Valentin D. Mihailetchi

Light intensity dependence of open-circuit voltage of polymer:fullerene solar cells

The open-circuit voltage Voc of polymer:fullerene bulk heterojunction solar cells is investigated as a function of light intensity for different temperatures. Devices consisted of a blend of a poly(p-phenylene vinylene) derivative as the hole conductor and 6,6-phenyl C61-butyric acid methyl ester as the electron conductor. The observed photogenerated current and Voc are at variance with classical p–n junction-based models. The influence of light intensity and recombination strength on Voc is consistently explained by a model based on the notion that the quasi-Fermi levels are constant throughout the device, including both drift and diffusion of charge carriers.

Ultimate efficiency of polymer/fullerene bulk heterojunction solar cells

We present model calculations to explore the potential of polymer/fullerene bulk heterojunction solar cells. As a starting point, devices based on poly(3-hexylthiophene) and 6,6-phenyl C61-butyric acid methyl ester (PCBM), reaching 3.5% efficiency, are modeled. Lowering the polymeric band gap will lead to a device efficiency exceeding 6%. Tuning the electronic levels of PCBM in such a way that less energy is lost in the electron transfer process enhances the efficiency to values in excess of 8%. Ultimately, with an optimized level tuning, band gap, and balanced mobilities polymeric solar cells can reach power conversion efficiencies approaching 11%.

Bimolecular recombination in polymer/fullerene bulk heterojunction solar cells

Bimolecular recombination in organic semiconductors is known to follow the Langevin expression, i.e., the rate of recombination depends on the sum of the mobilities of both carriers. We show that this does not hold for polymer/fullerene bulk heterojunction solar cells. The voltage dependence of the photocurrent reveals that the recombination rate in these blends is determined by the slowest charge carrier only, as a consequence of the confinement of both types of carriers to two different phases.

Origin of the light intensity dependence of the short-circuit current of polymer/fullerene solar cells

A typical feature of polymer/fullerene based solar cells is that the current density under short-circuit conditions (Jsc) does not scale exactly linearly with light intensity (I). Instead, a power law relationship is found given by Jsc∝Iα, where α ranges from 0.85 to 1. In a number of reports this deviation from unity is speculated to arise from the occurrence of bimolecular recombination. We demonstrate that the dependence of the photocurrent in bulk heterojunction solar cells is governed by the build-up of space-charge in the device as a consequence of a difference in electron- and hole mobility. We have verified this for an experimental model system in which the mobility difference can be tuned from one to three orders of magnitude by changing the annealing treatment.

Origin of the enhanced performance in poly(3-hexylthiophene): [6,6]-phenyl C61-butyric acid methyl ester solar cells upon slow drying of the active layer

The origin of the enhanced performance of bulk heterojunction solar cells based on slowly dried films of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester is investigated, combining charge transport measurements with numerical device simulations. Slow drying leads to a 33-fold enhancement of the hole mobility up to 5.0×10−7m2V−1s−1 in the P3HT phase of the blend, thereby balancing the transport of electrons and holes in the blend. The resulting reduction of space-charge accumulation enables the use of thick films (∼300nm), absorbing most of the incoming photons, without losses in the fill factor and short-circuit current of the device.

Thickness dependence of the efficiency of polymer:fullerene bulk heterojunction solar cells

We study the thickness dependence of the performance of bulk heterojunction solar cells based on poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene] as electron donor and [6,6]-phenyl C61 butyric acid methyl ester as electron acceptor. Typically, these devices have an active layer thickness of 100nm at which only 60% of the incoming light is absorbed. Increasing device thickness results in a lower overall power conversion efficiency, mainly due to a lowering of the fill factor. We demonstrate that the decrease in fill factor and hence device efficiency is due to a combination of charge recombination and space-charge effects.

Effect of metal electrodes on the performance of polymer:fullerene bulk heterojunction solar cells

An increase in the workfunction of the metal top electrode leads to a reduction of the open-circuit voltage, short-circuit current, and power conversion efficiency of organic bulk-heterojunction solar cells. It has been demonstrated that the photocurrent obtained from an active layer comprised of a blend of poly(2-methoxy-5-(3′,7′-dimethyloctyloxy)-p-phenylenevinylene) (OC1C10-PPV) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM), with lithium fluoride topped aluminum, silver, gold, or palladium electrodes, shows a universal behavior when scaled against the effective voltage across the device. Indeed, model calculations confirm that the dependence of the photocurrent on the effective voltage is responsible for the observed variation in performance of each different electrode. Consequently, for any given metal, only the device’s open-circuit voltage is required in order to be able to predict the remaining solar cell parameters.

Nitric acid pretreatment for the passivation of boron emitters for n-type base silicon solar cells

We have developed a simple method to passivate industrially produced boron-doped emitters for n-type base silicon solar cells using an ultrathin (∼1.5nm) silicon dioxide layer between the silicon emitter and the silicon nitride antireflection coating film. This ultrathin oxide is grown at room temperature by soaking the silicon wafers in a solution of nitric acid prior to the deposition of the silicon nitride antireflection coating film. The n-type solar cells processed in such a way demonstrate a conversion efficiency enhancement of more than 2% absolute over the solar cells passivated without the silicon dioxide layer.

Injection-limited electron current in a methanofullerene

The dark current of bulk-heterojunction photodiodes consisting of a blend of a methanofullerene (PCBM) as n-type electron acceptor and a dialkoxy-(p-phenylene vinylene) (OC1C10−PPV) as a p-type electron donor sandwiched between electrodes with different work functions has been investigated. With ohmic contacts for hole and electron injection, the dark current appears completely dominated by the electron current in the PCBM, as a result of a much higher electron mobility. This electron current is bulk space-charge limited. With Au as a high work function metal, the electron current becomes injection limited. The injection-limited electron current from the Au electrode into PCBM is explained within a thermally assisted hopping model. In spite of the presence of an injection barrier of about 0.76 eV, the injection-limited electron current from a Au electrode into PCBM still exceeds the bulk-limited hole current in OC1C10−PPV.

Modeling the photocurrent of poly-phenylene vinylene/fullerene-based solar cells

We have studied the photocurrent data of 20:80 wt% blends of poly(2-methoxy-5-(3’,7’-dimethyloctyloxy)-p-phenylene vinylene) (MDMO-PPV) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) bulk heterojunction solar cells. Two cases have been investigated: When only drift of charge carriers is taken into account, a voltage-independent photocurrent is expected, corresponding to the extraction of all generated charges. It is demonstrated that the experimental data are in disagreement with this prediction. However, when both drift and diffusion of charges are taken into account, the predicted photocurrent shows a different behavior: At low electric fields a linear behavior is predicted, which results from the diffusion of charges, followed by saturation at high fields. The agreement between the numerical result and the experimental data obtained from MDMO-PPV:PCBM cells is satisfactory when a charge carrier generation rate of G=1.6 × 1027 m-3s-1 is used, showing the importance of diffusion at low fields, i.e., near the open-circuit voltage.