C60 Acceptor Research Articles

Solution‐Grown Organic Single‐Crystalline Donor–Acceptor Heterojunctions for Photovoltaics

Organic single crystals are ideal candidates for high-performance photovoltaics due to their high charge mobility and long exciton diffusion length; however, they have not been largely considered for photovoltaics due to the practical difficulty in making a heterojunction between donor and acceptor single crystals. Here, we demonstrate that extended single-crystalline heterojunctions with a consistent donor-top and acceptor-bottom structure throughout the substrate can be simply obtained from a mixed solution of C60 (acceptor) and 3,6-bis(5-(4-n-butylphenyl)thiophene-2-yl)-2,5-bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4-dione (donor). 46 photovoltaic devices were studied with the power conversion efficiency of (0.255±0.095)% under 1 sun, which is significantly higher than the previously reported value for a vapor-grown organic single-crystalline donor-acceptor heterojunction (0.007%). As such, this work opens a practical avenue for the study of organic photovoltaics based on single crystals.

Solution‐Grown Organic Single‐Crystalline Donor–Acceptor Heterojunctions for Photovoltaics

AbstractOrganic single crystals are ideal candidates for high‐performance photovoltaics due to their high charge mobility and long exciton diffusion length; however, they have not been largely considered for photovoltaics due to the practical difficulty in making a heterojunction between donor and acceptor single crystals. Here, we demonstrate that extended single‐crystalline heterojunctions with a consistent donor‐top and acceptor‐bottom structure throughout the substrate can be simply obtained from a mixed solution of C60 (acceptor) and 3,6‐bis(5‐(4‐n‐butylphenyl)thiophene‐2‐yl)‐2,5‐bis(2‐ethylhexyl)pyrrolo[3,4‐c]pyrrole‐1,4‐dione (donor). 46 photovoltaic devices were studied with the power conversion efficiency of (0.255±0.095) % under 1 sun, which is significantly higher than the previously reported value for a vapor‐grown organic single‐crystalline donor–acceptor heterojunction (0.007 %). As such, this work opens a practical avenue for the study of organic photovoltaics based on single crystals.

Electronic Excitations in Push–Pull Oligomers and Their Complexes with Fullerene from Many-Body Green’s Functions Theory with Polarizable Embedding

We present a comparative study of excited states in push-pull oligomers of PCPDTBT and PSBTBT and prototypical complexes with a C60 acceptor using many-body Green's functions theory within the GW approximation and the Bethe-Salpeter equation. We analyze excitations in oligomers up to a length of 5 nm and find that for both materials the absorption energy practically saturates for structures larger than two repeat units due to the localized nature of the excitation. In the bimolecular complexes with C60, the transition from Frenkel to charge transfer excitons is generally exothermic and strongly influenced by the acceptor's position and orientation. The high CT binding energy of the order of 2 eV results from the lack of an explicit molecular environment. External polarization effects are then modeled in a GW-BSE based QM/MM approach by embedding the donor-acceptor complex into a polarizable lattice. The lowest charge transfer exciton is energetically stabilized by about 0.5 eV, while its binding energy is reduced to about 0.3 eV. We also identify a globally unbound charge transfer state with a more delocalized hole at higher energy while still within the absorption spectrum, which opens another potential pathway for charge separation. For both PCPDTBT and PSBTBT, the energetics are largely similar with respect to absorption and the driving force to form intermediate charge transfer excitations for free charge generation. These results support that the higher power conversion efficiency observed for solar cells using PSBTBT as donor material is a result of molecular packing rather than of the electronic structure of the polymer.

Geometric influence on intramolecular photoinduced electron transfer in platinum(II) acetylide-linked donor-acceptor assemblies.

A new donor-acceptor system, in which the electron donor triphenylamine (TPA) and the electron acceptor C60 are bridged through a cis- or trans-platinum(II) acetylide spacer have been prepared. Ground-state studies were conducted using electrochemistry and UV/Vis spectroscopy. Fluorescence studies suggested that charge transfer is the deactivation mechanism for the singlet excited state, and this was verified by transient absorption spectroscopy. Selective photoexcitation of 1 and 2 at 387 nm leads to a fast charge transfer between the TPA and C60, which gives rise to a radical ion-pair state (TPA(·+)-Pt-C60(·-)). Our results suggest that charge transfer is favored for the cis configuration while the presence of the trans configuration in the Pt(II) diacetylide results in a longer-lived charge separated states.

