Despite increasing applications in various fields, semiconductor nanocrystals (NCs) are now the most widely used materials for low-cost, high-efficiency photovoltaic cells (PV). These crystals can be synthesized and processed using solution-based techniques in various solar-powered energy models. These NCs can also be used in charge multiplication of single photons to improve power conversion efficiency. For example, by generating more power, the NC-size controlled energy gap can adjust the absorption spectrum that is best suited for solar radiation at 500nm.

Unfortunately, there are still significant challenges to successfully charge extraction/injection and carrier transport in terms of development methods. Due to the presence of some organic elements, silicon-based NCs also have poor environmental and photostability characteristics. Today's PV industry is primarily based on crystalline silicon (c-Si) cells; however, due to cost considerations, the PV market segment has begun utilizing amorphous silicon (a-Si), resulting in rapid market expansion.

Ⅰ.  Experimental Model

During this experiment, various devices were studied made of wide-gap CdSe NCs or narrow-gap PbS NCs. During fabrication, CdSe NCs capped with trioctylphosphine oxide (TOPO) and trioctylphosphine (TOP), precipitated solvents washed with methanol and later dried under an argon flow. As PbS are extremely prone when degraded to oxygen, all the fabrication processes were of the NCs while they were incorporated into the devices. The NCs made of PbS were synthesized and precipitated with toluene/ethanol and stored as a powder in dark at -37 °C in a glovebox refrigerator.

To make contact with the fabricated devices, the aluminium and indium tin oxide (ITO) were bonded using copper paint. The structure of this device was then covered with a glass slide and then sealed from all sides and later kept for solidification for twelve hours. For the test results, optical and PV fabricated outputs were studied under air with ambient conditions.

Ⅱ.  Hybrid a-Si/CdSe NC Structures

Fig 1: Cross-sectional structure of Si/CdSe

As shown in Fig 1, a PV structure made of Si and CdSe NCs is depicted. It consists of a layer of 90-150 nm thickness of pyridinecapped CdSe NCs that have been sandwiched between ITO and Si layers. For photocurrent measurements, the structure was made complete using an Al electrode. Higher resolution TEM images were taken and it could be observed that NC integrity was preserved during silicon film growth which was later confirmed by spectroscopic studies.

Fig 2: Size dependant energies of the NCs

As shown in fig 2, the energies of the conduction and valence band edges (denoted by CB and VB) for a silicon NC is the lowest. The induced confinement shifts can be quantized for bulk semiconductor band edges. This can be expressed as”

∆Ee(h) ) = (Eg - Eg,0)mh(e)(mh + me)-1 , 18 

From the above equation, where Eg,0  is the bulk semiconductor energy gap, me and mh are the electron and hole effective masses.

Ⅲ.  Hybrid a-Si/PbS NC Structures

In the following experiment comprising of CdSe NCs, the outcome after PV response was completely due to the NCs, and the photons in the a-Si film don't contribute to the photocurrent. The overall PV performance could be improved using structures, wherein PV response is made due to the charge carriers produced in the NC and a-Si layers.

In the above figure, electrons photogenerated in PbS NCs can be transferred into amorphous silicon and then collected at the Al electrode. Furthermore, alignment of the valence-band states at the PbS NC/a-Si interface sides hole transfer from silicon into the NCs. The PbS NC-based devices display PV performance which is significantly improved in comparison to that of structures consisting of CdSe NCs. One of the main factors contributing to this improvement is the increased spectral coverage. Especially, the use of narrow-gap NCs allows for extending the device's operational range into the IR region and thus efficiently harvesting low-photons. Due to which, this fabrication model outdid the earlier model in terms of performance. These devices also showed high quantum efficiencies in the visible region, which is a result of both a steep increase in absorbtivity and contribution from carriers generated in the a-Si layer. It was also noted in the I-V characteristics of PbS NC-based devices that the transformation from diode-like to Ohmic (low-resistance resistor) occurred once exposed to air within minutes. This rapid degradation in the PV performance is not accompanied by any drastic changes in the optical properties i.e. absorption spectra.

Another additional factor contributing to enhanced PV performance is better charge transport properties of PbS NC films when compared to CdSe NCs films. However, the change of original ligands with shorter, bifunctional molecules of EDT increases the photocurrent by ca.2 orders of magnitude. Due to this, EDT-treated NC films show photoconductivity which in comparison to that of a-Si is a few orders of magnitude lesser. One of the important features of hybrid NC/a-Si devices demonstrated in the experiment is great flexibility in controlling their electronic and optical properties that can be used to enhance PV characteristics.

Moreover, the potential enhancement in PV performance is the encapsulation of NCs into a-Si that separates them from the environment and improve their stability with regard to oxidation which is crucial in the case of lead salt NCs.

Ⅳ. Conclusion

In this experiment, functional PV structures that combine colloidal NCs with amorphous silicon is proposed. The response of these devices can be controlled by varying energies of electronic states in NCs either replacing the NC size or the composition. The structures containing nanoparticles of CdSe demonstrate PV response due to the NC device component based on the energy offsets at the a-Si/NC interface. When the NC size is changed, control on the efficiency of charge transfer across the a-Si interface could be achieved thus, generating photocurrent. This model can also be used to evaluate the field of magnetron sputtering, where fabrication methods for hybrid PV devices can be done using colloidal NCs.