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Transient photocurrent measurements on efficient polymer/fullerene solar cells based on a blend of the donor polymer PTB7 with the fullerene acceptor PC70BM are reported. In particular, we examine the light intensity dependence and voltage dependence of the turn-on and turn-off photocurrent dynamics of devices in response to a 200 μs square light pulse. At short circuit, subtle changes in the turn-on and turn-off dynamics are observed consistent with charge-density-dependent transport phenomena. As the working voltage is moved from short circuit to open circuit, we observe the appearance of an initial transient photocurrent peak a few microseconds after turn-on before the device settles to steady state. Furthermore, we observe only a weak dependence of the charge extraction dynamics on the working voltage, with the amount of charge extracted monotonically decreasing as the working voltage is moved from short circuit to open circuit. This collection of features is interpreted with the aid of numerical simulations in terms of charge trapping, with increased trap-assisted recombination closer to open circuit. The operation of devices fabricated with and without the solvent additive di-iodooctane is also compared. Charge trapping features are reduced for optimized devices fabricated with the solvent additive compared to devices fabricated without. The use of the solvent additive di-iodooctane in this system is therefore important in minimizing trap-assisted recombination.

Interfaces between donor and acceptor semiconducting polymers are critical to the performance of polymer light-emitting diodes and organic solar cells. Similarly, interfaces between a conjugated polymer and a dielectric play a critical role in organic thin-film transistors. Often, these interfaces are difficult to characterize with conventional methods. Resonant soft x-ray reflectivity (R-SoXR) is a unique and relatively simple method to investigate such interfaces. R-SoXR capabilities are exemplified by presenting or discussing results from systems spanning all three device categories. We also demonstrate that the interfacial widths between active layers can be controlled by annealing at elevated temperature, pre-annealing of the bottom layer, or casting from different solvent mixtures. The extension of R-SoXR to the fluorine K absorption edge near 698 eV is also demonstrated.

We report measurements of the turn-on and turn-off photocurrent dynamics as a function of applied voltage for efficient polymer/fullerene bulk heterojunction solar cells composed of poly[N-9″-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole) (PCDTBT): [6,6]-phenyl C71-butyric acid methyl ester (PC70BM) and 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):PC70BM blends. In particular we present evidence for charge trapping that facilitates recombination in these systems. For the PCDTBT:PC70BM system, an initial transient photocurrent peak 5–10 μs after turn-on is observed for operating voltages between 0.5 V and open-circuit. Furthermore, a long photocurrent tail is observed in the decay dynamics of PCDTBT:PC70BM devices with charge still being extracted hundreds of microseconds after turn-off. These features in the PCDTBT:PC70BM device are attributed to trapping and detrapping of charge on the microsecond time scale, with charge trapping facilitating recombination either through trap-assisted recombination or space-charge effects. For the PCPDTBT:PC70BM system, evidence for charge trapping is also observed albeit on a faster time scale. No initial transient photocurrent peak is observed, however the faster PCPDTBT:PC70BM decay dynamics show only a weak voltage dependence consistent with rapid trapping and recombination of charge. For both systems the amount of extracted charge as a function of applied voltage follows a similar form to the measured current-voltage curves providing evidence that photocurrent is hampered by the extraction, and not just the separation, of charge in these systems. The origin of charge trapping and the nature of recombination is discussed, along with the influence of additives on charge transport in the PCPDTBT:PC70BM system.

The device physics of inverted all-polymer solar cells based on a blend of the polymers poly(3-hexylthiophene) and poly(9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-hexylthiophen-5-yl)2,1,3-benzothiadiazole]-2′,2″-diyl) is investigated. In particular, the influence of active layer thickness on device parameters is investigated and compared to that of devices with a standard geometry. Increasing the active layer thickness from 70 to 480 nm is found to increase the open circuit voltage from 0.1 to 0.71 V and the external quantum efficiency (EQE) from 7% to 24%. In contrast, an optimum EQE of about 25% for the standard geometry is found for a film thickness of 70 nm, which decreases sharply with increasing active layer thickness. The shape of the EQE spectra of standard geometry devices also become severely distorted with increasing active layer thickness, with a minimum in EQE coinciding with the wavelength corresponding to maximum light absorption. In contrast, the shape of the EQE spectra of inverted devi...

