Band unpinning and photovoltaic model for P3HT:PCBM organic bulk heterojunctions under illumination Juan Bisquert a, * , Germà Garcia-Belmonte a , Antoni Munar a , Michele Sessolo b , Alejandra Soriano b , Henk J. Bolink b a Departament de Física, Universitat Jaume I, 12071 Castelló, Spain b Institut de Ciència Molecular, Universitat de València, Polígon La Coma s/n, 46980 Paterna, València, Spain article info Article history: Received 4 August 2008 In final form 8 September 2008 Available online 20 September 2008 abstract Capacitance analysis of P3HT:PCBM bulk heterojunction solar cells, in dark and under illumination, shows a linear Mott–Schottky characteristic at moderate reverse bias, indicating p-doping of the organic blend. The flatband potential under illumination is displaced negatively about 0.6 V with respect to dark conditions. A basic photovoltaic model is developed to explain this, in terms of electron transfer via sur- face states at the metal/organic interface. Surface states with a slow exchange kinetics, become charged under illumination, unpinning the band and decreasing the depletion layer at the electron extraction con- tact. This becomes a major factor limiting the performance of bulk heterojunction solar cells. Ó 2008 Elsevier B.V. All rights reserved. 1. Introduction Bulk heterojunctions formed by an interpenetrating blend of an optically active polymer and electron accepting molecules consti- tute a very promising route towards cheap and versatile solar cells [1]. The combination of poly(3-hexylthiophene) (P3HT) and [6,6]- phenyl C61-butyric acid methyl ester (PCBM) in organic blends has recently shown high photovoltaic performance [2]. P3HT dis- plays a relatively high hole mobility (10 4 to 10 3 cm 2 V 1 s 1 ) [3] and it was shown that P3HT:PCBM blends exhibit a remarkable improvement of performance when annealed, due to better order- ing of the polymer phase that enhances the hole conductivity [4,5]. Another important factor determining performance is the offset between the polymer HOMO level and the fullerene LUMO level, since that separation provides an upper limit to the photovoltage. Studies indicate the formation of a dipole as large as 0.6 eV at the interface between the two organic materials [6]. It is also recognized that P3HT can easily undergo p-doping when exposed to air or moisture [7–10], the doping level being re- lated to the oxygen concentration in the polymer film. The p-dop- ing opens the possibility of the formation of a Schottky barrier at the contact, which eventually may play an important role in carrier extraction. Recently, we showed that such a barrier can be detected by capacitance techniques in P3HT:PCBM blends [11], indeed a typical Mott–Schottky characteristic (MS, linear C 2 vs. V) is ob- served at reverse bias in the dark conditions. Previous studies [12] have found the MS characteristic in P3HT and the relation to the level of oxygen doping. It is well known that in a Schottky barrier the voltage across the metal–semiconductor interface is divided in two contributions, the space-charge region (SCR) in the semiconductor and the dipole layer in the surface. The latter layer may consist of a thin oxide layer in inorganic contacts [13,14] or a double layer in semiconductor–elec- trolyte contacts [15]. In general the dipole layer is characterized by a constant capacitance. Since this capacitance absorbs part of the volt- age, there is a shift of the conduction band energy at the interface, implying that the barrier for electron transfer, as indicated by the intercept of the MS plot, is less than the difference of work functions between the contacting materials. Surface states at the metal semi- conductor interface are known to determine many aspects of the electrical behavior of the Schottky barrier [16–18]. First, surface states produce a sheet of electrical charge that modifies the electro- static equilibrium of the interface. In addition, surface states, lying lower in energy than the conduction band states, often provide a preferential kinetic path for electron transfer from the semiconduc- tor to the contact material. An important effect of the presence of surface states was observed in the photoelectrochemistry of semi- conductor electrodes [15,19–21]. Under illumination, the concen- tration of minority carriers increases at the contact, with respect to the dark conditions. Thus at the same external voltage, the surface state acquires more charge under illumination, and the flatband po- tential determined from the MS plot changes [15]. In effect parallel MS plots are observed at different illumination light intensities [15,19–21]. Steady state models considering the band shift due to the surface state occupancy have been formulated [19,22]. Continuing our previous research on MS analysis of organic blends [11], we have analyzed the C – V characteristics of ITO/PED- OT:PSS/P3HT:PCBM/Al solar cells both in dark and under illumination and we observed a considerable (0.6 V) displacement 0009-2614/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2008.09.035 * Corresponding author. Fax: +34 964 728 066. E-mail address: bisquert@fca.uji.es (J. Bisquert). Chemical Physics Letters 465 (2008) 57–62 Contents lists available at ScienceDirect Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett