Batteries: thermoelectric and photoelectric – Photoelectric – Cells
Utility Patent
1998-09-08
2001-01-02
Chapman, Mark (Department: 1753)
Batteries: thermoelectric and photoelectric
Photoelectric
Cells
C438S487000
Utility Patent
active
06169246
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to photovoltaic devices and more particularly to thin film CdS/CdTe heterojunction solar cells comprising a zinc stannate (Zn
2
SnO
4
) buffer layer between the CdS window layer and a transparent conducting oxide (TCO) contact layer.
2. Description of the Prior Art
Cadmium telluride (CdTe) has long been recognized as a promising semiconductor material for thin-film solar cells due to its band gap of 1.44 eV, which is near optimum for solar energy absorption, and due to high direct absorption coefficient. CdTe is typically coupled with a second semiconductor material of different conductivity type, such as cadmium sulfide (CdS), to produce a high-efficiency heterojunction photovoltaic cell. Small-area CdS/CdTe heterojunction cells with efficiencies of photon energy to electrical energy conversion greater than 15% and commercial-scale modules with efficiencies of greater than 9% have been demonstrated. CdTe films have been produced using various deposition techniques, including close-space sublimation or “CSS” (U.S. Pat. No. 5,304,499, issued Apr. 19, 1994, to Bonnet et al.), spray deposition (e.g., J. F. Jordan,
Solar Cells,
23 (1988) pp. 107-113), and electrolytic deposition (e.g., B. M. Basol,
Solar Cells,
23 (1988), pp. 69-88).
A typical thin film solar cell device, including the CdS/CdTe heterojunction devices described above, may have an optically transparent substrate that provides structural support for the thin film layers of dissimilar semiconductor materials (e.g., CdS and CdTe) that comprise the junction and form the solar energy absorption media. Generally, optically transparent substrates are not electrically conductive, so a thin layer of transparent conductive oxide (TCO) is deposited between the substrate and the first semiconductor layer to function as a front contact current collector. A back contact of conductive film, usually a metal, is deposited on the CdTe.
A well known advantage of heterojunction solar cells, such as CdS/CdTe structures, is that they can have a relatively wide band gap in the front layer component of the cell (e.g., CdS with a bandgap of about 2.4 eV) which provides a window action that allows more of the electromagnetic solar radiation to pass through the front layer component and penetrate into the underlying direct band gap component, where the electromagnetic solar radiation is absorbed (e.g., CdTe with a bandgap of about 1.44eV), to create electron-hole pairs. However, the window layer component with its wider band gap does absorb some of the electromagnetic solar radiation, especially in the shorter wavelengths below about 500 nm before it can reach the underlying absorption layer. Therefore, that shorter wavelength, e.g., blue light energy is lost as heat instead of being usefully converted to electric current. Reducing the thickness of the window layer reduces this solar energy absorption in the window layer so that it can be absorbed in the underlying layer, yielding increased short-circuit current (J
sc
) and improved overall conversion efficiency of the device. In CdS/CdTe solar cells, such reduction in the thickness of the CdS window layer allows more of the shorter wavelength or blue solar radiation to reach and be absorbed by the CdTe layer, thus improving the blue spectral response of the device.
Unfortunately, reducing the thickness of the CdS film can also cause other problems that are detrimental to the electrical quality and performance of the heterojunction device. For example, the thinner the CdS film, the greater the probability of interface defects (commonly referred to as “pinholes”), which create localized TCO/CdTejunctions that result in reduced open-circuit voltage (V
oc
) and fill factor (FF). For example, creation of such localized TCO/CdTe junctions can lower the V
oc
of a CdS/CdTe heterojunction solar cell from a range of 800-850 mV down to a range of about 300-400 mV, depending on the severity and density of the pinholes, thickness of the CdS layer, and several other factors. Therefore, while thinner CdS window layers are desirable for obtaining higher solar energy conversion efficiency and higher J
sc
current technology is limited in how thin the CdS front or window layer can be made before the reduction of V
oc
and FF due to creation of pinholes as described above.
Another problem commonly associated with fabrication of thin film semiconductor devices is the formation of the back electrical contacts in a low resistance, ohmic manner to the CdTe layers. One conventional technique is to chemically etch the CdTe layer prior to deposition of the metallic back contact to form a tellurium rich p
+
conductivity region at the exposed surface of the CdTe. Then the back contact is either deposited on the etched surface of the CdTe layer using metals such as gold or nickel, or it is formed by applying a HgTe:Cu doped graphite paste. Unfortunately, conventional chemical etching is difficult to control and the CdTe layer is polycrystalline, so excessive chemical etching can preferentially etch grain boundaries in the polycrystalline CdTe, removing Cd to leave highly conductive Te channels extending through the CdTe layer to the CdS/CdTe junction, often corroding through the CdS layer and into close proximity to the TCO layer. Once the back contact is deposited, such channels can form highly conductive shunts that cause electrical short circuits between the front TCO contact and the back metal contact and reduce V
oc
, of the device. Therefore, while the chemical etching can enhance a desirable ohmic contact between the CdTe layer and the back contact layer, it can also have a detrimental effect on the V
oc
, thereby adversely affecting solar energy conversion efficiency of the device.
Before the chemical etching step described above, an annealing step, which typically involves heating the CdS/CdTe semiconductor heterojunction structure in a CdCl
2
atmosphere, is considered by persons skilled in the art to be almost essential to produce high-efficiency CdTe devices. Such annealing provides a number of benefits, including increased grain size, grain boundary passivation, improved CdS/CdTe interface alloying, and reduced lattice mismatch between the CdS and CdTe layers. Unfortunately, CdCl
2
heat treatment, like chemical etching, is difficult to control, and over-processing can significantly reduce both device performance and product yield. Moreover, it is believed that grain growth, although a generally desirable result of CdCl
2
heat treatment, can induce stress at the TCO/CdS interface, causing blistering or peeling of the semiconductor layers.
Most efforts to solve these problems in constructing high-efficiency CdS/CdTe semiconductor heterojunction devices have been directed to refining layer compositions, thicknesses, and processing control parameters to optimize a balance between the beneficial and adverse effects described above, although the U.S. Pat. No. 5,261,968 issued to Jordan addresses the problem of pinhole shunts between the TCO and CdTe layers through the CdS layer by interposing a low conductivity tin oxide layer between the high conductivity TCO layer and the CdS layer. In that patent, the TCO is a high conductivity tin oxide, while the interposed low conductivity tin oxide layer has its carrier concentration adjusted by a cadmium, zinc, or other metal dopant so that it provides an active junction with the CdTe layer in areas where flaws, such as pinholes, extend through the CdS window layer of the CdS/CdTe heterojunction structure. However, solutions to the problem of electrical short circuits through grain boundary shunts from chemical etching of the CdTe layer and to the problems of blistering and peeling between the TCO/CdS layers and other degradation from over processing in the annealing step have remained elusive prior to this invention.
SUMMARY OF THE INVENTION
Accordingly, it is a general object of this invention to provide a thin film CdS/CdTe heterojunction photovoltaic device havi
Coutts Timothy J.
Sheldon Peter
Wu Xuanzhi
Chapman Mark
Midwest Research Institute
Richardson Ken
White Paul J.
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