In general, compared with Topcon cells, heterojunction (HJT) cells are more suitable as the bottom cell for perovskite tandem solar cells.
1. HJT structure is inherently better suited for tandems
When pairing perovskite with an HJT bottom cell, the HJT front surface already uses a transparent conductive oxide (TCO), so the HJT production line requires no modification. By contrast, a Topcon cell has silicon nitride and aluminum oxide layers on its front that are insulating and nonconductive. For perovskite/Topcon tandems, those insulating layers must be removed or additional doping and passivation steps must be introduced to enable electrical contact.
2. Topcon's high-current advantage can be lost in tandem designs
Topcon's efficiency advantage driven by high current can be wasted in a tandem configuration. In commercial production, overall efficiencies of Topcon and HJT are similar, but their contributing parameters differ: HJT cells tend to have higher voltage and lower current, while Topcon cells have relatively lower open-circuit voltage but higher current. The main reason is that the HJT front-surface TCO transmits less light than the silicon nitride layer on Topcon. In a tandem cell, the front of the HJT remains TCO, while the Topcon front would need to be converted to a TCO as well. That removes Topcon's high-current advantage, so, in theory, a perovskite–Topcon tandem would have lower efficiency than a perovskite–HJT tandem.
However, perovskite–Topcon tandems remain of interest. In June 2022, researchers including Klaus Weber (Australian National University), Huanping Zhou (Peking University), and Pei Ting Zheng (JinkoSolar) fabricated monolithic perovskite/Topcon tandem devices using Topcon crystalline silicon as the bottom cell and a perovskite thin film as the top cell. The reported device efficiency was 27.6%.
3. Perovskite/HJT tandems: series stacking and higher output voltage
Perovskite and heterojunction silicon are well matched for tandem cells and can produce higher efficiencies than single-junction perovskite cells. Heterojunction refers to a structure where p-type and n-type semiconductors are formed on the same silicon substrate, creating a space-charge region (a PN junction) at their interface with rectifying behavior. In HJT cells, both crystalline and amorphous silicon coexist; the intrinsic amorphous layer provides better passivation, which increases open-circuit voltage and conversion efficiency.
In a tandem stack, materials are arranged from smallest to largest bandgap from top to bottom: the perovskite layer on top absorbs short to mid wavelengths, while longer wavelengths pass through to the HJT bottom cell. Through optical and stacking design, the tandem can produce a much higher combined voltage. In terms of contribution to conversion efficiency, the HJT bottom cell can contribute around 25%–26%, while the perovskite top layer typically adds an incremental 3%–5%.
Because both perovskite and silicon HJT are PN-structured, directly connecting them in series can create a reversed PN junction at the interface, causing voltage cancellation and blocking current. A transition layer or tunneling junction is therefore required. The transition layer must itself meet several conditions: it must be conductive, highly transparent, and have an appropriate thickness to electrically connect the two subcells while allowing light transmission.
