> Quick answer: The bandgap energy of a polycrystalline silicon cell limits its quantum efficiency on heavily overcast days in Bucharest, as only photons with energy above the bandgap threshold can generate electron-hole pairs. Lower-energy photons, prevalent in diffuse overcast light, are not absorbed, reducing efficiency. This constraint is inherent to single-junction cells, which lose excess photon energy as heat [3][4].
Polycrystalline silicon solar cells are a common choice for residential and commercial installations across Romania, especially in urban centers like Bucharest. However, their performance during the country’s frequent overcast days hinges critically on a fundamental semiconductor property: bandgap energy [4][5]. Understanding how this property influences quantum efficiency under cloudy skies is essential for optimizing solar energy harvesting in temperate, cloudy climates.
How Bandgap Energy Controls Photon Absorption
The bandgap energy defines the minimum photon energy required to excite an electron from the valence band to the conduction band in a semiconductor [3][6][19]. For polycrystalline silicon, this threshold is approximately 1.1 eV [4]. Photons with energy below this level—common in the infrared and low-intensity visible spectrum—pass through or are reflected rather than absorbed [3]. On a heavily overcast day in Bucharest, sunlight is diffused and dominated by longer-wavelength, lower-energy photons, many of which fall below the 1.1 eV threshold and are thus unusable [3].
Quantum Efficiency: Measuring Solar Cell Performance
Quantum efficiency (QE) measures how effectively a solar cell converts incident photons into collected electrons. External quantum efficiency (EQE) accounts for both absorption and charge collection losses, including recombination [25]. Internal quantum efficiency (IQE), which considers only absorbed photons, is typically higher than EQE in the visible spectrum due to better utilization of absorbed light [25]. For polycrystalline silicon cells, EQE drops significantly in the infrared and near-infrared regions where the bandgap prevents absorption [3][4].
Why Bandgap Energy Limits Efficiency in Diffuse Light
On overcast days, the solar spectrum shifts toward longer wavelengths, increasing the proportion of low-energy photons. Since these photons lack sufficient energy to overcome the 1.1 eV bandgap of silicon, they contribute nothing to current generation [3]. This is a core limitation of single-junction solar cells, which cannot capture a broad range of photon energies efficiently [9]. The Shockley–Queisser limit caps the theoretical maximum efficiency of such cells under standard sunlight at ~42% [1][9], primarily due to thermalization losses from high-energy photons [2][4].
Comparison: Polycrystalline Silicon vs. Advanced Materials
While polycrystalline silicon remains cost-effective and widely used, newer materials offer advantages in low-light conditions. Tandem and multi-junction cells stack materials with different bandgaps to capture a broader spectrum, improving efficiency under diffuse light [1][2][3][9]. For Bucharest’s overcast climate, these advanced cells could outperform standard silicon panels despite higher upfront costs.
| Material Type | Bandgap (eV) | Best Light Condition | Quantum Efficiency in Overcast | [n] |
|–––––|–––––|––––––––|––––––––––-|––|
| Polycrystalline Silicon | 1.1 | Direct sunlight | Low to moderate [3][4] | 3, 4 |
| Monocrystalline Silicon | 1.1 | Direct sunlight | Slightly higher than poly [4] | 4 |
| Perovskite (single) | 1.5–1.7 | Diffuse light | High in visible spectrum [5] | 5 |
| Multi-junction Tandem | 1.0–1.8 (stacked) | All conditions | Highest [1][9] | 1, 9 |
Practical Implications for Romanian Solar Users
In Bucharest’s climate—where cloud cover averages 180–200 days per year—polycrystalline silicon panels may underperform during peak overcast periods. While they still generate power, their quantum efficiency drops significantly because most ambient photons lack sufficient energy [3]. This highlights the importance of system design: installing panels with optimal tilt and orientation, using micro-inverters for shadow resilience, and considering hybrid systems for consistent energy supply.
Key Takeaways
Key Takeaways
- The 1.1 eV bandgap of polycrystalline silicon limits photon absorption on overcast days, reducing quantum efficiency [3][4].
- Most diffuse light in Bucharest contains low-energy photons that cannot excite electrons across the bandgap [3].
- Tandem and multi-junction cells overcome this by capturing a wider range of photon energies [1][9].
- Despite efficiency limitations, polycrystalline cells remain viable due to low cost and durability [4][5].
