Stacking Sequence and Acceptor Dependence of Photocurrent

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Stacking Sequence and Acceptor Dependence of Photocurrent Spectra and Photovoltage in Organic Two-Junction Devices Sixing Xiong, Fei Qin, Lin Mao, Bangwu Luo, Youyu Jiang, and Yinhua Zhou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b05380 • Publication Date (Web): 29 Jun 2017 Downloaded from http://pubs.acs.org on June 29, 2017

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ACS Applied Materials & Interfaces

Stacking

Sequence

Photocurrent

Spectra

and

Acceptor

and

Dependence

Photovoltage

in

of

Organic

Two-Junction Devices Sixing Xiong, † Fei Qin, † Lin Mao, † Bangwu Luo, † Youyu Jiang, † and Yinhua Zhou †,║,*

†

Wuhan National Laboratory for Optoelectronics, and School of Optical and Electronic

Information, Huazhong University of Science and Technology, Wuhan 430074, China ║

Research Institute of Huazhong University of Science and Technology in Shenzhen,

Shenzhen 518057, China

1

ACS Paragon Plus Environment


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ABSTRACT Both single-junction and tandem organic photovoltaic cells have been well developed. A tandem cell contains two junctions with a charge recombination layer (CRL) inserted between the two junctions. So far, there is no detailed report on how the device will perform that contains two junctions but without a CRL in between. In this work, we report the photocurrent spectra and photovoltage output of the devices that contains two bulk-heterojunctions (BHJ) stacked directly on top each other without a CRL. The top active layer is prepared by transfer printing. The photocurrent response spectra and photovoltage are found sensitive to stacking sequence and the selection of electron acceptors. The open-circuit voltage of the devices (up to 1.09 V) can be higher than the devices containing either junction layer. The new phenomenon in the new device architecture increases the versatility of the optoelectronic devices based on organic semiconductors.

KEY WORDS:

Organic optoelectronic devices; layer stacking sequence; transfer

printing; photovoltage; photocurrent response spectra

2


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1. INTRODUCTION Organic solar cells have been attracting great attention because they hold the great advantage of easy processing and excellent mechanical flexibility.1-4 In the past years, device structure, new active layer materials, phase segregation inside the active layer, and device physics have been intensively studied.5-13 With these efforts, power conversion efficiency of the organic cells has been rapidly enhanced to about 10-12%.14-19 As for the device structure, both single-junction and tandem organic solar cells have been extensively developed. A single-junction cell contains one junction, for example, a bulk-heterojunction (BHJ) that comprises a mixture of a donor and an acceptor that can be solution-processed. A tandem cell contains two junctions with a charge recombination layer (CRL) inserted between the two junctions.20-23 An efficient CRL has to be highly transparent, robust enough to protect the pre-deposited layers, and with large work function contrast on its top and bottom surfaces for electron and hole collection respectively.24-26 So far, there is no systematic report on the devices hold the structure that contains two junction layers but without a CRL in between. Since the polymer donor:fullerene BHJ layers are processed from the similar solvents, the pre-deposited BHJ layer underneath could possibly be re-dissolved or damaged during the coating of the top BHJ layer from solutions. Therefore, it is challenging to fabricate a device containing two junctions on top of each other, without a CRL inserted between the two junctions. Transfer-printing is a method for film preparation wherein a film is first deposited on a medium substrate, and transferred onto a target substrate. This technique avoids the washing or damage issues arising from the use of non-orthogonal solvents since the film is transferred onto the target surface in dry. Previously, we have demonstrated to transfer print a 3



