Interdigitated back contact (IBC) architecture can yield among the highest silicon wafer‐based solar cell conversion efficiencies. Since both polarities are realized on the rear side, there is a ...definite need for a patterning step. Some of the common patterning techniques involve photolithography, inkjet patterning, and laser ablation. This work introduces a novel patterning technique for structuring the rear side of IBC solar cells using the enhanced oxidation characteristics under the locally laser‐doped n++ back surface field (BSF) regions with high‐phosphorous surface concentrations. Phosphosilicate glass layers deposited via POCl3 diffusion serve as a precursor layer for the formation of local heavily laser‐doped n++ BSF regions. The laser‐doped n++ BSF regions exhibit a 2.6‐fold increase in oxide thickness compared to the nonlaser‐doped n+ BSF regions after undergoing high‐temperature wet thermal oxidation. The utilization of oxide thickness selectivity under laser‐doped and nonlaser‐doped regions serves two purposes in the context of the IBC solar cell, first patterning rear side and second acting as a masking layer for the subsequent boron diffusion. Proof‐of‐concept solar cells are fabricated using this novel patterning technique with a mean conversion efficiency of 20.41%.
An industrially viable novel patterning technique for fabrication of interdigitated back contact solar cells using the enhanced oxidation characteristics under laser‐doped back surface field regions is studied. The utilization of oxide thickness selectivity under laser‐doped and nonlaser‐doped regions serves two purposes, first patterning rear side and second acting as a masking layer for the subsequent boron diffusion.
A fundamental limitation of the photocurrent of solar cells based on a blend of poly(2‐methoxy‐5‐(3′,7′‐dimethyloctyloxy)‐p‐phenylene vinylene) (MDMO‐PPV) and 6,6‐phenyl C61‐butyric acid methyl ester ...(PCBM) is caused by the mobility of the slowest charge‐carrier species, the holes in the MDMO‐PPV. In order to allow the experimentally observed photocurrents electrostatically, a hole mobility of at least 10–8 m2 V–1 s–1 is required, which exceeds the observed hole mobility in pristine MDMO‐PPV by more than two orders of magnitude. However, from space‐charge‐limited conduction, admittance spectroscopy, and transient electroluminescence measurements, we found a hole mobility of 2 × 10–8 m2 V–1 s–1 for the MDMO‐PPV phase in the blend at room temperature. Consequently, the charge‐carrier transport in a MDMO‐PPV:PCBM‐based solar cell is much more balanced than previously assumed, which is a necessary requirement for the reported high fill factors of above 50 %.
The charge‐carrier transport in bulk‐heterojunction solar cells based on a poly(phenylene vinylene) (MDMO) and a methanofullerene (PCBM) is shown to be much more balanced than previously assumed. (The Figure shows mobility versus the square root of the electric field.) Space‐charge build‐up is therefore not limiting the photoresponse of the solar cell and the typically observed high fill factors of 60 % are explained.
The dependence of the performance of OC1C10‐PPV:PCBM (poly(2‐methoxy‐5‐(3′,7′‐dimethyloctyloxy)‐p‐phenylene vinylene):methanofullerene 6,6‐phenyl C61‐butyric acid methyl ester)‐based bulk ...heterojunction solar cells on their composition has been investigated. With regard to charge transport, we demonstrate that the electron mobility gradually increases on increasing the PCBM weight ratio, up to 80 wt.‐%, and subsequently saturates to its bulk value. Surprisingly, the hole mobility in the PPV phase shows an identical behavior and saturates beyond 67 wt.‐% PCBM, a value which is more than two orders of magnitude higher than that of the pure polymer. The experimental electron and hole mobilities were used to study the photocurrent generation of OC1C10‐PPV:PCBM bulk‐heterojunction (BHJ) solar cells. From numerical calculations, it is shown that for PCBM concentrations exceeding 80 wt.‐% reduced light absorption is responsible for the loss of device performance. From 80 to 67 wt.‐%, the decrease in power conversion efficiency is mainly due to a decreased separation efficiency of bound electron–hole (e–h) pairs. Below 67 wt.‐%, the performance loss is governed by a combination of a reduced generation rate of e–h pairs and a strong decrease in hole transport.
The performance of polymer–fullerene bulk‐heterojunction solar cells has been investigated as a function of fullerene content. Electron and hole mobilities were measured separately in the blend (see Figure). The maximum power conversion efficiency is achieved at a high fullerene fraction, where light harvesting is less effective, which is explained using numerical simulations including the measured electron and hole mobilities.
