(Keynote) High Efficiency Heterojunction c-Si Based Solar Cell and Module
Authors: Yamamoto Kenji , Konishi Katsunori , Adachi Daisuke 
Published: December 2020

1. INTRODUCTION

High-efficiency heterojunction (HJ) back-contact crystalline Si (c-Si) solar cells with record-breaking conversion efficiencies are reported.
We have so far focused our attention on the HJ technology using amorphous Si (a-Si) passivation layers, and we reported the fabrication of a large-area both-side-contact HJ c-Si solar cell with a record-breaking cell conversion efficiency of 25.1% in 2015 [1]. Using our HJ technology, we have achieved a record-breaking cell conversion efficiency of 26.7%, and a module conversion efficiency of 24.5% in c-Si solar cells [2].
Recent improvements of perovskite solar cells and c-Si solar cells have opened a path to realizing conversion efficiencies of over 30% by using a tandem structure solar cell. Recently, a 29.15%-efficiency perovskite-silicon tandem solar cell was reported [3].
In this paper, we report our state-of-the-art technologies for high-efficiency HJ c-Si solar cells and present perspectives for further improvement of conversion efficiency using tandem solar cells with high-efficiency HJ c-Si and perovskite solar cells based on our recent results.

2. HJBC solar cell

Heterojunction back-contact c-Si solar cells (HJBC solar cells) are fabricated using n-type CZ crystalline Si wafers. Figure 1 shows appearance and a typical schematic image of a cross section and of the HJBC solar cell.
To improve the conversion efficiency of the HJBC solar cell, we have improved our HJ technology with high-quality a-Si and a low-resistance electrode technology for a back-contact structure. By combining and optimizing the technologies, we developed a HJBC solar cell with a conversion efficiency of 26.3% in 2016, which was a record-breaking efficiency for Si solar cells [5].
In our preliminary experiments of the development of passivation technology, we have confirmed that further improvement of minority carrier lifetime is possible, which reduces the extrinsic recombination loss in the conversion efficiency by an absolute value of ~0.7% from ~1.2%. Small reductions in series resistance loss and optical loss are also possible by a small optimization. Therefore, it is reasonable to expect that a conversion efficiency of 27.1% is achievable by using our state-of-the-art technology [5]. Targeting a conversion efficiency of over 27%, we have improved our HJBC technologies. Now, we reach a conversion efficiency of 26.7% [2].
Besides the cell, using high-efficiency HJBC solar cells and newly developed technologies to minimize the resistance loss of intercell connection in the module and to raise the collection efficiency of the light radiated into the module, we achieved a conversion efficiency of 24.5% efficiency in a module with a size of a building-integrated photovoltaics (BIPV) product [2].

3. Perovskite/c-Si tandem solar cell

One of the low-cost solar cell candidates with efficiencies higher than the theoretical limit of Si is a combination of a perovskite solar cell and a c-Si solar cell. An organic-inorganic hybrid perovskite solar cell has rapidly progressed, and its conversion efficiency has reached 25.2% [3]. A perovskite solar cell is highly attractive as a top cell of a multijunction solar cell owing to its wide- and tunable-bandgap absorber. A record efficiency of 29.15% is reported for a tandem device comprising a perovskite component cell on a Si solar cell [3].
We have also developed perovskite solar cells and examined their multijunction structures with two-terminal and three-terminal tandem as shown in Fig. 2. Our derailed results can be found in ref 6-8. We believe that the improvement of the processes and materials of perovskite solar cells will lead to conversion efficiencies of over 30%.

Acknowledgments

The authors would like to thank Dr. Rudi Santbergen of Delft University of Technology for performing optical simulations of perovskite/c-Si tandem solar cells. This work was supported in part by the New Energy and Industrial Technology Development Organization (NEDO) under the Ministry of Economy, Trade and Industry of Japan.

Reference

D.Adachi et al., Appl. Phys. Lett. 107, 23350 (2015).

K. Yamamoto et al., Jpn. J. Appl. Phys 57, 08RB20 (2018).

https://www.helmholtz-berlin.de/pubbin/news_seite?nid=21020;sprache=en;seitenid=73236

M A Green et al., Prog Photovolt Res Appl. 28, 3 (2020).

Y, Yoshikawa et al., Nat. Energy 2, 17032 (2017).

R. Mishima et al. Appl. Phys. Express 10, 062301 (2017).

R. Santbergen et al., Optics Express. 24, A1288 (2016).

R. Santbergen et al., IEEE J. Photovolt. 9, 446 (2019).

Figure 1