No abstract

No abstract

No abstract

Here, correlated AFM and scanning Kelvin probe microscopy measurements with sub‐100 nm resolution on the phase‐separated active layer of polymer‐fullerene (MDMO‐PPV:PCBM) bulk heterojunction solar cells in the dark and under illumination are described. Using numerical modeling a fully quantitative explanation for the contrast and shifts of the surface potential in dark and light is provided. Under illumination an excess of photogenerated electrons is present in both the donor and acceptor phases. From the time evolution of the surface potential after switching off the light the contributions of free and trapped electrons can be identified. Based on these measurements the relative 3D energy level shifts of the sample are calculated. Moreover, by comparing devices with fine and coarse phase separation, it is found that the inferior performance of the latter devices is, at least partially, due to poor electron transport.

We present a numerical model for calculating current-voltage characteristics of polymer:fullerene bulk hetrojunction solar cells at different degrees of nanoscale phase separation. We show that the short-circuit current enhancement with finer phase separation is due to a reduction in bimolecular recombination caused by lateral movement of photogenerated electrons to the fullerene-rich phase. At high bias, vertical electron transport is enhanced and lateral movement is reduced, causing a significant field-dependent carrier extraction for coarse morphologies.

We present a combined numerical charge transport and morphology model to describe the current density-voltage ( j -V ) characteristics of three different, benchmark polymer:fullerene bulk heterojunction organic solar cells in which the device performance critically depends on the processing conditions or composition of the active layer. We fi nd that an accurate description of the j -V characteristics over a broad bias range can be obtained when the actual complex, three-dimensional (3D) phase separation is represented by a simplifi ed 2D or even 1D description. The morphological device model allows predicting the potential for increasing device performance by further optimizing the morphology. The optimal simplifi ed morphology consists of two, relatively thin alternating vertically oriented slabs, that allow for fast lateral separation of photocreated holes and electrons. This morphology can effectively be described as 1D.

The confi rmed power conversion efficiency (PCE) of solution-processed organic photovoltaics (OPV) with donoracceptor (D-A) bulk-heterojunction (BHJ) active layers has increased from 3.0 to 8.3% over the past 4 years. [ 1 ] This remarkable progress is in large part due to the tunability afforded in organic materials by seemingly simple modifi cations of their molecular structure, that led to the development of donor and acceptor materials with improved properties for solar energy conversion. Design rules for new materials to push toward PCEs of 10% were formulated by Scharber et al. in 2006, [ 2 ] which have led to two main areas of materials development: a) lowering the absorption onset of the conjugated polymer light-harvesting component, thereby improving overlap with the solar spectrum, to increase the short circuit current, J sc[ 3 ] and b) tuning the relative energetics of the ionization potential of the donor ( IP D ) and the electron affi nity of the acceptor ( EA A ) to improve the open-circuit voltage, V oc . [ 3a , 3b , 3e , 3g , 4 ] This paper pertains to the latter mechanism; we investigate a class of fullerene-based molecular acceptors that, when used in BHJ OPV devices, improve the V oc as compared to the prototypical acceptor PCBM, [ 4i , 4m ] and study the photoinduced dynamics of excitons and free carriers as a function of the chemical structure of the molecular acceptor, which is directly related to EA A .Although increased V oc , and in some instances enhanced PCE, have been demonstrated in polymer:fullerene OPV devices with acceptors with lower EA A , [ 4 ] in the case of the multiadducts of [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM) an undesirable reduction in photocurrent has been observed, [ 4f , 4g ] which has been attributed to a reduction in the electron mobility with increasing number of solubilizing side chains, particularly in the case of the tris-adduct. It is plausible that modifi cation of The effect of functionalization of the C 60 cage with multiple indene groups in relation to the dynamics of photogenerated species in blends with poly(3-hexylthiophene) (P3HT) and the performance of P3HT:indene-C 60 photovoltaic devices is reported. Despite the systematic decrease of the electron affi nity of the acceptor with the number of additions, exciton dissociation is effi cient in blends of P3HT with all three indene-C 60 derivatives. By replacing the prototypical acceptor [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM) with mono-indene-C 60 (ICMA) or a sample of a mixture of bis-indene-C 60 regioisomers (ICBA) the power conversion effi ciency is enhanced, predominantly due to an increase in the open-circuit voltage that originates from the lower electron affi nity of the indene-C 60 acceptor.The use of an acceptor sample that represents a mixture of tris-indene-C 60 (ICTA) regioisomers results in a reduction of the short-circuit current density, fi ll factor, and open-circuit voltage of the photovoltaic device. The electron mobility in ICTA domains is ca. a factor 10 l...

The electrostatic potential distribution across single, isolated, colloidal heterostructured nanorods (NRs) with component materials expected to form a p-n junction within each NR has been measured using scanning Kelvin probe microscopy (SKPM). We compare CdS to bicomponent CdS-CdSe, CdS-PbSe, and CdS-PbS NRs prepared via different synthetic approaches to corroborate the SKPM assignments. The CdS-PbS NRs show a sharp contrast in measured potential across the material interface. We find the measured built-in potential within an individual NR to be attenuated by long-range electrostatic forces between the sample substrate, cantilever, and the measuring tip. Surface potential images were deconvoluted to yield built-in potentials ranging from 375 to 510 meV in the heterostructured NRs. We deduce the overall built-in potential as well as the charge distribution across each segment of the heterostructured NRs by combining SKPM data with simulations of the system.