Bulk Heterojunction Solar Cells: Morphology and Performance Relationships

A new multi-functional conjugated polymer for use in high-performance bulk heterojunction solar cells

We report a new multi-functional copolymer, PDTP-DTBDT, containing DTP and DTBDT units. Surprisingly, the introduction of novel DTP and DTBDT units brings not only superior charge transfer properites but also charge transport characteristics for efficient bulk heterojunction solar cells.

Synthesis and photovoltaic properties of two new alkoxylphenyl substituted thieno[2,3-f]benzofuran based polymers

Two new alkoxylphenyl substituted thieno[2,3-f]benzofuran (TBFP)-based polymers (PTBFP-BT and PTBFP-BO) were designed and synthesized. Their structures were verified by nuclear magnetic resonance (NMR) spectroscopy, the molecular weights were determined by gel permeation chromatography (GPC) and the thermal properties were investigated by thermogravimetric analysis (TGA). The two polymers showed similar UV-Vis absorption spectra with a broad and strong absorption band from 300-750 nm in solid state. The resulting copolymers exhibited relatively deep highest occupied molecular orbital (HOMO) energy levels (-5.47 and -5.61 eV) for PTBFP-BT and PTBFP-BO, respectively. The device fabricated with PTBFP-BT : PC71BM (1 : 2) showed better balanced hole and electron mobility of 2.49 × 10(-4) cm(2) V(-1) s(-1) and 9.12 × 10(-4) cm(2) V(-1) s(-1), respectively, than those of PTBFP-BO based devices. The polymer solar cells (PSCs), based on the single layer device structure of ITO/PEDOT:PSS/PTBFP-BT : PC71BM (1 : 2, w/w)/ZrAcac/Al with 3 vol% 1,8-diiodooctane (DIO) as additive, showed a relatively high power conversion efficiency (PCE) of 6% under the illumination of AM 1.5G, 100 mW cm(-2), with a high fill factor (FF) of 0.69.

Fine Tuning Surface Energy of Poly(3-hexylthiophene) by Heteroatom Modification of the Alkyl Side Chains

Recent work has pointed to polymer miscibility and surface energy as key figures of merit in the formation of organic alloys and synergistic behavior between components in ternary blend solar cells. Here, we present a simple model system and first report of poly(3-hexylthiophene)-based random copolymers featuring either a semifluoroalkyl (P3HT-co-FHT) or oligoether (P3HT-co-MET) side chain, prepared via Stille polycondensation. Water drop contact angle measurements demonstrated that P3HT-co-FHT polymers reached a minimum surface energy of 14.2 mN/m at 50% composition of comonomers, while in contrast, P3HT-co-MET polymers increased as high as 27.0 mN/m at 50% composition, compared to P3HT at 19.9 mN/m. Importantly, the surface energy of the copolymers was found to vary regularly with comonomer composition and exhibited fine-tuning. Optical and electronic properties of the polymers are found to be composition independent as determined by UV-vis and CV measurements; HOMO energy levels ranged from 5.25 to 5.30 eV; and optical band gaps all measured 1.9 eV. Following this model, surface energy modification of state-of-the-art polymers, without altering desirable electronic and optical properties, is proposed as a useful tool in identifying and exploiting more alloying polymer pairs for ternary blend solar cells.

Photophysical Study of Polymer-Based Solar Cells with an Organo-Boron Molecule in the Active Layer

Our group previously reported the synthesis of four polythiophene derivatives (P1-P4) used for solar cells. The cells were prepared under room conditions by spin coating, leading to low efficiencies. However, after the addition of 6-nitro-3-(E)-3-(4-dimethylaminophenyl)allylidene)-2,3-dihydrobenzo[d]-[1,3,2] oxazaborole (M1) to their active layers, the efficiencies of the cells showed approximately a two-fold improvement. In this paper, we study this enhancement mechanism by performing ultrafast transient absorption (TA) experiments on the active layer of the different cells. Our samples consisted of thin films of a mixture of PC₆₁BM with the polythiophenes derivatives P1-P4. We prepared two versions of each sample, one including the molecule M1 and another without it. The TA data suggests that the efficiency improvement after addition of M1 is due not only to an extended absorption spectrum towards the infrared region causing a larger population of excitons but also to the possible creation of additional channels for transport of excitons and/or electrons to the PC₆₁BM interface.

