Hybrid Organic Inorganic Perovskites (HOIPs) have attracted a lot of attention since in 2009 a thin-film solar cell was produced with a PCE of almost 4%. Since then record after record in PCEs of perovskite solar cells using so-called 3D hybrid perovskites has been broken, reaching nowadays a PCE of more than 25%. The main drawback relates to the limited stability of performances by defects and/or ion mobility in the inorganic lattice. In an effort to improve stability and reduce hysteresis of the solar cells quasi 2D hybrid perovskites have been explored, quite often using besides methylammonium cations larger cations, e.g. butylammonium or more often phenylethylammonium cations. In recent years our research group is exploring the use of more complex and potentially functional larger cations composed of pyrene or carbazole subunits. We could demonstrate a substantial improvement of the thermal stability and under high relative humidity (77%) of active layers for perovskite solar cells composed of such quasi 2D hybrid perovskites. In this contribution, we discuss the basic mechanism of this stability effect. Furthermore, we present some recent results obtained using oligothiophene and a benzothieno[3,2-b]benzothiophene (BTBT) alkylammonium cation into the organic layer of a 2D layered lead iodide perovskite. Structural characterization, phase stability, and photoconductivity measurements of (n=1) and quasi 2D perovskites will be presented. Extraordinary high stability was observed for such layers under thermal stress (>240°C) and under high moisture conditions. Potential mechanisms and implications for alternative structures will be discussed.
Hybrid organic-inorganic perovskites (HOIPs) have received a lot of research attention over the past decade, related to the rapid increase in the power conversion efficiency of perovskite solar cells. The materials used for solar cells are mainly three-dimensional (3D) HOIPs, with a general formula of ABX3 with A being a small monovalent organic cation, B a divalent metal ion, and X a halide anion. More recently, the related material class of 2D HOIPs, with a general formula of (A*)2BX4, is receiving increased attention by combining a generally enhanced material stability compared to 3D HOIPs with a much higher degree of compositional flexibility. 2D HOIPs can accommodate bulkier organic cations (A*) with a conjugated organic core. Depending on the relative alignment between the frontier energy levels of the organic core and the inorganic framework, energy/charge transfer between the components of the hybrid is possible. We built in a carbazole derivative as the organic cation into a 2D HOIP. Through electron paramagnetic resonance experiments combined with computational calculations, we show that excitons generated in the inorganic layer undergo charge transfer at the organic−inorganic interface, resulting in a positive polaron delocalized over several carbazole moieties. In another material system, we incorporate an organic charge-transfer complex (CTC) with a pyrene derivative as the donor and TCNQ as the acceptor into the organic layer of a 2D HOIP. Based on time-resolved spectroscopy, we show that holes are transferred to the inorganic layer upon excitation of the CTC while electrons stay localized on TCNQ acceptor molecules.
KEYWORDS: Organic photovoltaics, Solar cells, Dielectric spectroscopy, Dielectrics, Heterojunctions, Renewable energy, Chemical engineering, Physics, Current controlled current source
Organic photovoltaics (OPV) show strong potential for a number of renewable energy applications because of some specifically appealing features (light weight, flexibility, color, …). Over the past decade, the power conversion efficiencies of organic solar cells have strongly risen to values surpassing the 10% threshold, mainly due to strong efforts in chemical engineering of the photoactive components, architectural device optimization and acquisition of fundamental insights in the underlying device physics. As part of the device optimization, the use of conjugated polyelectrolyte (CPE) interfacial layers has been introduced as a popular and powerful way to boost the inherent I-V characteristics. In the presented work, we applied impedance spectroscopy to probe the dielectric permittivity of a series of polythiophene-based CPE interlayer materials as a means to postulate design rules toward novel generation interfacial layers. The presence of ionic pendant groups grants the formation of a capacitive double layer, boosting the charge extraction and device efficiency. A counteracting effect is that the material’s affinity with respect to the underlying photoactive layer diminishes. To enhance the interlayer-photoactive layer compatibility, copolymer structures containing a certain amount of non-ionic side chains are found to be beneficial.
