Carbon-based organic semiconductors have many potential advantages like easy large-area preparation on flexible substrates, large variety of materials, and low cost. Organic solar cells have recently achieved significant progress and have crossed the 10% efficiency mark. In this talk, I will present an overview over the key features of solid-state organic solar cells and recent developments in the field. One central research area is the design of the bulk heterojunction active layer, requiring a nanoscale phase separation and optimized morphology to achieve efficient operation. I will also discuss highly efficient tandem structures with optimized electrical and optical properties, having the potential for approx. 20%.

Today numerous cyanine dyes that are soluble in organic solvents are available, driven by more than a century of research and development of the photographic industry. Several properties specific to cyanine dyes suggest that this material class can be of interest for organic solar cell applications. The main absorption wavelength can be tuned from the ultra-violet to the near-infrared. The unparalleled high absorption coefficients allow using very thin films for harvesting the solar photons. Furthermore, cyanines are cationic polymethine dyes, offering the possibility to modify the materials by defining the counteranion. We here show specifically how counterions can be utilized to tune the bulk morphology when blended with fullerenes. We compare the performance of bilayer heterojunction and bulk heterojunction solar cells for two different dyes absorbing in the visible and the near-infrared. Light-induced Electron Spin Resonance (LESR) was used to study the charge transfers of light induced excitons between cyanine dyes and the archetype fullerene C60. LESR results show good correlation with the cell performance.

Previously we have reported a series of perylene dimide (PDI) dimers as efficient solution-processible small molecule acceptors. In this paper we present a new solution-processible PDI trimer with triphenylamine as the romatic bridge. This trimer was synthesized by Suzuki coupling reaction and fully characterized with ¹H-NMR, ¹³C-NMR, TOF-MS, and elementary analysis. It exhibits a broad absorption band in the wavelength range from 450 to 650 nm with a peak round 533 nm and a maximum extinction coefficient of 9.61 × 10⁴ M^-1 cm^-1. Its lowest unoccupied and highest occupied molecular orbit (LUMO and HOMO) energies are −3.75 and −5.60 eV. When blend with the commercial P3HT, it gives an open-circuit voltage (V_oc) of 0.73 V, a short-circuit current-density (J_sc) of 0.60 mA/cm², a fill-factor (FF) of 51.0% and an efficiency of 0.22%. When utilizing the conjugated polymer of PBDTTT-C-T as the donor and 5% DIO as the additive, the best conventional cell yields 1.82% efficiency with a V_oc of 0.99 V, a J_sc of 3.44 mA/cm², and an FF of 53.0%.

We present a study of charge transfer and carrier dynamics in films of zinc phthalocyanine (ZnPc) and buckmisnsterfullerene (C₆₀) by investigated by time-resolved terahertz spectroscopy (TRTS). We compare terahertz photoconductivity dynamics in composite and multi-layered films of C₆₀ and ZnPc. The few picosecond terahertz photoconductivity dynamics arise from autoionization and recombination between C₆₀ molecules and cooling of hot photocarriers following from charge transfer between C₆₀ and ZnPc.

The ability to process amorphous or polycrystalline solar cells at low temperature (<150 °C) opens many possibilities for substrate choice and monolithic multijunction solar cell fabrication. Organometal trihalide perovskite solar cells have evolved rapidly over the last two years, and the CH3NH3PbX3 (X= Cl, I or Br) material is processed at low temperature. Until now however, the most efficient solar cells have employed 500 ºC sintered TiO2 compact layers as charge selective contacts. With our optimized formulation we demonstrate full sun solar power conversion efficiencies exceeding 16 % in an all low temperature processed solar cell.

Dye-Sensitized Solar Cells (DSSC) represents a reliable technology, ready for the market and able to compete with silicon solar cells for specific fields of application. Porphyrin dyes allow reaching high power conversion efficiency in conjunction with cobalt redox electrolytes due to larger open circuit potentials. The bigger size of the cobalt complexes compared to standard iodide/triiodide redox couple hampers its percolation through the meso-porous TiO₂ network, thus impairing the regeneration process. In case of porphyrin dyes mass transport problems in the electrolyte need to be carefully handled, due to the large size of the sensitizing molecule and the bulky cobalt complexes. Herein we report the study of structural variations on porphyrin sensitizers and their influence on the DSSC performance with cobalt based redox electrolyte.