The Effect of Processing Additives on Energetic Disorder in Highly Efficient Organic Photovoltaics: A Case Study on PBDTTT-C-T:PC71 BM

Energetic disorder, an important parameter affecting the performance of organic photovoltaics, is significantly decreased upon the addition of processing additives in a highly efficient benzodithiophene-based copolymer blend (PBDTTT-C-T:PC71 BM). Wide-angle and small-angle X-ray scattering measurements suggest that the origin of this reduced energetic disorder is due to increased aggregation and a larger average fullerene domain size together with purer phases.

Facile Preparation of Molybdenum Bronzes as an Efficient Hole Extraction Layer in Organic Photovoltaics

We proposed a facile and green one-pot strategy to synthesize Mo bronzes nanoparticles to serve as an efficient hole extraction layer in polymer solar cells. Mo bronzes were obtained through reducing the fractional self-aggregated ammonium heptamolybdate with appropriate reducing agent ascorbic acid, and its optoelectronic properties were fully characterized. The synthesized Mo bronzes displayed strong n-type semiconductor characteristics with a work function of 5.2-5.4 eV, matched well with the energy levels of current donor polymers. The presented gap states of the Mo bronzes near the Fermi level were beneficial for facilitating charge extraction. The as-synthesized Mo bronzes were used as hole extraction layer in polymer solar cells and significantly enhanced the photovoltaic performance and stability. The power conversion efficiency was increased by more than 18% compared with the polyethylene dioxythiophene:polystyrenesulfonate-based reference cell. The excellent performance and facile preparation render the as-synthesized solution-processed Mo bronzes nanoparticles a promising candidate for hole extraction layer in low-cost and efficient polymer solar cells.

Adding Amorphous Content to Highly Crystalline Polymer Nanowire Solar Cells Increases Performance

Polymer solar cells are fabricated with systematic variation of the phase purity. Photovoltaic tests demonstrate that devices with ca. 10% of mixed phases outperform pure-phase devices. Photophysical studies reveal the effects of mixed phase on charge generation and recombination. These results show a promising strategy for the optimization of organic electronic materials.

New generation solar cells: concepts, trends and perspectives

Organic, dye-sensitized and perovskite solar cell technologies have triggered widespread interest in recent years due to their very promising potential towards a high solar electricity future. A number of important milestones have marked the roadmap of each sector on the way to today's outstanding performances, but there still remains plenty of scope for further improvement. The most influential landmarks, together with basic concepts and future perspectives, are unraveled in this review.

Structure-property relationships: asymmetric alkylphenyl-substituted anthracene molecules for use in small-molecule solar cells

Two asymmetric anthracene-based organic molecules, NDHPEA and TNDHPEA, were prepared without or with a thiophene spacer between the anthracene and naphthalene units. These asymmetric oligomers displayed different degrees of coplanarity, as evidenced by differences in the dihedral angles calculated by using DFT. Differential scanning calorimetry and XRD studies were used to probe the crystallization characteristics and molecular packing structures in the active layers. The coplanarity of the molecules in the asymmetric structure significantly affected the crystallization behavior and the formation of crystalline domains in the solid state. The small-molecule crystalline properties were correlated with the device physics by determining the J-V characteristics and hole mobilities of the devices.

Effects of alkyl chain length on the optoelectronic properties and performance of pyrrolo-perylene solar cells

While the impact of alkyl side-chain length on the photovoltaic properties of conjugated polymers and their performance in bulk heterojunction (BHJ) solar cells has been studied extensively, analogous studies on pyrrolo-perylene-based polymers have not received adequate attention. To explore these effects, we synthesized two copolymers based on N-annulated pyrrolo-perylene and consisting of cyano group substituents on thiophene vinylene thiophene units with two different alkyl groups of 2-decyltetradecyl and 7-decylnonadecyl, and we studied them with regard to chemical structure and photovoltaic performance. UV-vis spectroscopy and cyclic voltammetry studies showed that variations in alkyl chain length affect crystallization, light absorption, and the highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels. These factors have a pronounced impact on the morphology of BHJ thin films and their charge carrier separation and transportation characteristics, which, in turn, influences photovoltaic properties.