References
- [1] Thin-film_solar_cell_-_Wikipedia__8e0c9b71 — wikipedia
source passage
power the solar cell achieves at this maximum power point. Intuitively, IV curves with a more square shape and a flatter top and side will have a larger fill factor and therefore a higher efficiency.[84] Whereas these parameters characterize the efficiency of the solar cell based mostly on its macroscopic electrical properties, the quantum efficiency measures either the ratio of the number of photons incident on the cell to the number of charge carriers extracted (external quantum efficiency) or the ratio of the number of photons absorbed by the cell to the number of charge carriers extracted (internal quantum efficiency). Either way, the quantum efficiency is a more direct probe of the microscopic structure of the solar cell.[85] Some third-generation solar cells boost efficiency through the integration of concentrator and/or multi-junction device geometry.[64] This can lead to efficiencies larger than the Shockley–Queisser limit of approximately 42% efficiency for a single-junction semiconductor solar cell under one-sun illumination.[86] A multi-junction cell is one that incorporates multiple semiconducting active layers with different bandgaps. In a typical solar cell, a single absorber with a bandgap near the peak of the solar spectrum is used, and any photons with energy greater than or equal to the bandgap can excite valence-band electrons into the conduction band to create electron-hole pairs. However, any excess energy above the Fermi energy will be quickly dissipated
- [2] Third-generation_photovoltaic_cell_-_Wikipedia__f5366eba — wikipedia
source passage
have been made to reduce the amount required. Moreover, it is mechanically fragile, which typically requires a sheet of strong glass to be used as mechanical support and protection from the elements. The glass alone is a significant portion of the cost of a typical solar module. According to the Shockley–Queisser limit, the majority of a cell's theoretical efficiency is due to the difference in energy between the bandgap and solar photon. Any photon with more energy than the bandgap can cause photoexcitation, but any energy above the bandgap energy is lost. Consider the solar spectrum; only a small portion of the light reaching the ground is blue, but those photons have three times the energy of red light. Silicon's bandgap is 1.1 eV, about that of red light, so in this case blue light's energy is lost in a silicon cell. If the bandgap is tuned higher, say to blue, that energy is now captured, but only at the cost of rejecting lower energy photons. It is possible to greatly improve on a single-junction cell by stacking thin layers of material with varying bandgaps on top of each other – the "tandem cell" or "multi-junction" approach. Traditional silicon preparation methods do not lend themselves to this approach. Thin-films of amorphous silicon have been employed instead, notably Uni-Solar's products, but other issues have prevented these from matching the performance of traditional cells. Most tandem-cell structures are based on higher performance semiconductors, notably gal
- [3] Multi-junction_solar_cell_-_Wikipedia__3adaf198 — wikipedia
source passage
photons must have enough energy to overcome the bandgap of the material. If the photon has less energy than the bandgap, it is not collected at all. This is a major consideration for conventional solar cells, which are not sensitive to most of the infrared spectrum, although that represents almost half of the power coming from the sun. Conversely, photons with more energy than the bandgap, say blue light, initially eject an electron to a state high above the bandgap, but this extra energy is lost through collisions in a process known as "relaxation". This lost energy turns into heat in the cell, which has the side-effect of further increasing blackbody losses.[15] Combining all of these factors, the maximum efficiency for a single-bandgap material, like conventional silicon cells, is about 34%. That is, 66% of the energy in the sunlight hitting the cell will be lost. Practical concerns further reduce this, notably reflection off the front surface or the metal terminals, with modern high-quality cells at about 22%. Lower, also called narrower, bandgap materials will convert longer wavelength, lower energy photons. Higher, or wider bandgap materials will convert shorter wavelength, higher energy light. An analysis of the AM1.5 spectrum, shows the best balance is reached at about 1.1 eV (about 1100 nm, in the near infrared), which happens to be very close to the natural bandgap in silicon and a number of other useful semiconductors. Cells made from multiple materials layers can
- [4] ShockleyQueisser_limit_-_Wikipedia__91c1e965 — wikipedia
source passage
