, P. Mulligan, J. Qiu, L. Cao, J.S. Huang, Electronhole diffusion lengths > 175 m in solution-grown CH3NH3PbI3 single crystals, Science 347 (2015) 967–970.
4. Conclusions In summary, we have demonstrated the successfully coupling of wide-spectral photoactive layer with perovskite to broaden the light response of inorganic CsPbBr3 solar cells. By fabricating integrated de vice with FTO/c-TiO2/m-TiO2/Cs0.91Rb0.09PbBr3/J61-ITIC/carbon configuration, an unprecedented efficiency of 10.34% and wide-spectral absorption to 780 nm is determined using the method reported here. Due to the thermal stability and hydrophobicity of J61-ITIC film, the optimized solar cell has improved long-term stability even under persistent attacks by temperature of 80 � C and 90% RH. Although the PCE is still lower than those of organic-inorganic hybrid PSCs, it is ex pected to further enhance the PCE of CsPbBr3 solar cells by discovering other advanced photoactive layers with longer wavelength absorbance and more matching energy levels. Acknowledgments This work was supported by the National Natural Science Foundation of China (61774139, 21503202, 61604143), the Fundamental Research Funds for the Central Universities (11618409, 21619311), and China Postdoctoral Science Foundation (55350315). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227151. References [1] https://www.nrel.gov/pv/assets/pdfs/best-research-cell-efficiencies.20190802. pdf. [2] D. Bai, H. Bian, Z. Jin, H. Wang, L. Meng, Q. Wang, S.( Frank) Liu, Temperatureassisted crystallization for inorganic CsPbI2Br perovskite solar cells to attain high stabilized efficiency 14.81%, Nano Energy 52 (2018) 408–415. [3] D.L. Bai, J.R. Zhang, Z.W. Jin, D.L. Bai, J.R. Zhang, Z.W. Jin, H. Bian, K. Wang, H. R. Wang, L. Liang, Q. Wang, S.Z.( Frank) Liu, Interstitial Mn2þ-driven high-aspectratio grain growth for low-trap-density microcrystalline film for record efficiency inorganic CsPbI2Br solar cells, ACS Energy Lett 3 (2018) 970–978. [4] Y.Q. Hu, F. Bai, X.B. Liu, Y.Q. Hu, F. Bai, X.B. Liu, Q.M. Ji, X.L. Miao, T. Qiu, S. F. Zhang, Bismuth incorporation stabilized α-CsPbI3 for fully inorganic perovskite solar cells, ACS Energy Lett 2 (2017) 2219–2227. [5] C.Y. Chen, H.Y. Lin, K.M. Chiang, W.L. Tsai, Y.C. Huang, C.S. Tsao, H.W. Lin, Allvacuum-deposited stoichiometrically balanced inorganic cesium lead halide perovskite solar cells with stabilized efficiency exceeding 11, Adv. Mater. 29 (2017) 1605290.
6
Y. Zhao et al.
Journal of Power Sources 440 (2019) 227151
[26] Q. Lin, A. Armin, R.C.R. Nagiri, P.L. Burn, P. Meredith, Electro-optics of perovskite solar cells, Nat. Photonics 9 (2015) 106–112. [27] S.Y. Ye, H.X. Rao, W.B. Yan, Y.L. Li, W.H. Sun, H.T. Peng, Z.W. Liu, Z.Q. Bian, Y. F. Li, C.H. Huang, A strategy to simplify the preparation process of perovskite solar cells by co-deposition of a hole-conductor and a perovskite layer, Adv. Mater. 28 (2016) 9648–9654. [28] P. Yadav, M.I. Dar, N. Arora, E.A. Alharbi, F.G.S. Zakeeruddin, M. Gr€ atzel, The role of Rubidium in multiple-cation-based high-efficiency perovskite solar cells, Adv. Mater. 29 (2017) 1701077. [29] M. Zhang, J.S. Yun, Q. Ma, J.H. Zheng, C.F.J. Lau, X.F. Deng, J.C. Kim, D. Kim, J. Seidel, M. Green, S.J. Huang, A. Ho-Baillie, High efficiency rubidium incorporated perovskite solar cells by gas quenching, ACS Energy Lett 2 (2017) 438–444. [30] Y.N. Li, J.L. Duan, H.W. Yuan, Y.Y. Zhao, B.L. He, Q.W. Tang, Lattice modulation of alkali metal cations doped Cs1-xRxPbBr3 halides for inorganic perovskite solar cells, Sol. RRL 2 (2018) 1800164. [31] W.J. Chen, J.W. Zhang, G.Y. Xu, R.M. Xue, Y.W. Li, Y.H. Zhou, J.H. Hou, Y.F. Li, A semitransparent inorganic perovskite film for overcoming ultraviolet light instability of organic solar cells and achieving 14.03% efficiency, Adv. Mater. 30 (2018) 1800855. [32] S.D. Stranks, V.M. Burlakov, T. Leijtens, J.M. Ball, A. Goriely, H.J. Snaith, Recombination kinetics in organic-inorganic perovskites: excitons, free charge, and subgap states, Phys. Rev. Appl. 2 (2014) 034007.
[33] K.Y. Lu, Y.J. Wang, J.Y. Yuan, Z.Q. Cui, G.Z. Shi, S.H. Shi, Lu Han, S. Chen, Y. N. Zhang, X.F. Ling, Z.K. Liu, L.F. Chi, J. Fan, W.L. Ma, Efficient PbS quantum dot solar cells employing a conventional structure, J. Mater. Chem. A 5 (2017) 23960–23966. [34] F.L. Cai, J.L. Cai, L.Y. Yang, W. Li, R.S. Gurney, H.N. Yi, A. Iraqi, D. Liu, T. Wang, Molecular engineering of conjugated polymers for efficient hole transport and defect passivation in perovskite solar cells, Nano Energy 45 (2017) 28–36. [35] H.J. Bin, Z.G. Zhang, G. Liang, S.S. Chen, L.C.L. Zhong, L.W. Xue, C. Yang, Y.F. Li, Non-fullerene polymer solar cells based on alkylthio and fluorine substituted 2Dconjugated polymers reach 9.5% efficiency, J. Am. Chem. Soc. 138 (2016) 4657–4680. [36] C. Liu, W.Z. Li, J.H. Chen, J.D. Fan, Y.H. Mai, R.E.I. Schropp, Ultra-thin MoOx as cathode buffer layer for the improvement of all-inorganic CsPbIBr2 perovskite solar cells, Nano Energy 41 (2017) 75–83. [37] Z. Wang, Z.J. Shi, T.T. Li, Y.H. Chen, W. Huang, Stability of perovskite solar cells: a prospective on the substitution of the A cation and X anion, Angew. Chem. Int. Ed. 56 (2017) 1190–1212. [38] S. Rodroguez-Hermida, M.Y. Tsang, C. Vignatti, K.C. Stylianou, V. Guillerm, J. P. Carvajal, F. Teixidor, C. Vinas, D. Choquesillo-Lazarte, C. Verdugo-Escamilla, I. Peral, J. Juanhuix, A. Verdaguer, I. Imaz, D. Maspoch, J.G. Planas, Switchable surface hydrophobicity-hydrophilicity of a metal-organic framework, Angew. Chem. Int. Ed. 55 (2016) 16049–16053.
7