Highly efficient, ultrathin (~400 nm) pure sulfide kesterite Cu 2 ZnSnS 4 (CZTS) solar cells have been realized by interface reaction route controlling and self-organized nano-pattern at the back contact. The Al 2 O 3 intermediate layer introduced at the Mo/CZTS interface can effectively inhibit the detrimental interfacial reaction between CZTS and Mo in the initial stage of sulfurization, and then turns into a self-organized nanopattern yielding a nanoscale opening for electrical contact. With this interface modification, the traditional issues of phase segregation (secondary phases) and voids at the back contact region can be well addressed, which greatly improves uniformity and reduces back contact recombination. As a result, this interface modification not only leads to beyond 8% ultrathin CZTS solar cells but also yields two certificated world record efficiencies: 9.26% for 0.237 cm 2 small area and 7.61% for 1 cm 2 standard kesterite CZTS solar cells (normal thickness).

Back contact modification plays an important role in improving energy conversion efficiency of Cu2ZnSnS4 (CZTS) thin film solar cells. In this paper, an ultrathin carbon layer is introduced on molybdenum (Mo)-coated soda lime glass (SLG) prior to the deposition of CZTS precursor to improve the back contact and therefore enhance CZTS solar cell efficiency. By introducing this layer, the short circuit current (Jsc) and device conversion efficiency increase for both nonvacuum (sol-gel) and vacuum (sputtering) methods. Specifically, for the sol-gel based process, Jsc increases from 13.60 to 16.96 mA/cm(2) and efficiency from 4.47% to 5.52%, while for the sputtering based process, Jsc increases from 17.50 to 20.50 mA/cm(2) and efficiency from 4.10% to 5.20%. Furthermore, introduction of this layer does not lead to any deterioration of either open circuit voltage (Voc) or fill factor (FF).

record 9.5% efficiency pure sulfide CZTS solar cell, [4] but it is still much lower than the S-Q limit and its CuInGaSe 2 (CIGS) counterpart. It has been identified that the main reason for the efficiency limitation is the open-circuit voltage (V oc ) deficit (E g /q −V oc , where q is the elemental charge) as a result of band fluctuation and nonradiative recombination. [5] In addition, unfavorable conduction band alignment, i.e., cliff-like conduction band offset, at the interface of CZTS absorber/ CdS has been reported to lead to high recombination, contributing to the loss of V oc . [6] The majority of electron-hole pairs are generated near the CZTS/CdS interface, [7] so the passivation of the CZTS/ CdS interface is of crucial importance in achieving high-performance CZTS solar cells. In CIGS solar cells, Al 2 O 3 was employed for better surface passivation. [8] In these papers, both field effect passivation by fixed negative charge and chemical passivation play a role in passivating the CIGS surface, and decrease surface recombination velocity. The thickness of Al 2 O 3 used in these studies is few tens of nanometers which are not suitable for the heterojunction passivation as such a thick layer would impede carrier flow at the junction area.We employed an ultrathin atomic layer deposited (ALD)-Al 2 O 3 layer, which is believed to effectively reduce interfacial recombination by reacting with the surface defects. [8a] It is known that, in ALD process, the precursor molecule and the reactant which are trimethylaluminum (TMA, Al(CH 4 ) 3 ) and H 2 O respectively, [9,10] are the hydrogen sources needed for chemical passivation. The passivation mechanism of ALD-Al 2 O 3 in the silicon solar cells can be divided into two categories: i) field effect passivation induced by negative fixed charges [11] and ii) chemical passivation by hydrogen. [12] For field effect passivation, it has been reported that a critical thickness is required to ensure this field effect. [13] Based on the benefits of ALD-Al 2 O 3 , the kesterite group at IBM has recently reported the passivation of CZTSSe/CdS interface by a thin ALD-Al 2 O 3 layer (less than 1 nm) showing enhanced performance due to surface passivation effects. [14] However, the detailed passivation route is unknown. The effect of ultrathin (less than 1 nm) Al 2 O 3 on interface passivation is unclear as negative fixed charges may not be involved in the passivation effect.

The ultrathin SnO2 intermediate layer deposited by a successive ionic layer adsorption and reaction (SILAR) method was introduced into the heterointerface between p-type Cu2ZnSnS4 (CZTS) absorber and n-type CdS buffer for interface defect passivation in kesterite thin film solar cells. CZTS solar cells with SnO2 intermediate layers show higher open circuit voltage (V oc) of 657 mV and fill factor (FF) of 62.8%, compared to its counterpart cells without the SnO2 intermediate layer, which have V oc of 638 mV and FF of 52.4%, resulting in improvement in the overall efficiency from 6.82% to 8.47%. The mitigation of the V oc deficit and the improvement of FF are believed to result from the integrated effects of CZTS/CdS heterointerface passivation, shunt blocking, and band alignments. The passivation effect is further affirmed by the improved carrier lifetime. Furthermore, external quantum efficiency profiles show a strengthened blue optical response which is contributed by the decrease of CdS thickness. This work provides a new insight into the CdS/CZTS interface passivation, shunt blocking, and optimization of band alignments, based on solution-processed SnO2.

Three kesterite thin-film solar cells, Cu2ZnSnSe4 (CZTSe), Cu2ZnSn(S,Se)4 (CZTSSe), and Cu2ZnSnS4 (CZTS), and based on low light intensity measurements, examined the possibility of using kesterite devices for indoor applications.