Implications of the device structure on the photo-stability of organic solar cells

Small molecule organic solar cells (OSCs) are systematically studied for their photo-stability in an inert N2 environment. In this work, 28-day dark, light-stress and heat-stress experiments are conducted on OSCs with strongly varied mixing ratios. Comparisons are made between the emerging, high performance Schottky device structure and the more widely studied (standard) BHJ structure. In both structures, light stress experiments result in a 10–15% loss in power conversion efficiency. Simultaneous heat-stress experiments demonstrate that these variations are not related to thermally induced changes. Photo-induced losses in open circuit voltages are observed for both device structures and are attributed to organic-electrode degradation. However, variations in the other photovoltaic parameters are more strongly dependent on the active layer composition and associated device structure. Schottky OSCs are shown to be slightly more resilient to variations in short circuit current compared to standard BHJ OSCs, but they suffer from losses in fill factor. Microsecond transient photocurrent and external quantum efficiency measurements are employed to show that these fill factor losses are due to increased recombination, associated with increased trap density by the exciton-induced degradation of the C60 acceptor. This effect may be problematic for all Schottky OSCs due to their use of mixed layers with very high C60 content. The choice of device architecture is thus shown to alter degradation mechanisms, and so it can have implications on the overall OSC photo-stability.

ChemInform Abstract: Macrocyclic Dyads Based on C60 and Perylenediimides Connected by Click Chemistry

Two sets of perylenediimide-[60]fullerene dyads PDI-C60 connected through 1,2,3-triazole units have been synthesized and characterized. The cyclic dyad PDICl4-C60 has four chlorine atoms in the 1,6,7,12-PDI positions, whereas the cyclic dyad PDIPip2-C60 has two piperidine units in the 1,7-PDI positions. On the other hand, PDICl4-C60 and PDIPip2-C60 dyads were synthesized as linear counterparts with the same substitution pattern. Also, a C60-PDICl4-C60 triad has been prepared. A small interaction between C60 and PDI moieties in the ground state was detected by UV/vis and electrochemical measurements in both PDI-C60 cyclic systems. The occurrence of photoinduced energy-transfer processes between PDI and C60 units was confirmed by time-resolved emission and transient absorption techniques. Femtosecond laser flash photolysis showed energy transfer from 1PDI* to C60 followed by intersystem crossing from 1C60* to 3C60* in the case of the PDICl4-C60 systems. The energy transfer rate for PDICl4-C60 in the cyclic dyad and the triad is one order of magnitude faster than that in PDICl4-C60 in the linear dyad. The fast energy transfer rate together with the enhanced molar absorption coefficient by perylenediimide functionalization will be highly beneficial for applications as acceptors in polymer solar cells. On the other hand, no electron transfer from the donor PDIpip2 units to the acceptor C60 moiety was detected in the case of PDIPip2-C60 dyads.

Exciton Management in Organic Photovoltaic Multidonor Energy Cascades

Multilayer donor regions in organic photovoltaics show improved power conversion efficiency when arranged in decreasing exciton energy order from the anode to the acceptor interface. These so-called "energy cascades" drive exciton transfer from the anode to the dissociating interface while reducing exciton quenching and allowing improved overlap with the solar spectrum. Here we investigate the relative importance of exciton transfer and blocking in a donor cascade employing diphenyltetracene (D1), rubrene (D2), and tetraphenyldibenzoperiflanthene (D3) whose optical gaps monotonically decrease from D1 to D3. In this structure, D1 blocks excitons from quenching at the anode, D2 accepts transfer of excitons from D1 and blocks excitons at the interface between D2 and D3, and D3 contributes the most to the photocurrent due to its strong absorption at visible wavelengths, while also determining the open circuit voltage. We observe singlet exciton Förster transfer from D1 to D2 to D3 consistent with cascade operation. The power conversion efficiency of the optimized cascade OPV with a C60 acceptor layer is 7.1 ± 0.4%, which is significantly higher than bilayer devices made with only the individual donors. We develop a quantitative model to identify the dominant exciton processes that govern the photocurrent generation in multilayer organic structures.