We have succeeded in harvesting energy in the NIR region by using Sn halide based perovskite materials. The cell has the following composition: F-doped SnO2 layered glass/compact titania layer/porous titania layer/Sn based perovskite material/ p-type polymer semiconductor. The edge of the incident photon to current efficiency (IPCE) edge reached 1040 nm. 4.18 % efficiency with open circuit efficiency (Voc):0.42 V, fill factor (FF): 0.5, short circuit current (Jsc): 20.04 mA/cm2 is reported.

We report on the impact of morphology onto charge carrier transport and photovoltaic properties in bulk heterojunction solar cells. The study is based on ternary blends of amorphous and semi-crystalline anthracene-containing poly(p-phenylene- ethynylene)-alt-poly(p-phenylene-vinylene) (PPE-PPV) copolymers, with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). Both copolymers exhibit the identical backbone, but bear different side-chain substitutions, which in turn yield changes in the conformation from semi-crystalline (AnE-PVab) to amorphous (AnE-PVba). Strongly phase separated domains are observed for binary AnE-PVab:PCBM blends, presumably dueto strong stacking tendency of AnE-PVab. On the other hand and due to good miscibility a fine-scaled homogeneously intermixed amorphous phase is obtained for binary AnE-PVba:PCBM blends. Upon ternary blending the phase separation between the polymers can readily be continuously tuned between coarse grained and fine-scaled. For each photoactive layer composition the dominant domain spacing was evaluated by resonant soft x-ray scattering (R-SoXS) and related to photovoltaic properties and charge carrier mobility. A with the AnE-PVba steadily decaying domain size and a strong correlation between phase separation and charge transport is demonstrated.

We report on a fully integrated roll-to-roll vacuum production process of small-molecule organic p-i-n tandem solar cells. The solar foils are prepared on a flexible PET substrate. Three different laser processes were developed to pattern the transparent bottom contact, the organic layers and the metal top contact. For the ramp up phase of the production tool, a simplified organic stack was developed to reach efficiencies above 5% with moderate complexity. The modules from Heliatek’s roll-to-roll production show efficiencies of up to 6.8% on the active area of 1033cm² with a fill factor of 65.4%. Lab modules with the same layer stack on smaller samples prepared in a batch to batch process reach about the same values in all electrical parameters proving the excellent scalability of small-molecule p-i-n tandem solar cells prepared by vacuum deposition.

Organolead halide perovskites are attracting considerable attention for applications in high performance and flexible hybrid photovoltaic devices. Low temperature solution-processed flexible hybrid solar cells with CH₃NH₃PbI₂Cl, using [6,6]-Phenyl C61 butyric acid methyl ester (PCBM) and Poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt- (benzo[2,1,3]thiadiazol-4,8-diyl)] (F8BT) as electron transport materials were fabricated on ITO coated plastic substrates in planar configuration. Poly(3,4-ethylenedioxythiophene):polystyrene sulfonate was employed as the electron blocking layer. Under standard AM 1.5G solar irradiation, these flexible solar cells yielded power conversion efficiencies of 5.14% and 7.05% with the electron transporting materials PCBM and F8BT, respectively.

The competition in the field of solar energy between Organic Photovoltaics (OPVs) and several Inorganic Photovoltaic technologies is continuously increasing to reach the ultimate purpose of energy supply from inexpensive and easily manufactured solar cell units. Solution-processed printing techniques on flexible substrates attach a tremendous opportunity to the OPVs for the accomplishment of low-cost and large area applications. Furthermore, tandem architectures came to boost up even more OPVs by increasing the photon-harvesting properties of the device. In this work, we demonstrate the road of realizing flexible organic tandem solar modules constructed by a fully roll-to-roll compatible processing. The modules exhibit an efficiency of 5.4% with geometrical fill factors beyond 80% and minimized interconnection-resistance losses. The processing involves low temperature (<70 °C), coating methods compatible with slot die coating and high speed and precision laser patterning.

Semitransparent organic photovoltaic (OPV) cells are promising for applications in transparent architectures where their opaque counterparts are not suitable. Manufacturing of large-area modules without performance losses compared to their lab-scale devices is a key step towards practical applications of this PV technology. In this paper, we report the use of solution-processed silver nanowires as top electrodes and fabricate semitransparent OPV modules based on ultra-fast laser scribing. Through a rational choice of device architecture in combination with high-precision laser patterning, we demonstrate efficient semitransparent modules with comparable performance as compared to the reference devices.

