Loss mechanisms inﬂuence on Cu 2 ZnSnS 4 /CdS-based thin ﬁlm solar cell performance Maykel Courel ⇑ , J.A. Andrade-Arvizu, O. Vigil-Galán Escuela Superior de Física y Matemáticas – Instituto Politécnico Nacional (IPN), C.P. 07738 México DF, Mexico article info Article history: Received 16 January 2015 Received in revised form 12 May 2015 Accepted 29 May 2015 Keywords: CZTS thin ﬁlms Solar cell modeling Loss mechanisms Trap-assisted tunneling CdS/CZTS interface recombination abstract One of the most important issues in kesterite Cu 2 ZnSnS 4 (CZTS)-based thin ﬁlm solar cells is low open circuit voltage, which is mainly related to loss mechanisms that take place in both CZTS bulk material and CdS/CZTS interface. A device model for CZTS/CdS solar cell which takes into account loss mechanisms inﬂuence on solar cell performance is presented. The simulation results showed that our model is able to reproduce experimental observations reported for CZTS/CdS-based solar cells with the highest conversion efﬁciencies, measured under room temperature and AM1.5 intensity. The comparison of simulation results to experimental observations demonstrated that among the different loss mechanisms, trap-assisted tunneling losses are the major hurdle to boost open circuit voltage. Under this loss mech- anism, a solar cell efﬁciency enhancement up to 10.2% with CdS donor concentration decrease was reached. Finally, the possible path toward a further solar cell efﬁciency improvement is discussed. Ó 2015 Elsevier Ltd. All rights reserved. 1. Introduction Kesterite Cu 2 ZnSnS 4 (CZTS) is a promising material for thin ﬁlm solar cell applications since it is based on abundant and low-toxic elements. Besides, its high absorption coefﬁcient (>10 4 cm 1 ) and band gap close to the optimal single-junction value (1.5 eV) make it attractive for replacing CdTe and Cu(In,Ga)(S,Se) 2 (CIGSSe) solar cell production. Although a CZTS-based solar cell efﬁciency of 8.4% [1] was reached, this number is still far behind the 20.8% [2] value that was achieved by analogous Cu(In,Ga)(S,Se) 2 (CIGSSe) solar cells. Among the device performance parameters, the general consensus is that low open circuit voltage (V oc ) in kesterite-based solar cells is the main limiting factor of this technology for reach- ing high solar cell efﬁciency [3–7]. The so-called V oc deﬁcit has been mainly attributed to aspects related to the material proper- ties, such as bulk defects, secondary phases formation and to inter- face optimization; however, a more detailed study related to the inﬂuence of the device loss mechanisms on the solar cell electrical parameters, including the V oc has not been carried out. In particular, there are some loss mechanisms that can take place in CZTS thin ﬁlm solar cells, with a fundamental role on V oc , such as majority carrier diffusion and/or thermionic emission, minority carrier radiative and non-radiative recombination, CdS/CZTS interface recombination and trap-assisted tunneling recombination. So far, the inﬂuence of each loss mechanism on CZTS solar cells performance has not been studied. Even more, the principal limiting factor for reaching high V oc values has not been clariﬁed yet. It is well known that CZTS thin ﬁlms are charac- terized by a high density of defect states which may introduce non-radiative recombination centers [8,9], dominating solar cell output. Taking into account that in certain regions of solar cells, such as space charge regions, large electric ﬁelds exist, the quan- tum tunneling of charge carriers from energy bands to traps and the reverse process may enclose an essential part of the whole cap- ture and emission mechanism. Thus, non-radiative recombination losses could be enhanced by tunneling processes. Nonetheless, up to now, the possible trap-assisted tunneling recombination effect on CZTS solar cell performance has not been taken into account. On the other hand, CdS/CZTS interface has been described by a cliff-like conﬁguration, which contributes to carrier losses [4,6,10]. Despite it is commonly assumed that CZTS non-radiative recombination and CZTS/CdS interface recombination are the most important loss mechanisms in this technology, a complete under- standing of its contributions on V oc is still unclear. Therefore, there is a need to understand each loss mechanism inﬂuence on CZTS-based solar cell performance which could help to improve solar cell efﬁciency. In this work, a device model for CZTS solar cell is presented where experimental observations (Quantum efﬁciency, Efﬁciency, J sc , FF and V oc ) reported for two solar cells with the highest conver- sion efﬁciencies are reproduced. A theoretical study of the most http://dx.doi.org/10.1016/j.sse.2015.05.038 0038-1101/Ó 2015 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail address: maykelcourel@gmail.com (M. Courel). Solid-State Electronics 111 (2015) 243–250 Contents lists available at ScienceDirect Solid-State Electronics journal homepage: www.elsevier.com/locate/sse