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The Q4 2008 edition of PV Sun Times^SM provides definitions of key PV concepts and terms, as well as advice for selecting the best type of TCO for your process.

Key PV Concepts and Terms

Exciton
An exciton is a mobile electron-hole pair created within the active layer of a solar cell by the collision of a photon with one of the outermost electrons of an atom.

Figure 1. Exciton (Click graphic to enlarge.)

Recombination
Ideally, after forming an exciton, an electron-hole pair separates. Occasionally, however, the electron falls back into a valence bond with an atom and energy is released as a phonon (heat) or a photon (light) instead of creating electrical current. This process is called recombination.

Conversion Efficiency
Conversion efficiency is the comparison of electrical power output to solar power input, and is expressed in the following equation:



P_m is the power at the max power point, E is the irradiance or light incident on the cell, and A is the cell area.

Quantum Efficiency (Internal and External)
Internal quantum efficiency (QE) concerns the number of photons that are absorbed versus the number of charge carriers (electrons and holes) produced. This can be improved by reducing recombination. External QE takes into account the amount of light that is transmitted or reflected and is therefore a lower value than internal QE.

Cell Efficiency
The percentage of photons that are converted into excitons is referred to as cell efficiency. Typically, this is lower than quantum efficiency due to recombination and other factors.

Panel Efficiency
The percentage of photons converted into useful electricity is referred to as panel efficiency. This is typically lower than cell efficiency due to ohmic losses in the electrical connections between cells.

Open Circuit Voltage (V_oc)
A solar cell’s maximum achievable voltage is called open circuit voltage. No power is produced at this point because current is zero.

Short Circuit Current (I_sc)
The maximum current a solar cell can produce is called short circuit current. No power is produced at this point because voltage is zero.

V_mp
V_mp is the actual voltage achieved by a solar cell at maximum power.

I_mp
I_mp is the actual current a solar cell can produce at maximum power.

Fill Factor
The maximum power theoretically possible, P_theoretical, is V_oc x I_sc. The maximum power actually possible, P_m, is V_mp x I_mp. Dividing P_m by P_theoretical provides the fill factor of the device, which is always less than 100% because P_m is always less than P_theoretical.

Figure 2. Fill factor

Are you eager to improve throughput and cell efficiency in your PV manufacturing operation?

AE’s solar experts, Ken Nauman and Doug Pelleymounter, offer advice for the solar industry’s most pressing manufacturing issues.

We’d love to hear from you! Please send your questions and comments to PVSunTimes@aei.com.

1. I’m starting a new solar line. What are the major TCO types I should consider?
2. Which of these is the highest performing? 
3. Which TCO materials typically pose the most problems?
4. Which TCO material is the most cost efficient?
5. Are there any additional manufacturing techniques I should consider for TCOs?

1. I’m starting a new solar line. What are the major TCO types I should consider?
   Answer: Congratulations and good luck with your new endeavor! To provide a little background for our other readers, the transparent conductive oxide (TCO) layer is the first deposited for multi-junction amorphous silicon and CdTe (cadmium-telluride) cells. For most CIGS cells, the TCO is deposited last. This layer is transparent to allow sunlight to enter the solar cell, and also must be electrically conductive.
   
   
   
   
   Figure 3. For most CIGS solar panels, the last layer deposited is the TCO, while for multi-junction amorphous silicon and CdTe panels, the first layer deposited is the TCO (Click graphic to enlarge.)
   
   
   
   There are five major types of TCO: ITO, IZO, AZO, SnO, and Cd₂SnO₄. It’s also possible to use a very thin layer of metal, such as silver, to serve the same function as a TCO. Each option has its own advantages and disadvantages, so when deciding what type of TCO is best for your manufacturing operation, it’s important to weigh your particular priorities for such issues as layer performance, cost, preferred deposition method, and ease of manufacturing.
   
   
   
   
   
2. Which of these is the highest performing?
   Answer: There is no clear winner in terms of TCO performance. It depends on your requirements for a number of parameters, including:
   
   
   • Sheet resistance (Ω/□)
   • Cost
   • Light transmission (T%)
   • Chemical Stability
   • Diffusion
   
   Table 1 below shows how four of the five major types of TCO generally compare to each other. All TCO types can be sensitive to sodium diffusion. Also, Cd₂SnO₄ provides good chemical stability, light transmission, and sheet resistance, but is very expensive and poses certain health hazards. It’s interesting to note that ITO is specifically used for TCOs in flat panel display manufacturing because that application requires low resistance and high light transmission.
   
   Table 1. TCO properties comparison
   
   
   
   
   
   
3. Which TCO materials typically pose the most problems?
   Answer: There are a number of potential challenges related to TCO manufacturing, but generally, reasonable solutions exist.
   
   Certain challenges are associated with conditions created by the TCO material itself. For example, cadmium is a heavy metal and therefore toxic, making it difficult and hazardous to deposit Cd₂SnO₄. Also, bonded ITO tends to produce nodules on the target that prevent complete target utilization, necessitating frequent process shutdowns for target replacement. However, DC pulsing lowers the probability of nodule formation, helping to maintain target quality over time so that you can utilize the target more fully. Although DC pulsing is quite effective here, the best process power option for alleviating this problem seems to be RF-superimposed DC. Higher target surface temperature also reduces or eliminates nodules. Please see Maintaining Target Quality in the Q4 2007 edition of AE’s Flat Panel Focus^® newsletter for more details about nodule formation.
   
