Improving the performance of perovskite solar cells by surface passivation

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Improving the performance of perovskite solar cells by surface passivation Wenbin Han , Guanhua Ren , Zhiqi Li , Minnan Dong , Chunyu Liu , Wenbin Guo PII: DOI: Reference:

S2095-4956(19)30894-0 https://doi.org/10.1016/j.jechem.2019.11.004 JECHEM 1000

To appear in:

Journal of Energy Chemistry

Received date: Revised date: Accepted date:

28 September 2019 26 October 2019 5 November 2019

Please cite this article as: Wenbin Han , Guanhua Ren , Zhiqi Li , Minnan Dong , Chunyu Liu , Wenbin Guo , Improving the performance of perovskite solar cells by surface passivation, Journal of Energy Chemistry (2019), doi: https://doi.org/10.1016/j.jechem.2019.11.004

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Highlight  The polymer modified HTLs is simplistic, low cost, and suitable for mass production. 

The polymer electrolyte modified HTLs reduce the interfacial defects and improve contact.



The polymer modified HTL can enhance the electron extraction by decreasing the injection barrier.

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Improving the performance of perovskite solar cells by surface passivation Wenbin Hana, Guanhua Rena, Zhiqi Lia, Minnan Donga, Chunyu Liua,b,*, Wenbin Guoa,* a

State Key Laboratory of Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun 130012, Jilin, China b

*

College of Materials Science and Engineering, Jilin University, Changchun 130012, Jilin, China

Corresponding authors. [email protected] (C. Liu), [email protected]. (W. Guo).

Abstract Although NiO has a perfect-aligned energy level with CH3NH3PbI3 perovskite such that it serves as a hole transport layer (HTL), NiO-based perovskite solar cells (PSCs) still suffer from low efficiency due to the poor interface contact between the perovskite layer and the NiO HTL, and haphazardly stacked perovskite grains. Herein,

poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-co-(1,4-benzo-{2,1’,3}-thiadiazole)]

(PFBT)

is

introduced

between the NiO and perovskite layers in the form of a polymer aggregate to enhance perovskite crystallinity and decrease the interface charge recombination between perovskite and NiO in PSCs, resulting in an improved performance. Moreover, PFBT modified perovskite films showed sharper, smoother, and more compact crystalline grains with fewer grain boundaries, which reduced the nonradiative recombination, as shown by analysis of the photoluminescence (PL) spectra. This study offers a simple strategy to achieve highly efficient PSCs with the incorporation of polymer semiconductor aggregates to passivate the interface between the perovskite and NiO layers.

Keywords: Perovskite solar cells; Hole transport; Interface passivation; Surface traps; Crystallinity

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1. Introduction Since the organic-inorganic hybrid perovskite material, which was possesses many versatile advantages, including a low-cost, high absorption coefficient, high charge carrier mobility, long charge carrier lifetime, and appropriate diffusion length [1–11], was discovered to be an excellent absorber in photovoltaic devices, the power conversion efficiency (PCE) of perovskite solar cells (PSCs) has developed rapidly from 4% to 25.2% within ten years [12,13]. Organic materials such as poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and polybis(4-phenyl) (2,4,6-trimethyl-phenyl)amine (PTAA) were typically used as hole transport materials (HTMs) in inverted p-i-n PSCs [14–17]. However, PEDOT:PSS-based devices usually suffer from low efficiencies and poor stabilities owing to the high energy levels of these devices, as well as their acidic nature and hygroscopicity. Despite the satisfactory performance of PTAA, the high-cost, low mobility, and complex device fabrication progress that requires an additional pre-treatment before depositing the perovskite has limited its further application [18,19]. In recent years, inorganic charge transport materials such as Cu2O, CuI, CuSCN, and NiO have been proposed to replace organic HTMs [20–24]. Most of these materials are characterized by their low-cost, simple preparation, and ease of deposition [25–29]. Among them, NiO is an easily prepared and cheap hole-transport layer (HTL) material with a suitable energy level that has attracted increased attention [24,30–34]. The valence band (VB) (-5.4 eV) of NiO is the same as the HOMO energy level of CH3NH3PbI3 (-5.4 eV), which enables a smaller open circuit voltage (Voc) loss. The high conduction band (CB) value of NiO can also effectively block the transfer of electrons and minimize the charge recombination loss [35–37]. It should be noted, however, that PSCs based on NiO HTLs usually demonstrate inferior device performances, which is attributed to the non-ideal crystallinity of perovskite and the poor contact between the perovskite layer and NiO [38–42]. Surface passivation has proved to 3