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polymer poly(3-hexylthiophene layer (P3HT) as the hole-blocking layer on top of the organic photoactive layer to suppress the reverse dark current and therefore enhance the detectivity of the organic photodetectors.27 In addition, we also have fabricated solar cells with transfer-printed both the active layer and the PEDOT:PSS electrode. The cells display comparable performance to those with spin-coated active layer and top electrode.28 Recently, we have also demonstrated large-area (10.5 cm2) solar cells with the transfer-printed top PEDOT:PSS electrode that yields high fill factor of 63% and power conversion efficiency over 6.5%.29 These results demonstrate the reliability of the transfer-printing the technique. In this work, we fabricate a new-type photovoltaic devices contain two BHJ layers that contact directly each other. The top BHJ layer was stacked on the underneath active layer via the transfer-printing technique. The two-junction devices exhibit new physical phenomenon that the photocurrent response spectra and photovoltage output are sensitive to stacking sequence and electron acceptors. The open-circuit voltage (VOC) of the two-junction devices reaches up to 1.09 V that is higher than the both single-junction reference devices with either of the BHJ junctions.

2. EXPERIMENTAL SECTION Device fabrication: Indium tin oxide (ITO) glass substrates (CSG Holding Co. Ltd., Shenzhen) were cleaned by ultrasonic baths of detergent in deionized water, deionized water, acetone and 2-propanol. Polyethylenimine ethoxylated (PEIE)2 was spin coated from a 2-propanol solution (IPA) solution at 5000 rpm for 1 min with a concentration of 0.1 wt.% and annealed at 100 °C for 5 min. The junction layer solutions include: P3HT: indene-C60 4


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bisadduct (ICBA) (1:1, weight ratio, total 40 mg/ml) was dissolved in chlorobenzene, P3HT: [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) (1:1, weight ratio, total 36 mg/ml) in chlorobenzene, poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b;4,5-b’]dithiophene2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl] (PTB7-Th, also known as PCE10): PC71BM (1:1.5, weight ratio, total 25 mg/ml) in a mixed solvent of chlorobenzene/1,8-diiodoctane (97:3 by volume) solution, and PDPP4T:PC71BM (1:2, weight ratio, total 9 mg/ml) in a mixed solvent of chloroform and o-dichlorobenzene (o-DCB) (92.5:7.5, volume ratio), respectively. The junction layer (JL) 1 was first deposited by spin-coating onto the ITO/PEIE substrate. The second junction layer (JL 2) was prepared by transfer printing. In brief, the JL2 solution was first spin-coated onto a clean silicon wafer. A piece of polydimethylsioxane (PDMS) was put down to adhere the JL2. The whole sample of silicon wafer/JL2/PDMS was dipped into deionized water for 10 s. The sample was taken out of the water and the JL2 was adhered on PDMS and peeled off from the silicon substrate. Meanwhile, the PDMS with JL2 was put down to the target surface of ITO/PEIE/JL1. The sample was transferred to glovebox and with a short-time (10 s) thermal annealing at 100 °C. PDMS is peeled off and the JL2 is left on the JL1 surface to finish the transfer printing. Here, the PDMS is used as the transfer medium. People might concern the PDMS residual may influence the solar cell performance. Actually, the surface of PEDOT:PSS that contacts with active layer for hole collection is the opposite side that contacts with the PDMS medium. The PDMS for PEDOT:PSS transfer printing will not affect the device performance. For the active layer, a short-time (10 s) air plasma treatment was done on the surface that contacts the PDMS. The surface was treated 5



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into the hydrophilic for the top PEDOT:PSS deposition, which indicates any residual has been removed since the PDMS is very hydrophobic. Thicknesses and coating information of these junction layers: P3HT:ICBA was spin coated at the speed of 1000 rpm for 40 s (about 200 nm), P3HT:PCBM at the speed of 1000 rpm for 60 s (about 200 nm), PCE10:PCBM at the speed of 1000 rpm for 60 s (about 100 nm) and PDPP4T:PCBM at the speed of 1000 rpm for 60 s (about 100 nm). Finally, these samples were transferred into the vacuum thermal deposition system (Mini-Spectros, Kurt J. Lesker), and a 7 nm-thick layer of MoO3 and a 70 nm-thick layer of Ag were evaporated to finish device fabrication. Device characterization: J-V characteristics were measured by using a Keithley 2400 source meter controlled by a LabVIEW program. The absorbance of the samples was measured by a UV-vis-NIR spectrophotometer (UV-3600, Shimadzu). The thickness of the film was characterized by a DEKTAK XT profilometer. The external quantum efficiency (EQE) spectra were measured by a standard system using a 150 W xenon lamp (Oriel) with a monochromator (Cornerstone 74004) as a monochromatic light source.

