Heterojunction solar cell
The selective contact material and the absorber have different band gaps, forming the carrier-separating heterojunctions that are analogous to the p-n junction of traditional solar cells.
[13][14] This presents challenges for electrode metallisation, as the typical silver paste screen printing method requires firing at up to 800 °C;[15] well above the upper tolerance for most buffer layer materials.
[30] Over 24 (mostly Chinese) manufacturers are beginning or augmenting their heterojunction production capacity, such as Huasun, Risen, Jingang (Golden Glass), LONGi, Meyer Burger and many more.
[40] The temperature sensitivity of solar cells has been inversely correlated to high open-circuit voltages compared to the absorber band gap potential,[41] as noted by Martin Green in 1982; "As the open-circuit voltage of silicon solar cells continues to improve, one resulting advantage, not widely appreciated, is reduced temperature sensitivity of device performance".
[43] SHJ production lines fundamentally do not use high temperature equipment such as diffusion or metal paste curing furnaces,[22] and on average have a lower power consumption per watt of fabricated cells.
[11][43] For these reasons, n-type wafers are strongly preferred for manufacturing, as inconvenient steps for improving bulk lifetimes are cut out and the risk of developing light-induced degradation is reduced.
[54][55] The higher price of n-type wafers is attributed to the smaller segregation coefficient of phosphorus in silicon whilst growing of doped monocrystalline ingots.
Due to stringent requirements for surface cleanliness for SHJ compared to PERC, the texturing and cleaning process is relatively more complex and consumes more chemicals.
The low-temperature paste composition compromises several factors in the performance and economics of SHJ, such as high silver consumption and lower grid conductivity.
A report by Wood Mackenzie (Dec 2022) predicts that TOPCon will be favoured over SHJ for new module production in the United States in light of the Inflation Reduction Act for this reason, citing a preferable balance between high efficiency and capital expenditure.
[66] In particular, rear-junction configurations are preferred in manufacturing as they allow for a greater proportion of lateral electron transport to transpire in the absorber rather than the front TCO.
Therefore, the sheet resistance of the front side is lowered and restrictions on TCO process parameters are relaxed, leading to efficiency and cost benefits.
Intrinsic amorphous silicon is deposited onto both sides of the substrate using PECVD from a mixture of silane (SiH4) and hydrogen (H2), forming the heterojunction and passivating the surface.
The buffer layer must be sufficiently thick to provide adequate passivation, however must be thin enough to not significantly impede carrier transport or absorb light.
[25] The dual purpose antireflection coating (ARC) and carrier transport layer, usually composed of Indium tin oxide (ITO), is sputtered onto both sides over the selective contacts.
For destructive interference antireflection properties, the TCO is deposited to the thickness required for optimum light capture at the peak of the solar spectrum (around 550 nm ).
[83] Through evaporation, a double-antireflection coating of magnesium fluoride (MgF2)[84] or aluminium oxide (Al2O3)[76] may be used to further reduce surface reflections, however this step is not currently employed in industrial production.
[63] These suffer major drawbacks including low grid conductivity and high silver consumption,[62][89] volatile production costs[22] or poor adhesion to the front surface.
Understanding the behaviour of defects, and how they interact with hydrogen over time and in manufacturing processes, is crucial for maintaining the stability and performance of SHJ solar cells.
[120] Staebler and Wronski found a gradual decrease in photoconductivity and dark conductivity of amorphous silicon thin films upon exposure to light for several hours.
It has been suggested that thermal annealing causes interstitial hydrogen to diffuse closer to the heterointerface, allowing greater saturation of dangling bond defects.
[128] Such a process may be enhanced using illumination during annealing, however this can cause degradation before the improvement in carrier lifetimes is achieved, and thus requires careful optimisation in a commercial setting.
[129] Illuminated annealing at high temperatures is instrumental in the Advanced Hydrogenation Process (AHP), an inline technique for defect mitigation developed by UNSW.
The Boron–Oxygen complex LID defect is a pervasive problem with the efficiency and stability of cheap p-type wafers and a major reason that n-type is preferred for SHJ substrates.
PID is primarily an electrochemical process causing corrosion[134] and ion migration[135] in a solar module and cells, facilitated by moisture ingress and surface contamination.
[136][137] Sodium ions, which are suspected to leach from soda-lime glass, are particularly problematic and can cause degradation in the presence of moisture (even without high electric potential).
In research, PID can be replicated in accelerated aging tests by applying high bias voltages to a sample module, especially in an environmental chamber.
[139] After long duration exposure to moisture, EVA can hydrolyse and leach acetic acid[140] with the potential to corrode the metal terminals[141] or surface[142] of a solar cell.
[12] The polymer backsheet, despite being more permeable to moisture ingress than glass–glass modules (which facilitates hydrolysis of EVA), is allegedly "breathable" to acetic acid and does not allow it to build up.
[142][144] Despite the higher cost, acetate-free and low water vapour permeability encapsulants such as polyolefin elastomers (POE) or thermoplastic olefins (TPO) show reduced degradation after damp-heat testing in comparison to EVA.
