Vladimir Dyakonov

Polymer–fullerene bulk heterojunction solar cells

Organic solar cells have the potential to be low-cost and efficient solar energy converters, with a promising energy balance. They are made of 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 better material stability and higher power conversion efficiencies. We will also briefly review processing and cost in view of the market potential.

Influence of nanomorphology on the photovoltaic action of polymer–fullerene composites

Composites of conjugated poly(3-hexylthiophene) (P3HT) and the fullerene derivative [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) demonstrate an efficient photogeneration of mobile charge carriers. Thermal annealing of P3HT:PCBM based devices gives rise to a significant increase of the photovoltaic efficiency, as follows from measurements of the external quantum efficiency and the current-voltage characteristics. Upon annealing, the absorption spectrum of the P3HT:PCBM composite undergoes a strong modification, whereas in the pure components it remains unchanged. The absorption of the annealed blends becomes stronger and red shifted in the wavelength region ascribed to P3HT, while the absorption due to the PCBM contribution does not change. Atomic force microscope measurements on P3HT:PCBM disclose some variation in morphology due to the crystallization of PCBM. The concentration of the PCBM clusters and their size (up to 500 nm) were found to be correlated with the amount of PCBM in the blend. We have studied the performance of photovoltaic devices with different weight ratios of P3HT:PCBM, namely, 1:3, 1:2, 1:1.5, 1:1, 1:0.9, 1:0.8, and 1:0.7. The photocurrent and the power conversion efficiency showed a maximum between 1:1 and 1:0.9. We conclude the variation in the absorption spectrum and the red shift to result from molecular diffusion of PCBM out of the polymer matrix upon annealing. The growth of the PCBM clusters leads to formation of percolation paths and, therefore, improves the photocurrent. Above a certain concentration, the PCBM crystals provide mechanical stress on the metal electrode, therefore possibly damaging the interface. Optimization of the composite weight ratio reveals the important role played by morphology for the transport properties of bulk heterojunction P3HT:PCBM based solar cells.

Role of the charge transfer state in organic donor-acceptor solar cells.

Charge transfer complexes are interfacial charge pairs residing at the donor-acceptor heterointerface in organic solar cell. Experimental evidence shows that it is crucial for the photovoltaic performance, as both photocurrent and open circuit voltage directly depend on it. For charge photogeneration, charge transfer complexes represent the intermediate but essential step between exciton dissotiation and charge extraction. Recombination of free charges to the ground state is via the bound charge transfer state before being lost to the ground state. In terms of the open circuit voltage, its maximum achievable value is determined by the energy of the charge transfer state. An important question is whether or not maximum photocurrent and maximum open circuit voltage can be achieved simultaneously. The impact of increasing the CT energy-in order to raise the open circuit voltage, but lowering the kinetic excess energy of the CT complexes at the same time-on the charge photogeneration will accordingly be discussed. Clearly, the fundamental understanding of the processes involving the charge transfer state is essential for an optimisation of the performance of organic solar cells.

S-shaped current-voltage characteristics of organic solar devices

Measuring the current-voltage characteristic of organic bulk heterojunction solar devices sometimes reveals an S-shaped deformation. We qualitatively produce this behavior by a numerical device simulation assuming a reduced surface recombination. Furthermore we show how to experimentally create these double diodes by applying an oxygen plasma etch on the indium-tin-oxide anode. Restricted charge transport over material interfaces accumulates space charges and therefore creates S-shaped deformations. Finally we discuss the consequences of our findings for the open-circuit voltage Voc.

Origin of the efficient polaron-pair dissociation in polymer-Fullerene blends.

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 delocalized charge carriers along conjugated polymer chain segments. We show that the resulting fast local charge carrier transport can indeed explain the high experimental quantum yields in polymer solar cells.