3. RESULTS AND DISCUSSIONS Device

structure

of

the

new

type

of

devices

with

two

junction

layers

(ITO/PEIE/JL1/JL2/MoO3/Ag) is shown in Figure 1a. The chemical structures of three different donor polymers and two acceptor fullerene derivatives used in this work are shown in Figure 1b. The three donor polymers are P3HT, PCE10, PDPP4T and the two acceptor materials are ICBA and PCBM. Their energy level is shown in Figure S1. These selected conjugated polymers and fullerene acceptors are widely used for photovoltaic devices.30-32 6


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The absorbance of the three donor materials are shown in Figure 1c. The absorption region of the P3HT, PCE10 and PDPP4T polymers ends at the wavelength of 650, 800, 900 nm, respectively.

3.1. Influence of stacking sequence on the photocurrent response and VOC Figure 2a shows the J-V characteristics of the single-junction cells and cells having two junctions with different stacking sequences. The cells with single layer of P3HT:ICBA (200 nm) and PCE10:PCBM (100 nm) display VOC of 0.83 V and 0.78 V, respectively. The VOC of the two-junction device with P3HT:ICBA (200 nm)/PCE10:PCBM (100 nm) (P3HT:ICBA is the bottom junction and PCE10:PCBM is the top junction) is 1.09 V, which higher than that of either single-junction cell (0.83 V and 0.78 V, respectively). It is also higher than that of the device with the junction layer of PCE10:ICBA layer (VOC is 0.92 V as shown in Figure S2). When reverse the stacking sequence, i.e. the PCE10:PCBM layer becomes the bottom junction and the P3HT:ICBA layer becomes the top junction. The device with the two junction layers of PCE10:PCBM (100 nm)/P3HT:ICBA (200 nm) displays a VOC of 0.8 V, which is between that of P3HT:ICBA-based (0.83 V) and PCE10:PCBM-based (0.78 V) single-junction devices (Table S1) . To further confirm the interesting phenomenon, we also tested another junction combination of PDPP4T:PCBM and P3HT:ICBA. Figure 2b shows the J-V characteristics of the corresponding single-junction cells and two-junction devices with different stacking sequences. The PDPP4T:PCBM (100 nm) single-junction layer device displays a VOC of 0.65 V. The two-junction cells with P3HT:ICBA (200 nm)/PPDPP4T:PCBM (100 nm) displays a VOC of 0.92 V that is also higher than that of the either of single-junction cells. When reverse 7



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the stacking sequence, the device exhibits a VOC of 0.74 V that is also between those of the two single-junction cells (Table S1). Therefore, these two examples both show that the VOC of the two-junction devices is sensitive to the stacking sequence. Furthermore, the VOC can be higher than that of either of the single-junction device. To exclude the possibility of the light illumination sequence on the junctions yielding the different VOC, we fabricated devices with both transparent electrodes that allow light could illuminate from both sides, and therefore change the light illumination sequence. Transparent conducting polymer poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is used as the top electrode.33-37 For the two-junction cells with P3HT:ICBA (200 nm)/PCE10:PCBM (100 nm) layer, the devices display similar VOC of 1.02 and 1.04 V when illuminated from different sides (Figure 3). The VOC of device with PEDOT:PSS electrode is comparable to that of the device with the evaporated MoO3/Ag electrode (1.09 V). The similar results are also observed for the P3HT:ICBA/PPDPP4T:PCBM case. The two-junction cells with P3HT:ICBA (200 nm)/PDPP4T:PCBM (100 nm) layer exhibits the similar VOC of 0.90 and 0.92 V when illuminated from different sides (Figure S3). The result indicates that the light illumination sequence don’t influence the open-circuit voltage in the two-BHJ structure. In addition, the short-circuit current (JSC) of the two-junction devices also exhibits dependence on the stacking sequence. The device with P3HT:ICBA/PCE10:PCBM layers shows a JSC of 6.06 mA/cm2, while the device with the PCE10:PCBM/P3HT:ICBA layers shows a JSC of 1.24 mA/cm2 (Figure 2a). The JSC of the two-junction devices is lower than that of either of the single-junction device. It is the same case for the P3HT:ICBA and 8


