Kesterite Cu2ZnSn(S,Se)4 solar cells fabricated from DMSO molecular solutions exhibit very different open circuit voltage (Voc) when tin precursor has different oxidation state (Sn2+ vs Sn4 ). Here, the grain...

The large open‐circuit voltage deficit (Voc,def) is the key issue that limits kesterite (Cu2ZnSn(S,Se)4, [CZTSSe]) solar cell performance. Substitution of Cu+ by larger ionic Ag+ ((Ag,Cu)2ZnSn(S,Se)4, [ACZTSSe]) is one strategy to reduce Cu–Zn disorder and improve kesterite Voc. However, the so far reported ACZTSSe solar cell has not demonstrated lower Voc,def than the world record device, indicating that some intrinsic defect properties cannot be mitigated using current approaches. Here, incorporation of Ag into kesterite through a dimethyl sulfoxide (DMSO) solution that can facilitate direct phase transformation grain growth and produce a uniform and less defective kesterite absorber is reported. The same coordination chemistry of Ag+ and Cu+ in the DMSO solution results in the same reaction path of ACZTSSe to CZTSSe, resulting in significant suppression of CuZn defects, its defect cluster [2CuZn + SnZn], and deep level defect CuSn. A champion device with an efficiency of 12.5% (active area efficiency 13.5% without antireflection coating) and a record low Voc,def (64.2% Shockley–Queisser limit) is achieved from ACZTSSe with 5% Ag content.

The limiting factor preventing kesterite (CZTSSe) thin film solar cell performance further improvement is the large open-circuit voltage deficit (Voc,def) issue, which is 0.345V for the current world record device with an efficiency of 12.6%. In this work, SnCl4 and SnCl2•2H2O are respectively used as tin precursor to investigate the Voc,def issue of dimethyl sulfoxide (DMSO) solution processed CZTSSe solar cells. Different complexations of tin compounds with thiourea and DMSO lead to different reaction pathways from solution to absorber material and thus dramatic difference in photovoltaic performance. The coordination of Sn 2+ with Tu leads to the formation of SnS and ZnS and Cu2S in the precursor film, which converted to selenides first and then fused to CZTSSe, resulting in poor film quality and device performance. The highest efficiency obtained from this film is 8.84% with a Voc,def of 0.391V. The coordination of Sn 4+with DMSO facilitates direct formation ofkesterite CZTS phase in the precursor film which directed converted to CZTSSe during selenization, resulting in compositional uniform absorber and high device performance. A device with active area efficiency 12.2% and a Voc,def of 0.344 V was achieved from Sn 4+ solution processed absorber. Furthermore, CZTSSe/CdS heterojunction heat treatment (JHT) significantly improved Sn 4+ device performance but had slightly negative effect on Sn 2+ device. A champion CZTSSe solar cell with a total area efficiency of 12.4% (active are efficiency 13.6%) and low Voc,def of 0.297 V was achieved from Sn 4+ solution. Our results demonstrate the preformed uniform kesterite phase enabled by Sn 4+ precursor is the key in achieving highly efficient kesterite absorber material. The lowest Voc-def and high efficiency achieved here shines new light on the future of kesterite solar cell.

The weak Sn–O coordination bonds in Sn(DMF)2Cl4 result in the formation of a kesterite phased (Cu2ZnSnS4) precursor film and thus fabrication of a highly efficient Cu2ZnSn(S,Se)4 absorber from DMF solution.