Accepted Manuscript

High performance thermal-treatment-free tandem polymer solar cells with high fillfactors

Shan-Ci Chen, Qingdong Zheng, Zhigang Yin, Dongdong Cai, Yunlong Ma

PII: S1566-1199(17)30199-4

DOI: 10.1016/j.orgel.2017.05.008

Reference: ORGELE 4079

To appear in: Organic Electronics

Received Date: 9 February 2017

Revised Date: 3 May 2017

Accepted Date: 4 May 2017

Please cite this article as: S.-C. Chen, Q. Zheng, Z. Yin, D. Cai, Y. Ma, High performance thermal-treatment-free tandem polymer solar cells with high fill factors, Organic Electronics (2017), doi: 10.1016/j.orgel.2017.05.008.

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High performance thermal-treatment-free tandem polymer solar

cells with high fill factors

Shan-Ci Chen, Qingdong Zheng*, Zhigang Yin, Dongdong Cai, Yunlong

Ma

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the

Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, P. R.

China

* Corresponding author.

Tel.: +86 591 63173282; fax: +86 591 63173282.

E-mail: qingdongzheng@fjirsm.ac.cn

Abstract

It is an effective way to enhance device performance of polymer solar cells (PSCs) by

using a tandem structure that combines two or more solar cells. For tandem PSCs, the

buffer layer plays an important role in determining the device performance. The most

commonly used buffer layers, such as PEDOT:PSS, TiOx, and ZnO, need thermal

treatments that are not beneficial for reducing the fabrication complexity and cost of

tandem PSCs. It is necessary to develop tandem PSCs fabricated by a

thermal-treatment-free process. In this paper, we report high performance

thermal-treatment-free tandem PSCs by developing PFN as buffer layers for both

subcells. A power conversion efficiency (PCE) of 10.50% and a high fill factor of

72.44% were achieved by stacking two identical PTB7:PC71BM subcells. When

adopting a rear PTB7-Th:PC71BM subcell, the highest PCE of 10.79% was further

obtained for the tandem devices. The thermal-treatment-free process is especially

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applicable to flexible devices, in which plastic substrates are usually used.

Keywords:

organic solar cells, PFN, tandem, PCE, thermal-treatment-free

1. Introduction

Polymer solar cell (PSC) is an excellent example of the third-generation solar cell

technologies with its promising advantages of low-cost, lightweight, large-area,

flexibility, and easy processing methods.[1-3] In the past decade, many efforts have

been devoted to improving the power conversion efficiency (PCE) such as the

innovation of donor or acceptor materials,[4-7] the morphology control of active

layers,[8, 9] the interface engineering and development of new device architectures

and device concepts.[10-13] Consequently, PCEs of state-of-the-art PSCs have been

rapidly enhanced in recent years.[14-20] Different with inorganic solar cells in which

electron-hole pairs are generated immediately upon light absorption, excitons

generated in PSCs upon light absorption have a short diffusion length in the range of

1–10 nm.[21] Only excitons that diffuse to electron donor-acceptor interfaces can

dissociate into holes and electrons. Restricted by this feature, the optimal film

thickness of the active layer of the state-of-the-art PSCs is normally less than 400

nm.[14-17, 22-27] The inherently thin active layer of PSCs is not enough to achieve

efficient light absorption compared with that of silicon-based solar cells which

typically employ an active layer thickness as great as 180-300 µm. The overall

absorption of the active layer for most high performance single-junction bulk

heterojunction (BHJ) devices is below 80% due to small film thickness.[23, 24] The

main challenge to achieve higher PCEs of PSCs is how to increase the amount of light

absorption while maintain efficient photo-generation of electrons and holes, and their

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collection efficiency at the respective electrodes.

In order to use solar radiation more effectively, a useful and universal strategy is to

make a tandem PSC in which two or more subcells are electrically connected in

series.[17, 28-32] Generally, two photoactive layers with complementary absorption

spectra are chosen for the two subcells in the tandem structure. On the other hand, the

absorption of incident photons is insufficient in a single-junction device and

increasing film thickness to elevate absorption will decrease the device performance.

