Unveil the Full Potential of Integrated-Back-Contact Perovskite Solar

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Unveil the Full Potential of Integrated-Back-Contact Perovskite Solar Cells Using Numerical Simulation Teng Ma, Qingwen Song, Daisuke Tadaki, Michio Niwano, and Ayumi Hirano-Iwata ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00044 • Publication Date (Web): 28 Feb 2018 Downloaded from http://pubs.acs.org on February 28, 2018

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ACS Applied Energy Materials

Unveil the Full Potential of Integrated-Back-Contact Perovskite Solar Cells Using Numerical Simulation †

‡

§

∥

†§

Teng Ma,*, Qingwen Song, Daisuke Tadaki, Michio Niwano, Ayumi Hirano-Iwata , †

Advanced Institute of Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai

980-8577, Japan ‡

School of Advanced Materials and Nanotechnology, Xidian University, No. 2 South Taibai

Road, Xi’an 710071, China §

Laboratory for Nanoelectronics

and Spintronics, Research

Institute of Electrical

Communication, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan ∥

Tohoku Fukushi University, Kunimi-ga-oka, Aoba-ku, Sendai 989-3201, Japan

Corresponding Author *E-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT: The technologies of perovskite solar cells (PSCs) have been developing rapidly. After 8 years of research, the quantum efficiency of PSCs based on the planar sandwich structure has been approaching 100% in the visible light region. In order to further improve the performance of PSCs, adopting integrated-back-contact (IBC) structure, which is expected to be able to reduce light loss, to the PSCs is a promising option. In this work, a numerical simulation method is, for the first time, used to verify the applicability of the IBC structure to PSCs. We have investigated the factors that may affect the power conversion efficiency of the IBC-PSCs, to demonstrate that IBC-PSCs are advantageous over the traditional sandwich PSCs when we use small contact width (≤5 µm) of the IBC-PSCs and reported characteristics of perovskite films (mobility≥10 cm2V-1s-1, lifetime≥1 µs) in the simulation. By optimizing the properties of perovskite films, we can fabricate IBC-PSCs with 11% of improved performance over that of the sandwich type PSCs. The present results provide guidelines for the design and fabrication of highly efficient IBC-PSCs.

KEYWORDS: numerical simulation, perovskite solar cells, integrated-back-contact structure, sandwich structure, light loss


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As a strong candidate of the 4th generation photovoltaics, the perovskite solar cells (PSCs) have demonstrated overwhelming growth in power conversion efficiency (PCE). The PCE of PSCs has caught up with and surpassed that of traditional thin-film solar cell technologies, such as copper indium gallium selenide (CIGS), amorphous silicon, in only 8 years.[1] And because the perovskite materials are composed of earth-abundant elements and are solution processible, the fabrication cost of PSCs can be significantly suppressed. These advantageous characteristics have been attracting much attention from scientists and engineers. Recently, most of researches on PSCs were based on a sandwich p-i-n or n-i-p structure, as shown in Fig.1a.[2-8] Researchers have improved the performance of the PSCs by fabricating high-quality perovskite films,[3-8] modifying the interfaces,[9-12] tuning the composition of the perovskite materials.[13-16] In the visible light region, the internal quantum efficiency of PSCs has been approaching 100%.[7,8,11,17] Therefore, it is difficult to further improve the PCE of PSCs based on the traditional sandwich structure. In order to break through the bottleneck, fundamental change to the structure is needed. As shown in Fig.1a, in the traditional sandwich structure, the incident light enters the solar cells from the upper side, through the glass substrate, transparent conductive oxide (TCO), and hole transporting layer (HTL) or electron transporting layer (ETL), and is finally absorbed by the perovskite layer. When the light transpass these layers, part of the photons would be reflected, scattered or absorbed, and would not contribute to the photocurrent of the PSCs. Though it is possible to use anti-reflection coatings to reduce the reflection at the glass surface, the fabrication cost of PSCs will increase. The widely used HTL and ETL materials, such as TiO2, NiO, usually show strong absorption around the UV region (300~400 nm). Therefore this part of light can hardly reach the perovskite layer. In Fig. S1, we show the absorption spectra of bare


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and TiO2 coated ITO substrates. We can see that more than 10% of light loss is observed at the visible light region. At the short wavelength region, the light loss resulted from the substrate is even higher. Moreover, in large area PSCs modules, the electrode fingers and the interconnection strips which are needed to reduce the series resistance in the sandwich types solar cells, may introduce extra shading loss to the system. Therefore, it is possible to further improve the PCE of PSCs by reducing both of the light loss and shading loss.

