LERL and rTOPcon

For the homojunction structure shown in Figure6.2.2, metal vias are required between the junction layer and front emitter to reduce the contact resistance. However, these vias also introduce shading. Hence the contact openings need to be relatively small and the contact resistivity needs to be sufficiently low to maintain low resistive losses. The full area diffusion on the surfaces also requires a careful balance - a heavier diffusion will increase the surface recombination current, while a lighter diffusion will increase the contact resistivity. Importantly, the bottom c-Si substrate needs to be annealed at minimum 400 °C to ensure

a high-quality TiOx electron transport layer for the perovskite top cell. Such an anneal will

significantly increase the contact resistivity, and therefore decrease the FF of the device. Therefore, this structure inevitably incurs significant FF and Voc losses. As shown in

Figure6.3.6, the surface recombination loss is close to 1/2 of its power output loss at MPP.

Substantial improvements may be achieved through heavy localized diffusions beneath the contacts on both surfaces, which allows decoupling of the contact resistivity and recombination in the diffused regions.193,217–219

Here we propose a Localized Emitter Rear Localized diffused (LERL) structure based on 1Ω·cm, p-type wafers for the monolithic perovskite/Si tandem bottom cell. The structure schematic can be found in Figure6.5.1. For the front side, since the conductivity of the majority carriers is relatively high when using a 1Ω·cm wafer, the lateral resistance loss is minimal in this case. A full area front diffusion is

therefore not necessary for the device, but only the localized heavy boron diffusion for front contact openings. This leads to three benefits: firstly, it saves a high-temperature processing

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82

step in cell processing. Secondly, the ALD deposited Al2O3 layer will provide excellent

surface passivation on the undiffused surface (<1fA/cm2_),220 _{which will further reduce the}

recombination loss and improve the Voc. Finally, a boron diffusion is known as a detrimental

fact for the bulk lifetime,221 _{hence, the omission of this step may be beneficial for bulk quality.}

On the rear side, the locally heavy diffused regions may be created through etch-back of a heavy POCl3 diffusion 222. This diffusion will not only provide a localized heavily area for

reduced contact resistivity but will also provide an impurity gettering effect to further improve the bulk quality.223

The rear side must have a full area diffusion to allow efficient collection of minority carriers from the bulk by minimising the lateral resistive loss. Light diffusion is sufficient with sheet resistance ~160Ω/□ to allow efficient extraction of minority carrier from bulk and good rear surface passivation (J0=30fA/cm2) on the random

pyramid texture.

The second structure we purpose is the reversed Tunnelling Oxide Passivating Contact (rTOPCon).224 _{The TOPCon structure has attracted a great deal of attention in the c-Si solar}

cell community for the last several years. The structure has the front side textured, locally diffused and passivated with SiNx to allow low parasitic absorption compared with the high

optical loss caused by a:Si in a rear contact heterojunction (HJT) device. It has its rear side covered with a SiOx/poly:Si stack to provide both full area passivation and contact. 225,226

Since the poly:Si stack deposition is a high-temperature process(>700 °C), the stack can

sustain its passivation quality following the potential high-temperature processing steps for the perovskite top cell deposition. In the tandem configuration, it is designed to be upside down to have a front planar side with the tunnelling oxide stack and is therefore referred to as reversed TOPCon. To date, boron doped SiOx/poly:Si stacks reported in the literature have

not shown better results for surface passivation compared with phosphorus doped stacks.206

This motivates the use of a rear junction rTOPCon structure, where the front planar side is passivated with a SiOx/poly:Si (phosphorus doped) stack. Since this is a front junction cell

fabricated with 0.5 Ω·cm resistivity wafer and providing low lateral resistive loss, a full area

diffusion at the rear textured side is not necessary, and a localized Boron diffusion is used instead. The fixed bulk lifetime is set similar to the LERL due to the impurity gettering effect

Chapter 6 - Optimization of the c-Si cell in monolithic perovskite/c-Si tandem devices

83 from the polysilicon at the front surface.227

With these modifications implemented, the simulated I-V results of such structures can be seen in Figure 6.5.2. Detailed parameters of the simulation can be found in Table 6.5.

