Beate Burkhart

Efficient Ternary Blend Bulk Heterojunction Solar Cells with Tunable Open-Circuit Voltage

To explore the potential of ternary blend bulk heterojunction (BHJ) photovoltaics as a general platform for increasing the attainable performance of organic solar cells, a model system based on poly(3-hexylthiophene) (P3HT) as the donor and two soluble fullerene acceptors, phenyl-C(61)-butyric acid methyl ester (PC(61)BM) and indene-C(60) bisadduct (ICBA), was examined. In all of the solar cells, the overall ratio of polymer to fullerene was maintained at 1:1, while the composition of the fullerene component (PC(61)BM:ICBA ratio) was varied. Photovoltaic devices showed high short-circuit current densities (J(sc)) and fill factors (FF) (>0.57) at all fullerene ratios, while the open-circuit voltage (V(oc)) was found to vary from 0.61 to 0.84 V as the fraction of ICBA was increased. These results indicate that the V(oc) in ternary blend BHJ solar cells is not limited to the smallest V(oc) of the corresponding binary blend solar cells but can be varied between the extreme V(oc) values without significant effect on the J(sc) or FF. By extension, this result suggests that ternary blends provide a potentially effective route toward maximizing the attainable J(sc)V(oc) product (which is directly proportional to the solar cell efficiency) in BHJ solar cells and that with judicious selection of donor and acceptor components, solar cells with efficiencies exceeding the theoretical limits for binary blend solar cells could be possible without sacrificing the simplicity of a single active-layer processing step.

Compositional Dependence of the Open-Circuit Voltage in Ternary Blend Bulk Heterojunction Solar Cells Based on Two Donor Polymers

Ternary blend bulk heterojunction (BHJ) solar cells containing as donor polymers two P3HT analogues, high-band-gap poly(3-hexylthiophene-co-3-(2-ethylhexyl)thiophene) (P3HT(75)-co-EHT(25)) and low-band-gap poly(3-hexylthiophene-thiophene-diketopyrrolopyrrole) (P3HTT-DPP-10%), with phenyl-C(61)-butyric acid methyl ester (PC(61)BM) as an acceptor were studied. When the ratio of the three components was varied, the open-circuit voltage (V(oc)) increased as the amount of P3HT(75)-co-EHT(25) increased. The dependence of V(oc) on the polymer composition for the ternary blend regime was linear when the overall polymer:fullerene ratio was optimized for each polymer:polymer ratio. Also, the short-circuit current densities (J(sc)) for the ternary blends were bettter than those of the binary blends because of complementary polymer absorption, as verified using external quantum efficiency measurements. High fill factors (FF) (>0.59) were achieved in all cases and are attributed to high charge-carrier mobilities in the ternary blends. As a result of the intermediate V(oc), increased J(sc) and high FF, the ternary blend BHJ solar cells showed power conversion efficiencies of up to 5.51%, exceeding those of the corresponding binary blends (3.16 and 5.07%). Importantly, this work shows that upon optimization of the overall polymer:fullerene ratio at each polymer:polymer ratio, high FF, regular variations in V(oc), and enhanced J(sc) are possible throughout the ternary blend composition regime. This adds to the growing evidence that the use of ternary blends is a general and effective strategy for producing efficient organic photovoltaics manufactured in a single active-layer processing step.

Origin of the Tunable Open-Circuit Voltage in Ternary Blend Bulk Heterojunction Organic Solar Cells

Ternary blend bulk heterojunction organic solar cells comprising either a polythiophene donor and two fullerene acceptors or two polythiophene donors and a fullerene acceptor are shown to have unique electronic properties. Measurements of the photocurrent spectral response and the open-circuit voltage show that the HOMO and LUMO levels change continuously with composition in the respective two-component acceptor or donor pair, consistent with the formation of an organic alloy. However, optical absorption of the exciton states retains the individual molecular properties of the two materials across the blend composition. This difference is attributed to the highly localized molecular nature of the exciton and the more delocalized intermolecular nature of electrons and holes that reflect the average composition of the alloy. As established here, the combination of molecular excitations that can harvest a wide range of photon energies and electronic alloy states that can adjust the open-circuit voltage provides the underlying basis of ternary blends as a platform for highly efficient next-generation organic solar cells.

Polymer-Based Solar Cells: State-of-the-Art Principles for the Design of Active Layer Components

Abstract The vision of organic photovoltaics is that of a low cost solar energy conversion platform that provides lightweight, flexible solar cells that are easily incorporated into existing infrastructure with minimal impact on land usage. Polymer solar cells have been a subject of growing research interest over the past quarter century, and are now developed to the point where they are on the verge of introduction into the market. Towards the goal of continuing to improve the performance of polymer solar cells, a number of avenues are being explored. Here, the focus is on optimization of device performance via the development of a more fundamental understanding of device parameters. The fundamental operating principle of an organic solar cell is based on the cooperative interaction of molecular or polymeric electron donors and acceptors. Here the state-of-the-art in understanding of the physical and electronic interactions between donor and acceptor components is examined, as is important for understanding future avenues of research and the ultimate potential of this technology.