(a)

ITO ETL Perovskite HTL Electrode

(b) ETL -

contact width

Perovskite HTL

+

-

+

ETL -

gap

Fig. 1 The structures of the sandwich type (a) and IBC type (b) PSCs. In silicon solar cells, the integrated back contact (IBC) structure was introduced to reduce the light loss and shading loss by integrating all the electrode at one side of the active layer, and have achieved the world record efficiency.[18] Because perovskite materials possess outstanding characteristics, such as high mobility and long charge lifetime, it will be possible to fabricate highly efficient IBC-PSCs. However, reported results of the IBC-PSCs were not as promising as we expected. Friends et al. have attempt to form IBC-PSCs using electrochemical deposited


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selective contacts.[19] But the PCE was quite low. Bach et al. formed the IBC electrode using photolithography process, and increased the PCE to 3.2%.[20] Yet the PCE was still not on par with that of sandwich type PSCs. In order to understand what has limited the performance of IBC-PSCs, and provide guidelines for the design and fabrication of the IBC-PSCs, we applied numerical simulation to investigate the effects of structural parameters, interfacial and bulk quality, on the performance of the IBC-PSCs. The structure of the IBC-PSCs used in this work is depicted in Fig. 1b. In this work, the basic perovskite material, CH3NH3PbI3 is used for the simulation. The optical, electrical properties of this material were extracted from other reports (see experimental details in Supplemental Information). NiO and TiO2, which have been widely used as charge selecting layers in sandwich type PSCs, are used as HTL and ETL respectively. The metal layers beneath the selective contacts, nickel and titanium, are used as the anode and the cathode, respectively. The ETL and HTL can be readily formed by oxidizing the surface of the metal electrodes. Since no further deposition or etch is needed, this process may help to reduce the fabrication cost of the IBC substrates. Incident lights enter the cell from the top surface and is immediately absorbed by the perovskite layer. Therefore, the light loss is significantly reduced compared to that of the traditional sandwich PSCs. Comparison of sandwich-type and IBC-type PSCs: In Fig. 2a, we show the calculated current density-voltage (J-V) curves of PSCs with the sandwich and the IBC structures. The calculated and reported performance parameters are summarized in Table 1. The short circuit current density (JSC), open circuit voltage (VOC), fill factor (FF) and PCE of the traditional sandwich PSCs are 21.7 mA/cm2, 1.04 V, 0.885 and 19.95%, respectively. The JSC, and PCE are comparable to those of the recent reports.[12,16,17] The VOC is slightly lower than that of the best


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published values,[16] since we use reported values for the charge transporting materials without further optimization. Because we used relatively low defect density (1014 cm-3) and long lifetime (10 µs), to calculate the J-V curve of the PSCs, the FF is slightly higher than other reported values.[2-4,7,9] Table 1. The reported performance parameters of sandwich-type PSCs and the calculated performance parameters of the sandwich and IBC PSCs with a 1 µm-thick CH3NH3PbI3 layer. The contact width and the gap for the IBC-PSCs are 1 µm and 200 nm, respectively. Type

Jsc (mA/cm2) Voc (V) FF

PCE (%)

Previous work12 22.5

1.04

0.73

17.1

Previous work16 22.8

1.11

0.77

19.6

Sandwich

21.7

1.04

0.885 19.95

IBC

24.3

1.06

0.880 22.77

Compared to the sandwich PSCs, the IBC-PSCs exhibit similar VOC and FF, suggesting that the IBC structure would not induce significant recombination in the perovskite bulk or at interfaces under ideal conditions (low defect density, long lifetime). Moreover, the IBC cells produced noticeably higher JSC (24.3 mA/cm2), leading to a largely improved PCE of 22.77%. We suppose that the increased JSC is mainly due to the reduced light loss in the IBC structure. In order to confirm these suppositions, we calculated the external quantum efficiency (EQE) of the PSCs based on the two different structures. As shown in Fig. 2b, the EQE of the sandwich cells is about 70%~90% in the absorption range, which is consistent with other reports.[3,4,7] In sandwich PSCs, the photons with short wavelength are partially reflect at the ITO surface (see


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Fig. S2), and then partially absorbed by the ETL layer (TiO2). Therefore, this part of light can

-2

(a)

Current density (mA•cm )

hardly reach the perovskite layer and contribute to the photocurrent. On the other hand, in the

25 20 15

IBC Sandwich

10 5 0 0.0

0.2

0.4

0.6

0.8

1.0

Voltage (V) 1.0

(b)