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PPDPP4T:PCBM-based two-junction devices. The lower current density and fill factor in the two-junction cells indicates the poor charge transport and severe charge recombination in junction layers or at the interfaces. To further understand the phenomenon, external quantum efficiency (EQE) characteristics of the devices were measured without light bias applied. The EQE curves of single layer device are shown in Figure S4. The absorbance of two-junction layers is slightly different when reversing the stacking layer sequence (Figure 4a and 4c). The EQE values and maximum peak positions are different when the stacking sequence was reversed in the two-junction devices. Another important feature is that the EQE spectra don’t cover the whole absorption region of the two junctions. Instead, the EQE spectra only cover the absorption region of the wider-bandgap junction layer. Here, the EQE only cut off at about 650 nm that corresponds to the absorption edge of the P3HT:ICBA (Figure 4b and 4d). The longer wavelength absorption of PCE10:PCBM or PPDPP4T:PCBM doesn’t contribute to extend the EQE response region. In addition, the integrated current based on EQE curves is lower than shot-circuit current measured by J-V measurement. The major reason is that the loss of longer wavelength absorption and the recombination in the two layer interface. For comparing with the properties of tandem cells, a tandem solar cell based on P3HT:ICBA and PCE10:PCBM junction layers and PEDOT:PSS CRL was fabricated.20 Figure 5 shows the J-V characteristics of the tandem cells. The tandem cell exhibits a VOC of 1.55 V that is close to the sum of that of the two single-junction cells. The VOC of the two-junction device (1.09 V) is higher than either of single-junction cells, but lower than the tandem solar cells (Figure 5a). In addition, EQE spectrum of the tandem cell was also measured without a light bias applied. The EQE spectra of the tandem and two-junction 9



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devices display similar shape. The spectral response region covers to about 650 nm that corresponds to the absorption region of P3HT:ICBA active layer (Figure 5b).The device with two-junction layers shows the features of the VOC (higher than either of the single-junction cells) and the EQE response region resemble the behavior of a tandem solar cell with a CRL between the junctions. These results indicate that the two-junction device with of P3HT:ICBA/donor:PCBM layers perform as “semi-tandem”.

3.2. Influence of acceptor on the photocurrent response and VOC So far, we have discussed the two-junction devices that have two different electron acceptors in the stacked junctions, i.e., ICBA and PCBM. For further study, we change the acceptors in the two junctions into the same one, i.e., PCBM. The P3HT:PCBM layer was employed as a junction to replace the previous P3HT:ICBA. The cells with the P3HT:PCBM (200 nm) delivers a VOC 0.56 V (Figure S5). For the P3HT:PCBM (200 nm)/PCE10:PCBM (100 nm)-based and PCE10:PCBM/P3HT:PCBM-based two-junction devices, they show VOC of 0.68 and 0.56 V, respectively (Figure S6). They are not higher than that of the PCE10:PCBM

single-junction

cell

(0.78

V,

Table

S1).

The

VOC

of

the

P3HT:PCBM/PCE10:PCBM-based two-junction device is 0.41 V smaller than that of the P3HT:ICBA/PCE10:PCBM-based

device

6a).

