5 research outputs found
Direct and charge transfer mediated photogeneration in polymer-fullerene bulk heterojunction solar cells
We investigated photogeneration yield and recombination dynamics in blends of
poly(3-hexyl thiophene) (P3HT) and
poly[2-methoxy-5-(30,70-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV)
with [6,6]- phenyl-C61 butyric acid methyl ester (PC61BM) by means of
temperature dependent time delayed collection field (TDCF) measurements. In
MDMO-PPV:PC61BM we find a strongly field dependent polaron pair dissociation
which can be attributed to geminate recombination in the device. Our findings
are in good agreement with field dependent photoluminescence measurements
published before, supporting a scenario of polaron pair dissociation via an
intermediate charge transfer (CT) state. In contrast, polaron pair dissociation
in P3HT:PC61BM shows only a very weak field dependence, indicating an almost
field independent polaron pair dissociation or a direct photogeneration.
Furthermore, we found Langevin recombination for MDMO-PPV:PC61BM and strongly
reduced Langevin recombination for P3HT:PC61BM.Comment: 4 pages, 3 figure
Photocurrent in Organic Solar Cells
A quite new approach to low-cost mass production of flexible solar cells are organic photovoltaics. Even though the device efficiencies increased rapidly during the last years, further imporvements are essential for a successful market launch. One important factor influencing the device efficiency is the photocurrent of a solar cell, which is defined as the difference between the current under illumination and in the dark. In case of organic bulk heterojunction (BHJ) solar cells it is â in contrast to inorganic devices â dependent on the applied bias voltage. The voltage dependence results in a reduced fill factor and thus an even more pronounced influence of the photocurrent on the device efficiency. It is therefore crucial to understand the underlying processes determining the photocurrent in order to be able to further improve the solar cell performance. In a first step the photocurrent of P3HT:PC61BM devices was investigated by a pulsed measurement technique in order to prevent disturbing influences due to device heating under continous illumination. The resulting photocurrent was hyperbolic tangent like and featured a point symmetry, whose origin and meaning were discussed. In addition, the photocurrent was described by a combined model of BraunâOnsager and SokelâHughes theory for field dependent polaron pair dissociation and charge extraction, respectively. After this macroscopic view on the photocurrent, the focus of this work moves to the more basic processes determining the photocurrent: charge photogeneration and recombination. In a comparative study the field-dependence of these was investigated by time-delayed collection field (TDCF) measurements for two well-known reference systems, namely P3HT:PC61BM and MDMO-PPV:PC61BM. It was possible to identify two different dominating scenarios for the generation of free charge carriers. The first one â via a thermalized charge transfer state (CTS) â is clearly influenced by geminate recombination and therefore less efficient. In the second scenario, the free charge carriers are either generated directly or via an excited, âhotâ CTS. In addition, clear differences in the nongeminate recombination dynamics of both material systems were found. Similar studies were also be presented with two modern low bandgap polymers which only differ by the bridging atom in the cyclopentadithiophene (PCPDTBT:PC71BM vs. Si-PCPDTBT:PC71BM). Such small changes in the chemical structure were already sufficient to affect the charge photogeneration as well as the morphology of the blend. These findings were set into relation to currentâvoltage characteristics in order to discuss the origin of the clear differences in the solar cell performance of both materials. Another crucial parameter limiting the solar cell efficiency is the builtin potential of a device. Within the range of semiconducting