Sub-nanometre resolution imaging of polymer-fullerene photovoltaic blends using energy-filtered scanning electron microscopy

The resolution capability of the scanning electron microscope has increased immensely in recent years, and is now within the sub-nanometre range, at least for inorganic materials. An equivalent advance has not yet been achieved for imaging the morphologies of nanostructured organic materials, such as organic photovoltaic blends. Here we show that energy-selective secondary electron detection can be used to obtain high-contrast, material-specific images of an organic photovoltaic blend. We also find that we can differentiate mixed phases from pure material phases in our data. The lateral resolution demonstrated is twice that previously reported from secondary electron imaging. Our results suggest that our energy-filtered scanning electron microscopy approach will be able to make major inroads into the understanding of complex, nano-structured organic materials.

Morphological characterization of organic photovoltaic active layers is restricted by the lack of accurate chemical mapping tools. Here, the authors demonstrate an energy-filtered scanning electron microscopy technique, which enables sub-nanometre resolution imaging of an organic photovoltaic blend.

A series of simple oligomer-like small molecules based on oligothiophenes for solution-processed solar cells with high efficiency

A series of acceptor-donor-acceptor simple oligomer-like small molecules based on oligothiophenes, namely, DRCN4T-DRCN9T, were designed and synthesized. Their optical, electrical, and thermal properties and photovoltaic performances were systematically investigated. Except for DRCN4T, excellent performances were obtained for DRCN5T-DRCN9T. The devices based on DRCN5T, DRCN7T, and DRCN9T with axisymmetric chemical structures exhibit much higher short-circuit current densities than those based on DRCN6T and DRCN8T with centrosymmetric chemical structures, which is attributed to their well-developed fibrillar network with a feature size less than 20 nm. The devices based on DRCN5T/PC71BM showed a notable certified power conversion efficiency (PCE) of 10.10% under AM 1.5G irradiation (100 mW cm(-2)) using a simple solution spin-coating fabrication process. This is the highest PCE for single-junction small-molecule-based organic photovoltaics (OPVs) reported to date. DRCN5T is a rather simpler molecule compared with all of the other high-performance molecules in OPVs to date, and this might highlight its advantage in the future possible commercialization of OPVs. These results demonstrate that a fine and balanced modification/design of chemical structure can make significant performance differences and that the performance of solution-processed small-molecule-based solar cells can be comparable to or even surpass that of their polymer counterparts.

A close look at charge generation in polymer:fullerene blends with microstructure control

We reveal some of the key mechanisms during charge generation in polymer:fullerene blends exploiting our well-defined understanding of the microstructures obtained in pBTTT:PCBM systems via processing with fatty acid methyl ester additives. Based on ultrafast transient absorption, electro-absorption, and fluorescence up-conversion spectroscopy, we find that exciton diffusion through relatively phase-pure polymer or fullerene domains limits the rate of electron and hole transfer, while prompt charge separation occurs in regions where the polymer and fullerene are molecularly intermixed (such as the co-crystal phase where fullerenes intercalate between polymer chains in pBTTT:PCBM). We moreover confirm the importance of neat domains, which are essential to prevent geminate recombination of bound electron-hole pairs. Most interestingly, using an electro-absorption (Stark effect) signature, we directly visualize the migration of holes from intermixed to neat regions, which occurs on the subpicosecond time scale. This ultrafast transport is likely sustained by high local mobility (possibly along chains extending from the co-crystal phase to neat regions) and by an energy cascade driving the holes toward the neat domains.

An electron acceptor challenging fullerenes for efficient polymer solar cells

A novel non-fullerene electron acceptor (ITIC) that overcomes some of the shortcomings of fullerene acceptors, for example, weak absorption in the visible spectral region and limited energy-level variability, is designed and synthesized. Fullerene-free polymer solar cells (PSCs) based on the ITIC acceptor are demonstrated to exhibit power conversion efficiencies of up to 6.8%, a record for fullerene-free PSCs.

