Light induced polarons in solid films of polymer-fullerene blends were studied by applying photoluminescence (PL), photo induced absorption (PIA) techniques as well as electron spin resonance (ESR). The materials used were poly(3-hexylthiophene) (P3HT) and poly-[2-methoxy, 5-(2'-ethyl-hexyloxy) phenylene vinylene] (MEH-PPV) as donors. As acceptors we used [6,6]-phenyl-C61-butyric acid methyl ester ([C60]PCBM) and various soluble C70-derivates: [C70]PCBM, diphenylmethano[70]fullerene oligoether (C70-DPM-OE), C70-DPM-OE2, and two fullerene dimers, C70-C70 and C60-C70 (all shown in figure 1). In all blends containing C70 we found typical signatures which were absent if [C60]PCBM was used as acceptor. Light-induced ESR revealed signals at g>=2.005, which we previously assigned to an electron localized on the C70 cage, the PIA measurements showed a new sub-bandgap absorption band at 0.92 eV, which we correspondingly ascribe to C70 radical anions formed in the course of photoinduced electron transfer from donor to acceptor.
For many years the faith of organic photovoltaics has been linked to the one of fullerene, since fullerene has been considered the electron-acceptor of choice in bulk heterojunctions solar cells. In the last years, the number of molecules that can be very competitive in replacing fullerene has increased significantly. In this work, we study by means of different theoretical methods five molecules that have demonstrated to work effectively as acceptors in organic heterojunctions. We discuss the comparison of simulated absorption spectra with the experimental spectra.
Recombination of photogenerated charge carriers in polymer bulk heterojunction (BHJ) solar cells reduces the short circuit current (Jsc) and the fill factor (FF). Identifying the mechanism of recombination is, therefore, fundamentally important for increasing the power conversion efficiency. Light intensity and temperature dependent current-voltage measurements on polymer BHJ cells made from a variety of different semiconducting polymers and fullerenes show that the recombination kinetics are voltage dependent and evolve from first order recombination at short circuit to bimolecular recombinat…
Organic solar cells have the potential to be low-cost and efficient solar energy converters, with a promising energy balance. They are made from carbon-based semiconductors, which exhibit favourable light absorption and charge generation properties, and can be manufactured by low temperature processes such as printing from solvent-based inks, which are compatible with flexible plastic substrates or even paper. In this review, we will present an overview of the physical function of organic solar cells, their state-of-the-art performance and limitations, as well as novel concepts to achieve a be…
The high recombination rate of photogenerated carriers is the bottleneck of photocatalysis, severely limiting the photocatalytic efficiency. Here, we develop a dipole-scheme (D-scheme for short) photocatalytic model and materials realization. The D-scheme heterojunction not only can effectively separate electrons and holes by a large polarization field, but also boosts photocatalytic redox reactions with large driving photovoltages and without any carrier loss. By means of first-principles and GW calculations, we propose a D-scheme heterojunction prototype with two real polar materials, PtSeTe…
Organic bulk-heterojunctions (BHJ) and solar cells containing the trimetallic nitride endohedral fullerene 1-[3-(2-ethyl)hexoxy carbonyl]propyl-1-phenyl-Lu3N@C80 (Lu3N@C80-PCBEH) show an open circuit voltage (VOC) 0.3 V higher than similar devices with [6,6]-phenyl-C[61]-butyric acid methyl ester (PC61BM). To fully exploit the potential of this acceptor molecule with respect to the power conversion efficiency (PCE) of solar cells, the short circuit current (JSC) should be improved to become competitive with the state of the art solar cells. Here, we address factors influencing the JSC in blend…
The major loss mechanism of photogenerated polarons was investigated in P3HT:PCBM solar cells by the photo-CELIV technique. For pristine and annealed devices, we find that the experimental data can be explained by a bimolecular recombination rate reduced by a factor of about ten (pristine) and 25 (annealed) as compared to Langevin theory. Aided by a macroscopic device model, we discuss the implications of the lowered loss rate on the characteristics of polymer:fullerene solar cells.