be an effective method to improve the interface contact between perovskite and the HTL, and it may serve to optimize the perovskite crystallization. Diethanol amine (DEA) and ferrocenedicarboxylic acid (FDA) have been employed to modify the surfaces of the sol-gel-processed NiO film to improve the contact between NiO and perovskite, as well as to improve the perovskite crystallization [39,41]. Moreover, a dilute PEDOT:PSS solution coated onto the NiO film was found to reduce the internal resistance and facilitate the hole extraction from perovskite to NiO, resulting in an increased fill factor (FF) [40]. The use of 4-bromobenzoic acid has also been determined to reduce the trap recombination and improve the HTL wettability, which enhanced the perovskite crystallization

and

resulted

in

leading

to

a

superior

PCE

of

18.4%

[42].

Additionally,

Li-bis-(trifluoromethanesulfonyl) imide (Li-TFSI) has been incorporated into the inverted PSCs as a modification layer to improve both the FF and PCE (from 0.75 and 14.46% to 0.79 and 17.09%, respectively) [43-45]. Herein, we successfully introduced poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-co-(1,4-benzo-{2,1’,3}-thiadiazole)] (PFBT), in the form of aggregates, to passivate the interface between the perovskite layer and HTL. We fabricated optimized inverted PSCs by spin-coating a diluted solution of PFBT onto the NiO layer and found that the device exhibited a significant improvement in FF and Voc in contrast to the control device, which resulted from the facilitated hole transport and improved MAPbI3 crystallization. A detailed characterization was performed to investigate the effects of PFBT on the enhancement of FF and PCE.

2. Experimental Nickel nitrate (Ni(NO3)2·6H2O, 99.9%) and 2-methoxyethanol (99.5%) were purchased from Aladdin. Lead iodide (PbI2, >99.99%), methylammonium iodide (MAI, ≥99.5%), and Bathophenanthroline (Bphen, 96%) were purchased from Xi’an Polymer Light Technology Corp. (China); [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM, 98%) was purchased from American Dye Source Inc., N, N-dimethylformamide (DMF, 99.9%), dimethyl sulfoxide (DMSO, 99.9%), and chlorobenzene (CB, 99%) were obtained from J & K Scientific. Ethanol 4

(99%) and tetrahydrofuran (THF, 99%) were purchased from Beijing Chemical Works (China). First, 290.79 mg of Ni(NO3)2·6H2O (1 mmol) was dissolved in 2-methoxyethanol (10 mL). After stirring at 50 °C for 1 h, 100 µL acetylacetone was added to the solution, which was then further stirred overnight at 25 °C. The PFBT solution was prepared by first dissolving the synthesized PFBT in THF to obtain a 1000 ppm solution. Next this solution was diluted produce solutions of 1 ppm, 2.5 ppm, 4 ppm, and 5 ppm, with the use of a solvent that was a mixture of DMF and THF (v:v = 1:1). Indium tin oxide (ITO) glass substrates were sequentially ultrasonically cleaned with acetone, ethanol and deionized water for 15 min, and then dried using nitrogen and treated in a UV-ozone oven for 10 min. A thin layer of NiO was fabricated by spin-coating a precursor solution onto the substrate at 3000 rpm for 45 s and annealing at 250 °C for 45 min in air. The PFBT solutions with different concentrations of 1 ppm, 2.5 ppm, 4 ppm and 5 ppm were spin-coated onto the NiO films at 4000 rpm for 8 s, and the fabricated films were named PF1, PF2.5, PF4, and PF5, respectively. To prepare the perovskite precursor solution, 461 mg of PbI2, 159 mg of MAI, and 78 mg of DMSO (molar ratio 1:1:1) were dissolved in 600 mg of DMF and stirred overnight at 30 °C. The MAPbI3 solution was filtered with a 0.22 µm PTFE filter and deposited onto the ITO/NiO/PFBT substrates at 4000 rpm for 30 s, followed by annealing at 100 °C for 10 min. After a spin-coating process of 10 s, 500 µL of diethyl ether was dropped onto the substrate to facilitate the crystallization of perovskite. Next PC61BM (20 mg/mL in CB) was spin-coated at 1000 rpm for 3 s and 5000 rpm for 30 s to provide the electron transport layer. Bphen (0.5 mg/mL in ethanol) was then spin-coated at 1000 rpm for 3 s and 3000 rpm for 30 s to provide the electrode modification layer. Finally, a 100 nm thick Ag film was thermally evaporated on top of the Bphen layer. The scanning electron microscopy (SEM) images were captured using an S-4800 scanning electron microscope at an acceleration voltage of 2 kV. The atomic force microscopy (AFM) images were captured using a Pico SPM instrument in tapping mode. The X-ray diffraction (XRD) patterns were found using a Rigaku Ultima IV 5