0.8

EQE

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.6

IBC Sandwich

0.4 0.2 0.0 300

400

500

600

700

800

Wavelength (nm) Fig. 2 J-V curves (a) and EQE (b) calculated based on IBC (black line) and sandwich (gray line) PSCs. IBC structure, the light loss is mainly due to the reflection at the perovskite surface. As a result, the EQE of the IBC-PSCs is higher in most of the absorption region. Even at the short wavelength region (300~380 nm), the EQE is higher than 75%. We can also notice a peak at 400


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nm in the EQE spectrum of the sandwich PSCs. This high EQE value is attributed to that the refractive index of TCO layer matched the refractive index of the TiO2 layer around 400 nm. And the TCO layer act as an anti-reflection layer and reduce reflection (also see Fig. S2). However, it is difficult to reduce reflection in the full absorption range of perovskite materials since the electrical, chemical, and optical properties of the layers should be optimized at the same time. Whereas in the IBC structure, we only need to consider the refractive index of the perovskite materials when designing the anti-reflection coating. Therefore, the IBC structure has advantage in light management over the sandwich structure. Effects of structural factors on the performance of IBC-PSCs: In PSCs, when light is absorbed by the perovskite layer, free charges generate. And the charges need to reach and to be collected by the electrodes to generate photocurrent. Because the contact width in IBC structures is normally much larger than the thickness of perovskite layer in sandwich structures, the charge recombination in the IBC-PSCs is expected to be much severer. In order to understand how and to what extent the structural parameters, such as the electrode width and the gap between the electrodes, affect the performance of the IBC-PSCs, we firstly fixed the gap between electrodes to 200 nm, and calculated the J-V characteristics of IBC-PSCs with different contact widths. As shown in Fig. 3a and Fig. S3a, there is no noticeable change of the VOC and the FF when the contact width varied from 1 µm to 100 µm; while the JSC significantly reduced from 24.3 mA/cm2 to 13.7 mA/cm2. As a result, the PCE also reduced from 22.7% to 12.8%. These results indicate that the increase of contact width mainly affect the JSC of IBC-PSCs. And the drop of PCE is mainly due to the decrease of JSC. Although it is shown that PSCs with smaller contact width will produce higher PCE, the fabrication cost of the IBC-PSCs may be too high for mass production, since the high-cost sub-micron photolithography process is needed to form the


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integrated back-contacts. In Fig 3a, we can also notice that the JSC and PCE only dropped by 0.7 mA/cm2 and 0.7% when the contact width increased from 1 µm to 5 µm. At this scale (5 µm), it is possible to form the integrated-electrodes by using low-cost fabrication process, such as printing technique.[21] Therefore, the IBC structure with a contact width of 5 µm may be a tradeoff between performance and fabrication cost. In the following simulations, the default value of

-2

(a)

Current density (mA•cm ) & PCE (%)

contact width is set to 5 µm if not otherwise specified.

25

1.1

20

1.0

15 0.9 10 Jsc Voc

5 0

2

4

PCE FF 6 8

1

2

0.8

Voc (V) & FF

0.7 100

4

6 8

10

-2

(b)

Current density (mA•cm ) & PCE (%)

Contact width (µm) 25

1.1

20

1.0

15 0.9 10 Jsc Voc

5 0 0.1

2

4 6

0.8

PCE FF 2

4 6

1

2

4

Voc (V) & FF

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.7

10

Gap (µm) Fig. 3 Performance parameters of IBC-PSCs with different contact widths (a) and gaps (b).


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Fig. 3b and Fig. S3b shows the effect of the dimension of the gap between the electrodes on the performance of IBC-PSCs. It is noticed that the VOC did not change much when we change the gap between electrodes. However, the JSC and PCE reduced from 23.6 mA/cm2 and 22.1 % to 4.7 mA/cm2 and 4.0 % as the gap varied from 0.1 µm to 50 µm. When comparing Fig. 3a with Fig. 3b, we can notice that the dimension of the gap shows more significant effect on the FF than the dimension of the contact width. This is due to the fact that the increase of the gap would lead to the reduction of electrical field between the electrodes, and further reduce the charge collection efficiency in solar cells.[22] Therefore, in order to achieve high efficiency using the IBC structure, we need to reduce the gap. Bach et al. have proposed a quasi-interdigitated electrode structure, in which two electrodes were separated by a thin insulating layer.[20] Since the gap is defined by the thickness of the insulating layer, the gap can be readily reduced to several hundred nanometers. By adopting this structure, we may ignore the effect of the gap to the IBC-PSCs according to our simulation results shown in Fig. 3b. Effects of bulk and interfacial defects on the performance of IBC-PSCs: In order to investigate the effects of the bulk defects on the performance of devices, we calculated the J-V curves of IBC-PSCs with different densities of defects. As shown in Fig. 4a and Fig. S4a, when the defect concentration is relatively low (1014~1017 cm-3), there is no noticeable difference in the J-V curve or the performance parameters. However, when the defect concentration is higher than 1018 cm-3, the VOC and FF quickly dropped from 1.06 V and 0.88 to 0.80 V and 0.64, as the defect concentration increase from 1017 to 1021 cm-3. This result indicate that the increase of defect concentration in perovskite film results in the increase of the recombination possibility during charge transporting process. At the same time, we can notice from Fig. 4a and Fig. S4a that the JSC slightly increased as the increase of defect concentration. This phenomenon can be


