# Influence of Hot Carrier and Thermal Components on Photovoltage Formation across the p-n Junction
## 1 Introduction  
Renewable energy such as an electric one generated by solar cells is a promising and environmentally friendly energy. To harvest it more extensively, one needs to reduce the production price of a solar cell and to increase its conversion efficiency. The conversion efficiency of a single p-n junction semiconductor solar cell is fundamentally constrained by the theoretical Shockley-Queisser limit [1]. In spite of multiple efforts (see, e.g., [2, 3]) devoted to revise the limit, the theory assumes that only photons having energy close to a semiconductor forbidden energy gap \(E_{g}\) are used effectively, photons with energy below \(E_{g}\) are not absorbed at all (so called below \(E_{g}\) loss), and the residual energy of the higher energy photons left over after the electron-pair generation is reckoned only through the process of carrier thermalization, i.e., through the lattice heating (thermalization loss). These two intrinsic loss processes are mainly responsible for low efficiency of a single junction solar cell. For example, in a silicon cell, the below \(E_{g}\) loss amounts to 17% and the thermalization loss comes to 41%, and in a GaAs cell these both are equal to 34% and 26%, respectively [2]. Hot carriers (HCs) in semiconductors are free carriers with energy higher than the average one. The carriers can be heated by means of a DC electric field, microwave or optical radiation. The photovoltaic community keeps a lively ongoing interest in hot carrier phenomena [4; 5; 6; 7; 8; 9; 10; 11]. For good operation of a hot carrier solar cell, fast relaxation of carrier energy needs to be extended and the hot carriers need to be extracted through the selective contacts before they cool down to the bandgap edges. Despite the advances in theory and materials synthesis, practically hot carrier solar cell remains unrealized. One way of experimental detection of carrier heating can be implemented by laser radiation with photons having energy lower than a semiconductor forbidden energy gap. Such intraband light absorption initiates rise of the hot carrier photovoltage across a semiconductor potential barrier (Figure 1a; process 2). This kind of photoresponse was investigated in Ge [12], Si [13] and GaAs [14] l-h junctions illuminated with CO\({}_{2}\) laser radiation (photon energy 0.12 eV). It was shown [14] that the photovoltage consisted of two components, the slow and the fast one. The fast one was attributed to the HC photoresponse, while the slow one was assumed to result from the semiconductor lattice heating, i.e., from the carrier thermalization. Actually, the process of thermalization is also based on the hot carrier phenomenon since the hot carriers supply the lattice with this extra energy while cooling down to the bandgap edges. Low photon energy laser pulses also gave rise to HC photovoltage across Ge [15], Si [16; 17], GaAs [18; 19; 20], InSb [21] and HgCdTe [22] p-n junctions. It is worth noting that in the case of the narrow bandgap semiconductors (InSb and HgCdTe) exposed to the CO\({}_{2}\) laser light pulses, the classical photovoltage induced by electron-hole pair generation was observed simultaneously with the HC photoresponse. Additionally, both these photovoltages were observed across Si p-n junction illuminated with 1.49 \(\upmu\)m-long laser radiation [19] and across GaAs p-n junction exposed to 1.06 \(\upmu\)m wavelength light [19; 20]. The polarities of these two photovoltages are opposite. High-energy photons also create hot carriers as the excess energy turns the photogenerated carriers into the hot ones [20; 23], and these show a two-sided influence on the open circuit voltage of a p-n junction [23]. This paper deals with all three effects, i.e., hot carriers, thermalization and classical photogeneration, simultaneously induced by a laser pulse across the GaAs p-n junction.