Low band gap liquid-processed CZTSe solar cell with 10.1% efﬁciency Santanu Bag, Oki Gunawan, Tayfun Gokmen, Yu Zhu, Teodor K. Todorov and David B. Mitzi * Received 7th January 2012, Accepted 23rd February 2012 DOI: 10.1039/c2ee00056c A low band gap liquid-processed Cu 2 ZnSn(Se 1x S x ) 4 (CZTSSe) kesterite solar cell with x z 0.03 is prepared from earth abundant metals, yielding 10.1% power conversion efﬁciency. This champion cell shows a band gap of 1.04 eV, higher minority-carrier lifetime, lower series resistance and lower Voc deﬁcit compared to our previously reported higher band gap (E g ¼ 1.15 eV; x z 0.4) cell with similar record efﬁciency. The ability to vary the CZTSSe band gap using sulfur content (i.e., varying x) facilitates the examination of factors limiting performance in the current generation of CZTSSe devices, as part of the thrust to achieve operational parity with CdTe and Cu(In,Ga)(S,Se) 2 (CIGSSe) analogs. Introduction Chalcogenide-based thin ﬁlm solar cells are expected to form the foundation of next generation photovoltaic (PV) technology. 1 Research in this ﬁeld has increased signiﬁcantly due to the promise of this technology to provide cleaner energy at a cost that is competitive with fossil fuels. Among these technologies, Cu(In,Ga)(S,Se) 2 (CIGSSe) and CdTe solar cells are already in production with record cell efﬁciencies of 20.3% and 17.3%, 2,3 respectively, while their commercial modules correspondingly reach efﬁciencies of as high as 15.7% and 13.5%. 3,4 However, the use of toxic Cd and the nonabundant and/or expensive elements In, Ga and Te already presents a concern with respect to meeting the multi-terawatt-scale production needed in order to satisfy global energy demand. 5,6 Recently, kesterite-structured copper zinc tin chalcogenide (sulﬁde/selenide) materials (collectively labeled CZTSSe) have emerged as a potential alternative to CIGSSe because of more earth abundant and less expensive constituents. The ﬁrst repor- ted CZTS (x ¼ 1) solar cell was vacuum deposited with power conversion efﬁciency of 0.66%. 7 By 2008, optimization of depo- sition conditions and annealing parameters led to efﬁciencies of as high as 6.8% for vacuum-based CZTS. 8 More recently, 9.7% (10.1% after optimization) 9,10 power conversion efﬁciency has been achieved for a mixed S and Se system using a hydrazine- based processing approach, which involves a combination of solution- and nanoparticle-based deposition routes. A 7.2% power conversion efﬁciency has also been demonstrated with devices prepared using a colloidal nanocrystal-based approach. 11 These early-stage development results point towards a substan- tial promise with respect to commercialization potential for CZTSSe, if efﬁciency can continue to be pushed beyond 10%. Control over band gap in chalcogenide-based solar cells is important in terms of tailoring the properties of the absorber layer to match the incident solar radiation spectrum and to otherwise optimize the device efﬁciency. 12–14 Like for the CIGSSe material, the band gap in CZTSSe can be tailored using the S:Se ratio. 15,16 A recent ﬁrst-principle calculation report shows that IBM T. J. Watson Research Center, 1101 Kitchawan Rd, Yorktown Heights, NY, 10598, USA. E-mail: dmitzi@us.ibm.com; Tel: (+1) 914-945-4176 Broader context Given the remarkable potential of photovoltaic (PV) technology to displace non-renewable electricity generation based on fossil fuels if associated manufacturing costs can be made competitive, there is a tremendous need for low-cost, environmentally benign materials and fabrication processes for preparing high-performance solar cells. In this regard, thin-ﬁlm heterojunction PV devices are particularly interesting because of their reduced material utilization and simple cell design. In thin-ﬁlm devices, a wide range of semiconducting materials can be used as the solar absorber, among which the kesterite Cu 2 ZnSn(Se 1x S x ) 4 (CZTSSe) family has been getting signiﬁcant recent attention due to its earth-abundant and less toxic constituents. To date, the highest efﬁciency CZTSSe solar cell is made by a hybrid solution-particle approach, where a ﬁnal heat treatment is done in a sulfur environment to ﬁne tune the band gap. Although the [S]:[Se] ratio in CZTSSe can be used to control the band gap and to optimize device performance, in practice, reproducibly controlling the [S]:[Se] ratio using a sulfur-enhanced heat treatment presents challenges because of the high volatility of sulfur. In this manuscript, we report a simpliﬁed approach to making a record 10.1% efﬁciency low band gap CZTSe solar cell, without any treatment in a sulfur rich atmosphere. This journal is ª The Royal Society of Chemistry 2012 Energy Environ. Sci. Dynamic Article Links C < Energy & Environmental Science Cite this: DOI: 10.1039/c2ee00056c www.rsc.org/ees PAPER Downloaded by Northwestern University on 07 March 2012 Published on 24 February 2012 on http://pubs.rsc.org | doi:10.1039/C2EE00056C View Online / Journal Homepage