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Loss mechanisms reduce the efficiency

Why is it that many crystalline silicon solar cells ‘only’ have an efficiency of 16% when converting solar radiation into usable electrical power and the resulting solar modules ‘only’ have an efficiency of 14%? What are the physical and technological limits to this efficiency?

In order that light can be converted into electricity, it has to penetrate into a solar cell. This consists of a semiconductor material that is characterised by its energy band gap Eg between the occupied and unoccupied electronic conditions. With crystalline silicon, the band gap energy is around 1.1eV, which corresponds to a characteristic light wavelength of 1.1 μm. Long-wave infrared light is let through the semiconductor while shortwave light is absorbed.

Various loss mechanisms reduce the yields from electrical energy. When a light particle (photon) is absorbed, a so-called electron hole pair is generated from the negatively charged electron and the positively charged hole. With high-energy short-wave light, the surplus energy hυ-Eg is released to the crystal lattice in the form of heat. Here, around 30% of the light energy is already lost for the electricity generation (1). The energy form the non-absorbed longwave photons from the solar spectrum is also lost (2). In the solar cell, the charge separation of the light-generated electron hole pairs creates an open terminal voltage Voc, which in turn is smaller than the band gap voltage Eg/q (3). The solar cell has an exponential current-voltage characteristic, which means that the maximum obtainable power point Vmppj mpp is smaller than the product of the open terminal voltage and the short-circuit current density jsc (4). In the n- and p-doped areas, Auger recombination takes place (5) in which the electron hole pairs are destroyed. These five basic loss mechanisms mean that the theoretical maximum efficiency of an ideal c-Si solar cell is 29%. The surfaces of real solar cells create an abrupt disruption of the crystal lattice that causes recombinationactive electronic conditions in the band gap. In real crystal, technology- determined microscopic crystal defects and contamination also occur that cause the recombination of electron-hole pairs (6). This volume recombination is much stronger in multicrystalline silicon than in monocrystalline silicon, reducing the efficiency of standard industry solar cells to between 17 and 18%. Optical reflection losses caused by non-optimal light coupling (7) and electrical conduction losses in the solar cells (8) further reduce the cell efficiency to around 16%. Further losses are created by the series connections and encapsulation of the solar cells in the module, which mean that the module itself achieves an efficiency of around 14%. The best small-sized c-Si solar cells currently achieve 25%, whereby 26% appears to be possible. In the long term, a module efficiency of around 24% can be expected. Further increases are possible through concentrating light (3). Research is also being conducted on manipulating the light spectrum (1) and (2) and the introduction of suitable heterojunctions (5).

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