Macrocyclic Dyads Based on C60 and Perylenediimides Connected by Click Chemistry

AbstractTwo sets of perylenediimide‐[60]fullerene dyads PDI‐C60 connected through 1,2,3‐triazole units have been synthesized and characterized. The cyclic dyad PDICl4‐C60 has four chlorine atoms in the 1,6,7,12‐PDI positions, whereas the cyclic dyad PDIPip2‐C60 has two piperidine units in the 1,7‐PDI positions. On the other hand, PDICl4‐C60 and PDIPip2‐C60 dyads were synthesized as linear counterparts with the same substitution pattern. Also, a C60‐PDICl4‐C60 triad has been prepared. A small interaction between C60 and PDI moieties in the ground state was detected by UV/vis and electrochemical measurements in both PDI‐C60 cyclic systems. The occurrence of photoinduced energy‐transfer processes between PDI and C60 units was confirmed by time‐resolved emission and transient absorption techniques. Femtosecond laser flash photolysis showed energy transfer from 1PDI* to C60 followed by intersystem crossing from 1C60* to 3C60* in the case of the PDICl4‐C60 systems. The energy transfer rate for PDICl4‐C60 in the cyclic dyad and the triad is one order of magnitude faster than that in PDICl4‐C60 in the linear dyad. The fast energy transfer rate together with the enhanced molar absorption coefficient by perylenediimide functionalization will be highly beneficial for applications as acceptors in polymer solar cells. On the other hand, no electron transfer from the donor PDIpip2 units to the acceptor C60 moiety was detected in the case of PDIPip2‐C60 dyads.

Small molecule-based tandem solar cells with solution-processed and vacuum-processed photoactive layers

A tandem solar cell device whose sub-cells are fabricated exclusively from small molecules (SMs) through both solution-processed and vacuum-processed deposition techniques is described. The front sub-cell's active layer consists of a bulk heterojunction (BHJ) DPP(TBFu)2:PC70BM device while the back cell has a typical bilayer structure employing a 2,4-bis[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl] squaraine (SQ) donor and a C60 acceptor.

Interplay between efficiency and device architecture for small molecule organic solar cells.

Small molecule organic solar cells (OSCs) have experienced a resurgence of interest over their polymer solar cell counterparts, owing to their improved batch-to-batch (thus, cell-to-cell) reliability. In this systematic study on OSC device architecture, we investigate five different small molecule OSC structures, including the simple planar heterojunction (PHJ) and bulk heterojunction (BHJ), as well as several planar-mixed structures. The different OSC structures are studied over a wide range of donor:acceptor mixing concentrations to gain a comprehensive understanding of their charge transport behavior. Transient photocurrent decay measurements provide crucial information regarding the interplay between charge sweep-out and charge recombination, and ultimately hint toward space charge effects in planar-mixed structures. Results show that the BHJ/acceptor architecture, comprising a BHJ layer with high C60 acceptor content, generates OSCs with the highest performance by balancing charge generation with charge collection. The performance of other device architectures is largely limited by hole transport, with associated hole accumulation and space charge effects.

Interface investigation of the alcohol-/water-soluble conjugated polymer PFN as cathode interfacial layer in organic solar cells

In this work, in situ ultraviolet photoelectron spectroscopy measurements were used to investigate the working mechanism of an alcohol-/water-soluble conjugated polymer poly [(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9–dioctylfluorene)] (PFN) as the cathode interfacial layer in organic solar cells from the view of interfacial energy level alignment. Fullerene (C60) was chosen as the model acceptor material in contact with PFN as well as two other cathode interfacial layers ZnO and TiO2 in the configuration of an inverted solar cell structure. Significant charge transfer between PFN modified ITO (indium tin oxide) electrode and C60 is observed due to the low work function of PFN. This results in the Fermi level of the substrate pinned very close to the lowest unoccupied molecular orbital of C60 as well as an additional electric field at the cathode/acceptor interface. Both of them facilitate the electron extraction from the acceptor C60 to the ITO cathode, as confirmed by the electrical measurements of the electron-only devices with PFN modification. The better electron extraction originated from the Fermi level pinning and the additional interface electric field are believed to contribute to the efficiency enhancement of the inverted organic solar cells employing PFN as cathode interfacial layer.

Control of Interface Order by Inverse Quasi-Epitaxial Growth of Squaraine/Fullerene Thin Film Photovoltaics

