The blend ratio effect on the photovoltaic performance and stability of poly (3-hexylthiophene):[6,6]-phenyl-C₆₁ butyric acid methyl ester (PCBM) and poly(3-octylthiophene):PCBM solar cells

Abstract

The blend ratio effect on the photovoltaic performance of poly(3-hexyl thiophene) (P3HT):PCBM and poly(3-octylthiophene) (P3OT) solar cells was systematically investigated. In addition to electrical characteristics, the morphology of the blends was examined using Atomic force microscopy (AFM) since the performance of the organic solar cells depends on the morphological organization of donor and acceptor compounds within the bulk heterojunction active layer. The results revealed that decreasing PCBM content, the photovoltaic performances of both organic solar cells improved. The optical microscope images of P3OT:PCBM blend showed that the morphology resembles a needle structure. Both the sizes and the amount of needles decreased with decreasing PCBM loading. An important part of understanding degradation mechanism is proper evaluation of organic solar cell performance as a function of time. In this study, ISOS-D-3 Damp testing was carried for all devices. ISOS-L-1 testing indicated that the efficiencies and short circuit currents of all devices have small variations except for P3OT:PCBM (1:0.6) ratios.

Introduction

Bulk heterojunction solar cells made from conjugated polymers and fullerenes have gained popularity and seen a rapid progression of improvements in the techniques as detailed in a series of reviews [1], [2], [3], [4], [5], [6] and special issues [7], [8], [9], [10]. Significant progress in BHJ solar cells has been made by the synthesis of low band gap conjugated polymers and optimization of the device preparation conditions, including the application of adjusting the volume fractions of the components [11], [12], [13], thermal annealing treatments [14], [15]and solvent annealing [16], [17]. During the last two years, progress in polymer/fullerene based solar cells was so rapid that new efficiency records were published frequently via news articles or web pages before full publications could follow, as seen with the recent 7.6% efficiency world record announced by Solarmer [18]. It has been shown that both the conditions for processing of these mixtures from solution and post production treatment can have a large influence on the performance of these devices, because they affect the morphology and phase separation of the active layer [19], [20].

Materials with high absorption coefficients are necessary for applications in polymer solar cells. This is due to the requirement that the film thickness of photoactive layer should normally not exceed some hundreds of nanometers (100–300 nm). The absorption maximum (λ_max) of P3HT and P3OT can be observed at 500 nm (2.48 nm) and 512 nm (2.42 eV), respectively. These λ_max values when correlated with the energy of π→π^⁎ band transition indicate more pronounced planarity of the backbone chains for P3OT than for P3HT. This is due to the fact that longer side groups restrict the number of possible conformations. P3OT chains can be well-planarized, giving more extended π conjugation than P3HT, which may increase the device performance. In addition, polymers with longer side-chains can be better dissolved by solvent molecules. Consequently P3OT chains with longer side-chains have more expanded structures in solutions. So fullerene can move freely during the film formation that can lead to better charge transport properties.

In this study, we investigate the blend ratio effect on photovoltaic performance of P3HT:PCBM and P3OT:PCBM solar cells and also morphology of blends was studied. The photovoltaic parameters and morphology of P3HT:PCBM and P3OT:PCBM blends are also compared.