Role of balanced charge carrier transport in low band gap polymer:Fullerene bulk heterojunction solar cells

Abstract

Lowering of the optical band gap of conjugated polymers in bulk heterojunction solar cells not only leads to an increased absorption but also to an increase of the optimal active layer thickness due to interference effects at longer wavelengths. The increased carrier densities due to the enhanced absorption and thicker active layers make low band gap solar cells more sensitive to formation of space charges and recombination. By systematically red shifting the optical parameters of poly[2-methoxy-5-(3 0 ,7and 6,6-phenyl C 61 -butyric acid methyl ester, we simulate the effect of a reduced band gap on the solar cell efficiencies. We show that especially the fill factor of low band gap cells is very sensitive to the balance of the charge transport. For a low band gap cell with an active layer thickness of 250 nm, the fill factor of 50% for balanced transport is reduced to less than 40% by an imbalance of only one order of magnitude. The efficiency of organic polymer:fullerene bulk heterojunction solar cell performance has been steadily increasing in the last years, going from 2.5% efficiency in 2001 1 to 3.5% in 2003, 2 up to 5.5% in 2007, 3,4 and recently an efficiency of 7.4% has been reported. 5 Part of the improvement originated from the refinement of existing production techniques to optimize the active layer morphology.6-8 The majority of the improvement, however, came from the development of low band gap polymers. 3,4,[9][10][11][12][13][14][15][16][17] By decreasing the bandgap of the donor, the amount of absorbed photon flux increases due to an enhanced overlap with the solar spectrum. A lowering of the polymer band gap can either be achieved by a lowering of the lowest unoccupied molecular orbital (LUMO) or by a raise of the highest occupied molecular orbital (HOMO). The lowering of the LUMO is limited by the energy offset needed for electron transfer to the acceptor, typically 0.4 eV. A raise of the HOMO on the other hand will lead to a decrease of the open circuit voltage V oc . A theoretical study by Koster et al. 18 showed that a decrease of the bandgap from 2.1 to 1.5 eV, by lowering the LUMO and keeping the HOMO in place, is expected to result in an increase in power conversion efficiency g from 3.5% to over 8%. This enhanced performance mainly originated from an increased short circuit current density J sc due to an increased absorption. However, an increased absorption because of a better spectral overlap with the solar sprectrum also leads to an increased concentration of electrons and holes in the solar cells. This enhancement of the carrier density then gives rise to an increase of the bimolecular recombination, which is proportional to the product of the electron and hole density. Furthermore, the higher carrier densities make the solar cells also more sensitive for the built-up of space charge, caused by an imbalanced charge transport. Space charge built-up is also dependent on the absorption profiles in the solar cell, which were not included in these calculations. To obtain more in...

Formulation strategies for optimizing the morphology of polymeric bulk heterojunction organic solar cells: a brief review

“…In OSCs, the charge carrier mobility is an important factor to optimize: both high mobility and a balance between electron and hole mobilities were shown to be beneficial. 36,[38][39][40][41] For some polymers, an increase in crystallinity is also generally accompanied by a modification in their UV-visible absorption spectra. For example, P3HT exhibits a red shift and the appearance of additional vibronic bands when the degree of crystallinity increases in the BHJ.…”

Origin of high fill factor in polymer solar cells from semiconducting polymer with moderate charge carrier mobility

“…In general, among all of the three device performance parameters, FF is the most sensitive one, which is closely related to the bulk properties of the photoactive layer and the interfacial properties between each layer. Specifically, as revealed by many experimental works and device modeling studies [14,15], the FF of PSCs is primarily limited by the charge carrier transporting/collection properties [15,16], charge recombination [17][18][19], and space charge effects [20,21], where charge carriers mobilities [22][23][24][25] and the balance between them [26] play important role. Recently Guo et al demonstrated high FF (up to 80%) in PSCs by using highly ordered polymer with enhanced hole mobility on the order of 10 À3 cm 2 /V s as electron donor [27], highlighting the importance of hole mobility [28] and optimized film morphology in PSCs.…”

Charge transport properties of MDMO PPV thin films cast in different solvents

“…Hence, the proper choice of solvent or solvent mixtures is one of the important factors for tailoring the film morphology for facilitating the charge transport and enhancing the efficiency of the fabricated devices. In earlier reports, influence of film morphology on photovoltaic response of MDMO PPV in combination with various acceptors has been widely investigated . Even though, very limited information is available on the basic mechanisms of charge transport in pristine MDMO PPV thin films cast in different solvents .…”