J. Phys. Chem. C 2009, 113, 5879–5885
5879
Incorporation of Thienylenevinylene and Triphenylamine Moieties into Polythiophene Side Chains for All-Polymer Photovoltaic Applications Guangyi Sang,†,‡ Erjun Zhou,† Yu Huang,†,‡ Yingping Zou,† Guangjin Zhao,†,‡ and Yongfang Li*,† Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China, and Graduate UniVersity of Chinese Academy of Sciences, Beijing 100039, China ReceiVed: January 1, 2009; ReVised Manuscript ReceiVed: February 10, 2009
We report all-polymer photovoltaic cells using poly(1,4-dioctyloxyl-p-2,5- dicyanophenylenevinylene) (DOCNPPV) as electron acceptor and a series of polythiophene (PT) derivatives P1 to P3 as electron donors. Among the polymer donors, P2 and P3 are new PT derivatives with functionalized and conjugated tri(thienylenevinylene) (TTV) and triphenylaminevinylene (TPAV) side chains, and they are characterized by absorption spectroscopy, cyclic voltammetry, and hole mobility, as well as morphology measurements. Photovoltaic results indicate that the device performance is sensitive to the presence of functionalized side chains within the molecular structure. To be specific, P1, without the functionalized side chains, yields the lowest power conversion efficiency (PCE) while P3, with both the TTV and TPAV side chains, shows a 2-fold increase in efficiency over P2 with only the TTV side chain, reaching a PCE of 0.44% under simulated AM 1.5 illumination at 100 mW/cm2. We attribute the enhancement of PCE to the improved absorption and enhanced hole mobility, as well as a better morphological structure of P3. 1. Introduction Since the discovery of photoinduced charge transfer (PCT) in composites of conjugated polymers and fullerenes in 1992,1 a large amount of effort has been given to exploit this key process in organic photovoltaics, aiming to fabricate low-cost, plastic, and high-efficiency solar cells. To date, the most efficient organic solar cells are fabricated with blends of regioregular poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). Power conversion efficiency (PCE) as high as 4-5% has been achieved.2-4 However, further efficiency improvement is severely held back by the inadequate photon harvesting of the sunlight. Particularly, fullerenes, which act as the acceptor in the photoactive layer, only absorb photons within a very limited range of the solar spectrum. A solution to this problem is using a polymer as the acceptor to construct all-polymer photovoltaic devices. This concept has some attractive advantages, for example, in a polymer blend, both active materials can exhibit a high optical absorption coefficient and are capable of covering the complementary part of the solar spectrum. Besides, for an all-polymer photovoltaic system, it is relatively easy to tune both components individually to optimize optical properties and charge transport, as well as charge collection processes, in device operation. During the past decade, several polymer electron accepters have been developed and actively involved in photovoltaic applications.5-7 Nevertheless, due to low stability when exposed in ambient environment, the choices of electron-accepting polymers are not as versatile as electron-donating ones. In addition, because of the low carrier mobility and small exciton diffusion length, the all-polymer solar cells have delivered rather low efficiencies. To date, the highest PCE of the all-polymer * To whom correspondence should be addressed. E-mail:
[email protected]. † Institute of Chemistry, Chinese Academy of Sciences. ‡ Graduate University of Chinese Academy of Sciences.
Figure 1. Molecular structure of DOCN-PPV.
solar cells is only 1.5-1.9%,6,8-10 far from that of the best PCEs of the polymer/fullerene solar cells. Therefore, development of the electron-accepting polymer is urgently needed and still remains a big challenge. In our previous work,11 a novel electron-accepting polymer DOCN-PPV (see Figure 1) was synthesized. Preliminary results revealed its potential role as an acceptor in photovoltaics (high electron affinity of 3.65 eV and a relatively broad absorption from 400 to 625 nm). In this work, we report all-polymer bulkheterojunction photovoltaics based on DOCN-PPV as the acceptor. For polymer solar cells, as we know, when the acceptor is fixed, the choice of suitable donors becomes vital. To maximize the PCE and investigate the relationship between the donor’s molecular structure and device performance, three polythiophene derivatives, P1, P2, and P3 (see Scheme 1) bearing functionalized side chains are deliberately chosen for comparison. Inspired by our group’s previous work and the work of others,12-15 conjugated tri(thienylenevinylene) (TTV) and triphenylaminevinylene (TPAV) moieties are introduced to functionalize the side chains of the polymers. Side chains containing these conjugated electron-rich groups can be regarded as a conjugation extension of the polymer backbone and we expect that the insertion of these functional groups could increase the
10.1021/jp9000048 CCC: $40.75 2009 American Chemical Society Published on Web 03/17/2009
5880 J. Phys. Chem. C, Vol. 113, No. 14, 2009
Sang et al.
SCHEME 1: Synthetic Route of Monomer C and Polymers P1 to P3
overall effective conjugation length and thus improve the sunlight absorption. Another consideration for choosing triphenylamine (TPA) is to enhance the hole mobility of the polymer, since TPA is a strong hole-transporting unit and the hole mobility is also a key concern in photovoltaic material design. 2. Experimental Section 2.1. Materials. Lithium diisopropylamide (LDA), Pd(PPh3)4, and (n-C4H9)3SnCl were purchased from Alfa and Aldrich. Toluene, THF, and DMF were freshly distilled prior to use. The following compounds were synthesized according to the literature procedures: (thiophen-2-ylmethyl)phosphonic acid diethyl ester (1),16 (2,5-dibromothiophene-3-ylmethyl)phosphonic acid diethyl ester (2),17 and 2,5-bis(tributylstannyl)thiophene (3).18 2.2. Instruments and Measurements. 1H NMR (400 MHz) spectra were measured on a Bruker spectrometer. Absorption spectra were taken on a Hitachi U-3010 UV-vis spectrophotometer. The molecular weight of polymers was measured by the GPC method, and polystyrene was used as a standard. Electrochemical cyclic voltammetry was conducted on a Zahner IM6e electrochemical workstation. Current-voltage (I-V) measurement of the PSCs was conducted on a computercontrolled Keithley 236 source measure unit. A Xenon lamp with an AM1.5 filter was used as the white light source, and the optical power at the sample was 100 mW/cm2. The measurement for the hole and electron mobilities was conducted in the dark by the space charge limited current (SCLC) method. The thickness of the active layer in the devices was determined by a surface profilometer (XP-2). To mimic the device fabrication, films for morphology study were spin-coated on PEDOT/
PSS-covered ITO glass. AFM images were obtained with a Digital Instruments nanoprobe atomic force microscope in the tapping mode. 2.3. Device Fabrication. The photovoltaic devices were fabricated with a bulk-heterojunction configuration. Poly(3,4ethylene dioxythiophene)/poly(styrene sulfonate) (PEDOT:PSS) was spin-coated on indium-tin oxide (ITO) coated glass followed by 150 °C baking in air for 40 min before the substrates were transferred to a nitrogen-filled glovebox. Photoactive layers were prepared via spin-coating a blend of polymer donor/ DOCN-PPV solution in 1:1 w/w ratio in chlorobenzene. The thickness of the photoactive layer is about 60 nm as measured by an Ambios Technology XP-2 surface profilometer. Finally, the cathode, composed of a Mg layer (∼15 nm) capped with an Al layer (∼100 nm), was evaporated through a shadow mask to define an active area of 4 mm2 in high vacuum (