pn-junctions, MottâSchottky analysis is an established method to determine the built-in potential. As it was originally derived for abrupt pn-junctions, its validity for organic BHJ solar cells â a bipolar, effective medium â was discussed. Experimental findings as well as the contradictions to MottâSchottky theory indicated, that a direct transfer of this method to organic photovoltaics is not appropriate. Finally, the results obtained in the framework of the MOPS-project (Massengedruckte Organische Papier-Solarzellen) will be presented, in which the first completely roll-to-roll printed paper solar cells were realized
Photostrom in Organischen Solarzellen
A quite new approach to low-cost mass production of flexible solar cells are organic photovoltaics. Even though the device efficiencies increased rapidly during the last years, further imporvements are essential for a successful market launch. One important factor influencing the device efficiency is the photocurrent of a solar cell, which is defined as the difference between the current under illumination and in the dark. In case of organic bulk heterojunction (BHJ) solar cells it is â in contrast to inorganic devices â dependent on the applied bias voltage. The voltage dependence results in a reduced fill factor and thus an even more pronounced influence of the photocurrent on the device efficiency. It is therefore crucial to understand the underlying processes determining the photocurrent in order to be able to further improve the solar cell performance. In a first step the photocurrent of P3HT:PC61BM devices was investigated by a pulsed measurement technique in order to prevent disturbing influences due to device heating under continous illumination. The resulting photocurrent was hyperbolic tangent like and featured a point symmetry, whose origin and meaning were discussed. In addition, the photocurrent was described by a combined model of BraunâOnsager and SokelâHughes theory for field dependent polaron pair dissociation and charge extraction, respectively. After this macroscopic view on the photocurrent, the focus of this work moves to the more basic processes determining the photocurrent: charge photogeneration and recombination. In a comparative study the field-dependence of these was investigated by time-delayed collection field (TDCF) measurements for two well-known reference systems, namely P3HT:PC61BM and MDMO-PPV:PC61BM. It was possible to identify two different dominating scenarios for the generation of free charge carriers. The first one â via a thermalized charge transfer state (CTS) â is clearly influenced by geminate recombination and therefore less efficient. In the second scenario, the free charge carriers are either generated directly or via an excited, âhotâ CTS. In addition, clear differences in the nongeminate recombination dynamics of both material systems were found. Similar studies were also be presented with two modern low bandgap polymers which only differ by the bridging atom in the cyclopentadithiophene (PCPDTBT:PC71BM vs. Si-PCPDTBT:PC71BM). Such small changes in the chemical structure were already sufficient to affect the charge photogeneration as well as the morphology of the blend. These findings were set into relation to currentâvoltage characteristics in order to discuss the origin of the clear differences in the solar cell performance of both materials. Another crucial parameter limiting the solar cell efficiency is the builtin potential of a device. Within the range of semiconducting pn-junctions, MottâSchottky analysis is an established method to determine the built-in potential. As it was originally derived for abrupt pn-junctions, its validity for organic BHJ solar cells â a bipolar, effective medium â was discussed. Experimental findings as well as the contradictions to MottâSchottky theory indicated, that a direct transfer of this method to organic photovoltaics is not appropriate. Finally, the results obtained in the framework of the MOPS-project (Massengedruckte Organische