The term “solar energy” refers to a wide variety of techniques for using the energy available as sunlight. Well-known examples are active and passive thermal solar energy and photovoltaic solar energy but, strictly speaking, hydropower, wind energy, and biomass are also forms of solar energy. Today, only hydropower is used in significant quantities, covering approximately 6% of the world's energy demand. Traditional use of biomass, mainly in developing countries, accounts for more than 10% of the total energy consumption, but is sometimes left out of statistics because it falls outside the category of organized and commercial use.

The global potential for solar energy is huge, since the amount of energy that reaches the earth's surface every year exceeds the total energy consumption by roughly a factor of 10,000. There are, however, various barriers to the large-scale use of solar energy technologies. Most technologies have in common that the power density of the generator is low; in other words, one needs large areas to generate significant amounts of energy. This is especially true for biomass, with typical conversion efficiencies (solar energy to chemical energy) of 1% or less. Further, many solar energy technologies have proved technically feasible, but have yet to be proved economically feasible. Last, but not least, the large-scale use of solar energy requires substantial modification of our global energy supply system, which is based largely on fossil fuels.

The vast majority of solar cells used in the field are based on single-crystal silicon. There are several reasons for this. First, by using this material, photovoltaic manufacturers can benefit from the economies of scale of the much larger microelectronics industry, where crystalline silicon also dominates. Since lower-quality silicon is acceptable for solar cells, cell manufacturers are able not only to benefit from large production volumes, but also to use off-grade material. The relatively high efficiencies that result from this material, its excellent reliability in the field, its almost complete lack of environmental problems, and recent laboratory progress strengthen the position of this technology.

Although there had been earlier work with what might be termed “cast multicrystalline silicon,” the first efficient crystalline silicon cells were reported in 1954. These cells displayed energy conversion efficiencies of about 6%, a substantial improvement over what had been previously demonstrated. Performance improved rapidly through the 1950s, with efficiencies up to about 15% demonstrated in the early 1960s. By this time, their use on spacecraft had become the major commercial purpose of the cells, with reliability a more important attribute than efficiency. The technology then stabilized for a decade. In the early 1970s, another burst of activity pushed the performance of laboratory cells to about 17%. At about this time, a simple processing sequence was developed, based on screen printing the metal contacts to the cell. Anisotropic chemical etching of the silicon to produce surface texture to reduce reflection loss was also implemented, to give the cell structure shown in Figure 1.

Worldwide environmental problems such as the greenhouse effect and acid rain have been caused by the human race's continuous reliance on the combustion of petroleum for fuel.

Solar energy, which is clean and practically unlimited, is expected to be a desirable alternate energy source to conventional power supplies, and demand for the photovoltaic system has increased throughout the world, especially in Europe and the United States.

Photovoltaic cells are probably the most effective method for capturing solar energy, since they are easy to use and are the most effective means of directly generating electricity.

Many kinds of solar cells have been developed in past years, especially since the first oil crisis in 1973. Among them, solar cells from cast multicrystalline silicon (also refereed to as (cast) polycrystalline silicon or semicrystalline silicon) are considered to be one of the most promising types, capable of achieving both high efficiency and low cost.

In 1975, Wacker proposed a new manufacturing method for silicon substrates, using the casting method. Since then, many organizations have been involved in the research and development of multicrystalline ingots and solar cells using multicrystalline silicon substrates.

Multicrystalline silicon substrates contain many kinds of defects compared to single-crystal silicon substrates, so the efficiency of multicrystalline silicon solar cells has been inferior to that of single-crystal cells. Recent research on multicrystalline silicon solar cells has resulted in substantial improvements and in the demonstration of high-efficiency cells.

Solar cells formed with thin silicon active layers (<100 μm thick) offer advantages over thick ingot-based devices. The advantages come in two forms: the first is the potential for higher conversion efficiency than that of conventional thick devices, and the second is a reduction in material requirements. The use of thin polycrystalline silicon for solar cells offers the potential of capturing the high performance of crystalline silicon while achieving the potential low cost of thin films. Experimental and theoretical studies initially uncovered the issues of grain size and thickness as limiting factors. Subsequent work added the issue of back-surface passivation. This article addresses the conditions required for the successful development of polycrystalline silicon into a high efficiency, low-cost, terrestrial product.