10

-2

(a)

25 20 3

Defect density (cm ) 15 10 5

10 10 10 10 10

0 0.0

0.2

14 18 19 20 21

0.4

0.6

0.8

1.0

Voltage (V) -2

(b)


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25 20 15 Surface recombination rate (cm/s) 1 10 100 10 1000 10000 5 0 0.0

0.2

0.4

0.6

0.8

1.0

Voltage (V) Fig. 4 J-V curves of IBC-PSCs with different defect densities (a) and surface recombination rates (b). attributed to the band narrowing effect induced by high defect concentration. However, the slight increase of JSC cannot compensate the significant reduction of VOC and FF. As a result, The PCE reduced from 22.0% to 11.8% as the defect density increased. In order to fabricate highly efficient IBC-PSCs, the defect concentration in the perovskite films should be lower than 1017 cm-3. In fact, perovskite single crystals grown by an antisolvent crystallization approach


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demonstrate low defect density on the order of 109 to 1010 cm-3.[23] And in multicrystalline perovskite thin films, the defect density is lower than 1017 cm-3.[24] Therefore, the effect of the bulk defect may be neglected in IBC-PSCs composed of high quality perovskite films. The interfacial defect may also affect the charge transporting process in IBC-PSCs. Here we use the parameter, surface recombination rate, to represent the recombination resulted by the surface defects. In sandwich structures, as shown in Fig. S5, the surface defect can hardly affect the performance of PSCs due to the short charge transporting path. However, for IBC-PSCs, the JSC, VOC and PCE significantly reduced from 23.7 mA/cm2, 1.06 V and 22.2% to 7.5 mA/cm2, 0.94 V and 5.7%, when the surface recombination rate at both sides of the perovskite layer increased from 1 cm/s to 10000 cm/s, as shown in Fig. 4b and Fig. S4b. In the IBC structure, the charge transporting path is much longer than that in the sandwich structures, and the surface defects distribute along the charge transporting path. Therefore, the IBC structure is more susceptible to the surface defects. When fabricating IBC-PSCs, we need to reduce surface defects and surface recombination rate. Previous reports have provided valuable knowledge about how to reduce surface recombination at the perovskite interfaces. Huang and colleagues has observed that perovskite materials tend to form small crystals and increase the surface defects on hydrophilic surfaces, whereas perovskite materials formed large crystal and reduced the surface defect on hydrophobic surfaces.[4] It is also reported that the surface defects could be passivated by coating the perovskite film with insulating or by treating the perovskite film in humid air.[25,26] And it is possible to reduce the surface recombination rate of the multicrystalline CH3NH3PbI3 film to 0.4 cm/s.[26] Effects of charge lifetime and mobility to the performance of IBC-PSCs: The charge lifetime and the mobility are closely related to the charge diffusion length in perovskite materials.[23,27,28]


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If the charge diffusion length is shorter than the contact width of the IBC device, charges generated in the perovskite layer may not be able to reach the electrodes before recombination. In order to understand how the charge lifetime and the charge mobility affect the performance of the IBC-PSCs, we firstly calculated the PCE of the IBC-PSCs with different combinations of contact width (1~100 µm) and lifetime (10-4~10-8 s). The result is illustrated as the 3-dimentional

Fig. 5 Variation of PCE of IBC-PSCs with different combinations of contact width and charge lifetime (a), and with different combinations of contact width and charge mobility (b). surface chart in Fig. 5a. It clearly demonstrates that the PCE is positively related to the lifetime, and it is negatively related to the contact width of IBC-PSCs. When the lifetime is long (100 µs)


