Nanoscale control of the network morphology of high efficiency polymer fullerene solar cells by the use of high material concentration in the liquid phase (original) (raw)
Related papers
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.
Polymer, 2013
Polymerefullerene bulk heterojunction (BHJ) solar cells have consistently been at the forefront of the growing field of organic photovoltaics (OPV). The enduring vision of OPV is the promise of combining a simple, low-cost approach with an efficient, flexible, lightweight platform. While efficiencies have improved remarkably over the last decade through advances in device design, mechanistic understanding, and evolving chemical structural motifs, steps forward have often been tied to a loss of simplicity and a deviation from the central vision of OPV. Within the context of active layer optimization, our focus is to target high efficiency while maintaining simplicity in polymer design and active layer processing. To highlight this strategy, this feature article focuses on our work on random poly(3hexylthiophene) (P3HT) analogs and their application in binary and ternary blend polymerefullerene solar cells. These random conjugated polymers are conceptually based on combining simple monomers strategically to influence polymer properties as opposed to the synthesis of highly tailored and synthetically complex monomers. The ternary blend approach further exemplifies the focus on device simplicity by targeting efficiencies that are competitive with complex tandem solar cells, but within the confines of a single active-layer processing step. These research directions are described within the broader context of recent progress in the field of polymerefullerene BHJ solar cells.
Solvent additives for tuning the photovoltaic properties of polymer–fullerene solar cells
We use solvent additives as a simple method to tune the photovoltaic performance of poly-3hexylthiophene (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) bulk heterojuncton solar cells. 1,2-dichlorobenzene (oDCB) was used as the reference solvent; chlorobenzene (CB) and 1,2,3,4-tetrahydronaphthalene (THN) were used as additives to influence film formation. An increase in the short circuit current and the power conversion efficiency of solar cells with blends cast from mixed solvents was observed. Blends prepared with THN, the highest boiling point solvent, resulted in the best device performance, while blends prepared with the pure reference solvent resulted in the lowest photocurrent. In-plane investigations of the morphology using transmission electron microscopy (TEM) revealed improved phase segregation for blends prepared with mixed solvents, and increased crystallinity in the P3HT phase is demonstrated using atomic force microscopy (AFM) coupled with Kelvin probe force microscopy (KPFM). Optical modeling reveals that the increase in the photocurrent is not due to changes in the optical properties of the blends. Electrical characterization reveals that the electron mobilities decrease slightly in blends cast from mixed solvents, corresponding to a decrease in the fill factor and an increase in P3HT crystallinity observed at the surface of the blend. The increase in the photovoltaic performance is discussed in terms of increased charge separation at the donoracceptor interface due to increased ordering in the P3HT phase induced by the solvent additives.
Thin Solid Films, 2008
This work investigates the correlations between the morphological characteristics of the active layers, comprised of poly(3-hexylthiophene) and [6,6]-phenyl-C 61 -butyric acid methyl ester, and the photovoltaic performance of polymer-based solar cells. The active layers were deposited by spincoating the polymer solutions under various conditions and, then, characterized by atomic force microscopy, X-ray diffraction, UV/Vis and Raman spectroscopy. Results of this study indicate that solar cells employing the slow-solvent-vapor-treatment blend films as the active layers exhibit the enhanced power conversion efficiency (3.0%), short-circuit (8.71 mA/cm 2 ) current and fill factor (0.59) than that of as-cast and fast-thermalannealing blend films.