It has been proposed that interface morphology affects the recombination rate for electrons and holes at donor-acceptor heterojunctions in thin film organic photovoltaic cells. The optimal morphology is one where there is disorder at the heterointerface and order in the bulk of the thin films, maximizing both the short circuit current and open circuit voltage. We show that an amorphous, buried functionalized molecular squaraine donor layer can undergo an "inverted" quasi-epitaxial growth during postdeposition processing, whereby crystallization is seeded by a subsequently deposited self-assembled nanocrystalline acceptor C60 cap layer. We call this apparently unprecedented growth process from a buried interface "inverse quasi-epitaxy" where the crystallites of these "soft" van der Waals bonded materials are only approximately aligned to those of the cap. The resulting crystalline interface hastens charge recombination, thereby reducing the open circuit voltage in an organic photovoltaic cell. The lattice registration also facilitates interdiffusion of the squaraine donor and C60 acceptor, which dramatically improves the short circuit current. By controlling the extent to which this crystallization occurs, the voltage losses can be minimized, resulting in power conversion efficiencies of ηP = 5.4 ± 0.3% for single-junction and ηP = 8.3 ± 0.4% for tandem small-molecule photovoltaics. This is a general phenomenon with implications for all organic donor-acceptor junctions. That is, epitaxial relationships typically result in a reduction in open circuit voltage that must be avoided in both bilayer and bulk heterojunction organic photovoltaic cells.

Semitransparent organic photovoltaics using a near-infrared absorbing cyanine dye

A selective near-infrared absorbing heptamethine cyanine dye (Cy7-P) electron donor was used for the fabrication of bilayer solar cells with the acceptor C60. Using a reflective metal top electrode, solar cell optimization resulted in an external power conversion efficiency (η) of 1.5% for layer thicknesses of ~20nm for Cy7-P and of ~40nm for C60, with using either PEDOT:PSS or MoO3 as anode and Alq3 as cathode buffer layers. Highly transparent devices were then fabricated by using silver/Alq3 cathodes. Average visible transmittance (450–670nm) values of, for example, 67.2% (η=0.7%), 62.1% (η=0.9%) or 50% (η=1%) were obtained for different thickness combinations of silver and Alq3. Optimized solar cells had a maximum transparency of 79.8% at 568nm. The operational stability under 1sun illumination was T80=30h, after which 80% of the initial cell performance was reached.

Improvement in power conversion efficiency and long-term lifetime of organic photovoltaic cells by using bathophenanthroline/molybdenum oxide as compound cathode buffer layer

We demonstrate response and lifetime improvements for organic photovoltaic (OPV) by using organic bathophenanthroline (Bphen) and inorganic molybdenum oxide (MoO3) as a compound cathode buffer layer (CBL). Not only power conversion efficiency (PCE) increases by 27% but also working and storage lifetimes extend at least 10 and 60 times respectively comparing with PV cell with 5nm thick Bphen as CBL. The optimized PV structure is ITO/CuPc (20nm)/C60 (40nm)/Bphen (2nm)/MoO3 (5nm)/Al and the CBL of the reference device is 5nm Bphen without MoO3 layer. The achievement of PV performance and lifetime improvement by introducing such a compound CBL is attributed to appropriate level alignment between C60, Bphen and MoO3, and stable inorganic MoO3 can prevent from superior oxygen and moisture diffusion into the C60 acceptor layer. The detailed working mechanism of the compound Bphen/MoO3 CBL in the OPV cells is also argued.

1% solar cells derived from ultrathin carbon nanotube photoabsorbing films

Using a carbon nanotube photoabsorbing film <5 nm in thickness, we demonstrate a 1% solar cell. Specifically, polymer wrapped, highly monochiral (7, 5) nanotubes are implemented in a bilayered heterojunction with acceptor C60. The nanotubes drive 63% of the conversion, several times stronger than previously demonstrated. Peak external quantum efficiency (QE) of 43% at the nanotube bandgap (1055 nm) and power conversion efficiency of 0.95% and 1.02% at 1.0 and 1.5 suns, respectively, are achieved. The high internal QE from the ultrathin layers suggests that nanostructured or multijunction cells exploiting multiple nanotube layers will be many times more efficient.

Large face to face tetraphenylporphyrin/fullerene nanoaggregates. A DFT study

Large face to face tetraphenylporphyrin/fullerene nanoaggregates containing up six C60 units and six tetraphenylporphyrin (H2TPP) or tetraphenylporphyrinato-zinc (TPP-Zn) moieties have been studied using dispersion corrected PBE/def2-SVP level of theory. It has been found that most important contribution to the binding energy between fragments comes from dispersion interactions. The binding energies for Zn containing nanoaggregates are slightly higher than those for metal free ones what is not related to the difference in dispersion contributions to the binding energy but comes from DFT term. Center to center distances for large nanoaggregates are shorter than those for the complexes H2TPP/C60 and TPP-Zn/C60 and this effect is more obvious for metal free nanoaggregates. According to the calculations, the band gap of nanoaggregates barely depends on its size being close to 2eV. The nature of electronic excitations in Zn containing nanoaggregates has strong charge transfer (CT) contribution and does not depend on nanoaggregate size, while for metal free nanoaggregate most of low energy excitations are not CT by nature. Ionization potentials and electron affinities of nanoaggregates depend strongly on their size. Polaron cations are uniformly delocalized over donor H2TPP or TPP-Zn units, while polaron anions are delocalized over acceptor C60 units. The reorganization energies calculated for hole and electron transport decreased linearly with 1/n where n is the number of repeating units in nanoaggregate.

