Solar Cells

The solar cell uses the same fundamental physical mechanism as the photodiode covered in Chapter 4 – absorption of a photon creates an electron-hole pair, separated by the electric field of a pn junction's depletion region – but pursues a fundamentally different goal. While the photodiode is optimized as a detector and typically operated in reverse bias to create a wide, high-field depletion region for high speed and sensitivity, the solar cell operates without external bias: it's meant to generate electrical power itself, not merely register a signal. Here the diode operates as a generator rather than a load – an operating regime referred to in the diode I-V plane as the fourth quadrant, where current and voltage have opposite signs relative to the usual diode convention.

Two limiting cases describe the illuminated cell's behavior. When the cell is short-circuited (terminal voltage U = 0), the short-circuit current I_sc flows – essentially the entire photogenerated current, since no opposing field prevents carriers from leaving the cell. When the cell is operated open-circuit instead (no current flow, open terminals), the open-circuit voltage U_oc builds up: photogenerated carriers accumulate at the contacts until the resulting opposing field exactly cancels the photo-induced diffusion current. Between these two extremes – short circuit and open circuit – lies the cell's entire usable operating range, where electrical power P = I·U is actually delivered to an external load.

Unlike the photodiode, the solar cell's active area is maximized rather than minimized: photodiodes are often small and fast, while solar cells are large (typically 15×15 cm or more for Si wafers) and optimized for maximum light collection. This area optimization brings its own challenges – particularly the lateral series resistance involved in transporting the generated current to the contacts, a topic explored further in the section on internal structure.

The classic crystalline silicon solar cell consists of a lightly p-doped base (typically boron-doped, wafer thickness 150–180 µm), into which a heavily n-doped emitter layer is diffused from the front (usually phosphorus, diffusion depth only 0.3–0.5 µm). This shallow, strongly asymmetric pn junction – the emitter much thinner and more heavily doped than the base – is designed so that as many photons as possible are absorbed at or near the depletion region, where charge separation is most efficient, while the thick base absorbs the bulk of the light across a wide spectral range.

The front side carries a contact grid of fine metal fingers (usually screen-printed silver paste, finger width often below 100 µm) plus several wider busbars that collect the current flowing laterally through the thin emitter layer. Two or three busbars were standard for a long time; today’s cells mostly use multi-busbar designs with nine to sixteen very thin wires, which shorten the path of the current through the emitter and therefore shade less area. This grid involves a fundamental trade-off: a denser, wider finger pattern reduces the emitter's series resistance but simultaneously shades more active area, blocking incident light – the optimal finger design balances electrical against optical losses. The back side, by contrast, usually carries a full-area aluminum or silver metallization that serves simultaneously as the back contact and as a reflector for unabsorbed light.

Two further structural features specifically boost efficiency. The anti-reflective coating (ARC, usually silicon nitride Si₃N₄, thickness on the order of a quarter-wavelength of the relevant spectral range) reduces front-surface reflection losses through destructive interference of the reflected light – uncoated silicon otherwise reflects over 30% of incident light due to its high refractive index (n ≈ 3.5), while ARC reduces this to a few percent. The surface is additionally often textured (for monocrystalline silicon usually pyramid-shaped via anisotropic alkaline etching along the (111) crystal planes), so that light reflected off one pyramid facet strikes a neighboring facet and gets a second chance at absorption instead of being lost entirely. On the back side, a back surface field (BSF) – an additional, more heavily doped p⁺ layer directly beneath the back contact – creates an internal field that keeps minority carriers away from the highly recombination-active metal interface, increasing effective collection.