One promising approach to realize both sufficient light harvesting and efficient charge

extraction is to stack two identical subcells in series forming the so-called

homo-tandem solar cells.[23, 24, 33-35] The active layers of two subcells can absorb

the sunlight independently, in consequence, the amount of light absorption is

enhanced while maintaining efficient photo-generation of the single-junction device.

So it is an effective way to maximize the photovoltaic efficiency of a given

active-layer system.

In a PSC, there are buffer layers between the active layer and the electrodes. The

buffer layers play a critical role in determining the performance of organic

photovoltaic devices.[10, 11] In tandem cells, they play another important role as they

are commonly part of the interconnecting layer (ICL) which serves as the charge

recombination zone between two subcells. Meanwhile, they are vital in realizing a

tandem structure since they protect the underlying layers against damage during the

device fabrication process. It is also important for industrial and commercial

considerations that the ICL can be processed under mild conditions. However, the

commonly used PEDOT:PSS and ZnO-based ICL materials require thermal

treatments up to 150 °C[17, 31, 32, 36] that may be detrimental to the underlying

organic layers. The high temperature procedure will also restrict their application in

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flexible devices and roll-to-roll manufacturing. In addition, the PEDOT:PSS-based

ICLs have encountered problems with strong acidity and optical loss. These features

are not favorable for industrial and commercial considerations of tandem PSCs. In

recent years, a group of water/alcohol soluble conjugated polymers has been

developed as an attractive electron injection/transport layers for PSCs. A

representative example is

poly[(9,9-bis(3’-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctyl

fluorene)] (PFN) which has been used as a buffer layer to simultaneously enhance

open-circuit voltage (VOC), short current density (JSC), and fill factor (FF) of PSCs.[16,

37] PFN can be deposited from solution without any further treatment, that making it

a good choice as ICL in tandem PSCs. Recently, an ICL comprising MoO3/Ag/PFN

was used for constructing high performance tandem PSCs.[38, 39] PFN was used as

an electron transporting layer (ETL) for the rear subcell in their works. However, for

the front subcell, the ETL used ZnO which needs high temperature treatment. It is

expected that a thermal-treatment-free tandem PSC can be achieved by using PFN as

an ETL for both subcells since its good performance in the single-junction PSCs. At

the same time, the fabrication complexity and cost of tandem PSCs will be reduced by

using PFN as ETLs for both subcells because no thermal treatment is needed in the

whole process. Surprisingly, little attempt has been made to fabricate

thermal-treatment-free tandem PSCs. In this work, we report a high performance

thermal-treatment-free tandem polymer solar cell with high FFs by using PFN as

buffer layers for both subcells. Unlike the reported buffer layer of PEDOT:PSS or

ZnO (which requires harsh thermal annealing at 120 to 200 °C), the buffer layers in

this work do not require any post-treatments. Consequently, the whole

thermal-treatment-free device fabrication process is realized for the tandem PSCs. The

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tandem PSC, obtained by stacking two identical PTB7:PC71BM subcells where PFN

was used as the ETL for devices with an inverted structure, exhibited a high PCE of

10.50% and a high FF of 72.44% (see Figure 1 for device and chemical structures).

Moreover, a higher PCE of 10.79% was further demonstrated by using a

PTB7-Th:PC71BM rear subcell instead of PTB7:PC71BM.

2. Experimental

2.1 Materials and Instruments

PTB7 and PTB7-Th were purchased from 1-Material. PC71BM was purchased from

American Dye Source. Zinc acetate dihydrate (99.9%), 2-methoxyethanol (99.8%),

and ethanolamine (99.5%) were purchased from Sigma-aldrich and MoO3 (99.9%)

from Alfa Aesar. PFN was purchased from Derthon. All the materials were used as

received.

2.2 Device Fabrication and Characterization.

For the single-junction devices, a layer of ZnO or PFN was deposited by spin

coating on top of the ITO glasses. ZnO films were fabricated on patterned ITO glasses

(∼15 Ω sq−1) by a facile sol−gel method. Firstly, the ITO glasses were cleaned by

ultrasonication sequentially in detergent, water, acetone, and isopropyl alcohol for 30

min each and then dried overnight in an oven. Then the ZnO precursor solutions (0.23

M in 2-methoxyethanol, ethanolamine as a stabilizer) were spin-coated on the top of

the ITO-glasses, which were pretreated by UV-O3 for 15 min. The films were first

annealed on a hot plate at 130 °C for 10 min. Then they were thermally annealed in an

oven (200 oC) for an hour. PFN films were fabricated in glovebox by spin-casting the

PFN solution (1 mg/ml in methanol, with small volume of acetic acid) with 3000 rpm.