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and the contact width is small (1 µm), the PCE can reach as high as 22.8%; on the other hand, when the lifetime is short (10 ns) and the contact width is large (100 µm), the PCE may decrease to 5%. Comparing to the PCE of the traditional sandwich structure (shown as the gray circles in Fig. 5a), the PCE of the IBC structure is more sensitive to the variation of lifetime. At the same time, we can notice that the IBC structure is clearly superior in performance than the sandwich structure when the devices have moderate contact width (≤5 µm) and lifetime (≥1 µs). It has been reported that the lifetime can reach 234 µs in perovskite single crystal and several microseconds in perovskite thin films.[28,29] Therefore, the IBC structure is highly possible to further improve the performance of PSCs. In Fig. 5b, we show the calculated PCE of the IBC devices with different combinations of charge mobility and contact width. The trend is similar to that shown in Fig. 5a. The IBC devices show superior performance over sandwich devices when the charge mobility is relatively high (≥10 cm2V-1s-1) and the contact width is relatively small (≤5 µm). Researchers have confirmed that the charge mobility in perovskite thin films can reach as high as 30 cm2V-1s-1.[30] Therefore, high performance is achievable based on the IBC structure. The grain boundaries in polycrystalline perovskite thin films may also affect charge lifetime, charge mobility, and performance of PSCs.[29,31] Since there are much more grain boundaries along the charge transporting path in the IBC structure than that in the sandwich structure, the IBC structure is more prone to the effect of grain boundaries. Therefore, it is very important to reduce the grain boundaries and increase the grain size of perovskite crystals to improve the charge lifetime, the mobility and the PCE of IBC-PSCs. The optimal situation is that the grain size is greater than the sum of the contact width and the gap. In this case, the two electrode is directly connected by a single crystal and charges do not need to go through any grain


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boundaries before being collected by one of the electrodes. There are several methods to increase the grain size of the perovskite thin-film, such as applying solvent annealing to perovskite thin films, using hydrophobic substrates, controlling the deposition conditions, and adding additive to the precursor solution of perovskite materials.[3-5,7,32] Previously, we have developed a vaporassisted high temperature process to increase the average grain area to more than 100 µm2.[33] When applying this method to fabricate the IBC devices with a moderate contact width (5 µm), the optimal charge transport and low fabrication cost could be achieved at the same time. Moreover, in large area solar cell modules, the IBC structure may show further advantage over the sandwich structure due to the reduced shading loss. In summary, we originally applied the numerical simulation to evaluate the applicability of IBC structure to PSCs. By comparing to the traditional sandwich structure, we demonstrated that the IBC structure is advantageous in light utilization and performance. We also investigated the factors that may affect the performance of IBC-PSCs. The results indicated that the structural parameters, including the contact width and the gap, are negatively related to the JSC and the PCE of IBC-PSCs, implying that the increased charge transporting distance reduced the charge collecting efficiency in the IBC structure. However, it is possible to reduce the gap by adopting the quasi-interdigitated electrode structure. It is also noticed that the performance of the IBCPSCs is, surprisingly, not so sensitive to the value of the contact width in a relatively wide range (1~5 µm). This enable us to fabricate the integrated electrode by using low-cost fabrication techniques, such as printing. We also illustrated that the defects in perovskite films and at the surface would not significantly affect the performance of the IBC-PSCs if the quality of the perovskite bulk and the interface is sufficiently high. Regarding the charge lifetime and the charge mobility in perovskite materials, the performance of the IBC-PSCs decreases more


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rapidly than that of the sandwich-PSCs as the lifetime or the mobility decreases. However, it is worth noting that when we use the reported parameters (lifetime≥1 µs, mobility≥10 cm2V-1s-1) of perovskite films and a relatively wide contact width (5 µm) of the IBC-PSCs in the simulation, the IBC structure shows apparent advantage over the sandwich structure. The results in this work unveil a new possibility to further boost the performance of PSCs, and provide valuable guidelines for the design and the fabrication of IBC-PSCs. ASSOCIATED CONTENT Supporting Information. Detailed experimental methods, parameters used for the simulations, and supplementary figures. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was partially supported by the CREST program “Development of Atomic or Molecular Two-Dimensional Functional Films and Creation of Fundamental Technologies for Their Applications” (grant number: JPMJCR14F3) of Japan Science and Technology Agency (JST). It was also partially supported by the cooperation research matching-funding from National Institute of Information and Communications (NICT), Japan. REFERENCES


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TOC GRAPHICS

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25 20 15

ETL -

10

+

ETL -

0.2

0.4

IBC

-

ITO ETL Perovskite HTL Electrode

5 0 0.0

Perovskite HTL

Sandwich +

0.6

0.8

1.0

Voltage (V)


22

Unveil the Full Potential of Integrated-Back-Contact Perovskite Solar

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