The ongoing depletion of fossil fuels has led to an intensive search for additional renewable energy sources. Solar-based technologies could provide sufficient energy to satisfy the global economic demands in the near future. Photovoltaic (PV) cells are the most promising man-made devices for direct solar energy utilization. Understanding the charge separation and charge transport in PV materials at a molecular level is crucial for improving the efficiency of the solar cells. Here, we use light-induced EPR spectroscopy combined with DFT calculations to study the electronic structure of charge …
In polymeric semiconductors, charge carriers are polarons, which means that the excess charge deforms the molecular structure of the polymer chain that hosts it. This effect results in distinctive signatures in the vibrational modes of the polymer. We probe polaron photo- generation dynamics at polymer:fullerene heterojunctions by monitoring its time-resolved resonance-Raman spectrum following ultrafast photoexcitation. We conclude that polarons emerge within 200 fs, which is nearly two orders of magnitude faster than exciton localisation in the neat polymer film. Surprisingly, further vibrati…
Blends of polymer and C$_{60}$-derived molecules are in the spotlight in recent years for application in organic photovoltaics, forming what is known as bulk-heterojunction active layers. The character of the heterojunction is determinant, with clear relevance of morphology and phase separation. To better understand the morphology of the systems, we present a classical molecular dynamics (CMD) simulation of polymer/fullerene (P3HT/C$_{60}$) blends, coming from different starting points, using the specifically designed Nanomol Force Field based on the Universal Force Field. We use not-so-short …
Organic solar cells (OSCs) containing an active layer consisting of a nanostructured blend of a conjugated polymer like poly(3-hexylthiophene) (P3HT) and an electron acceptor molecule have the potential of competing against silicon-based photovoltaic panels. However, this potential is unfulfilled primarily due to interrelated production and stability issues. The generally employed spin coating process for fabricating organic solar cells cannot be scaled up. Recently, He et al., have reported that the gelation of P3HT with [6,6]-phenyl-C61-butyric acid methyl ester (PC60BM) under sub-ambient co…
We investigated photogeneration yield and recombination dynamics in blends of poly(3-hexyl thiophene) (P3HT) and poly[2-methoxy-5-(30,70-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) with [6,6]- phenyl-C61 butyric acid methyl ester (PC61BM) by means of temperature dependent time delayed collection field (TDCF) measurements. In MDMO-PPV:PC61BM we find a strongly field dependent polaron pair dissociation which can be attributed to geminate recombination in the device. Our findings are in good agreement with field dependent photoluminescence measurements published before, supporting a scena…
Hybrid conjugated polymer/fullerene filaments based on MEH-PPV/PVP/PCBM are prepared by electrospinning, and their properties assessed by scanning electron, atomic and lateral force, tunnelling, and confocal microscopy, as well as by attenuated total reflection Fourier transform-infrared spectroscopy, photoluminescence quantum yield and spatially-resolved fluorescence. Highlighted features include ribbon-shape of the realized fibers, and the persistence of a network serving as a template for heterogeneous active layers in solar cell devices. A set of favorable characteristics is evidenced in t…
We investigate the mechanical properties of $π$-conjugated polymeric materials composed of regioregular poly(3-hexylthiophene) (P3HT) and fullerene C$_{60}$ using coarse-grained molecular dynamics simulations. Specifically, we perform tensile simulations of P3HT:C$_{60}$ composites with varied degrees of polymerization and C$_{60}$ mass fractions to obtain their stress-strain responses. Decomposition of stress tensor into kinetic energy and virial contributions indicates that the tensile moduli of the pure P3HT samples are greatly dependent on non-bonded interactions and on bonded interactions…
Colloidal zinc oxide (ZnO) nanoparticles are frequently used in the field of organic photovoltaics for the realization of solution-producible, electron-selective interfacial layers. Despite of the widespread use, there is a lack of detailed investigations regarding the impact of structural properties of the particles on the device performance. In this work, ZnO nanoparticles with varying surface-area-to-volume ratio were synthesized and implemented into polymer-fullerene bulk heterojunction solar cells with a gas-permeable top electrode. By comparing the electrical characteristics before and a…
We determined the dominant polaron recombination loss mechanism in pristine and annealed polythiophene:fullerene blend solar cells by applying the photo-induced charge extraction by linearly increasing voltage (photo-CELIV) method in dependence on temperature. In pristine samples, we find a strongly temperature dependent bimolecular polaron recombination rate, which is reduced as compared to the Langevin theory. For the annealed sample, we observe a polaron decay rate which follows a third order of carrier concentration almost temperature independently.