This is why the efficiency falls if the cell heats up. In fact this expression represents the thermodynamic upper limit of the amount of work that can be obtained from a heat source at the temperature of the sun and a heat sink at the temperature of the cell. Since the act of moving an electron from the valence band to the conduction band requires energy, only photons with more than that amount of energy will produce an electron-hole pair. In silicon the conduction band is about 1.1 eV away from the valence band, this corresponds to infrared light with a wavelength of about 1.1 microns. In other words, photons of red, yellow and blue light and some near-infrared will contribute to power production, whereas radio waves, microwaves, and most infrared photons will not.[10] This places an immediate limit on the amount of energy that can be extracted from the sun. Of the 1,000 W/m2 in AM1.5 sunlight, about 19% of that has less than 1.1 eV of energy, and will not produce power in a silicon cell. Another important contributor to losses is that any energy above and beyond the bandgap energy is lost. While blue light has roughly twice the energy of red light, that energy is not captured by devices with a single p-n junction. The electron is ejected with higher energy when struck by a blue photon, but it loses this extra energy as it travels toward the p-n junction (the energy is converted into heat).[10] This accounts for about 33% of the incident sunlight, meaning that, for silicon,
- [5] Perovskites_The_hottest_material_in_solar_cells_Laser__d2fb8c12 — magazine
source passage
and how photovoltaic semiconductors produce light. The solar spectrum is that of a blackbody of about 6000 K attenuated by atmospheric absorption (see Fig. 3). Photovoltaic junctions absorb photons with energy higher than the gap between their valence and conduction bands, but only the bandgap energy goes on to produce electric current. The remaining energy is lost as heat, so if a semiconductor with a 1 eV bandgap absorbs a 2 eV photon, 1 eV of energy produces current, but the other 1 eV produces heat. The fraction of the input light energy from a blackbody source (e.g., the sun) that a photovoltaic junction can convert into current depends on the semiconductor’s bandgap energy. The fraction of input light energy converted into usable electric current is highest for a 1.34 eV bandgap—higher than the 1.1 eV bandgap of silicon. The junction transmits photons that lack enough energy to excite a valence electron to the conduction band. If the photon has more energy than needed to excite an electron to the conduction band, the extra energy is lost. The bottom line is that, at best, a single photovoltaic junction can use only a third of the energy it absorbs to generate electric current—the remaining two-thirds of the energy. The way to beat the radiative limit is by stacking multiple photovoltaic junctions with different bandgaps so light can pass through more than one of them. The junction with the largest bandgap is put on the top of the stack so it absorbs the shortest wavelen
- [6] Theory_of_solar_cells_-_Wikipedia__238deba4 — wikipedia
source passage
# Theory of solar cells – Wikipedia Source: Blog/Web URL: https://en.wikipedia.org/wiki/Theory_of_solar_cells Author: Date: 2010-10-13 The theory of solar cells explains the process by which light energy in photons is converted into electric current when the photons strike a suitable semiconductor device. The theoretical studies are of practical use because they predict the fundamental limits of a solar cell, and give guidance on the phenomena that contribute to losses and solar cell efficiency. – Photons in sunlight hit the solar panel and are absorbed by semi-conducting materials. – Electrons (negatively charged) are knocked loose from their atoms as they are excited. Due to their special structure and the materials in solar cells, the electrons are only allowed to move in a single direction. The electronic structure of the materials is very important for the process to work, and often silicon incorporating small amounts of boron or phosphorus is used in different layers. – An array of solar cells converts solar energy into a usable amount of direct current (DC) electricity. When a photon hits a piece of semiconductor, one of three things can happen: – The photon can pass straight through the semiconductor — this (generally) happens for lower energy photons. – The photon can reflect off the surface. – The photon can be absorbed by the semiconductor if the photon energy is higher than the band gap value. This generates an electron-hole pair and sometimes heat depending on th
- [9] Undecided_with_Matt_Ferrell__How_Quantum_Dots_Solar_Panels_Could_Change_Everything__81JgczyzXy8 — youtube
source passage
to notice that there’s some big “ifs” and “mights” in that explanation. Solar panels actually aren’t all that efficient. Most commercial panels can only convert 15-23% of the light that hits them into usable electricity. And solar panels won’t get much more efficient, at least not the way we currently understand them. All the way back in 1961, scientists William Shockley and Hans-Joachim Queisser calculated that the maximum possible efficiency for a single junction solar cell to be about 30%. Thankfully, with modern advances in materials and engineering, that max efficiency now stands at a mighty… 33.7%. Oh yeah. Why is the Shockley-Queisser limit so low? In addition to all the hoops to jump through we mentioned earlier, there’s a lot of factors that limit how much juice we can squeeze out of a photon. Luckily there might be a few loopholes. For instance, Shockley and Queisser assumed there’s only one p-n junction. However, we could always add more semiconductors to create more band gaps. Remember that band gaps only generate energy from a photon if the photon has the same or greater energy level – they’re picky eaters. So combining a bunch of them together to form what’s called a multi-junction cell is a feasible way to break the Shockley-Queisser limit. It's kind of like making a wider net to catch more energy levels of photons… or maybe it's more like making a net with a bunch of back up nets behind it so less photons slip through? I’m stretching that metaphor … anyway, th
