Bulk Heterojunction versus Diffused Bilayer: The Role of Device Geometry in Solution p-Doped Polymer-Based Solar Cells Anna Loiudice, †,‡ Aurora Rizzo,* ,‡,§ Mariano Biasiucci, ⊥,# and Giuseppe Gigli †,‡,§ † Dipartimento di Matematica e Fisica “Ennio De Giorgi”, Universita ̀ del Salento, via Arnesano, 73100 Lecce, Italy ‡ CBN - Center for Biomolecular Nanotechnologies, Italian Institute of Technology, Energy Platform, Via Barsanti sn -73010 Arnesano, Lecce, Italy § NNL CNR-Istituto Nanoscienze, c/o Distretto Tecnologico, via per Arnesano km.5, 73100 Lecce, Italy ⊥ NNL-CNR Istituto di Nanoscienze, c/o Dip. Fisica Ed. G. Marconi, La Sapienza University, Roma, Italy # Center for Life NanoScience@LaSapienza, Italian Institute of Technology, Viale Regina Elena 295, Roma, Italy * S Supporting Information ABSTRACT: We exploit the effect of molecular p-type doping of P3HT in diffused bilayer (DB) polymer solar cells. In this alternative device geometry, the p-doping is accomplished in solution by blending the F 4 -TCNQ with P3HT. The p-doping both increases the film conductivity and reduces the potential barrier at the interface with the electrode. This results in an excellent power conversion efficiency of 4.02%, which is an improvement of ∼48% over the p-doped standard bulk heterojunction (BHJ) device. Combined V OC -light intensity dependence measurements and Kelvin probe force microscopy reveal that the DB device configuration is particularly advantageous, if compared to the conventional BHJ, because it enables optimization of the donor and acceptor layers independently to minimize the effect of trapping and to fully exploit the improved transport properties. SECTION: Energy Conversion and Storage; Energy and Charge Transport T he control of the p- and n-type doping with well-defined levels of impurity represents one of the most successful strategies for the development of efficient both inorganic 1 and organic optoelectronic devices. 2-8 The addition of impurities with appropriate electronic properties leads to a shift of the Fermi level toward the transport states and to a reduction of the ohmic losses. 9 In the field of small-molecule-based devices, it has been demonstrated that the molecular doping can raise the conductivity by many order of magnitudes, well above the conductivity of pure materials. 10-13 This principle has been efficiently exploited in different kinds of optoelectronic devices, such as organic light-emitting diodes (OLEDs), 14 organic light- emitting field-effect transistors (OLEFETs), 15 and organic photovoltaic devices. 16,17 Motivated by the success of the p-type doping concept in small-molecule devices, the interest in the doping of conjugated polymers has been recently manifested through experimental and theoretical studies on the system formed by the electron donor poly(3-exylthiophene) (P3HT) doped with tetrafluoro- tetracyanoquinodimethane (F 4 -TCNQ), which is one of the strongest and widely used molecular electron acceptors. 18,19 Although these reports have proven that upon doping, conductivity values increase up to 5 orders of magnitude over pristine P3HT, 19 the implementation of the doping technology in polymer solar cells remains still almost unexplored. Some attempts in this direction have been made by incorporating the strong electron acceptor, tetracyanoquinodimethane (TCNQ), in a standard BHJ device geometry. Such experiments have proven that the TCNQ acts as an electron trap if directly blended in the active layer, leading to enhanced recombination losses and to lower photovoltaic performances. 20 Given that the controlled doping in small-molecule devices relies on the ability to fabricate heterostructured stacks to correctly position the doped layers, 21 the difficulty to built multilayers by solution processing hinders the development of the doping concept in polymer devices. In this Letter, we report improved power conversion efficiency (PCE) by solution-based p-type doping of the donor species in polymer solar cells. To efficiently exploit the doping effect, we propose an alternative device fabrication strategy, which consists of a sequential coating of the donor (i.e., P3HT) and acceptor (i.e., [6,6]-phenyl-C 61 -butyric acid methyl ester, PCBM) species from orthogonal solvents and results in a DB device. The doping of P3HT with the strong electron acceptor, F 4 -TCNQ, occurs via solution coblending prior the deposition. We demonstrate that the DB device configuration is particularly advantageous because it enables Received: June 12, 2012 Accepted: July 3, 2012 Published: July 9, 2012 Letter pubs.acs.org/JPCL © 2012 American Chemical Society 1908 dx.doi.org/10.1021/jz300754p | J. Phys. Chem. Lett. 2012, 3, 1908-1915