Thin-film solar cells are a type of solar cell made by depositing thin layers of photovoltaic material onto a glass, plastic or metal substrate. Thin-film solar cells are typically much thinner than the wafers used in conventional crystalline silicon based solar cells. Thin-film solar cells are commercially made with one of several materials including cadmium telluride (CdTe), copper indium gallium diselenide (CIGS), and amorphous thin-film silicon (a-Si, TF-Si).
Solar cells are often classified into generations based on the kinds of light-absorbing layers used to make them. The most well-established or first-generation solar cells are made of single- or multi-crystalline silicon. This is the dominant technology currently used in most solar PV systems. Most types of thin-film solar cells are classified as second generation, made using thin layers of materials like amorphous silicon (a-Si), cadmium telluride (CdTe), copper indium gallium selenide (CIGS), or gallium arsenide (GaAs). Solar cells made with newer, less established materials are classified as third-generation or emerging solar cells. This includes some innovative thin-film technologies, such as perovskite, dye-sensitized, quantum dot, organic, and CZTS thin-film solar cells.
Thin-film cells have several advantages over first-generation silicon solar cells, including being lighter and more flexible due to their thin construction. This makes them suitable for use in building-integrated photovoltaics and as semi-transparent photovoltaic glazing material that can be laminated onto windows. Other commercial applications use rigid thin film solar panels (interleaved between two panes of glass) in some of the world's largest photovoltaic power stations. Additionally, the materials used in thin-film solar cells are typically produced using simple and scalable methods more cost-effective than first-generation cells, leading to lower environmental impacts like greenhouse gas (GHG) emissions in many cases. Thin-film cells also typically outperform renewable and non-renewable sources for electricity generation in terms of human toxicity and heavy-metal emissions.
Despite initial challenges with efficient light conversion, especially among third-generation PV materials, as of 2023 some thin-film solar cells have reached efficiencies of up to 29.1% for single-junction thin-film GaAs cells, exceeding the maximum of 26.1% efficiency for standard single-junction first-generation solar cells. Multi-junction concentrator cells incorporating thin-film technologies have reached efficiencies of up to 47.6% as of 2023.
Many thin-film technologies have been found to have shorter operational lifetimes and larger degradation rates than first-generation cells in accelerated life testing, which has contributed to their somewhat limited deployment. Globally, the PV market share of thin-film technologies remains around 5% as of 2023. However, thin-film technology has become considerably more popular in the United States, where CdTe cells alone accounted for 29% of new utility-scale deployment in 2021.
History
Early research into thin-film solar cells began in the 1970s. In 1970, Zhores Alferov's team at Ioffe Institute created the first gallium arsenide (GaAs) solar cells, later winning the 2000 Nobel prize in Physics for this and other work. Two years later in 1972, Prof. Karl Böer founded the Institute of Energy Conversion (IEC) at the University of Delaware to further thin-film solar research. The institute first focused on copper sulfide/cadmium sulfide (Cu2S/CdS) cells and later expanded to zinc phosphide (Zn3P2) and amorphous silicon (a-Si) thin-films as well in 1975. In 1973, the IEC debuted a solar-powered house, Solar One, in the first example of residential building-integrated photovoltaics. In the next decade, interest in thin-film technology for commercial use and aerospace applications increased significantly, with several companies beginning development of amorphous silicon thin-film solar devices. Thin-film solar efficiencies rose to 10% for Cu2S/CdS in 1980, and in 1986 ARCO Solar launched the first commercially available thin-film solar cell, the G-4000, made from amorphous silicon.
In the 1990s and 2000s, thin-film solar cells saw significant increases in maximum efficiencies and expansion of existing thin-film technologies into new sectors. In 1992, a thin-film solar cell with greater than 15% efficiency was developed at University of South Florida. Only seven years later in 1999, the U.S. National Renewable Energy Laboratory and Spectrolab collaborated on a three-junction gallium arsenide solar cell that reached 32% efficiency. That same year, Kiss + Cathcart designed transparent thin-film solar cells for some of the windows in 4 Times Square, generating enough electricity to power 5-7 houses. In 2000, BP Solar introduced two new commercial solar cells based on thin-film technology. In 2001, the first organic thin-film solar cells were developed at the Johannes Kepler University of Linz. In 2005, GaAs solar cells got even thinner with the first free-standing (no substrate) cells introduced by researchers at Radboud University.
This was also a time of significant advances in the exploration of new third-generation solar materials–materials with the potential to overcome theoretical efficiency limits for traditional solid-state materials. In 1991, the first high-efficiency dye-sensitized solar cell was developed, replacing the ordinary solid semiconducting (active) layer of the cell with a liquid electrolyte mixture containing light-absorbing dye. In the early 2000s, development of quantum dot solar cells began, technology later certified by the National Renewable Energy Laboratory in 2011. In 2009, researchers at the University of Tokyo reported a new type solar cell using perovskites as the active layer and achieving over 3% efficiency, building on Murase Chikao's 1999 work which created a perovskite layer capable of absorbing light.
