Influence of the isomeric composition of the acceptor on the performance of organic bulk heterojunction P3HT:bis-PCBM solar cells

We synthesized three isomeric subpopulations of bisadduct analogues of [6,6]-phenyl-C61-butyric acid methyl ester (bis-PCBM) via tether-directed control. Bulk heterojunction solar cells prepared using these isomers together with poly(3-hexylthiophene) (P3HT) resulted in an increase of Jsc from 72.4 to 79.6 A m-2, and an improvement in fill factor from 0.55 to 0.62, both with a Voc of 0.72 V resulting in an overall enhancement of the power conversion efficiency (PCE) from 2.9% to 3.5%, compared to conventional bis-PCBM.

The Use of Tethered Addends to Decrease the Number of Isomers of Bisadduct Analogues of PCBM

Roped in: The number of isomers present in the bisadduct analogue PCBM (bis-PCBM; PCBM=phenyl-C61-butyric acid methyl ester), a recently introduced acceptor for bulk heterojunction organic photovoltaic devices, has been reduced to seven by linking two tosylhydrazone addends with an ethylene glycol tether and reacting them with C60.

Perovskite Solar Cells on Polymer-Coated Smooth and Rough Steel Substrates

Fabricating efficient perovskite solar cells on steel substrates could enable easy building integration of this photovoltaic technology. Herein, an n–i–p perovskite solar cell is developed on steel substrates for top illumination. The optimized stack uses a Ti bottom electrode, covered with an indium tin oxide (ITO) interlayer and a SnO2 electron transport layer passivated by [6,6]-phenyl-C61-butyric acid. The active layer is a triple-cation perovskite. A thermally evaporated tris(4-carbazoyl-9-ylphenyl)amine)/MoO3 bilayer acts as hole transport layer. The transparent top contact consists of ITO with a MgF2 antireflective coating. Optical analysis shows small parasitic absorption and reflectance losses for this stack, which provides 15.9% power conversion efficiency when fabricated on glass. On steel, covered with a polyamide imide planarization coating to moderate the surface roughness (R p), the highest efficiency is 15.2% for high-gloss steel (R p≈ 200 nm), 14.9% for battery steel (R p≈ 500 nm), 14.2% for packaging steel (R p≈ 1500 nm), and 13.8% for construction steel (R p≈ 2500 nm). While the short-circuit current density and open-circuit voltage are invariant, the fill factor decreases with increasing R p due to increasing series resistance and decreasing shunt resistance. The yield of working devices remain high, also for the roughest substrates

Partially replacing Pb²⁺ by Mn²⁺ in hybrid metal halide perovskites: Structural and electronic properties

Tailoring the physical properties of hybrid lead metal halide APbX3 perovskites by means of compositional engineering is one of the key factors contributing to the development of highly efficient and stable perovskite solar cells. While the beneficial effects of partial ionic replacement at the A- and X-sites are largely demonstrated, partial replacement of Pb2+ is less explored. Here, we developed a solution-based procedure to prepare thin films of mixed-metal MAPb1-aMnaI3 perovskites. Although Mn2+ ions have a size that can potentially fit in the B-sites of MAPbI3, using a combination of structural and chemical analysis, we show that only less than 10% of Pb2+ can be replaced by Mn2+. A 3% replacement of Pb2+ by Mn2+ leads to an elongation of the charge carrier lifetimes as concluded from time-resolved PL measurements. However, by analysis of the time-resolved microwave conductance data, we show that the charge carrier mobilities are largely unbalanced, which is in accordance with density functional theory (DFT) calculations indicating that the effective mass of the hole is much higher than that of the electron. Increasing the concentration of Mn2+ in the precursor solution above 10% results in formation of amorphous Mn-rich domains in the film, while the perovskite lattice becomes depleted of Mn2+. These domains negatively affect the charge carrier mobilities and shorten the lifetime of photogenerated carriers. The resulting reduction in charge carrier diffusion lengths will severely limit the photovoltaic properties of solar cells prepared from these mixed metal halide perovskites.</p

Development of a Perovskite Solar Cell Architecture for Opaque Substrates

To date, substrate-configuration metal-halide perovskite solar cells (PSCs) fabricated on opaque substrates such as metal foils provide inferior efficiencies compared with superstrate-configuration cells on transparent substrates such as glass. Herein, a substrate-configuration PSC on planarized steel is presented. To quantify the differences between the two configurations, a 15.6%-efficient n–i–p superstrate-configuration PSC is transformed step wise into a substrate-configuration cell. Guided by optical modeling, the opaque Au electrode is replaced by a transparent MoO3/thin Au/polystyrene dielectric–metal–dielectric electrode. The semitransparent device affords efficiencies of 15.4% and 11.4% for bottom and top illumination, respectively. Subsequently, substrate-configuration PSCs with a metal bottom electrode are fabricated on glass and planarized steel, using a thin MoO3 interlayer between the Au bottom electrode and the SnO2 electron transport layer. The glass-based substrate-configuration cell provides 14.0% efficiency with identical open-circuit voltage and fill factor as the superstrate cell. The cell on planarized steel reaches 11.5% efficiency due to a lower fill factor. For both substrate-configuration cells, the lower short-circuit current density limits the efficiency. Optical modeling explains this quantitatively to be due to absorption and reflection by the top electrode and absorption by the organic hole transport layer