The PTB7:PC71BM blend (wt/wt, 1:1.5) was dissolved in

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chlorobenzene:1,8-diiodooctane (CB/DIO, v/v, 97/3) with a concentration of 25

mg/ml. The mixture was stirred for a few hours at 50 °C. For the solutions of

PTB7-Th:PC71BM, 2% DIO (vol%) was added. The solution containing

PTB7:PC71BM or PTB7-Th:PC71BM was spin-coated inside the glovebox. Finally, 10

nm MoO3 and 100 nm Ag layers were thermally deposited under high vacuum.

For the tandem devices, the front subcell was prepared as stated above using PFN

as an ETL. The ICL was fabricated by thermal evaporation of 10 nm of MoO3 and 1

nm of Ag followed by the spin coating of PFN solution inside the glovebox. The rear

subcell was fabricated after PFN deposition following the same steps as for the

single-junction cells. The active area of single-junction or tandem PSC was fixed at 4

mm2.

Solar cell characterization was performed under AM 1.5 G irradiation (100 mW

cm−2) from an Oriel Sol3A simulator (Newport) with a National Renewable Energy

Laboratory certified silicon reference cell. The devices were tested without a mask.

After a simple encapsulation by epoxy kits (general purpose, Sigma Aldrich) in the

glove-box, the PSCs were illuminated through their ITO sides. Current

density−voltage (J−V) curves were tested in air by a Keithley 2440 source

measurement unit. External quantum efficiency (EQE) spectra were measured on a

Newport EQE measuring system. The reflection of the device was measured to

evaluate the total absorption of the device, and the absolute absorption of the devices

was calculated by (100-R)%. The reflection was obtained by using Perkin-Elmer

Lambda 950 UV-vis spectrophotometer.

3. Results and Discussion

The photovoltaic performances of single-junction PSCs with ZnO or PFN as the

ETL were investigated. The J−V characteristics of these single-junction devices are

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shown in Figure 2. When using PTB7:PC71BM as an active layer, the device with

ZnO as the ETL yielded a Voc = 0.74 V, a Jsc = 17.06 mA/cm2 and an FF = 67.53%,

resulting in a PCE = 8.53% (Table 1). Mainly due to a more efficient photon harvest

in the devices, the devices based on the PFN ETL show superior performance over the

ZnO-based one. A higher PCE of 8.83% was obtained for the device based on PFN

which exhibited a Voc = 0.76 V, a Jsc = 17.29 mA/cm2 and an FF = 67.46%. This

improvement may be attributed to the unique interface modification effect of the

amino group in PFN which can tune the work function of ITO.[16] The PCE of

single-junction devices can be improved further by replacing PTB7 with another

low-bandgap polymer PTB7-Th. The resulting devices for both cases with ZnO and

PFN ETLs showed a larger Voc of 0.81 V and a higher Jsc, resulting in a higher PCE of

9.17% (for ZnO) and 9.42% (for PFN).

To optimize the device fabrication procedure for the tandem cells, we first

investigated the photovoltaic performance of tandem PSCs with the following

configuration: ITO/ZnO/PTB7:PC71BM/MoO3/metal/PFN/PTB7:PC71BM/MoO3/Ag.

It was found that the ultra-thin Ag layer is important for efficient electron–hole

recombination between the front and rear subcells.[38, 39] Besides Ag, other two

typical electrodes Al and Au were also used for the tandem devices in this work. The

corresponding photovoltaic parameters including the series resistance (Rs) and shunt

resistance (Rsh) are summarized in Table 2. In both cases of using Al or Au as an

intermediate layer in ICL, the devices showed inferior performance than that of the

devices based on Ag. It may be attributed to the suitable work function of Ag and its

maximum transparency lies in the maximum solar activity range.[40] This ultrathin

layer of silver is required to establish an ohmic contact that can help diminish the

contact resistance (reduced Rs, Table 2) or the current leakage (increased Rsh, Table 2).

