Ultra‐thin electron collectors based on nc‐Si:H for high‐efficiency silicon heterojunction solar cells

Abstract

Low parasitic absorption and high conductivity enable (n)-type hydrogenated nanocrystalline silicon [(n)nc-Si:H], eventually alloyed with oxygen [(n)nc-SiO x :H], to be deployed as window layer in high-efficiency silicon heterojunction (SHJ) solar cells. Besides the appropriate opto-electrical properties of these nanocrystalline films, reduction of their thickness is sought for minimizing parasitic absorption losses. Many strategies proposed so far reveal practical limits of the minimum (n)-layer thickness that we address and overcome in this manuscript. We demonstrated the successful application of an ultra-thin layer of only 3-nm-thick based on (n)nc-Si:H PECVD plasma growth conditions without the use of additional contact or buffer layers. For simplicity, we still name (n)nc-Si:H this ultra-thin layer and the solar cell endowed with it delivers a certified efficiency η of 22.20%. This cell shows a 0.61 mA/cm 2 overall J SC gain over the (n)a-Si:H counterpart mainly owing to the higher transparency of (n)nc-Si:H, while maintaining comparable V OC > 714 mV and FF > 80%. Our optimized (n)nc-Si:H layer yields low absorption losses that are commonly measured for (n)nc-SiO x :H films. In this way, we are able to avoid the detrimental effect that oxygen incorporation has on the electrical parameters of these functional layers. Further, by applying a MgF 2 /ITO double-layer anti-reflection coating, a cell with 3-nm-thick (n)nc-Si:H exhibits a J SC,EQE up to 40.0 mA/cm 2 . By means of EDX elemental mapping, we additionally identified the (n)nc-Si:H/ITO interface as critical for electron transport due to unexpected oxidation. To avoid this interfacial oxidation, insertion of a 2-nm-thick (n)a-Si:H on the 3-nm-thick (n)nc-Si:H contributes to FF gains of 1.4% abs. (FF increased from 78.6% to 80.0%), and showing further room for improvements. K E Y W O R D S(n)-type window layers, hydrogenated nanocrystalline silicon, hydrogenated nanocrystalline silicon oxide, opto-electrical properties, ultra-thin (n)-contact

Effects of (i)a‐Si:H deposition temperature on high‐efficiency silicon heterojunction solar cells

“…FF gain was reported by applying the HPTs before the n-layer deposition, which improved mainly the V OC of solar cells. 31 Therefore, as expected, for efficient collections of charge carriers, it is crucial to have (i)a-Si:H layers fostering the passivation quality. Nevertheless, pcontact is particularly more sensitive to the bulk quality of (i)a-Si:H layers than n-contact.…”

“…15 The detailed deposition conditions of doped layers can be found in our previous studies. 30,31 Different glass and n-type c-Si samples, as schematically illustrated in Figure 1, were prepared to evaluate the effects of the deposition temperature and HPTs on (i)a-Si:H layers and their further applications in SHJ solar cells. The abbreviations, stack descriptions and temperatures for layers and HPTs investigated in this study are given in Table 2.…”

“…The VHF treatment consists of highly H‐diluted SiH 4 plasma, which mainly modifies the microstructure of ( i )a‐Si:H layers rather than depositing ( i )nc‐Si:H, as no change in the thickness of ( i )a‐Si:H was detected via spectroscopic ellipsometry (SE) after 6 min of this VHF treatment 15 . The detailed deposition conditions of doped layers can be found in our previous studies 30,31 …”

“…The results are summarized in Figure 5. Note that the effects of HPTs on solar cells were discussed in our previous publications when optimizing either nor p-type nc-Si:H-based contact stack with (i)a-Si:H deposited at 180 C. 15,30,31 In those studies, we found that solar cell performances were improved after applying the HPTs mainly due to the H-rich and porous (i)a-Si:H layers formed after the HPTs (see Figure 3), where the H can largely suppress the surface recombination (see Figure 4A).…”

Silicon solar cells with passivating contacts: Classification and performance

“…More details on the intricacies of each of these passivating contact technologies is discussed extensively in other articles within this special issue on passivating contacts based silicon solar cells. For example, Zhao et al present strategies to improve the performance of SHJ solar cells 12 ; Glunz et al present an overview of the historical development of poly‐Si passivating contacts and their current status in both industry and research laboratories 5 ; Singh et al and Nandakumar et al focus on the industrial aspects of poly‐Si passivating contacts, 13,14 ; and Ibarra Michel et al review metal compound‐based passivating contacts 15…”