The separation of photogenerated polaron pairs in organic bulk heterojunction solar cells is the intermediate but crucial step between exciton dissociation and charge transport to the electrodes. In state-of-the-art devices, above 80% of all polaron pairs are separated at fields of below $10^7$V/m. In contrast, considering just the Coulomb binding of the polaron pair, electric fields above $10^8$V/m would be needed to reach similar yields. In order to resolve this discrepancy, we performed kinetic Monte Carlo simulations of polaron pair dissociation in donor--acceptor blends, considering deloc…
Mixed-dimensional van der Waals heterojunctions comprising polymer and twodimensional (2D) semiconductors have many characteristics of an ideal charge separation interface for optoelectronic and photonic applications. However, the photoelectron dynamics at polymer- 2D semiconductor heterojunction interfaces are currently not sufficiently understood to guide the optimization of devices for these applications. This manuscript is a report of a systematic exploration of the time-dependent photophysical processes that occur upon photoexcitation of a type-II heterojunction between the polymer PTB7 a…
Under open-circuit condition, the current is not extracted and the photogenerated carriers in principle disappear only by recombination. We study the open-circuit voltage $V_{\rm OC}$ and transient photovoltage under the effect of bulk recombination in a medium with energetic disorder by using the multiple trapping (MT) model. The key parameter in the MT model is the dispersion parameter $α$ given by the ratio of thermal energy to the characteristic energy of trap states. We show that $V_{\rm OC}$ depends linearly on the logarithm of the light intensity and the slope depends on the $α$ of the …
The electronic properties of co-evaporated mixtures (blends) of manganese phthalocyanine and the fullerene C$_{60}$ (MnPc:C$_{60}$) have been studied as a function of the concentration of the two constituents using two supplementary electron spectroscopic methods, photoemission spectroscopy (PES) as well as electron energy-loss spectroscopy (EELS) in transmission. Our PES measurements provide a detailed picture of the electronic structure measured with different excitation energies as well as different mixing ratios between MnPc and C$_{60}$. Besides a relative energy shift, the occupied elect…
In organic bulk heterojunction solar cells, the open circuit voltage ($V_\mathrm{oc}$) suffers from an ultra-high loss at low temperatures. In this work we investigate the origin of the loss through calculating the $V_\mathrm{oc}-T$ plots with the device model method systematically and comparing it with experimentally observed ones. When the energetic disorder is incorporated into the model by considering the disorder-suppressed and temperature-dependent charge carrier mobilities, it is found that for nonselective contacts the $V_\mathrm{oc}$ reduces drastically under the low temperature regim…
A polysquaraine low band gap polymer was synthesized by Yamamoto coupling of a monomeric dibromo indolenine squaraine dye. The resulting polymer has a weight average molar mass in the order of Mw ~30.000-50.000 and a polydispersity of ca. 1.7 as determined by gel-permeation chromatography (GPC). The electronic properties of monomer and polymer were investigated by cyclic voltammetry, absorption and emission spectroscopy. Owing to exciton coupling the absorption bands of the polymer are red-shifted and strongly broadened compared to the monomer squaraine dye. Bulk heterojunction solar cells wer…
We studied the recombination dynamics of charge carriers in organic bulk heterojunction solar cells made of the blend system poly(2,5-bis(3-dodecyl thiophen-2-yl) thieno[2,3-b]thiophene) (pBTCT-C12):[6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) with a donor--acceptor ratio of 1:1 and 1:4. The techniques of charge carrier extraction by linearly increasing voltage (photo-CELIV) and, as local probe, time-resolved microwave conductivity (TRMC) were used. We observed a difference in the initially extracted charge carrier concentration in the photo-CELIV experiment by one order of magnitude, w…
The active layer crystallinity has long been associated with favourable organic solar cells (OSCs) properties such as high mobility and Fill Factor. In particular, this applies to acceptor materials such as fullerene-derivatives and the most recent Non-Fullerene Acceptors (NFAs), which are now surpassing 19% of Power Conversion Efficiency. Despite these advantages are being commonly attributed to their 3-dimensional crystal packing motif in the single crystal, the bridge that links the acceptor crystal packing from single crystals to solar cells has not clearly been shown yet. In this work, we…
