Energy is one of the essential and crucial requirements in our day-to-day life. Coal, gas, fossil oils are only the naturally occurring energy sources. However, with increasing demand on energy there will be an energy crisis in near future, in fact, the chaos has already started. Developing countries, like India, whose population is increasing irresistibly, are going to affect more. Thus, researchers are already started to find an alternative source of energy (like Solar Cell). According to the law of conservation of energy, energy can neither be created nor destroyed, rather, it transforms from one form to another. Utilization of solar energy is an alternative way to solve the energy crisis problem.
Third generation solar cell based on light harvester materials sensitized TiO2 films have received much attention in research due to its low-cost effect as compared to the silicon-based solar cell. Initially different organic and inorganic dye molecules are used as a light harvester, which refers to the dye-sensitized solar cell (DSSC). However, recently inorganic and organic semiconductors have been chosen as sensitizer due to their exciting properties like higher extinction coefficient, optical tunability, long excited state charge carrier lifetime etc. Especially, lower band gap II-VI and IV-VI semiconductor materials, which absorb light in the visible and near infrared regions are widely used as light harvesters in the quantum dot solar cell (QDSC). Quantum Dots (QDs) are semiconductor nanocrystal having the size in the range of few nm. Due to smaller in size, the charge carriers (e– and h+) become confine in the size regime, which helps to engineer a device. Apart from this, some narrow band-gap ternary semiconductor materials like CuInS2, CuInSe2, CuInGaSe2, and of course, organic-inorganic perovskite have been extensively used as a sensitizer in the low-cost third generation solar cell. It has been observed that in ternary alloy QDs, the optical tunability not only depends on the size of the QDs but also on the composition of the constituents. Therefore, composition plays an extra degree of freedom towards the optical and photophysical properties of the ternary QDs. However, to enhance photocurrent efficiency it is necessary to optimize all the processes after characterization and measure the photovoltaic performance of a real cell.
In QDSC, the light harvesting materials are deposited on TiO2 sintered film. The mechanism of QDSC mainly involves in two major processes, however, there are different other pathways. Firstly, (Step 1) the electrons in QD are excited by absorbing solar radiation. The excited QDs inject electrons into the conduction band (CB) of the TiO2. Therefore, it is crucial to choose the QD whose CB is energetically higher than that of TiO2 for efficient electron transfer. The separated electrons are connected through a circuit to achieve the electricity. Secondly, (Step 2) the photo-excited hole in the valence band of the QD needs to be neutralized to complete the redox cycle. Polysulfide solution and Cu2S deposited ITO (Indium-doped tin oxide) are used as an electrolyte (hole neutralizer) and photocathode, respectively. The mechanistic pathways of a typical QDSC are schematically illustrated in Figure 1. II-VI inorganic semiconductor like CdS, CdSe and their alloy (CdSxSe1-x) can be used as photoanode in QDSC. These QDs materials are deposited on TiO2 sintered photo-anode individually for solar cell testing in terms of current versus voltage measurement. It is observed that CdS, CdSe, and CdSxSe1-x (x = 0.7) show 1.1 (±0.07)%, 3.36 (± 0.1)%, and 4.5 (± 0.12)% solar to electrical energy conversion efficiency, respectively. The reason for higher photocurrent conversion efficiency of CdSxSe1-x is due to higher light absorption in the solar region and higher excited state charge carrier lifetime. The CdS QDs absorb light at ~ 380-420 nm region while CdSe and CdSxSe1-x absorb a similar portion of the solar spectrum (550-600 nm). However, the higher performance of the CdSxSe1-x alloy is due to higher excited state electron lifetime, which helps to extract more electron in the external circuit. 
Related YouTube Video: