Improved photovoltaic performance of CdTe-based solar cells: Roles of using a hole-blocking layer and nanoscale imaging of barrier height at interfaces
Graphical abstract
Introduction
The development of sustainable, secure, and competitive energy sources is the topmost priority due to the world’s increasing energy demand and forthcoming severe climate change. The thin film photovoltaic (PV) technology based on Si (Battaglia et al., 2011), Cu(In, Ga)Se2 (Jackson et al., 2011), Cu2ZnSn(S, Se)4 (Todorov et al., 2013), CdTe (Kranz et al., 2013, Wu, 2004), CdSe (Abolghasemi et al., 2020), dye-sensitized (Yum et al., 2012), and organic materials (Shrotriya, 2009) have made significant contributions to renewable energy sources. Cadmium telluride (CdTe) is an alternative to other materials for use in thin films solar cells. Thin film CdTe-based solar cells are one of the leading contenders in terms of cost and environmental issues (Antonio, 2011, Fthenakis, 2004, Mariska de Wild-Scholten, 2011, Raugei et al., 2007). For instance, heterojunction solar cells of p-type CdTe and n-type CdS are extensively employed because of their suitable band alignments (Paudel et al., 2013). In general, a p-n junction is needed to fabricate a solar cell where the p-type material works as an absorber layer and the n-type material generates an electric field at the junction which can be used to separate out electron-hole pairs (de Moure-Flores et al., 2012, Wei and Zhang, 2002).
Recently, First Solar has announced a record 22.1% power conversion efficiency with the help of CdCl2 vapour treatment while theoretically it has 30% efficiency (Bosio et al., 2020). However, CdCl2 is completely soluble in water and thus, faces environmental issues at the end of their lifetime (Kumar and Rao, 2014). It may be mentioned that for a CdTe-based solar cell, it is difficult to achieve a high efficiency without a CdS layer because of short minority carrier lifetime, grain boundaries, impurity diffusion, interface passivation effects, high electron affinity of CdTe, and recombination of structural defects (Duenow et al., 2014). For instance, Crisp et al. have shown that the cell efficiency of the CdTe-based solar cell is below 5% without a CdS layer which is far below the maximum achieved efficiency of the same with a CdS layer (Crisp et al., 2014).
In the present scenario, we have studied the advantage of using a carrier-selective junction to decrease the recombination rate of photo-generated charge carriers (Singh et al., 2017) which should lead to a higher power conversion efficiency (PCE) of a PV cell. Recently, Battaglia et al. have reported an enhancement in the power conversion efficiency of Si solar cells using a MoO3 carrier selective layer (Battaglia et al., 2014). In fact, they have demonstrated that an MoO3 thin film shows the ability to selectively block the flow of one type of charge carriers via valence or conduction band barriers which depends on the band-bending (Battaglia et al., 2014). Due to a mismatch in the band gaps, band-bending at the interface of MoO3 and its neighbouring absorber layer takes place which in turn blocks both the charge carriers across the junction (Singh et al., 2020, Singh et al., 2017). Keeping this in mind and the fact that the fabrication of a hole-blocking CdTe-based solar cell is still lacking, we have used an ultrathin MoO3 film on the absorber layer to fabricate solar cells of this kind (Singh et al., 2020).
It is interesting to note that oxide semiconductors are also useful in various spheres of energy research. For instance, Zhang Singh et al have fabricated all oxide (WO3-NRs/Cu2O)-based heterojunction for photo-electrochemical water splitting (Zhang et al., 2017). In another report, a combination of oxide and nitride (Cu2O/InGaN) layers are used which reveal that a gradient in the energy band gap reduces the recombination rate of photo-induced electron-hole pairs (Alizadeh et al., 2020). Further, for visible light induced water splitting, Wei et al. have fabricated MoS2–CdS/WO3–MnO2 based heterostructure (Wei et al., 2020).
Another important aspect in fabricating a PV cell is the work function of the absorber layer. Since CdTe has a high work function, it is difficult to make an Ohmic back contact and the formation of a Schottky barrier takes place at the back contact (Bätzner et al., 2000). In order to overcome this problem is to decrease the work function of CdTe using suitable dopant materials. In the present study, the work function of the CdTe layer is lowered by Cu-doping and different metal back contacts (viz. Al and Mo) are used to check their influence on the efficiency of CdTe:Cu-based PV cells. In addition, a 10 nm-thick carrier selective layer (e.g. MoO3) is sandwiched between the CdTe:Cu absorber layer and the top transparent electrical contact in the form of ZnO:SnO2 (ZTO) (Singh et al., 2019a). Moreover, we have measured the relative nanoscale barrier height at each interface and shown the real-time photo-induced charge carrier dynamics across the ZTO/MoO3 and MoO3/CdTe:Cu interfaces using photo-KPFM (with high lateral resolution ~20 nm). Finally, we have measured the PCE of ZTO/CdTe:Cu/Mo heterojunction PV cells grown on pristine (pris)- and textured (txt)-Si substrates (Fig. 1) which show 3.3% and 4.1% cell efficiency, respectively. In contrast, after introducing a hole-blocking, MoO3, layer, a significant enhancement in the PCE values is observed, viz. 7.0% and 8.2% on pris- and txt-Si substrates, respectively. A comparative study shows that the ZTO/MoO3/CdTe:Cu/Mo/txt-Si hole-blocking solar cell shows the best power conversion efficiency (PCE) among all other heterostructures.
Section snippets
Experimental detail
Two different substrates namely pristine (pris)- and textured (txt)-Si were used to fabricate CdTe:Cu-based multi-junction hole-blocking solar cell having two different metal back contacts, e.g. Al and Mo. Prior to deposition, a p-Si(1 0 0) wafer was cut into slices (1 cm × 1 cm) and were ultrasonically cleaned in trichloroethylene, acetone, propanol, and de-ionized water for 5 min. each to remove the organic contaminations and air dried. Subsequently, the substrates were subjected to alkaline
Results and discussion
In order to find out a possible correlation between the charge transport and structural properties of different layers in all the heterostructures, AFM, SEM, EDX, and XRD studies are carried out after the growth of each individual layer. Fig. 2 (a)-(d) show the AFM images of Mo/pris-Si, CdTe:Cu/Mo/pris-Si, MoO3/CdTe:Cu/Mo/pris-Si, and ZTO/MoO3/CdTe:Cu/Mo/pris-Si, respectively. It is observed that the Mo film deposited on pris-Si substrate [Fig. 2 (a)] shows elongated columnar nanostructures
Conclusions
The study explores the possibility of enhancing the power conversion efficiency of CdTe-based solar cells by a suitable choice of the layer configurations. We have examined various configurations, viz. ZTO/CdTe:Cu/Mo/pris-Si, ZTO/MoO3/CdTe:Cu/Mo/pris-Si, ZTO/CdTe:Cu/Al/pris-Si, and ZTO/MoO3/CdTe:Cu/Al/pris-Si. In addition, we have studied the role of substrate texturing by replacing pris-Si substrates with txt-Si ones, while keeping all other layer configurations same. We have studied their
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Present address: Department of Materials Science and Engineering, Ajou University, Suwon 16499, South Korea.