- [19] Introduction to Materials Chemistry — book
source passage
how sunlight induces separation of electrons and holes in the region of the junction, and how this leads to the generation of an electric current. Figure 10.16. Schematic cross section of a dye-based solar cell. The nanocrystalline TiO2 semiconductor layer receives electrons from the adsorbed dye molecules, which are reactivated via the cycling of an couple. d. Inorganic Light-Emitting Diodes. These are devices based on a p-n junction that generates light from an electric current. Their operation can be understood in terms of the illustrations in Figure 10.13, 10.14, and 10.15. Assume the reverse of the scenario just illustrated in Figure 10.15 for a solar cell. Instead of using light photons to generate an electric current, an electric current is forced across the semiconductor junction. Current continues to flow as electrons combine with holes at the junction. However, for this to occur, electrons and holes must be concentrated in the same region to allow the electrons to fall from the conduction band to the valence band and, in so doing, combine with holes to emit energy. That energy can be in the form of light, the wavelength of which depends on the bandgap. Thus, alterations to the effective bandgap by changes in the composition of the semiconductor materials will generate light of different colors. This is why the color of light emitted from a gallium arsenide semiconductor depends on the amount of phosphorus present in the crystal. e. The Semiconductor Laser. A diode l
- [25] Quantum_efficiency_-_Wikipedia__5231ab40 — wikipedia
source passage
Internal quantum efficiency (IQE) is the ratio of the number of charge carriers collected by the solar cell to the number of photons of a given energy that shine on the solar cell from outside and are absorbed by the cell. The IQE is always larger than the EQE in the visible spectrum. A low IQE indicates that the active layer of the solar cell is unable to make good use of the photons, most likely due to poor carrier collection efficiency. To measure the IQE, one first measures the EQE of the solar device, then measures its transmission and reflection, and combines these data to infer the IQE. The external quantum efficiency therefore depends on both the absorption of light and the collection of charges. Once a photon has been absorbed and has generated an electron-hole pair, these charges must be separated and collected at the junction. A "good" material avoids charge recombination. Charge recombination causes a drop in the external quantum efficiency. The ideal quantum efficiency graph has a square shape, where the QE value is fairly constant across the entire spectrum of wavelengths measured. However, the QE for most solar cells is reduced because of the effects of recombination, where charge carriers are not able to move into an external circuit. The same mechanisms that affect the collection probability also affect the QE. For example, modifying the front surface can affect carriers generated near the surface. Highly doped front surface layers can also cause 'free carrie
power the solar cell achieves at this maximum power point. Intuitively, IV curves with a more square shape and a flatter top and side will have a larger fill factor and therefore a higher efficiency.[84] Whereas these parameters characterize the efficiency of the solar cell based mostly on its macroscopic electrical properties, the quantum efficiency measures either the ratio of the number of photons incident on the cell to the number of charge carriers extracted (external quantum efficiency) or the ratio of the number of photons absorbed by the cell to the number of charge carriers extracted (internal quantum efficiency). Either way, the quantum efficiency is a more direct probe of the microscopic structure of the solar cell.[85] Some third-generation solar cells boost efficiency through the integration of concentrator and/or multi-junction device geometry.[64] This can lead to efficiencies larger than the Shockley–Queisser limit of approximately 42% efficiency for a single-junction semiconductor solar cell under one-sun illumination.[86] A multi-junction cell is one that incorporates multiple semiconducting active layers with different bandgaps. In a typical solar cell, a single absorber with a bandgap near the peak of the solar spectrum is used, and any photons with energy greater than or equal to the bandgap can excite valence-band electrons into the conduction band to create electron-hole pairs. However, any excess energy above the Fermi energy will be quickly dissipated
have been made to reduce the amount required. Moreover, it is mechanically fragile, which typically requires a sheet of strong glass to be used as mechanical support and protection from the elements. The glass alone is a significant portion of the cost of a typical solar module. According to the Shockley–Queisser limit, the majority of a cell's theoretical efficiency is due to the difference in energy between the bandgap and solar photon. Any photon with more energy than the bandgap can cause photoexcitation, but any energy above the bandgap energy is lost. Consider the solar spectrum; only a small portion of the light reaching the ground is blue, but those photons have three times the energy of red light. Silicon's bandgap is 1.1 eV, about that of red light, so in this case blue light's energy is lost in a silicon cell. If the bandgap is tuned higher, say to blue, that energy is now captured, but only at the cost of rejecting lower energy photons. It is possible to greatly improve on a single-junction cell by stacking thin layers of material with varying bandgaps on top of each other – the "tandem cell" or "multi-junction" approach. Traditional silicon preparation methods do not lend themselves to this approach. Thin-films of amorphous silicon have been employed instead, notably Uni-Solar's products, but other issues have prevented these from matching the performance of traditional cells. Most tandem-cell structures are based on higher performance semiconductors, notably gal
