Influence of Chemical Structure on the Charge Transfer State Spectrum of a Polymer:Fullerene Complex

Charge transfer (CT) state properties at the donor:acceptor interface in organic photovoltaic materials are widely regarded as crucial in determining the charge separation efficiency of organic photovoltaic devices. In this work, we use time dependent density functional theory (TDDFT) with B3LYP/6-31g* to study the influence of chemical structure on excitation energies, oscillator strengths, and electronic structure. We vertically excite states in a series of polymer:fullerene blends, modeling each blend as an oligomer:fullerene pair in vacuo. Our method reproduces experimentally observed trends in CT state energy with chemical structure as measured by electroluminescence. For oligothiophene:PCBM (PCBM = [6,6]-phenyl C61-butyric acid methyl ester), we find that Coulomb binding tends to reduce in higher excited CT states. In the case of several isoindigo and diketopyrrolopyrrole (DPP) based donors, we find that the first excited state of the pair lies close in energy to the first singlet of the oligomers, and low excited states have hybrid, or incomplete charge transfer, character. The natures and energies of these states are dependent on the fullerene position. We discuss the effect of thiophene substitution in a DPP polymer on charge generation in terms of the calculated CT state properties and rationalize the observed charge separation efficiency of corresponding experimental systems in terms of these calculations

In Situ Measurement of Energy Level Shifts and Recombination Rates in Subphthalocyanine/C₆₀ Bilayer Solar Cells

Understanding the nature and impact of internal interfaces is critical to understanding the operation of nanostructured organic devices, such as organic photovoltaics. Here, we use transient optoelectronic analyses to quantify in situ the HOMO level shifts and changes in interfacial recombination rate that occur within thermally evaporated subphthalocyanine (SubPc)/C₆₀ bilayer solar cells as the SubPc evaporation source is varied. We show how such measurements can complement ex situ optical and physical techniques to access the functional impact of device modification, particularly with respect to the resulting device open-circuit voltage (<i>V</i>_OC). We are able to explain how subtle changes in SubPc deposition conditions lead to significant modification of interfacial energetics and recombination dynamics, which in turn cause substantial changes in <i>V</i>_OC

Understanding the Thickness-Dependent Performance of Organic Bulk Heterojunction Solar Cells: The Influence of Mobility, Lifetime, and Space Charge

We investigate the reasons for the dependence of photovoltaic performance on the absorber thickness of organic solar cells using experiments and drift-diffusion simulations. The main trend in photocurrent and fill factor versus thickness is determined by mobility and lifetime of the charge carriers. In addition, space charge becomes more and more important the thicker the device is because it creates field free regions with low collection efficiency. The two main sources of space-charge effects are doping and asymmetric mobilities. We show that for our experimental results on Si-PCPDTBT:PC₇₁BM (poly­[(4,40-bis­(2-ethylhexyl)­dithieno­[3,2-<i>b</i>:20,30-<i>d</i>]­silole)-2,6-diyl-<i>alt</i>-(4,7-bis­(2-thienyl)-2,1,3-benzothiadiazole)-5,50-diyl]:[6,6]-phenyl C71-butyric acid methyl ester) solar cells, the influence of doping is most likely the dominant influence on the space charge and has an important effect on the thickness dependence of performance

Quantitative Analysis of the Molecular Dynamics of P3HT:PCBM Bulk Heterojunction

The optoelectronic properties of blends of conjugated polymers and small molecules are likely to be affected by the molecular dynamics of the active layer components. We study the dynamics of regioregular poly­(3-hexylthiophene) (P3HT):phenyl-C61-butyric acid methyl ester (PCBM) blends using molecular dynamics (MD) simulation on time scales up to 50 ns and in a temperature range of 250–360 K. First, we compare the MD results with quasi-elastic neutron-scattering (QENS) measurements. Experiment and simulation give evidence of the vitrification of P3HT upon blending and the plasticization of PCBM by P3HT. Second, we reconstruct the QENS signal based on the independent simulations of the three phases constituting the complex microstructure of such blends. Finally, we found that P3HT chains tend to wrap around PCBM molecules in the amorphous mixture of P3HT and PCBM; this molecular interaction between P3HT and PCBM is likely to be responsible for the observed frustration of P3HT, the plasticization of PCBM, and the partial miscibility of P3HT and PCBM

Understanding the Thickness-Dependent Performance of Organic Bulk Heterojunction Solar Cells: The Influence of Mobility, Lifetime, and Space Charge

We investigate the reasons for the dependence of photovoltaic performance on the absorber thickness of organic solar cells using experiments and drift-diffusion simulations. The main trend in photocurrent and fill factor versus thickness is determined by mobility and lifetime of the charge carriers. In addition, space charge becomes more and more important the thicker the device is because it creates field free regions with low collection efficiency. The two main sources of space-charge effects are doping and asymmetric mobilities. We show that for our experimental results on Si-PCPDTBT:PC₇₁BM (poly­[(4,40-bis­(2-ethylhexyl)­dithieno­[3,2-<i>b</i>:20,30-<i>d</i>]­silole)-2,6-diyl-<i>alt</i>-(4,7-bis­(2-thienyl)-2,1,3-benzothiadiazole)-5,50-diyl]:[6,6]-phenyl C71-butyric acid methyl ester) solar cells, the influence of doping is most likely the dominant influence on the space charge and has an important effect on the thickness dependence of performance

