Yuancai Gong, Hao Xin, , and Liming Ding

1Key Laboratory for Organic Electronics andInformation Displays, Instituteof AdvancedMaterials, NanjingUniversity of Posts& Telecommunications, Nanjing 210023, China

2Center for Excellence in Nanoscience (CAS), Key Laboratory of Nanosystem and Hierarchical Fabrication (CAS), National Center for Nanoscience and Technology, Beijing 100190, China

As containingearth-abundantelements,kesterite (Cu2Zn-Sn(S,Se)4, CZTSSe) semiconductors have great potential to be low-cost and environmental-friendlyinorganic absorbers.However, the record power conversion efficiency (PCE) for CZTSSe solar cells is only12.6%[1,2], much lowerthan that for Cu(In,Ga)Se2(CIGS) solar cells (23.35%)[3]. The key issue for kesterite solar cells is the largeopen-circuit voltage deficit(Voc,def, the gap betweenVocand Shockley-Queisser limitVocSQ) or smallVocgain (Voc/VocSQ). TheVoc/VocSQis higher than 85% for high-performance CIGS solar cells but only 61%for current world-record CZTSSe device[2]. Many factors may cause theVoclossof kesterite: (1) the narrow phase stability makes it difficult to achieve highly uniform absorber composition, whichcan resultinbandgap fluctuation andsecondary phases; (2) the similar ionic size of Cu and Zn leads to high concentration ofCu–Znantisite defects(Cu–Zn disorder), which may cause electrostatic potential fluctuation and band tailing; (3) themulti-element composition and the variable valence of Sn lead to complicated defect property, causing serious recombinationinabsorberbulkandinterfaces[4−8]. Identify the most critical one and its origin is crucial for further improving device efficiency.

Cu–Zn disorder was once blamed to account for band tailing and limits kesteriteVoc, which has now been disapprovedby experimentsand theoretical calculations[9,10]."Order–disorder" transition experiments by Reyet al. indicated thatthe order level of kesteritecanbeimproved through long-time annealing at low temperature, which increasesthe bandgap(Fig.1(a))[11]. Bourdaiset al. reported that theVocof kesterite was indeed improved upon longtimeannealing(Figs.1(b) and 1(c)), and indicated that Cu–Zn disorder is not directly responsible for theVocdeficit. They suggested that thedefect clusters of[2CuZn+SnZn] might cause band tailing and limit theVoc[12]. Comparing the photocurrent and photoluminescence spectra for the CZTS films prepared under Sn-rich and Sn-poor conditions, Maet al. confirmed the band tailing of kesterite indeed comes from the high concentrationof[2CuZn+SnZn] defect clusters,not Cu–Zn disorder (Fig. 1(d))[13]. At the same time, their results also showed thattheconcentration of[2CuZn+SnZn] defect clusteras well as deepdonor statesSnZnand[CuZn+SnZn]formed under Sn-rich condition can be sufficiently suppressedunder Sn-poor condition (Fig. 1(d)). Considering that the state-of-the-art kesterite absorber materials are made under slightly Sn-poorcondition, to whatextent the band tailing limits deviceVocneeds further investigation.

Recently,Xinet al. found CZTSSe solar cellsmade from DMSO solution exhibited very differentVocwhen Sn precursor had different oxidation state (Sn2+vsSn4+)[14]. The device from Sn4+solution gave much highVocthanthatfrom Sn2+solution, yielding a record lowVocdeficit (0.297 V,Voc/VocSQ=63.7%). Investigationonsolution chemistry showed that different chemicalreactions ofSn2+and Sn4+in precursorsolution resulted in different composition in precursor films. The coordination of Sn2+with thiourea (Tu) led toahighlycrystalline precursor film containing various sulfide phases (Cu2–xS,ZnS, SnS, and CZTS), while the coordination of Sn4+with DMSO resultedin auniform precursorfilm with kesterite(CZTS) structure. The study on grain growth showed that the secondary sulfides in Sn2+precursor film first converted to selenides (Cu2–xSe, ZnSe, SnSe2)and thentook amulti-step fusion reaction to form the absorber film with secondary phase SnSe2existing near film surface (Fig. 2(a)), giving a very defectivesurface, while Sn4+film took adirect phase transformation from CZTS to CZTSSe without secondary phases (Fig.2(b)), presenting a clean surface[15]. Characterizations showed that both absorber films presented similar bulk electronic property with comparable bandgap fluctuation (σg, Fig. 2(e)), electrostatic potentialfluctuation(γopt, Fig.2(f)),andband tailing(EU,Fig.2(g)). ThelowVocofSn2+absorber mainly resulted from interface recombination due to high defect content nearthe surface(Fig.2(h)).Furthermore,low-temperature thermal annealing of CZTSSe/CdS films similarly reducedσgandγopt(Fig. 2), and improved Cu–Zn order level in both Sn2+andSn4+absorbers (Fig.2(d)). The much higherimprovement inVocof Sn4+device than Sn2+device (Fig. 2(c)) further confirmed the interface recombination is the primary factor limitingVoc.

Fig. 1. (Color online)(a) “Order–disorder” transition of kesterite uponthermal annealing. Reproduced withpermission[11], Copyright 2014, AIP PublishingLLC. (b) J–V curvesfor CZTSSe devices withPD(partially disordered) andPO (partially ordered)absorber. (c)Extraction of the bandgaps for CZTSSeabsorbersin (b)fromtheabsorption spectra.Reproducedwith permission[12],Copyright2016,Wiley-VCH. (d) Illustrationof the bandtail anddonor defectstatesforCZTS absorbersmadeunder Sn-rich andSn-poorconditions. Reproduced with permission[13],Copyright 2019,American ChemicalSociety.

TheVocdeficit for kesterite solar cells was ascribed to the deep defects on/near the absorber surface due to SnSe2involvedin the graingrowth. This workdemonstrates that the grain growth accompanied with a direct phase transformation is crucial for obtaining high-quality kesterite films. This approachis expected tofurtherboost the PCEfor kesterite solar cellsviawell engineering CZTSSe/CdS interface.

Acknowledgements

This work was supported by the National Key Research and Development ProgramofChina (2019YFE0118100) and theNationalNaturalScience FoundationofChina (U1902218,22075150). L. Ding thanks the National Key Research and Development Program of China (2017YFA0206600) and the National Natural Science Foundation of China (51773045,21772030, 51922032, 21961160720) for financial support.