# A Machine Learning Approach for Solar Power Technology Review and Patent Evolution Analysis
## 2 Literature Review
### Solar Power Cells and Energy Storage
Silicon heterojunction solar cells offer additional cost benefits compared to conventional crystalline silicon solar cells. [PERSON] et al. [PERSON] et al. (2018) analyzed the current cost breakdown of heterojunction designs using life-cycle costing and compared the results to conventional diffused junction monocrystalline silicon modules. The study showed that improvements in cell processing and module design results in a significant drop in production costs. The replacement of indium-tin-oxide was not found to contribute substantially to a reduction in module costs. Stand-alone PV systems require energy storage to supply continuous energy when there is insufficient or no solar radiation. Valve Regulated Lead Acid (VRLA) batteries are sometimes used but supplying a large burst of current such as motor startup degrades the battery plates and can destroy the battery. A method of supplying large amounts of constant current is to combine VRLA batteries with super capacitors to form a hybrid storage system where the super capacitor supplies instant power to the load [PERSON] et al. [PERSON] et al. (2018) proposed a type of solar battery material called 2D cyanimide-functionalized polyheptazine imide (NCN-PHI) which combines light harvesting and electrical energy storage within one single material. The charge storage of NCN-PHI is based on the photo reduction of the carbon nitride and the charge is stored by adsorption of alkali metal ions within the NCN-PHI layers. The photo reduced carbon nitride can thus be described as a battery anode working as a pseudo capacitor, which can store light-induced charge by trapping electrons for few  
Figure 4: Schematic representation of a hybrid system and key components. Figure 3: Schematic view of an on-grid system and key components. hours. The feasibility of light-induced electrical energy storage and release on demand by a single component light charged battery provides a unique solution for energy storage. [PERSON] and [PERSON] [34] in 2012 deployed a dataset of solar photovoltaic patents filed in Taiwan, Korea and China over the last 24 years (1984-2008). Their analysis of the knowledge in these patents resulted in a set of 12 International Patent Classification (IPC) technology categories. Commonalities in patterns of knowledge between solar photovoltaic and earlier industries are demonstrated. This study first identifies a comprehensive patent dataset for solar PV technologies then differentiates three generations using a three-stage patent extracting methodology. Scientific linkage is applied to investigate the development of knowledge flows for technologies such as solar cells and examines the causes and effects underlying the pursuit of this knowledge. By reviewing literature, the ontology of solar power is constructed. Solar power generation technology is divided into three parts, PV technology that uses the photoelectric effect to directly transform sunlight to electricity, concentrated solar power that heats water into steam to power machines such as power turbines, and storage systems (e.g., batteries) for uninterrupted supply of electricity when sunlight is not available. Figure 5 illustrates the concepts and newly derived technology structure identified by the comprehensive reviews. The ontology schema, as a structured knowledge map, is iteratively constructed mainly from literature reviews (and can be updated by the state-of-the-art patent reviews), as detailed descriptions of key solar technologies and their relationships.