(Figure

Meanwhile,

the

P3HT:PCBM/PDPP4T:PCBM-based two-junction device exhibits a VOC of 0.63 V, which is also between their P3HT:PCBM and PDPP4T:PCBM-based single-junction cells (0.56 and 0.65

V,

respectively,

Table

S1).

It

is

0.29

V

smaller

than

that

of

P3HT:ICBA/PPDPP4T:PCBM-based two-junction device (Figure 6b). As shown in the figure, when ICBA in P3HT:ICBA was replaced by PCBM, the VOC decrease a lot, but the JSC 10


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did not exhibit the difference. This phenomenon of the VOC decrease a lot may be ascribed to the difference of the energy level. Figure S1 shows the LUMO of ICBA is 3.7 eV and that of PCBM is 4.0 eV. When ICBA in P3HT:ICBA was replayed by PCBM, the built-in potential decrease, thus, the VOC decrease a lot. However, when ICBA in P3HT:ICBA was replayed by PCBM, the absorption of active layer remain the same, that is the JSC did not change. Furthermore, the characteristics of the device with PCE10:PCBM/PDPP4T:PCBM and PDPP4T:PCBM/PCE10:PCBM layer are shown in the Figure S7. The PCE10:PCBM/ PDPP4T:PCBM-based and PDPP4T:PCBM/PCE10:PCBM-based devices display almost identical VOC. Therefore, when the acceptor of ICBA is replaced by PCBM, the phenomenon that VOC of the double-junction cells is higher than that of either single-junction cells is no more observed. Figure 7 shows comparison of the absorption and EQE of two-junction films and cells with different acceptors. The two-junction films display similar absorption including the spectra shape and peak positions (Figure 7a and 7b), which is easy to understand since the main absorption of the films contributes by the polymer donors rather than the fullerene acceptors. However, their EQE spectra are quite different when different acceptors are used. As discussed in the Part 3.1, when ICBA is used as the acceptor in the bottom junction, the EQE spectra cut off at about 650 nm that corresponds to the P3HT absorption region (Figure 7c and 7d). When PCBM is used as the acceptor in the bottom junction, the EQE spectra are extended to the absorption region of the low-bandgap polymer (Figure 7c and 7d), i.e. to 800 nm for P3HT:PCBM/PCE10:PCBM and to 900 nm for P3HT:PCBM/PDPP4T:PCBM. In the structure of P3HT:PCBM/PCE10:PCBM, P3HT:PCBM/PDPP4T:PCBM and PCE10:PCBM/ 11



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PDPP4T:PCBM, one acceptor and two donor materials have been included. Their EQE spectra are similar to those of previously reported ternary solar cells and photodetectors.38-41 To understand the photocurrent response spectra and photovoltage are sensitive to the stacking sequence and the electron acceptors, energy level diagram is provided in the Figure 8. When the P3HT:ICBA junction is placed in the bottom (adjacent to the ITO/PEIE electron-collecting electrode), the electrons generated in the top junction and holes generated in the bottom junction are energetically unfavorable when transporting to the cathode and anode, respectively (Figure 8a). Under simulated AM1.5 irradiation, electrons and holes are generated insides the both junctions in the meantime. Charge carriers generated inside a junction transport across the other junction would promote the built-in the potential. This can presumably be the reason for the higher VOC in the two-junction solar cells than that of either corresponding single-junction solar cells. When reverse the sequence, the electrons generated in the top junction and holes generated in the bottom junction become energetically favorable when transporting to the electrodes (Figure 8b). The energy offset during the charge transport will not contribute to the built-in potential. Therefore, when reverse the stacking sequence, the higher VOC will not observed in the two-junction devices any more. When the electron acceptor of ICBA in the bottom junction is replaced by PCBM, the electrons generated inside the top junction can transport to the bottom cathode energetically favorable since both junctions contain the same electron acceptor of PCBM (Figure 8c). Therefore, higher VOC is not observed in the two-junction devices, either. Since the electron transporting generated in the top junction is not blocked, the EQE is extended to the spectral