   Another challenge inherent to the TCO material itself is AZO’s tendency to arc. It can produce hundreds or even thousands of arcs per second. Solar panel manufacturing can tolerate this level of arcing, especially if certain measures are employed during manufacturing. The use of a high-quality DC power supply with effective arc management capabilities is absolutely vital to processes depositing AZO. Pulsing provides an additional benefit by reducing the number or rate of arcs. In addition, correct output cable selection is critical. Improper selection may prevent the power supply from detecting and thus extinguishing arcs. Output cables must be shielded and have low inductance to prevent noise, high stored energy, and other problems. Please see What kind of output cables should I use for my power supply? in the Q4 2008 edition of Flat Panel Focus for more information.
   
   The problems described above are caused by properties of the TCO materials themselves. Other TCO-related problems are associated with the particular conditions of a given manufacturing operation. For example, TCO deposition traditionally is done at high temperatures in order to achieve sufficient layer conductivity. Deposition temperature is a concern for processes using flexible polymer substrates, which melt at high heat. Temperature is also an issue for the manufacturing of certain CIGS solar cells, in which the TCO layer is deposited last. Underlying doped layers may suffer from diffusion at the high temperatures required. Please see the Q2 2008 edition of PV Sun Times for more information on this phenomenon.
   
   An excellent solution exists for temperature-sensitive processes: RF-superimposed-DC or RF-superimposed-pulsed DC. These process power methods enable the deposition of a high-conductivity TCO while keeping temperatures low enough that diffusion does not occur. Please see AE’s Arc Handling in RF-Superimposed DC Processes application note for more information on these methods.
   
   
   
   
4. Which TCO material is the most cost efficient?
   Answer: Table 1 shows that as TCO sheet resistance decreases, materials cost increases. However, straight material cost doesn’t necessarily tell you which material will be most cost efficient in your manufacturing operation. Cost efficiency is a fairly complicated issue related to TCOs because each material has its own unique properties, and each process has its own requirements.
   
   For example, ITO is one of the most expensive TCO materials, but you don’t need to deposit as much compared to, for example, AZO, in order to achieve a comparable level of conductivity. Another benefit of a thinner TCO layer is that it transmits light better than a thicker layer and thus increases cell efficiency. However, ITO is also known to have problems with nodule growth on the target, which reduces target utilization and necessitates more frequent target replacement. This increases cost and downtime. Nevertheless, after all is said and done, because you don’t need as much material, your net cost for ITO may be comparable to that for a less expensive material. Also, the possible cell efficiency improvements it offers may outweigh the cost.
   
   
   
   
   Figure 4. AZO vs. ITO comparison—ITO’s higher material cost may be offset by the fact that you can deposit less and still achieve the same level of resistance as less expensive materials, such as AZO. A thinner TCO layer also leads to better light transmission and thus improved cell efficiency
   
   
   * In terms of material properties alone, the resistance of ITO is approximately ten times lower than that of AZO. Therefore, you can deposit much less ITO and produce a layer that performs comparably to a much thicker AZO layer. If a given ITO layer is five times thinner than the typical AZO layer, the resistance of the ITO layer is still approximately two times better (lower) than the AZO layer.
   
   
   
   All in all, figuring TCO cost efficiency has many variables. Please contact us if you’d like assistance determining the best TCO choice for your process.
   
   
   
   
   
5. Are there any additional manufacturing techniques to consider for TCOs?
   Answer: Yes, you might consider chemical etching, which can be performed on AZO and possibly other TCO materials to increase internal light reflection. This enhances quantum efficiency by creating a faceted surface that reflects light back into the solar cell. Without this faceting, the likelihood is greater that reflected photons would escape the cell without creating a useful collision with an electron. TCO texturing for internal light reflection increases quantum efficiency. (See Figure 5.) This means that more of the photons that enter the cell are used to actually generate electricity.
   
   
   
   
   Figure 5. Chemical etching creates a faceted surface on the TCO layer. This faceting increases internal light reflection and quantum efficiency by reflecting light back into the cell that otherwise would have escaped (Click graphic to enlarge.)
   
   
   Also consider adding barrier layers to your solar cell. Depending on where they are deposited within the solar stack, they can provide any of the following benefits:
   
   
   • Protection from external environment
   • Control of sodium diffusion (when SLG is used)
   • Shielding of underlying layers from laser scribing
   
   
   For barrier layers, the most common deposition method is AC dual-magnetron sputtering. However, PECVD is another viable option. It’s important to deposit a thick barrier layer (> 1 micron) that is pinhole-free. Depending on the purpose of your barrier, pinholes or insufficient thickness may allow contaminants to enter the solar cell, sodium to leach out, or laser scribing to damage underlying layers. Figure 6 shows a barrier layer deposited to curb sodium diffusion.
   
   
   
   
   Figure 6. The barrier layer between the soda lime glass and TCO in this solar cell curbs sodium diffusion (Click graphic to enlarge.)