Date Published: December 04, 2017
Publisher: John Wiley and Sons Inc.
Author(s): Pingqi Gao, Zhenhai Yang, Jian He, Jing Yu, Peipei Liu, Juye Zhu, Ziyi Ge, Jichun Ye.
By combining the most successful heterojunctions (HJ) with interdigitated back contacts, crystalline silicon (c‐Si) solar cells (SCs) have recently demonstrated a record efficiency of 26.6%. However, such SCs still introduce optical/electrical losses and technological issues due to parasitic absorption/Auger recombination inherent to the doped films and the complex process of integrating discrete p+‐ and n+‐HJ contacts. These issues have motivated the search for alternative new functional materials and simplified deposition technologies, whereby carrier‐selective contacts (CSCs) can be formed directly with c‐Si substrates, and thereafter form IBC cells, via a dopant‐free method. Screening and modifying CSC materials in a wider context is beneficial for building dopant‐free HJ contacts with better performance, shedding new light on the relatively mature Si photovoltaic field. In this review, a significant number of achievements in two representative dopant‐free hole‐selective CSCs, i.e., poly(3,4‐ethylene dioxythiophene):poly(styrenesulfonate)/Si and transition metal oxides/Si, have been systemically presented and surveyed. The focus herein is on the latest advances in hole‐selective materials modification, interfacial passivation, contact resistivity, light‐trapping structure and device architecture design, etc. By analyzing the structure–property relationships of hole‐selective materials and assessing their electrical transport properties, promising functional materials as well as important design concepts for such CSCs toward high‐performance SCs have been highlighted.
In the last few decades, the photovoltaic (PV) market has been dominated by crystalline silicon (c‐Si) solar cells (SCs) due to their overwhelming advantages in efficiency, cost, stability, and security. Recently, a record efficiency of 26.6% has been achieved combining the most successful heterojunctions (HJs) with interdigitated back contacts (IBCs).1 The IBC design is straightforwardly beneficial to light harvesting and passivation at the front side of the cell in comparison with the traditional double‐sided junctions. Another main reason for this efficiency progress can be ascribed to the ‘passivating contact’ or ‘carrier‐selective contacts’ (CSCs) technology enabled by HJs, which use intrinsic amorphous silicon (a‐Si) to passivate the Si surface and doped a‐Si to delivery hole/electron‐selective transport. Relying on the nearly ideal asymmetric band offset design at the CSC regions, the photoexcited electrons and holes can be efficiently separated and collected toward the opposite electrodes, and so, the devices with the HJ‐IBC design have extremely high open‐circuit voltages (Voc) and efficiencies. However, such SCs still introduce optical/electrical losses and technological issues due to parasitic absorption/Auger recombination inherent to the doped a‐Si films and complex processing in integrating discrete p+ and n+ HJ contacts to the back side of cell.2 These issues have motivated research into searching for alternative new functional materials as well as simplified deposition technologies whereby CSCs with c‐Si substrates, and thereafter IBC cells, can be formed directly via a dopant‐free manner.
Initial research on the PEDOT:PSS/Si HSCs came from Jabbour and co‐workers who used an organic PEDOT:PSS layer to replace the boron‐doped amorphous silicon carbide (a‐SiC:H) layer in microcrystalline silicon SCs and achieved a relatively high Voc of 0.88 V.21 Though the incipient efficiency (≈2.1%) was not satisfactory in comparison with traditionally diffused c‐Si homojunction SCs, the potential development of this kind of SCs still attracts wide attention due to their dopant‐free concept as well as their vacuum‐free, low‐temperature and solution‐based fabrication procedures. The concept was then transferred to n‐type c‐Si (n‐Si) substrates. The device configuration of PEDOT:PSS/n‐Si HSCs and the chemical structures of PEDOT and PSS are presented in Figure1a,b.22, 23 The device fabrication can be implemented by spin‐coating PEDOT:PSS on top of the n‐Si substrate and then depositing the front and rear electrodes. In this kind of CSC, the HTL layer of PEDOT:PSS plays multiple roles, including antireflection coating layer (ARC), charge extraction layer, electron‐blocking/hole‐transporting layer, surface passivation layer, etc. Therefore, the PEDOT:PSS layer shall satisfy the all‐around requirements in conductivity and WF as well as the contact property with the Si surface in order to achieve high‐performance SCs. Through extensive optimization of the photoelectric property of the PEDOT:PSS film, PEDOT:PSS/n‐Si hetero‐interface and n‐Si/rear electrode contact, the HSCs have achieved an efficiency over 16%.24, 25
HSCs featuring TMOs/n‐Si contacts have received great attention due to their advantages of low opto‐electrical losses and a high efficiency potential. At present, 22.5% efficiency has been achieved for the HSCs with an MoOx/a‐Si:H(i)/n‐Si/a‐Si:H(i)/a‐Si:H(n) core structure.18 In this review, we focus particularly on three typical TMOs (namely, MoOx, VOx, and WOx), in which the material characteristics including WF, conductivity, oxygen vacancy defects, bandgap, etc., and their effects on the performances of TMOs/n‐Si HSCs are thoroughly investigated.80, 81, 82, 83, 84, 85, 86, 87, 88, 89 In addition, the interfacial passivation quality of TMOs/Si is also studied. Although the transport mechanism of this kind of HSC is not clear completely, we attempt to provide a possible explanation of the carrier transport and hope the results in this section can help guide the development of high efficiency TMOs/n‐Si HSCs.
In this review, organic PEDOT:PSS and TMOs (i.e., MoOx, V2Ox, and WOx) have been fully investigated as promising candidates for efficient HTLs to construct dopant‐free and high efficiency Si‐based HSCs. Specifically, progress in research and innovations toward the enhanced performance of PEDOT:PSS/n‐Si HSCs has been comprehensively summarized, including transport mechanisms, PEDOT:PSS modification, work function tuning, advanced design for light‐trapping, interfacial passivation, and improvement of stability. TMOs were also seriously exploited by improving their work function, conductivity, interface passivation quality, and contact resistivity. Moreover, TMOs have been integrated into similar HJT and IBC HSCs, realizing the aim of dopant‐free fabrication methods and high efficiency. Although the current efficiencies of PEDOT:PSS/Si and TMOs/Si HSCs cannot contend with traditional high efficiency doped SCs, researchers continue to devote effort to the promising research field of dopant‐free heterocontacts. We hope that the presentation of the advanced methods and materials in this review may help to inspire a series of breakthroughs to further improve the efficiency of these devices and accelerate their practical applications.
The authors declare no conflict of interest.