Correlation between P3HT inter-chain structure and Jsc of P3HT:PC[70]BM blends for solar cells
Graphical abstract
Highlights
► Relation between Jsc and the interchains distance of the polymer in the active layer is shown. ► Lamellar structure and interchains of P3HT is affected when the PC[70]BM is increased. ► Peaks in μ-XRD analysis confirm the presence of nano-domains in the polymer active layer.
Introduction
Most bulk heterojunction BHJ organic solar cells use as active layer, a blend of a conjugated polymer and a fullerene. Currently, this kind of organic solar cell is the most promising for the accomplishment of a low cost, light-weight, large area, flexible, easily processed, and renewable energy source [1]. The main progress in bulk heterojunction organic solar cells has been achieved by introducing new materials with low band gap, process and new technologies [2], [3], [4], [5]. Power conversion efficiencies (PCE) above 10% were first reported in 2012 [6]. The PCE of such devices is predominantly influenced by the optical and electronic properties of the donor and acceptor materials and the morphology of the blend controlling the domain size [7], [8], [9].
Having domain sizes well defined in the bulk of the active layer helps to have a good performance on the charge carrier transport across of their respective materials (i.e. holes in donor material and electrons in acceptor material) and finally be collected in their respective metal contacts. Result of the foregoing is a reduction of the charge carrier recombination during the transport, situation that is desirable in the photovoltaic devices.
Although blends of Poly-(3-hexylthiophene-2,5-diyl) (P3HT):Phenyl C70 Butyric Acid Methyl Ester (PC[70]BM) have been much studied, not enough attention has been paid to the study of P3HT inter-chains with the PC[70]BM and to the relation between the short circuit current density (Jsc) and the morphology, since the process of the charge generation and separation should strongly depend on the interface properties, while the charge carrier transport will be determined by the bulk properties of the components of the active layer as was describe above.
In this paper we will show that the quantity of fullerene in the blend modifies the interchain distance-spacing (d100) in P3HT associated with the interdigitated alkyl chains, which affects the Jsc. The crystalline structure of the active layers were analyzed using micro-X-ray diffraction. This information in combination with the Braggs’ law was used to calculate the distance-spacing in P3HT. The findings in distance-spacing in all blends together with complementary electrical characterization were subsequently correlated with the performance of the organic solar cells.
Section snippets
Experimental
Indium tin oxide (ITO)-coated glass substrates (with nominal sheet resistance of 15 Ohm/square and 120 nm of thickness) were purchased from PsiOTec Ltd. Poly-(Ethylene dioxythiophene) doped with Poly-(Styrene Sulphonic acid) (PEDOT:PSS) FHC was acquired from Ossila Ltd., P3HT (melting point 238 °C, Mw ∼ 17500 g mol−1, 99.995% region-regularity) and PC[70]BM (Mw ∼ 1030.99 g mol−1) were purchased from Sigma–Aldrich. High-purity (99.99%) silver (Ag) pellets were obtained from Testbourne Ltd., and calcium
Results and discussion
The J–V characteristics of the devices were measured in a sealed capsule under inert nitrogen atmosphere immediately after removing from the vacuum system. The short circuit current, open circuit voltage (Voc), calculated fill factor (FF) and power conversion efficiency (η%) of the fabricated devices are summarized in Table 1. The extracted parameters of each device under illumination were done using an electrical circuit to model the measured J–V characteristics [11], [12], [13]. Fig. 2 shows
Conclusions
We show that the lattice constant between two lamellar structure of P3HT inside of the blend P3HT:PC[70]BM of bulk heterojunction composites is affected by the PC[70]BM amount in the blend. Increasing the amount of PC[70]BM increases the disorder of the interchains of the polymer in the blend and there is an increase in the charge carrier recombination. The micro-X-ray diffraction showed that the less interchain distance-spacing d100 in P3HT of 1.64 nm, corresponded to the blend S1 (1:0.5 wt.%),
Acknowledgements
This work was supported by the Spanish Ministry of Economy and Competitiveness (MINECO) under Grant number TEC2009-09551, TEC2012-34397, CONSOLIDER HOPE Project CSD2007-00007, Generalitat of Catalunya under Project 2009 SGR 549 and CONACYT in Mexico under Project 127978.
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