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Therefore we chose MoO3/Ag/PFN as the ICL in tandem cells for the further

optimization in this work. After optimizing the thickness of the active layer for the

both subcells (around 100 and 120 nm for the front and rear sub-cells, respectively),

the best performance tandem PSCs (defined as Tandem 0 hereafter) was obtained with

a PCE of 9.93% (Voc = 1.50 V, Jsc = 9.43 mA/cm2 and FF = 70.34%). This result is

better than that of the device with the same structure in the other report.[39] In this

kind of device, two subcells used different ETLs with each other. The layer of ZnO

which needs high temperature treatment was used as ETLs for the front subcell. In

order to construct a thermal-treatment-free tandem PSC, we adopted PFN as ETLs for

both subcells as there is no thermal-treatment needed. We fabricated tandem cells

based on the structure in Figure 1. Firstly, PTB7:PC71BM homo tandem cells were

constructed by stacking two identical PTB7:PC71BM subcells in series. The J–V

characteristics of tandem cells are shown in Figure 3. Device performance parameters

are summarized in Table 1. The best performance tandem device (Tandem 1 in Figure

3) displays a high PCE of 10.50% (with an average value of 10.34% based on over

eight cells), with Jsc = 9.65 mA/cm2, Voc = 1.50 V, and FF = 72.44%. Compared with

the controlled device which used the ZnO ETL for the front subcell, the PFN-based

devices (Tandem 1) show better performance with enlarged Jsc and FF. Compared

with the best performance of single-junction cells with PCE of 8.83%, the tandem

cells have achieved a ~15% enhancement in efficiency which mainly arises from the

essentially total light absorption (Figure 4a). This enhancement was confirmed by the

external quantum efficiency (EQE) results (Figure 4b). The EQE of the tandem device

is defined as the ratio of the total converted carriers by the two subcells to the sum of

the incident photons, and is estimated by measuring the photoresponse of the tandem

cell and then multiplying it by two to represent the total number of photons being

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converted to electrons.[23, 24, 34] The EQE of the whole tandem device increases

significantly compared to that of the single-junction device, especially in the region of

500–700 nm which is in agreement with the absorption spectrum of PTB7. The

integrated photocurrents from EQE spectra of single-junction and tandem devices are

16.51 mA/cm2 and 9.17 mA/cm2, which are in correspondence with the Jsc values

from the J−V measurements. To evaluate the quality of the PFN buffer layer, the

surface morphology of the layer was firstly examined by atomic force microscope

(AFM). The AFM height and cross-sectional images (Figure 5a) show a very small

root-mean-square (RMS) roughness of 0.313 nm, indicating a smooth and uniform

layer without the need of thermal-treatment. To investigate the vertical layer stacking

of the whole device, cross-sectional scanning electron microscopy (SEM) was

employed and the result is shown in Figure 5b. The two well-defined subcells and the

ICL are clearly observed, implying that the front subcell could be well protected by

the ICL.

As mentioned above, the PCE of single-junction devices based on

PTB7-Th:PC71BM is higher than that based on PTB7:PC71BM, since the blend films

PTB7-Th:PC71BM gives a wider absorption range toward longer wavelength and

slightly higher absorption coefficient in relative to that for PTB7:PC71BM.[14] It is

beneficial for light absorption of tandem cells to use PTB7-Th:PC71BM as an active

layer for the rear subcell. So we made another tandem PSCs with the following

configuration: ITO/PFN/PTB7:PC71BM/MoO3/Ag/PFN/PTB7-Th:PC71BM/MoO3 /Ag

(Tandem 2 in Figure 3). As expected, the resulting tandem cells exhibit a higher Jsc of

10.17 mA/cm2 than that of Tandem 1 which indicated enhanced light absorption.

Tandem 2 devices show a higher Voc than Tandem 1 devices since the Voc of

PTB7-Th:PC71BM-based single-junction cell is larger than that of

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PTB7:PC71BM-based one (0.81 V vs 0.76 V). Both types of tandem cells generate a

Voc that is equal to the Voc sum of the two single-junction cells, indicating the ICLs

work very well in both tandem cells.

