It was realized early in the history of Konarka that the ability to produce fibers that generate power from solar energy could be applied to a wide variety of applications where fabrics are utilized currently. These applications include personal items such as jackets, shirts and hats, to architectural uses such as awnings, tents, large covers for cars, trucks and even doomed stadiums, to indoor furnishings such as window blinds, shades and drapes. They may also be used as small fabric patches or fiber bundles for powering or recharging batteries in small sensors. Power generating fabrics for clothing is of particular interest to the military where they would be used in uniforms and body armor where portable power is vital to field operations. In strong sunlight these power generating fabrics could be used as a primary source of energy, or they can be used in either direct sunlight or low light conditions to recharge batteries. Early in 2002, Konarka performed a series of proof-of-concept experiments to demonstrate the feasibility of building a photovoltaic cell using dye-sensitized titania and electrolyte on a metal wire core. The approach taken was based on the sequential coating processes used in making fiber optics, namely, a fiber core, e.g., a metal wire serving as the primary electrode, is passed through a series of vertically aligned coating cups. Each of the cups contains a coating fluid that has a specific function in the photocell. A second wire, used as the counter electrode, is brought into the process prior to entering the final coating cup. The latter contains a photopolymerizable, transparent cladding which hardens when passed through a UV chamber. Upon exiting the UV chamber, the finished PV fiber is spooled. Two hundred of foot lengths of PV fiber have been made using this process. When the fiber is exposed to visible radiation, it generates electrical power. The best efficiency exhibited by these fibers is 6% with an average value in the 4-5 % range.

Charge concentration measurements for dye sensitized and polymer based photovoltaic cells are discussed and compared.

Time-of-flight (TOF) photocurrent measurements have been used to study charge transport in films of regioregular poly(3-hexylthiophene) (P3HT). Devices in which the P3HT film had been deposited directly onto an indium tin oxide (ITO) electrode produced high dark currents as a result of hole injection into P3HT from ITO. Photocurrent transients in such devices were disperse. It was found however, that these dark currents could be significantly reduced by inserting a dense TiO₂ layer between the ITO and the polymer film. The resulting devices gave non-dispersive transients with hole and electron mobilities in the range of 1 - 2 10^-4 cm² V^-1 s^-1 at room temperature. The mobility values were observed to be almost independent of film thickness over the range of 350 nm to 4.3 μm. Temperature dependence studies showed a weak dependence on temperature with a low energetic disorder parameter according to analysis using the Gaussian Disorder Model (GDM) of 71 meV.

Using a newly developed device model we have studied the effect of controlled thermal annealing on charge transport and photogeneration in bulk-heterojunction solar cells (BHJ) made from blend films of regioregular poly(3- hexylthiophene) (P3HT) and methanofullerene (PCBM). With respect to the charge transport, we demonstrate that the hole mobility in the P3HT phase of the blend is dramatically affected by thermal annealing. It increases more than three orders of magnitude, to reach a value up to ≈2×10^-8 m²/Vs after the annealing process, as a result of an improved crystallinity of the film. Slow drying leads to an additional 33-fold enhancement of the hole mobility even up to 5.0×10^{- 7} m²V^-1s^-1, thereby balancing the transport of electrons and holes in the blend. The resulting reduction of space-charge accumulation enables the use of thick films (~300 nm), absorbing most of the incoming photons, without losses in the fill factor and short-circuit current of the device. As a next step we performed model calculations to exploit the potential of polymer/fullerene bulk heterojunction solar cells. Lowering the polymeric band will lead to a device efficiency exceeding 6%. Tuning the electronic levels of PCBM in such a way that less energy is lost in the electron transfer process enhances the efficiency to values in excess of 8%. Ultimately, with an optimized level tuning, band gap and balanced mobilities polymeric solar cells can reach power conversion efficiencies approaching 11%.

The challenge to reversing the layer sequence of organic photovoltaics (OPVs) is to prepare a selective contact bottom cathode and to achieve a suitable morphology for carrier collection in the inverted structure. We report the creation of an efficient electron selective bottom contact based on a solution-processed Titania layer on top of Indium Tin Oxide. The use of o-xylene as the casting solvent creates an efficient carrier collection network with little vertical phase segregation, providing sufficient performance for both regular as well as inverted solar cells. We demonstrate inverted layer sequence OPVs with AM 1.5-calibrated power conversion efficiencies of over 3%.