diffractometer with Cu Kα radiation. The PL spectra were measured with a fluorospectrophotometer using a 150 W Xe lamp as an excitation source. The current density-voltage (J-V) characteristics were measured under illumination and in a dark environment using a Keithley 2400 source meter with AM1.5 simulated solar light in air. The external quantum efficiency (EQE) was determined with a Newport Oriel EQE measurement kit. UV-vis absorption spectra were measured using a UV-visible photometer-1700.

3. Results and discussion To investigate the effect of PFBT on the photovoltaic performance of PSCs, the planar inverted PSCs in this study

were

fabricated

with

a

device

architecture

of

ITO/NiO/PFn

(n

=

1,

2.5,

4,

or

5)/CH3NH3PbI3/PC61BM/Bphen/Ag. Various concentrations of the diluted PFBT solutions were spin-coated onto the NiO layer to modify the surface of the HTL. Fig. 1 shows the configuration (Fig. 1a) and a cross-sectional SEM image (Fig. 1b) of the PSCs. The molecular structures of the electron transport material (PCBM) and the passivation material (PFBT) are also shown in Fig. 1(c). PFBT has a polyfluorene skeleton without additional side chains, and it presents good chemical stability [46].

Fig. 1. (a) Schematic of the device structure and (b) cross-sectional SEM image of the fabricated device; (c) chemical structures of PCBM and PFBT.

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Fig. 2. AFM images of (a) bare NiO, (b) NiO/PF1, (c) NiO/PF2.5, (d) NiO/PF4, and (e) NiO/PF5 with areas of 5×5 µm. The effect of PFBT on the surface morphology of the NiO substrate was analyzed using AFM, as shown by the top-view images displayed in Fig. 2. As revealed in Fig. 2(a-e), the root-mean-square roughness (RMS) values of the bare NiO, NiO/PF1, NiO/PF2.5, NiO/PF4, and NiO/PF5 surfaces were 0.589 nm, 0.656 nm, 0.785 nm, 1.30 nm and 1.78 nm, respectively. The increase of the RMS values with the increasing PFBT concentration demonstrates the improved adhesion of PFBT on NiO. Fig. S1(a-e) present the corresponding 3D images of the surface morphologies of bare NiO HTL and NiO/PFn. It should be noted that sharp peaks in the visualized area exhibited lager and bulkier structures as the concentration of PFBT was increased, indicating that improved PFBT coverage on NiO was achieved. In order to explore more details of the surface topography, 3D surface images of NiO/PF4 and NiO/PF5 substrates within a smaller area of 1×1 µm were captured, and the results are revealed in Fig. S2. Notably, the surface morphology of NiO/PF5 shows perfect cladding with no obvious sharp columns, whereas the NiO/PF4 layer still demonstrates slight peaks. Based on these appealing results, it was assumed that the polymer PFBT maintains a distribution of dots or aggregates on the NiO surface at concentrations of 1 and 2.5 ppm while a PFBT film is formed at 4 and 5 ppm.

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Fig. 3. SEM images of the perovskite layers on (a) NiO, (b) NiO/PF2.5, (c) NiO/PF2.5, (d) NiO/PF4, and (e) NiO/PF5. It is well-known that the film formation and surface morphology of the perovskite absorber layer play important role in solar cell performance. To understand the efficacy of the PFBT passivation layer on the perovskite crystallization, we performed SEM measurements (Fig. 3a-e), wherein the top-view images of the MAPbI3 films on the NiO and NiO/PFn substrates are shown. As can be clearly seen, the perovskite film grown on the bare NiO substrate possesses a relatively randomly packed morphology with large clusters of small grains located close together, resulting in pinholes in the film structure. However, when a perovskite film is deposited on top of a modified NiO substrate, sharper, smoother, and more compact crystalline grains are shown without the presence of pinholes. It is also worth noting that, the SEM images of the different perovskite films in Fig. 3(b-e) are relatively similar. Fewer haphazardly-stacked grains can reduce the production of loose grain boundaries to suppress carrier recombination, the formation of pinholes, and prevent current leakage [36]. Nevertheless, the perovskite films deposited on the passivation layers do not show significantly enhanced average grain sizes compared to the film deposited on the bare NiO layer.