Advanced Energy Materials, 2014
conjugated polymer or small molecule as the donor material and a fullerene derivative as the electron acceptor. Efforts to raise the power conversion effi ciency by increasing the open-circuit voltage (V OC) have primarily focused on fi nding donor materials with lower-lying highest occupied molecular orbital (HOMO) levels [ 4 ] or fullerene derivatives with higher-lying lowest unoccupied molecular orbital (LUMO) levels. [ 5 ] Several research groups [ 6 ] have shown that a maximum of approximately 1.0 V for the V OC exists for effi cient OPV devices using fullerene derivatives as the electron acceptor. This limit is due to the inability to effi ciently split excitons on the fullerene molecule when the energy of the charge transfer (CT) state is less than 0.15 eV below the fullerene singlet excited state energy of 1.7 eV. In polymerfullerene systems where the fullerene has the smaller singlet energy, excitons formed in the polymer can reach the fullerenes via energy transfer, and ineffi cient hole transfer from the fullerenes to the polymer results in a large current loss in devices with V OC values exceeding 1.0 V. In order to relax this ceiling on the V OC and fi nd effi cient devices with voltages that can yield high effi ciencies in both single and multi-junction devices, new electron acceptors are needed with higher energy singlet excited states. [ 3b , 6d , 7 ] In addition to this restriction on the V OC , fullerene derivatives are relatively expensive [ 8 ] and C 60 derivatives do not absorb light well. One study [ 8a ] has shown that the PC 60 BM commonly used in bulk heterojunction (BHJ) organic solar cells could account for 12% of the overall OPV module cost and that C 70-based derivatives would be even more expensive. Current research into new electron acceptors [ 9 ] has covered an array of polymers [ 10 ] and small molecules. [ 11 ] While most devices prepared with these acceptors have effi ciencies near or below 2%, a few, including those based on evaporated devices incorporating halogenated boron subphthalocyanine molecules, [ 11p ] dimeric perylene diimide small molecules, [ 11r ] and solution-processed all-polymer devices based on naphthalene diimide [ 10a ] have achieved effi ciencies greater than 4%. V OC values approaching and even exceeding 1.0 V have been achieved in a few of these devices [ 11b , 11h ] but the typical values for the short-circuit current (J SC) and fi ll factor are There is a need to fi nd electron acceptors for organic photovoltaics that are not based on fullerene derivatives since fullerenes have a small band gap that limits the open-circuit voltage (V OC), do not absorb strongly and are expensive. Here, a phenylimide-based acceptor molecule, 4,7-bis(4-(N-hexyl-phthalimide)vinyl)benzo[c]1,2,5-thiadiazole (HPI-BT), that can be used to make solar cells with V OC values up to 1.11 V and power conversion effi ciencies up to 3.7% with two thiophene polymers is demonstrated. An internal quantum effi ciency of 56%, compared to 75-90% for polymer-fullerene devices, results from less effi cient separation of geminate charge pairs. While favorable energetic offsets in the polymer-fullerene devices due to the formation of a disordered mixed phase are thought to improve charge separation, the low miscibility (<5 wt%) of HPI-BT in polymers is hypothesized to prevent the mixed phase and energetic offsets from forming, thus reducing the driving force for charges to separate into the pure donor and acceptor phases where they can be collected.
Nano Letters, 2009
We demonstrate that intercalation of fullerene derivatives between the side chains of conjugated polymers can be controlled by adjusting the fullerene size and compare the properties of intercalated and nonintercalated poly(2,5-bis(3-hexadecylthiophen-2-yl)thieno[3,2-b]thiophene (pBTTT):fullerene blends. The intercalated blends, which exhibit optimal solar-cell performance at 1:4 polymer:fullerene by weight, have better photoluminescence quenching and lower absorption than the nonintercalated blends, which optimize at 1:1. Understanding how intercalation affects performance will enable more effective design of polymer:fullerene solar cells.
Acta Physica Polonica A, 2019
New method of polymer-fullerene composite crystallization was examined as a possible way for efficiency enhancement of polymer organic solar cells. Since the structure of bulk-heterojunction is crucial for its efficiency, there is a strong need for new methods that can control a crystallization process. In this work we studied the crystallization process taking place under thermal annealing in ambient conditions, as well as annealing in a solvent atmosphere. Two polymer compounds, P3HT and PTB7, performing as donor materials were used. The acceptor material in the fabricated bulk heterojunction was [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). The spin-coated layers were investigated by optical absorption and X-ray diffraction. We observed significant changes in the crystal structure of some of the annealed layers. The constructed solar cells were examined with use of current–voltage characterization method under AM1.5G sun irradiation, and by photocurrent spectroscopy. In the ...