Kevin J. Bergemann

Functionalized Squaraine Donors for Nanocrystalline Organic Photovoltaics

We study a family of functionalized squaraine (fSQ) donors for absorbing in the near-infrared (NIR) and green spectral regions. The NIR-absorbing materials are the symmetric molecules 2,4-bis[4-(N-phenyl-1-naphthylamino)-2,6-dihydroxyphenyl]squaraine (1-NPSQ), 2,4-bis[4-(N,N-diphenylamino)-2,6 dihydroxyphenyl]squaraine, and 2,4-bis[4-(N,N-dipropylamino)-2,6-dihydroxyphenyl]squaraine. The green light absorbing donors are asymmetric squaraines, namely, 2,4-bis[4-(N,N-diphenylamino)-2,6-dihydroxyphenyl]squaraine and 2-[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl]-4-diphenylamino]squaraine. Substitution of the arylamine groups enhances intermolecular packing, thereby increasing hole transport and the possibility of forming extended nanocrystalline junctions when annealed. Nanocrystalline solar cells based on fSQ and a C(60) acceptor have V(oc) = 1.0 V and fill factors 0.73 ± 0.01. Solar cells incorporating annealed 1-NPSQ films result in a power conversion efficiency of 5.7 ± 0.6% at 1 sun, AM1.5G illumination.

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.

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.

Measurement of exciton diffusion lengths in optically thin organic films

Spectrally resolved photoluminescence quenching (SR-PLQ) is a convenient and accurate method for measuring the exciton diffusion length of organic materials; however, the requirement of optically thick films demanded by this technique poses practical limitations to its implementation. Through simulations of the optical field and exciton dynamics, we extend SR-PLQ to the case of optically thin organic films; i.e., films whose thickness is comparable to or less than that of the optical absorption length across the entire optical absorption spectrum. This allows for the characterization of films whose thickness is comparable to that used in practical organic optoelectronic devices. Using this method, we measure the diffusion lengths of several squaraine donors, the acceptor 3,4,9,10 perylenetetracarboxylic dianhydride, and the relationship between the donor, boron subphthalocyanine thickness and diffusion length.

Nonideal Diode Behavior and Bandgap Renormalization in Carbon Nanotube p-n Junctions

The p-n junction diodes are formed by electrostatic doping using two gate electrodes positioned beneath individual, suspended single-walled carbon nanotubes (CNTs). These devices exhibit nearly ideal diode behavior within a small bias voltage range near 0 V. At higher bias (>\vert 0.2 V\vert), nonideal diode behavior is observed arising from Schottky contacts formed between the nanotube and its metal contact electrodes and the presence of electron tunneling between the N- and P-doped regions. We introduce a back-to-back diode model to explain the observed current versus voltage (I-V) characteristics. The reverse saturation current, parallel resistance, and open-circuit voltage dependence on gate voltage provide quantitative evidence for the theoretically predicted doping-induced bandgap shrinkage in CNTs. The minority carrier lifetimes are also estimated from this model.

Surprisingly High Conductivity and Efficient Exciton Blocking in Fullerene/Wide-Energy-Gap Small Molecule Mixtures

We find that mixtures of C60 with the wide energy gap, small molecular weight semiconductor bathophenanthroline (BPhen) exhibit a combination of surprisingly high electron conductivity and efficient exciton blocking when employed as buffer layers in organic photovoltaic cells. Photoluminescence quenching measurements show that a 1:1 BPhen/C60 mixed layer has an exciton blocking efficiency of 84 ± 5% compared to that of 100% for a neat BPhen layer. This high blocking efficiency is accompanied by a 100-fold increase in electron conductivity compared with neat BPhen. Transient photocurrent measurements show that charge transport through a neat BPhen buffer is dispersive, in contrast to nondispersive transport in the compound buffer. Interestingly, although the conductivity is high, there is no clearly defined insulating-to-conducting phase transition with increased insulating BPhen fraction. Thus, we infer that C60 undergoes nanoscale (<10 nm domain size) phase segregation even at very high (>80%) BPhen fractions.