Understanding the Effect of Donor Layer Thickness and a MoO₃ Hole Transport Layer on the Open-Circuit Voltage in Squaraine/C₆₀ Bilayer Solar Cells

Small molecule organic solar cells are becoming increasingly efficient through improved molecular design. However, there is still much to be understood regarding device operation. Here we study bilayer solar cells employing a 2,4-bis­[4-(<i>N,N</i>-diisobutylamino)-2,6-dihydroxyphenyl] squaraine (SQ) donor and fullerene acceptor to probe the effect of donor layer thickness and a MoO₃ electron transport layer on device performance. The thickness of SQ is seen to drastically affect the open-circuit voltage (<i>V</i>_OC) and fill factor (FF), while the short circuit current is not altered significantly. The fact that the <i>V</i>_OC of the bilayers with thin (6 nm) donor layers shows a strong dependence on the material and workfunction of the anode cannot be explained with a model for a perfect bilayer. Recombination of electrons from C₆₀ at the anode contact has to be possible to understand the strong effect of the anode workfunction. Using numerical simulations and a simple two-diode model we show that the most likely interpretation of the observed effects is that for thin SQ layers, the roughness of the interface is high enough to allow electrons in the C₆₀ to tunnel through the SQ to recombine directly at the anode. Thicker SQ layers will block most of these recombination pathways, which explains the drastic dependence of <i>V</i>_OC on thickness. Bulk-heterojunction devices were also fabricated to illustrate the effect of anode material on the <i>V</i>_OC

On the Differences between Dark and Light Ideality Factor in Polymer:Fullerene Solar Cells

Ideality factors are derived from either the slope of the dark current/voltage curve or the light intensity dependence of the open-circuit voltage in solar cells and are often a valuable method to characterize the type of recombination. In the case of polymer:fullerene solar cells, the ideality factors derived by the two methods usually differ substantially. Here we investigate the reasons for the discrepancies by determining both ideality factors differentially as a function of voltage and by comparing them with simulations. We find that both the dark and light ideality factors are sensitive to bulk recombination mechanisms at the internal donor:acceptor interface, as is often assumed in the literature. While the interpretation of the dark ideality factor is difficult due to resistive effects, determining the light ideality factor <i>differentially</i> indicates that the open-circuit voltage of many polymer:fullerene solar cells is limited by surface recombination, which leads to light ideality factors decreasing below one at high voltage

Understanding the Apparent Charge Density Dependence of Mobility and Lifetime in Organic Bulk Heterojunction Solar Cells

Energetic disorder in organic semiconductors leads to strong dependence of recombination kinetics and mobility on charge density. However, observed mobilities and reaction orders are normally interpreted assuming uniform charge carrier distributions. In this paper, we explore the effect of the spatial distribution of charge on the determination of mobility and recombination rate as a function of average charge density. Since the spatial gradient changes when the thickness of a device is varied, we study thickness series of two different polymer:fullerene systems and measure the charge density dependence of mobility and lifetime. Using simulations, we can show that the high apparent reaction orders frequently observed in the literature result from the spatial gradient of charge density at open circuit. However, the mobilities, measured at short circuit, are less affected by the gradients and therefore may show substantially different apparent charge density dependence than the recombination constants, especially for small device thicknesses

Analysis of the Relationship between Linearity of Corrected Photocurrent and the Order of Recombination in Organic Solar Cells

We address the claim that the dependence of the “corrected photocurrent” (defined as the difference between the light and dark currents) upon light intensity can be used to determine the charge recombination mechanism in an organic solar cell. We analyze a poly(3-hexylthiophene):[6,6]-phenyl C61-butyric acid methyl ester (P3HT:PCBM) device using corrected photocurrent and transient photovoltage experiments and show that whereas the corrected photocurrent is linear in light intensity the charge recombination rate scales superlinearly with charge carrier density. We explain this apparent discrepancy by measuring the charge carrier densities at different applied voltages and light intensities. We show that it is only safe to infer a linear recombination mechanism from a linear dependence of corrected photocurrent on light intensity under the following special conditions: (i) the photogenerated charge carrier density is much larger than the dark carrier density and (ii) the photogenerated carrier density is proportional to the photogeneration rate

Limits on the Fill Factor in Organic Photovoltaics: Distinguishing Nongeminate and Geminate Recombination Mechanisms

In this Letter, we present transient optoelectronic experimental studies of the recombination processes limiting the fill factor (FF) in three conjugated polymer:fullerene systems, poly­(3-hexylthiophene) (P3HT) and two lower-band-gap polymers that exhibit lower FFs poly­[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta­[2,1-<i>b</i>;3,4-<i>b</i>′]­dithiophene)-<i>alt</i>-4,7-(2,1,3-benzothiadiazole) (PCPDTBT) and poly­(2,7-(9,9-dioctylfluorene)-<i>alt</i>-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)) (APFO-3). Using transient absorption spectroscopy, charge extraction, and transient photovoltage experiments, we show that the lower FF observed for the PCPDTBT-based device results from enhanced nongeminate recombination even at short circuit, In contrast, we show that for APFO-3 devices, the FF is primarily limited by a voltage-dependent free charge generation, which we assign to a geminate recombination process