photons must have enough energy to overcome the bandgap of the material. If the photon has less energy than the bandgap, it is not collected at all. This is a major consideration for conventional solar cells, which are not sensitive to most of the infrared spectrum, although that represents almost half of the power coming from the sun. Conversely, photons with more energy than the bandgap, say blue light, initially eject an electron to a state high above the bandgap, but this extra energy is lost through collisions in a process known as "relaxation". This lost energy turns into heat in the cell, which has the side-effect of further increasing blackbody losses.[15] Combining all of these factors, the maximum efficiency for a single-bandgap material, like conventional silicon cells, is about 34%. That is, 66% of the energy in the sunlight hitting the cell will be lost. Practical concerns further reduce this, notably reflection off the front surface or the metal terminals, with modern high-quality cells at about 22%. Lower, also called narrower, bandgap materials will convert longer wavelength, lower energy photons. Higher, or wider bandgap materials will convert shorter wavelength, higher energy light. An analysis of the AM1.5 spectrum, shows the best balance is reached at about 1.1 eV (about 1100 nm, in the near infrared), which happens to be very close to the natural bandgap in silicon and a number of other useful semiconductors. Cells made from multiple materials layers can
This is why the efficiency falls if the cell heats up. In fact this expression represents the thermodynamic upper limit of the amount of work that can be obtained from a heat source at the temperature of the sun and a heat sink at the temperature of the cell. Since the act of moving an electron from the valence band to the conduction band requires energy, only photons with more than that amount of energy will produce an electron-hole pair. In silicon the conduction band is about 1.1 eV away from the valence band, this corresponds to infrared light with a wavelength of about 1.1 microns. In other words, photons of red, yellow and blue light and some near-infrared will contribute to power production, whereas radio waves, microwaves, and most infrared photons will not.[10] This places an immediate limit on the amount of energy that can be extracted from the sun. Of the 1,000 W/m2 in AM1.5 sunlight, about 19% of that has less than 1.1 eV of energy, and will not produce power in a silicon cell. Another important contributor to losses is that any energy above and beyond the bandgap energy is lost. While blue light has roughly twice the energy of red light, that energy is not captured by devices with a single p-n junction. The electron is ejected with higher energy when struck by a blue photon, but it loses this extra energy as it travels toward the p-n junction (the energy is converted into heat).[10] This accounts for about 33% of the incident sunlight, meaning that, for silicon,
and how photovoltaic semiconductors produce light. The solar spectrum is that of a blackbody of about 6000 K attenuated by atmospheric absorption (see Fig. 3). Photovoltaic junctions absorb photons with energy higher than the gap between their valence and conduction bands, but only the bandgap energy goes on to produce electric current. The remaining energy is lost as heat, so if a semiconductor with a 1 eV bandgap absorbs a 2 eV photon, 1 eV of energy produces current, but the other 1 eV produces heat. The fraction of the input light energy from a blackbody source (e.g., the sun) that a photovoltaic junction can convert into current depends on the semiconductor’s bandgap energy. The fraction of input light energy converted into usable electric current is highest for a 1.34 eV bandgap—higher than the 1.1 eV bandgap of silicon. The junction transmits photons that lack enough energy to excite a valence electron to the conduction band. If the photon has more energy than needed to excite an electron to the conduction band, the extra energy is lost. The bottom line is that, at best, a single photovoltaic junction can use only a third of the energy it absorbs to generate electric current—the remaining two-thirds of the energy. The way to beat the radiative limit is by stacking multiple photovoltaic junctions with different bandgaps so light can pass through more than one of them. The junction with the largest bandgap is put on the top of the stack so it absorbs the shortest wavelen
