Brian E. Lassiter

Open circuit voltage enhancement due to reduced dark current in small molecule photovoltaic cells

We demonstrate high open circuit voltage photovoltaic cells achieved by reducing the electron leakage current through the introduction of both organic and inorganic electron blocking layers between the donor layer and the anode contact. As an example, the blocking layers reduce the dark current in tin (II) phthalocyanine (SnPc)∕C60 solar cells with response across the visible and near infrared spectral region up to a wavelength of 1000nm, is decreased by two orders of magnitude compared to cells lacking the layers, resulting in a doubling of the open circuit voltage. The structure: indium tin oxide/electron blocker/SnPc (100A)∕C60 (400A)/bathocuproine (100A)∕Al, has a power conversion efficiency of (2.1±0.1)% at 1sun, standard AM1.5G solar illumination. This work demonstrates the importance of reducing dark current to achieve high organic thin film photovoltaic cell efficiencies.

A hybrid planar-mixed tetraphenyldibenzoperiflanthene/C70 photovoltaic cell

We describe a hybrid planar-mixed heterojunction (PM-HJ) organic photovoltaic cell based on tetraphenyldibenzoperiflanthene (DBP) and C70 with a power conversion efficiency of up to 6.4% ± 0.3%. Optimized cells consist of a DBP:C70 mixed layer at a volume ratio of 1:8 and a 9-nm thick C70 cap layer. The external quantum efficiency (EQE) in the visible of the PM-HJ cell is up to 10% larger than the mixed-HJ cell that lacks a C70 acceptor cap layer. The improvement in EQE is attributed to reduced exciton quenching at the MoO3 anode buffer layer surface. This leads to an internal quantum efficiency >90% between the wavelengths of λ = 450 nm and 550 nm, suggesting efficient exciton dissociation and carrier extraction in the PM-HJ cell. The power conversion efficiency under simulated AM 1.5G, 1 sun irradiation increases from 5.7% ± 0.2% for the mixed-HJ cell to 6.4% ± 0.3% for the PM-HJ cell, with a short-current density of 12.3 ± 0.3 mA/cm2, open circuit voltage of 0.91 ± 0.01 V, and fill factor of 0.56 ± 0.01.

Tandem organic photovoltaics using both solution and vacuum deposited small molecules

We demonstrate a tandem organic photovoltaic cell incorporating solution- and vacuum-deposited small molecules as the active layers. A blue and green-absorbing boron subphthalocyanine chloride:C70 graded heterojunction (HJ) sub-cell is combined with a green and red-absorbing functionalized squaraine/C70 bilayer HJ sub-cell, resulting in a tandem cell with a wavelength response from 350 nm to 800 nm. The efficiency of the cells depends on process conditions such as solvent annealing, resulting in nanocrystalline morphology that leads to improved charge and exciton transport compared with un-annealed cells. The incorporation of C70 in both sub-cells leads to an increase of short-circuit current by at least 30% compared to analogous cells using C60. The optimized power conversion efficiency of the tandem cell is 6.6% ± 0.1%, with an open-circuit voltage of 1.97 ± 0.1 V under simulated 1 sun, AM 1.5G illumination. The tandem cell voltage is equal to the sum of the constituent sub-cells, indicating that the tr...

A fullerene-based organic exciton blocking layer with high electron conductivity

We demonstrate the concentration dependence of C60 absorption in solid solutions of C60 and bathocuprione (BCP), revealing a nonlinear decrease of the C60 charge transfer (CT) state absorption. These blends are utilized to study the photocurrent contribution of the CT in bilayer organic photovoltaics (OPVs); 1:1 blends produce 40% less photocurrent. As exciton blocking electron transporting layers, the blends achieve power conversion efficiencies of 5.3%, an increase of 10% compared to conventional buffers.

Vertical orientation of copper phthalocyanine in organic solar cells using a small molecular weight organic templating layer

Ultrathin film material templating layers that force the morphology of subsequently grown electrically active thin films have been found to increase the performance of small molecule organic photovoltaic (OPV) cells. Here, we show that the electron-transporting material, hexaazatriphenylene-hexacarbonitrile (HAT-CN) can be used as a templating material that forces the copper phthalocyanine (CuPc) donor molecule to assume a vertical-standing morphology when deposited onto its surface on an indium tin oxide (ITO) electrode. For a device with HAT-CN as the templating buffer layer, the fill factor and short circuit current of CuPc:C60 OPVs were both significantly increased compared with cells lacking the HAT-CN template. This is explained by the reduction of the series resistance due to the improved crystallinity of CuPc grown onto the ITO surface.