12


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region of corresponding absorption of low-bandgap junctions (PCE10:PCBM and PDPP4T: PCBM) as shown in the Figure 7c and 7d. Therefore, the stacking sequence and the selection of electron acceptors are the keys to obtain the high VOC via two stacked junction layers without a CRL. It should be noted that the clean interface between the two junctions is also critical to obtain the high VOC. We have also fabricated quaternary devices where the active layer consists of the mixture of P3HT, PCE10, ICBA and PCBM. Their J-V characteristics and EQE spectra are illustrated in Figure S8. The EQE response spectra match the absorption. The VOC is not higher than neither of those the single-junction cells, which is different from the P3HT:PCBM/PCE10:PCBM-based two-junction device. This confirms importance of the clean interface between the two junctions to obtain the high VOC.

CONCLUSIONS In this work, we report photocurrent response spectra and photovoltage output of devices that contains two stacked junctions on top of each other without a charge recombination layer between. The photocurrent response spectra and open-circuit voltage are found sensitive to the stacking sequence of the junctions and electron acceptors used in the junctions. The open-circuit voltage can be higher than that of either single single-junction cell if the stacking sequence and the acceptor are properly selected. The high open-circuit voltage in the two-junction devices presumably comes from the increased built-in potential. The photocurrent response spectra can be changed via varying the acceptor and the stacking

13



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sequence that may provide new strategy to tune the spectral response for photodetector applications.

ACKNOWLEDGEMENTS The work is supported by the Recruitment Program of Global Youth Experts, the National Natural Science Foundation of China (Grant No. 51403071), the Fundamental Research Funds for the Central Universities, HUST (Grant No. 2016JCTD111), and Science and Technology Program of Shenzhen (JCYJ20160429182443609).

ASSOCIATED CONTENT Supporting Information The supporting Information is available free of charge on the ACS Publication website. Open-circuit voltage information of device, energy diagram of materials, J-V characteristics and

EQE

spectrum

of

a

PCE10:ICBA

single-junction

device,

P3HT:ICBA/PDPP4T:PCBM-based two-junction device with PEDOT:PSS top transparent electrode; J-V characteristics and EQE spectrum of a P3HT:ICBA:PCE10:PCBM quaternary device,

a P3HT:PCBM

single-junction

PCE10:PCBM/PDPP4T:PCBM-based

device,

two-junction

PDPP4T:PCBM/PCE10:PCBM and devices;

the

PDPP4T:PCBM/PCE10: PCBM and PCE10:PCBM/PDPP4T: PCBM film.

AUTHOR INFORMATION 14


absorbance

of

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Corresponding Author *E-mail: [email protected]

Author contributions S.X.X. and Y.H.Z. conceived the project. S.X.X. and F.Q. carried out the characterization and fabrication of the two-junction devices. S.X.X., L.M., and B.W.L., performed the device optimization. Y.Y.J. participated in the data analysis. Y.H.Z. directed the work. S.X.X. wrote the initial draft of the manuscript. All the authors revised and approved the final version the manuscript.

Notes The authors declare no competing financial interest.

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Figure 1 (a) Device structure of stacking two junction layer without a charge recombination layer; (b) Chemical structure of donor polymers and acceptor used in the active layers; (c) Absorbance of the three donor materials.

(a)

(b) MoO3/Ag Junction layer (JL) 1 Junction layer (JL) 2 Glass/ITO/PEIE PDPP4T 1.2

(c)

PCE10 P3HT

PCE10

PDPP4T

1.0 Absorbance. (a.u.)

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


0.8 0.6 0.4

P3HT 0.2 0.0

400

500 600 700 800 Wavelength (nm)