A big barrier impeding the expansion of large-scale power generation by photovoltaic (PV) systems was the high price of solar cell modules, which was more than $50/Wp (peak watts) by 1974. Therefore, cost reduction of solar cells is of prime importance. To achieve this objective, tremendous R&D efforts have been made over the past ten years in a wide variety of technical fields, from solar cell materials, cell structure, and mass production processes to photovoltaic systems. As a result, more than an order of magnitude in cost reduction has been achieved, and the module cost has come down to less than $5/Wp in a firm bid for the large-scale market. Two phases of technological innovation can be identified. The first innovation in progress is based on low-cost polycrystalline technologies applicable to well-developed single-crystalline silicon solar cell fabrication processes. The second remarkable innovation is a-Si:H (hydrogenated amorphous silicon) technology, which we will discuss.

We open our discussion with a brief overview of the present status of a-Si solar cell R&D efforts, with some new insights in device physics. Next, we discuss some new approaches and key technologies for improving solar cell efficiency with stabilized performance using new materials such as a-SiC:H (amorphous silicon carbide), μc-SiC:H (microcrystalline silicon carbide), and a-SiGe:H (amorphous silicon germanium). Also, the progress of conversion efficiency in various types of amorphous silicon solar cells is surveyed and summarized.

CuInSe2 and related chalcopyrite semiconductors are among the compound semiconductors that have been considered for thin solar cells for about the past 20 years. Recently, high efficiencies close to 17% have been achieved. This result could be the starting point for a new category of solar cells—high-performance thin-film cells—that would combine the high performance of single-crystal cells with possible low-cost thin-film processing.

The development of CuInSe2 cells started in 1974, when single-crystal cells with an efficiency of 12% were reported by a group at Bell Laboratories. Soon after, thin-film solar cells were demonstrated by Kazmerski et al. CuInSe2 thin films have been deposited by evaporating the CuInSe2 source material to completion and adding Se from a separate source. It was found that straight-forward evaporation of the compound does not generally lead to films with stoichiometric composition. By coevaporation of the elements, films with any desired composition can be obtained, provided there is appropriate process control. A “bilayer” recipe developed by Boeing, namely combining Cu-rich films and In-rich films, solved the problem of combining larger grains with suitable electronic properties. By this method, the first CuInSe2 thin-film solar cells with an efficiency exceeding 10% conversion efficiency were fabricated.

Alloying CuInSe2 with CuGaSe2 and CuInS2 considerably increases the potential for the innovative development of solar cells from these materials. The energy gaps covered by these alloys range from about 1 eV to about 2.4 eV The possibility of increasing the energy gap and achieving absorber layers with graded bandgaps has many advantages for the application of these materials in thin-film solar cell modules.

Cadmium telluride is currently the most promising material for high efficiency, low-cost thin-film solar cells. Cadmium telluride is a compound semiconductor with an ideal 1.45 eV bandgap for direct light-to-electricity conversion. The light absorption coefficient of CdTe is high enough to make a one-micrometer-thick layer of material absorb over 99% of the visible light. Processing homogenous polycrystalline thin films seems to be less critical for CdTe than for many other compound semiconductors. The best small-area CdTe thin-film cells manufactured show more than 15% conversion efficiency. Large-area modules with aperture efficiencies in excess of 10% have also been demonstrated. The long-term stability of CdTe solar cell structures is not known in detail or in the necessary time span. Indication of good stability has been demonstrated. One of the concerns about CdTe solar cells is the presence of cadmium which is an environmentally hazardous material.

While there are only two group IV semiconductors, Si and Ge, the III-V material system offers a nearly infinite number of single-crystal semiconducting compounds to choose from. For example, there are nine binary compounds resulting from combining the group III elements Al, Ga, or In with the group V elements P, As, or Sb. From this group, high-performance photovoltaic cells have been made from GaAs, GaSb, and InP. Using ternary III-V compounds, high-performance photovoltaic devices have been made with AlGaAs, GalnAs, GaAsP, and InGaP2. All of the above-cited photovoltaic cells have respectable energy conversion efficiencies of over 15%.