Table 6.5. Parameters for 3D device simulation in using Quokka3 with varies design structures.

Figure 6.5.1. Device schematic drawing for monolithic perovskite/c-Si tandem with the bottom cell structure using (a) LERL, (b) rTOPCon.

Figure 6.5.2. Device schematic drawing for monolithic perovskite/c-Si tandem with the bottom cell structure using (a) LERL, (b) rTOPCon.

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84

When the cell structure is changed from PERT to LERL, the rear surface recombination remains low due to the rear junction despite the reasonably high surface recombination rate on a textured surface. The front surface recombination loss is largely suppressed when applying an un-diffused surface passivated by a 20 nm Al2O3 film. However, we see a slight

increase of the hole transport loss at the front emitter due to lack of full area diffusion. As expected, by employing a localized heavy diffusion on both sides of the film, the resistive loss for contacts on both sides is significantly decreased, especially for the front side.

With the rTOPCon structure, we see a further increase in cell efficiency. The rear side is locally boron diffused and the undiffused area can be well passivated with Al2O3. In addition,

the front side is well passivated by a tunnelling oxide. Therefore, the recombination loss on both sides of the cell is largely suppressed. Other advantages are also apparent when we compare rTOPCon with the LERL structure. Firstly, rTOPCon is covered with a full area passivating front contact, which further reduces the reflection loss by the metal contact array (included in the simulation). The ITO recombination layer can be thinner because it is not required to have a low sheet resistance to distribute carriers laterally, but only acts as a recombination interface between top and bottom cells to recombine electrons and holes. Furthermore, the entire structure requires only one diffusion process, which reduces the thermal budget. The front side requires no patterning step to fabricate while the rear side localized diffusion and contact opening could be achieved by laser processing,[47–49]

which would allow the rTOPCon c-Si substrate to be fabricated on an industrial scale. Therefore, we believe that the rTOPCon structure has excellent potential for perovskite-silicon tandem cells and should be explored further.

6.6 Conclusion

Through systematic analysis and step-by-step optimisation of monolithic perovskite –

silicon tandem structures based on a silicon cell with a planar front side, our simulation results highlight possible pathways to improve the efficiency of the bottom Si cell. We find that the use of p-type wafers can result in a significant performance enhancement

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85

compared to n-type material regardless of device architecture, provided a similar bulk lifetime can be maintained. The optimum resistivity of the wafer is found to be relatively low (in the range 0.5-1 Ω·cm) for all the architectures investigated. Both front and rear

junction designs can offer excellent performance, with the optimal location dependent on the surface parameters, which in turn depend on the passivation of the junction technology employed. We propose two new structures which can further reduce the contact resistive loss and surface recombination losses. The new LERL and rTOPCon structures enable the realisation of n-i-p and p-i-n perovskite top cells respectively. Importantly, these structures are capable of the same excellent performance as optimised heterojunction cells while displaying a better temperature tolerance, which opens the way for a greater range of more stable, low loss inorganic selective carrier transport layers. With optimised design parameters, c-Si cell efficiencies of over 11% are achievable for both heterojunction and homojunction-based devices. In combination with a high-efficiency perovskite top cell, these cells with enabling the realisation of 30% efficient monolithic tandems.

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Conclusion and future work

In summary, while the debate around the origin of hysteresis behaviour in perovskite solar cell continues, there is a strong body of evidence that supports the notion that mobile ionic species are a key ingredient in hysteresis. The observed decoupling of transient change in Voc and photoluminescence intensity upon illumination or switching from short-circuited to

open circuited shows the generalised Planck radiation law appears to be violated in some perovskite solar cells. With the help of numerical simulation, such decoupling can be explained by the presence of a majority carrier bottleneck at one or both of the perovskite/transport layer interfaces. The accumulation of mobile ions at the interfaces under short circuit or equilibrium conditions can create such a majority carrier bottleneck when substantial interfacial recombination exists. Our work not only helps provide further evidence for the existence of mobile ions in the perovskite film and for their fundamental role in causing hysteresis, we also pointed out that by effective interfacial passivation, strong hysteresis behaviour can be moderated to fabricate devices with higher efficiency.