Besides high efficiency, long-term device stability is also a particularly important

property for tandem PSCs. Therefore, we further examined the shelf-life stability of

these tandem PSCs with the PFN as ETLs for both subcells. The devices were

encapsulated with epoxy and stored in air. The device stability of two tandem PSCs

with different donor polymers for rear subcells as a function of storage time under

ambient conditions is shown in Figure 6. Both tandem PSCs with PFN cathode

interfacial layers exhibit good device stability, and their PCEs can maintain at

approximately over 85% of their original values even after storage in air for 200 days.

As a comparison, the shelf-life stability of Tandem 0 device which using ZnO as the

ETL for the front subcell was tested together. As shown in Figure 6, the three tandem

devices are basically the same stable in the first several months.

4. Conclusions

In conclusion, thermal-treatment-free polymer tandem solar cells using PFN as

ETLs were demonstrated. A PCE of 10.50% was achieved using PTB7:PC71BM

active layers in both subcells. A high FF of 72.44%, which exceeding that of the

corresponding single-junction cells, was also achieved. A higher PCE of 10.79% was

obtained when using PTB7-Th:PC71BM active layer for the rear subcell. This type of

tandem devices showed both high efficiency and good device stability, indicating a

promising way to develop high performance organic solar cells towards real

applications. In the tandem devices in this work, both buffer layers can be processed

without any thermal treatment which is beneficial for the fabrication of flexible

organic solar cells.

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Acknowledgements

This work was financially supported by the National Natural Science Foundation

of China (Nos. 61325026 and 51503209), the Natural Science Foundation of

Fujian Province (No. 2015H0050), the Key Research Program of Frontier Sciences,

Chinese Academy of Sciences (CAS).

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Captions for Figures

Figures and Schemes

Figure 1. a) Device structure of the tandem cell. b) Chemical structures of the donor polymers

and the conjugated polyelectrolyte PFN.

Figure 2. J-V characteristics of single-junction cells with different ETLs and donor polymers.

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Figure 3. J-V characteristics of single-junction and tandem devices.

Figure 4. Absorption (a) and EQE curves of PTB7:PC71BM-based single-junction and homo-tandem

cells (Tandem 1).

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Figure 5. a) AFM height (top) and cross-section (bottom) images of PFN. b) Cross-sectional SEM

image of the homo-tandem device.

Figure 6. Normalized device PCE versus storage time. The samples were encapsulated with epoxy

and stored in air.

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Table 1. Device performances of single-junction and tandem cells using different ETLs or different

donor polymers.

Voc [V] Jsc [mA/cm2] FF [%] PCE (ave.) [%]

Single-junction

ZnO (PTB7) 0.74 17.06 67.53 8.53 (8.12 ± 0.37)

PFN (PTB7) 0.76 17.29 67.46 8.83 (8.69 ± 0.11)

ZnO (PTB7-Th) 0.81 17.81 63.84 9.17 (8.78 ± 0.22)

PFN (PTB7-Th) 0.81 17.80 65.22 9.42 (9.24 ± 0.12)

Tandem

ZnO (Tandem 0) 1.50 9.43 70.34 9.93 (9.72 ± 0.15)

PFN (Tandem 1) 1.50 9.65 72.44 10.50 (10.34 ± 0.13)

PFN (Tandem 2) 1.55 10.17 68.60 10.79 (10.43 ± 0.36)

Table 2. Device performances of tandem cells using different interconnecting metal layers.

Voc [V] Jsc [mA/cm2] FF [%] PCE (ave.) [%] Rs (Ω cm

2) Rsh (kΩ cm

2)

Ag 1.52 9.09 69.70 9.60 (9.38 ± 0.12) 12.6 3.1

Al 1.53 10.01 51.97 7.98 (7.42 ± 0.31) 161.5 1.0

Au 1.46 8.79 64.31 8.23 (8.02 ± 0.20) 30.8 3.0

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HighlightsHighlightsHighlightsHighlights

High performance thermaltreatmentfree tandem polymer solar cells are fabricated.

PFN processed at ambient temperatures was used as buffer layers for both subcells.

A highest PCE of 10.79% was achieved.

Tandem cells exhibit higher fill factors compared to the corresponding single cells.