Solar cells current-voltage

Fig. 5 Effect of the increase of the density of deep defects on the (a) total density of states, (b) the solar cell current voltage curve under illumination, (c) the current/voltage curve in the dark and (d) the ideality factor. For parameters, see Table 1...

The equivalent circuit diagram used to model solar cell current-voltage characteristics is shown at the top of Figure 1.1. The schematic energy level diagram of a DSSC at the bottom of Figure 1.1 shows the various charge transfer processes that occur in photoelectrochemical cells and relates these processes to current pathways via components of the model circuit. An illumination current density /l is induced upon photoexcitation of the... 

Figure 1.1 Simple equivalent circuit (top) for modeling solar cell current-voltage characteristics and energy level diagram (bottom) mapping the various charge transfer processes in a DSSC to the current pathways of the model circuit. The dominant mechanisms are described by a current density Jl induced upon photoexcitation and electron injection into the conduction band of the metal oxide semiconductor surface MO, linear (Jsh) and nonlinear (/jj) reverse current densities in parallel with photocurrent source and a series resistance to account for electrode and ionic resistances. In Section 1.2.2 M0 = Ti02, Sn02, X = Br, I.

Figure 1.8 Cell schematics for a regenerative solar cell based on (a) an n-type photoelectrode (b) ap-type photoelectrode. The top diagrams show the cell reactions under illumination, the middle diagrams the electronic energy levels and band bending, and the bottom diagrams the cell current-voltage (I-U) characteristics with the photoelectrode and counter electrode (CE) currents shown in the same quadrant. The maximum power point is located at the point on the current-voltage curve at which the rectangle of maximum area may be inscribed in this quadrant. The photovoltage V, the electron and hole quasi-Fermi levels E and fip and the solution Fermi level f o.R, the open-circuit potential Ugc of the photoelectrode and the standard redox potential 17 ° of the 0,R redox couple are also shown.

Under both short-circuit and open-circuit conditions, a solar cell produces no electric power, the power is consumed internally in the cell and is dissipated as heat. When a resistive load is connected to a cell in sunlight, a photogenerated voltage, F, is induced across the load and a current flows through it. The existence of requites that the flow of majority carriers be reduced from that in the open-circuit condition there must be a higher battier potential than in the open-circuit case (Fig. 2d). This higher barrier potential (V6 — ) indicates a smaller reduction from Since the photogenerated... 

The photogenerated current is in the same direction as /, but is always less than because the battier potential under load conditions is always less than F, which results in a larger flow of majority carriers than that in a short-circuited cell. Thus, when a solar cell is under load, the current and voltage are always less than and lU, respectively this condition is the curve-factor loss. Depending on the characteristics of the particularp—n junction and on the cell operating conditions, there is an optimal load resistance that maximizes the power output of the cell, ie, the product of its current and voltage. 

In inaccessible regions where an impressed current installation is not sufficiently close to a low voltage supply, the protection current can be supplied from batteries, thermogenerators, and if there is sufficient radiation from the sun, solar cells. Wind generators and diesel units, on the other hand, are less suitable because of the maintenance necessary for continuous operation. 

Photovoltaic systems for specific applications are produced by connecting individual modules in series and parallel to provide the desired voltage and current (Figure 4). Each module is constructed of individual solar cells also connected in series and parallel. Modules are typically available in ratings from a few peak watts to 250 peak watts. 

The optical properties of electrodeposited, polycrystalline CdTe have been found to be similar to those of single-crystal CdTe [257]. In 1982, Fulop et al. [258] reported the development of metal junction solar cells of high efficiency using thin film (4 p,m) n-type CdTe as absorber, electrodeposited from a typical acidic aqueous solution on metallic substrate (Cu, steel, Ni) and annealed in air at 300 °C. The cells were constructed using a Schottky barrier rectifying junction at the front surface (vacuum-deposited Au, Ni) and a (electrodeposited) Cd ohmic contact at the back. Passivation of the top surface (treatment with KOH and hydrazine) was seen to improve the photovoltaic properties of the rectifying junction. The best fabricated cell comprised an efficiency of 8.6% (AMI), open-circuit voltage of 0.723 V, short-circuit current of 18.7 mA cm, and a fill factor of 0.64. 

Fig. 3.17. Structure of the 9.35%-efflcient Basol solar cell, with open-circuit voltage 0.73 V, short-circuit current 20 mA cm, and fiU factor 0.64. (Reproduced from [86])...

FIG. 60. Current-voltage characteristics of a solar cell made at 65 MHz and 42 mW/cm-, The dashed line indicates the maximum-power point. 

The current-voltage I-V) characteristic of a solar cell is given by... 

Illumination of solar cells causes a reduction of efficiency and fill factor, as a result of light-induced creation of defects (Staebler-Wronski effect. Section 1.1.2.5). This reduction is halted after several hundred hours of illumination. The reduction is correlated with solar cell thickness. A large intrinsic layer thickness leads to a large reduction of efficiency and fill factor compared to a small intrinsic layer thickness. The solar cell properties can be completely recovered by annealing at about 150°C. The open circuit voltage and short circuit current decrease only slightly. 

Apart from recapture of the injected electrons by the oxidized dye, there are additional loss channels in dye-sensitized solar cells, which involve reduction of triiodide ions in the electrolyte, resulting in dark currents. The Ti02 layer is an interconnected network of nanoparticles with a porous structure. The functionalized dyes penetrate through the porous network and adsorb over Ti02 the surface. However, if the pore size is too small for the dye to penetrate, that part of the surface may still be exposed to the redox mediator whose size is smaller than the dye. Under these circumstances, the redox mediator can collect the injected electron from the Ti02 conduction band, resulting in a dark current (Equation (6)), which can be measured from intensity-modulated experiments and the dark current of the photovoltaic cell. Such dark currents reduce the maximum cell voltage obtainable, and thereby the total efficiency. 

The total efficiency of the solar cell depends on the current, voltage, and the fill factor of the cell as shown in Equation (9). Many groups have focused on the development of new sensitizers,... 

The PV characteristics of the CIGS2 thin-film solar cell on opaque Mo back contact, as measured at the NREL under AM 1.5 conditions, were as follows short-circuit current density sc of 20.88 mA/cm2, open-circuit voltage 1% of 830.5 mV, fill factor FF of 69.13%, and PV conversion efficiency // of 11.99%. 

He, I. Zhong, C. Huang, X. Wong, W.-Y. Wu, H. Chen, L. Su, S. Cao, Y., Simultaneous enhancement of open-circuit voltage, short-circuit current density, and fill factor in polymer solar cells. Adv. Mater. 2011, 23, 4636-4643. 

---