Critical role of domain crystallinity, domain purity and domain interface sharpness for reduced bimolecular recombination in polymer solar cells (original) (raw)
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Materials, 2018
Organic photovoltaic (OPV) devices, made with semiconducting polymers, have recently attained a power conversion efficiency (PCE) over 14% in single junction cells and over 17% in tandem cells. These high performances, together with the suitability of the technology to inexpensive large-scale manufacture, over lightweight and flexible plastic substrates using roll-to-roll (R2R) processing, place the technology amongst the most promising for future harvesting of solar energy. Although OPVs using non-fullerene acceptors have recently outperformed their fullerene-based counterparts, the research in the development of new fullerenes and in the improvement of the bulk-heterojunction (BHJ) morphology and device efficiency of polymer:fullerene solar cells remains very active. In this review article, the most relevant research works performed over the last 3 years, that is, since the year 2016 onwards, in the field of fullerene-based polymer solar cells based on the copolymers PTB7, PTB7-Th...
Nanotechnology, 2010
Despite the constant improvement of their power conversion efficiencies, organic solar cells based on an interpenetrating network of a conjugated polymer as donor and fullerene derivatives as acceptor materials still need to be improved for commercial use. In this context, we present a study on the optimization of solar cells based on poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) by varying a specific cell parameter, namely the concentration of the active layer components in the liquid phase before blend film deposition, in order to improve device performance and to better understand the relation between morphology and device operation. Our study shows a significant increase of the short-circuit current, open-circuit voltage and cell efficiency by properly choosing the formulation of the initial blend before film deposition. We demonstrate that the active layer morphology, which is strongly dependent on the initial material concentrations and the processing conditions, can greatly impact the electronic characteristics of the device, especially regarding charge recombination dynamics at the donor-acceptor interface. Our optimized P3HT:PCBM device exhibits both slow recombination and high photocurrent generation associated with an overall power conversion efficiency of 4.25% under 100 mW cm −2 illumination (AM1.5G).
We examine how thermal annealing affects the fullerene network in conjugated polymer bulk heterojunction (BHJ) solar cells. We begin by creating electron-only devices with a BHJ geometry by blending the fullerene derivative [6,6]-phenyl-C61-butyric-acid-methyl-ester (PCBM) with polystyrene (PS). These electron-only PS:PCBM diodes function even with a poly(ethylenedioxithiophene):poly(styrenesulfonate) (PEDOT:PSS) layer, indicating that PEDOT:PSS films do not serve as electron blocking layers. Atomic force microscopy shows that the degree of phase segregation in the PS:PCBM blend films is similar to that in the active layer of blends of PCBM with poly(3-hexylthiophene-2,5-diyl) (P3HT), so that the PS:PCBM blends provide a good model for the fullerene part of the BHJ network in P3HT:PCBM solar cells. We find that thermal annealing dramatically decreases the electron current that flows in the PS:PCBM diodes, suggesting that annealing leads to increased phase segregation that lowers the electron mobility on fullerenes in the BHJ geometry. We also find that annealing increases the photoluminesence of P3HT:PCBM blend films, indicating that thermal treatment produces increased phase segregation that leads to decreased exciton harvesting. The fact that annealing decreases both exciton harvesting and electron mobility implies that there is significant room to further improve polymer/fullerene photovoltaics by controlling the amount of phase segregation.
Synthetic Metals, 2018
Morphology of polymer:fullerene bulk heterojunctions (BHJ) displays beside suitable energy level alignment between donor and acceptor as well as a sufficient absorption one of the most crucial impacts on organic photovoltaic device efficiency and stability. In particular, the structural order of the polymer phase impacts the charge separation efficiency and transport. In this work we present a comprehensive study on the control of the polymeric order initiated already in solution by using fractions of anti-solvent isopropanol (IPA), and via adjusting the processing temperature for common solutions of an anthracene-containing poly(p-phenylene-ethynylene)-alt-poly(p-phenylene-vinylene) bearing statistically either linear (octyloxy) or branched (2-ethylhexyloxy) side-chains (AnE-PVstat) and phenyl-C61-butyric acid methyl ester (PCBM). Detailed analysis of photophysical data, absorption, and photoluminescence (PL) spectra, of deposited AnE-PVstat:PCBM thin films suggest the formation of J-aggregates for the copolymer to be in strong correlation with the processing formulation (presence of non-solvent IPA) and post-treatment parameters (annealing temperature). The findings for our system indicate that the addition of IPA increases the intermolecular interaction between the polymer chains, consistent with enhanced charge carrier (hole) mobility (μ h) in the final film reaching values as high as 10 −3 cm 2 V −1 s −1. The subtle control of polymer aggregation led to an overall variation in the solar cell device efficiency by about 50%, with optimal processing settings of IPA additive and a solution temperature of 15°C.
2012
The effect of phase separation of the donor-acceptor (DA) blend on the dominant recombination mechanism in polymer-fullerene [(poly(3-hexylthiophene) (P3HT) and phenyl-C 61 -butyric acid methyl ester (PCBM)] based bulk heterojunction (BHJ) cells has been investigated. Coarse (70-150 nm) and fine (20-25 nm) phase separated blends and corresponding devices were prepared using chlorobenzene (CB) and ortho-dichlorobenzene (1,2-DCB) as spin casting solvents respectively. Nanoscale mobility measurements indicated highly unbalanced charge transport in coarse morphology based (CB cast) devices. Linear dependence of short circuit current (J sc ) vs. light intensity (I) suggested first order monomolecular (MR) recombination in the fine phase separated devices (1,2-DCB cast) whereas sub-linearity suggested dominant role of bimolecular (BR) recombination in coarse phase separated devices (CB cast). Improved device efficiency of 1,2-DCB based devices (η ≈ 2.54 %) compared to CB (η ≈ 0.9 %) may be attributed to reduced BR recombination as a result of finer phase separation.
Soft Matter, 2010
The power conversion efficiency in a conjugated polymer-functionalized fullerene bulk heterojunction organic photovoltaic (OPV) device is dependent both on the electronic properties of the constituent materials and on the nanoscale morphology of the active semiconductor layer thin-film. Here we use in situ ellipsometry and grazing incidence X-ray scattering (GI-XS) to study molecular self-organization in poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) blend films in real time, during the drying process as they are cast from solution. We illustrate the evolution of the extinction coefficient from a solution to a solid, semi-crystalline state. We show that once the solvent fraction in the film falls below 50%, the P3HT undergoes rapid crystallization via heterogeneous nucleation; a process that is complete in seconds. We also evidence a rapid, dynamic self-annealing process that reduces the characteristic lamella spacing in the P3HT crystallites. The mechanistic understanding of film-formation demonstrated here is an important component in optimizing deposition processes suitable for large-area OPV manufacture. † Electronic supplementary information (ESI) available: 3D cartoon scheme of the in situ system set-up, methods of extracting film thickness and optical constants during the P3HT:PCBM film drying process via ellipsometry, GI-WAXS of P3HT:PCBM blend, and other additional results supporting the discussion. See