Eco-Friendly AgBiS2 Nanocrystal / ZnO Nanowire Heterojunction Solar Cells with Enhanced Carrier Collection
Authors: Xiao Yun , Wang Haibin , Shibayama Naoyuki , Kubo Takaya , Segawa Hiroshi 
Published: December 2020

Solution-process compatibility and size-dependent bandgap tunability of colloidal quantum dots (CQDs) make them low-cost promising materials for next generation solar cells. Among CQD-based solar cells, heterojunction devices formed with PbS CQDs and ZnO exhibit impressive solar cell performance. However, the hazardous impact of Pb is causing concern. In addition, researches on CQD-based solar cells fabricated using non-toxic and abundant elements are still very limited. Therefore, eco-friendly CQDs for solar cell have drawn much attention. AgBiS2 colloidal nanocrystals with a wide absorption band (0.4-1.2 μm) are one of the alternatives to PbS CQDs. However, the power conversion efficiencies of planar-type heterojunction solar cells using AgBiS2 nanocrystals (NCs) (ITO / ZnO dense layer / AgBiS2 NC layer/ hole transport layers (eg. PTB7:MoOx) / Ag) have been at most approximately 6% mainly due to the short carrier diffusion length of AgBiS2
[2].
In this work, thus, we paid our attention to nanowire (NW) structures, with the aim of elongation of carrier diffusion length, and also applied widely-used and less-expensive hole transport materials. We then fabricated NW-type solar cells (ITO / ZnO NW: AgBiS2 mixture / AgBiS2 overlayer / P3HT / Au) by a dip-coating method.
We independently tailored ZnO nanowire length and the active device layer thickness to study charge collection in AgBiS2 NW-type solar cell. ZnO NWs were synthesized by a hydrothermal method, and the length of ZnO NW was controlled by the reaction time [3]. As a reference, planar-type solar cells using dense ZnO layers with different thicknesses (NPSC) were also constructed.

Jsc
values of NPSCs increase as AgBiS2 layer thickness becomes thicker and reach to a maximum value of 12 mA/cm2 at 100 nm. And Jsc
then decreases with thickening AgBiS2 layers due to the limited carrier diffusion length of AgBiS2 layer. Whereas in the case of Jsc
of NWSCs, the highest Jsc of 20.51 mA / cm2 can be obtained at 200 nm-ZnO NWs. This Jsc
value is higher than previously-reported AgBiS2 solar cells (eg. JSC = 15.10 mA/cm2, VOC = 0.46 V, FF = 57%, and PCE = 3.99% [1]), and correspoinding PCE reached 3.97%, approximately doubled the value of planar cell (PCE = 2.06%).
Figure 1 shows the EQE spectra obtained with the best performing NWSC and NPSC. It can be seen that NWSC gives high EQE in almost all the spectral region studied. The ZnO NW / AgBiS2 heterojunction enabled achievement of more efficient charge carrier transportation than a planar ZnO NP / AgBiS2 cell. The cell gives a maximum EQE of 80% and shows a wide photocurrent response range of 300-1200 nm. In particular, the EQE is greatly enhanced in the near-infrared region from 35.6% (800 nm) in NPSC to 60.0% in NWSC, indicating improved carrier collection in near-infrared region for NWSC. NWSC also shows higher power conversion efficiency than NPSC because of the higher Jsc
of NWSC (inset table in Fig. 1). The length of ZnO NW dependence of NWSC performance was also studied to get a better understanding of diffusion mechanism in NWSC. The structure of NWSCs was optimized by varying nanowire length and active overlayer thickness, carrier transport mechanisms related to the AgBiS2 ZnO-NW blend region and planar AgBiS2 overlayer region were clarified. The solar cell with the 320 nm-thick active layer (200 nm-NW region plus 120 nm-overlayer region) gave the best performance.
These results show that ZnO nanowire / AgBiS2 nanocrystal mixtures are useful to achieve more efficient carrier collection. Long-term stability is equally important; at least 3-month air stabilty has been confirmed in NWSCs.
[1] M. Bernechea, N. C. Miller, G. Xercavins, et al. Nature Photonics 2016, 10, 521.
[2] H. Wang, T. Kubo, J. Nakazaki, T. Kinoshita, H. Segawa. J. Phys. Chem. Lett. 2013, 4, 2455.
[3] H. Wang, V. Gonzalez-Pedro, Kubo, T.; F. Fabregat-Santiago, J. Bisquert, Y. Sanehira, J. Nakazaki, H. Segawa. The Journal of Physical Chemistry C 2015, 49, 27265.

Figure 1