Understanding and design of efficient carrier-selective contacts for solar cells

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.

Perovskite‐Based Electron‐Selective Contacts for Silicon Solar Cells

“…The ZnTiO 3 film has a negligible ΔE C (0.07 eV) and a large ΔE V (2.19 eV) to c-Si, which is beneficial for the electrons' flow but blocks holes on the contact interface. According to our previous study, [2] the low WF, large E g , small ΔE C , as well as large ΔE V all suggest that ZnTiO 3 may be a good electron-selective contact for c-Si solar cells. The contact resistivity (ρ c ) of the n-type c-Si/SiO 2 /ZnTiO 3 /Al structure was extracted by current-voltage (I-V ) measurements according to the transfer length method (TLM), as illustrated schematically in Figure 2a.…”

“…The ZnTiO 3 film has a negligible Δ E C (0.07 eV) and a large Δ E V (2.19 eV) to c‐Si, which is beneficial for the electrons’ flow but blocks holes on the contact interface. According to our previous study, [ 2 ] the low WF, large E g , small Δ E C , as well as large Δ E V all suggest that ZnTiO 3 may be a good electron‐selective contact for c‐Si solar cells.…”

“…It is believed that the large asymmetry between electron and hole conductivity, together with a suitable band offset, is the key to realize efficient carrier‐selective contacts. [ 1,2 ] To date, all of the mainstream commercial crystalline silicon (c‐Si) solar cells, that is, passivated emitter and rear cell (PERC) cells, [ 3 ] tunnel oxide passivating contact (TPOCon) cells, [ 4 ] and heterojunction with intrinsic thin layer (HIT) cells, [ 5 ] utilize either heavily doped c‐Si films, poly‐Si films, or amorphous Si films as carrier‐selective contacts. Although they are efficient, any heavily doped Si suffers from parasitic optical absorption, due to their narrow bandgaps (<2 eV).…”

“…Furthermore, wide-bandgap materials are also beneficial in realizing the large asymmetry between electron and hole conductivity, thus realizing superior charge carrier selectivity. [2] Up to now, a lot of metal compounds, such as metal oxides, [10][11][12][13] fluorides, [14][15][16] nitrides, and oxynitride, [17][18][19] have been reported as efficient electron transport layers for c-Si solar cells. They typically have a small conduction band offset (ΔE C ) and large valance band offset (ΔE V ) with c-Si.…”

“…It has a low work function (4 eV), a small conduction band offset, and a large valance band offset with c-Si. Therefore, an ohmic contact with extremely low contact resistivity (<10 mΩ cm 2 ) is achieved in the contact of c-Si/ZnTiO 3 /Al. As a result, a power conversion efficiency over 20% is demonstrated in the c-Si solar cell featuring ZnTiO 3 /Al as electron-selective contacts and without advanced passivation technique, which is significantly improved compared with its counterpart (without ZnTiO 3 interlayer).…”