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Fig. 4. XRD patterns of all MAPbI3 films based on bare NiO and NiO/PFn. To quantificationally verify the passivation effect of PFBT on the crystallinity and grain size of MAPbI3, the XRD patterns of the perovskite films were obtained, as shown in Fig. 4. The two diffraction peaks at 14.48° and 28.48° are associated with the (110) and (220) crystal planes of perovskite. It has been reported that higher intensities of these peaks indicate the improved crystallinity of the perovskite [47]. As shown in Fig. 4, the PF2.5-based perovskite exhibits the highest intensity of all diffraction peaks, indicating that this concentration produced the best crystallinity among all perovskite films. Moreover, the full width at half maximum (FWHM) value of the (110) diffraction peak of the PF2.5-based perovskite was calculated to be 0.152° compared with NiO-based perovskite of 0.167°, which suggests the presence of a larger grain size in the former [32,34]. Table 1. Parameters of the devices fabricated with and without different concentrations of a PFBT modification layer. Device

Voc (V)

Jsc (mA/cm2)

FF (%)

Control PF1 PF2.5 PF4 PF5

1.065 1.085 1.097 1.090 1.081

20.57 21.17 21.73 21.47 20.92

69.73 70.05 74.12 72.11 70.72

a

PCE (%) 15.27 16.08 17.67 16.88 16.00

Average PCE (%) a 14.75±0.58 15.56±0.47 17.21±0.43 16.42±0.56 15.38±0.55

Average PCE including the standard errors. The statistics are based on 16 cells with different

substrates for each HTL. 9

Fig. 5. (a) J-V characteristics and (b) EQE spectra of the control device and modified devices with PFn modification layers; (c) UV-vis light absorption spectra of the perovskite films on NiO with and without PF2.5, and (d) the corresponding dark J-V curves of all devices. The devices fabricated with different concentrations of PFBT were labeled as the control device, Device PF1 (1 ppm), Device PF2.5 (2.5 ppm), Device PF4 (4 ppm), and Device PF5 (5 ppm). In order to exclude the influence of the solvent mixture of DMF and THF, devices without the inclusion of PFBT were fabricated using this solvent mixture. The performances of these devices did not differ from that of the control device. Fig. 5(a) presents the J-V curves of fabricated devices based on the bare NiO and NiO/PFn composite layers, as well as the corresponding photovoltaic parameters. The detailed photovoltaic parameters, including the short current density (Jsc), Voc, FF, and PCE, are summarized in Table 1. Every sixteen devices were fabricated for each of the aforementioned PFBT concentration. As the concentration increased from 0 to 2.5 ppm, a significant improvement in the performance of the PSCs was shown. The device efficiency decreased, however, when the concentration exceeded 2.5 ppm. It should be noted that the performance of Device PF2.5 is visibly improved in relation to several paraments compared to the other devices, demonstrating a Voc of 1.097 V, Jsc of 21.73 mA/cm2, FF of 10

74.12%, and PCE of 17.66%, in contrast to the control device, which possessed a Voc of 1.065 V, Jsc of 20.57 mA/cm2, FF of 69.73%, and PCE of 15.28%. The improvement of FF could due to the Lewis base properties of PFBT which is able to modify non-bonding orbital Pb2+ [48]. The performances of Devices PF1, PF4, and PF5, were reduced when compared with that of the optimal Device PF2.5. Fig. 5(b) displays the EQE curves of control device and modified devices, in which every curve shows the same tendencies in terms of their J-V characteristics. The EQE intensity of Device PF2.5 is higher than the other devices across a large range from 400 to 750 nm, and it corresponds to the highest Jsc in Fig. 5(a). The integrated Jsc values that were are estimated from the EQE spectra (19.80 mA/cm2 for the control device and 20.52 mA/cm2 for Device PF2.5) are consistent with the Jsc values obtained from the J-V curves. In order to investigate the reasoning behind the EQE enhancement across such a the wide spectral range, the UV-vis absorption spectra of the perovskite films deposited on NiO both with or without PFn were obtained. As depicted in Fig. 5(c), the light absorptions of the NiO/PFn-based perovskite layer devices are obviously higher than that of the bare NiO device, and this contributes to the improved EQE and Jsc value of the former devices. Neither a red shift nor a blue shift in the absorption diagram can be observed. Figure 5(d) shows the dark J-V characteristics of the devices using semi-log coordinates in which the dark current densities of the PFn devices are shown to be lower than that of the control device at both a reverse bias and a low positive bias. These lower values indicate a decreased leakage current, signifying that charge carrier recombination is suppressed by the PFBT interfacial aggregates, and this results in an increased photocurrent.

