Interfacial modification to improve inverted polymer solar cells Steven K. Hau,a Hin-Lap Yip,a Orb Acton,a Nam Seob Baek,a Hong Maa and Alex K.-Y. Jen*ab Received 12th May 2008, Accepted 14th August 2008 First published as an Advance Article on the web 1st October 2008 DOI: 10.1039/b808004f We report improved device performance of poly(3-hexylthiophene) (P3HT) and [6,6]phenyl C61 butyric acid methyl ester (PCBM)-based inverted bulk-heterojunction (BHJ) solar cells through the modified interface of the TiO2/BHJ with a series of carboxylic acid functionalized self-assembled monolayers (SAMs). The SAMs reduce the series resistance and improve the shunt resistance of the cell leading to increased fill factor and photocurrent density. Different aspects of device improvement can be affected depending on the nature of the SAMs. Modification with a C60-SAM shows the largest enhancement leading to a 35% improvement (h �� 3.78%) over unmodified inverted devices (h �� 2.80%). This SAM serves multiple functions to affect the photoinduced charge transfer at the interface to reduce the recombination of charges, passivation of inorganic surface trap states, improve the exciton dissociation efficiency at the polymer/TiO2 interface as well as a template to influence the overlayer BHJ distribution of phases, morphology and crystallinity leading to better charge selectivity and improved solar cell performance. Introduction Organic photovoltaic devices have attracted considerable interest as promising low-cost, solution processable alternatives to inorganic-based photovoltaic devices.1,2 The power conversion efficiency of polymer : fullerene based bulk-heterojunction devices has reached as high as 5%. The high perfomance was achieved through the optimization of phase segregation in the blend and the development of new materials to allow better p���n interfaces and balanced hole and electron charge transport.3���7 Until recently, most of the cells studied are based on the conventional device architecture which consists of a poly(3,4- ethylenedioxythiophene) : poly(styrenesulfonate) (PEDOT : PSS) hole-transporting layer and a bulk-heterojunction layer sandwiched between a high work function, hole collecting con- ducting transparent metal oxide electrode and a low work function, electron collecting metal electrode. However, the low work function metal electrode in this device geometry can be easily oxidized in air leading to deterioration in performance. To address this problem, a new device architecture using a titanium oxide or zinc oxide buffer layer between the organic active layer and the Al electrode has been used as a hole blocking and oxygen barrier layer to improve the device stability.8���11 The device performance can be further improved by the modification of the metal oxide/metal interface using a self-assembled mono- layer.12,13 Another way to circumvent this problem is to develop new device architectures that enable the use of a more air stable, high work function metal as the back electrode. Recently, inverted device geometries using high work function metals (Ag, Au) as the hole collecting electrode and metal oxides (TiOx, ZnO) as the electron collecting contact have been reported.14���19 We have already demonstrated improved ambient device stability using this inverted device architecture which retains over 80% of its original conversion efficiency after 40 days exposure to the atmosphere.20 Besides the advantage of using a less air-sensitive electrode, the inverted device geometry avoids the need for using PEDOT : PSS at the ITO interface which has been shown to degrade performance due to chemical instabilities at the inter- face.21 In addition, high work function metals offers the possi- bility for using non-vacuum techniques such as lamination or printing to deposit the top electrode.22,23 Another advantage of the inverted device geometry is that if an n-type metal oxide is utilized with a bulk-heterojunction blend, the metal oxide can provide additional interfaces for exciton dissociation which can lead to an increase in photocurrent generation.19 However, compared to the conventional device architecture, the inverted structures tend to have lower fill factors and photocurrent densities due to the un-optimized morphology of the bulk- heterojunction and poor charge selectivity at the electrode contact interfaces.16,18 The balanced charge transport and bulk resistance in each layer of an organic solar cell is extremely important to minimize charge recombination which will lead to the loss of perfor- mance.24���27 The resistance in each layer must be minimized not only in the active layers, but also at the interfaces between layers. Appropriate electrical contacts between interfaces are important in order to determine the short-circuit current density (Jsc), open- circuit voltage (Voc), and fill factor (FF) device characteristics of a solar cell. An ideal solar cell device should have low series resistance (Rs) and high shunt (parallel) resistance (Rsh) in order to optimize the device performance characteristics mentioned above. The series resistance reflects the ohmic loss in the entire device which is from a combination of the contact resistance and charge transfer rate at the interface as well as the bulk resistance of the active material. The shunt resistance reflects the loss of aDepartment of Materials Science and Engineering, University of Washington, Seattle, WA, 98195, USA. E-mail: ajen@u.washington.edu. Fax: (+1) 206-543-3100 Tel: (+1) 206-543-2626 bDepartment of Chemistry, University of Washington, Seattle, WA, 98195, USA This journal is �� The Royal Society of Chemistry 2008 J. Mater. Chem., 2008, 18, 5113���5119 | 5113 PAPER www.rsc.org/materials | Journal of Materials Chemistry

