For almost sixty years, solar energy for space applications has relied on inorganic photovoltaics, evolving from solar cells made of single crystalline silicon to triple junctions based on germanium and III-V alloys. The class of organic-based photovoltaics, which ranges from all-organic to hybrid perovskites, has the potential of becoming a disruptive technology in space applications, thanks to the unique combination of appealing intrinsic properties (e.g. record high specific power, tunable absorption window) and processing possibilities. Here, we report on the launch of the stratospheric mission OSCAR, which demonstrated for the first time organic-based solar cell operation in extraterrestrial conditions. This successful maiden flight for organic-based photovoltaics opens a new paradigm for solar electricity in space, from satellites to orbital and planetary space stations.Nevertheless, already in the fields of aerospace[3] and of organic and hybrid semiconductors [4,5], the specific power (W/kg) was proposed as a valid figure of merit to evaluate PV technologies for space missions. In this regard, Organic Solar Cells (OSCs) and hybrid organic-inorganic Perovskite Solar Cells (PSCs) -termed together as HOPV, Hybrid and Organic PhotoVoltaicsgreatly outperform their inorganic counterparts [4,5]. They represent two novel branches of PV technologies, which saw their rise during the last decade (last few years in the case of PSCs) thanks to their potentially very low production costs. The high absorbance of the photo-active layers in HOPVs allows for efficient light collection within a few hundred nanometers of material, which leads to thicknesses one or two orders of magnitude lower than those of inorganic thin PVs. The rest of the layers making up the solar cell stacks are either as thin as or thinner than the absorbers, and the only thickness (and hence mass) limitation comes from substrate and encapsulation, which can consist of micrometers thick flexible plastic foil [4,5]. The specific power reached to date for perovskite (23 kW/kg) [4] and organic (10 kW/kg)[5] solar cells is thus over 20

In disordered organic semiconductors, the transfer of a rather localized charge carrier from one site to another triggers a deformation of the molecular structure quantified by the intramolecular relaxation energy. A similar structural relaxation occurs upon population of intermolecular charge-transfer (CT) states formed at organic electron donor (D)-acceptor (A) interfaces. Weak CT absorption bands for D-A complexes occur at photon energies below the optical gaps of both the donors and the C acceptor as a result of optical transitions from the neutral ground state to the ionic CT state. In this work, we show that temperature-activated intramolecular vibrations of the ground state play a major role in determining the line shape of such CT absorption bands. This allows us to extract values for the relaxation energy related to the geometry change from neutral to ionic CT complexes. Experimental values for the relaxation energies of 20 D:C CT complexes correlate with values calculated within density functional theory. These results provide an experimental method for determining the polaron relaxation energy in solid-state organic D-A blends and show the importance of a reduced relaxation energy, which we introduce to characterize thermally activated CT processes.

Although a strong link between the molar mass of conjugated polymers and the performance of the resulting polymer:fullerene bulk heterojunction organic solar cells has been established on numerous occasions, a clear understanding of the origin of this connection is still lacking. Moreover, the usual description of molar mass and polydispersity does not include the shape of the polymer distribution, although this can have a significant effect on the device properties. In this work, the effect of molar mass distribution on photovoltaic performance is investigated using a combination of structural and electro-optical techniques for the state-of-the-art low bandgap copolymer PTB7. Some of the studied commercial PTB7 batches exhibit a bimodal distribution, of which the low molar mass fraction contains multiple homocoupled oligomer species, as identified by MALDI-TOF analysis. This combination of low molar mass and homocoupling drastically reduces device performance, from 7.0 to 2.7%. High molar mass batches show improved charge carrier transport and extraction with much lower apparent recombination orders, as well as a more homogeneous surface morphology. These results emphasize the important effect of molar mass distributions and homocoupling defects on the operation of conjugated polymers in photovoltaic devices.

In the field of polymer solar cells, improving photovoltaic performance has been the main driver over the past decade. To achieve high power conversion efficiencies, a plethora of new photoactive donor polymers and fullerene derivatives have been developed and blended together in bulk heterojunction active layers. Simultaneously, further optimization of the device architecture is also of major importance. In this respect, we report on the use of specific types of electron transport layers to boost the inherent I–V properties of polymer solar cell devices, resulting in a considerable gain in overall photovoltaic output. Imidazolium‐substituted polythiophenes are introduced as appealing electron transport materials, outperforming the currently available analogous conjugated polyelectrolytes, mainly by an increase in short‐circuit current. The molecular weight of the ionic polythiophenes has been identified as a crucial parameter influencing performance.