We systematically investigate the optical and electrical properties of ultrathin two-dimensional (2D) Ag nanogratings (NGs), and explore their use as plasmonic transparent conducting electrodes in molecular organic photovoltaics (OPVs). A large broadband and polarization-insensitive optical absorption enhancement in the CuPc (copper phthalocyanine): PTCBI (perylene tetracarboxylic bisbenzimidazole) active light-harvesting layers is demonstrated using ultrathin 2D Ag NGs, and is attributed to the excitation of surface plasmon resonances and plasmonic cavity modes.

Extensive investigations have been conducted in order to synthesize high quality Zinc oxide (ZnO) thin films for numerous applications. These methods are either expensive to make or result polycrystalline thin films with low optoelectronic properties. Here we demonstrated a simple and inexpensive method to grow high quality ZnO thin films by a mist chemical vapor assisted depositing (Mist-CVD) system for inverted polymer solar cell (IPSC) application. The IPSC performance fabricated by Mist-CVD grown ZnO thin films were compared with two different Zn precursors (Zinc acetylacetonate hydrate and Zinc acetate dehydrate). Variations in IPSC performance on the growth temperature and growth time of the ZnO thin films were prominently demonstrated. The surface morphology of the ZnO films was investigated using scanning electron microscopy, atomic force microscopy and correlated with IPSC performance. The IPSC performance using two different precursors has been compared thoroughly. A 24% increase in solar cell efficiency (contributed from 21% increase in fill factor and 151% increase in shunt resistance) was achieved using Zinc acetate dehydrate compare to Zinc acetylacetonate hydrate precursor. The transmittance of ZnO thin films was evaluated by transmission spectroscopy. High performance IPSC can be fabricated using this simple and inexpensive method by synthesizing high quality thin ZnO films.

Schottky-junction organic solar cells have been a topic of intense research in recent times due to their surprisingly high performance. Some aspects of their device physics are well understood but charge transport in neat-C₆₀ and donor-doped Schottky junction OSCs has not been studied in detail so far. In this study, charge transport in neat-C₆₀ OSCs is examined by studying the performance of these OSCs as a function of C₆₀ active layer thickness. Surprisingly, the fillfactor of the neat-C₆₀ Schottky OSC does not degrade for layer thicknesses between 20 nm – 80 nm indicating that charge transport is not an issue. However, the short-circuit current decreases significantly due to a reduction in the builtin voltage. The dissociation of excitons formed in C₆₀ aggregates is preferentially reduced. Devices with thicker C₆₀ layers and consequently, higher C₆₀ aggregation, were found to have greater recombination. Finally, charge mobility in C₆₀ films with aggregates is found to be lower than charge mobility in films with little to no aggregation.

Reproducibility in efficiency and lifetime of organic solar cells (OSCs) remains a major concern, especially with the development of more complex and modern multi-layer device architectures. In this work, OSCs are studied for their efficiency and photo-stability as a function of the quality of their thermally evaporated MoO_x hole extraction layer (HEL). To this end, the characteristics of the MoO_x film are demonstrated to change with repeat evaporation runs from the same source material. These variations have strong effects on polymer OSCs (p-OSCs), with an effective halving of the power conversion efficiency after only three MoO₃ evaporation runs. In contrast, vacuum deposited small molecule OSCs (sm-OSCs) appear to be unaffected by the history of the MoO₃ source material. sm- OSCs are instead shown to be prone to large changes in efficiency as a function of the delay time in between deposition of the MoO_x HEL and subsequent photo-active materials. Increased delay time between these deposition steps is also demonstrated to decrease the sm-OSC photo-stability. The results thus emphasize subtleties in materials deposition processes that can play a significant role in obtaining reproducible and scientifically relevant data.

