Abstract
Numerous efforts have been devoted to either understanding or fabricating efficient carrier-selective contacts, since they play a significantly crucial role in high-performance solar cells. In this work, we demonstrate that the asymmetry between electron conductivity and hole conductivity is important but far not enough for efficient carrier-selective contacts. Taking electron-selective contacts as examples, cell performances are greatly affected by the electron conductivity, the hole conductivity, and the work function of electron-selective materials, together with conduction and valance band offset between absorbers and electron-selective layers. When designing efficient electron-selective contacts, all these factors should be considered to mitigate carrier recombination and downward bending of the electron quasi-Fermi level within and around the contacts and to ensure low interfacial resistance for electrons. Although the absorber is crystalline silicon in the study, we believe that our understanding and design guidelines are also applicable to solar cells based on other absorbers.
Highlights
Carrier-selective contacts are one of the basic requirements for solar cells
Various materials have been explored as electron-selective layers in crystalline silicon (c-Si) solar cells by carefully controlling the physical parameters in a simulation software (AFORS-HET)
It is found that the ratio of σe to σh has a bigger influence on iVOC and VOC than the built-in electric field, suggesting stσhheao>tui1ltd0i−bs6emSs/oucrfmfeiccailetonVstelOylyCl)ar;eroglatehtee(derw.gtoi.,seσc,aetrh>reier1re0s−me8laeSyc/tecixmviistyta.taIennqouanid-lindbiertgiiuolimngi,bσolere downward bending in EF,e in the electron-selective contacts, resulting in the loss of VOC and fill factor (FF)
Summary
Carrier-selective contacts are one of the basic requirements for solar cells. They work to separate carriers generated in absorbers, realizing that electrons and holes are collected by two different terminals. Taking c-Si solar cells as an example, carrier-selective contacts can be realized by doping the c-Si substrate to form either a heavily p-type (p+) or heavily n-type (n+) thin layer at the surface, which is known for homojunction.. Carrier-selective contacts can be realized by externally overlying the c-Si substrate with a thin p+ or n+ polysilicon film and inserting an ultrathin silicon oxide between them.. Carrier-selective contacts can be realized by externally overlying the c-Si substrate with a thin p+ or n+ polysilicon film and inserting an ultrathin silicon oxide between them.2 Such a structure is demonstrated to have excellent carrier selectivity (especially for electron-selective contact) and high level of surface passivation. Heavily doped and hydrogenated amorphous silicon (a-Si:H) films, combining with intrinsic a-Si:H films, are widely employed as carrier-selective layers in c-Si solar cells and are proved to highly effective to realize high cell performance.
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