Time-Resolved Raman Spectroscopy of Polaron Pair Formation in Poly(3-hexylthiophene) Aggregates

The ultrafast formation of bound charge pairs, or polaron pairs (PPs), in mixed-order aggregates of poly(3-hexylthiophene) was investigated using femtosecond stimulated Raman spectroscopy (FSRS). Spectral dynamics in the carbon-carbon stretching region reveal a significant photoinduced depletion in steady-state features associated with lamellar-stacked, ordered polymer regions upon 500 nm photoexcitation; this is followed by the appearance of red-shifted features attributable to PPs that is delayed by a few hundred femtoseconds. PP features decay with concomitant recovery of the steady-state Raman depletion over a few picoseconds. The vibrational spectrum of the PP obtained exhibits a modest red shift (<15 cm(-1)) and lower Raman activity relative to steady-state features in the C═C stretching region but similar features in other regions. In total, this work demonstrates the potential of time-resolved Raman as a morphologically selective and structurally sensitive probe for tracking ultrafast charge separation and recombination dynamics within polymer regions of conjugated materials.

2,1,3-Benzothiadiazole-5,6-dicarboxylic imide--a versatile building block for additive- and annealing-free processing of organic solar cells with efficiencies exceeding 8%

A new photoactive polymer comprising benzo[1,2-b:3,4-b':5,6-d']trithiophene and 2,1,3-benzothiadiazole-5,6-dicarboxylic imide is reported. The synthetic design allows for alkyl chains to be introduced on both electron-rich and electron-deficient components, which in turn allows for rapid optimization of the alkyl chain substitution pattern. Consequently, the optimized polymer shows a maximum efficiency of 8.3% in organic photovoltaic devices processed in a commercially viable fashion without solvent additives, annealing, or device engineering.

Synthesis, optical properties, and electronic structures of nucleobase-containing π-conjugated oligomers

The molecular recognition properties of the nucleobases instruct the formation of complex three-dimensional architectures in natural and synthetic systems; relatively unexplored is their use as building blocks for π-conjugated materials where they might mutually tune electronic and supramolecular structures. Toward this goal, an introductory set (1a-d and 2a-d) of six purine-terminated and two pyrimidine-terminated π-conjugated oligomers has been synthesized and used to develop experimental electronic and photophysical structure-property trends. Unlike 2,2':5',2″-terthiophene (TTT) derivatives 2a-d, intramolecular charge transfer dominates oligomers 1a-d bearing a 4,7-bisthienylbenzothiadiazole (TBT) spacer due to the strong electron-accepting ability of its benzothiadiazole (BTD) ring. The resulting donor-acceptor-donor systems feature lower HOMO-LUMO gaps than the terthiophene-linked nucleobases (ΔE(g) ∼ 1.8 eV vs 2.4 eV based on electrochemical measurements), and the lowest so far for π-conjugated molecules that include nucleobases within the π-framework. Experiments reveal a dependence of photophysical and electronic structure on the nature of the nucleobase and are in good agreement with theoretical calculations performed at the B3LYP/6-31+G** level. Overall, the results show how nucleobase heterocycles can be installed within π-systems to tune optical and electronic properties. Future work will evaluate the consequences of these information-rich components on supramolecular π-conjugated structure.

Ternary morphology facilitated thick-film organic solar cell

We applied a ternary morphology to enhance light harvesting of a thick-film polymer solar cell.

Development of a donor polymer using a B ← N unit for suitable LUMO/HOMO energy levels and improved photovoltaic performance

We report a novel approach to tune the LUMO/HOMO energy levels of polymer donors by replacing a C–C unit with a B ← N unit. The polymer containing a B ← N unit exhibits lower LUMO/HOMO levels and a narrower bandgap, leading to an improved photovoltaic performance.

Asymmetrical squaraines for high-performance small-molecule organic solar cells with a short circuit current of over 12 mA cm−2

Asymmetrical squaraine dyes with two aryl groups directly linked to the squaric acid core were synthesized, and exhibited excellent photovoltaic performance.