Non-fullerene acceptors based on perylenediimides (PDIs) have garnered significant interest as an alternative to fullerene acceptors in organic photovoltaics (OPVs), but their charge transport phenomena are not well understood, especially in bulk heterojunctions (BHJs). Here, we investigate charge transport and current fluctuations by performing correlated low-frequency noise and impedance spectroscopy measurements on two BHJ OPV systems, one employing a fullerene acceptor and the other employing a dimeric PDI acceptor. In the dark, these measurements reveal that PDI-based OPVs have a greater …
In organic solar cells, photogenerated singlet excitons form charge transfer (CT) complexes, which subsequently split into free charge carriers. Here, we consider the contributions of excess energy and molecular quadrupole moments to the charge separation process. We investigate charge photogeneration in two separate bulk heterojunction systems consisting of the polymer donor PTB7-Th and two non-fullerene acceptors, ITIC and h-ITIC. CT state dissociation in these donor-acceptor systems is monitored by charge density decay dynamics obtained from transient absorption experiments. We study the el…
We present a first-principles investigation of the excited-state properties of electron acceptors in organic photovoltaics including C$_{60}$, C$_{70}$, [6,6]-phenyl-C$_{61}$-butyric-acid-methyl-ester ([C$_{60}$]PCBM), and bis-[C$_{60}$]PCBM using many-body perturbation theory within the Hedin's $G_0W_0$ approximation and an efficient Lanczos approach. Calculated vertical ionization potentials (VIP) and vertical electron affinities (VEA) of C$_{60}$ and C$_{70}$ agree very well with experimental values measured in gas phase. The density of states of all three molecules is also compared to phot…
In organic photovoltaics, the mechanism by which free electrons and holes are generated overcoming the Coulomb attraction is a currently much debated topic. To elucidate this mechanism at a molecular level, we carried out a combined electronic structure and quantum dynamical analysis that captures the elementary events from the exciton dissociation to the free carrier generation at polymer/fullerene donor-acceptor heterojunctions. Our calculations show that experimentally observed efficient charge separations can be explained by a combination of two effects: First, the delocalization of charge…
Understanding of degradation mechanisms in polymer:fullerene bulk-heterojunctions on the microscopic level aimed at improving their intrinsic stability is crucial for the breakthrough of organic photovoltaics. These materials are vulnerable to exposure to light and/or oxygen, hence they involve electronic excitations. To unambiguously probe the excited states of various multiplicities and their reactions with oxygen, we applied combined magneto-optical methods based on multifrequency (9 and 275 GHz) electron paramagnetic resonance (EPR), photoluminescence (PL), and PL-detected magnetic resonan…
We study the photoabsorption properties of photoactive bulk polymer/ fullerene/nanotube heterojunctions in the near-infrared region. By combining pump-probe spectroscopy and linear response time-dependent density functional theory within the random phase approximation (TDDFT-RPA) we elucidate the excited state dynamics of the $E_{11}$ transition within (6,5) and (7,5) single-walled carbon nanotubes (SWNTs) and combined with poly(3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl-C$_{61}$-butyric acid methyl ester (PCBM) in P3HT/PCBM/SWNT blended samples. We find the presence of a photoinduced …
Non-fullerene acceptor (NFA)-based ternary bulk heterojunction solar cells (TSC) are the most efficient organic solar cells (OSCs) today due to their broader absorption and quantum efficiencies (QE) often surpassing those of corresponding binary blends. We study how the energetics driving charge transfer at the electron donor:electron acceptor (D/A) interfaces impact the QE in blends of PBDB-T-2F donor with several pairs of lower bandgap NFAs. As in binary blends, the ionization energy offset between donor and acceptor (ΔIE) controls the QE and maximizes for ΔIE > 0.5 eV. However, ΔIE is not c…
Inclusion of heavy metal atoms in a polymer backbone allows transitions between the singlet and triplet manifolds. Interfacial dissociation of triplet excitons constitutes a viable mechanism for enhancing photovoltaic (PV) efficiencies in polymer heterojunction-based solar cells. The PV efficiency from polymer solar cells utilizing a ladder-type poly (para-phenylene) polymer (PhLPPP) with trace quantity of Pd atoms and a fullerene derivative (PCBM) is much higher than its counterpart (MeLPPP) with no Pd atom. Evidence is presented for the formation of a weak ground-state charge-transfer comple…
In photovoltaic diodes recombination of photogenerated electrons and holes is a major loss process. Biological light harvesting complexes (LHCs) prevent recombination via the use of cascade structures, which lead to spatial separation of charge-carriers1. In contrast, the nanoscale morphology and high charge densities in organic photovoltaic cells (OPVs) give a high rate of electron-hole encounters, which should result in the formation of spin triplet excitons, as in organic light emitting diodes (OLEDs)2. OPVs would have poor quantum efficiencies if every encounter lead to recombination, but …