Oxygen doping of P3HT:PCBM blends: Influence on trap states, charge carrier mobility and solar cell performance

Abstract We investigated the influence of oxygen on the performance of P3HT:PCBM (poly(3-hexylthiophene):[6,6]-phenyl C61 butyric acid methyl ester) solar cells by current–voltage, thermally stimulated current (TSC) and charge extraction by linearly increasing voltage (CELIV) measurement techniques. The exposure to oxygen leads to an enhanced charge carrier concentration and a decreased charge carrier mobility. Further, an enhanced formation of deeper traps was observed, although the overall density of traps was found to be unaffected upon oxygen exposure. With the aid of macroscopic simulations, based on solving the differential equation system of Poisson, continuity and drift-diffusion equations in one dimension, we demonstrate the influence of a reduced charge carrier mobility and an increased charge carrier density on the main solar cell parameters, consistent with experimental findings.

Temperature dependent characteristics of poly(3 hexylthiophene)-fullerene based heterojunction organic solar cells

Electrical and optical properties of poly(3-hexylthiophene-2.5diyl) (P3HT) used as the main component in a polymer/fullerene solar cell were studied. From the study of space-charge limited current behavior of indium-tin-oxide (ITO)/P3HT/Au hole-only devices, the hole mobility and density were estimated to range from 1.4×10−6 cm2/V s and 5.3×1014 cm−3 at 150 K to 8.5×10−5 cm2/V s and 1.1×1015 cm−3 at 250 K, respectively. The highest occupied to lowest occupied molecular orbital energetic difference was estimated from absorption spectrometry to be about 2.14 eV. Strong quenching of photoluminescence when the polymer was mixed with [6,6]-phenyl-C61 butyric acid methyl ester (PCBM), provided evidence of photoinduced charge transfer from P3HT to PCBM. Characterization of ITO/PEDOT:PSS/P3HT:PCBM/Al solar cells was done by analyzing the dependence of current density–voltage characteristics on temperature and illumination intensity. The main solar cell characteristics recorded at 300 K under 100 mW/cm2 white-ligh...

Electrical and optical characterization of poly(phenylene-vinylene) light emitting diodes

Abstract We investigated the electrical and optical properties of poly( p -phenylene-vinylene) and light emitting devices based on this polymer. At room temperature these devices can be well described by a Schottky diode configuration. Dark I–V characteristics reveal a good diode behaviour with a maximum rectification ratio of 10 6 . White light illumination yields an open-circuit voltage of more than 1 V and a power conversion efficiency of about 0.1%, suggesting the possible application as solar cells. Complex impedance gives information about acceptor dopant concentration, diffusion voltage and depletion width. At low temperatures the I–V characteristics become symmetrical and electroluminescence appears in both current directions, indicating that the Schottky junction is not necessary for the observation of electroluminescence in conjugated polymers.

Radiative efficiency of lead iodide based perovskite solar cells

The maximum efficiency of any solar cell can be evaluated in terms of its corresponding ability to emit light. We herein determine the important figure of merit of radiative efficiency for Methylammonium Lead Iodide perovskite solar cells and, to put in context, relate it to an organic photovoltaic (OPV) model device. We evaluate the reciprocity relation between electroluminescence and photovoltaic quantum efficiency and conclude that the emission from the perovskite devices is dominated by a sharp band-to-band transition that has a radiative efficiency much higher than that of an average OPV device. As a consequence, the perovskite have the benefit of retaining an open circuit voltage ~0.14 V closer to its radiative limit than the OPV cell. Additionally, and in contrast to OPVs, we show that the photoluminescence of the perovskite solar cell is substantially quenched under short circuit conditions in accordance with how an ideal photovoltaic cell should operate.

Trap distribution and the impact of oxygen-induced traps on the charge transport in poly(3-hexylthiophene)

The trap distribution in the conjugated polymer poly(3-hexylthiophene) was investigated by fractional thermally stimulated current measurements. Two defect states with activation energies of about 50 and 105 meV and Gaussian energy distributions were revealed. The first is assigned to the tail of the intrinsic density of states, whereas the concentration of the second trap is directly related to oxygen exposure. The impact of the oxygen induced traps on the charge transport was examined by performing photo-induced charge carrier extraction by linearly increasing voltage measurements that exhibited a strong decrease in the mobility with air exposure time.