Solar cells based on semi-random P3HT analogues containing dithienopyrrole: influence of incorporating a strong donor

Novel semi-random poly(3-hexylthiophene) (P3HT) based polymers P3HTT-DTP, P3HTT-BTD-DTP, P3HTT-TP-DTP and P3HTT-DPP-DTP containing the strong donor dithienopyrrole (DTP) as well as different acceptors (benzothiadiazole (BTD), thienopyrazine (TP) or diketopyrrolopyrrole (DPP)) were synthesized by Stille copolymerization and their optical, electrochemical, charge transport, and photovoltaic properties were investigated. All polymers (except for the all donor polymer P3HTT-DTP) show considerably broadened absorption compared to P3HT due to the donor-acceptor effect and their multichromophoric nature. The introduction of the strong donor DTP leads to increased HOMO energies and thus decreased open-circuit voltage (Voc) (compared to previously published semi-random polymers) as well as an amorphous character of P3HTT-DTP, P3HTT-BTD-DTP and P3HTT-TP-BTD resulting in low hole mobilities and moderate solar cell efficiencies (0.18% to 0.36%). The exception is P3HTT-DPP-DTP, which is semi-crystalline and has a high hole mobility of 1.4×10−4  cm2 ?startVend?−1 s−1 comparable to P3HT, as well as increased photocurrent (10.7  mA/cm 2 ) due to broad and uniform photoresponse up to 850 nm leading to a promising non-optimized device efficiency of 2.83%.

Medium area, flexible single and tandem junction solar cells based on roll coated semi-random copolymers

We report on medium area (1 cm2) slot-die coated organic photovoltaic devices (OPVs) of a recently developed semi-random copolymer of poly-3-hexylthiophene and diketopyrrolopyrrole (P3HTT–DPP-10%) mixed with phenyl-C61-butyric acid methyl ester ([60]PCBM). The devices were prepared using a compact laboratory roll-coater using only slot-die coating and flexographic printing under ambient conditions on a flexible ITO-free substrate. In order to overcome a low JSC and FF obtained for single junction devices, devices were also prepared in a tandem geometry making it possible to employ thinner junction films. Power conversion efficiencies of up to 1.36% and 1.31% were achieved for the tandem and single junction geometries, respectively.

Breakdown mechanisms and reverse current-voltage characteristics of organic bulk heterojunction solar cells and photodetectors

We investigate the reverse current-voltage characteristics and breakdown mechanisms of organic bulk heterojunction solar cells and photodetectors. Dark current and photo current measurements at different temperatures indicate that tunneling is the dominant mechanism at high reverse voltage. A band-to-band tunneling model that accommodates either Gaussian or exponential-parabolic density of states distributions is developed and used for simulations. At high reverse bias, the model explains the observed breakdown, which differs from the bias independent dark current behavior predicted by the Onsager-Braun model. At low reverse voltage, the incorporation of shunt resistance in the model provides good agreement between the measured reverse bias characteristics and simulations.

Efficiency limitations in organic bulk heterojunction solar cells

This paper presents a systematic study of key physical mechanisms in organic polymer/fullerene solar cells that determine the power conversion efficiency. The carrier loss at each step is calculated or measured. We compare poly(3-hexylthiophene) (P3HT)/[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and poly(3-hexylthiophene)-thiophene-thienopyrazine (P3HTT-TP)/PCBM solar cells. The latter polymer, P3HTT-TP, is a novel low-bandgap “semi-random” alkythiophene-based copolymer with a broader absorption spectrum than P3HT. Absorption, a significant loss factor for all polymer/fullerene solar cells, was studied using external quantum efficiency and absorption spectroscopy measurements and modeled with a multi-layer transfer-matrix theory. In addition, hole traps inside the polymers play an important role in polaron pair dissociation and free charge transport. It was found that this is particularly deleterious for P3HTT-TP/PCBM.

Photocurrent transients in polymer-fullerene bulk heterojunction organic solar cells

By measuring the transient photocurrents we study the charge carrier dynamics in regioregular poly(3-hexylthiophene) (P3HT): 6,6-phenyl-C61-butyric acid methyl ester (PCBM), and poly(3-hexylthiophene) thiophene thienopyrazine (P3HTT-TP):PCBM bulk heterojunction (BHJ) solar cells. The latter is based on a novel “semi-random” copolymer and exhibits broad spectral absorption. We observe that the charge carrier transport in P3HTT-TP:PCBM is more imbalanced than that of P3HT:PCBM. Voltage dependent photocurrent transients indicate that P3HT:PCBM has longer carrier lifetimes than P3HTT-TP:PCBM. Light intensity dependent open-circuit voltage measurements show stronger monomolecular recombination in P3HTT-TP:PCBM. The unfavorable charge carrier transport may be one of the reasons for the low power conversion efficiency of P3HTT-TP:PCBM BHJ solar cells.