Improved charge transport in P3HT:PCBM bulk heterojunction PV cell under ambient environment July T. Bell, Genene T. Mola n School of Chemistry & Physics, University of KwaZulu-Natal, Pietermaritzburg Campus, Private Bag X01, Scottsville 3209, South Africa article info Article history: Received 26 October 2013 Received in revised form 17 December 2013 Accepted 21 December 2013 Available online 30 December 2013 Keywords: Organic PV cells Polymer P3HT:PCBM bulk heterojunction abstract Enhanced current density has been observed from P3HT:PCBM bulk heterojunction solar cell after a well sonicated solution was diluted with fresh chloroform solvent. The preparation of the devices was carried out in a simple laboratory environment without glove box and clean room. As the result of such current surge in the devices the efficiency has risen more than double compared to those devices without dilution of the P3HT:PCBM solution. We have recorded an average power conversion efficiency of 4.02 (5)% under this condition. This is an encouraging development toward achieving low cost organic photovoltaic devices. & 2013 Elsevier B.V. All rights reserved. 1. Introduction Organic photovoltaic (OPV) cell has emerged as possible sub- stitute to the inorganic based solar cell because of its ease of processability, light weight, flexibility and cheaper device produc- tion cost [1–12]. However, the low power conversion efficiency (PCE) and fast degradation of the organic molecule under ambient environment are the major challenges slowing the realization of OPV in the energy market. Several semiconductor polymers have been chosen and investigated for the purpose of using as an active layer of OPV. Among others the regioregular poly(3-hexylthio- phene) (P3HT) and [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM) are the most studied and highly promising molecules which have brought light to the realization of OPV [6–14] (Fig. 1). The relative ease of the molecular structure of P3HT has made it possible computational modeling which led to better understand- ing of the charge transport phenomena in polymers [15]. The power conversion efficiency of organic photovoltaic cell based on P3HT/PCBM blend active layer has reached close to 9% [4,7]. Such high power conversion is often obtained in special device pre- paration methods using argon gas environment in glove box as well as clean room. However, under ambient laboratory condition the efficiency of P3HT:PCBM photovoltaic cell is often less than 2.5%. The use of special treatment such as clean room and glove box would certainly increase the cost of device preparation. Therefore, intensive research efforts are still in progress to make power conversion efficiency of the OPV higher and at the same time reduce the cost of device preparation. In line with these efforts the realization of high power conversion efficiency at ambient laboratory condition was highly sought. Despite the low power conversion efficiency and durability of the organic photovoltaic cell there are still encouraging reports that suggest that these challenges may be possible to overcome [7–14,17]. In the current experiment, we employed bulk hetero- junction device structure which is the most successful OPV design that produces the highest efficiencies to date. Post-device pre- paration heat treatment, the use of different solvent, clean room preparations, etc. have made tremendous improvement on the power conversion efficiency of the cells. The performance of P3HT: PCBM based PV cell depends on the crystallinity of the polymer chains and the miscibility of the two molecules for the creation of high donor/acceptor interfaces for the effective dissociation of the excitons. A number of investigations have been reported on the effect of solvent on the over all performance of the devices [18,19]. An article by Chen et al. [19] indicated that the use of solvents mixture in the preparation of photoactive layer of OPV has improved the PCE by more than 20%. Therefore, in this paper, we discuss the effect of dilution of sonicated polymer solution with fresh solvent (unsonicated solvent) in the preparation of OPV devices. 2. Experimental We employed unpatterned ITO coated glass substrate which was etched in acid solution at the concentration of 48% HCl: 48% H2O: 4% HNO3 by volume. The substrates are then cleaned using deionized water, acetone and isopropanol by 10 min ultrasonication. After drying Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/physb Physica B 0921-4526/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physb.2013.12.034 n Corresponding author. E-mail address: mola@ukzn.ac.za (G.T. Mola). Physica B 437 (2014) 63–66