We demonstrate novel plasmonic organic solar cells (OSCs) by embedding an easy processible nanobump assembly (NBA) for harnessing more light. The NBA is consisted of precisely size-controlled Ag nanoparticles (NPs) generated by an aerosol process at atmospheric pressure and thermally deposited molybdenum oxide (MoO₃) layer which follows the underlying nano structure of NPs. The active layer, spin-casted polymer blend solution, has an undulated structure conformably covering the NBA structure. To find the optimal condition of the NBA structure for enhancing light harvest as well as carrier transfer, we systematically investigate the effect of the size of Ag NPs and the MoO₃ coverage on the device performance. It is observed that the photocurrent of device increases as the size of Ag NP increases owing to enhanced plasmonic and scattering effect. In addition, the increased light absorption is effectively transferred to the photocurrent with small carrier losses, when the Ag NPs are fully covered by the MoO₃ layer. As a result, the NBA structure consisted of 40 nm Ag NPs enclosed by 20 nm MoO₃ layer leads to 18% improvement in the power conversion efficiency compared to the device without the NBA structure. Therefore, the NBA plasmonic structure provides a reliable and efficient light harvesting in a broad range of wavelength, which consequently enhances the performance of organic solar cells.

Processing additives are widely used to increase the efficiency of solution processed organic solar cells. We use the Hansen solubility parameters (HSPs) to investigate novel processing additives. The HSPs predict pyrrolidinone derivatives to be efficient processing additives for OSC systems based on poly(3-hexylthiophene)/[6,6]-phenyl-C61- butyric acid methyl ester (P3HT/PCBM). Two pyrrolidinone derivatives are identified: 1-methyl-2-pyrrolidinone and 1- benzyl-2-pyrrolidinone. The processing additives are introduced with various concentrations in the formulation of P3HT and PCBM solution. The electrical characterizations show that the two processing additives significantly increase the short circuit current and thus the power conversion efficiency of the OSCs. The results thus highlight HSPs as an effective and relatively straightforward tool that can be employed to optimize OSC morphology from a theoretical standpoint. Such a tool will be invaluable for identifying additives for novel high efficiency polymer species as they are synthesized, and thus to streamline the device fabrication and device optimization process.

Addition of a small fraction of high boiling point solvent into the host of donor/acceptor blend is one the best approach to control the morphology in order to enhance the power conversion efficiency of organic bulk heterojunction (BHJ) solar cell devices. Herein, we focus on the effect of two thiol-based additives (1,6-hexanedithiol (HDT) and 1,5-pentanedithiol (PDT)) on the charge dynamics of P3HT:PCBM blend system, studied by transient absorption spectroscopy (TAS) and correlated with the solar cell device performance. TAS reveals a more efficient charge generation and polaron formation in the systems with additives as compared to those without (NA systems), at the onset which persists up to few microseconds. The recombination dynamics also exhibits the reduced recombination losses on adding these additives in this system; however, there is marginal change of recombination dynamics in PDT added system with the control. These charge dynamics were validated using the analytical model proposed in our previous work and also correlated with improved device performance (η_NA = 0.9%, η_HDT = 2.7%, η_PDT = 1.6%).

Exciton transport plays an important role in the overall exciton dissociation process and must be optimized to yield high efficient OPV device. In this contribution, the influence of solvents and the nanoscale phase separations they caused on the glass transition temperatures (Tg) of P3HT-PCBM mixture were studied by reverse mapping mesoscale simulation results back to the molecular dynamics. Glass transition temperatures of P3HT-PCBM without solvent and with chloroform, dichlorobenzene, and chlorobenzene were obtained. Diffusion and reorientation ability of molecules and their subgroups at the temperature near Tg were also discussed.

Thermodynamic principles limit the conversion efficiency of a single bandgap organic photovoltaic (OPV) cell to 33%¹ . In order to increase efficiency, multiple OPV devices can be combined to cover a larger spectral range of the incident solar spectrum. The most common way of doing this is to mount multiple bandgap cells in tandem or series. However, stacked multijunction systems have limitations, such as current-matching constraints and optical quality of the OPV layer. A separated arrangement with spectrum splitting is a promising alternative to the stacked tandem approach. In this paper, two organic photovoltaic cells with complementary EQE curves are integrated into a holographic spectrum splitting module. The highest possible conversion efficiency of this two-cell combination is quantified assuming an ideal spectral filter as a reference. A spectrum splitting module is built, consisting of a reflective hologram oriented at an angle to split the incident beam into two spectral bands. The holographic beamsplitting system is assembled and studied under a solar simulator. Power output and conversion efficiency of the holographic spectrum splitting system is evaluated in terms of Improvement over Best Bandgap (IoBB) of the two-cell combination. The combined system has a measured improvement over its best single cell of 12.30% under a solar simulator lamp and a predicted improvement of 16.39% under sunlight.