The overall power conversion efficiency of organic solar cells depends on many factors, some of which such as photon absorption, charge carrier photogeneration, separation and transport are intrinsic properties of the active material. The use of low-bandgap conjugated polymers in polymer/fullerene bulk heterojunctions improves the spectral overlap between the polymer absorption and the solar irradiance spectrum, and is therefore a promising route toward increased light harvesting and higher power conversion efficiency of polymer photovoltaics. We present our studies on the optical and charge transport properties of a novel low-bandgap conjugated 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, with an optical energy gap of E_g=1.46 eV. The combination of steady-state and transient photoconductivity with photoinduced absorption measurements has allowed us to investigate the charge carrier photogeneration and charge transport mechanisms in pristine PCPDTBT and PCPDTBT:PCBM interpenetrating networks, and to compare them to the P3HT and P3HT:PCBM model systems. The picture of the photophysics of PCPDTBT:PCBM emerging from these studies is very similar to that of P3HT:PCBM blends. We discuss the potential of PCPDTBT as a new material for high efficiency polymer solar cells.

We show that time-resolved luminescence measurements at high excitation densities can be used to study exciton annihilation and diffusion, and report the results of such measurements on films of P3HT and MEH-PPV. The results fit to an exciton-exciton annihilation model with a time independent annihilation rate γ, which was measured to be γ = (2.8±0.5)×10^-8 cm³s^-1 in MEH-PPV and γ = (5.2±1)×10^-10 cm³s^-1 in P3HT. This implies much faster diffusion in MEHPPV. Assuming a value of 1 nm for the annihilation radius we evaluated the diffusion length for pristine P3HT in one direction to be 3.2 nm. Annealing of P3HT was found to increase the annihilation rate to (1.1±0.2)×10^-9 cm³s^-1 and the diffusion length to 4.7 nm.

One very important factor limiting the power conversion efficiency of the current state-of-the-art organic solar cells is the low energy conversion efficiency during the conversion process of an absorbed photon to an electron-hole pair collected at the electrodes. The absorption of a 2 to 3 eV photon typically leads to an open-circuit voltage of 0.5-0.6 V, representing approximately 80% energy loss. In this paper, we show that the open-circuit voltage of an organic donor-acceptor heterojunction cell is related to both the photocurrent and the dark current. Many factors, such as illumination intensity, organic heterojunction structure, electrode properties, operating temperature, can have significant impact on the open-circuit voltage. We also show that the conventional wisdom of using the "effective" gap of an organic donor-acceptor heterojunction to determine the maximum open-circuit voltage needs to be carefully re-examined. While the study shows that the open-circuit voltage in the copper phthalocyanine-C₆₀ heterojunction cell still has some room for improvement, ultimately new materials will have to be used to boost the power conversion efficiency of organic solar cells to the 20% regime.

The effect of multilayer barrier materials on the lifetime of organic photovoltaic cells has been investigated. For thin film encapsulated cells a protective layer was used to prevent damage during barrier layer deposition. No post deposition effects developed after dry box storage. In accelerated temperature and humidity lifetime testing the degradation of the encapsulated cells can be related to the loss of effective cell area. An extrapolation of the lifetime at room conditions has been quantitatively determined by comparing the cell degradation with the loss of Ca in a Ca-oxidation test. The results indicate a barrier permeation rate of 10^-4 gr/[m²* day] for these samples, corresponding to a lifetime of greater than 5000 hours. Routes to improvement of the OPV cell lifetime are discussed.

Modification of the interface of titanium dioxide/poly[2-(2-ethylhexyloxy)-5-methoxy-1,4,-phenylenevinylene] (TiO₂/MEH-PPV) nanocomposite photovoltaic devices with a lithium salt, Li[CF₃SO₂]₂N, is shown to result in a twofold increase in device efficiency. The devices are of the type ITO/TiO₂/MEH-PPV/Au. The TiO₂ layer is deposited by doctor blading a colloidal anatase paste, and the polymer is then spin-coated on top followed by thermal evaporation of gold contacts. Careful control of manufacturing conditions and use of a 35 nm polymer layer leads to a device efficiency of 0.48% for un-modified devices. The increased efficiency following Li treatment is the result of a 40% increase in both the short-circuit current and fill factor, while the open-circuit voltage remains unchanged. A maximum efficiency of 1.05% has been achieved under 80% sun illumination. This represents a record efficiency for this type of cell. Photoconductivity experiments show a substantial increase in conductivity of the TiO₂ layer following Li modification. Interfacial modification is done via a simple soaking procedure, and the effect of varying the concentration of Li[CF₃SO₂]₂N is discussed. We report investigations into optimization and the mechanism of such improvement, for example by varying processing parameters of the modification procedure or the ionic species themselves.

