Plastic solar cells bear the potential for large‐scale power generation based on materials that provide the possibility of flexible, lightweight, inexpensive, efficient solar cells. Since the discovery of the photoinduced electron transfer from a conjugated polymer to fullerene molecules, followed by the introduction of the bulk heterojunction (BHJ) concept, this material combination has been extensively studied in organic solar cells, leading to several breakthroughs in efficiency, with a power conversion efficiency approaching 5 %. This article reviews the processes and limitations that govern device operation of polymer:fullerene BHJ solar cells, with respect to the charge‐carrier transport and photogeneration mechanism. The transport of electrons/holes in the blend is a crucial parameter and must be controlled (e.g., by controlling the nanoscale morphology) and enhanced in order to allow fabrication of thicker films to maximize the absorption, without significant recombination losses. Concomitantly, a balanced transport of electrons and holes in the blend is needed to suppress the build‐up of the space–charge that will significantly reduce the power conversion efficiency. Dissociation of electron–hole pairs at the donor/acceptor interface is an important process that limits the charge generation efficiency under normal operation condition. Based on these findings, there is a compromise between charge generation (light absorption) and open‐circuit voltage (VOC) when attempting to reduce the bandgap of the polymer (or fullerene). Therefore, an increase in VOC of polymer:fullerene cells, for example by raising the lowest unoccupied molecular orbital level of the fullerene, will benefit cell performance as both fill factor and short‐circuit current increase simultaneously.
The origin of the enhanced performance of bulk heterojunction solar cells based on slowly dried films of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester is investigated, combining charge transport measurements with numerical device simulations. Slow drying leads to a 33-fold enhancement of the hole mobility up to 5.0×10−7m2V−1s−1 in the P3HT phase of the blend, thereby balancing the transport of electrons and holes in the blend. The resulting reduction of space-charge accumulation enables the use of thick films (∼300nm), absorbing most of the incoming photons, without losses in the fill factor and short-circuit current of the device.
The effect of iron contamination in multicrystalline silicon ingots for solar cells has been investigated. Intentionally contaminated p- and n-type multicrystalline silicon ingots were grown by adding 53 ppm by weight of iron in the silicon feedstock. They are compared to reference ingots produced from nonintentionally contaminated silicon feedstock. p-type and n-type solar cell processes were applied to wafers sliced from these ingots. The as-grown minority carrier lifetime in the iron doped ingots is about 1–2 and 6–20 μs for p and n types, respectively. After phosphorus diffusion and hydrogenation this lifetime is improved up to 50 times in the p-type ingot, and about five times in the n-type ingot. After boron/phosphorus codiffusion and hydrogenation the improvement is about ten times for the p-type ingot and about four times for the n-type ingot. The as-grown interstitial iron concentration in the p-type iron doped ingot is on the order of 1013 cm−3, representing about 10% of the total iron concentration in the ingot, and is reduced to below 1011 cm−3 after phosphorus diffusion and subsequent hydrogenation. The concentration of interstitial iron after boron/phosphorus codiffusion and hydrogenation is about 1012 cm−3, pointing out the reduced gettering effectiveness of boron/phosphorus codiffusion. The effect of the iron contamination on solar cells level is a decrease in the diffusion length in the top half of the ingots with a trend in agreement with Scheil’s model for segregation. This is, however, not the only impact of the iron. An increased crystal defect concentration in the top and bottom of the Fe doped ingots, compared to the reference ingots, is observed, which contributes considerably to the degradation of the solar cell performance.
We have developed a simple method to passivate industrially produced boron-doped emitters for n-type base silicon solar cells using an ultrathin (∼1.5nm) silicon dioxide layer between the silicon emitter and the silicon nitride antireflection coating film. This ultrathin oxide is grown at room temperature by soaking the silicon wafers in a solution of nitric acid prior to the deposition of the silicon nitride antireflection coating film. The n-type solar cells processed in such a way demonstrate a conversion efficiency enhancement of more than 2% absolute over the solar cells passivated without the silicon dioxide layer.
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