Oxygen induced degradation is one of the major problems in the field of organic photovoltaics. Photo-degradation impacts on performance of inverted bulk hetero junction poly(3-hexylthiophene) : phenyl-C61-butyric acid methyl ester (P3HT:PCBM) solar cells has been investigated by means of charge extraction by linearly increasing voltage (CELIV) and time of flight (ToF) methods. The irreversible loss in short circuit current (J_sc) can be attributed to a combination of adverse effects such as loss in mobility of the charge carrires, increase in trapping effect and sheilding of electric field by equilibrium carriers upon degradation.

Photovoltaic technology has powerful implications from commercial and national security standpoints. Due to the high material cost of silicon solar devices, inexpensive and lightweight polymer based solar is desirable to meet the demand for decentralized electrical power production in traditionally “off-grid” areas. Using a blend of Poly(3-hexylthiophene- 2,5-diyl) (P3HT), Phenyl-C61-butyric acid methyl ester (PCBM), and the low band-gap polymer Poly[2,6-(4,4-bis-(2- ethylhexyl)-4H-cyclopenta [2,1-b;3,4-b′]dithiophene)-alt-4,7(2,1,3-benzothiadiazole)] (PCPDTBT), we have fabricated devices with a wide spectral response and 3% power conversion efficiency in AM 1.5 conditions. Due to low absorptivity in the peak of the solar spectra (500nm), we have blended this previous polymer system with CdSe(ZnS) core (shell) quantum dots to improve absorption, and thus power conversion efficiencies. Devices were prepared with quantum dots having a peak absorbance at 560nm and an emission wavelength of 577nm, with device loading ranging from 0% to 2.7% by weight. The relationship between quantum dot concentration and device performance is discussed, along with the impact of quantum dot concentration on thermal resistance to morphology changes.

Patterned FTO electrodes for DSSCs were fabricated by a facile wet etching method. Pattern depth could be controlled by etching time. Most DSSCs with patterned FTO electrodes exhibited both larger open-circuit voltage and photocurrent density. Energy conversion efficiency gradually increased with longer etching time and achieved a highest value when etching time is 240s. An optimum pattern depth was required to acquire best DSSC performance. The improved device performance could be mainly attributed to more dye adsorption, enhanced light harvesting and scattering due to larger amount of TiO₂ nanoparticles filled in the pattern. More contact between TiO₂ nanoparticles and patterned FTO with larger surface area was also an advantage. It was revealed from Nyquist plots that the charge transfer impedance at the TiO₂/dye/electrolyte interface apparently influenced the magnitude of photocurrent density and device performance. Electron transfer became easier and higher performance was thereby obtained when a DSSC had a smaller interfacial impedance. This study has demonstrated obvious improvement in DSSC performance by surface patterning of FTO electrodes. With appropriate etching condition, the highest energy conversion efficiency of 7.71% was achieved, which was around 16 % higher than that of the DSSC with unpatterned FTO electrode.

Efficient conversion of solar energy to electricity in low-cost organic photovoltaic (OPV) devices requires the complex interplay between multiple processes and components over various length and time scales. Optimizing device morphology to ensure efficient exciton diffusion and charge transport as well as ensuring efficient charge photogeneration is necessary to achieve optimum performance in new materials. The conjugated polymer electron donor PFM (poly(9,9-diocetyluorene-co-bis-N,N-(4-methylphenyl)-bis-N,N-phenyl-1,4-phenylenediamine)) and electron acceptor F8BT (poly[(9,9-di-n-octyluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4,8-diyl)), comprise the novel triblock copolymer PFM-F8BT-PFM. This copolymer is designed to phase separate on the 20-30 nm scale, a domain size ideal for maximizing exciton collection at the donor-acceptor interface. Using steady-state and ultrafast spectroscopic characterization including high repetition rate transient absorption spectroscopy, the dynamics of charge and energy transfer of the component polymers and the triblock co-polymer have been investigated. The results demonstrate that for the homopolymers solvent dependent exciton transport processes dominate, while in the triblock copolymer solutions transient spectroscopy provides evidence for interfacial charge separation.