• PDF / 262,712 Bytes
• 7 Pages / 585 x 783 pts Page_size
• 98 Downloads / 139 Views

DOWNLOAD

REPORT

---

Richard D. Schallerb) Chemistry Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545

Sergey B. Lee Nanotech Institute, University of Texas at Dallas, Richardson, Texas 75083; and Plextronics, Inc., Pittsburgh, Pennsylvania 15238

Jeffrey M. Pietryga and Victor I. Klimov Chemistry Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545

Anvar A. Zakhidov Nanotech Institute, University of Texas at Dallas, Richardson, Texas 75083; and Physics Department, University of Texas at Dallas, Richardson, Texas 75083 (Received 28 February 2007; accepted 3 May 2007)

We investigated the photovoltaic response of nanocomposites made of colloidal, infrared-sensitive, PbSe nanocrystals (NCs) of various sizes and conjugated polymers of either regioregular poly (3-hexylthiophene) (RR-P3HT) or poly(2-methoxy-5-(2-ethylhexoxy)-1,4-phenylene vinylene) (MEH-PPV). The conduction and valence energy levels of PbSe NCs were determined by cyclic voltammetry and revealed type II heterojunction alignment with respect to energy levels in RR-P3HT for smaller NC sizes. Devices composed of NCs and RR-P3HT show good diode characteristics and sizable photovoltaic response in a spectral range from the ultraviolet to the infrared. Using these materials, we have observed photovoltaic response at wavelengths as far to the infrared as 2 ␮m (0.6 eV), which is desirable due to potential benefits of carrier multiplication (or multi-exciton generation) from a single junction photovoltaic. Under reverse bias, the devices also exhibit good photodiode responses over the same spectral region.

I. INTRODUCTION

Semiconductor nanocrystals (NCs) have several properties that make them attractive for use as the photoactive material in solar cells. First, the material band gap can be tuned over a large energy range simply via synthetic control over the NC-size.1 NCs that have an absorption onset in the near- to mid-infrared (IR) will also strongly absorb solar photons of higher energy. Furthermore, it has been found that NCs efficiently generate multiple excitons upon absorption of single photons of sufficient energy via carrier multiplication.2,3 Using this process one can appreciably increase the photovoltaic power conversion efficiency above the Shockley–Queisser apparent

Address all correspondence to these authors. a) e-mail: [email protected] b) e-mail: [email protected] DOI: 10.1557/JMR.2007.0289 2204

J. Mater. Res., Vol. 22, No. 8, Aug 2007

thermodynamic limit.4,5 Solution-processable materials such as semiconductor NCs combined with conjugated polymers can promote efficient charge transfer and make possible the production of low-cost, high-efficiency solar cells. NCs mixed with conducting polymers have been actively studied following the work of Greenham, Alivisatos, and co-workers.6–8 Progress in this direction has been made via study of the NC-polymer composite morphology9 by using polymers that are functionalized to attach to NC surfaces10 and by improvement of electron transport via incorporation of branched or elongated na