Hybrid Photovoltaic Cells and Related Methods

Abstract

Embodiments of the present invention involve photovoltaic (PV) cells comprising a semiconducting nanorod-nanocrystal-polymer hybrid layer, as well as methods for fabricating the same. In PV cells according to this invention, the nanocrystals may serve both as the light-absorbing material and as the heterojunctions at which excited electron-hole pairs split.

Description

CROSS-REFERENCE TO RELATED APPLICATION[0001]This application claims priority to and the benefits of U.S. Provisional Application Ser. No. 60 / 926,103, filed on Apr. 25, 2007, the entire disclosure of which is hereby incorporated by reference.FIELD OF THE INVENTION[0002]The present invention relates to solar cells and their fabrication, and in particular to nanorod-nanocrystal-polymer hybrid solar cellsBACKGROUND[0003]To create useful electrical current from electromagnetic radiation, photovoltaic (PV) cells must absorb incident radiation such that an electron is promoted from the valence band to the conduction band (leaving a hole in the valence band), and must be able to separate the electron and hole and deliver these charge carriers to their respective electrodes before they recombine.[0004]Many different strategies based on diverse materials have been employed, with varying degrees of success, to realize these basic behaviors with commercially satisfactory efficiency. Representat...

AI Technical Summary

Benefits of technology

[0028]One of the advantages of the invention lies in the use of small particles, the nanocrystals, as both light absorber and heterojunction. The resulting spatio-temporal proximity of exciton generation and splitting entails a significant reduction in recombination losses, compared, for example, with those of conventional polymer PV cells, and consequently in higher conversion efficiencies of photons into electricity. Embodiments of the invention offer the additional advantages of mechanical flexibility and low cost manufacturing processes.

Problems solved by technology

Creation of p-n junctions generally involves high-temperature processing in inert atmospheres to form very pure, crystalline silicon wafers, which are inflexible and expensive.

Because silicon is an indirect semiconductor, a relatively thick layer is typically needed to achieve a good level of absorption, which increases material costs further.

Unfortunately, excitons are very short-lived and can only travel about 10 nm before recombining.

Hence, any photon absorbed more than this diffusion length away from a heterojunction will be wasted.

There is no band-bending, so splitting of the exciton is more difficult.

The lack of selectivity at the electrode towards one particle or another means that the electrodes can make contact with both the donor and acceptor species.

For example, a bi-layer structure in which each layer has only one component results in a cell with poor performance.

The reason is that the lifetime of the excited state of the light-absorbing polymer is generally shorter than the transfer rate of the exciton to the interface, and, consequently, the majority of the excitons formed in the bulk of the polymer never reach the interface separating electrons and holes, resulting in loss of photocurrent.

This technology shows promise, but there are obstacles to overcome.

One problem is incomplete absorption of the incident radiation.

The polymer -which absorbs light very strongly and is referred to as a polymeric dye -has a large extinction coefficient (>100,000 M−1cm−1), but due to low exciton migration rates, the films must generally be thinner than 100 nm, which contributes significantly to incomplete absorption.

A problem associated with the light-absorbing polymer strategy is underutilization of available solar energy due to the narrow absorption bandwidth of typical polymers.

Unfortunately, it is difficult to disperse inorganic nanocrystals into a solution of monomers.

Ruthenium dye-sensitized, nanorod-based DSSCs tend to exhibit low efficiency, however, because the lower surface area does not accommodate enough dye to absorb all of the incident light.

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