charge carriers due to current leakage pathways and recombi- nation of charges in the bulk or at the interfaces. One potential approach to simultaneously improve the morphology and charge selectivity of the inverted devices is to modify the interface between the inorganic and organic layer with a self-assembled monolayer (SAM). SAMs have been shown to significantly change the interfacial properties of various oxide and metallic surfaces. They can be used to improve adhesion, compatibility, and charge transfer properties at the interface to reduce back charge recombination. In addition, they can also be used to control the upper layer growth mode and distribution of phases, passivate inorganic surface trap states, and shift the interfacial energy offset between donor���acceptor materials.28���33 However, the majority of the work using SAMs to modify the interface of organic solar cells are centered around dye-based carboxylic acids on TiO2 for dye-sensitized solar cells or for inorganic���organic heterojunction cells.34,35 There have been only a few attempts to use them to modify the interface of organic bulk-heterojunction cells.31 TiO2 has been shown to be a good hole blocking and electron selective contact in inverted solar cells.18 The lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) of TiO2 have been reported to be 4.4 eV and 7.6 eV, respectively.36 This allows TiO2 to function as a good interfacial layer between ITO and the bulk-heterojunction blend for inverted solar cell devices. Here we demonstrate the improved performance of P3HT : PCBM-based inverted bulk-hetero- junction solar cells through the appropriate SAM modification on the electron collecting TiO2 interface. The improved device performance is due to the reduction of series resistance and improved shunt resistance of the cell which can be attributed to the improvement of the following three aspects: 1) reduction of the contact resistance between the inorganic TiO2 layer and active organic layer by passivation of surface trap states 2) enhancement of the electronic coupling between the inorganic TiO2 and active organic layer to mediate better forward charge transfer and reduce back charge recombination at the interface, and 3) affect the upper organic layer growth mode and morphology. We show that different aspects of device improve- ment can be affected depending on the nature of the SAMs. Experimental Materials Regioregular poly(3-hexylthiophene) (P3HT) was purchased from Rieke Metals, Inc. and was used as received without further purification. The [6,6]phenyl C61 butyric acid methyl ester (PCBM) was purchased from American Dye Source Inc. (99.0% purity), and was used as received without further purification. Other chemicals were purchased from Aldrich and used as received unless otherwise specified. 2,20:50,2%-Terthiophene- 5-carboxylic acid was synthesized following a method similar to the reported one.37 1H NMR spectra (300 MHz) were recorded on a Bruker-300 FT NMR spectrometer with tetramethylsilane (TMS) as internal reference. Elemental analyses were determined at QTI (Whitehouse, NJ). ESI-MS spectra were obtained on a Bruker Daltonics Esquire Ion Trap Mass Spectrometer. For the synthesis of C60-substituted benzoic acid (Scheme 1), a mixture of 4-carboxybenzaldehyde (0.210 g, 1.40 mmol), C60 (0.202 g, 0.28 mmol), and N-methylglycine (0.125 g, 1.40 mmol) in chlorobenzene (60 mL) was refluxed overnight under a nitrogen atmosphere. The solvent was removed by rotary evaporation under reduced pressure. The crude product was purified by silica gel column chromatography with toluene to toluene���THF (2 : 1) as the eluents to afford a brown-yellow solid (0.238 g, 95%). 1H NMR (300 MHz, DMSO-d6): d 2.20 (s, 3H), 6.65 (s, 1H), 6.89 (s, 2H), 8.03 (d, J �� 8.4 Hz, 2H), 8.15 (d, J �� 8.4 Hz, 2H), 10.12 (s, 1H). C70H11NO2: calcd C 93.64, H 1.23, N 1.56 found C 93.45, H 1.31, N 1.62%. ESI-MS (m/z): calcd. 897.1 found 897.0. Device fabrication To fabricate solar cells, ITO-coated glass substrates (15 U , 1) were cleaned in an ultrasonic bath with detergent, deionized (DI) water, acetone, and isopropyl alcohol and then dried under a N2 stream. The substrates were then treated with oxygen plasma for 10 min. Titanium isopropoxide diluted in n-butyl alcohol was spun onto ITO at 3000 rpm ( 40���50 nm). The films were annealed at 450 C for 30 min to allow the growth of the crys- talline anatase regions. The different monolayers were formed by immersing the sample in 0.1 mM solutions of either C60, ter- thiophene, benzoic, or lauric acid in THF���ethanol (1 : 1) over- night. Samples were annealed at 140 C for 20 min under N2 to promote the chemical bonding of the SAM to the TiO2 surface. After, the samples were sonicated in THF���ethanol for 5 min to remove any physically absorbed SAM molecules. The substrates were then transferred into an argon-filled glove box. The active layer was spun from a 40 mg mL 1 solution of P3HT : PCBM (1 : 0.8 by weight) in 1,2-dichlorobenzene at 900 rpm for 60 s in an argon-filled glove box. The film was allowed to slowly dry in a covered Petri dish as described by Li et al.4 and then thermally annealed at 150 C for 10 min in the glove box. PEDOT : PSS (Baytron 4083) diluted with isopropyl alcohol and n-butyl alcohol was spun onto the active layer and annealed at 120 C for 10 min under nitrogen before a second PEDOT : PSS ( 30��� 40 nm) layer was spun on top and again annealed at 120 C for 10 min under nitrogen. To complete the device structure, a 100 nm silver electrode was thermally evaporated ( 10 6 Torr) on top. Top contact organic field effect transistors (OFETs) and capacitance���voltage (C���V) samples (prepared on the same substrate as OFET) as well as X-ray diffraction samples were fabricated on heavily n-doped silicon substrates with a 300 nm thick thermally grown SiO2 dielectric (from Montco Silicon Technologies, Inc.). Procedures for TiO2, self assembly of C60 molecules, and active layer preparation were performed in the same way as the solar cell fabrication. Pristine P3HT and PCBM films were fabricated by spin-coating 1 wt% solution in Scheme 1 Synthesis of C60-substituted benzoic acid. 5114 | J. Mater. Chem., 2008, 18, 5113���5119 This journal is �� The Royal Society of Chemistry 2008