Side chain structure affects the molecular packing and photovoltaic performance of oligothiophene-based solution-processable small molecules

Small molecules with alkyl side chains of different lengths were prepared with 2,2′-bithiophene, terthiophene and thiobarbituric acid as the central core, spacer and end-cap. Uniform, shorter chain lengths gave stronger intermolecular interactions, favoring crystallization.

The critical role of additives in binary halogen-free solvent systems for the general processing of highly efficient organic solar cells

Our work demonstrated that the use of the additive strategy for halogen-free solvent systems may provide a feasible route to address the critical environmental issues associated with large-scale manufacturing.

N,N-Diarylamino end-capping as a new strategy for simultaneously enhancing open-circuit voltage, short-circuit current density and fill factor in small molecule organic solar cells

N,N-Diarylamino end-capping strategy for asymmetrical squaraines with simultaneously enhanced V_oc, J_sc and FF in solution-processed small molecule organic solar cells.

S,N-Heteropentacene based small molecules with A–D–A structure for solution processed organic bulk heterojunction solar cells

SN(BTTh₂)₂ and SN(BTAOTh₂)₂, containing an electron rich planar S,N-heteropentacene flanked with alkoxy substituted and unsubstituted benzothiadiazole and end capped with hexyl-substituted bi-thiophene units, were designed and synthesized.

Hexacyclic lactam building blocks for highly efficient polymer solar cells

Four D–A copolymers based on new fused-ring acceptor units were developed, and an 8.18% PCE record was demonstrated for D–A copolymers using selenophene as the donor unit.

Organic solar cells based on bowl-shaped small-molecules

A supramolecular approach involving bowl-shape molecules as electron donors has been used for the preparation of small-molecule solar cells. The PCE values depend directly on the formation of the supramolecular complex.

Extended isoindigo core: synthesis and applications as solution-processable n-OFET materials in ambient conditions

Two isoindigo derivatives fused with benzothiophene (C20-DBTII) and benzofuran (C20-DBFII) heterocycles have been synthesized.

Effect of chain curvature on the performance of diketopyrrolopyrrole-based polymer solar cells

Two polymer semiconductors with different degrees of chain curvature are designed and synthesized. The curved polymer blended with PC₇₁BM exhibits a higher PCE of 5.3% than that of a linear polymer.

Rational selection of solvents and fine tuning of morphologies toward highly efficient polymer solar cells fabricated using green solvents

The polymer solar cells fabricated by the green solvent (anisole/diphenyl ether) exhibited an outstanding PCE of 8.37%.

Solution processed thick film organic solar cells

In this Review article, significant advances in materials development and processing methods toward efficient solution processed bulk-heterojunction thick film organic solar cells as well as the factors that determine the optimal active layer thickness are summarized.

Neat C₇₀-based bulk-heterojunction polymer solar cells with excellent acceptor dispersion

The replacement of common fullerene derivatives with neat-C70 could be an effective approach to restrain the costs of organic photovoltaics and increase their sustainability. In this study, bulk-heterojunction solar cells made of neat-C70 and low energy-gap conjugated polymers, PTB7 and PCDTBT, are thoroughly investigated and compared. Upon replacing PC70BM with C70, the mobility of positive carriers in the donor phase is roughly reduced by 1 order of magnitude, while that of electrons is only slightly modified. It is shown that the main loss mechanism of the investigated neat-C70 solar cells is a low mobility-lifetime product. Nevertheless, PCDTBT:C70 devices undergo a limited loss of 7.5%, compared to the reference PCDTBT:PC70BM cells, reaching a record efficiency (4.44%) for polymer solar cells with unfunctionalized fullerenes. The moderate efficiency loss of PCDTBT:C70 devices, due to an unexpected excellent miscibility of PCDTBT:C70 blends, demonstrates that efficient solar cells made of neat-fullerene are possible. The efficient dispersion of C70 in the PCDTBT matrix is attributed to an interaction between fullerene and the carbazole unit of the polymer.