When state-of-the-art bulk heterojunction organic solar cells with ideal morphology are exposed to prolonged storage or operation at elevated temperatures, a thermally induced disruption of the active layer blend can occur, in the form of a separation of donor and acceptor domains, leading to diminished photovoltaic performance. Toward the long-term use of organic solar cells in real-life conditions, an important challenge is, therefore, the development of devices with a thermally stable active layer morphology. Several routes are being explored, ranging from the use of high glass transition temperature, cross-linkable and/or side-chain functionalized donor and acceptor materials, to light-induced dimerization of the fullerene acceptor. A better fundamental understanding of the nature and underlying mechanisms of the phase separation and stabilization effects has been obtained through a variety of analytical, thermal analysis, and electro-optical techniques. Accelerated aging systems have been used to study the degradation kinetics of bulk heterojunction solar cells in situ at various temperatures to obtain aging models predicting solar cell lifetime. The following contribution gives an overview of the current insights regarding the intrinsic thermally induced aging effects and the proposed solutions, illustrated by examples of our own research groups.
In this Proceedings paper, we report on the synthesis of a family of polythiophene-based conjugated polyelectrolytes, both homopolymers and random copolymers varying in the building block ratio and counter ions, toward a better fundamental understanding of the structure-property relations of these ionic derivatives in organic photovoltaics. One of the ionic homopolymers was successfully implemented as a donor material in fully solution-processed efficient bi-layer solar cells (up to 1.6% PCE in combination with PC71BM) prepared by the low impact meniscus coating technique. On the other hand, these imidazolium-substituted polythiophenes were also applied as materials for electron transport layers (ETLs), boosting the I-V properties of PCDTBT:PC71BM solar cell devices up to average PCE values of 6.2% (~20% increase), which is notably higher than for previously reported ETL materials. Advanced scanning probe microscopy techniques were used to elucidate the efficiency enhancing mechanism.
This work is part of the inter-laboratory collaboration to study the stability of seven distinct sets of state-of-the-art organic photovoltaic (OPVs) devices prepared by leading research laboratories. All devices have been shipped to and degraded at the Danish Technical University (DTU, formerly RISO-DTU) up to 1830 hours in accordance with established ISOS-3 protocols under defined illumination conditions. In this work we present a summary of the degradation response observed for the NREL sample, an inverted OPV of the type ITO/ZnO/P3HT:PCBM/PEDOT:PSS/Ag/Al, under full sun stability test. The results reported from the combination of the different characterization techniques results in a proposed degradation mechanism. The final conclusion is that the failure of the photovoltaic response of the device is mainly due to the degradation of the electrodes and not to the active materials of the solar cell.
Seven distinct sets (n ≥ 12) of state of the art organic photovoltaic devices were prepared by leading research laboratories in a collaboration
planned at the Third International Summit on Organic Photovoltaic Stability (ISOS-3). All devices were shipped to DTU and characterized
simultaneously up to 1830 h in accordance with established ISOS-3 protocols under three distinct illumination conditions: accelerated full sun
simulation; low level indoor fluorescent lighting; and dark storage with daily measurement under full sun simulation. Three nominally
identical devices were used in each experiment both to provide an assessment of the homogeneity of the samples and to distribute samples for
a variety of post soaking analytical measurements at six distinct laboratories enabling comparison at various stages in the degradation of the
devices. Characterization includes current-voltage curves, light beam induced current (LBIC) imaging, dark lock-in thermography (DLIT),
photoluminescence (PL), electroluminescence (EL), in situ incident photon-to-electron conversion efficiency (IPCE), time of flight secondary
ion mass spectrometry (TOF-SIMS), cross sectional electron microscopy (SEM), UV visible spectroscopy, fluorescence microscopy, and
atomic force microscopy (AFM). Over 100 devices with more than 300 cells were used in the study. We present here design of the device
sets, results both on individual devices and uniformity of device sets from the wide range of characterization methods applied at different
stages of aging under the three illumination conditions. We will discuss how these data can help elucidate the degradation mechanisms as well
as the benefits and challenges associated with the unprecedented size of the collaboration.
Optimizing the post-production annealing conditions of polymer:fullerene bulk heterojunction solar cells is vitally
important, not only for fine-tuning the morphology - thus increasing the efficiency - but also for retaining the desired
morphology during long-term operation. However, optimal conditions for annealing temperatures and times can only be
chosen, once thermal transition temperatures and annealing kinetics of the blends are well-known. For instance, for
systems with glass transition temperatures (Tg) lower than the maximum device operation temperature of 80°C, the
mobility needed for morphology coarsening is present, leading to efficiencies decreasing in the course of time. Using
advanced fast-scanning thermal analysis techniques, the formation of nuclei and growth of crystals during heating or
cooling can be reduced or avoided, and thus, the fast crystallization processes occurring during annealing of the
polymer:fullerene blends can be followed. In this study, non-isothermal and isothermal crystallization kinetics of the
P3HT:PCBM (poly(3-hexyl thiophene: [6,6] -phenyl C61 - butyric acid methyl ester) and P3HT:bis-PCBM blends are
investigated and compared by using Rapid Heating Cooling Calorimetry (RHC).
