Most of the solar panels that I have seen do not have any mirrors, etc., but usually solar cookers have mirrors. What is the reason for solar panels not having focusing mirrors?
The simple answer is that the two devices work in completely different ways.
Solar cookers, as well as so-called 'solar thermal collectors', focus the light of the sun to heat something (a pot in a cooker, some oil or ceramics) and the heat is then transferred somewhere, where it generates electricity, usually by some steam engine. So, the more heat, the better.
Solar panels on the other hand use the photovoltaic effect, which directly converts light into electric energy. Light excites electrons to the conduction band and the current is then transmitted somewhere. Too much heat, however, destroys the materials used, so focusing might be a very bad idea.
Although the other answers are correct, I think they are missing the main point. Ordinarily, solar panels work regardless of their orientation. Their performance may be optimal when oriented directly toward the Sun, but if the Sun shines at 45°, 90°, or when it's cloudy, they still receive photons and still generate electricity.
Focussing mirrors, however, need to be directly exactly toward the Sun to be of use. To use them on a solar photovoltaic array would require the user to constantly rotate the array, requiring a Sun tracker and considerable mechanical overhead. For a cooker, that is used for perhaps half an hour or so, this is no problem. But solar PV arrays should operate all day, while the Earth rotates causing the relative position of the Sun to change.
Finally, sunlight is not scarce. If we need more electricity, it is usually cheaper to add more solar panels, than to go through the full overhead of adding focussing mirrors and infrastructure to rotate our entire array.
The concept of using mirrors to focus light on a photovoltaic cell has been considered.
The relative costs of the mirrors versus the cells needs to be considered.
Some light will be lost due to imperfect reflection and geometry.
Also, if the light is too intense the cell could be damaged or saturated.
Every so often there are products with some kind of focusing equipment. Some concepts use mirrors, and others use cheap plastic lenses.
Aside from the "damage" described in the other answers, there are a few other compelling reasons not to take this approach:
Cost: this might be less intuitive, but manufacturing and installing solar panels is a pretty straight forward process. In a lot of ways, solar panels are now commodity hardware modules. You want the the manufacturing process to be simple, the shipping to be simple, and the installation to be simple as well. Lets assume you could squeeze out an extra 10-20% more wattage (significantly more than this might cause overheating). At a current cost of 0.70 USD/watt on the low end for low end solar panels, it would potentially be cost effective for a large 200-watt panel (200*0.70 = 140 USD) if the wholesale cost of the mirrors would only add up to 20% the cost, so in this case $28 USD. My hunch is that the costs associated with installation and shipping would make this less competitive, but I can't speak to those numbers.
damage: I don't think that most PVs could handle more than 20% extra sunlight without additional temperature regulating hardware. You could add heat sinks and/or fans, but this equipment would add to your cost. Even if you could maintain a comfortable temperature, how would the lifetime of the PV cell change? (I'm not sure.) By the way, it also possible to harness the heat coming off of your PV cells and use that energy to provide hot water or heat your home. This would make the question a lot more complicated, so I won't go into it.
installation and maintenance: a flat structure is much easier to maintain, less chance of breaking, easier to clean. And if one mirror/panel breaks, gets dirty, or gets more sun than the others you're output is the same as it would be without mirrors because of #4:
shading and the effects of uneven exposure: wikipedia has a good piece on this, http://en.wikipedia.org/wiki/Photovoltaic_system#Shading_and_dirt , to summarize: "Photovoltaic cell electrical output is extremely sensitive to shading. The effects of this shading are well known. When even a small portion of a cell, module, or array is shaded, while the remainder is in sunlight, the output falls dramatically due to internal 'short-circuiting' (the electrons reversing course through the shaded portion of the p-n junction)." If the opposite is true, that one PV cell gets extra sunlight relative to the others, the same limiting effect would be observed. So if you put mirrors on one panel in an array and not the others, it would have no effect really.
space: this is not always a concern, but for many projects the goal is to cram as much wattage into the usable space. Mirrors are inherently less efficient than additional photovoltaic cells.
Although the shortfalls of focusing more light on the array have been described, a similar question is why you would not mount mirrors to reflect sunlight toward the array only when the incident angle is well off normal. This might provide some of the advantage of tracking the angle of the sun during the day. I think in this case the placement and size of the mirrors would not be practical for some of the same reasons as described above. They would be above the array and difficult to mount, would likely block the sun during part of the day, and would also likely reflect at a neighbor's house during part of the day. The people without any of these issues form too small of a market to see ads for big mirrors.
The problem with mirrors will be pointing them constantly to the panels. Constant Heat reduces the output of a panel and too much light will burn the panel and shorten it's life. If you added mirrors you would bake the panels in full sun.
And solar cookers have them because they are used for only a limited time and no damage is caused. Besides using mirrors would result in efficient cooking
Solar panels are semiconductors. The sunlight pushes charge carriers "up stream" - crossing the junction in the direction of higher energy.
When a semiconductor gets warm, charge carriers are excited into the conduction band - this causes leakage of the charge. The hotter the device the worse this gets - until you get to a point where you irreversibly change the material properties.
Here is a simile:
Imagine you are trying to move water from below the dam to above the dam so you can run a hydroelectric power station. The dam is the junction, the water is the charge carriers, and the incident light provides the energy to carry the water upstream. Heat makes the dam leaky - if you like it creates waves on the lake so water sloshes over the dam. And when the waves get too big the dam breaks.
Something else that I don't think has been mentioned - most solar panels are limited by the lowest power generated by any cell in the array. So, to boost the power by reflection, you would have to ensure that the reflected light covered the entire panel to gain anything from it. This of course reinforces what has been suggested - adding power by adding reflectors is not as simple as it sounds. It adds a great deal of complexity to the design of the system. Adding another panel is likely to be more economical both from the direct costs and labor/design costs.
Solar Cookers have to concentrate a volume of light into an area, thus providing more intensity/heat at that point (which depends on amount of photons).
Solar Panels work not by the amount of photons but by the frequency of photons.
Photon (quantum-mechanical) energy is proportional to the photion wave frequency (not the amount/number of photons). On the other hand photon intensity/luminosity IS proportional to the number of photons.
The photovoltaic effect is directly related to the photo-electric effect (a-la Einstein) and the relation of photon energy to photon frequency.
In this respect solar cookers need a way to concentrate more photons on a specific area, wheareas solar panels dont and that is also why solar panels can generate current even on cloudy day, unlike solar cookers.