Thin-Film Solar Cells: An Overview

Thin-Film Solar Cells: An
Overview

K. L. Chopra1, P. D. Paulson2*,y and V. Dutta1
1Photovoltaic Laboratory, Centre for Energy Studies, Indian Institute of Technology Delhi, Hauz Khas,
New Delhi-110 016, India

2Institute of Energy Conversion, Newark, Delaware 19716, USA

Thin film solar cells (TFSC) are a promising approach for terrestrial and space
photovoltaics and offer a wide variety of choices in terms of the device design and
fabrication. A variety of substrates (flexible or rigid, metal or insulator) can be used
for deposition of different layers (contact, buffer, absorber, reflector, etc.) using different
techniques (PVD, CVD, ECD, plasma-based, hybrid, etc.). Such versatility
allows tailoring and engineering of the layers in order to improve device performance.
For large-area devices required for realistic applications, thin-film device
fabrication becomes complex and requires proper control over the entire process
sequence. Proper understanding of thin-film deposition processes can help in achieving
high-efficiency devices over large areas, as has been demonstrated commercially
for different cells. Research and development in new, exotic and simple materials and
devices, and innovative, but simple manufacturing processes need to be pursued in a
focussed manner. Which cell(s) and which technologies will ultimately succeed commercially
continue to be anybody's guess, but it would surely be determined by the
simplicity of manufacturability and the cost per reliable watt. Cheap and moderately
efficient TFSC are expected to receive a due commercial place under the sun.

INTRODUCTION

The modern era of photovoltaic device technology reached its Golden Jubilee year in 2003.1 Since the

discovery of a p–n junction Si photovoltaic (PV) device2 reported in 1954, the science and technology

of PV devices (solar cells) and systems have undergone revolutionary developments. Today, the best

single crystal Si solar cells have reached an efficiency of 24 7%, compared with the theoretical maximum value

of 30%. Large-scale production of solar cells during the year 2002 worldwide3 stood above 500MWp, consisting

40% of single crystal Si and 51% multicrystalline Si cells and about 8% based on thin-film amorphous

Si solar cells. Cumulatively, about 2GWof solar cells are being used worldwide in a variety of applications,

ranging from severalMWof stand-alone / grid connected power stations to severalMWof low-power electronic

devices. A large number of countries, developing and developed, are seriously involved in creating and nurturing

the PV industries. Human life-style being a matter of power is a well-documented fact and the poor and

developing countries (ironically blessed with copious sun power) with limited conventional power sources,

particularly in remote areas, are increasingly turning to PV power for enhancing their development index.4 That

the sunrise PV industry worldwide is already a billon dollar industry leaves no doubt that it has matured fast to

be reckoned with economically in the world arena. However, its growth is limited largely by the ultimate cost of

the PV power. Despite tremendous progress in all aspects of production of Si-based solar cells and the rapid

decrease of production cost5 from $4 2/Wp in 1992 to $1 7/Wp in 2002, large-scale household applications

are not commercially viable as yet.With respect to single crystal Si technology, the single most important factor

in determining the cost of production is the cost of the 250–300 mm-thick Si wafer used for the fabrication of

solar cells. Unless a much thinner wafer, and thus less amount of Si, is used and the production process is made

cheaper and simpler, any further decrease in Si cells cost will be only by small increments.

The problem of high cost of Si was recognized right from the beginning. And it has also been recognized that

cheaper solar cells can be produced only if cheaper and more spuringly used materials and lower cost technologies

are utilized. The first thin/thick-film Cu2S/CdS cell6 was based on rather simple and cheap technology

known as the 'Clevite process',7 in which several mm-thick CdS film was deposited on to a metal or metallized

plastic substrate, then an acid etch of the CdS film followed by immersion in hot cuprous chlorides solution for

few seconds to topotaxially convert the CdS surface to Cu2S. Small-area cell efficiencies as high as 10% were

reported8 and produced commercially by several companies in USA and France. However, the rapidly rising

stabilized efficiency of cells based on the better-understood Si technology, compared with lower and questionable

stability of Cu2S/CdS cell, led to premature death of the latter. Nevertheless, extensive basic research on

the Cu2S/CdS PV devices has proved very useful for later developments in thin-film solar cells (TFSC).

The chance discovery of the possibility of doping amorphous hydrogenated Si (a-Si:H) films created a lot of

excitement in the PV industry.9 As a result of an enormous amount of basic and applied research by a very large

number of scientists and engineers worldwide, a major PV industry on megawatt scale, based on a-Si:H thin

films, grew rapidly in several countries. However, after several years of significant development and production

activities, a-Si:H has failed to challenge the supremacy of crystalline silicon, largely because of little cost

advantage, lower efficiency and relatively poor stability compared with crystalline Si cells.

The number of possible and viable thin/thick-film materials for solar cells is quite large. Some of the most

attractive candidates, based on a-Si:H, CdTe and CuInSe2 materials have been the subject of intense R&D and

exploration of manufacturing technologies for the last three decades. Despite all these efforts, which cell material

and which production technology will ultimately succeed in the commercially competitive field is still anybody's

guess. Several reviews on the physics, materials aspects,10 device characterization,11 performance and

manufacturing technologies12 of TFSC have been published in the recent past. Instead of focusing on these

issues, this review seeks to highlight the weaknesses in different thin-film solar cell devices and production

technologies so that a comparative analysis can be made.

STATUS OF DIFFERENT PHOTOVOLTAIC TECHNOLOGIES

To provide a commercial view of the current photovoltaic technologies, it is useful to compare the manufacturing
cost and the energy payback time. Figure 1 shows the average module manufacturing cost, average cost per
watt weighed by the production capacity, of thin-film modules in comparison with non-thin-film modules based
on data5 available in 2001. There has been a steady and rapid decline in the cost and one expects the cost comparison
of thin-film cells to become increasingly favorable. The average cost of thin-film module manufacturing
is reduced by 64% compared to 51% for non-thin-film modules. Clearly, thin-film solar cell technologies have
the potential for producing cheaper devices on a large scale.
Another major consideration in comparing different PV technologies is the energy payback period, which
refers to the number of years in which the electrical energy generated by the devices will be equal to the energy
required for production of these devices. Figure 2 shows a comparison for different types of solar cells and other
system components required for different applications.13 The figure gives two estimates for multicrystalline
modules to avoid the uncertainty related to the variation in the energy consumption estimates. The low estimate
is based on the lower end value for silicon purification and does not consider the primary crystallization step,
while the high estimates assume the high-energy end value for silicon that includes energy intensive primary

Figure 1. Manufacturing cost and production capacity projections for thin-film and non-thin-film modules based on the data

available in year 2001 (data from Reference 5)

Figure 2. Energy payback time for different PV technologies: A multicrystalline Si 1997 low; B multicrystalline Si 1997

high; C thin-film 1997; D multicrystalline Si 2007; E thin-film 2007, for different applications. Other system components are

also shown for comparison (data from Reference 13)

crystallization process. For grid-connected rooftop and array fields, the TFSC technology fare much better. On

the other hand, the multicrystalline estimates based on lower end value fare well for solar home systems because

of the higher balance of system (BOS) cost.

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