# Theory of solar cells – Wikipedia Source: Blog/Web URL: https://en.wikipedia.org/wiki/Theory_of_solar_cells Author: Date: 2010-10-13 The theory of solar cells explains the process by which light energy in photons is converted into electric current when the photons strike a suitable semiconductor device. The theoretical studies are of practical use because they predict the fundamental limits of a solar cell, and give guidance on the phenomena that contribute to losses and solar cell efficiency. – Photons in sunlight hit the solar panel and are absorbed by semi-conducting materials. – Electrons (negatively charged) are knocked loose from their atoms as they are excited. Due to their special structure and the materials in solar cells, the electrons are only allowed to move in a single direction. The electronic structure of the materials is very important for the process to work, and often silicon incorporating small amounts of boron or phosphorus is used in different layers. – An array of solar cells converts solar energy into a usable amount of direct current (DC) electricity. When a photon hits a piece of semiconductor, one of three things can happen: – The photon can pass straight through the semiconductor — this (generally) happens for lower energy photons. – The photon can reflect off the surface. – The photon can be absorbed by the semiconductor if the photon energy is higher than the band gap value. This generates an electron-hole pair and sometimes heat depending on th
to notice that there’s some big “ifs” and “mights” in that explanation. Solar panels actually aren’t all that efficient. Most commercial panels can only convert 15-23% of the light that hits them into usable electricity. And solar panels won’t get much more efficient, at least not the way we currently understand them. All the way back in 1961, scientists William Shockley and Hans-Joachim Queisser calculated that the maximum possible efficiency for a single junction solar cell to be about 30%. Thankfully, with modern advances in materials and engineering, that max efficiency now stands at a mighty… 33.7%. Oh yeah. Why is the Shockley-Queisser limit so low? In addition to all the hoops to jump through we mentioned earlier, there’s a lot of factors that limit how much juice we can squeeze out of a photon. Luckily there might be a few loopholes. For instance, Shockley and Queisser assumed there’s only one p-n junction. However, we could always add more semiconductors to create more band gaps. Remember that band gaps only generate energy from a photon if the photon has the same or greater energy level – they’re picky eaters. So combining a bunch of them together to form what’s called a multi-junction cell is a feasible way to break the Shockley-Queisser limit. It's kind of like making a wider net to catch more energy levels of photons… or maybe it's more like making a net with a bunch of back up nets behind it so less photons slip through? I’m stretching that metaphor … anyway, th
how sunlight induces separation of electrons and holes in the region of the junction, and how this leads to the generation of an electric current. Figure 10.16. Schematic cross section of a dye-based solar cell. The nanocrystalline TiO2 semiconductor layer receives electrons from the adsorbed dye molecules, which are reactivated via the cycling of an couple. d. Inorganic Light-Emitting Diodes. These are devices based on a p-n junction that generates light from an electric current. Their operation can be understood in terms of the illustrations in Figure 10.13, 10.14, and 10.15. Assume the reverse of the scenario just illustrated in Figure 10.15 for a solar cell. Instead of using light photons to generate an electric current, an electric current is forced across the semiconductor junction. Current continues to flow as electrons combine with holes at the junction. However, for this to occur, electrons and holes must be concentrated in the same region to allow the electrons to fall from the conduction band to the valence band and, in so doing, combine with holes to emit energy. That energy can be in the form of light, the wavelength of which depends on the bandgap. Thus, alterations to the effective bandgap by changes in the composition of the semiconductor materials will generate light of different colors. This is why the color of light emitted from a gallium arsenide semiconductor depends on the amount of phosphorus present in the crystal. e. The Semiconductor Laser. A diode l
Internal quantum efficiency (IQE) is the ratio of the number of charge carriers collected by the solar cell to the number of photons of a given energy that shine on the solar cell from outside and are absorbed by the cell. The IQE is always larger than the EQE in the visible spectrum. A low IQE indicates that the active layer of the solar cell is unable to make good use of the photons, most likely due to poor carrier collection efficiency. To measure the IQE, one first measures the EQE of the solar device, then measures its transmission and reflection, and combines these data to infer the IQE. The external quantum efficiency therefore depends on both the absorption of light and the collection of charges. Once a photon has been absorbed and has generated an electron-hole pair, these charges must be separated and collected at the junction. A "good" material avoids charge recombination. Charge recombination causes a drop in the external quantum efficiency. The ideal quantum efficiency graph has a square shape, where the QE value is fairly constant across the entire spectrum of wavelengths measured. However, the QE for most solar cells is reduced because of the effects of recombination, where charge carriers are not able to move into an external circuit. The same mechanisms that affect the collection probability also affect the QE. For example, modifying the front surface can affect carriers generated near the surface. Highly doped front surface layers can also cause 'free carrie