Solar Energy Scavenging Overview

Inexpensive Nanowire Solar Cells From Earth Abundant Materials

Peidong Yang (UCB) and Paul Alivisatos (UCB)

We have fabricated CdS-Cu2S core-shell nanowires based on a solution-based cation exchange reaction. The heterojunction prepared by this method is atomically well defined with low interface defects, enabling excellent charge separation and minimal minority carrier recombination. As a result, our nanowire PV device shows an excellent VOC, FF, and response to low light levels, as compared to both planar solar cells and to previously reported nanowire solar cells. For the first time, we will be able to demonstrate the nanowire solar cell with comparable overall conversion efficiency and better I-V characteristics than thin film solar cell.
We will develop new cation exchange chemistry to make core-shell nanowire solar cells from inexpensive, non-toxic, and abundant materials, such as ZnS-based nanowire. By introducing new copper precursors and processes, we hope to realize ZnS nanowire with thick and epitaxial Cu2S shell to efficiently convert solar energy into electricity. Furthermore, we are planning to make large area nanowire arrays on flexible substrate, which could potentially be used as low cost, high efficiency power source for integrated mobile system.

CdS-Cu2S core-shell nanowire PV devicesa. Schematic of the fabrication process..b. SEM image of a PV unit, c. Current-Voltage characteristic of a core-shell nanowire under 1-sun (AM 1.5G) illumination. d. Light intensity dependence of the photocurrent (ISC) and open circuit voltage (VOC) e. Wavelength dependence of the photocurrent.

Silicon Nanowire Raidal p-n Junction Photovoltaics

We have demonstrated a simple and scalable method to fabricate large-area silicon nanowire radial p-n junction photovoltaics and achieved efficiencies of between 5 and 6% for these nanowire array solar cells. We show that longer nanowires lead to both increased recombination and higher absorption, with the light-trapping effect dominating for 8 µm thin silicon absorbing layers. We quantitatively measure a maximum light trapping path length enhancement factor over the entire AM1.5G spectrum between 1.7 and 73, depending on the nanowire geometry, which agrees well with enhancement factors between 2 and 62 extracted from optical transmission measurements. This light-trapping ability is above the theoretical limit for a randomizing scheme, indicating that there may be photonic crystal enhancement effects present in our devices. This ordered vertical nanowire array geometry represents a viable path toward high-efficiency, low-cost thin-film solar cells by providing a way to reduce both the quantity and quality of the required semiconductor.

Direct Assembly of Nanoparticles and Organic Semiconductors for Hybrid Photovoltaic

Ting Xu (UCB) and Jean Fréchet (UCB)
The sun bathes the earth in over 7,000 times the energy required by humans to survive. Organic photovoltaic (PV) has advantages of easy processing, wide ranges of substrate selection and large-volume production and represent one viable means toward efficient, inexpensive sources of energy. It is critical to optimize the active layer morphology and to develop organic materials that absorb a large fraction of the solar spectrum.
We have developed a novel approach in the polymer-assisted hierarchical assembly of organic semiconductor to generate morphologies ideal for high efficiency photovoltaic fabrication. Using a model oligothiphene, we have assembled organic semiconductors over multiple length scales without compromising their built-in electronic properties and have developed routes to vertically align the microdomains to effectively transport charges to the electrodes, thereby achieving morphological control for the active layer of PV.

Charge Separation

C60/polymer blend show enormous promise as efficient, inexpensive, photovoltaics. Understanding the properties at the interface may hold the key to further improvements in efficiency. Grossman showed using quantum mechanical calculations, the reason why the interfacial charge separation process operate via an ultra-fast charge transfer process. Our findings demonstrated that the mechanism responsible for such efficient charge separation is quite unique, requiring a specific combination of electronic attributes in order to operate efficiently. This new understanding of the charge transfer mechanism may have broad implications in the future design of nanoscale solar cells. For a carbon nanotube, it was demonstrated that the CNT/polymer solar cells are unlikely to perform well with a mixed distribution of metallic and semiconducting tubes, due to the large charge transfer at the interface for the metallic case, and the absence of Fermi-level pinning or large built-in field across the junction.