Insufficient lifetimes of organic photovoltaics are manifested in a reduced photovoltaic response, which is a consequence of physical and chemical degradation of the photovoltaic device. To prevent degradation it is vital to gain detailed insight into the degradation mechanisms. This is possible by utilizing state-of-the-art characterization techniques such as TOF-SIMS, XPS, AFM, SEM, interference microscopy and fluorescence microscopy as well as isotopic labeling (¹⁸O₂ and H₂¹⁸O). By a combination of lateral and vertical analyses of the devices we obtain in-depth and in-plane information on the reactions and changes that take place in the various layers and interfaces. Examples will be presented that describe the advantages and disadvantages of various characterization techniques in relation to obtaining information on the degradation behavior of complete photovoltaic devices.

The mechanisms of exciton dissociation and migration in the conjugated polymer (poly(2-methoxy-5-(2'-ethyl)(hexyloxy)1,4-phenylenevinylene)(MEH-PPV) / CdSe nanoparticle hybrid materials were investigated by steady-state and time-resolved photoluminescence spectroscopy. Rapid exciton dissociation at the nanoparticle/polymer interfaces leading to quenching of the photoluminescence efficiency η and shortening of the measured lifetime τ_PL is observed. The excitons which contribute to the remaining luminescence in polymer will migrate to the lower energy sites with longer conjugated sequences in the composites. The result is evident from the observations of a redshift of the photoluminescence peak positions, a progressive decrease of the Huang-Rhys factor S and an increase in the nature radiative lifetime τ_R with increasing CdSe nanoparticle content. The solar cell based on the MEH-PPV / CdSe nanoparticle hybrid materials are fabricated and the transport mechanism of the device will also be discussed.

Photovoltaic devices based on the conjugated polymer (poly(2-methoxy-5-(2'-ethyl)(hexyloxy)1,4-phenylenevinylene) (MEH-PPV) / TiO₂ nanorods hybrid materials are investigated. It is found that efficient charge separation occurs at the interfaces of MEH-PPV/ TiO₂ nanorods, accompanying with a significant quench in the photoluminescence (PL) intensity and a decrease of PL lifetime with increasing TiO₂ nanorod concentrations. The device based on the MEHPPV/ TiO₂ nanorod hybrid material shows power efficiency about 2.2% at the incident light with a wavelength of 565 nm. The transport properties of the solar cells are further investigated and discussed.

Thermal annealing has been widely used to improve device performances of organic solar cells with regioregular (RR) poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) bulk heterojunction blends. Especially, short-circuit current density (Jsc) of the thermally-annealed device is significantly increased compared to that of the non-annealed one. The Jsc is proportional to the product of the carrier mobility and the number of photogenerated carriers which depends on the photocarrier generation efficiency and carrier recombination lifetime. Therefore, the enhanced Jsc implies that the thermal annealing can increase either the mobility and/or lifetime of the photogenerated carriers. In order to understand which parameter is more affected by thermal annealing, we compared the temperature dependence of the Jsc and carrier mobility of P3HT:PCBM (1:1, weight%) blend solar cells. The carrier mobility, measured from a time-of-flight photoconductivity (TOF-PC) measurement, increases from about 10^-5 cm²/Vs to the order of 10^-4 cm²/Vs as the temperature increases from 300 K to 360 K and then saturates above 360 K up to 400 K. This behavior is very similar to the temperature dependence of the current density of the P3HT:PCBM solar cell devices with the same blend ratio. Therefore, this correlation indicates that the thermal annealing increases the carrier mobility by improving morphological order of the blend film and thereby enhances the Jsc of the P3HT:PCBM blend solar cells.