We utilize a time-dependent drift-diffusion model incorporating electron trapping and field-dependent charge separation to explore the device physics of organic bulk-heterojunction solar cells based on blends of poly(3-hexylthiophene) (P3HT) with a red polyfluorene copolymer. The model is used to reproduce experimental photocurrent transients measured in response to a step-function excitation of light of varied intensity. The experimental photocurrent transients are characterized by (i) a fast rise of order 1 μs followed by (ii) a slow rise of order 10–100 μs that evolves into a transient peak at high intensity, (iii) a fast decay component after turn-off and (iv) a long-lived tail with magnitude that does not scale linearly with light intensity or steady-state photocurrent. The fast rise and decay components are explained by the transport of mobile carriers while the slow rise and decay components are explained by slower electron trapping and detrapping processes. The transient photocurrent peak at high ...

We have studied photocurrent transients in all-polymer bulk-heterojunction solar cells based on poly(3-hexylthiophene) and poly((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-hexylthien-5-yl)-2,1,3-benzothiadiazole]-2′,2″-diyl). By illuminating devices with square pulses of light of varying intensity, we reveal nonlinear photocurrent transients on the timescale of tens of microseconds. These microsecond photocurrent transients are attributed to the effects of trapping and detrapping of charges on this timescale, in particular, electrons. The buildup of trapped electrons results in the appearance of a peak in the photocurrent at high intensities at ∼10 μs after turn on. This trapped charge produces a local reduction in the strength of the internal electric field near the anode resulting in a net decrease in charge separation efficiency and an increase in the likelihood of bimolecular recombination due to increased and overlapping electron and hole densities. After turn off, a long photocurrent tail is obser...

The emission characteristics and external quantum efficiencies of ambipolar polymer light-emitting field-effect transistors are investigated as a function of applied voltage, current density, and ratio of hole to electron mobility. Green-emitting poly(9,9-di-n-octylfluorene-alt-benzothiadiazole) (F8BT) with balanced electron and hole mobilities and red-emitting poly((9,9-dioctylfluorene)-2,7- diyl-alt-[4,7-bis(3-hexylthien-5-yl)-2,1,3-benzothiadiazole]-2′,2″-diyl) (F8TBT) with strongly unbalanced hole and electron mobilities as semiconducting and emissive polymers are compared. The current-voltage and light output characteristics of the two types of light-emitting transistors were found to be fundamentally alike independent of mobility ratio. Device modeling allowing for a single (Langevin-type) charge recombination mechanism was able to reproduce the device characteristics for both cases but could not replicate the experimentally observed dependence of external quantum efficiency on current density. The ...

The dimensional dependence of micron-sized current inhomogeneities in poly[2-methoxy-5-(2′-ehtylhexyloxy)-1,4-phenylene vinylene]/1-(3-methoxycarbonyl)-propyl-1-1-phenyl-(6,6) C61 (MEH-PPV/PCBM) solar cells on processing conditions has been investigated. These current inhomogeneities typically consist of a central PCBM aggregate surrounded by a larger region of poor photocurrent production. For films spin coated from chlorobenzene, both the diameter of the current inhomogeneities and the size of the corresponding PCBM aggregates systematically increase with the time allowed for the film to dry. These observations indicate that the current inhomogeneities are a consequence of the nucleation and growth of the central PCBM cluster at the expense of PCBM from the surrounding area. In contrast to the results observed with chlorobenzene as the solvent, no micron-sized current inhomogeneities are observed using toluene as the solvent, despite the increased density of PCBM clusters in the film. The solubility of PCBM and the solvent volatility are identified as key parameters determining the formation of micron-sized current patterns. Also discussed is the influence of these current features on overall device performance, along with the influence of the film nanomorphology as processing conditions are changed.