p-i-n Perovskite Solar Cells on Steel Substrates

An efficient substrate-configuration p-i-n metal-halide perovskite solar cell (PSC) is fabricated on a polymer-coated steel substrate. The optimized cell employs a Ti bottom electrode coated with a thin indium tin oxide (ITO) interlayer covered with a self-assembled [2-(9H-carbazol-9-yl)ethyl]phosphonic acid monolayer as a hole-selective contact. A triple-cation perovskite is used as the absorber layer. Thermally evaporated C60and atomic layer deposited SnO2layers serve to create an electron-selective contact. The cells use an ITO top electrode with an antireflective MgF2coating. The optimized cell fabricated on a polymer-coated steel substrate reaches a power conversion efficiency of 16.5%, which approaches the 18.4% efficiency of a p-i-n reference superstrate-configuration cell that uses a similar stack design. Optical simulations suggest that the remaining optical losses are due to the absorption of light by the ITO top electrode, the C60layer, the Ti bottom electrode, and reflection from the MgF2coating in almost equal amounts. The major loss is, however, in the fill factor as a result of an increased sheet resistance of the top ITO electrode

Charge Separation and Recombination in Small Band Gap Oligomer-Fullerene Triads

Synthesis and photophysics of a series of thiophene-thienopyrazine small band gap oligomers, end-capped at both ends with C60, are presented. In these triads, a photoinduced electron transfer reaction occurs between the oligomer as a donor and the fullerene as an acceptor. Femtosecond photoinduced absorption has been used to determine the rates for charge separation and recombination. It was found that charge separation takes place within approximately 10 ps, and is situated close to the Marcus optimal region. Charge recombination is faster in o-dichlorobenzene (ODCB) (15-45 ps) than in toluene (90-730 ps), because in ODCB charge recombination takes place close to the optimal region. In toluene, the recombination is situated in the inverted region, with a much higher activation barrier. No signs of recombination into a triplet state were observed.

Charge separation and (triplet) recombination in diketopyrrolopyrrole–fullerene triads

Synthesis and photophysics of two diketopyrrolopyrrole-based small band gap oligomers, end-capped at both ends with C60 are presented. Upon photoexcitation of the oligomer, ultrafast energy transfer to the fullerene occurs (~0.5 ps), followed by an electron transfer reaction. Femtosecond transient absorption has been used to determine the rates for charge separation and recombination. Charge separation occurs in the Marcus normal region with a time constant of 18–47 ps and recombination occurs in the inverted regime, with a time constant of 37 ps to 1.5 ns. Both processes are faster in o-dichlorobenzene (ODCB) than in toluene. Analysis of the charge transfer rates by Marcus-Jortner theory leads to the view that the positive charge must be located on the thiophene/dithiophene unit closest to the fullerene. Approximately 14% of the charge transfer state was found to recombine into the low-lying triplet state of the oligomer for the smaller system in ODCB.

Improving the compatibility of fullerene acceptors with fluorene-containing donor-polymers in organic photovoltaic devices

Fluorene-containing PCBM analogs have been synthesized and tested with a polyfluorene copolymer, PF10TBT, in organic photovoltaic devices resulting in an increase of ~130 mV in the open circuit voltage compared to devices with PCBM as acceptor material.

Partially replacing Pb²⁺ by Mn²⁺ in hybrid metal halide perovskites: Structural and electronic properties

Tailoring the physical properties of hybrid lead metal halide APbX3 perovskites by means of compositional engineering is one of the key factors contributing to the development of highly efficient and stable perovskite solar cells. While the beneficial effects of partial ionic replacement at the A- and X-sites are largely demonstrated, partial replacement of Pb2+ is less explored. Here, we developed a solution-based procedure to prepare thin films of mixed-metal MAPb1-aMnaI3 perovskites. Although Mn2+ ions have a size that can potentially fit in the B-sites of MAPbI3, using a combination of structural and chemical analysis, we show that only less than 10% of Pb2+ can be replaced by Mn2+. A 3% replacement of Pb2+ by Mn2+ leads to an elongation of the charge carrier lifetimes as concluded from time-resolved PL measurements. However, by analysis of the time-resolved microwave conductance data, we show that the charge carrier mobilities are largely unbalanced, which is in accordance with density functional theory (DFT) calculations indicating that the effective mass of the hole is much higher than that of the electron. Increasing the concentration of Mn2+ in the precursor solution above 10% results in formation of amorphous Mn-rich domains in the film, while the perovskite lattice becomes depleted of Mn2+. These domains negatively affect the charge carrier mobilities and shorten the lifetime of photogenerated carriers. The resulting reduction in charge carrier diffusion lengths will severely limit the photovoltaic properties of solar cells prepared from these mixed metal halide perovskites.ChemE/Opto-electronic MaterialsRST/Neutron and Positron Methods in Material