In the 2010s and early 2020s, innovation in thin-film solar technology has included efforts to expand third-generation solar technology to new applications and to decrease production costs, as well as significant efficiency improvements for both second and third generation materials. In 2015, Kyung-In Synthetic released the first inkjet solar cells, flexible solar cells made with industrial printers. In 2016, Vladimir Bulović's Organic and Nanostructured Electronics (ONE) Lab at the Massachusetts Institute of Technology (MIT) created thin-film cells light enough to sit on top of soap bubbles. In 2022, the same group introduced flexible organic thin-film solar cells integrated into fabric.
Thin-film solar technology captured a peak global market share of 32% of the new photovoltaic deployment in 1988 before declining for several decades and reaching another, smaller peak of 17% again in 2009. Market share then steadily declined to 5% in 2021 globally, however thin-film technology captured approximately 19% of the total U.S. market share in the same year, including 30% of utility-scale production.
Theory of operation
In a typical solar cell, the photovoltaic effect is used to generate electricity from sunlight. The light-absorbing or "active layer" of the solar cell is typically a semiconducting material, meaning that there is a gap in its energy spectrum between the valence band of localized electrons around host ions and the conduction band of higher-energy electrons which are free to move throughout the material. For most semiconducting materials at room temperature, electrons which have not gained extra energy from another source will exist largely in the valence band, with few or no electrons in the conduction band. When a solar photon reaches the semiconducting active layer in a solar cell, electrons in the valence band can absorb the energy of the photon and be excited into the conduction band, allowing them to move freely throughout the material. When this happens, an empty electron state (or hole) is left behind in the valence band. Together, the conduction band electron and the valence band hole are called an electron-hole pair. Both the electron and the hole in the electron-hole pair can move freely throughout the material as electricity. However, if the electron-hole pair is not separated, the electron and hole can recombine into the lower-energy original state, releasing a photon of the corresponding energy. In thermodynamic equilibrium, the forward process (absorbing a photon to excite an electron-hole pair) and reverse process (emitting a photon to destroy an electron-hole pair) must occur at the same rate by the principle of detailed balance. Therefore, to construct a solar cell from a semiconducting material and extract current during the excitation process, the electron and hole of the electron-hole pair must be separated. This can be achieved in a variety of different ways, but the most common is with a p-n junction, where a positively doped (p-type) semiconducting layer and a negatively doped (n-type) semiconducting layer meet, creating a chemical potential difference which draws electrons one direction and holes the other, separating the electron-hole pair. This may instead be achieved using metal contacts with different work functions, as in a Schottky-junction cell.
In a thin-film solar cell, the process is largely the same but the active semiconducting layer is made much thinner. This may be made possible by some intrinsic property of the semiconducting material used that allows it to convert a particularly large number of photons per thickness. For example, some thin-film materials have a direct bandgap, meaning the conduction and valence band electron states are at the same momentum instead of different momenta as in the case of an indirect bandgap semiconductor like silicon. Having a direct bandgap eliminates the need for a source or sink of momentum (typically a lattice vibration, or phonon), simplifying the two-step process of absorbing a photon into a single-step process. Other thin-film materials may be able to absorb more photons per thickness simply due to having an energy bandgap that is well-matched to the peak energy of the solar spectrum, meaning there are many solar photons of the correct energy available to excite electron-hole pairs.
In other thin-film solar cells, the semiconducting layer may be replaced entirely with another light-absorbing material, for example an electrolyte solution and photo-active dye molecules in a dye-sensitized solar cell or by quantum dots in a quantum dot solar cell.
Materials
Thin-film technologies reduce the amount of active material in a cell. The active layer may be placed on a rigid substrate made from glass, plastic, or metal or the cell may be made with a flexible substrate like cloth. Thin-film solar cells tend to be cheaper than crystalline silicon cells and have a smaller ecological impact. Their thin and flexible nature also makes them ideal for applications like building-integrated photovoltaics. The majority of film panels have 2-3 percentage points lower conversion efficiencies than crystalline silicon,Datasheets of the market leaders: First Solar for thin film, Suntech and SunPower for crystalline silicon though some thin-film materials outperform crystalline silicon panels in terms of efficiency. Cadmium telluride (CdTe), copper indium gallium selenide (CIGS) and amorphous silicon (a-Si) are three of the most prominent thin-film technologies.
Second-generation thin-film materials
Cadmium telluride