Long alkyl chain ligands such as oleic acid (OLA) which cover the as-prepared PbS nanodots act as an insulating layer
that impedes efficient charge transfer in PbS nanodots:polymer hybrid solar cells. The replacement of OLA with tailored
ligands of an appropriate chain length is needed to achieve a noticeable enhancement of photovoltaic performance.
Several studies have centered on the ligand exchange prior to casting the PbS film1,2,3. However, this post synthesis
approach requires careful consideration for the choice of a ligand as clustering of the nanodots has to be avoided.
Recently, a new approach that allows direct chemical ligand replacement in a blended mixture of PbS:P3HT has been
demonstrated 4,5,6. In this contribution, the latter approach (post-fabrication) was compared with the post-synthesis ligand
exchange. We investigated the effect of the ligand exchange processes to the charge separation dynamics in the
P3HT:PbS blends by steady-state and time-resolved photoluminescence (PL). Hexanoic acid and acetic acid were used
as a short-length ligand for the post fabrication approach while decylamine, octylamine and butylamine were used for the
post-synthesis approach. As expected, decreasing the chain length of the ligand led to an increase of the P3HT
fluorescence quenching. The absence of enhancement of PbS luminescence due to energy transfer from P3HT and the
dependence of the quenching efficiency on the bulkiness of the ligands coating the QDs suggest that the quenching of the
P3HT fluorescence is dominated by electron transfer to PbS quantum dots (QDs). In addition, the fluorescence
quenching is also less prominent in the P3HT with higher regioregularity (RR) suggesting an enhanced phase separation
in the blend due to more densely packed nature of conjugated polymer with higher RR.
We focus upon the role of interfacial energetics and morphology in influencing the separation of CT states into dissociated charge carriers. In particular, we undertake transient optical studies of films comprising regioregular poly(3-hexylthiophene) (P3HT) blended with a series of perylene-3,4:9,10-tetracarboxydiimide (PDI) fullerene electron acceptors. For the PDI film series, we observe a close correlation between the PDI electron affinity and the efficiency of charge separation. This correlation is discussed in the context of studies of charge photogeneration for other organic donor/acceptor blend films, including other polymers, blend compositions, and the widely used electron phenyl-C61-butyric acid methyl ester(PCBM). Furthermore, we compare the charge recombination dynamics observed in films comprising P3HT blended with three fullerene derivatives: PCBM and two alternative pyrazolinofullerenes. Transient absorption data indicate that replacement of PCBM with either of the pyrazolinofullerene derivatives results in a transition from nongeminate to monomolecular (geminate) recombination dynamics. We show that this transition cannot be explained by a difference in interfacial energetics. However, this transition does correlate with nanomorphology data that indicate that both pyrazolinofullerenes yield a much finer phase segregation with correspondingly smaller domain sizes than observed with PCBM. Our results therefore provide clear evidence of the role of nanomorphology in determining the nature of recombination dynamics in such donor/acceptor blends.
Poly-3-AlkylThiophenes (P3ATs) with an n-alkyl chain length varying from C3 till C9 were synthesized by
using the Rieke method. Subsequently, these materials were used to make P3AT/PCBM blends which were investigated
in bulk heterojunction (BHJ) solar cells. The phase diagram of a P3H(exyl)T:PCBM blend was measured by means of
standard and modulated temperature differential scanning calorimetry (DSC and MTDSC). A single glass transition is
observed for all compositions. The glass transition temperature (Tg) increases with increasing PCBM concentration: from
12 °C for pure P3HT to 131 °C for pure PCBM. The observed range of Tg's defines the operating window for thermal
annealing and explains the long-term instability of both morphology and photovoltaic performance of P3HT:PCBM solar
cells. All regioregular P3ATs allow for efficient fiber formation in several solvents. The fibers formed are typically 15 to
25 nm wide and 0.5 to >4 μm long and mainly crystalline. By means of temperature control the fiber content in the
casting solution for P3AT:PCBM BHJ solar cells is controlled while keeping the overall molecular weight of the polymer
in the blend constant. In this way, fiber isolation and the use of solvent mixtures are avoided and with P3HT nanofibers,
a power conversion efficiency of 3.2 % was achieved. P3AT:PCBM BHJ solar cells were also prepared from P3B(utyl)T,
P3P(entyl)T and P3HT using the good solvent o-dichlorobenzene and a combination of slow drying and thermal
annealing. In this way, power conversion efficiencies of 3.2, 4.3, and 4.6 % were obtained, respectively. P3PT is proved
to be a potentially competitive material compared to P3HT.