Structural templating of multiple polycrystalline layers in organic photovoltaic cells

We demonstrate that organic photovoltaic cell performance is influenced by changes in the crystalline orientation of composite layer structures. A 1.5 nm thick self-organized, polycrystalline template layer of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) orients subsequently deposited layers of a diindenoperylene exciton blocking layer, and the donor, copper phthalocyanine (CuPc). Control over the crystalline orientation of the CuPc leads to changes in its frontier energy levels, absorption coefficient, and surface morphology, resulting in an increase of power conversion efficiency at 1 sun from 1.42 ± 0.04% to 2.19 ± 0.05% for a planar heterojunction and from 1.89 ± 0.05% to 2.49 ± 0.03% for a planar-mixed heterojunction.

Organic vapor phase deposition for the growth of large area organic electronic devices

We demonstrate that material utilization efficiencies of 50% and deposition nonuniformities ≤2.5% are achievable over substrate diameters of 200 mm using a simplified, organic vapor phase deposition (OVPD) system. The OVPD system is used to demonstrate doped electrophosphorescent organic light emitting diodes whose performance is comparable to those grown by vacuum thermal evaporation. Through continuum modeling, we demonstrate that analogous systems whose chamber dimensions are comparable to the substrate width are scalable to substrate sizes of at least 1500×1800 mm2 with deposition nonuniformities between 1.5% and 2.5%. These results indicate that OVPD is useful in the large area deposition of displays, lighting, and other organic electronic devices.

Understanding tandem organic photovoltaic cell performance

We develop a framework to understand the performance of tandem organic photovoltaic (OPV) cells consisting of a series-connected stack of an arbitrary number of sub-cells. The power conversion efficiency penalty, Δη, is defined as the loss incurred when the tandem cell is at its maximum power point (MPP) but one or more sub-cells are not operating at their individual MPPs. To minimize Δη, the current at the MPP for each sub-cell must be equal. We also develop a method to calculate the tandem cell spectral mismatch factor and fill factor, showing that they are related to both the fill factors and short circuit currents of all the constituent sub-cells. By including the current generated in the dark, exciton dissociation at the donor-acceptor heterojunction, and photoconductivity, along with current losses due to polaron-pair and bimolecular recombination, we simulate the operation of small molecule bilayer and mixed-layer sub-cells used in the tandem, and from these results derive the behavior of the integ...

Tandem organic photovoltaics incorporating two solution-processed small molecule donor layers

We develop a partially solution-processed small molecule tandem organic photovoltaic cell using an organic/inorganic interlayer structure that provides efficient charge recombination while protecting underlying layers from degradation due to attack from solvents applied during the deposition of subsequent sub-cells. Each sub-cell consists of a functionalized squaraine (fSQ) blend donor that is cast from solution, followed by evaporation of other functional layers. The first fSQ layer is cast from chloroform, while the second is cast from a tetrahydrofuran, thereby minimizing dissolution of the relatively insoluble, underlying fullerene layer that acts to protect the first donor layer. Solvent vapor annealing increases the sub-cell performance while decreasing the damage caused by spin-coating of the second fSQ layer, both of which result from increased film crystallinity that reduces the rate of solvent penetration. The tandem cell has a power conversion efficiency of 6.2% ± 0.3% and an open circuit volta...

Electron conducting buffer layers in organic photovoltaics

It is common to incorporate a cathode-side buffer layer in organic photovoltaic devices (OPVs) to mitigate damage from the evaporation of metal onto the underlying acceptor layer (e.g. C60), which can lead to exciton quenching and/or a barrier to charge extraction. Additionally, these materials can act as both an optical spacer and an exciton blocking layer. One class of buffer layers consists of a wide bandgap material (e.g. bathocuproine), that transports carriers via damage-induced midgap states. A second class of buffers consists of a material such as tris(acetylacetonato) ruthenium(III) (Ru(acac)3), which has a small highest occupied molecular orbital (HOMO) energy. In that case, electrons from the C60 and holes from the cathode recombine at the C60/Ru(acac)3 interface. In this work, we introduce a third class of buffer that, due to alignment of its lowest unoccupied molecular orbital with that of the acceptor, allows for low resistance transport of electrons between the acceptor and cathode. By utilizing 3,4,9,10 perylenetetracarboxylic bisbenzimidazole (PTCBI) as a buffer layer, we show improved fill factor in squaraine/C60-based devices without a loss in open-circuit voltage or photocurrent, leading to a >20% increase in power conversion efficiency. Although limited exciton transfer occurs from C60 to PTCBI, the short exciton diffusion length of PTCBI, coupled with the lack of loss at the C60/PTCBI interface, suggests that PTCBI also blocks excitons from quenching at metal-induced defects that are present in the absence of a buffer layer.