A primary strength associated with the III-V semiconductor material system lies therefore in the diversity of materials to choose from. Thus, a single-crystal III-V compound can be found with almost any bandgap ranging from 0.16 eV for InSb to 2.24 eV for GaP (or 3.5 eV for GaN). But not only can one choose a material with a particular bandgap, one can, more importantly, as a result of tetrahedral covalent bonding, also be assured that that material can be grown as a single crystal with either n- and p-type doping, and that junctions can be formed with nearly ideal rectifying characteristics. The implication this holds for solar cells of ideal diode characteristics is the achievement of theoretical limit open-circuit voltages and fill factors. An additional important feature associated with many of the III-V compounds is the direct-gap photon absorption which translates to a short optical absorption distance, with the result that excited-state carriers in a solar cell are created near the junction.

The best present-day single-bandgap solar cells have efficiencies around 20–25%. However, the Carnot efficiency of the earth-sun system is 95%, so there is considerable potential for improvement. The fundamental efficiency limitation in a conventional solar cell results from the tradeoff between a low bandgap which maximizes light absorption and hence output current and a high bandgap which maximizes output voltage. As a result, the maximum theoretical efficiency of a conventional solar cell is around 30% in unconcentrated sunlight at a bandgap close to that of GaAs.

The quantum-well solar cell is a novel approach to higher efficiency. In its simplest form, shown in Figure 1, it consists of a multiquantum-well (MQW) system in the undoped region of a p-i-n solar cell. For light with energy greater than the band-gap Eg, the quantum-well cell behaves like a conventional cell. However, light with energy below Eg can be absorbed in the quantum wells. Our studies show that if the material quality is good, the electrons and holes escape from the wells and contribute to a higher output current at a voltage between that of the barrier and well material. In AlGaAs/GaAs test devices, we have obtained efficiency enhancements of a factor of more than two when cells with quantum wells are compared with identical cells without wells.

The structure in Figure 1 is, of course, essentially similar to the MQW photodiode or modulator structure that operates in reverse bias, and the quantum-well laser that operates in forward bias beyond flat band.

In recent years, iron sulfide (FeS2), otherwise known as pyrite, has received some attention as a possible candidate for thin-film photovoltaic applications. This interest stems in part from the fact that it has a relatively large absorption coefficient of α > 105 cm−1 for photon energies larger than about 1.3 eV (see Figure 1), such that a thin layer of less than 1,000 Å should effectively absorb the visible part of the solar spectrum. The bandgap of pyrite of 0.95 eV is probably on the borderline of what might still be considered acceptable for a high-efficiency single-junction device, but can be considered ideal in combination with a 1.8 eV material for tandem structures. Another important advantage of this compound semiconductor is that it is composed of cheap, abundant, and non-toxic elements, the latter being of particular importance in times of growing environmental consciousness.

Pyrite is a naturally occurring mineral (also known as “fool's gold” because of its golden color) which shows both n- and p-type conduction. This could indicate possible electronic viability of this material. The study of synthetic crystals has been severely impeded by the fact that pyrite undergoes a peritectic decomposition into the more stable FeS and liquid sulfur at 743°C and that therefore the melt growth of high-purity large single-crystalline material is not possible. Nevertheless, it has been shown that polycrystalline pyrite layers could be prepared with different thin-film preparation techniques (see Table I), such that in principle a cost-effective large-scale application could be envisaged.

In a conventional p-n junction photovoltaic cell, made, for example, from silicon, the semiconductor assumes two roles at the same time: It harvests the incident sunlight and conducts the charge carriers produced under light excitation. In order to function at a good efficiency, the photons have to be absorbed close to the p-n interface. Electron-hole pairs produced away from the junction must diffuse to the p-n contact where the local electrostatic field separates the charges. To avoid charge carrier recombination during the diffusion, the concentration of defects in the solid must be small. This imposes severe requirements on the purity of the semiconductor material, rendering solid-state devices of the conventional type expensive.

We have developed a new molecular photovoltaic system for solar light harvesting and conversion to electricity. It is based on the spectral sensitization of a thin ceramic membrane by suitable transition-metal complexes. The film consists of nanometersize colloidal titanium dioxide particles sintered together to allow for charge carrier transport. When derivatized with a suitable chromophore, these membranes yield extraordinary efficiencies for the conversion of incident photons into electric current, exceeding 90% for certain transition metal complexes within the wavelength range of their absorption band. In this article, we discuss the underlying physical principles of these astonishing findings. Exploiting this discovery, we have developed a new type of photovoltaic device whose overall light-to-electric-energy conversion yield is 12–15% in diffuse day-light and 10% under direct (AM1.5) solar irradiation.