900

19


ICBA

PC71BM


Figure 2 Current density-voltage (J-V) characteristics of devices with single junction layer (ITO/PEIE/Junction Layer/MoO3/Ag) and two-junction device (ITO/PEIE/Junction Layer1/Junction Layer2/MoO3/Ag). (a) devices with junction layers of P3HT:ICBA, PCE10:PCBM, P3HT:ICBA/PCE10:PCBM and PCE10:PCBM/P3HT:ICBA; (b) devices with junction layers of P3HT:ICBA, PDPP4T:PCBM, P3HT:ICBA/PDPP4T:PCBM and PDPP4T:PCBM/P3HT:ICBA. ITO/PEIE/Junction layers (JL)/MoO3/Ag

(b)

5

ITO/PEIE/Junction layers (JL)/MoO3/Ag

2

2

5

Current density (mA/cm )

(a) Current density (mA/cm )

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

Page 20 of 27

0 JL2/JL1

1.09 V

-5 JL1/JL2

-10 JL1

-15 -0.2

JL1: P3HT:ICBA JL2: PCE10:PCBM

JL2

0.0

0.2

0.4

0.6

Voltage (V)

0.8

1.0

0 JL2/JL1

-5 JL1/JL2

-10 JL1

-0.2

JL1: P3HT:ICBA JL2: PDPP4T:PCBM

JL2

-15

1.2

0.92 V

0.0

0.2

0.4

0.6

Voltage (V)

20


0.8

1.0

1.2

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Figure 3 J-V characteristics of devices with transparent top electrode illuminated from different sides (ITO/PEIE/P3HT:ICBA/PCE10:PCBM/PEDOT:PSS).

4 2


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


ITO/PEIE/Junction layer (JL) 1/JL2/PEDOT:PSS Illumintaion from ITO electrode Illumination from PEDOT:PSS electrode

2 0 -2 -4 JL1: P3HT:ICBA

-6

JL2: PCE10:PCBM

-8 -0.2

0.0

0.2

0.4

0.6

0.8

Voltage (V)

21


1.0

1.2


Figure 4 Absorbance of different single junction layer and two-junction layers: (a) P3HT:ICBA, PCE10:PCBM, P3HT:ICBA/PCE10:PCBM and PCE10:PCBM/P3HT:ICBA; (b) P3HT:ICBA, PDPP4T:PCBM P3HT:ICBA/PDPP4T:PCBM and PDPP4T:PCBM/P3HT: ICBA. EQE spectra of two-junction devices: (c) ITO/PEIE/P3HT:ICBA/PCE10:PCBM/ MoO3/Ag and ITO/PEIE/PCE10:PCBM/P3HT: ICBA/MoO3/Ag; (d) ITO/PEIE/P3HT:ICBA/ PDPP4T:PCBM/MoO3/Ag and ITO/PEIE/ PDPP4T: PCBM/ P3HT:ICBA/MoO3/Ag.

0.2 0.0 32

4 P3HT:ICBA/PCE10:PCBM PCE10:PCBM/P3HT:ICBA

24 2

16

8

0 300

400

500

600

700

800

(d)

0

0.4 0.2 0.0 32

24 2

16

8

0 300

Wavelength (nm)

4 P3HT:ICBA/PDPP4T:PCBM PDPP4T:PCBM/P3HT:ICBA

2

0.4

0.6

400

500

600

700

800

Wavelength (nm)

22


Integrated Current (mA/cm )

0.6

P3HT:ICBA PDPP4T:PCBM P3HT:ICBA/PDPP4T:PCBM PDPP4T:PCBM/P3HT:ICBA

0.8

EQE (%)

0.8

1.0

Abssorbance(a.u.)

P3HT:ICBA PCE10:PCBM P3HT:ICBA/PCE10:PCBM PCE10:PCBM/P3HT:ICBA

2

(c)

(b)

1.0


Absorbance (a.u.)

(a)

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

Page 22 of 27

900

0

Page 23 of 27

Figure 5 (a) J-V characteristics of two-junction devices (ITO/PEIE/P3HT:ICBA/PCE10: PCBM/MoO3/Ag) and tandem device with a CRL between the junction (ITO/PEIE/P3HT: ICBA/PEDOT:PSS/PEIE/PCE10:PCBM/MoO3/Ag). (b) EQE spectra of a two-junction and a tandem device.