The synthesis of copolymers based on thiophene, benzothiadiazole and benzo-bis-thiadiazole are described. The polymers were obtained by employing Stille cross coupling polymerization. The polymers were characterized by NMR, size exclusion chromatography, UV-vis and ultraviolet photoelectron spectroscopy. The results obtained from UV-vis and ultraviolet photoelectron spectroscopy showed band gaps of 2.1-1.7 eV for polymers based on benzothiadiazole and 0.7 eV for polymers based on benzo-bis-thiadiazole. Furthermore the results showed that the band gap decreases with an increase in the number, n, of thiophenes in the polymer repeating unit (n= 1-4). Large area photovoltaic devices were prepared and the results of these devices are described.

The importance of nanocomposites materials such as carbon nanotubes-polymers composites for the efficient realization of innovative solar cells based on organic as well hybrid organic-inorganic solar cells is more and more evident. We present a study on the realization of dye sensitized solar cells (DSSC) and sublimation deposited solar cells, considering the impact of using nanocomposite materials in the different sections composing the cells. We discuss the effect of using poly-3,4-ethylene dioxythiophene/poly(styrene sulfonate) (PEDOT/PSS)-Carbon nanotube (CNT) blend as counterelectrode in DSSC on the cell efficiency and fill factor, also considering DSSC structures where low cost, innovative dyes are used. Nanocomposites can be used as solution processed or electropolimerized electrodes, where accurate control of nanotube dispersion is obtained through specific chemical treatment of Carbon nanotubes solubility. The use of Carbon based nanostructured material is also investigated in terms of their positive impact on the realization of organic solar cells on flexible substrates.

Thin film morphological studies using a number of techniques including AFM and XRD have revealed some interesting molecular self-assembled pattern in the synthesized −DBAB− block copolymer, and this is very different from the corresponding simple D/A blend. It was also observed that the thin film processing conditions affect the morphologies dramatically. The earlier observed much better photovoltaic properties of a −donor-bridge-acceptor-bridge type block copolymer versus the corresponding donor/acceptor blend has been attributed to morphological improvements in spatial domain.

Capacitance measurements of oxygen doped films of α or β iron phthalocyanine particles dispersed in a binder polymer polycarbonate (MK), with mixed (Al, Au) electrodes are studied. The capacitance of the cells changes in accordance with the morphological forms. A complete study of the space charge density as a function of temperature is carried out. The results obtained are in accordance with the model proposed for the dopant, oxygen. The low conductivity of the β iron phthalocyanine is due in large part to deep trap carriers presented in this phase.

The effect of side-chains on the molecular weight and the optical and electrical property of a low band gap copolymer poly{(9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-decyloxythien-2-yl)-2,1,3-benzothiadiazole]-5',5"-diyl} (PF-co-DTB) was studied. The decyloxy side-chains help to increase molecular weight (Mw = 115,000) and decrease the band gap (1.78 eV) as well as the oxidation potential (-5.4 eV). Zero-field mobility of 2×10^-5 cm²/Vs is measured in hole-only devices. Photovoltaic devices based on PF-co-DTB/fullerene bulk-heterojunction show power conversion efficiency of up to 1.6% under air mass 1.5G, 100 mW/cm² illumination. Side-chains effect on the photovoltaic devices studies show the trade-off between short circuit current increase and open-circuit voltage drop. Thermal annealing on device performance is also discussed.

To increase the absorption of sunlight in polymer solar cells a large active layer thickness is desired. This, however, is limited by the short charge carrier diffusion lengths in the active organic materials. Efficient light harvesting can be achieved in organic solar cells by using a tandem structure. However, fabricating a tandem structure for polymer solar cells presents its own difficulties. Since the polymer film is solution processed, spin-coating multiple layers in tandem can result in significant damage to the underlying layers. This problem can be overcome by fabricating separate PV cells and stacking them in tandem. Here, we report a multiple-device stacked structure where two polymer photovoltaic cells are stacked together with the help of a multi-layer semi-transparent electrode, made of lithium fluoride (LiF) / aluminum (Al) / gold (Au) metal layers. The semi-transparent electrode is used as the top contact in the bottom cell to efficiently transmit the unabsorbed photons to the upper cell. Maximum transparency of up to 80% is achieved for the semitransparent cathode. In the stacked structure, the open circuit voltage and the short circuit current are twice those of a single cell. As a result, power conversion efficiency of up to 2.6% is achieved, which is double than that of a single cell.