In the tandem work we presented, a new homojunction c-Si cell architecture suitable for perovskite/c-Si monolithic tandem devices is demonstrated. The design allows the use of a top perovskite device that can utilise TiOx based ETLs, which have to date delivered the

highest cell efficiencies but require high-temperature processing. We also implement, for the first time, a homojunction device in a perovskite/c-Si tandem that features double- sided passivation. A careful optimisation for the SiNx layer is carried out to reduce the

reflective loss of the bottom c-Si device. To achieve good repeatability and efficiency for the perovskite top device on 1 cm2

Conclusion and future work

87

the quadruple-cation perovskite deposition. By implementing these careful designs, a 22.5% efficient monolithic tandem device is realised.

In addition to the previous work, we find the bottom c-Si structure can be further improved to increase the efficiency of the monolithic tandem device. After a rigorous optoelectrical simulation of the c-Si in tandem structure, we find that double side textured substrates do not necessarily give a decisively better current collection compared to the single side textured substrates if the optical properties of the top perovskite cell are carefully managed. The optimum resistivity of the c-Si substrate is found to be relatively low. Further, it is shown that p-type wafers can result in a significant performance enhancement compared to n- type material regardless of device architecture, provided that comparably high lifetimes can be maintained. Two new device structures (LERL and rTOPCon) are presented which are capable of the same excellent device performance as optimised HJT cells while displaying a better temperature tolerance.

The bottleneck of our current tandem approach is the low photogeneration which is mostly the result of strong parasitic absorption in the spiro-MeOTAD layer at front. So far, spiro-MeOTAD has been used by the best performing n-i-p single junction perovskite solar cells, and its best replacement candidate PTAA also shows strong parasitic absorptions. Furthermore, most organic hole transport layers have a lower refractive index compared to the perovskite absorber, which will lead to a higher reflective loss when the device features a planar front surface. Inorganic HTLs such as NiOx, CuOx, CuSCN, and CuI can potentially solve the issue due to their

low parasitic absorption and better refractive index matching with perovskite absorber. However, depositing these inorganic materials on top of perovskite is problematic due to the incompatible processing conditions, e.g. temperature and solvents. CuSCN for the n-i-p device was recently implemented as an alternative HTL, resulting in efficiencies exceeding 20%. However, a thick rGO layer was also employed together with the CuSCN to provide better hole extraction.80

Due to its high absorption coefficient, the issues of parasitic absorption is not yet fully solved with this material. The route to unravelling this problem is still unclear and requires more work to be done.

Conclusion and future work

88

The second challenge of the future development of the perovskite/c-Si monolithic tandem device is its scalability. To be able to commercialize this particular product, a uniform, and pin- hole free perovskite thin film deposition method on an at least 4-inch size wafer requires to be developed. So far, a hybrid two-step deposition method combining sequential co-evaporation and spin-coating has been developed to show potential on the large-area device. However, the downside of the process is obvious. To be competitive in commercial solar cell market, the product needs to be relatively cheap. The high vacuum evaporation method is feasible for fabricating OLED product due to its high-value nature, but it will not work for PV products. The second promising method developed is the slot-die coating with vacuum flashing treatment. The method has demonstrated that the device can be fabricated on large panel planar perovskite modules. But this method can not be done on top of the textured surface, which is important for light harvesting on a tandem device.

The last challenging part is the device stability. Multiple mechanisms have been found can degrade the perovskite solar cell easily, including exposing to water, oxygen and UV. With advanced encapsulation methods, and introducing hydrophobic capping layers, the stability has much improved since 10 years ago. More recently, the metal electrode was found can diffuse into the perovskite layer and initiate degradation, but a few buffer layers have been found can fix this issue. For tandem devices, the stability issue becomes more problematic. As we discussed in section 4.3, high bandgap perovskite absorber can segregate into 2 different domains after heat treatment or long term exposure. Hence, there is still a long way before the perovskite/c- Si tandem cell become the convincinlly stable commercial product.

Overall, the speed of progress on perovskites has been impressive, and with so many groups now researching the technology, it is very that the technology will continue to improve rapidly.

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