(b)

2

Two-junction device Tandem solar cell 1.09 V

Tandem solar cell Two-junction device

60

2

5

6

50

1.55 V

EQE (%)

0

-5

40

4

30 20

2

10 0 300

-10 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

400

500

600

Wavelength (nm)

Voltage (V)

23




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


700

0 800


Figure 6 J-V characteristics of two-junction devices (ITO/PEIE/JL1/JL2/MoO3/Ag) with different

acceptors in the bottom junction: (a)

P3HT:PCBM/PCE10:PCBM and

P3HT:ICBA/PCE10:PCBM; (b) P3HT:PCBM/PDPP4T:PCBM and P3HT:ICBA/PDPP4T: PCBM.

(b) 4

ITO/PEIE/Junction layers (JL)/MoO3/Ag P3HT:PCBM/PCE10:PCBM P3HT:ICBA/PCE10:PCBM

2 1.09 V

0.69 V

0 -2

P3HT:PCBM/PDPP4T:PCBM P3HT:ICBA/PDPP4T:PCBM

2 0.63 V

0.92 V

0

-2

-4

-4

-6 -8 -0.2

ITO/PEIE/Junction layers (JL)/MoO3/Ag

2

2

4



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

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-6

0.0

0.2

0.4

0.6

0.8

1.0

-8 -0.2

1.2

0.0

0.2

Voltage (V)

0.4

0.6

Voltage (V)

24


0.8

1.0

1.2

Page 25 of 27

Figure 7 Absorbance of different single-junction layer and two-junction layers: (a) P3HT:ICBA, P3HT:PCBM, PCE10:PCBM, P3HT:ICBA/PCE10:PCBM and P3HT:PCBM/ PCE10:PCBM; (b) P3HT:ICBA, P3HT:PCBM, PDPP4T:PCBM, P3HT:PCBM/PDPP4T: PCBM and P3HT:ICBA/PDPP4T:PCBM; (ITO/PEIE/JL1/JL2/MoO3/Ag):

(c)

EQE spectra of different two-junction devices

P3HT:ICBA/PCE10:PCBM

and

PCE10:PCBM/

P3HT:ICBA; (d) P3HT:PCBM/PDPP4T:PCBM and P3HT:ICBA/PDPP4T:PCBM.

Absorbance(a.u.)

0.6 0.4 0.2 4

24

3

18 2 12 1

6 0 300

400

500

600

700

0.4 0.2

30 (d) 0.0

4 P3HT:ICBA/PDPP4T:PCBM P3HT:PCBM/PDPP4T:PCBM

2

P3HT:PCBM/PCE-10:PCBM P3HT:ICBA/PCE-10:PCBM

0.6

800

24

EQE(%)

30 0.0

0.8

2

0.8

P3HT:ICBA/PDPP4T:PCBM P3HT:PCBM/PDPP4T:PCBM P3HT:ICBA P3HT:PCBM PDPP4T:PCBM

1.0


Absorbance (a.u.)

(c)

(b)

P3HT:ICBA/PCE10:PCBM P3HT:PCBM/PCE10:PCBM P3HT:ICBA P3HT:PCBM PCE10:PCBM

1.0

3

18 2 12 1

6 0 300

0

400

500

600

700

Wavelength (nm)

Wavelength (nm)

25


800


(a)

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


900

0


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

Figure 8 Energy level diagrams of two-junction devices: (a) P3HT:ICBA/donor:PCBM; (b) donor:PCBM/ P3HT:ICBA; (c) P3HT:PCBM/donor:PCBM.

26


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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


ToC Graphic

MoO3/Ag Junction layer (JL) 1 Junction layer (JL) 2

VOC-JL1/JL2> max(VOC-JL1, VOC-JL2) Tunable photocurrent response spectra

Glass/ITO/PEIE

27


Stacking Sequence and Acceptor Dependence of Photocurrent

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