For an increased lifetime of polymer:fullerene bulk heterojunction (BHJ) solar cells, an understanding of the chemical and morphological degradation phenomena taking place under operational conditions is crucial. Phase separation between polymer and fullerene induced by thermal stress has been pointed out as a major issue to overcome. While often the effect of thermal stress on the morphology of polymer:fullerene BHJ is investigated in the darkness, here we observe that light exposure slows down fullerene crystallization and phase separation induced at elevated temperatures. The observed photo‐stabilizing effect on active layer morphology is quite independent on the polymer and is attributed to light‐induced dimerization of the fullerene. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 1209–1214

The low-energy edge of optical absorption spectra is critical for the performance of solar cells, but is not well understood in the case of organic solar cells (OSCs). We study the microscopic origin of exciton bands in molecular blends and investigate their role in OSCs. We simulate the temperature dependence of the excitonic density of states and low-energy absorption features, including low-frequency molecular vibrations and multi-exciton hybridisation. For model donor-acceptor blends featuring charge-transfer excitons, our simulations agree very well with temperature-dependent experimental absorption spectra. We unveil that the quantum effect of zero-point vibrations, mediated by electron-phonon interaction, causes a substantial exciton bandwidth and reduces the open-circuit voltage, which is predicted from electronic and vibronic molecular parameters. This effect is surprisingly strong at room temperature and can substantially limit the OSC's efficiency. Strategies to reduce these vibration-induced voltage losses are discussed for a larger set of systems and different heterojunction geometries.

Perovskite solar cells are well known to degrade under post-fabrication stress, a.o. due to humidity as a consequence of the hydrophilic property of the organic cation. On the other hand, it has been shown that the controlled addition of water molecules during the formation of the perovskite (while starting from water-free precursor materials) yields larger perovskite crystals with less defects, resulting in better device performance. One aspect still missing in this line of research is the water content of the perovskite precursors themselves: whereas most of them are prepared with anhydrous solvents as a precaution towards premature degradation, it is still unclear whether or not the precursors really need to be dry. In this paper, the impact of the perovskite precursor's water content up to 10 vol% is investigated, in the form of a detailed study regarding the opto-electronic and morphological properties of the resulting films and devices. It is found that only modest changes occur in the films that do not affect the final photovoltaic performance, thus relaxing the conditions for large-scale production of this upcoming photovoltaic technology.loss in V oc . Additionally, if trace amounts of water would at all be trapped inside the perovskite crystal, the band gap is expected to widen rather than shrink, or at least stay unaffected, as follows from molecular dynamics simulations by Mosconi et al.. 44 As the observed PL blue-shift also points to smaller grain sizes, we surmise that the thus induced inferior local charge transport causes increased non-geminate recombination, which in turn affects the V oc . 28, 42, 45 Figure 5: Photovoltaic parameters of perovskite solar cells with active layers prepared from precursors with different water content. Error bars represent the standard deviation based on 8 devices per condition. J sc values correspond with those estimated from EQE measurements within <5% (see Fig. S2).In summary, we have investigated the influence of water contamination in organometal halide perovskite precursors on the resulting perovskite films and solar cells. The small changes that do occur in terms of morphology and optical properties of the films are found not to have considerable influence on the photovoltaic performance of devices. Our findings thus demonstrate that to obtain decently performing perovskite solar cells, in principle no precautions need to be taken concerning the anhydrous quality of the used precursor solvent. This is a far-reaching outcome as the elimination of this redundant requirement relaxes the necessary conditions for the fabrication of perovskite-based opto-electronic devices, and brings perovskite technology one step closer to large-scale production and applications.In this paper, the impact of the water content (up to 10 vol%) in DMF-based precursors for organometal halide perovskites is investigated. It is found that only modest changes occur in the films that do not affect the final photovoltaic performance, thus relaxing the conditions for large-scale product...

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