Papier-Solarzellen) will be presented, in which the first completely roll-to-roll printed paper solar cells were realized.Ein relativ neuer Ansatz fuÌr eine guÌnstige Massenproduktion flexibler Solarzellen ist dabei die organische Photovoltaik. Obwohl die Wirkungsgrade in den letzten Jahren schnell anstiegen, sind weitere Verbesserungen fuÌr eine erfolgreiche MarkteinfuÌhrung dringend nötig. Ein wichtiger Faktor ist dabei der Photostrom einer Solarzelle, der als Differenz zwischen Hell- und Dunkelstrom definiert ist. Im Gegensatz zu anorganischen Solarzellen ist dieser im Falle der organischen âbulk heterojunctionâ(Heterogemisch, Abk.: BHJ) Solarzellen von der angelegten Spannung abhĂ€ngig. Dies fuÌhrt zu einer Reduzierung des FuÌllfaktors und so zu einem noch stĂ€rkeren Einlufss des Photostroms auf die Leistung der Solarzelle. Es ist daher Ă€uĂerst wichtig die grundlegenden, den Photostrom bestimmenden Prozesse zu verstehen, um die Leistung der organischen Solarzellen weiter steigern zu können. ZunĂ€chst wurde der Photostrom von P3HT:PC61BM Solarzellen mittels einer gepulsten Messmethode untersucht, die störende EinfluÌsse durch das ErwĂ€rmen der Probe unter kontinuierlicher Beleuchtung verhindern soll. Der resultierenden Photostrom wies einen dem Tangens Hyperbolicus Ă€hnlichen Verlauf auf und zeigte dabei eine Punktsymmetrie, deren Ursprung und Bedeutung im Verlauf dieser Arbeit genauer diskutiert werden. FuÌr die Beschreibung des spannungsabhĂ€ngigen Photostroms wird auĂerdem ein kombiniertes Modell vorgestellt, welches auf den Theorien von BraunâOnsager und SokelâHughes fuÌr die feldabhĂ€ngige Polaronenpaartrennung bzw. die LadungstrĂ€gerextraktion basiert. Nach der makroskopischen Betrachtung des Photostroms wird sich der Fokus dann auf die grundlegenden, den Photostrom bestimmenden Prozesse verschieben: Photogenerierung und Rekombination der LadungstrĂ€ger. Die FeldabhĂ€ngigkeit dieser Prozesse wurde dabei mittels time-delayed collection field (TDCF) Messungen an den beiden Referenz-Systemen P3HT:PC61BM und MDMO-PPV:PC61BM untersucht. Dadurch lieĂen sich neben deutlichen Unterschieden in der nichtgeminalen Rekombinationsdynamik freier LadungstrĂ€ger auch bei deren Photogeneration zwei unterschiedliche dominierende Prozesse identifizieren: Im ersten Szenario werden freie LadungstrĂ€ger uÌber einen relaxierten Ladungstransferzustand (âcharge transfer stateâ âCTS) generiert. Dieser Prozess ist jedoch durch einen deutlichen Einfluss der geminalen Rekombination stark feldabhĂ€ngig und somit weniger effizient. Im zweiten Szenario werden die freien LadungstrĂ€ger entweder direkt oder uÌber einen angeregten (âhotâ) CTS erzeugt. Ăhnliche Versuche wurden zudem fuÌr zwei neuartige Polymere mit niedrigen BandluÌcken prĂ€sentiert, die sich jeweils nur durch das BruÌckenatom im Cyclopentadithiophen unterscheiden (PCPDTBT:PC71BM im Vergleich zu Si-PCPDTBT:PC71BM). Dies hatte jedoch deutliche Auswirkungen auf die Photogeneration freier LadungstrĂ€ger und die Morphologie der aktiven Schicht. Die entsprechenden Ergebnisse wurden dann in Relation zu den StromâSpannungs-Kennlinien gesetzt, um die deutlichen Unterschiede in der Effizienz der Solarzellen zu diskutieren. Ein weiterer wichtiger, die Leistung einer Solarzelle begrenzender Parameter ist deren Diffusionsspannung (built-in potential, VBi). In der Physik halbleitender pn-Ăbergange ist die MottâSchottky Analyse eine etablierte Methode um VBi zu bestimmen. Diese wurde urspruÌnglich fuÌr abrupte pn-ĂbergĂ€nge hergeleitet, weshalb hier deren GuÌltigkeit fuÌr organische BHJ Solarzellen â und damit ein bipolares, effektives Medium â diskutiert wird. Die experimentellen Ergebnisse ebenso wie die WiderspruÌche zur MottâSchottky Theorie deuten darauf hin, dass eine direkte Ăbertragbarkeit dieser Methode auf organische BHJ Solarzellen nicht gegeben ist. AbschlieĂend werden noch die Ergebnisse des MOPS-Projekts (Massengedruckte Organische Papier-Solarzellen) prĂ€sentiert, in dessen Verlauf die ersten komplett auf Papier gedruckten Solarzellen entwickelt wurden