set of novel regioregular poly(3-hexylthiophene)-based random copolymers containing varying ratios of
ester functionalized alkyl side chains were synthesized using the Rieke method. The percentage of functionalized side
chain varied between 10 and 50 mol% for each copolymer. Using post-polymerization reactions, the ester functions in
the alkyl side chain were hydrolyzed to yield an alcohol or acid group. These groups are available for further
functionalization reactions, so a wide variety of secondary functionalities may be covalently attached to the conjugated
polymer. The copolymers were applied in polymer: fullerene bulk heterojunction solar cells (BHJSCs) with [6,6]-phenyl
-C61-butyric acid methyl ester (PCBM) as electron acceptor. The influence of side-chain functionalities on absorption,
device performance and layer morphology depends on the ratio and nature of the functionalized side chains. For a 9/1
copolymer, containing 10% of functionalized side chains, behaviour and efficiency in BHJSCs comparable to
P3HT:PCBM solar cells were observed.
Control of morphology is a key issue in order to improve the performance of organic bulk heterojunction solar
cells. Solar cells consisting of a blend of regioregular P3HT (poly(3-hexylthiophene)) and PCBM ([6-6]-phenyl
C61 butyric acid methyl ester) have demonstrated the highest efficiencies until now (up to 5 %). This
performance was achieved by applying a post-production annealing, which is considered to induce a dual
crystallization behavior. In order to control and tune the morphology, the phase behavior needs to be
described in terms of the underlying fundamental thermodynamics. Hence, it is essential to obtain a phase
diagram of the blend. In this study, the state diagram of P3HT:PCBM blends is measured by means of
standard and modulated temperature differential scanning calorimetry (DSC). For the first time, the glass
transition (Tg) of PCBM could be determined. All blends evidenced a single Tg, indicating an homogeneous
blend is formed. Phase separation is thus only induced from crystallization and no "intrinsic" phase separation
is occurring in the blend.
We have realized a light-emitting organic field-effect transistor (LEOFET). Excitons are generated at the interface
of an n-type and a p-type organic semiconductor heterostructure inside the transistor channel. The dimensions
and the position of the p-n heterostructure are defined by photolithography. The recombination region is several
microns from the metal electrodes. Therefore, the exciton quenching probability in this device is reduced.
Numerical simulations show that the recombination region can move within the transistor channel by changing
the biasing conditions.
This paper investigates the thermal stability of organic bulk heterojunction solar cells, with a special focus on the thermal ageing of both photovoltaic parameters and morphology of the active layer. The photovoltaic parameters of a set of bulk heterojunction solar cells were determined by IV-characterization and their bulk morphology was investigated with transmission electron microscopy (TEM). A link could be made between the degradation of the short circuit current under a thermal treatment and the corresponding change in bulk morphology. A possible improvement of the thermal stability of bulk heterojunction solar cells is presented through the use of a polymer with higher glass transition temperature.
Photophysical studies and photovoltaic devices on a low bandgap, high charge-carrier-mobility Poly(Thienylene
Vinylene) (PTV), prepared from a soluble precursor polymer synthesised via the 'dithiocarbamate route', are reported.
In composites with an electron acceptor ([6,6]-phenyl C61- butyric acid methyl ester (PCBM), a soluble fullerene
derivative) photoinduced absorption (PIA) characteristic for charged excitations together with photoluminescence (PL)
quenching are observed indicating photoinduced electron transfer. The "bulk heterojunction" photovoltaic devices using
PTV and PCBM composites show short circuit currents up to 4 mA/cm2 under AM 1.5 white-light illumination. The
photocurrent spectrum of the photovoltaic device shows an onset at about 1.65 eV (750 nm) which corresponds to the
absorption spectrum of the polymer.