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Fig. 6. (a) PL spectra of different perovskite films spin-coated onto NiO substrates that were modified using different concentrations of PFBT and (b) an energy level diagram for the devices examined in this study. Theoretical schematics for the charge transport in (c) the control device and the PSCs modified with (d) PF1, (e) PF2.5, (f) PF4, and (g) PF5. (h) Detailed remarks for PFBT and photogenic carrier transport process. Fig. 6(a) shows the PL spectra of the perovskite films that were spin-coated onto the bare NiO and NiO/PFn. It can is shown that the perovskite film without a passivation layer demonstrated the lowest intensity, while increased peak value of perovskite films based on modified HTLs were obtained. This can be partly attributed to the higher crystallinity and reduced nonradiative recombination of these films [35,37]. Fig. 6(b) depicts an energy diagram of the devices examined in this study. Typically, PFBT is not compatible with MAPbI3 in terms of their energy levels. The HOMO energy level of PFBT (-5.9 eV) is lower than that of perovskite (-5.4 eV), which is likely to block the migration of holes and results in a large accumulation of holes accumulation at this interface. On the other hand, the LUMO energy level of the polymer is close to that of CH3NH3PbI3, which may lead to the 12

recombination of the photogenerated electrons that flow from the perovskite layer and accumulated holes, resulting in current and voltage losses [39]. However, both the decreased dark-current and increased Voc values have a positive effect on the performance of the modified devices. Based on these results, we propose a possible mechanism such that the incomplete coverage of PFBT on NiO may passivate the electron traps instead of hindering hole transport and/or increasing charge recombination. Fig. 6(c) illustrates the distribution of defects and traps, as well as how they influence charge transport. As shown in Fig. 6(d-e), PFBT aggregates are deposited on NiO to fill surface defects and depress nonradiative recombination at lower concentrations, and the photogenerated holes are capable of being transported into the NiO HTL. The polymer aggregates tend to form a layer when further increasing the PFBT concentration, and this will lead to more severe interface charge recombination due to the aforementioned energy-level mismatch. Thus, it can be concluded that for Device PF1, the sparse polyfluorene aggregates fail to eliminate most of the defects and traps, resulting in an inferior device performance than the optimal device modified with PF2.5. However, for Devices PF4 and PF5, the polymer aggregates begin to form an ultrathin layer, which results in the accumulation of holes at the interface between perovskite and PFBT, leading to a loss of charge. Furthermore, we propose a method to solve the problem of the nonwetting perovskite solution on the deposited PFBT. DMF was used to dilute the existing PFBT solution (that was dissolved using THF) to improve the wettability interactions between this polymer and the perovskite precursor solution. After adding DMF, the contact angle was significantly decreased, making it easier for perovskite to grow. In general, this provides a simple but clever method way to solve the issue of contact between the perovskite precursor solution and the underlying substrates.

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4. Conclusions In summary, a simple and reliable method for of interface modification was achieved by incorporating PFBT polymer aggregates into inverted PSCs as a dual-functional layer that serves to passivate the interfacial defects of between the perovskite/HTL and improve the perovskite crystallinity. Improving the perovskite film quality can effectively suppress nonradiative recombination, leading to a reduced charge recombination loss. Meanwhile, the passivated interface defects can further minimize the charge recombination at the surface of NiO. Consequently, the optimized PSCs that incorporated PF2.5 demonstrated a significantly enhanced Voc of 1.097 V, FF of 74.12%, and PCE 17.66%, in comparison with the other tested devices. Our work provides a simple strategy by involving the use of polyfluorene as a modification layer to achieve high efficiency PSCs.

Acknowledgments The authors are grateful to the National Natural Science Foundation of China (61875072), National Postdoctoral Program for Innovative Talents (BX20190135), the Special Project of the Province-University Co-constructing Program of Jilin Province (SXGJXX2017-3), Project of Graduate Innovation Fund of Jilin University (101832018C018), and International Cooperation and Exchange Project of Jilin Province (20170414002GH, 20180414001GH) for the support to the work.

Statement The authors declare that there is no conflict of interest.

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The low cost and simple polymer modified is introduced between the anode and perovskite layer to reduce the interfacial defects and improve contact.

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