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

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1 Accepted Manuscript High performance thermal-treatment-free tandem polymer solar cells with high fill factors Shan-Ci Chen, Qingdong Zheng, Zhigang Yin, Dongdong Cai, Yunlong Ma PII: S (17) DOI: /j.orgel 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 thermaltreatment-free tandem polymer solar cells with high fill factors, Organic Electronics (2017), doi: / j.orgel This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

2 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 , P. R. China * Corresponding author. Tel.: ; fax: 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, TiO x, 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:PC 71 BM subcells. When adopting a rear PTB7-Th:PC 71 BM subcell, the highest PCE of 10.79% was further obtained for the tandem devices. The thermal-treatment-free process is especially 1

3 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 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 2

4 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 3

5 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 (V OC ), short current density (J SC ), 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 MoO 3 /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 4

6 tandem PSC, obtained by stacking two identical PTB7:PC 71 BM 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:PC 71 BM rear subcell instead of PTB7:PC 71 BM. 2. Experimental 2.1 Materials and Instruments PTB7 and PTB7-Th were purchased from 1-Material. PC 71 BM 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 MoO 3 (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-O 3 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 o C) 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:PC 71 BM blend (wt/wt, 1:1.5) was dissolved in 5

7 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:PC 71 BM, 2% DIO (vol%) was added. The solution containing PTB7:PC 71 BM or PTB7-Th:PC 71 BM was spin-coated inside the glovebox. Finally, 10 nm MoO 3 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 MoO 3 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 mm 2. 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 6

8 shown in Figure 2. When using PTB7:PC 71 BM as an active layer, the device with ZnO as the ETL yielded a V oc = 0.74 V, a J sc = ma/cm 2 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 V oc = 0.76 V, a J sc = ma/cm 2 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 V oc of 0.81 V and a higher J sc, 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:PC 71 BM/MoO 3 /metal/pfn/ptb7:pc 71 BM/MoO 3 /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 (R s ) and shunt resistance (R sh ) 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 R s, Table 2) or the current leakage (increased R sh, Table 2). 7

9 Therefore we chose MoO 3 /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% (V oc = 1.50 V, J sc = 9.43 ma/cm 2 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:PC 71 BM homo tandem cells were constructed by stacking two identical PTB7:PC 71 BM 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 J sc = 9.65 ma/cm 2, V oc = 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 J sc 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 8

10 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 nm which is in agreement with the absorption spectrum of PTB7. The integrated photocurrents from EQE spectra of single-junction and tandem devices are ma/cm 2 and 9.17 ma/cm 2, which are in correspondence with the J sc 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 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:PC 71 BM is higher than that based on PTB7:PC 71 BM, since the blend films PTB7-Th:PC 71 BM gives a wider absorption range toward longer wavelength and slightly higher absorption coefficient in relative to that for PTB7:PC 71 BM.[14] It is beneficial for light absorption of tandem cells to use PTB7-Th:PC 71 BM as an active layer for the rear subcell. So we made another tandem PSCs with the following configuration: ITO/PFN/PTB7:PC 71 BM/MoO 3 /Ag/PFN/PTB7-Th:PC 71 BM/MoO 3 /Ag (Tandem 2 in Figure 3). As expected, the resulting tandem cells exhibit a higher J sc of ma/cm 2 than that of Tandem 1 which indicated enhanced light absorption. Tandem 2 devices show a higher V oc than Tandem 1 devices since the V oc of PTB7-Th:PC 71 BM-based single-junction cell is larger than that of 9

11 PTB7:PC 71 BM-based one (0.81 V vs 0.76 V). Both types of tandem cells generate a V oc that is equal to the V oc 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:PC 71 BM 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:PC 71 BM 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. 10

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15 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. 14

16 Figure 3. J-V characteristics of single-junction and tandem devices. Figure 4. Absorption (a) and EQE curves of PTB7:PC 71 BM-based single-junction and homo-tandem cells (Tandem 1). 15

17 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. 16

18 Table 1. Device performances of single-junction and tandem cells using different ETLs or different donor polymers. Single-junction V oc [V] J sc [ma/cm 2 ] FF [%] PCE (ave.) [%] ZnO (PTB7) (8.12 ± 0.37) PFN (PTB7) (8.69 ± 0.11) ZnO (PTB7-Th) (8.78 ± 0.22) PFN (PTB7-Th) (9.24 ± 0.12) Tandem ZnO (Tandem 0) (9.72 ± 0.15) PFN (Tandem 1) (10.34 ± 0.13) PFN (Tandem 2) (10.43 ± 0.36) Table 2. Device performances of tandem cells using different interconnecting metal layers. V oc [V] J sc [ma/cm 2 ] FF [%] PCE (ave.) [%] R s (Ω cm 2 ) R sh (kω cm 2 ) Ag (9.38 ± 0.12) Al (7.42 ± 0.31) Au (8.02 ± 0.20)

19 Highlights High performance thermal treatment free 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.