Donor–Acceptor Shape Matching Drives Performance in Photovoltaics

AbstractWhile the demonstrated power conversion efficiency of organic photovoltaics (OPVs) now exceeds 10%, new design rules are required to tailor interfaces at the molecular level for optimal exciton dissociation and charge transport in higher efficiency devices. We show that molecular shape‐complementarity between donors and acceptors can drive performance in OPV devices. Using core hole clock (CHC) X‐ray spectroscopy and density functional theory (DFT), we compare the electronic coupling, assembly, and charge transfer rates at the interface between C60 acceptors and flat‐ or contorted‐hexabenzocorone (HBC) donors. The HBC donors have similar optoelectronic properties but differ in molecular contortion and shape matching to the fullerene acceptors. We show that shape‐complementarity drives self‐assembly of an intermixed morphology with a donor/acceptor (D/A) ball‐and‐socket interface, which enables faster electron transfer from HBC to C60. The supramolecular assembly and faster electron transfer rates in the shape complementary heterojunction lead to a larger active volume and enhanced exciton dissociation rate. This work provides fundamental mechanistic insights on the improved efficiency of organic photovoltaic devices that incorporate these concave/convex D/A materials.

The influence of donor material on achieving high photovoltaic response for organic bulk heterojunction cells with small ratio donor component

Authors demonstrated impact of series small ratio donors in C60 matrix on photovoltaic (PV) performance. A series of donor materials such as N′,N′-Di-1-naphthyl-N′,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB), 4,-4′-Bis(carbazol-9-yl) (CBP), 4,4′,4″-tris(N-3-methylphenyl-N-phenyl-amine)triphenyl-amine (m-MTDATA), copper phthalocyanine (CuPc) and 4,4,4-tris(n-carbazolyl-triphenyl-amine) (TCTA) were blended with fullerene (C60) by different ratio. It was found that although donor–acceptor (DA) interface in planar heterojunction (PHJ) structure increased charge separation probability at the near interface section, the PV response was stronger for bulk heterojunction (BHJ) with low-ratio donor doping into C60 matrix in which exciton dissociation can take place immediately after photon absorption without a diffusion progress. The power conversion efficiency (PCE) of BHJ-PV cell based on NPB donor reaches 2.25%, which is double of that of the PHJ cell. In terms of our series results we obtained that ΔEHOMO (HOMOC60–HOMOdonor) between C60 acceptor and donors would provide a maximal influence on achievement of a maximal PCE and an optimal ΔEHOMO locates around 0.8eV, which implies that dissociation of photo-exciton at C60 matrix needs feasible driving force. More detail mechanism was also argued.

Cyclotriveratrylene–carbazole cage for self-assembly of C60

A novel open-bowl pyramidal compound cyclotriveratrylene–carbazole (CTV–CZ) was designed and synthesized. Its absorption and emission spectroscopic properties in different organic solvents were characterized. Three absorption peaks of CTV–CZ around 235nm, 260nm and 290nm were observed. The fluorescence emission peaks were at about 353nm and 368nm using a solution of ∼10−7M CTV–CZ. The intermolecular interaction between CTV–CZ and fullerene (C60) was studied in detail by UV–vis absorption and fluorescence spectra. CTV–CZ could associate with C60 to form supramolecular complex with 1:1 molar ratio and the binding constant was estimated to be 5.0(6)×104M−1 by fluorimetry. The formation of inclusion complex for CTV–CZ and C60 was further confirmed by 1H NMR and cyclic voltammetry. The downfield shift of protons of CTV–CZ in its 1H NMR spectrum and the decrease of redox currents on addition of C60 showed that photo induced electron transfer (PET) occurred from the electron donor CTV and CZ units to the electron acceptor C60.

Temperature Activation of the Photoinduced Charge Carrier Generation Efficiency in Quaterthiophene:C60 Mixed Films

We measure photoinduced excitations in a dicyanovinyl end-capped methylated quaterthiophene derivative in blends with the electron acceptor C60, as already employed in organic photovoltaics. By usi...