Poly(p-Phenylene Vinylene) derivatives are synthesized mostly making use of the polymerization behavior of p-quinodimethane systems. Over the last forty years different synthetic routes have been developed, e.g. Wessling, Gilch, Xanthate and Sulphinyl route. For all these routes mechanistic studies are rather scarce and lead to a controversy between two possible mechanisms: anionic and radical polymerization. In this contribution it becomes clear that high molecular weight materials are associated with a self-initiated radical chain polymerization and low molecular weight materials are obtained via an anionic mechanism. This will be demonstrated for the model system in which a sulphinyl pre-monomer is polymerized in N-Methyl-Pyrrolidone. In this model system both these mechanisms are competing with each other. The observed effects on the product distribution of concentration of reagents, temperature and order in which the reagents are added, are consistent with the conclusion above. The question whether living polymerization can occur will be addressed for the radical mechanism. An experiment with a set of sequential polymerizations gives rise to an evolution of molecular weight consistent with the effect of simple dilution of the reaction medium. The conclusion is that a termination reaction is active, which can be identified as related to traces of oxygen. In these conditions the synthesis of block-type copolymers can not be achieved. For the anionic mechanism an argumentation against such possibility will be presented on the basis of relative acidities.
Optical absorption phenomena and in particular sub band gap absorption features are of great importance in the understanding of processes of charge generation and transport in organic pure and composite semiconductor films. To come towards this objective, an alternative and high sensitive spectroscopic approach is introduced to examine the absorption of light in pure and compound organic semiconductors. Because sub band gap absorption features are typically characterized by very low absorption coefficients, it is not possible to resolve them using common transmission and reflection measurements and high sensitive alternatives are needed. Therefore, a combination of photocurrent (Constant Photocurrent Method CPM/Fourier Transform Photocurrent Spectroscopy FT-PS) and photothermal techniques (Photothermal Deflection Spectroscopy PDS) has been used, increasing sensitivity by a factor of thousand, reaching detectable absorption coefficients ((E) down to 0.1 cm-1. In this way, the dynamic range of measurable absorption coefficients is increased by several orders of magnitude compared to transmission/reflection measurements. These techniques have been used here to characterize ground state absorption of thin films of MDMO-PPV, PCBM and a mixture of both materials in a 1:4 ratio, as typically used in a standard active layer in a fully organic solar cell. The spectra reveal defect related absorption phenomena and significant indication of existing interaction in the ground state between both materials, contrary to the widely spread conviction that this is not the case. Experimental details of the techniques and measurement procedures are explained.
A new precursor route towards conjugated polymers is presented. Whereas difficulties occurred for the preparation of poly(2,5-thienylene vinylene) (PTV) derivatives via the existing precursor routes, PTV has been synthesised via a new developed "dithiocarbamate route" in good yields and satisfactory molecular weight. Structural characterisations of the conjugated polymers reveal an optical band gap around 1.7 eV. Organic field effect transistors and organic based photovoltaic devices were made and the results are discussed. Solar cells were produced using a blend of the precursor polymer and PCBM at various ratios. The conversion of the precursor polymer towards the conjugated polymer was performed in situ in film spin-coated from the blend. Promising energy conversion efficiencies were observed which were still improved by thermal annealing of the device at 70°C.
Current state-of-the-art bulk hetero-junction organic photovoltaic devices will be discussed based on poly(2-methoxy-5-(3',7'-dimethyl-octyloxy))-p-phenylene vinylene, (MDMO-PPV), as an electron donor and (6,6)-phenyl-C61-butric-acid (PCBM)(a soluble C60 derivative) as electron acceptor. A brief review will be provided summarizing recent results on efficiency enhancement on morphological investigations. A significant increase in power conversion efficiency has been demonstrated for devices based on so-called 'sulphinyl' synthesized MDMO-PPV (ηAM1.5 = 2.9%) in comparison with devices based on 'Gilch' synthesized MDMO-PPV (ηAM1.5 = 2.5%). In order to understand the higher efficiency values obtained using a different solvent or a different MDMO-PPV-material, electrical and morphological investigations are being performed. Concerning the latter, it has been shown with various analytical techniques that the morphology of the blended photoactive films and also the power conversion efficiency of the corresponding photovoltaic devices are both simultaneously influenced by preparation conditions such as choice of the solvent and drying conditions.
The microstructure of MDMO-PPV:PCBM blends as used in bulk hetero-junction organic solar cells was studied by Atomic Force Microscopy (AFM) and Kelvin Force Microscopy (KFM) to image the surface morphology and by means of Transmission Electron Microscopy (TEM) to reveal images of the film's interior.
By introducing KFM, it was possible to demonstrate that phase separated domains have different local electrical properties than the surrounding matrix. Since blend morphology clearly influences global electrical properties and photovoltaic performance, an attempt to control the morphology by means of casting conditions was undertaken. By using AFM, it has been proven that not only the choice of solvent, but also drying conditions dramatically influence the blend structure. Therefore, the possibility of discovering the blend morphology by AFM, KFM and TEM makes them powerful tools for understanding today's organic photovoltaic performances and for screening new sets of materials.
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