In this work, we developed an in situ annealing process to crystallize boron-doped amorphous silicon a-Si(p + ) layers deposited by atmospheric pressure chemical vapour deposition (APCVD) to form ...boron-doped polycrystalline silicon poly-Si(p + ) layers. The influence of the temperature profiles during a-Si(p + ) inline deposition on structural, electrical, and passivation properties was studied in detail. The results show that a-Si(p + ) layers can be successfully crystallized by fine-tuning the temperature profiles in the postdeposition zones of the APCVD tool. It was observed that the hydrogenation processes during the fast firing play a significant role in enhancing the passivation quality as well as the electrical properties of the in situ annealed poly-Si(p + ) layers. The sheet resistance ( R sh ) and implied open circuit voltage ( iV oc ) of the best in situ annealed poly-Si(p + ) layers were found to be comparable to the ones that were ex situ annealed in the tube furnace at 950 <inline-formula><tex-math notation="LaTeX">^{\circ }</tex-math></inline-formula>C for 30 min. The sheet resistance of 200 <inline-formula><tex-math notation="LaTeX">\Omega</tex-math></inline-formula>/<inline-formula><tex-math notation="LaTeX">\square</tex-math></inline-formula> could be obtained on 150-nm thick poly-Si(p + ) layers with an ( iV oc ) of 718 mV. The use of this novel in situ annealing process to form poly-Si(p + ) layers opens a new horizon for a lean process sequence without the additional high-temperature annealing step for fabricating solar cells concepts based on passivating contact.
The edge recombination losses of crystalline silicon solar cells become significant when they are cut into smaller pieces to be assembled into modules. With the interdigitated pattern of doped ...<inline-formula><tex-math notation="LaTeX">p</tex-math></inline-formula> and <inline-formula><tex-math notation="LaTeX">n</tex-math></inline-formula> regions on the rear side, the interdigitated back contact (IBC) solar cells can be cut through different doped regions. In this study, the cutting losses in IBC solar cells are investigated and various cutting scenarios are studied. Through simulations and experimental measurements, it is found that the cut losses can be reduced by cutting through the back surface field rather than through the emitter. The losses under low light intensity are reduced to an even greater extent. When a 23% cell is cut into 1/3 pieces, the efficiency can be increased by 1.2%<inline-formula><tex-math notation="LaTeX">_\mathrm{rel}</tex-math></inline-formula> (cut related losses were improved from 2.0%<inline-formula><tex-math notation="LaTeX">_\mathrm{rel}</tex-math></inline-formula> to 0.8%<inline-formula><tex-math notation="LaTeX">_\mathrm{rel}</tex-math></inline-formula>) under standard 1-sun testing conditions, compared to cutting through the emitter. Under low light intensity of 0.25 sun, the improvement is around 2.4%<inline-formula><tex-math notation="LaTeX">_\mathrm{rel}</tex-math></inline-formula>. The improvement is mainly due to lower FF losses in the I-V characteristics, and this is further confirmed by Suns-<inline-formula><tex-math notation="LaTeX">V_{\text{oc}}</tex-math></inline-formula> and PL measurements. In the pFF analysis, the additional losses due to laser damage are also observed. This strategy of cutting through the BSF region in IBC solar cells can be quickly adopted in mass production without the need for additional processes or equipment and both module power and energy yield can be increased.
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.
A review of the processes and limitations that govern the device operation of polymer:fullerene bulk heterojunction solar cells is presented, with respect to the charge‐carrier transport and photogeneration mechanism. Dissociation of electron–hole pairs at the donor/acceptor interface is an important process that limits the charge generation efficiency (see figure).
The effect of controlled thermal annealing on charge transport and photogeneration in bulk‐heterojunction solar cells made from blend films of regioregular poly(3‐hexylthiophene) (P3HT) and ...methanofullerene (PCBM) has been studied. With respect to the charge transport, it is demonstrated that the electron mobility dominates the transport of the cell, varying from 10–8 m2 V–1 s–1 in as‐cast devices to ≈3 × 10–7 m2 V–1 s–1 after thermal annealing. The hole mobility in the P3HT phase of the blend is dramatically affected by thermal annealing. It increases by more than three orders of magnitude, to reach a value of up to ≈ 2 × 10–8 m2 V–1 s–1 after the annealing process, as a result of an improved crystallinity of the film. Moreover, upon annealing the absorption spectrum of P3HT:PCBM blends undergo a strong red‐shift, improving the spectral overlap with solar emission, which results in an increase of more than 60 % in the rate of charge‐carrier generation. Subsequently, the experimental electron and hole mobilities are used to study the photocurrent generation in P3HT:PCBM devices as a function of annealing temperature. The results indicate that the most important factor leading to a strong enhancement of the efficiency, compared with non‐annealed devices, is the increase of the hole mobility in the P3HT phase of the blend. Furthermore, numerical simulations indicate that under short‐circuit conditions the dissociation efficiency of bound electron–hole pairs at the donor/acceptor interface is close to 90 %, which explains the large quantum efficiencies measured in P3HT:PCBM blends.
The effect of controlled thermal annealing on charge transport and photogeneration in bulk‐heterojunction solar cells made from blend films of poly(3‐hexylthiophene) (P3HT) and methanofullerene is investigated. The hole mobility in the P3HT phase of the blend is dramatically affected by annealing (see Figure). An increase in this hole mobility is the most important factor leading to a strong enhancement of the efficiency.
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